New triphenylamine-based poly(amine-imide)s with carbazole-substituents for electrochromic applications

Article abstract of DOI:10.1016/j.orgel.2014.04.015

A new carbazole-derived triphenylamine-containing diimide-diacid monomer (5), 4,4′-bis(trimellitimido)-4″-N-carbazolytriphenylamine, is prepared by the condensation of 4,4′-diamino-4″-N- carbazolytriphenylamine (4) and two molar equivalents of trimellitic

Full text of DOI:10.1016/j.orgel.2014.04.015

Organic Electronics 15 (2014) 1422–1431
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Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
New triphenylamine-based poly(amine-imide)s with
carbazole-substituents for electrochromic applications
Yu Liu a, , Danming Chao b, Hongyan Yao b
⇑
a Institute of Metal Research Chinese Academy of Sciences, Shenyang 110016, China
b Alan G. MacDiarmid Lab College of Chemistry, Jilin University, Changchun 130023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
new carbazole-derived triphenylamine-containing diimide-diacid monomer (5),
4,40-bis(trimellitimido)-400-N-carbazolytriphenylamine, is prepared by the condensation
of 4,40-diamino-400-N-carbazolytriphenylamine (4) and two molar equivalents of trimellitic
anhydride (TMA). A series of new poly(amide-imide)s (PAIs) 7a–7d with carbazole-substi-
tuted triphenylamine units are prepared by direct polymerization from the new diimide-
diacid (5) and various aromatic diamines (6a–6d). The PAIs shows high glass transition
temperature between 269 and 297 °C, and high 5% weight loss temperature between
526 and 561 °C under nitrogen environment. Cyclic voltammograms of the PAIs films,
which are casted onto the indium–tin oxide (ITO)-coated glass substrate, exhibit two
reversible oxidation redox couples at 1.05–1.08 V and 1.38–1.46 V under an anodic sweep.
The PAI-7a film reveals excellent stability of electrochromic characteristics for the radical
cations generated, changing color from original pale yellowish neutral form to the green
and then to dark blue oxidized forms. Furthermore, the anodically coloring of PAI-7a shows
high coloration efficiency (CE = 205 cm2/C), high contrast of optical transmittance change
Received 25 February 2014
Received in revised form 9 April 2014
Accepted 11 April 2014
Available online 24 April 2014
Keywords:
Poly(amide-imide)
Synthesis
Electrochromic properties
Triphenylamine
Carbazole
(
D
T = 80% at 776 nm) and long-term redox reversibility.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
[2–9]. The properties of electrochromism are found not
only in conducting polymers, but also in variety of organic
and inorganic materials. The uses of conjugated polymers
as active layers in electrochromic devices become popular
because of the outstanding electrochromic properties such
as fast switching time, ease of synthesis, and wide range of
colors, which can be achieved through simple chemical
synthesis [10]. The difficulties in achieving satisfactory val-
ues for all these parameters at the same time stimulate the
development of new methods of electrochromic films
preparation, new materials and new components for the
devices [11].
Electrochromic materials, comprised redox-active spe-
cies, exhibit significant, lasting, and reversible changes in
color upon reduction or oxidation [1]. Color changes are
commonly between a transparent state, where the chro-
mophore only absorbs in the UV region, and a colored
state, or between two colored states in a given electrolyte
solution. sThe potential applications of electrochromic
materials include optical switching devices, automatic
anti-glazing mirrors, smart windows, large-area informa-
tion panels, electronic papers and chameleon materials
Aromatic polyimides (PIs) are well known as useful
high performance materials because of their excellent
thermal stability, mechanical, electric and optical proper-
ties [12–14]. However, the widespread applications of PIs
are often limited by processing difficulties because of their
⇑
Corresponding author. Address: Institute of Metal Research Chinese
Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China. Tel./fax:
+86 024 83978040.
E-mail address: yliu@imr.ac.cn (Y. Liu).
http://dx.doi.org/10.1016/j.orgel.2014.04.015
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
1423
poor solubility and high processing temperature, which are
caused by rigid polymer backbones and the strong inter-
chain interaction [15,16]. To overcome these problems,
typical approaches include the introduction of flexible
linkages, kinked or unsymmetrical structures, bulky pack-
ing-disruptive units, and bulky lateral groups into the
polymer backbone. In our previous work several soluble
fluorinated poly(ether imide)s with different pendant
groups have been explored, and the introduction of bulky
trifluoromethyl group into polymer backbones result in
an enhanced solubility and optical transparency [17].
