Substituent effects on electrochemical and electrochromic properties of aromatic polyimides with 4-(carbazol-9-yl)triphenylamine moieties

Article abstract of DOI:10.1002/pola.27103

Three series of aromatic polyimides with 4-(carbazol-9-yl)triphenylamine moieties were prepared from the polycondensation reactions of 4,4′-diamino-4″-(carbazol-9-yl) triphenylamine (1), 4,4′-diamino-4″-(3,6-di-tert-butylcarbazol-9-yl)triphenylamine (t-Bu-1), and 4,4′-diamino-4″-(3,6-dimethoxycarbazol-9-yl) triphenylamine (MeO-1), respectively, with various commercially available tetracarboxylic dianhydrides. In addition to high thermal stability and good film-forming ability, the resulting polyimides exhibited an ambipolar electrochromic behavior. The polyimides based on t-Bu-1 and MeO-1 revealed higher redox-stability and enhanced electrochromic performance than the corresponding ones based on 1 because the active sites of their carbazole units are blocked with bulky t-butyl or electron-donating methoxy groups. 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1172-1184 Three series of 4-(carbazol-9-yl)triphenylamine-based aromatic polyimides are prepared by polycondensation of corresponding diamines with various commercially available tetracarboxylic dianhydrides. All the polyimides afford flexible and tough films with high thermal stability. The polymer films show ambipolar redox and multicolored electrochromic behaviors. During the electro-oxidation processes, the polyimides with t-butyl or methoxy substituents on the pendent carbazole unit exhibit enhanced electrochemical stability and electrochemical performance, as compared with the coresponding parent polyimides, because the electrochemically active sites of carbazole are blocked. Copyright

Full text of DOI:10.1002/pola.27103

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Substituent Effects on Electrochemical and Electrochromic Properties of
Aromatic Polyimides with 4-(Carbazol-9-yl)triphenylamine Moieties
Hui-Min Wang, Sheng-Huei Hsiao
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
Received 19 December 2013; accepted 7 January 2014; published online 7 February 2014
DOI: 10.1002/pola.27103
ABSTRACT: Three series of aromatic polyimides with 4-(carbazol-
9-yl)triphenylamine moieties were prepared from the polyconden-
sation reactions of 4,40-diamino-4"-(carbazol-9-yl) triphenylamine
(1), 4,40-diamino-4"-(3,6-di-tert-butylcarbazol-9-yl)triphenylamine
(t-Bu-1), and 4,40-diamino-4"-(3,6-dimethoxycarbazol-9-yl)triphe-
nylamine (MeO-1), respectively, with various commercially avail-
able tetracarboxylic dianhydrides. In addition to high thermal
stability and good film-forming ability, the resulting polyimides
exhibited an ambipolar electrochromic behavior. The polyimides
based on t-Bu-1 and MeO-1 revealed higher redox-stability and
enhanced electrochromic performance than the corresponding
ones based on 1 because the active sites of their carbazole units
are blocked with bulky t-butyl or electron-donating methoxy
C
V
groups. 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2014, 52, 1172–1184
KEYWORDS: carbazole; electrochemistry; electrochromic poly-
mers; polyimides; redox polymers; triphenylamine
amine units are receiving considerable interest as ideal hole-
transporting materials in various optoelectronic device appli-
cations such as organic light-emitting diodes because of the
strong electron-donating and hole-transporting/injecting prop-
erties of triarylamine units.6 Additionally, triarylamines can be
easily oxidized to form stable radical cations, and the oxida-
tion process is always associated with a strong change of
coloration. Thus, many triarylamine-based polymers have
been investigated for electrochromic applications.7 Since
2005, Liou’s and our groups have developed several high-
performance polymer systems such as aromatic polyimides8
and polyamides9 with the triarylamine unit for potential elec-
trochromic applications. Our strategy was to synthesize the
triarylamine-containing monomers such as diamines and
dicarboxylic acids that were then reacted with the corre-
sponding comonomers via conventional polycondensation
techniques. The obtained polymers possessed a well-defined
structure, high thermal stability, and good mechanical proper-
ties. Most of them exhibited good solubility in many organic
solvents due to the incorporation of packing-disruptive, three-
dimensional triarylamine unit along the polymer backbone.
They could form uniform, transparent amorphous thin films
by simple solution processing, which is advantageous for their
ready fabrication of large-area, thin-film devices.
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 temper-
atures (Tg) and limited solubility in most organic solvents.
