Synthesis and electrochromic properties of aromatic polyetherimides based on a triphenylamine-dietheramine monomer

Article abstract of DOI:10.1002/pola.26686

A series of electroactive polyetherimides (PEIs) with triphenylamine (TPA) units were prepared from the polycondensation reactions of 4,4′-bis(p- aminophenoxy)triphenylamine with aromatic tetracarboxylic dianhydrides via a conventional two-step technique.

Full text of DOI:10.1002/pola.26686

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Synthesis and Electrochromic Properties of Aromatic Polyetherimides
Based on a Triphenylamine-dietheramine Monomer
Sheng-Huei Hsiao,1 Hui-Min Wang,1 Pei-Chi Chang,1 Yu-Ruei Kung,2 Tzong-Ming Lee2
1Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
2Division of Organic Opto-Electronic Materials & Applications, Industrial Technology Research Institute, Hsinchu 31040, Taiwan
Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
Received 10 January 2013; accepted 16 March 2013; published online 17 April 2013
DOI: 10.1002/pola.26686
ABSTRACT: A series of electroactive polyetherimides (PEIs) with
triphenylamine (TPA) units were prepared from the polycon-
densation reactions of 4,40-bis(p-aminophenoxy)triphenylamine
with aromatic tetracarboxylic dianhydrides via a conventional
two-step technique. The PEIs showed high thermal stability,
with glass-transition temperatures of 234–282 ꢀC and decompo-
sition temperatures in excess of 500 ꢀC. They showed well-
defined and reversible redox couples during both p- and
n-doping processes, together with multielectrochromic
behaviors. These polymers exhibited enhanced redox-stability
and electrochromic performance as compared with the corre-
sponding analogs without the phenoxy spacer between the
C
V
TPA and imide units.
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KEYWORDS: electrochemistry; electrochromic properties; polyi-
mides; redox polymers; triphenylamine
ferromagnetic coupling.4 Perhaps most commonly, triaryl-
amines have been used as the hole-transport layer in electro-
luminescent devices,5,6 because the two-layered organic
electroluminescent device using a diamine as a hole-transfer
layer was first reported by Tang and VanSlyke7 in 1987. Par-
ticularly, solution processability and good film formation
properties of triarylamine-based polymers may render them
suitable for the fabrication of large-area and flexible elec-
tronics based on spin-coating and inkjet printing.8 In addi-
tion, triarylamine-based polymers also show interesting
electrochromic behavior upon electrochemical oxidation.9 In
recent years, Liou and coworkers have carried out extensive
studies on the design and synthesis of triarylamine-based
high-performance polymers such as aromatic polyimides10
and polyamides11 for potential electrochromic applications.
The triarylamine-containing monomers such as diamines and
dicarboxylic acids could be easily prepared using a well-
established procedure, and they could react with the corre-
sponding comonomers through conventional polycondensa-
tion techniques, producing the desired triarylamine-based
polymers. In general, these polymers exhibit good solubility
to organic solvents due to the introduction of bulky, packing-
disruptive triarylamine moieties. Thus, they could be readily
solution-cast into flexible electrochromic films with good me-
chanical property and high thermal stability. Thus, incorpora-
tion of three-dimensional, packing-disruptive triarylamine
units into the polyimide backbone not only resulted in
INTRODUCTION Aromatic polyimides are characterized as
highly thermally stable polymers with a favorable balance
of physical and chemical properties. They are commercially
important materials used extensively as dielectric films and
coatings in a wide range of high technology applications.1
However, rigidity of the backbone and strong interchain
interactions result in high melting or glass-transition tempera-
tures (Tg) and limited solubility in most organic solvents.
Thus, polyimide processing is generally carried out via poly
(amic acid) precursor and then converted to polyimide by vig-
orous thermal or chemical cyclodehydration. This process has
inherent problems such as emission of volatile by-products
and storage instability of poly(amic acid) solution. To over-
come these problems, many attempts have been made to the
synthesis of soluble and processable polyimides in fully imi-
dized form while maintaining their excellent properties.2 The
majority of methods used for improving the solubility while
maintaining the high-temperature performance of polyimides
have involved the structural modifications of dianhydride and
diamine monomers. Typical approaches include the introduc-
tion of flexible linkages, kinked or unsymmetrical structures,
bulky packing-disruptive units, and bulky lateral groups into
the polymer backbone.3
Triarylamines are an important class of aromatic compounds
because they can form stable aminium radical cations. They
can be building blocks for high-spin polyradicals that showed
Additional Supporting Information may be found in the online version of this article.
C
V
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SCHEME 1 Synthetic route to the target TPA-dietheramine monomer 4.
