Synthesis and properties of new aromatic polyamides with redox-active 2,4-dimethoxytriphenylamine moieties

Article abstract of DOI:10.1002/pola.24124

A new triphenylamine-based diamine monomer, 4,4′-diamino-2″, 4″-dimethoxytriphenylamine (2), was synthesized from readily available reagents and was reacted with various aromatic dicarboxylic acids to produce a series of aromatic polyamides (4a-h) contain

Full text of DOI:10.1002/pola.24124

Synthesis and Properties of New Aromatic Polyamides with Redox-Active
2,4-Dimethoxytriphenylamine Moieties
SHENG-HUEI HSIAO,1 GUEY-SHENG LIOU,2 YI-CHUN KUNG,3 TZU-JUNG HSIUNG3
1Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10618, Taiwan
2Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
3Department of Chemical Engineering, Tatung University, Taipei 10451, Taiwan
Received 14 April 2010; accepted 8 May 2010
DOI: 10.1002/pola.24124
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A new triphenylamine-based diamine monomer,
4,40-diamino-200,400-dimethoxytriphenylamine (2), was synthesized
from readily available reagents and was reacted with various aro-
matic dicarboxylic acids to produce a series of aromatic poly-
amides (4a–h) containing the redox-active 2,4-dimethoxy-
substituted triphenylamine (dimethoxyTPA) unit. All the resulting
polyamides were readily soluble in polar organic solvents and
could be solution cast into tough and flexible films. These poly-
mers exhibited good thermal stability with glass transition temper-
atures of 243–289 ꢀC and softening temperatures of 238–280 ꢀC,
10% weight loss temperatures in excess of 470 ꢀC in nitrogen, and
char yields higher than 60% at 800 ꢀC in nitrogen. The redox behav-
iors of the polymers were examined using cyclic voltammetry
(CV). All these polyamides showed two reversible oxidation pro-
cesses in the first CV scan. The polymers also displayed low ioniza-
tion potentials as a result of their dimethoxyTPA moieties. In
addition, the polymers displayed excellent stability of electrochro-
mic characteristics with coloration change from a colorless neutral
state to green and blue-purple oxidized states. These anodically
coloring polyamides showed high green coloration efficiency (CE
¼ 329 cm2/C), high contrast of optical transmittance change
C
V
(DT% ¼ 84% at 829 nm), and long-term redox reversibility. 2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3392–
3401, 2010
KEYWORDS: cyclic voltammetry; electrochemistry; electrochrom-
ism; functionalization of polymers; polyamides; redox polymers;
spectroelectrochemistry; triphenylamine
INTRODUCTION Electrochromism consists in the formation of
new optical transitions in an electroactive surface film or an
electroactive solute as the result of electrochemical oxidation
or reduction.1 This is an intriguing phenomenon for which
there might be a wide range of applications, including optical
switching devices, smart windows, memory elements, large-
area information panels, electronic papers (e-papers), and cha-
meleon materials.2–9 The earliest electrochromic materials in
the visible region were the inorganic compounds such as tung-
sten trioxide (WO3), iridium dioxide (IrO2), and nickel oxides
(NiO, Ni2O3).10,11 Later, organic materials (i.e., viologens, metal-
lophthalocyanines, and conjugated polymers) have received
more attention than inorganics for electrochromic applications
because of the different colors observed with these compounds
while switching among their redox states.12–15 Among the avail-
able organic electrochromic materials, p-conjugated polymers
represented by polypyrrole and poly(3,4-ethylenedioxythio-
phene) (PEDOT) and their derivatives have received a great
deal of attention because of their ease of processability, fast
switching ability, high coloration efficiency and optical contrast
ratio, long-term redox stability, and wide range of colors.16–18
Aromatic polyamides are well-known high-performance poly-
mers with excellent mechanical and thermal properties.19
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.
These properties make them generally intractable or difficult
to process, thus restricting their wide-spread applications. To
overcome such a difficulty, polymer structure modification
becomes necessary. One of the common approaches for
increasing solubility and processability of aromatic polya-
mides without sacrificing high thermal stability is the intro-
duction of bulky, packing-disruptive groups into the polymer
backbone.20,21 We have demonstrated that aromatic polya-
mides containing bulky, propeller-shaped triphenylamine
(TPA) unit are amorphous, have good solubility in organic
solvents, and exhibit high mechanical property and thermal
stability.22 In recent years, TPA-based high-performance poly-
mers such as polyamides and polyimides with attractive
electrochromic properties have been reported in this study
and in the study by Liou et al.23 The electrochromic function
of these polymers originates from the electroactive TPA
Additional Supporting Information may be found in the online version of this article. Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
C
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3392–3401 (2010) 2010 Wiley Periodicals, Inc.
