Molecular design for tuning electronic structure of π-conjugated polymers containing fused dithienobenzimidazole units

Article abstract of DOI:10.1016/j.polymer.2016.11.013

Three dithienobenzimidazole derivative monomers (M1, M2, and M3) were prepared, where M3 was obtained by the oxidation of M1 and identified by the X-ray crystallographic analysis. π-Conjugated homopolymers (P1-0, P2-0, and P3-0) and copolymers (P1-2, P3-1

Full text of DOI:10.1016/j.polymer.2016.11.013

Polymer 107 (2016) 191e199
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Molecular design for tuning electronic structure of
p-conjugated
polymers containing fused dithienobenzimidazole units
,
Koji Takagi a *, Tomoharu Kuroda a, Masanori Sakaida a, Hyuma Masu b
a Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya, 466-8555, Japan
b Center for Analytical Instrumentation, Chiba University, 1-33 Yayoi, Inage, Chiba, 263-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Three dithienobenzimidazole derivative monomers (M1, M2, and M3) were prepared, where M3 was
Received 29 August 2016
Received in revised form
4 November 2016
Accepted 8 November 2016
Available online 9 November 2016
obtained by the oxidation of M1 and identified by the X-ray crystallographic analysis.
p-Conjugated
homopolymers (P1-0, P2-0, and P3-0) and copolymers (P1-2, P3-1, and P3-2) were synthesized by the
palladium-catalyzed coupling polymerizations of M1, M2, and M3. The absorption spectra of the refer-
ence compounds (fused R1 and non-fused R2), in conjunction with the optimized ground state structure,
certified the importance of the fused dithienobenzimidazole skeleton to increase the effective conju-
gation length of the polymers. On the basis of the absorption and emission spectra of the
p-conjugated
Keywords:
polymers in CHCl3, the influence of the thiophene-S,S-dioxide as well as the comonomer structure were
investigated to find out that P3-0 and P3-2 exhibited peak maxima at the relatively longer wavelength
region due to the donor-acceptor interaction. In addition, the protonation of the imidazole imine group
p-Conjugated polymer
Oxidation
Protonation
further tuned the optical properties of the
p-conjugated polymers by promoting the charge transfer
interaction along the polymer main chain, which was supported by the theoretical calculations in detail.
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
energy levels, and the favorable blend morphology. The incorpo-
ration of electron-rich and electron-deficient units, for many cases
Optoelectronic devices fabricated from organic materials have
many advantages over inorganic silicon-based devices owing to the
light-weight and flexible characteristics, low-cost and mild
manufacturing processes, and the finely tunable energy level of
in an alternating fashion, into the polymer main chain is a reliable
method to optimize the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy
levels by virtue of the intramolecular charge transfer interaction
[7e10]. These donor-acceptor architectures have been keenly
studied, giving rise to many high performance devices in the recent
decades.
frontier molecular orbitals (FMOs).
p-Conjugated polymers are
promising candidates with the potential application to large area
displays, wearable sensors, and stretchable memories. For example,
bulk heterojunction solar cells comprised of
p
-conjugated polymer
On the other hand, the introduction of
the polymer backbone determines not only the electronic structure
but also the intermolecular stacking structure to affect the
semiconducting properties of materials. The strong stacking
interaction leads to the good crystallinity and the high electronic
conductivity in some occasions, but it reduces the polymer solu-
bility, making the solution processability difficult in other occa-
p-extended systems into
donor and fullerene derivative acceptor materials have attracted
increasing attention in recent years [1e6]. However, at the present
stage, organic and polymer semiconductors do not have enough
performances to supersede silicon semiconductors because of the
low power conversion efficiency and the short device lifetime.
These parameters should be improved for the demand of
commercialization. Much efforts have been devoted to the devel-
p-p
p-p
sions. Thus the careful choice of p-extended systems including the
opment of
p
-conjugated polymers with the good solution pro-
attachment of solubilizing groups is much important to obtain
semiconducting polymers with the better performance [11].
Recently, in addition to fused ring systems consisting of the
electron-rich aromatic rings (thienothiophene [12], benzodithio-
phene [13], naphthodithiophene [11,14], and benzotrithiophene
[15,16]), those carrying both donor and acceptor constituents in one
cessability, the high charge carrier mobility, the broad light
absorption profile, the large absorption coefficient, the appropriate
* Corresponding author. Tel.: þ81 052 735 5264.
E-mail address: takagi.koji@nitech.ac.jp (K. Takagi).
http://dx.doi.org/10.1016/j.polymer.2016.11.013
0032-3861/© 2016 Elsevier Ltd. All rights reserved.

192
K. Takagi et al. / Polymer 107 (2016) 191e199
fused ring system have been investigated. For example, thiophene-
fused conjugated skeletons having the electron-accepting imide
were determined relative to quinine sulfate in 0.1 M H2SO4 with QY
of 0.55. Cyclic voltammetry (CV) measurements were performed on
with a potentiostat (Hokuto HZ-5000, Hokuto Denko). The working
electrode (Peek-coated platinum disk) was separated from the
counter electrode (platinum wire) and the Ag/Ag þ reference
electrode using a glass filter (G4). Polymer films were drop-cast on
to a platinum electrode from their CHCl3 solutions. All measure-
ments were performed at room temperature (25
trogen gas was used to degas the solutions before use and flowed
over the solutions during experiments.
group were installed in some
p-conjugated polymers [17e20].
