Novel blue and red electrochromic poly(azomethine ether)s based on electroactive triphenylamine moieties

Article abstract of DOI:10.1016/j.orgel.2009.11.009

Two series of red and blue electrochromic aromatic poly(azomethine ether)s (PAMEs) containing electroactive triphenylamine (TPA) moieties in the backbone were prepared from the high-temperature polycondensation reactions of newly azomethine-triphenylamine

Full text of DOI:10.1016/j.orgel.2009.11.009

Organic Electronics 11 (2010) 299–310
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Novel blue and red electrochromic poly(azomethine ether)s based on
electroactive triphenylamine moieties
*
Hung-Ju Yen, Guey-Sheng Liou
Functional Polymeric Materials Laboratory, Institute of Polymer Science and Engineering, National Taiwan University, 1 Roosevelt Road, 4th Sec.,
Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two series of red and blue electrochromic aromatic poly(azomethine ether)s (PAMEs)
containing electroactive triphenylamine (TPA) moieties in the backbone were prepared
from the high-temperature polycondensation reactions of newly azomethine–triphenyl-
amine (AM–TPA)-based biphenol monomers, 4,40-di(4-hydroxybenzylideneamino)tri-
phenylamine (3) and 4,40-di(4-hydroxybenzylideneamino)-400-methoxytriphenylamine
(4), with difluoro compounds, respectively. The obtained polymers were readily soluble
in many organic solvents and showed useful levels of thermal stability associated with
high softening temperatures (215–240 °C) and high char yields (higher than 64% at
800 °C in nitrogen). In addition, the polymer films showed reversible electrochemical
oxidation with high contrast ratio and unique anodic blue/red electrochromic
behaviors.
Received 18 September 2009
Received in revised form 22 October 2009
Accepted 3 November 2009
Available online 10 November 2009
Keywords:
Electrochemistry
Electrochromism
Polyethers
Azomethine
Triphenylamine
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
plored for applications in organic electronics, such as
light-emitting materials [6], pH sensors [7], and metal-col-
Electrochromic materials exhibited a reversible optical
change in absorption or transmittance upon electrochemi-
cally oxidized or reduced, such as transition-metal oxides,
inorganic coordination complexes, organic molecules, and
conjugated polymers [1]. In general, the investigation of
electrochromic materials had been directed towards opti-
cal changes in the visible region proved especially useful
for variable reflectance mirrors, tunable windows, and
electrochromic displays.
Poly(azomethine)s (PAMs) or poly(Schiff-base)s with
aromatic backbones were considered as high performance
polymers [2] due to their high thermal stability [3], excel-
lent mechanical strength [4], and good optoelectronic
properties [5]. In recent years, PAMs had also been ex-
lecting polymers [8]. However, their insolubility in com-
mon organic solvents limited their processability and
application [9]. One of the common approaches for increas-
ing the solubility and processibility of PAMs without sacri-
ficing high thermal stability was the introduction of
various substituted aromatic ring into the polymer
backbone [10], by using monomers with thiophene [11],
phenylquinoxaline ring [12], cardo-structure [13], tetraph-
enylethene [14], and others [15]. Though PAMs made by
vapor deposition of the monomers in a vacuum chamber
have also been used as electron transporting layers in poly-
mer light-emitting diodes [16], the preparation by solu-
tion-processable PAMs is beneficial for their fabrication
of large-area, thin-film photonic devices.
The anodic oxidation pathway of electroactive TPA
always associated noticeable color changed was well
reported, and the electrogenerated TPA cation radical could
be dimerized to form tetraphenylbenzidine (TPB), which
* Corresponding author.
E-mail address: gsliou@ntu.edu.tw (G.-S. Liou).
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.11.009

