Conjugated polymers based on benzodithiophene and arylene imides: Extended absorptions and tunable electrochemical properties

Article abstract of DOI:10.1016/j.polymer.2010.04.035

Three novel conjugated polymers have been designed and synthesized via the alternative copolymerization of the electron-donating monomer benzodithiophene (BDT) and three different electron-accepting monomers: perylene diimide (PDI), naphthalene diimide (NDI), and phthalimide (PhI). All obtained copolymers show good solubility in common organic solvents as well as broader absorptions in visible region and narrower optical band gaps compared to homopolymers from BDT units. It is found that the absorptions of the copolymers are red-shifted with increasing the electron-withdrawing ability of the co-monomer. In particular, the absorption edge of P(BDT-NDI) film extends to 760. nm, whereas that of P(BDT-PhI) film is only at 577. nm. Cyclic voltammograms of the three polymers disclose that P(BDT-PDI) and P(BDT-NDI) are typical n-type materials because PDI and NDI are strong electron-accepting groups, while P(BDT-PhI) is a stable p-type material where the weak electron-withdrawing monomer (PhI) is introduced. The results suggest that the absorption range and the electrochemical properties of the conjugated polymers can be tuned by appropriate molecule-tailoring, which will help exploring ideal conducting polymers for potential applications in polymer optoelectronics, especially in polymer solar cells.

Full text of DOI:10.1016/j.polymer.2010.04.035

Polymer 51 (2010) 2897e2902
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Conjugated polymers based on benzodithiophene and arylene imides: Extended
absorptions and tunable electrochemical properties
*
**
Jian Chen, Min-Min Shi , Xiao-Lian Hu, Mang Wang, Hong-Zheng Chen
MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P.R. China
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 January 2010
Received in revised form
Three novel conjugated polymers have been designed and synthesized via the alternative copolymeri-
zation of the electron-donating monomer benzodithiophene (BDT) and three different electron-
accepting monomers: perylene diimide (PDI), naphthalene diimide (NDI), and phthalimide (PhI). All
obtained copolymers show good solubility in common organic solvents as well as broader absorptions in
visible region and narrower optical band gaps compared to homopolymers from BDT units. It is found
that the absorptions of the copolymers are red-shifted with increasing the electron-withdrawing ability
of the co-monomer. In particular, the absorption edge of P(BDT-NDI) film extends to 760 nm, whereas
that of P(BDT-PhI) film is only at 577 nm. Cyclic voltammograms of the three polymers disclose that P
9
April 2010
Accepted 16 April 2010
Available online 27 April 2010
Keywords:
Conjugated polymers
Arylene imides
Absorptions
(
BDT-PDI) and P(BDT-NDI) are typical n-type materials because PDI and NDI are strong electron-
accepting groups, while P(BDT-PhI) is a stable p-type material where the weak electron-withdrawing
monomer (PhI) is introduced. The results suggest that the absorption range and the electrochemical
properties of the conjugated polymers can be tuned by appropriate molecule-tailoring, which will help
exploring ideal conducting polymers for potential applications in polymer optoelectronics, especially in
polymer solar cells.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
when some representative polymers are blended with fullerene
derivatives to form bulk heterojunction PSCs [15e17]. However,
Conjugated polymers have recently attracted considerable
attention because they can be applied to large-area, flexible opto-
electronic devices, such as polymer light-emitting diodes [1e5],
polymer solar cells (PSCs) [6e11], and organic thin-film transistors
most of the reported low band gap conjugated polymers are p-type
materials, while n-type materials in PSCs, fullerene derivatives in
particular, show little contributions to the sunlight absorption due
to their high band gaps. Furthermore, the open circuit voltage (VOC
)
[
12,13], etc., by low-cost solution-processing. Among these poten-
of a PSC is proportional to the difference between the HOMO of the
p-type material and the LUMO of the n-type material, thus it is
difficult to increase VOC once the n-type material is fixed [18].
