A new low band gap electrochromic polymer containing 2,5-bis-dithienyl-1H- pyrrole and 2,1,3-benzoselenadiazole moiety with high contrast ratio

Article abstract of DOI:10.1016/j.polymer.2011.10.017

A new low band gap electrochromic polymer containing of 3-(2,5-di-2-thienyl-1H-pyrrol-1-yl)-9-ethyl-9H-carbazole (SNSC) and 2,1,3-benzoselenadiazole (BSe) moiety has been reported. Structural identification of initial compounds and product were carried out by using FT-IR and ¹H NMR spectroscopy results. The resulting polymer, poly(SNSC-BSe), was completely soluble in various common organic solvents and its weight-average molecular weight (M_w) were 9420 (PDI: 1.22). According to AFM results, the RMS (root mean surface) roughness of poly-(SNSC-BSe) was found to be 48.6 nm. Besides, the thermogravimetric analysis presented that the poly(SNSC-BSe) is moderately stable against to thermal effect with the initial degradation temperature at 182 °C. The lowest energy transition band maxima of poly(SNSC-BSe) was 498 nm in thin film and the optical band gap calculated from the onset wavelength of the optical absorption was 1.77 eV. On the other hand, the electrochemical band gap calculated from oxidation and reduction onset values was 1.60 eV, respectively. Finally, the electrochromic performance of the polymer film represents a high contrast ratio in the NIR region (51%), fast response time of about 1 s, high coloration efficiency (274 cm² C^-1) and retained its performance by 94.6% even after 1000 cycles.

Full text of DOI:10.1016/j.polymer.2011.10.017

Polymer 52 (2011) 5772e5779
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
A new low band gap electrochromic polymer containing 2,5-bis-dithienyl-
1H-pyrrole and 2,1,3-benzoselenadiazole moiety with high contrast ratio
,
,
,
c
,
a,b,
Fatma Baycan Koyuncu a b, Emre Sefer a b, Sermet Koyuncu a , Eyup Ozdemir
*
**
a Department of Chemistry, Faculty of Sciences and Arts, Çanakkale Onsekiz Mart University, 17020 Çanakkale, Turkey
b Polymeric Materials Research Laboratory, Çanakkale Onsekiz Mart University, 17020 Çanakkale, Turkey
c Çan Vocational School, Çanakkale Onsekiz Mart University, 17400 Çanakkale, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A new low band gap electrochromic polymer containing of 3-(2,5-di-2-thienyl-1H-pyrrol-1-yl)-9-ethyl-
9H-carbazole (SNSC) and 2,1,3-benzoselenadiazole (BSe) moiety has been reported. Structural identifica-
tion of initial compounds and product were carried out by using FTeIR and 1H NMR spectroscopy results.
The resulting polymer, poly(SNSC-BSe), was completely soluble in various common organic solvents and
its weight-average molecular weight (Mw) were 9420 (PDI: 1.22). According to AFM results, the RMS (root
mean surface) roughness of poly-(SNSC-BSe) was found to be 48.6 nm. Besides, the thermogravimetric
analysis presented that the poly(SNSC-BSe) is moderately stable against to thermal effect with the initial
degradation temperature at 182 ꢀC. The lowest energy transition band maxima of poly(SNSC-BSe) was
498 nm in thin film and the optical band gap calculated from the onset wavelength of the optical
absorption was 1.77 eV. On the other hand, the electrochemical band gap calculated from oxidation and
reduction onset values was 1.60 eV, respectively. Finally, the electrochromic performance of the polymer
film represents a high contrast ratio in the NIR region (51%), fast response time of about 1 s, high coloration
Received 17 August 2011
Received in revised form
3 October 2011
Accepted 9 October 2011
Available online 15 October 2011
Keywords:
Low band gap polymer
Electrochromic materials
2,5-bis-dithienyl-1H-pyrrole
efficiency (274 cm2
C
e1) and retained its performance by 94.6% even after 1000 cycles.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
(OSCs) [6], organic field effect transistors (OFETs) [7], memories [8]
and energy storage [9]. A large number of applications exist based
on electrochromism, which relies on the reproducible switching
characteristics of low band gap polymers [10].
