Synthesis and unexpected electrochemical behavior of the triphenylamine-based aramids with ortho-and para-trimethyl-protective substituents

Article abstract of DOI:10.1002/pola.24326

Two series of new organosoluble polyamides with methyl-substituted triphenylamine (MeTPA) units showing anodically electrochromic characteristic were prepared from the phosphorylation polyamidation reaction of two diamine monomers, 4,4′-diamino-2″,4″,6″-trimethyltriphenylamine (Me₃TPA-diamine; 2) and 4,4′-diamino-4″- methyltriphenylamine (MeTPA-diamine; 2′), with various dicarboxylic acids, respectively. These polymers were readily soluble in many polar solvents and showed useful levels of thermal stability associated with relatively high glass-transition temperatures (T_g) (314-329 °C) and high char yields (higher than 62% at 800 °C in nitrogen). In addition, the polymer films showed reversible electrochemical oxidation, high coloration efficiency (CE), low switching time, and anodic green electrochromic behavior. The unexpected electrochemical behavior of higher oxidation potential and lower electrochemical stability of Me₃TPA-polyamides I than MeTPA corresponding polymers could be attributed to the higher steric hindrance of ortho-substituents in Me₃TPA moieties, thus made the resonance stabilization of cation radical much more difficult for the Me ₃-substituted phenyl ring.

Full text of DOI:10.1002/pola.24326

Synthesis and Unexpected Electrochemical Behavior of the
Triphenylamine-Based Aramids with Ortho- and
Para-Trimethyl-Protective Substituents
HUNG-JU YEN, SHIUE-MING GUO, GUEY-SHENG LIOU
Functional Polymeric Materials Laboratory, Institute of Polymer Science and Engineering, National Taiwan University,
No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan
Received 10 July 2010; accepted 18 August 2010
DOI: 10.1002/pola.24326
Published online 24 September 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Two series of new organosoluble polyamides with
methyl-substituted triphenylamine (MeTPA) units showing
anodically electrochromic characteristic were prepared from the
phosphorylation polyamidation reaction of two diamine mono-
mers, 4,40-diamino-200,400,600-trimethyltriphenylamine (Me3TPA-
diamine; 2) and 4,40-diamino-400-methyltriphenylamine (MeTPA-
diamine; 20), with various dicarboxylic acids, respectively. These
polymers were readily soluble in many polar solvents and
showed useful levels of thermal stability associated with rela-
tively high glass-transition temperatures (Tg) (314–329 ꢀC) and
high char yields (higher than 62% at 800 ꢀC in nitrogen). In addi-
tion, the polymer films showed reversible electrochemical oxi-
dation, high coloration efficiency (CE), low switching time, and
anodic green electrochromic behavior. The unexpected electro-
chemical behavior of higher oxidation potential and lower elec-
trochemical stability of Me3TPA-polyamides
I than MeTPA
corresponding polymers could be attributed to the higher steric
hindrance of ortho-substituents in Me3TPA moieties, thus made
the resonance stabilization of cation radical much more difficult
C
V
for the Me3-substituted phenyl ring.
2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 5271–5281, 2010
KEYWORDS: electrochemistry; high performance polymers; UV–
vis spectroscopy; polyamides
INTRODUCTION Electrochromic materials can be defined as
one where the reversible color change takes place from elec-
trochemically induced redox states. Color changes are com-
monly between a transparent state, where the chromophore
only absorbs in the ultraviolet (UV) region, and a colored
state which absorbs in the visible even near-infrared range in
a given electrolyte solution. Recently, the application of elec-
trochromic materials, such as antiglare back mirrors, smart
windows, and E-papers, have already been commercialized
nowadays.1–3 In general, most applications require electro-
chromic materials with a high contrast ratio (DT%), colora-
tion efficiency (CE),4 cycle life, and rapid response time.
The anodic oxidation pathways of TPA were well studied.19
The electrogenerated cation radical of TPA is not stable and
the radical electron could resonance to ortho and para posi-
tion. The chemical follow-up reaction produced tetraphenyl-
benzidine (TPB) by tail-to-tail coupling and the loss of two
protons per dimer. When the phenyl groups have substitu-
ents at the para position, the coupling reactions are greatly
prevented. It has been well established that incorporation of
electron-donating substituents at the para positions of triar-
ylamines affords stable radical cations.20,21
Wholly aromatic polyamides are known as highly thermally
stable materials, with a favorable balance of physical and
chemical properties. However, rigidity of the backbone and
strong hydrogen bonding result in high melting or glass-tran-
sition temperatures (Tg) and limited solubility in most or-
ganic solvents.5,6 These properties make them difficult to pro-
cess, thus restricting their applications. To overcome such a
dilemma, introduction of bulky and packing-disruptive
groups into the polymer backbone is a feasible approach7–14
;
triphenylamine (TPA) derivatives provide an avenue. Since
2002, we have developed TPA containing polyamides and
polyimides as high-performance polymers and hole-trans-
porting materials.15–18
In addition to para coupling of TPA, ortho coupling can also
take place.22 In this article, we, therefore, synthesized a
Additional Supporting Information may be found in the online version of this article. Correspondence to: G.-S. Liou (E-mail: gsliou@ntu.edu.tw)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 5271–5281 (2010) 2010 Wiley Periodicals, Inc.
