Luminescence

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Alexander Schinabeck • Michele Sessolo • Man-Chung Tang. Shi Tang • Daniel ... Markus J. Leitl, Daniel M. Zink, Alexander Schinabeck, Thomas Baumann,.
Topics in Current Chemistry Collections

Nicola Armaroli Henk J. Bolink Editors

Photoluminescent Materials and Electroluminescent Devices

Topics in Current Chemistry Collections

Journal Editors Massimo Olivucci, Siena, Italy and Bowling Green, USA Wai-Yeung Wong, Hong Kong Series Editors Hagan Bayley, Oxford, UK Kendall N. Houk, Los Angeles, USA Greg Hughes, Codexis Inc, USA Christopher A. Hunter, Cambridge, UK Seong-Ju Hwang, Seoul, South Korea Kazuaki Ishihara, Nagoya, Japan Barbara Kirchner, Bonn, Germany Michael J. Krische, Austin, Texas Delmar Larsen, Davis, USA Jean-Marie Lehn, Strasbourg, France Rafael Luque, Córdoba, Spain Jay S. Siegel, Tianjin, China Joachim Thiem, Hamburg, Germany Margherita Venturi, Bologna, Italy Chi-Huey Wong, Taipei, Taiwan Henry N.C. Wong, Hong Kong Vivian Wing-Wah Yam, Hong Kong Chunhua Yan, Beijing, China Shu-Li You, Shanghai, China

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Aims and Scope The series Topics in Current Chemistry Collections presents critical reviews from the journal Topics in Current Chemistry organized in topical volumes. The scope of coverage is all areas of chemical science including the interfaces with related disciplines such as biology, medicine and materials science. The goal of each thematic volume is to give the non-specialist reader, whether in academia or industry, a comprehensive insight into an area where new research is emerging which is of interest to a larger scientific audience. Each review within the volume critically surveys one aspect of that topic and places it within the context of the volume as a whole. The most significant developments of the last 5 to 10 years are presented using selected examples to illustrate the principles discussed. The coverage is not intended to be an exhaustive summary of the field or include large quantities of data, but should rather be conceptual, concentrating on the methodological thinking that will allow the non-specialist reader to understand the information presented. Contributions also offer an outlook on potential future developments in the field. More information about this series at http://www.springer.com/series/14181

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Nicola Armaroli • Henk J. Bolink Editors

Photoluminescent Materials and Electroluminescent Devices With contributions from Nicola Armaroli • Elisa Bandini • Andrea Barbieri Thomas Baumann • Larissa Bergmann • Marco Bettinelli Stefan Bräse • Henk J. Bolink • Felix N. Castellano • Mei-Yee Chan Takayuki Chiba • Ludvig Edman • Lidón Gil-Escrig Adam F. Henwood • Cheuk-Lam Ho • Aron J. Huckaba Maths Karlsson • Mohammad Khaja Nazeeruddin • Junji Kido Alan Kwun-Wa Chan • Markus J. Leitl • Yuan-Chih Lin Giulia Longo • Catherine E. McCusker • Filippo Monti Vakayil K. Praveen • Yong-Jin Pu • Abd. Rashid Bin Mohd Yusoff Alexander Schinabeck • Michele Sessolo • Man-Chung Tang Shi Tang • Daniel Volz • Vivian Wing-Wah Yam • Wai-Yeung Wong Hartmut Yersin • Daniel M. Zink • Eli Zysman-Colman

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Editors Nicola Armaroli Istituto per la Sintesi Organica e la Fotoreattività Consiglio Nazionale delle Ricerche Bologna, Italy

Henk J. Bolink Instituto De Ciencia Molecular Universidad De Valencia Paterna, Spain

Originally published in Top Curr Chem (Z) Volume 374 (2016), © Springer International Publishing Switzerland 2017

ISSN 2367-4067 ISSN 2367-4075 (electronic) Topics in Current Chemistry Collections ISBN 978-3-319-59302-9 ISBN 978-3-319-59304-3 (eBook) DOI 10.1007/978-3-319-59304-3 Library of Congress Control Number: 2017942980 © Springer International Publishing AG 2017 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer International Publishing AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

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Contents

Luminescence: The Never-Ending Story .............................................................. vii Nicola Armaroli, Henk J. Bolink Luminescent Metal-Containing Polymers for White Light Emission ................. 1 Cheuk-Lam Ho, Wai-Yeung Wong Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs) ............................................................................. 25 Adam F. Henwood, Eli Zysman-Colman Platinum and Gold Complexes for OLEDs .......................................................... 67 Man-Chung Tang, Alan Kwun-Wa Chan, Mei-Yee Chan, Vivian Wing-Wah Yam Phosphorescent Neutral Iridium (III) Complexes for Organic Light-Emitting Diodes .......................................................................................... 111 Abd. Rashid Bin Mohd Yusoff, Aron J. Huckaba, Mohammad Khaja Nazeeruddin Copper(I) Complexes for Thermally Activated Delayed Fluorescence: From Photophysical to Device Properties .......................................................... 141 Markus J. Leitl, Daniel M. Zink, Alexander Schinabeck, Thomas Baumann, Daniel Volz, Hartmut Yersin Materials Integrating Photochemical Upconversion ......................................... 175 Catherine E. McCusker, Felix N. Castellano Metal–Organic and Organic TADF-Materials: Status, Challenges and Characterization ............................................................................................ 201 Larissa Bergmann, Daniel M. Zink, Stefan Bräse, Thomas Baumann, Daniel Volz Perovskite Luminescent Materials ...................................................................... 241 Michele Sessolo, Lidón Gil-Escrig, Giulia Longo, Henk J. Bolink The Rise of Near-Infrared Emitters: Organic Dyes, Porphyrinoids, and Transition Metal Complexes ........................................................................ 269 Andrea Barbieri, Elisa Bandini, Filippo Monti, Vakayil K. Praveen, Nicola Armaroli

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Inorganic Phosphor Materials for Lighting ....................................................... 309 Yuan-Chih Lin, Maths Karlsson, Marco Bettinelli Organic Light-Emitting Devices with Tandem Structure ................................ 357 Takayuki Chiba, Yong-Jin Pu, Junji Kido Light-Emitting Electrochemical Cells: A Review on Recent Progres ............. 375 Shi Tang, Ludvig Edman

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Top Curr Chem (Z) (2016) 374:44 DOI 10.1007/s41061-016-0044-0 EDITORIAL

Luminescence: The Never-Ending Story Nicola Armaroli1 • Henk J. Bolink2

Published online: 24 June 2016 Ó Springer International Publishing Switzerland 2016

Luminescence has fascinated human beings since times immemorial, thanks to a variety of natural phenomena such as aurora borealis, lightening, or luminous animals of various sorts. A landmark in the history of luminescent materials was the discovery of the so-called Bolognian stone in 1603. Vincenzo Casciarolo, a cobbler and amateur alchemist, while trying to convert poor materials into gold, calcined a stone containing barium sulphate with coal and obtained luminescent barium sulfide. It was the first reported example of a phosphor, because the ‘‘magic’’ stone released light in the dark after exposure to sunshine. Probably, Mr. Casciarolo did not suspect that, after some centuries, this class of materials would become more important than gold itself in the daily life of people.

This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli, Henk Bolink. & Nicola Armaroli [email protected] & Henk J. Bolink [email protected] 1

Istituto per la Sintesi Organica e la Fotoreattivita`, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy

2

Instituto de Ciencia Molecular, Universidad de Valencia, Catedra´tico Jose´ Beltra´n 2, 46980 Paterna, Spain

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Virtually all of the electromagnetic spectrum is used to investigate the structure and properties of chemical systems, all the way from the c-rays (radiation chemistry) to radio frequencies (NMR). The tiny window of Visible light offers invaluable information regarding molecular electronic excited states, also providing snapshots of dynamic processes down to the femtosecond time scale. Moreover, visible light has a huge relevance for several technological applications that pervade our daily life. The very reason for this fact is extremely simple: the human eye is a prodigious sensor for these specific photons. Accordingly, if we want something to be immediately perceived by people, it has to emit light in the wavelength range between 380 and 700 nm and possibly all across this interval, because humans have evolved under the sun and have a strong preference for white light. Emission of visible radiation from matter can be prompted by several stimuli, as different as heat and acoustic waves. However, light and electricity—affording photoluminescence and electroluminescence, respectively—are by far the most utilized, and this volume is specifically dedicated to these two phenomena. In the last 20 years, the interest towards photoluminescence and electroluminescence has increased substantially, also as a consequence of the growing demand for efficient and sustainable solutions in the areas of analytical chemistry, lighting, and displays. However, surprisingly, the number of volumes dedicated to these topics is limited and somewhat outdated. We therefore accepted with enthusiasm the invitation to edit a topical collection for Topic in Current Chemistry as a nice opportunity to fill this gap and assemble an up-to-date resource in the field. This collection encompasses the discussion of the most established luminescent materials, such as organic polymers, transition metal complexes (iridium, copper, platinum, gold) and inorganic phosphors, with special emphasis on their use in emitting devices. Also contributions primarily focusing on electroluminescent devices—organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs)—and fundamental photophysical phenomena of technological interest—photochemical upconversion and thermally activated delayed fluorescence (TADF)—are discussed. Finally, luminescent perovskites and near-infrared emitters are illustrated as examples of materials with still limited or no technological applications, but with wide potential for development. We warmly thank the colleagues across Europe, Asia, and North America for the overwhelmingly positive response to our invitation and for the time and passion invested by them and their coworkers in writing these papers. We are confident that they contributed to a collection that will be an important reference text on institutional and personal libraries for several years to come within the wide scientific community working on luminescence. Finally, we wish to thank the reviewers, who carefully and without compensation checked the different chapters, ensuring that all the contributions are truly state-ofthe-art overviews of the respective topics and the Springer editorial staff for having effectively assisted us throughout the entire editing process.

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Nicola Armaroli

Henk J. Bolink

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Top Curr Chem (Z) (2016) 374:64 DOI 10.1007/s41061-016-0057-8 REVIEW

Luminescent Metal-Containing Polymers for White Light Emission Cheuk-Lam Ho1,2 • Wai-Yeung Wong1,2,3

Received: 24 February 2016 / Accepted: 22 July 2016 / Published online: 23 August 2016 Ó Springer International Publishing Switzerland 2016

Abstract This chapter focuses on the recent developments in luminescent metallopolymers. Synthetic routes to these polymers are briefly described and their applications in polymer white light-emitting diodes are discussed. Keywords Organic light-emitting diode  Phosphorescence  Polymer  Transition metal  White emission

1 Introduction Recently, research based on energy saving technologies is being given high priority. White organic light-emitting diodes (WOLEDs) have attracted great attention in both scientific and industrial communities during the past two decades because of their potential applications in flat panel displays and the next generation of solid-state lighting sources [1, 2]. WOLEDs are different from the

This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli, Henk Bolink. & Cheuk-Lam Ho [email protected] & Wai-Yeung Wong [email protected]; [email protected] 1

Department of Chemistry and Institute of Advanced Materials, Institute of Molecular Functional Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, People’s Republic of China

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HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen 518057, People’s Republic of China

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Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hunghom, Hong Kong, People’s Republic of China

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common light sources that are currently on the market in which the light is generated and emitted over a sizable area, ranging from a square centimeter in laboratory samples to over several square decimeters in current prototypes and potentially up to square-meter dimensions in future products [3]. Therefore, these organic light sources can provide very homogeneous illumination, even in tight spaces where it is very challenging to achieve using conventional, pointshaped light sources. This feature may allow WOLEDs to go into the lighting market as a complementary technology. In particular, WOLEDs based on polymers (WPLEDs) by using solution-based processing techniques, such as spin-coating and inkjet printing, have a great potential in manufacturing at low cost. In order to achieve white emission, mixtures of three primary colors, red, green, and blue (RGB) or two complementary colors, blue and orange or red are typically applied. A straightforward approach to WPLEDs is to blend two or three polymers [4] in the active layer for the realization of white-light emission. Other approaches such as consecutive evaporations of RGB light-emitting compounds, doping of fluorescent or phosphorescent dyes into blue lightemitting polymer, charge transfer exciplexes or excimers to achieve broad emission, etc., have also been used [5]. However, WPLEDs based on blending or doping systems may suffer from intrinsic phase separation during long-term device operation and spectral dependence on the applied voltage, which may lead to the decrease of color stability and device lifetime and pose a big challenge in display applications [6]. White electroluminescence (EL) from a single polymer that can display simultaneous RGB emission is an approach that has a great advantage over doping and blending systems to avoid phase separation, allow simple fabrication processes and scaling up in production [7, 8]. Therefore, there has been a flurry of research interest on developing single white-emitting polymers for WPLEDs. In another context, electrophosphorescent polymers have attracted much research interest since both of the singlet and triplet excitons can be harvested and the internal quantum efficiency of OLEDs of 100 % can be realized theoretically [9, 10]. There are many studies on electrophosphorescent polymers containing heavy metal complexes with broad emission covering the whole visible region from 400 to 700 nm as the emitting layer (EML) and these polymers have played a vital role in raising the efficiency of WPLEDs. In these polymers, the phosphorescent guests are either covalently bonded to the main chain of the polymer as a repeating unit or as a pendent group on the extended p-electron conjugated or non-conjugated backbones. In this chapter, recent progress on the development of different types of white light-emitting phosphorescent polymers and their advancement in optoelectronic performance are described.

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2 Iridium(III)-Based Polymers 2.1 Iridium(III)-Based Main Chain Conjugated Polymers In this type of polymer, blue- or/and green-emitting conjugated backbone and red Ir(III) phosphors were employed in order to get a wide EL spectrum. They can be synthesized by using brominated b-diketonate-coordinated Ir(III) complex, which could be copolymerized via Suzuki coupling with the main chain of conjugated polymers. In all of these cases, polyfluorenes were used as both the efficient blue emitter and the host in the WPLEDs due to their large band gap and high luminescence efficiency. The ancillary b-diketonate unit in the polymer main chain disturbs the p-conjugation of the polymer chain, leading to a shortening of the p-conjugation length of the molecules and hence a blue shift in their absorption spectra. Yang and coworkers took advantage of the built-in fluorenone defects [11], which usually appears due to the undesirable photo and thermal oxidation of polyfluorenes, as a stable green emission core to construct novel white-emitting Ir(III)-containing copolymers P1 [12]. In these polymers, the energy of blue-emitting polyfluorene could partially transfer to both Ir(III) complex and fluorenone to generate red and green emission separately, while the energy transfer from fluorenone to Ir(III) complex is negligible. Single active layer configuration with PEDOT:PSS on ITO as the hole-injecting bilayer electrode was employed for the device fabrication. Adjusting the loadings of fluorene monomer in P1 can fine-tune the color purity of the white emission. The content of the blue emissive fluorene should be kept higher than 99.9 mol% while simultaneously keeping the green and red emitters well proportioned in order to get a pure white light. Commission Internationale de l’Eclairage (CIE) coordinates of (0.32, 0.45), (0.30, 0.35), (0.28, 0.32) and (0.28, 0.32) were detected for P1-1 to P1-4-based devices, respectively. As the charge trapping on the fluorenone and Ir(III) complex units plays a dominant role in the EL process, the relative intensities of the green and red emission peaks in the EL spectra are dramatically enhanced as compared to their photoluminescence (PL) spectra. Therefore, a control of the content of lowenergy emission chromophores in the polymer is crucial for white-light generation. P1-1-based polymer light-emitting diodes (PLEDs) gave the maximum luminance efficiency (gL) of 5.5 cd A-1 with maximum luminance (Lmax) of 3361 cd m-2, however, a low amount of green emission was detected for this device due to the relative weakness of the blue emission. P1-3 achieved the highest efficiencies among all the white devices with gL of 3.25 cd A-1 and Lmax of 228 cd m-2. The voltage-independent white-light emission makes these copolymers attractive for display applications. Hwang and Shim reported that without the use of green segments, single chain polymer P2 can generate whitelight by combining red carbazole-substituted Ir(III) unit and blue polymer backbone [13]. The carbazole chromophore attached to the Ir(III) complex not only improves the stability of the polymers but is also able to donate electrons from its nitrogen lone pair to the metal center, raising the HOMO energy level

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to induce the red shift. P2 exhibited two strong emission bands in their PL spectra in the blue and red spectral regions with peak maxima at 453 and 642 nm, respectively, suggesting the presence of partial intra- and intermolecular energy transfer in the polymer chain between blue emissive fluorene and carbazole fragments and red emissive Ir(III) complexes. This eventually leads to a balanced white PL emission. The charge trapping effects of the Ir(III) complexes that result in the blue emission of P2-based devices was broader than that measured in their PL experiments, similar to the case in P1. The EL spectra of P2 covered the visible range from 400 to 750 nm with strong emission bands at 490 and 650 nm. Due to the well-balanced bluish green and red emissions of P2, an almost pure white-light CIE coordinates of (0.31, 0.32) was achieved. The white-light emission is stable and insensitive to the driving voltage. Unfortunately, very low gL was detected for this device, which suggested that triplet energy back transfer between Ir(III) complex and fluorene or/and carbazole segments may occur. The idea without the use of green emission segments may still be suitable for realizing white-light emitting polymers due to the easier controlled monomer ratios in the synthetic procedures but more efforts are needed in further optimizing the film morphology, layer thickness, or any device treatment conditions to boost the efficiency. By incorporating electron-deficient co-monomer benzothiadiazole (BT) unit into the polyfluorene backbone as a green emitter, Lee et al. reported a series of polymers P3 with or without the electron-transporting 1,3,4-oxadiazole unit in the main chain [14]. 1,3,4-Oxadiazole unit of high electron affinity can facilitate the electron-transportation and injection processes of the PLEDs, which result from more balanced holes and electrons within the polymers. Their PL spectra exhibited three characteristic peaks at 424, 442, and 514, and 425, 444, and 507 nm for P3-1 and P3-2, respectively. A partial energy transfer from the fluorene segment to the benzothiadiazole unit was realized and such kind of energy transfer was more efficient than that from fluorene to Ir(III) complex. The peak intensity difference in P3-1 and P3-2 is due to the less coplanarity between the adjacent fluorene and oxadiazole units in the latter one. The white EL from both polymers was relatively stable with the operating voltage. Taking advantage of the high triplet energy and good electron-blocking characteristics of poly(N-vinylcarbazole) (PVK), the devices using PVK as hole carrier injection and transportation layer exhibit better performance as compared to the commonly used hole-transporting materials PEDOT:PSS. Maximum gL and external quantum efficiency (gext) of 1.89 cd A-1 and 1.79 %, respectively, was achieved for oxadiazole-containing P3-2-based device. A further improvement of the performance of PLEDs is required for display applications.

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N O

O

C6H13

C6H13 x

C6H13

Ir

2 O

C6H13 y

m

P1-1 P1-2 P1-3 P1-4

P1

x = 0.05, y = 0.1 x = 0.01, y = 0.02 x = 0.005, y = 0.015 x = 0.005, y = 0.01

n

x, z n

RO

x, y

N C6H13

OR

O

RO

O Ir

N

OR

2 S

P2

x: y: z = 1: 0.98: 0.02

N

C8H17

C8H17

P3-1 y = 0 P3-2

C8H17

O

N

Ir

C8H17

O

xm

O

C8H17

N N y

n

C8H17 N

S

N z

o

P3 N

2

2.2 Iridium(III)-Based Side Chain Conjugated Polymers In this type of polymers, the Ir(III) complexes are attached as the pendant group and used as red/orange-emitting species. Yang and Cao have realized efficient white-light emission from a single polymer P4, which simultaneously consists of fluorescence- and phosphorescence-emitting species [15]. By introducing a small number of green light-emitting BT units into the blue light-emitting polyfluorene

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backbone and by attaching a small number of Ir(III) complexes to the N-position of the carbazole unit as red triplet emitters onto the side chain, simultaneous white-light emission can be obtained resulting from the partial energy transfer from fluorene to BT and Ir(III) complex. By tuning the contents of BT and Ir(III) complex, the RGB emission can be made more balanced to achieve a pure white light. The emission spectra of all the copolymers are broad, covering the whole visible range from 400 to 700 nm. The peaks at 425, 450, and 485 nm are attributed to the emission of the fluorene unit; the peak at 520 nm is due to the BT unit while the emission at 580 nm is from the Ir(III) complex. As the BT units are inserted into the polyfluorene backbone while the Ir(III) complexes are attached onto the side chain, the PL intensity of the green peak is stronger than that of the red. A more efficient energy transfer occurs from the fluorene to the BT units than to the Ir(III) complex due to the fact that energy transfer can only occur over a limited distance, as the average distance from the fluorene unit to the Ir(III) complex is greater than that to the BT units. The PLED devices are fabricated with the configuration ITO/PEDOT/PVK/P4/CsF/Al. By changing the content of the BT and/or Ir(III) complex, the EL spectra can be readily adjusted. Their EL spectra are stable as the driving voltage varies. Bias-independent white emission is important for practical applications, especially when the devices are used as the backlight of liquid crystal displays. By illustrating the difference in the peak intensities of the RGB emissions of their PL and EL spectra, a chargetrapping mechanism was confirmed in the EL process. A partial energy transfer also contributed to the white-light emission as the emission from the fluorene units appeared in all of the EL spectra even at a high Ir(III) complex content or at a low applied voltage. On the other hand, two processes were apparent to govern the recombination of holes and electrons for EL in the P4-based device. The holes and electrons underwent direct recombination on the main chain of the fluorene and BT units to produce blue and green light emission and in parallel, charges are trapped on the Ir(III) complexes followed by radiative recombination with red light emission from the triplet excited state of the Ir(III) complex. Although the device based on P4-4 gives the highest gL of 6.1 cd A-1, its color is a little bit green. Devices fabricated from P4-2 and P4-3 emit white light with CIE coordinates of (0.34, 0.33) and (0.32, 0.33), respectively, which are very close to the pure white-light point. The WPLEDs based on P4-6 and P4-8 show high gL of 4.7 and 4.6 cd A-1 with CIE coordinates of (0.38, 0.35) and (0.31, 0.34), respectively. The white-light emission of the devices made from copolymers is stable over the whole white-light region at different applied voltages, and the overall EL efficiencies decline slightly with increasing current density.

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N

N SN C8H17

C8H17 x

m

y n P4

2

O Ir O

C8H17

x = 0.005, y = 0.2 x = 0.01, y = 0.2 x = 0.03, y = 0.2 x = 0.05, y = 0.2 x = 0.01, y = 0.1 x = 0.01, y = 0.4 x = 0.01, y = 0.5 x = 0.03, y = 0.3

N

C8H17

N F3C

P4-1 P4-2 P4-3 P4-4 P4-5 P4-6 P4-7 P4-8

O Ir O

F3C

C8H17

2

P5-1 P5-2 P5-3 P5-4

C8H17

x = 0.1 x = 0.2 x = 0.3 x = 0.5

N

x n P5

Binary copolymers P5 with the same red-emitting Ir(III) complex attached to the side chain of polyfluorene were synthesized by the same research group but no acceptor conjugator was inserted in the main chain [16]. All their PL spectra possess two emission bands (440 and 580 nm), indicating that the partial energy transfer from fluorene segment to Ir(III) unit takes place, which is different from the case in P4, with energy transfer from fluorene to BT. A poorer performance is detected for P5-based WPLEDs as compared to those fabricated with P4. This is ascribed to the exciton confinement of the BT unit, which allows efficient singlet energy transfer from fluorene segment to BT unit and avoids the triplet quenching resulting from the higher triplet energy levels of phosphorescent green emitters than that of polyfluorene. A pure white-light emission of CIE coordinates (0.33, 0.32) can be

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obtained for P5-3 with maximum gL of 3.2 cd A-1 and Lmax of 4105 cd m-2. From the results of P4 and P5, it indicates that phosphorescence quenching is a key factor in the design of white light-emitting polyfluorene associated with triplet emitter. 1-x

x C6H13

C8H17

R=

C8H17

CH3

P6-1 x = 0.005 P6-4 x = 0.01 P6-2 x = 0.01 P6-5 x = 0.02 P6-3 x = 0.02 P6-6 x = 0.03

O

O

R=

Ir N

F

R

2 P6

A series of white light-emitting polymers P6 consisting of blue fluorescent polyfluorene that is covalently attached to a small amount of orange phosphorescent Ir(III) complex at the side chains have been developed [17]. Due to the extended pconjugation in the cyclometalating ligand in P6, the peaks due to the Ir(III) fragment are red-shifted in their PL spectra (kPL = 440, 590 nm) as compared to P4 and P5. The orange emissions in P6-1 to P6-3 are stronger than that in P6-4 to P6-6 at the same doping level. The best PLED result is based on the device with P6-1, with maximum gL of 4.49 cd A-1, maximum power efficiency (gP) of 2.35 lm W-1 and CIE coordinates of (0.46, 0.33). The CIE coordinates can be tuned to (0.34, 0.33) by using P6-4 as the emissive layer and are very close to those of the standard white light. However, the maximum gL and gP decreased to 0.81 cd A-1 and 0.42 lm W-1, respectively, with the Lmax of only 989 cd m-2. The reduction of the efficiencies may be attributed to the high triplet energy level of P6-4, which results in more effective phosphorescence quenching by energy transfer from the Ir(III) complex segment to the polyfluorene triplet states. By employing P7 with the charge-transporting triphenylamine and oxadiazole chromophores as pendant units and low concentrations of low-energy emitting red phosphor as the side group in a single polyfluorene copolymer, perception of whitelight in the human vision system was created [18]. In addition to the blue emissive backbone at 420 nm in its PL spectrum, a strong green emission band was detected at 520 nm due to the presence of BT segments and a minor contribution from the triplet emission of Ir(III) substituent at ca. 600 nm was also observed. The introduction of triphenylamine and oxadiazole moieties can significantly lower the energy barrier height for carrier injection from the electrodes and improve the carrier-transporting properties in its phosphor-doped PLEDs, leading to the improvement of EL performances. Its EL spectra cover the whole visible region with a full width at half-maximum (FWHM) of 205 nm in its device with single

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emissive layer showing balanced emission of the three primary colors. The CIE coordinates for the P7-based device are (0.34, 0.38) with a color rendering index (CRI) value of 82. White light was emitted at a voltage as low as 2.8 V, indicative of its low power consumption. Maximum gL and gP of 8.2 cd A-1 and 7.2 lm W-1, respectively, were measured. The gL of this device was retained over 6 cd A-1 with the Lmax of 1330 cd m-2 at a bias of 6.0 V. A slight drop of gL with increasing current density can be attributed to the triplet–triplet annihilation process as well as the dissociation of excitons by the electric field or effects related to the width and location of the recombination zone as a function of bias. The EL efficiency can be further improved if phosphors with higher triplet energies are covalently linked to a host polymer having a wide bandgap. For the above polymers, whitish lights were generated by using fluorescent/ phosphorescent hybrid with both singlet blue and triplet orange/red emitters. All phosphorescent polymers with triplet blue and orange/red emitters were difficult to achieve due to the lack of suitable hosts with the requisite high triplet energy levels, suitable HOMO and LUMO levels. Lee and Hwang reported all phosphorescent white polymers P8 based on tetraphenylsilane and carbazole host backbone with 0 blue-emitting FIrpic and red emissive bis[2-phenylquinoline-N,C2 ]iridium(III) picolinate (Phq)2Irpic. The triplet energy of the host polymer was found to be 2.67 eV, which is higher than that of the blue Ir(III) phosphor FIrpic (2.60 eV). Such high triplet energy ensures no back energy transfer from blue phosphor to the polymer backbone (Fig. 1). Both PL spectra of P8-1 and P8-2 in solution are dominated by the emission from the host polymer at 394 nm. The blue emission from FIrpic can be observed at around 470 nm while the red emission from (Phq)2Irpic cannot be observed, which indicates that the energy transfer from the host polymer to the (Phq)2Irpic unit was less effective than that to FIrpic. Owing to the high triplet energy of the host polymer, efficient energy transfer from the host backbone to the blue and red phosphors was observed in the film state. Three emission peaks at around 398, 473, and 577 nm were found and the latter two emissions became dominant in the film state PL spectrum. The reduction in the distance between blue and red phosphors favor Dexter energy transfer from FIrpic to (Phq)2Irpic, which results in the difference between the PL spectra in the solution and film states. The quantum efficiency of P8-1 and P8-2 was found to be 14.2 and 10.4 %, respectively. The drop in the quantum efficiency of P8-2 was mainly attributed to the triplet–triplet annihilation when the Ir(III) concentration was increased. Devices with the configuration of ITO/PEDOT:PSS/PVK/P8/TSPO1/ LiF/Al (TSPO1: 4-(triphenylsilyl)phenyldiphenylphosphine oxide) were fabricated by using P8. P8-2 exhibited better device performance as compared to P8-1, with gext of 1.07 %, gL of 2.26 cd A-1, gP of 0.96 lm W-1 and CIE coordinates of (0.33, 0.37). To further improve the performance of WPLEDs, electron-transporting 1,3bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene was doped into the emissive layer. A complete energy transfer from this material to P8 was noticed as the EL spectra of these optimized devices were similar to those of the pure P8. Due to the improved charge injection of the devices, the turn-on voltage (Von) was decreased as compared to the non-doped one. The device parameters were also improved, and the gext of P8-1-based device was increased from 0.69 to 1.72 %. The Reprinted from the journal

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nBu

Bun C8 H17 C8 H17

Bun

N N

N

x N N O

O

N

R = 2-ethylhexyl

nBu

x,y

N R

N

N R

N

P8-1 x = 0.500, y = 0.444, z = 0.050, m = 0.006 P8-2 x = 0.500, y = 0.418, z = 0.075, m = 0.007

O C8 H17 w

z C8 H17

x = y = 0.2496n, z = 0.0004n, w = 0.0004n

N

F

P8

O C8 H17

x,z

N

2

N

C8 H17

P7

Si

N R

N

S

y C8 H17

C8 H17

N N

N

R

Ir

N

O N

Ir

F

O

2

N R

O

N

N R

x,m n

N

O

Ir

N

2

O O

Fig. 1 Diagram illustrating the energy transfer processes in P8

best device performance was based on P8-1, with gL of 4.06 cd A-1, gP of 2.04 lm W-1 and CIE coordinates of (0.44, 0.41). This value of CIE coordinates is close to the standard CIE coordinates for warm white-light emission at (0.44, 0.40). This study provides a novel avenue for the design of polymer host material with high triplet energy for efficient white WPLEDs. Ding and Wang reported a novel series of all-phosphorescent single polymers P9 based on a fluorinated poly(arylene ether phosphine oxide) backbone simultaneously grafted with blue and yellow phosphors [19]. This polymer scaffold has the triplet energy of 2.96 eV, even higher than that of P8. The adjustment of the HOMO and LUMO levels to -5.7 and -2.3 eV, respectively, of the polymer backbone effectively facilitates the charge injection process in the P9-based device. Two strong peaks at 410 and 472 nm accompanied by a shoulder at 568 nm were found in the PL spectra of P9, which can be ascribed to the emissions from polymer backbone, FIrpic and bis[2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzimidazolate-jN,jC](acetylacetonato)-iridium(III) [(fbi)2Ir(acac)], respectively. As supported by the good overlap between the absorption spectra of the blue and red

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phosphors and the PL spectrum of polymer scaffold, efficient intramolecular Fo¨rster energy transfer from the polymer host to the Ir(III) complexes was confirmed in the solution state. However, in contrast to their PL behavior in solution, their thin-film emission from the polymer host almost disappeared, but only emissions from blue and red emitters were observed, which implies that both intra- and intermolecular energy transfer existed. Therefore, apart from the direct energy transfer from the polymer host to the blue and red phosphors, Dexter energy transfer from the blue emitter to the red phosphor was believed to occur. WPLEDs with the device structure of ITO/PEDOT:PSS/polymer/TPCz/LiF/Al was applied, in which TPCz (3,6-bis(diphenylphosphoryl)-9-[4-(diphenylphosphoryl)phenyl]-9H-carbazole)) acts as an electron-transporting material. The relative intensity of the red emission relative to the blue one in the EL spectrum turns out to be stronger than that in the PL spectrum, indicating the existence of charge trapping, especially at low operating voltages. With the synergistic effect of processes like Fo¨rster energy transfer from polymer host to FIrpic and (fbi)2Ir(acac), charge trapping on (fbi)2Ir(acac) and Dexter energy transfer from FIrpic to (fbi)2Ir(acac), simultaneous standard white EL emissions was produced. The CIE coordinates are bias-independent and insensitive to the contents of the Ir(III) complexes. These features are important for the enhancement of reliability and reproducibility during the fabrication of WPLEDs and keys for practical application. Device based on P9-4 showed the best performance among polymers P9, with gL of 15.2 cd A-1, gP of 6.7 lm W-1 and gext of 6.0 %. The author had also used 9,90 -spirobis(fluorene)-2,7-diylbis(diphenylphosphine oxide (SPPO13) as the electron-transporting layer in place of TPCz to construct WPLEDs, prominent efficiencies of 18.4 cd A-1, 8.5 lm W-1 and 7.1 % were obtained. A slow efficiency roll-off at high current density was indicated, and the gL of the device still remained as high as 14.2 cd A-1 even at a brightness of 1000 cd m-2. This is the highest performance ever reported for allphosphorescent WPLEDs, shedding light on the significance of WPLEDs. F

O P

O

F O

P

O 0.5 F

N

F

F

O O

P

N

P x

0.5-x-y O

P9-1 x P9-2 x P9-3 x P9-4 x P9-5 x

= 0.025; y = 0.006 = 0.05; y = 0.006 = 0.05; y = 0.007 = 0.075; y = 0.007 = 0.10; y = 0.01

P9

N

F

y

O C8H16 O

O O

O O

N

Ir

Ir N

F

O

F

C8H16 O

F

O

F

N 2

N 2

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2.3 Iridium(III)-Based Side Chain Non-Conjugated Polymers The color of EL from a polymer with multiple phosphorescent emitters may be affected by the energy transfer, such as Fo¨rster and Dexter energy transfers between different phosphors, like the case of P9. Therefore, isolation of the various emitters in the polymer by synthetic means can effectively minimize the energy transfer between different phosphors, so that the desired goal of multiple emission colors can be achieved. By incorporating two different colored phosphorescent Ir(III) emitters (green–blue and red emissive pendant heteroleptic Ir(III) complexes introduced randomly into each different block), bichromophoric block copolymers P10 were developed by Fre´chet et al. Due to the high triplet energy and outstanding charge-transport properties of triarylamine and oxadiazole, the introduction of these chromophores into the diblock copolymers would enable a more balanced transport of hole and electrons. The syntheses of P10 relied on the living free radical polymerization approach, in which each block was introduced to the polymeric system one by one. These polymers can deliver site isolation of the two emitters effectively to suppress energy transfer from blue to red emitters, which normally provides improved white color balance and efficiency of the resulting WPLEDs [20]. P10 have been used as the single active layer in a device with no additional material between anode and cathode. The device made from the phase separated copolymer P10-3 thin film is three times brighter than those devices made from copolymers P10-1 and P10-2 with a lower Von. P10-3 with a higher content of blue emitter shows markedly better gext ([1.5 %) for white emission than the lower molecular weight (MW) polymers P10-1 (gext * 0.3 %) and P10-2 (gext * 0.4 %). This is mainly attributed to the minimization of blue emission loss by suppressing the unnecessary energy transfer to the red dopant. This molecular approach may be extended to other WPLED systems to improve their device efficiency and should be versatile and broadly applicable. N O m

n

O N F

Ir N

N

N

N O N

P10-1 n = 10, m = 0.1 P10-2 n = 10, m = 0.5 P10-3 n = 10, m = 1.0

O N

P10

Ir N 2

F

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2.4 Iridium(III)-Based Chelating Polymers Similar to the polymers discussed above, the polymer backbone of this type of polymers is based on polyfluorene due to its excellent blue-light emitting polymer with large bandgap and good electronic properties. However, phosphor quenching of the triplet emitter by polyfluorene must be considered when a triplet emitter is introduced into polyfluorene. P11 with 4,7-bis-(9,9dioctyl-9H-fluoren-2-yl)benzo [1,2,5]thiadiazole (DFBT), polyfluorene, and Ir(III) complex with 2-naphthalene-pyridine as the cyclometalating ligand were prepared by Suzuki polycondensation [21]. Triplet energy back transfer from both of the DFBT and Ir(III) complex to polyfluorene is effectively well avoided due to the triplet energy levels of both DFBT and Ir(III) complex being lower than that of polyfluorene. Therefore, the triplet excitons would be mainly confined in the Ir(III) complex which can lead to efficient phosphorescent red emission, although energy may be lost to a certain extent owing to the triplet alignments of Ir(III) complex and DFBT (Fig. 2). The BT unit in the copolymer in fact behaves as a bridge to provide a more efficient energy transfer from the fluorene segment to the Ir(III) complex in the PL excitation process. By adjusting the monomer feed ratios, broadband white emission from this single polymer is realized. The EL spectra of P11 show balanced RGB emissions peaking at 625, 520, and 420/440 nm, respectively, and the overall emission is located very close to the ideal white point. The peak at 625 nm depends not only on the content of Ir(III) complex but also on that of the BT unit. The emission color of copolymers remains stable upon varying the applied voltage. Different excitation mechanisms were implied as there are a great difference between their PL and EL spectra; the EL spectra have a much

Fig. 2 Energy level scheme of polyfluorene, DFBT, and Ir(III) complex units of P11

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higher contribution from the BT unit and Ir(III) complex. It is believed that charge trapping makes a more important role in the EL process rather than photon absorption or transition only in the PL to induce the emission of polymers. The enhanced stability of the white emission of the single copolymer is obviously due to the sufficient isolation of green and red chromophores in the polyfluorene backbone. The pure WPLEDs based on P11-1 gave the peak gL and gext of 4.4 cd A-1 and 2.2 %, respectively, with the CRI value of 84. A better CRI value of 88 with the correlated color temperature (CCT) of ca. 5000 K was achieved by P11-4 with gL of 5.3 cd A-1 and Lmax of 9900 cd m-2, which represents good color quality of the device for displays and solid-state lighting. The Von of the device based on P11-4 is 5.3 V and is relatively low among PVK-based WPLEDs, which is important for reducing the power consumption for real applications. These devices possess a fairly slow roll-off in efficiency loss, which can be attributed to the reduction of long radiative lifetime of triplet excited states via chemical modification and therefore, they should be promising candidates for solid-state lighting purpose. By using 2-(20 -benzothienyl)pyridine as the cyclometalating ligand in P12, white emission was detected, in which the red emissions was further shifted bathochromically and found to be located at 660 and 720 nm [22]. More pure WPLEDs can be achieved based on P12-2 and P12-3 among the polymers P12. A better performance is realized for the former polymer with gext of 3.7 %, gL of 3.9 cd A-1 and CIE coordinates of (0.31, 0.32). The white-light emission from this polymer is stable at all applied voltages, and the EL efficiencies only decline slightly with increasing current density. However, because the peak at 660 nm is too red for WPLEDs, the efficiencies of WPLEDs are not as high as those based on P11. Hsu et al. developed new WPLEDs from phosphorescent single polymer system P13 by using blue-light emitting fluorene monomer copolymerized with red-light emitting phosphorescent dye and end-capped with a green-light emission dye, N-phenyl-1,8-naphthalimide [23]. The highest brightness in such device is 300 cd m-2 measured at a bias of 16 V and a current density of 2900 A m-2. However, due to the saturation effect of the emission capability of Ir(III) complex at higher current densities, which leads to changes in the CIE coordinates of the EL spectra, this causes a loss of color purity when the bias is raised.

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N SN C8H17

C8H17

C8H17

C8H17

x

O

C8H17

N

Ir

N

C8H17

y

O

n

x = 0.02, y = 0.1 P11-1 x = 0.04, y = 0.1 P11-2 x = 0.05, y = 0.1 P11-3 x = 0.03, y = 0.075 P11-4

P11

S x = 0.05, x = 0.1, x = 0.1, x = 0.1, x = 0.3, x = 0.005,

N C8H17

C8H17

O

Ir

O N

S

C8H17 N S N P12

O

S

N

N 0.5p C8H17

P12-1 P12-2 P12-3 P12-4 P12-5 P12-6

y n

xm C8H17

O

y = 0.02 y = 0.03 y = 0.04 y = 0.05 y = 0.03 y = 0.01

C8H17 m

Ir

m = 97, n = 2, p = 1

O O

O

N S P13

n

N O

0.5p

2.5 Iridium(III)-Based Dendritic Polymers White-emitting single polymers with star-shaped configuration are of increasing concern due to their superior optoelectronic properties, such as suppressed intermolecular interaction and enhanced solid-state luminescence [24–28]. Inspired by the star-like white EL polymers with a fluorescent donor–acceptor-donor structural unit as the orange emissive core and four polyfluorene chains as the blue emissive arms by Wang et al. [24], Wu and Yang first reported a new series of starshaped polymers P14 with a triphenylamine-based Ir(III) dendritic complex as the orange-emitting core and polyfluorene as the blue-emitting branching arms towards WPLED application [29]. These polymers were synthesized by the one-pot Suzuki polycondensation reaction. By controlling the feed ratio of Ir(III) complex core, the intensities of the blue and orange emissions can be finely tuned to realize dual color white light emission. When the polymers contain Ir(III) units at less than 0.08 mol%, a peak at about 430 nm was observed only in their PL spectra, which is a typical polyfluorene emission. A significant increase in the intensity of the orange emission signal at 570 nm was detected with a higher content of Ir(III)

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phosphor. Such orange emission can be attributed to the partial Fo¨rster energy transfer from the fluorene arms to the Ir(III) complex core. Single-layer WPLEDs were fabricated with the configuration of ITO/PEDOT:PSS/P14/CsF/Al. Similar to other single phosphorescent white-light emitting polymers, the charge trapping effect is dominated in their EL processes and results in an enhancement of the orange emission peak. The best device performance is based on the polymer with 0.08 mol% Ir(III) complex. This device turned on at a low voltage (Von = 4.5 V) with Lmax of 3160 cd m-2, gext of 1.76 % and gL of 1.69 cd A-1. Additionally, all the device exhibited slight efficiency roll-off with increasing current density and therefore the incorporation of blue-emitting fluorene species into orange-emitting Ir(III) core in a star-liked shape is a promising strategy to achieve stable white-light emission.

p

n

C6H13 C6H13

C6H13

C6H13

N

N N

C6H13

C6H13

N N

q

Ir N

C6H13

N

N

C6H13

C6H13

m

C6H13

s

N t

P14

C6H13 C6H13

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3 White Light-Emitting Polymers Based on Other Metals Besides Ir(III)-containing polymers, EL devices based on Pt(II) complexes are also of considerable interest due to their high emission quantum efficiencies. Due to the ligand effect, planar geometry around the Pt(II) ion and excimer formation, Pt(II) complexes can emit over a wide range of wavelengths, including white light [30–34]. Thompson and Fre´chet reported a series of multifunctional polymers P15 containing electron- and hole-transporting moieties and emissive Pt(II) complexes, which emit simultaneously from monomer (blue) and aggregate (orange) states [35]. These random terpolymers were prepared by polymerization of various ratios of monomers containing triphenylamine, oxadiazole, and b-diketonate units. The diketonate moieties were then metalated using [Pt-2-(40 ,60 -difluorophenyl)pyridinato-(l-Cl)]2. With the device configuration of ITO/PEDOT:PSS/polymer/BCP/ Alq3/LiF/Al, P15-based WPLEDs were prepared. Near-white lights were realized by the monomeric form for blue emission and the aggregate form for orange excimer emission simultaneously. The concentration of the Pt(II) complex will largely determine the extent of Pt–Pt interaction and therefore the colors of the PLEDs. The optimized device was based on P15-1 with gext of 4.6 % and CIE coordinates of (0.33, 0.50). In addition, the emission color of the obtained devices was invariant with the applied voltage, leading to a voltage-independent and goodquality near-white broad emission. Due to the relatively high complex loading, other devices likely suffer from the Fo¨rster losses. Further changes in the composition ratio could potentially tune the emission closer to pure white. N m

o

n

O

N N

O

m = 10, n = 1, o = 10 m = 6, n = 1, o = 6 m = 3, n = 1, o = 3 m = 10, n = 1, o = 2

O

O P15

N

P15 P15 P15 P15

Pt N

F F

Acetylenic polymers are found to be useful in many optoelectronic applications because of their thermal and chemical stability and ease of structural modification [36, 37]. Wong et al. reported conjugated Pt(II) ethynylene-linked polymer P16, which contains fluorene and electron-poor oxadiazole [32], and its optoelectronic properties were probed. This polymer was prepared by the metal alkynylation

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reaction via the CuI-catalyzed dehydrohalogenation. Simple PLEDs based on this phosphorescent polymer were prepared by spin-coating method to yield thin polymer films with good morphological properties. The optimized device turned on at 10 V with 1 wt % doping concentration. The best performance was realized at the gext of 0.15 %, gL of 0.58 cd A-1 and gP of 0.16 lm W-1 with Lmax of 480 cd m-2 at a current density of 17.6 cd m-2. The goal of white-light emission was achieved by simultaneous BCP fluorescence at 480 nm and polymer phosphorescence at 555 nm. Strong voltage dependence of the EL emission was observed. A decrease in performance was reported as a higher doping concentration was used, which suggested that less energy was transferred from PVK to P16 and the occurrence of concentration quenching effect. This work illustrates that the use of Pt(II) acetylides in the design of WPLEDs is possible, which is prone to show promises for improvement.

C6H13

C6H13

N N

C6H13

C6H13

O

PBu3 2

P16

Pt PBu3

n

Porphyrin derivatives show red light emissions with narrow half-peak width and they have been often used as red dopants in PLEDs. Zn(II) copolymers P17 containing BT and porphyrin derivatives as dopants to polyfluorene backbone were synthesized by Moon and coworkers using the Suzuki coupling reaction [38]. In these polymers, polyfluorene acts as the host, BT as the green emitter and porphyrin as the dopant. The porphyrin functionalities demonstrated strong Q-band absorption peak at around 600 nm and features as light-harvesting antenna component, and therefore they can produce red emission by effectively absorbing the energy in the blue and green regions. More effective Fo¨rster energy transfer from fluorene to porphyrin than to BT derivatives was found as the content of porphyrin increases. Both P17-1 and P17-2 gave attractive quantum efficiency of 0.79 in solution. Devices made in the ITO/PEDOT:PSS/P17/BaF2/Ba/Al structure by spin coating were applied to test for the optoelectronic performance of P17. Since the energy transfer from fluorene derivatives to dopants was partial, the overall EL spectrum became broad. A difference between the PL and EL spectra was indicated for both polymers. The green (562 nm) and red (630 nm) emissions were relatively higher than the blue emission (432 nm) in their EL spectra, which revealed that the BT and porphyrin worked as the charge trapping site. The CIE coordinates of P17-1 and P17-2 based devices were (0.29, 0.34) and (0.36, 0.34), respectively. The best device efficiencies were found to be gL = 0.66 and 0.43 cd A-1; gP = 0.29 and 0.17 lm W-1, respectively, for P17-1 and P17-2.

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OC6H13

C8H17

C8H17

N

1-x-y P17-1 x = 0.02, y = 0.05 P17-1 x = 0.02, y = 0.10

S

N

N N

x

Zn

N N

y

P17

OC6H13

Parallel to the aforementioned studies that employ luminescent Ir(III), Pt(II), and Zn(II)-containing polymers, less accessible Os(II)-emitting polymer P18 has been realized as an efficient white-light EL material [39]. This polymer was synthesized by covalent bonding of a red-emitting Os(II) complex Os(bpftz) (bpftz = 3-trifluoromethyl-5-(4-tert-butyl-2-pyridyl)triazolate) into the backbone of a bipolar polyfluorene copolymer incorporating appropriate amounts of green-emitting BT. The advantages of Os(II) over Ir(III) analogues as the emitting dopants for phosphorescent PLEDs include their shortened radiative lifetime which effectively reduces the extent of triplet–triplet annihilation for the devices when having a higher dopant concentration or under high current density. This is mainly due to the enhanced degree of the participation of heavy Os(II) metal atom in the lowest excitation triplet manifolds and the increased HOMO energy level as compared to Ir(III) ion, which make them more suitable to serve as the direct trapping sites. Energy transfer took place from the singlet excited state of polyfluorene to the singlet excited state of the Os(II) complex, followed by fast intersystem crossing and consequently, emission from its triplet excited state. A P18-based device possessing the configuration of ITO/PEDOT/P18/ TPBI/LiF/Al exhibited broadened emissions covering 400–700 nm region with the main peaks centered at 428, 518, and 614 nm, which lie within the ranges of the three primary colors. Direct charge trapping and recombination at the Os(II) moieties instead of Fo¨rster energy transfer was found to be the dominant mechanism responsible for the red EL emission. For the green EL emission, Fo¨rster energy transfer was the main operating process. White emission with the CIE coordinates of (0.37, 0.30) at a bias of 9 V was observed. In addition, the emission color of the obtained

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device showed minor color shift upon changing the applied voltage; CIE coordinates shifted slightly from (0.36, 0.29) to (0.39, 0.32) as the driving voltage increased from 7 to 13 V. This device turned on at 5.6 V with the gL of 10.7 cd A-1 and gext of 5.4 %. More than 80 % of the peak efficiency of 8.1 cd A-1 at the current density of 100 mA cm-2 was sustained with a brightness of 8094 cd m-2, which indicated that the use of Os(II) emitter in polymeric structure may effectively reduce the efficiency roll-off even at high current density and voltage. n-Bu

N

O

O

N

N

N

N

n-Bu

F3C

N

n-Bu

n-Bu N

S

N N N

N

Et

P C8H17

C8H17

x

C8H17

C8H17 x

C8H17

P18

x = (0.5-0.003)n, y = 0.002n, z = 0.004n

N

Et

C8H17 y

Et

P

Os N

Et N N N

C8H17

C8H17

z

CF3

4 Conclusions By merging the advantages of high EL efficiency of phosphorescent complexes and the solution processability of polymers, white-light emitting single electrophosphorescent polymers exhibit attractive photophysical, mechanical, and EL properties (Table 1). One of the most attractive features associated with WPLEDs is their flexibility and the possibility of large area fabrication. They are emerging clean light sources as they consume lower power as compared to the conventional lighting devices, such as fluorescent lamp and incandescent bulb, and therefore, could be a potential candidate to replace the existing lighting technology. However, before these polymers become commercialized, significant improvement on the device performance must be made. Development of new electrophosphorescent polymers with good charge transporting properties and suitable energy levels for charge injection, design of proper polymer backbone for blue electrophosphorescence as well as new fabrication technology for the multilayer WPLEDs with conventional materials are desirable to boost up the efficiency of the device. The lifetime of electrophosphorescent polymer-based WPLEDs is another critical issue that must be addressed. As more researchers work on this area, many intrinsic problems associated with the WPLEDs based on single phosphorescent polymers should be

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Top Curr Chem (Z) (2016) 374:64 Table 1 Comprehensive overview of various types of metallopolymers for WPLEDs gP (lm W-1)

gext (%)

gL (cd A-1)

Lmax (cd m-2)

CIE (x, y)

P1-3



2.24

5.50

3361

0.32, 0.45

P1-4



0.89

2.20

1268

0.30, 0.35

P1-5



1.35

3.25

1015

0.28, 0.32

P1-6



0.89

2.53

1059

0.28, 0.32

P2



0.04

0.05



0.31, 0.32

P3-1

0.32

1.68

1.76

362

0.31, 0.37

P3-2

0.46

1.79

1.89

498

0.33, 0.34

P4-1





2.80

2170

0.34, 0.30

P4-2





1.90

3585

0.34, 0.33

P4-3





1.80

2410

0.32, 0.33

P4-4





6.10

10,110

0.32, 0.44

P4-5





3.60

6280

0.26, 0.29

P4-6





4.70

5309

0.38, 0.35

P4-7





5.60

6440

0.44, 0.38

P4-8





4.60

6035

0.31, 0.34

P5-1





2.00

3407

0.27, 0.17

P5-2





2.70

1986

0.30, 0.30

P5-3





3.20

4105

0.33, 0.32

P5-4





4.80

5230

0.47, 0.35

P6-1





4.49

3316

0.44, 0.32

P6-2





3.59

3661

0.49, 0.36

P6-3





0.97

2137

0.51, 0.37

P6-4





0.81

989

0.31, 0.30

P6-5





0.83

638

0.36, 0.32

P6-6





0.38

464

0.45, 0.36

P7



3.70

8.50

11

0.34, 0.38

P8-1

0.51

0.69

1.55

309

0.43, 0.40

P8-2

0.96

1.07

2.26

315

0.33, 0.37

P9-1

6.0

5.6

13.8

2243

0.32, 0.39

P9-2

6.7

6.1

14.8

5782

0.28, 0.39

P9-3

6.0

5.5

13.7

4194

0.36, 0.41

P9-4

6.7

6.0

15.2

4331

0.30, 0.41

P9-5

5.5

4.8

12.7

4406

0.37, 0.43

P10-1



Ca. 0.30







P10-2



Ca. 0.40







P10-3



[1.50







P11-1



2.20

4.40

3700

0.33, 0.33

P11-2



1.50

3.10

3400

0.33, 0.36

P11-3



1.50

3.00

3100

0.36, 0.37

P11-4



2.70

5.30

9900

0.34, 0.36

P12-1



3.40

3.20

2623

0.38, 0.30

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Top Curr Chem (Z) (2016) 374:64 Table 1 continued gP (lm W-1)

gext (%)

gL (cd A-1)

Lmax (cd m-2)

CIE (x, y)

P12-2



3.70

3.90

4180

0.33, 0.34

P12-3



3.00

2.80

2204

0.37, 0.37

P12-4



3.50

3.30

3460

0.40, 0.40

P12-5



3.60

2.80

1896

0.46, 0.33

P13





0.10

300

0.33, 0.34

P14



1.76

1.69

3160

0.35, 0.33

P15-1



4.6





0.33, 0.50

P15-2



3.2





0.36, 0.48

P15-3



3.0





0.38, 0.50

P15-4



3.5





0.30, 0.43

P16

0.16

0.15

0.58





P17-1

0.29



0.66

936

0.29, 0.34

P17-2

0.17



0.43

726

0.36, 0.34

P18



5.4

10.7



0.37, 0.30

overcome in the near future and we can expect that a versatile organic-based display and lighting systems can be used which are as competitive as the existing technology concepts. Acknowledgments C.-L. Ho thanks Hong Kong Baptist University (FRG1/14-15/066 and FRG2/13-14/ 078) and the Science, Technology and Innovation Committee of Shenzhen Municipality (JCYJ20140818163041143) for their financial support. W.-Y. Wong is grateful to the financial support from Hong Kong Research Grants Council (HKBU203313), Areas of Excellence Scheme, University Grants Committee of HKSAR (Project No. AoE/P-03/08), Hong Kong Baptist University (FRG2/13-14/ 083) and the Science, Technology and Innovation Committee of Shenzhen Municipality (JCYJ20140419130507116).

References 1. Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K, Friend RH, Burns PL, Holmes AB (1990) Nature 347:539–541 2. Kraft A, Grimsdale AC, Holmes AB (1998) Angew Chem Int Ed 37:402–428 3. Gather MC, Ko¨hnen A, Meerholz K (2011) Adv Mater 23:233–248 4. Cheng G, Zhang YF, Zhao Y, Lin YY, Ruan CY, Liu SY, Fei T, Ma YG, Cheng YX (2006) Appl Phys Lett 89:043504 5. Liu J, Pei Q (2010) Curr Org Chem 14:2133–2144 6. Wang F, Wang L, Chen J, Cao Y (2007) Macromol Rapid Commun 28:2012–2018 7. Chen FC, He G, Yang Y (2003) Appl Phys Lett 82:1006–1008 8. Noh YY, Lee CL, Kim JJ, Yase K (2003) J Chem Phys 118:2853–2864 9. Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR (1998) Nature 395:151–154 10. Adachi C, Baldo MA, Thompson ME, Forrest SR (2001) J Appl Phys 90:5048–5051 11. Sun QJ, Fan BH, Tan ZA, Yang CH, Li YF (2006) Appl Phys Lett 88:163510 12. Zhang K, Chen Z, Yang C, Tao Y, Zou Y, Qin J, Cao Y (2008) J Mater Chem 18:291–298 13. Park MJ, Kwak J, Lee J, Jung IH, Kong H, Lee C, Hwang DH, Shim HK (2010) Macromolecules 43:1379–1386 14. Cho W, Karthikeyan NS, Kim S, Kim S, Gal YS, Lee JW, Jin SH (2013) Synth Met 175:68–74

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Top Curr Chem (Z) (2016) 374:64 15. Jiang J, Xu Y, Yang W, Guan R, Liu Z, Zhen H, Cao Y (2006) Adv Mater 18:1769–1773 16. Xu Y, Guan R, Jiang J, Yang W, Zhen H, Peng J, Cao Y (2008) J Polym Sci Part A Polym Chem 46:453–463 17. Mei C, Ding J, Yao B, Cheng Y, Xie Z, Geng Y, Wang L (2007) J Polym Sci Part A Polym Chem 45:1746–1757 18. Wu FI, Yang XH, Neher D, Dodda R, Tseng YH, Shu CF (2007) Adv Funct Mater 17:1085–1092 19. Shao S, Ding J, Wang L, Jing X, Wang F (2012) J Am Chem Soc 134:20290–20293 20. Poulsen DA, Kim BJ, Ma B, Zonte CS, Fre´chet JMJ (2010) Adv Mater 22:77–82 21. Chen Q, Liu N, Ying L, Yang W, Wu H, Xu W, Cao Y (2009) Polymer 50:1430–1437 22. Zhen H, Xu W, Yang W, Chen Q, Xu Y, Jiang J, Peng J, Cao Y (2006) Macromol Rapid Commun 27:2095–2100 23. Lee PI, Hsu SLC, Lee JF (2008) J Polym Sci Part A Polym Chem 46:464–472 24. Liu J, Cheng Y, Xie Z, Geng Y, Wang L, Jing X, Wang F (2008) Adv Mater 20:1357–1362 25. Lin Z, Lin Y, Wu C, Chow P, Sun C, Chow T (2010) Macromolecules 43:5925–5931 26. Chen L, Li P, Cheng Y, Xie Z, Wang L, Jing X, Wang F (2011) Adv Mater 23:2986–2990 27. Zou Y, Liu Y, Qin J, Yang C (2013) Chin J Polym Sci 31:938–945 28. Chen L, Li P, Tong H, Xie Z, Wang L, Jing X, Wang F (2012) J Polym Sci Part A Polym Chem 50:2854–2862 29. Zhu M, Li Y, Cao X, Jiang B, Wu H, Qin J, Cao Y, Yang C (2014) Macromol Rapid Commun 35:2071–2076 30. Ho CL, Chui CH, Wong WY, Aly SM, Fortin D, Harvey PD, Yao B, Xie Z, Wang L (2009) Macromol Chem Phys 210:1786–1798 31. Ho CL, Wong WY, Yao B, Xie Z, Wang L, Lin Z (2009) J Organomet Chem 694:2735–2749 32. Goudreault T, He Z, Guo Y, Ho CL, Zhan H, Wang Q, Wong KL, Fortin D, Yao B, Xie Z, Kwok WM, Wong WY, Harvey PD (2010) Macromolecules 43:7936–7949 33. Adamovich V, Brooks J, Tamayo A, Alexander AM, Djurovich PI, D’Andrade BW, Adachi C, Forrest SR, Thompson ME (2002) New J Chem 26:1171–1178 34. Andrade BWD, Brooks J, Adamovich V, Thompson ME, Forrest SR (2002) Adv Mater 14:1034–1036 35. Furuta PT, Deng L, Garon S, Thompson ME, Fre´chet JMJ (2004) J Am Chem Soc 126:15388–15389 36. Liu J, Lam JWY, Tang BZ (2009) Chem Rev 109:5799–5867 37. Lam JWY, Tang BZ (2005) Acc Chem Res 38:745–754 38. Song HJ, Kim DH, Lee TH, Moon DK (2012) Eur Polym J 48:1485–1494 39. Chien CH, Liao SF, Wu CH, Shu CF, Chang SY, Chi Y, Chou PT, Lai CH (2008) Adv Funct Mater 18:1430–1439

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Top Curr Chem (Z) (2016) 374:36 DOI 10.1007/s41061-016-0036-0 REVIEW

Luminescent Iridium Complexes Used in LightEmitting Electrochemical Cells (LEECs) Adam F. Henwood1 • Eli Zysman-Colman1

Received: 26 March 2016 / Accepted: 9 May 2016 / Published online: 6 June 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract Cationic iridium(III) complexes represent the single largest class of emitters used in light emitting electrochemical cells (LEECs). In this chapter, we highlight the state-of-the-art emitters in terms of efficiency and stability in LEEC devices, highlighting blue, green, yellow/orange, red and white devices, and provide an outlook to the future of LEECs. Keywords Light-emitting electrochemical cells  Phosphorescence  Iridium  Electroluminescence

1 Introduction Luminescent materials based on iridium(III) complexes have become the ‘‘go to’’ material when designing emitters for solid-state lighting (SSL) applications. Since the first report of a phosphorescent emitter in an organic light-emitting diode (OLED) [1], iridium complexes have come to be the most widely used class of emitters employed for this purpose by virtue of their efficient spin–orbit coupling (SOC) processes that relax the spin selection rule that otherwise forbids T1 to S0 transitions [2–4]. In an electroluminescent device, spin statistics necessitates that 75 % of excitons generated in the device are in the triplet state, while the remainder

This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli, Henk Bolink. & Eli Zysman-Colman [email protected]; http://www.zysman-colman.com 1

Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, UK

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are in the singlet state. Therefore, iridium complexes are able to harvest 100 % of the excitons. Aside from OLEDs, an alternative class of organic lighting device that is gaining attention is the light-emitting electrochemical cell (LEEC) [5–8]. In contrast to OLEDs, where the emitter is typically a charge-neutral chromophore, LEECs based on phosphorescent emitters utilize intrinsically charged ionic transition metal complexes (iTMCs) as the emitters in the device. The charged nature of the emitters confers a unique operating mechanism to the LEEC, whereby application of a bias leads to a slow migration of the ions in the device to the relevant electrodes. As this migration occurs, the barrier to charge injection drops significantly, meaning that only low work-function electrodes such as Au, Ag, or Al are required to operate these devices. Using air-stable electrodes thereby enables these devices to be solution processed and thus make them potential candidates for industrial scale processing. However, despite the promise of solution processing, to date the performance metrics of LEECs (device efficiency, stability) have remained some way short of their OLED counterparts, and significant improvements are needed if these devices are to become widely adopted in the future. In this chapter, we will first outline the use of the most frequently employed iridium complex in LEECs, [Ir(ppy)2(bpy)](PF6), 1, (where ppyH is 2-phenylpyridine and bpy is 2,20 -bipyridine) and demonstrate why its photophysical properties make it an attractive candidate for LEEC applications. We will next detail all the examples where this emitter has been reported in a device and how the performance of different devices changes as a function of study design and device architecture. We will then use the well-understood properties of this complex as a reference to contrast the performance of LEECs employing new emitters that differ from 1 through modulation of the substituents on the ligand scaffolds. In doing so, we will identify champion devices, in terms of efficiency and stability, categorized according to the emission color of the device, covering: blue, green, yellow/orange, red and white devices. We will provide insight into how the physical and chemical properties of the emitters employed in these devices confer such good performances, with a view to informing future molecular design of emitters for LEECs. Finally, we will conclude by offering a perspective on LEECs for the future.

Fig. 1 Structures of [Ir(ppy)2(bpy)](PF6), 1, and its tert-butyl analogue, [Ir(ppy)2(dtbubpy)](PF6), 2, which are widely studied iridium complexes employed in LEEC devices

123

PF6

N

N Ir

PF6

N N

Ir

N

N

1

2

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N N

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Top Curr Chem (Z) (2016) 374:36

2 [Ir(ppy)2(bpy)](PF6) 2.1 Syntheses One of the most extensively explored emitters for LEECs is the archetypal cationic iridium complex 1. Although not the first example of an iridium complex tested in a LEEC, (this distinction goes to its cousin, [Ir(ppy)2(dtbubpy)](PF6), 2, where dtbubpy is 4,40 -di-tert-butyl-2,20 -bipyridine) [9], the simple structure within this large family of cationic complexes makes it a useful reference to compare the performance of LEECs (Fig. 1). By far the most popular protocol for synthesizing this and related complexes is by the route shown in Scheme 1. The synthesis proceeds first by isolation of the ldichloro-bridged cyclometalated iridium dimer intermediate, following refluxing an iridium(III) salt in high boiling alcoholic solvents such as 2-ethoxyethanol, as first demonstrated by Nonoyama in 1974 [10]. Although Nonoyama demonstrated this synthesis using Na3[IrCl6] as the iridium source, IrCl3.nH2O is now by far the most popular source of iridium used for synthesizing these complexes. The dimer can then be easily cleaved in the presence of a neutral N^N ligand under mild conditions (such as refluxing DCM/methanol) [11], with the complex isolated as its chloride salt. While the chloride salts of these complexes do function LEECs, chloride anions have been implicated to negatively impact device stability (vide infra) [12] and thus typically the chloride anion is exchanged through a metathesis reaction for a tetrafluoroborate (BF4-) or most commonly a hexafluorophosphate (PF6-) anion instead. The performance of the LEEC can be severely impaired by the presence of trace impurities, such as chloride [12] or water [13]. Therefore, several groups have explored alternative synthetic protocols in order to minimize their presence. For example, the Housecroft group initially demonstrated that chloride-free [Ir(ppy)2(bpy)](PF6) could be isolated by performing the anion metathesis reaction with a mixture of NH4PF6 and AgPF6 [12]. Removal of AgCl by filtration over Celite is facile, leaving the PF6- anion as the only counterion in the sample as confirmed by elemental analysis. Device studies verified the merits of this synthetic route, with the chloride-free samples showing luminance levels roughly double that of samples containing just 1 % chloride. The group’s synthetic methodology has evolved since this initial report, with recent efforts demonstrating that cleaving the l-dichloro-

N

N

N IrCl3.nH2O + 2

a

Cl Ir

Ir N

Cl

N

N b

Ir

N N

N

Scheme 1 General protocol for the synthesis of [Ir(ppy)2(bpy)]?. a2-EtO-C2H4OH/H2O (4:1 v/v), 110 °C, N2, 19 h. b2,20 -bipyridyine (2.1 equiv.), CH2Cl2/MeOH (1:1 v/v), 40 °C, N2, 19 h

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bridged dimer with a silver salt in the presence of a weakly coordinating solvent allows for isolation of chloride-free solvento-complex intermediates, which can be further reacted with the ancillary ligand to afford the final complexes in excellent purity (and crucially, free of chloride counterions). Devices fabricated from complexes synthesized using this methodology showed exceptionally long-lived stability, with reported t1/2 values (time taken for the luminance to drop to half its maximum) in excess of 2800 h, even when operated at a notably high pulsed current density of 300 A m-2 [14]. These devices will be discussed in more detail in Sect. 5 (Scheme 2). Alternatively, the Baranoff group has explored using iridium(I) precursor materials in lieu of the more conventional iridium(III) chloride salts. Often, iridium dimers isolated from the IrCl3.nH2O reaction are not pure, but are nevertheless used without purification in the subsequent cleavage step. Purification of the final complex usually involves chromatography and/or recrystallization. However, purifying in this manner can be arduous or sometimes not possible at all. The Baranoff group has shown that the dimer [Ir(COD)(l-Cl)]2, bearing labile 1,4cyclooctadiene (COD) ligands, is more amenable to cyclometalation, with reactions proceeding in shorter times (usually 3 h) and affording much cleaner isolated dimers that facilitate the final purification process [15, 16]. They have also reported improved device performance based on materials synthesized in this manner [17] (Scheme 3). 2.2 Photophysics The relevant photophysical and electrochemical data for 1 is summarized in Table 1. In acetonitrile solution at room temperature 1 is an orange-yellow emitter

PF6 N

N

N Cl

N

R

N

N

i)

Ir

Ir

(a)

Ir

ii)

Cl

Ir

PF6

R

R

N

i)

Ir

R

N

R

H O

Ir

Cl

N

N

N

N

Cl

(b)

N

N

R

O H

Me

R ii)

PF6

N Ir

Me R

N N

N

Scheme 2 Two different protocols for isolating chloride-free [Ir(ppy)2(bpy)](PF6)-type complexes reported by Housecroft et al. [14]. a (i) 2,2’-bipyridine (2.0 equiv.), MeOH, MW 120 °C, 14 bar, 2 h; (ii) excess NH4PF6 and AgPF6, r.t., 1 h. b R is a 4-phenyl substituent or 4,6-diphenyl substituent (i) MeOH, AgPF6, r.t., 2 h; (ii) 2,20 -bipyridine (1.0 equiv.) MeOH, NH4PF6, r.t., 1 h

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R

Ir

Cl Cl

N Ir

Cl

a

+

Ir

Ir

R

R

R

N

N

N

Cl

N

R

Scheme 3 Protocol for synthesizing [Ir(ppy)2(l-Cl)]2-type dimer complexes from [Ir(COD)(l-Cl)]. R is either an unsubstituted ring or a 2,4-difluorophenyl substituted ring. a2-EtO-C2H4OH, 110 °C, N2, 3 h

Table 1 Relevant photophysical parameters for [Ir(ppy)2(bpy)](PF6), 1 References

1 kabs/nm [e (9104/M-1 cm-1)]a

265 [4.17], 310 [1.29], 375 [0.60], 420 [0.26]

[27]

kem(sol)/nma,b

605

[22]

kem(film)/nmc

587

[21]

UPL(sol)/%b,d

9

[22]

UPL(film)/%c,e

34

[21]

UPL(film)/%e

66

[23]

se/lsa,b

0.43

[21]

a

Measured in MeCN at 298 K

b

Measured under deaerated conditions

c

Film composition: 1:1 iridium complex to ionic liquid

d

Using Ru(bpy)3(PF6)2 as the standard (UPL = 9.5 % in MeCN), and scaled according to this value

e

Measured using an integrating sphere

with a broad, unstructured emission centered at 585 nm, with a triplet lifetime of 0.43 ls. This emission profile is characteristic of many [Ir(C^N)2(N^N)]? complexes, identified by Gu¨del as comprising a mixed charge transfer (CT, Fig. 2) triplet excited state consisting of CT transitions between the metal and the N^N ancillary ligand (metal-to-ligand charge transfer, 3MLCT) and between the phenyl groups of the C^N ligands and the N^N ancillary ligand (ligand-to-ligand charge transfer, 3LLCT) [18–20]. The spin density of the triplet state is thus delocalized over the entire complex (Fig. 3). Upon cooling to 77 K, the emission is hypsochromically shifted but remains unstructured. This rigidochromic blueshifting of the emission upon cooling is a further hallmark of the mixed CT nature of the emission, which is stabilized at ambient temperature by polar aprotic solvents such as MeCN. Aside from 3MLCT/3LLCT excited states, [Ir(C^N)2(N^N)]? complexes can also demonstrate structured emission profiles that are attributed to centered radiative decay (3LC). It is not uncommon for cationic iridium complexes to exhibit emission from a mixture of 3LC and 3MLCT/3LLCT states [3]. The photophysical properties of many iridium complexes in the solid state are very different to the solution state. In the case of 1, the emission energy in thin films

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Fig. 2 UV-Vis absorption spectra and emission spectra of 1 in MeCN. 77 K emission spectrum of 1 in 2-MeTHF

Fig. 3 DFT computed Kohn–Sham orbitals for the HOMO (left) and LUMO (middle) of 1. Spin density of the T1 state of 1 (right)

compared to MeCN solution is virtually unchanged (a somewhat rare phenomenon), but the UPL values differ dramatically. Reports of the UPL of 1 in MeCN have been somewhat variable ranging from 6 to 14 % [21, 22], but ultimately are rather low, while in the ‘LEEC’ film (containing the complex and an ionic liquid, IL, additive in a 1:1 molar ratio) or in a doped film (5 wt % in PMMA) the UPL values are substantially higher (34 % [21] and 66 % [23], respectively). The increased brightness in the solid state is plausibly attributed to rigidification of the local environment that inhibits molecular motions that otherwise non-radiatively deactivate the excited state. Although clearly a desirable feature, such effects are difficult to predict, with this rigidification phenomenon often in competition with self-quenching processes that lower the UPL. Self-quenching is particularly

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1.5 mA 1.5 mA

ITO/PEDOT:PSS/Ir:[EMIM][PF6] (1:1)/Al

ITO/PEDOT:PSS/Ir:IL (4:1)/Al

ITO/PEDOT:PSS/Ir:IL (1:1)/Al

ITO/PEDOT:PSS/Ir:[HMIM][PF6] (4:1)/Al

ITO/PEDOT:PSS/Ir:[HMIM][PF6] (1:1)/Al

ITO/PEDOT:PSS/Ir/Al

ITO/PEDOT:PSS/Ir ? 0.1wt% KPF6/LiF/Al

ITO/PEDOT:PSS/Ir ? 0.1wt% LiPF6/LiF/Al

ITO/PEDOT:PSS/Ir ? 0.33wt% LiPF6/LiF/Al

ITO/PEDOT:PSS/Ir ? 0.1wt% NH4PF6/LiF/Al

ITO/PEDOT:PSS/Ir/LiF/Al

ITO/PEDOT:PSS/Ir:IL (4:1)/Al

5

6

7

8

9

10

11

12

13

14

15

16

3.0 V

Reprinted from the journal

31

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ITO/PEDOT:PSS/Ir:IL (3:1)/Al

ITO/PEDOT:PSS/Ir:IL (4:1)/Al

ITO/PEDOT:PSS/Ir:IL (4:1)/Al

20

21

22

269 586 153 302 219

1.37a 0.06a 2.2a 0.37a 11.5a 4.6

0.7

7.2

49

77

0.003

Ca. 850

Ca. 255

ca. 260

ca. 450

ca. 900

375

334

80

1410

3030

4950

1560

615

0.05a

63

375 497

0.7

334

219

Lmax (cd m-2)

0.4a

7.2

70.2

ton (h)

Ca. 3.0

Ca 9.5

10.0

7.1

2.8

2.4

2.8

6.0

9.9

3.1

CE (cd A-1)

5.6

3.0

0.77

0.92

1.97

3.21

1.01

5.6

3.0

2.2

EQE (%)

16.3

8.7

1.9

2.2

3.8

5.8

2.3

6.1

14.1

5.6

16.7

8.6

4.9

4.6

16.3

8.7

6.1

PE (lm W-1)

7.8

70

167

199

137

37

295

668

5.5

134

4.3

103

4.1

81

7.8

69

668

t1/2 (h)

[39]

[39]

[38]

[12]

[12]

[21]

[21]

[37]

[37]

[37]

[37]

[37]

[36]

[36]

[36]

[36]

[36]

[36]

[36]

[23]

[23]

[23]

References

a

ton times defined as time to 100 cd m-2 luminance

Lmax is maximum luminance observed from device, kEL is electroluminescent emission maximum, ton defined as time to reach maximum luminance, CE is current efficiency, EQE is external quantum efficiency, PE is power conversion efficiency, t1/2 is the time taken for the device luminance to fall to half the maximum value

3.5 V at 333 K

3.5 V at 293 K

100 Am-2

ITO/PEDOT:PSS/Ir:IL (4:1) ? trace Cl-/Al

19

3.0 V 100 Am-2

ITO/PEDOT:PSS/Ir:IL (1:1)/Al

ITO/PEDOT:PSS/Ir:IL (4:1)/Al

17

18

3.0 V

1.5 mA

1.5 mA

1.5 mA

3.0 V

3.0 V

3.0 V

3.0 V

3.0 V

3.0 V

3.0 V

ITO/PEDOT:PSS/Ir:IL (1:1)/Al

ITO/PEDOT:PSS/Ir:[EMIM][PF6] (4:1)/Al

3

3.0 V

3.0 V

Bias

4

ITO/PEDOT:PSS/Ir/Al

ITO/PEDOT:PSS/Ir:IL (4:1)/Al

1

2

Device config.

