core.ac.uk · ACKNOWLEDGMENTS . I would like to express my gratitude to my supervisor, Assoc. Prof. Suresh Valiyaveettil for his guidance, constant support and encouragement throughout

SYNTHESIS, CHARACTERIZATION AND SELF-

ASSEMBLY OF STIMULI SENSITIVE MATERIALS

SATYANANDA BARIK

(M. Tech. IIT Kharagpur,

M. Sc. Utkal University, India)

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY

NATIONAL UNIVERSITY OF SINGAPORE

2009

ACKNOWLEDGMENTS

I would like to express my gratitude to my supervisor, Assoc. Prof. Suresh

Valiyaveettil for his guidance, constant support and encouragement throughout the

research project.

I sincerely thank Dr. Akhila, Dr. Raj, Dr. Aji, Dr. Vetri, Dr. Santhosh, Dr. Anideepthi,

Dr. Manoj, Dr. Jinu, Dr. Sindhu, Dr. Siva, Dr. Renu, Dr. Gayathri, Dr. Rajeev, Asha,

Sheeja, Hairong, Fathima, Haiyu, Balaji, Jhinuk, Pradipta, Narahari, Thirumal, Kavitha,

Yiwei, Nizar, and Kiruba for their cordiality, friendship and for exchanging knowledge

skills. Special thanks to Dr. Akhila who helped me first in laboratory to know more about

synthesis; Dr. Nurmawati, and Dr. Sindhu for helping me obtain the TEM images; Dr.

Jegadesan, and Sajini for their assistance in using the AFM. I am thankful to Ankur for

helping me in SEM and XRD.

My appreciation goes to Sheena and Karen, former Honors students and Radhika,

former M.Sc. student for their patience and assistance in performing synthesis.

I must acknowledge the technical assistance provided by the staff of the NMR, Mass

spectroscopy, Elemental Analyses and Thermal Analysis Laboratories at NUS.

Cheers to my buddies, Amarendu Da, Swopnil, Sujit, Manoj Manna, Ankur, Santosh,

and Pradipta, whose constant companion never failed to light up my days in Singapore.

The gratitude that is most difficult to express in words is towards my family. I

wholeheartedly thank my parents, brothers, sister, and sister-in-laws for their support and

encouragement. They loved me, taught me and showed me how to make sense of the

world. To my pillars of support, I dedicate this thesis.

I thank the National University of Singapore for granting the research scholarship.

i

TABLE OF CONTENTS Acknowledgments

i

Table of contents

ii

Summary

viii

Abbreviations and Symbols

x

List of Tables

xiv

List of Figures

xvi

List of Schemes

xxii

1 Introduction

1

1.1 Supramolecular Chemistry 2

1.2 Supramolecular Polymer Chemistry: An Overview 4

1.2.1 Fabrication of nanostructure materials 4

1.2.2 The macromolecular assembly 6

1.3 Block Macromolecular Self-assembly: A Recent Study 11

1.4 Precursor Copolymer Approach 17

1.4.1 Structural organization of precursor polymer 17

1.4.2 Controlled radical polymerization techniques 25

1.4.3 Electrochemical Nanolithography 26

1.5 Semiconducting Supramolecular Macromolecules 26

1.6 Photo-chromic Molecules and Morphosyntheses 38

1.7 Aim and Outline of This Thesis 43

1.8 Notes and References 45

ii

2 Synthesis and Self-assembly of Copolymers with Pendant Electroactive Units

61

2.1 Introduction 62

2.2 Experimental Section 63

2.2.1 Synthesis 63

2.3 Results and Discussion 66

2.3.1 Synthesis and characterization of copolymers (P1-P4) 66

2.3.2. Thermal properties 68

2.3.3. Optical properties 68

2.3.4. Nano-fiber morphology studies 70

2.3.5. Electrochemical nano-patterning using AFM 73

2.3.6. Electrochemical polymerization using CV 75

2.4 Conclusion 80

2.5 References 81

3 Diblock Copolymer Assemblies Through Changes in Amphiphilicity of Pendent electroactive Moiety

87

3.1 Introduction 88


3.2.1 Synthesis 89


3.3.1. Synthesis and characterization of block copolymer 89


iii

3.3.3. Optical Properties 95

3.3.4. Self-assembly of block copolymers 96

3.3.5 Electrochemical polymerization 101

3.4 Conclusion 107

3.5 References 108

4 Engineering Nano-architectures of Amphiphilic Dithienylethene (DTE): Synthesis and Characterization

112

4.1 Introduction 113

4.2 Experimental 113

4.2.1 Synthesis 113


4.3.1 Design, synthesis and characterization 114

4.3.2 Photochromism 118

4.3.3 Self-assembly and morphology 124

4.4 Conclusion 129

4.5 References 130

5 Regioregular Electro-active Carbazole End-Capped Oligo(p-phenylene): Synthesis, Characterization and Self-assembly Studies

134


5.2 Experimental 135

iv

5.2.1 Synthesis 135


5.3.1. Design, synthesis and characterization 136


5.3.3 Optical properties 141

5.3.4 Electrochemical properties 144

5.3.5 Self-assembly and microphase separation 145

5.4 Conclusion 150

5.5 References 151

6 Conjugated Polymer Network Self-assembled Films From Precursor Polymers: Cross-conjugated Poly (p-phenylene)

155



6.2.1 Synthesis 158


6.3.1 Synthesis and characterization 159

6.3.2 Optical properties 162

6.3.3 Thermal properties 164

6.3.5 Electropolymerization 165

6.3.6 Morphological characterization of electropolymerized film 168

6.4 Conclusion 169

6.5 References 170

v

7 Experimental Section 174

7.1 General Instrumentation 175

7.2 Synthesis of Compounds in Chapter 2 176

7.2.1 General procedure for synthesis of monomers (3, 5, 7, 9 and 11) 176

7.2.2 General procedure for free radical copolymerization (P1-P4) 179


7.3.1 Synthesis of monomer 6 181

7.3.2 Homopolymerization of 15 {PBMMA-Br (18)} 182

7.3.3 General procedure of atom transfer radical polymerization (ATRP) for block copolymer synthesis 183


7.4.1 General synthesis of intermediates 185

7.4.2 General procedure for diazo-compound synthesis 188

7.4.3 General procedure for the synthesis of azo-dithienylethene (DTE) molecules 189


7.5.1 Synthetic procedures of intermediates 191

7.5.2 General procedure for O-alkylation 191

7.5.3 General Procedure of boronicacid syntheses 192

7.5.4 General procedure for Suzuki coupling reaction 194

7.5.5 General procedure of Buchwald coupling for the synthesis of OLG1-OLG4 197


7.6.1 General procedure for selective bromination of thiophene 199

7.6.2 Synthesis of intermediates 200

7.6.3 General procedure of Wittig reaction 202

vi

vii

7.6.4 General procedure of Suzuki polymerization (P1-P3) 204

7.7 References 206

Appendix I

List of Publications 207

Summary

The focus of this thesis involves the design and synthesis of multidimensional (1D -

2D) organic macromolecules with redox or photo-active groups that assists in the self-

assembly process. Synthesis, characterization, and self-assembly of polymethacrylated

amphiphilic copolymers were achieved. Copolymers with varying spacers between the

backbone and the electroactive groups, the self-assembly properties were investigated.

The intermolecular interactions are important towards the self-assembly of polymers and

the target polymers gave nanofiber morphology in the solid state. The nature and

orientation of the pendant electroactive groups play a crucial role for the selective

morphogenesis. The electropolymerization of the electroactive groups led to the formation

of conjugated polymer network (CPN) in the polymer lattice. This concept was

investigated in Chapter two.

In Chapter three, the multi-functional amphiphilic block copolymers were

synthesized with pendant electroactive groups and polyhydroxylated moieties using ATRP

method. The block copolymers self-assembled from water/THF mixture through

microphase separation of polar and nonpolar blocks to give lamellar or vesicular

morphologies depending on the structure of the polymer backbone. The

electropolymerization of the groups on the side chain showed formation of conjugated

polymer network (CPN) on ITO.

In Chapter four, synthesis of photoactive (diazo) and photochromic (dithiaethylene)

moelcuels were discussed. The photochromism and self-assembly of the compounds were

explored. The formation of well defined nanorings and role of concentration and surface

viii

ix

on ring formation were investigated. The thermal stability of nano-rings through annealing

showed that the rings were stable at high temperature.

In Chapters five, a series of carbazole (electroactive) end-capped oligo- (p-phenylene)

was synthesized using Buchwald’s double amination reaction and characterized and

optical and electrochemical behaviour were investigated. The materials showed hole

transport characteristics. The molecular aggregations in THF/H2O solvent mixture were

investigated using electron microscopy. The structure-property correlation between

photophysical properties and crystalline domain formation of the homologous series of

oligomers are described.

In Chapter six, the roles of conjugated segment of soluble cross-conjugated polymer

poly (p-phenylene) with electroactive groups were discussed. The cross-conjugated poly-

(p-phenylene) with electron donor/acceptor (e.g. thiophene/carbazole) groups was

synthesized and photophysical/electrochemical properties were investigated to establish

the cross talking of electrons form the multiple branches of the molecule. The fabrication

of nanofibers from thiophene incorporated on the poly (p-phenylene) backbone was

demonstrated.

Details of the synthesis, characterizations of the intermediates and target compounds

along with the various other instrumentation techniques mentioned in this thesis are given

in Chapter seven.

ABBREVIATIONS AND SYMBOLS

1D One Dimensional

2D Two Dimensional

3D Three Dimensional

1H-NMR Proton Nuclear Magnetic Resonance

13C-NMR Carbon Nuclear Magnetic Resonance

Å Angstrom(s)

δ Chemical shift (in NMR spectroscopy)

Φ Fluorescence quantum yield

τ Measured fluorescence lifetime

τnrad Non-radiative lifetime

Γ Rate constant for fluorescence decay

τrad Radiative lifetime

λ Wavelength

θ Diffraction angle

ca. About

AFM Atomic Force Microscopy

b Broad

BuLi Butyllithium

CHCl3 Chloroform

CLSM Confocal Laser Scanning Microscope

conc. Concentrated

CV Cyclic Voltammetry

CP Conjugated Polymer

x

d Doublet

dd Double of Doublet

DCM Dichloromethane

DLS Dynamic Light Scattering

DP Degree of Polymerization

DMF Dimethylformamide

DMSO Dimethylsulfide

D2O Deuterium Oxide

EA Ethyl Acetate; Elemental Analysis

EI-MS Electron Impact Mass Spectrometry

ESIPT Excited State Intramolecular Proton Transfer

EtOH Ethanol

FS Femtosecond

GPC Gel Permeation Chromatography

HBC Hexa-peri-hexabenzocoronene

Hz Hertz

FT-IR Infrared Fourier Transform

hr Hour(s)

i.e. That is (Latin id est)

J Coupling constant

KBr Potassium Bromide

K2CO3 Potassium Carbonate

Ksv Stern-Volmer constant

LED Light Emitting Diode

xi

LB Langmuir-Blodgett

m Multiplet

m/z Mass/Charge

MeOH Methanol

mg Milligram(s)

mN/m milliNewton/meter

mL Milliliter(s)

mmol Millimol

MW Molecular Weight

NBS N-bromosuccinimide

nm nanometer

OBn O-Benzyloxy

OM Optical Microscope

OPA One Photon Absorption

OPP Oligo(para-phenylene)

PA Polyacetylene

PANI Polyaniline

Pd(0)P(Ph3)4 Tetrakis(triphenylphosphine)Palladium(0)

Pd/C Palladium on Carbon

PDI PolyDispersity Index

PF Polyfluorene

PL Photoluminescence

PPE Polyphenyleneacetylene

PPP Poly(p-phenylene)

xii

xiii

PPS Poly(phenylene sulphide)

PPV Polyphenylenevinylene

PPY Polypyrrole

PT Polythiophene

ps Picosecond

QY Quantum Yield

q Quartet

ROS Reactive Oxygen Species

RT Room Temperature

s Singlet; Second

SEM Scanning Electronic Microscope

t Triplet

tt Transit time

TCSPC Time-Correlated Single-Photon Counting

TEAC Trolox Equivalent Antioxidant Capacity

TEM Transmission Electron Microscope

TGA Thermo Gravimetric Analyzer

TMS Tetramethylsilane

THF Tetrahydrofuran

TLC Thin Layer Chromatogharphy

TPA Two Photon Absorption

UV-vis Ultra-Violet Visible spectroscopy

WSP Water Soluble Polymer

XRD X-Ray Diffraction

Table No.

LIST OF TABLES

Page No.

Chapter 1

Table 1.1 “Precursor polymers” based on single or binary tethered electroactive monomers.

19

Table 1.2 End-capped conjugated polymers and oligomers 28

Table 1.3 Cross-conjugated polymers and oligomers 32

Chapter 2

Table 2.1 Structural characteristics of copolymers P1-P4 67

Table 2.2 The absorption and emission wavelengths for copolymers P1-P4 in THF

70

Table 2.3 X-ray diffraction data for copolymer P2 72

Chapter 3

Table 3.1 Structural characteristics of polymers PFlMMA-b-PBMMA and PCzMMA-b-PBMMA

93

Table 3.2 X-ray diffraction data of block copolymer PCzMMA-b-PBMMA and PFlMMA-b-PBMMA

100

Chapter 4

Table 4.1 Photophysical properties of DTE-Ph and DTE-Naph in CHCl3 120

xiv

xv

Chapter 5

Table 5.1 Summary of physical measurements of OLG1-OLG4 143

Table 5.2. Observed and calculated reflections from X-ray diffraction data at room temperature of self-assembled OLG1

149

Chapter 6

Table 6.1 Physical properties of copolymers P1-P3 162

Figure No. LIST OF FIGURES

Page No.

Chapter 1

Figure 1.1 Schematic representations of nanostructure fabrication; lithography (a) and self-assembly (b).

5

Figure 1.2 Mesoscopic assemblies and Packing parameter representation (Redraw from Nature Materials 2005, 4, 729)

7

Figure 1.2.1 Schematic representations of bilayer supramolecular assemblies; micelles (a), vesicles (b) and lamellae (c).

9

Figure 1.3 Schematic representations of supramolecular assemblies; columnar stacking (a), helical (b) and tubular (c).

10

Figure 1.4 Schematic representations of copolymer morphologies. The volume fraction of one component increases from left to right. The morphologies are body-centered cubic spheres (BCC), hexagonally packed cylinders (H), gyroid (G), double gyroid (DG), lamellae (L).

12

Figure 1.5 Morphologies of PB-b-PEO in water. Four basic structural motifs—bilayers (B), Y-junctions (Y), cylinders (C), and spheres (S); cryo-TEM, micrographs (From left to right); vesicles, Y-junctions, and wormlike micelles.

14

Figure 1.6 Amphiphilic comb-dendronized block copolymers P(PEOMA)-b-DPS and its liquid crystalline property and self assembly in THF/MeOH solution.

16

Figure 1.7 Schematic representations of grafted precursor polymers; PMMA-g-PPY (a) and PMMA-g-PTh (b).

19

Figure 1.8 Photochromism; stilbene (1), diarylethenes with thiophene rings (2), and perfluorocyclopentene with thiophene rings (3).

41

Figure 1.9 Structure of dithienylethene-pyrene diad (a) and dithienylethene-pyrene-dithienylethene triad (b) showed 2D structural order.

42

xvi

Chapter 2

Figure 2.1 Molecular structure of copolymers (P1-P4). 63

Figure 2.2 FT-IR spectra of copolymer P1-P4. 67

Figure 2.3 TGA (A) and DSC (B) of copolymers; P1 (▲), P2 ( ), P3 ( ), and P4 ( ) at heating rate of 10 °C/min under N2 atmosphere.

68

Figure 2.4 Absorption (A) and emission spectra (B) of copolymers in THF solution; P1 (▲), P2 ( ), P3 ( ), and P4 ( ).

70

Figure 2.5 TEM images of nanofibres formed by self-assembly of copolymers (P2) in 0.01mg/mL.

71

Figure 2.6 X-Ray diffraction pattern of the copolymer (P2) film on glass slide.

72

Figure 2.7 Schematic representation of molecular self-assembly through aggregation of polymer chains and π – π stacking of the electroactive groups on the side chain.

73

Figure 2.8 Nanopatterning of polymer (P3) film at various tip bias (a) and

tip speed (b). Dot patterning on polymer film at various tip bias (c) and contact time (d).

74

Figure 2.9 CV for electrochemical polymerization (cross-linking) of spin coated polymer film in ITO substrate; P1 (A), P2 (B), P3 (C), and P4 (D) at the scan rate 50 mV/s.

76

Figure 2.10 Polymer free scan of polymer films; P1 (A), P2 (B), P3 (C), and P4 (D).

77

Figure 2.11 UV-vis spectra of electropolymerized thin film copolymers; P1 (A), P2 (B), P3(C), and P4 (D); precursor Polymers (▲) and electropolymerized thin films ( ).

78

Figure 2.12 FT-IR spectra of electrochemical cross-linking polymer film with precursor polymers; P1 (A), P2 (B), P3 (C), and P4 (D); precursor-polymer ( ) and after electropolymerization (▲).

79

xvii

Figure 2.13 Mechanism for the electropolymerization (cationic) and cross-linking of P2.

80

Chapter 3

Figure 3.1 Molecular structure of diblock copolymers (PCzMMA-b-PBMMA and PFlMMA-b-PBMMA).

89

Figure 3.2 GPC curve (a) and FT-IR spectra (b) of block copolymers in KBr; PFlMMA-b-PBMMA (▲), PCzMMA-b-PBMMA ( ).

93

Figure 3.3 Thermogravimetry analysis (TGA) of PFlMMA-b-PBMMA (▲) and PCzMMA-b-PBMMA ( ); protected (a) and deprotected (b) in N2 atmosphere with a heating rate of 10 ºC/min.

94

Figure 3.4 Absorption (a) and emission (b) spectra (excitation at 290 nm) of PCzMMA-b-PBMMA (▲) and PFlMMA-b-PBMMA ( ).

95

Figure 3.5 SEM micrographs of block copolymers; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

97

Figure 3.6 TEM images of block copolymers: PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

98

Figure 3.7 X-ray diffraction pattern of diblock copolymer; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b) on a glass slide.

99

Figure 3.8 Schematic diagram of microphase separation according to the amphiphilicity of block copolymers.

99

Figure 3.9 Ten cycles of CV for electrochemical polymerization of block copolymers; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b). Polymer free scans for block copolymers after electropolymerization of PCzMMA-b-PBMMA (c) and PFlMMA-b-PBMMA (d).

102

Figure 3.10 Linearity of current with number of scans of electropolymerization of block copolymers; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

103

xviii

Figure 3.11 Absorption and emission spectra of electropolymerized deposited film on ITO with precursor block copolymers; PCzMMA-b-PBMMA (a, b), PFlMMA-b-PBMMA (c, d). Precursor polymer (▲) and electropolymerised film ( ).

105

Figure 3.12 ATR-FTIR spectra of block copolymers PCzMMA-b-PBMMA (▲) & PFlMMA-b-PBMMA ( ); precursor polymer (a) and after electropolymerization (b).

105

Figure 3.13 Increase in absorption peak during electropolymerization (CV) of block copolymers at different number of cycles; 3 (▲), 5 ( ), 10 ( ), and 15 ( ); PCzMMA-b-PBMMA (A) and PFlMMA-b-PBMMA (B).

106

Figure 3.14 AFM images of conjugated polymer network film of 10 cycles on ITO; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

107

Chapter 4

Figure 4.1 Structural representation of amphiphilic dithienylethene DTE-Ph and DTE-Naph.

114

Figure 4.2 FT-IR spectra of target molecules; DTE-Ph ( ) and DTE-Naph (▲).

118

Figure 4.3 Absorption (A and C) and emission (B and D) spectra of dithienylethenes photochromism in chloroform solution; DTE-Ph (A and B) and DTE-Naph (C and D).

121

Figure 4.4 Change in the color upon irradiation of azo-thienylethenes; DTE-Ph (a) and DTE-Naph (b).

121

Figure 4.5 Change in the UV-vis absorption (A) spectra (recorded at 625-640 nm) and normalized emission (recorded at 595 nm) for chloroforms solution of DTE-Ph and DTE-Naph when irradiated with 356 nm light over 900s period [absorption (▲) and emission ( ) of DTE-Ph; absorption ( ) and emission ( )] of DTE-Naph. Normalized emission (B) as a function of normalized absorption for solution of DTE-Ph (▲) and DTE-Naph ( ) at 10-5 M.

122

Figure 4.6 Cyclic voltammogram of photochromic dithienylethene molecules; before (▲) and after ( ) irradiation of DTE-Ph (a)

123

xix

and DTE-Naph (b).

Figure 4.7 Photo-conductivity (I-V) characteristics of DTE-Ph (a) and DTE-Naph (b); (▲) before and ( ) after irradiation.

123

Figure 4.8 AFM images of azo-dithienylethenes on mica substrate; DTE-Naph (a) and DTE-Ph (b).

124

Figure 4.9 AFM images of azo-dithienylethenes on Si (111) substrate; DTE-Ph (a - b) and DTE-Naph (c - d).

126

Figure 4.10 AFM images of DTE-Naph at different concentration on Si (111) substrate; 1.2 x 10-3 (a) and 1.05x 10-8 M (b).

127

Figure 4.11 AFM images of DTE-Naph film on Si (111) surface annealed at 120 °C for 2 h

127

Figure 4.12 Absorption (a) and emission (b) spectra of DTE-Naph in chloroform; solution (▲) and nano-rings ( ) on Si (111) substrate.

128

Figure 4.13 1H NMR of – OH proton; DTE-Ph (a) and DTE-Naph (b).

129

Figure 4.14 Schematic representation of self-assembled DTE-Naph molecule into columnar stacks.

129

Chapter 5

Figure 5.1 Designed molecular structure of oligomers (OLG1-OLG4). 136

Figure 5.2 FT-IR spectra of target molecules OLG1 – OLG4. 140

Figure 5.3 Thermal properties of OLG1 (▲), OLG2 ( ), OLG3 ( ), and OLG4 ( ); TGA (A) and DSC (B) (2nd heating) at heating rate of 10 min/ ⁰C in N2.

141

Figure 5.4 Absorption and emission spectra of OLG1 (▲), OLG2 ( ), OLG3 ( ), and OLG4 ( ); THF solution (A) and thin film (B) in quartz surface.

142

Figure 5.5 CV curves of OLG1-OLG4 measured in CH2Cl2 containing n- 145

xx

Bu4NPF6 as supporting electrolyte at a scan rate of 50 mV/s.

Figure 5.6 Absorption (a) and emission (b) spectra of OLG1 in THF/water solution.

146

Figure 5.7 Scanning electron microscope (a, b) and transmission electron microscope (c, d) images of nano-rods formed by self-assembly of OLG1 in aqueous solution (THF: H2O).

147

Figure 5.8 XRD patterns of OLG1; powdered (▲) and self-assembly ( ) sample drop casted on glass cover slip

148

Figure 5.9 Proposed supramolecular assembly through side-chain association of OLG1.

150

Chapter 6

Figure 6.1 Structure of the target copolymers P1-P3. 157

Figure 6.2 FT-IR spectra of copolymers P1 (▲) and P2 ( ). 162

Figure 6.3 Absorption (a) and emission (b) spectra of polymers in THF solution; P1 ( ), P2 ( ), and P3 (▲).

163

Figure 6.4 Absorption (a) and emission (b) spectra of polymers in thin film; P1 ( ), P2 ( ), and P3 (▲).

164

Figure 6.5 Thermogravimetry analysis (TGA) of cross-conjugated copolymers; P1 (▲), P2 ( ), and P3 ( ) at heating rate of 10 °C/min under N2 atmosphere.

165

Figure 6.6 CV for electrochemical cross-linking film; P1 (a), P2 (b), and P3 (c) followed by their linearity curves P1 (d), P2 (e), and P3 (f).

167

Figure 6.7 AFM morphologies of polymers in THF on ITO substrate before electropolymerization; P1 (a), P2 (b), and P3 (c).

169

Figure 6.8 Tapping-mode AFM topography images of P2 after electropolymerized on ITO at scan rate of 100 mV/s; flatten nano-fibers (a-b).

169

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Scheme No.

LIST OF SCHEMES Page No.

Chapter 2

Scheme 2.1

General synthetic approach to monomers. 64

Scheme 2.2

General synthetic approach to copolymers (P1-P4). 65

Chapter 3

Scheme 3.1

Synthetic procedures for monomers. 90

Scheme 3.2 Synthetic procedures for homopolymer and block-copolymers. 91

Chapter 4

Scheme 4.1 General synthetic strategy for target DTE-Ph. 116

Scheme 4.2 General synthetic strategy for target DTE-Naph. 117

Chapter 5

Scheme 5.1 General synthetic strategy of compounds (28-35). 138

Scheme 5.2 General synthetic strategy of compounds (36-41). 139

Scheme 5.3 Synthesis of targets oligo (p-phenylene) OLG 1- OLG4.

139

xxii

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Chapter 6

Scheme 6.1 Synthetic procedures for thiophene based monomers (M1-M2). 158

Scheme 6.2 Synthetic procedures for monomer M3. 160

Scheme 6.3 Synthetic procedure for copolymers P1-P3.

161

Satyananda Barik National University of Singapore

Chapter 1

Introduction

1


Nature uses only the longest threads to weave her pattern, so that each small piece of her fabric reveals the organization of the entire tapestry. - Richard. P. Feynman

Molecules are created by the covalent bonding of atoms. A finite set of building

blocks is being utilized for the creation of infinite number of molecules. This diversity is

often achieved through non-covalent assembly of smaller units (atoms/molecules) into

larger and complex structures. This organizational science that transcends conventional

chemistry is termed ‘Supramolecular Chemistry’. This idea has attracted many scientists

to understand the organization and interactions. As a result, one of the ultimate aims of

supramolecular chemistry is to create self-assembled molecular materials.

1.1. Supramolecular Chemistry

Chemistry is a science of interactions and transformations of models. Chemical

synthesis is a prevailing tool to produce new molecules and materials with distinct

properties. The atomic or molecular arrangement in molecular chemistry was introduced

for the development of highly sophisticated complex structures.1a-b The importance of

supramolecular chemistry was recognized in 1987, when Nobel Prize were awarded to

Prof. Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen in recognition of their

pioneering work in this area. Particularly the development of selective "host-guest"

complexes (a host molecule recognizes and selectively binds a certain guest) was cited as

important contribution.1c Major developments in supramolecular chemistry over the last

three decades involved the design of systems, which were capable of generating well-

defined supramolecular entities via self-assembly under a given set of conditions.2

2

http://www.answers.com/topic/nobel-prize


Self-assembly and self-organization have recently been implemented in numerous of

organic and inorganic systems.3 Chemists utilize molecular synthesis (step-by-step

synthesis) to create new molecules with interesting functions.4a-b Supramolecular self-

assembly is a spontaneous process in which several molecular components organize into

ordered structures.4c In short, organic chemistry generates new molecules while self-

assembly generates ordered assemblies of molecules.5 Thus, supramolecular chemistry

helps to unravel the complexity of matter through self-organization. Formation of lipid

bilayers,6 self-assembled monolayers (SAMs) of molecules,7 molecular crystals,3a,8 phase

separated polymers,9 and quaternary structure of proteins,10 are a few examples of

molecular self-assembly. Self-assembly represent the most useful approach to generate

architectures with both structural and functional control at molecular level. In the past, the

study of supermolecules was aimed to mimic biological structures and functions.

Recently, this covers multidisciplinary areas of research. Recent developments in the

construction and organization of novel functional materials on the micro and nanoscale

dimensions lead to their potential applications in microelectronics,11 photonics,12 host-

guest chemistry13 and nanoscience.14-16

The field of supramolecular chemistry has branched into three significant areas; such

as (i) molecular recognition between a host and guest molecule;17 (ii) molecules built in

order to generate specific structures (crystal engineering),3a (iii) molecular assembly from

numerous molecules (molecular self-assembly).18 Molecular recognition provides logical

understanding of several biological processes, enzyme catalysis and biosynthetic pathways

in which selective recognition of a small guest molecule by a large host occurs.17 Crystal

engineering, on the other hand, is concerned with the design and synthesis of crystalline

3


materials with specific and tunable properties.3a However, molecular self-assembly deals

with the formation of polymolecular entities from spontaneous association of components

into a specific phase with more or less macroscopic or microscopic characteristics.19-22

1.2. Supramolecular Polymer Chemistry: An Overview

Supramolecular polymer chemistry involves molecular interactions and reorganization

process to generate interesting structures with new properties.23 Polymer technology can

be benefited from supramolecular chemistry to obtain new materials with controlled

internal dimensions ranging from nanometer to macroscopic scale. Supramolecular

polymer displaying constitutional diversity is determined by the choice of different

monomers. The exploration of supramolecular polymer chemistry gives access to wealth

of novel entities and functionalities.24

Supramolecules built from small molecules involve intermolecular interactions. On the

other hand, supramolecular association of macromolecules may be intermolecular or

intramolecular, involving recognition sites located either on the main chain or on the side-

chain. The direct manipulation of intermolecular interactions gives access to control the

self-organization in two- and three dimensions of macromolecular assemblies.25 Owing to

the presence of large functional groups, supramolecular polymers may be highly

continuity than conventional polymers.

1.2.1. Fabrication of nanostructure materials

Self-assembly is the most commonly used method for preparing nanostructure

materials. Lithography has been used to fabricate materials from macro to nanometer

4


dimensions (Figure 1.1a). A large number of lithographic methods; such as photo, UV-vis

and X-ray lithography have been developed predominantly for the electronic industry.26-27

Other lithographic methods include soft lithography,28 scanning probe lithography

(SPL),29 and edge lithography.30 Among several nanolithographic methods, SPL is one of

the easiest lithographic techniques for forming nanostructures. It involves the usage of a

sharpened tip of an atomic force microscope (AFM) or a scanning tunneling microscope

(STM). Nanostructures with the highest spatial resolution have been achieved using this

technique.29 Lithography finds a wide range of potential applications in electronic

devices,31 and biochips.32 It is difficult, however, to organize the nanostructures into well-

defined patterns for integrated and functional devices.

Molecular synthesis

Self-assembly(b)

(a)

Figure 1.1. Schematic representations of nanostructure fabrication; lithography (a) and self-assembly (b).

On the other hand, an enabling technique for nanotechnology, self-assembly replaces

top-down fabrication with the possibility of bottom - up fabrication.2,34-35 Self-assembly

deals with organizing atoms, molecules and macromolecules into (Figure 1.1b) ordered

structures. It is a straightforward and popular approach for fabricating materials with size

ranging from micro to nanoscale. The most intriguing feature of this approach is the

5


molecular level to nanoscale organization during the preparation process. This can be

easily controlled by tailoring the physicochemical properties of the components,

environment and dynamics of the process. In general, self-assembly processes occur near

or at equilibrium state. Such structure formation typically takes place in fluid phases, as it

relies on the mobility and interactions of the components. The selective interactions or

recognition between the components facilitate the transformation from a less ordered state

to a more favorable ordered state.36-40 They often arise from a process of phase separation

induced spatial periodicity.

1.2.2. The macromolecular assembly

Macromolecular assemblies leading to precise, three dimensional (3D) structures

through controlled assembly of non-covalently bonded polymers are of interest to material

scientists.41-45 In recent years, amphiphilic reagents like surfactants,48 phospholipids,49

amphiphilic oligomers/polymers50 have received considerable attention and has been

extensively reviewed. During the past decades, supramolecular chemistry has made

substantial progress in the construction of non-bonding macro-assemble from synthetic

macromolecules.1-3 There are three types of macromolecular architectures; namely linear,

cross-linked, and branched.46 Structure-controlled macromolecular architectures introduce

geometrical amplification, expression, and properties.47 The scientific development of

supramolecular assemblies in polymer science is now rapidly leading towards more

complex polymeric architectures.

Mesoscopic assemblies are defined as hierarchical supramolecular structures whose

ternary and higher assembly structures are controlled through solvophilic – solvophobic

interactions. The mesoscopic assemblies are formed by multiple non-covalent interactions

6


such as electrostatic interactions, hydrogen bonding, dipole – dipole interactions, and

hydrophobic associations.12a,51 The geometry of the molecule dictates the geometry of the

aggregate. Commonly adopted in the molecular level of mesoscopic assemblies described

from the geometric packing parameter, P = V/al, where, V is the tail volume per molecule,

‘a’ is the average cross-sectional area per amphiphilic molecule, and ‘l’ is the length of the

hydrophobic tail, used to predict the nature of the structure formed.52-53 The packing

parameter is not always constant and it depends on solution conditions such as

temperature and concentration. The pictorial representation of mesoscopic assemblies and

packing parameter (P) is shown in Figure 1.2.

Spherical Micelles V/al < 1/3

Cylindrical Micelles 1/3 < V/al < 1/2

Vesicles, flexible bilayer 1/2 < V/al < 1

Lamellae, planar bilayer V/al ≈ 1

Inverse Micelles V/al > 1

(Cross‐section)

Amphiphilic Molecule

Figure 1.2. Mesoscopic assemblies and Packing parameter representation (Redraw from Nature Materials 2005, 4, 729)53b

7


a) Micelles: The foundation of today’s bottom-up nanotechnology involves the

formation of aggregates of amphiphilic molecules into well ordered structures (micelles).

A typical micelle in aqueous solution forms an aggregation with the hydrophilic "head"

region in contact with surrounding solvent, sequestering the hydrophobic tail region in the

micelle centre (Figure 1.2.1a).52 The shape and size of a micelle is a function of the

molecular geometry of surfactant molecules as well concentration, temperature, pH,

and ionic strength of the solution. The process of forming micelle is known as

micellization and it forms a part of the phase behavior of many lipids.

b) Vesicles: Vesicles are the spherical shell structures comprising a bilayer of

amphiphiles. Any mismatch in the effective interfacial area per hydrophile compared with

hydrophobic groups lead to a vesicle structure (Figure 1.2.1b).52b, 53 It can be defined

from surfactant packing parameter p = V/al, where V is the volume per molecules, ‘a’ is

the effective cross-sectional area per molecule and ‘l’ is the chain length normal to the

interface. Vesicles are only observed for ½ < p < 1. Vesicles are formed because they get

rid of the edges of bilayers, protecting the hydrophobic chains from water, but they still

allow room for relatively small layers.

c) Lamellae: Hydrogen bonding in the aqueous media requires the integration of the

other non-covalent interactions such as hydrophobic interaction or aromatic stacking,

forming a bilayer polymer crystal called lamellar architecture.54 Lamellar structures are

fine layers, alternating between different materials by chemical effect. The schematic

representation of lamellar formation is shown in Figure 1.2.1.c. Species ‘A’ possesses

hydrogen bonding group, which is covalently connected to a hydrophilic head group via

spacer unit and species ‘B’ possesses a complementary hydrogen bonding group attached

8

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to the hydrophobic head unit. If the complementary hydrogen bonding is formed in the

water, the resultant hydrogen bond pair ‘B-A’ acquires higher amphiphilicity. The

supramolecular amphiphile ‘B-A’ thus generated is expected to hierarchically self-

assemble into the tetra-layer (supramolecular) lamellar structure.

d) Columnar: A characteristic feature of columnar structures is the formation one-

dimensional stacks of columns, which in turn arrange themselves on two-dimensional

lattices (Figure 1.3a). This can be exemplified by the hexa-alkoxy triphenylenes and their

self-assembly.55 The aromatic planes are stacked within their van Der Waals radius and the

column organized into a regular arrangement.

