Synthesis and spectroelectrochemical characterization of multi-colored novel poly(3,6-dithienylcarbazole) derivatives containing azobenzene and coumarin chromophore units

Article abstract of DOI:10.1016/j.electacta.2016.02.168

Azobenzene and coumarin units bearing four novel 3,6-linked thiophene-carbazole-thiophene type electroactive monomers were synthesized in order to observe the effects of strong chromophore units on the electrochemical and spectroelectrochemical behaviors of resulting π-conjugated conducting polymers (PTCbzAz, PECbzAz, PTCbzCo and PECbzCo). Electronic and optoelectronic properties of all polymers were investigated by cyclic voltammetry and in situ spectroelectrochemical studies, and their oxidation potentials, HOMO/LUMO energy levels, band gaps (E_g^op), optical contrasts (ΔT%) and switching times were reported in detail. It was observed that incorporation of strong chromophore units in polymer backbones resulted in poly(3,6-dithienylcarbazole) derivatives with different optical band gap values and different number of colored states. The optical band gap values for PECbzAz, PECbzCo, PTCbzAz, PTCbzCo and were calculated as 1.71 eV, 1.75 eV, 1.93 eV and 2.07 eV, respectively, from the onset of π-π?transition in polymer films' neutral state absorptions. All polymers exhibited multicolored electrochromic behavior upon extarnally applied potential. Reported electrochemical and optical features of PECbzAz, PECbzCo, PTCbzAz, PTCbzCo suggest that these poly(3,6-dithienylcarbazole) derivatives are promising candidates for multicolored electrochromic device applications.

Full text of DOI:10.1016/j.electacta.2016.02.168

Accepted Manuscript
Title: Synthesis and spectroelectrochemical characterization of
multi-colored novel poly(3,6-dithienylcarbazole) derivatives
containing azobenzene and coumarin chromophore units
˘
¨ ¨
Author: Deniz Yigit Serife O. Hacioglu Mustafa Gullu Levent
Toppare
PII:
S0013-4686(16)30470-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2016.02.168
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Accepted date:
7-1-2016
4-2-2016
24-2-2016
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Please cite this article as: Deniz Yigit, Serife O.Hacioglu, Mustafa Gullu,
Levent Toppare, Synthesis and spectroelectrochemical characterization
of multi-colored novel poly(3,6-dithienylcarbazole) derivatives containing
azobenzene
and
coumarin
chromophore
units,
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http://dx.doi.org/10.1016/j.electacta.2016.02.168
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Synthesis and spectroelectrochemical characterization of multi-colored
novel poly(3,6-dithienylcarbazole) derivatives containing azobenzene and
coumarin chromophore units
Deniz Yiğita*, Serife O. Hacioglub, Mustafa Güllüa* and Levent Topparebcde
a Department of Chemistry, Faculty of Science, Ankara University, Beşevler, 06100 Ankara, Turkey
b Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
c Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey
d Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey
e The Center for Solar Energy Research and Application, Middle East Technical University, 06800 Ankara, Turkey
GRAPHICAL ABSTRACT

HIGHLIGHTS



Poly(3,6-dithienylcarbazole) homopolymers containing strong chromophore units were
electrochemically synthesized.
Electrochemical and spectroelectrochemical behaviors of novel poly(3,6-dithienylcarbazole)
modified with azobenzene and coumarin were evaluated.
Poly(3,6-dithienylcarbazole) films exhibited multi-colored electrochromic characteristics.
Abstract
Azobenzene and coumarin units bearing four novel 3,6-linked thiophene-carbazole-thiophene
type electroactive monomers were synthesized in order to observe the effects of strong
chromophore units on the electrochemical and spectroelectrochemical behaviors of resulting
π-conjugated conducting polymers (PTCbzAz, PECbzAz, PTCbzCo and PECbzCo).
Electronic and optoelectronic properties of all polymers were investigated by cyclic
voltammetry and in situ spectroelectrochemical studies, and their oxidation potentials,
op
HOMO/LUMO energy levels, band gaps (Eg ), optical contrasts (ΔT%) and switching times
were reported in detail. It was observed that incorporation of strong chromophore units in
polymer backbones resulted in poly(3,6-dithienylcarbazole) derivatives with different optical
band gap values and different number of colored states. The optical band gap values for
PECbzAz, PECbzCo, PTCbzAz, PTCbzCo and were calculated as 1.71 eV, 1.75 eV, 1.93 eV
and 2.07 eV, respectively, from the onset of π-π* transition in polymer films’ neutral state
absorptions. All polymers exhibited multicolored electrochromic behavior upon extarnally
applied potential. Reported electrochemical and optical features of PECbzAz, PECbzCo,
PTCbzAz, PTCbzCo suggest that these poly(3,6-dithienylcarbazole) derivatives are promising
candidates for multicolored electrochromic device applications.
1. Introduction

π-Conjugated conducting polymers (CPs) are extremely organic electroactive materials for
commercial electronic devices due to their advantages of relatively low cost synthesis, easy
structural modification and perfect integration into many electronic devices. CPs have been
used in wide range of technological areas including electrochromic devices (ECDs) [1,2],
organic light emitting diodes (OLEDs) [3,4], field effect transistors (FETs) [5,6], sensors
[7,8], organic photovoltaics (OPVs) [9-11] and supercapacitors (SCs) [12-14], over the last
two decades. Especially, the use of CPs as electrochromic materials has been attracting great
deal of interest over inorganic counterparts since they exhibit excellent electronic and
electrochromic properties such as reversible and steady color changes applying low potentials,
high optical contrast in visible and near infra-red (NIR) regions, high coloration efficiency and
fast switching times [15-19].
Electrochromic conducting polymers are mostly colored in their neutral states and they
possess the ability to change color reversibly upon appropriate external bias. The apparent
change in spectral properties of conjugated conducting polymers is related to doping-undoping
processes on electroactive polymer chain [20,21]. Upon oxidation, new energy states arise in
between HOMO and LUMO energy levels (band gap, Eg) of conducting polymers owing to
formation of polarons and bipolarons termed as charge carriers. As a result of the structural
modification on polymer backbone, a second colored or transmissive state emerges [22,23].
The value of the band gap of a neutral state conducting polymer is most important factor for
manipulating electrochemical and spectroelectrochemical behaviors of the π-conjugated
polymers, as well as optical properties. In the context of controlling electronic and
optoelectronic features of conducting polymers, different directions including polymer
backbone and bond length alternation, planarity, interchain alternation, resonance effect,
diverse pendant groups and donor-acceptor concept have been developed until now [24-28].
Polymer backbone alternation, different pendant substituents and donor-acceptor theory