p-Conjugated polymers containing donor-acceptor moieties are
recently of interest because the built in intramolecular charge
transfers can facilitate ready arrangement of the electronic struc-
ture, leading to low band gap semiconducting polymers [1]. Since
the concept of valence and conduction band broadening based on
alternating electro-donor and electro-acceptor moieties in conju-
gated polymers was introduced in 1993 by Havinga et al. [2], they
have begun to attract interest in this type of polymeric material
rather have the space and now widely used for industrial applica-
tions such as molecular electronics [3], nonlinear optical devices
[4], organic light emitting diodes (OLEDs) [5], organic solar cells
The first studies about electrochromic materials in the visible
region started with the inorganic tungsten trioxide (WO3) and
iridium dioxide (IrO2) [11]. Because of the increased versatility of
organic materials such as viologens, metallophtalocyanines [12],
these compounds have received the brunt of attention for potential
electrochromic materials. Finally, construction of conducting
polymers that contain multiple redox-active chromophores are
important for the preparation of novel polymeric electrochromic
materials, which exhibit at least two distinct color states and may
give multiple colors, depending on the structure of the material
[10]. Besides, electrochromic polymers have potential for struc-
turally controllable HOMOeLUMO band gap by means of easily
tuning of colors and also processibility, high contrast ability and fast
response time [10]. Furthermore, these polymers can be switched
between their various color states many times without any
noticeable decline in performance, so they known to have excellent
switching reproducibility. Especially, the green-colored neutral
* Corresponding author. Çan Vocational School, Çanakkale Onsekiz Mart
University, 17400 Çanakkale, Turkey. Tel.: þ90 286 2180018; fax: þ90 286 2180533.
** Corresponding author. Department of Chemistry, Faculty of Sciences and Arts,
Çanakkale Onsekiz Mart University,17020 Çanakkale, Turkey.
E-mail addresses: sermetkoyuncu@hotmail.com (S. Koyuncu), eozdemir@comu.
edu.tr (E. Ozdemir).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.10.017

F. Baycan Koyuncu et al. / Polymer 52 (2011) 5772e5779
5773
electrochromic polymer required two absorption bands (red and
blue) are accomplished via donor-acceptor interaction. Owing to
this approach, Sonmez et al. reported on completing the additive
primary color space: red, green and blue (RGB), with the first
promising green-colored neutral conjugated electrochromic poly-
mer, a donor-acceptor based material which exhibited a high
degree of stability upon electrochemical switching [13]. Then,
Toppare et al. published the donor-acceptor green colored 4,7-
di(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)benzo[1,2,5]thiadia-
zole (BEDOT-BTD) and, the electrochemical synthesis and electro-
chromic properties have been reported [14]. Prepared insoluble
polymer film was revealed to be a neutral state green material
switching rapidly and reversibly to a transmissive light blue
oxidized state, an experimental result strongly supporting the
utility of the donor-acceptor theory in the design of neutral green
electrochromic polymers. Besides, Cihaner et al. have been reported
new neutral state green electrochromic polymers based on thio-
phene and/or EDOT units as donor parts and 2,1,3-
benzoselenadiazole as acceptor part, respectively [15]. They
observed that the Selenium (Se) atom having large size and less
electronegativity when compared to sulfur analogue published by
Toppare et al. would cause drastic changes in the electrochromic
performance of polymers. Finally, the same group was reported by
donor-acceptor approach for engineering soluble and processable
neutral state green conjugated polymers containing 2,1,3-
benzoselenadiazole as acceptor part which possess highly trans-
missive oxidized states, fast switching times, redox switching
stability and excellent optical contrasts [16].
achieved by the same groups [20]. Furthermore, an electroactive or
photoactive moiety are introduced along this SNS backbone to tune
the band gap and thus to gain useful properties [21]. Besides,
recently, opto-elelectronic applications of SNS based polymers have
been studied by Hyun et al. [22]. Alhough, there is a number of
studies about optoelectronic application of SNS based donor-
acceptor polymers, this is the first study about electrochromic
properties of these polymers.
