Novel starburst triarylamine-containing electroactive aramids with highly stable electrochromism in near-infrared and visible light regions

Article abstract of DOI:10.1021/cm103552k

A series of solution-processable near-infrared (NIR) electrochromic aromatic polyamides with starburst triarylamine units in the backbone were prepared by the phosphorylation polyamidation from a newly synthesized diamine monomer, N,N-bis[4-(4-methoxyphen

Full text of DOI:10.1021/cm103552k

ARTICLE
pubs.acs.org/cm
Novel Starburst Triarylamine-Containing Electroactive Aramids
with Highly Stable Electrochromism in Near-Infrared and
Visible Light Regions
Hung-Ju Yen, Hung-Yi Lin, and Guey-Sheng Liou*
Functional Polymeric Materials Laboratory, Institute of Polymer Science and Engineering, National Taiwan University, 1 Roosevelt
Road, fourth Sec., Taipei 10617, Taiwan
S Supporting Information
b
ABSTRACT: A series of solution-processable near-infrared (NIR) electro-
chromic aromatic polyamides with starburst triarylamine units in the
backbone were prepared by the phosphorylation polyamidation from a
newly synthesized diamine monomer, N,N-bis[4-(4-methoxyphenyl-40-
aminophenylamino)phenyl]-N0,N0-di(4-methoxyphenyl)-p-phenylenediamine,
and two aromatic dicarboxylic acids. These polymers were highly soluble in
many organic solvents and showed useful levels of thermal stability associated
with high glass-transition temperatures and high char yields (higher than 66%
at 800 °C in nitrogen). The polymer films showed reversible electrochemical
oxidation and electrochromism with high contrast ratio both in the visible
range and NIR region, which also exhibited high coloration efficiency (CE),
low switching time, and relatively high stability for long-term electrochromic
operation. At the first oxidation stage, the polyamide Ia thin film revealed high
coloration efficiency in NIR (CE = 290 cm2/C) region with reversible electroactive stability (more than 10000 times within 0.6%
loss relative to its initial injected charge). As the dication radical form of second oxidation stage, the polymer film still exhibited
excellent electrochromic/electroactive stability (more than 10 000 cyclic switches) with enhanced CE of 339 cm2/C.
KEYWORDS: polymeric materials, electronic materials, electrochemistry
’ INTRODUCTION
transmissive NIR electrochromic conjugated polymers, their
devices exhibited multicolor in neutral state and trasmissive
in the oxidized state. In addition to conjugated polymers,5
p-phenylenediamine-containing molecule is an interesting
anodic electrochromic system for NIR applications due to its
particular intramolecular electron transfer in the oxidized states.
Intramolecular electron transfer (ET) processes were studied
extensively in the mixed-valence (MV) systems,6 and usually
employed one-dimensional MV compounds contain two or more
redox states. According to Robin and Day,7 the p-phenylenediamine
cation radical has been reported as a symmetrical delocalized class III
structure with a strong electronic coupling (the electron is deloca-
lized over the two or more redox centers), leading an intervalence
charge transfer (IV-CT) absorption band in the NIR region.8
In order to be useful for electrochromic applications, some key
issues such as long-term stability, multiple colors within the same
material, rapid redox switching, high coloration efficiency (CE),
and high optical transmittance change (Δ%T) during operation
are indispensable and are playing important roles. Therefore, to
achieve a good combination of the above-mentioned parameters
is a crucial and ongoing issue. Reynolds’ group5e studied
Electrochromic materials exhibit a reversible optical change in
absorption or transmittance upon electrochemically oxidized or
reduced, such as transition-metal oxides, inorganic coordination
complexes, conjugated polymers, and organic molecules.1 De-
spite the fact that the electrochromic devices were mostly based
on inorganic oxides, nevertheless, the organic material has several
advantages over the former ones, such as processability, high
coloration efficiency, fast switching ability, and multiple colors
within the same material. Initially, investigation of electrochro-
mic materials has been directed toward optical changes in the
visible region (e.g., 400ꢀ800 nm), proved useful and variable
applications such as E-paper, optical switching devices, smart
window, and camouflage materials.2 Increasingly, attention of the
optical changes has been focused extending from the near-
infrared (NIR; e.g., 800ꢀ2000 nm) range to the microwave
region of the spectrum, which could be exploitable for optical
communication, data storage, and thermal control (heat gain or
loss) in buildings and spacecrafts.3 In recent years, NIR electro-
chromic materials including transition metal oxides WO3, or-
ganic metal complex (ruthenium dendrimer), and quinone-
containing organic materials have been investigated.4 Wang
and Wan4cꢀg made efforts on the quinone-containing electro-
chromic materials, which revealed high absorption in the NIR
upon electrochemical reduction. Reynold1i,5a reported color-to-
Received: December 14, 2010
Revised:
