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THESIS

HIGAN TE U AR

”Him“ 1m 1mm“1111711111111tn'riil '

3 1293 01712 6552

This is to certify that the

dissertation entitled

Synthesis and Characterization of
Substituted Polyphenylene Oligomers

presented by

Susan T. Pasco

has been accepted towards fulfillment
of the requirements for

 

 

Ph . D . degree in Chemistry

flat

Major professor

Date '/Z,//7l/? 7

MS U is an Affirmative Action/Equal Opportunity Institution 0- 12771

 

 

LIBRARY
Michigan State
University

 

 

 

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TO AVOID FINES return on or before date due.

 

MTE DUE DATE DUE DATE DUE

 

 

 

 

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SYNTHESIS AND CHARATERIZATION OF SUBSTITUTED POLYPHENYLENE
OLIGOMERS

By

Susan T. Pasco

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1 997

ABSTRACT

SYNTHESIS AND CHARATERIZATION OF SUBSTITUTED POLYPHENYLENE
OLIGOMERS

By

Susan T. Pasco

Due to their high thermal stability, conductivity in the doped state and large band
gaps, polyphenylenes are desirable materials for electronic applications but they are often
insoluble, intractable and cannot be synthesized reproducibly. Long alkyl side chains on
the monomer units impart solubility and processibility to the polymers, however, the
exact effect of the side chains on the solubility, crystallinity and optical properties of
polyphenylenes is poorly understood. We synthesized a series of 2,5-dialkyl substituted
exact-length polyphenylene oligomers to study the effect of side chains and oligomer
length on the properties of the oligomers and how these effects can be extrapolated to the
parent polymer. We synthesized oligomers of chain lengths ranging from 2 to 7 phenyl
rings with methyl, ethyl or hexyl side chains using the Suzuki coupling reaction in an
iterative scheme. The oligomers were characterized by Variable Temperature Nuclear
Magnetic Resonance (V TNMR), UV absorbance and fluorescence emission
spectroscopies, thermal analysis and optical microscopy. The VTNMR experiments
showed that the barrier to rotation around phenyl-phenyl bonds is 18.5 kcal/mol for the
methyl-substituted oligomers and ~20.5 kcal/mol for the ethyl- and hexyl-substituted

oligomers. The oligomer length had no effect on the rotational barrier. The optical

spectroscopy showed that the conjugation length for substituted polyphenylenes reached
a limiting value at less than 5 phenyl rings.

We also examined the solid state properties of the oligomers. The methyl-
substituted oligomers crystallize readily while the ethyl-substituted oligomers crystallize
slowly, over a period of 2-3 days. Solid state fluorescence of an ethyl-substituted glassy
oligomer shows a 2 band spectrum. After the oligomer crystallizes, the low energy band
decreases in intensity and the high energy band increases in intensity. We attribute the
two bands to fluorescence from the amorphous and crystalline forms of the oligomer. In
conjunction with optical microscopy and thermal analysis measurements, these
fluorescence experiments allowed us to propose a crystalline packing structure for ethyl-

substituted polyphenyls.

To my parents

iv

ACKNOWLEDGMENTS

First, I would like to thank my advisor, Professor Greg Baker, for his guidance
and for always showing me a "real world" view of chemistry. Thanks also go to my
committee members: Professors Gary Blanchard, Ned Jackson, Jeff Ledford and Simon
Garrett. Thanks especially to Professor Garrett for serving on very short notice. I would
also like to thank the past and present members of the Baker group: Jun Hou, Jun Qiao,
Jingpin, Yiyan, Micah, Chun, Cory, J. B., Tara, Mao, Gao and Tianqi, for always
providing a fun and interesting work environment — Go To Polybeersll

I would also like to thank Professor Matt Platz, without whom this degree would
not have been possible. He gave me the confidence and support I needed to come to
MSU and succeed. This degree also would not have been possible without the awesome
work of the Glass, Electronic and Machine shops — thanks for always knowing what I
needed, even if I didn't. I also must acknowledge Long Le and Kermit Johnson for all
their help on the NMRs. Thanks also to Lisa Dillingham, not only for all her hard work
in the Graduate Office but also for her friendship and for getting through orientation '96
together.

On a more personal level, there are many people I have met here who have made
Lansing feel like home to me and helped make graduate school a little easier. The

"Cheeters": Craig, Dave, Mike, Micah, Bill, Aimee, Rod, Danielle, Patti, and Suzanne,

thanks for cheering even after my 3rd strikeout of the game and for all the fun times on
and off the softball field. Rod and Danielle, my homebrew-drinking, sushi-eating
buddies- I can't wait for England and Scotland (Traquair House)! Shannon, whose daily
emails make Wyoming seem not so far away. Thanks for always being a great friend.
Per, for his constant friendship and support, especially during these last few months.
Thanksgiving next year? Micah, for all the hours gabbing in the lab when we should
have been working. Joy and Gary, for all the new recipes and fun conversations over the
past 4 1/2 years.

Matt, my friend and partner. Thanks for always being on my team. We've been
through a lot together and I can't wait for the adventures to come.

Finally, I want to thank my family: Mom, Dad and Bill for their love and support

throughout my academic career.

vi

TABLE OF CONTENTS

I. INTRODUCTION ........................................................................... 1
A. Aryl Couplings and Polymerizations .......................................... 4
1. Early Methods ............................................................ 4
2. Modern Aryl Coupling Methods ....................................... 8
B. Exact Length Conjugated Oligomers ........................................... 17
C. Optical Properties of PPPs and PPP Oligomers ............................... 19
D. Rotational Barriers in Polyphenylenes ......................................... 28
E. Side Chains ......................................................................... 32
11. RESULTS ................................................................................... 37
A. Synthesis ........................................................................... 38
B. Rotational Barriers ............................................................... 46
C. Optical Properties ................................................................. 57
l. UV-vis Absorbance ...................................................... 57
2. Fluorescence Emission ................................................... 68
3. Solid State Spectra ....................................................... 72

vii

D. Thermal Properties ............................................................... 75

E. Self-Assembled Monolayers ................................................... 89
III. DISCUSSION ............................................................................. 96
A. Electronic Properties of Poly(p-phenylene)s ................................. 96
B. Band Gap Engineering ........................................................... 97
C. Solubility and Crystallinity ...................................................... 102
1. Increased Ring Twist .................................................... 103
2. Rotational Barriers ....................................................... 105
3. Increased Entropy of Oligomers ....................................... 111
4. Steric Blocking ............................................................ 113
D. Effect of Side Chains on Morphology ......................................... 114
E. Design Rules ....................................................................... 117
F. Suggestions for Future Work .................................................... 118
1. Synthetic Methodology .................................................. 118
2. Self-Assembled Monolayers ............................................. 119
3. Crystallinity and Thermal Transitions ................................. 125
E. Summary ........................................................................... 126
IV. EXPERIMENTAL ........................................................................ 128
APPENDIX 1: lH NMR spectra of selected compounds ................................. 154
APPENDIX 11: Numbering of compounds ................................................ 165
BIBLIOGRAPHY ............................................................................... 167

viii

LIST OF TABLES

Table Page
Table 1. Nomenclature of unsubstituted oligophenyls .............................. 2
Table 2. Melting points of PPP oligomers ............................................. 33
Table 3. Abbreviations of oligomers used in the text ................................ 37
Table 4. Microsoft Excel macro for computing barriers with HyperC hem ........ 49
Table 5. Rotational barriers for alkyl-substituted terphenyls ....................... 50
Table 6. Rotational barriers for methyl-substituted oligomers ..................... 52
Table 7. UV-vis absorbance edge values for PPP oligomers ........................ 66
Table 8. Fluorescence kmax values for PPP oligomers ............................... 68
Table 9. Melting points of PPP oligomers ............................................ 81
Table 10. Solubilities and melting points for substituted PPP oligomers .......... 99
Table 11. Solubility in toluene of substituted quinquephenyls ....................... 105

ix

Figure
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.

Figure 11.

Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.

Figure 17.

LIST OF FIGURES

Page
Poly(para-phenylene) ............................................ 1
Numbering system in polyphenyls ............................. 2
A “hairy rod” ...................................................... 3
Oxidative-cationic polymerization of benzene ............... 4
Fittig Reaction ..................................................... 5
Wirth’s Ullmann coupling of substituted iodoaryls .......... 6
Diels Alder polymerization ...................................... 7
Tour’s synthesis of soluble PPP ................................. 7
Proposed mechanism for Yamamoto coupling ............... lO
Stille coupling reaction .......................................... 12
Suzuki coupling reaction ........................................ 12
Catalytic cycle of Pd(0) mediated Suzuki coupling .......... 13
Proposed mechanism for self-coupling of boronic acids. . .. 15
Synthesis of aryl boronic acids ................................. 16
Accelerated Suzuki coupling reaction ......................... 17
Linear conjugated oligomers for use as molecular wires... 18
19

Poly(p—phenylenevinylene) ......................................

Figure 18.

Figure 19.

Figure 20.

Figure 21.

Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.

Figure 35.

Figure 36.

Figure 37.

Schematic energy level diagram ................................ 20
Morse potential energy curves demonstrating the Franck-

Condon Principle ................................................. 22
Decrease in band gap for conjugated molecules .............. 23

Excited state structures of polyphenylene and

polyacetylene ...................................................... 24
Absorption spectra of PPP oligomers in chloroform. . . . . . 26
Twisted biphenyl ................................................. 27
Coalescing peaks in an NMR spectrum ........................ 29
Heitz’s methyl-substituted sexiphenyl ......................... 34
Traditional and accelerated Suzuki coupling reactions. 38
Synthesis of the first oligomers ................................. 39
Synthesis of TMS-functionalized materials ................... 4O
Bromination and iodination of 4,4’-bis(TMS)-TMB. . . . . 41
Incomplete reaction in accelerated Suzuki coupling. . . . . . 42
Synthesis of ethyl-substituted starting materials ............. 43
Synthesis of hexyl-substituted oligomers ..................... 45
The two diastereomeric rotational states of HMT ............ 46
Dihedral angles for substituted terphenyls ..................... 47

HyperChem calculation of rotational barriers for
substituted terphenyls ............................................ 48
Variable temperature NMR spectra of HMT ................. 51

Variable temperature NMR spectra of HET in the methyl

xi

Figure 38.

Figure 39.

Figure 40.

Figure 41.
Figure 42.
Figure 43.
Figure 44.
Figure 45.
Figure 46.
Figure 47.

Figure 48.

Figure 49.

Figure 50.
Figure 51.
Figure 52.
Figure 53.
Figure 54.
Figure 55.

Figure 56.

region ...............................................................
Actual and Simulated VTNMR spectra of HET ..............
Plot of T. versus temperature for HET ........................
UV-Vis absorbance spectra of methyl-substituted
oligomers .........................................................
UV absorbance spectra of ethyl-substituted oligomers. . .
UV absorbance spectra of hexyl-substituted oligomers. . . ..
UV absorbance spectra of substituted terphenyls ............
Plot of energy vs. oligomer length ..............................
Fluorescence spectra of methyl-substituted oligomers ......
Fluorescence spectra of ethyl-substituted oligomers. . . . . .
Fluorescence spectra of hexyl-substituted oligomers ........
Solid state UV absorbance spectra of ethyl-substituted
oligomers ..........................................................
Solid state fluorescence emission of ethyl-substituted
oligomers ..........................................................
DSC plot of HMT ................................................
DSC plot of OMQ ................................................
DSC plot of OEQ .................................................
DSC plot of DHQ ................................................
DSC plot of THS .................................................
DMA plot of OEQ ................................................

Solid state fluorescence of OEQ ................................

xii

53

55

56

59

61

62

63

67

69

70

71

73

74

76

77

78

79

80

84

85

Figure 57

Figure 58.

Figure 59.
Figure 60.
Figure 61.
Figure 62.
Figure 63.

Figure 64.

Figure 65.

Figure 66.

Figure 67.

Figure 68.
Figure 69.
Figure 70.

Figure 71.

Figure 72.
Figure 73.
Figure 74.
Figure 75.

Figure 76.

Variable temperature fluorescence spectra of OEQ .........
Plot of emission intensity vs. temperature for VT
fluorescence of OEQ .............................................
Structure of a self-assembled monolayer ......................
Synthetic scheme for SAMs ....................................
FT-IR spectra of SAMs on A200 fumed silica ...............
TGA plots of SAMs on silica ...................................
Grubbs’s synthesis of soluble polyacetylene ..................
UV absorbance spectra of Rehahn’s copolymers ............
Soluble ladder polymers .........................................
Comparison of a random coil polymer with a rigid-rod
polymer ............................................................
Side view of PPPs with different dihedral angles ............
Rotational barriers of substituted PPPs ........................
Measurement of crystallization rate of OEQ ..................
Photograph of crystalline HET under crossed polarizers. ..
Photograph of crystalline OEQ under crossed

polarizers. . ..

Schematic energy diagram for dissolution of PPPs ..........
DSC curves of substituted PPP derivatives ...................
Polyacetylenes solubilized by terminal t-butyl groups ......
Sandwich type morphology of 2,5-didodecyl PPP ...........

HyperChem depiction of proposed packing structure for

xiii

87

88

90

91

93

94

99

100

101

103

104

106

107

109

110

112

112

113

114

116

Figure 77.
Figure 78.
Figure 79.
Figure 80.
Figure 81.
Figure 82.
Figure 83.
Figure 84.
Figure 85.
Figure 86.
Figure 87.
Figure 88.
Figure 89.
Figure 90.
Figure 91.

Figure 92.

OEQ ................................................................
Proposed NLO chromophore .................................... 120
Proposed synthetic route to NLO chromophore .............. 121

PPP oligomer functionalized with a cross-linking group... 122

Thermal cross-linking of PPP oligomers ...................... 124
Chemical cross-linking of PPP oligomers ..................... 124
‘H NMR spectrum of TMB ..................................... 154
1H NMR spectrum of HMT ..................................... 155
‘H NMR spectrum of OMQ ..................................... 156
1H NMR spectrum of DMQ ..................................... 157
‘H NMR spectrum of TEB ....................................... 158
‘H NMR spectrum of HET ...................................... 159
'H NMR spectrum of OEQ ...................................... 160
‘H NMR spectrum of DEQ ...................................... 161
‘H NMR spectrum of 11m ...................................... 162
‘H NMR spectrum of DHQ ..................................... 163
'H NMR spectrum of THS ....................................... 164

xiv

I. INTRODUCTION

Polyphenylenes are a series of benzene rings connected by single bonds (Figure 1) in
an ortho, meta, or para fashion, or any combination of the three. The most commonly
studied phenylene is poly(para-phenylene) (PPP), since its large degree of conjugation
and rigid-rod structure make it a good candidate for many electronic, photonic and

structural applications.

n

Figure l: Poly(para-phenylene)

PPP is thermally stable to 400 °C in air and only 7% of the mass is lost after heating
to 900 °C under nitrogen. When doped with Ast, its conductivity is 500 S/cm,
approaching that of polyacetylene. Because of its high thermal and oxidative stability,
high mechanical strength1 and conductivity in the doped state,2 PPP has been explored
for use in light emitting diodes, as insulators for semiconductors and for metal catalyst
supports. Despite these desirable properties, PPP is an intractable, insoluble, highly

crystalline material that is difficult to synthesize reproducibly.

The established nomenclature for polyphenylenes is summarized in a review article

by Speight, Kovacic and Koch.3 The terms polyphenyl, poly(phenylene), oligophenyl,
and oligophenylene have all been used to describe the structure in Figure 1. The term
polyphenyl is usually reserved for molecules with a well-defined number of phenyl rings,
either substituted or unsubstituted, with the neighboring phenyl rings joined in an ortho,

meta or para arrangement. The most common unsubstituted para-linked oligomers are

Table l: Nomenclature of unsubstituted oligophenyls

 

Figure 2: Numbering system in polyphenyls. 2, 2’, 2”, 2’”, 5, 5’, 5”, 5’”

octamethquuaterphenyl

3
listed in Table 1. As shown in Figure 2, the numbering of the aromatic carbons of

polyphenylene begins at the carbon atom that links the first ring to an adjacent ring, and
each ring in turn is numbered fiom the preceding ring. Sites in each successive ring are
denoted by primes following the carbon number.

One difficulty in working with PPP is the inability to synthesize a soluble

structurally regular polymer of high molecular weight.4 An important aspect of PPP is
its rigid rod structure, which imparts many of its desirable properties but also leads to
poor solubility. The rigidity is caused by the para linkages, so it is imperative to find

polymerization or coupling reactions that ensure all para products. Several methods such

as synthesizing a soluble precursorsa6 or placing long alkyl side chains on the polymer
have been devised to overcome the synthetic difficulties in preparing PPP. The use of

large alkyl side chains to impart solubility on polyphenylenes has been widely

investigated. Rehahn, et al.7a8 first employed this strategy in the synthesis of the first

example of structurally regular, soluble, high molecular weight polyphenylenes. (Figure

Figure 3: A “hairy rod”

V

Q
+ 2n Cue:2
CuClz n + 2n CuCl + 2 HCI

Figure 4: Oxidative-cationic polymerization of benzene

3) Since the report of these “hairy rods”, the addition of side chains to polyphenylenes
has become a common route to soluble PPPs. While it is obvious that the side chains
impart solubility to PPP, the details of how solubility is achieved are poorly understood.
The goal of this research is to conduct a detailed study of exact length substituted poly(p-
phenylene) oligomers in order to define the side chain-solubility relationships. We chose
a series of 2,5-disubstituted, exact-length poly(p-phenylene) oligomers and systematically
investigated the effect these side chains have on the rotational barriers, optical properties,
thermal properties and solubility of the polymer. These oligomers were synthesized
using an iterative approach, so that coupling reactions can be used to assemble a series of
oligomers of increasing length from a library of common intermediates. The remainder
of this introduction summarizes the synthetic methods used for aryl couplings, including
polymerizations, as well as a background of the basic optical and structural analysis tools

used, and how these methods apply in this work.

A. Aryl couplings and polymerizations
1. Early Methods

Numerous reviews have been published on the synthesis and properties of

polyphenylenes-3,9,10 Until the late 1980’s, the most common method for synthesizing
polyphenylenes was oxidative-cationic polymerization.

Oxidative-cationic polymerizations of benzene to yield polyphenylenes (Figure 4)

5

were discovered and developed by Kovacic and the products are frequently referred to as
“Kovacic PPP” in the current literature. To be effective, the catalyst system should be a
good Friedel-Crafts catalyst and it must also be a good oxidizing agent. Other systems
that were effective in this polymerization were Ale in combination with MnOz, PbOz,
N02, benzoquinone, air or chloranil, but the reactions produced variable yields and
irregular structures. Interestingly, while benzene gave a polymeric product, biphenyl and
terphenyl yielded only sexiphenyls except upon heating, from which a polymer with meta
and para linkages resulted. Alkylated benzenes were poor substrates and gave a complex
mixture of low molecular weight ortha-linked molecules.

Another route to polyphenylenes is the coupling of halogenated aromatic
compounds. The Fittig reaction (Figure 5) has been known for over 100 years, but is not
commonly used for polymerizations because the anionic nature of the reaction results in a
large number of side products. However, it can be used to synthesize a symmetric biaryl
in reasonable yield. There are a few reports of the preparation of para-linked polymers
using the Fittig reaction, but since the melting points of the polymers are lower than that

of p-quaterphenyl, these are probably either branched polymers or very small oligomers.

Na 0
00' =H©+O
reflux,5h O

 

Figure 5: Fittig Reaction

6
CH3 CH3 CH3
Cu
3. 2 H l ——-——> H H
n 2n
H3C H3C H3C
CH3 CH3 CH3
C

b. H 1 + I 1 —”__. H H
n ”4..

H3C CH3 CH3

Figure 6: Wirth’s Ullmann coupling of substituted iodoaryls. a. even

oligomers. b. odd oligomers

For synthesizing oligomers such as biphenyl or quaterphenyl, a more useful coupling

reaction is the Ullmann reaction.1 1'13 This reaction is commonly used for synthesizing
symmetrical biphenyls, but it has been used in the synthesis of asymmetric oligophenyls.
Cross-coupling reactions usually yield a statistical mixture of symmetric and asymmetric

coupling products, but often these mixtures can be easily separated. A templating

scheme can also be used14 to facilitate cross-coupling reactions. Wirth et al15 used this
method in the first reported synthesis of exact length substituted polyphenylene
oligomers. They synthesized 2,5-dimethyl-substituted polyphenyls from biphenyl to
quinquephenyl by coupling iodoaryls (Figure 6).

The Diels-Alder reaction of bis-tetraphenylcyclopentadienones and bis-diacetylenes
has also been used to synthesize polyphenylenes. (Figure 7) This polymerization yielded
white amorphous polymers of molecular weights from 20,000 to 100,000. The Diels-
Alder reaction is not regiospecific, and thus some meta linkages are found in the polymer

backbone. These kinks, in addition to the aromatic rings attached to the phenylene

7

Ph Ph
0 Ph
9 O C + RCECQCECR E,
X
Ph Ph Ph 0

x=1or2

 

Figure 7: Diels Alder polymerization

backbone, contribute to the high solubility of the polymers.
Aryl lithium reagents have also been coupled to form polyphenylenes. Early
attempts to synthesize a polymer from phenyllithium in the presence of oxygen resulted

in a good yield (>65%) of biphenyl. However, oxygenated products are also often found

in this reaction. More recently, Tour and his group16‘18 synthesized a soluble
polyphenylene from 1-bromo-4-lithiobenzene (Figure 8). This instantaneous
polymerization is facilitated by the addition of HMPA to the lithiated bromobenzene

solution. The polymer is soluble because of several defects present in the polymer,

Br

 

Br Li Br
> ______. and/or O
m n m n
Br Br
TMSCI
-78 °C 86%

Br—QTMS

Figure 8: Tour’s synthesis of soluble PPP

8
namely phenylated rings capped with halogens. A mechanistic study18 suggests that

ortha-benzyne intermediates are responsible for the defects.

