Substituent effect on electrochemical and electrochromic behaviors of ambipolar aromatic polyimides based on aniline derivatives

Article abstract of DOI:10.1021/ma202128e

Three series of new ambipolar polyimides containing aniline derivatives with different degree of 4-methoxyphenyl substituent showing anodically/cathodically electrochromic characteristic were prepared from the polycondensation of three diamine monomers-4,4′-bis[(4-aminophenyl)amino] diphenylamine (3), 4,4′-bis[(4-aminophenyl)amino]-4″- methoxytriphenylamine (3′), and 4,4′-bis[4-aminophenyl(4- methoxyphenyl)amino]-4″-methoxytriphenylamine (3″)-with various commercially available tetracarboxylic dianhydrides. In addition to the high glass-transition temperatures (T_g, 246-339 °C), thermal stability, and solution processability, the resulting polyimide films exhibited ambipolar electrochromic behavior. The nonconjugated TPA-based polyimides III revealed more stable electrochemical oxidation behavior and better electrochromic reversibility than conjugated DPA-based corresponding polyimides I due to the additional phenyl ring providing the route to stabilize the cation radical formed during oxidation.

Full text of DOI:10.1021/ma202128e

ARTICLE
pubs.acs.org/Macromolecules
Substituent Effect on Electrochemical and Electrochromic Behaviors
of Ambipolar Aromatic Polyimides Based on Aniline Derivatives
Li-Ting Huang, Hung-Ju Yen, and Guey-Sheng Liou*
Functional Polymeric Materials Laboratory, Institute of Polymer Science and Engineering, National Taiwan University,
1 Roosevelt Road, 4th Sec., Taipei 10617, Taiwan
S Supporting Information
b
ABSTRACT: Three series of new ambipolar polyimides containing
aniline derivatives with different degree of 4-methoxyphenyl sub-
stituent showing anodically/cathodically electrochromic character-
istic were prepared from the polycondensation of three diamine
monomers—4,40-bis[(4-aminophenyl)amino]diphenylamine (3),
4,40-bis[(4-aminophenyl)amino]-400-methoxytriphenylamine (30),
and 4,40-bis[4-aminophenyl(4-methoxyphenyl)amino]-400-meth-
oxytriphenylamine (300)—with various commercially available
tetracarboxylic dianhydrides. In addition to the high glass-transi-
tion temperatures (Tg, 246ꢀ339 °C), thermal stability, and
solution processability, the resulting polyimide films exhibited
ambipolar electrochromic behavior. The nonconjugated TPA-
based polyimides III revealed more stable electrochemical oxida-
tion behavior and better electrochromic reversibility than conjugated DPA-based corresponding polyimides I due to the additional
phenyl ring providing the route to stabilize the cation radical formed during oxidation.
’ INTRODUCTION
switch between two or more different color states. Neverthe-
less, the long-term stability of these aniline-derivative polymers
was rarely investigated due to the switching time might be
elongated after cyclic scans and the environmental instability.10
In addition, the major drawbacks of all the different classes of
π-conjugated polymers are the issues of poor solubility, low
processability, and difficulty of switching from one distinct color
of neutral state to a highly transmissive and near colorless
of oxidative state,11 which largely limits the influence on the
demanding of highly transparent and colorless EC windows
without any applied potential at neutral state in applications.
For organic electroactive triphenylamine (TPA) materials,
the characteristic structure feature of TPA is the electroactive
site of the nitrogen center, which is linked to three electron-rich
phenyl groups in a propeller-like geometry. The anodic oxidation
behavior of TPA has been well studied.12 The electrogenera-
ted cation radical of TPA•+ is not stable and could form
tetraphenylbenzidine (TPB) by tail-to-tail coupling. When the
phenyl groups were incorporated by electron-donating substit-
uents at the para position of TPA, the coupling reactions could be
greatly prevented by affording stable cationic radicals.13 Intra-
molecular electron transfer (ET) processes were also investi-
gated extensively in the mixed-valence (MV) systems14 and us-
ually employed one-dimensional MV compounds contain two
or more redox states connected via π-bridge molecule. According
Electrochromic (EC) materials exhibit a reversible optical
change in absorption or transmittance upon electrochemically
oxidized or reduced; the EC effect has been known for quite
some time, and great strides have been made with transition-
metal oxides (e.g., WO3), inorganic coordinating complexes,
organic molecules, and electrodeposited π-conjugated polymers
films.1 Among the conjugated polymers, polyaniline (PANI)
has long been considered a strong contender in the field of
polymer electronics due to its variable color changes depend-
ing on the redox state, and its electrical properties can be
modified by p-doping (i.e., changing the oxidation state) or
by acidꢀbase chemistry (affecting the protonation states).2
However, PANI could not provide detailed understanding
of the relation between the structural disorder and the elec-
tronic transport. For this reason, oligoanilines with similar
but clearly defined molecular structures compared to poly-
mers have attracted much attention as theoretical models
and also could exhibit new properties or even find new ap-
plication.3 Similar to other conjugated polymers, the struc-
tural configuration and chain length of polymers have a great
effect on the optical and physical properties, such as the
oxidation potential of oligoanilines decreasing with increas-
ing their chain length.4 Recently, the electroactive polymers
with aniline oligomer have been widely studied, such as polya-
mide,5 polyimide,6 epoxy,7 dendrimer,8 and others.9 There
have been significant works published on these π-conjugated
polymers focused on their electrochemical behaviors and
electrochromic properties; in essence, these polymers could
Received: September 20, 2011
Revised:
November 10, 2011
Published: November 29, 2011
r
2011 American Chemical Society
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to Robin and Day,15 the cation radical has been reported as a
symmetrical delocalized class III structure with a strong
electronic coupling (the electron is delocalized over the two
or more redox centers), leading an intervalence charge
transfer (IV-CT) absorption band in the NIR region.16 Since
2005, our group has reported several TPA-containing elec-
trochromic polymers with interesting multicolor transi-
tions17 that showed good electrochromic reversibility in the
visible region and NIR range. Compared with the electro-
chromic π-conjugated polymers, the nonconjugated TPA-
based polymers exhibited highly transmissive in the neutral
state and multicolor in the oxidized states. In this article, in
order to investigate electrochemical and electrochromic
behaviors of the structurally related TPA-based polymers
and aniline-based polymers, we therefore synthesized a
novel diamine monomer, 4,40-bis[(4-aminophenyl)amino]-400-
methoxytriphenylamine (30) containing one 4-methoxyphenyl
substituted tertiary amino and two secondary amino moieties
and their corresponding aromatic polyimides. For comparison,
the structurally related two series of polyimides were also
prepared from the diamines 4,40-bis[(4-aminophenyl)amino]-
diphenylamine (3) with structure close to PANI and 4,40-bis-
[4-aminophenyl(4-methoxyphenyl)amino]-400-methoxytriphe-
nylamine (300).18
with 4-fluoronitrobenzene followed by hydrogen Pd/C-catalyzed
reduction according to the synthetic route outline in Scheme 1.
