Effect of photoisomerization on the electroactivity and electrochromic behavior of aniline pentamer-based polymers with azo chromophore as reversibly switchable pendant group

Article abstract of DOI:10.1016/j.polymer.2012.09.003

By an oxidative coupling polymerization approach, we have synthesized an electroactive polymer with good solubility containing alternating phenyl-capped aniline pentamer in the main chain and azo chromophores in the side chain. In this study, we present reversible example of photochemically induced switching of electroactivity by performing electrochemical CV in two ways: (i) in that the photochemical reaction switch can be photochemically reverted by reirradiation at a different wavelength (ii) in that a photoreaction is used that is thermally reversible. At the same time, electrochemical investigations of electrochromic properties of an aniline-pentamer-based electroactive poly(azine-azo) (EPAA) coating prepared by oxidative coupling polymerization are presented. The in-situ chemical oxidation of the reduced form of soluble, electroactive poly(azine-azo) (EPAA) in N-methyl-2-pyrrolidone (NMP) was monitored by UV-Visible absorption spectra. Moreover, the electrochromic performance of EPAA was investigated by measuring electrochromic photographs and UV absorption spectra.

Full text of DOI:10.1016/j.polymer.2012.09.003

Polymer 53 (2012) 4967e4976
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Polymer
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Effect of photoisomerization on the electroactivity and electrochromic behavior of
aniline pentamer-based polymers with azo chromophore as reversibly switchable
pendant group
Hsiu-Ying Huang, Jhong-Wei Jian, Yu-Ting Lee, Yi-Ting Li, Tsao-Cheng Huang, Jung-Hsiang Chang,
*
Lu-Chen Yeh, Jui-Ming Yeh
Department of Chemistry, Center for Nanotechnology at CYCU, Chung Yuan Christian University, Chung Li 32023, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
By an oxidative coupling polymerization approach, we have synthesized an electroactive polymer with
good solubility containing alternating phenyl-capped aniline pentamer in the main chain and azo
chromophores in the side chain. In this study, we present reversible example of photochemically induced
switching of electroactivity by performing electrochemical CV in two ways: (i) in that the photochemical
reaction switch can be photochemically reverted by reirradiation at a different wavelength (ii) in that
a photoreaction is used that is thermally reversible. At the same time, electrochemical investigations of
electrochromic properties of an aniline-pentamer-based electroactive poly(azine-azo) (EPAA) coating
prepared by oxidative coupling polymerization are presented. The in-situ chemical oxidation of the
reduced form of soluble, electroactive poly(azine-azo) (EPAA) in N-methyl-2-pyrrolidone (NMP) was
monitored by UVeVisible absorption spectra. Moreover, the electrochromic performance of EPAA was
investigated by measuring electrochromic photographs and UV absorption spectra.
Received 12 June 2012
Received in revised form
29 August 2012
Accepted 1 September 2012
Available online 7 September 2012
Keywords:
Electroactive polymer
Photoactive
Electrochromic
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction
in incident light or heat [6e8]. Materials of this kind are of consid-
erable theoretical and experimental interest and are well suited for
Among photoresponse materials azobenzene and related
compounds representone of the most widely studied systems, dueto
their ease of synthesis and facile detection of photoisomerization
[1,2]. The isomerization, which has first been described by Krollp-
feifferet al. [3], occursat theN]N double bondyielding two different
states, viz. the trans- and the cis-isomer. The two isomers exhibit
different chemical and physical properties and can be interconverted
reversibly by irradiation with light [4]. Interconversion between
trans and higher energy cis isomeric states can be effected both
photochemically (trans5cis) and thermally (cis5trans) with a high
degree of efficiency and an absence of competing side reactions [5,6].
That one configurational isomer is furnished photochemically while
the other is favored thermally makes it possible to effectively drive or
switch azobenzene modified species into a desired geometry and
polarity. As the trans-form is thermodynamically more stable, the
cis-form can also react to the trans-form thermally in the dark [5].
Polymers containing azobenzene moieties often display remarkable
photo- and thermo-regulated behaviour when subjected to changes
a variety of smart applications [9,10].