2. Modern Aryl Coupling Methods

Organometallic reagents have been widely used for aryl couplings. The first attempt
to use a Grignard-type reagent was by Ullmann, who tried to use magnesium instead of
copper to couple halobenzenes. This attempt was unsuccessful, but biaryls have been
successfully synthesized using metal halides such as CuClz, AgBr, MoCls, CoClz, CrCl3
and F eCl3 which oxidize the aryl Grignard, and form an intermediate (possibly radical?)
which couples to form the biaryl. Another early success was the coupling of aryl halides

with zero-valent nickel compounds such as Ni(COD)2 (COD = cyclooctadiene) to yield
biaryls.19 Yamamoto expanded on this work by studying several transition metal

catalysts for the catalytic coupling of Grignard reagents prepared from dihalobenzenes.20
For the polymerization of p-dibromobenzene, NiC12(bpy) and PdC12(bpy) were the most
effective catalysts, giving a 95% yield of PPP. The polymers were light yellow in color
and decomposed in air at 550 °C. Their thermal stability is comparable to that of a
polymer synthesized by the Kovacic method, but the color is considerably lighter,
indicating that there are fewer impurities in the polymer from halogens, traces of the
catalyst, or oxidizing agent. The Yamamoto PPPs show a high degree of crystallinity by
X-ray diffraction, indicating that the reaction is highly selective for coupling in a para-
fashion. The coupling mechanism proposed by Yamamoto is shown in Figure 9. The
first step involves formation of NiRsz followed by R2 loss and reaction of the resulting

unsaturated complex with R’-X to yield NiR’(X)L2, which is the active catalytic species

9
in the coupling. This species is alkylated with R-MgX and the resulting nickel compound

reductively eliminates R-R’ to regenerate NiR’(X)L2.

10

L. )3
Ni
L’ \q

R'X

L R—R

V

Nisz2 + 2 RMgX ———>

 

L

\

Ll

)5
NI
3t
RMgX
R'X
NiL2

. MQX
1., )8 2
Ni

L’ \5

Figure 9: Proposed mechanism for Yamamoto coupling

11
The Yamamoto reaction was the first catalytic reaction to show the specificity needed to

synthesize the exact length, para linked PPP oligomers that is the focus of our work. As
we will see throughout this Introduction, even one meta or ortho linkage can greatly
change the thermal stability, solubility and optical properties of PPP polymers or
oligomers. Although the Yamamoto coupling was a breakthrough for the polymerization
of bromobenzenes to polyphenylene, the reactivity of the Grignard reagent leads to

modest molecular weights. The polymerization of hexyl-substituted dibromobenzenes by

Rehahn7 resulted in a degree of polymerization of only 13. Newer coupling methods
including the Stille coupling and the Suzuki coupling, follow similar mechanisms but use
other organometallic reagents as different transmetallation agents. As we will see, the
judicious choice of a transmetallation agent can allow for several options in a coupling
scheme, and the scientist can choose based on the requirements of the reaction in
question.

Several investigators have used transition metal couplings involving zinc as the

transmetallation agent. Rieke21a22 used an activated zinc powder formed from Zan
and lithium naphthalide to selectively form a variety of organic halozinc compounds.

Aryl bromides gave the corresponding aryl zinc compounds in 90-100% yield, which

could then be polymerized with a Pd(0) catalyst. Jutand23 used activated zinc (formed

from zinc powder and acetic acid) as a reducing agent for the coupling of aryl halides.

Iyoda24 used zinc in the presence of tetraethylammonium iodide and a nickel catalyst to
couple substituted aryl halides. This combination of reagents presumably forms a more
active catalyst species, but the authors did not complete a detailed mechanistic study to

prove this theory.

12
The Stille coupling25 (Figure 10) is a palladium catalyzed coupling of aryl

triflates with organostannanes. This reaction is widely used,26'32 but a sometimes fails
for halobenzenes with electron-donating groups. This reaction is still tolerant of many

functional groups and is frequently used for coupling vinyl stannanes. The Stille
coupling has been conducted under microwave irradiation,33 improving the rate of the

reaction, and in the solid phase34 using an amide resin to produce biaryls in slightly
lower yields than the analogous solution phase syntheses.

Probably the most commonly used coupling method is the Suzuki reaction (Figure

 

R R
LiCl R R
PPh3
PdCl (PPh)
R R 2 32 R R

Figure 10: Stille coupling reaction

 

R R R R
Pd(PPh3)4 _
QBmHh + BrO NaZCO3 ,
R R R R

Figure 11: Suzuki coupling reaction

11), which involves the palladium catalyzed coupling of an aryl halide with an aryl

boronic acid. 35:36 The stability and low toxicity of boronic acids give the Suzuki

13

coupling distinct advantages compared to other coupling schemes. Both the highly
reactive Grignard reagent of the Yamamoto coupling and the toxic organotin species of
the Stille coupling are avoided. The stability of boronic acids allows Suzuki couplings to
be run under a variety of reaction conditions. As for all Pd(0) catalyzed reactions, the

most reactive halide is the aryl iodide followed by the bromide, and the chloride does not

couple.37
Steric hindrance can be a factor in the Suzuki coupling, but generally this reaction

is compatible with a wide range of fimctional groups. This coupling does not proceed

without the use of a base”, which is thought to aid the transmetallation of the boronic
acid moiety. Figure 12 demonstrates that a crucial difference between the catalytic cycle
for Suzuki coupling and those of most Pd-catalyzed reactions is that the oxidative

addition step is followed by a displacement of the halide ion from the Ar-Pd-X complex

Pd 0
Amt-”v ( ’ V Arx

Ar-Pd-Ar' Ar—Pd-X
e NaOH
B(OH)4 ArPd-OH
OH NaX

l
Ar'B(OH)2 MAr-EB—OH
OH

35
Figure 12: Catalytic cycle of Pd(0) mediated Suzuki coupling

14
by a base. In Figure 12, the organo-palladium hydroxide is more reactive than the

organo-palladium halide since the Pd—O bond is more polar than the Pd-Br bond. As a
result, the organo-palladium hydroxide species is much more electrophilic and therefore
facilitates the transmetallation step.

Although the Suzuki coupling is an excellent method for carbon-carbon bond
formation in aryl species, there are several limitations to this reaction. Sterically

hindered boronic acids, especially those substituted in the ortho position, undergo

coupling much slower than less hindered boronic acids”:40 A common side reaction in
the traditional Suzuki coupling is homocoupling of the aryl boronic acid. Homocoupling
is undesirable because it leads to lower yields, and the disruption of stoichiometry results

in low molecular weight products in polymerization reactions. In a mechanism proposed

by Moreno-Mafias and coworkers“, oxidative addition of the aryl-boron bond is
followed by a hybrid oxidative addition-transmetallation step to yield two aryl groups and
two boronic acid moieties bound to the palladium atom (Figure 13). The authors claim
that the metaboric acid O=B-OH is converted to bOrate under the aqueous alkaline
conditions usually present in the Suzuki coupling reaction. However, they reported no
evidence for this product in the mechanism, only for the intermediates formed, so it is not
certain that this pathway is correct. For slow coupling reactions, this self-coupling side

reaction can be significant.

15

L
L
@0le
Ar—Rd-B(0H)2 + 2
PdL4 ——» PdL2 + 2L Ar
2L
Pde (HO)pB-Pd—B(OH)2
(H2)
O=B—OH

Figure 13: Proposed mechanism for self-coupling of boronic acids

16
A large body of work targeted improved syntheses of sterically hindered biaryls

from boronic acid substrates. Boronic acids are typically synthesized from organo-

lithium or organo-magnesium reagents and a borate ester (Figure 14).42‘47 The most
common side products from a boronic acid synthesis are borinic acids, which are
molecules in which two aryl groups add to one borate ester and triphenyl borate

derivatives. Neither couple under the usual reaction conditions. Thompson and

Gaudino40, in their synthesis of 5-arylnicotinates, reported that these side products can
be minimized by using a large excess of a bulky borate such as triisopropyl borate, and
by extremely slow addition of the organometallic reagent to a concentrated borate
solution at cold temperatures. Electron withdrawing groups on aryl boronic acids cause

hydrolytic deboronylation that competes with coupling and decreases product

yield.39’48s49 Suzuki expanded on his original work by developing new conditions to
help eliminate these limitations. By using the esters of boronic acids and anhydrous
reaction conditions, they were able to couple aryl halides with a mesitylboronic acid, an
o-methoxyphenyl boronic acid and an o-benzaldehyde boronic acid in good yield, usually

about 80 %.

Recently, Novak, et al.50 developed an accelerated Suzuki coupling method

1.Mg orn—BuLi
8' 2. B(O-i-Pr)3_ 3(0le
3. H3O’ ' 0

Figure 14: Synthesis of aryl boronic acids

 

l7

Pd(OAc)2
01+ Q... =
N32003

THF, H20
2 h

 

Figure 15: Accelerated Suzuki coupling reaction

(Figure 15) which uses a phosphine-free palladium source. The reaction only takes 1 to 2
hours to complete as opposed to 24 hours for traditional Suzuki couplings. The
advantage of a phosphine-free catalyst is that it eliminates a side reaction, the transfer of
a phenyl group from the triphenyl phosphine ligand to the aryl halide. Novak’s method
works well for aryl iodides, but is very sensitive to steric hindrance in aryl bromide

substrates.
B. Exact Length Conjugated Oligomers

It is important to understand the optical properties of PPPs, since devices based on

conjugated organic molecules, particularly oligomers, have been the focus of much
interest recently.51 Examples of these molecules are phenylene-ethynylene oligomerssz,
oligothiophene553, poly(phenylacetylene) oligomer354,55, poly(phenylenevinylene)56
and polyphenylenes.” Exact length conjugated oligomers have found applications as
molecular wire558, thin film transistor559, and electroluminescent devices60. Exact

length oligomers serve as models for the intractable PPP,61'65 materials for second order

nonlinear optical applications, or to alter surfaces as self-assembled monolayers.

Tour, et. al.54a55 synthesized a 128A linear conjugated oligomer of poly(p-
phenylacetylene) that has been touted as a molecular wire (Figure 16). The synthesis

permits selective functionalization of the ends of the oligomer. These oligomer ends can

18
be converted to “molecular alligator clips” by attaching thiol acetates that can be

hydrolyzed to thiols. Upon exposure to gold surfaces gold thiolates are formed, creating
the link between two metal probes. With ethyl side groups, the chains are only soluble up
to the octamer, but by using a 3-ethylheptyl substituent the 16-mer is soluble. This side
chain introduces a stereogenic center, which affords many diastereomers. Tour reports
that racemization of the alkyl side chains provides solubility of the longer oligomer by
retarding crystallization, but the solubility may also be imparted by the steric bulk of the
branched chain.

Poly(p-phenylenevinylene) (PPV) oligomers (Figure 17) have also been studied

R R

8

Figure 16: Linear conjugated oligomers for use as molecular wires

as model compounds for the polymer“. Since poly(p-phenylenevinylene) becomes
insoluble with increasing chain length, these well-defined oligomers can be used to gain
insight into the structural and physical properties of the polymers such as the number of
accessible oxidation states. Well-defined short conjugated oligothiophenes were also

investigated as the active materials in organic field effect transistors and in light emitting
diodes (LEDs).62'65 Polyphenylene oligomers, which have been investigated as blue
LEDs, are good candidates for electronic materials because of their high thermal and

oxidative stability.

Figure 17: Poly(p-phenylenevinylene)

The precise length of these oligomers makes them especially attractive for
electronic applications. There is a narrow distribution of conjugation lengths, which
results in molecule-specific properties rather than averaged properties resulting from the

molecular weight distribution in polymers.

C. Optical Properties of PPPs and PPP Oligomers

The attractive electronic properties of exact length oligomers can be understood

by considering the basic principles of electronic spectroscopy (Figure 13).66,67 When a
molecule absorbs a photon at a certain wavelength, an electron is promoted from the
ground state (So) to one of a number of higher level excited states (8;, S2, S3, S4). The
molecule can then lose the energy either radiatively or nonradiatively. In Figure 18, the
curvy arrows indicate nonradiative processes, involving heat transfer, while the straight
arrows denote radiative processes, involving transfer of photons. If the molecule loses
the energy radiatively, fluorescence or phosphorescence occurs. Fluorescence is defined
as the radiative transition between two states of the same multiplicity and
phosphorescence is defined as the radiative transition between two states of different
multiplicities, caused by an intersystem crossing from an excited singlet state to an
excited triplet state followed by emission from the excited triplet state to the singlet

ground state.

S4
33

52

So

20

 

g Radiationless

Transitions-
Intemal conversion

 

 

Radiationless
Transition-

1

 

 

 

 

1

 

 

[Fluorescencej

 

l

%

 

 

 

Internal
Conversion

Triplet-triplet

 

-MWOSSing
1

Absorption
l

 

 

 

i

Figure 18: Schematic energy level diagram

mosphorescencej

l

 

21

Fluorescence spectra are seen at lower energy than absorbance spectra because
the transition is from the lowest vibrational level of the electronic excited state to the
ground state. The lowest energy absorption and the highest energy emission are often
due to the same transition, namely the 0-0 transition and thus the low energy edge of the
absorption band is usually structurally similar to the fluorescence spectrum, often in a
mirror-like pattern if the vibrational structures of the singlet ground and excited states are
similar. The displacement between these two bands is referred to as the “Stokes shifi”.
Similar ground and excited state geometries lead to small Stokes shifts, while significant
changes in geometry following excitation lead to large Stokes shifts. The Franck-Condon
principle states that electronic transition are vertical and between levels on the Morse
potential energy curves. If the minima in the Morse potential energy curves are not at the
same coordinates, the minimum energy for absorption will be larger than the difference in
energy between the lowest energy excited states and the ground state. When an excited
molecule relaxes through the vibrational states and fluoresces from the St state, a lower
energy transition results, and the difference in energy for the absorption and emission is

the Stokes shift.

22

 

 

 

 

 

 

 

 

 

 

Figure 19: Morse potential energy curves demonstrating the Franck-Condon

66
Principle

23
The length of the phenylene chain has an important effect on the optical spectra.

Theoretically, the longer the chain is, the smaller the band gap should become. Figure 20
shows how the gap between the TI and 1t* orbitals (called the band gap) decreases as more
p orbitals are in conjugation. The band gap is defined as the energy gap between the
HOMO and LUMO (or conduction band and valence band) in a molecular orbital

diagram. This gap can be measured by absorption spectroscopy by observing the optical

absorption edge. The band gap of PPP is about 2.7 eV, 68 polyacetylene (PA) is about
1.4 eV and a conducting metal has a band gap approaching zero. However, the band gaps
of most conjugated polymers are finite. The band gap energy (Eg) is inversely
proportional to the number of conjugated units in the polymer. This number, called the
“effective conjugation length”, corresponds to the size of the lowest energy excitation,
meaning the distance over which the excited state is delocalized. The conjugation length

is an effect of the amount of double bond character between n-electron-containing units

n*—— 7t*: 71*;
n— 1!: ”E
= M W

Figure 20: Decrease in band gap for conjugated molecules

so a molecule with an infinite effective conjugation length would have all its bonds

exactly the same length. The effective conjugation length for polyphenylene is not equal

24
to the length of the polymer because the steric interactions of the protons ortho to ring

junctions cause the rings to twist slightly out of alignment to a dihedral angle of about

23°.2 This twisting results in a decrease on the p-orbital alignment, causing decreased
overlap. Alkyl groups cause a twist of about 45° between the rings. Thus, oligomers that
are longer than the effective conjugation length have the same properties as PPP,
allowing the use of a processible oligomer in place of an intractable polymer in electronic
applications.

Figure 20 is adequate for describing the electronic structure of simple polyenes,
but is perhaps too simple for PPPs. We must consider aromaticity in polyphenyls to
describe the electronic structure of PPPs. The conjugation between each ring is
decreased relative to PA because the aromaticity of the individual phenyl rings prevents

electron delocalization along the polymer chain. Calculating the band gap for

polyphenylenes is actually quite complicated and has been studied extensively69'73, but
a complete explanation of these studies is not necessary to understand our work. We will
use a more simple model, treating the phenyl rings as individual conjugated wits and
examine how connected the units are to each other and how substituents affect that

connection. The excited state structures of a simple polyene and a polyphenylene are

C-C-C eC-C-C

 

(9

Figure 21: Excited state structures of polyphenylene and polyacetylene

25
represented in Figure 21. This diagram shows how the singlet excited state structure of a

PPP destroys the aromaticity and therefore 112 electron delocalization is not spread over a

large number of units.

The spectra of PPP oligomers contain two main bands, the K band and the B

band.74 The B band is attributed to the excitation of individual benzene rings, and is
affected by changes in substitution but not by differences in chain length. In benzene,
this band is found at 256 nm (hexane), but substituting the ring with methyl groups shifts
the B band to 261 nm for toluene, 266 for mesitylene and 272 for hexamethylbenzene.
The K band is polarized along the backbone of PPPs and is sensitive to changes in the
conjugation of the benzene ring. In benzene, the K band is found at 204 nm and in

biphenyl it shifts to 252 nm, obscuring the B band. Figure 22 illustrates the

bathochromic shift of the K band with increasing oligomer length.75

26

 

 

8
§

 

,8
§

 

Molecular extinction coefficients.

.8
EV

 

//
/ . (\\\\
4 , > \J_

2500 3009 3500
A, A.

«k/

 

 

 

 

 

 

 

 

 

Figure 21: Absorption spectra of PPP oligomers in chloroform. 1. biphenyl 2.

terphenyl 3. quaterphenyl 4. quinquephenyl 5. sexiphenyl (qualitative only)

27

Grem and Leising76 investigated the tunability or “band-gap engineering” of

LEDs designed from polyphenylenes. The band gap in conjugated polymers can be tuned

by introducing alkyl side chains on the rings, increasing the degree of twisting due to

ortho interactions. The band gap is directly related to the degree of n-overlap along the
polymer chain, and by increasing the twist angle, the n-overlap is decreased. (Figure 23).

The effect of alkyl chains can be observed in the UV absorption spectra. In 2,5-

dihexyl-substituted PPP, Km, = 318 nm.77a78 The calculated 714,,“ is ~344 nm for

unsubstituted PPP, but thin film samples exhibit Am“ at 379 nm.79 Both the calculated
and measured values of Km“ are at longer wavelengths, reflecting the better 1: overlap for

polyphenyls without ortho substituents.

 

Figure 23: Twisted biphenyl. a. side view, b. end-on view

28

D. Rotational Barriers In Polyphenylenes

One particularly interesting aspect of polyphenylenes is that by placing a
substituent on the main chain, the barrier to rotation around the phenyl-phenyl bond is
significantly increased. Barriers can often be characterized by Nuclear Magnetic
Resonance (NMR). Dynamic NMR is useful in determining rotational barriers if the two
rotamers are nonequivalent, meaning that distinct peaks will be present in the NMR
spectrum for a each rotamer as long as the rotation is slow on the NMR time scale. This
phenomenon can be caused by a steric effect, in which a bulky group prevents easy
rotation around a bond, or an electronic effect where conjugation makes free rotation
energetically unfavorable. A classic example is N,N-Dimethylformamide (DMF), which
typically shows two peaks for the two methyl groups at room temperature due to the
considerable double bond character of the C-N bond, but as the sample is heated, the two

peaks coalesce into one peak. A typical set of coalescing peaks is shown in Figure 24.

29

/ ‘.
/ a

1‘ ‘-\

1f 1\

| .

l 1

1 ‘1

‘1

l

I, ’

/ 1

. l
1' \

v‘: ‘ \
/ \x
\\
\ w
I , / ‘ / a A
/ \. /
; \ / \
’1' ‘. j “
If ‘

1 / \
1 / 1
I \l‘ I}, ‘1‘
l g' l
1 1‘ 1‘
I" | \1
(I: l I \1
1 ,1
LL _/ ,/ \
LL \
.‘l 1
11 11
111 11l
1 1
1 11
1‘ ’ 1
1 1 1
1 '1 1 l
1 1
, 1‘
7 \ L . L / \-

Figure 24: Coalescing peaks in an NMR spectrum

30

By measuring the spectrum at several temperatures and observing the temperature
where the signals for the two states coalesce, one can derive the rate constants and the

energy needed to cause the bond in question to rotate freely on the NMR time scale. The

Gutowsky-Holm approximationgoa81 is probably the most commonly equation used to
determine rotational barriers from NMR data. In practice, the coalescence point is
determined by gradually increasing the temperature of the NMR sample until the small
valley between the two coalescing peaks just disappears. The value of 61) varies with
the solvent used, but this variability is inconsequential, since the coalescence temperature
varies as well. Outlined below is a brief derivation of the Gutowsky-Holm
approximation for calculating rotational barriers.

The absolute rate theory developed by Eyring based on statistical thermodynamics

k _ K kBT e—AGI /RT

where k is the first order rate constant, k}; is the Boltzmann constant, K is the transmission
coefficient (the fraction of all molecules reaching the transition state that proceed forward
to product molecules), h is Planck’s constant and T is temperature. If k and T are known,

and we assume that K is unity, one can obtain the equation for AG’.

AG‘ = a1110.3 19+log(T/k)]
a = 1.914 x10'2 for AG:L in k] mol‘l

a = 4.575 x 10'3 for AGI in kcal mol'1

31

To determine the barrier to rotation from the coalescence temperature, It is replaced by

7:60/ J2 , where 61) is the frequency difference in Hertz between the two peaks at the

slow exchange limit, and the Gutowsky-Holm equation is obtained.

