A novel donor-acceptor polymeric electrochromic material containing carbazole and 1,8-naphtalimide as subunit

Article abstract of DOI:10.1016/j.electacta.2010.03.094

We report here the synthesis of a novel polymeric electrochromic material containing carbazole (Cbz)-donor and 1,8-napthalimide-acceptor as subunit. The band gap E_g was measured using UV-vis spectroscopy and compared with that obtained by cyclic voltammetry (CV). Due to intramolecular electron transfer from Cbz-donor to 1,8-napthalimide-acceptor, the fluorescence quenching was observed. When the spectro-electrochemical and electrochromic properties of polymer film were investigated, various tones of green color were obtained on the polymeric film. In the positive regime, the polymer film obtained thereby is dark green resulting from the association of carbazolylium cation radicals at oxidized state and then it can be bleached by electrochemical reduction. Besides, in the negative regime, yellowish green color of film converted to blue attributed to reduction of the 1,8-napthalimide moiety. Finally, the polymeric electrochromic exhibits multi-electrochromic behavior, high redox stability, high coloration efficiency and reasonable response time.

Full text of DOI:10.1016/j.electacta.2010.03.094

Electrochimica Acta 55 (2010) 4935–4941
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
A novel donor–acceptor polymeric electrochromic material containing
carbazole and 1,8-naphtalimide as subunit
a,∗
b
a,∗
Fatma Baycan Koyuncu , Sermet Koyuncu , Eyup Ozdemir
a
Department of Chemistry, Faculty of Sciences and Arts, C¸ anakkale Onsekiz Mart University, 17020 C¸ anakkale, Turkey
C¸ an Vocational School, C¸ anakkale Onsekiz Mart University, 17400 C¸ anakkale, Turkey
b
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here the synthesis of a novel polymeric electrochromic material containing carbazole
Received 9 February 2010
Received in revised form 25 March 2010
Accepted 30 March 2010
Available online 3 April 2010
(
Cbz)-donor and 1,8-napthalimide-acceptor as subunit. The band gap Eg was measured using UV–vis
spectroscopy and compared with that obtained by cyclic voltammetry (CV). Due to intramolecular elec-
tron transfer from Cbz-donor to 1,8-napthalimide-acceptor, the fluorescence quenching was observed.
When the spectro-electrochemical and electrochromic properties of polymer film were investigated, var-
ious tones of green color were obtained on the polymeric film. In the positive regime, the polymer film
obtained thereby is dark green resulting from the association of carbazolylium cation radicals at oxidized
state and then it can be bleached by electrochemical reduction. Besides, in the negative regime, yellowish
green color of film converted to blue attributed to reduction of the 1,8-napthalimide moiety. Finally, the
polymeric electrochromic exhibits multi-electrochromic behavior, high redox stability, high coloration
efficiency and reasonable response time.
Keywords:
Carbazole
1
,8-Napthalimide
Donor–acceptor polymer
Electrochromic polymer
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
-Conjugated polymers have attracted much attention because
cancer agents [15] and analgesics in medicine [16], photo-induced
electron transfer sensors [17,18], electroluminescent materials
[19,20], liquid crystal displays [21,22], light emitting diodes [23,24],
fluorescence switchers [25,26], and ion probes [27]. Moreover,
these properties are essential when employing such devices in real-
time and on-line analysis.
␲
of their promising electronic, opto-electronic and electrochemi-
cal properties [1]. These polymers have various application areas
such as photovoltaic devices, electroluminescent displays, emis-
sive layer in full-color video matrix displays, field-effect transistors,
active thin layer of various sensing devices, supercapacitors and
electrolytic-type capacitors, corrosion inhibitors, “smart windows”
that absorb sunlight in summer [2]. Among the ␲-conjugated
polymers, those containing carbazole deserve a great attention.