(a) (b)A

B

(c)

Figure 1.2.1. Schematic representations of bilayer supramolecular assemblies; micelles (a), vesicles (b), and lamellae (c). e) Helical: This superstructure is formed through the self-assembly of amphiphiles

with polar heads and hydrophobic tails with a chiral core. It is transferred to the core stack

forming helical structure (Figure 1.3b).56 For example, the metallo-mesogens twist

themselves into a chiral propeller like conformation whose handedness is attributed by the

chirality of the side chain.

f) Tubular: The linear micelle like building blocks triggering a longitudinal growth

in solution is called a tubular structure (Figure 1.3c). To form tubular stacks, the rigid

9


rods are held together in lengthwise to form a ring/channel, which further stacks to form

tubes. The channel, disk or ring is held together through intermolecular non-bonding

interactions between the side chains. This architecture assembly has been extensively

studied by Fenniri and co-workers.57

g) Liquid crystal: Liquid crystal is a phase of matter that has properties between a

conventional liquid and a solid crystal. There are many different types of LC phases

based on their different optical properties and appears with distinct textures. The

contrasting area in the texture corresponds to a domain, where the molecules are well

ordered and oriented in a different direction.58

Liquid crystals can be classified into two main categories: thermotropic liquid crystals,

and lyotropic liquid crystals.59 These two types of liquid crystals are distinguished by the

mechanisms drive their self-organization, but they are also similar in many ways. For a

molecule to display the characteristics of a liquid crystal, it must be rigid and rod-shaped.

This is accomplished by the interconnection of two rigid cyclic units, which leads to a

linear planar formation.

(a) (b) (c)

Figure 1.3. Schematic representation of supramolecular assemblies; columnar (a), helical (b), and tubular (c).

10

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http://en.wikipedia.org/wiki/Texture_(crystalline)


1.3. Block Macromolecular Self-assembly: A Recent Study

A macromolecule which is composed of blocks in a linear sequence is termed as a

copolymer. A block copolymer is a macromolecule composed of two or more distinct

section of different chemical species are chemically bonded together to a single molecular

structure. The resulting chain topologies can be linear, branched, or cyclic. Systematic

studies of these materials were possible through developments in polymerization

techniques.60 This allowed to implement the desired functionality for structural

organization, which helps the material to efficiently perform its function.

Physical properties of isolated copolymers are predominantly determined by their

chemical structures. However, these properties are significantly changed in solid state.

The interactions between polymer chains mainly depend on the polymer packing and

conformation of the pendant units attached to the polymer backbone. Inherent chemical

structure and nanostructure of the polymers are the two critical parameters controlling the

functional properties of polymers.61 The ordering or morphology, depend on the

experimental conditions, solvent, and the structure of copolymer.

The phase behavior of copolymers is of great interest from both scientific and

commercial point of view. The phase behaviour mostly results from the packing constraint

imposed by the connectivity of each blocks and the mutual repulsion of the disparate

blocks. This is termed as microphase separation. The copolymers of incompatible coil/rod

segments have been found to exist in a wide range of microphase separated

supramolecular structures. The most common morphologies are spheres, cylinders,

double diamond (DD), double gyroid (DG), and lamella (Figure 1.4).62 Different

11

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microphase separated morphologies can occur, which depends on the relative

compositions of components and the molecular weight of the copolymer. The aggregation

state of blocks (amorphous, liquid-crystalline, and crystalline) or the presence of solvents

(formation of micelles, vesicles) influence the morphology of the copolymer to a greater

extent.60 Recently, use of block copolymers as template to construct mesoscale structures

is of great interest to

BCC H G DG L

Figure 1.4. Schematic representations of copolymer morphologies. The volume fraction of one component increases from left to right. The morphologies are body-centered cubic spheres (BCC), hexagonally packed cylinders (H), gyroid (G), double gyroid (DG), lamellae (L).

The organization may vary, from simple dispersion with a preferential alignment, and

periodicity of each blocks. So creation of the necessary structural order with structural

elements covered a wide range of length scales. Directed self-organization process of

block copolymers facilitated the development of a wide variety of structures such as

micelles,63 vesicles,64 films,65 nanowires,66 network structures67 and nanoprisms.68

Surfactants (amphiphilic) and phase-separated block copolymers,69 are common tool for

various types of self-assembled materials. The amphiphilic and polyelectrolyte block

copolymers self-assemble into similar morphologies as they are observed in the bulk state.

In the field of block copolymers a lot of work has been done and extensively reviewed.60-

62

12


Fredrickson et al.70 discovered the first diblock copolymer polystyrene-b-polyisoprene

(PS-b-PI) and investigated the morphology using TEM. They observed three different

morphologies such as spheres, cylinders and lamellar structures. They explained that the

connectivity of different incompatible blocks restricted the assembly mode to specific

morphologies. These morphologies were observed within a relatively small composition

range between the blocks.

Eigenberg et al.71 proposed morphologies such as vesicles, micelles from polystyrene-

b-polyacrylic acid (PS-b-PAA) and polystyrene-b-polyethylene(oxide) (PS-b-PEO)

diblock copolymers in DMF/water mixture. The aggregation of diblock copolymer (PS-b-

PAA) into micelles with edge length of 200-600 nm was observed upon evaporation of the

solvents. The effect of organic solvent in the micelle formation is not completely

understood. It is expected that the hydrogen bonding of PAA block might have played a

role for micelle formation. A number of complex morphologies of block copolymers have

been reviewed by this group.

In a similar study, Bates et al.72 showed a novel morphology in aqueous solution of

PB-b-PEO diblock copolymer. Branched wormlike micelles containing Y-shaped

junctions and three dimensional (3D) network structures were investigated using TEM.

They demonstrated that, PEO block behaves an intermediate between bi-layer vesicles

and cylindrical micelles in the Diblock copolymer. Eventually the polymer rich network

phase separates from solution. The network structures were broken up by stirring or

sonication and fragments comprising Y-junctions, spherical caps and cylindrical loops

were formed (Figure 1.5).

13


R

O

O

Hm

m-1

PB-b-PEO

water

Figure 1.5 Morphologies of PB-b-PEO in water. Four basic structural motifs—bilayers (B), Y-junctions (Y), cylinders (C), and spheres (S); cryo-TEM, micrographs (From left to right); vesicles, Y-junctions, and wormlike micelles.72

The unique ability of amphiphilic molecules to self-organize at interfaces, leads to

modification of interfacial properties and enhances the compatibility of two phases.73 In

recent years, the demand for advanced materials with new properties has sparked the

development of novel functionality in a predictable and controllable way. The

development of new functional materials can be realized by varying the polymer chain

length, periodicity, and number of layers.74 Such intricate architecture facilitates the

properties, which could be tuned for specific applications such as sensors,75 enhance

electroluminescence,76 photoelectrochemistry76 and nano-devices.77

Recently, rod-rod, rod-coil, and coil-coil amphiphilic block copolymers78 have gained

much interest in supramolecular polymer self-assembly.79 The multi-component polymer

14


could be benefited from morphological structural ordering ability. These can form

continuous crystallizable domains type interconnected cylinders, lamellae, and matrix-

forming blocks. Nojima et al.80 investigated a diblock copolymer of polybutadiene-b-

polycaprolactone, in which the microphase separate into crystallizable domains above the

crystallization temperature of polycaprolactone. At lower temperature, it showed regular

lamellar microstructures and at higher temperature cross-linking of polybutadiene matrix

suppressed the morphological phase transition.

Similarly, Chen et al.81 reported a diblock copolymer of tightly packed dendrons on

the polymer backbone. They obtained dendronized polystyrene-block-polyethyleneoxide

grafted polymethacrylate (PS-b-PEOMA) using ATRP polymerization method (Figure

1.6). The comb-dendronized block copolymer exhibited hexagonal columnar liquid-

crystalline (LC) phase. The self-assembly behavior of block copolymer in solution was

also achieved. By changing the polarity of solvents, the copolymers showed various self-

assembled morphologies such as twisted strings, vesicles, and large micelles. In addition,

they proposed that the amphiphilic properties and the tendencies toward microphase

separation could be tuned by changing the structure and generation of dendronized block.

15


THF/MeOH

LC

Twisted string vesicles Large micelles

O

OO

O

*O

O

O

OC13H27

C13H27O

C13H27O

O

Om n

x

Figure 1.6 Amphiphilic comb-dendronized block copolymers P (PEOMA)-b-DPS with its liquid crystalline property and self assembly in THF/MeOH solution.81

Research in the field of semiconducting polymer/ oligomers were recently renowned

by awarding of the 2000 Nobel Prize in Chemistry to Heeger, Shirakawa and

MacDiarmid. The opportunity for supramolecular polymer chemistry is to improve the

functionalities of materials in the field of molecular electronics, which is very much

important to tune the properties of the system.82 Tuning electronic properties of conjugated

polymers along with the supramolecular morphology, is still a challenge. The electronic

functions could be either in the main chain of the blocks or in substituent. Due to the

chemical connectivity of the two blocks, they are capable of changing supramolecular

organization, leads to the development of dynamic devices.83-90

16


1.4. Precursor Copolymer Approach

The viability of the “precursor polymer” approach91 is based on a single or multiple

pendant electroactive (polymerizable) groups using various polymer backbones. This

involves tethering of an electroactive moiety to the polymer backbone resulting in the

formation of conjugated polymer network (CPN). Key advantage of precursor polymers

includes low friction co-efficient, rigidity, low surface energy, hydrophobicity, self-

organization, chemical inertness and thermal resistance. This makes them a better

candidate for electroluminescence devices.92 A variety of combination is possible for the

design of precursor polymer backbone and the electroactive monomer pendant unit in

order to form the CPN film. A major challenge in this area is to design materials with

tailored properties of high solid state order.

1.4.1. Structural organization of precursor copolymer

Supramolecular nanostructures derived from self-organizing molecules and

macromolecules are of great interest to achieve materials with interesting properties. In

general, self-organized structures were formed spontaneously without intervention.

Significant amount of work has been carried out in the synthesis and size control of

materials. Assembling them on substrates for useful purposes such as fabrication of

devices over a large area still remain as a challenge.93 The control over nanoscale

dimensions for the spatial arrangement of the building blocks remains as an important

objective for material scientists.

In the recent years, the demand for functional materials with new properties has

sparked the development of novel and innovative synthetic methods. For the synthesis of

17


precursor copolymer viability, one approach is to synthesize block copolymers with

conducting polymer block and soluble classical polymer block. François et al.94

discovered that a solution of a block or graft copolymer showed aggregates with excellent

thermal stability. Later on, Hallensleben et al.95 synthesized block and graft copolymers of

poly(methylmethacrylate) and poly(pyrrole)/poly(thiophene) (Figure 1.7). The authors

explored the precursor polymer approach to synthesize homogeneous conducting

polymers and the resulting polymers were stable in solution as aggregates. The films

casted from this solution reveals conductivity in the range of 0.2 – 0.4 S/cm.

Presently, carbazole, thiophene and fluorene copolymers are of interest due to their

electrochemical homopolymerization and copolymerization characteristics. Substituted

polycarbazole, polythiophene, and polyfluorene are of great interest since they are very

good hole and electron transporting material, which makes them a potential candidate for

optoelectronic devices. In the pioneer work of “precursor polymer” on optoelectronics,

Advincula et al.96 demonstrated CPN or cross-linked film formation approach from

dendrimers to polymers with tethering the backbone and electroactive monomer groups.

Multiple and precise placement of different components are not well-explored for

various applications. A number of homopolymers and copolymers with one type of

tethered electroactive monomer have been investigated in detail.96-110 Examples of multi-

component materials that could benefit from morphological structuring, the active layer of

a photovoltaic cell or a polymer light-emitting diode. An overview of the typical examples

of “precursor polymer” and polymerization methods are given in Table 1.1.

18


N

OO

NHN

H

H2C *

OCH3

O

* m n

(a)

OO

H2C *

OCH3

O

* m n

SS

S

(b)

Figure 1.7 Schematic representations of grafted precursor polymers; PMMA-g-PPY (a) and PMMA-g-PTh (b).95

Table 1.1 “Precursor polymers” based on single or binary tethered electroactive monomers.

Precursor Polymer Polymer Synthesis Method used Reference

N

CH CH2

N

HC CH2

S

S

CPN1

Chemical, electrochemical polymerization

97

N

CH CH2

S

n

N

HC CH2

S

S*

CPN2

Chemical and electrochemical polymerization

97a

NN

NN

CPN3

Self-assembly (Patterning) 92a

S S

N

S S

N

** n

CPN4

Electrochemical 98

S

OR

S

OR

** n CPN5

Electrochemical 99

19


(CH2)m

(CH2)m

** n CPN6

Chemical 96d

**

O

O

(CH2)5

(CH2)5

CH3

N

**

O

O(CH2)5

(CH2)5

CH3

N

* n

CPN7

Electrochemical 100

N

**

R1

n

N

**

R1

* * CPN8

Electrochemical 101

O

N

**

O

N

**

** n CPN9

Electrochemical 102

*

O

S n

*

O

S ** CPN10

Electrochemical 103

O

N

**

O

N

*

**

nCPN11

Electrochemical 103

20


N

O O

OOR

N

O O

OOR

** n

CPN12

Electrochemical 104

O

S

OCnF2n+1

O

S

OCnF2n+1

n* *CPN13

Electrochemical 105

O

OCnF2n+1

N

O

OCnF2n+1

Nn**

CPN14

Electrochemical 105

O

OCnF2n+1

O

OCnF2n+1

**n CPN15

Electrochemical 105

N

(CH2)2

OO

CH2CH2C*

HC *

ONH

CH2

SO3Na

N

(CH2)2

OO

CH2CH2C*

HC *

ONH

CH2

SO3Na

** CPN16

Chemical 106

N

(CH2)3

OO

CH2CH2C*

HC *

ONH

CH2

SO3Na

N

(CH2)3

OO

CH2CH2C*

HC *

ONH

CH2

SO3Na

** CPN17

Chemical 106

N

*

N

N

*

Br

n

N

*

N

N

*

Br

CPN18

Electrochemical 107

21


N

OO

**

O OH

n

N

OO

**

O OH

CPN19

Electrochemical 107

NN (CH2)m

N

N

(CH2 )m

N

N

(CH2 )m

n CPN20

Electrochemical 108

S

(CH2)11

SiO O Si

Si n

S

(CH2)11

SiO O Si

Si n

** m CPN21

Electrochemical 96c

S

O OS

OO

S

O OS

OO

**

n

CPN22

Self-assembly (Patterning) 109

*

O

S

O

N

*

*

O

S

O

N

*

*n CPN23

Electrochemical 103

22


NO

N

O

N

O N

O

O

OO

HO

NO

NO

OOH

NO O

OH

CPN24

Electrochemical 110

Typically, the donor-acceptor interface of an interpenetrating morphology is more

efficient and may be achieved through phase separated block copolymers. Therefore,

microphase separation of functional block copolymers consisting of an electron-

transporting and a hole-transporting block can be very advantageous for controlling the

structural ordering in PLEDs.

CPN of single pendant electroactive monomers (CPN1-CPN22): The compounds

CPN1 to CPN22, have a single electroactive monomer unit in the precursor polymers.

CPN1 reported by Biswas et al.97a and Valiyaveettil et al. 97b, comprise the chemical and

electerochemical approach towards the synthesis of conjugated polymer network. Biswas

et al.97a showed the formation of conducting composite comprising poly(N-

vinylcarbazole) (PNVC) and polythiophene by oxidative cross-linking through pendant

carbazole moieties in presence of anhydrous FeCl3 and thiophene monomer.

Polythiophene incorporated moiety of PNVC was characterized by SEM analyses and it is

revealed distinct morphological features of the composite. Valiyaveettil et al.97b described

generation of conducting nano-patterns using electrochemical nanolithography on an

ultrathin polymer film of an electroactive precursor polymer, PNVC with atomic force

microscope tip. Advincula et al.96 had investigated a series of CPN thin films from various

precursor polymers such as conjugated polymers (CPN3 and CPN9), polysiloxanes

23


(CPN21), poly(vinyl phenols) (CPN10-CPN11), polyvinyls (CPN6), polyelectrolytes

(CPN16-19), and dendrimer (CPN24). They have explored the film morphology of cross-

linked ultrathin film using AFM. They demonstrated the doping–dedoping reversibility to

tune the work function in a multilayer OLED device. Dendrimer CPN24 can be regarded

as core-ring structure, where carbazole arms towards periphery, due to the cross-linked

conjugation through 3, 6– carbazole linkage. A perfect stacking pattern was observed by

AFM. Syntheses of conjugated polymers with a wide span of band gaps were reported by

Reynolds et al.104 (CPN12) and Nicolas et al.107 (CPN13-15). It was found that

morphology of the functionalized 3, 4-propylenedioxypyrrole cross-linked films were

highly dependent on the electrochemical environment and pendant groups. To obtain

stable hydrophobic and lipophobic surfaces of conducting polymers, Nicolas et al.107

developed fluorinated polythiophene, polypyrrol, and polyfluorene (CPN13-15). Various

microstructures were obtained from the electropolymerization of electroactive groups with

different forms (flat or curved plate, mesh, rod, wire etc) and sizes.

CPN of binary pendant electroactive monomers (CPN23): Advincula et al.103

reported the first binary tethered pendant on a polymer backbone (CPN23). They

successfully synthesized CPN based precursor polymer incorporated with thiophene and

carbazole electroactive monomers onto a poly (4-vinylphenol) backbone. It was an

interesting study as both thiophene and carbazole monomers formed a unique copolymer

composition. Besides that it exhibited optoelectronic applications. Intramolecular and

intermolecular reaction between the individual polymer chains, form a transparent film

and the morphology of the film was correlated well with the deposition behavior.

24


1.4.2. Controlled radical polymerization technique

According to the principles laid out above, self-structuring may be achieved through

an amphiphilic diblock copolymer consisting of pendant tethered single or binary

precursor of electroactive monomers. For the formation of block copolymer, large arrays

of synthetic strategies are known to the chemists. The most prominent method is living

anionic/cationic polymerization.111 One of the drawbacks for this method is the necessity

to work under very strict conditions to avoid impurities such as water and oxygen.

Recently, this has been overcame by the development of the controlled/living radical

polymerization techniques.112 Four “living” free radical polymerization methods are often

employed, namely, stable nitroxide counter radicals113(nitroxide-mediated living radical

polymerization, NMRP), atom transfer radical polymerization (ATRP),114 reversible

addition-fragmentation chain transfer (RAFT)115 and initiator–chain transfer–

termination.116 The NMRP and ATRP techniques employ the principle of an equilibrium

between low concentration of active radicals and a rather large number of dormant

species. This suppresses bimolecular side reactions such as recombination or

disproportionation to certain extent. So the overall polymerization process shows “living”

characteristic and the products are formed with very low polydispersity.

The advantages of living free radical polymerizations are (a) linear increase of

molecular weight with time, (b) possibility of the formation of block copolymers by

reinitiating the polymerization in a different monomer solution, and (c) compatibility with

a wide variety of monomers, e.g., acrylates, styrenes and acrylonitrile.

25


1.4.3. Electrochemical Nanolithography

Nanolithography includes writing or printing on the surface at the nanoscopic scale.

The most prominent application of lithography is that of UV-mask lithography. Both

industry and academia are investing intensive efforts in developing UV and deep-UV

lithographic techniques, which can create submicron level features on a substrate.26-27 The

AFM based patterning technique is one among the many lithographic techniques, which

allow us to pattern the surface on nanometer scale.28

The electrochemical nanolithography is used to pattern the polymer/organic substrate

though electrochemical methods with the help of a biased AFM tip. In this method, the

spontaneous condensation of water meniscus between AFM tip and substrate is used as an

electrochemical cell for patterning purpose. For this, a negative bias voltage is applied to

AFM tip (cathode) and the electrochemical oxidation occurs on the polymer/organic

materials coated on a conducting electrode (acting as anode). This technique is used to

fabricate nanostructures by the oxidation and cross-linking of the precursor electroactive

polymer. Hence, we explore the various precursor polymeric materials for the fabrication

of functional nanoscale structures on silicon or ITO substrates.28-31

1.5. Semiconducting Supramolecular Macromolecules

Conjugated polymers have attracted much attention during the past few decades

because of their wide spread applications in organic displays and semiconductor

devices.117 A major challenge in this area is to design materials with tailored properties

with high solid state order and good conductivity.

26


The regioregularity of conjugated macromolecules118 is an important factor for

determining the higher order structures in their solid state. Incorporation of a conjugated

rod leads to self-assemble the macromolecule in to greater extent. The aggregation and the

properties can be tuned by selective components and relative block length. One approach

involves, self-assembling rigid components by attaching recognition groups to a rigid core

for the formation of rigid rod supramolecular systems. Another approach involves the

combination of incompatible segments. The anisotropic arrangement of rod blocks with

substituent leads to microphase separation. Jenekhe et al.119 investigated the effect of

supramolecular structure and morphology on the photophysical properties in polymer

systems. For this purpose they used rod–coil di- and triblock conjugated copolymers and

polymer blend. The authors investigated the electronic energy transfer from a light-

absorbing energy donating rod–coil system to a rigid-rod polymer. In most cases,

amphiphilic block macromolecules of rod-rod or rod-coil received considerable attention

in nanostructure fabrication.

Conjugated amphiphilic dumbbell-shaped macromolecules: Among the various

macromolecular entities, conjugated macromolecules with branched architectures are

successfully utilized for fabrication of different optical devices, transistors, photovoltaic

cells, lasers, sensors for chemical and biochemical species.83-79,120 Synthesizing functional

molecular wires with different effective conjugation lengths permits easier and more

reliable for device fabrication. An overview of typical end-capped block macromolecular

architectures is summarized in Table 1.2.

27


Table 1.2 End-capped conjugated polymer and oligomers

Category Structure Reference

OPP

4 OO

OO

O O O

OO

OPP1

121a

5

OR

RO

RO

O OO OR =OPP2

121b

OO

OO

O (CH2)21CH3O 11 2OPP3

121c

OOO

OOPP4

121d

OPV RO

RO

RO

NMe2

OMe

OC6H13

Bu2N

OPV1

122

OPE NN

OPE1

123

NN

OPE2

124

28


S SS

RR

RR

RR

R R

R

R R Rn

OPE3

125

Dendrimer

NN

N

NN

N

n

D1

126

NN

N

N

N

N

N

N

N

N

NN

N

N N

N

N

N

NN

NN

NN

N

NN

N

N

N

D2

127

End-capped Polymer (EP)

S NAr2Ar2N

EP1128

SN

Bu BuBu Bu

N n

EP2

129

Rigid rod macromolecules can generate various morphologies and their self-organized

mechanisms have been well studied.70-77 The shape and size of the aggregations have a

strong influence on photophysical properties of conjugated macromolecules. With this

29


respect, Lee et al.121 reported few rod-coil block molecules/ macromolecules for

fabrication of different supramolecular assemblies (OPP1-OPP4). The macrocyclic

molecule is formed from a hexa-p-phenylene rod and a chiral poly (ethyleneoxide) chain

that are fused together. The formation of cylindrical 1-D aggregates was confirmed using

TEM and dynamic light scattering (DLS). The cyclic dichromism (CD) confirmed the

helical architecture of self-assembled superstructures. However, tree shaped compound

OPP2 consisting of conjugated building blocks, p-phenylene and dendritic ether moieties

showed discrete rod nanostructures. The photophysical property demonstrated the

planarization of conjugated rod segments due to close packing in the lattice. Similar

strategy was used for compounds OPP3-OPP4. The presence of rod-coil-rod or coil-rod-

coil building blocks and the supramolecular structural variation was attributed to variation

of molecular length.

Oligo(phenylene ethylene) (OPE), oligo(phenylene vinylene) (OPV), dendrimers, and

conjugated block macromolecules with end-capped rod-coil segments have been

reported122-129 with emphasis on the synthesis and physical properties. The compounds

OPV1, OPE1-3, D1-2, and EP1-2 were synthesized and found to various optoelectronic

applications.

The dendritic PPV1 was synthesized with different surface functionalities.122 The

authors reported that the methodology could be used to a wide range for the synthesis of

asymmetrically conjugated structures. However, dendrimer D1-D2 were synthesized

considering the electroactive substitution on to a conjugated molecule. The authors

reported that the unique structure and properties make them suitable subjects for a wide

range of applications optoelectronics. Similar observations were reported in end-capped

polymer systems (EP1-EP2) consisting of electroactive amine groups.128-129 In EP2, the

30


photophysical properties (UV-Vis and PL) showed an increase of effective conjugation

length and suppressed the aggregation behaviour in ground state.129

These works showed that the carbazole / tripheylamine electroactive group containing

block macromolecules or dendrimer possess several attractive properties in materials

science.

Cross-conjugated amphiphilic macromolecules: The ability to tailor organic

molecular structures using organic synthetic techniques provides great versatility to

modulate the properties of macromolecules. The study of conjugated organic oligomers

with specific π−delocalization patterns can also provide the basis for the realization of

useful polymer-based materials. In order to facilitate the inter-chain charge transport,

strong π-π interaction between adjacent chains is highly desired and can be achieved

through extended conjugation.130 Cross-conjugation demonstrates that π-electron

delocalization takes place within each linearly conjugated segment, and π-electron

communication detected across the cross-conjugated backbone. As cross-conjugated

frameworks provide a novel platform for extended conjugated systems and many

interesting properties can be expected. Therefore investigations on the structural,

photophysical, electronic and morphological properties have been increasing in recent

years with potential applications in OLED, TFT, liquid crystal and photovoltaic devices.

An overview of the typical examples of cross-conjugated polymers/oligomers is given in

Table 1.3.

31


Table 1.3 Cross-conjugated polymer and oligomers

Category Structure Ref.

PPP, OPV

**

NN

C8H17

C8H17

NN

C8H17

C8H17

CPP1

131

**

NN

C8H17

C8H17

NN

C8H17

C8H17

CPP2

131

CPP3

132

OPE, OPV

CPP4

CF3

CF3

F3C

F3C

NR2

R2N

133

32


CPP5

OCH3

H3CO

OCH3H3CO 133

CPP6

Pr3Sii SiiPr3

SS

SS

SS

SS

CO2MeMeO2C

CO2MeMeO2C

MeO2CCO2Me

MeO2CCO2Me

133

OPV CPP7

OC8H17

OC8H17

OC8H17

C8H17O

N N

O

NN

O

134

CPP8

OC8H17

OC8H17

N N

O

OC8H17

C8H17O

NNO

134

33


CPP9

CN NC

NCCN

C8H17O

OC8H17

OC8H17

C8H17O

OC8H17

OC8H17

OC8H17

C8H17O

134

PPV

CPP10

135

CPP11

*

* n 136

CPP12

*

RO

N

OCH3

RO n

137

34


CPP13

S

SS

S

S

S

S

S

S

S

S

S

RO

OR

RO

OR

138

PPE

CPP14

RR

**n

139

CPP15

RR

*n

139

CPP16

RR

Et3Si

RR

R R

SiEt3 140

CPP17

S

SS

S

OCH3

OCH3

H3CO

H3CO

S

S

O

O

141

35


Cross conjugated

Polymer

CPP18

S

NN

SSS* *

CO2RRO2C

n

142

S* *

S

R

CPP19

143

CPP20

S

S

S

R

R

R

S R141

Poly(p-phenylene) (PPP), poly(p-phenylene vinylene) (PPV) and poly(p-phenylene

ethylene) (PPE) derivatives are good candidates for optoelectronics. Efficient charge

transport is essential to the optimization of properties of molecular and polymeric light-

emitting diodes.144 In oligomeric or polymeric molecules, charge transport occurs via

intra-chain and inter-chain routes. In intra-chain transport, charges are delocalized across

the polymer’s linear conjugated backbone. In inter-chain transport, charges move from

chain to chain through a hopping mechanism. In some of these oligomer and polymer

systems, two-dimensional (2D) charge delocalization can be achieved through π- stacking

between adjacent chains.

The cross conjugated macromolecular structures CPP1-CPP3, showed the hybrid

composition of PPP cross conjugated with OPV derivatives. Müllen et al.131 reported a

novel PPP with a 2, 2’-(p-phenylene)-bis(4,5-diphenylimidazole) as an additional

36


orthogonal chromophore (CPP1-CPP2). This polymer showed a blue-green emission in

both solution and in the solid state. UV-vis spectroscopy and cyclic voltammetry (CV)

revealed the optical and electronic properties of CPP1. The oxidation state of CPP1 with

potassium ferricyanide resulted in quinoid type cross-conjugate (CPP2) with a low band

gap of 1.6 eV. Broad absorption over entire UV-vis region made CPP2 suitable material

for light absorbing compounds in solar cells.145

Cross conjugates based on OPE, OPV and OPP hybrids are of great interest. 2D

molecules (CPP5-CPP6) had been synthesized with varying terminal groups by Bunz’s

group.133 Ground and excited state charge delocalization were altered by the nature of the

side groups. The redox and optical properties of the molecules CPP4-CPP6 were

investigated by cyclic and differential pulse voltammetries and UV-vis. The access to

multiple redox states rendered the molecules attractive candidate as wires, or possibly

transistors for molecular electronics.

Galvin’s group synthesized a series of OPV derivatives (CPP8-CPP9),134b with cross

conjugated structure attached to the central benzene core. The arms were chemically

modified by different donor and acceptor groups and arranged in central symmetry.

Significant π – π interactions were observed in concentrated solution and powder samples,

explaining the red shift in emission maxima. X-ray diffraction studies performed on

powdered samples revealed X-shaped molecules form stacks. Hwan Kin et al.138 reported

a conjugated molecule with oligothiophene (CPP13) as four arms connected to

phenylenevinylene core. CPP14 exhibited good film forming property. Crystallinity was

confirmed by well defined X-ray diffraction (XRD) patterns from uniform orientations of

molecules.

37


The cross conjugated systems have been reviewed in detail by Tykwinski.136

Photophysical studies of CPP11 indicated that the conjugation did not extend on length,

probably due to the non-planarity nature.136 There after, many polymers (CPP14-CPP16)

were synthesized with more planar structures, showed a red shift in absorption.139

Emission intensity also increased as a function of chain length.

The donor-acceptor cross conjugated oligomers was extended to polymeric systems

for the synthesis of CPP18142 and CPP19.143 The author showed a broad absorption,

covers from 300 to 700 nm.143 The emission spectra remained constant even at high

wavelength, confirmed the energy transfer of excitons from cross conjugated side chains.

It appeared to be great interest in the area of cross conjugated polymers. Carbazole-

containing molecules and polymers are known to be excellent hole-transporting materials

because of the electron-donating capabilities associated with the Nitrogen in the carbazole.

Recently, Liang et al.137 reported the luminescent properties of PPV cross conjugated

polymers that had carbazole moieties on the side chains (CPP12). The authors showed

this produced good film quality and thermal stability. They have also demonstrated that

the incorporation of hole-transporting functionality was effective for enhancing

electroluminescence characteristic. In addition, the carbazole substituents are expected to

perform the functions of solubilization, aggregation suppression, and energy transfer.

1.6. Photochromic Molecule and Morphosynthesis

Continuing to the development of new materials and devices at nanoscale dimensions,

research has been focused on multifunctional molecules. Incorporation of photoactive

components open up possibilities to produce self-assembled nanostructures for functional

molecular devices.146-149 The component can respond to external stimulus such as heat,

38


pressure or light which subsequently gives rise to changes in electronic, ionic, optical and

conformational properties. In 1950, Hirshberg150 suggested the term “photochromism”

[from the Greek words: phos (light) and chroma (color)] to describe the phenomenon. The

ability to modulate a given physical property by means of an external trigger is of great

importance to the development of molecular and supramolecular devices.151

An increasing effort has been directed towards the design of dynamic molecular

systems, which can undergo reversible changes between different states. In this context,

photochromic organic compounds are of particular interest. They represent a starting point

for the design of stable macromolecules, whose behavior can be controlled by light.152

Supramolecular assemble with well-defined size, shape, and morphology remains a

challenge, due to the multitude of co-operative intermolecular processes. The ordered

structures of photo-chromic compounds of diarylethenes are well studied. 153-154

The origin of diarylethenes with heterocyclic rings as photochromic molecules stems

from Stilbene (1A) (Figure 1.8). Stilbene was well-known to undergo a reversible cis-

trans photoisomerisation and a photocyclisation.156 However, as it is not thermally stable,

stilbene was never use in the optoelectronics industry.152a Kellogg et al.157 then discovered

that the thermal stability can be increased by substituting the phenyl rings of stilbene with

thiophene rings (2A). Ever since, derivatives of diarylethene with thiophene rings were

studied comprehensively by many means to develop suitable molecules for photonic

devices. The thermal stability of these derivatives is achieved mostly by the low energy

difference between the open and close ring isomer.158 The stability of the close ring state

can also be attributed to less bulky as well as electron-donating substituents to 2- and 2’-

position of the thiophene rings. The most widely studied diarylethene is the

39


perfluorocyclopentene derivative (3A).159-160 In the open ring state 3A, the π-electrons are

localized in the respective thiophene ring and in 3B, the π-electrons are delocalized

throughout the molecule. The open-ring structure (colourless) may be thought of as in the

‘OFF’ state, while the close-ring structure (coloured) may be thought of as in the ‘ON’

state.

Feringa et al.146 reported the design of the photoresponsive self-assembly molecules

using photochromic diarylethene derivatives. They showed, the differences in the

preferential helical structures between the open and close-ring form. The open-ring isomer

has a relatively loosely twisted conformation, where as the close-ring isomer has a stiff

planar structure. The geometrical structure change of diarylethene plays the difference in

rigidity and π – conjugation associated with photochromism using scanning tunneling

microscope (STM) imaging. They explained that the formation of chiral element arises

either from molecule – substrate interaction or from intermolecular interaction such as

hydrogen bonding and it creates the supramolecular nanostructures.

