Energy level tuning of polythiophene derivative by click chemistry-type postfunctionalization of side-chain alkynes

Article abstract of DOI:10.1002/pola.24443

A polythiophene derivative substituted with electron-rich alkynes as a side chain was synthesized using the Suzuki polycondensation reaction. The electron-rich alkynes underwent the "click chemistry"-type quantitative addition reaction with strong accepto

Full text of DOI:10.1002/pola.24443

Energy Level Tuning of Polythiophene Derivative by Click
Chemistry-Type Postfunctionalization of Side-Chain Alkynes
YUPING YUAN,1 TSUYOSHI MICHINOBU2,3
1Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguro-ku, Tokyo 152-8552, Japan
2Global Edge Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
3PRESTO, Japan Science and Technology Agency (JST), Japan
Received 11 September 2010; accepted 14 October 2010
DOI: 10.1002/pola.24443
Published online 11 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A polythiophene derivative substituted with electron-
rich alkynes as a side chain was synthesized using the Suzuki
polycondensation reaction. The electron-rich alkynes underwent
the ‘‘click chemistry’’-type quantitative addition reaction with
strong acceptor molecules, such as tetracyanoethylene (TCNE)
and 7,7,8,8-tetracyanoquinodimethane (TCNQ), resulting in the
formation of donor–acceptor chromophores. All polymers
showed excellent solubilities in the common organic solvents
as well as good thermal stabilities with their 5% decomposition
temperatures exceeding 230 ꢀC. The TCNE-/TCNQ-adducted
polymers displayed well-defined charge-transfer (CT) bands in
the low energy region. The CT energy of the TCNE-adducted
polymer was 2.56 eV (484 nm), which was much greater than
that of the TCNQ-adducted polymer [1.65 eV (750 nm)]. This
result was supported by the electrochemical measurements.
The electrochemical band gaps of the TCNE-adducted polymers
were much greater than those of the corresponding TCNQ-
adducted polymers. Furthermore, the highest occupied molecu-
lar orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels, determined from the first oxidation and
first reduction peak potentials, respectively, decreased with the
increasing acceptor addition amount. All these results suggested
that the energy levels of the polythiophene derivative can be
tuned by varying the species and amount of the acceptor mole-
C
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cules using this postfunctionalization method.
2010 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 225–233,
2011
KEYWORDS: charge transfer; conjugated polymers; electrochem-
istry; polythiophenes; postfunctionalization
INTRODUCTION Polythiophene derivatives have been exten-
sively investigated as an organic semiconducting polymer,
which can be used in many organic devices, such as field effect
transistors, photovoltaic cells, biosensors, and electrochromic
devices.1 To achieve a good device performance, it is important
to design polymer structures with energy levels suitable for a
specific application. One of the most straightforward and
promising approaches is the modification of the side-chain
groups of the polythiophenes.2 The introduction of electron-
donating or electron-accepting groups directly or via a p-
spacer into the polythiophene main chain dramatically altered
the energy levels. For example, it is reported that the introduc-
tion of electron-accepting side chains resulted in an enhanced
hole mobility and accordingly, good energy conversions in bulk
heterojunction solar cells.3
functionalization of polymers, as the desired moieties and
undesired moieties in a single polymer cannot be separated by
any methods.5 The most well-known click chemistry reaction
is the Cu(I)-catalyzed azide–alkyne cycloaddition reaction
(CuAAC), which forms a 1,4-triazole ring. There are indeed
numerous reports on the click postfunctionalization of poly-
thiophene derivatives by using CuAAC.6 For example, various
functional groups, such as electron-donating ferrocene and
electron-deficient benzoimide moieties, were efficiently
attached to the polythiophene derivatives.7 However, it is sug-
gested that CuAAC has a limitation regarding the application
to conjugated polymers. This is because the formed 1,4-tria-
zole rings do not serve as a powerful p-spacer for electronic
communication between the two substituents.8 Thus, the intro-
duction of electron-donating or electron-accepting groups
using CuAAC does not seem to significantly affect the energy
levels of the polythiophene main chains.
Click chemistry is the concept of achieving the target func-
tional materials using highly efficient addition reactions. This
definition suggests that the reactions should be performed
under mild conditions and yield no by-products.4 Therefore, it
is reasonable to apply click chemistry reactions to the post-
To solve this problem, we recently reported a new click
chemistry-type reaction.9 The reaction includes [2 þ 2]
cycloaddition between electron-rich alkynes and cyano-
Correspondence to: T. Michinobu (E-mail: michinobu.t.aa@m.titech.ac.jp)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 225–233 (2011) 2010 Wiley Periodicals, Inc.