The new monomer 4,40-bis[(4-aminophenyl)amino]-400-
methoxytriphenylamine (30) was synthesized by hydrogen
Pd/C-catalyzed reduction of the dinitro compound 4,40-bis-
[(4-nitrophenyl)amino]-400-methoxytriphenylamine (20) result-
ing from the triethylamine-mediated aromatic nucleophilic sub-
stitution reaction of 4,40-diamino-400-methoxytriphenylamine with
4-fluoronitrobenzene (Scheme 1). Synthesis and characterization
data of compounds 4,40-bis[(4-nitrophenyl)amino]diphenylamine
(2), 4,40-bis[(4-aminophenyl)amino]diphenylamine (3), 4,40-
bis[(4-nitrophenyl)amino]-400-methoxytriphenylamine (20), and
4,40-bis[(4-aminophenyl)amino]-400-methoxytriphenylamine (30)
were included in the Supporting Information (Figures S1ꢀS4)
and also agreed well with the proposed molecular structures.
Polymer Synthesis. Two series of polyimides I and II were
prepared by the polycondensation of diamines 3 and 30 with two
commercially available dianhydrides 4a and 4b via a conventional
two-step process (as shown in Scheme 2), respectively. In the
first step, the reaction mixtures became very viscous, indicating
the formation of poly(amic acid)s with high molecular weight.
The poly(amic acid) precursors could be chemically dehydrated
to the corresponding polyimides by treatment with acetic
anhydride and pyridine. Because of no secondary amine group
within monomer diamine 300, the series of polyimides III could be
prepared readily by one-step polymerization of diamine 300 and
dianhydrides 4 in m-cresol at 185 °C in the presence of isoquino-
line as the catalyst. These obtained polyimides had inherent
viscosities in the range of 0.33ꢀ1.34 dL/g with weight-average
molecular weights (Mw) and polydispersity (PDI) of 57 000ꢀ
143 000 Da and 1.54ꢀ2.35, respectively, relative to polystyr-
ene standards (Table S1). All polyimides could afford
tough and free-standing films. The formation of polyimides
was confirmed by IR and NMR spectroscopy (as shown
in Figures S5ꢀS7). The IR spectra of polyimides (IaꢀIIIa)
exhibited characteristic imide absorption bands at around 1781
(symmetrical CdO), 1720 (symmetrical CdO), 1380 (CꢀN),
and 738 cmꢀ1 (imide ring deformation). The structures of these
polyimides were also verified by NMR spectra, and Figure S6
1
shows a typical set of H and 13C NMR spectra of polyimides
IIa in DMSO-d6; all the peaks could be readily assigned to
the hydrogen and carbon atoms of the recurring unit. No
1
resonance peak appeared around 10 ppm in the H NMR
spectrum support the completed imidization. Assignments
of each carbon and proton were assisted by the two-
dimensional HꢀH COSY and CꢀH HSQC NMR spectra
shown in Figure S7 and agree well with the proposed mole-
cular structure of IIa.
Solubility. The solubility properties of polyimides IꢀIII in
some organic solvent were investigated qualitatively, and the
results are summarized in Table S2 of the Supporting In-
formation. The solubility behavior of these polyimides de-
pends on their chain packing and intermolecular interaction
affected by the rigidity, symmetry, and regularity of the
molecular backbone. Polyimides III derived from diamine
with higher steric hindrance and bulky phenyl groups showed
higher solubility not only in polar aprotic organic solvents
such as NMP, DMAc, and m-cresol but also in less polar
solvents like THF and CHCl3. Because of the strong hydrogen-
bonding interaction and less steric hindrance for close pack-
ing, the polymers I exhibited the lowest solubility among all the
three series of polyimides. The polyimides IꢀIII obtained from
’ RESULTS AND DISCUSSION
Monomer Synthesis. 4,40-Bis[(4-aminophenyl)amino]diphen-
ylamine (3) was prepared by the triethylamine-mediated aroma-
tic nucleophilic substitution reaction of 4,40-diaminodiphenylamine
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Scheme 1. Synthetic Routes to Diamine Compounds 3 and 30
Scheme 2. Synthesis and Structures of Polyimides IꢀIII
dianhydride 4b revealed better solubility than those of prepared
from 4a because of the additional contribution of the hexafluor-
oisopropylidene (ꢀC(CF3)2ꢀ) group in the polymer backbone
which reduce the intermolecular interactions and close pack-
ing. Thus, the excellent solubility makes these polymers as
potential candidates for practical applications by spin-coating or
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Figure 1. Cyclic voltammograms of 10ꢀ3 M (a) dinitro compounds 2, 20, and 200 and diamino compounds (b) 3, (c) 30, and (d) 300 in CH3CN
containing 0.1 M TBAP. Scan rate = 50 mV/s.
Table 1. Redox Potentials and Energy Levels of Compounds
oxidation (V) (vs Ag/AgCl)
HOMOb
LUMOc
Eg
d
code
Eonset
E1/2(ox,1)
E1/2(ox,2)
E1/2(ox,3)
E1/2(ox,4)
E1/2(ox,5)
λonset (nm)
2
0.49
0.48
0.54
0.11
0.14
0.19
0.60
0.59
0.63
0.19
0.23
0.27
0.86
0.88
0.85
0.40
0.44
0.47
1.37a
1.59a
1.40a
0.70
532
535
528
4.96
4.93
4.99
4.55
4.59
4.63
2.63
2.61
2.64
2.33
2.32
2.35
20
200
3
30
300
1.15a
1.10a
0.94
1.53a
1.45a
1.35
0.77
0.74
a Irreversible peak potential. b The HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV, E1/2 = 0.44 V).
c LUMO = HOMO ꢀ Eg. d Eg = 1240/λonset
.
inkjet-printing processes to afford high-performance thin films
for optoelectronic devices.
good thermal stability but also possessed high glass-transition
temperatures (Tg) in the range 246ꢀ339 °C. TMA curves of
polyimides Ia, IIa, and IIIa are shown in Figure S9. The
polyimide I series exhibited the higher Tg values that could
be attributed to the strong interaction of hydrogen bond-
ing between the secondary amine and carbonyl groups in the
polymer chains. Besides, the sulfone type polyimides revealed
the higher Tg due to their higher intermolecular force resul-
ted from sulfone group which could provide strong dipole
interactions relative to other connecting groups.
