A new multi-electrochromic 2,7 linked polycarbazole derivative: Effect of the nitro subunit

Article abstract of DOI:10.1016/j.orgel.2011.06.023

A new poly-2,7-di-2-thienyl-9H-carbazole derivative (poly(TCT-N)) bearing nitro (-NO₂) subunit has been reported. The electrochemical and optical properties of TCT-N monomer and its polymer have been compared to the 9-phenyl-2,7-di-2-thienyl-9H-carbazole (TCT) standard molecule. Cyclic voltammetry revealed that TCT-N and TCT have excellent polymerization activity; due to extended conjugation with the attached thiophene unit and also their low oxidation potentials. The oxidation potential of TCT-N is higher than that of TCT, because of electro-withdrawing effect of -NO₂ moiety. Besides, the TCT-N polymeric film prepared via electrochemical process exhibits a multi-electrochromic behavior compared with TCT standard molecule. The electroactive orange color of poly-TCT-N film converted respectively to green, turquoise and dark blue upon applied positive potentials. Consequently, the polymeric electrochrome exhibits multi-electrochromic behavior, high redox stability, high coloration efficiency and reasonable response time for electrochromic applications.

Full text of DOI:10.1016/j.orgel.2011.06.023

Organic Electronics 12 (2011) 1701–1710
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A new multi-electrochromic 2,7 linked polycarbazole derivative: Effect
of the nitro subunit
Fatma Baycan Koyuncu a,b, , Sermet Koyuncu b,c, Eyup Ozdemir a,b,
⇑
⇑
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:
Received 25 May 2011
Received in revised form 24 June 2011
Accepted 26 June 2011
Available online 14 July 2011
A new poly-2,7-di-2-thienyl-9H-carbazole derivative (poly(TCT-N)) bearing nitro (–NO2)
subunit has been reported. The electrochemical and optical properties of TCT-N monomer
and its polymer have been compared to the 9-phenyl-2,7-di-2-thienyl-9H-carbazole (TCT)
standard molecule. Cyclic voltammetry revealed that TCT-N and TCT have excellent poly-
merization activity; due to extended conjugation with the attached thiophene unit and also
their low oxidation potentials. The oxidation potential of TCT-N is higher than that of TCT,
because of electro-withdrawing effect of –NO2 moiety. Besides, the TCT-N polymeric film
prepared via electrochemical process exhibits a multi-electrochromic behavior compared
with TCT standard molecule. The electroactive orange color of poly-TCT-N film converted
respectively to green, turquoise and dark blue upon applied positive potentials. Conse-
quently, the polymeric electrochrome exhibits multi-electrochromic behavior, high redox
stability, high coloration efficiency and reasonable response time for electrochromic
applications.
Keywords:
Poly-2,7-carbazoles
Electrochemical polymerization
Electrochromic polymers
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
switching characteristics of conjugated polymers [9]. All
these applications usually require the modification of the
After polyacetylene as first conducting polymer which
has gained conductivity by using dopant ion was prepared
by Shirakawa et al. they have begun to attract interest in
this type of polymeric material rather have the space and
now widely used for industrial applications [1]. The poten-
tial applications include their use as molecular electronics
[2], nonlinear optical devices [3], organic light emitting
diodes (OLEDs) [4], organic solar cells (OSCs) [5], organic
field effect transistors (OFETs) [6], memories [7] and en-
ergy storage [8]. A large number of applications exist based
on electrochromism, which relies on the reproducible
monomer structure to tune the properties of the polymers
(electronic, optical, conductivity, etc.) with respect to de-
sired applications. In addition, the physical and chemical
properties of the conjugated polymers can efficiently be
controlled by some basic factors such as bond length alter-
nation, aromaticity, planarity, substitution and intermolec-
ular interaction [10]. For this purpose, functionalization of
the electroactive monomer structure before polymeriza-
tion and association of the conducting polymer with other
materials are widely used [11]. Besides, the attachment of
electro-withdrawing and/or electron-donating moieties
onto conjugated system is the most promising way to con-
trol the properties of the electroactive polymers [12]. For
instance, in spite of the rich chemistry of thiophene, only
a few polythiophene systems with electro-withdrawing
moieties such as nitro, carboxy or cyano on the thiophene
ring exist in the literature; however, a number of polythi-
ophene with alkyl and/or electron-donating substituents
⇑
Corresponding authors at: Department of Chemistry, Faculty of
Sciences and Arts, Çanakkale Onsekiz Mart University, 17020 Çanakkale,
Turkey. Tel.: +90 286 2180533; fax: +90 286 2180018 (F. Baycan
Koyuncu).