among other approaches can be considered as the most convenient ones to obtain unique
conducting polymers with better electrochemical and spectroelectrochemical properties.
The great majority of interest in electrochromic materials mainly focused on polythiophene,
polypyrrole and their derivatives owing to their low band gap, low oxidation potential, good
conductivity, high optical contrast and good environmental stability [29,30]. Furthermore,
polythiophene and polypyrrole derivatives exhibit high stability in their oxidized form and
multicolor electrochromism [31,32]. Because of their excellent electrochemical and
spectroelectrochemical features, they have been most commonly utilized as electroactive
layers in electrochromic device applications so far [33,34]. In addition to polythiophene and
polypyrrole derivatives, recently, carbazole containing conducting polymers have become
promising candidates for electrochromic materials because of various advantages such as high
charge carrier mobilities, easy formation of stable radicals, strong electron-donating
chromophore properties, high thermal and photochemical stabilities [35-37]. Carbazole-based
conducting polymers (Cbz) can be also easily functionalized at their (3,6-), (2,7-) and N-
positions. Moreover, conducting polymers based on 3,6-linked carbazole unit have interesting
multicolored electrochromic properties due to broken conjugation at their 3,6-positions. The
radical cations formed on the polymer chain via alternation of the applied potential do not
incorporate with one another due to the effective conjugation break [38-40]. Thus, 3,6-linked
Cbz-based electrochromic conducting polymers distinctively exhibit multiple colors in their
neutral, oxidized and further oxidized states. According to above considerations, the design
and synthesis of new precursor monomers containing 3,6-linked carbazole moetiy is believed
to contribute diversity of electrochromic materials with superior electrochemical and
spectroelectrochemical properties in terms of enlarging π-conjugated conducting polymers.
Herein, we report the synthesis, electropolymerization and electrochromic properties of four
novel 3,6-dithienylcarbazole type monomers bearing highly optically potent chromophore

groups in the UV-visible region (azobenzene and coumarin) as a pendant group. Conducting
polymers of these compounds were produced by electropolymerization and named as poly[9-
(2-4-(phenyldiazenyl)phenoxy)ethyl)-3,6-di(thiophen-2-yl)-9H-carbazole]
(PTCbzAz),
poly[3,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-(2-(4-(phenyldiazenyl)phenoxy)
ethyl)-9H-carbazole] (PECbzAz), poly[4-(2-(3,6-di(thiophen-2-yl)-9H-carbazol-9-yl)ethoxy)-
2H-chromen-2-one] (PTCbzCo) and poly[3,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-
9H-(carbazol-9-yl)ethoxy)-2H-chromen-2-one] (PECbzCo). As reported in our previous
publications, incorporation of a strong chromophore group (azobenzene, coumarin and
fluorescein) with the monomer skeleton showed interesting electrochemical and
spectroelectrochemical properties, as observed for terthienyl and 2,5-dithienylpyrrole (SNS)
derivatives [41-43]. In this work, the same strategy was used for modification of carbazole
units with azobenzene and coumarin groups via an ethylene bridge. It is anticipated that better
electrochemical and optical properties for resulting π-conjugated conducting polymers can be
obtained through the combination of carbazole ring and strong chromophore compounds since
carbazole itself is also highly effective electroactive and photoactive material. Poly(carbazole)
derivatives, however, have usually wider band gap energies. To the best of our knowledge, the
most commonly used method is functionalization of carbazole unit with various functional
electron-donating compounds via its 2,7- or 3,6-position in order to decrease their band gap
values.
For
this
purpose,
3,6-(dithiophenyl)
and
3,6-[di(3,4-
ethylenedioxythiophenyl)]carbazole derivatives containing strong chromophore groups have
been prepared by Stille cross-coupling reaction. It is also expected that these 3,6-linked
carbazole conducting polymers will exhibit multi colors for their neutral, polaronic and
bipolaronic species at various oxidation states due to the presence of conjugation-breaking at
3- and 6- position. The electronic and optical properties of homopolymers electrochemically
prepared on transparent ITO glass electrode surfaces, PTCbzAz, PECbzAz, PTCbzCo and

PECbzCo, were studied by using cyclic voltammetry and spectroelectrochemical techniques.
Besides, both the effects of the type of chromophore groups and 3,6-linked electron-donating
thiophene and 3,4-(ethylenedioxythiophene) units on electrochromic and electrochemical
behaviors of resulting conducting polymers were investigated in detail.
2. Experimental
2.1. Materials
Carbazole,
4-hydroxyazobenzene,
4-hydroxy-2H-chromen-2-one,
1,2-dichloroethane
(anhydrous, 99.8%), n-butyllithium solution (n-BuLi, 2.5 M in hexane), tributyltin chloride
(Bu3SnCl, 96%), 3,4-ethylenedioxythiophene (EDOT, 97%), 2-(tributylstannyl)thiophene
(reagent grade, 97%), bis(triphenylphosphine)palladium (II) chloride (≥99%), lithium
perchlorate (LiClO4, 99.999%), sodium perchlorate (NaClO4, ≥ 98.0%), N-bromosuccinimide
(NBS, 99%), N,N-dimethylformamide (DMF, anhydrous 99.8%), cesium carbonate (Cs2CO3,
99%), potassium hydroxide (KOH, ACS reagent, ≥ 85%, pellets), dichloromethane (DCM,
anhydrous, ≥ 99.8%) silica gel (SiO2, high-purity grade, pore size 60Å, 200-425 mesh particle
size), a platinum wire (Pt, 0.25 mm diameter, 99.999%), a silver wire (Ag, 0.05 mm diamater,
99.9%) and an indium tin oxide doped glass slide (ITO, slide resistance 8-12 Ω sq-1) were
purchased from Sigma Aldrich and used without any further purification. Prior to
electrochemical measurements, acetonitrile (ACN, HPLC grade) was refluxed with P2O5 for 8
h, distilled fractionally, finally stored over activated 4A molecular sieves under a nitrogen
atmosphere. 3,6-Dibromocarbazole [44] and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)
stannane [45] were synthesized according to previously described methods in the litrature.
2.2. Instrumentation
Microwave-assisted reactions were carried out using a CEM Discover S-Class single-mode
microwave synthesis instrument. Electrochemical data were collected using a Radiometer