SYNTHESIS OF THE TRIPHENYLAMINE-BASED ARAMIDS, YEN, GUO, AND LIOU
5271

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
novel diamine, 4,40-diamino-200,400,600-trimethyltriphenylamine
(Me3TPA-diamine; 2), and its derived polyamides containing
Me3TPA groups to investigate the electrochemical behavior
and the substituent effect on the electrochromic stability and
redox potential of the resultant polymers. The general prop-
erties such as solubility and thermal properties are
described. For a comparative study, electrochemical and elec-
trochromic properties of the present polyamides were also
compared with that of structurally related ones from 4,40-
diamino-400-methyltriphenylamine (MeTPA-diamine; 20)23 and
4,40-diaminotriphenylamine (200).24
stretch), 3040 cmꢁ1 (aromatic CAH stretch), 2917, 2854
cmꢁ1 (CH3 CAH stretch), 1650 cmꢁ1 (amide carbonyl), 1233
cmꢁ1 (asymmetric stretch CAOAC), and 1012 cmꢁ1 (sym-
metric stretch CAOAC).
1H-NMR (500 MHz, DMSO-d6, d, ppm): 10.12 (s, 2H), 8.03 (d,
J ¼ 8.5 Hz, 4H), 7.61 (d, J ¼ 8.5 Hz, 4H), 7.19 (d, J ¼ 8.5 Hz,
4H), 7.01 (s, 2H), 6.85 (d, J ¼ 8.5 Hz, 4H), 2.22 (s, 3H), 1.98
(s, 6H). Anal. Calcd. (%) for (C35H29N3O3)n (539.62)n: C,
77.90%; H, 5.42%; N, 7.79%. Found: C, 75.52%; H, 5.06%;
N, 7.02%. The other polyamides were prepared by an analo-
gous procedure.
EXPERIMENTAL
Materials
4,40-Diamino-400-methyltriphenylamine (20, mp ¼ 145–146
ꢀC) was synthesized by hydrazine Pd/C-catalyzed reduction
of 4,40-dinitro-400-methyltriphenylamine according to a previ-
ously reported procedure.23 Commercially available dicarbox-
ylic acids such as trans-1,4-cyclohexanedicarboxylic acid
(3a), 4,40-oxydibenzoic acid (3b), and 2,20-bis(4-carboxyphe-
nyl)hexafluoropropane (3c) were purchased from Tokyo
Chemical Industry and used as received. Anhydrous calcium
chloride (CaCl2) was dried under vacuum at 180 ꢀC for 8 h.
Tetrabutylammonium perchlorate (TBAP) was obtained from
ACROS and recrystallized twice by ethyl acetate under nitro-
gen atmosphere and then dried in vacuo before use. All other
reagents were used as received from commercial sources.
Preparation of the Film
A solution of polymer was made by dissolving about 0.65 g
of the polyamide sample in 15 mL of 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
at 160 ꢀC for 8 h. The obtained films were ꢂ65–80 lm in
thickness and were used for solubility tests and thermal
analyses.
