Mixed-valence class i transition and electrochemistry of bis(triphenylamine)-based aramids containing isolated ether-linkage

Article abstract of DOI:10.1002/pola.24819

A series of solution-processable electrochromic (EC) aromatic polyamides with bis(triphenylamine)ether (TPAO) units in the backbone were prepared by the phosphorylation polyamidation from a newly synthesized diamine monomer, bis(N-4-aminophenyl-N-4-methox

Full text of DOI:10.1002/pola.24819

Mixed-Valence Class I Transition and Electrochemistry of
Bis(triphenylamine)-Based Aramids Containing Isolated
Ether-Linkage
HUNG-JU YEN,1 SHIUE-MING GUO,1 GUEY-SHENG LIOU,1 JEN-CHIEH CHUNG,2 YU-CHANG LIU,2 YUNG-FANG LU,2
YU-ZHEN ZENG2
1Functional Polymeric Materials Laboratory, Institute of Polymer Science and Engineering,
National Taiwan University, 1 Roosevelt Road, 4th Sec., Taipei 10617, Taiwan
2Institute of Nuclear Energy Research, Atomic Energy Council, No. 1000, Wenhua Rd., Jiaan Village,
Longtan Township, Taoyuan County 32546, Taiwan
Received 16 May 2011; accepted 1 June 2011
DOI: 10.1002/pola.24819
Published online 7 July 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A series of solution-processable electrochromic
(EC) aromatic polyamides with bis(triphenylamine)ether (TPAO)
units in the backbone were prepared by the phosphorylation
polyamidation from a newly synthesized diamine monomer,
bis(N-4-aminophenyl-N-4-methoxyphenyl-4-aminophenyl)ether,
and various dicarboxylic acids. These polymers were highly
soluble in many organic solvents and showed useful levels of
thermal stability associated with high glass-transition tempera-
tures and high char yields (higher than 50 at 800 ꢀC in nitrogen).
The polymer films showed reversible electrochemical oxidation
and electrochromism with high contrast ratio in the visible
range, which also exhibited moderate coloration efficiency (CE),
low switching time, and good stability. Especially, the polya-
mides with two electroactive nitrogen centers only showed one-
stage oxidative coloring (no intervalence charge-transfer [IV-CT]
band was detected), implying the two electrons are simultane-
ously removed from the TPAO units on account of the ether-link-
age definitely isolated the two redox centers. The mixed-valence
(MV) Class I/II/III transition and electrochemistry of the synthe-
sized model compounds were investigated for the bridged triar-
ylamine system with various NAN distances and intramolecular
C
V
electron transfer (ET) capability.
2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 49: 3805–3816, 2011
KEYWORDS: electrochemistry; high performance polymers;
polyamides; polycondensation
INTRODUCTION Electrochromism involves electroactive mate-
rials that present a reversible change in optical properties
when the material is electrochemically oxidized or reduced.1
Recently, electrochromic (EC) utilization, such as antiglare
back mirrors, smart windows, and E-papers has already been
commercialized.2 In general, most applications require EC
materials with a high contrast ratio (DT%), coloration effi-
ciency (CE), cycle life, and rapid response time. EC materials,
such as transition-metal oxides, inorganic coordination com-
plexes, conjugated polymers, and organic molecules, have
been directed from optical changes in the visible-light (e.g.,
400–800 nm) to the near infrared (NIR; e.g., 800–2000 nm)
regions increasingly.3 According to Robin and Day,4 the
p-phenylenediamine-containing molecule is an interesting
anodic EC system for NIR applications due to its particular
intramolecular electron transfer (ET) in the oxidized states.
systems can be classified into three categories:4 Class I with
practically no coupling between the different redox states,
Class II with moderate electronic coupling, and Class III with
strong electronic coupling (the electron is delocalized over
the two redox centers). In recent experimental and theoreti-
cal studies,6 the N,N,N0,N0-tetraphenyl-p-phenylenediamine
(TPPA) cation radical has been reported as a symmetrical
delocalized Class III structure with a strong electronic cou-
pling, whereas N,N,N0,N0-tetraphenylbenzidine (TPB) cation
radical was demonstrated as a Class II structure with a
weakly electronic coupling. Among the numerous redox cen-
ters, triarylamine system is of great interest of investigating
hole-transfer processes between redox centers. In early
research, the MV character of bis(triarylamine) derivatives
with varying p-electron spacers has been investigated. The
distances of triarylamine redox centers vary from 0.5 to
2 nm. All radical cation species show rather strong interva-
lence charge-transfer (IV-CT) bands in the NIR region.7 In
our laboratory, we also have systemically developed TPPA
and TPB-containing aromatic polyamides,8 which revealed
Intramolecular ET processes have been studied extensively
in the mixed-valence (MV) systems.5 They usually used one-
dimensional MV compounds containing two or more redox
states connected via saturated or unsaturated molecule. MV
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
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3805–3816 (2011) 2011 Wiley Periodicals, Inc.
