Enhancement of redox stability and electrochromic performance of aromatic polyamides by incorporation of (3,6-dimethoxycarbazol-9-yl)-triphenylamine units

Article abstract of DOI:10.1002/pola.27001

New series aromatic polyamides with (carbazol-9-yl)triphenylamine units were synthesized from a newly synthesized diamine monomer, 4,4′-diamino- 4″-(3,6-dimethoxycarbazol-9-yl) triphenylamine, and aromatic dicarboxylic acids via the phosphorylation polyamidation technique. These polyamides exhibit good solubility in many organic solvents and can be solution-cast into flexible and strong films with high thermal stability. They show well-defined and reversible redox couples during oxidative scanning, with a strong color change from colorless neutral form to yellowish green and blue oxidized forms at applied potentials scanning from 0.0 to 1.3 V. They show enhanced redox-stability and electrochromic performance as compared to the corresponding analogs without methoxy substituents on the active sites of the carbazole unit. Copyright

Full text of DOI:10.1002/pola.27001

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Enhancement of Redox Stability and Electrochromic Performance of
Aromatic Polyamides by Incorporation of (3,6-Dimethoxycarbazol-9-yl)-
triphenylamine Units
Hui-Min Wang, Sheng-Huei Hsiao
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10618, Taiwan
Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
Received 18 September 2013; accepted 24 October 2013; published online 25 November 2013
DOI: 10.1002/pola.27001
ABSTRACT: New series aromatic polyamides with (carbazol-9-yl)
triphenylamine units were synthesized from a newly synthe-
sized diamine monomer, 4,40-diamino-400-(3,6-dimethoxycarba-
zol-9-yl) triphenylamine, and aromatic dicarboxylic acids via
the phosphorylation polyamidation technique. These polya-
mides exhibit good solubility in many organic solvents and
can be solution-cast into flexible and strong films with high
thermal stability. They show well-defined and reversible redox
couples during oxidative scanning, with a strong color change
from colorless neutral form to yellowish green and blue oxi-
dized forms at applied potentials scanning from 0.0 to 1.3 V.
They show enhanced redox-stability and electrochromic per-
formance as compared to the corresponding analogs without
methoxy substituents on the active sites of the carbazole unit.
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2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2014, 52, 272–286
KEYWORDS: carbazole; electrochemistry; electrochromic poly-
mers; polyamides; redox polymers; spectroelectrochemistry;
triphenylamine
been devoted to their synthesis and the investigation of their
electronic, optical, and materials properties.11 The wide-
spread use of triarylamines has its origin in the high stability
of the corresponding radical cations, that is, triarylamines
can reversibly be oxidized as long as the para-position of the
phenyl rings is protected.12 Electron-rich triarylamines can
be easily oxidized to form stable radical cations, and the oxi-
dation process is always associated with a noticeable change
of coloration. Thus, several triarylamine-based polymers
have been investigated for electrochromic applications.10 In
recent years, triarylamine-based condensation-type polymers
such as aromatic polyamides and polyimides have been
reported as a new and attractive family of electrochromic
materials because of high electroactivity of the triarylamine
unit and high thermal stability of the polymer backbone.13,14
It has also been reported in our previous publications15 that
the incorporation of electron-donating or bulky substituents
such as methoxy or t-butyl groups at the para-position of
phenyl rings of the triphenylamine (TPA) unit resulted in
stable TPA cationic radicals and decreased oxidation poten-
tial, leading to a significant enhancement on redox and elec-
trochromic stability of the prepared polyamides.
INTRODUCTION Electrochromism refers to the reversible
electromagnetic absorbance/transmittance and color change
resulting from the oxidation or the reduction of the material
in response to an externally applied potential by electrochemi-
cal means.1 One of the main uses of electrochromic materials
is in smart windows for cars and buildings and in antiglare
rear-view mirrors.2 Potential applications in information stor-
age,3 electrochromic displays,4 and adaptive camouflages5 can
also be envisioned. Among the different types of electrochro-
mic materials, conjugated polymers6 such as polyanilines, pol-
ypyrroles, polyselenophenes, polythiophenes, and in particular,
poly(3,4-ethylenedioxythiophene) and its derivatives7 attract
great interest because of mechanical flexibility, ease in band-
gap/color-tuning via structural control, and the potential for
low-cost processing for large-area devices. For efficient opera-
tion of an electrochromic device, it is necessary to take a
number of properties into consideration: electrochromic effi-
ciency, optical contrast, response time, stability, and durability.
The difficulty in achieving satisfactory values for all these
parameters at the same time stimulates the development of
new methods of preparation of electrochromic films, new
materials, and components for the devices.8
In recent years, carbazole derivatives have been widely used
as effective host materials in phosphorescent light-emitting
diodes because of their sufficiently high triplet energy and
Because of the fact that triarylamines play an important role
as hole conducting species in optoelectronic devices9 and for
electrochromic applications,10 numerous investigations have
Additional Supporting Information may be found in the online version of this article.