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167–169 C (by DSC, 10 C/min). IR (KBr): 1579 cmꢁ1, 1338
ꢀ
ꢀ
moieties, which can reversibly be oxidized as long as the
para-position of the phenyl rings is protected.
cmꢁ1 (ANO2 stretch). H-NMR (500 MHz, DMSO-d6, d, ppm)
1
(for the peak assignments, see Supporting Information Fig.
S2): 8.15 (d, J ¼ 9.2 Hz, 4H, Ha), 7.23 (d, J ¼ 8.6 Hz, 1H, Hc),
7.13 (d, J ¼ 9.2 Hz, 4H, Hb), 6.79 (d, J ¼ 2.6 Hz, 1H, He), 6.67
(dd, J ¼ 8.6, 2.6 Hz, 1H, Hd), 3.83 (s, 3H, AOCH3), 3.67 (s, 3H,
AOCH3). 13C-NMR (125 MHz, DMSO-d6, d, ppm) (for the peak
assignments, see Supporting Information Fig. S2): 160.44 (C8),
156.57 (C10), 151.41 (C4), 141.42 (C1), 130.90 (C6), 125.33
(C2), 124.53 (C5), 120.56 (C3), 106.42 (C7), 100.4 (C9), 55.89
(C11), 55.49 (C12).
TPAs with substituents in their para-position generally show
reversible one-electron oxidation behavior.24 As demonstrated
previously,23(e) polyamides on the basis of 4,40-diamino-400-
methoxytriphenylamine exhibit excellent electrochemical and
electrochromic stability, together with high coloration effi-
ciency, high optical contrast, and fast switching speed. Inspired
by the encouraging results, herein we synthesize 4,40-diamino-
200,400-dimethoxytriphenylamine (2) and its derived polya-
mides containing redox-active TPA units with electron-donat-
ing methoxy groups para- and ortho-substituted on the pend-
ent phenyl ring. The polyamides described in this work exhibit
comparable electrochemical and electrochromic stability in
comparison with that of the corresponding polyamides with
only one methoxy group para-substituted on the pendent phe-
nyl ring of the TPA unit.23(e) Additionally, it is interesting to
emphasize that the additional substitution of ortho methoxy
group on the TPA unit gives the resulting polyamides another
stable oxidized state; thus, two differently colored redox states
of the polymers can be accessed through electrochemical
oxidation.
Synthesis of 4,40-Diamino-200,400-dimethoxytriphenylamine (2)
In a 500-mL round-bottom flask, a mixture of 10 g (0.03
mol) of dinitro compound 1, 0.15 g of 10 wt % Pd/C, 10 mL
of hydrazine monohydrate, and 150 mL of ethanol was
heated at a reflux temperature for about 10 h. The resultant
solution was filtered to remove the catalyst, and the filtrate
was concentrated by rotary evaporation. The precipitate was
collected and dried in vacuum to give 7.92 g (yield 62%) of
pure 4,40-diamino-200,400-dimethoxytriphenylamine (2) as
pale-gray crystals; mp ¼ 131–132 ꢀC (by DSC, 10 ꢀC/min).
IR (KBr): 3219–3407 cmꢁ1 (NAH stretch). ANAL. Calcd for
EXPERIMENTAL
C
20H21N3O2 (335.40): C, 71.62%; H, 6.31%; N, 12.53%.
Found: C, 71.60%; H, 6.28%; N, 12.42%. 1H-NMR (500 MHz,
Materials
d, ppm, DMSO-d6) (for the peak assignments, see Fig. 1):
2,4-Dimethoxyaniline (Acros), 4-fluoronitrobenzene (Acros),
hydrazine monohydrate (TCI), 10% palladium on charcoal
(Pd/C) (Fluka), and cesium fluoride (CsF; Acros) were used
as received. The aromatic dicarboxylic acids that include ter-
ephthalic acid (3a) (TCI), isophthalic acid (3b) (TCI), 4,40-
biphenyldicarboxylic acid (3c) (TCI), 4,40-dicarboxydiphenyl
ether (3d) (TCI), 4,40-sulfonyldibenzoic acid (3e) (New Japan
Chemical Co.), 2,2-bis(4-carboxyphenyl)-1,1,1,3,3,3-hexafluor-
opropane (3f) (TCI), 1,4-napthalenedicarboxylic acid (3g)
(Wako), and 2,6-napthalenedicarboxylic acid (3h) (TCI) were
used as received. Triphenyl phosphite (TPP; Acros) was puri-
fied by distillation under reduced pressure. Commercially
obtained anhydrous calcium chloride (CaCl2) was dried
ꢀ
under vacuum at 180 C for 8 h before use. Tetrabutylammo-
nium perchlorate (TBAP; TCI) was recrystallized twice by
ethyl acetate under nitrogen atmosphere and then dried
in vacuo before use. Dimethyl sulfoxide (DMSO; Tedia), N,N-
dimethylacetamide (DMAc; Tedia), N,N-dimethylformamide
(DMF; Tedia), pyridine (Py; Wako), and N-methyl-2-pyrroli-
done (NMP; Fluka) were dried over calcium hydride for
24 h, distilled under reduced pressure, and stored over 4 Å
molecular sieves in a sealed bottle.
