Fine tuning of frontier orbital energy levels in dithieno[3,2-b:2′, 3′-d]silole-based copolymers based on the substituent effect of phenyl pendants

Article abstract of DOI:10.1016/j.polymer.2014.03.021

A series of dithieno[3,2-b:2′,3′-d]silole-based π-conjugated copolymers containing thieno[3,4-c]pyrrole-4,6-dione or thieno[3,4-b]thiophene units bearing 4-substituted phenyl pendants were synthesized and their thermal stability, optical properties and frontier orbital energy levels were systematically investigated. The introduction of electron-withdrawing substituents on the phenyl rings lowered their frontier orbital energy levels without deteriorating their thermal and optical properties. By replacing an electron-donating methoxy group with an electron-withdrawing trifluoromethyl group, both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital energy levels of the polymers were deepened by more than 0.3 eV. A relatively linear relationship was observed between the HOMO energy levels and the Hammett substituent constants.

Full text of DOI:10.1016/j.polymer.2014.03.021

Polymer 55 (2014) 2139e2145
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Polymer
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Fine tuning of frontier orbital energy levels in dithieno[3,2-b:20,30-d]
silole-based copolymers based on the substituent effect of phenyl
pendants
,
b
,
a
a
a
Tomoyuki Ikai a , Tomoya Kudo , Masahiro Nagaki , Tomoyuki Yamamoto ,
*
Katsuhiro Maeda a , Shigeyoshi Kanoh
,
b,
a,b
*
a Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
b Research Center for Sustainable Energy and Technology, College of Science and Engineering, Kanazawa University, Kakuma-machi,
Kanazawa 920-1192, Japan
a r t i c l e i n f o
a b s t r a c t
A series of dithieno[3,2-b:20,30-d]silole-based
p-conjugated copolymers containing thieno[3,4-c]pyrrole-
Article history:
Received 3 February 2014
Received in revised form
7 March 2014
Accepted 13 March 2014
Available online 20 March 2014
4,6-dione or thieno[3,4-b]thiophene units bearing 4-substituted phenyl pendants were synthesized and
their thermal stability, optical properties and frontier orbital energy levels were systematically investi-
gated. The introduction of electron-withdrawing substituents on the phenyl rings lowered their frontier
orbital energy levels without deteriorating their thermal and optical properties. By replacing an electron-
donating methoxy group with an electron-withdrawing trifluoromethyl group, both the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital energy levels of the polymers
were deepened by more than 0.3 eV. A relatively linear relationship was observed between the HOMO
energy levels and the Hammett substituent constants.
Keywords:
p-Conjugated copolymer
Electron-donor
HOMOeLUMO energy level
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
The following two approaches have been widely used to nar-
row the bandgap energies of p-conjugated polymers. The first was
Much attention has been paid to bulk heterojunction-type
polymer solar cells (BHJ-type PSCs) consisting of interpenetrating
to incorporate alternating electron-deficient units (A) and
electron-rich units (D) along the polymer main-chains, inducing
an intramolecular chargeetransfer interaction that effectively
narrows the bandgaps [21,22]. Among a huge number of D-A-type
conjugated polymers developed thus far, thieno[3,4-c]pyrrole-4,6-
dione (TPD) is one of the most useful A units [23e32] and BHJ-
type PSCs based on D-A-type conjugated polymers consisting of
alternating TPD and benzo[1,2-b:4,5-b0]dithiophene (BDT) units
have exhibited a high power-conversion efficiency (PCE) of 8.5%
[23]. The second approach is to use the stabilization of quinoidal
resonance structures in the polymer main-chains. Thieno[3,4-b]
thiophene (TT) is a representative unit used to stabilize the
quinoidal resonance structure. TT has been incorporated into
polymer backbones as a key component in electron donors [33e
networks of
p-conjugated polymers as electron donors and
fullerene derivatives as electron acceptors [1e8]. This is because
BHJ-type PSCs have potential advantages, such as their flexible
and lightweight characteristics and the low-cost manufacturing
of large-area devices using printing processes [9]. Regioregular
poly(3-hexylthiophene) (P3HT) has been used as a typical electron
donor in the active layers of BHJ-type PSCs, because of its solubility,
stability and compatibility with the electron acceptor [6,6]-phenyl-
C
61-butyric acid methyl ester (PC61BM) [10e13]. However, P3HTcan
only harvest ca. 23% of all solar photons because its absorption limit
is less than 650 nm [14]. Therefore, extensive research has been
conducted to develop narrow bandgap polymers that can harvest
more of the solar photons than P3HT [15e20].