Due to its superior properties such as exhibit high thermal
and photochemical stability, easily be functionalized at its (3,6-
Electrochromism can be defined as a reversible color change in a
material caused by an applied voltage. The first studies about elec-
trochromic materials started with inorganic semiconductors such
as tungsten trioxide (WO ) and iridium dioxide (IrO ), and then
3
2
organic small molecules such as viologens, metallophthalocyanines
and finally conducting polymers, received much attention for elec-
trochromic applications [28]. Among these molecules, conducting
polymers have potential for structurally controllable HOMO–LUMO
band gap by means of easily tuning of colors and also processi-
bility, high contrast ability and fast switching time [29]. The fist
report about electropolymerization of carbazole was published by
Ambose and Nelson [30], and also electrochromic properties of
poly-N-vinylcarbazole was studied by Monvernay et al. [31] in
1980s and Chevrot et al. [32–34] in 1990s.
In the most of the researches onto electrochromic materials,
donor and acceptor moieties are conjugated to the main chain of
the polymers [35–37]. In this study, we synthesized a novel donor
carbazole monomer (2-[4-(9H-carbazole-9-yl)phenyl]-1H-benzo-
izoquinoline-1,3(2H)-dione) containing 1,8-naphalimide acceptor
as pendant unit in three steps and then electro-polymerized onto
transparent ITO/glass surface to give multi-electrochromic polymer
film having reasonable redox stability, high coloration efficiency
)
, (2,7-) or N-positions to help polymer solubility and to tune
the optical and electrical properties, forming relatively stable
polarons (radical cations), photoconductivity, photorefractivity,
and electroluminescence, used as hole transport and blue emis-
sion materials for opto-electronic devices [3–7]. On the other hand
1
,8-naphthalimide derivatives as n-type semiconductors with rel-
atively high electron affinity and excellent transport property have
found application in a number of areas [8] including potential
photosensitive biological units [9], coloration of polymers [10,11],
fluorescent markers in biology [12], laser active media [13,14], anti-
∗ Corresponding authors. Tel.: +90 286 2180018; fax: +90 286 2180533.
E-mail addresses: fatmabaycan@hotmail.com (F.B. Koyuncu),
eozdemir@comu.edu.tr (E. Ozdemir).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.03.094

4
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F.B. Koyuncu et al. / Electrochimica Acta 55 (2010) 4935–4941
and reasonable response time. Owing to donor and acceptor moi-
ety independent from each other, various tones of green color
were obtained on the polymeric film upon applied positive poten-
tial. Further, intramolecular donor–acceptor interaction of the
CNI monomer and its polymer were investigated by using cyclic
voltammetry, UV–vis absorption and fluorescence spectroscopy.
0.1 M TBAPF6 by repetitive cycling at a scan rate of 100 mV/s.
The polymer was coated on platinum (0.02 cm ) or indium–tin
oxide (ITO, 8–12 ꢁ, 0.8 cm × 5 cm). A platinum wire was used as
a counter electrode and Ag wire as a reference. Electrochemically
prepared polymer film of CNI on ITO/glass surface was dedoped
electrochemically in monomer free-electrolyte solution and then
2
partly dissolved in CH Cl2 by using ultrasonic bath.
2
2
. Experimental
2.4. Instrumentation
2.1. Materials
FT-IR spectra were recorded by a Perkin Elmer FT-IR Spec-
Carbazole
(Aldrich),
1,8-naphtalicanhydride
(Aldrich),
−1
1
trum One by using ATR system (4000–650 cm ). H NMR (Bruker
hidrazinium hydroxide (Merck), Cu (Merck), K CO3 (Merck),
◦
2
Avance DPX-400) data were recorded at 25 C by using CHCl -d as
3
nitrobenzene (Merck), ethanol (Merck), pyridine (Merck), imida-
zole (Merck), 18-Crown-6 (Aldrich) and palladium on activated
carbon (10%, w/w) (Fluka) were used without further purification.
The syntheses and the characterizations of 9-(4-nitrophenyl)-9H-
carbazole (1) and 4-(9H-carbazol-9-yl)aniline (2) were previously
described by Koyuncu et al. [38]. Besides, 9-N-phenylcarbazole
solvent and TMS as internal standard.
