Fluorescent and electrochromic polymers from 2,8-di(carbazol-9-yl)dibenzothiophene and its S,S-dioxide derivative

Article abstract of DOI:10.1016/j.dyepig.2016.06.043

We report the synthesis and characterization of two carbazole-endcapped monomers, namely 2,8-di(carbazol-9-yl)dibenzothiophene (SCz) and 2,8-di(carbazol-9-yl)dibenzothiophene S,S-dioxide (SO₂Cz), and their derived polymers PSCz and PSO₂Cz prepared by both FeCl₃ oxidative coupling and electropolymerization processes. The dilute solutions of PSCz and PSO₂Cz prepared by chemical oxidative coupling exhibited fluorescent and solvatochromic behavior. Thin polymeric films could be robustly electrodeposited onto the surface of the ITO-glass substrate by repetitive cyclic voltammetry scanning of monomers SCz and SO₂Cz in an electrolyte solution between 0 and 1.8?V. The electrochemically generated polymer films exhibited two reversible oxidation redox couples due to successive oxidations of the biscarbazole unit. These polymer films also revealed good electrochemical and electrochromic stability, with coloration change from a colorless neutral state to yellow, green and blue oxidized states. Switching ability of the polymers was evaluated by kinetic studies upon measuring the percent transmittance (%T) at their maximum contrast point, indicating that the polymers exhibit moderate electrochromic performance.

Full text of DOI:10.1016/j.dyepig.2016.06.043

Dyes and Pigments 134 (2016) 51e63
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Fluorescent and electrochromic polymers from 2,8-di(carbazol-9-yl)
dibenzothiophene and its S,S-dioxide derivative
Sheng-Huei Hsiao*, Li-Chu Wu
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and characterization of two carbazole-endcapped monomers, namely 2,8-
di(carbazol-9-yl)dibenzothiophene (SCz) and 2,8-di(carbazol-9-yl)dibenzothiophene S,S-dioxide
(SO2Cz), and their derived polymers PSCz and PSO2Cz prepared by both FeCl3 oxidative coupling and
electropolymerization processes. The dilute solutions of PSCz and PSO2Cz prepared by chemical oxida-
tive coupling exhibited fluorescent and solvatochromic behavior. Thin polymeric films could be robustly
electrodeposited onto the surface of the ITO-glass substrate by repetitive cyclic voltammetry scanning of
monomers SCz and SO2Cz in an electrolyte solution between 0 and 1.8 V. The electrochemically
generated polymer films exhibited two reversible oxidation redox couples due to successive oxidations of
the biscarbazole unit. These polymer films also revealed good electrochemical and electrochromic sta-
bility, with coloration change from a colorless neutral state to yellow, green and blue oxidized states.
Switching ability of the polymers was evaluated by kinetic studies upon measuring the percent trans-
mittance (%T) at their maximum contrast point, indicating that the polymers exhibit moderate elec-
trochromic performance.
Received 13 May 2016
Received in revised form
24 June 2016
Accepted 26 June 2016
Available online 29 June 2016
Keywords:
Electrochemistry
Electropolymerization
Electrochromics
Dibenzothiophene
Carbazole
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
switching times, processability, and fine-tuning of the bandgap by
structural modification [14e19].
Functionalized conjugated polymers have attracted continuing
attention since they are potentially useful materials for a broad
range of applications such as photovoltaic devices [1], light-
emitting diodes (LEDs) [2], organic thin-film transistors [3], sen-
sors [4], and electrochromic devices [5e8]. Electrochromism con-
sists in the formation of new optical transitions in an electroactive
species subjected to reversible electrochemical oxidation/reduction
process [9]. There are three general classes of electrochromic ma-
terials; inorganic materials such as transition metal oxides and
Prussian Blue, molecular electrochromes such as viologens, and
conjugated polymers. Optically responsive conjugated polymer
systems that reveal electrochromism are particularly required for
their potential use in smart window and display technologies. Even
though many inorganic materials, especially tungsten oxides, have
been utilized over the past decades [10e13], the use of conjugated
polymers as active layers in electrochromic devices has received
enormous attention because of their high optical contrasts, fast
Carbazole derivatives are well-known hole-transporting and
photonic materials for the applications in optoelectronic devices
[20,21]. Carbazole can be easily functionalized at its (3,6-), (2,7-), or
N-positions, and then covalently linked into polymeric systems,
both in the main chain as building blocks and in a side chain as
subunit. Carbazole-containing polymers such as poly(N-vinyl-
carbazole), poly(2,7-carbazole)s, and poly(3,6-carbazole)s are
considered as a very important class of electroactive and photo-
active materials [22]. For instance, the excellent hole transporting
properties of poly(2,7-carbazole) derivatives make them highly
promising materials for electroluminescent devices, organic thin-
film transistors and photovoltaic cells [23e25]. It has been
demonstrated that poly(3,6-carbazole) derivatives exhibit inter-
esting electrochromic properties because of the conjugation breaks
that are present due to the inclusion of a 3,6-linkage [26]. N-
Substituted carbazole based polymers are generally colorless or
near-colorless at their neutral state and then can change colors at
their oxidized states, which is an important property for neutral
state colorless electrochromic device applications [27e29]. It has
been demonstrated that N-substituted carbazoles can form bis-
carbazoles upon anodic oxidation [30]. Therefore, various
* Corresponding author.
E-mail address: shhsiao@ntut.edu.tw (S.-H. Hsiao).
http://dx.doi.org/10.1016/j.dyepig.2016.06.043
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

