Nitrogen-Free Bifunctional Bianthryl Leads to Stable White-Light ...

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Article Cite This: ACS Omega 2018, 3, 1416−1424

Nitrogen-Free Bifunctional Bianthryl Leads to Stable White-Light Emission in Bilayer and Multilayer OLED Devices Samik Jhulki,*,† Saona Seth,† Shahnawaz Rafiq,‡ Avijit Ghosh,§,⊥ Tahsin J. Chow,*,§,⊥ and Jarugu Narasimha Moorthy*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Frick Chemistry Laboratory, Princeton University, Princeton, New Jersey 08544, United States § Institute of Chemistry Academia Sinica, Taipei, Taiwan 115 ⊥ Department of Chemistry, Tunghai University, Taichung, Taiwan 407 ‡

S Supporting Information *

ABSTRACT: White organic light-emitting diodes (WOLEDs) are at the center stage of OLED research today because of their advantages in replacing the high energy-consuming lighting technologies in vogue for a long time. New materials that emit white light in simple devices are much sought after. We have developed two novel electroluminescent materials, referred to as BABZF and BATOMe, based on a twisted bianthryl core, which are brilliantly fluorescent, thermally highly stable with high Td and Tg, and exhibit reversible redox property. Although inherently blue emissive, BABZF leads to white-light emission (CIE ≈ 0.28, 0.33) with a moderate power efficiency of 2.24 lm/W and a very high luminance of 15 600 cd/m2 in the fabricated multilayer nondoped OLED device. This device exhibited excellent color stability over a range of applied potential. Remarkably, similar white-light emission was captured even from a doublelayer device, attesting to the innate hole-transporting ability of BABZF despite it being non-nitrogenous, that is, lacking any traditional hole-transporting di-/triarylamino group(s). Similar studies with BATOMe led to inferior device performance results, thereby underscoring the importance of dibenzofuryl groups in BABZF. Experimental as well as theoretical studies suggest the possibility of emission from multiple species involving BABZF and its exciplex and electroplex in the devices. The serendipitously observed white-light emission from a double-layer device fabricated with an unconventional hole-transporting material (HTM) opens up new avenues to create new non-nitrogenous HTMs that may lead to more efficient white-light emission in simple double-layer devices.

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INTRODUCTION Lighting and display technologies based on organic lightemitting diodes (OLEDs) have captured a sizeable share of global market today. Much attention continues to be paid at present to the development of white organic light-emitting diodes (WOLEDs) owing to their prospect of replacing the conventional lighting based on fluorescent and incandescent bulbs.1 WOLEDs offer unrivaled advantages in terms of color quality (color rendering index and glare-free light that illuminates large area), opportunity for flexible devices, and so forth.2−5 However, stability and durability of white lightemitting devices coupled with manufacturing cost (material synthesis + device fabrication)the two major issuesmust be addressed before full-fledged commercialization of WOLED technologies can be made.2−6 Numerous reports on white-light emission from OLED devices have appeared, but they are largely based on fundamental strategies.2−5 For example, whitelight emission is commonly achieved by doping phosphors of complimentary colors in a common host matrix.2−5 An analogous doping method that involves mixing of fluorescent © 2018 American Chemical Society

and phosphorescent materials has also been shown to lead to white-light emission.2−5 Although such dopants are commercially available and are very efficient emitters, phosphorescent dopants based on heavy metals are scarce and are often costly. Also, such doping methods are nontrivial, as control over dopant ratio during sublimation is not easy. Furthermore, cascade energy transfer from the dopant possessing high band gap energy to the ones with low band gap energies is thermodynamically a downhill process, which may render optimization of dopant concentration an arduous task. Another approach for white-light emission entails casting of a multilayer device in which concurrent emission of different colors occurs from each layer, leading collectively to white light.2−5 This method too requires a careful optimization of thicknesses of the layers to sequester white light from such devices and is by no means a trivial job. It is, therefore, of paramount importance to Received: November 2, 2017 Accepted: January 19, 2018 Published: February 2, 2018 1416