ENERGY LEVEL TUNING OF POLYTHIOPHENE DERIVATIVE, YUAN AND MICHINOBU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
containing strong acceptor molecules, such as tetracyano-
ethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ)
derivatives, followed by the ring opening of the formed cyclo-
butene intermediate, resulting in donor–acceptor chromo-
phores.10 When dialkylanilino groups, ferrocene, and other spe-
cific donors are used as an activator of alkynes, the reactions
proceeds under mild conditions and the yields are quantitative.11
When compared with CuAAC, this class of reactions requires no
metal catalysts, and the energy levels of the products can be
tuned by the strength of the donor–acceptor interactions. Previ-
ously, we succeeded in controlling the lowest unoccupied molec-
ular orbital (LUMO) levels of an aromatic polyamine by applying
this click chemistry-type reaction to the postfunctionalization of
the side-chain alkynes.12 As more TCNE was added, the LUMO
levels were more significantly lowered. However, the energy
variation was not remarkable because of the presence of the
sp3-hybridized nitrogen atoms in the polymer main chain.
tials were referenced to the fericinium/ferrocene (Fcþ/Fc)
couple and used as an internal standard.
Monomer Synthesis
Trimethyl(thiophen-3-ylethynyl)silane (1)
In a 500-mL flask, 3-bromothiophene (10.0 mL, 107 mmol)
was dissolved into diisopropylamine (230 mL) and the solu-
tion was bubbled with argon for 30 min. After that, trime-
thylsilylacetylene (18.1 mL, 128 mmol), PdCl2(PPh3)2 (1.54
g, 2.14 mmol), and CuI (0.41 g, 2.14 mmol) were added to
the solution under nitrogen flow. The reaction mixture was
heated to 60 ꢀC for 8 h. After cooling to 20 ꢀC, the mixture
was passed through a short plug. Removal of the solvent
in vacuo followed by column chromatography (SiO2, hexane)
yielded the desired compound
yellow oil.
1 (15.6 g, 81%) as
1H NMR (300 MHz, CDCl3): d 0.24 (s, 9 H), 6.87 (q, J ¼ 3.0
Hz, 1 H), 6.98 (q, J ¼ 3.0 Hz, 1 H), 7.23 ppm (q, J ¼ 3.0 Hz,
1 H). 13C NMR (75 MHz, CDCl3): d 0.41, 93.87, 99.92, 122.34,
125.18, 129.62, 130.10 ppm. IR (neat): m 3106.6, 2959.0,
2893.9, 2147.4, 1516.8, 1410.0, 1360.8, 1254.1, 1155.6,
In this study, we applied this postfunctionalization method
to a polythiophene derivative substituted with the electron-
rich alkynes as a side chain, and report for the first time, the
remarkable large shift of the LUMO levels and control of
both the highest occupied molecular orbital (HOMO) and
LUMO levels. The maximum difference in the energy levels
caused by this postfunctionalization method was 2.39 eV.
1106.4, 942.9, 872.3, 843.5, 781.0, 758.4, 707.6, 628.6 cmꢁ1
.
[(2,5-Dibromothiophen-3-yl)ethynyl](trimethyl)silane (2)
In a 200-mL flask, 1 (4.00 g, 22.2 mmol) was dissolved into
ꢀ
anhydrous THF (40 mL) and cooled to ꢁ40 C. A hexane so-
EXPERIMENTAL
lution of n-butyllithium (22.3 mmol) was ꢀadded to the solu-
tion and the mixture was stirred at ꢁ40 C for 5 h. Then, a
THF solution of CF2BrCF2Br (22.6 g, 86.9 mmol) was added
Materials
All reagents were purchased from Kanto, Tokyo Kasei, Wako,
and Aldrich and used as received. Tetrahydrofuran (THF) was
distilled over sodium and benzophenone under N2 before use.
ꢀ
to the mixture at ꢁ40 C. After stirring for 0.5 h, an aqueous
solution of Na2S2O3 was added, and the product was
extracted three times with chloroform. The organic phase
was washed with brine and dried over MgSO4. After filtra-
tion and removal of the solvent, the crude product was puri-
fied by column chromatography (SiO2, hexane), yielding the
desired compound 2 (5.65 g, 75%) as brown oil.
