Synthesis, optical and electrochemical properties of novel D-π-A type conjugated polymers based on benzo[c][1,2,5]selenadiazole unit via alkyne module

Article abstract of DOI:10.1016/j.polymer.2013.09.017

Three novel donor-π-acceptor (D-π-A) type conjugated polymers P-1, P-2, and P-3 based on benzo[c][1,2,5]selenadiazole moiety and phenyl or naphthyl group via alkyne module were firstly prepared by Sonogashira-Hagihara reaction of various diiodo aryl compounds with the key monomer 4,7-diethynylbenzo[c] [1,2,5]selenadiazole (M-1), which was synthesized by a strategy of firstly introducing the trimethylsilylacetylene flexible group, and then introducing the selenium atom. The polymers displayed obvious absorption peaks at the region from 503 to 510 nm and narrow orange-red or red fluorescence in the range of 576-595 nm because benzo[c][1,2,5]selenadiazole unit can effectively reduce the lowest unoccupied molecular orbital (LUMO) energy levels of these polymers. The band gaps of these polymers can be tuned in the range of 1.37-1.76 eV by using different aryl donor groups. These findings indicate that these benzo[c][1,2,5]selenadiazole-based conjugated polymers can be developed for excellent fluorescent materials.

Full text of DOI:10.1016/j.polymer.2013.09.017

Polymer 54 (2013) 6158e6164
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis, optical and electrochemical properties of novel D-p-A type
conjugated polymers based on benzo[c][1,2,5]selenadiazole unit via
alkyne module
*
*
Duanqin Li, Hui Li, Miaochang Liu, Jiuxi Chen, Jinchang Ding, Xiaobo Huang , Huayue Wu
College of Chemistry & Materials Engineering, Wenzhou University, Wenzhou 325035, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel donor-
p
-acceptor (D-
p
-A) type conjugated polymers P-1, P-2, and P-3 based on benzo[c]
Received 8 March 2013
Received in revised form
12 August 2013
Accepted 11 September 2013
Available online 19 September 2013
[1,2,5]selenadiazole moiety and phenyl or naphthyl group via alkyne module were firstly prepared
by SonogashiraeHagihara reaction of various diiodo aryl compounds with the key monomer
4,7-diethynylbenzo[c] [1,2,5]selenadiazole (M-1), which was synthesized by a strategy of firstly intro-
ducing the trimethylsilylacetylene flexible group, and then introducing the selenium atom. The polymers
displayed obvious absorption peaks at the region from 503 to 510 nm and narrow orange-red or red
fluorescence in the range of 576e595 nm because benzo[c][1,2,5]selenadiazole unit can effectively
reduce the lowest unoccupied molecular orbital (LUMO) energy levels of these polymers. The band gaps
of these polymers can be tuned in the range of 1.37e1.76 eV by using different aryl donor groups. These
findings indicate that these benzo[c][1,2,5]selenadiazole-based conjugated polymers can be developed
for excellent fluorescent materials.
Keywords:
D-p-A conjugated polymer
Benzoselenadiazole
Low band gap
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
applied for the construction of low band gap conjugated polymers
[17e20]. 2,1,3-Benzoselenadiazole (BSe) has a similar chemical
In the past few decades, conjugated polymers with excellent
photoelectric properties have made great progress in a number of
materials science fields, including light-emitting diodes [1,2], field
effect transistors [3,4], solar cells devices [5e10] and fluorescent
structure as BT. But compared with BT, BSe can exhibit more
effective in lowering the band gaps of the conjugated polymers
because the selenium atom has a much larger size and is more
electron rich than the sulfur atom, which leads to the absorption
and emission red shift of BSe-based polymers [21e23]. The BSe-
based conjugated polymers can also exhibit interesting photo-
electric properties in their applications as light-emitting diodes and
solar cells [24e28].