EXPERIMENTAL
enhanced solubility but also led to new electronic functions
of polyimides such as electrochromic characteristics.12
Materials
Aniline (Acros), 4-iodoanisole (Acros), potassium carbonate
(K2CO3, Showa), triethylene glycol dimethyl ether (TEGDME,
Acros), copper (Cu, Acros), boron tribromide (BBr3, Acros),
sodium hydrogen carbonate (NaHCO3, Showa), p-fluoronitro-
benzene (Acros), 10% palladium on charcoal (Pd/C, Fluka),
and hydrazine monohydrate (TCI) were used as received. Di-
methyl sulfoxide (DMSO, Tedia), N,N-dimethylformamide
(DMF, Acros), N,N-dimethylacetamide (DMAc, Fluka), pyri-
dine (Py, Wako), and N-methyl-2-pyrrolidone (NMP, Tedia)
were dried over calcium hydride for 24 h, distilled under
reduced pressure, and stored over 4 Å molecular sieves in a
sealed bottle. The dietheramine monomer 4,40-bis(p-amino-
phenoxy)triphenylamine (4) was synthesized starting from
the Ullmann reaction between aniline and 4-iodoanisole by a
four-step reaction sequence. The synthetic details and char-
acterization have been included in the Supporting Informa-
tion. Commercially available tetracarboxylic dianhydrides
such as pyromellitic dianhydride (PMDA; 5a; Aldrich) and
3,30,4,40-benzophenonetetracarboxylic dianhydride (BTDA;
5c; Aldrich) were purified by recrystallization from acetic
It has been demonstrated that triarylamine-based polyimides
generally exhibited poor electrochemical and electrochromic
stability as compared with their polyamide analogs because
of the strong electron-withdrawing imide group, which
increases the oxidation potential of the triarylamine unit and
destabilizes the resultant amino radical cation upon
a spacer between the
triarylamine core and the imide ring may improve the elec-
trochemical and elcetrochromic stability of this kind of elec-
oxidation.10(b),11(c) Incorporating
troactive polymers. Thus,
a triphenylamine (TPA)-based
dietheramine monomer, namely 4,40-bis(p-aminophenoxy)tri-
phenylamine, was synthesized and led to a series of polye-
therimides (PEIs) with main-chain TPA and phenoxy groups.
By the incorporation of the phenoxy spacer between the TPA
and imide units, the resulting PEIs are expected to exhibit
enhanced electrochemical and electrochromic properties.
Although this dietheramine has been synthesized previously
and one of its derived polyimides has been studied for
potential applications in volatile polymeric memory devi-
ces,13 the electrochemical and electrochromic properties of
this kind polyimides have not been well understood.
anhydride.
3,30,4,40-Biphenyltetracarboxylic
dianhydride
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FIGURE 1 (a) 1H NMR, (b) 13C NMR, (c) HAH COSY, and (d) CAH HMQC spectra of the target dietheramine monomer 4 in DMSO-
d6.
(BPDA; 5b; Oxychem), 4,40-oxydiphthalic dianhydride (ODPA;
5d; Oxychem), 3,30,4,40-diphenylsulfonetetracarboxylic dia-
nhydride (DSDA; 5e; New Japan Chemical), and 2,2-bis(3,4-
dicarboxyphenyl)hexafluoropropane dianhydride (6FDA; 5f;
Hoechst Celanese) were heated at 250 ꢀC in vacuo for 3 h
before use. Tetrabutylammonium perchlorate, Bu4NClO4,
were recrystallized from ethyl acetate under nitrogen atmos-
phere and then dried in vacuo prior to use. All other
reagents and solvents were used as received from commer-
cial sources.
overnight. The poly(amic acid) in the form of solid film was
converted to PEI 6fꢀby successive heating under vacuum at
ꢀ
ꢀ
150 C for 1 h, 200 C for 1 h, and then 300 C for 1 h.
For the chemical imidization method, 4 mL of acetic anhy-
dride and 2 mL of Py were added to a poly(amic acid) solu-
tion obtained by a similar process as above, and the mixture
was heated at 100 ꢀC for 1 h to effect a complete imidiza-
tion. The homogenous polymer solution was poured slowly
into 150 mL of stirring methanol giving rise to light yellow
precipitate that was collected by filtration, washed thor-
oughly with hot water and methanol, and dried. A polymer
solution was made by the dissolution of about 0.5 g of the
polyimide sample in 10 mL of hot DMAc. The homogeneous
solution was poured into a 7-cm glass Petri dish, which was
Polymer Synthesis
PEIs 6a–6f were synthesized from TPA-dietheramine 4 and
aromatic dianhydrides 5a–5f by the conventional two-step
method via thermal or chemical imidization reaction. A typi-
cal procedure is as follows. The dietheramine monomer 4
(0.5085 g, 1.106 mmol) was dissolved in 9.5 mL of DMAc in
a 50-mL round-bottom flask. Then 5f, 6FDA (0.4915 g, 1.106
mmol) was added to the diamine solution in one portion.
Thus, the solid content of the solution is ꢁ10 wt %. The
mixture was stirred at room temperature for about 18 h to
yield a viscous poly(amic acid) solution. The inherent viscos-
ity of the resulting poly(amic acid) was 1.39 dL/g, measured
in DMAc at a concentration of 0.50 g/dL at 30 ꢀC. The pol-
y(amic acid) film was obtained by casting from the reaction
ꢀ
placed in a 90 C oven overnight for the slow release of the
solvent, and then the film was stripped off from the glass
ꢀ
substrate and further dried in vacuum at 160 C for 6 h.