AG:( 2 aT[9.972 + log(T/5u )]

The equation is only slightly sensitive to error in measurement; an error of i- 2 °C in
determining Tc results in an error of 0.12 kcal mol".
The barriers to rotation around the phenyl-phenyl bond of many biphenyl

derivatives have been studied by VTNMR and by computational methods. The first

application of this technique to biphenyls, reported by Meyer and Meyer”, examined the
energy barrier to inversion of 2,2’-bis(acetoxymethyl)biphenyl. The methylene protons

showed an AB quartet signal at room temperature which coalesced to a singlet at 94 °C in

C82, indicating a rotational energy barrier of 13 kcal/mol. Oki and Yamamoto83

determined that the barrier for 2,2’-diisopropy1biphenyl was greater than 27 kcal/mol, at

which point the NMR signals showed no broadening or coalescence. Bott, et al.84
studied the steric effects on the rotational barriers of 2,2’-disubstituted biphenyls by
introducing a prochiral group that can be monitored by NMR. The prochiral group is
necessary if the molecule is symmetric in either rotational state. They found that the
rotational barrier showed the expected linear increase with increasing van der Waals radii

of the substituents. More recently, the rotational barrier for oligothiophenes has been

studied,85'87 which is much lower than for phenylenes, because the ring angles are

smaller in thiophenes, so there is less steric interaction between adjacent rings.

32
Hindered rotation in aryl systems has been investigated due to its effect in

biologically active compounds88 and as potential models and/or building blocks for

devices.89 Charlton, et al.90 studied hindered rotation in arylnaphthalene lignans by
dynamic NMR and found that the barriers to rotation ranged from 16.9 to 21.5 kcal/mol
for the compounds they studied. This hindered rotation produces optically active

compounds due to the possibility of 2 or more possible rotational states. 1,1’-Binaphthyl
has a computed91 (PM3) barrier of 23.1 kcal/mo] which is almost in exact agreement
with the experimentally determined”,93 barrier of 22.5 kcal/mol. This feature has been

recently examined in liquid crystals94 and optically active polymers.95,96

NMR relaxation studies can also provide important information the amount of
motional freedom of a molecule. There are two mechanisms by which the nucleus can
relax, denoted by the time constants T1 and T2. T1, also called the spin-lattice relaxation
time, involves transfer of energy from the nucleus to the surroundings, or “lattice”. The
longer the T. time, the less efficient the relaxation. A very constrained molecule (by
steric effects or covalent bonding) will have a much longer T1 than a molecule which has
many motional degrees of freedom. T2, or spin-spin relaxation time, is the time constant

that represents the loss of energy from one nucleus to another.

E. Side Chains

Side chains are commonly placed on many types of molecules to increase their
solubility. Examples include phenacenes97, polyphenylene vinylene, polythiophenes,
polyacetylenes, polyphenylethynylenes. Wirth reported that the solubilities of several

polyphenylene oligomers in toluene correlated with their melting points; the more

33
soluble a compound, the lower its melting point. These results inspired some of the

research done for this thesis, which contains a full analysis of the exact effects of side

chains of differing lengths on oligomers of various lengths.

One significant effect of adding side chains is a change in melting point. Heitz98

synthesized a series of mono-methyl substituted poly(p-phenylene) oligomers and

showed that the addition of the methyl groups decreased the melting point. Wirth15
reported a similar melting point decrease for 2,5-dimethyl substituted oligomers. (Table
2) Interestingly, the melting points for the tetramethyl-substituted oligomers were higher

than those for the dimethyl-substituted oligomers, demonstrating that perhaps symmetry

1
Table 2: Melting points of PPP oligomers 5

 

 

 

 

n 2 3 4 5
{<3} 71 °C 215 °c 320 °C 395 T
H
CH3

54 183 266 309
Hsc "
H3C CH3

137 272 272 N/A
H3C CH3

 

 

 

 

 

 

 

34

plays a role in the melting point.

A number of polymers have been studied to determine the effect of the degree of
polymerization and/or side chain substitution on the thermal transitions of the polymer.
Heitz characterized a series of 2- and 3-methyl substituted, exact-length oligomers,
(Figure 25) and found that the Differential Scanning Calorimetry (DSC) plots of
oligomers containing up to 6 rings showed only one transition, a crystalline-isotropic
transition. However, when the number of phenyl rings was increased from 6 to 8, simple
melting at 142 °C was replaced by a smectic liquid crystal (L.C.) phase between 273 and
311 °C. Increasing the chain to 10 rings resulted in a smectic phase from 242 to 260 °C
and a nematic phase at temperatures greater than 260 °C with no isotropic phase reported.
Finally, when the number of rings was increased to 12, the oligomer showed only one

transition to a nematic phase at 298 °C. Heitz also examined oligomers with one or two

Figure 25: Heitz’s methyl-substituted sexiphenyl

meta linkages, and these compounds did not show any ordered phases. This work
suggests that for an ordered phase to occur, a polyphenyl must be at least 8 rings in length
and be completely linear.

McCarthy, et al99 synthesized didodecyl-substituted PPPs with molecular
weights ranging from 8000 amu to 137,000 amu as determined by gel permeation

chromatography (polystyrene standards). Using DSC, they showed that polymers with

35
molecular weights less than 44,000 amu showed only one transition, polymers with

molecular weights from 44,000 to 73,000 showed two transitions, and the 137,000 amu
polymer showed only one. By comparing their DSC results to polarized optical
microscopy, these investigators showed that the lower molecular weight polymers did not
show any liquid crystallinity, the middle polymers showed an LC. phase and an isotropic
phase, and the highest molecular weight polymer showed only a transition to a liquid
crystalline phase. These studies showed that liquid crystal phases formed at length :
width ratios (axial ratio) of 6 or greater. They assigned the geometry of the lower
molecular weight polymers to be “starlike” and the longer polymers to be of the “hairy-
rod” type. The difference in geometry for these two polymer types was also shown in a
viscosity study. For the longer hairy rods, the steady shear viscosity drops upon
formation of the mesophase while the viscosity of the shorter polymers remains constant.

This shear thinning effect is well known for liquid crystalline polymers.

Rehahn, schlfiter and Wegner77 used Suzuki coupling to synthesize 2,5-
disubstituted polyphenylenes with side chains ranging from one to 16 carbons. The
polymers having at least a six-carbon side chain were completely soluble in hot toluene.
When examined by DSC, these polymers showed two transitions, which the authors
named T1 (lower temperature) and T2 (higher temperature). The T1 transitions ranged
from 60 °C for the longest chain to 80 °C for the shortest chain, while the T2 transitions
ranged from 160 °C to 280 °C. The authors attributed the TI transition to side chain
melting, while T2 corresponded to the transition to the isotropic melt (as seen by
polarized optical microscopy). A small change in T1 with increasing side chain length

and a large change in T2 indicates that the nature of the side chains has a significant effect

36
on the polymer properties. In this paper, the authors were not able to determine the

nature of these transitions, or identify the phases between T1 and T2.

II. RESULTS

Throughout the Results and Discussion sections, the following abbreviations

for oligomers are used. (Table 3)

Table 3: Abbreviations of oligomers used in the text

HOH
n

R

N o nngs (n

 

37

38
A. Synthesis

All oligomers in this project were synthesized by a combination of accelerated
and traditional Suzuki coupling reactions. (Figure 26) As stated in the introduction,

these reactions were chosen for their general applicability to a wide variety of coupling

 

 

Pd(PPh3)4
N32C03 ‘

Oar + OB<OH)2 PhCH3 ' traditional
24h
Pd(OAC)2
Na CO

OI + @4wa TZHF 3 ’ accelerate
2h

Figure 26: Traditional and accelerated Suzuki coupling reactions

reactions, and their proven reliability in the synthesis of polyphenylenes. We devised an
iterative scheme in which, once a library of starting materials was synthesized, the

oligomers could be pieced together rather quickly. A similar scheme has been recently

reported by Liess, et al.100 Figure 27 shows the synthesis of the first methyl-substituted
oligomers.

The boronic acid synthesis was optimized by using a Grignard reagent as the aryl
nucleophile and triisopropyl borate as the borate ester. When an aryllithium reagent was
used, residual alkyl groups were ofien seen in the 1H NMR spectrum. One possible
source of these peaks is a butyl benzene derivative, caused by nucleophilic attack on

butyl bromide that was formed after the lithium-halogen exchange reaction. When the

39

 

Br B(OH)
CH3 1. Mg 0H3
H3C 2. r3(ocr13)3
3. 14* H3C
3a 7a

B(OH)2

CH3 CH3
”Willem QC“ Pdwpha)‘ O O
Nach3

Toluene H30 H3C

 

V

9a
B(OH)2

CH3 CH3 CH3
3"” ...... O O O
Na2C03

Toluene H3C H30 H3C

ll

 

10a

Figure 27: Synthesis of the first oligomers

40
Grignard reagent was used, these impurities disappeared. By using triisopropyl borate,

we reduced the amount of borinic acid in the product and achieved a purer crude product.

Oligomers 9a and 10a were quickly synthesized and the problem of obtaining the
quaterphenyl and quinquephenyl oligomers was approached. Shorter oligomers need to
be halogenated so that they can be coupled with aryl boronic acids to make longer

oligomers. We initially attempted the direct solid state bromination of TMB and

HMT,101 but this reaction resulted in an inseparable mixture of products, including those
resulting from benzylic bromination. We developed a new scheme in which the

trimethylsilyl (TMS) group serves as a masked halogen. Figure 28 outlines the synthesis

 

 

CHa CHa CHa
Br 1. n—BuLi, -78°C TMS 1. Mg TMS
Br 2. TMS-Cl, ~78°C*RT Br 2. B(OCH3)3 (HO)zB
HS CH; 3. H H3
Ba ' 8a

Figure 28: Synthesis of TMS-functionalized materials

of these new TMS-functionalized starting materials. The TMS group does not interfere

with Suzuki couplings, and the bis(TMS)-oligophenyl products can be transformed into
dibromo- or diiodooligophenyls by reaction with bromine102 or iodine

monochloride/AgBF4,103 respectively. (Figure 29) It is common practice to quench
bromination or iodination reactions with Na2S2O3, but residual thiosulfate in the
recrystallized product repeatedly deactivated the Pd catalyst used for coupling. Stannous

chloride or an aqueous potassium hydroxide solution gave satisfactory results.

41

 

CH3 CH3 1C1 CH3 CH3
AgBF,

(CHslsSi 0 811cm). ————-———> 1 O O 1

H3O H3C H3C H3C

11a 133
CH3 CH3

Brz

- . O O ..

H3C H3C

Figure 29: Bromination and iodination of 4,4’-bis(TMS)-TMB

Once this problem was solved, we attempted to couple 8a and 4,4'-dibromo-
2,2',5,5'-tetramethylbiphenyl (A in Figure 30) to synthesize bis-(TMS)-
octamethquuaterphenyl using the traditional Suzuki coupling, and obtained a mixture of
at least seven products, with the major fraction (~20%) being the product of coupling at
one ring of the dibromobiphenyl (Product B in Figure 30). All components were in
solution after the 24 hour reaction time, so solubility is not a problem with this particular
reaction. Longer reaction times and higher temperatures did not result in appreciable
formation of product. We then synthesized 13a and attempted the coupling reaction
again, since aryl iodides are more reactive than aryl bromides in the Suzuki coupling
reaction. We again obtained a similar mixture of many products. It is unclear what
caused this reaction to fail, considering that the steric requirements are the same for all of
the couplings with methyl-substituted phenyl rings.

We finally synthesized OMQ using Novak’s accelerated Suzuki coupling reaction

using Pd(OAc)2 as the catalyst.50 As a test reaction, we attempted to synthesize 9a by
this method from the aryl bromide but found that this reaction can only be used with aryl

iodides since the bromides are more sensitive to steric hindrance causing a slower

42

3

CH3 CH3 CH CH3 CH3 CH3
Pd(OAc)2
H30 H30 H30 H3C H3C

H3C

A 7| 8

X=Brorl

Figure 30: Incomplete reaction in accelerated Suzuki coupling

oxidative addition step in the catalytic cycle. Novak recommends acetone as the solvent
of choice due to its polarity and miscibility with water. In a reaction of 13a and 7a, the
almost exclusive product was that from the first coupling reaction (Figure 30). This
intermediate is insoluble in acetone and accounts for failure of the reaction. The catalyst
was not soluble in toluene, but when we used tetrahydrofuran as the solvent, the reaction
yielded almost exclusively OMQ. We also synthesized DMQ by this method. Both
OMQ and DMQ precipitated from solution as they were formed, so these longer
oligomers could not be prepared by this method. These oligomers could not be purified
by recrystallization, distillation or conventional column chromatography; instead they
were purified in small amounts using preparatory thin layer chromatography (TLC). We
found that it is imperative that the starting materials are of the highest possible purity, as
any impurities were very difficult to remove, even using prep TLC plate.

After we established the synthetic methodology for the exact length methyl-
substituted oligomers, we moved to the ethyl-substituted oligomers. Unfortunately, 2,5-
dibromo-l,4-diethylbenzene is not commercially available for a reasonable price, so we
synthesized the ethyl-substituted starting materials from ethyl benzene (Figure 31). This
synthesis proceeds in about 63% overall yield for 3b, with the Clemmenson reduction

being the yield limiting step. However, the starting materials are relatively inexpensive

CH3COC| I Zn1Hgl .
AIC|3 HCI
CH2C|2

 

 

2b
TMS
1. n- BULi 1) Mg
_. 2 TMSC' 2) 13104903
CH2CI2 =
B 3)H30+
l'2
l CH2Cl2 ' 3101112
6b 8c

Br 1) Mg B(OH)2
fl 2) 8(0-i-Pr)3 h
3)H30§

31)

Figure 31: Synthesis of ethyl-substituted starting materials

 

and the reactions are easy to run on a large scale. From these starting materials, the
ethyl-substituted oligomers were synthesized in the same manner as the methyl-
substituted compounds, except that the shorter oligomers (TEB and HET) could not be
synthesized directly by coupling 6b and 7b. Instead, they were first synthesized and

purified as the bis(TMS) compounds, and then the TMS groups were removed with

trifluoroacetic acid.104 We did not investigate why the coupling failed, but it is likely
due to impure starting materials.

The hexyl-substituted bromobenzenes were synthesized using literature

procedureleS. We chose the hexyl side chain because a hexyl substituent is the smallest
one that will make a PPP completely soluble in toluene. We synthesized 2-bromo-1,4-di-
n-hexylbenzene (6c) in the same manner as 6a and 6b. However 1,4-di-n-hexyl-5-
trimethylsilyl-2-phenylboronic acid (8c), was much more difficult to synthesize and
purify than 8a and 8b. When we used the usual procedure, the yield of boronic acid was

very low. The main side product was 2-trimethylsilyl-1,4-dihexylbenzene, suggesting

44
that the Grignard reagent was formed and then quenched by a proton source at some

point in the reaction. We did not conduct any deuterium studies to determine if the
quenching occurred during the Grignard formation or during the boronic acid addition,
but since the use of a less bulky borate increased the yield of boronic acid, the borate
addition step seemed the likely culprit. Even ensuring that the Grignard step went to
completion and by using trimethylborate instead of triisopropyl borate, the highest yield
of boronic acid we obtained was ~60%. This product was purified by column
chromatography using toluene to elute the side products followed by diethylether to elute
the boronic acid. We also found that it was very important to work up the reaction
promptly after hydrolyzing the borate ester since the boronic acid moiety partially
hydrolyzed to a phenol after prolonged stirring in water.

We chose only to synthesize the odd-numbered oligomers so we could work up to
a long chain length quickly. We were able to synthesize iodinated hexylbenzenes with a
mixture of 12 and H5106 in acetic acid. We found that the best method to make the long
hexyl oligomers was to add biphenyl boronic acid units instead of phenyl boronic acids.
This eases the separation of incomplete oligomers from the desired product because the
longer the oligomers are, the more similar their properties are, making them more

difficult to separate. Figure 32 shows the synthetic scheme for these oligomers.

EoEew=e cow—5:33-35.— .«e «magnum "mm «Ema

 

 

m .0 _. H. C
a: .8 a: on .8 a:
oOmFI 00va @0me @0me 009—..— oOmrI QOQI 009
e mooez c
00000. .02....00.
~65ch
mrImu 9100 9.100 mFIwO vaoo va00 9100 9.1.00
ecN 9“"
M45 @091 oOmFI L... .m @091 mOmf
N mAmIOOvm .N
fovm 1 m
92 .F
mFImO mfoo 916 9100
a: 9m 2.. cm

mOmFI oOmf

mOONmZ w:
O O 1 O . O
226%.”. m «fovm

mme 9100 9100 918

 

46
B. Rotational Barriers

As mentioned in the introduction, the two rotational isomers of HMT (Figure 33)
are diastereomers, and therefore show different NMR spectra. Knowing the magnitude of
the barriers to rotation might be important for understanding why some substituted

polyphenylenes are soluble and some are not.

©©©~——-©©©

Figure 33: The two diastereomeric rotational states of HMT

We measured the barriers to rotation around the bonds connecting phenyl rings in
a series of oligomers that have side chains of different lengths. We also measured the
rotational barriers for oligomers with the same side chain length but differing in the
number of rings in the oligomer to see if increased conjugation affects the barrier. To
examine whether it was feasible to determine the barriers experimentally, we calculated
the barriers by using the program HyperChem in conjunction with Microsofi Excel.
(Figure 35) Table 3 outlines the macro used to run HyperChem through Excel.

We calculated the barriers by first defining the dihedral angle 6) (Figure 34)
between the two rings. While fixing the 4 carbons that define the angle, the geometry of
the rest of the molecule was optimized by a molecular mechanics algorithm (using MM+
force field) followed by a molecular dynamics scheme. The total energy of the molecule
was calculated, then the angle was reset and the steps are repeated. We ran the entire

cycle twice for each computation, once “forward” (from -180° to +180°) and once

47

“backward” (from +180° to -180°) because the calculated geometries just past the barrier
maximum were not fully relaxed. Although the angle we defined requires that the phenyl
rings be perfectly planar and this is not necessarily true, this calculation provided us with
a good first approximation of what the rotational barriers looked like. The HyperChem

calculation is intended as a rough indicator of the barriers to rotation, and does not take

R'®1 F! I? I? 91 F! I?
R. R. i? F? R

0° 180°

Figure 34: Dihedral angles for substituted terphenyls

into consideration all the factors necessary to do a complete computational assessment of
the compounds. The curve for HHT should not be asymmetrical, but due to the large
number of conformations available to a molecule with large alkyl side groups, the
program is not capable of producing completely reproducible results. We chose to
calculate the barriers for terphenyls, since we experimentally determined the barriers for
the same compounds. The plots for biphenyls produce artificially low barriers, since the
steric barrier can be significantly decreased by the alkyl groups moving away from the
other phenyl ring. Terphenyl does still not provide an entirely accurate picture, but it

provides a reasonable result in a short period of time.

48

60.

 

50 ._
40 -
3O 2

20~ ‘.

Energy (kcal/mol)

10-

0-

 

'10 1 %
-180 -120 -60 0 60 1120 180

Dihedral Angle (degrees)

 

 

 

Figure 35: HyperChem calculation of rotational barriers for substituted terphenyls

49
Table 4: Microsoft Excel macro for computing barriers with HyperChem

 

Control-R Compute.Results

Channel =OpenFile()

=lfllSERROR(Channel))

= RETURNO

=END.|F()

Command =EXECUTE®hanneL"Lselect-noner)
=WHILE(NOT(ISBLANK(SELECTION0)))
=EXECUTE(ChanneI,"[query-response-has-tag(no)]")
=EXECUTE(ChanneI,flselection-target atoms]")
=EXECUTE(ChanneI,"Eelect-atom 3 1]")
=EXECUTE(ChanneI,"[select-atom 4 1]“)
=EXECUTE(Channel,"[select-atom 7 1]")
=EXECUTE(ChanneI,"[select-atom 8 1]“)
=EXECUTEjChanneI,"Lset-bond-torsion("&SELECTION0&")]")
NewChan =EXECUTE(ChanneI,"[menu-select-select-all]")
=EXECUTE(ChanneI,"[un-select-atom 3 1]”;
=EXECUTE(ChanneI,"[un-select-atom 4 fl“)
=EXECUTELChannel,"QJn-select-atom 7 1]“)
=EXECUTE(Channel,“[un-select-atom 8 1]")
=EXECUTE(ChanneI,"[ca|culation-method molecular-
mechanics]")
=EXECUTE(Channel,"[dynamics-run-time 0.51“)
=EXECUTE(Channel,'[dynamics-time-step 0001]")
=EXECUTE(ChanneI,"[do-molecuIar-dynamics]")
=EXECUTE(Channel,"[ogim-a|gorithm fletcherreeves]')
=EXECUTE§ChanneL"[periodic-boundariesmon")
=EXECUTE(Channel,"[screen-refresh-period 1]")
=EXECUTE(ChanneI,"[optim-max-cycles 5001")
=EXECUTE(Channel,"[do-optimizatiomD
=FORMULA.ARRAY(REQUEST(Channe|,“total-energy"),"rcj1]")
=SELECT("r[1]c")
=EXECUTE(ChanneI,"[select-none]")

=NEXTQ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

=TERMINATE(ChanneI)

=RETURNO

OpenFlle

=INITIATE(”HyperChem","System")

=IF(ISERROR(NewChan))

lF(ISERROR(EXEC(”c:\chem\ship\chem“,1 )))
RETURN(NewChan)

END.IF()

RETURNUNITIATE("HyperChem",“System"))

=END.|F()

=RETURN(NewChan)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

50
The results of the preliminary calculations indicate that in all of the substituted

terphenyls, both steric barriers (where the rings are coplanar and the two alkyl groups
must pass by each other) are greater than the electronic barrier (where the rings are
orthogonal and the conjugation between the two rings is completely broken). However,
for terphenyl, the electronic barrier is greater than the steric barrier, since only two
hydrogen atoms have to pass by each other. Table 5 shows the calculated and

experimental values obtained for the series of terphenyls.

