Synthesis and characterization of electroactive hyperbranched aromatic polyamides based on A2B-type triphenylamine moieties

Article abstract of DOI:10.1039/b910081d

A series of electrochromic aromatic hyperbranched polyamides with electroactive triphenylamine (TPA) units were prepared from the phosphorylation polyamidation reactions of a newly A₂B type monomer, 4-amino-4′,4″-dicarboxytiphenylamine, with AB

Full text of DOI:10.1039/b910081d

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www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and characterization of electroactive hyperbranched aromatic
polyamides based on A2B-type triphenylamine moieties†
*
Guey-Sheng Liou, Hung-Yi Lin and Hung-Ju Yen
Received 21st May 2009, Accepted 6th August 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b910081d
A series of electrochromic aromatic hyperbranched polyamides with electroactive triphenylamine
(TPA) units were prepared from the phosphorylation polyamidation reactions of a newly A2B type
monomer, 4-amino-40,400-dicarboxytiphenylamine, with AB monomer and end-capping agent,
respectively. These polymers were readily soluble in many organic solvents and showed useful levels of
thermal stability associatꢀed with high glass-transition temperatures (201–221 ꢀC) and high char yields
(higher than 71% at 800 C in nitrogen). The polymer films showed reversible electrochemical
oxidation with a high contrast ratio in the visible region, and exhibited high coloration efficiency (CE),
and high-level stability for an electrochromic operation. The polyamide P2 thin film revealed high
coloration efficiency (CE ¼ 426 cm2/C) and optical transmittance change (D%T ¼ 79%) in the visible
region with reversible electroactive stability (over 1000 times within 5% loss).
results reported to date from hyperbranched aromatic polyamide
Introduction
with TPA backbone structures.
Recently, dendritic macromolecules, such as dendrons, den-
drimers and hyperbranched polymers have received considerable
attention due to their unique structures, chemical and physical
properties.1 They are mainly classified into dendrimers with well-
defined structures and hyperbranched polymers having statisti-
cally branched architecture. Hyperbranched polymers resemble
dendrimers in many physical properties such as low viscosity,
high solubility, and lack of significant entanglement in the solid
state. In contrast, hyperbranched polymers are readily synthe-
sized by one-step polymerization of ABn monomers. This is
advantageous over the multistep synthesis of dendrimers because
of the rapid production of large quantities.2 Furthermore,
various functional groups can be introduced into hyperbranched
polymers and the properties can be tuned by the chemical
modification of end functional groups. A wide variety of
hyperbranched polymers such as polyesters,3 polyethers,4 poly-
phenylenes,5 poly(ether ketone)s6 and polyamides7 have been
reported in the literatures. In addition, the incorporation of tri-
phenylamine (TPA) groups into hyperbranched polyamine,
poly(3,4-ethylenedioxythiophene- didodecyloxybenzene), poly-
fluorene, polyphenylene, polyphenylenevinylene, and poly-
thiophene backbones were also investigated and utilized for
applications, such as solar cells8 and light-emitting9 materials.
However, there were just a few spectroelectochemical studies10 of
TPA-based hyperbranched polymers and no electrochromic
Photochromism is a reversible photoinduced transformation
of a molecule between two isomers exhibiting different absorp-
tion spectra. This phenomenon is of great interest for emerging
technologies, such as optical data storage and optical switching
for optoelectronic devices.11 Electrochromic materials exhibit
a reversible optical change in absorption or transmittance upon
electrochemically oxidized or reduced, such as transition-metal
oxides, inorganic coordination complexes, organic molecules,
and conjugated polymers.12 The investigation of electrochromic
materials has been directed towards optical changes in the visible
region (e.g., 400–800 nm) because the color changes are more
conveniently controlled and tuned by just adjusting the applied
potential, therefore, electrochromic polymers are especially
useful for antiglare back-mirrors, tunable windows, and elec-
trochromic displays.