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SCHEME 1 Monomer synthesis.
typical Class III and II transitions in MV systems,
respectively.
ing and spin-coating techniques. This is beneficial for their fab-
rication of large-area, thin-film EC devices.
In this contribution, we therefore synthesized the diamine
monomer, bis(N-4-aminophenyl-N-4-methoxyphenyl-4-amino-
phenyl)ether (3), and its derived aromatic polyamides con-
taining ether-linkage in the main chain with para-substituted
methoxy groups. In addition to the general properties (such
as inherent viscosities, solubility, and thermal properties),
the electrochemical and EC behaviors of these polymers
were also investigated. To demonstrate that the NIR absorb-
ance and MV system could be adjusted by the distance
between two electroactive nitrogen redox centers, electro-
chemistry and spectroelectrochemistry of the corresponding
model compounds M1, M2, and 2 were investigated and dis-
cussed herein.
Therefore, our strategy of this work is to design and synthesize
bis(triphenylamine)ether (TPAO)-based EC polymers with a
ether-linkage between two electroactive nitrogen centers. The
MV Class I character and behavior would be obtained in the re-
sultant electroactive polymers by using the ether-linkage as a
block and definitely isolated the two redox centers. These elec-
troactive polymers with high molecular weights and excellent
thermal stability should be readily prepared by using conven-
tional polycondensation.9 Because of the incorporation of
packing-disruptive TPAO units into the polymer backbone,
these novel polymers should also have excellent solubility in
various polar organic solvents; thus, transparent and flexible
polymer thin films could be easily prepared by solution-cast-
RESULTS AND DISCUSSION
urements were used to identify structures of the intermedi-
ate compound (1), dinitro compound (2), and the target dia-
mine monomer (3). The FTIR spectra of these three
synthesized compounds are illustrated in Supporting Infor-
mation Figure S1. The nitro groups of compound 2 exhibited
two characteristic bands at around 1586 and 1314 cmꢁ1
due to nitro asymmetric and symmetric stretching. After
reduced to diamine monomer (3), the characteristic absorp-
tion bands of the nitro group disappeared and the primary
amino group showed the typical absorption pair at 3443 and
Monomer Synthesis
The new aromatic diamine bis(N-4-aminophenyl-N-4-methox-
yphenyl-4-aminophenyl)ether (3) was synthesized by hydro-
gen Pd/C-catalyzed reduction of the dinitro compound bis(N-
4-methoxyphenyl-N-4-nitrophenyl-4-aminophenyl)ether (2)
resulting from the potassium carbonate-mediated aromatic
nucleophilic substitution reaction of bis(N-4-nitrophenyl-4-
aminophenyl)ether with 4-iodoanisole (Scheme 1). Elemental
analysis, Fourier transform infrared (FTIR), and NMR meas-
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SCHEME 2 Synthesis of polyamides by direct polycondensation reaction (60–80 lm). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
3365 cmꢁ1 (NAH stretching). 1H, 13C, HAH COSY, and CAH
HMQC NMR spectra of the compounds 1–3 are illustrated in
Supporting Information Figures S2–S6 and agree well with
synthesis and characterization have been described
previously.11
1
Solubility and Film Property
the proposed molecular structures. The H NMR spectra con-
The solubility behavior of polymers Ia–Ic was investigated
and the results are also listed in Supporting Information Table
S2. Most of the polyamides were readily soluble in polar
aprotic organic solvents such as N-methyl-2-pyrrolidinone
(NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylforma-
mide (DMF), and m-cresol. Thus, the excellent solubility makes
these polymers as potential candidates for practical applica-
tions by spin-coating or inkjet-printing processes to afford
high performance thin films for optoelectronic devices. As
mentioned earlier, the polyamides Ia–Ic could be solution cast
into flexible, transparent, and tough films. Their high solubility
firm that the nitro groups have been completely transformed
into amino groups by the high field shift of the aromatic pro-
tons and the resonance signals at around 4.96 ppm corre-
sponding to the amino protons.
Polymer Synthesis
According to the phosphorylation technique first described
by Yamazaki et al.,10 a series of novel polyamides (Ia–Ic)
with TPAO units in the backbone were synthesized from the
diamine (3) and various dicarboxylic acids via solution poly-
condensation using triphenyl phosphite and pyridine as con-
densing agents (Scheme 2). All the polymerization proceeded
homogeneously throughout the reaction and afforded clear
and highly viscous polymer solutions, which formed a tough,
fiberlike precipitate when slowly poured into stirring metha-
nol. The obtained polyamides had inherent viscosities in the
range of 0.43–0.63 dL/g with number-average molecular
weights (Mn) and polydispersity (PDI) of 109,000–151,200
Da and 1.76–1.81, respectively, relative to polystyrene stand-
ards (Supporting Information Table S1) and could be solu-
tion cast into flexible and tough films. The appearance and
quality of these films are also shown in Scheme 2. The for-
mation of polyamides was also confirmed by IR and NMR
spectroscopy. Supporting Information Figure S7 exhibits a
typical IR spectrum for polyamide Ib. The characteristic IR
absorption bands of the amide group were around 3317
(NAH stretching) and 1649 cmꢁ1 (amide carbonyl). Support-
TABLE 1 Redox Potentials and Energy Levels of Polyamides
Thin Film
(nm)
Oxidation
Potential (V)a
Eg
HOMO LUMO
Code kmax konset
E
E
Eonset (eV)b (eV)c
(eV)
ox
p;an
ox
p;ca
Ia
Ib
Ic
I0b
a
312 366
313 400
306 407
345 407
1.00 0.66 0.67
1.02 0.66 0.69
1.03 0.69 0.71
0.94 0.58 0.65
3.39 5.11
3.10 5.13
3.05 5.15
3.05 5.06
1.72
2.03
2.10
2.01
From cyclic voltammograms versus Ag/AgCl in CH3CN.