Table 5: Rotational barriers for alkyl-substituted terphenyls

 

 

 

 

 

 

 

 

 

 

Compound NMR HyperChem
(kcal/mol) (kcal/mol)
terphenyl not measured 6.9“
HMT 18.5 14.7”
HET 20.8 16.0”
HHT 21.4 18.7u
a. O = 90°
b. O = 0°

The calculated data show a big jump in the rotational barrier between the unsubstituted
and substituted terphenyls, but the effects of increasing the side chain length were minor.
To determine the values experimentally, we used a Varian VXR-500 spectrometer at 500
MHz to measure the coalescence temperature for the methyl resonances by taking 1H
NMR spectra at several temperatures. To ensure the reliability of the data, the samples
were equilibrated for 10 minutes at each temperature and the spectrometer was shimmed

and tuned before each spectrum was taken. Figure 36 shows spectra of HMT in the

51

 

 

 

 

 

 

 

 

 

MW
,1
70°C
2 10 2.00
ppm

Figure 36: Variable temperature NMR spectra of HMT

52
methyl region at various temperatures. The chemical shift associated with temperature

may be due to a change in the solvent density. The spectra shown in Figure 36 were
taken in toluene-d3 and the quintet at 2.09 ppm from residual toluene was used as a
reference peak. The coalescence temperature for HMT was 80 °C, which corresponds to
a rotational barrier of 18.5 kcal mol".

To determine if the barrier depended on chain length, we measured the barriers
for the entire series of methyl-substituted oligomers (Table 6). The barrier for TMB
cannot be measured because this molecule is symmetric and therefore the methyl groups
are magnetically equivalent. Bott measured the barrier for TMB using line shape analysis

at different temperatures of a biphenyl, using a prochiral group to monitor the bands.

Table 6: Rotational barriers for methyl-substituted oligomers

 

 

 

 

 

Compound Number of rings Rotational Barrier (kcal/moi)
TMB 2 19.484
HMT 3 18.5
OMQ 4 18.6
DMQ 5 18.3

 

 

 

 

 

It is fortunate that within experimental error (~0.3 kcal mol") the rotational barriers we
measured for all of the methyl-substituted oligomers are the same, and thus a given
oligomer in a series should be representative for all oligomers and polymers in that series.
We chose to examine the terphenyl oligomer from each series, since its spectrum is the
least complex and the chance for error is minimized. However, determining the

rotational barrier for an ethyl-substituted oligomer such as HET is not as simple as for

53

1 70°

1 30°

 

1.40 1.30 1.20 1.10

Figure 37: Variable temperature NMR spectra of HET in the methyl region

54

HMT. First, there are two alkyl sites that could be monitored, the methyl and the
methylene moieties, and secondly, the spectra are much more complicated due to the
coupling of these two groups. Figure 37 shows the variable temperature NMR spectra of
HET in the methyl region. It is difficult to determine the coalescence point visually,
since the coalescing peaks are triplets rather than singlets, and to our knowledge there are
no literature precedents for monitoring the coalescence of triplets. To simplify the
measurements, we employed a decoupling scheme in which the methylene protons were
irradiated so that the methyl protons appeared as singlets instead of triplets. We thus
determined that the coalescence temperature for HET is 130 °C in o-dichlorobenzene-d4.
To help confirm this result, we simulated the decoupled spectra using Microsoft Excel.
By using the center frequencies of the coalescing triplets and adjusting the line widths at
different temperatures, we achieved simulated spectra that closely approximate the actual
decoupled spectra. (Figure 38) Although we are confident that this method is valid for
measuring the rotational barrier of coupled systems, we further confirmed our result by
using an established method. A plot of the T1 spin-lattice relaxation time for an aromatic
proton against the temperature should change in slope at the point where the rings can
rotate freely. For the T1 measurements, the samples were deoxygenated by bubbling dry
nitrogen through the solution for at least 10 minutes immediately before the tube was
placed in the spectrometer. The samples were allowed to equilibrate for 10 minutes and
the spectrometer was shimmed and tuned as in the previous experiments. This
experiment was repeated several times, since there are many sources for error in a T1

measurement.

55

.5: cc 2.8% «22; Seesaw E; .25... "mm 8:3...—

 

 

 

ii:

5:
i
E
3..

Eng
0:. ONE. om... ow...

rlll.

600..

 

com _.

some

56

200

160

120

80

40

Temperature (°C)

Figure 39: Plot of T1 versus temperature for HET

57
Figure 39 shows the plot of T1 versus temperature. The trend lines are guides for the eye.

The slope changes at 120 °C, which is in good agreement with the results of the
coalescence experiment. Coalescence at 120 °C corresponds to a rotational barrier of

20.3 kcal mol”, while coalescence at 130 °C in o-dichlorobenzene, corresponds to a 20.8
kcal mol'1 barrier to rotation for HET. The rotational barrier for HHT, determined using
the same method, was 21.4 kcal mol", nearly the same as that of HET. Since an ethyl-
substituted PPP is only partially soluble in hot toluene, but a hexyl-substituted PPP is
completely soluble, we can conclude that rotational barriers do not significantly influence
the solubility of a substituted polymer. This topic of solubility will be discussed in

further detail in the discussion section.

C. Optical Properties

PPPs are often examined for use in optical devices such as Light Emitting Diodes
(LEDs) so it is important to understand how making these polymers soluble and
processible can affect the optical properties. We examined the solution and solid state
absorbance and fluorescence emission spectra of all oligomers synthesized. It is
important to note that the solutions used for all absorption spectra were 1 x 10‘4 M
solutions in cyclohexane. However, for clarity, the intensities of some fluorescence

spectra were normalized to account for the large differences in quantum yield.

1. UV-Vis Absorbance
Figure 40 illustrates the UV absorbance spectra for the methyl substituted

oligomers. These data show an approximately linear relationship between the number of

58
phenyl rings and the integrated intensity of the absorbance peak for the oligomer. DMQ

shows a slightly lower absorbance intensity due to the insolubility of the compound. By
examining the absorbance spectra, we can extract information about the order in the
system, which may be observed as line broadening or as a loss of structure in the spectra.
One possible source of disorder is the number of rotational isomers. The number of
rotational isomers possible increases geometrically with the number of phenyl rings in the
chain. Each phenyl ring can rotate in a positive or negative sense relative to an adjacent

ring, and since the calculated potential wells are symmetrical about O = 0, this rotation is

59

0.5 -

absorbance

 

. n=3
n=2
\
O \

I I I

210 230 250 270 290 310

wavelength (nm)

Figure 40: UV-Vis absorbance spectra of methyl-substituted oligomers

60
entirely random. Since these spectra behave as expected, showing an increase in the

absorbance and in Amax, we can conclude that the rotational isomers do not have a
significant affect on the order in the system.

The spectra of the ethyl-substituted oligomers also show an expected increase in
abosorbance intensity with an increasing number of phenyl rings (Figure 41). As the
number of rings increases, the structure of the absorbance bands becomes less distinct.
This could be caused by either the K band beginning to overtake the B band, or it could
be an effect of the increased disorder. Although the B band is not affected by increases in
conjugation and will not shift to lower energies with increasing oligomer length, it is
possible that the loss of structure is due to an increased disorder in the overall system, so
the band represents a combination of many differently configured benzene rings. By
comparing the spectra of the ethyl-substituted oligomers with those of the methyl-
substituted oligomers, the changes in the features are consistent with this hypothesis.

Figure 42 illustrates the spectra of the hexyl-substituted oligomers synthesized in
this study. These spectra cannot be directly compared to those of the ethyl- and methyl-
substituted oligomers, since the oligomer lengths are different, but the general trends can

be compared. The absorbance increases as expected with increasing oligomer length, and

61

absorbance

 

 

 

 

210 230 250 270 290 310

wavelength (nm)

Figure 41: UV absorbance spectra of ethyl-substituted oligomers

absorbance

 

62

 

n=7
1-
n=5
=3
0 I l 1 1‘ ‘i‘
210 230 250 270 290 310

wavelength (nm)

Figure 42: UV absorbance spectra of hexyl-substituted oligomers

63

absorbance

0 l I 1-- ‘ T I T
210 230 250' 270 290 310

wavelength (nm)

 

 

 

Figure 43: UV absorbance spectra of substituted terphenyls

64
the structure of the bands becomes less defined as there are more configurations available

to the molecule.

We also examined spectra that compare the different length side chains. (Figure
43) The spectrum of the unsubstituted terphenyl is obviously much different from that of
the substituted terphenyls. The K band completely obscures the B band, which can still
be seen in all the substituted terphenyl spectra. The K band is red-shifted because the
lack of ortho interactions allows the adjacent phenyl rings to be more nearly coplanar, so
that the degree of conjugation is greater than for the substituted terphenyls. By
extrapolating the absorption edge of the K band for each terphenyl, we can see more
clearly how the substitution affects the absorbance spectra. The absorption edges for
HMT, HET and HHT are ~270 nm,. indicating that the alkyl substituent does not have a
significant impact on the conjugation of an oligomer.

Another important piece of information we can extract from the absorbance
spectra is an idea of the effective conjugation length of a substituted PPP. The reported
km,“ of a dihexyl-substituted polymer is 318 nm. By plotting the energy of the absorption
edge versus l/n, where n is the number of phenyl rings, we can determine either the
effective conjugation length of a polymer, or predict the wavelength of the absorbance of
an unknown polymer. Table 7 lists the absorption edge values of the oligomers
synthesized and Figure 44 is a plot of their energy vs. 1/n. All the spectra taken in this

work are 10'5 M in cyclohexane, and the values (71mm) for the unsubstituted oligomers

were taken from the literature,106 which did not specify a solvent but the values seem to

be in accordance with our results.

65
Figure 44 provides information on how additional rings and different substituents

affect the spectra. The slope of the line indicates how each additional ring affects the
band gap of the oligomer, which reflects the amount of conjugation coplanarity of
adjacent groups. The y-intercept is the band gap for an infinite length oligomer. The
slope for PPP is steeper than those for the methyl, ethyl and hexyl derivatives, which all
have similar slopes. This is expected since the degree of conjugation is greater for an
unsubstituted PPP because of the smaller twist angle. The intercept for PPP is much
lower than for the substituted polyphenyls, as expected. The methyl-, ethyl- and hexyl-
substituted polyphenyls have similar plots. An increase in electron donation should result
in a decrease in band gap, or y-intercept. Therefore, we expect hexyl-substituted PPP to
have the lowest intercept, followed by the ethyl- and methyl-substituted PPPs. The
similarity between the three side chains indicates that inductive effects play a small role
in the band gap energy of polyphenyls. The diamond-shaped point on the y-axis
represents the reported value for 2,5-di-n-hexyl-PPP. This point does not correspond to
our data since Eg for this point is much lower than that predicted by Figure 44 for a 2,5-
di-n-hexyl substituted oligomer of infinite length. This reported value was probably
taken from a spectrum of a film of the polymer and thus cannot be compared with our

solution spectra.

Table 7: UV-vis absorption edge values for PPP oligomers

66

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

oligomer Max
Terphenyl 279 (Kmart)
Quaterphenyl 292 (kmax)
Quinquephenyl 299 (kmax)
Sexiphenyl 308 (lmax)
TMB 260.5
HMT 265.5
OMQ 267
DMQ 277.5
TEB 271
HET 270
OEQ 275.5
DEQ 276
HHT 268
DHQ 274.5
THS 276

 

 

67

 

 

6 . 0 methyl
I ethyl
5 5 q A hexyl
e R = H
A 5 -‘
> I
3 o 3
g; 45 A A t . .
E .
C: o
o 4 _ .
3.5 -
3 I l l
0 0.2 0.4 0.6
1/n

Figure 44: Plot of energy vs. oligomer length

68

2. Fluorescence emission

The emission spectra of the series of methyl-, ethyl-, and hexyl-substituted
oligomers are shown in Figures 45, 46 and 47. The spectra for the three sets of oligomers
follow the same trend, with the kmax values converging to their limiting values at 4-5

rings (Table 8). It is known that the fluorescence emission spectra of unsubstituted PPP
oligomers show a well-defined vibrational structure,107 presumably due to the quinoid

structure of the excited state.108 The study also showed that the emission bands shift to
longer wavelengths and show less structure with increasing chain length. Khanna’s data
suggest that the excited states of longer oligomers may be less planar in comparison to
smaller oligomers. The structureless shape of the emission bands shows that the structure
of the excited state of the molecule is not exactly planar, and this deviation from planarity
is probably caused by interaction of the side groups with each other. We did not attempt

to calculate any quantum yields for PPP oligomers.

Table 8: Fluorescence A..." values for PPP oligomers

 

 

 

 

 

 

 

 

 

 

 

 

 

Oligomer 3...,“
TMB 303
HMT 3 l 6
OMQ 327
DMQ 329
TEB 302
HET 3 15
OEQ 330
DEQ 332
DHB 289
HHT 322
DHQ 333
THS 336

 

 

 

 

normalized emission

69

 

 

 

 

%‘ =2 Aex=274 nm
\
280 330 380

wavelength(nm)

Figure 45: Fluorescence spectra of methyl-substituted oligomers

 

normalized emission

70

 

 

 

 

 

280 330 380
wavelength (nm)

Figure 46: Fluorescence spectra of ethyl-substituted oligomers

normalized emission

71

 

n=7

 

 

 

 

 

l

280 330 380 430

wavelength (nm)

Figure 47: Fluorescence spectra of hexyl-substituted oligomers

 

72
3. Solid state spectra

The solid state UV absorbance and fluorescence emission spectra of the ethyl-substituted
oligomers are shown in Figures 47 and 48. These films were created either from the melt
or from a solution cast from toluene, and the film thicknesses were not measured. The
same spectrum resulted regardless of film preparation method. Because of the imprecise
methods used to create the films, all the spectra are normalized. We did not examine the
methyl-substituted oligomers because all the films we created were too crystalline to
obtain a reasonable spectrum. Aside from OEQ, all the spectra look very similar to the
solution state spectra, indicating that the conformations in the solid state and in solution

are similar. These results will be analyzed in further detail in the discussion section.

73

 

 

 

 

 

if

Q

3

3

§

3 “ HET

l5.

3 TEB

l T M.
200 250 300 350

Figure 48: Solid state UV absorbance spectra of ethyl-substituted oligomers

74

 

9,8,:274 nm

normalized emission

 

 

 

 

 

280 330 380 430
wavelength (nm)

Figure 49: Solid state fluorescence emission of ethyl-substituted oligomers

75
D. Thermal Properties

As described in the introduction for alkyl-substituted PPPs, DSC and polarized
optical microscopy are useful tools for characterization of the solid state properties of
materials. Shown in Figures 50-54 are the DSC results for HMT, OMQ, OEQ, DHQ and
THS. Positive deflections from the baseline (endothermic) correspond to melting
temperatures, while negative deflections (exothermic) indicate crystallization or similar
disorder-order transitions. For all compounds studied, the phase transitions detected by
DSC were simple melting or crystallization events. Parallel observations using optical
microscopy confirmed the assignments and also showed that none of the compounds
formed thermotropic liquid crystalline phases.

We determined that all of the methyl-substituted oligomers are crystalline
compounds. The DSC plot of HMT is typical, showing a melting peak on the heating
curve at ~l85 °C, and crystallization on cooling at ~90 °C. Note that there is a large
hysteresis, which is characteristic of simple melting and crystallization. In contrast,
liquid crystalline transitions generally show small degrees of supercooling. We

confirmed these transitions by optical microscopy under crossed polarizers.

Heat Flow (mW)

70

65.

60.l

55 4

50

45:

40

35 -7

30

‘+ 4. . _ L __

76

heating

 

 

WV
I
cooling
50 100 1 50 200 250
Temperature °C

Figure 50: DSC plot of HMT

Heat Flow (mW)

 

01
0'1

01
O

77

heating

\

 

 

a— 7 a——— ~— .# - 7 T # —1>-— - -—- - 17*w 77.——

f /

cooling

1

50 100 150 200 250 300
Temperature (°C)

Figure 51: DSC plot of OMQ

78

70

65 heating
60
55

cooling
50

45

Heat Flow (mW)

40
35

30 A. A- 4.

0 -50 0 50 10 150
Temperature (°C)

0 T—' . ;._ ...-LL______--L___L v E. -..L. _. _-L_____ m
1
1
1
1
1
1
1

Figure 52: DSC plot of second heating and cooling scans of OEQ

79

70
65

60

55 heating

50 . \
45
4o 1 f

35

Heat Flow (mW)

30 1 - , -. L, .4. . .
-100 -50 0 50 100 150
Temperature (°C)

Figure 53: DSC plot of second heating and cooling scans of DHQ

80

70

65 1
60 1
55 1

50.

Heat Flow (mW)

45.
401
35‘

30 . 7
-100 0 100 200
Temperature (°C)

Figure 54: DSC plot of the second heating and cooling scans of THS

8 1
The DSC results show systematic trends in the melting point behavior. As shown

in Table 9, adding side groups of increasing length leads to progressively lower melting
points. The effect is dramatic. For example, adding hexyl groups to quinquephenyl
lowers the melting from 395 to 97. Within a series with identical side chains, the melting
point increases with the number of rings, as expected. TEB and HET were isolated as
oils and HET crystallized over the course of several months. DEQ decomposes at 235 °C
before melting. All of the members of this series show weak transitions in DSC
measurements. OEQ has particularly interesting thermal properties. OEQ forms a clear

glass that is stable for days. DSC scans of the OEQ glass show no first order thermal

 

 

 

 

 

 

 

transitions.
Table 9: Melting points of PPP oligomers
R\n 2 3 4 5 7
H 71 °C 215 °C 320 °C 395 °C ---
CH3 54 l 83 266 309 ---
C2H5 oil 60-61 1 10 235 --
C6H13 '1 oil 43 --- 97 140

 

 

 

 

 

Of the hexyl-substituted oligomers, DHB and HHT were isolated as oils, and
DHQ and THS as white powders. The DSC plot of DHQ (Figure 53) shows a broad
transition around 85 °C upon heating which we attributed to melting and confirmed by
optical microscopy. The reason that this value is different from the one in the Table is

probably because that value was taken using a melting point apparatus at a much slower

 

82

rate. There is also a small transition at about 15 °C that could be a sofiening or glass-
transition-like event. Since we did not do any microscopy below room temperature, we
could not confirm this transition. Upon cooling, this compound shows a transition at
about 32 °C which is probably due to crystallization. THS, however, displays a DSC plot
that is similar to that of OEQ, in that there are no discemable transitions.

Mechanical measurements can be sensitive indicators of weak thermal transitions
and relaxation phenomena in polymers. In many cases, these transitions are too weak to
be detected by DSC. In Dynamical Mechanical Analysis (DMA) measurements, an
oscillatory load is applied to the sample, and the in phase and out of phase components of
the response are measured. The mechanical response of glassy materials is primarily in
phase with the applied stress (elastic) while that of rubbery materials is generally out of
phase with the load (lossy). Typically, the data are reported in terms of E' and E", the
dynamic elastic and loss moduli. Carrying out DMA measurements on polymers as a
function of temperature maps thermally activated transitions such as molecular rotations
and glass transition temperatures, which show up as peaks in the E" or tan 6 spectrum.
The tan 6 spectrum is a plot of E"/E' versus temperature. DMA measurements of glassy
OEQ (Figure 55) show transitions at 15 and 30 °C presumably due to the onset of
disordering and ring rotation respectively. Neither transition is seen in the DSC scans.

OEQ also shows morphology dependent fluorescence behavior. As shown in
Figure 56, freshly prepared glassy films show a distinct 2-peak fluorescence spectrum.
With time, the spectrum evolves toward the solution phase results, a single peak centered
near 330 nm. Inspection of the aged film shows it is crystalline, and thus the shift in the

fluorescence spectrum must be associated with changes in the conformation of OEQ on

83
crystallization. Variable temperature measurements on glassy films also point to

structure-dependent fluorescence. As shown in Figures 57 and 58, the fluorescence peak
intensity decreases with temperature, with an abrupt change in slope near 30 °C. We
believe these data confirm our assignment of the 30 °C transition to ring librations, or a
change in the arrangement of the molecules, since changes in fluorescence for the

oligomers should be associated with changes the planarity of adjacent rings.

tan 5

84

 

1

 

J ikl

 

 

f .

-50 -40 -3

0-20 -10 0 10 20 30 40 50 60 70

Temperature (°C)

Figure 55: DMA plot of OEQ

 

normalized emissio

85

 

 

aged 5 months

/

   
  
   
   

freshly prepared film

/

 

    
 

l l
I l

330 380 430 480
wavelength (nm)

Figure 56: Solid state fluorescence of OEQ

86
A final experiment in this vein is again related to the aged solid state fluorescence

sample. When observing this sample under the microscope after aging, it appeared
crystalline with spherulite-like regions. By doing a more careful study, we see that the
sample begins to show crystallinity after one day, forming a solid phase reminiscent of a
nematic liquid crystal. We are attempting to monitor this crystallization by optical
microscopy by using a silicon photodiode detector in the camera mount. This detector is
read by a Hewlett-Packard multimeter, which is interfaced with a computer. The
program Instrument Basic can be used to record voltage measurements from the
multimeter at timed intervals. The voltage recorded reflects the amount of light passing
through the sample. We will analyze these results more thoroughly in the discussion

section.