VoltaLab PST 050 potentiastat/galvanostat in three-electrode cell configuration. An indium tin
oxide doped glass slide, a platinum wire and a silver wire served as the working electrode,
counter electrode and pseudo-reference electrode, respectively, in both electrochemical and
spectroelectrochemical studies. The silver wire pseudo-reference electrode was calibrated to
the ferrocene couple (+0.3 V vs Fc/Fc+). The homopolymers of carbazole derivatives were
electrochemically generated by the cyclic voltammetry technique (CV) in the presence of a
0.05 M monomer in 0.1 M solution of the NaClO4-LiClO4/ACN/DCM (95:5, v/v) supporting
electrolyte under a nitrogen atmosphere at 100 mV s-1 scan rate for 10 cycles. UV-Vis-NIR
spectra of the resulting conducting polymers were recorded on a Varian Cary 5000
spectrophotometer at a scan rate of 2000 nm min-1. The potentials in spectroelectrochemical
measurements were controlled by using a Solatron 1285 potentiastat/galvanostat. A Konica
Minalto CS-100 colorimeter in a Pantone Colorviewing Light PJC/CV L3 Light Box 3 was
used in order to perform color analysis of conducting polymer layer on ITO surfaces. FTIR
spectra of monomers were recorded on a Perkin-Elmer Spectrum 100 spectrometer. Ambient
temperature 1H NMR and 13C NMR spectra were recorded in CDCl3 on a Varion-Mercury 400
MHz (FT)-NMR spectrometer. All chemical shifts were given in ppm downfield from
tetramethylsilane (Me4Si). Mass spectra were recorded using a DI-2010 direct inlet probe on a
Shimadzu GC-MS QP2010 mass spectrometer operating at an ionization potential (EI) of 70
eV. Elemental analyses of the novel monomers were performed with a Leco CHNS-932
elemental analyzer.
2.3. Syntheses of monomers
The 3,6-dithienylcarbazole derivatives containing strong chromophore groups were
successfully synthesized via nucleophilic substitution reactions and Stille cross-coupling
reactions as represented in Scheme 1.
2.3.1. Synthesis of 3,6-dibromo-9-(2-chloroethyl)-9H-carbazole (3)

In a dry 100 mL one-necked round-bottomed flask, 3,6-dibromo-9H-carbazole (2) (1.81 g,
5.58 mmol) and grinded potassium hydroxide (KOH, 1.28 g, 23 mmol) was suspended in dry
1,2-dichloroethane (80 mL). 1,2-Dichloroethane was used as both reactive and solvent for this
reaction. The reaction mixture was refluxed at 85ºC overnight under nitrogen atmosphere. The
reaction suspension was poured into ice-water (200 mL) and extracted with dichloromethane
(3 x 30 mL). The organic phases were combined, washed with water and brine and dried over
anhydrous Na2SO4. Dichloromethane was removed on a rotary evaporator under reduced
pressure. The crude product was purified by column chromotography using hexane/ethyl
acetate (95:5, v/v) as eluent (Rf, 0.70) afforded (3) (1.7 g, 4.4 mmol, 79%, mp.: 152-154˚C) as
a white powder. FTIR ν/ cm-1 3079, 2953, 2921, 1741, 1623, 1568, 1471, 1352. 1H NMR (400
MHz, CDCl3, Me4Si) δH 4.03 (t, J = 5.2 Hz, 2H), 4.41 (t, J = 5.2 Hz, 2H), 7.32-7.34 (d, J = 8.8
Hz, 2H), 7.54-7.56 (dd, J = 2Hz and J = 8.4 Hz, 2H), 8.13 (d, J = 2 Hz, 2H). MS (EI)
calculated for C14H10Br2ClN m/z: 387.5, found m/z: 387 (M+, 50), 338 (100), 324 (5), 271 (5),
257 (22), 243 (4), 191.1 (10), 178.1 (15), 164.1 (25), 151.1 (20), 89.1 (5), 62 (3).
2.3.2. General procedure for microwave-assisted nucleophilic substitution reaction
3,6-Dibromo-9-(2-chloroethyl)-9H-carbazole (3) (1.06 g, 2.76 mmol), a chromophore
compound (3.31 mmol, hydroxyazobenzene or hydroxycoumarin derivative) and cesium
carbonate (Cs2CO3, 2.69 g, 8.28 mmol) were thoroughly mixtured and placed in a microwave
reaction vessel (30 mL). After adding dry ACN (3 mL), the eventual slurry reaction mixture
was heated at 100˚C under microwave irradiation (150 W single-mode power) for 15 min.
Then, the mixture was cooled to room temperature, poured into ice-water (150 mL) and
vigorously stirred for 30 min. The solid crude product was filtered under reduced pressure,
rinsed with alkaline solution and dried at room temperature. 3,6-Dibromocarbazole derivatives
containing chromophore groups were purified by colomn chromatography to obtain pure