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good hole-transporting ability.16 Carbazole can be easily
functionalized at its (3,6-), (2,7-), or N-positions, and then
covalently linked into polymeric systems, both in the main
chain as building blocks and in a side chain as subunit.17
Carbazole-containing polymers are considered as a very
important class of electroactive and photoactive materials
because of their unique properties, which allow various
optoelectronic applications such as photoconductive, electro-
luminescent, electrochromic and photorefractive materials.18
In a previous publication,19 Liou’s group studied the prepa-
ration and properties of a series of polyamides bearing (car-
bazol-9-yl)triphenylamine (CzTPA) units from the diamine
monomer 4,40-diamino-400-(carbazol-9-yl)triphenylamine (60)
and various dicarboxylic acids. The polyamides reveal good
redox stability during the first oxidation process, which is
associated with a noticeable change of the coloration. How-
ever, the second oxidation process of these polymers is not
reversible, possibly due to the electrochemical coupling of
carbazoles through the active C-3 and C-6 sites. In this work,
we synthesized a new CzTPA-based diamine monomer, 4,40-
diamino-400-(3,6-dimethoxycarbazol-9-yl)triphenylamine (6),
and its derived aromatic polyamides with main-chain TPA
and pendent 3,6-dimethoxycarbazole units. The incorporation
of electron-donating methoxy substituents on the active sites
of the pendent carbazoles is expected to increase the electro-
chemical and electrochromic stability of the resulting polya-
mides. For a comparative study, some properties of the
present polyamides are compared with those of structurally
related ones based on the parent CzTPA-diamine monomer 60.
sulfoxide (DMSO) was heated at 140 ꢀC for 18 h under a
nitrogen atmosphere. The mixture was poured into 200 mL
of methanol, and the precipitated compound was collected
by filtration and washed thoroughly by methanol and hot
water. The crude product was filtered and recrystallized
from DMF/methanol to afford 5.6 g (80% in yield) of orange
crystals with a mp of 217–219 ꢀC (by DSC at a scan rate of
2 C min21). FTIR (KBr): 1325, 1595 cm21 (ANO2 stretch).
ꢀ
1H NMR (500 MHz, DMSO-d6, d, ppm): 3.89 (s, 6H, AOCH3),
7.07 (dd, J 5 9.0, 2.0 Hz, 2H, Hb), 7.50 (d, J 5 9.0 Hz, 2H, Hc),
7.85 (d, J 5 2.0 Hz, 2H, Ha), 7.91 (d, J 5 8.9 Hz, 2H, Hd), 8.46
(d, J 5 8.9 Hz, 2H, He). 13C NMR (125 MHz, DMSO-d6, d,
ppm): 55.62 (AOCH3), 103.55 (C2), 110.85 (C5), 115.38 (C4),
124.32 (C1), 125.52 (C9), 125.82 (C8), 134.16 (C6), 143.47
(C10), 144.55 (C7), 154.45 (C3).
3,6-Dimethoxy-9-(4-aminophenyl)carbazole (4)
In a 250-mL three-neck round-bottomed flask equipped with
a stirring bar, 7.0 g (0.02 mol) of compound 3 and 0.15 g of
10% Pd/C were dissolved/suspended in 160 mL of ethanol.
The suspension solution was heated to reflux under a nitro-
gen atmosphere, and 3 mL of hydrazine monohydrate was
added slowly to the mixture, then the solution was stirred at
reflux temperature. After a further 16 h of reflux, the solu-
tion was filtered to remove Pd/C, and the filtrate was cooled
under a nitrogen atmosphere to grow colorless crystals. The
products were collected by filtration and dried in vacuo at
ꢀ
ꢀ
80 C; yield 5 4.2 g (66%), mp 5 131–134 C (by DSC at 2
ꢀC min21). FTIR (KBr): 3350, 3465 cm21 (ANH2 stretch).
1H NMR (500 MHz, DMSO-d6, d, ppm): 3.87 (s, 6H,
AOCH3), 5.37 (s, 2H, ANH2), 6.79 (d, J 5 8.6 Hz, 2H, He),
7.01 (dd, J 5 9.0, 2.0 Hz, 2H, Hb), 7.15 (d, J 5 8.6 Hz,
2H, Hd), 7.16 (d, J 5 9.0 Hz, 2H, Hc), 7.77 (d, J 5 2.0 Hz,
2H, Ha). 13C NMR (125 MHz, DMSO-d6, d, ppm): 55.60
(AOCH3), 103.08 (C2), 110.29 (C5), 114.57 (C9), 115.01
(C4), 122.47 (C1), 125.12 (C7), 127.28 (C8), 136.23 (C6),
148.07 (C10), 153.19 (C3).
EXPERIMENTAL
Monomer Synthesis
According to a reported procedure,20 3,6-dibromo-9H-carba-
zole (1) was prepared from the bromination of carbazole with
N-bromosuccinimide (NBS). Following a procedure published
in literature,21 synthesis of 3,6-dimethoxycarbazole (2) was
achieved by direct methoxide displacement of bromine from 1
by using sodium methoxide (MeONa) and CuI in DMF. The
synthetic details and characterization data of compounds 1
and 2 are included in the Supporting Information.
3,6-Dimethoxy-9-(4-nitrophenyl)carbazole (3)
In a 250-mL one-neck round-bottomed flask equipped with a
stirring bar, a mixture of 9.0 g (0.04 mol) of 3,6-dimethoxy-
carbazole, 5.6 g (0.04 mol) of 4-fluoronitrobenzene, and 6.1
g (0.04 mmol) of cesium fluoride (CsF) in 40 mL of dimethyl
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4,40-Dinitro-400-(3,6-dimethoxycarbazol-9-
yl)triphenylamine (5)
In a 250-mL one-neck round-bottomed flask equipped with a
stirring bar, a mixture of 6.36 g (0.02 mol) of compound 4,
5.7 g (0.02 mol) of 4-fluoronitrobenzene, and 6.1 g (0.02
Hd), 7.76 (d, J 5 2.0 Hz, 2H, Ha). 13C NMR (125 MHz, DMSO-
d6, d, ppm) (for the peak assignments, see Fig. 1(b)]: 55.60
(AOCH3), 103.11 (C2), 110.37 (C8), 114.81 (C13), 115.03
(C4), 116.38 (C9), 122.61 (C1), 126.62 (C7), 126.76 (C5),
127.70 (C12), 135.33 (C11), 135.91 (C6), 145.98 (C14), 148.69
(C10), 153.28 (C3).
