Highly redox-stable and electrochromic aramids with morpholinyl-substituted triphenylamine units

Article abstract of DOI:10.1002/pola.27972

A novel morpholinyl-substituted, triphenylamine-based diamine monomer, namely 4,4′-diamino-4"-(4-morpholinyl)triphenylamine, was synthesized and polymerized with various aromatic dicarboxylic acids via the phosphorylation polyamidation reaction leading to a series of electroactive aromatic polyamides (aramids). All aramids were readily soluble in polar organic solvents and could be solution cast into tough and flexible films with high thermal stability. Cyclic voltammograms of the aramid films on the indium-tin oxide-coated glass substrate exhibited a pair of reversible oxidation waves with very low onset potentials of 0.36 - 0.41 V (vs. Ag/AgCl) in acetonitrile solution. The polymer films showed reversible electrochemical oxidation accompanied by strong color changes with high coloration efficiency, high contrast ratio, and rapid switching time. The optical transmittance change (Δ%T) at 650 nm between the neutral state and the fully oxidized state is up to 90%.

Full text of DOI:10.1002/pola.27972

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Highly Redox-Stable and Electrochromic Aramids with
Morpholinyl-Substituted Triphenylamine Units
Sheng-Huei Hsiao,1 Ying-Hsiu Hsiao,2 Yu-Ruei Kung3
1Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
2Department of Chemical Engineering, Tatung University, Taipei 10451, Taiwan
3Department of Opto-Electronic Nano Hybrid Materials Research, Material and Chemical Research Laboratories, Industrial
Technology Research Institute, Hsinchu 31040, Taiwan
Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
Received 7 October 2015; accepted 28 October 2015; published online 27 November 2015
DOI: 10.1002/pola.27972
ABSTRACT: A novel morpholinyl-substituted, triphenylamine-
based diamine monomer, namely 4,40-diamino-400-(4-morpholi-
nyl)triphenylamine, was synthesized and polymerized with var-
ious aromatic dicarboxylic acids via the phosphorylation
polyamidation reaction leading to a series of electroactive aro-
matic polyamides (aramids). All aramids were readily soluble
in polar organic solvents and could be solution cast into tough
and flexible films with high thermal stability. Cyclic voltammo-
grams of the aramid films on the indium-tin oxide-coated glass
substrate exhibited a pair of reversible oxidation waves with
very low onset potentials of 0.36 2 0.41 V (vs. Ag/AgCl) in ace-
tonitrile solution. The polymer films showed reversible electro-
chemical oxidation accompanied by strong color changes with
high coloration efficiency, high contrast ratio, and rapid switch-
ing time. The optical transmittance change (D%T) at 650 nm
between the neutral state and the fully oxidized state is up to
C
V
90%.
2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2016, 54, 1289–1298
KEYWORDS: electrochemistry; electrochromic polymers; mor-
pholine; polyamides; redox polymers; triphenylamine
expanding the application of the aromatic polyamides as
optically active materials, luminescent, and electroactive
films, gas or ion-exchange membranes, etc. by incorporating
new chemical functionalities in the polyamide backbone or
lateral structure.11–13
INTRODUCTION Wholly aromatic polyamides are character-
ized as highly thermally stable polymers with a favorable
balance of physical and chemical properties.1 Fibers obtained
from anisotropic solutions of these high-performance materi-
als have been used in applications where high thermal stabil-
ity and mechanical strength are required. However, rigidity
of the backbone and strong hydrogen bonding results in high
melting or glass-transition temperatures (Tgs) and limited
solubility in most organic solvents. These properties make
them generally intractable or difficult to process, thus
restricting their applications. To overcome such a difficulty,
polymer structure modification becomes necessary. One
approach to solving this problem has been the incorporation
of bulky groups in the main chain or as the pendent
groups.2–7 It has also been reported that aromatic polya-
mides bearing the propeller-shaped triphenylamine (TPA)
unit in the main chain were amorphous and easily soluble in
organic solvents and they could be solution-cast into flexible
films with good mechanical property and high thermal stabil-
ity.8–10 Moreover, polyamides can be endowed with new
functions by changing the chemical structure and composi-
tion. There is continuously a huge effort directed toward
TPA derivatives and polymers are well-known for their elec-
troactive and photoactive properties that may find optoelec-
tronic applications as photoconductors, hole-transporters,
and light-emitters.14,15 TPAs can be easily oxidized to form
stable radical cations as long as the para-position of the phe-
nyl rings is protected, and the oxidation process is always
associated with a strong change of coloration. In the past
decade, a lot of high-performance polymers (typically, aro-
matic polyamides and polyimides) carrying the redox-active
TPA unit have been prepared and evaluated for electrochro-
mic applications.16–24 As reported in the pioneering works
by Nelson and Adams et al.,25,26 unsubstituted TPA under-
goes dimerization to tetraphenylbenzidine (TPB) after the
formation of an unstable monocation radical. This is accom-
panied by the loss of two protons per dimer and the dimer
is more easily oxidized than TPA and also can undergo fur-
ther oxidations in two discrete one-electron steps to give
Additional Supporting Information may be found in the online version of this article.