87

 

 

 

 

emission intensity

 

 

 

 

290 340 390 440 490

 

wavelength (nm)

 

 

Figure 57: Variable temperature fluorescence spectra of OEQ

88

Emission Intensity

 

0 '1 fl ’ i ' _'T'— i —— 1— g' '* 2T2, ”m —1

0 20 40 60 80
Temperature (°C)

Figure 58: Plot of emission intensity vs. temperature for VT fluorescence of OEQ

89
E. Self-assembled monolayers

This section presents the preliminary results obtained while exploring PPP
oligomers for use as self-assembled monolayers (SAMs). This project was approached
from two angles: 1) The ability of PPP oligomers to form a SAM, and 2) The use of PPPs
in a SAM as a non-linear optical device. We wished to study monolayers on a silicon

surface, which has a native oxide coating of about 20 A. The hydroxyl groups on the

oxide surface can be coupled to triethoxysilyl terminated PPP oligomers.109’1 10 (Figure
59) To utilize this chemistry, we synthesized PPP oligomers that were functionalized

with a bromine atom at one end of the oligomer. This bromide was converted to a

triethoxysilyl group by the Barbier-Grignard reaction.111 (Figure 60)

We developed synthetic routes to functionalized PPP oligomers for thermally
stable SAMs, and nonlinear optical (NLO) devices. The synthetic schemes for the NLO
chromophore will be presented in the Discussion section, since they are more
appropriately classified as “fliture work”. We first attempted to characterize monolayers
on Si wafers by ellipsometry and reflectance Fourier transform infrared spectroscopy
(FT-IR). Neither technique gave satisfactory results, probably because the monolayers
we are examining are estimated to be on the order of 3-9 A thick, while those commonly
studied by these methods in the literature are at least ~20 A thick. The ellipsometry
measurements were not reproducible from sample to sample, and the thicknesses did not

increase in a linear fashion as would be expected for increasing oligomer length. There

90

Surface group
Alkyl group

lnterchain van der Waals and : :
electrostatic interaction

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Surface-active headgrou Chemisorption
pL\“’ é at the surface
§ 11 E
/ [F E
g M ’ 11 E
/ Self-Assembly (R E
/
¢ 11 E
11 E
2

 

Figure 59: Structure of a self-assembled monolayer

91

 

R R R R
Pd(PPh3)4
(OH)2 ... Br TMS v‘ 0 O TMS
N82C03
R R R R
Br2
R MeOH

TMS <
Pd(PPh3)4 ”ASHE
R R R Na2CO3

 

 

 

 

Biz
MeOH
11
R R R R R R
1. Mg
000* poems
2. Si(OEt)4
R R R R R R
H
'OHor
R R R
—0
/e000
u / '
SlOz /—O
/—OH R R R
/

 

 

 

Figure 60: Synthetic scheme for SAMs

92

are three possible explanations for this result. One possibility is that the monolayers were
not stable and desorbed before the measurements were taken. The second possibility is
that the oligomers are not all oriented perpendicular to the silicon surface but some or all
of them are lying parallel to the surface. It is also possible that the surface attachment
was inefficient, resulting in a low surface density of organic molecules. Some suggested
improvements to this experiment will be explored in the Discussion section.

The reflectance FT-IR experiments were also unsuccessful. We determined that

this method of examining the monolayers, while an excellent tool for long alkyl

SAMS,10921 12'1 15 is not appropriate for such short oligomers. The amount of signal
that arises from the alkyl groups in our monolayers is too small in comparison to the
extraneous hydrocarbons in the instrument to be detected.

Perhaps a more appropriate method of conducting preliminary studies on these
monolayers is to form them on substrates with much larger surface area, such as fumed
silica. F umed silica contains 2 mmol of hydroxyl groups per gram, so that even if a small
percentage of the hydroxyl groups react, there will still be a perceptible amount of

monolayer present. We did not attempt to measure the amount of monolayer on the

silica. By following the method of Hou116, we attached benzene and the methyl
substituted benzene, biphenyl and terphenyl oligomers to fumed silica. FTIR spectra of
pressed pellets of the functionalized silica gel are shown in Figure 61. As the oligomers
increase in length, the intensities of the alkyl peaks in the spectrum increase relative to

the peaks arising from the silica.

93

 

r 'T "T' ‘7 ——" '_ T— 7—‘—1__'_—_‘ "1‘ "“"T

4000 3500 3000 2500 2000 1500

waven um bers (cm '1)

Figure 61: F T-IR spectra of SAMs on A200 fumed silica

94

 

100 1 \
99 i .
98 .
97 1
,,\° 1
u 1
c 1
g 96 .,
g 1
95 1
1
1
94 i
931 --. 4 — ‘

benzene
DMB

TMB

HMT

100 200 300 400 500 600 700 800 900

Tem peratu re (°C)

Figure 62: TGA plots of SAMs on silica

95
Figure 62 shows the thermogravimetric analysis (TGA) plots of the monolayers

on fumed silica. The samples were dried at 150 °C at < 100 mtorr for at least 12 h before
TGA scans were taken. Once the samples were placed in the TGA sample pan, they were
heated at 110 °C under nitrogen for about 30 minutes. These plots show the expected
increase in weight loss with increasing percentage of organic material. These scans were
run under nitrogen, so it is possible that cross-linking reactions may have occurred.
There seem to be two periods of weight loss for the oligomers, the first is a gentle slope
from ~200-400 °C, and the second is the steep slope occurring after 400 °C. Alkylated
and unalkylated oligomers behave similarly upon heating in the TGA analyzer. We did
not conduct an analysis of the gas emitted from the burning sample. A useful experiment
to help identify the source of the two weight loss periods would be to remove the sample

from the pan and examine the FTIR spectrum afier each period.

III. DISCUSSION

From the data presented in the Results section, we learned that PPP oligomers are
reasonable models for PPP. For substituted oligomers, the conjugation lengths are
relatively short, so by examining a series of oligomers we can draw conclusions about the
effects of these substituents on a polymer. The important issues to be addressed are the
solubility and processibility of the polymers and the band gap, both of which are
important for application of PPPs in devices.

A. Electronic Properties of PPPs

PPPs are commonly explored for use in LEDs, so it is important that a polymer or
oligomer should emit light at the desired wavelength, and be processible and oxidatively
stable. Substituents alter the electronic properties of the oligomer or polymer by severely
decreasing the conjugation between phenyl rings because of the increased twist angle
between the rings. The decreased conjugation can be observed as an increase in the band
gap (Eg) of the molecule, which results in a blue shifi in the absorbance and emission
spectra. There is also evidence for some inductive effect, especially for strongly
electron-donating or electron-withdrawing groups, but in dialkyl-substituted PPPs and

oligomers, the twist angle dominates the band gap. We can determine which effect

96

97
dominates by examining the plot of E vs. 1/n (Figure 44). The trendlines for all the

substituted oligomers are nearly parallel, indicating that the effect of adding another ring

is equivalent for an oligomer with any alkyl substituent.

B. Band Gap Engineering

Adding substituents to a PPP can also be used to “tune” a molecule to absorb or emit
light at a given wavelength. The solution UV absorbance spectra show a blue shifl of
about 40 nm when two alkyl substituents are added to each ring of a terphenyl. For

devices such as light emitting diodes which must emit light in the visible range of the

spectrum, this is a disadvantage, but there are reports1 17‘] 19 of PPPs substituted with
alkoxy groups which decrease the band gap and allow a blue-violet emission from a solid
film.

The effective conjugation length is often defined as the polymer length at which
the optical properties converge to their limiting value, but we assume from the plot of E
vs. l/n that E continues to decrease with increasing chain length and does not actually
reach a limiting value. However, for our purposes, we define effective conjugation
length to be the oligomer length at which the band gap does not increase enough by
adding another phenyl ring to observe a change in properties (i.e. for use in a light
emitting diode if the luminescence of a polymer asymptotically approaches 430 nm and
the luminescence of a decamer is 420 nm, we can say that the effective conjugation
length of the polymer is 10 units, because the human eye cannot distinguish between 420

nm and 430 nm wavelength light). The effective conjugation length of an unsubstituted

PPP has been estimated to be ~20 ringle6, by extrapolating from the Km,“ values in the

98
UV absorbance spectra. However, since it is unknown if a film of PPP has a degree of

polymerization higher than 20, this number may not be valid. The longest oligomer
measured, p-sexiphenyl, showed a longest wavelength absorption at 308 nm in solution
and 345 nm in the solid state. We estimate the effective conjugation length in a dialkyl-
substituted PPP to be 6-7 rings, increasing the band gap of the polymer, so there is an
obvious decrease in the desirable electronic properties found in unsubstituted PPP. It
seems that the solubility and low band gaps are inversely related, but a number of
investigators have devised means to retain conjugation while increasing the solubility and
processibility of a polymer. One method is to place substituents on selected rings instead

of every ring, thus imparting less solubility but retaining more conjugation. In an early

study, Wirthlzoexamined a series of quaterphenyls and quinquephenyls with varying
side chains on only the terminal rings. A table from this paper is reproduced here (Table
10). We can see that the addition of only one ring has a huge effect on the solubility in
toluene and melting points of these oligomers, but these oligomers are much more
processible than the unsubstituted oligomers and their optical properties remain virtually
unchanged. This study also showed that an increase in length of a linear side chain has
an optimum value, n-butyl in this case. To further increase the solubility of the
quinquephenyl derivative, the authors used long branched alkyl groups to obtain

solubilities of over 500 g/L with a 9-heptadecyl substituent. This strategy is similar to

that employed by Gorman, Ginsburg and Grubbs121 in their synthesis of soluble

polyacetylenes (Figure 63)

99

Table 10: Solubilities and melting points for substituted PPP oligomers

R

R

R

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solubility Melting Solubility Melting
R

[g/L] [mMol/L] point (°C) [g/L] [mMol/L] point (°C)
H 0.12 0.39 320 <0.005 <0.013 395
Methyl 3.7 11.1 213 0.01 0.24 313
Ethyl 7.5 20.7 194 0.18 0.41 291
n-Butyl 43 103 165 0.48 0.97 268
n-Hexyl 46 97 157 0.52 0.94 259
n-Octyl 48 90 150 0.55 0.91 253
(PAS). The polymers are synthesized by ring opening metathesis polymerization of

cyclooctatetraene (COT) with one substituent on the COT ring.

By altering that

substituent, the authors were able to tune the properties of the polymer, eventually

obtaining a soluble PA with electronic properties very similar to the unsubstituted PA.

..

catalyst
————>

R

/ / / /

n

Figure 63: Grubbs’s synthesis of soluble polyacetylene

 

100

 

 

lol

. 71 ‘7
(h)
(C)
112.7
id)
1: 1,1
lel
1.7 : i
2.7 : l

 

' 2601 280' 360 '
Wavelength in nm

 

Figure 63: UV absorbance spectra of Rehahn’s c0polymers. The feed

ratios are listed on the right.

 

101
Using a similar idea, Rehahn, et al.77 synthesized PPPs by introducing varying ratios of

monomers with and without side chains. They saw an expected increase in the
wavelength of the lowest energy band with an increasing fraction of unsubstituted
monomers in the feed. (Figure 64)

Several groups have synthesized PPPs with planarizing moieties connecting

adjacent phenyl rings and forcing them into a planar conformation. These polymers,

nicknamed “ladder polymers” (Figure 65) such as Grimme, et al.'s106 substituted
fluorene-based polymer, can be solubilized by placing alkyl groups on either the phenyl
rings or the planarizing groups. These polymers retain the conjugation present in an

unsubstituted PPP, but they are soluble, processible and can be characterized by NMR

and gel permeation chromatography (GPC). In fact, Lamba and Tour122 calculated the
twist angle between phenyl rings for their imine-bridged ladder polymer to be less than

1°.

 

 

 

 

Figure 65: Soluble ladder polymers

To summarize, if a certain conjugation length is desired, there are several options
for tuning the band gap of PPPs and similar rigid rod polymers. A substitution on every

ring provides the most processibility, but causes the greatest decrease in conjugation. By

102
synthesizing a molecule with a solubilizing group on only some rings, we can increase

the conjugation but retain solubility. Placing alkyl groups on only the ends of molecules
also increases solubility but with retention of the optical properties of an unsubstituted
PPP. However, this strategy is only feasible for oligomers of less than 5 or 6 phenyl
rings. Finally, substituted ladder polymers offer the most conjugation, some even higher
than unsubstituted PPP, while retaining processibility.

Thermal stability is important in electronic applications such as coatings for

integrated circuit devices because these devices often run at high temperatures. PPP is

stable up to ~660-675 °C in nitrogen and about 400 °C in air.10 When substituents are
placed on the rings, the stability decreases somewhat because of the reactivity of the alkyl
groups. Oligomers are not suitable for these applications since they tend to sublime at
fairly low temperatures even though the molecular structure is still intact. Their volatility
is however useful for preparing LEDs and organic transistors. Common thermally

induced side reactions include cross-linking and bond cleavage.

C. Solubility And Crystallinity

To completely understand how side chains impart solubility to PPPs, we must
first understand why unsubstituted PPPs are insoluble. Polyphenylene is insoluble due to
its high heat of fusion, rigid-rod geometry, large aspect ratio, and contributions from 112-112
interactions. The rigid rod geometry of PPP means that there are fewer conformations
available to the polymer chains, thus the molecules can easily pack together. Once the

rods are aligned, the energy to “pry” them apart is much greater than for flexible

103
polymers, since the rigid rod structure requires all the monomers to be separated at once,

rather than sequentially. The TH: intermolecular attraction and strong polarizability of
PPP rods also increases the energy needed to separate the polymer chains. This concept
can be illustrated by comparing polyphenylene to polystyrene. (Figure 66) Atactic
polystyrene is more soluble than isotactic polystyrene because of its random coil
structure. Isotactic polystyrene, a poorly soluble polymer, has a more regular structure
and readily crystallizes. By studying how those properties that make polyphenylene
insoluble can be disrupted, we can understand what makes a hexyl-substituted
polyphenylene soluble. Several types of disruptions are possible: 1) twisting between

rings, 2) varying the barrier to rotation around bonds connecting rings, 3) increasing the

 

 

 

 

/

 

 

 

 

 

 

1

Atactic polystyrene (random coil) Polyphenylene (rigid rod)

Figure 66: Comparison of a random coil polymer with a rigid-rod polymer

entropy by introducing functionality on the rings, or 4) simply sterically blocking
interactions between the polymer chains by the side groups. In each case, we assume that
factors that lead to inefficient packing also lead to increased solubility.
1. Increased ring twist

The increased twisting between rings induced by ortho interactions of the alkyl

chains causes polymer chains to be less planar, making it more difficult for the individual

104

polymer molecules to pack in geometries that have appreciable n-overlap with adjacent

chains. (Figure 67) According to literature4 and our HyperChem calculations, an
unsubstituted PPP has a dihedral angle of ~23° in the gaseous (and solution) state and
~10° in the solid state. A dialkyl substituted PPP has a dihedral angle of ~45° in the
gaseous/solution state. It is fairly certain that this increase in ring twist causes some
degree of increased solubility in the polymers, especially since the ring rotation is entirely
random. Each ring may twist in either of two directions with respect to the previous ring
creating many different diastereomeric polymers, analogous to the polystyrene example.
In Figure 67, the picture on the lefi represents an alkyl-substituted PPP, the straight lines
indicating a side view of a planar benzene ring, and the picture on the right represents an
unsubstituted PPP, having a dihedral angle of 23°. The substituted PPP, having the larger
dihedral angle, has increasingly poor 7r-1t overlap between each polymer chain than the
PPP with a smaller dihedral angle. However, this phenomenon alone does not account
for the increase in solubility between the three types of substituted oligomers synthesized

in this research and the unsubstituted PPP and its oligomers (Table l 1).

 

><
><

 

><><><

Figure 67: Side view of PPPs with different dihedral angles a. 45° b. 23°

105

Table 11: Solubility in toluene of substituted quinquephenyls

 

 

 

 

 

 

Oligomer Solubility (g/L) Solubility (moi/L)
Quinquephenyl 0.005 1.6 x 10"
DMQ 0.240 0.0005
DEQ 66 0.10
DHQ 116 0.09

 

 

 

 

2. Rotational Barriers

Rotational barriers have the possibility of offering insight to the mechanism of
crystallization. (Figure 68) Since molecules in the same conformation pack together and
crystallize more easily, it is possible that the barrier to rotation around the phenyl-phenyl
bond can be a limiting step to crystallization and an important factor in determining the
solubility of a PPP. The larger the barrier, the lower the likelihood that the molecules can
adopt the same conformation and crystallize. However, our measurements of the actual
barriers showed that there was a significant difference between the methyl-substituted
oligomer and the ethyl-substituted oligomers, but a negligible difference between the
ethyl- and hexyl-substituted oligomers. Remembering that an ethyl-substituted PPP is
insoluble while a hexyl-substituted PPP is completely soluble in hot toluene, the
magnitude of the barrier cannot alone explain solubility. It may, however, be responsible
for the slow crystallization rates of many oligomers.

As implied above, the oligomers with longer side chains are less crystalline than
those with short or no side chains. While it is probably accurate to generalize that

oligomers disubstituted with short side chains are more crystalline than oligomers

106

 

rot

 

 

Figure 68: Rotational barriers of substituted PPPs

 

107

disubstituted with longer side chains, we believe that there are other factors that
contribute to crystallinity. First, by examining the DSC scans of THS and DHQ, we can
see that DHQ shows apparent melting and crystallization peaks, while THS does not,
displaying a plot very similar to that of OEQ. As noted earlier OEQ typically forms a
glass at room temperature and crystallizes very slowly. To measure the rate of
crystallization of OEQ, we equipped a microscope with a silicon photodiode detector and
a hot stage to detect crystal formation. With the sample under crossed polarizers,
crystallization causes a greater fraction of the viewable area to become birefiingent, and
more light is allowed through to the detector. Figure 69 shows the results from two trials
of the crystallization of OEQ. By examining the plot, we see that it takes approximately

two days for the entire viewable area to become crystalline. For HMT, the viewable area

0'12 1 trial 2
0115 * tnal1

0.11 -
0.105 ,

0.1 .
0.095 .
0.09 1
0.085 .

0.08 “A-__._ , , ._ j
0 1440 2880 4320 5760

minutes

volts

 

 

Figure 69: Measurement of crystallization rate of OEQ

108
becomes crystalline in a matter of minutes or seconds, depending on the crystallization

temperature. The increase in birefringence is consistent with a model of constant linear
growth from a nucleation site. Since one must know the number of sites and the optical
constants for the crystals, we did not attempt to fit the data. HMT crystallized as

spherulites (Figure 70), while OEQ crystallizes with a nematic-like structure. (Figure 71)

 

 

 

 

 

Figure 70: Photograph of crystalline HET under crossed polarizers

110

 

 

 

 

 

 

Figure 71: Photograph of crystalline OEQ under crossed polarizers

111
3. Increased entropy of oligomers

In agreement with Goldfinger, et al.123, it is possible that the increase in entropy
caused by the addition of longer side chains is the driving force for solubilization. Figure
72 illustrates the possible fates of a crystal. By measuring AHfus, we can calculate AS for
the transition from the equation AG = AH - T AS . If AS is small, indicating that the
average geometry of the crystal is similar to the average geometry of the solvated
molecule, then AHfus is also small. The overall energy of dissolution can be reduced to
AGmix when AHfus is zero. The reduction in 112-1t interactions between polymer chains that
is induced by long side chains allows the polymers to adopt geometries similar to their
solution geometries. This is shown by the data presented in Rehahn’s paper.77 This
paper demonstrates that by increasing the side chain length on polymers of the same
molecular weight, the first thermal transition, attributed to side chain melting, decreases
by only 20 °C while the second transition, attributed to the transition into the isotropic

melt, decreases by 120 °C with increasing side chain length. (Figure 73)

112

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

féyrgts)’ AH,“ > M elt AGmix = Solution
ICPAT Cp'AT le'A
Crystal Glass Solution
(RT) (RT) (RT)
AG

 

Figure 71: Schematic energy diagram for dissolution of PPPs

 

Side chain:
C6
C7

 

 

I I I I fi
100 0 100 200 300
Temperature in °C

 

 

 

Figure 72: DSC curves of substituted PPP derivatives

1 13
4. Steric blocking

Finally, the increase in solubility could be due to a simple steric interaction or
blocking between the main chains of the polymer. This is probably the case in soluble
polyacetylenes which have 01, 0) tert-butyl groups. (Figure 74) This type of steric

blocking is unlikely to play a role in substituted PPPs.

X

Figure 74: Polyacetylenes solubilized by terminal t-butyl groups

Actually, the increase in solubility is probably due to a combination of all of these
theories. The solubility of a molecule or polymer can be related to its crystallinity within

a series. The dissolution of a solid in a solvent is equal to the energy of melting followed

by the energy of mixing for the two liquids,124 so assuming the AGmix values are similar,
the solubility of these oligomers can be directly related to their melting points. The
melting points of the oligomers we synthesized decrease with increasing side chain
length, and increase with increasing main chain length and their solubilities increase
accordingly, in agreement with the thermodynamic arguments presented in the previous
section.

It is our belief that a combination of the factors listed above contribute to the
increased solubility of the PPPs substituted with longer chains. The chains often “get in

the way” of packing by moving around and not allowing the main chains to come close

114
enough together to crystallize. The longer side chains also increase the entropy of the

polymer while decreasing its heat of fusion per gram. The increase in twist angle
between rings and the increase in rotational barrier prevents the main chains from being

in a common configuration, thus impeding crystallization.

D. Effect of side chains on morphology

McCarthy, et al.99 examined the morphology of polymer films prepared from the
liquid crystalline melt or by solution casting, and found that regardless of preparation
method, all the films of the high molecular weight 2,5-didodecyl-substituted PPP showed

some level of orientation as determined by X-ray diffraction. They showed that there is a

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 75: Sandwich type morphology of 2,5-didodecyl PPP (reproduced from

99
McCarthy, et al. )

115
strong tendency for the side chains to segregate from the main chains, forming a

sandwich type structure. (Figure 75) The layer spacing showed a linear decrease with
increasing temperature, probably due to ordering of the side chains. At lower
temperatures there are defects present in the side chain packing as seen by the weak,
diffuse reflections seen in the diffraction pattern.