52
S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
carbazole-containing electrochomic polymeric films have been
prepared via the electrochemical coupling of carbazole units
[31e39].
(C4), 141.17 (C7). Calcd for C24H22N2S (370.52): C, 77.80%; H, 5.98%;
N, 7.56%. Found: C, 77.68%; H, 5.89%, N, 7.50%.
Dibenzothiophene and its S,S-dioxide derivatives containing
diarylamine segments have been reported as potential hole-
transporting and emitting materials [40e44]. Conjugated co-
polymers containing electron-deficient dibenzothiophene-S,S-di-
oxide moiety are of particular interests on account of their high
efficiency and excellent spectral stability [45,46]. The synthesis and
optical and electrochromic properties of dibenzothiophene (or
dibenzothiophen-S,S-dioxide) and 3-hexylthiophene (or EDOT)
based conjugated polymers have been reported in literature
[47e51]. However, to our knowledge, there is little information
about the synthesis and electrochromic properties of electroactive
polymers from dibenzothiophene or dibenzothiophene-S,S-dioxide
containing carbazole derivatives so far. In this study, two carbazole
end-capped monomers containing dibenzothiophene (SCz) or
dibenzothiophene-S,S-dioxide (SO2Cz) as the core unit are syn-
thesized and their polymers are successfully synthesized from both
chemically oxidative polymerization and electrochemical poly-
merization techniques. The electrochemical, electrochromic, and
luminescent properties of these polymers are also described herein.
Two referenced compounds with tert-butyl blocking groups on
their carbazole units, namely 2,8-di(3,6-di-tert-butylcarbazol-9-yl)
dibenzothiophene (StBCz) and 2,8-di(3,6-di-tert-butylcarbazol-9-
yl)dibenzothiophene-S,S-dioxide (SO2tBCz), are also synthesized
and characterized as a contrast experiment.
a
S
2
1
3
b
d
8
e
9
6
4
N
7
c
5
N
10
f
g
12
11
2.2.2. Synthesis of 2,8-di(carbazol-9-yl)dibenzothiophene-S,S-
dioxide (SO2Cz)
In a 100-mL three-neck round-bottomed flask equipped with a
stirring bar under nitrogen atmosphere, mixture of 2,8-
a
dibromodibenzothiophene-S,S-dioxide (3.74 g, 0.01 mol), carba-
zole (4.18 g, 0.025 mol), K2CO3 (5.53 g, 0.04 mol), and Cu (1.34 g,
0.021 mol) in TEGDME (15 mL) was heated at 180 ꢀC with stirring
for 24 h. The reaction mixture was filtered while hot, and the filtrate
was poured into 200 mL methanol. The precipitated product was
collected by filtration (1.15 g, 21% yield) and then recrystallized
from toluene to give white crystals with a melting point of
392e395 ꢀC. FT-IR (KBr): 1300 and 1162 cmꢁ1 (sulfonyl asymmetric
2. Experimental details
and symmetric stretching). 1H NMR (500 MHz, CDCl3,
d, ppm): 7.31
(t, J ¼ 8.0 Hz, 4H, Hf), 7.44 (t, J ¼ 8.0 Hz, 4H, He), 7.58 (d, J ¼ 8.0 Hz,
4H, Hd), 7.96 (dd, J ¼ 8.5, 2.0 Hz, 2H, Hb), 8.25 (d, J ¼ 8.0 Hz, 4H, Hg),
8.34 (d, J ¼ 8.5 Hz, 2H, Ha), 8.80 (d, J ¼ 2.0 Hz, 2H, Hc). 13C NMR
2.1. Materials
2,8-Dibromodibenzothiophene
(1),
2,8-
(125 Hz, CDCl3, d
, ppm): 110.11 (C8),120.52 (C11),120.75 (C10),121.91
dibromodibenzothiophene-S,S-dioxide (2) and 3,6-di-tert-butyl-
9H-carbazole (3) were synthesized according to literature methods
[52e54]. Dibenzothiophene (Acros), carbazole (Acros), N-bromo-
succinimide (NBS, Acros), N,N-dimethylformamide (DMF, Tedia),
hydrogen peroxide 35% (H2O2, Shimakyu’s Pure Chemicals), acetic
acid (Tedia), 2-chloro-2-methylpropane (tert-butyl chloride)
(Acros), aluminum chloride (Acros), dichloromethane (CH2Cl2,
Fischer Chemical), potassium carbonate (K2CO3, Showa), copper
(Cu, Acros), triethylene glycol dimethyl ether (TEGDME, Acros),
iron(III) chloride (FeCl3, Fischer Chemical), nitrobenzene (Acros),
18-crown-6 (TCI), and 1,2-dichlorobenzene (Acros) were used
without further purification. Tetrabutylammonium perchlorate
(Bu4NClO4) (Arcos) was dried in vacuo prior to use. All other re-
agents were used as received from commercial sources.
(C5), 123.15 (C12), 123.95 (C2), 126.48 (C9), 129.01 (C3), 132.94 (C6),
135.51 (C1), 139.53 (C7), 142.57 (C4). Calcd for C24H22N2SO2
(402.52): C, 71.61%; H, 5.51%; N, 6.96%. Found: C, 71.52%; H, 5.45%,
N, 6.88%.
O
1
O
a
S
2
3
b
d
8
e
9
6
4
N
7
c
5
N
10
f
g
12
11
2.2. Monomer synthesis
2.2.1. Synthesis of 2,8-di(carbazol-9-yl)dibenzothiophene (SCz)
In a 100-mL three-neck round-bottomed flask equipped with a
stirring bar under nitrogen atmosphere,
a mixture of 2,8-
dibromodibenzothiophene (3.42 g, 0.01 mol), carbazole (4.18 g,
0.025 mol), K2CO3 (5.53 g, 0.04 mol), and Cu (1.34 g, 0.021 mol) in
TEGDME (15 mL) was heated at 180 ꢀC with stirring for 24 h. The
reaction mixture was hot filtered, and the filtrate was poured into
200 mL ethanol. The light yellow crystals (2.68 g, 52% yield;
mp ¼ 290e293 ꢀC) was collected by filtration and vacuum dried. 1H
2.3. Polymer synthesis
2.3.1. Electrochemical polymerization
Electrochemical polymerization was carried out with a CH In-
struments 750A electrochemical analyzer. The polymers were
synthesized from 1 ꢂ 10ꢁ4 M monomer solutions in 0.1 M Bu4N-
ClO4/CH2Cl2 via repetitive cyclic voltammetry (CV) scanning be-
tween 0 and 1.8 V at a scan rate of 50 mV sꢁ1 for ten cycles. The
polymer was deposited onto the surface of the working electrode
(ITO/glass surface, polymer films area about 0.8 cm ꢂ 1.25 cm), and
the film was rinsed with plenty of acetone for the removal of
inorganic salts and other organic impurities formed during the
NMR (500 MHz, CDCl3,
d
, ppm): 7.31 (t, J ¼ 7.0 Hz, 4H, Hf), 7.42 (t,
J ¼ 8.0 Hz, 4H, He), 7.43 (d, J ¼ 7.0 Hz, 4H, Hd), 7.73 (dd, J ¼ 8.5,
2.0 Hz, 2H, Hb), 8.15 (d, J ¼ 8.5 Hz, 2H, Ha), 8.17 (d, J ¼ 7.5 Hz, 4H,
Hg), 8.34 (d, J ¼ 2.0 Hz, 2H, Hc). 13C NMR (125 MHz, CDCl3,
d, ppm):
109.54 (C8), 120.08 (C10), 120.38 (C11), 120.59 (C5), 123.42 (C12),
124.35 (C2), 126.06 (C9), 126.58 (C3), 134.94 (C6), 136.59 (C1), 139.24