The arylene imides units are attractive candidates as elec-
tron-accepting co-monomers of low band gap D-alt-A polymers
by virtue of their extremely facile synthesis and readily varied
substitution at the nitrogen, allowing manipulation of polymer
solubility, packing and morphology. Moreover, when inserting
into polymer backbones, the strongly electron-withdrawing
imide groups would largely alter band energy levels, even
change the semiconductive behavior from p-type to n-type
tial applications, PSCs are especially attractive as promising
renewable energy sources. For conjugated polymers used in PSCs,
a low band gap is of crucial importance for sunlight harvest since
the maximum photon flux of the solar irradiation locates at ca.
6
80 nm [14]. The copolymers alternating electron-rich and elec-
tron-deficient monomers (D-alt-A polymers) are suggested as
materials with a narrow band gap and a broad absorption, and
more than 5% power conversion efficiency (PCE) has been achieved
[
19e23]. Nevertheless, only
a
few published examples of
-systems
conjugated polymers contain imide-functionalized
p
*
Corresponding author. Tel./fax: þ86 571 8795 3733.
such as thiophene imides [24], bithiophene imide [25], iso-
thianaphthene imide [26], phthalimide [27,28] and arylene imide
[19,23]. Herein we designed and successfully synthesized three
** Corresponding author. Tel./fax: þ86 571 8795 2557.
E-mail addresses: minminshi@zju.edu.cn (M.-M. Shi), hzchen@zju.edu.cn (H.-Z.
Chen).
0
032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.04.035

2898
J. Chen et al. / Polymer 51 (2010) 2897e2902
new polymers, P(BDT-PDI), P(BDT-NDI) and P(BDT-PhI), with
arylene imide units electronically conjugated along the backbone
by alternative copolymerization of the electron-donating mono-
mer benzodithiophene (BDT) and three different electron-
accepting monomers of perylene diimide (PDI), naphthalene
diimide (NDI), and phthalimide (PhI), respectively. The benzo-
dithiophene (BDT) unit was utilized because of its planarity,
stability and solubility [29]. In addition, its homopolymer (PBDT)
was reported as a good semiconductor material [30e32]. All
obtained polymers show broad visible absorption bands and
narrow optical band gaps. Cyclic voltammograms display that P
2.3.2. 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-
b:4,5-b ]dithiophene (3)
0
Compound 2 (0.45 g, 1.0 mmol) was dissolved in 15 mL of
anhydrous THF under N
2
protection. The solution was cooled down
ꢀ
to ꢁ78 C by a liquid nitrogeneacetone bath, and t-butyllithium
solution (1.9 mL, 2.5 mmol, 1.3 M in hexane) was added dropwise.
The reaction mixture was stirred at this temperature for 1 h and
allowed to warm to RT over 3 h, at which point it was stirred for an
ꢀ
additional hour. Then the mixture was cooled to ꢁ78 C, and tri-
methyltin chloride solution (3.0 mL, 3.0 mmol, 1 M in THF) was
added dropwise. The mixture was allowed to warm to RT and
stirred overnight. The reaction was quenched with 50 mL of cold
water and extracted with hexane three times. The organic extrac-
tion was washed by water twice and then dried by anhydrous
(
(
BDT-PDI) and P(BDT-NDI) are typical n-type materials while P
BDT-PhI) is a stable p-type material.
2 4
Na SO . After removing the solvent under vacuum, the residue was
2
. Experimental
dissolved in hexane and quickly passed through a column pre-
treated with triethylamine. After removing the solvent under
vacuum, recrystallization of the residue from methanol yielded
2.1. Instrument
1
compound 3 (0.67 g, yield 86%) as colorless needle crystals. H NMR
1H NMR spectra were recorded on a Bruker DMX-300 nuclear
(
2
300 MHz, CDCl
H), 1.28e1.49 (m, 16H), 0.85e1.07 (m, 12H), 0.44 (s, 18H).