Measurements
Infrared (IR) spectra were recorded on a Horiba FT-720
FTIR spectrometer. 1H and 13C NMR spectra were measured
on a Bruker AVANCE 500 FT-NMR system with tetramethylsi-
lane as an internal standard. The inherent viscosities were
determined with a Cannon-Fenske viscometer at 30 ꢀC.
Wide-angle X-ray diffraction (WAXD) measurements were
ꢀ
polymer solution onto a glass Petri dish and drying at 90 C
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SCHEME 2 Synthesis of PEIs 6a–6f.
performed at room temperature (ca. 25 ꢀC) on a Shimadzu
XRD-6000 X-ray diffractometer with a graphite monochroma-
tor (operating at 40 kV and 30 mA), using nickel-filtered Cu-
Ka radiation (k 5 1.5418 Å). The scanning rate was 2ꢀ/min
over a range of 2h 5 10–40ꢀ. Thermogravimetric analysis
(TGA) was performed with a Perkin-Elmer Pyris 1 TGA.
Experiments were carried out on ꢁ4–6 mg of samples
heated in flowing nitrogen or air (flow rate 5 40 cm3/min)
1.25 cm2) was used as a working electrode. A platinum wire
was used as an auxiliary electrode. All cell potentials were
taken with the use of a home-made Ag/AgCl, KCl (sat.) refer-
ence electrode. Ferrocene was used as an external reference
for calibration (10.48 V vs. Ag/AgCl). Voltammograms are
presented with the positive potential pointing to the left and
with increasing anodic currents pointing downwards. Spec-
troelectrochemistry analyses were carried out with an elec-
trolytic cell, which was composed of a 1 cm cuvette, ITO as a
working electrode, a platinum wire as an auxiliary electrode,
and Ag/AgCl reference electrode. Absorption spectra in the
spectroelectrochemical experiments were measured with an
Agilent 8453 UV–vis diode array spectrophotometer.
ꢀ
at a heating rate of 20 C/min. DSC analyses were performed
on a Perkin-Elmer Pyris 1 DSC at a scan rate of 20 ꢀC/min
in flowing nitrogen. Thermomechanical analysis (TMA) was
determined with a Perkin-Elmer TMA 7 instrument. The
TMA experiments were carried out from 50 to 400 ꢀC at a
ꢀ
scan rate of 10 C/min with a penetration probe 1.0 mm in
diameter under an applied constant load of 10 mN. Softening
temperatures (Ts) were taken as the onset temperatures of
probe displacement on the TMA traces. Electrochemistry was
performed with a CHI 750A electrochemical analyzer. Cyclic
voltammetry (CV) was conducted with the use of a three-
electrode cell in which ITO (polymer films area about 0.8 3
RESULTS AND DISCUSSION
Monomer Synthesis
The target aromatic dietheramine monomer, 4,40-bis(p-ami-
nophenoxy)triphenylamine (4), was synthesized starting
from aniline and 4-iodoanisole by a four-step reaction
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TABLE 1 Inherent Viscosity and Solubilitya Behavior of PEIs Prepared via Thermal (2T) or Chemical (2C) Imidization
ginh (dL/g)b
Polyimide
Solventsc
DMSO
Polymer Code
PAAd
1.59
NMP
DMAc
DMF
m-Cresol
THF
6a-T
6a-C
6b-T
6b-C
6c-T
6c-C
6d-T
6d-C
6e-T
6e-C
6f-T
2
2
2
2
12
2
2
12
2
2
2
12
2
12
2
12
2
12
12
12
12
12
12
1
1.82
1.36
1.61
1.53
1.39
2
2
12
12
11
2
12
12
12
2
12
2
12
2
12
2
2
1.10e
12
2
12
2
12
2
2
0.82e
2
11
12
11
2
12
12
11
2
12
12
1
12
12
1
12
2
12
1
0.90
2
12
12
1
12
1
12
1
12
1
6f-C
0.65
11
11
a
c
The qualitative solubility was tested with 10 mg of a sample in 1 mL
of stirred solvent. 11: soluble at room temperature; 1: soluble on heat-
ing; 12: partially soluble; 2: insoluble even on heating.
Solvent:NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide;
DMF: N,N-dimethylformamide; DMSO: dimethyl sulfoxide; THF:
tetrahydrofuran.
b
d
Inherent viscosity measured at a concentration of 0.5 dL/g in DMAc at
PAA 5 Poly(amic acid).
Inherent viscosity measured at a concentration of 0.5 dL/g in NMP at
e
30 ꢀC.
