Synthesis and characterization of thiadiazolo [3,4-g]quinoxaline based π-conjugated copolymers with tunable band gaps

Article abstract of DOI:10.14233/ajchem.2013.14934

A series of alternating conjugated copolymers based on thiadiazolo[3,4-g] quinoxaline units and different electron-donating units, such as fluorene, benzene and thiophene, with donor (D)-acceptor (A) alternating structures have been synthesized by palladium catalyzed Sonogashira condensation polymerization. The resulting copolymers P1, P2 and P3 were characterized by NMR, IR, gel permeation chromatography, thermogravimetric analysis and differential scanning calorimetry. Their optical and electronic properties can be facilely fine-modulated by adjusting the structures of different aromatic or heteroaromatic blocks. UV-visible absorption and cyclic voltammetry measurements indicate that all these copolymers have low band gaps due to the strong interaction between the donor and thiadiazolo[3,4- g]quinoxaline segments. Polymer P1 exhibits highest HOMO energy level, which can be expected to obtain high open-circuit voltage (Voc) from the fabricated PSCs. Polymer P3 based on thiophene and thiadiazolo[3,4-g]quinoxaline show smallest band gap and best absorption of sun light even in near-infrared (NIR) region. Preliminary studies imply that these copolymers can be used as efficient polymer donor materials in polymer solar cell applications.

Full text of DOI:10.14233/ajchem.2013.14934

Asian Journal of Chemistry; Vol. 25, No. 13 (2013), 7499-7504
http://dx.doi.org/10.14233/ajchem.2013.14934
Synthesis and Characterization of Thiadiazolo[3,4-g]quinoxaline
Based π-Conjugated Copolymers with Tunable Band Gaps
*
*
FENG TAO, ZHENG WANG, ZUOJIA LI, KAI LI, YING LI and QIANG PENG
College of Chemistry, Sichuan University, Chengdu 610064, P.R. China
Corresponding authors: Fax: +86 028 85413601; Tel: +86 028 85413601; E-mail: profliying@sina.com; qiangpengjohnny@yahoo.com
Received: 8 December 2012; Accepted: 1 July 2013) AJC-13737
*
(
A series of alternating conjugated copolymers based on thiadiazolo[3,4-g]quinoxaline units and different electron-donating units, such as
fluorene, benzene and thiophene, with donor (D)-acceptor (A) alternating structures have been synthesized by palladium catalyzed
Sonogashira condensation polymerization. The resulting copolymers P1, P2 and P3 were characterized by NMR, IR, gel permeation
chromatography, thermogravimetric analysis and differential scanning calorimetry. Their optical and electronic properties can be facilely
fine-modulated by adjusting the structures of different aromatic or heteroaromatic blocks. UV-visible absorption and cyclic voltammetry
measurements indicate that all these copolymers have low band gaps due to the strong interaction between the donor and thiadiazolo[3,4-
g]quinoxaline segments. Polymer P1 exhibits highest HOMO energy level, which can be expected to obtain high open-circuit voltage
(Voc) from the fabricated PSCs. Polymer P3 based on thiophene and thiadiazolo[3,4-g]quinoxaline show smallest band gap and best
absorption of sun light even in near-infrared (NIR) region. Preliminary studies imply that these copolymers can be used as efficient
polymer donor materials in polymer solar cell applications.
Key Words: Thiadiazolo[3,4-g]quinoxaline, Low band gap, Optical absorbance, Electronic properties, Polymer solar cells.
efficient photoinduced intramolecular charge transfer occurs
from the donor to the acceptor units on photoexcitation, which
INTRODUCTION
The development of efficient photovoltaic cells for
1
6
lead to a lower energy absorption band . The interesting
renewable solar energy conversion is one of the most important
ways to address the growing global energy needs. Polymer
solar cells have attracted extensive research interest recently
because of their potential advantages of low cost, light weight,
intramolecular charge transfer structures in D-A copolymers
17
can give rise to higher hole mobility . In the past few years,
power conversion efficiency values up to 9 % were obtained
in bulk-heterojunction solar cells by using low band gap
1
-5
flexibility, ease of processing and fabrication . In the past
decade, the regioregular poly(3-hexylthiophene) (P3HT) was
extensively investigated as a polymer donor in typical bulk-
1
8-20
copolymers as electron donors
.