C
V
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TPB1• and finally the quinoidal TPB12. Quantitative data
have been obtained for several 4-substituted TPAs in the
form of second-order coupling rate constants, and it was
generally found that electron-donating substituents such as
methoxy group tended to stabilize the cation radicals while
electron-withdrawing groups such as nitro group had the
opposite effect.27 Thus, several highly redox-stable, electro-
chromic polyamides and polyimides with methoxy-
substituted TPA units have been reported in literature.28–35
4-fluoronitrobenzene, and 20.7 g (0.15 mol) of K2CO3 in
100 mL of DMSO was heated with stirring at 120 8C for 20 h.
After cooling, the mixture was poured into 400 mL mixed solu-
tion of ethanol/water (1:1), and the formed yellow crystals
were collected by filtration with an yield of 30.4 g (97%) and a
melting point of 149 2 151 8C. IR (KBr): 1583, 1319 cm21
(ANO2 str.). 1H NMR (500 MHz, DMSO-d6, d, ppm): 8.07
(d, J 5 9.5 Hz, 2H, Hd), 7.03 (d, J 5 9.5 Hz, 2H, Hc), 3.74
(t, J 5 4.9 Hz, 4H, Ha), 3.41 (t, J 5 4.9 Hz, 4H, Hb). 13C NMR (125
MHz, DMSO-d6, d, ppm): 154.94 (C3), 137.29 (C6), 125.56 (C5),
112.53 (C4), 65.66 (C1), 46.39 (C2).
On the other hand, dialkylamino substituents are even better
electron pair donors than the corresponding alkoxy substitu-
ents and stabilize a radical cation to which they are attached.
In a previous publication,36 we have demonstrated that poly-
amides with extremely electron-donating dimethylamino-TPA
moieties exhibit greatly lowered oxidation potentials and
enhanced electrochemical and electrochromic stability. As a
continuation of our efforts in developing electrochromic
materials with long-term stability, rapid redox switching, and
high optical contrast between their bleached and colored
states, herein we synthesize a new diamine monomer, 4,40-
diamino-400-(4-morpholinyl)triphenylamine, and its derived
aramids containing the electroactive TPA unit with electron-
donating 4-morpholinyl group para substituted on the pend-
ent phenyl ring. The effect of incorporating the heterocyclic
morpholinyl substituent on the electrochemical and electro-
chromic properties of the aramids will be investigated.
4-(4-Morpholinyl)aniline (2)
In a 500 mL round-bottom flask, 14.6 g (0.07 mol) of 4-(4-
nitrophenyl)morpholine (1), 0.15 g of 10 wt % Pd/C, 10 mL
hydrazine monohydrate and 140 mL of ethanol was stirred
at a reflux temperature for 10 h. The solution was filtered
hot to remove Pd/C and then allowed to cool to room tem-
perature, and the filtrate was concentrated by rotary evapo-
ration. The precipitate was collected and dried in vacuum to
give 8.9 g (yield 71%) of amine 2 as purplish red crystals
with a melting point of 132 2 134 8C. IR (KBr): 3330,
3225 cm21 (ANH2 str.). 1H NMR (500 MHz, DMSO-d6, d,
ppm): 6.68 (d, J 5 8.8 Hz, 2H, Hd), 6.51 (d, J 5 8.8 Hz, 2H,
Hc), 4.55 (s, 2H, ANH2), 3.69 (t, J 5 4.7 Hz, 4H, Ha), 2.87 (t,
J 5 4.7 Hz, 4H, Hb). 13C NMR (125 MHz, DMSO-d6, d, ppm):
142.30 (C3), 142.17 (C6), 117.49 (C5), 114.70 (C4), 66.24
(C1), 50.55 (C2).