Herein, we synthesized a new low band gap polymer bearing
electron-rich
3-(2,5-di-2-thienyl-1H-pyrrol-1-yl)-9-ethyl-9H-
carbazole (SNSC) and electron-deficient 2,1,3-benzoselenadiazole
(BSe) moieties via Kumada coupling reactions (Scheme 1). Ther-
mogravimetric analysis and GPC results exhibit that synthesized
polymer is thermally stable and exhibits a low poly-dispersity
index (PDI), respectively. Optoelectronic properties of poly(SNSC-
BSe) were investigated by using cyclic voltammetry, UVevis
absorption and photoluminescence (PL) spectroscopy. Surface
morphology of poly(SNSC-BSe) film on to ITO/glass surface used via
spray- coating process determined by the AFM is uniform with the
roughness of 48.6 nm. Finally, we demonstrate that electrochromic
properties of this film exhibits a high contrast ratio (ΔT ¼ 51% at
950 nm), a fast response time (about 1 s), a high redox stability, and
a high coloration efficiency (274 cm2 Ce1).
2. Experimental
2.1. Materials
On the other hand, recently, 2,5-di-(2-thienyl)-1H-pyrrole (SNS)
derivatives [17] have very useful properties for optoelectronic
applications such as i) the oxidation potentials of the SNS deriva-
tives is lower (about þ0.7 V vs. SCE) then that of many of other
electroactive materials, ii) the functionalization of the ter-
heteroatom unit by the Paal-Knorr reaction seemed an attractive
one step procedure for the introduction of various bridges in to the
monomer, iii) provided that good quality of the films poly-SNS can
easily be generated on ITO/glass surface thus high electrochromic
performance would be obtained [18]. Toppare et al. and Cihaner
et al. have been reported in various electrochromic polymers based
on SNS moiety [19]. The electrochemical co-polymers of SNS with
EDOT-moiety were also obtained and color-tunning was partially
All chemicals purchased from Aldrich, Merck and Fluka and used
without further purification. 4-Bis(2-thienyl)butane-1,4-dione [23],
3-(2,5-di-2-thienyl-1H-pyrrol-1-yl)-9-ethyl-9H-carbazole (SNSC)
[21b], 2,1,3-benzoselenadiazole (BSe) [24], 4,7-dibromo-2,1,3-
benzoselena diazole (diBr-BSe) [24] were prepared from previ-
ously published procedures (Scheme 1).
2.2. Synthesis of 3-[2,5-di(5-bromo-2-thienyl)-1H-pyrrole-1-yl]-9-
ethyl-9H-carbazole (diBr-SNSC)
SNSC (1.42 g, 3.3 mmol) was dissolved in CH2Cl2 (30 ml) and
SiO2 (5.0 g) mixture and then added N-bromosuccinimide (NBS)
(0.7 g, 4 mmol). The mixture was stirred for 24 h at room
Scheme 1. Synthetic route to poly(SNSC-BSe).

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F. Baycan Koyuncu et al. / Polymer 52 (2011) 5772e5779
temperature. At the end of the reaction, SiO2 was filtered and
organic phase was extracted with water for 3 times. Then organic
phase was dried with MgSO4 and solvent was removed. The
product was purified by column chromatography (silica gel, 2/1;
hexane/CH2Cl2). Yield:1.23 g, 64%.
FTeIR (cmꢁ1): 3083, 3059 (CeH aromatic); 2921, 2851, (CeH
aliphatic); 1625, 1595, 1490, (C]C); 793, (CeBr) dH (400 MHz, CDCl3;
Me4Si): 8.00e6.56, (m, 13H, CeH aromatic’);4.39, (m, 2H, eNeCH2);
1.49, (t, 3H, eCH3). dC (100 MHz; CDCl3; Me4Si) 143.14, 141.26,
136.92, 132.15, 126.04, 125.58, 124.18, 121.35, 118.92, 118.14, 116.38,
115.04, 113.61, 110.98, 109.16, 108.73, 113.18, 108.12, 37.58, 13,51.
2.3. Synthesis of poly(SNSC-BSe)
DiBr-SNSC (1 g, 1.7 mmol) was dissolved in new distilled THF
(30 ml) at 0 ꢀC. Then 2 M (3.5 mmol) CH3MgBr in diethyl ether
solution added as dropwise. After 1 h, diBr-BSe (0.58 g, 1.7 mmol) in
10 ml of THF was slowly added in the mixture. Then Ni(dppp)Cl2
(10 mg) was added to this solution as catalyst and the reaction was
refluxed for 5 h. After that, bromo(phenyl)magnesium (0.1 g) and
bromobenzene (0.1 g) were added to the reaction mixture respec-
tively and the reaction was refluxed for 2 h. Finally, the mixture was
cooled to room temperature and then poured into a large amount of
methanol (200 ml), and the residue was obtained by filtration.