Electrochemical Properties. The electrochemical behavior of
compounds 2ꢀ200 and 3ꢀ300 series and the polyimide IꢀIII
series was investigated by cyclic voltammetry (CV). Compounds
were dissolved in dry acetonitrile (concentration 10ꢀ3 mol/L),
and the polymers were conducted by cast film on an indiumꢀ
tin oxide (ITO)-coated glass substrate as working electrode
Thermal Properties. The thermal properties of polyimides
recorded by TGA and TMA are summarized in Table S3.
Typical TGA curves of polyimides Ia, IIa, and IIIa are
depicted in Figure S8. All the prepared polyimides exhibited
good thermal stability with insignificant weight loss up to
350 °C under nitrogen or air atmosphere. The 10% weight loss
temperatures of these polymers in nitrogen and air were
recorded in the range of 460ꢀ630 and 450ꢀ585 °C, respec-
tively. The amount of carbonized residue (char yield) of these
polymers in nitrogen atmosphere was more than 58% at
800 °C. The high char yields of these polymers can be ascribed
to their high aromatic content. The lowest Td value of polyi-
mide I series could be explained in terms of the less aromatic
segment in its backbone. These polymers not only showed
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Figure 2. Cyclic voltammograms of the polyimide films (a) Ia, (b) IIa, (c) IIIa, and (d) IIIc on an ITO-coated glass substrate in 0.1 M TBAP
acetonitrile (for anodic process) and DMF (for cathodic process) solution at a scan rate of 50 and 100 mV/s.
containing 0.1 M of tetrabutylammonium perchlorate (TBAP)
as an electrolyte under a nitrogen atmosphere. The oxidation
and reduction measurements of the polymer films were mea-
sured in acetonitrile (CH3CN) and DMF, respectively. The
typical CV diagrams for compounds 2ꢀ200 and 3ꢀ300 are
depicted in Figure 1, and quantitative details are summarized
in Table 1. There are three and five oxidation redox couples for
compounds 2ꢀ200 (Figure 1a) and monomers 3ꢀ300, respec-
tively. As the substituent changed from electron-withdrawing
nitro group to the electron-donating amino group, monomers
3ꢀ300 exhibited lower oxidation potentials than those of com-
pounds 2ꢀ200, indicating the inductive effect of the substituent
on the oxidation reaction. Furthermore, replacing hydro-
gen atom by incorporation of methoxy-substituted phenyl
groups into the electroactive nitrogen site of amino moieties
resulted in increasing the applied potential values in the first
oxidation stage, while these extra phenyl rings with electron-
donating methoxy groups could provide routes for stabiliz-
ing the cation radical formed during oxidation procedure,
thus enhancing the electrochemical reversibility. In the case
of polymer system as shown in the Figure 2, all the polyimides
underwent a three-stage one-electron oxidation originating
from amine units, and only one-electron reduction induced
by the aromatic-carbonyl π-system of the imide functional
group could be observed from the imide ring of DSDA polyi-
mides IaꢀIIIa and 6FDA polyimides IbꢀIIIb, indicating
that the conjugation across the imide was interrupted by
the sulfonyl and hexafluoroisopropylidene linkages. All the
DSDA polyimides IaꢀIIIa showed the lower reduction po-
tential values than that of the 6FDA polyimides IbꢀIIIb, imply-
Figure 3. Cyclic voltammetric diagrams of polyimide (a) Ia, (b) IIa,
ing that the sulfonyl bridge has more electron-withdrawing
capability. The CV curve of BPDA polyimide IIIc (Figure 2d)
and (c) IIIa films on an ITO-coated glass substrate over cyclic scans in
0.1 M TBAP/CH3CN at a scan rate of 50 mV/s.
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Table 2. Redox Potentials and Energy Levels of Polyimides
thin films
oxidationa
E1/2(ox2)
reductionb
code
λonset
Eonset
E1/2(ox1)
E1/2(ox3)
Eonset
E1/2(re1)
E1/2(re2)
Egopt/eVc
EgEC/eVd
HOMOe
LUMOe
Ia
733
727
563
560
560
557
564
0.28
0.27
0.34
0.33
0.40
0.38
0.37
0.42
0.40
0.48
0.46
0.52
0.51
0.51
0.70
0.68
0.78
0.78
0.74
0.74
0.74
1.29c
1.22c
1.63c
1.43c
1.12
ꢀ0.78
ꢀ1.02
ꢀ0.78
ꢀ1.04
ꢀ0.73
ꢀ0.94
ꢀ0.69
ꢀ0.98
ꢀ1.21
ꢀ1.05
ꢀ1.23
ꢀ0.98
ꢀ1.20
ꢀ0.95
1.69
1.71
2.20
2.21
2.21
2.23
2.20
1.48
1.69
1.61
1.77
1.58
1.79
1.54
4.78
4.76
4.84
4.82
4.88
4.87
4.87
3.30
3.07
3.23
3.05
3.30
3.08
3.33
Ib
IIa
IIb
IIIa
IIIb
IIIc
1.29c
1.12
ꢀ1.43
a Versus Ag/AgCl in CH3CN. E1/2: average potential of the redox couple peaks. b Versus Ag/AgCl in DMF. c Bandgaps calculated from absorption edge
opt
of the polymer films: Eg = 1240/λonset.
d EgEC: electrochemical band gap is derived from the difference between the HOMO and LUMO levels.
e The HOMO and LUMO energy levels were calculated from E1/2ox and E1/2re value of CV curves and were referenced to ferrocene (4.8 eV; E1/2 = 0.44
V in CH3CN; E1/2 = 0.52 V in DMF).
exhibited two quasi-reversible one-electron reduction couples
occurred at E1/2 = ꢀ0.95 and ꢀ1.43 V, corresponding to the
formation of radical anion and dianions, respectively. Meanwhile,
the first-stage reduction process occurred at a lower applied
potential than polyimides IIIa and IIIb. The decreased red-
uction potential of polyimide IIIc could be attributed to the
strong electron coupling between two phthalimide groups.
This phenomenon could also be found in Kapton synthesized
from 4,40-oxidianiline (ODA) and pyromellitic dianhydride
(PMDA) with two reduction couples at E1/2 = ꢀ0.83 and
ꢀ1.34 V.20 The difference between the first and second reduc-
tion process of Kapton from E1/2 value (ΔE) was QJ;0.51 V,
which was larger than polyimide IIIc (ΔE = 0.48), reveal-
ing the PMDA segment possessed stronger electron transfer -
than BPDA.14b The continuous CV scanning was used to
investigate the donor effect on the electrochemical stability (p-
doping), and the results are depicted in Figure 3. Because the
4-methoxyphenyl group can stabilize the cation radicals, poly-
imide IIIa exhibited the best reversible CV behavior in the
second oxidation stages. The redox potentials of the poly-
imides as well as their respective highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) (versus vacuum) are calculated and summarized
in Table 2.