300
H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
was more easily oxidized than the original TPA moieties [17].
Recently, we have reported some organo-soluble and high Tg
aromatic polymers bearing TPA units in the main chain
[18]. Because of the incorporation of bulky and three-dimen-
sional TPA units along the polymer backbone, all the poly-
mers were amorphous with good solubility in many
aprotic solvents and exhibited excellent thin-film-forming
capability.
He), 7.29 (t, J = 8.4 Hz, 2H, Hb), 7.75 (d, J = 8.4 Hz, 4H,
Hg), 8.48 (s, 2H, –CH@N–), 10.09 (s, 2H, –OH). 13C NMR
(DMSO-d6, d, ppm): 115.6 (C10), 122.2 (C7), 122.7 (C1),
123.3 (C3), 124.4 (C6), 127.7 (C5), 129.5 (C2), 130.5
(C11), 144.9 (C8), 146.7 (C9), 147.3 (C4), 158.6 (–CH@N–
), 160.4 (C12). Anal. Calcd (%) for C32H25N3O2 (483.56):
C, 79.48; H, 5.21; N, 8.69. Found: C, 79.34; H, 5.36; N,
8.55. ESI-MS: calcd for (C32H25N3O2)+: m/z 483.6; found:
m/z 482.6.
In this contribution, we therefore synthesized the azo-
methine–triphenylamine
(AM–TPA)-based
aromatic
OH
HO
poly(azomethine ether)s (PAMEs) from the new (AM–
TPA)-based diol monomers, 4,40-di(4-hydroxybenzylide-
neamino)-400-triphenylamine (3) and 4,40-di(4-hydroxyb-
enzylideneamino)-400-methoxytriphenylamine (4), with
difluoro compounds, respectively. Incorporation of pre-
formed TPA units into PAMEs allowed the retention of
some of the favorable properties of polymers such as
high thermal stability and glass transition temperature,
whereas the ether linkages increase the solubility and
enhance the film-forming ability. In addition to the gen-
eral properties (such as inherent viscosities, solubility,
and thermal properties), the electrochemical and electro-
chromic behaviors of these polymers were also
investigated.
12
11
g
N
N
9
C
C
8
10
f
H
H
e
7
5
N
6
d
4
c
3
b
2
a1
2.2.2. 4,40-Di(4-hydroxybenzylideneamino)-400-
methoxytriphenylamine (4)
99.7 % in yield; mp of 293–295 °C (by DSC). 1H NMR
(300 MHz, DMSO-d6, d, ppm): 3.73 (s, 3H, –OCH3), 6.86
(d, J = 8.4 Hz, 4H, He), 6.91–6.96 (m, 6H, Ha + Hc), 7.04 (d,
J = 8.4 Hz, 4H, Hd), 7.73 (d, J = 8.4 Hz, 4H, Hf), 8.46 (s, 2H,
–CH@N–), 10.08 (s, 2H, –OH). 13C NMR (DMSO-d6, d,
ppm): 55.3 (–OCH3), 115.1 (C2), 115.6 (C10), 122.1 (C7),
122.9 (C6), 127.0 (C3), 127.8 (C5), 130.4 (C11), 140.0 (C4),
145.4 (C8), 145.8 (C9), 156.0 (C1), 158.1 (–CH@N–), 160.4
(C12). Anal. Calcd (%) for C33H27N3O3 (513.59): C, 77.17;
H, 5.30; N, 8.18. Found: C, 77.15; H, 5.46; N, 8.15. ESI-
MS: calcd for (C33H27N3O3)+: m/z 513.6; found: m/z 512.6.
2. Experimental section
2.1. Materials
4,40-Diaminotriphenylamine [19] (1) (mp: 186–
187 °C), 4,40-diamino-400-methoxytriphenylamine [20] (2)
(mp: 150–152 °C) and 2,5-bis(4-fluorophenyl)-1,3,4-oxa-
diazole [21] (5b) (mp: 205–206 °C) were synthesized
according to a previously reported procedure. 4,40-Di-
fluoro-benzophenone (5a) (ACROS) was purified by
OH
HO
recrystallization.
Tetrabutylammonium
perchlorate
12
11
N
N
(TBAP) (ACROS) was recrystallized twice from ethyl ace-
tate under a nitrogen atmosphere and then dried in va-
cuo prior to use. All other reagents were used as
received from commercial sources.
f
9
C
H
C
e
8
7
10
H
d
5
N
4
c
6
b
a
3
2
1
2.2. Monomer synthesis
OCH3
2.3. Polymer synthesis
The general procedure is as follows: To a solution of
30.10 mmol of diamino compound was stirred in 85 mL
of ethanol and heated to 70 °C under nitrogen atmo-
sphere. After the diamino compound was dissolved,
90.40 mmol of 4-hydroxybenzaldehyde was added and
the mixture was then heated to reflux. After a further
15 h of reflux, the solution was cooled and poured slowly
into 300 ml of stirred methanol, and the product was col-
lected by filtration and dried in vacuo at 80 °C to give yel-
low powders.
The synthesis of PAME Ia was used as an example to
illustrate the general synthetic procedure. To a two necked
50 ml glass reactor was charged with diol monomer 3
(0.97 g, 2 mmol), 5a (0.44 g, 2 mmol), and 4 ml N-methyl-
2-pyrrolidinone (NMP), 2 ml of toluene, and an excess of
potassium carbonate (0.41 g, 3 mmol). The reaction mix-
ture was heated at 150 °C for 3 h to remove water during
the formation of phenoxide anions, then heated at 170 °C
for 1 h, 180 °C for 3 h, and finally heated at 190 °C for
1 h. After the reaction, the obtained polymer solution was
poured slowly into 400 ml of acidified methanol/water
(v/v = 1/1). The precipitate was collected by filtration,
washed thoroughly with hot water and methanol, and
dried under vacuum at 100 °C. Reprecipitations of the
polymer by NMP/methanol were carried out twice for fur-
2.2.1. 4,40-Di(4-hydroxybenzylideneamino)triphenylamine
(3)
96.9 % in yield; mp of 259–260 °C (by DSC). 1H NMR
(300 MHz, DMSO-d6, d, ppm): 6.87 (d, J = 8.4 Hz, 4H, Hf),
7.00–7.03 (m, 7H, Ha + Hc + Hd), 7.20 (d, J = 8.4 Hz, 4H,