EXPERIMENTAL
Materials
Morpholine (TCI), 4-fluoronitrobenzene (Acros), 10% palla-
dium on charcoal (Pd/C) (Fluka), potassium carbonate
(K2CO3) (Showa), cesium fluoride (CsF) (Acros), triphenyl
phosphite (TPP) (Acros), dimethyl sulfoxide (DMSO), and
hydrazine monohydrate (TCI) were used as received from
commercial sources. N,N-Dimethylacetamide (DMAc) (Tedia),
N,N-dimethylformamide (DMF) (Tedia), pyridine (Py) (Wako)
and N-methyl-2-pyrrolidone (NMP) (Tedia) were dried over
calcium hydride for 24 h, distilled under reduced pressure,
and stored over 4 Å molecular sieves in a sealed bottle. The
commercially available aromatic dicarboxylic acids such as
terephthalic acid (5a) (Wako), isophthalic acid (5b) (Wako),
4,40-biphenydicarboxylic acid (5c) (TCI), 4,40-dicarboxydi-
phenyl ether (5d) (TCI), bis(4-carboxyphenyl) sulfone (5e)
(New Japan Chemicals Co.), 2,2-bis(4-carboxyphenyl)hexa-
fluoropropane (5f) (TCI), 1,4-naphthalenedicarboxylic acid
(5g) (Wako), and 2,6-naphthalenedicarboxylic acid (5h)
(TCI) were used as received. Commercially obtained calcium
chloride (CaCl2) (Wako) was dried under vacuum at 180 8C
for 8 h prior to use. Tetra-n-butylammonium perchlorate
(TBAP) was obtained from Acros and recrystallized twice
from ethyl acetate and then dried vacuo before use.
4,40-Dinitro-400-(4-morpholinyl)triphenylamine (3)
In a 250 mL round-bottom flask equipped with a stirring
bar, a mixture of 12.5 g (0.07 mol) of 4-morpholinoaniline
(2), 19.8 g (0.14 mol) of 4-fluoronitrobenzene, and 21.3 g
(0.14 mol) of CsF in 60 mL of DMSO was heated with stir-
ring at 120 8C for 20 h. After cooling, the mixture was
poured into 400 mL mixed solution of ethanol/water (1:1),
and the dark red precipitate was collected by filtration.
Recrystallization from DMF/H2O yielded 24.7
g of the
desired dinitro compound (3) as red crystals in 84% yield
with a melting point of 186 2 188 8C. IR (KBr): 1579,
1338 cm21 (2NO2 str.). 1H NMR (500 MHz, DMSO-d6, d,
ppm): 8.16 (d, J 5 9.2 Hz, 4H, Hf), 7.18 (d, J 5 9.2 Hz, 4H,
He), 7.13 (d, J 5 9.0 Hz, 2H, Hd), 7.05 (d, J 5 9.0 Hz, 2H, Hc),
3.75 (t, J 5 4.7 Hz, 4H, Ha), 3.17 (t, J 5 4.7 Hz, 4H, Hb). 13C
NMR (125 MHz, DMSO-d6, d, ppm): 151.69 (C7), 149.84 (C3),
141.53 (C10), 135.03 (C6), 128.46 (C5), 125.41 (C9), 121.60
(C8), 116.24 (C4), 65.99 (C1), 47.79 (C2).
Monomer Synthesis
4-(4-Nitrophenyl)morpholine (1)
In a 250 mL round-bottom flask equipped with a stirring bar, a
mixture of 13.1 g (0.15 mol) of morpholine, 21.2 g (0.15 mol) of
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SCHEME 1 Synthetic route to the target diamine monomer 4: (a) p-fluoronitrobenzene, K2CO3, DMSO, 120 8C, 20 h; (b) hydrazine,
Pd/C, EtOH, reflux, 10 h; (c) p-fluoronitrobenzene, CsF, DMSO, 120 8C, 20 h; (d) hydrazine, Pd/C, EtOH, reflux, 20 h.
monomer 4, 0.2326 g (1.40 mmol) of terephthalic acid (5a),
1.4 mL of TPP, 1.4 mL of NMP, 0.4 mL of pyridine, and 0.2 g of
calcium chloride (CaCl2). The reaction mixture was heated
with stirring at 120 8C for 3 h. The resulting viscous 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. The polyamide exhibited inherent viscosity of 0.86
dL/g, measured on 0.5 g/dL in DMAc. IR (film): 3298 (amide
NAH str.), 1653 cm21 (amide C@O str.). The other polyamides
were prepared by an analogous procedure.
4,40-Diamino-400-(4-morpholinyl)triphenylamine (4)
Measurements
Infrared (IR) spectra were recorded on a Horiba FT-720 FT-IR
spectrometer. Elemental analyses were run in a Heraeus Vario
EL III CHNS elemental analyzer. 1H and 13C NMR spectra were
measured on a Bruker AVANCE 500 FT-NMR system with tet-
ramethylsilane as an internal standard. The inherent viscos-
ities were determined with a Cannon-Fenske viscometer at 30
8C. Wide-angle X-ray diffraction (WAXD) measurements were
performed at room temperature (ca. 25 8C) on a Shimadzu
XRD-6000 X-ray diffractometer with a graphite monochroma-
tor (operating at 40 kV and 30 mA), using nickel-filtered Cu-
Ka radiation (k 5 1.5418 Å). The scanning rate was 28/min
over a range of 2h 5 10 2 408. Thermogravimetric analysis
(TGA) was performed with a Perkin-Elmer Pyris 1 TGA.