Thus, polyimide processing is generally carried out via pol-
y(amic acid) precursor and then converted to polyimide by
vigorous 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 overcome these problems, many attempts have been
made to the synthesis of soluble and processable polyimides
in fully imidized form while maintaining their excellent prop-
erties.2 The majority of methods used for improving the sol-
ubility while maintaining the high-temperature performance
of polyimides have involved the structural modifications of
dianhydride and diamine monomers. Typical approaches
include the introduction of flexible linkages, kinked or
unsymmetrical structures, bulky packing-disruptive units,
and bulky lateral groups into the polymer backbone.3
Triarylamine4 and carbazole5 derivatives are well known as
their photo and electroactive properties that find optoelec-
tronic applications as photoconductors, hole-transporters,
light-emitters, and memory devices. Polymers bearing triaryl-
Previous studies have demonstrated that aromatic polya-
mides and polyimides containing 4-(carbazol-9-yl)triphenyl-
amine (CzTPA) segments show attractive electrochemical and
C
V
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SCHEME 1 Synthetic routes to diamine monomers 1, t-Bu-1 and MeO-1.
electrochromic properties.10 These polymers reveal good
redox stability during the first oxidation process at a lower
applied potential, and the oxidation process is accompanied
with a noticeable change of absorption in the visible and
near-infrared regions. However, the second oxidation process
of these polymers at a higher applied potential is not revers-
ible, possibly because of the electrochemical coupling of car-
bazoles through the active C-3 and C-6 sites.11 This may
result in undesired decreased device performance, localized
color defects, and heterogeneous color contrast of this kind
of electrochromic materials. It has been reported12 that the
incorporation of bulky groups such as tert-butyl or triphenyl-
silyl group on the electrochemically active sites (C-3 and C-
6) of carbazole leads to enhanced electrochemical and mor-
phological stability of the carbazole derivatives. As reported
in our recent publications,13,14 redox-stable aromatic polya-
mides and polyimides with valuable electrochromic proper-
ties have been prepared by using 4,40-diamino-4"-(3,6-di-
tert-butylcarbazol-9-yl)triphenylamine (t-Bu-1) as the dia-
mine component. Very recently, we have also reported the
synthesis and characterization of 4,40-diamino-4"-(3,6-dime-
thoxycarbazol-9-yl)triphenylamine (MeO-1) and its derived
polyamides.15 The polyamides showed an enhanced electro-
chemical and electrochromic stability due to the substitution
of active sites of the carbazole unit with electron-donating
methoxy groups. Turning our attention to enhancing electro-
chemical and electrochromic stability of the CzTPA-based
polyimides, we have synthesized a series of aromatic polyi-
mides from the polymerization reactions of diamine MeO-1
with aromatic dianhydrides. The electrochemical and electro-
chromic properties of these polyimides were compared
with those of structurally similar polyimides derived from
t-Bu-1 and 4,40-diamino-4"-(carbazol-9-yl)triphenylamine (1).
The effects of incorporating methoxy or tert-butyl groups on
the C-3 and C-6 positions of pendent carbazole units on the
properties of these polyimides were investigated.
EXPERIMENTAL
Materials
4,40-Diamino-4"-(carbazol-9-yl)triphenylamine (1) (mp 5
192–195 ꢀC),10(a) 4,40-diamino-4"-(3,6-di-tert-butylcarbazol-
13
ꢀ
9-yl)triphenylamine (t-Bu-1) (mp 5 273–274 C), and 4,40-
diamino-4"-(3,6-dimethoxycarbazol-9-yl)triphenylamine (MeO-1)
(mp 5 131–132 ꢀC)15 were synthesized according to the
previously reported procedures. Their synthetic routes are
shown in Scheme 1. Details of the synthesis and characteri-
zation data of these CzTPA-based diamine monomers have
been described in our previous publications.10(a),13,15 Accord-
ing to a literature procedure,16 the model compounds, 9-
phenylcarbazole (M1), 3,6-di-tert-butyl-9-phenylcarbazole
(M2), and 3,6-dimethoxy-9-phenylcarbazole (M3), were syn-
thesized via Ullmann CAN coupling reaction of iodobenzene
with carbazole, 3,6-di-tert-butyl-9H-carbazole and 3,6-dime-
thoxy-9H-carbazole, respectively, by using copper powder
and K2CO3 in tri(ethylene glycol) dimethyl ether (TEGDME).
N,N-Dimethylacetamide (DMAc) was dried over calcium
hydride for 24 h, distilled under reduced pressure, and
stored over 4 Å molecular sieves in a sealed bottle. Commer-
cially available tetracarboxylic dianhydrides such as pyromel-
litic dianhydride (PMDA; 2a; Aldrich) was purified by
recrystallization from acetic anhydride. The other dianhy-
drides that included 3,4,30,40-biphenyltetracarboxylic dianhy-
dride (BPDA; 2b; Oxychem), 4,40-oxydiphthalic dianhydride
(ODPA; 2c; Oxychem), and 2,2-bis(3,4-dicarboxyphenyl)hexa-
fluoropropane dianhydride (6FDA; 2d; Hoechst Celanese)
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SCHEME 2 Synthesis of polyimides.
were heated at 250 ꢀC in vacuo for 3 h before use. Tetra-n-
butylammonium perchlorate (Bu4NClO4, TCI) was recrystal-
lized twice from ethyl acetate and then dried in vacuo before
use. All other reagents were used as received from commer-
cial sources.
