Electrochemically fabricated electrochromic films from 4-(: N -carbazolyl)triphenylamine and its dimethoxy derivative

Article abstract of DOI:10.1039/c6ra08528h

A carbazolyl-substituted triphenylamine derivative, 4-(N-carbazolyl)triphenylamine (TPACz), was synthesized and used to build a fluorescent and electrochromic polymeric film (PTPACz) on the ITO-glass surface by electrochemical oxidative coupling reactions

Full text of DOI:10.1039/c6ra08528h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Electrochemically fabricated electrochromic films
from 4-(N-carbazolyl)triphenylamine and its
dimethoxy derivative†
Cite this: RSC Adv., 2016, 6, 43470
*
Sheng-Huei Hsiao and Hui-Min Wang
A carbazolyl-substituted triphenylamine derivative, 4-(N-carbazolyl)triphenylamine (TPACz), was synthesized
and used to build a fluorescent and electrochromic polymeric film (PTPACz) on the ITO-glass surface by
electrochemical oxidative coupling reactions. The electrodeposited PTPACz film showed blue emission
(lmax ¼ 424 nm) and exhibited reversible electrochemical redox processes and stable color changes upon
electro-oxidation, which can be switched by potential modulation. The remarkable electrochromic
behavior of the film is clearly interpreted on the basis of spectroelectrochemical studies. A dimethoxy-
substituted derivative of TPACz, namely 4,40-dimethoxy-400-(N-carbazolyl)triphenylamine (MeOTPACz), was
also synthesized and characterized for a comparative study. Repetitive cyclic voltammetry scanning of
MeOTPACz in an electrolyte/acetonitrile solution resulted in an electroactive dimeric film of (MeOTPACz)2
on the electrode surface. Electrochromic devices using PTPACz or (MeOTPACz)2 as an active layer were
also fabricated as preliminary investigations for electrochromic applications.
Received 3rd April 2016
Accepted 28th April 2016
DOI: 10.1039/c6ra08528h
www.rsc.org/advances
During the past decade, Liou's group has carried out extensive
studies on the design and synthesis of triarylamine-based high-
Introduction
performance polymers such as aromatic polyamides and poly-
imides for potential electrochromic and memory applica-
tions.15,16 In general, these polymers exhibit good solubility to
organic solvents due to the introduction of bulky, packing-
disruptive triarylamine moieties. Therefore, the polymeric
layers used in the devices can be easily prepared by simple spin-
coating or drop-casting methods. As a class of excellent electro-
chromic and memory materials, these polymers have various
useful properties such as easily forming relatively stable polarons
(radical cations), high carrier mobility, high thermal stability,
and good mechanical properties. Recently, we have reported that
aromatic polyamides and polyimides containing 4-(N-carbazolyl)
triphenylamine (TPACz) segments show attractive electro-
chemical and electrochromic properties, and incorporating
sterically hindered tert-butyl or electron-donating methoxy
groups at the electrochemically active C-3 and C-6 sites of the
carbazole unit leads to a greatly enhanced redox and electro-
chromic stability of the polymers upon electro-oxidation.17 In
addition to visible absorption, the polarons of these TPACz-based
polymers also show strong absorption in near-infrared (NIR)
region due to the intervalence-charge transfer (IV-CT) between
the TPA and carbazole redox centers.18,19 Thus, the reported
TPACz-containing polymers could be candidates for the appli-
cations in organic visible/NIR electrochromic devices.
Electrochromism is the phenomenon displayed by an electro-
active species that changes color reversibly in response to an
applied electrical charge.1 A vast number of electrochromic
materials have been studied, and transition metal oxides,2
molecular dyes,3 and p-conjugated polymers4 are the most
frequently investigated components. Some organic–inorganic
coordination polymers have also received great interest for elec-
trochromic applications.5 In general, p-conjugated polymers
show several advantages over the other types of electrochromics,
such as high coloration efficiency, rapid switching speeds, high
optical contrasts, ease of processing, and the possibility to tune
the properties through chemical modication.6 Potential appli-
cations of electrochromic materials include optical switching
devices, electrochromic displays, sunglasses, antiglare rear-view
mirrors, smart windows for cars or buildings, data storage,
electronic papers and adaptive camouage.7 Recent high-prole
commercialization of electrochromics includes the Boeing 787
Dreamliner windows manufactured by Gentex.8
Carbazole and triarylamine derivatives and polymers are well-
known for their electroactive and photoactive properties that may
nd optoelectronic applications in electrophotography, electro-
luminescent diodes, eld-effect transistors, solar cells, memory
devices, and electrochromic or electrouorochromic devices.9–14
As reported in the pioneering works by Nelson and Adams
et al., unsubstituted triphenylamine (TPA) undergoes dimeriza-
tion (coupling) to tetraphenylbenzidine (TPB) aer the formation
of an unstable monocation radical.20 This is accompanied by the
Department of Chemical Engineering and Biotechnology, National Taipei University of
Technology, Taipei 10608, Taiwan. E-mail: shhsiao@ntut.edu.tw
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra08528h
43470 | RSC Adv., 2016, 6, 43470–43479
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
loss of two protons per dimer and the dimer is more easily
oxidized than TPA and also can undergo further oxidations in two
discrete one-electron steps to give TPB+c monocation and nally
the quinoidal TPB2+. Quantitative data have been obtained for
several 4-substituted TPAs in the form of second-order coupling
rate constants, and it was generally found that electron-donating