37]. Particularly,
p-conjugated polymers containing TT and BDT
units (PTB-based polymers) have exhibited a maximum PCE of
over 9% [38e45]. The extended absorption properties of these
narrow bandgap polymers directly contributed to increasing the
short-circuit current density (Jsc).
To enhance the PCEs, the open-circuit voltages (Voc) of PSCs
should be increased along with improving the Jsc. It is well known
* Corresponding authors. Graduate School of Natural Science and Technology,
Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. Tel./fax: þ81 76
234 4781.
E-mail addresses: ikai@se.kanazawa-u.ac.jp (T. Ikai), maeda@se.kanazawa-u.ac.jp
(K. Maeda).
http://dx.doi.org/10.1016/j.polymer.2014.03.021
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

2140
T. Ikai et al. / Polymer 55 (2014) 2139e2145
that the Voc is closely related to the difference between the highest
occupied molecular orbital (HOMO) of the electron donors and the
lowest unoccupied molecular orbital (LUMO) of the electron ac-
ceptors [46,47]. Therefore, deepening the HOMO energy levels of
narrow bandgap polymers is a useful strategy to develop efficient
PSCs with high Voc and Jsc values. Recently, the HOMO and LUMO
energy levels of PTB-based polymers were effectively deepened by
introducing electron-withdrawing substituents on the phenyl rings
that were connected to the TT units through an ester linkage [48].
An advantage of this method is that the frontier orbital energy levels
of the polymers can be precisely controlled by tuning the electron-
withdrawing ability of the substituents on the phenyl rings.
2.3. Synthesis of TPD-based monomers
2.3.1. 1,3-Dibromo-5-(4-trifluoromethylphenyl)thieno[3,4-c]
pyrrole-4,6-dione (TPD1)
Acetic anhydride (7.6 mL) was added to 1 (0.50 g, 1.52 mmol)
under a nitrogen atmosphere. The mixture was heated to 140 ꢀC for
12 h and then cooled to room temperature. The volatile compounds
were removed in vacuo and the crude product was used in the next
step without purification. After the brown solid was dissolved in
toluene (1 mL), 4-trifluoromethylaniline (294 mg, 1.83 mmol) was
added to the solution and the mixture was heated under reflux for
24 h. After removing the volatile components, thionyl chloride was
added to the residue and the reaction mixture was stirred at 90 ꢀC
for 12 h. The volatile components were removed in vacuo and the
residue was extracted with chloroform. The organic phase was
dried with anhydrous sodium sulfate and the solvent was removed
under a reduced pressure. The crude product was purified by col-
umn chromatography with silica gel using chloroform as the eluent.
The product was recrystallized from ethanol to afford TPD1 as an
off-white solid with a yield of 81%. 1H NMR (500 MHz, CDCl3, rt):
In the present study, we synthesized dithieno[3,2-b:20,30-d]
silole-based
p-conjugated polymers containing either TPD or TT
units as the comonomer units, which possessed 4-substituted
phenyl pendants. The influence of the substituents on the phenyl
pendants was systematically investigated by measuring the ther-
mal stability, optical properties and frontier orbital energy levels.
The DTS unit was used as the common building unit because
DTS-based polymers have a higher photostability than the corre-
sponding polymers containing BDT units and are more practical for
use as electron donors in PSCs [49].
d
7.76 (d, J ¼ 8.0 Hz, 2H), 7.55 (d, J ¼ 8.0 Hz, 2H). 13C NMR (125 MHz,
CDCl3, rt):
d 158.73, 134.55, 133.86, 130.14, 126.42, 126.26, 126.23,
114.90. Anal. Calcd for C13H4Br2F3NO2S: C, 34.31; H, 0.89; N, 3.08.
Found: C, 34.11; H, 0.95; N, 3.06.