Measurements
Fourier transform infrared (FT-IR) spectra were recorded on
a PerkinElmer Spectrum 100 Model FT-IR spectrometer. Ele-
mental analyses were run in a Heraeus VarioEL-III CHNS ele-
mental analyzer. 1H-NMR spectra were measured on a
Bruker AVANCE-500 FT-NMR using tetramethylsilane as the
internal standard, and peak multiplicity was reported as fol-
lows: s, singlet; d, doublet. X-Ray single-crystal diffraction
experiments were carried on a Norius Kappa CCD four-circle
diffractometer equipped with graphite-monochromated Mo-
Ka radiation. The structures were solved by direct methods
using SHELXL-97 software (Sheldrick, 1997). Wide-angle X-
ray diffraction (WAXD) measurements were performed at
room temperature (ca. 25 ꢀC) on a Shimadzu XRD-7000 X-
ray diffractometer (40 kV, 20 mA), using graphite-mono-
chromatized Cu-Ka radiation. The inherent viscosities were
determined at 0.5 g/dL concentration using Tamson TV-2000
viscometer at 30 ꢀC. Gel permeation chromatographic (GPC)
analysis was performed on a Lab Alliance RI2000 instrument
(one column, MIXED-D from Polymer Laboratories) con-
nected with one refractive index detector from Schambeck
SFD Gmbh. All GPC analyses were performed using a poly-
mer/N,N-dimethylformamide (DMF) solution at a flow rate
of 1 mL/min at 70 ꢀC and calibrated with polystyrene
Polymer Synthesis
The synthesis of polyamide Ib was used as an example to
illustrate the general synthetic route used to produce the
polyamides. A mixture of 380.9 mg (1.20 mmol) of the dia-
mine monomer (2), 310.9 mg (1.20 mmol) of 4,40-oxydiben-
zoic acid (3b), 110.0 mg of calcium chloride, 1.00 mL of
triphenyl phosphite, 0.50 mL of pyridine, and 1.00 mL of N-
methyl-2-pyrrolidinone (NMP) was heated with stirring at
105 ꢀC for 3 h. The obtained polymer solution was poured
slowly into 300 mL of stirred water giving rise to a stringy,
fiber-like precipitate that was collected by filtration, washed
thoroughly with hot water and methanol, and dried under
vacuum at 100 ꢀC; yield: 0.646 g (99%). Reprecipitations of
the polymer by N,N-dimethylacetamide (DMAc)/methanol
were carried out twice for further purification. The inherent
viscosity and weight-average molecular weights (Mw) of the
obtained polyamide Ib was 0.68 dL/g (measured at a con-
centration of 0.5 g/dL in DMAc at 30 ꢀC) and 151,900 dal-
tons, respectively. The FT-IR spectrum of Ib (film) exhibited
characteristic amide absorption bands at 3312 cmꢁ1 (NAH
5272
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 1 Synthetic routes for diamine monomers 2 and 20, and model compounds M-3Me and M-Me.
standards. Thermogravimetric analysis (TGA) was conducted
with a PerkinElmer Pyris 1 TGA. Experiments were carried
out on ꢂ6–8-mg film samples heated in flowing nitrogen or
counter electrode, and a Ag/AgCl cell as the reference elec-
trode. CE (g) determines the amount of optical density
change (dOD) at a specific absorption wavelength induced as
a function of the injected/ejected charge (Q; also termed as
electroactivity), which is determined from the in situ experi-
ments. CE is given by the equation: g ¼ dOD/Q ¼ log[Tb/
Tc]/Q, where g (cm2/C) is the CE at a given wavelength, and
Tb and Tc are the bleached and colored transmittance values,
respectively. The thickness of the polyamide thin films was
measured by a-step profilometer (Surfcorder ET3000,
Kosaka Lab., Japan).
3
ꢀ
air (flow rate ¼ 20 cm /min) at a heating rate of 20 C/min.
DSC analyses were performed on a PerkinElmer Pyris 1 DSC
at a scan rate of 10 ꢀC/min in flowing nitrogen (20 cm3/
min). Electrochemistry was performed with a CH Instru-
ments 611B electrochemical analyzer. Voltammograms are
presented with the positive potential pointing to the left and
with increasing anodic currents pointing downwards. Cyclic
voltammetry (CV) was conducted with the use of a three-
electrode cell in which indium-tin oxide (ITO, area of poly-
mer films ꢂ0.5 ꢃ 1.1 cm2) was used as a working electrode.
A platinum wire was used as an auxiliary electrode. All cell
potentials were taken by using a homemade Ag/AgCl, KCl
(sat.) reference electrode. Spectroelectrochemical experi-
ments were carried out in a cell built from a 1-cm commer-
cial UV-visible cuvette using Hewlett–Packard 8453 UV–vis
diode-array spectrophotometer. The ITO-coated glass slide
was used as the working electrode, a platinum wire as the
RESULTS AND DISCUSSION
Monomer Synthesis
The new monomer Me3TPA-diamine (2) was synthesized by
hydrazine Pd/C-catalyzed reduction of the dinitro compound
(1) resulting from the sodium hydride-mediated aromatic
nucleophilic substitution reaction of 2,4,6-trimethylaniline
with 4-fluoronitrobenzene (Scheme 1). Elemental analysis,
SYNTHESIS OF THE TRIPHENYLAMINE-BASED ARAMIDS, YEN, GUO, AND LIOU
5273

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 X-ray structures of dinitro compounds (a) 1 and (b) 10. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
FT-IR, and NMR spectroscopic techniques were used to iden-
tify structures of the intermediate dinitro compound (1) and
the targeted diamine monomer (2). The FT-IR spectra of
these two synthesized compounds are illustrated in Figure
S1 in Supporting Information. The nitro groups of compound
(1) exhibited two characteristic absorption bands at around
1578 and 1307 cmꢁ1 due to NO2 asymmetric and symmetric
stretching. After reduction to diamine monomer (2), the
characteristic absorption bands of the nitro group disap-
peared and the primary amino group showed the typical
absorption pair at 3401 and 3326 cmꢁ1 (NAH stretching).
