Highly stable anodic green electrochromic aromatic polyamides: Synthesis and electrochromic properties

Article abstract of DOI:10.1039/b613140a

A 4-methoxy-substituted triphenylamine containing the aromatic diamine, 4,4′-diamino-4″-methoxytriphenylamine (2), was synthesized by the caesium fluoride-mediated condensation of p-anisidine with 4-fluoronitrobenzene, followed by palladium-catalyzed hydr

Full text of DOI:10.1039/b613140a

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www.rsc.org/materials | Journal of Materials Chemistry
Highly stable anodic green electrochromic aromatic polyamides: synthesis
and electrochromic properties{
Cha-Wen Chang,a Guey-Sheng Liou*a and Sheng-Huei Hsiaob
Received 11th September 2006, Accepted 16th November 2006
First published as an Advance Article on the web 18th December 2006
DOI: 10.1039/b613140a
A 4-methoxy-substituted triphenylamine containing the aromatic diamine, 4,49-diamino-
40-methoxytriphenylamine (2), was synthesized by the caesium fluoride-mediated condensation of
p-anisidine with 4-fluoronitrobenzene, followed by palladium-catalyzed hydrazine reduction of
the dinitro intermediate. A series of new polyamides with pendent 4-methoxy-substituted
triphenylamine (TPA) units having inherent viscosities of 0.27–1.39 dL g21 were prepared via the
direct phosphorylation polycondensation of various dicarboxylic acids and the diamine (2). All
the polymers were readily soluble in many organic solvents, such as N-methyl-2-pyrrolidinone
(NMP) and N,N-dimethylacetamide (DMAc), and could be solution-cast into tough and flexible
polymer films. These aromatic polyamides had useful levels of thermal stability associated with
their relatively high softening temperature (242–282 uC), 10% weight-loss temperatures in excess
of 470 uC, and char yields at 800 uC in nitrogen higher than 60%. The hole-transporting and
electrochromic properties are examined by electrochemical and spectroelectrochemical methods.
Cyclic voltammograms of the polyamide films cast onto an indium-tin oxide (ITO)-coated glass
substrate exhibited reversible oxidation at 0.73–0.79 V versus Ag/AgCl in acetonitrile solution,
and revealed excellent stability of electrochromic characteristics with a color change from
colorless to green at applied potentials ranging from 0.00 to 1.05 V. These anodically polymeric
electrochromic materials not only showed excellent reversible electrochromic stability with good
green coloration efficiency (CE = 374 cm2 C21) but also exhibited high contrast of optical
transmittance change (DT %) up to 85% at 787 nm and 30% at 391 nm. After over 1000 cyclic
switches, the polymer films still exhibited excellent stability of electrochromic characteristics.
materials (i.e., viologens, metallophthalocyanines and conju-
Introduction
gated polymers) have received more attention than inorganics
Electrochromism can be defined as the reversible change in
for electrochromic applications because of the different colors
optical properties of a material resulting from electrochemi-
observed with these compounds while switching among their
redox states.1,5,6 Although conjugated polymers (CPs) in
cally induced redox states. Color changes are commonly
between a transparent state, where the chromophore only
particular have several advantages over inorganic compounds,
absorbs in the UV region, and a colored state, or between two
i.e., outstanding coloration efficiency, fast switching ability,
colored states in a given electrolyte solution. The electro-
high stability, fine-tuning of the band gap (and the color)
chromic material may exhibit several colors and be termed
through chemical structure modification, multiple colors with
the same material,7 thin film flexibility and cost effectiveness,
polyelectrochromic. Electrochromic anti-glare car rear-view
mirrors have already been commercialized, with other
proposed applications of electrochromic materials including
their use in controllable light-reflective or light-transmissive
devices for optical information and storage, sunglasses,
protective eyewear for the military, controllable aircraft
canopies, and glare-reduction systems for offices and smart
windows for use in cars and buildings.1–3
they have not received enough attention in terms of applica-
tions. One of the main reasons for this was the deficient third
leg (green) of color space in conjugated polymers (CPs). Until
the first report about a green-colored neutral conjugated poly-
mer (CP) was published,8 researches on conjugated polymers
(CPs) as cathodic electrochromics mainly focused on red and
blue colors. For green color, at least two chromophores are
required that absorb at red and blue wavelengths in the neutral
form, and should also deplete together when the polymer is
oxidized.
