A new processable donor-acceptor polymer displaying neutral state yellow electrochromism

Article abstract of DOI:10.1016/j.polymer.2013.09.033

We report here the synthesis of a new solution processable neutral state yellow polymeric electrochromic material containing 2,5-bis-dithienyl-1H-pyrrole (SNS)-donor and 1,8 naphthalimide-acceptor (SNS-NI) as a subunit. The electrochemical and optical properties were investigated via cyclic voltammetry (CV), UV-Vis absorption and fluorescence emission measurements, respectively. Besides, electrochromic performance of poly(SNS-NI) has been compared to the both the film preparation method and poly(1-phenyl-2,5-dithiophen-2-ylpyrrole) [poly(SNS-P)] as a standard polymer. In the poly(SNS-NI), yellow color of the polymer film at neutral state converted to green and then dark blue upon the polymer film fully oxidized in the positive regime. SNS-NI polymer film prepared via spin casting process exhibits a high contrast ratio in the near-IR region (ΔT% = 56% at 890 nm), a response time of about 1 s, high coloration efficiency (299 cm² C^-1) and retained its performance by 98.6% even after 5000 cycles. Finally, the results clearly indicate that both electronic nature of the molecule and film preparation method have a major impact on electrochromic performance of these polymers.

Full text of DOI:10.1016/j.polymer.2013.09.033

Polymer 54 (2013) 6283e6292
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
A new processable donoreacceptor polymer displaying neutral state
yellow electrochromism
Eda Oguzhan a , Hakan Bilgili , Fatma Baycan Koyuncu b, Eyup Ozdemir a,b,
,
b
c
a,
b,d,
*
Sermet Koyuncu
a
Department of Chemistry, Faculty of Sciences and Arts, Çanakkale Onsekiz Mart University, 17020 Çanakkale, Turkey
Polymeric Materials Research Laboratory, Çanakkale Onsekiz Mart University, 17020 Çanakkale, Turkey
Solar Energy Institute, Ege University, 35100 Bornova, Izmir, Turkey
b
c
d
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:
Received 10 August 2013
Received in revised form
We report here the synthesis of a new solution processable neutral state yellow polymeric electro-
chromic material containing 2,5-bis-dithienyl-1H-pyrrole (SNS)-donor and 1,8 naphthalimide-
acceptor (SNSeNI) as a subunit. The electrochemical and optical properties were investigated via
cyclic voltammetry (CV), UVeVis absorption and fluorescence emission measurements, respectively.
Besides, electrochromic performance of poly(SNSeNI) has been compared to the both the film
preparation method and poly(1-phenyl-2,5-dithiophen-2-ylpyrrole) [poly(SNSeP)] as a standard
polymer. In the poly(SNSeNI), yellow color of the polymer film at neutral state converted to green and
then dark blue upon the polymer film fully oxidized in the positive regime. SNSeNI polymer film
17 September 2013
Accepted 18 September 2013
Available online 25 September 2013
Keywords:
Electrochromic materials
Morphology effect
prepared via spin casting process exhibits a high contrast ratio in the near-IR region (
D
T% ¼ 56% at
890 nm), a response time of about 1 s, high coloration efficiency (299 cm2
C
ꢀ1
) and retained its
2,5-Di-(2-thienyl)-1H-pyrrole
performance by 98.6% even after 5000 cycles. Finally, the results clearly indicate that both electronic
nature of the molecule and film preparation method have a major impact on electrochromic perfor-
mance of these polymers.
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction
inhibitors, and “smart windows” that absorb sunlight [4e14]. For
device applications, it is especially important to control the elec-
Especially in the last quarter century, the field of conducting
polymers has grown enormously in both scientific and commercial
areas because of their promising electronic, optoelectronic and
electrochemical properties [1,2]. Conducting polymers can be ob-
tained from suitable oxidizable monomers chemically or electro-
chemically [3]. These polymers have various application areas such
as photovoltaic devices, electroluminescent displays, emissive
layers in full-color video matrix displays, superadsorbents, ion
sensors, field-effect transistors, active thin layers of various sensing
devices, supercapacitors and electrolytic-type capacitors, corrosion
tronic band structure of the polymer to achieve a band gap of the
desired magnitude and frontier orbitals, HOMO (highest occupied
molecular orbital) and LUMO (lowest unoccupied molecular
orbital) (or band edges), of the proper energies [15]. As a result,
among the conducting polymers, low band gap polymers are very
important [16].
Electrochromism is defined as a reversible optical change in a
material under external applied potentials [17,18]. Polymeric
electrochromic materials show different redox states that derive
distinct absorption bands at different wavelengths [19]. If more
than two redox states are present in the visible region, this type
of materials exhibit multielectrochromic behavior. Although the
first studies concerning electrochromic materials involved inor-
ganic semiconductors [20,21] and organic small molecules [22],
conducting polymers received increasing attention in the course
of time, due to their superior features such as their ability to
*
Corresponding author. Çan Vocational School, Çanakkale Onsekiz Mart Uni-
versity, 17400 Çanakkale, Turkey. Tel.: þ90 286 4167705; fax: þ90 286 4167706.
