Synthesis of Conjugated Polymers

SYNTHESIS OF CONJUGATED POLYMERS

A Dissertation

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy

Chao Wang

May, 2013 SYNTHESIS OF CONJUGATED POLYMERS

Chao Wang

Dissertation

Approved: Accepted:

Advisor Department Chair Dr. Li Jia Dr. Coleen Pugh

Committee Member Dean of the College Dr. Matthew L. Becker Dr. Stephen Z.D. Cheng

Committee Member Dean of the Graduate School Dr. Chrys Wesdemiotis Dr. George R. Newkome

Committee Member Date Dr. Abraham Joy

Committee Member Dr. Xiong Gong

ABSTRACT

Conjugated polymers (CPs) are a unique class of materials because they possess the electronic properties of semi-conductors and processability of polymers. Their electronic and optical properties can be controlled by their chemical structures and conjugation length.

To enhance the electron affinity of CPs, a series of boron-containing CPs were synthesized with arylene-borylene units. One of the boron-containing polymer had indeed shown the ability of quenching the photoluminescence of poly(3-hexylthiophene) (P3HT), indicating possible exciton separation and charge transfer.

Next, a ketone group was introduced to polythiophene to lower its LUMO level and increase the electron affinity. Regioregular poly(3-heptanoylthiophene) (PHOT) was synthesized. The polymerization was realized by Ni-catalyzed Kumada coupling with a chain-growth mechanism. Schlenk equilibrium played a key role in the generation of the monomer for the polymerization. PHOT was fully characterized by various techniques.

However, only p-channel activity was observed in field-effect transistors (FETs) for

PHOT. The hole mobility was one order lower than that of P3HT. Photovoltaic devices with an active layer of 1:1 blend of PHOT and PC71BM had a low power conversion efficiency of ~0.5%.

iii Finally, the synthesis of graphene nanoribbons (GNRs) was attempted which could be potentially used as a good charge carrier. To prevent intramolecular and intermolecular side reactions during Scholl reaction, the precursors, oligo-naphthalenes with alkoxy groups capping the 2, 3, 6, and 7-positions had been synthesized. Intramolecular ring fusion via the Scholl reations was tested, but no expected GNRs were formed.

iv ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my advisor, Professor Li Jia, for his guiadance and encouragement throughout the course of my doctoral research. I am deeply moved by his passion for research and his knowledge of science. It is extremely lucky for me to be able to work with him so closely that I have leaned about various aspects of organic synthesis and polymer chemistry. I would always remember his teachings about critical thinking.

I would like to thank Professor Matthew L. Becker, Professor Chrys Wesdemiotis,

Professor Abraham Joy and Professor Xiong Gong for serving on my doctoral committee and for their valuable suggestions.

I am indebted to all the members of Jia’s group. In particular, I would like to thank

Dr. Shaohui Lin and Dr. Jianfang Chai for their kind help during my graduate studies. I have benefited a lot from their experience and technique of organic synthesis, which have greatly stimulated my synthetic skill.

I dedicate this dissertation to my parents for their endless love, support and encouragement.

TABLE OF CONTENTS

Page

LIST OF TABLES ix

LIST OF FIGURES x

LIST OF SCHEMES xiii

CHAPTER

I. CONJUGATED POLYMERS AND THEIR APPILICATIONS 1

1.1 History of Conjugated Polymers 1

1.2 Synthesis of Conjugated Polymers 3

1.2.1 Conjugated Polymers via Electropolymerization 3

1.2.2 Boron Containing Conjugated Polymers from Non-Catalyzed Step-Growth Polymerization Synthesis 6

1.2.3 Transition Metal-Catalyzed Coupling Reactions 11

1.2.4 Application of Transition Metal-Catalyzed Coupling Reactions in Step-Growth Polymerization 13

1.2.5 Chain-Growth and Livingness of Polycondensation of Aryls 15

1.2.6 Head-to-Tail Regioregular Poly(3-Alkylthiophene) (rr-P3ATs) via Transition Metal-Catalyzed Chain-Growth Polymerization 16

1.2.7 Functionalization of Regioregular Polythiophenes 25

1.2.8 Other types of Conjugated Polymers via Transition Metal- Catalyzed Chain-Growth Polymerization 26

vi 1.2.9 Attempted Transition Metal-Catalyzed Polymerization of Electron-Acceptor Materials 33

1.3 Application of Conjugated Polymers in Organic Solar Cells 35

1.3.1 Working Principles of Organic Solar Cells 35

1.3.2 Device Structure Improvement of Organic Solar Cells 37

1.3.3 Materials Challenges for OPV 40

1.4 Summary and Outlook 42

II. SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF BORON- CONTAINING CONJUGATED POLYMERS 43

2.1 Introduction 43

2.2.1 Synthesis 46

2.2.2 Solution and Solid State Optical Properties 51

2.2.3 Cyclic Voltammetric Study 54

2.2.4 Detection of Charge Transfer by Photoluminescence Quenching 55

2.3 Conclusion 58

2.4.1 Materials 59

2.4.2 Measurements 60

2.4.3 Synthesis 61

III. SYNTHESIS AND CHARACTERIZATION OF REGIOREGULAR POLYTHIOPHENE WITH ELECTRON-WITHDRAWING GROUPS 65

3.1 Introduction 65

3.2.1 Synthesis of Monomer Precursors 67

3.2.2 Generation of Monomers 68

3.2.3 Polymer Synthesis 69

3.2.4 Cyclic Voltammetry 74 vii 3.2.5 Optical Absorption and Photoluminescence Spectra 75

3.2.6 Thermal and Morphological Properties 77

3.2.7 Charge Transport and Photovoltaic Properties 79

3.2.8 Attempted Synthesis of rr-Poly(3-(1,1-difluoroalkyl)thiophene) 84

3.3 Conclusions 85

3.4.1 Materials 86

3.4.2 Measurements 87

3.4.3 Synthesis 91

IV. ATTEMPT OF SYNTHESIS OF GRAPHENE NANORIBBONS 99

4.1.1 Physical Properties of Graphene and Graphene Nanoribbons 99

4.1.2 Successes in Fabrication and Synthesis of GNRs 101

4.1.3 Development of Solution Synthesis of PAHs and Attempted Synthesis of GNRs 104

4.2.1 Attempted Synthesis of PPN via Intermolecular Scholl Reaction 109

4.2.2 Attempted Synthesis of PPN via Intramolecular Scholl Reaction 110

4.2.3 Attempted Synthesis of PPA Model Compounds 115

4.3 Conclusions 118

4.4.1 Materials 119

4.4.2 Measurements 120

4.4.3 Synthesis 121

V. SUMMARY 136

REFERENCES 138

APPENDIX 159 viii LIST OF TABLES

Table Page

1.1 Summary of side chains and terminal groups on rr-P3ATs 26

2.1 Comparison of reduction potentials of boranes 45

2.2 UV-Visible absorption data 53

3.1 The solubility of PHOT in different solvents 73

3.2 Electrical parameters of the thin film transistors of PHOT 81

3.3 J-V characteristics and PCE of BHJ cells under different conditions 83

4.1 Failed polymerization. (Isomers of monomers are omitted for clarity.) 112

LIST OF FIGURES

Figure Page

1.1 Coupling region-isomers of 3-alkylthiophene dimers 5

1.2 Regioirregular P3AT in non-planar and rr-P3AT in planar structure 6

1.3 Orbital overlapping in vinylboranes 7

1.4 1H NMR spectra of (a) regiorandom (1:1:1:1, HT-HT:HT-HH:TT-HT:TT-HH) P3HT and (b) regioregular (98.5%) P3HT 19

1.5 13C NMR spectra of (a) regiorandom (1:1:1:1, HT-HT:HT-HH:TT-HT:TT-HH) P3HT and (b) regioregular (98.5%) P3HT 19

1.6 MALDI-TOF mass spectrum of rr-P3HT 22

1.7 Mn and Mw/Mn values of rr-P3HT as a function of the molar ratio of converted monomer to catalyst 23

1.8 Bilayer OPV cell 35

1.9 BHJ OPV cell 38

1.10 Ideal structure of a BHJ solar cell 39

1.11 Self-assembly of block polymers into the ideal structure of a BHJ solar cell 41

2.1 Aryborylene units structure 44

1 1 2.2 A H NMR of polymer 2 in CDCl3. B H NMR of compound 2-M in CDCl3 47

1 2.3 H NMR spectrum of polymer 3 in DMF-d7. Peaks with star are due to deuterated solvents and end groups. B 1H NMR spectrum of compound 3-M in CDCl3. 48

1 1 2.4 A H NMR spectrum of 4 in 1:2 CD3CN:d-THF. B H NMR spectrum of compound 4-M in CDCl3. 50

x 2.5 GPC trace of polymer 4 51

2.6 UV-Vis spectra of model compounds and polymers in the presence of a coordinated Lewis base and in the absence of a coordinated Lewis base 52

2.7 Cyclic voltammograms of model compounds and polymers 54

2.8 Comparison of LUMO energies 55

2.9 Photoluminescence spectra of pure rr-P3HT, a blend of 3 and rr-P3HT, and a blend of 4 and rr-P3HT. These samples were excited at 530 nm 58

3.1 Isomers after quenching 69

3.2 1H NMR of PHOT with assignments. The HT/HH ratio was estimated to be ~96:4 using the α-methylene protons at shown in the inset to the right 71

3.3 A. Total conversion of monomers as a function of time and temperature. B. Plot of Mn vs total conversion of 9a and 9b. The number over each point was the PDI of the particular sample 72

3.4 A. Cyclic voltammograms of PHOT (solid) and rr-P3HT. B. Comparison of HOMO and LUMO levels of PHOT and rr-P3HT 74

3.5 Chemical structure of Polymer A 75

3.6 A. UV-Vis absorption spectra of PHOT (solid) and rr-P3HT in chloroform. B. UV-Vis absorption spectra of PHOT (solid) and rr-P3HT as spin-cast film 75

3.7 A. Solution photoluminescence spectra of PHOT (solid) and rr-P3HT in chloroform solution. The spectra are normalized. B. Thin film photoluminescence spectra of PHOT (solid), rr-P3HT, and their 1:1 blend (dot). The spectra were not normalized for comparison of the emission intensity 76

3.8 A. TGA thermogram of PHOT. B. DSC thermogram of PHOT 77

3.9 A. WAXD powder pattern of PHOT annealed at various temperatures. B. Change of d-spacing after annealing 78

3.10 Inter-chain distance and possible π-stacking of PHOT 79

3.11 The device structure of field effect transistor 80

3.12 Output and transfer characteristics of the thin film transistor of PHOT after thermal annealing at 150 ºC 80

xi 3.13 UV-Vis absorption spectrum and TEM image of a BHJ cell with an active layer of PHOT:PC71BM blend in 1:1 weight ratio 82

3.14 Current density-voltage curves of a BHJ cell with an active layer of PHOT:PC71BM blend in 1:1 weight ratio 83

3.15 Current density-voltage curves of a BHJ cell with an active layer of PHOT:P3HT blend in 1:1 weight ratio 84

4.1 Charge carrier mobility of different materials 101

4.2 A. Overview STM image after cyclodehydrogenation, showing straight N=7 GNRs. B. Raman spectrum (532 nm) of straight N=7 ribbons 103

4.3 b) HRTEM of GNRs. c) STM image of GNRs 106

4.4 1H NMR of 18 114

4.5 MALDI-TOF spectrum and GPC trace of 18 114

4.6 1H NMR of boron-doped nanographene 118

xii

LIST OF SCHEMES

Scheme Page

1.1 Mechanism of electropolymerization of heterocycles 3

1.2 Hydroboration polymerization of ArBH2 7

1.3 Haloboration polymerization of diyne monomers 8

1.4 Polycondesation of aryldimethoxyborane using Grignard and organolithium reagents 8

1.5 Tin-boron exchange polymerization of bis(trimethylstannyl) monomers 9

1.6 Synthesis of conjugated organoborane oligomers 10

1.7 Hydroboration polymerization of 5,10-dihydroboranthrene 10

1.8 Eqaution of coupling reaction 12

1.9 General mechanism of coupling reactions 12

1.10 Preparing polyacetylenes from prepolymer 13

1.11 Soluble polyacetylene from monosubstituted cyclooctatetraene 14

1.12 Polythiophene from Stille coupling reaction 15

1.13 Regio-irregular Poly(thienylenevinylene)s via Stille coupling reaction 15

1.14 Synthetic routes of rr-P3ATs 17

1.15 Mechanism of chain-growth condensation polymerization of rr-P3ATs 21

1.16 Synthesis of 100% HT regioregular P3HT 23

1.17 Synthesis of monodisperse HT oligothiophene 25

1.18 Synthesis of poly(p-phenylene) via chain-growth polycondensation 27 xiii 1.19 Strategies of synthesis of poly(p-phenylene)-b-poly(3-alkylthiophene) 28

1.20 Synthesis of poly(p-phenylene) via Suzuki polycondensation 28

1.21 Synthesis of poly(N-alkylpyrrole) via chain-growth polycondensation 29

1.22 Synthesis of poly(p-phenylene)-b-poly(N-hexylpyrrole) 29

1.23 Synthesis of polyfluorenes via Suzuki polycondensation 30

1.24 Synthesis of polyfluorenes via lithiated fluorene monomer 30

1.25 Synthesis of polyfluorene via GRIM 31

1.26 Synthesis of polycarbazole via GRIM 31

1.27 Synthesis of poly(meta-pyridine) via GRIM 32

1.28 Synthesis of poly(para-pyridine) via GRIM 33

1.29 Synthesis of n-type conjugated polymer from acceptor monomers 34

1.30 Working principles of OPV cells. A. Light absorption and excitons generation. B. Excitons diffusion. C. Charge separation. D. Charge transportation 36

2.1 Synthesis of compound 1, 2-M and polymer 2 46

2.2 Synthesis of compound 3-M and polymer 3 47

2.3 Synthesis of compound 4-M and polymer 4 49

2.4 Illustration of photoluminescence quenching of excited states 56

3.1 Synthesis of monomer precursor 67

3.2 Schlenk equilibrium of monomer generation 68

3.3 Polymer synthesis 70

3.4 Polymerization mechanism 73

3.5 Synthetic route of rr-poly(3-(1,1-difluoroalkyl)thiophene) 85

4.1 Arm-chair GNRs (PPN and PPA) and zig-zag GNR 100

4.2 Bottom-up synthesis of GNRs 102 xiv 4.3 Proposed mechanism of surface-assisted GNR synthesis 103

4.4 Two synthetic routes toward the HBC core molecule. (Functional groups around the HBC core are omitted for clarity.) 104

4.5 GNR from polyaromatic precursor 105

4.6 Synthesis of linear GNRs from hexaphenylbenzene type prepolymer 106

4.7 Intermolecular side reactions of Scholl reaction of o-terphenyl 107

4.8 100% conversion via Scholl reaction of substituted o-terphenyl 107

4.9 Attempted synthesis of PPN derivative 108

4.10 Intramolecular side reaction under Scholl reaction and the formation of a five-member ring 108

4.11 Synthesis of monomer 14 119

4.12 Unfavorable intermolecular reaction 110

4.13 Synthesis of monomer precursors 15 110

4.14 Synthesis of model compounds 16 and 17 111

4.15 Synthesis of polymer precursor 18. X and Y are functional groups 112

4.16 Synthesis of polymer precursor using PhPdP(t-Bu)3Br as initiator 113

4.17 Synthesis of different model precursors 116

4.18 Carbocation rearrangement of anthracenes 117

4.19 Synthesis of boron-doped nanographene 118

CHAPTER I

CONJUGATED POLYMERS AND THEIR APPLICATIONS

1.1 History of Conjugated Polymers

The first reported conjugated polymer can be traced back to the mid-19th century.

Letheby reported the formation of polyaniline as an electrochemical oxidation product of aniline in acidic media.1 In 1965, Australian scientists Weiss, Willis and co-workers

reported iodine-doped oxidized polypyrrole blacks with resistivity as low as ~ 1 Ω/cm.2,3

These findings invalidate the previous concept that organic molecules are insulators or at most weakly conducting materials and opened the door of conductive polymer research.

The discovery of well-defined polyacetylene in the 1974 by Shirakawa and Ikeda accelerated interest of conjugated polymers in the fields of microelectronics.4 Although

the first polymerization of acetylene was reported by Natta and co-workers in 1958,5 the work did not attract much attention. In addition to the many side reactions such as the formation of benzene — a cyclic trimer of acetylene, the heterogeneous Ziegler-Natta catalysts could only form an intractable solid. With the discovery of a new catalyst system, Ti(OC4H9)4-Al(C2H5)3 (Al/Ti=3-4), the homogeneous polymerization of

acetylene became feasible. According to Shirakawa, a polyacetylene thin film was

initially obtained by a fortuitous error.6 One student made a wrong calculation and used a

concentration of the Ziegler-Natta catalyst nearly 1000 times greater than that usually

used. As a result, a silvery polyacetylene film was formed on the surface of the reaction

solution. After thermal annealing at various temperatures, the cis- and trans- contents could be changed. Shortly after this discovery, Shirakawa, Heeger and Macdiarmid published two seminal papers in 1977 and 1978 on the oxidation of polyacetylene with halogens, resulting in a 108 fold increase in conductivity.7,8 The conductivity of this doped material could approach the conductivity of silver. Based on these discoveries, the three are awarded the Nobel Prize in Chemistry in 2000.

In general, conjugated polymers are defined as polymer chains with carbon or other

2 main group atoms in the sp pz hybridized configuration. They are semiconductors that can be doped to become conducting or even metallic materials.9 After several decades of

study since their discovery, conjugated polymers have been demonstrated to combine the

electronic properties of metals and semiconductors with the ease of processing and

mechanical advantages of polymers.

A wide range of electrical and optical devices have been demonstrated using

conjugated polymers. Light-emitting devices have been fabricated which are as bright as

fluorescent lamps at very low applied voltages.10-12 Organic photovoltaic cells using

conjugated polymers have shown high solar energy conversion and are on the verge of

industrial production.13-15 The advances in ink-jet printing technology will change the

way in which electronic devices are manufactured.16-18 The development of Polymer

organic field effect transistors (OFET) has surged in the last two decades to produce

flexible displays with low-cost.10,19 In addition, conjugated polymers can be used in

applications which require special mechanical properties such as flexibility and

stability.20-22 The field of conjugated polymers has become one of the most interesting

areas of interdisciplinary science and technology.

1.2 Synthesis of Conjugated Polymers

Conjugated polymers can be synthesized from electropolymerization, non-catalyzed

polymerization, transition metal-catalyzed coupling reactions and etc.

1.2.1 Conjugated Polymers via Electropolymerization

Until 1980s, although chemical syntheses are the adequate methods of preparation of

oligomers with conjugated structures, most conjugated polymers have been prepared by

electrochemical polymerization.23,24 For example, poly(p-phenylene)s, polypyrroles and

polythiophenes have been prepared by cathodic route involving electro-reduction25,26 and oxidative anodic electropolymerization24,27,28 of the corresponding monomers. Compared

to other chemical syntheses of conjugated polymers, electropolymerization of the

monomer presents several distinct advantages such as the absence of catalysts, control of

film thickness, direct grafting onto electrode surface etc.23,24 Scheme 1.1 represents the

mechanism of electropolymerization of five-membered heterocycles.29

Scheme 1.1 Mechanism of electropolymerization of heterocycles.

Monomer is oxidized into its radical cation form at the beginning. Because the

electron-transfer reaction is faster than the diffusion of the monomer, more cation

radicals are continuously formed near the electrode surface. After coupling of two radicals to produce a dihydro dimerized dication, a dimer is formed after further loss of two protons and rearomatization. Due to the applied potential, the dimer is more easily

oxidized than the monomer into its radical cation form and further couples with another

monomeric radical. Electropolymerization proceeds in such a fashion until the polymers

become insoluble in the electrolytic medium and precipitate onto the electrode surface.

Since the first synthesis of poly(3-alkylthiophene) in 1986, this method has attracted extensive interest owing to the improvement in solubility compared to the unsubstituted polythiophenes.30-33 However, contradictory results have been reported concerning the

effects of the alkyl chain length on the solubility34,35 which might be attributed to the regioregularity and cross-linking of the polymers.36,37 The properties of polymers depend

on the electro-synthesis conditions36-39 and the regio-selectivity is missing.23

Due to the asymmetric structure of the 3-substituted thiophene molecule, three

relative orientations are available when two of these thiophene rings are coupled between

2- and 5- positions (Figure 1.1). The first orientation is 2,2’ or head-to-head (HH)

coupling, the second is 2,5’ or head-to-tail (HT) coupling and the last is 5,5’ or tail-to-tail

(TT) coupling. And the orientation becomes much more complicated for substituted

oligo-thiophenes and polythiophenes.

Figure 1.1 Coupling region-isomers of 3-alkylthiophene dimers.

The contamination of the HH couplings will not only cause the loss of the regioregularity, but also introduce a sterically twisted geometry in the polymer backbone

which results the loss of π-conjugation of the whole system. This is due to the increased

steric hindrance between the alkyl substituents on the 3-postions of the thiophene rings

and the lone electron pairs of the sp2 sulfur atoms (Figure 1.2). These repulsive

interactions lead to increased bandgaps and decreased intermolecular packing. In

comparison, regioregular HT coupled P3ATs (rr-P3ATs) which consist of near-perfect

structures, thus can easily access planar polymer backbones and form well-organized

packing geometry. These structures provide efficient interchain charge carrier pathways

which lead to high charge mobility and other desirable physical properties for electronic

applications.40 The contradictory observations of conjugation length and conductivity of

electropolymerized poly(3-alkylthiophene)s36-39 can be explained by this fact. As the

result, it is important to synthesize polymers with structure control.

Figure 1.2 Regioirregular P3AT in non-planar and rr-P3AT in planar structure.

1.2.2 Boron Containing Conjugated Polymers from Non-Catalyzed Step-Growth

Polymerization Synthesis

Conjugated polymers that contain boron are synthesized from step-growth

polymerization and was first reported by Chujo and co-workers in 1998.41 Developments over the past decade prove that stable boron-containing conjugated polymers of high molecular weight can be achieved readily.42 The incorporation of boron, which is an

element with high Lewis acidity resulting from the vacant p-orbital of boron atom, into π-

conjugated systems leads to extension of the π-conjugation via the empty orbitals of

boron atoms. This concept has been explored theoretically much earlier by Good and

Ritter. 43 Their study predicts that the π-orbitals of the vinyl group in low molecular

weight vinylborane derivatives overlap conjugatively with the vacant p-orbital of the

boron atom (Figure 1.3). Thus, the fully aromatic boron-containing polymers are

expected to possess unusual electronic states with positive holes built in. Poirier predicts

that poly-boroles will show fairly low bandgaps because of their quinoidal structures in

the ground state.44 Yamabe also predicts that poly(bora-acetylene)s would show metallic

conductivity without dopping.45 Due to these intriguing theoretic predictions of the organo-boron π-conjugated system, increasing efforts have been devoted to the preparation of boron-containing conjugated polymers.

Figure 1.3 Orbital overlapping in vinylboranes.

As mentioned above, synthesis of boron-containing conjugated polymers was first

achieved by Chujo group via hydroboration polymerization of bifunctional alkynes with

41,46 sterically hindered strong electron acceptor arylboranes ArBH2 (Scheme 1.2). The bulky aryl groups on boron prevent nucleophilic attack, resulting in good stability in air.

The polymers are highly fluorescent, and the polymer derived from diethynylpyridine exhibits white emission.

Scheme 1.2 Hydroboration polymerization of ArBH2.

Haloboration polymerization of aromatic diyne monomers with bromo-

diphenylborane has been achieved by Chujo and co-workers (Scheme 1.3).47 The

7 resulting polymers are soluble in common organic solvents and stable enough for GPC analysis which are around several thousands. Efficient red-shifts of the maximum absorption peaks of polymers relative to the respective monomers indicate the π- conjugation through the p-orbital of the boron atoms.

Scheme 1.3 Haloboration polymerization of diyne monomers.

Poly(p-phenylene-borane)s are prepared by polymerization of aryldimethoxy- boranes lithiated diacetylenes or bifunctional Grignard reagents generated in situ as shown in Scheme 1.4.48,49 These polymers are expected to be stable to air and heat due to the bulky aromatic groups.

OR OR

ArB(OMe)2 /Mg Br Br B Ar n RO RO OR OR Ar ArB(OMe)2 Li Li B n RO RO

Ar= or R= C8H17 or C12H25

Scheme 1.4 Polycondesation of aryldimethoxyborane using Grignard and organolithium reagents.

Synthesis of boron-containing conjugated polymers via tin-boron exchange

polymerization under mild conditions of bis(trimethylstannyl) monomers has been

reported by Jäkle and co-workers (Scheme 1.5).48,50 Using this route, polymerization

could be conducted in non-coordinating solvents to eliminate possible side reactions of

ether cleavage or Lewis acid/base complex formation.

C H C6H13 6 13 Ar n ArBBr2 S B S SnMe3 n Me Sn S 3 S n C H C6H13 6 13

C H C6H13 C H C6H13 C H C6H13 6 13 6 13 6 13 Br

BBr2 SnMe3 B Br2B + Me3Sn n

F F

S S NPh2 Ar= F F F

Scheme 1.5 Tin-boron exchange polymerization of bis(trimethylstannyl) monomers.