substituents such as methoxy group tended to stabilize the cation
radicals while the electron-withdrawing groups like nitro group
had the opposite effect. On the other hand, the electrochemical
oxidation of carbazole and N-substituted derivatives was rst
studied by Ambrose and Nelson.21a They further studied
systematically 76 ring-substituted carbazoles about their
substituent effects using electrochemical and spectroscopic
techniques.21b For the N-phenylcarbazoles (NPCs) with both the 3
and 6 carbazole ring positions unprotected, these compounds
underwent an initial one-electron oxidation to generate a very
reactive cation radical; two of these then coupled at the 3 posi-
tions to yield a N,N0-diphenyl-3,30-bicarbazyl. This species is then
oxidized more easily than the corresponding carbazole to a stable
dication. The basic difference between TPAs and NPCs is that the
carbazoles are planar across two benzene rings while the TPAs
have all three rings at angles to each other. Due to the planarity of
the carbazole system and the attendant high electron density at
the reactive sites, the coupling rate of NPCs has been found to be
much faster than for the TPAs. The TPA or carbazole electro-
chemically oxidative dimerization reaction has been employed
efficiently to fabricate electroactive polymer lms with potential
applications in electronic and optoelectronic devices.22
Electrochromic devices have been extensively prepared using
solution processable electroactive polymers.23 This is in part
because accurate assessment of optical and electrochemical
polymer properties rely on solution based techniques. Electro-
chromic devices also could be assembled using electrochromic
polymers electro-deposited on ITO glass electrodes.24 Electro-
chemical polymerization provides the unique advantage to
combine both synthesis and direct fabrication of electroactive
polymer lms on the electrode surface. This procedure signi-
cantly shortens the experimental time and avoids the solubility
issues oen encountered with conventional chemical methods,
thus enlarging the scope of candidate polymers for electro-
chromic applications. As a continuation of our recent efforts in
developing electrochromic materials by electrochemical
synthesis, herein we study the electrochemical polymerization
of a novel simple monomer TPACz and the electrochemical,
electro-optical, and electrochromic properties of its electro-
deposited polymer lms (PTPACz) on ITO-glass substrate. For
a comparative study, an analogous derivative MeOTPACz with
methoxy groups substituted on the active sites of the TPA unit
was also synthesized, and its electrochemical behavior was
investigated. Oxidative coupling reactions between MeOTPACz
molecules only produced a dimer (MeOTPACz)2. This work
provides a model to design TPACz derivatives capable to form
electrochemically active polymers with potential applications in
electronic and optoelectronic devices. Single layer electro-
chromic devices based on PTPACz and (MeOTPACz)2 are also
constructed and characterized. These devices show good elec-
trochromic stability and high optical contrast ratios.
Experimental
Materials
According to the reported procedure,15b N-(4-aminophenyl)
carbazole was prepared by the cesium uoride-mediated
aromatic nucleophilic substitution reaction of 4-uoroni-
trobenzene with carbazole followed by Pd/C-catalyzed hydrazine
reduction in reuxing ethanol. Iodobenzene (TCI), 4-iodoanisole
(ACROS), copper powder (ACROS), potassium carbonate (K2CO3,
SHOWA), triethylene glycol dimethyl ether (TEGDME, ACROS),
iron(III) chloride (ACROS), nitrobenzene (ACROS), chloroform
(TEDIA) and tetrahydrofuran (THF, TEDIA) were used without
further purication. All electrochemical and spectroelec-
trochemical studies were performed using acetonitrile (ACN,
Tedia) solutions containing tetrabutylammonium perchlorate
(TBAP, TCI) as the supporting electrolyte. TBAP was recrystallized
twice from ethyl acetate under nitrogen atmosphere and then
dried in vacuo before use. Acetonitrile was dried over calcium
hydride for 24 h, distilled under reduced pressure, and stored
˚
over 4 A molecular sieves in a sealed bottle. All other reagents
were used as received from commercial sources.
Monomer synthesis
4-(N-Carbazolyl)triphenylamine (TPACz). A mixture of N-(4-
aminophenyl)carbazole (8.5 g, 33 mmol), iodobenzene (16.3 g,
80 mmol), copper powder (5.1 g, 80 mmol), potassium
carbonate (22.1 g, 160 mmol), and TEGDME (30 mL) was stirred
under nitrogen atmosphere at 180 ꢀC for 24 h. The brown
precipitate was collected by ltration and washed thoroughly by
water. The crude product was ltered and recrystallized from
acetonitrile to afford 8.3 g (61% in yield) of brown crystals with
an mp of 172–174 ꢀC (by DSC at a heating rate of 2 ꢀC minꢁ1). 1H
NMR (500 MHz, CDCl3, d, ppm): 7.14 (t, J ¼ 7.5 Hz, 2H, Hi), 7.27
(d, J ¼ 7.5 Hz, 4H, Hg), 7.31 (d, J ¼ 8.8 Hz, 2H, Hf), 7.33 (t, J ¼ 7.5
Hz, 2H, Hb), 7.37 (t, J ¼ 7.5 Hz, 4H, Hh), 7.44 (d, J ¼ 9.5 Hz, 2H,
He), 7.47 (t, J ¼ 7.5 Hz, 2H, Hc), 7.50 (d, J ¼ 7.5 Hz, 2H, Hd). 13
C
NMR (125 MHz, CDCl3, d, ppm): 109.8 (C5), 119.7 (C3), 120.2
(C2), 123.2 (C1), 123.5 (C14), 124.0 (C9), 124.7 (C12), 125.8 (C4),
127.8 (C8), 129.4 (C13), 131.4 (C7), 141.1 (C6), 147.1 (C10), 147.5
(C11). Crystal data: pale brown crystal grown during the slow
crystallization in acetonitrile, 0.40 ꢂ 0.34 ꢂ 0.14 mm, mono-
˚
clinic C2/c with a ¼ 27.8714(16), b ¼ 9.7479(5), c ¼ 19.7603(9) A;
a ¼ 90ꢀ, b ¼ 126.516(3)ꢀ, g ¼ 90ꢀ, where density of crystal Dc ¼
1.264 Mg mꢁ3 for Z ¼ 8 and V ¼ 4314.7(4) A . The molecular
3
˚
structure of TPACz by single crystal X-ray analysis is shown in
Fig. 1.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 43470–43479 | 43471

View Article Online
RSC Advances
Paper
Fig. 1 Molecular structures of TPACz and MeOTPACz by single crystal X-ray analysis.
