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SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF 3D SUBSTITUTED HETEROHELICENES AND THEIR DERIVATIVES

Ying Hu

A Dissertation Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY December 2008 Committee: Douglas C. Neckers, Advisor Paul Morris Graduate Faculty Representative Thomas H. Kinstle Michael A. J. Rodgers John R. Cable

© 2008 Ying Hu All Rights Reserved

iii

ABSTRACT

Douglas C. Neckers, Advisor

Light-emitting fluorophores 1-10b have been synthesized. Helical compounds end-capped with different electron-rich and electron-poor aryl moieties are evaluated. Photocyclization of 10a was investigated, and the X-ray crystal structure of 4 obtained. We demonstrate that the optical properties of all of the new compounds, and by extension many conjugated materials, can be tuned over the entire blue range (400-480 nm), below. The charge-separated state of 8 was observed in the absorption and emission spectra. DFT (B3LYP/ 6-31G*) calculations identified the optimum geometry of 8. The distance between the electron-donating and electron-withdrawing group in 8 is 20 Å through the bond. The helical structure of the T5H bridge causes the distance of donor and acceptor to be as short as 7.06 Å. Rapid charge separation and slow charge recombination was detected by femtosecond pump-probe absorption measurements. A through bond electron transfer mechanism is preferred, although the distance through space is much shorter than the thru-bond distance. The nonlinear optical properties of BDT, T5H, 1, 6, 7, 9 and 11 as well as Rhodamine B have been investigated. Rhodamine B in methanol was used for calibration. π-Centers of BDT and T5H were studied. Compounds 1, 6, 7, 9 and 11 possess either donor-π (D-π) or donor-π-acceptor (D-π-A) structures. Two-photon absorption cross-sections related to the third-order nonlinear susceptibility were measured using Z-scan techniques. Molecular structure and nonlinear optical property

iv

relationship was formulated. Compound 9 has a two-photon absorption cross-section of 103 GM when excited at 720nm.

v

To my most loved ones – my husband, daughter and parents

vi TABLE OF CONTENTS Page CHAPTER 1. BACKGROUND AND INTRODUCTION ..................................................

1

1.1. Organic Helical Polycyclic Compounds: Helicene.............................................

1

1.2. Thiahelicenes and Their Derivatives...................................................................

3

1.3. Goals and Achievements of the Project ..............................................................

5

1.4. References ...........................................................................................................

6

CHAPTER 2. SYNTHESIS AND PHOTOPHYSICAL PROPERTIES..............................

11

2.1. Introduction.........................................................................................................

11

2.2. Synthesis .............................................................................................................

12

2.3. Experimental .......................................................................................................

14

2.3.1. Materials...............................................................................................

14

2.3.2. Instruments ...........................................................................................

14

2.3.3. Synthesis...............................................................................................

15

2.4. Results and Discussion........................................................................................

20

2.4.1. Photocyclization ...................................................................................

20

2.4.2. Crystal Structure...................................................................................

22

2.4.3. Absorption and Emission Spectra ........................................................

24

2.5. Conclusions.........................................................................................................

26

2.6. References ...........................................................................................................

27

CHAPTER 3. TRANSIENT SPECTROSCOPY AND GEOMETRY OPTIMIZATION ...

29

3.1. Introduction.........................................................................................................

29

3.2. Experimental .......................................................................................................

30

vii 3.2.1. Materials ..............................................................................................

30

3.2.2. Ultrafast Spectrometry .........................................................................

30

3.2.3. UV-visible Absorption, Excitation and Fluorescence Spectroscopy ...

31

3.3. Results and Discussion........................................................................................

31

3.3.1. Solvent Effect .......................................................................................

31

3.3.2. Femtosecond Transient Absorption Measurements .............................

34

3.3.3. Geometry Optimization........................................................................

35

3.4. Conclusions.........................................................................................................

38

3.5. References ...........................................................................................................

38

CHAPTER 4. NONLINEAR OPTICAL PROPERTY AND STRUCTURE−PROPERTY CORRELATION

............................................................................................................

40

4.1. Introduction.........................................................................................................

40

4.1.1. Nonlinear Optical Effect ......................................................................

41

4.1.2. The Third-Order NLO Materials .........................................................

42

4.1.3. Tow-Photon Absorption.......................................................................

45

4.1.4. Z-Scan Technique ................................................................................

46

4.2. Z-Scan Setup .......................................................................................................

47

4.3. Results and Discussion........................................................................................

48

4.4. Conclusions .........................................................................................................

54

4.5. References ...........................................................................................................

54

APPENDIX A. List of Abbreviations and Symbols............................................................................ APPENDIX B:

59

viii (1) 1H and 13C NMR Spectra of 1-10b (Figure B1-B11) ..........................................

61

(2) Thin Film Fluorescence Spectra of 1, 3 and 7 (Figure B12) ...............................

71

(3) IR Spectra of 1-10b (Figure B13-B23) ...............................................................

71

(4) Data Obtained from Crystal Structure Determination for 4 .................................

77

APPENDIX C: (1) Transient Absorption Spectra (Figure C1) ..........................................................

82

(2) Optimized Geometry Structures and Calculated HOMO, LUMO (Figure C2-C13) ............................................................................................................

83

(3) Fluorescence Lifetimes (Figure C14-C25) ..........................................................

86

APPENDIX D: Z-Scan Traces and Theoretical Fit with Equation 5 of 1, 6 and 11 (Figure D1-D3)

90

ix

LIST OF FIGURES/TABLES Figure

Page

1.1 A Dissymmetrical Cylindrical Helix with a C2 Axis .................................................

1

1.2

Numbering of the [6]Helicene Skeletons...................................................................

2

1.3

Photoconversion of Stilbene to Phenanthrene ...........................................................

2

1.4

The Preferential Product: an Isomer of [7]Helicene ..................................................

3

1.5

Heterohelicenes Representatives ...............................................................................

4

1.6

Mechanism of Photo-Oxidation Cyclization .............................................................

4

1.7

Thiahelicene Derivatives Found in Literature ...........................................................

5

2.1

Chemical Structures of Benzo[1,2-b:4,3-b’]dithiophene (BDT) and Trithia[5]helicene (T5H) Derivatives......................................................................................................

11

2.2

1

H NMR Profile of Photocyclization of 10a .............................................................

21

2.3

Photocyclization Data Profile of 10a.........................................................................

22

2.4

X-Ray Crystal Structure of 4 .....................................................................................

23

2.5

Packing Pattern of 4 ...................................................................................................

23

2.6

Crystals of 4 under UV irradiation.............................................................................

24

2.7

Normalized Absorption and Emission Spectra of BDT and T5H in DCM...............

25

2.8 Normalized Fluorescence Spectra of Compounds 1-10b in DCM ............................

26

3.1

Normalized Absorption and Emission Apectra of (a) T5H, (b) 1, (c) 3 and (d) 8 in Hexane (HEX), Dichloromethane (DCM) and Acetonitrile (ACN) ..........................