Entry

Table 2 Summary of LEECs reported employing 1

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problematic for LEECs since, unlike OLEDs, the emitters are not normally doped into host matrices; typically, the emissive layers are neat films or contain small amounts of IL doped into the host emitter (4:1 and 1:1 weight by weight ratios are the most common configurations), leading to films that show lower UPL compared to solution and thus lower device efficiencies as well. Furthermore, the emission energies of these complexes can change substantially in the solid state. Small red shifts frequently occur due to effects such as aggregate formation but on occasion substantial red-shifting (as high as 71 nm, or 2037 cm-1) [24] or even blue-shifting have been reported [25, 26]. The example given here for 1 demonstrates some of these effects, but they will be revisited in multiple instances throughout this chapter. Aside from characterizing its photophysical parameters, it is important to determine the electrochemical properties of these complexes as well. Normally, this is done in order to estimate the energies of the HOMO and LUMO levels [28, 29]. These values are particularly important in the context of OLEDs so that the energy levels of the emissive material can properly align with those of the host materials and charge transport layers. For LEECs, this consideration is only important when aligning the energy levels of host–guest systems [30–32], but not for more traditional ‘single emitter’ devices since the emitters here also carry out the role of charge transport. The dual charge transport/light-emitting role of the iTMCs in the device is the most important contributing factor for explaining why LEECs are invariably less stable than their OLED counterparts. Thus, an important feature to look for when characterizing the electrochemical properties of the emitter is for reversible oxidation and reduction waves since good reversibility suggests that the emitter might be more resilient to electrochemical degradation when operating in the device. With the support of DFT calculations (Fig. 3), the nature of the oxidation and reduction of 1 has been assigned. The oxidation is ascribed to the IrIII/IV redox couple combined with contribution from the phenyl rings of the cyclometalating ligands. The degree of reversibility of this redox couple depends on the magnitude of the contribution from the C^N ligands; greater contributions results in a greater degree of irreversible electrochemistry. The reduction is assigned to a highly reversible bpy0/1- redox couple, which is believed to be an important factor in giving devices based on 1 impressive stability metrics (t1/2 = 668 h, vide infra) [23]. 2.3 Devices To the best of our knowledge, 1 represents the most investigated iTMC emitter in an LEEC. The performances of all of the devices using 1 have been summarized in Table 2. The device architectures are somewhat more complex than the simplest reported LEECs, which themselves are single layer devices comprised of a neat film of emissive iTMCs sandwiched between a transparent conducting anode, usually indium tin oxide (ITO), and an air-stable cathode such as gold, silver, or aluminium. The ability to use air-stable, high-work-function electrodes is an advantageous characteristic of LEECs compared to OLEDs.

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Upon application of an external bias to the LEEC, there is a large initial barrier to charge injection. As migration of the ions in the emissive layers progresses, an electric double layer forms and the barrier to injection drops significantly until eventually charge injection at very low driving voltages (typically ca. 3 V) becomes facile. A charge-hopping mechanism ensues akin to that found in an OLED and emission is realized upon radiative decay of the formed exciton. Although these single-layer devices readily generate light, various groups have shown in the last decade that small modifications to the device architecture can yield vastly improved LEEC performance. Figure 4 depicts these modifications in what can now be considered the most popular device architecture for LEECs, bearing two crucial features that differentiate it from the early reports. As mentioned, LEECs do not require charge-injecting layers to function, but nevertheless the ITO anode is invariably coated with PEDOT:PSS (an electrically conducting mixture of poly(3,4ethylenedioxythiophene) and poly(styrenesulfonate)) since it facilitates the formation of uniform iTMC thin films on the ITO substrate and it improves hole injection. Devices fabricated in the absence of PEDOT:PSS are prone to forming crystallinelike domains within the film, which can have deleterious effects on the device performance and batch-to-batch reproducibility [33–35]. These two-layer devices nevertheless can give good performance, provided that the optoelectronic properties of the iTMC are also favorable. Entry 1 effectively demonstrates this principle with the device based on 1 exhibiting a remarkably long lifetime of 668 h. The authors attribute this stability to the relatively large calculated 3MC-T1 energy gap for 1 [23]. Theory and experimental observations have implicated 3MC states in an elongation of the Npyridyl-Ir bond of the C^N ligands of [Ir(C^N)2(N^N)]? complexes, which accounts for the efficient nonradiative quenching resulting from these states [40]. In addition, work on ruthenium(II) complexes has suggested that this bond lengthening/breaking process within the 3MC state introduces a free coordination site that allows small molecules such as water to coordinate to the metal, quenching the emission and leading to degradation products within the device [13, 41]. Thus, devices employing complexes with a small 3MC-T1 energy gap tend to not be stable unlike the case with 1 where the device stability is enhanced. In addition, the reversible electrochemistry in 1 results in its capacity to act as an effective charge transport material and thereby resist electrochemical degradation processes that also impact device lifetimes. Aside from the addition of PEDOT:PSS, an IL additive is also normally doped into the emissive layer to enhance charge mobility and to reduce turn-on times (ton,

Fig. 4 Typical device architecture of a LEEC employing 1 as the emissive layer

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defined as the time taken to reach maximum luminance under constant bias) [42]. The most common IL used in LEECs is 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6]. Entries 1–3, Table 2, demonstrate the remarkable differences in LEEC performance produced by just varying the ratio of 1 and [BMIM][PF6] in an otherwise identical device configuration [23]. In the absence of IL, an extremely long ton of more than 70 h is observed. This is attributed to low ionic mobility of the PF6- anions, possibly due to the formation of microcrystalline domains in the film. Addition of IL circumvents this problem, improving ionic conductivity and thus charge transport in the device in addition to disrupting any possible crystallite formation. Entries 2 and 3, Table 2, show the performances of two devices with different weight ratios of iTMC to IL (4:1 and 1:1, respectively). Both devices show improved ton times (7.2 and 0.7 h, respectively) as well as improved EQEs (3.0 and 5.6 %, respectively); where EQE is the external quantum efficiency, defined as the ratio of electrons injected into the device to photons outcoupled from the device. However, the addition of IL comes at the considerable cost of device stability as measured by its lifetime, t1/2 (t1/2, defined as the time taken for the device to reach half of its maximum luminance). In the absence of IL, this device lasts for up to 668 h, but lifetimes are dramatically reduced for the devices constituting iTMC:IL ratios of 4:1 (69 h) and 1:1 (7.8 h). This further reduction of device stability with increasing amounts of IL is representative of the behavior in LEECs, regardless of iTMC emitter, and illustrates the trade-off between t1/2 and ton that has been a significant challenge to overcome. The compromise between attaining good device performance (EQEs, ton) and reasonable device lifetimes has meant the most popular iTMC:IL ratio employed has been 4:1 weight by weight. Significant efforts have been expended to overcome this compromise in device performance. The most successful strategy is based on application of a pulsed current driving method in place of the more established constant voltage method and will be discussed multiple times in the following sections of this chapter. The majority of the remaining entries in Table 2 illustrate other approaches researchers have taken to tackle this issue, which will be discussed here. Entries 4–10 summarize a study undertaken by Bolink et al. [36] to elucidate how using different ILs impact ton. It should be noted that in this study they define ton as the time taken for the LEEC to reach 100 cd m-2, as opposed to maximum luminance in the device. The study uses ILs comprised of imidazolium cations of differing N-alkyl chain length: 1-ethyl-, 1-butyl- and 1-hexyl-3-methylimidazolium, each as their hexafluorophosphate salts. They demonstrated that the ethyl analogue, which is the most conducting IL, demonstrated the fastest ton times (0.4 h for 4:1 iTMC:IL ratio). Although the t1/2 value was lowest for this IL, it was reasoned that the higher luminance values observed for this device were the major contributing factor for this shortened t1/2 and that actually the overall device stability had not been significantly impacted even when compared with the control device bearing no IL. Nevertheless, despite this study, [BMIM][PF6] is still the most popular choice of ionic liquid. In a similar guise, entries 11–15 and Table 2, summarize the recent contribution by Slinker et al. [37] where they explored the effect on the response time of doping

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in inorganic salt additives. They reasoned that the large size of the iTMC cations renders the complexes to be essentially stationary in the device, meaning that upon initial application of a bias, the cation density at the cathode is initially much lower than the anion concentration at the anode, leading to an imbalance of charge injection into the device. However, by doping in small amounts of alkali metal cation hexafluorophosphate salts, they demonstrated that indeed fast ton times under constant current conditions can be achieved, with the LiPF6 additive showing the best performance due to the small size of the Li? cation. Crucially, this also led to more balanced charge injection that ultimately improved not just response times, but also luminance values and EQEs. Aside from functioning as a useful standard for exploring new device physics, 1 also serves as a helpful reference compound for chemists to compare the performance of devices operating with new emitters. However, as alluded to above, reproducibility across devices can be a challenge. Entries 1 and 10, 2 and 16, and 3 and 17 reproduce each other well. However, comparison of entries 2 and 3 with the corresponding devices in entries 6 and 7 demonstrates considerable variation in device performance. The device in entry 6 in particular shows significant differences in the luminance (269 cd m-2) and t1/2 (103 h) values compared with its counterpart LEEC in entry 2 (334 cd m-2 and 69 h). Housecroft and co-workers have recently addressed this issue, demonstrating that the poor batch-to-batch reproducibility of these devices was attributable at least in part to trace Cl- in the sample [12]. As outlined above, using silver salt-assisted syntheses they were able to isolate 1 with improved purity, leading to devices with superior luminance levels (ca. 900 cd m-2, entry 18) compared with those containing trace chloride impurities (ca 450 cd m-2, entry 19). High-purity samples are crucial for achieving good device performance, and indeed aside from the presence of Cl-, trace water has also been implicated in impacting device performances of ruthenium-based LEECs [13]. Entries 20-22 are beyond the scope of this discussion, with the relevant references discussing the device physics surrounding the peculiar operational mechanism of LEECs. They are included for the reader’s reference to illustrate additional examples reported where 1 has been the constituent emitter in the LEEC [38, 39].

3 Blue To date, attaining simultaneously efficient, stable, and deep-blue-emitting LEECs remains the most pressing issue for LEEC development. The challenge of obtaining high-performance blue-emitting devices is well-known for both organic and inorganic light-emitting devices, and while it has largely been addressed in the former case, this topic is still the source of very active research for OLEDs. For example, a recent report detailed the performance of a new champion blue OLED, which showed simultaneously deep-blue emission and high device efficiency (EQE = 10.1 %) [43]. However, this efficiency value still falls well below the efficiencies reported for red or green (EQE * 30 %), or even sky-blue OLEDs (EQE [20 %) [4]. Reprinted from the journal

35

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123

123

452, 480

492

493

4a

4b

5

6

3

4

5

6

477, 500

9

10

9

10

36

[email protected]

3

0.001

13

54

24

40

100

20

20

UPL (%)

37

16.1

15.4

113

15

39

1700

8

23

39

Lmax (cd m-2)

8.7

0.6

0.2

4.7

18.3

8.4

0.51

0.65

CE (cd A-1)

7.6

3.4

4.6

14.4

0.21

0.28

EQE (%)

1.95

18

11

32.1

PE (lm W-1)

15.83

29.8

24.3

0.017

2.17

t1/2 (h)

0.26

0.41

0.43

0.24

0.22

0.25

0.20

0.20

0.33

0.20

CIE (x)

0.48

0.53

0.53

0.40

0.41

0.46

0.41

0.36

0.45

0.28

CIE (y)

486, 512

556

560

500

474, 494

497

567

460, 490, 526

460, 486

kEL (nm)

[51]

[50]

[49, 50]

[48]

[47]

[46]

[33]

[45]

[44]

[44]

References

b

a

Measurement in 2-MeTHF

Measurement in DCM

kPL is the solution-state emission maximum in MeCN, UPL is the photoluminescence quantum yield in deaerated solution, Lmax is the maximum luminance observed from device, CE is the current efficiency, PE is the power conversion efficiency, t1/2 is the time taken for the device luminance to fall to half the maximum value, kEL is the electroluminescence emission maximum

480, 509

472, 501

7

8

7

8

440

472, 490

489

452, 480

3

3a

1

kPL (nm)

2

Complex

Entry

Table 3 Summary of LEECs employing blue-emitting iTMCs

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In the case of LEECs, the situation is more dire. To date, no LEEC has even been reported emitting blue light close to the ‘ideal deep blue’ CIE coordinate (CIE: 0.15, 0.06, as defined by the European Broadcast Union, EBU), let alone with good efficiency. Furthermore, the stability of these sky-blue LEECs is demonstrably inferior than their OLED counterparts; device lifetimes are often in the tens of hours at best, compared with thousands of hours reported for yellow or orange LEECs. Given that blue is a necessity in attaining white light from a typical RGB color combination, overcoming this issue is of pressing concern. A summary of the relevant performance metrics of blue LEECs discussed herein is given in Table 3. 3.1 Efficiency The widely accepted paradigm to achieve blue emission requires that the HOMO be stabilized with electron-withdrawing groups located on the phenyl ring of the C^N ligands and the LUMO be destabilized with electron-donating groups located on the ancillary N^N ligand. The most commonly used electron-withdrawing groups used are fluorine atoms [52] with examples of complexes shown in Fig. 5. Further blueshifting of the emission can be achieved by incorporating electron-rich heterocycles within the ligand frameworks of either the N^N (3) or C^N (4) ligands [33, 44, 45]. This strategy is most effective when this structural modification occurs within the ancillary ligand, as exemplified by the greater blue shift in emission observed for 3 (kPL = 451, 484 nm in MeCN) compared to 4 (kPL = 492 nm in DCM). The blue emission in solution observed for 3 translates to its performance in the device, with CIE coordinates in the sky blue of (CIE: 0.20, 0.28). Despite being reported in 2008, this LEEC nevertheless remains the bluest reported for any iridium emitter to date. However, the emission observed for this device is still a long way from the ideal ‘deep-blue’ coordinates required in RGB devices (CIE 0.15, 0.06). Furthermore, this device functions in the absence of ionic liquid, which results in essentially impractical turn on times (ton = 7.1 h). When an ionic liquid dopant is added, the response time shortens dramatically (ton = 1.1 h) but the observed color is also greatly red-shifted (CIE 0.33, 0.45). Aside from the device based on 3 achieving the bluest emission reported to date, the performance of 3 in the device is relatively poor, with low efficiencies and brightness levels reported for both the IL

F

N

N F F

Ir

N N

N

PF6

F

PF6

F F

N Ir

N

N

N N

N

F

F

4

3

Fig. 5 Blue-emitting iridium complexes bearing pyrazole-type ligands

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free device (EQE = 0.28 %, Lmax = 39 cd m-2) and IL-doped device (EQE = 0.21 %, Lmax = 23 cd m-2). By contrast, although complex 4 displays an emission profile that is strongly redshifted compared to 3, it is a much more efficient emitter in the device. Indeed, this high efficiency has made it a favored choice of emitter either for blue-emitting LEECs [30, 33, 53], or as the blue component in white LEECs [45, 54–56]. The performance of this emitter in white LEECs will be revisited in Sect. 7. As a blue/blue-green emitter, the device reported based on 4 displays extraordinarily high device efficiencies (EQE = 14.4 %, P. E. = 32.1 lm W-1). These values can vary significantly, depending on the device architecture. For example, the first reported LEEC employing this complex gave a comparably low overall EQE of 4.4 %, using a typical device architecture of ITO/PEDOT:PSS/Ir/Al. However, it is worth noting that this device displays very high brightness for a LEEC (entry 4, Lmax = 1700 cd m-2) [33]. Since then, Wong in particular has explored different means by which charge injection and transport can be improved using this complex as an emitter. For example, it was shown that by doping small amounts (up to 1.0 wt %) of a pure organic NIR emitting laser dye, 3,30 -diethyl2,20 -oxathiacarbocyanine iodide, DOTCI, (Fig. 6) into the emissive layer of complex 4, higher device efficiencies could be obtained (EQE = 12.8 % for 0.01 wt % DOTCI) than without any dopant (EQE = 9.06 % for the pristine device DOTCI) [57]. The intrinsic hole transporting properties of 4 leads to the formation of the charge recombination zone near the cathode, which facilitates exciton quenching. This charge imbalance can be mitigated by doping in DOTCI. DOTCI has a much higher HOMO than 4 and therefore impedes hole transport but has a similar LUMO that thereby keeps electron mobility balanced. Furthermore, the poor spectral overlap between the emission of 4 and the absorption of DOTCI results in minimal quenching of the iridium-based emission by energy transfer to the guest, which would otherwise negatively impact the efficiency of the device. Aside from exploring dopants, Wong has also used 4 to study the effects of incorporation of additional layers to the LEEC architecture to further balance charge transport. The archetypal device based on 4 (ITO/PEDOT:PSS/4/Ag) gave reasonably good efficiencies (EQE = 8.5 %). However, the efficiency of the device could be further improved by incorporating a high work function cathode within the device (ITO/PEDOT:PSS/4/Ca/Ag, EQE = 9.6 %) and adding a hole injecting layer as well (ITO/PEDOT:PSS/TPD/4/Ca/Ag, EQE = 10.5 %). It should be noted that the strong hole transporting characteristics of 4 meant that addition of only the hole injecting layer actually impeded device efficiencies (ITO/PEDOT:PSS/TPD/4/ Ag, EQE = 6.8 %), providing further evidence of the necessity of balancing charge injection and transport in the device (Fig. 7). Fig. 6 Organic dopant used by Wong to improve the efficiencies of 4

N

O I

N

O DOTCI

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F

F N

N F F

Ir

N N

F F

N

PF6

N Ir

N

N

N

F

Ph Ph

N N

Ph

N

F 5

6

Fig. 7 Iridium complexes bearing pyridylimidazole ancillary ligands

A different strategy that has been used to improve device efficiencies is by adding steric bulk to the complex to inhibit intermolecular quenching processes. Complexes 5 and 6 demonstrate this strategy. Both complexes are blue-green emitters in acetonitrile solution (kPL = 489 nm for 5 and 472, 490 nm for 6), with 6 blue-shifted due to the additional pyrazole rings incorporated within the cyclometalating ligands. The most important structural difference between 5 and 6 is the presence of the trityl group on the ancillary ligand of 6. This bulky unit serves to increase the molecular spacing between emissive molecules in the film, which leads to reduced excited-state self-quenching that negatively impacts the device efficiencies. Indeed, despite 6 being moderately blue-shifted both in solution and in the device (kEL = 474, 494 nm, CIE 0.22, 0.41) compared to 5 (kEL = 497 nm, CIE 0.25, 0.46), the LEEC with 6 shows greatly improved efficiencies (CE = 8.4 cd A-1 for 5 and 18.3 cd A-1 for 6; EQE = 3.4 % for 5 and 7.6 % for 6). Finally, complex 7 represents the unpredictability in designing new blue emitters for devices. For the ancillary ligand, this complex uses N-heterocylic carbenes (NHCs), which are very strongly r-donating heterocycles that can strongly destabilize the LUMO, invoking a significant blue shift in the emission. The potency of these heterocycles is well known, with near-UV emission having been reported for iridium complexes containing multiple NHCs within the ligand frameworks

F N

PF6

N

F F

Ir N

N

N N N N

F

7

Fig. 8 Blue-emitting iridium complex bearing an NHC ancillary ligand

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[43, 58]. Furthermore, the cyclometalating ligand incorporates a nitrogen ring into the 5-position of the cyclometalating ring. This nitrogen acts as a strongly rwithdrawing unit that serves to stabilize the HOMO in concert with the fluorine rings in the 4,6-positions. Thus, in solution, 7 is the bluest emitter reported to date to have been incorporated into a LEEC (kPL = 440 nm)—significantly bluer than complexes 3 to 6. Unfortunately, for all the effort to blue shift the emission of this complex, ultimately the color of the device is red-shifted, even in comparison with the devices discussed above (kEL = 500 nm, CIE 0.24, 0.40) (Fig. 8). 3.2 Stability Although the examples above have demonstrated that high efficiencies are possible for sky-blue LEECs, the two most significant issues still to be addressed with these LEECs is their lack of deep-blue color and crucially their poor stability. There are many reasons for the poor stability of these devices, but one factor in particular thought to be contributing to poorer device performance is the presence of Caryl-F bonds on the cyclometalating ligands. It has been posited that the highly electron deficient C^N ligands make them susceptible to chemical degradation by nucleophilic aromatic substitution of the fluorine substituent. A study by Bolink and co-workers[16a] on the stability of fluorine-containing green-emitting iridium complexes will be outlined in more detail in Sect. 4, but it is worth noting here that they demonstrated that of the four complexes studied, the complex bearing four fluorine substituents was far less stable than others bearing just two fluorine atoms, providing indirect evidence that indeed (multiply) fluorinated aromatic rings can be implicated in the electrochemical degradation of the emitter in the device. Similar degradation processes are believed to be operative in OLEDs [59, 60], but the harsher environment in the emissive layer of a LEEC means that this effect is more pronounced in this class of electroluminescent device. Thus, there is interest in designing new emitters that emit blue light without the need for fluorine substituents that might negatively impact the stability. In addition, there is interest in adopting hydrophobic substituents within the ligand framework to impede nucleophiles from coordinating to the iridium center and quenching the emission. These two strategies are exemplified by complexes 8 and 9, with 9 in particular representing an all-in-one effort to achieve blue emission without impacting the device stability. Like 7, complexes 8 and 9 use NHCs within the ancillary ligand to destabilize the LUMO of these complexes. Despite both complexes bearing just ppy as the C^N ligands, they are both blue-green emitters in acetonitrile solution (kPL = 472, 501 nm for 8 and 477, 500 nm for 9), with emission strongly blue-shifted and more ligand-centered compared to 1 (kPL = 605 nm) [49, 50] thereby demonstrating the feasibility of blue-shifting emission without using fluorine. Aside from the fluorine-free cyclometalating ligands, 9 also adopts the common intramolecular p-stacking strategy for improving the stability of the emitter in the device, utilizing a pendent phenyl substituent on the N^N ancillary ligand. This ring is predisposed to form an intramolecular p-stacking interaction with the phenyl ring of one of the C^N ligands, enveloping the iridium core in a supramolecularly caged hydrophobic scaffold that shields it from adventitious attack from prospective

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nucleophiles in the device that degrade the emitter [14, 61, 62]. This substitution pattern is more common for six-membered ring systems, such as the ligand 6-phenyl-2,20 -bipyridine (see Sect. 5 for examples), since the intramolecular pstacking distance is usually shorter than in the case of five-membered rings such as the imidazolium ring in 9, and is thus more effective at shielding the iridium core (Fig. 9). The results of combining a fluorine-free ligand scaffold with an intramolecular pstacking interaction do appear to improve the stability of the emitter. The devices with both 8 and 9 are much longer lived (t1/2 = 24.3 h for 8 and 29.8 h for 9) than any of the LEECs with other blue/blue-green emitters discussed so far (e.g., t1/2 = 2.17 h for 4), pointing to some extent to the merits of this strategy. However, it is important to note that the devices based on these materials are also greatly redshifted. While they are blue-green in solution, the devices are essentially yellowgreen (kEL = 560 nm for 8 and 556 nm for 9; CIE: 0.43, 0.53 for 8 and 0.41, 0.53 for 9), which is at least partly accountable for the improved device lifetimes. In addition, although there is a slight improvement in the device lifetime of the LEEC with 9 over that with 8, this effect is not as pronounced as for some examples that will be discussed below. This is because the intramolecular p-stacking interaction in this complex is not as strong as it is for others based on 6-membered ring systems. Finally, the use of complex 10, a structurally related analogue of 9, also attempts to combine strategies for improving stability with strategies for achieving blue emission. The ancillary ligand in this instance contains a pyrazole with coordination through the nitrogen. Complex 10, like 9, is fluorine-free and has an intramolecular p-stacking ring. This complex is red-shifted in MeCN solution compared to 8 or 9 (kPL = 480, 509 nm) but, surprisingly, is much bluer in the device, essentially retaining its solution-state emission characteristics (kEL = 486, 512 nm; CIE: 0.26, 0.48). The blue-shifted emission appears to impact the stability, however, with a lower device lifetime (t1/2 = 15.83 h) compared to the LEECs with 8 or 9. The lower device stability for the device with 10 is probably due in part to its higher brightness compared to the LEECs with 8 or 9 (Lmax = 15.4 cd m-2 for 8, 16.1 cd m-2 for 9 and 37.0 cd m-2 for 10). Nevertheless, although these complexes are the most stable among blue-green LEECs, none of them come close to

PF6

N Ir N

N

N N N N

8

PF6

PF6

N

Ir

N N

N

9

Ir N

N N N

10

Fig. 9 Iridium complexes bearing NHC ancillary ligands

Reprinted from the journal

41

[email protected]

123

123

32

548

18

19

9

10

42

[email protected]

54

190

157

837

1095

1046

1066

1028

757

ca. 20

52

Lmax (cd m-2)

6.1

5.7

6.4

9.6

10.4

9.8

9.8

28.2

38

CE (cd A-1)

2.2

2.3

1.95

2.90

2.99

2.92

2.85

8.2

14.9

7.1

EQE (%)

7

8.6

3.08

5.3

5.3

5.4

5.2

17.1

39.8

26.2

PE (lm W-1)

356

223

0.01

13.2

55

48.3

59.8

98

9

12

t1/2 (h)

0.47

0.44

0.31

0.39

0.42

0.39

0.38

0.38

0.30

0.35

CIE (x)

0.52

0.55

0.57

0.55

0.55

0.56

0.57

0.57

0.45

0.57

CIE (y)

570

555

ca. 540

555

552

558

554

554

525

535

kEL (nm)

[21]

[21]

[66]

[25]

[25]

[25]

[25, 65]

[65]

[64]

[63]

References

kPL is the solution-state emission maximum in MeCN, UPL is the photoluminescence quantum yield in deaerated solution, Lmax is the maximum luminance observed from device, CE is the current efficiency, PE is the power conversion efficiency, t1/2 is the time taken for the device luminance to fall to half the maximum value, kEL is the electroluminescence emission maximum

559

62

41

554

515

16

17

59

52

69

7

555

555

552

8

13

14

4

5

70

28

UPL (%)

15

13

3

512

535

kPL (nm)

6

11

12

1

2

Complex

Entry

Table 4 Summary of LEECs employing green-emitting iTMCs

Top Curr Chem (Z) (2016) 374:36

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Top Curr Chem (Z) (2016) 374:36

commercially relevant stability requirements or even to some of the stability metrics reported for yellow or orange devices (thousands of hours).

4 Green Green emitters, like sky-blue emitters, have also been shown to achieve high efficiencies but relatively low stabilities. Many green-emitting complexes also contain fluorinated cyclometalating ligands, which certainly account for their shorter device lifetimes compared to yellow or orange LEECs. A summary of the emitters discussed in this section is given in Table 4. 4.1 Efficiency Most LEECs reported, even now, employ constant voltage-driving methods as a means of powering the device. Such a method results in generally slow turn-on times, but good performance metrics in terms of brightness and efficiency have been reported. Two of the best performing LEECs were reported employing complexes 11 and 12. The crucial design feature of 11 is that it contains a bulky 4,5-diaza-9,90 spirobifluorene ancillary ligand [63]. The bulk of this ligand ensures that intermolecular quenching in the solid state is minimized. The photoluminescence quantum yield in the neat film (UPL = 31 %) is in fact not measurably different compared to the quantum yield in solution (UPL = 28 %). Ultimately, it is this high neat film quantum yield that accounts for the very good device efficiency (EQE = 7.1 %; PE = 26.2 lm W-1). Similarly, complex 12 employs the bulky 4,40 -di-tert-butyl-2,20 -bipyridine ancillary ligand. In this case, the quantum yields are even higher (UPL = 70 % in solution and 72 % in the film used for the device), giving device efficiencies that are extraordinarily high (EQE = 14.9 %) [64]. Both of these devices are driven at a constant voltage (2.8 V for 11 and 3.0 V for 12). This driving method leads to some drawbacks, including long device turn-on times (ton = 1.5 h for 11 and 0.8 h for 12) and also relatively poor stability (t1/2 = 12 h for 11 and 9 h for 12) for both devices. However, it is worth noting that a pulsed current LEEC (which will be elaborated on below) based on 12 has been

PF6

F

N

N F F

Ir

PF6

F

N

F F

N

Ir

N N

N

N F

F 11

12

Fig. 10 Efficient green-emitting devices based on constant voltage-driving conditions

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reported, demonstrating much lower efficiencies (EQE = 2.83 %) than reported under constant voltage [17] (Fig. 10). In recent years an alternative driving method has become prevalent for operating these devices. Using a pulsed current driving method, high device efficiencies are also possible, but not at the expense of the stability or turn-on times of the devices. Arguably the champion green LEEC published to date is based on complex 13, which uses a pulsed-current driving method to operate the device [65]. In the report, the authors explored driving the devices under a variety of different conditions, including varying the duty cycles from 25 % to 100 % (the latter of which correlates to operation under constant current) and also the average pulsed current density, from 18.75 to 150 A m-2. After much optimization, it was found that a 75 % duty cycle with a pulsed-current density of 25 A m-2 led to the best overall device performance. Crucially, it was found that high device efficiencies (EQE = 8.2 %, CE = 28.2 cd A-1, PE = 17.1 lm W-1) were possible, without adversely affecting the stability (t1/2 = 98 h) or the turn on time (ton = 0.2 s). Indeed, these metrics make this LEEC the best overall performer, certainly when comparing the turn-on times and stabilities of the constant voltage LEECs employing 11 and 12. 4.2 Stability Complex 13 has also been reported as part of a larger study into the stability of iridium complexes bearing fluorinated C^N ligands.[16a] As identified in Sect. 3, such complexes are expected to be unstable, due to the reactivity of such aromatic rings bearing fluorine substituents. To unequivocally study this, Baranoff et al. synthesized complexes 13–16, and studied their performance in the LEEC. All four complexes were designed to have similar photophysical properties (kPL = 552–555 nm, UPL = 52–69 %) and similar device properties (EQE = 2.85–2.99 %, Lmax = 1028–1095 cd m-2), such that the stability data would be directly comparable. They demonstrated that complex 16, bearing four fluorine atoms, shows greatly reduced device lifetimes (t1/2 = 13.2 h) compared with the other three complexes (t1/2 = 48.3–59.8 h for complexes 13–15). The device lifetimes for complexes 13–15 are by comparison rather long for green emitters; indeed, only 18 and 19 are longer lived green emitters in the device (Fig. 11).

PF6

Ir

N

Ir

N

N

PF6

F

N

N F F

PF6

F

N N

F

N

Ir

N

N

14

15

F 13

PF6

F N

N N

F F

Ir

N N

N F 16

Fig. 11 Multiply fluorinated green-emitting iridium complexes. The tetra-fluorinated complex displays much faster device degradation than the bis-fluorinated complexes

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To address issues with the stability of fluorine-containing iridium complexes, there has been interest in developing so-called fluorine-free ligands that are capable of achieving blue-shifted emission. Complex 17, for example, was one of a number of complexes reported utilizing a 2,30 -bipyridine as the cyclometalating ligand to achieve a similar effect as that of the dFppy ligand, with the non-coordinating nitrogen acting to inductively withdraw electron density away from the metal centre, thereby stabilizing the HOMO energy and blue-shifting the emission as a result [66]. Although the stability of the devices based these emitters was not significantly improved (t1/2 = 0.01–2.5 h) it is worth noting that the CIE coordinates of 17 (CIE: 0.31, 0.57) are the closest to pure green (CIE 0.30, 0.60) that have been reported so far. Complexes 18 and 19 provide a good comparison with 13. The electroluminescence of these complexes is only slightly red-shifted (CIE 0.44, 0.55 for 18 and 0.47, 0.52 for 19; kEL = 555 nm for 18 and 570 nm for 19) compared to 13 (CIE 0.38, 0.57, kEL = 554 nm), but they show greatly improved stabilities (t1/2 = 98 h for 13, 223 h for 18 and 356 h for 19). This is attributed to the methyl groups in the ortho- position with respect to the pyridyl nitrogens, which act in a similar fashion to the intramolecularly p-stacking phenyl rings for 9 and 10. It is plausible also that the lack of fluorine substituents appended to complexes 18 and 19 also adds to their stability in the device (Fig. 12).

5 Yellow/orange Moving from green emitters to yellow/orange leads to a pattern becoming apparent: as the color of the device shifts from blue to yellow the efficiency of the devices generally decreases, but the stability improves. Indeed, emitters of these colors surpass all others in terms of stability, with the most stable devices reported to date emitting in this color regime (Table 5).

PF6

N MeO MeO

Ir N

N

N N N

PF6

PF6

Ir

N N N

Ir

N

N

N

17

18

19

N N

Fig. 12 High stability green emitters

Reprinted from the journal

45

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123

123

46

[email protected]

32

33

14

15

645

611

600

583

593

623

574

595

595

570

568

605

595

588

kPL (nm)

2

4

13

43

5

26

2

3

3

47

59

23

7

9

UPL (%)

261

676

1024

684

650

183

130

105

70

290

79

105

16.45

284

395

Lmax (cd m-2)

0.7

2.2

3.5

6.5

3.6

8.2

4.0

3.1

9.7

14.7

13.2

CE (cd A-1)

3.4

0.3

1.1

4.0

4.0

5.5

9.16

6.1

6.1

EQE (%)

8.6

0.46

3.3

10.1

17.1

18.7

26.91

15.3

PE (lm W-1)

2000

[14]

[14]

[72]

[2800

589

[14]

0.44 1204

2800

0.54

[70]

[69]

[68]

[61]

[67]

[47]

[53, 63]

[62]

[46]

References

[62]

594

577

566

580

588

kEL (nm)

[71]

0.44

0.50

0.48

0.47

CIE (y)

950

0.55

0.49

0.51

0.53

CIE (x)

[4000

110

2000

1300

3000

660

t1/2 (h)

kPL is the solution-state emission maximum in MeCN, UPL is the photoluminescence quantum yield in deaerated solution, Lmax is the maximum luminance observed from device, CE is the current efficiency, PE is the power conversion efficiency, t1/2 is the time taken for the device luminance to fall to half the maximum value, kEL is the electroluminescence emission maximum

30

31

12

13

29

29

10

11

27

28

8

9

25

26

6

7

23

24

4

22

3

5

20

21

1

2

Complex

Entry

Table 5 Summary of LEECs employing yellow/orange-emitting iTMCs

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Top Curr Chem (Z) (2016) 374:36

5.1 Efficiency Efficiencies reported for yellow/orange LEECs tend not to be as high as for green LEECs, although several examples of complexes with comparable efficiencies have been reported. For example, the external quantum efficiencies reported for the devices using 20 and 21 are two of the highest (EQE = 6.1 % for both 20 and 21) reported for this color to date [46, 62]. The origin of the high efficiency for 20 is not explained. In solution this complex is not especially emissive (UPL = 9 % in MeCN) and no thin film PL data is reported. Complex 21 is also poorly emissive in solution (UPL = 7 % in MeCN) but in this instance the value reported for the UPL in the film is much higher (UPL = 47 % in a film of iridium complex and [BMIM][PF6] in 4:1 molar ratio), which accounts for the good device performance. As with other complexes previously discussed, the performance of 21 in the device is attributed to the presence of the bulky hydrophobic substituents on the complex, which contribute to decreased quenching of the excitons formed in the device. An added benefit of the substitutions on the ancillary bipyridine ligand is that they improve the stability of the emitter in the device, with a very good device lifetime compared to many other LEECs reported in the literature (t1/2 = 660 h). This result is in contrast with the LEECs using complexes 9 and 10, wherein the shielding of the iridium centre by the intramolecular p-stacking interaction was mitigated somewhat by the use of five-membered pyrazole and imidazolium rings (Fig. 13). The highest efficiency yellow/orange device reported to date is complex 22, which is the fluorine-free analogue of complex 11, using the same 4,5-diaza-9,90 spirobifluorene as the diimine ligand [53, 63]. Steric bulk of this ligand remains an important factor in preventing intermolecular quenching in the device by increasing the spacing between the chromophores, leading to a reasonably bright emitter in solution (UPL = 23 %) and solid state (UPL = 33 % in the ‘LEEC’ film containing the iridium complex and [BMIM][PF6] in a 1.3:1.0 molar ratio). Its initial device efficiency was reported to be 7.1 % using a simple LEEC architecture [ITO/ Ir:[BMIM][PF6] (1.3:1.0 molar ratio)/Ag]. However, studies on improving the carrier injection efficiency of the device have since led to a record quantum efficiency (EQE = 9.16 %) reported for a yellow/orange device, based on a related device architecture [ITO/PEDOT:PSS/Ir:[BMIM][PF6]/Ag. This improved

PF6

PF6

N Ir

N N N

Ir

N

N

N

N N

OC10H21 OC10H21

21

20

Fig. 13 Yellow/orange emitters showing high device efficiencies

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performance is likely to be due to the fact that 22 has preferred electron-transporting characteristics [53], and thus PEDOT:PSS, which is effective as a hole injecting layer, helps to balance charge transport in the device. It is curious to note however, that analogues of 22, using the same diazafluorenyltype ligand (23 and 24) are in fact brighter in solution than 22 (UPL = 59 % and 47 %, respectively), but display poorer performance in the device [67]. These emitters were designed to explore strategies for improving the turn-on time of the LEEC, with the charged groups appended to 24 anticipated to increase the rate of ion separation in the emissive layer similar to that previously demonstrated by Zysman-Colman et al. [73]. and thus more quickly lower the barrier to charge injection into the device. This effect is achieved (ton = 1.1 h for 23 and 0.2 h for 24) but at the detriment of the performance of the emitter in the device (EQE = 5.5 % for 23 and 4.0 % for 24), suggesting that even minor changes to functionality peripheral to the electronics of the emitter can nevertheless have a significant effect on the efficiency (Fig. 14). 5.2 Stability Complex 25 is the first example reported of a complex containing the intramolecular p-stacking motif alluded to previously. Within 25, there is a short centroid-to˚ between the phenyl ring on the bpy and one of the centroid distance of 3.48 A cyclometalating phenyl rings. This tight interaction maintains the structural integrity of the inner coordination sphere, even when the anti-bonding e.g., orbitals of the MC states are populated, and thus inhibits potential nucleophiles from coordinating to the metal centre upon population of the 3MC states. The devices reporting this emitter were operated under constant voltage and two values have been reported for the device lifetime based on this emitter (t1/2 = ca. 1300 h [36, 62, 68] or 3000 h [61]), with the longer value resulting from operating the device with a pre-biasing method (Fig. 15). Although the intramolecular p-stack is an effective strategy for increasing device stability, it does have limits: complex 26, with two incorporated p-stacking phenyl rings on the ancillary ligand results in poorer device performance compared to that with 25 [68]. Devices based on 25 and 26 both show long lifetimes (t1/2 = 1300 h for 25 and 26). However, the luminance values of 25 (Lmax = 110 cd m-2) are higher than for 26 (Lmax = 70 cd m-2). Devices with different luminance levels are not necessarily comparable in terms of stability, since brighter devices intrinsically PF6

PF6

N

N Ir

N N

Ir

3(PF6)

N N

Ir

N

N

N

N

22

23

24

N

N

N N N

N

Fig. 14 High efficiency yellow/orange emitter (22) and related complexes (23 and 24)

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Top Curr Chem (Z) (2016) 374:36 PF6

N

N

N Ir

PF6

N N

N

Ir

N

N

25

26

PF6

N Ir

N N

N N

N

27

Fig. 15 Champion stable LEECs operated at constant voltage

degrade more quickly. Thus, to compare devices of different luminance levels it has been argued that considering the total photon flux emitted from the device once the luminance reaches 1/5 of the maximum value, Et1=5 , is a more accurate assessment of its stability. In this instance, 25 showed higher Et1=5 values (13.6 J) than 26 (6.9 J), and thus it was concluded to be the more stable emitter. It was rationalized that the although the additional p-stacking ring further shields the metal centre, in order to maximize the dual p-stacking interaction there is a distortion of the inner coordination the sphere of the complex. This distortion in turn makes the MC states more thermally accessible, thus promoting exciton quenching and making the complex more susceptible to degradation reactions in the device. An alternative strategy designed to protect the iridium from adventitious attack of small molecule nucleophiles is shown for complex 27 [69]. Here, the methyl groups appended to the pyrazole rings add an additional steric shield to the metal centre similar to complexes 18 and 19. This strategy confers excellent stability to the LEEC with the device with 27 showing higher lifetimes (t1/2 = 2000 h) than that reported using 25 as the reference emitter (t1/2 = 1290 h) (Fig. 16). In general, hydrophobic substituents appear to improve the stability of the emitter in the device. For example, the phenyl rings on the 5,50 -positions of the ancillary ligand of complex 28 do not form an intramolecular p-stacking motif. Nevertheless, the hydrophobicity of these rings appear to confer good stability to the device, with

PF6

N N

Ir

N

N

28

Fig. 16 Stable LEEC based on a hydrophobic iridium complex

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good brightness (Lmax = 130 cd m-2) and a reasonable device lifetime (t1/2 = 110 h) [70] without impacting greatly the emission color compared to 1. Given the aforementioned benefits of pulsed current LEECs, it is plausible that pulsed current LEECs based on complexes 25–28 would perform even better. Complex 29, which is the methoxy analogue of 21, is a good example of the contrasting performances of an emitter in a LEEC under constant voltage and pulsed current conditions. Under constant driving voltage, the device was reported to show good stability (t1/2 = 950 h) [62], but this is well below the value reported for this emitter under pulsed current conditions (t1/2 = 4000 h) [71]. Indeed, this latter lifetime is the longest reported for any iridium-based LEEC to date. The long lifetime in the device is coupled with higher brightness (Lmax = 650 cd m-2 under pulsed current vs. 183 cd m-2 under constant voltage), although the efficiency under constant voltage is higher (CE = 8.2 cd A-1 under constant voltage versus 3.6 cd A-1 for pulsed current). The merits of the pulsed current driving method are exemplified by the device with 30. To the best of our knowledge, this complex is the only emitter reported with a device lifetime of greater than one thousand hours (t1/2 = 2000 h) that does not have an intramolecular p-stacking motif. The device with this complex also shows a higher efficiency than that with 29 (CE = 6.5 cd A-1) under pulsed current driving [72] (Fig. 17). Finally, complex 31 represents arguably the most stable emitter reported to date [14]. The values reported for complexes 31–33 are from operating the devices at an exceptionally high current density of 300 A m-2. Typically, pulsed-current LEECs are operated at average current densities of 50–100 A m-2 (29 was operated at an average of 185 A m-2 and 30 at 100 A m-2); however, at these current densities no discernible degradation of the devices could be observed and thus a much higher average current density was required. The high stability of the device at these current densities for all three complexes (t1/2 = 2800 h for 31, 1204 h for 32, and [2800 h for 33) is attributed to the silver-assisted synthesis of these complexes to ensure they are free of chloride impurities, which as discussed above, are detrimental to the performance of these complexes in the device. Since no Et1/5 values are reported it is difficult to discern which of the three complexes is the most stable; however, the much higher luminance value for 31

PF6

PF6

N

N Ir

N

Ir

N

OMe

N

N

N

N

N

N

OMe 29

30

Fig. 17 High stability emitters operated under pulsed-current conditions

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(Lmax = 1024 cd m-2) compared with 32 (Lmax = 676 cd m-2) and 33 (Lmax = 261 cd m-2) suggest that it is the champion emitter in terms of device stability. It is surprising however, that 31 is the simplest of these structures, with no intramolecular p-stacking motif. These results illustrate that there are still challenges for correlating the structure of an emitter to its performance in the device (Fig. 18).

6 Red Although the challenge of designing blue emitters is still the greatest for LEECs (no LEEC so far has been reported to even achieve deep-blue emission, let alone with good device performance), only a small number of LEECs have been reported to have CIE coordinates close to the ideal red value (CIE 0.66, 0.33) and, like the blue LEECs reported to date, these devices all show poorer stability compared with yellow/orange LEECs (Table 6). 6.1 Efficiency Although heterocycles such as pyrazoles and imidazoles are rarely used for red emission (due to their strong r-donating character and their tendency to induce blue-shifting in the emission compared with pyridyl rings), two of the best redemitting devices nevertheless utilize such heterocycles. Complex 34 utilizes a phenylpyrazole-type cyclometalating ligand but compensates for its blue-shifting effect by incorporation of the highly conjugated 2,20 -biquinoline ancillary ligand to red shift the emission [33]. Similarly, the LUMO destabilizing capabilities of the imidazole ring contained within the ancillary ligand of 35 are also compensated by the annelated benzene to form the benzimidazole and by the appended quinoline ring [46]. Both of these complexes are red emitters (kPL = 624 nm for 34 and 627 nm for 35) in MeCN solution but they are only poorly emissive (UPL = 0.68 % for 34 and 3 % for 35), presumably as a function of the energy gap law. Photoluminescence quantum yield data in the solid state is not reported for either of these emitters so it is not possible to correlate these to the device performances. However, high efficiencies are reported for both devices, particularly the device

Ir

N

Ir

6

N

N

N

PF

PF6

PF6

N

N N

N

Ir

N

N

N

31

32

33

N

Fig. 18 Champion-pulsed current LEEC for stability (31) and related analogues (32 and 33)

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51

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123

123

52

[email protected]

42

9

687

687

608

666

619

573

556

627

624

kPL (nm)

1

2

6

2.6

55

58

24

3

0.68

UPL (%)

ca. 35

ca. 35

626

14

217

154

70

7500

Lmax (cd m-2)

0.68

0.83

2.7

2.5

6.29

13.1

1.6

CE (cd A-1)

1.67

0.08

3.27

2.74

9.51

2.6

7.4

EQE (%)

0.71

0.87

2.56

10

PE (lm W-1)

37

0.52

25

6.3

9.83

8.17

t1/2 (h)

0.69

0.71

0.59

0.68

0.65

0.50

0.59

0.66

0.67

CIE (x)

0.29

0.28

0.41

0.33

0.34

0.41

0.40

0.33

0.32

CIE (y)

660

630

607

666

644

616

624

650

635

kEL (nm)

[77]

[77]

[76]

[75]

[74]

[24]

[24]

[46]

[33]

References

kPL is the solution-state emission maximum in MeCN, UPL is the photoluminescence quantum yield in deaerated solution, Lmax is the maximum luminance observed from device, CE is the current efficiency, PE is the power conversion efficiency, t1/2 is the time taken for the device luminance to fall to half the maximum value, kEL is the electroluminescence emission maximum

40

41

7

8

38

39

5

6

36

37

3

4

34

35

1

2

Complex

Entry

Table 6 Summary of LEECs employing red-emitting iTMCs

Top Curr Chem (Z) (2016) 374:36

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Top Curr Chem (Z) (2016) 374:36

based on 34, (EQE = 7.4 % for 34 and 2.6 % for 35), making them among the best red devices reported to date. In addition, the color of both devices essentially coincides with the pure red CIE coordinate (CIE 0.67, 0.32 for 34 and 0.66, 0.33 for 35 (Fig. 19). One of the intrinsic issues with blue emitters is that the emission is frequently red-shifted in the device. For red emitters, this feature can act as an advantage, exemplified by complexes 36 and 37 [24]. In solution these complexes emit yellow light (kPL = 573 nm for 36 and 556 nm for 37), but in neat film (kPL = 627 nm for 36 and 625 nm for 37) and in the device (kEL = 624 nm for 36 and 616 nm for 37) the emission is strongly red-shifted. The authors attribute this red shift to the possible formation of excimers in the condensed phase, due to strong p–p intermolecular stacking interactions observed in the crystal structures of 36 and 37. Crucially, this red shift in emission observed in the LEEC is accompanied with impressive device performance, particularly for 36, which shows the highest device efficiency of any red or yellow/orange device reported to date (EQE = 9.51 % for 36 and 2.74 % for 37). This high efficiency certainly qualifies 36 as the champion red-emitting device reported to date. However, it is worth noting that the CIE coordinates of these devices (CIE 0.59, 0.40 for 36 and 0.50, 0.41 for 37) are blueshifted compared to the pure red CIE coordinates (CIE: 0.66, 0.33) required for RGB color coordinates (Fig. 20). Finally, complex 38 is an interesting red emitter [74]. The extended conjugation of the perylenediimide unit was used to achieve deep-red emission, while also functioning as an electron-transporting moiety to balance the hole conducting capabilities of the iridium component. It was found that the iridium essentially does not contribute to the photophysics of the compound. The short emission lifetime (se = 3.0 ns) and high quantum yield (UPL = 55 %) for a red emitter (kPL = 619 nm) point instead towards fluorescence directly from the perylenediimide chromophore, an assignment supported by theoretical calculations that implicated only the perylenediimide unit in the electronics of the HOMO or LUMO. Thus, the iridium in this case acts only as an appended charged unit to enable this complex to function in the LEEC. Crucially, the short emission lifetime is suggested to help in circumventing nonradiative quenching pathways in which typical triplet emitters are susceptible,

PF6

N

N

N

N Ir

PF6

N

Ir

N

N N

N

N

34

35

N

Fig. 19 High efficiency, red-emitting iridium complexes

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Top Curr Chem (Z) (2016) 374:36 Fig. 20 Champion red-emitting device (36) and its analogue (37)

PF6

PF6

N

N N

Ir

Ir

N N

N

O N

36

N N N

O

37

leading to a good efficiency (EQE = 3.27 %) for a red-emitting device (CIE 0.65, 0.34). Although a useful feature, it is unclear if this compound is purely a singlet emitter, or whether it is in fact harvesting triplets as well, an important feature of typical phosphorescent iridium complexes. In this instance, it is possible that this strategy is in fact wasting the triplets generated in the emissive layer, defeating the object of utilizing an iridium-based material in the first place (Fig. 21). 6.2 Stability Complex 39 employs a 2,5-dipyridyl(pyrazine) ancillary ligand which, demonstrating the opposite trend to complex 17, shows red-shifted emission as a result of the electron-withdrawing nature of the non-coordinating nitrogen on the pyrazine ring [75]. Thus, in this instance, the LUMO is strongly stabilized. Further narrowing of the HOMO–LUMO gap comes by way of the non-coordinating pyridyl ring, which extends the conjugation on the ancillary ligand, red-shifting the emission further, both in solution (kPL = 666 nm) and the device (CIE 0.68, 0.33; kEL = 666 nm). Although the efficiency of the device is low (EQE = 0.08 %), the lifetime Fig. 21 Deep-red-emitting iridium complex with good device efficiency

PF6

N Ir

O

N N

N

O

N

O

O O

38

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O

N O

O

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Top Curr Chem (Z) (2016) 374:36

(t1/2 = 6.3 h) is rather long for a deep-red-emitting device. As with the examples below, much shorter device lifetimes compared with yellow/orange seems to be a general feature of deep-red-emitting LEECs (Fig. 22). A number of red emitting intramolecularly p-stacked complexes have been reported but few of them demonstrate any appreciable stability when compared to their yellow/orange analogues. Indeed, even in the case of the most stable of the redemitting complex bearing an intramolecularly p-stacking motif, 40, the device lifetime (t1/2 = 25 h) is still very short compared with many other devices employing intramolecularly p-stacked complexes [76]. Although the luminance levels for this device are good (Lmax = 626 cd m-2), the yellow-emitting devices utilizing structurally related complexes 32 and 33 are brighter and of course significantly longer lived. Clearly the stability of deep-red emitters is still lagging some way behind other devices reported to date (Fig. 23). An alternative strategy reported for improving the stability of this class of emitters is that shown in a comparative study between complexes 41 and 42. By covalently tethering the emitter to a polymer backbone, it was suggested that this should lead to a more uniform distribution of the complex within the emissive layer, reducing aggregate formation and increasing the spatial distribution of the emitters within the device, thereby improving the stability. Indeed, this appears to be the case, with a significant enhancement in the device lifetime with 42 (t1/2 = 37 h) compared to the device with 41 (t1/2 = 0.52 h). Although in absolute terms the lifetime of 42 is still poor, it is nevertheless the longest of any red device (CIE 0.69, 0.29) reported to date, suggesting that this is a viable, underexplored strategy for improving the stability of the device, as well as highlighting similar challenges in achieving stable red LEECs as discussed for achieving stable blue LEECs (Fig. 24).

7 White Given the paucity of charged blue emitters, examples of white LEECs in the literature are subsequently scarce. Such devices broadly fall into one of two categories: host–guest systems, where a blue emitting host material is doped with a red emitter, or a multi-stack device where multiple emissive layers are combined

Fig. 22 Iridium complex with good stability for a deep-red LEEC

PF6

N Ir N

N N

N N

39

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Top Curr Chem (Z) (2016) 374:36 Fig. 23 Intramolecularly pstacked deep-red-emitting iridium complex

PF6

N N

Ir

N

S

N

40

PF6

N Ir

n(PF6)

O

Ir

N

N

N

OEt

N

O

OEt

N

O

41

OEt

N N

O O

42

O O n

Fig. 24 Deep-red-emitting iridium complex and its corresponding polymer, which shows greatly enhanced stability

within a single device to achieve white light. Both families of devices will be discussed. Although in these publications it is common for multiple configurations to be reported, we will discuss only the devices in a particular report that demonstrate CIE coordinates closest to the ideal white coordinate (CIE 0.33, 0.33), which is the ultimate goal of white LEECs, regardless of the performance of other devices in the report. Likewise, often these devices are operated at multiple biases, leading to significant changes in the color and the performance. Again we will consider only the driving voltage/current that achieves the ‘whitest’ emission. The values are summarized in Table 7. 7.1 Host–guest LEECs The typical host–guest device is that based on 43, a blue-green emitter (kPL = 491 nm in the neat film), and 44, which is a deep-red emitter (kPL = 672 nm in the neat film) [78]. A small amount of 44 (0.4 wt %) doped into 43 ensures a partial energy transfer (although not stated in this case, other references on white

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Ca. 400

845

3.7

7.0

100 A m-2

6

7

8

7.2

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8.5

0.41

13.4

11.4

11.2

5.8

CE (cd A-1)

12.5

6.3

7.3

5.6

5.2

3.3

EQE (%)

27

12.8

13.5

10

5.5

PE (lm W-1)

0.17

0.62

2.83

0.4

t1/2 (h)

0.43

0.33

0.41

0.44

0.39

CIE (y)

0.38

0.32

0.47

0.34

time dependent

0.32

0.33

0.37

0.40

0.35

CIE (x)

475, 580

Ca. 500, 600

Ca. 500, 600

Ca. 480, 550, 600

495, 600

497, 590

488, 612

kEL (nm)

70

80

81

80

CRI

[80]

[79]

[45, 54]

[56]

[55]

[47]

[46]

[78]

References

Lmax is the maximum luminance observed from device, CE is the current efficiency, PE is the power conversion efficiency, t1/2 is the time taken for the device luminance to fall to half the maximum value, kEL is the electroluminescence emission maximum, CRI is the color rendering index value of the device

32

20.2

3.6

3.3

7.9

4

3.2

3

31

43

Lmax (cd m-2)

5

3.3

3.5

1

2

Bias/V

Entry

Table 7 Summary of white LEECs

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emitting LEECs describe this kind of energy transfer as Fo¨rster in nature) [55] from the excited blue host to the red emitting guest, resulting in emission from both chromophores, and giving white light as a result. Despite being the first reported example of a white light-emitting LEEC based on a host–guest system, the device showed decent efficiencies (EQE = 3.3 %), and crucially the color of the device was very close to the ideal white point (CIE 0.35. 0.39) (Fig. 25). An improvement in the device performance of the white LEEC is documented in entries 2 and 3 in Table 7. A device utilizing 5 as the host and 35 as the guest (1: 0.002 molar ratio) achieved greatly improved efficiencies compared to entry 1 (EQE = 5.2 %, CE = 11.2 cd A-1) [46], but with CIE coordinates that are modestly red-shifted (CIE: 0.40, 0.44) from the white point. Changing the host from 5 to 6 achieves a small increase in the efficiency (EQE = 5.6 %, CE = 11.4 cd A-1) and a slight blue shift in the CIE coordinates (CIE 0.37, 0.41) [47]. However, the brightness of the device based on 6 (Lmax = 7.9 cd m-2) is greatly diminished compared with 5 (Lmax = 31 cd m-2) (Fig. 26). The most commonly used donor complex in host–guest LEEC systems is 4 and these devices tend to offer improved performance (entry 4). The guest in the LEEC need not be an iridium complex. For instance, the device shown in entry 4 has an EQE of 7.3 % and employs a highly emissive (UPL = 90 % in ethanol) red-emitting dye compound, sulforhodamine 101 (SR 101) as an organic guest molecule. The high photoluminescence quantum yield of SR 101 is not typical of many redemitting systems; indeed, the previously discussed red-emitting phosphorescent dopants 35 (UPL = 3 % in MeCN) and 44 (UPL = 20 % in MeCN) show much lower quantum yields in solution than SR 101. The LEEC produces white light with CIE coordinates coinciding with the white point (CIE 0.33, 0.33). In addition, although the value is still short (t1/2 = 2.83 h) the device lifetime reported for this device is the longest for white LEECs reported to date [55] However, this is probably due at least in part to the very low luminance levels reported for this device (Lmax = 7.2 cd m-2), particularly compared with the devices shown in entries 1 and 2 (Fig. 27). Taking the doping strategy further, Su and co-workers[38c] have demonstrated that these LEECs can be ‘double-doped’, with one dopant providing the red-light component required for white light and the other dopant acting to improve charge transport in the emissive layer, as demonstrated by entry 5. In this case, complex 22

Fig. 25 Host-guest LEEC containing 43 as the blue/green host component and 44 as the red dopant. Device: ITO/43 (80.5 wt %), 44 (0.4 wt %), [BMIM][PF6] (19.1 wt %)/Ag, entry 1

PF6

F N F F N

N

N Ir

PF6

N N

Ir

N

N

43

44

N N

F

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Top Curr Chem (Z) (2016) 374:36 F

F

PF6

F F

N

N

Ir

F F

N

N

N Ph Ph

N

Ir N

F

Ir

Ph

N

N

N

PF6

PF6

N

N N

N

N

6

35

N

F 5

Fig. 26 Host-guest LEECs containing 5 or 6 as the blue/green host component and 35 as the red dopant. Device: ITO/PEDOT:PSS/5: [BMIM][PF6]: 35 (1: 0.35: 0.002 molar ratio)/Al, entry 2. Device: ITO/ PEDOT:PSS/6: [BMIM][PF6]: 35 (1: 1: 0.008 molar ratio)/Al, entry 3

PF6

F N F F

Ir N

N

N

N

O

N N

SO3

N

F

SO3H 4

SR 101

Fig. 27 Host-guest LEEC containing 4 as the blue/green host component and SR 101 as the red dopant. Device: ITO/PEDOT:PSS/4 (79.5 wt %), SR 101 (0.5 wt %), [BMIM][PF6] (20 wt %)/Ag, entry 4

is doped into the emissive layer along with 44. As mentioned, complex 22 is an effective electron transporting material, acting to improve carrier mobilities throughout the emissive layer and thus improve the efficiency. Indeed, the efficiency of the device (EQE = 6.3 %), entry 5, is almost double the efficiency of the control device utilizing only 4 and 44 in the emissive layer (EQE = 3.2 %).

F N F F

N N

PF6

N

N

N Ir

N

PF6

PF6

N

Ir

N

Ir

N

N

N

4

22

44

N N

F

Fig. 28 Double-doped host–guest LEEC containing 4 as the blue/green host component, 22 as an orange-emitting electron transporting material and 44 as the red dopant. Device: ITO/PEDOT:PSS/4 (79.85 wt %), 22 (0.05 wt %), 44 (0.10 wt %), [BMIM][PF6] (20.00 wt %)/Al, entry 5

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Furthermore, since the orange emission of 22 in the solid state (kPL = 593 nm) falls in between the emission of the host complex 4 (kPL = 492 nm) and the red-emitting dopant 44 (kPL = 672 nm), the overall color of the device is also not negatively impacted (CIE 0.32, 0.43) compared with the control device utilizing only 4 and 44 (CIE: 0.33, 0.42) [56] (Fig. 28). 7.2 Multilayer LEECs Multilayer LEECs comprise much more varied device architectures than host–guest LEECs. For example, entry 6 is an example where a typical blue-green LEEC employing 4 as the emitter is capped on top of the ITO anode with a color conversion layer (CCL). This CCL is comprised of a transparent inert film doped with a small quantity red dye, such as DCJTB, and produces red light following photoexcitation by the blue-emitting component of the LEEC. Control of the doping concentration controls the relative contribution of blue and red light, and thus enables the generation of white light. Furthermore, this device achieves much higher efficiencies (EQE = 12.5 %) than the values reported for the host–guest LEECs in entries 1–5. However, an issue with this class of device is that since the chromophores generating white light are not excited simultaneously—as in the case of a typical host–guest device—the color of the device is strongly dependent on the amount of blue light output from the device. Since LEECs characteristically show variation in the luminance as a function of time, this ultimately leads to strong fluctuations in the CIE coordinates of the device with time. The degree of variability is dependent on the concentration of dye present in the CCL, with 0.3 wt% showing the least change in CIE coordinates, but nevertheless the coordinates are not consistent over a 0.5 h period [45, 54] (Fig. 29). Since the performance of many white LEECs is ultimately governed by the performance of the blue-emitting iTMC, the device shown in entry 7 addresses this issue by doing away with a blue-emitting iTMC altogether, instead adopting a conjugated polyfluorene polymer CB02 (the full structure is not disclosed, appearing in the report exactly as shown in Fig. 30) as the blue-emitting

F N F F

CN

N Ir

N

NC

PF6

N N

N

N

F 4

DCJTB

Fig. 29 Multilayer LEEC employing 4 as the blue/green component and DCJTB as the red-emitting dopant in the color conversion layer. Device: glass/photoresist film (99.7 wt %), DCJTB (0.3 wt %)/ITO/ PEDOT:PSS/4 (80.0 wt %), [BMIM][PF6] (20.0 wt %)/Ag, entry 6

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Top Curr Chem (Z) (2016) 374:36 Fig. 30 Multilayer LEEC employing 25 as the orangeemitting layer and CB02 as the blue-emitting layer. Device: ITO/PEDOT:PSS/25, [BMIM][PF6]/CB02/Ba/Ag, entry 7

R

PF6

R

N Ir Ar n

k

CB02

N N

N

25

component. The device conveniently takes advantage of the ‘orthogonal’ solubility profiles of the iTMC 25 (spin-coated from MeCN) and CB02 (spin-coated from mesitylene), meaning that each layer can be sequentially deposited from solution without impacting the integrity of previously deposited layer, before encapsulation within the electrodes. In contrast to the device in entry 6, both emitters are excited electrically, with electrons injected into the CB02 layer via the cathode and holes into the iTMC layer via the anode, recombining towards the interface of the two emissive layers. Since the CB02 layer is not charged, this device does not display typical ‘LEEC behavior’, showing no J-V dependence as a function of time. This probably also accounts for the somewhat high driving voltages utilised for this device (7.0–10.0 V) compared with those discussed previously. Such high voltages also account for the much higher brightness for this device (Lmax = ca. 400 cd m-2 at 7.0 V) compared with other white LEECs but this also appears to impact the efficiency, with only a relatively low current efficiency reported (CE = 0.41 cd A-1). Nevertheless, by controlling the relative thickness of the emissive layers (and thus the amount of light generated from each emitter), almost ideal white light can be achieved for this device (CIE 0.32, 0.34) [79].

Fig. 31 Multilayer LEEC employing 2 as the yellow emitting layer and 45 as the blue-emitting layer. Device: ITO/PEDOT:PSS/PVK (43.5 wt %), OXD-7 (43.5 wt %), 45 (8.7 wt %), [THA][BF4] (4.3 wt %)/Au/MoO3/2 (80.0 mol %), [BMIM][PF6] (20.0 mol %)/Al, entry 8, where PVK is poly(vinylcarbazole), OXD-7 is 1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene and [THA][BF4] is tetrahexylammonium tetrafluoroborate

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Arguably the most complex device is that of entry 8, which adopts a ‘tandem LEEC’ architecture to achieve white light. In this architecture, two sub-devices emitting yellow (2) and blue (‘FIrpic’, complex 45) light are stacked on top of each other, with an air stable middle electrode employed to separate the two devices. As with entry 7, entry 8 borrows from well-established OLED literature to circumvent the issues with blue-emitting iTMCs, utilizing instead the well-known sky-blueemitting FIrpic complex. The blue LEEC adopts a typical OLED-type emissive layer, with the neutral complex 45 doped into a host of poly(N-vinylcarbazole) (PVK), which has hole transporting characteristics, and 2,20 -(1,3-phenylene)bis[5(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7, Fig. 31), which is an electron transporting material. An ionic liquid dopant (tetrahexylammonium tetrafluoroborate, [THA][BF4]) confers characteristic LEEC-type behavior, with the control device, ITO/PEDOT:PSS/PVK (43.5 wt %), OXD-7 (43.5 wt %), 45 (8.7 wt %), [THA][BF4] (4.3 wt %)/Au, showing strong variations in luminance as a function of time. The use of a semi-transparent Au cathode allows for deposition of the second yellow-emitting component. In order to preserve the integrity of the Au surface, a thin MoO3 interlayer is also deposited. The second emissive layer of 2 is then added, followed by capping the device with an Al cathode. Whereas entry 7 does not show normal LEEC behavior, the tandem LEEC, like the control based on 45, does show features typical of LEEC devices: upon applying a constant current, the driving voltage of the device drops rapidly, as ions in the emissive layers migrate to form the dynamically doped zones that facilitate the injection of electrons and holes. This phenomenon is common for conventional LEECs, but this is the only example of it being demonstrated in a double-stacked ‘tandem LEEC’ based on iridium. Overall, the performance of the device is very high: the luminance levels reported for this device (Lmax = 845 cd m-2) are by far the highest reported for a white LEEC, while the efficiency (CE = 8.5 cd A-1) is comparable to some of the best host–guest LEECs described above. However, the CIE coordinates are somewhat shifted from the ideal white coordinate to ‘warm white’ (CIE 0.38, 0.47).