Another common approach for increasing solubility and
processability of PIs without sacrificing high thermal
stability is the development of various copolyimides.
Replacement of polyimides by copolyimides such as
poly(amide-imide)s (PAIs) may be useful in modifying
the intractable character of polyimides [18,19]. Because
of the PAIs possess both amide and imide groups in theirs
mainchain. This class of polymers combines the advanta-
ges of polyamides and polyimides and offers good compre-
hensive properties such as high thermal property and
processability.
In recent years, triarylamine-based PAIs with attractive
electrochromic properties have been reported in G.S. Liou’s
laboratory [20]. Triarylamine derivatives are well-known
for their electroactive and photoactive properties, which
make them be used as photoconductors, hole-transporters,
and light-emitters. Polymers bearing triarylamine units are
being received considerable interest as ideal hole-trans-
porters, and light-emitting diodes (OLEDs) because of the
strong electron-donating and hole-transporting/injecting
properties of triarylamine units [21]. What’s more, triaryl-
amine can be easily oxidized to form stable radical cations,
and the oxidation process is always associated with a
strong change of coloration [22]. Except triarylamine, car-
bazole is another well-known hole-transporting and
light-emitting unit. From structural point of view, carba-
zole differs from diphenylamine in its planar structure
because it can be further imagine as the bonded diphenyl-
amine; the thermal stability of materials with the incorpo-
ration of carbazolyl units therefore is improved. In
addition, carbazole could be easily functionalized at its
3,6-, 2,7-, or N-positions and then covalently linked to
polymeric systems, both in the main chain as building
blocks and in a side chain as pendant groups [23]. Thus,
incorporation of three-dimensional, packing-disruptive tri-
arylamine and carbazole units into the poly(amide-imide)
backbone not only enhance the solubility but also form
some new electronic factions of poly(amide-imide) such
as electrochromic characteristics.
ubility
and
excellent
thermal
properties.
The
electrochemical and electrochromic properties of these
films prepared by casting the PAIs onto an indium–tin
oxide (ITO)-coated glass substrate are also described
herein.
1.1. Measurements
IR spectra (KBr) were measured on a Nicolet Impact 410
Fourier transform infrared spectrometer. 1H NMR and 13C
NMR were recorded on a Bruker 510 NMR spectrometer
(500 MHz) with tetramethyl silane as a reference. Elemen-
tal analyses were performed on an Elemental Analyses
MOD-1106. Gel permeation chromatograms (GPC) using
polystyrene as a standard were obtained on a Waters 410
instrument with a DMF as an eluent at a flow rate of
1 mL/min. Inherent viscosity was determined on an
Ubblohde viscometer in thermostatic container with the
polymer concentration of 0.5 g/dL in NMP at 30 °C. Differ-
ential scanning calorimetry (DSC) measurements were
performed on a Mettler Toledo DSC 821e instrument at a
heating rate of 20 °C/min under nitrogen. Thermal gravi-
metric analyses (TGA) were determined in nitrogen atmo-
sphere using a heating rate of 10 °C/min and polymers
were contained within open aluminum pans on a PERKIN
ELMER TGA-7. The mechanical tests in tension were car-
ried out using a SHIMADZU AG-I at a constant crosshead
speed of 10 mm/min. Wide-angle X-ray diffraction
(WAXD) measurements were carried out at room temper-
ature using a Rigaku/max-rA diffractometer equipped with
a Cu Ka radiation source. Voltammograms were presented
with the potential pointing to the left and with increasing
anodic currents pointing downwards. Cyclic voltammetry
was performed with the use of a three-electrode cell in
which ITO (polymer films area about 0.4 cm ꢀ 2.5 cm)
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 homemade Ag/AgCl, KCl (sat.) reference
electrode. The spectroelectrochemical cell was composed
of a 1 cm cuvette, ITO as a working electrode, a platinum
wire as an auxiliary electrode and Ag/AgCl reference elec-
trode. Absorption spectra were measured by a Shimadzu
UV 3101-PC spectrophotometer.