32

3.2

Transient Absorption Spectra of 8 in DCM ...............................................................

34

3.3

Kinetic Decay of 8 at 535 nm (left) and Growth at 484 nm (right) in DCM .............

35

x 3.4

Kinetic Decay of 8 at 535 nm (left) and Growth at 484 nm (right) in ACN..............

35

3.5

Optimized Geometry Structures of 8 Calculated at B3LYP/6-31G* Level...............

36

3.6

Calculated HOMO (left) and LUMO (right) of 8 ......................................................

36

4.1

Chemical Structures of Compounds Investigated in NLO Study ..............................

40

4.2

(a) KDT and (b) KTP crystals....................................................................................

43

4.3

1D (left) and 2D (right) Third-Order Nonlinear Optical Organic Chromophores .....

44

4.4

Europium (63) Salts ...................................................................................................

45

4.5

Schematic Representation of Z-Scan Setup ...............................................................

48

4.6

Z-Scan Traces (red dot) and Theoretical Fit with Eqn. 12 (black square) of 10 mM methanol solution of Rhodamine B at 720 nm ..........................................................

4.7

4.8

49

Z-Scan Traces (red dot) and Theoretical Fit with Eqn. 12 (black square) of 10 mM DCM solution of 7 (a) and 1.4 mM DCM solution of 9 (b) at 720 nm .....................

51

Flowchart of Compounds...........................................................................................

53

xi LIST OF TABLES Tables

Page

2.1 Photophysical Properties of 1-10b in DCM............................................................... 3.1

25

UV absorption, emission, emission lifetimes, and quantum yields in various solvents 33

3.2 HOMO-LUMO Energy of T5H, 1, 3 and 8...............................................................

37

4.1

52

Results of Z-Scan Measurements...............................................................................

vi

ACKNOWLEDGMENTS My sincere thanks go to my advisor Dr. Douglas C. Neckers for the guidance, encouragement and support through the years of my graduate studies. He has given me much freedom to develop my interests and enabled me to grow as a scientist. Special thanks go to Dr. Brigitte Wex for being my mentor. Her valuable time and knowledge are highly appreciated. I am thankful to Dr. Thomas Kinstle for consulting on organic chemistry matters, Dr. Michael A. Rodgers and Dr. John Cable for helpful discussions and valuable comments. I thank Dr. Paul Morris for serving as my committee member. Thanks are also due to Dr. Evgeny Danilov for help with nonlinear optical study and fruitful discussions and Dr. Xichen Cai for help with transient spectroscopic studies. I acknowledge all of Dr. Neckers’ group members for providing a friendly atmosphere in the lab. Nora Cassidy and Alita Frater are greatly appreciated for making many aspects of my life at the Center much easier. I thank Dr. J. Romanowicz and Dr. D.Y. Chen for assisting with mass spectrometry and NMR experiments, respectively. I would also like to acknowledge Larry Ahl, Craig Bedra and Doug Martin for technical support. The Ohio laboratory for Kinetic Spectroscopy and Wright Photoscience Laboratory are appreciated for providing necessary instrumentation. I am thankful to the McMaster Endowment for support in form of a Fellowship. I would like to thank my friends Janet Holton and David Bartholomew for their care and kindness through all my years in Bowling Green. I wish to express my deepest gratitude to my family for their dedication and many years of support: my dearest husband Dapeng Zhou, my lovely daughter Yayue Grace Zhou, my parents and sisters.

1 CHAPTER 1. BACKGROUND AND INTRODUCTION 1.1. Organic Helical Polycyclic Compounds: Helicene. Conjugated organic molecules have attracted interest because of their potential applications in the development of optoelectronic devices or as photoactive materials. Linear and nonlinear polycyclic compounds have been studied in organic light-emitting diodes (OLED), organic field-effect transistors (OFET), and nonlinear optics (NLO). In linear π-conjugated systems the polyacenes1-7, polyheteroacenes and their derivatives8-10 have been extensively studied. Applications, however, are limited due to their instability and poor solubility. Helicenes, a name introduced by M. S. Newman in 195611, are interesting helical-shaped aromatic systems. Molecules formed by ortho-condensed aromatic rings, if the number of rings is five or more, steric interactions cause the conjugated aromatic molecules to distort into helices that are intrinsically chiral structures (Figure 1.1). Helicenes are inherently dissymmetric chromophores. A twofold symmetry axis, C2, is perpendicular to its cylindrical helix.

C2

Figure 1.1. A dissymmetrical cylindrical helix with a C2 axis. From a practical point of view, helicenes as unique three-dimensional (3D) polyaromatic systems are chemically stable and soluble in common organic solvents. Helicenes represent an attractive objective for research in various branches of chemistry. They have been intensively studied owing to their chiropitical and nonlinear optical properties. Enantiomerically enriched helicenes have been used as chiral catalysts12 and ligands13,14 in asymmetric synthesis.

2 “Carbohelicenes” contain only carbon atoms in their skeleton. The first reported synthesis of carbohelicenes is the twelve-step synthesis of [6]helicene described by Newman and Lednicer in 1956. Newman’s synthesis and resolution of [6]helicene opened the way to the study this class of synthetic compounds. He suggested numbering the carbohelicene skeletons as shown in Figure 1.2. Attempts to use the same scheme to prepare higher numbered carbohelicenes were unsuccessful. In 1964, Wood and Mallory described the procedure for the photo-conversion of stilbene to phenanthrene.15 Using this procedure, carbohelicenes could be prepared in fair to excellent yields by the photoinduced oxidation cyclization of appropriate 1,2-diarylethenes (Figure 1.3).

Figure 1.2. Numbering of the [6]helicene skeletons. hv

Figure 1.3. Photoconversion of stilbene to phenanthrene. Martin and coworkers improved synthetic routes to [6], [7] and even [13], [14]carbohelicenes.16 Isomer formation cannot be avoided at some stage of these synthetic processes (Figure 1.4) due to the facts that the position ortho to the ethene moiety are identical and that the differences in reactivity in the ground and excited states are small.17,18 Alternative approaches have recently emerged. At least two of these approaches have been found to be more

3 general, such as the Diels-Alder reaction of aromatic vinylethers with p-benzoquinone, developed by Katz,19 and the intramolecular [2+2+2]cyclization of aromatic triynes in the presence of Ni0 or Co0 complexes.20-22

hv

Figure 1.4. The preferential product: an isomer of [7]helicene. Experimental efforts were complemented by many theoretical studies. Furche et al.23 and Autschbach et al.24 carried out time-dependent density functional theory calculations to investigate electronic circular dichroism spectra of helicenes. The electronic properties of helicenes, such as the electronic conductance of the growing helical system, was studied by Treboux et al.25 The local stability of [n]acenes, [n]phenacenes, and helicenes (n=1-9) was addressed by Portella et al.;26 Very recently, Rulíšek et al. performed density function theory (DFT) and DFT-D methods to calculate physicochemical properties of helicenes (n= 1-14).27 1.2. Thiahelicenes and Their Derivatives. The presence of heteroatoms in the helicene significantly alters molecular properties (Figure 1.5), though they have received less attention than carbohelicenes. In the monoaza[5]helicene, it has been shown that the position of the N atom has an important effect on the structural and spectroscopic properties of the molecules.28-30 The relative phosphorescence/fluorescence ratio can be markedly tuned by changing the position of the N atom.