8 Conclusions and Outlook LEEC research is burgeoning owing to their attractively simple device architectures and facile processing from solution. These two features make these devices appealing for industrial applications, overcoming some of the fabrication drawbacks linked to OLEDs, which rely on costly multilayer device architectures that are typically fabricated by vacuum deposition methods. In spite of the intrinsic simplicity of LEECs, recent developments in areas from the device architectures (host–guest systems, charge transport layers), to the synthesis of iTMCs (particularly silver-assisted methods) and methods of device operation (pulsed current LEECs) have culminated in reports demonstrating high device efficiencies (EQEs [10 %) and stabilities (t1/2 of thousands of hours). However, even with these advances, the best performing LEECs fall well short of

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their OLED counterparts, with stabilities in LEECs in particular at least an order of magnitude lower. Thus, the principal goal for researchers is still to address issues with respect to LEEC stability. In particular, LEECs still perform particularly poorly in the blue, deep-red, and white. There is therefore a dire need of further advances if these devices are to approach the performance metrics of leading OLEDs. In addition to more established strategies such as incorporating hydrophobic units within the ligand scaffold, several different approaches have begun to emerge that may tackle the difficulties present with devices of these colors. These include polymer-iTMC LEECs to the use of neutral blue emitters to overcome the lack of stable and efficient blue iTMCs. If the challenges relating to LEEC stability and the development of deep-blue devices can be overcome then the future of LEECs will be bright indeed. Acknowledgments EZ-C acknowledges the University of St Andrews for financial support. We would like to thank the Engineering and Physical Sciences Research Council for financial support for Adam Henwood: EPSRC DTG Grants: EP/J500549/1; EP/K503162/1; EP/L505097/1. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Top Curr Chem (Z) (2016) 374:36 48. Meier SB, Sarfert W, Junquera-Herna´ndez JM, Delgado M, Tordera D, Ortı´ E, Bolink HJ, Kessler F, Scopelliti R, Gra¨tzel M, Nazeeruddin MK, Baranoff E (2013) J Mater Chem C 1:58. doi:10.1039/ c2tc00251e 49. Zhang F, Duan L, Qiao J, Dong G, Wang L, Qiu Y (2012) Org Electron 13:1277. doi:10.1016/j.orgel. 2012.03.017 50. Zhang F, Duan L, Qiao J, Dong G, Wang L, Qiu Y (2012) Org Electron 13:2442. doi:10.1016/j.orgel. 2012.06.050 51. He L, Duan L, Qiao J, Zhang D, Wang L, Qiu Y (2011) Chem Commun 47:6467 52. Lowry MS, Goldsmith JI, Slinker JD, Rohl R, Pascal RA, Malliaras GG, Bernhard S (2005) Chem Mater 17:5712 53. Liao CT, Chen HF, Su HC, Wong KT (2012) Phys Chem Chem Phys 14:9774. doi:10.1039/ c2cp40739f 54. Lu J-S, Chen H-F, Kuo J-C, Sun R, Cheng C-Y, Yeh Y-S, Su H-C, Wong K-T (2015) J Mater Chem C 3:2802. doi:10.1039/c4tc02258k 55. Su H-C, Chen H-F, Chen P-H, Lin S-W, Liao C-T, Wong K-T (2012) J Mater Chem 22:22998 56. Su H-C, Chen H-F, Shen Y-C, Liao C-T, Wong K-T (2011) J Mater Chem 21:9653 57. Liao C-T, Chen H-F, Su H-C, Wong K-T (2011) J Mater Chem 21:17855. doi:10.1039/C1JM13245H 58. Darmawan N, Yang CH, Mauro M, Raynal M, Heun S, Pan J, Buchholz H, Braunstein P, De Cola L (2013) Inorg Chem 52:10756. doi:10.1021/ic302695q 59. Sivasubramaniam V, Brodkorb F, Hanning S, Loebl HP, van Elsbergen V, Boerner H, Scherf U, Kreyenschmidt M (2009) J Fluor Chem 130:640. doi:10.1016/j.jfluchem.2009.04.009 60. Zheng Y, Batsanov AS, Edkins RM, Beeby A, Bryce MR (2012) Inorg Chem 51:290. doi:10.1021/ ic201655n 61. Bolink HJ, Coronado E, Costa RD, Ortı` E, Sessolo M, Graber S, Doyle K, Neuburger M, Housecroft CE, Constable EC (2008) Adv Mater 20:3910 62. Costa RD, Ortı´ E, Bolink HJ, Graber S, Housecroft CE, Constable EC (2010) Adv Funct Mater 20:1511. doi:10.1002/adfm.201000043 63. Su HC, Fang FC, Hwu TY, Hsieh HH, Chen HF, Lee GH, Peng SM, Wong KT, Wu CC (2007) Adv Funct Mater 17:1019. doi:10.1002/adfm.200600372 64. Bolink HJ, Coronado E, Costa RnD, Lardie´s N, Ortı` E (2008) Inorg Chem. doi:10.1021/ic801587n 65. Tordera D, Frey J, Vonlanthen D, Constable E, Pertega´s A, Ortı´ E, Bolink HJ, Baranoff E, Nazeeruddin MK (2013) Adv Energy Mater 3:1338. doi:10.1002/aenm.201300284 66. Evariste S, Sandroni M, Rees TW, Roldan-Carmona C, Gil-Escrig L, Bolink HJ, Baranoff E, Zysman-Colman E (2014) J Mater Chem C 2:5793. doi:10.1039/C4TC00542B 67. Su H-C, Chen H-F, Wu C-C, Wong K-T (2008) Chem—Asian J. doi:10.1002/asia.200800020 68. Costa RD, Ortı´ E, Bolink HJ, Graber S, Housecroft CE, Neuburger M, Schaffner S, Constable EC (2009) Chem Commun (Camb). doi:10.1039/b821602a 69. Costa RD, Ortı´ E, Bolink HJ, Graber S, Housecroft CE, Constable EC (2010) J Am Chem Soc. doi:10.1021/ja1010674 70. Sun L, Galan A, Ladouceur S, Slinker JD, Zysman-Colman E (2011) J Mater Chem 21:18083 71. Tordera D, Meier S, Lenes M, Costa RD, Ortı´ E, Sarfert W, Bolink HJ (2012) Adv Mater 24:897. doi:10.1002/adma.201104047 72. Tordera D, Pertega´s A, Shavaleev NM, Scopelliti R, Ortı´ E, Bolink HJ, Baranoff E, Gra¨tzel M, Nazeeruddin MK (2012) J Mater Chem 22:19264. doi:10.1039/c2jm33969b 73. Zysman-Colman E, Slinker JD, Parker JB, Malliaras GG, Bernhard S (2008) Chem Mater 20:388 74. Costa RD, Cespedes-Guirao FJ, Orti E, Bolink HJ, Gierschner J, Fernandez-Lazaro F, Sastre-Santos A (2009) Chem Commun (Camb). doi:10.1039/b905367k 75. Hasan K, Donato L, Shen Y, Slinker JD, Zysman-Colman E (2014) Dalton Trans. doi:10.1039/ C4DT02100B 76. Graber S, Doyle K, Neuburger M, Housecroft CE, Constable EC, Costa RD, Orti E, Repetto D, Bolink HJ (2008) J Am Chem Soc 130:14944. doi:10.1021/ja805900e 77. Rodriguez-Redondo JL, Costa RD, Orti E, Sastre-Santos A, Bolink HJ, Fernandez-Lazaro F (2009) Dalton Trans:9787 78. Su H-C, Chen H-F, Fang F-C, Liu C-C, Wu C-C, Wong K-T, Liu Y-H, Peng S-M (2008) J Am Chem Soc 130:3413. doi:10.1021/ja076051e 79. Sessolo M, Tordera D, Bolink HJ (2013) ACS Appl Mater Interfaces 5:630. doi:10.1021/am302033k 80. Akatsuka T, Roldan-Carmona C, Orti E, Bolink HJ (2014) Adv Mater 26:770. doi:10.1002/adma. 201303552

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Top Curr Chem (Z) (2016) 374:46 DOI 10.1007/s41061-016-0046-y REVIEW

Platinum and Gold Complexes for OLEDs Man-Chung Tang1 • Alan Kwun-Wa Chan1 Mei-Yee Chan1 • Vivian Wing-Wah Yam1



Received: 28 January 2016 / Accepted: 14 June 2016 / Published online: 1 July 2016 Ó Springer International Publishing Switzerland 2016

Abstract Encouraging efforts on the design of high-performance organic materials and smart architecture during the past two decades have made organic light-emitting device (OLED) technology an important competitor for the existing liquid crystal displays. Particularly, the development of phosphorescent materials based on transition metals plays a crucial role for this success. Apart from the extensively studied iridium(III) complexes with d6 electronic configuration and octahedral geometry, the coordination-unsaturated nature of d8 transition metal complexes with square-planar structures has been found to provide intriguing spectroscopic and luminescence properties. This article briefly summarizes the development of d8 platinum(II) and gold(III) complexes and their application studies in the fabrication of phosphorescent OLEDs. An in-depth understanding of the nature of the excited states has offered a great opportunity to fine-tune the emission colors covering the entire visible spectrum as well as to improve their photophysical properties. With good device engineering, high performance vacuum-deposited OLEDs with external quantum efficiencies (EQEs) of up to 30 % and solution-processable OLEDs with EQEs of up to 10 % have been realized by modifying the cyclometalated or pincer ligands of these metal complexes. These impressive demonstrations reveal that d8 metal complexes are promising candidates as phosphorescent materials for OLED applications in displays as well as in solid-state lighting in the future.

This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli, Henk Bolink. & Vivian Wing-Wah Yam [email protected] 1

Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China

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Keywords Organic light-emitting devices  Metal complexes  Phosphorescence  Platinum  Gold

1 Introduction and Scope Electroluminescence (EL) phenomenon from an organic material was firstly demonstrated by Pope et al. in 1963 by applying a direct current (DC) through a single crystal of anthracene [1]. Under the application of the DC voltage at 50–1000 V, the EL was generated from the radiative recombination of excitons, i.e. bound excited states of electrons and holes injected from the conduction band and valence band, respectively, of the single crystal by an electrostatic coulombic force. Later, Helfrich and Schneider reported the capture of the first image of recombination radiation in anthracene crystals and Roberts et al. further employed Langmuir–Blodgett technique to process anthracene into a thin film and applied it in the fabrication of organic light-emitting diodes (OLEDs) in 1965 [2, 3]. However, an extremely high driving voltage of these OLEDs limited their development for practical applications. It was only in 1987 that Tang and VanSlyke reported an efficient OLED based on a bilayer structure, in which an aromatic diamine was used as hole-transporting material and tris(8-hydroxyquinoline)aluminium (Alq3) was used as both the light-emitting and electron-transporting material [4]. High external quantum efficiency (EQE) of up to 1 % and power efficiency (PE) of 1.5 lm W-1 were achieved at a driving voltage of less than 10 V [4]. This pioneering concept was further extended to the development of polymeric light-emitting devices (PLEDs) based on poly(p-phenylenenvinylene) (PPV) by Burroughs et al. in 1990 [5]. In 1998, Baldo et al. used 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) as emissive material and demonstrated a very encouraging EQE of up to 4 % [6]. Since then, organometallic compounds based on transition metal centers, such as iridium(III) [7–15], rhenium(I) [16–28], ruthenium(II) [29–35], osmium(II) [36–42], copper(I) [43, 44], gold(I) [45, 46], platinum(II) [47–54], and more recently, gold(III) [55–66], have attracted extensive attention in the past few decades and are considered highly attractive candidates for full-color display technologies and energy-saving solid-state lighting. It has been shown that the presence of a heavy metal center can effectively lead to a strong spin–orbit coupling and thus promotes an efficient intersystem crossing from the singlet excited state to the lower-energy triplet excited state, harvesting both singlet and triplet excitons for light emission at room temperature via phosphorescence in order to realize an internal quantum efficiency (IQE) of up to 100 %. This can ideally result in a fourfold improvement in the IQE, compared to the singlet emitters with theoretical maximum IQE of 25 % due to the spin statistics of singlet and triplet excitons in a ratio of 1:3. In view of this advantage, the maximum EQE of OLEDs can be greatly improved by using metal phosphors [67–70]. Amongst these metal phosphors, iridium(III) complexes are one of the most important classes of phosphorescent materials due to their relatively short excited state lifetimes, high luminescence quantum yields in both solution and solid states, respectable solubility in common organic solvents, and high thermal and chemical

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stabilities. These properties allow them to be easily fabricated into thin films either by vacuum evaporation or solution-processing techniques. In addition, the emission energies of these complexes are readily tunable from saturated blue to saturated red by changing the electronic properties of the cyclometalated ligands or modifying their auxiliary ligands [71]. These distinct properties have enabled them to be widely investigated and employed as phosphorescent materials in the fabrication of OLEDs in last two decades. With the continuous development, encouraging progresses on the design and synthesis of highly luminescent iridium(III) complexes, as well as smart device architecture have boosted up the EQEs to its theoretical limit of 30 % [72, 73]. Apart from the state-of-art development in the d6 iridium(III) systems and other d6 metal complex systems such as those of rhenium(I) [15–28], ruthenium(II) [29–35], and osmium(II) [36–42], numerous efforts have been reported in the exploration of new classes of phosphorescent metal complexes for OLED applications, especially d8 metal complexes. Of particular interest is the platinum(II) system, which has been demonstrated as potential material for optoelectronic devices, including organic solar cells [74–79] and OLEDs [80–84], due to its synthetic versatility, broad absorption, and emission tunability and chemical stability [71]. The nature of the ligands on the metal complexes has been shown to affect greatly the excited state properties. Systematic modifications of the chelating ligands on the metal complexes can effectively tune the relative energies of the metal d-orbitals of platinum and the p and p* orbitals of the ligands, which in turn can vary the luminescence energies, luminescence quantum yields, as well as the charge transporting properties of these metal complexes [60, 85]. Owing to the square planar geometry of platinum(II) complexes, the metal center can accommodate a variety of monodentate [86–93], bidentate [94–98], tridentate [99–106], and tetradentate [107] nitrogen-containing donor ligands with interesting luminescence properties. Indeed, the first triplet emitter employed in OLED application was featured by PtOEP [6, 108]. The luminescence quantum yield of this complex could achieve up to 0.5 in solution with an excited-state lifetime of 90 ls. An optimized OLED based on this complex as phosphorescent dopant showed an EQE of up to 4 % at low current density [5]. Detailed investigations on this class of metal complexes have further provided an indepth understanding of the key factors governing the luminescence efficiencies of platinum(II) complexes. On the other hand, unlike the relatively mature platinum(II) systems, other d8 metal centers such as gold(III) and palladium(II) have been less explored and only a very limited number of luminescent gold(III) complexes [55–66], and more recently, palladium(II) complexes [109] have been demonstrated in the literature. It is believed that the presence of low-energy d–d ligand field excited states would quench the luminescence excited state through thermal equilibration or energy transfer. This limitation has been overcome by Yam et al., in which the incorporation of strong r-donating ligands on the gold(III) center [55], which would render the metal center less electrophilic as well as result in the enhancement of luminescence properties by raising the energy of the d–d states [60]. These concepts have been further proved by the introduction of strongly r-donating aryl [110], alkynyl [62], boroxinato [111], dicarbanionic [112], and N-heterocyclic carbene ligands into the gold(III) center [63]. Based on these approaches, highly emissive cyclometalated Reprinted from the journal

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gold(III) complexes with luminescence quantum yields of up to 58 % in the solution state and 75 % in the solid state have been realized [66, 113]. In addition, the coordinate-unsaturated nature of d8 platinum(II) and gold(III) complexes with square planar geometry has been found to show unique spectroscopic and luminescence functionality as a consequence of their propensity to form aggregates, excimers, and exciplexes via non-covalent metal–metal interactions in the solid state thin films [60, 114–134]. These interesting photophysical properties of the d8 square planar metal complexes have opened up the design of white OLEDs (WOLEDs) by the combination of three primary colors (i.e. red, green, and blue) or based on single emitter, especially in the platinum(II) based systems [120–138]. In this chapter, we will give a comprehensive description focusing on the photophysical and electrochemical properties of several classes of platinum(II) and gold(III) complexes and their use as triplet emitters and dopants in phosphorescent OLEDs (PHOLEDs).

2 Development of Luminescent Platinum(II) Complexes 2.1 Platinum(II) Complexes with Bidentate N-Donor Ligands Platinum(II) complexes with bidentate diimine (N^N) ligands, such as 2,20 bipyridine and 1,10-phenanthroline derivatives, have attracted worldwide attention over the past few decades with numerous reports and investigations on their unique features [94–106, 139–143]. For the monomeric species, the nature of the emissive states can be altered by the incorporation of different ancillary ligands, such as cyanide [94–106], halide [94–106, 139, 140], alkynyl [141], and thiolate [142]. There are in general several possible excited states, i.e. metal-centered (MC) states, intraligand (IL) states, metal-to-ligand charge transfer (MLCT) states, and ligandto-ligand charge transfer (LLCT) states that could be exhibited by the complex system [94–106, 139–143]. Moreover, the use of smaller size and less sterically hindered ligands would facilitate the formation of ground state or excited state 1D oligomeric aggregated species [142, 143]. As a result, the metal-metal-to-ligand charge transfer (MMLCT) excited state would become accessible in some bidentate platinum(II) complexes with significant Pt(II)–Pt(II) interactions to give a diverse array of emission properties [142, 143]. The introduction of strong-field ligands into the platinum(II) metal center could raise the non-emissive d–d states to higher energies, thus becoming thermally less accessible, leading to the improvement of the photophysical properties. For example, with the introduction of cyano ligand, [Pt(5,5-Me2-bpy)(CN)2] was reported to emit in solution state at room temperature with emission quantum yield (Uem) = 1 9 10-3, while the corresponding chloro-complex was found to be nonemissive under the same condition [144]. Another class of the platinum(II) diimine complexes, such as [Pt(N^N)(C:CR)2], has been reported by Che and co-workers in 1994. This class of complexes was reported to possess rich luminescence properties in the solution state [103]. The structureless emission bands observed in the solution state were suggested to be derived from the 3MLCT [dp(Pt) ? p*(diimine)] excited state [103]. Detailed investigations on a complementary series of

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Top Curr Chem (Z) (2016) 374:46 Fig. 1 Molecular structures of bis(arylalkynyl)platinum(II) diimine complexes reported by Eisenberg and co-workers. From Ref. [105]

[Pt(N^N)(C:CR)2] complexes, with the simultaneous fine-tuning of electronic properties of the diimine ligands and alkynyl ligands, have then been established by Eisenberg and co-workers (Fig. 1) [105]. The emissive state was confirmed to be derived from the 3MLCT [dp(Pt) ? p*(diimine)] excited state, with certain degree of mixing of arylalkynyl-to-diimine 3LLCT [p(arylalkynyl) ? p*(diimine)] character [105]. The highly luminescent [Pt(N^N)(C:CR)2] complexes with tunable emission energies have rendered them to be a suitable candidate as triplet emitter in OLED applications. Later, Che and co-workers used the [Pt(diimine)(C:CPh)2] complexes as phosphorescent dopants in the emissive layer in single- and double-layer solution-processable OLEDs in 2001 [81, 142]. The complexes were doped into poly(N-vinylcarbazole) (PVK) thin films [81, 142]. The EL spectra for the 10 wt% doped single-layer devices were similar to their corresponding thin film photoluminescence (PL) spectra, confirming that a complete energy transfer occurred from PVK to the platinum(II) complexes. The single-layer EL device showed a reasonable EL performance with maximum luminance of 620 cd m-2 at a driving voltage of 30 V and a peak PE of 0.10 lm W-1 at luminance of 25 cd m-2 (Fig. 2). Recently, Yam and co-workers reported a new class of unsymmetric bipyridine alkynylplatinum(II) complexes prepared by a post-click reaction (Fig. 3) [145]. The phosphorescence of this class of complexes in degassed dichloromethane originated from the 3MLCT excited state with mixing of a 3LLCT character. In contrast, they were found to exhibit relatively weak emission bands in degassed tetrahydrofuran and in the solid state. The weaker emission bands were suggested to be attributed to the mixing of triplet sigma-bond-to-ligand charge transfer (3SBLCT) state into the 3 MLCT/3LLCT emissive state, similar to that observed in the arylplatinum(II) terpyridine system previously reported by McMillin and co-workers [146]. More interestingly, emission enhancement properties were observed upon addition of water into the tetrahydrofuran media [145]. This class of unsymmetric bipyridine platinum(II) alkynyl complexes had also been used as phosphorescent dopants in the fabrication of solution-processable OLEDs. Consistent with the PL spectra in the

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Fig. 2 Molecular structures of [Pt(N^N)(C:CR)2] and their selected EL and PL spectra. Reproduced with permission from Ref. [81]. Copyright 2001 Wiley– VCH

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Fig. 3 Molecular structures of unsymmetric bipyridine platinum(II) alkynyl complexes. From Ref. [145]

thin-film studies, the EL spectra of the devices exhibited broad structureless emission bands at 511 nm with a maximum current efficiency (CE) of 18.4 cd A-1 and a PE of 8.0 lm W-1, corresponding to an EQE of up to 5.8 %, which was relatively comparable to those of solution-processable PHOLEDs based on other platinum(II) systems [147, 148]. 2.2 Platinum(II) Complexes with Tridentate N-Donor Ligands In addition to the bidentate N-donor platinum(II) complexes, the use of tridentate Ndonor ligands in the design and synthesis of the square-planar system has been extensively investigated. The coordination mode and the rigidity provided by the tridentate ligands can offer additional stability to the platinum(II) system. Unnecessary reductive elimination due to cis-coordinating monodentate ligands can also be avoided. Moreover, the conjugated tridentate N-donor ligands can facilitate the planar geometry of the complex system to avoid the non-radiative decay caused by molecular distortion and thus enhance the luminescence properties [161]. Apart from the employment of the structural modification on the tridentate ligands to minimize the non-radiative decay pathways via the d–d ligand field states, Yam and co-workers first reported a novel class of alkynylplatinum(II) terpyridine complexes (Fig. 4), which exhibited intense luminescence in acetonitrile solution at room temperature [149]. The intense emission was assigned as originating from an admixture of the 3MLCT [dp(Pt) ? p*(tpy)] excited state and alkynyl-toterpyridine 3LLCT [p(alkynyl) ? p*(tpy)] excited state. By the incorporation of the strong r-donating alkynyl ligand, the group suggested that the energy of the dr* orbitals would be raised to cause an increase of the d–d splitting that resulted in a higher-lying d–d ligand field excited state. Additionally, the lowering of the energy of the 3MLCT/3LLCT state would be resulted due to the destabilization of the

Fig. 4 Molecular structures of alkynylplatinum(II) terpyridine complexes reported by Yam and co-workers. From Ref. [149]

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highest occupied molecular orbital (HOMO) level, which was a bonding combination of the pp(alkynyl)–dp(Pt) orbitals. The rise in energy of the non-emissive ligand field excited state and the simultaneous lowering of the energy of the emissive 3MLCT/3LLCT state would both contribute to the enhancement of their luminescence properties [149–151]. Notably, the introduction of the alkynyl ligand into this class of complexes significantly improved their solubility in solution, further allowing versatile studies to be conducted to further investigate the spectroscopic behaviors of the complex in solution state. Inspired by the rich and diverse photophysical properties of the alkynylplatinum(II) terpyridine system, various research groups have further used the complex system for versatile applications, such as pH sensing, biolabeling, molecular recognition, hydrogen gas generation, and dye-sensitized solar cells [126, 152–159]. 2.3 Cyclometalated Platinum(II) Complexes The incorporation of cyclometalating ligands to the platinum(II) metal center could further enhance the luminescence properties as well as the EL performance of OLEDs [160–162]. Cyclometalation commonly refers to the coordination of a polydentate ligand to a metal center through covalent metal–carbon bond [162]. The cyclometalation process would also involve the deprotonation of the aromatic protons and leads to anionic cyclometalating ligands. The anionic carbon atom that coordinates to the metal center is a very strong r-donor that gives rise to a very high energy ligand field splitting, rendering the non-radiative d–d ligand field excited state much higher in energy, which in turn becomes less competitive than other emissive states to facilitate the luminescence of the complex system. The formation of the strong metal–carbon bond also enhances the stability of the metal complex system to make it more processable for OLED applications. In addition, the planar and rigid nature of the cyclometalating ligands can suppress the non-radiative decay pathway via molecular distortion [161, 162]. Indeed, a variety of cyclometalated platinum(II) complexes had been proved to be strongly luminescent in the solution state under ambient conditions, leading to worldwide interest of cyclometalated platinum(II) complexes as the triplet emitters in OLEDs [160–162]. 2.3.1 Cyclometalated Platinum(II) Complexes with Bidentate Ligands The early examples of cyclometalated platinum(II) complexes were with the bidentate 2-phenylpyridine (ppyH) ligand, which coordinated to the platinum(II) Fig. 5 Molecular structures of cis-[Pt(N^C-thpy)2] and cis[Pt(N^C-thpy-5-SiMe3)2] complexes. From Refs. [167, 168]

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metal center through pyridine nitrogen and the anionic ortho phenyl carbon. The system involved both strong r-donating and p-accepting characters from the anionic carbon and the pyridine ring, respectively, while the chelating platinum– carbon–nitrogen five-membered ring was responsible for the rigidity of the system [162]. Balzani, von Zelewsky and Maestri in the 1980s demonstrated one complex system with bidentate ligand, cis-[Pt(thpy)2] [thpyH = 2-(2-thienyl)pyridine], which gave yellow luminescence at 580 nm in solution with an excited state lifetime of 2.2 ls. A number of other heterocycles had also been incorporated into the cyclometalating ligand to give structurally related platinum(II) complexes that were emissive at low temperature glass. In general, the emissions were attributed to originate from a 3MLCT state [163–165]. The application of cis-[Pt(thpy)2] as phosphorescent emitters in OLEDs had been explored by Thompson and co-workers [166]. To ensure complete energy transfer between the host and the phosphor, a mixed host consisting of PVK and 2-(4biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) was used and a reasonable EL performance with CE of 6.0 cd A-1 and EQE of 2.2 % were achieved [166]. The related cis-[Pt(N^C-thpy-5-SiMe3)2] was further investigated by Cocchi and coworkers in 2004 (Fig. 5) [167, 168]. These complexes were found to be thermally unstable in spite of their strong luminescence in degassed solution with Uem of ca. 0.35. In view of this, OLEDs based on this class of complexes were fabricated by spin-coating. A device based on cis-[Pt(N^C-thpy-5-SiMe3)2] showed a single peak emission band that originated from the cis-[Pt(N^C-thpy-5-SiMe3)2] complex, suggesting a complete energy transfer from the N,N0 -bis(3-methylphenyl)-N,N0 bis(phenyl)-benzidine (TPD) host to the platinum(II) complex. Surprisingly, a high EQE of 11.5 % was realized [167]. The superior EL performance compared to the analogous platinum(II) porphyrin devices was ascribed to a shorter excited state lifetime of this cyclometalated complex [167, 168]. The synthesis of structurally related cyclometalated complexes of [Pt(N^C)(N^N)]?, [Pt(N^C)(O^O)], [Pt(N^C)(CO)(Cl)] and [Pt(N^C)(CO)(SR)] was achieved by the addition of different donor ligands [129–131]. Kvam and co-workers conducted in-depth studies on the emission properties of [Pt(ppy)Cl2]-, [Pt(ppy)(en)]?, [Pt(ppy)(bpy)]? , and [Pt(bpy)(phen)]2? with an assignment of 3MLCT state for the emission in dimethylformamide solution and verified the assignment with their corresponding electrochemical data [129]. The strong-field cyclometalated N^C ligand was anticipated to facilitate the luminescence properties of the complexes by shifting the non-emissive d–d states to high energy and to become less thermally accessible [129–131]. However, the major challenge of these classes of complexes ([Pt(ppy)L]n?/-) for the OLED applications was associated with their charged nature, which made them not suitable to be fabricated by vapor deposition processes. With the use of strong-field anionic spectator ligands such as acetylacetonate, charge neutral complexes could be achieved [132]. The systematic study of versatile cyclometalated [Pt(N^C)(O^O)] (O^O = acetylacetonate) and [Pt(N^C)(dpm)] (dpm = dipivolylmethane) complexes was performed and reported by Thompson and co-workers [48, 130, 132, 133]. The study of the N^C ligands included the systematic variation of substituents in either the phenyl or the pyridine rings of Reprinted from the journal

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ppy, as well as extending the aromatic structures. The emission energies were found to be significantly perturbed by such structural modification and the excited states rationalized to be predominantly 3MLCT [dp(Pt)/ p(N^C) ? p*(N^C)] character, with the support from electrochemical data and density functional theory (DFT) calculations [48, 134]. Bre´das and coworkers also employed DFT and time-dependent DFT (TDDFT) calculations to investigate the geometric and electronic structures and the optical properties of the phosphorescent platinum compounds [169]. It was found that the role of Pt– Pt interactions determines the excimer formation while the intermolecular p–p interactions are more decisive to the optimal excimer geometry and the magnitude of the phosphorescence energy [169]. More interestingly, the [Pt(N^C)(O^O)] system was found to be susceptible to self-quenching and excimer formation because of its planar geometry [130]. In the solid state, excimer emission was observed upon the appropriate packing of molecules and resulted in aggregation in the ground state. The interactions might involve the overlap of the dz2 orbitals of two neighboring platinum metal centers, leading to [dr*(Pt2) ? p*(N^C)] MMLCT excited states. Thompson and Forrest had further investigated the use of this complex as a single phosphorescent dopant to test its possibility of emissions from both the monomer (blue) and excimer (red) states simultaneously [134]. For [Pt(N^C-F2ppy)(acac)], an undesired residual emission from the 4,40 -N,N0 -dicarbazole-biphenyl (CBP) host was observed at a low doping level (1.5 %) in addition to the monomeric platinum(II) complex [134]. On the other hand, the analogous [Pt(F2ppy)(O^O-mhpt)] was found to provide the appropriate steric bulkiness for a balanced monomer/excimer emission [133]. At 10 % doping concentration of such complex, devices exhibited a maximum EQE of 3.3 % and white light emission with Commission International de L’Eclairage (CIE) coordinates of (0.39, 0.43) [133]. In 2006, Wong and co-workers had designed and synthesized a new class of ambipolar [Pt(ppy)(acac)] emitters, in which the platinum(II) emitter was covalently linked to oxadiazole and triarylamine moieties as electron- and hole-transporting units, respectively [170, 171]. These complexes were capable to be sublimable and readily formed neat emissive layer devices by vacuum deposition. In particular, orange-emitting OLEDs were prepared with the structure of indium-tin oxide (ITO)/copper phthalocyanine (CuPc)/[Pt(ppy)(acac)]/Ca/Al with a maximum CE of 1.2 cd A-1 and maximum luminance of 1065 cd m-2 at 14 V [170, 171]. More importantly, these complexes were found to display a mixed 3MLCT/intraligand charge transfer (ILCT) emission that could widen the EL spectrum [172]. By adjusting the applied voltage, the ratio of ILCT and 3MLCT emission energies could be fine-tuned to achieve WOLEDs based on a single dopant as emissive material. Researchers had further extended this work to incorporate various donors, such as fluorene [173], triphenylamine [174–176], and carbazole [177, 178], as well as acceptors, such as dimesitylboron [174] and fluorinated groups [173], to further improve the device performance. The introduction of the dimesitylboron unit at the 4-position of the phenyl ring or the 5-position of the pyridine ring of the ppy ligand by Wang and co-workers was found to result in a red shift in emission energies [174]. More importantly, a device based on one of the dimesitylboron-containing

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platinum(II) complexes with the configuration of ITO/molybdenum(VI) oxide (MoO3)/CBP (35 nm)/platinum(II) complex:CBP (15 nm)/2,20 ,200 -(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBI):[Pt-PPY-B5] (10 nm)/TPBI (65 nm)/LiF/Al featured an outstanding EL performance with EQE exceeding 20.0 % and PE close to 80 lm W-1. The high luminescence quantum yield of the phosphor, excellent electron-transporting properties, and steric bulkiness offered by the triarylboron group were suggested to be the keys for this success [174]. Strassner and coworkers had also reported a special class of bidentate cyclometalated bdiketonate platinum(II) complexes with 1-phenylimidazole NHC cyclometalating ligands and investigated their photophysical properties [179]. The structured emission bands in the blue region and the relatively long lifetime in tens of microseconds suggested the emissive origin to be the triplet p–p* excited state of the cyclometalating ligand [179]. The complexes were then fabricated into OLED devices with different doping concentrations (6, 8, 10, and 12 %) and gave the CIE coordinates of (0.162, 0.314) as green–blue color emitter [179]. Wang and coworkers also reported another interesting class of Pt(II) acetylacetonate complexes by incorporating BMes2-functionalized NHC moieties as the N^C cyclometalated ligand [180]. These NHC-chelate Pt(II) compounds displayed intense blue to bluishgreen phosphorescence in solution (Uem = 0.41 - 0.87) and in solid state (Uem = 0.86 - 0.90). Highly efficient EL devices based on these Pt(II) compounds were realized with a remarkable high EQE of 17.9 % [180]. More recently, the triphenylsilane and triphenylgermane substituents had been incorporated into the ppy ligand, in which the sp3-Si/Ge was found to show a certain extent of electrondonating and electron-accepting properties (Fig. 6) [181]. Both the charge carrier injection and carrier-transporting properties in [Pt(ppy-Si)(acac)] or [Pt(ppyGe)(acac)] were suggested to be improved [181]. The bulky triphenylsilane and triphenylgermane groups were also found to enhance the rigidity of molecules in order to facilitate radiative pathways, and thus resulted in high luminescence quantum yields. Devices based on [Pt(ppy-Ge)(acac)] showed both monomeric and

Fig. 6 Molecular structures of [Pt(N^C)(acac)] complexes with different donor–acceptor moieties. From Refs. [174, 181]

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excimeric emission bands in the EL spectrum. At a doping concentration of 8 %, a color rendering index (CRI) of ca. 90 at the brightness level [15,000 cd m-2, CIE coordinates of (0.33, 0.36), and correlated color temperature (CCT) of 5340 K were reported [181]. In addition to the cyclometalated N^C system, the coordination of bidentate anionic N-donor ligands composed of pyridine ring with 5-membered heterocyclic azole ring (imidazole, pyrazole, or triazole) to the platinum(II) center has received increasing attention in recent years [120–125]. The electron-transporting properties of these complexes are facilitated as a result of the strong electron-withdrawing ability of the nitrogen-containing heterocycles. Upon deprotonation, the anionic nitrogen donor ligand should feature similar properties as the cyclometalated ligand, serving as a strong field ligand to avoid the population of non-emissive d–d excited state. Chi and co-workers had explored the chemistry of platinum(II) azolate complexes with detailed investigations on their luminescence properties and applications in PHOLEDs [121–123]. A number of [Pt(N^N)2] complexes were shown to exhibit tunable emission energies by structural modification of the ligands (Fig. 7). Complexes with bulky t-butyl substituents were reported to show intense 3 MLCT emission in degassed solution at room temperature with Uem = 0.8 and s = 2.4 ls, respectively [122, 123]. On the contrary, related complexes without bulky substituents displayed very weak and short-lived emission under the same conditions, which was attributed to excited-state deactivation driven by selfquenching [122, 123]. In addition, these complexes were found to be brightly emissive in the solid state thin films at room temperature. The emissions were suggested to originate from a 3MMLCT [dr* ? p*] excited state, as supported by the observed columnar packing of the complex molecules in the crystal structure ˚ [122]. Devices were fabricated using with short Pt(II)–Pt(II) contacts of 3.442 A [Pt(N^N)2] (N^NH = 3-trifluoromethylpyrazoles) as a phosphorescent dopant and CBP as host material for OLEDs [122]. Using this 3MMLCT emitter, respectable EQE of 6.0 %, and CE of 19.7 cd A-1 were achieved [122]. The same research group also investigated the incorporation of isomeric isoquinolyl-indazole ligands (1-iqdzH and 3-iqdzH) into the platinum(II) center as bidentate ligand to give the [Pt(N^N-iqdzH)Cl2] complexes [123]. However, these complexes were found to be non-emissive, requiring further substitution of the chloro ligand by the picolinate or 3-trifluoromethyl-5-(2-pyridyl)-pyrazolate to give [Pt(N^NiqdzH)(N^O)] and [Pt(N^N-iqdzH)(N^N-CF3-ppz)] to give tunable green to red 3 MLCT phosphorescence (Uem = 0.6) in fluid solution at room temperature. These

Fig. 7 Selected examples of platinum(II) azolate complexes. (R = CF3 and R’ = pyridine). From Ref. [121–123]

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were attributed to the stronger ligand field strength of the picolinate ligands. At 6 % dopant concentration, a maximum EQE of 5.7 % at 5 V was recorded. More importantly, such devices exhibited satisfactory EL performance even at high brightness, in which a maximum luminescence of 20,296 cd m-2, an EQE of 4.9 %, CE of 12.2 cd A-1 , and a PE of 6.1 lm W-1 were recorded [122, 123]. Qiao and co-workers recently reported a structurally related bis[3-trifluoromethyl-5-(2-pyridyl)-1,2-pyrazolato]platinum(II) complex. In the photophysical studies, its emission quantum yield in N,N0 -dicarbazolyl-3,5-benzene (MCP) thin film was found to remain almost unchanged at doping concentrations ranging from 10 to 100 %, indicating that the emission quenching behavior was negligible (Fig. 7, R = CF3) [182]. By taking this advantage, a non-doped red-emitting device with the configuration of ITO/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC)/platinum(II) complex: MCP/tris-(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl) borane (3TPYMB)/LiF/Al was fabricated and showed an extremely high EQE of 31.1 %, a maximum CE of 47.1 cd A-1 and a PE of 35.0 lm W-1, in which the EQE value was the highest among those obtained by non-doped devices. Meanwhile, another structurally related bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II) [Pt(ptp)2] (Fig. 7, R0 = pyridine) had been demonstrated to give tunable emission colors from blue-green to yellow and orange-red in the solid state thin films upon increasing the doping concentration from 3 to 45 % in CBP [183]. The origin of emissions was also found to be switchable from the monomer 3MLCT/3IL structured emission to the 3MMLCT Gaussian-shaped emission due to Pt–Pt interactions. Recently, [Pt(ptp)2] was also observed to give a moderate to high electron mobility on the order of 10-4 cm2 V-1 s-1, attributed to the electrondeficient pyridine and triazole moieties in the bidentate ligands [184]. The electron– hole recombination was proved to be confined within a narrow region near the holetransporting layer/[Pt(ptp)2] interface. Non-doped OLEDs based on this complex showed a high EQE of up to 20.0 % with a relatively low turn-on voltage at 2.2 V. Short triplet excited lifetime was found to be the key success of this system [184]. 2.3.2 Cyclometalated Platinum(II) Complexes with Tridentate Ligands Although bidentate cyclometalated platinum(II) complexes perform interesting photophysical properties, the d–d excited states of d8 platinum(II) complexes with bidentate ligands are usually susceptible to molecular distortion, shifting the molecule away from the ideal ground state D2h or C2v symmetry to a D2 or C2 symmetry with the plane of the bidentate cyclometalating ligand being twisted relative to the ancillary ligands [132, 161, 162]. Such distortion would provide the pathway for non-radiative decay that lowered the luminescent properties of the metal complex. Tridentate cyclometalating ligand systems with more rigid binding towards the platinum(II) metal center when compared with those with mono- and bidentate ligands could in principle reduce the extent of molecular distortion. Indeed, the cyclometalated [Pt(C^N^N)]?, [Pt(C^N^C)] and [Pt(N^C^N)]? complexes have been extensively investigated for their potential applications in OLEDs since the past decade. These classes of tridentate cyclometalated platinum(II) complexes had been demonstrated to display a variety of emissive excited states, Reprinted from the journal

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including MLCT, IL, excimeric, and MMLCT excited states [132, 161, 162]. It was found that the relative energy of these excited states was strongly affected by subtle changes in the local environment in which the platinum(II) complex was located. 2.3.3 Cyclometalated Platinum(II) Complexes with C^N^N Ligands In 1990, Constable and co-workers first reported the coordination of 6-phenyl-2,20 bipyridine (phbpy) to platinum(II) metal center as tridentate C^N^N ligand [185]. Later, the same group reported a related series of cyclometalated platinum(II) complexes with 6-(2-thienyl)-2,20 -bipyridine as the C^N^N tridentate ligand, [Pt(thbipy)X], where thbipy = 6-(2-thienyl)-2,20 -bipyridine and X = Cl, P(O)(OMe)2 or acetylacetone [186]. The luminescent properties of this system were further explored by Che and co-workers [187]. In their photophysical studies, the room-temperature solution luminescence of [Pt(phbpy)Cl] was assigned as originating from a 3MLCT excited state with emission maximum at 565 nm and an excited state lifetime of 0.51 ls [187]. The introduction of pendent aryl groups in the 4-position of the central pyridine ring (R = Ph, p-Cl–C6H4, p-Me–C6H4, pMeO–C6H4–, or 3,4,5-(MeO)3–C6H2–) enhanced the emission quantum yield by two to three times without significant variation in the emission energy, suggesting that there was limited electronic communication between the pendent aryl group and the planar phbpy unit [187]. The same group further carried out comprehensive studies on the synthesis and structural, spectroscopic, and electrochemical properties of a number of mononuclear, dinuclear, and trinuclear cyclometalated

Fig. 8 Molecular structures of cyclometalated platinum(II) C^N^N carbonyl, isocyano, phosphine, and alkynyl complexes. From Refs. [188, 192]

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platinum(II) C^N^N complexes with various types of auxiliary ligands such as carbonyl [188], isocyanide [153], phosphine, and alkynyl [189–191], as well as the extension of p-conjugation of the cyclometalated ligand [192] to enhance the luminescence (Fig. 8). In 2007, the cyclometalated [Pt(C^N^N)] system reported by Che and co-workers showed intense and tunable phosphorescence, together with their high thermal stability and charge neutrality, allowing them to be a favorable candidate as EL dopants for OLED applications [50]. Selected complexes were doped and fabricated as the emissive layer in the multilayer vapor-deposited OLEDs. The emission energies of the EL spectra were found to be color-tunable, ranging from saturated yellow to red, similar to their corresponding counterparts in the solution state [50]. Orange-emitting OLEDs based on [(C^N^N)Pt(PPh)3](ClO) exhibited a maximum luminance of 37,000 cd m-2 and CE of 15.4 cd A-1 with CIE coordinates of (0.37, 0.58) [50, 192, 193]. Meanwhile, high performance WOLED was prepared by combining the orange-yellow emission from the chloroplatinum substituted [Pt(C^N^N)] complex [C^N^N = 3-(60 -(200 -naphthyl)-20 -pyridine)-isoquinoline] and blue emission from the fluorescent 9,10-bis-(b-naphthyl)anthrene material [50]. Such device based on [{(CF3)2C6H3-C^N^N}PtCl] showed high EQE and PE of 11.8 % and 18.4 lm W-1, respectively, along with a balanced white emission with CIE coordinates of (0.30, 0.32) and CRI of 73 at 6 V (Fig. 9) [50]. These superior EL efficiencies were attributed to the use of hybrid phosphorescence/fluorescence dualemitting architecture, which harvests both high-energy singlet excitons from the blue fluorescent material and low-energy triplet excitons from the platinum(II) C^N^N complex [50]. More recently, the same group reported another related series of platinum(II) complexes with fluorene-containing C^N^N cyclometalated ligands, in which an extensive intermolecular p–p interaction between the fluorene-containing C^N^N ligands was observed in the X-ray structure [194]. Upon irradiation, the complexes exhibited vibronic-structured emission bands, which were assigned as originating from a mixture of 3IL p–p* and 3MLCT excited states. In addition, the emission energy was found to be red-shifted upon increasing the fluorene units, rationalized by the increase of p-conjugation of the system. PHOLEDs based on selected fluorene-containing platinum(II) complexes were fabricated by both vacuumevaporation and solution-processing techniques. The vacuum-deposited device

Fig. 9 Left Molecular structures of cyclometalated C^N^N chloroplatinum(II) complexes. Right EL spectrum of the fabricated WOLED. Reproduced with permission from Ref. [50]. Copyright 2007 Wiley– VCH

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demonstrated a satisfactory EL performance with maximum CE of up to 14.7 cd A-1 and maximum brightness of 27,000 cd m-2, while those of the solution-processed OLEDs using PVK as host material were 9.2 cd A-1 and 3500 cd m-2, respectively [194]. This work represents the first demonstration of fluorene-containing C^N^N platinum(II) complexes as suitable phosphorescent dopants in both vacuum-deposited and solution-processable devices [194]. 2.3.4 Cyclometalated Platinum(II) Complexes with C^N^C Ligands In addition to C^N^N, the platinum(II) C^N^C system has drawn extensive interest for their potential applications in OLEDs [195–199]. The first synthesis and characterization of the platinum(II) C^N^C complexes of 2,6-diphenylpyridine was reported by Rourke and co-workers in 1999, and the complexes were found to be highly robust with high decomposition temperatures of above 320 °C [195]. Che and co-workers also reported a related class of cyclometalated platinum(II) C^N^C complexes with 2,6-diphenylpyridine as the tridentate ligand and incorporated with different ancillary ligands, such as 2,6-dimethylphenylisocyanide, tricyclohexylphosphine, triphenylphosphine, pyrazine, bis(dicyclohexylphosphino)methane (dcpm), and bis(diphenylphosphino)methane (dppm) and bis(diphenylphosphinomethyl)-phenylphosphine (dpmp), to study their photophysical properties [196]. The broad and structureless emission bands of the platinum complexes in the solid state were assigned to the 3p–p* excimeric emissions. Interestingly, one of the complexes with bis(diphenylphosphino)methane (dppm) bridging ligand was demonstrated to show vapochromism. The incorporation of other auxiliary ligands, such as isocyanide and pyridine, into the platinum C^N^C motif was also studied by the same group to demonstrate their rich photophysical properties [196]. Meanwhile, Yam and co-workers incorporated the P- and N-donor crown ether ligands into the platinum C^N^C motifs to create chemosensors for various metal ions [197]. In 2006, Che and co-workers further reported a related series of mononuclear and dinuclear bis-cyclometalated platinum(II) C^N^C complexes with 2,6-di-(20 -naphthyl)-4-R-pyridine as the tridentate ligand, where R = H, C6H5, C6H4–Br-4, or C6H3-F2-3,5 (Fig. 10) [198]. This class of dinuclear [{Pt(C^N^C)}2(l-dppm)] system was also shown to exhibit a reversible vapoluminescence response upon exposure to volatile organic compounds.

Fig. 10 Molecular structures of mononuclear and dinuclear bis-cyclometalated platinum(II) complexes with 2,6-di-(20 -naphthyl)-4-R-pyridine (R = H, C6H5, C6H4–Br-4, or C6H3-F2-3,5). From Ref. [198]

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Top Curr Chem (Z) (2016) 374:46 Fig. 11 Selected molecular structures of cyclometalated C^N^C platinum(II) complexes. From Ref. [199]

More recently, Che and co-workers extended their work to synthesize a related class of [(C^N^C)Pt(L)] (L = DMSO or C:N-Ar) with the tridentate C^N^C ligand derived from carbazole, fluorene, or thiophene moieties as shown in Fig. 11 [199]. The emission origins were attributed to the 3MLCT/3IL excited state with the luminescence quantum yields of ca. 2–26 %. The emission energies could also be tuned by the variation of the p-conjugation of the C^N^C ligand. Given the high luminescence quantum yields, good solubility, tunable emission energies, and high thermal stability ([300 °C), the complexes were used as phosphorescent dopants in the preparation of OLEDs. By using vacuum deposition technique, one of the complexes was demonstrated to be a good candidate for red-emitting OLEDs with a maximum EQE of 12.6 %, a low turn-on voltage of 4.0 V and CIE coordinates of (0.65, 0.35) at 1000 cd m-2 [199]. As this class of complexes had good solubility and displayed relatively high emission quantum yields in most of the common organic solvents, they had also been employed to fabricate OLEDs by spin-coating method, in which organish-red-emitting devices with emission peak maxima in the range of 596–616 nm were observed [199]. 2.3.5 Cyclometalated Platinum(II) Complexes with N^C^N and N^N^N Ligands Apart from the extensive studies on the cyclometalated platinum(II) C^N^N [185–194] and C^N^C complexes [195–199], cyclometalated platinum(II) complexes of 1,3-dipyridylbenzene (N^C^N) were first demonstrated by Ca´rdenas, Echavarren and co-workers in 1999 [200]. The study of the luminescence property of this system was first reported by Williams and Weinstein in 2003 [201]. This class of complexes emitted strongly in degassed dichloromethane at room temperature, achieving a high luminescence quantum yield of 68 %. The strong luminescence behavior was associated with the strong ligand field exerted by the N^C^N ligand due to the short Pt–C bond. Thus, the energy of the non-emissive ligand field state would become higher-lying and the non-radiative deactivation would be less likely to occur [201]. In addition, this platinum(II) N^C^N system displayed strong green phosphorescence in dichloromethane solution at room temperature with Uem from 0.4 to 0.6, in which the emission bands were assigned as originating from the 3IL [p? p*(N^C^N)] excited state. More importantly, the

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Fig. 12 Left Molecular structures of cyclometalated N^C^N platinum(II) complexes. Right Schematic diagram of the employed OLED structure. Reproduced with permission from Ref. [202]. Copyright 2007 Wiley–VCH

excited state lifetimes in the solid state at room temperature and 77 K were found to be very similar, suggesting that N^C^N coordination could effectively deactivate pathways of potential thermal non-radiative decay. The emissions could also be steadily tuned from greenish-blue to yellow by increasing the electron-donating ability of the substituents at the 5-position of the central phenyl ring of the N^C^N ligand [202]. Taking advantage of the high luminescence quantum yields and the ready tunability of the emission color of this class of complexes, cyclometalated platinum(II) N^C^N complexes of 1,3-dipyridylbenzene were used as the phosphorescent dopants for OLED applications and had been shown to display much better EL performance when compared to those devices based on the C^N^N or C^N^C systems (Fig. 12) [202]. High performance OLEDs were realized by doping the platinum complexes into a CBP host, in which external EQEs of 4–16 % and CEs of 15–40 cd A-1 were achieved using the device configuration of ITO/ TPD:polycarbonate (PC)/CBP/platinum(II) complex:CBP:3,5-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (OXA)/OXA/Ca (Fig. 12). Such devices gave negligible CE roll-off even at high current densities [202]. Extension of the work was done by the same group by using different fabricating conditions and different N^C^N ligands to optimize the photoluminescence quantum yield (PLQY) and OLED performance [203–206]. Upon increasing the dopant concentrations in solution, the cyclometalated N^C^N platinum(II) complexes of 1,3-dipyridylbenzene formed excimers, which gave an intense emission at around 700 nm with Uem close to 0.3 [203]. The combination of both monomer and excimer emissions of the platinum complexes doped at appropriate concentrations in polymer matrices yielded WOLEDs with an EQE of 6.5 % [203]. At the same time, by using platinum complexes as neat emissive layer, highperformance, near-infrared-emitting OLEDs could also be developed [204, 205]. Recently, the related tridentate N^C^N pyrazole-containing platinum(II) complexes were also designed and synthesized (Fig. 13) [206]. The poor p-accepting ability of the pyrazoles compared to the pyridines led to the destabilization of the 3IL excited state, resulting in a blue emission. However, the bite angle between the cyclometalating ligand and the platinum center was highly strained, causing a certain degree of molecular distortion. These, together with the less competitive and higher-energy emissive state, would lower the luminescence quantum yield than

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Fig. 13 Molecular structures of cyclometalated N^C^N platinum(II) complexes with different heterocycles. From Ref. [211]

Fig. 14 a Molecular structures of cyclometalated N^C^N platinum(II)-bizmb complexes. Normalized EL spectra of devices based on b [Pt(bzimb)Cl] and c [Pt(bzimb)(C:C-pyrene)]. Reproduced with permission from Refs. [138, 213]. Copyright 2011 The Royal Society of Chemistry; Copyright 2013 Wiley–VCH

those of other platinum(II) N^C^N complexes [207–209]. In 2006, a platinum(II) complex with 5-methyl-1,3-bis-(benzothiazol-2-yl)benzene (bzthb) as the tridentate ligand had been reported and showed yellow phosphorescence in solution [210]. This emission band was attributed to the 3IL [p ? p*(N^C^N)] excited state. By replacing the pyridine rings with azaindoles in the N^C^N ligand (Fig. 13), weak luminescence was observed in low-temperature glass, in spite of the relief of angle strain [211]. Recently, Yam and co-workers reported a new class of luminescent cyclometalated platinum(II) complexes with 2,6-bis(N-alkylbenzimidazol-20 -yl)benzene (bzimb) as the N^C^N ligand (Fig. 14) and demonstrated an intense green emission with PLQY of 19 % in dichloromethane solution, which increased to 45 % in solidstate thin film [213]. The vibronic-structured emission bands of the complexes were Reprinted from the journal

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assigned as the metal-perturbed 3IL [p ? p*] excited state. Multilayer PHOLEDs based on the complexes showed bright green EL with high CE and EQE of 38.9 cd A-1 and 11.5 %, respectively [213]. The high EQE of this complex system was one of the highest values ever reported for green-emitting PHOLEDs with platinum(II) systems. To optimize further the metal complex system for high-efficiency PHOLEDs, Yam and co-workers extended their efforts to design a new series of cyclometalated alkynylplatinum(II) complexes based on the 2,6-bis(N-alkyl-benzimidazol-20 yl)benzene as the N^C^N ligand (Fig. 14) [138]. By functionalizing the paraposition in the central phenyl ring of the bzimb ligand, the emission energy from the 3 IL [p ? p*] excited state could be fine-tuned from green to yellow. Vacuumdeposited OLEDs with the configuration of ITO/a-naphthylphenylbiphenyldiamine (NPB)/tris(4-carbazoyl-9-ylphenyl)amine (TCTA)/6 % platinum(II) complex:MCP/ 6 % platinum(II) complex:3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H1,2,4-triazole (TAZ)/bis(2-methyl-8-quinolinato)-4-phenylphenolate)aluminum(III) (BAlq)/LiF/Al were fabricated, in which the platinum complexes were doped in both MCP and TAZ host materials to form dual emissive layers. The optimized devices exhibited a maximum CE of 40.3 cd A-1 and EQE of 11.8 %. It was also found that the incorporation of the alkynyl ligand could improve both luminescence quantum yield and solubility of the system [138]. Interestingly, the incorporation of a pyrenylalkynyl ligand to the system exhibited dual emissions in both the green and the red regions, and a white emission was generated upon variation of the dopant concentration (Fig. 14) [138]. The device doped with 5 % of the platinum N^C^N complex of pyrenylalkynyl ligand gave a balanced white-light emission with CIE coordinates of (0.35, 0.39), which is very close to those of pure white light at (0.33, 0.33). This is different from the cases in other platinum(II) systems where white light emission was obtained upon dopant aggregation at high concentrations [138]. Very recently, Yam and co-workers further reported a related series of cyclometalated platinum(II) complexes with the N^C^N ligands of 2,6-bis(benzoxazol-20 -yl)benzene (bzoxb), 2,6-bis(benzothiazol-20 -yl)benzene (bzthb), and 2,6bis(N-alkylnaphthoimidazol-20 -yl)benzene (naphimb) as depicted in Fig. 15 [212]. Their vibronic-structured emission bands mainly originated from the 3IL [p ? p*(N^C^N)] state with mixing of a 3MLCT [dp(Pt) ? p*(N^C^N)] excited state [212]. The change of heteroatom from nitrogen to oxygen and to sulphur in the heterocycles in the N^C^N ligands resulted in a red shift in emission of the complexes, which was attributed to the electronegativity difference of the heteroatoms. The extension of p-conjugation by changing the benzimidazole to the naphthoimidazole heterocycles in the N^C^N ligands also gave rise to drastic red shift in the emission colors. As supported by computational studies, the variation of the heterocycles, modification of the p-conjugation, and changing of the paraaryl substituents of the N^C^N ligands could fine-tune the emission energies [212]. Some selected platinum(II) complexes were also used as phosphorescent dopants for PHOLEDs, in which a saturated yellow emission with CIE coordinates of (0.50, 0.49), maximum CE of 16.9 cd A-1 and EQE of 6.9 % were achieved (Fig. 15) [212]. It is worth highlighted that the EL spectra were found to be independent of the dopant concentration.

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Fig. 15 a Normalized emission spectra of cyclometalated N^C^N platinum(II) complexes of different heterocycles and the CIE coordinates of the yellow-emitting device. b CE and EQE of the devices as a function of current density. Reproduced with permission from Ref. [212]. Copyright 2013 Wiley–VCH

Apart from the N^C^N ligand, De Cola and co-workers isolated a series of neutral platinum N^N^N complexes comprising two deprotonated triazole heterocycles in the tridentate ligands, together with pyridine as the ancillary ligand (Fig. 16) [127]. To minimize the intermolecular interactions of these platinum(II) complexes, bulky groups such as tolyl and adamantyl groups were introduced to the N^N^N ligands. An intense green emission with over 70 % luminescence quantum

Fig. 16 Left Molecular structures of platinum(II) triazole complexes [PtII(N^N^N)(py)]. Right EL spectra of the OLED device. Reproduced with permission from Ref. [127]. Copyright 2011 American Chemical Society

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yields in degassed chloroform solution were observed for this class of triazolecontaining platinum(II) complexes. The adamantyl-substituted platinum(II) complex in neat and 10 wt% doped poly(methyl methacrylate) (PMMA) films displayed structured monomeric emission spectra, similar to that recorded in the solution state. It should be mentioned that the tolyl-substituted derivatives in PMMA films showed a structureless and broadened emission band, suggesting that the adamantyl group played an important role in minimizing the intermolecular aggregation. At a doping concentration of 6 % in MCP matrix, a green-emitting OLED based on the adamantyl-substituted platinum(II) complex gave a maximum CE and PE of 15.2 cd A-1 and 7.01 lm W-1, respectively [127]. The same research group also extended the work to a more conjugated 2,6-bis(tetrazolyl)pyridine as the N^N^N ligand and demonstrated the possibility to make use of these materials to create the gelating nanofibers and EL films [128]. The microscopic studies revealed that the emissive material was comprised of a 3D network of fibers with 90 % PLQY in the gel state [128]. More importantly, the PLQY was almost unchanged in the doped PMMA thin films. An optimized OLED showed the maximum CE of 13.2 cd A-1 and PE of 12.7 lm W-1, at the brightness level of 1 cd m-2 [128]. Later, Strassert and co-workers reported two similar unsymmetrical platinum(II) complexes bearing tridentate triazole N^N^N ligands [147]. Although only one adamantyl group was attached to the triazolyl moiety, the intermolecular interactions between platinum(II) complexes could still be suppressed [147]. The ancillary pyridine ligand could be replaced by bulky triphenylphosphine ligand to further minimize the intermolecular Pt(II)–Pt(II) interactions. Solution-processable green-emitting PHOLEDs based on the unsymmetrical platinum(II) complex were fabricated [147]. These PHOLEDs achieved satisfactory performance with a maximum PE of 16.4 lm W-1 and CE of 15.5 cd A-1, corresponding to the EQE of 5.6 % [147], comparable to those of devices based on its vacuum-deposited analogs [213]. 2.3.6 Cyclometalated Platinum(II) Complexes with Tetradentate Ligands Apart from the cyclometalated platinum(II) complexes with bidentate and tridentate ligands, researchers have also explored the possibility of synthesizing cyclometalated platinum(II) complexes with tetradentate ligands. In addition to the first demonstration of PtOEP, Che and co-workers reported platinum complexes, [(N^N^O^O)Pt], with the bis(phenoxy)diimine as the tetradentate ligand in 2003 [107]. The tetradentate quinoline-containing ligands bis(20 -phenol)-bipyridine and bis(20 -phenol)-phenanthrolines were suggested to improve the electron mobilities and steric hindrance, which would be advantageous for OLEDs regarding charge combination and the suppression of self-quenching activity, respectively. The [(N^N^O^O)Pt] complexes exhibited intense orange emissions at 580–590 nm in dichloromethane solution, which were tentatively assigned as the mixed 3MLCT [dp(Pt) ? p*(diimine)] and 3LLCT [pp(phenoxide) ? p*(diimine)] excited states. These emitters were also employed as phosphorescent dopants in the fabrication of multilayer OLEDs, giving rise to two emission bands at 453 and 540 nm with CIE coordinates of (0.33, 0.47) [175].

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Fig. 17 Molecular structures of tetradentate Schiff base platinum(II) complexes. From Ref. [214]

Afterwards, Che and co-workers further extended the work to develop tetradentate platinum(II) complexes with N,N0 -bis(salicylidene)-1,2-ethylenediamine (salen) as the ligand (Fig. 17) [214]. The platinum(II) complexes bearing the Schiff base as the tetradentate ligand exhibited yellow to orange emissions in acetonitrile solution at 298 K, with emission maximum peaking at ca. 550 nm and Uem of ca. 0.2. The corresponding device showed satisfactory performance with maximum CE, PE, and EQE of 31 cd A-1, 14 lm W-1 , and 11 %, respectively. Combining this yellowish-orange emission with a blue fluorescence, WOLEDs with peak PE of 0.79 lm W-1 and CIE coordinates of (0.33, 0.35) were obtained [214]. More recently, the same group structurally modified the tetradentate Schiff base platinum(II) complexes and attempted to correlate the molecular structures with their photophysical properties [215]. The extension of p-conjugation of the Schiff base ligands resulted in a red-shifted emission band to ca. 649 nm in the red region. Measurements of the emission decay times in the temperature range from 130 to 1.5 K gave total zero-field splitting parameters of the emitting triplet state of 14–28 cm-1. High-performance yellow- to red-emitting OLEDs based on these platinum(II) Schiff base complexes had been fabricated, demonstrating the highest CE of up to 31 cd A-1 and a long operational lifetime of up to 77,000 h at 500 cd m-2, which were much higher than those of devices based on the earlier analogues [215]. Generally, structural distortion in luminescent materials leads to severe nonradiative decay, lowering the luminescence quantum yields. In 2010, Huo and coworkers reported a new series of triphenylamine-based tetradentate phosphorescent emitters, [Pt(C^N)2], with highly rigid molecular skeletons [216]. These tetradentate [Pt(N^C^C^N)] complexes displayed an intense red phosphorescence at 613 nm in solution [216]. Later, Fujikake and co-workers designed a related series of tetradentate [Pt(N^C^C^N)] complexes with more bulky substituents attached to the ligand in order to minimize their self-quenching (Fig. 18) [217]. The optimized PHOLED yielded a saturated red emission with excellent CIE coordinates of (0.66, 0.34), low driving voltage, high efficiency, and high stability. A maximum EQE of

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Fig. 18 Molecular structures of tetradentate [Pt(N^C^C^N)] complexes. From Ref. [217]

over 19 %, a maximum PE of 30 lm W-1, and an estimated half-life of the optimized PHOLED of *10,000 h at an initial luminance of 1000 cd m-2 were noted [217]. Recently, Che and co-workers developed another new series of tetradentate [Pt(O^N^C^N)] complexes by employing the rigid and conjugated carbazole heterocycles as the molecular framework [218]. Because of the rigidity of the tetradentate ligand, the platinum(II) complexes showed an intense yellow emission with luminescence quantum yields of up to 0.86 in dichloromethane solution. With a simple device configuration of ITO/NPB/platinum(II) complex:MCP/BAlq/LiF/Al, excellent EL performance of high CE and PE of 74.9 cd A-1 and 52.1 lm W-1 were obtained. These CE and PE values were some of the highest values reported so far [218]. The complex could also be used to prepare two-color WOLEDs by the combination of monomer and excimer emissions. Superior EL performance with a respectably high EQE of up to 16 % was achieved [218]. Meanwhile, Li and coworkers reported another series of carbazole-containing tetradentate [Pt(C^N^N^C)] and [Pt(C^C^N^C)] complexes with rigid structure as shown in Fig. 19 [219, 220]. The complexes were found to exhibit high PLQY of up to 0.85 in PMMA thin films. The [Pt(C^N^N^C)] complex with a more conjugated isoquinoline component was selected to fabricate OLEDs and showed a red emission with a peak EQE of 12.5 %, while the less conjugated [Pt(C^C^N^C)] with the pyrazole counterparts displayed a blue emission with a much higher peak EQE of 23.7 % at 100 cd m-2 [219]. Taking its narrow EL spectrum and emission maximum at ca. 450 nm into account, the EL performance of this blue-emitting device was one of the best blue-emitting devices

Fig. 19 Molecular structures of carbazole-containing tetradentate [Pt(O^N^C^N)] and [Pt(C^N^N^C)] complexes. From Refs. [219, 220]

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Fig. 20 Molecular structures of imidazole-containing tetradentate [Pt(N^C^C^N)] complexes and their corresponding emission spectra at room temperature. Reproduced with permission from Ref. [221]. Copyright 2014 Wiley–VCH

Fig. 21 Left Molecular structures of a symmetrical and b unsymmetrical N-heterocyclic-carbenecontaining tetradentate platinum(II) complexes. From Refs. [224–227]

based on platinum(II) complexes and was comparable to the best iridium(III) based blue-emitting OLEDs reported [220]. More recently, Li and co-workers elaborated the work to change the carbazole heterocycles to other azole moieties in the tetradentate platinum(II) complexes [221, 222]. Tetradentate [Pt(N^C^C^N)] complexes with imidazole and pyrazole units were found to give a blue emission with a very high luminescence quantum yield of 0.64 in solution and 0.81 in doped PMMA films (Fig. 20). As a result of the significant molecular distortion from the ideal planar geometry caused by the linking oxygen atoms, emission quenching and excimer emission formation due to intermolecular interactions between complex molecules could be suppressed [221]. Devices based on the [Pt(N^C^C^N)] complex with the imidazole unit displayed bluish-green light with an extraordinarily high EQE of 23.1 % and a PE of 48.8 lm W-1 [223]. Replacing the pyridine ring and the bridging oxygen atom in the tetradentate complex with a pyrazole or an imidazole unit, the emission peaks were red-shifted from 470 to 490 nm. More importantly, dramatically high

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performance was achieved by doping 4 % tetradentate [Pt(N^C^C^N)] complex with both imidazole and pyrazole units; in particular, the devices showed a peak EQE as high as 26.7 % and PE of up to 68.3 lm W-1 [221]. The superior performances of these devices could be attributed to the contribution from their unique electronic features originated from the imidazole and pyrazole components. Meanwhile, Che and co-workers reported a new series of tetradentate bis(Nheterocyclic carbene)-based platinum(II) complexes, which served as efficient blue triplet emitters (Fig. 21a) [224, 225]. Upon doping of 1 wt% of this class of complexes into PMMA thin films, intense blue phosphorescence peak ranging from 434 to 449 nm with Uem from 0.24 to 0.29 was resulted. Devices based on this class of complexes showed a maximum EL performance at the doping concentration of 4 wt%, in which a peak EQE of 15 % and a PE of 16.6 lm W-1 were recorded [224, 225]. The emission color was found to be deviated from the deep-blue region with CIE coordinates of (0.19, 0.22). The potential of using bis-NHC tetradentate platinum(II) complexes for preparing two-color WOLEDs was also investigated. By combining this blue emission with yellow emission from another platinum(II) triplet emitter, an impressively high PE of 55.2 lm W-1 and a CE of 87.8 cd A-1 were realized [224, 225]. Li and co-workers had also synthesized a series of tetradentate platinum(II) complexes bearing an unsymmetrical carbene moiety (Fig. 21b) [226, 227]. The carbene-based [Pt(C^N^C^C)] complex was found to emit blue light with emission peak at 452 nm when doped into PMMA films. The blueemitting OLED based on this complex exhibited a remarkable EL performance with a peak EQE of 23.7 %, a PE of 26.9 lm W-1 and CIE coordinates of (0.14, 0.15) [227]. The same group had also replaced the carbazolyl pyridine moieties by the phenoxy pyridine in the complex to further blue-shift the EL spectrum [226]. In particular, devices based on such complex exhibited a more saturated blue color with better CIE coordinates of (0.15, 0.10). In addition, by modification of the molecular structure by replacing the phenoxy pyridine with the imidazolyl benzene, a symmetric carbene-based tetradentate [Pt(C^C^C^C)] complex was synthesized [227]. The complex showed a red shift in emission spectrum, which was attributed to the destabilized HOMO level by the weakening of the Pt–C(Ph) bond due to the effect of the Pt–C(NHC) bond at the trans position [227]. Such OLED doped with 2 wt% of the symmetrical NHC-[Pt(C^C^C^C)] complex gave a sky-blue emission with CIE coordinates of (0.12, 0.24).