These -conjugated polymers showed a potential utility as the field
p
effect transistor and photovoltaic cell materials. Low band gap
oligomers and polymers consisting of thiophene-fused boron
dipyrromethane (BODIPY) repeat units were also synthesized
[21,22]. The large overlap between HOMO and LUMO is realized to
effectively lower the band gap energy, and the strong electron-
accepting ability of BODIPY decreases the HOMO energy level to
improve the air stability of the materials.
1
ꢀC) and ni-
We have previously reported the synthesis of
p-conjugated
2.3. Monomer syntheses
polymers based on a fused dithienobenzimidazole unit in the main
chain, and found that the substitution pattern of the fused ring
system as well as the chemical structure of the comonomer affect
2.3.1. 5,8-Dibromo-2-(p-octyloxyphenyl)-1-pentyldithieno
[30,20:3,4:200,300:5,6]benzimidazole (M1)
the optoelectronic characteristics of the
p-conjugated polymers
Step 1: To a EtOH solution (10 mL) of benzo[1,2-b:6,5-b0]
dithiophene-4,5-dione (97 mg, 0.44 mmol) were added 4-
octyloxybenzaldehyde (0.15 g, 0.62 mmol) and ammonium ace-
tate (0.17 g, 2.2 mmol), and the reaction mixture was heated to
reflux overnight. After solvents were removed, CH2Cl2 was added
and washed with water. The organic phase was dried over MgSO4,
and the crude product was purified by SiO2 column chromatog-
raphy (gradually changing the solvent composition from CH2Cl2 to
ethyl acetate, Rf ¼ 0.80) to give 2-(p-octyloxyphenyl)dithieno
[30,20:3,4:200,300:5,6]benzimidazole as a colorless solid (0.17 g, 90%
[23]. Imidazole has been utilized as the building unit of the
p-
conjugated molecules and polymers [24,25], and the trans-
formation to imidazolium cation imparts new properties to the
molecules owing to its ionic character and strong electron-
accepting nature [26e29]. In this paper, we describe the chemical
modification of the dithienobenzimidazole-containing
p-conju-
gated polymers for further tuning of the electronic structure (Fig.1).
As a result, the control of FMO energy levels was possible by the
choice of the comonomer, the oxidation of the thiophene ring, and
the protonation of the imidazole moiety.
yield). Mp 236e238 ꢀC; 1H NMR (CDCl3)
d ppm 0.84e0.93 (m, 3H),
1.21e1.43 (m, 10H), 1.77e1.83 (m, 2H), 3.99 (brs, 2H), 6.94e7.01 (m,
2H), 7.50 (d, J ¼ 4.89 Hz, 2H), 7.84 (brs, 2H), 8.04 (d, J ¼ 7.09 Hz, 2H).
Step 2: To a THF solution (100 mL) of 2-(p-octyloxyphenyl)
dithieno[30,20:3,4:200,300:5,6]benzimidazole (4.2 g, 9.5 mmol) was
added NaH (55% oil suspension) (0.69 g, 29 mmol), and the reaction
mixture was heated to reflux for 1 h. 1-Iodopentane (2.3 g,
11 mmol) was added, and the reaction mixture was heated to reflux
overnight. After solvents were removed, CH2Cl2 was added and
washed with water. The organic phase was dried over MgSO4, and
the crude product was purified by SiO2 column chromatography
(hexane:ethyl acetate ¼ 1:3, Rf ¼ 0.50) to give 2-(p-octylox-
yphenyl)-1-pentyldithieno[30,20:3,4:200,300:5,6]benzimidazole (R1,
reference compound) as a colorless solid (3.0 g, 61% yield). Mp
2. Experimental
2.1. Materials
[1,3-Bis(diphenylphosphino)propane]dichloronickel(II)
[Ni(dppp)Cl2] was purchased from Tokyo Chemical Industry. Tet-
rakis(triphenylphosphine)palladium(0)
[Pd(PPh3)4],
9,9-
dihexylfluorene-2,7-diboronic acid, and i-PrMgCl solution (2.0 M
in tetrahydrofuran (THF)) were purchased from Aldrich. n-Butyl-
lithium solution (n-BuLi, 1.6 M in hexane) was purchased from
Kanto Chemical. Benzo[1,2-b:6,5-b0]dithiophene-4,5-dione [30],
1,2-di(thiophen-3-yl)ethane-1,2-dione [30], benzo[2,1-b:5,6-b0]
dithiophene-4,5-dione [31], and 3,30-dihexyl-5,50-bis(tributyl-
stannyl)-2,20-bithiophene [32] were prepared as reported previ-
ously. All reactions were performed under dry nitrogen atmosphere
unless otherwise noted.