Electrochemical measurements were carried out by Bio-Logic
SP50 potentiostat/galvanostat system. The electrochemical cell
consists of an Ag wire as reference electrode (RE), Pt wire as
counter electrode (CE) and platinum as working electrode (WE)
immersed in 0.1 M TBAPF6 as the supporting electrolyte. All mea-
surements were carried out under argon atmosphere. To calculate
the HOMO–LUMO energy levels, oxidation and reduction potential
onsets of CNI monomer and polymer were used and the potentials
(
NPC) and N-phenyl-1,8-napthalimide (NPN) were synthesized via
similar procedures.
2
.2. Synthesis of (2-[4-(9H-carbazole-9-yl)phenyl]-1H-benzo-
izoquinoline-1,3(2H)-dione)
CNI)
◦
+
were calibrated to the ferrocene redox couple E (Fc/Fc ) = +0.41 V
39]. UV–vis absorption spectra were measured by Analytic Jena
[
(
Speedcord S-600 diode-array spectrophotometer. The optical band
gap (Eg) of products was calculated from their absorption edges
4
-(9H-Carbazol-9-yl)aniline (2) (1.03 g, 4 mmol), 1,8-
[
40]. Fluorescence spectra were recorded on a PTI QM1 fluorescence
naphtalicanhydride (0.79 g, 4 mmol), and Zn(CH COO) ·H O
3
2
2
spectrophotometer.
(
0.2 g, 1 mmol) were added into 30 ml dry pyridine and refluxed
Spectro-electrochemical measurements were carried out to use
absorption spectra of this polymer film under applied voltage. The
spectro-electrochemical cell includes a quartz cuvette, an Ag wire
under argon atmosphere. After 6 h, this mixture was cooled to
room temperature and poured into HCl solution (2 M, 250 ml). The
resulting precipitate (1.11 g) was collected by filtration, washed
with NaHCO3 solution (100 mL) and water (50 mL), and then the
residue was crystallized from chloroform-hexane (1:1; v:v). Yield:
.31 g, 75%, light brown product. UV–vis (ꢀmax)(CHCl ): 258, 284,
96, 345. IR (cm ): (C–H, aromatic) 3059; (C O imide) 1704,
666; (C C aromatic) 1552, 1487. H NMR (CHCl -d): ı ppm, 8.75
(RE), Pt wire counter electrode (CE) and ITO/glass as transparent
working electrode (WE). These measurements were carried out in
the 0.1 M TBAPF6 as supporting electrolyte in CH CN.
3
1
2
1
(
6
3
Colorimetry measurements were performed by using Konica
Minolta CS-200 chromometer with viewing geometry as recom-
mended by CIE. According the CIE system, the color is made up of
three attributes; luminance (L), hue (a), and saturation (b). These
parameters were measured at neural, intermediate, fully oxidized
and reduced state of the electrochromic polymer on the ITO/glass
surface.
−1
1
3
ꢀ
ꢀ
ꢀ
ee
d, 4H, Ar − H ); 8.35 (d, 4H, Ar − H ); 8.18 (d, 4H, H ); 7.90 (t,
ee
ii
ꢀ ꢀ ꢀ ꢀ
H, H ); 7.85 (d, 4H, H ) 7.63 (d, 4H, H ); 7.59 (d, 4H, H ); 7.51
bb ff dd
aa
ꢀ
ꢀ
(
t, 6H, H ); 7.36 (t, 6H, H ) (Scheme 1).
gg
hh
2
.3. Synthesis of poly-(2-[4-(9H-carbazole-9-yl)phenyl]-1H-
benzo-izoquinoline-1,3(2H)-dione)
3. Results and discussion
(
PCNI)
3
.1. Synthesis and characterization
Electrochemical polymerization was carried out from
CH CN–CH Cl (3/1; v/v) solution of 2.0 × 10−
3
M
CNI and
3
2
2
The initial compounds (1 and 2) were synthesized and fully
characterized by Koyuncu et al. in the previous studies [38]. The
final product, CNI (3), was synthesized from condensation reaction
between 2 and 1,8-napthalic anhydride presence of Zn(CH COO)2
3
and pyridine. Naphtalimide (NPN) and carbazole (NPC) standard
compounds were synthesized by the similar procedures. The struc-
tures of all compounds were also fully identified by using FT-IR and
1
H NMR spectra (Scheme 2).