General Measurements
1H NMR and 13C NMR spectra were measured on a JEOL
model AL300 spectrometer at 20 ꢀC. Chemical shifts are
reported in ppm downfield from SiMe4, using the solvent’s
residual signal as an internal reference. The resonance multi-
plicity is described as s (singlet), br (broad), d (doublet), t
(triplet), q (quartet), and m (multiplet). Infrared (IR) spectra
were recorded on a JASCO FT/IR-4100 spectrometer. MALDI-
TOF-MS spectrum was measured on an Applied Biosystems
model Voyager-DE STR in the reflect mode. Gel permeation
chromatography (GPC) was measured on a Shodex system
equipped with polystyrene gel columns using THF as an elu-
ent at a flow rate of 1.0 mL minꢁ1. Relative molecular
weights were determined by comparison with the calibrated
standard polystyrenes. Elemental analysis was performed on
a Perkin-Elmer 2400-SeriesII CHNS/O Analyzer. UV/vis spec-
tra were recorded on a JASCO V-670 spectrophotometer
using a 10-mm-thick cuvette. Thermogravimetric analysis
(TGA) was carried out on a Seiko SII TG 6220 under nitro-
gen flow at the scanning rate of 10 ꢀC minꢁ1. Electrochemi-
cal measurements of the polymer thin films were carried out
1H NMR (300 MHz, CDCl3): d 0.25 (s, 9 H), 6.93 ppm (s, 1
H). 13C NMR (75 MHz, CDCl3): d 0.42, 96.96, 99.86, 110.88,
116.90, 125.35, 132.33 ppm. IR (neat): m 3303.3, 3096.0,
2959.1, 2899.4, 2158.1, 1647.4, 1525.1, 1413.0, 1324.9,
1249.9, 1175.2, 1114.0, 1022.4, 985.1, 937.4, 845.7, 757.3,
699.5, 652.1, 563.7 cmꢁ1
.
2,5-Dibromo-3-ethynylthiophene (3)
To a 200-mL flask, 2 (1.56 g, 4.60 mmol), K2CO3 (1.91 g,
13.8 mmol), and methanol (30 mL) were added, and the
mixture was stirred at 20 ꢀC for 3 h. The mixture was
diluted with dichloromethane and the organic phase was
washed three times with water. After drying over Na2SO4,
the solution was filtered. Removal of the solvent in vacuo
and column chromatography (SiO2, hexane) yielded the
desired compound 3 (1.17 g, 95%) as a scarlet solid.
at 20 ꢀC in dehydrated CH3CN containing 0.1
M
1H NMR (300 MHz, CDCl3): d 3.30 (s, 1 H), 6.96 ppm (s, 1
H). 13C NMR (75 MHz, CDCl3): d 76.12, 81.68, 110.96,
117.08, 123.97, 132.22 ppm. IR (KBr): m 3286.8, 3082.4,
2114.9, 1647.7, 1516.8, 1410.0, 1319.9, 1172.3, 1016.3,
(nC4H9)4NClO4 in a classical three-electrode cell. The work-
ing, reference, and auxiliary electrodes were a glassy carbon
disk electrode (2 mm in diameter), Ag/Agþ/CH3CN/
(nC4H9)4NClO4, and a platinum wire, respectively. All poten-
909.6, 827.8, 671.9, 615.1, 565.9 cmꢁ1
.
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ARTICLE
4-[(2,5-Dibromothiophen-3-yl)ethynyl]-N,
N-dihexadecylaniline (4)
In a 200-mL flask, 3 (1.42 g, 5.40 mmol) and 4-iodo-N,N-
dihexadecylaniline (2.4 g, 5.4 mmol) were dissolved in diiso-
propylamine (70 mL), and the solution was bubbled with ar-
gon for 30 min. PdCl2(PPh3)2 (66 mg, 0.094 mmol) and CuI
(32 mg, 0.20 mmol) were added under nitrogen flow, and
the mixture was stirred at 20 ꢀC for 18 h. The mixture was
passed through a short plug. Removal of the solvent in vacuo
followed by column chromatography (SiO2, hexane) yielded
the desired compound 4 (2.78 g, 64%).