March 1, 2011
Published: March 15, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Route to 9Ph4OMe-diamine Monomer 2
poly(3,4-alkylenedioxy thiophene) films and found that their
electrochromic devices retained 90% after 180,000 switching
cycles with a high contrast ratio of 75%. The EDOT derivative
reported by Wudl’s group5b,c exhibited high CE and stability after
5000 switching cycles but only with a contrast ratio of 57%. Since
2005, our group has reported several triphenylamine containing
electrochromic polymers with interesting color transitions9 that
showed good electrochromic reversibility in the visible region
and NIR range. However, most of the polymers could only reveal
less than two stages of electrochromism because of the difficulty
and complexity in increasing electroactive nitrogen atoms within
triarylamine-containing structures. Therefore, our strategy is to
design and synthesize the NIR electrochromic starburst triaryla-
mine-based materials with the incorporation of electron-donat-
ing substituents at the para-position of phenyl groups, thus could
greatly prevent the coupling reactions by affording stable cationic
radicals and lowering the oxidation potentials.10 The advantage
of more electroactive sites and better processability than the con-
ventional linear polymer was due to the introduction of starburst
triarylamine derivatives into polymer system. The resultant
electroactive polymers with high molecular weights and excellent
thermal stability should be readily obtained by using conven-
tional polycondensation. Because of the incorporation of pack-
ing-disruptive starburst triarylamine units into the polymer
backbone, these novel polymers should also show excellent
solubility in various polar organic solvents, thus transparent
and flexible polymer thin films could be prepared easily by
solution casting and spin-coating techniques. This is beneficial
for their fabrication of large-area, thin-film electrochromic devices.
In this contribution, we therefore synthesized the diamine
monomer, N,N-bis[4-(4-methoxyphenyl-40-aminophenylami-
no)phenyl]-N0,N0-di(4-methoxyphenyl)-p-phenylenediamine
(2), and their corresponding aromatic polyamides containing
starburst triarylamine units with para-substituted methoxy
groups. The incorporation of electron-donating methoxy
substituents is expected to reduce the oxidation potential asso-
ciated with enhanced electrochemical and electrochromic stabi-
lity of the result polyamides. Thus, we anticipate that these novel
polyamides should be excellent in many aspects such as electro-
chemical stability with multiple electrochromic switchings, op-
tical response times, coloration efficiency, and high optical
contrast in both NIR and visible-light regions.
’ RESULTS AND DISCUSSION
Monomer and Polymer Synthesis. The new starburst dia-
mine monomer 2 was synthesized by hydrazine Pd/C-catalyzed
reduction of the dinitro compound 1 derived from N,N-bis(4-
bromophenyl)-N0,N0-di(4-methoxyphenyl)-p-phenylenediamine
and 4-methoxy-40-nitrodiphenylamine by Ullman reaction
(Scheme 1). Elemental analysis, FT-IR, and NMR spectroscopy
were used to identify structures of the intermediate dinitro
compound 1 and the target diamine monomer 2. The FT-IR
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Scheme 2. Synthesis of Aromatic Polyamides I; Photograph Shows Appearance of the Flexible Films (thickness ≈ 60μm)
ARTICLE
spectra of these two synthesized compounds are illustrated in
Figure S1 in the Supporting Information. The nitro groups of
compound 1 showed characteristic bands at around 1587 and
1311 cmꢀ1 due to NO2 asymmetric and symmetric stretching.
After reduction to diamine monomer 2, the characteristic
absorption bands of the nitro group disappeared and the primary
amino group revealed the typical absorption pair at 3446
Information. These polyamides were highly soluble not only in
polar aprotic organic solvents such as N-methyl-2-pyrrolidinone
(NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylforma-
mide (DMF), dimethyl sulfoxide (DMSO) but also in less polar
solvents such as tetrahydrofuran (THF) and CHCl3. Thus, the
excellent solubility makes these polymers as potential candidates for
practical applications by spin-coating or inkjet-printing processes to
afford high performance thin films for optoelectronic devices. As
mentioned above, the polyamides I could be solution cast into
flexible, transparent, and tough films. The appearance and quality of
these films are also shown in Scheme 2. Their high solubility and
amorphous properties can be attributed to the incorporation of
bulky, three-dimensional starburst triarylamine moiety along the
polymer backbone, which results in a high steric hindrance for close
packing, and thus reduces their crystallization tendency.
1
and 3365 cmꢀ1 (NꢀH stretching). The H NMR and 13C
NMR spectra of the intermediate dinitro compound 1 and
diamine monomer 2 are illustrated in Supporting Information
(Figure S2ꢀS4) and agree well with the proposed molecular
structures. Thus, the results of all the spectroscopic and ele-
mental analyses suggest the successful preparation of the target
diamine monomer.