2. Experimental
2.1. Materials
2.3.2. 1,3-Dibromo-5-(4-fluorophenyl)thieno[3,4-c]pyrrole-4,6-
dione (TPD2)
The anhydroussolvents (toluene, N,N-dimethylformamide(DMF)
and tetrahydrofuran (THF)), the common organic solvents, aniline
and p-anisidine were purchased from Kanto (Tokyo, Japan). Acetic
anhydride, 2-bromothiophene, 2-(tributylstannyl)thiophene and
n-octylamine were bought from Tokyo Kasei (TCI, Tokyo, Japan).
Thionyl chloride and 4-fluoroaniline were purchased from
Wako (Osaka, Japan). Chlorobenzene was obtained from Kishida
(Osaka, Japan). Tetrakis(triphenylphosphine)palladium(0) and 4-
trifluoromethylaniline were purchased from Nacalai (Kyoto, Japan).
The thieno[3,4-b]thiophene-based monomers (TT1eTT4) [48], 2,5-
dibromo-3,4-thiophenedicarboxylic acid (1) [50] and 4,4-bis(2-
ethylhexyl)-2,6-bis(trimethyltin)-dithieno[3,2-b:20,30-d]silole (DTS)
[51] were prepared according to the procedures in the literature.
The title compound was prepared from 1 and 4-fluoroaniline in
the same way as TPD1 and obtained in 53% yield. 1H NMR
(500 MHz, CDCl3, rt):
NMR (125 MHz, CDCl3, rt):
d
7.34 (m, 2H), 7.17 (t, J ¼ 10.0 Hz, 2H). 13C
d 161.19, 161.18, 134.11, 128.31, 127.38,
116.20, 114.41. Anal. Calcd for C12H4Br2FNO2S: C, 35.59; H, 1.00; N,
3.46. Found: C, 35.76; H, 1.22; N, 3.48.
2.3.3. 1,3-Dibromo-5-phenylthieno[3,4-c]pyrrole-4,6-dione (TPD3)
The title compound was prepared from 1 and aniline in the same
way as TPD1 and obtained in 85% yield. 1H NMR (500 MHz, CDCl3,
rt):
d
7.50 (t, J ¼ 7.7 Hz, 2H), 7.42 (t, J ¼ 7.4 Hz, 1H), 7.36 (m, 2H). 13
C
NMR (125 MHz, CDCl3, rt):
d 159.27, 134.21, 131.41, 129.11, 128.56,
126.41, 114.15. Anal. Calcd for C12H5Br2NO2S: C, 37.24; H, 1.30; N,
3.62. Found: C, 37.04; H, 1.39; N, 3.64.
2.2. Measurements
2.3.4. 1,3-Dibromo-5-(4-methoxyphenyl)thieno[3,4-c]pyrrole-4,6-
dione (TPD4)
The 1H and 13C NMR spectra were measured in CDCl3 at room
temperature and 55 ꢀC with a JEOL ECA-500 spectrometer (JEOL,
Tokyo, Japan). The molecular weights and distributions of the
polymers were estimated using size-exclusion chromatography
(SEC) equipped with a Shodex KF-805L column (Showa Denko,
Tokyo, Japan), a JASCO PU-2080 Plus HPLC pump (Jasco, Tokyo,
Japan), a JASCO UV-970 UV/VIS detector at 650 nm and a JASCO CO-
1560 column oven, where THF was used as the eluent. The mo-
lecular weight calibration curve was obtained with polystyrenes
(Tosoh). Thermogravimetric analysis (TGA) was conducted using a
TG/DTA6200 (SII NanoTechnology Inc., Chiba, Japan) with a heating
rate of 10 ꢀC/min under a flow of nitrogen. The UV-vis-NIR spectra
of the spin-coated films prepared on glass substrates from a chlo-
robenzene solution (5 mg/mL) were measured using a JASCO V-570
spectrometer. The HOMOs of the polymers were measured using
a photoelectron spectrometer AC-2 (Riken Keiki, Tokyo, Japan)
by measuring the ionization potential of polymer films in air.