Poly(SNSC-BSe) was then subjected to a Soxhelet extraction in
methanol to remove residual monomer and other impurities. Pure
product was dried at 60 ꢀC under vacuum. Yield: 0.67 g, 44%.
UVeVis (nm) 270, 297, 333, 500 FTeIR (cmꢁ1): 3083, 3061 (CeH
aromatic); 2975, 2927, (CeH aliphatic); 1615 (C]N), 1594, 1490, (C]
C); dH (400 MHz, CDCl3; Me4Si): 8.00e6.51, (m, CeH aromatic’);4.39,
(m, 2H, eNeCH2); 1.49, (t, 3H, eCH3). dC (100 MHz; CDCl3; Me4Si)
162.41, 148.62, 144.16, 140.34, 136.48, 135.13, 134.02, 131.71, 129.14,
128.45, 127.16, 125.08, 124.36, 118.88, 116.12, 114.88, 113.18, 109.91,
108.45, 37.30, 13,46.
Fig. 1. 1H-NMR spectrum of poly(SNSC-BSe ).
of products were calculated from their low energy absorption edges
(
lonset) (Eg ¼ 1241/lonset) [26]. Photoluminescence spectra were
recorded on a PTIQM1 fluorescence spectrophotometer.
Conductivity measurements were carried out by an Entek FPP-
460 four point probe electrometer from polymer film surface.
Iodine doping was carried out by exposure of the film to iodine
vapor at atmospheric pressure and room temperature in a desic-
cator [27].
AFM measurements were carried out at room temperature and
ambient conditions by using Ambios QScope 250 instrument. The
non-contact mode (wave mode) was used to take topographic
images. 20 mm scanner equipped with silicon tips with 10 nm tip-
curvature and ITO coated glass substrate was used for measure-
ments. The system is covered with acoustic chamber to prevent
electromagnetic noises which may effect the measurements
Spectro-electrochemical measurements were carried out to
consider absorption spectra of this polymer film under applied
potential [28]. The spectro-electrochemical cell consisted of
a quartz cell, an Ag wire (RE), Pt wire counter electrode (CE) and
ITO/glass as transparent working electrode (WE). Measurements
were carried out in the 0.1 M TBAPF6 as supporting electrolyte in
acetonitrile.
Colorimetry measurements were performed by using Analytic
Jena UVeVis spectrophotometer consisted of chromameter module
with viewing geometry as recommended by CIE. According the CIE
system, the color is made up of three attributes; luminance (L), hue
(a), and saturation (b) [29]. These parameters were measured at
neural and oxidized state of the poly(SNSC-BSe) on the ITO/glass
surface.
2.4. Instrumentation
FTeIR spectra were recorded by a Perkin Elmer FTeIR Spectrum
One by using ATR system (4000e650 cme1). 1H NMR and 13C NMR
(Bruker Avance DPX-400) spectra were recorded at 25 ꢀC in
deuterated chloroform and TMS as internal standard. Thermal data
were obtained by using TA Q50-TGA and TA Q1000-DSC instru-
ments. The TGA measurements were performed between 20 ꢀC and
900 ꢀC (in N2, rate 10 ꢀC/min). The number-average molecular
weight (Mn), weigh taverage molecular weight (Mw) and poly-
dispersity index (PDI) values were determined by Aggilent GPC-
addon. For GPC measurements, an SGX (100 Å and 7 nm diameter
loading material) 3.3 mm i.d. ꢂ 300 mm as column, DMF/methanol
(v/v, 4/1, 0.4 ml/min) mixture as the eluent, polystyrene as standard
and a refractive index as detector (at 25 ꢀC) were used.
Cyclic voltammetry (CV) technique used for electrochemical
measurements was performed using Biologic SP50 potentios-
tategalvanostat system. The electrochemical cell includes an Ag/
AgCl as reference electrode (RE), Pt wire as counter electrode (CE)
and glassy carbon as working electrode (WE) immersed in 0.1 M
TBAPF6 as the supporting electrolyte. CV measurements were
carried under argon atmosphere. HOMO and LUMO energy levels of
polymer were calculatþed according to the inner reference ferrocene
redox couple Eꢀ(Fc/Fc ) ¼ þ0.41 V (vs. Ag/AgCl) in acetonitrile by
using the formula EHOMO ¼ ꢁe(EoxeEFc) þ (ꢁ4.8 eV) [25]. Onset
values of oxidation potentials were taken into account while
calculating HOMOeLUMO energy levels.