E-mail addresses: fatmabaycan@hotmail.com (F. Baycan Koyuncu),
eozdemir@comu.edu.tr (E. Ozdemir).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.06.023

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F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
have been reported [13]. These attachments, based on the
electronic properties of the neutral semiconducting form of
conjugated systems, have had a great effect on the chemis-
try of these systems. This new formation has created a
strong change in point of view regarding the molecular
engineering of the electronic properties of materials de-
rived from conjugated systems and hence on the structural
control of their band gap [10].
trobiphenyl (1), 2,7-dinitro-9H-carbazole (2), 2,7-diami-
no-9H-carbazole (3), 2,7-diiodo-9H-carbazole (4) were
prepared from previously published procedures [19].
2.2. Synthesis of 2,7-di-2-thienyl-9H-carbazole (5)
Compound 4 (0.5 g, 0.95 mmol) was dissolved in acetic
anhydride (20 mL) and added boron trifluoride diethyl
etherate (0.15 mL) with refluxing for 1 h to yield 9-acetyl-
2,7-diiodocarbazole as a residue. A mixture of 9-acetyl-
2,7-diiodocarbazole (0.44 g, 1 mmol) 2-thiopheneboronic-
acid (0.27 g, 2 mmol), 20 mL THF and 3 M 5 mL of Na2CO3
solution was stirred at room temperature for 2 h under ar-
gon atmosphere. Then Pd(PPh3)4 (0.02 g) was added to this
solution as catalyst and the reaction was refluxed for 24 h.
The mixture was cooled to room temperature and then
poured into a large amount of methanol (500 mL), and the
mixture of 9-acetyl-2,7-di-2-thienylcarbazole was ob-
tained by filtration. Finally the residue was dissolved in
THF (50 mL), DMSO (10 mL), H2O (5 mL), and then KOH
(9.72 g, 0.173 mol) was added, and the mixture was re-
fluxed for 24 h. The reaction mixture was cooled to room
temperature, neutralized by HCl, and then poured into
water to give the final product. Remaining solid was crys-
tallized in methanol two times. The solid was dried at
50 °C under vacuum. Yield: 0.34 g, 87%, brown color product.
FT-IR (cmꢀ1): 3397 (–NH); 3063, (C–H aromatic); 1605,
(C@C aromatic), 685 (thienyl C–H) (1H NMR, CHCl3-d,
In particular, thiophene and carbazole containing poly-
mers have been of recent attraction due to their interesting
electrochemical and optical behavior, and electrochromic
properties [14]. Using these materials in electrochromic
applications that possess the ability to reversibly color
change by altering redox state has emerged [15]. Besides,
polycarbazole is well-known as a good hole transport and
photonic material for optoelectronic devices [16]. Polycar-
bazoles are known as not only an efficient short wavelength
emitter but also as a strong electro-donating (p-type) chro-
mophore [17]. Another advantage of carbazole is that it can
be easily functionalized at its (3,6-) [18], (2,7-) [19] or N-
positions[20]andthencovalentlylinkedintopolymericsys-
tems, both in the main chain [21] as building blocks and in a
side chain as pendant unit [22]. Furthermore, poly(3,6-car-
bazole)s show interesting electrochromic properties be-
cause of the conjugation breaks that are present due to the
inclusion of a 3,6-linkage. These broken conjugation lengths
generate radical cations, which are separated from one an-
other and do not combine. Therefore, the ability of
poly(3,6-carbazole) derivatives to form two distinct oxida-
tion states further ability to exhibit many colors on the
ITO/glass surface upon applied positive potentials [15a,23].