4 O HN

N

S

NH O

5-aza[5]helicene

O

S

trifuryl[5]helicene

dicarbonzal[6]helicene

S

trithia[5]helicene

Figure 1.5. Heterohelicenes representatives. Due to the unique 2-position of thiophene and the electron rich S atoms, replacing benzenes with thiophenes has beneficial effects on the nonlinear optical properties. Thiophene, known for over 100 years, has been detected in a number of environmental sources.31,32 Although thiophene’s chemical behavior resembles that of benzene, it has many different structural and chemical features.33 Simultaneous with Martin’s work, Wynberg’s group proved that the presence of thiophene in a helicene significantly alters the molecular properties: there is one, and only one, position in the thiophene ring available for photocyclic ring closure and no other isomer can form. The mechanism of photo-oxidation cyclization they proposed is shown in Figure 1.6.34 H H S H

I2

hv

hv

H

S

S

S H

H

S

S H

H

S

S H

H

Figure 1.6. Mechanism of photo-oxidation cyclization. The presence of thiophene makes thiahelicenes stable enough to be synthesized in good yield via photo-oxidation cyclization.35,36 A Japanese group has used the same methodology to synthesize thiohelicenes with higher numbers of rings (up to 15).37 Thia[15]helicene is the first member in the series with a three-layered structure. Single crystal X-ray diffraction on thia[9]

5 and [11]helicene have been presented by the Caronna group from Italy.33,38 The preparation of higher numbered thiahelicene is difficult due to their poor solubility in organic solvents. In order to enhance the solubility or optical nonlinearity, substituents on the aromatic skeleton are introduced. Novel thiahelicene derivatives have been synthesized as shown in Figure 1.7.39-46 R1 R2 S

S

S

R5

R5

R5 S

S

S R3

S R3

R4

R1= CHO, R2 = R3 = R4 = H R1= R2 = H, R3 = R4 = n-C3H7 R1= R2 = H, R3 = R4 = n-C8H17 R1= R2 = R3 = H, R4 = n-C3H7 R1= R2 = R3 = H, R4 = n-C4H9 R1= H, R2 = COCF3, R3 = R4 = n-C3H7 R1= H, R2 = CHO, R3 = R4 = n-C3H7 R1= H, R2 = CO2Et, R3 = R4 = n-C3H7 R1= R2 = COCF3, R3 = R4 = n-C3H7 R1= R2 = CHO, R3 = R4 = n-C3H7 R1= R2 = CO2Et, R3 = R4 = n-C3H7 R1= R2 = Me, R3 = R4 =H R1= R2 = I, R3 = R4 =H R1= R2 = Br, R3 = R4 =H R1= R2 = TMS, R3 = R4 =H R1= R2 = TiPrS, R3 = R4 =H

S

R5

R4

R5 = n-C8H17, R3 = R4 =H R5 = n-C8H17, R3 = R4 = n-C8H17

R7OC S

S

S

R6 R6 R7 =

NH OH

,

NH OH

S

S

R6 = OH R6 = OMe

Figure 1.7. Thiahelicene derivatives found in literature. 1.3. Goals and Achievements of the Project. Nonlinear polycyclic helicene compounds used as NLO materials have been studied both theoretically and experimentally.47-49 They serve as model

6 compounds for understanding the chemical and physical properties of related materials as well as for the investigation of structure-property correlations among similar electronic materials. Our particular interest was in probing structural features to affect useful functions and properties of materials of aryl substituted thiahelicenes in that these compounds are essential toward the rational design and the optimization of functional organic helical conjugated materials. Several π-conjugated helical compounds based on benzo[1,2-b:4,3-b’]dithiophene (BDT) and trithia[5]helicene (T5H) were synthesized, and their structural-functional property relationships investigated.50-52 BDT and T5H emit in the UV region at 333 nm, 365 and 379 nm respectively. By carefully choosing the number of aromatic substituents and end-capped moieties, blue-emitting compounds can be obtained. Though BDT itself is planar, aryl substituents increase the overcrowding of the molecules and cause the structures to become helical. The helical structure was verified by X-ray crystallographic analysis taken on one of those compounds. Structures of several compounds were optimized by DFT calculations (B3LYP 631G*). The third-order nonlinear optical (NLO) responses of these compounds manifested by their two-photon absorption (TPA) cross-sections σ2 have been studied. Femtosecond spectroscopy was employed to obtain information about the intramolecular electron transfer.

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11 CHAPTER 2. SYNTHESIS AND PHOTOPHYSICAL PROPERTIES 2.1. Introduction. Many alkyl thiahelicenes have been reported (Figure 1.4), but aryl thiahelicenes have not been prepared. Aryl substituents have the ability to self-assemble which is a valuable feature for materials used in organic optoelectronic devices. Optical nonlinearity in these materials may be enhanced by introducing electron-rich and electron-poor moieties at strategic positions. Therefore, the synthesis of thiahelicenes end-capped with electron-rich and electron-poor aryl moieties is proposed to be potential material for nonlinear optics. R1

R2 S

S

1. R1 = 4-methoxyphenyl, R2 = H 2. R1 = a-naphthyl, R2 = H 3. R1 = 4-cyanophenyl, R2 = H 4. R1 = R2 = a-naphthyl 5. R1 = 4-acetylphenyl, R2 = H 6. R1 = 4-methoxyphenyl, R2 = (E)-2-thienylethenyl

R3

R4

S

S

S 7. R3 = 4-methoxyphenyl, R4 = H 8. R3 = 4-methoxyphenyl, R4 = 4-cyanophenyl 9. R3 = 4-methoxyphenyl, R4 = 4-acetylphenyl

Figure 2.1. Chemical structures of benzo[1,2-b:4,3-b’]dithiophene (BDT) and trithia[5]helicene (T5H) derivatives. 2-(4-Methoxyphenyl)benzo[1,2-b:4,3-b’]dithiophene (1), 2-(α-naphthyl)benzo-[1,2-b:4,3b’]dithiophene (2), 2-(4-cyanophenyl)benzo[1,2-b:4,3-b’]dithiophene (3), 2,7-di-(α-naphthyl)benzo[1,2-b:4,3-b’]dithiophene (4), 2-(4-acetylphenyl)benzo[1,2-b:4,3-b’]dithiophene (5), and 2-