The first studies and commercial interests in electrochromic
materials started with inorganic compounds such as tungsten
trioxide (WO3) and iridium dioxide (IrO2).4 Later, organic
Wholly aromatic polyamides are characterized as highly
thermally stable polymers with a favorable balance of physical
and chemical properties. However, rigidity of the backbone
and strong hydrogen bonding results in high melting or
glass-transition temperatures (Tgs) and limited solubility in
most organic solvents.9,10 These properties make them
generally intractable or difficult to process, thus restricting
aDepartment of Applied Chemistry, National Chi Nan University, 1
University Road, Puli, Nantou Hsien 54561, Taiwan, Republic of China.
E-mail: gsliou@ncnu.edu.tw
bDepartment of Chemical Engineering, Tatung University, 40
Chungshan North Rd, 3rd Sec., Taipei 10452, Taiwan, Republic of China
{ The HTML version of this article has been enhanced with color
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their applications. To overcome such a difficulty, polymer-
structure modification becomes necessary. One of the
common approaches for increasing solubility and pro-
cessability of polyamides without sacrificing high thermal
stability is the introduction of bulky, packing-disruptive
groups into the polymer backbone.11–18 We have reported
Synthesis of monomer
4,49-Dinitro-40-methoxytriphenylamine (1). A mixture of
15.20 g (0.10 mol) of caesium fluoride in 90 mL dimethyl
sulfoxide (DMSO) was stirred at room temperature. To the
mixture 5.02 g (0.04 mol) of p-anisidine and 11.68 g (0.082 mol)
of 4-fluoronitrobenzene were added in sequence. The mixture
was heated with stirring at 120 uC for 24 h. The mixture was
poured slowly into 300 mL of stirred methanol, and the
precipitated orange–red powder was collected by filtration and
washed thoroughly with methanol : water (1 : 1). The product
was filtered to afford 13.07 g (89% in yield) of orange–red
some
TPA-containing
polyamides
and
polyimides
from N,N9-bis(4-aminophenyl)-N,N9-diphenyl-1,4-phenylene-
diamine,19,20 N,N9-bis(4-carboxyphenyl)-N,N9-diphenyl-1,4-
phenylenediamine,21
and
2,4-diaminotriphenylamine,22
respectively. Because of the incorporation of bulky, three-
dimensional TPA units along the polymer backbone, all the
polymers were amorphous and exhibited good solubility in
many aprotic solvents with excellent thin film-forming
capability. Recently, we have initiated a study to obtain
TPA-containing anodic electrochromic polymers which
exhibited green light in the oxidized state and were transparent
in the neutral state.23–26
powder. Mp: 211–212 uC measured by DSC at 10 uC min21
.
IR (KBr): 1578, 1313 cm21 (–NO2 stretch). 1H NMR
(DMSO-d6, d, ppm): 3.79 (s, 3H, OCH3), 7.06 (d, 2H, Hc), 7.17
(d, 4H, Hb), 7.21 (d, 2H, Hd), 8.16 (d, 4H, Ha). 13C NMR
(DMSO-d6, d, ppm): 55.6 (OCH3), 116.0 (C7), 121.9 (C2), 125.7
(C3), 129.4 (C6), 137.0 (C5), 141.9 (C1), 151.9 (C4), 158.4 (C8).
Anal. Calcd for C19H15N3O5 (365.35): C, 62.46%; H, 4.14%;
N, 11.50%. Found: C, 62.52%; H, 4.67%; N, 11.71%.
The anodic oxidation pathways of TPA are well studied.27
The electrogenerated cation radical of TPA+ is not stable and
?