The acceptor blocks that have been used for D-A low band
gap alternating copolymers include 2,1,3-benzothiadiazole
6
heterojunction solar cells . However, the power conversion
efficiencys of these devices using P3HT are limited at 4-5 %,
because P3HT has a relatively large band gap and a highest
2
1
22
23
BT) , thieno[3,4-b]pyrazine (TP) , quinoxaline (QX) and
(
2
4
pyrazino[2,3-g]quinoxaline (PQX) . Thiadiazolo[3,4-g]quino-
xaline (TQX) is also an interesting electron-accepting skeleton
with outstanding strong electron-withdrawing property of four
7-10
occupied molecular orbital (HOMO) energy level .After that,
conjugated polymers featuring electron donor (D)-acceptor
25
imine groups .A few D-A alternating copolymers containing
TQX as the acceptor unit have been reported to have low band
gaps of 0.90-1.50 eV, depending on the electron-donating ability
of donor blocks and the bulkiness of side group of the acceptor
(
A) units have been developed due to their tunable electronic
properties, ambipolar charge transport abilities and enlarged
spectral absorption ranges. Their band gaps and energy levels
can be easily tuned by selecting appropriate electron-rich and
2
5-28
1
1-13
unit . Although some research groups have synthesized
TQX-based low band gap copolymers and reported the photo-
voltaic properties of those copolymers, the power conversion
electron-deficient blocks . To achieve enhanced absorption
and suitable energy levels, it is necessary to design and prepare
14
low band gap copolymers with D-A structures . Especially
for multijunction polymer solar cells, it is still a challenge to
develop low band gap copolymers for absorbing near-infrared
2
5-28
efficiencies were still below 1 % . So it is still necessary to
design and prepare more TQX-based conjugated copolymers and
study the relationship between their structures and properties.
15
(NIR) light .As we know, the push-pull interaction can induce

7
500 Tao et al.
Asian J. Chem.
In this work, we synthesized three novel conjugated
mixture of N(Et) /THF (25 mL/25 mL) was degassed at room
3
copolymers using TQX acceptor blocks through palladium
catalyzed Sonogashira condensation polymerization. The fluo-
rene, benzene and thipophene moieties were chosen as donor
blocks because of their different electron-donating power. The
long alkyl or alkoxyl side chains were attached onto the donor
skeletons to enhance the solubility of the resulting copolymers
for easy wet-processes of device fabrications (Scheme-I). The
incorporation of triple-bond into the polymer backbones could
be expected to isolate the large donor and acceptor blocks,
avoiding the twisty and giving rise to the worse coplanarity of
neighboring units compare to single- or double-bond.
temperature for 1 h and injected into the mixture and then
2-methyl-3-butyn-2-ol (1.79 g, 21.28 mmol) was injected.
After complete addition, the reaction mixture was refluxed
for 10 h and then cooled to room temperature. The resulting
mixture was filtered, while the solvent was evaporated under
reduced pressure. The product was purified by silica gel
column chromatography with petroleum ether/ethyl acetate
(v/v, 8/1) as the eluent. After recrystallization from toluene,
1
1.97 g of 2 was obtained as a pale yellow solid (42.8 %). H
NMR (400 MHz, CDCl
40H), 1.66 (s, 12H), 1.91-2.00 (m, 4H), 7.37-7.41 (m, 4H),
.59-7.61 (m, 2H).
,9-Didodecyl-2,7-diethynyl-9H-fluorene (3): Under an
argon atmosphere, a solution of 2 (1 g, 1.5 mmol) and KOH
0.34 g, 6.0 mmol) in toluene (15 mL) was heated at 100 ºC
3
): δ 0.86-0.88 (m, 6H), 1.01-1.27 (m,
7
9
(
N
N
N
N
for 8 h and then the mixture was cooled to room temperature.
After removal the solvent under reduced pressure, the pure
product was purified by a silica gel column chromatography
Donor
n
S
with petroleum ether as the eluent to give 0.46 g of 3 as a
1
yellow solid (55.7 %). H NMR (400 MHz, CDCl
3
): δ 0.87-
0
2
.89 (m, 6H), 1.03-1.28 (m, 40H), 1.92-1.95 (m, 4H), 3.15 (s,
H), 7.46-7.49 (m, 4H), 7.62-7.64 (m, 2H).