3 Luminescent Gold(III) Complexes for OLED Applications 3.1 Cyclometalated Gold(III) Complexes with C^N^C Ligands Compared to the isoelectronic d8 platinum(II) systems, research works on the development of phosphorescent gold(III) emitters for OLED applications are relatively less explored, while the related gold(I) complexes, especially those of polynuclear nature, are well known to show strong luminescence behaviour [228]. As mentioned in the introduction, the presence of low-energy d–d ligand field excited states would quench the luminescence state through thermal equilibration or

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energy transfer. The incorporation of strong r-donating ligands can raise the energy of the d–d ligand field states and thus improves the luminescence quantum yields of gold(III) complexes [55–60, 228]. The first demonstration on the design and synthesis of strongly luminescent cyclometalated alkynylgold(III) complexes, [Au(R–C^N(R0 )^C–R)(C:C–R)] (R–HC^N^CH = 2,6-diphenylpyridine or substituted 2,6-diphenylpyridine; R = alkyl, aryl or substituted aryl; R0 = aryl or substituted aryl), at room temperature was carried out by Yam and co-workers [56–60], based on their earlier strategy of introducing strong r-donating aryl and alkyl ligands to the gold(III) diimine complexes to give room-temperature phosphorescent gold(III) complexes [55]. In general, this class of cyclometalated alkynylgold(III) complex showed strong absorption bands at ca. 360–420 nm that were assigned as IL p–p* transitions of the tridentate C^N^C ligands, with mixing of some charge transfer character from the alkynyl moiety to the pyridine unit. An additional absorption tail was also observed for some alkynylgold(III) complexes, assignable to a LLCT transition arising from the presence of the electron-rich substituent on the alkynyl moiety. These assignments were further confirmed by DFT calculations at the B3LYP level of theory [61]. From the calculation results, the transitions at the low-energy absorption region at ca. 370 nm and 380 nm were found to resemble those observed in the experimental UV–visible absorption spectra, corresponding to the metal-perturbed IL transitions and the metal-perturbed LLCT transitions respectively [61]. Upon irradiation, all complexes were found to emit strongly with tunable emission color from sky-blue to red in dichloromethane solution and in the solid state at room temperature. By modifying the cyclometalated or alkynyl ligands, the emission energies could be fine-tuned and covered the entire visible light region. Interestingly, the emission energies of this class of complex were found to be red-shifted upon increasing the doping concentration from 5 to 50 % in the solid state PMMA or MCP thin films, possibly due the presence of the dimeric or excimeric emission arising from the p–p stacking of the cyclometalated C^N^C ligands, as supported by the observation of such p–p stacking in the crystal packing of [Au(C^N^C)(C:C–C6H4N(C6H5)2-p)] and [Au(2,5-F2C6H3-C^N^C)(C:C–C6H4N(C6H5)2-p)] (Figs. 22, 23) [57, 58]. Unlike the gold(I) complexes, which have been used as phosphorescent dopants in OLEDs [45], gold(III) complexes have not been employed in OLED fabrication. It was only in 2005 that luminescent gold(III) complexes were first used as phosphorescent dopant for OLED fabrication [57], taking advantage of their high thermal stability Fig. 22 Molecular structures of cyclometalated alkynylgold(III) complexes. From Refs. [57, 58]

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Fig. 23 Crystal packing of [Au(C^N^C)(C:C–C6H4N(C6H5)2-p)]. Hydrogen atoms are omitted for clarity. From Ref. [57]

Fig. 24 Molecular structures of cyclometalated alkynylgold(III) complexes. Reproduced with permission from Ref. [59]. Copyright 2013 American Chemical Society

and high decomposition temperature [350 °C. In particular, Yam and co-workers successfully demonstrated the capability of gold(III) complexes for OLED applications [57, 58]. Multilayer PHOLEDs with the configuration of ITO/TPD/ [Au(C^N^C)(C:C–C6H4N(C6H5)2-p)]:CBP/2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP)/Alq3/LiF/Al were fabricated. The optimized device showed intense EL band from 500 to 580 nm upon increasing the dopant concentration from 1 to 100 %. Such red shifts in emission energies were in line with the PL spectra in the solid-state thin films, probably due to the better packing and strong molecular interaction of the tridentate C^N^C ligand [57]. At doping concentration of 6 wt% of [Au(C^N^C)(C:C–C6H4N(C6H5)2-p)], a maximum EQE of 5.5 % and CE of 17.6 cd A-1 were recorded. Furthermore, the white EL could be simply achieved by combining the orange emission of [Au(C^N^C)(C:C–C6H4N(C6H5)2-p)] and the blue emission of the hole-transporting TPD by controlling the bias voltage [57]. In order to further improve the EL performance of the bis-cyclometalated alkynylgold(III) systems, a systematic modification was conducted on the bis-

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cyclometalated ligands, as well as on the alkynyl ligands [59]. It was found that the introduction of more conjugated and more rigid aryl-substituted diphenylpyridines and alkynyltriarylamine ligands onto the gold(III) metal center could generally improve their luminescence quantum yields [58, 59]. For instance, by replacing 4-methoxyphenylalkynyl with 4-diphenylaminophenylalkynyl onto the same gold(III) precursor complex [Au{C^N(2,5-Me2C6H3)^C}Cl] to yield [Au{C^N(2,5Me2C6H3)^C}(C:C–C6H4-OCH3-p)] and [Au{C^N-(2,5-Me2C6H3)^C}(C:C–C6H4N(C6H5)2-p)] (Fig. 24), respectively, where HC^N(2,5-Me2C6H3)^CH = 2,6diphenyl-4-(2,5-dimethylphenyl)pyridine [59], the emission quantum yield of [Au{C^N-(2,5-Me2C6H3)^C}(C:C–C6H4N(C6H5)2-p)] in the dichloromethane solution demonstrated a threefold enhancement when compared to that of [Au{C^N(2,5-Me2C6H3)^C}(C:C–C6H4-OCH3-p)] [59]. In addition, the incorporation of more rigid and conjugated 2,6-diphenyl-4-(2,5-difluorophenyl)-pyridine onto the gold(III) metal center in [Au(2,5-F2C6H3-C^N^C)(C:C–C6H4N(C6H5)2p)] yielded a higher luminescence quantum yield of 0.34 in doped PMMA thin films [58], higher than that of unsubstituted diphenylpyridine [Au(C^N^C)(C:C– C6H4N(C6H5)2-p)] (Uem = 0.22) [60]. The higher luminescence quantum yield of [Au(2,5-F2C6H3-C^N^C)(C:C–C6H4N(C6H5)2-p)] has made it a suitable candidate as phosphorescent dopant in OLEDs. In particular, devices based on [Au(2,5F2C6H3-C^N^C)(C:C–C6H4N(C6H5)2-p)] showed intense EL spectra, in which the peak maximum was slightly red-shifted from 528 to 532 nm upon increasing the dopant concentration from 2 to 8 wt% [58]. As expected, the device performance based on [Au(2,5-F2C6H3-C^N^C)(C:C–C6H4N(C6H5)2-p)] was better than that based on [Au(C^N^C)(C:C–C6H4N(C6H5)2-p)]. The optimized device with [Au(2,5-F2C6H3-C^N^C)(C:C–C6H4N(C6H5)2-p)] exhibited a maximum CE of 37.4 cd A-1, PE of 26.2 lm W-1, and a high EQE of 11.5 %, and were comparable to those of the Ir(ppy)3-based devices [9, 229]. In light of interest in the use of phosphorescent materials for solution-processable OLEDs, further extension of the work has been made to the design and synthesis of phosphorescent dendrimers through the incorporation of the gold(III) complexes into a dendritic structure [64, 65]. Yam and co-workers demonstrated the first report of the design and synthesis of gold(III) dendrimers. In particular, carbazole-based and triphenylamine-based dendritic alkynylgold(III) complexes were synthesized. Efficient solution-processable OLEDs were achieved by doping gold(III) complexes into MCP as an emissive layer by spin-coating. In particular, a high EQE of up to 7.8 % was recorded [65]. In addition, the incorporation of hole-transporting moieties, such as carbazole or triphenylamine, in the dendritic structure to form higher generation dendrimers suppressed the intermolecular interactions and reduced the bathochromic shift of the emission, similar to the cases for iridium(III) dendrimers [230–233]. More importantly, the emission energies could be effectively tuned from green to saturated red by a delicate design on the cyclometalated tridentate ligands as well as the dendrimer generations (Figs. 25, 26) [64, 65]. In 2014, Yam and co-workers reported a new class of red-emitting gold(III) dendrimers based on 5-fluoro-1,3-bis-(4-fluorophenyl)isoquinoline (4-F–C^N(5-F– C9H4)^C–F-4) ligand (i.e. [Au{4-FC^N(5-FC9H4)^CF-4}(C:C–C6H4N(C6H5)2p)], [Au{4-FC^N-(5-FC9H4)^CF-4}{C:C–TPA–(TPA)2}] and [Au{4-FC^N(5Reprinted from the journal

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Fig. 25 Molecular structures of triphenylamine-based dendritic alkynylgold(III) complexes. Reproduced with permission from Ref. [64]. Copyright 2014 Wiley–VCH

Fig. 26 Molecular structures of carbazole-based dendritic alkynylgold(III) complexes. Reproduced with permission from Ref. [65]. Copyright 2013 Wiley–VCH

FC9H4)^CF-4}{C:C–TPA–(TPA)6}]) [64]. This class of complex showed similar absorption and emission properties to the structurally related gold(III) complexes [60], in which the absorption bands at wavelength B370 nm were mainly attributed to spin-allowed IL p ? p* transitions of the triphenylamine units and the less intense vibronic-structured band at 438–458 nm was tentatively assigned to a metalperturbed IL p ? p* transition of the cyclometalated ligand with charge transfer character from the phenyl ring to the quinolinyl unit. The absorption tail beyond 470 nm was assigned as an admixture of IL and LLCT transitions. Upon excitation, the emission origins of these complexes are found to be dependent on the dendrimer generations. A vibronic-structured band with emission maximum at 612 nm was observed for [Au{4-FC^N(5-FC9H4)^CF-4}(C:C–C6H4N(C6H5)2-p)], which was assigned to a metal-perturbed 3IL state, while the broad and structureless emission band of [Au{4-FC^N(5-FC9H4)^CF-4}{C:C–TPA–(TPA)2}] and [Au{4-FC^N(5FC9H4)^CF-4}{C:C–TPA–(TPA)6}]) with emission maximum at 685 nm was assigned as originating from a 3LLCT state. The change in the emission origin from a 3IL state in complex [Au{4-FC^N(5-FC9H4)^CF-4}(C:C–C6H4N(C6H5)2-p)] to

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a 3LLCT state in complexes [Au{4-FC^N(5-FC9H4)^CF-4}{C:C–TPA–(TPA)2}] and [Au{4-FC^N(5-FC9H4)^CF-4}{C:C–TPA–(TPA)6}]) was revealed by a distinctive change in the emission lifetime from the long-lived 3IL state of [Au{4-FC^N(5-FC9H4)^CF-4}(C:C–C6H4N(C6H5)2-p)] (25.9 ls) to those of [Au{4-FC^N(5-FC9H4)^CF-4}{C:C–TPA–(TPA)2}] (0.8 ls) and [Au{4-FC^N(5-FC9H4)^CF-4}{C:C–TPA–(TPA)6}]) (1 ls). Their emission properties in MCP were also studied; in particular, the emission energies were strongly dependent on the dendrimer generations and dopant concentrations. Consistent with that in the solution state, complex [Au{4-FC^N(5-FC9H4)^CF-4}(C:C–C6H4N(C6H5)2-p)] featured a vibronic-structured emission band, the band shape and energy of which were independent of the dopant concentration. On the other hand, the PL spectra of [Au{4-FC^N(5-FC9H4)^CF-4}{C:C–TPA–(TPA)2}] and [Au{4-FC^N-(5-FC9H4) ^CF-4}{C:C–TPA–(TPA)6}]) showed a concentration dependence. An additional Gaussian-shape red emission band with peak maximum at *650 nm was observed when the concentrations of [Au{4-FC^N(5-FC9H4)^CF-4} {C:C–TPA–(TPA)2}] and [Au{4-FC^N(5-FC9H4)^CF-4}{C:C–TPA–(TPA)6}]) increased up to 25 % in MCP thin films. This red emission band is possibly due to the excimeric emission arising from the p–p stacking of the 4-F–C^N(5-F–C9H4)^C–F-4 ligand in the solid state thin films. Notably, the optimized devices made with [Au{4-FC^N(5FC9H4)^CF-4}{C:C–TPA–(TPA)2}]) gave a high EQE of 3.62 % and a saturated red emission with CIE coordinates of (0.64, 0.36), very close to the National Television System Committee (NTSC) standard for red emission [12]. In addition, a very low turn-on voltage of less than 3 V was obtained. These results have opened up a new avenue for the development of red light-emitting phosphorescent dopants for solution-processable OLEDs based on alkynylgold(III) dendrimers [64]. Carbazole-based dendrimers with high generation of up to 3 had been successfully synthesized and characterized by Yam and co-workers (Fig. 26) [65]. This class of complex was highly soluble in common organic solvents. In their photophysical studies, the absorption bands at 242–350 nm were mainly attributed to spin-allowed IL p ? p* transitions of the carbazole units. Similar to the reported alkynylgold(III) compounds, the weaker vibronic-structured absorption band was assigned to a metal-perturbed IL transition of the cyclometalated (2,5-F2–C6H3– C^N^C) ligand with charge transfer character from the phenyl ring to the pyridine unit, and the absorption tail at 430–500 nm was assigned as an admixture of IL and LLCT transitions [65]. Upon excitation, broad structureless emission bands were observed at ca. 620–695 nm for carbazole-based gold(III) complexes. The incorporation of higher generation dendrimers causes a blue shift in emission [i.e. [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)2}] (695 nm) < [Au(2,5-F2C6H3– C^N^C){C:C–TPA–(tBu2Cbz)6}] (646 nm) < [Au(2,5-F2C6H3–C^N^C){C:C– TPA–(tBu2Cbz)14}] (620 nm)] [234]. The lower-lying HOMO energy in the higher generation dendrimers was attributed to the electron-withdrawing nature of the carbazole dendron with negative inductive effect of the electronegative nitrogen atom. In the solid-state thin films, the emission energies were found to be red-shifted upon increasing the dopant concentration in PMMA. Interestingly, smaller shifts in emission energies for [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)6}] and [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)14}] were observed in their PL Reprinted from the journal

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spectra, possibly due to the increase in the dendrimer generations that effectively alter the degree of intermolecular interactions, leading to a fine-tuning of the emission color. Solution-processable OLEDs based on this class of gold(III) dendrimers were also prepared, in which a maximum CE of 24.0 cd A-1 and a PE of 14.5 lm W-1 were achieved [65]. Notably, a saturated yellow emission with excellent CIE coordinates of (0.50, 0.49) was realised for devices based on [Au(2,5F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)2}]. Very recently, the EQE was further boosted up to 10.1 % by changing the solution matrix from chloroform to toluene in the spin-coating process [66]. These results were comparable or even superior to those of solution-processable OLEDs based on other transition metal centers [11, 235]. More importantly, the second generation dendrimer did not result in a degradation of the device performance. The yellowish-green-emitting OLEDs based on [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)6}] showed a high EQE and CE of 7.0 % and 21.9 cd A-1, respectively. Indeed, a poorer performance was commonly observed for the higher-generation iridium(III) dendrimers due to a lower hole mobility [236]. On the other hand, the driving voltage of devices based on [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)14}] was even smaller than those of devices with [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)2}] and [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)6}], implying the peripheral carbazole moieties were actively involved in the charge transport process [65]. Another novel class of bipolar alkynylgold (III) complexes was recently reported by the same group by replacing hole-transporting units, such as carbazole or triphenylamine moieties, with electron-transporting units, such as benzimidazole or substituted-benzimidazole moieties, as the peripheral groups (Fig. 27) [66]. In the photophysical studies of [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(Me-DPBI)2}] and [Au(2,5-F2C6H3–C^N^C){C:C–C6H4N(C6H3-Me-3)2–(Me-DPBI)2}], additional absorption bands were observed at around 330–350 nm when compared to the

Fig. 27 Molecular structures of bipolar cyclometalated alkynylgold(III) complexes. Reproduced with permission from Ref. [66]. Copyright 2014 American Chemical Society

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structurally related complex [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)2}], assignable to p ? p* transitions from the electron-donating triphenylamine moiety to the electron-accepting benzimidazole moiety [237]. The blue shift of this charge transfer band in [Au(2,5-F2C6H3–C^N^C){C:C–C6H4N(C6H3-Me-3)2–(MeDPBI)2}] was attributed to the lower p-conjugation between donor–acceptor pairs by the introduction of the methyl group onto the diarylamine moiety. Remarkably, the introduction of a methyl group into the diarylamine unit could rigidify the molecular structure and improved the PLQY in the solid-state thin films. A record high PLQY of up to 75 % was recorded for [Au(2,5-F2C6H3–C^N^C){C:C– C6H4N(C6H3-Me-3)2–(Me-DPBI)2}]. More importantly, highly efficient solutionprocessable PHOLEDs based on bipolar alkynylgold(III) complexes as phosphorescent dopants were prepared. As compared to the structurally related complex (i.e. [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)2}]), the incorporation of benzimidazole moieties could significantly enhance the EL performance. Particularly, the optimized devices doped with [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(MeDPBI)2}] (i.e. 20 %) and [Au(2,5-F2C6H3–C^N^C){C:C–C6H4N(C6H3-Me-3)2– (Me-DPBI)2}] (i.e. 10 %) showed maximum CEs of 30.4 and 33.5 cd A-1, respectively, corresponding to maximum EQEs of 9.5 and 10.0 % [66]. The dramatic improvement on EQEs was believed to be due to the bipolar character of the alkynylgold(III) complexes. It should be highlighted that the EQEs of devices doped with [Au(2,5-F2C6H3–C^N^C){C:C–C6H4N(C6H3-Me-3)2–(Me-DPBI)2}] could remain as high as 8.8 and 7.0 % at a luminance of 1000 and 5000 cd m-2, respectively. These corresponded to small EQE roll-off of 1 and 21 %, respectively [66]. It was not the case for devices doped with [Au(2,5-F2C6H3–C^N^C){C:C– TPA–(tBu2Cbz)2}], where a poor performance with larger EQE roll-offs of 18 % at L = 1000 cd m-2 and 65 % at L = 5000 cd m-2 were noted [239]. As confirmed by the current–voltage characteristics of hole-only and electron-only devices, the performance improvement was ascribed to balanced hole and electron currents, and thus a broadened recombination zone and a reduced triplet–triplet annihilation. This explained why a smaller efficiency roll-off was observed for devices doped with [Au(2,5-F2C6H3–C^N^C){C:C–TPA–(Me-DPBI)2}] and [Au(2,5-F2C6H3– C^N^C){C:C–C6H4N(C6H3-Me-3)2–(Me-DPBI)2}], as compared to [Au(2,5F2C6H3–C^N^C){C:C–TPA–(tBu2Cbz)2}] [238]. These findings clearly demonstrate that bipolar alkynylgold(III) complexes are potential candidates as phosphorescent dopants for solution-processable PHOLEDs [66]. Fig. 28 Molecular structures of fluorine-containing cyclometalated alkynylgold(III) complexes. Reproduced with permission from Ref. [113]. Copyright 2013 Wiley–VCH

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Recently, Che and co-workers reported a new series of alkynylgold(III) complexes by incorporating fluorene moiety as one r-donating ligand in the cyclometalated C^N^C ligands (Fig. 28) [113, 239]. Such modifications not only induced a change of the lowest-energy excited triplet states from ILCT to ligand centered p–p* states in this system, but also enhanced luminescence quantum yields and prolonged the excited state lifetime of up to 58 % and 200 ls, respectively. In the photophysical studies, this class of complex showed intense absorption bands at 250–350 nm and lower energy absorption shoulders at 370–440 nm, assignable to ligand-centered transitions localized on the C^N^C ligands. Upon irradiation, all complexes exhibited vibronic-structured emission bands with peak maximum at ca. 540 nm in dichloromethane solution at room temperature. The vibrational progressional spacings of about 1300 cm-1 were characteristic of the C:C and C:N stretching frequencies of the C^N^C ligand, where these emissions were assigned as originating from metal-perturbed IL transition of the C^N^C ligand. These assignments had been further confirmed by TDDFT calculations. It is worth noting that the luminescence quantum yields of this class of alkynylgold(III) complexes were in the range of 0.40–0.58 [113]. In addition, satisfactory device performances were obtained. Some of the alkynylgold(III) complexes were selected to prepare PHOLEDs by vacuum deposition or solution-processable technique [239]. With a dopant concentration of 7 wt% of a cyano-containing alkynylgold(III) complex in MCP as emissive layer, a yellow-emitting OLED with high EQE of 20.3 %, CE of 56 cd A-1 and PE of 62.8 lm W-1 was realized. These values were comparable to the best OLEDs based on iridium(III) or platinum(II) emitters [240, 241]. Meanwhile, solution-processable OLEDs based on one of these alkynylgold(III) complexes demonstrated a maximum EQE of 8.4 % with stable color spanning a wide range of driving voltages. However, this class of gold(III) complexes inevitably suffered from severe efficiency roll-offs. For instance, the EQE at the brightness level of 1000 cd m-2 showed a drop of 91.7 %. This severe efficiency roll-off was attributed to the considerably long excited state lifetime ([170 ls) of this complex system. This long excited state lifetime would definitely lead to the accumulation of triplet excitons within the emissive layer, and hence resulted in severe triplet–triplet annihilation [242–244]. Nevertheless, these findings demonstrated the realization of the high performance OLEDs based on fluorene-containing alkynylgold(III) complexes. In order to compare their emission energies and device efficiencies more easily, the key photophysical and EL data for the representative platinum(II) and alkynylgold(III) complexes aforementioned are summarized in Table 1.

4 Conclusions In this chapter, we have summarized the development of d8 platinum(II) and gold(III) complexes and their application studies in the fabrication of PHOLEDs. Taking the advantage of the square planar geometry, the platinum metal center can accommodate a variety of monodentate, bidentate, tridentate, and tetradentate nitrogen-containing donor ligands to synthesize different platinum(II) complexes

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0.60

0.19

0.20

641

578

466, 580

527

595

532, 592

589

491

506

PtOEP

cis-[Pt(thpy)2]

[Pt(F2ppy)(O^O)]

{Pt[B(Mes)2ppy](O^O)}

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[Pt(N^N)2] (N^NH = 3trifluoromethylpyrazoles)

[{(CF3)2C6H3–C^N^N}PtCl]

[(Th-C^N^C)Pt(C:N–C6H4–Me2)]

[(dpyb-N^C^N)PtCl]

[(bzimb–N^C^N)PtCl]

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528

636

515

[Au(C^N^C)(C:C–C6H4-N(C6H5)2-p)]

[Au(2,5-F2C6H3–C^N^C)(C:C– C6H4N(C6H5)2-p)]

[Au{4-FC^N(5-FC9H4)^C-F-4}{C:C– TPA–(TPA)2}]

550

470

[Pt(O^N^N^O-salen)]

[Pt(pyridine-pyrazole–N^C^C^N)]

0.16

0.34

0.22

0.84

0.60

0.26

0.02

0.24

0.98

0.02

0.18

Abem

Complex

kapeak/ nm

ITO/PEDOT:PSS/complex(20 wt%):MCP/3TPYMB/ TmPyPB/LiF/Al

ITO/NPB/complex(6 wt%):CBP/BAlq/LiF/Al

ITO/TPD/complex(6 wt%):CBP/BCP/Alq3/LiF/Al

ITO/HATCN/NPD/TAPC/complex(12 wt%):26mCPy/ DPPS/BmPyPb/LiF/Al

ITO/NPB/complex(4 wt%):Bepp2/LiF/Al

ITO/NPB/ TCTA/complex(6 wt%):CBP/complex(6 wt%):TAZ/ BAlq/Alq3/LiF/Al

ITO/TPD:PC/CBP/complex(6 wt%):CBP: OXA/OXA/ Ca

ITO/NPB/TCTA/complex(6 wt%):mCP/BAlq/Alq3/ LiF/Al

ITO/NPB/complex(5 wt%):CBP/NPB/DNA/BCP/Alq3/ LiF/Al

ITO/NPB/complex(20 wt%):CBP/BCP/Alq3/LiF/Al

ITO/MoO3/CBP/complex(8 wt%):CBP/ TPBI:complex(8 wt%)/TPBI/LiF/Al

ITO/NPD/complex(10 wt%):CBP/BCP/Alq3/LiF/Al

ITO/PVK:PDB:complex(4 wt%)/Mg:Ag/Ag

ITO/complex(6 wt%):NPD:CuPC/Alq3/Mg

Device structure

3.3

37.4

17.6

64.7

31.0

38.9

40.2

13.4

26.4

19.7

64.8

11.3

6.0



Max. CE/ cd A-1

Table 1 Selective photophysical and electroluminescence data for the representative platinum(II) and alkynylgold(III) complexes

3.4

26.2

14.5

53.5

14.0

27.2



10.5

18.4

6.4

79.3

8.1



0.2

Max. PE/ lm W-1

3.6

11.5

5.5

23.1

11.0

11.5

16.5

12.6

11.8

5.9

20.9

3.3

2.2

1.3

Max. EQE/%

[64]

[58, 60]

[57, 60]

[219]

[212]

[211]

[200]

[197]

[50]

[122]

[173]

[130]

[166]

[6]

Refs.

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123

123

532

540, 579

[Au(2,5-F2C6H3–C^N^C){C:C– C6H4N(C6H3Me-3)2-(Me-DPBI)2}]

[fPh_Au_C2fb]

b

a

0.57

560

[Au(2,5-F2C6H3–C^N^C){C:C–TPA– (tBu2Cbz)2}]

ITO/PEDOT:PSS/PVK/complex(10 wt%):OXD-7/ TmPyPb/TPBi/LiF/Al

ITO/PEDOT:PSS/complex(10 wt%):MCP/3TPYMB/ TmPyPB/LiF/Al

ITO/PEDOT:PSS/complex(20 wt%):MCP/3TPYMB/ TmPyPB/LiF/Al

Device structure

Emission quantum efficiency in solution or solid-state thin film

Emission peak measured in solution or solid-state thin film

0.61

0.78

Abem

kapeak/ nm

Complex

Table 1 continued

22.6

33.6

22.4

Max. CE/ cd A-1

30.0

8.7

17.7

Max. PE/ lm W-1

9.2

10.0

7.8

Max. EQE/%

[113, 237]

[66]

[65, 66]

Refs.

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with interesting luminescence properties. With an in-depth understanding of the nature of the excited states, the emission color can be readily fine-tuned to span the entire visible spectrum, as well as to improve the EL performance. Highly efficient vacuum-deposited OLEDs with EQEs of up to 30 % were realized. Meanwhile, the incorporation of strong donor ligands is found to be an effective means to raise the low-energy d–d ligand field, yielding another interesting class of luminescent gold(III) complexes. Recent advances on the employment of dendritic structure into gold(III) system have further opened up a new avenue for the fabrication of highperformance solution-processable OLEDs. Rational design on the cyclometalated ligands and/or dendrons not only can tune the emission colors of the dendritic gold(III) complexes from blue to red, but also improves the charge-transporting properties, as exemplified by the successful demonstration of high-performance solution-processable OLEDs with EQEs of up to 10 %. Looking ahead, rational design and development of platinum(II) and gold(III) complexes with high color purity is highly desirable. Of particular importance for blue-emitting OLEDs is that excimeric emission must be eliminated to generate pure blue emission. As aforementioned, the EL spectra are found to show significant spectral shifts upon dopant aggregation to give a broad red-shifted excimeric emission of these square-planar metal complexes. Strategies to avoid excimeric emission need to be developed. Novel device structure is also highly desirable to further boost up the performance of OLEDs based on platinum(II) and gold(III) complexes. The development of new hosts and charge-transporting materials, especially those that are solution-processable, are necessary to match the energies of these metal complexes. With the continuous advances on the molecular design and OLED device architecture, it is anticipated that the d8 metal complexes will have a bright future for various OLED applications from head-mounted micro displays, state-of-the-art mobile phone displays to the large-area light sources for general illumination. Acknowledgments V.W.W.Y. acknowledges support from the University of Hong Kong under the URC Strategic Research Theme on New Materials. This work was supported by the grants from the University Grants Committee Areas of Excellence Scheme (Project no. AoE/P-03/08), the Research Grants Council Theme-Based Research Scheme (Project No. T23-713/11), and ANR/RGC Joint Research Scheme (Project No. A-HKU704/12).

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Top Curr Chem (Z) (2017)375:39 DOI 10.1007/s41061-017-0126-7 REVIEW

Phosphorescent Neutral Iridium (III) Complexes for Organic Light-Emitting Diodes Abd. Rashid Bin Mohd Yusoff1,2 • Aron J. Huckaba1 • Mohammad Khaja Nazeeruddin1

Received: 21 January 2016 / Accepted: 22 February 2017 Ó Springer International Publishing Switzerland 2017

Abstract The development of transition metal complexes for application in lightemitting devices is currently attracting significant research interest. Among phosphorescent emitters, those involving iridium (III) complexes have proven to be exceedingly useful due to their relatively short triplet lifetime and high phosphorescence quantum yields. The emission wavelength of iridium (III) complexes significantly depends on the ligands, and changing the electronic nature and the position of the ligand substituents can control the properties of the ligands. In this chapter, we discuss recent developments of phosphorescent transition metal complexes for organic light-emitting diode applications focusing solely on the development of iridium metal complexes. Keywords Organic light-emitting diodes  Fluorescence  Phosphorescence  Iridium  Triplet emitters  Transition metal complexes

1 Introduction Organic light-emitting diodes (OLEDs) have attracted significant attention due to their potential applications in solid-state lighting and flat panel displays owing to their various advantages such as low operating voltages, high brightness, and This article is part of the topical collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’: edited by Nicola Amaroli and Henk Bolink. & Mohammad Khaja Nazeeruddin [email protected] 1

Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, E´cole Polytechnique Fe´de´rale de Lausanne, 1951 Sion, Switzerland

2

Advanced Display Research Center, Department of Information Display, Kyung Hee University, Dongdaemoon-gu, Seoul 130-701, Korea

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contrast [1–5]. Phosphorescent iridium (III) complexes, in particular, have been commonly employed as dopants in OLEDs owing to their high quantum efficiency and a wide range of tunable emission colors [6–21]. The advantage of Ir atoms is enhanced spin–orbit coupling (SOC), which promotes efficient intersystem crossing from the excited singlet state to the triplet resulting radiative decay. The radiative process of triplet excited states to a singlet ground state is termed phosphorescence. In cyclometallated Ir complexes, the excited triplet state is responsible for phosphorescence, which is a combination of the LC and the MLCT excited triplet state, which is a mixed (triplet) excited state (Fig. 1). Although phosphorescent iridium (III) provides high-efficiency OLEDs, efficiency roll-off ratios are too large and attributed to the deterioration of chargecarrier balance and increases in nonradiative quenching processes such as triplet– triplet annihilation (TTA), triplet–polaron annihilation (TPA), as well as electricfield-induced dissociation of excitons at high current density [22, 23]. Another factor that probably affects the efficiency roll-off is the unbalanced hole and

Fig. 1 Luminescence process in OLEDs

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electron mobility of most charge transport materials, in which the hole mobility is 2–3 orders of magnitude higher compared to that of electron mobility. Therefore, to obtain high-efficiency phosphorescent OLEDs featuring low-efficiency roll-off, one must use ambipolar host materials or else one must synthesize Iridium (III) dopants with excellent electron mobility [24–30].

2 Photophysics of Cyclometalated Iridium (III) Complexes Cyclometalated iridium (III) complexes have attracted considerable interest because of their phosphorescence high quantum yields at room temperature, and tunable emission color through ligand modification [29, 30]. The relatively long phosphorescent lifetime and high quantum yields are due to the close proximity of the ligand-centered (3LC) and metal-to-ligand charge transfer (3MLCT) excited states, and the large spin–orbit coupling (SOC) constant of iridium (III), which induce a strong mixing of the charge-transfer character [31, 32]. Owing to these various advantages, we will next review the photophysical process associated with the cyclometalated iridium (III) complexes. 2.1 Photoexcitation to the Excited States Photoexcitation initiates various photophysical processes; for instance, electronic interactions through the Ir core favor the delocalization of frontier molecular orbitals of cyclometalated iridium (III) complexes over the entire molecule. Figure 2 demonstrates the chemical structure and isodensity plot of the frontier molecular orbitals of fac-tris(2-phenylpyridinato)iridium (III) (fac-Ir(ppy)3) [33], where the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) were acquired using the time-dependent density functional theory (TD-DFT) [34]. In principle, TD-DFT is a frequently used method for predicting excited state properties of molecules, in which the resulting time-dependent absorption, fluorescence, IR, and resonance Raman spectra can be assigned by TDDFT excited state calculations. For Ir(ppy)3, the triplet state is determined from electronic transitions from the HOMO to the LUMO, where at

Fig. 2 Chemical structure of fac-Ir(ppy)3 and isodensity plots of the frontier molecular orbitals (Reprinted with permission from [33]. Ó American Chemical Society)

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least two electronic transitions metal-to-ligand (MLCT) Ir ? ligand, and ligand centered (LC) transitions take place. Moreover, strong spin–orbit-coupling from the Ir metal enabled intersystem crossing from singlet to triplet MLCT and LC transitions, resulting in four different electronic states: the singlet and triplet MLCT (1MLCT and 3MLCT) or LC (1LC and 3LC) transition states. Generally, 1LC transitions are higher in energy compared to that of 1MLCT transitions. This is because the exchange energy of a LC transition is commonly superior compared to a MLCT transition. One can simplify the energetic ordering of these four transitions as: 1LC [ 1MLCT [ 3MLCT [ 3LC. Due to significant overlap with the phosphorescence spectrum, the 3LC transitions are not normally observed from the absorption spectrum. The classification of transitions as MLCT or LC is usually determined by luminescence lifetimes and band shape, where the active ligand is identified by electrochemical measurements, and both can be confirmed through computational methods. Although in some cases solid-state spectroscopic methods have to be applied, especially when the excited states of the MLCT and LC are located close to each other, room-temperature solution spectra are usually sufficient for such a classification. The low-lying triplet state is also sometimes referred to as a mixture of MLCT and LC transition states. Several reports of calculations have shown that the excited state is localized onto a single ligand. For instance, from TD-DFT calculation, the assumed C3 symmetry indicates that the A substate is lower in energy than the E substates, i.e., Ds [ 0. When TDDFT is used to optimize the geometry with the complex constrained to the T1 electronic state, it is found that the C3 symmetry is broken, and the excitation is localized to a single ligand [35–43], which indicates a hybrid triplet state (Fig. 3). Moreover, Matsushita and coworkers demonstrated through multi-configurational self-consistent field orbitals (MC-SCF) and second-order configurational interactions (SOCIs) that fac-Ir(ppy)3 is lower in energy than mer-Ir(ppy)3 by 7.7 (MP2) and 6.7 (MCSCF) kcal/mol, which is consistent with the fact that only fac-Ir(ppy)3 is used as a phosphorescent molecule [38]. According to the author (38), phosphorescence in fac-Ir(ppy)3 is provided by the electronic transition from the fifth excited spin-mixed state (SM5) to the ground state (SM0) (see ref [42]), where SM1–SM6 originate from T1 (3E) and are close in energy to each other. SM5 is calculated to have a T1 adiabatic component of 25% (square of its CI coefficient). The emission energy and the transition dipole moment (TDM) from SM5 to SM0

Fig. 3 Changes in the excitation spectra and transition densities calculated as fac-Ir(ppy)3 moves from the S0 to the T1 geometries; calculated from scalar relativistic TDDFT. Transition density (product of initial and final states of an excitation) of the first triplet excited state, T1, of fac-Ir(ppy)3 at four different geometries (Reprinted with permission from [43]. Ó American Chemical Society)

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were calculated to be 479 nm (20,866 cm-1) and 0.856 [ebohr] (SBKJC basis set) and 449 nm (22,290 cm-1) and 0.906 [ebohr] (iMCP basis set) [38] even though these emission wavelengths are somewhat underestimated in comparison with the corresponding experimental reports [44–46]. While phosphorescence results from a mixture of the two triplet states [6], Lamansky and coworkers reported that the emissive excited state, on the other hand, is mostly 3(p–p) or 3MLCT, depending on the energies of those states. The same situation is present in the C^N2Ir(LX) complexes, where ppy2Ir(acac), tpy2Ir(acac), and bzq2Ir(acac) have high-energy 3(p–p)* states and provide unstructured and mainly 3MLCT emission. The other complexes reported in their study provide phosphorescence spectra with a reasonable degree of vibronic fine structure and significant Stokes shifts, consistent with predominantly 3(p–p)* C^N-based emission. By changing the C^N ligands in cyclometalated iridium (III) complexes, they reported green to red electrophosphorescence with high gext. Furthermore, Seo and coworkers reported an unusually large positive solvatochromic shift in excited state intramolecular proton transfer (ESIPT) keto fluorescence originating from the creation of an intramolecular charge-transfer (ICT) state after the ESIPT process [47]. The introduction of a conjugative electron acceptor in 2-(2‘-hydroxyphenyl)benzoxazoles (HBO) caused a strong positive solvatochromism in ESIPT keto emission. This unique spectral change was attributed to the consecutive ESIPT/ ICT process in the acceptor-substituted HBO compounds (Fig. 4). In contrast, Chou and coworkers demonstrated the remarkable dual excitation behavior of proton-transfer versus charge-transfer fine-tuned by dielectric as well as hydrogen-bonding perturbation [48]. Tsuboyama and coworkers also reported MLCT phosphorescence along with a relatively short phosphorescence lifetime [49]. They developed a series of facial homoleptic cyclometalated iridium(III) complexes with general structure Ir(III)(C–N)3, where (C–N) is a monoanionic cyclometalating ligand: 2-(5-methylthiophen-2-yl)pyridinato, 2-(thiophen-2-yl)-5trifluoromethylpyridinato, 2,5-di(thiophen-2-yl)pyridinato, 2,5-di(5-

Fig. 4 a Plot of absorption (circles) and emission maxima in enol (triangles) and keto (squares) forms of HBOCE as a function of solvent polarity parameter. n and e are the refractive index and dielectric constant of solvent, respectively. b Proposed consecutive photophysical process (Reprinted with permission from [47]. Ó American Chemical Society)

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methylthiophen-2-yl)pyridinato, 2-(benzo[b]thiophen-2-yl)pyridinato, 2-(9,9dimethyl-9H-fluoren-2-yl)pyridinato, 1-phenylisoquinolinato, 1-(thiophen-2-yl)isoquinolinato, or 1-(9,9-dimethyl-9H-fluoren-2-yl)isoquinolinato (Fig. 5). Luminescence properties of all their complexes at 298 K in toluene are as follows: quantum yields of phosphorescence Up = 0.08–0.29, emission peaks kmax = 558–652 nm, and emission lifetimes s = 0.74–4.7 ls. Bathochromic shifts of the Ir(thpy)3 family [the complexes with 2-(thiophen-2-yl)pyridine derivatives] are observed by introducing appropriate substituents, e.g., methyl, trifluoromethyl, or thiophen-2-yl. However, Up of the red emissive complexes (kmax [ 600 nm) becomes small, caused by a significant decrease of the radiative rate constant, kr. In contrast, the complexes with the 1-arylisoquinoline ligands are found to have marked red shifts of kmax and very high Up (0.19–0.26). These complexes are found to possess dominantly 3MLCT (metal-to-ligand charge transfer) excited states and have kr values approximately one order of magnitude larger than those of the Ir(thpy)3 family. An organic light-emitting diode (OLED) device that uses Ir(1phenylisoquinolinato)3 as a phosphorescent dopant produces very high efficiency (external quantum efficiency gex = 10.3% and power efficiency 8.0 lm/W at 100 cd/m2) and pure-red emission with 1931 CIE (Commission Internationale de L’Eclairage) chromaticity coordinates (x = 0.68, y = 0.32). In fact, various strategies have been proposed to tune the excited-state energies of iridium (III)complexes [49–60]. Basically, LC transitions arouse and tuned through careful controlling of ligand structure with bandgap theory gives an idea of LC energy tuning; for instance ligands with long conjugation, lengths produce a

Fig. 5 Ir complexes used in Tsuboyama’s study (Reprinted with permission from [49]. Ó American Chemical Society)

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bathochromic shift of the absorption spectrum. The introduction of the larger ligand introduces a large bathochromic shift to an absorption peak. The manipulation of the MLCT transition energy without altering LC transition was on the other hand reported by Nazeeruddin and coworkers [50]. In their study, systematic tuning of the t2g orbital energy was carried out by using cyanide, thiocyanate, and isocyanate ancillary ligands whose field strength are cyanide [ thiocyanate [ isocyanate (Fig. 6). The resultant complexes were found to emit blue, green, and yellow phosphorescence, respectively. Depending on the metal, its oxidation state as well as the nature of the ligandfield (LF), another interesting electronic transitions namely intra ligand charge transfer (ILCT), can also be observed. Vogler and coworkers were among the first to report ILCT, which is responsible for the solvatochromic behavior within the tetradentate ligand, mediated by nickel ions [61]. Intraligand charge transfer spectrum of biacetyl-bis-(mercaptoethylimine)-nickel (II). Inspired by this work, various groups reported the ILCT transition, which is commonly found in ligands with donor–acceptor electronic structures [62–69]. The ILCT results reported above are useful for the design of visible-light-harvesting transition-metal complexes with long-lived triplet excited states along with possible applications as triplet sensitizers for various photophysical processes, such as TTA upconversion, photovoltaics, photocatalysis, etc. 2.2 Excited-State Photophysics Figure 7 illustrates a typical photophysics process of iridium (III)complexes. In brief, when the photon is absorbed, it promotes the LC and MLCT transitions states, where these two transition states are associated since these two share LUMO energy

Fig. 6 Emission spectra of TBA[Ir(ppy)2(CN)2] (1), TBA[Ir(ppy)2(NCS)2] (2), and TBA[Ir(ppy)2(NCO)2] (3) in degassed dichloromethane solution at 298 K (Reprinted with permission from [50]. Ó American Chemical Society)

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ˆ N) Ir(III) complexes, and b the construction Fig. 7 a Excited-state photophysics of cyclometalated tris (C of excited states via molecular orbital interactions: MC metal-centered ligand-field state, LC ligandcentered state, MLCT metal-to-ligand charge-transfer state, LL’CT ligand-to-ligand charge-transfer state, ILCT intraligand charge-transfer state, IC internal conversion, ISC intersystem crossing, IVR intramolecular vibrational redistribution. L and L’ denote different ligands of a heteroleptic Ir(III) complex

level placed on the cyclometalating ligand. The internal conversion process takes place at a timescale of [100 [70] fs from the high-lying 1LC to the low-lying 1 MLCT state. Due to the fact that the 1MLCT state is strongly influenced by the Ir metal that induces a spin–orbit-coupling effect, the resulting intersystem crossing takes place from 1MLCT to the 3MLCT [70] at an ultrafast timeframe of \100 fs [71, 72], which indicates that ISC transition process is not a rate-determining step in the photophysical process. The strength of spin–orbit coupling is directly proportional to the fourth power of the atomic number of the metal; thus the heavier the metal, the stronger the spin–orbit coupling and the higher the emission efficiency [73]. It is worth noting that one-electron spin–orbit coupling constants (fc) are as follow: Pt(III) [ Iridium (III) [ Os(III) [ Rh (III) [ Ru (III) [74, 75]. The 3MLCT substates are assigned to be nondegenerate due to the symmetryreducing influence of the ligand structures and the local environment. In the case of fac-Ir(ppy)3, three 3MLCT substates lie at 19 693 cm-1 (507.79 nm, I ? 0), 19 712 cm-1 (507.31 nm, II ? 0), and 19 863 cm-1 (503.45 nm, III ? 0) were produced with a zero-field splitting (ZFS) energy of 170 cm-1 [33] (Fig. 8a). Yersin and coworkers reported that the radiative time constant of the highestlying III substate was ca. 750 ns, whereas that of the lowest-lying I substate was 145 ls [76, 77] (Fig. 8b). With average lifetimes of cyclometalated iridium (III)complexes in the range of 100–101 ls, hence, the III electronic substate was thought to dominate in the radiative transition. It is predicted that high-lying III substate hybridized significantly with singlet excited states. Moreover, Powell and coworkers reported that, for the lowest excited states, there are no significant differences between the exact and approximate wavefunctions and energies [42, 78]. This means that the key radiative properties of the complex are accurately reproduced in the configuration interaction singles (CIS) approximation. These

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Fig. 8 a Energy-level schemes and emission decay times for Ir(ppy)3 in CH2Cl2 (c & 2 9 10-5 mol/l) for magnetic fields of B = 0T. b Energy levels for the three lowest triplet substates I, II, and III and decay times of Ir(ppy)3. The states I, II, III are assigned to be substates of one 3MLCT-state. HT and FC represent Herzberg–Teller and Franck–Condon active vibrational modes, respectively (Reprinted with permission from [33]. Ó American Chemical Society and Elsevier)

results also show that there are broad ranges of parameter values for which these approximations can reproduce the energies of the higher excited states. This holds because the hybridization between the metal and the ligand (tH and tL) is smaller than the other energy scales of the problem. The errors introduced by making the CIS approximation mostly effect the high energy states, which do not play a significant role in, e.g., the phosphorescence. Thus, the authors conclude that electronic correlations do not play an important role in determining the optoelectronic properties of these complexes. Comparing their results with an experiment shows that chemical substitutions that increase the energy of the lowest singlet state rapidly suppress the radiative rate as kTR * (ES1 - ET4)4. This provides a clear design principle for designing highly efficient phosphorescent complexes. The initially excited system, after crossing from the 1MLCT to the 3MLCT states, cascades down into the lowest-lying 3MLCT state, initially populating substate III. The femtosecond dynamics arise from equilibration of the substates. The electronic relaxation to the lower two sub-states occurs by internal conversion (IC). The energy released in this process, and in ISC, is dissipated by intramolecular vibrational energy redistribution (IVR) that involves energy transfer from hot vibrational modes to the lower frequency modes. IVR on a femtosecond timescale has also been reported for ruthenium complexes [46, 79, 80] (Fig. 9). Although the photophysical process of iridium(III) is rather complex, it permits facile phosphorescence color tuning. This is because phosphorescent transition energy is determined in part by the HOMO to LUMO energy gap, and the ligand with most electron density in the 3MLCT state among the three ligands dominates the phosphorescence transition. It has been demonstrated that the emission wavelengths can be controlled by adjusting the HOMO and LUMO gap using donor–acceptor substituents on the ligands [57, 59, 60, 81].

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Fig. 9 Population equilibration among the three triplet sublevels of the lowest excited state in Ir(ppy)3. The PL decays, s = 230 fs and s = 3 ps, are assigned to the redistribution of the initially (\100 fs) populated highest III sublevel due to the equilibration of the population via IVR and vibrational cooling (values shown are calculated from Boltzmann statistics). The observed steady state emission occurs on the microsecond timescale from substate III, fed by the populations from the two lower substates due to thermal effects

2.3 Photophysics in a Condensed State A photophysical process, which is defined as the physical outcome resulting from the electronic excitation of a molecule or systems of molecules by electromagnetic radiation (photon), is often influenced by its vicinity media. Accordingly, rigidochromism and solvatochromism commonly induce phosphorescence emission spectra shifts. The spectra shifting can be understood from the basis of dipolar stabilization of the triplet-excited state. In fact, phosphorescence emission shifts could potentially be induced by intermolecular interactions among the excited iridium (III) complexes. As a matter of fact, iridium (III) complexes are prone to intermolecular interactions since they have a long excited-state lifetime. Complicated intermolecular interactions, which make color balancing using multiple dopants problematic, are basically associated with excimers and exciplexes, which are usually found in condensed states. D’Andrade and coworkers were among the first to report employing triplet excimers in developing white OLED [82]. Inspired by the fact that energy transfer between dopants is remarkably reduced since excimers lack a bound ground state (i.e., no excimer absorption is observed), the authors were successful in developing efficient and bright electrophosphorescent WOLED with a high color rendering index (CRI) of 78, a maximum external quantum efficiency gext of 4% corresponding to 9.2 cd/A and a maximum luminance of 31,000 cd/m2 at 16.6 V, which covers a broad spectrum from a single emitter using blue monomer emission and red excimer emission. The excitation spectra are identical for the monomer and excimer emissions, indicating the formation of an intermediate excited monomer before excimer formation in photoluminescent (PL) emission [83]. Based on theoretical work on phosphorescent excimers, the design of new device architectures, and the development of novel emissive materials have been achieved, and a white device exceeding 20% has now been demonstrated [84, 85].

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Fig. 10 A possible mechanism of phosphorescence emission in state I (a) and state II (b) (Reprinted with permission from [86]. Ó The Royal Society of Chemistry)

Another interesting phenomenon attributed to intermolecular packing resulting in a switch from the non-emissive 3LX excited state to the emissive 3MLLCT transition is called aggregate-induced phosphorescent emission (AIPE), and the photophysical processes of AIPE differ slightly from the photophysical processes found in excimers and exciplexes [86]. By analyzing the influence of molecular packing on photophysical properties with the support from theoretical DFT calculations, they found that AIPE could be explained by a new mechanism of 3 MLLCT-mediated phosphorescent emission, that is, the formation of excimers by p–p stacking of adjacent pyridyl rings of ppy ligands can significantly change the excited state properties of iridium (III)complexes (Fig. 10). Despite limited efforts made to investigate this phenomenon, Shan and coworkers observed AIPE in cationic iridium (III) complexes [87, 88]. In this study, the cationic iridium complexes contained dendritic ligand (L2) with AIPE characteristics, namely, [Ir(ppy)2(L2)]PF6 (ppy = 2-phenylpyridine, L2 = 4,7-bis(30 ,60 -di-tert-butyl-6-(3,6di-tert-butyl-9H-carbazol-9-yl)-3,90 -bi(9H-carbazol)-9-yl)-1,10-phenanthroline). Although the cationic iridium complex shows AIPE, the PLQY of the neat film is relatively low (10%). Very recently, the same group demonstrated similar work, which was completed with the aim of increasing the PLQY [88]. They introduced another dendritic ligand, 3,8-bis(30 ,60 -di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9yl)-3,90 -bi(9H-carbazol)-9-yl)-1,10-phenanthroline (L4). Compared to their previous work, the carbazole dendrimers was introduced, which lead to a decrease in photochemical degradation and increased hydrolytic stability. As a result, the author successfully obtained high PLQY of 16.2% in a neat film, which is the highest among the cation iridium (III)complexes with AIPE activity.

3 Iridium (III) Phosphorescent Emitters To date, the iridium complexes are the most studied phosphorescent materials. This is due to the several distinctive features that iridium possesses, including stability of the octahedral geometry in both oxidized and reduced states, shorter triplet lifetime

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(*ls) to avoid triplet–triplet annihilation, thus ensuring high efficiency in the fabricated OLEDs, flexible color tunability, and high quantum yields. 3.1 Structures and Syntheses The iridium (III) ion possesses a d6 electron configuration, and the iridium (III) complexes feature an octahedral conformation with three bidentate or two tridentate ligands. According to Albrecht [89], cyclometalation is defined as the transition metal-mediated activation of a C-R bond to create a metallocycle forming of a metal–carbon (M-C) r bond (see Fig. 11). The process consists of two successive steps; (1) initial coordination of the metal center via donor group and (2) intermolecular activation of the C–R bond that closes the cyclometalation. Hence, those ligands that form M–C r bond are referred to cyclometalating ligands. In addition, cyclometalated Ir complexes can be categorized as homoleptic when three bidentate ligands of the same structure are integrated to the iridium (III) ion that has a d6 electron configuration or heteroleptic when different ligands are involved in the iridium (III) complexes. Consequently, the bis-cyclometalated Ir complexes holding the structure of (C^N)2Ir(L^X) are defined as heteroleptic while the tris-cyclometalated Ir complexes having featuring structure of Ir(C^N)3) is homoleptic. Figure 12 demonstrates the frequently used scheme to produce both homoleptic and heteroleptic complexes. There are two-step processes to synthesis the iridium (III)complexes where the first step is called the Nonoyama process (route a, Fig. 12), which produces a chloride-bridged dinuclear iridium (III)dimer [Ir(C^N)2l-Cl]2. The [Ir(C^N)2-l-Cl]2 dimer can be fragmented by the ancillary ligands to produce (C^N)2Ir(L^X) or charged bis-cyclometalated complexes with transN,N configuration of the C^N ligands. Either the mer-Ir(C^N)3 or fac-Ir(C^N)3, which are known to be kinetically or thermodynamically favored isomers, respectively, can be produced by replacing the chlorides in the [Ir(C^N)2-l-Cl]2 with a third cyclometalating ligand. Tamayo and McDonald found a stepwise procedure involving the synthesis of a pure mer-isomer followed by photochemical isomerization of this mer- to the fac- isomer was necessary to synthesize pure fac[Ir(C,N)2(C0 ,N0 )] complexes [90, 91]. It was also found that the kinetic examination of the mer-to-fac isomerization of homoleptic trisphenylpyridine iridium(III) complexes supports the proposed mechanism and the kinetic analysis supported the theory that an iridium(III) alkoxide intermediate is formed. Or else, the facIr(C^N)3 complexes can also be synthesized using Ir(acac)3 precursor or taking route e in the above scheme [92].

Fig. 11 The transition metal-mediated activation of a C–R bond to create a metallacycle forming of a new metal–carbon (M–C) r bond

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Fig. 12 Schematic representation of the synthetic strategies utilized for the synthesis of cyclometalated iridium(III) complexes. H–C^N: cyclometalating ligand

3.2 Color Tuning of Iridium (III) Complexes The heavy metal complexes involving 5 d metals Pt and Ir are able to induce intersystem crossing because of strong spin–orbit coupling, resulting in a mixingsinglet and triplet-excited states (Fig. 13). The spin-forbidden nature of radiative relaxation from the triplet-excited state then becomes allowed, resulting in high phosphorescent quantum yields useful for OLED applications. Color tuning of iridium (III) complexes can be realized considering several aspects such as (1) altering cyclometalating ligand framework, since it is well accepted that the emission color of an Ir(III) complex correlates highly with the conjugation length of

Fig. 13 Energetic closeness and overlap between 3MLCT and 3LC states result in the mixed lowest excited state (T1) for transition-metal complexes, especially for iridium (III)complexes (Reproduced with permission from Ref. [93]. Copyright 2013 Elsevier)

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its C^N ligand and the electron donating and/or accepting nature of each ligands, (2) substituent effect in the ligands, (3) position of the substituent on the ligand, and (4) ligand field strength. Subsequently, the optical and electronic properties as well as the EL performance of the resultant Ir complexes for OLED applications can be systematically tuned accordingly. The chemical structure of the cyclometalating ligands that are involved in the lowest-energy triplet excited state play an important role in determining the emission energy and quantum efficiency of both homoleptic and heteroleptic Ir complexes. The involved ligands can either be the ligand-centered state 3LC or the 3 MLCT triplet state or a mixture of both. Lamansky and coworkers demonstrated by changing the C^N ligands in cyclometalated Ir complexes (Fig. 14a), green to red electrophosphorescence alongside high gext was obtained. Moreover, high phosphorescence efficiencies and lifetimes less than 10 ls demonstrated in record highperformance OLEDs operating from the green to the red [94]. Figure 14a also shows all four complexes with vibronic fine structure in their emission spectra, as expected for ligand-based transitions. The lowest energy emission is observed for the benzoxazole (bo) complex. Substitution of S for O in a ligand (bo ? bt) leads to a 30-nm red shift, due to the higher polarizability and donor strength of sulfur relative to oxygen, in this ligand-based excited state. Increasing the size of the ligand p system is expected to bathochromic shift electronic transitions, as is observed in converting a phenyl group to a naphthyl group (bo ? bon), which leads to a 60-nm red shift. The effects of the naphthyl and sulfur substitutions are nearly additive, leading to an 80-nm red shift when comparing bo to absn complexes.

Fig. 14 a Solution photoluminescence spectra of bo2Ir(acac), bt2Ir(acac), bon2Ir(acac), and absn2Ir(acac). The structures of the individual C^N ligands are shown above the corresponding spectrum. b Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of OLEDs and phosphorescence spectra of C^N2Ir(LX) complexes. The CIE coordinates for OLEDs with ppy2Ir(acac):CBP, bt2Ir(acac):CBP, and btp2Ir(acac)Ir:CBP are shown relative to the fluorescencebased devices, coumarine6:Alq3 and DCJT:Alq3 on the left. The CIE coordinates of the phosphorescence spectra of many of the C^N2Ir(LX) complexes prepared here are shown to the right. The NTSC standard coordinates for the red, green, and blue subpixels of a CRT are at the corners of the black triangle (Reproduced with permission from Ref. [6]. Copyright 2001 American Chemistry Society)

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Meanwhile, Fig. 14b shows the Commission Internationale de L’Eclairage (CIE) coordinates for the three OLEDs. The btp2Ir(acac)-doped OLED gives a saturated red emission that has CIE coordinates close to the National Television Standards Committee (NTSC) recommended red for a cathode ray tube (CRT). Green emission from ppy2Ir(acac) is very similar to Ir(ppy)3-based OLEDs and a common fluorescence-based green OLED (Coumarin6: Alq3). The three complexes used to fabricate OLEDs were chosen to be representative of the family of C^N2Ir(LX) phosphors. Figure 14b shows the CIE coordinates of the solution phosphorescent spectra of all of the C^N2Ir(acac) complexes. All of the C^N2Ir(acac) complexes are expected to give OLEDs with efficiencies similar to those reported for the ppy, bt, and btp complexes. Adachi and coworkers reported an introduction of the electronwithdrawing fluorine substituent, and the resulting complex leads to an increase of the triplet exciton energy, hence, a blue shift of the phosphorescence compared with that of Ir(ppy)3 [94]. Another work by Gu and coworkers demonstrated a simple method for tuning the color emission of metal complexes by changing the structures of the ancillary ligands, where the introduction of electron-donating and electronwithdrawing groups remarkably influence the photophysical properties of Ir(dfppy)2(LX) complexes [95]. In the Ir complexes, their HOMOs (highest occupied molecular orbital) were demonstrated to be located mainly on the Ir center and the C-related phenyl segments of C^N ligands (denoted as C-ring here), but their LUMOs (lowest unoccupied molecular orbital) are primarily distributed on the pyridine moieties (denoted as N-ring here) [96]. Therefore, it is generally considered that the substitution of an electron-donating group (EDG) on the C-ring will induce a raised HOMO level (destabilized HOMO), and hence a decreased HOMO–LUMO gap resulting red-shifted emission of the complex; while the grafting of an electronwithdrawing group (EWG) on the C-ring will endow the complex with a lowered HOMO level (stabilized HOMO), and hence a blue-shifted emission [97]. Zhou and coworkers have independently introduced electron-withdrawing main-group moieties to tune the color emission of Ir complexes with enhanced electroninjection/electron-transporting properties, which is essential for high-performance color tunable OLED [98]. Despite the fact that the grafting of an EWG-like trifluoromethyl on the C-ring at either the meta- or para-site relative to the Ir atom was found to induce a blue-shifted emission [50], in some cases, it was observed, e.g., when pentafluorophenyl, carborane, sulfonyl, dimesitylboron, or formyl was introduced to the meta-position (with respect to the Ir ion) of the C-ring, it would endow the complex with a red- rather than a blue-shifted emission, regardless of its EWG nature [36, 38, 62–69]. Similarly, although it has been demonstrated that the substitution of an EDG of diphenylamino on the C-ring at either the meta- or parasite relative to the Ir center would result in a red-shifted emission [51], in a few reports, it has been revealed that the introduction of an electron-donating –OCH3 or –CH3 group into the meta-site on the C-ring will endow the complex with blueshifted rather than red-shifted phosphorescence [35, 70]. More recently, it was observed that the para-substitution of an EWG of dicyanovinyl at the C-ring will have a negligible effect on the emission color of the complex; but its meta-grafting would lead to drastically red-shifted phosphorescence [38]. Hence, it is clear from Reprinted from the journal

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the literature review that the rational molecular design of iridium (III) complexes with the desired emission color is not straightforward [71, 72].

4 Iridium (III) Phosphors for OLEDs We have seen significant attention and application of phosphorescent cyclometalated iridium (III) complexes. It is of interest since it has a quasi-octahedral geometry permitting the introduction of specific ligands in a controlled manner. Ir complexes have stable and accessible oxidation and reduction states, the photophysical and electrochemical properties of Ir complexes can be tuned in a predictable way, and cyclometalated Ir complexes possesses the highest triplet quantum yields. The development of new iridium complexes is an area of activity of many research groups. Thompson and coworkers synthesized a neutral emissive cyclometalated Ir(III) complexes [99] suitable as a phosphorescent dopant in OLEDs. Grushin and coworkers reported a series of Ir(III) complexes with fluorinated 2-arylpyridines ligands [100] and Cheng and coworkers, with substituted 2-phenylbenzothiazoles ligands [101]. Both of them showed that the complexes exhibit excellent processing and electroluminescent properties and emissive colors, which can be fine-tuned via controlling the nature and position of the substituents on the aromatic rings and ligands. 4.1 Green-Emitting Iridium (III) Phosphors Dedeian and coworkers have demonstrated a procedure to synthesize fac-tris-orthometalated complexes of iridium (III) with (Hppy) and with substituted 2-phenylpyridine (R-Hppy) ligands [92]. Inspired by this work, various iridium (III) phosphor red-, green-, yellow-, and orange-emitting complexes (Fig. 15) have been synthesized over the years. These complexes, featuring various color emission, were synthesized by varying the conjugation of cyclometalating ligands including incorporating substituents and altering ancillary ligands. For example, greenemitting phosphorescent fac-Ir(ppy)3 was first synthesized by altering ancillary ligands features high phosphorescence quantum yield of 0.4, short emission lifetime of 1.90 ls and emission peak at 494 nm [92]. Watanabe and coworkers developed a high-performance green PhOLEDs employing double emissive layers [102]. In this study, the emissive layer consists of wide energy-gap materials (CBP and TCTA) served as hosts for fac-Ir(ppy)3. Chemically doped polymers (MCC-PC1020) and 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC) were spin-casted as the hole injection- and hole transport layers, respectively. On the other hand, they used 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP) as an electron transport layer (ETL) and metal-doped electron injection layer (EIL) was formed by the co-evaporation of BCP and Cs. The optimized PhOLEDs exhibited high power efficiency of 97 lm/W at a brightness of 100 cd/m2 (3.1 V) and an external quantum efficiency of 27% corresponding to 95 cd/A.

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Fig. 15 Chemical structures of some typical red-, green-, yellow-, and orange-emitting iridium complexes

The same group further improved the fac-Ir(ppy)3 green PhOLEDs performance by introducing a novel ETL, bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM) [103]. The device demonstrated low drive voltages, which were 2.5 V at 100 cd/m2 and 2.9 V at 1000 cd/m2. High external quantum efficiencies of 29% at 100 cd/m2 and 26% at 1000 cd/m2 were also observed, which lead to the ultrahigh power efficiencies of 133 lm/W at 100 cd/m2 and 107 lm/W at 1000 cd/m2. Tanaka and coworkers attribute this excellent device performance to a significantly reduced barrier height for electron injection from the cathode and high electron mobility for novel ETL (which is ten times higher than that of Alq3). Today, most of the Ir(III) phosphors bear three bidentate chelates such as cyclometalated 2-phenylpyridine (ppy) in the prototypical example [Ir(ppy)3] [104–106]. Nevertheless, Williams and Haga have independently demonstrated a class of Ir(III) complexes with formula [Ir(dpyx)(ppy)Cl] (dpyxH = 4,6-dipyridylxylene) using both bidentate and tridentate cyclometalating ligands [107–112]. Despite that a huge effort has been put forward, studies on the bis-tridentate iridium (III) complexes have met with only limited success [113]. It is worth mentioning that among all the Ir complexes of the three primary emitting colors, the overall performance of the green-emitting Ir complexes and their PhOLEDs are the best, and some of the green

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Ir complexes have been utilized to fabricate large-scale OLEDs products in industry. 4.2 Red-Emitting Ir(III) Phosphors In 2004 and 2005, several groups developed new iridium complexes for deep red emission, bis(2,3-diphenylquinoxaline)iridium(III) acetylacetonate [(dpqx)2Ir(acac)], which demonstrates a maximum emission peak at around 670 nm in solution and around 680 nm in resulting devices, showing a high quantum yield [114–116]. However, in order to improve luminance efficiency, it is required to tune the emission from deep-red emission to the orange-red region. Amongst numerous developed red-emitting iridium complexes, the tris- and bis-cyclometalated iridium complexes based on the 2-phenylquinoline and 2-phenylisoquinoline cyclometalating ligands are among the frequently used. Compared to that of 2-phenylpyridinebased green-emitting iridium complexes, the 2-phenylquinoline and 2-phenylisoquinoline Ir complexes demonstrate red-shifted phosphorescence by extending the p-conjugation in the main cyclometalating ligands. Moreover, iridium complexes with 2-phenylisoquinoline ligands illustrate a red shift in phosphorescence relative to its isomers. Various structural modifications on these red Ir complexes were proposed, either by introducing substituents on the phenylquinoline and phenylisoquinoline ligands or replacing the phenyl ring with other aryl groups. Based on this argument, Kim and coworkers developed iridium (III)bis(2-phenylquinoline)acetylacetonate [(phq)2Ir(acac)]-based red dopants featuring sterically crowded alkyl moieties on their main ligands as well as ancillary ligands, where they obtained red spectra with significantly narrower full width at half-maximum [117]. The fabricated red PhOLED exhibited high EQEs up to 24.6% in case of the dopant with a fully methylated main ligand and a sterically crowded tmd ancillary ligand. The addition of an electron-donating methyl group to the metallated phenyl ring (mphq) gave a bathochromic shift, while the additional introduction of methyl group to the quinoline ring (mphmq ligand) led to a hypsochromic shift com-pared to the spectral range of the mphq ligand. In addition, the change of ancillary ligand to a tmd moiety from an acac moiety results in significant improvement of device efficiency. Chen and coworkers prepared a novel cyclometalating ligand framework and corresponding iridium complex (tmq)2Ir(acac) [11] (Fig. 16). BIQS shows a relative low-lying LUMO that facilitates electron injection, leading to a significantly lower operating voltage and higher current density. Also, the material exhibits suitable singlet and triplet energies to provide efficient energy transfer to deep-red emitters. The deep-red emitter (tmq)2Ir(acac) shows a very sharp emission band with a proper emission maximum resulting in very high luminous efficiency. The application of BIQS as the host for deep-red iridium complexes (piq)2Ir(acac) and (tmq)2Ir(acac) gave idealized deep-red PhOLEDs. The best performance device of the BIQS/(tmq)2Ir(acac) system shows a brightness of 58,688 cd/m2 and a maximum external quantum efficiency, current efficiency, and power efficiency of 25.9%, 37.3 cd/A, and 32.9 lm/W, respectively. To reduce the power consumption, it is still a great challenge to design and synthesize ideal host and dopant materials for deep-red electroluminescent (EL)

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Fig. 16 Synthesis and structures of BIQS and (tmq)2Ir(acac) (Reproduced with permission from Ref. [11]. Copyright 2011 Wiley–VCH-Verlag)

devices to improve the current and power efficiencies. Kwon and coworkers also reported a narrow bandgap beryllium complex as the host material and bis(2phenylquinoline)(acetylacetonate)iridium (Ir(phq)2(acac)) as the red dopant for the devices, thus achieving an external quantum efficiency (EQE) of up to 21% with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.62, 0.37) [118]. To facilitate charge injection, Chien and coworkers reported a fluorine-based bipolar host material with a maximum EQE of 19.9% and CIE coordinates of (0.64, 0.36) [119]. Reports for high-efficiency deep-red PhOLEDs with the CIE coordinate x C 0.67 are still rare. The efficiency and brightness of deep-red PhOLEDs are hard to improve because of the energy gap law and the drop in luminous flux in the deepred region [120]. Yang and coworkers optimized PhOLEDs based on a bipolar triphenylamine/oxadiazole hybrid material as the host to realize a very high EQE of 21.6% and a maximum power efficiency (PE) of 16.1 lm/W [121]. We reported an efficient deep-red EL device that utilized bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide (BCPO) as the host and tris[1-phenylisoquinolinato-C2,N]iridium(III) (Ir(piq)3) as the dopant to achieve an EQE of 17.0%, and a PE of 20.4 lm/W along with CIE coordinates of (0.67, 0.33) [122]. Most recently, Kido and coworkers developed a series of host materials containing building blocks of carbazole and arylenes. The best performance of the resultant deep-red PhOLEDs exhibited an EQE of 18.4% and a PE of 20.3 lm/W with CIE coordinates of (0.67, 0.33) [123]. Two key issues for the phosphorescent dopant of an idealized deep-red PhOLED with a CIE x C 0.67 are the emission wavelength and the bandwidth of the emission. Based on the chromaticity diagram, a spectrally pure monochromatic light with a CIE x C 0.67 should have a wavelength of C612 nm. For an actual OLED, the EL spectrum is a band structure rather than monochromatic light. If the EL spectrum is symmetric and the emission maximum of the device is kept at 612 nm, its CIE x should be lower than 0.67 because human eyes are much more sensitive to wavelengths shorter than 612 nm and less sensitive to wavelengths longer than

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612 nm in the EL spectrum. Thus, to achieve a CIE x of 0.67, the emission maximum of the device needs to shift to more than 612 nm, depending on the bandwidth. A larger emission bandwidth requires a longer emission maximum in order to maintain the same CIE x value of 0.67 [124, 125]. However, as the emission shifts to longer wavelength, the luminous efficiency decreases rapidly [126]. As a result, in the search for a supreme deep-red dopant with a CIE x = 0.67, the dopant should have an emission maximum close to 612 nm and the emission bandwidth as narrower as possible to reach the highest luminous efficiency. 4.3 Yellow- and Orange-Emitting Iridium (III) Phosphors Besides the three main phosphorescent materials, red, green, and blue, yellow and orange phosphorescent materials are employed in developing white devices are also receiving huge attention [127–130]. This is because to fabricate white OLEDs, two colors (blue and yellow) of dopants have various advantages compared to that of three colors (red, green, and blue) dopants regarding enhancing device efficiency and the color rendering index (CRI)/color temperature index [77]. Thus, developing highly efficient yellow and orange complexes is urgently desired. In 2001, Lamansky and coworkers first introduced Ir(bt)2acac [131]. Ever since, many efforts have been devoted to obtaining more and better yellow complexes, for instance with novel ligand skeletons as OLED dopants [89]. Despite these efforts, most yellow iridium (III)complexes are developed by focusing on the main ligand to tune the color emission. Owing to steric and electronic effects, functional groups on the main ligand sometimes render the synthesis of the indispensable intermediates of chloride-bridged iridium (III)dimers unrealistic. Therefore, a different route to tune the color emission via ancillary ligands was successfully demonstrated by You and coworkers [59]. This approach is quite promising because the triplet energy of ancillary ligands is lower than the energy of MLCT. Also, Chang and coworkers proposed a color-tuning method with ancillary ligands and successfully prepared novel green-emitting phosphorescent dopants [132]. Very recently, Chao and coworkers were also able to demonstrate color tuning from green to yellow by varying the ancillary ligands [133]. Another interesting work by Wang and coworkers was incorporating electrondonating (CH3, OCH3) and -withdrawing groups (F) into the 6-position of the benzothiazole moiety in the ligands. Organic light-emitting diodes using these iridium complexes as doped emitters demonstrated orange electrophosphorescence with excellent performances. A significantly higher brightness of 95 800 cd/m and a maximum luminance efficiency of 87.9 cd/A and power efficiency (46.0 lm/W) were obtained [134]. Prior to this record high brightness, the same group also successfully incorporated a strong electron-withdrawing CF3 or F in the 6-position of the benzothiazolyl moiety of the 2-phenylbenzothiazole ligand; the resultant iridium complexes (CF3-bt)2Ir(acac) and (F-bt)2Ir(acac) demonstrated significantly improved performance in their orange OLEDs, with a maximum luminance efficiency gL of 76 cd/A and a peak power efficiency (gp) of 45 lm/W. In conclusion, research on emission color adjustment associated with ancillary ligands is attractive and relevant to both fundamental research and practical

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applications. However, studies on this color-tuning methodology and its applications in yellow OLEDs are still scarce. 4.4 Blue-Emitting Iridium (III) Phosphors A few important factors have to be taken into account in developing highperformance blue phosphorescent emitters. The most vital criteria are to improve the contribution of the MLCT in the lowest-lying triplet manifold [79, 107]. The direct involvement of the metal dp orbital enhances the coupling of the orbital angular momentum to the electron spin, such that the T1 ? S0 transition would have a large first-order spin–orbit coupling term, which would result in a drastic decrease in the radiative lifetime and hence the possibility of increasing the quantum yield. Compared with the highly efficient and stable red and green iridium (III) complexes, development of blue and deep blue iridium (III) emissive for PhOLEDs are still limited and challenging. The commonly used dopant, bis(40 ,60 -difluorophenylpyridinato) iridium(III) picolinate (FIrpic) has been proven to be an excellent material for greenish-blue PhOLEDs [135, 136]. Limited improvements have been made by substituting picolinate with other ancillary ligands such as tetrakis(1-pyrazolyl)borate [56, 137, 138], tetrazolate [139], pyridyl azolates [55], picolinate N-oxide [140], or N-phenyl pyrazole [141, 142] to push the emission into the blue region. Recently, Chiu and coworkers developed blue heteroleptic iridium (III) complexes using ancillary phosphine chelate [143–145]. In one of their studies, PhOLEDs realizing phosphorescent dopant [Ir(fppz)2(P^N)] demonstrated maximum efficiencies of 6.9%, 8.1 cd/A, and 4.9 lm/W, together with a true-blue chromaticity CIEx,y = 0.163, with 0.145 recorded at 100 cd/m2 and other ingenious molecular designs [146, 147]. Also, the use of high-field-strength ligands such as Nheterocyclic carbenes [148–151] also resulted in a shift towards higher energy of the emission and an increase in the blue phosphorescent efficiency. Hsieh and coworkers synthesized five blue-emitting heteroleptic iridium dicarbene complexes (Fig. 17) using N-heterocyclic carbenes and pyridyl azolates ligands for which the blue phosphorescent emission can easily be tuned between 453 and 490 nm by varying the heteroleptic N^N ligand. The LUMOs of these complexes are mainly located on the N^N ligand, while the HOMOs are on the two carbene ligands and the iridium center. The devices using [Ir(mpmi)2(pypz)], [Ir(fpmi)2(pypz)] and [Ir(fpmi)2(tfpypz)] as the dopant emitters showed good-toexcellent performance with external quantum efficiencies of 15.2, 14.1, and 7.6% and with CIEx,y values of (0.14, 0.27), (0.14, 0.18), and (0.14, 0.10), respectively [152]. Tamayo and coworkers on the other hand prepared blue phosphorescent iridium (III)complexes by modifying difluorophenylpyridine (dfppy) ligands, where they were able to shift to even higher energy region relative to that of a homoleptic blue-green complexes, fac-Ir(dfppy)3 [46]. Besides, tris-cyclometaled blue or deep-blue iridium complexes using nonpyridine-based ligands that have higher triplet energies than dfppy ligand, such as phenylpyrazole [153], phenyltriazole [154], pyridylazolate [155], or Reprinted from the journal

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Fig. 17 HOMO and LUMO surfaces of the heteroleptic iridium carbene complexes 1–5 obtained from DFT calculations

Fig. 18 a Phenylpyrazole ligand. b Phenyltriazole ligand. c Pyridylazolate ligand (Reproduced with permission from Ref. [153–155]. Copyright 2006 American Chemical Society, and Wiley–VCH-Verlag, respectively)

imidazolphenathridine [156, 157] have been prepared (Fig. 18). Recently, Lee and coworkers reported two emissive Ir(III) complexes with fluorine-substituted 2,30 bipyridine (dfpypy), namely fac-[Ir(dfpypy)3 [158] and (3,5-difluoro-4-cyanophenyl)pyridine cyclometalates, FCNIr [159–161]. These iridium complexes were developed and found to exhibit deep-blue phosphorescence with high emission quantum yields in fluid solution at room temperature. Furthermore, remarkable EQE (above 20%) was achieved in deep-blue phosphorescent OLEDs using FCNIr as dopant [162]. In addition, all ancillary ligands must have strong metal–ligand bonding interaction so that the d–d excited states or other unspecified quenching states are strongly destabilized to prevent thermal population to these higher-lying deactivating states [150, 154]. Incorporation of multiple strong electron-withdrawing groups is an effective strategy to enlarge the band gap of iridium complexes by stabilizing the HOMO levels. Phenylpyridine derivatives with cyano group (FCNIr or FCNIrpic) [162, 163] along with two fluorine units further shifted the color coordinate of blue PHOLEDs in this way. To obtain deep-blue phosphorescent emitters, using the above-described concept, fluorine-substituted 2,30 -bipyridine

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(dfpypy) as cyclometalated ligand represents a good candidate for replacing the dfppy ligand was developed by Park and coworkers [164]. This main ligand has an electron-donating group such as a methyl group in one of the pyridyl moieties and might further contribute to making the LUMO levels of the complex raised, and thus lead to the large HOMO–LUMO gap to give the saturated blue emission. Employing the same concept, Jeon and coworkers used a heteroleptic FCNIrpic with picolinic acid ancillary ligand as a deep-blue dopant and their fabricated blue PhOLEDs exhibiting EQE of 25.1% and low roll-off (23.1% at 1000 cd/m2) along the color coordinates of (0.14, 0.17) [162], which is the best efficiency value reported in the deep-blue PHOLED up to now [162]. Despite all advancements to date, management of the charge transport properties and energy levels of the organic materials are of important for the development of high-performance color-tunable blue PhOLEDs. This approach could prove valuable for future PhOLEDs applications in displays and lighting. Acknowledgments This work was supported by the Swiss Federal Office for Energy, and the European Commission H2020-ICT-2014-1, SOLEDLIGHT project, Grant agreement No: 643791 and the Swiss State Secretariat for Education, Research and Innovation (SERI).