Particularly, the electrochemical properties of structurally
related polyaniline (PANI) had long been studied in the acidic
solution.21 In order to get more insight into the solvent effect
on the mechanism of these structurally related polymers in this
study, we therefore investigated the electrochemical behaviors of
PANI in 1 M HCl and 0.1 M TBAP/CH3CN system, respec-
tively. PANI films synthesized by electrochemical deposition
were carried out using 0.1 M aniline in a solution of 0.5 M sulfuric
acid at constant potential (0.9 V vs Ag/AgCl) for 6 min, the
doped green PANI could be readily deposited onto working
electrode and washed by water several times to remove aniline
monomer, then dipped in 5 wt % Na2CO3(aq) for 30 s (chang-
ing color from green to blue), and washed completely with
water to remove salts. Finally, the dedoped blue PANI film was
quickly dipped in acetone to remove water and dried under a
nitrogen atmosphere. As the obtained PANI films were at fully
oxidized state, we held on ꢀ0.2 V for 1 min before CV
scanning. It is worth to note that the fully oxidized PANI film
could not be reduced in nonacidic solution; thus, the acidic
electrolytes were used before anodic scanning and washed by
Figure 4. Absorption spectra change of the PANI films (∼750 nm in
thickness) on the ITO-coated glass substrate (deposited area: 1.2 cm ꢁ
0.6 cm) in 1 M HCl at various applied potentials between (a) ꢀ0.20 and
1.00 and (b) ꢀ0.20 and 0.40 (V vs Ag/AgCl).
water to remove HCl. The electrochemical behaviors of
PANI are shown in Figure S10, the CV diagrams of PANI in
acid electrolytes revealed three reversible oxidation redox
couples (Figure S10a), and the redox reaction for PANI is
postulated in Scheme S1. The first and third oxidized stages
associated with the oxidation of the electroactive nitrogen site.
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lower reversibility due to the stable imine (CdN) structure
could not be reduced by regaining protons from nonacidic
electrolytes.
Measurement of n Electrons Transfered for PANI. Polyani-
line was known as its color changes depending on the redox
states. However, some results reported previously described
the PANI oxidation processes losing two electrons and protons
in each oxidized states.21c In order to confirm the mechanism
of how many electrons n were transferred for each step by
the OTTLE techniques, a series of potentials, Eapplied, were
sequentially impressed (potentiostated) across the transparent
cell containing a PANI film of the redox couple. At each applied
potential, the ratio of the concentrations of oxidized to neutral
forms [O]/[N] of the couple adjusted by electrolysis at the
transparent electrode to the value required for adherence to
the Nernst equation:12,22
O þ neꢀ þ mHþ T R
ꢀ
ꢁ
0:059 ½Oꢂ
ln
m
ꢀ 0:059 pH
n
Eapplied
¼
E0 þ
n
½Rꢂ
where n represents the number of reacting electrons and m
represents the number of reacting protons. However, the oxida-
tion potential for this reaction was independent of pH because of
the pH value was zero in 1 M HCl. For the reaction, the Nernst
equation is
ꢀ
ꢁ
0:059
½Oꢂ
½Nꢂ
Eapplied ¼ E0 þ
log
n
where Eapplied and E0 represent the actual and standard poten-
tials, respectively, O and N represent the oxidized and neutral
species, and 0.059 represents the ratio of the gas constant to
the Faraday constant at room temperature. The [O]/[N] value
corresponding to each Eapplied to be obtained from absorbance
changes measured by recording spectra after equilibrium is
achieved for each potential.
Figure 4a shows the optical absorption and color changes
for PANI at various applied potentials in 1 M HCl. The ab-
sorption peaks at 313 and 408 nm are characteristic for PANI.
After one-electron oxidation (ꢀ0.20 to 0.40 V), the original
peak at 313 nm decreased and the 408 nm and a broad band at
around 1084 nm increased gradually. The first oxidation
reversibility was 98% based on the absorbance at 313 nm.
When the potential was adjusted to more positive values
(0.40ꢀ0.60 V) corresponding to the second stage oxidation,
the characteristic new peaks appeared at 805 and 1084 nm
increased with the decreasing at 408 nm, and the second
oxidation reversibility was 90%. For the third oxidative state
(0.60ꢀ1.00 V), the peak at 1084 nm decreased obviously with
a reversibility of only 80%, implying the lower stability of this
product at third oxidation stage. The PANI film switches from
a pale yellowish neutral state to the green/bluish green and
then blue oxidized states. The inset curve in Figure 4b shows a
typical plot of Eapplied vs log(An ꢀ A0)/(Af ꢀ An) correspond-
ing to 1084 nm (IV-CT band) for the first coloring stage. The
slope of the straight-line plot is 56.7 mV, which compares
Figure 5. Absorption spectra of 5 ꢁ 10ꢀ4 M dinitro compounds (a) 2,
(b) 20, and (c) 200 in CH3CN solution containing 0.1 M TBAP at applied
potentials of different oxidative stages (V vs Ag/AgCl).
The second redox couple (E1/2 = 0.51 V) in anodic scan could
be attributed to the formation of quinones (mostly benzo-
quinone) as a consequence of hydrolysis in water. It has
been reported that the loss of electrochemical response was
accompanied by a large increased frequency, indicating the
products formed by hydrolysis of the oxidized structures
(changing imine into benzoquinone) were soluble.21c The
PANI film gradually degraded while the potential was increa-
sed more than 0.85 V and lost its electrochemical proper-
ties after continuous 10 cyclic scans both in the acidic and
nonacidic systems (Figure S10). The latter system exhibited
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Scheme 3. Postulated Redox Behavior of the Dinitro Compounds 2, 20, and 200
favorably with the Nernstian value of 59.1 mV for a one-
electron process.
ring. After one-electron oxidation, the original peak decreased
with a broad band at NIR region increased gradually. 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, and the
phenomenon was consistent with the classified by Robin
and Day.14 When the potential was adjusted to more positive
values corresponding to the second electron oxidation, the
characteristic new peak at around 650ꢀ850 nm appeared. It is
worth noting that the NIR region absorbance of compound 2
decreased more obviously than others, which might be attrib-
uted to the deprotonation from these secondary amino groups
thus eliminated the resonance of the dications and decreased
the IV-CT absorbance. The possible resonance forms of
compounds 2ꢀ200 are proposed in Scheme 3. As the applied
potentials increased to the third oxidation state, the NIR
wavelength decreased. Because of the incorporation of the
methoxyphenyl group at secondary amine, the cation radicals
of compound 200 could be stabilized effectively to show the
Spectroelectrochemistry and Electrochromic Properties.