30 ꢀC.
sequence as depicted in Scheme 1. The intermediate com-
pound 4,40-dimethoxytriphenylamine (1) was synthesized via
Ullmann reaction between aniline and 4-iodoanisole by using
copper powder and K2CO3 in TEGDME. According to a litera-
ture method,14 demethylation of the dimethoxy compound 1
with boron tribromide gave 4,40-dihydroxytriphenylamine
(2). Then, 4,40-bis(p-nitrophenoxy)triphenylamine (3) was
prepared by the nucleophilic aromatic fluoro-displacement
reaction of p-fluoronitrobenzene with compound 2 in the
presence of K2CO3. The target dietheramine monomer 4 was
prepared by hydrazine Pd/C-catalyzed reduction of dinitro
compound 3. FTIR, 1H NMR, and 13C NMR spectroscopic
techniques were used to identify structures of the intermedi-
ate compounds 1, 2, and 3 and the target dietheramine
monomer 4. Figure S1 (see Supporting Information) illus-
trates FTIR spectra of all the synthesized compounds. The
methyl groups of compound 1 give rise to a symmetric
stretching vibration at about 2834 cm21 and an asymmetric
stretch at about 2929 cm21. After demethylation to com-
pound 2, the characteristic absorption of the phenol group
previous publication,13 where the intermediate dinitro com-
pound 3 has been prepared by a different synthetic route.
Only a slight difference in chemical shifts of the resonance
peaks was observed due to the use of different test solvents.
1
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 5.0 ppm corresponding to the amino protons. Assign-
ments of each carbon and proton assisted by the 2-D NMR
spectra are also indicated in these spectra, and they are in
good agreement with the proposed molecular structure of 4.
Polymer Synthesis
PEIs 6a–6f were prepared in conventional two-step method
by the reactions of dietheramine 4 with various aromatic
dianhydrides (5a–5f) to form poly(amic acid)s, followed by
thermal or chemical cyclodehydration (Scheme 2). As shown
in Table 1, the poly(amic acid) precursors had inherent vis-
cosities in the range of 1.36–1.82 dL/g. The molecular
weights of these poly(amic acid)s were sufficiently high to
permit the casting of flexible and tough poly(amic acid)
films, which were subsequently converted into tough PEI
films by stage-by-stage heating to elevated temperatures.
The transformation from poly(amic acid) to a polyimide
could also be carried out via chemical cyclodehydration by
using acetic anhydride and Py. All the PEIs showed the char-
acteristic absorption bands of the imide ring near 1780
(asymmetric C@O stretching) and 1732 cm21 (symmetric
C@O stretching). Typical IR spectra of PEI 6c and its poly
(amic acid) precursor are illustrated in Supporting Informa-
tion Figure S5. The 1H NMR spectrum of a representative
PEI 6f is illustrated in Figure 2. All the aromatic protons in
(AOH stretching) appears in the region of 3100–3500 cm21
.
The nitro groups (ANO2 asymmetric and symmetric stretch-
ing) of compound 3 give two strong bands at around 1589
and 1344 cm21. After reduction, the characteristic absorp-
tions of the nitro group disappear and the amino group shows
the typical NAH stretching absorption pair at 3463 and 3365
cm21 as shown in the IR spectrum of compound 4. The NMR
spectra of all the intermediate compounds are included in
Supporting Information Figures S2–S4. 1H NMR, 13C NMR, and
two-dimensional (2-D) NMR spectra of the target TPA-diether-
1
amine 4 are compiled in Figure 1. The H and 13C NMR spec-
tra of 4 are essentially identical to those reported in a
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FIGURE 2 (a) 1H NMR and (b) HAH COSY spectra of PEI 6f in CDCl3.
the dietheramine moiety resonated in the region of d 6.98–
8.02 ppm, and the protons in the 6FDA component appeared
at a lower field of 7.88–8.02 ppm. Assignments of each pro-
ton, assisted by the 2-D NMR spectroscopy, are in good
agreement with the structure of the repeating unit.
backbone, which results in a high steric hindrance for close
packing, and thus reduces their crystallization tendency.
The solubility behaviors of all the PEIs prepared by both of
thermal and chemical imidization methods are summarized
in Table 1. All the thermally imidized samples did not reveal
good solubility to the tested solvents. However, the PEIs pre-
pared by the chemical imidization method exhibited an
increased solubility as compared with those by the thermal
imidization method. The chemically imidized samples of the
PEIs derived from less stiff dianhydrides such as 6c to 6f
are soluble at least in NMP at room temperature. PEIs 6e
and 6f are also soluble in DMAc. The lower solubility of the
Polymer Properties
Basic Characterization
As evidenced by their WAXD patterns (see Supporting Infor-
mation Figure S6), the PEIs exhibited an amorphous nature.