The repeated scan electropolymerization behavior of a solution
−
3
of 2.0 × 10 M of CNI was examined by using both on the plat-
inum disc and ITO/glass surface in 0.1 M TBAPF /CH CN–CH Cl
2
6
3
2
(
0
3/1; v/v) as supporting electrolyte. With the cyclic scans between
ox
and 1.4 V, the new redox couple a half wave potential (E
) of
p,1/2
0
.97 V was occurred and then the reversible peaks were increased
after each successive cycle which clearly indicates the formation
of polymer on the working electrode surface (Fig. 1). The polymer
film of CNI on ITO/glass surface was also dedoped electrochem-
ically in monomer free-electrolyte solution and it can be partly
soluble in CH Cl2 by using ultrasonic bath for UV-absorption and
2
Scheme 1.
CV measurements.

F.B. Koyuncu et al. / Electrochimica Acta 55 (2010) 4935–4941
4937
Scheme 2. Synthetic route to CNI: (a) Cu, K2CO3, nitrobenzene, 24 h reflux; (b) Pd/C, hidraziniumhidroxide, EtOH, 24 h reflux; (c) pyridine, imidazole, 6 h reflux.
3.2. Electrochemical properties
were found to be 2.39 and 1.83 eV, respectively. Furthermore, the
dramatically decreasing of the band gap results from enlargement
to ␲–␲ conjugation of carbazole-donor moiety and donor–acceptor
interaction. According to these results, mechanism of electrochem-
ically oxidation and reduction is proposed in Fig. 2.
In order to determine electrochemical behavior and calculate
HOMO–LUMO energy gap of any organic compound, cyclic voltam-
metry (CV) is a useful method. During the cathodic scan region,
a cyclic voltammogram of CNI exhibits characteristic reversible
reduction peak with a half wave potential (Ered ) of −1.28 V
m,1/2
3.3. Optical properties
attributed to one-electron stepwise reduction process of naphthal-
imide moiety. In comparison to the NPN as a standard molecule, due
to donor–acceptor interaction, the reduction waves of CNI slightly
shifted positively.
The optical properties of the CNI and its electrochemical
polymerization product (PCNI) were investigated by UV–vis spec-
troscopy recorded in CH Cl2 (liquid phase) (Fig. 3). The UV–vis
2
In the positive regime, the CNI exhibits one irreversible oxi-
spectrum of CNI exhibits two main absorption peak at about 296
and 345 nm attributed to carbazole and naphthalimide moiety,
respectively. Due to having ␲–␲ conjugation over the polymer
backbone, a new broad band was observed at about 435 nm in
the polymer–liquid phase. On the other hand, all absorption bands
of PCNI on the ITO/glass surface was more broadened than liquid
phase due to strong ␲–␲ interactions in solid state. The optical
band gap values of CNI and PCNI calculated from maximum absorp-
tion band edge of liquid phase were found to be 3.40 and 2.44 eV,
respectively.
ox
dation peak (Em ) of carbazole moiety at 1.33 V. Due to the
1
,8-napthalimide unit on the CNI has an electron-withdrawing
character, oxidation of carbazole causes a positive shift on the
potential when compared to carbazole standard molecule (NPC).
At the end of the polymerization, the oxidation peak was broad-
ened and shifted to lower potential. Besides, the electrochemical
HOMO–LUMO band gap of CNI monomer and its polymer calcu-
lated from the onset potential of oxidation and reduction peaks
Fig. 4 shows fluorescence spectra of the NPN, CNI and PCNI
molecules in CH Cl . The lowest energy band of absorption spectra
2
2
of the CNI was the same as that of the NPN reference compound.
This proves that there are no intra- or inter-molecular interac-
tion between carbazole-donor and 1,8-napthalimide-acceptor in
the ground state. Besides, NPN reference compound exhibits two
emission bands in fluorescence spectra at ꢀmax = 351 and 367 nm
when exited at 345 nm. The fluorescence intensity of CNI monomer
was weak in comparison with that of NPN reference compound.
Further, due to donor character that increases with the polymer-
ization and thus forming the poly-conjugated backbone, polymer
emission band was also declined when exited at 345 nm. This pho-
toluminescence quenching probably occurs due to photo-induced
electron transfer between carbazole-donor and 1,8-naphtalimide-
acceptor in the exited state [41].