Since 2,7-carbazoles were theoretically found to have
smaller energy gaps than the 3,6-carbazoles, the small en-
ergy gaps of these monomers are expected to contribute to
a lowering of the band gaps in the resulting polymers
[17b–e]. Due to easily synthesized, there are many reports
about poly(3,6-carbazole)s [24]. For instance, direct oxida-
tive polymerization or electropolymerization of carbazoles
resulted in just oligomeric materials linked at the 3,6- posi-
tions [23,25]. On the other hand, the synthesis of poly(2,7-
carbazole)s was not straightforward due to unreactive the
2 and 7 positions of carbazole. Several approaches have
been developed to overcome the barrier to obtaining vari-
ous poly(2,7-carbazole) derivatives [19].
0
ppm): d, 11.96, (s, 1H, N–H); 7.94 (d, 2H, Ar-Hff ); 7.62 (d,
0
0
0
2H, Ar-Hcc ); 7.55, (s, 2H, Ar-Hdd ); 7.39, (d, 2H, Ar-Haa );
0
0
7.14, (m, 4H, Ar-Hff , Ar-Hbb ).
Hf'
Hf
He'
He
S
S
Ha
Ha'
N
H
Hb
Hd'
Hd
Hb'
Hc
Hc'
Herein, we synthesized a new poly(2,7-carbazole) deriv-
ative bearing electro-withdrawing –NO2 substituent that
couldeasilybe polymerizedelectrochemicallytogivestable,
electroactive and electrochromic materials (Scheme 1).
When compared to the poly-9-phenyl-2,7-di-2-thienyl-
9H-carbazole (poly(TCT)) standard molecule, as a result of
theattachmentof–NO2 moietytothestructure, it gain many
useful properties such as multi-electrochromic behavior,
high redox stability, and high coloration efficiency.
2.3. Synthesis of 9-(4-nitrophenyl)-2,7-di-2-thienyl -9H-
carbazole (TCT-N)
Compound
5 (0.18 g, 0.54 mmol), K2CO3 (0.15 g,
1.0 mmol), Cu powder (0.05 g) and 1-iodo-4-nitrobenzene
(0.135 g, 0.54 mmol) were added to 30 mL nitrobenzene
and heated to reflux temperature for 48 h under argon
atmosphere. Then the reaction mixture was cooled, filtered
through a pad of celite and nitrobenzene was removed un-
der vacuum. Remaining solid was dissolved in DMF and
poured into cold water. The precipitated product was fil-
tered, recrystallized from a mixture of chloroform/hexane
(1/3; v/v) dried at 60 °C under vacuum. Yield: 0.1 g, 41%, yel-
low color product.
2. Experimental
2.1. Materials
FT-IR (cmꢀ1): 3093, 3068, (C–H aromatic); 1594 (C@C
All chemicals purchased from Aldrich, Merck and Fluka
and used without further purification. 2-Azido-4,40-dini-
aromatic); 1522, 1340 (–NO2), (1H NMR, CHCl3-d, ppm):
0
0
d, 8.18, (d, 2H, Hhh ); 7.89 (d, 2H, Ar-Hee ); 7.57 (d, 2H, Ar-

F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
1703
0
0
0
Hcc ); 7.37, (d, 2H, Ar-Hgg ); 7.35, (d, 2H, Ar-Haa ); 7.28, (d,
was directly coated onto platinum disk (0.02 cm2) or in-
dium–tin oxide (ITO, 8–12
the film was rinsed with acetonitrile to remove from elec-
trolyte salt.
0
0
0
2H, Ar-Hff ); 7.22, (s, 2H, Ar-Hdd ); 7.13, (dd, 2H, Ar-Hbb ).
X
, 0.8 cm ꢁ 5 cm) and then
He'
He
Hd'
Hd
S
S
2.6. Instrumentation
Ha
Ha'
FT-IR spectra were recorded by a Perkin Elmer FT-IR
Spectrum One by using ATR system (4000ꢀ650 cmꢀ1). 1H
NMR (Bruker Avance DPX-400) spectra were recorded at
25 °C in deuterated chloroform and TMS as internal
standard.
N
Hb
Hf'
Hf
Hb'
Hc
Hc'
Hg
Hg'
Hh'
Cyclic voltammetry (CV) technique used for electro-
chemical measurements was performed using Biologic
SP50 potentiostat–galvanostat 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/acetonitrile as
the supporting electrolyte. CV measurements were carried
under argon atmosphere. HOMO and LUMO energy levels
of polymer were calculated according to the inner reference
ferrocene redox couple E (Fc/Fc+) = +0.41 V (vs. Ag/AgCl) in
acetonitrile by using the formula EHOMO = ꢀe(EoxꢀEFc) +
(ꢀ4.8 eV) [26]. Onset values of oxidation potentials were
taken into account while calculating HOMO energy levels.