the film was stripped off from the glass substrate and fur-
ther dried in vacuum at 160 C for 6 h. The IR spectrum of
ꢀ
MeO-3d (film) exhibited characteristic imide absorption
bands at 1783 (asymetrical C@O stretch) and 1732 cm21
(symmetrical C@O stretch). 1H NMR (500 MHz, CDCl3, d,
ppm) [for the peak assignments, see Fig. 2(c)]: 3.94 (s, 6H,
AOCH3), 7.05 (d, J 5 7.0 Hz, 2H, Hc), 7.36–7.39 (m, 12H,
Hd 1 Hf 1 Hg 1 Hh), 7.49 (d, J 5 8.0 Hz, 2H, He), 7.55 (s, 2H,
Ha), 7.87 (d, J 5 7.5 Hz, 2H, Hj), 7.97 (s, 2H, Hk), 8.05 (d,
J 5 8.0 Hz, 2H, Hi).
Polymer Synthesis
A typical example of the synthesis of polyimide MeO-3d is
described as follows: The diamine monomer MeO-1 (0.530
g, 1.06 mmol) was dissolved in 9.5 mL of dried DMAc in a
50-mL round-bottom flask. Then 6FDA (0.470 g, 1.06 mmol)
was added into 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 24 h to yield a
highly viscous poly(amic acid) solution. The inherent viscos-
ity of the resulting poly(amic acid) was 1.80 dL g21,ꢀmeas-
ured in DMAc at a concentration of 0.5 g dL21 at 30 C. For
the thermal imidization process, about 5 g of the obtained
poly(amic acid) solution was transferred to a 7-cm glass
Petri dish, which was placed overnight in a 90 ꢀC oven for
the slow release of the casting solvent. The poly(amic acid)
in the form of solid film was converted to polyimide MeO-
3d by suꢀccessive heating under vacuum at 150 ꢀCꢀ for 30
Measurements
Infrared spectra were recorded on a Horiba FT-720 FT-IR
spectrometer, and the 1H and 13C NMR spectra on a 500
MHz instrument (Bruker AVANCE-500 FT-NMR system) using
DMSO-d6 as the solvent and tetramethylsilane as the internal
standard. Elemental analyses were run in a Heraeus VarioEL-
III CHNS elemental analyzer. The inherent viscosities of the
polymers were determined at 0.5 g/dL concentration using
ꢀ
an Ubbelohde viscometer at 30 C. Thermogravimetric analy-
ses (TGA) were conducted with a TA instruments Q50 TGA,
at a heating rate of 20 ꢀC min21 in flowing nitrogen or air
(flow rate 5 20 cm3 min21). DSC analyses were performed
on a PerkinElmer DSC4000 differential scanning calorimeter
(DSC) at a scan rate of 20 ꢀC min21 under nitrogen.
Ultraviolet-visible (UV–vis) spectra of the polymer films were
recorded on an Agilent 8453 UV–visible spectrometer. Elec-
trochemistry was performed with a CH Instruments 750A
electrochemical analyzer. Voltammograms are presented with
the positive potential pointing to the left and with increasing
anodic currents pointing downwards. Cyclic voltammetry
(CV) was conducted with the use of a three-electrode cell in
which ITO (polymer films area about 1 cm2, 0.8 3 1.25 cm2)
was used as a working electrode. A platinum wire was used
as an auxiliary electrode. All cell potentials were taken by
using an ALS RE-1B (Ag/AgCl, 3M NaCl) reference electrode.
ꢀ
min, 200 C for 30 min, 250 C for 30 min, and 300 C for 1
h. For the chemical imidization method, 2 mL of acetic anhy-
dride and 1 mL of pyridine were added to the remaining
poly(amic acid) solution, and the mixture was heated at 100
ꢀC for 1 h to effect a complete imidization. The homogenous
polymer solution was poured slowly into 200 mL of stirring
methanol giving rise to a reddish brown precipitate that was
collected by filtration, washed thoroughly 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 9-cm glass Petri dish, which was placed in a 90 ꢀC
oven overnight for the slow release of the solvent, and then
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TABLE 1 Inherent Viscosity and Solubility Behavior of the Chemically Imidized Polyimides
Solubilityb in Various Solventsc
ginh (dL g21
)
NMP
DMAc
DMF
DMSO
m-Cresol
THF
a
Polymer Code
3a
1.93
1.74
1.69
1.50
1.36
1.72
1.63
2.64
1.52
1.46
0.91
1.80
12
12
1h
12
2
12
2
12
2
2
2
3b
1h
1h
1h
12
1h
11
11
12
1h
1h
11
2
3c
1h
11
12
2
1h
11
12
2
1h
1h
12
2
2
3d
11
12
1h
11
12
2
t-Bu-3a
t-Bu-3b
t-Bu-3c
t-Bu-3d
MeO-3a
MeO-3b
MeO-3c
MeO-3d
11
11
12
1h
11
11
12
12
12
11
11
11
12
12
12
11
1
11
11
12
12
12
11
1
12
12
12
1
11
11
a
c
Inherent viscosity of the poly(amic acid) precursors measured at a
concentration of 0.5 dL g21 in DMAc at 30 ꢀC.
