Novel anodic electrochromic aromatic polyamides with multi-stage oxidative coloring based on N,N,N′,N′-tetraphenyl-p-phenylenediamine derivatives

Article abstract of DOI:10.1039/b810750e

A series of novel aromatic polyamides with pendent 4,4′-dimethoxy- substituted triphenylamine (TPA) units were prepared via the direct phosphorylation polycondensation from a new dicarboxylic acid monomer, N,N-bis(4-carboxyphenyl)-N′,N′-di(4-methoxyphenyl)-1, 4-phenylenediamine (4), and various aromatic diamines. These polyamides were amorphous with good solubility in many organic solvents, such as N-methyl-2-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc), and could be solution-cast into flexible polymer films. They had excellent levels of thermal stability associated with their relatively high glass-transition temperatures (233-308 °C). These polymers exhibited strong UV-vis absorption bands at 351-363 nm in NMP solution. Their photoluminescence spectra showed maximum bands around 450-504 nm. The hole-transporting and electrochromic properties are examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the polyamide 6g prepared from the dicarboxylic acid monomer (4) with a structurally similar diamine monomer N,N-bis(4-aminophenyl)-N′, N′-di(4-methoxyphenyl)-1,4-phenylenediamine (5g) exhibited four reversible oxidation redox couples in acetonitrile solution at E_onset = 0.35, E_1/2 = 0.64, 0.84, and 0.99 V, respectively. After over 3000 cyclic switches for green color, the films of polyamide 6g still showed excellent continuous cyclic stability of electrochromism.

Full text of DOI:10.1039/b810750e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Novel anodic electrochromic aromatic polyamides with multi-stage oxidative
coloring based on N,N,N0,N0-tetraphenyl-p-phenylenediamine derivatives†
a
Cha-Wen Chang and Guey-Sheng Liou
b
*
Received 24th June 2008, Accepted 10th September 2008
First published as an Advance Article on the web 22nd October 2008
DOI: 10.1039/b810750e
A series of novel aromatic polyamides with pendent 4,40-dimethoxy-substituted triphenylamine (TPA)
units were prepared via the direct phosphorylation polycondensation from a new dicarboxylic acid
monomer, N,N-bis(4-carboxyphenyl)-N0,N0-di(4-methoxyphenyl)-1,4-phenylenediamine (4), and
various aromatic diamines. These polyamides were amorphous with good solubility in many organic
solvents, such as N-methyl-2-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc), and could be
solution-cast into flexible polymer films. They had excellent levels of thermal stability associated with
their relatively high glass-transition temperatures (233–308 ꢀC). These polymers exhibited strong UV-
vis absorption bands at 351–363 nm in NMP solution. Their photoluminescence spectra showed
maximum bands around 450–504 nm. The hole-transporting and electrochromic properties are
examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the
polyamide 6g prepared from the dicarboxylic acid monomer (4) with a structurally similar diamine
monomer N,N-bis(4-aminophenyl)-N0,N0-di(4-methoxyphenyl)-1,4-phenylenediamine (5g) exhibited
four reversible oxidation redox couples in acetonitrile solution at Eonset ¼ 0.35, E1/2 ¼ 0.64, 0.84, and
0.99 V, respectively. After over 3000 cyclic switches for green color, the films of polyamide 6g still
showed excellent continuous cyclic stability of electrochromism.
or more redox states connected via a s- or p-bridge molecule.