sensors [11e13], etc. Among the diverse classes of organic
p-sys-
tems, donor- -acceptor (D- -A) type conjugated polymers have
p
p
attracted an increasing interest due to low band gaps in recent
years, they can show a high tendency for absorbing visible wave-
length photons as well as enhancing the intramolecular charge
transfer (ICT), leading to good charge carrier mobilities [14,15]. A
very common strategy for the design of low-band gap polymers is
based on alternating electron-rich donor (D) and electron-poor
Among the D-p-A systems, poly(aryleneethynylene)s (PAEs) are
a class of conjugated polymers, in which aromatic or hetero-
aromatic groups are linked by alkyne modules. The triple bonds in
the polymer main chain provide extended conjugation and rigidi-
fied backbones for pronounced interchain interactions [5]. They
reportedly show better photostability than poly(arylene vinylene)s
(PAVs) [29], and are widely used in the fields of fluorescent sensors
[11,13] and organic electronics [1,5]. As a further extension of our
research on PAE type of fluorescent materials [30e32], a series of
acceptor (A) fragments along the
p-conjugated backbone. This
derives from the fact that D groups raise the HOMO energy and A
groups lower the LUMO energy, which led to the decrease of the
polymer band gap energy [16].
Recently, the acceptor benzothiadiazole (BT) ring has received
great attention since its electron-withdrawing ability can be
D-p-A conjugated polymers based on BSe moiety and aryl unit via
alkyne module are designed to gain narrow band gap conjugated
polymers with long absorption and emission wavelength, and for
fluorescent material applications. But to the best of our knowledge,
these kinds of conjugated polymers have not been reported. This is
likely because dihalo compounds of benzoselenadiazole, such as
* Corresponding authors.
E-mail
addresses:
xiaobhuang@wzu.edu.cn,
xiaobhuang@hotmail.com
(X. Huang), huayuewu@wzu.edu.cn (H. Wu).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.09.017

D. Li et al. / Polymer 54 (2013) 6158e6164
6159
4,7-dibromobenzoselenadiazole (4) and 4,7-diiodo-benzoselena-
2.3. Preparation of compound M-1 (Scheme 1)
diazole (5) are hard to participate in the Sonogashira-Hagashira
coupling reaction due to the poor reactivity and solubility influ-
enced by the selenium atom, and a similar unsuccessful example
has recently been reported [33].
To solve the problems of synthesizing conjugated polymers
containing BSe, the key monomer diethynylbenzoselenadiazole
(M-1) was synthesized by a strategy of firstly introducing the tri-
methylsilylacetylene flexible group, and then introducing the se-
A mixture of 3,6-dibromobenzene-1,2-diamine (3) (266 mg,
1.0 mmol), Pd(PPh3)2Cl2 (70.1 mg, 0.10 mmol), CuI (19.1 mg,
0.10 mmol), PPh3 (26.2 mg, 0.10 mmol) and trimethyl silyl acetylene
(0.86 mL, 6.00 mmol) was dissolved in 10 mL of Et3N and 10 mL of
THF. The reaction mixture was stirred at 80 ꢀC for 12 h under N2
atmosphere. The solution was cooled to room temperature and
then the solvent was removed under reduced pressure, and the
residue was extracted with CHCl3. The organic layer was washed
with water and then brine, dried over anhydrous Na2SO4, and then
evaporated in vacuum to dryness. The residue was purified by silica
gel column chromatography (petroleumether/ethyl acetate) (30:1,
v/v) to give compound 6 as yellow solids in 45% yield. MS (EI): m/z:
300 [Mþ]. Compound 6 was unstable in air and need be used
immediately for the synthesis of compound 7.
lenium atom. In this way, a series of D-p-A conjugated polymers
incorporating BSe moiety as an acceptor and aryl group as a donor,
such as phenyl and naphthyl group in the main chain backbone
were successfully synthesized by SonogashiraeHagihara coupling
reaction. Optical and electrochemical properties of these polymers
were investigated by UVevis absorption, fluorescence spectra and
cyclic voltammetry (CV) analyses. These polymers displayed
obvious absorption peaks at the region from 503 to 510 nm and
narrow orangeered or red fluorescence in the range of 576e
595 nm. And the resulting polymers show tunable band gaps in the
range of 1.37e1.76 eV. The results show that the optical and elec-
trochemical properties of the copolymers can be easily tuned by
introducing different electron rich groups, which is beneficial for
preparation of good fluorescent materials.