3
):
d
¼ 7.52 (s, 2H), 4.17e4.22 (d, 4H), 1.74e1.88 (m,
resonance spectroscope. Absorption spectra were obtained on
a Shimadzu UV-2450 UVevis spectrophotometer. Mass spectra
were gotten on a VG 70-SE MS spectroscope under the electron
impact (EI) mode. Elemental analyses were carried out on a LECO
0
.3.3. N,N -bis(2-ethylhexyl)-1,7-dibromo-3,4,9,10-perylene
2
diimide (5)
9
32 CHNS elemental analyzer. The molecular weight of polymers
A
mixture of 1,7-dibromoperylene-3,4,9,10-tetracarboxy-
was determined on Waters 1525/2414 gel permeation chromatog-
raphy (GPC), and polystyrene was used as a standard. Thermogra-
vimetric analysis (TGA) was performed on a WCT-2 thermal balance
2
dianhydride (4) (1.65 g, 3.0 mmol) and 2-ethylhexylamine (H NEH,
1
.55 g, 12 mmol) in propionic acid (40 mL) was heated to reflux at
ꢀ
ꢀ
ꢁ1
140 C for 12 h under a N atmosphere. The resulting mixture was
2
under a N atmosphere at a heating rate of 10 C min . Electro-
2
cooled and poured into water (200 mL), filtrated, and washed with
water until the filtrate reached neutrality. The crude solid was dried
chemical cyclic voltammetry was conducted on a CHI 600A elec-
trochemical workstation with Pt disk, Pt plate, and SCE as working
electrode, counter electrode, and reference electrode, respectively,
ꢀ
at 70 C under vacuum. After being purified by column chroma-
ꢁ
1
tography on silica gel using a mixture of CH Cl /petroleum ether
2
2
in
a
0.1 mol L
tetrabutylammonium hexafluorophosphate
(
1:1) as eluent, product 5 (2.08 g, yield 90%) was obtained as a deep
(
Bu
4
NPF ) CH Cl
6
2
2
solution.
1
red powder. H NMR (300 MHz, CDCl
3
):
d
¼ 9.47e9.52 (d, 2H), 8.93
(s, 2H), 8.69e8.73 (d, 2H), 4.11e4.20 (m, 4H), 1.90e2.00 (m, 2H),
2.2. Materials
1.27e1.46 (m, 16H), 0.85e0.99 (m, 12H). C40H40Br N O : Calcd, C
2
2 4
62.19, H 5.22, N 3.63; found, C 62.34, H 5.31, N 3.35.
0
Benzo[1,2-b:4,5-b ]dithiophene-4,8-dione (1) [14] and 1,7-
dibromoperylene-3,4,9,10-tetracarboxydianhydride (4) [33] were
synthesized according to the procedures reported in the literature.
All reagents, unless otherwise specified, were obtained from
Aldrich, Acros, and TCI Chemical Co. and used as received. All the
solvents were freshly distilled prior to use.
2
.3.4. 2,6-Dibromonaphthalene-1,4,5,8-tetracarboxydianhydride
(7)
A solution of dibromoisocyanuric acid (DBI) (1.47 g, 5.1 mmol) in
oleum (30 mL, 20% SO ) was added at RT to a solution of 1,4,5,8-
naphthalenetetracarboxylic dianhydride (6) (1.43 g, 5.0 mmol) in
3
oleum (30 mL, 20% SO
3
) over a course of 1 h. The resulting mixture
ꢀ
was stirred at 40 C for 5 h and then cautiously poured into crushed
ice (200 g). The mixture was diluted with water (200 mL) and then
stirred at RT for 1 h. The precipitate was collected on a Buchner
funnel, washed with water and methanol, and dried under vacuum,
leading to a bright yellow solid (1.89 g, yield 83%). The crude
product 7 was used for the next step without further purification.
2.3. Synthesis of the monomers
The synthesis routes of the monomers are shown in Scheme 1.
