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thesis entitled

Synthetic Routes to the Alkanethiol Tethered Chromphores Pyrene and
Perylene and Determination of around and Excited State Isomerization Barriers
for the Oligothiophene: 3', 4'-Dibutyl-2,2':5',2"-Terthiophene

presented by
Lee A. DeNitt

has been accepted towards fulfillment
of the requirements for

Master of Science degree in Chemistry

 

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Maj/or profesér

 

Date '3/ I ‘l’i’é/Fg

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SYNTHETIC ROUTES TO THE ALKANETHIOL TETHERED
CHROMOPHORES PYRENE AND PERYLENE
AND
DETERMINATION OF GROUND AND EXCITED STATE ISOMERIZATION
BARRIERS FOR THE OLIGOTHIOPHENE: 3',4'-DIBUTYL-2,2':5'2"-
TERTHIOPHENE

By

Lee Alan DeWitt

A THESIS

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

MASTER OF SCIENCE

Department of Chemistry

1 993

ABSTRACT

SYNTHETIC ROUTES TO THE ALKANETHIOL TETHERED
CHROMOPHORES PYRENE AND PERYLENE
AND
DETERMINATION OF GROUND AND EXCITED STATE ISOMERIZATION
BARRIERS FOR THE OLIGOTHIOPHENE: 3',4'-DIBUTYL-2,2':5',2"-
TERTHIOPHENE

By

Lee Alan DeWitt

This abstract is comprised of two parts. First, the synthetic routes to the
alkanethiol tethered chromophores pyrene and perylene are discussed, and the
eventual use of these tethered chromophores as imbedded probes in self assembled
monolayers is reviewed. These syntheses involve the Friedel-Crafts acylation of
pyrene and perylene with co-bromoalkanoyl chlorides, the subsequent reduction of
the keto-carbonyl group, and the transformation of the bromo functionality to the
thiol. Secondly, the intrinsic limitations to regularity in soluble polythiophenes are
studied through the determination of the rotational isomerization of a
polythiophene oligomer, 3',4'-dibutyl-2,2':5',2"-terthiophene, in the ground and
first excited singlet states. The So barrier to rotation is measured to be 19.7
kcal/mol using 1H-NMR and the 81 barrier to rotation is determined to be 4.2
kcal/mol using fluorescence lifetime measurements. The significance of these
results is discussed in the context of understanding the structure/property
relationships in poly(alkylthiophenes).

This is dissertation is dedicated with love to my family,
and especially to Ruth.

iii

ACKNOWLEDGEMENTS

First I want to thank Dr. Blanchard. Under his guidance I have gained the tools to
become an independent researcher, and have learned to use them in a manner

which can lead to the answers to our questions while having an absurd amount of

fun along the way.

I'd like to thank to thank the members of our group, for their inspirations and
genuine thoughtfulness. I'll especially remember the awesome perserverance of

Ying and Selezion, which I constantly use as a motivation.

I am indebted to Jim Roberts for countless stimulating conversations on the
philosophical nature of chemistry and other meaningful topics, usually lasting well

into the morning hours.

Special thanks go out to Mikey, Réf, and Shim ("we're half way there"). Without
their friendship, camraderie, and arm-twisting, I certainly would be less (more?)

sane today.

I also must thank Ruth. It was her love and support that enabled me to pursue this
degree in the first place, and without her this would not have been possible.

iv

TABLE OF CONTENTS

LIST OF TABLES vii
LIST OF SCHEMES viii
LIST OF FIGURES ix
INTRODUCTION 1

l. A STUDY OF ORGANIC MODIFIED INTERFACES 1

2. A STUDY OF THE STRUCTURE/PROPERTY RELATIONSHIPS
OF ELECTRICALLY CONDUCTING AND NONLINEAR

OPTICAL CONJUGATED POLYlvaRS 7

3. LITERATURE CITED ll

SYNTHESIS OF ALKANETHIOL TETHERED CHROMOPHORES 14

1. INITIAL REACTIONS AND PITFALLS OF SYNTHESES 24

2. CONCLUSIONS 27

3. LITERATURE CITED 28
DETERMINATION OF GROUND AND EXCITED STATE ISOMERIZATION

BARRIERS FOR 3',4'-DIBUTYL-2,2':5',2"-TERTHIOPHENE 29

1. INTRODUCTION 29

2. EXPERIMENTAL 32

3. RESULTS AND DISCUSSION 43

4. CONCLUSIONS 6O

5. LITERATURE CITED 61

TABLE OF CONTENTS, CONT'D.

APPENDIX A: INFRARED SPECTRA OF THE VARIOUS COMPOUNDS

APPENDIX B: ULTRAVIOLET SPECTRA OF THE VARIOUS
COMPOUNDS

APPENDIX C: 1H--NMR SPECTRA OF THE VARIOUS COMPOUNDS

APPENDIX D: MASS SPECTRA OF THE VARIOUS COMPOUNDS

65

79

84

90

TABLE 1

LIST OF TABLES

vii

57

LIST OF SCHEMES

Scheme 1. Conversion fi'om the carboxylic acid to the acid chloride.
Scheme II. Friedel-Crafis acylation of pyrene.

Scheme III. Clemmensen reduction of the carbonyl group.

Scheme IV. Treatment of the alkyl bromide with thiourea.

Scheme V. Formation of the ethyl ester of l-pyrenebutyric acid.

Scheme VI. Reduction of the ethyl ester of pyrenebutyric acid to the alcohol.

Scheme VII. Treatment of l-pyrenebutanol with thiourea.

Scheme VIII. Conversion of lZ-bromododecanoic acid to the acid
chloride via oxalyl chloride.

Scheme 1X. Friedel-Crafis acylation of perylene.

Scheme X. Treatment of the 12-bromo-l-perylenyldodecanoate with
thioacetic acid.

Scheme XI. Attempted reduction of the thioacetate carbonyl group.

18

19

20

20

22

23

23

25

25

26

26

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

LIST OF FIGURES

An illustration of an alkanethiol tethered probe imbedded in a self

assembled monolayer of alkanethiols.

The structures of monoalkylated head to tail and head to head
a-coupled polythiophene.

The structure of 3',4'-dibutyl-2,2':5',2"-terthiophene. The arrows

indicate the free rotation of the bridging o-bonds.
Structure of the desired alkanethiol tethered chromophores.

The structures of pyrene and perylene and the possible
positions of acylation.

Reaction schemes used to obtain the thiol fimctional group.

The structures of head to tail and head to head a-coupled
monoalkylated polythiophenes.

The structure of 3',4'-dibutyl-2,2':5',2"-terthiophene, and the
the free rotation of the bridging o-bonds.

Absorption and emission spectra of 10‘4 M 3',4'-dibutyl-
2,2':5',2"-terthiophene in l-butanol.

Figure 10. Schematic of the TCSPC spectrometer. Abbreviations:

BS = beam splitter; FOC = fiber optic coupler; L = focusing
lens; SHG = frequency doubling crystal; CL = cylindrical
lens; CF = color filter, P0 = polarizer; GT = glan-taylor
prism; SC = polarization scrambler. Other abbreviations
are defined in the text.