To a solution of compound 6 (56.1 mg, 0.19 mmol) in refluxing
ethanol (20 mL) was added a solution of SeO2 (21.6 mg, 0.2 mmol)
in hot water (8 mL). The mixture was heated under reflux for 2 h.
The solvent was evaporated and the residues were extracted with
CHCl3. The organic layer was washed with water and then brine,
dried over anhydrous Na2SO4, and then evaporated in vacuum to
dryness. The residues were purified by silica gel column chroma-
tography with petroleum ether as an eluant to give compound 7 as
yellow solids in 80% yield. 1H NMR (CDCl3, 500 MHz):
0.33 (s, 18H); 13C NMR (CDCl3, 125 MHz):
158.0, 132.8, 118.2, 102.3,
d 7.62 (s, 2H);
2. Experimental part
d
99.5, ꢁ1.2; Anal. calcd for C16H20N2SeSi2: C, 51.18; H, 5.37; N, 7.46.
2.1. Materials
Found: C, 51.10; H, 5.43; N, 7.38.
A solution of compound 7 (50 mg, 0.13 mmol) was dissolved in
THF (10 mL) and MeOH (10 mL), and then K2CO3 (18 mg,
0.133 mmol) was added to the reaction mixture. After being stirred
at room temperature for 1 h, the solvent was evaporated and the
residues were extracted with CHCl3. The organic layer was washed
with water and then brine, dried over anhydrous Na2SO4, and then
evaporated in vacuum to dryness to give yellow solids M-1 in 85%
All solvents and reagents were commercially available and
analytical-reagent-grade. THF and Et3N were purified by distilla-
tion from sodium in the presence of benzophenone. 4,7-
Dibromobenzo[c] [1,2,5]-thiadiazole (2), 3,6-dibromobenzene-
1,2-diamine (3), 4,7-dibromobenzo[c] [1,2,5]selenadiazole (4) and
4,7-diiodobenzo[c] [1,2,5]selenadiazole (5) could be prepared
from 2,1,3-benzothiadiazole (1) according to the literature re-
ported by Myashi and co-workers [34]. 1,4-Dibutoxy-2,5-
diiodobenzene (M-2) [35], 1,2-dibutoxy-4,5-diiodobenzene (M-3)
[36] and 1,4-dibromo-2,3-dibutoxy-naphthalene [37] could be
prepared according to reported literature from corresponding
starting materials.
yield. 1H NMR (CDCl3, 500 MHz):
d
7.68 (s, 2H); 3.68 (s, 2H); 13C
NMR (CDCl3, 125 MHz): 159.1, 133.5, 118.5, 85.2, 79.4; Anal. calcd
d
for C10H4N2Se: C, 51.97; H, 1.74; N, 12.12. Found: C, 51.90; H, 1.71; N,
12.06.
2.4. Preparation of compound M-2 (Scheme 1)
2.2. Measurements
M-2 was prepared according to reported literature [35]. 1H NMR
(CDCl3, 500 MHz):
d
7.17 (s, 2H), 3.93 (t, J ¼ 6.5 Hz, 4H), 1.81e1.76
The 1H NMR and 13C NMR spectra were recorded in solution of
CDCl3 on Bruker DPX 300 or DRX 500 NMR spectrometer with
tetramethylsilane (TMS) as the internal standard. The chemical
shift was recorded in ppm and the following abbreviations were
used to explain the multiplicities: s ¼ singlet, d ¼ doublet,
m ¼ multiplet, br ¼ broad. FT-IR spectra were taken on a Nexus 870
FT-IR spectrometer. EI mass spectra were recorded on Agilent
5975C DIP/MS mass spectrometer. C, H and N of elemental analyses
were performed on an Elementar Vario MICRO analyzer. UVevis
absorption was recorded on Shimadzu UV-1700 spectrometer and
fluorescence spectra were recorded on a RF-5301PC fluorometer.
TGA was performed on a PerkineElmer Pyris-1 instrument under
N2 atmosphere. Molecular weight was determined by GPC with
Waters-244 HPLC pump and THF was used as solvent and relative
to polystyrene standards. The electrochemical measurements were
carried out in anhydrous CH2Cl2 with 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) as the supporting electrolyte at a
scan rate of 0.02 V/s at room temperature under the protection of
nitrogen. A gold disk was used as working electrode, platinum wire
was used as counter electrode, and Ag/AgCl (3 M KCl solution) was
used as reference electrode.