could be dimerized to form tetraphenylbenzidine by tail to tail
coupling with loss of two protons per dimer. When the phenyl
groups were incorporated by electron-donating substituents at
the para-position of TPA, the coupling reactions were greatly
prevented affording stable cationic radicals.28,29 In this article,
we therefore synthesized a diamine, 4,49-diamino-40-methoxyl-
triphenylamine (2), and its derived polyamides containing TPA
groups with an electron-rich pendent 4-methoxy phenyl ring
which permits tuning of the solubility and redox potential of
the polymers. The general properties such as solubility and
thermal properties are described. The electrochemical and
electrochromic properties of these polymers are also described
herein and are compared with those of structurally related
ones from 4,49-diaminotriphenylamine.30
4,49-Diamino-40-methoxytriphenylamine (2). In a 250 mL
three-neck round-bottom flask equipped with a stirring bar
under nitrogen atmosphere, 7.34 g (0.020 mol) of nitro
compound (1) and 0.13 g of 10% Pd/C were dissolved/
suspended in 100 mL of ethanol. The suspension solution
was heated to reflux, and 7 mL of hydrazine monohydrate was
added slowly to the mixture, then the solution was stirred at
reflux temperature. After a further 9 h of reflux, the solution
was filtered to remove Pd/C, and the filtrate was cooled to
precipitate. The product was collected by filtration and dried
in vacuo at 80 uC to give 4.88 g (80% in yield) of light-green
Experimental
Materials
4,49-Diaminotriphenylamine (mp = 186–187 uC) was synthe-
sized by hydrazine Pd/C-catalyzed reduction of 4,49-dinitro-
triphenylamine resulting from the condensation of aniline with
4-fluoronitrobezene in the presence of caesium fluoride (CsF)
needles. Mp: 150–152 uC measured by DSC at 10 uC min21
.
IR (KBr): 3454, 3339 cm21 (N–H stretch). 1H NMR
(DMSO-d6, d, ppm): 3.65 (s, 3H, OCH3), 4.82 (s, 4H, NH2),
6.48 (d, 2H, Ha), 6.68 (d, 2H, Hb), 6.69–6.73 (m, 4H, Hc + Hd).
13C NMR (DMSO-d6, d, ppm): 55.4 (OCH3), 114.5 (C7), 115.0
(C2), 121.2 (C6), 125.8 (C3), 137.7 (C4), 143.2 (C5), 144.5 (C1),
153.0 (C8). Anal. Calcd for C19H19N3O (305.37): C, 74.73%;
H, 6.27%; N, 13.76%. Found: C, 74.54%; H, 6.19%; N, 13.63%.
according to
a
previously reported procedure.30 N,N-
Dimethylacetamide (DMAc) (Tedia), N,N-dimethylformamide
(DMF) (Acros), dimethyl sulfoxide (DMSO) (Tedia),
N-methyl-2-pyrrolidinone (NMP) (Tedia), pyridine (Py)
(Tedia), and triphenyl phosphite (TPP) (Acros) were used
without further purification. Commercially available dicar-
boxylic acids such as adipic acid (Ia) (Showa), succinic acid
(Ib) (Acros), trans-1,4-cyclohexanedicarboxylic acid (Ic),
terephthalic acid (Id), isophthalic acid (Ie), 2,6-naphthalenedi-
carboxylic acid (If), 4,49-biphenyldicarboxylic acid (Ig),
4,49-oxydibenzoic acid (Ih), 4,49-sulfonyldibenzoic acid (Ii),
and 2,2-bis(4-carboxyphenyl)hexafluoropropane (Ij) were pur-
chased from TCI and used as received. Tetrabutylammonium
perchlorate (TBAP) (Acros) was recrystallized twice by ethyl
acetate under a nitrogen atmosphere and then dried in vacuo
prior to use. All other reagents were used as received from
commercial sources.
Polymer synthesis
The synthesis of polyamide IId is used as an example to
illustrate the general synthetic route. The typical procedure
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was as follows. A mixture of 0.382 g (1.25 mmol) of 4,49-
diamino-40-methoxytriphenylamine (2), 0.208 g (1.25 mmol) of
terephthalic acid (Id), 0.15 g of calcium chloride, 1.25 mL of
TPP, 0.6 mL of Py, and 1.25 mL of NMP was heated with
stirring at 105 uC for 3 h. The polymer solution was poured
slowly into 300 mL of stirring methanol giving rise to a stringy,
fiber-like precipitate that was collected by filtration, washed
thoroughly with hot water and methanol successively,
and dried under vacuum at 100 uC; yield: 0.538 g (99%).