ꢀ
mol) of CsF in 70 mL of DMSO was heated at 140 C for 18
h under a nitrogen atmosphere. The mixture was poured
into 300 mL of stirred methanol slowly, and the precipitated
compound was collected by filtration and washed thoroughly
by methanol and hot water. The crude product was filtered
and recrystallized by DMF/methanol to afford 8.1ꢀg (72% in
yield) of brown needles with an mp of 298–301 C (by DSC
Polymer Synthesis
The synthesis of polyamide 8b is used as an example to
illustrate the general synthetic route. A mixture of 0.3504 g
(0.7 mmol) of the diamine monomer 6, 0.1105 g (0.7 mmol)
of 4,40-dicarboxydiphenyl ether (7b), 0.15 g of calcium chlo-
ride, 0.7 mL of triphenyl phosphite (TPP), 0.2 mL of pyri-
at a scan rate of 2 C min21). FTIR (KBr): 1309, 1580 cm21
ꢀ
ꢀ
dine, and 1.5 mL of NMP was heated with stirring at 120 C
(ANO2 stretch).
for 3 h. The obtained highly viscous polymer solution was
poured slowly into 150 mL of stirring methanol giving rise
to a stringy, fiber-like precipitate that was collected by filtra-
tion, washed thoroughly with hot water and methanol, and
dried under vacuum at 100 ꢀC. Reprecipitations from DMAc
into methanol were carried out twice for further purification.
The inherent viscosity of the obtained polyamide 8b was 0.52
dL g21, measured at a concentration of 0.5 g dL21 in DMAc–5
wt % LiCl at 30 ꢀC. The IR spectrum of 8b (film) exhibited
characteristic amide absorption bands at 3300 cm21 (NAH
stretch) and 1655 cm21 (amide carbonyl stretch).
1H NMR (500 MHz, DMSO-d6, d, ppm): 3.89 (s, 6H, AOCH3),
7.07 (dd, J 5 8.9, 2.0 Hz, 2H, Hb), 7.35 (d, J 5 9.1 Hz, 4H,
Hf), 7.42 (d, J 5 8.9 Hz, 2H, Hc), 7.50 (d, J 5 8.5 Hz, 2H, He),
7.71 (d, J 5 8.5 Hz, 2H, Hd), 7.83 (d, J 5 2.0 Hz, 2H, Ha),
8.24 (d, J 5 9.1 Hz, 4H, Hg). 13C NMR (125 MHz, DMSO-d6, d,
ppm): 55.62 (AOCH3), 103.28 (C2), 110.85 (C5), 115.26 (C4),
122.70 (C12), 123.40 (C1), 125.58 (C13), 127.50 (C8), 128.60
(C9), 132.29 (C7), 134.99 (C6), 138.02 (C10), 142.19 (C14),
151.49 (C11), 153.84 (C3).
1H NMR (500 MHz, DMSO-d6, d, ppm) (for the peak assign-
ments, see Fig. 2): 3.87 (s, 6H, AOCH3), 7.03 (d, J 5 8.9 Hz,
2H, He), 7.20 (three overlapped doublets, 10H, Hb 1 Hf 1
Hi), 7.31 (d, J 5 8.9 Hz, 2H, Hd), 7.44 (d, J 5 8.8 Hz, 2H, Hc),
7.79 (s, 6H, Ha 1 Hg), 8.05 (d, J 5 8.6 Hz, 4H, Hh), 10.27 (s,
2H, amide).
Preparation of the Polyamide Films
A solution of polymer was made by dissolving about 0.5 g of
the polyamide sample in 10 mL of hot DMAc. The homogene-
ous solution was poured into a 9-cm glass Petri dish, which
was placed in a 90 ꢀC oven for 5 h to remove most of the
solvent; then the semidried film was further dried in vacuo
4,40-Diamino-400-(3,6-dimethoxycarbazol-9-
yl)triphenylamine (6)
ꢀ
at 150 C for 8 h. The obtained films were about 30–50 lm
In a 1-L three-neck round-bottomed flask equipped with a
stirring bar and a nitrogen inlet, 2.8 g (0.005 mol) of dinitro
compound 5 and 0.1 g of 10% Pd/C were dissolved/sus-
pended in 500 mL of ethanol. The suspension solution was
heated to reflux under a nitrogen atmosphere, and 3 mL of
hydrazine monohydrate was added slowly to the mixture,
then the solution was stirred at reflux temperature. After a
further 16 h of reflux, the solution was filtered to remove
Pd/C, and the filtrate was cooled under a nitrogen atmos-
phere to grow colorless crystals. The products were collected
by filtration and dried in vacuo at 80 ꢀC; yield 5 1.5 g
(60%), mp 5 228–330 ꢀC (by DSC at 2 ꢀC min21). FTIR
(KBr): 3360, 3455 cm21 (NAH stretch).
in thickness and were used for X-ray diffraction measure-
ments, solubility tests, and thermal analyses.