1H NMR (300 MHz, CDCl3): d 0.88 (t, J ¼ 6.9 Hz, 6 H), 1.26 (s,
52 H), 1.56 (t, J ¼ 10.5 Hz, 4 H), 3.27 (t, J ¼ 7.8 Hz, 4 H), 6.55
(d, J ¼ 9.3 Hz, 2 H), 6.98 (s, 1 H), 7.35 ppm (d, J ¼ 9.3 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d 14.12, 22.69, 26.99, 27.09, 27.14,
29.36, 29.50, 29.60, 29.66, 29.68, 29.70, 31.92, 53.41, 75.21,
79.82, 107.53, 110.42, 111.05, 113.91, 131.97, 132.98, 137.58,
148.20 ppm. IR (KBr): m 2926.4, 2851.1, 2205.3, 1606.7,
1535.9, 1511.3, 1497.6, 1465.3, 1425.1, 1399.1, 1368.3,
1294.5, 1248.3, 1191.5, 1148.3, 1115.9, 1014.5, 931.6, 823.8,
811.6, 800.8 cmꢁ1. MALDI-TOF MS (matrix: dithranol): m/z:
calcd for C44H71Br2NSþ: 805.1; found 806.6 [M þ H]þ.
SCHEME 1 Synthesis of alkyne-substituted thiophene mono-
mer. (a) HCBCSiMe3, PdCl2(PPh3)2, CuI, (iPr)2NH, 60 ꢀC, 81%;
(b) 1. nBuLi, THF, ꢁ40 ꢀC, 2. CF2BrCF2Br, THF, ꢁ40 ꢀC, 75%; (c)
K2CO3, methanol, 95%; (d) 4-iodo-N,N-dihexadecylaniline,
PdCl2(PPh3)2, CuI, (iPr)2NH, 20 ꢀC, 64%.
P1
A mixture of 4 (161 mg, 0.200 mmol), 2,20-bithiophene-5,50-
dibronic acid bis(pinacol) ester (86.2 mg, 0.200 mmol), and
Pd(PPh3)4 (0ꢀ.010 g, 0.010 mmol) in THF (10 mL) was
stirred at 20 C for about 0.5 h under argon. Then, an aque-
ous solution of K2CO3 (0.60 mmol; purged with argon) was
was added under nitrogen, and the mixture was stirred at
160 ꢀC for 4 h. After cooling to 20 ꢀC, the solvent was
removed in vacuo, yielding the green solid P3 (x ¼ 1; 37.5
mg, 100%).
ꢀ
1
added, and the mixtꢀure was further stirred at 80 C for 36 h.
GPC (THF): Mn ¼ 7200 (Mw/Mn ¼ 2.01). H NMR (300 MHz,
After cooling to 20 C, the reaction mixture was poured into
an excess amount of methanol. The precipitate was collected
by filtration, washed with methanol twice, and dried under
vacuum, yielding a red solid (130 mg, 80%).
CDCl3): d 0.89 (s, 6n H), 1.25 (br, 52n H), 1.65 (br, 4n H),
3.28 (br, 4n H), 6.53–7.99 (m, 13n H). IR (KBr): m 2923.7,
2852.1, 2202.4, 2179.5, 1737.1, 1580.3, 1515.6, 1464.3,
1401.6, 1360.5, 1262.5, 1180.5, 1019.8, 908.4, 795.9, 719.8,
542.4 cmꢁ1. ELEM. ANAL calcd for (C64H79N5S3)n: C 75.77, H
7.85, N 6.90; found: C 68.5, H 7.07, N 7.17%.
1
GPC (THF): Mn ¼ 6600 (Mw/Mn ¼ 1.80). H NMR (300 MHz,
CDCl3): d 0.88 (br, 6n H), 1.25 (br, 52n H), 1.58 (br, 4n H),
3.28 (br, 4n H), 6.52–7.83 ppm (m, 9n H). IR (KBr): m
2931.5, 2852.3, 2194.2, 1737.1, 1614.8, 1523.4, 1466.7,
1401.5, 1368.2, 1187.9, 1126.8, 812.4, 791.7, 726.0, 629.6,
RESULTS AND DISCUSSION
The key monomer, 4-[(2,5-dibromothiophen-3-yl)ethynyl]-
N,N-dihexadecylaniline 4, was prepared by the repeated
Sonogashira cross-coupling and the silyl-deptrotection proto-
col (Scheme 1). Starting from the commercially available 3-
bromothiophene, the bromo group was substituted by trime-
thylsilylacetylene under the Sonogashira conditions to yield
1. Subsequently, the 2,5-positions of the thiophene ring were
selectively brominated according to a reported procedure.13
Deprotection of the trimethylsilyl group of 2 with K2CO3 fol-
lowed by the Sonogashira coupling with 4-iodo-N,N-dihexa-
decylaniline afforded the desired monomer 4. It should be
noted that the introduction of a dialkylanilino group must
be preceded by the bromination reaction, as aniline rings are
readily brominated. The reaction yields of each step are
moderate to good, and the total yield of the four steps
amounted to 37%. The chemical structure of 4 was fully
characterized by 1H and 13C NMR, IR, and MALDI-TOF MS
analyses. As an example, the identified 1H NMR spectrum of
4 measured in CDCl3 is shown in Figure 1.