LUMO energy levels of TCT and poly-TCT were calculated
by subtracting the optical band gap from HOMO level.
UV–Vis spectra were recorded by Analytic Jena-Speed-
cord S600 spectrophotometer. The absorption spectra of
monomer and polymer were recorded in dichloromethane.
The optical band gap (Eg) of products were calculated from
Hh
N+
O-
O
2.4. Synthesis of 9-phenyl-2,7-di-2-thienyl -9H-carbazole
standard molecule (TCT)
2,7-Di-2-thienyl-9H-carbazole (0.09 g, 0.27 mmol),
K2CO3 (0.075 g, 0.05 mmol), Cu powder (0.05 g) and iodo-
benzene (0.2 g, 0.49 mmol) were added to 20 mL nitroben-
zene and heated to reflux temperature for 48 h under
argon atmosphere. Then the reaction mixture was cooled,
filtered through a pad of celite and nitrobenzene was re-
moved under vacuum. Remaining solid was dissolved in
DMF and poured into cold water. The precipitated product
was filtered, recrystallized from a mixture of chloroform/
hexane (1/2; v/v) dried at 60 °C under vacuum. Yield:
0.08 g, 70%, light yellow color product.
their low energy absorption edges (konset) (Eg = 1241/konset
)
[27].
FT-IR (cmꢀ1): 3088, 3045, (C–H aromatic); 1592, 1450
Spectro-electrochemical measurements were carried
out to consider absorption spectra of this polymer film un-
der applied potential [28]. The spectroelectrochemical cell
consisted of a quartz cell, an Ag wire (RE), Pt wire counter
electrode (CE) and ITO/glass as transparent working elec-
trode (WE). Measurements were carried out in the 0.1 M
TBAPF6 as supporting electrolyte in acetonitrile.
(C@C aromatic); (1H NMR, DMSO-d6, ppm): d, 7.98, (d, 2H,
0
0
0
Hff ); 7.52 (d, 2H, Ar-Haa ); 7.41 (t, 2H, Ar-Hhh ); 7.37, (d, 2H,
Ar-Hgg ); 7.24, (d, 2H, Ar-Hcc ); 7.16, (d, 2H, Ar-Hii ); 7.13, (s,
2H, Ar-Hdd ); 6.97, (d, 2H, Ar-Hbb ); 6.77, (d, 2H, Ar-Hbb ).
0
0
0
0
0
0
f
e
Colorimetric measurements were performed by using
Konica Minolta CS-200 chromameter with viewing geome-
try 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-TCT-N and poly-
TCT on the ITO/glass surface.
S
S
c
d
N
b
a
g
h
3. Results and discussion
i
3.1. Synthesis and characterization
The new 2,7-carbazole derivative bearing –NO2 pendant
was synthesized in six steps. Starting from 2-amino-4,40-
dinitrobiphenyl, 2,7-dinitro functionalized carbazoles typ-
ically obtained from the Cadogan ring-closure reactions
were firstly synthesized by Morin and Leclerc [19]. Reduc-
tion of nitro group of this intermediate product was per-
formed by using Pd/C and hydrazine monohydrate, a
different reducing conditions used by Morin and Leclerc
2.5. Electrochemical polymerization
Electrochemical polymerization of TCT-N and TCT were
carried out in a dichloromethane solution of 2.0 ꢁ 10ꢀ3 M
monomers and 0.1 M TBAPF6 by repetitive cycling at a scan
rate of 100 mV/s. A platinum wire was used as a counter
electrode and Ag wire as a pseudo reference. The polymer

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F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
O2
N
a
NO2
b
NO2
O2
N
NO2
O2
N
N
H
N3
NH2
2
1
c
S
S
d
e
g
H2N
NH2
I
I
N
H
N
H
N
H
5
4
3
f
S
S
S
S
N
N
N+
TCT
O-
O
TCT-N
Scheme 1. (a) (i) NaNO2, H2SO4, AcOH 0 °C, 2 h, (ii) NaN3, r.t., 18 h; (b) Kerosene, 250 °C, 6 h; (c) Pd/C, N2H5OH, EtOH, reflux, 24 h; (d) (i) NaNO2, AcOH, 0 °C,
2 h, (ii) KI, r.t.,18 h; (e) (i) Acetic anhydride, BF3-Et2O, reflux, 1 h, (ii) 2-thiopheneboronic acid, Pd(Ph3P)4, THF-Water (4:1), Na2CO3, reflux 24 h, (iii) KOH,
THF–DMSO–Water (10:2:1), reflux, 24 h; (f) Cu powder, 18-Crown-6, K2CO3, 1-iodo-4-nitrobenzene, nitrobenzene, reflux, 24 h; (g) Cu powder, 18-Crown-6,
K2CO3, iodobenzene, nitrobenzene, reflux, 24 h.
