Synthesis of electroactive polystyrene derivatives para-substituted with π-conjugated oligothiophene via postgrafting functionalization

Article abstract of DOI:10.1002/pola.24531

Two electroactive polystyrene derivatives para- substituted with π-conjugated oligothiophene, poly(5-hexyl-5″-(4-vinylphenyl)-2, 2′:5′,2″-terthiophene) (PH3TS), and poly(5-hexyl- 5″″-(4-vinylphenyl)-2,2′:5′,2″:5″, 2″′:5″′,2″″-quinquethiophene) (PH5TS) hav

Full text of DOI:10.1002/pola.24531

Synthesis of Electroactive Polystyrene Derivatives Para-Substituted with
p-Conjugated Oligothiophene via Postgrafting Functionalization
ZHAOKANG HU, ELSA REICHMANIS
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Dr. NW, Atlanta, Georgia 30332
Received 15 November 2010; accepted 29 November 2010
DOI: 10.1002/pola.24531
Published online 3 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Two electroactive polystyrene derivatives para-
ing of the hexyl terminal groups attached to the pendant
oligothiophene units, no obvious chain aggregation was
observed for both PH3TS and PH5TS even in a poor solvent
environment. When compared with conventional linear conju-
gated polymer systems, the concept of grafting electroactive
units as pendant side chains via postfunctionalizing aliphatic
polymers might offer a strategy to precisely synthesize new
00
substituted with p-conjugated oligothiophene, poly(5-hexyl-5 -
0 0 00
(
4-vinylphenyl)-2,2 :5 ,2 -terthiophene) (PH3TS), and poly(5-
0000
0
0
00 00 000 000 0000
hexyl-5 -(4-vinylphenyl)-2,2 :5 ,2 :5 ,2 :5 ,2 -quinquethiophene)
PH5TS) have been successfully synthesized via the Stille cou-
(
pling reaction between tributyltin postfunctionalized poly(4-(2-
thiophenyl)styrene) (PTS) and bromo-oligothiophene. The
effect of the chain length of the pendant oligothiophenes on
properties of the resulting polymers including solubility, ther-
mal stability, optical absorption, and electroactivation energy
levels has been studied by using a variety of techniques such
as thermogravimetric analyzer, differential scanning calorime-
try, UV–Vis, Fluorescence, and cyclic voltammetry. With shield-
electroactive polymer materials for a number of organic elec-
tronic applications. VC 2011 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 49: 1155–1162, 2011
KEYWORDS: electroactive; graft polystyrene; oligothiophene;
postfunctionalization
2
0
21
22
INTRODUCTION Electroactive polymers have attracted con-
siderable attention in view of both academic and a wide
range of applications including organic light-emitting diodes,
Suzuki reaction, the Stille coupling, the Wittig reaction,
2
3
and others. These advanced synthetic methods allowed a
great deal of flexibility to introduce a variety of complex
new building blocks such as fluorene,
isothianaphthene,
thiadiazole, and many others into polymer backbones.
1
–5
24–26
27,28
organic field effect transistors, and photovoltaic devices.
carbazole,
2
9,30
31–33
Realization of these supermolecular architectures with well
defined dimensions and functions is one of the current chal-
lenges of research in the field of organic semiconducting
cyclopenta dithiophene,
benzo-
3
4
35–40
Besides all the effort made to build supermolecular archi-
tectures with conjugated systems onto linear polymer back-
bones, an alternative design strategy was to introduce con-
jugated systems as pendant side chains. Instead of linear
main chain p-conjugated polymers with rigid aromatic
backbones, aliphatic polymer backbones were thought to
provide enhanced solubility and mechanical flexibility
allowing for necessary stacking along pendant p-conjugated
oligomers. Monomers functionalized with semiconducting
moieties can be directly (co)polymerized to form electroac-
tive polymers via a variety of polymerization methods such
6
materials. It is essentially important to precisely build
supermolecular architectures through the techniques of
organic synthesis to afford desired performance attributes
such as good solubility, long-term thermal stability, oxidative
stability, and enhanced p-stacking for superior electrical
7
,8
conductivity.