E-mail address: skoyuncu@comu.edu.tr (S. Koyuncu).
0
032-3861/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.09.033

6284
E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
form durable films and also the tunable nature of their HOMOe
LUMO band gap, resulting from changes in molecular architec-
ture [19].
(2 M, 100 ml) and then water (50 ml), and finally the product was
purified by column chromatography (silica gel, 3/1; chloroform/
hexane). The purified product was dried at 60 C in vacuum oven.
ꢁ
Recently, 2,5-bis-dithienyl-1H-pyrrole (SNS) containing elec-
troactive polymers have been found to be very important for
electrochromic applications [23e25]. Along the SNS backbone,
electroactive or photoactive moieties are placed to tune the band
gap and thus to gain useful properties. On the other hand, 1,8-
naphthalimide (NI) derivatives are n-type materials and have a
relatively high electron affinity and excellent transport properties.
Owing to these properties, they are used as n-type semi-
conductors in various fields such as laser active media [26e28],
coloration of polymers [29], photo-induced electron transfer
sensors [30], electroluminescent materials [31], liquid crystal
displays [32], light emitting diodes [33e35], and ion probes [36].
Moreover, they are also used as potential photosensitive biolog-
ical units [37], anticancer agents [38,39] and analgesics [40] in
medicine.
Yield: 89%; 0.25 g.
ꢀ
1
FT-IR (cm ):3059 (CeH, aromatic); 1704, 1666; (C]O imide);
1
1552, 1487 (C]C aromatic). H NMR (CHCl
Ar-H ); 8.48 (d, 1H, Ar-H ); 8.06 (d, 1H, Ar-H
);7.62 (m, 1H, Ar-H
3
-d): d ppm, 8.66 (d, 1H,
i
m
j
); 7.69 (d, 1H, Ar-
0
H
l
k
); 7.58 (d, 2H, Ar ꢀ H ); 7.36 (t, 2H,
dd
0
0
ff
currhskip0ptAcurrhskip0ptr ꢀ H ); 7.45 (t, 2H, Ar ꢀ H ); 6.63 (d,
ee
0
0
2H, currhskip0ptAcurrhskip0ptr ꢀ H ); 6.80 (d, 2H, Ar ꢀ H ); 6.41
cc
bb
0
þ
(s, 2H, Ar ꢀ H ); MALDI-ToF (m/z): [M ] calcd. for C
8
2 2 2
H17BrN O S ,
aa
581.50; found, 581.44.
Ha
Ha'
Hb
Hb'
Hc
Hc'
In this work, we synthesized a new SNS type donoreacceptor
monomer [6-(dihexylamine)-2-[4-(2,5-di-2-thienyl-1H-pyrrole-1-
N
N
S
S
yl)phenyl]-1H-benzo[de]isoquinoline-1,3(2H)-dione]
(SNSeNI)
He
He'
Hd
Hd'
containing 1,8-naphthalimide electroneacceptor with
a
a
branched alkyl chain as a subunit. The polymer prepared via an
electrochemical process can be soluble in the chloroform and then
recolating to ITO/glass surface via a spin casting process to give a
multi-electrochromic polymer film having high redox stability, a
high coloration efficiency and a fast response time. Further,
intramolecular electronic interactions in the donor acceptor SNSe
NI monomer and its polymer have been investigated by using
cyclic voltammetry (CV), UVeVis absorption and fluorescence
spectroscopy. Finally, there are only a very limited number of ex-
amples of processable neutral state yellow electrochromic mate-
rial in the literature [41,42], we believe the highly soluble
synthesized material may be very important for electrochromic
applications.
Hf
O
Hf'
O
Hý
Hm
Hk
Hj
Br
Hl
2
. Experimental
.1. Materials
Thiophene
2
.3. Synthesis of 6-(dihexylamine)-2-[4-(2,5-di-2-thienyl-1H-
pyrrole-1-yl)phenyl]-1H-benzo[de]isoquinoline-1,3(2H)-dione
SNSeNI)
2
(
(Aldrich),
4-bromo-1,8-naphtalicanhydride
(
(
(
Aldrich), hidrazinium hydroxide (Merck), Cu (Merck), K
2
CO
3
6-Bromo-2-[4-(2,5-di-2-thienyl-1H-pyrrole-1-yl)phenyl]-5,6-
dihidro-1H-benzo[de] isoquinoline-1,3(2H)-dione (4) (0.095 g,
0.163 mmol), dihexylamine (0.302 g 1.63 mmol) and 30 ml DMA
Merck), ethanol (Merck), pyridine (Merck), AlCl (Merck), NaHCO
3
3
Merck), 4-nitroaniline (Aldrich), 18-Crown-6 (Fluka), succinyl
dichloride (Aldrich) and palladium on activated carbon (10%, w/w)
Fluka) were supplied from Aldrich, Merck and Fluka, and were
2 3
stirred in a 100 ml flask and then CuI, (0.9 g) K CO (1.55 g,
(
11.2 mmol) and 18-Crown-6 (0.02 g) were added. The reaction
mixture was refluxed under argon atmosphere for 48 h. Then this
mixture was filtered through cellite to remove the cupper im-
purities. After that, the solution was poured into cold water and
the precipitate was collected by filtration. The product was pu-
rified by column chromatography (silica gel, 2/1; chloroform/
used without further purification. The syntheses and character-
izations of 1,4-bis(2-tiyenil)butan-1,4-dione (1) [43], 1-(4-nitro-
phenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole (2) [44] and 4-(2,5-
di(thiophen-2-yl)-1Hpyrrol-1-yl)aniline (3) [45], 1-phenyl-2,5-
dithiophen-2-ylpyrrole (SNSeP) as a standard molecule 1-(4-
nitrophenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole [46] were previ-
ously described in the literature.