H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
H2N
NH2
301
OH
N
2
+
C
O
H
R
1, 2
OH
HO
N
N
C
H
C
H
Condensation
N
EtOH, reflux
3
yield: 96.9 %
yellow powders
mp: 259-260 o
C
R
3, 4
yield: 99.7 %
4
-OCH3
R:
-H
yellow powders
mp: 293-295 o
C
1, 3 2, 4
Scheme 1. Synthesis of AM–TPA-based diol monomers.
ther purification. The inherent viscosity of the obtained
PAME Ia was 0.20 dl/g (measured at a concentration of
0.5 g/dl in NMP at 30 °C). Anal. Calcd (%) of Ia for
(C47H37N3O3)n (691.81)n: C, 81.60; H, 5.39; N, 6.07. Found:
C, 79.70; H, 5.61; N, 5.90. Anal. Calcd (%) of IIa for
(C48H39N3O4)n (721.84)n: C, 79.87; H, 5.45; N, 5.82. Found:
C, 77.97; H, 5.33; N, 5.89. The other PAMEs were prepared
by an analogous procedure.
HO
OH
12
N
N
11
f
9
C
C
e
8
7
10
H
H
d
5
N
c
6
4
b
a
3
2
1
OCH3
10.0
Fig. 1. 1H NMR and 13C NMR spectra of diol monomer 4 in DMSO-d6.

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H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
OH
HO
12
11
N
N
f
9
C
C
e
8
7
10
H
H
d
5
N
c
6
4
b
a
3
2
1
OCH3
Fig. 2. H–H COSY and C–H HMQC spectra of diol monomer 4 in DMSO-d6.
OH
HO
N
N
C
C
H
H
F
F
+
Ar
5
N
R
3, 4
O
O
Ar
N
N
C
C
H
H
Polycondensation
n
N
K2CO3, NMP, Toluene
R
I, II
-OCH3
-H
3, I 4, II
R:
Ar:
O
N
N
O
a
b
Scheme 2. Synthesis of aromatic PAMEs.

H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
303
2.4. Preparation of the poly(azomethine-ether) films
intoaglassPetridish,whichwasplacedina90 °Covenfor5 h
to remove most of the solvent; then the semidried film was
further dried in vacuo at 160 °C for 8 h. The obtained films
were used for solubility tests and thermal analyses.
A solution of polymer was made by dissolving the poly-
ethersample inNMP. Thehomogeneoussolutionwaspoured
Table 1
Inherent viscositya and solubility behavior of PAMEs.
Polymer
ginh
Solubility in various solventb
(dl/g)
NMP
DMAc
DMF
DMSO
m-cresol
THF
CHCl3
Ia
Ib
IIa
IIb
0.20
0.24
0.32
0.27
++
++
++
++
++
++
++
++
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
++
++
++
++
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
+ꢀ
a
Measured at a polymer concentration of 0.5 g/dl in NMP at 30 °C.
The solubility was determined with a 2 mg sample in 2 ml of a solvent. ++, Soluble at room temperature; +, soluble on heating; +ꢀ, partially soluble or
b
swelling; ꢀ, insoluble even on heating.
O
O
12
13
14
g
11
N
N
O
h
9
16
C
H
C
H
8
10
f
15
i
N
N
e
7
5
n
N
6
d
4
c
3
b
2
a1
Fig. 3. 1H NMR spectrum of PAME Ib in CDCl3.
Fig. 4. TGA and TMA curves of PAME Ia.