From these results, we can propose a crystalline structure for the ethyl-substituted
PPP oligomers that accounts for the solution-like fluorescence of the films and the slow
crystallization. Based on space-filling models, there are two likely packing arrangements
for 2,5-dialkyl substituted PPPs. The first, a raft-like arrangement, is analogous to that
proposed by McCarthy. The second (Figure 76) stacks molecules in a staggered
arrangement. The second arrangement would be more favorable for PPPs with short side
chains. In both cases the rings are nearly orthogonal to each other. If McCarthy’s
structure were also true for our oligomers, then we would have observed a red shift in the
solid film fluorescence spectra, but instead the fluorescence results point to a twist angle
between the rings of 50-60°, similar to the solution conformation. The solid state spectra
of OEQ support this theory, since as the molecule crystallizes, the fluorescence kmax

undergoes a blue shift. The raft-like arrangement allows for easy solution of the

oligomers and may be the main source of solubility.

116

 

End-on view Side view

Figure 76: HyperChem depiction of proposed packing structure for OEQ

1 17
E. Design Rules

By examining what we have learned about the effects of substituents on PPP
oligomers and how they may apply in general to rigid-rod or disc-like molecules, we can
describe a set of “design rules” to help predict the properties of PPPs. The rules are
based on the conjugation length desired, how the side chains will affect the properties of
the molecule, and what role the aspect ratio plays. These “design rules” will enable us to
design molecules for a specific purpose, and they are derived from a combination of our
results and those found in the literature.

The side chains on a rigid rod molecule affect more than just the solubility, as
they can also impart other properties to the molecules such as liquid crystallinity and
chirality and they can determine the degree of crystallinity in a molecule. In examining
our oligophenylenes by optical microscopy, we observed great differences in the degree
of crystallinity and the rate of crystallization in oligomers that were the same length but

had differing side chains. We have not yet identified a liquid crystalline phase in any of

our oligomers, which is reasonable since McCarthy et al.99 reported that the minimum

axial ratio (lengthzwidth) for a main chain liquid crystalline polymer is about 6, while we

calculated the axial ratios for our oligomers to be around 2. Witteler, et al.78 also noted
that disubstituted PPPs showed liquid crystalline behavior only above a certain molecular

weight (about 40,000). For methyl substituted oligomers, a chain length of 8 rings is

necessary for liquid crystal formation.98
The conjugation length of an oligomer or polymer can be controlled by properly
spacing solubilizing groups along the backbone of the molecule. Our results show that

while the solubility and processibility of a substituted PPP oligomer can be greatly

118
increased by the addition of side chains, the electronic properties do not vary significantly

in response to PPP length or side chain length. Therefore the band gap can only be
adjusted by changing the number of contiguous unsubstituted phenyl rings in the
backbone, or by placing electron-donating or withdrawing groups on the phenyl rings.

To obtain a polymer or oligomer with properties appropriate for some application
or study, these “design rules” can help predeterrnine a starting point for optimization of
the desired property. A combination of the right molecular weight (or chain length),
conjugation length and side chains can provide a large range of molecules that are

suitable for study or application.

F. Suggestions for Future Work
1. Synthetic Methodology

An important topic that we did not explore in this research was why some of the
coupling reactions were successful and others were not. Determining the reason for this
apparent discrepancy would be a valuable contribution to the scientific literature. It is
possible that these reactions can indeed be completed, but for some reason, such as an
impurity in one of the starting materials, they did not work through our attempts. The
synthesis of OMQ is particularly puzzling since we could only synthesize this compound
through Novak’s accelerated Suzuki coupling and not through the traditional Suzuki
coupling. The steric constraints are the same for both OMQ and TMB and the electronic
effects of the side groups should be similar for both reactions.

If experimental errors such as impurities in the starting materials can be ruled out,

then the difference in reactivity can probably be attributed to the differences in reaction

119
between a dibromobenzene and a dibromobiphenyl. A simple mechanistic study could

help determine the cause for this difference. The first approach should be to determine
all products formed in the reaction of the purified starting materials. As stated in the
results section, the major product isolated in this study was 4-bromo-
hexamethylterphenyl, so perhaps there is an electronic reason for the inability of the Pd
catalyst to perform the oxidative addition step twice. It is also possible that the reaction
simply proceeds at a much slower rate than for dibromobenzenes. This hypothesis can be
tested by simply taking samples from a refluxing solution of the dibromobiphenyl and
Pd(PPh3)4 to monitor the progress of the oxidative addition by 1H NMR.

It seems likely that this experiment will provide an answer to the problem of

synthesizing OMQ, since it has been reported125 that the rate determining step for the
Pd(PPh3)4 coupling of an aryl bromide is the oxidative addition step, while for an aryl
iodide it is the transmetallation step. This study also noted that differences in boronic
acids synthesized in different batches, containing different amounts of trace impurities,
showed an effect on the rate. In this research, we also noted that the greater the steric
hindrance in the boronic acid, the more carefully it needed to be purified in order for the
reaction to work. Thus a scheme in which the boronic acid is rigorously purified should

be adapted for all compounds to ensure consistent results.

2. Self-Assembled Monolayers
The results section outlined the progress to date on the study of PPP oligomers as
self-assembled monolayers (SAMs). One problem in the characterization of these

monolayers on Si wafers was obtaining reproducible ellipsometry data, which could be

120
linked to the way they were prepared. Since we proved that the monolayers formed on

fumed silica, monolayers on Si wafers could be prepared in the same manner, using a
small amount of triethyl amine to initiate the reaction. After the reaction is complete, the
monolayer should be rinsed thoroughly with an organic solvent to remove any excess
triethoxysilane, and then heated under vacuum to promote polymerization of the
siloxanes. This procedure should ensure a more stable monolayer that will provide

reproducible ellipsometry results

N(CH3)2

O O O
OZN

Figure 77: Proposed NLO chromophore

One particularly interesting application of these monolayers is their use in a
nonlinear optical (NLO) device. For a molecule to exhibit second order NLO activity, it
must be noncentrosymmetric and have a permanent dipole, so a nitroaniline derivative
was selected. (Figure 77) The synthesis of this chromophore is quite challenging since it
requires incorporation of three functional groups, and all the molecules must be identical,
i.e. the nitro group and the amino group cannot be interchanged. We attempted two
synthetic schemes that were unsuccessfiil before devising a final scheme that is likely to
succeed. (Figure 78) 2-Amino-4—bromo-5-nitrobenzoic acid is a known compound, so
the first five steps in the synthesis are not likely to introduce any major problems in this

synthetic route. The methylation of the amine in the presence of a carboxylic acid has

121

NC: ~02 N02

NH N ‘
2 3’2 _ H2 1) NaNOz, H230, ‘ C”
T 2) CuCN '
B e

 

 

 

 

 

H2804,H20
180°C
”02 HN03,l-12SO4 N02
C02 140°C ’Jij/COZH
B B
No2
1)NH40H.sealednbe
2)1-1+
NH: B(OH)2
N(CHs)2
c H NC
02 c1131 _ 00211 O (”3’2
3 MeOH/NaOH ‘ = O O CO2H
B Pd(PPh3)4
N02 N02 02M
Bra
HgO
PhN02

 

N(CHs)2 mgsnOSKORh N(CH3)2
.—.—.45110R)3 < ”an
02N

 

Pd(PPhah

Figure 78: Proposed synthetic route to NLO chromophore

122

111)
—CO

000
/\

 

SH

Figure 79: PPP oligomer functionalized with a cross-linking group

also been cited in the literature, and the following Suzuki coupling should also not pose
any problems since the reaction is tolerant to a number of functional groups. The
conversion of a benzoic acid to an aryl bromide is a questionable reaction, but we tested
this reaction on a model compound and the amount of product detected in the reaction
mixture was appreciable (< 50%), and the product should be easily separated from the
solvent and side products by distillation.Unfortunately, the most worrisome step is the
final Stille coupling reaction. We have not had much success with the Stille coupling in
our lab, but by testing various reaction conditions on model compounds, the reaction
should be optimized and yield the desired NLO chromophore.

Another application of SAMs of PPP oligomers is as a thermally stable monolayer
coating. This can be accomplished by simply incorporating a cross-linkable unit into the
oligomer structure. An example of such a molecule is shown in Figure 79. This
molecule contains a reactive o-quinodimethane functionality that will rapidly react with

an adjacent unfunctionalized oligomer to link the two together by a six-membered ring.

123
(Figure 80) This cross-linking will thermally stabilize the monolayer, preventing

desorption and breakdown of the oligomer structure. This particular monomer is
terminated with a thiol as opposed to a siloxane, since this monolayer is designed to form
on gold instead of silicon, but the scheme is still valid for the silicon monolayers. An
alternative cross-linking scheme is to synthesize monolayers functionalized with

methoxymethyl groups, which when treated with acid, lose methanol and form a benzylic

X
30 l He
0113
3C I If,
Cl’b
H

CHzOCI'b H‘

1‘bOCHQ

9000

/'\

 

Figure 81: Chemical cross-linking of PPP oligomers

X

C 0

C113
0 heat
——-——>

C O
CHa

H

Figure 80: Thermal cross-linking of PPP oligomers

 

124

 

I H

0129,09 - CH30H

0
cruqcm

I
\OO
—£!.>.
/

 

————§

 

 

 

 

~—

 

 

 

125
cation which can add to a neighboring oligomer via electrophilic aromatic substitution.

(Figure 81) This cross-linking scheme is likely to be “messier”, but the oligomers will be

easier to synthesize.

3. Crystallinity and Thermal Transitions

We began some interesting work on the relationship between the oligomer length,
identity of the side chains and the crystallinity and thermal transitions of these oligomers.
Completing this study will help round out the design rules discussed above, and provide a
valuable contribution to the scientific literature on this topic. The first project that should
be completed is to determine the identity of the two transitions in the DMA scan of OEQ.
It would also be useful to examine DMA plots of some other oligomers to look for
similarities and differences in the plots such as THS, which shows a similar DSC plot to
OEQ, and also oligomers such as HMT which have well defined and characterized
crystallization schemes. Assuming that the two transitions are due to the main chain and
the side chains respectively, there are two experiments that can confirm or deny this
hypothesis. First, transitions in the main chain should be evident in the optical spectrum
either as a shift of km” or as a change in intensity, so continuing variable temperature
fluorescence experiments similar to those described in the Results section will help us
determine if one of the transitions is due to movement of the main chain. Second, solid
state NMR experiments may be able to tell us something about the side chain movement.
Preliminary results showed that the ethyl group have a strong signal in the CP/MAS
NMR spectrum, and side chain movement should be manifested as either a change in the

line shape or intensity, or in the T. spin lattice relaxation time of either the protons or the

126
carbons in the ethyl groups of OEQ. We have attempted these experiments, but we have

not yet been able to find a definite, reproducible result yet. Some experiments with
model compounds may be helpful in determining the correct procedures for these tests. It

is reported that oligophenyls have unusually long relaxation times (biphenyl is reported to .

have a T1 value of 910 $1126), so it is possible that some of the difficulties encountered in
this experiment were due to this unusual characteristic. Substituted oligophenyls have a
much shorter T1 time, because the relatively efficient motions of an amorphous or less

crystalline compound shorten the relaxation time.

E. Summary

This thesis describes the synthesis and characterization of a series of exact-length
dialkyl substituted PPP oligomers. We synthesized the oligomers through an iterative
approach using a combination of traditional and accelerated Suzuki coupling reactions.
By determining the barrier to rotation around the single bonds connecting phenyl rings,
we realized that twist angles between rings and rotational barriers are not the sole causes
for an observed increase in solubility. We theorize that the increase in solubility is due to
a combination of the twist angle, rotational barrier, increase in entropy from the longer
side chains, and the more amorphous state of functionalized oligomers.

We also examined the optical properties of these oligomers to determine if they
have any usefulness in devices and how the side chains affect the optical and electronic
properties of the oligomers and related polymers. We estimate the effective conjugation

length to be about 5-6 rings, which is much shorter than that reported for unsubstituted

127
PPPs. There is very little difference between oligomers of the same length with different

side chains (except H).

By analyzing these results, we can propose a set of “design rules” that can be used
to design appropriate molecules with desired properties. The main considerations are
chain length (or molecular weight), conjugation length, and the nature of the side chains,
if any. Once the crystallization data is complete, one will be able to define the molecular
properties even more specifically.

Although the oligomers synthesized in this study are not suitable for use in
organic LEDs, they have a number of potential applications aside from being models for
a polymer. We began the investigation of PPP oligomers as self-assembled monolayers
and propose their use in a nonlinear optical device or as a thermally stable coating on

silicon.

128

IV. EXPERIMENTAL

General: Toluene, tetrahydrofuran (THF), methanol and carbon tetrachloride were
purchased from Mallinckrodt; dichloromethane was purchased from EM Science and
diethylether was purchased from CCI, Inc. All solvents were used as received except
THF and toluene, which were dried and deoxegenated by distillation first from CaH2 then
from sodium benzophenone ketyl. Deionized water was deoxygenated by bubbling
nitrogen through it for at least 2 h. Magnesium, stannous chloride, acetyl chloride, mossy
zinc and mercuric chloride were purchased from Mallinckrodt and used as received
excepted acetyl chloride, which was distilled under nitrogen before each use. Palladium
acetate, tetrakis(triphenylphosphine) palladium, triisopropyl borate, trimethyl borate, n-
butyllithiurn, trimethylsilyl chloride, ethylbenzene, 2,5-dibromo-p-xylene, silver
tetrafluoroborate, and iodine monochloride were purchased from Aldrich Chemical
Company and used as received. Bromine was purchased from Fisher Scientific.
Aluminum chloride and concentrated hydrochloric acid were purchased from EM
Science. p-Xylene and sodium bicarbonate were purchased from Baker. Magnesium
sulfate and sodium carbonate were purchased from CCI, Inc. Reactions requiring inert
conditions were conducted under argon or nitrogen. Preparatory thin layer

chromatography (TLC) plates were 1000 pm thick silica gel with fluorescent indicator

129
(silica gel GF) purchased from Analtech, Inc. All reactions were stirred magnetically

miless otherwise indicated. UV absorption spectra were taken using a Unicarn
Spectrophotometer and fluorescence emission spectra were taken using a Hitachi F -4500
Fluorimeter. The solutions were 1x10“1 M in spectrophotometric grade cyclohexane
(Spectrum Chemical) for the fluorescence experiments and 1 x 10'5 M in
spectrophotometric grade cyclohexane for the UV absorption experiments. Fluorescence
emission spectra were taken by exciting the solution at 274 nm and recording the
emission spectrum from 275 to 500 nm. Routine 1H and 13C spectra were taken at 300
MHz and 75.43 MHz respectively, using either a Varian VXR-300 NMR Spectrometer or
a Varian-Gemini NMR Spectrometer. NMR data are reported in parts per million (ppm).
1H and ’3 C spectra taken in CDCl3 (Isotec, Inc.) are referenced to residual CHC13 at 7.24
or 77.0 ppm, respectively. The reported melting points are uncorrected, and were
determined by either optical microscopy (by observing the point at which the sample is
no longer birefringent under crossed polarizers) or using an Electrotherrnal Melting point
apparatus. Melting points for boronic acids were not taken because they are

irreproducible due to dehydration reactions that occur during heating.

Dynamic NMR Experiments: Dynamic 1H NMR experiments were conducted at 500
‘MHz using a Varian (V XR-500) NMR Spectrometer with the temperature controlled
using an FTS Systems air-jet. VTNMR experiments for ethyl and hexyl- substituted
oligomers were conducted in o-dichlorobenzene-d4 (Aldrich) from 20 to 140 °C and
experiments for methyl-substituted oligomers were taken in toluene-d3 (Cambridge

Isotope Laboratories) from 20 to 80 °C. The methyl groups on the ethyl- and hexyl-

130
substituted oligomers were decoupled during the variable temperature experiments. The

barriers to rotation were calculated by observing spectra at various temperatures to

determine the coalescence temperature and then applying the Gutowsky-Holm

approximation. 30.3 1

Optical Microscopy: All optical microscopy was conducted on a Nikon microscope
equipped with crossed-polarizers and a photomicrographic attachment. The sample
temperature was controlled by a Mettler FP82HT hot stage which was controlled by a

Mettler FP90 central processor.

Solid State Fluorescence Experiments: Solid state fluorescence spectra were obtained
from films of oligomers on 2 cm by 1 cm quartz slides. The films were created by two
methods. In the first method, a small amount of powdered oligomer was placed between
two slides and the compound was heated to 20 degrees above the melting point. After
holding at that temperature for one minute, the films were flash cooled by placing the
slide into a dewar filled with liquid nitrogen. Flash cooling prevented crystallization of
the compounds and minimized light scattering from the sample. Compounds that
decomposed at their melting points were spin cast from a concentrated toluene solution.
The slides were placed in the fluorimeter at approximately 45° to the incident beam.

For the variable temperature fluorescence experiments, the samples were prepared
in the same manner and stored in liquid nitrogen until the measurements were taken. The
temperature was varied using a home-built sample holder. The copper holder had a

window sized for the sample, and was equipped with a heater and thermocouple. The

131
sample was cooled by running dry nitrogen gas through a copper heat exchanger, and

then into the sample holder through a tunnel in the holder. The gas was vented into the
sample chamber to purge the chamber of air and to prevent condensation on the sample.
Each sample was allowed to equilibrate at each temperature for 10 minutes before a

spectrum was taken.

Solid State UV Absorbance Experiments: For solution spectra, the samples were
prepared as described above. For solid state spectra, the samples were prepared as for the
fluorescence experiments. The spectra were taken by scanning air as the background,
then scanning a blank quartz slide, then scanning the sample. The quartz absorbance was
manually subtracted from the sample spectrum. The samples were placed in the cuvette

holder at a 45° angle to the incident beam.

4-Ethylacetophenone(1). Compound 1 was prepared according to literature

procedurele7 from 20.0 g (0.149 moi) of ethylbenzene, 15.7 g (0.118 mol) of AlCl3,
and 8.15 g (0.104 mol) of acetyl chloride. The product was purified by vacuum
distillation (bp 70-78 °C @ 360 mtorr) (lit.128 bp 116-117 °C @ 130 mtorr) to yield 13.8
g (99%) as a clear, colorless oil.

‘H NMR: 5 7.90 (d, 2H), 7.30 (d, 2H), 2.70 (quartet, 2H), 2.60 (s, 3H), 1.25 (t, 3H).

1,4-Diethylbenzene (2b). The synthesis of 2b was adapted from Read and Woodl29
using 5.00 g (0.034 mol) of l and the following workup. The reaction mixture was held

at reflux temperature and was stirred with a mechanical stirrer for 24 h. After cooling to

132
room temperature, the reaction mixture was poured into a separatory funnel, diethyl ether

was added and the layers were separated. The aqueous layer was extracted with diethyl
ether (3x25 mL) and the combined organic layers were washed with saturated NaHCO3
until the washings were neutral to litmus paper. The organic layers were dried (MgSO4),

filtered and concentrated. The yellow oil was purified by vacuum distillation (34 °C @

640 mtorr) (lit.130 bp 181-182 °C) to yield 3.20 g (71%) as a clear colorless oil.

‘H NMR: 8 7.10 (s, 4H), 2.60 (quartet, 4H), 1.20 (t, 6H).

p-Di-n-hmlbenzene (2c). This compound was synthesized according to the literature
procedure105 bp 117 °C @ 146 mtorr (lit.105 bp 134 °C @ 99 mtorr)

‘H NMR: 8 7.08 (s, 4H), 2.55 (t, 4H), 1.60 (m, 4H), 1.30 (m, 12H), 0.78 (t, 6H).

2,5-Dibromo-I,4-diethylbenzene (4b). The synthesis of compound 4b was adapted from

Rehahn, et al.105 To a 500 mL three-necked round-bottomed flask equipped with an
addition funnel and an outlet to a KOHM) trap were added 20.0 g (0.149 mol) of 2b and
50 mL of methylene chloride. The apparatus was rigorously shielded from light and
cooled to 0 °C. In the dark, 52.5 g (0.328 mol) of bromine in 50 mL of methylene
chloride were added to the addition funnel. The bromine solution was added dropwise to
the reaction mixture over 20 minutes and allowed to warm slowly to room temperature.
After 36 h, an additional 23.0 g of Br2 in 25 mL of methylene chloride were added and
the mixture was allowed to stir for an additional day. With the reaction still protected
from light, 100 mL of an aqueous KOH (20% w/w) solution were added and the reaction

was stirred until no orange color remained. The light yellow solution was poured into a

133
separatory funnel and the layers were separated. The aqueous layer was extracted with

CH2Cl2 (3x40 mL) and the combined organic layers were washed once with water, dried
(MgSO4), filtered and concentrated to yield a yellow oil which crystallized from a small
amount of absolute ethanol to yield 21.2 g (49%) as colorless needles. mp 30-32 °C.
(lit.131 mp 33-35 °C)

'H NMR: 6 7.35 (s, 2H), 2.70 (quartet, 4H), 1.20 (t, 6H)

2,5-Dibromo-I,4-di-n-hexylbenzene (4c). Compound 4c was synthesized from 20.0 g
(0.081 mol) of 2c and 52.0 g (0.325 mol) of bromine in the same manner as 4b. The
crude product was recrystallized from ethanol to yield 29.1 g (89%) as a white powder.
mp 4243 °C (lit.105 mp 33 °C)

1H NMR: 5 7.34 (s, 2H), 2.62 (t, 4H), 1.60 (m, 4H), 1.30 (m, 14H), 0.90 (m, 6H)

13C NMR: 6 144.3, 133.7, 123.1, 35.5, 31.6, 29.8, 29.0, 22.6, 14.1.