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Top Curr Chem (Z) (2016) 374:25 DOI 10.1007/s41061-016-0019-1 REVIEW

Copper(I) Complexes for Thermally Activated Delayed Fluorescence: From Photophysical to Device Properties Markus J. Leitl1 • Daniel M. Zink2 • Alexander Schinabeck1 Thomas Baumann2 • Daniel Volz2 • Hartmut Yersin1



Received: 7 December 2015 / Accepted: 7 March 2016 / Published online: 28 April 2016 Ó Springer International Publishing Switzerland 2016

Abstract Molecules that exhibit thermally activated delayed fluorescence (TADF) represent a very promising emitter class for application in electroluminescent devices since all electrically generated excitons can be transferred into light according to the singlet harvesting mechanism. Cu(I) compounds are an important class of TADF emitters. In this contribution, we want to give a deeper insight into the photophysical properties of this material class and demonstrate how the emission properties depend on molecular and host rigidity. Moreover, we show that with molecular optimization a significant improvement of selected emission properties can be achieved. From the discussed materials, we select one specific dinuclear complex, for which the two Cu(I) centers are four-fold bridged to fabricate an organic light emitting diode (OLED). This device shows the highest efficiency (of 23 % external quantum efficiency) reported so far for OLEDs based on Cu(I) emitters. Keywords Thermally activated delayed fluorescence  TADF  Phosphorescence  Fluorescence  OLED  Emitter  Triplet harvesting  Singlet harvesting  Emission properties  Electroluminescence  Cu(I)  Copper

This article is part of the topical collection ‘‘Photoluminescent Materials and Electroluminescent Devices. & Daniel Volz [email protected] & Hartmut Yersin [email protected] 1

Institut fu¨r Physikalische Chemie, Universita¨t Regensburg, Universita¨tsstr. 31, 93053 Regensburg, Germany

2

Cynora GmbH, Werner-von-Siemensstraße 2-6, Building 5110, 76646 Bruchsal, Germany

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1 Introduction Organic light emitting diodes (OLEDs) are capable of converting electrical energy into light. This property may be used to substitute established lighting technologies, such as light bulbs, energy-saving lamps, or fluorescent tubes. The possibility of making thin, flexible, and large-sized OLEDs inspired scientists and engineers to create entirely new applications: semi-transparent or bendable displays and light emitting labels for packaging already exist as prototypes. However, to turn these prototypes into products, new materials are required. Especially regarding light emitting materials, there is much room for improvement in terms of efficiency, realization of deep-blue emitting devices, sustainability, and cost efficiency. In this chapter, we introduce organo-metallic emitter materials that potentially satisfy all these requirements. These materials are based on Cu(I) complexes that exhibit certain key properties allowing to harvest all excitons, singlets and triplets, that are generated in the emission layer for the generation of light. Hereby, the underlying photophysical process is the singlet harvesting effect [1–3] that is based on the molecular mechanism of thermally activated delayed fluorescence (TADF) [4]. 1.1 Exciton Harvesting Mechanisms and Historical Developments The properties of electroluminescent devices depend essentially on the chemical and physical nature of the emitter material. In this section, we briefly introduce three emission mechanisms that are currently being used in OLEDs: fluorescence, phosphorescence, and thermally activated delayed fluorescence (TADF) [1, 2]. 1.1.1 Fluorescent Emitters The initial launch of OLEDs was realized by Van Slyke and Tang in 1987. Their OLED prototype had a quantum efficiency of 1 %, a brightness of 1,000 cd m-2, and a turn-on voltage of less than 10 V [5, 6]. In this device, tris(8-hydroxyquinolinato)aluminum or short Alq3, a fluorescent metal complex, was used as emitter material. In the following years, many other fluorescent molecules have been applied as emitters in OLEDs [7–10]. Some examples for such purely fluorescent materials are given in (Fig. 1 top). Soon after the realization of OLEDs with small molecules1, Burroughes et al. published work on the first OLED with a polymeric emitter in 1990, using poly-phenylene-vinylene (PPV) [11]; other conjugated polymers have also been successfully applied as emitters (Fig. 1 bottom) [12–17]. However, it was soon realized that only a limited efficiency is achievable with these conventional fluorescent materials because of spin statistics: There are two different types of excitons formed during OLED operation, singlet and triplet excitons, which are generally formed in a 1:3 ratio [18–20]. As demonstrated in Fig. 2, fluorescent materials can only use singlet excitons for the generation of light. 1

‘Small molecule’ is a widely-used, yet ambiguous term. It is most often used to distinguish isolated molecules with a molecular weight of less than 1,000 Da from polymers.

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NC

NC

CN

O

O

N O

O

N

NMe2

Al N

CN

N

DCM1

DCM2

[6]

[6]

O Ph

Ph

Ph

Alq3

Ph

Ph

[5,7,10] Ph

Ph

Ph

Rubrene

DPVBi

[7,10]

[7,10]

OC10H21 ∗

∗ ∗

OC10H21 OC10H21



OC8H17



OC10H21 OMe ∗ x

y

z

PPV

"Super Yellow"

MEH-PPV

[11]

x : y : z = 1 : 12 :12

[14-15,17]

[16]

Fig. 1 Examples of small molecule (top) and polymer (bottom) fluorescent OLED materials [5–7, 10, 11, 14–17]

Hereby, the emission originates from the spin-allowed S1 ? S0 transition. Triplet excitons cannot be harvested, as the T1 ? S0 transition is strongly spin-forbidden in this material class. Consequently, 75 % of all excitons are lost for the generation of light and transferred into heat [19]. Nevertheless, fluorescent materials are still used today in electroluminescent devices, especially, to achieve stable deep blue-emitting OLEDs due to the lack of satisfying alternatives [21]. 1.1.2 Phosphorescent Emitters To overcome the issue of losing 75 % of the excitons, organo-metallic compounds containing heavy metals such as iridium and platinum were investigated as emitter materials. These so-called phosphorescent materials or triplet emitters exhibit strong Reprinted from the journal

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Top Curr Chem (Z) (2016) 374:25 Fig. 2 Exciton harvesting mechanism for conventional fluorescent materials. Only singlet excitons (25 %) can be used for the generation of light, as all triplet excitons (75 %) are lost and transferred into heat

spin–orbit coupling (SOC) with respect to the lowest excited states. This changes the emission and exciton harvesting mechanism: First, SOC leads to a fast relaxation from the populated lowest excited singlet to the triplet state by means of fast intersystem crossing (ISC). The ISC time for these emitters is less than about 100 fs [22, 23]. Second, the T1 ? S0 transition that is normally spin-forbidden for purely organic molecules becomes by orders of magnitude more allowed [1] and thus, can efficiently generate photons. Hence, both singlet and triplet excitons can be used for the emission of light (compare Fig. 3). Since all excitons are harvested in the triplet state, this mechanism is called the triplet harvesting effect [1, 2, 19, 24]. As a consequence, phosphorescent emitters are able to reach an internal quantum efficiency of up to 100 % [25]. The use of phosphorescent organo-metallic emitters represented the birth of modern OLED emitters in 1998 [24, 26, 27]. The first working examples used the platinum complex PtOEP [24] (Fig. 4). Nowadays, the most efficient phosphorescent materials are based on Ir(III) and also Pt(II) complexes. Several selected examples are displayed in Fig. 4, such as FIrpic (sky-blue light emitter), Ir(ppy)3 (green) [1, 21, 23, 25, 26, 28, 29], Ir(dm-2-piq)2(acac) (red) [1], I(piq)3 (red) [30, 31] Pt(O^N^C^N) (green) [32], and PtON7-dtb (deep blue) [33, 34] and related complexes [35, 36]. 1.1.3 TADF Emitters The most recent developments in this field represent emitters that show thermally activated delayed fluorescence (TADF). Unlike conventional fluorescent or phosphorescent materials, TADF-materials are designed to exhibit a very small energy splitting DE(S1 - T1) between the first excited singlet and triplet state. Because of this, up-intersystem crossing (up-ISC) or reverse ISC (RISC) from T1 to

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Top Curr Chem (Z) (2016) 374:25 Fig. 3 Schematic diagram to illustrate the triplet harvesting mechanism. All excitons, singlets and triplets, can be used for the generation of light. Prior to emission, all excitations are harvested in the first excited triplet state T1

S1 is possible. The reverse ISC processes are thermally activated at ambient temperatures, which means that DE(S1 - T1) should not be much larger than around 1,000 cm-1 (&120 meV) so that the thermal energy kBT of & 210 cm-1 at ambient temperature is still sufficient to distinctly thermally populate the S1 from the T1 state. Since the spin-allowed S1 ? S0 transition from the thermally activated singlet state to the singlet ground state possesses much larger oscillator strength compared to the spin-forbidden T1 ? S0 transition, emission occurs as delayed fluorescence from the singlet state S1 and shows a significantly shorter (radiative) emission decay time than the triplet state. Unlike phosphorescent materials, which harvest both exciton types, singlets and triplets, in the triplet state, TADF emitters harvest the excitation in the singlet state and hence, are often referred to as singlet harvesting materials [1–3]. Two important material classes were identified that exhibit TADF. One class is based on Cu(I) emitters. The corresponding mechanism is displayed in Fig. 5, right. For these compounds, the processes of ISC are rather fast (order of 10 ps for downISC [41, 42]) due to SOC induced by the Cu(I) center(s). Hence, significant prompt S1 ? S0 florescence is not observed. Properties of this class of emitters are in the focus of this article (see below). A second class of TADF materials is based on purely organic molecules. The corresponding molecular effect was already discovered in 1961 by Parker and Hatchard [4]. However, it took decades until it was realized that TADF emitters based on organic molecules can be highly interesting for application in electroluminescent devices. Mainly, this can be attributed to the fact that despite the TADF mechanism, emission decay times can be as long as several milliseconds and thus, these emitters caused problems regarding OLED efficiency at higher current densities due to roll-off effects [43]. These long decay times are dictated by the long ISC times for organic molecules [44]. However, it was recently demonstrated that Reprinted from the journal

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N Pt

N

Pt

N

O

O

N

N

Pt N

N

N

F

F

PtOEP

Pt(Me4-salen)

Pt(dpt)(cc-phF2)

[24,37]

[38]

[40]

t

t

N

O Ir O

2 Ir(dm-2-piq)2(acac) [1]

Bu

Bu

N

N Bu

N

t

Pt

Pt O N

t

N

O

[Pt(O^N^C^N)] [32]

Bu

N

PtON7-dtb [33] F

Ir

Ir

N

O

Ir N

N

N 3

O

F

2

3

Ir(ppy)3

Ir(piq)3

"FIrpic"

[27,31,37]

[30-31]

Ir(2,4-dFppy)2(pic) [39]

Fig. 4 Examples of phosphorescent platinum and iridium OLED materials [1, 24, 27, 30–33, 37–40]

these materials can be strongly optimized in this regard. Accordingly, the latest generation of organic TADF materials can exhibit suitably short decay times [45– 49]. In addition, it was shown that with these materials highly efficient OLEDs can be realized [45–48]. In Fig. 6, selected examples are displayed for illustration.

2 Luminescent Cu(I) Emitters: A Brief Overview More than 30 years ago, luminescent copper(I) complexes started to gain the interest of the scientific community. Beginning in the late 1970s and 1980s, McMillin investigated mononuclear phenanthroline complexes in various studies

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Fig. 5 Graphical illustration of the singlet harvesting mechanism based on the molecular TADF effect [1–3]. All excitons, singlets and triplets, can be used for the generation of light. The excitations are harvested in the first excited singlet state S1 prior to emission. The mechanism is displayed for purely organic (left) as well as metal-organic (right) emitter materials. Note the differences with respect to the occurrence of the prompt fluorescence. Both material classes usually exhibit phosphorescence at low temperatures

R

R

R

R

N NC

N

CN

N

CN CN

NC

N

N

N

N R

R R

4CzIPN [45]

N

NC

N

R

4CzTPN

R=H

4CzTPN-Me

R = CH3

4CzTPN-Ph

R = phenyl

mCNA [49]

[45]

Fig. 6 Examples for purely organic TADF materials [45, 49]

and, in the course of his results, proposed that the emission stems from two different excited electronic states, which is nowadays confirmed and known as TADF [50]. A large variety of luminescent copper(I) compounds has been investigated since that time and their first successful application as emitting materials in OLEDs was reported in 1999 [51, 52]. Subsequently, the number of scientific investigations Reprinted from the journal

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using this new kind of TADF emitters is increasing rapidly and a short overview of several important structure classes together with examples with respect to OLED applications is given in this section (compare also Sect. 4). A comprehensive review concerning an overview of Cu(I) compounds can be found in the literature [53]. 2.1 Mononuclear Compounds Mononuclear complexes can be grouped into three main categories: cationic tetrahedrally coordinated (Fig. 7), neutral tetrahedrally coordinated (Fig. 8), as well as trigonally coordinated compounds (Fig. 9). With respect to an application as OLED emitters, in particular, phenanthroline, bipyridyl-, pyrazolyl- as well as tetrazolyl-based [Cu(N^N)(P^P)] complexes have been reported with external quantum efficiencies in electroluminescent devices of around 15 % [54–60]. Neutral complexes with tetrahedral coordination are an important class of mononuclear compounds (Fig. 8). A steadily increasing number of copper(I) complexes, which feature an anionic ligand in order to compensate the positive charge of the copper ion instead of a counterion, bear witness to the great potential of this class of materials. Neutral complexes offer the advantage that they are lacking the counterion, which might have unexpected effects at high electric fields in the +

+ R N

Ph2P Cu

N R

R1

R3 N O

Ph2P Cu

BF4-

N

Ph2P

R3

R2

O

BF4-

Ph2P

Cu(pop)(phen)

R=H

Cu(pop)(tmbpy)

R1 = Me; R2 = Me; R3 = Me

Cu(pop)(dmp)

R = Me

Cu(pop)(dmbpy)

R1 = H; R2 = H; R3 = Me

Cu(pop)(dmbp)

R = Bu

Cu(pop)(6-mebpy) R1 = Me; R2 = H; R3 = H [55,62-63]

[54,64] +

+

R N

Cu N N N NH

N

Ph2P X

Y

BF4-

N

Ph2P

Ph2P Cu

N

O

BF4-

Ph2P

R R

(DPEPhos)Cu(PyrTetH)

R = H; X = O; Y = H2

Cu(pop)(pypz)

(Xantphos)Cu(PyrTetH)

R = H; X = O; Y = CMe2

Cu(pop)(pympz)

R = CH3

(PTEPhos)Cu(PyrTetH)

R = Me; X = O; Y = H2

Cu(pop)(pytfmpz)

R = CF3

R=H

[56]

[61]

Fig. 7 Examples of mononuclear cationic copper(I) complexes [54–56, 61–64]

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R R

B

N

Ph2P

N

Cu

N N

Ph2P

N

Cu

O N N N N

Ph2P

X

Y

Ph2P R

Cu(pz2BH2)(pop)

R=H

(DPEPhos)Cu(PyrTet) R = H; X = O; Y = H2

Cu(pz2Bph2)(pop)

R = Ph

(Xantphos)Cu(PyrTet) R = H; X = O; Y = CMe2 (PTEPhos)Cu(PyrTet)

[3,65]

R = Me; X = O; Y = H2

[61]

R2 P Cu P R2

N N Ph B Ph N N

Cu(dppb)(pz2Bph2)

R = Ph

Cu(dppb-F)(pz2Bph2)

R = 3,5-difluorobenzene

Cu(dppb-CF3)(pz2Bph2)

R = 3,5-di(trifluoromethyl)benzene

[3,65]

Fig. 8 Examples of mononuclear neutral copper(I) complexes [3, 61, 65]

R2 P

N Cu

X

N B

Cu N

P R2

(LMe)CuX

R = Me; X = Cl, Br, I

(LEt)CuBr

R = Et; X = Br

(LiPr)CuBr

R = iPr; X = Br

(LiPr)CuX

R = m-toluene; X = SPh

N

CH3 CH3

(IPr)Cu(py2-BMe2) [69-70]

[68,73]

Fig. 9 Examples of trigonally coordinated Cu(I) complexes [68–70, 73]

operating OLED device. A large number of anionic ligands such as borates, tetrazolates, or thiolates have been used so far to synthesize emitter complexes, along with blue-light emitters [3, 65], with outstanding photoluminescence quantum yields (PLQY) [3, 61] as well as high device efficiencies (of almost 18 %) of external quantum efficiencies (EQE) [66–68]. Finally, the class of mononuclear complexes consisting of the trigonally threecoordinated copper(I) ions should be mentioned. Here, a monocoordinating anionic ligand can be used to compensate the positive charge of the copper ion instead of a

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bidentate anionic ligand (Fig. 9). Halide as well as thiolate complexes have been reported to show high PLQY values of 71 % together with device EQEs of over 20 % [68]. Alternatively, the charge of the Cu(I) ion can be compensated by using a monodentate neutral carbene and a bidentate negatively charged boron ligand [69– 72]. 2.2 Dinuclear Compounds Dinuclear halide- or pseudo halide-bridged Cu(I) complexes form a further large group of frequently brightly emitting materials. Various compounds featuring monodentate or chelating phosphines as well as nitrogen-containing ligands are known and have been tested in OLEDs. In 2007, the first devices using iodo-bridged complexes ([Cu(l-I)(1,2-bis[diphenylphosphino]benzene)]2) (Fig. 10) were reported [74, 75]. In most cases, such compounds were synthesized classically by solution-based reactions. However, dinuclear CuI-based complexes featuring the general formula L2Cu2I2 (L = pyridine derivative) can also be synthesized via the evaporation of CuI together with the organic ligand. This new strategy paves the way for OLED fabrication using the well-established sublimation technique, which is often not successful for compounds with high molecular weight or insufficient thermal stability. This approach has been proven to enable device construction with EQEs of up to 15.7 % [76, 77]. Further, it was shown that Cu(I) complexes with chelating ligands exhibiting N and P donors can be synthesized [78]. With this strategy, it was possible to develop blue-emitting compounds (Fig. 10 bottom), which is particularly interesting

X

N

Cu2I2(PPh3)2(Ph)2

X=I

[79-80]

Ph2 P

Ph2 P

X Cu

Cu P Ph2

R N

Cu2(µ-Cl)2(dppb)2

X = Cl

Cu2(µ-Br)2(dppb)2

X = Br

Cu2(µ-I)2(dppb)2

X=I

X

P Ph2

[74,81]

X

R N

[Cu(µ-Cl)(PNMe2)]2

X = Cl; R = Me2

[Cu(µ-Br)(PNMe2)]2

X = Br; R = Me2

P Ph2

[Cu(µ-I)(PNMe2)]2

X = I; R = Me2

[Cu(µ-I)(PNpy)]2

X = I; R = py

Cu

Cu P Ph2

X = Cl

Cu2Br2(PPh3)2(Ph)2 X = Br

N

X

Ph3P

Cu2Cl2(PPh3)2(Ph)2

PPh3 Cu

Cu

X

[78]

Fig. 10 Examples of dinuclear halide-bridged complexes with P and N ligands [74, 78–81]

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N

N Ph2P

I

Ph3P

PPh2

Cu Cu I PPh N 2

I

PPh3 Cu I PPh2 N

Cu

Ph2P

I Cu N

I

PPh2 Cu PPh2

Cu2I2(MePyrPHOS)(MePyr(Ph3)2P)2

Cu2I2(MePyrPHOS)(PPh3)2

Cu2I2(MePyrPHOS)(dpph)

[82-84]

[88]

[94]

Fig. 11 Examples of NHetPHOS-type Cu(I)-emitters [82–84, 88, 94]

considering the lack of efficient alternatives for blue fluorescent emitters for OLEDs. A different class of dinuclear complexes exhibits a butterfly-shaped copperhalide core. The two copper atoms are bridged by one P^N ligand. A fourth coordination site of each copper ion is represented by phosphine ligands (Fig. 11) [82–84]. A related structure has been presented in Refs. [75, 85]. By introducing additional substitutions, such as solubility enhancing or crosslinking precursor groups, the complex properties can be strongly modified [86–93]. For example, the emission of these compounds can be tuned from deep blue to red by modifications of the bidentate P^N ligand [84]. The complexes show extraordinarily high photoluminescence quantum efficiencies and also very high EQE values up to 23 % when applied in OLEDs [94, 95]. Photophysical properties of these types of compounds are discussed in Sect. 3.2, while an OLED stack with this recordefficiency will be presented in Sect. 4.

3 Photophysical Properties—Case Studies The vast majority of Cu(I)-compounds that have been used in electroluminescent devices are either mononuclear or dinuclear copper complexes with bridging halides and chelating P ligands [56, 60, 66, 68, 94–98]. For this reason, we will focus on examples of these two families in the following sections by presenting case studies. First, we will investigate the impact of specific molecular features on the excitedstate properties of mononuclear Cu(I) complexes. Second, we will investigate examples of dinuclear, NHetPHOS-type emitters (compare compounds 5 and 6, below) [93], which have been used in OLED devices recently. 3.1 Mononuclear Compounds: Rigidity and Emission Properties A structure motif often found for Cu(I) compounds is represented by complexes with pseudo-tetrahedral coordination of two bidentate ligands [1–3, 62–66, 96, 97, 99–110]. Frequently, these complexes suffer from nonradiative deactivations since strong geometry changes can occur upon excitation. In this section, we focus on

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Fig. 12 Structures of the investigated compounds 1 [62, 63], 2 [62, 63], and 3 [102]. The intrinsic rigidity of the complexes due to sterical interactions of the two ligands increases from left to right [62, 102]

these mechanisms and show that reducing the extent of such distortions can significantly enhance emission quantum yields. Two different strategies are promising in order to control and limit excited-state distortions. The first approach is based on an optimization of the matrix material (environment) to limit excitedstate distortions of the embedded or doped emitter material. The second approach is based on modifying the chemical structure of the compounds so that geometry distortions upon excitation are reduced by sterical interactions of the ligands. In this latter strategy, excited-state distortions are already hindered at a molecular level. In this case study, a series of the three compounds [Cu(dmbpy)(pop)]BF4 (1), [Cu(tmbyp)(pop)]BF4 (2), and [Cu(dmp)(phanehphos)]PF6 (3) is investigated [62, 102]. The corresponding structures are displayed in Fig. 12. Hereby, dmbpy = 4,40 dimethyl-2,20 -bipyridine, tmbpy = 4,40 ,6,60 -tetramethyl-2,20 -bipyridine, pop = bis[2-(diphenylphosphino)-phenyl]ether, dmp = 2,9-dimethyl-1,10-phenanthroline, and phanephos = 4,12-bis(diphenylphosphino)-[2.2]paracyclophane. 3.1.1 Emission and Rigidity Effects In this section, we will discuss and compare properties of the three compounds displayed in Fig. 12. At first, we focus on photophysical properties of [Cu(dmbpy)(pop)]BF4 (1) in different environments. In Fig. 13, emission data are displayed for the compound dissolved in ethanol (EtOH) and compared to data of the powder material. In addition, absorption spectra of [Cu(dmbpy)(pop)]BF4 as well as of the free ligands pop and bpy recorded in the same solvent are shown2. For complex 1, intense absorption bands are observed in the wavelength range between 230 and 330 nm. These bands are also present in the spectra of the free ligands, which indicates that they result from ligand-centered transitions. In contrast, complex 1 exhibits an absorption band between 330 and 450 nm, which 2

Bipyridine (bpy) was used instead of dmbpy. However, the absorption spectrum of the methylated ligand is not expected to deviate significantly from that of bpy.

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Fig. 13 Absorption spectra of the compound [Cu(pop)(dmbyp)]BF4 (1) and of the ligands pop and bpy recorded in EtOH. Emission spectra are displayed for the compound as powder and dissolved in deoxygenated EtOH solution. The samples were excited at kexc = 350 nm. All spectra were recorded at ambient temperature [62]

does not occur in the spectra of the free ligands. Consequently, this band is assigned to result from metal-to-ligand charge transfer (MLCT) transitions. This assignment is supported by density functional theory (DFT) calculations, which are frequently and successfully applied for investigating electronic states of transition metal complexes [62, 65, 67–69, 72, 81, 84, 87, 102, 111–113] and, in particular [114, 115]. The calculations for 1 and 2 show that the highest occupied molecular orbital (HOMO) is largely located at the copper center and is mainly of 3d character, whereas the lowest unoccupied molecular orbital (LUMO) is distributed on the bipyridine moiety of the complex (Fig. 14). Furthermore, timedependent density functional theory (TDDFT) calculations reveal that the first excited singlet S1 and triplet state T1 are determined by transitions between these two frontier orbitals, which clearly underlines the 1,3MLCT character of these states. At ambient temperature, the powder of compound 1 exhibits relatively weak orange luminescence (kmax = 575 nm) under illumination with UV light. The corresponding emission spectrum as displayed in Fig. 13 is broad and featureless, which is in agreement with the charge transfer character of the emitting state(s). The emission quantum yield is moderate, amounting to UPL = 9 %. When complex 1 is dissolved in EtOH, the emission is red-shifted to kmax = 655 nm and the quantum yield decreases significantly to UPL \1 %. This behavior is a consequence of the pronounced MLCT character of the emitting states. Since on excitation a significant amount of charge is transferred from the Cu(I) center to the bipyridine ligand, the copper center is partially oxidized towards Cu(II). Furthermore, Cu(II) prefers a planar coordination compared to the tetrahedral configuration of Cu(I). Thus, the charge transfer in the Cu(I) complexes is generally connected with a substantial molecular reorganization [41, 103, 116].

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Fig. 14 Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of [Cu(pop)(dmbpy)]? (1) displayed for the optimized triplet state (T1) geometry. Calculations were performed on the B3LYP/def2-SVP level of theory. Iso-contour values were set to 0.05. Hydrogen atoms were omitted for clarity. Note the pronounced spatial shift from HOMO to LUMO. For completeness it is noted that the HOMO and LUMO contour plots are largely similar for compound 2

For the investigated compound, this reorganization is represented by a flattening distortion from the pseudo-tetrahedral ground state to a more planar excited-state configuration. This can also be seen from the DFT calculations [62]. Such distortions are driven by a lowering of the excited-state energies which represents a red shift of the emission energies. Furthermore, strong distortions from the ground to the excited-state geometry lead to a pronounced increase of non-radiative deactivations to the ground state and thus, to a lowering of the emission quantum yield. This behavior can be explained in a model in which the potential energy surfaces of the excited and ground states can come close or even intersect (compare Fig. 15). This leads to an increased overlap of the vibrational wavefunctions of the excited and the ground state and therefore, enlarges the corresponding Franck– Condon factors that govern the non-radiative processes [44]. The tendency of excited-state distortions to occur is especially pronounced in non-rigid environments, such as fluid solutions. In contrast, in more rigid environments such as powders, these distortions are partly suppressed. Consequently, it is expected that the quantum yield strongly depends on the rigidity of the environment. Indeed, in a rigid environment the quantum yield is substantially higher than in a non-rigid, fluid environment (compare also Table 1 and Refs. [3, 62, 102]). As discussed above, a rigidity increase of the environment provides an effective strategy to reduce excited-state distortions and, as a consequence, non-radiative deactivations. This process enhances emission quantum yields. However, frequently, molecular distortions that occur as a consequence of excitation can be more efficiently reduced by introducing sterically demanding groups at the ligands, i.e., within the molecules themselves. An impressive demonstration for this approach is illustrated by the series of the compounds 1, 2, and 3. Compound 1 can be modified by adding two methyl groups at the 6,60 positions of the bipyridine ligand, giving compound 2 (Fig. 12). This modification should limit distortions that can occur upon excitation due to the sterical interaction of the

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Fig. 15 Simplified illustration of the dependence of the emission energy on the degree of distortion occurring in the excited state. In the case of Cu(I) compounds with two bidentate ligands, the distortion coordinate mainly represents the dihedral angle between the two ligands [62]. Solid lines represent radiative transitions, dotted lines refer to resonant ISC and/or internal conversion processes

methyl groups of the modified bipyridine and the phenyl groups of the pop ligand. Consequently, the non-radiative deactivation to the ground state should be reduced and the emission quantum yield should increase. Furthermore, as excited-state distortions result in a red-shifted emission, a less pronounced distortion is expected to induce a smaller red shift. Indeed, comparing the two compounds shows that both effects, the increase of the emission quantum yield and the blue shift of the emission, are observed experimentally. In the powder phase, the emission maximum of compound 2 lies at kmax = 555 nm (18,020 cm-1) compared to that of compound 1 with kmax = 575 nm (17,390 cm-1). More importantly, the powder of compound 2 exhibits an emission quantum yield of UPL = 55 % compared to UPL = 9 % found for the powder of compound 1. These trends are also observed for the compounds dissolved in EtOH solution, giving quantum yields of UPL = 6 % for 2 and UPL \ 1 % for 1. For completeness, it is noted that also the electron-donating character of the methyl groups might have an influence on the emission energy. This would result in a shift of the LUMO to higher energy and consequently also to a blue shift of the emission. However, in the absorption spectra of both compounds (not displayed) only a slight blue shift of the MLCT absorption band from compound 1 to 2 is observed. In emission, a shift of similar energy would be expected. However, the observed shift of the emission energy is significantly larger and therefore, is mainly rationalized by the rigidity effect and not by an electron-donating effect of the methyl groups. Compound 3 represents an example in which possible distortions on excitation are even more hindered due to the interaction of the bulky phanephos and the dimethylphenantroline ligand. Consequently, the emission quantum yield is even Reprinted from the journal

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higher than for compound 2, amounting to 80 % in the powder phase. Importantly, also in fluid dichloromethane solution the quantum yield is very high, amounting to 40 % compared to 9 % for 2 and to \1 % for 1. This strongly indicates that geometry distortions upon excitation can effectively be reduced at a molecular level. An overview of the emission parameters of the three compounds is given in Table 1. Interestingly, the occurrence of geometry distortions even in the powder material has another important consequence. Typically, powder samples are not well-suited to investigate molecular emission properties, as these can be significantly masked by intermolecular effects, for example, by energy transfer to quenching impurities. However, the geometry distortions of the Cu(I) compounds are sufficient (even in the powder phase) to lower the excited-state energy to such an extent that the condition for energy transfer to adjacent non-excited molecules is not fulfilled. Therefore, the excitation may be regarded as trapped at the initially excited emitter molecule (self-trapped) [1–3, 117]. Consequently, even powder samples can be used well to study emission properties of such Cu(I) complexes. In the following, this self-trapping effect is the basis for the studies of temperature-dependent emission properties of compounds 1, 2, and 3 using powder samples.

Table 1 Emission data of the compounds Cu(pop)(dmbyp)BF4 (1), Cu(pop)(tmbyp)BF4 (2), and Cu(dmp)(phanephos)PF6 (3) measured in solution and powder, respectively Compound:

Cu(pop)(dmbyp)BF4 (1)

Cu(pop)(tmbyp)BF4 (2)

Cu(dmp)(phanephos)PF6 (3)

Reference:

[62]

[62]

[102]

Temperature (K):

300

77

300

77

300

77

Solution kmax (nm)

655

605

575

535

558

548

s (ls)

0.02

16

2.5

73

10

130

UPL (%)

\1

6

40

60

kr (s-1)

2.4 9 104

4.0 9 104

0.5 9 104

knr (s-1)

38 9 104

6.0 9 104

0.3 9 104

Powder kmax (nm)

575

595

555

575

530

562

s (ls)

a

a

11

87

14

240

UPL (%)

9

55

47

80

70

kr (s-1)

5.0 9 104

0.5 9 104

5.7 9 104

0.3 9 104

knr (s-1)

4.1 9 104

0.6 9 104

1.4 9 104

0.1 9 104

For compounds 1 and 2 the solution data were recorded for the complexes dissolved in ethanol (EtOH), while compound 3 was dissolved in dichloromethane (DCM). kmax represents the wavelength at the emission peak, s is the emission decay time, and UPL is the photoluminescence quantum yield. The radiative rate kr and nonradiative rate knr were calculated according to kr = UPL s-1 and knr = (1 - UPL) s-1 a

Strongly deviating from a monoexponential decay behavior

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3.1.2 Low-Temperature Phosphorescence—High-Temperature TADF For a deeper understanding of the photophysical properties of the complexes, we investigated powder samples in the temperature range between 77 and 300 K. As the photophysical behavior of compound 1 is mainly governed by non-radiative processes, we want to focus the discussion on compounds 2 and 3. At T = 77 K, the emission decay time of compound 2 amounts to s(77 K) = 87 ls and is assigned to be a phosphorescence originating from the lowest excited triplet state T1. With increasing temperature to T = 300 K, the decay time decreases by a factor of about 8 to s(300 K) = 11 ls, while the emission quantum yield stays almost constant over the whole temperature range [UPL(77 K) = 47 % and UPL(300 K) = 55 %]. This allows us to determine the radiative rates kr according to kr = UPL s-1, giving kr(77 K) = 0.5 9 104 s-1 and kr(300 K) = 5.0 9 104 s-1. Thus, the radiative rate increases by a factor of ten with temperature increase. This behavior is paralleled by a blue shift of the emission from kmax(77 K) = 575 nm to kmax(300 K) = 555 nm, corresponding to an energy difference of 630 cm-1 (compare Fig. 16). Both effects, the increase of the radiative rate and the blue shift of the emission upon heating can be rationalized by a simple TADF model (Fig. 18, see below). At low temperature, only emission from the lowest excited triplet state T1 is observed. With increasing temperature, a thermal population of the energetically higher-lying first excited singlet state S1 occurs. As the spin-allowed S1 ? S0 transition carries significantly larger allowedness than the spin-forbidden T1 ? S0 transition, population of the S1 state results in a drastic reduction of the decay time. Also, the blue-shifted emission at higher temperatures can be explained by this model, as the energy of the S1 state is higher than that of the T1 state. Such a mechanism corresponds to a thermally activated delayed fluorescence (TADF). The energy separation DE(S1 - T1) between the first excited singlet and triplet state can

Fig. 16 Emission spectra of compounds 1 [62], 2 [62], and 3 [102] as powders at different temperatures. The blue shift of the emission maximum with increasing temperature is clearly visible

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Fig. 17 Emission decay time versus temperature for powders of compounds 2 [62] and 3 [102]. The gray solid line represents a fit according to Eq. (1) and the values given in the figure represent the data as obtained by the fitting procedure. The decay time at a given temperature is monoexponential in the entire temperature range

roughly be estimated from the shift of the emission spectra with temperature increase3 from T = 77 to 300 K amounting to DE(S1 - T1) = 630 cm-1 for compound 2. A more accurate approach for the determination of the emission parameters is represented by an investigation of the temperature dependence of the emission decay time. The obtained data can be fitted by a modified Boltzmann relation according to the following equation: [1, 2, 118]. h i 3 þ exp  DEðSkB1TT1 Þ h i sð T Þ ¼ ð1Þ 3sðT1 Þ1 þ sðS1 Þ1 exp  DEðSkB1TT1 Þ In this equation, DE(S1 - T1) represents the energy separation between the first excited singlet S1 and triplet T1 state, s(S1) and s(T1) the intrinsic emission decay times of the individual states, and kB the Boltzmann constant. If Eq. (1) is used to fit the data points displayed in Fig. 17, these molecular parameters can be determined. From the fitting procedure, a value of DE(S1 - T1) = 720 cm-1 is obtained, which is in good agreement with the value resulting from the spectral shift (630 cm-1). For the decay time of the first excited singlet state, a value of s(S1) = 160 ns was found. Such a value is in agreement with the singlet nature of this state, however, being connected with a rather low oscillator strength of the S1 ? S0 transition. An emission originating as a prompt fluorescence was not found. This is due to the competing and significantly faster ISC process from the S1 to the T1 state, which has been reported for other Cu(I) complexes to be on the order of 10 ps [116, 119, 120]. It is noted that Eq. (1) can only be applied if the states participating in the emission process are in a thermal equilibrium [121, 122]. In the temperature range between 3 It is remarked that an estimation of DE (S1 - T1) from the spectra is only possible if both states, S1 and T1, result from transitions between the same molecular orbitals. For the investigated compounds, this condition is fulfilled.

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Fig. 18 Energy level diagram for compounds 2 and 3. The energy splittings DE(S1 - T1) and the phosphorescence decay time s(T1) were obtained with the data from Fig. 17 by applying Eq. (1). The TADF decay time at ambient temperature was determined according to sðT1 Þ1 þ sðTADFÞ1 ¼ sð300 KÞ1 . Hereby, s(300 K) = s(phos. ? TADF) and s(T1) = s(phos.). Note that for compounds 2 and 3, s (phos) [[ s(TADF) and therefore, s(phos. ? TADF) & s(TADF). A prompt fluorescence was not observed

77 and 300 K this condition is fulfilled, which is indicated by the strictly monoexponential decay behavior found for the entire temperature range. Similar investigations have been performed for compound 3 [102]. It was found from the fitting procedure that the singlet–triplet energy splitting amounts to DE(S1 - T1) = 1,000 cm-1, which is in good agreement with the spectral shift occurring on heating from T = 77 to 300 K of 1,070 cm-1. Furthermore, the fitting procedure gives values for the intrinsic decay times of the S1 and T1 states amounting to s(S1) = 40 ns and s(T1) = 240 ls, respectively. Interestingly, an alternative approach to determine s(S1) from absorption spectra based on the Strickler–Berg relationship gives a value of s(S1) & 80 ns. Taking into account the fundamentally different nature of these two independent methods and the connected errors, both values are in good agreement [102]. It is noted that the determination of the singlet–triplet energy splitting via the indirect method based on the measurement and fitting of the temperature dependence of the emission decay is not suitable for compound 1, as in this case the excited state deactivation is mainly governed by non-radiative processes, which are strongly temperature-dependent. However, the energy separation between the S1 and T1 state can be estimated from the spectral shift to DE(S1 - T1) = 580 cm-1 for the powder material (compare Fig. 16). This value is similar to the one found for compound 2. As complex 1 and 2 exhibit similar structures differing only by two methyl groups, similar shifts are expected to occur. Although these groups have a strong impact on the emission quantum yields, they do not seem to strongly alter the electronic structures of the compounds.

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Top Curr Chem (Z) (2016) 374:25 Table 2 Properties of the excited S1 (1MLCT) and T1 (3MLCT) states for powders of compounds 1, 2, and 3 [Cu(pop)(dmbyp)BF4 1 [62]

[Cu(pop)(tmbyp)]BF4 2 [62]

[Cu(dmp)(phanephos)]PF6 3 [102]

580

630

1,070

DE(S1 - T1) (cm-1)

720

1,000

s(S1) (ns)

160

40

s(T1) (ls)

84

240

Compound: Reference: Spectra DE(S1 - T1) (cm-1) Fit

The energy splitting DE(S1 - T1) was determined by the spectral shifts occurring between 77 and 300 K and by an indirect method, based on the measurement and fitting of the temperature-dependent emission decay time. In addition, the intrinsic decay times s(S1) and s(T1) are given

For all investigated compounds, the energy splitting DE(S1 - T1) between the first excited singlet and triplet state is very small compared, for example, to conventional purely organic molecules, for which the singlet–triplet energy splitting is frequently of the order of many 103 cm-1 [44]. Also, for transition metal complexes, e.g., Pt(II) complexes, larger values have been reported [22, 123]. The rather small singlet–triplet energy splitting found for the investigated Cu(I) complexes is rationalized by the pronounced charge transfer character of the T1 (3MLCT) and S1 (1MLCT) states. For the investigated compounds, the HOMO is mainly located on the copper center, whereas the LUMO is mainly distributed over one of the ligands. Consequently, the spatial overlap of these frontier orbitals is small. This results in a small exchange interaction of the two involved electrons and thus, in a small DE(S1 - T1) singlet–triplet splitting. Interestingly, the small spatial overlap of HOMO and LUMO results also in a smaller oscillator strength of the S1 ? S0 transition. Thus, it is expected that with decreasing DE(S1 - T1), the singlet decay time s(S1) increases. Indeed, this trend is displayed when comparing compounds 2 (s(S1) = 160 ns, DE(S1 - T1) = 720 cm-1) and 3 (s(S1) = 40 ns, DE(S1 - T1) = 1,000 cm-1). (see also [3, 65]). The results are summarized in Table 2 for the compounds investigated in this section. Furthermore, in Fig. 16, for compounds 2 and 3 energy level diagrams visualize the singlet-triplet splittings and the decay times of the phosphorescence s(phos), the thermally activated delayed fluorescence s(TADF), and give the decay times (at ambient temperature) for the combined emission s(phos ? TADF). Compare also Ref. [69]. 3.2 Dinuclear Compounds: Stability and Photophysical Properties Cu(I) complexes with two metal centers have also gained high attention recently [111]. Frequently, these complexes exhibit a structure in which the two Cu(I) centers are bridged by two halides [74–84]. However, these bridges are often unstable, especially in fluid environments [78, 124, 125]. On the other hand, bridging the two copper centers with an additional bidentate ligand can significantly enhance the

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Fig. 19 Increasing stabilization by introduction of bridges. Structures of compounds 4 [78], 5 [82, 83, 91], and 6 [94]

stability. A relatively stable structure type has been proposed in Refs. [82–84]. Furthermore, in this complex class, the HOMO is often not just localized on one copper center but is delocalized over the entire copper halide core. On excitation, an oxidation of the entire core and not only of one copper center is occurring, which helps to limit excited state distortions [94]. Two representatives of this class and a typical Cu(I) dimer without an additional bridge are discussed in this section, in particular with regard to their photophysical properties. Moreover, it will be demonstrated in a case study, presented in Sect. 4, that one of the compounds represents an excellent emitter for highly efficient OLEDs. 3.2.1 Photophysical Introduction The three compounds [Cu(l-I)(PNMe2)]2 (4), Cu2I2(MePyrPHOS)(PPh3)2 (5) and Cu2I2(MePyrPHOS)(dpph) (6) (MePyrPHOS = 2-Diphenylphosphino-4-methylpyridin, dpph = 1,6-Bis diphenylphosphino hexan) represent dinuclear Cu(I) complexes in which the two copper centers are bridged by two iodine ions. In compounds 5 and 6, the copper centers are further bridged by a P^N ligand and in the case of compound 6 by an additional alkyl bridge representing a fourth bridge (Fig. 19). Accordingly, within the series of the three complexes, the stability is enhanced from compound 4 to 6. At ambient temperature, the powder of compound 4 exhibits a bright blue emission with a maximum at kmax = 464 nm, an emission quantum yield of UPL = 65 %, and an emission decay time of s = 4.6 ls. For compound 5, a bright green emission is seen with a maximum at kmax = 511 nm, an emission quantum yield being close to UPL = 100 %, and a decay time of s = 5.0 ls is found, while compound 6 exhibits an emission maximum at kmax = 519 nm and a quantum yield

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Top Curr Chem (Z) (2016) 374:25 Table 3 Emission data of powder of [Cu(l-I)(PNMe2)]2 (4), Cu2I2(MePyrPHOS)(PPh3)2 (5) and Cu2I2(MePyrPHOS)(dpph) (6) Compound:

[Cu(l-I)(PNMe2)]2 4

Cu2I2(MePyrPHOS) (PPh3)2 5

Cu2I2(MePyrPHOS) (dpph) 6

Reference:

[78]

This work

This work

Temperature (K):

300

77

300

77

300

77

Powder kmax (nm)

464

471

511

520

519

558

s (ls)

4.6

270

5.0

20

24

109

UPL (%)

65

100

97

100

88

76

kr (s-1)

14 9 104

0.4 9 104

19 9 104

5 9 104

3.7 9 104

0.7 9 104

knr (s-1)

7.6 9 104

&0

0.6 9 104

&0

0.5 9 104

0.2 9 104

kmax is the wavelength maximum of the emission, s the emission decay time, and UPL is the photoluminescence quantum yield. The radiative rates kr and nonradiative rates knr were calculated according to kr = UPL s-1 and knr = (1 - UPL) s-1, respectively

of UPL = 88 %. However, the emission decay time is significantly longer amounting to s = 24 ls (Table 3). In the following we want to focus on the brightly emitting compounds 5 and 6. A detailed discussion of the properties of compound 4 can be found in Ref [78]. To gain a preliminary insight into the electronic structures of the compounds, DFT and TDDFT calculations were performed on the optimized T1 state geometry. The calculations reveal that for compound 5, the HOMO is located at both copper centers to similar amounts (Cu(1) 27 %, Cu(2) 14 %). Furthermore, a significant contribution to the HOMO is also located at the bridging iodines (I(1) 18 %, I(2) 16 %). In contrast, the LUMO is localized at the pyridine moiety of the bridging P^N ligand (Fig. 20). The first excited singlet S1 and triplet T1 states result from transitions between these frontier orbitals. Thus, we can assign these states as being of (metal ? halide)-to-ligand charge-transfer [(M ? X)LCT] character (compare also Ref. [84]). For compound 6, the situation is slightly different. Although, the iodine contribution to the HOMO is similar [I(1) 22 %, I(2) 16 %], the contribution of the copper centers is different compared to 5, as a significant amount to this molecular orbital is contributed by only one copper center [Cu(1) 39 %, Cu(2) 3 %]. Again, the LUMO is located at the pyridine moiety of the P^N ligand. Also here, the S1 and T1 states result from HOMO–LUMO transitions, which again allows assigning these states as being of (M ? X)LCT character. TDDFT calculations based on the T1 state geometry give singlet–triplet splittings for compound 5 of DE(S1 - T1) = 510 cm-1 and for compound 6 of 640 cm-1. These values may be regarded as an orientation for the amount of splitting, while the experimentally determined values are presented in the next section.

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Fig. 20 Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of Cu2I2(MePyrPHOS)(PPh3)2 (5) and Cu2I2(MePyrPHOS)(dpph) (6) displayed for the optimized triplet state (T1) geometry. Calculations were performed on the B3LYP/def2-SVP level of theory. Iso-contour values were set to 0.05. Hydrogen atoms were omitted for clarity. Note that in the case of compound 6 essentially only one Cu(I) center is involved in the HOMO, whereas for compound 5 both copper centers contribute with largely similar amounts

Fig. 21 Emission spectra of compounds [Cu(l-I)(PNMe2)]2 (4) [78], Cu2I2(MePyrPHOS)(PPh3)2 (5) [82, 83, 91], and Cu2I2(MePyrPHOS)(dpph) (6) [94] as powders at different temperatures. The blue shift of the emission maximum with increasing temperature is clearly visible

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Fig. 22 Emission decay time versus temperature for powders of compounds 5 and 6. The gray solid line represents a fit according to Eq. (1). Fit data are also displayed. Note the different time scales for the decay time axes

3.2.2 Thermally Activated Delayed Fluorescence Frequently, emission spectra measured at different temperatures might already give a first indication of an occurrence of TADF due to a blue shift of the spectra with temperature increase. Such spectra are displayed in Fig. 21. Indeed, for compounds 5 and 6, the blue shift is obvious. On the other hand, a determination of DE(S1 - T1) based only on this spectral shift might be misleading as the shift can easily be masked by temperature-induced broadening effects, in particular, for small singlet–triplet gaps. Therefore, a better access to this gap (and other important photophysical parameters) is gained from the dependence of the emission decay time on temperature (compare Sect. 3.1.2). The results for powders of compounds 5 and 6 are displayed in Fig. 22. For compound 6, it is found that the energy splitting amounts to DE(S1 - T1) = 830 cm-1 (which is somewhat larger than predicted by TDDFT calculations), whereas for compound 5 it is with DE(S1 - T1) = 270 cm-1, distinctly smaller than predicted by the calculations. These deviations might be related to short-comings of the TDDFT approach with regard to the description of chargetransfer-state energies and/or to the fact that the calculations were performed for the gas phase but not for the relatively rigid crystalline environment, for which a different molecular geometry might exist compared to the one assumed for the model calculations. The singlet–triplet energy splitting DE(S1 - T1) for compound 5, amounting to DE(S1 - T1) = 270 cm-1, is remarkably small and to the best of our knowledge represents the smallest value that has been reported so far. Consequently, the thermal population of the singlet state S1 from the T1 state reservoir should be highly effective at ambient temperature and result in a very pronounced shortening of the TADF decay time. The resulting energy level diagrams and decay times for compounds 5 and 6 are summarized in Fig. 23. However, at ambient temperature, the emission decay time for a TADF system is not solely determined by the singlet–triplet gap but also by the intrinsic decay s(S1) of the first excited singlet state S1 [compare Eq. (1)]. For compounds 5 and 6, these

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Fig. 23 Energy level diagrams for compounds 5 and 6. The energy splittings DE(S1 - T1) and the phosphorescence decay time s(T1) were obtained with the data from Fig. 22 by applying Eq. (1). The TADF decay time at ambient temperature was determined by applying sðT1 Þ1 þ sðTADFÞ1 ¼ sð300 KÞ1 . Hereby, s(300 K) = s(phos. ? TADF) and s(T1) = s(phos.). Note that in contrast to the situation shown in Fig. 18, phosphorescence and TADF decay times, especially for compound 5, are not very different. Consequently, the phosphorescence channel also contributes to the emission at ambient temperature [69, 126]. A prompt fluorescence was not observed

Table 4 Properties of the excited S1 (1(M ? X)LCT) and T1 (3(M ? X)LCT) states Compound (powder):

[Cu(l-I)(PNMe2)]2 4

Cu2I2(MePyrPHOS)(PPh3)2 5

Cu2I2(MePyrPHOS)(dpph) 6

Reference:

[78]

This work

This work

Spectra DE(S1 - T1) (cm-1)

240

340

1,350

DE(S1 - T1) (cm-1)

460

270

830

s(S1) (ns)

210

570

190

Fit

The energy splitting DE(S1 - T1) was determined from the spectral shift and from the temperature dependence of the emission decay time, respectively. In addition, the intrinsic decay times s(S1) and s(T1) of the S1 and T1 state, respectively, are given

decay times amount to s(S1) = 570 ns (5) and s(S1) = 190 ns (6), respectively (Table 4). In particular, the s(S1) value for compound 5 is unusually long for a spin-allowed transition. This value is of similar size as that found for spinforbidden T1 ? S0 transitions for Ir(III) complexes. For example, Ir(ppy)3 (ppy = phenylpyridine) exhibits an ambient-temperature triplet decay time of s(T1) = 1.4 ls [1, 28]. Moreover, the ‘‘spin-forbidden’’ transition from the triplet sublevel III to the S0 ground state exhibits an even shorter decay time of Reprinted from the journal

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s(III) = 200 ns. Accordingly, it is by a factor of almost three shorter than that of the spin-allowed S1 $ S0 transition of compound 5. The occurrence of a long S1 decay time for compounds with a very small DE(S1 - T1) value is, however, not unexpected. As already discussed in the previous section, a small spatial overlap of HOMO and LUMO does not only give a small singlet–triplet splitting, but also a low oscillator strength for the S1 $ S0 transition. Consequently, the costs for a small singlet–triplet gap are paid by a long singlet decay time—as displayed by our experimental results. Furthermore, such long intrinsic singlet decay times explain also why no prompt fluorescence is observed. Compared to singlet decay times of several 100 ns, the S1 ? T1 ISC time of 10 ps represents a much faster and therefore dominating decay route [116, 119, 120]. Consequently, at low temperature, the S1 state is dominantly depopulated via ISC before prompt fluorescence can occur. However, at higher temperatures up-ISC processes are very fast and therefore, a favorite situation is given for an effective thermal activation (up-ISC) of the S1 state from the long-lived T1 reservoir.

4 Electroluminescence with Cu(I) Compounds An OLED device consists of a number of layers, such as anode, hole-injection layer, hole transporting layer, emission layer, electron transporting layer, electron injection layer, and cathode [21, 127, 128]. An example is discussed below. To obtain an optimized electron and hole transport and charge carrier recombination, an adjustment of the redox potentials of the respective layers and of the emitter is required. Here, we want to restrict ourselves only to considerations with respect to the emission layer that consists of the host material and the Cu(I) complex emitter. 4.1 Introductory Remarks In most cases, the HOMOs of Cu(I)-compounds are either located on a coppercentered orbital or—in the case of dinuclear complexes—on a Cu2X2-localized orbital. For this reason, most Cu(I)-emitters have HOMO energies that lie in the range between -5.0 and -5.4 eV [88, 94]. The LUMO energies are somewhat more spread among the different Cu(I)-compounds, because these values are dictated by the respective ligands [84]. The determination is often difficult, because many Cu(I)-emitters cannot be measured with standard methods of, for example, cyclic voltammetry [129], and due to the fact that indirect determination of the LUMO energy based on a known HOMO energy and the optical transition energy may be faulty, especially, in the case of Cu(I)-emitters that frequently exhibit very broad emission spectra and large Stokes shifts. This latter issue has been discussed above with respect to the ‘‘self-trapping’’ behavior [1–3, 117]. As a rule of thumb, the LUMO energy of many Cu(I)-emitters lies between -3.0 and -2.2 eV, at least for dinuclear compounds [88]. The ‘‘bandgap’’ DE(S1 - S0) of Cu(I) complexes is often large. For instance, even for green-emitting Cu(I) emitters, the singlet energies are relatively high as

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displayed by the very broad TADF emission spectra. These energies can be estimated from the high-energy flanks of the emission spectra and it is found that, for green emitters, they lie near 2.8 eV (compare the spectra reproduced in Fig. 21 and in Refs. [3, 58, 69, 78, 102, 126]). The information given above is important, for example, for the selection of an adequate host material. In particular, its triplet energy has to be higher than the singlet energy of the TADF emitter. This is (coarsely) fulfilled, for example, for the host material PYD2 (E(T1) = 2.93 eV). The chemical structure of this molecule is displayed in Fig. 25 (see below) [58]. Furthermore, it is required that the HOMO and LUMO energies of the host fit to the corresponding values of the Cu(I) based emitters. This is also fulfilled for the PYD2 host material, exhibiting HOMO and LUMO energies at -5.9 and -2.2 eV, respectively [130]. Frequently, Cu(I) emitters are applied in rather high doping concentrations of much more than 10 % in the host materials. This is advantageous for the construction of efficient OLED devices due to the absence of concentration quenching, as has been proposed in Refs. [1, 3, 65, 69, 78, 102, 126, 131, 132]. On the other hand, the emitters can represent effective traps for holes in the emission layer or even significantly contribute to the charge transport [133]. This requires that the emitters are very stable against redox reactions in order to achieve a high operational stability in OLED devices. The fact that many publications showed problems concerning measurements of reversible oxidation or reduction potentials [109, 129, 134–136] suggests that this is an open issue concerning the development of devices with long-term stability. 4.2 Literature OLED Examples Indeed, a number of valuable investigations using Cu(I) emitters in OLEDs have been carried out. However, it is not in the scope of this contribution to present those details. Here, we will only refer to examples reported in these studies. Early stageinvestigations were carried out with tetranuclear compounds [51, 52]. Subsequently, compounds as displayed in Fig. 24, for example, were applied. In particular, compound (LMe)Cu(Br) could be vacuum sublimed and reached, as already shown in 2011, a very high efficiency of 65.3 cd/A (EQE 21.3 %) [98]. 4.3 OLED Case Study—Solution-Processed Device Achieving nearly 100 % Internal Quantum Efficiency In Sect. 3.2, we presented photophysical properties of dinuclear compounds. According to these studies, compound 5 should be best-suited for an OLED application. However, compound 6 (Fig. 19) shows several advantages [94]. Presumably, due to its four-fold bridged Cu(I) centers, it exhibits a high thermal stability (no decomposition up to T = 290 °C in nitrogen atmosphere derived from thermo-gravimetric analysis (TGA)). Moreover, when doping compound 6 into the PYD2 host an emission quantum yield of 92 % is found, being slightly higher than the value registered for the powder material with UPL = 88 % (Table 3). Probably, this is induced by the specific thin-film morphology, an effect that has recently been Reprinted from the journal

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Top Curr Chem (Z) (2016) 374:25 R R

R R

R

R

P

P Cu P R

N N

Cu

B

N N

X R

P R

R R R Cu(dppb)(Ph2Bpz2)

R=H

(LMe)CuX

Cu(dppb-F)(Ph2Bpz2)

R=F

(LEt)CuBr

R = Et; X = Br

Cu(dppb-CF3)(Ph2Bpz2) R = CF3

(LiPr)CuBr

R = iPr;X = Br

[66]

[68]

t

t

Bu

R = Me; X = Cl, Br, I

Bu

Ph i

Bu2

P

N Cu

i

Bu2

P

i

P Bu2

N

i

N

Cu

Cu

N

P Bu2

Ph2 P C P C Ph2

Ph = BH t

t

Bu

Bu

[Cu(PNP-tBu)]2

Cu(dmp)(carb)

[111]

[97,137]

Fig. 24 Chemical structures of selected Cu(I) emitters for highly efficient OLEDs [66, 68, 97, 111, 137]

found also for complexes with similar structures [88, 90]. Moreover, good film formation properties and little crystallization tendency are observed even when preparing neat thin films of compound 6. The OLED device was fabricated after extensive optimization leading to the stack architecture, as displayed in Fig. 25 with layer thicknesses given in the figure caption. The hole injection layer (PEDOT:PSS (AI4803)) was spin-coated onto the indium tin oxide (ITO) substrate and baked at 140 °C for 30 min in air. The hole transport layer PLEXCORE [138, 139] was spin-coated onto PEDOT:PSS and annealed at 180 °C for 30 min under nitrogen for crosslinking. Then, the emitting layer was spin-coated from toluene solution on top of the hole transporting layer and annealed to dry. All other layers were thermally evaporated under vacuum [94]. Note that the emission layer exhibits a relatively high doping concentration of the emitter without showing concentration quenching, as was already proposed earlier [1, 3, 102, 132]. Both, 3TPYMB and PLEXCORE UT-314 were chosen due to their high triplet energies in order to prevent quenching by energy transfer [58].

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Fig. 25 Energy level diagram for the device ITO (130 nm) // PEDOT:PSS (30 nm) // UT-314 (45 nm) // compound 6/PYD2 (30/70 [wt%]) (27 nm) // 3TYPMB (15 nm) // LiF(2 nm) // Al (100 nm) and molecular structures of the materials used [94]

Figure 26 displays the current efficiency versus electro-luminance of the optimized device. The turn-on voltage was 2.6 V and the maximum brightness 10,000 cd/A was achieved at 10 V. The current efficiency amounts to 71 cd/A at 100 cd/m2, while the peak current efficiency reaches even 73 cd/A (at &40 cd/m2) with 63 lm/W, corresponding to an external quantum efficiency of 23 %. This is the highest value reported for OLEDs so far for solution- and vacuum-processed emission layers based on Cu(I) emitters. It is remarked that no outcoupling-

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Fig. 26 Left current efficiency vs. luminance (EL). Right electroluminescence spectrum as obtained with the device according to Fig. 25

enhancing strategies were applied. This value is comparable to the state-of-the-art efficiency of vacuum sublimed devices based on Ir(III) emitters. For completeness, it is remarked that the device shows a pronounced roll-off of the current efficiency with increasing luminance. This behavior can be attributed to a loss of charge balance as most of the hole transport does not occur via the host material but via the emitter itself. At higher current densities, the relatively long emission decay of the emitter amounting to 24 ls could become problematic and lead to saturation effects [43].

5 Conclusion The development and understanding of new luminescent materials was stimulated by their potential application in electroluminescent devices. Since more than 15 years research in this respect was focused on phosphorescent Ir(III) and Pt(II) compounds [35]. Since about 5 years, Cu(I) complexes and metal-free TADF emitters became interesting, as these materials might replace high-cost rare metal compounds that are currently most-often applied in commercial OLEDs. Both phosphorescent and TADF emitters can harvest all excitons that are generated after the electron–hole recombination process, i.e., according to the triplet harvesting [1, 2, 19, 24, 140] and the singlet harvesting mechanism [1–3], respectively. Obviously, the required progress in material development can only proceed if a deep scientific research concerning structure–property relations is carried out. Thus, in this contribution, we presented—after an introduction to the mentioned mechanisms—a detailed insight into the photophysics of Cu(I) compounds and how their properties can deliberately be modified. Accordingly, we present an emitter compound representing a multi-bridged dinuclear Cu(I) complex (compound 6) that opens the door to very efficient OLEDs. With this material, we obtained an external quantum efficiency of 23 %, which represents the highest value reported so far for Cu(I)-based OLEDs. However, although these investigations are

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very promising, further developments have to be carried out to optimize the device performance, especially with respect to more sophisticated architectures on the device-level and a shortening of the TADF decay time (at high emission quantum yield) on the material level. A new promising approach based on a combined TADF and triplet-state emission at high spin–orbit coupling may open the required progress [69, 126, 141]. Acknowledgments The authors thank the German Ministry for Education and Research (BMBF) for funding in the scope of the cyCESH project (FKN 13N12668). The authors (T.B., D.V., D.M.Z.) gratefully acknowledge the collaboration with the groups of Prof. Franky So (NCSU), Prof. Christopher Barner-Kowollik (KIT), Prof Clemens Heske (KIT, UNLV), Prof. Uli Lemmer (KIT), and Prof. Stefan Bra¨se (KIT), as well as the scientific division of CYNORA.

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Top Curr Chem (Z) (2016) 374:19 DOI 10.1007/s41061-016-0021-7 REVIEW

Materials Integrating Photochemical Upconversion Catherine E. McCusker1 • Felix N. Castellano1

Received: 11 January 2016 / Accepted: 14 March 2016 / Published online: 29 March 2016 Ó Springer International Publishing Switzerland 2016

Abstract This review features recent experimental work focused on the preparation and characterization of materials that integrate photochemical upconversion derived from sensitized triplet–triplet annihilation, resulting in the conversion of low energy photons to higher energy light, thereby enabling numerous wavelengthshifting applications. Recent topical developments in upconversion include encapsulating or rigidifying fluid solutions to give them mechanical strength, adapting inert host materials to enable upconversion, and using photoactive materials that incorporate the sensitizer and/or the acceptor. The driving force behind translating photochemical upconversion from solution into hard and soft materials is the incorporation of upconversion into devices and other applications. At present, some of the most promising applications of upconversion materials include imaging and fluorescence microscopy, photoelectrochemical devices, water disinfection, and solar cell enhancement. Keywords Photochemical upconversion  Triplet–triplet Annihilation  Wavelength shifting  Light emitting materials  Solid state upconversion  Soft photonic materials

This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli, Henk Bolink; please follow CAP workflow & Felix N. Castellano [email protected] 1

Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA

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1 Introduction to Photochemical Upconversion 1.1 Introduction Photon upconversion afforded via sensitized triplet–triplet annihilation (TTA) is a regenerative photochemical process resulting in the net frequency upconversion of light. Sensitized TTA and photon upconversion, first introduced by Parker and Hatchard in the early 1960s, incorporated the exclusive use of organic sensitizers and acceptors [1–3]. By selectively exciting the sensitizer chromophore, these authors were able to observe blue-shifted emission (fluorescence) characteristic of the acceptor chromophore, concomitant with nonlinear excitation light power dependence. They concluded this upconverted emission was due to the interaction of two excited triplet acceptors, which resulted in one excited singlet acceptor and one ground state acceptor. These initial systems suffered from low efficiency and further advancement in this field was not to be realized for several decades. During the last 13 years there has been a resurgence of interest in the field of sensitized photon upconversion prompted by the change to inorganic (transition metal containing) sensitizers featuring unity to near quantitative intersystem crossing (ISC) yields [4–6]. Much of this initial work has been developed in fluid solution, where the sensitizer and acceptor can diffuse freely. With few exceptions, it has been during the last 8 years that upconversion in numerous host materials and media has made significant progress. The process of photon upconversion is displayed pictorially in Fig. 1. Sensitizer molecules absorb light and are promoted into a singlet excited state, which intersystem crosses into a triplet excited state. When the triplet sensitizer encounters an acceptor it can undergo a Dexter-type collisional triplet–triplet energy transfer (TTET), forming the triplet excited state of the acceptor. When two triplet acceptors meet they can engage in TTA (essentially a second TTET step), resulting in one excited singlet acceptor and one ground state acceptor. The excited singlet acceptor can then fluoresce, returning to the ground state by the emission of this higher High Energy Emission

Low Energy Absorption 3

A

A*

A A

1

S*

3

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1

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S*

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S

A

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ISC = Intersystem Crossing TTET = Triplet-Triplet Energy Transfer

= Ground State Sensitizer

= Ground State Acceptor

TTA = Triplet-Triplet Annihilation

= Excited State Sensitizer

= Excited State Acceptor

Fig. 1 Pictorial illustration of sensitized photon upconversion. The orange circles are ground state sensitizers, the yellow circles are excited state sensitizers, the blue hexagons are ground state acceptors, and the green hexagons are excited state acceptors

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Fig. 2 Qualitative Jablonski diagram illustrating the relative energy levels of sensitizer and acceptor required for photochemical upconversion

energy photon. The relative energetics of these states is important to realize successful photon upconversion. The Jablonski diagram in Fig. 2 presents the energies of the relevant states. Most important is that the sum of the energy of two triplet acceptors must be greater than or equal to the energy of the resultant excited singlet state (3A* ? 3A* C 1A*) otherwise the annihilation reaction cannot produce sufficient energy to form the emitter singlet state. For favorable (exothermic) energy transfer, the acceptor triplet state must be lower in energy than the sensitizer triplet state, and for true upconversion (i.e., the acceptor emission is located at a higher frequency with respect to the sensitizer absorption) the acceptor singlet state must be higher in energy than the sensitizer singlet state. To meet these requirements, the acceptor should have a large singlet–triplet gap and the sensitizer should have a small singlet–triplet gap. Given these energetic criteria, the acceptor is generally possesses p-p* excited states with large values of 2J. 1.2 Quantifying Photon Upconversion As illustrated in Fig. 2, two absorbed photons are required for every one emitted photon, implying that the maximum yield for this process is 50 % based on absorbed photons. As a result, the upconverted emission does not have a simple linear dependence on light intensity [7]. The concentration of triplet acceptors, and therefore, the intensity of the upconverted fluorescence, is a function of two competing rates (Eq. 1), where kT is the intrinsic first order and pseudo-first order decay pathways for 3A* and kTT is the second order TTA rate constant. d½3 A t ¼ kT ½3 A   kTT ½3 A 2t dt

ð1Þ

In the weak annihilation limit, kT is much larger than kTT[3A*], and first order decay to the ground state is the dominant relaxation pathway for 3A*. In this limit, the upconverted emission intensity can be expressed as shown in Eq. 2 where NF is

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the upconverted fluorescence intensity and UF is the fluorescence quantum yield of the acceptor. NF ¼

/F kTT ½3 A 20 2kT

ð2Þ

In this limit, NF is proportional to [3A*]2, which means it has a quadratic dependence on excitation light intensity, i.e., proportional to the square of the triplet acceptor population. In the strong annihilation limit, kTT[3A*] is much larger than kT and TTA becomes the dominant relaxation pathway for 3A*. In this limit, the upconverted emission intensity can be expressed as Eq. 3, where NF is proportional to [3A*], and therefore, has a linear dependence on excitation light intensity. NF ¼ /F ½3 A 0

ð3Þ

The strong annihilation limit is where the efficiency of TTA becomes maximized, and therefore, in any application it is desirable for the device or process to operate under these conditions. Therefore, the system will favor the strong annihilation limit if one minimizes kT and maximizes kTT[3A*]. The intrinsic decay rate of 3A* cannot be changed, but also included in the kT term is the pseudo-first order quenching of 3 A* by trace dissolved oxygen. Complete exclusion of oxygen from the system is the best way to minimize kT. Assuming that TTET quenching has already reached saturation, maximizing kTT[3A*] is afforded by increasing the concentration of 3A*, either by increasing the absorbance of the sensitizer or the intensity of the excitation light. The power dependence of the upconverted emission signal is one metric that is used to judge the performance of an upconversion composition. When the

Fig. 3 Representative example of the incident light power dependence on upconverted emission intensity, plotted on a logarithmic scale

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upconverted emission intensity of a system is plotted vs. incident excitation power density on a logarithmic scale, as shown in Fig. 3, regions that are in the weak annihilation limit will have a slope of 2, regions in the strong annihilation limit will have a slope of 1, and intermediate regions, where kT & kTT[3A*], will have slopes varying between 2 and 1 depending upon their relative proportions. The threshold intensity (Ith) is the crossover point between the weak annihilation limit and the strong annihilation limit, defined as the crossing point between the slope = 1 line and the slope = 2 line on a logarithmic scale. A system or device has to operate above this threshold power density to reach the maximum upconversion efficiency possible for the given composition. Upconversion quantum yield (UUC) is another useful metric to evaluate upconversion systems. Conceptually, UUC is the product of the yields of all the steps involved in the upconversion process, as shown in Eq. 4 where UISC is the intersystem crossing yield in the sensitizer, UTTET is the triplet–triplet energy transfer yield, UTTA is the triplet–triplet annihilation yield, and UF is the fluorescence quantum yield of the acceptor. /UC ¼ /ISC  /TTET  /TTA  /F

ð4Þ

Practically, the upconversion quantum yield is often measured relative to a known standard using Eqs. 5 [6] or 6 [8] where Ustd is the quantum yield of the standard, A is the absorbance of the standard or upconversion sample, I is the integrated emission intensity of the standard and upconversion samples, and g is the refractive index of the medium. The factor of two is included to scale the maximum UUC to 1 rather than 0.5.     Astd IUC gUC 2 /UC ¼ 2/std ð5Þ AUC Istd gstd  /UC ¼ 2/std

1  10Astd 1  10AUC

   IUC gUC 2 Istd gstd

ð6Þ

Under optically dilute conditions (A B 0.1) the simplified Eq. 5 can be used, and under more concentrated conditions the expanded Eq. 6, where 1–10-A is the fraction of light absorbed by the sample, must be used. The challenge of this approach, especially for non-solution measurements, is choosing an appropriate, well-established quantum yield standard. An alternative to using a separate quantum yield standard is to use the prompt fluorescence of the acceptor as an internal standard. This method, developed by Schmidt and co-workers [9], uses pulsed laser excitation to directly measure UTTA (Eq. 7). /TTA ¼

2Fd Ep kd Fp Ed k p

ð7Þ

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prompt fluorescence (kp). The integrated emission intensity of the prompt fluorescence (Fp) is then compared to the integrated intensity of the total upconverted emission (Fd). This model assumes that all of the laser pulse energy (Ep or Ed) is absorbed by the sample and that the quantum yield of the acceptor is same in prompt fluorescence and upconverted (delayed) fluorescence. Using either method, UTTA and UUC will be dependent on excitation power, increasing with increasing excitation intensity until the strong annihilation limit is achieved. It is also difficult to compare directly measurements performed with continuous wave (cw) excitation and pulsed excitation sources. A pulsed excitation source will have a considerably larger peak power than a cw source with the same average power. For example, a laser with a 10 ns pulse and 10 Hz repetition rate will have a peak power of 100 kW while the average power is only 10 mW.