(m, 4H), 1.57e1.51 (m, 4H), 0.98 (t, J ¼ 7.5 Hz, 6H); 13C NMR (CDCl3,
125 MHz):
d 152.9, 122.8, 86.3, 70.1, 31.3, 19.3, 13.9.
2.5. Preparation of compound M-3 (Scheme 1)
M-3 was prepared according to reported literature [36]. 1H NMR
(CDCl3, 500 MHz):
d
7.25 (s, 2H), 3.93 (t, J ¼ 7.0 Hz, 4H), 1.80e1.75
(m, 4H), 1.52e1.44 (m, 4H), 0.97 (t, J ¼ 7.5 Hz, 6H); 13C NMR (CDCl3,
125 MHz):
d 149.7, 123.5, 95.8, 69.2, 31.1, 19.2, 13.8.
2.6. Preparation of compound M-4 (Scheme 1)
1,4-Dibromo-2,3-dibutoxy-naphthalene was synthesized ac-
cording to previous reports [37]. 1,4-Dibromo-2,3-dibutoxy-naph-
thalene (4.0 g, 9.3 mmol) was dissolved in anhydrous THF (50 mL),
n-BuLi (11.2 mL, 2.5 mol Lꢁ1 in hexanes, 28.0 mmol) was added by
syringe injection at room temperature under N2 atmosphere. After
the reaction mixture was stirred for 5 h, I2 (7.12 g, 28.0 mmol in
40 mL of THF) was added to the above solution at ꢁ78 ꢀC under N2
atmosphere. The reaction mixture was gradually warmed to room
temperature and stirred overnight. The reaction was quenched

6160
D. Li et al. / Polymer 54 (2013) 6158e6164
TMS
Pd(PPh3)2Cl2
TMS
TMS
Br
Br
Br
Br
NH2
Br2
NaBH4
EtOH
HBr
THF, Et3N, CuI
N
N
H2N
NH2
N
N
H2N
S
S
6
3
1
2
SeO2
EtOH
SeO2
EtOH
TMS
Br
Br
I
N
N
I
K2CO3
CuI, KI
Se
DMSO
N
EtOH, THF
N
N
N
N
N
Se
Se
Se
M-1
4
5
TMS
7
I
I2, HIO3, H2SO4
AcOH, CCl4
n-BuBr,KOH
n-BuO
OBu-n
n-BuO
OBu-n
HO
OH
EtOH
I
M-2
I
OBu-n
OH
OH
OBu-n
OBu-n
I2, HIO3, H2SO4
AcOH, CCl4
n-BuBr
K2CO3, CH3CN
I
OBu-n
M-3
Br
I
OH
OH
OBu-n
OBu-n
OBu-n
OBu-n
OBu-n
n-BuBr
Br2
n-BuLi
I2, THF
K2CO3, CH3CN
AcOH
OBu-n
Br
I
M-4
Scheme 1. Synthetic procedures of M-1, M-2, M-3 and M-4.
with saturated Na2S2O3 solution (45 mL). After removal of the
solvent under reduced pressure, the residue was extracted with
ethyl acetate (2 ꢂ 50 mL). The combined organic layers were
washed with water and brine, and then dried over anhydrous
Na2SO4. After removal of solvent under reduced pressure, the crude
product was purified by column chromatography (petroleum ether/
ethylacetate) (50:1, v/v) to afford pale yellow solids M-4 (2.29 g) in
days under N2. The mixture was cooled to room temperature, and
then was filtered through a short silica gel column eluted with THF.