90e92 ꢀC; 1H NMR (CDCl3)
d ppm 0.84e0.91 (m, 5H), 1.25e1.38 (m,
13H), 1.46e1.54 (m, 2H), 1.82e1.93 (m, 4H), 4.05 (t, J ¼ 6.60 Hz, 2H),
4.50 (s, 2H), 7.06 (d, J ¼ 8.56 Hz, 2H), 7.50 (d, J ¼ 5.38 Hz, 1H), 7.55
(d, J ¼ 5.38 Hz, 1H), 7.63e7.67 (m, 2H), 7.70 (d, J ¼ 5.62 Hz, 1H), 8.04
(d, J ¼ 5.38 Hz, 1H); 13C NMR (CDCl3)
d ppm 13.9, 14.1, 22.1, 22.7,
2.2. Instrumentations
26.1, 28.6, 29.2, 29.4, 29.7, 30.2, 31.8, 46.0, 68.2, 114.7, 119.9, 122.1,
122.9, 123.4, 124.4, 124.6, 128.9, 130.0, 130.3, 131.1, 136.4, 151.3,
160.1; Anal Calcd for C30H36N2OS2: C, 71.39%; H, 7.19%; N, 5.55%; S,
12.71%, Found C, 72.52%; H, 8.12%; N, 4.81%; S, 11.28%.
Microwave reactions were performed on a Biotage Initiator 8 in
the normal absorption level. 1H and 13C nuclear magnetic reso-
nance (1H-NMR and 13C-NMR) spectra were recorded on a Bruker
AvanceIII HD 400 FT-NMR spectrometer in CDCl3. Melting points
(Mp) were determined on a Yanagimoto micro melting point
apparatus MP-500D and were uncorrected. High resolution elec-
trospray ionization mass spectra (HR ESI-MS) were performed on a
Waters Synapt G2 HDMS in the positive mode. Elemental analyses
(EA) were performed on a Elementar vario EL cube. Gel permeation
chromatography (GPC) analyses were carried out on a Shodex 104
system using tandem LF-404 columns (THF as an eluent, flow
rate ¼ 1.0 mL/min, 40 ꢀC) equipped with an ultravioletevisible
(UVevis) detector (Shimadzu SPP-20A). Number-averaged molec-
ular weight (Mn) and molecular weight distribution (Mw/Mn) were
determined on the basis of a calibration curve made from standard
polystyrene samples and ethylbenzene. UVevis and fluorescence
spectra were recorded on a Shimadzu UV-1650 spectrophotometer
and a Shimadzu RF-5300 spectrofluorometer, respectively, using a
1 cm quartz cell. Fluorescence quantum yields (QYs) in solution
Step 3: To a N,N-dimethylformamide (DMF) solution (20 mL) of
2-(p-octyloxyphenyl)-1-pentyldithieno[30,20:3,4:200,300:5,6]benz-
imidazole (0.10 g, 0.20 mmol) was added N-bromosuccinimide
(NBS) (70 mg, 0.42 mmol), and the reaction mixture was stirred at
room temperature overnight. After solvents were removed, CHCl3
was added and washed with water. The organic phase was dried
over MgSO4, and the crude product was purified by SiO2 column
chromatography (CH2Cl2, Rf ¼ 0.50) followed by recrystallization
from hexane/CHCl3 to give M1 as a colorless solid (0.10 g, 77% yield).
Mp 145e147 ꢀC; 1H NMR (CDCl3)
d ppm 7.99 (s, 1H), 7.61 (d,
J ¼ 8.7 Hz, 2H), 7.60 (s, 1H), 7.05 (d, J ¼ 8.7 Hz, 2H), 4.42 (t, J ¼ 7.6 Hz,
2H), 4.05 (t, J ¼ 6.5 Hz, 2H), 1.90e1.80 (4H), 1.56e1.20 (14H),
0.94e0.81 (6H); 13C NMR (CDCl3)
d ppm 160.1, 151.7, 135.5, 132.4,
129.9, 129.5, 129.3, 128.4, 128.3, 126.2, 123.3, 122.7, 122.3, 116.0,
115.4, 113.1, 113.0, 68.2, 45.8, 31.9, 29.9, 29.3, 28.6, 26.1, 22.7, 22.0,
13.1; Anal Calcd for C30H34Br2N2OS2: C, 54.38%; H, 5.17%; N, 4.23%;
S, 9.68%, Found: C, 54.41%; H, 5.28%; N, 4.09%; S, 9.62.

K. Takagi et al. / Polymer 107 (2016) 191e199
193
Fig. 1. Overview of dithienobenzimidazole-containing
p-conjugated polymers.