ꢁ
hexane) and finally was dried at 60 C in vacuum oven. Yield:
80%, 0.09 g.
ꢀ
1
FT-IR (cm ): 3093 (CeH aromatic); 2916 (CeH aliphatic); 1702,
1660 (C]O imide); 1508, 1387, (C]C aromatic); H NMR (CHCl
1
2.2. Synthesis of 6-bromo-2-[4-(2,5-di-2-thienyl-1H-pyrrole-1-yl)
3
-d,
phenyl]-5,6-dihidro-1H-benzo[de] isoquinoline-1,3(2H)-dione
ppm):
d
8.78, (d, 1H, Ar-H
g
); 8.43, (d, 1H, Ar-H
l
); 8.23, (t, 1H, Ar-H );
i
0
0
7
.88, (d, 2H, Ar ꢀ H ); 7.61, (d, 1H, Ar-H
m
); 7.45, (d, 2H, Ar ꢀ H );
ff
ee
0
0
4
-Bromo-1,8-naphtalicanhydride (0.533 g, 1.92 mmol), was
7.39, (d, 2H, Ar ꢀ H ); 7.32, (d, 2H, Ar ꢀ H ); 7,04, (d, 2H,
hh
aa
0
0
0
added into 40 ml dry pyridine and refluxed under argon atmo-
sphere for 30 min. After 30 min, [4-(2,5-di-2-thienyl-1H-pyrrole-1-
yl)phenyl]amine (0.155 g, 0.481 mmol) was added by small por-
Ar ꢀ H ); 6,93, (d, 2H, Ar ꢀ H ); 6.61, (d, 2H, Ar ꢀ H ); 3.46, (m,
cc
bb
dd
13
4H, NeCH
2
); 2.24e1.22, (m 16H, eCH
2
e); 0.86, (t, 6H, eCH
3
);
C
3
NMR (CHCl -d, ppm): 157.77, 157.01, 144.85, 136.92, 134.25, 134.07,
132.78, 132.01, 130.91, 128.44, 127.36, 123.66, 121.81, 119.08, 118.88,
ꢁ
tions and this mixture was stirred at 120 C for 8 h. After that the
reaction mixture was cooled to room temperature and poured into
HCl aqueous solution (3 M, 500 ml). The resulting precipitate
116.76, 115.44, 109.22, 103.77, 51.34, 31.77, 29.55, 27.71, 22.55, 13.96;
þ
MALDI-ToF (m/z): [M ] calcd. for C42
685.83.
43 3 2 2
H N O S , 685.94; found,
(
3
1.11 g) was collected by filtration, washed with NaHCO solution

E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
6285
Hd
Hd'
2.4. Synthesis of poly-6-(dihexylamine)-2-[4-(2,5-di-2-thienyl-1H-
pyrrole-1-yl)phenyl]-1H-benzo[de]isoquinoline-1,3(2H)-dione
Hc
Hc'
(poly-SNSeNI)
Hb
Hb'
N
N
Electrochemical polymerization was carried out from a CH
3
CNe
S
S
ꢀ3
CH
2
Cl
2
(2/1; v/v) solution of 2.0 ꢂ 10 M SNSeNI and 0.1 M TBAPF
6
He
He'
Ha
Ha'
by repetitive cycling at a scan rate of 100 mV/s at a potential be-
tween 0 and 1.2 V for 50 times. The polymer was coated on plat-
inum (0.02 cm ) or indiumetin oxide (ITO, 8e12 U, 0.8 cm ꢂ 5 cm).
2
Hf
O
Hf'
O
A platinum wire was used as a counter electrode and Ag wire as a
reference. An electrochemically prepared polymer film of SNSeNI
on ITO/glass surface (Fig. 1a) was dedoped electrochemically in
monomer free-electrolyte solution by applying ꢀ0.4 V for 1 min.