The polymer solution is concentrated by evaporation of the sol-
vents under reduced pressure, and the polymer is precipitated in
methanol (30 mL). The resulting polymer was filtered and washed
with methanol several times. Further purification could be con-
ducted by dissolving the polymer in THF to precipitate in methanol
again. The polymer was dried in vacuum to give 65.0 mg as red
solids in 65% yield. P-1 spectroscopic data: 1H NMR (500 Hz,
47% yield. 1H NMR (CDCl3, 500 MHz):
d 8.15e8.11 (m, 2H), 7.51e7.48
(m, 2H), 4.11 (t, J ¼ 6.5 Hz, 4H),1.94e1.88 (m, 4H),1.64e1.56 (m, 4H),
1.03 (t, J ¼ 7.5 Hz, 6H); 13C NMR (CDCl3, 125 MHz):
d
152.2, 132.6,
CDCl3): d 7.76e7.64 (m, 2H), 7.17e7.02 (m, 2H), 4.12e3.97 (m, 4H),
132.5, 127.3, 97.8, 73.9, 32.5, 19.5, 14.1.
1.88e1.80 (m, 4H), 1.62e1.49 (m, 4H), 1.01e0.98 (m, 6H); FT-IR (KBr,
cmꢁ1): 2914, 2352, 1554, 1368, 1215, 1096; Anal. calcd for
C24H22N2O2Se: C, 64.14; H, 4.93; N, 6.23. Found: C, 62.78; H, 4.85; N,
6.07.
2.7. Preparation of P-1, P-2 and P-3 (Scheme 2)
By using the same procedure as for the preparation of P-1,
polymer P-2 was obtained as red solids in 60% yield from M-1 and
A mixture of M-1 (57.8 mg, 0.25 mmol), M-2 (118.3 mg,
0.25 mmol), 10 mol% Pd(PPh3)4 (29 mg, 0.025 mmol) and 20 mol%
CuI (10.0 mg, 0.05 mmol) was dissolved in the mixed solvents of
30 mL THF and 15 mL Et3N. The solution was stirred at reflux for 2
M-3. P-2 spectroscopic data: 1H NMR (500 Hz, CDCl3):
d
7.77e7.65
(m, 2H), 7.16e7.03 (m, 2H), 4.03e3.97 (m, 4H), 1.83e1.80 (m, 4H),
Pd(PPh3)4
Ar
Ar
I
I
+
n
THF, Et3N, CuI
N
N
N
N
Se
Se
M-2/M-3/M-4
M-1
OBu-n
OBu-n
OBu-n
OBu-n
Ar
OBu-n
n-BuO
=
P-1
P-2
P-3
Scheme 2. Synthetic procedures of P-1, P-2 and P-3.

D. Li et al. / Polymer 54 (2013) 6158e6164
6161
1.53e1.50 (m, 4H), 1.01e0.96 (m, 6H); FT-IR (KBr, cmꢁ1): 2968,
2365,1558,1374,1232,1086; Anal. calcd for C24H22N2O2Se: C, 64.14;
H, 4.93; N, 6.23. Found: C, 62.43; H, 5.02; N, 6.38.
Byusing thesame procedure asfor the preparation of P-1, polymer
P-3 was obtained as red solids in 70% yield from M-1 and M-4. P-3
spectroscopic data: 1H NMR (500 Hz, CDCl3):
d 7.75e7.70 (m, 2H),
7.55e7.50 (m, 2H), 7.47e7.43 (m, 2H), 4.39e4.09 (m, 4H), 2.04e1.91
(m, 4H), 1.68e1.60 (m, 4H), 1.05e1.00 (m, 6H); FT-IR (KBr, cmꢁ1):
2954, 2358, 1550, 1398, 1222, 1096; Anal. calcd for C28H24N2O2Se: C,
67.33; H, 4.84; N, 5.61. Found: C, 65.16; H, 4.75; N, 5.37.