Fig. 1. Repeated potential scans of CNI between 0.0 and 1.4 V in 0.1 M TBAPF6 in
CH3CN/CH2Cl2 (3/1; v/v), scan rate 100 mV/s.

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F.B. Koyuncu et al. / Electrochimica Acta 55 (2010) 4935–4941
Fig. 3. UV–vis absorption spectra of (a) CNI (b) PCNI in CH2Cl2.
Fig. 2. Cyclic voltammogram of CNI (a) and polymer (b) in 0.1 M TBAPF6 in CH2Cl2,
scan rate 100 mV/s.
Finally, optical and electrochemical band gap values are not
in agreement for the CNI and its polymer since photoactive cen-
ter and electroactive center are independent from each other
in these molecules. Thus, these differences are supported to
a possible photo-induced electron transfer from the carbazole-
donor to 1,8-naphtalimide-acceptor in the exited state (Fig. 5 and
Table 1).
Fig. 4. Fluorescence spectra of (a) NPN reference (b) CNI and (c) PCNI in CH2Cl2
(
equal concentrations).
process, the band intensified at 825 nm indicated the formation of
polarons on the carbazole polymer backbone and thus yellowish
green color of the film (L: 69; a: −19; b: 48) at neutral state turned
into dark green (L: 34; a: −27; b: 21). Due to carbazole derivatives
are very stable in the polaronic state, the reduction process of the
oxidized PCNI film is slowly then the oxidation process.
3.4. Spectro-electrochemical properties
The electro-optical properties of PCNI film were investigated.
Upon the positive potential applied to PCNI film, the valance band-
conduction band (␲ → ␲* transition) at about 370 and 402 nm did
not change too much between 0.0 and 1.8 V. During the oxidation
It is very difficult to prepare of green polymer films which can
absorb in two different regions (blue and red) in electromagnetic
Fig. 5. Molecular orbital (MO) energy level diagram for CNI monomer and its polymer.

F.B. Koyuncu et al. / Electrochimica Acta 55 (2010) 4935–4941
4939
Table 1
ꢀ
HOMO and LUMO energy levels, electrochemical (E ) and optical band gap (Eg) values of NPC and NPN reference compounds and, CNI monomer and its polymer.
g
ꢀ
Molecule
Reduced group and peak
potentials (V)
Oxidized group and
peak potentials (V)
Carbazole (ring)
HOMO (eV)
LUMO (eV)
E g , electrochemical
Eg, optical band
gap (eV)
band gap (eV)
1,8-Napthalimide (C O)
a
–
1.29
−5.56
−2.02
–
3.54
b
−
−
1.32 and −1.38 reversible
–
−6.52
−3.13
–
3.39
1.25 and −1.31 reversible
1.33
−5.61
−3.22
2.39
3.40
−
1.21 and −1.31 reversible
1.02, 1.10 reversible
−5.29
−3.46
1.83
2.44
a
Calculated by the subtraction of the optical band gap from the HOMO level of NPC.
Calculated by the subtraction of the LUMO level from the optical band gap of NPN.
b
spectrum. Up to date, absorption of blue and red regions was suc-
ceeded only by donor and acceptor moieties conjugated to the main
chain of the polymers which have green color [35–37]. However,
on the synthesized structure, CNI, donor and acceptor moieties are
not conjugated with each other, so each moiety can act freely. 1,8-
Napthalimide absorbs the blue region and the band is not affected
by p-doping process. On the other hand, carbazole absorbs the
red region in the oxidized state. Therefore, many different tones
of green color could be observed on the ITO glass surface by the
oxidation of the neutral film (Fig. 6).