1H and 13C NMR spectra of the dinitro compound (1) and
diamine monomer (2) are illustrated in Figure S2 and Figure
S3, respectively, and agree well with the proposed molecular
structures. The molecular structures of compounds (1) and
(10) were further confirmed by single-crystal X-ray diffrac-
tion (Fig. 1). The single crystal of (1) was acquired by slow
crystallization of a DMF solution. As shown in Figure 1, the
molecular structures both display a propeller-shaped confor-
mation of the TPA core. Notably, the twist of N2AC7 bond in
ortho-methyl-substituted compound 1 was more than that in
compound 10 due to the steric hindrance of ortho-dimethyl-
substituents on the phenyl ring. The dihedral angle of the
ortho-dimethyl-substituted phenyl ring in TPA core is about
77.1ꢀ. The model compounds M-3Me and M-Me were pre-
pared from the condensation of diamine 2 and 20 with two
equivalent amounts of benzoic acid as shown in Scheme 1,
and their IR spectroscopic data can be found in the ESI (Fig-
ure S4).
acids (3a–3c), respectively, (Scheme 2). The polymerization
was carried out via solution polycondensation using tri-
phenyl phosphite and pyridine as condensing agents. All
polymerization reactions proceeded homogeneously and
gave high molecular weights. The obtained polyamides had
inherent viscosities in the range of 0.40–1.41 dL/g with
weight-average molecular weights (Mw) and polydispersity
(PDI) of 44,700–151,900 daltons and 1.49–2.12, respectively,
relative to polystyrene standards (Table S1). All the high mo-
lecular weights polymers could afford transparent and tough
films via solution casting. The structures of the polyamides
were confirmed by IR spectroscopy. As shown in Figure S5, a
typical IR spectrum for polyamide Ib exhibited characteristic
absorption bands of the amide group at around 3312 cmꢁ1
(NAH stretch) and 1650 cmꢁ1 (amide carbonyl). A structur-
ally related polyamide IIIb derived from diamine 200 is used
for comparison, and the preparation of polymer IIIb has
been described previously.27
Basic Characterization
The solubility properties of polymers I and II were investi-
gated qualitatively, and the results are also listed in Table S2.
Most of the polyamides were readily soluble in polar aprotic
organic solvents, such as NMP, DMAc, DMF, dimethyl sulfox-
ide (DMSO), and m-cresol. Thus, the excellent solubility
makes these polymers as potential candidates for practical
applications by spin-coating or inkjet-printing processes to
afford high-quality thin films for optoelectronic devices. The
WAXD patterns of the polyamides given in Figure S6 indi-
cated that the polymers were essentially amorphous. Their
high solubility and amorphous properties can be attributed
to the incorporation of bulky and noncoplanar Me3TPA moi-
ety along the polymer backbone, which results in a high
steric hindrance for close packing and thus reduces their
crystallization tendency.
Polymer Synthesis
According to the phosphorylation technique described by
Yamazaki,25,26 two series of new polyamides I and II with
main-chain Me3TPA and MeTPA units were synthesized from
the diamine monomers 2 and 20 with three dicarboxylic
5274
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 2 Synthesis of aromatic polyamides Ia-Ic and IIa-IIc. The photographs show appearance of the flexible films (thickness:
65–80 lm). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
The thermal properties of polyamides were examined by
TGA and DSC, and the thermal behavior data are summar-
ized in Table S3. Typical TGA curves of polyamide Ic and IIc
are shown in Figure S7. All the prepared polyamides exhib-
ited good thermal stability with insignificant weight loss up
to 450 ꢀC under nitrogen or air atmosphere. The 10%
weight loss temperatures of these polymers in nitrogen aꢀnd
air were recorded in the range of 450–535 and 460–540 C,
respectively. The concentration of carbonized residue (char
yield) of these polꢀymers in a nitrogen atmosphere was more
than 62% at 800 C. The high char yields of these polymers
can be ascribed to their high aromatic content. The glass-
transition temperatures (Tg) of polyamides I and II could be
easily measured by the DSC thermograms; they were
observed in the range of 314–329 and 293–313 ꢀC, respec-
tively, depending on the stiffness of the polymer chain. DSC
curves of polyamide Ic and IIc are shown in Figure S8. The
higher Tg in polyamide I series could be attributed to the
introduction of Me3TPA moieties, which hindered the rota-
tion of ortho-dimethyl-substituted phenyl ring and the mov-
ability of the polymer backbone. All the polymers indicated
no clear melting endotherms up to the decomposition tem-
peratures on the DSC thermograms, which supports the
amorphous nature of these polyamides.