Irie et al.147 demonstrated fundamental properties and applications of heterocyclic

dithienylethene molecules as optical memory devices and as switches. They synthesized

polyethyleneglycol (PEG) substituted dithienylethene molecules and investigated their

photochromic and self-assembly properties. The molecules were dispersed in common

organic solvents and self-assembled into nanostructures in water. In the similar

architecture, introduction of the parallel and antiparallel structural conformation of

dithienylethene molecule showed different nanostructures. The molecules in the

antiparallel conformation have an extended rod-like structure, where the parallel

conformation had a cubic structure.

40


1A 1B

S S S S2A 2B

S S

F2F2F2

S S

F2F2

F2

3A 3B

hν1

hν1

hν1

hν2

hν2

hν2

OFF ONColourless Coloured

Figure 1.8 Photochromism; stilbene (1), diarylethenes with thiophene rings (2), and perfluorocyclopentene with thiophene rings (3).

Terminally functionalized polyenes have been the subject of great interest in

supramolecular chemistry. Lehn et al. reported the synthesis and characterization of a

series of novel dithienylethene photochromic molecules bearing the electron donor/

acceptor and electroactive moieties.148 Due to the push-pull characteristic, the electron

conduction and non-linear properties were controlled. Presence of electron donor or

acceptor groups, change the π – electron density and hence electronic properties.

In another approach, drastic improvements in material properties were achieved by

employing a chromophore material in the molecular design of the dithienylethene

photochromic molecule. Matsuda et al.149 reported the synthesis of a diarylethene-pyrene

diad and diarylethene-pyrene-diarylethene triad (Figure 1.9), which showed a high

molecular ordering in solution state. In this study, the authors showed the two dimensional

(2D) ordering of the molecular structures and new morphological ordering upon

irradiation with UV light. The self-assembled structures were readily formed due to the

41


coexistence of adsorption and desorption processes,162 hydrogen bonding,163 and charge

transfer interactions.164

S S

F F FF

FF

C8H17O OO

(a)

S S

F F FF

FF

C8H17O OO S

S

FFFF F

F

OC8H17

O

O

(b)

Figure 1.9 Structure of dithienylethene-pyrene diad (a) and dithienylethene-pyrene-dithienylethene triad (b) showed 2D structural order.163

In principle, any type of physical behavior dependent on the electronic communication

between the end groups can be controlled photochemically. “Azobenzene” is a very

convenient photoactive molecule to control the self-assembled structures because it

exhibits large conformational change along with cis-trans isomerization.161 Thus emphasis

was on synthesis and characterization of diazo-based dithienylethene-based molecules and

their self-assembly. Because, they are considered to be thermally stable (p–type)

photochromic compounds, which can be used in optical memory media, switching

devices, and display materials.155

42


1.7. Aim and Outline of This Thesis

Controlled self-assembly of organic macromolecule has attracted increasing attention

in view of their potential utility for the fabrication of nano-structured materials.

Contemporary science aims at designing materials with specific properties and soft matter

offers a viable route to realize the goals due to huge structural space present in organic

systems. The major objectives of this thesis are-

1. To implement the functionalities in small molecules and macromolecules using

facile synthetic strategies to show non-covalent interactions

2. To investigate the self-assembly of the random/block macromolecules

3. To understand the influence of dimensionality, higher or lower symmetry and

molecular interactions that assists the formation of supramolecular assemble

The scope of the thesis is to study various aspects of amphiphilic macromolecules with

electroactive and photoactive moieties. Self-assembly to generate ordered nanostructures

on surfaces can be either manipulated by interaction with external stimuli such as light or

applied voltage.

Chapter 2 describes the design and synthesis of amphiphilic random precursor

copolymers with two different electroactive monomers and the influence of organization

of polymer chains in thin film formation. The structural conformation of the copolymers

in solution and the microphase separation/morphology in the solid-state is investigated

using TEM and XRD analysis. The electrochemical studies reveal that the electroactive

molecules can be successfully electropolymerized to form of stable CPN. It is further

confirmed from nano-patterning, FT-IR, UV-Vis and PL studies.

43


The copolymer system was further extended to amphiphilic block copolymers

consisting of hydroxylated polymethacrylate and pendant electroactive polymethacrylate.

In Chapter 3, the block copolymer was synthesized utilizing “living” free radical

polymerization technique (ATRP). The effect of amphiphilicity of block copolymer, on

microphase separation or molecular self-assembly is studied using SEM, TEM and XRD.

The electrochemical studies are carried out for electropolymerization of electroactive

monomer units to get stable CPN thin film. The cross-linked polymer network thin-film is

characterized by UV-vis, FT-IR, fluorescence and the morphological characterization is

carried out by AFM.

In order to understand the role of weak interactions such as π-π interactions with

assisted from hydrogen bond, a series of amphiphilic dithienylethene based on

photochromic molecules are designed. In Chapter 4, amphiphilic diazo-dithienylethene

(DTE) molecules were synthesized with 2-phenol/2-Naphthol functionalities. The photo-

chromism of the synthesized molecules was studied using UV-vis, PL, CV, and photo-

conductivity (I-V) measurements. Self-organization studies are carried out by AFM in

order to manifest the role of functionality in self-assembly. The effect of second photo-

active (diazo) group on self-assembly of compound well demonstrated. Supramolecular

self-assembly effect on concentration, temperature and substrate are also explored.

Self-assembly of rigid rod structures has been extensively investigated. However, not

many reports describe the influence of substituent groups on the self-assembly of

dumbbell molecules. At the outset, Chapter 5 prompted the design and synthesis of a

series of end-capped OPP amphiphilic molecules with electroactive groups. The molecular

ordering and self-assembled behavior is investigated using SEM, TEM and XRD. The

44


studies focused on how the strong π-π interaction between polymer backbones affected the

overall photophysical properties and self-assembly.

Since cross- conjugated structure can largely delocalize the π-electron, this idea can

be applied to design extended conjugated system to have interesting self-assembly

properties. Chapter 6 described the synthesis of a series of cross-conjugated PPP with

various electroactive (donor/acceptor) moieties, cross conjugated length, and symmetry.

The cross conjugated PPPs with different pendant structures are investigated

comprehensively using UV-Vis, fluorescence and the overall photophysical properties are

also explored. A stable CPN thin film is formed on ITO upon electropolymerization. Self-

assembling properties are investigated using AFM. It is anticipated that the self-assembly

would be controlled primarily by weak Vander Waals forces and π-π interactions.

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159. Irie, M.; Miyatake, O.; Uchida, K. J. Am. Chem. Soc. 1992, 114, 8715.

160. Hohlneicher, G.; Muller, M.; Demmer, M.; Le, J.; Penn, J.H.; Gan, L.X.; Loesel,

P.D. J. Am Chem. Soc. 1988, 110, 4483.

59


60

161. (a) Murakami, H.; Kawabuchi, A.; Matsumoto, R.; Ido, T.; Nakashima, N. J. Am.

Chem. Soc. 2005, 127, 15891; (b) Yang, W.-Y.; Ahn, J.-H.; Yoo, Y.-S.; Oh, N.-K.;

Lee, M. Nat. Mater. 2005, 4, 399; (c) Sakai, H.; Orihara, Y.; Kodashima, H.;

Matsumura, A.; Ohkubo, T.; Tsuchiya, K.; Abe, M. J. Am. Chem. Soc. 2005, 127,

13454; (d) Yagai, S.; Nakajima, T.; Karatsu, T.; Saitow, K.; Kitamura, A. J. Am.

Chem. Soc. 2004, 126, 11500.

162. (a) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De

Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Müllen, K. Acc. Chem.

Res. 2000, 33, 520; (b) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.;

Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 317.

163. (a) Eichhorst-Gerner, K.; Stable, A.; Moessner, G.; Declerq, D.; Valiyaveettil, S.;

Enkelmann, V.; Müllen, K.; Rabe, J. P. Angew. Chem. Int. Ed. 1996, 35, 1492; (b)

Puigmarti-Luis, J.; Minoia, A.; Uji-i, H.; Rovira, C.; Cornil, J.; De Feyter, S.;

Lazzaroni, R.; Amabilino, D. B. J. Am. Chem. Soc. 2006, 128, 12602.

164. Samori, P.; Yin, X.; Tchebotareva, N.; Wang, Z.; Pakula, T.; Jackel; Watson, M.

D.; Venturini, A.; Müllen, K.; Rabe, J. P. J. Am. Chem. Soc. 2004, 126, 3567.


61

Chapter 2

Synthesis and Self-assembly of Copolymers with Pendent Electroactive Units

Barik, Satyananda; Vallyaveettil, Suresh; “Design, Synthesis and Self-assembly of

Organic Macromolecules” Poly. Mater. Sci. & Eng. 2006, 95, 1105-1106.

Barik, Satyananda; Valiyaveettil, Suresh, “Synthesis and Self-assembly of Copolymers

with Pendent Electroactive Units” Macromolecules 2008, 41(17), 6376-6386.


62

2.1. Introduction

The self-assembly of copolymers into nanostructure aggregates has received

considerable attention in research due to their ability to form different morphologies in

bulk or in solution.1-10 Polymer aggregates in the form of hollow spheres, lamella and

hollow cylinders were observed from the self-organization of rod-coil polymers depending

on the solvent polarity.11 Self-assembly of copolymers into nanostructure aggregates

reported to date include zero12, one-13 and two dimensional14 assemblies based on different

theromodynamic/kinetic interactions like hydrogen bonding,15 metal-ligand coordinate

bonding,16 aromatic π - π interaction,17 and hydrophobic effect.18 In the case of polymers

with aromatic moieties on the side chain, the main challenge in assembling large aromatic

molecules into 1D structure lies in balancing the molecular assembly for growth along the

π-stacking direction against the lateral association of polymer backbone or side chain alkyl

units.20 Advincula et al.21 have demonstrated viability of the “precursor polymer”

approach based on single/binary pendant electroactive monomer groups (e.g. carbazole,

thiophene) on the polymer backbone, which can be polymerized electrochemically for the

device applications. The composition of copolymer plays a significant influence on size,

shape and the stability of the supramolecular assemblies.22

In this chapter, we investigated the supramolecular assembly of a few amphiphilic

copolymers with interesting binary pendant electroactive blocks. We have incorporated a

series of electropolymerizable groups on the polymer backbone and varied the length and

nature (hydrophobic/hydrophilic) of the spacer groups (Figure 2.1). Bulky aromatic side

chains were incorporated along the polymer backbone to control the aggregation and

morphology in the polymer lattice through π-stacking. The target polymer structures


involve a relatively flexible polymethacrylate chain and rigid electroactive pendant

aromatic units on the side chain, which can be polymerized electrochemically. We chose

groups such as carbazole, thiophene, and fluorene as side chains because of their potential

applications as hole/electron transport materials in optoelectronic material. The length and

flexibility of the spacer between the side chain and polymer backbone plays an important

role in controlling the self-assembly inside the polymer lattice.

OO

N

O

O

(CH2)2

* *

x y

P3

OO

N

O

O

(CH2)11

* *

x y

P4

O

OO

*x y

P2

O

S

(CH2)2

O

O

* *

xy

P1

N

O

(CH2)2

N

CH2

CH2

O3

Figure 2.1. Molecular structures of copolymers (P1-P4).

2.2. Experimental Section

2.2.1 Synthesis

The monomers and polymers (P1-P4) were prepared using synthetic routes shown in

Scheme 2.1 and Scheme 2.2. Complete experimental procedures for monomer and

polymer synthesis and characterization details are given in Chapter 7.2

63


N

H

i (n=2)iii (n=11)

N

OH

(CH2)n

2 (n=2)4 (n=11)

ii

N

OO

(CH2)n

3 (n=2)5 (n=11)

N

H

iv

6

ii

7

O

9

OOH

1

1

ii

8

11

10

ii

N

O

CH2

CH2

O3

N

H

CH2

CH2

O3

O

H

S

(CH2)2O

O

S

(CH2)2

Scheme 2.1. General synthetic approach to monomers, (i) 2-bromoethanol, KOH, DMSO, triethyl benzyl ammonium chloride, rt, 88%; (ii) triethylamine, THF, 0 °C, 50-80%; (iii) 11-bromo 1-undecanol, benzene, 50% NaOH, triethyl benzyl ammonium chloride, reflux 4 h, 82%; (iv) chloro-(ethoxy-ethoxy) ethanol, DMSO, KOH, rt, 62%.

64


N N

7

O

O

9

OO

11

O

OO

(CH2)2

3

O

9

O

O

O

* *

xy

P1

N

OO

(CH2)2

3

OO

N

O

O

(CH2)2

* *

x y

S

O

OO

*x y

P2

O

CH2

CH2

O3

(a)

P3

(CH2)2

S

(CH2)2

N

O

(CH2)2

N

CH2

CH2

O3

(a)

O

9

O

N

OO

(CH2)11

5

O

O

N

O

O

(CH2)11

* *

x y

P4

(a)

(a)

Scheme 2.2. General synthetic approach to copolymer (P1-P4), (a) toluene, AIBN (5 mol% of total monomer concentration), 60 ⁰C, 2 days, 60-72%.

65


66

2.3. Results and Discussion

2.3.1 Synthesis and characterization of copolymers (P1-P4)

The primary synthetic goal was to synthesize methacrylated monomers with

electroactive units in the side chain. A few multi-component copolymers with

hydrophobic rigid aromatic units attached to the polymer backbone, were designed and

synthesized. Here, the monomers and polymers (P1-P4) were prepared using synthetic

routes shown in Scheme 2.1 and Scheme 2.2.23b-h So the synthesis began with the reaction

of inexpensive and commercially available starting materials with electroactive units like

carbazole (1), 9-fluorenol (8) and thienylethanol (10) (Scheme 2.1). N-(2-hydroxyethyl)

carbazole (2) and 9-(11-hydroxyundecyl) carbazole (4) of carbazole derivatives were

synthesized from carbazole.23c, 23f The corresponding monomers with methacrylate

substituent 3, 5, 9 and 11 were synthesized by esterification reaction of hydroxy group

containing electroactive units (2, 4, 8 and 10) with methacryloyl chloride in THF and

triethylamine at room temperature.23c,g Synthetic scheme for copolymers (P1-P4) were

shown in Scheme 2.2. The polymerization of all methacrylate monomers for their

homopolymer and copolymers were carried out using standard procedures reported in

literatures.23

The starting materials and the polymers were characterized by 1H-, 13C-NMR and

elemental analysis to confirm product formation and purity. As spacer length increases,

the peaks in the NMR spectra become sharper and multiplets were better resolved.16 The

physical properties of the copolymers P1-P4 are listed in Table 2.1. The molecular

weights of the polymers were measured by means of gel permeation chromatography

(GPC) using THF as eluant and polystyrene as the standard (Table 2.1). All copolymers


were soluble in common organic solvents such as THF, chloroform, dichloromethane,

chlorobenzene, and toluene. The FT-IR spectra of the copolymers were given in Figure

2.2. The copolymers P1, P3, and P4 containing carbazole ring shows strong carbonyl band

(υ ~ 1726 cm-1) and the corresponding ester linkage (υ ~ 1631 cm-1), -N-CH2- bands for

carbazole groups, and aromatic C-N stretching at υ ~1253 cm-1.

Table 2.1. Structural characteristics of copolymers P1-P4

Copolymer Molecular Weight Thermal Analysis

Mn Mw PD Td Tg

P1 6496 10535 1.62 250 84

P2 20200 32000 1.67 207 116

P3 6692 10567 1.57 225 89

P4 16874 28076 1.66 253 82

500 1000 1500 2000 2500 3000 3500 40001726

Tran

smita

nce

Wavenumber(Cm-1)

P1

P2

P3

2930

743

11491631

1443

P4

Figure 2.2. FT-IR spectra of copolymer P1-P4

67


2.3.2 Thermal properties

Thermal properties of the polymers were investigated using thermogravimetric analysis

(TGA) and differential scanning calorimetry at a heating rate 10 °C/min under nitrogen

atmosphere (Figure 2.3). The thermograms indicated that the copolymers P1-P4 are

thermally stable up to 250 °C. The first onset degradation temperatures were in the range

of 200 - 250 °C (Figure 3A). The glass transition temperatures of these copolymers

ranged from 82 - 116 °C (Figure 3B). Decrease in Tg of the copolymer was observed with

increase in spacer length owing to the increase in flexibility of the polymer. The

copolymer P2, with no spacer showed a maximum Tg (116 °C), whereas P4 with C11-

spacer linkage showed lowest Tg (82 °C).

200 400 600 8000

50

100

Wei

ght (

%)

Temperature (oC)40 60 80 100 120 140

Hea

t Flo

w

Temperature (oC)

(A) (B)

Figure 2.3. TGA (A) and DSC (B) of copolymers; P1 (▲), P2 ( ), P3 ( ), and P4 ( ) at heating rate of 10 °C/min under N2 atmosphere.

2.3.3 Optical properties

The representative absorption and emission spectra of copolymers in THF are shown

in Figure 2.4. The copolymers do not exhibit any new bands other than the corresponding

68


69

chromophore in the absorption spectra, indicating that there is no interaction between the

chromophores in their ground state. The spectrum of P1, which contains only carbazole

unit, exhibited four strong absorption peaks at 266, 294, 330 and 343 nm, and the relevant

carbazole homopolymers showed peaks at 260, 294, 330 and 344 nm with significant

overlap. The absorption band at 294 nm corresponds to the S0 - S1 transition of carbazole17

units with a shoulder at 330 nm. The absorption and emission wavelengths for the

copolymers P1-P4 in THF are summarized in Table 2.2. The P2, which contains fluorene

and thiophene chromophores on the backbone showed two strong absorptions at 240 and

275 nm, as compared to the thiophene and fluorene incorporated homopolymers,

respectively. The copolymer P3 and P4 containing fluorene and carbazole pendant units,

showed five absorption peaks at 260, 270, 294, 330 and 344 nm corresponding to each

chromophores.

It is expected that, the interaction between the neighboring chromophores along a

polymer chain induce fluorescence quenching through various mechanisms.23a The

fluorescence spectra of P1-P4 in THF are shown in Figure 2.4B with the excitation

wavelength corresponding to the maximum absorption wave length. The P1 containing

only carbazole units showed emission maxima at 353 and 366 nm. By introducing other

chromophores on the polymer backbone, the emission and electrochemical properties can

be significantly influenced. Incorporation of fluorene and thiophene groups on the

polymer backbone caused a red shift in the emission maxima of the copolymers. The red

shift in the emission maxima observed for all copolymers indicates possible interaction

between the substituent and change in conformation of the polymer backbone.


250 300 350 4000.0

0.5

1.0

1.5

Abs

orba

nce(

a.u.

)

Wavelength(nm)

A

400 500 600 700-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

(a.u

.)

Wavelength (nm)

B

Figure 2.4. Absorption (A) and emission (B) spectra of copolymers in THF solution; P1 (▲), P2 ( ), P3 ( ), and P4 ( ). Table 2.2. The absorption and emission wavelengths for copolymers P1-P4 in THF

Copolymer Solution (THF) Position of new absorption peaks

appeared after electropolymerization of

a thin film λmax (nm) λems (nm) λmax (nm)

P1 266, 294, 330, 343 350, 366 301, 535

P2 236, 274 488, 513, 520 329, 545

P3 236, 266, 294, 330, 344 432, 444 301, 349, 440

P4 266, 294, 330, 344 402, 420 229, 348, 455

2.3.4 Nano-fiber morphology studies

70

The polymers P1-P4 would be capable of forming solvent dependant supramolecular

self-assembly. Transmission electron microscopy (TEM) has been an effective tool for

characterizing supramolecular self-assembly at nanometer dimensions.24-26 For this

purpose, the polymers were dissolved in different solvents such as THF, DCM, and CHCl3


at a concentration of 0.01mg/mL. A thin film was obtained on TEM grid through drop

casting of the solution. The TEM images obtained from the THF solution of the

copolymer P2 is shown in Figure 2.5, which suggests that P2 forms hollow tubes with 10

- 12 nm in diameter (Figure 2.5). In Figure 2.5A, tubes were seen with a focal point. The

formation of one dimensional (1D) hollow tubes could be explained using polymer chain

aggregation through π-π stacking of side chain units. All copolymers formed nanofibers

from THF solutions and were fully characterized.

The formation of hollow elongated nanofibrils is expected to be due to a particular

molecular ordering, which can be characterized using X-ray diffraction. Figure 2.6 shows

the wide angle X-ray diffraction pattern of the dried polymer film on a glass substrate. The

sharp diffraction patterns (Table 2.3) indicate the highly crystalline nature of the polymer

in solid state.

Figure 2.5. TEM images of nanofibres formed by self-assembly of copolymer (P2) in 0.01mg/mL

71


10 20 30 400

20

40

60

80

100

120

140

(0 -4

4)

(-2 -4

3)

(1 -5

1)

(-2 3

2)

(0 -1 3)

(-2-2

0)(110

)

Inte

nsity

2θ Figure 2.6. X-Ray diffraction pattern of the copolymer (P2) film on glass slide

Table 2.3. X-ray diffraction data for copolymer P2

Diffraction Angle (2θ) º

ahkl dobs (Å) dcalc (Å) b

Thickness of crystallites (nm)

13.83 (110) 6.65 6.41 2.1

21.21 (-220) 4.18 4.18 3.6

29.76 (0-13) 2.98 3.0 11.0

34.97 (-232) 2.56 2.56 7.0

36.35 (1-51) 2.47 2.47 11.5

39.89 (-2-43) 2.25 2.25 11.5

43.63 (0-4-4) 2.07 2.07 11.5

aDeduced from International Centre for Diffraction Data 2006 (JCPDF Data).

bCalculated from Scherrer’s equation26b

The nanofibers were formed by the self-assembly of polymers, facilitated π-π

interaction and hydrophobic interactions among the polymer chains. The d–spacing

(calculated using Bragg’s law) correspond to the π- π interaction inferred from the sharp

peak at 3.0 Å (2θ = 29.8⁰), which is close to the typical distance (~ 3.5 Å) for an effective 72


π-π stacking between the aromatic molecules.26 Considering the hydrophobic interaction

between the polymer chains, the broad peak with d-spacing of 6.41 Å (2θ = 13.83⁰), might

be due to close packing of long alkyl chains. The breadth of the diffraction peaks can be

used to measure thickness of crystallites using the Scherrer equation26b (Table 2.3).

Thickness of the 2D-crystallite calculated using the Scherrer equation is TEM

micrographs are consistent to each other (Figure 2.5). The expected conformation of the

polymer chains in the nanofibers can be depicted as shown in Figure 2.7.

Figure 2.7. Schematic representation of molecular self-assembly through aggregation of polymer chains and π – π stacking of the electroactive groups on the side chain. 2.3.5 Electrochemical nano-patterning using AFM

The patterning ability of all polymers was studied using AFM-assisted electrostatic

lithography27 and change in conductivity after cross linking of the side chain

chromophores was explored using conductive atomic force microscope (C-AFM)

technique. In order to do the nanopatterning, polymer film was prepared by spin coating a

solution of copolymers in chlorobenzene (0.5 wt %) on Si (100) substrate at 4000 rpm.

The film was then annealed for 2 h at 100 ⁰C and the thickness of the film was measured

73


as 98 nm using AFM. Various nanostructures were created on a polymer film via

patterning at different applied bias and tip speed. Figure 2.8a shows that the AFM image

of the line patterning on the polymer (P3) film with various tip bias of -7V, -8V, -9V, -

10V and -11V at a tip speed of 0.5 µm/s. Here, the width of the nanopattern was

increased with the increase of applied bias and pattern width extended rapidly at higher

voltages of -10 V and -11V. Also nanolines were drawn at various tip speeds of 1.0 µm/s,

0.5 µm/s, 0.1 µm/s and 0.05 µm/s at a tip bias of -9V (Figure 2.8b). The broadening of

pattern at higher voltage and a slow tip speed demonstrated easy patternability of the

polymer films due to presence of electroactive groups on the polymer backbone. Figure

2.8c and Figure 2.8d shows the dot patterning on the polymer film at various tip bias with

contact time of 2s and at different time intervals at a constant voltage of -9V, respectively.

Figure 2.8. Nanopatterning of polymer (P3) film at various tip bias (a) and tip speed (b). Dot patterning on polymer film at various tip bias (c) and contact time (d)

It is expected that cross-links can be formed between the electroactive monomer units

on the side chain during the patterning. The conductivity of these nanopatterns formed

through electrochemical process is measured using C-AFM, where the patterned region

74


75

showed a higher conductivity than the unpatterned region. The cross-linked networks were

formed by oxidation of electropolymerizable groups, such as carbazole / carbazole-

fluorene/ fluorene-thiophene rings on the polymer backbone.28 Such electropolymerization

can also be monitored using cyclic voltammetry (CV).

2.3.6 Electrochemical polymerization using CV

The polymers were electropolymerized using cyclic voltammetry in a three-electrode

cell. The cell was equipped with Ag/AgCl /3.8 M KCl as a reference electrode and

platinum as a counter electrode. The ITO was used as working electrode and the substrate.

In the three cell compartments, 0.1 M tetrabutylammoniumhexafluorophosphate (TBAP)

was used as supporting electrolyte. The carbazole/fluorene/thiophene incorporated

precursor polymers, P1-P4 were spin coated onto ITO substrate and used for the

experiment. Cyclic voltammograms of the polymer (P1-P4) films on ITO substrate with a

scan rate of 50 mV/s are given in Figure 2.9. The oxidation onset for spin coated polymer

film of P1 is 0.77 V and the corresponding reduction peak is 0.43 V (Figure 2.9 A) which

are similar to the reported values for electropolymerization of carbazole29. The current

increases gradually for the subsequent cycles.

For P3 and P4 (Figure 2.9 C- D), two anodic oxidation peaks (Epa1 and Epa2) were

observed in the range of 1.1 - 1.45 V with their corresponding reduction peaks (Epc1 and

Epc2) potentials in the range of 0.78 - 1.18 V, after 10 cycles. It is interesting that after 5

cycles, the Epc1 value did not increase but the Epc2 value constantly increased,

suggesting the formation of polyfuorene (oxidation potential of fluorene is 1.25 V).29h

Since the statistical ratio of carbazole to fluorene unit in the precursor polymer is 1:1, the

formation of copolymer with carbazole-fluorene repeating units is expected after


electropolymerization. On the basis of electron density, intermolecular coupling involving

2, 7- positions predominate over 3, 6- positions on fluorene unit.30 Similarly, the 3, 6-

position is expected to predominate over 2, 7- positions of the carbazole units.

B

C D

0.0 0.4 0.8 1.2 1.6-100.0µ

-50.0µ

0.0

50.0µ

100.0µ

150.0µ

200.0µC

urre

nt (μ

A)

Potential (E Vs Ag/AgCl)

1st Cycle

11th Cycle

0.0 0.4 0.8 1.2 1.6-10.0µ

0.0

10.0µ

20.0µ

30.0µ

40.0µ

50.0µ

60.0µ

Cur

rent

(μA

)Potential (E Vs Ag/AgCl)

1st Cycle

8th Cycle

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-100.0µ

-50.0µ

0.0

50.0µ

100.0µ

150.0µ

200.0µ

250.0µ

300.0µ

Cur

rent

(μA

)


1st Cycle

10th Cycle

-0.5 0.0 0.5 1.0 1.5-0.002

-0.001

0.000

0.001

0.002

0.003

Cur

rent

(mA

)


1st Cycle

12th CycleA

Figure 2.9. CV for electrochemical polymerization (cross-linking) of spin coated polymer film in ITO substrate; P1 (A), P2 (B), P3 (C), and P4 (D) at the scan rate 50 mV/s.

76

In the same manner, copolymerization of thiophene and fluorene units on P2 was

achieved (Figure 2.9-B). In case of P2, oxidation peaks were absent in the first cycle but

two oxidation peaks (Epa1 and Epa2) at 1.34 V and 1.75 V with corresponding reduction

peaks (Epc1 and Epc2) at 1.25 V and 1.61 V, respectively, appeared during repeated

scans, which indicates that first cycle is different from the second cycle with the

possibility of the cross-linking. A precursor polymer free scan using the already cross-


linked polymer was performed and showed characteristic oxidation and reduction peaks

(Figure 2.10 A-D), with no further changes in the peak position or intensity during

additional scans. This also indicated complete electropolymerization of all accessible side

chains.

0.0 0.4 0.8 1.2 1.6-100.0µ

-50.0µ

0.0

50.0µ

100.0µ

150.0µ

200.0µ

Cur

rent

(μA

)


-0.5 0.0 0.5 1.0 1.5

-0.002

0.000

0.002

Cur

rent

(mA

)


0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-100.0µ

0.0

100.0µ

200.0µ

300.0µ

Cur

rent

(μA

)


0.0 0.4 0.8 1.2 1.6-10.0µ

0.0

10.0µ

20.0µ

30.0µ

40.0µ

50.0µ

60.0µ

Cur

rent

(μA

)


A B

C D

Figure 2.10. Polymer free scan of polymer films; P1 (A), P2 (B), P3 (C), and P4 (D).

The electropolymerized thin films on ITO substrate were further characterized by UV-

vis and FT-IR spectroscopic investigations. In the absorption spectra, a new broad band

appeared at a long wavelength region after electropolymerization (Figure 2.11). For

copolymer P1 with carbazole units, an absorption peak at 301 nm and a shoulder at 345

nm were observed, which could be attributed to the π-π* transition in polycarbazole.32a A

broad peak around 535 nm appeared to be due to the formation of highly cross linked

77


copolymer. In case of P2, the observed red shift of absorption peaks at 301 and 330 nm

indicated a high degree of conjugation in polymer lattice. The new peak at 555 nm is also

attributed to the formation of fluorene and thiophene oligomers inside the polymer

lattice.32c-d

300 450 600 7500.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Abs

orba

nce

(a.u

)

Wavelength (nm)

300 450 600 7500.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

(a.u

.)

Wavelength (nm)

D

A

300 450 600 750

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Abs

orba

nce

(a.u

.)

Wavelength (nm)

C300 450 600 750

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Abs

orba

nce

(a.u

.)Wavelength (nm)

B

Figure 2.11. UV-vis spectra of electropolymerized thin film copolymers; P1 (A), P2 (B), P3 (C), and P4 (D); precursor polymer (▲) and electropolymerized thin film ( ).

Similarly, in case of P3 and P4, the peak due to the π-π* transition appeared at 301

nm. The new peak appeared around 455 nm is attributed to the polymerization of

carbazole and fluorene. The absence of an absorption peak at 540 nm indicates that the

individual carbazole polaronic transition is not seen from the cross linked carbazole

units.32b-e

78


800 1600 2400 Wavenumber (Cm-1)

Tran

smitt

ance

(%)

843750 800 850 900

-15

-10

-5

0Tr

ansm

itanc

e (a

. u.)

Wavenumber (Cm-1)

D

800 1600 2400Wavenumber (Cm-1)

843

Tran

smitt

ance

(%)

700 750 800 850 900-30

-25

-20

-15

-10

-5

0

5

Tran

smita

nce

(a.u

.)

Wavenumber (Cm-1)

C


842

Tran

smitt

ance

(%)

700 750 800 850 900

-20-15-10-505

Tran

smita

nce

(a.u

.)

Wave Number (Cm-1)

A

700 1400 2100 2800Wavenumber (Cm-1)

840

Tran

smitt

ance

(%)

700 750 800 850 900-15

-10

-5

0

Wavenumber (Cm-1)

Tran

smita

nce

(a.u

.)

B

Figure 2.12. FT-IR spectra of electrochemical cross-linking polymer film with precursor polymers; P1 (A), P2 (B), P3 (C), and P4 (D); precursor-polymer ( ) and after electropolymerization (▲).

The FT-IR spectra of P1-P4 after electropolymrization are shown in Figure 2.12 A-D.

Comparing with the four precursor copolymers, the electropolymerized polymers showed

cross-linking because of the appearance of extra peaks in the FT-IR spectrum at 840 cm-1.

The observed sharp peaks are due to the C-H out- of- plane bending vibrations of 1, 2, 3, 4

- substituted aromatic units.31 A schematic representation of the mechanism of cross-

79


linking through electropolymerization is shown in Figure 2.13,27b-27e in which the anodic

oxidation leads to the formation of radical cations generated through the electroactive

moieties. It appears that electron rich substituent on the polymer backbone activate the

electropolymerization of the side chains to form a conjugated polymer network. Work is

in progress in our lab to find the degree of polymerization and conformation of the

electropolymerized chains inside the polymer lattice.

27

S

80

S 25

S

H

H

H

HH

H

S

H

H

S

H

H

S

H

S

H

-4e

H

H

S

S

SH

H

Further cross linking

Figure 2.13. Mechanism for the electropolymerization (cationic) and cross-linking of P2.

2.4. Conclusion

A series of methacrylate-based copolymers containing aromatic electroactive units were

prepared using free radical polymerization and investigated the cross-linking through

electropolymerization. For all copolymers, glass transition temperature decreased with

increasing the spacer length between the rigid aromatic units. The optical properties of the

polymers showed little electronic interactions between the chromophore groups in solution

or in thin films. The nanotubes of copolymers obtained from solution through self-

assembly were fully characterized. The precursor polymers were electropolymerized using

cyclic voltammetry (CV) and AFM. The anodic electropolymerization of precursor


81

polymer film was confirmed through the C-C linkage in the FT-IR spectra, changes in

absorption maxima and increase in conductivity of the polymerized regions. The variation

of polymer nanopatterning features with respect to the applied bias and tip speed were

investigated. Electropolymerization and cross linking ability of the electroactive precursor

polymer (P1 – P4) can be used for design and fabrication of potential devices.

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87

Chapter 3

Diblock Copolymer Assemblies Through Changes in Amphiphilicity of Pendent

Electroactive Moiety

Barik, Satyananda; Valiyaveettil, Suresh, “Synthesis, characterization and self-assembly studies of a new series of amphiphillic diblock copolymer with pendant electroactive moiety” Polym. Prep. Am. Chem. Soc. Div. Polym. Chem. 2008, 49(2), 361-362 Barik, Satyananda; Valiyaveettil, Suresh, “Diblock Copolymer Assemblies Through Changes in Amphiphilicity of Pendent electroactive Moiety” Submitted


88

3.1. Introduction

Amphiphilic block copolymers with hydrophobic block and hydrophilic block are

interesting owing to their wide applications in drug encapsulation,1 adhesives,2 coatings,3

thin film formation,4 and unique characteristic of supramolecular self-assembly. A wide

variety of chemical approaches have been developed to explore polymeric nanostructures

formed from amphiphilic block copolymers and their potential applications.5 Block

copolymer nanostructures were primarily controlled by structural factors such as polarity

of each blocks,6 relative lengths of blocks,7 and the molecular weight.8 Recently, block

copolymers with electroactive and hydrophilic blocks have been extensively studied due

to their unusual morphologies and photophysical behavior in solution and in solid state.9-12

We have obtained some interesting results from copolymers (Chapter 2), with

electroactive groups. This inspired us to design copolymers with electroactive block and

polyhydroxylated blocks and investigate their structure – property relationship.