Reprecipitations from DMAc into methanol were carried out
twice for further purification. The inherent viscosity of the
obtained polyamide IId was 0.79 dL g21, measured at a
concentration of 0.5 g dL21 in NMP at 30 uC. The IR
spectrum of IId (film) exhibited characteristic amide absorp-
tion bands at 3310 (N–H stretching), 1654 cm21 (amide
carbonyl). Anal. Calcd for (C27H21N3O3)n (435.47)n: C,
74.47%; H, 4.86%; N, 9.65%. Found: C, 73.52%; H, 4.98%;
N, 9.48%. The other polyamides were prepared by an
analogous procedure.
wire as an auxiliary electrode at a scan rate of 50 mV s21
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 downwards. The spectroelectrochemical cell
was composed of a 1 cm cuvette, ITO as a working electrode, a
platinum wire as an auxiliary electrode, and a Ag/AgCl
reference electrode. Absorption spectra in spectroelecto-
chemical analysis were measured with a HP 8453 UV-vis
spectrophotometer. Photoluminescence spectra were measured
with
a Jasco FP-6300 spectrofluorometer. Fluorescence
quantum yields (WF) of the samples in NMP were measured
by using quinine sulfate in 1 M H2SO4 as a reference standard
(WF = 0.546).31 All corrected fluorescence excitation spectra
were found to be equivalent to their respective absorption
spectra.
Results and discussion
Monomer synthesis
Preparation of the films
An aromatic diamine with a pendent 4-methoxy-substituted
triphenylamine group, 4,49-diamino-40-methoxytriphenyl-
amine (mp = 150–152 uC), was prepared through the CsF-
mediated nucleophilic displacement reaction of p-anisidine
with 4-fluoronitrobenzene, followed by hydrazine Pd/C-
catalyzed reduction.32,33 The synthetic routines are outlined
in Scheme 1. Due to the inherent electron-donating nature
arising from the nitrogen atom through the conjugated
aromatic p-anisidine, the compound (1) could be readily
obtained in high purity and good yield (89%). 4,49-Diamino-
40-methoxyltriphenylamine (2) could be obtained by the
catalytic reduction of compound (1) with hydrazine mono-
hydrate or under a hydrogen atmosphere. Elemental analysis,
IR, and 1H and 13C NMR spectroscopic techniques were used
to identify the structures of the intermediate compounds (1)
and (2). The transformation of nitro to amino groups could be
monitored by changes in the IR spectra. The nitro groups
of compound (1) gave two characteristic bands at around
1579 cm21 and 1312 cm21 (–NO2 asymmetric and symmetric
stretching). After reduction, the characteristic absorptions of
the nitro group disappeared and the amino group showed the
A solution of the polymer was made by dissolving about 0.5 g
of the polyamide sample in 10 mL of DMAc or NMP. The
homogeneous solution was poured into a 9 cm glass Petri dish,
which was placed in a 90 uC oven for 5 h to remove most of the
solvent; then the semi-dried film was further dried in vacuo at
170 uC for 8 h. The obtained films were about 40–60 mm thick
and were used for X-ray diffraction measurements, solubility
tests, and thermal analyses.
Measurements
Infrared spectra were recorded on a PerkinElmer RXI FT-IR
spectrometer. Elemental analyses were run in an Elementar
1
Vario EL-III. H and 13C NMR spectra were measured on a
Bruker AV-400 FT-NMR system and referenced to the
DMSO-d6 signal, and peak multiplicity was reported as
follows: s, singlet; d, doublet; t, triplet; m, multiplet. The
inherent viscosities were determined at 0.5 g dL21 concentra-
tion using a Tamson TV-2000 viscometer at 30 uC. Wide-angle
X-ray diffraction (WAXD) measurements were performed at
room temperature (ca. 25 uC) on a Shimadzu XRD-7000 X-ray
diffractometer (40 kV, 20 mA), using graphite-monochroma-
tized Cu-Ka radiation. Ultraviolet-visible (UV-vis) spectra of
the polymers were recorded on a Varian Cary 50 Probe
spectrometer. Thermogravimetric analysis (TGA) was con-
ducted with a PerkinElmer Pyris 1 TGA. Experiments were
carried out on approximately 6–8 mg film samples heated in
flowing nitrogen or air (flow rate = 20 cm3 min21) at a heating
rate of 20 uC min21. Thermomechanical analysis (TMA) was
conducted with a PerkinElmer TMA 7 instrument. The TMA
experiments were conducted from 50 to 350 uC at a scan rate of
10 uC min21 with a penetration probe 1.0 mm in diameter
under an applied constant load of 10 mN. Softening tempera-
tures (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 poten-
tiostat and a BAS X-Y recorder with ITO (polymer film area
about 0.7 cm 6 0.5 cm) as a working electrode and a platinum
Scheme 1
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Fig. 1 (a) 1H NMR and (b) 13C NMR spectra of compound 2 in
DMSO-d6.