Electrochemical Properties
The electrochemical properties of the polyamides were
investigated by CV conducted by film cast on a ITO-coated
glass slide as working electrode in anhydrous acetonitrile
(CH3CN) using 0.1 M of TBAP as an electrolyte under nitro-
gen atmosphere. The typical CV for polyamides Ib (with
2,4,6-trimethyl-substituted), IIb (with 4-methyl-substituted),
and IIIb (without substitutions) are shown in Figure 2 for
comparison. There is one reversible oxidation redox couples
at half-wave potential (E1/2) value of 0.91 (Eonset ¼ 0.78) V
for polyamide IIIb (without substitution) in the first oxida-
tion CV scan [Fig. 2(a)]. After scanning for several cycles, a
new pair of redox peaks with E1/2 at 0.77 and 0.92 V corre-
sponding to TPB structure was observed indicating the for-
mation of 4,40-diaminobiphenyl linkages (as shown in
Scheme 3). The similar observation was also described in
our previous study.28 To prevent the irreversible coupling
SYNTHESIS OF THE TRIPHENYLAMINE-BASED ARAMIDS, YEN, GUO, AND LIOU
5275

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 Cyclic voltammetric dia-
grams of polyamide (a) IIIb, (b) Ib,
and (c) IIb films on a ITO-coated
glass substrate over cyclic scans
and ferrocene (inset) in 0.1 M TBAP/
CH3CN at a scan rate of 50 mV/s.
reactions, we tried to design and prepare the polyamides
with electron-donating methyl substituents not only at para
position but also at ortho position of phenyl rings in a pro-
peller-like TPA geometry. The resulting polyamides showed
reversible oxidation redox couples at the E1/2 value of 0.89
(Eonset ¼ 0.75) V for polyamide Ib, and 0.83 (Eonset ¼ 0.70)
para-methyl protection) still exhibited good electrochemical
stability. Furthermore, the Me3TPA-based polyamides
I
revealed higher oxidation potential than polyamide II even
though three electron donating methyl groups were attached
to the phenyl ring of the Me3TPA unit. This unexpected phe-
nomenon could be attributed to steric hindrance resulted
from the large twisting of the ortho-dimethyl-substituted
phenyl ring (ꢂ77.1ꢀ), thus the n electrons of the highest
occupied molecular orbital (HOMO) on the N atom has lower
energy level, which means the orbital overlap and electron
delocalization between p-orbital of N atom and p-orbital of
Me3-substituted phenyl ring is low and less resonance when
oxidized from its neutral state of Me3-TPA moiety. The CV
V
for polyamide IIb, respectively. Both trimethyl- and
methyl-substituted TPA structures could effectively prevent
the obtained polyamides from taking place irreversible cou-
pling reaction after 40 continuous cyclic scans. However,
polyamide Ib (with ortho- and para-methyl protection) grad-
ually lost redox reversibility after more preceding CV scans.
On the contrary, the corresponding polyamide IIb (with only
5276
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 3 TPA-derived polyamide without para-substitution IIIb could be dimerized to form TPB moiety by tail to tail coupling.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
curve of polyamide Ib with higher oxidative potential was
also in agreement with the results of 1 and M-3Me (Figure
S9), emphasizing the less resonance stabilization of cation
radical than that in mono-methyl-substituted polyamide IIb.
Analog ortho-substitution effect was also mentioned in the
literatures.29,30 The other polyamides showed similar CV
curves to that of Ib and IIb, respectively. The redox poten-
tials of the polyamides as well as their HOMO and lowest
unoccupied molecular orbital (vs. vacuum) are shown in
Table 1. The HOMO level or called ionization potentials (vs.
vacuum) of polyamides Ia–Ic and IIa–IIc were estimated
from the onset of their oxidation in CV experiments as 5.16–
5.24 and 5.11–5.19 eV (on the basis that ferrocene/ferroce-
nium is 4.8 eV below the vacuum level with Eonset ¼ 0.36 V).