Experiments were carried out on ꢀ4 2 6 mg of samples
heated in flowing nitrogen or air (flow rate 5 40 cm3/min) at
a heating rate of 20 8C/min. DSC analyses were performed on
a Perkin-Elmer Pyris 1 DSC at a scan rate of 20 8C/min in flow-
ing nitrogen. Thermomechanical analysis (TMA) was deter-
mined with a Perkin-Elmer TMA 7 instrument. The TMA
experiments were carried out from 50 to 350 8C at a scan rate
of 10 8C/min with a penetration probe 1.0 mm in diameter
under an applied constant load of 10 mN. Softening tempera-
tures (Ts) were taken as the onset temperatures of probe dis-
placement on the TMA traces. Electrochemistry was
performed with a CH Instruments 600c electrochemical ana-
lyzer. Voltammograms are presented with the positive poten-
tial pointing to the left and with increasing anodic currents
pointing downwards. Cyclic voltammetry (CV) was conducted
with the use of a three-electrode cell in which indium-tin oxide
(ITO) was used as a working electrode. A platinum wire was
used as an auxiliary electrode. All cell potentials were taken
with the use of a home-made Ag/AgCl, KCl (sat.) reference
electrode. Ferrocene was used as an external reference for
In a 500 mL round-bottom flask, 12.6 g (0.03 mol) of dinitro
compound 3, 0.15 g of 10 wt % Pd/C, 9 mL hydrazine mono-
hydrate and 200 mL of ethanol was stirred at a reflux temper-
ature for 20 h. The solution was filtered hot to remove Pd/C
and then allowed to cool to room temperature, and the filtrate
was concentrated by rotary evaporation. The precipitate was
collected and dried in vacuum to give 10.0 g (yield 93%) of
diamine 4 as white needles with a melting point of 213–215
8C. IR (KBr): 3356, 3209 cm21 (ANH2 str.). 1H NMR (500 MHz,
DMSO-d6, d, ppm): 6.75 (d, J 5 9.1 Hz, 2H, Hc), 6.68 (two over-
lapped doublets, 6H, Hd 1 He), 6.49 (d, J 5 8.7 Hz, 4H, Hf), 4.79
(s, 4H, -NH2), 3.69 (t, J 5 4.5 Hz, 4H, Ha), 2.95 (t, J 5 4.5 Hz, 4H,
Hb). 13C NMR (125 MHz, DMSO-d6, d, ppm): 144.50 (C10),
144.07 (C7), 142.24 (C3), 137.44 (C6), 125.46 (C8), 120.69
(C5), 116.35 (C4), 114.73 (C9), 66.12 (C1), 49.46 (C2). ANAL.
Calcd for C22H24N4O (360.45) : C, 73.31%; H, 6.71%; N,
15.54%. Found : C, 73.17%; H, 6.65%; N, 15.31%.
Synthesis of Polyamides
The synthesis of polyamide 6a is used as an example to illus-
trate the general synthetic route used to produce the polya-
mides. A 50 mL round-bottom flask equipped with a magnetic
stirrer was charged with 0.5046 g (1.40 mmol) of diamine
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SCHEME 2 Synthesis of polyamides 6a26h.
calibration (10.44 V vs. Ag/AgCl). Spectroelectrochemistry
analyses were carried out with an electrolytic 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 refer-
ence electrode. Absorption spectra in the spectroelectrochemi-
cal experiments were measured with an Agilent 8453 UV-
Visible diode array spectrophotometer.
the nitro group disappeared and the primary amino group
showed the typical NAH stretching absorptions in the region
of 3200 2 3500 cm21. The 1H and 13C NMR spectra of com-
pounds 1 to 4 are collected in Supporting Information Figures
S2 and S3, respectively. Assignments of each hydrogen and
carbon in the diamine monomer 4 are assisted by the two-
dimensional (2-D) NMR spectra (see Supporting Information
Figs. S4 and S5), and these spectra agree well with the pro-
posed molecular structure of the targeted diamine monomer.