1,4-Bis-(3-hydroxy-3-methyl-1-butynyl)-2,5-bis(2-
O
O
O
Donor
3
ethylhexyloxy)-benzene (4): 1,4-Dibromo-2,5-bis-(2-
2
C12H25 C12H25
O
S
ethylhexyloxy)benzene (2.62 g, 5.33 mmol) and catalytic
P1
P2
P3
amount of Pd(PPh
neck round-bottom flask, followed with three cycles of vacuum
and argon.After degassing at room temperature for 1 h, N(Et)
THF (20 mL/30 mL) was injected. Upon injecting 2-methyl-
-butyn-2-ol (1.79 g, 21.3 mmol) to the flask, the reaction
3 2 2 3
) Cl , CuI and PPh were added to a three-
Scheme-I: Structures of the low band gap copolymers P1, P2 and P3
3
/
EXPERIMENTAL
3
All reagents were purchased from Aldrich or Acros
Chemical companies and were used without further purifica-
tion. All the solvents were purified according to the standard
methods prior to use. 2,7-Dibromo-9,9-didodecyl-fluorene
mixture was refluxed for 18 h and then cooled to room tempe-
rature. Subsequently, the mixture was separated by filtration
and thw solvent was removed under reduced pressure to afford
the crude product. The pure product was then purified by silica
gel column chromatography with petroleum ether/ethyl acetate
29
30
(1), 1,4-dibromo-2,5-bis(2-ethylhexyloxy)benzene and 3,4-
dimethoxythiophene were prepared according to the similar
procedures reported before.
31
(
v/v, 8/1) as the eluent to afford 1.60 g of 4 as a light yellow
1
solid (60.4 %). H NMR (400 MHz, CDCl
3
): δ 0.90-0.97 (m,
2H), 1.31-1.47 (m, 18H), 1.63 (s, 12H), 1.80-2.05 (broad,
H), 3.81-3.86 (m, 4H), 6.87 (s, 2H).
,4-Bis-(2-ethylhexyloxy)-2,5-diethynyl-benzene (5):
1
Characterizations: H NMR spectra were recorded on a
1
2
Bruker DRX 400 spectrometer with CDCl
tetramethylsilane (TMS) as the internal standard. Number-
average (M ), weight-average (M ) molecular weights and
polydispersity indices (M /M ) of the copolymers were
3
as the solvent and
32
1
n
w
Under an argon atmosphere, a solution of 4 (0.14 g, 0.28 mmol)
and KOH (0.063 g, 1.12 mmol) in toluene (10 mL) was heated
at 100 ºC for 3 h and then cooled to room temperature. After
removal the solvent under reduced pressure, the pure product
was purified by silica gel column chromatography with petro-
w
n
measured on a PL-GPC model 210 chromatograph at 25 ºC,
using THF as the eluent and standard polystyrene as the refer-
ence. TGA measurements were performed on a Perkin-Elmer
-1
series seven thermal analyzer at a heating rate of 20 ºC min
leum ether/CH
as a colourless viscous liquid (50.3 %). H NMR (400 MHz,
CDCl ): δ 0.88-0.95 (m, 12H), 1.25-1.71 (m, 18H), 3.24 (s,
2H), 3.77 (dd, 4H, J = 6.0 Hz), 6.88 (s, 2H).
2 2
Cl (v/v, 8/1) as the eluent to give 0.054 g of 5
and under a nitrogen atmosphere. UV-visible spectra in solutions
and thin films were taken on a Shimadzu UV 2100 UV-visible
recording spectrophotometer. The cyclic voltammetry measu-
rements were recorded on a computer-controlled EG & G
potential/galvanostat model 283 at room temperature.
1
3
1
33
3
,4-Bis-(2-ethylhexyloxy)-thiophene (6): 3,4-Dimethoxy-
thiophene (2.0 g,13.9 mmol), 2-ethylhexan-1-ol (7.7 g, 55.5
mmol) and dry toluene (50 mL) were added to a three-neck
round-bottom flask. The flask was equipped with a constant
Synthesis of monomers
9
,9-Didodecyl-2,7-bis-(3-hydroxy-3-methyl-1-butynyl)-
fluorene (2): 2,7-Dibromo-9,9-didodecyl-fluorene 1 (2.0 g,
.03 mmol), catalyst amount of Pd(PPh Cl , CuI and PPh
3
2
pressure funnel full with CaCl to absorb 2-ethylhexan-1-ol.