4,40-Dimethoxy-400-(N-carbazolyl)triphenylamine
(MeOT- Electrochemical polymerization/dimerization
PACz). A mixture of N-(4-aminophenyl)carbazole (8.5 g, 33 mmol),
4-iodoanisole (18.7 g, 80 mmol), copper powder (5.1 g, 80 mmol),
potassium carbonate (22.1 g, 160 mmol), and TEGDME (30 mL)
Electrochemical polymerization or dimerization was performed
with a CH Instruments 750A electrochemical analyzer. The
polymers were synthesized from 1.0 ꢂ 10ꢁ3 M TPACz or
MeOTPACz in 0.1 M TBAP/ACN solution via cyclic voltammetry
repetitive cycling at a scan rate of 150 mV sꢁ1 during ten cycles.
The polymer was deposited onto the surface of the working
electrode (platinum disc or ITO/glass surface, polymer lms
area about 0.8 cm ꢂ 1.25 cm), and the lm was rinsed with
plenty of acetone for the removal of inorganic salts and other
organic impurities formed during the process.
ꢀ
was stirred under nitrogen atmosphere at 180 C for 24 h. The
brown precipitate was collected by ltration and washed thor-
oughly by water. The crude product was ltered and recrystallized
from acetonitrile to afford 8.5 g (55% in yield) of brown crystals
with an mp of 126–127 ꢀC (by DSC at a heating rate of 2 ꢀC minꢁ1).
1H NMR (500 MHz, CDCl3, d, ppm): 3.84 (s, 6H, –OCH3), 6.92 (d, J
¼ 8.8 Hz, 4H, Hh), 7.12 (d, J ¼ 8.7 Hz, 2H, Hf), 7.20 (d, J ¼ 8.8 Hz,
4H, Hg), 7.30 (m, 2H, Hb), 7.33 (d, J ¼ 8.7 Hz, 2H, He), 7.43 (m, 4H,
Hc + Hd), 8.16 (d, J ¼ 7.7 Hz, 2H, Ha). 13C NMR (125 MHz, CDCl3, d,
ppm): 55.49 (–OCH3), 109.84 (C5), 114.87 (C13), 119.53 (C3), 120.19
(C2), 120.58 (C9), 123.08 (C1), 125.74 (C4), 126.96 (C12), 127.72 (C8),
129.39 (C7), 140.62 (C11), 141.26 (C6), 148.14 (C10), 156.22 (C14).
Crystal data: pale brown crystal grown during the slow crystalli-
zation in acetonitrile, 0.44 ꢂ 0.24 ꢂ 0.14 mm, monoclinic C2/c
Fabrication of electrochromic devices (ECDs)
The PTPACz polymer and (MeOTPACz)2 dimer lms were elec-
trodeposited on the ITO-coated glass substrate by the electro-
chemical oxidative coupling method described above. A gel
electrolyte based on PMMA (Mw: 120 000) and LiClO4 was
plasticized with propylene carbonate to form a highly trans-
parent and conductive gel. PMMA (1 g) was dissolved in dry ACN
(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.
ꢀ
˚
with a ¼ 9.2535(2), b ¼ 13.8327(3), c ¼ 19.4070(5) A; a ¼ 90 , b ¼
90ꢀ, g ¼ 90ꢀ, where density of crystal Dc ¼ 1.258 Mg mꢁ3 for Z ¼ 4
3
˚
and V ¼ 2484.03(10) A . The molecular structure of MeOTPACz by
single crystal X-ray analysis is shown in Fig. 1.
Instrumentation
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
43472 | RSC Adv., 2016, 6, 43470–43479
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
an internal standard. X-ray single crystal diffraction experiment N-(4-aminophenyl)carbazole with iodobenzene and 4-iodoani-
was carried out on a Norius Kappa CCD four-circle diffractom- sole, respectively. IR, NMR spectroscopic techniques and X-ray
eter equipped with graphite-monochromated Mo-Ka radiation. crystal analysis were used to conrm the structures of the ob-
The structure was solved by direct methods using SHELXL-97 tained monomers. IR spectra of TPACz and MeOTPACz are
soware. Ultraviolet-visible (UV-vis) spectra of the polymer included in ESI Fig. S1.† The methoxy groups of MeOTPACz
lms were recorded on an Agilent 8453 UV-visible spectrometer. present characteristic absorption (–OCH3, sp3 C–H stretching)
The absorption spectra of monomers and polymers were in the range 2837–2950 cmꢁ1. No absorption bands due to the
recorded in CH2Cl2 or as electrodeposited thin lms on ITO/ NH2 stretching of N-(4-aminophenyl)carbazole around 3200–
1
glass substrate. The optical band gaps (Eg) of monomers and 3400 cmꢁ1 can be observed in Fig. S1.† The H and 13C NMR
polymers were calculated from their absorption edges. Photo- spectra of TPACz and MeOTPACz are illustrated in ESI Fig. S2
luminescence (PL) spectra were measured with a Varian Cary and S3,† respectively. Assignments of each proton and carbon
Eclipse
uorescence
spectrophotometer.