The detailed synthetic processes are as follows.
0
2
.3.1. 4,8-Bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]dithiophene (2)
0
Benzo[1,2-b:4,5-b ]dithiophene-4,8-dione (1) (2.2 g, 10 mmol)
0
and zinc dust (1.43 g, 22 mmol) were put into a 100 mL flask. NaOH
solution (40 mL, 20%) was added, and the mixture was heated to
reflux for 1 h. Then, 2-ethylhexyl bromide (5.8 g, 30 mmol) and
a catalytic amount of tetrabutylammonium bromide (TBAB) were
added into the flask. After being refluxed for 8 h, the reactant was
2.3.5. N,N -bis(2-ethylhexyl)-2,6-dibromo-1,4,5,8-naphthalene
diimide (8)
A mixture of compound 7 (2.13 g, 5.0 mmol), 2-ethylhexylamine
(1.94 g, 15 mmol), and acetic acid (50 mL) was stirred at 120 C for
2 h under N protection. Upon cooling to RT, the mixture was
2
ꢀ
poured into 150 mL of cold water, and extracted by CH
2 ꢂ100 mL). The organic extraction was dried with anhydrous
Na SO and evaporated in vacuo. Column chromatography on silica
gel using a mixture of CH Cl and hexane (1:1) as the eluent yielded
compound 2 (3.66 g, yield 82%) as a light yellow oil. H NMR
300 MHz, CDCl ):
¼ 7.46e7.51 (d, 2H), 7.35e7.40 (d, 2H),
.14e4.23 (br, 4H),1.76e1.87 (m, 2H),1.32e1.53 (m, 16H), 0.89e1.07
2
Cl
2
poured into 200 mL cold water, filtrated, and washed with water
and methanol. After drying under vacuum, the resulting reddish
solid was purified by column chromatography on silica gel using
a mixture of CH Cl /petroleum ether (1:1) as eluent. And the
2 2
resulting orange powder was further purified by recrystallization
(
2
4
2
2
1
(
4
3
d
from hexane to obtain product 8 (1.10 g, yield 34%) as slight yellow
1
crystals. H NMR (300 MHz, CDCl
3
):
d
¼ 9.00 (s, 2H), 4.12e4.18 (q,
þ
(
m, 12H). MS (EI): Calcd, 446.2; found (M þ 1) , 446.9.
4H), 1.88e2.00 (m, 2H), 1.23e1.43 (m, 16H), 0.83e0.98 (m, 12H).

J. Chen et al. / Polymer 51 (2010) 2897e2902
2899
O
O
OEH
OEH
OEH
1. Zn, NaOH, 100 oC, 1 h
. BrEH, TBAB, 8 h
S
S
. t-BuLi, THF, -78 o
C
S
1
Sn
Sn
2
3
2. SnMe Cl
S
S
S
OEH
1
2
EH
3
O
N
O
O
O
O
O
O
O
H2NEH, propionic acid
40 oC, 12 h
Br
Br
2
, I
2 2 4
, H SO
Br
Br
1
Br
5 oC, 24 h
8
O
O
O
O
O
O
O
O
N
O
4
EH
5
8
EH
N
O
O
O
O
O
O
O
O
O
O
Br
DBI, oleum
0 oC, 5 h
H NEH, acetic acid
Br
2
4
1
20 oC, 2 h
Br
Br
O
O
O
N
7
O
O
6
EH
N
EH
O
O
O
O
O
O
O
O
Br
2
, I
2
, oleum
2
H NEH, acetic acid
6
5 oC, 36 h
120 oC, 2 h
Br
Br
Br
Br
11
9
10
2
Scheme 1. Synthesis routes of the monomers. EH stands for 2-ethylhexyl, and H NEH is the abbreviation of 2-ethylhexylamine.
C
5
30
H
36Br
2
N
2
O
4
: Calcd, C 55.57, H 5.60, N 4.32; found, C 55.81, H
(20 mL), compound 3 (0.77 g, 1.0 mmol) and compound 5, 8 or
11 (1.0 mmol) was put into a 50 mL three-necked flask. The
.39, N 4.17.
