Synthesis and optoelectronic properties of conjugated polymers based on a dithienobenzimidazole unit in the main chain

Article abstract of DOI:10.1002/pola.27013

Three conjugated polymers with the dithienobenzimidazole (DTBIm) unit (P1, P3, and P4) and one conjugated polymer with the dithienobenzoxazole unit (P2) were synthesized by the cross-coupling polymerization. The absorption maxima showed a red-shift in the order of P3 (406 nm), P2 (426 nm), P1 (438 nm), and P4 (450 nm), which was studied in detail using the frontier molecular orbital calculation of the model compounds. The energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the DTBIm unit-containing conjugated polymers were estimated by the cyclic voltammetry. The transformation from DTBIm (P4) to dithienobenzimidazolium (P4') was also carried out to shift the absorption maxima of P4' (454 nm) by promoting the intramolecular charge transfer between the DTBIm and thiophene units. 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 401-409 The synthesis and optoelectronic characterization of conjugated polymers consisting of a dithienobenzimidazole unit are investigated using UV-visible/fluorescence spectra and the cyclic voltammetry in conjunction with the frontier molecular orbital calculation of model compounds. Optoelectronic properties of conjugated polymers are influenced by the structure of the fused-ring system and the alternating building block, and they can be modified by the transformation into the cationic dithienobenzimidazolium unit. Copyright

Full text of DOI:10.1002/pola.27013

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Synthesis and Optoelectronic Properties of Conjugated Polymers Based
on a Dithienobenzimidazole Unit in the Main Chain
Koji Takagi, Masanori Sakaida, Kazuma Kusafuka, Takuya Miwa
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology,
Gokiso, Showa-ku, Nagoya, 466-8555 Japan
Correspondence to: K. Takagi (E-mail: takagi.koji@nitech.ac.jp)
Received 26 July 2013; accepted 31 October 2013; published online 17 November 2013
DOI: 10.1002/pola.27013
ABSTRACT: Three conjugated polymers with the dithienobenzi-
midazole (DTBIm) unit (P1, P3, and P4) and one conjugated
polymer with the dithienobenzoxazole unit (P2) were synthe-
sized by the cross-coupling polymerization. The absorption
maxima showed a red-shift in the order of P3 (406 nm), P2
(426 nm), P1 (438 nm), and P4 (450 nm), which was studied in
detail using the frontier molecular orbital calculation of the
model compounds. The energy levels of the highest occupied
molecular orbital and the lowest unoccupied molecular orbital
of the DTBIm unit-containing conjugated polymers were esti-
mated by the cyclic voltammetry. The transformation from
DTBIm (P4) to dithienobenzimidazolium (P4’) was also carried
out to shift the absorption maxima of P4’ (454 nm) by promot-
ing the intramolecular charge transfer between the DTBIm and
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thiophene units.
2013 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2014, 52, 401–409
KEYWORDS: calculations; charge transfer; conjugated polymers;
fused-ring system; imidazole
phene4 and benzotrithiophene5) but also those carrying both
donor and acceptor constituents in one fused-ring system
have been investigated. For example, thiophene-fused conju-
gated skeletons having the electron-accepting imide group
were installed in some conjugated polymers.6 These conju-
gated polymers showed a potential utility as the FET and
PVC materials owing to the tunability of the energy levels,
the solubility, and the solid-state packing structure.