In this chapter, we designed and synthesized amphiphilic diblock copolymers consists

of a hydrophilic (hydroxyl polymethacrylate) block and a hydrophobic (electroactive

aromatic) block (Figure 3.1). Bulky aromatic side chains were incorporated along the

polymer backbone to control the aggregation and morphology in polymer lattice. Further,

the electroactive unit can undergo cross-linking to afford a conjugated polymer network

(CPN) on solid substrate, which may be useful for various applications.


O

OHHO

O

O n *

N

(CH2)6

O

O

m

PCzMMA-b-PBMMA

O

OHHO

O

O n*

O

O

m

PFlMMA-b-PBMMA

Figure 3.1. Molecular structures of diblock copolymers (PCzMMA-b-PBMMA and PFlMMA-b-PBMMA)


3.2.1 Synthesis

The monomers and block copolymers (PCzMMA-b-PBMMA and PFlMMA-b-

PBMMA) were prepared using synthetic routes shown in Scheme 3.1 and Scheme 3.2.

Complete experimental and characterization details of monomers and block copolymer

synthesis (ATRP) are given in Chapter 7.3.


3.3.1 Synthesis and Characterization of Block Copolymers

Bulky aromatic (hydrophobic) side chains were incorporated along the block

copolymer backbone to control the aggregation and morphology in the polymer lattice.

Carbazole and fluorene were chosen as side chains owing to their potential applications as

89


hole/electron transport material or optoelectronic material. Further, the electroactive units

can undergo cross-linking to afford a conjugated polymer network (CPN) on solid

substrate.

HO OO

TsO OO12

OH

O H

O

O H

OO

O

OO

OH

13 14 O

OO

OO

15

OH OO

9

NH

N

(CH2)6

OH

16 17

(i)

(ii) (iii) (iv)

(iv)

(v) (iv) N

(CH2)6

OO

8 Scheme 3.1. Synthetic procedures for monomers; (i) p-TsCl, THF, 0 ºC, 12 h, 98%; (ii) 2, K2CO3, DMF, 18-crown-6, 75 ºC, 5 days, 55%; (iii) THF/EtOH, NaCNBH3, AcOH, rt, 12h 65%; (iv) methacryloyl chloride, triethylamine, THF, rt, 12h, 75-80%; (v) KOH, DMSO, 1-bromohexanol, rt, 24 h, 87%.

90


O

OO

O

O

Br

x

18

O

OO

OO

15

O

O

Br

x

18

O

OO

O

O

Br

x

18

17O

O

O

O

O n *

N

(CH2)6

OO

m

O

O

9 O

OO

OO n

*

OO

m

(i)

(ii) (iii)

(ii) (iii)

19

20

(PBMMA-Br)

N

(CH2)6

OO

O

OHHO

O

O n *

N

(CH2)6

OO

m

PCzMMA-b-PBMMA

O

OHHO

O

O n*

O

O

m

PFlMMA-b-PBMMA

O

O

O

Scheme 3.2. Synthetic procedures for homopolymer and block-copolymers; (i) CuBr, PMDETA, Ethyl 2-bromoisobutyrate, toluene, 75 ºC, 80%; (ii) CuCl, PMDETA, anisole, 75 ºC, 70-80%; (iii) THF, 10% HCl, 70 ºC, 2 h, 90-92%.

Commercially available solketal was reacted with tosyl chloride to give the protected

compound 12, which was than reacted with 3-hydroxy benzaldehyde to give the

91


92

corresponding aldehyde 13, followed by reduction with sodiumcyanoborohydride to yield

alcohol 14. The monomers with methacrylate substituent 15, 9, and 17 were prepared from

the corresponding alcohols 14, 8 and 16, respectively, using methacryloyl chloride and

triethylamine as a base in THF at room temperature. Homopolymerization of 15 was

carried out in toluene under nitrogen atmosphere using ethyl-2-bromoisobutyrate as the

initiator and CuBr/PMDETA as the catalyst system. The homopolymer 18 (macro

initiator) was precipitated from large excess of hexane, filtered, dissolved in THF and

reprecipitated for a few times. The macroinitiator 18 was used to copolymerize the

fluorenyl/ carbazolyl methacrylates to afford the diblock copolymers. (Scheme 3.2)

The diblock copolymers PCzMMA-b-PBMMA and PFlMMA-b-PBMMA were

synthesized using atom transfer radical polymerization (ATRP) in toluene and

CuCl/HMTETA catalyst system with macroinitiator 18 for one day. The reaction was

stopped by diluting with THF and the polymer was precipitated from excess hexane. The

diblock copolymers were readily soluble in solvents such as chloroform and

tetrahydrofuran. Deprotection of the dioxalane groups was achieved using 10 % aqueous

HCl solution in THF at 70 ºC for 2 h. The molecular weight of diblock copolymers was

determined using GPC with polystyrene as standards. The GPC curves are shown in

Figure 3.2a. The PDI of the block copolymers was narrow indicating the formation of

well-defined block structures.

The chemical structures of the diblock copolymers PCzMMA-b-PBMMA and

PFlMMA-b-PBMMA were confirmed by 1H NMR, 13C NMR and FT-IR spectroscopy.

The FT-IR spectra are shown in Figure 3.2b. Both copolymers contain the strong C=O

stretching band at ~1724 cm-1 and broad C–O band due to the ester group was also visible

at ~1150 cm-1. The sharp stretching frequency at 1250 cm-1 for PCzMMA-b-PBMMA is


due to the N–C bond stretching, which is absent in PFlMMA-b-PBMMA block

copolymer. These results strongly suggest the formation of well-defined block copolymer

with pendant electroactive units through ATRP polymerization. The molecular weights

and distributions for block copolymers PFlMMA-b-PBMMA & PCzMMA-b-PBMMA

and other physical properties are summarized in Table 3.1.

Table 3.1. Structural Characteristics of polymers PFlMMA-b-PBMMA and PCzMMA-b-PBMMA

Polymer Mna

(macroinitiator) Mna Mwa n:mb

(mol%) Decomposition Temperaturec

(Td) °C

Glass Transition Temperature d

(Tg) °C

PFlMMA-b-PBMMA

8033 32,994 36,775 22:47 250 92

PCzMMA-b-PBMMA

8033 21,598 23,749 12:36 337 51

aDetermined by GPC (THF, Polystyrene standard); bDetermined from NMR spectroscopy; cDetermined from TGA (N2 atmosphere, 10 ºC/min scan rate); dDetermined from DSC (N2 atmosphere, 5 °C/min scan rate)

PFlMMA-b-PBMMAMw= 36,775Mn= 32, 994

PCzMMA-b-PBMMAMw= 23,749Mn= 21, 598

600 1200 1800 2400 3000 3600Wavenumber (Cm-1)

Tran

smita

nce

(%)

1720

14561151

1318

(b)(a)

Figure 3.2. GPC curve (a) and FT-IR spectra (b) of block copolymers in KBr; PFlMMA-b-PBMMA (▲), PCzMMA-b-PBMMA ( )

93


3.3.2 Thermal Properties

The thermal properties of block copolymers were determined by thermogravimetric

analysis (TGA). Figure 3.3 represents the TGA curves of the protected and deprotected

block copolymers under N2 atmosphere and showed good thermal stability. The

deprotected hyroxylated copolymers were stable until 300 ºC (PCzMMA-b-PBMMA and

PFlMMA-b-PBMMA). The deprotected copolymers showed only single stage

decomposition where, the observed stability of block copolymer bearing carbazole moiety

can be explained by the presence of spacer linkage between the polymer backbone and the

chromophore unit. It was notice that the residual weights of the deprotected copolymers

were reduced to zero in the TGA thermograms around 500 ºC, indicating the complete

degradation of the polymer chain (Figure 3.3b). However, protected block copolymers

(13 and 14) showed two degradation temperatures and was stable up to 250 ºC (Figure

3.3a). The glass transition temperature (Tg) of block copolymers PCzMMA-b-PBMMA

and PFlMMA-b-PBMMA were 51 °C and 92 °C, respectively.

300 600 900Temperature (oC)

0

50

100

Wei

ght (

%)

200 400 600 8000

20

40

60

80

100

Wei

ght (

%)

Temperature (oC)

(a) (b)

Figure 3.3. Thermogravimetry analysis (TGA) of PFlMMA-b-PBMMA (▲) and PCzMMA-b-PBMMA ( ); protected (a) and deprotected (b) in N2 atmosphere with a heating rate of 10 ºC/min.

94


3.3.3 Optical Properties

The normalized absorption and emission spectra of PCzMMA-b-PBMMA and

PFlMMA-b-PBMMA in THF are shown in Figure 3.4. The absorption spectra of both

the block copolymers are consistent with the fact that carbazoyl /fluorenyl moieties were

incorporated as a pendant in the block copolymers (Figure 3.4 a). The absorption band of

PCzMMA-b-PBMMA below 350 nm is attributed to the absorption by the π-π*, n-π*,

and benzenoid transitions of carbazole and phenyl groups. The absorption band of

PFlMMA-b-PBMMA below 300 nm is attributed to the absorption by the π-π*, n-π*

transitions of fluorene groups.13 The block copolymers did not exhibit any new bands in

the absorption spectra, indicating that there was no observable interaction between the

electroactive units on the polymer backbone.

All block copolymers exhibited the emission characteristic of monomers (Figure

3.4b), peaks at 353 and 366 nm for PCzMMA-b-PBMMA and 315 nm for PFlMMA-b-

PBMMA, implying that the intermolecular exciplex can be ruled out in solution. This

supports the previous conclusion that the chromophores were not in the stacked form.14

300 4000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)300 360 420 480

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

(a.u

.)

Wavelength (nm)

(a) (b)

Figure 3.4. Absorption (a) and emission (b) spectra (excitation at 290 nm) of PCzMMA-b-PBMMA (▲) and PFlMMA-b-PBMMA ( ).

95


96

3.3.4 Self- assembly of Block Copolymers

As the synthesized block copolymers can be regarded as amphiphiles with

electroactive (carbazole/fluorene) groups as a hydrophobic block and polyhydroxylated as

hydrophilic block. Morphological studies of block copolymers were carried out using

scanning electron microscope (SEM), and transmission electron micrograph (TEM). The

self assembly of block copolymers was studied in a mixture of good and bad solvent

mixture such as THF: H2O or dioxane: H2O.

In a typical procedure, the water was progressively added to the THF or dioxane

solution of block copolymers (0.05 mg/mL) and the resulting colloid was drop casted for

characterization using scanning electron microscope and transmission electron microscope

at room temperature. Notably, the optimized THF: water (3:2 v/v) mixture gave

interesting nanostructures as compare to dioxane: water mixture.

The polymer films for SEM analysis were prepared by drop casting of polymer

solution on glass cover slip and solvent was allowed to evaporate slowly under ambient

conditions. Field emission scanning emission microscope (FE-SEM) images of block

copolymer PCzMMA-b-PBMMA and PFlMMA-b-PBMMA are shown in Figure 3.5a

and Figure 3.5b, respectively. SEM micrographs revealed that block copolymer

PCzMMA-b-PBMMA forms nanorods with a width of 90 nm on glass surface (Figure

3.5a). The magnified image of nanorods showed a cylindrical morphology. However;

under the same condition, the block copolymer PFlMMA-b-PBMMA formed spherical

structures (Figure 3.5b). The magnified SEM micrograph showed a spherical morphology

with a size range from 100 nm to 200 nm.


Transmission electron microscopy (TEM) has been an effective tool for characterizing

supramolecular self-assembly at nanometer dimension. A thin film was obtained on Cu-

grid through drop-casting of polymer solution and the TEM images of block copolymers

are shown in Figure 3.6. As shown in Figure 3.6a, TEM images of block copolymer,

PCzMMA-b-PBMMA showed in-plane order in which rod shaped structures

(hydrophobic) are regularly arrayed in a flexible chain matrix. The domain distances were

measured to be approximately 3.3 to 5.0 nm. However, TEM images of block copolymer

PFlMMA-b-PBMMA turned to achieve vesicular assemblies of 80 nm in diameter

(Figure 3.6b). From the TEM image, the thickness of the vesicular membrane was

estimated to be around 25 nm.

Figure 3.5. SEM micrographs of block copolymers; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

The formation of such structures by diblock copolymers can be explained with respect

to their chemical architecture. Hydrogen bonding and π – π stacking are strongly involved

in self-assembly process. Water is a good solvent for hydrophilic hydroxy-

benzylmethacrylate block but a non-solvent for π-conjugated aromatic

(carbazole/fluorene) block. On increasing water content, the solubility of

PCzMMA/PFlMMA block was distinctly reduced, which than lead to the formation of

97


aggregates. However, in case of PCzMMA-b-PBMMA, the long alkyl or alkoxy group

(spacer) tends to assemble in a lamellar or double lamellar morphology (Figure 3.6a)

from THF/H2O mixture. The polymer structure is dominated by packing of rod – coil

(PCzMMA – PBMMA) blocks.15 The micrograph shows grain structure, within which

cylindrical axes are oriented nearly parallel to each other. The bilayer vesicles and lamella

structure of block copolymers are schematically represented in Figure 3.8.

0.2 μm

(a)

0.1 μm

(b)

Figure 3.6. TEM images of block copolymers; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

For wide angle X-ray diffraction studies, the sample was prepared by casting the

solution (THF: H2O) of block copolymers on a glass cover slip. The X-ray diffraction

pattern of PCzMMA-b-PBMMA showed a number of sharp reflections including several

equidistant peaks, indicating the existence of a long-range order in the lattice (Figure

3.7a). The highly crystalline nature of the block copolymer PCzMMA-b-PBMMA in

solid state can be seen from X-ray data. The d-spacing (calculated using Brag’s Law) at

3.15 Å correspond to the π - π interaction.17 Considering the hydrophilic interaction on the

block copolymer chains, the sharp peak with d-spacing of 4.66 Å (2θ = 19.0 °) might be

due to the close packing of hydrophilic groups. The interdomain separation distance was

found to be slightly higher, which is consistent, with those obtained from TEM (Figure 98


3.6 a-b). The self- assembly and inter layer distance revealed a lamella type structure for

diblock copolymer PCzMMA-b-PBMMA.

4.2 8.4 12.6 16.8 21.0 25.2

Inte

nsity

2θ

12 18 24 30 36 42

Inte

nsity

2θ

(1 0 0)

(0 1 1)

(1 1 1)

(1 1

-2) (1

1 2

) (0 1

3)

(0 2

0)

(0 2

-1)

(1 -2

-1)

(2 -1

1)

(0 2

2)

(a) (b)(1 0 0)

(2 0

0)

(1 1

0)

(2 1

0)

(1 1

1)

(2 1

1)

(1 3

0) (4 0

0)

(2 0

2)

Figure 3.7. X-ray diffraction pattern of diblock copolymer; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b) on a glass slide.

Figure 3.8. Schematic diagram of microphase separation according to the amphiphilicity of block copolymers.

99


The wide angle X-ray diffraction pattern of self-assembled PFlMMA-b-PBMMA

sample is shown in Figure 3.7b. The block copolymer PFlMMA-b-PBMMA exhibit

diffraction peaks at small region 2θ of 3.3 ° corresponding to the d-spacing of 3.2 nm (π –

π interactions). The small broad peak in the small angle region contains diffraction peaks

of d200 and d110. The peaks in the higher angle region, 2θ= 31.4 °, 33.8 °, and 34.3 ° (d111,

d211, and d130, respectively) confirms the strong H-bonding. The detailed diffraction

patterns are explained in Table 3.2.

Table 3.2. X-ray diffraction data of block copolymer PCzMMA-b-PBMMA and PFlMMA-b-PBMMA

Diffraction angle(2θ) deg. PCzMMA-b-PBMMA PFlMMA-b-PBMMA

PCzMMA-b-PBMMA

PFlMMA-b-PBMMA

hkla dobs (Ǻ) dcalc (Ǻ) hkla dobs (Ǻ) dcalc. (Ǻ)

17.5 3.3 (100) 5.1 5.05 (100) 32.0 31.8

19.0 5.3 (011) 4.65 4.66 (200) 16.0 16.5

28.5 7.7 (111) 3.13 3.13 (110) 12.0 11.2

30.8 11.3 (11-2) 2.88 2.9 (210) 7.7 7.7

31.4 12.6 (112) 2.79 2.84 (111) 7.2 7.0

32.0 14.4 (013) 2.74 2.79 (211) 5.8 6.1

33.8 16.0 (020) 2.67 2.65 (130) 5.2 5.5

34.3 19.8 (02-1) 2.58 2.6 (400) 4.4 4.4

38.2 22.0 (1-2-1) 2.33 2.35 (202) 4.0 4.0

38.7 (2-11) 2.32 2.32

39.4 (022) 2.28 2.28

a

Deduced from International Centre for Diffraction Data 2006 (JCPDF Data). 100


101

3.3.5 Electrochemical Polymerization

Electropolymerization of the block copolymers was investigated using cyclic

voltammetry in a three-electrode cell. The cell was equipped with Ag/AgCl /3.8 M KCl as

reference electrode and platinum as counter electrode. The ITO substrate was used as

working electrode and the substrate. In the three cells compartment, 0.1 M

tetrabutylammoniumhexafluorophosphate (TBAP) was used as supporting electrolyte. The

block copolymer solutions (10 wt% in DCM) were drop casted onto ITO substrate and

used for the electropolymerization to prepare conjugated polymer network using cyclic

voltammetry at a scan rate of 50 mV/s (Figure 3.9).

Figure 3.9 a-b shows the CV traces for the block copolymers PCzMMA-b-PBMMA

and PFlMMA-b-PBMMA, respectively, with the polymer free scan in Figure 3.9 c-d.

For block copolymer PCzMMA-b-PBMMA, oxidation potential of the tethered carbazole

monomer was observed to occur at 0.9 V on the first cycle. Starting from second anodic

scan (Figure 3.9a), one new anodic peak at 1.05 V was observed with corresponding

reduction peak at 0.85 V. These results are in good agreement with the reported values for

the formation of radical cation units of carbazole.18 Subsequent cycles showed the same

onset values of oxidation and reduction peaks with increase in current values (Figure

3.10a), suggesting the electropolymerization of carbazole unit. This was further confirmed

from the polymer free scan (Figure 3.9c) of the cross-linked polymer film, which

preserved the anodic and cathodic peak potentials.

However, for PFlMMA-b-PBMMA, oxidation potential of the tethered fluorene

monomer was observed to occur at 1.24 V on the first three cycles where the shape of

redox peaks was not pronounced and much broader. Starting from fourth anodic scan


(Figure 3.9b), two anodic peaks (Epa1 and Epa2) appeared at 1.24 V and 1.43 V and

reduction peaks (Epc1 and Epc2) at 0.9 V & 1.10 V, which corresponds to the formation

of radical cation and bication species of the dimer unit, respectively.19 The anodic and

cathodic peak potentials with subsequent cycles reveals an increase in current values

(Figure 3.10b), suggesting the formation of polyfluorene through cross-linking. The

polymer free scan (Figure 3.9d) of cross-linked polymer films also preserved the redox

potential peaks.7

0.3 0.6 0.9 1.2 1.5-2.0µ

0.0

2.0µ

4.0µ

6.0µ

8.0µ

Cur

rent

(μA

)

Potential (E Vs Ag/AgCl)0.0 0.2 0.4 0.6 0.8 1.0 1.2

-80.0µ

-40.0µ

0.0

40.0µ

80.0µ

Cur

rent

(μA

)


(d)(c)0.0 0.3 0.6 0.9 1.2 1.

-1.0µ

0.0

1.0µ

2.0µ

3.0µ

4.0µ

5.0µ

5

Cur

rent

(μA

)


1st Cycle

8th Cycle

0.0 0.2 0.4 0.6 0.8 1.0

-60.0µ

-40.0µ

-20.0µ

0.0

20.0µ

40.0µ

60.0µ

80.0µ

Cur

rent

(μA

)


1st Cycle

10th Cycle(a) (b)

Figure 3.9. Ten cycle CV for electrochemical polymerization of block copolymers; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b). Polymer free scans for block copolymers after electropolymerization of PCzMMA-b-PBMMA (c) and PFlMMA-b-PBMMA (d).

102


0 2 4 6 8 10 12

-60

-45

-30

-15

0

15

30

45

60C

urre

nt (μ

A)

Number of Scans

R= 0.9881

R= -0.9927

0 2 4 6 8

-60

-45

-30

-15

0

15

30

45

60

Cur

rent

(μA

)

Number of Scans

R= -0.9820

R= 0.9739(a) (b)

Figure 3.10. Linearity of current with number of scans of electropolymerization of block copolymers; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

The electropolymerized thin films on ITO substrate were further characterized by Uv-

Vis, fluorescence, and FT-IR spectroscopic measurements. The cross-linked film was

washed with acetonitrile and acetone (3 times each) and performed the measurements. The

UV-Vis spectra of cross-linked films for both block copolymers PCzMMA-b-PBMMA

(Figure 3.11a) and PFlMMA-b-PBMMA (Figure 3.11c) showed single absorption

maximum at higher wavelength compared to the precursor one. This confirms the cross-

linking between the electroactive units to form conjugated oligomers/polymers. For block

copolymer PCzMMA-b-PBMMA, the absorption peak at 300 nm wavelength can be

assigned as polaron bonding to π* conduction band and the 520 nm peak correspond to

transition in bonding to antibonding state i.e. π - π* transition.20 In case of block

copolymer PFlMMA-b-PBMMA, the observed red shift of absorption peak at 320 nm

indicates a high degree of cross-linked conjugated moieties on the polymer and the new

peak at 530 nm attributes to the formation of conjugated polymer network with fluorene

unit. Similarly, for the emission spectra of the electropolymerized films, a red shift on

emission maxima as compared to the precursor polymer was observed (Figure 3.11b and

103


104

d). Further, the electropolymerization of precursor polymer was confirmed from FT-IR

spectroscopy of electropolymerized cross-linked films. The electropolymerized polymers

showed peak due to cross-linking in the ATR FT-IR spectrum (Figure 3.12b). The broad

peak at 846 cm-1 may be due to the C-H out- of- plane bending vibrations of 1, 2, 3, 4 -

substituted aromatic units.21

To observe the rate of cross-linking of electroactive units, absorption spectra were

measured using cross-linked films at a scan rate of 50 mV/s and showed a linear increase

with increasing the number of CV cycles. It indicates an increase in crosslinks with

increasing number of CV scans (Figure 3.13). The 520 nm peak can be assigned to the π -

π* transition of the newly formed polycarbazole. The absorption spectra in Figure 3.13 B

(PFlMMA-b-PBMMA) shows a similar increase in the peak values at 550 nm due to π -

π* transition of polyfluorene.

From the measurements, the band gap, Eg, could be calculated and compared to the

different electroactive species present in the film. The calculated Eg for the cross-linked

block copolymers were 1.76 eV (PCzMMA-b-PBMMA) and 1.64 eV (PFlMMA-b-

PBMMA) based on the extrapolation of the onset of absorbance in the UV-vis spectra.

However, their exact quantitative contribution to the cross-linked structure in the film is

not easily distinguishable from this experiment.


300 450 600 7500.0

0.4

0.8

Nor

mal

ized

Abs

orba

nce

Wavelength (nm)

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Abs

orba

nce

Wavelength (nm)400 480 560

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

Wavelength (nm)

300 400 5000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

Wavelength (nm)

(a)

(d)(c)

(b)

Figure 3.11. Absorption and emission spectra of electropolymerized deposited film on ITO with precursor block copolymers; PCzMMA-b-PBMMA (a, b), PFlMMA-b-PBMMA (c, d). Precursor polymer (▲) and electropolymerized film ( )


Tran

smitt

ace

(%)

846

846


Tran

smitt

ance

(%)

(a) (b)

Figure 3.12. ATR-FTIR spectra of block copolymers PCzMMA-b-PBMMA (▲) and PFlMMA-b-PBMMA ( ); precursor polymer (a) and after electropolymerization (b).

105


300 400 500 600 700 8000.0

0.1

0.2

0.3

Abs

orba

nce

(a.u

.)

Wavelength (nm)300 400 500 600 700 800

0.0

0.1

0.2

0.3

Abs

orba

nce

(a.u

.)

Wavelength (nm)

A B

Figure 3.13. Increase in absorption peak during CV electropolymerization of block copolymers at different number of cycles; 3 (▲),5 ( ),10 ( ), and 15 ( ); PCzMMA-b-PBMMA (A) and PFlMMA-b-PBMMA (B).

The surface morphology and molecular orientation of cross-linked films after

electropolymerization on ITO substrate has been studied using AFM measurements. The

electrochemical cross-linking of thiophene monomer showed both three-dimensional (3D)

and two dimensional (2D) nucleations, according to earlier report.19 Figure 3.14 shows

tapping mode AFM images of a cross-linked polymer film of PCzMMA-b-PBMMA and

PFlMMA-b-PBMMA. In case of block copolymer PCzMMA-b-PBMMA, at low

concentration and 10 cycles cross-link shows a unique morphology. These pyramidal

structures were of 250 nm in length and 100 nm in width with an interlayer separation of

3.5 Å, which was similar to aromatic stacked distance. Such feature was observed for

conjugated polymer network (CPN) and is presumably related to the overall hydrophobic-

hydrophilic balance properties.22 However, the block copolymer PFlMMA-b-PBMMA,

gave disordered ring type nanostructures after electropolymerization (Figure 3. 14b).

These structures were of 25 nm in thickness and 80-100 nm internal diameters.

Interestingly, the internal diameters are close to the size of the self-assembled vesicles

106


from PFlMMA-b-PBMMA and electropolymerization presumably occurs along the

vesicle membrane. Thus, the cross-linking process clearly influences the assembly size

and shape of the polymer morphology.

Figure 3.14. AFM images of conjugated polymer network film of 10 cycles on ITO; PCzMMA-b-PBMMA (a) and PFlMMA-b-PBMMA (b).

3.4. Conclusion

We have synthesized a new class of amphiphilic precursor block copolymers

(PCzMMA-b-PBMMA and PFlMMA-b-PBMMA) with polyhydroxy block and

electropolymerizable carbazole/fluorene block through atom transfer radical

polymerization (ATRP). Macrophase separated morphologies from hydrophilic and a

hydrophobic block was obtained by changing the polarity of the solvent mixtures.

Morphology of self-assembled structures has been indentified from SEM, TEM and X-ray

107


108

diffraction data. The cross-linked block copolymers were prepared electrochemically on

an ITO substrate with the formation of conjugated polycarbazole or polyfluorene network

without decomposition of polymer backbone. The cyclic voltammogram, FT-IR and UV-

vis spectra have indicated the formation of cross-linked polymer network (CPN). The

electrochemical oxidation of block copolymers showed a higher degree of aggregation on

ITO, forming square pyramidal nanostructure of PCzMMA-b-PBMMA and irregular ring

type nanostructures of PFlMMA-b-PBMMA, for cross-linked films.

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Joanicot, M.; Zasadzinski, J. A.; Weitz, D. A. Macromolecules 2004, 37, 2215; (b)

Mertoglu, M.; Laschewsky., A.; Skrabania, K.; Wieland, C. Macromolecules 2005,

38, 3601.

21. (a) Brunner, K.; van Haare, J. A. E. H.; Langeveld-voss, B.-M. W.; Schoo, H, F, M.;

Hofstraat, J. W.,van Dijken, A. J. Phys. Chem. B 2002, 106, 6834; (b) Lemmer, U.;

Henu, S.; Mahrt, R. F.; Hopmeire, M.; Sienger, U.; Gobel, E. O.; Müllen, K.;

Bassler, H. Chem. Phys. Lett. 1995, 240, 373.

22. (a) Lakowicz, J. R.; “Principle of Fluorescence Spectroscopy”; Kluwer

Academic/Plenum Publishers, New York, 1999; (b) Watkins, D. M.; Fox, M. A. J.

Am. Chem. Soc. 1994, 116, 6441; (c) Watkins, D. M.; Fox, M. A. J. Am. Chem. Soc.

1996, 118, 4344.


Chapter 4

Engineering Nano-architecture of Amphiphilic Dithienylethene (DTE):

Synthesis and Characterization Barik, Satyananda, Karen, Goh H.K.; Vadukumpully, Sajini; Valiyaveettil, Suresh; “Synthesis, Characterization and Self-Assembly of Azo-Aromatic Based Diarylethene: A Photochromic Molecule for Molecular Electronics” Submitted

112


4.1. Introduction

Engineering of supramolecular architectures for the generation of well-defined

nanostructures remain a challenge due to the multitude of co-operative intermolecular

processes.1 Synthetically well organized molecules with respect to the position of non-

covalent interactive sites, amphiphilicity, and molecular shape play key roles in self-

assembly.2 Presence of stimuli responsive units allow us to create functional materials,

which could be used for dynamic molecular devices.3-7 The most common strategy

applied for extended π – conjugated system is the self-assembly through favorable π –

stacking interactions.8-13 Construction of three-dimensional (3D) nanostructures can be

regulated via sterric interactions13 or by hydrogen bonding motifs.9

In this chapter, incorporation of photochromic moieties in molecular building blocks

to generate stimuli responsive material has been explored. We have designed and

synthesized dithienylethene (DTE) derivatives having diazo (-N=N-) side-chains DTE-Ph

and DTE-Naph (Figure 4.1) and characterized their molecular assemblies in solution.


4.2.1 Synthesis

The synthetic strategy for targets DTE-Ph and DTE-Naph are given in Scheme 4.1

and Scheme 4.2, respectively. Complete experimental and characterizations of target azo-

dithienylethene compounds and intermediates were given in Chapter 7.4

113


OC12H25N N

HO

C12H25OOH

NN

S S

F FF

F

F

F

N N

HOOH

NN

DTE-Naph

S S

F FF

F

F

F

DTE-Ph

Figure 4.1. Structural representation of amphiphilic dithienylethene DTE-Ph and DTE-Naph.


4.3.1 Design, synthesis and characterization

To employ the hydrophobic interactions for self-assembly, molecule is required to

have amphiphilic side chains and an aromatic core. Bearing the requirements for

molecular switch and self-assembly characteristics in mind, the target amphiphilic

diarylethene molecules are designed such that dithienylperfluorocyclopentene serve as the

photoswitching unit and azo-benzene/azo-naphthene derivative controls the self-

assembly. The thiophene rings have methyl groups at 2 and 2’- positions to prevent

oxidation. The two target molecules also have electron donating diazo groups attached to

5 and 5’- positions to amplify the molar absorptivities and absorption maxima. There is

114


also a hydroxyl (-OH) group, ortho to the diazo bridge, forming an intramolecular

hydrogen bonds between -OH group and the lone pair on N- atom. The difference

between the two target molecules is that one is based on azo-benzene, whereas, the other

is based on naphthalene derivative. As such, we expect the photophysical and self-

assembly properties to differ.

Schemes 4.1 and 4.2 show the synthesis schemes of target compounds DTE-Ph and

DTh-Naph, respectively. The synthesis starts with selective dibromination of 2-

methylthiophene at the 3 and 5 - positions. Thiophene methyl group at the 2- position of

thiophene ring activate the 3 and 5 positions due to stability of the intermediates during

the reaction. Sonogashira coupling of the dibrominated product (21) using Pd (0) catalyst

produced the desired product 22 with high yield and high purity. The

diarylperfluorocyclopentene (23) moiety was prepared by lithium-halogen exchange

reaction of 22 with n-BuLi followed by the nucleophilic substitution reaction with C5F8.

The reaction was carried out at -78 ºC to prevent THF from undergoing a reverse [2+3]

cycloaddition reaction, forming ethylene and the enolate of the acetaldehyde.

Deprotection of 23 was carried out using tetra-butyl ammonium fluoride (TBAF) in THF.

The deprotected product was used immediately in the next step after purification.

115


S S Br Pd(PPh3)4, CuI, TMSA, 45 oCMe

Br

(i) n-BuLi(ii) C5F8Me Br2

Dioxane, rtEt3N/THF THF, -78 oC

TBAF

THF, rt

21 22

23 24

47%80% 57%

80%

DTE-Ph

N

N

HO

OC12H25

Br

OH

OH

NaOH, ethanolC12H25Br

(i) NaNO2, HCl, 0 oC

(ii)4-bromoaniline26

25

Pd(PPh3)4, CuI

Et3N / THF, 50 oC35%

reflux44%

85%

SMe

Br

TMS

OC12H25

HO

N N

HO

OC12H25

NN

OH

C12H25O

N N

HO

OC12H25

NN

OH

C12H25ODTE-Ph (C)

UVVisible

24

S

H

S

H

F FF

F

F

F

S

TMS

S

TMS

F FF

F

F

F

S S

F FF

F

F

F

S S

F FF

F

F

F

Scheme 4.1. General synthetic strategy for target DTE-Ph

116


OH(i) NaNO2, HCl, 0 0C

(ii) 4-bromoanilineOH

NN Br

24

88 %

N N

HOOH

NN

N N

HOOH

N N

DTh-Naph

UVVisible

Pd (PPh3)4, CuI

Et3N/ THF50 0C40 %

S S

F FF

F

F

F

S S

F FF

F

F

F

27

DTh-Naph (C)

Scheme 4.2. General synthetic strategy for target DTE-Naph

The azo-dye was synthesized from the most common and frequently used azo

coupling reaction using sodium nitrite at zero degree temperature. Hence, the

regioselectivity of the reaction is enhanced owing to the presence of alkyl group at the

para- position. For the synthesis of bromo azo-compounds (26 and 27), electrophilic

attacks occurred at the alpha position of phenol/2-naphthol and are much more stable due

to resonance stability. The target molecules of azo-dithienylethene was obtained by

117


Sonogashira coupling reaction of diacetylene (24) and bromo azo-compound (26 or 27) in

presence of Pd (0) and CuI, resulting DTE-Ph or DTE-Naph with a reasonable yield.

The deprotected alkyne was added dropwise to the diazo reaction mixture to prevent the

formation of diacetylene coupled product. The target and intermediate molecules were

characterized using FT-IR, 1H-NMR, 13C-NMR, elemental analysis and HR-MS/

MALDI-TOF. The target amphiphilic molecules of dithienylethene, DTE-Ph and DTE-

Naph were soluble in common organic solvents. The FT-IR spectra of the azo-

derivatives of dithienylethene is given in Figure 4.2, which shows strong diazo band (υ ~

1434 cm-1) where the diazo group is in trans-form. Azobenzene derivative of

dithienylethene molecules shows a strong C-N stretching at υ ~1274 cm-1.

600 1200 1800 2400 3000 3600Wavenumber (Cm-1)

Tran

smitt

ance

(%)

Figure 4.2. FT-IR spectra of target molecules; DTE-Ph ( ) and DTE-Naph (▲).

4.3.2 Photochromism

The photoinduced ring closure of amphiphilic diarylethenes DTE-Ph and DTE-Naph

was monitored by the photochromic reactivity under UV lamp in chloroform solution.