Wholly aromatic polyamides can be readily synthesized by
direct polycondensation of diacids and diamines in the presence
of condensing agents such as triphenyl phosphite and pyridine.13
They are characterized as highly thermally stable polymers with
a favorable balance of physical and chemical properties.
However, the rigidity of the backbone and strong hydrogen
bonding result in high melting or glass-transition temperatures
and limit solubility in most organic solvents.14 These properties
make them generally intractable or difficult to process, thus
restricting their applications. Fortunately, hyperbranched
aromatic polyamides are soluble in polar aprotic organic
solvents, and even in tetrahydrofuran (THF).15
Functional Polymeric Materials Laboratory, Institute of Polymer Science
and Engineering, National Taiwan University, No.1, Sec. 4, Roosevelt Rd.,
Taipei, 10617, Taiwan. E-mail: gsliou@ntu.edu.tw
In 1952, Flory reported a one-step polymerization of AB2 type
monomers and copolymerization of AB and AB2 type mono-
mers.16 Then, some reports showed a ‘‘AB2 + AB’’ approach
about copolymers of AB and AB2 type monomers.17 The intro-
duction of the AB monomer allows to control the content of
branching units. Therefore, our strategy was to synthesize the
A2B-type monomer with the electroactive TPA moiety.
In addition to the high solubility of the resulting A2B-type
† Electronic supplementary information (ESI) available: Monomer
synthesis, table of thermal and optical properties, IR spectra of
monomers and polyamides, 1H and 13C NMR spectra of monomers,
GPC curves of hyperbranched polyamides, TGA and TMA curves of
hyperbranched polyamide P3, UV-visible transmission spectra of
polyamide films, cyclic voltammetric diagrams of polyamide P2,
calculation of optical switching time of polyamide P2 film. See DOI:
10.1039/b910081d
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hyperbranched polyamide, the chemical modification of end
functional groups by incorporating para-methoxy-substituted
TPA groups could lower the oxidation potentials and afford
different electrochromic characteristics.18 Furthermore, the
content of branching units could also be tuned by copolymeri-
zation of the A2B and AB-type monomers. Because of the
incorporation of packing-disruptive TPA units along the poly-
mer backbone, all of the polymers exhibited good solubility in
polar organic solvents. The transparent and amorphous thin
films could be prepared by spin-coating and inkjet-printing
techniques that are beneficial for their fabrication of large-area
thin-film optoelectronic devices.
The TPA-based novel A2B-type monomer, 4-amino-40,400-
dicarboxytiphenylamine (3), was therefore synthesized in this
contribution, then a series of electrochromic aromatic hyper-
branched polyamides could be prepared by phosphorylation
polyamidation reactions of the newly A2B-type monomer with
AB monomer and end-capping agent, respectively. The general
properties such as inherent viscosities, molecular weights, solu-
bility, and thermal properties are described. The electrochemical,
electrochromic, and photophysical properties of these polymers
are also investigated.
Preparation of the film
A solution of the polymer was made by dissolving about 0.55 g of
the hyperbranched aromatic polyamide sample in 8.0 mL of
DMAc. The homogeneous solution wꢀas poured into a 9-cm glass
Petri dish, which was placed in a 90 C oven for 3 h to remove
most of the solvent; then the semidried film was further dried in
ꢀ
vacuo at 170 C for 7 h. The obtained films were used for solu-
bility tests and thermal analyses.