Since the discovery of the insoluble polythiophene by a Ni-
9
catalyzed reaction for the first time in 1980, numerous poly-
1
0–12
thiophene derivatives with variable alkyl side chains,
1
3
14
molecular weights,
and region-regularities
have been
4
1
synthesized via a variety of advanced metal-catalyzed reac-
as free radical polymerization, living anionic polymeriza-
tion, atomic transfer radical polymerization, reversible
addition fragmentation chain transfer polymerization, and
nitroxide mediated polymerization. Particularly for these
1
5
42
43
tions such as the Chen and Rieke route and the Grignard
1
6,17
44
metathesis developed separately by McCullough et al.
and Yokozawa.
1
8,19
45
Later on, to achieve polymers with
desired properties such as low-band gap and enhanced pla-
narity facilitating p-stacking, other synthetic methods have
been extensively applied to build complex chemical struc-
tures onto polythiophene-based backbones such as the
side chain graft polymers, the formation of crystallinity
along pendant conjugated systems was thought to benefit
p-stacking and was advantageous for charge carrier
4
6
mobility.
Additional Supporting Information may be found in the online version of this article. Correspondence to: E. Reichmanis (E-mail: elsa.reichmanis@
chbe.gatech.edu)
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1155–1162 (2011) V
2011 Wiley Periodicals, Inc.
C
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ꢀ
ꢀ
C at a heating rate of 10 C/min. UV–Vis absorption spectra
To date, most aliphatic polymers grafted with semiconduct-
ing cores as pendant side chains have been obtained via
direct polymerization of electroactive monomers. Issues such
as steric hindrance built in these bulky monomers had to be
taken into significant consideration when high molecular
were recorded on an Agilent 8453 UV–Vis Spectrophotome-
ter. Fluorescence spectra were recorded on a Fluorolog 3,
model FL3-12 by Horiba Jobin Yvon. The detector was a pho-
tomultiplier tube model R928P by Hamamatsu. The excita-
tion lamp was a 450-W Xe lamp. The spectra were fully cor-
rected for the instrumental response. Gel permeation
chromatography (GPC) was performed on a Waters 1515 Iso-
cratic HPLC system with a Waters 2489 UV/Vis detector at
4
7
weight polymers were desired. Although some attempts
have been made to attach electroactive small monomers
such as oligothiophenes to polymer backbones via physical
4
8,49
forces such as hydrogen bonds and ionic interactions,
ꢀ
the resulting supermolecular complex was thought not to be
very thermally stable. Little work has been done to obtain
polymers chemically grafted with electroactive pendant side
chains via postfunctionalizing prepolymers. To our best
35 C, using a Styragel HR 5E THF 4.6*300 mm column with
HPLC grade THF as the mobile phase at a flow rate of 0.3
mL/min. All data were processed with software Breeze 2.
Cyclic voltammograms (CV) were acquired at a scan rate of
50 mV/s using a BAS 100 W potentiostat. Measurements
were performed using polymer thin films drop-cast from
CH Cl solution onto a Pt working electrode and a 0.1 M so-
5
0
knowledge, only the Reynolds’ group has recently reported
the synthesis of regioregular electroactive polyolefins via
postfunctionalizing polyolefin using sequenced p-conjugated
chromophores via the Suzuki reaction. Because of the limita-
tion of the experimental design, the grafted chromophores
aligned parallel to the resulting polyolefin backbone. Herein,
we extended this area of research by synthesizing a series of
polystyrene derivatives para-substituted with p-conjugated
oligothiophene segments via a metal-mediated postgrafting
functionalization. With relatively small steric hindrance, a
thiophene-modified styrene was readily free radically poly-
merized to form a poly(4-(2-thiophenyl)styrene) (PTS) pre-
cursor. The high reactivity of a-protons in the pendant thio-
phene units of the PTS precursor provided a wide range of
flexibility, so that it could be postfunctionalized with many
reactive groups such as bromide, tributyltin, and boronic
acid. Specifically in this study, a stannylated derivative of
PTS (Sn-PTS) was synthesized with a very high yield
2
2
lution of Bu NPF in dry acetonitrile as the electrolyte. The
4
6
auxiliary electrode was a Pt wire and the pseudo-reference
electrode was a silver wire anodized in 0.1 M aqueous KCl.
Ferrocene was used as internal reference.