1.3 Scope of Review With a few exceptions, much of the earlier, pre-2010 work in the sensitized upconversion field has been largely evaluated in room temperature solutions, where the sensitizer and acceptor can diffuse freely [6]. This review will cover more recent developments that have been made over the last 5 years on sensitized upconversion in non-solution environments. These advances are largely in the areas of encapsulating or rigidifying fluid solutions to give them mechanical strength, using inert host materials, and using photoactive materials that incorporate the sensitizer and/or the acceptor. The driving force behind translating upconversion from solution into materials is the incorporation of upconversion into devices and other applications. Some of the most promising applications of upconversion materials at

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present include imaging and fluorescence microscopy, photoelectrochemical devices, water disinfection, and solar cells.

2 Upconversion in Fluids 2.1 Encapsulation In photochemical upconversion both the TTET and the TTA are diffusional processes, and therefore, are highly sensitive to the viscosity of the surrounding environment. Encapsulating a fluid solution of sensitizer and acceptor in a shell allows the low viscosity and favorable photophysical properties of organic solution to be maintained while garnering the versatility of a solid material along with aqueous and biocompatibility. If the shell has low oxygen permeability, it provides a mechanism to protect the encapsulated solution from oxygen, which quenches both the sensitizer and acceptor triplet states and thereby increases the power threshold for saturation. Emulsions of hexadecane, containing PdOEP (sensitizer), perylene (acceptor), and polymer precursors, could be thermally polymerized to form polystyrene, poly(methyl methacrylate) (PMMA), or poly(styrene/acrylic acid)-copolymer shells around a liquid hexadecane core containing PdOEP and perylene [10]. The formed core–shell nanocapsules were 150-200 nm in diameter and could be dispersed in water and taken up by HeLa cells. The TTET efficiency between the PdOEP and perylene was reduced compared to solution, as evidenced by the residual phosphorescence. Despite these issues, the upconverted emission intensity was sufficient to visualize HeLa cells with confocal microscopy using cw laser excitation (514 nm, *100 W/cm2). While these capsules remained intact in aqueous suspension and when incubated in cells, the fragility of the capsule prevented any dry applications such as thin film formation. Embedding the nanocapsules in poly(vinyl alcohol) PVOH polymer fibers mitigated their fragility and formed a lightweight solid that undergoes upconversion under ambient conditions and is visible to the naked eye [11].

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An alternative to thermal polymerization is to use photoinduced polymerization. A solution of PtTPTBP and BPEA in ethoxylated trimethylopropane triacrylate (ETPTA), emulsified in water and irradiated with UV light in a microfluidic channel, formed an ETPTA resin shell around a liquid ETPTA monomer core [12]. The capsule size could be tuned from 210–450 lm by varying the flow rates of the oil and water phases through the microfluidic device and the shell thickness could be varied with irradiation time. Capsules made with this method could be dispersed in water or deposited as a monolayer on a surface. The thinnest shells showed the highest initial upconverted emission intensity, but were susceptible to leakage and quenching by oxygen when exposed to air. Thicker shells were more robust and oxygen stable, but showed decreased upconversion signal and increased phosphorescence from the PtTPTBP trapped in the solid shell. The Kim group has made more recent advances in UV cured capsules with double [14] and triple [13] layered capsules designed to isolate the chromophores in the liquid core of the capsules. The sensitizer and acceptor are dissolved in mineral oil with polyisobutylene added as an oxygen scavenger (anti-oxidant). The doublelayered capsules surround the mineral oil center with an ETPTA shell and the triple layered capsules have a water layer between the oil core and ETPTA shell (Fig. 4). The water layer between the oil and outer shell effectively prevents diffusion of the chromophores from the core into the rigid shell, as evidenced by a significant decrease in sensitizer phosphorescence. These capsules were also highly stable, with the triple layered particles showing \10 % decrease in upconversion intensity after storage in water, under ambient conditions for over 2 months. A unique approach to encapsulation by Svagan et al. is to use cellulose shells [15]. A hexadecane solution of a mixture of benz- and naphthannulated porphyrin sensitizers and t-butylethynyl substituted perylene (TBEP) acceptor was encapsulated in a shell of cellulose nanofibers, and for additional protection from oxygen diffusion, the capsules were imbedded under a thick (8.8 lm) layer of cellulose nanofibers to form a paper which undergoes upconversion with moderate excitation power densities (\500 mW/cm2, cw excitation). In this upconverting paper, the residual sensitizer phosphorescence was not quenched over 5 h under a 20.5/79.5 oxygen/nitrogen atmosphere. However, the upconverted emission decreased 40 % over 5 h, compared to 80 % over 1.5 h for the unprotected capsules, showing the cellulose nanofibers offer some degree of oxygen protection, but significantly less than rigid polymer shells.

Fig. 4 Schematic illustration of a the microfluidic device used to form triple-layered capsules and b the triple layered upconverting capsules. Adapted from Ref. [13]

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2.2 Organogels An alternative to encapsulation of fluids inside rigid shells is to provide the fluid solution mechanical stability through gel formation. Gels offer an alternative way to combine the low viscosity of a liquid with the stability and versatility of a solid material. In organogel formation, the gel or crosslinked polymer acts as a host to a liquid solution resulting in a macroscopic solid with microscopic liquid properties. The formation and properties of organogels have been well studied and reviewed [16, 17]. In one example a sorbitol-based gelator (DMDBS) was used with PdTPP and DPA in a tetralin solution [18]. The DMDBS forms a gel consisting of entangled fibril bundles with large volume of interstitial space when heated above 120 °C and cooled to room temperature. The triplet lifetime of PdTPP and the bimolecular quenching constant between the PdTPP and DPA remains unchanged in the gel compared to the tetralin solution, showing the chromophores are still in a solution environment. The TTA quantum yield (UTTA) also remains the same in the gel and liquid solutions, 7 % at one-sun illumination. The gel could be heated and spin coated to form a thin film, but upconversion was quenched on exposure to oxygen. A gel formed by crosslinking PVOH with hexamethylene diisocyanate (HMDI) (2.5–5 % w/w) in a solution of PdMesoIX and DPA in DMF/DMSO resulted in moldable, free standing gels which undergo photon upconversion even when prepared and measured under ambient conditions [19]. Gels prepared and measured under air free conditions had an upconversion quantum yield (UUC) of approximately 14 % (kex = 543 nm, 180 mW/cm2, cw excitation). This quantum yield decreased significantly when the gels were prepared and measured under ambient conditions, but the upconverted emission was still visible to the eye. When the gel was frozen (77 K) the upconverted emission disappeared and only PdMesoIX phosphorescence was observed, confirming that upconversion is the result of molecular diffusion in the room temperature gel. To gain additional oxygen resistance N,N0 bis(octadecyl)-L-boc-glutamicdiamide (LBG) was used a gelator [20]. LBG creates a gel by forming a network of interconnecting, vesicle-like nanofibers with the upconverting solution inside the fiber (Fig. 5). Solutions of PtOEP and DPA in DMF were gelled with LBG by heating to uniformly dissolve all components and cooling to room temperature. This method yielded moldable, freestanding gels that showed no significant decrease in UC intensity even after 25 days of air exposure. Upconversion also occurs at very low power density, with an Ith of 1.48 mW/cm2 in the aerated gel. The upconversion was also thermally reversible, heating to 90 °C above the gel temperature, decreases the upconversion efficiency and cooling to 25 °C recovers the upconversion intensity. This is reproducible over many heating and cooling cycles, but prolonged (\2 h) heating in air leads to a permanent decrease in upconversion efficiency presumably due to increased oxygen diffusion into the hot solution. This gelation method was also shown to be applicable to a range of different sensitizer and acceptor combinations, spanning from yellow to UV emission (Fig. 6). These gels show impressive oxygen stability and a low power threshold, but, as they are solution based, long-term use over months or years without solvent loss might be a potential issue. Reprinted from the journal

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Fig. 5 Illustration of upconverting organogel formed from N,N0 bis(octadecyl)-L-boc-glutamicdiamide (LBG). Reprinted with permission from Duan P, Yanai N, Nagatomi H, Kimizuka N (2015) J. Am. Chem. Soc. 137:1887–1894. Copyright 2015 American Chemical Society

Fig. 6 Digital photographs of sensitizer and acceptor combinations in LBG organogels. Adapted from Ref. [20]

3 Inert Substrates 3.1 Soft Materials A flexible, rubbery polymer is the material environment that is closest to a fluid solution. The sensitizer and acceptors are guests inside the polymer matrix and

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(slow) diffusion can occur through the polymer. The first and most successful examples of upconversion in solid materials used low Tg polymers such as ethylene oxide and epichlorohydrin copolymers and thermoplastic polyurethanes [21, 22]. More recent examples embed the sensitizer and acceptor in a soft polyurethane polymer (ClearFlex 50) [23] and poly butylacrylate elastomer [24]. Previous polymer work has relied on soaking or dissolving the preformed polymer in a solution of sensitizer and acceptor followed by removal of solvent. The advantage of these newer polymer systems is that the sensitizer and acceptor can be mixed with the polymer precursors prior to polymerization. This allows for precise control over the concentration of both components and ensures they are evenly distributed throughout the final polymer. Additionally, any desirable shape or size can be constructed in these instances. Both of the final polymers films show impressive upconversion quantum yields (UUC = 22 % for ClearFlex 50 and 17 % for poly butylacrylate) at moderate cw excitation intensities (200 mW/cm2 for ClearFlex 50 and 100 mW/cm2 for poly butylacrylate) under ambient conditions. 3.2 Rigid Materials Moving from a flexible polymer into a rigid material offers some challenges. In a rigid environment, molecular diffusion isn’t a viable mechanism for TTET or TTA. Instead, one needs to rely on energy/exciton diffusion, which requires higher chromophore concentrations to overcome short exciton diffusion lengths. Initial work demonstrated that upconversion was indeed possible in rigid films, but the efficiency was much less than in fluid solution [25, 26]. Lee and co-workers dissolved PdOEP (0.005–0.5 wt%) and DPA (25 wt%) in molten PMMA and pressed it into a 100–180 lm thick film, which was rapidly cooled to form an optically transparent glass with a uniform distribution of chromophores [27]. Decreased upconversion efficiency was observed in films with higher loadings of PdOEP, as seen in Fig. 7. This is presumably due to the aggregation of the PdOEP in the polymer and subsequent formation of low-energy triplet trap states. Films were stable when stored in the dark under ambient conditions for up to 3 months. In the 0.05 % w/w PdOEP film, nearly linear power dependence (slope = 1.2) was observed at cw excitation power densities as low as 34 mW/cm2. Linear power dependence in upconversion requires significant control of experimental parameters, particularly in fluid solution, but conditions exist in the solid state that make it more likely to be observed. In the PMMA film, molecular oxygen was excluded due to the low oxygen permeability of PMMA and in a rigid solid the nonradiative decay of the sensitizer and acceptor was significantly restricted. These factors, combined with high concentration of DPA (500x [PdOEP]), decrease kT and increase kTT[3A*], which lowered the power threshold required to achieve the strong annihilation limit. A theoretical study has simulated triplet–triplet annihilation in two different arrangements of sensitizers and acceptors in an inert host [28]. In the first arrangement, the sensitizer and acceptor are randomly distributed through the host matrix and in the second arrangement the sensitizers are clustered in islands surrounded by acceptors. The model simulates the creation of the sensitizer excited state, triplet diffusion via a hopping mechanism, TTET between sensitizer and Reprinted from the journal

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Fig. 7 a Digital photograph of, from left to right, neat PMMA and films containing 25 wt% DPA and 0.005, 0.01, 0.05, 0.1, and 0.5 wt% PdOEP. b Emission spectra of the PdOEP/DPA containing films with 543 nm excitation (340 mW/cm2), wt% of PdOEP is given in the legend. Adapted from Ref. [27]

Fig. 8 Illustration of the a random and b clustered arrangements of sensitizers (red) and acceptors (blue). c Optimal upconversion efficiencies (vopt) vs. excited state creation rate (Cc) for the random (red) and clustered (blue) arrangements. Symbols represent different sets of simulation parameters. Adapted from Ref. [28]

acceptor, and TTA of two triplet acceptors as well as the non-productive TTA of two sensitizers, TTA of one sensitizer and one acceptor, and the decay of both triplet states. In both arrangements, and under all simulation conditions, the upconversion quantum efficiency (vopt) decreased as the rate of excited state creation (Cc) increased. In the high creation rate region, TTA between sensitizer triplets that result in the non-productive loss of sensitizer triplets, becomes the dominant decay pathway. In the low creation rate region, however, the clustered arrangement outperforms the random arrangement under all the tested conditions (Fig. 8). This is because, in the random distribution, many acceptor triplets were

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formed in isolation and decayed before they encountered another acceptor triplet, whereas in the clustered distribution acceptor triplets were localized around the sensitizer islands. This result suggests that, especially under low light conditions, an ordered arrangement of sensitizer and acceptor is superior to a random distribution. A caveat of this result is that the simulations did not take into account the aggregation of sensitizers and formation of triplet trap states that are observed in experimental devices. 3.3 Nanoparticles One solution to short exciton diffusion lengths in solid materials is to use nanoparticles rather than rigid films. In an example from Monguzzi and co-workers, PtOEP and DPA were imbedded in pre-formed polystyrene nanoparticles 16 nm in diameter by soaking in a solution containing PtOEP and DPA then rinsing and removing excess organic solvent [29]. The dye loading was, on average, one PtOEP molecule and 50 DPA molecules per nanoparticle as measured by UV–visible absorption. The polystyrene and stabilizing surfactant are both effective barriers against oxygen, upconversion could be observed in aqueous suspensions and thin films exposed to air. The residual photoluminescence decay of the PtOEP was biexponential, indicating the PtOEP was in two local environments, one where TTET to the DPA was possible (8.9 ls component) and one where TTET to DPA wasn’t possible (71.2 ls component). These results suggested that *75 % of the excited PtOEP chromophores successfully undergo TTET to the DPA. Kinetic analysis of the upconverted DPA emission showed a rise time of 16.4 ls, approximately twice the rate of TTET from PtOEP to DPA. This is a good indication that the 3DPA* is formed by energy transfer from the 3PtOEP*. Power dependence measurements using a pulsed excitation source also found that the strong annihilation limit can be reached at low excitation intensities (5.6 mW/cm2). Most interestingly, this study found no differences in performance between aqueous suspensions and thin films of the nanoparticles, showing that each particle acts independently and increasing the concentration of particles has no negative effect on performance. This offers a mechanism to increase chromophore concentration (and light absorption) without aggregation and trap state formation. Taking a different approach to photochemical upconversion with nanoparticles, the Morandeira group studied upconversion on the surface of nanocrystalline ZrO2 films with the hopes of transferring the technology to TiO2 and generating photocurrent rather than emission from TTA. Physisorbing PtOEP and DPA onto sintered films of ZrO2 nanoparticles lead to a very weak (UUC = 6 9 10-4 %) upconverted emission signal under non-coherent, low intensity (8 mW/cm2) illumination due to aggregation of PtOEP and poor orientation of the DPA on the nanoparticle surface [30]. This poor efficiency was improved by covalently linking the DPA to the to the ZrO2 surface through a carboxylate anchor, forcing them to align perpendicular to the ZrO2 surface [31, 32]. The PtOEP sensitizer was dissolved in butyronitrile, and the film and sensitizer solution were sealed under an inert atmosphere. This device architecture showed improved upconversion efficiency (UUC = 0.04 %) under non-coherent, low intensity (8 mW/cm2) Reprinted from the journal

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illumination, and power dependence measurements showed that the device deviated from the weak annihilation limit at intensities \10 mW/cm2. However, the relatively thick pathlength of the device (50 lm) meant that only a small fraction (*3 %) of the excited PdOEP was close enough to the surface to undergo TTET with the surface bound DPA. Anchoring the sensitizer to the nanoparticle surface, rather than relying on diffusion, could potentially lead to further performance improvements.

4 Photoactive Substrates 4.1 Thin Films Rather than embedding sensitizer and acceptor materials in an inert material, using neat films of sensitizer and acceptor materials represents one method to improve triplet exciton diffusion through the material. Karpicz and co-workers doped thin films of DPA with PtOEP and studied them with transient absorption spectroscopy to probe triplet exciton diffusion [33]. The authors found that ISC in PtOEP was faster than the 100 fs experimental resolution, much faster than what was observed in PdTPTBP [34]. Also, crystallization of the DPA and concommitant formation of PtOEP domains lead to slow energy transfer between the PtOEP and DPA. Interestingly, unlike other studies [27, 34, 35], there was no evidence of non-productive TTA within the PtOEP domains. Rather than the traditional inorganic sensitizer, Baldo and co-workers used an organic semiconductor (4CzTPN-Ph) to sensitize a thin film of DPA [36]. The advantage of the 4CzTPN-Ph sensitizer is that it has a comparatively small singlet–triplet gap (DEST \800 cm-1), meaning ISC occurs without a heavy atom and less excitation energy is lost through ISC. A device formed of a 50 nm film of DPA, topped with a 20 nm film of 4CzTPN-Ph did produce upconverted emission when excited at 532 nm. The efficiency was limited by both the poor absorption of the 4Cz-TPNPh, which only absorbed 6.8 % of the excitation light, and poor energy transfer to the DPA (UTTET = 9.1 %). Despite the low efficiency, the magnetic field dependence of the upconverted emission was able to confirm that the emission was likely a result of TTA. 4.2 Sensitizer and Acceptor Materials A strategy in upconverting materials is to incorporate photoactive components, either the sensitizer or acceptor into the host matrix rather than using an inert material. Jankus and co-workers used the commercially available fluorescent poly(paraphenylene vinylene) polymer, super yellow, doped with PdTPTBP, looking towards applications in organic photovoltaic devices [34]. Toluene solutions of the super yellow and PdTPTBP were drop cast to form thin films that were 4 wt% PdTPTBP. Ultrafast transient absorption studies of the films reveals intersystem crossing in the PdTPTBP, with a time constant of 5.7 ps, as well as TTET to the polymer, with a time constant of 930 ps. Nanosecond studies

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revealed, however, that a majority of the excited PdTPTBP was decaying through self-TTA rather than TTET to the polymer. This indicates that the sensitizer was indeed aggregated in the polymer. This was supported by the broadened absorption spectrum of the PdTPTBP in super yellow compared to solution. Kinetic analysis of the delayed fluorescence showed that the TTET yield (UTTET) was at best 24 % and the UUC was 6 %. Building on their previous work on PMMA films [27] Weder, Simon and coworkers have developed a method to incorporate DPA into the PMMA polymer [37]. Methacrylate-substituted DPA (DPAMA) was co-polymerized with methyl methacrylate, yielding polymers with a DPA content that ranged from 8 to 54 wt%. Glassy films with 0.05 wt% PdOEP were formed with thickness ranging from 50–250 lm. The optimal DPA content, as measured by upconverted emission intensity, was found to be 34 wt% due to self-quenching to the DPA fluorescence at higher concentrations. The optimal film (34 wt% DPA and 0.05 wt% PdOEP) also proved to be both air and photo-stable, showing stable upconverted emission over 3.5 h of continuous irradiation, after 4 months of aging. The advantage of incorporating the DPA into the polymer backbone is that a higher concentration of DPA can be used without phase separation or crystallization of the DPA in the polymer film, allowing for quantitative TTET from the PdOEP to the DPA. Another way to incorporate chromophores into a solid material without aggregation is to use metal–organic frameworks (MOFs). A zinc-based MOF, using 4,40 -(anthracene-9,10-diyl)dibenzoate (ADB) as the bridging ligand was synthesized and sensitized with PtOEP [38]. In the MOF structure, the ˚ , as anthracene cores of the ADB are stacked co-facially and separated by 9 A shown in Fig. 9. This arrangement allows for efficient triplet exciton diffusion along the MOF lattice without aggregation. When the ADB containing MOFs are dispersed in a deaerated solution containing PtOEP as a sensitizer upconverted emission at 440 nm can be observed with 532 nm excitation. The MOF could also be sensitized by exchanging carboxylates on the MOF surface with PdMesoIX. The resulting MOF was imbedded in PMMA, resulting in

Fig. 9 a Crystal structure of the synthesized Zn-ADB MOF b Arrangement of ADB ligands in the MOF structure. Adapted from Ref. [38]

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stable photon upconversion with a threshold power density of \1 mW/cm2. These sensitized and encapsulated MOF structures offer an exciting combination of excitation powers accessible by solar irradiance and reasonable quantum efficiencies (UUC = 1.9 % with 5 mW/cm2 excitation). Rather than being detrimental to upconversion, aggregation can also be used to enhance the upconversion process [39]. Cyano-substituted 1,4-distyrylbenzene (CN-DSB) was used as a triplet acceptor for PtOEP. In solution TTET from the PtOEP to the CN-DSB is quantitative; however, no upconverted emission is observed and no long-lived CN-DSB triplet excited state can be detected by nanosecond transient absorption spectroscopy. Crystalline thin films of CN-DSB doped with PtOEP did, however, produce upconversion under an inert atmosphere (Fig. 10) at relatively low excitation power densities (Ith = 14.5 mW/cm2). In solution, the triplet state of CN-DSB is deactivated very rapidly through a conformational twisting around a C=C bond. In the solid state, the rigid environment prevents twisting and the resulting long-lived CN-DSB triplet states can undergo annihilation. These results show that acceptors that undergo aggregation-induced emission (AIE) are also viable candidates for photon upconversion and offer a way to work at high acceptor concentrations without enhancing non-productive acceptor decay pathways. The natural extension of incorporating the acceptor into a material is to incorporate both species into the same material. In one example, PdMesoIX and DPA–(CH2OH)2 were used as cross-linkers in poly(mannitol-sebacate) as shown in Fig. 11 [40]. This resulted in a rubbery polymer with 0.05 wt% PdMesoIX and 34 wt% DPA which exhibited blue fluorescence when excited at 543 nm (32–320 mW/cm2). However, in the crosslinked polymer there was also a significant amount of unquenched phosphorescence from PdMesoIX indicating inefficient TTET between the sensitizer and acceptor. The polymer used is a soft elastomer, meaning molecular diffusion is possible through the polymer matrix, linking the sensitizer and acceptor to the polymer eliminates molecular diffusion. The TTET and TTA processes must therefore rely on triplet exciton diffusion through the polymer.

Fig. 10 a Schematic illustration of aggregation induced upconversion with CN-DSB. In solution the triplet state is deactivated through cis–trans isomerization before it can undergo TTA. In the solid state isomerization is restricted and the triplet states undergo TTA. b Upconverted emission spectra of PtOEP and CN-DSB in degassed THF solution (blue) and in a solid film (red). Adapted from Ref. [39]

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Fig. 11 Incorporation of PdMesoIX and DPA-(CH2OH)2 into a poly(mannitol-sebacate) copolymer. Adapted from Ref. [40]

5 Applications and Device Integration 5.1 Photovoltaic Devices Photon upconversion has long been recognized as a potential mechanism for solar cells to use sub-bandgap photons, potentially exceeding the imposed ShockleyQueisser limit [41–47]. Incorporating an upconverting layer into a single junction photovoltaic device increases the maximum theoretical efficiency from *30 to *45 % under 1 sun illumination and allows for wider bandgap materials to be used, as shown in Fig. 12 [41, 43]. Schmidt, Lips, and co-workers have successfully incorporated photon upconversion layers into amorphous silicon [48–50], organic [48], and dye sensitized [51] photovoltaic devices. In these devices, air-free solutions of a red-absorbing sensitizer, PQ4Pd or PQ4PdNA, and rubrene acceptor were sealed in a 1 cm pathlength cuvette and mounted on the backside of the device

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Fig. 12 The energy conversion limits for a single junction photovoltaic device (dashed line) and an upconverting single junction photovoltaic device (solid line) under 1 sun illumination as a function of bandgap. Reprinted with permission from [41]. Copyright 2013 Society of Photo Optical Instrumentation Engineers

Fig. 13 Diagram illustrating the incorporation of an upconversion solution into a dye-sensitized solar cell. Reprinted with permission from Nattestad A, Cheng YY, MacQueen RW, Schulze TF, Thompson FW, Mozer AJ, Fu¨ckel B, Khoury T, Crossley MJ, Lips K, Wallace GG, Schmidt TW (2013) J. Phys. Chem. Lett. 4:2073–2078. Copyright 2013 American Chemical Society

as shown in Fig. 13. In this arrangement, long wavelength light not absorbed by the photovoltaic device passes through to be absorbed by the upconversion solution. The upconverted emission is reradiated and can be absorbed by the photovoltaic material. These devices all showed small but significant increases in photocurrent, ranging from 2.25 9 10-3 to 0.275 mA/cm2 and plots of incident photon-to-current efficiency vs. wavelength showed the increased photocurrent is due to light absorbed by the upconversion solution. While using a cuvette of upconverting solution is not practical device integration, these studies show that even with nonideal integration upconversion can yield real and measurable improvements in photovoltaic devices of all types.

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5.2 Photocatalysis The idea of photocatalysis is similar to photovoltaic devices, but in this case the energy of the absorbed light performs a chemical reaction rather than generating electricity. Direct excitation of wide bandgap semiconductors such as TiO2 and WO3 produces an oxidizing valence band hole and reducing conduction band electron, capable of many useful chemical transformations such as water oxidation and water detoxification. Unfortunately, UV light is needed to excite these semiconductors, meaning they use very little of the available sun’s solar spectrum. Photon upconversion can be an asset in these applications by converting visible light into UV light that can be absorbed by the semiconductor. As a proof-of-principle, Khnayzer et al. used an air-free solution of PdOEP and DPA to sensitize a WO3 photoanode as shown in Fig. 14 [52]. Under non-coherent visible illumination (k [ 500 nm 32 mW/cm2) the upconversion solution produces Anti-Stokes fluorescence between 400 and 450 nm, which is stochastically collected by the WO3 electrode. With a small positive bias (0.9 V vs. Ag/AgCl) the WO3 electrode was capable of oxidizing water and producing a photocurrent. Control

Fig. 14 a Schematic illustration of an upconversion-powered photoelectrochemical cell. b Normalized photoaction spectra of the photoelectrochemical cell with a degassed (red circles) and aerated (black squares) upconversion solution compared to the absorption spectrum of the PdOEP sensitizer (blue triangles). Adapted from Ref. [52]

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experiments using a non-upconverting, air saturated solution of PdOEP and DPA did not produce photocurrent. A photoaction spectrum of the device in the visible region also matches the absorption spectrum of the PdOEP sensitizer. Together these experiments show that the upconversion solution is able to convert visible light into UV light, which is absorbed by the photoanode to generate photocurrent. The Kim group expanded on this result by using upconverting microcapsules to sensitize platinum-loaded WO3 nanoparticles [53]. Microcapsules containing PdOEP and DPA were suspended with the Pt/WO3 nanoparticles in an aqueous solution and irradiated with a 532 nm laser. The photochemical formation of hydroxyl radicals was detected by the formation of the emissive 7-hydroxycoumarin from coumarin. Control experiments without the upconverting microcapsules resulted in no hydroxyl radical formation. Similarly, Vaiano and co-workers coated upconverting silica nanoparticles containing PdOEP and DPA with n-doped TiO2 nanoparticles. Under green or white LED excitation the particles were capable of decolorizing many common dyes such as methylene blue, crystal violet, and Rhodamine B. This type of semiconductor based photochemistry is important in both solar fuels photoelectrochemistry, where the semiconductor electrode will catalyze fuel-forming reactions and in solar water detoxification, where the semiconductor particles will generate reactive oxygen species such as hydroxyl radicals which then go on to degrade organic and inorganic pollutants and deactivate pathogens in drinking water. 5.3 Fluorescence Imaging Fluorescence imaging, particularly in biological samples, is another application where photon upconversion has great potential to be useful. Using an upconverting dye rather than a traditional fluorescent dye allows for an anti-Stokes detection window, which minimizes or eliminates background sample fluorescence. Upconversion also allows the excitation to shift into the biological window (* 650 1300 nm) where the light penetration depth into tissue is the largest while keeping the emission in the more easily detected visible region. Nanoscale upconversion systems, either encapsulated solutions or solid nanoparticles have proven to be readily used in imaging applications. As mentioned previously, Wohnhaas et al. have used polystyrene-acrylic acid copolymer nanocapsules containing PdOEP and perylene in a hexadecane solution to visualize HeLa cells. The capsules were small enough that they were taken into the cells after incubation [10]. Li and co-workers developed biocompatible silica nanoparticles containing PdOEP and DPA as the sensitizer and acceptor [54]. Dispersed in water these particles had an upconversion quantum yield of 4.5 % (kex = 532 nm, 260 mW/cm2) and showed no photobleaching after 30 min of continuous irradiation. These upconverting nanoparticles were used to stain live HeLa cells and perform in vivo lymphatic imaging in a live mouse. To avoid problematic aggregation with more red absorbers and emitters the authors shifted from solid nanoparticles to nanocapsules with liquid cores [55]. A water stable and bio-compatable shell of dextran and bovine serum albumin contained sensitizers and acceptors in soybean oil. The red absorbing PtTPTBP could be combined with green

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Fig. 15 In vivo fluorescence imaging of live mouse after subcutaneous injection of BD-G containing nanocapsules under different excitation power densities (kex = 635 nm). The signal-to-noise ratio (SNR) is given for each power density. Reprinted with permission from Liu Q, Yin B, Yang T, Yang Y, Shen Z, Yao P, Li F (2013) J. Am. Chem. Soc. 135:5029–5037. Copyright 2013 American Chemical Society

(BD-G) or yellow (BD-Y) emitting boron dipyrromethene dyes to form red-to-green and red-to-yellow upconverting nanocapsules. Aqueous suspensions of these nanocapsules under ambient conditions showed quantum yields of 1.7 % (BD-G) and 4.8 % (BD-Y) under moderate excitation intensities (kex = 635 nm, 106 mW/ cm2). As with the previously reported Si nanoparticles, the nanocapsules were capable of in vivo fluorescence imaging in live mice with excitation intensities as low as 12 mW/cm2 (Fig. 15). 5.4 Other Emerging Applications While the above applications are the ones most commonly studied with photochemical upconversion they are not the only viable candidates. Similar to biological imaging, photodynamic therapy is a technique which can benefit from the incorporation of photon upconversion. In photodynamic therapy a drug or protodrug is activated by the absorption of light. Upon absorption of light the drug can produce reactive singlet oxygen species, form an activated drug that can damage cells/DNA or some combination of both. The difficulty of photodynamic therapy is that the visible wavelengths of light often needed for this chemistry have very limited penetration through tissue. Incorporating upconversion into photodynamic therapy allows for red or NIR excitation. Bonnet and co-workers have created upconverting PEGylated liposomes containing PdTPTBP and perylene as the sensitizer and acceptor [56]. These upconverting liposomes were mixed with ruthenium-functionalized liposomes, which contained a photoactivatable ruthenium complex [Ru(tpy)(bpy)(SRR’)]2? (tpy = 2,20 :60 ,200 -terpyridine, bpy = 2,20 -bipyridine, SRR0 = thioether-cholesterol ligand). Irradiation with blue light cleaves the Ru–S bond and frees [Ru(tpy)(bpy)(H2O)]2? from the liposome. Irradiation of the mixture of liposomes with red light (kex = 630 nm) lead to the formation of [Ru(tpy)(bpy)(H2O)]2? in solution (Fig. 16). This photo-induced release strategy is well suited to photodynamic therapy agents, or could be used to use to encapsulate and release traditional drugs in selected areas or the body.

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Fig. 16 a Schematic illustration of photon upconversion in the liposome bilayer followed by photorelease of [Ru(tpy)(bpy)(H2O)]2?. b Absorption spectra of liposome mixture during photolysis showing the formation of [Ru(tpy)(bpy)(H2O)]2? at 490 nm. Blue dashed line is at t = 0 and red solid line is at t = 240 min. Adapted from Ref. [56]

It is well known that the sensitized photon upconversion process is highly sensitive to oxygen, making it an ideal oxygen sensor. Upconverting solutions containing PtTPTBP or PdTPTBP as the sensitizer and Solvent Green 5 (SG-5) were adsorbed into porous glass beads and imbedded into an oxygen permeable polymer (silicon or Teflon AF) [57]. Because the upconversion device has two modes of emission, upconverted fluorescence and residual phosphorescence from the sensitizer it is capable of accurately detecting oxygen over a much wider dynamic range (pO2 trace—40 kPa) than traditional oxygen sensors. At low oxygen concentrations (\0.2 kPa O2) the upconverted fluorescence shows significant quenching, and nearly quantitatively quenched by 1 kPa O2. The residual phosphorescence signal is much less sensitive; the emission signal is detectable at concentrations as high as 40 kPa O2. Unfortunately, the SG-5 also proved susceptible to photodegradation by singlet oxygen, and devices degraded under constant illumination in the presence of oxygen. Moving from a perylene-type acceptor to one more resistant to degradation in the presence of oxygen is a relatively simple way to improve the long-term stability of these materials.

6 Future Directions To further improve upon photochemical upconversion in materials, and its incorporation into devices and other applications there are several factors to consider. For all types of usages, with the exception of oxygen sensing, exclusion of oxygen is critical for both maximizing upconversion efficiency and long-term stability. This has been achieved either through oxygen impermeable materials, or by the addition of oxygen scavengers. In biological applications such as imaging and photodynamic therapy, water and biological compatibility as well as low toxicity represent important mandated properties. Long-term stability over months or years is not required for these applications, especially when used in living

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subjects. Ideally, after the imaging or drug delivery process has been completed, the upconverting materials would ideally break down and/or be excreted from the body. Further development of materials with small particle sizes that can cross membranes into cells and materials with target-specific coatings for localized drug delivery or imaging represent desirable goals for future research endeavors. In biological applications, low power thresholds are not critical because ambient light (or solar photons) isn’t the desired excitation source, but using excitation wavelengths suitable for biological tissue windows is important. For integration into solar cell and photocatalysis devices, long-term stability over years of operation appears crucial. This potentially limits the use of fluidic-based materials such as organogels. In order for these devices to operate using the solar spectrum as the excitation source, a low power threshold is required to get the maximum upconversion efficiency out of low excitation power densities. In photovoltaic devices the sensitizer absorption should be in the red and NIR, below the bandgap of the photovoltaic material, and the acceptor emission should be efficiently absorbed by the photovoltaic material (i.e., within the bandgap). In photocatalytic upconversion applications combinations of sensitizers and acceptors that convert the visible light of the solar spectrum into UV light that can sensitize wide-bandgap semiconductors are also needed. One major difficulty of incorporating upconversion into materials is need for molecular diffusion. As of yet, none of the reported solid upconversion materials come close to matching the upconversion quantum yields that have been reported in solution or soft polymers, where molecular diffusion is readily enabled. These solid materials must rely on triplet exciton diffusion for both TTET and TTA, meaning concentrations of sensitizer and acceptor used must be significantly higher than in solution. Theoretical studies suggest that the best way to overcome this is to have ordered domains of sensitizer and acceptor so that TTET and TTA can occur at or near the interface between sensitizer and acceptor domains. In experimental devices, however, sensitizer aggregation and trap state formation, as well as acceptor selfquenching ultimately limit material performance. Considering all of the work completed on upconverting materials there has been a very limited selection of sensitizer and acceptor chromophores that have been evaluated. With few exceptions the sensitizers have remained in the Pt(II) and Pd(II) porphyrin family and the acceptors are mostly anthracene derivatives although other polycyclic aromatic hydrocarbons have been investigated. Diversifying the selection of chromophores used in photochemical upconversion materials should enable marked expansion of excitation and emission wavelengths while minimizing aggregation in sensitizers and/or acceptors in addition to improving oxygen stability of the requisite compositions. The recent observation of triplet-triplet energy transfer from semiconductor nanocrystals to molecular acceptors [58] implies new classes of triplet sensitizers are immediately available for incorporation in upconversion schemes. Acknowledgments This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0011979. Some of the work on solid-state upconversion performed in this laboratory was supported by the Air Force Office of Scientific Research (FA9550-13-1-0106).

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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

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Top Curr Chem (Z) (2016) 374:22 DOI 10.1007/s41061-016-0022-6 REVIEW

Metal–Organic and Organic TADF-Materials: Status, Challenges and Characterization Larissa Bergmann2 • Daniel M. Zink2 • Stefan Bra¨se1,3 • Thomas Baumann2 • Daniel Volz2

Received: 17 November 2015 / Accepted: 21 March 2016 / Published online: 7 April 2016  Springer International Publishing Switzerland 2016

Abstract This section covers both metal–organic and organic materials that feature thermally activated delayed fluorescence (TADF). Such materials are especially useful for organic light-emitting diodes (OLEDs), a technology that was introduced in commercial displays only recently. We compare both material classes to show commonalities and differences, highlighting current issues and challenges. Advanced spectroscopic techniques as valuable tools to develop solutions to those issues are introduced. Finally, we provide an outlook over the field and highlight future trends. Keywords OLED  Copper(I)  Organic  Singlet harvesting  Thermally activated delayed fluorescence

This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli and Henk Bolink. & Stefan Bra¨se [email protected] & Thomas Baumann [email protected] & Daniel Volz [email protected] 1

Institute of Toxicology and Genetics, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany

2

CYNORA GmbH, Werner-von-Siemensstraße 2-6, building 5110, 76646 Bruchsal, Germany

3

Institute of Organic Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany

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1 Introduction: Thermally Activated Delayed Fluorescence 1.1 History Emitting materials for organic light-emitting diodes (OLEDs) have been a significant focus of academic and industrial research in the last few years. In this respect, thermally activated delayed fluorescence (TADF) represents a promising concept to harvest both singlet and triplet excitons without the use of heavy and usually rare (low abundance in the earths crust) metals such as iridium or platinum [1]. Excellent device performance based on TADF—instead of phosphorescence— with devices exhibiting more than 20 % external quantum efficiency (EQE) [2, 3] has been demonstrated with metal-free materials [4–6] or copper complexes [7, 8]. This puts both organic and copper-containing emitters on par with modern iridium materials in terms of efficiency. Thermally activated delayed fluorescence (Figs. 1, 2, 3) has first been reported for the organic molecule eosin [9] in the early 1960s (‘‘E-type’’ delayed fluorescence), but later also investigated in fullerenes [10], and tin porphyrin and copper(I) complexes [11, 12] (Fig. 1). For eosin, Parker and Hatchard had discovered a second band of long-lived emission besides phosphorescence and, due to its identical contour with the nanosecond fluorescence band, assigned this to a delayed fluorescence from the same excited singlet state. However, the TADF mechanism in eosin is only partly effective, as indicated by a rather large energy separation between the lowest excited singlet and triplet states of around 3000 cm-1 [9]. Also, the early copper(I) complexes, for which McMillin proposed an emission mechanism of two excited states in thermal equilibrium, exhibited only low efficiencies that are far from suitable for practical use in OLED devices [11, 13]. In

Fig. 1 Milestones from the history of TADF materials: eosine [9], bis(phenanthroline) copper(I) complexes [13], fullerene derivatives [14], and tin(IV) porphyrin complex [12]

R

R

2 Na COOBr

Br

N

N Cu

N -

O

O

O Br

Br

N R

R [Cu(dmp)2 ]+, R = H [Cu(bcp)2]+, R = Ph

N F N Sn N F N

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N

O

P

P

O

S

N

PPT N

N

exciplex from two compounds

m-MTDATA

N NC

CN

N

Ph2P

N N

PPh2 I Cu Cu I N PPh2

Fig. 2 The first purely organic TADF materials applied in OLEDs were based on exciplex emission from an electron-donating and an electron-accepting material such as m-MTDATA and PPT [19], but recently, research has focused on the promising approach of combining acceptor and donor moieties in one molecule. 4CzIPN is a highly efficient TADF material with bright green emission [6]. Furthermore, copper(I) complexes have gained increasing interest as TADF materials due to their large abundance compared to the Ir(III) or Pt(II) compounds in common use today—here represented by a dinuclear copper(I) complex [7]

1996, Berberan-Santos reported on the delayed fluorescence in fullerene derivatives [10], and later derived rate equations to describe the time-resolved processes of the TADF mechanism [14]. It was not until 2009 that TADF achieved a breakthrough in the design of emitting materials for OLEDs, when a Sn(IV)-porphyrin complex was applied to an optoelectronic device [12], and shortly afterwards a copper(I) complex was shown to give an external quantum efficiency of 16.1 % in an OLED by using both singlet and triplet excitons [8]. Nowadays, particularly designed organic materials with donor–acceptor structures or in copper(I) complexes with intrinsically moderate spin–orbit coupling are used to achieve a small energy separation between the lowest excited singlet and triplet states (Fig. 2). For organic materials, a good thermally activated delayed fluorescence (TADF) efficiency is accomplished by breaking the conjugation between the donor and acceptor moieties of the molecule, which can either be approached by two separate organic materials giving exciplex emission, or lately, by

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Top Curr Chem (Z) (2016) 374:22 Fig. 3 Mechanism of thermally activated delayed fluorescence: In contrast to fluorescent materials, both singlet and triplet excitons can be used for the generation of light due to a reasonable intersystem crossing and small energy separations DEST. In TADF emitters a thermal up-conversion from the triplet T1 back to singlet state S1 is observed at ambient temperature, giving rise to thermally activated delayed fluorescence, also called singlet harvesting technology. This ensures short emission decay times despite small spin–orbit coupling

designing a material with both moieties in one molecule. Device records of up to 31 % external quantum efficiency (EQE) for green emission comprising an organic TADF material support the success of this latter approach [15]. Also with copper(I) complexes, EQEs of up to 23 % have been reported for a solutionprocessed, green device, which exceeds the performance of comparable Iridium(III)-based devices [16]. The modular design of copper(I) complexes to tune the emission color, photoluminescence efficiencies as well as solubility [17], and their low susceptibility to concentration quenching (reduced quenching even when aggregates are formed during solution processing [18],) makes TADF copper(I) materials the ideal candidates for solution-processing techniques. 1.2 Basic Concepts of TADF and Material Design 1.2.1 Harvesting Excitons Via Delayed Singlet Emission It is not within the scope of this article to explain the different exciton harvesting mechanisms in detail. We recommend the following reference for more information [20]. In summary, TADF enables the use of both singlet and triplet excitons, thus exceeding the EQE limit of OLEDs with fluorescent materials by a factor of 4 (the spin statistic giving 25 % singlet and 75 % triplet excitons, see also Fig. 3). In efficient TADF emitters, a small energy separation DEST between the lowest excited singlet and triplet states and a sufficient intersystem crossing (or reverse intersystem crossing, respectively) between these states enable the delayed emission. In contrast to triplet harvesting materials based on the heavy-metals Ir(III) or Pt(II), purely organic materials or copper(I) complexes exhibit only small spin–orbit coupling (e.g., nC = 32 cm-1, nCu = 857 cm-1 versus nPt = 4481 cm-1,

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nIr = 3909 cm-1, with the spin–orbit-coupling constant n) [21], resulting in slow intersystem crossing (ISC) and a largely forbidden T1 ? S0 transition (phosphorescence). Indeed, the long-lived triplet state facilitates the back-transfer of excitons to the lowest excited singlet state. According to Berberan-Santos, TADF occurs when there is a ‘‘reasonably high probability of S1 ? T1 intersystem crossing and reasonably high probability of subsequent S1 / T1 back intersystem crossing’’ [22], i.e., a certain population of the triplet state, which has to be stable and longlived, and a small energy gap between the states S1 and T1 to ensure sufficient backtransfer (Fig. 3). To interpret the photophysical properties of TADF materials and to derive material design rules, the mechanism of TADF has to be thoroughly understood: In an OLED, both singlet and triplet excitons are formed, as indicated in Fig. 3. The TADF emitter can either deactivate via spontaneous fluorescence S1 ? S0 (singlet excitons) or intersystem-cross from the triplet T1 ? S1, followed by delayed fluorescence (triplet excitons, Fig. 3). The extent of these transitions depends on the ratio of their decay rates to each other, i.e., in organic TADF materials the intersystem crossing is a sluggish process and thus both spontaneous fluorescence on the ns-timescale and delayed fluorescence on a longer timescale are observed, while for copper(I) complexes with an ISC process of several picoseconds only delayed emission will be detected and no such spontaneous fluorescence on the nstimescale will be observed. This is due to the fast equilibration between S1 and T1. In Sect. 3, we will discuss ultrafast processes on the ps-timescale, which can found for copper(I). After residing in the triplet reservoir, the excitons deactivate to the singlet ground state or, at sufficiently high temperatures, can overcome the small energy gap DEST by thermal activation (kBT) and transfer back to the excited singlet by reverse intersystem crossing (RISC), from where they finally deactivate via delayed fluorescence. By TADF, both singlet and triplet excitons can be used for the generation of light via the excited singlet state S1. While TADF is a widely used concept that describes a photophysical mechanism, occasionally the term singlet harvesting is used when referring to OLED with TADF emitters. 1.2.2 Minimizing the Singlet–Triplet Splitting DEST The determining factor for efficient TADF is a small energy separation DEST between the lowest excited singlet S1 and the triplet state T1. This facilitates a high proportion of delayed fluorescence as well as short emission decay times, which are crucial for a small roll-off in OLED devices. In order to reduce DEST, the exchange integral between S1 and T1 has to be minimized, which can be achieved by a spatial separation of the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) on a donor and acceptor part of the molecule.1 A small overlap of frontier orbitals is usually accomplished via charge-transfer (CT) emission. Although no clear limit can be

1 It needs to be said that there are also organic TADF emitters in which DEST is not controlled by this, for example due to symmetry-related mechanisms [127].

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Top Curr Chem (Z) (2016) 374:22 Table 1 Performance limitations for TADF emitters Organic emitters

Cu(I) emitters

Spin orbit coupling constant

Low

Medium

Rate-limiting factor for ISC

Non-adiabatic coupling

Spin orbit coupling

Limiting factor for short emission decay times

(reverse) ISC

Oscillator strength S1 ? S0

defined, energy splittings DEST between the excited singlet and triplet state above 0.2 eV (1600 cm-1) usually shut off the TADF [23]. Table 1 gives an overview on various limiting factors for the efficiency of TADF emitters. As mentioned above, spin–orbit coupling (SOC) is much more effective for copper than for carbon. Nonetheless, efficient TADF emitters with a short excited-state lifetime have been realized with metal-free materials. The reason for this is not yet fully understood, but sometimes explained with so-called vibronic or non-adiabatic coupling of electronic and nuclear vibrational motions in molecules, which is supposed to result in a moderate intersystem and reverse intersystem crossing when the energy separation DEST is small [24]. In fact, even the substitution of organic materials with heavy elements such as bromine with a large SOC constant does not significantly change the excited state dynamics [25]. While the intersystem crossing in metal-free TADF emitters is mostly limited by nonadiabatic coupling, the spin–orbit coupling is more relevant for Cu(I) emitters, which leads to the observation that even Cu(I) compounds with rather large DEST can show efficient TADF. However, the oscillator strength for the transition S1 ? S0 is lower for Cu(I) compounds than for organic materials, also seen from the slightly weaker CT absorption band of Cu(I) complexes when taking the correlation of the molar absorbance and the oscillator strength for a given electronic transition into account. For example, the observable extinction coefficient of the charge transfer transition from S1 / S0 of a typical organic TADF emitter often is in the order of 105 M cm-2 [6], while for typical Cu(I) complexes, only 103 M cm-2 to 104 M cm-2 are found with UV VIS spectroscopy [8]. This indicates significant differences in the oscillator strength for the transition S1 ? S0, too. Thus, the ratelimiting factor for short emission decay times is rISC in the case of purely organic materials, while the oscillator strength for S1 / S0 controls the emission decay times for luminescent Cu(I) complexes because the ISC and rISC are much faster compared to metal-free systems. For organic TADF materials, reducing the overlap of frontier orbitals is achieved by breaking the conjugation between the donor and acceptor moieties of the molecule, visualized by time-dependent DFT calculations (Fig. 5). Since in organic materials the spin–orbit coupling (SOC) is weak, the intersystem crossing S1 ? T1 and the fluorescence S1 ? S0 are competitive processes in the nanosecond order, with their ratio controlling the extent of TADF versus spontaneous fluorescence. A strong charge-transfer character with small overlap of the frontier orbitals not only minimizes DEST, but also induces a mixing of the singlet and triplet states and thus

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HOMO

N

N NPh

N

LUMO

N N

Ph2 P I Cu Cu I N Me2

PhN

Ph2 P N Me2

Fig. 4 The key to achieving a low DEST is to have a situation where HOMO and LUMO are well separated. In organic molecules, this is achieved by using donor- and acceptor-moieties known from host and charge transport materials, while Cu(I) complexes usually have metal or metal-halide units as donors and ligands as acceptors

Fig. 5 Highest occupied and lowest unoccupied natural transition orbitals (NTO) for the S1 state of 4CzIPN, calculated by time-dependent DFT, indicate its charge-transfer character, with the carbazole moieties as electron donors and the phthalonitrile backbone as the acceptor [5]

N CN

NC N

N N

enhances intersystem crossing for a high TADF efficiency. Typical electrondonating moieties used in organic TADF materials are amine-type structures such as carbazole and its derivatives, arylamines, phenoxazine and acridine, while phthalonitrile, ketone and triazine structures are applied as electron-acceptors (see also Sect. 2). As in the highly efficient 4CzIPN, the donor and acceptor moieties can be linked in the ortho-position to break the conjugation (Fig. 4), but also the spiroconjunction or a twist due to steric hindrance are commonly used motifs to ensure a strong charge-transfer character. By deliberate design of the molecule structure,

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organic TADF materials with energy separations DEST down to 0.02 eV and short emission decay times of the delayed component of only 1.0 ls in a host matrix were reported [12, 27, 28]. Also for many luminescent copper(I) complexes the emission was found to be of TADF character due to often [29] small energy separations DEST around 0.1–0.2 eV [26, 30–32], controlled by the complex structure and the respective ligands. In these complexes, the emission is usually of a distinct metal-to-ligand charge-transfer (MLCT) character, where the metal center(s) acts as an electron donor and the ligand(s) as acceptor moieties with spatially separated frontier orbitals per se. Predicting the localization of frontier orbitals and thus deriving design rules for a small exchange integral between S1 and T1 is challenging, since this strongly depends on the complex geometry (mononuclear or polynuclear, tetrahedral or trigonal), the energetic states of applied ligands and/or halides, and their orientation [33]. However, the stronger spin–orbit coupling in copper(I) complexes compared to purely organic TADF materials leads to fast intersystem crossing in the picosecond regime, which then prevails the spontaneous fluorescence. Because of this, TADF is possible even for relatively large energy separations DEST, which gives more flexibility for material design. In general, luminescent copper(I) complexes are obtained by applying N^N, P^P, N^P or carbene ligands (see also Sect. 3). In the blue-emitting copper(I) complex [(pz2BR2)Cu(POP)] (Table 4), the HOMO is localized primarily on the copper(I) center and partially on the P-atoms of the bis(phosphine), while the LUMO lies on the arylether backbone of the P^P ligand, thus ensuring a spatial separation of frontier orbitals. Dependent on the substituent R of the pyrazolyl-borate, the energy separations DEST vary between 0.10 and 0.16 eV, while the TADF emission decay times are between 13 and 22 ls. In recent years, highly efficient copper(I) complexes with photoluminescence quantum yields (PLQY) up to 100 % and short excited-state lifetimes down to 1 ls have been reported [34–36]. By thermally activated delayed fluorescence these copper(I) materials can harvest both triplet and singlet excitons for the emission of light and achieve internal quantum efficiencies of 100 % [7, 8]. Besides the TADF efficiency, also the luminescence efficiency is an important factor for the design of emitting materials. To achieve high PLQYs, the nonradiative processes have to be minimized; especially for the long-lived triplet states. A few design rules can be applied such as high steric demand of the ligands to induce rigid complex geometries and suppress excited state distortions [30], or avoidance of CH2- and CH3-groups to prevent vibrational quenching [37]. To sum up, the TADF mechanism enables short emission decay times in the low microsecond regime for emitting materials of small or moderate spin–orbit coupling, and enables the use of both singlet and triplet excitons in OLEDs to achieve theoretical internal quantum yields of 100 %.

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Top Curr Chem (Z) (2016) 374:22 Table 2 Status quo of the processing methods for TADF emitters as of the year 2015 Organic emitters

Cu(I) emitters

Evaporation

???

??

Coating

?

???

Printing

-

??

1.3 Status Quo: Processing Techniques for TADF Emitters 1.3.1 Vacuum Versus Solution Processing Besides the performance, the processing of materials is of vital importance for the development of commercial OLEDs. While currently used vacuum deposition techniques offer some advantages such as the possibility to produce OLED stacks with well-defined interfaces and a high reproducibility, these techniques go hand in hand with major drawbacks such as high material intensity, and the produced devices are prone to defects that can render them unusable (as detailed in Sects. 1 and 2) [38]. Hence, new techniques are developed based on wet processing, for example ink-jet printing or slot-die coating. Printing as well as coating are not only environment-friendly, economical and fastprocessing techniques, which are suitable for large scale fabrication and incorporation into roll-to-roll fabrication lines but also offer lower production costs by limiting wastage of materials, faster processing speeds, ambient condition processing and roll-to-roll production (as detailed in Sect. 1.3) [1]. While vacuum processing is considered the only viable commercial processing method at present, solution-processing techniques are expected to become more important for the realization of large-area lighting panels and for micro-structured display applications. Table 2 gives an overview on the current state-of-the-art processing techniques for both organic and Cu(I)-based TADF emitters. At this point, only limited results on the use of industrial solution-processing techniques of TADF-materials are available. Most scientific literature describes vacuum-processed devices, while few researchers investigated solution-processed devices prepared by spincoating. Often, TADF materials are only moderately soluble, which does not allow for careful ink engineering, required for more sophisticated, industrial printing and coating techniques. As a rule of thumb, a material needs at least a minimum solubility of 10 g L-1 in a medium-to-high boiling point solvent (bp 100–200 C) such as xylene, anisole, or—within some boundaries—toluene. Not all TADF emitters are suitable for vacuum processing due to a low thermal stability. A thermal stability of more than 250 C and a relatively high volatility are required. This is usually achieved by using materials with a limited molecular weight of up to 1200 g mol-1 for thermal evaporation. Many copper(I) complexes are much heavier, easily surpassing 2000 g mol-1. Even when low-weight materials

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are used, the cleavage of bonds (organic) [39] or the formation of thermodynamic traps and byproducts (copper) [40] can prevent the vacuum processing of materials. Besides obvious criteria such as material stability during processing, as well as the device operation, new materials for mass market applications have to face and overcome previously mentioned challenges: They have to be more efficient as current state-of-the-art emitting compounds, and they should be processable from solution. As mentioned above, two classes of materials are currently believed to have the potential to be the next generation emitter materials: pure organic materials and materials based on copper. High photoluminescence quantum yield values and particularly a short-excited states lifetime are crucial for both classes of materials in order to achieve highly efficient devices with only a small efficiency roll-off behavior. 1.3.2 Processing of Organic Materials Many purely organic TADF-emitters are poorly soluble in solvents commonly used for solution-processing techniques. Until today, there are only a few results reported in the literature on the solution processing of metal-free TADF emitters, which is related to the rather high solubility that is required for industrial-scale wet processing. With one of the most commonly used emitters, 4CzIPN [6], an external quantum efficiency (EQE) of over 26 % has been achieved for a vacuum-processed device [41]. Solution-processed devices with the same emitter yielded 15 % EQE [42] and 20 % EQE [43], respectively. In those studies, the maximum solubility of 4CzIPN was reported to be 10 mg mL-1 in solvents such as dichloromethane and tetrahydrofuran, both being too volatile to be used in industrial printing. One strategy is to develop tailor-made materials for solution-processed TADF-devices, e.g., by modifying existing structures with alkyl substituents [44] or designing entirely new emitter structures, which are intrinsically more soluble due to reduced intermolecular interactions [45]. However, the solubility that has been achieved with these strategies is too low to allow for ink-development for coating or printing techniques. More importantly, only highly volatile solvents have been used so far, which cannot be used in advanced solution-processing approaches for mass production. Another common approach to generating more soluble, morphologically stable OLED materials is the transformation of small-molecule systems to polymers. Very recently, Nikalaenko and co-workers published the first organic TADF-type polymer (Fig. 6). In this work, the TADF-unit is generated by connecting a triazine-type acceptor-monomer A and a triarylamine-type donormonomer D in a conjugated polymer backbone. The authors use an additional backbone spacer B to dilute the emitting species in the block A–B acting as host, in which the emitting block A–D is embedded. A promising efficiency of 10 % EQE was achieved. Apart from the polymeric TADF emitter discussed above, organic materials are easily processable by vacuum deposition techniques. However, a high thermal stability as well as volatility is required. Especially sulfones, which are used as acceptor moieties in efficient deep-blue-emitting materials, seem to show some drawbacks in terms of thermal stability (Sect. 2).

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1.3.3 Processing of Cu(I) Materials For the application in OLEDs, copper(I) complexes can either be processed by vapor deposition or from solution e.g., with coating or printing techniques. For deposition from the gas phase, many copper emitters exhibit high molecular weights, which often render them not volatile enough for sublimation. Nevertheless, various Cu(I)-emitters are suitable for vacuum processing [46, 47]. Apart from a low molecular weight, preferably lower than 1000 g mol-1, there are some guidelines for sublimability: •

• •

Use of chelating ligands. Cu(I) exhibits an extremely rich structural chemistry. In order to prevent the undesired formation of side products or thermodynamic traps, chelating or bridging ligands are crucial to obtaining kinetically stable structures. It is known that monodentate ligands may be extruded upon heating under low pressure. Use of rigid structures. Most Cu(I) complexes that are suitable for vacuum processing feature rigid, tight structures without flexible side chains. Use of neutral compounds with a small dipole moment. It is obvious that evaporation of charged compounds requires more thermal energy than the evaporation of neutral compounds. This being said, the polarity, dipole moment, and polarizability of the emitter should be small to minimize the intermolecular interactions in the bulk material, which need to be compensated during the evaporation. C8H17

N N N

a

N

c

N C8H17

backbone B

donor D C12H25

b

acceptor A a : b : c = 27 : 30 : 3

-A-B-A-B-A-B-D-A-D-A-D-A-B-A-B-A-B-A-

Fig. 6 Organic TADF-type copolymer from Nikolaenko and co-workers

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The solution processing technique can be considered state-of-the-art for Cu(I) compounds. Unlike organic emitters, most efficient Cu(I) emitters are also soluble in a broad array of polar and nonpolar solvents [17]. This enabled not only very efficient, solution processed Cu(I) OLED devices with more than 23 % EQE (processed by coating techniques), but also inkjet-printed devices with EQEs of up to 16 % [13]. Potential issues arise from the vulnerability of the emitting materials towards water/oxygen and potential degradation. Furthermore, the rich structural variety of Cu(I) complexes can also turn out to be disadvantageous when the molecular structures, the local structure found in single crystal phases, is not necessarily the same as in solution or amorphous films: Copper(I) complexes may in fact dissociate or undergo ligand exchange reactions to form other complex motifs or even mixtures of several complex species in solution. This is especially detrimental for processing techniques from the liquid phase. One needs to ensure that structurally stable complexes are used in order to prevent the formation of several species, an aspect that will be further discussed in Sect. 3.

2 Organic Materials: Design Principles and Challenges 2.1 Essential Donor Structures In Sect. 1.2, we introduced the concept of spatial separation of HOMO and LUMO as key for the realization of efficient organic TADF emitters, which is achieved by linking donor- and acceptor moieties in a geometry that prevents conjugation between these units. This general concept is not entirely new: During the last decade, the same approach was used to create so-called ambipolar host materials, in which both holetransporting donor- and electron-transporting acceptor moieties are present [48–52]. Donors and acceptors are often conjugated in these compounds, though, resulting in rather large DEST-values. In this section, we will start with the introduction of commonly used donor structures, which are mainly based on diphenylamine-type moieties. In Sects. 2.2– 2.4, we will highlight several noteworthy examples of organic emitters showing TADF, organized by the respective acceptor unit. In a closing section, we will summarize some issues on the theoretical prediction of TADF for organic materials (Sect. 2.5). Basic donor structures are summarized in Fig. 7. All known donor structures found in modern organic TADF emitters contain the diphenyl-amine moiety due to its strong electron-donating character. It might be functionalized with a tert-butyl group to decrease molecular packing or slightly shift the emission color [53, 54]. Often, the two phenyl rings of the diphenylamine unit are fused by introduction of a bridging atom (O, S, N, CR2) to form a ring-system, such as in acridine [55], phenazine [4], phenothiazine [56], or phenoxazine [57] or—most commonly—are directly connected to form a carbazole ring [5].

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diphenylamine-type Ph N N

N

O

S

N

N

N

carbazole-type Ph

N

N

Ph

N

Ph N N

N

PhN

N

N

Fig. 7 Basic donor moieties in organic TADF emitters

Figure 8 gives a schematic overview of three concepts for the design of TADF emitters. The most commonly used among these is the multi-donor approach: Often, more than one donor unit is present and the donor–acceptor ratio ranges from 1:1 to 4:1 [6, 58]. So far, no systematic reason to favor a certain donor–acceptor ratio has been presented. It has been shown that for some systems, an increasing number of donors seems to be favorable [59–61], while the way the donors and acceptors are connected is also of great importance [6]. Another approach is the fusion of two small TADF emitters to a larger one via a single bond between the acceptors [59]. Lastly, in some cases the introduction of 1,4-phenylene as a spacer has been proven to be favorable [62]. This will be further discussed in Sect. 2.3. 2.2 Arylnitriles as Acceptor Moieties Arylnitriles are used as acceptor moieties in the most efficient and stable TADFemitters so far. The work of Adachi et al. initially presented a class of several

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Fig. 8 Important concepts for the design of TADF emitters

N

N

acceptor

dual core emitters

N

N

acceptor

acceptor

spacer-systems

N

N

acceptor

acceptor

carbazole-substituted benzonitriles [6]. The substitution pattern of the acceptor was varied broadly: phthalonitrile (PN) and isophthalonitrile (IPN) as acceptors were compared in terms of emission color, photoluminescence efficiency, and TADF effectiveness. Also, the substitution with two and four carbazole moieties was investigated, and furthermore the carbazole units were substituted with alkyl and aryl substituents. Out of all these potential modifications, tetrakis-N-carbazoylisophthalonitrile (4CzIPN) gave the best performance: The material emits a bright green with a PLQY of more than 80 % and exhibits an excited state lifetime of a few microseconds (delayed component) [6]. In another study, 4CzTPN (TPN = terepthalonitrile) [25] was synthesized, which is slightly less efficient than 4CzPN. The most efficient OLED device with this compound showed an external quantum efficiency (EQE) of more than 30 % [63]. In an optimized device, promising stability values were reported: at a starting luminance of 1000 cd m-2, a LT90 value of more than 200 h was demonstrated in 2013 [64]. Several other strategies to modify this highly efficient structure have been investigated, e.g., for di-carbazoyl-isophthalonitrile (DCzIPN) and its dual-cored dimer DDCzIPN (see Sect. 2.2) or by the use of cyano-pyridines instead for benzonitriles [65] (Fig. 9).

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CN Cz

NC

Cz

Cz

CN

CN

Cz

NC

Cz

Cz

CN

CN

Cz

Cz

Cz

CN Cz

DCzIPN and DDCzIPN

4CzIPN

CN Cz

Cz

Cz Cz

N

Cz

Cz

Cz

Cz

4CzTPN

Cz

Cz

Cz

NC

N

CN Cz Cz

CN

2CzPN

Cz

CN

CN CN

4CzPN

Cz N

Cz

CN

Cz =

Ph

CN

pyridine-type TADF emitters

Fig. 9 Arylnitrile-type TADF emitters

2.3 Aryltriazines as Acceptor Moieties Another important class of acceptor moieties besides arylnitriles (2.2) are 1,3,5triazines (Fig. 10). Basically, triazine as a strong electron-accepting unit has been investigated in combination with all donor-moieties of Fig. 7 to design TADF emitters [28, 57, 58, 60, 61, 66–71]. For a concise review of this class of materials, we suggest the literature [72]. Thereby, the number of donors attached to the triazine core was varied between one and three, as can be seen in Fig. 10 [61]. The substitution pattern determines the efficiency, excited state lifetime, and color of the TADF molecules. As we discuss in Sect. 2.5, the general understanding of the structure–property relations needs to be improved though: Many of the great successes in organic TADF emitters seem to be a result of empiric optimization and the synthesis and screening of many different molecules rather than a deep understanding of the underlying mechanisms. Usually, phenyl or biphenyl substituents on the triazine-unit function as innocent/ spectator groups, which are easily synthetically accessible, but do not affect the emission color too much. To further vary the emission characteristics of the TADF emitters, phenyl spacers between the acceptor and donor moieties were introduced and the photophysical properties of the emitting materials examined [61]. While there are some examples for phenyl-free triazine-type TADF materials, this approach does not necessarily lead to emitters, but also potentially host materials when the HOMO–LUMO overlap is too large, resulting in a larger DEST [69]. So Reprinted from the journal

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N

donor

N

donor

N

N

N

donor N

donor N

N NPh

N

donor N

N N

N

N

PhN

donor

Ph PIC-TRZ

Fig. 10 1,3,5-Triazine as TADF-emitters. For donors, refer to Fig. 9

far, there is no rational explanation for this behavior. Further research should be directed towards a more accurate description of the electronic properties of TADF emitters with quantumchemical methods, which will in turn improve the predictability of TADF properties, especially for radiative and non-radiative processes. In a recent investigation by J. Y. Lee and co-workers, the substitution pattern was further varied from the C3-symmetric tris-(4-carbazoylphenyl)-triazine shown in Fig. 10 to 1,2,3-tris-carbazoyl-phenyl-diphenyltriazine [73]. This led to one of the most efficient examples of this material family with more than 25 % EQE in a skyblue TADF OLED. For a concise review of this class of materials, we refer to the literature [72]. 2.4 Biaryl Sulfones as Acceptor Moieties Biaryl sulfones are a commonly used acceptor for TADF emitters [71, 74]. Amongst the materials, two of the most efficient, deep-blue TADF emitters are found, which can be synthesized by fusing 4,40 -diphenylsulfone with tert-butyl carbazole (tBuCz) dimethoxy carbazole (DMOC), or dimethyl dihydroacridine (DMAC). Apart from the various open biaryl sulfones, the two phenyl rings might be fused to form a five-membered ring system, as shown in structure 2 in Fig. 11 [75]. In another study with cyclic biarylsulfones, a C=O bridge was introduced to form thioxanthones, yielding emitters with more than 20 % EQE [76]. Apart from the symmetric, 4,4’-bis-substitued diphenyl sulfones, Lee and co-workers created a derivative with only one donor unit in the 4-position of the biaryl sulfone,

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Top Curr Chem (Z) (2016) 374:22 O O S

O O S

Fig. 11 Sulfone-type acceptors. For donors, refer to Fig. 9 donor

donor

donor

O O S

O O S donor

donor

donor

donor O

O O S Ph

N

+ ePh

O S

N O

arylsulfinate anion

arylcarbazoyl radical

Fig. 12 Degradation mechanism in sulfone-type materials. Upon reduction, the sulfur-carbon bonds are significantly weakened, which may lead to dissociation and formation of arylsulfinate as well as a radical [78]

respectively 1,3-di-carbazoyle benzene [41]. Surprisingly, this material does not show TADF, but was successfully used as a host for 4CzIPN. Another study showed various asymmetric combinations of diphenylsulfone as an acceptor fused with one or two units of tert-butyl-carbazole [77]. The authors’ results suggest that the 4,40 derivatives give the most efficient examples for this family. Apart from the relative distance between the S1 and T1 states, local states seem to also affect the overall photophysical performance of the emitters, which will be discussed in Sect. 2.5 [75]. Recent results suggest that the sulfone-type TADF emitters can exhibit an intrinsic instability, limiting their suitability in OLED devices (Fig. 12). It was demonstrated that these compounds are unstable upon reduction, i.e., the emitting materials are unstable towards electrons in the running device. In the reduced form of the sulfone, the S–C bond is weakened due to the stabilization of an anionic sulfinate leaving group [78]. This is reflected in the degradation of the materials in electron-only devices as well as photo-instability. 2.5 The Prediction of TADF Properties and Outlook In the previous sections, we reviewed common donor- and acceptor moieties of TADF emitters. However, the search for more structural motifs to increase emission efficiencies, reduce excited-state lifetimes of the TADF emission, and to enhance the stabilities (see Sect. 2.4) is not finished. Recently, new examples for TADFemitters have been reported, with aryl ketones [79] or aryl boranes [80] as acceptor units. Despite the great, mostly empiric success in designing TADF emitters with high PLQYs and proving their suitability in OLED devices with EQEs up to 30 % [63], it Reprinted from the journal

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should be said that the theoretical understanding of organic TADF emitters needs to be greatly improved. So far, no clear design rules or theories exist to rationalize the large photophysical differences between slightly different core structures. As of today, it is possible to predict if a molecule shows TADF at all based on DEST, and what the (approximate) color will be, but the theoretical understanding is not yet good enough to predict efficiency and the excited-state lifetime of the delayed decay processes, which are of great relevance for any application of TADF materials in OLEDs. Earlier in this section, we mentioned that slight changes in the structures of an emitter family can suppress thermally activated delayed fluorescence completely, yielding non- or only weakly luminescent molecules. By small changes in the structure, the excited-state lifetime of the delayed fluorescence may be increased by up to three orders of magnitude, as it was shown for sulfone- or triazine-type emitters [72]. Also, even the relative ratio of spontaneous versus delayed fluorescence in photophysical experiments does not give a hint on the effectiveness of the TADF mechanism in an OLED device, as has been seen for a heptazine-based emitter, which shows only very weak delayed fluorescence when excited by UV light, but exhibits high performance in an OLED with an external quantum efficiency of 18 % clearly assigned to TADF [53]. The basic, most popular concepts that were introduced in Sect. 1.2 mainly rely on minimizing DEST while still keeping a reasonably high oscillator strength. The lack of a straightforward explanation of the impact of calculated or measured DEST values on observables like efficiency and excited-state lifetime (see Sects. 2.2 and 2.3) suggests that important parameters have yet to be identified. Local triplet states on the donor and acceptor moieties may also affect the photophysical properties of TADF emitters [75, 81], especially when the triplet energy of the isolated moieties (DET1–S0) is close to the triplet energy of the full molecule. Recent work also set the stage for an improved theoretical description of TADF molecules [82–84], but more work needs to be done to achieve a comprehensive understanding of the basic structure–property relations behind intersystem crossing, energy gap, and excited-state lifetime.