After washing with acetonitrile to remove the unreacted mono-
mers and/or oligomeric species, the polymer film was dissolved in
chloroform (Fig. 1b). This operation was repeated for ten times and
obtained polymer (0.05 g) was used in structural optical and elec-
trochemical characterizations.
Hg
Hl
Hh
Hý
N
Hm
ꢀ
1
FT-IR (cm ): 3086, 3061 (CeH aromatic); 2918 (CeH aliphatic);
1
1
699, 1654 (C]O imide); 1504, 1412, 1366, (C]C aromatic);
NMR (CHCl -d, ppm): 8.77, (d, 1H, Ar-H ); 8.41, (d, 1H, Ar-H ); 8.24,
); 7.86e6.52, (m, 13H, Ar-H); 3.44, (m, 4H, NeCH );
.22e1.21, (m 16H, eCH e); 0.84, (t, 6H, eCH ); GPC: M 13,480;
H
3
d
g
l
(
2
t, 1H, Ar-H
i
2
2
3
w
PDI: 1.26.
CH
3
CH
3
Fig. 1. a) Structure of the SNSeNI polymer, b) electrochemically prepared SNSeNI polymer film on ITO glass surface, c) SNSeNI polymer film in CH
polymer film in dichloromethane under 366 nm.
2 2
Cl under daylight, d) SNSeNI

6286
E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
NH2
O
S
(1)
S
Cl
(a)
O
O
+
+
Cl
S
O
NO 2
S
S
(b)
a) AlCl , CH Cl , r.t., 24h
3
2
2
N
b) Toluene, PTSA, reflux, 24h
c) Pd/C, N H OH, reflux, 24h
2
5
S
S
d) pyridine, Zn(CH COO) , reflux, 24h
N
3
2
O
N
O
e) dihexylamine, DMA, CuI, K CO , 18-crown-6
(5)
2
3
(
2)
N
(
e)
NO 2
(
c)
S
S
CH3
O
CH3
N
S
S
O
O
N
(
d)
O
N
O
+
(
3)
(
4)
Br
NH 2
Br
Scheme 1. Synthetic route to SNSeNI donoreacceptor monomer.
2.5. Instrumentation
g
The optical band gap (E ) of products was calculated from their
absorption edges [1]. Fluorescence spectra were recorded on a PTI
QM1 fluorescence spectrophotometer.
AFM measurements were carried out at room temperature and
ambient conditions by using an Ambios QScope 250 instrument.
The non-contact mode (wave mode) was used to take topographic
FT-IR spectra were recorded by a Perkin Elmer FT-IR Spectrum
ꢀ1
1
13
One by using an ATR system (4000e650 cm ). H and C NMR
ꢁ
(
Bruker Avance DPX-400) data were recorded at 25 C by using
CHCl3-d as a solvent and TMS as an internal standard. The number-
average molecular weight (M ), weight average molecular weight
) and polydispersity index (PDI) values were determined via gel
n
images. A 20 mm scanner equipped with silicon tips with 10 nm tip-
(M
w
curvature and an ITO coated glass substrate was used for mea-
surements. The system is covered with an acoustic chamber to
prevent electromagnetic noises which may affect the
measurements.
Spectroelectrochemical measurements were carried out to use
absorption spectra of this polymer film under applied potential
[48]. The spectroelectrochemical cell includes a quartz cuvette, an
Ag wire (RE), Pt wire counter electrode and ITO/glass as a trans-
parent working electrode (WE). These measurements were carried
permition chromatography by using Agilent GPC-addon. For GPC
ꢀ
measurements, an SGX (100 A and 7 nm diameter loading material)
3
.3 mm i.d. ꢂ 300 mm as a column, THF as the eluent, polystyrene
ꢁ
as a standard and a refractive index as a detector (at 25 C) were
used.
Electrochemical measurements were carried out using a Bio-
logic SP-50 electrochemical workstation. The electrochemical cell
consists of an Ag wire as a reference electrode (RE), a Pt wire as a
counter electrode and platinum disk as a working electrode (WE)
6
out in 0.1 M TBAPF as a supporting electrolyte in acetonitrile.
immersed in 0.1 M TBAPF
6
as the supporting electrolyte. All mea-
Colorimetry measurements were performed by using an Ana-
lytic Jena Specord S600 UVeVis spectrophotometer, which con-
sisted of a chromameter module (standard illuminator D65, field of
surements were carried out under argon atmosphere. To calculate
the HOMOeLUMO energy levels, oxidation and reduction potential
onsets of SNSeNI monomer and polymer were used and the po-
tentials were calibrated to the ferrocene redox couple E(Fc/
Fcþ) ¼ 0.48 V [47]. UVeVis absorption spectra were measured using
an Analytic Jena Speecord S-600 diode-array spectrophotometer.