12 (4-methoxyphenyl)-7-[(E)-2-(2-thienyl)ethenyl]benzo[1,2-b:4,3-b’]dithiophene (6) are BDT derivatives with aryl groups attached at the one/both terminal thiophenes. 2-(4-Methoxyphenyl)trithia[5]helicene (7), 2-(4-cyanophenyl)-11-(4-methoxyphenyl)trithia[5]helicene (8), and 2-(4acetylphenyl)-11-(4-methoxyphenyl)trithia[5]helicene (9) are T5H derivatives with electron-rich and/or electron-poor aryl group attached. Each compound is assigned a unique number (Figure 2.1), which will be used throughout the manuscript. Their photophysical properties were recorded. 2.2. Synthesis. Scheme 2.1.a Synthesis of BDT derivatives. (a) Br

S

(b) R1

S

O R1

S

(c) (d)

R1

R1

S

S

10a. R1 = α−naphthyl (e)

S

S

R2

10b. R1 = R2 = α−naphthyl (e) R2

R1 S

S

2. R1 = α−naphthyl, R2 = H 4. R1 = R2 = α−naphthyl a

Reagents and conditions: (a) R1B(OH)2, toluene, (C4H9)4NOH, Pd(PPh3)4, reflux,

overnight. (b) n-BuLi, DMF, -78°C, 40 min. (c) diethyl(2-thienylmethyl)phosphonate, NaH, DME, 0°C for 30 min, reflux 3 h. (d) TiCl4, Zn, THF, -18°C for 1 h, reflux 4 h. (e) I2, propylene oxide, hv, 1 day. Photocyclization of cis-stilbene-like olefins followed by in situ iodine oxidation is a crucial step used for the synthesis of higher numbered helicenes.1,2 This reaction requires dilute

13 solutions - typically in toluene. More importantly, the products act as filters for the incident radiation.3 Application to higher numbered (n≥7) helicenes and heterohelicenes is limited due to poor solubility and the small scale of this synthetic approach. Suzuki coupling is an efficient method for introducing aryl groups using a palladium catalyst to cross couple an organoboronic acid with an aryl halide. Theoretically, there are two approaches to the synthesis of 1-9, i.e. Suzuki coupling followed by Wittig/McMurry reaction or Wittig/McMurry reaction followed by Suzuki coupling. Compounds 2 and 4 were synthesized by the former approach in overall yield of 27-47%, Scheme 2.1. Decomposition products were detected by NMR spectroscopy during photochemical cyclization (step e, Scheme 2.1). When the latter approach was applied, the synthesis of 2 was achieved in ~80%, Scheme 2.2. 2-Arylthiophenes were susceptible to rearrangement on irradiation.4 Photolysis of 2-(αnaphthyl)thiophene in dilute solution, for example, yielded 3-(α-naphthyl)-thiophene as the only product.5-7 Photolysis of 3-(α-naphthyl)thiophene in dilute solution lead to slow decomposition in the absence of migration of the naphthalene ring.5-7 In contrast, 2- and 3-arylbenzo[b]thiophenes showed no significant migration of the aryl group or decomposition upon photolysis.8,9 In this work, 2-arylbenzo[1,2-b:4,3-b’]dithiophenes showed no evidence of group migration as verified by gas chromatography, NMR, and UV absorption spectroscopies, as well as X-ray crystallography. In this case, conjugation might stabilize carbons 2 and 3 of the thiophene moiety toward rearrangement by rigidifying the structure and delocalizing the electrons.

14 Scheme 2.2.a Synthesis of T5H derivatives. I

(a) S

R1

S

S

S

R2

R1

(b)

S

S

1. R1 = 4-methoxyphenyl 2. R1 = α−naphthyl 3. R1 = 4-cyanophenyl 5. R1 = 4-acetylphenyl

11. R1 = 4-methoxyphenyl, R2 = CHO 6. R1 = 4-methoxyphenyl, R2 = (E)-2-thienylethenyl

(c)

(d) R1 S

R3 S

S 8. R1 = 4-methoxyphenyl, R3 = 4-cyanophenyl 9. R1 = 4-methoxyphenyl, R3 = 4-acetylphenyl a

R1

(a)

S

I

R1 S

(e)

S 12. R1 = 4-methoxyphenyl

S

S

S 7. R1 = 4-methoxyphenyl

(a) DME, K2CO3 (2M), Pd(PPh3)4, R3B(OH)2, reflux, overnight. (b) n-BuLi, DMF, -78 °C,

40 min. (c) diethyl(2-thienylmethyl)phosphonate, NaH, DME, 0 °C for 30 min, reflux 3 h. (d) I2, propylene oxide, hv, 1 day. (e) n-BuLi, I2, DME, -78 °C, 1 h. 2.3. Experimental. 2.3.1. Materials. Solvents were dried and freshly distilled following usual protocols prior to use.10 Reagents were used as received from commercial suppliers. Reactions that required anhydrous conditions were carried out under argon in oven-dried glassware. Standard grade silica gel (60 Å, 32-63 μm) and silica gel thin layer chromatography plates were purchased from Sorbent Technologies. Distilled dichloromethane (DCM) was used for recording UV-visible spectra. 2.3.2. Instruments. Melting points were determined using a Thomas capillary melting point apparatus and are uncorrected. Mass spectra were recorded on a Shimadzu GCMS-QP5050A instrument equipped with a direct probe (ionization 70 eV). A Bruker spectrometer (working

15 frequency 300.0 MHz for 1H and 75.0 MHz for 13C) was used to record the NMR spectra using CDCl3 and C6D6 as solvents. Chemical shifts relative to TMS at 0.00 ppm are reported in parts per million (ppm) on the δ-scale for 1H NMR. For 13C NMR, chemical shifts were reported in the scale relative to CDCl3 at 77.00 ppm and C6D6 at 128.00 ppm. Absorption and fluorescence spectra were recorded on a Shimadzu UV-2401 spectrophotometer and a Fluorolog-3 spectrometer, respectively. All measurements were carried out under ambient conditions. 2.3.3. Synthesis. 2-(α-Naphthyl)thiophene, 5-(α-naphthyl)thiophene-2-carbaldehyde, BDT, T5H and 2-iodobenzo[1,2-b:4,3-b’]dithiophene were synthesized following literature procedures.11-15 The NMR and IR spectra of 1-10b are included in Appendix B. General procedure for Wittig reaction: To a stirred, ice-cooled solution of diethyl(2-thienylmethyl)phosphonate (2.3 g, 10 mmol) in dimethoxyethane (DME) (40 mL) was added sodium hydride (60% dispersion, 0.52 g, 12 mmol). The mixture was stirred for 30 min in an ice bath and then a solution of 5-(αnaphthyl)thiophene-2-carbaldehyde (2.4 g, 10 mmol) in DME (10 mL) was added dropwise over a period of 30 min. The mixture was slowly warmed and refluxed for 3 h. After quenching the reaction by addition of ice-water (50 mL), the mixture was extracted with ether. The extract was washed with water, dried over anhydrous Na2SO4, and evaporated. The residue was recrystallized from cyclohexane or purified by silica gel column chromatography. (E)-1-{2-[5-(α-Naphthyl)thienyl]}-2-(2-thienyl)ethene (10a). Recrystallization from cyclohexane gave pure yellow solid, yield: 42%, mp 95-96 °C. 1H NMR (300 MHz, CDCl3, 25°C): δ 7.0 (m, 1H), 7.06 (d, 1H, J = 4 Hz), 7.09 (s, 2H), 7.11 (d, 1H, J = 4 Hz), 7.14 (d, 1H, J = 4 Hz), 7.20 (d, 1H, J = 4 Hz), 7.47-7.54 (m, 3H), 7.59 (d, 1H, J = 7 Hz), 7.85-7.91 (m, 2H), 8.28-8.31 (m, 1H); 13C NMR (75 MHz, CDCl3): δ 121.4, 121.5, 124.4, 125.3, 125.7, 126.0,