RESULTS AND DISCUSSION
Synthetic Strategies
Styrene has been successfully copolymerized with 2-(4-vinyl-
phenyl)thiophene to form a copolymer. But as an insulator,
the incorporation of styrene into the oligothiophene graft
polystyrene severely deteriorated the performance of the
material as organic semiconductor. We also attempted to
0
00
0
copolymerize styrene with 5-hexyl-5 -(4-vinylphenyl)-2,2 :
0
00 00 000
5 ,2 :5 ,2 -quaterthiophene (4TS). Because of the strong
steric hindrance from this bulky monomer, a homopolystyr-
ene with unreacted 4TS instead of a copolymer was
obtained. To avoid steric hindrance that prevented a direct
free radical polymerization of oligothiophene-modified styryl
monomers to form high molecular weight polystyrenes
grafted with oligothiophenes, 2-(4-vinylphenyl)thiophene (5)
was first synthesized and polymerized to form PTS in tolu-
(>96%) and further reacted with a bromo-modified oligo-
thiophene to incorporate electroactive oligothiophene units
as pendant side chains perpendicular to the polystyrene
backbone. By this method, a series of electroactive graft
polystyrenes were synthesized with variable oligothiophene
chain length and terminal side groups, so as to systemati-
cally study how fundamental changes on the chemical struc-
tures of the pendant oligothiophene side chains affect poly-
mer properties such as solubility, optical absorption, and
electroactivation energy levels.
ꢀ
ene at 70 C with AIBN as an initiator. With highly reactive
protons at the a-position of the pendant thiophene units
associated in the polystyrene main chains, the PTS precursor
could be easily modified to produce a stannylated derivative
(
Sn-PTS) as shown in Scheme 1. The oligothiophene units
EXPERIMENTAL
were then grafted onto the postfunctionalized polystyrene as
the pendant side chains via the Stille reaction between Sn-
PTS and a bromo-modified oligothiophene compound. Specif-
ically, two oligothiophene compounds have been synthesized
in this study including 5-bromo-5 -hexyl-2,2 -bithiophene (1)
and 5-bromo-5 -hexyl-2,2 :5 ,2 :5 ,2 -quaterthiophene (4).
Compound 1 can be easily obtained with a very high
yield (>95%) by functionalizing 5-hexyl-2,2 -bithiophene
with N-bromosuccinimide (NBS) at room temperature. To
obtain compound 4, 5-hexyl-2,2 :5 ,2 :5 ,2 -quaterthiophene
(3) was first synthesized via the Stille reaction between 5-
(tri-n-butylstannyl)-5 -hexyl-2,2 -bithiophene (2) and 5-
1
H NMR (400 MHz) spectra were taken with a Varian Mer-
cury 400 at room temperature. Elemental analyses were per-
formed by Atlantic Microlabs using a LECO 932 CHNS ele-
mental analyzer. Mass spectra were taken on a Micromass
AutoSpec. The samples were run in EI mode with a mass re-
solution of 1000. The thermal stability was investigated with
a Perkin-Elmer Pyris 1 thermogravimetric analyzer (TGA).
Polymer powders were predried in a vacuum oven at 60 C
overnight and then heated from 30 to 500 C in a nitrogen
atmosphere with a heating rate of 1 C/min. Differential
0
0
0
00
0
0
00 00 000
0
ꢀ
ꢀ
0
0
00 00 000
ꢀ
0
0
scanning calorimetric (DSC) measurements were performed
using a Perkin-Elmer Diamond DSC. The samples were first
0
bromo-2,2 -bithiophene. Compound 3 was then reacted with
NBS to yield compound 4. The bromo-modified oligothio-
phene compound 1 or 4 were finally grafted onto the PTS
ꢀ
ꢀ
heated to 300 C and cooled down to 25 C at a rate of 80
ꢀ
C/min. Thermographs were then recorded from 25 to 300
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, DMF, 90 C; (ii) AIBN, Toluene, 70 ꢀC; (iii) (1) n-BuLi, THF, ꢁ78 ꢀC,
ꢀ
SCHEME 1 Synthetic route of PH3TS and PH5TS. (i) Pd(PPH
3 4
)
(
2) Bu SnCl; and (iv) NBS, DMF, room temperature.
3
backbone to form two different grafted polymers poly(5-
ingly once the reaction was completed. When compared with
the spectrum of PTS, the peak (e) at 7.01 ppm in the spec-
trum of PH3TS slightly move upfield to 6.91 ppm with much
enhanced peak intensity, indicating the successful grafting of
the bithiophene segments onto PTS.