ꢁ
with 10 observer) with viewing geometry as recommended by CIE
[49]. According to the CIE system, the color is made up of three
attributes; luminance (L), hue (a), and saturation (b). Platinum co-
balt DIN ISO 621, iodine DIN EN 1557, and Gardner DIN ISO 6430 are

E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
6287
6 3 2 2
Fig. 2. Repeated potential scans of (a) SNSeNI and (b) SNSeP in 0.1 M TBAPF in CH CN/CH Cl (2/1; v/v), scan rate 100 mV/s.
1
the references of colorimetric measurement. These parameters
were measured for the neutral and oxidized states of the polymers
on the ITO/glass surface.
compounds were also fully identified by using FT-IR and H NMR
13
spectra. Moreover, SNSeNI monomer was also identified from
NMR and MALDI-ToF results. On the other hand, according to GPC
analysis, the number-average molecular weight (M ), weight-
average molecular weight (M ) and polydispersity index (PDI) of
the poly(SNSeNI) were found to be 10,650; 13,480; 1.26, respec-
tively. The M value corresponded to degree of polymerization of 10
C
n
3
. Results and discussion
w
3.1. Synthesis and characterization
n
and thus poly(SNSeNI) exhibits narrow molecular weight distri-
bution (Scheme 1).
The initial compounds (1, 2 and 3) and SNSeP as a standard
molecule were synthesized and fully characterized according to the
literature [43e46]. The compound 4 was synthesized from a
condensation reaction between 3 and 4-bromo-1,8-naphthalic
anhydride in the presence of pyridine. The monomer, SNSeNI (5),
was synthesized from the reaction between 4 and dihexylamine in
Repeated scan electrochemical polymerization behavior of a
ꢀ3
solution of 2.0 ꢂ 10 M of SNSeNI and SNSeP standard molecule
were examined by using both on the platinum disc and/or ITO/glass
6 3 2 2
surface in 0.1 M TBAPF /CH CNeCH Cl (2/1; v/v) as a supporting
electrolyte. Potentiodynamic electrochemical polymerization of
SNSeNI was carried out by using repetitive cycling at a potential
between 0 and 1.2 V. A new redox couple with a half wave potential
(E) of 0.67 V was observed and the reversible peaks increased after
each successive cycle which clearly indicates the formation of
2 3
the presence of DMA, K CO , CuI and 18-Crown-6. After the syn-
thesis of monomer, the polymer of SNSeNI was prepared by using
electrochemical process and then easily obtained from the film
surface via solubility in chloroform. The structures of all
2 2
Fig. 3. UVeVis absorption and fluorescence spectra of (a) SNSeP, (b) SNSeNI, (c) poly-SNSeNI in CH Cl . and (d) poly-SNSeNI on transparent film surface.

6288
E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
Table 1
HOMO and LUMO energy levels, electrochemical ðE Þ and optical band gaps (E
0
g
g
) values of SNSeP, poly-SNSeP, SNSeNI and poly-SNSeNI.
0
Molecules
Reduction potential (V) Oxidation potential (V) HOMO (eV) LUMO (eV) E , electrochemical band gap (eV)
g
E , optical band gap (eV)
g
ox
a
e
E
E
¼ 0:71
¼ 0:61
ꢀ4.95
ꢀ4.69
ꢀ1.70
ꢀ2.45
e
3.25
2.24
m;a
ox
m;c
Semi-reversible
ox
E
¼ 0:51
m;on
ox
a
e
E
E
¼ 0:58
¼ 0:50
e
p;a
ox
p;c
Semi-reversible
ox
E
¼ 0:25
p;on
red
ox1
p;a
E
E
¼ ꢀ1:38
¼ ꢀ1:30
E
E
¼ 0:81
¼ 0:52
ꢀ5.09
ꢀ3.19
1.90
2.44
m;c
red
m;a
ox1
p;c
Reversible
Semi-reversible
red
ox
E
¼ ꢀ1:25
E
¼ 0:65
m;on
p;on
red1
p;c
ox1
p;a
E
E
E
E
¼ ꢀ1:41
¼ ꢀ1:35
¼ ꢀ1:55
¼ ꢀ1:49
E
E
¼ 0:56
¼ 0:50
ꢀ4.75
ꢀ3.14
1.61
2.27
red1
p;a
ox1
p;c
red2
p;c
Reversible
red2
ox
E
¼ 0:31
p;a
p;on
Reversible
red
E
¼ ꢀ1:30
p;on
a
Calculated by the addition of the optical band gap to the HOMO level.
polymer on the working electrode surface (Fig. 2a). Due to the
electron withdrawing effect of the naphthalimide moiety, electro-
chemical polymerization of SNSeNI requires higher potentials
compared with SNSeP as a standard molecule. Because of this ef-
fect, electrochemical polymerization of SNSeP was carried out by
repetitive cycling at a potential between 0 and 1.0 V and exhibited a
new redox couple with a half wave potential (E) of 0.54 V (Fig. 2b).
Prepared polymer films on ITO/glass surface were dedoped elec-
trochemically in monomer free-electrolyte solution to equilibrate
the redox behavior after washing with acetonitrile to remove the
unreacted monomers and/or oligomeric species. Then, poly(SNSe
NI) on the ITO glass surface can be highly dissolved in chloroform.