S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
53
Scheme 1. Synthetic routes to compounds SCz, SO2Cz, StBCz and SO2tBCz.
process.
2.5. Instrumentation and measurements
Infrared (IR) spectra were recorded on a Horiba FT-720 FT-IR
spectrometer. 1H and 13C NMR spectra were measured on a Bruker
AVANCE 500 FT-NMR system with tetramethylsilane as an internal
standard. Elemental analyses were carried out with a Heraeus
VarioEL III elemental analyzer. DSC analyses were performed on a
Perkin-Elmer DSC 4000 at a scan rate of 20 ꢀC minꢁ1 under nitro-
gen. Ultravioletevisible (UVeVis) spectra of the polymer films were
recorded on an Agilent 8453 UVeVisible spectrometer. Gel
permeation chromatography (GPC) analysis was carried out on a
Waters chromatography unit interfaced with a Waters 2410
2.3.2. Chemical oxidative polymerization
PSCz and PSO2Cz could also be prepared by the chemical
oxidative polymerization of SCz and SO2Cz, respectively, with FeCl3
as an oxidant. In a 50-mL two-necked round-bottom flask equipped
with a stirring bar under nitrogen atmosphere, a mixture of SCz
(0.514 g, 1 mmol), FeCl3 (0.404 g, 2.5 mmol) in nitrobenzene (2 mL)
was stirred at room temperature for 24 h. The reaction mixture was
poured into methanol containing 10% hydrochloric acid. The pre-
cipitate was collected and washed thoroughly with aqueous
ammonium hydroxide. The crude polymer (0.43 g, 84% yield) was
re-dissolved in chloroform. After removing the insoluble fraction by
filtration, the filtrate was condensed and poured into methanol. The
resulting precipitate was washed with methanol and dried under
reduced pressure to yield the desired polymer PSCz (0.28 g, 54%
yield). PSO2Cz (88% yield for the crude product; 42% for the soluble
part) was prepared from monomer SO2Cz by a similar procedure.
refractive index detector. Two Waters 5 mm Styragel HR-2 and HR-4
columns (7.8 mm I.D. ꢂ 300 mm) were connected in series with
NMP as the eluent at a flow rate of 0.5 mL minꢁ1 at 40 ꢀC and were
calibrated with polystyrene standards. Electrochemistry was per-
formed with a CHI 750A electrochemical analyzer. Voltammograms
are presented with the positive potential pointing to the left and
with increasing anodic currents pointing downwards. Cyclic vol-
tammetry (CV) was conducted with the use of a three-electrode cell
in which ITO (polymer films area about 0.8 cm ꢂ 1.25 cm) was used
as a working electrode. A platinum wire was used as an auxiliary
electrode. All cell potentials were taken with the use of a home-
made Ag/AgCl, KCl (sat.) reference electrode. Ferrocene was used
as an external reference for calibration (þ0.48 V vs. Ag/AgCl).
Spectroelectrochemistry analyses were carried out with an elec-
trolytic cell, which was composed of a 1 cm cuvette, ITO as a
working electrode, a platinum wire as an auxiliary electrode, and a
Ag/AgCl reference electrode. Absorption spectra in the spec-
troelectrochemical experiments were measured with an Agilent
8453 UVeVisible photodiode array spectrophotometer. Photo-
luminescence (PL) spectra were measured with a Varian Cary
Eclipse fluorescence spectrophotometer. Fluorescence quantum
yields (FF) values of the samples in NMP were measured by using
2.4. Fabrication of electrochromic device
Electrochromic PSCz films were prepared by electro-
polymerization of SCz onto an ITO-coated glass substrate
(20 ꢂ 30 ꢂ 0.7 mm, 50e100
U
cmꢁ2) by a procedure as described
above. A gel electrolyte based on PMMA (Mw: 120000) and LiClO4
was plasticized with propylene carbonate to form a highly trans-
parent and conductive gel. PMMA (1 g) was dissolved in dry
acetonitrile (4 mL), and LiClO4 (0.1 g) was added to the polymer
solution as supporting electrolyte. Then propylene carbonate (1.5 g)
was added as plasticizer. The mixture was then gently heated until
gelation. The gel electrolyte was spread on the polymer-coated side
of the electrode, and the electrodes were sandwiched. Finally, an
epoxy resin was used to seal the device.