Their amorphous properties can be attributed to the incorpo-
ration of packing-disruptive TPA unit along the polymer
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TABLE 2 Thermal Properties of PEIs
Td at 5% Weight
Td at 10% Weight
Loss (ꢀC)d
Loss (ꢀC)d
Char Yield
(wt %)e
Polymera Code
Tg (ꢀC)b
Ts (ꢀC)c
N2
Air
N2
Air
6a
6b
6c
6d
6e
6f
282
246
247
234
264
258
282
242
241
231
260
257
563
589
562
597
510
569
544
576
570
598
524
552
590
609
585
610
543
590
587
612
606
622
567
579
62
64
63
59
54
65
a
d
All the polymer films were heated at 300 ꢀC for 1 h to DSC and TGA
Decompositiontemperature at which a 5 or 10% weight loss was
experiments.
recorded by TGA at a heating rate of 20 ꢀC/min and a gas flow rate of
20 cm3/min.
b
Midpoint temperature of the baseline shift on the second DSC heating
e
trace (rate 5 20 ꢀC/min) of the sample after quenching from 400 to 50 ꢀC
(rate 5 200 ꢀC/min) in nitrogen.
Residual wt % at 800 ꢀC at a scan rate 20 ꢀC/min in nitrogen.
c
Softening temperature measured by TMA with a constant applied load
of 10 mN at a heating rate of 10 ꢀC/min.
thermally cured samples may be attributed to the presence
of partial interchain crosslinking or an aggregation of the
polymer chains during thermal curing process.
yields of these polymers can be ascribed to their high aro-
matic content. The glass-transition temperatures (Tg) of all
the polymers were observed in the range of 234–282 ꢀC by
DSC (Supporting Information Figure S7) and decreased with
decreasing rigidity and symmetry of the aromatic tetracar-
boxylic dianhydride. The decreasing order of Tg generally
correlated with that of chain flexibility. For example, PEI 6d
from ODPA showed the lowest Tg of 234 ꢀC because of the
presence of flexible ether linkage between the phthalimide
units. The softening temperatures (Ts; may be 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 dis-
placement on the TMA trace. A typical TMA thermogram for
PEI 6f is illustrated in the inset of Figure 3. In most cases,
the Ts values of the PEIs obtained by TMA are comparable to
the Tg values measured by the DSC experiments.
Thermal Properties
The thermal stability and phase-transition temperatures of
these PEIs were determined by TGA, DSC, and TMA techni-
ques. The thermal behavior data of PEIs 6a–6f are included
in Table 2. Typical TGA and TMA curves of the representa-
tive 6f are shown in Figure 3. All of the PEIs exhibited a sim-
ilar TGA pattern with no significant weight loss below 500
ꢀC in air or nitrogen atmosphere. The decomposition temper-
atures (Td) at which a 10% weight-loss temperatures of the
aromatic PEIs in nitrogen and air were recorded in the range
of 543–610 ꢀC and 567–622 ꢀC, respectively. The amount of
carbonized residue (char yield) of these polymers in nitrogen
atmosphere was more than 54% at 800 ꢀC. The high char
Electrochemical Properties
The electrochemical properties of the PEIs were investigated
by CV conducted for the cast films on an ITO-coated glass
substrate as working electrode in dry acetonitrile (CH3CN;
for the anodic scan) or DMF (for the cathodic scan) contain-
ing 0.1 M of Bu4NClO4 as an electrolyte under nitrogen
atmosphere. The first CV scans of the 6a–6f series PEIs are
depicted in Figure 4, and the data are summarized in Table
3. As shown in Table 3, the oxidation half-wave potentials
(E1/2ox) were recorded in the range of 0.96–0.99 V. One pair
of reversible redox waves are observed for all the PEIs in
the oxidation region, which are attributed to the oxidation of
TPA units. These PEIs also show two or three quasireversible
waves in the reduction region because of the reduction of
imide segments. Redox reactions for the diimide systems
shown in Supporting Information Figure S8 represent a pos-
sible distribution of electron density for the reduced forms
and describe by other resonance forms, which contribute to
the charge delocalization. Reduction of the imide groups may
FIGURE 3 TMA and TGA curves of PEI 6f with a heating rate of
10 and 20 ꢀC/min, respectively.
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FIGURE 4 Cyclic voltammetric diagrams of the cast films of PEIs (a) 6a, (b) 6b, (c) 6c, (d) 6d, (e) 6e, and (f) 6f on an ITO-coated
glass substrate in 0.1 M Bu4NClO4 acetonitrile (for anodic process) and DMF (for cathodic process) solution at a scan rate of 50
and 100 mV/s, respectively.