According to the phosphorylation technique described by
Yamazaki,11 these novel polyamides I with starburst arylamine
moieties were synthesized from the diamine monomer 2 and two
aromatic dicarboxylic acids 3 (Scheme 2). The polymerization
was carried out via solution polycondensation using triphenyl
phosphite and pyridine as condensing agents. All polymerization
reactions proceeded homogenously and gave high molecular
weights. The obtained polyamides had inherent viscosities in the
range of 0.27ꢀ0.36 dL/g with weight-average molecular weights
(Mw) and polydispersity (PDI) of 69 800ꢀ131 000 Da and
1.85ꢀ2.13, respectively, relative to polystyrene standards (see
Table S1 in the Supporting Information). All the polymers with
high molecular weights could afford transparent and tough films
via solution casting. The structures of the polyamides were
confirmed by elemental analysis, NMR, and IR spectroscopy.
The 1H NMR spectra of the polyamide Ia and Ib were illustrated
in Figure S5 in the Supporting Information and agree well with
the proposed molecular structures. As shown in Figure S6 in the
Supporting Information, a typical IR spectrum for polyamides
exhibited characteristic absorption bands of the amide group at
around 3307 (NꢀH stretch) and 1669 cmꢀ1 (amide carbonyl).
Solubility and Film Property. The solubility properties of
polymers I were investigated qualitatively at 5% w/v concentration
and the results are also listed in Table S2 in the Supporting
Thermal Properties. The thermal properties of polyamides
were examined by TGA and DSC, and the thermal behavior data are
summarized in Table S2 in the Supporting Information. Typical
TGA and DSC curves of polyamides Ia and Ib in both air and
nitrogen atmospheres are shown in Figure S7 in the Supporting
Information. All the prepared polyamides exhibited good thermal
stability with insignificant weight loss up to 400 °C in nitrogen or air
atmosphere even with the introduction of methoxy groups. The
10% weight loss temperatures of these polymers in nitrogen and air
were recorded in the range of 480ꢀ510 and 495ꢀ510 °C, respec-
tively. The carbonized residue (char yield) of these polymers in a
nitrogen atmosphere was more than 66% at 800 °C. The high char
yields of these polymers can be ascribed to their high aromatic
content. The glass-transition temperatures (Tg) of polyamides I
could be easily measured in the DSC thermograms; they were
observed in the range of 224ꢀ270 °C, depending upon the stiffness
of the polymer chain. The lower Tg value of Ia can be explained in
terms of the flexible ether linkage in its diacid component.
Electrochemical Properties. The electrochemical properties
of compounds 1, 2, and the polyamides were investigated by
cyclic voltammetry (CV). The compounds were dissolved in
anhydrous dichloromethane (CH2Cl2) solution (Conc.: 1 ꢁ
10ꢀ4 mol/L), and the polymers were conducted by cast film
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Figure 1. Cyclic voltammograms of (a) 1 ꢁ 10ꢀ4 M dinitro compound 1, (b) 1 ꢁ 10ꢀ4 M diamino compound 2, (c) ferrocene in CH2Cl2 solution
containing 0.2 M TBAP at scan rate of 50 mV/s, and (d) differential pulse voltammograms of 1 ꢁ 10ꢀ4 M dinitro compound 1 in CH2Cl2 solution
containing 0.2 M TBAP. Scan rate, 5 mV/s; pulse amplitude, 50 mV; pulse width, 50 ms; pulse period, 0.2 s.
Figure 2. (a) Cyclic voltammetric diagrams of polyamide Ia and Ib films on an ITO-coated glass substrate and ferrocene (inset) in 0.1 M TBAP/
CH3CN at a scan rate of 50 mV/s. (b) Cyclic voltammograms of polyamide Ia on an ITO-coated glass substrate over cyclic scans in CH3CN solution
containing 0.1 M TBAP at scan rate of 25 mV/s.
on an indiumꢀtin oxide (ITO)-coated glass slide as working
electrode in anhydrous acetonitrile (CH3CN), using 0.1 M of
tetrabutylammonium perchlorate (TBAP) as a supporting elec-
trolyte in a nitrogen atmosphere. The typical CV diagrams
for compounds 1 and 2 are shown in Figure 1. There are
four reversible oxidation and one reduction redox couples for
compound 1, and five reversible oxidation redox couples for
compound 2, respectively. As the substituent varies from elec-
tron-withdrawing nitro group to electron-donating amino group,
the oxidation potential of amino compound 2 was lower than
that of compound 1, indicating the electronic effect of the
substituent on the oxidation reaction. In the case of polymer
system as shown in the Figure 2 and Figure S8 in the Supporting
Information, the CV of polyamides I reveal three reversible and
one irreversible oxidation processes. During the electrochemical
oxidation of the polyamide thin films, the color of polymer
film changed from colorless to yellow, purple, bluish green and
then to deep blue. Because of the high electrochemical stability
and good adhesion between the polyamide thin film and ITO
substrate, polyamide Ia exhibited reversible CV behavior by
continuous 10 000 cyclic scans both in the first and second
oxidation stages. The redox potentials of the polyamides as well
as their respective highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) (versus
vacuum) are calculated and summarized in Table 1. The HOMO
level or called ionization potentials (versus vacuum) of poly-
amides I could be estimated from the onset of their oxida-
tion in CV experiments as 4.67ꢀ4.68 eV (on the basis that
ferrocene/ferrocenium is 4.8 eV below the vacuum level with
Eonset = 0.36 V).