14

15

17

3O

31

35

40

Figure 11. A typical response function and fluorescence lifetime for the
time correlated single photon counting spectrometer. Counts
are shown on a linear scale.

Figure 12. The 1H-NMR spectrum of DBTT in perdeuterotoluene at 25°C.

Figure 13 a,h. The 1H-NMR spectra of the aromatic region of DBTT in
perdeuterotoluene at 85 (a) and 90 (b) degrees.

Figure 14. AMI calculated results for transition energies and change
in dipole moment for three rotameric forms of DBTT.

Figure 15. Dependence of DBTT fluorescence lifetimes on the solvent
viscosity.

Figure 1A. The Infrared spectrum of compound 1.
Figure 2A. The Infrared spectrum of compound 2.
Figure 3A. The Infrared spectrum of compound 4.

Figure 4A. The Infrared spectrum of compound 5 via Clemmensen
reduction of compound 4.

Figure 5A. The Infrared specturrn of compound 5 via treatment of 4
with Zn, HCl(g).

Figure 6A. The Infrared spectrum of compound 7.
Figure 7A. The Infrared spectrum of compound 8.
Figure 8A. The Infi'ared spectrum of compound 9.
Figure 9A. The Infrared spectrum of compound 10.
Figure 10A. The Infrared spectrum of compound 12.
Figure 11A. The Infrared spectrum of compound 14.

Figure 12A. The Infrared spectrum of compound 15.

X

42

46

48

52

55

65

66

67

68

69

7O

71

72

73

74

75

76

Figure 13A.

Figure 14A.

Figure 15A.
Figure 16A.

Figure 17A.

Figure 18A.

Figure 19A.
Figure 20A.
Figure 21A.
Figure 22A.
Figure 23A.

Figure 24A.

Figure 25A.

Figure 26A.
Figure 27A.

Figure 28A.

The Infrared spectrum of compound 16.

The Infrared spectrum of the perylene compound
afier sulfide bond cleavage.

The Ultraviolet spectrum of pyrene.
The Ultraviolet spectrum of compound 4.

The Ultraviolet spectrum of compound 5 via
Clemmensen reduction of 4.

The Ultraviolet spectrum of compound 5 via
treatment of 4 with Zn, HCl(g).

The Ultraviolet spectrum of compound 7.

300 MHz lH—NMR spectrum of Cambridge 99%+ CDCl3.

300 MHz 1H—NMR spectrum of compound 1.
300 MHz lH-NMR spectrum of compound 4.
300 MHz 1H-NMR spectrum of compound 5.

300 MHz IH-NMR spectrum of compound 7,
acetonitrile fraction.

300 MHz 1H-NMR spectrum of compound 7,
hexane fraction.

The mass spectrum of compound 4.
The mass spectrum of compound 5.

The mass spectrum of compound 7.

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

Chapter 1
Introduction

My research as a graduate student has consisted of two fairly divergent themes.
The first involves organic synthetic chemistry, and the second is comprised of
work on the dynamic properties of a polythiophene oligomer. Therefore, this
introduction is composed of two portions: the first describing the synthesis of
tethered chromophores for the ultrafast spectroscopic study of self-assembled
monolayers, and the second outlines the work done on the structure/property
relationships of electrically conducting and nonlinear optical polymers, or more

specifically, an oligomer of polythiophene.

I. A Study of Organic-Modified Interfaces: Well defined interfacial systems
are of great importance to many sciences, including chemistry
(chromatography), biology (membranes), physics (fiictional processes), and
materials science (corrosion). Similar to Langmuir-Blodgett films, Self
Assembled Monolayers (SAMs) consist of a highly ordered environment of
organic molecules adsorbed onto a substratem The ease of fabricating these
monolayers has promoted their use as a tool to study the properties of organic-
modified interfaces The chemical identity of the adsorbed molecules, as well
as the physical nature of the stacking within the monolayer determine
interfacial properties such as wettability, conductivity, lubrication, and
molecular selectivity. [2’3] Much is known about the properties of SAMs on a

macroscopic level, but our aim is to gain a better understanding of the
monolayer's molecular level structure, and the effect differing molecular scale
structures have on the macroscopic properties of the monolayer. Previous
methods of obtaining information on SAMs have relied on techniques such as
infi'ared spectroscopy,m electrochemistry,[3‘7] X-Ray photoelectron
spectroscopy,[81 mass spectrometrylg’w] contact angle measurements}1 1]
ellipsometryprm scanning tunneling microscopy,[12’13] helium
difi‘raction,[l4'16] and X-ray diffractionlm Among these, only the approach
using electrochemistry has used an actual imbedded probe within the
monolayer to interrogate the monolayer environment. The use of a tethered
probe will allow us to obtain direct molecular scale information about the
monolayer. The goal of this research will be to eventually use the reorientation
dynamics of a tethered probe molecule imbedded within the self-assembled
monolayer to determine the restrictions imposed by the monolayer itself. We
will also measure the fluorescence lifetimes of the tethered chromophores to »
determine the dynamic electronic response of the molecules comprising the
monolayer. The goal of this research is to gain an understanding of the effect
the molecular scale environment has on the macroscopic properties, so that

monolayers with enhanced properties may be constructed in a rational manner.

The interfacial system itself is comprised of a substrate and an adsorbed
organic molecule. We will study aliphatic hydrocarbons adsorbed onto a gold
substrate.“'l7] The reorientation dynamics of a tethered probe molecule

imbedded in the monolayer will enable us to elucidate the shape of its local

[13'24] This shape is swept out as the probe molecule

environment.
"swivels" about it's tethered axis on the gold substrate. Similar to solvent-
dependent reorientation dynamics, the reorientation times of the tethered probe
molecule will give information about the immediate restrictions imposed by

neighboring aliphatic hydrocarbons.

This is also the first study in which the depth of the monolayer studied will be
controlled. By varying the length of the chromophore's tether, we are able to
control the height of the chromophore within the monolayer, and therefore
obtain information about the local environments at different depths.

Current knowledge of SAMs

Much knowledge of alkanethiol self-assembled monolayers on gold has been
discovered in recent years due to the intense interest in these monolayers.
Alkanethiols are readily adsorbed onto a gold surface from solution, typically
in ethanol.[1'l7] The alkanethiol forms a stable bond with the gold surface,
exhibiting a free energy of adsorption (AGoabs) of -5 kcal/mol.[25] Recent
work has shed light on the kinetics of formation of these monolayers?” The
monolayers form within one minute after immersion of the gold substrate in the
alkanethiol solution. The kinetics of formation depend on the concentration of
the thiol. The kinetics of adsorption follow a Langmiur adsorption isotherm,
but only over a limited range of concentrations. At low thiol concentrations,

there is a negative deviation from Langmuir behavior. At high thiol

concentrations, there is also a negative deviation, and there appears to be a

rate-limiting transition state in the formation of the monolayer.

The infrared spectrum of aliphatic hydrocarbons near 2900 cm'1 shows the
absorption due to the methylene stretchm Depending on whether the
hydrocarbon is crystalline or in solution, the bands in this region are shifted by
10-15 cm' 1. Recent kinetic data show that after initial monolayer mass
equilibrium is reached, on the order of a minute, there is then an annealing
process that occurs on the order of hours to days.[25] Over this period of time,
the shift of the bands shows that the monolayer environment achieves an

equilibrium exhibiting a more crystalline-like structure.