Electroactive polymers have been classified as a new class of
materials in the past decades and have attracted extensive research
activity as a result of their broad spectrum of potential commercial
applications in electronic [11], optical [12] and biological [13]
research fields. Among these conducting polymers, polyaniline
occupies special place among the electroactive polyconjugated
polymers due to its stability to the action of the environmental
conditions, and because of the easy and cheap method of its
synthesis and the unique properties [14,15]. It has a broad spectrum
of probable use as the anticorrosion covering [16,17], in the accu-
mulator batteries [18e20], for the separation of gases and hardly
separable liquids [21e23], as bio and chemosensor [24e26], etc.
Research activities have been focused particularly on the synthesis
of aniline oligomers with well-defined structures and end-groups
because of their good solubility and ability to undergo further
polymerization. The synthesis of a number of aniline oligomers has
been reported [27,28]. Recently, electroactive aniline-oligomer-
derivative polymers have evoked great research attention [29e
35]. For example, the preparation and electrochemical behaviour
of electroactive polymer have been reported by Wang et al. [29,31]
and Wei et al. [30,32], they developed oxidative coupling
* Corresponding author.
E-mail address: juiming@cycu.edu.tw (J.-M. Yeh).
0032-3861/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.09.003

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H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
polymerization to couple oligoaniline and 1,4-phenylenediamine
[32e34]. The resulting polymers contain well-defined conjugated
segments and provide an opportunity for developing a better
understanding of the structureeproperty relationships and the
conduction mechanism of conjugated polymers; this under-
standing has been limited by the complex molecular structures of
these polymers and their poor solubility in organic solvents.
In the series of polyconjugated electroactive polymers the
substances containing aromatic azo-groups in the main chain and
in the side groups [36,37] are studied rather scarcely. Recently,
Wang et al. prepared electroactive polymers containing azo-groups
in the main chain and side chain, which exhibited electrochemical
properties, high dielectric constants, and electrochromic behavior
[38,39]. Synthesis of electroactive polymer containing the amino
and the azogroups in the main polymer chain is actuality because
the polymers obtained combine specific features of polyaniline and
polyazo-arylenes and may exhibit new valuable properties. With
the purpose of obtaining such polymers we for the first time
studied oxidative coupling polymerization of Azine-Azo with
phenyl-capped aniline pentamer under the ammonium persulfate.
In this work, we have synthesized a novel electroactive polymer
with good solubility containing alternating phenyl-capped aniline
pentamer in the main chain and azo chromophores in the side
chain by oxidative coupling polymerization. UVeVis spectra were
used to monitor the photoisomerization mainly owing to the
photoisomerization derived from azo chromophores in the side
chain. Moreover, we present reversible example of photochemically
induced switching of electroactivity by performing electrochemical
CV in two ways: (i) in that the photochemical reaction switch can
be photochemically reverted by reirradiation at a different wave-
length (ii) in that a photoreaction is used that is thermally revers-
ible. The electrochromic performance of EPAA was investigated by
measuring electrochromic photographs and UV absorption spectra.
at 0e5 ꢀC. Then a water solution (5 mL) of sodium nitrite (0.70 g,
10.1 mmol) was added slowly for 10 min. The mixture was stirred at
0e5 ꢀC for further 60 min. A yellow transparent diazonium salt
solution was obtained. A coupling solution was prepared as follows:
aniline (0.93 g, 10.1 mmol) and HCl (1 N, 10 mL) was dissolved in
30 mL of water under vigorous stirring at 0e5 ꢀC. Then the diazo-
nium salt solution was added dropwise to the coupling solution
with the temperature of 0e5 ꢀC. The system was kept being stirred
at 5 ꢀC for 3 h. Then the final solution was added slowly to 30 mL of
NH3 (1 N) and a yellow-orange precipitate of the azo-compound
formed. The precipitate was filtered and washed with water con-
taining a little amount of sodium hydrogen carbonate (pH ¼ 8). The
precipitate was collected by filtration, washed with deionized
water three times, and dried under vacuum. Compound was ob-
tained as yellow-orange crystal (1.32 g, yield: 71.3%). The charac-
teristic analytical data involved are as follows: LC-MS (Mw ¼ 197)
m/z 195.9 [M]ꢁ. 1H NMR (400 MHz, DMSO-d6):
d
¼ 6.10 (s, 1H, e
NH2), 6.66e6.69 (d, 2H, ArH), 7.41e7.53 (m, 3H, ArH), 7.65e7.75 (d,
4H, ArH). Elemental analysis: Calculated (%): C 73.09, H 5.58, N
21.31; Found (%): C 72.91, H 5.98, N 20.93.