16 126.1, 126.5, 126.6, 127.7, 128.0, 128.1, 128.4, 128.5, 131.7, 132.4, 133.9, 140.8, 142.5, 142.7; HRMS (EI): m/z [M]+ Calcd for C20H14S2, 318.0537; Found, 318.0536. 2-(4-Methoxyphenyl)-7-[(E)-2-(2-thienyl)ethenyl]benzo[1,2-b:4,3-b’]dithiophene (6). Silica gel column chromatography with hexane : DCM = 1:1 as eluent produced pure 6, yield: 71%, mp 220-222 °C, 1H NMR (300 MHz, CDCl3): δ 3.88 (s, 3H), 6.97 (m, 1H), 7.00 (m, 1H), 7.03 (m, 1H), 7.12 (m, 1H), 7.18 (d, 2H, J = 3 Hz), 7.23 (m, 1H), 7.60 (s, 1H), 7.65 (m, 1H), 7.68-7.74 (m, 4H); 13C NMR (75 MHz, CDCl3) : δ 55.4, 114.5, 116.3, 118.1, 118.9, 121.1, 121.9, 123.7, 124.9, 126.7, 127.1, 127.7, 127.8, 134.9, 135.5, 135.7, 135.9, 142.1, 142.7, 144.9, 159.9; HRMS (EI): m/z [M+] Calcd for C23H16OS3, 404.0363; Found, 404.0368. (E)-1,2-Di-{2-[5-(α-naphthyl)thienyl]}ethane (10b). To a stirred solution of 5-(αnaphthyl)thiophene-2-carbaldehyde (2.8 g, 10 mmol) in THF (100 mL) was added titanium(IV) chloride (3 mL, 3 mmol) over a period of 30 min at -18°C (dry ice with ethanol/water 1 : 4). After stirring at this temperature for 30 min, zinc powder (4 g, 60 mmol) was added in small portions over a period of an additional 30 min. The mixture was stirred at -18°C for 30 min, warmed to room temperature, and refluxed for 4 h. The reaction was quenched by addition of ice-water (50 mL), the resulting solid was collected by filtration and dried. The solid was dissolved in DCM (70 mL) and the insoluble inorganic material was removed by filtration. The filtrate was evaporated and the residue was recrystallized from cyclohexane to give orange solid 10b, yield: 60%, mp 156-158 °C, 1H NMR (300 MHz, CDCl3, 25°C) δ 7.13-7.16 (m, 6H), 7.487.55 (m, 6H), 7.60 (d, 2H, J = 7 Hz), 7.85-7.92 (m, 4H), 8.28-8.31 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 121.4, 125.3, 125.7, 126.1, 126.5, 126.6, 127.9, 128.1, 128.4, 128.5, 131.7, 132.3, 133.9, 140.8, 142.8; HRMS (EI): m/z [M+] Calcd for C30H20S2, 444.1006; Found, 444.1006. General Procedure for Photocyclization

17 Compounds 10a-b or 6 (0.5 mmol) were dissolved in toluene (300 mL) and exposed to 300 nm light from a UV lamp in a Rayonet reactor or a photoreactor for 1 day. I2 (0.25 g, 0.1 mmol) and propylene oxide (3 mL) were added to the solution and air was bubbled through. The solvent was evaporated and the residue recrystallized from hexane or purified by silica gel column chromatography. 2-(α-Naphthyl)benzo[1,2-b:4,3-b’]dithiophene (2). Yellow solid, yield: 64%, mp 91-93 °C, 1H NMR (300 MHz, CDCl3, 25°C): δ 7.50-7.57 (m, 4H), 7.67-7.71 (m, 2H), 7.82-7.83 (m, 3H), 7.89-7.93 (m, 2H), 8.31-8.34 (m, 1H); 13C NMR (75 MHz, CDCl3): δ 118.3, 118.7, 121.9, 122.2, 125.2, 125.8, 126.2, 126.6, 126.7, 128.4, 128.6, 128.9, 131.9, 132.4, 133.9, 134.6, 135.1, 136.7, 136.9, 142.5; HRMS (EI): m/z [M+] Calcd for C20H12S2, 316.0381; Found, 316.0382. 2,7-Di-(α-naphthyl)benzo[1,2-b:4,3-b’]dithiophene (4). Yellow solid, yield: 78%, mp 148-150 °C, 1H NMR (300 MHz, CDCl3, 25°C): δ 7.51-7.57 (m, 6H), 7.71 (d, 2H, J = 7 Hz), 7.86 (s, 2H), 7.88 (s, 2H), 7.90-7.95 (m, 4H), 8.34-8.37 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 118.4, 122.2, 1253, 125.8, 126.2, 126.7, 128.4, 128.6, 129.0, 131.9, 132.5, 133.9, 135.1, 137.2, 142.7; HRMS (EI): m/z [M]+ Calcd for C30H18S2, 442.0850; Found, 442.0843. 2-(4-Methoxyphenyl)trithia[5]helicene (7). Silica gel column chromatography with hexane/DCM (1:2), yellow solid, yield: 80%, mp 152-153 °C, 1H NMR (300 MHz, CDCl3, 25°C): δ 3.88 (s, 3H), 7.02 (d, 2H, J = 8 Hz), 7.73-7.76 (m, 3H), 7.81 (d, 1H, J = 8 Hz), 7.87 (d, 1H, J = 8 Hz), 7.93 (d, 1H, J = 8 Hz), 8.02 (d, 1H, J = 8 Hz), 8.41-8.43 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 55.4, 114.6, 118.6, 119.0, 119.2, 120.7, 121.0, 124.4, 126.2, 127.1, 127.8, 130.4, 134.7, 135.8, 136.9, 137.0, 137.4, 138.1, 144.3; HRMS (EI): m/z [M]+ Calcd for C23H14OS3, 402.0207; Found, 402.0208.