3 Cu(I) Complexes: Molecular Structure and Emission Dynamics 3.1 Status and Challenges Luminescent copper(I) complexes as emitting materials for organic light-emitting devices have been in the focus of extensive research in recent years, especially after a device with an outstanding external quantum efficiency of 16.1 % had been reported by Deaton and Peters in 2010 [8]. This result showed that both singlet and triplet excitons were used for the generation of light, despite only a moderate spin– orbit coupling of copper(I). As presented in Sect. 1.2, luminescent copper(I) complexes often exhibit relatively small energy gaps DEST between the lowest excited singlet and triplet states of around 0.05–0.18 eV [85]. Sufficient intersystem crossing and reverse intersystem crossing, respectively, facilitate the back-transfer of excitons from the excited triplet to the singlet state and thus TADF, even for

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relatively large DEST. Nevertheless, the TADF efficiency of Cu(I) complexes is strongly controlled by the energy separation DEST, which should be minimized by a spatial separation of HOMO and LUMO. It has also been shown recently that in cases where DEST is very large, efficient phosphorescence also can occur in Cu(I) compounds [29]. Whereas in organic molecules (Sect. 2), this is achieved by using various donor- and acceptor moieties, in most Cu(I)-based materials the HOMO is located on a copper- or copper-halide center, while the LUMO resides on a ligand of low-lying non-populated p-orbitals. Thus, the emission is characterized by a metal(?halide)-to-ligand charge transfer with the metal centers as electron donors and the ligands as electron-accepting units. However, it is difficult to predict the localization of frontier orbitals for a new complex class so far, and thus material design rules are difficult to define for a small energy gap DEST. Experience shows that the latter mainly depends on the complex structure, e.g., the local geometry around the copper center(s) and used ligands or halides. In Fig. 13, several complex structures of copper(I) are shown, divided into different subclasses that are defined by the number of copper atoms in the individual complexes. Generally, plenty of donor atoms can be used in order to saturate the coordination sphere from the ligand side, thus leading to a large diversity of different complexes, which cannot fully be outlined herein. To narrow it down, for luminescent complexes donating-elements such as nitrogen, phosphorous or sulfur, and less often carbenes, are used in the ligand structures and thus will be the focus of this overview.

Mononuclear complexes + L L Cu BF4L L

+ L L Cu L L

L L Cu L L

BF4-

L L Cu L L

L Cu X L Dinuclear complexes L Cu

P Cu

X Cu L X

X X

N

P

P

Cu

Cu

P

N

R

X L L Cu Cu X L L

X L Cu L Cu X L

X X

X L L Cu Cu X L L

P Cu P

R

Fig. 13 Overview of different copper complex classes and typical motifs therein

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Various mono-, di-, tri- as well as tetranuclear complexes are known so far, featuring either several mono- or bidentate ligands, which can act either as chelating or bridging ligands between two copper(I) centers, or a combination thereof in their ligand sphere. A precise prediction of the formed complex motif is difficult, since it depends on the biting angle of the ligands, the stoichiometry of ligands and copper salts, or simply on thermodynamic reasons. Mono- as well as dinuclear complexes have been extensively studied as emitting materials for OLED applications, while luminescent tri- or tetranuclear compounds have been investigated for their photophysical properties, but are afflicted with some disadvantages such as low solubility, low thermal stability and often a lack of sublimability. The huge classes of mononuclear as well as dinuclear complexes can further be divided into different subclasses: Mononuclear complexes appear as cationic or neutral structures of tetrahedral or trigonal planar geometry, while dinuclear complexes can be divided based on planar- or butterfly-shaped copper halide cores. As mentioned above, the large structural variety even of these two complex classes can be ascribed to the flexible coordination numbers (2, 3, 4) and geometries (linear, trigonal, tetrahedral) [86, 87] of the copper(I) ion together with halide ions of likewise flexible coordination numbers, as well as ligands, which can act in a mono- or bidentate and chelating or bridging manner. A few highlights shall be emphasized here in order to show the high potential of copper compounds: One of the basic principles for highly efficient devices is to use emitting materials of high PLQYs close to 100 %, which can be fulfilled by various compounds based on mononuclear as well as dinuclear complexes reported since 2010 [30, 88, 89] [17, 26, 90, 91]. Furthermore, in 2011, Hashimoto et al. used a trigonal-planar copper complex to fabricate a device with an EQE of 21.3 %, and in 2013 Igawa et al. reported on OLEDs with a series of neutral mononuclear complexes featuring EQEs of up to 17.7 % [35, 36, 92]. In 2015, our group reported on a highly modular emitter system based on a dinuclear complex, with which OLED devices of EQEs of 23 % were achieved. This is comparable to state-of-theart devices based on Ir(III) emitters, fabricated by vacuum deposition [16]. In the following Sects. 3.2 and 3.3, the most common copper(I) materials and their photophysical properties are organized according to the number of copper atoms. Although copper(I) complexes are a highly promising class of materials based on more abundant starting materials than the nowadays used phosphorescent materials for OLEDs, several aspects have to be approached to lead these materials to industrial application: (I)

(II)

As already discussed in Sect. 1.3 (Sects. 1.3.1 and 1.3.3), the materials have to be stable for processing both by vapor deposition as well as by solution techniques. Besides this chemical and thermal stability during the manufacturing of OLED devices, the copper(I) complexes are also required to be photochemically and redox-stable during operation of the device. Byproducts or chemically reactive degradation products act as charge carrier traps and lead to short device lifetimes. It is crucial to decrease the excited-state lifetime down to ideally 1 ls or less. For this, a deeper understanding of the emission dynamics on a fast

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time scale as well as of the TADF mechanism is required. We present a first-of-its-kind case study for mononuclear compounds in Sect. 3.5. Interestingly, it was found that the processing technique affects the photophysical properties of the copper(I) complexes, although it was proven that the compounds are still intact. To investigate such phenomena, which can be mainly ascribed to morphology effects, we present x-ray absorption spectroscopy at the Cu K edge as a potent tool in an additional case study in Sect. 3.6.

(III)

3.2 Mononuclear Copper(I) Complexes Mononuclear copper(I) complexes are usually constructed from two bidentate ligands, with the coordination atoms nitrogen or phosphorous, thus reducing the tendency of ligand dissociation or exchange reactions during synthesis or processing and increasing the stability of the material, while complexes with monodentate ligands are also known, but are less common (Fig. 14). The overall charge of the complexes is controlled by its ligands and/or halides, and in the case of cationic motifs a counter-ion is required, which is believed to be disadvantageous for the

N N

Ph2P Cu

N

O

Ph2P

N H N B H N N

Ph2 P Cu P Ph2

PPh3

N Cu

PPh3

X

CF3 1

N

2

Ph2P

N N N N B N N N N

Cu N

Ph2P

Ph2P Cu

N

Ph2P Cu

Ph2P

N

O

Ph2P

N Ph B Ph N N

Cu

O

N N Me B Me N N

Cu

8

N N

Ph2 P

N

N Ph B Ph N N

Cu P Ph2

Ph2P 9

O

Ph2P

7

6

N

X =Cl (3) Br (4) I (5)

10

11

Fig. 14 Several mononuclear complexes investigated with respect to their DEST values

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application in OLED devices as a charge carrier trap. In the last few years, research mainly focused on the development of neutral mononuclear copper(I) complexes to avoid these issues. In order to achieve an efficient TADF and to reduce excited-state lifetimes, a small separation between the first excited singlet S1 and triplet T1 states DEST is required. In the following, a correlation of experimentally gained values for DEST and excited-state lifetimes for mononuclear copper(I) compounds (and in Sect. 3.3 for binuclear complexes) will be shown. Although there are plenty of studies about the photophysical properties of luminescent, mononuclear copper(I) complexes, only a few can be taken into account for this comparison since there is only one method to experimentally estimate DEST so far: In contrast to purely organic TADF materials, in copper(I) complexes the shape of the emission spectra seem to be not only controlled by the excited states but also by morphological effects [93]. Thus, the so-far used method to measure DEST indirectly is decay time measurement versus temperature and fitting of the graph to an equation derived from a two-state model in thermal equilibrium, as reported by Yersin [26, 30, 85, 94, 95]. Ideally, this is done from 1–2 K to 300 K to guarantee distinguishing between a (TADF) fluorescence from the singlet state and a phosphorescent emission from the triplet state. A detailed derivation of this method along with the theoretical background is part of Sect. 3.5. A detailed study on the excited-state lifetimes of the TADF processes in dependence of the temperature was first reported in 2011 by the group of Yersin et al.: The blue-emitting copper(I) complex 7 [Cu(pop)(pz2Bph2)] (pop = bis(2(diphenylphosphino)phenyl)ether, pz2BPh2 = bis(pyrazol-1-yl)biphenylborate) was investigated thoroughly, giving an excited state lifetime around twenty microseconds and an energy gap DEST of around 0.12 eV [26]. Since then, only a few more studies on mononuclear copper complexes, which are depicted in Fig. 14, have been reported by the groups of Lu, Kato, as well as Yersin. Complex 1, based on the anionic 2-pyridyl pyrazolate as N^N ligand along with bis[2-diphenylphosphino)phenyl]ether (POP) as a chelating P^P counterpart, was investigated by the group of Can-Zhong Lu in 2013. They determined the energy gap DEST to be around 1371 cm-1 (0.17 eV) and an excited-state lifetime of 23 ls [89]. Neutral mononuclear copper complex 2 based on an anionic borate as N^N ligand and POP as chelating P^P ligand was investigated by Czerwieniec et al. revealing a DEST distance of 1300 cm-1 accompanied by an emission lifetime of 20 ls [26]. A series of complexes based on triphenylphosphine, 4-methylpyridine, and the different halides Cl (3), Br (4) as well as I (5) were reported by Ohara et al. in 2014 [96]. They measured the excited-state lifetime of those complexes in dependence of the temperature in a range of 77–300 K. They found that DEST values between 940 and 1170 cm-1 correlated with short emission lifetimes of 9.4 ls (940 cm-1, 0.12 eV) for complex 3 and 15 ls (1070 cm-1, 0.13 eV) and 9.5 ls (1170 cm-1, 0.15 eV) for complexes 4 and 5, respectively [97]. Furthermore, Czerwieniec et al. reported DEST distances of 1000 cm-1 (0.12 eV) for complexes 6 and 7 based on phenanthroline and phanephos as well as bis[2-diphenylphosphino)phenyl]ether and a borate, respectively. Excited state lifetimes of 14 ls as well as 22 ls were measured for these two complexes at 300 K [85].

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Even smaller DEST values of 650 cm-1 (0.08 eV), 720 cm-1 (0.09 eV), and 740 cm-1 (0.09 eV) were found by Czerwieniec et al., Linfoot et al., and Leitl et al. for complexes 8, 9, and 10 resulting in short emission lifetimes of 13 ls (8) and11 ls (9, 10) [29, 85, 94]. The shortest excited-state lifetime of 3.3 ls accompanied by the smallest DEST distance of only 370 cm-1 (0.05 eV) was recently found by Czerwieniec et al. during the investigation of complex 11 in 2015 [85]. To sum up, while there is an obvious correlation between the energetical separation of the first excited triplet and singlet state and the corresponding excited state lifetime, it is difficult to derive an absolute correlation. Of course, it needs to be taken into account that the excited-state lifetime is also strongly affected by nonradiative processes and any comparisons based on excited-state lifetime between different samples from different sources are only valid if the PLQY is at least roughly comparable. Ideally, such comparisons are based on analysis of radiative and non-radiative rates for the different processes [37]. In general, the smaller DEST, the shorter the emission lifetime is of the respective compound. 3.3 Binuclear Copper(I) Complexes A common complex class of binuclear copper(I) complexes is constructed of a copper halide core Cu2X2 with X = Cl, Br, I coordinated by two N^P or P^P ligands (Fig. 15). The formation of a distinct complex motif is controlled mainly by the steric hindrance of the applied ligand [98]. As for the mononuclear complexes in Sect. 3.2, a similar trend for DEST distances and excited-state lifetimes becomes obvious for dinuclear copper(I) complexes, although there are even less studies reported so far. Tsuboyama et al. investigated Ph2 P

Ph2 P

I Cu

Cu

P Ph2

I

P Ph2

12

Me2 N

Cu

Cu P Ph2

Ph2 P

X

N Me2

X

Ph2P Cl Cu N

N Cu Cl PPh2

16

X = Cl (13) Br (14) I (15) Fig. 15 Dinuclear complexes investigated with respect to their DEST values

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the dinuclear halide-bridged complex [Cu(l-I)dppb]2 (12, dppb = 1,2bis[diphenylphosphino]benzene) and was able to determine the DEST to be around 700 cm-1 (0.09 eV) accompanied by an emission lifetime of only 4 ls [99]. Slightly smaller DEST values between 460 cm-1 (0.06 eV) and 570 cm-1 (0.07 eV) have been found by Leitl et al. in a study of dinuclear halide-bridged complexes 1315 based on chelating aminophosphane ligands. These relatively small distances come along with short emission lifetimes in the range of 4.1–6.6 ls, thus confirming the trend that was found for mononuclear complexes [100]. As demonstrated above, the emission decay time seems to be strongly correlated to the energy separation DEST: the smaller DEST the shorter the emission decay times. However, the lifetimes achieved so far are still longer than the 1 ls found for Ir(ppy)3. In order to overcome this limit, Yersin et al. found a possibility to open up an additional radiative pathway, in particular from the lowest excited triplet state. While for most copper(I) compounds known so far, this T1 ? S0 path is largely ineffective due to weak spin orbit coupling, the phosphorescence from the T1 state can contribute significantly to the emission in these complexes if the spin–orbitcoupling is large and induces a short radiative T1 decay time [95]. The Cu2Cl2(N^P)2 complex (16), based on two copper(I) atoms along with two terminal, non-bridging chlorine anions and two 2-(diphenylphosphino)-6methylpyridines as bridging N^P ligands, features a TADF decay time of 11 ls along with a measured phosphorescence decay time of 42 ls, resulting in an overall emission decay time of 8.3 ls. It has not only been found that for this compound 20 % of the radiation is a phosphorescent emission while 80 % is a TADF-only contribution, but also that the additional phosphorescence path reduces the overall emission decay time by about 20 % [95]. To sum up, based on these experimental values the correlation between DEST and emission decay time is valid and can be used as a design criterion for future emitting compounds. 3.4 Emission Dynamics on the Fast Time-Scale In contrast to photophysical studies on organic TADF compounds [6, 72], there are only few publications about the electronic processes occurring after excitation in luminescent copper(I) compounds that are characterized by TADF. Photophysical methods to investigate the TADF behavior of luminescent materials are of high importance to gain a deeper understanding of the structure–property relationships and draw conclusions for the design of efficient emitters. The methods applied for luminescent copper(I) complexes so far are based on the two-state model proposed by McMillin in 1980, wherein the singlet and triplet excited states are in thermal equilibrium, with the long-lived triplet state serving as exciton reservoir for delayed fluorescence [11, 13]. A first hint on the TADF behavior of a material is given by a red-shift of the emission and a strong increase of the emission decay times when cooling down, until reverse intersystem crossing is completely disabled and phosphorescence occurs (Fig. 16). Further confirmation is given by time-resolved photoluminescence measurements at ambient temperature,

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Fig. 16 Emission decay times of the neutral copper(I) complex [Cu(pop)(pz2Bph2)] (see also Sect. 3.2) plotted against temperature in the range 25–300 K [26]. A strong increase of the emission lifetime from 13 to 480 ls is observed below 175 K, corresponding to emission of different origins, namely delayed fluorescence from the singlet and phosphorescence from the triplet state. By fitting Eq. (3) to the experimental data, one obtains a lifetime of the singlet of 120 ns and an energy gap of 800 cm-1 as fitting parameters

which show the same emission spectra for the prompt and delayed fluorescence, thus arising from the same excited state [8]. The common method used to investigate TADF recently is temperaturedependent emission decay time measurement as reported by Yersin, which will be explained in the next section [20, 26, 31, 100]. Two states of different energy—here denoted as singlet and triplet—which are in fast thermal equilibrium (i.e., kISC  kSr ? kSnr and kRISC  kTr ? kTnr), can be understood as steady-state (after an initial time period) and thus described by Boltzmann statistics: [11]   NS gS DEST kRISC ¼  exp ð1Þ K¼  NT gT kB T kISC K equilibrium constant, N population of the respective states S or T, g degree of degeneracy of the excited state S or T, DEST energy gap between the excited singlet and triplet states S and T, kB: Boltzmann constant, T absolute temperature. The temperature-dependent averaged decay time of this system can be expressed by term (2) (obtained from the rate equation for the total population N of the system), wherein KS = kSr ? kSnr and KT = kTr ? kTnr [101, 102]. If a thermal population of the three substates of the triplet is assumed, one can use gT = 3 and gS = 1 for the degeneracy factors g. By introduction of sS = (KS)-1 and sT = (KT)-1 as the lifetimes of the fitted excited singlet and triplet states S1 and T1, Eq. (2) can be simplified to term (3).   ST gT þ gS  exp  DE kB T   save ¼ ð2Þ ST gT KT þ gS KS  exp  DE kB T

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save

  ST 3 þ exp  DE kB T   ¼ DEST 3 1 sT þ sS  exp  kB T

ð3Þ

The experimental data of emission decay times versus temperature can be fitted by Eq. (3), as shown in Fig. 16, and the two fit parameters energy separation DEST and emission decay time of the spontaneous fluorescence sS (which cannot be measured directly, as discussed above) are obtained. The energy splitting between the excited singlet and triplet state as well as the triplet decay rates control the overall, experimentally observable TADF lifetime save, thus for efficient emitting materials DEST values below 0.2 eV (1600 cm-1) are favored. To further understand the interplay of all radiative and non-radiative transitions in the copper(I) TADF system, examinations on the non-emissive transitions intersystem crossing S1 ? T1 and reverse intersystem crossing T1 ? S1 are of high importance. So far, ISC rates have been reported only for the hardly emissive copper(I) bis(imine) complexes in solution, which were studied by time-resolved emission spectroscopy, fluorescence upconversion, and transient absorption spectroscopy, determining the ISC rates to 7–15 ps [103–109]. Siddique et al. first reported on intersystem crossing time constants of 13–16 ps for [Cu(N^N)2]? complexes with phenanthroline and bipyridine ligands, by using time-correlated single photon counting as well as DFT calculations [103]. The experimentally observed time constants thus fit well to the formerly observed kinetics in transient absorption measurements. The assignment of observed kinetics from the initially formed excited state S1 to either structural changes or intersystem crossing has been subject to discussion, and only in 2007 did Tahara et al. make unambiguous assignments feasible by using the method of fluorescence upconversion. A short emission decay time in the order of hundreds of femtoseconds was found for the structural reorganization of the copper(I) bis(imine) complexes after excitation, depending on their steric hindrance [110], and a slower time constant of 7.4 ps (N^N = 2,9-dimethyl-1,10-phenanthroline) could be attributed to intersystem crossing (see also Fig. 5) [111]. Also, Chen et al. determined the intersystem crossing rate to be 10-15 ps by fluorescence upconversion combined with transient absorption results [108]. For the copper bis(imine) complexes a prompt fluorescence S1 ? S0 is observed subsequent to excitation into the higher lying singlet state S2 and deactivation to the S1 (the deactivation path being dependent on the excitation wavelength). However, large energy gaps DEST and quenching of the triplet states in solution prevent efficient back transfer, and thus TADF at ambient temperature [11]. Recently, we investigated the intersystem crossing rate in a TADF-efficient copper(I) complex for the application in OLEDs [112]. The highly luminescent complex [(PyrTet)Cu(DPEPhos)] (Fig. 17) [90], with PyrTet = 5-(2-pyridyl)tetrazolate and DPEPhos = bis(2-(diphenylphosphino)phenyl)ether, was chosen as a model compound. Owing to the very low intensity of prompt fluorescence in contrast to strong, long-lived delayed fluorescence, detection of emission from the excited singlet state and its deactivation by ISC was not possible by the presented methods, namely

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Ph2P

N Cu N N N N

O Ph2P

Fig. 17 Picosecond luminescence decay dynamics of [(PyrTet)Cu(DPEPhos)] in neat film after excitation. The solid line represents a fit to a monoexponential decay with a time constant of 27 ps [113]. The instrument-response function is depicted as dotted line with 2.5 ps full-width half-maximum

fluorescence upconversion and transient absorption spectroscopy, and thus a different method had to be established. Furthermore, the compound was aimed for further study in film, which better represents the processes in the solid, thin-film state of an OLED. The intersystem crossing process of the complex [(PyrTet)Cu(DPEPhos)] is more readily recognized in picosecond time-resolved emission data by recording the luminescence decay dynamics with a streak camera. The presence of long-lived delayed fluorescence and back population of the excited singlet state S1 by RISC challenges the experimental methods used to determine ISC and the assignment of the observed dynamics. Time-resolved photoluminescence spectra support the interpretation of kinetics and especially the assignment of the intersystem crossing rate: Emission spectra in the time windows of 0-20 and 40–60 ps show a fast decrease of emission intensity, reflecting the picosecond luminescence decay, while the normalized emission spectra agree well with the spectrum for the long-lived TADF background. The observed decay can be fitted to a single exponential with a time constant of 27 ps, when convolved with the instrument response function, representing intersystem crossing (Fig. 17) [113]. This method for the determination of intersystem crossing rates of luminescent copper(I) complexes—also in the solid state—represents a tool to screen various complex classes, different ligands for a given complex structure, as well as the effect of the environment on the emission properties of complexes in host–guest systems. For the above discussed copper(I) bis(imine) complexes it was proposed that ligands of different steric hindrance and thus the extent of flattening distortion in the excited state control the ISC [103, 114, 115], while different solvents do not seem to play a significant role [103, 108, 116] Throughout various complex classes, the intersystem crossing is strongly controlled by the effective spin–orbit coupling, i.e., SOC only affects closely lying excited singlet and triplet states of different

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metal d-orbital characters [24, 31, 95]. The comparison of intersystem crossing rates of different complex classes could thus lead to a better understanding of the effective spin–orbit coupling and—due to the increased understanding—eventually to material design for short emission decay times. Although the determination of the intersystem crossing rate in these highly emissive compounds is challenging, it is very important for the understanding of the excited state mechanisms in TADF materials as well as for material design. 3.5 Assessing the Molecular Structure of Cu(I) Complexes with X-ray Absorption Spectroscopy at the Cu K Edge When considering the application of luminescent Cu(I) compounds in material science due to their favorable emission properties, problems arise concerning the characterization of the materials upon processing. For the application in OLEDs, processing of Cu(I)-complexes from solution e.g., by coating or printing techniques, is advantageous. Therefore, the emitting material is processed from bulk powders into amorphous, thin films with a matrix material. Whereas the deployed crystalline material can be characterized by single crystal x-ray diffraction, proofing the preservation of the molecular structure in the amorphous thin film upon solutionprocessing is challenging. The comparison of photophysical emission spectra of the deployed bulk material and the operating device can only give a first hint as to whether a change in the molecular structure has to be expected. Furthermore, as the morphologies, packing, and dipole moments of the environment in the bulk and the thin film are different due to the matrix material and possibly residual solvent molecules [93], changes in the shape and position of the visible emission band can be expected even if the molecular structure is preserved upon processing. In case of the formation of different species upon processing, there would not be a detectable difference found in elemental analysis and the change in the vibrational bands in the IR spectra might be not significant, especially since the shape of many IR spectra is dominated by ligand modes, while copper-halide modes are usually not detectable in the standard range of this method (4000–400 cm-1). It was previously reported that the characterization of Cu(I) complexes by NMR can be problematic [63, 64], mainly due to a plethora of NMRactive nuclei e.g., 31P, 63Cu, 65Cu, and 127I. As the NMR-active copper and iodine nuclei exhibit a nuclear spin greater than S ¼ 12 and a quadrupolar momentum, the nuclear energy levels split upon the application of a magnetic field according to ð2S þ 1Þ. The NMR signals of quadrupolar nuclei are usually wider than those with S ¼ 12 due to rapid quadrupolar relaxation. This may lead to broad, ill-structured signals with complex multiplet coupling, even when studying solid samples with MAS-NMR techniques [117]. X-ray absorption spectroscopy (Fig. 18) offers an alternative method with which to characterize non-crystalline Cu(I) complexes: XAS with hard energetic x-ray radiation is a well-established method for the characterization of the coordination environment of metal atoms in non-crystalline materials. This method was previously used to characterize the molecular structure of Cu(I) complexes in

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Fig. 18 Schematic illustration of XAS measurement: (1) incoming X-ray photon and ejected electron, (2) interference of outgoing photoelectron wave and scattered photoelectron, (3) normalized XAS spectrum, (4) extraction of the extended X-ray fine structure (EXAFS) by subtracting a background function, and (5) Fourier transformation of the EXAFS, which contains information on the nearestneighbor atoms, in particular on the distance, the number, the element and the structural disorder of the environment. This figure has been published before [118]

amorphous thin films formed by co-deposition of a ligand and CuI [119–123] and is also suitable for the investigation of amorphous thin film samples of the luminescent Cu(I) complexes. The molecular structure of copper(I) complexes in the amorphous emissive layer of an OLED can be assessed by x-ray absorption spectroscopy at the Cu K edge. Information on the oxidation state as well as on the coordination number and symmetry of the copper ion can be extracted from the XANES region. (light grey part in Fig. 19). Typically, Cu(I) complexes show low energy peaks in the region between 8983.0 and 8986.0 eV, which are assigned as 1s ? 4px and 1s ? 4py,z transitions, and one peak at 8990.0 eV [124]. Cu(II) complexes show intense peaks in the region between 8986.0 and 8988.0 eV, which are assigned to 1s ? 4p transitions, and a pre-edge peak at 8979.0 eV, which corresponds to 1s ? 3d transitions [124]. The coordination number of Cu(I) and the symmetry of the ligands around the central atom influence the energy position of the 1s ? 4p transitions in Cu(I) complexes [86, 125]. The degenerated px,y,z orbitals of a Cu(I) ion in tetrahedral coordination split for a Cu(I) ion in a distorted coordination environment and for a three- and twofold coordinated Cu(I) ion according to the ligand field theory [86, 124] Analysis of the EXAFS region (dark grey part in Fig. 19) provides further information on the local environment of the Cu(I) centers, e.g., nearest-neighbor distances and coordination numbers. Commonly, known structural parameters of

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Fig. 19 Exemplary Cu K XAS spectrum. The data has been published before [16]. The onset-part around 9000 keV (left rectangle) marks the so-called X-ray absorbtion near edge structure (XANES) region, while the high-energy part of the spectrum (right rectangle) is known as extended X-ray absorbtion fine structure (EXAFS). The arrow marks a trace impurity of zinc. Because of the similar excitation energy of the Zn K edge, this is visible at higher energies

Cu(I) complexes, derived by singly crystal X-ray diffraction, are used to set up a starting model for fitting of the EXAFS data. A particularly interesting approach to fabricate vacuum-processed OLEDs with copper emitters has been introduced by Thompson and co-workers in 2011 [122]. Bypassing the inability of many copper complexes to be evaporated without the occurrence of chemical changes, they used ligands that also feature host properties and co-evaporated them together with copper precursors such as copper iodide. This approach led to very efficient, durable OLEDs. However, due to the rich structural chemistry of copper iodide, the chemical structure of the obtained species could not be predicted and could also not be determined with classic analytical methods such as single crystal x-ray diffraction or powder diffraction. Using XAS however, the authors were able to reveal that there are in fact several species formed during the evaporation process. The dominant emitter, Cu2I2(mCPy)4, is indeed not identical with one of the reference materials that have been synthesized and crystallized by conventional methods and seems only to be accessible via the vacuum co-deposition approach. Recently, we introduced NHetPHOS copper(I) complexes as highly favorable emitting material for OLEDs [17, 91, 126]. We demonstrated an OLED device with an internal quantum efficiency close to 100 % [16]. As it has been found for other copper emitters, NHetPHOS complexes show different photophysical properties after being processed [93], such as color shifts in the order of 30 nm, as well as significant differences in excited-state lifetime and PLQY, which raises the question of whether the molecular structure derived by single crystal X-ray diffraction is retained in non-crystalline solid state. This question has been answered in two recent studies: [16, 117] Overall, the XANES spectrum of amorphous powder and

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single crystal samples were in very good agreement with each other. Interatomic distances and coordination numbers derived from the single crystal x-ray diffraction analysis of the corresponding crystalline samples were used to set up the starting structures for the fit of the EXAFS data. The analysis suggested that the studied complexes retain their principle structure even when preparing thin films from solution and offer the opportunity to be used as emitters in solution-processed optoelectronic devices. The same approach was also used to characterize the molecular structure of Cu(I) complexes in amorphous thin films, which were formed by co-deposition of a ligand and a copper(I) halide as precursors. Liu et al. analyzed the XANES and EXAFS region of codeposited films of an isoquinoline ligand (L) and CuI, and concluded that a dimeric complex Cu2I2(L)4 was the dominant species in the amorphous film. Red emissive OLEDs based on the codeposited ligand (L) and CuI (5:1) as emissive layer yielded an EQE of 3.6/1.4 % at and a photoluminescence efficiency of 4.8/1.1 lm W-1 at 1/100 cd m-2 [120]. To sum up, XAS at the Cu K edge is a valuable method for determining coordination geometries, distances, and coordination numbers of the nearest neighbors of the metal center, as well as oxidation states for non-crystalline copper(I) complexes as emitting materials in OLEDs.

4 Conclusion: Future Challenges for TADF Emitters In this work, we introduced organic and metal–organic TADF-materials from a conceptual point-of-view. We described the main merits of the different material classes known so far (Sect. 1). It is fair to say that TADF materials are one of the most promising material classes and will help in solving current material issues in organic light emitting devices. Even though the materials are on par with state-of-the-art commercial materials in terms of efficiency and have reached a promising operational stability level, there is still much to be learned. To solve the open question that have been addressed in Sects. 2 and 3, joint efforts of synthetic chemistry, photophysics, theoretical chemistry, as well as engineers are required. Table 3 lists some important issues to be solved.

Table 3 Open questions for organic and metal organic TADF emitters Organic emitters

Cu(I) emitters

Understanding

How to understand and control the excited state lifetime?

How to further decrease the excited state lifetime?

Application

How to realize solution-processed and printed OLEDs with organic TADF materials?

How to increase the operational stability of OLEDs with Cu materials?

Performance

How to realize deep-blue TADF?

How to realize deep-blue TADF?

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Top Curr Chem (Z) (2016) 374:22 Table 4 The blue-emitting copper(I) complexes [(pz2BR2)Cu(POP)] are characterized by metal-to-ligand charge-transfer excitations facilitating—together with a moderate spin–orbit coupling—TADF by spatial separation of the frontier orbitals Substituent R=

DEST (eV)

s (ls)

PLQY

H

0.16

20

0.45

Pz

0.12

22

0.90

Ph

0.10

13

0.90

The moderate spin–orbit coupling of the copper(I) center, compared to the very weak SOC in purely organic TADF emitters, ensures TADF emission for a broader range of energy separations DEST

For both material families, the understanding needs to be increased. As we discussed in Sect. 2.5, it is now clear that the singlet–triplet splitting has an important impact on TADF properties. Nevertheless, it is not yet clear how exactly the molecules need to be constructed to have a very high efficiency and a low excited-state lifetime at the same time, especially for organic emitters. For copper emitters, the variation of the excited-state lifetime is much smaller, due to the increased impact of spin orbit coupling on the ISC-RISC equilibria. Here, the question can be narrowed down to one main task: to increase the oscillator strength of the S1–S0 transition. From an application point-of-view, increasing the stability is an overallimportant question, but seems to be particularly pressing for Cu(I) emitters. Inkjetprinting of TADF OLEDs has not yet been demonstrated with organic materials, which requires the development of well-soluble materials and an understanding of the impact of morphology and aggregation on the device performance. Last, the Holy Grail of OLED research, that being the realization of efficient AND stable, deep-blue devices, is a task that could be solved with TADF materials to the high efficiency that matches the best phosphorescent emitters in addition to a rather low bandgap S1–S0. Acknowledgments The authors thank the German Ministry for Education and Research (BMBF) for funding in the scope of the cyCESH project (FKN 13N12668). Funding through the Deutsche Forschungsgemeinschaft (TRR88, B2; SFB 1176) is acknowledged. We gratefully acknowledge the collaboration with the groups of Prof. Ifor Samuel (University of St. Andrews), Prof. Franky So (NCSU), Prof. Christopher Barner-Kowollik (KIT), Prof. Clemens Heske (KIT, UNLV), Prof. Uli Lemmer (KIT), and Manuela Wallesch (KIT) as well as the scientific division of CYNORA and the synchrotron facilities of KIT, Angstromquelle Karlsruhe (ANKA).

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Top Curr Chem (Z) (2016) 374:22 117. Volz D, Wallesch M, Grage SL, Go¨ttlicher J, Steininger R, Batchelor D, Vitova T, Ulrich AS, Heske C, Weinhardt L, Baumann T, Bra¨se S (2014) Labile or stable: can homoleptic and heteroleptic PyrPHOS-copper complexes be processed from solution? Inorg Chem 53:7837–7847. doi:10.1021/ic500135m 118. Wallesch M, Bra¨se S, Baumann T, Volz D (2015) X-ray absorption spectroscopy: towards more reliable models in material sciences. In: So F, Adachi C, Kim J-J (eds) Proceedings of SPIE. p 956609 119. Liu Z, Qiu J, Wei F, Wang J, Liu X, Helander MG, Rodney S, Wang Z, Bian Z, Lu Z, Thompson ME, Huang C (2014) Simple and high efficiency phosphorescence organic light-emitting diodes with codeposited copper(I) emitter. Chem Mater 26:2368–2373. doi:10.1021/cm5006086 120. Ni T, Liu X, Zhang T, Bao H, Zhan G, Jiang N, Wang J, Liu Z, Bian Z, Lu Z, Huang C (2015) Red emissive organic light-emitting diodes based on codeposited inexpensive Cu(I) complexes. J Mater Chem C 3:5835–5843. doi:10.1039/C5TC00727E 121. Liu Z, Qayyum MF, Wu C, Whited MT, Djurovich PI, Hodgson KO, Hedman B, Solomon EI, Thompson ME (2011) A codeposition route to Cu(I)-pyridine coordination complexes for organic light-emitting diodes. J Am Chem Soc 133:3700–3703. doi:10.1021/ja1065653 122. Liu Z, Qayyum MF, Wu C, Whited MT, Djurovich PI, Hodgson KO, Hedman B, Solomon EI, Thompson ME (2011) A codeposition route to Cu(I)—pyridine coordination complexes for organic light-emitting diodes. J Am Chem Soc 133:3700–3703. doi:10.1021/ja1065653 123. Zhang S, Turnbull GA, Samuel IDW (2012) Highly efficient solution-processable europiumcomplex based organic light-emitting diodes. Org Electron 13:3091–3096. doi:10.1016/j.orgel. 2012.09.006 124. Kau LS, Spira-Solomon DJ, Penner-Hahn JE, Hodgson KO, Solomon EI (1987) X-ray absorption edge determination of the oxidation state and coordination number of copper. Application to the type 3 site in Rhus vernicifera laccase and its reaction with oxygen. J Am Chem Soc 109:6433–6442. doi:10.1021/ja00255a032 125. Pickering IJ, George GN, Dameron CT, Kurz B, Winge DR, Dance IG (1993) X-ray absorption spectroscopy of cuprous-thiolate clusters in proteins and model systems. J Am Chem Soc 115:9498–9505. doi:10.1021/ja00074a014 126. Volz D, Baumann T, Flu¨gge H, Mydlak M, Grab T, Ba¨chle M, Barner-Kowollik C, Bra¨se S (2012) Auto-catalysed crosslinking for next-generation OLED-design. J Mater Chem 22:20786. doi:10. 1039/c2jm33291d 127. Sato T, Uejima M, Kaji H, Tanaka K, Adachi C (2014) A light-emitting mechanism for organic light-emitting diodes: molecular design for inverted singlet-triplet structure and symmetry-controlled thermally activated delayed fluorescence. J Mater Chem C 00:1–9. doi:10.1039/ C4TC02320J

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Top Curr Chem (Z) (2016) 374:52 DOI 10.1007/s41061-016-0051-1 REVIEW

Perovskite Luminescent Materials Michele Sessolo1 • Lido´n Gil-Escrig1 Giulia Longo1 • Henk J. Bolink1



Received: 26 March 2016 / Accepted: 6 July 2016 / Published online: 27 July 2016 Ó Springer International Publishing Switzerland 2016

Abstract We describe recent progress on the luminescent properties of hybrid organic inorganic metal halide perovskites and the LEDs employing them. Keywords Hybrid perovskite  Light-emitting diode  Electroluminescence  Photoluminescence  Organic material  Lead halide

1 Introduction Organic–inorganic hybrid materials crystallizing with the perovskite structure (hence named hybrid perovskites) comprise a large class of compounds with interesting optical, electronic, and structural properties. Investigations towards these compounds have been ongoing since the beginning of the last century [1], but were mainly focused on the structural and magnetic properties of the compounds [2, 3]. Seminal work on the optical properties of hybrid perovskites was presented by Ishihara et al. [4, 5], who described the excitonic properties of layered perovskites. A step forward in the understanding and applications of this class of materials was the demonstration of electroluminescence [6, 7] and, almost at the same time, of conducting and semiconducting hybrid perovskites [8–10]. This variety of properties originates from the isolated organic and inorganic constituents of the perovskite. The inorganic components have important optical and electronic properties, together with mechanical and thermal stability. Organic moieties can tune such properties, adding This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli, Henk Bolink. & Henk J. Bolink [email protected] 1

Instituto de Ciencia Molecular, Universidad de Valencia, C/Catedra´tico J. Beltra´n 2, Paterna 46980, Spain

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the possibility of simple thin-film deposition at low temperature and hence on any type of substrate. Hybrid perovskites owe their name to the general three dimensional (3D) perovskite structure AMX3, where M is a metal cation, X is generally a halide anion, and A is an organic cation. The cation M is usually a divalent metal with stable octahedral coordination, the most common ones being Pb2?, Sn2?, and Ge2? because the corresponding halides have interesting electronic structure. Lately, few studies have also demonstrated perovskites based on trivalent ions such as Bi3? and Sb3? , in which metal vacancies compensate for the extra charge [11]. The solid state structure is formed by an extended network of MX6 octahedra intercalated with organic cations (A) balancing the charge (Fig. 1a). Extended 3D hybrid perovskites can form only if the organic cation fits in the cavity delimited by four corner-sharing octahedra. Practically, only methylammonium (MA) and formamidinium (FA) fulfil such space limitation. Among these compounds, MAPbI3 was recently applied in high efficiency perovskite solar cells, which are responsible for the renewed interest towards this class of materials. In only 6 years from the first demonstration of a solar cells sensitized with a perovskite, certified power conversation efficiencies exceeding 20 % have been reported [12–23]. When the size of the organic cations increases, the structure cannot accommodate them and it collapses into a low dimensional material formed by sheets of corner sharing MX6 octahedra separated by organic layers. The most interesting and widely studied structures are 2D layered compounds composed of a single layer MX6 inorganic sheet, oriented along the \100[ direction of the corresponding 3D perovskites, alternating with organic ammonium cations (Fig. 1b). The general formula for these 2D perovskites is (R-NH3)2MX4, where R is an aliphatic or aromatic substituent. The organic layer in \100[ oriented 2D perovskites can be either a bilayer of mono-ammonium cations or a single layer of diammonium cations (Fig. 1c) [25]. The layered perovskite family is not restricted solely to this structure, and thicker sheets of inorganic perovskites can be incorporated as well as oriented along different crystal direction (\110[ and \111[). Here, however, we will report only on the photoluminescent

3D (a)

2D (b)

AMX3 MX6 (Oh)

(c)

(R-NH3)2MX4 A (MA+, FA+)

R-NH3+

(NH3-R-NH3)MX4 +NH

+ 3-R-NH3

Fig. 1 Schematics of a extended 3D perovskites with general formula AMX3, and of the two possible types of \100[ oriented 2D perovskites, comprising b mono-ammonium or c diammonium organic cations. Adapted from Ref. [24]

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properties and applications of extended 3D and \100[ oriented 2D perovskites, as these are the most studied materials in the available literature. Also, due to the limited stability of Ge2? compounds, which tends to oxidize even when processed in inert atmosphere, we will limit our discussion to the more stable and widely studied tin and especially lead perovskites.

2 Optical Properties of 2D Perovskites The optical and electronic properties of 2D perovskites originate from the peculiar structures in which the organic and inorganic components are oriented and assembled. They can be thought of as multi-quantum well structures, where semiconducting inorganic sheets are periodically alternated with insulating organic layers (Fig. 2). Upon irradiation with photon energy higher than the metal halide bandgap (Eg), electrons and holes are formed in the conduction and valence band, respectively, and can bind to form excitons. Because of the low dielectric constant and the wide energy gap of the organic sheets, excitons are confined in the metal halide wells, with binding energy on the order of hundreds of millivolts [5]. Interestingly, the exciton binding energy can be directly tuned by exchanging the organic dielectric layer [26]. As a reference, the metal halide alone typically has binding energy on the order of tens of millivolts. The multi-quantum well structure gives an additional degree of freedom in the design of tailored luminescent materials, since the Eg and the relative band alignment between the inorganic and organic components, will determine the global material properties. An important feature of these multi-quantum well structures is the ability to self-assemble from simple solutions of the organic and inorganic salts, (R-NH3)2MX4

(a)

Energy

(b) Conducon bands

Eg (MX4)

Eg (R-NH3) Valence bands

Fig. 2 Schematics of a the \100[ oriented 2D perovskites structure and b the corresponding flat band energy diagram where the metal halide semiconducting sheet has a much lower bandgap than to the insulating organic ammonium layer. Adapted from Ref. [24]

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leading to highly crystalline thin films, as clearly visible in the X-ray diffraction pattern of a series of spin-coated films of (C4H9NH3)2PbX4 with different halide anions (Fig. 3a). The excitonic character dominates the photoluminescent properties of such materials, leading to very narrow emission spectra (due to high oscillator strength) and the small Stokes shift between absorption and emission (Fig. 3b). A striking property of 2D perovskites such as those characterized in Fig. 3 is the possibility to tune the optical absorption and luminescence by simple substitution of the halide anion. As a general trend, by increasing the halogen electronegativity, the M–X bonds become more ionic, the bandgap of the metal halide widens and the absorption and photoluminescence (PL) spectra blue-shift. This effect is clearly visible in Fig. 3b, where the PL spectra of the (C4H9NH3)2PbX4 can be shifted from the green to the blue and finally to the ultraviolet region of the spectra by substitution with I, Br, and Cl, respectively. But the versatility of hybrid perovskite is not limited to the full substitution of a single component, and mixed halide can also be prepared simply by controlling the stoichiometry in the precursor solution. In this way, it is possible to solution-process semiconducting thin films with the desired bandgap energy simply by exchanging the halide composition. A demonstration of such bandgap flexibility is reported in Fig. 4, where the excitation and PL spectra for a series of phenethylammonium lead halide perovskite is presented as a function of the type and relative amount of halide anion. In particular, by increasing first the Br content in the (RNH3)2Pb(BrxI4-x) compound and then the chloride in the (RNH3)2Pb(ClxBr4-x), one can continuously tune the PL emission from about 2.4 eV (corresponding to the pure, fully substituted iodide perovskite) to about 3.2 eV, with approximately 200 meV step depending on the halide stoichiometry [28]. Another peculiarity of this family of compounds is the flexibility in the choice of the organic cations, whose dielectric constant and chemical nature affect the optical and mechanical property of the resulting 2D perovskite. Ahmad et al. recently prepared a large series of aryl- and alkyl-

Fig. 3 a XRD patterns for spin-coated thin films of (C4H9NH3)2PbX4, where (a) X = Cl, (b) X = Br, and (c) X = I. The X-ray reflection indices are given for the X = I data and are the same for each sample [27]. b Room-temperature UV–vis absorption spectra for the same set of samples [24]. In each spectrum, the arrow indicates the position of the exciton absorption peak (with the wavelength in parentheses). In curve (c), the corresponding photoluminescence (PL) spectrum (kexc = 370 nm) is indicated by the dashed curve. Note the small (*15 nm) Stokes shift between the absorption and emission peaks for the excitonic transition

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Fig. 4 PL and excitation spectra for a (RNH3)2Pb(BrxI4-x) and b (RNH3)2Pb(ClxBr4-x) films measured at room temperature. Excitation energies of the incident light beam for the PL measurement are indicated in the figure. The organic cation RNH3? is phenethylammonium [28]

ammonium lead iodide perovskite (Fig. 5), demonstrating that thin films with welldefined crystallinity and optical properties can be readily obtained by simple selection of the organic moiety [29]. Variation of the organic component induces structural reorganization within the inorganic network, changing the electronic bandgap and the associated exciton binding energy. In general, long and narrow molecules are expected to favor the perovskite formation over bulky organic cations, whose steric interaction would destabilize the 2D perovskite. Importantly, stabilization and destabilization of the layered structure can also originate from the interaction or steric hindrance between the organic cations, giving an additional degree of freedom in the design of 2D materials [30, 31]. While in the previous examples the organic cations are spectating components only influencing the perovskite structure and the optical properties of the inorganic sheets, they can be chosen in order to participate actively in the optical absorption and emission properties of the material. In particular, narrow-bandgap

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- Methylphenethylamine (MP) -Phenylethylamine (PE) 2-Cyclohexyl-ethylamine (CH) Heptylamine (C7) Nonylamine (C9) Decylamine (C10) Dodecylamine (C12) Tetradecylamine (C14) Oleylamine (OL)

Fig. 5 Room-temperature exciton absorption and PL spectra of various organic moieties (R cyclic and long carbon chain) based hybrid perovskite (R-NH3)2PbI4 thin films. The name and structure of the organic cation is reported for clarity. Adapted from Ref. [29]

organic cations such as conjugated molecules or dyes can be intercalated into the inorganic structure [32, 33]. In this way, different charge transfer mechanisms (as depicted in Fig. 6) can take place, significantly modifying the perovskite optical properties. The organic cation bandgap and the relative energy band alignment can be chosen such that, following optical absorption in the inorganic sheets, the excitation will be transferred to the organic cations, which will in turn determine the photoluminescent properties of the materials (Fig. 6a). Otherwise, the cation can introduce a certain degree of energy level misalignment, such that charge transfer states from the organic cations to the inorganic sheets and vice versa can be obtained (Fig. 6b, c). These relaxation mechanisms could have interesting properties in photoconducting devices such as solar cells or photodiodes. An early and unique example of this strategy has been presented by Mitzi et al., who reported the synthesis and optical characterization of the (AEQT)PbX4 perovskite (X = Cl, Br, I, Fig. 6d), where AEQT is a diammonium-substituted oligothiophene chromophore [32, 33]. This hybrid material shows the optical absorption features of both the organic and inorganic components, and the emission can be tuned by exchanging the halide anion in the system, and hence the relative energy level alignment between the chromophore and the inorganic layer. Notably, they demonstrated electroluminescent devices with the (AEQT)PbCl4 as the active layer, with light emission coming solely from the oligothiophene moiety.

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(R-NH3)2MX4 e-

(a)

LUMO

(d)

CB

h HOMO

h+ VB

e-

(b) h

h+ e-

(c)

h h+

Fig. 6 a–c Schematics of the possible charge transfer mechanism occurring between the inorganic and organic sheets, depending on the energy level alignment. d Crystal structure of (AEQT)PbBr4 viewed down the b axis. The unit cell outline is shown by the dashed lines [32]

3 Optoelectronic Properties of 3D Perovskites The origin of the recent renewed interest towards 3D perovskite is due to the successful application of compounds such as MAPbI3 in photovoltaics. One of the most important features of hybrid perovskites is indeed the high optical absorption, which allows the preparation of efficient solar cells with active layers as thin as few hundred nanometers. The calculated absorption coefficient (e [ 105 cm-1) is comparable to or even higher than those of inorganic semiconductors such as GaAs

Fig. 7 Crystal structures and unit lattice vectors on the (00l) plane of the a tetragonal and b cubic phases of 3D hybrid perovskites [36]

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and amorphous silicon and results from the unique direct-gap p–p transitions [34]. The analysis of the temperature-dependent optical absorption allows identifying the phase transitions of the perovskite structure [35]. At low temperatures the MAPbI3 adopts an orthorhombic structure, characterized by an excitonic, steep absorption edge at *1.7 eV. Increasing the temperature above 120 K results in a red-shift of the absorption edge, corresponding to the rearrangement to a tetragonal phase, which is stable at room temperature (Fig. 7a). A broadening of the absorption takes place near room-temperature, leading to the distinctive, gradual onset of the MAPbI3 absorption between 1.6 and 1.7 eV. There is an additional phase transition above 330 K, leading to the cubic perovskite structure. By substituting the iodide anions with bromide, there is a shift in the absorption edge at high energy (*2.3 eV). While not of large interest for single-junction solar cells, MAPbBr3 has been studied for light-emission applications [37] due to the sharp luminescence peak in the green region of the visible spectra. MAPbBr3 adopts a cubic structure at room temperature (Fig. 7b) and shows a steep absorption with an increased excitonic character compared to the corresponding iodide perovskite. Analogous to 2D perovskites, the bandgap (and hence the absorption/emission spectra) of 3D compounds can be conveniently tuned by partial halide exchange from the pure iodide to the pure bromide material (Fig. 8a). Increasing substitution with chloride further enlarged the bandgap to cover the entire visible spectra, with the pure MAPbCl3 having a bandgap of *3.1 eV (Fig. 8b). The 3D MAPbX3 family of compounds perfectly exemplifies the versatility of hybrid perovskites, where substitution of a single component (the halide anion) permits selecting at will the optical property of the material. From the spectra in Fig. 8, one can also note how the excitonic contribution to the optical absorption, as seen by the sharp peak at band edge, increases from iodide to bromide and chloride. The pure MAPbCl3 was found to have an exciton binding

Fig. 8 a Absorption coefficient of mixed MAPb(I1-xBrx)3 perovskite thin films with increasing bromide content [38]. b Absorption spectra of mixed bromide-chloride perovskite films with increasing chloride content [39]. The inset shows photographs of the same film series

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energy of about 50 meV, more than double the value for MAPbBr3 (21 meV) [39]. This is a result of the stronger ionic character of the Pb-X bond, and it is a sign of an augmented exciton binding energy (EB) when moving from iodide to chloride. This has large implications in the design of electroluminescent devices, since the recombination mechanisms from free carriers are substantially different compared to those present in excitonic materials. Whether the photogenerated species exist at room temperature as excitons or free carriers in 3D perovskite is still subject of debate. Exciton binding energies for the MAPbI3 have also been reported in the range between 20 and 50 meV [40, 41], which does not exclude a priori the existence of excitons at room temperature (kBT = 25.6 meV). On the other hand, a recent study brought a new estimation of the dielectric constant for these compounds, reported to be higher than 70. By using this value, an EB on the order of only 2 meV was calculated, which would exclude the existence of excitons at room temperature [42]. Interestingly, MAPbI3 thin films do not show substantial PL at low excitation intensity [43], while intense PL can be observed by increasing the excitation power. This phenomena has been studied in solution-processed samples, where PL quantum yields (PLQYs) up to 70 % have been obtained at high excitation intensity ([1 W cm-2) [44]. The lower PLQY at low excitation was ascribed to the presence of defects through which non-radiative recombination can occur, i.e. PL becomes dominant over recombination at high fluence after the defects are filled. This behavior suggests a non-excitonic nature of the photogenerated charges (at least for MAPbI3), since high exciton densities would lead to a high probability of exciton– exciton annihilation, with a consequent drop of the quantum yield. A recent study on the trap states of MAPbI3 perovskite films gave important insights about the optoelectronic properties of these materials [45]. Through ultraviolet photoelectron spectroscopy (UPS), Wu et al. clearly identify an additional density of states above the valence band maximum (VBM), extending towards the Fermi level. These low density states with a broad energy distribution were associated with charge carrier (hole) traps on the surface of hybrid perovskite thin films. This effect would also be responsible for the generally low PLQY observed in 3D and 2D perovskite thin films, at least when measured at low excitation intensity. In electroluminescent devices, the low PLQY means that high current densities are needed to produce significant light emission, which leads to a decrease in power conversion efficiency of the LED. Hence, the importance of developing strategies to substantially enhance the photoluminescence of hybrid perovskites.

4 Enhancing the Photoluminescence of Hybrid Perovskites Highly photoluminescent materials are widely investigated for their potential applications in displays, lighting, and lasers. While the optical absorption and luminescence of hybrid perovskites have been studied in detail, the quantification of the photoluminescence processes has been explored only scarcely. As for inorganic semiconductors, one of the most promising strategies for a substantial PLQY enhancement is the preparation of nanostructured materials, such as quantum dots Reprinted from the journal

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(QDs). The spatial confinement of the excitons/excited states, together with a passivation of the surface of the material has been proven to substantially increase the luminescence yield. This phenomena was first observed for 2D hybrid perovskites obtained in a dielectric matrix such as poly(methyl methacrylate) (PMMA) [46, 47]. Although the PLQY was not estimated, the exciton binding energy and the PL intensity were observed to increase as a consequence of the nanoparticle size and the dielectric confinement of the polymer. Later, Kojima et al. showed that the PL of the MAPbBr3 3D perovskite could be enhanced by casting the precursor solution onto a porous alumina scaffold [48]. The porous alumina acts as a template by limiting the growth of the MAPbBr3 crystals, resulting in highly luminescent, green-emitting hybrid thin films (Fig. 9). The template approach was recently used to obtain highly photoluminescent mixed Br/Cl perovskites with tunable emission color, characterized by excited state lifetime on the order of hundreds of nanoseconds, an important feature in applications such as light-emitting devices [49]. An alternative way to obtain quantum-sized hybrid perovskite is the direct synthesis of nanoparticles (NPs) with tailored size and hence controlled properties. Schmidt et al. reported the preparation of stable dispersions of 6 nm-sized MAPbBr3 NPs, using octadecylammonium as the capping ligand [42]. Thin films obtained by spin-coating the NPs dispersion showed high photoluminescence with an associated quantum yield of about 20 %. Through modified synthetic conditions, the same group later demonstrated that surface states could be further passivated by insertion of a complementary weak Lewis base, and PLQYs up to 82 % in solution were obtained [50]. As in the case of bulk or polycrystalline samples, the emission wavelength can be easily tuned over the entire visible spectra by employing mixed halide systems (Fig. 10a, b) [51]. Importantly, in contrast to inorganic semiconductors, the optical bandgap of 3D perovskite NPs is weakly size-dependent, and the excitonic features that have sometimes being assigned to quantum-confined excitons in MAPbBr3 are instead characteristic of bulk 3D crystals [53]. In particular, the observed strongly blue-shifted emission was proven to raise from the formation of low dimensional perovskite platelets, whose structure and optical properties are analogous to bulk 2D perovskites [54–56]. Essentially, a reduction of the dimensionality of MAPbBr3

Fig. 9 In situ synthesis of luminescent MAPbBr3 nanoparticle under UV-light irradiation. A colorless MAPbBr3 precursor solution (left) is uniformly dropped onto a mesoporous alumina thin film. During spin-coating, the quick vaporization of the organic solvent triggers the crystallization of highly luminescent lead bromide perovskite nanostructures within the mesoporous alumina [48]

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Fig. 10 a Optical photograph of mixed halide MAPb(XxY1-x)3 (X, Y being I, Br, or Cl) perovskite suspensions under visible and UV light, with b the correspondent PL spectra [51]. c PL spectra for CsPbBr3 as a function of the particle size and d with partial or complete substitution with Cl (blue) or I (red) [52]

results in the formation of oriented platelets rather than in smaller, nanocrystalline 3D materials. Nevertheless, the recent breakthroughs in the PLQY ([90 %) [57], points at perovskite NPs as the most promising material for light-emission applications. Another class of promising nanostructured materials is the analogous inorganic perovskite, where a Cs? cation replaces the methylammonium intercalated in the inorganic framework. CsPbX3 NPs have optical properties similar to the hybrid perovskites, i.e. PLQY [90 % [58], tunable bandgap (weakly due to size effect, strongly through halide exchange, Fig. 10c, d) [52, 59, 60] and show enhanced thermal stability due to the absence of organic components. As in the case of MAPbX3, the high PLQY of CsPbX3 QDs result from negligible electron- or hole-trapping pathways, when passivated with proper capping ligands [61]. An alternative way to obtain highly photoluminescent perovskites is the use of a dielectric matrix that is solution-processed in conjunction with the perovskite precursors, similar to what was mentioned previously for 2D perovskites in PMMA matrix. The matrix acts as a scaffold that controls and limits the growth of the perovskite crystals. Several approaches have been used in the preparation of nanostructured perovskite thin films, in particular, small molecular weight organic molecules [62, 63], polymer dielectrics [64, 65], and metal oxide NPs suspensions [66]. While a strong reduction of the PLQY is often observed when NPs suspensions are processed into thin films, the use of an inert matrix during the crystallization Reprinted from the journal

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prevents aggregation, leading to PLQY values as high as 40 % in the solid state [66]. Moreover, there is a significant advantage in terms of synthetic ease, since the size and shape of the perovskite nanocrystal can be simply controlled by the relative amount of matrix added to the precursor solution [62, 66].

5 Thin Film Deposition Methods One of the main features of hybrid perovskites is the possibility to obtain high quality (both optically and electronically) semiconducting thin films with simple solution deposition methods. These deposition methods have the advantage of lowcost processing and are compatible with roll-to-roll fabrication. The perovskite precursors solutions are obtained by dissolving the inorganic and organic compounds (the metal halide salt and the desired organic ammonium halide) in solvents such as N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), cbutyrolactone (GBL), or acetonitrile. The perovskite can be deposited on a substrate by one-step deposition, usually by spin-coating (Fig. 11a), even though other wet coating methods (dip-, blade-, spray-coating) can lead to equivalent results. The film formation relies on the evaporation of the solvent, and this process may not leave enough time for the organic–inorganic compound to assemble in an ordered fashion. The size and connectivity among the perovskite crystals will determine the charge transport property of the thin-film [67]. For this reason the

Fig. 11 Schematics of the different solution deposition methods for hybrid perovskite thin-films, shown here in the case of methylammonium lead halides. Adapted from Ref. [67]

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reaction kinetics needs to be rigorously controlled in order to achieve the desired morphology and hence to maintain consistent optical and electronic properties [68]. A post - annealing process (80–150 °C) is usually required for a quantitative conversion of the precursors into a polycrystalline perovskite film. The quality of the layer is strongly dependent on the substrate used, and on the polarity of its surface. It is possible to increase or diminish the polarity of the surface by prespinning with surface tension modifiers (e.g. surfactants), by plasma or UV-O3 pretreatment. On the other hand, the size of the perovskite crystals depends on the solvent used and on its vapor pressure. The one-step deposition technique is the most common coating method used to prepare perovskite thin-film for photovoltaic and light-emitting devices. The first reported LEDs with organic–inorganic perovskites were prepared by making use of this method [69, 70]. Hybrid perovskite materials tend to rapidly crystallize when the solvent evaporates due to the strong ionic interaction between the inorganic anion and the organic cation. This behavior limits the control over the film formation during the one-step deposition technique. An interesting strategy to overcome this limitation is the casting of a nonpolar solvent (such as toluene or chlorobenzene) on top of a perovskite layer during its formation, i.e. while the precursors are reacting and drying on the spin-coater [15]. The non-polar solvent modifies the crystallization kinetics of the strongly ionic precursors, enabling the formation of very dense and homogeneous perovskite films

Fig. 12 Schematics of the different vacuum deposition methods for hybrid perovskite thin-films, shown here in the case of methylammonium lead halides. Adapted from Ref. [67]

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[15, 63, 71–73]. While this method has been widely applied for the deposition of 3D perovskites layers, it might be helpful also to obtain flat and well-defined 2D hybrid perovskite surfaces. A better kinetic control over the crystallization process can be obtained by sequential deposition [14]. The latter consists in the formation of the perovskite layer by intercalation of the organic moieties into a preformed inorganic framework. In this approach the inorganic layer is (spin-)coated on the substrate (Fig. 11b) and the organic cation is subsequently introduced either by dip- or spincoating (Fig. 11c, d) [14, 74–76]. The layer is usually annealed in order to promote the interdiffusion of the layers to obtain highly crystalline perovskite films. Besides solution processing, vacuum deposition has also been used to prepare hybrid perovskite thin films for optoelectronic applications. Vacuum methods have the advantage of a better control over the film stoichiometry and thickness, lead to very flat and homogeneous surfaces, in addition to high compositional purity [77]. Importantly, vacuum methods are intrinsically additive, meaning that multilayer perovskite devices could be easily assembled. As for solution-based perovskite deposition methods, also vacuum techniques can be classified into two main approaches, the dual-source vapor deposition and the sequential vapor deposition. In the dual-source vapor deposition the organic cation and the metal halide are simultaneously thermally evaporated in a high vacuum chamber, where they condense and react on a substrate placed above the molecular sources (Fig. 12a). The stoichiometry of the layer is controlled by adjusting the evaporation rate of the two components, giving real time control over the film growth [77]. Dual-source vapor deposition has been used for both 2D and 3D hybrid perovskites [78–80], demonstrating the flexibility of the method. As in the case of the solution-processing, also for vacuum methods the formation of the inorganic framework can be decoupled from the reaction and intercalation with the ammonium halide, by performing a sequential deposition (Fig. 12c) [81]. Again, both 2D and 3D hybrid perovskite thin films can be readily obtained through this process [81, 82]. Of course, hybrid sequential methods, where the organic and the inorganic moieties are deposited one by solution and the other by vacuum deposition, have also been demonstrated [83, 84]. A third vacuum deposition method that has been applied in the preparation of perovskite thin-films is flash evaporation, also known in the literature as singlesource thermal ablation [85, 86]. In this technique the perovskite is deposited as a powder or as a thin film on a tantalum foil, which is then connected to two electrodes in a high vacuum chamber. A large current passes through the foil and causes the instantaneous evaporation of the material that condenses onto a substrate (Fig. 12b). The advantage of this process is the possibility of rapidly (few seconds) depositing perovskite thin films maintaining the initial stoichiometry of the material. A wide variety of 2D perovskite films with interesting optical properties have been prepared with this method [29, 87]. More recently, flash evaporated MAPbI3 thin films have been integrated in efficient solar cells [88]. All the above mentioned methods can be combined in order to obtain materials with desired thickness, properties, and composition. A representative example has been recently demonstrated by Gil-Escrig et al., who prepared mixed halide

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Fig. 13 On the top, process schematics with the deposition steps used to prepare the MAPb(I1– xBrx)3 thin film series and, on the bottom, the correspondent evolution of the surface morphology as monitored by scanning electron microscopy (SEM, scale bar 500 nm) [89]

MAPb(I1-xBrx)3 perovskite thin films and integrated them in photovoltaic and lightemitting diodes [89]. Mixed iodide-bromide perovskite were obtained by initial dual-source vacuum deposition of a pure MAPbI3 layer (Fig. 13). A layer of PbBr2 was deposited on top and converted to the MAPbBr3 perovskite by sequential spin-coating with MABr solutions. The mixed phase was finally obtained by annealing the MAPbI3/ MAPbBr3 bilayer, with the I/Br ratio being controlled by the relative thickness of the two perovskite films.

6 Hybrid Perovskite Light-Emitting Diodes The perovskite LEDs presented so far closely resemble organic light-emitting diodes (OLEDs), in terms of both device structure and materials used. They are built on transparent substrates (glass or plastic foils) coated with a transparent conducting oxide (TCO), either indium tin oxide (ITO) or fluorine doped tin oxide (FTO). In general, two types of device architecture have been explored, the p-i-n and n-i-p, whose names depends on the polarity of the bottom transparent electrode (Fig. 14). In general, p-i-n diodes (Fig. 14a) make use of organic semiconductors as the charge transport materials. They consist of an ITO-coated glass substrate working as the anode, where holes are injected into the device. An organic hole injection layer (HIL), most notably poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS), is used to smooth the ITO surface and to have an ohmic contact with the top layer. A hole transport layer (HTL, also electron-blocking) can be deposited on top of the HIL, to ensure charge confinement within the electroactive material. The perovskite film is then deposited on top and capped with an electron transport layer (ETL) that can also efficiently block the holes injected from the anode. In this configuration, electrons must be injected into the lowest unoccupied molecular orbital (LUMO) of the ETL material, implying that the cathode must have a low work function (WF). Hence, while p-i-n diodes are extremely versatile devices since they benefit from the wide library of materials developed for OLEDs [90–92], they must be rigorously encapsulated to avoid rapid

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(a)

Perovskite

n i

HTL (oponal) HIL TCO

p +

Low WF Metal ETL

(b)

high WF Metal HTL Perovskite MOx ETL TCO

Substrate (glass)

+ p i n -

Substrate (glass)

p-i-n

n-i-p

Fig. 14 Schematics of device structures commonly adopted in the preparation of perovskite LEDs

oxidation of the cathode. n-i-p devices, on the other hand, partially overcome this limitation using an inverted structure (Fig. 14b), with the bottom transparent electrode functioning as the cathode and the top metal contact being the anode. In this way, high work function and hence stable metals can be used, potentially enhancing the device stability over moisture and oxygen. Whereas this strategy has been intentionally explored to enhance the stability of OLEDs in the past [93], the development of n-i-p perovskite LEDs comes from the successful application of this device structure in record efficiency perovskite solar cells [20]. In order to efficiently inject electrons, an n-type metal oxide (MOx, notably TiO2 or ZnO) film is commonly deposited on top of the bottom TCO. Because of the high temperature annealing usually employed in the preparation of MOx layers, the more thermally stable FTO is used. After the deposition of the perovskite active material, an organic HTL is deposited, sometimes in combination with a thin MoO3 layer in order to ensure ohmic charge injection from the top anode into the organic semiconductors. In the following sections, we will review the published work on perovskite LED, using both 2D and 3D hybrid materials.

Fig. 15 a Device structure and b energy diagram of the ITO/PAPI/OXD7/MgAg device. Adapted from Ref. [69]

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7 Electroluminescence from 2D Hybrid Perovskites Layered 2D hybrid perovskites have been extensively studied for their interesting optical absorption and photoluminescence; nevertheless, their electroluminescent properties have been scarcely explored. The first report on electroluminescence from 2D hybrid perovskites was published by Era et al. [69]. In this work an optoelectronic device was prepared by spin-coating the perovskite on an ITO-coated glass substrate, followed by vacuum deposition of an ETL and a Mg/Ag cathode (Fig. 15). The perovskite used as the emitting material was the phenethylammonium lead iodide (C6H5C2H4NH3)2PbI4 (PAPI), while the electron transport material was the 1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7). The low-dimensionality of the phenethylammonium lead iodide perovskite permits the formation of stable excitons with large binding energy and intense photoluminescence even at room temperature. However, electroluminescence was only observed when the device was driven at liquid nitrogen temperature. In these conditions, luminance levels up to 10,000 cd/m2 at a current density of 20,000 A/m2, were obtained with an applied bias of 24 V. When the temperature was increased up to 200 K, the electroluminescence intensity dramatically decreased. The intense emission observed at low temperature was believed to arise from the confinement of

Fig. 16 On the left: chemical structure of a series of layered perovskites. On the right: corresponding electroluminescence spectra (solid lines) and photoluminescence spectra (dotted lines) at 110 K. Adapted from Ref. [95]

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the holes at the perovskite-ETL interface (PAPI/OXD-7) due to the low-lying highest occupied molecular orbital (HOMO) of the OXD-7 compared to the perovskite valence band (Fig. 15b). On the other hand, no blocking interface exists for the injected electrons, likely causing charge recombination at the anode/PAPI interfaces, and hence low power conversion efficiency. When the device was measured at room temperature, instead, thermal quenching of the exciton occurred, with the consequent reduction of the electroluminescence intensity [69, 70]. In fact, as shown by previous works, the photoluminescence intensity (and, by extension, the electroluminescence) of PAPI was found to be almost constant until 250 K, while it drastically decreases at higher temperatures [94, 95]. The reduction of the photoluminescence intensity followed an exponential decay with the form exp(Ea/ kbT), with an activation energy Ea of about 220 meV, in good agreement with the exciton binding energy calculated for this perovskite. The authors suggested that quenching at high temperature is phonon-mediated, increasing the probability of non-radiative recombination paths, hence decreasing both the photoluminescence and electroluminescence intensities. Hattori et al. prepared very similar devices (ITO/emitting perovskite/OXD-7/MgAg) using three layered perovskites with different organic cations, phenethylammonium (Fig. 16a), cyclohexenylethylammonium (Fig. 16b) and phenylbutylammonium (Fig. 16c) [95].