Monomer Synthesis
Synthesis of 4,40-Dinitro-200,400-dimethoxytriphenylamine (1)
2,4-Dimethoxyaniline (15.32 g, 0.10 mol) and cesium fluoride
(30.38 g, 0.20 mol) were dissolved in dimethyl sulfoxide
(DMSO, 100 mL) in a 500-mL round-bottomed flask. Then,
4-fluoronitrobenzene (28.3 g, 0.20 mol) was added, and the
ꢀ
reaction solution was heated at 120 C for 18 h. The mixture
was allowed to cool and poured into 1 L of MeOH/water (2:1)
to precipitate orange-yellow crystals. The product was col-
lected and dried to afford 31.2 g (yield 76%) of pure 1; mp ¼
FIGURE 1 (a) 1H-NMR and (b) 13C-NMR spectra of the diamine
monomer 2 in DMSO-d6. [Color figure can be viewed in the
online issue, which is available at www.interscience.wiley.com.]
NEW REDOX-ACTIVE POLYAMIDES, HSIAO ET AL.
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6.90 (d, J ¼ 8.6 Hz, 1H, Hc), 6.58 (d, J ¼ 2.7 Hz, 1H, He), 6.51
(d, J ¼ 8.7 Hz, 4H, Hb), 6.47 (dd, J ¼ 8.7, 2.7 Hz, 1H, Hd),
6.41 (d, J ¼ 8.7 Hz, 4H, Ha), 4.59 (s, 4H, ANH2), 3.74 (s, 3H,
AOCH3), 3.60 (s, 3H, AOCH3). 13C-NMR (125 MHz, d, ppm,
DMSO-d6) (for the peak assignments, see Fig. 1): 157.34
(C8), 156.31 (C10), 142.34 (C1), 138.6 (C4), 129.61 (C6),
129.53 (C5), 121.82 (C3), 114.55 (C2), 105.32 (C7), 100.28
(C9), 55.52 (C11), 55.14 (C12).
Synthesis of Polyamides
The synthesis of polyamide 4a is used as an example to
illustrate the general synthetic route used to produce the
polyamides. A 50-mL round-bottom flask equipped with a
magnetic stirrer was charged with 0.5031 g (1.50 mmol) of
diamine momomer 2, 0.2492 g (1.50 mmol) of terephthalic
acid (3a), 1.5 mL of TPP, 2.5 mL of NMP, 0.8 mL of pyridine,
and 0.15 g of calcium chloride (CaCl2). The reaction mixture
was heated with stirring at 110 ꢀC for 3 h. The resulting
polymer solution was poured slowly into 200 mL of stirring
methanol giving rise to a stringy, fiber-like precipitate that
was collected by filtration, washed thoroughly with hot
water and methanol, and dried. IR (film): 3298 cmꢁ1 (amide
NAH stretch), 1653 cmꢁ1 (amide C¼¼O stretch).
SCHEME 1 Synthetic route to the dimethoxyTPA-based dia-
mine monomer 2.
temperatures of probe displacement on the TMA traces.