619.3, 526.7 cmꢁ1
.
P2 (x ¼ 1)
To a solution of P1 (35.7 mg, 0.0440 mmol per repeat unit)
in 1,2-dichloroethane (20 mL), TCNE (5.68 mg, 0.0440
mmol) was added, and the mixture was stirred at 20 ꢀC for
5 min. Removal of the solvent in vacuo yielded the brown
solid P2 (x ¼ 1; 41.4 mg, 100%).
1
GPC (THF): Mn ¼ 6100 (Mw/Mn ¼ 2.40). H NMR (300 MHz,
CDCl3): d 0.87 (br, 6n H), 1.23 (br, 52n H), 1.63 (br, 4n H),
3.36 (br, 4n H), 6.67–7.68 ppm (m, 9n H). IR (KBr): m
2924.5, 2852.2, 2216.5, 1606.3, 1486.4, 1464.5, 1414.1,
1367.1, 1337.2, 1294.9, 1260.6, 1210.2, 1183.7, 1112.2,
802.1, 693.6, 616.7 cmꢁ1. ELEM. ANAL calcd for (C58H75N5S3)n:
C 74.23, H 8.06, N 7.46; found: C 71.1, H 7.57, N 7.46%.
P3 (x ¼ 1)
To a solution of P1 (30 mg, 0.037 mmol per repeat unit) in
1,2-dichlorobenzene (20 mL), TCNQ (7.55 mg, 0.0370 mmol)
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
the 3-alkyne moiety of the thiophene monomers partially
prevents the polymerization mechanism.17 In this study, we
used the Suzuki polycondensation between 4 and the 2,20-
bithiophene-5,50-dibronic acid bis(pinacol) ester to prepare
the precursor polymer P1 (Scheme 2). After the polymeriza-
ꢀ
tion at 80 C for 36 h, P1 was obtained as a red powder in
80% yield. The obtained polymer showed very good solubil-
ities in the common organic solvents, such as THF, dichloro-
methane, benzene, and somewhat in hexane, because of the
two long alkyl chains. Therefore, the chemical structure was
1
characterized by H NMR and IR spectroscopies (vide infra).
The number-average molecular weight (Mn) and polydisper-
sity (Mw/Mn) of P1, determined by GPC (eluent: THF), were
6600 and 1.80, respectively. As the polymerization results
were reproducible, there were no significant side reactions
in this case.
FIGURE 1 1H NMR spectrum of 4 in CDCl3 at 20 ꢀC.
Subsequently, the postfunctionalization of the side-chain
alkynes of P1 was attempted by using the TCNE/TCNQ addi-
tion reactions. To estimate the progress of the reactions as
well as the presence of side reactions, the UV–vis–NIR spec-
tra were monitored during the stepwise addition of the
acceptor molecules. The absorption spectrum of P1 in 1,2-
dichlorobenzene possessed the longest wavelength absorp-
tion (kmax) at 488 nm (2.54 eV) as a shoulder peak. When a
TCNE solution in 1,2-dichloroethane was added, the
There is, to the best of our knowledge, only a limited num-
ber of reports on the polymerization of 3-alkyne-substituted
thiophene monomers.14 For example, Yamamoto and co-
workers15 synthesized a series of polythiophene derivatives
by the Stille and Suzuki polycondensation. Li and co-
workers16 also reported the synthesis of such polythiophene
derivatives by the Grignard metathesis method. However, it
is also suggested that the coordination of a nickel catalyst to
SCHEME 2 Synthesis of polythio-
phene derivative and the postfunc-
tionalization by TCNE/TCNQ addition.
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ARTICLE
FIGURE 2 UV–vis–NIR spectral change of P1 during the titration of (a) TCNE in 1,2-dichloroethane and (b) TCNQ in 1,2-dichloro-
benzene. The titration experiments of TCNE and TCNQ were performed at 20 ꢀC and 160 ꢀC, respectively. All spectra were taken at
20 ꢀC.
intensities of the longer wavelength absorptions at about
484 and 425 nm quickly increased, while the shorter wave-
length band at 347 nm decreased [Fig. 2(a)]. The increased
absorptions were charge-transfer (CT) bands originating
from the intramolecular donor–acceptor interactions,
because these bands displayed solvatochromic behaviors
from 480 and 426 nm in CH2Cl2 to 456 and 410 nm in hex-
acceptor moieties were introduced by the TCNQ addition.