304
H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
Table 2
Thermal properties of PAMEs.
Polymera
Ts (°C)b
Rw800 (%)d
T5d (°C)c
T1d0 (°C)c
N2
Air
N2
Air
Ia
Ib
IIa
IIb
215
219
227
240
490
480
460
450
500
480
475
460
540
510
505
485
550
515
525
495
75
68
64
65
a
The polymer film samples were heated at 300 °C for 1 h prior to all the thermal analyses.
b
c
Softening temperature measured by TMA with a constant applied load of 10 mN at a heating rate of 10 °C/min.
Temperature at which 5% and 10% weight loss occurred, respectively, recorded by TGA at a heating rate of 20 °C/min and a gas flow rate of 30 cm3/min.
Residual weight percentages at 800 °C under nitrogen flow.
d
Fig. 5. Cyclic voltammogram of: (a) Ia and (b) IIa films on ITO-coated glass substrate in CH3CN (oxidation) and DMF (reduction) solutions containing 0.1 M
TBAP at scan rate of 50 and 100 mV/s, respectively. The inset showed a linear plot of the anodic current grown at 0.80 V vs. the number of scans and
ferrocene, respectively.

H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
305
2.5. Measurements
0.5 g/dl concentration using Tamson TV-2000 viscometer
at 30 °C. Thermogravimetric analysis (TGA) was conducted
with a PerkinElmer Pyris 1 TGA. Experiments were carried
out on approximately 6–8 mg film samples heated in flow-
ing nitrogen or air (flow rate = 20 cm3/min) at a heating
rate of 20 °C/min. Softening temperatures (Ts) were taken
as the onset temperatures of probe displacement on the
TMA traces. Electrochemistry was performed with a CH
Instruments 611B electrochemical analyzer. Voltammo-
Elemental analyses were run in a Heraeus VarioEL-III
CHNS elemental analyzer. Electrospray ionization (ESI)
mass spectra were measured on LCQ Advantage mass spec-
trometer. NMR spectra were measured on a Bruker AC-300
MHz spectrometer in CDCl3 and DMSO-d6, and peak multi-
plicity was reported as follows: s, singlet; d, doublet; m,
multiplet. The inherent viscosities were determined at
O
O
Ar
N
N
-1 e- (Ox.)
C
C
Resonance
N
N
H
H
N
n
H
N
2
H
Dimerization
-1 e- (Ox. 1)
-1 e- (Ox. 2)
N
N
N
N
N
N
Resonance
Resonance
N
N
N
N
Scheme 3. Proposed reaction for electropolymerization of PAME I.
Table 3
Redox potentials and energy levels of PAMEs.
Index
Oxidation/Va
Reduction/Vb
Epeak
EHOMO (eV)c
ELUMO (eV)c
Eg (eV)d
E1/2
Eonset
Eonset
1st
2nd
Ia
Ib
IIa
IIb
1.03
1.03
0.87
0.86
0.82
0.83
0.70
0.69
ꢀ1.33
ꢀ1.31
ꢀ1.28
ꢀ1.27
ꢀ2.04
ꢀ2.07
ꢀ2.06
ꢀ2.05
ꢀ0.83
ꢀ0.79
ꢀ0.81
ꢀ0.77
5.26
5.27
5.14
5.13
3.61
3.65
3.63
3.67
1.65
1.62
1.51
1.46
a
b
c
From cyclic voltammograms vs. Ag/AgCl in CH3CN. E1/2: average potential of the redox couple peaks.
From cyclic voltammograms vs. Ag/AgCl in DMF.
The energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV; onset = 0.36 V).
d
Difference between EHOMO and ELUMO
.