DOI: 10.1021/acsomega.7b01712 ACS Omega 2018, 3, 1416−1424

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ACS Omega develop materials that lead to white-light emission in simple devices, which often exploit multispecies emission involving exciplex, electroplex, excimer, and so forth.2−5,7−14 Recent literature reveals an exalting interest in bifunctional, that is, hole transporting as well as emissive, materials that simplify device engineering and minimize production cost, as they eliminate the requirement of casting separate layers for hole transport and emission;15−26 when separate materials are employed for hole transport and emission, their highest occupied molecular orbital (HOMO) energies should match to allow efficient transport of holes from the former to the latter, which is essential for good device performance. A single material that serves for hole transport and emission not only precludes such issues, but also simplifies the device configuration, paving way for easy fabrication. In general, almost all bifunctional materials reported so far contain “nitrogen” atom(s) as part of di-/triarylamine and carbazole, which are responsible for hole transport.15−26 This is due to the ease with which an electron can be removed from the nitrogen atom and the stabilization of the resultant radical cation. However, the high ionization potential has been implicated to be responsible for poor photooxidation stability, which diminishes the quality and durability of the devices.27,28 We were, therefore, motivated to develop bifunctional materials that do not contain di-/triarylamine and/or carbazole as holetransporting functional groups. Notably, such materials are rare, and rarer are those that are emissive in addition to being hole transporting.29−34 Furan30 and benzodifuran32 derivatives have been investigated as hole-transporting materials (HTMs) in the recent OLED literature. By contrast, dibenzofuran-based compounds are generally employed as host materials in phosphorescent OLED devices.35−38 We surmised that it should be of interest to examine the hole-transporting property of a dibenzofuran-functionalized bianthryl derivative, in addition to inquiring into how a dibenzofuran group modifies the fluorescence property of the parent bianthryl. Moreover, dibenzofuran, being a rigid heterocycle, should endow the resultant material with good thermal and morphological stabilities; notably, the anthracene units in bianthryl are nearly orthogonal and hence should prevent aggregation-caused quenching.39−44 Accordingly, we designed BABZF in which the bianthryl is functionalized at 10 and 10′ positions with two dibenzofuryl groups. An analogous fluorescent material, that is, BATOMe, functionalized at the same positions with 3,5dimethoxyphenyl groups, was also designed, cf. Figure 1, to compare the results with BABZF. We report herein the synthesis, physical properties of BABZF and BATOMe, and serendipitous observation of white-light emission from multilayer devices fabricated thereof. Remarkably, the emitted white color is very stable over a broad range of applied potential. The bifunctional, that is, hole transporting as well as emissive, property of BABZF has been demonstrated by the fabrication of double-layer devices, which also lead to white-light emission.

Figure 1. Chemical structures of bianthryl-based compounds.

phenylboronic acid to get hold of BABZF and BATOMe, respectively. Photophysical Properties. Ultraviolet−visible (UV−vis) absorption and fluorescence spectra of the bianthryls, that is, BABZF and BATOMe, recorded in dilute dichloromethane (DCM) solutions (ca. 1 × 10−5 M) are shown in Figure 2. The absorption spectra for both cases are dominated by the structured features in the range of 325−425 nm, which are typical of bianthryl chromophore. BABZF shows marginally different absorption in the range of 275−325 nm, arising due presumably to the absorption of two dibenzofuran moieties. Fluorescence spectra of both bianthryls, for excitation at 340 nm, are similar and structureless with λmax at 447 nm. The photoluminescence (PL) spectra of the materials in the vacuum-sublimed thin films are given in Figure S7. As can be seen, both BABZF and BATOMe have identical PL emission profiles. A comparison of their PL spectra in DCM and thin films suggests that the latter are broader as well as red-shifted, which suggests intermolecular interactions in the solid state. Fluorescent quantum yields of BABZF and BATOMe in DCM determined relative to anthracene as the standard are 92.1 and 89.0%, respectively, whereas the same in the thin-film state are only 11.8 and 8.3%, respectively (Table 1). The reduction in the photoluminescence quantum efficiency by almost an order of magnitude from DCM solutions to neat thin film suggests that aggregation-caused quenching is operative in the solid state. Electrochemical Properties. Electrochemical properties of BABZF and BATOMe were studied by cyclic voltammetry (CV) in DCM in the presence of n-Bu4NPF6 as the supporting electrolyte in a typical three-electrode setup consisting of a Ag/ AgCl reference electrode, a glassy carbon working electrode, and a Pt wire counter electrode. Both the bianthryls display reversible oxidations in anodic sweep but exhibit no propensity for reduction in cathodic sweep within the potential window in DCM (Figure S1). HOMO energies of both the compounds determined from their half-cell oxidation potentials (E1/2 ≈ 1.29 V) relative to the Fc/Fc+ couple as the standard were similar, ca. 5.64 eV; note that ferrocene has a HOMO energy of 4.8 eV with respect to vacuum. Lowest unoccupied molecular orbital (LUMO) energies calculated by subtraction of the molecular HOMO−LUMO energy gaps from the HOMO energies were 2.68 and 2.65 eV for BABZF and BATOMe,