RESULTS AND DISCUSSION
Monomer Synthesis
The new TPA-based diamine monomer, 4,40-diamino-400-(4-
morpholinyl)triphenylamine (4), was prepared by a four-step
reaction sequence outlined in Scheme 1. In the first step, 4-
(4-nitrophenyl)morpholine (1) was obtained by the nucleo-
philic aromatic displacement of 4-fluoronitrobenzene with
morpholine using potassium carbonate as the base. Then,
the nitro compound 1 was reduced to 4-(4-morpholinyl)ani-
line (2) by hydrazine monohydrate and Pd/C catalyst in
refluxing ethanol. In the third step, 4,40-dinitro-400-(4-mor-
pholino)triphenylamine (3) was synthesized by the cesium
fluoride (CsF)-promoted N,N-diarylation reaction of the
TABLE 1 Inherent Viscosity and Solubility Behavior of
Polyamides
Solubility in Various Solventsb
a
Polymer ginh
Code
(dL/g) NMP DMAc DMF DMSO m-Cresol THF
6a
6b
6c
6d
6e
6f
0.86
0.45
0.92
0.68
0.40
0.68
0.80
0.78
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
amino
compound
2
with
two
equivalent
4-
12
12
2
fluoronitrobenzene. Finally, the nitro groups of compound 3
were reduced by the same technique used in the second
step to give the targeted diamine monomer 4. FT-IR, 1H
NMR and 13C NMR spectroscopic techniques were used to
confirm the structures of all the synthesized compounds
124. The FT-IR spectra of compounds 124 are included in
the Supporting Information Figure S1. The nitro groups of
compounds 1 and 3 gave two characteristic bands at around
1580 and 1330 cm21 (ANO2 asymmetric and symmetric
stretching). After reduction, the characteristic absorptions of
6g
6h
2
a
Inherent viscosity measured at a concentration of 0.5 dL/g in DMAc at
30 8C.
b
The solubility was determined with a 10 mg sample in 1 mL of a sol-
vent. Solubility: 1: soluble at room temperature; 12: partially soluble;
2: insoluble even on heating. Solvent: NMP: N-methyl-2-pyrrolidone;
DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO:
dimethyl sulfoxide; THF: tetrahydrofuran.
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TABLE 2 Thermal Properties of Polyamidesa
Properties of Polyamides
Organo-Solubility and X-Ray Diffraction Data
Td at 10 wt %
lossd (8C)
The solubility properties of polyamides 6a26h in several
organic solvents at 10% (w/v) are summarized in Table 1.
All the polyamides exhibited excellent solubility in a variety of
solvents such as NMP, DMAc, DMF, DMSO, and m-cresol. The
high solubility of these aramids can be attributed in part to the
incorporation of bulky, three-dimensional (4-morpholinyl)TPA
groups, which retard dense chain packing and lead to a
decreased chain–chain interaction. Therefore, the good solubil-
ity makes these polymers potential candidates for practical
applications in solution processing. All of the polyamides
6a26h could afford flexible and tough films, and they were
amorphous in nature as evidenced by wide-angle X-ray diffrac-
tion (WAXD) patterns (see Supporting Information Fig. S7)
Polymer
Code
Char
b
c
Tg (8C)
Ts (8C)
In N2
In Air
Yielde (%)
6a
6b
6c
6d
6e
6f
288 (295)
284 (290)
295 (302)
287 (283)
293 (296)
278 (288)
293 (295)
295 (307)
285
280
545
500
550
543
506
584
520
560
545
511
577
570
531
578
539
563
74
75
68
75
70
68
72
77
f
–
284
292
270
291
–
6g
6h
Thermal Properties
a
The polymer film samples were heated at 300 8C for 30 min prior to
The thermal properties of polyamides were examined by
DSC, TMA, and TGA techniques. The relevant thermal behav-
ior data are summarized in Table 2. The glass-transition tem-
peratures (Tg) of polyamides 6a26h were in the range of
278 2 295 8C by DSC. The lower Tg values of 6b and 6f can
be explained in terms of the flexibility and low rotation bar-
rier of its diacid moiety. As compared to the corresponding
all the thermal analyses.
b
The sample were heated from 50 to 400 8C at a scan rate of 20 8C/min
followed by rapid cooling to 50 8C at 2200 8C /min in nitrogen. The mid-
point temperature of baseline shift on the subsequent DSC trace (from
50 to 400 8C at heating rate 20 8C/min) was defined as Tg. Values indi-
cated in parentheses are data of analogous 60 series polyamides with
the corresponding diacid residue as in the 6 series polymers.
c
Softening temperature measured by TMA using a penetration method.
d
Decomposition temperature at which a 10% weight loss was recorded
parent 60 series polyamides, the
6 series polyamides
by TGA at a heating rate of 20 8C/min.
e
revealed a slightly lower Tg possibly due to the internal plas-
ticization effect of the 4-morpholinyl substituents. The soft-
ening 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. A typical TMA thermogram for
polyamide 6b is illustrated in Supporting Information Figure
S8. The Ts values of the polyamides are in the range from
278 2 292 8C. In most cases, the Ts values of the polyamides
obtained by TMA are comparable to the Tg values measured
by the DSC experiments. The thermal stability of polyamides
was evaluated by TGA in both air and nitrogen atmospheres.
