Synthesis and characterization of conjugated polymers based on benzoselenadiazole

Article abstract of DOI:10.14233/ajchem.2015.17885

A multistep synthesis of the electron-poor 4,7-di(thiophen-2-yl)benzo[c][1,2,5]selenadiazole are presented. The new deficient acceptor has good solubility in organic solvents to permit an appropriate coating process. P1 was synthesized by direct oxidative polymerization under the reaction condition with FeCl₃ and CH₃NO₂. The spectra of polymers P1 to P2 show homogenous absorptions across most of the visible spectrum (450-650 nm). Solutions of P1 show a dark purple-black colour due to the lack of absorption in the far blue and red regions. And the solution of P2 exhibits a bright orange-red colour due to the increased reflection/transmission of red light. Both of the polymers were characterized by IR, UV-visible and cyclic voltammetry.

Full text of DOI:10.14233/ajchem.2015.17885

Asian Journal of Chemistry; Vol. 27, No. 7 (2015), 2427-2430
A
SIAN
J
OURNAL OF HEMISTRY
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http://dx.doi.org/10.14233/ajchem.2015.17885
Synthesis and Characterization of Conjugated Polymers Based on Benzoselenadiazole
1,*
1
2
XIAOXIA SUN , XIAOLONG LEI and YU HU
1Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, P.R. China
2Experimental Chemistry Center, Nanchang University, Jiangxi 330031, P.R. China
*Corresponding author: Fax: +86 791 83826894; Tel: +86 791 83805183; E-mail: sunxiaoxia77@126.com
Received: 24 April 2014;
Accepted: 27 July 2014;
Published online: 30 March 2015;
AJC-17050
A multistep synthesis of the electron-poor 4,7-di(thiophen-2-yl)benzo[c][1,2,5]selenadiazole are presented. The new deficient acceptor
has good solubility in organic solvents to permit an appropriate coating process. P1 was synthesized by direct oxidative polymerization
under the reaction condition with FeCl3 and CH3NO2. The spectra of polymers P1 to P2 show homogenous absorptions across most of the
visible spectrum (450-650 nm). Solutions of P1 show a dark purple-black colour due to the lack of absorption in the far blue and red
regions. And the solution of P2 exhibits a bright orange-red colour due to the increased reflection/transmission of red light. Both of the
polymers were characterized by IR, UV-visible and cyclic voltammetry.
Keywords: π-Conjugated polymers, Donor-acceptor, [ 2,1,3]-Benzoselenadiazole, Dual-band-absorbing polymers.
thieno-[3,4-b]pyrazine and silole have emerged as useful
heterocycles units for conducting a variety of conjugated
polymers for photovoltaic application8. A wide range of
conjugated polymers and related oligomers contain hetero
atoms with available lone pairs of electrons. DA type polymers
generally show two distinct optical absorption bands, which
can be modulated as a function of the composition of donor
and acceptor moieties in the main chain9. The tuning of the
dual-band absorption character of DA type polymers provides
an elegant method of modifying spectral characteristics to
achieve desired colours.
INTRODUCTION
Conjugated polymer semiconductors are finding growing
applications in electronics and optoelectronics, including light-
emitting diodes, photovoltaic cells, thin film transistors and
electro chromic cells1. Their molecular structures contain
electron rich and electron deficient moieties linked via a
π-conjugated bridge. In recent years, a number of narrow-band
gap π-conjugated polymers absorbing at longer wavelengths
than wide-band gap all-donor parents (e.g., P3HT, MEHPPV
and MDMO-PPV) have been synthesized2-4. With the concept
of band gap-engineering, the idea that various colours can
subsequently be accessed and taken advantage of. For example,
in recent years, the DA approach has proven particularly useful
in the synthesis of non emissive polymer electro chromes,
extending the palette of colours available for electro chromic
display technologies5. In excitonic solar cells, excit on disso-
ciation occurs almost exclusively at the interface between two
materials of differing electron affinities i.e., the electron donor
and electron acceptor6. To generate an effective photocurrent
in these organic solar cells, an appropriate donor-acceptor pair
and device architecture must be selected.
With the perspective of solution-processed conjugated
heterocyclic unit based 2,1,3-benzothiadiazole, we report the
new synthesis of poly(4,7-di(thiophen-2-yl)benzol[c][1,2,5,]-
selenadiazole).
EXPERIMENTAL
Chemical reagents were purchased from Acros Corp and
used without further purification. All solvents used were spec-
troscopic grade and were purified by distillation before use.
Synthesis of dibromobenzo[c][1,2,5]thiadiazole (M1):
Bromine was added drop wise over a period of 5 min to a
vigorously stirred suspension of benzo[c][1,2,5]thiadizole in
hydrobromic acid (30 mL) at 120 °C. The reaction was left to
stir for 36 h during which a canary yellow precipitate formed.
Water (100 mL) was added, the suspension left to stir for 10
min, then filtered and washed with a little water. The yellow
One of the most widely applied strategies to make narrow
band gap donor polymers is the synthesis of an alternating
copolymer from electron-rich (donor) and electron-deficient
(acceptor) units in their backbone. For the incorporation of
conjugated heterocyclic units can greatly influence the pro-
perties of conjugated polymers7. Quinoxaline, thiophene,