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Table 1. Redox Potentials and Energy Levels of Polyamides
thin films (nm)
oxidation potential (V)a
polymer
λmax
λonset
Eonset
E1/2(ox1)
E1/2(ox2)
E1/2(ox3)
λmax
Eg (eV)b λonset
HOMO (eV)c
LUMO (eV) Eonset
Ia
Ib
349
345
410
412
0.24
0.23
0.39
0.39
0.61
0.59
1.01
1.01
1.34
1.27
3.02
3.01
4.68
4.67
1.66
1.66
a From cyclic votammograms vs Ag/AgCl in CH3CN. E1/2: average potential of the redox couple peaks. Ep: peak potential of the irreversible fourth
oxidation steps. b The data were calculated from polymer films by the equation: Eg = 1240/λonset (energy gap between HOMO and LUMO). c The
HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV ; onset = 0.36 V).
was adjusted to more positive values (0.61ꢀ0.86 V) corresponding
to the second electron oxidation, the characteristic new peaks at 597
and 1036 nm appeared and the second oxidation reversibility was
99%. For the third oxidative state (0.86ꢀ1.35 V), the peak at
1036 nm decreased slightly with a reversibility of 81%. It shows the
first, second, and third oxidative states are reversible and highly
stable. When the applied potential was adding to 1.50 V, the
characteristic peaks new bands at 891 nm appeared, but only 52%
absorbance remained implying that the oxidation product
(tetracation) was not stable. The same experiment for polyamide
Ia film exhibited the first, second, third, and fourth oxidation
reversibility up to 100, 99, 83, and 59%, respectively.
The typical spectroelectrochemical spectra and three-dimen-
sional % transmittance-wavelength-applied potential correlations
of polyamide Ia film is presented in Figure 4. In the neutral form
(0 V), the film exhibited strong absorption at around 346 nm,
characteristic for triarylamine, but it was almost transparent in
the visible region. Upon oxidation (increasing applied voltage
from 0 to 0.55 V), the intensity of the absorption peak at 346 nm
gradually decreased while a new peak at 447 nm and a broad IV-
CT band centered around 1170 nm in the NIR region gradually
increased in intensity. We attribute the spectral change in visible-
light range to the formation of a stable monocation radical Ia1þ in
the TPA center of starburst triarylamine moiety. Furthermore,
the broad absorption in NIR region was the characteristic result
due to IV-CT excitation between states in which the positive
charge is centered at different nitrogen atoms, which was
consistent with the phenomenon classified by Robin and Day.6
As the more anodic potential to 0.80 V corresponding to Ia2þ
,
the absorption bands (346 and 447 nm) decreased gradually with
new broad bands centered at around 583 and 1100 nm. By
further applying positive potential value up to 1.10 V correspond-
ing to Ia3þ, the characteristic absorbance at 643 and 824 nm
appeared, and the strong IV-CT band at 1079 nm in the NIR
region still could be observed. When the applied potential was
added to 1.45 V, the absorption bands at 1079 nm decreased
gradually with a new broad band centered at around 729 nm. The
disappearance of NIR absorption band can be attributable to the
further oxidation of Ia3þ species to the formation of Ia4þ in the
starburst triarylamine segments. The observed UVꢀvis-NIR
absorption changes in the polyamide Ia film at various potentials
are reversible and associated with strong color changes. The
other polyamide Ib showed similar spectral change to that of Ia.
From the figure shown in Figure 4, the polyamide Ia film switches
from a transmissive neutral state (colorless; Y: 90; x, 0.313; y,
0.331) to the first-oxidized state (yellow; Y: 50; x, 0.398; y,
0.461), second-oxidized state (purple; Y: 21; x, 0.278; y, 0.271),
third-oxidized state (bluish-green; Y: 20; x, 0.252; y, 0.315), and a
fully oxidized state (deep blue; Y; 7; x, 0.208; y, 0.229). The film
colorations are distributed homogeneously across the polymer film
Figure 3. Absorption spectral change of 1 ꢁ 10ꢀ4 M dinitro compound
1 in CH2Cl2 solution containing 0.2 M TBAP at various applied
potentials between 0.0 and 1.50 (V vs Ag/AgCl).