118


Figure 4.3 shows the absorption spectrum of DTE-Ph (1.13 × 10-5 M) and DTE-Naph

(4.81 × 10-5 M) in chloroform. As expected, the results demonstrated that the molecules

exhibit photochromism. Upon irradiation with 365 nm UV-light, there is an immediate

decrease in the intensity of absorption band at 374 nm (DTE-Ph), 325 and 506 nm (DTE-

Naph) and an increase in the intensity of new band at 629 nm (DTE-Ph) and 640 nm

(DTE-Naph). It corresponds to the disappearance of the open ring isomer. With

increasing irradiation time the new absorption peak intensity increases, which confirm the

formation of the close ring isomer.21 These changes account for the change in color of the

solution from orange to green for DTE-Ph and pink to red for DTE-Naph (Figure 4.4).

Clean isobestic point at 498 nm and 551 nm were recorded for DTE-Ph and DTE-Naph,

respectively. This suggested the occurrence of two isomers (open and close ring) was in

equilibrium, with no detectable photodegradation of the DTE derivatives. Similar pattern

was observed in thin films drop casted on quartz surface. The complete photophysical

data are given in Table 4.1.

As predicted, there is no energy overlaps between the absorbance of the open- ring

and close- ring isomer of DTE-Ph and DTE-Naph (Figure 4.3B and D). This implies

that, the intermolecular quenching of the phenylacetylene azo-benzene/azo-naphthalene

excited state (through energy transfer) should be efficient only in the case of close-rings,

resulting in a decrease in the emission.22 Conversion of open-ring isomer to its closed-

ring counterpart results in efficient fluorescence quenching and no emission was

observed. This argument is reinforced by the data shown in Figure 4.5A, where the

absorbance corresponding to the close-ring isomer at 630/ 640 nm plotted against

irradiation time. The emissions of close- ring isomers are quenched 98 % relative to the

119


open- ring isomers of DTE-Ph and DTE-Naph (Figure 4.5 B).

Table 4.1. Photophysical properties of DTE-Ph and DTE-Naph in CHCl3

Compound Solvent Solution Thin film

λmax/ nm λems./ nm ε/cm-1M-1 λmax/ nm λems./ nm

DTE-Ph

(open-ring)CHCl3 375, 460 438 141055 396, 478 554

DTE-Ph

(close-ring)CHCl3 375, 460, 630 438 400, 480, 650 549

DTE-Naph

(open-ring)CHCl3 324, 507 585 23712 345, 530 655

DTE-Naph

(close-ring)CHCl3 324, 507, 640 585 345, 520, 645 686

The redox potentials of open-ring and close-ring isomers of DTE-Ph and DTE- Naph

molecules were obtained by cyclic voltammetry (Figure 4.6), with Ag/AgCl as an

internal reference. The cyclic voltammetry of opened-ring isomer DTE-Ph is quite

similar to that of open-ring isomer of DTE-Naph, where an irreversible oxidation

potential was observed at 1.5 V. This could be attributed to the oxidation of

dithienylethene moiety of the molecule. However, close-ring isomer of DTE-Ph and

DTE-Naph showed reversible oxidation potentials 1.5 V and 1.42 V, respectively.

120


400 500 6000

5

10

15

20

Inte

nsity

(a.u

.)

Wavelength (nm)

300 450 600 7500.0

0.5

1.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)

300 450 600 7500.0

0.4

0.8

1.2

1.6

Abs

orba

nce

(a.u

.)

Wavelength (nm)

UV Vis.

UV Vis.

A B

UV Vis.

C

525 600 675 7500

2

4

6

8

Inte

nsity

(a.u

.)

Wavelength (nm)

UV Vis.

D

Figure 4.3. Absorption (A and C) and emission (B and D) spectra of dithienylethenes photochromism in chloroform solution; DTE-Ph (A and B) and DTE-Naph (C and D).

Figure 4.4. Change in the color upon irradiation of azo-thienylethenes; DTE-Ph (a) and DTE-Naph (b) at different time intervals.

121


0 200 400 600 8000.0

0.3

0.6

0.9

0.0

0.3

0.6

0.9

F/ F

0

Abs

orba

nce

Time/sec0.0 0.3 0.6 0.9

0.0

0.2

0.4

0.6

0.8

1.0

F/ F

0

A/ A0

A B

Figure 4.5. Change in the UV-vis absorption (A) spectra (recorded at 625-640 nm) and normalized emission (recorded at 595 nm) for chloroforms solution of DTE-Ph and DTE-Naph when irradiated with 356 nm light over 900s period [absorption (▲) and emission ( ) of DTE-Ph; absorption ( ) and emission ( )] of DTE-Naph. Normalized emission (B) as a function of normalized absorption for solution of DTE-Ph (▲) and DTE-Naph ( ) at 10-5 M.

Electrical properties of photochromic molecules DTE-Ph and DTE-Naph were also

investigated. Photocurrent of photochromic molecules in opened and closed isomers were

characterized using four probe photoconductivity measurement. I-V characteristics of

DTE-Ph and DTE-Naph thin film on quartz substrate are summarized in Figure 4.7.

Before UV irradiation, no conductivity was observed for the samples of DTE-Ph till an

applied voltage of 86 V (Figure 4.7a). However, at 86 V a small but sharp increase in

conductivity was observed as can be seen from the graph. This could be due to the

electro-polymerization of the molecule at this particular voltage. For the UV irradiated

sample, the conducting current increases sharply to a value of 0.1 nA at a voltage range 0-

5 V above which the rate of increase remains constant. However, at around 86 V, a very

small rise in conductivity was noted. The electrical properties of DTE-Naph thin film

deposited on Quartz before and after UV irradiation is summarized in Figure 4.7b. For

open- ring isomer, the sample did not give measurable values of current. But for the

122


close- ring structure, the current increased drastically to a level of 0.7 nA at a low voltage

of approximately 3 V, after which a small increase was observed. At 100 V, the current

reaches a value of 0.96 nA. This confirms that the π – conjugation gets extended from the

donor group through the dithienylethene to the acceptor unit.

-0.5 0.0 0.5 1.0 1.5

0.0

2.0µ

4.0µ

6.0µ

8.0µ

Cur

rent

(μA

)

Potential (V)-0.5 0.0 0.5 1.0 1.5

-2.0µ

0.0

2.0µ

4.0µ

6.0µ

8.0µ

10.0µ

12.0µ

14.0µ

Cur

rent

(μA

)Potential (V)

(a) (b)

Figure 4.6. Cyclic voltammogram of photochromic dithienylethene molecules; before (▲) and after ( ) irradiation of DTE-Ph (a) and DTE-Naph (b).

0 20 40 60 80 100

0

1x10-1

2x10-1

3x10-1

4x10-1

5x10-1

6x10-1

7x10-1

Cur

rent

(nA

)

Voltage (V)0 20 40 60 80 10

0.0

0.2

0.4

0.6

0.8

1.0

0

C

urre

nt (n

A)

Voltage (V)

(a) (b)

Figure 4.7. Photo-conductivity (I-V) characteristics of DTE-Ph (a) and DTE-Naph (b); (▲) before and ( ) after irradiation.

123


4.3.3 Self-assembly and morphology

Under the ambient conditions, well-designed molecular building blocks usually self-

assemble into interesting supramolecular architectures.23 Samples for investigating the

self-assembly were prepared by drop casting photochromic molecules from chloroform

solution onto a freshly cleaved mica or Si substrate. The morphologies of the film were

characterized using atomic force microscopy (AFM). Factors such as effect of surface,

concentration of solutions, and temperature on the self-assembly, were examined.

Figure 4.8 shows AFM images of dithienylethene derivatives (DTE-Ph and DTE-

Naph) by drop casting a 10-5 M solution in chloroform (CHCl3) on a freshly cleaved mica

surface. For DTE-Naph, ring type structures were observed with uniform size. Cross-

sectional analysis of the respective rings showed that each ring has dimensions of 2 μm in

diameter and 827 nm thickness of the peripheries (Figure 4.8a). However, in the case of

DTE-Ph, deformed broken ring-type structures were observed (Figure 4.8 b).

(a) (b)

Figure 4.8. AFM images of azo-dithienylethenes on mica substrate; DTE-Naph (a) and DTE-Ph (b).

124


Figure 4.9 shows the AFM images of dithienylethene derivatives drop casted thin

films from chloroform solution with same concentration on hydrophobic Si (111)

substrate. The rings formed by DTE-Naph were similar to those observed on mica

surface. Size distribution and number of rings formed in certain areas were strongly

dependent on the substrate used. In Si (111) substrate, the rings were formed in bangle

shape (Figure 4.9 a-b) with similar diameter (1.2 μm) and thickness (82 nm). By

comparing the images of DTE-Naph on both substrates, it can be seen that the individual

rings were formed on Si surface, which was confirmed from the AFM images (Figure

4.9b). On the mica surface, DTE-Ph self-assemble into discontinuous ring aggregates,

similar structure was observed on a Si (111) surface. Figure 4.9 c-d shows the typical

structures formed from DTE-Ph molecule. The high magnification image (Figure 4.9d)

suggests the formation of fused ring type morphology, which might be explained by

amphiphilic character of the DTE-Ph molecules.

Driving force for the formation of ring architecture can be attributed to the orientation

of transition dipoles, hydrogen bonds, and π – π interactions of the molecules on the

substrate. In case of DTE-Naph due to absence of long alkyl chains, the intra-

intermolecular hydrogen bonding was more (from 1H NMR the –OH proton appears at δ

= 16.1 ppm) prominent (Figure 4.13). The molecular aggregation self-assembled and

giving rise to well define ring architectures.

125


(c) (d)

(b)(a)

82 nm 82 nm 82 nm

Figure 4.9. AFM images of azo-dithienylethenes on Si (111) substrate; DTE-Ph (a - b) and DTE-Naph (c - d).

Influence of concentration and thermal stability on the molecular aggregate of DTE-

Naph was also explored (Figure 4.10). In this study it is observed that rings formed by

DTE-Naph are quite comparable even at a low concentration (10-8 M). At higher

concentration (10-3 M) the rings tend to aggregate into columnar stacks (Figure 4.10a).

Thermal stability of the supramolecular nanostructures was studied by thermal

annealing of film at 120 °C for 2 h. Figure 4.11 shows the AFM images of DTE-Naph

nano-rings on Si (111) substrate annealed at 120 °C for 2 h. The magnified image of

126


annealed film (Figure 4.11b) shows that the ring structures were formed from number of

rings stacked each other in columnar manner. We have proposed that the driving force for

ring aggregation is determined by the columnar aggregation in solution, followed by

deposition on to the substrate.

(a) (b)

Figure 4.10. AFM images of DTE-Naph at different concentration on Si (111) substrate; 1.2 x 10-3 (a) and 1.05x 10-8 M (b).

(a) (b)

Figure 4.11. AFM images of DTE-Naph film on Si (111) surface annealed at 120 °C for 2 h.

127


Precise arrangement of DTE-Naph molecules within the rings was determined from

the optical properties of the rings. The absorption spectrum of rings of DTE-Naph is

shown in Figure 4.12a along with the spectrum from the solution. The spectroscopic

features of DTE-Naph rings are different from those in solution. Spectral changes

include a red-shift from solution (505 nm) to the ring (530 nm), indicating a more planar

structure. Similar red shift on the emission maxima was observed from solution (574 nm)

to nano-rings on the substrate (677 nm). Thus, we propose that DTE-Naph aggregate into

columnar stacks in which the azo-naphthol tail tends to form strong H-bonds along the

column.25

Schematic representation of ring nanostructure formation from DTE-Naph has been

represented in Figure 4.14. In a polar solvent (CHCl3), rings are formed via

intermolecular hydrogen bond assisted π-stacking of adjacent naphthalene moieties.

540 600 660 720 7800.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

(a.u

.)

Wavelength (nm)300 360 420 480 540 600 660 720 7800.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

(a.u

.)

Wavelength (nm)

(a) (b)

Figure 4.12. Absorption (a) and emission (b) spectra of DTE-Naph in chloroform; solution (▲) and nano-rings ( ) on Si (111) substrate.

128


11 12 13 14 15 16ppm

10 11 12 13 14 15 16ppm

12.3

92

16.2

63(a) (b)

Figure 4.13. 1H NMR of –OH proton; DTE-Ph (a) and DTE-Naph (b).

SMe

S

F F FFFF

MeH

NN N

NOH O

SMe

S

FFFF

FF

Me

NNNN

OHO H

SMe

S

F F FF

FF

Me

NN N

NOH O

H

SMe

S

FFFF

FF

Me

NNNN

OHO H

Self-assemblyOn silicon substrate

CHCl3

SMe

S

F F FFFF

MeH

NN N

NOH O

Figure 4.14. Schematic representation of self-assembled DTE-Naph molecule into columnar stacks.

4.4. Conclusion

In this study, we have successfully synthesized amphiphilic dithienylethene (DTE)

derivatives with diazo substitution to achieve their photochromic properties and

129


supramolecular self-assembly. The close-ring isomer generated from UV irradiation

showed a high current–voltage response than that of open-ring isomer. We have shown

that it is possible to construct well-defined nano- rings of supramolecular structures with

a high degree of internal order. This concept gives us a new strategy for the internal

ordering of DTE derivatives by H-bonding assisted π – π interactions.

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18. (a) Kurukawa, S.; Tahara, K.; De Schryver. F. C.; De Feyter, S. Angew Chem. Int.

Ed. 2007, 46, 2831; (b) Ichimura, K.; Oh, S. –K.; Nakagawa, M. Science 2000, 288,

1624.

19. (a) Murakami, H.; Kawabuchi, A.; Matsumoto, R.; Ido, T.; Nakashima, N. J. Am.

Chem. Soc. 2005, 127, 15891; (b) Yang, W. Y.; Ahn, J.-H.; Yoo, Y,-S.; Oh, N,-K.;

Lee, M. Nat. Mater. 2005, 4, 399; (c) Kadota, S.; Aoki, K.; Nagano, S.; Seki, T. J.

Am. Chem. Soc. 2005, 127, 8266.

20. (a) Jordan, B. J.; Ofir, Y.; Patra, D.; Caldwell, S. T.; Kennedy, A; Joubanian, S.;

Rabani, G.; Cooke, G.; Rotello, V. M. Small, 2008, 4 (11), 2074; (b) Zhang, X. J.;

Zhang, X. H.; Shi, W. S.; Meng, X. –M., Lee, C. S.; Lee, S. T. Angew. Chem. Int. ed.

2007, 46, 1525.

21. (a) Finden, J.; Kunz, T. K.; Branda, N. R.; Wolf, M. O. Adv. Mater. 2008, 1-5, 9999;

(b) Osuka, A.; Fujikane, D.; Shinmori, H.; Kobatake, S.; Irie, M. J. Org. Chem. 2001,

66, 3913; (c) Matsuda, K.; Irie, M. J. of Photochemistry and Photobiology C:

Photochemistry Reviews 2004, 5, 169; (d) Kose, M.; Shinoura, M.; Yokoyama, Y.;

Yokoyama, Yusushi, Y. J. Org. Chem. 2004, 69, 8403.

22. (a) Lim, S. J.; Seo, j.; Park, S. Y. J. Am. Chem. Soc. 2006, 128, 14542; (b)

Fukaminato, T.; Sasaki, T.; Kawai, T.; Tamai, N.; Irie, M. J. Am. Chem. Soc. 2004,

126, 14843; (c) Sheng, X.; Peng, A.; Fu, H.; Yao, J.; Liu, Y.; Wang, Y. J Mater. Res.

2007, 22 (6), 1558.

132


133

23. (a) Arai, R.; Uemura, S.; Irie, M.; Matsuda, K. J. Am. Chem. Soc. 2008, 130, 9371; (b)

Yagai, S.; Karatsu, T.; Kitamura, A. Chem. Eur. J. 2005, 11, 4954.

24. (a) Lehn, J. –M. Science 2002, 295, 2400; (b) Elemans, J. A. A. W.; Rowan, A. E.;

Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2662.

25. (a) Lesen, M. C.; Takazawa, K.; Elemans, J. A. A. W.; Jeukens, C. R. L.P.N.;

Christianen, P. C. M.; Maan, J. C.; Rowan, A. E.; Nolte, R. J. M. Chem. Eur. J. 2004,

10, 831; (b) Ribo, J. M.; Bofill, J. M.; Crusats, J.; Rubires, R. Chem. Eur. J. 2001, 7;

2733; (c) Cho, H. S.; Jeong, D. H.; Cho, S.; Kim, D.; Matsuzaki, Y.; Tanaka, K.;

Tsuda, A.; Osuka, A. J. Am. Chem. Soc. 2002, 124, 14642.


Chapter 5

Regioregular Electroactive Carbazole End-capped Oligo (p-phenylene): Synthesis,

Characterization and Self-assembly Studies

Barik, Satyananda; Valiyaveettil Suresh; “Regioregular Electro-active Carbazole End-Capped Oligo- (p-phenylene): Synthesis, Characterization and Self-assembly Studies” Communicated

134


5.1. Introduction

Self-assembly of conjugated rod building blocks offers opportunities to explore

functions and properties of interesting materials.1-8 Poly (p-phenylene) is a widely

investigated polymer owing to its interesting electronic properties, high thermal stability

and non-linear optical properties.1 The aggregation and properties of rigid molecules can

be tuned by the functional groups and relative length of respective blocks.3 Although rod-

like molecules have been studied, the design of aromatic rod segments with laterally

extended aromatic chains remains a challenge.3-10 The end-capped building blocks has a

strong tendency in the formation of controlled supramolecular nanostructure through

steric repulsions.

In this Chapter, a new class of regioregular carbazole end-capped oligo-p-phenylene

molecules was synthesized (Figure 5.1) and fully investigated their self-assembled

properties. Substituents on the backbone also influence the self-assembly of molecular

chains. It is anticipated that the microphase separation between rigid and flexible

segments could lead to the formation of crystalline materials.


5.2.1 Synthesis

The synthetic strategy for targets OLG1 – OLG4 was given in Scheme 5.1 - 5.3.

Complete experimental and characterization of intermediates and targets are given in

Chapter 7.5

135


OLG-2

N N

OC6H13

C6H13O

H3CO OR

OCH3RO

R= 2-ethyl hexyl

N N

OC6H13

C6H13O

RO OR

ORRO

R= 2-ethyl hexylOLG-1

NN

O

O

OLG3

N

N

N

N

NN

O

O

N

N

N

OLG4

N

Figure 5.1 Designed molecular structure of oligomers (OLG1-OLG4).


5.3.1 Design, synthesis and characterization

The syntheses of intermediates and target molecules OLG-OLG4 are shown in

Scheme 5.1-5.3. Our synthesis started with carbazole, 4-bromobiphenyl-1-ol,

hydroquinone and 4-N, N-diphenylamino-1-bromobenzene. Carbazole trimer (31) was

synthesized through iodination (28) and acetylation of carbazole (29) followed by

Ullmann condensation (30) and deprotection (31) (Scheme 5.1).11 We adopted the acetyl

group for the protection of amines, due to the high thermal stability of the amide bond

compared to that of Boc (t-butoxycarbonyl) group,which is a traditional protecting group

for amines. The biphenyl boronicacids (33a-b) were obtained from the starting material

4-bromobiphenyl-1-ol by O-alkylation with 2-ethylhexyl bromide or iodomethane

136


followed by boronicacid preparation. The compound 33a-b undergoes Suzuki coupling

reaction with 3, 6-dibromocarbazole to afford di- (34a) or mono- (34b) coupling products

selectively. The monocoupling product (34b) further undergoes Suzuki coupling reaction

in same condition to afford product 35. Importantly, the 3,6-bis[4’-(N, N-diphenylamino)-

1’-phenyl]carbazole (37) (Scheme 5.2) was obtained by palladium catalyzed direct

Suzuki coupling of boronicacid 36 and 3, 6-dibromocarbazole with 95 % yield.

Hydroquinone was O-alkylated with 1-bromohexane to 1, 4-di-O-hexylbenzene (38)

followed by bromination in bromine and acetic acid to afford a quantitative yield of 2, 5-

dibromo-1,4-di-O-hexylbenzene (39).13 The corresponding boronicacid (40) was

synthesized under low temperature in Bu-Li and tri-isopropylborate with isolated yield

72%. The extended aromatic structure 41, was obtained by palladium-catalyzed Suzuki

cross- coupling of compound 40 with 4-bromo-1-iodobenzene in selective solvent

dimethylethane (DME) with a yield of 82%.9d The target oligomer (OLG1-OLG4) were

obtained via Buchwald double amination reaction using palladium catalyst.12 The

dibromo benzene trimer (41) was treated with aromatic heterocyclic amine (31, 34a, 35,

37), palladium (II) acetate, tri-tertButyl phosphine and sodium tert-butoxide in toluene

under reflux afforded corresponding oligomers (OLG1-OLG4) (Scheme 5.3) in a

moderate yield. Structures of the intermediates and oligomers (OLG1-OLG4) were

indentified using FT-IR, 1H & 13C NMR spectroscopy, elemental analysis and molecular

weight by MALDI-TOF and FAB-MS analysis.

137


HN

HN

II

N

II

O

N

NN

O

N

NN

H

i ii iii

iv

Br OH Br OR

R= 2-ethylhexyl

(HO)2B OR

R= 2-ethylhexyl

HN

OR

OMe

R1=R2=

v vi

vii viii

28 29

30 31

32a 33a

34a

32b R= Me 33b R= Me

34b R1 =

R1 R2

, R2 = Br

R = 2-ethylhexyl

HN

MeOOR

R = 2-ethylhexyl

35

Scheme 5.1 General synthetic strategy of compounds 28 - 35; (i) KI, KIO3, AcOH, 30 min.58%; (ii) Ac2O, BF3-Et2O, 30 min.63%; (iii) carbazole, Cu2O, DMAc, 160 °C, 36 h; (iv) KOH, H2O, THF/DMSO, 2 h, 33%; (v) K2CO3, DMF, 2-ethylhexylbromide/ MeI, reflux, 80 °C, 24 h, 75%; (vi) n-BuLi, THF, -78 °C, B(O-iPr)3, -78 °C- rt, 21 h, 2M HCl, 40-43%; (vii) 3,6-dibromocarbazole, THF, PTC, K2CO3 (2M), Pd[PPh3]4, 80 °C , 24 h, 40-52%; (viii) 34b, 33b, THF, 2M K2CO3, Pd[PPh3]4(0), 80 °C, 24 h, 76%.

138


OH

HO

OR'

R'O

Br Br

OR'

R'O

B BHO

HO

OH

OHiii iv

Br Br

OC6H13

C6H13O

R' = C6H13 R' = C6H13

3637

38

i

v

NBr N(HO)2B

HN

NN

39 40

41

i ii

OR'

R'O R' = C6H13

Scheme 5.2 General synthetic strategy of compounds 36-41; (i) THF, -78 °C, n-Bu-Li, B(O-iPr)3, 2M HCl, 65%; (ii) 3, 6-dibromocarbazole, THF, 2M K2CO3, PTC, Pd[PPh3]4, 80 °C, 24 h, 92%; (iii) NaOH, EtOH, C6H13Br, 70 °C, 24 h, 76%; (iv) Br2, AcOH, 80 %; (v) 4-bromo iodobenzene, DME, Na2CO3, H2O, Pd[PPh3]4, 80 °C, 24 h, 92%.

Br Br

OC6H13

C6H13O

N

RR'

H

Toluene, Pd(OAc)2PtBu3, NaOtBuReflux, 18 h

OC6H13

C6H13O

N

R

R'

N

R

R'

OLG1, OLG2, OLG3, OLG4

OMe

OOLG1: R = R' =

OOLG2: R = R' =

OLG3: R = R' = N

41 31, 34a, 35, 37

OLG1: R = R' =N

Scheme 5.3 Synthesis of target oligo (p-phenylene) OLG 1- OLG4, 40-74% 139


1000 2000 3000 4000

% T

rans

mita

nce

Wavenumber (Cm-1)

OLG2

OLG1

OLG4

OLG3

Figure 5.2 FT-IR spectra of target molecules OLG1 – OLG4.

The FT-IR spectra of OLG1-OLG4 showed corresponding peaks due to N-C

stretching (1240 cm-1) (Figure 5.2). The peak at 1600 cm-1 showed C=C stretching band

of aromatic units and the C-H aromatic stretching bands was observed around 3000 cm-1.

The strong band at 800 - 810 cm-1 for OLG1 and OLG2, shows C-H out-of-plane, which

was due to para-substituted benzenes and are absent for OLG3-OLG4.


The thermal properties of the 4, 4’-disubstituted end-capped oligo(p-phenylenes)

(OLG1-OLG4) were investigated by thermogravimetric analysis (TGA) and differential

scanning calorimetry (DSC). The TGA and DSC results are shown in Figure 5.3 and

summarized in Table 5.1. Thermogravimetric analyses showed that all the oligomers

were thermally stable with a decomposition temperature above 380 °C under nitrogen.

The DSC curves of the oligomer OLG1-OLG3 exhibit, a clear glass transition 140


temperature of 270, 145, and 212 °C, respectively. The OLG4 does not show any

crystallization temperature. These results support the observation that on increasing the

end-capped conjugation gives a clear glass transition temperature due to undisturbed π-

conjugation.

50 100 150 200 250 300

-0.6

-0.4

-0.2

0.0

Hea

t Flo

w

Temperature (0C)

B

200 400 600 800

40

50

60

70

80

90

100

Wei

ghtlo

ss (%

)

Temperature (οC)

(A)

Figure 5.3 Thermal properties of OLG1 (▲), OLG2 ( ), OLG3 ( ), and OLG4 ( ); TGA (A) and DSC (B) (2nd heating) at heating rate of 10 min/ °C in N2.


The optical properties of molecules OLG1-OLG4 in THF solution are depicted in

Figure 5.4 and summarized in Table 5.1. The UV-vis spectra showed absorption maxima

at 315, 314, 340 and 292 & 340 nm for OLG1, OLG2, OLG3 and OLG4, respectively

(Figure 5.4A). The red shift of 20-25 nm relative to the corresponding p-phenylenes

indicates the formation of a highly extended π-electron delocalization system through the

carbazole end-caps.12 The absorption band corresponds to the π - π* electron transition of

the conjugated backbone. The OLG3 shows a 30 - 40 nm red shift compared to other

oligomers, due to the presence of different electroactive (triphenylamine) units. However,

these absorption maxima were considerably red shifted in comparison to the PPP 141


polymers indicating the formation of a highly extended π-electron delocalization.3,13 The

HOMO-LUMO energy gaps (Egopt)14of OLG1-OLG4 were estimated from the onset

absorption edge (Table 5.1).

The emissions spectra of OLG1, OLG2 and OLG4 showed the fairly constant

maxima at 396 nm, while OLG3 gave a red shift of 10-15 nm (Figure 5.4A). The

emission spectra obtained are identical, suggesting that energy or excitation can

efficiently transfer from the peripheral electroactive unit to the phenylene rod segment.

Therefore, the red shift should be attributed to the intermolecular charge transfer through

formation of intermolecular aggregates.15 This may suggest that at low concentration,

more aggregation should occur in our oligomer systems because of low molecular weight.

Furthermore, all the fluorescence peaks have a red shift compared to the corresponding

values in polymer systems, which is again due to high aggregation with more ordered

structure.

300 400 5000.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

(a.u

.)

Wavelength (nm)

Nor

mal

ized

Inte

nsity

(a.u

.)A

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

Nor

mal

ized

abs

orba

nce

(a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

(a.u

.)B

Figure 5.4 Absorption and emission spectra of OLG1 (▲), OLG2 ( ), OLG3 ( ), and OLG4 ( ); THF solution (A) and thin film (B) in quartz surface.

Both the absorption and emission spectra of thin films (Figure 5.4B) of the end-

capped oligophenylens (OLG1-OLG4) appear to be red shifted with respect to solutions

142


and thus demonstrate existence of intermolecular interaction between the molecules. The

fluorescence quantum yields (Фn) of the end-capped oligomers OLG1- OLG4 in

dichloromethane (CH2Cl2) solution range from 0.52 to 0.62 (Table 5.1). The fluorescence

quantum yields of oligomers were identical except for OLG3, which was higher due to

the presence of triphenylamine electroactive block. The low quantum yield may be

attributed to the planar π-conjugated structures, particularly vulnerable to π – stacking.

Introduction of end-capped units to the phenylene rod skeleton perturbs the planarity and

alters the emission properties of the phenylene core.

Table 5.1 Summary of physical measurements of OLG1-OLG4

Oligo

mers

aUV-

vis

λmax

bEmi-

ssion

λems.

UV-vis

λonset

cBand

gap

Egopt

(eV)

dE1/2

(Oxd)

eV

eHOMO

eV

fLUMO

eV

gEgel

eV

hФ

n

iTdec

°C

jTg

°C

OLG1 315 395 488 2.54 1.09 5.89 3.44 2.45 52 405 270 OLG2 315 395 513 2.41 1.25 6.05 3.64 2.41 54 390 145 OLG3 340 408 509 2.43 0.71 5.51 3.08 2.43 62 387 198 OLG4 292,

340 395 512 2.42 1.26 6.06 3.64 2.42 58 370 148

a Measured in dilute THF solution b Excited at the absorption maxima c Calculated from the onset of absorption spectra (Eg

opt = 1240/λonset)

d Measured in a three-electrode system fitted with a platinum rod counter electrode, ITO working electrode and a SCE reference electrode in CH2Cl2 containing 0.1 M n-Bu4NPF6 as supporting electrolyte in acetonitrile at a scan rate of 50 mV/s under N2.

e Calculated from empirical formula: HOMO = Eonset + 4.44 eV f Calculated by LUMO = HOMO - Eg

opt g Determined by Eg

elc = HOMO – LUMO h Determined in CH2Cl2 solution (A < 0.05) at room temperature using quinine sulphate

solution in 0.01 M H2SO4 (Фstd = 0.54) i Determined by thermal gravimetric analyzer with a heating rate of 10 deg/min under

N2. j Determined by differential scanning calorimeter from remelt after cooling with a

heating rate of 10 deg/min under N2.

143


5.3.4 Electrochemical properties

In order to investigate the electrochemical behavior of 4, 4’-disubstituted oligo(p-

phenylenes) OLG1-OLG4, cyclic voltammetry (CV) measurements were performed in

CH2Cl2 containing 0.1 M n-Bu4NPF6 as supporting electrolyte at a scan rate of 50 mV/s.

The results are shown in Figure 5.5 and summarized in Table 5.1. All the end-capped

carbazole rod structures exhibit low reversible one/two electron oxidations with E1/2 from

0.7 to 1.27 V. For OLG1, OLG2 and OLG4, the redox potentials are of 1.23, 1.23 and

1.27 V, respectively, which corresponds to the removal of electrons from the interior

carbazole moieties forming radical tetracation and hexylcations. The OLG4 showed a

higher redox potential due to the presence of carbazole trimer unit at the end. Ambipolar

end-capped carbazole OLG3, exhibits electrochemical behavior with four-electron anodic

redox couples (E1/2 ~ 0.7 eV) and another two-electron anodic redox couples (E1/2 ~

0.92 eV). Comparing the redox potential of the oligomer systems, both oxidation and

reduction potentials were affected by the structural change of the end-capped oligo p-

phenylenes. The oxidation and reduction potentials of OLG3 are a bit smaller than those

of OLG1, OLG2 and OLG4 suggests the incorporation of electroactive block makes

backbone more susceptible for oxidation. Analogous to the spectroscopic results, the

oxidative potentials are progressively shifted to lower energies with the increasing length

of the π-conjugated system. A potential difference in OLG3 predicts the radical cation

could effectively delocalize the charge along the phenylene backbone to triarylamine end-

caps. These results are consistent with the red-shifts in the absorption spectra.16

The HOMO and LUMO energy levels of the oligomers were determined from the

onset position of oxidation and are summarized in Table 5.1. In general, with the

144


incorporation of triarylamine end-caps, the HOMO energy level moved up to ~ 5.51 eV

as estimated by electrochemical method. The LUMO was determined by the difference of

HOMO and the optical energy. Such a high HOMO energy level greatly reduces the

energy level for hole injection from ITO (Ф = 5.0 eV) to the phenylene rod. This suggests

the oligomers can be used as hole transport materials.

-0.5 0.0 0.5 1.0 1.5

-50.0µ

0.0

50.0µ

100.0µ

150.0µ

Cur

rent

(μA

)


Oxd-OLG4

-0.4 0.0 0.4 0.8 1.2-2.0µ

-1.0µ

0.0

1.0µ

2.0µ

3.0µ

Cur

rent

(μA

)


Oxd-OLG3

-0.5 0.0 0.5 1.0 1.5

-20.0µ

0.0

20.0µ

40.0µ

60.0µ

80.0µ

Cur

rent

(μA

)

Potential (E Vs Ag/Agcl)

Oxd-OLG2

-0.5 0.0 0.5 1.0 1.5 2.0-100.0µ

0.0

100.0µ

200.0µ

300.0µ

400.0µ

Cur

rent

(μA

)


Oxd-OLG1

Figure 5.5 CV curves of OLG1-OLG4 measured in CH2Cl2 containing n-Bu4NPF6 as supporting electrolyte at a scan rate of 50 mV/s.

5.3.5 Self-assembly and microphase separation

Diblock molecules OLG1-OLG4 is considered as novel class of amphiphilic molecules

and can self-assemble into an aggregates when dissolved in a particular solvent.17 The

145


aggregation behaviors of these molecules were subsequently studied in water by adding

the THF solution of the compound into ultrapure water (final THF/water ratio of 1: 9 v/v).

After the solution was allowed to equilibrate over 24 h, the aggregates were investigated

using UV-vis and fluorescence spectra, scanning electron microscope (SEM), and

transmission electron microscopy (TEM). The absorption spectrum of OLG1 in THF

solution exhibits a broad transition with a maximum at 315 nm (Figure 5.6). However, in

aqueous solution of OLG1 both the absorption and the emission maximum were red

shifted with respect to the THF solution and the fluorescence were significantly quenched

with the added water concentration, indicative of aggregation of the conjugated rod core

segments.18

400 500 6000

100

200

300

400

Inte

nsity

(a.u

.)

Wavelength (nm)

OLG1 10% 20% 40% 50% 70% 80%

300 400 500 6000.0

0.5

1.0

1.5

2.0

2.5

Abs

oeba

nce

(a.u

.)

Wavelength (nm)

OLG1 10% 20% 40% 50% 80% 70% 90%

(a) (b)

Figure 5.6 Absorption (a) and emission (b) spectra of OLG1 in THF/water solution.