2.3. Synthesis of phenyl-capped aniline pentamer (AP)
AP could be easily synthesized by oxidative coupling of N,N0-
diphenyl-1,4-phenylenediamine (1.20 g) and 4-phenyl-p-phenyl-
enediamine (2.21 g) with ammonium persulfate (1.05 g) as oxidant
as shown in Scheme 1(a). Then, the AP product (1.21 g, 35% yield)
was dispersed into a stirring mixture solution (12 mL hydrazine
hydrate in 120 mL of 1.0 M ammonium hydroxide) and stirred for
24 h. The AP was reduced to the leucoemeraldine state. Finally, it
was filtered using a Buchner funnel and water aspirator, and the
filter was washed with distilled water several times, followed by
drying under dynamic vacuum at 40 ꢀC for 24 h. The detailed
characterizations for the AP were listed as follows: LC-MS
(Mw ¼ 440.2) m/z 441.2 [M]þ. FTIR (KBr, cmꢁ1): 3388 (s, yNH),
2. Experimental
1595 (s,
1308 (s,
y
C]C of quinoid rings), 1516 (vs,
yC]C of benzenoid rings),
2.1. Materials and instrumentation
y
C]N), 817 (m, para-substitution of benzene ring). 1H NMR
(400 MHz, DMSO-d6):
d
¼ 6.6e7.1 (m, 22 H, ArH), 7.9 (s, 2H, eNH).
Aniline (SigmaeAldrich) was doubly distilled prior to use. 2-
Chloro-4,6-diamino-1,3,5-triazine (Aldrich), 4-phenyl-p-phenyl-
enediamine (98%, Aldrich), sodium nitrite (NaNO2, SigmaeAldrich),
N,N0-diphenyl-1,4-phenylenediamine (Aldrich), ammonium persul-
fate (APS, 98%, Merck), N-methyl-2-pyrrolidone (NMP, 99%, Merck),
hydrochloric acid (37%, Riedel-deHaën), ammonium hydroxide (30%,
Riedel-deHaën), and acetone (99%, Acros) were used as received
without further purification. All of the chemicals were of reagent
grade unless otherwise stated. Fourier transform infrared spectra
were collected using an FTIR spectrometer (JASCO FT/IR-4100) at
room temperature. The chemicalstructure of theoligoaniline andthe
electroactive poly(azine-azo) (EPAA) were determined by 1H NMR
spectroscopy on a Bruker 400 spectrometer, using deuterated
dimethyl sulfoxide (DMSO) as the solvent. Molecular weight of EPAA
was determined by gel permeation chromatography (GPC), Waters-
150CV using N-methyl-2-pyrrolidone (NMP) as eluant. Calibration
was accomplished with monodispersed polystyrene (PS) standards.
UVeVisible absorption spectra were collected using a UVeVisible
spectrometer (JASCO V-650). Electrochromic characterization of
EPAA was carried out using an AutoLab (PGSTAT302N) electro-
chemical workstation in an aqueous 1.0 M H2SO4 solution.
Elemental analysis: Calculated (%): C 81.79, H 5.49, N 12.72; Found
(%): C 81.38, H 4.98, N 12.39. The MS, FTIR and 1H NMR spectrum of
AP were shown in Figs. 1e3 respectively.
2.4. Synthesis of Azine-Azo [40]
2-Chloro-4,6-diamino-1,3,5-triazine(0.7 g) wasdissolvedin35mL
of NMP and 1 mL of 1 M HCl was added. The mixture was stirred at
40 ꢀC for 30 min, 4-aminoazobenzene (1.0 g) was then added, and the
mixturewasheatedat100ꢀCfor5 h, afterwhichitwascooledtoroom
temperature as shown in Scheme 1(b). It was neutralized with
a solution of 1 M NaOH. Ayellow solid was filtered off and dried under
vacuum (1.5 g, 84% yield). LC-MS (Mw ¼ 306.1) m/z 307.0 [M]þ. FTIR
(KBr, cmꢁ1): 3395 (m, yNH2), 1496 (vs,
(m, N]N),1308 (s,
1H NMR (400 MHz, DMSO-d6):
y
C]C of benzenoid rings), 1406
C]N), 812 (m, para-substitution of benzene ring).