Spectroelectrochemistry. Spectroelectrochemical experi-
ments were used to evaluate the optical properties of the
electrochromic films. For the investigations, compound 1 and
polyamide film were prepared in the same manner as described
above, and a homemade electrochemical cell was built from a
commercial ultraviolet (UV)ꢀvisible cuvette. The cell was placed in
the optical path of the sample light beam in a UVꢀvisꢀNIR
spectrophotometer, which allowed us to acquire electronic absorp-
tion spectra under potential control. UVꢀvisꢀNIR absorbance
curves correlated to electrode potentials of compound 1is presented
in Figure 3. The absorption peaks at 337 and 412 nm are
characteristic for compound 1. After one-electron oxidation
(0.00ꢀ0.61 V), the original peaks decreased and a broad band at
around 1240 nm increased gradually. The first oxidation reversibility
was 99% based on the absorbance at 337 nm.12 When the potential
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Figure 4. Electrochromic behavior (left) at applied potentials of (a) 0.00, (b) 0.55, (c) 0.80, (d) 1.10, (e) 1.45 (V vs Ag/AgCl), and 3D
spectroelectrochemical behavior (right) from 0.00 to 1.45 (V vs Ag/AgCl) of polyamide Ia thin film (∼120 nm in thickness) on the ITO-coated glass
substrate in 0.1 M TBAP/CH3CN.
extremely high optical transmittance change (Δ%T) of 85% at
1100 nm for purple coloring at the second oxidation stage, and 85%
at 729 nm for blue coloring at the fourth oxidation stage, respec-
tively. Because of the apparent high electrochromic contrast, optical
switching studies were investigated more deeply to manifest the
outstanding electrochromic characteristics of these obtained novel
anodically electrochromic materials.
Electrochromic Switching Studies. For electrochromic
switching studies, polymer films were cast on ITO-coated glass
slides in the same manner as described above, and chronoampero-
metric and absorbance measurements were performed. While the
films were switched, the absorbance at the given wavelength was
monitored as a function of time with UVꢀvisꢀNIR spectroscopy.
Switching data for the representative cast film of polyamide Ia
were shown in Figures S9 and S10 in the Supporting Information
and Figure 5. 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 Figure S9a in the
Supporting Information, when the potential was switched be-
tween 0 and 0.55 V, polyamide Ia thin film revealed switching
time of 2.06 s for coloring process and 0.90 s for bleaching. When
the switched potential was set between 0 and 0.80 V, thin film Ia
required 1.82 s for coloration and 1.06 s for bleaching (Figure S9b
in the Supporting Information). The polyamides switched quickly
between the highly transmissive neutral state and the colored
oxidized state. Two movies of electrochromic switches between
neutral and oxidation states with a cycle time of 6 s were provided
in the Supporting Information. The amount of injected/ejected
charge (Q) were calculated by integration of the current density
and time obtained from Figure S9c in the Supporting Information
as 1.679 and 1.677 mC/cm2 for oxidation and reduction process
at the first oxidation stage, respectively. The ratio of the charge
density was 99.9%, indicated that charge injected/ejected was
Figure 5. Electrochromic switching between (A) 0 and 0.55 V and (B) 0
and 0.80 V (vs Ag/AgCl) of polyamide Ia thin film (∼120 nm in thickness)
on the ITO-coated glass substrate (coated area: 1.2 cm ꢁ 0.5 cm) in 0.1 M
TBAP/CH3CN with a cycle time of 30 s. (a) Current consumption and (b)
absorbance change monitored at the given wavelength.
and survive for more than thousands of redox cycles. The polymer Ia
shows good contrast both in the visible and NIR regions with an
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Table 2. Optical and Electrochemical Data Collected for
Coloration Efficiency Measurements of Polyamide Ia
b
cycling timesa δ%T1170 δOD1170 Q (mC/cm2)c η (cm2/C)d decay (%)e
1
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
60
60
60
59
59
59
59
59
58
58
58
0.487
0.487
0.484
0.482
0.479
0.479
0.474
0.473
0.470
0.470
0.466
1.679
1.679
1.680
1.678
1.675
1.675
1.674
1.672
1.672
1.671
1.669
290
290
288
287
286
286
283
283
281
281
279
0
0
0.69
1.03
1.38
1.38
2.41
2.41
3.10
3.10
3.80
a Switching between 0.00 and 0.55 (V vs Ag/AgCl). b Optical density
change at 1170 nm. c Ejected charge, determined from the in situ
experiments. d Coloration efficiency is derived from the equation: η =
δOD1170/Q. e Decay of coloration efficiency after cyclic scans.
Figure 6. Potential step absorptometry during the continuous cycling
test by switching potentials between (a) 0 and 0.55 V, (b) 0 and 0.80 V
(vs Ag/AgCl), and (c) absorptometry for 1 switching cycle of polyamide
Ia thin film (∼120 nm in thickness) on the ITO-coated glass substrate
(coated area: 1.2 cm ꢁ 0.5 cm) in 0.1 M TBAP/CH3CN with a cycle
time of 5 h and 5 min for coloring and bleaching processes, respectively.