Fluorescence were assisted by the two dimensional H–H COSY and C–H
quantum yields (FF) of the samples in different solvent were HMQC spectra given in ESI Fig. S4 and S5,† and the results
measured by using quinine sulfate in 1 N H2SO4 as a reference agreed well with the proposed molecular structures of TPACz
standard (FF ¼ 54.6%). Electrochemistry was performed with and MeOTPACz. The molecular structures of TPACz and
a CHI 750A electrochemical analyzer. Cyclic voltammetry was MeOTPACz were also conrmed by single-crystal X-ray acquired
conducted with the use of a three-electrode cell in which ITO from the single crystal obtained by slow crystallization of an
(polymer lms area about 0.8 cm ꢂ 1.25 cm) was used as acetonitrile solution, and the bond length and angle data are
a working electrode. A platinum wire was used as an auxiliary summarized in ESI Table S1.† As shown in Fig. 1, TPACz and
electrode. All cell potentials were taken with the use of a ALS RE- MeOTPACz display a coplanar structure of the carbazolyl unit
1B (Ag/AgCl, 3 M NaCl) reference electrode. Ferrocene was used and a propeller-shaped conformation of the triphenylamine
as an external reference for calibration (+0.48 V vs. Ag/AgCl). core.
Spectroelectrochemical experiments were carried out in a cell
built from a commercial UV-visible cuvette using an Agilent
Electrochemical oxidative coupling
8453 UV-visible diode array spectrophotometer. The cell was
composed of a 1 cm cuvette, ITO as a working electrode,
a platinum wire as an auxiliary electrode, and a Ag/AgCl refer-
ence electrode. Photographs of the polymer lms were taken by
using a Ricoh R7 digital camera.
The electrochemical oxidative coupling reactions of monomers
were carried out on the ITO glass slides in 0.1 M TBAP/ACN
solution containing 1.0 ꢂ 10ꢁ3 M TPACz or MeOTPACz via
repetitive cycling at a potential scan rate of 150 mV sꢁ1. Fig. 2(a)
displays the successive cyclic voltammograms (CV) of the TPACz
solution between 0 and 1.8 V. In the rst anodic half-cycle, two
peaks are observed at Epa ¼ 1.07 and 1.55 V, which are attrib-
uted to the oxidation of TPA and carbazole unit, respectively,
Results and discussion
Monomer synthesis
The synthetic route of monomers is outlined in Scheme 1. resulting in formation of oligomers/polymer in the vicinity of
TPACz and MeOTPACz were prepared by Ullmann reaction from electrode surface. In the reverse scan, two peaks are detected at
Scheme 1 Synthetic route to monomers TPACz and MeOTPACz.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 43470–43479 | 43473

View Article Online
RSC Advances
Paper
involved in very fast electrochemical reactions that produced an
MeOTPACz dimer [(MeOTPACz)2] via the ring–ring coupling
reaction between the carbazole units (Scheme 2). The dimer
seemed to be the major product during the oxidation process of
MeOTPACz, due to the high stability of the oxidized states
(bicarbazylium cations).
Optical and electrochemical properties
Because the polymer lm of PTPACz prepared by the electro-
chemical polymerization is insoluble in organic solvents, we
prepared some of the PTPACz sample via the chemical oxidative
polymerization of PTPACz using FeCl3 in nitrobenzene (see
ESI†). Fig. S6† shows the absorption and emission proles of
the monomers and polymers, together with their PL images on
exposure to an UV light in both solution and solid states. The
relevant absorption and PL data are collected in Table 1. These
compounds and their derived polymers exhibited strong UV-vis
absorption bands at 289–347 nm in CH2Cl2 solutions, assign-
able to the p–p* and n–p* transitions of the carbazole and
triphenylamine moieties in the monomer or polymer backbone.
The solid state emission spectrum was similar to that recorded
from the CH2Cl2 solution. Although the solid-state emission of
PTPACz was found to be slightly red shied (ca. 13 nm) from
that in solution, the emission remained in the blue region. This
polymer in dilute CH2Cl2 solution exhibited a maximum blue
PL emission at 417 nm with PL quantum yield of 28.7%. The
red-shi of PL maxima of polymer PTPACz means an extended
conjugation length in comparison with the monomer TPACz.
The UV-vis absorption spectra of monomers TPACz and
polymer PTPACz both in CH2Cl2 and the polymer lms PTPACz
in solid state on an ITO electrode are shown in Fig. S7.† The UV-
vis spectrum of TPACz exhibits two main absorption peaks at
about 294 and 305 nm. Due to having extended p conjugation
over the polymer backbone, a new broad band was observed at
about 347 nm in the polymer. The red-shi and broadening of
the absorption prole of the polymer lm could be attributed to
the aggregation of the polymer chains, resulting in a different
energy level distribution and enhanced interchain p–p inter-
actions in the solid state. The optical band gap values of PTPACz
calculated from maximum absorption band edge of solid state
was found to be 2.81 eV.
The electrochemical behavior of the electrodeposited poly-
mer and dimer lms was investigated by CV in a monomer-free
TBAP/ACN solution. The quantitative details are summarized in
Table 1. As shown in Fig. 3, the CV diagram of PTPACz reveals
two reversible oxidation redox couples at half-wave potentials
(E1/2) of 0.88 V and 1.47 V. The rst anodic peak has been
assigned to the one-electron oxidation of the triphenylamine
units into the radical cation. When higher potentials are
reached, dicationic forms might be obtained in the polymer.