induce increased quinoid character due to charge separation
by the ring structure to minimize the electron–electron
repulsion. The CV curve of PMDA PEI 6a [Fig. 4(a)] shows
that the pyromellitimide groups undergo two quasireversible
one-electron reductions, which occurred at Epc 5 20.88 and
21.45 V. The first reduction corresponds to formation of
radical anions, and the second reduction relates to formation
of dianions (see Supporting Information Figure S8). Similar
result was observed with the BPDA PEI 6b [Fig. 4(b)]; how-
ever, the first reduction process (Epc 5 21.26 V) occurred at
a more negative potential than observed for the PMDA PEI
6a. Two separated reduction peaks peculiar to the BPDA dii-
mide segment indicate a facile electronic communication
between the two phthalimide groups. It is worth to note that
the benzophenone diimide group of the BTDA PEI 6c
revealed three reversible redox couples [Fig. 4(c)]. It was
proposed that the first two electroreduction processes occur
for the BTDA diimide units, leading to the radical-anion
(Epc 5 21.06 V) and diradical-dianion form (Epc 5 21.30 V),
respectively. The benzophenone moiety allows
a third
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TABLE 3 Optical and Electrochemical Properties of PEIs
Oxidation
Potential (V)a
Bandgap
(eV)c
Energy level
(eV)e
Thin Film (nm)
Reduction Potential (V)b
Abs
Abs
OX
onset
OX
red
red1
red2
red3
opt
ec
Polymer Code kmax
konset
E
E1/2
Eonset
E1/2
E1/2
E1/2
Eg
Eg
HOMO LUMO
6a
6b
6c
6d
6e
6f
296
299
299
296
298
296
305
606
557
583
531
547
506
590
0.88
0.89
0.85
0.88
0.78
0.90
1.00
0.99
0.98
0.98
0.98
0.97
0.96
1.08
20.57
20.81
20.73
21.06
20.75
20.95
21.02
20.67
21.09
20.88
21.25
20.84
21.10
20.79
21.26
21.28
21.14
–
–
–
2.04
2.22
2.12
2.33
2.26
2.45
2.10
1.66 5.35
2.07 5.34
1.86 5.34
2.23 5.34
1.81 5.33
2.06 5.32
1.64 5.51
3.69
3.27
3.48
3.11
3.52
3.26
3.87
21.68
–
–
–
–
21.03
–
60d
–
a
e
ox
The HOMO and LUMO energy levels were calculated from E1/2 and
Verus Ag/AgCl in CH3CN. E1/2 5 average potential of the redox couple
red
peaks.
b
E1/2
values of CV curves and were referenced to ferrocene (4.8 eV rel-
Versus Ag/AgCl in DMF.
ative to the vacuum energy level).
c
Bandgaps calculated from absorption edge of the polymer films:
Egopt 5 1,240/konset
.
d
Egec, electrochemical band gap is derived from the difference between
HOMO and LUMO values.
electroreduction to the proposed radical–trianion form
(Epc 5 21.86 V) as depicted in Supporting Information Fig-
ure S8. Two pair of redox waves were observed for the im-
ide reduction of ODPA polyimide 6d [Fig. 4(d)], DSDA
polyimide 6e [Fig. 4(e)], and 6FDA polyimide 6f [Fig. 4(f)]
during the negative CV scanning, even though the conjuga-
tion across the imide group is disrupted by the ether, sul-
fonyl, and hexafluoroisopropylidene linker.
FIGURE 5 (a) Absorption spectra of the thin film of PEI 6d on an ITO electrode at various potentials from 0 to 1.14 V, (b) spectral
change from 1.14 back to 0.74 V, (c) repetitive cyclic voltammograms for 6d, and (d) the second spectroelectrochemical series of
the 6d thin film.
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SCHEME 3 Proposed coupling reaction between the TPA units and the subsequent oxidation reactions in the PEIs.
When comparing the CV diagram of PEI 6d with that of
analogous 60d (Supporting Information Fig. S9), it is found
that incorporation of a phenoxy group between the TPA
and imide groups results in a decreased oxidation potential.
This is likely attributed to the decreased electron-withdraw-
ing effect of the imide group caused by the insertion of the
phenoxy spacer and possibly also to the electron-donating
nature of the phenoxy group. Furthermore, it was found
that all the PEIs revealed new redox patterns upon repeti-
tive scanning over the voltage range from 0 to 1.1 V. An
example of PEI 6d is shown in Figure 5(c). For the first
positive potential scan, we observed an oxidation peak at
about 1.14 V. From the first reverse negative scan, we
detected one cathodic peak at 0.82 V together with a
shoulder at 0.74 V. After the second scan, a new oxidation
shoulder appeared at 0.84 V that was complementary an-
odic process of the shoulder at 0.74 V in the first reverse
negative scan. In the meantime, the oxidation peak revealed
a decreased current and moved to a lower anodic potential
(1.01 V). The observation of a new oxidation couple in the
second potential scan implies that the TPA radical cations
were involved in fast electrochemical/chemical reactions
that produced a structure that was easier to oxidize than
was the parent TPA unit. In addition, it was also found that
the value of the anodic current intensity at 0.84 V slightly
increased with the number of the scan. This is a typical ox-
idation wave of the tetraphenylbenzidine (TPB) group, indi-
cating the occurrence of the oxidative coupling between
TPA units (Scheme 3). The electro-oxidation of TPA was
extensively studied in the pioneering works reported by
Adams, Nelson, and coworkers.15 All those studies con-
cluded that the radical cation formed by oxidation is
involved in a dimerization reaction that produced TPB. The
TPB has a lower oxidation potential than TPA because of
its more extended p-electron delocalization.
Spectroelectrochemistry and Electrochromic Switching
When the PEI films were electrochemically oxidized or
reduced, a strong color change of them was observed. Their
electrochromic properties were further studied by the means
of electrochromic absorbance spectra with a potential step
scan on the polymer films coated on an ITO electrode via a
combination of a potentiostat and an UV–vis-NIR spectrome-
ter. In this case, a three-electrode configuration was still
used for applying potential to the polymer films. During the
test, the electrochromic absorbance spectra were recorded.