In the negative regime, anion radicals were formed on the car-
bonyl groups of the 1,8-naphthalimide moiety during the reduction
process. Upon applying potential between −1.0 and −1.6 V, both
intensity of the band at about 402 nm decreased and hypsochromi-
cally shifted. On the other hand, the new broad band at about
Switching ability of the PCNI film was extracted by analysis of
changes in transmittance of the absorption band at 825 nm with
respect to time while a constant switching the potential between
−1.0 and 1.8 V with a residence time of 10 s. Double potential step
chronoamperometry technique was carried out to estimate the
response time of the polymer film. During the switching, the %
transmittance at the wavelength of maximum contrast was mea-
sured by using a UV–vis spectrophotometer. The optical contrast
measured as the difference between neutral and oxidized states
(%ꢂT) were found to be 25% for 825 nm. While the oxidation
response time was calculated as 3.2 s, reduction response time was
found to be over 5 s (Fig. 6). Optical activity of the film was retained
by 82.7% even after 2000 cycles, namely exhibits reosanable redox
stability. Although having a relatively high response time, PCNI
film show reversible redox behavior, resistance to over oxida-
tion, this material can be useful for electrochromic applications
(Fig. 8).
7
12 nm was observed and intensified by applied negative poten-
tial. Therefore, during the reduction process, yellowish green color
L: 68; a: −24; b: 42) of the film turned into blue (L: 47; a: −5; b:
16) (Fig. 7).
(
−
The coloration efficiency (CE) is a key parameter for the elec-
trochromic materials and this can be calculated by using the

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F.B. Koyuncu et al. / Electrochimica Acta 55 (2010) 4935–4941
Fig. 8. Electrochromic switching, optical absorbance monitored at 825 nm for PCNI
film between −1.0 and +1.8 V.
4. Conclusion
A novel donor–acceptor monomer containing carbazole and
,8-napthalimide as pendant unit was synthesized in three steps
1
Fig. 6. Electronic absorption spectra and the colors of the PCNI on ITO/glass in 0.1 M
TBAPF6/CH3CN at various applied potentials from 0.0 to +1.8 V.
and then its polymer was coated onto Pt or transparent ITO/glass
working electrode surface via potentiodynamic process at con-
tinuous cycles between 0 and 1.4 V. Electrochemical and optical
band gap values of these compounds were determined via the
results of their CV and UV–vis measurements. The band gap values
are not in agreement for the both CNI monomer and its polymer
due to photo-induced electron transfer between Cbz-donor and
equations given below [42]:
ꢀ
ꢁ
ꢂOD
Qd
Tcolored
CE =
and ꢂOD = log
Tbleached
1
,8-naphtalimide-acceptor in the excited state. Furthermore, the
fluorescence quenching occurred by the carbazole substitution on
the naphtalimide moiety supports the estimated electron transfer
mechanism. Further, electrochromic characterization of the poly-
mer film was carried out by spectro-electrochemical analysis and
electrochromic switching. During the oxidation process, the band
intensified at 825 nm indicated the formation of polarons on the
poly-carbazole backbone. Thus, yellowish green color of the film at
reduced state turned into dark green. While %ꢂT were found to be
where Qd is the injected/ejected charge between neutral and oxi-
dized state, Tcolored and Tbleached are the transmittance in the
oxidized and neutral states, respectively.
2
−1
by
Using this equation, CE was measured as 268 cm C
chronoamperometry while switching the PCNI film between −1.0
and 1.8 V. In comparison with the other polymeric electrochromic
materials, coloration efficiency of PCNI film is better than that of
many others [43–45].
2
5% at 825 nm, the oxidation–reduction response times were calcu-
lated as 3.2 s and over 5 s, respectively. Due to carbazole derivatives
are very stable in the polaronic state, the reduction process of the
oxidized PCNI film is slowly then the oxidation process. Therefore,
many different tones of green color could be observed on the ITO
glass surface. On this kind of structures, donor and acceptor moi-
eties are not conjugated with each other so, each moiety can act
freely and it is possible to obtain a wide range of color scale and even
three main colors (red, green, blue) on the same polymer film via
changing applied voltage. Although having high oxidation poten-
tial and reasonable transmittance change (contrast) comparing to
the donor–acceptor green poly-thiophene derivatives synthesized
earlier, PCNI is an alternative green polymer for the electrochromic
applications.
Acknowledgement
We gratefully acknowledge the supports from C¸ anakkale
Onsekiz Mart University Grants Commission (Project Number:
2
009/17).
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