54
S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
dibromodibenzothiophene (1) was prepared from bromination of
dibenzothiophene with N-bromosuccinimide (NBS) in DMF, the
oxidation of compound 1 with hydrogen peroxide in acetic acid
gave 2,8-dibromodibenzothiophene-S,S-dioxide (2). Friedel-Crafts
alkylation of carbazole with tert-butyl chloride in the presence of
aluminum chloride gave 3,6-di-tert-butyl-9H-carbazole (3). The
target monomers SCz and SO2Cz and their tert-butyl derivatives
StBCz and SO2tBCz were synthesized by the Ullmann CeN coupling
reactions of dibromo compounds 1 and 2 with carbazole and 3 by
using copper powder (Scheme 1). The synthetic details and char-
acterization data of the tert-butyl derivatives are included in the
Supplementary Information file. The structures of all the synthe-
sized compounds were confirmed by FTIR, 1H and 13C NMR ana-
lyses. Fig. S1 (Supplementary Information) illustrates the FT-IR
spectra of the synthesized monomers SCz and SO2Cz together
with their starting materials. The IR spectra of compounds 2 and
SO2Cz gave characteristic bands of sulfonyl groups at around 1300
and 1162 cmꢁ1 (eSO2e asymmetric and symmetric stretching). The
secondary amino group of carbazole showed
a sharp NeH
stretching absorption at 3419 cmꢁ1, which disappeared after Ull-
mann coupling reaction. The IR spectra of the referenced com-
pounds StBCz and SO2tBCz and their starting materials are
compiled in Fig. S2. The IR spectra of compounds 2 and SO2tBCz
gave characteristic bands of sulfonyl groups at around 1300 and
1162 cmꢁ1 (eSO2e asymmetric and symmetric stretching). Com-
pounds 3, StBCz and SO2tBCz showed additional aliphatic CeH
stretching absorptions around 2956 cmꢁ1 due to the presence of
tert-butyl groups. Fig. 1 illustrates the 1H NMR spectra of SCz and
SO2Cz. 13C NMR and two-dimensional (2-D) NMR spectra of SCz,
SO2Cz, StBCz and SO2tBCz are summarized in the Supplementary
Information (Figs. S3eS6). Assignments of all proton and carbon
signals were assisted by the 2-D NMR spectra. These spectra agree
well with their proposed molecular structures. In the proton NMR
spectra, the aromatic protons in the central core (i.e., protons Ha to
Hc) of compounds SO2Cz and SO2tBCz resonated at a more
downfield region than those of compounds SCz and StBCz due to
the strong electron-withdrawing nature of the sulfonyl group. The
resonance peaks appearing at a high field region (about 1.47 ppm)
and their integration values in the proton NMR spectra, together
with the resonance signals of the carbon-13 NMR spectra at 31 and
34 ppm confirmed the presence of tert-butyl groups in StBCz and
SO2tBCz. Thus, all the IR and NMR spectra confirmed the successful
syntheses of the desired compounds.
Fig. 1. 1H NMR spectra of SCz and SO2Cz in CDCl3.
quinine sulfate in 1 N H2SO4 as a reference standard (FF ¼ 0.546).
3. Results and discussion
3.1. Monomer synthesis
According
to
well-known chemistry
[52e54],
2,8-
Fig. 2. First (in red) and second (in black) CV diagrams of 1 ꢂ 10ꢁ4 M solutions of (a) SCz and (b) SO2Cz in 0.1 M Bu4NClO4/CH2Cl2 at a scan rate of 50 mV sꢁ1. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version of this article.)

S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
55
Fig. 3. Repetitive CV diagrams of 1 ꢂ 10ꢁ4 M solutions of (a) SCz, (b) SO2Cz, (c) StBCz and (d) SO2tBCz in 0.1 M Bu4NClO4/CH2Cl2 at a scan rate of 50 mV sꢁ1. The first CV curves are
marked in red.
3.2. Polymer synthesis
peaks. This behavior suggests that the oxidative coupling of the
radical cations of SCz produced a continuous build-up of an elec-
troactive and conductive layer on the electrode. Similar results
were observed for SO2Cz (Fig. 3(b)). The electrodeposited PSCz and
PSO2Cz polymer films are proposed to be formed by the carbazole-
carbazole coupling reactions. In contrast, StBCz and SO2tBCz
showed reversible and almost the same oxidation behavior upon
repeated scanning (Fig. 3(c) and (d)). The electrochemical stability
and non-polymerizability of these two compounds can be attrib-
uted to the blocking of the active sites of carbazole by the tert-butyl
group.
3.2.1. Electrochemical polymerization
The electrochemical property of the dicarbazole compounds
was probed by cyclic voltammetry (CV). Fig. 2 presents the CV di-
agrams of SCz and SO2Cz in 0.1 M Bu4NClO4/CH2Cl2. For the first
positive potential scan, an oxidation peak at ca. 1.60 V and 1.70 V
was observed. From the first reverse negative potential scan, two
cathodic peaks were detected. In the second scan, a new oxidation
peak appeared at 1.17 V and 1.26 V, respectively. That was the
complementary anodic process of the cathodic peak at a lower
potential. The observation of a new oxidation couple in the second
potential scan implies that the SCz and SO2Cz radical cations were
involved in very fast electrochemical reactions that produced a
substance that was easier to oxidize than was the parent SCz and
SO2Cz. The additional peak is most likely resulted from coupling
reaction of C3 and C6 of the carbazole. Fig. 3(a) displays the suc-
cessive cyclic voltammograms (CV) of the SCz solution between
0 and 1.8 V at a potential scan rate of 50 mV sꢁ1. In the first anodic
half-cycle, one oxidation peak is observed at Epa ¼ 1.60 V, which is
attributed to the oxidation of carbazole unit, resulting in formation
of oligomers/polymer in the vicinity of electrode surface. In the
reverse scan, two peaks are detected at Epc ¼ 0.91 and 1.20 V, which
correspond to reduction of film deposited in the preceding anodic
scan. In the second and subsequent voltammetric cycles, the anodic
peaks gradually shifted to higher potentials and the cathodic peaks
shifted to lower potentials with the increasing intensity of the
3.2.2. Chemical oxidative polymerization
PSCz and PSO2Cz could also be readily prepared by chemical
oxidative polymerizations of SCz and SO2Cz with FeCl3 as an
oxidant. By using the reaction conditions shown in the Experi-
mental details, the reactions generally resulted in a large insoluble
fraction. For the chemical polymerization of SCz, an 84% yield of the
polymer with 35% insoluble fraction was obtained. In the case of
SO2Cz, an 88% yield of the polymer with 52% insoluble fraction was
obtained. The high yields and the formation of a great amount of an
insoluble fraction indicate that these two monomers are highly
reactive and more than 3 positions were reacted leading to the gel
formation. The solubility of the CHCl3-insoluble polymers was also
tested in highly polar organic solvents such as NMP and DMAc.
They were insoluble in all the available organic solvents and even in
concentrated sulfuric acid. These results implied that the insoluble