The solution was also used for the spin casting process of the
polymer. It is concluded that the good solubility is also advanta-
geous for the other optoelectronic applications such as OPVs,
OLEDs, OFETs.
3.2. Optical properties
The optical properties of SNSeP, SNSeNI and its electrochemical
polymerization product [poly(SNSeNI)] were investigated by UVe
Vis absorption and fluorescence spectroscopy recorded in both
dichloromethane as a liquid phase and on ITO/glass surface as a
solid state (Fig. 3). In the UVeVis spectrum of SNSeP, characteristic

E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
6289
triple bands at about 242, 285 and 325 nm attributed to SNS de-
rivatives were clearly observed (Fig. 3a) [50]. On the other hand, in
the UVeVis spectrum of SNSeNI, the bands at about 330 and
3
45 nm were attributed to transitions of the naphthalimide moiety
[51]. Further, a new charge transfer band at about 405 nm was
attributed to a strong interaction between the donor tert-amine
moieties to the directly attached naphthalimide-acceptor chro-
mophore. Due to increasing
p p conjugation over the SNS polymer
ꢀ
backbone, this band greatly intensified and became bathochromi-
cally shifted. It is concluded that this charge transfer band of SNSe
NI was overlapped with the conjugated SNS moiety absorption
band of poly(SNSeNI). On the other hand, the low energy ab-
sorption band of poly(SNSeNI) on the ITO/glass surface was
observed to be more broadened than that of the liquid phase owing
to stronger
p p interactions in the solid state. The optical band gap
ꢀ
values of all compounds, calculated from maximum absorption
band edges of solid state absorptions are given in Table 1.
Considering fluorescence spectra of all compounds, electronic
interactions between SNS-donor and naphthalimide acceptor are
discussed by comparison to UVeVis absorption and electro-
chemical measurements. When SNSeP as a standard molecule was
excited at 325 nm, a blue (in web version) emission band was
observed at 420 nm (Fig. 3a). On the other hand, a bathochromic
shift was observed in the fluorescence spectrum of SNSeNI. An
explicit difference in emission behaviors of the SNS-P and SNSeNI
can be deduced from the attached naphthalimide chromophore to
the SNS main chain. Besides, poly(SNSeNI) has a strong emission
band at the same wavelength (510 nm) with a shoulder at 548 nm
Fig. 4. Cyclic voltammogram of (a) SNSeP (b) SNSeNI and (c) poly-SNSeNI in 0.1 M
TBAPF
6
in CH
2
Cl
2
, scan rate 100 mV/s.
ox
ox
ox
at E
¼ 0:81V and E
¼ 0:51V; E
¼ 0:66V vs. Ag wire, and
m;a
m;c
m;1=2
this oxidation potential is higher than that of SNSeP (semi
reversible; E
Ag wire) (Fig. 4a and b). According to this result, it can be deduced
that the SNSeP backbone has a more electron rich nature than
SNSeNI which contains naphthalimide as an electron withdrawing
moiety in a similar system. Due to extended conjugation after
ox
ox
ox
¼ 0:71V and E
¼ 0:61V; E
¼ 0:66V vs.
m;a
m;c
m;1=2
when excited at its absorption maxima (
l
max ¼ 448 nm) (Fig. 3c).
This increase effect was also observed in the oxidative polymeri-
zation of pyrene by Li et al. [52]. The results proved that electron
transfer at the excited state from the LUMO of the poly-SNS-donor
main chain to the LUMO of naphthalimide-acceptor subunits. On
the other hand, the emission band of poly(SNSeNI) was slightly
polymerization, the oxidation potentials were observed as revers-
ox
ible and at lower potentials (reversible;
E
¼ 0:56V and
m;a
quenched on the transparent film surface because of a strong
p
ep
ox
ox
E
¼ 0:50V; E
¼ 0:53V vs. Ag wire) when compared to the
m;c
m;1=2
interaction along the polymer backbone in the solid state. There-
fore, as a result of containing both electro-donor and electro-
acceptor moieties and also having strong emission and good solu-
bility, poly(SNSeNI) has the potential to be used in single compo-
nent OLEDs.