Scheme 2 Synthesis of polyamides by direct polycondensation
typical N–H stretching absorption pair in the region of 3300–
3500 cm21. Fig. 1 illustrates the 1H NMR and 13C NMR
spectra of diamine (2). Assignments of each carbon and proton
are also given in these figures, and agree well with the
reaction.
reaction and afforded clear and highly viscous polymer
solutions, which precipitated in a tough, fiber-like form when
the resulting polymer solutions were slowly poured into
stirring methanol. As shown in Table 1, the obtained
polyamides had inherent viscosities (ginh) in the range 0.27–
1.39 dL g21, and could be solution-cast into flexible and tough
films indicating the formation of high molecular weight
polymers.
1
proposed molecular structure. The H NMR spectra confirm
that the nitro groups have been completely transformed into
amino groups by the high field shift of the aromatic protons
and the resonance signals at around 4.8 ppm corresponding to
the amino protons.
Polymer synthesis
The elemental analyses were in a good agreement with the
proposed structures. The formation of polyamides was also
confirmed by IR and NMR spectroscopy. Fig. 2 shows a
typical IR spectrum for polyamide IId. The characteristic IR
absorption bands of the amide group were around 3310 (N–H
stretching) and 1654 cm21 (amide carbonyl). Fig. 3 shows a
According to the phosphorylation technique first described by
Yamazaki and coworkers,34,35 a series of novel polyamides
(IIa–IIj) with pendent 4-methoxy-substituted triphenylamine
units were synthesized from the diamine (2) and various
dicarboxylic acids via solution polycondensation using TPP
and Py as condensing agents as shown in Scheme 2. All the
polymerization proceeded homogeneously throughout the
typical set of H and 13C NMR spectra of polyamide IId in
1
DMSO-d6; all the peaks could be readily assigned to the
Table 1 Inherent viscosities and thermal properties of polyamides
Td at 5% weight loss/uCc
Td at 10% weight loss/uCc
Char yield (wt%)d
N2
Polymer
ginha/dL g21
Ts/uCb
N2
Air
N2
Air
IIa
IIb
IIc
IId
IIe
IIf
IIg
IIh
IIi
IIj
a
0.27
0.53
0.68
0.79
0.44
0.93
1.39
0.60
0.72
0.39
165
170
259
248
252
282
276
242
277
263
350
390
415
465
445
440
470
455
455
440
345
400
395
430
445
455
470
455
465
470
375
400
445
515
485
470
505
495
485
500
375
415
445
465
495
495
520
500
485
515
35
25
40
63
67
66
65
69
64
62
Measured at a polymer concentration of 0.5 g dL21 in NMP at 30 uC. Softening temperature measured by TMA with a constant applied
b
load of 10 mN at a heating rate of 10 uC min21
.
Decomposition temperature, recorded via TGA at a heating rate of 20 uC min21 and a gas-
c
flow rate of 30 cm3 min21
.
Residual weight percentage at 800 uC in nitrogen.
d
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Fig. 2 IR spectrum (film) of polyamide IId.
hydrogen and carbon atoms of the recurring unit. The TPA-containing polyamides do not form
a
well-packed
resonance peaks appearing at 10.33 ppm in the 1H NMR
spectrum and at 164.3 ppm in the 13C NMR spectrum also
support the formation of amide linkages.
structure. The amorphous nature of these polyamides was
reflected not only in their good solubility but also agreed with
the UV-vis data of the polymers, which show that the UV-vis
absorption peaks of the polymers do not shift from the
solutions to the films, implying that there is no significant p–p
interaction aggregation between the polymer chains.
Basic characterization
The solubility behavior of polyamides was tested qualitatively,
and the results are summarized in Table 2. All the polyamides
were highly soluble in polar solvents such as NMP, DMAc,
DMF, and DMSO, and the enhanced solubility could be
attributed to the introduction of the bulky pendent methoxy-
substituted TPA moiety into the repeat unit. Thus, the
excellent solubility makes these polymers potential candidates
for practical applications by spin- or dip-coating processes.