Results and discussion
Monomer synthesis
The synthesis procedure of A2B monomer and two intermediate
compounds was outlined in Scheme 1. 4,40-Dicyano-400-nitro-
triphenylamine 1 was synthesized from the nitroaniline and
4-fluorobenzonitrile by a nucleophilic substitution reaction, and
4,40-dicarboxy-400-nitrotriphenylamine
2
could be readily
prepared by hydrolysis of compound 1 in an alkaline solution. A
new A2B monomer 3, 4-amino-40,400-dicarboxytiphenylamine,
was synthesized by hydrogen Pd/C-catalyzed reduction of the
compound 2 and shown in Scheme 1. Elemental analysis, IR, and
1H and 13C NMR spectroscopic techniques were used to identify
the structures of the intermediate compound 1, 2, and A2B
monomer 3. The FT-IR spectra of the synthesized compounds
1–3 are illustrated in Fig. S1.† The dinitrile compound 1 showed
a cyano characteristic band at 2219 cmꢁ1 (–ChN stretch) and two
characteristic bands at 1582, 1323 cmꢁ1 (–NO2 stretch), respec-
tively. After hydrolysis and reduction, the characteristic absorp-
tions of the cyano and nitro group disappeared, and the primary
amino group showed the typical absorption pair at 3450 and
3365 cmꢁ1 with C]O and O–H stretching absorptions at 1686
and 2700–3600 cmꢁ1, respectively. Fig. S2 and S3 illustrate the 1H
NMR and 13C NMR spectra of the compounds 1–3, respectively.
The 13C NMR spectra of compound 2 confirm that the cyano
group was completely converted into the carboxylic acid group by
the appearance of a carbonyl carbon peak at 166.85 ppm instead
of cyano carbon at 118.80 ppm. Other important evidence is the
shifting of the carbon resonance signal of C2 adjacent to the cyano
or carboxyl group. The C2 of compound 1 resonated at a higher
field (107.25 ppm) than the other aromatic carbons because of the
anisotropic shielding by the electron of the cyano group. After
hydrolysis, the resonance peak of C2 shifted to a lower field
(127.45 ppm) because of the lack of anisotropic shielding. In 1H
NMR spectra of compound 2, a broad characteristic peak around
12–13 ppm (–COOH) proved that the cyano groups were con-
verted into the carboxylic acid groups. Besides, the shifting of Hd
from 8.14 to 6.61 ppm could be used to judge the successful
reduction of A2B monomer 3.
Experimental
Materials
4-Amino-40,400-dimethoxytriphenylamine18b (B-OMe2) (mp: 133–
134 ꢀC) and 4-amino-40-carboxy-400-methoxytiphenylamine19
(AB-OMe) (mp: 256–258 ꢀC) were synthesized according to
a
previously reported procedure. Tetrabutylammonium
perchlorate (TBAP) (ACROS) was recrystallized twice from 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 polyamides P1–P3 was shown as following the
synthetic route. P1: a mixture of 0.52 g (1.5 mmol) of 4-amino-
40,400-dicarboxytiphenylamine (3), 0.18 g of calcium chloride
(CaCl2), 1.2 mL of triphenyl phosphate (TPP), 0.6 mL of
pyridine, and 1.0 mL of N-methyl-2-pyrrolidinone (NMP) was
ꢀ
heated with stirring at 70 C for 3 h. P2: 0.35 g (1.0 mmol) of
monomer 3, 0.12 g of CaCl2, 0.8 mL of TPP, 0.4 mL of
ꢀ
pyridine, and 1.0 mL of NMP was heated with stirring at 70 C
for 1.5 h, followed by adding 0.32 g (1.0 mmol) of 4-amino-
40,400-dimethoxytriphenylamine (B-OMe2) and heated with
ꢀ
stirring at 105 C for 3 h. P3: 0.35 g (1.0 mmol) of monomer 3,
0.33
g
(1.0 mmole) of 4-amino-40-carboxy-400-methoxy-
tiphenylamine (AB-OMe), 0.12 g of CaCl2, 0.8 mL of TPP, 0.4
mL of pyridine, and 1.0 mL of NMP was heated with stirring
at 70 ꢀC for 3 h. Then, the obtained polymer solutions were
poured slowly into 150 mL of stirred methanol giving rise to
the precipitate that was collected by filtration, washed thor-
Polymer synthesis
According to the phosphorylation technique described by
Yamazaki,13
the
hyperbranched
aromatic
polyamide
ꢀ
oughly with hot water and methanol, and dried at 100 C. The
homopolymer P1 and the copolymer P3 were prepared by direct
polycondensation of A2B monomer 3 and copolymerization of
A2B monomer 3 and AB monomer AB-OMe in the presence of
triphenyl phosphite and pyridine as condensation agents. The
hyperbranched polymer P2 was synthesized by the reaction of
yields of P1–P3 were 0.40 g (81%), 0.61 g (94%), and 0.58 g
(93%), respectively. Finally, reprecipitation from N,N-dime-
thylacetamide (DMAc) into methanol was carried out twice for
further purification.