2428 Sun et al.
Asian J. Chem.
solid was dried in vacuo for 24 h to give the pure product. (25
g, 91.3 %) 1H NMR (400 MHz, CDCl3, ppm): δ-aromatic H,
7.73 (s, 2H)
chromatography (silica gel, petroleum ether as eluent) to a
red solid (0.54 g, 81 % yield). H NMR (400 MHz, CDCl3,
ppm) δ-aromatic H, 8.02 (d, 2H), 7.80 (s, 2H), 7.45 (d, 2H),
7.20 (t, 2H). 13C NMR (100 MHz, CDCl3, ppm): δ 126.1, 127.2,
127.5, 127.7, 139.6, 158.2.
1
Synthesis of 3,6-dibromobenzene-1,2-diamine: Dibro-
mobenzo[c][1,2,5]thiadiazole (2.29 g, 10 mmol) and sodium
borohydride (5.7 g, 150 mmol) were added to ethanol at 0 °C
and stirred at room temperature for 36 h. The solution was
added water and ethyl acetate then was separated by separatory
funnel. The organic layer was dried over MgSO4 then concen-
trated under reduced pressure and purified by chromatography
on a silica column eluting with petroleum ether/ethyl acetate
(5:1, v/v) to afford a yellow solid (1.9 g, 71 % yield). 1H NMR
(400 MHz, CDCl3, ppm) δ-aromatic H, 6.86 (d, 2H), 3.92 (s,
4H); 13C NMR (100 MHz, CDCl3, ppm): 133.73,123.27, 109.70.
Synthesis of dibromobenzo[c][1,2,5]selenadiazole: To a
solution of 3,6-dibromobenzene-1,2-diamine (1.33 g, 5 mmol)
in ethanol and the solution of selenium dioxide (0.67 g, 6
mmol) was added at 60 °C for 24 h. The solution was extracted
by ethyl acetate and the organic layer was dried over MgSO4
then concentrated under reduced pressure to a yellow solid
Synthesis of polymer: To a suspension of eight equivalent
of anhydrous FeCl3 (4 mmol) in 5 mL CH3NO2 was added
drop wise a solution of one equivalent of the monomer (0.5
mmol) dissolved in 3 mL of CHCl3. After stirring the mixture
at room temperature for 3 h, 100 mL of methanol was added
to precipitate the polymer. The mixture was put in the freezer
overnight. After filtration, the polymer was washed with
CH2Cl2 and methanol, respectively and dried under vacuum
for 24 h. Polymer 1; dark purple powder with 46 % yield.
Reactions were monitored by thin-layer chromatography
on plates coated with 0.25 mm silica gel 60 F254. NMR spectra
were obtained with a Bruker AV-400 spectrometer with
teramethylsilane (TMS) as internal reference and CDCl3 as
solvent.Absorption spectra were obtained with anAgilent 8453
UV/visible spectrometer. The number-average molecular
weight (Mn) and molecular weight distribution (Mw/Mn) of
the resulting polymers were determined by using a waters 1515
gel permeation chromatograph (GPC) which was performed on
an HP 1100 HPLC, equipped with a refractive-index detector
(Waters 2414), using three Styragel HR 2, HR 4, HR 5 of 300
× 7.5 mm columns (packed with 5 mm particles of different
pore sizes). Electrochemical measurements of these polymers
were performed with a Model 263 potentiostat-galvanost at
(EG&G Princeton Applied research) electrochemical work-
station under computer control at room temperature.
1
(1.6 g, 96 % yield). H NMR (400 MHz, CDCl3, ppm): δ-
aromatic H, 7.51 (s, 2H).
Synthesis of 4,7-di(thiophen-2-yl)benzo[c][1,2,5]-
selenadiazole (M2): Under the protection of the adjacent in
Ar2 bromine thiophene (6 g, 6.1 mmol) dropped to the magne-
sium (1.8 g, 12 mmol), with constant stirring for 0.5 h. Then
trimethylborate (4.6 g, 7.32 mmol) was dropped to the mixture
slowly and the solution was stirred at -78 °C. The hydrochloric
acid (2 mol/L) was added to the solution, stirred for 0.5 h. The
solution was extracted with ethyl acetate. The organic layer
was dried over MgSO4 then filtrated to afford a white solid.
To a solution of dibromobenzo[c][2,1,3]selendiazole (0.68 g,
2 mmol) in freshly distilled THF was added Pd(PPh3)4(0.1 g,
0.08 mmol) and thiophene boric acid (0.75 g, 5.8 mmol) and
the mixture was stirred at room temperature for 0.5 h under
argon environment. Then the mixture was stirred at 80 °C for
32 h. The solvent was evaporated and purified by column
RESULTS AND DISCUSSION
To prepare black-to-transmissive electrochromic polymers
via direct oxidative polymerization, we demonstrate a facile
synthetic route as shown in Fig. 1. The decificent acceptor
monomer of M1 was prepared in good yields over four steps
starting from the readily commercially available material. The
Br
Br
Br
Br
Br
2
Br
Br
NaBH
SeO
2
4
HBr
N
N
N
N
N
N
H N
NH
2
2
Se
S
1
S
3
2
B(OH)
2
S
S
FeCl
S
3
n
S
CH NO
3
2
S
N
N
Se
N
N
Se
M2
P1
S
S
n
S
S
N
N
N
N
S
S
M1
Fig. 1. Synthesis of poly(4,7-di(thiophen-2-yl)benzol[c][1,2,5,]selenadizole)
P2