2-Bromo-I,4-diethylbenzene (3b). The synthesis of compound 8b from 11.1 g (82.9
mmol) of diethylbenzene is identical to that of 4b, except that only 1.2 equivalents of
bromine were used. More bromine (12.3 g) was added after 36 h to compensate for
evaporation of bromine through the outlet to the KOH trap. The product was purified by
vacuum distillation (bp 90-100 °C @ 384 mtorr) to afford 16.1 g (91%) as a clear
colorless liquid.

1H NMR: 8 7.35 (s, 1H), 7.15 (d, 1H), 7.05 (d, 1H), 2.7 (quartet, 2H), 2.6 (quartet, 2H),
1.2 (t, 6H).

13C NMR: 6 143.6, 133.0, 131.9, 129.3, 127.0, 124.1, 28.9, 28.0, 15.4, 14.3.

134
HRMS: calc. for C10H13Br 212.0201, found 212.0201

Alternate Synthesis for Monobromination of Aromatics:132 a) 2-Bromo-I,4-
diethylbenzene (3b): An ice-chilled solution of bromine (4.78 g, 29.9 mmol) in 5 mL of
DMF, prepared by adding the bromine dropwise to DMF in a jacketed pressure-
equalizing addition funnel, was added dropwise to a light-protected ice-chilled solution of
diethylbenzene (1.00 g, 7.26 mmol) in 10 mL of DMF. After the addition was complete,
the reaction mixture was allowed to stir for an additional 2 h. The mixture was quickly
poured into an iced solution of Na2SO3 (l9g/L) and extracted with pentane. The
combined organic layers were dried over MgSO4, filtered and concentrated to yield 1.23

g (80.4%) of a yellow oil. The product can be vacuum distilled as above.

b) 2-Bromo-I,4—di-n-he.1qylbenzene (3c):This compound was synthesized from 10.00 g
(41.0 mmol) of di-n-hexylbenzene and 26.0 g (163 mmol) of bromine. The reaction was

monitored by 1H NMR and was stirred overnight at room temperature. The product was
distilled under vacutun (bp 128 °C @ 83 mtorr, lit.7bp 158-161 °C @ 10 mtorr) to yield
12.5 g (94%) as a colorless oil.

1H NMR: 8 7.34 (s, l H), 7.10 (d, 1H), 7.00 (d, 1H), 2.65 (t, 2H), 2.50 (t, 2H), 1.60 (m,

4H), 1.30 (m, 12H), 0.90 (m, 6H).

2-Bromo-p-xylene (3a). The synthesis of compound 3a from 100 g (0.943 mol) of p-

xylene and 62.4 g (0.391 mol) of bromine is identical to that of 3b except that p-xylene

135
was used as the solvent. The crude product was purified by distillation (bp 195-201 °C)

(lit.133bp 203-204 °C) to yield 49.9 g (69%) as a colorless liquid.

'H NMR: 6 7.40 (s, 1H), 7.12 (d, 1H), 7.00 (d, 1H), 2.38 (s, 3H), 2.30 (s, 3H).

2,5-Diiodo-I,4-diethylbenzene (5b). Compound 5b was synthesized according to the
literature procedure[Suzuki, 1971 #111] from 10.0 g (74.6 mmol) of 2b, 6.80 g (29.9
mmol) of H5106 and 15.2 g (59.7 mmol) of iodine. The crude product was purified by
two recrystallizations from acetone to yield 18.6 g (64%) as white needles. mp 70-71 °C.
‘H NMR: 6 7.60 (s, 2H), 2.60 (quartet, 4H), 1.15 (t, 6H)

13C NMR: 6 145.8, 138.6, 100.3, 33.1, 14.4.

2,5-Diiodo-I,4-di-n-hexylbenzene (5c). Compound 5c was synthesized from 5.00 g
(20.3 mmol) of 2c, 1.85 g (8.13 mmol) of H5106, and 4.10 g (16.2 mmol) of iodine to
yield 3.12 g (31%) as white needles. mp 53-54 °C

1H NMR 6 7.60 (s,2H), 2.60 (t, 4H), 1.50 (m, 4H), 1.30 (m, 12H), 0.90 (t, 6H)

13C NMR 6 144.7, 139.2, 100.4, 39.8, 31.6, 30.1, 29.0, 22.6, 14.1.

General Procedure for 2-bromo-I,4-dialkyl-5(trimethylsilyl)benzene: a) 2-Bromo-5-
(trimethylsib'D-p-rvlene (6a). To a 500 mL round bottomed flask fitted with a Schlenk
vacuum adapter were added 30.0 g (0.114 mol) of 2,5-dibromo-p-xylene (4a). The flask
was placed under an argon atmosphere and 80 mL of tetrahydrofuran were added. After
cooling the solution in a dry ice/acetone bath, 107 mL (1.60 M, 0.171 mol) of n-BuLi

were added dropwise via a syringe. After stirring for 4 h, 24.7 g (0.227 mol) of

136
trimethylsilyl chloride were added dropwise via a syringe and the mixture was allowed to

warm to room temperature. A white precipitate formed (probably LiCl) which dissolved
when 75 mL of water were added to the reaction. The layers were separated and the
aqueous layer was extracted with diethylether (3 x 50 mL). The combined organic layers
were dried over MgSO4, filtered and concentrated to yield a yellow oil. The oil was
distilled (bp 95-105 °C @ 520 mtorr) to afford 27.9 g (95%) as a clear colorless oil.

1H NMR: 6 7.32 (s, 1H), 7.24 (s, 1H), 2.37 (s, 3H), 2.34 (s, 3H), 0.30 (s, 9H)

13C NMR: 6 142.6, 137.5, 136.6, 133.9, 133.3, 126.1, 22.3, 22.0, -0.3.

HRMS calc. for C. 1H17BrSi 258.0283, found 258.0259

2-Bromo-1,4-diethyl-5-(trimethylsilyl)benzene (6b). The synthesis of this compound
from 10.0 g (34.2 mmol) of 4b is as described for 6a. The crude product was purified by
vacuum distillation (bp 90 °C @ 335 mtorr) to yield 7.71 g (79%) as a clear colorless oil.
1H NMR: 6 7.35 (s, 1H), 7.25 (s, 1H), 2.70 (m, 4H), 1.20 (m, 6H), 0.30 (s, 9H)

13C NMR: 6 149.2, 137.2, 135.5, 134.0, 132.0, 125.9, 29.0, 28.2, 16.2, 14.5, 0.3.

HRMS calc. for C13H21BrSi 284.0598, found 284.0600

2-Bromo-1,4-di-n-hexyl-S-(trimethylsilyl)benzene (6c). Compound 6c was synthesized
from 10.0 g (24.8 mmol) of 4c. The crude product was purified by vacuum distillation
(bp 175 °C @ 60 mtorr) to yield 9.30 g (94%) as a clear colorless viscous oil.

1H NMR: 6 7.35 (s, 1H), 7.23 (s, 1H), 2.65 (m, 4H), 1.60 (m, 4H), 1.37 (m, 12H), 0.90 (t,

6H), 0.30 (s, 9H)

137
13C NMR: 6 148.0, 138.3, 137.1, 136.2, 132.5, 125.9, 35.8, 35.6, 32.4, 31.8, 31.7, 30.1,

29.6, 29.2, 22.7, 22.6, 14.1, 0.4

HRMS: calc. for C21H37BrSi 398.1830, found 398.1830

General Procedure for replacing trimethylsilyl (TMS) group with bromine a) 4-
Bromo-2,2',5,5 '-tetramethylbiphenyl ([90): Compound 19a was synthesized from 10.0 g
(35.4 mmol) of 4-TMS-2,2’,5,5’-tetramethy1bipheny1 and 6.80 g (42.5 mmol) of bromine
using a procedure was adapted from Walker, et. (21.102 which uses methanol as the
solvent. We used a mixture of dichloromethane/methanol to increase the solubility of the
starting material. The TMS-terminated compound was dissolved in the minimum amount
of dichloromethane at 0°C and then the required amount of methanol was added.
Compound 19a was isolated as a light yellow oil which crystallized from ethanol to yield
8.00 g (78%) as white needles. mp 34-34.5 °C

lH NMR: 6 7.40 (s, 1H), 7.15 (d, 1H), 7.05 (d, 1H), 6.95 (s, 1H), 6.86 (s, 1H), 2.37 (s,
3H), 2.31 (s, 3H), 2.00 (s, 6H)

13C NMR: 6 141.0, 140.3, 135.3, 135.0, 134.7, 133.2, 132.5, 131.5, 129.8, 129.7, 128.0,
123.1, 22.3, 20.9, 19.3, 19.0.

HRMS: calc. for CmHnBr 290.0495, found 290.0488

b) 4-Bromo-2,2 ',5,5 '-tetrahe.xylbiphenyl (19c): This compound was synthesized from
2.50 g (4.45 mmol) of 18c and 0.850 g (5.34 mmol) of bromine to yield 2.52 g (100%) of
the product as a light yellow oil. This product was used directly without further

purification.

138
‘H NMR: 6 7.40 (s, 1H), 7.15 (d, 1H), 7.08 (dd, 1H), 6.93 (s, 1H), 6.86 (d, 1H), 2.60 (m,

4H), 2.25 (m, 4H), 1.60 (m, 4H), 1.20 (m, 28H), 0.80 (m, 12H)
1“(2 NMR: 6140.5, 1402,1397, 139.6, 138.5,137.7, 132.5, 131.6, 129.7, 128.7, 127.4,
122.8, 35.6, 35.4, 32.7, 32.6, 31.7, 31.6, 31.5, 31.4, 31.0, 30.7, 30.0, 29.2, 29.1, 29.0,
22.6, 22.5, 14.1, 14.0

HRMS: calc. for C36H57Br 570.3630, found 570.3622

c) 4-Brorno-2,2 ',2 ",5,5 ',5 "-hexamethylterphenyl (22a): This compound was
synthesized from 5.00 g (12.9 mmol) of 21a to yield 4.21 g (83%) as a white powder. mp
182.5-183.5 °C

1H NMR: 67.45, s, 1H; 67.15, d, 1H; 67.05, m, 2H; 66.95, m, 3H; 62.39, s, 3H; 62.35, s,
3H; 62.03, m, 12H

13C NMR: 6141.8, 141.2, 139.1, 135.4, 134.8, 133.2, 132.6, 132.5, 131.8, 131.7, 130.7,

130.4, 130.1, 130.0, 129.6, 121.8, 123.1, 22.3, 22.2, 20.9,, 19.4, 19.3, 19.2, 19.0

General procedure for replacing T MS with iodine: a) 4,4 '-Diiodo-2,2 ',5,5 '-
tetramethylbiphenyl (13a). This procedure was adapted from Jacob, et al. 103 with the
following modifications: To a 100 mL round-bottomed flask were added 0.210 g (0.565
mmol) of 11a, 0.144 g (0.739 mmol) of AgBF4, 50 mL of methanol and 20 mL of
methylene chloride. The solution was cooled to 0°C and a solution of ICl (0.101 g, 0.622
mmol) in methanol (0.54 mL) was prepared and added dropwise to the reaction mixture.
This solution was allowed to warm to room temperature and stirring was continued

overnight. The reaction was quenched using 30 mL of a solution (20% w/w) of SnClz in

139
methanol and stirred for 2 h. The reaction mixture was then partitioned between CH2C12

and water. The layers were separated and the aqueous layer was extracted with CHzClz
(3 x 30 mL). The combined organic layers were washed with aqueous KOH (20% w/w)
and then dried (MgSO4), filtered and concentrated. The crude product was purified by
recrystallization from ethanol to yield 0.210 g (76%) as white needles. mp 94-95 °C A
small portion was further purified by preparatory TLC (silica, hexane) to obtain pure
white needles. mp 102-103 °C (lit.15mp 110 °C)

'H NMR: 5 7.70 (s, 2H), 6.90 (s, 2H), 2.35 (s, 6H), 1.95 (s, 6H)

b)4,4 "-Diiodo-2,2 ',2 ",5,5 ',5 "-hexamethylterphenyl (14a). Compound 14a was
synthesized fiom 1.00 g (2.18 mmol) of 12a. The crude product was recrystallized twice
from benzene/ligroin to obtain 0.560 g (46%) of the pure product. mp 254-255 °C
(lit.15mp 254-255 °C)

‘H NMR: 5 7.7 (s, 2H), 7.0 (d, 2H), 6.9 (d, 2H), 2.4 (s, 6H), 2.0 (s, 6H)

c)4,4 '-Diiodo-2,2 ',5,5 '-tetraethylbiphenyl (13b): Compound 13b was synthesized from
0.50 g (1.22 mmol) of 11b. The crude product was isolated as a colorless oil to yield
0.450 g (72%) and was used without further purification.

1H NMR: 6 7.7 (s, 2H), 6.9 (s, 2H), 2.68 (quartet, 4H), 2.25 (m, 4H), 1.2 (t, 6H), 1.0 (t,
6H)

13C NMR: 6 143.4, 141.4, 140.2, 138.9, 129.4, 99.4, 33.5, 28.4, 15.0, 14.6.

HRMS: calc. for C20H2412 517.9968, found 517.9963

140
d)4,4"-Diiodo-2,2',2",5,5 ', "-hexaethylterphenyl (14b). Compound 14b was

synthesized from 0.500 g (0.920 mmol) of 12b. The crude product was isolated as a
white powder and was used without further purification to yield 0.540 g (90%)

1H NMR: 6 7.7 (s, 2H), 7.0 (s, 2H), 6.95 (d, 2H), 2.7 (quartet, 4H), 2.3 (m, 8H), 1.2 (t,
6H), 1.0 (m, 12H)

13C NMR: 6 143.3, 141.8, 141.7, 141.3, 141.2, 139.0, 138.9, 138.8, 138.7, 138.6, 138.5,
129.9, 129.8, 129.2, 99.1, 99.0, 33.6, 25.7, 25.5, 25.4, 15.3, 15.2, 14.9, 14.8, 14.6

HRMS: calc. for C30H3612 650.0907, found 650.0933

e)4,4"-Diiodo-2,2',2",5,5', "-hexahex;ylterphenyl (14c). Compound 14c was
synthesized from 1.50 g (1.71 mmol) of 12c. The crude product was recrystallized from
ethanol to yield 1.40 g (83%) as a white powder. mp 61 .5-63 °C

IH NMR: 6 7.6 (s, 2H), 6.9 (d, 2H), 6.9 (d, 2H), 2.6 (m, 4H), 2.2 (m, 811), 1.1-1.6 (m,
48H), 0.8 (m, 18H)

l3C NMR: 6 142.0, 141.3, 141.1, 140.6, 140.5, 139.4, 139.3, 138.9, 137.2, 137.1, 130.8,
130.6, 130.0, 99.0, 40.3, 32.6, 32.4, 31.7, 31.6, 31.5, 31.0, 30.9, 30.8, 30.7, 30.5, 30.3,
29.3, 29.2, 29.1, 29.0, 22.6, 22.5, 14.1

HRMS: calc. for C54H3412 986.4662, found 986.4667

General procedure for Pd(OAc)2 coupling: a) 2,2 ',2",2"',5,5 ',5 ' ',5 "'-
0ctamethquuaterphenyl (15a). This coupling procedure was adapted from Wallow and

Novak.50 To a 50 mL Schlenk flask were added 0.200 g (0.433 mmol) of 1311, 0.220 g

(0.991 mmol) of 7a and 0.114 g (1.08 mmol) of Na2C03. The flask was placed under an

141
Ar atmosphere by 3 pump-fill cycles. Pd(OAc)2 (0.001g) was placed in a Schlenk tube

under an Ar atmosphere. THF (30 mL) was added to the catalyst and the catalyst
solution was transferred to the reaction flask via a cannula. Water (10 mL) was added to
the reaction flask via syringe and the reaction mixture was stirred at reflux for 2 h. Upon
cooling to room temperature, the mixture was poured into a separatory funnel and the
layers were separated. The aqueous layer was extracted with CHzClz (3x30 mL) and the
combined organic layers were dried over MgSO4. The solution was heated before
filtering to ensure complete dissolution of the product and was concentrated to yield
0.122 g as an off-white powder. A small portion was purified by preparatory TLC
(silica/hexane). mp 260-261 °C (lit.15 mp 264-266 °C)

1H NMR: 6 7.15 (d, 2H), 7.05 (m, 4H), 6.98 (m, 4H), 2.34 (s, 6H), 2.08 (2 singlets,
12H), 2.03 (s, 6H).

13C NMR: 6141.5, 140.3, 134.8, 132.8, 132.6, 130.7, 130.6, 130.1, 129.6, 127.7, 20.9,

19.4, 19.3.

b)2,2',2",2"',2"",5,5',5 ",5"',5'"'-Decamethquuinquephenyl (16a). Compound 16a was
synthesized from 0.200 g (0.354 mmol) of 14 to yield 0.185 g as a white powder. A
small portion was purified by preparatory TLC (silica/hexane). mp 315 °C (lit.15 mp
307-309 °C)

1H NMR: 6 7.15 (d, 2H), 7.06 (m, 6H), 7.0 (m, 4H), 2.34 (s, 6H), 2.08 (m, 18H), 2.04 (s,
6H).

13C NMR: 6141.6, 140.3, 134.8, 132.8, 130.7, 130.6, 130.5, 130.2, 130.1, 129.6, 127.7,

21.0, 19.5, 19.3.

142

c) 4,4 "-Bis(trimethylsilyI)-2,2 ',2 ",5,5 ',5 "-hexaethylterphenyl (12b). This compound was
synthesized from 1.00 g (2.59 mmol) of 5 and 1.46 g (5.70 mmol) of 8b. The crude
product was recrystallized from ethanol to yield 0.79 g (56%) as a white fluffy powder.
mp 139-141 °C

1H NMR: 6 7.35 (d, 2H), 7.05 (d, 2H), 7.03 (s, 1H), 7.0 (s, 1H), 2.74 (quartet, 4H), 2.36
(m, 8H), 1.21 (2 triplets, 6H), 1.02 (m, 12H), 0.35 (s, 18H).

13C NMR: 6 146.7, 142.0, 141.9, 139.7, 139.6, 138.4, 138.3, 138.2, 138.1, 136.2, 136.1,
134.4, 134.3, 129.6, 129.4, 129.2, 129.1, 28.5, 26.0, 25.9, 25.7, 16.5, 16.4, 15.6, 15.4,
15.3, 15.2, 0.6.

HRMS: calc. for C36H54Si2 542.3764, found 542.3762

d) 2,2 ',2",2"',5,5 ',5",5"'-0ctaethquuaterphenyl (15b). Compound 15b was
synthesized from 0.200 g (0.390 mmol) of 13b and 0.151 g (0.850 mmol) of 7b. The
crude product was recrystallized from ethanol to yield 0.050 g (24%) as a white powder.
mp 110-112 °C

lH NMR: 6 7.00-7.23 (m, 10H), 2.65 (quartet, 14H), 2.40 (m, 12H), 1.24 (2 triplets, 6H),
1.04 (m, 18H)

13C NMR: 6141.1,140.9,139.8, 139.7, 139.3, 138.7, 138.6, 138.5, 138.4, 129.5, 129.3,
128.0, 127.9, 126.7, 126.6, 28.4, 25.9, 25.8, 15.6, 15.5, 15.4, 15.2, 15.0

HRMS: calc for C40H50 530.3912, found 530.3920

143
e) 2,2 ',2' ',2"',2' "',5,5 ',5",5"',5 "' '-Decaethquuinquephenyl (16b). Compound 16b was

synthesized from 0.200 g (0.308 mmol) of 14b to yield 0.215 g of a white powder. A
small portion was recrystallized from ethanol. mp 235-237 °C (dec.)

1H NMR: 6 7.00-7.30 (m, 12H), 2.70 (quartet, 4H), 2.40 (m, 16H), 1.25 (t, 6H), 1.10 (m,
24H)

13C NMR: 6 140.9, 139.7, 139.3, 138.7, 138.5, 129.6, 129.5, 129.4, 128.0, 127.9, 126.7,

28.4, 25.9, 15.6, 15.5, 15.4, 15.3

HRMS: calc. for C50H62 662.4852, found 662.4852

1) 4,4 "-Bis(trimethylsilyl)-2,2',2",5,5 ',5 "-hexahexylterphenyl (12c). Compound 12c
was synthesized from 1.52 g (3.00 mmol) of 5c and 2.32 g (6.40 mmol) of 8c. The
reaction was monitored by lH NMR and was allowed to reflux for 29 h. The crude
product was crystallized from ethanol to yield 1.67 g (64%) as a white powder. mp 67-
68.5 °C.