Measurements
Infrared spectra were recorded on a Horiba FT-720 FTIR
spectrometer. Elemental analyses were run in a Heraeus
VarioEL-III CHNS elemental analyzer. 1H and 13C NMR spec-
tra were measured on a Bruker AVANCE-500 FT-NMR using
tetramethylsilane as the internal standard. The inherent vis-
cosities were determined at 0.5 g dL21 concentration using
ꢀ
Cannon-Fenske viscometer at 30 C. Weight-average molecu-
lar weight (Mw) and number-average molecular weight (Mn)
were obtained via gel permeation chromatography (GPC) on
the basis of polystyrene calibration using Waters 2410 as an
apparatus and NMP as the eluent. Wide-angle X-ray diffrac-
tion (WAXD)ꢀmeasurements were performed at room tempera-
ture (ca. 25 C) on a Shimadzu XRD-6000 X-ray diffractometer
(40 kV, 20 mA), using graphite-monochromatized Cu-Ka radia-
tion. Thermogravimetric analysis (TGA) was conducted with a
1H NMR (500 MHz, DMSO-d6, d, ppm) (for the peak assign-
ments, see Fig. 1(a)]: 3.86 (s, 6H, AOCH3), 5.03 (s, 4H,
ANH2), 6.59 (d, J 5 8.6 Hz, 4H, Hg), 6.76 (d, J 5 8.9 Hz, 2H,
He), 6.95 (d, J 5 8.6 Hz, 4H, Hf), 7.00 (dd, J 5 8.9, 2.0 Hz,
2H, Hb), 7.20 (d, J 5 8.8 Hz, 2H, Hc), 7.21 (d, J 5 8.9 Hz, 2H,
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FIGURE 1 (a) 1H NMR, (b) 13C NMR, (c) H-H COSY, and (d) C-H HMQC spectra of diamine monomer 6 in DMSO-d6. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
PerkinElmer Pyris 1 TGA. Experiments were carried out on
approximately 3–5 mg film samples heated in flowing nitrogen
troelectrochemistry analyses were carried out with an electro-
lytic cell, which was composed of a 1 cm cuvette, ITO as a
working electrode, a platinum wire as an auxiliary electrode,
and a Ag/AgCl reference electrode. Absorption spectra in the
spectroelectrochemical experiments were also measured with
an Agilent 8453 UV–visible diode array spectrophotometer.
or air (flow rate 5 20 cm3 min21) at a heating rate of 20 C
ꢀ
min21. DSC analyses of the polyamides were performed on a
21
ꢀ
PerkinElmer Pyris 1 DSC at a scan rate of 20 C min under
a nitrogen flow (20 cm3 min21). Thermomechanical analysis
(TMA) of the polymer film was conducted with a PerkinElmer
TMA 7 instrument. The TMA experiments were conducted
RESULTS AND DISCUSSION
21
ꢀ
ꢀ
from 50 to 350 C at a scan rate of 10 C min with a pene-
tration 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.
Absorption spectra were measured with an Agilent 8453 UV–
visible diode array spectrophotometer. Electrochemistry was
performed with a CH Instruments 611c electrochemical ana-
lyzer. 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 with the use of at an ALS RE-1B (Ag/
AgCl, 3M NaCl) reference electrode. Ferrocene was used as an
external reference for calibration (10.44 V vs. Ag/AgCl). Vol-
tammograms are presented with the positive/negative poten-
tial pointing to the right/left with increasing anodic/
decreasing cathodic current pointing upward/downward. Spec-
Monomer Synthesis
The new aromatic diamine monomer, 4,40-diamino-400-(3,6-
dimethoxycarbazol-9-yl)triphenylamine (6), was synthesized
starting from carbazole by a reaction sequence as shown in
Scheme 1. 3,6-Dibromocarbazole (1) was synthesized by bro-
mination of carbazole with NBS. Direct methoxide displace-
ment of bromine from compound 1 by using MeONa and CuI
in DMF gave 3,6-dimethoxycarbazole (2). The intermediate
compound, 3,6-dimethoxy-9-(4-nitrophenyl)carbazole (3), was
synthesized by nucleophilic aromatic fluoro-displacement reac-
tion of p-fluoronitrobenzene with compound 2 in the presence
of CsF. Reduction of the nitro group of compound 3 by means
of hydrazine and Pd/C gave 3,6-dimethoxy-9-(4-aminophenyl)-
carbazole (4). The target diamine monomer 6 was prepared
by hydrazine Pd/C-catalyzed reduction of 4,40-dinitro-400-(3,6-
dimethoxy-carbazol-9-yl)triphenylamine (5) resulting from the
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FIGURE 2 (a) IR, (b) 1H NMR, and (c) aromatic portion of the H-H COSY spectrum of polyamide 8b in DMSO-d6. [Color figure can
be viewed in the online issue, which is available at wileyonlinelibrary.com.]
CsF-assisted N,N-diarylation reaction of compound 4 with two
equivalent amount of p-fluoronitrobenzene. The structures of
all the synthesized compounds were confirmed by IR and
NMR analyses. The FTIR spectra of compounds 1–6 are shown
in Supporting Information Figure S1. The IR spectrum of com-
pound 1 shows the secondary NAH stretching absorption at
3423 cm21. After methoxylation, the IR spectrum of dime-
thoxy compound 2 gave rise to a symmetric ACH3 stretching
absorption at about 2830 cm21 and an asymmetric ACH3
stretching absorption at about 2940 cm21. The nitro groups
of compounds 3 and 5 gave two characteristic bands at
around 1580–1590 and 1300–1330 cm21, respectively (ANO2
asymmetric and symmetric stretching). After reduction to 4
and 6, the characteristic absorptions of the nitro group disap-
peared and the primary amino group showed the typical
absorption pair in the range 3400–3200 cm21 due to NAH
stretching. The 1H and 13C NMR spectra of the intermediate
compounds 1 to 5 are also shown in Supporting Information
SCHEME 1 Synthetic pathway of the target diamine monomer 6.