Introduction
According to Robin and Day,20 MV systems can be classified
into three categories: class I with practically no coupling between
the different redox states, class II with moderate electronic
coupling, and class III with strong electronic coupling (the
electron is delocalized over the two redox centers). Recently, an
experimental and theoretical study of the N,N,N0,N0-tetraphenyl-
p-phenylenediamine cation radical has been reported and
a symmetrical delocalized class III structure was proposed.21 The
redox properties, ion-transfer process, electrochromism, and
photoelectrochemical behavior of N,N,N0,N0-tetrasubstituted-
1,4-phenylenediamine are important for technological applica-
tions.22–26
Electrochromism involves electroactive materials that present
a reversible change in optical properties when the material is
electrochemically oxidized or reduced. A single-species electro-
chromic material which exhibits several colors could be termed as
‘‘polyelectrochromic’’.1 Electrochromic properties have proved
especially useful or promising for the construction of mirrors,2
displays,3–5 windows,6–10 and earth-tone chameleon materials.11–13
On the basis of this concept, electrochromic rear-view and side-
view mirrors have been recently commercialized in the auto-
motive industry.14,15 Anodic electrochromic materials are those
that exhibit different colors depending upon their oxidation
states. One of the interesting multicolor systems is that based
upon the N,N,N0,N0-tetraphenyl-p-phenylenediamine moiety.16
Therefore the intramolecular electron transfer and electronic
coupling effects in the oxidized states are important in the design
of new N,N,N0,N0-tetraphenyl-p-phenylenediamine based poly-
mers for electrochromic devices.
In order to be useful for applications, electrochromic materials
must exhibit long-term stability, multiple colors with the same
material, and large changes in transmittance (large D%T)
between their bleached and colored states.27 Our strategy was to
synthesize
N,N,N0,N0-tetrasubstituted-1,4-phenylenediamine
monomers such as diamines and dicarboxylic acids into which
phenyl groups were incorporated by electron-donating substitu-
ents at the para-position of TPA; coupling reactions were largely
prevented by affording stable cationic radicals and lowering the
oxidation potentials.28–31 The corresponding polymers with high
molecular weights and high thermal stability could be obtained
through conventional polycondensation techniques. Because of
the incorporation of packing-disruptive, propeller-shaped TPA
units along the polymer backbone, most of these polymers
exhibited good solubility in polar organic solvents and uniform,
transparent amorphous thin films could be obtained by solution
casting and spin-coating methods. This is advantageous for their
ready fabrication of large-area, thin-film devices.
Intramolecular electron transfer (ET) processes have been
studied extensively in mixed-valence (MV) systems.17–19 They
usually employ one-dimensional MV compounds containing two
aDepartment of Applied Chemistry, National Chi Nan University, 1
University Road, Puli, Nantou Hsien, 54561, Taiwan, Republic of China
bInstitute of Polymer Science and Engineering, National Taiwan
University, Taipei, 10617, Taiwan, Republic of China. E-mail: gsliou@
ntu.edu.tw
† Electronic supplementary information (ESI) available: Further
synthesis details; NMR, COSY, HMQC spectra; DSC curves; IR
spectra; tables of solubility, viscosity and GPC data; WAXD pattern;
TMA and TGA plots; UV-vis spectra; CVs. See DOI: 10.1039/b810750e
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In this article, we therefore synthesized a new diacid, N,N-
bis(4-carboxyphenyl)-N0,N0-di(4-methoxyphenyl)-1,4-phenylene-
diamine (4), and its derived polyamides containing one or two
N,N,N0,N0-tetraphenyl-p-phenylenediamine units that have
multiple redox potentials to produce multiple colors. The general
properties such as solubility and thermal properties are
described. The electrochemical and electrochromic properties of
these polymers are also investigated herein and are compared
with those of structurally related ones from model compounds
M1, M2 and polyamides M3 and M4.32
nucleophilic substitution reaction of 4-nitroaniline with iodo-
anisole36 followed by hydrazine Pd/C-catalytic reduction
according to the synthesis route outlined in Scheme 1.