The type of substrate also affects the structure of the monolayer. The gold
surface roughness plays an important role in the quality of the monolayer
formed. [5 1 Cyclic voltarnmetry and scanning tunneling microscopy results
have shown that etched bulk gold fihns, exhibiting visible roughness, form
monolayers with lower defect densities than those formed from polished or
annealed gold films. Although the etched films exhibit macroscopic roughness
due to many uneven surfaces, these surfaces themselves are smooth on a
microscopic level, providing a better substrate for an ordered monolayer. The
formation of monolayers is a very interesting phenomenon that is still far fi'om
understood. Studies of formation of monolayers from solutions of mixed thiols
(differing chain lengths) have shown that the ratio of thiols in solution does not

correspond to the ratio of the thiols adsorbed onto the monolayer.[26] The

studies show that this control is likely not to be kinetic, because of the
favorability of longer chain thiols over their shorter counterparts. [8]

Once the monolayers are formed on a Au(111) surface, they adopt a
hexagonally close-packed lattice,[14] with the sulfur atoms aligning with the
Au(l 1 l) lattice, [26] and the terminal CH3 groups forming a lattice with spacing
of 5 A116] The hydrocarbon chains align themselves in an all-trans
configuration with a tilt of ~ 30° with respect to the surface normal, the trans
segments being rotated by ~ 55° from the plane encompassing the chain axis
and the surface normal. [27] A schematic of the orientation of adsorbed
molecules is shown in figure 1.

 

 

Figure 1. An illustration of an alkanethiol tethered probe imbedded in a self assembled
monolayer of alkanethiols.

Self-assembled monolayers of alkanethiols on gold are already finding
applications in the area of electrochemistry, for instance, to study long range
electron transfer from ferrocene-terminated alkanethiols to the gold surface. [7]
Monolayers of mixed alkanethiols have been used to assemble a nanoporous
surface which forms an electron-retarding framework, and a much shorter thiol,
which creates defects in the passivating surface.[4] This mixed monolayer can
then be used for molecular recognition by allowing electron transfer from some

molecules, but not others, based on the molecule's size and structure.

My contributions to this project lie in obtaining tethered chromophores in a
timely, efficient, and cost-effective manner that will serve the purpose of
allowing us to excite and interrogate these chromophores to obtain information
about the monolayers. We decided to concentrate our efforts on the
chromophores pyrene and perylene. These two chromophores were chosen
because they have been widely studied and are well understood?“ A search
of commercially available tethered pyrene and perylene derivatives quickly
showed that they are prohibitively expensive. For this reason, along with the
need to establish a knowledge base of these syntheses within our group, we
decided to synthesize these tethered chromophores ourselves. To interrogate
different depths of the monolayer, alkanethiol substituted chromophores of
different lengths would have to be synthesized. For an alkanethiol monolayer
such as octadecanethiol (C13H37SH), probing the monolayer near the gold
surface would require imbedding 4-pyrene-1-butanethiol within the monolayer

to obtain information about the local environment near the bottom of the

monolayer. lZ-pyrene-l-dodecanethiol would tell us about the environment
near the upper portion of the monolayer, and so on. Similar analogs of the
perylene moiety also would be synthesized to provide complementary
information with a different chromophore shape and different spectroscopic

properties.

II. A Study of the Structure/Property Relationships of Electrically
Conducting and NonLinear Optical Conjugated Polymers. Isomerization of
labile organic molecules in the solution phase has been studied widely because
this conformational variability can affect a range of important physical and
chemical properties. Much of this work has been invested in understanding the
role isomerization plays in the reaction- and photochemistry of labile
compounds. Our interest lies in gaining a fundamental understanding of the
structure/property relationships for conducting and nonlinear optical
conjugated polymers. Several classes of conjugated polymers are synthesized
in solution and the incorporation of rotational defects into the polymer is
therefore determined, at least in part, by the isomerization barriers exhibited by

its constituent oligomers.

Many polymers that contain an extensive conjugated n-electron system become
conductive when doped, [29] and exhibit large nonlinear optical responses in
their undoped form.[3O] The discovery of conductivity in conjugated polymers
was made when polyacetylene was doped with bromine and iodine.[31]

Polyacetylene is not an ideal conducting polymer because its conductive

response is sensitive to the presence of oxidizing agents and the defect density
along individual polymer chains can be controlled to only a limited extent.
These materials limitations have created interest in more stable conjugated
polymers,[32'34] with polythiophenes being one example. While polythiophene
is more stable than polyacetylene toward oxidative degradation, it is equally
insoluble, limiting its prospects for processability. To make polythiophenes
soluble, and therefore processable, 3-alkylthiophenes have been used as
monomers.[35'38] This class of polythiophenes offers a significant solubility
enhancement, but also exhibits structural irregularity due to the occurrence of

both head-to-head and head-to—tail (Jr-coupling during polymerization.

R

S /\ S /\
\/ s \/ s

R

head to tail head to head

Figure 2. The structures of monoalkylated head to tail and head to head or-coupled
polythiophene.

Poly(3 -alkylthiophene) can exhibit on the order of 15 % of this type of
defect. [39] Recently, several ways have been discovered to overcome this

intrinsic limit to structural regularity in soluble polythiophenes.[40'44] One

approach uses preformed 3'4'-dibutyl-2,2':5'2"-terthiophene (DBTT) as the

monomer unit in the synthesis of poly(alkylthiophene).[43t44]

/\ S /\
s \/ s

Figure 3. The structure of 3'4'-dibutyl-2,2':5'2"-terthi0phene. The arrows indicate the flue
rotation of the bridging o-bonds.

By introducing two alkyl chains on the central thiophene unit, while leaving
the terminal thiophene rings without alkyl substituents, the possibility of head-
to-head or head-to-tail coupling is obviated, enhancing the regularity of the
structure. Nonetheless, there still remains potential ambiguity in the
regiochemistry of the oligomer due to rotation about the DBTT bridging 0-
bonds. The purpose of this work is to understand the rotational freedom
intrinsic to the oligomer so that we may ultimately understand the limits to
ordering attainable in the synthesis of poly(DBTT) and polythiophenes in
general.

Our efforts in determining the degree of regularin in the oligomer DBTT are
aimed at both the ground and first excited singlet states. These results will give

10

information about the regularity of DBTT ultimately used for electrically

conducting polymer efforts (ground state), and information from the excited
state will be useful in the realm of nonlinear optics.

We use 1H NMR to determine the ground state barrier to rotation about the
bridging o-bonds, thereby determining the intrinsic limit to regularity in
poly(DBTT) synthesized from the DBTT monomer. The regularity of the
oligomer backbone in the excited state is determined by using time correlated
single photon counting. Through this technique, we can examine reorientation
times and solvent dependent lifetimes. These data allow us to extract an

excited state barrier to rotation about the bridging o-bonds for the oligomer.

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16. N. Camillone; C. E. Chidsey; G. Y. Liu; T. M. Putvinski; G. Scoles; J.
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29. T. A. Skotheim, Ed.; Handbook of Conducting Polymers, Vols. 1 and 2,
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13

32. s. Chao; M. s. Wrighton; J. Am. Chem. Soc., 109, 6627, (1987).