Typical TGA thermograms for polyamide 6f are illustrated in
Residual weight percentages at 800 8C under nitrogen flow.
No discernible Ts was observed.
f
Polymer Synthesis
According to the phosphorylation technique described by
Yamazaki et al.37 a series of novel TPA-based aromatic polya-
mides, 6a26h, with morpholinyl para-substituted on the
pendent phenyl ring were prepared from the diamine 4 and
various aromatic dicarboxylic acids (5a25h) by the direct
polycondensation reaction with TPP and pyridine as con-
densing agents (Scheme 2). All the polymerizations pro-
ceeded homogeneously throughout the reaction and afforded
clear, highly viscous polymer solutions. The polymers pre-
cipitated in a tough, fiber-like form when the resulting poly-
mer solutions were slowly poured with stirring into
methanol. These polyamides were obtained in almost quanti-
tative yields, with inherent viscosity values in the range of
0.40 2 0.92 dL/g, as summarized in Table 1. All the polymers
can be solution-cast into flexible and tough films, and this is
indicative of the formation of high molecular weight poly-
mers. Structural features of these polyamides were verified
by FTIR spectra based on characteristic absorption bands
observed around 3298 (amide NAH stretching), 1653 (car-
bonyl CAO stretching), and 1279 cm21 (amine CAN stretch-
ing). Figure S6 (Supporting Information) illustrates a typical
FTIR spectrum of the representative polyamide 6a.
FIGURE 1 TGA curve of polyamide 6f with a heating rate 20 8C/
min. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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SCHEME 3 Proposed oxidation pathways of the (4-morpholinyl)TPA segment in the 6 series polyamides.
Figure 1. The decomposition temperatures (Td) at 10%
weight losses in nitrogen and air atmosphere read from the
original TGA thermograms are also summarized in Table 2.
All of the polymers exhibited good thermal stability; the Td
values at a 10% weight loss were recorded in the range of
500 2 584 8C in nitrogen and 511 2 578 8C in air, respec-
tively. The amount of carbonized residues (char yield) at 800
8C in nitrogen for all polyamides was in the range from 68
to 77 wt %. The high char yields of these polyamides can be
attributed to their high aromatic content.
E
1/2 5 0.86 V for polyamide 6d0 in the oxidative scan. Poly-
amide 6d revealed lower onset oxidation potential
a
(Eonset 5 0.40 V) in comparison with its parent analog
6d0 (Eonset 5 0.72 V). This indicates that the first oxidation
wave in the CV diagram of polyamide 6d is associated with
its morpholinyl groups. It was also found that the film
changed color from colorless to green and then to deep blue
because of electrochemical oxidation of polymer 6d. The oxi-
dative and electrochromic reversibility of 6d is maintained
on repeated scanning between 0.0 and 1.2 V (vs. Ag/AgCl).
This result confirms that para-substitution of the morpho-
linyl group on the TPA unit lends considerable stability to
both the cation radical and dication quinonediimine species,
as shown in Scheme 3. The other polyamides showed similar
CV diagrams to that of 6d. As shown in Figure 3, after 500
consecutive cyclic scans between 0 and 0.67 V polyamide 6d
still retained a high electrochemical stability. The redox
potentials of the various polyamides as well as their respec-
tive HOMO and LUMO potentials (vs. vacuum) are included
in Table 3. The E1/2 and Eonset values of the external ferro-
cene/ferrocenium (Fc/Fc1) redox standard in MeCN were
recorded at 0.44 and 0.37 V (vs. Ag/AgCl), respectively.