The microphase separation of end-capped oligo-(p-phenylene) (OLG1) in THF: water

solution was conferred from scanning electron microscopy (SEM), and transmission

electron microscopy (TEM). Figure 5.7a-b shows the SEM micrographs of OLG1 casted

on a glass slide, which clearly shows the cylindrical (rice grain) morphologies with the

uniform diameter of 4 - 5 nm and length of 100 nm. The morphology of the oligomer

146


(OLG1) was further conferred from transmission electron microscope (TEM). The

sample was prepared by drop casting of solution (THF: H2O) on a copper grid surface.

The TEM micrographs of OLG1 are shown in Figure 5.7c-d. The molecule has shown

the same nano-rod structures with counter length of 90-95 nm and 4 -5 nm width (Figure

5.7d). Because of the bulky dendrons grafted on the rigid backbone, the oligomers hold a

segmented structure to adopt cylindrical (nano-rod) morphology. Considering the

extended molecular length (hyperchem energy minimized) of OLG1, the diameter of the

rice-grain nanostructure corresponds to the hydrophobic (π-π) stacking of molecules. The

cylindrical aggregate consists of aromatic cores surrounded by hydrophilic chains in

which the rod segments stack on top of each other.

Figure 5.7 Scanning electrom microscope (a, b) and transmission electron microscope (c, d) images of nano-rods formed by self-assembly of OLG1 in aqueous solution (THF: H2O).

147


Wide angle X-ray diffraction technique was used to investigate the supramolecular

nanostructures of OLG1. Figure 5.8 shows the X-ray diffraction pattern of powdered

oligomer and self-assembled film on a glass cover slip. The X-ray diffraction pattern of

self-assembled OLG1 includes several equidistant peaks, indicative of the existence of

long-range-order structure on the lattice (Table 5.2).19 In powdered form, the molecule

showed a broad diffraction peak (2θ = 28.3°).

The reflections observed for self-assembly film were indexed as a 2D primitive

rectangular structure with lattice parameters a = 7.4 Å, b = 9.94 Å, and c = 10.8 Å. The

diffraction pattern is dominated by sharp peak at low angle (2θ = 4.3 º, d = 20.2 Å) with

shoulder peaks, indicating the existence of a 2D oblique columnar assemblies.20 The 2D

supramolecular assembly of OLG1 is schematically depicted in Figure 5.9. The spacing

of reflections ranging from 2.0 to 35.0 Å were all elucidated and listed in the Table 5.2.

10 20 30 402θ (deg.)

Inte

nsity

(a.u

.)

(110)(002)

(012)(120)

(022)(220)(132)

(-223)

(042)

(311)

(150)

(410)

(-101)(210)

(200)(-123)

Figure 5.8 XRD patterns of OLG1; powdered (▲) and self-assembly ( ) sample drop casted on glass cover slip.

148


Table 5.2 Observed and calculated reflections from X-ray diffraction data at room temperature of self-assembled OLG1

Diffraction Angle(2θ) (deg.) a h k l d obs.( Å) d calc. (Å)

2.5 1 1 0 32.4 35.0

3.4 2 0 0 28.0 25.66

4.3 2 1 0 21.0 20.26

7.3 4 1 0 12.10 12.03

14.2 -1 0 1 6.57 6.23

16.3 0 0 2 5.35 5.42

18.7 0 1 2 4.70 4.72

22.0 1 2 0 4.11 4.02

24.3 0 2 2 3.63 3.65

30.3 2 2 0 2.94 2.94

31.8 -1 2 3 2.82 2.81

34.6 1 3 2 2.55 2.59

36.8 -2 2 3 2.43 2.44

39.2 0 4 2 2.25 2.29

41.5 3 1 1 2.23 2.16

46.8 1 5 0 1.92 1.93

a Deduced from International for Diffraction Data 2006 (JCPDS Data)

The 2-dimensional nanorod structure obtained in this study is likely formed via π-

stacking. The d-spacing corresponds to the π-stacking of OLG1 inferred from the peak

value of 2θ = 24.3º, is calculated to be 3.65 Å, which is quite close to the reported value

(~3.5 Å) for an effective π-π stacking between the aromatic molecules.21 The stability of 149


such molecular assemblies originated due to interplay of the side-chain association and

the strong π-π interaction (Figure 5.9).

Figure 5.9 Proposed supramolecular assembly through side-chain association of OLG1.

5.4. Conclusion

We have presented a facile synthetic procedure for the preparation of a series of 4, 4’-

disubstituted end-capped p-phenylenes by palladium catalyzed Buchwald’s double

amination reaction. All the end-capped materials were characterized by 1H NMR, 13C

NMR, matrix-assisted laser desorption ionization times-of-flight (MALDI-TOF) mass

spectroscopy and elemental analysis. The carbazole acts as a hole transporting moiety and

physical properties of all oligomers were investigated. The optical and electrochemical

investigations reveal the electronic interaction between the carbazole and the backbone

phenylene/dendronized π-conjugated substituent in the oligomers. The thermal properties

of these materials were enhanced with the presence of conjugated carbazole unit. In

aqueous solution, it was observed that the target oligomers were self-assemble into

cylindrical nano-rods (rice-grain) and was characterized from TEM, SEM, and WXRD

study.

150


5.5. References

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Bredas, J. L.; Chance, R. R.; Elsenbaumer, R. L.; Shacklette, L. W. Chem. Rev.

1982, 82, 209; (c) G. Tourillion, G. in Handbook of conducting polymers (Ed:

Skotheim, T. A.) Deker, New York 1986.

2. (a) Segura, J. L.; Martin, N. J. Mater. Chem. 2000, 10, 2403; (b) Kraft, A.;

Grimsdale, A. C.; Holmes, A. B.; Angew. Chem. Int. Ed. 1998, 37, 402; (c) Bunz,

U. H. F. Acc. Chem. Res. 2001, 34, 998; (d) Breen, C. A.; Deng, T.; breiner, T.;

Thomas, E. L.; Swager, T. J. Am. Chem. Soc. 2003, 125, 9942; (e) Berresheim, A.

J.; Muller, M.; Mullen, K. Chem.. Rev. 1999, 99 (9), 1747.

3. (a) Bae, J.; Choi, J. –H.; Yoo, Y. –S.; Oh, N. –K.; kim, B. –S.; Lee, M. J. Am.

Chem. Soc. 2005, 127, 9668; (b) Ryu, J. –H.; Huang, Z.; Lee, M. Angew. Chem.

Int. Ed. 2006, 45, 5304; (d) Yang, W. –Y.; Ahn, J. –H.; Yoo, Y.-S.; Oh, N.-K.;

Lee, M. Nature Mater. 2005, 4, 399; (e) Yang, W. –Y.; Lee, E.; Lee, M. J. Am.

Chem. Soc. 2006, 128, 3484; (f) Yoo, Y.-S.; Choi, J.-H.; Song, J.-H.; Oh, N. –K.;

Zin, W.-C.; Park, S.; Chang, T.; Lee, M. J. Am. Chem. Soc. 2004, 126, 6294.

4. Gellman, S. H. Acc. Chem. Res. 1998, 31, 173.

5. (a) Ryu, J.-H.; Oh, N.-K.; Lee, M. Chem. Commun. 2005, 1770; (b) Hoger, S.

Chem. Eur. J. 2004, 10, 1320; (c) Zhao, D.; Moore, J. S. Chem. Commun. 2003,

807.

6. Percec, V.; Dulcey, A. E.; Balagurusamy, V. S. K.; Miura, Y.; Smidrkal, J.;

Peterca, M.; Nummelin, S.; Edlund, U.; Hudson, S. D.; Heiney, P. A.; Duan, H.;

Magnov, S. N.; Vinogradov, S. A. Nature 2004, 430, 764.

151


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1793; (b) Petitjiean, A.; Nierengarten, H.; Van Dorsselaer, A.; Lehn, J. –M.

Angew. Chem. Int. Ed. 2004, 43, 3695; (c) Kim, H. –J.; Zin, W.-C.; Lee, M. J. Am.

Chem. Soc. 2004, 126, 7009; (d) Ryu, J.-H.;Bae, J.; Lee, M. Macromolecules

2005, 38, 2050.

8. Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. Rev. 2001, 101, 3869.

9. (a) Hill, J. P.; Jin, W.; Kosake, A.; Fukushima, T.; Ichihara, H.; Shimomura, T.;

Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481; (b) Elemans,

J. A.A. W.; Slangen, R. R. J.; Rowan, A. E.; Nolte, R. J. M. J. Org. Chem. 1999,

64, 7009; (c) Kim, T.; Arnt, L.; Atkins, E.; Tew, G. N. Chem. Eur. J. 2006, 12,

2423; (d) Li, Z. H.; Wong, M. S. Org. Lett. 2006; 8(7), 1499; (e) Li, Z. H.; Wong,

M. S.; Tao, Y.; Fukutani, H. Org. Lett. 2007, 9(18), 3659.

10. (a) Lee, M.; Yoo, Y. S. J. Mater. Chem. 2002, 12, 2161; (b) Lee, M.; Cho, B. –K.;

Zin, W.-C. Chem. Rev. 2001, 101, 3869.

11. Kimoto, A.; Cho, J.-S.; Higuchi, M.; Yamamoto, K. Macromolecules 2004, 37,

5531.

12. (a) Venkatesh, C.; Sundaram, G. S. M.; Ila, H.; Junjappa, H. J. Org. Chem. 2006,

71, 1280; (b) Rataboul, F.; Zapf, A.; Jackstell, R.; Harkal, S.; Riermeier, T.;

Monsees, A.; Dingerdissen, U.; Beller, M. Chem. Eur. J. 2004, 10, 2983; (c)

Bedford, R. B.; Betham, M. J. Org. Chem. 2006, 71, 9403; (d) Cabello-Sanchez,

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2030; (e) Bedford, R. B.; Cazin, C. S. J. Chem. Commun. 2002, 2310.

13. (a) Baskar, C.; Lai, Y.H.; Valiyaveettil, S. Macromolecules 2001, 34, 6255; (b)

Liu, B.; Yu, W.L.; Lai, Y.H.; Huang, Wei. Macromolecules 2002, 35, 4975; (c)

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Balanda, P.B.; Ramey, M.B.; Reynolds, J.R. Macromolecules 1999, 32, 3970; (d)

Lee, S.A.; Hotta, S.; Nakanish, F. J. Phys. Chem. A 2000, 104, 1827.

14. (a) Schlüter, A. D. J. Polym. Sci. A: Polym. Chem. 2001, 39, 1533; (b) Rehan, M.;

Schlüter, A.-D.; Wegner, G. Polymer 1989, 30, 1060; (c) Young H. Kim and

Owen W. Webster; Macromolecules 1992, 25, 5561; (d) Ravindranath, R.;

Ajikumar, P. K.; Advincula, R. C.; Knoll, W.; Valiyaveettil, S. Langmuir 2006,

22, 9002; (e) Ravindranath, R.; Vijila, C.; Ajikumar, P. K.; Hussain, F.S. J.; Ng,

K. L.; Wang, H.; Jin, C. S.; Knoll, W.; Valiyaveettil,S. J. Phys. Chem. B 2006,

110, 25958.

15. Wong, M. S.; Li, Z. H. Pure. Appl. Chem. 2004, 76, 1409.

16. (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A.J.; Wudl, F. Science 1992, 258,

1474; (b) Jenekhe, S. A.; Osaheni, J. Science 1994, 265, 765; (c) Onoda, M.;

Tada, K.; Zakhidov, A. A.; Yoshino, L. Thin Solid Films 1998, 331, 76; (d) Peng,

K.Y.; Chen, S.A.; Fann, W.S. J. Am. Chem. Soc. 2001, 123, 11388.

17. (a) Promarak, V.; Punkvuang, A.; Jungsuttiwong, S.; Saengsuwan, S.;

Sudyoadsuk, T.; Keawin, T. Tetrahedron Letters 2007, 48, 3661; (b) Rodriguez, J.

G.; Lafuente, A.; Rubio, L.; Esquivias, J. Tetrahedron Letters 2004, 45, 7061.

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Fukushima, T.; Kosaka, A.; Ishii, N.; Aida, T. J. Am. Chem. Soc. 2007, 129, 719;

(d) Kim, B.-S.; Hong, D.-J.; Bae, J.; Lee, M. J. Am. Chem. Soc. 2005, 127, 16333;

(e) Xu, J.; Zubarev, E. R. Angew. Chem. Int. Ed. 2004, 43, 5491; (f) Niece, K. L.;

Hartgerink, J. D.; Donners, J. J. J. M.; Stupp, S. I. J. Am. Chem. Soc. 2003, 125,

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154

7146; (g) Ornatska, M.; Peleshanko, S.; Genson, K. L.; Rybak, B.; Bergman, K.

N.; Tsukruk, V. V. J. Am. Chem. Soc. 2004, 126, 9675.

19. (a) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148; (b) Hoeben,

F. J. M.; Shklyarevskiy, I. O.; Pouderoijen, M. J.; Engelkamp, H.; Schenning, A.

P. H. J.; Christianen, P. C.. M.; Maan, J. C.; Meijer, E. W. Angew. Chem. Int. Ed.

2006, 45, 1232; (c) Bae, J.; Choi, J. –H.; Yoo, Y. –S.; Oh, N.-K; Kim, B, -S.; Lee,

M. J. Am. Chem. Soc. 2005, 127, 9668; (d) Vmessmore, B. W.; Hulvat, J. F.;

Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2004, 126, 14452; (e) Varghese, R.;

George, S. J.; Ajayaghosh, A. Chem. Commum. 2005, 593.

20. (a) Yamamoto, T.; Fang, Q.; Morokita, T. Macromolecules 2003, 36, 4262; (b)

Yasuda, T.; Yamamoto, T. Macromolecules 2003, 36, 7513.

21. (a) Bushey, M. L.; Hwang, A.; Stephens, P. W.; Nuckolls, C. J. Am. Chem. Soc.

2001, 123, 8157; (b) Balakrishnan, K.; Datar, A.; Zhang, W.; Yang, X.; Naddo,

T.; Huang, J.; Zuo, J.; Yen, M.; Moore, J. S.; Zang, L. J. Am. Chem. Soc. 2006,

128, 6576.

22. (a) Ajayaghosh, A.; Gerge, S. J. Am. Chem. Soc. 2001, 123, 5148; (b) Endo, K.;

Ezubara, T.; Koyanagi, M.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1997,

119, 499; (c) Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkovsky,

E. B.; Grubbs, R. H. Angew. Chem. Int. Ed. 1999, 38, 2741; (d) Struijk, C. W.;

Sieval, A. B.; Dakhorst, J. E. J.; Van Dijk, M.; Kimkes, P.; Warman, J. M.;

Zuihof, H.; Sudholter, E. J. R. J. Am. Chem. Soc. 2000, 122, 11057.


Chapter 6

Conjugated Polymer Network Self-assembled Films from Precursor Polymers: Cross-

conjugated Poly (p-phenylene) Barik, Satyananda; Valiyaveettil, Suresh; “Cross-conjugated Poly (p-Phenylene): A side chain conjugated Polymer for molecular electronics” Manuscript under preparation

155


6.1. Introduction

Over the last decade a significant amount of research has focused on conjugated

polymers because of their potential applications in light-emitting devices,1 photovoltaic

cells2 and field effect transistors,3 combined with attractive material properties such as

low weight and ease of processing.4 Recently cross conjugated polymers are designed and

developed to fine tune the electrical and optical properties in polymers5 and dendrimers6.

Extended conjugation in these macromolecules leads to strong intermolecular interactions

via π-π stacking. It is well-known that a balanced and efficient charge injection/transport

for both carrier types (electron and hole) is essential for high device efficiency.

Poly (p-phenylene) (PPP)7-8 is a blue emitting material with large band gap, and its

electronic, optoelectronic properties could be easily modified through proper

functionalization. The introduction of long alkoxy chains onto the polymer backbone not

only makes these materials more soluble but also increases the p-type strength. Intensive

research activities on the physical properties and synthesis of PPP – conjugated polymers

have shown these materials to be promising candidates for applications in stable

optoelectronic devices.9 Recently, our group focused on the synthesis and characterization

of a series of dialkoxy-phenylene copolymers.10 Dialkoxyphenylene – pyridine

copolymers exhibited blue emission,10a and their photophysical properties were controlled

by protonation of pyridine ring, intramolecular hydrogen bonding and metal ion.11 The

study of carbazole-based conjugated polymers have attracted interest for preparing

functional materials, such as photorefractive materials,12 and hole-transporting

materials.13 This is due to their inherent electron-donating nature, excellent

photoconductivity and unique nonlinear optical properties.14 Thus carbazole incorporated

156


polymers are a potential candidate for tuning the optical and electrical properties of light

emitting and semiconducting organic materials. Electropolymerization of conjugated

polymer with electropolymerizable carbazole group, gave conjugated polymer network.15a

In this chapter, synthesis and characterization of a donor-acceptor copolymer with

electro-optically active monomers incorporating on the side chain of the polymer

backbone is discussed in detail (Figure 6.1). The electroactive monomers

thiophene/carbazole moieties in the side chain were polymerized or the synthesis of

conjugated polymer network (CPN). Substituent on the backbone have been designed to

incorporate strong π – π stacking and weak Vander Waals interactions and to control the

self-assembly of polymer chains in the lattice. It is anticipated that the length and lateral

position of the cross-conjugated monomer groups on the polymers could have significant

effect on the self-assembly process.

*

S

C6H13

SN2S

C6H13

S

S

C6H13

SN

2S

C6H13

S

*

SSS

* *

OR

RO

S

S

* *

OR

RO

m

m

n

n

SS

SS

nm

P1

P2

P3

R = C12H25

R = C12H25

Figure 6.1. Structures of the target copolymers P1-P3.

157



6.2.1 Synthesis

The synthetic strategy for the polymer synthesis is given in Scheme 6.1- 6.3. Complete

experimental and characterization details of intermediates, monomers (M1-M3) and

polymer (P1-P3) synthesis were given in Chapter 7.6.

S

OH

S

OH

BrBr S

H

SSS B(OH)2NBS

CHCl3, AcOHrt, 4d, 82%

THF, K2CO3 (2M)PTC, Pd[o], 72%

Br

Br

Br

Br

Br

Br

Br

Br

(EtO)2OP

PO(OEt)2

Br2, AcOH NBS, AIBN

AcCN, reflux12 h, 50-58%

DMF, PO(OEt)3

reflux, 38-43%

OH

HO

OC12H25

OC12H25

OC12H25

C12H25O

BrBr

OC12H25

C12H25O

B(OH)2(HO)2BK2CO3, EtOH

C12H25Br, 12 h

Br2, AcOH THF, n-BuLi,

16 h, 65%-78 0, B(OiPr)3

Br

Br

Br

Br

Br

PO(OEt)2

THF, KOtBurt, 12 h

Br

Br

SSS

Br

Br

S

SS

M1

M2

O

S

H

SS

O

SS

4243

44

45b

45a 46a

46b

4748 49

43

80%

75%

80%

46b

46a

S

64%

72%

Scheme 6.1. Synthetic procedures for thiphene based monomers (M1-M2)

158



6.3.1 Synthesis and characterization

3-thiophene carboxaldehyde, p-xylene, hydroquinone, 3-hexyl thiophene boronicacid

and 2, 2’-dibromo bithiophene were used as starting materials. 2,5-di-[2-thiophene] 3-

thiophenecarboxaldehyde (43),16 mono and di-Wittig ylides (46a-b),17 2,5-dibromo-1,4-

didodecylbenzene (48)9 and the corresponding diboronicacid (49)10 were synthesized

according to the reported procedures. To achieve further extended conjugation in P1, and

P2, tetrathiophene (50) was used in P3. The compound 50 was synthesized from Suzuki

coupling of dibromo-bithiophene and 3-hexyl thiopheneboronicacid. Selective

bromination of 50 using NBS and chloroform18 followed by Suzuki coupling with 5-

formyl 2-thiopheneboronicacid produced the monobromo compound (52) according to

reported procedures. The electroactive (carbazole) unit in 53 was introduced by

Bucchwald’s coupling reaction with carbazole.19 Aldehydes (43 and 53) and Wittig ylides

(46a-b) were coupled to generate the dibromo monomers (M1-M3) according to the

reported procedure.17 Finally, the target copolymers P1-P3 were synthesized by Suzuki

polymerization of dibromo monomers (M1-M3) and boronicacids (49 or 1, 4-benzene

diboronicacid). The polymers were purified by reprecipitation from chloroform and

excess methanol (5-times), filtered and dried under vacuum for overnight.

The chemical structures of the copolymers P1-P3 were confirmed by 1H NMR, 13C

NMR, FT-IR spectroscopy, elemental and thermogravimetry analyses. FT-IR spectra are

shown in Figure 6.2. The strong and broad -C=C- band due to the vinyl group was visible

at ~1466 cm-1. The observed sharp band at 1010 cm-1 in P1 and P2 is due to the O-C bond

stretching. The molecular weight distributions for block copolymers P1-P3 and other

159


physical properties are summarized in Table 6.1

SS BrBr

S B(OH)2

C6H13

SS

SS

C6H13

C6H13

(ii)(i)

SS

SS

C6H13

Br

C6H13

Br(i)

S B(OH)2

OS

SS

S

C6H13

Br

C6H13

SO

HN

(iii)

SS

SS

C6H13

C6H13

NSO

(iv)

Br

Br

SS

S

C6H13

SS

SS

C6H13

S

C6H13

C6H13

2

2

NN

50

51 52

53

46a

M3

Scheme 6.2. Synthetic procedures for monomer M3; (i) THF, K2CO3 (2M), PTC, Pd [0], 92%; (ii) NBS, CHCl3, rt, 78%; (iii) toluene, Pd (OAc)2, PBu3-BF4, reflux, 12 h, 62%; (iv) THF, KOtBu, rt, 12 h, 52%.

160


OC12H25

C12H25O

B(OH)2(HO)2B

SSS

* *

OC12H25

C12H25O

* *

OC12H25

C12H25O

P1

P2

SS S

SSS

a

a

n

n

m

m

B(OH)2(HO)2Ba

SC6H13

S

N

2

S

C6H13

S

S C6H13

S

N

2

S

C6H13

SP3

n

m

M1

M2

M3

Scheme 6.3. Synthetic procedure for copolymers P1-P3; (a) THF, K2CO3 (2M), PTC, Pd [0], 80 °C, 72 h, 52-67%.

161


500 1000 1500 2000 2500 3000Wavenumber (cm-1)

Tran

smitt

ance

(%)

Figure 6.2. FT-IR spectra of copolymers P1 (▲) and P2 ( ).

Table 6.1. Physical properties of copolymers P1-P3

Polymer aMn aMw aPDI λmax (nm) λ onset (nm)

Egopt

(ev)bTd ( C)

Solution Thin Film

P1 8076 13,568 1.68 320 324 499 2.48 268

P2 10,508 11,875 1.13 338 325 510 2.43 317

P3 11,401 15,164 1.33 366 460 660 1.87 296

aDetermined by GPC (THF, Polystyrene standard); bDetermined from TGA (N2 atmosphere, 10 ºC/min scan rate).


The normalized UV-vis and PL spectra of the polymer (P1-P3) in solution and in

solid state are shown in Figure 6.3 and Figure 6.4, respectively. All characterizations

were done in THF. All polymers showed absorption maxima between 320 and 470 nm,

which corresponds to the π – π* transition. A red shift in λmax (Δλmax ~ 25 nm) was also

observed compared to the linear PPPs without conjugated side chain.20 The λmax in P2 is

much more red shifted (338 nm) as compared to that in P1 (320 nm) due to the presence

162


of symmetrical cross-conjugation in P2. The absorption maxima are expected to shift to

higher wave length region with increasing conjugation. This was indeed observed in

polymer P3. It showed highest absorption maxima at 466 nm (Figure 6.3a). Absorption

spectra of P1-P3 were broad indicating possible aggregation in solution. The polymer P1-

P3 showed emission maxima from 460 nm to 550 nm. P1 showed a blue shift in the

emission maximum (461 nm) compared to that of P2 (490 nm), owing to its lack of

planarity caused by asymmetrical cross-conjugated thiophene units. The HOMO-LUMO

energy gap (Egopt) of P1-P3 were estimated from the onset absorption edge (Table 6.1).21

It is well known that, the intermolecular interactions like photon-induced electron

transfer or formation of excimer, exciplexes and aggregates become more efficient in

solid state. The solid-state UV-vis and PL spectra of P1-P3 showed similar pattern as in

solution state with red shift of the absorption and fluorescence maxima (Figure 6.4a-b).

P2 showed a blue shift in the emission maximum (515 nm) as compared to that of P1

(545 nm), indicating its lack of planarity caused by symmetrical cross-conjugated

thiophene units. P3 showed a maximum red-shift in solid state of 600 nm compared to the

polymer P1-P2.

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

(a.u

.)

Wavelength (nm)400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

(a.u

.)

Wavelength (nm)

(a) (b)

Figure 6.3. Absorption (a) and emission (b) spectra of polymers in THF solution; P1 ( ), P2 ( ), and P3 (▲).

163


450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

(a.u

.)

Wavelength (nm)

(a) (b)

280 350 420 490 560 630 700 7700.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

(a.u

.)

Wavelength (nm)

Figure 6.4. Absorption (a) and emission (b) spectra of polymers in thin film; P1 ( ), P2 ( ), and P3 (▲).


Thermogravimetric analysis (TGA) was done using a heating rate of 10 ºC/min up to

1000 ºC under N2 atmosphere. All polymers (P1-P3) showed reasonably good thermal

stability (Figure 6.5 and Table 6.1). The decomposition temperature falls in the same

range as the previously reported linear PPP polymer.9-10 It is interesting to note that P1,

P2, and P3 have almost same thermal stability in spite of the structural differences. P1

and P2 contain tri-thiophene cross-conjugated asymmetrical and symmetrical

architecture, whereas, P3 contains cross-conjugated system with thiophene end-capped

carbazole moiety. Therefore, it is concluded that all these groups decompose/degrade

almost at the same temperatures.

164


90 180 270 360 450 540 630 7200

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (oC)

Figure 6.5. Thermogravimetry analysis (TGA) of cross-conjugated copolymers; P1 (▲), P2 ( ), and P3 ( ) at heating rate of 10 °C/min under N2 atmosphere.

6.3.4 Electropolymerization

Electrochemical polymerization and cross-linking of side-chain substituted carbazole

or thiophene units are well studied in Chapter 2 and Chapter 3. The polymers thus

obtained demonstrated potential ability to form thin films with unique optical,

electrochemical and morphological properties. The cross-linked structures were

facilitated by oxidation of electropolymerizable groups. This involves a three-electron

transfer process via the 3, 6-position of carbazole unit or 2, 5–position of thiophene unit

forming conjugated polymer network film. The intermediate is believed to be based on a

radical cation which rapidly reacts via coupling-deprotonation to form the dimer.

Subsequent cycles lead to higher oligomeric species and further cross-linking as

evidenced by a lowering of the oxidation potential and increase of charge with each

succeeding cycle. Similar behaviour was shown by polymers P1-P3 while

165


electropolymerizing on ITO to obtain cross-conjugated polymer film. Cyclic voltammetry

(CV) measurements were carried out using an electrolyte solution of 0.1 M

tetrabutylammoniumhexafluorophosphate (Bu4NF6PO4) dissolved in acetonitrile, in

which the precursor polymer was not soluble. The oxidation and reduction potentials of

the parent PPP were not in the range of the applied potentials for electropolymerization.

The oxidation can only happen between the thiophene or carbazole groups within the

potential range used for experiments. Therefore the cross-linking of the thiophene or

carbazole monomer units occurs during the electropolymerization without affecting the

polymer backbone.

Cyclic voltammograms of the cross-linked films of P1-P3 on ITO substrates with a

scan rate of 100 mV/s are given in Figure 6.6. The oxidation onset for 10 layers of P1 is

1.07 V and the corresponding reduction peak is 0.79 V (vs Ag/AgCl) (Figure 6.6a). The

first cycle is entirely different from the second cycle with development of a 1.40 V

oxidation peak and 0.28 V reduction peak. The current increased with the number of

cycles. The observed peak values for doping (1.40 V) and dedoping (0.89, 0.28 V) for P1

is slightly higher than previously reported thiophene incorporated films.15d

In a similar conjugated system, P2, backbone with thiophene cross-conjugated PPP,

the oxidative doping was observed at about 1.3 – 1.4 V, followed by another current

increase at about 1.4 V during the anodic scan of the cycle (Figure 6.6b). Similarly, the

dedoping onset of current was observed at 0.93 V and 0.29 V with subsequent scans

confirming the cross-conjugated structure formation.

166


0.00 0.31 0.62 0.93 1.24 1.55-116.00µ

-58.00µ

0.00

58.00µ

116.00µ

174.00µ

232.00µ

290.00µ

10st Cycle

Cur

rent

(μA

)

Potential (V)

1st Cycle

(a)

0.0 0.5 1.0 1.5

-220.00µ

0.00

220.00µ

440.00µ

660.00µ

Potential (V)

Cur

rent

(μA

)

1st Cycle

15th Cycle(b)

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.2m

-800.0µ

-400.0µ

0.0

400.0µ

800.0µ

1.2m

1.6m

Cur

rent

(μA

)

Potential (V)

(c) 20th Cycle

0.0 1.6 3.2 4.8 6.4 8.0 9.6 11.2-12

-10

-8

-6

-4

-2

0

2

Cur

rent

(μA

)

Number of Scan

R= 0.97939

R= -0.99505

(d)

0 2 4 6 8 10 12 14 16

-1.56

0.00

1.56

3.12

Cur

rent

(μA

)

Number of Scan

R = -0.99617

R = 0.99182(e)

0 3 6 9 12 15 18-2.72

-1.36

0.00

1.36

2.72

4.08

Cur

rent

(μA

)

Number of Scan

R = -0.99827

R = 0.99472(f)

Figure 6.6. CV for electrochemical cross-linking film; P1 (a), P2 (b), and P3 (c) followed by their linearity curves P1 (d), P2 (e), and P3 (f).

In the case of P3, the electropolymerizable group carbazole is in the cross-conjugated

“precursor polymer”. The oxidation onset for spin casted film was 0.93 V and the

167


corresponding reduction peak was 0.78 V (vs Ag/AgCl) (Figure 6.6c). The oxidation

peak was almost similar to that of previously reported electropolymerized carbazole

unit.15 On increasing the number of electropolymerized scans (cycles), the current

increases along the same peak potentials (anodic, cathodic) (Figure 6.6c). It conferred the

formation of cross-conjugated polymer network thin film on ITO.

The CV gives clear evidence of the electropolymerization of the thiophene and

carbazole units in P1-P3. This was further confirmed from the linearity curve of increase

in current with number of scans (Figure 6.6d-f).

6.3.4 Morphological characterization of electropolymerized film

The morphologies of the polymer films before and after electropolymerization on the

ITO substrate were studied using atomic force microscopy (AFM). The morphologies of

polymers P1-P3 in THF solution are shown in Figure 6.7. The morphologies exhibit

ultrathin regular layers on a substrate for P1 and irregular aggregates for P2 and P3.

Figure 6.8 shows the morphologies of the film after electropolymerization of P2. The

film was extensively washed with acetonitrile, water and acetone before recording the

AFM image. The roughness of the film measured in different areas of the conjugated

polymer network films is less than 7 nm with complete coverage.

The AFM images in Figure 6.8 showed unique nanostructures (nano-fibers) of P2

after electropolymerization on ITO substrate at a scan rate of 100 mV/s for 15 cycles.

These nano-fibers have a width of 5-10 nm after cross-linking. The self-assembly

depends on the interplay between the cross-linking of thiophene units on

electropolymerization. Due to the symmetrical nature of P2, the number of thiophene

units of cross-conjugated polymer is constant. On electropolymerization, the cross-linked

168


film showed extened sexithiophene systems, which are symmetrically aligned in a

perpendicular direction. Most probably, the tilt angle of cross-linked molecules

(sexithiophene) relative to the surface normal becomes lower as length increases.

Figure 6.7. AFM morphologies of polymers in THF on ITO substrate before electropolymerization; (a) P1, (b) P2, and (c) P3.

(a) (b)

Figure 6.8. Tapping-mode AFM topography images of P2 after electropolymerized on ITO at scan rate of 100 mV/s; flatten nano-fibers (a-b).

From the AFM data in combination with the molecular modeling stimulations, the

fibrillar structures were formed from the conjugated core of parallel π-stacked thiophene

units.

6.4. Conclusion

A series of symmetrical and asymmetrical cross-conjugated “precursor polymer” PPP

derivatives have been synthesized and fully characterized. All polymers incorporated with

169


multiple functional groups on the conjugated side chain are thermally stable. Optical

studies indicated an extended conjugated π-electron system with collective response

arising from the respective chromophores. Electropolymerization was facilitated without

decomposing the PPP backbone leading to the formation of conjugated polymer network

film. Present studies on the self-assembly of cross-conjugated sexithiophenes have shown

that the inter-cross-linking leads to interactions (π – π stacking between conjugated

segments). Such molecules may have applications in electro-optical, sensing and other

nanoscience and functional materials.

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CCCHHHAAAPPPTTTEEERRR 777

EEEXXXPPPEEERRRIIIMMMEEENNNTTTAAALLL SSSEEECCCTTTIIIOOONNN

174


7.1. General Instrumentations

All reagents were purchased from Aldrich, Fluka, Merck and TCI and were used

without further purification unless mentioned in this thesis. Tetrahydrofuran (THF) was

distilled under N2 over sodium/benzophenone prior to use. N,N´-dimethylformamide

(DMF) was dried over activated molecular sieves (4 Å, Aldrich). Toluene,

dichloromethane, hexane, acetone and diethyl ether were purchased from J. T. Baker and

purified by distillation. The necessary chemicals were purchased from Sigma Aldrich,

Merck, Lancaster, Scientific Resource, and Sino Chemicals. Silica gel 60 (0.040–0.063

mm) purchased from Merck was used for all column chromatography purifications. AIBN

was recrystallized from methanol. 1H (300 MHz) and 13C NMR (75.4 MHz) spectra was

recorded using Bruker ACF 300 instrument. FT-IR spectra were recorded using BIO-RAD

FT-IR spectrophotometer. Thermogravimetric analyses (TGA) were recorded on a TA-

SDT2960 at a heating rate of 10 °C min-1 under N2 environment. Differential scanning

calorimetry (DSC) thermograms were recorded using a TA-DSC 2920 at a heating rate of

5 °C min-1 under nitrogen atmosphere. Gel permeation chromatographic (GPC) analyses

were performed with a Waters 2696 separation module equipped with a Water 410

differential refractometer HPLC system and Waters Styragel HR 4E columns using THF

as eluent and polystyrene as standard. X-ray powder diffraction studies were performed

using a D5005 Siemens X-ray diffractometer with Cu Kα (1.54 Å) radiation at 40 kV and

40 mA. The morphology of the gels and the calcium carbonate crystals were studied using

a JEOL JSTM 220A scanning electron microscope (SEM). The samples were mounted on

copper stubs using double-sided carbon tapes and sputter coated with gold before

measurement. Transmission electron microscopy (TEM) investigations were done on a

175


JEOL 2010 machine at an accelerating voltage of 200 kV. A NanoMan AFM system with

Au-coated Si3N4 contact tips was used for all nanopatterning experiments. I-V response

measurements were performed on the nanopatterned area using conductive AFM (CAFM),

in which a current sensor with a range of µA to fA was used. Cyclic voltammogram

studies were performed using an Autolab PGSTAT30 Potentistat/Galvanostat with a

standard three electrode electrochemical cell containing a 0.1 M tetrabutylammonium

hexafluorophosphate solution in acetonitrile at room temperature under nitrogen

atmosphere using a scanning rate of 50-100 mV/s. A platinum/ITO working electrode,

platinum counter electrode and Ag/AgCl /SCE (saturated KCl aqueous solution) reference

electrode were used.