6.47 (s, 4H, eNH2), 7.50e7.58 (m, 3H,
y
y
d
ArH), 7.79e7.84 (d, 4H, ArH), 8.04e8.07 (d, 2H, ArH), 9.38(s,1H, eNH).
Elementalanalysis: Calculated (%): C 58.81, H 4.61, N 36.58; Found (%):
C 58.71, H 4.74, N 36.12. The MS, FTIR and 1H NMR spectrum of Azine-
Azo were shown in Figs. 1e3 respectively.
2.5. Preparation of electroactive poly(azine-azo) (EPAA)
2.2. Synthesis of 4-aminoazobenzene
EPAA was prepared by simultaneously dissolving 1.0
g
Aniline (0.92 g, 10 mmol) was added dropwise to a solution of
concentrated HCl (37%, 3 mL) in deionized water (30 mL). The
mixture was stirred in an ice bath to keep the reaction temperature
(2.2 mmol) of AP and 0.7 g (2.2 mmol) of Azine-Azo that contained
30 mL of NMP and 1 mL of 1 M HCl was added. Subsequently,
a solution containing 0.52 g of APS and 1 mL of 1.0 M aqueous HCl

H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
4969
Scheme 1. Synthesis route of the (a) AP (b) Azine-Azo and (c) EPAA.
was added dropwise while stirring at 60 ꢀC for 12 h. A black product
was then precipitated by pouring the obtained solution into 200 mL
of distilled water with continuous stirring for 0.5 h. The mixture
was filtered and washed with distilled water and acetone several
times. The as-obtained product was then dried under dynamic
vacuum at 30 ꢀC for 24 h as shown in Scheme 1(c). The typical yield
of as-prepared EPAA powder was ca. 81%. The EPAA characteriza-
tion is given as follows: GPC: The number-average molecular
weight (Mn) is 1.1 ꢂ 104, weight-average molecular weight (Mw) is
2.8 ꢂ 104, and molecular weight distribution of the polymer is 2.53.
50 mV sꢁ1. The viscous NMP solution containing EPAA was subse-
quently cast onto a working electrode of a platinum foil (surface
area ¼ 1 cm2), and the organic solvent was evaporated to produce
a dense thin solid film with a thickness of 30 ꢃ 2
mm. All electro-
chemical cyclic voltammetric measurements were performed in
a double-wall jacketed cell. The electrochemical CV measurement
was repeated at least three times to ensure repeatability and
reproducibility.
(2) In solutions
FTIR (KBr, cmꢁ1): 3336 (m, yNH), 1600 (s, yC
1534, 1498 (vs, yC of benzenoid rings), 1415 (m,
C]N), 812 (m, para-substitution of benzene ring). 1H NMR
(400 MHz, DMSO-d6): 6.42 (s, eNH),
] of quinoid rings),
C
]
C
yN]N), 1303 (s,
Photoresponsive electrochemical experiments were conducted
in a single-compartment, three electrode cell, which has a small
quartz window for introducing the irradiation light. The glassy
carbon electrode was used as a working electrode. An Ag/AgCl,
saturated KCl and a Pt wire were employed as the reference and
auxiliary electrodes, respectively. All the CVs were carried out at
a scan rate of 50 mV/s, in 0.1 M aqueous sodium perchlorate
solution, buffered to pH 5.0 with BrittoneRobinson buffer, was
employed as the electrolyte. All cyclic voltammograms (CV) were
taken using a VoltaLab 40 (PGZ 301) potentiostat.
y
d
d
¼ 6.78e7.32 (m, ArH),
7.45e8.09 (m, ArH), 7.73e7.86 (s, eNH), 9.36 (s, eNH). The FTIR and
1H NMR spectrum of EPAA were shown in Figs. 2 and 2 respectively.
2.6. Reduction of electroactive poly(azine-azo) (EPAA)
The obtained EPAA (0.4 g) was dispersed into a solution of 4 mL
hydrazine hydrate in 40 mL 1.0 M ammonium hydroxide and stirred
for 24 h. The reaction mixture was then filtered, washed with
distilled water several times, and dried under dynamic vacuum at
30 ꢀC for 24 h. Finally, the EPAA was reduced to the leucoemer-
aldine oxidation state (0.37 g, 92%).