Ultraviolet–visible (UV–vis) spectra of the polymer films
were recorded on a Jasco UV/VIS V530 spectrometer. Photo-
luminescence (PL) spectra were obtained on a Varian Cary
Eclipse fluorescence spectrophotometer. Electrochemistry
was performed with a CH Instruments 600C electrochemical
analyzer. Voltammograms are presented with the positive
potential pointing to the left and with increasing anodic cur-
rents pointing downward. Cyclic voltammetry (CV) was con-
ducted with the use of a three-electrode cell in which ITO
(polymer films area about 0.7 cm ꢃ 1.4 cm) was used as a
working electrode. A platinum wire was used as an auxiliary
electrode. All potentials were reported with respect to Ag/
AgCl electrode at room temperature under a nitrogen atmos-
phere. Ferrocene was used as an external reference for cali-
bration (þ0.44 V vs. Ag/AgCl). An Agilent 8453 UV-Visible
photodiode array spectrophotometer was used to conduct
the spectroelectrochemical experiments. For the spectroelec-
trochemical studies, the polymer film cast on an ITO-coated
glass slide was immersed in 0.1 M TBAP/MeCN solution
using a UV-cuvette as a single-compartment cell equipped
Preparation of the Polyamide Films
A polymer solution was made by the dissolution of about
0.7 g of the polyamide 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 the film was stripped
off from the glass substrate and further dried in vacuum at
ꢀ
160 C for 6 h. The obtained films were about 0.09-mm thick
and were used for X-ray diffraction measurements, solubility
tests, and thermal analyses.
Measurements
Infrared (IR) spectra were recorded on a Horiba FT-720 FT-
IR spectrometer. Elemental analyses were run in a Heraeus
1
VarioEL III CHNS elemental analyzer. H- and 13C-NMR spec-
tra were measured on a Bruker AVANCE 500 FT-NMR system
with tetramethylsilane as an internal standard. The inherent
viscosities were determined with a Cannon-Fenske viscome-
ter at 30 ꢀC. Wide-angle X-ray diffraction measurements
were performed at room temperature (ꢂ25 ꢀC) on a Shi-
madzu XRD-6000 X-ray diffractometer with a graphite mono-
chromator (operating at 40 kV and 30 mA), using nickel-fil-
tered Cu-Ka radiation (k ¼ 1.5418 Å). The scanning rate was
2ꢀ/min over a range of 2y ¼ 10ꢀ–40ꢀ. DSC analyses were
performed on a Perkin–Elmer Pyris 1 DSC at a scan rate of
20 ꢀC/min in flowing nitrogen. Thermogravimetric analysis
(TGA) was performed with a Perkin–Elmer Pyris 1 TGA.
Experiments were carried out on approximately 4–6 mg of
samples heated in flowing nitrogen or air (flow rate ¼ 40
with
a Pt counter electrode and a Ag/AgCl reference
electrode.
RESULTS AND DISCUSSION
Monomer Synthesis
According to a well-established synthetic method,25 the tar-
get TPA-based diamine monomer, 4,40-diamino-200,400-dime-
thoxytriphenylamine (2), was successfully synthesized by the
CsF-mediated double N-arylation reactions of 2,4-dimethox-
yaniline with two equivalent 4-fluoronitrobenzene, followed
by catalytic reduction of the intermediate dinitro derivative
1 using Pd/C and hydrazine in refluxing ethanol (Scheme 1).
The synthesized compounds 1 and 2 were fully character-
ized using elemental analysis and FTIR, 1H-NMR, and 13C-
NMR spectroscopy. FTIR spectra of these two compounds
are shown in Supporting Information Figure S1. The nitro
3
ꢀ
cm /min) at a heating rate of 20 C/min. Thermomechanical
analysis (TMA) was determined with a Perkin–Elmer TMA 7
instrument. The TMA experiments were carried out from 50
to 350 ꢀC at a scan rate of 10 ꢀC/min with a penetration
probe 1.0 mm in diameter under an applied constant load of
10 mN. Softening temperatures (Ts) were taken as the onset
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SCHEME 2 Synthesis of dimethoxy-
TPA-based polyamides.
groups of compound 1 gave two characteristic bands at
around 1579 and 1338 cmꢁ1 (ANO2 asymmetric and sym-
metric stretching). After reduction, the characteristic absorp-
tions of the nitro group disappeared and the primary amino
group showed the typical NAH stretching absorptions in the
of these polymers showed characteristic amide absorptions
around 3300 cmꢁ1 (amide NAH stretching) and 1650 cmꢁ1
(carbonyl C¼¼O stretching).