The ionization potential of the polymers was estimated using the
plots of the photoelectron quantum yield from the polymer solid
surface against the incident photon energy. The threshold of the
photoelectron quantum yield was correlated with the ionization
potential.
The title compound was prepared from 1 and p-anisidine in the
same way as TPD1 and obtained in 22% yield. 1H NMR (500 MHz,
CDCl3, rt):
d
7.27 (d, J ¼ 9.0 Hz, 2H), 6.98 (d, J ¼ 9.0 Hz, 2H), 3.83
159.71, 159.52, 134.59,
(s, 3H). 13C NMR (125 MHz, CDCl3, rt):
d
127.79, 124.46, 114.55, 113.80, 55.54. Anal. Calcd for C13H7Br2NO3S:
C, 37.44; H, 1.69; N, 3.36. Found: C, 37.31; H, 1.89; N, 3.31.
2.3.5. 1,3-Dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione (TPD5)
The title compound was prepared from 1 and n-octylamine in
the same way as TPD1 and obtained in 75% yield. 1H NMR
(500 MHz, CDCl3, rt):
d
3.59 (t, J ¼ 7.3 Hz, 2H), 1.63 (t, J ¼ 7.3 Hz, 2H),
1.4-1.2 (m, 10H), 0.87 ppm (t, J ¼ 6.8 Hz, 3H). 13C NMR (125 MHz,
CDCl3, rt): d 160.35, 134.76, 112.88, 38.79, 31.73, 29.07, 28.21, 26.75,
22.58, 14.06. Anal. Calcd for C14H17Br2NO2S: C, 39.74; H, 4.05; N,
3.31. Found: C, 39.85; H, 3.95; N, 3.29.
2.4. Synthesis of polymers
2.4.1. Copolymerization of DTS and TPD-based monomers
Polymerization was carried out in a dry Schlenk flask under
a nitrogen atmosphere in a similar manner to that previously

T. Ikai et al. / Polymer 55 (2014) 2139e2145
2141
Table 1
Polymerization results and thermal and optical properties of the polymers.
Run
Polymer
Yield (%)a
Mn (104)b
PDIb
Td5 (ꢀC)c
lonset (nm)
Eogpt (eV)d
HOMO (eV)e
LUMO (eV)f
1
2
3
4
5
6
7
8
9
PDTSTPD1
PDTSTPD2
PDTSTPD3
PDTSTPD4
PDTSTPD5
PDTSTT1
PDTSTT2
PDTSTT3
PDTSTT4
30
34
40
25
41
87
67
69
76
1.1
1.3
1.5
2.3
2.5
1.6
1.7
1.6
1.4
1.8
1.5
1.7
1.8
1.7
2.5
2.7
2.5
2.4
435
435
430
432
428
348
380
386
367
742
733
729
729
719
913
908
900
899
1.67
1.69
1.70
1.70
1.72
1.36
1.36
1.38
1.38
ꢁ5.88
ꢁ5.56
ꢁ5.42
ꢁ5.38
ꢁ5.38
ꢁ5.40
ꢁ5.20
ꢁ5.09
ꢁ5.07
ꢁ4.21
ꢁ3.87
ꢁ3.72
ꢁ3.68
ꢁ3.66
ꢁ4.04
ꢁ3.84
ꢁ3.71
ꢁ3.69
a
MeOH and hexane insoluble part.
b
c
d
e
f
Determined by SEC (eluent: THF, PSt standards).
The 5% weight-loss temperatures with a heating rate of 10 ꢀC/min in N2.
Calculated from Eg ¼ 1240/lonset
.
Measured by photoelectron spectroscopy in air.
Calculated from LUMO ¼ Eg þ HOMO.
reported [28]. Each TPD-based monomer was purified by recrys-
tallization before use. A typical polymerization procedure is
described below.