The wide-angle X-ray diffraction (WAXD) patterns of the
polyamides given in Fig. 4 indicate that the polymers were
essentially amorphous, revealing that the methoxy-substituted
The thermal properties of the polyamides were investigated
by TGA and TMA. The results are summarized in Table 1.
Typical TGA curves of representative polyamide IIj in both
air and nitrogen atmospheres are shown in Fig. 5. All the
aromatic polyamides exhibited good thermal stability with
insignificant weight loss up to 430 uC in nitrogen. Their 10%
weight-loss temperatures in nitrogen and air were recorded at
475–505 and 465–520 uC, respectively. The carbonized residue
(char yield) of these aromatic polymers was more than 63% at
800 uC in nitrogen atmosphere. The high char yields of these
polymers could be ascribed to their high aromatic content. The
softening temperature (Ts) values of the polymer films were
Table 2 Solubilitya of polyamides
Solvents
Polymer NMP DMAc DMF DMSO m-cresol THF CHCl3
IIa
IIb
IIc
IId
IIe
IIf
IIg
IIh
IIi
IIj
a
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
+
++
+
+
+
+
++
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
Qualitative solubility was tested with 1 mg of a sample in 1 mL of
stirred solvent. ++, soluble at room temperature; +, soluble on
heating; ¡, partially soluble; S, swelling; —, insoluble even on
heating.
Fig. 3 (a) 1H NMR and (b) 13C NMR spectra of polyamide IId in
DMSO-d6.
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Fig. 6 TMA curve of polyamide IIi with a heating rate of 10 uC min21
.
IIb, and IIc in NMP (concentration: 1 6 1025 mol L21).
Aromatic–aliphatic polyamides IIa–IIc and aromatic poly-
amides IId–IIj in NMP solution exhibited fluorescence
emission maxima at 394–396 nm and 444–460 nm with
quantum yields ranging from 1.85% for IIa to 3.63% for IIc,
respectively. The blue shift and higher fluorescence quantum
yield of aromatic–aliphatic polyamides IIa–IIc compared
with aromatic polyamides IId–IIj could be attributed to the
effectively reduced conjugation and capability of charge
transfer complex formation by aliphatic diacids with the
electron-donating diamine moiety in comparison with that of
the strongly electron-accepting aromatic diacids. The cutoff
wavelengths (absorption edge) from UV-vis transmittance
spectra were in the range 358–447 nm. Because of the lower
capability of charge transfer, polyamides IIa–IIc showed a
light color and high optical transparency with cutoff wave-
lengths in the range 358–360 nm.
Fig. 4 WAXD pattern of polyamide films.
The electrochemical behavior of the polyamide II series was
investigated by cyclic voltammetry conducted by film cast on
an ITO-coated glass substrate as the working electrode in
dry acetonitrile (CH3CN) containing 0.1 M of TBAP as an
electrolyte under nitrogen atmosphere. The typical cyclic
voltammograms for TPA-containing polyamide IIh (with
4-methoxy-substituted) and IIh9 (without 4-methoxy-substi-
tuted) are shown in Fig. 8 for comparison. Electrochemical
oxidation (or p-doping) of polymer IIh starts at about 0.62 V
(Eonset) with a reversible oxidation process at E1/2 (average
potential of the redox couple peaks) 0.76 V versus Ag/AgCl.
After 1000 continuous cyclic scans between 0.0 to 1.05 V, the
polymer films still exhibited excellent electrochemical stability.
The oxidation of polyamide IIh occurred at the nitrogen center
of the TPA unit, which is incorporated by electron-donating
methoxyl substituents at the para-position of phenyl rings in a
propeller-like geometry. The introduction of electron-donating
methoxyl substituents into the para-position of TPA not
only greatly stabilize the cationic radicals but also lower the
oxidation potentials of the electroactive polyamides II as
compared with the corresponding polyamides II9 without
methoxyl substituents. The energy of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) levels of the investigated polyamides could be
determined from the oxidation onset or half-wave potentials
Fig. 5 TGA thermograms of polyamide IIj at
20 uC min21
a scan rate of
.
determined from the onset temperature of the probe displace-
ment on the TMA trace, and observed in the range 165–282 uC,
depending on the structure of diacid component, and
decreased with decreasing rigidity of the polymer backbone,
or with the introduction of flexible linkages in the polymer
main chain. A typical TMA thermogram for polyamide IIi is
illustrated in Fig. 6.