2. Experimental
2.1. Materials
All the reagents were purchased from commercial
sources and used as received. Carbazole (Acros), 4-fluoro-
nitrobenzene (Acros), trimellitic anhydride (TMA) (TCI),
triphenyl phosphite (TCI), 10% palladium on charcoal (Pd/
C, TCI), and hydrazine monohydrate (TCI) were used as
received. N,N-dimethylformamide (DMF), N-Methyl-2-pyr-
rolidone (NMP) and pyridine were purified by distillation
under reduced pressure over calcium hydride and stored
over 4 Å molecular sieve. 1,4-Bis(4-amino-2-trifluorom-
ethylphenoxy) benzene (6FAPB) was synthesized in our
laboratory according to the literature. 4,40-Oxydianiline
(ODA) (TCI) and 1,4-bis(4-aminophenoxy) benzene (APB)
In some reports, the triphenylamine cationic radical of
the first electron oxidation is not stable. However, the cou-
pling reactions that afford stable cationic radical are
greatly prevented when the phenyl groups are incorpo-
rated by electron-donating substituents at the para posi-
tion of triarylamines [24,25]. In this contribution, we
reported the synthesis of novel carbazole-derived triphen-
ylamine-containing aromatic dicarboxylic acid monomer,
4,40-bis(trimellitimido)-400-N-carbazolytriphenylamine (5),
and its derived PAIs bearing electron-rich pendent triphen-
ylamine and carbazole groups. The PAIs exhibited good sol-

1424
Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
(TCI) were used without further purified. Bu4NClO4, was
recrystallized from ethyl acetate under nitrogen atmo-
sphere and then dried in vacuum prior to use.
135.80, 128.38, 127.74, 126.47, 126.42, 122.77, 120.83,
119.97, 116.81, 115.40, 110.14.
2.2.5. Synthesis of 4,40-bis(trimellitimido)-400-N-
carbazolytriphenylamine (5)
2.2. Monomer synthesis
A flask was charged with 4.4 g (0.01 mol) of 4,40-dia-
mino-400-N-carbazolyltriphenylamine
(4),
3.84 g
2.2.1. Synthesis of N-(4-nitorphenyl) carbazole (1)
According to the synthesis procedure reported previ-
ously [26], compound 1 (mp: 208–210 °C) was prepared
by nucleophilic fluoro-displacement reaction of p-fluoroni-
trobenzene with carbazole in the presence of anhydrous
potassium carbonate.
(0.02 mol) of trimellitic anhydride, and 60 ml of glacial
acetic acid. The heterogeneous mixture was stirred for
1 h and then refluxed at 140 °C for 12 h. The reaction mix-
ture was cooled to precipitate a yellowish brown powder
which was rinsed with methanol to remove acetic acid.
The product was filtered several times with methanol
and dried in vacuum at 50 °C. Yield, 87 %, Tm: 274–
276 °C. IR (KBr): 3500–2400 cmꢁ1 (OH stretch); 1780,
1722 cmꢁ1 (imide C@O). 1H NMR (500 MHz, DMSO-d6, d,
ppm):13.7(s, 2H, ACOOH), 8.42 (dd, 2H), 8.32 (s, 2H),
8.25 (d, 2H), 8.10 (d, 2H), 7.62 (d, 2H), 7.46 (m, 8H), 7.40
(d, 2H), 7.35 (d, 4H), 7.29 (t, 2H). 13C NMR (126 MHz,
DMSO-d6, d, ppm):166.9, 166.3, 147.0, 146.3, 140.7,
135.9, 135.9, 135.4, 132.6, 132.4, 129.1, 128.6, 127.3,
126.7, 125.8, 124.5, 124.3, 123.8, 123.1, 121.0, 120.5, 110.3.
IR (KBr): 1580, 1312 cmꢁ1 (NO2 stretch). 1H NMR
(500 MHz, DMSO-d6, d, ppm): 7.36 (t, 2H), 7.48 (t, 2H),
7.56 (d, 2H), 7.98 (d, 2H), 8.28 (d, 2H), 8.50 (d, 2H).
2.2.2. Synthesis of N-(4-aminophenyl) carbazole (2)
To a refluxed solution of compound 1 (1.44 g, 5.00 mol)
and Pd/C (0.0720 g) in ethanol (10 mL) was added drop-
wise hydrazine monohydrate. The mixture was refluxed
for 4 h and cooled to room temperature. Pd/C was removed
by filtration through Celite and the filtrate was concen-
trated to give compound 2 as a clear viscous liquid
(1.24 g) in 94 % yield. IR (KBr): 3460, 3379 cmꢁ1 (NH2
stretch). 1H NMR (500 MHz, DMSO-d6, d, ppm): 8.19
(d, 2H), 7.40 (t, 2H), 7.27–7.21 (m, 4H), 7.18 (d, 2H), 6.81
(d, 2H), 5.35 (s, 2H).