3. Results and discussion
3.1. Synthesis and characterization
In our previous study, SNSC as donor unit prepared via the Paal-
Knorr reaction was carried out between 3-amino-9-ethylcarbazole
Table 1
Thermal behavior and molecular weigh distribution of poly(SNSC-BSe).
TGA and DSC
GPC
UVeVis spectra were recorded by Perkin Elmer Lambda35
spectrophotometer. The absorption spectra of monomer and poly-
mer were recorded in dichloromethane. The optical band gap (Eg)
Ton
Tg
Wmax
T
Te
Mn
Mw
PDI
poly(SNSC-BSe)
182
60.1
336
32%
7700
9420
1.22

F. Baycan Koyuncu et al. / Polymer 52 (2011) 5772e5779
5775
Fig. 2. AFM image of poly(SNSC-BSe) onto ITO/glass surface.
and 1,4-dithiophene-2-yl butane-1,4-dione prepared from thio-
phene and succinyl chloride in dry toluene in presence of p-tol-
uenesulfonic acid as catalyst [21b]. Then the product, SNSC, was
brominated with NBS in dry dichloromethane in presence of SiO2.
On the other hand BSe as acceptor unit was synthesized by using
SeO2 and o-phenylenediamine in ethanol-water mixture (10:1)
[24]. BSe was brominated with liquid bromine in H2SO4 in the
presence of Ag2SO4. The reaction yields were found to be 84 and
52%, respectively. The donor-acceptor co-polymer was performed
by Kumada type aryl-aryl coupling reaction. Therefore, SNSC was
activated with the Mg by using CH3MgBr and then and Ni(dppp)Cl2
as catalyst added the reaction mixture. The light brown colored
solution turned into purple by the formation of poly(SNSC-BSe).
Finally, bromobenzene and bromo(phenyl)magnesium were added
for end group closing and the yield was found to be 44%. The co-
polymer was soluble in polar protic solvents such as DMSO, DMF
NMP and partly soluble in CHCl3 and THF. Besides, it was insoluble
in methanol, ethanol and toluene. Structural identification of these
compounds was carried out by using FTeIR, and 1H NMR. Signifi-
cant changes were observed in the spectral properties of the initial
compounds and the products. SNSC was fully characterized in our
previous study [21b]. In FTeIR spectrum of diBr-SNSC occurred
aromatic CeBr vibration band was observed at 789 cmꢁ1. On the
other hand, characteristic amine band attributed to o-phenyl-
enediamine was removed by the accomplishment of the synthesis
of BSe. In the next step, aromatic CeBr band was observed at
829 cmꢁ1. After the polymerization, whole CeBr peaks attributed to
donor and acceptor moieties were removed on the spectrum. In the
FTeIR spectrum of poly(SNSC-BSe), the peaks were not observed as
broad because of both obtained co-polymer has not high molecular
weight distribution and not included cross-linking due to using
regioregular synthetic process (see supporting information).
Molecular structure of co-polymer was also identified from its
1H NMR spectra recorded in CHCl3-d (Fig. 1). In the 1H NMR spec-
trum of poly(SNSC-BSe), characteristic singlet signals at 7.82 and
6.61 ppm attributed to phenyl ring of BSe-acceptor unit and pyrrole
moiety of SNSC-donor unit were clearly observed, respectively.
Furthermore, other signals attributed to thiophene and carbazole
groups were observed at their expected ppm values. Phenyl end
group protons were observed lower intensity than polymer main
chain protons. Due to inductive effect of nitrogen atom of carbazole
moiety, NeCH2 signal was observed as multiplet at higher ppm
than that of eCH3 end group. Finally, these results clearly indicate
that all reactions were completed successfully.
Fig. 3. Change in the electrical conductivity of poly(SNSC-BSe) during the process of
Fig. 4. UV-Vis absorption and fluorescence spectra of poly(SNSC-BSe) in
dichloromethane.
iodine doping (done at atmospheric pressure and 25 ꢀC).

5776
F. Baycan Koyuncu et al. / Polymer 52 (2011) 5772e5779
chain so its solubility is low and it shows uniformly dispersed
nodular structure. The AFM image of the poly(SNSC-BSe) shows
agglomerated non-uniform structure. Because of these agglomer-
ates, the roughness of poly(SNSC-BSe) is high. If the donor acceptor
conjugated polymer have the longer side alkyl chain, solvation is
promoted with increase in the length of the alkyl side-chain,
leading to expanded chain structure with longer
p conjugation.