Qualitative solubility was tested with 10 mg of a sample in 1 mL of a
NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMF:
N,N-dimethylformamide; DMSO: dimethyl sulfoxide; THF: tetrahydrofuran.
b
stirred solvent. 11: soluble at room temperature; 1h: soluble on
heating; 12: partial soluble on heating; 2: insoluble even on heating.
Ferrocene was used as an external reference for calibration
(10.44 V vs. Ag/AgCl). Spectroelectrochemical experiments
were carried out in a cell built from a 1 cm commercial
UV-visible cuvette using an Agilent 8453 UV–Visible diode
array spectrophotometer. The ITO-coated glass slide was
used as the working electrode, a platinum wire as the coun-
ter electrode, and a Ag/AgCl cell as the reference electrode.
Colorimetric measurements were obtained using a Minolta
CS-100A Chroma Meter and the results are expressed in
terms of lightness (L*) and color coordinates (a*, b*).
these polyimides were characterized by IR and NMR
analysis. Typical IR spectra of polyimides 3d, t-Bu-3d, and
MeO-3d are illustrated in Figure 1. All polyimides exhibited
characteristic imide group absorptions around 1783 and
1732 cm21 (typical of imide carbonyl asymmetrical and
symmetrical stretch), 1380 cm21 (CAN stretch), and 1100
RESULTS AND DISCUSSION
Polymer Synthesis
Three series of aromatic polyimides (3a–d, t-Bu-3a-d, and
MeO-3a-d) with CzTPA units were prepared in conventional
two-step method by the reactions of equal molar amounts of
diamine 1, t-Bu-1, and MeO-1, respectively, with various aro-
matic dianhydrides (2a-2d) to form poly(amic acid)s, fol-
lowed by the thermal or chemical cyclodehydration
(Scheme 2). As shown in Table 1, the poly(amic acid) precur-
sors had inherent viscosities in the range of 0.91-2.64 dL
g21. 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 polyimide films by stage-by-stage heating to ele-
vated temperatures. For the organosoluble polyimides such
as 3d, t-Bu-3c, t-Bu-3d, and MeO-3d, flexible and strong
films could also be cast directly from their DMAc solutions.
The transformation from poly(amic acid) to a polyimide
could also be carried out via chemical cyclodehydration by
using acetic anhydride and pyridine. Structural features of
FIGURE 1 IR spectra of polyimides 3d, t-Bu-3d, and MeO-3d.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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in the 6FDA component appeared at a lower field of 7.87–
8.05 ppm. No resonance peak appeared around 10 ppm in
the 1H NMR spectrum support the completed imidization.
Assignments of each proton were assisted by the two dimen-
sional H-H COSY NMR spectra shown in Figure 3 and agree
well with the structure of the repeating unit. As can be seen
from the NMR spectrum shown in Figure 2(c), the electron-
donating methoxy group makes the carbazole protons Ha
and Hc of MeO-3d move significantly to the high fields.
Solubility
The solubility properties of the polyimide samples prepared
via the chemical imidization are also listed in Table 1. The
polyimides (3d, t-Bu-3d, and MeO-3d) derived from 6FDA
(2d) showed higher solubility than the other ones in the
same series because of the additional contribution of the
hexafluoroisopropylidene (AC(CF3)2A) fragment in the poly-
mer backbone which interferes the intermolecular interac-
tions and close packing. They are soluble not only in polar
aprotic organic solvents, such as NMP, DMAc, DMF, and
DMSO, but also in less polar solvents like THF. For compari-
son, the MeO-3 or t-Bu-3 series polyimides showed a
slightly enhanced solubility compared to the corresponding
3 ones without the methoxy or tert-butyl substituent. This
may be a result of increased free volume and decreased
interchain interactions caused by the methoxy or tert-butyl
substitution. Therefore, the good solubility makes these
polyimides potential candidates for practical applications in
spin-coating and inkjet-printing techniques to afford high
performance thin films for optoelectronic devices.