2
solution was flushed with N for 10 min, and then 30 mg of Pd
2
.3.6. 3,6-Dibromophthalic anhydride (10)
A mixture of phthalic anhydride (9) (4.0 g, 27 mmol), oleum
(PPh
3
)
4
was added into the flask. The solution was further
ꢀ
flushed with N
under N
methanol (100 mL). The precipitate was dissolved in CHCl
2
for 20 min, and then heated to 110 C for 48 h
(
(
30 mL, 20% SO
3
), bromine (3.1 mL, 9.6 g, 60 mmol), and iodine
2
. The raw product was collected by precipitating in
and
ꢀ
0.1 g, 0.4 mmol) was stirred at 65 C for 36 h. The resulting mixture
3
was cooled and cautiously poured into crushed ice (200 g), then
extracted by CHCl
quickly passed through a silica column to remove the metal
catalyst. Then, the polymer was subjected to Soxhlet extractions
3
(100 mL ꢂ 3). The organic extraction was
concentrated by rotary evaporation to produce a brown solid. The
crude product was recrystallized twice from acetic acid to give
with methanol, acetone, hexane, and CHCl
polymer was obtained as a solid from the CHCl
3
in succession. The
fraction by
3
1
compound 10 (1.4 g, yield 17%) as colorless crystals. H NMR
rotary evaporation. The solid was dried under vacuum for 1 day
(
300 MHz, CDCl
3
):
d
¼ 7.85 (s, 2H).
to get the final product.
2
.3.7. N-(2-ethylhexyl)-3,6-dibromophthalimide (11)
A mixture of compound 10 (1.53 g, 5.0 mmol), 2-ethylhexyl-
2
.4.1. P(BDT-PDI)
Purple solid powder, yield 63.7%. 1H NMR (300 MHz, CDCl
3
):
amine (0.97 g, 7.5 mmol), and acetic acid (30 mL) was stirred at
d
¼ 8.87e8.95 (br, 2H), 8.30e8.47 (br, 4H), 7.83e7.90 (br, 2H),
ꢀ
120 C for 2 h under N
2
protection. After most of the acetic acid was
4
0
.07e4.42 (br, 8H), 1.77e2.05 (br, 4H), 1.10e1.51 (br, 32H),
evaporated under reduced pressure, the crude product was purified
by column chromatography on silica gel using a mixture of CH Cl
petroleum ether (1:4) as eluent. After recrystallization from
.72e1.04 (br, 24H). GPC: M
w
¼ 5.6 K, M ¼ 4.8 K, PDI ¼ 1.2. TGA:
n
2
2
/
ꢀ
T
d
¼ 387 C.
hexane, compound 11 (1.94 g, yield 93%) was obtained as colorless
1
2.4.2. P(BDT-NDI)
Black solid powder, yield 75.3%. 1H NMR (300 MHz, CDCl
):
3
crystals. H NMR (300 MHz, CDCl
3
):
d
¼ 7.65 (s, 2H), 3.55e3.61 (d,
2
H), 1.76e1.89 (m, 1H), 1.20e1.41 (m, 8H), 0.80e0.97 (m, 6H).
NO : Calcd, C 46.07, H 4.59, N 3.36; found, C 46.19, H 4.30,
d
¼ 8.89e9.01 (br, 2H), 7.58e7.71 (br, 2H), 3.87e4.39 (br, 8H),
C
16
H
19Br
2
2
1
.76e2.06 (br, 4H), 1.15e1.52 (br, 32H), 0.73e1.12 (br, 24H). GPC:
N 3.47.
ꢀ
M
w
¼ 6.4 K, M
n
¼ 4.5 K, PDI ¼ 1.4. TGA: T
d
¼ 382 C.