The CV curve of dimer (MeOTPACz)2 is essentially identical to
that shown in Fig. 2(b). According to the order of oxidation
peaks of polymer PTPACz and dimer (MeOTPACz)2, we propose
a possible sequence of reaction under electrochemical oxida-
tion which is shown in ESI Scheme S1.† The HOMO (highest
occupied molecular orbital) energy levels of the investigated
Fig. 2 CV diagrams of the electrodeposition of (a) TPACz and (b)
MeOTPACz on ITO-glass, using 1.0 ꢂ 10ꢁ3 M monomer solutions in
0.1 M TBAP/ACN solution at 150 mV sꢁ1 during 10 cycles.
Epc ¼ 0.84 and 1.23 V, which correspond to reduction of lm
deposited in the preceding anodic scan. In the second and
subsequent voltammetric cycles, the anodic peaks gradually
shied to higher potentials and the cathodic peaks shied to
lower potentials with the increasing intensity of the peaks. At
the end of CV scan, a visible polymer lm was robustly depos-
ited on the working electrode surface. Aer being immersed in
water for a certain period of time, a free-standing polymer lm
could be detached from the ITO-glass surface. The electro-
deposited lm of monomer TPACz (coded with PTPACz) is
proposed to possess a crosslinked polymer structure formed by
the possible TPA–TPA, carbazole–carbazole, and TPA–carbazole
coupling reactions. The electrochemical property of MeOTPACz
is completely different [Fig. 2(b)]. It is possible to identify three
anodic peaks at Epa ¼ 0.88, 1.35 and 1.63 V and three cathodic
peaks at Epc ¼ 0.55, 1.07 and 1.27 V in the rst CV scan. As the
CV scan continued, no signicant increase in intensity of these
peaks was observed and no polymer lm grew up on the elec-
trode surface. The rst oxidation process of MeOTPACz
occurred at a lower applied voltage than that of TPACz because
of the electron-donating methoxy groups substituted at the 4,40-
positions of the TPA unit. The subsequent second and third
anodic peaks indicate that the carbazole radical cations were
43474 | RSC Adv., 2016, 6, 43470–43479
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Scheme 2 Synthetic route of polymer PTPACz and dimer (MeOTPACz)2.
Table 1 Optical and electrochemical properties of monomers, dimer and polymer
In solution
As solid lmd
Oxidation potential
Energy levels
absa
PLb
c
abs
abs
PL
Ox1
Ox2
Ox3
f
lmax
lmax
FPL
lmax
lonset
lmax
E
E
E
1/2
Eg
HOMOg
(ev)
LUMOh
(ev)
1/2
1/2
Index
(nm)
(nm)
(%)
(nm)
(nm)
(nm)
(V)
(V)
(V)
(eV)
TPACz
PTPACz
MeOTPACz
(MeOTPACz)2
294, 305
294, 347
294
392
417
395
408
4.5
28.7
3.1
298, 311
298, 360
298, 310
298, 310
364
441
375
390
—
424
—
0.96
0.88
0.71
0.71
1.42
1.47
1.30e
1.20
—
—
—
1.46
3.41
2.81
3.30
3.18
5.31
5.24
5.07
5.07
1.91
2.17
1.77
1.89
294
2.2
—
a
b
c
Measured in dilute solutions in CH2Cl2 at a concentration of about 1 ꢂ 10ꢁ5 mol Lꢁ1
.
Excited at the absorption maximum. The uorescent
d
quantum yield was calculated in an integrating sphere with quinine sulfate as the standard (FPL ¼ 54.6%). Drop-coated from CH2Cl2 solution.
e
f
g
Irreversible peak potential (Epa). Optical band gap obtained from Eg ¼ 1240/ledge
.
The HOMO energy levels were calculated from E1/2 and
h
were referenced to ferrocene (4.8 eV). LUMO ¼ HOMO ꢁ Eg.
monomers, dimer and polymer were calculated from the half- absorption at wavelength around 306 nm, characteristic for p–
wave potentials of the rst oxidation wave (EO1/x21) and by p* transitions, but it was almost transparent in the visible and
comparison with ferrocene (4.8 eV). These data together with near infrared (NIR) regions. Upon oxidation of the PTPACz lm
absorption spectra were then used to obtain the LUMO (lowest (increasing applied voltage from 0 to 1.00 V), the absorption
unoccupied molecular orbital) energy levels (Table 1).
peak at 465 nm and a broadband from 800 nm extended to 1100
nm in the NIR region grew up. Since the potentials examined
are similar to the rst anodic process, the spectral changes are
assigned to the radical cation (polaron) formation arising from
the oxidation of the TPA unit. The absorption band in the NIR
region may be attributed to an intervalence charge transfer
(IVCT) between states in which the positive charge is centered at
different amino centers (TPA and carbazole). The IVCT
phenomenon of the family of triarylamines has been reported
Spectroelectrochemical and electrochromic properties
Spectroelectrochemistry were performed on the electro-
deposited polymeric and dimeric lms on ITO glass to clarify
their electronic structures and optical behaviors upon oxida-
tion. The result of polymer PTPACz lm is presented in Fig. 4(a)
as the UV-vis-NIR absorbance curves correlated to electrode
potentials. In the neutral form, at 0 V, PTPACz exhibited strong
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 43470–43479 | 43475

View Article Online
RSC Advances
Paper
intensity during this process. The observed electronic absorp-
tion changes in the lm of PTPACz at various potentials are fully
reversible and are associated with strong color changes; indeed,
they even can be seen readily by the naked eye. As shown in
Fig. 4(a) inset, it can be seen that the lm of PTPACz switches
from a transmissive neutral state (nearly colorless) to a highly
absorbing semi-oxidized state (pale yellow or yellowish green)
and a fully oxidized state (blue). In Fig. 4(b) are depicted the
spectral changes of (MeOTPACz)2 during the oxidation in 0.1 M
TBAP/ACN. The dimeric lm showed strong absorption at
around 310 nm in the neutral form (0 V). Upon oxidation
(increasing applied voltage from 0 to 1.0 V), a new absorption
shoulder at 390 and a peak at 724 nm appeared due to the
formation of a stable monocation radical of the dimethoxy-
substituted TPA unit. Upon further oxidation at applied volt-
ages to 1.8 V, the absorption band at 724 nm increased gradu-
ally and a new broad band centered at around 520 nm appeared.
The dimer (MeOTPACz)2 changed color from a colorless neutral
state to light green semi-oxidized states and then to russet at the
fully oxidized state.