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Scheme 1 Synthetic route to A2B monomer.
Scheme 2 Synthesis of hyperbranched polyamides.
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Table 1 Inherent viscosity, molecular weights, and solubility of polyamides
Solubility in various solventsd
a
b
Code
hinh /dL gꢁ1
Mw
Mn
PDIc
NMP
DMAc
DMF
THF
CHCl3
CH3CN
P1
P2
P3
0.19
0.24
0.22
13300
42600
34400
6500
28300
20600
2.05
1.51
1.67
++
++
++
++
++
++
++
++
++
++
++
++
—
—
—
—
—
—
a
Measured at a polymer concentration of 0.5 g dLꢁ1 in NMP at 30 ꢀC. b Average molecular weights relative to polystyrene standard in NMP by GPC.
c
Polydispersity Index ¼ Mw/Mn. d The solubility was determined with a 1 mg sample in 1 mL of a solvent. ++, soluble at room temperature; –, insoluble
even on heating. CHCl3: chloroform.
according to the following equation: DB ¼ (D + T)/(D + T + L).
In the proton NMR spectra (Fig. 1), polyamide P1 showed three
kinds of amide (–NH) peaks which can be attributed to dendritic
(D1), linear (L1), and terminal (T1) units, respectively. DB of P1
and P3 were calculated as 0.71 and 0.42, respectively, based on
the integral intensity of quantitative 1H NMR data. The
unusually high DB value of P1 might be because the terminal
strong electron withdrawing group, 4-amido-40,400-di(carboxylic
acid)TPA, increased the reactivity of the nearby acid group, thus
increasing the number of dendritic units (as shown in Scheme 4).
The lower DB of P3 was attributed to the copolymerization of
A2B monomer 3 with AB monomer AB-OMe which increased
the L unit as well as decreased the branching units.
Basic characterization
Fig. 1 1H NMR spectra of (a) P1, (b) P2, and (c) P3.
The solubility properties of the hyperbranched polyamides
P1–P3 were investigated qualitatively, and the results are also
listed in Table 1. Most of the polyamides are readily soluble
in polar aprotic organic solvents such as NMP, DMAc,
N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO),
and tetrahydrofuran (THF). Thus, the excellent solubility makes
these polymers potential candidates for practical applications by
spin-coating or inkjet-printing processes to afford high perfor-
mance thin films for optoelectronic devices. Their high solubility
and amorphous properties can be attributed to the incorporation
of bulky, three-dimensional TPA moiety along the polymer
backbone, which results in a high steric hindrance for close
packing, and thus reduces their crystallization tendency.
polymer P1 with amino monomer B-OMe2 as the end-capping
agent as shown in Scheme 2. The obtained polyamides had
inherent viscosities in the range of 0.19–0.24 dL gꢁ1 with weight-
average molecular weights (Mw) and polydispersity index (PDI)
in the range of 13300–42600 Da and 1.51–2.05, respectively,
relative to polystyrene standards (Table 1). The GPC curves of
P1–P3 were shown in Fig. S4.† The formation of polyamides was
also confirmed by IR and NMR spectroscopy. Fig. S5† shows IR
spectra for P1–P3, and the typical polyamide P1 exhibited broad
bands around 3650–3100 cmꢁ1 (–OH stretch) and 1686 cmꢁ1
(amide carbonyl stretch). After the end-capping reaction with
B-OMe2, polyamide P2 showed the characteristic amide bands
around 3311 (–NH stretch) and 1670 cmꢁ1 (amide carbonyl
stretch) but the absorption of carboxylic acid group disappeared.