Optical and electrochemical properties
The optical and electrochemical properties of the polyamides
were investigated by UV-vis and photoluminescence spectro-
scopy, and cyclic voltammetry. The results are summarized in
Table 3. The UV-vis absorption of these polymers exhibited
strong absorption at 262–370 nm in NMP solutions, which are
assignable to a p–p* transition resulting from the conjugation
between the aromatic rings and nitrogen atoms. The UV-vis
absorption of methoxy-substituted TPA-based polyamide
films IIa–IIj also showed similar single absorbance at
264–377 nm in the solution state. Fig. 7 shows the UV-vis
absorption and photoluminescence spectra of polyamides IIa,
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Table 3 Optical and electrochemical properties for polyamides
Solution l/nma
film l/nm
Abs. Abs.
E1/2/V (vs. Ag/AgCl in CH3CN)
HOMOf/eV
Ege/eV E1/2
LUMOg/eV
Abs.
PL
WF
PL
max onset maxd E1/2
c
Index max
max
(%)b
l0
Eonset
Eonset
E1/2
Eonset
IIa
IIb
IIc
IId
IIe
IIf
IIg
IIh
IIi
263
263
262
364
348
370
364
342
364
354
336
396
394
396
445
444
454
451
446
460
446
477
1.85
2.18
3.63
0.23
0.29
0.13
0.17
0.29
0.15
0.34
2.30
360 264
360 265
358 264
431 364
394 344
447 377
432 361
394 345
444 361
398 356
391 334
b
371
367
362
454
417
466
446
407
488
427
409
395
399
390
496
512
469
502
509
507
492
492
0.76
0.75
0.79
0.73
0.75
0.77
0.75
0.76
0.78
0.79
0.85
0.60
0.60
0.60
0.58
0.59
0.62
0.60
0.62
0.65
0.65
0.71
3.34
3.37
3.43
2.73
2.97
2.66
2.78
3.05
2.54
2.90
3.03
5.08
5.07
5.11
5.05
5.07
5.09
5.07
5.08
5.10
5.11
5.16
4.96
1.74
1.70
1.68
2.32
2.10
2.43
2.29
2.03
2.56
2.21
2.13
1.62
1.59
1.53
2.21
1.98
2.32
2.18
1.93
2.47
2.11
2.04
4.96
4.96
4.94
4.95
4.98
4.96
4.98
5.01
5.01
5.07
IIj
IIh9
a
Spectra in NMP (1 6 1025 mol L21). The quantum yield in dilute solution was calculated in an integrating sphere with quinine sulfate as
c
the standard (WF = 0.546). The cutoff wavelength from the UV-vis transmission spectra of polymer films. Excited at the absorption
d
e
maximum for both the solid and solution states. The data were calculated by the equation: Eg = 1240/lonset of polymer film. The HOMO
f
g
energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV). LUMO = HOMO – Eg.
Fig. 7 UV-vis absorptions and photoluminescence (PL) spectra of
polyamides IIa, IIb, IIc in NMP (1 6 1025 M). Quinine sulfate
dissolved in 1 M H2SO4 (aq.) with a concentration of 1 6 1025 M as
the standard (WF = 0.546).
and the onset absorption wavelength of polymer films, and the
results are also listed in Table 3. For example, the oxidation
half-wave potential for polyamide IIh has been determined as
Fig. 8 Cyclic voltammograms of (a) polyamides IIh and IIh9 film
0.76 V (Eonset = 0.62 V) vs Ag/AgCl. The external ferrocene/
onto an indium-tin oxide (ITO)-coated glass substrate over 1000 cyclic
ferrocenium (Fc/Fc+) redox standard E1/2 (Fc/Fc+) is 0.48 V vs
scans (b) ferrocene in CH3CN containing 0.1 M TBAP at scan rate =
0.05 V s21
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 IIh has been evaluated
to be 5.08 eV.
up at 391 and 787 nm, respectively. The new spectral patterns
were assigned as those of the cationic radical polyamide IIh,
formed by electron removal from the lone pair on the nitrogen
atom on the TPA structure causing the structure to flatten off
and delocalise, so can absorb at longer wavelengths.