Electrochromic switching studies for PTPACz were per-
formed to monitor the percent transmittance changes (D% T) as
a function of time at their absorption maximum (lmax) and to
determine the response time by stepping potential repeatedly
between the neutral and oxidized states. The active area of the
polymer lm on ITO glass is about 1 cm2. Fig. 5 depicts the
optical transmittance as a function of time at 740 nm by
applying square-wave potential steps of 14 s between 0 and 1.4 V
for the rst 10 cycles. The polymer lm still retained reasonable
electroactivity aer hundreds of cycles. 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. The polymer PTPACz exhibited DT% up to
68% at 740 nm for blue coloring and required 4.4 s for the
Fig. 3 Cyclic voltammograms of PTPACz and the (MeOTPACz)2 dimer
in monomer-free 0.1 M TBAP/ACN solution at scan rate 50 mV sꢁ1
.
in literature.18,19 Upon further oxidation at applied voltages to
1.8 V, the dication (bipolaron) band at 740 nm formed.
Concurrently, the absorption peak at 465 nm decreased in
Fig. 5 (a) Electrical current response and (b) transmittance change as
Fig. 4 Spectroelectrograms of (a) PTPACz and (b) (MeOTPACz)2 films a function of time for the deposited film of PTPACz on the ITO-coated
on ITO-glass substrate in 0.1 M TBAP/ACN solution at applied glass slide (active area about 1 cm2) in 0.1 M TBAP/ACN by applying
potentials between 0 and +1.8 V.
a potential step between 0.0 and 1.4 V (versus Ag/AgCl).
43476 | RSC Adv., 2016, 6, 43470–43479
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
V and 2.4 V, respectively), the color changed to yellow, green,
and deep blue, respectively, the same as those were already
observed in the spectroelectrochemical experiments. The ECD
based on dimer (MeOTPACz)2 was also fabricated as shown in
Fig. 7. The device exhibited
a russet-pink color while
(MeOTPACz)2 is in the oxidized state. When the potential was
subsequently set back at 0.0 V, the polymer and dimer lm
turned slowly back to original color.
Conclusions
Two carbazole and triphenylamine-containing monomers
TPACz and MeOTPACz were readily synthesized from the Ull-
mann reaction of N-(4-aminophenyl)carbazole with iodo-
benzene and 4-iodoanisole, respectively. Redox-active and
electrochromic polymer lm could be robustly deposited on the
electrode surface in an electrolyte solution by the electro-
chemical polymerization of TPACz via the arylamine oxidative
coupling reactions. However, MeOTPACz formed a dimer lm
during the CV scanning because the active sites of its TPA unit
are blocked with methoxy groups. Both lms revealed good
electrochemical stability and multi-electrochromic behavior,
exhibiting pale yellow, yellowish green, blue, or russet colors,
according to their oxidation state. The synthesized TPACz
derivatives are promising materials for visible/NIR electro-
chromic devices because they could be easily prepared from
cheap starting materials and could be facilely fabricated into
electrochromic lms via electrochemical polymerization.
Fig. 6 (a) Pictures of sandwich-type ITO-coated glass electrochromic
device, using PTPACz as active layer. (b) Schematic illustration of the
assembled ECD.
Acknowledgements
We thank the Ministry of Science and Technology of Taiwan for
nancial support of this work.
References
1 (a) P. M. S. Monk, R. J. Mortimer and D. R. Rosseinsky,
Electrochromism: Fundamentals and Applications, VCH,
Weinheim, Germany, 1995; (b) P. M. S. Monk,
R. J. Mortimer and D. R. Rosseinsky, Electrochromism and
Electrochromic Devices, Cambridge University Press,
Cambridge, UK, 2007.
Fig. 7 (a) Photos of sandwich-type ITO-coated glass electrochromic
device, using (MeOTPACz)2 as active layer. (b) Schematic illustration of
the assembled ECD.
coloring process and 1.3 s for the bleaching process. The elec-
2 (a) C. G. Granqvist, Handbook of Inorganic Electrochromic
Materials, Elsevier, Amsterdam, 1995; (b) G. A. Niklasson
and C. G. Granqvist, J. Mater. Chem., 2007, 17, 127–156; (c)
D. T. Gillaspie, R. C. Tenent and A. C. Dillion, J. Mater.
Chem., 2010, 20, 9585–9592.
trochromic coloring efficiency (CE) of blue coloring (h ¼
DOD750/Q) of the PTPACz lm was calculated to be 158 cm2 Cꢁ1
.
Electrochromic devices
Finally, we fabricated the single layer electrochromic cells as
preliminary investigation for their electrochromic applications.
The polymeric and dimeric lms were electrodeposited onto
ITO-coated glass, thoroughly rinsed, and then dried. Aer-
wards, the gel electrolyte was spread on the polymer-coated side
of the electrode and the electrodes were sandwiched. To prevent
leakage, an epoxy resin was applied to seal the device. Fig. 6
shows the pictures and schematic illustration of the assembled
electrochromic device (ECD) using the PTPACz lm as electro-
active layer. When the voltage was applied (from 0.0 to 1.8 V, 2.0
3 (a) P. M. S. Monk, The Viologens: Physicochemical Properties,
Synthesis and Applications of the Salts of 4,40-Bipyridine,
John Wiley and Sons, Chichester, UK, 1998; (b) M. Li,
Y. X. Wei, J. M. Zhang, D. Zhu and C. Y. Xu, Org. Electron.,
2014, 15, 428–434.