b
The data were calculated from polymer films by the equation: Eg
1240/konset (energy gap between HOMO and LUMO).
¼
c
The HOMO energy levels were calculated from cyclic voltammetry and
were referenced to ferrocene (4.8 eV; onset ¼ 0.36 V).
1
ing Information Figure S8 depicts a typical set of H and 13C
spectra of polyamide Ib in dimethyl sulfoxide-d6 (DMSO-d6);
all the peaks could be readily assigned to the hydrogen
atoms of the recurring unit. The resonance peak appearing
at 10.15 ppm in the 1H NMR spectrum supports the forma-
tion of amide linkages. The structurally related polyamide I0b
shown in Table 1 is used for comparison studies and the
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FIGURE 1 Cyclic voltammetric diagrams of polyamide (a) Ib and (b) I0b films on an ITO-coated glass substrate over cyclic scans
and ferrocene (inset) in 0.1 M TBAP/CH3CN at a scan rate of 50 mV/s.
and amorphous nature can be attributed to the incorporation
of bulky, three-dimensional (3D) TPAO moiety along the poly-
mer backbone, which results in a high steric hindrance for
close packing thus reduces their crystallization tendency.
ing two electroactive nitrogen centers revealed only one re-
versible oxidation redox (Eonset ¼ 0.69 V), implying that two
electrons are almost simultaneously removed from the two
definitely separated triarylamine units with independent
behavior, to form two radical cations on account of the block
of ether-linkage between the two redox centers, which is
similar to I0b [with (OMe)TPA units]. The lower oxidation
potential of polymer I0b (Eonset ¼ 0.65 V) could be attribut-
able to the electron-donating methoxy- and amino-substitu-
ents at the para-position of phenyl rings. After hundred con-
tinuous cyclic scans between 0.00 and 1.13 V, the polymer
films still exhibited good electrochemical stability.
Thermal Properties
The thermal properties of the polyamides were investigated by
thermogravimetric analysis (TGA) and differential scanning cal-
orimetry (DSC) and are summarized in Supporting Information
Table S3. Typical TGA curves of representative polyamide Ib in
both air and nitrogen atmospheres are shown in Supporting
Information Figure S9. All the aromatic polyamides exhibited
good thermal stability with insignificant weight loss up to 440
ꢀC in nitrogen. Their 10% weight-loss temperatures in nitro-
gen and air were recorded at 465–520 and 475–525 ꢀC,
respectively. The carbonized residue (char yield) of these aro-
matic polymers was more than 53% at 800 ꢀC in nitrogen
atmosphere. The high char yields of these polymers could be
ascribed to their high aromatic content. The glass-transition
temperature (Tg) could be easily measured by the DSC thermo-
grams; they were observed in the range of 258–265 ꢀC as
shown in Supporting Information Figure S10, depending on
the stiffness of the polymer chain. All the polymers indicated
no clear melting endotherms up to the decomposition
temperatures on the DSC thermograms, which supports the
amorphous nature of these polyamides.
The other polyamides (Ia and Ic) showed similar CV curves
to that of Ib. The redox potentials of the polyamides as well
as their respective highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO; vs.
vacuum) are summarized in Table 1. The HOMO level or
called ionization potentials (vs. vacuum) of polyamides Ia–Ic
are estimated from the onset of their oxidation in CV experi-
ments as 5.11–5.15 eV (on the basis that ferrocene/ferroce-
nium is 4.8 eV below the vacuum level with Eonset ¼ 0.36 V).
Figures 2 and 3 show CV and differential pulse voltammetry
(DPV) of the model compounds M1, M2, and 2, and the
results are summarized in Table 2. For comparison, all meas-
urements of model compounds were obtained under the
same conditions. In CV scans (Fig. 2), we observed two
Electrochemical Properties
ox;1
reversible oxidation redox steps for M1 (E
¼ 0.93 V and
The electrochemical behavior of the polyamides was investi-
gated by cyclic voltammetry (CV) conducted by film cast on
an ITO-coated glass substrate as the working electrode in
dry acetonitrile (CH3CN) containing 0.1 M of tetrabutylam-
monium perchlorate (TBAP) as an electrolyte. The typical
CVs for polyamides Ib and I0b are shown in Figure 1 for
comparison. The polyamide Ib [with (OMe)2TPAO units] hav-
p;an
ox;2
E
¼ 1.15 V), and only one reversible oxidation redox step
p;an
ox
ox
for M2 (E
¼ 1.07 V) and 2 (E
¼ 1.16 V), respectively,
p;an
p;an
indicating the effect of NAN distance between two redox
center on oxidation potential and intramolecular ET process.