Meanwhile, the color of the IIh film changed from colorless
to green with high contrast of optical transmittance change
(DT%) up to 85% at 787 nm and 30% at 391 nm (as shown in
Fig. 9).
Electrochromic characterization
Electrochromism of the polyamide thin films was examined by
an optically transparent thin-layer electrode (OTTLE) coupled
with UV-vis spectroscopy. The electrode preparation and
solution condition were identical to those used in cyclic
voltammetry. The typical electrochromic transmittance spectra
of polyamide IIh are shown in Fig. 9. When the applied
potentials increased positively from 0.00 to 0.95 V, the peak of
absorption at 345 nm, characteristic for the neutral form
polyamide IIh, decreased gradually, while two new bands grew
The color switching times were estimated by applying a
potential step, and the absorbance profiles were followed
(Fig. 10). The switching time was defined as the time required
to reach 90% of the full change in absorbance after the
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Table 4 Optical and electrochemical data collected for polyamide IIh
coloration efficiency measurements
Cycle
DA
Q/mC cm22
g/cm2 C21
Decay (%)
1
0.852
0.866
0.867
0.891
0.867
0.873
0.863
0.847
0.835
0.844
0.851
2.28
2.45
2.46
2.55
2.49
2.53
2.51
2.47
2.44
2.56
2.58
374
353
352
349
348
345
344
343
342
330
330
0
100
200
300
400
500
600
700
800
900
1000
5.6
5.9
6.7
7.0
7.8
8.0
8.3
8.6
11.7
11.8
coloring efficiency [g = DA/Q where DA is the optical
absorbance change, and Q (minicoulombs of charge per
square centimetre, mq cm22) is the injected/ejected charge as
a function of electrode area for switching an electrochromic
device from one state to another] ranging from 374 cm2 C21
for the first cycle to 330 cm2 C21 for the thousandth cycle and
decay of the polyamide IIh were calculated36 and the results
are summarized in Table 4.
Fig. 9 Electrochromic behavior of polyamide IIh thin film (in
CH3CN with 0.1 M TBAP as the supporting electrolyte) at 0.00 V
(&), 0.50 V (%), 0.60 V ($), 0.70 V (#), 0.80 V (m), 0.95 V (n).
switching of the potential. Thin films from polyamide IIh
would require 4.5 s at 0.95 V for switching absorbance at 391
and 787 nm and 1.5 s for bleaching. The color switching times
of polyamide IIh are also shown in Fig 10. After over 1000
cyclic scans, the polymer films still exhibited good stability
of electrochromic characteristics. The high electrochromic
Conclusions
A series of highly stable anodic green electrochromic poly-
amides with high contrast of optical transmittance change
(DT%) up to 85% at 787 nm and 30% at 391 nm have been
readily prepared from the diamine, 4,49-diamino-40-methoxy-
triphenylamine, and various aromatic dicarboxylic acids via
direct phosphorylation polycondensation. Because of the
incorporation of electron-donating methoxy substituents at
the para-position of TPA, not only the electrochemical
oxidative coupling reactions could be greatly prevented but
also lower the oxidation potentials of the electroactive
polyamides. In addition to the enhanced solubility and
excellent thin film formability, high Ts values, and good
thermal stability, these anodically polymeric electrochromic
materials also showed excellent continuous cyclic stability of
electrochromic characteristics with good coloration efficiency
from 374 cm2 C21 for first cycle to 330 cm2 C21 for
thousandth cycle, changing color from the colorless neutral
form to the green oxidized forms when scanning potentials
increased positively from 0.00 to 0.95 V. After over 1000 cyclic
switches, the polymer films still exhibited excellent reversibility
of electrochromic characteristics. Thus, the 4-methoxy-sub-
stituted TPA-based polyamides could be good candidates as
anodic electrochromic materials due to their proper oxidation
potentials, excellent electrochemical and thermal stability.
Acknowledgements
The authors are grateful to the National Science Council of the
Republic of China for financial support of this work.
Fig. 10 Potential step absorptometry and current consumption of
polyamide IIh (in CH3CN with 0.1 M TBAP as the supporting
electrolyte) by applying a potential step (0.00 V ) 0.95 V, coated area:
1 cm2) and cycle time of 10 s for coloration efficiency from 374 cm2 C21
(1st cycle) to 330 cm2 C21 (1000th cycle).
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