Under the assumption that the HOMO energy level for the
ferrocene standard was 4.80 eV with respect to the zero
Electrochemical Properties
The electrochemical properties of the polyamides were
investigated by CV conducted for the cast films on an ITO-
coated glass slide as working electrode in anhydrous acetoni-
trile (MeCN), using 0.1 M of TBAP as a supporting electro-
lyte. For comparison, referenced polyamides 6a026h0 were
prepared from dicarboxylic acids 5a25h and 4,40-diaminotri-
phenylamine in a similar synthetic procedure used for the
6 series polyamides. The typical cyclic voltammograms for
polyamides 6d and 6d0 are compared in Figure 2. There are
two reversible oxidation redox couples at half-wave
potentials (E1/2) of 0.52 and 0.88 V, respectively, for polyam-
ide 6d and only one reversible oxidation redox couple at
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FIGURE 2 Cyclic voltammograms of (a) ferrocene and the films of polyamides (b) 6d and (c) 6d0 on an ITO-coated glass substrate
in MeCN containing 0.1 M TBAP with a scanning rate of 100 mV/s. Polyamide 6d was determined in two different potential ranges:
0 2 0.8 and 0 2 1.2 V. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
TABLE 3 Electrochemical Properties of the Polyamides
E1/2 (V)b (vs. Ag/AgCl)
First Second
HOMO (eV)d
LUMO (eV)e
Code
Eonset (V)a
Eg (eV)c
Eonset
E1/2
Eonset
E1/2
6a
6b
6c
6d
6e
6f
0.37
0.39
0.38
0.40
0.40
0.41
0.41
0.36
0.50 (0.87)f
0.55 (0.85)
0.54 (0.86)
0.52 (0.86)
0.55 (0.88)
0.56 (0.88)
0.56 (0.85)
0.53 (0.85)
0.88
0.90
0.91
0.88
0.93
0.92
0.91
0.91
2.66
2.82
2.71
2.97
2.58
2.82
2.83
2.65
4.80
4.82
4.81
4.83
4.83
4.84
4.84
4.79
4.86
4.91
4.90
4.88
4.91
4.92
4.92
4.89
2.14
2.00
2.10
1.86
2.25
2.02
2.01
2.14
2.20
2.09
2.19
1.91
2.33
2.10
2.09
2.24
6g
6h
a
d
Onset oxidation potentials from cyclic voltammograms.
The HOMO energy levels were calculated from Eonset or E1/2 values
b
c
Oxidation half-wave potentials from cyclic voltammograms.
Energy band gap calculated from absorption edge of the polymer film
and were referenced to ferrocene (4.8 eV).
e
LUMO 5 HOMO 2 Eg.
Values in parentheses are data of the parent polyamides 60 having the
f
according to the following equation: Eg 5 1240/kabs,onset
.
corresponding diacid residue as in the 6 series.
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FIGURE 5 Optical switching for polyamide 6d at kmax 5 906 nm
as the applied voltage was stepped between 0 and 0.55 V (vs.
Ag/AgCl).
FIGURE 3 Cyclic voltammograms of the cast films of (a) poly-
amide 6d film on an ITO-coated glass substrate over 500 cyclic
scans and (b) ferrocene in MeCN containing 0.1 M TBAP at a
scan rate of 100 mV/s. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
lectrochemistry. For these investigations, the polyamide film
was cast on an ITO-coated glass slide (a piece that fit in the
commercial UV–visible cuvette), and a homemade electro-
chemical cell was built from a commercial UV–visible cuvette.
The cell was placed in the optical path of the sample light
beam in a commercial diode array spectrophotometer. This
procedure allowed us to obtain electronic absorption spectra
under potential control in a 0.1 M TBAP/MeCN solution. The
result of the polyamide 6d film is presented in Figure 4 as a
series of UV-vis absorbance curves correlated to electrode
potentials. In the neutral form, at 0 V, the film exhibited
strong absorption at wavelength around 342 nm, characteris-
tic for p2p* transitions of TPA and other aromatic compo-
nents, but it was almost transparent in the visible region.
Upon oxidation of the polyamide 6d film (increasing applied
voltage from 0 to 0.80 V), the intensity of the absorption
vacuum level, the HOMO energy levels for polyamides
6a26h were estimated to be 4.86 2 4.92 eV and 4.79 2 4.84
eV calculated from E1/2 and Eonset, respectively. The lower
ionization potential could suggest an easier hole injection
into films from ITO electrodes in electronic device
applications.