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H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
grams were presented with the positive potential pointing
to the left and with increasing anodic currents pointing
downwards. Cyclic voltammetry (CV) was conducted with
the use of a three-electrode cell in which ITO (polymer
films area about 0.5 cm ꢁ 1.6 cm) was used as a working
electrode. A platinum wire was used as an auxiliary elec-
trode. All cell potentials were taken by using a homemade
Ag/AgCl, KCl (sat.) reference electrode. Spectroelectro-
chemical experiments were carried out in a cell built from
a 1 cm commercial UV–visible cuvette using Hewlett–
Packard 8453 UV–Visible diode array and Hitachi U-4100
UV–vis-NIR spectrophotometer. The ITO-coated glass slide
was used as the working electrode, a platinum wire as the
counter electrode, and Ag/AgCl cell as the reference elec-
O
O
O
O
N
N
N
N
O
C
H
C
C
C
H
H
H
N
N
O
n
n
N
N
R
R
- e-
+ e-
- e-
+ e-
O
O
O
O
N
N
O
N
N
C
H
C
C
C
H
H
H
N
N
O
n
n
N
N
R
R
- e-
+ e-
- e-
+ e-
O
O
O
O
N
N
O
N
N
C
H
C
C
H
C
H
H
N
N
O
n
N
n
N
R
R
Scheme 4. Mechanism of the electrochemical reduction for PAMEs in an aprotic medium.
Fig. 6. Electrochromic behavior (left) at applied potentials of: (a) 0, (b) 0.60, (c) 0.70, (d) 0.80, (e) 0.90, (f) 1.00, (g) 1.10, (h) 1.25, (i) 1.40 (V vs. Ag/AgCl), and
3-D spectroelectrochemical behavior (right) from 0.50 to 1.40 (V vs. Ag/AgCl) of electropolymerized PAME Ia thin film (ꢂ160 nm in thickness) on the ITO-
coated glass substrate in 0.1 M TBAP/CH3CN.

H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
307
trode. The thickness of the PAME thin films was measured
by alpha-step profilometer (Kosaka Lab., Surfcorder
ET3000, Japan). Colorimetry measurements were obtained
using a Minolta CS-100A Chroma Meter. The color coordi-
nates were expressed in the CIE 1931 Yxy color spaces.
diol monomer around 10 ppm disappeared for PAMEs con-
firming the completion of polymerization.
3.3. Polymer properties
3.3.1. Basic characterization
The solubility behavior of the PAMEs was investigated
qualitatively, and the results were also summarized in Ta-
ble 1. The TPA-based PAMEs showed good solubility in
polar organic solvents such as NMP, N,N-dimethylacet-
amide (DMAc), and m-cresol. The enhanced solubility
made these polymers as potential candidates for practical
applications by spin-coating or inkjet-printing processes
to afford high performance thin films for optoelectronic
devices.
The thermal properties of PAMEs were examined by
TGA and TMA and the typical TGA and TMA curves of rep-
resentative PAME Ia were shown in Fig. 4, and the data
were also listed in Table 2. All the prepared PAMEs exhib-
ited good thermal stability with insignificant weight loss
up to 400 °C under nitrogen or air atmosphere. The 10%
weight loss temperatures of these polymers in nitrogen
and air were recorded in the range of 485–540 and 495–
550 °C, respectively. The concentration of carbonized resi-
due (char yield) of these polymers in a nitrogen atmo-
sphere was more than 64% at 800 °C. The high char yields
of these polymers can be ascribed to their high aromatic
content. The softening temperatures (Ts) of the polymer
film samples were determined by the TMA method with
a loaded penetration probe. They were obtained from the
onset temperature of the probe displacement on the TMA
traces and recorded in the range of 215–240 °C, depending
upon the stiffness of the polymer chain.
3. Results and discussion
3.1. Monomer Synthesis
The new AM–TPA-based diol monomers 3 and 4 were
synthesized by condensation reaction of the diamino com-
pounds 1 and 2 with 4-hydroxybenzaldehyde, respectively
(Scheme 1). Elemental analysis, MS, and NMR spectro-
scopic techniques were used to identify structures of the
target diol monomers. Fig. 1 illustrates the 1H NMR and
13C NMR spectra of the diol monomer 4. The 1H and 13C
NMR chemical shift of CH@N at 8.46 and 158.1 ppm,
respectively, were consistent with the previous reports
(Fig. 1) [6b,18d]. The 1H NMR chemical shift of Ar–OH
shifts to low field at 10.08 ppm due to its acidity increased
by the introduction of azomethine moiety. Assignments of
each carbon and proton were assisted by the two-dimen-
sional NMR spectra shown in Fig. 2, and these spectra
agreed well with the proposed molecular structure.
3.2. Polymer synthesis
A series of newly TPA-based PAMEs were synthesized
from the biphenol monomers 3 and 4 with corresponding
difluoro compounds 5 (Scheme 2). The polymerization
was carried out via potassium carbonate-mediated nucleo-
philic substitution reaction. All polymerization reactions
proceeded homogenously and the obtained PAMEs had
inherent viscosities in the range of 0.20–0.32 dl/g (Table 1).
All the polymers could afford transparent and tough films
via solution casting implying the formation of polymers
with high molecular weight. Typical 1H NMR spectrum of
PAME Ib is illustrated in Fig. 3, and the hydroxyl group of
3.3.2. Electrochemical properties
The electrochemical properties of the PAMEs were
investigated by cyclic voltammetry (CV) conducted for
the cast film on an indium–tin oxide (ITO)-coated glass
substrate as working electrode in anhydrous CH3CN and
Fig. 7. Electrochromic behavior (left) at applied potentials of: (a) 0, (b) 0.75, (c) 0.85, (d) 0.95, (e) 1.05, (f) 1.10 (V vs. Ag/AgCl), and 3-D
spectroelectrochemical behavior (right) from 0.70 to 1.10 (V vs. Ag/AgCl) of PAME IIa thin film (ꢂ160 nm in thickness) on the ITO-coated glass substrate
in 0.1 M TBAP/CH3CN.