Fig. 17 a Device structure. b Room temperature current–voltage (open squares) and electroluminescence-voltage (filled circles) for the device a. c Room temperature electroluminescence (solid line) and photoluminescence (dotted line, kexc = 360 nm) spectra of (AEQT)PbCl4. Adapted from Ref. [33]

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The PLQY of these three perovskites was found to be rather high, and hence they were expected to lead to intense electroluminescence. The first two materials (a–b) showed better device performances compared to the previous report: 3 cd/A (12000 cd/m2 with current density of 4000 A/m2 at 24 V) for the phenethylammonium lead iodide and 9.6 cd/A (4800 cd/m2 with current density of 500 A/m2 at 24 V) for the cyclohexenethylammonium lead iodide. However, the device characterization was performed at 110 K, since exciton quenching was observed, again, at room temperature. Electroluminescence spectra of the three perovskites correspond well to the excitonic emission observed by photoluminescence measurements (Fig. 16) [95]. Electroluminescence studies were also performed by Mitzi et al. on 2D system employing the oligothiophene derivative AEQT as the organic cation, whose optical properties have been illustrated before in this chapter [32, 33]. Interestingly, in the corresponding chloride perovskites (AEQT)PbCl4, the photoluminescence spectrum is dominated by the oligothiophene emission, while in the iodide compound, also the excitonic peak of the inorganic layer emission appears. This is due to the narrower bandgap of the lead iodide, resulting in only partial charge transfer to the organic dye. The authors prepared diodes essentially following the previously published embodiment. They deposited the dye-containing perovskite by flash evaporation (also called single-source thermal ablation) on top of ITO-coated quartz substrates, subsequently covering it with an OXD-7 ETL and a Mg/Ag cathode (Fig. 17a) [33]. Notably, these perovskite LEDs showed bright green electroluminescence even at room temperature. The turn-on voltage was much lower compared to previous reports (5.5 V), with maximum power conversion efficiency of 0.1 lm/W at 8 V. The electroluminescence spectrum coincides with the photoluminescence one (Fig. 17c), coming solely from the cationic dye. It is worth noting that this is the first report where intense electroluminescence was obtained at room temperature and also where light-emission was coming from the organic part of the perovskite. There are other reports where photoactive molecules have been inserted into layered perovskite structures, i.e. naphthalene [82, 96–98], naphtylmethylammonium [99], azobenzene [82, 100, 101], N-(3-aminopropyl)imidazole, pyrene [102], polyacetylenes [103], but in those works the electroluminescence was not investigated. Creating a hybrid of inorganic halide with organic dyes still remains a very promising path to obtain efficient photoluminescence and electroluminescence from 2D perovskites.

8 Electroluminescence from 3D Hybrid Perovskites As for 2D materials, 3D perovskites can be easily solution-processed at low temperature, and their optical bandgap can be tuned from the visible to the infrared regions [18, 36, 104, 105], making them attractive materials for low-cost and largearea optoelectronic applications. Also, this class of material offers remarkable PLQY and carrier diffusion lengths up to 175 lm. In light of the above, 3D perovskites are considered potential candidates for future novel optoelectronic

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devices beyond solar cells, such as for lasers, photodetectors, and LEDs [37, 106–110]. The first report of room temperature electroluminescence from 3D perovskite LEDs was published as recently as in August 2014 [37]. Tan et al. showed infrared emitting LEDs using an n-i-p configuration (ITO/TiO2/Al2O3 (1 nm)/MAPbI3-xClx/ F8/MoO3/Ag; Fig. 18a), where the thin Al2O3 was used to reduce luminescence quenching and the F8, poly(9,90 -dioctyl-fluorene), is used as the HTL. The partial chloride substitution in the perovskite is used to promote carrier diffusion as compared to the pure iodide compounds, but has negligible effect on the optical bandgap of the material [111]. They observed peak EL with radiance of 13.2 Wsr-1 m-2 at a current density of 363 mA cm-2, corresponding to an external quantum efficiency (EQE) of 0.76 %. In the same work, the authors demonstrated also the first p-i-n LEDs, with the structure ITO/PEDOT:PSS/MAPbBr3/F8/Ca/Ag device. The use of the MAPbBr3 perovskite shifted the emission to green EL (centered at 520 nm), with an associated EQE of 0.1 %. The lower EQE observed

Fig. 18 a Flat band energy diagram of an n-i-p infrared LED using the CH3NH3PbI3-xClx perovskite as the emitting material. b Absorption (black), normalized EL (green, solid), and normalized PL (green, dashed) spectra of MAPbBr3 perovskite. Normalized EL spectrum of the MAPbBr2I mixed halide perovskite is shown in red. Inset image: Uniform green and red electroluminescence from p-i-n diodes using the two compounds [37]. c Cross-sectional electron microscope images of a p-i-n MAPbBr3 LEDs using a modified PEDOT:PSS HIL (Buf-HIL), and d luminance versus voltage characteristics for a series of LEDs with increasing Buf-HIL work function [114]

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for the green LED can be due to the energy mismatch for the hole injection from the PEDOT:PSS HIL (-5.0/-5.2 eV) into the MAPbBr3 valence band (-5.9 eV) [112]. Red-emitting (630 nm) diodes were also obtained, this time by employing the mixed MAPbBr2I in the same p-i-n configuration. Interestingly, the EQE was found to increase with increasing current density, demonstrating a need for high charge densities to achieve efficient radiative recombination. This agrees with previous observations of high PLQY only at high excitation intensity [113]. The same effect was observed by Gil-Escrig et al., who studied the EL properties of efficient p-i-n solar cells based on the CH3NH3PbI3 perovskite [115]. They showed how, by current driving the device using square waves with 50 % duty cycle at 100 Hz, they were able to increase substantially the net current densities without damaging the device. Other works have exploited a device configuration commonly employed for in photovoltaics. Jaramillo-Quintero et al. prepared infrared LEDs based on CH3NH3I3-xClx using a compact layer of TiO2 and Spiro-OMeTAD as electron and hole injecting layers [116]. Significant EQE values were obtained (0.42 % in average, with a record of 0.48 %) with a very low turn-on voltage. Almost at the same time, Kim et al. showed bright p-i-n perovskite LEDs with the structure ITO/Buf-HIL/perovskite/TPBI/Al (Fig. 18c), where the ETL TPBI is

Fig. 19 a EL spectra from perovskite LEDs using MAPbBr3 (green circles) and MAPbI1.25Br1.75 (red squares) perovskite semiconductors; inset shows PL from the mixed halide MAPbI1.25Br1.75 films [117]. b EL spectra of blue and green LEDs obtained with MAPbBr1.08Cl1.92 (blue circles) and MAPbBr1.86Cl1.14 (green squares), respectively [118]. c Device structure and flat band energy diagram of n-i-p perovskite LEDs. d Normalized EL spectra of a series of MAPb(BrxCl1-x)3 perovskite thin film based LEDs with different chloride/bromide ratios as indicated and measured at 77 K [119]

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2,20 ,200 -(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), and Buf-HIL is a modified PEDOT:PSS film [114]. As mentioned before, an energy barrier for the hole injection is present at the PEDOT:PSS/perovskite interface. By adding a perfluorinated polymeric acid (NafionÒ) to the commercial PEDOT:PSS suspension, they were able to enlarge the Buf-HIL work function down to -5.95 eV, essentially matching the perovskite valence band energy. Moreover, with increasing Buf-HIL work function, the PL intensity of a MAPbBr3 film was observed to rise substantially. Through this strategy, they observed increasingly high luminance values (Fig. 18d) and were able to observe EL also from the wide-band gap (3.1 eV), blue-emitting, pure MAPbCl3 perovskite. However, the device EQE was still low due to the presence of an energy barrier, this time for the electron injection, from the aluminum cathode into the LUMO of the TPBI. The possibility to tune the perovskite bandgap, and hence the emission wavelength, has also been exploited in LEDs. In a series of works, Kumawat et al. studied the optoelectronic properties of a series of 3D perovskites partially exchanging the halide X from the pure iodide progressively to the pure chloride MAPbX3 [117, 118]. They were able to modulate the EL spectra from the red to the blue part of the spectra, as depicted in Fig. 19a, b. The device structure used a fullerene as the ETL and PEDOT:PSS as the HIL, which resulted in a modest EQE, independently of the perovskite composition. The authors ascribed this effect to a significant quenching of the electrically generated excited states by the two transport materials. Interestingly, they observed the PL intensity increasing with increasing perovskite bandgap. Unfortunately, the corresponding LEDs did not perform efficiently, probably due to the consequent increase of the energy barrier for the charge injection. Sadhanala et al. also investigated the possibility of simple color tuning by modifying the perovskite bandgap from the green to the blue region of the visible spectrum [119]. They overcame the challenge of incorporating chloride into perovskite structures containing bromide using a mixed solvent approach and an organic source of lead. The perovskite family MAPb(BrxCl1-x)3 was used in the fabrication of LEDs with tunable color and very narrow spectral features (Fig. 19d). Despite the rather complicated device structure (Fig. 19c), that should ensure efficient charge injection and confinement into the perovskite emitter, very low EQE were obtained and blue EL was observed only at low temperature (77 K). In perovskite photovoltaics, the effect of the film morphology (homogeneity and crystallinity) has soon been identified as a key factor leading to high power conversion efficiency devices [67, 120, 121]. On the other hand, the film structure– property relation in perovskite LEDs has not yet been fully investigated nor understood. In general, pinhole-free, flat, and highly luminescent perovskite films are desired since they would result in a homogeneous field distribution and lower non-radiative recombination in the device. As described before, one way to enhance the PLQY of perovskite films is the deposition of nanostructure films, achieved by co-deposition of a templating matrix, such as PMMA or Al2O3 nanoparticles [46–48, 66]. Li et al. prepared single-layer perovskite LEDs using a composite active layer obtained by blending MAPbBr3 and poly(ethylene oxide) (PEO) [64]. They showed that not only were the optical properties of the perovskite enhanced by

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blending the material with PEO, but also the film morphology and connectivity could be substantially improved. By integrating the composite thin-film into devices, they obtained very bright ([4000 cd/m2) even though not yet efficient LEDs. In a similar fashion, Li et al. deposited a blend of perovskite and a dielectric polyimide precursor (PIP), forming perovskite nanocrystals in a thinfilm matrix of PIP [65]. As shown in Fig. 20a, the pure MAPbBr3 perovskite film consists of crystals of approximately 250 nm in dimension, with voids essentially dominating the film structure. In composite films, the crystal size decreases with increasing PIP polymer concentration down to about 60 nm (Fig. 20b, c). At this concentration, the perovskite crystals are embedded in a uniform polymer layer. The PIP formed a pinhole-free charge-blocking layer, while still allowing the embedded perovskite crystals to form electrical contact with the adjacent layers. This composite film was used as the emitting material in a p-i-n LED, with PEDOT:PSS and F8 as the hole and electron injection layers, respectively. Through the increase in the blending ratio of PIP, the current density decreases at each driving voltage and the luminance is enhanced (Fig. 20d, e), thereby leading to a substantial enhancement in device efficiency (Fig. 20d, e), with maximum EQE of 1.2 %. As shown in Fig. 20f, the F8 contribution to the electroluminescence spectrum could be observed between 400 and 500 nm for the pure MAPbBr3 devices. The F8 emission most likely arises from hole injection directly from PEDOT:PSS into the F8, due to the not homogeneous perovskite layer. In contrast, F8 electroluminescence was substantially diminished (but not fully quenched) in the PIP/perovskite LEDs, with EL coming mainly from the perovskite film. Most of the LEDs reported so far employed F8 as a wide-band gap charge transport and blocking layer in combination with the perovskite. Unfortunately, as in the previous example, perovskite LEDs with F8 show parasitic blue emission due to the formation of excitons in the polymer across the perovskite-free voids [122].

Fig. 20 Top view SEM image of a pure MAPbBr3, b 1/10 PIP/perovskite, and c 1/2 PIP/perovskite films on PEDOT:PSS-coated silicon (scale bar is 1 lm). d Current density (solid line) and luminance (dashed line) versus voltage characteristics for perovskite LEDs with different PIP content. e EQE versus current density characteristics for the same device series. f EL spectra of F8 LED, pure perovskite, and PIP/ perovskite blend LEDs. Adapted from Ref. [65]

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For this reason, several investigations explored the use of metal oxides (in n-i-p diodes) as ohmic contact to the perovskite emitters. In particular, by applying ethanolamine or polyethyleneimine (PEI) on top of the metal oxide surface (either ZnO or TiO2), efficient charge injection and enhanced device performances were obtained [109, 123]. While EQE up to 2 % have been achieved through this method, the perovskite LEDs were unstable due to the very high current densities even at low applied bias. A step forward in the development of hybrid perovskite LEDs has been recently presented by Lee et al. [63]. In this work, the authors tackled and overcame all the device and material limitations discussed above. They first developed a method to reduce effectively exciton quenching, by a modified MAPbBr3 emission layer. They controlled the perovskite thin-film morphology with a method called nanocrystal pinning, i.e. by casting a non-solvent during spin-coating of the organic/inorganic precursors. In particular, MAPbBr3 compact and homogeneous nanostructured layers could be fabricated by casting pure chloroform (S-NCP, Fig. 21a) or chloroform-TPBI solutions (A-NCP, Fig. 21b), onto the spinning layers during the formation of MAPbBr3. The MABr:PbBr2 stoichiometry ratio was optimized to 1.05:1, since a small excess of MABr was found to passivate efficiently the perovskite and hence significantly enhance the PLQY. Additionally, the excess MABr favorably reduces the ionization energy (IE) of the MAPbBr3 thin film, facilitating hole injection from the anode (Fig. 21d). Following a previously described method [114], PEDOT:PSS was modified with a perfluorinated polymeric acid in order to enlarge the work function and achieve ohmic injection into the perovskite emitting layer (the modified PEDOT:PSS was called self-organized conducting polymer, SOCP). By using an ITO-free p-i-n

Fig. 21 SEM images of perovskite layers obtained with a ratio of MABr:PbBr2 = 1.05:1, a with S-NCP, and b with A-NCP. c Cross-sectional SEM image of a perovskite LED and d corresponding energy band diagram, showing a decrease in IE with increasing MABr content. e Current efficiency and f luminance of LEDs based on S-NCP and A-NCP MAPbBr3 emissive layers [63]

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device with the structure SOCP/MAPbBr3/TPBI/LiF/Al, very bright EL was achieved. High current efficiency (21.4 cd/A) was obtained with S-NCP perovskite, while the LEDs based on A-NCP MAPbBr3 had a maximum efficiency of 42.9 cd/A (Fig. 21e). The latter value corresponds to an impressive EQE of 8.53 %, far higher compared to any previous reports of perovskite LEDs. This work has reduced the efficiency gap between perovskite LEDs and polymer or quantum dot LEDs and highlights the huge potential of hybrid perovskite in the development of novel efficient and simple optoelectronic devices.

9 Summary Hybrid perovskites are revolutionizing the way semiconducting materials can be prepared and assembled in optoelectronic devices. For the first time, researchers have the possibility to prepare high mobility, highly absorbing, and luminescent semiconductors with simple solution processing, without having to rely on commercial sources or complicated deposition tools. The optical properties of this class of materials are not yet fully disclosed, and more studies need to be performed in order to picture clearly a relation between the chemical composition, the crystalline structure, and the photophysics of such compounds. The development of efficient perovskite LEDs will benefit from the already published thorough investigations on the photovoltaic properties of the materials. Considering the variety of possible materials, from 2D to 3D, the virtual infinite color emission and ionization energies, the potential of hybrid perovskites in electroluminescent applications is enormous. As for perovskite solar cells, the device structure and the energy levels alignment, and especially the perovskite morphology, needs to be carefully tuned in order to control and enhance the performance of LEDs. Acknowledgements We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) via the Unidad de Excelencia Marı´a de Maeztu MDM-2015-0538 and MAT2014-55200, PCIN-2015-255 and the Generalitat Valenciana (Prometeo/2012/053). M.S. thanks the MINECO for the post-doctoral (JdC) contract.

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Top Curr Chem (Z) (2016) 374:47 DOI 10.1007/s41061-016-0048-9 REVIEW

The Rise of Near-Infrared Emitters: Organic Dyes, Porphyrinoids, and Transition Metal Complexes Andrea Barbieri1 • Elisa Bandini1 • Filippo Monti1 Vakayil K. Praveen2 • Nicola Armaroli1



Received: 22 April 2016 / Accepted: 20 June 2016 / Published online: 19 July 2016  Springer International Publishing Switzerland 2016

Abstract In recent years, the interest in near-infrared (NIR) emitting molecules and materials has increased significantly, thanks to the expansion of the potential technological applications of NIR luminescence in several areas such as bioimaging, sensors, telecommunications, and night-vision displays. This progress has been facilitated by the development of new synthetic routes for the targeted functionalization and expansion of established molecular frameworks and by the availability of simpler and cheaper NIR detectors. Herein, we present recent developments on three major classes of systems—i.e., organic dyes, porphyrinoids, and transition metal complexes—exhibiting the maximum of the emission band at k [ 700 nm. In particular, we focus on the design strategies that may increase the luminescence efficiency, while pushing the emission band more deeply in the NIR region. This overview suggests that further progress can be achieved in the near future, with enhanced availability of more robust, stronger, and cheaper NIR luminophores. Keywords Near-infrared luminescence  Organic dyes  Porphyrinoids  Transition metal complexes

This article is part of the Topical Collection ‘‘Photoluminescent Materials and Electroluminescent Devices’’; edited by Nicola Armaroli and Henk Bolink. & Vakayil K. Praveen [email protected] & Nicola Armaroli [email protected] 1

Istituto per la Sintesi Organica e la Fotoreattivita`, Consiglio Nazionale delle Ricerche (ISOFCNR), Via Gobetti 101, 40129 Bologna, Italy

2

Photosciences and Photonics Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram, Kerala 695019, India

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1 Introduction Luminescence is a physical phenomenon of wide-ranging and increasing importance [1]. Life itself on our planet is sustained by light emission from the outer shell of the sun as a result of a complex chain of absorption/emission events triggered by c-rays emitted upon nuclear fusion processes [2]. In our modern daily lives, we take advantage of several light emitting devices, for instance when we switch on artificial lights, watch the screen of a telephone or a computer, or check out at the supermarket. Luminescence can be defined as the generation of light by matter through the formation of transient electronically excited states which, partially or totally, deactivate through emission of electromagnetic radiation in the visible (Vis) and near-infrared (NIR) spectral regions. Such states can be produced through a variety of stimuli such as light (photoluminescence), voltage (electroluminescence), heat (thermoluminescence), electrons (cathodoluminescence), chemical potential (chemiluminescence), mechanical action (piezo/triboluminescence), acoustic waves (sonoluminescence), and ionizing radiation (radioluminescence). Each of these subfields entails extensive scientific exploration, sometimes at the frontier of knowledge, and a wealth of technological applications ranging from lighting to telecommunications and from lasers to the investigation of biological environments [3]. In the latter area, luminescence has become a very successful analytical tool due to the exceptional sensitivity and low toxicity compared to, for instance, methods based on radionuclides [4, 5]. In the area of photochemical sciences, luminescence is perhaps the most powerful tool to trace the generation and fate of electronic excited states produced by light stimulation [6]. This affords key kinetic information to optimize functional supramolecular systems or nanomaterials for specific applications such as solar energy conversion. The vast majority of luminescent molecules and materials produced and investigated in the last decades emits in the visible spectral region, from violet to red [6]. This trend has been driven by several factors: (1) the main practical applications (e.g., light sources, displays) require an emission output detectable by the human eye; (2) plenty of robust and cheap photodetectors for Vis detection are available, making basic scientific instrumentation generally affordable; (3) according to the energy gap law, luminescence quantum yields tend to decline by decreasing the emission energy [7], therefore NIR detection is typically more challenging than Vis; (4) high sensitivity detectors for low-energy NIR light need to be cooled down at cryogenic temperatures to limit background noise, hence NIR instrumentation is typically more complex and substantially more expensive to buy and maintain compared to Vis apparatuses. In recent years, a marked change has occurred and NIR luminescence has moved out from obscurity to become a quickly expanding and increasingly popular field of investigation [8–10]. This has been driven by the availability of more affordable and sensitive NIR detectors and, even more, by the expansion of some technological fields in which NIR luminescence is a perfect tool for analytical detection and transmission of information. This applies in particular to imaging in biological

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environments [10], which are much more transparent to NIR than Vis radiation and to telecommunications via fiber optics, which transmit NIR photons with very high efficiency [11]. We present herein three selected classes of NIR emitting molecules (i.e., with kmax C 700 nm) that have emerged in the last two decades, namely organic dyes, porphyrinoids, and transition metal complexes. Our aim is to highlight recent trends in molecular design to enhance emission performance, which may drive future developments in a field of research that is clearly poised to expand in the years to come. The reader may refer to recent literature for other established or emerging classes of NIR emitting materials and molecules, such as metal complexes of trivalent lanthanide ions (e.g., Yb, Nd, Er) [11], fused polycyclic aromatic compounds [12], core-enlarged perylene dyes [13], semiconducting single-walled carbon nanotubes [14, 15], quantum dots and other nanomaterials [16, 17], which are not examined in the present review.

2 Organic Emitters Molecules of fascinating colors are popular as dyes and are widely used in staining. Recently, extensive use of dyes in advanced applications, such as ink-jet printing, imaging, and in electronics, has expanded substantially. This trend helped chemists to widen the search of next-generation functional dyes. In particular, a great deal of interest is concentrated on dyes whose absorption and/or emission lies within the NIR spectral region ranging from 750 to 2000 nm, which may lead to advanced applications in night-vision target identification, information security display, bioimaging, and sensors [18–22]. Besides this, it is well admitted that the materials used in solar cells should have good light-harvesting capability starting from the UV–Vis spectral range to the NIR range, as sunlight possesses 50 % of its radiation energy in the IR region. Though NIR-emissive organic materials (emitting beyond 750 nm) are far less common than NIR-absorbing organic materials, they are well known for their potential applications in telecommunications, displays, and bioimaging [18–22]. They are particularly interesting for the latter use, because biological environments have typically no autofluorescence in the NIR and tissues are virtually transparent in the window between 750 and 900 nm. In this section, we review some of the recent developments related to NIR fluorescent organic molecules. 2.1 Cyanines The basic structural motif of classical cyanine dyes is a polymethine chain bridging two aromatic nitrogen-containing heterocycles (Fig. 1a) [20, 21]. One of the merits of cyanine dyes is that their optical properties can be tuned from Vis to NIR by changing the number of methine groups in the bridging chain [18, 19, 21, 22]. The addition of each vinylene group introduces nearly a 100-nm bathochromic shift, thus the emission wavelength of heptamethine cyanine reaches well into the NIR region

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(a) X N R

N R

n

(b) SO3H

O3S R N

N

Ph

NC

O

N

EM

= 0.17

1b: R = HN Ph

1

O

1a: R = Cl max = 803 nm,

CHNH2 NBu4 CN

NC

max

= 757 nm,

EM

= 0.47

max

= 936 nm,

EM

= n.r.

CN

CN

2

Fig. 1 a General structure of cyanine dyes, b chemical structures of NIR-emitting cationic and anionic cyanines. Data for cationic and anionic cyanines, respectively, refer to aqueous and CH2Cl2 solution at room temperature

[18, 19, 21, 22]. Cyanine dyes, characterized by narrow absorption band and high molar extinction coefficients ([105 M-1 cm-1), are weakly luminescent due to the flexibility of the polymethine bridge that undergoes isomerization in the excited state [18, 19, 21]. Introduction of chlorocyclohexenyl group as a part of the polymethine chain (1a, Fig. 1b) has been found to stiffen the backbone, thereby improving quantum yield (kmax = 803 nm, UEM = 0.17) as well as photostability [20, 21, 23]. The replacement of the chloro group of cyclohexenyl moiety with substituted amine enables heptamethine cyanine dye 1b (Fig. 1b) to display a large Stokes shift (155 nm) and NIR emission with very good quantum yield (kmax = 757 nm, UEM = 0.47) due to the excited state intramolecular charge transfer occurring between the donor and acceptor moieties in the dye [23]. The photostability of these dyes is further improved by reducing the electron density at the amine moieties by introducing electron-withdrawing groups such as the acetyl residue exhibiting superior performance in in vivo studies compared to other commercial NIR dyes [23]. Introduction of electron-donating groups at the N-atom of aromatic heterocycles is also found to improve the photostability of NIR-emitting cyanine dyes [24]. Furthermore, intramolecular crosslinking of the 1,10 position (Natom of aromatic heterocycle) of classical heptamethine dyes has been found to enhance the photo and thermal stabilities [25]. Thus, cyanines are considered an excellent platform in the design of probes useful for the detection and imaging of various analytes such as metal ions, anions, pH, enzymes, thiols, reactive oxygen, and nitrogen species [18, 19, 22].

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In general, cyanine dyes are cationic, an exception of this is the anionic dye featuring tricyanofuran terminal groups 2 (Fig. 1b) [26]. Compared to the analogues cationic cyanine dye, the anionic dye shows red-shifted absorption (kabs = 900 nm) and emission (kmax = 915 nm) with very good thermal stability, which makes it suitable for bioimaging applications. 2.2 Pyrrolopyrrole Cyanines Pyrrolopyrrole cyanine (PPCy) dyes are new members of the family of cyanine-type dyes but, unlike the parent molecules, they are nonionic [27–30]. PPCy systems show good absorption in the NIR region but, however, are nonluminescent due to torsional vibration mediated nonradiative decay of the excited state [27, 28]. This issue has been circumvented by stiffening the chromophore through chelation with BF2 and BPh2, which strongly reduces radiationless decay pathways and makes them highly fluorescent (Fig. 2) [27, 28]. The luminescence quantum yield values observed for the dyes 3a and 3b (UEM = 0.32–0.69) are exceptional for NIR fluorophores. Studies have shown that the optical properties of PPCy dyes can be tuned in a wide spectral range by changing the terminal heteroaromatic substituents [28]. Interestingly, some of the derivatives of PPCy dyes such as the BF2 and BPh2 chelated bis(pyrrolopyrrole) cyanines 4a and 4b (Fig. 2) exhibit strong fluorescence in the spectral region close to 1 lm [29]. The design of asymmetric NIR-emitting BF2 chelated PPCy has allowed biofunctionalization of the dye and its utilization for live cell imaging [30]. 2.3 Squaraines Squaraines belong to the class of polymethine dyes with resonance-stabilized zwitterionic structures [31–38]. Squaraine dyes are characterized by a donor– C12H25O

OC8H17

C12H25O CN

CN R N B R N A

A N R

B N R

NC

3

N N R

B N R

C12H25O

OC8H17

C12H25O R R N B N NC

3b: R = Ph

max

= 708 - 805 nm

EM

= 0.32 - 0.69

max

= 749 - 881 nm

EM

= 0.32 - 0.62

S

CN

N N R B R

S

OC12H25 C12H25O

OC12H25

4

A = Aromatic Heterocycles 3a: R = F

C12H25O

OC12H25

4a: R = F

4b: R = Ph

max

= 924 nm

EM

= n.r.

max EM

OC12H25 R R N B N N NC OC12H25

OC12H25

= 966 nm = n.r.

Fig. 2 Chemical structure of NIR-emitting BF2 and BPh2-chelated PPCy dyes. All the data refer to CHCl3 solution at room temperature

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acceptor–donor type substitution pattern that consists of electron-rich aromatic or heterocyclic moieties on both sides of an electron-deficient oxocyclobutenolate core. In general, the optical properties of squaraine dyes are quite similar to cyanine dyes. However, the unique combination of photostability and optical properties that reach the NIR region make squaraines attractive for nonlinear optics, molecular recognition, biolabeling and imaging, photodynamic therapy, bulk heterojunction, and dye-sensitized solar cells [31, 32, 34–39]. Although squaraine dyes show intense and sharp optical features in solution, the strong propensity to aggregate and the susceptibility of the central cyclobutene ring to undergo chemical attack are considered as drawbacks. The physical encapsulation of squaraine dyes in tetralactam macrocycles has been found to overcome these issues [34]. Based on this approach, a variety of stable NIR-emissive squaraine dyes that can be used in in vivo imaging has been developed [34]. It has been demonstrated that the optical properties of squaraine dyes can be pushed to the NIR region by extending the conjugation length as well as increasing the strength of the donor moiety conjugated with the central core [32–37]. The recent finding of the unusual halogen effect in the NIR-emitting properties of squaraines is particularly interesting [40, 41]. The dicyanovinyl functionalized squaraines 5 and 6 (Fig. 3) exhibit a bathochromic shift in absorption and emission properties with an increase in molar extinction coefficient and fluorescence quantum yield depending on substituents H \ Cl \ Br \ I (Fig. 3). This trend is against the common perception of the effect of heavy atoms on the fluorescence properties of squaraines, which are known to quench the luminescence by favoring the intersystem crossing through strong spin–orbit coupling [36]. Detailed optical, crystallographic, and computational studies showed that the high-fluorescence quantum yield values of squaraines even in the NIR region are due to the stiffening of the structure, as their central four-membered ring restrains the conjugated backbone of the molecule in the cisoid conformation (Fig. 3) [40, 41]. The observed halogen effects on the optical properties is correlated with the polarizability of halogen substituents, which, in fact, determine the transfer of electron density from the substituents to the chromophore core. NC Bu

N

CN

O

5

X

NC Bu

N

CN

X

X O N H25C12

N C12H25

6

X

5a: X = H

max

= 698 nm,

EM

= 0.37

6a: X = H

max

= 890 nm,

EM

= 0.10

5b: X = Cl

max

= 708 nm,

EM

= 0.47

6b: X = Cl

max

= 913 nm,

EM

= 0.11

5c: X = Br

max

= 710 nm,

EM

= 0.47

6c: X = Br

max

= 916 nm,

EM

= 0.12

5d: X = I

max

= 713 nm,

EM

= 0.58

6d: X = I

max

= 922 nm,

EM

= 0.17

Fig. 3 Chemical structure of NIR-emitting dicyanovinyl functionalized squaraines. All the data refer to CH2Cl2 solution at room temperature

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2.4 BODIPYs Borondipyrromethenes (BODIPYs) can be considered as rigid cross-conjugated cyanine dyes (Fig. 4). These dyes display narrow and structured optical features with high molar extinction coefficients and relatively small Stokes shifts. The spectroscopic properties of these dyes have been found less influenced by environment factors such as solvent polarity and pH [42–46]. BODIPYs are well appreciated for their ability to fluoresce with high quantum yields approaching unity [42–46]. This excellent emission performance of BODIPY dyes is attributed to the presence of boron atoms in the polymethine chain, which introduces rigidity to the molecule and strongly limits non-radiative decays of the excited state through trans–cis isomerization and twisting. The combination of properties of BODIPY dyes makes them a useful platform to design NIR-emitting molecules [18, 19, 22, 46]. In general, the pristine BODIPY core exhibits the lowest energy absorption and emission features at around 500 nm [42, 43]. Over the years, a number of strategies have been developed to shift the optical properties of BODIPYs in the far-red and NIR regions (Fig. 4) [12, 19, 22, 42, 44, 47, 48]. These approaches mainly aim at extending the conjugation length and lowering the resonance energy, including (1) attachment of a styryl or ethynyl phenyl substituent to the 3 and/or 5-positions, (2) fusion of a rigid aromatic ring to the pyrrole unit, (3) replacement of meso carbon with a nitrogen atom to form aza analogues, and (4) introduction of polycyclic aromatic compound at the meso position [12, 19, 22, 38, 42, 44, 47, 48]. Apart from these methods, the attachment of thiophene at 1-, 3-, 5-, and 7-positions of the aza-BODIPY has demonstrated to red shift the absorption (kabs = 733 nm) and emission (kmax = 757 nm) in comparison to the phenyl-substituted derivative (kabs = 643 nm, kmax = 673 nm) [49]. The origin of the bathochromic shift is ascribed to the reduction of torsion angles and the increase of electron donation to the aza-BODIPY core, when the thienyl group replaced the phenyl substituents. Grafting of short oligomeric thiophene units to the 3,5-positions of BODIPY has shown to tune the emission maxima of the dye from 640 (UEM = 0.78) to 769 nm (UEM = 0.14) in CH2Cl2 solution [50]. 2.5 Rhodamines Rhodamine dyes are excellent orange-red-emitting chromophores having good photostability [51, 52]. These dyes have been used extensively for the design of fluorescent probes and labeling agents [22, 51, 52]. Recently, few new approaches have been developed to shift their emission maxima in the NIR region with a view

BODIPY core

Fig. 4 Different molecular approaches used to shift the optical properties of BODIPY in the NIR region

aza-BODIPY

meso 1 2 3

8

N N B F 4 F

7

extension of -conjuation

N 6

5

R

N N B F F

R

functionalization

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Top Curr Chem (Z) (2016) 374:47 Fig. 5 Chemical structures of Si-rhodamine 7 and Changsha dyes 8 showing NIR emission

COOH

N

max

N CF3COO

Si

R

N R

7

8

SiR700

Changsha Dyes

= 712 nm,

EM

O

N ClO4

= 0.12

to utilize them in bioimaging [53–55]. The chemical modifications on the xanthene core such as replacement of oxygen with silicon has been found to be an effective method to develop NIR-emitting rhodamine analogues (7, Fig. 5) [53, 54]. The bathochromic shift of the optical properties of Si-rhodamine derivatives 7 (Fig. 5) compared to the classical rhodamine dyes is attributed to the relatively low lying lowest unoccupied molecular orbital (LUMO) energy levels. This effect is facilitated by r*–p* conjugation, which results from the r* orbital of silicon– carbon (exocyclic methyl groups) and the p* orbital of the adjacent conjugated carbon atoms. In a different approach, a new series of dyes known as Changsha NIRs (8, Fig. 5) have been developed. The objective is the combination of the spirocyclization-based luminescence on–off switching exhibited by classical rhodamines with merocyanines, which are NIR dyes [55]. Changsha NIRs 8 exhibit strong emission in the NIR region (kmax = 721–763 nm) with relatively good quantum yield values (UEM = 0.29–0.56) in ethanol. 2.6 Donor–Acceptor Substituted Chromophores An appealing strategy to develop chromophores with optical properties in the NIR region is linking electron donor (D) and acceptor (A) molecular units through a conjugated p-spacer (Fig. 6) [19, 21]. The D-p-A-p-D substitution pattern has been found to enhance the electronic communication between the donor and acceptor moieties and thus considerably reduce the band gap energy [19, 21, 56–59]. The NIR optical features of such chromophores are determined by the strength of the donor–acceptor units, the choice of conjugated p-spacer, and the combination thereof. The most commonly used donor units includes diarylamine, thiophene, and fluorene, and the acceptor units are mainly heterocycles such as derivatives of benzo bis(1,2,5-thiadiazole) and diketopyrrolopyrrole. Electron-rich p-spacers such as thiophene and pyrrole are preferable over phenyl because they facilitate quinonoid structure formation and intramolecular charge transfer. In Fig. 6 are reported three families of molecules (9–11), which exemplify this strategy [56].

3 Porphyrinoids Porphyrins are conjugated tetrapyrrolic macrocycles, which exhibit remarkable chromophoric, luminophoric, and electrochemical properties [60]. They are considered ‘‘the pigments of life’’ due to their wide presence in biological

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Top Curr Chem (Z) (2016) 374:47 C8H17O

Fig. 6 Chemical structures of NIR-emitting D-p-A-p-D chromophores. All the data refer to toluene solution at room temperature

OC8H17 N

X

N

N

N N

C8H17O

N

Y

OC8H17

9

9a: X, Y = S

max

= 1065 nm,

EM

= 0.071

9b: X = Se, Y = S

max

= 1120 nm,

EM

= 0.028

9c: X, Y = Se

max

= 1230 nm,

EM

= 0.018

N

X

N

S

S C8H17

N

C8H17

N

Y

C8H17

C8H17

10 10a: X, Y = S

max

= 1055 nm,

EM

= 0.185

10b: X = Se, Y = S

max

= 1120 nm,

EM

= 0.046

10c: X, Y = Se

max

= 1285 nm,

EM

= 0.019

OC8H17

C8H17O N N

X

N

N C8H17O

N

S

S N

Y

OC8H17

11

11a: X, Y = S

max

= 1125 nm,

EM

= 0.053

11b: X = Se, Y = S

max

= 1295 nm,

EM

= 0.011

11c: X, Y = Se

max

= 1360 nm,

EM

< 0.01

environments where, for instance, serve as light harvesting units or oxygen carriers. As robust and versatile molecular platforms, they can be easily functionalized to afford multichromophoric systems [61]. Moreover, they can undergo a variety of supramolecular interactions, via p-stacking [62], charge-transfer [63], and metal– ligand interactions [64], occurring both in solution and on surfaces [65]. Due to this outstanding combination of properties, it is not surprising that porphyrins have been one of the most investigated classes of molecules over the last decades. Porphyrin-related systems are generally termed ‘‘porphyrinoids’’, a vast family of molecules that includes both naturally occurring and, to an increasing extent, synthetic macrocyclic molecules with pyrrole units [66, 67]. The remarkable and strongly tunable chromophoric and luminophoric properties of porphyrinoids, as a

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N NH

N

NH

N

N

NH

N

NH HN

meso

HN N

N N

HN

N

HN

N

HN

NH

N

NH

N N

Porphyrin

Chlorin

Bacteriochlorin

Corrole

Phthalocyanine

Fig. 7 The basic structure of the most important porphyrinoids. For porphyrins, the conventional names of the ring positions in which substituents can be inserted are indicated. The structural variations with respect to the pristine porphyrin structure are highlighted in blue

function of the specific chemical structure and connectivity, make them interesting for photonic, analytical, and therapeutic applications [68]. In particular, some ‘‘free base’’ and metallated porphyrinoids are among the strongest NIR emitters available to date, and their number continues to increase. The basic skeleton of the most important families of porphyrinoids is depicted in Fig. 7. 3.1 Standard and Extended Porphyrins The photophysical properties of porphyrins—both free base and metallated—have been investigated since the 1960s [69]. Typically, they exhibit a very intense absorption feature between about 400 and 450 nm (the so-called Soret band, or B-band) that corresponds to the transition to the second singlet excited state (S0 ? S2). At lower energy (500–700 nm), an envelope of weaker characteristic absorption features occurs, related to the S0 ? S1 transitions (Q bands). Both bands have a p–p* character and their very different intensities and energies are explained in detail by the so-called Gouterman four orbital model [69]. The prototypical free base tetraphenyl porphyrin (H2TPP) exhibits fluorescence in the visible spectral region (kmax = 646 nm) and chemical substitution do not remarkably affect this behavior [70]. Upon complexation with closed-shell diamagnetic ions—e.g., Zn(II), Cd(II), Mg(II), Al(III)—[71, 72] fluorescence in the visible spectral region is also observed, with moderate-to-low quantum yields. H2TPP and related metalloporphyrins also exhibit phosphorescence bands in the NIR region (e.g., ZnTPP: kmax = 776 nm, toluene matrix) [73] but they are typically weak and observable only in rigid matrix at cryogenic temperatures [71], hence their interest is limited. The triplet excited state lifetime is in the range of ms and, in solution, can only be detected by transient absorption upon removal of oxygen [74]. Taking advantage of heavy atom effects, the porphyrin triplet excited state lifetime can be shortened below the ms threshold. Accordingly, triplet emission at room temperature is observable in porphyrins of second- and third-row transition metals, such as Ru(II) and Os(II), which saturate their octahedral coordination cage by coordinating ligands in the apical positions. A standard ruthenium porphyrin system exemplifying the general photophysical behavior of this class of compounds is [Ru(TPP)(CO)(py)] 12 (Fig. 8) [75], which exhibits weak phosphorescence in

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ClC6F5

O CO N

N N N Ru N

O

L N

N

L

N

N

Ir

C6F5

Pt N

N

C6F5 N O

N C6F5 O N L=

12 max EM

= 776 nm,

= 0.001,

N

13

= 20 µs

max EM

14

= 760 nm,

= 0.3,

= 23 µs

max EM

= 866 nm,

= 0.06,

= 21 µs

Fig. 8 Selected examples of NIR-emitting Ru(II) [75, 76], Ir(III) [78] and Pt(II) porphyrins [81]. Ruporphyrins are faint emitters, Ir-porphyrins are rare, Pd-porphyrins can afford good yields both with ring modifications (this example, trans-porphodilactone) and ring expansion (see below). Photophysical data are in solution at 298 K, in toluene (Ru- and Ir-porphyrin) and CH2Cl2 (Pt-porphyrin)

solution at room temperature (kmax = 726 nm, UEM = 0.001, s = 30 ls in deaerated toluene) [76]. In these compounds, ring-centered and charge transfer (metal-to-ring) excited states are very close in energy, and the lowest-lying one depends on specific ring substituents or axial ligands [77]. Os(II) porphyrins behave similarly to Ru(II) analogues, with weak NIR triplet emission at room temperature (0.01) [74]. Despite the fact that Ir(III) complexes and porphyrins have been widely investigated in recent years, iridium porphyrins are not very common, primarily because they are difficult to synthesize. One example of strongly emitting NIR Ir(III)-porphyrin 13 has been reported (Fig. 8); this compound exhibits kmax = 760 nm, UEM = 0.3, s = 23 ls in deaerated toluene [78]. Corroles— smaller analogues of porphyrins with a direct C–C link between two pyrrole rings [79] (Fig. 7)—can also bind Ir(III). However, NIR emission from iridium corroles is very weak, probably due to enhanced non-radiative deactivation favored by the more distorted structure [80]. The most relevant NIR-emitting porphyrins are those based on Pt(II) and Pd(II), the former ones being probably the most successful NIR-emitting transition metal complexes in general [9]. Pt(II)- and Pd(II)-porphyrins are square planar, without additional axial ligands. They are relatively easy to synthesize and chemically modify, exhibit good photo and thermal stability, and are typically well soluble. Chemical modifications of standard Pt(II)- and Pd(II)-porphyrins can afford nearinfrared emission with reasonable quantum yields [82], as also recently demonstrated with porphodilactones 14 (Fig. 8) [81]. Further progress can be expected in this area; meanwhile, the most effective strategy to obtain strong NIR luminescence is still the expansion of the ring pconjugation [83]. This approach has been fruitfully proved by extending the conjugation of the pyrrole rings in Pt(II)-porphyrins and by investigating the effect Reprinted from the journal

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N

N

N Pt

N

N

N

15 max

max

= 53 µs

N

N

N

17

= 891 nm

EM = 0.15,

N Pt

16

= 770 nm

EM = 0.49,

N

N Pt

= 12.7 µs

max

= 1022 nm

EM = 0.08,

= 3.2 µs

Fig. 9 p-extended Pt(II) porphyrins with progressively NIR-shifted emission in degassed toluene solution. The photoluminescence quantum yield is maximized with two trans aryl substituents in the meso positions instead of four. Shifting photoluminescence to the NIR region is promoted by enhancing the number of fused benzene units on the pyrrole rings (from left to right)

N

N Pt

N max

= 773 nm,

EM

= 0.35,

= 30 µs (toluene)

max

= 765 nm,

EM

= 0.70,

= 53 µs (methylcyclohexane)

N

18

Fig. 10 Pt(II)-tetraphenyltetrabenzoporphyrin, a molecule successfully used to make electricity-to-light and light-to-electricity conversion devices

of the number and type of substituents in the meso positions [84]. As indicated in Fig. 9, these systems can achieve emission quantum yields has high as 0.49 (770 nm) (15) and emission bands down to 1022 nm (UEM = 0.08) (17), with lifetimes in the 10-6–10-5 s range. The emission performance of the same metalloporphyrin may differ with the solvent. An example is Pt(II)-tetraphenyltetrabenzoporphyrin 18 (Fig. 10), a molecule with a combination of remarkable properties (chemically stable, nonionic, strong triplet NIR emitter with large Stokes’ shift) that make it ideal as photoactive component in different types of optoelectronic devices. It has been used to make a NIR-emitting OLED with an external quantum efficiency above 6 % [85] and a luminescent solar concentrator with projected light-to-electricity power efficiency above 6.5 % [86]. This is probably the most promising and useful NIR emitter reported to date in terms of optoelectronic applications.

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N

N Pd

F N

max

= 803 nm,

EM

= 0.23,

F N

= 297 µs

19 F

Fig. 11 Pd(II)-meso-tetra(4-fluorophenyl)tetrabenzoporphyrin. The data are in toluene solution at room temperature

Some examples of b-substituted [81] and p-extended [9] Pd(II)-porphyrins which emit in the NIR region have been also reported. Typically, when compared to Pt(II) analogues, Pd(II)-porphyrins exhibit red-shifted emission bands, lower phosphorescence quantum yields and longer lifetimes [81, 83, 87, 88]. The strongest NIRemitting Pd(II)-porphyrin 19 with the lowest energy luminescence band reported to date is depicted in Fig. 11 [88]. It entails the use of fluorinated substituents, a strategy that affords enhanced luminescence performance and better stability. 3.2 Porphyrins with Expanded Rings or Modified Pyrrole Connectivity In recent years, a great deal of work has been made in the synthesis of porphyrintype systems with modified structure. Two main strategies have been adopted: (1) expansion of the macrocycle using more than four pyrrole units (e.g., hexaphyrin, Fig. 12) and (2) change of the linking mode between the pyrrole units, to get to a variety of tetrapyrrolic porphyrin variants (Fig. 12). Expanded porphyrins containing up to ten pyrrole units have been synthesized [89, 90]. These p-conjugated aromatic macrocycles typically exhibit good solubility and can bind one or even two metal ions. Due to their remarkable chromophoric and photophysical properties, they can be used as dyes, sensors, two-photon absorbing materials, or photosensitizers for photodynamic therapy [91]. They exhibit an

N N

NH

N

N

N

NH N

HN

N

N

HN

N

NH HN

HN N

NH

N

N N

N

Hexaphyrin

N-confused

N-fused

neo-confused

neo-confused

Porphyrin

Porphyrin

Porphyrin

Corrole

Fig. 12 An example of expanded porphyrin with a six-membered ring and four porphyrinoids with modified connectivities between the pyrrole rings. The so-called norrole (far right) is a corrole isomer with a C–N bond between two pyrrole units

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C6F5

C6F5

C6F5

HN N

C6F5

C6F5

N

Zn

C6F5

C6F5

N N

N

N

N

N

NH HN

C6F5 C6F5 HN

NH

O

HN

N

C6F5

C6F5 C6F5

20 max EM

21

= 705 nm,

= 0.06,

= n.r.

max EM

22

= 741 nm,

= 0.10,

= n.r.

max EM

= 900 nm,

= 0.003,

= n.r.

Fig. 13 Porphyrinoids with modified connectivity between pyrrole rings exhibiting good luminescence quantum yields (20 and 21) [93, 96] and neo-fused hexaphyrin 22 with weak emission at 900 nm [97]. All the data refer to CH2Cl2 solution at room temperature

electronic absorption profile extending into the red and near-infrared spectral region and several of them are reported to exhibit NIR luminescence [92]. However, due to strong conformational flexibility, they are prone to non-radiative deactivation and emission quantum yields are poor (\1 % [93]) and often not reported, presumably due to signal weakness [92, 94]. As far as tetrapyrrole porphyrin rings with modified connectivity are concerned (Fig. 12), some are reported to be poor NIR emitters (UEM = 10-4–10-6) [95], but others exhibit good luminescence such as those depicted in Fig. 13 [93, 96]. These molecules have fluorinated substituents in the meso positions, a successful strategy to reduce non-radiative deactivations proposed also for other porphyrin systems [88]. More recently, the two-ring modification approaches have been merged to obtain ‘‘expanded’’ and ‘‘confused’’ porphyrinoids such as 22 (Fig. 13), which emits at 900 nm with very low quantum yield [90, 97]. 3.3 Bacteriochlorins Chlorins (also termed dihydroporphyrins) are macrocycles made of three pyrroles and one 1-pyrroline unit, which normally exhibit luminescence in the red-spectral region. This large family of compounds includes chlorophylls, i.e., magnesium containing chlorins, which constitute the central photosensitive pigments in the chloroplasts of plants and algae. Porphyrinoids made of two alternate 1-pyrroline and pyrrole units are called bacteriochlorins (or tetrahydroporphyrins), their name being related to their role as NIR light-harvesting molecules in photosynthetic bacteria (Fig. 7). By moving from porphyrins to chlorin to bacteriochlorins, the lowest energy absorption features (Q bands) move from the yellow-orange, to the red and then to the NIR spectral region [98]. Natural bacteriochlorins are not particularly stable and tend to be oxidized to chlorins or porphyrins. However, recently, new synthetic strategies have afforded stable bacteriochlorins, opening the

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Top Curr Chem (Z) (2016) 374:47 24a M = H2:

max

= 805 nm,

EM

= 0.15,

= 3.2 ns

24b M = Zn:

max

= 825 nm,

EM

= 0.17,

= 3.6 ns

Ph

N

N M N

O OMe

N O

N

N

N M N

N

23

O N

23a M = H2:

max

= 716 nm,

EM

= 0.14,

= 4.0 ns

23b M = Zn:

max

= 725 nm,

EM

= 0.10,

= 3.4 ns

O

Ph

24

Fig. 14 Simple bacteriochlorins 23 and their bisimide analogues 24 with lower energy and stronger NIR emission. The geminal dimethyl residues on the reduced pyrrole units impart stability by preventing dehydrogenation. Photophysical data are in toluene at room temperature

N

HN O

NH

NH

N

EM

= 0.19,

NH

HN

25 = 760 nm,

HN O

N

max

N

N N

26 = 5.7 ns

Fig. 15 A phenylethynil-substituted bacteriochlorin 25 (5-methoxy-15-(2-phenylethynyl)-8,8,18,18tetramethyl-2,12-di-p-tolylbacteriochlorin) with NIR fluorescence in toluene at room temperature, and a related free-base chlorin dyad 26. The emission properties of the latter, upon excitation of the chlorin moiety, are exactly the same as those of the bacteriochlorin model, indicating quantitative energy transfer in toluene solution at room temperature

route to an expanding class of NIR-emitting dyes with kmax [ 700 nm, in which the pristine bacteriochlorine core bears a variety of aliphatic and aromatic fragments [99], also with several coordinating metals, such as Zn(II), Mg(II), and Pd(II) [100]. The simplest synthetic free base 23a and Zn(II)-bacteriochlorins 23b (Fig. 14) exhibit NIR fluorescence at 716–725 nm, which is substantially red shifted (805–825 nm) and even enhanced upon fusion of the pyrrole moieties with fivemembered bisimide rings (24a–b, Fig. 14) [101]. A synthetic bacteriochlorin 25 with moderately strong NIR fluorescence is depicted in Fig. 15 [102]. This molecule has been linked to red-emitting free base

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(26, Fig. 15) or Zn(II)-chlorin analogues and quantitative chlorin ? bacteriochlorin energy transfer has been observed [103]. Due to strong red absorption and NIR emission, these porphyrinoid dyads are prototypes for molecular imaging probes in biological environments. 3.4 Porphyrin Tapes Following a seminal paper by Crossley and Burn [104], extended p-conjugated porphyrin systems have become a popular field of research, primarily with the aim of obtaining synthetic organic materials with unusual optical and electronic properties. A landmark result in this area was the synthesis of soluble meso–meso, b-b,b-b triply linked zinc(II)-oligoporphyrins arrays 27 (Fig. 16), containing up to 12 fused monomeric units [105]. The lowest energy absorption features of these molecules extend linearly as a function of the monomeric units, reaching 2500 nm for the longest dodecameric system. This reveals a surprisingly long effective conjugation length (ECL) with no saturation of electronic properties, as typically observed with most organic conjugated systems of similar size. Only the smallest dimeric tape of the series (28, Fig. 16) is found to exhibit NIR luminescence well above 1000 nm, but with minuscule fluorescence quantum yield and ultrashort excited state lifetime [106, 107]. Interestingly, deuteration of dimeric Zn(II) porphyrin tapes does not improve the emission performance, suggesting that the observed ultrafast deactivation rate and related poor luminescence quantum yield not only depends on the Franck–Condon factor and the energy gap law, but accessible intersections of S0 and S1 potential energy surfaces must occur [108]. An effective strategy to promote NIR emission is to make rigid planarized paraphenylene-bridged tape-like porphyrin dimers, such as those depicted in Fig. 17. In particular, non-conjugation (29, CPh2 link) enables emission quantum yield of 10 % at 736 nm, which is progressively weakened and shifted to longer

N

Ar1 N

Ar1 N Ar2

N Zn N

N Zn

N

N Zn N N n

N Ar1

N

Ar

Ar

Ar1

Ar2

N

N

N

Zn

N Ar1

N

N

N

O

O

28

O

O O

O Ar1

O

Ar

Ar

O

N Zn

27

t-Bu

Ar1=

Ar = t-Bu

Ar2=

max

= 1080 nm,

EM

= 0.00035,

= 5.7 ns

Fig. 16 Left b-b-linked zinc(II)-oligoporphyrin tapes 27 (n = 0, 1, 2, 3, 4, 6, 10). Right 28, the only member of this family of compounds that exhibit (faint) luminescence. Photophysical data are in toluene solution at room temperature

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(a)

(b)

Ar

Ar

Ar

Ph Ph

N

N Ph

N Ph

Zn N

N

N

N

N Zn

Ph

N

Zn

N

N

N

Ph

Ar

Ar

29

max = 736 nm,

EM = 0.10,

Ar

= 1.4 ns

max

= 721 nm,

EM

Ar

Ar

32 = 0.08, = 1.5 ns

Ar

O N

N

N Ph

Zn N

N

N

N

N Zn

Ph

N

Zn

N

N

N

anti

O

max

Ar

30

= 960 nm,

EM

Ar

= 0.017,

Ar

= 0.25 ns

max

= 829 nm,

= 0.25 ns

syn Ph

Zn

N

N

N

N

Zn

N Zn N

N

N

N Ar

= 0.08,

Ar

N

N

33

EM

Ar

N Ph

Ar

31

34

Ar

Ar

no luminescence

t-Bu

max

= 839 nm,

EM

= 0.13,

= 0.25 ns

Ar = t-Bu

Fig. 17 a Para-phenylene-bridged Zn–porphyrin tapes, which can exhibit remarkable NIR emission in toluene solution at 298 K. b Molecular tapes obtained by one Zn–porphyrin unit fused with one or two pyrene moieties (CH2Cl2, 298 K)

wavelengths with cross-conjugation 30 (keto link), until disappearance of the luminescence when full conjugation occurs 31 (C–C link) [109] (Fig. 17a). Even larger NIR emission quantum yields have been achieved via fusion of Zn– porphyrin rings with pyrene units (32–34, Fig. 17b). Besides good NIR emission, these molecules exhibit broad absorption and have good solubility; accordingly, they are of potential interest for organic photovoltaics, bioimaging, and photodynamic therapy [110]. Similar N-annulated perylene-fused systems exhibit photoluminescence quantum yield above 5 % at 800 nm (not shown) [111]. 3.5 Linear Porphyrin Oligomers Several linear porphyrin oligomers connected through bridges linking only the meso positions have been prepared. Systems with phenylene bridges are typically non-

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emitting [112], whereas the direct C–C linkage affords arrays with fluorescence bands little affected with respect to the porphyrin monomer and occurring in the visible range [107]. In such arrays, the porphyrin units are perpendicularly arranged, therefore p-conjugation is disrupted and the lowest energy transition is virtually unaffected, even though exciton coupling brings about the splitting of the Soret band. Interestingly, NIR luminescence is observed when these linear porphyrin arrays 35 bear methanofullerenes as terminal units enabling face-to-face arrangements (Fig. 18), due to charge transfer (porphyrin ? fullerene) interactions [113, 114]. Emission maxima are placed between 950 and 1050 nm and the intensity progressively decreases with solvent polarity, which indicates electron transfer in the Marcus inverted region [114]. The emission quantum yields have not been determined due to signal weakness and the lifetimes are in the range 500–110 ps. Anyway, such NIR emission is the most convenient spectroscopic tool for detecting such peculiar interactions. The most interesting porphyrin arrays in terms of NIR luminescence are those connected through meso–meso alkyne bridges, as first demonstrated by Therien and coworkers using acetylene linkers [115]. They synthesized six Zn(II) porphyrin linear arrays (with 2, 3, or 5 subunits) with electron donating and/or accepting groups on the free meso position. The absorption spectra are progressively redshifted as a function of the number of chromophores, showing that the acetylene

N

N

N

N

N

Zn

N

Ar

Ar

Ar

N

N

N

N

Zn Ar

Ar O

n

N Zn

N

Ar O

n = 0, 1, 2

O

O

35

O O

O O

t-Bu Ar = t-Bu

35a n=0

35b n=1

35c n=2

max

= 953 nm,

= 0.8 ns (toluene)

max

= 1050 nm,

= 0.4 ns (THF)

max

= 956 nm,

= 0.7 ns (toluene)

max = 1040 nm,

= 0.3 ns (THF)

max

= 945 nm,

= 1.1 ns (toluene)

max

= 1053 nm,

= 0.4 ns (THF)

Fig. 18 Linear porphyrin arrays 35 exhibiting charge transfer emission in the NIR in apolar solvents (toluene, THF)

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R2

R1 =

N

N

N

N

Zn N

N

N

R1

N

N

N

N

R1

37 DDDDD:

N

R2 max

= 883 nm,

= 0.14,

O

O

O

O

O

N

N

Zn N

R2

N

N

Zn

R2 EM

O

R1 N

N

Zn N

O

R2 N

N

Zn

O R2 =

= 1.13 ns

R2

N

Zn

N

R1

EM = 0.22,

R2 N

N

N

R2 max = 806 nm,

O

Zn

N

R1

N

N

Zn

36 DDD:

O

R1

N

N

R1

= 0.45 ns

Fig. 19 Linear porphyrin arrays connected through acetylene bridges exhibiting remarkable NIR emission in THF

linker warrants effective electronic conjugation. Irrespective of the length and of the specific sequence of donors or acceptors, these systems exhibit good NIR emission, particularly when electron donor residues are used [115]. In Fig. 19 are reported the arrays exhibiting the highest luminescence quantum yield and the lowest-energy emission band. Oligomeric conjugates based on butadyine bridges have also been synthesized, but emission quantum yields are typically lower due to larger structural flexibility, which promotes non-radiative deactivations [116]. For instance, the linear array 38 schematized in Fig. 20 exhibits fluorescence at 830 nm with photoluminescence quantum yield of 0.08 [117, 118]. These longer systems allow the preparation of a variety of beautiful open and closed Zn-porphyrin nanorings 39, by means of templated syntheses based on hexaphenylbenzene 40 equipped with terminal pyridine centers that bind the Zn ions [116, 119, 120] (Fig. 20). As a consequence of the bending of the p-conjugated system and of the intrinsic high symmetry, such nanorings 39 exhibit uniquely broad NIR emission bands extending between 800 and over 1200 nm, which are strongly affected also by the presence/absence of the templating unit. However, emission quantum yields are below 1 % [118]. 3.6 Phthalocyanines Phthalocyanines are analogues of tetrabenzoporphyrins bearing N atoms in the ring meso position instead of C atoms (Fig. 7); accordingly, they are also termed tetrabenzotetraazaporphyrins. Similarly to porphyrins, they are characterized by two groups of absorption bands in the UV–Vis region, but their position, separation, and intensity is substantially different. The B band, corresponding to the S0 ? S2 Reprinted from the journal

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Top Curr Chem (Z) (2016) 374:47 Ar N N Zn N N

N

Ar

C8H17O

OC8H17

N Hex3Si

N N Zn N N Ar

N Ar

N Ar Zn N N

N

N

N

N

N Zn

N

Ar

N

SiHex3 6 N

C8H17O

OC8H17

Ar

N Ar Zn N N

Ar N

N N Zn N Ar

Ar

N

N N Zn N N Ar

38

39 39 40: 39:

max

> 900 nm,

max = 900 nm,

40 EM

= 0.0012,

EM = 0.0043,

= 250 ps = n.r.

Fig. 20 Linear 38 and nanoring Zn-porphyrin hexameric arrays 39 connected through butadyine bridges. Both exhibit NIR emission in toluene (1 % pyridine), but the unique structural properties of the nanoring favors the lowering of the excited state energy and the luminescence quantum yield drops below 1 %. Right the hexadentate template 40 used for the synthesis of the nanoring

transition, is placed at about 350 nm, whereas the Q band—red-shifted and substantially increased in intensity—is located in the region between 600 and 700 nm [121]. The strong absorption in the red spectral region makes phthalocyanines widely utilized green–blue dyes and excellent candidates as photosensitizers for solar cells [122], photodynamic therapy agents [123], and several other applications [121]. Phthalocyanines can host a wide number of metal ions in the core and may undergo extensive chemical functionalization. This affords a remarkable tuning of the optical properties as a function of the nature of the central metal, the nature and position of the peripheral substituents, the sequential addition of fused benzene rings, and the deviation from planarity that may ensue [121, 124]. As far as luminescence is concerned, phthalocyanines exhibit a p–p* fluorescence band with typically very small Stokes’ shift (100–200 cm-1) relative to the Q-band absorption features, which is attributed to the strong rigidity of the macrocyclic ring [121]. Free base and Zn(II)-phthalocyanines are the most investigated systems, but only the former show emission maxima systematically above 700 nm and can be classified as NIR fluorophores, although some examples of NIR-emitting Zn(II)-phthalocyanines have been also reported [121, 125]. In Fig. 21 are depicted some examples of free base (41, 42) and Zn(II)-phthalocyanines (43, 44), which are strongly emitting in the NIR region in THF solution. Electron-withdrawing and electron-donating substituents for free base and, respectively, Zn(II)-phthalocyanines have been used to enhance the emission output [125]. However, rationale emission tuning criteria cannot be drawn, because a clear correlation between the electronic features of the substituents and the fluorescence quantum yield has not been established [121].

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Top Curr Chem (Z) (2016) 374:47 NO2 N N

N N

HN

N

N

N N

NH

NO2

HN N N

NH

O2N

N

N

41

42

O2N max

= 705 nm,

= 0.60,

EM

BuS

= 6.9 ns

max

= 702 nm,

SBu

BuS

EM

t-Bu

= 7.4 ns

BuS

SBu

t-Bu

N

N

N N

BuS

N Zn

N

N Zn

N

N N

N

t-Bu

SBu BuS

SBu

SBu

t-Bu

44

43 EM

SBu

N

BuS

= 733 nm,

N N

N

N

max

= 0.79,

= 0.12,

= 3.2 ns

max

= 727 nm,

EM

= 0.22,

= 3.1 ns

Fig. 21 Selected examples of strongly emitting free base and Zn(II)-phthalocyanines in THF solution [125]. Very few other examples with similar performance can be found in Ref. [121]

Several phthalocyanines of heavy metal ions—Rh(III), Pd(II), Ir(III), Pt(II)— exhibit phosphorescence in solution at room temperature between 950 and 1100 nm [121, 126], often in combination with fluorescence between 700 and 730 nm [127]. However, both emission quantum yields are rather low (\1 %) even in the best cases of Pt(II) and Pd(II) phthalocyanines [121, 127]. Also naphthalocyanines of the same heavy metals listed above exhibit fluorescence and phosphorescence quantum yields below 1 %, with triplet emission at k [ 1230 nm and down to 1340 nm for Pd(II)-based systems [128]. An interesting alternative to tetrabenzoporphyrins and phthalocyanines has been proposed by Borisov et al., who were able to synthesize ‘‘intermediate’’ azatetrabenzoporphyrin structures with both C and N atoms in the meso positions of the ring and then make Pt(II) 45 and Pd(II) 46 complexes [129]. These compounds exhibit remarkable phosphorescence in the NIR at room temperature in toluene, lower than Pt(II) and Pd(II) meso-tetraphenyltetrabenzoporphyrins (18, Fig. 10), but much stronger than related phthalocyanines. The strongest emitters of the series are reported in Fig. 22.

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N N

M N

45

M = Pt:

46

M = Pd:

max

= 844 nm,

EM

= 0.40,

= 22 µs

= 0.08,

= 213 µs

N max

= 875 nm,

EM

N

Fig. 22 The strongest emitting Pt(II)- and Pd(II)-azatetrabenzoporphyrin reported by Borisov et al. [129]. Photophysical data are in oxygen-free toluene at 298 K

4 Transition Metal Complexes Phosphorescent transition metal complexes are a versatile class of emitters and offer several important advantages over traditional fluorescent materials. For instance: the T1 ? S0 phosphorescence enables the harvesting of both singlet and triplet excitons in electroluminescent devices (OLEDs, LECs) to achieve the maximum internal quantum efficiency; they exhibit large Stokes’ shifts because of the S1 ? T1 intersystem crossing, induced by the relatively large spin–orbit coupling of the transition metal center; the emissive excited state has long lifetime, ranging from ls to ms. The most common excited states of phosphorescent transition metal complexes have a charge transfer nature, including metal-to-ligand (MLCT), ligandto-ligand (LLCT), intraligand (ILCT), metal–metal-to-ligand (MMLCT) charge transfer. The most effective way for tuning the phosphorescence of transition metal complexes involves the modification of the ligands. Several design factors, such as changing the p-conjugation length, altering the substituents, e.g., by including donor–acceptor push–pull systems, or adding heterocycles, can be considered. In the case of multinuclear complexes also the effect of intramolecular metal–metal interactions, leading to red-shifted emission from 3MMLCT excited states, should be taken into account. Intermolecular interactions, such as hydrogen bonding, p–p stacking, or charge transfer can be present in condensed phase. These can lead to the formation of excimers, exciplexes, or aggregates that can result in red-shifted emission. Up to now, there have appeared only a few reviews on the NIR phosphorescence of transition metal complexes [9, 130, 131]. 4.1 Complexes of d6 Metal Centers 4.1.1 Re(I) Re(I) complexes of the type [Re(N^N)(CO)3L]n? (where N^N = diimine ligand, L monodentate ligand, and n = 0, 1) exhibit room temperature (RT) phosphorescence and have been widely investigated [132, 133]. They usually display a structureless red emission at about 600 nm with small quantum yields (\10-3) typically from 3 MLCT excited states. The emission band can be tuned by introducing electronwithdrawing or -donating groups on the diimine ligand. Using this strategy,

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complexes with emission bands close to 800 nm have been prepared by Meyer and coworkers [134–136]. The introduction of the acetylide as the monodentate ligand into the d6 metal diimine system raises the energy of the metal centered d–d states. This improves the population of the 3MLCT state and induces a red shift of the transition energies, as demonstrated by the research group of Yam [137]. Strong emitting rhenium tricarbonyl complexes have been prepared by Rillema and coworkers using phenanthroline derivatives and 2,6-dimethylphenylisocyanide ligands [138, 139]. These represent the Re(I) complexes with the highest quantum yield prepared so far (UEM = 0.83 and s = 20.2 ls). However, the emission wavelength is significantly blue shifted (kmax = 510 nm). More recently, a series of deep-red to NIR-emitting Re(I) complexes 47a–e with good quantum yield (kmax = 680–710 nm and UEM = 0.01–0.07 in CH2Cl2 solution at RT) have been prepared by Yam and coworkers [140]. Here, the metal center was coordinated by a bipyridine moiety inserted into a D–p–A–p–D structure (Fig. 23). The observed luminescence has been attributed to an intraligand charge transfer (3ILCT) from the

R

R

N

N N

N R

OC

Re

OC

Br

R

CO

47

R=H

a

N

c

b

e

d

47a:

max

= 680 nm,

EM

= 0.06,

< 0.1 µs

47b:

max

= 701 nm,

EM

= 0.01,

< 0.1 µs

47c:

max

= 686 nm,

EM

= 0.04,

= 0.11 µs

47d:

max

= 703 nm,

EM

= 0.06,

< 0.1 µs

47e:

max

= 708 nm,

EM

= 0.07,

< 0.1 µs

Fig. 23 Chemical structures of some of the best-performing NIR-emitting Re(I) complexes [140]. Photophysical data are in CH2Cl2 at 298 K

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triarylamine to the bipyridine. To the best of our knowledge, these are the brightest NIR-emitting Re(I) complexes reported in the literature. 4.1.2 Os(II) and Ru(II) Polypyridine complexes of Os(II) and Ru(II) have similar photophysical behavior with emissive excited states of 3MLCT character and an octahedral coordination geometry [141–145]. The energy of Os(II) luminescence is lower with respect to that of the Ru(II) analogues, because of the significantly more negative oxidation potential of the Os metal center [143, 144]. Although Os(II) complexes with high quantum yield in the visible region have been reported [143, 146], the luminescence quantum yield quickly drops below 10-2 in the NIR. This is due to the so-called ‘‘energy gap law’’ [7, 147], which forecasts an exponential increase of the nonradiative deactivation rate constant as the energy gap between the excited emitting state and the ground state decreases: knr ¼ aebDE : Moreover, the excited state lifetime of Os(II) complexes is shorter than that of the Ru(II) homologues, because of the stronger spin–orbit coupling (SOC) in the former, which facilitates the intersystem crossing (ISC) from the triplet excited state to the singlet ground state. The emission of Os(II) complexes can be shifted in the NIR by using diimine ligands with electron-withdrawing substituents or with extended p-conjugation. Notably, using the latter strategy, weak emission bands maximizing up to 1270 nm has been reported by Kol and coworkers (UEM \ 10-4 and s \ 4 ns) for a series of [Os(eilatin)]2? derivatives 48a–c (Fig. 24) [148–150]. Recently, Wu and coworkers prepared a series of neutral Os(II) complexes 49a–d (Fig. 25) using two isoquinolyl triazolate chromophores and ancillary phosphine ligands both in trans and cis arrangement [151]. These complexes exhibit a strong deep-red and NIR phosphorescence emission (kmax = 602–805 nm, UEM = 0.04–0.15 and s = 0.16–0.76 ls) of MLCT character mixed with more localized ligand centered ILCT/LLCT transitions (Fig. 25). This series of chargeneutral and volatile Os(II) complexes, make them ideal for the fabrication of NIRemitting OLED via thermal vacuum deposition. The photochemistry and photophysics of Ru(II) polypyridyl complexes is mainly dominated by emitting excited states with MLCT character, suffering from the presence of close-lying d–d metal centered (MC) excited states that can lead to photosubstitution reactions and subsequent non-radiative deactivation [141, 142]. The spin–orbit coupling induced by the heavy metal atom causes large singlet– triplet mixing and, accordingly, these excited states usually possess triplet multiplicity. Several strategies have been applied to shift the emission of Ru(II) complexes into the NIR. Beside the polypyridyl ligand modifications mentioned above for the Os(II) complexes, which lower the energy of the LUMO, the destabilization of the metal t2g orbitals with strong r-donating ligands proved to be particularly effective [152]. This has been accomplished, for instance, with monodentate isothiocyanates –NCS 50 or cyclometalating ligands 51 (Fig. 26)

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N

N N

N

N Os

N

N

N

N

N

N

N

N N

48b,c

Os N N N N

N

N

N

N

N

N

N

N

N

= N

48a eilatin

48a:

= 1087 nm,

max

b) iso-eilatin

EM

= 1.4x10-4,

= n.r.

c) dibenzoil eilatin

< 0.1 µs

48b:

max

= 1240 nm,

EM