1H NMR: 6 7.30 (s, 2H), 7.03 (d, 2H), 6.97 (d, 2H), 2,70 (t, 4H), 2.30 (m, 8H), 1.05-1.65
(m, 48H), 0.85 (m, 18H), 0.35 (s, 18H)

13C NMR: 6145.3, 142.0, 141.8, 139.6, 137.0, 136.9, 136.0, 135.1, 130.3, 130.0, 36.0,
35.9, 33.1, 33.0, 32.8, 32.7, 32.5, 31.9, 31.7, 31.6, 31.3, 31.2, 30.8, 29.6, 29.5, 29.3, 29.2,
29.1, 22.7, 22.5, 14.1, 0.70

HRMS: calc. for C60H1028i2 878.7520, found 878.7515

144
g) 2,2 ',2",2" ',2"",5,5',5",5 "',5""-Decahexquuinquephenyl (16c). Compound 16c was

synthesized from 0.424 g (0.852 mmol) of Se and 1.00 g (1.87 mmol) of 20c. The crude
product was recrystallized from isopropyl alcohol to yield 0.650 g (65%) as a white
powder. mp 95-97 °C

1H NMR: 6 7.00-7.20 (m, 12H), 2.60 (m, 4H), 2.36 (m, 16H), 1.00-1.70 (m, 80H), 0.80
(m, 30H)

13C NMR: 6 141.1, 140.9, 139.6, 139.5, 138.0, 137.3, 137.1, 130.1, 130.0, 128.7, 128.5,

127.0, 35.5, 33.1, 33.0, 32.9, 31.7, 31.5, 31.2, 30.9, 30.7, 29.4, 29.2, 29.1, 29.0, 22.6,
22.5, 14.1, 14.0

HRMS: calc for C90H142 1223.1110, found 1223.1110.

h)2,2 ',2 ",2 "',2 "",2 ""',2 """,5,5 ',5 ",5 "',5 "",5 ""',5 """-Tetradecahexylseptaphenyl
(17c). Compound 17c was synthesized from 0.840 g (0.852 mmol) of 14c and 1.00 g
(1.87 mmol) of 20c. The crude product was recrystallized from ligroin and run through a
column of silica gel (cyclohexane) to yield 0.300 g (21%) as a shiny white solid. mp
138-140 °C

1H NMR: 6 7.00-7.20 (m, 16H), 2.60 (m, 4H), 2.30 (m, 24H), 1.00-1.70 (m, 120H), 0.80
(m, 34H)

13C NMR: NMR 6 140.9, 139.7, 138.1, 137.4, 130.2, 128.6, 127.0, 35.5, 32.9, 31.7,
31.5, 31.2, 30.9, 29.5, 29.3, 29.0, 22.6, 14.1, 14.0

HRMS: calc. for C126H193 1712.5530, found 1712.5557

145
2,2 ',2",5,5 ',5 "~Hexaethylterphenyl (10b): This compound was synthesized from 12b

according to Bennetau, et al.104 mp 60-61 °C

1H NMR: 6 7.22 (dd, 2H), 7.14 (dd, 2H), 7.04 (d, 2H), 7.01 (d, 2H), 2.70 (quartet, 4H),
2.35 (m, 8H), 1.25 (t, 6H), 1.05 (m, 12H).

13C NMR: 6 141.0, 140.9, 140.9, 139.7, 139.3, 139.2, 138.4, 138.4, 129.4, 129.3, 129.2,

128.0, 127.9, 126.7, 126.6, 28.4, 25.9, 25.8, 25.7, 15.6, 15.5, 15.4, 15.2, 15.2, 15.1.

HRMS: calc. for C30H33 398.2974, found 398.2973

2,2 ',5,5 '-T etraethylbiphenyl (9b). Compound 9b was synthesized analogously to 10b
from 0.310 g (0.760 mmol) of 11b.

1H NMR: 6 7.20 (d, 2H), 7.13 (d, 2H), 6.94 (s, 2H), 2.60 (quartet, 4H), 2.31 (m, 4H),
1.22 (t, 6H), 1.00 (t, 6H)

13C NMR: 6 140.9, 139.0, 129.2, 127.9, 126.7, 28.3, 25.8, 15.5, 15.2

HRMS: calc for C20H26 266.2035, found 266.2011

General procedure for boronic acid synthesis:40 a) 1,4-Diethyl-2-phenylboronic acid
(7b). To a dry 50 mL Schlenk flask containing 0.860 g (35.0 mmol) of dry magnesium
metal turnings were added 5.00 g (23.5 mmol) 3b in ~15 mL of THF. This reaction was
stirred at reflux for 1 h and then cooled to room temperature. A second flask was
prepared containing 8.83 g (46.9 mmol) of tn'isopropylborate in 60 mL of THF. This
solution was cooled in a dry ice/acetone bath and the Grignard reagent was added
dropwise to the solution via a cannula. The mixture was allowed to warm to room

temperature. After stirring for an additional 2 h, 60 mL of 2N HCl were added and

146
stirring was continued for 1 h. The mixture was poured into a separatory funnel and the

layers were separated. The aqueous layer was extracted with diethylether (3x40 mL), and
the combined organic layers were dried over MgSO4, filtered and concentrated to afford a
white solid suspended in a light yellow oil. This mixture was then dried under vacuum to
yield 3.75 g (90%) as a white powder. All boronic acid products were used directly for
coupling reactions unless otherwise indicated.

1H NMR: 6 8.10 (s, 1H), 7.30 (s, 1H), 7.20 (s, 1H), 3.20 (quartet, 2H), 2.70 (quartet, 2H),
1.30 (t, 3H), 1.20 (t, 3H).

l3C NMR: 6 143.7, 138.6, 128.3, 100.4, 33.6, 27.8, 15.4, 14.7.

b) 1,4-Diethyl-5-(trimethylsilyD-Z—phenylboronic acid (8b). Compound 8b was
synthesized from 4.00 g (14.0 mol) of 6b to yield 3.10 g (88%) as an oily white solid.
1H NMR: 6 8.10 (s, 1H), 7.40 (s, 1H), 3.20 (quartet, 2H), 2.80 (quartet, 2H), 1.35 (t, 3H),
1.30 (t, 3H), 0.35 (s, 9H).

13’C NMR: 6 148.8, 146.4, 143.0, 136.7, 135.4, 129.3, 28.6, 28.5, 17.8, 16.3, 0.3.

c)1,4-Dirnethyl-2-phenylboronic acid (7a). Compound 7a was synthesized from 5.00 g

(27.0 mmol) of 6a to yield 3.85 g (95%) as a white powder.
1H NMR (CD3OD ): 6 7.05 (m, 3H), 2.25 (d, 6H).

13C NMR (CD30D): 5 132.9, 130.6, 130.3, 21.8, 22.0.

d) 1,4-Dimethyl-5-(trimethylsilyD-Z-phenylboronic acid (8a). Compound 8a was

synthesized from 10.0 g (38.9 mmol) of 6a to yield 8.37 g (97%) as an off-white powder.

147
‘H NMR: 5 7.95 (s, 1H), 7.37 (s, 1H), 2.75 (s, 3H), 2.50 (s, 3H), 0.30 (s, 9H).

13C NMR: 6 143.3, 141.9, 139.8, 138.0, 136.5, 22.6, 22.5, -0.3.

e) 1,4-Dihexyl-5-(trimethylsib’l)-2-phenylboronic acid (8c). Compound 8c was
synthesized from 5.62 g (14.2 mmol) of 6c, 0.520 g (21.0 mmol) of magnesium and 12.0
g (56.6 mmol) of trimethylborate. The Grignard reaction was monitored by 1H NMR and
was allowed to reflux for 3 h. After work-up, the crude product was purified by column
chromatography (silica gel) using toluene as the solvent to remove side products and then
switching to diethyl ether to elute the boronic acid. This reaction yielded 3.40 g (66%) of
a colorless oil which solidified upon standing.

1H NMR: 6 8.00 (s, 1H), 7.35 (s, 1H), 3.15 (t, 2H), 2.70 (t, 2H), 2.65 (m, 4H), 1.20 —-
1.45 (m, 12H), 0.95 (t, 3H), 0.80 (t, 3H), 0.35 (s, 9H)

13C NMR: 6 146.9, 145.1, 142.4, 137.1, 136.0, 36.1, 35.2, 33.3, 32.9, 31.9, 29.8, 29.3,

22.7, 14.1, 14.0, 0.4

1) 2,2 ',5,5 '-T etrahewIJ-biphenylboronic acid (20c). This compound was synthesized
from 7.00 g (12.0 mmol) of 19c. It was purified analogously to Sc to yield 2.26 g (34%).
1H NMR: 6 8.20 (s, 1H), 7.00-7.20 (m, 3H), 6.90 (s, 1H), 3.40 (m, 1H), 3.15 (m, 1H),

2.60 (t, 2H), 2.40 (m, 4H), 1.00-1.80 (m, 32H), 0.80 (m, 12H).

General procedure for Pd(PPh3)4-catalyzed coupling of aryl halides and aryl boronic

acids: a) 2,2 ',2 ",5,5 ', "-Hexamethylterphenyl (10a). This procedure was adapted from

Miyaura, et (11.38 To a 50 mL Schlenk flask fitted with a reflux condenser were added

148
0.500 g (1.89 mmol) of 411, 0.600 g (4.00 mmol) of 7a, and 1.06 g (10.0 mmol) of

Na2C03, The flask was purged with argon, and 15 mL of water were added to the flask
containing the starting materials. To a second Schlenk flask was added 0.050 g of
Pd(PPh3)4 and 30 mL of toluene.. The catalyst solution was transferred to the reaction
flask via a cannula and the heterogeneous reaction mixture was stirred vigorously at
reflux for 24 h. The reaction mixture was cooled to room temperature and transferred to
a separatory filnnel and the layers were separated. The aqueous layer was extracted with
low boiling petroleum ether (3x25mL) and the combined organic layers were dried
(MgSO4), filtered through a short pad of silica gel/Celite and concentrated to yield an off-

white powder. The crude product was recrystallized from ethanol to yield 0.420 g (78%)
ofa white powder. mp 183-185 °c (lit.15mp 182-183 °C)

1H NMR: 6 7.15 (d ,2H), 7.05 (dd, 2H), 6.98 (d, 2H), 6.95 (d, 2H), 2.33 (s, 6H), 2.06 (s,
3H), 2.05 (s, 3H), 2.01 (s, 6H)

13C NMR: 6 141.5, 140.4, 134.8, 132.8, 130.5, 130.1, 129.6, 127.7, 21.0, 19.3

b)2,2',5,5 '-Tetramethylbiphenyl (9a). This product was synthesized from 0.700 g (4.70
mmol) of 7a and 0.800 g (4.30 mmol) of 3a. The crude product was isolated as a yellow
oil which was crystallized from ethanol to yield 0.340 g (35%) as colorless needles. mp
50.0-50.5 °c (lit.15mp 53-54 °C)

1H NMR: 6 7.14 (d, 2H), 7.05 (d, 2H), 6.90 (s, 2H), 2.30 (s, 6H), 2.00 (s, 6H).

13C NMR: 6 141.6, 134.8, 132.6, 129.9, 129.6, 127.7, 20.9, 19.3.

149
c) 4,4 '-Bis(trimethylsilyl)-2,2 ',5,5 '-tetramethylbiphenyl (11 a). Compound 11a was

synthesized from 2.00 g (7.78 mmol) of 6a and 1.90 g (8.56 mmol) of 8a. The crude
product was recrystallized from ethanol to yield 2.01 g (66%) as white needles. mp 185-
185.5 °C

1H NMR: 6 7.30 (s, 2H), 6.90 (s, 2H), 2.40 (s, 6H), 2.00 (s, 6H), 0.35 (s, 18H).

13C NMR: 6 142.3, 140.4, 136.7, 135.9, 131.7, 130.6, 22.4, 19.4, 0.0.

HRMS: calc. for C22H34Si2 354.2199, found 354.2195

d) 4,4 ' '-Bis(trimethylsilyI)-2,2 ',2 ",5,5 ',5 "-hexamethylterphenyl (12a). Compound 12a
was synthesized from 5.55 g (25.0 mmol) of 8a and 3.00 g (11.4 mmol) of 4a. The crude
product was purified by washing with ethanol until the washings were colorless to yield
1.95 g (38%) as a white powder. mp 241-243 °C

1H NMR: 6 7.35 (s, 2H), 6.90 (s, 4H), 2.40 (s, 6H), 2.10 (d, 6H), 2.00 (s, 6H), 0.35 (s,
18H).

13C NMR: 6 142.2, 140.4, 140.2, 136.6, 135.9, 132.6, 131.9, 130.9, 130.8, 130.5, 22.4,
19.4, 19.3, 0.0.

HRMS: calc. for C30H42Si2 458.2825, found 458.2792

e) 4-(TrimethylsilyI)-2,2 ',5,5 '-tetramethylbiphenyl: The crude product was synthesized
from 10.9 g (42.5 mmol) of 6a and 7.00 g (46.7 mmol) 7a. The product was
recrystallized from absolute ethanol to yield 10.9 g (91%) as a white flaky solid. mp 60-

61°C

150
'H NMR: 5 7.31 (s, 1H), 7.15 (d, 1H), 7.05 (d, 1H), 6.92 (s, 1H), 6.90 (s, 1H), 2.42 (s,

3H), 2.32 (s, 3H), 2.03 (s, 6H), 0.35 (s, 9H).

13C NMR: 6 142.4, 141.4, 140.4, 136.7, 135.8. 134.8, 132.5, 131.8, 130.7, 129.9, 129.6,

127.7, 22.4, 20.9, 19.4, 19.3, 0.0.

HRMS: calc. for C15H26Si 282.1804, found 282.1804

1) 4-(TrimethylsilyI)-2,2 ',5,5 '-tetrahe.§ylbiphenyl (18c). Compound 18c was synthesized
from 3.00 g (8.52 mmol) of 4c and 3.39 g (9.38 mmol) of 8c. The crude product was run
through a short column of silica gel (hexane) to remove traces of catalyst and then
distilled under vacuum (bp 249-250 °C @ 610 mtorr) to yield 2.71 g (57%) of a viscous
colorless oil.

11*] NMR: 6 7.29 (s, 1H), 7.15 (d, 1H), 7.03 (d, 1H), 6.92 (broad s, 2H), 2.65 (t, 2H), 2.55
(t, 2H), 2.28 (m, 4H), 0.90-1.60 (m, 44H), 0.34 (s, 9H)

13C NMR: 6 145.2, 142.0, 140.9, 139.4, 137.7, 136.7, 136.0, 135.0, 130.1, 129.8, 128.6,
127.1, 35.9, 35.5, 32.9, 32.8, 32.6, 31.9, 31.8, 31.6, 31.5, 31.1, 29.7, 29.3, 29.2, 29.0,
22.7, 22.6, 22.5, 14.1, 0.7

HRMS: calc. for C39H66Si 562.4934, found 562.4930

g) 4-(Trimethylsilyl)-2,2',2",5,5 ',5 "-hexamethylterphenyl (21a): This compound was
synthesized from 5.00 g (17.3 mmol) of 19a and 4.91 g (19.0 mmol) of 8a. The crude
product was recrystallized from absolute ethanol to yield 6.41 g (96%) as a white powder.

A small portion was recrystallized for analysis. mp. 180-180.5 °C.

151
‘H NMR: 5 7.35 (s, 1H), 7.17 (d, 1H), 7.09 (d, 1H), 7.00 (m, 4H), 2.45 (s, 3H), 2.35 (s,

3H), 2.05 (m, 12H), 0.39 (s, 9H)
l3c NMR: 5 140.4, 135.9, 132.6, 132.5, 130.8, 130.5, 130.1, 130.0, 129.6, 127.7, 22.4,
20.9, 19.4, 19.3, 0.0

HRMS: calc. for C27H34Si 386.2430, found 386.2445

h) 4-(Trimethylsilyl)-2,2',2",5,5 ', "-hexahexylterphenyl (21c). Compound 21c was
synthesized from 1.00 g (1.76 mmol) of 19c and 0.70 g (1.93 mmol) of 8c. The crude
product was purified by first running it through a short column of silica gel/hexane and

then recrystallizing from ethanol to yield 0.56 g (3 9%).

‘H NMR: 5 7.30 (s, 1H), 7.18 (d, 1H), 7.09 (d, 1H), 7.00 (m, 4H), 2.70 (t, 2H), 2.60 (t,
2H), 2.35 (m, 8H), 1.00 -— 1.50 (m, 48H), 0.80 (m, 18H), 0.35 (s, 9H)

13C NMR: 5145.3, 142.0,141.8, 141.1, 140.9,139.8,~139.6, 139.5, 138.0, 137.1, 137.0,
136.9, 136.0, 135.9, 135.1, 135.0, 130.3, 130.1, 130.0, 128.6, 128.5, 127.0, 36.0, 35.9,
35.6, 35.5, 33.2, 33.0, 32.8, 32.7, 32.6, 32.5, 31.9, 31.8, 31.7, 31.6, 31.4, 31.3, 31.2, 31.1,
31.0, 30.8, 30.7, 29.8, 29.7, 29.5, 29.4, 29.3, 29.2, 29.1, 22.6, 15.3, 14.1, 14.0, 0.7

HRMS: calc. for C57H94Sl 806.7125, found 806.7131

2,2 ',2",5,5 ',5 "-Hexahexylterphenyl (10c): This compound was synthesized according to
Rehahn, et (117,8 mp 42-43 °C (111.7 mp 47 °C)
1H NMR: 6 6.90-7.20 (complex aromatics, 8H), 2.60 (2 triplets, 4H), 2.30 (m, 8H), 1.60

(quartet, 4H), 1.00-1.50 (m, 44H), 0.80 (m, 18H).

152

General procedure for triethoxysibrl-terminated oligomers:111 a)2-(triethoxysilyl)-1,4-
dimethylbenzene (23a). To a 250 mL three-necked round bottom flask fitted with a
condenser and an addition funnel was added 1.97 g (81.1 mmol) of magnesium, 56.3 g
(270 mmol) of tetraethylorthosilane (T1303) and 75 mL of THF. A small iodine crystal
was added and the reaction mixture was heated to just below reflux temperature. The
reaction was cooled slightly and 10.00 g (54.05 mmol) of 2-bromo-p-xylene in 25 mL of
THF was added dropwise. Upon completion of the addition the reaction was heated at
reflux for l h, and then cooled to room temperature. The condenser and addition funnel
were removed and a distillation apparatus was attached. The THF was distilled under
argon from the reaction mixture at atmospheric pressure. To the remaining residue were
added 100 mL of freshly distilled hexanes (from CaH2) to precipitate magnesium salts.
This slurry was filtered under argon through a glass frit. The hexane was evaporated in
vacuo and the remaining liquid was fractionally distilled under vacuum. The product was
collected at 156°C @ 33 torr to yield 10.34 g (71%) as a clear colorless oil and was
stored in a dessicator.

1H NMR: 6 7.51 (s, 1H), 7.15 (d, 1H), 7.07 (d, 1H), 3.85 (quartet, 6H), 2.45 (s, 3H), 2.30
(s, 3H), 1.25 (t, 9H)

13C NMR: 6 141.3, 137.0, 133.7, 131.2, 129.6, 129.4, 58.4, 21.8, 20.9, 18.1

2981 NMR: 5 -56.4

153
b) 4-(triethoxysilyl)-2,2 '-5,5 '-tetramethylbiphenyl (24a). Compound 24a was

synthesized from 2.00 g (6.92 mmol) of 19a. The product was distilled (153-154 °C @
220 mtorr) to yield 1.89 g (74%) as a clear colorless oil.

1H NMR: 6 7.62 (s, 1H), 7.18 (d, 1H), 7.10 (d, 1H), 6.95 (s, 2H), 3.95 (quartet, 6H), 2.52
(s, 3H), 2.38 (s, 3H), 2.07 (2 singlets, 6H), 1.32 (t, 9H)

l3C NMR: 6 143.8, 141.3, 137.8, 134.7, 132.4, 131.7, 130.6, 129.9, 129.6, 127.7, 58.5,
21.8, 20.8, 19.2, 18.2

2“$1 NMR: 5 -56.37

c) 4-(triethoxysilyI)-2,2 '2 ",-5,5 ',5 ' '-hexamethylterphenyl (25a). Compound 25:! was
synthesized from 1.00 g (2.54 mmol) of 22a. The product was distilled (165-170 °C @
200 mtorr) to yield 0.64 g (52%) as a white solid.

1H NMR: 6 7.60 (s, 1H), 7.15 (d, 2H), 7.08 (d, 2H), 7.00 (m, 1H), 3.95 (quartet, 6H),
2.50 (s, 3H), 2.35 (s, 6H), 2.10 (m, 9H), 1.30 (t, 9H)

13C NMR: 6 143.8, 141.5, 141.4, 140.5, 140.4, 140.3, 140.2, 137.9, 134.8, 132.8, 132.7,
132.5, 132.4, 131.9, 130.9, 130.8, 130.6, 130.3, 130.5, 130.1, 129.6, 127.7 58.6, 21.9,

20.9, 19.4, 19.3, 18.2.

APPENDIX I: 1H NMR spectra of selected compounds

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APPENDIX 11: Numbering of compounds

R = CHzCH3 5b
R = C6H13 SC

31(CH3)3

O R
R
B(OH)2
R = CH3 88
R = CH2CH3 8b
R = C6H13 8C

R

R = CH3 153
R = CH2CH3 15b
R = Call]; 156

R = CH3
R = CH2CH3
R = C6H13

31(CH3)3

OR
R
Br

R = CH3 6a
R = CHzCH3 6b
R = C5H13 6c

R

O

R

R = CH3 93
R = CH2CH3 9b

R

O.

R

R = CH3 163
R = CH2CH3 161)
R = Can 16c

Br

0 R
R

Br

R = CH3 4a
R = CH2CH3 4b
R = C5H13 4C

B(OH)2

.b”

R = CH3 . 7a
R = CH2CH3 7b
R = C6H13 76

R
E: 73
R
R=CH3 108

R = CH2CH3 10b
R = C6H13 100

R
E 7
R

R = C6H13 17C

166

R R
(CH3)3Si O Si(CH3)3 (CH3)3Si O Si(CH3)3
2 3
R R
R=CH3 11a R=CH3 12a
R = CH2CH3 1“) R = CH2CH3 12b
R = C6H13 12C

CH3 CH3 CHa

R R
OOX OOOX
R R

H3C H3C H3C

R = CH3,. X = SI(CH3)3 188 X = SI(CH3)3 218
R = C6H13, X = Si(CH3)3 180 X = Si(0CHzCH3)3 258
R = CH3,. X = SI(OCH2CH3)3 248 X = Br 228
R = CH3,. X = Br 19a
R = C6H13, X = Br 19c
R = C5H13, X = B(OH)2 20¢
CH3
0 Si(OCH20H3)3
CH3

23a

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