2.4. General synthetic procedures of polymers
2.4.3. P(BDT-PhI)
Orange solid powder, yield 41.5%. 1H NMR (300 MHz, CDCl
):
3
The synthetic route to the polymers is shown in Scheme 2.
d
¼ 8.16e8.52 (br, 2H), 7.39e7.64 (br, 2H), 3.55e4.45 (br, 6H),
The three polymers were prepared by the same procedure
through the coupling reaction between compound 3 and arylene
imide (compound 5, 8, or 11). A mixture of anhydrous toluene
1.97e2.36 (br, 3H), 1.09e1.69 (br, 24H), 0.71e0.95 (br, 18H). GPC:
M
ꢀ
w
¼ 40.2 K, M
n
¼ 20.7 K, PDI ¼ 1.9. TGA: T
d
¼ 368 C.

2900
J. Chen et al. / Polymer 51 (2010) 2897e2902
EH
N
EH
O
O
O
N
O
OEH
OEH
Br
S
n
Br
S
O
O
N
O
O
O
N
O
5
EH
EH
P(BDT-PDI)
EH
N
EH
N
Pd(PPh ) , toluene
3
4
OEH
OEH
O
O
o
OEH
OEH
S
110 C, 48 h
Br
Sn
+
Sn
S
n
S
Br
S
3
N
N
O
O
O
O
EH
8
EH
P(BDT-NDI)
EH
EH
N
O
N
O
O
O
OEH
S
Br
Br
S
n
1
1
OEH
P(BDT-PhI)
Scheme 2. Synthetic route to three polymers. EH stands for 2-ethylhexyl.
3
. Results and discussion
3.2. Optical properties
3.1. Synthesis and characterization of the polymers
Fig. 2 shows the normalized absorption spectra of the three
polymers solutions in CHCl and the corresponding spin-coated
3
The general synthetic strategies for the monomers and poly-
films on quartz substrates. The absorption spectrum of P(BDT-PDI)
in solution displays an absorption band ranging from 350 to
693 nm with the centered absorption peak at 519 nm. The broad
visible absorption should be attributed to the conjugated polymer
main chains copolymerized by the PDI units with BDT groups. For
a comparison, P(BDT-NDI) shows a broader absorption band
extending to 750 nm with the absorption peak at 629 nm. This
extension could be attributed to the large electron affinity of the
mers are outlined in Schemes 1 and 2. Compound 3 was obtained
by introducing flexible alkyl side chains and trimethylstannyl
groups into compound 1. Dibromation of perylene dianhydride 4
was achieved in concentrated sulfuric acid directly with bromine
and a catalytic amount of iodine, while phthalic anhydride 9 was
brominated in oleum in the same way. On the other hand, the
bromination of naphthalene dianhydride 6 was achieved using
dibromoisocyanuric acid (DBI) in oleum, which is known as one of
the most powerful brominating agents. The reactions of
compounds 4, 7, 10 with 2-ethylhexylamine in organic acid gave
arylene imide compounds 5, 8, and 11 with moderate yield,
respectively. Copolymers of P(BDT-PDI), P(BDT-NDI) and P(BDT-
PhI) were synthesized through a Stille coupling reaction of
compound 3 and compounds 5, 8, and 11, respectively. Toluene was
NDI unit, comparable to that of far more
p-extended PDI system
[34]. It is worth noting that NDI-Br can be easily isolated as pure
2
used as the solvent while Pd(PPh
obtained copolymers were completely soluble in common organic
solvents, such as THF, CHCl , chlorobenzene and toluene.