FIGURE 2 1H NMR spectra of polyimides (a) 3d, (b) t-Bu-3d,
and (c) MeO-3d in CDCl3 (* the solvent peak). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
Thermal Properties
The thermal stability and phase transition temperatures of
these polyimides were recorded by TGA and DSC. The ther-
mal behavior data of the polyimide are summarized in Table
2. Typical TGA curves of polyimides 3d, t-Bu-3d, and MeO-
3d are depicted in Figure 4. All the prepared polyimides
exhibited good thermal stability with insignificant weight
loss up to 400 ꢀC under nitrogen or air atmosphere. The
10% weight loss temperatures of these polymers in nitrogen
and air were recorded in the range of 529–652 and 492–634
and 720 cm21 (imide ring deformation). Representative 1H
NMR spectra of polyimides 3d, t-Bu-3d, and MeO-3d in
DMSO-d6 are illustrated in Figure 2, and the resonance peaks
are well assigned to the repeating structure of the polymer
backbone. All the aromatic protons in the CzTPA moiety
resonated in the region of d 7.05–7.58 ppm, and the protons
FIGURE 3 H-H COSY NMR spectra of polyimides (a) 3d, (b) t-Bu-3d, and (c) MeO-3d in CDCl3 (* the solvent peak). [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 2 Thermal Properties of Polyimidesa
Td at 10 wt %
scan, three cathodic peaks were detected. In the second posi-
tive potential scan, a new oxidation shoulder appeared in the
range of 10.90–1.10 V that was the complementary anodic
process of the cathodic peak at a lower potential. The obser-
vation of the new redox couple in the second potential scan
implies that the carbazole radical cations were involved in
very fast electrochemical reactions that produced a new
structure that was easier to oxidize than was the parent car-
bazole. This can be further evidenced by the CV of model
compound M1. As shown in Figure 5(d), in the first oxida-
tion scan, a sharp peak at Ep,a 5 11.50 V is observed. In the
reverse scan, two reduction waves at Ep,c 5 11.21 and
10.99 V are observed, respectively. In the continuous scans,
an oxidation wave at Ep,a 5 11.10 V gradually grows, indica-
tive of a new species formation. As reported by Ambrose
and co-workers in their pioneering work11 devoted to anodic
oxidation of carbazole and various N-substituted carbazoles,
ring-ring coupling is the predominant pathway, the carbazole
radical cation yielding 3,30 (6)-bicarbazyls. Similar results
have also been reported in recent publications.13–15 In con-
trast, model compounds M2 and M3 exhibit a reversible
redox wave at E1/2 5 11.23 and 11.01 V, respectively, and
continuous cycling does not generate any new wave in
0.0 ꢁ 11.50 V region [Figure 5(e,f)]. Therefore, it is possible
to prevent the irreversible coupling reactions by incorporat-
ing bulky tert-butyl or electron-donating methoxy substitu-
ents at the electrochemically active C-3 and C-6 sites on the
carbazole unit of the CzTPA segment. As shown in Figure
5(b,c), polyimide t-Bu-3d shows two reversible oxidation
redox couples at the E1/2 5 11.05 V (Eonset 5 10.88 V) and
11.28 V, and MeO-3d shows two reversible oxidation redox
couples at the E1/2 5 10.90 V (Eonset 5 10.78 V) and 11.19
V. The shift to lower oxidation potentials for polyimide MeO-
3d is conceivably due to the electron-donating effect of the
methoxy groups. The other polyimides show a similar CV
behavior and their oxidation potentials are compiled in
Table 3. As illustrated in Figure 6, model compounds M1,
M2, and M3 revealed oxidation peaks at Ep.a 5 1.50, 1.40,
Loss (ꢀC)c
Polymer
Char yield
(%)d
code
Tg (ꢀC)b
In N2
In air
3a
2e
640
652
648
642
568
583
581
564
552
577
571
529
627
634
627
576
552
583
560
565
498
525
514
492
72
74
73
72
66
70
66
63
68
71
62
59
3b
325
303
310
3c
3d
e
t-Bu-3a
t-Bu-3b
t-Bu-3c
t-Bu-3d
MeO-3a
MeO-3b
MeO-3c
MeO-3d
–
336
320
333
320
335
306
324
a
The polymer film sample were heated at 300 ꢀC for 1 h prior to all the
thermal analyses.
b
The sample were heated from 50 to 400 ꢀC at a scan rate of 20 ꢀC/min
followed by rapid cooling to 50 ꢀC at 2200 ꢀC min21 in nitrogen. The
midpoint temperature of baseline shift on the subsequent DSC trace
(from 50 to 400 ꢀC at heating rate 20 ꢀC min21) was defined as Tg.
c
Decomposition temperature at which a 10% weight loss was recorded
by TGA at a heating rate of 20 ꢀC min21
d
.
Residual weight percentages at 800 ꢀC under nitrogen flow.
e
No discernible transition was detectable by DSC.
ꢀC, respectively. The amount of carbonized residue (char
yield) of these polymers in nitrogen atmosphere was more
ꢀ
than 59% at 800 C. The high char yields of these polymers
can be ascribed to their high aromatic content. As expected,
the incorporation of methoxy or tert-butyl groups into the
polymer backbones leads to a slight decrease in thermal sta-
bility. Except for 3a and t-Bu-3a, other polyimides showed
discernible glass-transition temperatures (Tg) of 305–336 ꢀC
by DSC. The t-Bu-3 series showed the highest Tg values
ꢀ
(320–336 C) because of the decreased conformational flexi-
bility caused by the bulky tert-butyl substituents.