7.2. Chapter 2

The experimental procedures for the synthesis of intermediates and target polymers are

described as Scheme 2.1 in Chapter 2.

7.2.1 General procedure for synthesis of monomers (3, 5, 7, 9 and 11)1

The hydroxyl group containing electroactive compounds [2-(N-carobazolyl) ethanol

(2), 11-(N-carbazolyl) undecanol (4), 9-flurenol (8), and 2-(3-thienyl) ethanol (10)

(Scheme 2.1 and pp. 64] were esterified with methacryloyl chloride in dry THF and in

presence of triethylamine at 0 °C (Scheme 2.1, pp. 64). The reaction mixture was stirred at

room temperature for overnight. Solvent was removed under vacuum and the residue was

dissolved in saturated solution of NaHCO3. The product was extracted in ether and the

organic layer was washed three times with water (3×10 mL) and dried over Na2SO4. After

176


removal of solvent in vacuum, the crude product was purified using column

chromatography in hexane and ethyl acetate as eluent.

2-(carbazolyl-N-ethyl) methacrylate (3) (Scheme 2.1, pp. 64)

White solid (Yield = 80 %). 1H NMR (300 MHz, CDCl3, δ ppm), 1.80 (s, 3H,-CH3), 4.61

(t, 2H, O-CH2), 4.56 (t, 2H, N-CH2), 5.48 (s, 1H cis-vinylene), 5.93 (s, 1H trans-vinylene),

7.25 (m, 2H, Cz), 7.45 (m, 4H, Cz), 8.08 (d, 2H, Cz). 13C NMR (75.4 MHz, CDCl3, δ

ppm): 18.2, 41.6, 62.4, 108.6, 119.2, 120.4, 123.0, 125.7, 126.3, 135.7, 140.4, 167.2. FT-

IR (KBr, cm-1): 1718.4 (C=O), 1635.6 (C=C), 3054, 1600, 749 (carbazole ring), 2979,

1465 (Alkyl); EI-MS: 279.1 (M+); Elemental analysis: Calculated. C, 77.40; H, 6.13; N,

5.01; Found: C, 77.32; H, 6.15; N, 4.97.

3

N

(CH2)2

OO

9-(carbazole-N-undecyl) methacrylate (5) (Scheme 2.1, pp. 64)

White solid (Yield = 80 %). 1H NMR (300 MHz, CDCl3, δ ppm): 8.19 (d, 2H, Cz), 7.36

(m, 4H, Cz), 7.31 (m, 2H, Cz), 6.22 (s, 1H trans-vinylene), 5.64 (s, 2H cis-vinylene), 4.33

(t, 2H, O-CH2-CH2-), 4.22 (t, 2H, O-CH2), 2.05 (m, 3H, -CH3), 1.90 (m, 2H, CH2), 1.34

(m, 16H, -CH2-); 13C NMR (75.4 MHz, CDCl3, δ ppm): 18.2, 25.9, 27.2, 28.5, 28.9, 29.1,

29.3, 29.4, 43.0, 64.7, 108.6, 118.6, 120.2, 122.7, 125.0, 125.5, 136.5, 140.3, 167.4; EI-

MS: 405.2 (M+); Elemental analysis: Calculated. C, 79.96; H, 8.70; N, 3.45; Found: C,

79.84; H, 8.72; N, 3.43.

177


5

N

(CH2)11

OO

[2-(2-carbazol-9-yl-ethoxy)-ethyl] methacrylate (7) (Scheme 2.1, pp. 64)

Yellow solid (yield = 50 %). 1H NMR (300 MHz, CDCl3, δ ppm): 8.06 (d, 2H, Cz), 7.43

(m, 4H, Cz), 7.23 (m, 2H, Cz), 6.08 (s, 1H, trans-vinylene), 5.52 (s, 1H, cis-vinylene),4.46

(t, 2H, O-CH2-CH2-O), 4.16 (t, 2H, O-CH2-CH2-N), 3.81 (t, 2H, N-CH2-CH2-), 3.54 (t,

2H, CH2), 1.91 (s, 3H, CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm): 18.3, 43.1, 63.8, 69.1,

69.3, 70.59, 71.04, 108.8, 119.0, 120.2, 122.9, 125.6, 125.7, 136.1, 140.6, 167.3; EI-MS:

367.20 (M+); Elemental analysis: Calculated. C, 71.91; H, 6.86; N, 3.81; Found: C, 71.92;

H, 6.88; N, 3.82.

7

NCH2

CH2

OO

3

9-fluorenyl methacrylate (9) (Scheme 2.1, pp. 64)

Yellow oil (Yield = 72 %). 1H NMR (300 MHz, CDCl3, δ ppm): 7.67 (d, 2H, Fl), 7.56 (m,

4H, Fl), 7.37 (m, 2H, Fl), 6.86 (s, 1H, Fl), 6.16 (s, 1H, trans-vinylene), 5.62 (s, 1H, cis-

vinylene), 2.01 (s, 3H, -CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm): 18.4, 75.4, 120.0,

125.9, 126.8, 127.8, 129.4, 137.8, 167.2; EI-MS: 250.1 (M+); Elemental analysis:

Calculated. C, 81.58; H, 5.64; O, 12.78; Found: C, 81.56; H, 5.68.

178


OO

9

Thienylethyl methacrylate (11) (Scheme 2.1, pp. 64)

White solid (Yield = 60 %). 1H NMR (300 MHz, CDCl3, δ ppm): 1.97 (s, 3H, CH3), 3.01

(t, 2H, C-CH2-), 4.37 (t, 2H, O-CH2-CH2-), 5.59 (S, 1H, cis-vinylene), 6.13 (s, 1H, trans-

vinylene), 7.01 (m, 1H, Th), 7.29 (m, 2H, Th); 13C NMR (75.4 MHz, CDCl3, δ ppm):

18.2, 29.5, 64.4, 121.4, 125.5, 128.2, 136.2, 138.0, 167.2; EI-MS: 196.12 (M+); Elemental

analysis: Calculated. C, 61.20; H, 6.16; O, 16.30; Found: C, 61.22; H, 6.17.

S

OO

11

7.2.2 General procedure for free radical copolymerization (P1-P4)

The radical copolymerization of methacrylic monomers with pendant electroactive

groups was carried out under N2 atmosphere. A solution of monomers in 20 mL of dry

toluene was purged with N2 for 30 min. The free radical initiator AIBN (5 mol% of total

monomer concentration) was added and stirred under N2 for another 30 min. The

polymerization was started by heating the mixture at 60 °C for 2 days (Scheme 2.2, pp.

65). The light yellow color reaction mixture was cooled to room temperature and

precipitated in 300 mL methanol with vigorous stirring. The resulting solid was collected

179


by filtration and reprecipitated from hexane for three times and dried under vacuum at 100

°C.

P1: Yellowish amorphous solid, yield 60 %, 1H NMR (300 MHz, CDCl3, δ ppm): 7.9 (b,

2H, Cz), 7.25 (m, 4H, Cz), 7.07 (b, 2H, Cz), 4.67 (b, 2H, O-CH2), 4.2 (b, 2H, O-CH2-CH2-

O), 3.55 (b, 2H, N-CH2), 1.52 (b, 3H, CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm): 43.0,

69.2, 70.2, 70.7, 70.9, 108.8, 119.2, 120.2, 125.5, 125.7, 140.1, 140.4. FT-IR (KBr, cm-1):

3458, 3052, 2922, 2858, 1728, 1631, 1487, 1328, 1253, 1149, 751, 721, 613, 557.

Elemental Analysis for (C39H42N2O6) n: Calculated. C, 73.77; H, 6.19; N, 4.53. Found: C,

73.24; H, 6.19; N, 5.58.

P2: Grey amorphous solid, yield 65 %. 1H NMR (300 MHz, CDCl3, δ ppm): 7.57(b, 4H,

Fl), 7.48-6.90 (m, 8H, Fl,Th), 6.46 (s, 1H, Fl), 4.32 (m, 2H, O-CH2), 2.99 (m, 2H, O-CH2-

CH2-), 2.10 (b, 6H, CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm): 22.6, 25.5, 29.0, 44.7,

67.9, 89.4, 119.8, 121.6, 125.7, 126.7, 127.6, 128.1, 129.5, 137.7, 137. 9, 141.0, 181.2,

190.3, FT-IR (KBr, cm-1): 3443, 3103, 3067, 2948, 1726, 1450, 1586, 1147, 974, 758,

744. Elemental Analysis for (C27H26SO4) n, Calculated. C, 72.62; H, 5.87; S, 7.28; O,

14.33. Found: C, 72.43; H, 6.20; S, 7.35.

P3: White amorphous solid, yield 72 %. 1H NMR (300 MHz, CDCl3, δ ppm): 7.87 (b, 2H,

Cz), 7.52 (b, 2H, Fl), 6.90-7.15 (m, 8H, Fl, Cz), 6.48 (s, 1H, Fl), 4.35 (m, 2H, O-CH2),

3.72 (m, 2H, -N-CH2), 1.57 (s, 6H, CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm): 14.8,

45.4, 108.5, 119.1, 119.8, 120.3, 122.8, 125.6, 126.8, 127.6, 129.4, 140, 140, 180.8. FT-IR

(KBr, cm-1): 3441, 3061, 2986, 2929, 1727, 1602, 1557, 1455, 1147, 745. Elemental

Analysis for (C35H31NO4) n, Calculated. C, 79.33; H, 5.90; N, 2.67. Found: C, 78.92; H,

5.87; N, 2.20.

180


P4: White amorphous solid, yield 67 %. 1H NMR (300 MHz, CDCl3, δ ppm): 7.98 (b, 2H,

Cz), 7.09-7.43 (m,10H, Fl & Cz), 6.36 (s, 1H, Fl), 4.10 (b, 2H, -O-CH2), 3.79 (b, 2H, -N-

CH2), 2.05 (b, 6H, CH3), 0.86-1.89 (m, 16H); 13C NMR (75.4 MHz, CDCl3, δ ppm): 27.2,

28.8, 29.3, 42.9, 108.5, 118.5, 120.2, 122.7, 125.4, 140.3, 141. FT-IR (KBr, cm-1): 3445,

3061, 2927, 2852, 1726, 1624, 1455, 1328, 1149, 973, 746. Elemental Analysis, for

(C44H53NO4) n, Calculated. C, 79.37; H, 5.90; N, 2.64; Found: C, 79.02; H, 7.12; N, 1.66.

7.3. Chapter 3

The experimental methods for the synthesis of intermediates and block copolymers are

described as Scheme 3.1-2 in Chapter 3.

7.3.1. Synthesis of Monomer 15

3-(oxymethyl-2, 2-dimethyl-1, 3-dioxolane)] benzylmethacrylate (15)1 (Scheme 3.1, pp.

90)

The procedure for the synthesis of 15 is similar to that of monomer synthesis described in

Chapter 2 (3, 5, 7, 9, and 11). The compound 15 was obtained as colorless oil with yield

of 75 %. 1H NMR (300 MHz, CDCl3, δ ppm): 1.40 (s, 3H, -CH3), 1.46 (s, 3H, -CH3), 1.81

(s, 3H, -CH3), 3.86 (m, 4H, -CH2), 4.14 (m, 1H, -CH), 5.15 (s, 2H, -CH2O), 5.58 (s, 1H,

=CH), 6.15 (s, 1H, =CH), 6.85 (m, 3H, Ar-H), 7.24 (m, 1H, Ar-H). 13C NMR (75.4 MHz,

CDCl3, δ ppm): 167.0 (C=O), 158.6, 137.6 (-C=CH2), 136.0, 129.5, 125.7 (=CH2), 120.5,

114.0, 109.6 (Ar-C), 103.4 (-C-), 73.8 (-CH-), 68.6 (-CH2-O), 66.6 (-CH2), 66.0 (-CH2),

26.6 (CH3), 25.2 (CH3), 17.8 (CH3). FT-IR (KBr, cm-1): 2986, 1726, 1678, 1635, 1587,

1491, 1449, 1372, 1040, 943, 844, 783, 695. MS-ESI (M+): 306 m/z. Elemental analysis

for C17H22O7: Calculated. C, 66.65; H, 7.24; Found: C, 66.84 H, 7.03.

181


O

OO

OO

15

7.3.2. Homopolymerization of 15 {PBMMA-Br (18)} (Scheme 3.2, pp. 91)

3-(oxymethyl-2, 2-dimethyl-1, 3-dioxolane)] benzylmethacrylate (15) (0.96 g, 3.1

mmol) in toluene (10 mL) was transferred to a Schlenk flask and purged with nitrogen.

CuBr (0.004 g, 0.027 mmol), N, N, N , N , N - pentamethyldiethylenetriamine (PMDETA)

(0.1 mL, 0.005 mmol), were added to the flask. The flask was degassed and refilled with

nitrogen by three freeze/pump/thaw cycles. Ethyl 2–bromoisobutyrate (0.05 mmol) was

added to the flask before polymerization at 75 °C. At different intervals, the samples were

taken out by a nitrogen purged syringe and diluted with THF. These samples were injected

to GPC to check the molecular weights. Monomer, initiator and catalyst ratio was

maintained as 100:1:1. The polymers were precipitated from hexane, filtered and dried. 1H

NMR (300 MHz, CDCl3, δ ppm): 1.32 (s, 3H, -CH3), 1.42 (s, 3H, -CH3), 1.82 (3H, -CH3),

3.83 (m, 4H, -CH2), 4.15 (broad, 1H, -CH), 4.47 (broad, 2H, -CH2O), 6.81 (broad 1H, Ar-

H), 6.97 (broad, 2H, Ar-H), 7.12 (broad, 1H, Ar-H). 13C NMR (75.4 MHz, CDCl3, δ ppm):

158.5, 142.7, 129.3, 119.3, 113.4, 112.6 (Ar-C), 109.6 (-C-), 73.8 (-CH-), 68.4 (-CH2OH),

66.4 (-CH2-), 64.2 (-CH2-), 26.5 (-CH3), 25.1 (-CH3). FT-IR (KBr, cm-1): 2934, 2881,

1728, 1587, 1491, 1451, 1371, 1269, 1157, 1049, 930, 851, 783, 692. GPC; Mn, 8033.

182


7.3.3. General procedure for atom transfer radical polymerization (ATRP) for

block copolymer synthesis (Scheme 3.2, pp. 91)

Macroinitiator (PBMMA-Br) (18) (0.96 g, 0.2 mmol) in anisole (10 mL) was

transferred to a Schlenk flask and purged with nitrogen. CuCl (0.04 g, 0.40 mmol) and

PMDETA (0.1 mL, 0.005 mmol) were added to the flask. The flask was degassed and

refilled with nitrogen by three freeze-pump-thaw cycles. Nitrogen purged monomer (17)

(1.28 g, 40 mmol) was added to the catalyst and polymerization was carried out at 75 ºC.

Similar reaction was repeated for compound (9) (1.38 g, 40 mmol). Monomer to

homopolymer ratio was 200 : 1. Monomer, initiator and catalyst ratio was maintained at

100 : 1 : 1. The polymers 19 and 20 were precipitated from hexane, filtered and dried.

Protected Block Copolymer 19: 1H NMR (300 MHz, CDCl3, δ ppm): 0.87 ( t, 3H, -CH3),

1.25 (broad, 2H, -CH2), 1.43 (s, 1H, -CH-), 1.70 (b, 2H, -CH2-CH2-), 1.85 (b, 2H, -CH2),

3.72 (s, 6H, -CH3), 3.76 (broad, 2H, N-CH2), 4.02 (broad, 2H, -CH2-O-), 4.21 (b, 4H, O-

CH2), 6.97 (broad, 1H, Ar-H), 7.10 (b, 1H, Ar-H), 7.18 (broad, 1H, Ar-H), 7.41 (b, 4H,

Cz-H), 8.07 (b, 2H, Cz-H); 13C NMR (75.4 MHz, CDCl3, δ ppm): 206.2 (-C=O), 158.5,

142.7, 140.3, 129.3, 125.6, 125.2, 122.8, 119.3, 118.7, 113.4, 112.6 (Ar-C), 109.6 (-C-),

108.5, 73.7 (-CH-), 69.4 (-CH2-O-), 67.4 (-O-CH2-), 67.2 (-CH2-), 64.5, 60.3 (-N-CH2),

26.5 (-CH3), 42.8, 30.6, 26.9, 25.1 (-CH3), 25.5, 24.7, 18.1. Elemental analysis:

Calculated; C, 73.14; H, 7.67; N, 2.13; Found; C, 73.03; H, 7.71; N, 2.04.

Protected Block Copolymer 20: 1H NMR (300 MHz, CDCl3, δ ppm): 0.88 (t, 3H, -CH3),

1.25 (broad, 1H, -CH-), 2.38 (broad, 6H,-CH3), 5.0 (broad, 2H, -CH2), 5.12 (broad, 2H, -

CH2-O), 6.89 - 7.22 (broad, 5H, Ar-H), 7.46 (broad, 4H, Fl-H), 7.80 (broad, 2H, Fl-H).

13C NMR (75.4 MHz, CDCl3, δ ppm): 206.8 (-C=O), 158.5, 142.7, 141.8, 129.3, 129.4,

183


127.8, 126.8, 125.9, 120.0, 119.3, 113.4, 112.6 (Ar-C), 109.6 (-C-), 75.4 (-CH-), 73.8 (-

CH-), 68.4 (-CH2OH), 66.4 (-CH2-), 64.2 (-CH2-), 26.5 (-CH3), 25.1 (-CH3), 18.4.

Elemental analysis: Calculated; C, 73.53; H, 6.88; Found; C, 73.48; H, 6.91.

Deprotection of block copolymers:

The precursor block copolymers (19/20) were dissolved in 10 mL THF. 2.5 ml of 2M

HCl solution was added and the reaction mixture stirred at 70 °C for 2 h. The mixture was

then cooled down, concentrated and precipitated from excess amount of hexane. The solid

was then filtered and dried. The corresponding block copolymer of PCzMMA-b-

PBMMA and PFlMMA-b-PBMMA was afforded.

PCzMMA-b-PBMMA: 0.1 g (yield = 90 %). 1H NMR (300 MHz, CDCl3, δ ppm): 0.87

(broad, 2H, -CH2), 1.16 (s, 1H, -CH-), 1.70 (b, 2H, -CH2-CH2-), 1.80 (b, 2H, -CH2), 2.16

(s, 3H, -CH3), 3.84 (broad, 2H, N-CH2), 4.03 (broad, 2H, -CH2-O-), 4.23 (b, 4H, O-CH2),

6.85 (broad, 1H, Ar-H), 7.04 (b, 1H, Ar-H), 7.18 (broad, 1H, Ar-H), 7.36 (b, 2H, Cz-H),

7.42 (b, 2H, Cz-H), 8.07 (b, 2H, Cz-H); 13C NMR (75.4 MHz, CDCl3, δ ppm): 206.9 (-

C=O), 158.5, 142.7, 140.3, 129.3, 125.6, 125.2, 122.8, 120.3, 118.7, 113.4, 112.6 (Ar-C),

108.2, (-C-), 64.8 (-CH2-O-), 64.7 (-O-CH2-), 64.5 (-CH2-CH2-) 60.3 (-N-CH2), 44.8, 44.2,

31.9, 28.6, 26.8, 25.9, (-CH2), 14.1, 14.0 (-CH3). Elemental analysis: Calculated; C, 72.24;

H, 7.82; N, 2.22; Found; C, 72.03; H, 7.71; N, 2.14

PFlMMA-b-PBMMA: 0.1 g (yield = 92 %). 1H NMR (300 MHz, CDCl3, δ ppm): 0.88 (b,

1H, -CH-), 1.43-1.67 (b, 2H, -CH2-), 2.04 (s, 3H,-CH3), 5.05 (b, 2H, -O-CH2-), 5.12

(broad, 2H, -CH2-O), 6.54 (b, 1H, Fl-H9), 6.95- 7.16 (broad 5H, Ar-H), 7.46 (broad, 4H,

Fl-H), 7.72 (broad, 2H, Fl-H); 13C NMR (75.4 MHz, CDCl3, δ ppm): 206.9 (-C=O), 158.5,

142.7, 141.8, 129.3, 129.4, 127.8, 126.8, 125.9, 120.0, 119.3, 113.4, 112.6 (Ar-C), 109.6

184


(-C-), 75.4 (-CHFl), 66.4 (-CH2OH), 64.8 (-CH2-), 64.2 (-CH2-), 26.5 (-CH-), 18.4 (-CH3).

Elemental analysis: Calculated; C, 72.51; H, 7.01; Found; C, 72.08; H, 6.91

7.4. Chapter 4

The experimental procedures for the synthesis of intermediates and target molecules

are described as Scheme 4.1-2 in Chapter 4.

7.4.1 Synthesis of Intermediates2-4

3, 5-dibromo-2-methylthiophene (21)2 (Scheme 4.1, pp. 116)

To a stirred solution containing 2-methylthiophene (10 mL, 110 mmol) in 250 mL of

dioxane, was slowly added bromine (11.15 mL, 220 mmol). The mixture was stirred at

room temperature for 2 h and refluxed at 100 °C for another 2 h to complete the evolution

of HBr. The reaction mixture was cooled and was quenched with 100 mL of water. The

layers were separated and the organic layer was washed with 0.5 M NaOH and water. The

combined aqueous layer was extracted with diethyl ether. The combined organic layer was

dried over Na2SO4 and the solvent was evaporated under reduced pressure. The crude

product was purified by vacuum distillation to give di-brominated thiophene 21 as a pale

yellow solution (13.29 g, 47 %); bp 55 – 60 °C / 0.45 mmHg (lit. bp 48 – 55 °C / 0.35 –

0.45 mmHg); 1H NMR (300 MHz, CDCl3, δ ppm) 2.33 (s, 3H, Ar-CH3), 6.85 (s, 1H, Ar-

H). 13C NMR (75.4 MHz, CDCl3, δ ppm) 14.8, 108.5, 108.7, 131.9, 136.0; MS (M+): 256

m/z.

S BrMe

Br 21 185


3-bromo-2-methyl-5-trimethylsilylethy-nyl-thiophene (22)3 (Scheme 4.1, pp. 116)

A degassed solution of 21 (5.14 g, 20.1 mmol) in 100 mL of Et3N was added

PdCl2(PPh3)2 (0.576 g, 0.82 mmol) and CuI (0.311 g, 1.63 mmol). After 30 min,

trimethylsilylacetylene (TMSA) (2.23 mL, 16.12 mmol) was added slowly. The resulting

mixture was stirred at 45 °C for 7 h. The reaction was cooled to room temperature and

quenched with saturated solution of NH4Cl. The organic layer was diluted with CH2Cl2

and washed with a saturated solution of NH4Cl. The combined aqueous layer was

extracted with CH2Cl2. The combined organic layer was dried over Na2SO4 and the

solvent was evaporated under reduced pressure. The crude product was purified over flash

column chromatography (hexane; Rf = 0.74) to yield acetylene 22 as a yellow solid (4.38

g, yield 80 %). 1H NMR (300 MHz, CDCl3, δ ppm) 0.23 (s, 9H, Si (CH3)3), 2.36 (s, 3H,

Ar-CH3), 7.02 (s, 1H, Ar-H). 13C NMR (75.4 MHz, CDCl3, δ ppm) -0.23, 14.8, 96.5, 99.6,

108.7, 120.7, 134.8, 136.4; Elemental analysis for C10H13BrSSi: Calculated; C, 43.95; H,

4.80; Found; C, 43.57; H, 4.82; MS (M+): 273 m/z.

22

SMe

Br

TMS

1,2-bis(5-(4-trimethylsilylethynyl)-2-methylthiophen-3-yl)perfluorocyclopentene (23)3

(Scheme 4.1, pp. 116)

A solution of acetylene 22 (0.2 g, 7.32 mmol) in 30 mL of THF was cooled to -78 °C

under a nitrogen atmosphere. n-BuLi (1.6 M in hexane, 4.6 mL, 7.32 mmol) was added

dropwise. After 2 h, perfluorocyclopentene (0.51 mL, 3.66 mmol) was added slowly. The

reaction mixture was stirred at -78 °C for 2 h. The reaction was allowed to warm to room

186


temperature. After an additional 2 h, the reaction was diluted with diethyl ether and

washed with dilute HCl (1 %) and water. The combined aqueous layer was extracted with

diethyl ether. The combined organic layer was dried over Na2SO4 and the solvent was

evaporated under reduced pressure. The crude product was purified over flash column

chromatography (hexane; Rf = 0.55) to yield the diarylethene 23 as a yellow-green semi-

solid (2.35 g, 57 %). 1H NMR (300 MHz, CDCl3, δ ppm) 0.25 (s, 18H, Si(CH3)3), 1.88 (s,

6H, ArCH3), 7.19 (s, 2H, ArH). 13C NMR (75.4 MHz, CDCl3, δ ppm) -0.28, 14.4, 96.2,

100.1, 119.3, 120.6, 121.8, 124.6, 132.0, 136.1, 143.3; Elemental analysis for

C25H26F6S2Si2: Calculated; C, 53.55; H, 4.67; Found; C, 53.48; H, 4.47; MS: m/z (M+)

560.

23S

TMS

S

TMS

F FF

F

F

F

1, 2-bis (5-(4-ethynyl)-2-methylthiophen-3-yl) perfluorocyclopentene (24)3 (Scheme

4.1, pp. 116)

To a solution of diarylethene 23 (0.54 g, 0.96 mmol) in 15 mL of THF under nitrogen

atmosphere was added, tetra-n-butylammonium fluoride (TBAF) (1.39 mL, 4.81 mmol).

The reaction was stirred at room temperature and allowed to go to completion

(approximately 2 h). The reaction was then diluted with CH2Cl2 and washed with water.

The combined aqueous layer was extracted with CH2Cl2. The combined organic layer was

dried over Na2SO4 and the solvent was evaporated under reduced pressure. The crude

187


product was purified over flash column chromatography (hexane; Rf = 0.41) to yield

terminal alkyne 24 as a yellow-green semi-solid (0.3 g, 75 %). 1H NMR (300 MHz, CDCl3,

δ ppm) 1.90 (s, 6H, ArCH3), 3.35 (s, 2H, CH), 7.22 (s, 2H, Ar-H). 13C NMR (75.4 MHz,

CDCl3, δ ppm) -0.28, 14.4, 96.2, 100.1, 119.3, 120.6, 121.8, 124.6, 132.0, 136.1, 143.3;

MS: m/z (M+) 416.

4S

H

S

H

F FF

F

F

F

24

7.4.2 General procedure for diazo-compound synthesis4

4-bromoaniline was dissolved in a solution of conc. HCl and water, cooled to 0 °C and

stirred for 5 h until completion of diazonium salt (orange solution obtained). This solution

was added dropwise at 0 °C to a solution of phenol/naphthol dissolved in ethanol and 20%

aq. NaOH. Care was taken to ensure the temperature did not exceed 5 °C during the

addition. The mixture was stirred at 8 °C for another 2 h. A 2M HCl solution was then

added to the reaction mixture to precipitate out the diazo-dye. The mixture was then

washed with water and dried under vacuum (hexane/DCM= 2: 1; Rf = 0.81)

4-bromo-2’-hydroxy-5’-dodecyloxyazobenzene (26) (Scheme 4.1, pp. 116): 0.12 g

(yield = 85 %), 1H NMR (300 MHz, CDCl3, δ ppm) 0.88 (t, 3H, CH3), 1.27 (m, 12H, alkyl

H), 1.83 (m, 2H, CH2), 4.00 (t, 2H, O-CH2), 6.96 (m, 2H, Ar-H), 7.41 (d, 1H, Ar-H), 7.67-

7.73 (dd, 4H, Ar-H) 13C NMR (75.4 MHz CDCl3, δ ppm): 14.1, 2.68, 26.0, 29.2, 29.3,

29.4, 29.5, 29.6, 29.7, 31.9, 68.9, 115.4, 118.8, 122.7, 123.5, 132.6; Elemental analysis for

188


C24H33BrN2O2: Calculated; C, 62.47; H, 7.21; N, 6.07; Br, 17.32; Found; C, 62.28; H,

6.97; N, 6.02; MS: m/z = 460.17

N

N

HO

OC12H25

Br

26

4-bromo-1’-phenylazo-2’hydroxynaphthalene (27) (Scheme 4.2, pp. 117): 0.15 g (yield

= 88 %). 1H NMR (300 MHz, CDCl3, δ ppm) 6.86 (d, 1H, aryl H), 7.61 (m, 6H, naphthol

H), 7.72 (d, 1H, Ar H), 8.53 (d, 1H, Ar H), 16.04 (s, OH) 13C NMR (75.4 MHz, CDCl3, δ

ppm): 120.0, 121.7, 124.4, 125.93, 128.6, 128.9, 132.6, 140.2; Elemental analysis for

C16H11BrN2O: Calculated; C, 58.74; H, 3.39; N, 8.56; Br, 24.42; Found; C, 58.58; H, 3.17;

N, 8.42; MS: m/z = 327.1

OH

NN Br

27

7.4.3 General Procedure for the synthesis of azo-dithienylethene (DTE) molecules

To a degassed solution of bromo azo-compounds (26/27) (0.57 g, 1.23 mmol) in a

mixture of Et3N/THF (10 mL: 10 mL), Pd (PPh3)4 (0.244 g, 0.587 mmol) and CuI (0.022

mg, 0.117 mmol) were added. After 30 min, a solution of 24 (0.244 g, 0.687 mmol)

dissolved in minimal amount of THF was added slowly. The resulting mixture was stirred

at 50 °C for 20 h. The reaction was cooled to room temperature and quenched with

saturated solution of NH4Cl. The organic layer was diluted with CH2Cl2 and washed with

a saturated solution of NH4Cl. The combined aqueous layers were extracted with CH2Cl2.

189


The combined organic layers were dried over Na2SO4 and the solvent was evaporated

under reduced pressure. The crude product was purified using flash column

chromatography using hexane: DCM as eluent to obtain pure products.

DTE-Ph (Scheme 4.1, pp. 116): A light red solid of 0.3 g (60 % yield)

(Hexane/DCM=2:1; Rf = 0.68). FT-IR (ν cm-1): 2292, 2852, 2200, 1594, 1494, 1467, 1341,

1270, 1196, 1136, 1041, 985, 899, 844, 738, 533. 1H NMR (300 MHz, CDCl3, δ ppm)

0.87 (t, 4H, - CH3), 1.27 (m, 8H, -CH2-), 1.80 (2H, -CH2), 1.98 (s, 6H thiophene CH3),

4.00 (t, 4H, -O-CH2), 6.95 (m, 2H, Ar H), 7.31 (s, 1H, Ar H), 7.64 (d, 4H, Ar H), 7.86 (d,

4H, Ar H), 12.40 (s, 2H, -OH); 13C NMR (75.4 MHz, CDCl3, δ ppm) 14.1, 14.6, 22.7,

26.0, 29.3, 29.3, 29.4, 29.5, 29.6, 29.6, 29.6, 31.9, 68.9, 84.3, 93.7, 115.4, 118.8, 121.5,

122.9, 125.0, 125.0, 131.8, 132.4, 137.2, 44.0, 147.3, 150.1, 152.5; HR-MS: m/z (M+)

1176.5 Elemental analysis calculated (%) for C67H74F6N4O4S2: C:66.50; H:6.26; F:10.74;

N:3.79; S: 7.19; Found: C, 65.96; H, 6.12; N, 3.67; F, 11.02

DTE-Naph (Scheme 4.2, pp. 117): A red solid of 0.15 g (46 % yield) (Hexane/EA = 2:1;

Rf = 0.32). FT-IR (ν cm-1): 3452.5, 3051.0, 2921.6, 2850.8, 1725.7, 1621.1, 1552.5,

1502.6, 1434.5, 1274.6, 1207.7, 1136.6, 984.8, 899.9, 832.1, 742.6, 513.9; 1H NMR (300

MHz, CDCl3, δ ppm) 1.97 (s, 3H, CH3), 6.81 (d, 2H, ArH), 7.29 (s, 2H, ArH), 7.33 (d, 2H,

ArH), 7.42 (s, 2H, Ar H), 7.55 (s, 2H, ArH), 7.61 (d, 2H, ArH), 7.7 (s, 2H, ArH), 8.51 (s,

2H, ArH), 16.26 (s, 2H, -OH); 13C NMR (75.4 MHz, CDCl3, δ ppm): 14.4, 82.4, 92.7,

96.2, 100.1, 119.3, 120.02, 120.5, 121.6, 121.8, 124.4, 124.6, 125.8, 128.6, 128.7, 132.1,

132.6, 136.0, 140.2 143.2, MS m/z (M+) 908. Elemental analysis calculated (%) for

C51H30F6N4O2S2: C: 61.85 H: 4.22, F: 5.29, N: 2.61, S: 3.76; Found: C, 61.58; H, 4.21; F,

5.23, N, 2.48.

190


7.5 Chapter 5

The experimental procedures for the synthesis of target compounds and intermediates

are described as Scheme 5.1-5.3 in Chapter 5.

7.5.1 Synthesis of intermediates (Scheme 5.1, pp. 138)

7.5.2 General procedure for O-alkylation6

In a dried 100 mL round bottom flask potassium carbonate (11.1 g, 80.0 mmol) was

dissolved in N, N-dimehylformamide (DMF) (70 mL) and 4-(4-bromophenyl) phenol (5.0

g, 20.07 mmol) was added to the mixture followed by potassium iodide (1.7 g, 17.7

mmol). The reaction mixture was stirred for 1 h and alkylhalide (2-ethylhexylbromide or

iodomethane) (26.4 mmol) was added drop wise and heated to reflux for over night. The

reaction mixture was cooled to room temperature and poured in to 500 mL water. The

product was extracted in dichloromethane (3 × 100 mL). The organic layer was washed

with water (2 × 100 mL) and dried over anh.Na2SO4. The solvent was evaporated in

reduced pressure and the crude product was purified by column chromatography in silica

gel using hexane: dichloromethane (8: 2) as eluent to afford the O-alkylated compounds

32a-b with isolated yield of 75 %.