4 (a) S. A. Sapp, G. A. Sotzing, J. L. Reddinger and
J. R. Reynolds, Adv. Mater., 1996, 8, 808–811; (b) A. Kumar,
D. M. Welsh, M. C. Morvant, F. Piroux, K. A. Abboud and
J. R. Reynolds, Chem. Mater., 1998, 10, 896–902; (c)
L. B. Groenendaal, G. Zotti, P.-H. Aubert, S. M. Waybright
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 43470–43479 | 43477

View Article Online
RSC Advances
Paper
and J. R. Reynolds, Adv. Mater., 2003, 15, 855–879; (d)
R. M. Walczak and J. R. Reynolds, Adv. Mater., 2006, 18,
1121–1131.
G.-S. Liou, Chem. Mater., 2009, 21, 4062–4070; (e)
L.-T. Huang, H.-J. Yen and G.-S. Liou, Macromolecules,
2011, 44, 9595–9610.
5 (a) F. S. Han, M. Higuchi and D. G. Kurth, J. Am. Chem. Soc., 16 (a) C.-J. Chen, H.-J. Yen, W.-C. Chen and G.-S. Liou, J. Mater.
2008, 130, 2073–2081; (b) A. Maier, A. R. Rabindranath and
B. Tieke, Adv. Mater., 2009, 21, 959–963; (c) C.-J. Yao,
Y.-W. Zhong and J.-N. Yao, Inorg. Chem., 2013, 52, 10000–
10008.
6 (a) P. M. Beaujuge and J. R. Reynolds, Chem. Rev., 2010, 110,
268–320; (b) C. M. Amb, A. L. Dyer and J. R. Reynolds, Chem.
Mater., 2011, 23, 397–415; (c) A. L. Dyer, E. J. Thompson and
Chem., 2012, 22, 14085–14093; (b) C.-J. Chen, Y.-C. Hu and
G.-S. Liou, Polym. Chem., 2013, 4, 4162–4171; (c) H.-J. Yen,
C.-J. Chen and G.-S. Liou, Adv. Funct. Mater., 2013, 23,
5307–5316; (d) H.-J. Yen, J.-H. Wu and G.-S. Liou, High
Performance Polyimides for Resistive Switching Memory
Devices, in Electrical Memory Materials and Devices, RSC
Polymer Chemistry Series, 2015, ch. 4, pp. 136–166.
J. R. Reynolds, ACS Appl. Mater. Interfaces, 2011, 3, 1787– 17 (a) H.-M. Wang and S.-H. Hsiao, J. Polym. Sci., Part A: Polym.
1795; (d) G. Gunbas and L. Toppare, Chem. Commun., 2012,
48, 1083–1101; (e) L. Beverina, G. A. Pagani and M. Sassi,
Chem. Commun., 2014, 50, 5413–5430.
7 (a) P. M. Beaujuge, S. Ellinger and J. R. Reynolds, Nat. Mater.,
2008, 7, 795–799; (b) C. Ma, M. R. Taya and C. Y. Xu, Polym.
Chem., 2014, 52, 272–286; (b) S.-H. Hsiao, H.-M. Wang and
S.-H. Liao, Polym. Chem., 2014, 5, 2473–2483; (c)
H.-M. Wang and S.-H. Hsiao, J. Mater. Chem. C, 2014, 2,
1553–1564; (d) H.-M. Wang and S.-H. Hsiao, J. Polym. Sci.,
Part A: Polym. Chem., 2014, 52, 1172–1184.
Eng. Sci., 2008, 48, 2224–2228; (c) R. Baetens, B. P. Jelle and 18 M. B. Robin and P. Day, Adv. Inorg. Radiochem., 1967, 10,
A. Gustavsen, Sol. Energy Mater. Sol. Cells, 2010, 94, 87–105; 247–422.
(d) G. Sonmez and H. B. Sonmez, J. Mater. Chem., 2006, 16, 19 (a) C. Lambert and G. Noll, J. Am. Chem. Soc., 1999, 121,
2473–2477; (e) Y. Kondo, H. Tanabe, H. Kudo, K. Nakano
and T. Otake, Materials, 2011, 4, 2171–2182; (f) S. Beaupre,
A. C. Breton, J. Dumas and M. Leclerc, Chem. Mater., 2009,
21, 1504–1513; (g) H. T. Yu, S. Shao, L. J. Yan, H. Meng,
8434–8442; (b) A. V. Azeghalmi, M. Erdmann, V. Kriegisch,
G. Noll, R. Stahl, C. Lambert, D. Leusser, D. Stalke,
M. Zabel and J. Popp, J. Am. Chem. Soc., 2004, 126, 7834–
7845.
Y. W. He, C. Yao, P. P. Xu, X. T. Zhang, W. P. Hu and 20 (a) E. T. Seo, R. F. Nelson, J. M. Fritsch, L. S. Marcoux,
W. Huang, J. Mater. Chem. C, 2016, 4, 2269–2273.
8 A. P. Weidner, U.S. Pat., 7450294, 2008.
9 M. Thelakkat, Macromol. Mater. Eng., 2002, 287, 442–461.
10 (a) Y. Shirota, J. Mater. Chem., 2000, 10, 1–25; (b) Y. Shirota, J.
D. W. Leedy and R. N. Adams, J. Am. Chem. Soc., 1966, 88,
3498–3503; (b) R. F. Nelson and R. N. Adams, J. Am. Chem.