0
0
0
0
00
hexyl-5 -(4-vinylphenyl)-2,2 :5 ,2 -terthiophene) (PH3TS) or
0
000
0
0
00 00 000 000 0000
poly(5-hexyl-5 -(4-vinylphenyl)-2,2 :5 ,2 :5 ,2 :5 ,2 -quin-
quethiophene) (PH5TS) via the Stille reaction in a similar
way.
The polymerization and grafting process was effectively
1
monitored by H NMR spectra as shown in Figure 1. Take
the synthesis of PH3TS as an example; the broad single
peaks upfield of 1.44 and 1.84 ppm were associated with the
ACH ACHA structure in the PTS backbone. Three peaks
2
centered at 6.55, 7.01, and 7.21 ppm were assigned to the
aromatic protons in the benzene and thiophene rings. Addi-
tionally, the peaks at 0.94 and 1.12 ppm were assigned to
the hydrogens in Me
2
C(CN)A endgroups in AIBN segments.
Other peaks at 1.28 and 1.61 ppm resulted from unidentified
5
1
contaminants and have been mentioned by others. On the
postfunctionalization of PTS to form Sn-PTS, four distinct
peaks appeared at 0.89, 1.12, 1.35, and 1.58 ppm as charac-
teristic of the formation of the tributyltin groups at the a-
positions of the pendant thiophene rings. The integration of
1
the H NMR spectrum of Sn-PTS indicated that >96% of the
a-position protons in PTS have been successfully stanny-
lated. It is noteworthy that a complete conversion of Sn-PTS
into PH3TS was achieved when an excess of 1 was added in
the Stille coupling reaction. As seen in Figure 1, all the peaks
associated for the tributyltin groups in Sn-PTS completely
disappeared and were replaced by the series of peaks at
0
.95, 1.37, 1.71, and 2.79 ppm, which were assigned to the
1
terminal hexyl groups in the newly formed PTH3T. Mean-
FIGURE 1 H NMR spectra of polymers PTS, Sn-PTS, and
while, the peaks for the aromatic protons changed accord-
2 2 4
PH3TS in C D Cl .
ELECTROACTIVE POLYSTYRENE DERIVATIVES, HU AND REICHMANIS
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Molecular Weights and Thermal Properties of the
Polymers PTS, PH3TS, and PH5TS
to a melting process (with decomposition) at temperatures
in the range of 220–300 C. The endothermic transition dis-
ꢀ
appeared on the second heating cycle and this was partially
interpreted as the thermal decomposition of the small
amount of the unreacted tributyltin groups on the terminal
thiophene units, which was consistent with the observation
by TGA. An exothermic transition was repeatedly observed
on the cooling process, which was thought to be associated
with reaggregation of oligothiophene segments. No crystalli-
zation peak was observed for both PH3TS and PH5TS,
which probably indicated that the side chains aligned only in
random combinations and orientations. Morphologies of PTS,
PH3TS, and PH5TS were further investigated by X-ray dif-
ꢀ
ꢀ
TDSC ( C)
Polymers
M
n
M
w
PDI
T
d
( C)
PTS
13,000
19,700
28,100
26,300
42,600
63,400
2.02
2.16
2.25
325
325
325
155 (T
n/a
220–300
g
)
PH3TS
PH5TS
Molecular weights and polydispersity indices (PDIs) of the
resulting polymers were identified by GPC as listed in Table
1. The dry PTS was a white powder (M
n
¼ 13,000 and PDI
ꢀ
¼
2.01) and soluble in common organic solvents such as
fractometry (XRD) in a scan range of 5–30 (2h; Supporting
dichloromethane (DCM) and tetrahydrofuran (THF) to form
clear colorless solutions. The resulting polymer PH3TS
obtained from the postfunctionalization of Sn-PTS was a yel-
Information Figure S1). No apparent diffraction peak was
observed at small angle regions for all three polymers, which
was consistent with the observation by DSC and further con-
firmed that the polymers were amorphous. A broad peak
low powder (M ¼ 19,700 and DPI ¼ 2.16). PH3TS demon-
n
ꢀ
(
particularly for PH5TS) was observed at 2h ¼ 19.5 , and its
strated good solubility in common organic solvents such as
diethyl ether, THF, DCM, and chloroform. The polymer
PH5TS was a reddish powder. When compared with its
counterpart PH3TS, the overall solubility of PH5TS was
deteriorated by the incorporation of two extra thiophene
units onto the pendant side chains. As a result, it partially
intensity increased as the chain length of the grafted oligo-
thiophene units increased. The d spacing corresponding to
dissolved in THF, CHCl , or 1,2,4-tri-chlorobenzene (TCB) to
3
form a reddish suspension. Filtration of the PH5TS suspen-
sion formed in CHCl resulted in a clear orange solution and
3
a brown solution obtained from the suspension in TCB.