2.8. Electrochromic performance
Electrochromic characterization of the EPAA film was carried
out using an AutoLab (PGSTAT302N) Electrochemical Workstation
in an aqueous 1.0 M H2SO4 solution. A compact, three-electrode
electrochemical cell using Ag/AgCl as the reference electrode con-
nected to an AutoLab system that fits in a quartz cell (including
indium tin oxide (ITO) as the working electrode and Pt wire as an
auxiliary electrode) was placed in the UVeVisible sample holder.
The absorption of the resulting thin films was monitored at
different oxidation states (for different voltages) by setting the UVe
Vis detector wavelength to detect across 250e900 nm.
2.7. Electrochemical cyclic voltammetry (CV) and photoresponsive
electrochemical measurements [41,42]
(1) In films
To study the redox behavior of EPAA, sample was examined
systematically by electrochemical cyclic voltammetry (CV) in
100 mL of 1.0 M H2SO4 in the 0.0e1.0 V range at a scan rate of

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H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
Fig. 1. Mass spectra of (a) AP and (b) Azine-Azo.
Fig. 2. FTIR spectra of (a) AP and (b) Azine-Azo (c) EPAA.

H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
4971
Fig. 3. 1H NMR spectra of (a) AP and (b) Azine-Azo (c) EPAA.
3. Results and discussion
leucoemeraldine oxidation state in NMP solution by UVeVisible
absorption spectroscopy as shown in Fig. 4(a). Subsequently, trace
amounts of the oxidant, (NH4)2S2O8, was introduced into the EPAA
solution. It should be noted that upon oxidation, the color of the
EPAA solution varied from dark blue to purple. Initially, one
absorption band was visible at 305 nm, which was associated with
3.1. Polymer synthesis and characterization
The synthetic routes for the preparation of oligoaniline, Azine-
Azo, EPAA are shown in Scheme 1. Fig. 2(c) shows the FTIR
spectra of the obtained EPAA materials. For example, in both of the
FTIR spectra, the characteristic peak found at 3336 cmꢁ1 was
attributed to the NeH stretching modes. Moreover, characteristic
peaks at 1600 cmꢁ1 and 1534 cmꢁ1 were designated as the
stretching modes of N]Q]N and NeBeN respectively. (Q repre-
sents the quinoid ring, and B represents the benzene ring struc-
ture.) The characteristic peak at 1303 cmꢁ1 was assigned to C]N
stretching. The absorption band found at 812 cmꢁ1 corresponded to
the out-of-plane CeH deformation mode. The azo group in both the
EPAA polymer and the Azine-Azo monomer manifested itself by the
peak at 1405e1435 cmꢁ1 which corresponded to the stretching
vibration mode of such a group [43,44]. These changes in the
characteristic peaks of EPAA indicated that EPAA was successful
synthesized.
the
absorption band was at 570 nm, which attributed to the transition
pe
p* transition of the conjugated ring system [45], second
pbepq from benzene ring to quinoid ring.
Upon the addition of trace amounts of the oxidant, slow
oxidation of the aniline pentamer segments in EPAA was observed.
Further oxidation caused hypsochromic shifts of all peaks pre-
sented in the spectrum. The 302 nm absorption band detected by
UVeVisible spectroscopy underwent a blue shift from 302 to
291 nm, accompanied with a decreasing in intensity. At the same
time, the 570 nm peak intensity increased after its intensity
reached a maximum, lmax of a second absorption began to undergo
hypsochromic shifted from 570 to 550 nm with the oxidation
process.
One possible interpretation of this result is presented as follows.
During the continuous oxidation of EPAA in the emeraldine
oxidation state with each aniline pentamer segment contained only
one quinoid ring, which showed a second absorption at a longer
wavelength. Subsequently, it was oxidized to the second emer-
aldine oxidation state with each aniline pentamer segment
3.2. Chemical oxidation of EPAA
In order to study sequential oxidation process of the conjugated
aniline pentamer components, the polymer was reduced to the

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H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
Fig. 5. Cyclic voltammetry measurements of electroactive EPAA thin film on ITO-
coated glass electrodes in aqueous H2SO4 (1.0 M) at a scan rate of 50 mV sꢁ1
.
emeraldine base II (two quinoid rings in oligoaniline segment, EBII),
and pernigraniline base (PNB), as shown in Fig. 4(b).