4,40-Diaminotriphenylamine32b (5d; mp ¼ 186–187 ꢀC), 4,40-dia-
mino-400-methoxytriphenylamine31 (5e; mp ¼ 150–152 ꢀC), N,N-
bis(4-aminophenyl)-N0,N0-diphenyl-1,4-phenylenediamine37 (5f;
mp ¼ 245–247 ꢀC), and N,N-bis(4-aminophenyl)-N0,N0-di(4-
methoxyphenyl)-1,4-phenylenediamine (5g; mp ¼ 87–89 ꢀC)38
were synthesized by the nucleophilic fluoro-displacement reac-
tion of 4-fluoronitrobenzene with aniline, p-anisidine, 4-amino-
triphenylamine, and 4-amino-40,400-dimethoxytriphenylamine,
respectively, followed by palladium-catalyzed hydrazine reduc-
tion. Commercially available aromatic diamines such as
p-phenylenediamine (5a), 4,40-oxydianiline (5b), and 9,9-bis
(4-aminophenyl)fluorene (5c) were purchased from TCI and used
as received. Commercially obtained anhydrous calcium chloride
Experimental
Materials
According to well-known chemistry,33–35 4-aminotriphenylamine
ꢀ
(mp ¼ 148–149 C) was prepared by the aromatic nucleophilic
ꢀ
(CaCl2) was dried under vacuum at 180 C for 8 h. Tetrabutyl-
amination of 4-nitrofluorobenzene and diphenylamine in
N,N-dimethylformamide (DMF) in the presence of sodium
hydride, followed by reduction by means of hydrazine and Pd/C
in refluxing ethanol. 4-Amino-40,400-dimethoxytriphenylamine
(2) was prepared by the potassium carbonate-mediated aromatic
ammonium perchlorate (TBAP) (ACROS) was recrystallized
twice from ethyl acetate under nitrogen atmosphere and then
dried in vacuo before use. All other reagents were used as received
from commercial sources.
Scheme 1 Synthesis of monomers.
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Scheme 2 Synthesis of polyamides by direct polycondensation reaction.
Preparation of the films
Cu-Ka radiation. Ultraviolet-visible (UV-vis) spectra of the
polymers were recorded on a Varian Cary 50 Probe spectrom-
eter. Thermogravimetric analysis (TGA) was conducted with
a Perkin-Elmer Pyris 1 TGA. Experiments were carried out on
approximately 4–6 mg film samples heated in flowing nitrogen or
air (flow rate ¼ 30 cm3/min) at a heating rate of 20 ꢀC/min.
Thermomechanical analysis (TMA) was conducted with a Per-
kin-Elmer TMA 7 instrument. The TMA experiments were
A solution of polymer was made by dissolving about 0.60 g of the
polyamide sample in 10 mL of N,N-dimethylacetamide (DMAc).
The homogeneous solution was poured into a 9 cm glass Petri
dish, which was placed in a 90 ꢀC oven for 5h to remove most of
the solvent, then the semi-dried film was further dried in vacuo at
170 ꢀC for 7 h. The obtained films were about 50–70mm thick and
were used for solubility tests and thermal analyses.
ꢀ
ꢀ
conducted from 50 to 350 C at a scan rate of 10 C/min with
Table 1 Thermal properties of polyamides
Td at 5% Td at 10%
Measurements
Infrared spectra were recorded on a PerkinElmer RXI FT-IR
spectrometer. Elemental analyses were run in an Elementar
Vario EL-III. 1H and 13C NMR spectra were measured on
a Bruker AV-300 and 400 FT-NMR system, and referenced to
the CDCl3-d1 and DMSO-d6 signals, and peak multiplicity was
reported as follows: s, singlet; d, doublet. The inherent viscosities
were determined at 0.5 g/dL concentration using a Tamson TV-
2000 viscometer at 30 ꢀC. Gel permeation chromatographic
(GPC) analysis was performed on a Lab Alliance RI2000
instrument (one column, MIXED-D from Polymer Laborato-
ries) connected with one refractive index detector from Scham-
beck SFD Gmbh. All GPC analyses were performed using
a polymer/DMF solution at a flow rate of 1 mL/min at 70 ꢀC and
calibrated with polystyrene standards. Wide-angle X-ray
diffraction (WAXD) measurements were performed at room
temperature (ca. 25 ꢀC) on a Shimadzu XRD-7000 X-ray
diffractometer (40 kV, 20 mA), using graphite-monochromatized
weight
loss/ꢀCc
weight
loss/ꢀCc
Char yield (wt%)d
Polymer Tg/ꢀCa Ts/ꢀCb N2 Air N2 Air N2
6a
6b
6c
6d
6e
6f
266
269
308
262
255
256
233
260
259
294
257
254
243
229
465 480 490 510 68
485 480 515 530 66
485 480 525 535 66
480 480 515 540 71
485 480 520 520 71
490 480 525 520 75
490 475 530 520 73
6g
a
Midpoint temperature of the baseline shift on the second DSC heating
trace (rate ¼ 20 ꢀC/min) of the sample after quenching from 400 to 50 ꢀC
b
(rate ¼ 200 ꢀC/min) in nitrogen. Softening temperature measured by
TMA with a constant applied load of 50 mN at a heating rate of 10
c
ꢀC/min. Decomposition temperature, recorded via TGA at a heating
d
rate of 20 ꢀC/min and a gas-flow rate of 30 cm3/min. Residual weight
percentage at 800 ꢀC in nitrogen.