Polymer Properties
Organo-Solubility and Film Morphology
1
region of 3200–3400 cmꢁ1. The H- and 13C-NMR spectra of
the dinitro derivative 1 are included in Supporting Informa-
The qualitative solubility properties of polyamides 4a–h in
several organic solvents at 10% (wt/vol) are summarized in
Table S1. All polymers exhibited excellent solubility in a vari-
ety of solvents such as NMP, DMAc, DMF, DMSO, and m-cre-
sol at room temperature. The high solubility can be attrib-
uted in part to the incorporation of packing-disruptive
dimethoxyTPA groups, which retard dense chain packing and
lead to a decreased chain–chain interaction. Therefore, these
polymers have the advantage of allowing inexpensive solu-
tion processing techniques, such as spin-coating and printing
methods, to be used for the preparation of large-area devi-
ces. All the polymers could afford flexible and tough films,
and they were amorphous in nature as evidenced by wide-
angle X-ray diffraction patterns. The mechanical property
and amorphous nature of these polymers are important for
the further development of flexible optoelectronic devices.
tion Figure S2. Figure 1 illustrates the 1H- and 13C-NMR
1
spectra of the diamine monomer 2 in DMSO-d6. The H-NMR
spectra clearly reveal that the nitro groups have been com-
pletely reduced to the amino groups by the high-field shift of
the aromatic protons Ha and Hb and by the resonance signal
at 4.6 ppm corresponding to the primary aromatic amino
protons. Assignments of each proton and carbon atoms are
assisted by the 2D NMR spectra as shown in Supporting
Information Figure S3, and these spectra agree well with the
proposed molecular structures of 1 and 2.
Polymer Synthesis
According to the phosphorylation technique developed by
Yamazaki et al.,26 a series of novel TPA-based polyamides
4a–h, with dimethoxy ortho- and para-substituted on the
pendent phenyl ring, were prepared by the direct polycon-
densation reaction of diamine 2 with various aromatic dicar-
boxylic acids (3a–h) using triphenyl phosphite (TPP) and
pyridine as condensing agents (Scheme 2). All the polymer-
izations proceeded homogeneously throughout the reaction
and afforded clear, highly viscous polymer solutions. The
polymers precipitated in a tough, fiber-like form when the
resulting polymer solutions were slowly poured into stirred
methanol. These polyamides were obtained in almost quanti-
tative yields, with inherent viscosities in the range of
0.52–1.42 dL/g, as shown in Supporting Information Table
S1. All the polymers can be solution cast into flexible and
tough films, and this is indicative of the formation of high-
molecular-weight polymers. Structural features of these poly-
amides were verified by FTIR spectroscopy. The FTIR spectra
Thermal Properties
DSC, TMA, and TGA were used to investigate the thermal
properties of all polyamides. The thermal behavior data of
polyamides 4a–h are summarized in Table 1. The polyamides
showed high glass transition temperatures (Tgs) in the range
of 243–289 ꢀC by DSC. The Tg order corresponds to the
decreasing order of stiffness and polarity of the polymer
backbones. The relatively lower Tg values of polyamides 4b
and 4d can be explained in terms of the flexibility and low
rotation barrier of its diacid moiety. The highest Tg value of
polyamide 4c can be explained by the presence of rigid
biphenylene unit in the diacid moiety that stiffens the poly-
mer backbone.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Thermal Properties of Polyamides
Td at 5 wt % Lossc
Td at 10 wt % Lossc
(ꢀC)
(ꢀC)
Polymer Code
In N2
In Air
In N2
In Air
Char Yieldd (%)
a
Tg (ꢀC)
b
Ts (ꢀC)
4a
4b
4c
4d
4e
4f
259
243
289
254
278
277
260
263
264
238
280
252
276
269
261
269
444
437
460
443
449
455
441
439
455
437
463
439
470
494
465
481
481
472
496
480
471
500
477
473
497
483
508
482
502
549
510
530
65
68
73
68
65
61
69
71
4g
4h
a
c
The samples were heated from 50 to 400 ꢀC at a scan rate of 20 ꢀC/
min followed by rapid cooling to 50 ꢀC at ꢁ200 ꢀC/min in nitrogen. The
midpoint temperature of baseline shift on the subsequent DSC trace
(from 50 to 400 ꢀC at heating rate 20 ꢀC/min) was defined as Tg.
Decomposition temperature at which a 5% or 10% weight loss was
recorded by TGA at a heating rate of 20 ꢀC/min.
d
Residual weight percentages at 800 ꢀC under nitrogen flow.
b
Softening temperature measured by TMA using a penetration method.
The softening temperatures (Ts) of the polymer films were
determined with TMA by the penetration method. The Ts
value was read from the onset temperature of the probe dis-
placement on the TMA curve. Figure 2 shows a typical TMA
thermogram of polyamide 4e. The Ts values of the 4 series
polyamides are in the range from 238 to 280 ꢀC. In most
cases, the Ts values obtained by TMA are comparable to the
Tg values measured by the DSC experiments.
emission maximum around 445–468 nm (in the blue region),
with a very low fluorescence quantum yields of 0.33%–
0.77%. The absorption spectra of the polyamides in the solid
state are similar (a little red shifted) to those measured in
NMP solution. Optical band gap (Eg) determined from the
absorption edge of the solid-state spectra of polyamides are
found to be 2.58–3.02 eV.