The reaction solution was refluxed for 12 h and then cooled to
room temperature. After removal the solvent under reduced
pressure, the pure product was purified by a silica gel column
3
3
)
2
2
were added to a three-neck round-bottom flask and the
mixture was degassed with nitrogen three times. In addition, a

Vol. 25, No. 13 (2013)
Synthesis and Characterization of Thiadiazolo[3,4-g]quinoxaline Based π-Conjugated Copolymers 7501
chromatography with petroleum ether/CH
2
Cl
2
(v/v, 10/1) as
the eluent to give 3.4 g of 6 as a light yellow vicous liquid
stirring for 30 min, the precipitate was collected by filtration
and washed repeatedly with water and then the crude product
was Sohxlet extracted with acetone for 24 h to remove oligo-
mers. The resulting copolymer was collected by filtration and
dried under vacuum (Scheme-II).
1
(
72.4 %). H NMR (400 MHz, CDCl
.29-1.52 (m, 18H), 1.71-1.79 (m, 2H), 3.87 (dd, 4H, J = 6.0
Hz), 6.16 (s, 2H).
,5-Dibromo-3,4-bis-(2-ethylhexyloxy)-thiophene (7):
3
): δ 0.88-0.94 (m, 12H),
1
34
2
Spectral data
Under an argon atmosphere, N-bromosuccinimide (NBS) (4.12
g, 23.16 mmol) was dissolved in dry DMF (30 mL). Then the
mixture was added dropwise to a mixture of 3,4-bis-(2-
ethylhexyloxy)thiophene 6 (3.15 g, 9.26 mmol) in dry DMF
-
1
P1: Dark blue solid.Yield: 82.5 %. IR (KBr pellet, cm ):
437, 2921, 2850, 2186, 1652, 1560, 1456, 1371, 1260, 1095,
3
1
0
7
1
022, 897, 799, 760, 692. H NMR (400 MHz, CDCl
3
, δ/ppm):
.84-0.88 (m, 6H), 1.16-1.68 (m, 40H), 1.86 (s, 4H), 6.99-
.88 (m, 16H).
(100 mL) in the dark. The reaction mixture was stirred at room
temperature for 12 h and poured into water (100 mL). The
organic material was extracted with ethyl ether three times.
The combined organic extracts were washed with water and
-1
P2: Dark blue solid.Yield: 80.6 %. IR (KBr pellet, cm ):
439, 2957, 2922, 2857, 2185, 1628, 1455, 1408, 1370, 1263,
3
1
1
114, 1025, 899, 800, 762, 693. H NMR (400 MHz, CDCl
3
,
dried over anhydrous MgSO
reduced pressure, the pure product was purified by a silica gel
column chromatography with petroleum ether/CH Cl (v/v,
2/1) as the eluent to give 1.97 g of 7 as a colourless viscous
4
.After removal the solvent under
δ/ppm): 0.82-0.96 (m, 12H), 1.26-1.95 (m, 18H), 4.06-4.10
m, 4H), 6.98 (s, 2H), 7.35-7.47 (m, 6H), 7.82 (m, 4H).
P3: Dark green solid.Yield: 75.9 %. IR (KBr pellet, cm ):
3431, 2956, 2924, 2168, 1627, 1499, 1446, 1367, 1259, 1151,
(
2
2
-
1
1
1
liquid (42.8 %). H NMR (400 MHz, CDCl
3
): δ 0.91-0.95 (m,
2H), 1.29-1.52 (m, 16H), 1.62-1.68 (m, 2H), 3.93-3.95 (m,
H).
1
1
0
7
3
032, 802, 722, 693. H NMR (400 MHz, CDCl , δ/ppm):
1
4
.86-0.88 (m, 6H), 1.25-2.03 (m, 18H), 3.72-3.74 (m, 4H),
.44-7.49 (m, 4H), 7.53-7.55 (m, 2H), 7.65-7.69 (m, 4H).