Spectroelectrochemical experiments were used to evaluate the
optical properties of these electrochromic materials. For the
investigations, dinitro compounds 2ꢀ200 and polyimide films
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
absorption spectra under potential control. UVꢀvisꢀNIR ab-
sorbance curves correlated to applied potentials of these dinitro
compounds are presented in Figure 5. The absorption peaks at
296 and 407 nm are characteristic for the πꢀπ* transition of the
aromatic phenyl ring and charge transfer complex formation of
compound 2, respectively. It revealed a red shift from 296 to
309 nm (from compound 2 to compound 200), owing to
increasing the conjugated length by incorporating the phenyl
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Figure 6. Electrochromic behavior (left) at applied potentials of (a) 0.00, (b) 0.60, (c) 0.90, and (d) 1.50 (V vs Ag/AgCl) and 3D spectro-
electrochemical behavior (right) from 0.00 to 1.50 (V vs Ag/AgCl) of polyimide Ia thin film (∼250 nm in thickness) on the ITO-coated glass substrate
(coated area: 1.1 cm ꢁ 0.5 cm) in 0.1 M TBAP/CH3CN.
Figure 7. Electrochromic behavior (left) at applied potentials of (a) 0.00, (b) 0.70, (c) 1.20, and (d) 1.70 (V vs Ag/AgCl) and 3D spectro-
electrochemical behavior (right) from 0.00 to 1.70 (V vs Ag/AgCl) of polyimide IIa thin film (∼250 nm in thickness) on the ITO-coated glass substrate
(coated area: 1.1 cm ꢁ 0.5 cm) in 0.1 M TBAP/CH3CN.
highest reversibility of 100, 98, and 97% at the first, second,
and third oxidation stages, respectively.
305 nm gradually decreased while a new peak at 424 nm and a
broad IV-CT band centered around 1088 nm in the NIR region
gradually increased due to the formation of a monocation
radical of the diphenylamine moiety. As mentioned before,
the broad absorption in NIR region was the result due to IV-
CT excitation between states in which the positive charge is
centered at different nitrogen atoms. As the more anodic
potential to 0.90 V corresponding to Ia2+, the absorption bands
(305, 424, and 1088 nm) decreased gradually with a new broad
band centered at around 790 nm. Similar to the compound 2, the
The typical spectroelectrochemistry and three-dimentional %
transmittance-wavelength-applied potential correlations of poly-
imide IaꢀIIIa films are presented in Figures 6ꢀ8, respectively.
The Ia film exhibited strong absorption at around 305 nm,
characteristic of the πꢀπ* transition of the aromatic phenyl rings
in the neutral form (0.00 V) with a color of pale green in the
visible region. Upon oxidation (increasing applied voltage
from 0.00 to 0.60 V), the intensity of the absorption peak at
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Figure 8. Electrochromic behavior (left) at applied potentials of (a) 0.00, (b) 0.70, (c) 1.05, and (d) 1.40 (V vs Ag/AgCl) and 3D spectro-
electrochemical behavior (right) from 0.00 to 1.40 (V vs Ag/AgCl) of polyimide IIIa thin film (∼200 nm in thickness) on the ITO-coated glass substrate
(coated area: 1.1 cm ꢁ 0.5 cm) in 0.1 M TBAP/CH3CN.
disappearance of NIR absorption band could be attributable
to the formation of imine structure by the further oxidation of
second electroactive nitrogen atom. Further increasing poten-
tial up to 1.50 V corresponding to Ia3+, the characteristic
absorbance at 790 nm increased slightly. From the results
shown in Figure 6, the polyimide Ia film could be switched
from a pale green neutral state (0.00 V) to the light green first
oxidized state (0.60 V), light bluish green second oxidized state
(0.90 V), and deep bluish green fully oxidized state with a extre-
mely low optical transmittance change (ΔT %) of 12% at 1088 nm
for light green coloring at first oxidation stage and 57% at 790 nm
for deep bluish green coloring at the third oxidation stage.
The spectroelectrochemical behavior of the IIa film
is shown in Figure 7. In the neutral form, the polymer film
withhigh transparency and pale yellowish exhibited a strong
band at 314 nm, characteristic of triarylamine absorption. Upon
oxidation of the IIa film (increasing electrode potential from
0.00 to 0.70 V), the intensity of the absorption band at 314 nm
gradually decreased, whereas new peaks at 419 and 1032 nm
appeared in the visible and NIR regions, respectively, relating to
formation of a stable monocation radical IIa1+. Further oxidation
of IIa1+ species to the formation of IIa2+, the absorption bands
(314 and 419 nm) decreased gradually with new broad band
centered at around 897 nm, and the IV-CT band in the NIR
region decreased slightly. When the applied potential was
increased to fully oxidized form corresponding to IIa3+, the ab-
sorption band at 897 nm decreased slightly with the increasing
absorption intensity at around 694 nm. The polyimide IIa film
at various potentials was associated with strong color changes
switching from a pale yellow at neutral state to green at semi-
oxidized states and then to dark blue at fully oxidized state.
In addition, the polymer IIa showed good contrast both in
the visible and NIR regions with a high optical transmittance
change (ΔT %) of 81% at 897 nm for bluish green coloring
at second oxidation stage and 76% at 694 nm for blue coloring at
the third oxidation stage, respectively.
The electrochromic results of the IIIa film upon oxida-
tion (p-doping) are depicted in Figure 8. The polymer film
exhibited a strong absorption at around 308 nm at the neutral
form with high transparency in the visible region. Similar to
the process mentioned above, upon oxidation of the IIIa film
(from 0.00 to 0.70 V), the absorbance at 308 nm gradually
decreased, while a new peak at 438 nm and a broad band with
maximum absorption at 1090 nm in the NIR region gradually
increased in intensity. As the more anodic potential to 1.05 V,
the absorption bands at the 438 nm gradually decreased while
IV-CT absorption broad band still increased with a slightly
shift from 1090 to 1070 nm. By further applying potential up
to 1.40 V, the characteristic absorbance at 793 nm increased
obviously with greatly decreasing NIR absorption band could
be attributed to the further oxidation of IIIa2+ species to
the formation of IIIa3+ in the triarylamine units. Meanwhile,
the color of the film changed from very pale yellowish of neu-
tral state to the yellowish-green/lawn-green semioxidation
states and purple fully oxidation state. From the inset shown in
Figure 8, the polymer IIIa exhibited highly optical transmit-
tance changes (ΔT %) of 66% at 1070 nm for lawn-green
coloring at the second oxidation stage and 77% at 793 nm for
purple coloring at third oxidation stage, respectively. The film
colorations were distributed homogeneously across polymer
film and survived for more than 100 redox cycles. Comparing
the optical transmittance differences between these three series
of polymers, the TPA-based polyimides revealed higher con-
trast than diphenylamine (DPA)-based polyimides.