This route is different from the above methodologies for that various aromatic

structures could be easily introduced into the conjugated system simply by changing the

bis(trimethylstannyl) monomers. As the result, the whole polymers are color tunable from blue to dark-red luminescence, indicating the relative HOMO and LUMO energy levels are tunable, too. From this method, it would be also possible to synthesize monodisperse oligomer with a specific conjugation length as shown in Scheme 1.6 by repeating a number of reactions.51 A gradual decrease in the bandgap is evident in the absorption and

emission spectra with the increasing molecular weight. Higher molecular weight

organoborane oligomers are highly fluorescent with quantum yield up to 95% and with

emission maxima in the blue region.

Scheme 1.6 Synthesis of conjugated organoborane oligomers.

The π-conjugated diorganoboron polymers poly(p-phenylenevinlyene-diboraanthracene), which are structurally related to poly(p-phenylene-anthracene), have been reported by Jäkle group and Jia group independently (see Chapter II).52,53 These polymers

are prepared from hydroboration polymerization of bifunctional alkynes with ladder-like

polymeric dibora-anthracene (Scheme 1.7).

Scheme 1.7 Hydroboration polymerization of 5,10-dihydroboranthrene.

The interesting two-electron-three-center B-H-B bonds structure of diboraanthracene

precursor is confirmed according to single crystal X-ray diffraction studies. Although 1,4-

diethynylbenzene furnished polymer is air sensitive and only sparingly soluble in non-

coordinating solvents, the hexyloxy substituted dialkynylbenzene derivative has good

solubility in common organic solvents and is stable enough for the matrix-assisted laser

desorption ionization- time-of-flight (MALDI-TOF) mass spectrometric analysis. The

thin film of this boron-containing polymer emits strong green light at 518 nm.

1.2.3 Transition Metal-Catalyzed Coupling Reactions

Coupling reactions between two hydrocarbon species are important organic

transformations in organic chemistry. Coupling reactions between two aryls, aryls and

vinyls, two vinyls, etc. are often catalyzed by transition metals. In these reactions, usually

one of the specie is an organic halide (RX), and the other one is a main group

organometallic compound (R’M). After losing the MX as a salt, the newly formed species

RR’ with a new carbon-carbon bond is regarded as a coupled product (equation 1.1).

Common coupling reaction types includes: Ullmann reaction54, Kumada coupling55,56,

Heck reaction57-59, Sonogashira coupling60,61, Negishi reaction62,63, Stille coupling64-66,

Suzuki coupling67-70, Hiyama coupling71,72, Buchwald-Hartwig reaction73-75 and etc.

Because of the development of these reactions, Richard F. Heck, Ei-ichi Negishi and

Akira Suzuki received the 2010 Nobel Prize in Chemistry.76 Highly selective catalysts

and mild reaction conditions are being developed. These reactions have been applied to

polymer synthesis.

Scheme 1.8 Eqaution of coupling reaction.

The detailed mechanisms of the individual types of reactions can be complicated and

different from one another. However, a simplified general mechanism can be described as

shown in Scheme 1.9.

Scheme 1.9 General mechanism of coupling reactions.

The metal catalyst is inserted into one organic halide (RX) bond to form its oxidative state first. The other molecule (R’B) then will undergo trans-metallation and add onto the

same metal center. Eventually, after reductive elimination of the two species to regenerate the metal catalyst, the coupled product RR’ is formed. The unsaturated organic

compounds have higher reactivity since they couple more easily with the metal catalyst center, for example: vinyl-vinyl > phenyl-phenyl > vinyl-alkyl > alkyl-alkyl.77 Nickel and

palladium are mostly used as the catalyst. Other factors such as complex ligands, leaving

groups and operating conditions are also crucial for the coupling reactions.19,78-80

1.2.4 Application of Transition Metal-Catalyzed Coupling Reactions in Step-Growth

Polymerization

Since transition metal-catalyzed coupling reactions have advantages in coupling between two aryls, aryls and vinyls, two vinyls, etc. Schrock and co-workers have developed a number of well-defined ring opening metathesis step-growth polymerization

(ROMP) initiators of the type M(CH-t-Bu)(NAr)(O-t-Bu)2, which allows better control over the polyacetylene synthesis (Scheme 1.10).81,82 Using these systems, polyacetylene

prepolymers are obtained through step-growth ring opening metathesis. The prepolymer

decomposes spontaneously into polyacetylene film at room temperature (half-life: 20 h).

Furthermore, the use of these catalytic systems allows the introduction of functionalized

end-groups to the polymer chain (R in Scheme 1.10).82

Scheme 1.10 Preparing polyacetylenes from prepolymer.

However, irrespective of the method adopted for the synthesis of polyacetylene, it is

an insoluble, infusible and generally intractable material.83 In order to obtain soluble

polyacetylene analogues, substituted acetylenes have been polymerized to polymers with

side groups attached.84-87 The electrical conductivity of all substituted polyacetylene

analogues is significantly lower than that of polyacetylene itself.88 This is attributed to

steric repulsion between adjacent side groups that causes the polymer chains to twist and

loses planarity.89 Grubbs and co-workers demonstrated that the ROMP of monosubstituted cyclooctatetraene derivatives leads to partially substituted polyacetylenes that are soluble and highly conjugated (Scheme 1.11).85,87 The molecular weight can be up to

345,000 and polydispersity index (PDI, depending on the ratio of Mw/Mn) between 1.7

and 7.6.

Scheme 1.11 Soluble polyacetylene from monosubstituted cyclooctatetraene.

Besides polycondensation of vinyls, transition metal-catalyzed step-growth

polymerization of aryls has also been developed. For example, Stille coupling reaction

has been used extensively in conjugated polymer synthesis due to its advantages of good

tolerance of different functional groups and mild reaction conditions.19 Yang and co-

workers employed Stille coupling polymerization to prepare a polythiophene with fewer alkyl side chains than typical poly(alkylthiophene)s in order to reduce the electron-

donating effect of the alkyl side chains (Scheme 1.12).90 Coupling conditions using the

Pd(PPh3)4 catalyst in toluene give polythiophene with acceptable molecular weight and a

broad PDI near 2.

Scheme 1.12 Polythiophene from Stille coupling reaction.

McCullough and co-workers have prepared a series of regional irregular

poly(thienylene-vinylene)s via Stille coupling (Scheme 1.13),91 while the regioregular

polymer is obtained via Heck reaction conditions. Polymers are prepared in relatively

high molecular weights up to 15,000 and PDIs between 1.80 and 2.46.

Scheme 1.13 Regio-irregular Poly(thienylenevinylene)s via Stille coupling reaction.

In general, conjugated polymers prepared from Stille and Heck coupling reactions are from polycondensation of two A—A and B—B type monomers (A represents functional tin group or proton and B represents halogen atom). It follows a step-growth mechanism, leading to broad PDI and low regioselectivity.

1.2.5 Chain-Growth and Livingness of Polycondensation of Aryls

As described above, conjugated polymers can be synthesized via polycondensation methods, such as electrochemical approaches and transition metal-catalyzed polycondensation reactions. The molecular weight, the distribution and the regioregularity of these polymers are difficult to control. Under classical Flory’s

assumption for step-growth polymerization, all the end groups of monomers, oligomers and polymers in the reaction mixture react equally, resulting in an uncontrolled molecular weight and broad distribution. However, this is not essentially true for all condensation polymerization which has already been demonstrated by nature for the synthesis of perfectly mono-dispersed biopolymers such as proteins and DNA. It is because their mechanisms of condensation polymerization could be converted from step-growth to chain-growth. In recent years, it is found that chain-growth condensation polymerization could be realized by Kumada-Tamao coupling polymerization with a Ni catalyst and

Suzuki-Miyaura coupling polymerization with a Pd catalyst.92 If polycondensation could

proceed by a chain-growth mechanism, the ability of a growing polymer chain to

terminate could be removed and living polymerization will be possible.93,94

1.2.6 Head-to-Tail Regioregular Poly(3-Alkylthiophene) (rr-P3ATs) via Transition Metal-

Catalyzed Chain-Growth Polymerization

The control synthesis of different regioregular polythiophenes has been tested in the

past decades and is the most well-established. Well-defined rr-P3ATs can be prepared by

one of the following methods as shown in Scheme 1.14: McCullough,95 Rieke,96

Grignard metathesis (GRIM),97 Yokozawa,98 Iraqi,99 Guillerez100 and Ozawa.101 Among these strategies, GRIM and Yokozawa’s methods are most popular.

McCullough R o R R i. LDA/THF, -40 C Ni(DPPP)Cl + 2 Br S Br ii. MgBr2OEt2,-60to BrMg S Br S MgBr -40 oC, 1h 98% 2%

Rieke

R R R Zn Ni(DPPE)Cl2 + o Br Br S Br THF, -78 Ctort,4h BrZn S Br S ZnBr

90% 10% GRIM

R R R i-PrMgBr Ni(DPPP)Cl + 2 R Br Br S Br THF, rt, 1h BrMg S Br S MgBr S n 85% 15% Yokozawa R= Hexyl R R i-PrMgBr Ni(DPPP)Cl2 BrMg I S Br THF, rt, 1h S Br

Iraqi 100% R R i.LDA/THF,-80to-40oC, 1h Pd(PPh3)4 I o Bu3Sn Br S Br ii. Bu3SnCl, -80 Ctort,o/n S

Guillerez o i. LDA/THF, -40 C, 1h R R o ii. B(OMe)3,-40 Ctort,o/n O Pd(OAc)2,K2CO3 B S I S I iii. Dialchol, rt, 0.5 h O

Ozawa R Herrmann's catalyst o S Br THF, Cs2CO3, 120 C, 12h

Scheme 1.14 Synthetic routes of rr-P3ATs.

The key synthetic strategy feature is the regioselective metallation of the regiospecific monomers, which generates the corresponding intermediaes 2-bromo-5- metalo-3-alkylthiophenes (Scheme 1.14). Although a small portion of the undesirable 5- bromo-2-metalo-3-alkylthiophenes is generated in the case of the three Ni catalyzed methods, the desired intermediate reacts preferentially to produce fully HT arranged rr-

P3ATs. All these synthetic methods for the production of rr-P3ATs yield similar materials

that are not chemically and physically different. In all these methods, GRIM, Yokozawa

and Ozawa’s routes have the advantage over the two palladium catalyzed methods in

terms of easy accessibility to the rr-P3ATs. They do not require extra purification of the

monomers or precursors and the whole polymerization can be handled in one pot and an

economical way.

The first synthesis of rr-P3ATs is reported by McCullough and co-workers in 1992

(Scheme 1.14).95 Through their route, a regio-specific 2-bromo-5-bromomagnesio-3- alkylthiophene is generated with an overall 98% yield. The polymerization is then

engaged in situ by Kumada coupling reactions using catalytic amount of Ni(DPPP)Cl2

(DPPP= 1,3-diphenylphosphinopropane). Purified rr-P3ATs has around 70% yield and afford HT-HT regioregularity of above 98% which was identified by 1H NMR spectrum.

The number average molecular weight (Mn) is typically around 20,000 to 40,000 which

could be controlled by the feed ratio between monomers and nickel catalyst, and the PDI

is around 1.4 to 1.5.40,95,102 In the same year, Rieke and co-workers reported an approach

to rr-P3ATs through Negishi coupling reactions (Scheme 1.14).96,103 In this strategy, 2,5-

dibromo-3-alkylthiophene is treated with highly reactive zinc to yield a mixture of

isomeric intermediates in a ratio of 9:1. The polymerization is then realized in situ using

catalytic amount of Ni(DPPE)Cl2 (DPPE=1,3-diphenylphosphinoethane) and gives rr-

P3ATs with around 75% yield. The resulting polymers have the Mn about 24,000 to

40,000 and PDI around 1.4. Only one sharp band for the vinyl group in 1H NMR spectrum and four triad structures in 13C NMR spectrum clearly demonstrate the high

regioregularity of the polymer (Figure 1.4 and Figure 1.5).

Figure 1.4 1H NMR spectra of (a) regiorandom (1:1:1:1, HT-HT:HT-HH:TT-HT:TT-HH)

P3HT and (b) regioregular (98.5%) P3HT. Reproduced with permission from Ref. 96.

Figure 1.5 13C NMR spectra of (a) regiorandom (1:1:1:1, HT-HT:HT-HH:TT-HT:TT-HH)

P3HT and (b) regioregular (98.5%) P3HT. Reproduced with permission from Ref. 96.

UV-Vis data shows that rr-P3HT has longer wavelength of maximum absorption either in solution (chloroform, 456 nm) or in the solid state (film, 560 and 610 nm) when

compared with the values of other regiorandom P3HT (427 nm in chloroform and 438 nm

as film).95,96,104,105 This indicates the increased effective conjugation through the polymer

chain, apparently due to complete HT regularity. The physical properties between

regioregular and regiorandom polymers are quite different.106

In 1999, a facile and economical synthetic route for rr-P3ATs was reported by

McCullough group which is now known as Grignard Metathesis (GRIM).107 This method does not require expensive chemicals used in previous strategies but a cheap Grignard reagent (R’MgBr) to treat 2,5-dibromo-3-alkylthiophenes to form a mixture of

intermediates in a ratio of 85:15 (Scheme 1.14). Although the ratio of desirable to

undesirable isomers is relatively higher compared to either the McCullough or the Rieke

method, polymerization in situ by Kumada coupling reactions using catalytic amount of

Ni(DPPP)Cl2 still generate rr-P3ATs with over 99% high HT regioregularity. Another

advantage of this method is that the reaction can be realized at room temperature in one

pot without any further purification, which eliminates the precaution of handling at low

temperatures. The resulting polymers typically have the Mn about 24,000 to 35,000 and

PDIs between 1.2 and 1.4.

However, recent research by Luscombe and co-workers demonstrates that the high

degree of regio-control from GRIM polymerization should not simply be explained by

steric hindrance largely preventing formation of HH bithiophene coupling. The actual reason is the inability of 5-bromo-2-metalo-3-alkylthiophenes to polymerize. It is believed to be due to the lack of ortho-stablization group in the propagating nickel

complex, preventing oxidative addition of the nickel into the Ar-Br bond.108 Additionally,

regioregular polyselenophenes could be also prepared through GRIM method.109 The

resulting polymer has a broader optical absorption range and 80 nm red-shift compared to

that of rr-P3HT.

The GRIM method is later advanced by Yokozawa and co-workers (Scheme 1.14) by treating 2-bromo-5-iodo-3-alkylthiophenes with stoichiometric Grignard reagent which

generate the desirable isomer as the only starting material quantitatively.98,110 A chain-

growth condensation polymerization mechanism has been proposed as shown in Scheme

1.15. They have pointed out that the existence of excess Grignard reagents is harmful for the polymerization. It is because that the remaining Grignard reagents would terminate the on-going chain ends and broad up the PDI of the product, which is commonly seen in

GRIM method.

Scheme 1.15 Mechanism of chain-growth condensation polymerization of rr-P3ATs.

At the beginning, two monomers undergo coupling reaction with the generation of a

zero-valent Ni complex after reductive elimination step. Once the TT dimer has been

formed, the Ni(0) complex does not diffuse freely to the reaction mixture but is inserted

intramolecular into the Ar-Br bond at the chain end. Another monomer reacts with the Ni

catalytic center, followed by the reductive elimination and intramolecular oxidative

insertion to the next Ar-Br bond. Growth continues in such a fashion and a regioregular

HT polymer is formed, with the only regio-mistake happens at the beginning dimmer

position.

After quenching with HCl, the resulting polymer has one end group as bromine atom

and the other is a hydrogen atom, which is consistent with the MALDI-TOF spectrum

containing only one series of peaks (Figure 1.6). As a result, polymers from this method

typically have the Mn up to 28,700 and can be controlled by the feed ratio (Figure 1.7).

The PDI can be as low as 1.10. These evidences support the living chain-growth

polycondensation mechanism.

Figure 1.6 MALDI-TOF mass spectrum of rr-P3HT. Reproduced with permission from

Figure 1.7 Mn and Mw/Mn values of rr-P3HT as a function of the molar ratio of converted

American Chemical Society.

It should be pointed out again that polymerization from 5-bromo-2-iodo-3-

alkylthiophenes did not produce any polymeric material. There is no problem for the

near-complete conversion with Grignard reagent, but the bithiophene in the first step was

not formed in any detectable amount after addition of catalyst.108 Therefore 2-bromo-5-

magnesio-3-alkylthiophenes is the only polymerizable monomer.

Recently, 100% regioregular polythiophene have been achieved by initiation of 2-

bromo-5-magnesiobromo-3-alkylthiophenes with an externally added initiator by

Luscombe and co-workers as shown in Scheme 1.16.111 This route eliminates the TT regio-mistake formed in Yokozawa’s method and all the polymer chains are initiated at

the same time.

Scheme 1.16 Synthesis of 100% HT regioregular P3HT.

Stille and Suzuki palladium catalyzed coupling polycondensation are alternative strategies to afford rr-P3ATs (Scheme 1.14). These methods require purification of the corresponding organometallic monomers before polymerization. Iraqi and co-workers explored the synthesis of rr-P3ATs through Stille coupling reactions, using 3-alkyl-2-

iodo-5-(tri-n-butylstannyl)thiophene as monomers.99 rr-P3ATs with regioregularity above

96% are readily achieved, and the Mn around 10,000 to 16,000 with PDI around 1.2 to1.4.

Suzuki reaction using 3-alkyl-2-iodo-5-boronatothiophene has been reported by Guillerez

and co-workers (Scheme 1.14) to give rr-P3ATs with 96-97% regioregularity and Mn up

to 27,000.100

Recently, Ozawa group successfully employed polymerization of 2-bromo-3-

hexylthiophene as monomer using Herrmann’s catalyst and special ligands (Scheme 1.14)

at elevated temperature.101 This dehydrohalogenative polycondensation affords rr-P3ATs

with regio-regularity about 96-98% and Mn around 24,000 to 30,600. Compared to other methods, their products have much broader PDI, but the yield is readily above 98% to

nearly quantitative.

Although all the above methods give rr-P3ATs with controlled molecular weights

and their PDI can be as low as 1.10, the final product is still not a monodisperse

macromolecule. Mori and co-workers recently developed a strategy that 100% HT

oligothiophene can be realized by single-step extension from regioselective metalated 3-

substituted thiophene using the Knochel-Hauser base (TMPMgCl•LiCl).112 Treatment of

3-substitued thiophene with TMPMgCl•LiCl generates a 5-metalo-3-substitutedthiophene selectively due to the steric hindrance as shown in Scheme 1.17. Subsequent addition of the 2-bromo-3-substituted thiophene and the nickel catalyst leads to the formation of the

24 corresponding HT dimer. Longer regioregular oligothiophene is obtained by repeating via the similar protocol. This facile method provides a tool toward the synthesis of monodisperse regioregular oligothiophene with different functional groups or alternating block copolymers.

Scheme 1.17 Synthesis of monodisperse HT oligothiophene.

1.2.7 Functionalization of Regioregular Polythiophenes

The development of synthesis of rr-P3ATs has led to the success in producing a wide range of functional regioregular polythiophenes not only at the 3-position of the thiophene rings,113-122 but also at the end position of polymer chains as summarized in

Table 1.1.123,124 Due to the nature of chain-growth polymerization mechanism, well- defined block polymers are easily accessible.125,126

Table 1.1 Summary of side chains and terminal groups on rr-P3ATs.

Side chains Terminal Groups

1.2.8 Other types of Conjugated Polymers via Transition Metal-Catalyzed Chain-Growth

Polymerization

The methodologies for the synthesis of rr-P3ATs have been demonstrated to be applicable to other aromatic π-conjugated. Yokozawa and co-workers first apply the

GRIM polycondensation synthesis of poly(p-phenylene) which contain no heteroatoms in the aromatic rings. However, under similar conditions, polymers obtained in this method

all give low molecular weights and broad PDI.79 To solve the problem, they find that the

existence of LiCl is essential to optimize the chain growth polycondensation (Scheme

1.18). The resulting polymers have the Mn in range of 9,200 to 35,000 which is controlled

by the feed ratio of monomer to Ni catalyst.

Scheme 1.18 Synthesis of poly(p-phenylene) via chain-growth polycondensation.

Additionally, block copolymers of poly(p-phenylene) and polythiophene are

affordable through this method. This first example of the successive chain-growth

polycondensation for the synthesis of block copolymers consisting of these two different

π-conjugated systems is reported by Yokozawa group.127 The sequence of addition of

monomers is important to yield copolymers with PDI control. A well-defined copolymer

is obtained by first synthesizing the poly(p-phenylene) block and subsequent

polythiophene block, and a broad PDI copolymer is generated with the reverse order

(Scheme 1.19). The broad up of the PDI is believed due to the difficulty of cross-coupling from poly(p-phenylene) pre-polymer to thiophene monomers and the intramolecular oxidative insertion into the terminal Ar-Br bond.

Scheme 1.19 Strategies of synthesis of poly(p-phenylene)-b-poly(3-alkylthiophene).

Chain-growth polycondensation through Suzuki coupling of p-phenylene monomers has also been investigated. The polymerization is initiated with a special initiator PhPd(P- t-Bu3)Br as shown in Scheme 1.20. Polymers with Mn up to 21,000 and PDI below 1.5 are obtained.128

Scheme 1.20 Synthesis of poly(p-phenylene) via Suzuki polycondensation.

The GRIM polymerization of N-alkylpyrrole has been investigated by Yokozawa

129 group. Under similar conditions for the systhesis of rr-P3ATs using Ni(DPPP)Cl2 catalyst, a polymer with PDI ~1.26 is obtained together with low molecular weight oligomers. However, polymerization with Ni(DPPE)Cl2 and excess DPPE ligand afford

high molecular weight polymer with much narrower PDI (~1.11), without the formation

of any by-products as shown in Scheme 1.21. The Mn could be controlled by the feed ratio of monomers to catalyst which is in the range of 2,800 to 12,400.

Scheme 1.21 Synthesis of poly(N-alkylpyrrole) via chain-growth polycondensation.

The block copolymers are also available by first polymerizing p-phenylene

monomers with Ni(DPPE)Cl2 in the presence of LiCl and subsequent addition of N-

hexylpyrrole monomers to afford desired block copolymer with a narrow PDI ~1.16

(Scheme 1.22). The copolymerization in the reverse order offers a broader PDI ~1.38

which could be explained using similar analogs as for the synthesis of poly(p-phenylene)-

b-poly(3-alkylthiophene).

Scheme 1.22 Synthesis of poly(p-phenylene)-b-poly(N-hexylpyrrole).

The synthesis of well-defined polyfluorene was first reported via Suzuki-Miyaura

coupling polymerization. Yokozawa and co-workers found that the desired polymer can

be obtained from chain-growth mechanism (Scheme 1.23),80 instead of the commonly seen step-growth mechanism in Suzuki-Miyaura coupling polymerization due to the large

130 fluorene units. The use of the special initiator PhPdP(t-Bu)3Br is the key to achieve

high Mn (7,700-17,700) and low PDI (1.33-1.39). The MALDI-TOF analysis of the mass

spectrum indicates that all the polymers bore the phenyl group at one end. These

evidences clearly demonstrate that the polymerization is initiated by PhPdP(t-Bu)3Br.

Since this special initiator is not easily accessible, an economical way to generate it in situ from Pd2(dba)3/ P(t-Bu)3/PhI without separation or purification is applied for the

Suzuki chain-growth polycondensation. Comparable results are obtained for different

fluorene monomers.131

Scheme 1.23 Synthesis of polyfluorenes via Suzuki polycondensation.

After this report, Carter and co-workers investigated the polymerization of lithiated fluorene monomer which is generated by the reaction of the dibromofluorene with t-BuLi

in situ as shown in Scheme 1.24.132

Scheme 1.24 Synthesis of polyfluorenes via lithiated fluorene monomer.

Similar to the GRIM polycondensation of polythiophene, the excess t-BuLi is

harmful to the polymerization which would decrease the molecular weight of the final

product due to chain termination. The polyfluorenes made using this method have the Mn in the range of 16,800 to 32,700 and the PDI from 1.92 to 2.91. Therefore, a step-growth polymerization mechanism is proposed for this route.

Geng and co-workers reported the GRIM polymerization of fluorene monomers with

133 Ni(DPPP)Cl2. The bromo-iodo-fluorene is treated with i-PrMgCl•LiCl to generate the

monomer. The polymerization goes rapidly at 0 oC and finishes within 10 min as shown

in Scheme 1.25. Polyfluorenes with high molecular weight (Mn ~ 18,800 to 86,000) and

PDI between 1.49 and 1.77 are obtained. The MALDI-TOF spectrum of the product shows an evident amount of Br/Br ended polymers, which is deduced to be formed by intermolecular chain transfer after reductive elimination and is attribute to be the reason for the broad PDI.

Scheme 1.25 Synthesis of polyfluorene via GRIM.

The GRIM has also been applied to the synthesis of polycarbazole.134 Unlike the

cases for polythiophenes and polyfluorenes, the magnesium halogen exchange of

dibromo-N-alkylcarbazole with i-PrMgCl•LiCl is considerably slowly and n-Bu3MgLi is needed for effective magnesium halogen exchange as shown in Scheme 1.26. The polymerization carried out in this method results in a polycarbazole with Mn up to 26,000

and PDI around 1.23.

Scheme 1.26 Synthesis of polycarbazole via GRIM.