[19]. 2,7-Diiodo-9H-carbazole (4) was synthesized in two
steps. In the first step, 2,7-diamino-9H-carbazole (3) was
converted to diaazonium salt and then to give 4 adding
KI to this solution [19]. After that, the diiodo compound
coupled with thiophene was carried out by using Suzuki
type coupling reaction to get 2,7-dithienyl-9H-carbazole
(5). Consequently, the electroactive monomers (TCT-N
and TCT) were synthesized by Ullman type condensation
of 5 and iodophenyl derivatives in nitrobenzene in the
presence of CuI as catalyst and K2CO3. The reaction yields
of TCT-N and TCT were found to be 41% and 70%,
respectively.
After completion of the synthetic works, chemical
structure of TCT-N was identified by FT-IR, and 1H NMR.
Significant changes were observed in the spectral proper-
ties of the initial compounds and the products. In FT-IR
spectrum of 1, characteristic triple bond vibration exerted
by azide (–N3) groups was observed at 2123 cmꢀ1 and also
characteristic –NH2 vibrations at about 3200–3400 cmꢀ1
were disappeared. In the next step, as a result of the ring
closure reaction, characteristic N–H vibration band for
secondary amines of obtained carbazole derivative was
observed at 3339 cmꢀ1. Then, 3 obtained from reduction
of 2 was identified from its characteristic –NH2 bond
vibration at 3306 and 3205 cmꢀ1. In the FT-IR spectra of
4 characteristic –NH2 vibration attributed to 3 disap-
peared. The FT-IR spectrum of 5 obtained with the attached
thienyl moiety on the structure exhibits characteristic
strong peak at 685 cmꢀ1 (for
a-hydrogens of thiophene
rings). Finally, TCT-N synthesized from Ullmann condensa-
tion of F10 with 1-iodo-4-nitrobenzene, their characteris-
tic unsymmetric and symmetric –NO2 vibrations were
observed at 1522 and 1340 cmꢀ1, respectively. Also char-
acteristic –NH vibration for secondary amines was re-
moved in this spectrum (see Electronic Supplementary
information).
In the 1H NMR spectrum of TCT-N monomer, the pheny-
lene proton signals neighboring –NO2 moiety were ob-
served at higher ppm (8.09 ppm) than that of other
proton signals due to electro-withdrawing effect of –NO2
substituent. Besides characteristic thienyl b-H doublet sig-
nal was observed at 7.12 ppm. Furthermore, other signals
attributed to thiophene and carbazole moieties were ob-
served as expected. Finally, FT-IR and 1H NMR spectros-
copy results proved that all chemical reactions were
completed successfully (Fig. 1).
Electrochemical polymerization was carried out in a
reaction medium containing 2.0 ꢁ 10ꢀ3 M TCT-N or TCT
monomer and 0.1 M TBAPF6/dichloromethane via repeti-
tive cycling at a scan rate of 100 mV sꢀ1. The electroactive

F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
1705
Fig. 1. 1H NMR spectrum of TCT-N.
Fig. 2. Repeated potential scan of TCT-N between 0 and 1.45 V (a) TCT (b) between 0 and 1.45 V in 0.1 M TBAPF6/dichloromethane, scan rate 100 mV/s, vs.
Ag/AgCl.