Spectroelectrochemical and Electrochromic Properties
Following the electrochemical tests, the optical properties of
the electrochromic films were evaluated by using spectroe-
FIGURE 4 Spectral change of 6d thin film on the ITO-coated
glass substrate (in MeCN with 0.1 M TBAP as the supporting
electrolyte) along with increasing of the applied voltage: 0 (᭿),
0.50 (•), 0.60 (᭡), 0.65 (᭢), 0.80 (᭜), 0.95 (w), 1.00 (᭺), 1.05 (᭝),
1.10 (᭞), and 1.20 V (᭛) vs. Ag/AgCl couple as reference. The
inset shows the photographic images of the film at indicated
applied voltages. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
FIGURE 6 (a) Potential step absorptometry and (b) current con-
sumption of the polyamide 6d film on to the ITO-coated glass
substrate (coated area: 1 cm2) during the continuous cycling
test by switching potentials between 0 and 0.55 V (vs. Ag/AgCl)
with a pulse width of 6 s. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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TABLE 4 Coloration Efficiency of Polyamide 6d
The switching time was calculated at 90% of the full switch
because it is difficult to perceive any further color change
with naked eye beyond this point. The polyamide switched
rapidly (within three seconds) between the highly transmis-
sive neutral state and the colored oxidized states. Thin film
of polyamide 6d required only 2.7 s at 0.55 V for coloring
and 1.4 s for bleaching, reflecting the different reaction rates
between the neutral and oxidized forms of the film of 6d. As
shown in Figure 6, the absorbance changes at 906 nm reflect
the switch in current, and the kinetics of the charge trans-
port process can be referenced to the coloration response
Cyclesa
dOD906
Q (mC/cm2)c
g (cm2/C)d
Decay (%)e
b
1
0.182
0.195
0.192
0.194
0.186
0.179
0.177
0.171
0.168
0.167
0.161
0.614
0.679
0.671
0.686
0.654
0.633
0.627
0.611
0.599
0.601
0.582
296
287
286
284
284
283
282
280
280
278
277
0
100
200
300
400
500
600
700
800
900
1000
3.0
3.4
4.1
4.1
4.4
4.7
5.4
5.4
6.1
6.4
time. The electrochromic coloration efficiencies (g 5 dOD906
/
Q) after various switching steps of the film of polyamide 6d
are summarized in Table 4. The electrochromic film of 6d
was found to exhibit high coloration efficiencies up to
296 cm2/C at 906 nm, and to retain near 94% of its optical
response after 1000 coloring/bleaching cycles. Therefore, the
electrochromic switching behavior appears to be a highly
reversible process.
a
Times of cyclic scan by applying potential steps: 0.00 $ 0.55 V (vs.
Ag/AgCl) with a pulse width of 6 s.
b
Optical density change at 906 nm.
Ejected charge, determined from the in situ experiments.
Coloration efficiency is derived from the equation: g 5 dOD906/Q.
Decay of coloration efficiency after cyclic scans.
c
d
e
CONCLUSIONS
A new TPA-based aromatic diamine monomer, 4,40-diamino-
400-(4-morpholinyl)triphenylamine, was successfully synthe-
sized in high purity and good yield. A new family of 4-
morpholinylTPA-functionalized aromatic polyamides were
readily prepared form the newly synthesized diamine mono-
mer with various aromatic dicarboxylic acids by the phos-
phorylation polyamidation reaction. All the polymers were
amorphous with good solubility in many polar aprotic sol-
vents and could afford flexible and strong films with high
thermal stability by solution casting. Incorporating the mor-
pholinyl substituent on the TPA unit greatly lowered the oxi-
dation potentials of the polyamides. In addition, these
polymers also revealed multi-colored electrochromic proper-
ties, high coloration efficiency, good electrochemical, and
electrochromic stability, and rapid switching times. Thus,
these characteristics suggest that the present polyamides
have great potential for optoelectronics applications.
peak at 342 nm gradually decreased while a new peak at
402 nm and a broad band having its maximum absorption
wavelength at 906 nm gradually increased in intensity. We
attribute this spectral change to the formation of a stable
monocation radical of the morpholinyl-TPA moiety. The
absorption band in the near infrared (NIR) region can be
attributed to an intervalence charge-transfer (IV-CT) between
states in which the positive charge is centered at different
amino centers (morpholine and TPA units).38,39 As the
applied potential became more anodic to 1.20 V, the absorp-
tion bands of the cation radical decreased gradually in inten-
sity, with the formation of a new strong absorption band
centered at around 650 nm. This spectral change can be
attributable to the formation of
a
dication in the
morpholino-TPA segment of the polyamide (see Scheme 3).
The observed UV–vis absorption changes in the film of 6d at
various potentials are fully reversible and are associated
with strong color changes; indeed, they even can be seen
readily by the naked eye. From the inset shown in Figure 4,
it can be seen that the film of 6d switches from a transmis-
sive neutral state (colorless) to a highly absorbing semioxi-
dized state (green) and a fully oxidized state (deep blue).
The film colorations are distributed homogeneously across
the polymer film and survive for more than hundreds of
redox cycles. The polyamide 6d shows a good optical con-
trast in the visible region, with an extremely high optical
transmittance change (D%T) of 90% at kmax 5 650 nm.
ACKNOWLEDGMENT
The authors thank the Ministry of Science and Technology of
Taiwan for the financial support.
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