Electrochemical Properties
The electrochemical behavior of model compounds M1-M3
and the polyimides were investigated by CV. The model com-
pounds were measured in anhydrous acetonitrile (CH3CN)
solution (ꢁ 1 3 1023 M) containing 0.1 M Bu4NClO4 as the
electrolyte, and the polymers were measured by using the
solution-cast films on an indium tin oxide (ITO)-coated glass
substrate as working electrode which were immersed in a
0.1 M Bu4NClO4/acetonitrile solution. The repetitive CV dia-
grams for model compounds M1-M3 and representative pol-
yimides 3d, t-Bu-3d, and MeO-3d are illustrated in Figure 5
for comparison. There are one reversible oxidation redox
couples at half-wave potential (E1/2) value of 11.08 V
(Eonset 5 10.93 V) and one irreversible oxidation redox wave
at E1/2 5 11.40 V for polyimide 3d in the first oxidation
CV scan [Fig. 5(a)]. In the first reverse negative potential
FIGURE 4 TGA thermograms of polyimides 3d, t-Bu-3d, and
MeO-3d in both air and nitrogen atmospheres at a scan rate of
20 ꢀC min21. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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FIGURE 5 Repeated anodic CV diagrams of the cast films of polyimides (a) 3d, (b) t-Bu-3d, and (c) MeO-3d on the ITO-coated glass
substrate and the solutions (ꢁ 1 3 1023 M) of model compounds (d) M1, (e) M2, and (f) M3 in 0.1 M Bu4NClO4/CH3CN at a scan
rate of 50 mV/s. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
and 1.12 V, respectively. Based on the oxidation potentials of
these model compounds, we propose the possible oxidative
order of the redox centers for polyimides 3d, t-Bu-3d and
MeO-3d (see Fig. 6). It is interesting to note that the
N-phenylcarbazole unit might oxidize earlier than the triphe-
nylamine core in polyimide MeO-3d because of the electron-
donating methoxy substituents.
nance forms due to the charge delocalization. Reduction of
the imide groups may induce increased quinoid character
due to charge separation by the ring structure to minimize
the electron–electron repulsion. The CV curve of PMDA polyi-
mide MeO-3a [Fig. 7(a)] shows that the pyromellitimide
groups undergo two quasi-reversible one-electron reduc-
tions, which occurred at Ep,c 5 21.06 and 21.60 V, respec-
tively. The first reduction corresponds to formation of radical
anions, and the second reduction relates to formation of dia-
nions.17 Two separated reduction peaks peculiar to the pyro-
mellitimide diimide segment indicate a facile electronic
communication between the two imide units. Similar result
was observed with the BPDA polyimide MeO-3b [Fig. 7(b)];
however, the first reduction process (Epc 5 21.24 V) occurred
at a more negative potential than observed for the PMDA
These polyimides also show one or two quasi-reversible
waves in the negative side because of the reduction of imide
segments. Typical cathodic CV diagrams of polyimides
MeO-3 series are depicted in Figure 7. Redox reactions
for the diimide systems shown in the bottom of Figure 7
represent a possible distribution of electron density for the
reduced forms, which can also be described by other reso-
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TABLE 3 Redox Potentials and Energy Levels of Polyimides
Thin
Oxidation
Potentiala (V)
Ox1
Reduction
Bandgapc
(eV)
Energy
Levelsd (eV)
films (nm)
Potentialb (V)
Ox2
Red1
Red2
opt
eg
Polymer code
kmax
konset
Eonset
E1/2
E1/2
Eonset
E1/2
E1/2
Eg
Eg
HOMO LUMO
3a
299, 320
299, 331
299, 328
299, 330
299, 315
299, 336
299, 331
299, 348
313, 360
312, 345
313, 347
312, 330
419
387
385
383
420
393
393
391
418
397
396
391
0.89
0.91
0.90
0.93
0.92
0.90
0.88
0.88
0.74
0.75
0.75
0.78
1.05
1.07
1.06
1.08
1.01
1.02
1.03
1.05
0.92
0.91
0.90
0.90
1.40e
20.57
20.81
21.04
20.90
20.55
20.82
21.05
20.92
20.51
20.87
20.97
20.88
20.65
21.09
21.25
21.10
20.67
21.10
21.25
21.11
20.80
21.14
21.31
21.16
21.26
2.96
3.20
3.22
3.23
2.95
3.16
3.16
3.17
2.98
3.08
3.10
3.12
1.78
2.24
2.39
2.26
1.76
2.20
2.36
2.24
1.80
2.13
2.29
1.86
5.41
5.43
5.42
5.44
5.37
5.38
5.39
5.41
5.28
5.27
5.26
5.31
3.63
3.19
3.03
3.18
3.61
3.18
3.03
3.17
3.48
3.14
2.97
3.12
3b
1.38e
1.36e
1.40e
1.29
1.30
1.31
1.28
1.20
1.19
1.19
1.19
21.28
3c
–
3d
–
t-Bu-3a
t-Bu-3b
t-Bu-3c
t-Bu-3d
MeO-3a
MeO-3b
MeO-3c
MeO-3d
21.29
21.31
–
–
21.37
21.32
–
–
a
d
Ox1
Versus Ag/AgCl in CH3CN. E1/2 5 average potential of the redox couple
The HOMO and LUMO energy levels were calculated from E1/2
and
Red1
peaks.
b
E1/2
values of CV curves and were referenced to ferrocene (4.8 eV;
Versus Ag/AgCl in DMF.