Because of enlargement of conjugation and increasing the solu-
p
bility, the uniform structure could be observed on the film surface.
So the quality of the film could be improved by the reducing RMS
roughness value [6g,h].
3.3. Conductivity
Conductivity is related to both intermolecular and intra-
molecular charge transfer on the polymer backbone. Therefore
these results have shown that the dopant interacts with the double
bond in the polymer backbone and forms a polaronic state (radical
cation) as an electron is transferred from the double bond to the
dopant creating a hole or a positive carrier at the double bond site.
These holes or positive carriers are responsible for the electrical
conduction in these materials [27a]. Conductivity measurements of
poly(SNSC-BSe) was carried out with an electrometer using a four
point probe technique. Fig. 3 exhibits the electrical conductivity
behavior results of thꢀe co-polymer under iodine vapor doping in
varying periods at 25 C. First of all, the conductivity was found to
be approximately 10ꢁ6 S/cm and then an increase was observed
(10ꢁ3 S/cm) until 96 h. When the co-polymer film exposed to
excessive iodine vapor, the conductivity was decreased to 10ꢁ4 S/
cm due to structural deterioration. Li et al. have been prepared
conductive polythiophene microparticles via interfacial synthesis
method with FeCl3 as oxidant [27b]. Owing to decrease thiophene
concentration a fixed FeCl3/thiophene molar ratio at 0 ꢀC hexane/
nitromethane biphase system, the obtained polythiophene exhibits
Fig. 5. Cyclic voltammogram of BSe (a) SNSC (b) and poly(SNSC-BSe) (c) in the 0.1 M
TBAPF6/dichloromethane, scan rate 100 mV/s, vs. Ag/AgCl.
According to GPC analysis, the number-average molecular
weight (Mn), weight-average molecular weight (Mw) and poly-
dispersity index (PDI) values were found to be 7700; 9420; 1.220,
respectively. The Mn value corresponded to degree of polymeriza-
tion of 10e15 and thus poly(SNSC-BSe) exhibits narrow molecular
weight distribution.
TGA and its derivative curves (DTG) were obtained under
dynamic scans and analyzed in order to determine thermal stability
trends of the poly(SNSC-BSe) (see supporting information). The
measured temperature characteristics determined from TGA and
DTG curves such as, Ton (decomposition temperature onset), Wmax
T
(temperature for maximum mass loss) Te (% carbine residue of
900 ꢀC) are presented in Table 1. While poly(SNSC-BSe) is thermally
stable up to 182 ꢀC, 32% carbine residues are formed at 900 ꢀC. Due
to poly(SNSC-BSe) is donor acceptor conjugated system, donor and
acceptor moieties interacted with each other at the neutral state
and its thermal stability is lower than that of polythiophene
synthesized by Li et al [27b]. On the other hand the glass transition
temperature (Tg) measured by DSC results was found to be 60.1 ꢀC.
According to all these results, poly(SNSC-BSe) was found to be
moderately stable against to thermal effect.
a steadily increased conductivity from 10ꢁ12 to 10ꢁ2 S cmꢁ1
.
Comparing to this results, 10ꢁ3 S/cm value obtained from pol-
y(SNSC-BSe) by iodine doping is almost appropriate for various
applications.
3.4. Optical and electrochemical properties
3.2. Surface morphologies
The UVeVis absorption spectra of poly(SNSC-BSe) was recorded
in dichloromethane (Fig. 4). The UVeVis spectrum of the co-
polymer exhibits two main absorption peaks at about 297 and
The morphologies of film surfaces of the poly(SNSC-BSe) was
studied by AFM. Fig. 2 represents the images of the co-polymer,
prepared by using spray-casting method. As a typical film for
coated by the method, rough films were obtained. While the
thickness of the film was calculated as 102.8 nm, the RMS (root
mean surface) roughness of poly-(SNSC-BSe) is found to be
48.6 nm. The film properties can be affected by the solubility and
structure of the polymer system. Poly(SNSC-BSe) has not long alkyl
333 nm attributed to SNSC and BSe moieties of the pe
p* transition.