3. Results and discussion
3.1. Synthesis and feature of the conjugated polymers
The synthetic procedures of M-1, M-2, M-3, M-4 and P-1, P-2, P-3
are outlined in Schemes 1 and 2, respectively. We first designed key
intermediate compound 4,7-bis(2-(trimethylsilyl)ethynyl)-benzo[c]
[1,2,5]selenadiazole (7) for the synthesis of the monomer 4,7-
diethynylbenzo[c] [1,2,5]selenadiazole (M-1). Theoretically com-
pound 7 could be synthesized from 4,7-dibromobenzoselenadiazole
(4) with trimethylsilylacetylene via SonogashiraeHagihara coupling
reaction, but we failed and found that a similar unsuccessful example
has recently been reported [33]. The coupling reaction could not be
carried out between diiodo compound 5 and trimethylsilylacetylene,
which may be due to the reason that the selenium atom reduces the
reactivity and solubility of dihalo compounds. Therefore we chose
the second synthesis procedure as shown in Scheme 1. Compound 7
could be obtained by the treatment of 3,6-bis(2-(trimethylsilyl)
ethynyl)benzene-1,2-diamine (6) using SeO2 in hot ethanol. Herein,
compound 6 synthesized by the SonogashiraeHagihara coupling
reaction of 3,6-dibromobenzene-1,2-diamine (3) with trimethylsi-
lylacetylene was unstable in air and need be used immediately for
the synthesis of compound 7. M-1 could be obtained in 85% yield by
deprotection of compound 7 using K2CO3 in the mixed solvent of THF
and methanol. 1,4-Dibromo-2,3-dibutoxy-naphthalene synthesized
according to previous reports [37] was first lithiated with n-BuLi at
room temperature, and then followed by I2 at ꢁ78 ꢀC to afford 2,3-
dibutoxy-1,4-diiodonaphthalene (M-4) in 47% yield.
M-1, M-2, M-3 and M-4 could be served as the monomers for the
synthesis of the target polymers. In this paper, a typical Sonoga-
shiraeHagihara reaction condition was applied to the synthesis of
these polymers. The polymerization was easily carried out by the
diiodo monomers with M-1 in the presence of a catalytic amount of
Pd(PPh3)4 and CuI with Et3N under N2. The molecular weight and
polydispersity index (PDI) of the polymers measured by gel
permeation chromatography (GPC) were Mn ¼ 5610, Mw ¼ 8580
and PDI ¼ 1.53 for P-1, Mn ¼ 4420, Mw ¼ 5850 and PDI ¼ 1.32 for P-
2, and Mn ¼ 4250, Mw ¼ 5320 and PDI ¼ 1.25 for P-3. The GPC
results of three polymers show the moderate molecular weight.
Fig. 1. TGA curves of P-1, P-2 and P-3.
3.2. Photophysical properties
The UVevis absorption and fluorescence spectra of the resulting
polymers were recorded in CHCl3 as shown in Figs. 2 and 3,
respectively, and the data are collected in Table 1. As is evident from
Fig. 2, three polymers exhibit two distinct bands and have well-
resolved absorption peaks with large molar absorption co-
efficients in the range of 300 nme520 nm, which may be attributed
that the low band gaps of these conjugated polymers promote the
tendency for absorbing visible wavelength photons. One band sit-
uated at a shorter wavelength 341e350 nm region is assigned to the
localized
pep* transition, and the other band at a longer wave-
length around 503e510 nm can be regarded as the absorption of
the conjugated structure arisen from the intramolecular charge
transfer (ICT) band between electron-rich donors and electron-
deficient acceptors [38]. Herein, the BSe unit acts as an electron
acceptor and aryl group as an electron donor. The optical band gaps
(Egopt, Table 1) of three polymers estimated from the UVevis onset
absorptions are 2.10 eV, 2.12 eV and 2.15 eV for P-1, P-2 and P-3,
respectively. It can be found that incorporating BSe unit into the
poly(phenylene ethynylene) (PPE) main chain (P-1) results in
obvious absorption red shift (about 20 nm) in comparison with its
sulfur analog (483 nm) reported by Yang and co-workers [39].
The D-p-A conjugated polymers show good solubility in some
common solvents, such as toluene, THF, CHCl3, and CH2Cl2, which
can be attributed to the flexible n-butyl group substituents on aryl
units as side chains of the polymers. Thermogravimetric analyses
(TGA) of these polymers were carried out under a N2 atmosphere at
a heating rate of 10 ꢀC/min. As shown in Fig. 1, although the
repeating units of P-1, P-2 and P-3 are the similar polymer
composition and backbone chain structure, their TGA plots are
different. The TGA curves reveal that the degradation temperature
(Td) of 5% weight loss of P-1, P-2 and P-3 was 270 ꢀC, 310 ꢀC and
290 ꢀC, respectively, which indicates an excellent thermal stability
of these materials. There is a total loss of about 75.4%, 46.0% and
60.0% for P-1, P-2 and P-3 when heated to 700 ꢀC, respectively.