2.3.2. 5,8-Dibromo-2-(p-octyloxyphenyl)-1-pentyldithieno
[20,30:3,4:300,200:5,6]benzimidazole (M2)
organic phase was washed with water, dried over MgSO4, and the
crude product was purified by SiO2 column chromatography
(CHCl3, Rf ¼ 0.23) to give M3 as a yellow solid (0.10 g, 26% yield). Mp
This compound was synthesized from benzo[2,1-b:5,6-b0]
dithiophene-4,5-dione in a similar manner to M1. Mp 104e106 ꢀC;
58e61 ꢀC; 1H NMR (CDCl3)
d ppm 0.83e0.93 (m, 6H), 1.19e1.37 (m,
1H NMR (CDCl3)
d
ppm 7.63 (d, J ¼ 8.2 Hz, 2H), 7.60 (s, 2H), 7.03 (d,
12H), 1.46e1.53 (m, 2H), 1.80e1.90 (m, 4H), 4.06 (t, J ¼ 6.60 Hz, 2H),
J ¼ 8.8 Hz, 2H), 4.33 (t, J ¼ 7.8 Hz, 2H), 4.03 (t, J ¼ 6.6 Hz, 2H),
4.45 (s, 2H), 7.08 (d, J ¼ 8.56 Hz, 2H), 7.59e7.67 (m, 3H), 7.98 (s, 1H);
1.94e1.74 (4H), 1.53e1.31 (14H), 0.95e0.82 (6H); 13C NMR (CDCl3)
13C NMR (CDCl3)
d ppm 12.8, 13.1, 20.9, 21.6, 25.0, 27.4, 28.1, 28.2,
d
ppm 160.4, 152.4, 134.4, 132.3, 130.3, 130.2, 130.0, 129.9, 129.4,
28.3, 28.7, 30.8, 45.2, 67.3, 75.7,114.0,117.5, 119.5,120.6,120.7,124.3,
127.1, 128.6, 130.0, 130.1, 131.5, 155.1, 160.0; Anal Calcd for
129.3,126.8,126.5,123.7,123.4,122.0, 121.8, 121.6,116.1, 115.6,112.5,
111.5, 68.2, 31.9, 31.3, 29.4, 29.3, 26.1, 22.7, 22.1, 13.4; Anal Calcd for
C30H34Br2N2O3S2: C, 51.88%; H, 4.93%; N, 4.03%; S, 9.23%, Found C,
C
30H34Br2N2OS2: C, 54.38%; H, 5.17%; N, 4.23%; S, 9.68%, Found: C,
51.92%; H, 5.18%; N, 3.83%; S, 9.06%; HR ESI-MS Calcd for
C
54.44%; H, 5.21%; N, 4.07%; S, 9.94%.
30H35Br2N2O3S2 [MþH]þ: 695.0435, Found: 695.0431.
2.3.3. 5,8-Dibromo-2-(p-octyloxyphenyl)-1-pentyldithieno
[30,20:3,4:200,300:5,6]benzimidazole-S,S-dioxide (M3)
2.4. Synthesis of reference compound (R2)
To a CH2Cl2 solution (40 mL) of M1 (0.37 g, 0.55 mmol) were
added 35% H2O2 solution(5.9 mL, 54 mmol) and 55% HCO2H solu-
tion (23 mL, 0.11 mol) at 0 ꢀC, and the reaction mixture was stirred
at 30 ꢀC for 18 h. After the mixture was poured into saturated
NaHCO3 solution, an aqueous phase was extracted with CH2Cl2. The
Step 1: To a EtOH solution (2 mL) of 1,2-di(thiophen-3-yl)
ethane-1,2-dione (0.20 g, 0.90 mmol) were added 4-
octyloxybenzaldehyde (0.32 g, 1.4 mmol), ammonium acetate
(2.1 g, 27 mmol), and
L
-proline (16 mg, 0.14 mmol). The reaction
mixture was heated to reflux overnight. After solvents were

194
K. Takagi et al. / Polymer 107 (2016) 191e199
removed, CH2Cl2 was added and washed with water. The organic
phase was dried over MgSO4, and the crude product was purified by
SiO2 column chromatography (CHCl3, Rf ¼ 0.1) to give 2-(p-octy-
3. Results and discussion
3.1. Synthesis
loxyphenyl)-4,5-di(thiophene-30-yl)benzimidazole as
a
yellow
ppm
solid (0.37 g, 95% yield). Mp 168e170 ꢀC; 1H NMR (CDCl3)
d
Monomers M1 and M2 were prepared from the corresponding
benzodithiophene-4,5-dione derivatives [30,31] by the imidazole
ring formation, N-alkylation, and bromination sequences (Fig. 2).
The synthesis of M3 was then attempted by oxidizing M1 under
various conditions. The oxidation using 3-chloroperbenzoic acid in
CH2Cl2 at room temperature gave a complex mixture, while the
reaction using H2O2 in CH3CO2H at refluxing temperature did not
consume M1. The oxidation using NaBO3$4H2O in CH3CO2H at 50 ꢀC
also resulted in the recovery of M1. Finally, M3 was successfully
obtained by the reaction with H2O2 in HCO2H/CH2Cl2 at room
temperature and purified by SiO2 column chromatography. The HR
ESI-MS and elemental analysis of the crude mixture revealed that
not bithiophene-S,S,S0,S0-tetraoxide but bithiophene-S,S-dioxide
solely existed. In this reaction condition, the imidazole ring was
probably protonated by formic acid to decrease the electron density
0.84e0.95 (m, 3H), 1.22e1.49 (m, 11H), 1.80 (s, 2H), 3.99 (t,
J ¼ 6.60 Hz, 2H), 6.95 (d, J ¼ 8.80 Hz, 2H), 7.27e7.64 (m, 5H), 7.80 (d,
J ¼ 8.80 Hz, 2H), 9.08e9.34 (m, 1H); 13C NMR (CDCl3)
d ppm 14.1,
22.7, 26.0, 29.2, 29.4, 31.8, 68.1, 76.7, 114.7.