Due to the formation of SNSC-donor and BSe-acceptor conjugated
structure, the typically broad charge transfer band was observed at
about 500 nm in the UVeVis absorption spectrum [2]. The excita-
tion of the co-polymer at its absorption maxima (lmax ¼ 500 nm)
resulted in an emission band with moderate intensity at 604 nm
that is a Stokes’ shift of 104 nm (Fig. 4). The absorption bands of
Table 2
HOMO and LUMO energy levels, electrochemical (Eg0 ) and optical band gaps (Eg) values of BSe, SNSC initial compounds and poly(SNSC-BSe).
Molecules
Reduction potential (V)
Oxidation potential (V)
HOMO (eV)
LUMO (eV)
Eg0 ; Electrochemical
Eg, Optical
band gap (eV)
band gap (eV)
red
BSe
E
E
E
-
E
E
E
¼ ꢁ1.32
-
ꢁ6.90b
ꢁ3.21
e
3.69
m;c
red
m;a
red
¼ ꢁ1.23 reversible
¼ ꢁ1.18
m;on
ꢁ2.03a
ꢁ3.46
e
3.07
1.77
ox
ox
SNSC [21b]
poly(SNSC-BSe)
E
E
E
¼ 0.80 irreversible E
¼ 0.71
ꢁ5.10
ꢁ5.06
m;a
ox1
p;a
m;on
red
ox1
¼ ꢁ1.10
¼ 0.78 E
¼ 0.95 E
¼ 0.72 reversible
1.60
p;c
p;c
red
p;a
ox2
p;a
ox2
p;c
ox
¼ ꢁ1.00 reversible
¼ ꢁ0.93
¼ 0.87 reversible E
¼ 0.67
p;on
red
p;on
a
Calculated by the addition of the optical band gap to the HOMO level
Calculated by the subtraction of the LUMO level from the optical band gap.
b

F. Baycan Koyuncu et al. / Polymer 52 (2011) 5772e5779
5777
Table 3
calculated according to the onset values of their oxidation and
reduction potentials (Table 2 and 3).
Electrochromic parameters for the poly(SNSC-BSe) film.
The spectro-electrochemical properties of the poly(SNSC-BSe)
film were determined upon doping process (Fig. 6). In our previous
study [21b], poly(SNSC) does not consist of BSe-acceptor moiety in
the main chain. For this reason, just one strong band at 445 nm
corresponding to light brown color film was observed in the
absorbtion spectrum. On the other hand, the poly(SNSC-BSe) film
exhibits a strong band at 522 nm due to charge transfer between
SNSC-donor and BSe-acceptor part at the neutral state, thus it is
violet color corresponding to the absorption area. The optical band
gap (Eg) of poly(SNSC) and poly(SNSC-BSe) was found to be 2.22
and 1.77 eV from the onset of absorption. The band gap of pol-
y(SNSC-BSe) is somewhat higher than the Eg of poly(4,7-di-2-
thienyl-2,1,3-benzoselenadiazole) (PTSeT) (1.46 eV). Reason for
this difference, the synthesis of PTSeT was performed via electro-
chemical process by Cihaner and Algi. On the other hand, owing to
the method used for the synthesis of poly(SNSC-BSe), regioregular
polymer structure was obtained. Thus, band gap of poly(SNSC-BSe)
was higher than PTSeT due to the lack of cross-link structure. On
the other hand, the absorption band at 522 nm reached the
minimum intensity and finally the band diminished completely
upon oxidation. Besides, the broad band intensified at about
740 nm (0e0.7 V) and the more broad band about 950 nm
(0.7e1.1 V) indicated the formation of polarons and bipolarons on
the co-polymer film, respectively (Fig. 6). Owing to two oxidation
process between 0.5 and 1.1 V, NIR region, above 700 nm, was
strongly absorbed at the fully oxidized state and thus the high
contrast ratio could be observed on the electrochromic polymer
film. Based on foregoing results, the purple color of the film with
Commision Internationale de I’Eclairage (CIE) color parameters
(Luminance L ¼ 34, hue a ¼ 5, and saturation b ¼ ꢁ10) turned into
light blue (L: 46; a: ꢁ5; b: ꢁ6), upon oxidation (Fig. 6).