Thermal properties of all three polymers were investigated
by TGA and DSC. Decomposition temperature (T ) defined as
d
the temperature at which 5% weight lose was detected by
TGA. As shown in Figure 2(a), the thermal traces of PH3TS
and PTS appeared very similar at low temperature regions,
ꢀ
with the same T
3
d
at 325 C. PH5TS also demonstrated T
d
at
ꢀ
25 C. But a closer observation revealed that the initial
weight loss began at 231 C for PH5TS, much lower than
those for PTS and PH3TS. Pronounced weight loss was not
ꢀ
ꢀ
observed until 275 C for the latter polymers. It was
hypothesized that the early weight loss was caused by the
thermal decomposition of a small amount of unreacted tribu-
tyltin groups attached on the PH5TS chains. Given the rela-
tively poor solubility of the resulting polymer PH5TS in
dimethylformamide (DMF), it was possible that the fraction
of PH5TS with relatively higher molecular weight began to
precipitate out from the reaction solution before all of the
tributyltin groups could be consumed by compound 4.
Glass transition temperatures were studied by DSC. With one
extra thiophene unit attached to each phenyl group, the PTS
chains appeared to be less flexible with the restricted chain
mobilization, which was demonstrated by an increased glass
ꢀ
transition temperature at 152 C, much higher than that of
polystyrene (ꢂ100 ꢀC). Even higher glass transitions were
expected for the polymers PH3TS and PH5TS grafted with
more rigid thiophene units. However, there was no clear evi-
dence of any usual glass behaviors as detected by DSC for
both polymers and, hence, the polymers were assumed to
involve highly hindered segmental motions. PH5TS gave rise
FIGURE 2 TGA (a) and DSC (b) curves of all three studied
polymers.
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The p–p* transition in single side chain conformation was
revealed by the UV–Vis absorption spectra in dilute solution.
To study the intrachain and interchain interactions, the poly-
mer solutions were studied by fluorescence spectroscopy. As
shown in Figure 4, these polymers showed fluorescence
emissions in the visible region on excitation at a certain
wavelength (275 nm for PTS and 375 nm for PH3TS and
PH5TS). Emission peaks were observed at 411, 485, and
514 nm for the polymers PTS, PH3TS, and PH5TS, respec-
tively. The red shifting trend was in agreement with the ob-
servation in the UV–Vis spectra [Fig. 3(a)]. An apparent dif-
ference in the shape of the emission peak of PTS was
observed when compared with those of PH3TS and PH5TS.
The peak of PTS was split into two subpeaks at 387 and
409 nm with a shoulder at 431 nm. This was consistent
with a similar poly(vinyl thiophene) system previously
4
7
reported by others, and the complex pattern was thought
to be caused by the combination of the intrachain and inter-
chain interactions between the pendant thiophene groups.
This could be the case for PTS with the ‘‘naked’’ pendant thi-
ophene units. Without shielding from the bulky hexyl termi-
nal groups, thiophene units could easily interact intramolecu-
larly and intermolecularly with other pendant thiophene
units. However, it is believed to be difficult for PH3TS and
PH5TS to form strong intrachain/interchain interactions in a
similar fashion. In both cases, the oligothiophene units are
shielded by the hexyl terminal groups, which prevent the
pendant oligothiophene units from aggregating and forming
strong intrachain/interchain interactions.
To verify this assumption, the UV–Vis spectra were taken for
the PH3TS chloroform solutions with the addition of metha-
nol in the range of 0–83 wt % as a poor solvent. Generally,
polymer chains tend to aggregate in a poor solvent system,
which is demonstrated by a red shifted absorption maximum
FIGURE 3 UV–Vis absorption spectra of (a) Polymers PTS,
PH3TS, and PH5TS in CHCl
(ꢂ0.1 mg/mL) and (b) PH3TS in
CHCl /methanol cosolvent solutions.