Figure 9 shows cathodically electrochemical behaviors of
these ambipolar electrochromic polyimides IIIa and IIIc
films upon reduction (n-doping) scanning. The radical anion
of polyimide IIIa which appeared at ꢀ0.75 and ꢀ1.35 V
exhibited an increasing absorption between 400 and 600 nm
and a strong broad absorption band with a peak at 730 nm.
Meanwhile, the color of the film changed from pale yellow to
red (Figure 9a). For the polyimide IIIc, it exhibited a strong
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time of 1.29 s for coloring process and 4.64 s for bleach-
ing. When the switched potential was set between 0.00 and
0.93 V, PANI required 1.72 s for coloration and 1.80 s for
bleaching. It is noteworthy that the coloring of PANI pro-
ceeded during the anodic oxidation faster than the bleaching
in the reverse cathodic reduction process, which is very
different from nonconjugated polymer system. This phenom-
enon was also described and discussed in the previous
reporte.23 The amount of extracted/injected charge (Qr)
was calculated by integration of the current density and time
obtained from Figure S11c as 7.535 and 7.241 mC/cm2 for
oxidation and reduction process at the first oxidation stage,
respectively, and the ratio of charge density was 96.1%.
Similarly, the ratio of charge density could be obtained only
80.4% at the fully oxidation stage, indicating lower stability of
PANI during the electrochemical reaction at the fully oxida-
tion stage. Color efficiency, CE (η = δOD/Q), and injected
charge (Q) within continuous switching steps in both acidic
and nonacidic solutions were examined and summarized in
Tables S4 and S5 of the Supporting Information. Comparing
the first oxidation stage in these two different solvent systems,
the PANI film in 1 M HCl revealed higher decay than in 0.1 M
TBAP/CH3CN. Although it could keep the maximum co-
loring absorbance but the optical contrast value declined
obviously after 50 cycles, and the switching time was also
much elongated (Figure 10a). This result was similar to early
studies.10 Furthermore, even the cycle time was increased
from 60 to 150 s in acidic system to give more enough time
to return to the original state, but the switching and bleach-
ing time still elongated and optical contrast value decreased
more obviously than before (Figure S12). In the fully oxidized
state, the PANI film in 1 M HCl exhibited highly optical
contrast (Figure 10b). However, the coloring efficiency de-
creased obviously due to the degradation of polymer film
caused by oxidation under the high applied potential. In
contrast, the electrochromic performance of PANI in non-
acidic solution was quite unstable at the fully oxidized state.
It could not bleach color to the original absorbance even
only after 1 cycle, and the phenomenon might be attributed
to no protons could be received from a nonacidic system
(Figure 10d). Therefore, the conjugated PANI could obtain
protons from acidic solution to return to original state after
oxidation, while it may degrade gradually due to the acidic
aqueous solution.
The typical switching results for these representative films
of polyimides Ia, IIa, and IIIa are shown in Figures S13 and
S14 of the Supporting Information and Figure 11, respec-
tively. Among the three series, the polyimide IIIa (Figure 11)
showed the fastest response time than others. When the ap-
plied potential was adjusted to the first oxidation stage from
0.00 to 0.70 V, the coloring and bleaching time were less than
5 and 2 s, respectively, with 99% of the ratio of the charge
density. While the switched potential was set between 0.00
and 0.94 V, the polyimide IIIa thin film required almost the
same times for coloration and bleaching with 96% of the ratio
of the charge density. The electrochromic stability of the
polyimide Ia, IIa, and IIIa films was also determined by
measuring the optical change as a function of the number of
switching cycles (Figures 12ꢀ14). The coloration efficiency
at different switching steps is summarized in Tables 3 and 4.
After continuous 100 cyclic scanning at the first oxidation
stage, the polyimide Ia film reduced electroactivity rapidly
Figure 9. Optical spectra of polyimides (a) IIIa and (b) IIIc thin
films (∼260 nm in thickness) on the ITO-coated glass substrate
(coated area: 1.1 cm ꢁ 0.5 cm) in 0.1 M TBAP/DMF at various
applied potentials versus Ag/AgCl. The inset shows the n-doping
electrochromic photographic images of the film at indicated applied
voltages.
absorption band at 320 nm with an increasing absorption
between 400 and 650 nm due to the formation of radical
anion IIIc1ꢀ (at ꢀ1.10 V). As shown in the inset of Figure 9b,
the radical anion state of polyimide IIIa was yellow in color.
Further reduction to applied potential ꢀ1.80 V resulted in the
dianion state with broad absorption maxima at 564 and
782 nm, and the color of polyimide IIIa film turned to green
in the second reduction.
Electrochromic Switching Studies. Electrochromic switch-
ing studies of these resulting polyimide films were performed
to monitor the absorbance changes as a function of time at
their absorption maximum and to determine the response time
by stepping potential repeatedly between the neutral and oxi-
dized states. First, structurally related PANI was investigated
systematically to manifest the electrochemical and electrochro-
mic characteristics both in the acidic and nonacidic solution,
respectively, and the results are summarized in Figure S11 of
the Supporting Information and Figure 10. The response 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 S11a,b, when the potential was
switched between 0.00 and 0.40 V, PANI revealed response
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Figure 10. Electrochromic switching between (A) ꢀ0.2 and 0.40 V and (B) ꢀ0.2 and 1.00 V (vs Ag/AgCl) of PANI film (∼770 nm in thickness)
deposited on the ITO-coated glass substrate (deposited area: 1.2 cm ꢁ 0.6 cm) in 1.0 M HCl with a cycle time of 60 s. (C) ꢀ0.2 and 0.70 V and (D) ꢀ0.2
and 1.50 V (vs Ag/AgCl) of PANI film (∼700 nm in thickness) in 0.1 M TBAP/CH3CN with a cycle time of 60 s. (a) Current consumption and
(b) absorbance change monitored at the given wavelength.
(Figure 12a). On the contrary, the polymer IIIa was found
to exhibit high CE up to 171 cm2/C at 1090 nm and to
retain their electroactivity after switching 100 times between
0.00 and 0.70 V with only slightly decay of 0.58% (Figure 14a
and Table 3). As the applied switching potential increased
to the second oxidation stage, the polymer IIIa still re-
vealed good electrochromic characteristics, and a higher CE
(168 cm2/C at 1070 nm) could be obtained with only 1.19%
decay of its electroactivity after 30 cycles (Figure14b and Table 4).