The electrochemical properties of these polyamides were
investigated by cyclic voltammetry (CV) for the cast film on
an ITO-coated glass substrate as working electrode in dry
acetonitrile (MeCN) containing 0.1 M of TBAP as electrolyte
salt under nitrogen atmosphere. One pair of reversible redox
waves was observed in the first CV scan of these polymers.
Figure 3 shows a representative CV curve for polyamide 4d
recorded at a scan rate of 50 mV/s. The CV diagram indi-
cates two well-defined redox waves, with half-wave poten-
tials (E1/2) of 0.7 and 1.2 V. The first peak, at 0.82 V, can be
ascribed to oxidation of the electron-rich nitrogen atom in
The thermal stability of polyamides was evaluated by TGA in
both air and nitrogen atmospheres. The Figure 2 inset
depicts a typical set of TGA curves for polyamide 4e in air
and in nitrogen. The decomposition temperatures (Td) at 5%
and 10% weight losses in nitrogen and air atmosphere were
taken from the original TGA thermograms and are given in
Table 1. All the polymers exhibited good thermal stability;
the Tds of polyamides 4a–h at a 10% weight loss were in
the range of 471–500 ꢀC in nitrogen and 482–549 ꢀC in air.
It is worthy to mention that in all cases, these polyamides
revealed a delayed decomposition in air than in nitrogen.
The reason is not clear at present. The amount of carbonized
ꢀ
residues (char yield) at 800 C in nitrogen for all polyamides
was in the range of 61–73 wt %. The high char yields of
these polyamides can be attributed to their high aromatic
content. Thus, the thermal analysis results revealed that
these polyamides exhibited excellent thermal stability, which
in turn is beneficial to increase the service time in device
application and enhance the morphological stability to the
spin-coated film.
Optical and Electrochemical Properties
The optical properties of the dimethoxyTPA-based poly-
amides 4a–h were investigated by UV–vis and photolumines-
cence (PL) spectroscopy, and the relevant data are presented
in Table 2. These polymers exhibited strong UV–vis absorp-
tion bands at 278–382 nm in NMP solution, assignable to
the p–p* transitions resulting from the conjugated dimethox-
yTPA segment. Their PL spectra in NMP solution showed
FIGURE 2 TMA and TGA curves of polyamide 4e at a heating
rate of 10 ꢀC/min and 20 ꢀC/min, respectively. [Color figure can
be viewed in the online issue, which is available at www.
interscience.wiley.com.]
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TABLE 2 Optical and Electrochemical Properties of Polyamides
Oxidation Potential (V vs. Ag/AgCl
in MeCN)
HOMOc
(eV)
LUMOc
(eV)
In Solutiona
As Film
d
e
f
b
Abs
max
PL
max
Abs
max
Abs
Index
k
(nm)
k
(nm) UF (%)
k
(nm)
k (nm) E1/2
Epa
Eonset Eg (eV) E1/2 Eonset E1/2 Eonset
onset
4a
4b
4c
4d
4e
4f
283, 363 461
346 449
294, 361 459
337 455
278, 382 468
0.53
0.77
0.37
0.41
0.47
0.42
0.39
0.33
363
344
359
338
384
350
311
373
458
427
453
410
481
437
436
471
0.69 (0.73)g (0.87)h 0.80, 1.28 0.56 2.71
5.05 4.99 2.34 2.28
5.05 4.99 2.15 2.09
5.05 4.99 2.31 2.25
5.06 5.01 2.04 1.99
5.08 5.01 2.50 2.43
5.09 5.02 2.25 2.18
5.08 5.02 2.24 2.18
5.06 5.00 2.43 2.37
0.69 (0.75) (0.85)
0.69 (0.75) (0.86)
0.70 (0.76) (0.86)
0.72 (0.78) (0.88)
0.73 (0.79) (0.88)
0.72 (0.74) (0.85)
0.70 (0.77) (0.85)
h
0.83, 1.31 0.56 2.90
0.82, 1.29 0.56 2.74
0.82, 1.30 0.58 3.02
0.86, 1.34 0.58 2.58
0.84, 1.36 0.59 2.84
0.86, 1.35 0.59 2.84
0.81, 1.31 0.57 2.63
352
313
448
445
4g
4h
a
301, 373 461
Measured in NMP (1 ꢃ 10ꢁ5 mol/L).