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Chapter 2

Synthesis of Alkanethiol Tethered Chromophores

One of the first options encontered when planning the synthesis of the
alkanethiol tethered chromophores is whether to begin with the pre-existing
ring structure and attach to it the appropriate alkanethiol tail, or to begin with
the aliphatic tail and subsequently build the ring structure. Pictured in figure 3

are the structures of the desired compounds.

(CHzln—SH
(CH2)n_SH

   

Figure 4. Structure of desired alkanethiol tethered chromophores.

Depending on the chromophore, one has to be careful which strategy to use. In
my syntheses, working with pyrene and perylene has allowed me to use the
pre-existing ring structures, and concentrate on alkanethiol attachment to these
rings. Other chromophores, however, are not stable enough to withstand the
reactions needed to attach the aliphatic hydrocarbon tail. For instance,
tetracene decomposes when attempts at acylation are made. Fortunately, the

chromophores pyrene and perylene retain their structural integrity when

14

15

undergoing Friedel-Crafts acylation. Because of this stability, and the relative
ease of acylation compared to synthesis of the ring systems pyrene and
perylene, I chose Friedel-Crafts acylation to begin building the aliphatic moiety
needed to attach the chromophore to the interface substrate. Fortunately, there
exists a specific reactive position for both pyrene and perylene such that for
acylation, there is substitution predominantly at a single ring position. For
pyrene, classical Friedel-Crafts acylation occurs only at the l-position, with
intermolecular acylations being the only routes to acylation at the 2 and 4
positionsm Acylations of perylene give more derivatives such as double
substitutions, but less than 5 % of the mono-acylated products are at the l and
2 positionsm The predominant product is acylated at the 3-position.

Q8 ‘2 H.

Pyrene Perylene

       

Figure 5. The structures of pyrene and perylene and the possible positions of acylation

This reaction selectivity increases the ease of purification when separating
positional isomers. It is also very beneficial in that it greatly increases the
yield of the desired product. It is necessary to use a single isomer for our
monolayer studies because the different positional isomers would provide

different information about the restrictive environment of the self-assembled

16

monolayer; the reorientation times depend on the volume swept out by the

rotating probe chromophore, and thus its shape.

The next step in formulating the synthetic procedure was the need for a straight
chain aliphatic hydrocarbon having the thiol functionality as one end group,
and the acyl chloride as the other. No such compounds were found to be
commercially available. There are, however, several possible routes to the
mercaptan functional group. Some of these routes include the hydrolysis of
thiol esters, reduction of sulfonyl halides, reduction of disulfides, addition of
hydrogen sulfide to alkenes, the action of hydrogen sulfide on alcohols, the
alkylation of sodium hydrogen sulfide, and the hydrolysis of s-
alkylthiouronium salts.[3] These many reaction pathways to the thiol
functional group can be seen in fig. 6.

17

H20
COSR W or OH_ RCOzH R SH

R8020 ——(“)——> nsu
assn—(Hl—p 2RSH

(CH3)2C=CH2 + H28 —->(CH3)3CSH

ROH + H25 —380—,-—>2 RSH + H20

RX + NaSH ————->RSH + NaSH

csmHm i6 RscmH2)=NH2+x-

RSC(NH)2= NH2+X-M—) RSH + (NHch)x

Figure 6. Reaction schemes used to obtain the thiol functional groupm

l8

Halogens are one of the functional groups that lead to thiols in high yield. [3]
Primary bromo-functionalized alkanes give alkanethiols in high yields.[3] The
good yield, along with recent similar work that has shown thiol synthesis from
the bromo functionality suggested the use of m-bromoalkanoyl chlorides as
starting materialsm The only similar compound widely available in different
chain lengths are co-bromocarboxylic acids, which are suitable starting
materials. Here I will show the complete synthesis of an alkanethiol tethered
chromophore using the representative compound l2-bromododecanoic acid.
Limited data are available on percent yields of reactions due to purification

problems and the microscale nature of the syntheses.

Treatment of lZ-bromododecanoic acid 1 with thionyl chloride in heptane
yields 12-bromo-dodecanoyl chloride 2 quantitatively after refluxing for 4

hours.[5]

jg)
H c1 ‘0 if 0
HO-C-(CH2)11—Br ————) Cl-C-(CH2)lI—Br + A
1 2 or on

Scheme 1. Conversion from the carboxylic acid to the acid chloride.

The acid chloride is washed with dilute aqueous sodium bicarbonate solution to

remove the starting material and thionyl byproduct. [6] The resulting brown oil

19

is then washed with water, and dried over sodium sulfate. IR spectroscopy

shows complete conversion to the acid chloride.

For the acylation, the lZ-bromododecanoylchloride 2 is taken up in methylene
chloride, and pyrene 3 and anhydrous aluminum trichloride are added. This

solution is refluxed for 24 hours.[5]

ii

C—(CHz) Br
inn), ,3. . @gQ Aces Q86) 1r

2 3 4
Scheme II. Friedel-Crafis acylation of pyrene

The solid formed on solvent removal is taken up in hexane, filtered, and
washed with acetonitrile to give 12-bromo-l-pyrenyldodecanoate 4 in 37 %

yield.

The lZ-bromo-l-pyrenyldodecanoate 4 is dissolved in THF, and treated with
Zn(Hg) and HCl(g) under reflux for 3 hours.[5] The solvent is removed under
reduced pressure, and the resulting yellow oil is taken up in hexane and washed

with distilled water, then with brine.

20

ii

an O
Q Q “0(a) ) (QQ

Scheme III. Clemmensen reduction of the carbonyl group.

Mass spec, IR and UV spectroscopy show reduction of the carbonyl group to
the methylene group to give lZ-bromo- l-pyrenyldodecane 5.

The lZ-bromo-l-pyrenyldodecane 5 is dissolved in ethanol, excess thiourea is

added, and the solution is refluxed for 12 hours to give the thiouronium salt
5151

Br-

s )1“:
(CH2)12-Br /£\ (CH2)12—S+=C\NH2
Q80 ““2 We ego

 

Scheme IV. Treatment of the alkyl bromide with thiourea.

Aqueous sodium hydroxide is added, and the solution is allowed to reflux for

another 3 hours. [5]

21

 

The thiol 7 separates from the aqueous layer as a brown oil. The aqueous layer

is acidified to efiect a separation, the organic phase is removed, and solvent is
removed under reduced pressure. The thiol is dissolved in hexane, washed
with water, dried over sodium sulfate, and extracted with acetonitrile. After
several washings with acetonitrile, the hexane fraction is saved, and the NMR
and mass spectrometry data show that 12-mercapto-1-pyrene-dodecane 7 is

present.

A different reaction scheme is employed to obtain the shorter analog, l-pyrene-
butanethiol 12, because an inexpensive tethered pyrene starting material is
available. l-pyrene-butyric acid 8 is dissolved in ethanol, and HCl(g) is
bubbled into the solution for 3 hours, or until the temperature of the reaction

vessel begins to decrease.

22

(Gibb—fi—0H (CH ) J—ogt
Q as. O 23

Scheme V. Formation of the ethyl ester of l-pyrenebutyric acid.