Spectroelectrochemical and Electrochromic Properties
Spectroelectrochemical investigation was used to evaluate
the optical properties of the electrochromic materials, and
the polyamide film was cast on an ITO-coated glass slide,
and a homemade electrochemical cell was built from a
TABLE 1 Redox Potentials and Energy Levels of Polyamides
Oxidation Potential
Thin Film (nm)
(V)a
kmax
konset
E1/2
Eonset
Eg (eV)b
HOMO (eV)c
LUMO (eV)
Ia
Ib
Ic
306
338
343
314
343
353
370
404
413
375
413
423
0.84
0.89
0.93
0.80
0.83
0.88
0.72
0.75
0.80
0.67
0.70
0.75
3.35
3.07
3.00
3.30
3.00
2.93
5.16
5.19
5.24
5.11
5.14
5.19
1.81
2.12
2.24
1.81
2.14
2.26
IIa
IIb
IIc
a
c
From cyclic votammograms versus Ag/AgCl in CH3CN. E1/2: Average
The HOMO energy levels were calculated from cyclic voltammetry and
were referenced to ferrocene (4.8 eV; onset ¼ 0.36 V).
potential of the redox couple peaks.
b
The data were calculated from polymer films by the equation: Eg
1240/konset (energy gap between HOMO and LUMO).
¼
SYNTHESIS OF THE TRIPHENYLAMINE-BASED ARAMIDS, YEN, GUO, AND LIOU
5277

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 Electrochromic behavior (left) at applied potentials of (a) 0, (b) 0.70, (c) 0.75, (d) 0.80, (e) 0.90, (f) 0.95, (g) 1.00, and (h)
1.05 (V vs. Ag/AgCl) and 3D spectroelectrochemical behavior (right) from 0.00 to 1.05 (V vs. Ag/AgCl) of polyamide Ib thin film
(ꢂ120 nm in thickness) on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN.
commercial UV–vis cuvette. The cell was placed in the opti-
cal path of the sample light beam in a UV–vis-NIR spectro-
photometer to acquire electronic absorption spectra under
potential control in a 0.1 M TBAP/CH3CN solution. UV–vis-
NIR absorbance curves correlated to applied potentials and
three-dimensional transmittance-wavelength-applied poten-
tial correlation of Ib film were depicted in Figure 3. The Ib
film exhibited strong absorption at ꢂ338 nm, characteristic
for TPA unit in the neutral form (0 V) with almost color-
less and transparent in the visible region. On oxidation
(increasing applied voltage from 0 to 1.05 V), the intensity
of the absorption peak at 338 nm gradually decreased
while two new peaks grew up at 403 and 868 nm gradu-
ally in intensity due to the formation of a stable monoca-
tion radical of the Me3TPA unit. From the inset shown in
Figure 3, the polyamide Ib film switches from a transmis-
sive neutral form to a highly visible-light absorbing oxi-
dized green form with a high optical transmittance change
(DT%) of 84% at 868 nm and 58% at 403 nm, respec-
tively. In addition, the spectroelectrochemical behavior of
polyamide IIb film was shown in Figure 4, and the peak of
characteristic absorbance at 343 nm for polyamides IIb
decreased gradually with increasing applied potential posi-
tively from 0 to 1.00 V, whereas new bands grew at 401
and 807 nm additionally. This new spectrum was assigned
as that of the monocation radical MeTPA, which revealed
the color of bluish-green after oxidation. From the inset
shown in Figure 4, the polyamide IIb film switches from a
transparent neutral state to a highly visible light absorbing
oxidized state showing highly optical transmittance change
(DT%) of 87% at 807 nm and 59% at 401 nm. Besides,
the distribution of electrochromic coloration across the
polymer film was very homogeneous and stable even after
continuous 100 redox cycles. Furthermore, when compared
the cutoff wavelengths (absorption edge; konset) of polymer
films between Ib with and IIb in neutral state, we
observed ꢂ5–10-nm blue shift for polyamide Ib, indicating
a shorter conjugation length for polyamide Ib than IIb.
Therefore, the possible resonance forms for polyamide I
and II was proposed in Scheme 4.
FIGURE 4 Electrochromic behavior (left) at applied potentials of (a) 0, (b) 0.65, (c) 0.70, (d) 0.80, (e) 0.85, (f) 0.90, (g) 0.95, (h) 1.00
(V vs. Ag/AgCl), and 3D spectroelectrochemical behavior (right) from 0.00 to 1.00 (V vs. Ag/AgCl) of polyamide IIb thin film (ꢂ145
nm in thickness) on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN.
5278
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 4 Resonance stabilization forms of cation radical of I and II series. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
The color switching times were estimated by applying a
potential step, and the absorbance profiles were followed.
The switching time of polyamide Ib shown in Figure S10
was defined and calculated as the time required to reach
90% of the full switch absorbance, because it is difficult to
perceive any further color change with naked eye beyond
this point. The electrochromic stability of the polyamide Ib
and IIb films was determined by measuring the optical
change as a function of the number of switching cycles (Fig.
5). The CE (g ¼ dOD/Q) at different switching steps were
summarized in Table 2. After continuous 15 cyclic scanning,
the polyamide Ib film reduced electroactivity gradually. On
the contrary, the polymer IIb still exhibited good stability of
electrochromic characteristics. These results were also con-
sistent in the above-mentioned assumptions of the lower res-
onance capability due to large twist dihedral angle of inside
N2AC7 bond in Me3TPA-based polyamide I series.