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RESULTS AND DISCUSSION Synthesis. Both of the target bianthryl-based compounds BABZF and BATOMe were synthesized in high yields with Suzuki coupling as the key reaction (Scheme 1). Accordingly, 9,9′-bianthryl was synthesized following the literature-reported procedure and was subjected subsequently to dibromination to obtain 10,10′-dibromo-9,9′-bianthryl.45 The latter was reacted with dibenzo[b,d]furan-2-ylboronic acid and 3,5-dimethoxy1417


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ACS Omega Scheme 1. Synthetic Routes to the Target Bianthryl-Based Compounds

Figure 2. Normalized absorption (a) and fluorescence (b) spectra of BABZF and BATOMe in DCM. The fluorescence spectra were recorded for the excitation at 340 nm.

Table 1. Photophysical, Thermal, and Electrochemical Characterization Data of the Bianthryls substrate

λmax (UV)a (nm)

Egb (eV)

λmax (PL)a soln (nm)

Φfl solnc/thin filmd (%)

HOMOe/LUMOf (eV)

Tgg/Tmh/Tdi (°C)

BABZF BATOMe

360, 380, 402 360, 379, 401

2.96 2.99

445 447

92.1/11.8 89.0/8.3

5.64/2.68 5.64/2.65

193/436/457 153/-h/406

Absorption and fluorescence spectra were recorded in dilute DCM solutions (ca. 10−5 M). bMolecular HOMO−LUMO energy gaps were calculated from red-edge absorption onset values using the formula E = hc/λ. cQuantum yields were determined for excitation at 340 nm relative to anthracene as the standard. dObtained by using an integrating sphere for excitation at 340 nm. eHOMO energies were determined from oxidation potentials in the CV spectra. fLUMO energies were calculated by subtracting the molecular HOMO−LUMO energy gaps from HOMO energies. g From DSC. hFrom TGA. iNot observed. a

high temperature of ca. 436 °C, whereas BATOMe decomposed without melting. Furthermore, BABZF and BATOMe exhibited high glass-transition temperatures (Tgs) of 193 and 153 °C, respectively (Figure S3). High thermal stability of the compounds presumably owes origin to the rigidity of the bianthryl core. Electroluminescence Properties. We fabricated the following two devices to investigate electroluminescence (EL) properties of the bianthryls: (A) ITO/NPB (40 nm)/bianthryls (10 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm) and (B) ITO/m-MTDATA (40 nm)/BABZF (10 nm)/TPBI (40 nm)/ LiF (1 nm)/Al (150 nm), where indium tin oxide (ITO)

respectively (cf. Table 1). The molecular HOMO−LUMO energy gaps were in turn determined from the red-edge absorption cutoffs. Thermal Properties. Good thermal stability is an important prerequisite for a material to be applied in OLED devices because the material must withstand significant heating effects during sublimation as well as Joule heating. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to investigate thermal properties of the bianthryls. Both BABZF and BATOMe displayed very high decomposition temperatures (Tds) of 457 and 406 °C, respectively (Figure S2). BABZF was found to melt at a very 1418


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Figure 3. Current density vs voltage (a) and luminance vs voltage (b) profiles for the devices of configuration A.

functions as the anode, N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) serve as HTMs, 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (TPBI) functions as an electron-transporting material (ETM), and LiF/Al as the composite cathode; of course, the emissive layer in each case was cast with the bianthryls. I−V−L characteristics and EL spectra of the devices fabricated with bianthryls in configuration A are shown in Figures 3 and 4, respectively. EL data for all the devices are collected in Table 2.