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F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
Fig. 3. UV–Vis absorbtion spectra of TCT-N (a) and TCT (b) inset: poly-TCT-N (a) and poly-TCT (b) in dichloromethane.
polymers were directly coated onto the working electrode
(platinum disk or ITO/glass surface). Potentiodynamic elec-
trochemical polymerization of TCT-N was carried out repet-
itive cycling at a potential between 0 and 1.45 V exhibited a
new redox couple at 0.89–1.31 V (Eopx;1=2 = 1.12) (Fig. 2a).
Due to presence of –NO2 moiety, electrochemical polymer-
ization of TCT-N requires to reach higher potentials com-
pared with TCT standard molecule. Because of this reason,
the electrochemical polymerization of TCT was carried
out by repetitive cycling at a potential between 0 and
icant bathochromic shift is attributed to the –NO2 electro-
withdrawing effect. According to these results, the optical
band gap values of TCT and TCT-N were found to be 3.13
and 2.63 eV. On the other hand, the lowest energy transi-
tion band (kmax) of poly(TCT-N) was observed at about
415 nm. This significant bathochromic shift is attributed
to resulting the polymer with extended
p-system. Due to
TCT-N consist of –NO2 moiety, the molecule require for
higher potential than that of TCT and thus reduced both
the length of chain and degree of conjugation with respect
to that of the TCT. Because of this effect, 20 nm red shift of
kmax was observed in the kmax of TCT. Finally, for poly(TCT-
N), the onset absorption is 580 nm corresponding to an en-
ergy band gap of 2.13 eV. Besides, the poly-TCT has an on-
set value of 615 nm (equal to a band gap of 2.01 eV)
corresponding to a 35 nm red shift with respect to that of
the poly(TCT-N).
The electrochemical properties of TCT-N and TCT mono-
mer and their polymers were investigated by cyclic voltam-
metry (CV). An explicit difference in redox behavior of the
TCT-N and TCT can be deduced from the CV which was
depicted in Fig. 4. During CV scan in the anodic regime of
1.40 V exhibited two new redox couples at 0.85–0.95 V
ox1
p;1=2
(E
= 0.90 V), and 1.08–1.22 V (Eopx;12=2 = 1.15 V) (Fig. 2b).
These two redox couple may be attributed to polaronic
and bipolaronic formation of the occurred polymer chain
by the electrochemical process [30]. Due to electro-with-
drawing effect of the –NO2 substituent on the poly(TCT-
N), the two redox states could not be observed at this po-
tential of 1.40 V. In addition, all peaks were increased cur-
rent density after each successive cycle clearly indicated
the formation of poly(TCT-N) and/or poly(TCT) deposited
onto the surface of the working electrode. Prepared poly-
mer films on ITO/glass surface was dedoped electrochemi-
cally in monomer free-electrolyte solution to equilibrate
the redox behavior after washing with an acetonitrile to re-
move the unreacted monomers and/or oligomeric species.
Then, the polymers on ITO/glass surface partly dissolved
in dichloromethane by using ultrasonic bath for UV–Vis
absorption measurements.
TCT-N and TCT monomer, TCT-N exhibited an irreversible
ox
m;a
oxidation peak at E = 1.46 V vs. Ag/AgCl during an anodic
scan, and this oxidation potential is higher than that of TCT
ox
m;a
ox
m;c
ox
m;1=2
(semi reversible;
E
= 1.25 and
E
= 1.11 V;
E
=
1.19 V vs. Ag/AgCl). As expected, this result indicated that
TCT has a more electron rich nature than TCT-N when
attaching the –NO2 moiety in a similar system. On the
other hand, to record the cyclic voltammogram of
poly(TCT-N) and poly(TCT), the polymer films prepared
by repetitive potential cycling were rinsed in a mono-
mer-free-electrolyte solution. Due to the extended conju-
gation in the polymers backbone, the oxidation potentials
were observed as reversible and at lower potential with
compared to corresponding monomers. On the other hand,
in the cathodic regime, cyclic voltammogram of TCT-N
exhibits characteristic reversible reduction couple with
3.2. Optical and electrochemical properties
The UV–Vis absorption spectra of TCT-N and TCT mono-
mers and polymers were recorded in dichloromethane
(Fig. 3). The lowest energy bands of TCT-N and TCT stan-
dard molecule at about kmax = 350 nm attributed to thio-
phene-carbazole-thiophene (TCT)
p-conjugated system.