CONCLUSIONS
Two series of novel organo-soluble and thermally stable elec-
trochromic aromatic polyamides containing electroactive
methyl-substituted TPA moieties in the backbone were pre-
pared from the diamine monomers, Me3TPA-diamine and
MeTPA-diamine, with various aromatic dicarboxylic acids,
respectively. However, the introduction of more electron-
donating methyl groups at ortho position as Me3-substituted
TPA into the polymer main chain could not effectively
SYNTHESIS OF THE TRIPHENYLAMINE-BASED ARAMIDS, YEN, GUO, AND LIOU
5279

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Electrochromic switch-
ing between (A) 0 and 1.02 V (vs.
Ag/AgCl) of polyamide Ib thin film
(ꢂ165 nm in thickness) on the
ITO-coated glass substrate (coated
area: 1.1 ꢃ 0.5 cm2) and (B) 0 and
0.96 V of polyamide IIb thin film
(ꢂ165 nm in thickness and coated
area: 1.0 ꢃ0.5 cm2) in 0.1 M TBAP/
CH3CN with a cycle time of 40 s.
(a) Current consumption and (b)
absorbance change monitored at
the given wavelength. [Color fig-
ure can be viewed in the online
issue, which is available at
wileyonlinelibrary.com.]
TABLE 2 Optical and Electrochemical Data Collected for Coloration Efficiency Measurements of Polyamides Ib and IIb
Decay (%)e
Q (mC/cm2)c
g (cm2/C)d
Ib
IIb
Cycling Timesa
dOD868
dOD807
Ib
IIb
Ib
IIb
Ib
IIb
b
b
1
3
0.808
0.742
0.662
0.594
0.542
0.503
0.751
0.751
0.740
0.731
0.728
0.722
5.90
5.92
5.78
5.64
5.58
5.54
3.34
3.34
3.30
3.29
3.27
3.27
136
125
114
104
97
225
225
224
222
222
221
0
0
8.08
16.18
23.53
28.68
33.82
0
6
0.44
1.33
1.33
1.78
9
12
15
a
90
c
Switching between 0 and 1.02 for Ib, 0 and 0.96 for IIb (V vs. Ag/
Ejected charge, determined from the in situ experiments.
Coloration efficiency is derived from the equation g ¼ dOD/Q.
Decay of coloration efficiency after cyclic scans.
d
e
AgCl).
b
Optical density change at the given wavelength.
5280
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
16 Liou, G. S.; Hsiao, S. H.; Ishida, M.; Kakimoto, M.; Imai, Y.
stabilize the cationic radicals of oxidized form comparing
with the corresponding polymers with Me-substituted TPA
units due to large twist (ꢂ77.1ꢀ) of N2-C7 bond in Me3TPA
structure. The unexpected electrochemical behavior of higher
oxidation potential and lower electrochemical stability of
Me3TPA-polyamides I than MeTPA corresponding polymers
was due to the higher steric hindrance of ortho-substituents
in Me3TPA moieties, thus made the resonance stabilization of
cation radical more difficult for the Me3-substituted phenyl
ring.
J Polym Sci Part A: Polym Chem 2002, 40, 3815–3822.
17 Liou, G. S.; Hsiao, S. H. J Polym Sci Part A: Polym Chem
2003, 41, 94–105.
18 (a) Hsiao, S. H.; Chen, C. W.; Liou, G. S. J Polym Sci Part A:
Polym Chem 2004, 42, 3302–3313; (b) Liou, G. S.; Lin, K. H. J
Polym Sci Part A: Polym Chem 2009, 47, 1988–2001; (c) Kung, Y.
C.; Liou, G. S.; Hsiao, S. H. J Polym Sci Part A: Polym Chem
2009, 47, 1740–1755; (d) Hsiao, S. H.; Liou, G. S.; Wang, H. M. J
Polym Sci Part A: Polym Chem 2009, 47, 2330–2343.
The authors are grateful to the National Science Council of
the Republic of China for financial support of this work,
and also to Ting-Shen Kuo of National Taiwan Normal Uni-
versity for giving helps in measuring X-ray single-crystal
diffraction.
19 (a) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.;
Leedy, D. W.; Adams, R. N.
J Am Chem Soc 1966, 88,
3498–3503; (b) Otero, L.; Sereno, L.; Fungo, F.; Liao, Y. L.; Lin,
C. Y.; Wong, K. T. Chem Mater 2006, 18, 3495–3502; (c) Kuoro-
sawa, T.; Chueh, C. C.; Liu, C. L.; Higashihara, T.; Ueda, M.;
Chen, W. C. Macromolecules 2010, 43, 1236–1244.