3 4
) was used as a catalyst. The
3
1
H NMR spectra of three polymers exhibit the chemical shift of
arylene-H at 8.1e9.0 ppm and thienyl-H at 7.4e7.9 ppm. Both
imine-H of the polymers are located at 4.4 ppm. The molecular
weights of the polymers were determined by GPC with polystyrene
as standard. The weight-average molecular weights (M
w
) are 5.6,
6
.4 and 40.2 K with a polydispersity index (PDI) of 1.2, 1.4 and 1.9
for P(BDT-PDI), P(BDT-NDI) and P(BDT-PhI), respectively. As
revealed by the TGA analyses (Fig. 1), the onset decomposition
temperature (T
about 387, 382 and 368 C under N
d
) of P(BDT-PDI), P(BDT-NDI) and P(BDT-PhI) is
ꢀ
2
protection, respectively, mainly
due to the leaving of alkoxyl groups. The thermal stability of these
three polymers is apparently good enough for their applications in
PSCs and other optoelectronic devices.
Fig. 1. TGA curves of the polymers.

J. Chen et al. / Polymer 51 (2010) 2897e2902
2901
3
Fig. 2. UVevis absorption spectra of three polymers in CHCl solutions (A) and thin films (B).
2
,6-diastereoisomers, enabling the synthesis of a regioregular
voltammograms of P(BDT-PDI), P(BDT-NDI) and P(BDT-PhI) in
CH Cl . The electrochemical data are calculated in Table 1. For P
polymeric backbone [35], while the isolation of PDI-Br
2
2
2
regioisomers is difficult. Therefore, compared to PDI-based poly-
mers, P(BDT-NDI) should own stronger DeA interactions with
a more delocalized p-conjugating system along the polymer main
chain, leading to, a lower band gap. In comparison with P(BDT-PDI)
and P(BDT-NDI), P(BDT-PhI) in solution shows a blue-shift with an
absorption peak at 470 nm, which is due to the relatively weak
electron affinity of the PhI unit with only one imide group.
There are some noticeable differences in the absorption spectra
between the polymer solutions and thin films (Fig. 2 and Table 1).
The main absorption peaks of polymer films appear at 549 nm for P
(BDT-PDI) and P(BDT-NDI), there are quasi-reversible reduction and
oxidation peaks at the negative potential range, while no obvious
redox waves at the positive potential range are detected, indicating
that these two polymers are typical n-type materials, mainly
because of the strong electron-accepting ability of PDI and NDI
groups. Oppositely, P(BDT-PhI) shows quasi-reversible reduction
and oxidation waves only at the positive potential range, suggesting
that it is an intrinsic p-type semiconductor because the weak
electron-withdrawing monomer (PhI) is introduced.
In the negative potential region, the E1/2 values for the first
redox waves are ꢁ0.55 V vs SCE for P(BDT-PDI), and ꢁ0.75 V for P
(BDT-NDI), respectively. In the positive potential region, the E1/2
value for the first redox wave is 0.77 V vs SCE for P(BDT-PhI). From
(
BDT-PDI) and 508 nm for P(BDT-PhI), 30 and 38 nm red-shifted
compared to those of polymer solutions, respectively. This is
because of the higher conjugation along polymer backbones and
the stronger interchain interaction in the condensed solid films. On
the contrary, the absorption peak of P(BDT-NDI) film appears at
E1/2 of the polymers, the HOMO and LUMO energy levels of the
polymers can thus be calculated [36].
6
04 nm, which is blue-shifted by 25 nm compared to that of the
The LUMO energy levels were estimated to be ꢁ4.18 eV for P
(BDT-PDI) and ꢁ3.99 eV for P(BDT-NDI), which are close to that of
the commonly used [6,6]-phenyl-C61-butyric acid methyl ester
solution. Nevertheless, its absorption edge shows the same red-
shift as P(BDT-PDI) and P(BDT-PhI). The absorption band-edge of
the polymer films extend to 713, 760 and 577 nm, which are red-
(PCBM) and other n-type conjugated polymers reported to date.
opt
shifted by 20, 10, and 8 nm compared to those in solutions,
From the LUMO energy level and the optical band gap ðE Þ; the
g
opt
respectively. Thus, the optical band gaps ðE Þ are calculated as
HOMO energy levels of P(BDT-PDI) and P(BDT-NDI) were deduced
to be ꢁ5.92 and ꢁ5.62 eV, respectively. The results indicate that
these two polymers could be used as electron acceptors in all
polymer solar cells and other polymer optoelectronic devices.