As a typical example, the result of the 6d film upon electro-
oxidation (p-doping) is presented in Figure 5(a) as a series
of UV–vis-NIR absorption curves correlated to electrode
potentials. In the neutral form, PEI 6d exhibited strong
absorption at wavelength around 307 nm, characteristic for
TPA, but it almost transmissive in the visible region. When
the applied voltage was stepped from 0 to 1.14 V, the inten-
sity of the absorption peak around 307 nm decreased gradu-
ally, and new peaks at 362 and 753 nm gradually increased
in intensity. Consequently, the film turned from colorless to
blue. We attribute these spectral changes to the formation of
a stable cation radical of the TPA moiety. As shown in Figure
5(b), when the applied voltage returned from 1.14 to 0.74 V,
the absorbance peak at 362 and 753 nm was decreased dra-
matically while a new absorption band at 474 nm appeared.
Meanwhile, the film changed color from blue to pale orange.
For the explanation of this spectral change, repetitive CV dia-
grams of 6d are also included in Figure 5(c). After the first
electrochemical series of PEI 6d was recorded from 0 to
1.14 V and then back to 0 V, we reapplied the electrode volt-
age and recorded its absorption profile. As shown in Figure
5(d), an additional pale orange oxidized state has been
observed in the re-scan film at applied voltage of 0.84 V. The
film showed a multicolored electrochromism from colorless
neutral state to pale orange and blue oxidized states. This
phenomenon persisted for several subsequent scans. The
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FIGURE 6 Spectroelectrochemistry of the films of PEIs (a) 6a, (b) 6b, and (c) 6d on an ITO electrode in 0.1 M Bu4NClO4/DMF at var-
ious applied potentials.
new pale orange oxidized state may be attributed to the oxi-
dation of the benzidine unit resulted from the coupling reac-
tion of TPA units, as shown in Scheme 3. As shown in
Supporting Information Figure S9, a pale orange coloring
state was also observed in the second spectroelectrochemical
series of the referenced polyimide 60d. This result indicated
the occurrence of the oxidative coupling between the TPA
units of 60d in the electrochemical oxidation process,
although the oxidation waves could not be well-resolved in
the CV analysis.
Figure 6 shows the spectral changes of the cast films of PEIs
6a, 6b, and 6d upon electroreduction (n-doping). The radical
anion of PEI 6a, which appears at potential 20.88 V, exhibits
a strong band at 318 nm and an increased absorption
between 657 and 720 nm. As shown in the inset of Figure
6(a), the radical anion form of polyimide 6a is light green in
color. Further reduction at potentials negative to 21.45 V
results in the two-electron reduced (dianion) state with a
new strong absorption at 557 nm, and the film turns to a
pink color during the second-step reduction. The spectral
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FIGURE 7 Potential step absorptiometry of the cast films of PEI 6d and the referenced 60d on the ITO-glass slide (coated area – 1
cm2; in CH3CN with 0.1 M Bu4NClO4 as the supporting electrolyte) by applying a potential step: (a) optical switching for 6d at
potential 0.0 Vꢀ1.10 V and cycle time 12 s, monitored at kmax 5 753 nm; (b) 60d at potential 0.0 Vꢀ1.28 V and cycle time 14 s,
monitored at kmax 5 768 nm.
changes associated with the electroreduction reactions of the
pyromellitimide unit are very similar to that of standard
PMDA-ODA polyimide (ODA: 4,40-oxydianiline) reported by
Mazur et al.16 This result reaffirms that the diamine residue
has very little direct influence on the reduction of the dii-
mide moiety as reported in literature.16 The radical anion of
PEI 6b formed at 21.26 V reveals a new absorption band in
the visible region at about 486 nm, and the film appears as
yellow-green in color [Fig. 6(b)]. As the film was further
charged with electrons at21.46 V, the absorption intensity at
486 nm decreased, and two new bands emerged at about
625 and 859 nm, and the film turned to a blue color during
the second-step reduction. Figure 6(c) illustrates the spectral
and coloration changes of PEI 6d on electroreduction. How-
ever, the color change (colorless to light yellow) is not strong
as that observed for PEIs 6a and 6b. All the polyimide films
showed a quick optical contrast loss after about 10 full
switches. It is believed that a little unwanted delamination
or dissolution of the anionic products in the DMF/electrolyte
solution is the main cause of the unstable switching behav-
ior. The electrochemical and optical properties of the PEI
films on electrochemical reduction are summarized in the
Supporting Information Table S1.