Typical 1H NMR spectra of P1–P3 in DMSO-d6 were shown in
Fig. 1. The multiple peaks around 10.0 ppm were attributed to
the amide groups formed during polymerization. The carboxylic
acid group around 13 ppm was not observed for P2, indicating
that the reaction between the carboxylic acid groups on homo-
polymer P1 and end-capping agent B-OMe2 was complete.
The thermal properties of polyamides were examined by TGA,
TMA and DSC, and the thermal behavior data are summarized
in Table S1.† Typical TGA and TMA curves of polyamide P3 are
shown in Fig. S6.† All the prepared polyamides exhibited good
thermal stability with insignificant weight loss up to 500 ꢀC under
nitrogen or air atmosphere. The 10% weight loss temperatures
of these polymers in nitroꢀgen and air were recorded in the range
of 505–550 and 510–535 C, respectively. The concentration of
carbonized residue (char yield) of these polymers in a nitrogen
ꢀ
atmosphere was more than 71% at 800 C. The high char yields
of these polymers can be ascribed to their high aromatic content.
The glass-transition temperatures (Tg) of polyamides P1–P3
could be easily measured by the DSC thermograms; they were
Degree of branching (DB)
TPA-based hyperbranched polymers from A2B type monomer
were composed of linear (L), dendritic (D), and terminal (T) units
as shown in Scheme 3. The DB for hyperbranched polymers has
been described as the ratio of the sum of D and T units versus the
total units (L, D, and T units),3a and is generally calculated
ꢀ
observed in the range of 193–211 C, depending upon the stiff-
ness of the polymer chain. All the polymers indicated no clear
melting endotherms up to the decomposition temperatures on
the DSC thermograms, which supports the amorphous nature of
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Scheme 3 Possible structural units of the hyperbranched polyamides.
Scheme 4 Terminal (left) and linear (right) units of the hyperbranched polyamides.
these polyamides. The softening temperatures (Ts) (may be
Optical properties and electrochemical properties
referred to as apparent Tg) 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. In most cases, the Ts
values obtained by TMA are comparable to the Tg values
measured by the DSC experiments.
The optical properties of the hyperbranched polyamides P1–P3
were investigated by UV-vis and photoluminescence (PL) spec-
troscopy. The results are summarized in Table S2.† These soluble
polymers P1–P3 exhibited maximum UV-vis absorption bands at
358–363 nm (in THF solution; conc.: 10ꢁ5 mol/L) and 361–365 nm
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(in film state) due to the p–p* transitions resulting from the
conjugation between the aromatic rings and nitrogen atom.
Fig. 2 shows UV-vis absorption and PL behavior of polyamides
P1–P3 in THF solution and thin film states. The polyamide P1
exhibited blue PL emission maximum at 455 nm in THF solution
with quantum yield of 4.0% referenced by 9,10-diphenylan-
thracene (FF ¼ 0.90).20 As shown in Table S2,† the red shift PL
emission of P2 (505 nm) could be attributed to the incorporation
of end-capped B-OMe2 groups, resulting a more extended
conjugation length. The cutoff wavelengths (absorption edge; l0)
from the UV-vis transmittance spectra showed light-color and
high optical transparency with cutoff wavelength in the range of
420–423 nm. (Fig. S7).†
The electrochemical properties of the hyperbranched poly-
amides were investigated by cyclic voltammetry (CV) conducted
for the cast film on an indium–tin oxide (ITO)-coated glass slide
as working electrode in anhydrous acetonitrile (CH3CN), using
0.1 M of tetrabutylammonium perchlorate (TBAP) as a sup-
porting electrolyte under a nitrogen atmosphere. The typical CV
for polyamides P1–P3 are shown in Fig. 3 and exhibited
a reversible oxidation redox peak for P1 at the half-wave
potential (E1/2) around 1.23 V. There are additional redox peaks
for P2 (E1/2 ¼ 0.78) and P3 (E1/2 ¼ 0.85) due to the end-capped
B-OMe2 unit and AB-OMe comonomer, respectively. During the
electrochemical oxidation scanning of the polyamide P2 thin
films, color of the film changed from colorless to green and then
to deep purple. Because of the high electrochemical stability and
good adhesion between the polyamide thin film and ITO
substrate, polyamide P2 exhibited reversible CV behavior by
continuous 1000 cyclic scans within the first oxidation stage
(Fig. S8).† The oxidation potentials of these polyamides as well
as their highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) are summarized in
Table 2. The HOMO level of polyamides P1–P3 could be esti-
mated from the onset of their oxidation in CV experiments as
5.03–5.44 eV (on the basis that ferrocene/ferrocenium is 4.8 eV
below the vacuum level with Eonset ¼ 0.36 V).