INTRODUCTION Due to the low-cost fabrication process and
the flexible large-area product, conjugated polymer-based
optoelectronic devices such as the field effect transistor
(FET), the light-emitting diode (LED), and the photovoltaic
cell (PVC) are promising candidates for the next-generation
microelectronics.1 Homopolymers and copolymers consisting
of thiophene derivatives exhibit a variety of intermolecular
interactions as represented by the van der Waals, the p–p
stacking, and the sulfur–sulfur interactions, which make
them to be applicable for the aforementioned devices.2 The
easy fine-tuning of the optoelectronic characteristics of con-
jugated polymers by the precision molecular design is also
the great advantage over inorganic materials. In the case of
the bulk heterojunction PVC using conjugated polymers as
the hole transporting material, the intramolecular charge
transfer (ICT) between the alternatively arranged electron-
rich (donor) and electron-deficient (acceptor) monomers
improves the photo-electronic conversion efficiency of the
device.3 On the other hand, the introduction of the donor
and acceptor units in the fused-ring system results in the
strong intramolecular ICT interaction. This approach also
promotes the crystal-like packing of conjugated polymers
including the planar and rigid fused-ring system, which
would be desirable for the development of semiconducting
polymer materials. Recently, not only the fused-ring system
consisting of the electron-rich aromatic rings (benzodithio-
Benzimidazole has an electron-withdrawing imine nitrogen,
and its homopolymer was shown to be electrochemically
active regarding the reduction (Yamamoto et al.).7 Conju-
gated polymers consisting of thiophene (donor) and (benz)
imidazole (acceptor) units were also synthesized, and the
ICT structure along the polymer main chain was confirmed
resulting in the red-shift of the absorption spectra (Toba
et al.).8(b) Liu and Chen synthesized a kind of photochromic
dithienylethenes with the imidazole bridge, and revealed that
the photochromism and fluorescence behaviors are influ-
enced by the ICT between thiophene and imidazole.9 To the
best of our knowledge, however, there is no precedent for
the rigid p-conjugated chromophore having thiophene and
(benz)imidazole in one fused-ring system and related conju-
gated polymers.10 In this contribution, we report the synthe-
sis and optoelectronic properties of conjugated polymers
based on a dithienobenzimidazole (DTBIm) unit in the main
Additional Supporting Information may be found in the online version of this article.
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2013 Wiley Periodicals, Inc.
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FIGURE 1 Chemical structure of conjugated polymers.
chain (Fig. 1, P1, P3, and P4). The influences of the chemical
structure of DTBIm and comonomer units on the absorption,
emission, and electrochemical characteristics were investigated
using the ultraviolet-visible (UV-Vis) and fluorescence (FL)
spectra, the cyclic voltammetry (CV) in conjunction with the
frontier molecular orbital (FMO) calculation. The optical prop-
erties were also compared with those of P2 bearing the dithie-
nobenzoxazole unit. The oligomeric and polymeric materials
including the benzimidazolium unit were also demonstrated to
have a good photoluminescent properties.11 Therefore, the qua-
ternization of the imine nitrogen was performed (P4’) to mod-
ify the electronic structure of the conjugated polymer.
nol (EtOH), dry toluene, and silica gel (SiO2 for column chro-
matography, 60N) were purchased from Kanto Chemical
(Tokyo, Japan). Other solvents were dried following to the
standard methods and distilled before use; 9,9-dihexylfluor-
ene-2,7-diboronic acids and tetrakis(triphenylphosphine)
palladium(0) (Pd(PPh3)4) were purchased from Sigma-
Aldrich (St. Louis, MO); 2,7-bis(trimethylsilyl)benzo[1,2-b:
6,5-b0]dithiophene-4,5-dione (1),12 benzo[2,1-b:5,6-b0] dithio-
phene-4,5-dione (2),13 and 3,30-dihexyl-5,50-bis (tributyl-
stannyl)-2,20-bithiophene14 were synthesized according to
the previous reports.
Characterization
EXPERIMENTAL
1H and 13C nuclear magnetic resonance (1H NMR and 13C
NMR) spectra were measured on Bruker (Billerica, MA)
AVANCE 200 and 600 FT-NMR spectrometers in CDCl3 and
DMSO-d6 using tetramethylsilane (d 0.00 in ppm) as an
Materials
All reactions were performed under dry nitrogen atmosphere
unless otherwise noted. Dry tetrahydrofuran (THF), dry etha-
402
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internal reference compound. Infrared (IR) spectra were
measured on a Jasco (Tokyo, Japan) FT-IR 460Plus spectro-
photometer in the attenuated total reflection (ATR) method.
Melting points (Mp) were determined on a Yanaco (Kyoto,
Japan) micro melting point apparatus MP-500D. Electrospray
ionization mass spectra (ESI-MS) were obtained on a Waters
(Milford, MA, USA) Synapt G2 HDMS. Elemental analyses
(EA) were performed on an Elementar (Hanau, Germany)
Vario EL cube. Gel permeation chromatographic (GPC) analy-
ses were carried out on a Shodex (Tokyo, Japan) GPC-104
system using tandem LF-404 columns (THF as an eluent,
flow rate 5 1.0 mL/min, 40 ꢀC) equipped with an UV detec-
tor (Shimadzu (Kyoto, Japan) SPP-20A). Number-averaged
molecular weight (Mn) and molecular weight distribution
(Mw/Mn) were determined on the basis of a calibration curve
made from standard polystyrene samples. UV-vis and FL
spectra were measured on a Shimadzu UV-1650PC spectro-
photometer and a Shimadzu RF-5300PC spectrofluorometer,
respectively, using a 1 cm quartz cell. Relative fluorescence
quantum yields (QYs) in solution were determined using qui-
nine sulfate in 0.1 M H2SO4 with QY of 0.55. Electrochemical
measurement for polymers were carried out on a BAS
(Tokyo, Japan) 2323 bipotentiostat using a platinum disk
working electrode, a platinum wire counter electrode, and a
Ag/Ag1 reference electrode in a standard three-electrode
cell. The polymer film cast onto a platinum disk was
immersed in a nitrogen-saturated 0.1 M CH3CN solution of
tetrabutylammonium hexafluorophosphate (Bu4NPF6), and
the voltage-current characteristic was obtained at the scan
rate of 100 mV/s.