52
3
peak. But as shown in Figure 3(b), there was no obvious
3
change observed for the absorption maximum or the shape
of the absorption peak. It seems that no obvious aggregation
of the PH3TS chains in the chloroform/methanol system
occurs to affect the intrachain/interchain interactions
the wide angle XRD peak was 4.8 Å. The broad peak
appeared to be very similar to that found on a thin film cast
1
4
from a regiorandom P3HT. It probably indicated that p-
stacking occurred between some adjacent oligothiophene
side chains even though PH5TS, overall, appeared to be
amorphous.
The normalized UV–Vis absorption spectra of PTS, PH3TS,
and PH5TS in chloroform solution (ꢂ0.1 mg/mL) are
reported in Figure 3. All of the spectra were characterized
by a broad band with an absorption maximum reflecting the
intense p–p* transition within the conjugated oligothiophene
side chains. As expected, PH5TS, with the longest conjugated
side chain, exhibited a more red shifted peak with an
absorption maximum at 407 nm, whereas PH3TS and PTS
exhibited absorption maxima at 383 and 293 nm, respec-
tively. For comparison, the UV absorption spectrum was
0
0
00 00 000
taken for a solution of compound 3, 5-hexyl-2,2 :5 ,2 :5 ,2 -
quaterthiophene. It was understandable that compound 3
exhibited an absorption maximum of 390 nm in solution,
lying in between those of the polymers PH3TS and PH5TS.
3
FIGURE 4 Fluorescence spectra of polymers in CHCl .
ELECTROACTIVE POLYSTYRENE DERIVATIVES, HU AND REICHMANIS
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
found to be reversible. The onset of the oxidation occurred
þ=0
at ꢂ0.46 V (vs. FeCp2 ) above which the anodic current
quickly increased and a peak appeared at 0.64 V (vs.
þ=0
FeCp2 ). It was noteworthy that the anodic peak current
was nearly unchanged within subsequent scan cycles, show-
ing good electrochemical stability of PH3TS during the oxi-
dation process. As for PH5TS, because of its limited solubil-
ity in DCM, a uniform film with known thickness could
hardly be deposited onto the platinum electrode. It resulted
in an irreversible oxidation process with decreased anodic
current in subsequent scan cycles. Additionally, a shoulder
instead of an anodic peak during the oxidation process was
þ=0
observed with the maximum at ꢂ0.50 V (vs. FeCp ), but a
2
pronounced corresponding cathodic peak was observed at
þ=0
0
5
.36 V (vs. FeCp2 ). The reduction peak at 0.65 V in Figure
(b) has not been identified. It could be a result of a trace
amount of unreacted tributyltin terminated thiophene units.
By Comparing the CVs of these two polymers, it is suggested
that the oxidation potential of the grafted oligothiophene
units decreased as its conjugation length increased, which
5
3
was in agreement with previous reports.
The redox potential of the ferrocenium/ferrocene was
assumed to be an absolute energy level of ꢁ4.80 eV to vac-
5
4
uum. As a result, the highest-occupied molecular orbital
(
HOMO) energy levels were calculated using the equation
EHOMO ¼ ꢁeðEox þ 4:8ÞðeVÞ
where Eox is the half-wave oxidation potential versus
þ=0
FeCp2 . Because both polymers could barely be reduced,
their lowest-unoccupied molecular orbital (LUMO) energy
levels were roughly estimated from the HOMO energy levels
opt
and optical bandgaps (Eg ) using the equation
EHOMO ¼ ELUMO þ E go pt
FIGURE 5 CV for (a) PH3TS and (b) PH5TS.
As shown in Table 2, the HOMO levels were calculated to be
significantly. This confirmed our assumption that strong
steric hindrance from the bulky hexyl terminal groups
shielded the oligothiophene segments, which isolated each
polymer chain as well as the pendant oligothiophene side
chain and prevented them from forming pronounced intra-
ꢁ
5.34 and ꢁ5.22 eV for PH3TS and PH5TS. Accordingly, the
LUMO levels were ꢁ2.74 and ꢁ2.85 eV. PH3TS, with a
deeper HOMO energy level, demonstrated a better oxidative
stability when compared with PH5TS. It was interesting to
compare the HOMO energy level of PH5TS with that of a
previously reported polymethacrylate grafted with quinque-
chain/interchain interactions even in
environment.