1
target precursor compounds. The pure compounds were characterized by FTIR and H NMR
techniques and found to be satisfactory.
3,6-Dibromo-9-(2-4-(phenyldiazenyl)phenoxy)ethyl)-9H-carbazole (4a)
This carbazole derivative was prepared with the same procedure described above using 3,6-
dibromo-9-(2-chloroethyl)-9H-carbazole (3) and 4-hydroxyazobenzene. The residue was
subjected to column chromatography on silica gel eluting with hexane/ethyl acetate (80:20,
v/v) (Rf, 0.65) to obtain 4a as orange solid (1.22 g, 2.23 mmol, 81.5%, mp.: 168-169˚C). FTIR
1
ν/ cm-1 3064, 3043, 2939, 2875, 1600, 1581, 1473, 1359, 1246. H NMR (400 MHz, CDCl3,
Me4Si) δH 4.38 (t, J = 7.6 Hz, 2H), 4.66 (t, J = 7.5 Hz, 2H), 6.86-6.87 (dd, J = 8.7 Hz and J =
2.4 Hz, 2H), 7.37-7.39 (d, J = 8.8 Hz, 2H), 7.42-7.5 (m, 3H), 7.57-7.59 (dd, J = 8.4 Hz and J =
2 Hz, 2H), 7.82-7.86 (m, 4H), 8.12 (d, J = 2 Hz, 2H).
4-(2-(3,6-Dibromo-9H-carbazole-9-yl)ethoxy)-2H-chromen-2-one (4b)
This derivative was also prepared with the same procedure described for 4a using 3,6-
dibromo-9-(2-chloroethyl)-9H-carbazole (3) and 4-hydroxy-2H-chromen-2-one. The crude
product was subjected to column chromatography on silica gel eluting with hexane/ethyl
acetate (70:30, v/v) (Rf, 0.50) to obtain 4b as orange solid (1.11 g, 2.18 mmol, 79.5%, mp.:
1
147-150˚C). FTIR ν/ cm-1 3080, 2952, 2890, 1666, 1604, 1509, 1463, 1352, 1241. H NMR
(400 MHz, CDCl3, Me4Si) δH 4.57 (t, J = 7.5 Hz, 2H), 4.98 (t, J = 7.6 Hz, 2H), 5.82 (s, 1H),
7.15-7.25 (m, 1H), 7.55-7.59 (m, 1H), 7.29-7.31 (d, J = 8.7 Hz, 2H), 7.66-7.69 (dd, J = 8.4 Hz
and J = 2 Hz, 2H), 7.79-7.81 (d, J = 8.8 Hz, 2H), 8.48 (d, J = 2 Hz, 2H).
2.3.3. General procedure for Stille cross-coupling reactions of 3,6-dibromocarbazole
derivatives

In a dry 100 mL three-necked round-bottomed flask fitted with a condenser and argon inlet,
3,6-dibromocarbazole derivative (4a or 4b, 0.45 mmol) and 2-(tributylstannyl)thiophene
(0.5g, 1.35 mmol) or tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl) stannane (0.58 g, 1.35
mmol) were dissolved in dry DMF (25 mL). The solution was intensely purged with argon at
room temperature for 20 min and tetrakis(triphenylphosphine)palladium (0) (Pd(PPh3)4,
0,0289 g, 0.025 mmol) was added. The reaction suspension was heated at 110 ºC for 24 h
under inert atmosphere. Then, the reaction mixture was cooled to room temperature and
poured into ice-water (200 mL). The aqueous layer was washed with saturated KF solution (3
x 80 mL) to destroy the residual catalyst and tributylstannyl compounds, and extracted with
chloroform (3 x 40 mL). The organic phases were combined, washed with water and brine and
dried over anhydrous Na2SO4. After removal of solvent by evaporation under reduced
pressure, the residue was subjected to column chromatography using SiO2 as solid phase and
hexane/dichloromethane mixture as eluent to afford a pure 3,6-linked carbazole-thiophene
1
monomer. The chemical structure of monomer was characterized by FTIR, H NMR, 13C
NMR, mass spectrometry techniques and elemental analysis.
9-(2-(4-(Phenyldiazenyl)phenoxy)ethyl)-3,6-di(thiophen-2-yl)-9H-carbazole (TCbzAz)
This monomer was synthesized via Stille cross-coupling reaction of 3,6-dibromo-9-(2-4-
(phenyldiazenyl)phenoxy)ethyl)-9H-carbazole (4a) and 2-(tributylstannyl)thiophene. The
crude product was purified by column chromatography using hexane/dichloromethane (90:10,
v/v) as eluent (Rf, 0.65) to afford a bright orange solid (0,19 g, 0.35 mmol, 78%, mp.: 138-
1
140˚C). FTIR ν/ cm-1 3068, 3039, 2934, 2864, 1599, 1580, 1483, 1360, 1241. H NMR (400
MHz, CDCl3, Me4Si) δH 4.37 (t, J = 7.6 Hz, 2H, ethylene bridge-CH2-), 4.66 (t, J = 7.5 Hz,
2H, ethylene bridge-CH2-), 6.86 (dd, J = 8.7 Hz and J = 2.4 Hz, 2H, Ar-H), 7.09-7.49 (m, 9H,
Th-H, Ar-H), 7.37-7.75 (dd, J = 9.8 Hz and J = 2 Hz, 2H, Cbz-H), 7.81-7.85 (m, 4H, Ar-H),

8.32 (d, J = 2 Hz, 2H, Cbz-H). 13 C NMR (400 MHz, CDCl3, Me4Si) δC 42.6, 66.2, 109,
114.6, 117.9, 122.2, 122.5, 123.4, 123.8, 124.7, 126.4, 128, 129, 130, 140.4, 145.4, 147.2,
160. MS (EI) calculated for C34H25N3OS2 m/z: 555.7, found m/z: 555 (M+, 5), 519 (2), 473
(30), 422 (15), 357 (10), 317 (5), 262 (55), 183 (10), 139 (5), 77 (100). Anal. Calcd. for
(C34H25N3OS2): C, 73.48%; H, 4.53%; N, 7.56%. Found: C, 73.65%; H, 4.12%; N, 7.31%.
3,6-Bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-(2-(4-phenyldiazenyl)phenoxy)ethyl)-
9H- carbazole (ECbzAz)
This monomer was synthesized with the same procedure explained for TCbzAz using 3,6-
dibromo-9-(2-4-(phenyldiazenyl)phenoxy)ethyl)-9H-carbazole
(4a)
and
tributyl(2,3-
dihydrothieno[3,4-b][1,4]dioxin-5-yl) stannane. The residue was subjected to column
chromatography using hexane/dichloromethane (95:5, v/v) as eluent (Rf, 0.50) to afford a pale
orange solid (0,25 g, 0.38 mmol, 82%, mp.: 150-151˚C). FTIR ν/ cm-1 3100, 3063, 2921,
1
2871, 1598, 1582, 1473, 1362, 1245. H NMR (400 MHz, CDCl3, Me4Si) δH 4.24-4.7 (m,
12H, EDOT-CH2- and ethylene bridge –CH2-), 6.27 (s, 1H, Th-H), 6.86-6.9 (t, J = 6.8 Hz, 2H,
Ar-H), 7.35-7.51 (m, 7H, Ar-H and Cbz-H), 7.83-7.86 (m, 4H, Ar-H), 8.14 (d, J = 2 Hz, 2H,
Cbz-H). 13 C NMR (400 MHz, CDCl3, Me4Si) δC 29.7, 42.6, 64.5, 64.8 64.9, 66.2, 96.6, 109,
110, 112.5, 114.5, 118, 122, 123, 124, 125, 128, 129, 130, 130.5, 139, 152.6, 160.4. MS (EI)
calculated for C38H29N3O5S2 m/z: 617.7, found m/z: 611 (2), 549 (25), 471 (10), 444 (5), 422
(5), 338 (25), 273 (5), 259 (10), 164 (12), 105 (8), 77 (100). Anal. Calcd. for (C38H29N3O5S2):
C, 67.94%; H, 4.35%; N, 6.26%. Found: C, 67.65%; H, 4.02%; N, 6.61%.
4-(2-(2-(3,6-Dithiophen-2-yl)-9H-carbazole-9-yl)ethoxy)-2H-chromen-2-one (TCbzCo)
This monomer was prepared by following general procedure using 4-(2-(3,6-dibromo-9H-
carbazole-9-yl)ethoxy)-2H-chromen-2-one (4b) and 2-(tributylstannyl)thiophene. The crude