DTS (111 mg, 0.15 mmol), TPD1 (68 mg, 0.15 mmol) and Pd(PPh3)4
(14 mg, 12 mmol) was placed in a Schlenk flask under a nitrogen
dichloromethane for 12 h in a Soxhlet apparatus to remove any
by-products and oligomers. After the polymer was collected in
chloroform, the chloroform solution was passed through Celite to
remove the metal catalyst. The solution was concentrated and
poured into hexane. The precipitate was collected by centrifugation
and dried in vacuo to yield PDTSTPD1, which was a dark purple solid
atmosphere. Anhydrous toluene (2.7 mL) and DMF (0.27 mL) were
added with a syringe and the mixture was heated to 110 ꢀC. After
(32 mg, 30%). 1H NMR (500 MHz, CDCl3, 55 ꢀC):
d 8.7e7.4 (br, 6H),
42 h, a solution of 2-(tributylstannyl)thiophene (14 mg, 38
m
mol) in
2.0e0.5 (br, 34H). Anal. Calcd for (C37H40F3NO2S3Si)n: C, 62.42; H,
5.66; N, 1.97. Found: C, 62.12; H, 5.73; N, 1.93.
0.27 mL of toluene/DMF (10:1, v/v) was added with a syringe and the
reaction mixture was heated to 110 ꢀC for 5 h. A solution of 2-
In the same way, PDTSTPD2, PDTSTPD3, PDTSTPD4 and
PDTSTPD5 were synthesized with their corresponding monomers.
The polymerization results are summarized in Table 1.
Spectroscopic data for PDTSTPD2. 1H NMR (500 MHz, CDCl3,
bromothiophene (6 mg, 38 mmol) in 0.27 mL of toluene/DMF (10:1,
v/v) was then added and the reaction mixture was further heated at
110 ꢀC for an additional 8 h to complete the end-capping reaction.
The reaction mixture was cooled to room temperature and poured
into methanol (MeOH). The resulting polymer was collected by
centrifugation and washed with hexane for 12 h and then with
55 ꢀC):
d 8.7e8.0 (br, 2H), 7.7e6.9 (br, 4H), 2.0e0.5 (br, 34H). Anal.
Calcd for (C36H40FNO2S3Si)n: C, 65.32; H, 6.09; N, 2.12. Found: C,
65.55; H, 6.11; N, 2.10.
Scheme 1. Synthesis and structures of monomers and polymers.

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T. Ikai et al. / Polymer 55 (2014) 2139e2145
Fig. 1. Absorption spectra of the polymers in chlorobenzene (a,b) and in the film states (c,d).
Spectroscopic data for PDTSTPD3. 1H NMR (500 MHz, CDCl3,
55 ꢀC):
8.7e8.0 (br, 2H), 7.6e7.1 (br, 5H), 1.8e0.7 (br, 34H). Anal.
by-products and oligomers. After the polymer was collected in
d
chlorobenzene, the chlorobenzene solution was passed through
Celite to remove the metal catalyst. The solution was concentrated
and poured into hexane. The precipitate was collected by centri-
fugation and dried in vacuo to yield PDTSTT1, which was a dark blue
Calcd for (C36H41NO2S3Si)n: C, 67.14; H, 6.42; N, 2.18. Found: C,
66.76; H, 6.78; N, 2.09.
Spectroscopic data for PDTSTPD4. 1H NMR (500 MHz, CDCl3,
55 ꢀC):
d
8.7e8.0 (br, 2H), 7.4e7.0 (br, 4H), 4.1e3.6 (br, 3H), 1.7e0.5
solid (100 mg, 87%). 1H NMR (500 MHz, CDCl3, 55 ꢀC):
d 8.0e6.8 (br,
(br, 34H). Anal. Calcd for (C37H43NO3S3Si)n: C, 65.93; H, 6.43; N,
2.08. Found: C, 65.51; H, 6.11; N, 2.05.
7H), 2.0e0.5 (br, 34H). Anal. Calcd for (C38H41F3O2S4Si$0.5H2O)n: C,
60.69; H, 5.63. Found: C, 60.74; H, 5.63.