Recently, the synthesis of polypyridine via GRIM was examined by Yokozawa and co-workers as shown in Scheme 1.27.135 meta-substitued pyridine monomer is generated

quantitatively by treatment of the corresponding bromo-iodo-pyridine molecule with

o stoichiometry i-PrMgCl•LiCl at 0 C. The so-formed polypyridines have a Mn in range of

5,000 to 10,000 and PDI around 1.22 to1.34. The Mn is controlled by the feed ratio of

monomers to catalyst. MALDI-TOF analysis of mass spectrum shows that the

polypyridines uniformly have a Br at one end and an H at the other end. All these results confirm that the polymerization follows a chain-growth mechanism.

Scheme 1.27 Synthesis of poly(meta-pyridine) via GRIM.

However, if para-substitued pyridine monomer is used, polymer with broad PDI (>4) is obtained.136 Although the product has a broad distribution, MALDI-TOF is still able to

clarify that major products have Br at both ends. This result is different from the above

case and in polythiophene synthesis which one end of the polymer is Br and the other is

H. Therefore, it was proposed that disproportionation happens continually through the

whole polymerization process (Scheme 1.28). The tendency of disproportionation in the polymerization of para-substituted pyridine monomer is ascribed to the coordination of

the Ni catalyst center in the Ar-Br bond to the nitrogen atom of pyridine ring on the other

chain end. Suzuki-Miyaura polycondensation of the same para-substituted pyridine unit

has the similar disproportionation problem.137

Scheme 1.28 Synthesis of poly(para-pyridine) via GRIM.

1.2.9 Attempted Transition Metal-Catalyzed Polymerization of Electron-Acceptor

Materials

Up till now, the main challenge of transition metal-catalyzed polymerization is the

synthesis of n-type (electron accepting) conjugated polymers partially due to the use of

the Grignard reagents or active Zinc to the functional groups. Thus, most n-type

conjugated polymers are synthesized via Stille19,138,13919,138,139 coupling reactions19,138,139 containing base-sensitive heterocycles or electron-withdrawing groups. Due to the weak

π-donation of the n-type polymer backbone to the metal catalyst center, polymerization of acceptor monomers may not facilitate intramolecular transfer insertion to the Ar-Br bond.

Although successive chain-growth polymerizations of monomers with high π-donating

33 ability have been achieved as above sections, monomers with low π-donating ability are retarded to be polymerized.127-129,140

Recently, Kirity and co-workers have demonstrated that Ni catalyst not only can undergo intramolecular transfer on pyridine, but also on a good acceptor material—

141 naphthalenediimide. The polymers so-formed have Mn in the range of 10,400 to 25,000 and PDI around 1.3 to 1.7. Meanwhile, Huch and co-workers have confirmed that Pd catalyst can experience intramolecular transfer on benzothiaziazole with polymers having

136 PDI < 1.27 and Mn about 3,300 to 7,400 (Scheme 1.29). MALDI-TOF shows that the polymer has one end as the aromatic group from the initiator and the other end is Br/H, indicating the polymerization follows the chain-growth polymerization mechanism.

Scheme 1.29 Synthesis of n-type conjugated polymer from acceptor monomers.

Their strategies are to combine two, three or more aromatic monomers together which consist strong acceptor and weak π-donating aromatic rings, though the weak π- donating arenes will slightly increase the HOMO levels of the whole conjugated polymer.

Even the conjugation length of the repeating units is increased, the intramolecular

catalyst transfer will still take place.142 These efforts open the door of the synthesis of n-

type conjugated polymers from acceptor monomers.

1.3 Application of Conjugated Polymers in Organic Solar Cells

Conjugated polymers are unique semiconductors and can be used in organic solar

cells.

1.3.1 Working Principles of Organic Solar Cells

Solar energy is an abundant energy source.143 Organic photovoltaic (OPV)

potentially have the advantage of low cost of manufacture and easy processability.144

They are generally considered to be one of the viable methods for solar energy conversion.

A typical organic solar cell contains an electron-donor material (or p-type material) and an electron-acceptor material (or n-type material). The simplest and original device reported by Tang143 has a bilayer structure as shown in Figure 1.8. The process of

converting solar energy by OPV can be conceptually divided into four main events as

shown in Scheme 1.30.

Figure 1.8 Bilayer OPV cell.

First, photons are absorbed by both donor (p-type) and acceptor (n-type) materials,

leading to the formation of an excited state. This excited state is generally accepted as an

exciton, which is the bound state of an electron and hole that are associated with each

other by electrostatic Coulombic force.145 It exists in semi-conductors as an electrically

neutral quasi-particle. The so-formed excitons could transport energy without carrying

charges in both donor and acceptor domains.146 Due to the short lifetime of exciton which is only around 100 ps, the exciton diffusion lengths in conjugated polymers and in organic semiconductors are usually estimated to be around 10-20 nm.144

A B

C D

Scheme 1.30 Working principles of OPV cells. A. Light absorption and excitons

generation. B. Excitons diffusion. C. Charge separation. D. Charge transportation.

To prevent the decay of excitons via radiative or non-radiative pathways, strong local

electric fields as well as interfaces are necessary for efficient charge dissociation. For

example, the lowest unoccupied orbital (LUMO) of the acceptor material must be lower than that of the donor material so that the energy offset between the LUMOs (ΔELUMO) is

larger than the electrostatic Coulombic force of the photo-generated excitons in the p-

type material. Similarly, it is required that the energy offset between the highest occupied

orbitals (HOMOs) is larger than the Coulombic binding energy of excitons generated in

the acceptor material for efficient charge separation. It has been proved that the photo-

induced charge separation at interfaces happens on a time scale around 100 fs, which is

much faster than other competing decay processes.

After separation, the generated charges need to be transported to the appropriate

electrodes within their lifetime which require donor and acceptor materials to have high

charge mobilities. This step is driven by the gradient in the chemical potentials of

electrons and holes created by the difference between the HOMO level of the donor and

the LUMO level of the acceptor. This internal electrical field also impacts on the

144 maximum open circuit voltage (Voc). Eventually, the charge carriers are collected from the device through elective contacts and electricity is generated in the outside circuit.

1.3.2 Device Structure Improvement of Organic Solar Cells

The efficiencies of the four events determine the overall power conversion efficiency

(PCE). The problem of bilayer structure is mainly due to the conflicting requirements for

light absorption and for excitons to reach the interfaces. Even for materials like

conjugated polymers which have very high absorption coefficient exceeding 105 cm-1,147 it still needs around 100 nm thickness to fully absorb the solar light. However, only excitons generated within the distance of 10-20 nm from the interface can reach the heterojunction boundary, resulting in the loss of absorbed photons far away from the interface. Expectedly, the device fabricated using bilayer structure usually has a PCE lower than 1%.143

Solar cell device design advanced from bilayer construction to bulk heterojunction

(BHJ) as shown in Figure 1.9.13

Figure 1.9 BHJ OPV cell.

The BHJ structure is composed of a blend of donor and acceptor materials in bulk

volume. The main goal of utilizing this morphology is to increase the interfacial area

between donor and acceptor phases. In such penetrating networks, each interface is

within a distance less than the exciton diffusion length (10-20 nm) to ensure efficient

charge dissociation.144 Furthermore, tandem (multi-junction) solar cells structure can be

adopted which consisting of several p-n junctions and each junction is tuned to absorb a

different wavelength range of solar light to achieve a higher efficiency.13 An increased

PCE is observed that polymer/fullerene derivatives BHJs have by far been the most

successful material combination with reported power conversion efficiency over 9%.148

Theoretically, an maximum PCE of 11.7% for single BHJ cell has been calculated and an

ultimate efficiency of about 20% is possible through the optimum configuration of

tandem cell structures.149

The conceptually ideal structure of a BHJ solar cell is shown in Figure 1.10. The

donor and acceptor materials are interdigitated with a length scale around 10 nm which is

below the exciton diffusion length. The interspaced phases will ensure high mobility

charge transportation with reduced charge recombination as in common BHJ structure.

The pure thin layers of donor and acceptor at the collecting electrodes will minimize the

losses by recombination as diffusion barriers. However, such well-organized

nanostructure is hard to achieve technically. Attempts by nanostructure templates such as

nano-imprint lithography have been tried,150,151 but the final structure is still far away

from the ideal configuration and the resulting PCE is usually lower than 1%.

Besides the cell structures, solvent effects, thermal annealing, additives and morphologies are all important factors in cell fabrication.152

Figure 1.10 Ideal structure of a BHJ solar cell.

1.3.3 Materials Challenges for OPV

All polymer solar cells (PSCs) have unique advantages over polymer/bucky balls

BHJs which have the best performance so far for the following reasons. First, semi-

conducting conjugated polymers have broad overlap with solar spectrum and high

absorption coefficients of UV-Vis light in the spectral region, while fullerene derivatives

only absorb lights with a very limited range (typically < 400 nm). Although C70 derivatives provide enhanced absorption in the blue region compared to C60 derivatives

due to the breakdown of symmetry,153 it is difficult to increase the absorption of bucky balls into red and near-infrared (NIR) region. Second, bucky balls and their derivatives such as phenyl-C61-butyric acid methyl ester (PCBM) have intrinsic high exciton binding energy and morphology fabrication problems which lower the overall PCE.154

Third, n-type acceptor conjugated polymers have a large potential for fine adjustment of energy levels which is crucial for the development of high performance PSCs, simply by varying their repeating structures. Theoretically, it is required that the acceptor should have a low-lying LUMO to overcome the Coulombic binding energy for more efficient photo-induced charge separation at the donor/acceptor interfaces. However, an excessively low LUMO in fullerene derivatives will result in a decreased Voc and PCE of

polymer solar cells.147,155 A significant number of fullerene derivatives have been

synthesized to improve the HOMO/LUMO levels.155-158 Despite all these valuable efforts,

only small shifts (< 200 mV) of the LUMO level of derivatized C60 have been achieved

by attaching electron-donating groups to the bucky ball.156,159 Fullerene derivatives have

shown limited change of relative energy levels due to the absence of various chemical

159 structures, resulting in a low Voc in OPV. As the result, careful control of the energy

levels is necessary to maximize the overall conversion efficiency. Fourth,

polymer/polymer blends offer increased stability and superior flexibility which can be

processed via large scale solution film coating.160 Additionally, all polymer solar cells

can be made using the strategy of block polymers methods to solve the problems of

undesirable morphological properties such as large phase separation,161,162 inhomogeneous internal phase composition163-165 and poor crystallinity in polymer/bucky

balls BHJs.166,167 By careful control of the molecular weight and distribution of each

block, the co-polymer will be self-assembled into a desirable lamella structure to form the

ideal interspaced nanostructure as shown in Figure 1.11. Due to this morphological

control for excitons diffusion and charge separation,114,134 increased conversion efficiency

would be expected. Although much less researches have been devoted to all-polymer solar cells, promising improvements in efficiency around 2.7% have been reported recently by Mori and co-workers.168

Figure 1.11 Self-assembly of block polymers into the ideal structure of a BHJ solar cell.

1.4 Summary and Outlook

Conjugated polymers are one of the most attractive conducting materials due to their unique set of chemical and physical properties. Significant efforts have been devoted for exploring their potential applications in optical and electronic fields.

Although remarkable progress has been made in the research of developing new conjugated polymers, many challenges remain unmet. For example, conjugated polymers with relative low HOMO and LUMO levels are rare, and their charge mobilities are in general inferior to inorganic semiconductors or even organic small molecular semiconductors (see Chapter IV). The present thesis work seeks to use rational concepts and methodologies for tuning relative energy levels and increasing charge mobilities of conjugated polymers to develop novel n-type acceptor materials.

CHAPTER II

SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF BORON-CONTAINING

CONJUGATED POLYMERS∗

2.1 Introduction

Design and synthesis of π-conjugated organic molecules and polymers with high electron affinity continue to be a research challenge because of the demand of n-type

semiconducting materials for photovoltaic, optical and electronic applications.169 As discussed in Chapter I, for organic solar cells, the lowest unoccupied orbital (LUMO) of the n-type material must be lower than that of the p-type material by a minimal margin so that the energy offset between the LUMOs (ΔELUMO) is larger than the Coulombic

binding energy of the photo-generated electron-hole pairs in the p-type material,145 i.e.,

ΔELUMO > Ec-p. This proves not an easy task when regioregular poly(3-hexylthiophene)

(rr-P3HT), which has the desirable optical absorption range and high charge carrier

mobility for photovoltaic application, are used as the p-type material in a heterojunction

cell.144 For example, the LUMO of the cyano-substituted poly(phenylene vinylene) (CN-

PPV), which is normally used as the n-type material, is actually higher in energy than that of rr-P3HT.170,171 Not surprisingly, CN-PPV does not quench the photoluminescence of

rr-P3HT.172 In fact, only fullerenes and their derivatives, of which the exceptionally high

electron affinities arise from the topological characteristics of their π-orbitals and their

surface curvatures,173,174 have been shown to be able to quench the photoluminescence of

rr-P3HT.175-177 As the result, the all conjugated polymer photovoltaic cells commonly use a p-type polymer other than rr-P3HT. A polymeric material with adequate electron affinity to separate the electron–hole pair in rr-P3HT would be therefore very desirable from the point of photovoltaic application as well as in the general context of organic optical and electronic devices.

The concept of using the empty p-orbital of boron to gain electron affinity has been applied to both small molecular or macromolecular conjugated systems in the past.41,48,52,178-190 In all of the boron-containing systems that have been investigated for

this purpose, arylborylene units (A) are inserted into the π-conjugated carbon backbone.

Figure 2.1 Aryborylene units structure.

In comparison to A, the diborylene molecules of the type B (X = o,o-phenylene,

vinylene, S, or NH.)191-198 are an interesting series of π-conjugated boranes with

dissimilar π-bond connectivity compared to A. These electron-deficient diboranes have

been investigated previously as ligands for electron-rich transition metals,184,193,199-207 but electrochemical data concerning their single electron-accepting ability are rare. Only two

9,10-diboroanthracene derivatives are studied electrochemically.208 Even if the reduction

potential of 9,10-diiodo-9,10- diboroanthracene (-0.45 V vs SCE) is discounted, which

might conceivably involve iodide, 9,10-dimethyl-9,10-diboroanthracene is reduced at a

more positive potential than a number of highly electron-accepting representative boranes

as shown in Table 2.1.209-211 These evidences indicate that the diborylenes B may possess

exceptionally high electron affinity.

Table 2.1 Comparison of reduction potentials of boranes.

Structure LUMO (eV) Structure LUMO (eV)

-3.26 -3.01

-3.20 -2.60

a All values are referenced against the saturated calomel electrode (SCE).212

Here, several 9,10-diboroanthracene-derived polymers as well as small molecules

that served as models for better characterization and understanding of the corresponding

polymers were synthesized. The reversible coordination of Lewis base solvents with the boron-containing rigid polymers proves a property that the π-conjugated polymers could

be dissolved and processed into thin films. The electron affinities of the polymers and

small molecules were evaluated using cyclic voltammetry. One of them had a LUMO appreciably lower in energy than that of rr-P3HT and effectively quenched the

photoluminescence of rr-P3HT in the solid thin film state.

2.2.1 Synthesis

+2HSiEt toluene +2BrSiEt (1) BrB BBr 3 rt HB BH 3

toluene + 2 1 rt B B (2)

2-M

toluene B B (3) 1 + rt n 2

Scheme 2.1 Synthesis of compound 1, 2-M and polymer 2.

9,10-Dihydro-9,10-diboroanthracene (1) was prepared in nearly quantitative yield by

reaction of 9,10-dibromo-9,10-diboraanthracene and Et3SiH in toluene at room temperature (equation 1). Compound 1 was then used as the hydroboration reagent for the synthesis of polymer 2 and its small molecular analog 2-M. Reaction of 1 with phenylacetylene proceeds cleanly at room temperature to give exclusively the anti-

Markovnikov cis-addition product 2-M (equation 2). The structure of 2-M was fully characterized by 1H, 13C and 11B NMR spectroscopies and elemental analysis. Polymer 2

was prepared under identical conditions as 2-M and was isolated as a yellow powder

(equation 3).

A B

1 1 Figure 2.2 A H NMR spectrum of polymer 2 in CDCl3. B H NMR spectrum of

compound 2-M in CDCl3.

Assignments of the 1H NMR signals of 2 could be made by comparing the 1H NMR

spectra of 2-M and 2 (Figure 2.2). Both 2 and 2-M were very sensitive to moisture. This

precluded the possibility of determining the molecular weight of 2 by GPC.

Scheme 2.2 Synthesis of compound 3-M and polymer 3.

Polymers 3 and its small molecule analog 3-M were synthesized by the reaction of

9,10-dibromo-9,10-dihydro-9,10-diboroanthracene with the corresponding stannyl

11 reagent (equations 4 and 5). The B NMR resonance of 3-M appears at 65 ppm in CDCl3

but shifted upfield to 30 ppm in THF-d8, indicating THF-coordination to boron in the

THF solution. The coordinated THF was labile. 1H NMR spectroscopy (Figure 2.3)

showed that the base-free 3-M was recovered after the solid sample of THF/3-M adduct

was evacuated overnight under vacuum at room temperature or left open to the nitrogen

atmosphere in the glove box overnight.

A B

* *

1 Figure 2.3 A H NMR spectrum of polymer 3 in DMF-d7. Peaks with star are due to

1 deuterated solvents and end groups. B H NMR spectrum of compound 3-M in CDCl3..

1 The structure of polymer 3 was supported by its H NMR spectrum in DMF-d7 and by comparison of the spectrum with that of 3-M in Figure 2.3. The 11B NMR spectrum of

3 in DMF-d7 showed a broad peak at 0 ppm, indicating the boron atoms were tetra- coordinate in DMF. Polymer 3 was also moderately soluble in THF but only sparsely

soluble in toluene. The tendency for the coordinated THF to dissociate could be easily

1 assessed by H NMR spectrum in DMF-d7. Like 3-M, 3 easily lose THF when allowed to

dry at the ambient temperature under one atmosphere pressure of nitrogen in the glove

box. The lability of the coordinated Lewis base solvent was an important attribute to the boron-containing polymers because it allowed the polymers to be processed by solution processes without losing their electron accepting ability. Although much less sensitive to

air than 2, polymer 3 still must be manipulated under the protection of an inert atmosphere. GPC analysis was not attempted due to its sensitivity to air. The number average degree of polymerization for 3 prepared under the conditions specified in the experimental section could be estimated to be ~24 (i.e., the average number of repeat

1 n units is ~12) by comparing the H NMR spectrum integrations of the –Sn Bu3 end group

and the aromatic protons assuming that each chain was terminated on statistical average

n by one –Sn Bu3 end group and one -Br end group.

Scheme 2.3 Synthesis of compound 4-M and polymer 4.

Polymer 4 and the small molecule model 4-M were synthesized in a similar manner

as 3 and 3-M (equations 6 and 7), but the reactions were carried out at higher

temperatures. The more reactive trimethylstannyl reagents instead of tri-n-butylstannyl

reagents were used to achieve higher conversion. Compound 4-M was formed essentially

quantitatively under these conditions and was fully characterized by 1H (Figure 2.4 B),

13C, 19F, and 11B NMR spectroscopy and elemental analysis. Again, polymer 4 was prepared using the same procedure as its small-molecule analog 4-M was.

A B

d-THF d-THF

CD3CN

1 Figure 2.4 A H NMR spectrum of 4 in 1:2 CD3CN:d-THF. Peaks with star are due to

1 deuterated solvents and grease. B H NMR spectrum of compound 4-M in CDCl3..

The solubility of 4 was poor in normal single organic solvents, but dissolved well in

the acetonitrile/THF mixture in 1:2 volume ratio. The 1H (Figure 2.4 A), 19F and 11B

NMR spectra were consistent with the proposed polymeric structure especially when

compared with the NMR spectra of 4-M. The number average molecular weight was

1 estimated to be 4,500 according to H NMR end group analysis using the –SnMe3 end group (δ 0.45 ppm) and the aromatic resonances in the chain (δ 7.37 and 7.71 ppm).

Although not indefinitely stable, polymer 4 was stable enough to stand moisture and oxygen for GPC analysis. The GPC sample was first dissolved in the 2:1 mixture of THF and CH3CN and then diluted with CHCl3. The Mn value determined by GPC (Figure 2.5)

was 5,050 relative to polystyrene standards with PDI=1.14. The narrow polydispersity

was unexpected for a polymer synthesized by step-growth polymerization but may be

attributable to the low solubility of 4 in refluxing toluene, i.e., the molecular weight was

controlled by the solubility, which likely progressively decreased as the molecular weight

increased.

Figure 2.5 GPC trace of polymer 4.

Coordination and departure of the Lewis base to 4-M and 4 was studied. After removal of the solvents from the CH3CN/THF solution of 4-M, CH3CN was the only

1 observable coordinated Lewis base according to H NMR spectrum in CDCl3. Each boron Lewis acid center coordinated with approximately one CH3CN. The coordinated

CH3CN was rather labile. After the solid 4-M/CH3CN adduct was left to dry in the

atmosphere of the glove box for 1-2 days, CH3CN was completely lost. Because 4 could only be dissolved in the CH3CN/THF mixture, the departure of CH3CN from 4 was

confirmed by elemental analysis in addition to the evidence provided by the above

experiments using 4-M as a model compound. The 4/CH3CN adduct was left in the glove

box for two days, and then shipped for elemental analysis. The elemental analysis results

indicated that only 4.2% of the boron Lewis acid centers retained CH3CN. The above

results confirmed that coordinative solvents could be used for processing the polymers

and these solvents could be easily removed afterwards.

2.2.2 Solution and Solid State Optical Properties

The optical absorption spectra of the model compounds 2-M – 4-M and polymers 2 –

4 were shown in Figure 2.6.

Figure 2.6 UV-Vis spectra of model compounds and polymers in the presence of a

coordinated Lewis base and in the absence of a coordinated Lewis base.

The solid films were semi-crystalline and scatter light, rendering the apparent non- zero absorption value below the absorption bands. The highest wavelength absorption maxima were summarized in Table 2.2. The first absorption maxima were at 352 nm and

384 nm for 2-M and 2, respectively, in CH2Cl2. The red shift was likely the result of an

increase in the effective length of π-conjugation through the polymer backbone in

comparison to the small molecule. The UV-visible spectrum of 2 in the solid state was

obtained with a thin film drop-casted on the inside of the quartz cuvette from the CH2Cl2 solution of 2. The solid state and solution spectra were very similar.

Table 2.2 UV-Visible absorption data.

Compound λmax (nm) in CH2Cl2 λmax (nm) in THF λmax (nm) in solid

2-M 352 290 N/A 2 384 N/A 387 3-M 350 291 361 3 N/A 293 362 4-M 369 303 383 4 N/A 303 372 Due to the poor solubility of polymer 3 and 4 in non-coordinating solvents, their

solution UV-visible spectra could only be obtained in coordinating solvents. The UV-

visible spectra of 3 and 4 free of coordinated Lewis base were obtained using their thin

solid films by taking advantage of the fact that 3 and 4 readily lose the coordinated

solvents in the solid state as discussed above. The thin film samples were first left in the

glove box for at least two days to allow the complete departure of the coordinated Lewis

base at 40 oC. The absorption maximum of 3 was observed at 362 nm in the solid state. In

comparison, the first absorption maximum of 3 in THF solution was observed at 295 nm.

The absorption maximum of 4 also shifted from 372 nm in the thin film state to 303 nm

in the CH3CN/THF solution. The absorption shifts was attributed primarily to the

coordination of the Lewis base solvents because shifts of the same magnitude could be observed when comparing the THF and chloroform solutions of 3-M and the

CH3CN/THF and chloroform solutions of 4-M. Furthermore, the UV-visible spectra of the thin films of 3 and 4 did not change after being annealed at 90 ºC for 2 hours.

2.2.3 Cyclic Voltammetric Study

Cyclic voltammetry is a type of potentiodynamic electrochemical measurement base

on the onset potentials of reduction and oxidation. The reduction potentials were

measured using cyclic voltammetry to evaluate the electron affinity of the electron-

deficient polymers and the small molecules. The cyclic voltammograms were shown in

Figure 2.7.

Figure 2.7 Cyclic voltammograms of model compounds and polymers.

The Lewis-base free films of 3 and 4 were prepared by drop-casting on the flat head formed by the platinum electrode and its plastic cladding. The reductions of 3 and 4 occurred at much more negative potentials in a coordinating solvent than in the Lewis base-free state as expected due to solvent coordination.211

The LUMO energies of 2, 3, and 4 could be estimated from the onset potential of

their reduction waves in their Lewis base-free forms according to an empirical scale factor that relates the SCE potential to vacuum (typically 4.4 eV).170,171,213 The LUMO

170,171 levels of 2, 3, and 4 were compared with those of C60 and rr-P3HT in Figure 2.8.

Apparently, 2, 3, and 4 were less electron accepting than C60. However, the LUMO level

of 4 was lower than that of rr-P3HT by an appreciable margin, and the LUMO of 3 also

appeared somewhat lower than that of rr-P3HT.

-3.3 0 6 -3.4 -3.45 -3.50 -3.5 2 P3HT -3.56 3 -3.6 -3.68 -3.7 4 -3.8 -3.9 -3.94 -4 C60 -4.1 LUMO energy (eV vs vacuum)

Figure 2.8 Comparison of LUMO energies.