56
S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
Fig. 4. Absorbance and photoluminescence (PL) spectra of (a) PSCz and (b) PSO2Cz in solution and solid film. (c) Photographs were taken under illumination of a 365 nm UV light.
polymers exhibited a crosslinked structure. The soluble portions of
PSCz and PSO2Cz were used for GPC, FT-IR, DSC, and UVevis ab-
sorption measurements. As shown in Table S1, the soluble PSCz and
PSO2Cz had Mw values of 5000 and 3000, respectively, indicating
they are low-molecular-weight oligomers.
3.3. Optical properties
All the monomers and polymers were examined by UVeVis
absorption and photoluminescence (PL) spectroscopy. Fig. 4 shows
the absorption and emission profiles of the monomers and poly-
mers, together with their PL images on exposure to an UV light. The
relevant absorption and PL data are collected in Table S2. Com-
pounds SCz and SO2Cz and their derived polymers exhibited strong
UVeVis absorption bands at 289e294 nm in CH2Cl2 solutions,
Fig. S7 illustrates the FT-IR spectra of polymers PSCz and
PSO2Cz. These spectra are essentially very similar to those of their
monomers. As compared to PSCz, PSO2Cz gave additional charac-
teristic bands of sulfonyl groups at around 1309 and 1182 cmꢁ1
(eSO2e asymmetric and symmetric stretching). The DSC thermo-
grams of the monomers and the polymers prepared by the chem-
ical oxidative polymerization method are shown in Fig. S8.
Monomers SCz and SO2Cz showed a sharp melting endotherm at
291 and 293 ꢀC, respectively. The polymers PSCz and PSO2Cz dis-
played glass transition at 301 and 302 ꢀC, respectively. No melting
endotherms were observed in their DSC curves up to 450 ꢀC. The
UVeVis absorption spectra of monomers SCz and SO2Cz and
polymers PSCz and PSO2Cz in CH2Cl2 and as solid films on the ITO-
glass are shown in Fig. S9. The monomers showed absorption
maxima at 293 and 289 nm, with absorption onsets at 309 and
317 nm, respectively. The polymers showed absorption maxima at
294 and 290 nm and absorption onsets at 366 and 411 nm,
respectively. The red-shift of absorption maxima and onsets of the
polymer films might be attributed to higher conjugation length in
comparison with the monomers and/or the aggregation of the
assignable to the pep* and nep* transitions of the SCz and SO2Cz
moieties. Solid film absorption spectra of PSCz and PSO2Cz were
similar to those in solution, with a slight red-shift of ca. 15 nm. The
monomer SCz and polymer PSCz exhibited blue PL emission with
PL quantum yield of 0.48% and 1.54% at the maximum peaks of 386
and 426 nm, and the monomer SO2Cz and polymer PSO2Cz
exhibited bright blue and yellowish green PL emission with a PL
quantum yield of 7.77% and 4.05% at the maximum peaks of 471 and
528 nm, respectively. The monomer SO2Cz and polymer PSO2Cz
with the electron-withdrawing sulfonyl group exhibited higher
fluorescence quantum yields in comparison to SCz and PSCz. This
can be attributed to the dipolar nature of the former ones. The
dibenzothiophene-S,S-dioxide moiety has been demonstrated to be
beneficial for increasing electron affinity and PL efficiency [55].
It is well known that dibenzothiophene-S,S-dioxide derivatives
exhibited environment sensitive solvatochromic behavior in which
the relative intensity of emission bands is dependent on the solvent
polarity. In order to further investigate the solvatochromic prop-
erties, we investigated the absorption and fluorescence of PSCz and
PSO2Cz in some organic solvents with different polarity. Fig. 5
polymer chains and enhanced inter-chain
pep* stacking interac-
tion in the film state.