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a penetration probe 1.0 mm in diameter under an applied
constant load of 50 mN. Softening temperatures (Tss) were taken
as the onset temperatures of probe displacement on the TMA
traces. Cyclic voltammetry was performed with a Bioanalytical
System Model CV-27 potentiostat and a BAS X–Y recorder with
ITO (polymer films area about 0.7 cm ꢁ 0.7 cm) was used as
a working electrode and a platinum wire as an auxiliary electrode
at a scan rate of 50 mV/s against a Ag/AgCl reference electrode in
a solution of 0.1 M tetrabutylammonium perchlorate (TBAP)/
acetonitrile (CH3CN). Voltammograms are presented with the
positive potential pointing to the left and with increasing anodic
currents pointing downward. The spectroelectrochemical cell
was composed of a 1 cm cuvette, ITO as a working electrode,
a platinum wire as an auxiliary electrode, and an Ag/AgCl
reference electrode. Absorption spectra in spectroelec-
trochemical analysis were measured with a HP 8453 UV-visible
spectrophotometer. Photoluminescence spectra were measured
with a Jasco FP-6300 spectrofluorometer. Fluorescence quantum
yields (FF) values of the samples in NMP were measured by using
quinine sulfate in 1 N H2SO4 as a reference standard (FF
¼
0.546).39,40 All corrected fluorescence excitation spectra were
found to be equivalent to their corresponding absorption
spectra.
Results and discussion
Polymer synthesis
A series of novel aromatic polyamides 6a–6g having the TPA
unit in the main chain and pendent 4,40-dimethoxy-substituted
TPA groups were prepared from the newly synthesized dicar-
boxylic acid (4) (Scheme 1) and various aromatic diamines 5a–5g
by the phosphorylation polycondensation reaction using tri-
phenyl phosphite and pyridine as condensing agents
(Scheme 2).41,42 All the polymerizations proceeded homoge-
neously throughout the reaction and afforded clear, highly
viscous polymer solutions. All the polymers precipitated in
a tough, fiber-like form when the resulting polymer solutions
were slowly poured with stirring into methanol. These poly-
amides were obtained in almost quantitative yields, with inherent
viscosity values in the range of 0.24–0.83 dL/g. The formation of
polyamides was also confirmed by IR and NMR spectroscopy.
Fig. S5 (ESI†) shows a typical IR spectrum of polyamide 6b. The
characteristic IR absorption bands of the amide group were
around 3414 (N–H stretching) and 1654 cmꢂ1 (amide carbonyl).
Table 2 Optical properties of polyamides
Solution l/nma
Film l/nm
Polymer Abs lmax PLb lmax FF (%)c l0 Abs lmax Abs lonset PLb lmax
d
6a
6b
6c
6d
6e
6f
362
351
357
355
360
363
356
504
499
499
498
492
483
450
1.36
1.43
2.22
1.16
1.18
0.95
0.74
415 354
409 348
405 348
419 350
417 357
419 354
419 359
431
426
430
432
436
434
447
512
513
511
511
508
514
516
1
Fig. S6† shows a typical set of H and 13C NMR spectra of
polyamide 6b in DMSO-d6; all the peaks could be readily
assigned to the hydrogen and carbon atoms of the recurring unit.