Vol. 27, No. 7 (2015)
Synthesis and Characterization of Conjugated Polymers Based on Benzoselenadiazole 2429
first step, benzo[c][1,2,5]thiadizole was brominized in HBr.
This reaction, was filtrated, washed and dried, the yield was
above 90 %. Here, the acceptor dibromobenzo[c][1,2,5]thia-
diazole was first reacted with NaBH4, the product was extracted
with ethyl acetate and dried. Then the 3,6-dibromobenzene-
1,2-diamine was added in ethanol solution containing two
oxidation of selenium to produce the final compound with
high yield (96 %). The monomer M1 was synthesized as bright
red solid by palladium-catalyzed Suzuki coupling reaction by
dibromobenzo[c][2,1,3]selendiazole and two electron-rich
thiophene boric acid. Finally, P1 was synthesized by direct
oxidative polymerization under the reaction condition with
FeCl3 and CH3NO2.
P2 exhibits a bright orange-red colour due to the increased
reflection/transmission of red light.
Electrochemical properties of polymers: The electro-
chemical behaviour of the copolymers was investigated by
cyclic voltammetry. Cyclic voltammograms of the polymers
in degassed anhydrous dichloromethane containing 0.1 mol/L
Bu4NBF4 and a potential scan rate of 100 mV/s. We could
record only one p-doping and one n-doping process of the
copolymers. The onsets of oxidation processes of the random
copolymers were 0.75V for P1 and 0.91V for P2, respectively.
These results indicated that the oxidation process of the
polymer P1 occurred at a lower potential, as compared with
that of the P2. As is stated above, the onsets of the n-doping
processes of the two polymers are -0.89 V for P2 and -0.08 V
for P1, which are the reductions of the benzothiadiazole and
benzoselenadizole moieties (Fig. 3).
IR spectra of P1: IR spectrum of the polymer shows a
ν(C-C) peak of disubstituted thiophe neat 1045 cm-1. ν(C=C)
and ν(C-H) absorption peaks of terminal thiophene at about
1741 cm-1 and about 1029 cm-1, 1093 cm-1, respectively, are
not observable.
Optical properties: The photophysical characteristics of
the monomers have been investigated by UV-visible absorption
in dilute chloroform solution. The absorption spectra for
monomer 1, monomer 2, polymer 1 and 2 were measured in
CHCl3 (1.0 × 10-5 mol l-1) at room temperature and showed on
Fig. 2.
(a)
3 mA cm–1
1.6
M1
M2
P1
P2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
-1.0
-0.5
0
0.5
1.0
1.5
2.0
Potential (V)
(b)
P2
1.5
0.0
300 350 400 450 500 550 600 650 700 750 800 850 900
Wavelength (nm)
Fig. 2. UV-visible absorption spectra of M1, M2, P1 and P2 in CHCl3 solution
-1.5
In solution, the λmax absorption of the monomer 1 and 2
observed at 300 nm due to π-π* transition. The absorption
band centered at 440 nm due to the formation of charge transfer
from thiophene unit to benzothiadiazole unit. The difference
in absorption maxima was relatively small but M1 exhibits a
weak absorption peak at 478 nm and was red-shifted to 31 nm
compared to M2 which shows a weak absorption peak at
447 nm. A broad spectral absorption is evident ranging
from approximately 400 nm to greater than 650 nm for two
polymers. Unlike the spectra of typical donor-acceptor
polymers, which generally have two distinct absorption bands,
the spectra of polymers P1 to P2 show homogenous absorptions
across most of the visible spectrum (450-650 nm). The solu-
tions of P1 show a dark purple-black colour due to the lack of
absorption in the far blue and red regions. And the solution of
-3.0
-1.5
-1.0
-0.5
Potential (V) vs. Ag/AgCl
Fig. 3. Cyclic voltammograms of the polymers (a) P1 and (b) P2
0
0.5
1.0
1.5
2.0
According to the reported method, the highest occupied
molecular orbitals (HOMO) and the lowest unoccupied
molecular orbitals (LUMO) energy levels could be estimated
by using the energy level of ferrocene as a reference. HOMO
and LUMO levels calculated according to an empirical formula
(HOMO) = -e(Eox + 4.8) eV and LUMO = -e(Ered + 4.8) eV10
are also listed in Table-1. Based on the HOMO and LUMO