Step 2: To a THF solution (5 mL) of 2-(p-octyloxyphenyl)-4,5-
di(thiophene-30-yl)benzimidazole (0.1 g, 9.5 mmol) was added
NaH (55% oil suspension) (33 mg, 0.69 mmol), and the reaction
mixture was heated to reflux for 1 h. 1-Iodopentane (54 mg,
0.27 mmol) was added, and the reaction mixture was heated to
reflux overnight. After solvents were removed, CH2Cl2 was added
and washed with water. The organic phase was dried over MgSO4,
and the crude product was purified by SiO2 column chromatog-
raphy (hexane: ethyl acetate ¼ 1:1, Rf ¼ 0.78) to give 2-(p-octy-
loxyphenyl)-1-pentyl-4,5-bi(thiophene-30-yl)benzimidazole (R2)
as an orange oil (98 mg, 85% yield). 1H NMR (CDCl3)
d
ppm 1.05 (s,
of the fused p-electron system, and one thiophene ring was
4H), 1.25e1.48 (m, 17H), 3.82 (s, 2H), 4.01 (t, J ¼ 6.60 Hz, 2H), 6.99
(d, J ¼ 8.80 Hz, 2H), 7.10e7.18 (m, 3H), 7.31 (dd, J ¼ 3.06,1.34 Hz,1H),
7.40 (dd, J ¼ 2.93, 1.22 Hz, 1H), 7.50 (dd, J ¼ 4.89, 2.93 Hz, 1H), 7.56
oxidized to further decrease the electron density of another thio-
phene ring and inhibit the complete oxidation, i.e., the formation of
bithiophene-S,S,S0,S0-tetraoxide. The single crystal of M3 was pre-
pared by the vapor diffusion method using CHCl3 (good solvent)
and CH3OH (poor solvent), and subjected to the X-ray crystallo-
graphic analysis (space group ¼ P-1, Z ¼ 2), revealing that the
thiophene ring more distant from the N-pentyl group is oxidized
(Fig. 3). The torsion angle between the imidazole ring (A) and the 4-
octyloxyphenyl ring (B) is 45ꢀ, and the fused dithienobenzimida-
zole unit has a completely planar structure. On the other hand, in
addition to fused reference compound R1 that is the synthetic
precursor of M1, non-fused reference compound R2 was prepared
(d, J ¼ 8.80 Hz, 2H); 13C NMR (CDCl3)
d ppm 12.7, 13.1, 20.8, 21.6,
25.0, 27.4, 28.2, 28.4, 29.2, 30.8, 43.8, 67.1, 113.6, 118.7, 122.5, 123.6,
125.4, 125.5, 128.5, 129.4, 130.2, 135.0, 146.6, 158.6; Anal Calcd for
C
30H38N2OS2: C, 71.10%; H, 7.56%; N, 5.53%; S, 12.65%, Found C,
71.85%; H, 7.70%; N, 4.67%; S, 10.62%.
2.5. Polymerization
2.5.1. Homopolymerization (typical procedure)
A microwave vial containing M1 (0.10 g, 0.15 mmol), bis(tri-n-
butyltin) (92 mg, 0.16 mmol), CuI (0.86 mg, 0.16 mmol), Pd(PPh3)4
(8.3 mg, 7.5 mmol), THF (3.0 mL), and N-methylpyrrolidone (NMP)
to investigate the effect of the fused p-conjugated skeleton. R2 was
obtained from 1,2-di(thiophen-3-yl)ethane-1,2-dione [30] by the
imidazole ring formation and N-alkylation reactions. The structure
and purity of these low molecular weight compounds were abso-
lutely confirmed by NMR spectra and elemental analyses.
(1.5 mL) was heated under the microwave irradiation condition at
120 ꢀC for 48 h. After CHCl3 was added to dissolve precipitates and
washing with 1 M HCl, the organic phase was dried over MgSO4.
The solution was concentrated and poured into MeOH to obtain P1-
Initially, the synthesis of P1-0 was investigated using R1 or M1
as a monomer. When the oxidation coupling polymerization of R1
was conducted using FeCl3 in CHCl3 at 0 ꢀC for 48 h, insoluble
products in organic solvents such as CHCl3 and THF were obtained.