Then the solution was washed with water and dried over
MgSO4. Solvents were removed by the rotary evaporator to
dryness. The obtained crude products were purified by
recrystallization from hexane/CHCl3 to give a yellow solid
ꢀ
(550 mg, 77% yield). mp: 86–89 C.
ESI-MS CALCD. for C25H25Br2N2OS2 (M1H1): 592.9754, found:
592.9845. 1H NMR (200 MHz, CDCl3) d 7.95 (d, J 5 8.7 Hz,
2H), 7.72 (s, 2H), 6.96 (d, J 5 8.7 Hz, 2H), 3.99 (t, J 5 5.8 Hz,
2H), 1.85–1.29 (12H), 0.90 (3H). IR (cm21, ATR) 2919, 2850,
1613, 1496, 1465, 1361, 1246, 1178, 827.
5,8-Dibromo-2-(p-octyloxyphenyl)-1-pentyl-
dithieno[30,20:3,4:200,300:5,6]benzimidazole (5)
To a THF solution (11 mL) of 4 (650 mg, 1.10 mmol) was
added NaH (55% oil suspension) (144 mg, 3.30 mmol), and
the reaction mixture was heated to reflux for 1 hr. After 1-
iodopentane (0.784 mg, 3.96 mmol) was added, the reaction
mixture was heated to reflux for 12 hr. Then the solution
was washed with aqueous Na2S2O3 and dried over MgSO4.
Solvents were removed by the rotary evaporator to dryness.
The obtained crude products were purified by SiO2 column
chromatography (CH2Cl2, Rf 5 0.23) followed by recrystalli-
zation from hexane/CHCl3ꢀto give a colorless solid (355 mg,
49% yield). mp: 145–147 C.
EA 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. 1H NMR (200 MHz, CDCl3) d 7.99 (s, 1H), 7.61 (d,
J 5 8.7 Hz, 2H), 7.60 (s, 1H), 7.05 (d, J 5 8.7 Hz, 2H), 4.42 (t,
J 5 7.6 Hz, 2H), 4.05 (t, J 5 6.5 Hz, 2H), 1.90–1.80 (4H),
1.56–1.20 (14H), 0.94–0.81 (6H). 13C NMR (50 MHz, CDCl3)
d 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. IR
(cm21, ATR) 2918, 1614, 1464, 1365, 1255, 1171, 669.
Theoretical Calculation
Theoretical calculations were performed using the Gaussian
09W (Revision C.01) package of programs.15
Syntheses
5,8-Bis(trimethylsilyl)-2-(p-octyloxyphenyl)
dithieno[30,20:3,4:200,300:5,6]benzimidazole (3)
2,7-Dibromobenzo[1,2-b:6,5-b0]dithiophene-4,5-dione (6)
To a CH2Cl2 solution (30 mL) of 1 (1.10 g, 3.02 mmol) was
added Br2 (340 mL, 6.64 mmol), and the reaction mixture
was stirred at room temperature for 30 min. Then the solu-
tion was washed with aqueous Na2S2O3 and dried over
MgSO4. Solvents were removed by the rotary evaporator to
dryness. The obtained black solid (723 mg, 63% yield) was
used without purification.
To an EtOH solution (24 mL) of 1 (1.00 g, 2.74 mmol) were
added 4-octyloxybenzaldehyde (0.900 g, 3.84 mmol) and
ammonium acetate (1.06 g, 13.7 mmol), and the reaction
mixture was heated to reflux for 12 hr. Then water and
CH2Cl2 were added and an aqueous phase was extracted
with CH2Cl2. The combined organic phase was dried over
MgSO4 and solvents were removed by the rotary evaporator
to dryness. The obtained crude products were purified by
SiO2 column chromatography (CH2Cl2, Rf 5 0.50) to give a
1H NMR (200 MHz, CDCl3) d 7.47 (s, 2H). IR (cm21, ATR)
3081, 1654, 1509, 1409, 1273, 1167, 1054, 949, 668.