nation. Both BABZF and BATOMe lead to white-light emission in the device configuration A. Although blue emission is expected from all compounds based on their fluorescence properties, emission of white light from the devices fabricated with BABZF and BATOMe is intriguing and a surprise. The EL spectra of the corresponding devices span a significant part of the visible region (cf. Figure 4). In fact, the EL captured for the BABZF-based device A has a full width at half-maximum of 184 nm with the CIE coordinates of (0.28, 0.33) for the emitted light, which are close to those of pure white-light emission (0.33, 0.33). In contrast, the BATOMe-based device in configuration A emits white light with the CIE coordinates of (0.21, 0.29). Notably, the BABZF-based device performs significantly better than the one fabricated with BATOMe (cf. Table 2). For example, the efficiency parameters for the BABZF-based device A are three times higher when compared to those for the BATOMe-based device A (Table 2). Insofar as the maximum brightness is concerned, the BABZF-based device produces a white light of intensity 15 600 cd/m2, whereas the same fabricated with BATOMe is limited to 2290 cd/m2. A similar device of configuration B in which mMTDATA was employed as a HTM instead of NPB fabricated to affirm the results obtained in device A. Indeed, BABZFbased device B produced white light, albeit in relatively poor efficiencies with slightly different CIE coordinates (cf. Table 2). Next, we analyzed whether or not the emitted white color is stable in the usable potential range, as the color stability of a WOLED is a measure of the quality of the device.2 In general, emissions from more than one species cumulatively lead to white light, and hence relative emission intensities of the involved species at different voltages determine the color stability of the WOLED device over a range of potential. In Figure 5, the variation of CIEx and CIEy coordinates with

Figure 4. EL spectra captured at 8 V for the devices of configuration A fabricated with BABZF and BATOMe. Note the broad feature of the emission spectra.

As can be seen from Table 2, the turn-on voltages are low (ca. 3.0−4.0 V), which suggest that the energy levels of the employed materials are nicely matched for the injection of holes and electrons, their transport, and subsequent recombi-

Table 2. EL Data for the Bianthryl-Based Compounds in Nondoped OLED Devices substrate BABZF

BATOMe

devicea

Vonb

ηexc

ηpd

ηle

Lmaxf

fwhmg

A B C A

3.0 3.0 4.0 3.5

0.97 0.43 0.86 0.31

1.78 0.70 1.48 0.42

2.24 1.21 1.89 0.63

15 600 6780 3990 2290

184 132 160 140

CIEh (x, y) 0.28, 0.34, 0.28, 0.21,

0.33 0.44 0.36 0.29

a

A−C refer to the device configurations: (A) ITO/NPB (40 nm)/BABZF or BATOMe (10 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm), (B) ITO/m-MTDATA (40 nm)/BABZF (10 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm), and (C) ITO/BABZF (60 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm). bTurn-on voltage (V). cMaximum external quantum efficiency (%). dMaximum power efficiency (lm/W). eMaximum luminance efficiency (cd/A). fMaximum luminance achieved (cd/m2). gFull width at half-maximum (nm) measured at 8 V. h1931 chromaticity coordinates measured at 8 V. 1419


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been realized for the first time.29−34 It is noteworthy that such a white-light emission from simple double-layer fluorescent devices has been observed in only few instances, and the results obtained with BABZF are superior to those reported earlier.7,9−11 Fabrication of similar devices with the methoxysubstituted compound, that is, BATOMe, to demonstrate the hole-transporting property led to very poor results, presumably because of its inferior hole-injecting and hole-transporting attributes. What is the origin of white-light emission? It is intriguing that the observed light emission from the devices is white, although the material itself is inherently blue-emissive. Intuitively, it must be due to the emissions from multiple species that collectively lead to broad emission. Let’s endeavor to understand the origin of broad emission. First, the emission due to NPB in device A may be ruled out because of the absence of its λmax (EL) at ca. 412 nm.46 On the basis of the same consideration, emission due to m-MTDATA may also be discounted in device B. The λmax (EL) of TPBI is well below 400 nm;47 thus, the emission from TPBI cannot contribute to the observed EL spectra. The remaining possibilities are emissions from BABZF and its excimer, exciplex with TPBI, or electroplex. The peak at 448 nm for devices A−C overlaps with the emission spectrum of the vacuum-sublimed thin film of BABZF (Figure 7). Therefore, the peak at 448 nm is