The HOMO energy level of P(BDT-PhI) was estimated to be
ꢁ5.51 eV, 0.4 eV lower-lying than that of poly(3-hexylthiophene)
(P3HT), indicating a higher stability against oxidation and thus
g
1.74, 1.63 and 2.15 eV for P(BDT-PDI), P(BDT-NDI) and P(BDT-PhI),
respectively. All of three polymers show broader visible absorption
bands and narrower optical band gaps than those of the homo-
polymer PBDT in film, which exhibits the absorption peak at
4
50 nm with the optical band gap of about 2.2 eV [30]. The broader
absorption bands and the lower optical band gaps of the polymers
are favorable for their application in polymer solar cells.
3.3. Electrochemical properties
The electrochemical redox behaviors of the polymers were
investigated by cyclic voltammetry (CV). Fig. 3 shows the cyclic
Table 1
Optical and electrochemical properties of the polymers
soln
max
film
max
filmesoln
max
opt
g
a
Polymers
l
l
Δl
E
;
E1/2 (V vs LUMO HOMO
(
nm)
(nm)
(nm)
(eV)
SCE)
(eV)
(eV)
c
c
P(BDT- 519
PDI)
P(BDT- 629
NDI)
P(BDT- 470
PhI)
549
30
1.74
ꢁ0.55
ꢁ4.18
ꢁ5.92
604
508
ꢁ25
1.63
2.15
ꢁ0.75
ꢁ3.99
ꢁ3.36
ꢁ5.62
ꢁ5.51
b
38
0.77
a
Calculated from the absorption edge of the polymer films, E
g
¼ 1240/
edge
l .
b
c
opt
Calculated from LUMO ¼ HOMO þ E
g
:
opt
g
2 2
Fig. 3. Cyclic voltammograms of three polymers in CH Cl solution. The scan rate is
Calculated from HOMO ¼ LUMO ꢁ E
:
ꢁ
1
5
0 mV s .

2
902
J. Chen et al. / Polymer 51 (2010) 2897e2902
[5] Lee JF, Hsu SLC. Polymer 2009;50:2558.
a larger VOC for polymer solar cell application. The LUMO level of P
[
[
[
[
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2
molecular energy level of conjugated polymers could be adjusted
by appropriate molecule-tailoring.
4
. Conclusion
[
[
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J
Three low band gap conjugated polymers based on benzodi-
thiophene and arylene imide units, P(BDT-PDI), P(BDT-NDI) and P
BDT-PhI), have been successfully synthesized by Stille coupling
reaction. All obtained polymers are completely soluble in common
organic solvents, thus applicable to solution-processing. All of the
three copolymers show broader visible absorption bands and nar-
rower optical band gaps than those of the homopolymer of BDT
unit. The LUMO energy levels of P(BDT-PDI) and P(BDT-NDI) are
2
[
[
[
[
[
(
13] Zou YP, Wu WP, Sang GY, Yang Y, Liu YQ, Li YF. Macromolecules
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ꢁ
4.18 and ꢁ3.99 eV, respectively, suggesting that these two poly-
mers are typical n-type polymers which could be used as electron
acceptors in all polymer solar cells. The HOMO energy level of P
[17] Hou JH, Chen HY, Zhang SQ, Chen RI, Yang Y, Wu Y, et al. J Am Chem Soc
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[
2
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Adv Mater 2006;18:789.
thus an advantage for polymer solar cell applications and other
polymer optoelectronic devices.
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2
This work was supported by the National Natural Science
Foundation of China (nos. 20774083, 50990063, 51011130028) and
Zhejiang Province Natural Science Foundation (no. Y407101). The
work was also partly supported by Science Foundation of Chinese
University and the Opening Project of State Key Laboratory of
Crystal Material (Shandong University).
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