product was purified by column chromatography using hexane/dichloromethane (70:30, v/v)
as eluent (Rf, 0.65) to afford a bright yellow solid (0,15 g, 0.29 mmol, 65%, mp.: 165-168˚C).
1
FTIR ν/ cm-1 3102, 3053, 2956, 2850, 1713, 1700, 1608, 1566, 1438, 1355, 1234. H NMR
(400 MHz, CDCl3, Me4Si) δH 4.61 (t, J = 7.5 Hz, 2H, ethylene bridge-CH2-), 4.99 (t, J = 7.6
Hz, 2H, ethylene bridge-CH2-), 5.85 (s, 1H, Coumarine –C=CH), 7.09-7.32 (m, 4H, Th-H and
Ar-H), 7.5-7.82 (m, 8H, Cbz-H, Th-H and Ar-H), 7.83 (d, J = 4.8 Hz, 2H, Cbz-H), 8.59 (d, J =
2 Hz, 2H, Cbz-H). 13 C NMR (400 MHz, CDCl3, Me4Si) δC 42.1, 68.8, 91.5, 111.2, 115.4,
117, 118.4, 123.2, 123.5, 126.3, 129.1, 129.4, 129.5, 132.1, 132.7, 133.4, 140.9, 145.3, 153.3,
162, 165.2. MS (EI) calculated for C31H21NO3S2 m/z: 519.6, found m/z: 519 (M+, 25), 475 (3),
437 (65), 375 (35), 344 (75), 310 (5), 275 (15), 262 (100), 228 (5), 165 (5), 137 (3), 121 (12).
Anal. Calcd. for (C31H21NO3S2): C, 71.65%; H, 4.07%; N, 2.70%. Found: C, 71.28%; H,
3.97%; N, 2.66%.
4-(2-(3,6-Bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9H-carbazole-9-yl)ethoxy)-2H-
chromen-2-one (ECbzCo)
This monomer was synthesized via Stille cross-coupling reaction of 4-(2-(3,6-dibromo-9H-
carbazole-9-yl)ethoxy)-2H-chromen-2-one
(4b)
and
tributyl(2,3-dihydrothieno[3,4-
b][1,4]dioxin-5-yl) stannane. The residue was subjected to column chromatography using
hexane/dichloromethane (60:40, v/v) as eluent (Rf, 0.45) to afford a pale yellow solid (0,23 g,
0.36 mmol, 81.5%, mp.: 154-156˚C). FTIR ν/ cm-1 3103, 3065, 2955, 2872, 1706, 1685, 1621,
1
1505, 1473, 1362, 1235. H NMR (400 MHz, CDCl3, Me4Si) δH 4.18-4.34 (m, 8H, EDOT -
CH2-), 4.56 (t, J = 7.5 Hz, 2H, ethylene bridge –CH2-), 4.94 (t, J = 7.4 Hz, 2H, ethylene bridge
–CH2-), 5.8 (s, 1H, Coumarine –C=CH-), 6.55 (s, 2H, Th-H), 7.09-7.27 (m, 4H, Cbz-H and
Ar-H), 7.51-7.84 (m, 4H, Cbz-H and Ar-H), 8.45 (d, J = 2 Hz, 2H, Cbz-H). 13 C NMR (400
MHz, CDCl3, Me4Si) δC 41.9, 64.4, 64.6, 65.1, 65.4, 68.5, 91.3, 97.3, 98, 110.8, 112.1, 112.5,

115.2, 116.8, 118.3, 122, 123.1, 124.2, 125.1, 129.3, 133.1, 139, 153, 161.7. MS (EI)
calculated for C35H25NO7S2 m/z: 635.7, found m/z: 601 (5), 513 (60), 437 (10), 387 (5), 338
(100), 273 (15), 260 (40), 191 (50), 164 (60), 121 (65), 89 (35), 77 (48). Anal. Calcd. for
(C35H25NO7S2): C, 66.13%; H, 3.96%; N, 2.20%. Found: C, 65.93%; H, 3.45%; N, 2.14%.
3. Results and discussion
3.1. Synthesis
Controlling the spectroelectrochemical properties and tuning the band gaps (Eg) of π-
conjugated conducting polymers are crucial and possible by the design and synthesis of novel
electroactive monomers. In this context, combination of strong chromophore compounds such
as azobenzene, coumarin and fluorescein with different monomer units has been explored.
This incorporation approach resulted with conjugated polymers having low band gaps,
multicolor electrochromism, fast switching times and high optical contrast, as observed for
poly(terthienyl) and poly(2,5-dithienylpyrrole) (PSNS) derivatives in our previous studies. In
the present paper, a new kind of 3,6-thiophene-linked carbazole monomers containing
azobenzene and coumarin chromophore units were designed for electrochromic applications
and their electrochemical and spectroelectrochemical behaviors were investigated in detail for
the first time. It is well known that carbazole structure has ability to form stable radicals and
hence, conducting polymers containing carbazole moiety create stable polarons or dipolarons
more effectively on the polymer backbones. Regarding their prominent features, carbazole
based monomers were especially preferred for this study in order to increase the
doping/undoping efficiency and stability of novel conducting polymer derivatives during
oxidation/reduction processes. New 3,6-linked carbazole monomers were synthesized via a
two-step synthetic strategy, as illustrated in Scheme 1. The first step of reaction pathway
involves the microwave-assisted nucleophilic substitution reactions of 3,6-dibromo-9-(2-