Therefore, three resulting polymers can provide a desirable thermal
property for photoelectric materials.
Fig. 2. UVevis absorption spectra of P-1, P-2 and P-3 (1 ꢂ 10ꢁ5 mol/L in CHCl3).

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D. Li et al. / Polymer 54 (2013) 6158e6164
P-3
P-2
P-1
-3
-2
-1
0
1
2
Potential vs Fc+/Fc (V)
Fig. 3. Fluorescence spectra of P-1, P-2 and P-3 in CHCl3. Inset: fluorescence photos of
three polymers in CHCl3 under a 365 nm UV lamp.
Fig. 4. Cyclic voltammograms of P-1, P-2 and P-3.
In this paper three BSe-based polymers also show longer absorp-
tion wavelength than the corresponding BT polymers due to the
larger size and the weaker electron-withdrawing character of se-
lenium, which is also consistent with Cao and co-workers’ reports
[23,24]. The UV absorptions of these polymers in thin films (Fig. S1)
show red shifts more than 30 nm relative to those measured in
CHCl3 solution, respectively. It can be attributed to a higher con-
jugated structure stacking of the repeating unit in the solid than in
the solution [40]. These polymers display red fluorescence centered
at 596 nm, 586 nm, and 578 nm (Table 1), respectively by using a
commercially available UV lamp (365 nm) (Insert of Fig. 3), which
have about 35 nm red shift than the corresponding BT polymer
(561 nm) [39]. But these red polymers have low fluorescence
quantum yields (VPL ¼ 0.04e0.11) compared with BT polymer using
Rhodamine B as the fluorescence reference, which could be due to
the fact that the narrow energy band gap may increase the prob-
ability of singlet excitons decaying to the ground state in a non-
radiative way [41].
scan rate of 0.05 V/s at room temperature under the protection of
nitrogen. The redox potentials were calibrated using a ferrocenee
ferrocenium (FceFcþ) couple as the internal potential standard, and
under our experimental conditions, E (Fcþ/Fc) ¼ 0.42 V versus Ag/
AgCl. The HOMO (highest occupied molecular orbital) and LUMO
(lowest unoccupied molecular orbital) energy levels of these
polymers can be estimated according to the equations of
HOMO ¼ ꢁ(4.8 þ Eooxnset), LUMO ¼ ꢁ(4.8 þ Eroendset). On the basis of
these onset potentials, the HOMO energy levels of P-1, P-2 and P-3
were determined to be ꢁ5.10 eV, ꢁ5.33 eV and ꢁ5.36 eV, and the
LUMO energy levels were ꢁ3.73 eV, ꢁ3.67 eV and ꢁ3.60 eV,
respectively. The electrochemical band gaps (Eg) of P-1, P-2 and P-3
are 1.37 eV, 1.66 eV and 1.76 eV, respectively. It can be observed that
the LUMO energy levels of P-1, P-2 and P-3 are little difference
because they have the same electron-deficient acceptor, whereas
donors have much effect on their HOMO levels, such as 2,5-
dialkoxyphenyl group (P-1) with strong electron-donating ability
and small steric hindrance results in an obvious negative shift of the
onset potential of the oxidation wave compared with 3,4-
dialkoxyphenyl group (P-2) and 2,3-dialkoxynaphthyl group
(P-3), and thus an increase in the HOMO level. Among the three
polymers, P-1 has the lowest band gap (1.37 eV) due to its most
3.3. Electrochemical properties
Cyclic voltammetry (CV) analyses of three polymers were per-
formed to examine their electrochemical behavior and measure
their energy levels (Fig. 4, Table 1). Three-electrode electrochemical
cell was used with a gold disc electrode as working electrode, a
platinum wire as counter electrode and an AgCl/Ag (in 3 M aq. KCl)
as reference electrode. The electrochemical measurements were
carried out in anhydrous DCM with 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) as the supporting electrolyte at a
extended
p-conjugation as demonstrated from its UVevis ab-
sorption spectra, which is also identical to the result of density
functional theory (DFT) calculations (Fig. 5). The band gaps of these
BSe-based polymers can be effectively tuned by the modification of
the different substituted moieties at the well-defined molecular
level, which can change and control the internal charge transfer
from an electron-rich moiety to an electron-deficient moiety.