Removal of the solvent leaves a dark brown oil containing the ethyl ester 9 in
quantitative yield. The oil is dissolved in methylene chloride, and washed with
sodium carbonate solution to remove the acid starting material and dried over
magnesium sulfate. IR spectroscopy, as well as NMR, shows conversion to the

ethyl ester.

The ethyl ester of l-pyrene-butyric acid 9 is treated with excess lithium
aluminum hydride as a slurry in THF, and refluxed for 5 hours, quantitatively
reducing the ester to the alcohol 10151

23

*W—Ls 1...... > *nnwn

Scheme VI. Reduction of the ethyl ester of pyrenebutyric acid to the alcohol.

 

Excess thiourea is dissolved in 6M hydrochloric acid by gentle heating, and the
1-pyrene-butanol 10 is added. [71

Cl‘

ego 2 + Wt, i» ego “W

10 11
Scheme VII. Treatment of l-pyrenebutanol with thiourea.

When the reaction is complete, the reaction vessel is allowed to cool to room
temperature. A solution of sodium hydroxide is added to the reaction mixture.

The thiouronium salt 11 separates as a brown oil.

24

Cl'
/NH2

+
(CH2)¢-—S=C
O “*2

090

ll 12

 

 

Methylene chloride is added to the vessel, and the organic layer is separated
from the aqueous phase, and dried with sodium sulfate to give a brown-yellow

oil containing l-pyrene-butanethiol 12.
Initial reactions and pitfalls for the syntheses

A preliminary test reaction scheme was employed to examine the feasibility of
the pathway from the carboxylic acid to the thiol. I began with l-hexadecanoic
acid, and dissolved it in ethanol, then bubbled HCl(g) into the solution for 3
hours. This gave the ethyl ester of the hexadecanoic acid. The ester was
reduced to the corresponding alcohol by treatment with lithium aluminum
hydride.[5] From the alcohol, I added HBr and thiourea, and refluxed for 13
hours. A solution of sodium hydroxide was added, and reflux was continued

for another 3 hours to give the l-hexadecanethiol.

25

Perylene attempts

A strategy similar to the one employed to synthesize the tethered pyrene was
used to synthesize the perylene analog. lZ-bromododecanoic acid 1 was
quantitatively converted to the acid chloride 2 by treatment with oxalyl
chloride. [51

o
HO—iilJ—(Cflzhl—Br + cr-fi—fi—cr ——) Cl—fi—(Cthl-Br
2

1
Scheme VIII. Conversion of lZ-bromododecanoic acid to the acid chloride via oxalyl
chloride.

This reaction was less efficient than thionyl chloride, in that the oxalyl chloride
conversion would have to be refluxed for at least 24 hours, whereas the thionyl
chloride took only 3 hours. Perylene 13 was then acylated to produce 14 in 37
% yield. [51

Cl—fi—(Cflzm-Br + jfl,
(9%)

Scheme IX. Friedel-Crafis acylation of perylene.

 

26

The 12-bromo-l-perylenyldodecanoate 14 was treated with thioacetic acid,
then sodium hydroxide solutionm

“W + ctr—t-.. _, a, 41,,

14
Scheme X. Treatment of the lZ-bromo-l-perylenyldodecanoatc with thioacetic acid.
Zinc dust was added to the thioacetate-ketone 15, and HCl(g) was bubbled
through the solutionm The intent was to reduce the carbonyl group adjacent

to the perylene, but the selectivity of this reaction was insufl'rcient and both the

keto carbonyl group and the thioacetate carbonyl group were reduced.

8©8 I_(CH2)n—s——'d—CH3 m Helm) 808 CH,—(CH,),.—s—cnz—crr3

1s 16
Scheme XI. Attempted reduction of the thioacetate carbonyl group.

From here I was unable to convert the sulfide to the thiol.

27

I also experienced dificulty with the reduction of the carbonyl moiety in the
synthesis of lZ-mercapto- l-pyrene-dodecane 12. It appears that reduction by
treatment with zinc dust and HCl(g) reduces the ring structure as well as the
carbonyl. This is evidenced by the near disappearance of the aromatic proton
resonances in the 1H NMR spectrum, as well as the loss of features in the
ultraviolet spectrum of the compound, and the shift in the infrared spectrum of
the absorption band at 844 cm'l. The milder Clemmensen reduction leaves the
ring structure undisturbed.

Conclusions

For the synthesis of alkanethiol tethered pyrene, the general synthetic
procedure described above is a practical procedure that serves our purpose of
providing an alkanethiol tethered chromophore in a timely and cost efl‘ective
manner. The overall yield should be on the order of 30°/o, with the acylation
reaction yielding roughly 60% product,[1] and the thiolation reaction yielding
nearly 50% product. [3]

N

Literature Cited

. Y. E. Gerasirnenko ; S. I. Didenko; Zh Org. Khim, 12, 1546, (1976).
. H. E. Ziegler; J. Org. Chem, 31, 2977, (1966).

. R B. Wagner; H. D. Zook; Synthetic Organic Chemistry, John Wiley and
Sons, New York, (1953)

D. M. Collard; M. A. Fox; Langmuir, 7, 1192, (1991).

. T. W. G. Solomons; Organic Chemistry, Fourth Edition, John Wiley and
Sons, (1988).

D. D. Penin; W. L. F. Armarego; D. R Penin; Purification of Laboratory
Chemicals, lst Edition, Pergammon Press, Ltd., London, (1966).

. R. L. Frank; P. V. Smith; J. Am. Chem. Soc., 68, 2103, (1946).

C. E. D. Chidsey; C. R. Bertozzi; T. M. Putvinski; M. J. Mujsce; J. Am.
Chem. Soc. 112, 4301, 1990.

28

Chapter 3

Determination of Ground and Excited State Isomerization Barriers for
3'4'-dibutyl-2,2':5',2"-terthiophene

Introduction

Isomerization of labile organic molecules in the solution phase has been
studied widely because this conformational variability can affect a range of
important physical and chemical properties. Much of this work has been
invested in understanding the role isomerization plays in the reaction- and
photochemistry of labile compounds. Our interest lies in gaining a
fundamental understanding of the structure/property relationships for
conjugated and nonlinear optical polymers. Of the many possible types of
defects present in conjugated polymers, rotational defects are thought to be the
most abundant for systems such as polypyrroles and polythiophenes.
Rotational defects produce "bends" and "kinks" along the polymer backbone
which interrupt and weaken the extent of n-conjugation in a given polymer
chain. The extent of conjugation in these polymers determines their utility, and
defects along the backbone diminish both their nonlinear optical response and
charge carrier mobility. Several classes of conjugated polymers are
synthesized in solution and the incorporation of rotational defects into the
polymer is therefore determined, at least in part, by the isomerization barriers

exhibited by the constituent oligomers.