Thus, P3 (x ¼ 1) exhibited much broader original aromatic
ring protons as well as relatively sharp quinone protons. The
acceptor additions were also confirmed by the IR spectra
(Fig. 4). The precursor polymer P1 displayed a weak peak at
2194.2 cmꢁ1, which was ascribed to the alkyne vibration of
the side chains. After the TCNE and TCNQ additions, this
peak completely disappeared. The TCNE-adducted polymer
P2 (x ¼ 1) showed a new and stronger peak ascribed to the
cyano vibration at 2216.5 cmꢁ1. The TCNQ-adducted poly-
mer P3 (x ¼ 1) similarly displayed the cyano peak at 2202.4
cmꢁ1 together with a small side band at 2179.5 cmꢁ1. The
absence of the cyano peaks of TCNE and TCNQ, which were
observed at 2257 cmꢁ1 and 2227 cmꢁ1, respectively, indi-
cated no residual unreacted acceptor molecules in the poly-
mer samples. The position and relative intensities of these
peaks were consistent with previous reports.18 The nitrogen
contents of P2 (x ¼ 1) and P3 (x ¼ 1) determined by ele-
mental analysis also revealed good agreement with the theo-
retical values.
ꢀ
ane. The spectral change continued at 20 C until an equiva-
lent amount of TCNE relative to the polymer repeat unit was
added. The presence of the isosbestic point at 382 nm indi-
cated the absence of any undesired side reactions. All these
results suggest the occurrence of the click postfunctionaliza-
tion of P1. When compared with the compact acceptor mole-
cule of TCNE, the postfunctionalization using the TCNQ addi-
tion required heating. Based on the TGA of P1 and the
products (vide infra), the reaction temperature was deter-
ꢀ
mined to be 160 C. Similar to the TCNE addition, a new CT
band of lower energy was observed at 750 nm and the in-
tensity gradually increased with the increasing amount of
TCNQ addition up to 1.0 eq. [Fig. 2(b)]. Despite the higher
reaction temperature, a smooth postfunctionalization without
any side reactions was suggested from the isosbestic point
at 376 nm.
It is reported that the construction of donor–acceptor chro-
mophores by the postfunctional TCNE/TCNQ additions some-
times improves the thermal and chemical stabilities of poly-
mers.18 To estimate the decomposition temperatures of the
obtained polymers, TGA meaꢁsu1rements were performed at
Based on the titration experiments of TCNE and TCNQ, the
fully postfunctionalized polymers P2 and P3 with x ¼ 1
were readily prepared from P1 in a click chemistry fashion.
The GPC profiles of P2 (x ¼ 1) and P3 (x ¼ 1) suggested
almost the same molecular weights with slightly wider poly-
dispersities compared with P1, which were probably caused
by the decreased solubilities of the polymers in the eluent
and the concomitant adhesion to the polystyrene gel col-
umns (Table 1). The 1H NMR spectrum of P1 displayed rea-
sonable peaks ascribed to the hexadecyl chains and aromatic
protons [Fig. 3(a)]. The latter peaks were complicated
because of the uncontrolled regioregularity of the side-chain
positions. After the TCNE addition, the alkyl chain protons of
P2 (x ¼ 1) did not significantly change, whereas the aro-
matic ring protons became equivocal probably because of
the steric hindrance of the 1,1,4,4-tetracyanobutadiene
(TCBD) acceptor moieties. This effect was more significant
when the cyclohexa-2,5-diene-1,4-diylidene-expanded larger
ꢀ
the heating rate of 10 C min under flowing nitrogen. The
TABLE 1 Summary of Molecular Weights,
Decomposition Temperatures, and Optical Properties
of Polythiophene Derivatives
Td5%
kmax
kend
a
a
Mn
6600 1.80
Mw/Mn
(ꢀC)b [nm (eV)]c
[nm (eV)]c
P1
252
262
236
488 (2.54)d
484 (2.56)d
750 (1.65)
578 (2.15)
810 (1.53)
1116 (1.11)
P2 (x ¼ 1) 6100 2.40
P3 (x ¼ 1) 7200 2.01
a
Measured by GPC (eluent: THF).
b
c
Temperature at which 5% weight loss occurred on heating.
Measured in 1,2-dichloroethane.