3,4-Bis-(2-ethylhexyloxy)-2,5-bis(3-hydroxy-3-methyl-
-butynyl)-thiophene (8): 2,5-dibromo-3,4-bis-(2-ethyl-
1
RESULTS AND DISCUSSION
hexyloxy)thiophene 7 (2.65 g, 5.32 mmol), catalyst amount
of Pd(PPh Cl , CuI and PPh were added to a three-neck
round-bottom flask and the mixture was degassed with
nitrogen three times. In addition, a mixture of N(Et) (25 mL)
3
)
2
2
3
Synthesis and characterization:The chemical structures
of the resulting copolymers was unambiguously confirmed
1
by H NMR and IR analysis. The prepared copolymers exhibit
3
and THF (25 mL) was degassed at room temperature for 1 h
and injected into the mixture, followed by injecting 1.79 g
good solubility in common organic solvents, such as THF,
3 2 2
CHCl , CH Cl and so on. Molecular weights of the copolymers
(
21.28 mmol) of 2-methyl-3-butyn-2-ol. The mixtures were
are determined by GPC, using THF as the eluent and poly-
styrene as the standards. The GPC data show that weight-
refluxed for 12 h and then cooled to room temperature. After
filtration and evaporation, the pure product was purified by a
silica gel column chromatography with petroleum ether/ethyl
w
average molecular weights (M ) of P1, P2 and P3 are 18000
g/mol with polydispersity indice (PDI) of 1.9, 7800 g/mol with
polydispersity indice of 1.4 and 11500 g/mol with polydis-
persity indice of 1.9, respectively. Polymer P1 and P3 have
relatively higher molecular weight than that of polymer P2
because of the steric hindrance induced by the 2-ethylhexyloxy
side chains on phenyl ring (Table-1).
acetate (v/v, 4/1) to give 0.82 g of 8 as a yellow vicous liquid
1
30.5 %). H NMR (400 MHz, CDCl
(
3
): δ 0.88-0.94 (m, 12H),
1
4
.28-1.57 (m, 16H), 1.58 (s, 12H), 1.60-1.66 (m, 2H), 4.11-
.20 (m, 4H).
,4-Bis-(2-ethylhexyloxy)-2,5-diethynylthiophene (9):
3
Under an argon atmosphere, 3,4-bis-(2-ethylhexyloxy)-2,5-
bis-(3-hydroxy-3-methyl-1-butynyl) thiophene 8 (0.10 g, 1.98
mmol) and KOH (0.044 g, 7.94 mmol) were dissolved in
toluene (10 mL) and heated at 100 ºC for 3 h and then cooled
to room temperature.After removal the solvent under reduced
pressure, the pure product was purified by a silica gel column
TABLE-1
MOLECULAR WEIGHTS AND THERMAL
PROPERTIES OF THE COPOLYMERS
a
a
b
T (°C) T (°C)
Polymer Yield (%)
Mw
PDI(M /M )
w
n
d
g
P1
P2
P3
82.5
80.6
75.9
18000
7800
1.9
388
169
1.4
1.9
362
337
165
-
chromatography with petroleum ether as the eluent to give
0
11500
1
.50 g of 9 as a yellow solid ( 65.1 %). H NMR (400 MHz,
a
Molecular weights and PDI were measured by GPC
T is 5% weight-loss temperatures under a nitrogen atmosphere
b
CDCl
m, 2H), 3.39 (s, 2H), 4.17-4.24 (m, 4H).
Synthesis of polymers: Monomer (3, 5 or 9) (0.10
mmol), 4,9-dibromo-6,7-diphenyl-[1,2,5]thiadiazolo [3,4-
g]quinoxaline (0.10 mmol), Pd(PPh Cl (0.00876 mmol), CuI
0.00333 mmol) and PPh (0.048 mmol) were added to a 25
3
): δ 0.88-0.94 (m, 12H), 1.26-1.56 (m, 16H), 1.62-1.68
d
(
Thermal properties: Thermal stability of the copolymers
was investigated with thermogravimetric analysis. As shown
in Fig. 1a, The TGA curves reveal that the thermal decompo-
)
3 2
2
(
3
d
sition temperature (T , 95 wt % residue) of P1, P2 and P3 are
mL three-necked flask and the mixture was degassed with
nitrogen three times. In addition, toluene (6 mL) and 2 mol/L
3
88 ºC, 362 ºC and 337 ºC in the inert nitrogen atmosphere,
respectively. The results indicate that these polymers have good
thermal stability, which are adequately suitable for avoiding
the deformation and degradation of the polymeric active layer
in polymer solar cells. The relatively higher thermal decom-
position temperature of P1 than those of P2 and P3 is mostly
2 3
K CO (3 mL) were degassed at room temperature for 1 h and
then injected into the mixture, respectively. The reaction was
then stirred at 100 ºC for 6 h and then cooled to room tempe-
rature. The reactant was poured into methanol (250 mL). After

7
502 Tao et al.
Asian J. Chem.