Data of analogous polyamides 400 having the corresponding diacid
residue as in the 4 series.
b
Band gap calculated by the equation: Eg ¼ 1240/konset of polymer
film.
c
The HOMO energy levels were calculated from cyclic voltammetry and
were referenced to ferrocene (4.8 eV). LUMO ¼ HOMO ꢁ Eg.
d
The quantum yield was calculated in an integrating sphere with qui-
nine sulfate as the standard (UF ¼ 54.6%).
e
Half-wave potential of the first oxidation redox couple.
f
Anodic peak potentials.
Data of analogous polyamides 40 having the corresponding diacid res-
g
idue as in the 4 series.
the TPA core. The second peak, at 1.30 V, can be attributed
to oxidation of methoxy groups, causing radical recombina-
tion and formation of a quinoid structure (see Scheme 3).
The oxidation of the polymer film of 4d is accompanied by
color changes. In the neutral state, the film is almost color-
less. Applying a potential of 0.8 V, where the first oxidation
takes place, the color is changed to green. If the potential is
further increased to 1.3 V, where the second oxidation
occurs, the color of the film turns to blue-purple. The first
oxidation step is fully reversible. After 2000 oxidative cyclic
scans between 0 and 1.0 V at scan rate of 100 mV/s, the
polymer film still preserved superior redox reversibility
(Fig. 4). However, the second oxidation process seemed to be
not very reversible as that of the first one. As shown in Fig-
ure 5, the second redox wave of polymer 4d showed a slight
shift of the anodic peak and a slight decrease in current den-
sity after five cyclic scans in a potential range of 0–1.4 V at
scan rate of 50 mV/s. Figure 6 compares the CV curves of
structurally similar polyamides 4d, 4d0, and 4d00. As
expected, the onset oxidation potential of 4d is lower as
compared with those of 4d0 and 4d00 because of dimethoxy
substitution on the pendent phenyl ring of the TPA unit.
From polyamide 4d, one can also observe a second oxidation
process at 1.30 V, which yields a dication, whereas the corre-
sponding 4d0 and 4d00 analogues show only a single wave.
This result indicated that the presence of the second elec-
tron-donating methoxy substituent stablilizes the quinonei-
mine dicationic states, leading to the occurrence of the sec-
ond oxidation process.
dation onset potentials (Eonset) or half-wave potentials (E1/2)
and the onset absorption wavelengths, and the results
are listed in Table 2. The external ferrocene/ferrocenium
(Fc/Fcþ) redox standard E1/2 is 0.44 V versus Ag/AgCl in
MeCN. Assuming that the HOMO energy for the Fc/Fcþ
standard was 4.80 eV with respect to the zero vacuum level,
we can estimate the HOMO energy levels for polyamides 4a–
h to be in the range of 4.99–5.02 eV and 5.05–5.09 eV calcu-
lated from Eonset and E1/2, respectively. The lower ionization
FIGURE 3 The first CV scan of the cast film of polyamide 4d on
an ITO-coated glass substrate in 0.1 M TBAP/MeCN solution
(scan rate ¼ 50 mV/s). E1/2 values are indicated by blue dashed
lines. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
The energy levels of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
the investigated polyamides can be determined from the oxi-
NEW REDOX-ACTIVE POLYAMIDES, HSIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 3 Proposed anodic oxi-
dation pathways of the polyamides.
potential could suggest an easier hole injection into films
from ITO electrodes in electronic device applications.
nitrogen atom on the TPA unit. On this oxidized state,
polyamide 4d absorbed in the regions below 450 nm and
above 550 nm with a valley (absorption minimum) at
Spectroelectrochemical and Electrochromic Properties
The spectroelectrochemical properties of the polymer thin
films were monitored by a UV–vis spectrometer at different
applied potentials. The electrode preparations and solution
conditions were identical to those used in CV. The typical
absorption spectral changes of polyamide 4d are shown in
Figure 7. The polyamide 4d started to be oxidized at
about 0.6 V. When the applied potential was gradually
raised from 0.6 to 0.8 V, the absorption intensity at 337
nm slightly dropped, while a new peak at 397 nm and a
broadband having its maximum absorption wavelength at
829 nm appeared. The spectral changes were clearly due
to the formation of the radical cationic states of the poly-
amide 4d by electron removal from the lone pair on the
450–550 nm, reflecting
a green color. As the applied
potential was adjusted to 1.1 V or above, another absorp-
tion band originated from the dicationic dimethoxyTPA
moiety (see Scheme 3) showed up around 614 nm with a
concomitant decrease of absorption intensity at 397 nm.