These results were also consistent with the above-mentioned
assumption that the incorporating 4-methoxyphenyl moie-
ties into the nitrogen atom of amino groups could effectively
stabilize the cation radicals by the additional extra resonance
routes.
the extra phenyl rings as the additional routes for resonance.
Furthermore, the lower electrochemical stability of the DPA-
based polyimides I than the TPA-based polyimides III also could
be attributed to the unrecoverable protons in the amino groups
after removed in the nonacidic solution.
’ EXPERIMENTAL SECTION
Materials. 4,40-Diaminodiphenylamine19 (1, mp = 158ꢀ160 °C)
was purified by a previously reported procedure. 4,40-Diamino-
400-methoxytriphenylamine12b,24,25 (10, mp = 150ꢀ152 °C) and 4,40-
bis[4-aminophenyl(4-methoxyphenyl)amino]-400-methoxytriphenylamine18
(300) were synthesized according to a previously reported procedure.
Synthesis and characterization data of compounds 4,40-bis[(4-nitrophenyl)-
amino]diphenylamine (2), 4,40-bis[(4-aminophenyl)amino]diphenyla-
mine (3), 4,40-bis[(4-nitrophenyl)amino]-400-methoxytriphenylamine
(20), and 4,40-bis[(4-aminophenyl)amino]-400-methoxytriphenylamine
(30) are included in the Supporting Information (Figures S1ꢀS4).
Commercially available aromatic tetracarboxylic dianhydrides such as
3,30,4,40-diphenylsulfonetetracarboxylic dianhydride (DSDA) (4a) (TCI),
2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) (4b)
(Chriskev), and 3,30,4,40-biphenyltetracarboxylic diandydride (BPDA)
(Oxychem) (4c) were purified by vacuum sublimation. Commercially
obtained anhydrous calcium chloride (CaCl2) was dried under vacuum
at 180 °C for 8 h. Tetrabutylammonium perchlorate (TBAP) (Acros)
was recrystallized twice by ethyl acetate under a nitrogen atmosphere
and then dried in vacuo prior to use. All other reagents were used as
received from commercial sources.
’ CONCLUSIONS
Three series of novel ambipolar electrochromic aromatic
polyimides containing electroactive aniline-based diphenylamine
or triphenylamine derivatives were prepared from the diamine
monomers 4,40-bis[(4-aminophenyl)amino]diphenylamine,
4,40-bis[(4-aminophenyl)amino]-400-methoxytriphenylamine, and
4,40-bis[4-aminophenyl(4-methoxyphenyl)amino]-400-methoxy-
triphenylamine, with various tetracarboxylic dianhydrides.
Introduction of the methoxy-substituted phenyl groups into
the nitrogen atom position not only stabilized its radical
cations but also enhanced the solubility of the resulting
polyimides. In addition to the excellent thermal stability, these
polymers also revealed valuable electrochromic characteristics
in both visible and NIR regions. The series of conjugated
DPA-based polyimides could not effectively stabilize the
cationic radicals of oxidized form comparing to the corre-
sponding nonconjugated TPA-based polymers due to lack of
Polymer Synthesis. IꢀII Series of Polyimides. The synthesis of
polyimide IIa was used as an example to illustrate the general synthetic
route used to produce the II series of polyimides (IIaꢀIIb). The
solution of 0.24 g (0.50 mmol) of diamine 30 in 2.3 mL of DMAc was
mechanically stirred at room temperature. Until the diamine monomer
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Figure 12. Electrochromic switching between (A) 0 and 0.58 V and (B)
0 and 0.83 V (vs Ag/AgCl) of polyimide Ia thin film (∼260 nm in
thickness) on the ITO-coated glass substrate (coated area: 1.1 cm ꢁ
0.5 cm) in 0.1 M TBAP/CH3CN with a cycle time of 60 s. (a) Current
consumption and (b) absorbance change monitored at the given
wavelength.
Figure 11. Calculation of optical response time at (a) 1090 nm and (b)
1070 nm at the applied potential, and (c) currentꢀtime curves of
polyimide IIIa thin film (∼300 nm in thickness) coated on the ITO-
coated glass substrate (coated area: 1.1 cm ꢁ 0.5 cm) in 0.1 M TBAP/
CH3CN.
Hd + He), 6.94 (s, 2H, Hb), 6.87 (s, 6H, Ha + Hc), 3.69 (s, 3H,
ꢀOCHH3). 13C NMR (125 MHz, DMSO-d6, δ, ppm): 166.1 (C20),
166.0 (C19), 155.0 (C12), 145.3 (C8), 144.7 (C4), 141.8 (C5), 141.0
(C9), 137.0 (C1), 136.3 (C16), 134.3 (C15), 133.3 (C13), 128.4 (C2),
125.4 (C10), 124.9 (C14), 124.0 (C7), 122.7 (C17), 122.0 (C18), 119.8
(C3), 115.2 (C6), 114.9 (C11), 55.3 (ꢀOCH3). Anal. Calcd (%) for
C47H31N5O7S (809.84): C, 69.71%; H, 3.86%; N, 8.65%. Found: C,
67.13%; H, 4.18%; N, 8.82%.
III Series of Polyimides. Furthermore, in addition to IIIa and IIIb, the
diamine 300 was chosen to react with BPDA (4c) to obtain the polyi-
mide IIIc which would reveal more interesting muti-electrochromism
behaviors during both p- and n-doping processes. The synthesis of polyi-
mide IIIa was used as an example to illustrate the general synthetic
route used to produce the III series of polyimides (IIIaꢀIIIc). Into a
50 mL round-bottom flask were added 0.35 g (0.50 mmo) of diamine 300,
0.18 g (0.50 mmol) of dianhydride DSDA (4a), 0.12 mL of isoquinoline,
and 2 mL of m-cresol. The reaction mixture was stirred at room temp-
erature in a nitrogen atmosphere for 5 h. Then, the reaction temperature
was increased to 185 °C for 15 h. After the imidization reaction, the
mixture was cooled to room temperature; the viscous polymer solu-
tion then was poured slowly into 100 mL of methanol with stirring.
A yellowish brown precipitate was washed thoroughly with hot metha-
nol, collected by filtration, and dried in vacuum at 80 °C for 15 h.