In addition, DPV diagrams of these model compounds
(Fig. 3) also display two one-electron oxidation redox for M1
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and only one two-electron oxidation redox for both M2 and
2, respectively. Because of the lack or absence of electro-
chemical splitting, neither electrostatic effects nor electron
coupling could be observed for both M2 and 2 in CV and
DPV; the further investigation was performed by spectroelec-
trochemical measurements.
Measurement of n Electrons Transferred for Each Step
of Model Compounds
Spectroelectrochemistry of model compounds12 was exam-
ined by an optically transparent thin-layer electrode (OTTLE)
coupled with UV–vis–NIR spectroscopy. The concentration of
5 ꢂ 10ꢁ4 M CH3CN solution was prepared and a 100-mesh
platinum film was used as a working electrode. To measure
the amount of n electrons transferred for each step by the
OTTLE techniques,
a series of potentials (Eapplied) are
sequentially impressed (potentiostated) across the transpar-
ent cell containing a solution of the redox couple. At each
applied potential, the ratio of the concentrations of oxidized
form to neutral one, [O]/[N], of the couple adjusts by elec-
trolysis at the transparent electrode to the value required for
adherence to the Nernst equation:
0:059
½Oꢃ
½Nꢃ
Eapplied ¼ Eꢀ þ
log
n
FIGURE 3 Differential pulse voltammetric diagrams of model
compounds (a) M1, (b) M2, and (c) 2 (10-3 M) in 0.1 M TBAP/
CH3CN at a scan rate of 50 mV/s. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
where Eo is a formal oxidation potential. Each applied poten-
tial is maintained until the equilibrium value of [O]/[N] has
been established throughout the thin layer of solution
adjacent to the platinum film electrode by electrolysis and
diffusion. The optical transparency of the electrode enables
the [O]/[N] value corresponding to each Eapplied to be
obtained from absorbance changes measured by recording
spectra directly through the transparent electrode and the
thin layer of solution after equilibrium is achieved for each
potential.
Figures 4–6 depict the spectral changes for model com-
pounds M1, M2, and 2 at various electrode potentials. M1 is
TABLE 2 Electrochemical Properties of Model Compounds M1,
M2, and 2
Oxidation (V; vs. Ag/AgCl in CH3CN)a
ox;1
p;an
ox;1
p;ca
ox;
p;an
ox;2
Code
E
E
Eonset
E
E
p;ca
M1
M2
2
0.93 (1eꢁ)
1.07 (2eꢁ)
1.16 (2eꢁ)
0.77
0.91
0.98
0.79
0.93
0.98
1.15 (1eꢁ)
1.01
–
–
–
–
FIGURE 2 Cyclic voltammetric diagrams of 10-3 M model com-
pounds (a) M1, (b) M2, and (c) 2 in 0.1 M TBAP/CH3CN at a
scan rate of 50 mV/s. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
a
From cyclic voltammograms versus Ag/AgCl in CH3CN.
b
1eꢁ ¼ one-ET, 2eꢁ ¼ two-ET; calculated from the integral of the DPV.
MIXED-VALENCE CLASS I TRANSITION AND ELECTROCHEMISTRY, YEN ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE
4 Absorption spectral
change of compound M1 (5 ꢂ 10-4
M) in 0.1 M TBAP/CH3CN at Eapplied
of (a) 0, (b) 0.81, (c) 0.84, (d) 0.86, (e)
0.88, (f) 0.90, (g) 0.92, (h) 0.94, (i)
1.00, (j) 1.03, (k) 1.06, and (l) 1.13
(V vs. Ag/AgCl) and the oxidation
pathway. [Color figure can be viewed
in the online issue, which is available
at wileyonlinelibrary.com.]
used as an example to illustrate how many electrons are
transferred for each step. The absorption peaks at 295 and
408 nm are characteristic for M1 (Fig. 4). After first stage of
one-electron oxidation (0.00–1.00 V), the original peaks
decreased gradually, and a broad and intense new band
around 992 nm appeared. When the potential was adjusted
to more positive values (1.00–1.13 V) corresponding to the
second stage of one-electron oxidation, the new peak
appeared at 793 nm. The spectral change of M1 was consist-
ent with the reported MV Class III transition.7 The inset in
Figure 4 shows a typical plot of Eapplied versus log(An ꢁ A0)/
(Af ꢁ An) corresponding to 992 nm (IV-CT band of Class III)
for the first oxidized stage. The color of compound M1 solu-
tion switches from brown (neutral state) to green (semioxi-
dized state) and blue (fully oxidized state). The slope of the
straight-line plot is 61.3 mV, which compares favorably with
the Nernstian value of 59.1 mV for a one-electron process.