For the cyclic voltammetry curve of EPAA, the first oxidation
peak (470 mV) corresponded to the transition from EBI to EBII, the
second peak (680 mV) corresponded to the transition from EBII to
PNB. On the other hand, in the case of EPAA, a typical two-pair
redox peak was found, which corresponded to the transition
between three different oxidation states. This included the emer-
aldine base I, emeraldine base II and pernigraniline forms of the
aniline pentamer in EPAA.
3.4. Electrochromic characteristics
Electrochromism has been broadly defined as a reversible
optical change in a material induced by an external voltage. Among
the various electrochromic polymers, PANI has multiple colored
forms depending on the oxidation states, including LB, emeraldine
base, and pernigraniline base [48,49]. In previous literature, the
PANI film switched from a pale yellowish neutral state to the green/
bluish green and then blue oxidized states [15]. The electro-
chromism of the EPAA thin film was examined by means of ITO-
coated glass oxidized at different potentials coupled with UVeVis
spectroscopy, as shown in Fig. 6. For in-situ UVeVis spectra
measurements, the electrode bearing the EPAA thin film was held
in a 1.0 M H2SO4 aqueous solution in a quartz cuvette, with Ag/AgCl
as the reference electrode and a Pt wire as an auxiliary electrode.
Fig. 7 shows the 3-D spectra for the absorbanceewavelength-
applied potential correlations of EPAA. From Figs. 6 and 7, one
can see that the peak for the characteristic absorbance at 620 nm
gradually increased with an increasing in the applied potential
from 0.00 to 0.8 V. This strong absorbance resulted from the
delocalization of the radical cation (polaron) along the doped
electroactive structure [47]. While being oxidized at 1.00 V, EPAA
showed a much lower intensity and a blue shift of the polaron band
because the benzoid ring was gradually replaced with the quinoid
ring [47,50]. These electrochromic properties conformed to the
cyclic voltammetry data (Fig. 5). During a complete redox cycle, we
also observed a continuous color change (inset of Fig. 6). The color
of the EPAA thin film changed from green (at 0 mV), to Cyan (at ca.
470 mV), to indigo (at ca. 680 mV). In conclusion, as a multicolor
electrochromic material with color changing from green to indigo,
EPAA would have a wider application in an electrochromic device
similar to polyaniline materials [15].
Fig. 4. (a) UVeVis spectra monitoring the chemical oxidation of the EPAA in the leu-
coemeraldine oxidation state and (b) a schematic presentation of the oxidation state
change of aniline-pentamer-based EPAA.
containing two quinoid rings, which corresponded with the
increase in intensity of the second absorption. After the second
absorption peak reached maximum intensity, it exhibited a blue
shift. This blue shift is indicative of the conversion past the emer-
aldine base state to the pernigraniline oxidation state, which is in
agreement with the two pairs of redox peaks in the cyclic vol-
tammogram as shown in Fig. 5. The chemical oxidation process of
EPAA was similar to that of oligoaniline [46].
3.3. Electroactivity of EPAA coatings
Electrochemical cyclic voltammetric (CV) studies have been
widely used to characterize the redox properties of electroactive
polymers. In this study, the polymers were characterized by cyclic
voltammetry using a typical three-electrode electrochemical cell.
As shown in Fig. 5, the cyclic voltammetry curve of EPAA shows
a two-pair redox peak (470 and 680 mV) [47]. It should be noted
that three different oxidation states of EPAA can be found: emer-
aldine base I (one quinoid ring in oligoaniline segment, EBI),

H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
4973
Fig. 6. The electrochromic behavior of the EPAA thin film on ITO-coated glass electrodes at different oxidation potentials in a 1.0 M H2SO4 solution.