4-(4-bromophenyl)-O-2-ethylhexylbenzene (32a): 1H NMR (300 MHz, CDCl3, δ ppm):

7.54-7.39(m, 4H), 6.95 (m, 4H), 3.87 (d, 2H, OCH2), 1.75 (h, 2H, CH2CH2CH3), 1.53-

1.38 (m, 6H),1.35 (m, 1H), 0.91 (t, 6H, CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm):

159.1, 139.7, 132.0, 131.6, 128.1, 127.8, 120.6, 114.8, 70.5, 39.3, 30.4, 29.0, 23.8, 22.9,

14.0, 11.0; EI-MS: 360.1 (m/z); Elemental analysis calculated (%) for C20H25BrO: C,

66.48; H, 6.97; Br, 22.11; O, 4.43; Found: C, 66.54; H, 6.92.

191


Br OR

R= 2-ethylhexyl32a

4-(4-bromophenyl)-O-methylbenzene: 1H NMR (300 MHz, CDCl3, δ ppm) 7.47 (dd,

4H), 7.40 (d, 2H), 6.96 (d, 2H), 3.85 (s, 3H). 13C NMR (75.4 MHz, CDCl3, δ ppm) 159.4,

139.7, 132.5, 131.7, 128.3, 128.0, 120.8, 114.3, 55.4; EI-MS: 261.9 (m/z); Elemental

analysis calculated (%) for C13H11BrO: C, 59.34; H, 4.21; Br, 30.37; O, 6.08; Found: C,

59.28; H, 4.32.

Br OR

32b R= Me

7.5.3 General Procedure of Boronicacid Syntheses7 (Scheme 5.1, pp. 138)

To a 100 mL three-necked flask containing a solution of brominated compounds (5.50

mmol) in 50 mL of dry THF equipped with a magnetic stirrer, a N2 purge and -78 °C

acetone-dry ice bath were dropwise added 1.6 M of n-Butyl lithium (7.3 mL, 11.62 mmol)

while maintaining a good stirring. After stirring for 1.5 h, tri-isopropylborate (5.4 mL,

23.24 mmol) was added dropwise and keep the stirring from -78 °C to room temperature

for 22 h. The reaction mixture was cooled to 0 °C and 2M HCl was added carefully till

acidic (checked by litmus paper) and stirred for 2 h. The solution was extracted in excess

of diethyl ether and the organic layer was washed in brine solution and dried over Na2SO4.

The evaporation on organic solvent in reduced pressure gave a crude gray solid which was

purified by recrystalizing from hexane.

4’-(O-2-ethylhexyl)-4-biphenyl boronicacid (33a): The white powered of boronicacid

was obtained with yield of 0.8 g (40 %). 1H NMR (300 MHz, DMSO-d6, δ ppm): 8.02 (s,

192


2H, B-OH), 7.82 (d, 2H), 7.56 (dd, 4H), 6.99 (d, 2H), 3.88 (d, 2H, -O-CH2-), 1.65 ( h, 2H),

1.46-1.39 (m, 8H), 1.31(m, 1H), 0.87 (t, 6H); 13C NMR (75.4 MHz, DMSO-d6, δ ppm):

159.0, 141.6, 135.1, 132.6, 128.1, 125.4, 115.2, 70.3, 30.2, 28.8, 23.6, 22.8, 14.3, 11.2;

Elemental analysis for C20H27BO3; Calculated: C, 77.63; H, 8.34; B, 3.31; O, 14.71;

Found: C, 77.58; H, 8.23.

(HO)2B OR

R= 2-ethylhexyl33a

4’-(O-methyl)-4-biphenyl boronicacid (33b): The white powered of boronicacid was

obtained with isolated yield of 1.0 g (43 %). 1H NMR (300 MHz, DMSO-d6, δ ppm): 8.0

(s, 2H, B-OH), 7.83 (d, 2H), 7.57 (dd, 4H), 6.99 (d, 2H), 3.8 (s, 3H, OCH3); 13C NMR

(75.4 MHz, DMSO-d6, δ ppm): 159.5, 141.7, 135.2, 132.9, 128.2, 125.5, 114.8, 55.6

Elemental analysis for C13H13BO3; Calculated: C, 68.47; H, 5.75; B, 4.74; O, 21.0; Found:

C, 67.98; H, 5.23.

(HO)2B OR

33b R= Me

4-(N,N-diphenylamino)-1-phenyl boronicacid (36): The white powered 4.2 g (Yield of

65 %). 1H NMR (300 MHz, DMSO-d6, δ ppm): 7.82 (s, 2H, B-OH), 7.58 (d, 2H), 7.21 (t,

4H), 7.0-7.12 (m, 6H), 6.88 (d, 2H); 13C NMR (75.4 MHz, DMSO-d6, δ ppm): 149.0,

145.3, 134.8, 127.4, 124.3, 121.2, 120.8; Elemental analysis for C18H16BNO2; Calculated:

C, 74.77; H, 5.58; B, 3.74; N, 4.84; Found: C, 74.72; H, 5.60; N, 4.82.

193


36

N(HO)2B

7.5.4 General procedure for Suzuki coupling reaction8 (Scheme 5.1-2, pp. 138-9)

To a vertical three-neck RB flask equipped with a condenser, the dibromocompound

(3.07 mmol), boronicacid (6.92 mmol), anhydrous THF (40 mL) and 2N aqueous

potassium carbonate solution (20 mL) were added. The flask was degassed three times

before the catalyst, tetrakispalladiumtriphenylphosphine (5 mol %) was added in the

absence of light under N2 atmosphere. The whole set up was covered with alluminium foil

and was heated to 80 °C for 24 h. The reaction mixture was than cooled to room

temperature and poured into water and extracted in dichloromethane (3 × 50 mL). The

combined organic later was dried over anhydrous Na2SO4. The removal of solvent gives

crude product which was purified by column chromatography in silica gel using hexane :

DCM (7: 3) as eluent.

3, 6-di [4’-O-2-ethylhexyl biphenyl] carbazole (34a): 1.23 g, and isolated yield of 52 %.

1H NMR (300 MHz, CDCl3, δ ppm): 8.40 (s, 1H, NH), 8.08 (8, 2H, Cz), 7.80-7.59 (m,

16H), 7.47 (d, 2H), 7.0 (d, 2H), 3.91 (d, 2H, OCH2), 1.72 (h, 2H, -CH2CH2CH3), 1.58-

1.42 (m, 8H), 1.38 (m, 1H), 0.94 (t, 6H, CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm):

159.9, 140.1, 139.3, 139.0, 133.0, 132.6, 127.8, 127.4,126.9, 125.4, 124.0, 118.6, 114.8,

110.9; EI-MS: 727.4 (m/z), Elemental analysis for C52H57NO2; Calculated: C, 85.79; H,

7.89; N, 1.92; Found: C, 85.62; H, 7.82; N, 1.83.

194


HN

ORR1=R2 =34a

R1 R2

R = 2-ethylhexyl

3-Bromo, 6- [4’-O-methylbiphenyl] carbazole (34b): 1.14 g, and the isolated yield of

43 %. 1H NMR (300 MHz, CDCl3, δ ppm): 11.5 (s, 1H, NH), 8.6 (s, 1H, Cz4), 8.48 (s, 1H,

Cz), 7.70-7.85 (m, 8H), 7.34 (s, 2H), 7.0 (d, 2H), 3.81 (s,3H, O-CH3), 13C NMR (75.4

MHz, CDCl3, δppm): 159.9, 140.1, 139.3, 139.0, 133.0, 132.6, 127.8, 127.4,126.9, 125.4,

124.0, 118.6, 114.8, 110.9, 55.4; EI-MS: 429.0 (m/z), Elemental analysis for C52H57NO2;

Calculated: C, 70.10; H, 4.24; N, 3.27, Br, 18.66; Found: C, 70.12; H, 4.21; N, 3.25.

HN

OMe34b R1 =

R1 R2

R2 = BrR = 2-ethylhexyl

3- [4’-O-2-ethylhexyl biphenyl] 6- [4’-O-methylbiphenyl] carbazole (35): 1.24 g, and

isolated yield of 76 %. 1H NMR (300 MHz, CDCl3, δ ppm): 8.4 (s, 2H, Cz), 8.14 (s, 1H,

NH), 7.73 (dd, 4H), 7.68 (d, 4H), 7.53 (d, 4H), 7.51 (d, 2H) 7.0 (dd, 2H), 3.91 (d, 2H, O-

CH2), 3.81 (s,3H, O-CH3), 1.73 ( h, 1H), 1.25-1.47 (m, 8H), 0.92 (dt, 3H, CH3); 13C NMR

(75.4 MHz, CDCl3, δ ppm): 159.9, 140.1, 139.3, 139.0, 133.0, 132.6, 127.8, 127.4,126.9,

125.4, 124.0, 118.6, 114.8, 110.9, 70.3, 55.4, 30.2, 28.8, 23.6, 22.8, 14.3, 11.2; EI-MS:

629.3 (m/z), Elemental analysis for C45H43NO2; Calculated: C, 85.81; H, 6.88; N, 2.22. O,

5.08; Found: C, 85.78; H, 6.93; N, 2.17.

195


HN

OR

OMe

35

R1 =

R1 R2

R2 =

R = 2-ethylhexyl

3,6-bis[4’-(N,N-diphenylamino)-1’-phenyl]carbazole (37): An isolated yield of 92 %.

1H NMR (300 MHz, CDCl3, δ ppm) 8.30 (s, 2H) 8.1 (s, 1H), 7.67 (d, 2H), 7.58 (d, 4H),

7.45 (d, 2H), 7.29 (t, 8H), 7.14-7.19 (m, 12H), 7.0 (t, 4H); 13C NMR (75.4 MHz, CDCl3, δ

ppm) 148.0, 147.2, 140.0, 137.1, 133.2, 130.1, 127.2, 126.1, 125.8, 124.7, 124.0, 123.1,

119.2, 111.6,. EI-MS; 653.4 (m/z); Elemental analysis for C48H35N3; Calculated: C, 88.18;

H, 5.40; N, 6.43; Found: C, 88.11; H, 5.32; N, 6.46.

37

HN

NN

2, 5-dioxyhexyl-1, 4- di(4’-bromobenzene) phenylene (41): 1.34 g, and isolated yield of

92 %. 1H NMR (300 MHz, CDCl3, δ ppm): 7.54-7.45 (m, 4H), 7.34 (d, 4H), 6.92 (s, 2H),

3.89 (t, 2H, OCH2), 1.65 (p, 2H, CH2CH2CH2), 1.37-1.1.24 (m,6H), 0.86 (t, 3H, CH3); 13C

NMR (75.4 MHz, CDCl3, δ ppm): 150.1, 137.1, 133.8, 133.6, 132.6, 132.2, 131.1, 131.0,

129.8, 129.0, 128.6, 128.5, 121.1, 115.88; EI-MS: 588.1 (m/z); Elemental analysis for

C30H36Br2O2, Calculated C, 61.24, H, 6.17, Br, 27.16, O, 5.44; Found: C, 61.68; H, 6.21.

196


Br Br

OC6H13

C6H13O41

7.5.5 General procedure of Buchwald coupling for the synthesis of OLG1-OLG4

(Scheme 5.3, pp. 139)

To a solution of dibromo compound 41 (0.1 g, 0.1 mmol) in 10 mL of toluene was

added followed by 30 mol % of palladium acetate (0.011 g), 15 mol % of tBu3PHF4 (0.008

g), 60 mol % of NaOtBu (0.098 g) and amine (3, 6-substituted carbazole) (0.27 g, 2.25

mmol) in N2 atmosphere for 1 h. The reaction mixture was evacuated and refilled with N2,

repeated this for three times and refluxed at 110 °C for 18 h (Scheme 5.3). The resulting

reaction mixture was poured into water and the product was extracted with DCM (25 mL×

2). The combined organic layer was washed with brine and dried over anhydrous Na2SO4.

The solvent was removed under reduced pressure and the crude product was purified

through column chromatography using hexane: DCM as eluent to get the final product

OLG1-OLG4.

1,4-di-{4”,4”-[3’,6’-bis(4”-O-2ethylhexylbiphenyl)-N,N’-carbozolyl]-phenyl}-2,5-

dihexyloxyphenylene (OLG1): Light yellow solid with the isolated yield of 62 %. 1H

NMR (300 MHz, CDCl3, δ ppm) 8.49 (d, 2H), 7.92 (d, 2H), 7.81 (d, 2H), 7.60-7.78 (m,

20H), 7.19 (s, 2H), 7.0 (d, 2H), 4.1 (t, 2H, O-CH2-hexyl), 3.90 (d, 2H, O-CH2 ethylhexyl),

1.71-1.85 (m, 3H), 1.25-1.49 (m, 16H), 0.90-0.98 (m, 9H). 13C NMR (75.4 MHz, CDCl3,

δ ppm), 159.0, 150.4, 140.8, 140.1, 139.2, 137.5, 136.4, 133.2, 133.1, 131.1, 130.1, 127.9,

127.5, 127.1, 126.3, 125.5, 124.2, 118.7, 116.0, 114.9, 110.3, 70.6, 69.7, 39.4, 31.5, 30.6,

29.7, 29.4, 29.1, 25.9, 23.9, 23.1, 22.6, 14.1, 11.1; HR-MS (MALDI-TOF) (M+): 1881.38

197


m/z; Elemental analysis for C139H148N2O6; Calculated: C, 85.49; H, 7.92; N, 1.49; O, 5.10.

Found: C, 85.37; H, 8.06; N, 1.42.

1,4-di-{4”,4”-[3’-(4”-O-2ethylhexylbiphenyl)-6’-(4”-O-methylbiphenyl)--N,N’

carbozolyl] phenyl}-2,5-dihexyloxyphenylene (OLG2): Yellow solid with the isolated

yield of 40 %. 1H NMR (300 MHz, CDCl3, δ ppm) 8.58 (s, 2H), 8.11 (d, 2H), 7.83 (d, 2H),

7.61-7.81 (m, 20H), 7.20 (s, 2H), 7.02 (d, 2H), 4.1 (t, 2H, O-CH2-CH2-), 3.91 (d, 2H, O-

CH2-CH-), 3.88 (s, 3H, O-CH3), 1.78-1.85 (m, 3H), 1.27-1.49 (m, 16H), 0.92-0.97 (m,

9H). 13C NMR (75.4 MHz, CDCl3, δ ppm), 159.2, 150.4, 140.8, 140.2, 140.1, 139.2, 139.1,

137.5, 136.4, 133.4, 133.2, 133.1, 131.1, 130.1, 128.0, 127.9, 127.6, 127.5, 127.1, 127.08,

126.3, 125.5, 124.2, 118.7, 116.0, 114.9, 114.3, 111.0, 110.3, 70.6, 69.7, 55.3, 39.4, 31.5,

30.6, 29.7, 29.4, 29.1, 25.9, 23.9, 23.1, 22.6, 14.1, 11.1; HRMS (MALDI-TOF) (M+):

1684.98 m/z; Elemental analysis for C120H120N2O6; Calculated: C, 85.47; H, 7.27; N, 1.66;

O, 5.69. Found: C, 85.31; H, 7.32; N, 1.68.

1,4-di-{4”,4”-[3’,6’-bis(4”-N,N-diphenylamino-1”-phenyl)-N,N’-carbozolyl]phenyl}-

2,5-dihexyloxyphenylene (OLG3): Light yellow solid with the isolated yield of 58 %. 1H

NMR (300 MHz, CD2Cl2, δ ppm) 8.46 (s, 4H), 7.93 (d, 2H), 7.74 (d, 2H), 7.62-7.72 (m,

20H), 7.31 (t, 6H), 7.16-7.23 (m, 4H), 7.0 (d, 2H), 4.04 (t, 2H, O-CH2-), 1.85 (p, 2H, -

CH2-), 1.30-1.38 (m, 6H), 0.90 (t, 3H). 13C NMR (75.4 MHz, CD2Cl2, δ ppm), 150.0,

147.8, 146.3, 140.5, 135.9, 131.0, 129.5, 129.2, 127.9, 126.1, 125.1, 124.2, 122.78, 118.1,

110.3, 69.7, 31.5, 29.7, 29.3, 22.6, and 13.8; HRMS (MALDI-TOF) (M+): 1732.88 m/z;

Elemental analysis for C126H104N6O2; Calculated: C, 87.26; H, 6.04; N, 4.85; O, 1.85.

Found: C, 87.09, H, 6. 12; N, 4.77.

198


1,4-di-{4”,4”-[3’,6’-bis-(N”,N”-carbazolyl)-N,N’-carbozolyl]phenyl}-2,5-

dihexyloxyphenylene (OLG4): White solid with the isolated yield of 74 %. 1H NMR

(300 MHz, CD2Cl2, δ ppm) 8.32 (d, 2H), 8.15 (d, 2H), 8.01 (d, 2H), 7.79 (dd, 4H), 7.67 (d,

2H), 7.41-7.65 ( m, 6H), 7.24-7.30 (m, 4H), 4.10 (t, 2H, O-CH2-), 1.78 (p, 2H, -CH2-),

1.33-1.35 (m, 6H), 0.87 (t, 3H). 13C NMR (75.4 MHz, CDCl3, δ ppm), 150.5, 141.8,

140.7, 139.2, 131.3, 130.4, 126.6, 126.2, 125.9, 124.0, 123.2, 120.3, 119.7, 116.1, 112.02,

111.4, 109.71, 69.7, 31.5, 29.7, 25.9, 22.6, 14.08; HRMS (MALDI-TOF) (M+): 1420.68

m/z; Elemental analysis for C102H80N6O2; Calculated: C, 86.17; H, 5.76; N, 5.91; O, 2.25.

Found: C, 86.19, H, 6.63; N, 6.02.

7.6 Chapter 6

The experimental methods for the synthesis of copolymers, monomers, and

intermediates are described as Scheme 6.1-2 in Chapter 6.9-11

7.6.1 General procedure for selective bromination of thiophene9 (Scheme 6.1, pp.

158)

To a 100 mL two-necked flask containing a solution of thiophene derivative (4.0 g,

35.66 mmol) in acetic acid/CHCl3 mixture (140 mL, 1:1 v/v) was added slowly NBS

solution ( 13.3 g, 74.88 mmol) in nitrogen atmosphere. The reaction was stirred at room

temperature for 4 days. The reaction mixture was poured into water and extracted in

dichloromethane. The organic layer was washed with dil. NaOH (1M) and water twice

each. The organic layer was dried over Na2SO4 and solvent was removed under reduced

pressure. The crude product was recrystalized from hexane.

199


2, 5-dibromo thiophene 3-carboxaldehyde (42): An isolated yield of 82 %. 1H NMR

(300 MHz, CDCl3, δ ppm): 9.79 (s, 1H, CHO), 7.33 (s, 1H, Ar H). 13C NMR (75.4 MHz,

CDCl3, δ ppm): 183.2, 139.3, 128.6, 124.2, 113.3. EI-MS: 269.8 (m/z).

S

OH

BrBr

42

2”’, 5”’ dibromo 3”, 3” dihexyl- tetra-thiophene (51): An isolated yield of 78 %. 1H

NMR (300 MHz, CDCl3, δ ppm): 7.36 (d, 2H, ArH), 7.01 (d, 2H, Ar H), 6.89 (d, 2H, Ar

H), 2.68 (2H, Ar-CH2-CH2-), 1.63 (p, -CH2-CH2-CH2-), 1.31 (m, 6H), 0.86 (t, -CH3); 13C

NMR (75.4 MHz, CDCl3, δ ppm): 142.2, 139.1, 133.8,136.4, 127.8, 112.1, 32.9, 31.4,

29.3, 22.7, 14.8; EI-MS: 656.1 (m/z)

SS

SS

C6H13

Br

C6H13

Br

51

7.6.2 Synthesis of Intermediates (Scheme 6.1-2, pp. 158-160)

Synthesis of compounds 43 and 52: The compound 43 and 52 were synthesized from

General Suzuki coupling reaction, as followed in Chapter 5.8

2, 5-dithiophene- 3- thiophenecarboxaldehyde (43): 2.23 g, and the isolated yield of

72 %. 1H NMR (300 MHz, CDCl3, δ ppm): 10.07 (s, 1H, -CHO), 7.33 (s, 1H, ArH), 7.48

(d, 1H, ArH), 7.28-7.31 (m, 2H, ArH), 7.21 (d, 1H, ArH), 7.15 (dd, 1H), 7.05 (dd, 1H,

ArH); 13C NMR (75.4 MHz, CDCl3, δ ppm): 185.1, 145.9, 137.7, 136.8, 135.5, 132.1,

200


129.2, 128.7, 128.3, 128.0, 125.8, 124.9, 122.4; Elemental analysis for C13H8OS3,

Calculated. C, 56.49, H, 2.92, S, 34.8; Found: C, 56.23; H, 2.82; EI-MS: 276.0 (m/z).

S

H

SS

O

43

5”’ bromo 3”, 3” dihexyl- 2-folmyl pentathiophene (52): An isolated yield of 62 %. 1H

NMR (300 MHz, CDCl3, δ ppm): 9.95 (s, 1H, Ar-CHO), 7.76 (d, 1H, ArH), 7.21 (d, 1H,

Ar H), 7.11-7.18 (m, 4H, Ar H), 6.96 (d, 1H, Ar H), 6.89 (s, 1H, Ar H), 2.69 (t, 4H, -CH2),

1.59 (p, 4H, CH2-CH2), 1.29-1.57 (m, 12H), 0.87 (t, 6H, -CH3), 13C NMR (75.4 MHz,

CDCl3, δ ppm): 187.1, 148.9, 143.2, 141.0, 138.7, 138.4, 138.1, 138.0, 137.3, 136.8, 133.5,

129.9, 128.4, 126.1, 112.4, 32.2, 30.8, 28.4, 28.1, 22.4, 14.7; Elemental analysis for

C33H35BrOS5; Calculated. C, 57.29, H, 5.13, Br, 11.62, S, 23.31; Found: C, 57.12; H, 5.2;

EI-MS: 688.2 (m/z).

SS

SS

C6H13

Br

C6H13

SO

52

Buchwald coupling10 for the synthesis of 5”’-carbazolyl- 3”, 3” dihexyl-2-folmyl

pentathiophene (53) (Scheme 6.2, pp. 160)

To a solution of monobromo compound 52 (0.2 g, 0.29 mmol) in 10 mL of toluene

was added 10 mol % of palladium acetate (0.007 g), 30 mol % of (tBu)3PHF4 (0.012 g), 60

mol % of NaOtBu (0.083 g) and carbazole (0.058 g, 0.34 mmol) in N2 atmosphere for 1 h.

The reaction mixture was evacuated and refilled with N2 three times and than refluxed at

110 °C for 18 h. Then the resulting reaction mixture was poured into water and the

201


product was extracted with DCM (25 mL× 2). The combined organic layer was washed

with brine and dried over anhydrous Na2SO4. The solvent was removed under reduced

pressure and the crude product was purified by silica gel column chromatography using

hexane: DCM as eluent to get the final product 53. 1H NMR (300 MHz, CDCl3, δ ppm):

9.86 (s, 1H, Ar-CHO), 8.10 (d, 2H, Cz), 7.66 (d, 1H, Ar), 7.44 (d, 2H, Cz), 7.30 (d, 2H,

Cz), 7.21 (d, 1H, ArH), 7.17 (m, 4H, Ar H), 7.1 (d, 1H, Ar H), 7.0 (s, 1H, Ar H), 2.78 (t,

4H, Ar-CH2-), 1.69 (p, 4H, CH2-CH2-), 1.41-1.48 (m, 12H, -CH2-), 0.89 (t, 6H, -CH3);

13C NMR (75.4 MHz, CDCl3, δ ppm): 182.4, 141.6, 141.0, 138.8, 137.48, 137.4, 136.8,

136.6, 134.9, 134.3, 133.6, 129.1, 128.3, 127.3, 127.1, 127.0, 126.3, 124.2, 124.1, 124.0,

123.6, 120.7, 120.2, 110.3, 31.6, 30.4, 29.2, 22.6, 14.1; EI-MS: 773.02 (m/z).

SS

SS

C6H13

C6H13

NSO

53

7.6.3 General procedure of Wittig reaction for synthesis of monomers (M1-M3)11

(Scheme 6.1-2, pp. 158-160)

The monomers M1 – M3 were synthesized using the common Wittig rearrangement

reaction. The Wittig ylides (46a-b) and dialdehydes (43, 53) of electroactive group were

mixed together and made slurry in THF. The mixture was stirred under N2 for 30 min. To

the mixture KOtBu solution (1M) was added and stirred for over night at room

temperature. The reaction was quenched with 1M HCl solution and extracted by DCM.

The organic layer was washed with water and brine solution twice and dried under

Na2SO4. The solvent was removed under reduced pressure and crude product was purified

by column chromatography using hexane and DCM as eluent.

202


Monomer M1: Yield = 72 %. 1H NMR (300 MHz, CDCl3, δ ppm): 7.33 (s, 1H, Ar H),

7.48 (d, 1H, Ar H), 7.28-7.31 (m, 2H, Ar H), 7.21 (d, 1H, vinylene), 7.15 (dd, 1H, H-Ar),

7.08 (dd, 1H, Ar H), 6.74 (s, 1H, H-Ar), 6.69 (s, 1H, Ar H), 6.61 (d, 1H, vinylene), 2.37 (s,

3H, Ar-CH3); 13C NMR (75.4 MHz, CDCl3, δ ppm): 137.1, 136.6, 135.9,134.6, 133.7,

132.1, 129.2, 128.7, 128.3, 128.0, 126.4, 125.8, 124.9, 122.4, 122.3, 22.4; EI-MS: 521.9

(m/z); Elemental analysis for C21H14Br2S3, Calculated C, 48.29, H, 2.70, Br, 30.59, S,

18.42; Found: C, 47.98; H, 2.42.

Br

Br

SSS

M1

Monomer M2: Yield = 64 %. 1H NMR (300 MHz, CDCl3, δ ppm): 7.72 (s, 1H, Ar H),

7.46 (d, 1H, Ar H), 7.34-7.41 (m, 2H, Ar H), 7.21 (d, 1H, vinylene), 7.21 (dd, 1H, Ar H),

7.08 (dd, 1H, Ar H), 6.61 (d, 1H, vinylene); 13C NMR (75.4 MHz, CDCl3, δ ppm): 137.1,

136.6, 135.9,134.6, 133.7, 132.1, 129.2, 128.7, 128.3, 126.4, 125.8, 122.4, 122.3; EI-MS:

780.0 (m/z); Elemental analysis for C34H20Br2S6, Calculated C, 52.31, H, 2.58, Br, 20.47,

S, 24.64; Found: C, 52.18; H, 2.31.

Br

Br

SSS

SS S

M2

Monomer M3: Yield 52 %.1H NMR (300 MHz, CDCl3, δ ppm): 8.11 (d, 2H, Cz), 7.82 (s,

1H, Ar H), 7.59 (d, 1H, Ar H), 7.44 (dd, 2H, Cz), 7.30 (dd, 2H, Cz), 7.17 (d, 4H, vinylene),

203


7.14 (d, 1H, Ar H), 7.08 (s, 1H, Ar H), 7.06-7.08 (m, 4H, Ar H), 2.77 (t, 4H, Ar-CH2-),

1.67 (p, 4H, -CH2-CH2-), 1.34-1.48 (m, 12H, -CH2-), 0.86 (t, 6H, -CH3); 13C NMR (75.4

MHz, CDCl3, δ ppm): 141.6, 141.0, 138.8, 137.5, 137.4, 136.8, 136.6, 134.9, 134.3, 133.4,

133.6, 129.1, 128.6, 128.3, 127.3, 127.1, 127.0, 126.3, 124.2, 124.1, 124.0, 123.6, 122.2,

120.7, 120.2, 117.8, 110.3, 33.6, 31.2, 29.5, 22.2, 14.0; EI-MS: 1775.2 (m/z); Elemental

analysis for C98H90Br2N2S10; Calculated C, 66.27, H, 5.11, Br, 9.0, N, 1.58, S, 18.05;

Found: C, 66.08; H, 4.96; N, 1.52.

Br

Br

SS

S

C6H13

SS

SS

C6H13

S

C6H13

C6H13

22

N N

M3

7.6.4 General procedure of Suzuki polymerization (P1-P3) (Scheme 6.3, pp. 161)

The dibromo monomers (M1 - M3), diboronicacid (49 or 1, 4 benzene diboronicacid),

and (PPh3)4Pd(0) (5 mol %) were dissolved in a mixture of toluene and 2M K2CO3 (3: 2

v/v) with cetyltrimethyl ammoniumbromide (30 mol%) as phase transfer catalyst (PTC).

The solution mixture was degassed thrice under N2 atmosphere and refluxed with vigorous

stirring for 72 h at 80 °C. The resulting solution was cooled and poured into methanol

solution (excess). The precipitate was filtered followed by washing with water and acetone

alternatively for 3-5 times. The obtained solid was purified by dissolving it in CHCl3 and

precipitating from excess methanol. The purification procedure was repeated 5 times and

polymer was dried in vacuum oven at 100 °C to get pure polymers (P1-P3).

P1: Green solid with yield 67 %. 1H NMR (300 MHz, CDCl3, δ ppm): 7.58-7.67 (broad,

4H, central Ar-H, OAr-H), 7.3-7.46 (broad, 7H, Th-H), 7.04 (broad, 1H, vinylene-H), 6.95

204


(broad, 1H, vinylene-H), 3.93 (broad, 2H, ArOCH2CH2), 2.33 (broad,3H, CH3), 1.80

(broad, 2H, ArOCH2CH2), 1.24 (broad, (CH2)3CH3), 0.88(t, CH3).; 13C NMR (75.4 MHz,

CDCl3, δ ppm): 151.9, 147.5, (OAr-C), 142.0, 137.1, 136.9, 134.8, 128.6, 128.5, 128.4,

128.0, 133.6, 131.3, 128.5, 127.5, 126.2, 125.5, 123.2, 112.0, 114.0, 68.4, 31.8, 29.6, 27.2,

25.7, 22.5, 22.0 (Ar-CH3), 14.7, (Alk-C); Elemental analysis: calculated, C, 75.88; H,

8.24; O, 3.96; S, 11.92; Found, C, 75.24; H, 8.18; S, 11.78.


4H, Ar-H, OAr-H), 7.3-7.46 (broad, 7H, Th-H), 7.04 (broad, 1H, vinylene-H), 6.95

(broad, 1H, vinylene-H), 3.93 (broad, ArOCH2CH2), 1.80 (broad, ArOCH2CH2), 1.24

(broad, (CH2)3CH3), 0.88 (t, CH3).; 13C NMR (75.4 MHz, CDCl3, δ ppm): 151.9, 147.5,

(OAr-C), 142.0, 137.1, 136.9, 134.8, 128.6, 128.5, 128.4, 128.0, 133.6, 131.3, 128.5,

127.5, 126.2, 125.5, 123.2, 112.0, 114.0, 68.4, 31.8, 29.6, 27.2, 25.5, 22.7, 14.7, (Alk-C);

Elemental analysis: calculated, C, 72.13; H, 6.81; O, 3.00; S, 18.05; Found, C, 71.98; H,

6.72; S, 18.08.


8H, Cz-H), 7.3-7.46 (broad, 16H, Th-H, Ar- H), 7.04 (broad, 1H, vinylene-H), 6.95 (broad,

1H, vinylene-H), 2.70-2.89 (broad, 2H, Th-CH2CH2), 1.60-1.77 (broad, 2H, Th-CH2CH2),

1.25-1.47 (broad, 6H, (CH2)3CH3), 0.88 (t, 3H, CH3); 13C NMR (75.4 MHz, CDCl3, δ

ppm):, 144.2, 138.4, 138.0, 137.3, 136.6, 136.0, 135.9, 134.8, 133.3, 130.0, 129.5, 128.4,

127.9, 127.8, 127.0, 133.4, 130.4, 122. 120.1, 111.1, 105.9, 32.8, 29.6, 28.9, 25.7, 22.7,

14.1, (Alk-C); Elemental analysis, calculated, C, 75.53; H, 6.21; N, 1.80; S, 16.46; Found,

C, 75.42; H, 6.12; S, 16.28.

205


206

7.7 References

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5. (a) Kimoto, A.; Cho, J.-S.; Higuchi, M.; Yamamoto, K. Macromolecules 2004, 37,

5531.

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LLIISSTT OOFF PPUUBBLLIICCAATTIIOONNSS 1. Barik, Satyananda; Valiyaveettil, Suresh, “Synthesis and Self-assembly of

Copolymers with Pendant Electroactive Units” Macromolecules 2008, 41(17), 6376-6386.

2. Barik, Satyananda; Valiyaveettil, Suresh; “Diblock Copolymer Assemblies through Changes in Amphiphilicity of Pendant Electroactive Moiety” Submitted

3. Barik, Satyananda; Valiyaveettil Suresh; “Regioregular Electro-active Carbazole End-Capped Oligo- (p-Phenylene): Synthesis, Characterization and Self-assembly Study” Submitted.

4. Barik, Satyananda, Karen, Goh H.K.; Vadukumpully, Sajini; Valiyaveettil, Suresh; “Synthesis, Characterization and Self-Assembly of Azo-Aromatic Based Diarylethene: A Photochromic Molecule for Molecular Electronics” Submitted.

5. Barik, Satyananda; Valiyaveettil, Suresh; “Cross-conjugated Poly (p-Phenylene): A side chain conjugated Polymer for molecular electronics” Under Revision.

6. Barik, Satyananda; Valiyaveettil, Suresh, “Synthesis, characterization and self-assembly studies of a new series of amphiphillic diblock copolymer with pendant electroactive moiety” Polym. Prep. Am. Chem. Soc. Div. Polym. Chem. 2008, 49(2), 361-362. Poster presentation at 236th ACS National Meeting, Philadelphia, PA, United States, Aug 17-21, 2008

7. Barik, Satyananda; Vallyaveettil, Suresh; “Design, synthesis and self assembly of organic macromolecules” Polym. Mater. Sci. Eng. 2006, 95, 1105-1106. Oral presentation at 232nd ACS National Meeting, San Francisco, CA, United States, Sept 10 – 14, 2006

8. Barik, Satyananda; Jegadesan,S.; Valiyaveettil, S.; * “Synthesis, Optical properties and self-assemble study of Acrylated polymers with electroactive side-chain unit” Poster presentation at SICC4-2005, National University of Singapore, Dec. 8-10 (2005).

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core.ac.uk · ACKNOWLEDGMENTS . I would like to express my gratitude to my supervisor, Assoc. Prof. Suresh Valiyaveettil for his guidance, constant support and encouragement throughout