Spectroscopic data for PDTSTPD5. 1H NMR (500 MHz, CDCl3,
Using the same method, PDTSTT2, PDTSTT3, PDTSTT4 and
PDTSTT5 were synthesized with their corresponding monomers.
The polymerization results are summarized in Table 1.
Spectroscopic data for PDTSTT2. 1H NMR (500 MHz, CDCl3,
55 ꢀC):
d 8.2e6.8 (br, 2H), 4.1e3.2 (br, 2H), 2.1e0.5 (br, 49H). Anal.
Calcd for (C38H53NO2S3Si)n: C, 67.11; H, 7.86; N, 2.06. Found: C,
67.22; H, 7.69; N, 2.00.
55 ꢀC):
d 8.3e6.8 (br, 7H), 2.0e0.4 (br, 34H). Anal. Calcd for
2.4.2. Copolymerization of DTS and TT-based monomers
(C37H41FO2S4Si$0.4H2O)n: C, 63.46; H, 6.02. Found: C, 63.42; H, 6.13.
Polymerization was carried out in a dry Schlenk flask under a
nitrogen atmosphere using a similar manner to that previously
reported [48]. Each TT-based monomer was purified by recrystal-
Spectroscopic data for PDTSTT3. 1H NMR (500 MHz, CDCl3,
55 ꢀC):
d 8.2e6.9 (br, 8H), 1.9e0.5 (br, 34H). Anal. Calcd for
(C37H42O2S4Si$0.6H2O)n: C, 64.79; H, 6.35. Found: C, 64.75; H, 6.29.
lization prior to use.
described below.
A
typical polymerization procedure is
Spectroscopic data for PDTSTT4. 1H NMR (500 MHz, CDCl3,
55 ꢀC):
d 8.3e6.8 (br, 7H), 4.0e3.7 (br, 3H), 2.0e0.5 (br, 34H). Anal.
DTS (116 mg, 0.16 mmol), TT1 (76 mg, 0.16 mmol) and Pd(PPh3)4
(7.4 mg, 6.4 mol) were placed in a Schlenk flask under a nitrogen
Calcd for (C38H44O3S4Si$0.8H2O)n: C, 63.44; H, 6.39. Found: C,
63.42; H, 6.16.
m
atmosphere. Anhydrous toluene (2.56 mL) and DMF (0.64 mL) were
added with a syringe and the mixture was then heated to 120 ꢀC.
After 12 h, the reaction mixture was cooled to room temperature
and poured into a large amount of MeOH. The resulting polymer
was collected by centrifugation and washed with MeOH followed
by hexane for 12 h each in a Soxhlet apparatus to remove any
2.5. Computational study
The density functional theory (DFT) calculations were per-
formed at the B3LYP/6-31G (d,p) level [52,53] using the Gaussian 03
package. The model compounds, which consisted of either a TPD or

T. Ikai et al. / Polymer 55 (2014) 2139e2145
2143
Fig. 2. Plots of experimental (closed circle) and calculated (open circle) HOMO energy levels of PDTSTPD- (a,c) and PDTSTT- (b,d) based polymers against the Hammett constant
s of
the substituents.
a TT unit and two DTS units bearing methyl groups instead of
2-ethylhexyl groups, were used for the computational study as
simplified models.
3.2. Thermal properties
The thermal stabilities of these polymers were investigated by
TGA under nitrogen atmosphere (Supporting information,
a
Fig. S1). Both types of polymers demonstrated good thermal sta-
bility with 5% weight-loss temperatures (Td5) above 300 ꢀC
(Table 1), which is adequate for application in PSC devices. Partic-
ularly, the Td5 values for the PDTSTPD-based polymers were around
430 ꢀC and much higher than those for the corresponding PDTSTT-
based polymers. The superior thermal stability of the PDTSTPD-
based polymers was likely derived from the imide linkage in the
TPD unit, which is more robust than the ester linkage in the TT unit.
In addition, the Td5 values for the PDTSTPD-based polymers bearing
phenyl pendants on the TPD unit were slightly higher than that for
PDTSTPD5 bearing alkyl pendants [28], so replacement of alkyl
group with phenyl group seems to enhance thermal stability.