2.2.4 Detection of Charge Transfer by Photoluminescence Quenching

Quenching of the photoluminescence (PL) of p-type conjugated polymers such as rr-

P3HT in the presence of C60 or its derivatives had been attributed to the electron transfer from the excited state of the p-type materials to fullerenes.214 This charge transfer process

was the milestone of the organic bulk heterojuction photovoltaic cells. Interestingly, beside C60 and some of its derivatives, no other n-type materials had been reported that could quench the photoluminescence of rr-P3HT before this work.53 As one of the

motivations of this research was to find a polymeric material to replace the fullerene

derivatives in photovoltaic applications, the photoluminescence studies were carried out

to prove the possible electron transfer from the excited rr-P3HT to 3 or 4 since both have

a relative low-lying LUMO.

After photo-induced generation of exciton in rr-P3HT, it would either undergo

charge separation or decay via radiative or non-radiative path ways to give out energy as

the form of fluorescence and heat as shown in Scheme 2.4. If the energy offset was large

enough to overcome the exciton binding energy, possible electron transfer from LUMO

level of donor to LUMO level of acceptor would dominate over the decay process,

resulting in decreased photoluminescence intensity.

Scheme 2.4 Illustration of photoluminescence quenching of excited states.

Before carrying out the photoluminescence experiments, whether there was any

Lewis acid/Lewis base interaction between the boron-containing species 3 and 4 with rr-

P3HT was first probed. No changes of 1H NMR chemical shifts were observed for the

mixtures of 3-M and rr-P3HT or 4-M and rr-P3HT compared to the spectra of the pure

individual compounds, indicating the absence of the Lewis acid-base interaction.

Thin films of rr-P3HT, the 4/ rr-P3HT blend, and the 3/ rr-P3HT blend were coated

on 2x2 cm2 ITO substrates. The UV-visible spectra of the blends on ITO were simple

additions of the individual spectra of the blend components. These indicated no

significant mixing of the electronic wave functions in the ground state. The

photoluminescence experiments were carried out using the films coated on 2x2 cm2 silicon wafers. The polymer film thicknesses on the silicon wafers were approximately 1

μm, well above the thickness required for the maximum absorption and thus the highest photoluminescence of rr-P3HT (that thickness was ~160 nm in our hands).

The photoluminescence intensity of rr-P3HT in its blend with 3 was only marginally lower than that of pure rr-P3HT as shown in Figure 2.9. Although the lack of quenching may be attributed to a number of reasons, the insufficient energy difference between the

LUMOs of 3 and rr-P3HT to effectively separate the Coulombically bounded electron– hole pairs (excitons) in rr-P3HT appeared an easy explanation to cite. In comparison, the photoluminescence of rr-P3HT in its blend with 4 was quenched to <1% of the level of the pure rr-P3HT sample. This indicated a strong interaction between the two components in the excited states and rapid electron transfer from the excited rr-P3HT to

4. Also worth noting was that the ~0.18 eV energy offset between rr-P3HT and 4 was apparently enough to overcome the Coulombic binding energy of the rr-P3HT exciton, consistent with the notion that the exciton binding energies of well conjugated macromolecules were often rather low and the exciton binding energy was reported to be around 0.2 eV for rr-P3HT.154,215 Although the detailed mechanisms216-221 involved in the non-radiative decay of the excitons was unknown at the present stage, to the best of our knowledge, 4 was the first reported polymer that had been demonstrated to completely

57 quench the photoluminescence of rr-P3HT when this work was published.53,222 Recently, some other n-type polymers had been synthesized to quench the photoluminescence of rr-

P3HT. Mixture of these n-type conjugated polymers and rr-P3HT was used as polymer/polymer blend solar cells.151,160,166,168,223-226

Figure 2.9 Photoluminescence spectra of pure rr-P3HT, a blend of 3 and rr-P3HT, and a blend of 4 and rr-P3HT. These samples were excited at 530 nm.

2.3 Conclusion

Several electron-deficient polymers containing the 9,10-diboroanthracene unit had been synthesized and characterized. Electrochemical study showed that they had relatively high electron affinity. Only when the approach of utilizing the empty p-orbital of boron was combined with the approach of utilizing the electron withdrawing substituent, a polymer had with a LUMO appreciably lower than that of rr-P3HT.

Quenching of the photoluminescence of rr-P3HT by 4 demonstrated the electron transfer from the excited state of rr-P3HT to 4 indeed occurs in the solid state.

However, all these boron-containing conjugated polymers all absorbed less than 400

nm, resulting in limited overlap of the solar spectrum. To be a suitable organic solar cell material, it was desirable to introduce other functional groups into the polymer backbone to increase its absorption range, especially in the 400-800 nm range. Due to the instability of the 9,10-diboroanthracene unit, polymer 2 and 3 could only be handled in glove box, and underwent rapid decomposition in atmosphere. Although polymer 4 had the lowest

LUMO and seems to be the most stable, it was still sensitive to moisture and oxygen to some degree. As the result, even other functional groups could be introduced into the polymer backbone, the instability issue made these boron-containing polymers not suitable for further device fabrication.

To sum up, it was urgent to find some robust materials with low energy levels and

high charge mobilities to be the suitable candidates of n-type materials. This leads me to

the next project which is the synthesis of regioregular polythiophene with electron-

withdrawing groups.

2.4.1 Materials

All experiments were conducted under nitrogen using Schlenk techniques or in a

glove box. Anhydrous THF, hexanes, diethyl ether and toluene, rr-P3HT were purchased from Aldrich and used without further purification. Chloroform was refluxed over CaH2, distilled, and stored over molecular sieves. Deuterated tetrahydrofuran was stirred over a

Na/K alloy, vacuum-transferred, and stored in the glove box. All other deuterated solvents were refluxed over CaH2, distilled, and stored over molecular sieves in the glove

227 228 229 box. Compounds 1,4-C6H4(SnMe3)2 , 1,4-C6F4(SnMe3)2 , 1,4-C6H4(SnBu3)2 , 9,10-

230 230 231 dibromo-9,10-dihydro-9,10-diboraanthracene , C6H5SnBu3 and C6F5SnMe3 were prepared according to literature methods.

2.4.2 Measurements

NMR spectra were obtained on either a Varian Mercury 300 MHz or a Varian Inova

400 MHz instruments. All 1H and 13C NMR spectra were referenced with respect to

residual proton in the deuterated solvent. 19F NMR spectra were referenced with respect

to tetrafluoro-p-xylene (-146.21 ppm). 11B NMR spectra were recorded on a Varian Inova

400 MHz instrument and referenced with respect to boron-trifluoride diethyl ether.

Elemental analysis was carried out by Galbraith Laboratories, Inc.

Ultraviolet-visible (UV) spectra were acquired on a Hewlett-Packard 8453 diode-

array spectrophotometer. Photoluminescent spectra were obtained on a HORIBA Jobin

Yvon NanoLog spectrofluorometer. The solid film of 2 was prepared by drop-casting its

THF solution on the inside of the quartz cuvette. The solid films of 3 and 4 were prepared

by spin-coating on quartz or ITO glass wafers for the UV-visible spectroscopic

measurements and on silicon wafers for the photoluminescence measurements. The

spectra were recorded after the sample were left open to the atmosphere in the glove box

(~ 35 ºC) overnight to allow the coordinated Lewis base to escape from the film.

Cyclic voltammetric 232 measurements were performed under a nitrogen atmosphere

+ - in a gas-tight three-electrode electrochemical cell. n-Bu4N PF6 (0.1 M) was used as the

supporting electrolyte. A platinum wire was used as the counter electrode. A silver wire

60 was used as a quasi-reference electrode and is calibrated with the ferrocene/ferrocenium redox couple. The working electrode was a platinum rod with a plastic cladding, which render a flat area large enough for drop-casting thin films. The polymer films were prepared by first dissolving the polymer in the appropriate solvent and then drop-casting onto the electrode. The film was allowed to dry for 24 hours. All the preparation steps were carried out in the glove box under nitrogen. The electrochemical cell was then assembled and taken out the glove box to be connected with the potentiostat for the cyclic voltametric measurement.

Gel permeation chromatography was performed using a Waters 150C system equipped with a refractive index detector. The relative molecular weight was determined at 35 ºC using monodisperse polystyrene standards as the references and chloroform as eluent phase.

2.4.3 Synthesis

Preparation of 1

o At −78 C, Et3SiH (1.8 g, 15.6 mmol) in toluene (10mL) was added into a suspension of 9,10-dibromo-9,10-dihydro-9,10- dibora-anthracene (2.0 g, 6.0 mmol) in toluene (30 mL) under nitrogen. The mixture was allowed to warm to room temperature and stirred for 24 hours. The colorless precipitate was isolated by filtration and washed with hexane

(2× 10 mL). Dried under vacuum gave as white solid (0.98 g, 93%). 1H NMR (300 MHz,

13 THF-d8): δ 4.67 (br, B–H, 2 H), 7.16 (t, J = 3.9 Hz, 4 H), 7.60 (t, J = 4.1 Hz, 8 H). C

1 11 1 { H} NMR (75 MHz, THF-d8): δ 127.8, 136.6, 152.6 (br, B–C). B { H} NMR (400

MHz, THF-d8): δ 26. Anal. Calc. for C12H10B2 (175.62): C 82.00, H 5.71; found: C 81.51,

H 5.53.

Preparation of 2

A suspension of 1 (1.00 g, 5.70 mmol) and 1,4-diethynylbenzene (0.700 g, 5.70 mmol) in toluene (60 mL) was stirred under nitrogen for 24 hours at room temperature.

The yellow precipitate was collected by filtration and washed with hexane. Dried under

1 vacuum gave as yellow powder (1.45 g, 85%). H NMR (300 MHz, CDCl3): δ 7.30–8.04

(several broad signals, B-C6H4, B-CH=CHC6H4CH=CH-, 12 H), 8.16 (br, B-C6H4, 4 H).

11 1 B { H} NMR (400 MHz, CDCl3): δ 30.

Preparation of 2-M

A suspension of 1 (0.50 g, 2.80 mmol) and phenylacetylene (0.64 g, 6.30 mmol) in

toluene (30 mL) was stirred under nitrogen for 14 hours at room temperature. All volatile

components were removed under vacuum. The solid residue was dissolved in hexane at

room temperature. Recrystallization at −32 oC gave as white solid (0.66 g, 63%). 1H

NMR (300 MHz, CDCl3): δ 7.32–7.45 (m, 6H), 7.48 (d, J = 18.2 Hz, 2 H), 7.60 (dd, J =

5.3, 3.4 Hz, 4 H), 7.7 (d, J = 6.8 Hz, 4 H), 7.80 (d, J = 18.2 Hz, B-CH, 2 H), 8.18 (dd, J =

13 1 5.3, 3.4 Hz, 4 H). C { H} NMR (75 MHz, CDCl3) : δ 118.4, 127.8, 128.9, 129.4, 131.5

11 1 (br), 131.8, 136.3, 138.4, 146.6 (br), 151.6. B { H} NMR (400 MHz, CDCl3): δ 63.

Anal. Calc. for C28H22B2 (379.62): C 88.51, H 5.80; found: C 88.32, H 5.45.

Preparation of 3

A solution of 9,10-dibromo-9,10-dihydro-9,10-diboroanthracene (1.0 g, 3.0 mmol)

and 1,4-C6H4(SnBu3)2 (1.96 g, 3.0 mmol) in toluene (40 mL) was refluxed under nitrogen

for 12 hours. The precipitate was collected by filtration and washed with hexane. Dried

1 under vacuum gave as yellow powder (0.68 g, 91%). H NMR (300 MHz, DMF-d7): δ

11 1 7.0 (C6H4, 4 H), 7.24 (BC6H4B, 4 H), 7.41 (C6H4, 4 H). B { H} NMR (400 MHz, DMF-

d7): δ13.

Preparation of 3-M

A solution of 9,10-dibromo-9,10-dihydro-9,10-diboraanthracene (0.67 g, 2.0 mmol)

and C6H5SnBu3 (1.47 g, 4 mmol) in toluene (30 mL) was refluxed under nitrogen for 12 hours. After the mixture was cooled to room temperature, all volatile components were removed under vacuum. The residue was dissolved in toluene (10 mL). Hexane (15mL) was then layered on top of the toluene solution. Recrystallization at −32 oC gave as pale

1 yellow solid (0.43 g, 65%). H NMR (300 MHz, CDCl3): δ 7.51 (m, 10 H), 7.62 (dd, J =

13 1 5.9, 3.0 Hz, 4 H), 7.83 (dd, J = 5.3, 3.0 Hz, 4 H). C { H} NMR (75 MHz, CDCl3) : δ

127.2, 128.2, 132.1, 132.9, 140.2, 143.5 (br, B–C), 146.2 (br, B–C). 11B {1H} NMR (400

MHz, CDCl3): δ 65. Anal. Calc. for C24H18B2 (327.62): C 87.91, H 5.49; found: C 87.62,

H 5.42.

Preparation of 4

A solution of 9,10-dibromo-9,10-dihydro-9,10-diboraanthracene (1.0 g, 3.0 mmol)

and 1,4-C6F4(SnMe3)2 (1.43 g, 3.0 mmol) in toluene (40 mL) was sealed in a thick-walled

glass tube with a teflon screw cap and was heated at 120 oC under nitrogen for 12 hours.

The precipitate was collected by filtration and washed with toluene. Dried under vacuum

1 gave as pale yellow powder (0.87 g, 90%). H NMR (300 MHz, CH3CN-d3/THF-d8 in 1:2

19 volume ratio): δ 7.37 (C6H4, 4 H), 7.71 (C6H4, 4 H). F NMR (CH3CN-d3/THF-d8 in 1:2

11 1 volume ratio): δ −133.58. B { H} NMR (CH3CN-d3/THF-d8 in 1:2 volume ratio): δ −4.

Preparation of 4-M

A solution of 9,10-dibromo-9,10-dihydro-9,10-diboraanthracene (0.67 g, 2 mmol)

and C6F5SnMe3 (1.33 g, 4 mmol) in toluene (30 mL) was sealed in a thick-walled glass

tube with a teflon screw cap and was heated at 120 oC under nitrogen for 12 hours. After

the mixture was cooled to room temperature, all volatile components were removed

under vacuum. The residue was dissolved in toluene (10 mL) at 80 oC. The solution was

left at room temperature overnight to allow the initial crystallization. Further

recrystallization at −32 oC gave as pale yellow solid (0.72 g, 71%). 1H NMR (300 MHz,

19 CDCl3): δ 7.60–7.75 (m, 8 H). F NMR (CDCl3) : δ −129.8 (dd, J = 24.4, 9.8 Hz, o-C6F5,

4F), −153.0 (t, J = 20.1 Hz, p-C6F5, 2F), −161.3 (ddd, J = 23.8, 24.0, 9.8 Hz, m-C6F5, 4F).

13 1 C { H} NMR (75 MHz, CDCl3) : δ 137.7 (d, JC–F = 251), 141.6 (d, JC–F = 254 Hz),

11 1 145.5 (d, JC–F = 245 Hz), 135.4, 140.7, 113.9 (br), 144.0 (br). B { H} NMR (400 MHz,

CDCl3): δ 65. Anal. Calc. for C24H8B2F10 (507.93): C 56.74, H 1.58; found: C 56.42, H

1.53.

CHAPTER III

SYNTHESIS AND CHARACTERIZATION OF REGIOREGULAR POLYTHIOPHENE

WITH ELECTRON-WITHDRAWING GROUPS∗

3.1 Introduction

The seminal works of McCullough95 and Rieke96 on the Ni-catalyzed synthesis of

poly(3-alkylthiophene)s with high HT regioregularity rr-P3ATs have led to the widespread use of these polymers as p-type materials in organic photovoltaics (OPVs) and field-effect transistors (FETs) as described in Chapter I. Since the Ni-catalyzed polymerization is living,92,98,125,233,234 p/n-block co-polythiophenes are attractive synthetic

targets for the purpose of achieving morphological control in ambipolar devices such as

bulk heterojunction (BHJ) solar cells.

However, only a relatively small number of polythiophenes with electron-

withdrawing substitutents have been synthesized, and an even smaller number of them

have the HT regioregularity. Collard and coworkers235,236 and Rikukawa and coworkers237 reported poly(3-fluoroalkylthiophene)s with high HT regioregularity (rr-P3FATs) and very high electron affinities. The helical conformation of the fluoroalkyl side chains slightly twists the adjacent thiophene rings away from coplanar conformation. This

feature renders rr-P3FATs highly emissive, but the extent of π-conjugation in the ground

state is reduced compared to rr-P3ATs. Pomerantz and coworkers reported the synthesis

of polythiophenes with ester side chains via the Ni-catalyzed polycondensation but only

obtained low molecular weight polymers in low yields.238 They also reported ester-

substituted polythiophenes with HH regioregularity via Ullman-type coupling.239,240

Several examples of polythiophenes with electron-withdrawing cyano and ester substituents on alternating thiophene repeat units have been reported.241-247

The use of the less reactive stannyl reagents or zinc reagents for transmetallation

apparently avoided the potential side reaction between the metallated thiophene with the ester and cyano substituents. Functionalization of rr-P3ATs at the 4-position of the thiophene rings after polymerization have also been reported,248 but the UV-visible

absorption of the 3,4-disubstituted polymers is significantly blue-shifted compared to the

parent rr-P3ATs, indicating a highly sterically encumbered backbone.

In view of the need for n-type conjugated polymers and the general usefulness of

controlling the electronic structure of π-conjugated polymers for optical and electronic

applications,169,249-253 it appears worthwhile to further examine polythiophenes with

electron-withdrawing substituents.

Here, the synthesis and studies of a representative member of the regioregular

poly(3-alkanoylthiophene)s (PHOT) were developed. The target polymer had been

synthesized via the Ni-catalyzed Kumada polycondensation, taking advantage of

protection-deprotection chemistry of the ketone and ketal functional groups.

Cyclic voltammetry was used to evaluate the energy levels of the HOMO and LUMO.

Optical absorption and emission spectroscopies of the new polymer were studied. The

new polymer as an active component in field-effect transistors (FETs) and BHJ solar cells

were also evaluated.

3.2.1 Synthesis of Monomer Precursor

Organic carbonyl functionalities reacted with Grignard reagents at room temperature

or below.254 The inherent incompatible reactivity required protection of the electron- withdrawing acyl substitutent in the monomer in order to carry out the Ni-catalyzed polycondensation. The synthetic route to monomer precursor 7 was outlined in Scheme

3.1.

Scheme 3.1 Synthesis of monomer precursor.

Most synthetic steps were straightforward. An extended reaction time was necessary

to ensure the complete conversion of 6 to 7. Otherwise, the mono-brominated

intermediate 8b (Figure 3.1), which could be isolated if one equivalent of NBS was used

(see Experimental Section), was difficult to remove. All these molecules, 5, 6 and 7 were

purified by distillation to ensure the high purity.

3.2.2 Generation of Monomers

Reaction of 7 with one equivalent of i-PrMgCl was first attempted in THF at various temperatures from -40 ºC to room temperature. The reaction mixture was quenched with water and analyzed by 1H NMR spectrum. The persistence of 7 in the quenched mixture even after a prolonged reaction time indicated that the products and reactants were in equilibrium. About 70% of 7 was converted to the two regio-isomeric Grignard reagents

(equation 3.1) as shown in Scheme 3.2.

Scheme 3.2 Schlenk equilibrium of monomer generation.

The reaction temperature or the use of t-BuMgCl did not significantly shift the equilibrium. When the equilibrium mixture was directly subjected to Ni (DPPP)Cl2 for polymerization, catalyst poisoning was evident as no polymeric product was obtained at all. The detrimental effect of excess alkyl Grignard reagent was reported by Yokozawa and coworkers.110 The equilibrium observed here apparently was not a serious problem in the previously reported syntheses of other polythiophene derivatives due to the higher reactivity of those monomers.

The equilibrium involving an alkyl Grignard and an aryl halide had recently been studied by Straub and Knochel.255 They reported that addition of LiCl and 1,4-dioxane,

2+ which coordinates to the Mg ion, shifts the Schlenk equilibrium to i-Pr2Mg•LiCl and

allows the exchange reaction to go to completion. The methodology fortunately proved

applicable to overcome the problem in our case. In the presence of LiCl and 1,4-dioxane,

7 was completely converted to the thiophene magnesium species 9a and 9b by one

equivalent of i-PrMgBr (equation 3.2). The ratio of 9a and 9b was estimated to be ~1:4

according to the ratio of 8a and 8b in their hydrolysis product determined by 1H NMR

spectrum integration.

Figure 3.1 Isomers after quenching.

3.2.3 Polymer Synthesis

Synthesis of the target polymer 11 (PHOT) was outlined in Scheme 3.3. The polymerization was catalyzed by Ni(DPPP)Cl2 in THF using the mixture of 9a and 9b as the monomers in-situ generated following the method detailed above. The reaction was run first at 0 ºC for 1 h and then heated at 40 ºC overnight. Polythiophene 10 was obtained essentially in near quantitative yield. Deprotection of 10 following the common protocol for small molecule ketals did not occur readily in chloroform/water mixtures likely because of the very unfavorable partition coefficients of organic polymers in the

aqueous phase. Addition of 1,4-dioxane proved necessary for deprotection to proceed to

any substantial extent. Still, the deprotection procedure must be repeated or preferably

performed three times to cleanly convert 10 to PHOT 11.

The aromatic region in the 1H NMR spectrum of PHOT was diagnostic of the

completeness of deprotection (Figure 3.2). The spectrum also demonstrated the relatively

high regioregularity of PHOT. A maximum of four peaks were possible in the aromatic

region corresponding to the HT-HT, TT-HT, HT-HH, and TT-HH triads in the 1H NMR

spectrum of PHOT. Only one major peak was observed, indicating that one triad,

presumably HT-HT, was dominant. The four triads were not completely resolved as only

two minor peaks are observable. We therefore used the α-methylene protons to estimate

the HT/HH dyad ratio, which was typically > 95:5.

Scheme 3.3 Polymer synthesis.

The high HT regioregularity despite the ~1:4 regioisomeric ratio in the monomer

mixture was previously explained by McCullough and coworkers97 on the basis of the

difference in reactivity of the regioisomeric monomers in their study of rr-P3HT synthesis. A similar explanation was likely applied here. To confirm it, the progress of the polymerization by periodically sampling the reaction mixture was monitored by quenching the samples with methanol, and analyzing them by 1H NMR spectrum.

After the polymerization was started at 0 ºC, the minor isomer 9a was depleted

within 1 h, while the major isomer 9b was intact after 1 day (Figure 2 A). When the

polymerization temperature was raised to 40 ºC, the consumption of 9b started and

reached completion in ~10 h. Therefore, the product of the polymerization was

essentially composed of two stereo-regular blocks arising from 9a and 9b, respectively.

The HH region-chemical mistakes were more likely to be located at the block junction

(Scheme 3.4) than the rest of the polymer chain with the exception of perhaps the

terminus of the chain.

HT HH

Figure 3.2 1H NMR spectrum of PHOT with assignments. The HT/HH ratio was

estimated to be ~96:4 using the α-methylene protons at shown in the inset to the right.

The number average degree of polymerization (Mn) increased linearly with the total conversion of 9a and 9b (Figure 2.3 B). Mn of 20,000 could be easily achieved. The

polydispersity index (PDI) was narrow before 9b polymerizes but broadened afterward. It

was tentatively suggested that the polymerization was quasi-living and attribute the broad

molecular weight distribution to the slow initiation of the second stereo-regular block. In

other words, the first enchainment of 9b at the block junction after the complete

consumption of 9a, where the region-chemical mistake was made as discussed above

(Scheme 3.4), was particularly slow compared to the subsequent 9b enchainment. The

slow rate of this step could be understood by the high steric hindrance of both reactants

involved in this step.

A B

25000 1.79 80 20000 1.94 1.93 1.98 60 15000 2.13 40 Polymerization Polymerization 10000 (g/ mol) (g/ of 9a of 9b n

M 1.54

Conversion (%) 20 5000 1.08 0 oC 40 oC 0 0 0 1020304050 0 20406080100 Time (h) Conversion (%)

Figure 3.3 A. Total conversion of monomers as a function of time and temperature. B.

Plot of Mn vs total conversion of 9a and 9b. The number over each point was the PDI of

the particular sample.

Scheme 3.4 Polymerization mechanism.

PHOT polymer had good solubility in chlorinated solvents and THF. It also had certain solubility in other solvents (Table 3.1).

Table 3.1 The solubility of PHOT in different solvents.

Solvent Boiling point Solubility chloroform 61 oC above 10 mg/mL at RT

1,2-dichlorobenzene 180 oC 9.0 mg/mL at 100 oC

Methanesulfonic acid 167 oC (10 mm Hg) Less than 0.05 mg/mL at b.p.

DMF 153 oC Less than 0.1 mg/mL at b.p.

NMP 203 oC Around 0.90 mg/mL at b.p.

Phenol 181 oC Above 2.1 mg/mL at b.p.

2-chloroethanol 130 oC Less than 0.1 mg/mL at b.p.

1,2,4-trichloro benzene 215 oC Above 6.8 mg/mL at b.p.

pyridine 115 oC Around 2.1 mg/mL at b.p.

trifluoroethanol 78 oC Less than 0.1 mg/mL at b.p.

3.2.4 Cyclic Voltammetry

Cyclic voltammetry was used to evaluate the HOMO and LUMO levels of PHOT.