absorption maximum (kmax) and to determine the response
time by stepping potential repeatedly between the neutral
and oxidized states. The active area of the polymer film on
ITO glass is about 1 cm2. Figure 7 depicts the optical trans-
mittance at 753 nm as a function of time by applying
square-wave potential steps between 0.00 and 1.10 V for a
resident time of 12 s for PEI 6d and at 768 nm between
0.00 and 1.28 V for a resident time of 14 s for the refer-
enced polyimide 60d. The PEI film of 6d almost showed no
optical contrast loss during the first 10 switching cycles;
however, the 60d film showed a moderate optical contrast
loss after 10 full switches. The response time was calculated
at 90% of the full-transmittance change, because it is diffi-
cult to perceive any further color change with naked eye
beyond this point. As shown in Figure 7(a), PEI 6d attained
90% of a complete coloring and bleaching in 3 and 1 s,
respectively. The optical contrast measured as DT% of PEI
6d between neutral colorless and oxidized blue states was
found to be 67% at 753 nm. Figure 7(b) indicates that the
referenced polyimide 60d attained 90% of a complete color-
ing and bleaching in 2.6 and 0.8 s, respectively. The optical
contrast measured as DT% of polyimide 60d between neutral
colorless and oxidized blue states was found to be 65% at
768 nm. The electrochromic coloring efficiency (CE) for the
blue coloring (g 5 ᭝OD753/Q) of the PEI 6d was estimated
to be 178 cm2/C, but that for the blue coloring
(g 5 ᭝OD768/Q) of the polyimide 60d was estimated to be
only 51 cm2/C (Table 4).
To obtain a partially crosslinked film of PEI 6d, the film of
6d on ITO-glass was repeatedly scanned between 0.00 and
1.10 V at 100 mV/s for 10 cycles. After that, the spectroelec-
trochemistry and electrochromic switching of the electro-
chromic film was investigated. The spectroelectrochemical
behavior of the polyimide film was found to be very similar
to that shown in Figure 5(d). Electrochromic switching stud-
ies for the PEIs were performed to monitor the percent
transmittance changes (DT%) as a function of time at their
As mentioned earlier, the PEI 6a also exhibited a strong colo-
ration change upon electrochemical reduction. Supporting In-
formation Figure S10 shows the optical transmittance
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TABLE 4 Electrochromic Properties of PEIs 6a, 6d, and 60d
Response Timeb
DODc
Qd (mC/cm2)
d
CEe (cm2/C)
a
Polymer
kmax (nm)
D%T
tc (s)
tb(s)
6d-p-doping
60d-p-doping
6a-n-doping
753
768
558
67
65
95
3
1
0.376
0.363
2.093
2.11
7.10
11.23
178
51
2.6
5.2
0.8
2.3
186
a
Wavelength of absorption maximum.
b
c
Time for 90% of the full-transmittance change.
Optical Density (DOD) 5 log[Tbleached/Tcolored], where Tcolored and
Tbleached are the maximum transmittance in the oxidized and neutral
states, respectively.
d
Qd is ejected charge, determined from the in situ experiments.
Coloration efficiency (CE) 5 DOD/Qd.
e
C. Huang, K.-R. Lee, J.-Y. Lai, C.-S. Ha, Prog. Polym. Sci. 2012,
37, 907–974.
changes at kmax 5 558 nm of PEI 6a while switching between
its colorless (neutral) and pink (reduced) states in 0.1 M
Bu4NClO4/DMF by applying a potential step between 0.00
and 21.45 V (vs. Ag/AgCl). The electrochromic properties of
the 6a film during n-doping processes are also summarized
in Table 4. In general, the cathodically coloring switching is
less stable than the anodically coloring switching for this PEI
film. A faster decay in optical contrast and an earlier delami-
nation of the polymer film were usually observed for these
PEIs during the cathodically switching experiments.
2 For review papers: see (a) S. J. Huang, A. E. Hoyt, TRIP 1995,
3, 262–271; (b) J. de Abajo, J. G. de la Campa, Adv. Polym. Sci.
1999, 140, 23–59; (c) M. Ding, Prog. Polym. Sci. 2007, 32, 623–
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3 For selected examples: see (a) S.-H. Hsiao, K.-H. Lin, J.
Polym. Sci. Part A: Polym. Chem. 2005, 43, 331–341; (b) S. M.
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3289; (g) M. Calle, A. E. Lozano, J. G. de la Campa, J. de Abajo,
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CONCLUSIONS
Herein, we synthesized and characterized a series of electro-
active PEIs from 4,40-bis(p-aminophenoxy)triphenylamine
and various aromatic dianhydrides. Insertion of a phenoxy
spacer between the TPA unit and the imide ring decreases
the oxidation potentials and increases the electrochemical
stability of the polyimides. All the PEIs exhibit an ambipolar
electrochemical and electrochromic behavior. When the poly-
mer film was subjected to a repeated cyclic scan between 0
and 1.1 V, an electrochemical coupling reaction between the
TPA units occurred which may lead to a partially crosslinked
film. The PEI films exhibit reversible electrochemical oxida-
tion accompanied by strong color changes that can be
switched through modulation of the applied potential. Thus,
these characteristics suggest that these PEIs may find appli-
cations in electrochromic devices.
4 (a) A. Ito, H. Ino, K. Tanaka, K. Kanemoto, T. Kato, J. Org.
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