H
OH
KOH,C12H25Br
Br
Br
Br
Br
OH
HO
Toluene/H O
2
Pd(PPh
3
)
2
Cl
2
, PPh
, CuI
3
C12H25
C12H25
N(Et)
3
/THF
C12H25
C12H25
1
2
Br
Br
N
N
N
S
N
KOH
P1
Toluene
Pd (PPh ) Cl , CuI, PPh
3
2
2
3
C12H25
C
12
H
25
K
2
CO
3
/ Toluene
3
Br
N
N
N
N
S
O
O
O
O
Br
H
OH
KOH
Br
HO
OH
P2
Pd(PPh ) Cl , PPh , CuI
Pd (PPh
3
)
2
Cl
2
, CuI, PPh
3
Br
3 2
2
3
Toluene
K CO / Toluene
N(Et) /THF
3
O
O
2
3
4
5
O
O
O
O
H CO
OCH3
O
O
3
HO
NBS
DMF
H
OH
Br
Br
HO
OH
Pd(PPh
3
)
2
Cl
2 3
, PPh , CuI
S
S
S
Toluene
S
N(Et)
3
/THF
8
7
6
Br
Br
N
N
S
O
O
N
N
KOH
Toluene
P3
S
Pd (PPh ) Cl , CuI, PPh
3 2 2 3
9
K CO / Toluene
2 3
Scheme-II: Synthesis routes of the monomers and low band gap copolymers
because of the higher molecular weights and rigid properties
of fluorene donor skeleton. The differential scanning calori-
metry (DSC) curves of copolymers are shown in Fig. 1b. The
DSC analysis reveals that both P1 and P2 have high glass
(
a)
1
00
P1
P2
P3
transition temperatures (T
g
) of 169 ºC and 165 ºC. However,
90
80
70
the T of P3 is not obviously observed.
g
Optical properties: UV-visible spectra were measured
both in chloroform solutions and as thin films on quartz plates.
The spectra are depicted in Fig. 2. The spectroscopic data of
the copolymers are summarized in Table-2. As shown in Fig.
2
a, the absorption peak maxima of the copolymer P1, P2 and
P3 are 656, 667 and 714 nm in CHCl solution, respectively.
3
*
The absorption peaks can be assigned to the π-π transition of
the delocalized structures resulted from the alternating D-A
backbones in these copolymers. Obviously, the absorption
spectra of P1 and P3 exhibited broader and stronger absor-
ption than that of P2, which can be attributed to the stronger
D-A interaction in the polymer backbones.
60
50
0
100
200
300
400
500
600
700
o
Temperature ( C)

Vol. 25, No. 13 (2013)
Synthesis and Characterization of Thiadiazolo[3,4-g]quinoxaline Based π-Conjugated Copolymers 7503
TABLE-2
OPTICAL AND ELECTROCHEMICAL DATA OF THE COPOLYMERS
α
λ max, sol
λmax, film
b opt
E g (eV)
c
E
c
ec
g
E (eV)
Polymer
onset, ox (V)
E
onset, red (V)
HOMO (eV)
LUMO ( eV)
(
nm)
(nm)
P1
P2
P3
656
667
714
665
693
1.59
1.45
1.27
0.96
0.87
0.75
-0.78
-0.79
-0.76
-5.35
-5.26
-5.14
-3.61
-3.60
-3.63
1.74
1.66
1.51
760
a
c
-5
-1
b
; E
opt
1
Oxidation and reduction onset potential measured by cyclic voltammogram
× 10 mol L in CHCl
3
g
were calculated from the edge of absorption band of the copolymer films
The thin solid films of the copolymers were obtained by
spin-coating their chloroform solution. As shown in Fig. 2b,
the absorption peak maxima of the copolymer P1, P2 and P3
are 665, 693 and 760 nm in films. The absorption maximum
of P1, P2 and P3 thin films in visible region are red-shifted by
(b)
P1
P2
P3
0
0
0
0
.6
.4
.2
.0
9, 26 and 47 nm in comparison with those from their solutions.
The results indicate that there are strong intermolecular inter-
action and more aggregated configuration formed in solid
9
,35,36
state.
enhanced absorption into the near-infrared region. The optical
Especially the polymer P3 exhibits a significantly
opt
band gaps (Eg ) are estimated to be 1.59, 1.45 and 1.27 eV
for P1, P2 and P3, respectively, from the band edge of the
UV-visible absorption spectra of the copolymer films.