Although kmax of TCT-N and TCT are the same, the absorp-
tion band of TCT-N is broadened than that of TCT. Thus,
TCT-N has an onset value of 472 nm corresponding to a
76 nm red shift with respect to that of the TCT. This signif-
red
m;c
red
m;a
red
m;1=2
potentials of E = ꢀ0.93 V ꢀE = ꢀ0.85 V and E
=
ꢀ0.89 V which reflects the reduction process of –NO2

F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
1707
The spectro-electrochemical properties of the poly(TCT-
N) film were determined upon doping process (Fig. 5) and
compared to poly-TCT (Fig. 6) in order to get further sup-
port for the effect of –NO2 electro-withdrawing moiety.
In the neutral state, the poly(TCT-N) had one maximum
absorption at 418 nm (kmax, p–
p⁄ transition). The optical
band gap (Eg) of poly(TCT-N) was found to be 2.18 eV from
the onset of absorption, which is somewhat higher than
the Eg of poly-TCT (2.06 eV). Upon oxidation, the intensity
of the kmax was simultaneously decreased up to a certain
first and second broad bands intensified at about 762 (0–
1.0 V) and 1000 nm (1.0–1.6 V) indicated the formation of
polarons and bipolarons on poly(TCT-N) film, respectively
(Fig. 5). Thus, NIR region, above 700 nm, was strongly ab-
sorbed at the fully oxidized state. Based on foregoing re-
sults, the orange color of the film with Commision
Internationale de I’Eclairage (CIE) color parameters (Lumi-
nance L = 60, hue a = 7, and saturation b = 57) turned into
green (L: 54; a: ꢀ20; b: 43), turquoise (L: 42; a: ꢀ16; b:
2), and dark blue (L: 38; a: ꢀ2; b: ꢀ10), respectively, upon
oxidation (Fig. 5). On the other hand, when the intensity of
Fig. 4. Cyclic voltammogram of TCT-N (a) TCT (b) in the 0.1 M TBAPF6/
dichloromethane, scan rate 100 mV/s, vs. Ag/AgCl.
moiety. In comparison to the monomer, the reduction cou-
ple of poly(TCT-N) broadened and slightly shifted posi-
tively (Epre;cd = ꢀ0.91 V ꢀ Ered = ꢀ0.77 V and Ered = ꢀ0.84 V)
the
p
–p⁄ transition band of poly(TCT) film (kmax = 422 nm)
decreased, a new broad absorption band at about 790 nm
started to intensify due to the polaron formation (0–
0.8 V). Upon further oxidation, a broad band between 400
and 1000 nm also intensified (1–1.4 V). Thus all visible
and near-IR regions were absorbed by the poly(TCT) film
at fully oxidized state. The yellow color of the film with
CIE color parameters (L: 75; a: ꢀ11; b: 64) turned into dark
gray (L: 39; a: ꢀ6; b: 13), upon oxidation (Fig. 6).
p;a
m;1=2
and the conjugated backbone is still sufficiently conduct-
ing to allow electron transfer at potentials. Besides,
HOMO–LUMO energy levels of TCT-N and poly(TCT-N)
were calculated according to the onset values of their oxi-
dation and reduction potentials. The LUMO energy levels of
TCT and poly(TCT) were calculated by adding the optical
band gap from their HOMO levels (Table 1). As a result of
the data in Table 1, optical and electrochemical properties
of the molecules were greatly influenced by the addition of
electro-withdrawing –NO2 as subunit in the structure.
Double step chronoamperometry technique was used to
monitor the changes in the electro-optical responses dur-
ing switching. Electrochromic parameters of the polymer
Table 1
HOMO and LUMO energy levels, electrochemical (E0 ) and optical band gaps (Eg) values of TCT-N and TCT standard molecules.