Fig. 2 UV-vis absorptions and PL spectra of P1–P3 in film and solution
(10ꢁ5 M in THF) states.
Spectroelectrochemical and electrochromic properties
Spectroelectrochemical investigation was used to evaluate the
optical properties of the electrochromic materials, and the
polyamide film was cast on an ITO-coated glass slide and
a homemade electrochemical cell was built from a commercial
ultraviolet (UV)-visible cuvette. The cell was placed in the optical
path of the sample light beam in a UV-vis-NIR spectropho-
tometer to acquire electronic absorption spectra under potential
Fig. 3 Cyclic voltammetric diagrams of polyamide P1–P3 films on an
ITO-coated glass substrate and ferrocene (inset) in 0.1 M TBAP/CH3CN
at a scan rate of 50 mV sꢁ1
.
Table 2 Electrochemical properties of polyamides
Oxidationa
Polymer
Eonset
E1/2(ox1)
E1/2(ox2)
Egb/eV
HOMOc
LUMO
P1
P2
P3
1.00
0.59
0.74
1.23
0.78
0.85
—
1.24
1.23
2.95
2.93
2.93
5.44
5.03
5.18
2.49
2.10
2.25
a
From cyclic votammograms versus Ag/AgCl in CH3CN. E1/2: average potential of the redox couple peaks. b The data were calculated from polymer
films by the equation: Eg ¼ 1240/lonset (energy gap between HOMO and LUMO). c The HOMO energy levels were calculated from cyclic voltammetry
and were referenced to ferrocene (4.8 eV; onset ¼ 0.36 V).
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radical of P2 increased gradually. From the inset shown in Fig. 4,
the polyamide P2 film switches from transparent neutral state
(colorless; Y: 85; x, 0.313; y, 0.332) to a highly absorbing semi-
oxidized state (green; Y: 40; x, 0.266; y, 0.406) and a fully
oxidized state (deep purple; Y; 2; x, 0.244; y, 0.112). The distri-
bution of coloration across the polymer film was very homoge-
neous and still stable even after more than hundreds of redox
cycles. The polymer P2 also shows good contrast in the visible
region with an extremely high optical transmittance change
(D%T) of 79% at 755 nm for green coloring and 69% at 530 nm
for deep purple coloring, respectively.
For electrochromic switching studies, polymer films were cast
on ITO-coated glass slides in the same manner as described
above, and chronoamperometric and absorbance measurements
were performed. While the films were switched, the absorbance at
the given wavelength was monitored as a function of time by
UV-vis-NIR spectroscopy. The switching time of polyamide P2
shown in Fig. S9† was calculated at 90% of the full switch
because it is difficult to perceive any further color change with the
naked eye beyond this point. As depicted in Fig. S9(a),† P2 thin
film revealed switching time of 2.12 s at 0.90 V for the coloring
process at 755 nm and 1.21 s for bleaching. The amounts of
Q were calculated by integration of the current density and time
obtained from Fig. S9(b)† as 1.724 and 1.662 mC cmꢁ2 for an
oxidation and reduction process at the first oxidation stage,
respectively. The ratio of the charge density was 96.4%, indicated
that charge injection/extraction was reversible during the elec-
trochemical reactions. The electrochromic stability of the poly-
amide P2 film was determined by measuring the optical change
as a function of the number of switching cycles shown in Fig. 6.