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H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
DMF containing 0.1 M of TBAP as an electrolyte under
nitrogen atmosphere for oxidation and reduction measure-
ments, respectively. The typical CV for PAMEs Ia (without
4-methoxy-substituted) and IIa (with 4-methoxy-substi-
tuted) are shown in Fig. 5 for comparison, the results re-
vealed one oxidation and two reduction processes for all
the PAMEs. The PAME IIa (with 4-methoxy-substituted)
exhibited lower oxidation potential than the correspond-
ing Ia (without 4-methoxy-substituted) [20]. The cyclic
voltammogram of PAME Ia indicated that TPA without 4-
methoxy-substituted unit was irreversibly oxidized at ap-
plied potential range from 0 to 1.4 V upon repetitive scan-
ning. A new pair of redox peaks corresponding to TPB was
observed around 0.80 V from the second scan in Fig. 5a
indicating the TPB formation during oxidation of the TPA
moieties of PAME Ia (Scheme 3). As shown in the inset of
Fig. 5a, the plot of the anodic current at 0.80 V was linearly
proportional to the number of scans confirming the occur-
rence of the oxidative coupling at the para-position of TPA
in PAME I main chains. As summarized in Table 3, the
reduction behavior of PAMEs Ia, IIa and Ib, IIb were attrib-
uted the methanone and oxadiazole groups in the polymer
main chain, respectively. In addition, the second reduction
stage around ꢀ2.05 V could be assigned to the imine
(CH@N) groups in the polymer main chains [22], and the
mechanism for the electrochemical reduction of PAMEs
could be depicted as in Scheme 4. During the electrochem-
ical oxidation scanning of the PAME thin films, the color of
O
O
Ar
N
N
C
H
C
H
N
n
OCH3
- 1e- (Ox)
+ 1e-
O
O
O
O
Ar
Ar
N
N
N
N
C
H
C
H
C
H
C
H
Resonance
N
N
n
n
OCH3
OCH3
Resonance
Resonance
O
O
O
O
Ar
Ar
N
N
N
N
C
H
C
C
H
C
H
H
N
N
n
n
OCH3
OCH3
Resonance
O
O
Ar
N
N
C
H
C
H
N
n
OCH3
Scheme 5. Mechanism of the electrochemical oxidation for PAMEs II.