3. Results and discussion
3.1. Synthesis
The synthetic routes for the monomers and polymers are shown
in Scheme 1. Four TPD-based monomers bearing various phenyl
groups (TPD1eTPD4) were easily prepared in one pot from
commercially available aniline derivatives and the key precursor 1,
which was synthesized using modified procedures from the liter-
ature [50]. The copolymerizations of DTS and TPD1eTPD4 by Stille
cross-coupling were carried out using Pd(PPh3)4 as the catalyst in a
toluene/DMF mixture (10/1, v/v) at 110 ꢀC (Table 1). For comparison,
the previously reported polymer, PDTSTPD5 (run 5 in Table 1)
containing n-octyl chains instead of phenyl pendants, was also
prepared [28]. In addition, PDTSTT-based polymers were also syn-
thesized from DTS and TT1eTT4 comonomers (run 6e9 in Table 1)
using a similar method as the PDTSTPD-based polymers. Both the
PDTSTPD- and PDTSTT-based polymers exhibited good solubility in
THF, chloroform and chlorobenzene. The molecular weights (Mn)
and distributions (PDI) of the obtained polymers were determined
by SEC in THF. All of the polymers had similar molecular weights.
Thus, the structural modification of the phenyl groups with TPD
and TT units had little effect on the polymerizability of these
monomers.
3.3. Optical properties
Fig. 1 shows the absorption spectra of the polymers in chloro-
benzene and in the film state. The absorption spectrum of each
polymer in solution is very similar to that in the solid state, sug-
gesting that both types of polymers possess a rigid-rod conforma-
tion in both states. The absorption bands of the PDTSTPD-based
polymers with phenyl pendants were located at 500e750 nm. This
was consistent with the spectrum of the previously reported
PDTSTPD5 polymer with alkyl chains on the TPD unit [28]. The
optical bandgaps were estimated based on the absorption onset

2144
T. Ikai et al. / Polymer 55 (2014) 2139e2145
wavelength (lonset) of the polymer films ðEgopt ¼ 1240=lonsetÞ and
are summarized in Table 1. Compared with PDTSTPD5, the
PDTSTPD-based polymers bearing phenyl pendants exhibited
somewhat narrower Egopt values, depending on the type of sub-
stituent used. As the electron-withdrawing ability of the substitu-
ent increased, the Egopt values tended to become narrower. These
results were consistent with our previous report [48]. This showed
levels and the Hammett substituent constants, an electron donor
polymer with optimal energy levels can be purposefully prepared
by considering the energy levels of applied electron-acceptors.
Detailed investigations of the photovoltaic properties of the PSCs
produced from these polymers are currently in progress and will
be reported in due course.
that the phenyl groups could contribute to elongation of the
p-
Acknowledgments
conjugation length in the polymer main-chains. The PDTSTT-based
polymers also demonstrated a similar tendency in their absorption
spectra, although the replacement of the TPD unit with the TT unit
resulted in broadening and large redshifts in the absorption
spectra. This is probably due to the stabilization of the quinoidal
resonance structure in the main chains.
We thank Prof. K. Takahashi and Prof. T. Yamagishi (Kanazawa
University) for allowing us to use their AC-2 and TGA instruments,
respectively. This work was supported by the Environment
Research and Technology Development Fund (B-0807) of the
Ministry of the Environment, Japan.
3.4. Energy levels
Appendix A. Supplementary data
The HOMO energy levels of the polymers were estimated with a
photoelectron spectrometer by measuring the ionic potential of the
polymer films in air (Supporting information, Fig. S2). The LUMO
energy levels were calculated from the values of the HOMO energy
levels and the EgoptðLUMO ¼ Egopt þ HOMOÞ. The HOMO and LUMO
energy levels for the polymers are listed in Table 1. The HOMO en-
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2014.03.021.
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4. Conclusion
A
series of dithieno[3,2-b:20,30-d]silole-based
p-conjugated
polymers consisting of thieno[3,4-c]pyrrole-4,6-dione or thieno
[3,4-b]thiophene units bearing 4-substituted phenyl pendants
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