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SCHEME 2 Synthesis of aromatic polyamides 8a–8d. Photographs show the appearances of their solution-cast films. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Figures S2–S6. 1H NMR, 13C NMR, and two-dimensional (2D)
pyridine as condensing agents (Scheme 2). All the polymer-
izations proceeded homogeneously throughout the reaction
and afforded clear and highly viscous polymer solutions, and
the products precipitated in a tough, fiber-like form when
the resulting polymer solutions were slowly poured into stir-
ring methanol. As shown in Table 1, the obtained polyamides
had inherent viscosities in the range of 0.52–0.57 dL g21. All
the polyamides could be solution-cast into flexible and tough
films (see the photos shown in Scheme 2), indicating high
molecular weight polymers. The GPC measurement of these
polyamides showed weight-average molecular weights (Mw)
in the range of 43,000–57,000 and polydispersity index
(Mw/Mn) of 2.46–2.49. The formation of polyamides was also
confirmed by IR and NMR spectroscopy. Typical IR and NMR
spectra for polyamide 8b are given in Figure 2. The charac-
teristic IR absorption bands of the amide group appeared
NMR spectra of the target diamine monomer 6 are compiled
1
in Figure 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 sig-
nals at around 5.0 ppm corresponding to the amino protons.
Assignments of each carbon and proton assisted by the 2D
NMR spectra are also indicated in these spectra, and they
agree well with the proposed molecular structure of 6.
Polymer Synthesis
According to the phosphorylation technique first described
by Yamazaki et al.,22 a series of novel aromatic polyamides
8a–8d with (3,6-dimethoxycarbazol-9-yl)-substituted TPA
units were synthesized from the diamine monomer 6 with
various aromatic dicarboxylic acids 7a–7d using TPP and
TABLE 1 Inherent Viscosity, Molecular Weights and Solubility Behavior of Polyamides
a
Polymer Code
ginh (dL g21
)
GPC Datab
Solubilityd in Various Solventse
Mn
Mw
PDIc
NMP
DMAc
DMF
DMSO
m-Cresol
THF
8a
8b
8c
8d
0.57
0.52
0.54
0.56
17,500
17,500
23,000
20,500
43,000
43,500
57,000
50,500
2.46
2.49
2.48
2.46
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
1h
12
1h
1h
12
12
12
11
a
d
Inherent viscosity measured at a concentration of 0.5 dL g21 in DMAc
Solubility: 11: soluble at room temperature; 12: partially soluble;
– 5 wt % LiCl at 30 ꢀC.
b
1h: soluble on heating.
e
Calibrated with polystyrene standards, using NMP as the eluent at a
Solvent: NMP, N-methyl-2-pyrrolidone; DMAc, N,N-dimethylaceta-
constant flow rate of 0.5 mL min21 at 40 ꢀC.
c
mide; DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; THF,
tetrahydrofuran.
Polydispersity Index (Mw/Mn).
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TABLE 2 Thermal Properties of Polyamides
b
c
d
Polymera Code
Tg (ꢀC)
Ts (ꢀC)
Td at 10 wt % Loss (ꢀC)
Char Yielde (%)
In N2
In Air
8a
8b
8c
8d
296
277
308
296
295
277
306
295
507
512
492
508
496
504
480
507
70
72
66
72
a
c
The polymer film samples were heated at 300 ꢀC for 1 h prior to all
Softening temperature measured by TMA using a penetration method.
Decomposition temperature at which a 10% weight loss was recorded
d
the thermal analyses.
b
The samples were heated from 50 to 400 ꢀC at a scan rate of 20 ꢀC
by TGA at a heating rate of 20 ꢀC min21
e
.
min21 followed by rapid cooling to 50 ꢀC at –200 ꢀ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.
Residual weight percentages at 800 ꢀC under nitrogen flow.
around 3300 (NAH stretching) and 1655 cm21 (amide car-
bonyl). With the help of 2D NMR spectroscopy, all the peaks
could be readily assigned to the hydrogen atoms in the
repeating unit. The resonance peak appearing at 10.27 ppm
in the 1H NMR spectrum also supports the formation of
amide linkages.
bility in less polar solvents like THF because of the additional
contribution of the hexafluoroisopropylidene (AC(CF3)2A)
fragment in the polymer backbone. The high solubility of
these polyamides can be attributed in part to the introduction
of bulky, packing-disruptive 3,6-dimethoxy-CzTPA units in the
polymer structure. Thus, the excellent solubility makes these
polymers potential candidates for practical applications by
simple spin- or dip-coating processes to afford high perform-
ance thin films for optoelectronic devices. As it was men-
tioned earlier, all the polyamides could be solution-cast into
transparent, flexible, and strong films. The WAXD studies of
these film samples indicated that all the polymers were essen-
tially amorphous (Supporting Information Fig. S7).
Polymer Properties
Basic Characterization
The solubility behavior of polyamides was tested qualitatively,
and the results are also listed in Table 1. All the polyamides
were readily soluble in dipolar organic solvents such as NMP,
DMAc, DMF, and DMSO. Polyamide 8d also showed good solu-
TABLE 3 Redox Potentials and Energy Levels of Polyamides
Absorptiona
kmax (nm) konset (nm)
Oxidationb (V)
Eg (eV)
HOMOd (eV)
LUMOd (eV)
c
Polymer Code
OX1
E
OX2
Eonset
E
1=2
1=2
8a
8b
8c
312
312
313
312
343
429
400
466
406
400
0.64
0.65
0.65
0.66
0.75
0.76
0.76
0.77
0.78
0.90
1.04
1.04
1.05
1.06
1.27
2.89
3.10
2.72
3.05
3.10
5.08
5.09
5.09
5.10
5.19
2.20
1.99
2.37
2.05
2.09
8d
80d
a
Measured as thin films.
b
From cyclic votammograms versus Ag/AgCl in CH3CN. E1/2: Average
potential of the redox couple peaks.
c
The data were calculated from polymer films by the equation: Eg
5
1240/konset (energy gap between HOMO and LUMO).
d
The HOMO energy levels were calculated from cyclic voltammetry
and were referenced to ferrocene (4.8 eV; onset 5 0.36 V); LUMO 5
HOMO - Eg.