2430 Sun et al.
Asian J. Chem.
TABLE-1
HOMO AND LUMO ENERGY LEVEL VALUES DETERMINED BY DFT CALCULATIONS
Polymer
Eoxon (V)
Eredon (V)
HOMO (eV)
LUMO (eV)
Egec (ev)
P1
P2
0.75
0.91
-0.08
-0.89
-5.55
-5.71
-4.72
-3.91
0.83
1.80
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energy level, the band gaps (Eg) of the two polymers were
1.80 eV for P2 and 0.83 eV for P1 (Table-1).
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Conclusion
In conclusion, we have demonstrated a new short and rela-
tively efficient synthesis of 4,7-di(thiophen-2-yl)benzo[c]-
[1,2,5]selenadizole which makes these classes of molecules
much more readily available for use as components in new
materials for organic electronics applications. The synthesis
of a series conjugated copolymers of thiophene and benzothia-
diazole and benzoselendiazole was shown on the paper. How-
ever, the effect is more pronounced for the LUMO, which resides
predominantly near the acceptor fragment. This differential
influence over the HOMO and the LUMO is responsible for
the observed decrease in band gap energy
ACKNOWLEDGEMENTS
The authors are grateful for the financial support of Science
Fund of the Technology Office of Jiangxi, China (20122BA-
B203017 and 20132BBE50024).
9. P. Shi, C.M. Amb, E.P. Knott, E.J. Thompson, D.Y. Liu, J. Mei, A.L.
Dyer and J.R. Reynolds, Adv. Mater., 22, 4949 (2010).
10. A.K. Agrawal and S.A. Jenekhe, Chem. Mater., 8, 579 (1996).
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