corresponding SNSeNI monomer (Fig. 2c). On the other hand, in
the cathodic regime, the cyclic voltammogram of SNSeNI exhibits a
red
characteristic reversible reduction couple with potentials of E
¼
m;c
red
red
ꢀ1:38V and E
¼ ꢀ1:30V; E
¼ ꢀ1:34V which reflects the
m;a
m;1=2
one electron stepwise reduction process of the naphthalimide
moiety. Upon comparison to the monomer, the reduction couple of
red
poly(SNSeNI) slightly shifted negatively (E
¼ ꢀ1:41V and
3
.3. Electrochemical properties
p;c
red
red
E
¼ ꢀ1:35V; E
¼ ꢀ1:38V). In addition, the HOMOeLUMO
p;a
m;1=2
energy levels of all compounds were calculated according to the
onset values of their oxidation and reduction potentials. The LUMO
energy level of SNSeP was calculated by adding the optical band
gap from their HOMO levels (Table 1). As a result of the data in
Table 1, it can be concluded that the optical and electrochemical
properties of the molecules were greatly influenced by the addition
The electrochemical properties of SNSeP as a standard molecule
and SNSeNI monomer and the corresponding polymerepoly(SNSe
NI) were examined by cyclic voltammetry (CV). An explicit differ-
ence in redox behavior of the SNSeP and SNSeNI can be deduced
from the CV which is depicted in Fig. 4. During CV scanning in the
anodic regime, SNSeNI exhibited a semi-reversible oxidation peak
Fig. 5. AFM images of (a) poly (SNSeNI)-1 prepared via electrochemical process and (b) poly(SNSeNI)-2 prepared via spin casting process.

6290
E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
6 3
Fig. 6. Electronic absorption spectra and the color changes of (a) poly-SNSeP and (b) poly-SNSeNI on ITO/glass surface in 0.1 M TBAPF /CH CN at various applied potentials.
of the electron-withdrawing naphthalimide as a subunit to the SNS
main chain.
surface) roughness was found to be 49.8 nm. The film surface can
be greatly affected by the structure of the polymer system and
preparing method. Poly(SNSeNI)-1 shows a uniformly dispersed
nodular structure. The AFM image of this polymer film exhibits an
agglomerated non-uniform structure (Fig. 5a). On the other hand,
the second poly(SNSeNI) film prepared via the spin casting process
is better than that of the electrochemically prepared film (Fig. 5b).
Electrodeposited polymers can produce randomly grown nodular
morphologies whereas on comparison, the spin casting process can
be seen as a more controllable method. The RMS (root mean sur-
face) roughness which was found to be 22.6 nm is two times lower
than that of electrochemically prepared film. When we investigated
3.4. Surface morphologies
The morphologies of film surfaces of poly(SNSeNI) were studied
by AFM. We compared morphologies of poly(SNSeNI) film pre-
pared by both electrochemical and spin casting processes. Both of
the film thicknesses were arranged as approximately 100 nm
(Fig. 5). Fig. 5a exhibits the images of a poly(SNSeNI)-1 film pre-
pared using the electrochemical method. As typical for coating by
this method, rough films were obtained. The RMS (root mean
Fig. 7. Electrochromic switching, optical absorbance monitored at 410 and 890 nm for poly-SNSeP (a) and poly-SNSeNI (b) films between 0 and 1.0 V.

E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
6291
Table 2
Electrochromic parameters for the poly-SNSeP and poly-SNSeNI.
ꢀ1
Molecules
Optical contrast change (
10 nm 890 nm
T: 18% T: 23%
D
T%)
Response time (s) Optical activity
after 5000 cycles (%)
CIE color parameters
Coloration efficiency, (cm2
C
)
4
Luminance (L); hue (a); saturation (b)
Poly-SNSeP
D
D
Oxidation
4.1
44.2
Neutral state
L: 61; a: 5; b: 49
Oxidized state
89
T
T
neut: 61%
oxi: 79%
T
T
neut: 95%
oxi: 72%
Reduction
2
.2
L: 56; a: ꢀ6; b: ꢀ10
Poly-SNSeNI
D
T%: 26%
DT%: 56%
Oxidation
72.4; 98.6a
Neutral state
258; 299a
L: 64; a: 6; b: 43
Intermediate state
L: 46; a: ꢀ11; b: 31
Oxidized state
T
T
neut: 41%
oxi: 67%
T
T
neut: 92%
oxi: 36%
2.1; 1.2a
Reduction
a
1.4; 0.6
L: 30; a: ꢀ4; b: ꢀ18
a
These values attributed to SNSeNI polymer film prepared via spin coating process.
the electrochromic performance of both films, it was observed that
the film preparing method has a profound effect on the electro-
chromic performance.