ꢀ
yellow solid (1.04 g, 65% yield). mp: 229–231 C.
1H NMR (200 MHz, CDCl3) d 8.17–7.72 (4H), 6.93 (d,
J 5 8.3 Hz, 2H), 3.96 (t, J 5 5.8 Hz, 2H), 1.76 (m, 2H), 1.30
(10H), 0.89 (3H), 0.36 (s, 18H). IR (cm21, ATR) 2924, 2856,
1614, 1493, 1458, 1248, 1179, 978, 833.
5,8-Dibromo-2-(p-octyloxyphenyl)dithieno
[30,20:3,4:200,300:5,6]benzoxazole (7)
To an EtOH solution (12 mL) of 6 (500 mg, 1.32 mmol)
were added 4-octyloxybenzaldehyde (0.443 g, 1.89 mmol)
and ammonium acetate (500 mg, 6.48 mmol), and the reac-
tion mixture was heated to reflux for 12 hr. After cooling to
room temperature, the precipitate was collected by the filtra-
tion. The obtained crude products were purified by SiO2 col-
umn chromatography (CHCl3, Rf 5 0.85) followed by
5,8-Dibromo-2-(p-octyloxyphenyl)dithieno
[30,20:3,4:200,300:5,6] benzimidazole (4)
To a CHCl3 solution (150 mL) of 3 (700 mg, 1.21 mmol) was
added N-bromosuccinimide (452 mg, 2.54 mmol), and the
reaction mixture was stirred at room temperature for 12 hr.
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recrystallization from hexane/CHCl3 to give a pale yellow
solid (197 mg, 25% yield).
column chromatography (CH2Cl2, Rf 5 0.25) followed by
recrystallization from hexane/CHCl3 to give a colorless solid
(779 mg, 73% yield).
mp: 148–150 ꢀC. EA CALCD. for C25H23Br2NO2S2: C, 50.60%;
H, 3.91%; N, 2.36%; S, 10.81%, Found: C, 50.39%; H, 3.81%;
N, 2.14%; S, 10.93%. 1H NMR (200 MHz, CDCl3) d 8.03 (d,
J 5 8.6 Hz, 2H), 7.67 (s, 1H), 7.44 (s, 1H), 6.94 (d, J 5 8.6 Hz,
2H), 4.00 (t, J 5 6.5 Hz, 2H), 1.83 (2H), 1.48–1.33 (10H),
0.91 (3H). 13C NMR (50 MHz, CDCl3) d 161.7, 142.1, 133.9,
129.0, 128.9, 124.4, 122.3, 122.1, 119.1, 114.7, 114.3, 68.3,
31.9, 29.4, 29.3, 26.1, 22.7, 14.2. IR (cm21, ATR) 2923, 2849,
1615, 1498, 1361, 1245, 1171, 963, 830.
mp: 104–106 ꢀC. EA CALCD. for C30H34Br2N2OS2: C, 54.38%;
H, 5.17%; N, 4.23%; S, 9.68%, Found: C, 54.44%; H, 5.21%;
N, 4.07%; S, 9.94%. 1H NMR (200 MHz, CDCl3) d 7.63 (d,
J 5 8.2 Hz, 2H), 7.60 (s, 2H), 7.03 (d, J 5 8.8 Hz, 2H), 4.33 (t,
J 5 7.8 Hz, 2H), 4.03 (t, J 5 6.6 Hz, 2H), 1.94–1.74 (4H),
1.53–1.31 (14H), 0.95–0.82 (6H). 13C NMR (50 MHz, CDCl3)
d 160.4, 152.4, 134.4, 132.3, 130.3, 130.2, 130.0, 129.9,
129.4, 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. IR (cm21, ATR) 2920, 2856, 1614, 1464,
1375, 1255, 1171, 909, 803, 669.
2-(p-Octyloxyphenyl)dithieno[20,30:3,4:300,200:5,6]
benzimidazole (8)
To an EtOH solution (32 mL) of 2 (800 mg, 3.63 mmol)
were added 4-octyloxybenzaldehyde (1.19 g, 1.89 mmol) and
ammonium acetate (1.40 g, 18.2 mmol), and the reaction
mixture was heated to reflux for 12 hr. After cooling to room
temperature, the precipitate was collected by the filtration.