g
0
Molecules
Reduced group and its
peak potential (V)
Oxidized group and its
peak potential (V)
HOMO (eV)
LUMO (eV)
Eg , Electrochemical
band gap (eV)
Eg, Optical
band gap (eV)
–NO2
TCT (Ring)
ox
red
TCT-N
ꢀ5.64
ꢀ3.63
2.01
2.63
E
¼ 1:46
E
E
¼ ꢀ0:93
¼ ꢀ0:85
m;a
m;c
red
m;c
Irreversible
ox
m;on
E
¼ 1:25
reversible
red
m;on
E
¼ ꢀ0:76
TCT
–
ꢀ5.43
ꢀ5.30
ꢀ2.30a
ꢀ3.68
–
3.13
2.18
ox
E
E
¼ 1:25
¼ 1:11
m;a
ox
m;c
Semi-reversible
ox
E
E
E
¼ 1:04
¼ 1:31
¼ 0:89
m;on
ox
p;a
ox
p;c
red
p;c
Poly-TCT-N
1.62
E
E
¼ ꢀ0:91
¼ ꢀ0:77
red
p;a
reversible
reversible
red
m;on
ox
m;on
E
¼ 0:91
E
¼ ꢀ0:71
Poly-TCT
–
ꢀ5.12
ꢀ3.06a
–
2.06
ox1
E
E
¼ 0:95
¼ 0:85
p;a
ox1
p;c
reversible
ox2
E
E
¼ 1:22
¼ 1:08
p;a
ox2
p;c
reversible
ox2
m;on
E
¼ 0:73
a
Calculated by the addition of the optical band gap to the HOMO level.

1708
F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
Fig. 5. Spectro-electrochemical behavior of poly-TCT-N film on the ITO/glass surface.
Fig. 6. Spectro-electrochemical behavior of poly-TCT film on the ITO/glass surface.
films were analyzed by changes that occurred in the trans-
mittance (increments and decrements of the absorption
band at 420 and 1000 nm with respect to time) while
switching the potential step wisely between neutral and
oxidized states with a residence time of 10 s [28a]. The
transmittance in the oxidized and neutral states, respec-
tively. Using this equation, CE of poly(TCT-N) and
poly(TCT) was measured to be 264 and 126 cm2 Cꢀ1 by
chronoamperometry, while switching the polymer films
between oxidized and reduced states. The results clearly
show that the coloration efficiency of poly(TCT-N) is two
times greater than that of poly(TCT). Toppare et al. re-
ported the multi-electrochromic polymer of thiophene-
pyrrole-thiophene (SNS) analogue containing electro-with-
drawing –NO2 substituent [31]. Based on previous results,
it can be easily seen that the prepared polymer film of TCT-
N has much better electrochromic performance than that
of its SNS analogue and poly(TCT) standard.
percentage transmittance changes (%DT) of poly(TCT-N)
between the neutral (at 0 V) and oxidized states (at
1.6 V) were found to be 22% for 420 nm and 57% for
1000 nm (Fig. 5). For poly(TCT), these values were found
as 8% for 420 nm and 55% for 1000 nm (Fig. 6). Besides,
the oxidation and reduction response times were mea-
sured to be 4.3 and 1.3 s for poly(TCT-N) and 0.8 and 1.8
s for poly(TCT) (Fig. 7). Optical activities of the poly(TCT-
N) and poly(TCT) polymer film were retained by 92% and
77% even after 1000 cycles of operation, respectively
(Fig. 7). Although the response times were relatively slow,
the polymer film of TCT-N was more stable than that of
TCT upon repetitive cycles.
4. Conclusion
Herein, we have reported the synthesis of a new multi-
electrochromic 2,7 linked thiophene-carbazole-thiophene
(TCT) derivative containing an electro-withdrawing –NO2
substituent. It has been determined that the optical and
the electrochemical properties of the monomer were
On the other hand, the coloration efficiency (CE) was
calculated by using CE =
DOD/Qd and DOD = log (Tcolored/
Tbleached), where Qd is the injected/ejected charge between
neutral and oxidized states, Tcolored and Tbleached are the

F. Baycan Koyuncu et al. / Organic Electronics 12 (2011) 1701–1710
1709
Fig. 7. Electrochemical switching, optical absorbance monitored at 1000 nm for poly-TCT-N (a) and poly-TCT (b) film between 0 and 1.6 V.
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–NO2 as subunit in the structure. TCT-N monomer was
electrochemically polymerized onto transparent ITO/glass
surface to give a quite stable multi-electrochromic poly-
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Acknowledgements
We gratefully acknowledge the supports from Çanakk-
ale Onsekiz Mart University Grants Commission (Project
Number: 2009/17) and also grateful to Scientific and Tech-
nical Research Council of Turkey (TUBITAK, project #:
109T516) for financial support.
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