The potential-axis intercept is 0.89 V, which is in good agree-
ment with values of 0.85 V obtained by CV. Other model
compounds M2 and 2 revealed slopes of 62.9 mV for one-
electron process and 38.0 mV for two-electron process. In
Figure 5, owing a broad IV-CT characteristic band around
1367 nm from the cation radical state, compound M2 could
be claimed as the Class II system, and the color of compound
M2 solution switches from brown (neutral state) to red
(semioxidized state) and blue (fully oxidized state).7 For
compound 2, the introduction of ether-linkage definitely sep-
arated two redox centers with independent behavior, and the
two electrons on TPAO units are simultaneously removed to
form two radical cations. Furthermore, no IV-CT band could
be detected in Figure 6 during oxidation, indicating that
there is no stable single radical cation existing in TPAO units,
and these two triarylamine in TPAO units should be oxidized
at the same time with a color switches from light-brown
(neutral state) to bluish-green (oxidized state) of the solu-
tion. The result suggests that the introduction of ether-
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FIGURE 5 Absorption spectral change of compound M2 (5 ꢂ 10-4 M) in 0.1 M TBAP/CH3CN at Eapplied of (a) 0, (b) 0.90, (c) 0.94, (d)
0.98, (e) 1.02, (f) 1.04, (g) 1.06, (h) 1.09, and (i) 1.14 (V vs. Ag/AgCl) and the oxidation pathway. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
linkage is an effective method for the purpose of tuning the
Class I/II transition.
radical cation resonance structures after an electron removal
from the lone pair on the nitrogen atom of the TPA moiety,
thus shifted the absorption to longer wavelength. The UV–
vis–NIR absorption changes in the polyamide Ib film at
various potentials were fully reversible associated with
strong color changes. The other polyamides showed similar
spectral change to that of Ib. From the inset shown in
Figure 7, the polyamide Ib film switches from highly
transmissive neutral state (colorless: L*, 99; a*, 0; and b*,
0) to a coloring oxidized state (deep bluish-green: L*, 50;
a*, -24; and b*, -4). The colorations were homogeneously
distributed across the polymer film and survived for more
than hundreds of redox cycles. Polyamide Ib exhibited
high contrast of optical transmittance change (DT%) up
to 87% at 781 nm and 54% at 390 nm. The absence of
Spectroelectrochemical and EC Properties
Electrochromism of the polyamide thin films was examined
by an OTTLE coupled with UV–vis–NIR spectroscopy. The
preparation of electrode and solution was identical to those
used in CV. UV–vis–NIR absorbance curves corresponding to
applied potentials and 3D transmittance–wavelength–applied
potential correlation of Ib film are depicted in Figure 7 and
Supporting Information Figure S11. The peak of absorption
at 314 nm, characteristic for the neutral form of polyamide
Ib, decreased gradually when the applied potentials
increased positively from 0 to 1.15 V, and two new bands
grew up at 390 and 781 nm due to the more planar TPA
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Information Fig. S12). 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. As depicted in Supporting Information Figure S12,
when the potential was switched between 0 and 0.99 V,
polyamide Ib thin film revealed switching time of 3.14 s
for coloring and 1.04 s for bleaching process. As depicted
in Figure 8, the EC stability of the polyamide Ib film was
determined by measuring the optical changes as a function
of the number of switching cycles and plotted every 10th
cycle. The EC CE (g ¼ dOD/Q) and injected charge (elec-
troactivity) after various switching steps were monitored
and are summarized in Supporting Information Table S4.
The EC film of Ib was found to exhibit high CE up to 166
cm2/C at 781 nm, and showed only 4.8% decay of its
electroactivity after 100 cycles (Fig. 8).
Furthermore, a single layer EC cell was fabricated as prelimi-
nary investigation (Fig. 9). The polyamide films were spray-
coated onto ITO-glass and then dried. Afterward, the gel
electrolyte was spread on the polymer-coated side of the
electrode and the electrodes were sandwiched. To prevent
leakage, an epoxy resin was applied to seal the device. As a
typical example, an EC cell based on polyamide Ib was fabri-
cated. The polymer film is colorless in neutral form. When
the voltage was increased (to a maximum of 1.0 V), the color
changed to bluish-green due to electro-oxidation, the same
as that was already observed for the solution spectroelectro-
chemical experiments. When the potential was subsequently
set back at 0 V, the polymer film turned back to original col-
orless. We believe that optimization could further improve
the device performance and fully explore the potential of
these EC polyamides.
FIGURE 6 Absorption spectral change of dinitro compound 2
(5 ꢂ 10-4 M) in 0.1 M TBAP/CH3CN at Eapplied of (a) 0, (b) 1.07,
(c) 1.09, (d) 1.10, (e) 1.12, (f) 1.14, and (g) 1.18 (V vs. Ag/AgCl)
and the oxidation pathway. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
IV-CT band in the TPAO-based polymers was confirmed
because of the introduction of ether-linkage, which was
demonstrated by the spectroelectrochemical investigation
of model compound 2.