S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
57
in dilute solution in various solvents, together with PL images of
their solutions. The absorption and PL emission data of SCz and
StBCz are summarized in Table S4. The solution absorption spectra
of SCz in various solvents are very similar (absorption lmax: ca
294 nm), and its emission wavelengths are not very sensitive to the
solvent polarity (382 nm in toluene; 392 nm in DMSO). The solu-
tions of StBCz in various solvents shows absorption peaks at around
298 nm and PL emission maxima from 390 nm in toluene to 409 nm
in DMSO. The variation of emission wavelengths between toluene
and DMSO is in within 20 nm, indicating low polar character of
these two compounds. Fig. 6 shows the normalized PL spectra of
SO2Cz and SO2tBCz in dilute solution in various solvents, together
with fluorescence images of their solutions. The absorption and PL
emission data of SO2Cz and SO2tBCz are also listed in Table S4. Their
solution absorption spectra in different solvents are similar, with
little shift in the peak maximum (absorption lmax: 290e294 nm).
This clearly indicates that the solvent polarity exerts little effect on
their ground-state electronic transitions. In contrast, the PL emis-
sion spectra of these two compounds show strong solvent-polarity
dependence, revealing a dominant broad emission band that un-
dergoes remarkable bathochromic shifts with an increase of the
solvent polarity. The emission color changes from purple-blue in
toluene (PL lmax ¼ 425e443 nm) to yellow-green in DMSO (PL
lmax ¼ 508e530 nm). The emission wavelength increases as the
solvent polarity increases. This clearly indicates that these two
molecules permit strong intramolecular charge transfer from the
donor (carbazole) to acceptor (dibenzothiophene-S,S-dioxide) unit.
It is obvious that the compounds stabilized in the excited state by
the polar solvent molecules will display red-shifted emission.
3.4. Electrochemical properties
Cyclic voltammetry (CV) experiments were conducted to
investigate the electrochemical behavior of the monomers. The
oxidation and reduction cycles of the monomers were measured in
dichloromethane (CH2Cl2) containing 0.1 M of Bu4NClO4 as the
supporting electrolyte and saturated Ag/AgCl as the reference
electrode under a nitrogen atmosphere. The CV curves of SCz,
StBCz, SO2Cz and SO2tBCz are shown in Fig. 7. SO2Cz and SO2tBCz
display a quasi-reversible reduction wave (Epc ~ ꢁ1.93 V vs. ferro-
cene) due to the electron-deficient dibenzothiophene-S,S-dioxide
segment. SCz and SO2Cz exhibit a quasi-reversible oxidation wave
(Epa ~1.60 and 1.70 V, respectively, vs. ferrocene). SO2Cz shows a
slightly higher oxidation potential than that of SCz due to the
electron-withdrawing sulfonyl group. The reverse negative poten-
tial scans of these two compounds show two cathodic peaks
attributed to the in-situ coupling reaction of C-3 and C-6 of the
carbazole. StBCz and SO2tBCz reveal a reversible oxidative wave at
lower potentials (Epa ~1.37 and 1.53 V, respectively, vs. ferrocene)
attributable to the tert-butyl substitution on the C-3 and C-6 po-
sitions of the carbazole. The reverse negative potential scans of
these two compounds show only one corresponding cathodic peak
at 1.12 and 1.28 V because the active sites of their carbazole units
are blocked with bulky tert-butyl groups. The derived oxidation and
reduction potentials of the monomers as well as their respective
highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) (vs. vacuum) are summarized in Table 1.
The electrochemical behavior of the electrodeposited polymer
films was investigated by CV in a monomer-free Bu4NClO4/CH3CN
solution. The quantitative details are also summarized in Table 1. As
shown in Fig. 8, these two polymer films showed two oxidation
peaks attributed to their polaronic and bipolaronic states, respec-
tively. The half-wave oxidation potentials (E1/2) were recorded at
1.07 V and 1.30 V for PSCz and at 1.17 V and 1.42 V for PSO2Cz. The
slightly higher oxidation voltages of the latter one could be
Fig. 5. Normalized PL spectra the dilute solutions of (a) PSCz (b) PSO2Cz (ca.
1 ꢂ 10ꢁ5 M) in toluene and DMSO. Photographs were taken under illumination of a
365 nm UV light.
shows the normalized PL spectra of PSCz and PSO2Cz of dilute
solutions in toluene and DMSO, together with fluorescence images
of their solutions. The absorption and PL emission data of PSCz and
PSO2Cz in various solvents are summarized in Table S3. The solu-
tion absorption spectra of PSCz are similar, with little shift in the
peak maximum (absorption lmax ~ 294 nm). This clearly indicates
that the solvent polarity exerts little effect on its ground-state
electronic transition. In contrast, the PL emission spectra of PSCz
show strong solvent-polarity dependence, revealing a dominant
broad emission band that undergoes a remarkable bathochromic
shift in highly polar DMSO. The emission color changes from
purple-blue in toluene (PL lmax ¼ 402 nm) to bright blue in DMSO
(PL lmax ¼ 458 nm). PSO2Cz showed a similar solvatochromic
phenomenon, with emission color changing from blue in toluene
(PL lmax ¼ 460 nm) to yellow-green in DMSO (PL lmax ¼ 528 nm). It
was also found that PSO2Cz exhibited a larger solvatochromic effect
in emission when compared to PSCz because the former has a
stronger donor (carbazole)-acceptor (sulfone) pair. The sol-
vatochromic effect could be attributed to the fast intramolecular
charge-transfer (ICT) process resulting in a large change of dipole
moment in the excited state. Such solvatochromic behavior is
associated with the stabilization of the polar emissive excited states
by the polar solvents.
We also investigated the absorption and fluorescence of small
molecules SCz, SO2Cz, StBCz and SO2tBCz in solvents with different
polarity. Fig. S10 shows the normalized PL spectra of SCz and StBCz

58
S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
Fig. 6. Normalized PL spectra the dilute solutions of (a) SO2Cz and (b) SO2tBCz (ca. 1 ꢂ 10ꢁ5 M) in solvents of various polarities. Photographs were taken under illumination of a
365 nm UV light.
Fig. 7. Cyclic voltammograms of (a) SCz, (b) SO2Cz, (c) StBCz and (d) SO2tBCz (1 ꢂ 10ꢁ4 M) in 0.1 M Bu4NClO4/CH2Cl2 solutions at a scan rate of 50 mV sꢁ1
.