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Thermal Properties
The thermal properties of all the polyamides were investi-
gated by TGA, DSC, and TMA techniques. The pertinent data
are summarized in Table 2. Typical TGA curves of a repre-
sentative polyamide 8d in both air and nitrogen atmospheres
are illustrated in Supporting Information Figure S8. All the
polymers exhibited good thermal stability with insignificant
weight loss up to 480 ꢀC in both air and nitrogen atmos-
pheres. The decomposition temperatures (Td) at a 10%
weight-loss of the polyamides in nitrogen and air were
recorded in the range of 492–512 and 480–507 ꢀC, respec-
tively. The mount of carbonized residue (char yield) of these
polymers in nitrogen atmosphere was more than 66% at
800 ꢀC. The high char yields of these polymers can be
ascribed to their high aromatic content. The glass-transition
temperatures (Tg) of these polyamides were observed in the
ꢀ
range of 277–308 C by DSC. Polyamide 8b showed the low-
ꢀ
est Tg (277 C) among this series polyamides because of the
presence of flexible ether linkage in its diacid component.
The softening temperatures (Ts) of the polymer films were
determined from the onset temperature of the probe dis-
placement on the TMA trace. In most cases, the Ts values of
the polyamides obtained by TMA are comparable to the Tg
values measured by the DSC experiments. Overall, the ther-
mal analysis results reveal that these polyamides exhibit
good thermal stability.
Electrochemical Properties
The electrochemical behavior of the polyamides was investi-
gated by CV conducted for the cast film on an ITO-coated
glass substrate as working electrode in dry acetonitrile
(CH3CN) containing 0.1 M of tetrabutylammonium perchlo-
rate (Bu4NClO4) as an electrolyte under nitrogen atmos-
phere. The derived oxidation potentials are summarized in
Table 3. The representative cyclic voltammograms of polya-
mides 8d (with methoxy substituents on the active sites of
the carbazole unit) and 80d (without methoxy groups on the
carbazole unit) are illustrated in Figure 3 for comparison.
Polyamide 8d exhibits two reversible oxidation redox cou-
ples with half-wave potentials (E1/2) of 0.76 and 1.03 V dur-
ing the oxidative scan, corresponding to successive one
electron removal from the TPA and carbazole moieties. Poly-
amide 8d exhibited excellent redox reversibility when
repeatedly scanning between 0.0 and 1.3 V at a scan rate of
50 mV s21. The high electrochemical stability of polyamide
8d can be attributable to the fact that the active sites (the C-
3 and C-6 positions) of the carbazole are blocked with
methoxy groups. The referenced polyamide 80d shows higher
oxidation potentials as compared to polyamide 8d, with E1/2
values of 0.90 and 1.27 V in the first CV scan. Without block-
ing the electrochemically active sites of the carbazole, the
oxidation process of 80d is not reversible. As shown in Fig-
ure 3(b), two oxidation peaks were observed at about 1.02
and 1.52 V in the first positive potential scan of 80d. From
the first reverse negative potential scan, we detected three
cathodic peaks at 1.28, 1.01, and 0.77 V. In the second scan,
two new oxidation peaks appeared at about 1.25 and 1.42 V,
which was the complementary anodic process of the
FIGURE 3 Repeated CV diagrams of the cast films of polya-
mides (a) 8d and (b) 80d on the ITO-coated glass substrate in
0.1 M Bu4NClO4/CH3CN at a scan rate of 50 mV s21. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
cathodic peak at 1.01 and 1.28 V. The observation of two
new oxidation couples in the second scan indicates that the
carbazole radical cations were involved in very fast electro-
chemical reactions that produced a new structure that was
easier to oxidize than was the parent carbazole. As reported
by Ambrose et al. in their pioneering works23 devoted to
anodic oxidation of carbazole and other N-substituted deriva-
tives, ring-ring coupling is the predominant decay pathway.
One possible coupling reaction of carbazolium radical cations
to biscarbazole shown in Scheme 3 can be used to explain
the irreversible oxidation process occurring in polyamide
80d. Thus, in the second CV curve of 80d, the second anodic
peak corresponds to one-electron oxidation of the biscarba-
zole units to form radical cations, followed by oxidation to
the dicationic species. Thus, the introduction of electron-
donating methoxy group not only greatly prevents the cou-
pling reaction but also slightly lowers the oxidation poten-
tials of the present polyamides.
The HOMO (highest occupied molecular orbital) energy lev-
els of the investigated polyamides were calculated from the
oxidation onset potentials (Eonset) and by comparison with
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SCHEME 3 The anodic oxidation pathways of polyamides 8d and 80d. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
ferrocene (4.8 eV; onset 5 0.36 V). These data together with
absorption spectra were then used to obtain the LUMO (low-
est unoccupied molecular orbital) energy levels (Table 3).
According to the HOMO and LUMO energy levels obtained,
these polyamides appear to be appropriate as hole injection
and transport materials.