The obtained crude products were washed with water and
hexane to give a green solid (1.50 g, 95% yield).
Poly[(2-(p-octyloxyphenyl)-1-pentyl-
dithieno[30,20:3,4:200,300:5,6]benzimidazole-5,8-diyl)-
alt-(9,9-dihexylfluorene-2,7-diyl)] (P1)
To a THF solution (2 mL) of 5 (100 mg, 0.151 mmol) and
9,9-dihexylfluorene-2,7-diboronic acids (63.7 mg, 0.151
mmol) were added 2 M aqueous Na2CO3 (0.5 mL) and
Pd(PPh3)4 (17.4 mg, 15.1 mmol), and the reaction mixture
was heated to reflux for 3 days. The solution was poured
into saturated NH4Cl and an aqueous phase was extracted
with CHCl3. The combined organic phase was dried over
MgSO4 and the concentrated solution was poured into MeOH
to give a brown solid (62 mg, 49% yield).
ESI-MS Calcd. for C25H27N2OS2 (M1H1): 435.1565, Found:
435.1633. 1H NMR (200 MHz, CDCl3) d 8.18 (d, J 5 8.1 Hz,
2H), 8.01 (d, J 5 5.1 Hz, 2H), 7.75 (d, J 5 4.8 Hz, 2H), 7.11 (d,
J 5 8.1 Hz, 2H), 4.06 (t, J 5 6.1 Hz, 2H), 1.75 (2H), 1.44–1.28
(10H), 0.87 (3H). IR (cm21, ATR) 2921, 2852, 1611, 1459,
1248, 1182, 1042, 839, 709.
1H NMR (600 MHz, CDCl3) d 8.09–6.61 (12H), 4.58 (2H),
4.09 (2H), 2.45–0.47 (50H). Mn 5 6100, Mw/Mn 5 1.63.
2-(p-Octyloxyphenyl)-1-pentyl-dithieno[20,30:3,4:300,200:5,6]
benzimidazole (9)
Poly[2-(p-octyloxyphenyl)dithieno[30,20:3,4:200,300:5,6] benzoxa-
zole-5,8-diyl]-alt-(9,9-dihexylfluorene-2,7-diyl)] (P2), Poly[2-(p-
octyloxyphenyl)21-pentyl-dithieno[20,30:3,4:300,200:5,6] benzim-
idazole-5,8-diyl]-alt-(9,9-dihexylfluorene-2,7-diyl)] (P3), and
Poly[2-(p-octyloxyphenyl)-1-pentyl-dithieno[20,30:3,4:300,200:5,6]
benzimidazole-5,8-diyl]-alt-(3,30-dihexyl-2,20-bithiophene-5,50-d
iyl)] (P4) were likewise synthesized by the palladium-catalyzed
cross-coupling polymerization, and the structure was confirmed
by the 1H NMR spectra. The number-averaged molecular weight
and the molecular weight distribution were estimated from the
GPC analyses.
To a THF solution (21 mL) of 8 (1.00 g, 2.30 mmol) was
added NaH (55% oil suspension) (301 mg, 6.90 mmol), and
the reaction mixture was heated to reflux for 1 hr. After 1-
iodopentane (0.547 g, 2.76 mmol) was added, the reaction
mixture was heated to reflux for 12 hr. Then the solution
was washed with aqueous Na2S2O3 and dried over MgSO4.
Solvents were removed by the rotary evaporator to dryness.
The obtained crude products were purified by SiO2 column
chromatography (CH2Cl2, Rf 5 0.10 then ethyl acetate,
Rf 5 0.95) to give a yellow solid (820 mg, 73% yield).
mp: 94–96 ꢀC. ESI-MS CALCD. for C30H38N2OS2 (M1H1):
505.2347, Found: 505.2418. 1H NMR (200 MHz, CDCl3) d 7.84
(d, J 5 5.4 Hz, 1H), 7.77 (d, J 5 5.4 Hz, 1H), 7.68 (d, J 5 8.6 Hz,
2H), 7.51 (d, J 5 2.5 Hz, 1H), 7.48 (d, J 5 2.4 Hz, 1H), 7.04 (d,
J 5 8.7 Hz, 2H), 4.47 (t, J 5 7.7 Hz, 2H), 4.04 (t, J 5 6.5 Hz, 2H),
Poly[1-methyl-2-(p-octyloxyphenyl)-3-pentyl-
dithieno[20,30:3,4:300,200:5,6]benzimidazolium-5,8-diyl]-alt-
(3,30-dihexyl-2,20-bithiophene-5,50-diyl)] (P40)
To a CH3CN/CHCl3 (19 mL/38 mL) solution of P4 (30 mg,
35.8 mmol) was added iodomethane (167 mL, 2.68 mmol),
and the reaction mixture was heated to reflux for 12 hr. Sol-
vents were removed by the rotary evaporator to dryness.