Table 3. Optical and Electrochemical Data Collected for
Coloration Efficiency Measurements of Polyamide Ia
cycling
timesa
decay
δ%T1100 δOD1100 Q (mC/cm2)c η (cm2/C)d (%)e
b
1
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
85
85
85
85
84
84
84
84
84
83
83
0.888
0.886
0.880
0.875
0.869
0.861
0.853
0.845
0.837
0.830
0.822
2.622
2.621
2.622
2.621
2.619
2.617
2.615
2.612
2.610
2.607
2.605
339
338
336
334
332
329
326
324
321
318
316
0
0.29
0.88
1.47
2.06
2.95
3.83
4.42
5.31
6.19
6.78
electroactive aromatic polyamides with the potential for commer-
cial applications. Considering the retained contrast ratio and
electroactive stability during electrochromic operation, these
novel polyamides show highly electrochromic stability compared
with other electrochromic polymers.5
’ CONCLUSIONS
A series of novel NIR electrochromic aromatic polyamides
with starburst triarylamine units were readily prepared from the
newly synthesized diamine monomer, N,N-bis[4-(4-methoxy-
phenyl-40-aminophenylamino)phenyl]-N0,N0-di(4-methoxyphe-
nyl)-p-phenylenediamine, with various aromatic dicarboxylic
acids via the phosphorylation polyamidation. Introduction of
highly electron-donating starburst triarylamine group to the
polymer main chain not only stabilizes its radical cations but
also leads to good solubility and film-forming properties of the
polyamides. In addition to high Tg and good thermal stability, all
the obtained polymers also reveal valuable electrochromic char-
acteristics such as high contrast ratio both in the visible range and
NIR region, low switching times, high coloration efficiency, and
highly electrochromic/electroactive reversibility. Thus, these
characteristics suggest that these novel starburst triarylamine-
containing electrochromic aromatic polyamides have great po-
tential for practical applications in both visible and NIR region.
a Switching between 0.00 and 0.80 (V vs Ag/AgCl). b Optical density
change at 1100 nm. c Ejected charge, determined from the in situ
experiments. d Coloration efficiency is derived from the equation: η =
δOD1100/Q. e Decay of coloration efficiency after cyclic scans.
highly reversible during the electrochemical reactions. As shown
in Figure S10 in the Supporting Information and Figure 5, the
electrochromic stability of the polyamide Ia film was determined
by measuring the transmittance and optical changes as a function
of the number of switching cycles and plotted every 1000th cycle.
The electrochromic CE (η = δOD/Q) and injected charge
(electroactivity) after various switching steps were monitored
and summarized in Tables 2 and 3. The electrochromic film of Ia
was found to exhibit high CE up to 290 cm2/C at 1170 nm, and to
retain more than 99% of their electroactivity after switching
10,000 times between 0 and 0.55 V (Figure 5A). As the applied
switching potential increased to 0.80 V, a higher CE (339 cm2/C
at 1100 nm) could be obtained and showed only 0.7% decay of its
electroactivityafter10,000cycles(Figure5B).Additional,Figure6
show the long-term stability measured by keeping 5 h for each
coloring process at applied potentials of 0.55 and 0.80 V,
respectively, and the relatively high electrochromic stability was
observed, which reinforces and affords positive proof of these NIR
’ EXPERIMENTAL SECTION
Materials. N,N-Bis(4-bromophenyl)-N0,N0-di(4-methoxyphenyl)-
p-phenylenediamine13 and 4-methoxy-40-nitrodiphenylamine12 were
synthesized according to a previously reported procedure. Commercially
available aromatic dicarboxylic acids such as 4,40-oxidibenzoic acid (3a) and
2,20-bis(4-carboxyphenyl)hexafluoropropane (3b) were purchased from
Tokyo Chemical Industry (TCI) Co. and used as received. Commercially
obtained anhydrous calcium chloride (CaCl2) was dried under vacuum at
180 °C for 8 h. Tetrabutylammonium perchlorate (TBAP) (Acros) was
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recrystallized twice by ethyl acetate in a nitrogen atmosphere and then dried
in vacuo prior to use. All other reagents were used as received from
commercial sources.