29

30

Many polymers that contain an extensive conjugated n-electron system become
conductive when doped,[1] and exhibit large nonlinear optical responses in
their undoped formm The field of conducting polymers was initiated with the
discovery of conductivity in doped polyacetylene filmsm Polyacetylene is not
an ideal conducting polymer because its conductive response is sensitive to the
presence of oxidizing agents and the defect density along individual polymer
chains can be controlled to only a limited extent. These materials limitations
have created interest in more stable conjugated polymers, [4'6] with
polythiophenes being one example. While polythiophene is more stable than
polyacetylene toward oxidative degradation, it is also insoluble, limiting its
prospects for processability. To make polythiophenes soluble, and therefore
processable, 3-alkylthi0phenes have been used as monomers.[7'l°] This class
of polythiophenes offers a significant solubility enhancement, but also exhibits
structural irregularity due to the occurrence of both head-to-head and head-to-
tail cr-coupling during polymerization (figure 7).

R

S /\ S /\
\/ s \/ s

R

head to tail head to head

Figure 7. The structures of head to tail and head to head (rt-coupled monoalkylated
polythiophenes

31

Poly(3 -a]kylthiophene) can exhibit on the order of 15 % of this type of
defect.[1 1] Recently, several ways have been discovered to overcome this
intrinsic limit to structural regularity in soluble polythiophenes.[12'16] One
approach uses preformed 3',4'-dibutyl-2,2':5'2"-terthiophene (DBTT)[16] as the
monomer unit in the synthesis of poly(alkylthiophene).[16] By introducing two
alkyl chains on the central thiophene unit, while leaving the terminal thiophene
rings without alkyl substituents, the possibility of head-to-head or head-to-tail
coupling is obviated, enhancing the regularity of the structure (Figure 8).

/\ S /\
s \/ s

Figure 8. The structure of 3',4'-dibutyl-2,2':5',2"-terthiophene, and the free rotation of the
bridging o-bonds.

Nonetheless, there still remains potential ambiguity in the regiocherrristry of the
oligomer due to rotation about the o-bonds that connect the individual
thiophene rings in DBTT. The purpose of this work is to understand the
rotational freedom intrinsic to the oligomer so that we may ultimately know the
limits to ordering attainable in the synthesis of poly(DBTT), and, more
generally, substituted polythiophenes. We have measured the rotational

isomerization barriers for individual DBTT thiophene rings in the ground state

32

using 1H NMR and in the first excited singlet state using fluorescence lifetime
measurements. We find that the So barrier to rotation for DBTT is 19.7
kcal/mol, and its 8 1 barrier to rotation is 4.2 kcal/mol. The data show
collectively that rotational defects arising from the solution phase
polymerization of DBTT occur predominantly at the bonds joining individual
terthiophene oligomers and not within the primarily all-anti DBTT unit. [57]

Experimental

Chemicals. 3',4'-dibutyl-2,2':5',2"-terthiophene was synthesized and purified
according to a published procedurellsrml Briefly, tetrabromothiophene was
selectively debrominated with n-butyllithium to give 3,4-dibromothi0phene,
which was converted to 3,4-dibutylthi0phene by coupling with n-
butylmagnesium bromide in the presence of a catalyst. Treatment with
tetramethylammonium tribromide in a 1:1 mixture of acetic acid and
dichloromethane resulted in 2,5-dibromo-3,4-dibutylthiophene. Coupling with
thiophenemagnesimn bromide and a catalyst resulted in the formation of 3',4'-
dibutyl-2,2':5;,2"-terthiophene. All solvents used for the fluorescence lifetime
measurements were purchased from Aldrich Chemical Company as either
reagent grade or spectroscopic grade, and were used without further

purification.

WK 1H-NMR spectra of DBTT in perdeuterotoluene were taken on a 300
MHz Gemini VXR NMR Spectrometer. Temperature was varied between

33

20°C and 95°C. Uncertainty in the sample temperature was determined to be i
0.2°C.

Steady state optical spectroscopies. The absorption spectrum reported here
was measured on a Beckrnan DU-64 spectrophotometer. The resolution of the
measurement was ~1 run over the wavelength range studied. The spontaneous
emission spectrum of DBTT was measured using a Perkin-Elmer LS-S
fluorescence spectrophotometer. Data were acquired using 3 nm detection and
excitation bandwidths. These data were digitized on a J andel Scientific
digitizing tablet, and are shown in Fig. 9.

34

Figure 9. Absorption and emission spectra of 10‘4 M 3',4'-dibutyl-2,2':5'2"-
terthiophene in l-butanol.

(10

35

 

 

l

 

 

 

 

250

300

350 400 450

Wavelength (nm)

36

Time Correlated Single Photon Counting Spectrometer. A schematic of the
spectrometer used in this work is shown in Fig. 10. A CW mode-locked
Nd:YAG laser (Quantronix 416) produces ~9 W average power at 1064 nm
with 100 ps pulses at 80 MHz. The 1064 nm pulse train is frequency doubled
using a type II KTP crystal to produce ~900 mW average power at 532 nm.
The 532 nm light is used to excite a cavity dumped dye laser (Coherent 702-2).
The dye laser is cavity dumped at a repetition rate of 4 MHz and can produce a
output between 550 nm and ~1000 nm, depending on the dye and optics used.
For these experiments, pulses were generated at 602 nm using Rhodamine 6G
(Eastman Kodak), at 650 nm using DCM (Exciton), at 700 nm using LBS-698
(Exciton), and at 750 nm using LDS-751 (Exciton). The output of this laser is
between 60 and 125 mW average power depending on the output wavelength,
with pulses that exhibit a 5 ps FWHM autocorrelation trace. The output from
the dye laser is divided using a 90/10 beam splitter, with 90 % of the light
being sent to an angle tuned type I LiIO3 crystal (2m) to produce 301 nm,
325 nm, 350 nnr, or 375 nm light. The fundamental is separated from the
second harmonic light using color filters and the polarization of the UV pulses
is controlled using a fused silica half-wave rhomb (CV1). Typical average UV
power at the sample is 51 mW. The second harmonic output of the Nd:YAG
laser can also be frequency doubled using an angle tuned type I KDP crystal
(2m) to excite the sample at 266 nm. For all experiments the fluorescence
from the sample is collected at 90° with respect to the incident light, and the
polarization of the emitted light is selected using a Glan-Taylor prism. The

polarization of this selected light is subsequently scrambled, and wavelength

37

selection is carried out using a subtractive double monochromator (American
Holographic DB 10-S). We use a subtractive double monochromator to
eliminate dispersion-induced time broadening of the collected lightlm The
detector, a cooled two stage microchannel plate photomultiplier (MCP-PMT,
Hamamatsu R2809U-07), has a rise time of 156ps, and a transit time spread of
42 ps FWHM. The signal fi'om the MCP-PMT is sent directly to one channel
of the quad constant fiaction discriminator (CFD, Tennelec TC454), where it is
processed for input to the time to amplitude converter/biased amplifier (TAC,
Tennelec TC864). The system is operated in reverse mode, where the signal
channel is input to the TAC as the start channel. We operate the TAC in
reverse mode to avoid electronic dead time limitations which can result in
temporal distortions.[18’21] For the reference channel, the remainder of the
divided dye laser output (~10%) is coupled into a 2m optical fiber and routed
to a custom-made fiber optic delay lineml This delay line consists of fiber
optic loops of several different lengths to allow adjustable time ofi‘set between
the signal and reference channels. This optical delay system allows us to
measure fluorescence lifetimes ranging from us to the instrument response
function of 25 ps FWHM. The optically delayed laser output is incident on an
avalanche photodiode (PD, Hamamatsu 82381), and the signal fi'om the PD is
input to a second channel of the quad CFD. The output of the reference CFD
channel is delayed electronically (Tennelec TC412A) and sent to the TAC as
the stop signal. TAC output counts are observed using a ratemeter (Tennelec
TC525) and a 100 MHz oscilloscope (T ektronix 2230), and are sent to a
multichannel analyzer (MCA, Tennelec PCA-II) for collection. The two

38

channels of the quad CFD we use have been modified by the manufacturer for
use with MCP-PMT and other fast electro-optic devices. The sample cell
(3mm x 3mm) we used was made from two adjacent UV grade silica faces and
two adjacent blackened silica faces (N SG Precision) to minimize reflections of
both the incident radiation and the fluorescence.[l7] A typical response

function and fluorescence lifetime signal for this system is shown in Figure 11.