The color switching times were estimated by chronoam-
perometric and absorbance measurements (Supporting
FIGURE 7 EC behavior (left) by increasing the applied voltage to 1.10 (V vs. Ag/AgCl), and 3D spectroelectrochemical behavior
(right) from 0.00 to 1.10 (V vs. Ag/AgCl) of polyamide Ib thin film (ꢄ70 nm in thickness) on the ITO-coated glass substrate in 0.1 M
TBAP/CH3CN. The potential was varied in 50 mV interval. The inset shows the color change of the polymer film at indicated poten-
tials. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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(CaCl2) was dried under vacuum at 180 ꢀC for 8 h. TBAP
was obtained from ACROS and recrystallized twice by ethyl
acetate under nitrogen atmosphere and then dried in vacuo
before use. All other reagents were used as received from
commercial sources.
FIGURE 8 EC switching between 0 and 0.99 V (vs. Ag/AgCl) of
polyamide Ib thin film (ꢄ80 nm in thickness) on the ITO-coated
glass substrate (coated area: 1.6 cm ꢂ 0.6 cm) 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 figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
Bis(N-4-nitrophenyl-4-aminophenyl)ether (1)
A mixture of 9.20 g (90.92 mmol) of triethylamine in 80 mL
DMSO was stirred at room temperature. To the mixture, 6.01
g (30.01 mmol) of 4,40-oxydianiline and 10.72 g (75.21
mmol) of 4-fluoronitrobenzene were added in sequence. The
mixture was heated with stirring at 120 ꢀC for 72 h. The
mixture was poured into stirred methanol/water, and the
precipitated orange-red powders was collected by filtration
and washed thoroughly with methanol/water (1:2). The
product was filtered to afford 10.14 g (76% in yield) of or-
CONCLUSIONS
A series of anodic bluish-green EC polyamides with MV Class
I transition have been readily prepared from the diamine,
bis(N-4-aminophenyl-N-4-methoxyphenyl-4-aminophenyl)ether
(3), and various dicarboxylic acids via direct phosphorylation
polycondensation. In addition to the good solubility and
high thermal stability, these anodically polymeric EC materi-
als also revealed highly continuous cyclic stability of EC
characteristics with good CE, changing color from the color-
less to bluish-green after oxidized. Spectroelectrochemical
investigations also demonstrated that these polymers can be
used as potential anodic coloring materials in the develop-
ment of EC devices.
ange-red powders with a mp of 103–106 ꢀC (measured by
13
ꢀ
ꢀ
DSC at 10 C/min; ref. ; 163 C).
FTIR (KBr): 3328 cmꢁ1 (secondary NAH stretch), 1586 and
1312 cmꢁ1 (ANO2 stretch). 1H NMR (500 MHz, DMSO-d6, d,
ppm): 9.27 (s, 2H, ANH), 8.08 (d, 4H, J ¼ 9.3 Hz, Hd), 7.28
(d, J ¼ 8.9 Hz, 4H, Ha), 7.08 (d, J ¼ 8.9 Hz, 4H, Hb), 7.01 (d,
J ¼ 9.3 Hz, 4H, Hc). 13C NMR (125 MHz, DMSO-d6, d, ppm):
By the introduction of ether-linkage into TPAO unit, the
resulting polymers containing two separated redox centers
revealed independent electrochemical behavior, and two
electrons within TPAO moiety are simultaneously removed to
form two radical cations without intramolecular ET. The con-
firmation of lacking electron coupling was further demon-
strated by OTTLE measurements of model compounds by
Nernst equation. The MV Class I/II/III transition and electro-
chemistry of bridged triarylamines were investigated in the
aspect of small molecule and polymer systems.
EXPERIMENTAL
Materials
N,N0-Bis(4-aminophenyl)-N,N0-di(4-methoxylphenyl)-1,4-phen-
ylenediamine (M1)8(e) and N,N0-bis(4-aminophenyl)-N,N0-
di(4-methoxylphenyl)-4,40-biphenyldiamine (M2)8(g) were
synthesized according to a previously reported procedure.
Commercially available dicarboxylic acids such as trans-1,4-
cyclohexanedicarboxylic acid (3a), 4,40-oxydibenzoic acid
(3b), and 2,20-bis(4-carboxyphenyl)hexafluoropropane (3c)
were purchased from Tokyo Chemical Industry and used as
received. Commercially obtained anhydrous calcium chloride
FIGURE 9 (a) Photographs of single-layer ITO-coated glass EC
device, using polyamide Ib as active layer. (b) Schematic dia-
gram of polyamide EC device sandwich cell.