The obtained crude products were purified by precipitating
into hexane to give a red solid (30 mg).
2.01–1.74 (4H), 1.51–1.31 (14H), 0.95–0.83 (6H). IR (cm21
ATR) 2922, 2852, 1608, 1466, 1250, 1176, 1028, 833, 713.
,
5,8-Dibromo-2-(p-octyloxyphenyl)-1-pentyl-
dithieno[20,30:3,4:300,200:5,6]benzimidazole (10)
To a CHCl3 solution (180 mL) of 9 (750 mg, 1.48 mmol) was
added N-bromosuccinimide (555 mg, 3.12 mmol), and the
reaction mixture was stirred at room temperature for 12 hr.
Then the solution was washed with water and dried over
MgSO4. Solvents were removed by the rotary evaporator to
dryness. The obtained crude products were purified by SiO2
RESULTS AND DISCUSSION
Syntheses
DTBIm monomer
5 was synthesized in the following
sequence, step 1: the condensation cyclization of
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TABLE 1 Polymerization Results
Yield/%a
Mn
Mw/Mn
b
b
Code
P1
P2
P3
P4
49
76
71
96
6100
3600
7800
14500
1.63
1.42
1.81
1.77
a
After precipitation into MeOH.
Estimated from GPC analysis (THF, PSt standard).
b
2,7-bis(trimethylsilyl)benzo[1,2-b:6,5-b0]dithiophene-4,5-dio-
ne (1) with 4-octyloxybenzaldehyde, step 2: the
trimethylsilyl-bromo exchange reaction using N-bromosucci-
nimide, and step 3: the N-alkylation at the 1-position (Sup-
porting Information Scheme S1). On the other hand, the
dithienobenzoxazole monomer 7 was prepared when 1 was
initially treated with Br2 and then the condensation cycliza-
tion with 4-octyloxybenzaldehyde was performed. Hence
monomers 5 and 7 were selectively obtained by choosing
the timing of the bromination. Another DTBIm monomer 10,
being the structural isomer of 5, was synthesized in three
steps by the condensation cyclization of benzo[2,1-b:5,6-
b0]dithiophene-4,5-dione (2) with 4-octyloxybenzaldehyde,
the N-alkylation at the 1-position, and the bromination using
N-bromosuccinimide. In this reaction sequence, 10 was
selectively prepared through the formation of the DTBIm
derivative 8. The monomer structure and purity were con-
firmed by the NMR spectra and the elemental analyses.
The polymerizations were carried out by the Suzuki-Miyaura
(P1, P2, and P3) or Migita-Kosugi-Stille (P4) cross-coupling
reaction using Pd(PPh3)4 as the catalyst (Supporting
Information Scheme S2). The number-averaged molecular
weight of polymers ranged from 3600 to 14,500, and the
molecular weight distribution were 1.42–1.81 (Table 1). The
molecular weight of P2 was low since the product was pre-
cipitated from the solution during the course of the polymer-
ization owing to the low solubility. Other polymers bearing
the DTBIm unit in the main chain had a better solubility in
THF, CHCl3, and toluene. The N-methylation at the 3-position
of the neutral P4 was also conducted with iodomethane in
order to promote the ICT interaction between the electron-
donor (thiophene) and electron-acceptor (dithienobenzimida-
zolium) segments. The conversion to imidazolium salt was
75%, which was calculated by comparing the integral ratio of
the methylene proton signals adjacent to the nitrogen and
oxygen atoms with that adjacent to the thiophene ring in the
1H NMR spectrum of P4.’
FIGURE 2 UV-Vis (a) and FL (b) spectra of P1 (circle), P2 (trian-
gle), P3 (square), and P4 (diamond) in the CHCl3 solution.