39

Figure 10. Schematic of the TCSPC spectrometer. Abbreviations: B8 = beam
splitter; FOC = fiber optic coupler; L=focusing lens; SHG = frequency
doubling crystal; CL = cylindrical lens; CF = color filter; P0 = polarizer; GT =
glan-taylor prism; SC = polarization scrambler. Other abbreviations are
defined in the text.

 

 

mmHDmEOU

 

 

 

mEOOm
radomo

 

 

 

 

 

mmbmdz
-mrdosm

 

_l meow

 

 

 

40

 

 

<02

 

 

 

 

 

Odom.

 

 

 

 

 

was

 

 

 

EEO

 

 

 

 

 

QmU

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H35 ZOE
9.5 do:
mmmobam 4
DEC amen AV OSCeZ
Om I/"\\/)
I I I I Home; as
\ 0 0 0 s a
me as La

MMAADOU
~ OF M

>450

 

><Amfl ><Amfl

 

 

 

 

 

41

Figure 11. A typical response function and fluorescence lifetime for the time
correlated single photon counting spectrometer. Counts are shown on a linear
scale.

42

 

 

 

 

 

 

0
Shane
ace 0

o an
Wot) hr
in . _
.U .0
no no
no 4.
2 1...

1000 —

$550

500 -

400 600 300 1000 1200 1400
Time (ps)

200

43

Data Analysis. Lifetimes reported are for data deconvoluted from the
instrumental response function using software written by Snyder and Demas.
The lifetimes we report are averages and standard deviations of at least 9

individual determinations for each solvent.
Results and Discussion

It is not uncommon for the excited state isomerization surface of a molecule to
be different than that for its ground state. The promotion of an electron to an
excited state can have a large effect on the equilibrium structure of a
molecule.[23'27] Well studied examples of molecules that exhibit state-
dependent isomerization surfaces are sfilbenel28'36] and the cyanines,[37'4°]
and this behavior is also known for polymers such as polyphenylene or
poly(phenylenevinylene).[1] In this work we focus on the state-dependent
rotational isomerization barrier of DBTT. Our results show that the first
excited singlet state barrier to rotation is significantly smaller than the ground

state barrier.

Ground state barrier measurement. 1H NMR spectra of DBTT were taken
using a 300 MHz spectrometer. At a sample temperature of 25°C, DBTT in
perdeuterotoluene produced the spectrum shown in Fig. 12. The splitting and
shifting of the peaks corresponding to the aromatic protons located between 6.8
and 7.2 ppm was observed to depend on the temperature of the sample.

Between 85°C and 90°C we are able to distinguish the coalescence of two

44

distinct chemical environments for the aromatic protons on the outer thiophene
rings (Figs. 13 a,b). The temperature-dependent shifis seen for the resonances
at 8~7 .05 ppm are for the 4 and 3" protons. These data show that, in addition
to splitting, the local environments of these protons change in an absolute sense

with temperature.

45

Figure 12. The 1H-NMR spectrum of DBTT in perdeuteroluene at 25°C.

46

Figure 12.

47

Figures 13 a,b. The 1H-NMR spectra of the aromatic region of DBTT in
perdeuterotoluene at 85 (a) and 90 (b) degrees.

7.1

 

 

 

 

 

 

7.1

 

48

 

 

7.0 6.9 6.8

 

l r l r l

7 .0 6.9 6.8
chemical shift (ppm)

6.7

6.7

49

We believe this is because of temperature-dependent changes in the motional
fi'eedom of the n-butyl groups at the 3' and 4' positions. The different
environments responsible for the splitting of these proton resonances arise from
difl‘erent rotameric forms of the oligomer. The splitting seen for the 4 and 3"
protons and for the 2 and 5" protons (8~6.87 ppm) coalesce at the same
temperature. From these linewidths and the observed coalescence temperature

we can calculate the ground state barrier to ring rotation in DBTT.[41]

now" = 4.575x10'3Tc[9.972 +l°gwi§£li
V

where Tc is the coalescence temperature, 8v = 4.65 Hz is the full width at half
maximum of the coalesced resonance at Tc, and AGiSO’" is in kcal/mol. From
our data we find AGiSO’" = 19.7:I:0.3 kcal/mol for So DBTT. While this is a
large torsional barrier, it is not inconsistent with that of other organic
molecules, such as trans stilbenel28'33] and DODCI[37] which exhibit
isomerization about their polyene bonds, or several other molecules where the
isomerization involves rotation of adjacent ring structures.[42'45] Other
experimental data from which the isomerization barrier of different thiophene

oligomers can be inferred are also consistent with our measurement.[46‘48]

Reorientation times. It is important to consider any possible conformer-
specific solvation effects on DBTT because such an effect could alter the
distribution of structures in solution and thus have an effect on the measured
torsional barriers. In addition, semi-empirical AMI calculations indicate that

both the optical and dipolar properties of the three conformers shown in Fig. 14

50

are difierent enough that the reorientation dynamics of 11 may be discernible
fiom those of I and III. The change in dipole moment upon excitation
predicted for H suggests that we should consider excited state complexation to
the solvent, [49'54] while the same is not true for I or III. In addition, the 80-81
origin of II is predicted to be blue shifted by ~250 meV from the origins for I
and 111, indicating the contribution of multiple rotarners to the absorption and

emission profiles measured for DBTT.

51

Figure 14. AMI calculated results for transition energies and change in dipole
moment for three rotameric forms of DBTT.

 

52

 

TERTHIOPHENE: 3 CONFORMERS

So-Sl

I 0110 3......
S S S

 

11 MO 356m“

111 S 389nm

l?

+.08

+.26

-. 12

 

 

53

It is important to establish the presence or absence of conformation-dependent
dynamics in DBTT to address the role that site-specific solvation effects play
in determining its isomerization barriers. We investigated this possibility by
measuring the excitation and emission wavelength dependence of the
orientational relaxation time of DBTT in l-butanol. We excited DBTT at
301nm, 325nm, 350nm, and 375nm, and collected fluorescence decays for
polarizations parallel and perpendicular to the excitation polarization at 410nm,
435nm, and 454nm. Reorientation times for DBTT were measured for all
combinations of the four excitation wavelengths with the three emission
wavelengths. We observed no dependence of reorientation time on either the
excitation or emission wavelength, to within our experimental uncertainty. For
DBTT in 1-butanol, 1’01- = 128113 ps. If there is any conformation-dependence
in the interaction between DBTT and the surrounding medium, it will be too

weak to affect the isomerization of the oligomer measurably.