MIXED-VALENCE CLASS I TRANSITION AND ELECTROCHEMISTRY, YEN ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
152.9 (C5), 151.2 (C1), 137.5 (C8), 135.3 (C4), 126.2 (C7),
123.2 (C3), 119.5 (C6), 112.8 (C2).
the mixture was stirred vigorously under hydrogen atmos-
phere at room temperature until the theoretical amount of
hydrogen was consumed. The time to reach this stage is
about 36 h. The mixture was filtered to remove Pd/C, and
the filtrate was poured into 300 mL of water and washed by
MeOH. The precipitated product was collected by filtration
ꢀ
and dried in vacuo at 65 C to give 2.80 g (92% in yield) of
ꢀ
grayish powder with a mp of 83–86 C (measured by DSC of
ꢀ
10 C/ min).
FTIR (KBr): 3443, 3365 cmꢁ1 (NAH stretch). 1H NMR (500
MHz, DMSO-d6, d, ppm): 6.91 (d, J ¼ 9.0 Hz, 4H, Ha) , 6.84–
6.80 (m, 8H, Hb þ Hc), 6.78–6.75 (m, 8H, He þ Hd), 6.54 (d, J
¼ 8.8 Hz, 4H, Hf), 4.96 (s, 4H, ANH2), 3.70 (s, 6H, AOCH3).
13C NMR (125 MHz, DMSO-d6, d, ppm): 154.51 (C12), 150.47
(C1), 145.42 (C8), 144.30 (C9), 141.19 (C4), 136.07 (C5),
126.91 (C6), 124.49 (C2), 121.16 (C11), 118.94 (C10), 114.84
(C7), 114.55 (C3), 55.10 (AOCH3). Anal. Calcd (%) for
Bis(N-4-methoxyphenyl-N-4-nitrophenyl-4-aminophenyl)
ether (2)
To a solution of 27.48 g (62.11 mmol) of dinitro compound
1, 30.53 g (130.45 mmol) of 4-iodoanisole, 22.56 g (163.23
mmol) of powdered anhydrous potassium carbonate, 10.10 g
(158.93 mmol) of copper powder, and 5.11 g (19.33 mmol)
of 18-crown-6-ether were stirred in 105 mL of o-dichloro-
benzene under nitrogen atmosphere. After heated at 180 ꢀC
for 48 h, the solution was filtered and poured into 1000 mL
of MeOH. The precipitated brown powder was collected by
filtration and purified by DMF/ethyl acetate. The product
was filtered to afford 21.47 g (53% in yield) of bright yellow
powder with a mp of 193–194 ꢀC (measured by DSC at 10
ꢀC/min).
C38H34N4O3 (594.70): C, 76.75; H, 5.76; N, 9.42. Found: C,
76.19; H, 6.00; N, 9.22. Electro-spray ionization mass spec-
trometry (ESI-MS): Calcd for (C21H23N3)þ, m/z 594.70;
found, m/z 594.60.
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 594.7 mg (1.00 mmol) of the dia-
mine monomer (2), 258.2 mg (1.00 mmol) of 4,40-oxydiben-
zoic acid (4b), 52.0 mg of calcium chloride, 0.90 mL of tri-
phenyl phosphite, 0.50 mL of pyridine, and 0.8 mL of 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, fiberlike precipitate that was
collected by filtration, washed thoroughly with hot water
and methanol, and dried under vacuum at 100 ꢀC; yield:
829.0 mg (99%). Reprecipitations of the polymer by DMAc/
methanol were carried out twice for further purification. The
inherent viscosity and Mn of the obtained polyamide Ib were
0.51 dL/g (measured at a concentration of 0.5 g/dL in DMAc
1
FTIR (KBr): 1586, 1314 cmꢁ1 (ANO2 stretch). H NMR (500
MHz, DMSO-d6, d, ppm): 8.04 (d, J ¼ 9.4 Hz, 4H, Hf), 7.34 (d,
J ¼ 8.9 Hz, 4H, Ha), 7.28 (d, J ¼ 8.9 Hz, 4H, Hc), 7.13 (d, J ¼
8.9 Hz, 4H, Hb), 7.04 (d, J ¼ 8.9 Hz, 4H, Hd), 6.71 (d, J ¼ 9.4
Hz, 4H, He), 3.78 (s, 6H, AOCH3). 13C NMR (125 MHz, DMSO-
d6, d, ppm): 157.70 (C12), 154.37 (C1), 153.80 (C8), 140.31
(C5), 138.14 (C9), 137.30 (C4), 128.95 (C10), 128.58 (C2),
125.59 (C7), 120.15 (C3), 115.47 (C11), 115.07 (C6), 55.29
(AOCH3). Anal. Calcd (%) for C38H30N4O7 (654.67): C, 69.72;
H, 4.62; N, 8.56. Found: C, 69.70; H, 4.60; N, 8.74. Electron
impact mass spectrometry (EI-MS): Calcd for (C21H19N3O4)þ,
m/z 654.67; found, m/z 654.20.
ꢀ
at 30 C) and 134,800 Da, respectively.