2.2.6. Polymer synthesis
Utilizing compound 5 as diimide-diacid monomer, four
kinds of PAIs were synthesized by polycondensation with
diamine monomers ODA (6a), APB (6b), 6FAPB (6c) and
4,40-diamino-400-N-carbazolytriphenylamine (6d), respec-
tively. The resulting PAIs were abbreviated to 7a-7d
successively.
In a typical experiment, PAI-7a, which derived from dii-
mide-diacid 5 and ODA (6a) was prepared as follows: a
mixture of 0.7888 g (1 mmol) of the diimide-diacid mono-
mer 5, 0.2002 g of ODA (6a), 0.15 g of anhydrous calcium
chloride, 1.2 ml of triphenyl phosphite (TPP), 0.4 ml of pyr-
idine, and 3.5 ml of NMP was heated with stirring at 120 °C
for 3 h. The resulting polymer solution was poured slowly
into 200 ml of stirred methanol giving rise to a tough,
fiber-like precipitate that was collected by filtration, and
washed thoroughly with hot water and methanol and
dried.
2.2.3. Synthesis of 4,40-dinitro-400-N-carbazolyltriphenylamine
(3)
A
mixture of 9.041 g (0.035 mol) of compound 2,
10.12 g (0.072 mol) of 4-fluoronitrobenzene, 7.245 g
(0.0525 mol) of anhydrous potassium carbonate, 80 mL of
dry N,N-dimethylformamide (DMF) was in a 250-mL,
three-necked, round-bottomed flask equipped with
a
mechanical stirrer, a Dean–Stark strap, a reflux condenser
and a nitrogen purge. The mixture was refluxed at 150 °C
with stirring for 10 h under a nitrogen atmosphere. After
being poured into water (500 mL), the red precipitate
was collected by filtration and dried under vacuum. The
crude product was crystallized from DMF to obtain red
crystals. Yield, 70 %, Tm: 282 °C. IR (KBr): 1578,
1311 cmꢁ1 (NO2 stretch). 1H NMR (500 MHz, DMSO-d6, d,
ppm): 8.50 (d, 4H), 8.03 (d, 2H), 7.87 (d, 2H), 7.72 (m,
8H), 7.57 (m, 4H).
A solution of the polymer was obtained by dissolving
about 1.0 g PAI sample in 10 ml of NMP. The homogeneous
solution was poured onto a 6 cm glass Petri dish, which
was placed in a 90 °C overnight for the slow release of
the solvent, and then the film was stripped from the glass
substrate and further dried in vacuum at 180 °C for 5 h. The
2.2.4. Synthesis of 4,40-diamino-400-N-
carbazolyltriphenylamine (4)
thickness of the films were about 70–80 lm and the films
In a 250 mL round-bottom flask equipped with a stir-
ring bar, a mixture of 5.5 g of dinitro compound 3, 0.05 g
were used for X-ray diffraction measurements, solubility
tests and thermal analyses. Other PAIs were synthesized
in an analogous procedure. The characterization of the tar-
get PAIs are listed as follows.
of 10
% Pd/C, 5 mL of hydrazine monohydrate, and
100 mL of ethanol was heated at reflux for 10 h and cooled
to room temperature. Pd/C was removed by filtration
through Celite. The crude product was crystallized from
ethanol under nitrogen and dried in vacuum at 70 °C. Yield,
90 %, Tm: 110–112 °C. IR (KBr): 3467, 3455 cmꢁ1 (NH2
stretch). 1H NMR (500 MHz, DMSO-d6, d, ppm): 8.17 (d,
2H), 7.39 (t, 2H), 7.28 (d, 2H), 7.22 (m, 4H), 6.95 (d, 4H),
6.76 (d, 2H), 6.58 (d, 4H), 5.01 (s, 4H, -NH2). 13C NMR
(126 MHz, DMSO-d6, d, ppm): 149.68, 146.67, 141.18,
PAI-7a: IR (KBr, cmꢁ1): 3349 cmꢁ1 (NAH stretching),
1779, 1723 cmꢁ1 (C@O stretching), 1374 (CAN stretching).
Anal. Calcd for (C60H36N6O7)n: C, 75.62%, H, 3.81%, N, 8.82%.