Electrochemical properties: The electrochemical redox
behaviour of the copolymers was investigated by cyclic
voltammetry. Cyclic voltammetry measurements were per-
-
0.2
9
0
120
150
180
210
240
o
Temperature ( C)
Fig. 1. (a) TGA and (b) DSC curves of the copolymers measured at a heating
rate of 10 ºC/min under nitrogen
-1
formed at a scan rate of 100 mV s in a anhydrous acetonitrile
-1
solution with 0.1 mol L tetrabutylammonium perchlorate
(n-Bu NClO ) as the supported electrolyte at room temperature.
(a)
4
4
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
P1
P2
P3
The working electrode was a Pt disk, the counter electrode
was a Pt wire and the reference electrode was a saturated
calomel electrode. The cyclic curves of the copolymer thin
films are shown in Fig. 3 and all the electrochemical data are
summarized in Table-2. The onsets of oxidation potential (Eonset, ox)
of P1, P2 and P3 occurred at 0.96, 0.87 and 0.75 V, respec-
tively, while the corresponding onsets of reduction potential
(Eonset, red) appeared at -0.78 V, -0.79 V and -0.76 V, respec-
tively. Eonset, ox and Eonset, red are the onset potentials for oxidation
and reduction potential relative to the SCE reference electrode,
respectively. The HOMO and LUMO energy levels of poly-
mers P1, P2 and P3 can be estimated by using the empirical
3
00
400
500
600
Wavelength (nm)
700
800
900
equations: I
p
(HOMO) = -(Eonset, ox + 4.39) eV; E
a
(LUMO) =
ec
37
= Eonset, ox -Eonset, red. From these
-
(Eonset, red + 4.39) eV; E
g
equations, the HOMO energy levels of P1, P2 and P3 were
estimated to be -5.35, -5.26 eV and -5.14 eV, respectively. On
the other hand, the LUMO energy levels of P1, P2 and P3
(b)
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
P1
P2
P3
were estimated to be -3.61, -3.60 and -3.63 eV, respectively.
ec
The electrochemical band gaps (E ) of the P1, P2 and P3
g
were calculated to be 1.74, 1.66 and 1.51 eV. Polymer P3 has
ec
the relative lower E
g
among these three copolymers, which
may be attributed to the stronger D-A interaction in the
polymer backbone between thiophene ring and TQX unit.
The lower band gap can afford improved absorbance even in
NIR region. It is noted that polymer P1 exhibits high HOMO
energy level, which can be expected to obtain high open-
oc
circuit voltage (V ) from the fabricated polymer solar cells
using P1 as a donor material. The electrochemical band gaps
ec
4
00
500
600
Wavelength (nm)
700
800
900 1000
g
(E ) values of these polymers are also agreed with above
opt
optical band gaps (Eg ) values obtained from the UV-visible
absorption.
Fig. 2. UV-visible absorption spectra of the polymers solutions in CHCl
3
solutions (a) and films on a quartz plate (b)

7
504 Tao et al.
Asian J. Chem.
1
1
0
0
0
0
.5
.2
.9
.6
.3
.0
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P3
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-0.6
-0.9
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A series of donor (D)-acceptor (A) alternating copolymers,
consisting of electron-deficient thiadiazolo[3,4-g]quinoxaline
units and different electron-donating units, such as fluorene,
benzene and thiophene, have been synthesized by palladium
catalyzed Sonogashira condensation polymerization. Their
optical and electrical properties can be facilely fine-modu-
lated by adjusting the structures of different aromatic or
heteroaromatic blocks. UV-visible absorption and cyclic
voltammetry measurements show that all these copolymers
have low band gaps due to the strong interaction between the
donor and TQX segments. Polymer P1 exhibits deepest HOMO
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circuit voltage (Voc) from the fabricated polymer solar cells.
Polymer P3 based on thiophene and TQX show smallest band
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ACKNOWLEDGEMENTS
(
The work was financially supported by NSFC (No.
(
2
0802033, 21272164), Program for NCETU (No. NCET-10-
170), Key Foundation for Applied Basic Research Program
0
(
of Sichuan Province (No. 2012JY0010), Scientific Research
Foundation for ExcellentYouth Scholars (No. 2012SCU04B01)
and Recruit Talents of Sichuan University (No. YJ2011025).
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Organomet. Chem., 691, 4028 (2006).
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