spin coating process has a better performance than that of the
electrochemically prepared film (poly(SNSeNI)-2). The oxidation
and reduction response time decreased from 2.1 s to 1.4 s (for
poly(SNSeNI)-1) to 1.2 and 0.6 s (for poly(SNSeNI)-2), respec-
tively. While the optical activity of the poly(SNSeNI)-1 film was
only retained by 72.4%, the value for poly(SNSeNI)-2 was pre-
served by a factor of 98.6% even after 5000 cycles of operation. On
the other hand, the coloration efficiency (CE) is one of the most
important parameters for the electrochromic application. CE was
3.5. Spectroelectrochemical properties
The spectro-electrochemical properties of the SNSeNI film were
investigated using a doping process (Fig. 6) and compared to pol-
y(SNSeP) as a standard in order to get further support for the effect
of the naphthalimide electron-withdrawing moiety. When a posi-
tive potential was applied to the poly(SNSeNI) film, the intensity of
calculated by using CE ¼
DOD/Q
d
and
DOD ¼ log(Tcolored/Tbleached),
where Q is the injected/ejected charge between neutral and
d
the valance band-conduction band (
55 nm decreased between 0.0 and 1.2 V. During the oxidation
process, the band intensified at 750 nm (between 0 and 0.7 V) and
95 nm (between 0.7 and 1.2 nm) which indicated the formation of
polarons on the SNS polymer backbone and thus the yellow (in web
version) color of the film (L: 64; a: 6; b: 43) in the neutral state
turned green (in web version) (L: 46; a: e11; b: 31) and dark blue
p
ꢀ
p
* transition) at about
oxidized states, Tcolored and Tbleached are the transmittance in the
oxidized and neutral states, respectively. Using these equations,
while switching the polymer film (coated onto ITO glass surface
4
2
8
with an active area of 1 cm ), CE of poly(SNSeP) and poly(SNSe
2
ꢀ1
NI)-1 was determined to be 89 and 258 cm C respectively. The
film forming method also improves the coloration efficiency. The
coloration efficiency of the poly(SNSeNI)-2 film was found to be
2
ꢀ1
(
in web version) (L: 30; a: e4; b: ꢀ18), respectively. Thus, in the NIR
region, above 700 nm, strong absorption due to the fully oxidized
state was observed. On the other hand, the intensity of the
transition band of poly(SNSeP) (
max ¼ 411 nm) decreased, and the
absorption band between 500 and 1000 nm increased between
and 1.0 V. Thus all visible and near-IR regions were absorbed by
299 cm C (Table 2). These results clearly show that the CE value
of poly(SNSeNI) is approximately three times greater than that of
poly(SNSeP) as a standard molecule. By consideration of all the
data, it can be clearly seen that the electrochromic performance of
the polymer was greatly affected by the electron-withdrawing
naphthalimide unit and the film preparation method. It can be
concluded that the enhanced performance of poly(SNSeNI) is due
to its self-aggregation by the spin coating process and the electron-
withdrawing effect of the naphthalimide moiety on the SNS-donor
structure. Finally, the poly (SNSeNI) film having a yellow color in
the neutral state exhibits multielectrochromic behavior, low
response time, reversible redox behavior, resistance to over
oxidation and low driving voltage. Because of all these properties, it
can reasonably be concluded that this material can be useful for
electrochromic applications.
p p*
ꢀ
l
0
the poly(SNSeP) film in the fully oxidized state. The orange (in web
version) color of the film with CIE color parameters (L: 61; a: 5; b:
4
(
9) turned into dark gray (L: 56; a: ꢀ6; b: ꢀ10), upon oxidation
Fig. 6).
In order to monitor the changes in the electro-optical responses
during electrochemical switching, we used the double step chro-
noamperometry technique. The electrochromic parameters of the
polymer films were investigated by the changes that occurred in
the transmittance (increments and decrements of the absorption
band at 410 and 890 nm with respect to time) while switching the
potential step wisely between neutral and oxidized states with a
residence time of 10 s (Fig. 7). The percentage transmittance
4. Conclusion
changes (
D
T%) of poly(SNSeP) between neutral (at 0 V) and
Herein, we have reported solution processable neutral state
yellow SNSenaphthalimide type donor acceptor materials. The
optical and electrochemical properties were greatly influenced by
the presence of a naphthalimide acceptor as a subunit attached on
the SNS-donor structure. In addition, the electrochromic perfor-
mance of the polymer film was greatly influenced by the attached
naphthalimide unit and the film preparation method. In compari-
son to the SNSeP standard reference and an electrochemically
prepared polymer film of SNSeNI, the polymer film prepared via
the spin coating process was determined to have a much better
electrochromic performance in terms of multi-electrochromic
behavior, high resistance to over oxidation, high contrast ratio in
the near-IR region and a fast response time. Owing to these useful
oxidized states (at 1.0 V) were found to be 18% for 410 nm and 21%
for 890 nm. For poly(SNSeNI), these values were determined as
2
6% for 410 nm and 56% for 890 nm. It is clearly seen that the
contrast change ability was greatly improved by attaching naph-
thalimide as an acceptor subunit on the SNS main chain. In addi-
tion, the oxidation and reduction response time and optical activity
against switching were compared for the poly(SNSeP) film and the
poly(SNSeNI)-1 prepared by the electrochemical process and
poly(SNSeNI)-2 prepared by the spin casting process. The oxida-
tion and reduction response times were measured as 4.1 and 2.2 s
for poly(SNSeP). On the other hand, when we compare with the
film preparation method, the poly(SNSeNI)-2 film prepared via the

6292
E. Oguzhan et al. / Polymer 54 (2013) 6283e6292
properties, poly(SNSeNI) is a good candidate for electrochromic
applications. Furthermore, containing both electron-donor and
electron-acceptor moieties, and having strong green emission and
good solubility, poly(SNSeNI) may have applications in single
component OLEDs.
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