18 2-Iodo-11-(4-methoxyphenyl)trithia[5]helicene (12). A 2.5 M solution of n-BuLi in hexane (0.44 mL, 1.1 mmol) was added dropwise under stirring to a solution of 2-(4methoxyphenyl)trithia[5]helicene 7 (0.4 g, 1 mmol) in 20 mL of dry THF at -78 °C. The solution was stirred for 10 min at -78 °C and for 15 min at room temperature. The resulting light yellow solution was cooled at -78 °C, and a solution of I2 (0.27 g, 1.05 mmol) in 5 mL of dry THF was added dropwise. After 2 h, the reaction was quenched with aqueous Na2SO3 (5%, 20 mL) and warmed to room temperature. The THF was removed under reduced pressure. The yellow material that precipitated was filtered and washed with water. The crude compound was dried under vacuum and used without further purification. General Procedure for Suzuki Reaction: Under argon, a mixture of 2-iodobenzo[1,2-b:4,3-b’]dithiophene (1 mmol, 316 mg), aryl boronic acid (1.05 mmol), and Pd(PPh3)4 (0.03 mmol, 3.0%) in a mixed solvent of DME (3 mL) and aqueous K2CO3 (2 M, 1 mL) was refluxed for overnight. Then, water (5 mL) was added. After separation, the organic phase was washed twice with water (2 x 5 mL) and the collected aqueous phases were extracted twice with 5 mL DCM. The organic phases were collected, dried over anhydrous Na2SO4, and the solvent was removed under reduced pressure. The residue was subjected to silica gel chromatography to obtain the pure products. 2-(4-Methoxyphenyl)benzo[1,2-b:4,3-b’]dithiophene (1). Elution with hexane/DCM (1:1) gave yellow solid, yield: 77%, mp 168-169 °C, 1H NMR (300 MHz, CDCl3): δ 3.87 (s, 3H), 6.97 (m, 1H), 7.00 (m, 1H), 7.56 (d, 1H, J = 8 Hz), 7.72-7.70 (m, 3H), 7.76 (m, 2H), 7.80 (s, 1H); 13C NMR ( 75 MHz, CDCl3): δ 55.4, 114.4, 116.4, 118.4, 118.5, 121.9, 126.4, 127.2, 127.8, 134.4, 135.7, 135.8, 136.7, 144.7, 159.8; HRMS (EI): m/z [M]+ Calcd for C17H12OS2, 296.0330; Found, 296.0338.

19 2-(α-Naphthyl)benzo[1,2-b:4,3-b’]dithiophene (2). Elution with hexane/DCM (10:1) gave a yellow solid 89%. 2-(4-Cyanophenyl)benzo[1,2-b:4,3-b’]dithiophene (3). Elution with hexane/DCM (1:1) gave yellow solid, yield: 83%, mp 181-183 °C, 1H NMR (300 MHz, CDCl3, 25°C): δ 7.62 (d, 1H, J = 5 Hz), 7.71-7.75 (m, 3H), 7.78 (d, 1H, J = 8 Hz), 7.85 (m, 2H), 7.88 (m, 1H), 8.03 (s, 1H); 13

C NMR (75 MHz, CDCl3): δ 29.7, 111.4, 118.4, 119.8, 120.0, 121.7, 126.7, 127.2, 132.8,

134.8, 136.9, 138.7, 141.9; HRMS (EI): m/z [M]+ Calcd for C17H9NS2, 291.0176; Found, 291.0175. 2-(4-Acetylphenyl)benzo[1,2-b:4,3-b’]dithiophene (5). Elution with hexane/DCM (1:1) gave yellow solid, yield: 78%, mp 175-176 °C, 1H NMR (300 MHz, CDCl3): δ 2.65 (s, 3H), 7.61 (d, 1H, J = 5 Hz), 7.75 (d, 1H, J = 5 Hz), 7.80 (s, 1H), 7.83 (s, 1H), 7.85 (m, 1H), 7.88 (m, 1H), 8.02 (m, 1H), 8.04-8.05 (m, 3H); 13C NMR (75 MHz, CDCl3): δ 197.3, 143.0, 138.8, 137.0, 136.8, 136.4, 135.5, 134.8, 129.2, 127.0, 126.3, 121.8, 119.6, 119.2, 118.5, 26.6; HRMS (EI): m/z [M]+ Calcd for C18H12OS2, 308.0330; Found, 308.0332. 2-(4-Cyanophenyl)-11-(4-methoxyphenyl)trithia[5]helicene (8). Bright yellow solid was precipitated from DCM. The yield starting from 7 was 73%; mp 295-296 °C, 1H NMR (300 MHz, CDCl3): δ 3.90 (s, 3H), 6.98 (d, 2H, J = 9 Hz), 7.70-7.75 (m, 4H), 7.83 (d, 1H, J = 8 Hz), 7.88-7.97 (m, 5H), 8.46 (s, 1H), 8.79 (s, 1H); HRMS (EI): m/z [M]+ Calcd for C30H17ONS3, 503.0472; Found, 503.0467. 2-(4-Acetylphenyl)-11-(4-methoxyphenyl)trithia[5]helicene (9). Bright yellow solid was precipitated from DCM. The yield starting from 7 was 69%; mp 289-291 °C, 1H NMR (300 MHz, CDCl3): δ 2.66 (s, 3H), 3.89 (s, 3H), 6.98 (d, 2H, J = 9 Hz), 7.76 (d, 2H, J = 8 Hz), 7.83 (d,

20 1H, J = 9 Hz), 7.88 (d, 1H, J = 8 Hz), 7.93-7.98 (m, 4H), 8.02-8.05 (d, 2H, J = 7 Hz), 8.51 (s, 1H), 8.80 (s, 1H); HRMS (EI): m/z [M]+ Calcd for C31H20O2S3, 520.0625; Found, 520.0629. 5-(4-Methoxyphenyl)benzo[1,2-b:4,3-b’]dithiophene-2-carbaldehyde (11). A n-BuLi 2.5 M hexane solution (0.44 mL, 1.1 mmol, 1.1 equiv) was added dropwise under stirring to a solution of 2-(4-methoxyphenyl)benzo[1,2-b:4,3-b’]dithiophene 1 (296 mg, 1 mmol), in dry THF (25 mL) at –78°C. The solution was stirred for 5 min at –78°C and 15 min at room temperature. The resulting blue solution was cooled at –78°C, treated with dry DMF (0.1 mL, 1.3 mmol). After 2 h at –78°C, the solution was warmed to room temperature, and quenched with a saturated aqueous solution of NH4Cl (5 mL). The THF was removed under reduced pressure, the crude material was taken up with DCM (40 mL), and washed with a saturated aqueous solution of NH4Cl (2 x 10 mL). The organic phases were dried over anhydrous Na2SO4, the solvent was removed under reduced pressure, and the crude material was purified by flash column chromatography on silica gel with hexane/DCM (1:1) as eluent to provide yellow solid, 96% yield, mp 198-200°C; 1H NMR (300 MHz, CDCl3): δ 3.88 (s, 3H), 6.99 (m, 1H), 7.01 (m, 1H), 7.70 (m, 1H), 7.73 (m, 1H), 7.77 (d, 1H, J = 8 Hz), 7.85 (s, 1H), 7.91 (d, 1H, J = 8 Hz), 8.39 (s, 1H), 10.17 (s, 1H); 13C NMR (300 MHz, CDCl3): δ 55.5, 114.6, 116.1, 118.6, 122.7, 126.6, 127.9, 131.8, 133.5, 136.3, 137.3, 140.8, 143.0, 146.8, 184.2; HRMS (EI): m/z [M]+ Calcd for C18H12O2S2, 324.0279; Found, 324.0282. 2.4. Results and Discussion 2.4.1. Photocyclization. Photocyclization of 10a, the precursor of 2, was carried out in C6D6 in an NMR tube. The solution was irradiated under 350 nm light and the proton NMR spectra recorded every 10 min. Selected spectra are shown in Figure 2.2. Precursor 10a may occur as cis and trans isomers. Trans 10a was the major product in step c Scheme 2.1. Thus trans 10a was