20 (a) Hagopian, L.; Kohler, G.; Walter, R. I. J Phys Chem 1967,
71, 2290–2296; (b) Ito, A.; Ino, H.; Tanaka, K.; Kanemoto, K.;
Kato, T. J Org Chem 2002, 67, 491–498.
REFERENCES AND NOTES
1 Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Rosseinsky,
Electrochromism: Fundamentals and Applications; VCH: Wein-
heim, 1995.
21 (a) Chang, C. W.; Liou, G. S.; Hsiao, S. H. J Mater Chem
2007, 17, 1007–1015; (b) Liou, G. S.; Chang, C. W. Macromole-
cules 2008, 41, 1667–1674. (c) Hsiao, S. H.; Liou, G. S.; Kung, Y.
C.; Yen, H. J. Macromolecules 2008, 41, 2800–2808; (d) Chang,
C. W.; Chung, C. H.; Liou, G. S. Macromolecules 2008, 41,
8441–8451; (e) Chang, C. W.; Liou, G. S. J Mater Chem 2008,
18, 5638–5646; (f) Chang, C. W.; Yen, H. J.; Huang, K. Y.; Yeh,
J. M.; Liou, G. S. J Polym Sci Part A: Polym Chem 2008, 46,
7937–7949; (g) Yen, H. J.; Liou, G. S. Chem Mater 2009, 21,
4062–4070; (h) Liou, G. S.; Chang, H. W.; Lin, K. H.; Su, Y. O. J
Polym Sci Part A: Polym Chem 2009, 47, 2330–2343.
2 Green, M. Chem Ind 1996, 17, 641–647.
3 Bach, U.; Corr, D.; Lupo, D.; Pichot, F.; Ryan, M. Adv Mater
2002, 14, 845–848.
4 Sonmez, G.; Meng, H.; Wudl, F. Chem Mater 2004, 16,
574–580.
5 Cassidy, P. E. Thermally Stable Polymers, Marcel Dekker,
New York, 1980.
6 Yang, H. H. Aromatic High-Strength Fibers; Wiley: New York,
1989.
22 Kvarnstrom, C.; Petr, A.; Damlin, P.; Lindfors, T.; Ivaska, A.;
¨
Dunsch, L. J Solid State Electrochem 2002, 6, 505–512.
7 Imai, Y. High Perform Polym 1995, 7, 337–345.
8 Imai, Y. React Funct Polym 1996, 30, 3–15.
23 Wheeler, E. L. U.S. Patent 3,277,174, Oct. 4, 1966.
24 Oishi, Y.; Takado, H.; Yoneyama, M.; Kakimoto, M.; Imai, Y.
9 Hsiao, S. H.; Li, C. T. Macromolecules 1998, 31, 7213–7217.
J Polym Sci Part A: Polym Chem 1990, 28, 1763–1769.
10 Liou, G. S. J Polym Sci Part A: Polym Chem 1998, 36,
25 Yamazaki, N.; Higashi, F.; Kawabata, J. J Polym Sci Polym
1937–1943.
Chem Ed 1974, 12, 2149–2154.
11 Eastmond, G. C.; Paprotny, J.; Irwin, R. S. Polymer 1999, 40,
26 Yamazaki, N.; Matsumoto, M.; Higashi, F.
J Polym Sci
469–486.
Polym Chem Ed 1975, 13, 1373–1380.
12 Eastmond, G. C.; Gibas, M.; Paprotny, J. Eur Polym Mater
27 Su, T. H.; Hsiao, S. H.; Liou, G. S. J Polym Sci Part A: Polym
1999, 35, 2097–2106.
Chem 2005, 43, 2085–2098.
13 Reddy, D. S.; Chou, C. H.; Shu, C. F.; Lee, G. H. Polymer
28 Yen, H. J.; Liou, G. S. Org Electron 2010, 11, 299–310.
2003, 44, 557–563.
29 Nishiumi, T.; Nomura, Y.; Chimoto, Y.; Higuchi, M.; Yama-
14 Myung, B. Y.; Ahn, C. J.; Yoon, T. H. Polymer 2004, 45,
moto, K. J Phys Chem B 2004, 108, 7992–8000.
3185–3193.
30 Wang, B. C.; Liao, H. R.; Chang, J. C.; Chen, L.; Yeh, J. T.
15 Liou, G. S.; Hsiao, S. H.; Ishida, M.; Kakimoto, M.; Imai, Y. J
J Lumin 2007, 124, 333–342.
Polym Sci Part A: Polym Chem 2002, 40, 2810–2818.
SYNTHESIS OF THE TRIPHENYLAMINE-BASED ARAMIDS, YEN, GUO, AND LIOU
5281