S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
59
Table 1
Redox potentials and energy levels of monomers and the electrodeposited polymer films.
Polymer code (as film)
Absorption (nm)
Oxidation potential (V)a
Reduction potential
(V)a
Bandgap (eV)b,c
Energy level (eV)d
ox
red
lmax
lonset
Eonset
E
Eonset
E
Eogpt
Eegc
HOMO
LUMO
1/2
1/2
PSCz
PSO2Cz
Monomer code (in CH2Cl2)
319
306
381
421
0.90
0.98
1.07, 1.30
1.17, 1.42
e
e
3.26
2.95
e
5.43
5.53
2.17
2.58
ꢁ1.54
ꢁ1.69
2.86
SCz
293
289
297
294
309
317
307
311
1.32
1.49
1.25
1.30
1.38
1.49
1.13
1.41
e
e
e
e
e
e
e
5.74
5.85
5.61
5.66
e
SO2Cz
StBCz
SO2tBCz
ꢁ1.55
e
ꢁ1.75
e
3.21
e
2.61e
e
ꢁ1.54
ꢁ1.75
3.05
2.61e
a
vs. Ag/AgCl in CH3CN. E1/2 ¼ average potential of the redox couple peaks.
b
c
The data were calculated from polymer films by the equation: Eogpt ¼ 1240/lonset (optical energy gap between HOMO and LUMO).
Eegc, electrochemical band gap is derived from the difference between first E and E values.
The HOMO energy levels were calculated from first E values and were referenced to ferrocene (4.8 eV relative to the vacuum energy level; E ¼ 0.44 V in CH3CN):
ox
red
1/2
1/2
d
ox
ox
1/2
1/2
HOMO ¼ first E þ 4.8e0.44 (eV); LUMO ¼ HOMO e Egopt
.
ox
1/2
e
LUMO ¼ HOMO e Eegc
.
explained by the strong electron-withdrawing nature of the sulfone
group. The reduction of the dibenzothiophene-S,S-dioxide core of
PSO2Cz occurs at a notably negative potential (Epc ~ ꢁ1.95 V)
relative to ferrocene/ferrocenium (Fc/Fcþ). The energy levels of
HOMO and LUMO of these two polymers were estimated from the
E1/2 values. Assuming that the HOMO energy level for the Fc/Fcþ
standard is 4.80 eV with respect to the zero vacuum level, the
HOMO levels for PSCz and PSO2Cz were calculated to be 5.43 and
5.53 eV (relative to the vacuum energy level), whereas the values
for the LUMO levels lay at about 2.17 and 2.58 eV. Redox reactions
for PSCz shown in Scheme S1 represent a possible distribution of
electron density for the oxidized forms and describe by other
resonance forms, which contribute to the charge delocalization.
polymer PSCz exhibited strong absorption at 319 nm correspond-
ing to p-p* transitions in the polymer backbone, but it was almost
transparent in the visible and near-IR regions. Consequently, the
film is almost colorless. The optical band gap of polymer PSCz was
estimated to be 3.26 eV from the onset of the
p-p* transition at
381 nm. When the applied potential was increased to about 1.15 V,
the spectra displayed an absorption peak at ca. 427 nm and a
broadband that extended to the near-IR region, which we assigned
to the formation of biscarbazole radical cations (polarons). The
absorption band in the near-IR region may be attributed to an inter-
valence charge transfer (IV-CT) between states in which the posi-
tive charge is centered at different amino centers (biscarbazole).
The IV-CT phenomenon of the family of triarylamines with multiple
amino centers has been reported in literature [56]. Upon further
oxidation at applied potential to 1.43 V, the dication (bipolaron)
band at 762 nm appeared, and the absorption at 427 nm decreased
in intensity. The observed spectral changes of the PSCz film were
fully reversible upon varying the applied potential. In addition, they
were associated with significant color changes (from colorless to
greenish-yellow and to blue) that were homogeneous across the
ITO glass and easy to detect with the naked eye (Fig. 9 insets).
Fig. S11 illustrates the spectral and coloration changes of PSO2Cz
upon electro-reduction. When the applied potential was decreased
to about ꢁ1.93 V, the spectra displayed the absorption peaks at 538,
819 and 904 nm. Meanwhile, the color of the film changes from
3.5. Spectroelectrochemical properties
Spectroelectrochemistry was used to study the changes in the
absorption spectra and the information about the electronic
structures of the polymers as a function of the applied potential.
The electrogenerated polymer films on ITO glass were switched
between 0 and 1.43 V (for PSCz) or 1.60 V (for PSO2Cz). The spectral
changes of all the polymer films upon potential variation are
compiled in Fig. 9. Their spectroelectrochemical behaviors are very
similar. We use PSCz as an example to explain the spectroelec-
trochemical behavior of these two polymers. In the neutral form,
Fig. 8. Cyclic voltammograms of electrochemically synthesized (a) PSCz and (b) PSO2Cz polymer films on the ITO-coated glass slide in 0.1 M Bu4NClO4/CH3CN at a scan rate of
50 mV sꢁ1
.