Assignments of each carbon and proton are also assisted by the
two-dimensional NMR spectra shown in Fig. S7† and the spectra
agree well with the proposed molecular structure of polyamide
6b. The resonance peaks appearing at 10.13 ppm in the 1H NMR
spectrum and at 164.9 ppm in the 13C NMR spectrum also
support the formation of amide linkages.
6g
a
b
Polymer concentration of 10ꢂ5 mol/L in NMP. They were excited at
the absmax for both the solid and solution states. The quantum yield
in dilute solution was calculated in an integrating sphere with quinine
c
d
sulfate as the standard (FF ¼ 0.546). The cutoff wavelength (l0) from
the UV-vis transmission spectra of polymer films (thickness: 1–3 mm).
Table 3 Electrochemical properties of polyamides and model compounds
Oxidation potential/V (vs. Ag/AgCl in CH3CN)
1st
2nd
E1/2
3rd
E1/2
4th
E1/2
Polymer
E1/2
Eonset
Egapa/eV
HOMOb Eonset/eV
LUMOc Eonset/eV
6a
6b
6c
6f
6g
60bd
0.61
0.61
0.60
0.62
—
0.52
0.54
0.52
0.49
0.35
0.36
0.99
0.98
0.98
—
0.64
0.84
—
—
—
—
0.84
—
—
—
—
0.98
0.99
—
2.88
2.91
2.88
2.86
2.77
2.95
4.88
4.90
4.88
4.85
4.71
4.72
2.00
1.99
2.00
1.99
1.94
1.77
0.48
Oxidation potential/V (vs. Ag/AgCl in CH3CN)
Polymer
E1/2
Eonset
M1
M2
M3
M4
0.78
0.82
0.82
1.07
0.70
0.73
0.68
0.90
a
b
The data were calculated from polymer film by the equation: gap ¼ 1240/lonset
.
The HOMO energy levels were calculated from cyclic voltammetry
and were referenced to ferrocene (4.8 eV). c LUMO ¼ HOMO ꢂ Egap
as in the 6 series.
.
The value is that of analogous polyamide (60b) with the corresponding -Ar- unit
d
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Basic characterization
spectroscopy, and cyclic voltammetry. The results are summa-
rized in Tables 2 and 3. The UV-vis absorption spectra of these
polymers exhibited strong absorption bands at 351–363 nm in
NMP solution, which are assignable to a p–p* transition
resulting from the conjugation between the aromatic rings and
nitrogen atoms. In the solution photoluminescence spectra,
polyamides exhibited fluorescence emission at (450–504 nm). The
polymer films were measured for optical transparency using UV-
The solubility behavior of polyamides was tested qualitatively,
and the results are summarized in Table S1.† All the polyamides
were highly soluble in polar solvents such as NMP, DMAc,
DMF, and dimethyl sulfoxide (DMSO), and the enhanced
solubility could be attributed to the introduction of the bulky
pendent 4,40-dimethoxy-substituted TPA moiety into the repeat
unit. Thus, the excellent solubility makes these polymers poten-
tial candidates for practical applications by spin- or dip-coating
processes. These polymers could afford transparent, flexible and
tough films. In addition, 6c exhibited better solubility compared
to polyamides 6a and 6b. This result implies that bulky lateral
fluorene groups can provide enhanced solubility because of
decreased packing density and crystallinity. The wide-angle
X-ray diffraction studies of the polyamides in Fig. S8† indicated
that these polymers were essentially amorphous.
The obtained polyamides had inherent viscosities in the range
of 0.24–0.83 dL/g, and the weight-average molecular weights
(Mw) and number-average molecular weights (Mn) values were
recorded in the range of 34 000–67 000 and 18 000–35 000,
respectively, relative to standard polystyrene (Table S2†).
The thermal properties of all the polyamides were investigated
by TGA, DSC and TMA. The results are summarized in Table 1.