neck round-bottom flask equipped with a stirring bar in a nitrogen
atmosphere, 730.9 mg (0.76 mmol) of dinitro compound (2) and 29.0
mg of 10% Pd/C were dissolved/suspended in 10 mL of ethanol and
15 mL of THF. The suspension solution was heated to reflux, and 0.4 mL
of hydrazine monohydrate was added slowly to the mixture, and the
solution was then stirred at reflux temperature for 10 h, the solution was
filtered to remove Pd/C. After concentrated the filtrate, the product was
washed thoroughly with ethanol and dried in vacuo at 80 °C to afford
603.4 mg (88% in yield) of bright yellow powders with a mp of
254ꢀ256 °C (measured by DSC at 10 °C/min) FT-IR (KBr
pellet, cmꢀ1): 3446, 3365 (NꢀH stretch), 1239, 1035 (-C-O-C-
1
stretch). H NMR (DMSO-d6, δ, ppm): 3.68 (s, -OCH3), 3.69 (s,
-OCH3), 4.98 (s, -NH2), 6.51 (d, 4H), 6.65 (d, 4H), 6.70ꢀ6.80 (m,
12H), 6.83 (d, 8H), 6.91 (d, 8H). 13C NMR (DMSO-d6, δ, ppm): 55.40
(C1þC22), 115.00, 115.11, 120.67, 122.65, 123.45, 124.65, 125.05,
125.53, 127.41, 136.13, 140.20, 141.02, 141.29, 141.96, 142.57,
144.10, 145.79, 154.87, 155.20. Anal. Calcd for C58H52N6O4
(897.07): C, 77.65%; H, 5.84%; N, 9.37%. Found: C, 77.31%; H,
5.75%; N, 9.34%. ESI-MS: calcd for (C58H52N6O4)þ, m/z 897.1; found,
m/z 896.5.
N,N-Bis[4-(4-methoxyphenyl-40-nitrophenylamino)phenyl]-N0,N0-
di(4-methoxyphenyl)-p-phenylenediamine (1). In a 100 mL three-neck
round-bottom flask equipped with a stirring bar in a nitrogen atmosphere, 2.80
g (4.41 mmol) of N,N-bis(4-bromophenyl)-N0,N0-di(4-methoxyphenyl)-p-
phenylenediamine, 3.02 g (12.35 mmol) of 4-methoxy-40-nitrodiphenyla-
mine, 4.89 g (35.39 mmol) of powdered anhydrous potassium carbonate, 1.12
g (17.64 mmol) of copper powder, and 0.23 g (0.88 mmol) of 18-crown-6-
ether were stirred in 40 mL of o-dichlorobenzene at 160 °C for 24 h in a
nitrogen atmosphere. The copper and inorganic salts were then removed by
filtration of the hot reaction mixture, and the product was precipitated into
ethanol. The product was recrystallized by ethyl acetate to give 2.79 g (66% in
yield) of dark red solid with a mp of 231ꢀ233 °C (measured by DSC at
10 °C/min). FT-IR (KBr pellet, cmꢀ1): 1587, 1311 (-NO2 stretch), 1243,
1034 (-C-O-C- stretch). 1H NMR (DMSO-d6, δ, ppm): 3.72 (s, -OCH3),
3.77 (s, -OCH3), 6.69 (d, 4H), 6.77(d, 2H), 6.89 (d, 4H), 6.97ꢀ7.04 (m,
14H), 7.19 (d, 4H), 7.25 (d, 4H), 8.02 (d, 4H). 13C NMR (DMSO-d6, δ,
ppm): 55.40 (C1), 55.54 (C22), 115.15, 115.25, 115.72, 120.83, 123.65,
125.86, 126.70, 127.25, 128.00, 129.00, 137.54, 138.22, 138.86, 140.30, 145.40,
145.54, 154.11, 155.86, 157.93. Anal. Calcd for C58H48N6O8 (957.04): C,
72.79%; H, 5.06%; N, 8.78%. Found: C, 72.19%; H, 5.13%; N, 8.66%. ESI-
MS: calcd for (C58H48N6O8)þ, m/z 957.0; found, m/z 956.4.
Polymer Synthesis. The synthesis of polyamide Ia was used as an
example to illustrate the general synthetic route used to produce the
polyamides. A mixture of 577.1 mg (0.64 mmol) of the diamine
monomer 2, 166.1 mg (0.64 mmol) of 4,40-oxidibenzoic acid (3a), 20
mg of calcium chloride, 0.51 mL of triphenyl phosphite, 0.32 mL of
pyridine, and 0.51 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
methanol 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. Reprecipitations of the polymer by N,N-
dimethylacetamide (DMAc)/methanol were carried out twice for
further purification. The inherent viscosity and weight-average molec-
ular weights (Mw) of the obtained polyamide Ia was 0.27 dL/g
(measured at a concentration of 0.5 g/dL in DMAc at 30 °C) and
69,800 Da, respectively. The FT-IR spectrum of Ia (film) exhibited
characteristic amide absorption bands at 3306 cmꢀ1 (NꢀH stretch),
3036 cmꢀ1 (aromatic CꢀH stretch), 2998ꢀ2834 cmꢀ1 (CH3 CꢀH
stretch), 1670 cmꢀ1 (amide carbonyl), 1240 cmꢀ1 (asymmetric stretch
CꢀOꢀC), 1036 cmꢀ1 (symmetric stretch CꢀOꢀC). 1H NMR
(DMSO-d6, δ, ppm): 3.70 (s, -OCH3), 6.74ꢀ6.99 (m, 32H), 7.17 (d,
4H), 7.62 (d, 4H), 8.00 (d, 4H), 10.14 (s, 2H, amide-NH). Anal. Calcd
(%) of Ia for (C74H64N6O7)n (1149.34)n: C, 77.33; H, 5.61; N, 7.31.