chloroethyl)-9H-carbazole (3) with 4-hydroxyazobenzene or 4-hydroxy-2H-chromen-2-one in
order to bring together carbazole moiety and a chromophore unit through an alkyl bridge.
Then, Stille cross-coupling reactions were successfully carried out by using 2-
(tributylstannyl)thiophene or tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl) stannane in
the presence of Pd(PPh3)4 to obtain desired 3,6-thiophene-linked carbazole monomers, 9-(2-4-
(phenyldiazenyl)phenoxy)ethyl)-3,6-di(thiophen-2-yl)-9H-carbazole] (TCbzAz), 3,6-bis(2,3-
dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-(2-(4-(phenyldiazenyl)phenoxy)ethyl)-9H-carbazole
(ECbzAz),
4-(2-(3,6-di(thiophen-2-yl)-9H-carbazol-9-yl)ethoxy)-2H-chromen-2-one
(TCbzCo) and 3,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9H-(carbazol-9-yl)ethoxy)-
2H-chromen-2-one (ECbzCo). The chemical structures of novel 3,6-thiophene-linked
1
carbazole derivatives were confirmed by FTIR, H NMR, 13C NMR, mass spectrometry
techniques and elemental analysis.
3.2. Electrochemistry
In order to study electrochemical redox behavior of novel monomers and produce their
corresponding conducting polymers, TCbzAz, ECbzAz, TCbzCo and ECbzCo were initially
subjected to cyclic voltammetry (CV) in 0.1 M supporting electrolyte solution containing
equimolar amounts of a sodium perchlorate (NaOCl4) and lithium perchlorate (LiOCl4) in
acetonitrile/dichloromethane (ACN/DCM) (95:5, v:v) on indium tin oxide (ITO) coated glass
slides. The cyclic voltammograms were recorded by scanning potential in between 0.0 V and
1.2 V for TCbzAz and ECbzAz, between 0.0 V and 1.3 V potential range for TCbzCo and
ECbzCo at a scan rate of 100 mV s-1 under nitrogen atmosphere. During repeated potential
scanning, all monomers exhibited typical irreversible oxidation behavior versus Ag wire
pseudo-reference electrode as represented in Fig.1. In the first cycle of oxidative
electrochemical polymerizations, irreversible monomer oxidation peaks of TCbzCo, TCbzAz,
ECbzCo and ECbzAz were centered at +0.96 V, +0.98 V, +0.76 V and +0.77 V, respectively.

After the monomer oxidations, reversible redox waves appeared with an increase in the
current density after each succussive oxidation/reduction cycle and the resulting conducting
polymers were regularly grown on ITO electrode surfaces. As seen from cyclic
voltammograms in Fig.1a and Fig.1c, coumarin moiety containing monomers TCbzCo and
ECbzCo revealed slightly lower oxidation potentials than those of their azobenzene included
analogues; TCbzAz and ECbzAz. This trend can be explained by weak inductive effect of
adjacent chromophore units over monomer chains. The carbonyl (-C=O) and etheric (-C-O-C-
) functional groups in the structure of coumarin more increase resonance stabilization of the
chromophore unit more by distributing electron density in the hetereoaromatic system
compared to azobenzene moiety. This effect relatively decreases the electron withdrawing
property of coumarin unit as a pendant group and increases electron densities on the monomer
skeletons of coumarin containing monomers. Therefore, TCbzCo and ECbzCo were slightly
easily oxidized than TCbzAz and ECbzAz. In addition, 3,4-ethylenedioxythiophene (EDOT)
bearing 3,6-linked carbazole monomers (+0.76 V for ECbzCo and +0.77 V for ECbzAz)
distinctly exhibited lower monomer oxidation potentials than those for thiophene bearing ones
(+0.96 V for TCbzCo and +0.98 V for TCbzAz) as expected owing to EDOT’s better
electron donating character. The thicknesses of the electrochemically deposited conducting
polymer films on ITO surfaces were calculated as 0.80 µm for PTCbzAz 0.55 µm for
PTCbzCo, 0.15 µm for PECbzCo and 0.05 µm for PECbzAz. The corresponding deposition
charges for each polymer film were also calculated from the last cycle of the cyclic
voltammograms as; 37.75 mC (PTCbzAz), 24.57 mC (PTCbzCo), 6.5 mC (PECbzCo) and
2.34 mC (PECbzAz).
In order to investigate doping/dedoping properties of resulting electroactive polymers, single
scan cyclic voltammograms of PTCbzCo, PTCbzAz, PECbzCo and PECbzAz were
recorded in a monomer free 0.1 M NaClO4-LiClO4/ACN solution. All polymers have a