The voltammogram of thin film of PHOT was shown in Figure 3.4 A along with that of

rr-P3HT for comparison. The LUMO and HOMO energy levels were estimated from the

onset potential of the reduction and oxidation waves, respectively, taking the SCE

potential as 4.4 eV below vacuum (see experimental section).

A B P3HOT -2 P3HT -2.76 -3 -3.30

-4

Current (A.U.) Current -5 -5.15 -5.64

-2 -1 0 1 2 (eV, vs Vacuum) Energy Level Voltage (V vs SCE)

Figure 3.4 A. Cyclic voltammograms of PHOT (solid) and rr-P3HT.256 B. Comparison of

HOMO and LUMO levels of PHOT and rr-P3HT.

The LUMO and HOMO levels of PHOT were lower than those of rr-P3HT by ~0.5 eV in the solid thin film (Figure 3.4 B). Interestingly, the LUMO levels of the polythiophenes with carboxyl side chains on every other thiophene ring, for example polymer A, were also ~0.5 eV lower than rr-P3HT (Figure 3.5).241 Therefore, the electron

affinity of the polythiophene did not increase after doubling the density of electron- withdrawing groups despite the fact that the electron-withdrawing abilities of acyl and

carboxyl functionalities were very similar according to Hammet substituent constants.257,258 The HOMO level of PHOT did seem somewhat lower than polymer A by

~0.1-0.2 eV.

Figure 3.5 Chemical structure of Polymer A.

3.2.5 Optical Absorption and Photoluminescence Spectra

The visible absorption maximum of PHOT in chloroform solution was at 442 nm and

shifted to 543 nm in a spin-casted film (Figure 3.6). The absorption spectra of PHOT and

rr-P3HT were almost identical in both the solid state and in solution. The introduction of

the acyl group in the side chain hence did not obviously alter the bandgap or hinder the

effective conjugation of the polythiophene backbone.

A B 1.0 1.0

0.5 0.5

0.0 0.0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 Normalized Absorbance (A.U.) Absorbance Normalized Wavelength (nm) (A.U.) Absorbance Normalized Wavelength (nm)

Figure 3.6 A. UV-Vis absorption spectra of PHOT (solid) and rr-P3HT256 in chloroform.

B. UV-Vis absorption spectra of PHOT (solid) and rr-P3HT256 as spin-cast film

The PL emission maximum of PHOT was slightly red-shifted compared to rr-P3HT in chloroform solution (Figure 3.7 A). The difference was more pronounced for the spin- cast thin films (Figure 3.7 B). A possible rationale was that the 3-acyl substituent was included in the π-system in the excited state more than in the ground state. The difference may also had been exaggerated by the nonlinear response of the photo-detector above

750 nm wavelength. The inherent difficulty of quantifying luminescence in the near- infrared region was manifested by the inconsistency of the PL spectra of rr-P3HT films that had appeared in the literature.259-262

A 1.0 B 4x106 0.8 3x106 0.6

6 0.4 2x10

0.2 1x106 PL Intensity (A.U.) 0.0 0

Normalized PL Intensity (A. U.) PL Intensity Normalized 450 500 550 600 650 700 750 800 550 600 650 700 750 800 850 Wavelength (nm) Wavelength (nm)

Figure 3.7 A. Solution photoluminescence spectra of PHOT (solid) and rr-P3HT in chloroform solution. The spectra are normalized. B. Thin film photoluminescence spectra of PHOT (solid), rr-P3HT, and their 1:1 blend (dot). The spectra were not normalized for comparison of the emission intensity.

To test whether any photo-induced electron-transfer would happen between PHOT and rr-P3HT, the PL spectrum of the thin film of PHOT/rr-P3HT blend in 1:1 weight ratio was recorded. As mentioned above, in order to make a valid comparison of the photoluminescence intensity, we paid special attention to running the experiments under identical conditions using the same instrument. As shown in Figure 3.7 B, some PL

76 quenching occurred but the quenching efficiency was low. Since cyclic voltammetry revealed a convincing 0.5 eV offset at both the HOMO and LUMO levels between the two polymers. The low quenching efficiency was attributed to the morphology of the blend or other factors that might contribute to the kinetics of the electron-transfer process at the heterojunction interface.

3.2.6 Thermal and Morphological Properties

Thermal decomposition of PHOT was noticeable as the temperature approaches ~300

ºC under a nitrogen environment according to thermogravimetric analysis (TGA) (Figure

3.8 A). Decomposition was accompanied by severe volume expansion or foaming, causing the ups and downs in the thermogram beyond 300 ºC. No thermal transitions could be observed on the heating scan in differential scanning calorimetry (DSC) experiments (Figure 3.8 B). However, a glass transition-like feature could be observed at

~269 ºC on the cooling scan after the sample was heated to 290 ºC.

A B 5 100 T 4 g 90 3 2 80 1 70

Weight (%) 0 Heat Flow (W/g) 60 -1 -2 100 200 300 400 50 100 150 200 250 300 Temperature (oC) Temperature (oC)

Figure 3.8 A. TGA thermogram of PHOT. B. DSC thermogram of PHOT.

The WAXD experiment on a drop-cast film of PHOT showed that the as-casted film

had very low crystallinity. Only one broad Bragg diffraction could be observed,

corresponding to a d-spacing of 1.96 nm at room temperature (Figure 3.9 A). The

intensity and number of diffraction did not change after the sample was annealed at 150

and 200 ºC for 1 h. The samples annealed at 240, 260, and 280 ºC, however, displayed

additional and progressively more intense diffraction peaks. The d-spacings of these

diffractions for the sample annealed at 280 ºC were 2.11, 1.03, and 0.69 nm at room

temperature.

A B 2.15 1000 Not annealed 2.1 o

(100) Annealed at 240 C ) Annealed at 260oC 2.05 o Annealed at 280 C 2 500

-spacing (nm 1.95 d 1.9 Intensity (A. U.) (200) (300) 1.85 0 none 240 260 280 51015 annealing temperature (oC) 2θ (Degrees)

Figure 3.9 A. WAXD powder pattern of PHOT annealed at various temperatures. B.

Change of d-spacing after annealing.

The 1:2:3 d-spacing relationship indicated that the observed diffractions were the progressively higher order diffractions due to the same set of periodic structures similar to what is observed in the rr-P3AT series.40,103,126,263-265 The d-spacings increased after

annealing according to the primary diffraction (Figure 3.9 B). If we apply the diffraction

assignments of the rr-P3AT series to PHOT, the observed d-spacings correspond to the

inter-chain periodicity of the lamellar structure. This result was consistent with the d- spacing of rr-P3HT which was around 1.57 nm,266 considering there was one more

78 carbonyl group on the side chains on PHOT. The diffraction in the π-stacking direction

(010) was never observed in PHOT (Figure 3.10). rr-P3HT had sharp peak corresponded to the periodicity of π-stacking distance around 0.39 nm.267 A comparable WAXD peak was not found or negligible for PHOT.

Figure 3.10 Inter-chain distance and possible π-stacking of PHOT.

3.2.7 Charge Transport and Photovoltaic Properties

Thin film transistors were fabricated as in Figure 3.11 to evaluate the charge transport properties of PHOT. After charge injection at source, current would change by tuning the gate voltage. The carrier mobility was calculated from the transfer

1/2 characteristics (Ids vs Vgs) in the saturation region. No n-channel activity was observed, but the transistors showed fair current modulation and saturation when they were operated in the p-channel mode. Figure 3.12 showed the typical output and transfer characteristics.

Figure 3.11 The device structure of field effect transistor.268

-120 103 8

-100 Vgs = -80 V -4 2 10 6 -80 ) /10 1/2 1

10 (A

-60 4 (nA) (nA)

-60 V 1/2 ) ds ds I

-I 0 -40 ds 10

2 (-I -20 -40 V -1 0 V, -20 V 10 0 0 0 -20 -40 -60 -80 20 0 -20 -40 -60 -80

Vds (V) Vgs (V)

Figure 3.12 Output and transfer characteristics of the thin film transistor of PHOT after thermal annealing at 150 ºC.

The average hole mobility increased from 7.3×10-6 cm2/Vs without annealing to

2.7×10-4 cm2/Vs after thermal annealing at 150 °C, then decreased to 7.9×10-5 cm2/Vs after annealing at 250 °C (Table 3.2). Films deposited from dichlorobenzene showed slightly lower mobility (Table 3.2). The on/off ratio was on the order of 103.

Table 3.2 Electrical parameters of the thin film transistors of PHOT.

2 Solvent Ta (°C) Average Mobility (cm /Vs) Vt (V) On/off ratio

Chloroform N/A 7.3×10-6 3.1 >102

Chloroform 100 2.8×10-5 -17 >102

Chloroform 150 2.7×10-4 3.9 >103

Chloroform 200 1.6×10-4 -1.8 >103

Chloroform 250 7.9×10-5 -9.2 >103

Hot ODCB 150 1.8×10-4 -7.1 >103

Ta: Annealing temperature.

Unlike rr-P3HT, the super-linear increase of currents in output characteristics (Ids vs

Vds) suggested that there was a contact resistance between gold electrode and the polymer semiconductor. The contact resistance was likely from the energy barrier due to the low- lying HOMO level of PHOT (5.64 eV in Figure 3.4 B) compared to rr-P3HT (5.15 eV in

Figure 3.4 B). The work function of gold was believed to be around 5.1~5.47 eV,269 which was consistent with the cyclic voltammetry results. The hole mobility (~3×10-4

2 cm /Vs) that obtained for PHOT (Mn ~ 13.6 kDa, PDI~1.8) was similar to the hole

-4 2 mobility (~4-5×10 cm /Vs) of rr-P3HT with comparable molecular weight (Mn ~ 13.4

kDa, PDI~ 1.8).270 A sudden drop was observed for the device annealed at 280 ºC,

possibly signaling the onset of thermal decomposition (Td ~ 300 ºC).

The lack of mobility improvement with device annealing temperature did not

necessarily contradicted the WAXD study because effective π-stacking had been

indicated to be of principal importance in achieving high mobility,271 and the diffraction

peak due to π-stacking was not observed in the films of PHOT after annealing at various temperatures from 150-280 ºC.

The photovoltaic properties of PHOT were evaluated by using it as a p-type material in BHJ solar cells and using PC71BM as the electron acceptor. The standard BHJ device

structure (ITO/PEDOT:PSS/Active layer/LiF/Al) was used.

Absorption spectrum and morphology of the active layer 90 nm thick composed of

PHOT and PC71BM in 1:1 weight ratio were similar to those of the rr-P3HT:PC71BM

blend (Figure 3.13). TEM images of the active layers from actual devices showed that

donor and acceptor materials were uniformly mixed. However, commonly observed rr-

272 P3HT fibrilar nanostructures in rr-P3HT:PC71BM blend thin films were missing. This

may be attributed to the retarded ability to form self-assembled nanostructures of PHOT.

Figure 3.13 UV-Vis absorption spectrum and TEM image of a BHJ cell with an active

layer of PHOT:PC71BM blend in 1:1 weight ratio.

The typical J-V characteristics were shown in Figure 3.14. PCE results of BHJ cells

under different conditions were summarized in Table 3.3.

Table 3.3 J-V characteristics and PCE of BHJ cells under different conditions.

2 Fabrication condition Jsc (mA/cm )Voc (V) FF PCE (%) PCEave (%)

1200 rpm 2.18 0.65 0.38 0.53 0.44

1400 rpm 0.86 0.63 0.32 0.17 0.15

1200 rpm, SA 0.99 0.64 0.32 0.20 0.15

1400 rpm, SA 1.15 0.65 0.32 0.24 0.15

ODCB, Vacuum 2.05 0.41 0.31 0.26 0.22

ODCB, TA 1.7 0.60 0.34 0.34 0.30

ODCB/DIO, Vacuum 0 0 0 0 0

ODCB/DIO, TA 0.52 0.46 0.32 0.08 0.05

OCDB/CHCl3, Vacuum 0 0 0 0 0

OCDB/CHCl3, TA 0 0 0 0 0

a SA stands for the solvent annealing. Active layers were coated from chloroform solution.

Devices were thermal annealed at 100 oC for 5 min. TA stands for thermal annealing at

110 oC. DIO stands for 1,8-diiodooctane.

10 Dark ) 2 Under Illumination

Current Density (mA/cm -5 -1.0 -0.5 0.0 0.5 1.0 1.5 Voltage (V)

Figure 3.14 Current density-voltage curves of a BHJ cell with an active layer of

PHOT:PC71BM blend in 1:1 weight ratio.

The best device had a power conversion efficiency (PCE) of 0.53 %. However, the

2 fill factor (FF = 0.38) and short circuit current (Jsc = 2.18 mA/cm ) were low. The open-

circuit voltage (Voc = 0.65 V) was comparable to the highest (Voc = 0.66 V) achieved in

273 devices with rr-P3AT:PC71BM active layers in our hands and was consistent with the

low HOMO energy level of PHOT as depicted (Figure 3.14). This was because Voc relied on the difference between the HOMO of the p-type donor material and the LUMO of the n-type acceptor material. The poor photovoltaic performance of rr-P3HT:PC71BM was likely due to the poor hole charge mobility in PHOT, and insufficient percolation between donor and acceptor phases due to poor self-assembly of PHOT. Attempts were also made using PHOT as an n-type material with rr-P3HT as the p-type material, but the polymer/polymer BHJ devices showed negligible PCEs (Figure 3.15).

Figure 3.15 Current density-voltage curves of a BHJ cell with an active layer of

PHOT:P3HT blend in 1:1 weight ratio.

3.2.8 Attempted Synthesis of rr-Poly(3-(1,1-difluoroalkyl)thiophene)

The synthesis of rr-poly(3-(1,1-difluoroalkyl)thiophene) had also been attempted as outlined in Scheme 3.5. After retro-analysis, 3-(1,1-difluoroalkyl)thiophene was chosen

as the starting synthetic target. It was easy to make a 3-(1,1-difluoroalkyl)benzene from

phenyl alkyl ketone. However, it was impossible to generate the precursor via similar

chemistry from 1-(thiophen-3-yl)heptan-1-one. Many reactions had been attempted, and

no expected product was formed according to 19F NMR spectrum.

Scheme 3.5 Synthetic route of rr-poly(3-(1,1-difluoroalkyl)thiophene).

1-(thiophen-3-yl)heptan-1-one was then protected into dithiol form (Scheme 3.5).

After treating with reactive fluorinating reagents such as Deoxo-Fluor®, the expected

molecule could be detected as the only fluorinated product. As the result, it would be

possible to synthesize the rr-poly(3-(1,1-difluoroalkyl)thiophene) using a similar method

as the polymerization of rr-P3HT or rr-PHOT. However, compared to regioregular

poly(3-fluoro-alkylthiophene)s (rr-P3FAT), this polymer might still have a similar helical

237 conformation and limited interchain π-stacking due to the –CF2– adjacent. As the result, rr-poly(3-(1,1-difluoroalkyl)thiophene) should have low charge carrier mobility.

3.3 Conclusions

In summary, rr-PHOT polymer had been synthesized and investigated as a

representative member of regioregular head-to-tail poly(3-alkanoylthiophene)s. The

HOMO and LUMO levels of PHOT were lower than those of rr-P3HT by ~0.5 eV. Red-

shift of the UV-visible absorption maximum of PHOT in the solid state compared to the

solution confirmed effective backbone conjugation along the polymer chain. WAXD

indicated that PHOT had low crystallinity and particularly lacked order in the π-stacking

direction in the solid state. The higher electron affinity of PHOT than rr-P3HT suggested

that it could be potentially used as an n-type material. However, PHOT did not display

any electron-transport activity in FET devices. The average field-effect hole mobility of

PHOT was ~3×10-4 cm2/Vs. The lack of electron mobility and the relative low hole

mobility in PHOT could be likely, not in small part, attributable to the lack of π-stacking

order. Improvement of the crystallinity of PHOT was prevented by its very high glass

transition temperature in proximity to its decomposition temperature. Photovoltaic

devices with an active layer of 1:1 blend of PHOT and PC71BM had a power conversion efficiency of ~0.5%. The PCE was partially attributed to the low hole mobility of the

PHOT polymer. As the result, a conjugated polymer with robust structure which has good packing ability for high charge mobilities is crucial in OPV. This leads me to the next project which is the attempt to synthesis of graphene nanoribbons.

3.4.1 Materials

The anhydrous solvents, [1,3-bis(diphenylphosphino)propane]d nickel(II) chloride

(Ni(DPPP)Cl2), p-toluenesulfonic acid monohydrate, magnesium turnings, triethylamine

(TEA), N-bromosuccinimide (NBS), 2,2-dimethyl-1,3-propanediol, isopropylmagnesium chloride (2.0 M in THF), 3-cyanothiophene, Deoxo-Fluor® and trifluoroacetic acid (TFA)

were purchased from Alfa Aesar, Aldrich or Acros. They were used without purification.

Lithium chloride (LiCl) from Acros Organics was flame-dried under vacuum. [6,6]–

phenyl-C71 butyric acid methyl ester (PC71BM, >99.0%) was purchased from American

Dye Source (Quebec, Canada) and used as received. Poly(3,4-ethylene-

dioxythiophene):poly(styrene sulfonate) (PEDOT; Baytron P VP AI 4083) was purchased

from H. C. Stark (Newton, MA). Deuterated chloroform (CDCl3) and deuterated benzene

(C6D6) were purchased from Cambridge Isotope Laboratories. CDCl3 was kept over

molecular sieves, and C6D6 was stirred over a Na/K alloy, vacuum-transferred, and stored in a glove box. rr-P3HT (HT content = ~96%, Mn = 12,000, Mw = 14,400) was

synthesized following the GRIM procedure reported by McCullough et al.233

3.4.2 Measurements

1H and 13C NMR spectra were obtained on a Varian Mercury 300 MHz instrument.

Chemical shifts were referenced internally using solvent peaks.

Cyclic voltammetric 232 measurements were performed under a nitrogen atmosphere

in a gas-tight three-electrode electrochemical cell at 20 ºC and recorded on a BAS-100-W

n + - electrochemical apparatus from Bioanalytical systems, Inc. Bu4N PF6 (0.1 mol/L in acetonitrile) was used as the supporting electrolyte. A platinum wire was used as the counter electrode. A silver wire was used as a quasi-reference electrode and is calibrated

with ferrocene dissolved in the electrolyte solution. The electrodes were polished and

washed with deionized water and acetone before each test. The working electrode was a

glassy carbon rod with a plastic cladding, which render a flat area large enough for drop-

casting thin films. Polymer films were prepared by casting one drop of a dilute polymer

solution (~0.1 mg/mL) onto the working electrode and allowing it to dry. Each polymer

sample was measured at least three times, and the average data were reported. The

LUMO and HOMO energy levels were estimated from the onset potentials of the

reduction or oxidation waves, respectively.274 The SCE potential was taken to be 4.4 eV

below the vacuum energy level.251

Ultraviolet-visible (UV-Vis) spectra were acquired on a Hewlett-Packard 8453 diode- array spectrophotometer. Photoluminescence (PL) spectra were obtained on a HORIBA

Jobin Yvon NanoLog spectrofluorometer. Polymer films were prepared by spin-coating chloroform solutions on quartz wafers for the UV-Vis measurements and on silicon wafers for the PL measurements. A quartz cuvette was used for solution samples.

Size exclusion chromatography (SEC) was performed using a TOSOH EcoSEC

HLC-8320 GPC instrument equipped with a differential refractometer. Two Varian

Resipore columns (300 x 7.5 mm) were used in tandem as the stationary phase, and chloroform was used as the mobile phase at a flow rate of 0.5 mL/min at 45.0 ºC.

Calibrations were conducted with polystyrene standards from Varian.

The fabrication and characterization of thin film transistors were done as follows.

Thin film transistors with standard bottom-contact/bottom-gate device architecture was

fabricated and tested under inert atmospheres.275 Silicon substrates with 200–300 nm

thermal oxide and patterned gold source/drain electrodes were used. Channel widths and

lengths were 800–1000 μm and 20–100 μm, respectively. Surface of silicon dioxide was

treated with octyltrichlorosilane to minimize possible charge-trapping sites. The film of

PHOT was deposited by spin-coating of its chloroform or dichlorobenzene solutions onto

the substrates. Devices were then dried and annealed under inert conditions.

Thermogravimetric analysis (TGA) was carried out on a TGA Q500 V20.7

instrument (TA Instruments). The sample was heated at the rate of 5 oC/min under nitrogen protection. During the decomposition of PHOT, the sample expended dramatically and likely foamed. The escaped substance from the sample holder touched the interior housing of the sample holder. This always happened even when a minimal amount of 7 (5 mg) was used.

Wide angle X-ray diffraction (WAXD) was measured as follows. The samples were prepared by drop-casting the chloroform solution of PHOT (5 mg/mL) onto silver-plated aluminum foil. After evaporation under ambient conditions, the samples were annealed under nitrogen at the designated temperatures for 24 h. WAXD powder pattern was taken at room temperature with a Rigaku MultiFlex 2 kW tube-anode X-ray (Cu Kα radiation) generator coupled to a diffractometer. The samples were scanned at a 1o/min scanning

rate. The peak positions were calibrated using silicon powder in the high-angle region

(>15o) and silver behenate in the low-angle region (<15o). Background scattering was

subtracted from the sample pattern.

Output (Ids vs. Vds) and transfer (Ids vs. Vg) characteristics of the devices were

measured using a HP4145B semiconductor parameter analyzer (Yokogawa Hewlett-

Packard). Field-effect mobility was calculated from the standard equation for saturation

2 region in metal-oxide-semiconductor field-effect transistors: Ids = µ(W/2L)Ci(Vg – Vt) , where Ids is drain-source current, µ is field-effect mobility, W and L are the channel width and length, Ci is the capacitance per unit area of the gate insulator, Vg is the gate voltage,

and Vt is the threshold voltage.

Differential scanning calorimetry (DSC) was performed with a DSC Q2000

instrument (TA Instruments). The samples were sealed in a Hermetic Aluminum pan. The

process was carried out in a quadruple cycle under nitrogen: first heating up, subsequent

cooling down, second heating up and final cooling down, in the temperature range from

25 to 290 ºC. The rate of heating and cooling was controlled as 10 ºC/min. The glass

transition temperature was determined from the middle point of the transition in the curve

of the second cooling cycle.

Fabrication and Characterization of PV Cells were done as follows. ITO-coated glass substrates (10 Ω/square, Shanghai B. Tree Tech, P. R. China) were cleaned sequentially with acetone, deionized water and isopropyl alcohol in an ultrasonic bath, and completely dried in a vacuum oven at 120 °C. A 50 nm PEDOT:PSS layer was spin-coated on top of

ITO and dried at 150 °C for 10 min under vacuum. The chloroform solutions of poly(3- alkanoylthiophene)s (PHOT) and PC71BM (15 mg/mL and 30 mg/mL, respectively) were

prepared and mixed to make a solution with PHOT/PC71BM weight ratio of 1:1. The

blend solution was filtered with a 0.45 μm PTFE filter and then spin-coated on top of the

PEDOT:PSS layer in a glove box to make the active layer with a thickness of around 80

nm. The devices were annealed at (110 ± 10) °C on a hot plate for 5 min. The devices

were loaded in a thermal evaporator (BOC Edwards, 306), where a cathode consisting of

1.0 nm lithium floride and 80 nm aluminum was deposited through a shadow mask under

high vacuum (8×10-7 Torr). Five bulk heterojunction photovoltaic cells were produced

per substrate, with an active area of 4.0 mm2 each. The current density-voltage (J-V)

curves were measured using a HP4155A semiconductor parameter analyzer (Yokogawa

Hewlett-Packard) under laboratory ambient air condition. AM1.5 illumination at 100

mW/cm2 was provided by a filtered Xe lamp and calibrated by using an NREL-calibrated

Si diode.

3.4.3 Synthesis

Synthesis of 1-(thiophen-3-yl)heptan-1-one (5).

Magnesium turnings (13.5 g, 563 mmol) and anhydrous diethyl ether (50 mL) were charged into a pre-dried 500 mL three-neck flask under nitrogen. The flask was placed in a water bath at room temperature. Iodine (0.54 g, 2 mmol) was added into the above mixture. 1-Bromohexane (74 mL, 527 mmol) dissolved in anhydrous diethyl ether (120 mL) was added dropwise into the flask while the reaction was vigorously stirred. After the addition was finished, the reaction was allowed to continue for another 2 h. The solution was then filtered into another pre-dried three-neck flask under nitrogen. The three-neck flask was placed in an ice bath. 3-Cyanothiophene (50 g, 458 mmol) dissolved in anhydrous diethyl ether (60 mL) was added dropwise into the above solution while it was stirred. The reaction was allowed to continue for 24 h at room temperature. Then, the solution was poured to a 1000-mL beaker charged with ice (75 g), ammonium chloride

(30 g), and sodium chloride (30 g). The organic layer was decanted out, and the aqueous phase was extracted by diethyl ether (100 mL x 2). The combined organic phase was washed with a saturated sodium bicarbonate solution (150 mL) and a saturated saline solution (150 mL) and dried over anhydrous sodium sulfate. Distillation (85 ºC/85 milliTorr) gave 5 as a colorless liquid, which slowly crystallized over a long time at room

1 temperature (59.4 g, 66% yield). H NMR (300 MHz, CDCl3): δ 8.04 (dd, J = 3.0 and 1.2

Hz, 1 H), 7.56 (dd, J = 5.1 and 1.2 Hz, 1 H), 7.32 (dd, J = 5.1 and 3.0 Hz, 1 H), 2.874 (t,

J = 2.5 Hz, 2 H), 1.73 (m, 2 H), 1.34 (m, 6 H), 0.90 (t, J = 6.6 Hz, 3 H). 13C {1H} NMR

(75 MHz, CDCl3): δ 195.2, 142.7, 131.8, 127.3, 126.4, 40.2, 31.9, 29.2, 24.6, 22.7, 14.3.