a
poor solvent
5
5
thiophene as pendant side groups. Both polymers were
grafted with oligothiophenes consisting of five thiophene
rings. PH5TS possessed a deeper HOMO level than the quin-
quethiophene grafted polymethacrylate. The aliphatic struc-
tures on polymer backbones rarely affected the HOMO levels
Electrochemical Properties
To verify the oxidative stability of the studied polymers by
measuring their electroactivation energy levels, CV were
obtained in a three electrode cell using a platinum electrode
at a scan rate of 50 mV/s and Ag/AgCl reference electrode
in an anhydrous and nitrogen-saturated acetonitrile solution
of the grafted polymers because they did not contribute to
55
the p-conjugated system.
Therefore, the benzene ring
between the quinquethiophene side chains and the PH5TS
of 0.1
M
tetrabutylammonium hexafluorophosphate
(
Bu NPF ). To obtain accurate redox potentials, ferrocene
4
6
TABLE 2 Optical and Electrochemical Properties of Polymers
was used as an internal reference. In this study, thin layers
of PH3TS and PH5TS films were drop-cast deposited onto
the platinum electrode from their DCM solutions. As shown
in Figure 5, only oxidation peaks were revealed for both
polymers in the CVs. The oxidation process of PH3TS was
opt
g
þ=0
1=2
kabs(nm)
E
(eV)
E
E
HOMO
E
LUMO
PH3TS
PH5TS
383
424
2.60
2.37
0.54
0.42
ꢁ5.34
ꢁ5.22
ꢁ2.74
ꢁ2.85
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ARTICLE
backbone was believed to enhance the p-conjugation and
further delocalize the electron density along the entire conju-
gated side chain system, thus deepening the HOMO energy
level of PH5TS. Considering that deeper HOMO energy level
generally engenders material with higher oxidative stability,
the enhanced p-conjugation along the grafted oligothiophene
moieties could be expected to afford better air stability for
use as electroactive materials.
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Electronic Materials: The Oligomer Approach; M u¨ llen, K.;
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Summary of the Fundamental Research Needs in Organic
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CONCLUSIONS
10 Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J Am Chem Soc
2
004, 126, 3378–3379.
We have shown a rational molecular strategy to design and
synthesize electroactive polystyrenes grafted with p-conju-
gated oligothiophenes in variable side chain lengths. Both
studied graft polymers demonstrated similar thermal stabil-
ity. PH3TS showed better solubility and enhanced air stabil-
ity with a deeper HOMO energy level, whereas its counter-
part, PH5TS, showed a red shifted UV–Vis absorption
maximum and a lower optical bandgap. The presence of the
benzene ring on the pendant side chains enabled delocaliza-
tion of electron density along the entire p-conjugated side
chains and lowered the HOMO energy levels, which was
thought to result in more oxidatively stable materials. The
hexyl terminal groups on the pendant side chains enhanced
the overall solubility of the graft polymers. They were also
found to shield the pendant oligothiophene units and pre-
vent them from aggregating even in a poor solvent environ-
ment. The presented design principal should be applicable to
other organic semiconductors and thus provides a versatile
method to synthesize conjugated graft polymers with good
solution processibility and electrochemical properties, which
may potentially lead to fabrication of high-performance or-
ganic electronic devices.
11 Wu, P.-T.; Xin, H.; Kim, F. S.; Ren, G.; Jenekhe, S. A. Macro-
molecules 2009, 42, 8817–8826.
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8 Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J Am Chem Soc
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19 Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules
2004, 37, 1169–1171.
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The authors thank Stephen Barlow for his help with the cyclic
voltammetry measurements, Mariacristina Rumi for her help
with the Fluorescence measurements, Xuyang He for her help
with the gel permeation chromatography measurements,
Zhengzhi Zhou for his help with the X-ray diffractometry
experiments at the Georgia Institute of Technology, and Yapei
Wang at the University of North Carolina at Chapel Hill for his
help with the differential scanning calorimetric tests. This
research was supported by the American Chemical Society
Petroleum Research Fund (PRF 49158ND7).
21 Hou, J.; Chen, T. L.; Zhang, S.; Huo, L.; Sista, S.; Yang, Y.
Macromolecules 2009, 42, 9217–9219.
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Lett 2001, 79, 2133–2135.
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