Shoulder.
d
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FIGURE 3 1H NMR spectra of (a) P1,
(b) P2 (x ¼ 1), and (c) P3 (x ¼ 1) in
CDCl3 at 20 ꢀC.
onset decomposition temperatures of all the polymers
exceeded 180 ꢀC, which ensured no decomposition during
the postfunctionalization (Fig. 5). The TGA curve of the
TCNE-adducted polymer P2 (x ¼ 1) actually showed a
slightly enhanced thermal stability compared with the pre-
tion of donor–acceptor chromophores in the polythiophene
side chains by the postfunctionalization lowered the polymer
band gap. The band gap lowering was more significant when
the expanded acceptor of TCNQ was used. Thus, the kend of
P3 (x ¼ 1) was 1116 nm, corresponding to the optical band
gap of 1.11 eV.
curosor polymer P1. Thꢀe 5% weight loss temperature (Td5%
)
ꢀ
of P2 (x ¼ 1) was 262 C, which was 10 C higher than that
of P1 (Table 1). The Td5% of P3 (x ¼ 1) was, on the contrary,
lower than that of P1.
To verify the optical band gaps and determine the absolute
energy levels, cyclic voltammograms (CVs) were measured in
CH3CN with 0.1 M (nC4H9)4NClO4 at 20 ꢀC for the polymer
thin films cast on a glassy carbon electrode. Several post-
functionalized polymers P2 and P3 with different acceptor
addition amounts were prepared to elucidate the relation-
ship between the added acceptor amount (x) and the abso-
lute energy levels. The CV curves of all polymers are shown
in Figure 6, and the peak top values are summarized in
Although the kmax values of P1 and P2 (x ¼ 1) were almost
the same, the end absorption (kend) values showed a clear
difference between these two polymers. The kend of P1
measured in 1,2-dichloroethane was 578 nm (2.15 eV), while
P2 (x ¼ 1) displayed the bathochromically shifted kend value
at 810 nm (1.53 eV). This result suggested that the forma-
Table 2. P1 showed the first oxidation peak potential (Eox1
)
at 0.412 V (vs. Fc/Fcþ), which was ascribed to the N,N-
FIGURE 4 IR spectra of (a) P1, (b) P2 (x ¼ 1), and (c) P3 (x ¼ 1)
FIGURE 5 TGA curves of P1, P2 (x ¼ 1), and P3 (x ¼ 1) at the
(KBr pellet).
heating rate of 10 ꢀC minꢁ1 under flowing nitrogen.
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ARTICLE
FIGURE 6 Cyclic voltammograms
of polymer thin films (a) P1, (b)
P2 (x ¼ 0.33), (c) P2 (x ¼ 0.66), (d)
P2 (x ¼ 1), (e) P3 (x ¼ 0.33), (f) P3
(x ¼ 0.66), and P3 (x ¼ 1) in
CH3CN with 0.1 M (nC4H9)4NClO4
at 20 ꢀC, at the scanning rate of
0.1 V sꢁ1
.
dihexadecylaniline moieties. Oxidation and reduction of the
polythiophene main chain were detected at 1.242 V and
ꢁ2.318 V, respectively. A stepwise TCNE addition to P1 grad-
ually changed the oxidation and reduction potentials. The
Eox1 anodically shifted with the increasing TCNE addition
amount. For example, P2 (x ¼ 0.33) and P2 (x ¼ 0.66)
TABLE 2 Redox Potentials and Energy Levels of Polythiophene Derivativesa
Ered3
Ered2
Ered1
Eox1
Eox2
HOMO
(eV)b
LUMO
(eV)b
(V)
(V)
(V)
(V)
(V)
P1
ꢁ2.318
ꢁ1.708
ꢁ1.178
ꢁ0.828
ꢁ0.136
ꢁ0.059
0.066
0.412
0.502
0.585
0.862
0.486
0.531
0.796
1.242
1.102
1.042
ꢁ5.21
ꢁ5.30
ꢁ5.39
ꢁ5.66
ꢁ5.29
ꢁ5.33
ꢁ5.60
ꢁ2.48
ꢁ3.09
ꢁ3.62
ꢁ3.97
ꢁ4.66
ꢁ4.74
ꢁ4.87
P2 (x ¼ 0.33)
P2 (x ¼ 0.66)
P2 (x ¼ 1)
P3 (x ¼ 0.33)
P3 (x ¼ 0.66)
P3 (x ¼ 1)
a
ꢁ2.168
ꢁ1.422
ꢁ1.214
ꢁ0.481
ꢁ0.396
ꢁ0.290
ꢁ1.958
ꢁ1.728
1.071
0.867
All potentials were represented as the peak top value versus Fc/Fcþ.