tendency to be p-doped when the polymer layers were cycled between different positive
potentials, as illustrated in Fig.2. The reversible redox couples were determined at +1.0
V/+0.61 V for PTCbzCo, at +0.58 V/+0.37 V for PTCbzAz, at +0.51 V/+0.33 V and +0.82
V/+0.64 V for PECbzCo and at +0.48 V/+0.36 V and +0.80 V/+0.70 V for PECbzAz.
The cyclic voltammograms of PTCbzCo, PTCbzAz, PECbzCo and PECbzAz were also
recorded at different scan rates (between 50 and 300 mV s-1) in a monomer free medium in
order to examine the scan rate dependence of doping/dedoping peak currents of the conducting
polymer films. A linear relationship between the peak currents and scan rates proves that
electroactive polymer films were well adhered and the doping/dedoping processes were non-
diffusion controlled (Fig. 3). HOMO energy levels of novel conducting polymers were
calculated from oxidation onset values in cyclic voltammograms as -5.40 eV (PTCbzCo), -
5.35 eV (PTCbzAz), -4.85 eV (PECbzCo) and -5.21 eV (PECbzAz). The onset values are
generally determined by taking the interaction between the baseline and tangent line drawn to
the increasing part of current. On the other hand, LUMO energy levels were calculated from
op
HOMO energy levels and optical band gaps (Eg ) which are calculated from π-π* transition
onset in UV-Vis spectrum since all polymer films revealed only p-doping property. LUMO
energies were calculated as -3.33 eV for PTCbzCo, -3.42 eV for PTCbzAz, -3.50 eV for
PECbzCo and -3.10 eV for PECbzAz, as summarized in Table 1. The reference electrodes
were calibrated to Fc/Fc+ (+0.30 V) and HOMO-LUMO energies were calculated relative to
the vacuum level (NHE is -4.75 eV versus vacuum).
To evaluate the long-term redox stabilities of conducting polymers films, single scan cyclic
voltammograms of PTCbzCo, PTCbzAz, PECbzCo and PECbzAz were recorded in a
monomer free 0.1 M NaClO4-LiClO4/ACN electrolyte/solvent couple for 500 cycles. As
seen in Fig. 4, especially PTCbzAz and PECbzCo exhibited good stability and redox activity
after 500 cycles.

3.3. Spectroelectrochemistry
UV-vis-NIR absorption spectra of PTCbzCo, PTCbzAz, PECbzCo and PECbzAz were
recorded in a monomer free 0.1 M NaClO4-LiClO4/ACN solution upon gradually increasing
external bias in order to evaluate the optical behaviors and spectral responses of corresponding
conducting polymers. Prior to spectroelectrochemical analysis, all polymer films coated
electrochemically on ITO surfaces were subjected to -0.3 V to reduce them into their neutral
states and remove any trapped charge or dopant ions on the polymer chains during
electrochemical polymerization processes. The in situ spectroelectrochemical studies were
performed between -0.3 V and +1.1 V for PTCbzCo, -0.3 V and +1.2 V for PTCbzAz, 0.0 V
and +1.0 V for PECbzCo and -0.2 V and +1.0 V for PECbzAz using a UV-vis-NIR
spectrophotometer coupled to cyclic voltammetry. As can be seen in Fig.5a and Fig.5b,
PTCbzCo exhibited a sharper absorption maximum in the visible region centered at 410 nm
whereas PTCbzAz revealed broad absorption maxima at 370 nm with a shoulder at 415 nm in
the neutral state. Substitution of 3,4-ethylenedioxythiophene (EDOT) units on the conjugated
main chain instead of thiophene also resulted in relatively bathochromically shifted absorption
maxima for PECbzCo (λmax= 470 nm) and PECbzAz (λmax= 437 nm) compared to PTCbzCo
and PTCbzAz. The maximum absorption peaks (λmax) are due to the conducting polymers’
high energy π-π* transition. Hence, the bathochromic (red) shift for PECbzCo (λmax= 470 nm)
and PECbzAz (λmax= 437 nm) can be attributed to intramolecular π-interaction (π-orbital
overlap) between coumarin chromophore unit and 3,6-linked EDOT-carbazole-EDOT
polymer backbone is more effective than those of 3,6-linked thiophene-carbazole-thiophene in
PTCbzCo and PTCbzAz.
Upon stepwise oxidation, new absorption bands in NIR region appeared at around 630/1010
nm for PTCbzCo and 640/1100 nm for PTCbzAz as intensity of absorption bands in the
visible region diminished. These new electronic transitions with lower energies at longer

wavelengths prove the formation of polarons (radical cations) and bipolarons (dications) on
the polymer films. On the other hand, it was observed that there was no decrease in the neutral
state absorption intensities of PECbzCo and PECbzAz during the formation of polaronic
absorption bands at 850 nm and 590 nm/950 nm were determined for PECbzCo and
PECbzAz, respectively (Fig. 5c and Fig. 5d). This tendency is probably a result of
intersection of the polaron bands of PECbzCo and PECbzAz with those of the neutral
absoprtion peaks. Spectroelectrochemical studies also showed that the incorporation of strong
chrmophore units (azobenzene and coumarin) in thiophene-carbazole-thiophene polymer
backbone resulted in multicolored property. Colorimetry studies were also performed in order
to identify conducting polymer films’ colors and L, a, b coordinates for their different states.
The L, a, b coordainates and colors of PECbzCo, PECbzAz, PTCbzCo and PTCbzAz films
upon different potentials are given in Fig. 6.
op
The optical band gaps (Eg ) for PECbzCo, PECbzAz, PTCbzCo and PTCbzAz were
calculated by utilizing the onsets of the lowest energy π-π* transitions on the neutral state
absorption spectra (Egop = 1241/λmax) as 1.75 eV, 1.71 eV, 2.07 eV and 1.93 eV, respectively
(Table 1). Low optical band gap values for PECbzCo and PECbzAz can be explained by
planar conformity. The 3,6-linked EDOT-carbazole-EDOT polymer backbone in PECbzCo
and PECbzAz is more planar and rigid conjugated chain owing to the fact that the presence of
EDOT units provides extra planarity, compared to thiophene-carbazole-thiophene skeleton.
Hence, π-orbital overlapping between polymer chains and chromophore units in PECbzCo
and PECbzAz are more effective than those of PTCbzCo and PTCbzAz. The enhaced
possibility of π-orbital interactions gave rise to a distinctive decrease in band gap values of
PECbzCo and PECbzAz. These results observed for novel poly(3,6-dithienylcarbazole) type
conducting polymer derivatives (PECbzCo, PECbzAz, PTCbzCo and PTCbzAz) were
compared with poly(2,5-dithienylpyrrole) (PSNS) (PTPTCo and PTPTAz) derivatives,