Figure 5. Variation of CIEx and CIEy with voltage for device A.

voltage for device A fabricated with BABZF is shown. Clearly, the CIE coordinates remain steady with variation in the applied potential, attesting to the stability of the emitted white light in the potential range of 3−10 V. As mentioned at the outset, bifunctional, that is, hole transporting as well as emissive, materials are of tremendous commercial importance because of their dual role, which is important in economizing production cost. Although benzodifuran derivatives32 have been reported to serve as HTMs in OLED devices, dibenzofuran compounds have never been investigated as HTMs, to the best of our knowledge. Given that both NPB-based device A and m-MTDATA-based device B produce similar white-light emission, we wondered if elimination of the NPB/m-MTDATA layer and its replacement by the BABZF layer of same thickness would also lead to whitelight emission. Accordingly, we fabricated a double device of configuration C: ITO/BABZF (60 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm) to examine the dual property (hole transport + emission) of BABZF. To our delight, white-light emission (CIE ≈ 0.28, 0.36) similar to that observed in device A was indeed captured (cf. Figure 6). The efficiency parameters are, however, slightly inferior to those obtained from multilayer device configurations (devices A and B, Table 2, Figure S4). This can be understood from slightly uphill hole injection barrier, arising as a consequence of deeper HOMO energy of BABZF relative to that of NPB (5.6 vs 5.4 eV) (cf. Figure S5). Be this as it may, the fact that white-light emission can indeed be captured from a simple double-layer device with an unconventional non-nitrogenous, yet bifunctional HTM has

Figure 7. EL spectra captured from the devices of configurations A−C fabricated with BABZF; PL spectrum of BABZF in the vacuumsublimed thin film is also shown for comparison. Note that the EL emission profiles in all the devices are dissimilar. The inset shows the photographs of the light emanating from the devices.

Figure 6. EL spectrum (a) and I−V−L characteristics (b) for the devices of configuration C. 1420


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Figure 8. (a) Energy level diagram and the corresponding transition energies of BABZF, TBPI, and the 1:1 complex of BABZF/TPBI calculated at the B3LYP/6-311G(d,p) level of theory. Exciplex energy was calculated by performing excited-state optimization of the 1:1 complex using timedependent density functional theory/B3LYP/6-311G(d,p) functional. (b) Energy level diagram of the BABZF/TPBI heterojunction and the possible emission pathways.

unambiguously assigned to the singlet emission from BABZF. There have been earlier reports about the occurrence of dual emission from the locally-excited (LE) and the twisted intramolecular charge-transfer (TICT) state of bianthracene.14,48 We performed single-point energy scan of the excited and ground electronic states along the torsional coordinate bridging the two anthracene rings in BABZF using Gaussian 09.49 We did not observe two minima on either side of the Franck−Condon geometry at a torsion of ca. 20° as has been reported for anthracene.45,48 On the contrary, the excited-state potential was flat and parallel to the ground-state potential until steric interactions took over at larger torsions (Figure S6). Such a flat nature could predict the existence of dual-state emission. However, TICT contribution would hardly increase in the device configuration, as LE and TICT states lie very close to each other along the energy and torsion dimensions. This is the main reason that the emission spectra resemble significantly those of the solution phase and thin film (Figure S7). Thus, any further red emission from the device due to the LE/TICT state interplay can easily be ruled out. The energy diagram of the BABZF/TPBI heterojunction is shown in Figure 8a. Because of a high injection barrier for holes (ca. 0.8 eV), some holes will be blocked by TPBI and hence accumulate at the interface. Similarly, an electron injection barrier of ca. 0.2 eV will result in some electrons being blocked by BABZF and accumulated at the interface, whereas other electrons will tunnel through the interface into the BABZF layer. This tunneling process should result in the filling of LUMO of BABZF, which eventually results in the radiative emission characteristic of BABZF peaked approximately at 448 nm.50 Presumably, the origin of the broadened emission spectra of the devices is due to the excimers/exciplexes/electroplexes formed at the interface with TPBI. To examine the possibility of excimer formation, fluorescence spectra of BABZF were recorded in concentrated DCM solution (ca. 1 × 10−3 M) as well as in the thin-film state. In both cases, no emission at longer wavelengths characteristic of excimer was observed, which rules out the possibility of contribution of excimer to the observed EL spectra. We recorded fluorescence spectra of a DCM solution of 1:1 mixture of BABZF and TPBI, which also did not reveal any longer wavelength absorption that could be attributed to exciplex. However, it is possible that they are weakly emissive, and their emission is subdued by the emission from BABZF. For the possible exciplex emission at the interface of BABZF and TPBI layers, eight different configurations of the BABZF