3.5. Photochemical behavior of the Azine-Azo monomer and
electroactive poly(azine-azo) (EPAA) polymer
chromophore. The maximum absorption peak centered at 359 nm is
attributed to the * transition, (see curve A of Fig. 8(a)). Irradia-
pep
tion with UV light for 1 min caused the absorption peak at 359 nm
While the spectrum of the trans-isomer of azo-compound
obviously decreased (lmax changes from 359 to 348 nm), while
shows an intense
a weaker (forbidden) ne
of the cis-isomer is characterized by a more intense ne
p
e
p
* transition band around
* band around
¼ 440 nm, the spectrum
* transition
l
¼ 360 nm and
a broad absorption at around 440 nm (nep*, curve B of Fig. 8(a))
p
l
with an isosbestic point at 422 nm was developed, indicating the
occurrence of trans to cis photoisomerization. And from Fig. 8(a)
curve (C) we can see that the cis isomer reconverted to the trans
isomer again under visible illumination for 1 min, which resulted in
a recovery of the spectral features of trans isomer. It is seen that the
recovered absorbance of trans-azobenzene is slightly lower than
that before irradiation. However, in Fig. 8(b), spectra have taken
before and after irradiation as well as during the thermal back
reaction were shown. Under these conditions (in the dark room
temperature), the back reaction took about 30 min for completion. It
is obvious that cis/trans isomerization thermally reversible is
more easily reduced than photochemically reverted. These results
demonstrate that Azine-Azo monomer is thermodynamically
compound. From the result described above, the obtained monomer
retained the photoisomerization characteristics of azobenzene [51].
In order to study its trans5cis isomerization, the polymer in the
emeraldine oxidation state without processing was dissolved in
NMP solution and UVeVis spectra changes were monitored during
the ultraviolet irradiation. This photoinduced trans5cis isomeri-
zation was analyzed in the EPAA as presenting in Fig. 9. The poly-
mer absorption band at 387 nm due to azopolymer decreased
gradually, while the absorption band at 580 nm due to aniline
pentamer remained unchanged. Under this aniline pentamer band
was hidden the smaller band at 440 nm due to the cis form of
azopolymer. As same as monomer, Fig. 9(a) was photochemically
reverted and Fig. 9(b) was thermally reversible results. That
cis/trans isomerization thermally reversible is more easily
reduced than photochemically reverted. These results demonstrate
that EPAA is thermodynamically compound, too. These character-
istics are due to the presence of a trans5cis azo isomer of EPAA.
Moreover, this trans5cis behavior of EPAA can also be further
identified by electrochemical cyclic voltammetry (CV) approach, as
shown in Fig. 10.
p
band around
light of
l
¼ 440 nm [4]. Irradiation of the trans form with UV
a
suitable wavelength induces a trans/cis photo-
isomerization reaching a photostationary state. The back cis/trans
isomerization can occur either via a thermal reaction, because the
trans-form is thermodynamically more stable, or via a photoin-
duced isomerization by irradiation into the nep* band of the cis
form. We present reversible example of photochemically induced
switching in two ways: (i) in that the photochemical reaction
switch can be photochemically reverted by reirradiation at
a different wavelength (ii) in that a photoreaction is used that is
thermally reversible. The result of the isomerization processes was
monitored by UVeVis spectroscopy.
Fig. 8 showed that the Azine-Azo monomer displayed photo-
isomerization behavior in NMP solution in a sealed quartz cuvette
under alternate exposed to UV lamp (365 nm) for 1 min and visible
lamp (420 nm) for 1 min. The spectra were typical of azobenzene
3.6. Photoresponsive electrochemical behavior of EPAA solutions
Azobenzene molecules in organic solution undergo reversible
Fig. 7. 3-D spectroelectrochemical behavior of the EPAA thin film on an ITO-coated
glass electrode between 0.00 and 1.00 V (vs Ag/AgCl).
trans5cis isomerization under light illuminations [52,53].

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H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
Fig. 8. UVeVis spectra of Azine-Azo in NMP solution (a) as a function of different UV irradiation and (b) as a function of time in dark, at room temperature.
However, such a photochromic property is often restricted in films
because the closely packed film structure cannot provide enough
free volume to allow the trans to cis photoisomerization which is
accompanied by a volume-increased structural change [53,54]. This
makes it difficult to conduct the electrochemistry studies of cis-
azobenzene in such molecular assemblies. Scheme 2 illustrates
this concept schematically. As the configuration of azobenzene
much higher than the reduction potential of azobenzene, the
electrochemical reduction of azobenzenes to hydroazobenzene
could be eliminated, just as shown in other literature [58e61].