Found: C, 74.12%, H, 3.54%, N, 8.55%. 1H NMR (500 MHz,
DMSO-d6, d, ppm): 10.66 (s, 2H, NH-CO), 8.57 (s, 2H),
8.47 (d, 2H), 8.25 (d, 2H), 8.14 (d, 2H), 7.85 (d, 4H), 7.63
(d, 2H), 7.52 (d, 4H), 7.47(s, 4H), 7.40 (dd, 6H), 7.30
(s, 2H), 7.08 (d,4H).

Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
1425
PAI-7b: IR (KBr, cmꢁ1): 3349 cmꢁ1 (NAH stretching),
1779, 1723 cmꢁ1 (C@O stretching), 1374 (CAN stretching).
Anal. Calcd for (C66H40N6O8)n: C, 75.85%, H, 3.86%, N, 8.04%.
Found: C, 73.96%, H, 3.61%, N, 7.95%. 1H NMR (500 MHz,
DMSO-d6, d, ppm): 10.67 (s, 2H, NH-CO), 8.57 (s, 2H),
8.47 (d, 2H), 8.25 (d, 2H), 8.15 (d, 2H), 7.84 (d, 4H), 7.63
(d, 2H), 7.51 (d, 4H), 7.47(s, 4H), 7.37 (dd, 6H), 7.30 (s,
2H), 7.07 (s, 8H).
diamine compound 4 with two molar equivalents of TMA
in refluxing glacial acetic acid.
N-(4-Aminophenyl) carbazole 2 was prepared by the
condensation of carbazole with 4-fluoronitrobenzene fol-
lowed by hydrazine Pd/C catalytic reduction. The double-
coupling reaction of the conjugate base of compound 2
with 4-fluorobenzonitrile produced dinitro compound 3,
and diamine compound 4 underwent reduction with Pd/C
catalyst in ethanol. The targeted diimide-diacid 5 was pre-
pared by the condensation of 4 with two molar equivalents
of TMA in refluxing glacial acetic acid. The synthetic route
is shown in Scheme 1. Elemental analysis, IR, 1H and 13C
NMR spectroscopic techniques were used to identify the
structures of the intermediate compounds 1, 2, 3, 4 and
the diimide-diacid monomer 5. The transformation of
amino to carboxyl functional groups could be monitored
by the change of IR spectra (Fig. 1). The IR spectrum of
the diimide-diacid monomer 5 shows absorption bands
at 3500–2400 cmꢁ1 (OH stretching), 1780 cmꢁ1 (imide,
symmetric C@O stretching) and 1722 cmꢁ1 (asymmetric
imide C@O stretching).
Fig. 2 illustrates the HAH COSY spectra of diimide-dia-
cid monomer 5. Assignments of each proton and carbon are
also given in these figures, and agree well with the pro-
posed molecular structure. In HAH COSY spectra, the cor-
related pairs of AB doublets at 7.62/7.40 ppm and 7.46/
7.35 ppm can be easily assigned to the protons 5–8 on
the triphenylamine core. The aromatic protons 10
appeared at the most downfield (8.42 ppm) owing to the
strong electron-withdrawing effect of the C@O group as a
doublet. Accordingly, the connected doublet signal at
8.10 ppm is assigned to the protons 9. The complicated res-
onances in the region of 7.50–7.46 ppm are assigned to the
interconnected protons 3 (as a triplet), protons 4 (as a dou-
PAI-7c: IR (KBr, cmꢁ1): 3350 cmꢁ1 (NAH stretching),
1779, 1723 cmꢁ1 (C@O stretching), 1379 (CAN stretching).
Anal. Calcd for (C68H38F6N6O8)n: C, 69.15%, H, 3.24%, N,
7.12%. Found: C, 67.89%, H, 3.31%, N, 7.33%. 1H NMR
(500 MHz, DMSO-d6, d, ppm): 10.88 (s, 2H, NH-CO), 8.59
(s, 2H), 8.48 (d, 2H), 8.34 (d, 2H), 8.25 (d, 2H), 8.15 (d,
4H), 7.63 (d, 2H), 7.52 (d, 4H), 7.47(s, 4H), 7.37 (dd, 6H),
7.30 (s, 2H), 7.18 (s, 2H), 7.14 (s, 4H).
PAI-7d: IR (KBr, cmꢁ1): 3349 cmꢁ1 (NAH stretching),
1779, 1723 cmꢁ1 (C@O stretching), 1373 (CAN stretching).
Anal. Calcd for (C78H48N8O6)n: C, 78.51%, H, 4.05%, N, 9.39%.