containing the same chromophore units in order to have a better understanding of the
influence of the type of monomer backbone on electronic and optical properties of
corresponding polymers [43]. It was seen that the poly(3,6-dithienylcarbazole) derivatives
have conspicuously lower band gap values (1.75 eV, 1.71 eV, 2.07 eV and 1.93 eV) than those
for poly(2,5-dithienylpyrrole) derivatives (2.44 eV and 2.88 eV). Also, PECbzCo, PECbzAz,
PTCbzCo and PTCbzAz revealed intermediate colored states with a wide color scale
different from their poly(2,5-dithienylpyrrole) analogues. These results proved that structural
designs for the 3,6-linked carbazole derivatives were accomplished in terms of improving
electronic and spectral features of conducting polymers.
3.4. Kinetic Studies
Kinetic measurements together with spectroelectrochemical experiments were performed in
order to observe percent transmittance changes (ΔT%) and to calculate switching times
required for the transition period between neutral and oxidized states. The novel π-conjugated
conducting polymer films were initially subjected to chronoabsorptometry studies in a
monomer free solution containing 0.1 M NaClO4-LiClO4/ACN and the percentage
transmittances of PECbzCo, PECbzAz, PTCbzCo and PTCbzAz were recorded for their
dominant wavelenghts in visible and NIR regions as a function of time upon applied external
bias within 5 s time intervals. As depicted in Fig.7a and Fig.7c, PTCbzCo films revealed
optical contrast values (ΔT%) of 44% at 1010 nm, 11% at 630 nm and 18% at 410 nm while
the transmittance changes (ΔT%) for PECbzCo were calculated as 31% at 850 nm and 14% at
470 nm. As seen, PTCbzCo and PECbzCo exhibited the superior optical contrast
characteristics in NIR region due to multicolor formation between their doped/states in the
UV-Vis region. On the other hand, PTCbzAz (22% at 1100 nm, 11% at 640 nm and 6% at
415 nm) and PECbzAz (10% at 950 nm and 7% 590 nm) showed lower optical contrast
values than PTCbzCo and PECbzCo in both UV-Vis and NIR regions (Fig.7b and Fig.7d).

The significant differences in optical contrast values are due to the difficulty in charge
injection/ejection processes for PTCbzAz and PECbzAz compared to those of PTCbzCo and
PECbzCo. In other words, PTCbzAz and PECbzAz can switch rather slowly between their
extreme states.
The switching times for PECbzCo, PECbzAz, PTCbzCo and PTCbzAz were calculated by
utilizing kinetic studies for both visible and NIR regions. The switching times were
determined as 0.7 s at 850 nm and 470 nm for PECbzCo while PECbzAz revealed switching
times of 2.4 s (950 nm) and 3.3 s (590 nm). PTCbzCo has switching times of 1.3 s, 1.9 s and
2.1 s at 1010 nm, 630 nm and 410 nm, respectively. Similary, the switching times for
PTCbzAz at 1100 nm, 640 nm and 415 nm were found to be 3.0 s, 2.4 s and 2.8 s. When
strong chromophore units containing multicolored conducting polymer films are compared in
terms of their switching abilities, PECbzCo and PTCbzCo functionalized with coumarin unit
switch faster than PECbzAz and PTCbzAz derivatives bearing an azobenzene unit.
4. Conclusions
Four novel 3,6-linked thiophene-carbazole-thiophene type monomers bearing azobenzene and
coumarin units as the pendant groups were designed and synthesized successfully in order to
have a better comparison and understanding the effect of chromophore units on
electrochromic properties of conjugated conducting polymers. PECbzCo, PECbzAz,
PTCbzCo and PTCbzAz polymers were electrochemically synthesized on ITO coated glass
slides and characterized in terms of their electrochemical and optical behaviors. The oxidation
op
potentials, HOMO/LUMO energy levels, optical band gap values (Eg ), optical contrast
(ΔT%) and switching times of the novel π-conjugated conducting polymers were evaluated by
utilizing cyclic voltammetry and in situ spectroelectrochemical studies. The results revealed
that electronic and optical properties of conducting polymers were remarkably affected by the

type of strong chromophore group. The coumarin unit containing polymers PECbzCo and
PTCbzCo exhibited better electrochemical and optical characteristics in terms of band gaps,
optical contrast and switching times than their analogues bearing an azobenzene unit as the
pendant group; PECbzAz and PTCbzAz. All electrochemical and spectroelectrochemical
studies also proved that the incorporation of strong chromophore units with 3,6-linked
thiophene-carbazole-thiophene polymer backbone resulted in lower band gaps and
multichromic properties with a wide color scale compared to poly(2,5-dithienylpyrrole)
(PSNS), poly(2,5-di-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-ylthienyl) and poly(2,2:5,2-
terthien-3-yl) derivatives functionalized with the same chromophore units in our previous
published papers [41-43]. In the lights of results in hand, it may be inferred that the novel 3,6-
linked thiophene-carbazole-thiophene type conducting polymers are good candidates and have
the potential to be used in different electrochromic applications.
Acknowledgements
M. Güllü and D. Yiğit greatfully thank Scientific and Technological Research Council of
TURKEY-TÜBİTAK (110T071) for their generous financial support.
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Fig.1. Repetitive cyclic voltammograms of (a) TCbzCo, (b) TCbzAz, (c) ECbzCo and
ECbzAz in 0.1 M NaClO4/LiClO4/ACN/DCM on an ITO electrode
(d)

Fig.2. Single scan cyclic voltammograms of (a) PTCbzCo, (b) PTCbzAz, (c) PECbzCo and (d)
PECbzAz in 0.1 M NaClO4/LiClO4/ACN/DCM monomer free solution

Fig.3. Scan rate dependence of (a) PTCbzCo, (b) PTCbzAz, (c) PECbzCo and (d) PECbzAz
conducting polymer films

Fig.4. The stability of the electrochemically synthesized conducting polymer films (a)
PTCbzCo, (b) PTCbzAz, (c) PECbzCo and (d) PECbzAz

Fig.5. Electronic absorption spectra of (a) PTCbzCo film between -0.3 V and +1.1 V, (b)
PTCbzAz film between -0.3 V and +1.2 V, (c) PECbzCo film between 0.0 V and +1.0 V and
(d) PECbzAz film between -0.2 V and +1.0 V on an ITO coated slide in a momomer-free in
0.1 M NaClO4/LiClO4/ACN/DCM electrolyte-solvent couple

Fig.6. The colors and chromaticity coordinates of PTCbzCo, PTCbzAz, PECbzCo and
PECbzAz films at their neutral and various oxidized states