glass substrates and mounted in a spectroelectrochemical
cell. Figure 4(a) presents the UV–vis-NIR absorption spectra
of polyamide 8d film at various applied potentials. In the
neutral form, polyamide 8d exhibited strong absorptions at
312 and 343 nm, characteristic for p–p* transitions, but it
was almost transparent in the visible and NIR regions. The
band gap of polyamide 8d was estimated to be 3.10 eV from
the onset of the p–p* transition at 400 nm. When the
applied potential was gradually increased to 0.9 V, the
absorption of p–p* transition decreased while a new absorp-
tion peak at 424 nm grew up and a broad absorption cen-
tered at 771 nm in the visible region together with a broad
band from 900 nm extending into the NIR region beyond
Spectroelectrochemistry
Following the electrochemical tests, the optical properties of
the electrochromic films were evaluated by using spectroe-
lectrochemistry at different applied potentials. The electrode
preparations and solution conditions were identical to those
used in CV. The polymers were drop-coated as films on ITO-
280
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FIGURE 4 Spectroelectrochemistry of the cast films of polyamides (a) 8d and (b) 80d on the ITO-coated glass substrate in 0.1 M
Bu4NClO4/CH3CN at various applied potentials (vs. Ag/AgCl); Optical change in %T as a function of applied potential for the indi-
cated absorption wavelengths for polyamides (c) 8d and (d) 80d. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
1100 nm gradually increased in intensity. As the potentials
examined are similar to the first anodic process, the spectral
changes are assigned to the radical cation (polaron) forma-
tion arising from the oxidation of TPA unit. The absorption
band in the NIR region may be attributed to an intervalence
charge transfer between states in which the positive charge
is centered at different amino centers (TPA and carbazole).24
Upon further oxidation at applied voltages to 1.3 V, the
intensity of the absorption peak at 424 nm gradually
decreased while the absorption peak at 800 nm gradually
increased in intensity with a formation of a new strong
absorption band centered at about 922 nm. The observed
electronic absorption changes in the film of 8d at various
potentials are fully reversible and are associated with strong
color changes; indeed, they even can be seen readily by the
naked eye. The film of polyamide 8d switches from a trans-
missive neutral state (nearly colorless) to a highly absorbing
semioxidized state (yellowish green) and a fully oxidized
state (blue). For comparison, the spectroelectrochemical
series of polyamide 80d are shown in Figure 4(b). The
change of % transmittance for the absorption maxima of pol-
yamides 8d and 80d at various applied electrode potentials
are depicted in Figure 4(c,d), respectively. It can be seen that
polymer 8d revealed a higher optical contrast (83%) in the
NIR regions for blue coloring at the second oxidation stage
than the referenced polymer 80d (65%) without the methoxy
substituents on the carbazole unit at 800 nm when the
applied potential was set at 1.3–1.4 V. This result may be
explained by the irreversible second oxidation process asso-
ciated with polyamide 80d. Therefore, the introduction of the
methoxy group at the active sites of the carbazole unit not
only enhances the redox stability but also improve the elec-
trochromic contrast of these polymers.
As described earlier, the original structure of 80d might
transform to a partially crosslinking structure via the oxida-
tive coupling of the carbazoles (Scheme 3). In order to
obtain a partially crosslinking film of polyamide 80d, the film
of 80d on ITO-glass was repeatedly scanned between 0.0 and
1.5
V
at 50 mV s21 for 10 cycles. After that, the
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FIGURE 5 (a) Spectral changes the crosslinking film of polyamide 80d on the ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN
at various applied potentials. (b) The CV diagram for the first and second scans. (c) The possible electro-oxidation order for the
amino centers in the repeating unit of crosslinked polyamide 80d. (d) The color changes of the polymer films at indicated applied
electrode potentials. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 6 Potential step absorptometry of the cast films of polyamides 8d and 80d 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: (left) optical switching for polyamide 8d at
potential 0.0 V ( 1.0 V and cycle time 10 s, monitored at kmax 5 424 and 771 nm; (right) 80d at potential 0.0 V ( 1.1 V and cycle time
10 s, monitored at kmax
5 414 and 800 nm. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
282
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spectroelectrochemical and electrochromic properties of the
polymer film were investigated. After the first CV scan
between 0.0 and 1.5 V, polyamide 80d displayed three peaks
at around 0.96, 1.25, and 1.42 V in the subsequent CV scans
[Fig. 5(b)]. The spectroelectrochemical behavior of the cross-
linking polyamide 80d film is shown in Figure 5(a). When
the applied potentials increased positively from 0 to 0.96,
1.25, and 1.42 V, respectively, corresponding to the first, sec-
ond, and third electron oxidation, the characteristic absorp-
tion bands at 298 and 337 nm for neutral-form polyamide
80d decreased gradually and bathochromically shifted to 410
nm, accompanied by the concomitant formation of new
broad long-wavelength absorption bands centered around
794, 538, and 760 nm, respectively. The film showed a mul-
ticolored electrochromism from colorless neutral state to
pale green, purple, and orange oxidized states [Fig. 5(d)].
The possible oxidation order of the redox centers of polyam-
ide 80d is proposed in Figure 5(c).
Electrochromic Switching Studies
For the electrochromic switching studies, polymer films were
cast on ITO-coated glass slides in the same manner as
described above, and chronoamperometric and absorbance
measurements were performed. While the films were
switched, the absorbance at selected wavelengths was moni-
tored as a function of time with UV–vis-NIR spectroscopy.
Figure 6 depicts the optical transmittance at 424 and 771
nm as a function of time by applying square-wave potential
steps between 0 and 1.0 V for a resident time of 10 s for
polyamide 8d and at 414 and 800 nm between 0 and 1.1 V
for a resident time of 12 s for the referenced polyamide 80d.
The response time was calculated at 90% of the full-
transmittance change, because it is difficult to perceive any
further color change with naked eye beyond this point. As
shown in Figure 6, polyamide 8d attained 90% of a complete
coloring and bleaching in 3.0 and 1.0 s, respectively. The
optical contrast measured as DT% of polyamide 8d between
FIGURE 7 Potential step absorptometry of the cast films of polyamides 8d and 80d 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: (left) optical switching for polyamide 8d at
potential 0.0 V ( 1.3 V and cycle time 13 s, monitored at kmax 5 800 and 922 nm; (right) 80d at potential 0.0 V ( 1.4 V and cycle time
14 s, monitored at kmax
5 800 and 572 nm. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 4 Electrochromic Properties of Polyamides 8d and 80d
a
c
b
c
Doping
E
Pulse Width
(s)
kmax
DT
t0.9
t0.9
Qd (mC
gd (cm2
C21
Neutral
State
Oxidized
State
Polymer
Processes
(v)
(nm)
(%)
(s)
(s) DODb
cm22
)
)
8d
First p-doping 1.0
Second p-doping 1.3
10
13
424
771
800
922
67
55
83
79
3.0
5.5
3.2
4.0
1.0 0.592
1.0 0.347
1.0 0.841
1.0 0.850
3.01
197
115
215
217
3.91
80d
First p-doping 1.1
Second p-doping 1.4
10
14
414
800
800
572
69
65
65
42
3.6
5.0
3.0
2.2 0.650
2.3 0.456
5.0 0.456
3.61
9.21
184
129
50
10.7 6.2 0.237
26
a
c
Wavelength of absorption maximum.