21 purified and chosen as the starting material. During irradiation trans 10a isomerized to cis 10a and then photocyclized to form the desired product 2. At t = 0 min, only the starting trans 10a was observed, and the vinylic protons appeared around 7.11 ppm as a singlet. The formation and photocylization of cis 10a was confirmed by appearance and disappearance of peaks in the 6.35-6.45 ppm region. At t = 570 min, the signal of the proton at position 2 of thiophene changed into a doublet shown at 7.08 ppm. Because the double bond was easily oxidized, peaks above 9.50 ppm were assigned to aldehydic by-products formed by oxidation. Figure 2.3 shows a plot of the change in percentage of starting material, intermediates, product and by-products. At the beginning of the reaction trans 10a photoisomerizes to cis 10a, and immediately forms a 1:1 photostationery state of both isomers. Both isomers remain in 1:1 ratio until the end of the photocyclization.

Figure 2.2. 1H NMR profile of photocyclization of 10a. In order to avoid solubility problems, 7-9 were synthesized by a combination of both approaches (Scheme 2.2). Attempts to purify 12 have continually failed due to the similar

22 polarities of 7 and 12. The Suzuki cross coupling of crude 12 with the corresponding boronic acids afforded 8 and 9, which were recrystallized from DCM as bright yellow solids. The yields over these 2 steps were 69-73%. Noteworthy, the synthesis of 8, starting from 3, did not provide the desired product. The synthesis was attempted following the same scheme as the synthesis of 1; however, and the synthesis of corresponding aldehyde failed. This observation is attributed to the deactivation of the 2’ position of BDT by the terminal electron-deficient cyanophenyl group. As anticipated, the electron donating nature of the methoxyphenyl group stabilized the reaction intermediates and afforded 8 in satisfactory yield.

trans 10a cis 10a product 2 by-product

100

Percentage (%)

80 60 40 20 0 0

100

200

300

400

500

600

700

Time (min)

Figure 2.3. Photocyclization data profile of 10a. 2.4.2. Crystal Structure. Slow recrystallization of 4 from benzene resulted in wedge-like crystals. Figure 2.4 and Figure 2.5 show the molecular structure and the packing obtained by single-crystal X-ray crystallography. Data for single-crystal X-ray crystallography is included in Appendix B. Although both naphthalene and BDT are planar, the steric bulk of the naphthyl arms cause a twisting in 4. It has a C2 molecular symmetry with the 2-fold axis bisecting the central ring of the molecules.

23

Figure 2.4. X-ray crystal structure of 4.

Figure 2.5. Packing pattern of 4. The angle between the two naphthalene rings is 114°. Molecules in the crystal are welloriented and exist as a racemic mixture. Pi-stacking is observed between neighboring

24 naphthalene rings and is also observed between neighboring BDT rings. The 57° angle may be the reason for the absence of interaction between naphthalene and BDT. Figure 2.6 depicts crystals of 4 under UV light.

Figure 2.6. Crystals of 4 under UV irradiation. 2.4.3. Absorption and emission spectra. Table 2.1 summarizes the results of the absorption and emission spectra of compounds 1-10b. Generally, π-conjugated systems exhibit strong π-π* electronic absorption in the UV-vis region. Compounds 1-10b fluoresce under UV light, and most are blue emitters. The absorption and emission spectra of parent BDT and T5H are shown in Figure 2.7. Similar absorption spectra due to the same heterocyclic backbone were observed for 1-6. 79 also showed similar absorption spectra. Compounds 2 and 4 with one or two naphthyl substituents exhibited absorption maxima at 307 nm and 333 nm, respectively. While 1, 3 and 5 with phenyl moieties absorb at 333 nm, 346 nm and 350 nm suggesting that the chain extension at the 2- and 7-position of BDT does not improve the π-orbital overlap further in the ground state as their structures are non planar. The electron rich methoxyphenyl group helps lower the ground state energy slightly more than the electron-poor cyano- and acetylphenyl groups do.

25 Table 2.1. Photophysical Properties of 1-10b in DCM

a

λmax a (nm)

εb (x104 M-1cm-1)

λF,maxc (nm)

Φ Fd

1

333

3.0

379

0.55

2

307

2.4

405

0.17

3

346

3.2

413

0.22

4

333

2.7

423

0.25

5

350

3.2

430

0.12

6

376

4.8

445

0.28

7

295,357

3.5, 4.1

409

0.12

8

323,383

4.9, 4.3

469

0.09

9

324,380

5.4, 4.9

477

0.07

10a

370

3.8

453

0.25

10b

388

4.2

475

0.19

λmax = wavelength of absorption maximum; b ε = molar decadic absorption coefficient;

λF,max = wavelength of fluorescence maximum; d ΦF = fluorescence quantum yield.

Absorption of BDT Emission of BDT Absorption of T5H Emission of T5H

1.0 0.8 Intensity

c

Compound

0.6 0.4 0.2 0.0 250

300

350

400

450

500

550

Wavelength (nm)

Figure 2.7. Normalized absorption and emission spectra of BDT and T5H in DCM.

26 A fine emission-color tuning of 1-10b in the region of 380 nm-480 nm is observed in the fluorescence spectra as shown in Figure 2.8. Comparing the emission maxima of 1 (379 nm), 3 (413 nm), and 5 (430 nm) with those of parent BDT (333 nm) shows that aryl substituents introduce an overall red-shift of emission. The electron-rich aryl group, methoxyphenyl, introduces a red-shift of 46 nm, while the electron-deficient aryl groups, cyanophenyl and acetylphenyl, introduce red-shifts of more than 80 nm in DCM. Similarly, the emission maxima of 7 (409 nm), 8 (469 nm), and 9 (477 nm) show the same trend when compared with T5H (363 nm). The fluorescence quantum yields of 1 and 7 are 0.55 and 0.12, respectively, compared to that of BDT (0.08) and T5H (0.02).2 This implies that the electron-rich aryl moiety affords a smaller red-shift in fluorescence, while quantum yields of fluorescence increase dramatically. Fluorescence quantum yields of compounds 1 (0.64), 3 (0.56), and 7 (0.07) were also measured in the solid state. The solid-state fluorescence spectra are provided in the Appendix B.