Soc., 1968, 90, 3925–3930; (c) S. C. Creason, J. Wheeler and
R. F. Nelson, J. Org. Chem., 1972, 37, 4440–4446.
Mater. Chem., 2005, 15, 75–93; (c) Y. Shirota and 21 (a) J. F. Ambrose and R. F. Nelson, J. Electrochem. Soc., 1968,
H. Kageyama, Chem. Rev., 2007, 107, 953–1010.
115, 1159–1164; (b) J. F. Ambrose, L. L. Carpenter and
R. F. Nelson, J. Electrochem. Soc., 1975, 122, 876–894.
22 (a) L. Otero, L. Sereno, F. Fungo, Y.-L. Liao, C.-Y. Lin and
K.-T. Wong, Chem. Mater., 2006, 18, 3495–3502; (b)
J. Natera, L. Otero, L. Sereno, F. Fungo, N.-S. Wang,
Y.-M. Tsai, T.-Y. Hwu and K.-T. Wong, Macromolecules,
2007, 40, 4456–4463; (c) J. Natera, L. Otero, F. D'Eramo,
L. Sereno, F. Fungo, N.-S. Wang, Y.-M. Tsai and
K.-T. Wong, Macromolecules, 2009, 42, 626–635; (d)
M. I. Mangione, R. A. Spanevello, A. Rumbero, D. Heredia,
G. Marzari, L. Fernandez, L. Otero and F. Fungo,
Macromolecules, 2013, 46, 4754–4763; (e) S. Koyuncu,
B. Gultekin, C. Zafer, H. Bilgili, M. Can, S. Demic, I. Kaya
and S. Icli, Electrochim. Acta, 2009, 54, 5694–5702; (f)
F. Baycan Koyuncu, Electrochim. Acta, 2012, 68, 184–191; (g)
C. Xu, J. Zhao, M. Wang, Z. Wang, C. Cui, Y. Kong and
X. Zhang, Electrochim. Acta, 2012, 75, 28–34; (h) S.-H. Hsiao
and J.-W. Lin, Macromol. Chem. Phys., 2014, 215, 1525–
1532; (i) S.-H. Hsiao and J.-W. Lin, Polym. Chem., 2014, 5,
6770–6778.
11 J. V. Grazulevicius, P. Strohriegl, J. Pielichowski and
K. Pielichowski, Prog. Polym. Sci., 2003, 28, 1297–1353.
12 (a) J.-F. Morin, M. Leclerc, D. Ades and A. Siove, Macromol.
Rapid Commun., 2005, 26, 761–778; (b) N. Blouin and
M. Leclerc, Acc. Chem. Res., 2008, 41, 1110–1119; (c) P.-L.
T. Boudreault, S. Beaupre and M. Leclerc, Polym. Chem.,
2010, 1, 127–136.
13 (a) W.-Y. Lee, T. Kurosawa, S.-T. Lin, T. Higashihara, M. Ueda
and W.-C. Chen, Chem. Mater., 2011, 23, 4487–4497; (b)
A.-D. Yu, T. Kurosawa, Y.-C. Lai, T. Higashihara, M. Ueda,
C.-L. Liu and W.-C. Chen, J. Mater. Chem., 2012, 22, 20754–
20763; (c) A.-D. Yu, T. Kurosawa, M. Ueda and W.-C. Chen,
J. Polym. Sci., Part A: Polym. Chem., 2014, 52, 139–147.
14 (a) M.-Y. Chou, M.-K. Leung, Y.-L. O. Su, C.-L. Chiang,
C.-C. Lin, J.-H. Liu, C.-K. Kuo and C.-Y. Mou, Chem. Mater.,
2004, 16, 654–661; (b) H.-J. Niu, H.-Q. Kang, J.-W. Cai,
C. Wang, X.-D. Bai and W. Wang, Polym. Chem., 2011, 2,
2804–2817; (c) L.-N. Ma, H.-J. Niu, J.-W. Cai, P. Zhao,
C. Wang, Y.-F. Lian, X.-D. Bai and W. Wang, J. Mater.
Chem. C, 2014, 2, 2272–2282; (d) J.-H. Wu and G.-S. Liou, 23 (a) C. M. Amb, P. M. Beaujuge and J. R. Reynolds, Adv. Mater.,
Adv. Funct. Mater., 2014, 24, 6422–6429.
2010, 22, 724–728; (b) A. L. Dyer, E. J. Thompson and
J. R. Reynolds, ACS Appl. Mater. Interfaces, 2011, 3, 1787–
1795; (c) G. Hizalan, A. Balan, D. Baran and L. Toppare, J.
Mater. Chem., 2011, 21, 180–1809; (d) H.-J. Yen and
G.-S. Liou, Polym. Chem., 2012, 3, 255–264; (e) S.-H. Hsiao,
H.-M. Wang, P.-C. Chang, Y.-R. Kung and T.-M. Lee, J.
15 (a) S.-H. Cheng, S.-H. Hsiao, T.-H. Su and G.-S. Liou,
Macromolecules, 2005, 38, 307–316; (b) G.-S. Liou,
S.-H. Hsiao and H.-W. Chen, J. Mater. Chem., 2006, 16,
1831–1842; (c) C.-W. Chang, G.-S. Liou and S.-H. Hsiao, J.
Mater. Chem., 2007, 17, 1007–1015; (d) H.-J. Yen and
43478 | RSC Adv., 2016, 6, 43470–43479
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
Polym. Sci., Part A: Polym. Chem., 2013, 51, 2925–2938; (f)
H.-M. Wang and S.-H. Hsiao, J. Polym. Sci., Part A: Polym.
Chem., 2014, 52, 272–286.
24 (a) S. Koyuncu, O. Usluer, M. Can, S. Demic, S. Icli and
N. S. Saricici, J. Mater. Chem., 2011, 21, 2684–2693; (b)
M. Icli-Ozkut, Z. Oztas, F. Algi and A. Cihaner, Org.
RSC Advances
Electron., 2011, 12, 1505–1511; (c) G. Gunbas and
L. Toppare, Chem. Commun., 2012, 48, 1083–1101; (d)
E. Karabiyik, E. Sefer, F. Baycan Koyuncu, M. Tonga,
E. Ozdemir and S. Koyuncu, Macromolecules, 2014, 47,
8578–8584; (e) B. Karabay, L. C. Pekel and A. Cihaner,
Macromolecules, 2015, 48, 1352–1357.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 43470–43479 | 43479