E
1/2 5 0.44 V in CH3CN; E1/2 5 0.52 V in DMF).
The second oxidation redox couple is irreversible.
c
e
Bandgaps calculated from absorption edge of the polymer films:
Egopt 5 1240/konset. Egec, electrochemical band gap is derived from the
difference between HOMO and LUMO values.
polyimide MeO-3a. Only one pair of redox waves was observed
for the imide reduction of ODPA polyimide MeO-3c [Fig. 7(c)]
and 6FDA polyimide MeO-3d [Fig. 7(d)] during the negative
CV scanning, indicating that the conjugation across the imide is
interrupted by the ether and hexafluoroisopropylidene bridges.
If two phthalimide groups were connected by a nonconjugated
bridge, the generated radicals in each part could not be
coupled to form a more stable quinoid dianion. With ether or
hexafluoroisopropylidene as the connector, reduction of each
phthalimide group occurred at nearly the same position. All
the 6FDA polyimides showed the lower reduction potentials
than that of the ODPA polyimides due to the electron-
withdrawing nature of the hexafluoroisopropylidene linker.
p-p* transitions of the carbazole and triphenylamine units,
but it was almost transparent in the visible and NIR regions.
Upon oxidation (increasing applied voltage from 0.0 to 1.1
V), the intensity of the absorption band at 296–330 nm
gradually decreased while a new peak at 412 nm and a
broad band from 800 nm extended to 1100 nm in the NIR
region gradually increased in intensity. These absorptions
can be attributed to the formation of the aminyl radical cati-
ons. Because of the intervalence charge transfer (IVCT) inter-
actions with the adjacent amino groups, the aminyl radical
cations usually show absorption in the NIR region. The IVCT
phenomenon of the family of triarylamines has also been
reported in the literature.18 As the anodic potential was fur-
ther increased to 1.5 V, the absorption bands of the cation
radical decreased gradually with a new strong absorption
band centered at around 768 nm. The decrease of NIR
absorption intensity can be attributable to the further oxida-
tion of monocation radical species to the formation of dica-
tion in CzTPA segments. The other two polyimides t-Bu-3d
and MeO-3d showed a similar spectral change to that of 3d.
The observed UV–vis-NIR spectral changes in the polyimide
films of t-Bu-3d and MeO-3d at various potentials are fully
reversible and are associated with strong color changes.
From the photos shown in Table 4, these polyimides show a
color change from a nearly colorless neutral state to a pale
green semi-oxidized state and a blue fully oxidized state dur-
ing p-doping. However, the spectra and color changes of the
film of polyimide 3d are irreversible during the electro-
oxidation process. After the first spectroelectrochemical
series of 3d was recorded from 0.0 to 11.5 V and then back
to 0.0 V, we reapplied the electrode voltage and recorded
it absorption profile. As shown in Figure 8(b), we observed
The energy levels of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
Ox1
the corresponding polymers were estimated from the E1/2
and E1/2
Red1
values. Assuming that the HOMO energy level
for the ferrocene/ferrocenium (Fc/Fc1) standard is 4.80 eV
with respect to the zero vacuum level, the HOMO and LUMO
values for all polyimides were calculated to be in the range
of 5.26–5.44 eV and 2.97–3.63 eV, respectively.
Spectroelectrochemical and Electrochromic Properties
The electro-optical properties of the polymer films were
investigated using the changes in electronic absorption spec-
tra at various applied voltages. The results of the cast films
of representative polyimides 3d, t-Bu-3d and MeO-3d upon
electro-oxidation (p-doping) are presented in Figure 8 as a
series of UV-vis-NIR absorption profiles correlated to elec-
trode potentials. In the neutral form, polyimide 3d exhibited
strong absorptions at 296 and 330 nm, characteristic of the
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FIGURE 6 First anodic CV scans of the cast films of polyimides 3d, t-Bu-3d, and MeO-3d on the ITO-coated glass substrates and
model compounds M1-M3 (ꢁ 1 3 1023 M) in 0.1 M Bu4NClO4/CH3CN solution at a scan rate of 50 mV/s. The right shows the possi-
ble order of the oxidation reactions in the CzTPA segments of these polyimides. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
a
new absorption profile of 3d at 11.6 V, with the
The polyimide films also exhibited a strong coloration
change upon electrochemical reduction. Figure 9 shows the
spectral changes of the cast films of polyimides MeO-3a and
MeO-3b upon electro-reduction (n-doping). The radical
anion of polyimide MeO-3a, which appears at potential
21.05 V, exhibits a strong band at 312 nm and an increased
absorption between 654 and 723 nm. As shown Table 4, the
radical anion form of polyimide MeO-3a is light green in
color (L*: 67; a*:26; b*: 4). Further reduction at potentials
negative to 21.60 V results in the two-electron reduced
(dianion) state with a new strong absorption at 564 nm, and
the film turns to a pink color (L*: 53; a*: 25; b*: 217) during
the second-step reduction. The spectral changes associated
with the electroreduction reactions of the pyromellitimide
unit are very similar to that of standard PMDA-ODA polyi-
mide (ODA: 4,40-oxydianiline).17 This result reaffirms that the
diamine residue has very little direct influence on the
longer-wavelength absorption maximum shifting to 756 nm.