Optical density (DOD) 5 log[Tbleached/Tcolored], where Tbleached and Tcol-
Qd is ejected charge, determined from the in situ experiments.
Coloration efficiency g 5 DOD/Qd.
b
d
are the maximum transmittance in the neutral and oxidized state,
ored
respectively.
neutral colorless and oxidized green states was found to be
67% at 424 nm. The referenced polyamide 80d attained
90% of a complete coloring and bleaching in 3.6 and 2.2 s,
respectively. The optical contrast measured as DT% of poly-
amide 80d between neutral colorless and oxidized green
states was found to be 69% at 800 nm. The amount of
injected/ejected charge (Qr) were calculated by integration
of the current density and time obtained from Figure 6 as
3.01 and 2.96 mC cm22 for oxidation and reduction proc-
esses of polyamide 8d at the first oxidation stage, respec-
tively. The ratio of the charge density was 98.3%,
indicating that charge injected/ejected was highly reversible
during the electrochemical reactions. While the switched
potential was adjusted to the second oxidation stage from
0.0 to 1.3 V, the polyamide 8d thin film required almost
the same times for coloration and bleaching with 93.3% of
the ratio of the charge density (Fig. 7). The electrochromic
CE (g 5 dOD/Q) and injected charge (electroactivity) after
various switching steps were monitored and summarized in
Table 4. After continuous 100 cyclic scanning at the first
oxidation stage, the film of polyamide 8d only showed
5.0% decay on coloration efficiency, which is much lower
than that of corresponding 80d analog. As the applied
switching potential increased to the second oxidation stage,
the polyamide 8d still showed little optical contrast loss
during switching 30 cycles and a higher CE for the blue
coloring (215 cm2 C21 at 922 nm) (Table 5). However, the
80d film showed a moderate optical contrast loss at 800
nm after 10 full switches (Fig. 7). Therefore, the introduc-
tion of the methoxy group at the electrochemically active C-
3 and C-6 sites of the carbazole unit enhances the redox
and electrochromic stability of these polymers.
TABLE 5 Optical and Electrochemical Data Collected for Coloration Efficiency Measurements of Polyamide 8d and 80d
Cyclea
DT (%)
DODb
Q (mC cm22 c
)
g (cm2 C21 d
)
Decay(%)e
80d
8d
80d
8d
80d
8d
80d
8d
80d
8d
1
67
67
67
66
66
65
65
64
64
64
63
69
68
68
67
66
65
65
64
64
64
63
0.592
0.592
0.590
0.586
0.582
0.580
0.576
0.570
0.562
0.558
0.555
0.650
0.642
0.640
0.631
0.621
0.609
0.597
0.583
0.575
0.565
0.553
3.01
3.02
3.01
3.00
2.99
2.99
2.99
2.98
2.97
2.97
2.96
3.61
3.61
3.60
3.59
3.58
3.58
3.56
3.54
3.53
3.53
3.50
197
196
196
195
194
194
193
191
189
188
187
184
178
177
176
174
170
167
164
163
160
158
0
0
10
20
30
40
50
60
70
80
90
100
0.5
0.5
1.0
1.5
1.5
2.0
3.0
4.0
4.6
5.0
3.3
3.8
4.3
5.4
7.6
9.2
10.9
11.4
12.5
13.6
a
c
Switching between 0 and 1.0 V for 8d and 0 and 1.1 V for 80d (vs. Ag/
Ejected charge, determined from the in situ experiments.
Coloration efficiency is derived from the equation: g 5 DOD/Q.
Decay of optical density change after cyclic scans.
d
e
AgCl).
b
Optical density change at 424 nm for 8d and 414 nm for 80d.
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8 (a) C.-G. Wu, M.-I. Lu, S.-J. Chang, C.-S. Wei, Adv. Funct.
Mater. 2007, 17, 1063–1070; (b) F. S. Han, M. Higuchi, D. G.
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CONCLUSIONS
A
series of novel carbazole and triphenylamine-
functionalized aromatic polyamides were readily prepared
from the newly synthesized aromatic diamine monomer 4,40-
diamino-400-(3,6-dimethoxycarbazol-9-yl)triphenylamine with
various aromatic dicarboxylic acids via the phosphorylation
polyamidation reaction. Because of the introduction of three-
dimensional triphenylamine units and bulky 3,6-dimethoxy-
carbazole pendent groups in polymer backbone, all the
polymers were amorphous, had good solubility in many polar
aprotic solvents, and exhibited excellent film-forming ability.
All the obtained polyamides revealed good electrochemical
and electrochromic stability along with multielectrochromic
behavior and enhanced NIR absorption upon oxidation. By
substitution of the electrochemically active C-3 and C-6 sites
of the carbazole unit with electron-donating methoxy groups,
the new polyamides exhibit greatly enhanced electrochemical
stability and electrochromic performance in comparison with
previously reported analogs without methoxy substituents on
the carbazole moiety. Such prominent features make these
processable polymers amenable for optoelectronic applica-
tions such as OLEDs and electrochromic devices.
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ACKNOWLEDGMENT
The authors thank the National Science Council of Taiwan
for the financial support.
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