The electrochromic CE (h ¼ dOD/Q) at different switching steps
are summarized in Table 3.21 The electrochromic behavior of P2
film exhibited high CE up to 426 cm2/C at 755 nm within 2% of
CE loss after switching 100 times between 0 and 0.90 V, and
retained 96% of their electroactivity even after further 1000
switching cycles. Considering the oxidation potentials and elec-
trochromic stability, the novel hyperbranched polyamide showed
the comparable coloration efficiency with other anodic electro-
chromic polymers.18
Fig. 4 Electrochromic behavior of polyamide P2 thin film (ꢂ160 nm in
thickness) on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN at
applied potentials of (a) 0, (b) 0.60, (c) 0.70, (d) 0.80, (e) 0.90, (f) 1.00, (g)
1.05, (h) 1.15, (i) 1.25, (j) 1.35, (k) 1.45, (l) 1.55, (m) 1.60, (n) 1.65 (V vs.
Ag/AgCl).
Fig. 5 3-D spectroelectrochemical behavior of the P2 thin film on the
ITO-coated glass substrate in 0.1 M TBAP/CH3CN from 0 to 1.65 V
(vs. Ag/AgCl).
control in a 0.1 M TBAP/MeCN solution. The UV-vis-NIR
absorbance curves of P2 film correlated to applied potentials are
depicted in Fig. 4, and the three-dimensional transmittance-
wavelength-applied potential correlation of this sample is
revealed in Fig. 5. The P2 film exhibited strong absorption at
around 365 nm, characteristic of the TPA unit in the neutral form
(0 V), being almost colorless and transparent in the visible region.
Upon oxidation (increasing applied voltage from 0 to 1.05 V), the
intensity of the absorption peak at 365 nm gradually decreased
while a new peak at 755 nm gradually increased in intensity due
to the formation of a stable cation radical P2+c. As the anodic
potential increased to 1.65 V, an additional new absorption band
centered at around 530 nm corresponding to another cation
Fig. 6 (a) Current consumption and (b) electrochromic switching
between 0 and 0.90 V (vs. Ag/AgCl) and absorbance change monitored at
750 nm of polyamide P2 thin film (ꢂ160 nm in thickness) on the ITO-
coated glass substrate (coated area: 1.6 ꢃ 0.5 cm) in 0.1 M TBAP/
CH3CN with a cycle time of 20 s.
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Table 3 Optical and electrochemical data collected for coloration efficiency measurements of polyamide P2
Cycling timesa
dOD755
Qc/mC cmꢁ2
hd/cm2/C
Decaye/%
b
1
0.734
0.733
0.731
0.730
0.728
0.727
0.726
0.725
0.723
0.721
0.720
1.724
1.724
1.724
1.724
1.723
1.723
1.723
1.722
1.721
1.721
1.721
426
425
424
423
423
422
421
421
420
419
418
0
10
20
30
40
50
60
70
80
90
100
0.23
0.47
0.70
0.70
0.94
1.17
1.17
1.41
1.64
1.88
a
b
c
Switching between 0.00 and 0.90 (V vs. Ag/AgCl). Optical density change at 755 nm. Ejected charge, determined from the in situ experiments.
Coloration efficiency is derived from the equation: h ¼ dOD755/Q. e Decay of coloration efficiency after cyclic scans.
d
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Conclusion
A series of novel electroactive TPA-based hyperbranched
aromatic polyamides were readily prepared from the phosphory-
lation polyamidation reaction of the newly synthesized A2B
monomer, 4-amino-40,400-dicarboxytiphenylamine, with AB
monomer and end-capping agent, respectively. Because of the
TPA-based hyperbranched structure, these polyamides showed
good solubility and high thermal stability. In addition, the
obtained polymers also revealed valuable electrochromic char-
acteristics such as high contrast in the visible region, high
coloration efficiency, and high-level electrochromic/electroactive
reversibility.
Acknowledgements
The authors are grateful to the National Science Council of the
Republic of China for financial support of this work.
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