Found: C, 77.07%, H, 4.13%, N, 9.72%. 1H NMR (500 MHz,
DMSO-d6, d, ppm): 10.69 (s, 2H, NH-CO), 8.58 (s, 2H),
8.49 (d, 2H), 8.26 (d, 4H), 8.15 (d, 4H), 8.01 (d, 2H), 7.88
(m, 4H), 7.51(d, 2H), 7.47(m, 4H), 7.38 (dd, 6H), 7.30 (s,
2H), 7.18 (s, 2H), 7.12 (s, 2H).
3. Results and discussion
3.1. Monomer synthesis
The new diimide-diacid monomer 5 containing carba-
zole and triphenylamine groups was synthesized by the
synthetic route outlined in Scheme 1. The targeted dii-
mide-diacid
5 was prepared by the condensation of
Scheme 1. Synthesis of monomer.

1426
Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
series of novel PAIs 7a–g were synthesized from the poly-
condensation reactions of diimide-dicarboxylic acid 5 with
various aromatic diamines 6a–d by using triphenyl phos-
phite (TPP) and pyridine as condensing agents (Scheme 2).
The polymerization proceeded homogeneously throughout
the reaction and afforded clear, highly viscous polymer
solution. All the PAIs precipitated in a tough, fiber-like
from when the resulting polymer soutions were slowly
poured under stirring into methanol.
As shown in Table 1, the inherent viscosities of the
intermediate PAIs ranged from 0.78 dL/g to 0.96 dL/g. The
weight-average molecular weight (Mws) and number-aver-
age molecular weights (Mns) were recorded in the ranges
of 46,000–70,000 and 27,000–39,000, relative to polysty-
rene standards. The molecular weights of all the PAIs were
sufficiently high to permit the casting of flexible and tough
PAIs films.
Fig. 1. FTIR spectra of diamine monomer 4 and diimide-diacid monomer
The formation of PAIs was confirmed with elemental
analysis and IR spectroscopy. As shown in Fig. 4, the IR spec-
tra of these PAIs exhibited characteristic imide absorption
bands at around 1779 cmꢁ1 (imide carbonyl asymmetrical
stretching), 1723 cmꢁ1 (imide carbonyl symmetrical
stretching), 1374 cmꢁ1 (CAN stretching), together with
some strong absorption bands in the region of 1100–
1300 cmꢁ1 due to the CAO and CAF multiple stretching. In
addition to IR spectra, the elemental analysis results of these
PAIs also generally agree with the calculated values for the
proposed structures. The 1H NMR spectra of PAI 4a are
shown in Fig. 5. The amide group appeared in 10.66 ppm.
The Hi close to the imide ring appeared at the farthest down-
field region of the spectrum because of the resonance. The
proton Hh shifted to a higher field because of the electron-
donating property of amine group. The above results
5.
blet) and protons 7 (as a doublet), in which the triplet of
protons 3, doublet of protons 4 and the doublet of protons
7, is also connected with the triplet signal at 7.29 ppm aris-
ing from protons 2 and doublet signal at 7.35 ppm arising
from protons 8. Fig. 3 shows the 13C NMR spectra of dii-
mide-diacid monomer 5, the carbonyl carbon atoms (Co
and Cv) appeared in the downfield (166.9 ppm) of the spec-
trum and the carbon atom (Ce) appeared in the upfield
(110.3 ppm).
3.2. Polymer synthesis
According to the phosphorylation polyamidation tech-
nique first described by Yamazaki and co-workers [27], a
Fig. 2. HAH COSY spectra of diimide-diacid monomer 5 in DMSO-d6.

Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
1427
Fig. 3. 13C NMR spectra of diimide-diacid monomer 5 in DMSO-d6.
Scheme 2. Synthesis of the PAIs.
Table 1
Inherent viscosity, GPC data and solubility of the PAIs.
Polymer
g
(dL/g)a
GPC datab
Solventsc
NMP
Mw ꢀ 104
Mn ꢀ 104
PDI
DMAc
DMF
DMSO
m-Cresol
THF
CHCl3
PAI-7a
PAI-7b
PAI-7c
PAI-7d
0.85
0.92
0.96
0.78
5.4
6.2
7.0
4.6
3.4
3.9
3.7
2.7
1.6
1.6
1.9
1.7
++
++
++
++
++
++
++
++
+
+
++
++
++
+
++
++
+
+
++
++
–
–
+
–
–
–
–
++: Soluble at room temperature; +: soluble on heating. : partially soluble on heating.
a
Measured at a polymer concentration of 0.5 g/dL in NMP at 30 °C.
Relative to polystyrene standard, using DMF as the eluent.