These changes in electronic absorption spectra of 4d on
further oxidation were nicely reflected by color change
from green to a blue-purple color. These results indicated
the occurrence of a two-step oxidation sequence during
the oxidation of 4d.
observed for other polyamides of this series.
A similar spectral change was
The film color of polyamide 4d turned into green and blue-
purple with high contrast of optical transmittance change
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ARTICLE
FIGURE 4 Repetitive CV scanning of the polyamide 4d film on
an ITO electrode in 0.1 M TBAP/MeCN solution over the poten-
tial range from 0 to 1.0 V at a scan rate of 100 mV/s. [Color fig-
ure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
FIGURE 6 Cyclic voltammograms of the cast films of polya-
mides 4d, 4d0, and 4d00 on an ITO-coated glass substrate in
0.1 M TBAP/MeCN solution at a scan rate of 50 mV/s. [Color
figure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
tance change decrement or increment of the absorption
band at 337, 397, and 829 nm with respect to time while
the potential was switched between the neutral (ꢁ0.5 V)
and the first oxidized states (þ0.8 V) with a residence time
of 14 s. During this measurement, % transmittance (%T) at
the wavelength of maximum contrast was determined by
using a UV–vis spectrophotometer. The optical contrast
measured as the difference between %T at neutral and oxi-
dized states (DT) were found to be 34%, 54%, and 84% for
337, 397, and 829 nm, respectively (Fig. 9). The electrochro-
mic coloration efficiency (CE; g) can be calculated via optical
density using the following equation:27
(DT%) up to 84% at 829 nm, 68% at 614 nm, and 54% at
397 nm as shown in Figure 7. The color switching times
were estimated by applying a potential step, and the absorb-
ance profiles were followed. The switching time was defined
as the time that was required to reach 90% of the full
change in absorbance after switching potential. Thin films
from polymer 4d would require 1.7 s at 0.8 V for switching
absorbance at 829 nm and 1.0 s for bleaching as shown in
Figure 8.
The long-term stability of the film of 4d was determined by
measuring the optical absorbance change as a function of
the number of switching cycles. We extracted the electro-
chromic parameters of the 4d film by analysis of transmit-
g ¼ DOD=Q
where DOD is the optical absorbance change and Q (mC/
cm2) is the injected/ejected charge during a redox step. On
the basis of this equation, the CE of polyamide 4d was found
to be 329 cm2/C, which was higher than that of many conju-
gated polymers.18 No significant change in redox response of
the polymer was observed at the end of 1000 cycles. Its op-
tical activity retained by 80% even after 1000 cycles, namely
exhibits high redox stability.
CONCLUSIONS
A series of new redox-active polyamides has been prepared
from 4,40-diamino-200,400-dimethoxytriphenylamine with vari-
ous aromatic dicarboxylic acids. Incorporation of 2,4-dime-
thoxytriphenylamine units to the polymer main chain not
only functionalizes the polyamides with stable redox proper-
ties but also leads to enhanced solubility and good thin-film
formability. Aside from high Tg and Ts values and good ther-
mal stability, the polymers also revealed stable electrochro-
mic properties, changing color from colorless or pale yellow
neutral state to green or blue-purple oxidized state. The
FIGURE 5 Repetitive CV scanning of the polyamide 4d film on
the ITO/glass electrode in 0.1 M TBAP/MeCN solution over the
potential range from 0 to 1.5 V at a scan rate of 50 mV/s. [Color
figure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
NEW REDOX-ACTIVE POLYAMIDES, HSIAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 Spectroelectrochemical series of polyamide 4d (in
MeCN with 0.1 M TBAP as supporting electrolyte) at various
applied potentials (vs. Ag/AgCl). The photographs show the
color changes of the film in electrochemical cell at indicated
electrode voltages.
FIGURE 9 Optical transmittance changes for polyamide 4d film
on ITO-glass substrate (active area ꢂ1 cm2) at 337, 397, and
829 nm in 0.1 M TBAP/MeCN while the polymer was switched
between ꢁ0.5 and 0.8 V with a switch time of 14 s.
polymer films exhibited high coloration efficiency, high opti-
cal contrast ratio, reasonable response times, and long-term
redox and electrochromic reversibility. Thus, these polymers
may find optoelectronic applications as new hole-transport-
ing and electrochromic materials because of their proper
HOMO values and stable electrochromic behavior.
The authors thank the National Science Council of Taiwan
(ROC) for financial support through the research grant NSC 97-
2221-E-027-113-MY3.
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