The FTIR spectrum of Ib (film) exhibited characteristic amide
absorption bands at 3317 cmꢁ1 (NAH stretch), 3040 cmꢁ1
(aromatic CAH stretch), 2928, 2829 cmꢁ1 (CH3 CAH stretch),
1
1649 cmꢁ1 (amide carbonyl). H NMR (500 MHz, DMSO-d6, d,
ppm): 10.14 (s, 2H, ANH), 8.00 (d, J ¼ 8.0 Hz, 4H, Hg), 7.63 (s,
J ¼ 8.4 Hz, 4H, Ha), 7.16 (d, J ¼ 8.0 Hz, 4H, Hh), 7.00–6.90 (m,
20H), 3.72 (s, 6H, AOCH3). 13C NMR (125 MHz, DMSO-d6, d,
ppm): 164.18 (C¼¼O), 158.48, 155.48, 151.67, 143.56, 143.23,
140.23, 133.47, 130.34, 129.87, 126.17, 123.91, 122.45,
121.56, 119.34, 118.30, 114.89, 55.14 (AOCH3). Anal. Calcd
(%) for (C52H40N4O6)n (816.90)n: C, 76.45; H, 4.94; N, 6.86.
Found: C, 75.04; H, 4.95; N, 6.85. The other polyamides were
prepared by an analogous procedure.
Bis(N-4-aminophenyl-N-4-methoxyphenyl-4-aminophenyl)
ether (3)
In a 100-mL two-neck round-bottomed flask equipped with
a stirring bar, 3.03 g (4.63 mmol) of dinitro compound 2 and
0.30 g of 10% Pd/C were dispersed in 42 mL of DMF, and
Preparation of the Polyamide Films
A solution of the polymer was made by dissolving 0.8 g of
the polyamide sample in 12 mL of DMAc. The homogeneous
solution was poured into a 9-cm glass Petri dish, which was
placed in a 70 ꢀC oven for 12 h to remove most of the
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ARTICLE
solvent, and then the semidried film was further dried in
vacuo at 160 C for 8 h. The obtained films were about 60–
80 lm in thickness and were used for solubility tests, and
thermal analyses.
amount of optical density change (dOD) at a specific absorp-
tion wavelength induced as a function of the injected/ejected
charge (Q; also termed as electroactivity) which is deter-
mined from the in situ experiments. CE is given by the equa-
tion: 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 alpha-step profi-
lometer (Kosaka Lab., Surfcorder ET3000). Colorimetric
measurements were obtained by JASCO V-650 spectropho-
tometer and the results are expressed in terms of lightness
(L*) and color coordinates (a* and b*).
ꢀ
Fabrication of the EC Device
EC polymer films were prepared by dropping solutions of
the polyamide Ib (4 mg/mL in DMAc) onto a ITO-coated
glass substrate (20 mm ꢂ 30 mm ꢂ 0.7 mm, 50–100 X/
square). The polymers were drop coated onto an active area
(ca. 20 mm ꢂ 20 mm) then dried in vacuum. A gel electro-
lyte based on poly(methyl methacrylate) (PMMA) (Mw:
350,000) and LiClO4 was plasticized with propylene carbon-
ate to form a highly transparent and conductive gel. PMMA
(3 g) was dissolved in dry acetonitrile (15 g), and LiClO4
(0.3 g) was added to the polymer solution as a supporting
electrolyte. Then, propylene carbonate (5 g) was added as
plasticizer. The gel electrolyte was spread on the polymer-
coated side of the electrode, and the electrodes were sand-
wiched. Finally, an epoxy resin was used to seal the device.
The authors gratefully acknowledge the support of this
research through the Institute of Nuclear Energy Research,
Atomic Energy Council and the National Science Council
(NSC98-2113-M-002-005-MY3) of Taiwan. C. W. Lu at the
Instrumentation Center, National Taiwan University, for CHNS
(EA) analysis experiments and C. H. Ho at the Instrumentation
Center, Department of Chemistry, National Taiwan Normal Uni-
versity, for the measurement of 500 MHz NMR spectrometer
are also acknowledged.
Measurements
FTIR spectra were recorded on a PerkinElmer Spectrum 100
Model FTIR spectrometer. Elemental analyses were run in a
Heraeus VarioEL-III CHNS elemental analyzer. 1H NMR spec-
tra were measured on a Bruker AVANCE-500 FT-NMR using
tetramethylsilane as the internal standard, and peak multi-
plicity was reported as follows: s, singlet; d, doublet. ESI-MS
and EI-MS spectra were measured on JEOL JMS-700 and Fin-
nigan TSQ 700 mass spectrometers, respectively. The inher-
ent 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) connected with one refractive index
detector from Schambeck SFD Gmbh. All GPC analyses were
performed using a polymer/DMF solution at a flow rate of
1 mL/min at 70 ꢀC and calibrated with polystyrene stand-
ards. TGA was conducted with a PerkinElmer Pyris 1 TGA.
Experiments were carried out on ꢄ6–8-mg film samples
heated in flowing nitrogen or air (flow rate ¼ 20 cm3/min)
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