b
HOMO and LUMO levels were estimated from Eox1 and Ered1 values, respectively, based on the assump-
tion of Fc/Fcþ ¼ ꢁ4.80 eV.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 Relationship between
the amount of the added acceptor
molecules, (a) TCNE and (b) TCNQ,
and the HOMO and LUMO levels,
determined by the Eox1 and Ered1
,
respectively.
displayed the N,N-dihexadecylaniline-centered Eox1 at 0.502
V and 0.585 V, respectively. However, the Eox2 originating
from the polythiophene main chain cathodically shifted from
1.102 V for P2 (x ¼ 0.33) to 1.042 V for P2 (x ¼ 0.66).
These two oxidation peaks were fused into a single broad
peak with the Eox1 of 0.862 V for P2 (x ¼ 1). In contrast to
the oxidation peaks, the origin of the first reduction peak
potential (Ered1) was changed from the polythiophene main
chain to the TCBD moieties when TCNE was added to P1.
Thus, the reduction of P1 was facilitated as more TCNE was
added. The Ered1 of P1 detected at ꢁ2.318 V bathochromi-
cally shifted to ꢁ1.708 V, ꢁ1.178 V, and ꢁ0.828 V for P2
with x ¼ 0.33, 0.66, and 1, respectively. The TCBD moieties
of P2 displayed the multiple reduction peaks with an
improved reversibility in the CVs. The addition of TCNQ
instead of TCNE resulted in similar changes in the electro-
chemical behaviors. The oxidations of the N,N-dihexadecyla-
niline moieties and polythiophene main chain were detected
at different potentials when the TCNQ addition amount (x)
was 0.33 and 0.66, whereas a single broad peak was
observed at 0.796 V for P3 (x ¼ 1). The bathochromic (posi-
tive) shift in the Ered1 from P1 to P3 (x ¼ 0.33) was signifi-
cant because of the expanded acceptor moiety. A slight bath-
ochromic shift in the Ered1 was further recorded from
ꢁ0.136 V for P3 (x ¼ 0.33) to ꢁ0.056 V for P3 (x ¼ 0.66)
and 0.066 V for P3 (x ¼ 1). It should be noted that the Ered1
values of P3 suggest a0 stronger electron-accepting power
than TCNQ with the Eo of ꢁ0.25 V.19 This was probably
because of the efficient communication of the multiple
acceptor moieties through the planar conjugated main
chains, such as polythiophenes.
corresponding P2. The electrochemical band gaps of P2 (x ¼
1) and P3 (x ¼ 1) were 1.69 V and 0.86 V, respectively,
which were in fair agreement with the corresponding optical
band gaps determined from the kend values [1.53 eV for P2
(x ¼ 1) and 1.11 eV for P3 (x ¼ 1)]. As a result, the order of
the band gaps decreased in the order of P1 > P2 > P3.
CONCLUSIONS
It was demonstrated that the click chemistry-type postfunc-
tionalization using the high-yielding addition reactions
between electron-rich alkynes and TCNE/TCNQ acceptor
molecules lowers the energy levels of the polythiophene de-
rivative. The data suggested that the careful selection of the
species and amount of the added acceptor molecules enables
fine control of the polymer energy levels. In this study, there
was apparently an efficient communication between the
side-chain groups because of the highly conjugated polythio-
phene main chain. Accordingly, the extent of the decrease in
the LUMO levels was much greater compared with the previ-
ous report for an aromatic polyamine.12 The LUMO level
decrease of the polythiophene derivative on the TCNE and
TCNQ additions amounted to 1.49 eV and 2.39 eV, respec-
tively. These results provide a promising prospect for apply-
ing this methodology to other conjugated polymers and
building super acceptor polymers. Further interests also
include the regioregularity control of the polythiophene
derivatives.
This work was supported, in part, by a Grant-in-Aid for Sci-
entific Research and the Special Coordination Funds for Pro-
moting Science and Technology from MEXT, Japan, the Mur-
ata Science Foundation, and the Association for the Progress
of New Chemistry.
The HOMO and LUMO energy levels of all the polymers were
calculated from the Eox1 and Ered1 values, respectively, based
on the assumption of Fc/Fcþ ¼ ꢁ4.80 eV.20 The energy lev-
els were plotted versus the added acceptor amount (x; Fig.
7). The data clearly indicated that both the HOMO and
LUMO levels decreased as more acceptor molecules were
added (Table 2). The extent of the decrease in the LUMO lev-
els was more significant compared with the decreased
HOMO levels. Accordingly, the electrochemical band gaps,
calculated from the difference between the HOMO and LUMO
levels, reasonably decreased after the postfunctionalization.
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