Qualitative solubility was determined with as 10 mg of polymer in 1 mL of solvent.
b
c
demonstrate that the diimide-diacid monomer (5) hold a
good polymerization activity to form PAIs.
triphenylamine and carbazole units along the polymer
backbone, which resulted in a high steric hindrance for
close packing, and thus reduced their crystallization ten-
dency. The amorphous structure endowed the obtained
PAIs with good solubility.
The solubility of synthesized PAIs was tested in a vari-
ety of organic solvents at 1.0% (w/v) and the results are
summarized in Table 1. All the PAIs showed high solubility
WAXD experiments were conducted in an attempt to
evaluate the morphological structure of our synthesized
PAIs. As shown in Fig. 6, the curves of all the PAIs were
broad and without obvious peak features, indicating their
amorphous structure. Their amorphous properties could
be attributed to the incorporation of packing-disruptive

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Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
in the repeat unit. The excellent solubility makes these
PAIs potential candidates for practical applications in spin-
or dip-coating processes. Thus, all these PAIs can be readily
processed from solution.
Flexible and tough films can be obtained via solvent
casting. These films were subjected to tensile testing, and
the results are given in Table 2. The tensile strengths, elon-
gations to break, and initial moduli of these films were in
the ranges of 76–85 MPa, 12–19% and 2.0–2.4 GPa,
respectively.
3.3. Thermal properties of the PAIs
DSC was used to evaluate the thermal properties of the
PAI films. The thermal behavior data of all the PAIs are also
listed in Table 3. Generally, the decreasing order of Tg cor-
related with both molecular packing and chain conforma-
tion (chain rigidity and linearity) of the PAIs. The Tg
values of these PAIs were in the range of 269–297 °C. The
Tg value of PAI-7c is lower than PAI-7a because of the intro-
duction of two flexible ether segments group in diamine
monomer, which could increase the polymer chain mobil-
ity. Besides the trifluromethtyl groups could decreased the
interchain interaction and chain packing density. These
groups also lead to an internal plasticization in addition
to the geometry and free volume factors.
Fig. 4. FTIR spectra of the PAIs.
Thermal stabilities of the PAIs were evaluated by TGA
under nitrogen with a 5% weight loss and 10% weight loss
for comparison. The results are summarized in Table 3. The
decomposition temperatures at a 5% weight loss and 10%
loss of PAIs were in the range of 526–561 °C and 565–
580 °C, respectively. All the PAIs exhibited excellent ther-
mal stability, and no obvious decomposition was observed
below 500 °C. The amount of carbonized residue (char
yield) of these polymers in nitrogen atmosphere was more
than 65% at 800 °C. The high char yields of these PAIs could
be ascribed to their high aromatic content. The outstanding
thermal stability was also an advantage in various
applications.
Fig. 5. 1H NMR spectra of PAI-7a in DMSO-d6.
3.4. Electrochemical characteristics
The electrochemical properties of the PAIs were investi-
gated by cyclic voltammetry (CV) conducted for the cast
films on an ITO-coated glass substrate as wording elec-
trode (CH3CN) in dry acetonitrile containing 0.1 M of TBAP
as an electrolyte under nitrogen atmosphere. CV data for
the PAIs are given in Table 4, and typical representative
cyclic voltammogram of PAI-7a is shown in Fig. 7. The PAIs
7a–7d revealed two reversible redox couples at half-wave
potential of 1.05–1.08 V (Eox1 1/2) and 1.38–1.46 V
(Eox2 1/2), respectively, under an anodic sweep. The first
electron removal of PAI-7a was assumed to occur at the
nitrogen atom on the main chain triphenylamine unit,
which was more electron-rich than the nitrogen atom on
the pendent carbazolyl moiety at Ep,a = 1.72 V. Thus, the
first stable cationic radical of PAI+ should be formed by
the first oxidation at the nitrogen atom of triphenylamine
unit. The energy levels of the HOMO and LUMO of the
investigated PAIs can be estimated from the oxidation
onset (Eonset) or half-wave potentials (E1/2), and the results
Fig. 6. Wide-angle X-ray diffractograms of the PAIs.
in aprotic polar solvents such as NMP, DMAc, DMF, DMSO
and m-Cresol due to the introduction of propeller-shaped
triphenylamine core and bulky pendent carbazole group

Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
1429
Table 2
Mechanical properties of PAIs.