Synthesis of 2-hexyl-2-(thiophen-3-yl)-5,5-dimethyl-1,3-dioxane (6).

Compound 5 (59.0 g, 301 mmol), 2,2-dimethylpropane-1,3-diol (45 g, 431 mmol),

benzene (150 mL) and p-toluenesulfonic acid monohydrate (2.2 g, 13 mmol) were added

into a 500 mL three-neck flask with a Dean-Stark apparatus. The mixture was refluxed

for 24 h, during which time water was collected in the Dean-Stark apparatus and removed.

Then, benzene was removed under vacuum, and the resulting oil was dissolved in diethyl

ether (100 mL) and transferred into a separatory funnel. The ether solution was washed

successively with a saturated sodium bicarbonate solution (150 mL) and a saturated

saline solution (150 mL) and dried over anhydrous sodium sulfate. Distillation (68 ºC/50

1 milliTorr) gave 6 as a colorless oil (59.4 g, 70% yield). H NMR (300 MHz, CDCl3): δ

7.30 (dd, J = 5.1 and 3.0 Hz, 1 H), 7.19 (dd, J = 3.0 and 1.2 Hz, 1 H), 6.99 (dd, J = 5.1 and 1.2 Hz, 1 H), 3.47 (AB-spin system, 4 H), 1.74 (m, J = 2.5 Hz, 2 H), 1.36 (m, 2 H),

13 1 1.24 (m, 9H), 0.85 (t, J = 6.6 Hz, 3 H), 0.61 (s, 3 H). C { H} NMR (75 MHz, CDCl3): δ

142.6, 127.4, 125.7, 123.3, 100.9, 72.0, 44.3, 32.0, 30.2, 29.6, 23.2, 22.8, 22.1, and 14.3.

Synthesis of 2-(2,5-dibromothiophen-3-yl)-2-hexyl-5,5-dimethyl-1,3-dioxane (7).

Compound 6 (29.3 g, 104 mmol), NBS (22.2 g, 125 mmol), and DMF (150 mL) were added into a 500-mL three-neck flask under nitrogen in a water bath. After the mixture was stirred for 72 h at room temperature, diethyl ether (200 mL) was added into the mixture. The organic phase was washed successively with a saturated sodium bicarbonate

solution (150 mL) and a saturated saline solution (150 mL) and dried over anhydrous

sodium sulfate. Distillation (175 ºC/50 milliTorr) gave 7 as a light yellow oil (40.6 g,

1 88.9%). H NMR (300 MHz, CDCl3): δ 6.84 (s, 1 H), 3.44 (AB-spin system, 4 H), 1.74

(m, 2 H), 1.40 (m, 2 H), 1.24 (m, 9 H), 0.87 (t, J = 6.9 Hz, 3 H), 0.67 (s, 3 H). 13C {1H}

NMR (75 MHz, CDCl3): δ 140.8, 131.9, 111.3, 109.1, 100.7, 72.2, 42.5, 32.0, 30.0, 29.5,

23.0, 22.8, 22.1, 14.3.

Synthesis of 2-(5-bromothiophen-3-yl)-2-hexyl-5,5-dimethyl-1,3-dioxane (8b).

Compound 5 (2.13 g, 7.5 mmol) and NBS (1.61 g, 9.0 mmol) were successively added into a pre-dried 100 mL three-neck flask under nitrogen. The flask was placed in a water/ice bath. The mixture was dissolved in DMF (30 mL) and stirred for 8 h at 0 ºC. A saturated sodium dicarbonate aqueous solution (50 mL) was added into the mixture. The aqueous solution was extracted with diethyl ether (20 mL x 3). The organic phase was combined and washed with saturated saline solutions (50 mL x 2) and dried over anhydrous sodium sulfate. After ether was removed by distillation, the resulting crude product was distilled under dynamic vacuum to give 8b as a colorless oil (2.32 g, 85%

1 yield). H NMR (300 MHz, CDCl3): δ 7.08 (d, J = 1.5 Hz, 1 H), 6.94 (d, J = 1.8 Hz, 1 H),

3.44 (AB spins, 4 H), 1.70 (m, 2 H), 1.33 (m, 2 H), 1.22 (m, 9 H), 0.85 (t, J = 6.6 Hz, 3

H), 0.63 (s, 3 H).

Reaction of 7 and i-PrMgCl.

Compound 7 (0.540 g, 1.23 mmol) was loaded into a 25-mL Schlenk flask in a glove box and was dissolved in THF (10 mL). The flask was removed from the glove box and placed into a bath at a certain temperature from -40 ºC to room temperature. i-PrMgCl

(2.0 M solution in THF, 0.612 mL, 1.22 mmol) was injected into the flask with an air-

tight syringe under a stream of nitrogen. The reaction was kept at the desired temperature for several hours. Aliquots of the reaction mixture were withdrawn and quenched with methanol. After the volatile components were evaporated under reduced vacuum, the nonvolatile residual was analyzed by 1H NMR spectrum. The ratio of 8b and its

regioisomer 2-(2-bromothiophen-3-yl)-2-hexyl-5,5-dimethyl- 1,3-dioxane (8a) was

determined by the areas of the peaks at δ 7.08 and 6.93 ppm for 8b and 7.24 and 6.87

ppm for 8a. The assignment of the isomers was made based on the magnitude of the

coupling constants (1.5 Hz and 5.7 Hz, respectively). The same method for product

analysis was used when the reaction was performed in the presence of LiCl and 1,4-

dioxane.

General procedure of polymerization and isolation of polymer 10.

All glass apparatus were dried in an oven prior to use. Air-tight syringes were used for addition of reagents and sampling the reaction mixture. All operations were carried out under nitrogen protection. Flamed dried LiCl (0.094 g, 2.22 mmol), anhydrous 1,4- dioxane (2 mL), and anhydrous THF (4 mL) were loaded into a 25 mL Schlenk flask in the glove box. I-PrMgCl (2.0 M solution in THF, 2.1 mL, 4.15 mmol) was then added via a syringe. After the flask was stirred for 3 h in an ice/water bath, a solution of 7 (1.830 g,

4.16 mmol) in 6 mL THF was added. The reaction was stirred for another 7 h in ice/water bath. Next, the solvent was removed under vacuum. The solid residual was dissolved in

20 mL THF at 0 ºC. Then Ni(DPPP)Cl2 (0.032 g) and THF (5 mL) was added

successively into the above flask. The mixture was stirred at 0 ºC for 2 h and at 40 ºC for

1 day. Methanol was then added to quench the reaction. The solvent was removed the

solid residual was extracted with a Soxhlet extractor with methanol and chloroform. The

product 10 (0.95 g, 81.7%) was collected from the chloroform extraction after removal of

1 the solvent. H NMR (300 MHz, CDCl3): δ 7.18 (1 H), 3.40 (4 H), 1.94 (2 H), 1.44 (2 H),

13 1 1.25 (9 H), 0.87 (3 H), 0.65 (3 H). C { H} NMR (75 MHz, CDCl3): δ 137.8, 133.4,

131.8, 101.2, 72.1, 43.5, 32.1, 30.1, 29.8, 23.2, 22.9, 22.3, 14.3. SEC against polystyrene

standards: Mn = 9,090, Mw/Mn = 1.70.

In the experiments where the progress of the polymerization was monitored, an excess of 7 (1.1 eq vs i-PrMgCl) was added in order to use it as an internal reference for

quantification. Small aliquots of the reaction mixture was withdrawn via a syringe

periodically and quenched with methanol. After evaporation of methanol, the solid

residual was extracted by CDCl3, and the conversions were determined according to the

1H NMR spectrum integrations of the aromatic resonances belonging to 10, 8a, and 7.

The same samples after dilution were used for SEC analysis.

Synthesis of polymer 11 (PHOT).

Polymer 10 (950 mg) was dissolved in chloroform (15 mL) and 1,4-dioxane (4 mL).

TFA (3 mL) and water (6 mL) was added into the solution. The mixture was vigorously

stirred at 40 ºC for 24 h. The volatile components were completely removed with a rotary

evaporator. The solid residual was subjected to the above process again. If the reaction

scale was larger, a third repetition might be necessary. The final solid residual was

extracted with a Soxhlet extractor with methanol, hexanes and chloroform. The product

(530 mg, 80%) was collected from the chloroform extracted after removal of the solvent.

1 H NMR (300 MHz, CDCl3): δ 7.91 (s, 1 H), 2.96 (t, 2 H), 1.76 (m, 2 H), 1.34 (m, 6 H),

13 0.90 (t, 3 H). C NMR (300 MHz, CDCl3): δ 197.2, 142.1, 136.1, 133.2, 131.5, 42.4,

31.9, 29.1, 24.5, 22.7, 14.2. SEC against polystyrene standards: Mn = 7,939, Mw/Mn =

1.74. Calcd for C452H577BrO41S41 (M = 8060.96): C, 67.25; H, 7.21%. Found: C, 66.87;

H, 7.37%.

Synthesis of 2-hexyl-2-(thiophen-3-yl)-1,3-dithiolane (12).

5 (9.4 g, 48.0 mmol), benzene (150 mL), ethane-1,2-dithiol (5.1 g, 54.1 mmol),

boron trifluoride diethyl etherate (1.5 g, 10.6 mmol) and p-toluenesulfonic acid

monohydrate (1.1 g, 6.5 mmol) were added into a 300 mL three-neck flask with a Dean-

Stark apparatus. The mixture was refluxed for 24 h, during which time water was

collected in the Dean-Stark apparatus and removed. Then, benzene was removed under

vacuum, and the resulting oil was dissolved in diethyl ether (100 mL) and transferred into

a separatory funnel. The ether solution was washed successively with a saturated sodium

bicarbonate solution (120 mL) and a saturated saline solution (120 mL) and dried over

anhydrous sodium sulfate. Flash column chromatography (SiO2, 7:1 hexanes:

dichloromethane) gave 12 as a colorless oil (11.9 g, 91% yield). 1H NMR (300 MHz,

CDCl3): δ 7.38 (dd, J = 5.1 and 3.0 Hz, 1 H), 7.28 (dd, J = 3.0 and 1.2 Hz, 1 H), 6.14 (dd,

J = 5.1 and 1.2 Hz, 1 H), 3.44 (m, 4 H), 2.32 (t, J = 6.0 Hz, 2 H), 1.29 (m, 8 H), 0.86 (t, J

13 1 = 6.0 Hz, 3 H). C { H} NMR (75 MHz, CDCl3): δ 147.3, 127.0, 126.0, 122.2, 70.2,

44.9, 39.2 31.6, 29.3, 27.8, 22.5 and 14.0.

Synthesis of 1-(2,5-dibromothiophen-3-yl)heptan-1-one (13).

7 (2.2 g, 5.0 mmol), DI water (3 mL), TFA (0.5 mL) and chloroform (6 mL) were

added into a 100-mL single-neck flask under nitrogen in a water bath. After the mixture

was stirred for 24 h at 50 oC, diethyl ether (60 mL) was added into the mixture. The

organic phase was washed successively with a saturated sodium bicarbonate solution (50 mL) and a saturated saline solution (60 mL) and dried over anhydrous sodium sulfate.

Flash column chromatography (SiO2, 7:1 hexanes: dichloromethane) gave 13 as a

1 colorless oil (1.3 g, 74%). H NMR (300 MHz, CDCl3): δ 7.31(s, 1 H), 2.88 (t, J = 4.8 Hz,

2 H), 1.69 (m, 2 H), 1.33 (m, 4 H), 0.90 (t, J = 6.0 Hz, 3 H).

Synthesis of 2-(2,5-dibromothiophen-3-yl)-2-hexyl-1,3-dithiolane (14).

13 (7.0 g, 19.8 mmol), benzene (150 mL), ethane-1,2-dithiol (5.6 g, 59.4 mmol),

boron trifluoride diethyl etherate (1.5 g, 10.6 mmol) and p-toluenesulfonic acid

monohydrate (1.1 g, 6.5 mmol) were added into a 300 mL three-neck flask with a Dean-

Stark apparatus. The mixture was refluxed for 24 h, during which time water was

collected in the Dean-Stark apparatus and removed. Then, benzene was removed under

vacuum, and the resulting oil was dissolved in diethyl ether (100 mL) and transferred into

a separatory funnel. The ether solution was washed successively with a saturated sodium

bicarbonate solution (120 mL) and a saturated saline solution (120 mL) and dried over anhydrous sodium sulfate. Flash column chromatography (SiO2, 6:1 hexanes:

dichloromethane) gave 14 as a colorless oil (5.1 g, 60% yield). 1H NMR (300 MHz,

CDCl3): δ 7.07 (s, 1 H), 3.35 (m, 4 H), 2.24 (t, J = 6.0 Hz, 2 H), 1.24 (m, 8 H), 0.85 (t, J

= 6.0 Hz, 3 H).

Synthesis of 3-(1,1-difluoroheptyl)thiophene (15).

12 (0.46 g, 1.7 mmol), Deoxo-Fluor® (0.65 g, 2.9 mmol), dichloromethane (4 mL) and anhydrous ethanol (40 μL) were added into a 100 mL BOLA flask. The misture was stirred at room temperature for 8 hours and then was warmed to 85 oC for 3 hours. The

reaction was quenched with saturated aqueous sodium bicarbonate (10 mL). The mixture

was extracted with diethyl ether (15 mL), washed with a saturated saline solution (20 mL)

and dried over anhydrous sodium sulfate. Flash column chromatography (SiO2, 8:1

hexanes: dichloromethane) gave 15 as a colorless oil (0.08 g, 22% yield). A good 1H

19 19 NMR spectrum was not got, but its F NMR spectrum was clean. F NMR (CDCl3) : δ

−90.0 (t, J = 16.3 Hz, 2F). This result is consistent with the literature value of (1,1- difluoroalkyl)- thiophenes.276

CHAPTER IV

ATTEMPT OF SYNTHESIS OF GRAPHENE NANORIBBONS

4.1.1 Physical Properties of Graphene and Graphene Nanoribbons

Since the discovery by Novoselov and Geim,277 graphene has attracted significant

attention as a novel two-dimensional (2-D) carbon allotrope. Its unique intrinsic

properties are different from the conventional three-dimensional (3-D) materials. The

widespread fascination about this 2-D atomic thin sheet arises from its extraordinary

charge carrier mobility, mechanical strength and elasticity.278-280 However, due to the lack

of a bandgap, graphene has limited applications to optical and electronic areas.281

Graphene nanoribbon (GNR), defined as narrow strips of graphene with widths below 10 nm, has finite bandgaps and is a semiconductor.282,283 The extraordinary charge mobility and the small size in comparison to the scaling limit of the complementary metal–oxide–semiconductor (CMOS) devices284,285 make the semiconducting GNRs the

suitable materials for microelectronic and optoelectronic applications.249,286

However, to use GNRs for building functional devices, the bandgap must be

precisely controlled. The bandgap of GNR is a keen topic of theoretical investigations in

the past decade. A number of studies have demonstrated that the bandgap of GNRs is depending on not only the ribbon width but also the topography and edge structure.

GNRs can be either in zig-zag or arm-chair configuration (Scheme 4.1). GNRs can be

divided into three subclasses in arm-chair configuration based on the topological

connection of the conjugated π-electron system.287,288 The bandgap gradually decreases

with the increase of ribbon width within each subclass,174,175 and the change in bandgap

is abrupt and substantial across the topological subclasses. For example, poly(peri- naphthalene) (PPN) belongs to the subclass with the lowest bandgap, and poly(peri- anthracene) (PPA) belongs to the subclass with the highest bandgap (Scheme 4.1).

Scheme 4.1 Arm-chair GNRs (PPN and PPA) and zig-zag GNR.

Depending on the specific theoretical method used, the bandgaps of PPN and PPA

are calculated to be 0.3~1.7 and 1.5~3.7 eV,289-293 respectively. Regardless of the method

used, all theoretical studies predicted a difference of more than 1 eV in the bandgaps, with the narrower PPN having a lower bandgap than PPA. For various edge structures in between the arm-chair and zigzag edge (Scheme 4.1), they have been expected to generate different bandgap characteristics.291 In short, these theoretical studies

demonstrate that atomic precision of the structure of GNRs must be achieved to control

the bandgap. Recently, Mullen and co-workers have synthesized atomic precisely PPA292 ribbon and use angle-resolved photoelectron spectroscopy measurement to reveal its bandgap of 2.3 eV.294

100

4.1.2 Successes in Fabrication and Synthesis of GNRs

A number of ways to make GNRs have been reported in the literature by using

chemical,295-297 sono-chemical,298,299 lithographic methods,300-302 and through the

unzipping of carbon nanotubes (CNTs).303-308 These methods can all be categorized as

top-down approaches. The GNRs synthesized from these methods have been proved to

exhibit much higher charge carrier mobility than conjugated polymers or small molecules,

which is around 40 to 1, 500 cm2/Vs (Figure 4.1).299,307 The on/off ratio of FET devices

from these ribbons could exceed 104.308-310

Figure 4.1 Charge carrier mobility of different materials.

Recently, researchers at IBM and the University of California-Riverside have succeeded in making the narrowest ever GNR from top-down method in 2012.311 Each nanoribbon has a width of just 10 nm, which is theoretically predicted to have a bandgap of about 0.2 eV. However, up till now, the reliable production of GNRs smaller than 10 nm with chemical precision control remains a considerable challenge. More importantly, none of the above top-down methods have any control over the topography and edge structure. In addition to the poor structural control, GNRs made through these ways are

101

difficult or costly to scale up. As a result, chemical synthesis through bottom-up strategy

is of the most interest (Scheme 4.2).

Scheme 4.2 Bottom-up synthesis of GNRs.

The accurate bottom-up synthesis of GNRs has recently been achieved by Mullen and co-workers in 2010.312,313 They reported a simple way for the production of atomic precise GNRs with different topologies and widths, using surface-assisted coupling

approaches. Polymer precursors are synthesized into linear or kinked polyphenylenes and undergo subsequent cyclodehydrogenation to afford the corresponding ribbon.314,315 The width and topology are defined by the structure of the starting molecules with specific design. Take the synthesis of PPA for example (Scheme 4.3). 10,10’-dibromo-9,9’- bianthryl molecules are first sublimed and deposited onto a gold surface to remove the halogen atoms, yielding a biradical species. After thermal activation, the biradical species diffuse across the gold surface and undergo radical addition reactions to form linear polymer chains. These polyaromatic precursors are conformationally restricted on the gold surface. As a result, the intramolecular cyclodehydrogenation reaction becomes much more selective in further thermal activation step. A surface-assisted fully coupled

PPA is established with evidences from scanning tunneling microscope (STM) and

Raman spectrum shown in Figure 4.2.

102

Scheme 4.3 Proposed mechanism of surface-assisted GNR synthesis.

A B

Figure 4.2 A. Overview STM image after cyclodehydrogenation, showing straight N=7

GNRs. B. Raman spectrum of straight N=7 ribbons. Reproduced with permission from

However, because the key steps in the synthesis are surface-assisted, the quantity of so-formed GNRs is limited that only one layer of GNRs on gold surface can be made.

Further physical characterizations and device fabrication are not possible. Therefore, solution synthesis appears to be the only way to meet the strict requirement of atomic precision and is potentially scalable for practical applications.

In fact, successful bottom-up synthesis of graphene sheets or giant polycyclic aromatic hydrocarbons (PAHs) has paved ways for the synthesis of GNRs. Remarkable discotic PAHs have been synthesized using standard reactions in solution.316-318 A

103

solution-phase synthesis that allows access of atomically defined GNRs in substantial

quantity is urgent.

4.1.3 Development of Solution Synthesis of PAHs and Attempted Synthesis of GNRs

Early synthetic attempts of small graphene molecules date back to the pioneering

works by Clar and Scholl at the beginning of last century.319-326 Subsequently, more

efficient and milder reaction conditions as well as characterization techniques have been

developed, leading to the expansion of graphene molecules.327-335 The most investigated

molecules of this type are Hexa-peri-hexabenzocoronene (HBC) and its derivatives, a

discotic PAH consisting of 42 carbon atoms (Scheme 4.4). A number of experimental

protocols have been developed for the oxidative dehydrogenation synthesis of HBCs. Of

the many oxidizing system, Scholl reaction (Lewis acids and oxidants) using FeCl3 and

+ DDQ/H or Sc(OTf)3 (DDQ= 2,3-dichloro-5,6- dicyanobenzoquinone) appear to have

met the most success.318,336 Functionalized HBC derivatives could be synthesized in

solution with high yields (>90%) as shown in Scheme 4.4 and have been employed as

advanced materials in varies applications from batteries and transistors to chemical sensors and solar cells.337-341

Scheme 4.4 Two synthetic routes toward the HBC core molecule. (Functional groups

around the HBC core are omitted for clarity.)

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The protocol for the synthesis of HBC or graphene molecules has been applied to build GNRs from linear polyphenylene precursors. Mullen and co-workers develop a route which involves first synthesis of polyaromatic precursors and then stepwise dehydrogenative cyclization of these polyaromatic precursors via the Scholl reaction

(Scheme 4.5). The aromatic groups bear side groups R (alkyl, alkoxy and etc.), which should engender solubility in common solvents and subsequently solution processability.

Although other routes can be conceived, Mullen’s route appears the most feasible. One of the critical issue is to achieve high molecular weight precursor polymers, which could eventually deliver GNRs with a large aspect ratio.342

Scheme 4.5 GNR from polyaromatic precursor.

A type of linear GNRs is achieved by oxidative cyclo-dehydrogenation of polymer

containing hexaphenylbenzene subunits via Scholl reaction as shown in Scheme 4.6.297

The formation of the GNRs is confirmed by transmission electron microscopy (TEM) and

STM (Figure 4.3). However, the 1H NMR spectrum of the ribbon have aromatic peaks in

a broad envelop of chemical shifts (δ 7.0~7.9 ppm), indicating the incompletely fusion of

phenyl rings. Estimation of cyclodehydrogenation efficiency from MALDI-TOF reveals

an average number of 94%.

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Scheme 4.6 Synthesis of linear GNRs from hexaphenylbenzene type prepolymer.

Despite the progress achieved in the bottom-up synthesis of graphene macromolecules and GNRs by solution chemical methods, several drawbacks still need to be resolved. Intramolecular oxidative cyclo-dehydrogenations under Scholl reaction conditions suffer from intrinsic problems of reactivity and regioselectivity, and the mechanism is still not clear yet.343-345 For instance, the Scholl reaction of o-terphenyl

gives a mixture of PAHs of various sizes as shown in Scheme 4.7 due to unfavorable

intermolecular reactions.346

106

Scheme 4.7 Intermolecular side reactions of Scholl reaction of o-terphenyl.

Unlike atomic precise synthesis of PPA, the polyaromatic precursors are conformationally restricted on the gold surface and the intramolecular cyclodehydrogenation is much more selective (Scheme 4.3). To ensure 100% conversion of the intramolecular ring fusion product in solution, it is necessary to introduce some alkyl or alkoxy groups to block the active sites to suppress the intermolecular couplings (Scheme

4.8).343 The presence of alkoxy groups at the o,p- positions also accelerates

intramolecular ring fusion because of their electron-donating effect.343,346

Scheme 4.8 100% conversion via Scholl reaction of substituted o-terphenyl.

Furthermore, intramolecular side reactions should be taken into consideration as well.

Polyphenylene precursor may not be graphenized due to incomplete reactions,

unexpected migrations/rearrangements of precursors, or lack of regioselectivity.347-349

Different substitution and steric hindrance of the precursors also make the intramolecular

Scholl reaction less predictable.342 Mullen group did try to synthesize the PPN derivative

through a polyperylene polymer via Scholl reaction in 2002 (Scheme 4.9).350 By the careful design of the repeating units, the intermolecular side reactions are mostly suppressed by introduction of the bulky phenoloxy groups. However, only partial ring

107 fusions are observed with the formation of quarterylene units, which is deduced simply by UV-Vis absorption change.

Scheme 4.9 Attempted synthesis of PPN derivative.

It is not realized until their work toward the synthesis of terylene in 2006 that reveals the existence of another intramolecular side reaction (Scheme 4.10).351 Starting from naphthyl-perylene, a five-member ring is formed preferentially under the exact same

Scholl condition as in Scheme 4.9. In short, both intramolecular and intermolecular side reactions should be taken into account.

Scheme 4.10 Intramolecular side reaction under Scholl reaction and the formation of a five-member ring.

To sum up, in view of the need for atomic precise GNRs of scalable quantities, it is highly desirable to further develop a solution synthesis of GNRs. This led to my attempt to make well-defined GNRs, especially the PPN derivative. The strategy is divided into two parts. First, the synthesis of PPN is attempted via intermolecular Scholl reaction directly with equivalent reactive site strategy but failed due to intermolecular side reactions. Afterwards, oligo-naphthalenes precursor with alkoxy groups capping the 2, 3,

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6 and 7- positions have been synthesized via Suzuki polycondensation with the help of a highly reactive catalyst. The purpose of capping the 2, 3, 6 and 7-positions is mainly to prevent both intra-molecular and intermolecular side reactions and promote ring fusion.