FMO calculation using the model compounds (Fig. 3). The
geometry optimizations were executed with the density func-
tional theory (DFT) employing the B3LYP/6–31G(d) basis
set. The time-dependent DFT method at the B3LYP/6–
311 1 G(d,p) level was used to calculate the one electron
transition energy and the oscillator strength (f). The dihedral
angle between the DTBIm and benzene units of M1, the
model compound of P1, is 52ꢀ due to the steric repulsion of
the N-methyl group at the 1-position (Supporting Informa-
tion Fig. S17A). Both the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) of M1 are relatively localized on the DTBIm unit.
The p–p* transition corresponding to the S0–S1 transition
Optoelectronic Properties
The UV-Vis and FL spectra of P1–P4 were obtained in the
CHCl3 solution (1025 M) (Fig. 2), and the results are sum-
marized in Supporting Information Table S1. The absorption
and emission maxima of P1 with the DTBIm unit showed a
red-shift as compared with those of P2 with the dithienoben-
zoxazole unit. These results were further investigated by the
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M3 is almost similar to M1 because the dihedral angle
between the DTBIm and benzene units of M3 is 51ꢀ (Sup-
porting Information Fig. S17C). The FMO calculation was
performed using M3 by the time-dependent DFT method to
gain insight into the FMO energy levels (Supporting Informa-
tion Fig. S19). The p–p* transition corresponding to the
S0–S1 transition involves the electronic transition from
HOMO-1 to LUMO with f 5 0.50. The theoretical absorption
maximum of M3 (321 nm) is shorter rather than M1, which
would be stemmed from the shallower LUMO energy level.
Both the absorption (449 nm) and emission (541 nm) max-
ima of P4 having the bithiophene unit as the alternating
building block were observed at the longest wavelength
region among four polymers. From the FMO calculation of
the model compound M4 (Supporting Information Fig. S20),
the p–p* transition corresponding to the S0–S1 transition
involves the electronic transition from HOMO to LUMO. The
theoretical absorption maximum observed at 422 nm with
f 5 0.90 can be explained by the occurrence of the ICT inter-
action between the electron-donor (thiophene) and electron-
acceptor (DTBIm) units. The ICT interaction in M4 was also
demonstrated by the FMO calculation of 2,7-dithiophene-20-
yl-benzo[2,1-b:3,4-b0]dithiophene because its theoretical
absorption maximum corresponding to the HOMO–LUMO
transition is 408 nm with f 5 0.99 (Supporting Information
Fig. S21).
Subsequently, the UV-Vis and FL spectra of P1, P3, and P4
were collected in the thin-film obtained by spin-coating their
CHCl3 solutions. The spectra shifted to the longer wavelength
in the order of P3, P1, and P4. (Fig. 4). The absorption max-
ima showed a red-shift from those in the CHCl3 solution
except for P3 (Supporting Information Table S1) to indicate
the stacking interaction of polymer chains. The shift widths
were relatively small because P1 has the hexyl chains on the
fluorene unit perpendicular to the main chain and P4 has
the head-to-head connection at the bithiophene unit. The
onset wavelength of the UV-vis spectra showed a larger red-
shift than the absorption maxima.
FIGURE 3 FMOs of M1 (left) and M2 (right) along with the
energy levels of HOMO (solid line), LUMO (broken line), and
LUMO 1 1 (dotted line). The FMO energy levels concerning the
S0–S1 transition are only shown above the lines. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
involves the electronic transition from HOMO to LUMO with
f 5 0.45. On the other hand, the dithienobenzoxazole and
benzene units exist in the almost planar fashion (the corre-
sponding dihedral angle was 0.5ꢀ) in the case of the model
compound M2 (Supporting Information Fig. S17B). The FMO
of M2 at the ground state (HOMO) therefore spread over the
whole molecule. The FMO energy levels of M2 generally shift
to the negative side rather than M1 owing to the strong
electron-withdrawing character of the oxazole ring. The p–p*
transition corresponding to the S0–S1 transition involves the
electronic transition from HOMO to LUMO 1 1 (f 5 0.89).
Accordingly, the theoretical absorption maxima of M1 (363
nm) and M2 (338 nm) nicely verified the trend of experi-
mental results of P1 (kmax 5 432 nm) and P2 (kmax 5 426
nm). The relative QYs of P1 (17%) and P2 (10%) were low
probably because of the heavy-atom effect of sulfur in the
DTBIm unit.