The strong coordination interaction between the Fe3þ ion and the
imidazole moiety might result in the formation of physical network
gel. The Yamamoto and Kumada coupling polymerizations of M1
using nickel catalysts proceeded, however, the number-averaged
molecular weight of products remained around 1500. In contrast,
the Stille coupling polymerization using the (Bu3Sn)2, CuI, and
Pd(PPh3)4 system in THF/NMP or in chlorobenzene at 120 ꢀC under
the microwave irradiation condition for 48 h successfully gave
soluble P1-0 in 88% yield as a reddish solid. P2-0 (a yellow solid)
and P3-0 (a reddish solid) were likewise synthesized from M2 and
M3, respectively. The Suzuki coupling polymerization of M3 with
9,9-dihexylfluorene-2,7-diboronic acid in THF/2 M Na2CO3 solution
at the refluxing temperature for 72 h gave P3-1, and the Stille
coupling polymerization of M3 with 3,30-dihexyl-5,50-bis(tributyl-
stannyl)-2,20-bithiophene in toluene at the refluxing temperature
for 72 h gave P3-2. P1-2 was synthesized from M1 and 3,30-dihexyl-
5,50-bis(tributylstannyl)-2,20-bithiophene as previously reported
[23]. These copolymers showed a better solubility in organic sol-
vents than the homopolymers due to the presence of hexyl chains
in the comonomer units. P1-0 and P3-0 were partially soluble in
THF, which is the GPC eluent, owing to the rigid framework of main
chain. All polymers were isolated by the reprecipitation of the
CHCl3 soluble fraction into MeOH. The number-averaged molecular
0 as a reddish solid (67 mg, 88% yield). 1H NMR (CDCl3)
d ppm
0.76e1.58 (22H), 1.77e2.22 (5H), 3.88e4.57 (4H), 7.06 (3H),
7.36e8.03 (4H). Other homopolymers (P2-0 and P3-0) were ob-
tained in a similar manner.
2.5.2. Copolymerization (typical procedure)
To a THF solution (2.0 mL) of M3 (0.10 g, 0.14 mmol) and 9,9-
dihexylfluorene-2,7-diboronic acid (58 mg, 0.14 mmol) were
added Pd(PPh3)4 (16 mg, 14 mmol) and 2 M Na2CO3 solution (0.5 mL,
0.96 mmol), and the reaction mixture was heated to reflux for 72 h.
After CHCl3 was added to dissolve precipitates and washing with
1 M HCl, the organic phase was dried over MgSO4. The solution was
concentrated and poured into MeOH to obtain P3-1 as a reddish
solid (41 mg, 36% yield). 1H NMR (CDCl3)
d ppm 0.54e1.47 (43H),
1.80e2.25 (8H), 4.09 (2H), 4.49e4.68 (2H), 7.12 (2H), 7.36e8.30
(10H). Other copolymers (P1-2 and P3-2) were obtained in a similar
manner.
2.6. Theoretical calculation
Theoretical calculations were performed using the Gaussian
09 W (Revision C.01) package of program. The ground state struc-
ture was optimized with density functional theory (DFT) calcula-
tion at the B3LYP/6-31G(d,p) level of theory. Time dependent (TD)-
DFT calculations were then performed at the same level to estimate
the vertical excitation energies and oscillator strengths (f).
weight and the molecular weight distribution of the p-conjugated

K. Takagi et al. / Polymer 107 (2016) 191e199
195
Fig. 2. Chemical structure of monomers and reference compounds (for Ar and R groups, See Fig. 1).
polymers are summarized in Table 1. Unfortunately, the molecular
weights were rather low, which might be ascribed to the coordi-
nation of the imidazole to the palladium complex to decrease the
catalytic activity in the polymerization.
compounds R1 and R2 were measured in CHCl3 solution (Fig. S1, a).
The absorption maximum of fused R1 exhibited a clear red-shift of
61 nm as compared with non-fused R2. The DFT calculations were
performed for optimizing the ground state structures of 2-(p-
methoxyphenyl)-1-methyldithieno[30,20:3,4:200,300:5,6]benzimid-
azole (R1′) and 2-(p-methoxyphenyl)-1-methyl-4,5-bi(thiophene-
30-yl)benzimidazole (R2′). Therein, the long alkyl substituents were
replaced by the methyl group to reduce the computation time. As a
result, the torsion angles between imidazole ring (A) and the
thiophene ring (C) of R1′ are 0.8ꢀ and 0.3ꢀ indicating the almost
planar structure, while those of R2′ are 14.4ꢀ and 129.4ꢀ reflecting
the remarkably twisted conformation (Fig. S1, b). These results
agree well with the UVevis absorption data, and the fused dithie-
3.2. Optical properties
At the beginning, the UVevis absorption spectra of reference
nobenzimidazole unit incorporated in the p-conjugated polymer is
expected to increase the effective conjugation length.