Excited state barrier measurement. Dilute (~10'4 M) solutions of DBTT in
normal alcohols methanol through l-octanol and l-decanol were excited at
301nm, and fluorescence was collected at 410nm, 435nm, and 454nm to ensure
the contribution of only one excited electronic state to our data. The lifetimes
were observed by collecting at the magic angle (54.7°) to eliminate rotational
diffusion contributions to the data. The fluorescence lifetime of the oligomer

depends linearly on the solvent viscosity (see Fig. 15).

54

Figure 15. Dependence of DBTT fluorescence lifetimes on the solvent
viscosity.

1.. (08)

190

180

170

160

150

140

130

120

55

 

 

 

 

 

 

 

I 1 l 1 J r I r I

 

0.2 4 6 8 IO

Solvent Vrscosity (cP)

56

We can understand this behavior as follows. The most eflicient relaxation
pathway, which determines the excited state lifetime, is accessible to all of the
conformers through rotation about the o-bonds. One possibility is that the 90°
conformer exhibits emcient relaxation to the ground state surface, as is seen
for stilbene,[28'33] although the 81 isomerization surfaces of stilbene and
DBTT differ substantially. Another is that one of the conformers is coupled
more strongly to the ground state than the others and the lifetime of the excited
state is mediated by the time it takes the molecule to isomerize to that
particular conformation. Regardless of the exact identity of the dominant
relaxation pathway, as the individual thiophene rings rotate, they encounter
two distinct barriers to rotation, one intrinsic to the molecule and one imposed
by the solvent cage. The portion of the barrier contributed by the solvent can
be controlled by varying the solvent viscosity. As the viscosity is increased,
the lifetimes increase due to the larger effective barrier to thiophene ring
rotation. The lifetimes we measure for DBTT range from 124:1:3 ps in
methanol to l77:I:11 ps in l-decanol (Table I).

57

Table 1. Fluorescence lifetime data for DBTT in primary n-alcohols. The
reported uncertainties are standard deviations for a minimum of nine
determinations in each solvent.

 

 

 

solvent viscosity (cP) 111 (ps)
methanol 0.576 124i3
ethanol 1.032 132i3
l-propanol 1.796 133:1:3
l-butanol 2.377 136:1:2
l-pentanol 3. 160 136:1:6
l-hexanol 4.146 l4li7
l-heptanol 5.427 143i7
l-octanol 6.878 151:1:5
l-decanol 10.524 177i11

 

 

 

 

These data on DBTT are consistent with work on stilbene showing that the rate
of cis-trans isomerization is slowed with increasing solvent viscosity. [55 ’56]
Extrapolation of the lifetime dependence on viscosity to zero viscosity yields
the intramolecular barrier to rotation. Regression of our data shows a zero

viscosity lifetime of 123122 ps.
1
km E 117:0 = A exp('Al-‘%u.)
11

For A = 1013 (appropriate for the alcohols), AB = 4.2 kcal/mol. The
uncertainty in this barrier height is ultimately determined by our knowledge of

 

A. We estimate an uncertainty of :I:0.4 kcal/mol based on an uncertainty of a

factor of two in A. This result is also consistent with measurements on

58

cyanines, where an excited state isomerization barrier of 3-5 kcal/mol is

observed. [37]

Our data also provide insight into internal conversion in DBTT. Exciting
DBTT in butanol at 266 nm yields a lifetime of l48:I:2 ps, significantly longer
than the same solution excited at 350 nm (tfl = 136:I:2 ps). The ~12 ps
difference is the time required for the excitation to relax to the S1 surface from

the higher excited state, presumably S2.

Comparison to literature data on thiophene oligomers. There have been
several other reports in the recent literature that provide information relevant to
estimating the ground state torsional barrier for thiophene oligomers. A recent
AMI computational study on alkylthiophene short oligomers has indicated a
ground state torsional barrier of ~3 kcal/mol for a sterically strained (head-to-
head) dialkylbithiophene, with lower barriers for the less hindered (head-to-tail
and tail-to-tail) coupled ring systems.[461 We have calculated this barrier for
DBTT using the AMI Hamiltonian with configuration interaction, and our
results indicate a ground state torsional barrier of ~3.7 kcal/mol for DBTT.

One possible explanation for the consistently low ground state torsional
energies predicted by the AMI methods is that the bonds joining the thiophene
rings are predicted to have a bond order lower than that observed
experimentally. Resonance contributions to the inter-ring bonds that increase

their bond order would serve to make the system more resistant to torsional

59

motions, but such a resonance structure would involve the formation of a
biradical. It is also possible that the spatial extent of the sulfur orbitals is not
modeled well by the AM 1 parameterization. Regardless of the reasons for the
difference between calculated and experimental results, we can conclude that

AMI calculations do not model thiophene torsional barriers well.

Experimental information on the conformation and motional fi'eedom of
thiophene rings in oligomeric polythiophenes suggests that the individual
thiophene rings of the oligomer are rotated at ~40° with respect to one another
in chloroform solution, although direct experimental evidence for this assertion
is not provided.[481 The steric interactions between the thiophene sulfur and
the n-butyl tails of DBTT yield an equivalent result. The lowest energy
crystalline conformation of DBTT is all-anti with the terminal thiophene rings
rotated 20° with respect to the center 3,4-dibutylthiophene ring. The
temperature-dependence of the linear optical response of di-n—
hexylterthiopnene oligomers has also been reported recently. [47] No direct
attempt is made to extract the torsional barrier energy from these data, although
the authors note that at the melting point of the hexarner, bi-DHTT, there is no
abrupt change in the absorption maximum. This finding suggests that the bi-
DHTT oligomer largely retains its solid state conformation until 46° above the
melting point, where the onset of a blue shift in the absorption maximum is
noted. Thus, even for larger oligomers with more labile substituents, there is a

substantial barrier to torsional motion.

60

Conclusions

We have determined a 19.7 kcal/mol isomerization barrier in the ground state
and a 4.2 kcal/mol barrier to rotation in the first excited singlet state of DBTT.
These results demonstrate that the DBTT oligomer can circumvent many of the
defects associated with using the alkylthiophenes for polymerization, but there
necessarily still remains some structural irregularity in poly(DBTT) resulting
from a distribution of orientations about the bridging o-bonds between
oligomers. This degree of disorder for poly(DBTT) is comparatively small, but
not insignificant, as even low defect densities in n—conjugation can diminish the
conductive and nonlinear optical properties of the polymer. Synthesis of more
highly ordered precursors will be required to increase the conductivity of
doped polythiophene films and to optimize the nonlinear optical response of
undoped forms. In addition to enhanced structural regularity, such an oligomer
would require an asymmetric polymerization condition to avoid statistical

[13]

disorder over a longer length scale. We note that the use of longer

oligomers for synthesis of polythiophenes may not prove useful because of the
apparently lower barriers to intramolecular rotation exhibited by these

compoundsmrm

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APPENDICES

APPENDIX A: INFRARED SPECTRA OF THE VARIOUS COMPOUNDS

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