Polymer
Tensile strength (MPa)
85
78
76
82
Elongation at break (%)
Initial modulus (GPa)
PAI-7a
PAI-7b
PAI-7c
PAI-7d
14
19
18
12
2.4
2.2
2.2
2.0
Table 3
Thermal properties of the PAIs.
Polymer
DSC
TGA
a
Tg (°C)
Tb
(°C)
Tb
(°C)
10%
Char yieldc (%)
5%
PAI-7a
PAI-7b
PAI-7c
PAI-7d
297
277
269
275
541
550
526
561
570
578
565
580
75
71
68
70
a
Baseline shift in the second heating DSC traces, with a heating rate of
20 °C/min in nitrogen.
b
Temperature at 5% and 10% weight loss were recorded by TGA at a
heating at 10 °C/min in nitrogen.
c
Residual weight (%) when heated to 800 °C.
are listed in Table 4. For example, the Eonset value for PAI-7a
has been determined 0.92 eV. The external ferrocene/ferro-
cenium (Fc/Fc+) redox standard E (Fc/Fc+) was 0.48 V vs Ag/
AgCl in CH3CN. Assuming that the HOMO energy level of
the Fc/Fc+ standard is 4.80 eV with respect to the zero vac-
uum level, the HOMO energy level of PAI-7a has been eval-
uated to be 5.24 eV. The high-lying HOMO energy level and
reversible electrochemical oxidation of these PAIs suggest
that they have potential for use as hole injection and trans-
port materials in the organic light-emitting devices.
Fig. 7. Repeated CV of PAI-7a films on the ITO-coated glass substrate in
0.1 M Bu4NClO4 at a scan rate of 100 mV/s.
3.5. Electrochromic characteristics
Electrochromism of the PAIs thin films was determined
by optically transparent thin-layer electrode (OTTLE) cou-
pled with a UV–vis spectroscopy. The electrode prepara-
tions and solution conditions were identical to those
used in cyclic voltammetry. The typical electrochromic
absorption spectrum of PAI-7a is shown in Fig. 8. When
the applied potentials increased positively from 0 V to
1.68 V, the peak of characteristic absorbance at 322 nm
for PAI-7a decreased gradually while three new bands
grew up at 421 nm and 776 nm. The film of PAI-7a
Fig. 8. Spectroelectrochemistry of the cast film of PAI-7a on the ITO-
coated glass substrate in 0.1 M Bu4NClO4 at various applied potentials.
Table 4
Electrochemical properties of the PAIs.
Oxidation potentiala(V)
Eg (eV)
HOMOc (eV)
LUMOd (eV)
b
Index
Thin film (k/nm)
ox1
1=2
ox2
Abs. max
Abs. onset
Eonset
E
E
1=2
PAI-7a
PAI-7b
PAI-7c
PAI-7d
322
310
318
315
421
415
411
409
0.92
0.90
0.95
0.96
1.05
1.06
1.08
1.06
1.43
1.46
1.44
1.38
2.58
2.60
2.61
2.59
5.24
5.22
5.27
5.28
2.66
2.62
2.66
2.69
a
b
c
Oxidation half-wave potentials from cyclic voltammograms.
The data were calculated by the equation: Eg = 1240/konset of polymer film.
The HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV).
d
LUMO = HOMO–Eg.

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Y. Liu et al. / Organic Electronics 15 (2014) 1422–1431
phenylamine (5) with various aromatic diamines (6a–d).
The incorporation of triphenylamine and carbazole substit-
uents to the polymer main chain, not only facilely tunes
the electrochromic behaviors by adjusting the oxidation
potentials, but also enhances the solubility. Besides high
Tg and Td values and good thermal stability, the PAIs also
revealed stable electrochromic properties, changing color
from original pale yellowish neutral form to the green
and then to dark blue oxidized forms. The PAI-7a film
exhibited high coloration efficiency, high optical contrast
ratio, long-term redox and electrochromic reversibility.
Thus, these PAIs might have optoelectronic applications
such as new hole-transporting and electrochromic materi-
als because of their proper HOMO values and stable elec-
trochromic behavior.
Acknowledgements
Fig. 9. (a) Current densities during switching studies. (b) Optical trans-
mittance change monitored at 412 nm and 763 nm for PAI-7a thin film on
the ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN solution.
We thank the National Science Foundation of China
(51203172) and Liaoning Province Doctor Startup Fund
(20121067) for the financial support.
switches from a transmissive neutral state (pale yellowish)
to a high absorbing semioxidized state (green) and fully
oxidized state (dark blue).
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