The UVevis absorption and fluorescence emission spectra of
three homopolymers P1-0, P2-0, and P3-0 were then measured in
CHCl3 (Fig. 4, left). The absorption spectrum of P3-0 was particu-
larly broad. P3-0 might be composed of segments with various
effective conjugation lengths because the thiophene and thio-
phene-S,S-dioxide rings are supposed to be not regularly arranged
Table 1
Yield, number-averaged molecular weight, and molecular weight distribution of
conjugated polymers.
p-
Yield (%)a
Mn (DP)b
Mw/Mn
b
Code
P1-0
P2-0
P3-0
P1-2
P3-1
P3-2
88
43
89
49
36
59
4000 (7.5)c
3300 (6.2)
6700 (11.9)c
7300 (8.4)
9000 (10)
8000 (8.9)
1.64
1.34
1.88
1.77
1.93
2.50
a
Isolated yield after precipitating the CHCl3 soluble fraction into MeOH.
Estimated by GPC (THF as an eluent, calibrated by polystyrene standard sam-
b
ples). DP ¼ Degree of polymerization calculated based on the Mn values.
c
Fig. 3. Single crystal X-ray structure of M3 in a stick model.
Only soluble part in THF.

196
K. Takagi et al. / Polymer 107 (2016) 191e199
Fig. 4. UVevis absorption (closed marker) and fluorescence emission (open marker) spectra in CHCl3 (10ꢁ5 M). Left: P1-0 (circle), P2-0 (triangle), and P3-0 (square). Right: P3-1
(diamond) and P3-2 (bar).
along the polymer main chain [33]. Two bromine atoms in mono-
mer M3 must have a different electronic character; namely, one is
substituted on the thiophene ring and another is substituted on the
thiophene-S,S-dioxide ring. It can be considered, however, that the
Stille coupling polymerization does not proceed in the regio-
controlled manner because of the small difference between two
bromides in the reactivity. The absorption maximum wavelengths
of P1-0, P2-0, and P3-0 were 497 nm, 392 nm, and 569 nm,
respectively. Because the thiophene rings in P1-0 are connected
structure (two thiophene rings and a fluorene unit) to P1-1.
Because the absorption and emission maxima of P3-1 were
observed at 485 nm and 550 nm, respectively, the electronic in-
fluence of thiophene-S,S-dioxide included in the fused dithieno-
benzimidazole unit is prominent to obviously increase the effective
conjugation length. P3-2 exhibited the absorption and emission
spectra at the longest wavelength region (lab ¼ 510 nm and
lem ¼ 602 nm) among the synthesized
p-conjugated polymers,
reflecting the enhanced donor-acceptor interaction. Fig. S3 visual-
izes the fluorescence emission color of the selected copolymers in
CHCl3 which can cover the wide optical wavelength region.
The DFT calculations were performed for the model compounds
of the repeat units of P3-1 and P3-2, where the long alkyl sub-
stituents were replaced by the methyl group. The optimized ground
state structure indicates that P3-1′ has larger torsion angles (28ꢀ
and 25ꢀ) than P3-2′ (17ꢀ and 15ꢀ) between thiophene or thiophene-
S,S-dioxide rings (C) and phenyl or thienyl rings (D) (Fig. 5). On the
basis of the TD-DFT calculation, important electronic transitions are
always from HOMO to LUMO, and these FMO surfaces are distrib-
uted throughout the fused dithienobenzimidazole unit. The
alkoxypheny group at the 2-position of the imidazole ring has little
electronic interaction with the fused dithienobenzimidazole unit.
The vertical excitation energy and oscillator strength of P3-1′ are
2.93 eV (423 nm) and 0.62, respectively, while those of P3-2′ were
2.72 eV (455 nm) and 0.74. The calculated excitation energies are
estimated at the relatively shorter wavelength region, however, the
low energy shift from P3-1 to P3-2 is nicely reproduced by the
theoretical calculation.
with each other at the 2,5-positions, the
p-conjugation is effec-
tively expanded as compared with P2-0. As for P3-0, the charge
transfer interaction between the electron-donor (thiophene) and
the electron-acceptor (thiophene-S,S-dioxide) possibly results in
the wide conjugation system. The emission maximum wavelengths
exhibited a red-shift in the order of P2-0 (488 nm), P1-0 (554 nm),
and P3-0 (601 nm). The fluorescence quantum yield of P2-0 was
10%, while those of P1-0 and P3-0 were not available because of the
large overlap of the absorption spectra with the emission (small
Stokes shifts), leading to the weak fluorescence intensity. Table 2
summarizes the absorption and emission maximum wavelengths
along with the optical band gap energies calculated from the onset
of UVevis absorption spectra.
The UVevis absorption and fluorescence emission spectra of P3-
1 and P3-2 were likewise measured in CHCl3 (Fig. 4, right). P1-1
(See Fig. 1) composed of two thiophene rings and the fluorene unit
in the main chain showed the absorption and emission maxima at
449 nm and 541 nm, respectively, as we reported in the previous
paper [23]. P3-1 is also composed of the similar main chain
The electrochemical properties of polymers were evaluated by
the CV measurement. Polymer films were prepared by the drop-
cast on to a platinum electrode from their CHCl3 solutions. In all
cases, the irreversible oxidation CV curves were obtained (Fig. S6).