Therefore, the scanning potential is confined from 0 to þ1.0 V in the
CV experiment, which is much higher than ꢁ0.6 V. In this study, we
present reversible result of photochemically induced switching of
electroactivity by performing electrochemical CV in two ways: (i) in
that the photochemical reaction switch can be photochemically
reverted by reirradiation at a different wavelength (ii) in that
a photoreaction is used that is thermally reversible. Fig. 10(a)
showed the cyclic voltammetric (CV) of EPAA in aqueous solutions
under photochemical reaction switched by reirradiation at
a different wavelength. Two pairs of waves from the redox of EPAA
were observed from each curve. Curve A was the before irradiation
case, corresponding to the voltammetric behavior of trans-EPAA.
When exposing the sample to UV light, a clear cut Faradaic
response was observed curve B. This was attributed to the elec-
trochemical reduction-oxidation of UV-created cis-EPAA isomers.
The photochromic property of EPAA molecules provided an alter-
native approach for corroborating the above reactions. In curve C of
Fig. 10(a), the EPAA was exposed to visible illumination after being
irradiated with UV light. Since the UV-generated cis-EPAA isomers
were mostly reconverted into trans-EPAA by the visible illumina-
tion, the Faradaic response was greatly decreased, in nice agree-
ment with our prediction. By repeating the UV and visible
irradiations, the increasing and decreasing of electrochemical
response was also changed reversibly, indicative of the reversible
trans-cis photoisomerization of EPAA molecules. A preliminary
explanation of those changes was given below: the trans-cis
changes from trans to cis,
a most probable angle of ca. 10ꢀ close to planar. The result causes
the * interaction increased between the benzene ring C and D
q moves to the high angle region with
pep
for EPAA as shown in Scheme 2. This concept is based on the fact
that the cross-sectional area of cis-azobenzene is larger than that of
trans-azobenzene. This means that trans-to-cis isomerization gives
rise to an increasing in the cross-sectional area of azobenzene. Free
volume means not only that there are actual voids, but also that
additional space can be created easily by changing the tilt angle of
the molecules. When there is enough free volume, photo-
isomerization will proceed as shown in the left part. On the other
hand, if the molecules are closely packed in the film, the molecules
will not photoisomerize [55]. To understand the electrochemical
behavior of trans5cis isomerization EPAA was used as solution
which could be researched in aqueous solutions.
Previous studies demonstrated that the azobenzene moiety was
also electrochemical activity [56,57]. Thus, in order to investigate
the influence of photochromic in azobenzene groups on electro-
chemical behavior of the EPAA, it is necessary to avoid the reduc-
tion of azobenzene groups. Fujishima et al. showed that the
reduction potential of azobenzene to hydroazobenzene was lower
than ꢁ0.6 V (vs SCE) [56,57]. Thus, when the scanning potential was
Fig. 9. UVeVis spectra of EPAA in NMP solution (a) as a function of different UV irradiation and (b) as a function of time in dark, at room temperature.

H.-Y. Huang et al. / Polymer 53 (2012) 4967e4976
4975
Fig. 10. Cyclic voltammetry measurements of electroactive EPAA in 0.1 M aqueous sodium perchlorate solution, buffered to pH 5.0 with BrittoneRobinson buffer (a) as a function of
different UV irradiation and (b) as a function of time in dark, at room temperature.
reversible switching of EPAA was possible without changing
temperature or composition. Moreover, the electrochromic
performance of EPAA was investigated by measuring electro-
chromic photographs and UV absorption spectra. The color of the
EPAA thin films changed from green (at 0.0 V), to Cyan (at ca. 0.4 V),
to indigo (at ca. 0.8 V). The electroactive poly(azine-azo) showed
photoactive and electroactive properties, suggesting that it has
great potential for many applications, such as for use as photo
materials and in electrochromic devices.
Acknowledgment
This research was supported by the National Sciences Council of
Republic of China under Grant number of NSC 98-2113-M-033-001-
MY3 and 99-EC-17-A-08-S1-143. We thank Prof. Yih Kuang-Hway,
(Department of applied cosmetology and Graduate Institute of
cosmetic Science at Hungkuang University) for his help with pho-
toisomerization UVeVis measurements.
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