Different Scholl reactions have been tested on these oligomers for the intramolecular ring fusion to form the final GNRs. Model precursors with anthracene units have also been prepared and tested under various Scholl reactions.

4.2.1 Attempted Synthesis of PPN via Intermolecular Scholl Reaction

Compound 14 was chosen as the monomer (Scheme 4.11). 12 was synthesized using a modified route according to literature.352 The synthesis of PPN was first tried using 14 as monomer through Scholl reactions directly. By blocking the unwanted reactive sites using alkoxy substituents, the remaining reactive sites in the D2h symmetric 2, 3, 6, and 7- tetrasubstituted naphthalenes are all equivalent.

Br OH Br OR RO OR i ii 85% 87% Br OH Br OR RO OR 12 13 14 i. C6H13Br, K2CO3, KI, Methyl Ethyl Ketone, Reflux, 24 h ii. C6H13ONa, DMF, CuBr, Reflux, 24 h Scheme 4.11 Synthesis of monomer 14.

+ After treating with Scholl reagents like DDQ/H or Sc(OTf)3 in dichloromethane at room temperature or 0 oC, the crude product contained much less aromatic resonances compared to alkyl hydrogen on side chains according to 1H NMR spectrum. This indicated the consumption of monomers and the formation of highly conjugated macromolecules. However, MALDI-TOF spectra did not support the formation of the

109

expected GNRs, but usually with a very complicated spectrum. It should be pointed out

that the use of 14 to form the all capped precursors did not absolutely exclude the

undesirable intermolecular Scholl reaction in Scheme 4.12. This intermolecular reaction

can be expected to be much sensitive to the steric congestion imposed by the 2, 3, 6 and

7-substituents and the adjacent naphthyl group. Structural errors caused by such

intermolecular Scholl reactions can be expected to be rare, if exist at all, due to the steric

and entropic factors. Practically, highly diluted condition might help to suppress this

intermolecular side reaction, but experimental results with 100 times dilution did not

show obvious improvement.

Scheme 4.12 Unfavorable intermolecular reaction.

4.2.2 Attempted Synthesis of PPN via Intramolecular Scholl Reaction

The precursors 15 could be prepared from 14 in about 78% yield (Scheme 4.13).

Scheme 4.13 Synthesis of monomer precursors 15.

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According to Mullen’s route, model compounds of polyaromatic precursors are prepared. Formation of graphene molecules are demonstrated with high efficiency and selectivity via Scholl reaction.297,353,354 Therefore, model compounds 16 and 17 had been prepared from 15 (Scheme 4.14) from Suzuki coupling reactions. As discussed in the previous sections, it was important to block all the possible reactive 2, 3, 6, 7- positions of naphthalene units to suppress both intramolecular and intermolecular side reactions.

Therefore, model compounds 16 and 17 could not form the respective graphene molecules via various Scholl reactions.

(HO)2B B(OH)2 15 OR RO OR RO OR OR RO OR OR OR RO OR OR

+ +

RO OR RO OR OR RO OR OR OR RO OR OR 16 17

R= -C6H13

Scheme 4.14 Synthesis of model compounds 16 and 17.

As the result, in order to further prevent all known side reactions as discussed above, the synthesis of PPN derivative have to be divided into two stages. A polymer precursor

18 should be first synthesized as shown in Scheme 4.15 in order to suppress both intramolecular and intermolecular side reactions. Additionally, all these alkoxy sites would promote the final ring fusion reaction for that they are regarded as activating groups of o,p- positions in Scholl reactions, even the resulting PAHs are steric hindered.343

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Scheme 4.15 Synthesis of polymer precursor 18. X and Y are functional groups.

Table 4.1 Failed polymerizations. (Isomers of monomers are omitted for clarity.)

Monomers Catalysts Solvents Temp. and Time

o Ni(dppp)Cl2 THF 0 C to reflux Ni(dppe)Cl2 THF/Dioxane 2 days

o Ni(dppp)Cl2 THF 0 C to reflux PdCl2 Dioxane 2 days NiBr2 DME

Ni(COD)2 THF r.t. to reflux DMF 3 days

o Pd(PPh3)4 THF 60 C to reflux Pd(OAc)2 DME 3 days Pd2(DBA)3 Toluene Benzene

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However, the synthesis of polymer 18 was not easy, even the oligomers were hard to

make. A number of polymerizations with various monomers, catalysts, solvents,

temperature and reaction time were tested as summarized in Table 4.1. For all these

reactions, 1H NMR spectrum showed negligible formation of oligomers. MALDI-TOF

analysis confirmed that only very limited amount of dimer and trimer were formed.

Recently, Yokozawa group has reported the synthesis of polyphenylenes via Suzuki

coupling chain-growth polycondensation using a highly reactive initiator PhPdP(t-Bu)3Br as described in Chapter I.80 Different from other commonly used catalysts such as

Pd(PPh3)4, only one bulky phosphine ligand is bonding to the metal center due to steric

hindrance. The Pd center of this special catalyst has 14 electrons, 2 electrons fewer than

using other ligands. It renders this special initiator less stable and much more reactive.

19 were synthesized from 15 after lithiation, substitution with B(OMe)3 and

hydrolysis steps and purified by recrystallization. Upon treating 19 with PhPdP(t-Bu)3Br and additives (Scheme 4.16), formation of oligomer precursors 18 was observed. 1H

NMR spectrum (Figure 4.4), GPC and MALDI-TOF spectrum (Figure 4.5) all support the formation of oligomers.

Scheme 4.16 Synthesis of polymer precursor using PhPdP(t-Bu)3Br as initiator.

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Figure 4.4 1H NMR spectrum of 18.

Figure 4.5 MALDI-TOF spectrum and GPC trace of 18.

Although oligomers 18 were successfully formed, the 30% overall conversion was till low. One of the main reasons was that monomers 19 were unreactive and not stable under the reaction condition. It was found that about 50% monomers would be decomposed under the same condition except the addition of initiator. The change of boronic acid groups of 19 into boronic ester groups had been tested to increase the

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stability of the monomers. However, these molecules were still not stable and apparently

less reactive, resulting in a similar or lower overall yield. After column separation,

molecules smaller than trimer were removed. The highest molecular weight of the

oligomers could be up to 9,000 from MALDI-TOF analysis. As discussed above, the

intramolecular and intermolecular side reactions would be essentially suppressed for

these oligomers.

Further ring fusion reactions were tested based on these oligomers. However, no

expected ribbons were identified after various Scholl reactions or other special conditions

(see experimental section). Usually the residue was too complicated to interpret or the

starting oligo-naphthalenes were still intact after the reaction. Mullen group has proved

that the energy bandgap of rylenes decreases with increasing naphthalene units, and it

obeyed the one-particle-in-the-box behavior (i.e., E∝ L-2, where E is the energy gap and L

is the length of the molecule in the longest dimension).355 Hexarylene with six repeating

naphthalene units already showed a UV-Vis maximum absorption peak around 900 nm.

However, no absorption peaks corresponding to the formation of rylene units were

observed in the NIR region. 1H NMR and MALDI-TOF spectra did not support any expected GNR.

4.2.3 Attempted Synthesis of PPA Model Compounds

Although very few researches had been devoted to the synthesis of GNRs with anthracene units to form PPA ribbons, reactions towards bisanthene and teranthene using solution chemistry were reported.356,357 However, the synthetic routes involved are quite

115 complicated which could be more than twenty steps. Hence, it will be interesting to develop the synthesis of these ribbons via simple chemistry like Scholl reactions.

Several model compounds 20-23 with anthracene units and its boron/nitrogen derivatives were synthesized as shown in Scheme 4.17. It should be noticed that the 9,

10- positions of anthracene unit were quite sensitive to Scholl condition. For example, directly treating 9,10'-bianthracene with Scholl reagents led to no ring fusion to PAH of bisanthene, but 10,10'-dichloro-9,9'-bianthracene as a clean product (See experimental section). As the result, these active sites should be blocked in advance.

Scheme 4.17 Synthesis of different model precursors.

The above model precursors were synthesized via nucleophilic substitution or metal- catalyzed coupling reactions. Subsequently, these molecules were subjected to various

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+ Scholl conditions like DDQ/H , DDQ/Sc(OTf)3, FeCl3/MeNO2/CH2Cl2, AlCl3/CuCl2 or special conditions like I2/toluene, K/THF and NaO(t-Bu)/DBN/ethanolamine. However,

no expected fully coupled products were found after all these reactions. This might be

attributed to the low reactivity of the molecules themselves and side reactions caused by

the uncapped 2, 3, 6, 7- positions of the anthracene subunits. As pointed out by King and

co-workers in Scheme 4.18, a rearrangement reaction could also complicate the outcome

of the Scholl reactions.358 Additionally, although the PPA GNR made by Mullen and co-

workers with anthracene units as building blocks is stable in atmosphere, small graphene

molecules like teranthene is proved to be unstable and have a half-life only about one

day.356

+ +

Scheme 4.18 Carbocation rearrangement of anthracenes.

Recently, Yamaguchi and co-workers reported the synthesis of a boron-doped nanographene (Scheme 4.19).359 The precursor has a similar backbone structure with 20. The

9 positions of anthracene units are protected by the bulky mesityl groups at 1,8- positions to suppress the chlorination from FeCl3. 4,5- positions are activated by electron-donating groups at para- positions. The α-H of diborapentacene also has higher reactivity

compared to that of diboraanthracene in 20. These two improvements lead to the yield of

the air-stable boron-doped nanographene as a deep purple solid, which has good

solubility in chlorinated solvents. Its structure has been characterized by NMR

spectroscopy (Figure 4.6), mass spectrometry and X-ray crystallography.

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Scheme 4.19 Synthesis of boron-doped nanographene.

4.3 Conclusions

Several precursors to GNRs with naphthalene and anthracene units were synthesized.

However, the solution ring fusion chemistry was not selective. Without capping all the 2,

3, 6, and 7- positions, intermolecular couplings would inevitably happen, together with the other possible intramolecular side reactions like carbocation rearrangements and etc.

Eventually, the all sites blocked oligo-naphthalenes were prepared to exclude both

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intramolecular and intermolecular side reactions. These oligomers could only be made

via Suzuki polycondensation with a highly reactive initiator. The low overall yield was

because of the instability of the monomers under basic condition. However, upon treating

with different Scholl reagents, no expected ribbons were formed from these oligo-

naphthalenes. This might be explained from the similar phenomenon that alkoxy-

functionalized HBCs from the corresponding hexaphenylbenzene were not obtainable.

Oxidative cyclization using Scholl reactions often led to variable results with byproducts

due to quinine formation,360 alkoxy migration361 and indenofluorene formation.362,363

4.4.1 Materials

The anhydrous solvents, [1,3-bis(diphenylphosphino)propane]nickel(II) chloride

(Ni(DPPP)Cl2), PdCl2, Pd(OAc)2, Ni(COD)2, hexanol, sodium, N-bromosuccinimide

(NBS), tri-t-butyl phosphine, 1-bromohexane, tetrabutylammonium bromide, methyl

ethyl ketone, potassium iodide, potassium carbonate, naphthalene-2,3-diol, naphthalene-

2,7-diol, acetic acid, tin powder, isopropyl magnesium chloride (2.0 M in THF), t-BuLi in

hexane (1.6 M in hexane), n-BuLi in hexane (1.6 M in hexane), 9,10-dibromoanthracene,

bromine, and sodium bicarbonate, sodium chloride, concentrated hydrochloric acid, TFA,

methane sulfonic acid, DDQ, 1,2-dichlorobenzene were purchased from Alfa Aesar,

Aldrich or Acros. They were used without purification. Lithium chloride (LiCl) from

Acros Organics was flame-dried under vacuum. Deuterated chloroform (CDCl3) and deuterated benzene (C6D6) were purchased from Cambridge Isotope Laboratories. CDCl3 was kept over molecular sieves, and C6D6 was stirred over a Na/K alloy, vacuum-

119 transferred, and stored in a glove box. FeCl3 was purified by refluxing in thionyl chloride overnight and followed by removal of all the volatile. Nitromethane, nitrobenzene were stirred over CaH2, distilled and stored in a storage flask. 9,10-dibromo-9,10-dihydro-

230 53 80 9,10-diboraanthracene , 9,10'-bianthracene and PhPdP(t-Bu)3Br were synthesized according to the literatures.

4.4.2 Measurements

1H and 13C NMR spectra were obtained on a Varian Mercury 300 MHz instrument.

Chemical shifts were referenced internally using solvent peaks. Ultraviolet-visible (UV-

Vis) spectra were acquired on a Hewlett-Packard 8453 diode-array spectrophotometer.

Photoluminescence (PL) spectra were obtained on a HORIBA Jobin Yvon NanoLog spectrofluorometer. Polymer films were prepared by spin-coating chloroform solutions on quartz wafers for the UV-Vis measurements and on silicon wafers for the PL measurements. A quartz cuvette was used for solution samples.

Size exclusion chromatography (SEC) was performed using a TOSOH EcoSEC

HLC-8320 GPC instrument equipped with a differential refractometer. Two Varian

Resipore columns (300 x 7.5 mm) were used in tandem as the stationary phase, and chloroform was used as the mobile phase at a flow rate of 0.5 mL/min at 45.0 ºC.

Calibrations were conducted with polystyrene standards from Varian.

MALDI-TOF mass spectra were acquired with Bruker Ultraflex-III TOF/TOF mass spectrometer (Bruker Dlatonics, Inc., Billerica, MA) equipped with a Nd:YAG laser (355 nm). All spectra were measured in positive reflection mode. The instrument was

120

calibrated using external polystyrene or PMMA standards at the molecular weight under consideration. A solution of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2- propenylidene]- malononitrile (DCTB, Santa Cruz Biotechnology, Inc., >99%) which was used as matrix was prepared in chloroform at a concentration of 20 mg/mL. Solutions of sodium trifluoroacetate or silver trifluoroacetate, which were used as cationizing salt, were prepared in methanol/chloroform (v/v= 1/3) at a concentration of 10 mg/mL. All the samples were dissolved in chloroform. Matrix and cationizing salt were mixed in a ratio of 10/1 (v/v). The sample preparation involved depositing 0.5 micro liter of matrix and salt mixture on the wells of a 384-well ground-steel plate, allowing the samples to air-dry, depositing 0.5 micro liter of sample on the matrix spot, and adding another 0.5 micro liter of matrix and salt mixture on the dry sample spot. After evaporation of solvent, the target plate was inserted into the MALDI source. The laser was adjusted and attenuated to minimize undesired polymer fragmentation and to optimize the peak intensity.

4.4.3 Synthesis

Synthesis of 1,3,6-tribromonaphthalene-2,7-diol

Naphthalene-2,7-diol (10 g, 62 mmol) and acetic acid (250 mL) were added into a

500-mL three-neck flask under nitrogen. Bromine (10.5 mL, 205 mmol) was added dropwise through an equal-pressure addition funnel. The mixture was allowed to reflux for 60 hours. After cooled to room temperature, DI water (100 mL) was added and the solid was collected by filtering through a Büchner funnel. Drying under vacuum gave

1,3,6-tribromonaphthalene-2,7-diol as a white solid (21 g, 85%). 1H NMR (300 MHz,

121

CDCl3): δ 7.89 (s, 1H), 7.89 (s, 1 H), 7.88 (s, 1 H), 7.60 (s, 1 H), 6..24 (s, 1 H), 5.88 (s, 1

H).

Synthesis of 3,6-dibromonaphthalene-2,7-diol (12)

1,3,6-Tribromonaphthalene-2,7-diol (20 g, 50.4 mmol), acetic acid (200 mL) and tin

turnings (9.6 g, 75.6 mmol) were added into a 500-mL three-neck flask under nitrogen.

The mixture was allowed to reflux for 96 hours. After cooled to room temperature, DI water (200 mL) was added into the flask. The mixture was then filtered through a

Büchner funnel. Drying under vacuum gave 12 as a white solid (13.8 g, 86%). 1H NMR

(300 MHz, CDCl3): δ 7.89 (s, 1H), 7.87 (s, 2 H), 7.24 (s, 2 H), 5.67 (s, 2 H).

Synthesis of 2,7-dibromo-3,6-bis(hexyloxy)naphthalene (13)

3,6-Dibromonaphthalene-2,7-diol ( 12 g, 37.7mmol), potassium carbonate (20.4 g,

150 mmol), tetrabutylammonium bromide ( 1.1 g, 3.4 mmol), 1-bromohexane (13.7 g,

83.0 mmol) and methylethylketone (120 mL) were added into a 300-mL three-neck flask under nitrogen. The mixture was allowed to warm to reflux overnight. After cooled to room temperature, the mixture was filtered through a Büchner funnel. The liquid phased was rotavapored to remove methylethylketone. The solid was extracted with ethyl acetate

(120 mL). Then the organic phase was washed successively with a saturated sodium bicarbonate solution (100 mL) and a saturated saline solution (100 mL) and dried over anhydrous sodium sulfate. Recrystallization from hexanes/chloroform gave 13 as a

1 yellow crystal (16.7 g, 91%). H NMR (300 MHz, CDCl3): δ7.87 (s, 2 H), 7.01 (s, 2 H),

4.10 (t, J = 4.8 Hz, 4 H), 1.91 (m, J = 4.5 Hz, 4 H), 1.55 (m, 4 H), 1.39 (m, 8H), 0.93 (t, J

= 4.5 Hz, 6 H).

122

Synthesis of 2, 3, 6, 7-tetrakis(hexyloxy)naphthalene (14)

Dry hexanol (70 mL) and sodium ( 2.3 g, 100 mmol) were successively added into a

250-mL Schlenk flask with vigorous stirring until bubbling ceased. 2,7-dibromo-3,6-

bis(hexyloxy)naphthalene ( 15.0 g, 30.9 mmol), copper bromide ( 14.0 g, 73.3 mmol) and

anhydrous DMF (15 mL) were added into the flask. The mixture was allowed to reflux

overnight. After cooled to room temperature, DI water (100 mL) was added into the flask

to quench the reaction. The mixture was extracted with chloroform (150 mL). Then the

organic phase was washed successively with a saturated sodium bicarbonate solution

(100 mL) and a saturated saline solution (100 mL) and dried over anhydrous sodium

sulfate. Recrystallization from hexanes/chloroform gave 14 as a white or colorless crystal

1 (12.4 g, 76%). H NMR (300 MHz, CDCl3): δ 7.02 (s, 4H), 4.07 (t, J = 4.5 Hz, 8 H), 1.88

(m, J = 4.2 Hz, 8 H), 1.52 (m, 8 H), 1.37 (m, 16H), 0.93 (t, J = 4.2 Hz, 12 H). 13C {1H}

1 NMR (75 MHz, CDCl3): δ 148.13, 124.5, 108.0, 69.1, 31.6, 29.2, 25.8, 22.6 and 14.0. H

NMR (300 MHz, C6D6): δ7.13 (s, 4 H), 3.96 (t, J = 4.2 Hz, 8 H), 1.81 (m, J = 5.1 Hz, 8

H), 1.50 (m, 8 H), 1.30 (m, 16 H), 0.91 (t, J = 4.2 Hz, 12 H). 13C {1H} NMR (75 MHz,

C6D6): δ 148.9, 125.2, 108.3, 68.8, 31.8, 29.6, 26.1, 22.8 and 14.0.

Synthesis of 1,4-dibromo-2,3,6,7-tetrakis(hexyloxy)naphthalene and 1,5-dibromo-

2,3,6,7-tetrakis(hexyloxy)naphthalene (15)

2,3,6,7-Tetrakis(hexyloxy)naphthalene ( 11.0 g, 20.8 mmol), NBS ( 8.2 g, 46.0 mmol), chloroform (40 mL) and acetic acid (15 mL) was added into a 100-mL flask. The solution was allowed to warm to 60 oC overnight. Then the mixture was extracted with

diethyl ether (100 mL). The organic phase was washed successively with a saturated

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sodium bicarbonate solution (100 mL), a saturated saline solution (100 mL) and dried

over anhydrous sodium sulfate. Recrystallization from hexanes/dichloromethane gave 15

as a white crystal (13.3 g, 93%). These two isomers had identical 1H and 13C spectra in

1 CDCl3, but could be differentiated by solution NMR spectroscopy in C6D6. H NMR

(300 MHz, CDCl3): δ 7.51 (s, 2 H), 4.13 (t, J = 6.0 Hz, 4 H), 4.08 (t, J = 6.0 Hz, 4 H),

1.89 (m, 8 H), 1.55 (m, 8 H), 1.39 (m, 16 H), 0.93 (t, J = 4.5 Hz, 12 H). 13C {1H} NMR

(75 MHz, CDCl3): δ 151.2, 146.5, 125.2, 115.2, 107.7, 73.6, 68.8, 31.72, 31.57, 30.2,

1 29.2, 25.8, 22.66, 22.63, 14.09 and 14.02. H NMR (300 MHz, C6D6): δ7.81 (s, 1 H),

7.75 (s, 1 H), 4.21 (t, J = 6.0 Hz, 4 H), 4.11 (t, J = 6.0 Hz, 4 H), 3.91 (t, J = 6.0 Hz, 4 H),

3.81 (t, J = 6.0 Hz, 4 H), 1.89 (m, 8 H), 1.69 (m, 8 H), 1.35 (m, 48 H), 0.92 (m, J = 6.0

13 1 Hz, 24 H). C { H} NMR (75 MHz, C6D6): δ 150.5, 149.9, 147.8, 146.1, 124.8, 124.6,

114.5, 114.1, 107.1, 106.7, 73.1, 72.3, 67.5, 67.4, 30.7, 30.5, 29.3, 28.1, 24.8, 24.7, 21.7,

21.6, 12.9 and 12.8.

Synthesis of 1-bromo-2, 3, 6, 7-tetrakis(hexyloxy)naphthalene

2,3,6,7-Tetrakis(hexyloxy)naphthalene ( 3.2 g, 6.1 mmol), NBS ( 1.2 g, 6.7 mmol), chloroform (40 mL) and acetic acid (15 mL) was added into a 100-mL flask. The mixture was allowed to warm to 45 oC overnight. Then the mixture was extracted with diethyl

ether (100 mL). Then the organic phase was washed successively with a saturated sodium

bicarbonate solution (100 mL) and a saturated saline solution (100 mL) and dried over anhydrous sodium sulfate. Flash column chromatography (SiO2, 7:1 hexanes:

dichloromethane) gave 1-bromo-2,3,6,7-tetrakis(hexyloxy)naphthalene as a white solid

1 (2.3 g, 63%). H NMR (300 MHz, CDCl3): δ 7.46 (s, 1 H), 7.02 (s, 2 H), 4.15 (t, J = 3.6

Hz, 2 H), 4.08 (m, 6 H), 1.90 (m, J = 4.5 Hz, 8 H), 1.55 (m, 8 H), 1.39 (m, 16 H), 0.94 (t,

124

13 1 J = 4.5 Hz, 12 H). C { H} NMR (75 MHz, CDCl3): δ 150.6, 149.3, 148.9, 145.1, 126.6,

123.2, 115.1, 107.9, 107.5, 107.2, 73.5, 69.02, 68.98, 68.8, 31.7, 31.59, 31.57, 31.56, 30.3,

29.3, 29.1, 25.8, 25.78, 25.74, 25.71, 22.64, 22.59, 14.05 and 13.98.

Synthesis of 2',3',6',7'-tetrakis(hexyloxy)-1,1':4',1''-ternaphthalene and 2',3',6',7'- tetrakis(hexyloxy)-1,1':5',1''-terbenzobenzene (16)

15 (1.35 g, 1.97 mmol), naphthalen-1-ylboronic acid (1.05 g, 6.1 mmol), Pd(PPh3)4

(30 mg), potassium carbonate (510 mg, 3.75 mmol) ) and anhydrous THF (15 mL) were loaded into a 25 mL Schlenk flask in the glove box. Under nitrogen, degassed water (1 mL) was added via a syringe. The mixture was refluxed for 3 days. The solvent was removed under vacuum. The solid residual was dissolved in 30 mL ethyl acetate and washed successively with water (20 mL), a saturated sodium bicarbonate solution (20 mL) and a saturated saline solution (20 mL) and dried over anhydrous sodium sulfate. The solvent was removed by rotavapor. Flash column chromatography (SiO2, 7:1 hexanes:

dichloromethane) gave 16 as a yellowish viscous liquid (1.09 g, 71%). The 1H NMR

spectrum was hard to interpret due to the existence of regio- and configurational-isomers.

MALDI-TOF confirmed the structure by m/z 780.54 [M]+ (calcd. m/z = 780.51).

Synthesis of 2,7-bis(hexyloxy)naphthalene

Naphthalene-2,3-diol (5 g, 31 mmol) , potassium carbonate (20.4 g, 150 mmol),

tetrabutylammonium bromide ( 1.1 g, 3.4 mmol), 1-bromohexane (11.3 g, 68 mmol) and

methyl ethyl ketone (120 mL) were added into a 300-mL three-neck flask under nitrogen.