Only P3-0 showed an irreversible reduction CV curve. The HOMO
energy level (EHOMO) is estimated from the onset oxidation poten-
tial (Eonset) using the following equation: HOMO ¼ ꢁ(Eonse þ 4.73)
eV, where the energy level is calibrated against the ferrocene/fer-
rocenium couple (Fc/Fcþ measured as 0.08 V vs Ag/Agþ) and 4.8 eV
is the energy level of Fc under vacuum. Table 2 summarizes the
Table 2
Optical and electrochemical properties of polymers.
Code
labs (nm)a
lem (nm)b
Eg (eV)c
EHOMO (eV)d
ELUMO (eV)e
P1-0
P2-0
P3-0
P1-2
P3-1
P3-2
497
392
569
449
485
510
554
488
601
541
550
602
2.00
2.48
1.77
2.23
2.15
1.98
ꢁ5.32
ꢁ5.06
ꢁ5.51
ꢁ5.46
ꢁ5.73
ꢁ5.69
ꢁ3.32
ꢁ2.58
ꢁ3.74 (ꢁ3.86)f
ꢁ3.23
ꢁ3.58
ꢁ3.71
experimental EHOMO as well as the LUMO energy level (ELUMO
)
a
Absorption maximum wavelength in CHCl3 (10ꢁ5 M).
calculated from the optical band gap (Eg) in the UVevis absorption
spectra. The ELUMO for P3-0 was alternatively estimated from the
onset reduction potential, showing good agreement with that
calculated from the optical band gap. Among three homopolymers,
P3-0 shows the ELUMO value at the most negative level, which can
b
c
d
e
f
Emission maximum wavelength in CHCl3 (10ꢁ5 M).
Optical band gap energy calculated from the onset of UVevis absorption spectra.
Estimated from the onset oxidation potential.
LUMO is estimated by LUMO-HOMO ¼ Eg.
Estimated from the onset reduction potential.

K. Takagi et al. / Polymer 107 (2016) 191e199
197
Fig. 5. Ground state (GS) structure, HOMO surface, and LUMO surface of P3-1′ (top) and P3-2′ (bottom).
be stemmed from the strong electron-accepting characteristic of
the thiophene-S,S-dioxide unit. On the other hand, P3-1 has the
lower EHOMO value and the higher ELUMO one then P3-2, which are
in good accordance with the trend observed in the DFT calculation;
EHOMO and ELUMO of P3-1 are ꢁ5.41 eV and ꢁ2.12 eV, EHOMO and
ELUMO of P3-2 are ꢁ5.26 eV and ꢁ2.25 eV.
shows the UVevis absorption and fluorescence emission spectra of
P1-2 and P3-2 before and after adding the excess amounts of TFA,
which leads both polymers to exhibit several nm (in absorption)
and ca. 20 nm (in emission) red-shifts of the spectra. P1-2c (proton-
adduct of P1-2) has the absorption and emission maxima at 452 nm
and 564 nm, respectively, while P3-2c (proton-adduct of P3-2) has
them at 519 nm and 620 nm, respectively. It is noteworthy that the
N-protonation causes the larger red-shifts than the N-methylation
[23], which is likely originated from the smaller steric hindrance of
Finally, the tuning of optical properties were investigated by the
protonation of the fused dithienobenzimidazole unit. The imine
(C]N) groups of P1-2 and P3-2 were protonated using trifluoro-
acetic acid (TFA) in CHCl3. Judging from the rough titration exper-
iment, the spectral change was saturated by adding 50 equivalent of
TFA to the dithienobenzimidazole unit in P1-2, while 1000 equiv-
alent of TFA was required to completely protonate the C]N group
in P3-2. This difference might be stemmed from the decreased
electron density of the imine nitrogen of P3-2 due to the electron-
withdrawing characteristic of the thiophene-S,S-dioxide ring. Fig. 6
the proton, less disturbing the p-conjugation, in comparison to the
methyl group. Furthermore, the red-shifts observed for P1-2c and
P3-2c are in contrast with the result reported by Carter et al. [34],
where poly(2-alkyl-benzimidazole-alt-9,9-dihexylfluorene) exhib-
ites a blue-shift by the proton doping using TFA. Although the
torsion angle between the imidazole and the aryl group at the 2-
position must increase upon protonating the C]N groups in P1-2
Fig. 6. UVevis absorption (closed marker) and fluorescence emission (open marker) spectra of P1-2 (left) and P3-2 (right) in CHCl3 (10ꢁ5 M) before (circle) and after (square)
adding TFA (50 and 1000 equiv. relative to the C]N group for P1-2 and P3-2, respectively).

198
K. Takagi et al. / Polymer 107 (2016) 191e199
and P3-2, the enhanced charge transfer interaction between the
electron-donor (the bithiophene comonomer unit) and the
electron-acceptors (the dithienobenzimidazolium and dithieno-
benzimidazolium-S,S-dioxide cations) might result in the red-shifts
of the spectra in our case. On the other hand, no obvious red-shifts
were observed for P3-1 bearing the weakly electron-donating flu-
orene comonomer (Fig. S2).
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