Cyclic voltammetry (CV) was utilized to evaluate the FMO
energy levels of conjugated polymers. Polymer films were
drop cast onto a platinum electrode from their CHCl3 solu-
tions. The well-resolved peak could not be obtained in the
cathodic scan, and therefore only the oxidation CV curves are
shown in Supporting Information Figure S22. The HOMO
energy level (EHOMO) was estimated from the onset oxidation
potential (Eonset) using the following equation: EHOMO 5 2
(Eonset 1 4.73) eV, where the energy level was calibrated
against the ferrocene/ferrocenium couple (Fc/Fc1 measured
as 0.07 V vs. Ag/Ag1). Table 2 summarizes the experimental
EHOMO and the LUMO energy level (ELUMO) obtained by
applying optical band gaps (BGopt) in the UV spectra. The
EHOMO value of P3 was observed at the lower oxidation
potential relative to that of P1 in consistent with the results
of the FMO calculation (EHOMO(M1) 5 25.37 eV and
EHOMO(M3) 5 25.23 eV). On the other hand, the EHOMO value
of P4 was comparable to that of P3 contrary to the
The absorption (406 nm) and emission (456 nm) maxima of
P3 markedly blue shifted from those of P1, and the relative
QY of P3 was decreased to 9%. These results indicate that
P3 has the shorter effective conjugation length than P1. The
optimized ground state geometry of the model compound
406
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FIGURE 5 UV-Vis (open mark) and FL (closed mark) spectra of
P4 (circle) and P4’ (square) in the CHCl3 solution.
theoretical prediction (EHOMO(M4) 5 25.18 eV). This is prob-
ably because the bithiophene segment in P4 has the head-to-
head connection to increase the distortion between DTBIm
and thiophene units.
Finally, the UV-Vis and FL spectra of neutral P4 and cationic
P4’ were measured in order to clarify the influence of the
quaternization on the optical properties (Fig. 5). Upon the N-
methylation of the imine nitrogen, both the absorption and
emission maxima of P4’ showed small but unambiguous red-
shift as compared with those of P4 to be observed at 454
and 553 nm, respectively. These results can be ascribed to
the enhanced ICT interaction between the electron-donor
(thiophene) and electron-acceptor (dithienobenzimidazo-
lium) units in the main chain of P4’, which was studied by
the FMO calculation. From the optimized ground state struc-
ture of the model compound M4’, the dihedral angle between
the dithienobenzimidazolium and benzene units largely
increases to 87ꢀ as compared with that of M4 (52ꢀ) due to
the steric repulsion of the two methyl groups (Supporting
Information Fig. S18). Both the HOMO and LUMO energy lev-
els apparently shift to the negative side as expected from the
cationic character of the compound (Fig. 6). In addition, the
HOMO does not exist on the imidazolium cation moiety. The
p–p* transition corresponding to the S0–S1 transition
involves the electronic transition from HOMO to LUMO. The
theoretical absorption maximum of M4’ is 432 nm with
f 5 0.72 which is red-shifted from that of M4 (422 nm) in
accordance with the experimental results. The relative QYs of
P4’ decreased to 4% that can be attributed to the external
heavy-atom effect of the iodide counter anion.11(d) In the
spin-coated thin-film, the absorption and emission maxima
of P4’ actually shifted to the longer wavelength (Supporting
Information Fig. S23), however, the shift widths were smaller
FIGURE 4 UV-Vis (a) and FL (b) spectra of P1 (circle), P3
(square), and P4 (diamond) in the thin-film.
TABLE 2 Electrochemical Characteristics
b
c
Code
E
HOMO/eVa
ELUMO/eV
BGopt/eV
P1
P3
P4
25.73
25.66
25.66
23.24
23.03
23.49
2.49
2.63
2.17
a
Estimated from CV curves.
b
c
Calculated from EHOMO and optical band gaps.
Estimated from UV-Vis spectra.
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FIGURE 6 FMOs of M4 (left) and M4’ (right) along with the
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CONCLUSIONS
Four conjugated polymers consisting of a dithienobenzimida-
zole (DTBIm) or dithienobenzoxazole unit in the main chain
were synthesized by the cross-coupling polymerization. From
the ultraviolet/fluorescence spectra and the cyclic voltamme-
try, the optoelectronic properties of conjugated polymers
could be tuned by the structure of the fused-ring system and
the alternating building block, which was carefully exempli-
fied by the frontier molecular orbital calculation of the
model compounds. The intramolecular charge transfer inter-
action between the DTBIm and thiophene units was
enhanced by the quaternization of the imidazole ring.
ꢀ
€
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10 During the progress of our research, Ragogna et al. have
reported the preparation of thiophene-fused benzimidazolium
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ACKNOWLEDGMENTS
This work was financially supported by the Tokuyama Science
Foundation.
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