The fully oxidized film became pale orange with a signifi-
cantly decreased optical contrast. The film could not be
switched to blue coloring oxidized state anymore.
Apparently, a new product was formed via the intermolecu-
lar oxidative coupling of the carbazole units during the first
potential scan. On the contrary, the spectral changes in the
second spectroelectrochemical series of polyimides t-Bu-3d
and MeO-3d are almost the same with those observed in
the first series. The optical contrast measured as ᭝T% of
t-Bu-3d and MeO-3d between neutral colorless and oxidized
blue states was found to be 88% at 771 nm and 86% at 817
nm, respectively. Thus, the introduction of tert-butyl or
methoxy group at the electrochemically active C-3 and C-6
sites of the carbazole unit greatly enhances the redox and
electrochromic stability of these polyimides.
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FIGURE 7 Cathodic CV diagrams of the cast films of polyimides (a) MeO-3a, (b) MeO-3b, (c) MeO-3c, and (d) MeO-3d on the ITO-
coated glass substrate in 0.1 M Bu4NClO4/DMF at a scan rate of 100 mV/s. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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FIGURE 8 Spectral changes of the cast films of polyimides (a) 3d, (c) t-Bu-3d, and (d) MeO-3d on the ITO-coated glass substrate in
0.1 M Bu4NClO4/CH3CN at various potentials; (b): spectral changes of the 3d film in the second potential scan. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
TABLE 4 Images and Color Coordinates of Electrochromic Polyimide Films at Indicated Applied Voltages
Polymers
3d
t-Bu-3d
MeO-3d
E (V)
0.0
1.2
1.5
0.0
1.15
1.4
0.0
1.05
1.3
Colors
L*
a*
b*
95
21
2
55
2
49
21
24
96
22
1
57
3
49
26
96
1
53
22
6
37
25
0
23
215
3
222
Polymers
MeO-3a
MeO-3b
E (V)
0.0
21.05
21.60
0.0
21.25
21.45
Colors
L*
a*
b*
96
222
3
67
26
4
53
25
96
1
60
3
53
22
6
217
3
20
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FIGURE 9 Spectral changes of the cast films of polyimides (a) MeO-3a and (b) MeO-3b on an ITO-glass substrate in 0.1 M Bu4N-
ClO4/DMF at various applied potentials. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.
com.]
reduction of the diimide moiety as reported in literature.17
The radical anion of polyimide MeO-3b formed at 21.25 V
Liaw, K.-L. Wang, Y.-C. Huang, K.-R. Lee, J.-Y. Lai, C.-S. Ha,
Prog. Polym. Sci. 2012, 37, 907–974.
reveals a new absorption band in the visible region at about
495 nm, and the film appears as yellow-orange in color
(L*: 60; a*: 3; b*: 220). As the film was further charged with
electrons at 21.45 V, the absorption intensity at 495 nm
decreased slightly, and two new bands emerged at about
643 and 873 nm, and the film turned to a green-blue color
(L*: 53; a*: 22; b*: 26) during the second-step reduction.
3 (a) S.-H. Hsiao, K.-H. Lin, J. Polym. Sci. Part A: Polym. Chem.
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CONCLUSIONS
Three series of aromatic polyimides with CzTPA units have
been successfully prepared from the polycondensation
reactions of CzTPA-based diamine monomers 1, t-Bu-1,
and MeO-1, respectively, with commercially available tetra-
carboxylic dianhydrides. Incorporating bulky tert-butyl or
electron-donating methoxy substituents at the electro-
chemically active C-3 and C-6 sites of the carbazole unit
prevents the coupling reactions of carbazoles and leads to
a greatly enhanced redox and electrochromic stability of
the resulting polyimides upon electro-oxidation. In addi-
tion to good thermal stability and film-forming ability, the
t-Bu- and MeO-series polyimides also revealed valuable
electrochromic characteristics in both visible and NIR
regions. Thus, these polyimides may find optoelectronic
applications as new hole-transporting and electrochromic
materials.
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