Found: C, 75.35; H, 5.39; N, 7.13. The other polyamide Ib were
prepared by an analogous procedure. The FT-IR spectrum of Ib
(film) exhibited characteristic amide absorption bands at 3307 cmꢀ1
(NꢀH stretch), 3036 cmꢀ1 (aromatic CꢀH stretch), 2998ꢀ2834 cmꢀ1
(CH3 CꢀH stretch), 1669 cmꢀ1 (amide carbonyl), 1242 cmꢀ1
(asymmetric stretch CꢀOꢀC), 1037 cmꢀ1 (symmetric stretch
CꢀOꢀC). 1H NMR (DMSO-d6, δ, ppm): 3.68 (s, -OCH3),
6.72ꢀ6.96 (m, 32H), 7.48 (d, 4H), 7.61 (d, 4H), 7.99 (d, 4H), 10.31
(s, 2H, amide-NH). Anal. Calcd (%) of Ib for (C77H64F6N6O6)n
(1283.36)n: C, 72.06; H, 5.03; N, 6.55. Found: C, 70.80; H, 4.81;
N, 6.61.
Preparation of the Polyamide Films. A solution of polymer was
made by dissolving about 0.6 g of the polyamide sample in 9 mL of
DMAc. The homogeneous 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 about 60 μm 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.
Elemental analyses were run in a Heraeus VarioEL-III CHNS elemental
N,N-Bis[4-(4-methoxyphenyl-40-aminophenylamino)phenyl]-N0,
N0-di(4-methoxyphenyl)-p-phenylenediamine (2). In a 100 -mL three-
1
analyzer. H NMR spectra were measured on a Bruker AC-300 MHz
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ARTICLE
spectrometer in DMSO-d6, using tetramethylsilane as an internal reference,
and peak multiplicity was reported as follows: s, singlet; d, doublet. 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 carried out on a Waters chromatography unit interfaced
with a Waters 2410 refractive index detector. Two Waters 5 μm Styragel
HR-2 and HR-4 columns (7.8 mm I. D. ꢁ 300 mm) were connected in
series with NMP as the eluent at a flow rate of 0.5 mL/min at 40 °C and
were calibrated with polystyrene standards. Thermogravimetric analysis
(TGA) was conducted with a PerkinElmer Pyris 1 TGA. Experiments were
carried out on approximately 6ꢀ8 mg film samples heated in flowing
nitrogen or air (flow rate = 20 cm3/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 Instruments 611B electrochemical analyzer. Voltam-
mograms are presented with the positive potential pointing to the left and
with increasing anodic currents pointing downward. Cyclic voltammetry
(CV) was conducted with the use of a three-electrode cell in which ITO
(polymer films area about 0.5 cm ꢁ1.2 cm) 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 experiments were carried out in a cell built from
a 1 cm commercial UVꢀvisible cuvette using Hewlett-Packard 8453
UVꢀvisible diode array, Jasco V-570 UVꢀvis/NIR, and Hitachi U-4100
UVꢀvisꢀNIR spectrophotometer. The ITO-coated glass slide was used as
the working electrode, a platinum wire as the counter electrode, and an Ag/
AgCl cell as the reference electrode. CE (η) determines the amount of
optical density change (δOD) at a specific absorption wavelength induced as
a function of the injected/ejected charge (Q) which is determined from the
in situ experiments. CE is given by the equation: η=δOD/Q= log[Tb/Tc]/
Q, where η (cm2/C) is the coloration efficiency 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
profilometer (Kosaka Lab., Surfcorder ET3000, Japan). Colorimetry mea-
surements were obtained using a Minolta CS-100A Chroma Meter. The
color coordinates are expressed in the CIE 1931 Yxy color spaces.
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’ ASSOCIATED CONTENT
S
Supporting Information. Two movies of electrochromic
b
switches between neutral and oxidation states. Table: inherent
viscosity, molecular weights, solubility behavior, and thermal
properties. Figure: NMR of monomers, IR of monomers and
polyamide, TGA, DSC traces and CV of polyamides, calculation
of optical switching time of polyamides. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: gsliou@ntu.edu.tw.
’ ACKNOWLEDGMENT
The authors are grateful to the National Science Council of the
Republic of China for financial support of this work. And also to
C.-W. Lu of the Instrumentation Center, National Taiwan
University, for CHNS (EA) analysis experiments. C. H. Ho
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Chem. Ed. 1975, 13, 1373.
(12) Liou, G. S.; Lin, H. Y. Macromolecules 2009, 42, 125.
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