All the polymers exhibited good thermal stability with insignifi-
ꢀ
cant weight loss up to 450 C in nitrogen. The 10% weight-loss
temperatures of the polyamides in nitrogen and air were recorded
in the range of 490–530 and 510–540 ꢀC, respectively. The
amount of carbonized residue (char yield) of these polymers in
nitrogen atmosphere was more than 66% at 800 ꢀC. The high char
yields of these polymers can be ascribed to their high aromatic
content. All the polymers indicated no clear melting endotherms
up to the decomposition temperatures on the DSC thermograms.
This result also supports the amorphous nature of these TPA-
containing polymers. The Tgs of all the polymers were measured
to be in the range of 233–308 ꢀC by DSC. 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 trace. In all cases, the Ts values obtained by TMA
are comparable to the Tg values measured by the DSC experi-
ments. The typical TMA thermogram for polyamide 6a and TGA
curves for polyamide 6b are shown in Fig. S9.†
Fig. 1 Cyclic voltammograms of (a) ferrocene (b) polyamides 6g, 6b and
60b in CH3CN containing 0.1 M TBAP at scan rate ¼ 0.05 V/s.
Comparing the thermal properties data of polyamides 6 in
Table 1, one will find that polyamide 6c showed the highest Tg
and Ts due to the attachment of bulky lateral fluorene groups
thus restricting the segmental mobility. In addition, polyamide
6d (without 4-methoxy substitution) revealed a slightly higher Tg
and Ts than polyamide 6e (with 4-methoxy substitution). It
means that the bulky di(4-methoxyphenyl) substituent in poly-
amide 6e increasing the steric hindrance for close chain packing,
as well as an enhanced fractional free volume between polymer
chains. Similar tendencies were observed with polyamides 6f and
6g; i.e., polyamides 6f showed the higher Tg and Ts than poly-
amide 6g.
Fig. 2 Electrochromic behavior of polyamide 6g thin film (in CH3CN
with 0.1 M TBAP as the supporting electrolyte) at 0.00 (-), 0.65 (C),
0.75 (:), 0.95 (,), and 1.14 (B) (V vs. Ag/AgCl). 6g+ and 6g2+: (solid
symbols with black arrows); 6g3+ and 6g4+: (hollow symbols with gray
arrows).
Optical and electrochemical properties
The optical and electrochemical properties of the polyamides
were investigated by UV-vis and photoluminescence
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Scheme 3 The simplified redox process of polyamide 6g from its neutral state, radical cation state to dication, trication and tetracation states.
vis spectroscopy, and the cutoff wavelengths (absorption edge;
an ITO-coated glass substrate as the working electrode in dry
CH3CN containing 0.1 M of TBAP as an electrolyte under
nitrogen atmosphere. The typical cyclic voltammograms for
polyamides 6g, 6b and 60b are shown in Fig. 1. There are four
l0) were in the range of 405–419 nm (as shown in Fig. S10†).