1 2 3 4 5 6 7 8 9 10a 10b

1.0

Intensity

0.8 0.6 0.4 0.2 0.0 350

400

450

500

550

600

650

700

Wavelength (nm)

Figure 2.8. Normalized fluorescence spectra of compounds 1-10b in DCM. 2.5. Conclusions. Two series of blue-emitting aryl substituted helical compounds were successfully synthesized. Compound 2 was prepared by two approaches. The photocyclization of

27 10a was carried out in an NMR tube and followed by 1H NMR spectroscopy. Trans and cis 10a immediately formed an equilibrium 1:1 photostationery state. Single crystal X-ray crystallographic analysis carried out on 4 shows the molecules exist in the crystal as a racemic mixture. From the packing pattern, interaction between aryl substituents was observed and their contribution to the self-assembly confirmed. The photophysical properties of all the new compounds were studied. By carefully choosing end-capped moieties, emission can be well tuned in the blue region of about 400-480 nm.

2.6. References. ______________________________ (1)

Caronna, T.; Sinisi, R.; Catellani, M.; Malpezzi, L.; Meille, S. V.; Mele, A. Chem.

Commun. 2000, 1139. (2)

Caronna, T.; Catellani, M.; Luzzati, S.; Malpezzi, L.; Meille, S. V.; Mele, A.;

Richter, C.; Sinisi, R. Chem. Mater. 2001, 13, 3906. (3)

Larsen, J.; Bechgaard, K. J. Org. Chem. 1996, 61, 1151.

(4)

Turro, N. J. Modern Molecular Photochemistry; The Benjamin/Cummings

Publishing Company, Inc.: Menlo Park, CA, 1978. (5)

Wynberg, H. Acc. Chem. Res. 1971, 4, 65.

(6)

Wynberg, H.; Kellogg, R. M.; Van Driel, H.; Beekhuis, G. E. J. Am. Chem. Soc.

1966, 88, 5047. (7) 3487.

Wynberg, H.; Van Driel, H.; Kellogg, R. M.; Buter, J. J. Am. Chem. Soc. 1967, 89,

28 (8)

Kitamura, T.; Morizane, K.; Taniguchi, H.; Fujiwara, Y. Tetrahedron Lett. 1997,

38, 5157. (9)

Kitamura, T.; Zhang, B.; Fujiwara, Y. Tetrahedron Lett. 2002, 43, 2239.

(10)

Riddick, J. A.; Bunger, W. B. Organic Solvents:Physical Properties and

Methods of Purification, 3rd, Wiley-Interscience: New York, 1970. (11)

IJpeij, E. G.; Beijer, F. H.; Arts, H. J.; Newton, C.; de Vries, J. G.; Gruter, G.-J. M.

J. Org. Chem. 2002, 67, 169. (12)

Maiorana, S.; Papagni, A.; Licandro, E.; Annunziata, R.; Paravidino, P.;

Perdicchia, D.; Giannini, C.; Bencini, M.; Clays, K.; Persoons, A. Tetrahedron 2003, 59, 6481. (13)

Nakayama, J.; Fujimori, T. Heterocycles 1991, 32, 991.

(14)

Baldoli, C.; Bossi, A.; Giannini, C.; Licandro, E.; Maiorana, S.; Perdicchia, D.;

Schiavo, M. Synlett 2005, 7, 1137. (15)

Caronna, T.; Catellani, M.; Luzzati, S.; Malpezzi, L.; Meille, S. V.; Mele, A.;

Richter, C.; Sinisi, R. Chem. Mater. 2001, 13, 3906.

29 CHAPTER 3. TRANSIENT SPECTROSCOPY AND GEOMETRY OPTIMIZATION 3.1. Introduction. Investigation of photoinduced energy or electron transfer in a designed chromophore is important work in photochemistry. The objective is to mimic natural photoprocesses as well as to further the understanding of processes in opto-electronic devices. Electron transfer occurs through bond or through space and depends on the character of the linker molecule connecting the donor and the acceptor.1-3 Rigid, flexible, conductive, and nonconductive variations of linkers have been widely investigated for different purposes, such as for in light-harvesting chromophores, photovoltaic materials and solar cells .4-9 In the 1970s, new generations of laser flash spectroscopic, conductometric, and magnetic techniques were employed to identify and study key intermediates in electron transfer.10 The direct observation of radical ion intermediates in reactions was achieved by the application of time-resolved laser flash photolysis.11 Due to its helical structure, trithia[5]helicene (T5H) can serve as a rigid bridge between electron-donating and electron-accepting groups. In this work, the spectral pattern, lifetime and quantum yield of fluorescence of 2-(4-methoxyphenyl)benzo[1,2-b:4,3-b’]dithiophene (1) was studied. The electron-rich methoxyphenyl group doesn’t affect any significant solvatochromic properties. However, compounds with electron-poor aryl groups, 2-(4-cyanophenyl)benzo[1,2b:4,3-b’]dithiophene (3) and 2-(4-cyanophenyl)-11-(4- methoxyphenyl)trithia[5]helicene (8), exhibit a large solvatochromic effect. This suggests that polar solvents stabilize the excited state causing the bathochromic shift. Therefore, the excited state of compounds with electron-poor aryl moieties might be more polar ─ or even charge-separated ─ compared to compounds with an electron-rich aryl moiety. Photophysical properties of T5H, 1, 3 and 8 have been investigated using steady state and kinetic state measurements. The lifetimes (τ) and kinetics of decay

30 processes of radicals are presented. Detailed information about the radical species is obtained using ultrafast transient spectroscopy. DFT calculations at the B3LYP/ 6-31G* level were performed on these compounds to optimize their structures. The structure-property relationships are discussed. 3.2. Experimental. 3.2.1. Materials. Compounds T5H, 1, 3 and 8 were synthesized according to Scheme 2.1 and 2.2. Solvents (e.g. hexane, dichloromethane, and acetonitrile) purchased from Aldrich were either spectrophotometric or HPLC grade, or purified by distillation. 3.2.2. Ultrafast Spectrometry. The femtosecond time-resolved apparatus at the Ohio Laboratory for Kinetic Spectrometry at Bowling Green State University, USA, has been described elsewhere.12,13 A Spectra-Physics Hurricane system was used as the laser source. The output of the system consisted of pulses at 800 nm, 1 mJ, ca. 100 fs (FWHM) at a repetition rate of 1 kHz. The output from the Hurricane was split (85 and 15%) into two beams. The pump beam passed through an optical chopper (DigiRad C-980) rotating at a frequency of 100 Hz, and was focused, with the spot size of about 2 mm, into the sample cell where it was overlapped with the probe beam at an angle of ca. 5°; the relative polarizations of the pump and probe beams were set at the magic angle. The pump power was ~0.3 mW at 400 nm. The other (probe) beam passed through a computer-controlled delay line (Newport Corp. MTL 250 PP 1 250 mm linear positioning stage) that provided an experimental time window of about 1.6 ns with a step resolution of 6.6 fs, then was focused into a 3 mm thick sapphire plate (Crystal Systems, Inc., HEML UX grade) to generate a white light continuum (effective useful range, 460–800 nm), and then was focused into the sample. The energy of the probe pulse was