and TPBI molecules were sampled through energy optimization at a lower level of Gaussian to choose the most favorable geometry of the exciplex (Figure S8).14 Configurations in which BABZF and TPBI were aligned face-to-face in an H-aggregate fashion are energetically most stable of all. Excited-state energy optimization51 of the most stable configuration predicts an energy gap of ca. 2.25 eV (550 nm) between the electronic ground and first excited states. We propose that the EL peak at ca. 570 nm is most likely due to the exciplex formation between BABZF and TPBI of the type TPBI*BABZF at the interface between the HTM and the ETM (Figure 8a). This exciplex is formed as a consequence of quantum mechanical mixing between the LE complex and the charge-transfer complex of BABZF and TPBI. As briefly mentioned above, the potential barriers between BABZF and TPBI result in the accumulation of electrons and holes in the interface region accompanied by simultaneous tunneling of the electrons, which increases significantly with increasing the applied electric field. This leads to cross recombination of the charge carriers with electrons primarily in the LUMOs of TPBI and holes in the HOMOs of BABZF, which results in the emission band slightly shifted toward the longer wavelength region peaking approximately at 480 nm, known as an electroplex emission.50,52 This electroplex emission band lies nearer to the emission band of BABZF because the LUMOs of TPBI and BABZF have a very small energy gap of ca. 0.2 eV, which does not give rise to a larger shift in the emission band of the electroplex. The multiple bands in the EL spectra of the device can thus be ascribed to the monomolecular emission from BABZF peaking at ca. 450 nm, an electroplex emission at ca. 480 nm, and the low-energy exciplex emission peaking at ca. 570 nm. It is noteworthy that the efficiencies of emission from the involved species are different in different devices, which ultimately determine the CIE coordinates (cf. Table 2).

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CONCLUSIONS Two novel electroluminescent materials, that is, BABZF and BATOMe, based on a twisted bianthryl core were designed and synthesized; these are thermally stable with high Tds and Tgs. Both compounds exhibit a reversible redox property and are brilliantly fluorescent with high quantum yields of emission. In the fabricated nondoped devices, BABZF leads to white-light emission (CIE ≈ 0.28, 0.33) with a moderate power efficiency of 2.24 lm/W and a very high luminance of 15 600 cd/m2. The device exhibited excellent color stability over a range of applied 1421


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BATOMe as a yellowish solid, yield 2.23 g (91%); IR (KBr) cm−1: 3062, 3004, 2929, 2836, 1590, 1451, 1415, 1365; 1H NMR (CDCl3, 500 MHz): δ 3.90 (s, 12H), 6.71 (s, 2H), 6.80 (s, 4H), 7.14−7.17 (m, 4H), 7.23 (d, J = 8.55 Hz, 4H), 7.33− 7.36 (m, 4H), 7.91 (d, J = 8.60 Hz, 4H); 13C NMR (CDCl3, 100 MHz): δ 55.5, 100.1, 109.4, 125.3, 125.5, 127.0, 127.3, 129.8, 131.3, 133.4, 137.8, 141.1, 160.8; ESI-MS+ m/z: [M + H]+ calcd for C44H35O4, 627.2535; found, 627.2532.

potential. Experimental as well as theoretical studies suggest the possibility of multispecies emission involving BABZF and its exciplex and electroplex in the devices. Remarkably, a similar white-light emission was also captured in a simple double-layer device, which is unprecedented. The results attest to the innate hole-transporting ability of BABZF, despite the fact that it does not contain any traditional hole-transporting di-/triarylamino group(s) in its structure. Similar studies with BATOMe led to inferior device performance results, suggesting the importance of dibenzofuran group in BABZF. The findings described herein may serve as a foundation for the creation of new nonnitrogenous HTMs, which may lead to more efficient white light-emission in simple double-layer devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01712. 1 H and 13C NMR spectral reproduction, TGA, DSC, CV, PL and EL profiles, and device efficiency plots (PDF)