reversible oxidation redox couples for polyamide 6g at Eonset
¼
0.35, E1/2 ¼ 0.64, 0.84, and 0.99 V corresponding to successive
one electron removal from the nitrogen atoms at both
N,N,N0,N0-tetraphenyl-1,4-phenylenediamine structures in each
repeating unit to yield two stable delocalized radical cations,
polyamide+ and polyamide3+, from N atoms on the pendent and
main chain TPA groups of the diamine, and two stable quino-
noid-type dications, polyamide2+ and polyamide4+, from N
atoms on the pendent and main chain TPA groups of the diacid,
respectively. Fig. S11† indicates electrochemical behavior for the
model compounds and polyamides (M1, M2, M3, and M4),
which have reversible oxidation redox couples at E1/2 ¼ 0.78
The electrochemical behavior of the polyamides 6 series was
investigated by cyclic voltammetry conducted with film cast on
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(Eonset ¼ 0.70), 0.82 (Eonset ¼ 0.73), 0.82 (Eonset ¼ 0.68), 1.07
(Eonset ¼ 0.90), respectively, to prove the oxidation order of
nitrogen atoms for polyamide 6g, and the results are summarized
in Table 3. Comparing these electrochemical data, we found that
polyamide 6g (with 4-methoxy substitution) was much more
easily oxidized than polyamide 6f (without 4-methoxy substitu-
tion), and the first electron removal for polyamide 6g could be
assumed to occur at the pendent 4,40-dimethoxytriphenylamine
group, which is electron-richer than the N atom on the main
chain TPA unit. Similar tendencies were observed with poly-
imides 6d and 6e; i.e., polyamide 6e was much more easily
oxidized than polyamide 6d. Because of the good stability of the
films and excellent adhesion between the polymer and ITO
substrate, these polyamides exhibited great reversibility of elec-
trochromic characteristics by continuous cyclic scans between 0.0
and 1.20 V, changing color from the original pale yellowish to
green, and then to blue. The energies of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) levels of the investigated polyamides can be
determined from the oxidation onset potentials and the onset
absorption wavelength, and the results are listed in Tables 2 and
3. For example (Fig. 1), the oxidation onset potential for poly-
amide 6g has been determined as 0.35 V vs. Ag/AgCl. The
external ferrocene/ferrocenium (Fc/Fc+) redox standard E1/2 is
0.44 V vs. Ag/AgCl in CH3CN. Assuming that the HOMO
energy for the Fc/Fc+ standard is 4.80 eV with respect to the zero
vacuum level, the HOMO energy for polyamide 6g has been
evaluated to be 4.71 eV.
Fig. 3 Cyclic voltammograms of (a) ferrocene, (b) polyamide 6g film on
an indium-tin oxide (ITO)-coated glass substrate over 3000 cyclic scans in
CH3CN containing 0.1 M TBAP at scan rate ¼ 0.08 V/s.
Electrochromic characterization
Electrochromism of the polyamide thin films was examined by
an optically transparent thin-layer electrode (OTTLE) coupled
with a UV-vis spectroscopy. The electrode preparations and
solution conditions were identical to those used in cyclic vol-
tammetry. The typical electrochromic absorption spectra of
polyamide 6g are shown as Fig. 2. When the applied potentials
increased positively from 0 to 0.65, 0.75, 0.95, 1.14 V, respec-
tively, corresponding to the first, second, third and fourth
electron oxidation, the peak of characteristic absorbance at 359
nm for neutral form polyamide 6g decreased gradually, while
four new bands grew at 1030, 999, 831 and 800 nm, respec-
tively. The new spectral patterns were assigned as those of the
cationic radicals and quinonoid-type dications, the products
obtained by electron removal from the lone pair of the nitrogen
atom on two different p-phenylenediamine structures. Accord-
ing to the electron density of the four nitrogen atoms on two
different p-phenylenediamine structures in each repeating unit,
the anodic oxidation pathway of polyamide 6g was postulated
as in Scheme 3.
Fig. 4 Electrochromic behavior of polyamide 6b thin film (in CH3CN
with 0.1 M TBAP as the supporting electrolyte) at 0.00 (-), 0.65 (C),
0.75 (:), 0.85 (A;), and 0.95 (,), 1.05 (B), 1.15 (O), 1.20 (>) (V vs.
Ag/AgCl). 6b+: (solid symbols with black arrows) and 6b2+: (hollow
symbols with gray arrows).
decreased gradually. Two new bands grew at 425 and 963 nm due
to the first stage oxidation. When the potential was adjusted to
a more positive value of 1.20 V, corresponding to the second step
oxidation, the peak of characteristic absorbance decreased
gradually and a new band grew at 675 nm. Meanwhile, the film
changed from colorless to green and then to a blue oxidized form.
Polymer 6b exhibited high contrast of optical transmittance
change (DT %) to 74% at 963 nm for green and 89% at 675 nm for
blue (as shown in Fig. 4). Thus, this will be a good approach for
facile color tuning of the electrochromic behavior by attaching
TPA units to the polymer main chain and/or as pendent groups.