Table 1
Optical and electrochemical data of P-1, P-2 and P-3.
a
a
b
c
d
e
onset
ox
onset
red
UVevis [nm]
P-1 350, 510
P-2 341, 505
P-3 343, 503
lem
(
lex) [nm]
E
[eV]
HOMOb [eV]
E
[eV] LUMOb [eV]
Eg [eV] HOMOc [eV] LUMOc [eV]
Eg [eV] Eg [eV] VPL
595 (468)
586 (497)
578 (503)
0.30
0.53
0.56
ꢁ5.10
ꢁ5.33
ꢁ5.36
ꢁ1.07
ꢁ1.13
ꢁ1.20
ꢁ3.73
ꢁ3.67
ꢁ3.60
1.37
1.66
1.76
ꢁ5.48
ꢁ5.80
ꢁ5.82
ꢁ3.10
ꢁ3.15
ꢁ3.06
2.38
2.65
2.76
2.10
2.12
2.15
0.08
0.04
0.11
a
set
ox
set
red
E
and E
were determined from the onset potentials of the oxidation and reduction waves.
b
HOMO energy level was calculated from the onset of oxidation wave; LUMO energy level was calculated from the onset of the first reduction wave; the energy levels were
calculated by using the following equations: HOMO ¼ ꢁ(4.8 þ Eooxnset), LUMO ¼ ꢁ(4.8 þ Eroendset), Eg ¼ LUMO-HOMO.
c
DFT quantum mechanical calculations (B3LYP/6-31 þ G**).
d
Eg was calculated from the absorption of UVevis spectra in CHCl3 solution: band gap energy [eV] ¼ 1240/wavelength [nm].
e
Quantum yields were determined with Rhodamine B as the fluorescence reference in ethanol (V ¼ 0.97).

D. Li et al. / Polymer 54 (2013) 6158e6164
6163
Fig. 5. Structures and molecular orbital diagrams for the LUMO and HOMO of the designed model compounds 1, 2 and 3 from DFT calculations.
3.4. Molecular orbital calculations
Acknowledgments
To gain a better insight into the geometric and electronic
structure, we performed theoretical calculation analysis on the
model molecules 1, 2 and 3 constituting the corresponding repeat
units. All calculations were carried out with the Gaussian 03,
Revision C. 02 program [42], using the B3LYP method and
6-31 þ G** basis set. Herein, all the alkyl chains were replaced by
methyl groups in the calculation for simplicity. Fig. 5 displays the
LUMO and the HOMO of model 1, model 2 and model 3, and the
calculated HOMO, LUMO and Eg are listed in Table 1. As shown in
Fig. 5, the LUMO of the model compounds is mainly centered on the
BSe core because it is an electron acceptor. In comparison with
model 1 and model 2, model 3 shows a lower HOMO level, which
should be attributed to the steric effects of naphthyl group and the
HOMO isosurfaces are well spread over the whole of the conjugated
backbones except model 3. Because of the strong donoreacceptor
interaction between 2,5-dialkoxyphenyl and BSe unit, the HOMO-
LUMO band gaps of model 1 is smaller than those of model 2
with 3,4-dialkoxyphenyl moiety and model 3 with a naphthyl
segment. Further, the order of the absorption maxima of P-1, P-2
and P-3 was identical to that of the band gaps of the designed
model compounds, that is, the labs is in the order of P-1 > P-2 > P-3
and the band gaps are in the order model 1 < model 2 < model 3.
This work was supported by the National Natural Science
Foundation of China (No. 21204066 and 21272176), Zhejiang Pro-
vincial Natural Science Foundation (No. Y4110141) and Common-
weal Project of Science and Technology Department of Zhejiang
Province (No. 2012C23030 and 2011R09002-03).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.09.017.
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