H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
309
the film changed from yellow to red and blue for PAMEs I
and II, respectively. The redox potentials of the PAMEs as
well as their respective highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
(on the basis of ferrocene/ferrocenium is 4.8 eV below the
vacuum level with Eonset = 0.36 V) estimated from the on-
set of their oxidation or reduction in CV experiments were
summarized in Table 3.
absorption at 493 nm. As the anodic potential higher than
1.00 V, the absorption peak at 717 nm corresponding to the
formation of a dication of TPB unit grew up. From the inset
shown in Fig. 6, the PAME Ia film switched from a trans-
missive neutral state (light-yellow; Y: 74; x, 0.372; y,
0.444) to a highly absorbing oxidized state (red; Y: 13; x,
0.601; y, 0.351) with an high optical transmittance change
(
D
%T) of 84%.
On the other hand, the spectroelectrochemical behavior
of PAME IIa film shown in Fig. 7 also exhibited strong
absorption at around 405 nm in the neutral form (0 V).
Upon oxidation (increasing applied voltage from 0 to
1.10 V), the intensity of the absorption peak at 405 nm de-
creased while a new peak at 725 nm gradually increased in
intensity due to the formation of a stable monocation rad-
ical of the AM–TPA unit. Furthermore, the broadened
absorbance band was because of the monocation radical
of AM–TPA unit having more extended conjugation length
than TPA (Scheme 5), which resulted different and unique
coloration from other TPA-based electrochromic materials
[20,23]. From the inset shown in Fig. 7, the PAME IIa film
switches from a transmissive neutral state (light-yellow;
Y: 70; x, 0.374; y, 0.453) to a highly absorbing oxidized
state (blue; Y: 10; x, 0.246; y, 0.305) with an high optical
3.3.3. Spectroelectrochemical and electrochromic properties
Spectroelectrochemical experiments were used to eval-
uate the optical properties of the electrochromic films. The
PAME film was cast on an ITO-coated glass slide, and was
placed in the optical path of the light beam from a UV–
vis-NIR spectrophotometer, which allowed us to acquire
electronic absorption spectra under potential control in a
0.1 M TBAP/MeCN solution. The UV–vis-NIR absorbance
curves correlated to applied potentials and three-dimen-
sional transmittance-wavelength-applied potential corre-
lation of after-oxidized Ia film were depicted in Fig. 6.
The strong absorption at around 396 nm, characteristic
for the charge-transfer complex (CTC) formation between
the electron-donating TPB group and the strongly elec-
tron-accepting AM group in the neutral form (0 V), de-
creased and new peaks at 493 and 717 nm grew up
steadily upon electrochemical oxidation (increasing ap-
plied voltage from 0 to 1.00 V). The one-electron oxidation
transmittance change (D%T) of 85% at 725 nm. Besides,
the distribution of coloration across these polymer films
was very homogeneous with high electrochemical
stability.
product,
a stable monocation radical TPB, exhibited
Fig. 8. (a) Current consumption and (b) electrochromic switching and absorbance change monitored for PAME thin films (ꢂ160 nm in thickness) on the
ITO-coated glass substrate (coated area: 1.6 cm ꢁ 0.5 cm) in 0.1 M TBAP/CH3CN.

310
H.-J. Yen, G.-S. Liou / Organic Electronics 11 (2010) 299–310
(b) G. Zotti, A. Randi, S. Destri, W. Porzio, G. Schiavon, Chem. Mater.
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were cast on ITO-coated glass slides in the same manner
as described above, and chronoamperometric and absor-
bance measurements were performed. While the films
were switched, the absorbance at the given wavelength
was monitored as a function of time with UV–vis-NIR spec-
troscopy (Fig. 8). The switching time was calculated at 90%
of the full switch because it is difficult to perceive any fur-
ther color change with naked eye beyond this point. The
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polymer films still exhibited good stability of electrochro-
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4. Conclusion
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Two series of blue and red electrochromic aromatic
PAMEs containing electroactive TPA moieties in the back-
bone were prepared from the potassium carbonate-medi-
ated nucleophilic substitution reaction of newly
azomethine–triphenylamine (AM–TPA)-based biphenol
monomers with difluoro compounds. Introduction of elec-
tron-donating TPA groups to the polymer main chain not
only afford high Tg and good thermal stability but also
leads to good solubility of the PAMEs. All the obtained
polymers revealed valuable electrochromic characteristics
such as high contrast in visible region and unique blue/
red electrochromic behavior, indicating the incorporation
of azomethine groups into TPA-based polymers is a new
approach for tuning the coloration changed.
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Acknowledgement
The authors are grateful to the National Science Council
of the Republic of China for its financial support of this
work.
(b) G.S. Liou, S.H. Hsiao, N.K. Huang, Y.L. Yang, Macromolecules 39
(2006) 5337–5346;
(c) G.S. Liou, S.H. Hsiao, W.C. Chen, H.J. Yen, Macromolecules 39
(2006) 6036–6045;
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(2008) 2800–2808;
(g) C.W. Chang, C.H. Chung, G.S. Liou, Macromolecules 41 (2008)
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