The mixture was allowed to reflux overnight. After cooled to room temperature, the

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mixture was filtered through a Büchner funnel. The liquid phased was rotavapored to

remove methyl ethyl ketone. The solid was extracted with ethyl acetate (120 mL). The

organic phase was washed successively with a saturated sodium bicarbonate solution

(100 mL) and a saturated saline solution (100 mL) and dried over anhydrous sodium

sulfate. Flash column chromatography (SiO2, 7:1 hexanes: dichloromethane) gave 2,7-

1 bis(hexyloxy)naphthalene as a yellow oil (9.1 g, 89%). H NMR (300 MHz, CDCl3):

δ8.26 (m, 4 H), 7.57 (m, 4 H), 4.13 (t, J = 6.6 Hz, 4 H), 1.92 (m, 4 H), 1.58 (m, 4 H), 1.39

13 1 (m, 8 H), 0.94 (t, J = 6.9 Hz, 6 H). C { H} NMR (75 MHz, CDCl3): δ 150.1, 130.2,

127.2, 126.9, 116.4, 74.5, 31.7, 30.2, 25.7, 22.6 and 14.0.

Synthesis of 1-bromo-2,7-bis(hexyloxy)naphthalene

2,3-Bis(hexyloxy)naphthalene (6.2 g, 18.9 mmol), NBS (7.40 g, 41.6 mmol), acetic acid (15 mL) and chloroform (50 mL) were added into a 100 mL flask. The mixture was warmed to 50 oC and stirred for overnight. Diethyl ether (60 mL) was added into the

mixture. The organic phase was washed successively with a saturated sodium bicarbonate

solution (150 mL) and a saturated saline solution (50 mL) and dried over anhydrous

sodium sulfate. Recrystallization from hexanes/chloroform gave 1-bromo-2,7-

1 bis(hexyloxy)naphthalene as a white solid (7.7 g, 84%). H NMR (300 MHz, CDCl3):

δ7.65 (m, 2 H), 7.30 (m, 2 H), 7.10 (s, 1 H), 4.09 (t, J = 6.6 Hz, 4 H), 1.93 (m, 4 H), 1.55

13 1 (m, 4 H), 1.36 (m, 8 H), 0.91 (t, J = 6.9 Hz, 6 H). C { H} NMR (75 MHz, CDCl3): δ

149.5, 129.3, 126.2, 123.9, 107.9, 68.8, 31.6, 29.1, 25.7, 22.6 and 14.0.

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Synthesis of (2,7-bis(hexyloxy)naphthalen-1-yl)boronic acid

1-Bromo-2,7-bis(hexyloxy)naphthalene (2.1 g, 5.16 mmol) ) was dissolved in THF

(25 mL) in a 100 mL Schlenk flask. The mixture was cooled to -78 oC. 1.6 M n-BuLi

(3.23 mL, 5.17 mmol) was added dropwise via syringe over 30 min. The homogeneous

solution was allowed to gradually warmed up to room temperature and stirred for another

2 hours. After cooling to 0 oC, 1.0 M HCl (10 mL, 10 mmol) was added dropwise. The

mixture was stirred vigorously overnight. The organic phase combined with ethyl acetate

(75 mL) was washed successively with water (50 mL X2), a saturated sodium bicarbonate solution (100 mL) and a saturated saline solution (100 mL) and dried over anhydrous sodium sulfate. Flash column chromatography (SiO2, 3:1 hexanes:

dichloromethane) gave (2,7-bis(hexyloxy)- naphthalen-1-yl)boronic acid as a waxy

yellowish solid (0.69 g, 36%). This compound was not stable in air. 1H NMR (300 MHz,

CDCl3): δ8.35 (s, 1 H), 7.84 (d, J = 9.0 Hz, 1 H), 7.67 (d, J = 9.0 Hz, 1 H), 7.10 (d, J =

9.0 Hz, 1 H), 7.07 (d, J = 9.0 Hz, 1 H), 4.22 (t, J = 6.0 Hz, 2 H), 4.12 (t, J = 6.0 Hz, 2 H),

1.87 (m, 4 H), 1.55 (m, 4 H), 1.37 (m, 8 H), 0.92 (t, J = 6.9 Hz, 6 H).

Synthesis of 2,2',2'',3',6',7,7',7''-octakis(hexyloxy)-1,1':4',1''-ternaphthalene and

2,2',2'',3',6',7,7',7''-octakis(hexyloxy)-1,1':5',1''-terbenzobenzene (17)

15 (1.75 g, 2.55 mmol), (2,7-bis(hexyloxy)naphthalen-1-yl)boronic acid (2.23 g, 6.0 mmol), Pd(PPh3)4 (34 mg), potassium carbonate (580 mg, 4.26 mmol) ) and anhydrous

THF (15 mL) were loaded into a 25 mL Schlenk flask in the glove box. Under nitrogen,

degassed water (1 mL) was added via a syringe. The mixture was refluxed for 3 days.

The solvent was removed under vacuum. The solid residual was dissolved in 30 mL ethyl

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acetate and washed successively with water (20 mL), a saturated sodium bicarbonate

solution (20 mL) and a saturated saline solution (20 mL) and dried over anhydrous

sodium sulfate. The solvent was removed by distillation. Flash column chromatography

(SiO2, 7:1 hexanes: dichloromethane) gave 17 as a yellowish viscous liquid (1.87 g, 62%).

The 1H NMR spectrum was hard to interpret due to the existence of regio- and

configurational-isomers. MALDI-TOF confirmed the structure by m/z 1180.88 [M]+

(calcd. m/z = 1180.87).

Synthesis of (4-bromo-2,3,6,7-tetrakis(hexyloxy)naphthalen-1-yl)boronic acid and

(5-bromo-2,3,6,7-tetrakis(hexyloxy)naphthalen-1-yl)boronic acid (19)

15 (2.302 g, 3.35 mmol) was dissolved in THF (25 mL) in a 100 mL Schlenk flask.

The mixture was cooled to -78 oC. 1.6 M n-BuLi (2.10 mL, 3.36 mmol) was added

dropwise via syringe over 30 min. The homogeneous solution was then allowed to

gradually warmed up to room temperature and stirred for another 2 hours. After cooling

to 0 oC, 1.0 M HCl (10 mL, 10 mmol) was added dropwise. The mixture was stirred

vigorously overnight. Then the organic phase combined with ethyl acetate (75 mL) was

washed successively with water (50 mL X2), a saturated sodium bicarbonate solution

(100 mL) and a saturated saline solution (100 mL) and dried over anhydrous sodium

sulfate. Recrystallization from hexanes/ether gave 19 as a white solid (0.94 g, 43%). 1H

NMR (300 MHz, CDCl3): δ 8.25 (s, 1 H), 7.64 (s, 1 H), 6.57 (s, 2 H), 4.11 (m, 8 H), 1.88

(m, 8 H), 1.55 (m, 8 H), 1.38 (m, 16 H), 0.93 (m, 12 H).

General Procedure of Polymerization and Isolation of Oligomer 18

19 (230 mg, 0.353 mmol), Cs2CO3 (400 mg, 1.22 mmol), PhPdP(t-Bu)3Br (8 mg,

0.017 mmol) and anhydrous THF (5 mL) were loaded into a 25 mL Schlenk flask in the

128

glove box. Under nitrogen, degassed water (1 mL) was added via a syringe. The mixture

was stirred at 60 oC for 2 days. The solvent was removed under vacuum. The solid

residual was dissolved in 30 mL ethyl acetate and washed successively with water (20

mL), a saturated sodium bicarbonate solution (20 mL), a saturated saline solution (20 mL

X2) and dried over anhydrous sodium sulfate. The solvent was removed by distillation.

The solid was loaded on a silica gel. 10:1 hexanes: dichloromethane was first used to

wash off small molecules, and the oligomers were collected by rinsing with 2:1 hexanes:

dichloromethane. The 1H NMR spectrum was hard to interpret due to the existence of

regio- and configurational-isomers. Both GPC and MALDI support the formation of

oligomers with highest molecular weight up to 9,000.

General Procedures of Ring-fusion and Isolation of 18

Method A. 18 (1 equiv) was dissolved in dichloromethane (10 mL) in a 25 mL

Schlenk flask. A constant stream of nitrogen was bubbled into the solution via a syringe.

A solution of FeCl3 or MoCl5 (4 equiv) in anhydrous nitromethane was added dropwise.

Throughout the whole reaction, nitrogen stream was bubbled through the mixture to

remove the HCl generated in situ. The reaction was stirred overnight and quenched by adding methanol (20 mL). The mixture was extracted with dichloromethane and concentrated. After removal of the solvent, a black solid was formed. If needed, the residue was further purified by column chromatography (chloroform:hexanes= 3:1).

Method B. 18 (1 equiv), Sc(OTf)3 (1.5 equiv) and DDQ (1.5 equiv) were dissolved in

10 mL anhydrous toluene, nitromethane or nitrobenzene. The mixture was stirred at 110

oC for 24 h under nitrogen. After cooling to room temperature, the mixture was poured

129 into water and the product was extracted with THF. The organic layer was separated. The solvent was removed under reduced pressure to give a black residue. If needed, the residue was further purified by column chromatography (chloroform:hexanes= 3:1).

Method C. 18 (1 equiv), AlCl3 (2 equiv) and CuCl2 (2 equiv) were dissolved in chloroform (10 mL) in a 25 mL Schlenk flask. The reaction was stirred overnight and quenched by addition of methanol (20 mL). The mixture was extracted with dichloromethane and concentrated. The solvent was removed under reduced pressure to give a black residue.

Method D. 18 (1 equiv), AlCl3 (2 equiv) and chlorobenzene (50 equiv) were flushed with nitrogen and stirred in a 25 mL Schlenk flask at 80 oC for 12 h. The mixture was cooled to room temperature and quenched with diluted HCl (10%, 15 mL). The resulting two-phase system was cooled and filtered under suction. The precipitate from the filter and the reaction flask were rinsed with dichloromethane. The solid product was collected from the filter paper and dried under vacuum.

Method E. 18 (1 equiv), methane sulfonicacid (MSA, 5 equiv) or trifluoro- aceticacid

(TFA, 5 equiv) and 2,3-dichloro-5,6- dicyanobenzoquinone (DDQ, 1.5 equiv) were dissolved in 10 mL anhydrous dichloromethane, nitrobenzene or dichlorobenzene (DCB).

The mixture was stirred at room temperature or at 110 oC for 24 h under nitrogen. After cooling to room temperature, the mixture was poured into water and the product was extracted with ethyl acetate. The organic layer was separated. The solvent was removed

130

under reduced pressure to give a black residue. If needed, the residue was further purified

by column chromatography (chloroform:hexanes= 3:1).

Method F. 18 (1 equiv), potassium tert-butoxide (3 equiv), DBU (3 equiv) and

ethanolamine (4 mL) were added in a 25 mL Schlenk flask. The mixture was heated to

140 oC for 12 h. Then the same amount of potassium tert-butoxide, DBU and ethanolamine were added and the mixture was kept at 140 oC for another 12 h. After cooled to room temperature, the mixture was poured into 1M HCl (50 mL), filtered and washed with DI water. The dark solid was collected.

Method G. 18 (1 equiv) and freshly cut potassium (10 equiv) were dissolved in THF

(15 mL) in a 25 mL storage flask. The mixture was stirred vigorously at 90 oC for 24 H.

The reaction was quenched by addition of an excess of iodine solution in THF. The

mixture was poured into water (75 mL) and extracted with ethyl acetate (15 mL X2). The

organic phase was separated and dried under vacuum and the residue was collected. If

needed, the residue was further purified by column chromatography (chloroform:hexanes

= 3:1).

Synthesis of 9-bromo-10-hexylanthracene

9,10-Dibromoanthracene (4.66 g, 13.9 mmol) and anhydrous diethyl ether (30 mL)

were mixed in a 100 mL Schlenk flask. The mixture was cooled to -78 oC. t-BuLi in

hexane (1.6 M, 8.67 mL, 13.9 mmol) was added via a gas-tight syringe. 3 hours later, 1-

bromohexane (2.52 g, 15.3 mmol) was added. The mixture was allowed to warm to room

131

temperature and stirred overnight. A saturated sodium bicarbonate aqueous solution (50

mL) was added into the mixture. The aqueous solution was extracted with diethyl ether

(30 mL x 3). The organic phase was combined and washed with saturated saline solutions

(50 mL x 2) and dried over anhydrous sodium sulfate. After ether was removed by

distillation, the resulting crude product was recrystallized from chloroform/hexanes to

give 9-bromo-10-hexylanthracene as a yellow solid (2.55 g, 54% yield). 1H NMR (300

MHz, CDCl3): δ 8.64 (d, J = 9.0 Hz, 2 H), 8.31 (d, J = 9.0 Hz, 2 H), 7.58 (m, 5 H), 3.61 (t,

J = 6.0 Hz, 2 H), 1.86 (m, 2 H), 1.62 (m, 2 H), 1.43 (m, 4 H), 0.97 (t, J = 6.0 Hz, 3 H).

Synthesis of 5,10-bis(10-hexylanthracen-9-yl)-5,10-dihydroboranthrene (20)

9-Bromo-10-hexylanthracene (1.2 g, 3.52 mmol) and anhydrous THF (30 mL) were

added into a 100 mL Schlenk flask. The mixture was cooled to -78 oC. t-BuLi in hexane

(1.6 M, 2.20 mL, 3.52 mmol) was added via a gas-tight syringe. The mixture was allowed

to warm to room temperature and stirred overnight. 9,10-dibromo-9,10-dihydro-9,10-

diboraanthracene (0.58 g, 1.74 mmol) was added into the flask. The mixture immediately

formed a bright red color solution together with lots of precipitates. After the mixture was

stirred for another 8 hours, all volatile components were removed under vacuum.

Recrystallization from ether/hexanes gave 20 as a red solid (0.25 g, 21%). 20 is not stable

1 in air. H NMR (400 MHz, C6D6): δ 8.32 (d, J = 8.0 Hz, 4 H), 7.63 (d, J = 8.0 Hz, 4 H),

7.43 (m, 4 H), 7.33 (m, 4 H), 7.23 (m, 4 H), 7.15 (m, 4 H), 3.64 (t, J = 6.0 Hz, 4 H), 1.89

(m, 4 H), 1.62 (m, 4 H), 1.39 (m, 8 H), 0.88 (t, J = 6.0 Hz, 6 H).

132

Synthesis of 10,10'-dichloro-9,9'-bianthracene

9,10'-Bianthracene (100 mg, 0.28 mmol) and anhydrous chloroform (15 mL) were

added into a 100 mL Schlenk flask. FeCl3 (366 mg, 2.26 mmol) in anhydrous

nitromethane (5 mL) was added dropwise over a period of 30 minutes. Then a stream of

nitrogen was bubbled through the solution and the mixture was kept stirring vigorously

for 8 hours. After quenching with methanol (10 mL), a saturated sodium bicarbonate

aqueous solution (30 mL) was added into the mixture. The aqueous solution was

extracted with diethyl ether (30 mL x 3). The organic phase was combined and washed

with saturated saline solutions (50 mL x 2) and dried over anhydrous sodium sulfate.

Flash column chromatography (SiO2, 8:1 hexanes: dichloromethane) gave 10,10'- dichloro-9,9'-bianthracene as a yellow solid (98 mg, 82% yield). The above procedure was under a typical Scholl reaction condition, but only chlorinated products were

1 obtained. H NMR (300 MHz, CDCl3): δ 8.71 (d, J = 9.0 Hz, 4 H), 7.60 (t, J = 6.0 Hz, 4

H), 7.20 (t, J = 6.0 Hz, 4 H), 7.11 (d, J = 9.0 Hz, 4 H).

Synthesis of 10,10'-dibromo-9,9'-bianthracene

9,10'-Bianthracene (1.0 g, 2.8 mmol) and chloroform (40 mL) were added into a 100 mL flask. Bromine (1.0 g, 6.25 mmol) was added into the flask dropwise at 0 oC. Then the mixture was allowed to warm to room temperature and stirred overnight. A saturated sodium bicarbonate aqueous solution (30 mL) was added into the mixture. The aqueous solution was extracted with diethyl ether (30 mL x 3). The organic phase was combined and washed with saturated saline solutions (50 mL x 2) and dried over anhydrous sodium sulfate. Flash column chromatography (SiO2, 8:1 hexanes: dichloromethane) gave 10,10'-

133

dibromo-9,9'-bianthracene as a yellow solid (1.23 g, 85% yield). 1H NMR (300 MHz,

CDCl3): δ 8.71 (d, J = 9.0 Hz, 4 H), 7.60 (t, J = 6.0 Hz, 4 H), 7.20 (t, J = 6.0 Hz, 4 H),

7.11 (d, J = 9.0 Hz, 4 H).

Synthesis of 9',10-dihexyl-9,10'-bianthracene (21)

9',10-Dibromo-9,10'-bianthracene (1.5 g, 2.9 mmol), Ni(DPPP)Cl2 (0.10 g, 0.18

mmol) and anhydrous diethyl ether (40 mL) were added into a 250 mL Schlenk flask.

Hexyl magnesium bromide364 in diethyl ether (0.11 M, 40 mL, 4.4 mmol) was added into

the flask via a cannula. The mixture was stirred overnight at room temperature. A

saturated sodium bicarbonate aqueous solution (30 mL) was added into the mixture. The

aqueous solution was extracted with diethyl ether (30 mL x 3). The organic phase was

combined and washed with saturated saline solutions (50 mL x 2) and dried over

anhydrous sodium sulfate. Flash column chromatography (SiO2, 8:1 hexanes:

dichloromethane) gave 21 as a yellow solid (0.66 g, 44% yield). 1H NMR (300 MHz,

CDCl3): δ 8.49 (d, J = 9.0 Hz, 4 H), 8.19 (d, J = 9.0 Hz, 4 H), 7.49 (m, 4 H), 7.03 (m, 4

H), 3.64 (t, J = 9.0 Hz, 4 H), 2.05 (m, 4 H), 1.76 (m, 4 H), 1.38 (m, 8 H), 0.92 (t, J = 6.0

13 1 Hz, 6 H). C { H} NMR (75 MHz, CDCl3): δ 136.1, 131.6, 129.4, 127.9, 125.2, 125.1,

124.7, 31.9, 31.7, 30.2, 28.5, 22.8 and 14.2.

Synthesis of 10,10''-dihexyl-9',10'-dihydro-9,9':10',9''-teranthracene (22)

9-Bromo-10-hexylanthracene (458 mg, 1.34 mmol), 9,10-dihydroanthracene (122 mg,

0.67 mmol), Sodium t-butoxide (200 mg), Palladium acetate (3 mg), tri-t-butyl phosphine

(0.07 mL) and anhydrous THF (10 mL) were added into a 25 mL Schlenk flask. The

mixture was microwave to 60 oC for 5 hours. After cooled to room temperature, the

134 solution was filtered by a short alumina column. Solvent was removed by distillation.

The product 22 was hard to separate by flash column chromatography and was identified by NMR spectroscopy with a conversion around 50%. MALDI-TOF confirmed the structure by m/z 679.17 [M]+ (calcd. m/z = 679.19).

Synthesis of 5,10-di(acridin-10(9H)-yl)-5,10-dihydroboranthrene (23)

5,10-Dibromo-5,10-dihydroboranthrene (69 mg, 0.21 mmol), triethylamine (0.2 mL),

9,10-dihydroacridine (76 mg, 0.419 mmol) and toluene (10 mL) were added into a 25 mL

Schlenk flask. The mixture was refluxed overnight. After cooled to room temperature, solvent was removed. The crude was a mixture and hard to separate. The product 23 was identified by the two new doublets around 4.0 ppm.

135

CHAPTER V

SUMMARY

A series of Boron-containing conjugated polymers have been successfully

synthesized and well characterized by means of 1H, 13C NMR spectroscopy, cyclic voltammetry, UV-Vis absorption, and photoluminescence quenching experiments. One of the polymers has been proved to be able to quench the photoluminescence of rr-P3HT, suggesting the possible charge separation and electron transfer from Boron-containing

polymer to rr-P3HT.

Considering the stability and charge mobility are important features for conjugated

polymers in photo-electron applications, a regioregular polythiophene (PHOT) with

strong electron-withdrawing groups has been successfully synthesized. 1H, 13C NMR spectroscopies demonstrate the polymer has a regioregularity over 95%. Cyclic voltammetry results show PHOT has LUMO and HOMO levels both lowered by around

0.5 ev than rr-P3HT, indicating that it could be potentially used as an n-type material.

However, WAXD indicates that PHOT lacks order in the π-stacking direction in the solid state. PHOT does not exhibit any electron-transport activity in FET devices, but a low

average field-effect hole mobility. The lack of electron mobility and the relative low hole mobility in PHOT could be likely attributable to the lack of π-stacking order.

Photovoltaic devices of PHOT and PC71BM have a low power conversion efficiency.

136

To further increase to charge mobility of a conjugated polymer, a number of soluble

GNRs are chosen as the synthetic targets. However, the successful ring-fusion reactions

are not well-established and only Scholl reaction has been proved by Mullen’s group to

be the most feasible. To prevent the possible intermolecular and intramolecular side reactions in Scholl reactions, oligo-naphthalenes capping the 2, 3, 6, and 7-positions are synthesized. The oligomers can be achieved through Suzuki polycondensation with a highly reactive initiator. Although various Scholl reactions have been tested, no expected ribbon has been achieved.

137

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158

APPENDIX

SUPPORTING NMR SPECTRA i g,h f e d i h g f e d

O c S b a b ca

300M 1H NMR spectrum of compound 5.

159

e c O

d S

a e

13 300M C NMR spectrum of compound 5 in CDCl3.

160

e O

g O S

f a

300M 1H NMR spectrum of compound 6.

161

p o i,j n m h l k f,g p o n m e l k

g O e d c j O h S f b a i b d a c

300M 13C NMR spectrum of compound 6.

162

h g Br d O

f O S

1 300M H NMR spectrum of 8a in CDCl3.

163

g f Br

c O

e O S

d Br

1 300M H NMR spectrum of 7 in CDCl3.

164

p o i,j n m h l k f,g p o n m e l k Br e O c g d c a

j O

hi S f b a Br b d

13 300M C NMR spectrum of 7 in CDCl3.

165

c O n

e O S

1 300M H NMR spectrum of compound 10 in CDCl3.

166

n a

O c

d d S

1 300M H NMR spectrum of compound 11 in CDCl3.

167

e k d,h,i,j g f b,c k j i h g f Br c O

e O S b a d I a

300M 1H NMR spectrum of 2-(2-bromo-5-iodothiophen-3-yl)-2-hexyl-5,5-dimethyl-1,3- dioxane in CDCl3.

168

k g,h,i,j f d,e k j i h g f e

S c

S S d b a b a c

1 300M H NMR spectrum of 2-hexyl-2-(thiophen-3-yl)-1,3-dithiolane in CDCl3.

169

S e

g S S

13 300M C NMR spectrum of 2-hexyl-2-(thiophen-3-yl)-1,3-dithiolane in CDCl3.

170

c F S

1 300M H NMR spectrum of 3-(1,1-difluoroheptyl)thiophene in CDCl3.

171

F F S

19 300M F NMR spectrum of 3-(1,1-difluoroheptyl)thiophene in CDCl3.

172

1 300M H NMR spectrum of compound 13 in CDCl3.

173

g e,f d c b g f e d c b a O O

O O a

1 500M H NMR spectrum of compound 14 in CDCl3.

174

i h g f e d i h b g f e d b a

O O a c

O O c

13 300M C NMR spectrum of compound 14 in CDCl3.

175

g,g' e,e',f,f' d,d' c,c' g' g f f' b,b' e' e d d' c c' b'

b Br a O O

O O Br a

1 300M H NMR spectrum of 15 in CDCl3.

176

Br O O

O O Br

1 300M H NMR spectrum of 15 in C6D6.

177

k k

j j

g f Br f

O O d

O O c Br

13 500M C NMR spectrum of 15 in CDCl3.

178

Br O O

O O Br

13 500M C NMR spectrum of 15 in C6D6.

179

R R

H R R 6

1 300M H NMR spectrum of 16 in CDCl3.

180

k,k' i,i',j,j' h,h' g,g' f,f' k j' k' j i i' h' h g g' f' f B(OH)2 e O O a d b c c b a d e

1 300M H NMR spectrum of (2,7-bis(hexyloxy)naphthalen-1-yl)boronic acid in CDCl3.

181

R R R R 13 H R R 6 R R R= OC

1 300M H NMR spectrum of 17 in CDCl3.

182

B(OH)2 b O O

O O c Br a b a

1 300M H NMR spectrum of 19 in CDCl3.

183

1 300M H NMR spectrum of 9,9'-bianthracene in CDCl3.

184

a b c

Br d c b a Br d

1 300M H NMR spectrum of 10,10'-dibromo-9,9'-bianthracene in CDCl3.

185

1 300M H NMR spectrum of 21 in CDCl3.

186

300M 13 C NMRspectrumof

d,e

21 inCDCl c f h i a k g i,j b j m c f 3 l k . m

187 de n h n g

l a

j h,i g f e j i h g e f d c b a Br a,d c b

1 300M H NMR spectrum of 9-bromo-10-hexylanthracene in CDCl3.

188

l j,k i h l k j i h g g f e d c

b B a

B d,e f a c b

1 400M H NMR spectrum of compound 20 in CDCl3.

189

1 300M H NMR spectrum of compound 22 in CDCl3.

190

N B g

f B

e N g cd b e,f a c,d e

1 300M H NMR spectrum of compound 23 in CDCl3.

191