60
S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
performed to monitor the percent transmittance changes (D%T) as a
function of time at their absorption maximum and to determine the
response time by stepping potential repeatedly between the
neutral and oxidized states. The active area of the polymer film on
ITO glass is about 2 cm2. Fig. 10 depicts the optical transmittance as
a function of time, current densities monitored and the optical
transmittance changes for the PSCz film at 427 and 762 nm by
applying square-wave potential steps between 0 and 1.15 V for a
pulse width of 13 s and between 0 and 1.43 V for a pulse width of
14 s. The response time was calculated at 90% of the full-
transmittance change, because it is difficult to perceive any
further color change with naked eye beyond this point. As shown in
Fig. 11, the PSCz film attained 90% of a complete greenish-yellow
coloring and bleaching in 5.5 and 1.3 s, respectively. The optical
contrast measured as
D%T of PSCz between neutral colorless and
oxidized greenish-yellow states was found to be 44% at 427 nm. The
PSCz film attained 90% of a complete blue coloring and bleaching in
7.3 and 1.5 s, respectively. The optical contrast measured as D%T of
PSCz between neutral colorless and oxidized blue states was found
to be 61% at 762 nm. The electrochromic coloring efficiency (CE) for
the greenish-yellow coloring (
h
¼
DOD427/Q) was estimated to be
122 cm2
C
ꢁ1, and that for the blue coloring (
h
¼
DOD762/Q) of the
PSCz film was estimated to be 45 cm2 Cꢁ1 (Table 2).
Fig. S12 depicts the optical transmittance as a function of time,
current densities monitored and the optical transmittance changes
for the PSO2Cz film at 425 and 747 nm by applying square-wave
potential steps between 0 and 1.28 V for a resident time of 13 s
and between 0 and 1.60 V for a resident time of 15 s. As shown in
Fig. S13, the PSO2Cz film attained 90% of a complete greenish-
yellow coloring and bleaching in 4.7 and 1.0 s, respectively. The
optical contrast measured as
D%T of PSO2Cz between neutral
colorless and oxidized greenish-yellow states was found to be 39%
at 425 nm. The PSO2Cz film attained 90% of a complete green col-
oring and bleaching in 7.7 and 1.5 s, respectively. The optical
Fig. 9. Spectral changes of the polymer films (a) PSCz and (b) PSO2Cz on an ITO-coated
glass in 0.1 M Bu4NClO4/CH3CN at various applied potentials (vs. Ag/AgCl).
contrast measured as D%T of PSO2Cz between neutral colorless and
oxidized green states was found to be 53% at 747 nm. The elec-
trochromic coloring efficiency (CE) for the greenish-yellow coloring
colorless to pink.
(
h
¼
D C
OD425/Q) was estimated to be 116 cm2 ꢁ1, and that for the
green coloring (
h
¼
DOD747/Q) of the PSO2Cz film was estimated to
3.6. Electrochromic switching studies
Electrochromic switching studies for the polymer film were
be 38 cm2 Cꢁ1 (Table 2).
Fig. 10. Electrochromic switching of PSCz (a) 427 nm and (b) 762 nm thin films on the ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN for 10 cycles.

S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
61
Fig. 11. Current densities monitored and optical transmittance changes for polymer PSCz on the ITO-glass slide in 0.1 M Bu4NClO4/CH3CN while the potential was switched (a)
between 0.0 V and 1.15 V at 427 nm with a pulse width of 13 s and (b) between 0.0 V and 1.43 V at 762 nm with a pulse width of 14 s.
3.7. Electrochromic device of PSCz
dibenzothiophene-S,S-dioxide monomers SCz and SO2Cz were
synthesized and used as the building blocks of fluorescent and
electrochromic polymers via oxidative polymerization using FeCl3
as catalyst or electrochemical polymerization onto the ITO/glass
substrate. The polymeric thin films were successfully deposited
onto the surface of the working electrode by repetitive cyclic vol-
tammetry (CV) scanning of compounds SCz and SO2Cz in Bu4NClO4/
CH3CN solution between 0 and 1.8 V. However, the compounds
StBCz and SO2tBCz with tert-butyl groups attached on the reactive
sites of the carbazole units did not give any polymer films on the
electrode upon repetitive CV scanning, indicating the electro-
chemical polymerization occurred through the coupling reaction
between the carbazole units at the 3- or 6-position. The electro-
chemically generated PSCz and PSO2Cz films exhibited two
reversible oxidation redox couples due to successive oxidations of
the biscarbazole unit. These polymer films also revealed excellent
electrochemical and electrochromic stability, with coloration
change from a colorless neutral state to greenish-yellow, blue and
green oxidized forms. These properties make the prepared PSCz
and PSO2Cz films promising materials for electrochromic devices.
Using the schematic diagram illustrated in Fig. 12, a simple
electrochromic sandwich cell using PSCz as the electroactive layer
was constructed. The polymer film was electrodeposited onto ITO-
coated glass and then dried. Afterwards, the gel electrolyte was
spread on the polymer-deposited side of the electrode and the
electrodes were sandwiched under atmospheric condition. To
prevent leakage, an epoxy resin was applied to seal the device. The
color changes of the electrochromic device upon oxidation are
illustrated in Fig. 12. The polymer film is colorless in neutral form.
When the voltage was increased (to a maximum of 3.20 V), the
color changed to yellowish green and green due to electro-
oxidation. When the potential was subsequently set back at 0 V,
the polymer film turned back to original colorless. We believe that
optimization could further improve the device performance and
fully explore the potential of these electrochromic polymers.
4. Conclusions
Two
carbazole-endcapped
dibenzothiophene
and
Table 2
Electrochromic properties of polymers.
Response timeb
D
ODc
Qdd (mC cmꢁ2
)
CEe (cm2 Cꢁ1
)
a
Polymer
lmax (nm)
D%T
tc (s)
tb(s)
PSCZ
427
762
425
747
44
61
39
53
5.5
7.3
4.7
7.7
1.3
1.5
1.0
1.5
0.280
0.459
0.320
0.344
2.29
10.2
2.74
9.05
122
45
116
38
PSO2CZ
a
Wavelength of absorption maximum.
b
c
Time for 90% of the full-transmittance change.
Optical Density (
OD) ¼ log[Tbleached/Tcolored], where Tcolored and Tbleached are the maximum transmittance in the oxidized and neutral states, respectively.
Qd is ejected charge, determined from the in situ experiments.
Coloration efficiency (CE) ¼ OD/Qd.
D
d
e
D

62
S.-H. Hsiao, L.-C. Wu / Dyes and Pigments 134 (2016) 51e63
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Acknowledgment
We gratefully acknowledge the supports from the Ministry of
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