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EXPERIMENTAL SECTION Materials. ITO-coated glass slides (thickness 0.7 mm; resistance 11 Ω) and NPB, TPBI, LiF and Al, and HPLC grade solvents employed for cleaning the ITO-coated glasses, and carrying out other measurements such as UV−vis, fluorescence, CV, and so forth were procured from commercial sources. Device fabrications and photophysical, thermal, electrochemical, and EL characterizations were carried out as described elsewhere.24,25 Synthetic Procedures and Characterization Data. Synthesis of BABZF. A mixture of toluene (15 mL), EtOH (10 mL), and water (5 mL) contained in a two-necked round bottom flask was degassed thoroughly by bubbling N2 gas for 10 min. To it were added 10,10′-dibromo-9,9′-bianthryl (1.0 g, 1.95 mmol), dibenzo[b,d]furan-2-ylboronic acid (1.65 g, 7.80 mmol), K2CO3 (1.08 g, 7.8 mmol), and Pd(PPh3)4 (0.45 g, 0.39 mmol). The resultant mixture was heated at 110 °C for 2 d. At the end of this period, organic solvents were stripped off in vacuo and the crude mixture was extracted with chloroform three times. The combined organic extract was dried over anhyd Na2SO4 and filtered. The solvent was subsequently removed under reduced pressure to obtain a crude mixture, which was purified by silica gel column chromatography using a mixture of chloroform and pet ether (v/v 1:10 to 7:10) to obtain BABZF as an off-white solid, yield 1.22 g (92%); IR (KBr) cm−1: 3059, 2922, 1587, 1475, 1449, 1357; 1H NMR (CDCl3, 500 MHz): δ 7.19−7.22 (m, 4H), 7.30−7.37 (m, 8H), 7.41 (t, J = 7.15 Hz, 2H), 7.56 (td, J1 = 7.45 Hz, J2 = 1.15 Hz, 2H), 7.72 (d, J = 8.00 Hz, 2H), 7.75 (dd, J1 = 8.00 Hz, J2 = 1.70 Hz, 2H), 7.87−7.90 (m, 6H), 8.00 (d, J = 7.45 Hz, 2H), 8.26 (d, J = 1.15 Hz, 2H); 13C NMR (CDCl3, 100 MHz): δ 111.7, 111.9, 120.9, 123.0, 123.6, 124.1, 124.6, 125.4, 125.57, 125.61, 127.13, 127.14, 127.3, 127.5, 130.4, 130.5, 131.4, 133.47, 133.54, 137.6, 155.8, 156.8; ESI-MS+ m/z: [M]+ calcd for C52H30O2, 686.2246; found, 686.2245. Synthesis of BATOMe. A mixture of toluene (15 mL), EtOH (10 mL), and water (5 mL) contained in a two-necked round bottom flask was degassed thoroughly by bubbling N2 gas for 10 min. To it were added 10,10′-dibromo-9,9′-bianthryl (2.0 g, 3.9 mmol), 3,5-dimethoxyphenylboronic acid (2.12 g, 11.7 mmol), K2CO3 (1.6 g, 11.7 mmol), and Pd(PPh3)4 (0.68 g, 0.6 mmol). The resultant mixture was heated at 110 °C for 2 d. At the end of this period, organic solvents were stripped off in vacuo and the crude mixture was extracted with chloroform three times. The combined organic extract was dried over anhyd Na2SO4 and filtered. The solvent was subsequently removed under reduced pressure to obtain a crude mixture, which was purified by silica gel column chromatography using a mixture of chloroform and pet ether (v/v 1:10 to 1:2) to obtain

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.J.). *E-mail: [email protected] (T.J.C.). *E-mail: [email protected] (J.N.M.). ORCID

Samik Jhulki: 0000-0003-1318-8666 Jarugu Narasimha Moorthy: 0000-0001-9477-5015 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.N.M. is thankful to Science and Engineering Research Board (SERB), New Delhi for generous financial support. S.J. thanks CSIR, New Delhi for a senior research fellowship, and S.S. is grateful to CSIR for SPM fellowship. We thankfully acknowledge the support by the Scientific Instruments Facility at the Institute of Chemistry, Academia Sinica for optoelectronic device fabrications and testing. We thank the anonymous reviewers for their insightful suggestions.

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