The color switching times were estimated by applying
a potential step, and the absorbance profiles were followed
(Fig. 5 and 6). The switching time was defined as the time
required to reach 90% of the full change in absorbance after the
switching of the potential. Thin film from polyamide 6b required
2.54 s at 0.88 V for switching absorbance at 425 and 963 nm and
1.37 s for bleaching. When the potential was set at 1.20 V, thin
Meanwhile, the complementary color of the 6g film changed
from the original pale yellowish to green, and then to blue (as
shown in Fig. 2) due to different oxidation states. After over 3000
cyclic switches to the green color, the films of polyamide 6g
showed excellent continuous cyclic stability of electrochromism
(as shown in Fig. 3). The electrochromic characteristics of
polyamide 6b are also shown in Fig. 4. When the applied
potentials increased positively from 0 to 0.85 V, the peak of
transmittance at 357 nm, characteristic of neutral polyamide 6b,
5644 | J. Mater. Chem., 2008, 18, 5638–5646
This journal is ª The Royal Society of Chemistry 2008

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100 cycles) and blue (h ¼ DOD675/Q) (154 cm2/C for 1 cycle to
141 cm2/C for 30 cycles) and decay of the polyamides 6b were also
calculated,43 and the results are summarized in Table 4.
Conclusion
A series of new polyamides 6a–6g having TPA units both in the
polymer main chain and as pendent groups have been readily
prepared from the aromatic dicarboxylic acid, N,N-bis(4-
carboxyphenyl)-N0,N0-di(4-methoxyphenyl)-1,4-phenylenediamine
(4) and various aromatic diamines. Polymer 6b exhibited high
contrast of optical transmittance change (DT%) up to 74% at
963 nm for green and 89% at 675 nm for blue. After over 3000
cyclic switches for green color, the films of polyamide 6g showed
excellent continuous cyclic stability of electrochromism. By
incorporating electron-donating methoxy substituents at the
para-position of N,N,N0,N0-tetraphenyl-1,4-phenylenediamine,
the oxidation potentials of the electroactive polyamides were
lowered; this could be a good approach for facile color tuning of
the electrochromic behavior due to the different multi-stage
oxidation potentials. Attaching bulky electron-donating TPA
units to the polymer main chain and/or as pendent groups can
disrupt the coplanarity of aromatic units in chain packing, which
increases the between-chains spaces or free volume, thus most of
the polymers were amorphous with good solubility in many polar
aprotic solvents. In addition to good thermal stability, all the
obtained polyamides also revealed stability of electrochromic
characteristics by electrochemical and spectroelectrochemical
methods, changing color from the original pale yellowish to
green, and then to blue. Thus, 4,40-dimethoxy-substituted TPA-
based polyamides could be good candidates as anodic electro-
chromic materials due to their useful oxidation potentials,
excellent electrochemical stability, and thin film formability.
Fig. 5 (a) Current consumption and (b) potential step absorptometry of
polyamide 6b (in CH3CN with 0.1 M TBAP as the supporting electrolyte)
by applying a potential step (0.00 V $ 0.88 V) (coated area: 1 cm2) and
cycle time 20s for coloration efficiency from 250 cm2/C (1st cycle) to 239
cm2/C (100th cycle).
Fig. 6 (a) Current consumption and (b) potential step absorptometry of
polyamide 6b (in CH3CN with 0.1 M TBAP as the supporting electrolyte)
by applying a potential step (0.00 V $ 1.20 V) (coated area: 1 cm2) and
cycle time 20s for coloration efficiency from 154 cm2/C (1st cycle) to 141
cm2/C (30th cycle).
Acknowledgements
The authors are grateful to the National Science Council of the
Republic of China for financial support of this work.
film 6b required 4.79 s for coloration at 675 nm and 1.74 s for
bleaching. After over 500 cyclic switches for green color, the films
of polyamide 6b showed excellent continuous cyclic stability of
electrochromism. The high electrochromic coloration efficiency
of green (h ¼ DOD963/Q) (250 cm2/C for 1 cycle to 239 cm2/C for
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