Optical Contrast
Change (ΔT%)
Response
time (s)
Optical
activity
after 1000 Luminance (L)
cycles (%) hue (a)
saturation (b)
CIE color
parameters
522 nm 950 nm
poly
(SNSC-Bse) Tneut:52%
Toxi:81% Toxi: 44%
ΔT:29%
ΔT:51%
Oxidation 1.4 94.6
Neutral state
L: 34; a:5;
b:ꢁ10
T
neut:95%
Reduction 0.8
Oxidized state
L: 46; a: ꢁ5;
b: ꢁ6
poly(SNSC-BSe) on the ITO/glass surface are more broadened than
the solution of co-polymer most probably because of the strong
pe pe
p* interactions. This type of interaction is attributed to p*
staking and packing effect at the solid state. Finally, for poly(SNSC-
BSe), the onset absorption is 701 nm corresponding to an energy
band gap of 1.77 eV.
The electrochemical properties of poly(SNSC-BSe) were inves-
tigated by cyclic voltammetry (CV). An explicit difference in redox
behavior of the poly(SNSC-BSe) and its initial compounds, SNSC
and BSe, can be deduced from the CV which is depicted in Fig. 5.
During CV scan in the anodic regime of SNSC and poly(SNSC-BSe),
SNSC exhibited an irreversible oxidation peak at E
Ag/AgCl during an anodic scan, and this oxidation potential is
ox
¼ 0.80 V vs.
m;a
ox
higher than that of poly(SNSC-BSe) (reversible; E
¼ 0.78 and
m;a
ox
E
¼ 0.72 V; Eox
¼ 0.75 V vs. Ag/AgCl). As expected, this result
m;c
m;1=2
indicated that SNSC has a more electron rich nature than pol-
y(SNSC-BSe) when attaching the BSe-acceptor moiety in a conju-
gated system. But the conjugation extended in the co-polymer
backbone and thus the oxidation potentials were observed as
reversible and at lower potential with compared to corresponding
initial compound (SNSC). On the other hand, in the cathodic regime,
cyclic voltamogram of BSe-acceptor structure exhibits character-
Double step chronoamperometry technique was used to the
changes in the electro-optical responses during switching. Elec-
trochromic switching studies for the poly(SNSC-BSe) was per-
formed to monitor the percent transmittance changes (%DT) as
istic
reversible
¼ ꢁ1.32 VeE
reduction
couple with
potentials
of
red
red
red
a function of time at their absorption maximum (lmax) and to
determine the response time by stepping potential repeatedly
between the neutral and oxidized states. The active area of the
polymer film on ITO glass is about 1 cm2. Fig. 7 depicts the optical
transmittance as a function of time to increments and decrements
of the absorption band at 498 and 950 nm by with respect to time of
10 s between 0 and 1.0 V. The percentage transmittance changes
E
¼ ꢁ1.24 V and E
¼ ꢁ1.28 V vs. Ag/AgCl.
m;c
m;a
m;1=2
Compared with the BSe-acceptor structure, the reduction redox
couple of poly(SNSC-BSe) broadened and slightly shifted positively
red
red
red
(E
¼ ꢁ1.10 VeE
¼ ꢁ1.00 V and E
¼ ꢁ1.05 V vs. Ag/AgCl)
p;c
p;a
m;1=2
and the conjugated backbone is still sufficiently conducting to
allow electron transfer at the potentials. Besides, HOMOeLUMO
energy level of poly(SNSC-BSe) and its initial compounds
Fig. 6. Spectro-electrochemical behavior of poly(SNSC-BSe) film on the ITO/glass surface.

5778
F. Baycan Koyuncu et al. / Polymer 52 (2011) 5772e5779
Fig. 7. Electrochromic switching, optical absorbance monitored at 522 and 950 nm for poly(SNSC-BSe) film between 0 and 1.1 V.
(DT%) of poly(SNSC-BSe) between the neutral (at_0 V) and oxidized
Appendix. Supplementary material
states (at 1.1 V) were found to be 29% for 522 nm and 51% for
950 nm (Fig. 7). Besides, the oxidation and reduction response
times were measured to be 1.4 and 0.8 s for poly(SNSC-BSe).
Optical activity of the co-polymer film was retained by 94.6% even
after 1000 cycles of operation. Finally, poly(SNSC-BSe) exhibits
reversible redox behavior, fast response time, high resistance to
over oxidation, and especially high contrast ratio at the NIR region
and can thus be useful for electrochromic applications. On the other
hand, the coloration efficiency (CE) was calculated by using
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.polymer.2011.10.017.
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