The inherent viscosity and weight-average molecular weights (Mw)
of obtained polyimide IIIa was 0.33 dL/g (measured at a concentration
of 0.5 g/dL in NMP at 30 °C) and 66 000 Da, respectively. The FT-IR
spectrum of IIIa (film) exhibited characteristic imide absorption bands
dissolved completely, using liquid nitrogen with an ice bath to freeze the
solution. And then, 0.18 g (0.50 mmol) of dianhydride DSDA (4a) was
added in one portion (15 wt % solid content). The mixture was
mechanically stirred and kept at low temperature (ca. ꢀ10 °C) for
about 2.5 h to yield a viscous poly(amic acid) solution. The poly(amic
acid) was subsequently converted to polyimide IIa via a chemical
imidization process by addition of pyridine 0.25 mL (3.10 mmol) and
acetic anhydride 0.29 mL (3.10 mmol), and the mixture was heated
at 120 °C for 2 h to effect complete imidization. The resulting poly-
mer solution was poured into 100 mL of methanol, giving a yellowish
brown precipitate, which was washed thoroughly with methanol, and
collected by filtration. For I series of polyimides (IaꢀIb), it just only
changed the solvent as NMP and added the salts (CaCl2) in the
procedure to prevent the side reaction and disrupt the hydrogen-
bonding interaction. The inherent viscosity and weight-average molec-
ular weights (Mw) of obtained polyimide IIa were 1.30 dL/g (measured
at a concentration of 0.5 g/dL in NMP at 30 °C) and 122 000 Da,
respectively. The FT-IR spectrum of IIa (film) exhibited characteristic
imide absorption bands at around 1779 (asymmetrical CdO), 1721
(symmetrical CdO), 1382 (CꢀN), and 739 cmꢀ1 (imide ring defor-
1
mation). H NMR (500 MHz, DMSO-d6, δ, ppm): 8.56 (s, 4H, Hi +
Hh), 8.25 (s, 2H, ꢀNH), 8.13 (s, 2H, Hg), 7.16 (s, 4H, Hf), 7.04 (s, 8H,
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Figure 14. Electrochromic switching between (A) 0 and 0.70 V and
(B) 0 and 0.94 V (vs Ag/AgCl) of polyimide IIIa thin film (∼300 nm in
thickness) on the ITO-coated glass substrate (coated area: 1.1 cm ꢁ
0.5 cm) in 0.1 M TBAP/CH3CN with a cycle time of 60 s. (a) Current
consumption and (b) absorbance change monitored at the given
wavelength.
Figure 13. Electrochromic switching between (A) 0 and 0.71 V and
(B) 0 and 1.10 V (vs Ag/AgCl) of polyimide IIa thin film (∼300 nm
in thickness) on the ITO-coated glass substrate (coated area: 1.1 cm ꢁ
0.5 cm) in 0.1 M TBAP/CH3CN with a cycle time of 60 s. (a) Current
consumption and (b) absorbance change monitored at the given
wavelength.
heated in flowing nitrogen or air (flow rate = 20 cm3/min) at a
heating rate of 20 °C/min. Thermal mechanical analysis (TMA)
was conducted with a TA Instruments TMA Q400. The TMA
experiments were conducted from 40 to 400 °C at a scan rate of
10 °C/min with a film/fiber probe under an applied constant load of
5 mN. Electrochemistry was performed with a CH Instruments
611B electrochemical analyzer. Voltammograms are presented with
the positive potential pointing to the left and with increasing anodic
currents pointing downward. Cyclic voltammetry (CV) was con-
ducted with the use of a three-electrode cell in which ITO (polymer
films area about 0.5 cm ꢁ 1.1 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 UVꢀvis cuvette using a Hewlett-
Packard 8453 UVꢀvis diode array spectrophotometer and a Hitachi
U-4100 UVꢀvisꢀNIR spectrophotometer. The ITO-coated glass
slide was used as the working electrode, a platinum wire as the
counter electrode, and a 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 ex-
tracted/injected charge (Qr; also termed as electroactivity) 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, Q is the injected charge,
and Tb and Tc are the bleached and colored transmittance values,
at around 1783 (asymmetrical CdO), 1720 (symmetrical CdO), 1380
(CꢀN), and 724 cmꢀ1 (imide ring deformation). Anal. Calcd (%)
for C61H43N5O9S (1022.09): C, 71.68%; H, 4.24%; N, 6.85%. Found: C,
71.40%; H, 4.61%; N, 6.40%.
Preparation of the Polyimide Films. A solution of the polymer
was made by dissolving about 0.2 g of the polyimide sample in 6 mL of
NMP. The homogeneous solution was poured into a 5 cm glass Petri
dish, which was heated in oven at 80 °C for 5 h to remove most of the
solvent; then the semidried film was further dried in vacuo at 160 °C for
10 h. The obtained films were about 40ꢀ90 μm thick 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 spectro-
meter. Elemental analyses were run in a Heraeus VarioEL-III
CHNS elemental analyzer. 1H and 13C NMR spectra were measured
on a Bruker AVANCE-500 FT-NMR using tetramethylsilane as
the internal standard, 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
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Table 3. Optical and Electrochemical Data Collected for Coloration Efficiency Measurements of Polyimide IaꢀIIIa
Q (mC/cm2)c
η (cm2/C)d
decay (%)e
b
b
b
δOD1088
δOD1032
δOD1090
cycling timesa
Ia
IIa
IIIa
Ia
IIa
IIIa
Ia
IIa
IIIa
Ia
IIa
IIIa
1
10
20
30
40
50
60
70
80
90
100
0.176
0.165
0.163
0.151
0.147
0.143
0.134
0.127
0.121
0.117
0.106
0.497
0.489
0.468
0.455
0.444
0.432
0.423
0.416
0.404
0.397
0.388
0.571
0.571
0.571
0.571
0.571
0.571
0.571
0.570
0.570
0.568
0.568
4.329
4.328
4.325
4.325
4.326
4.324
4.323
4.323
4.321
4.320
4.320
4.504
4.503
4.501
4.501
4.501
4.501
4.498
4.498
4.498
4.497
4.497
3.334
3.334
3.334
3.334
3.334
3.334
3.334
3.333
3.334
3.334
3.334
41
38
37
35
34
33
31
29
28
27
24
110
109
104
101
99
171
171
171
171
171
171
171
171
171
170
170
0
0
0
7.31
9.76
0.91
5.45
8.18
0
0
14.6
0
17.1
19.5
24.4
29.3
31.7
34.1
41.5
10.0
0
96
12.7
14.5
16.4
18.2
20.0
21.8
0
94
0
92
0
90
0
88
0.58
0.58
86
a Switching between 0 and 0.58 V for Ia, 0 and 0.71 V for IIa, and 0 and 0.70 V for IIIa (V vs Ag/AgCl). b Optical density change at the given wavelength.
c Ejected charge, determined from the in situ experiments. d Coloration efficiency is derived from the equation η = δOD/Q. Decay of coloration
e
efficiency after cyclic scans.
Table 4. Optical and Electrochemical Data Collected for Coloration Efficiency Measurements of Polyimides IaꢀIIIa
Q (mC/cm2)c
η (cm2/C)d
decay (%)e
b
b
b
δOD790
δOD897
δOD1070
cycling timesa
Ia
IIa
IIIa
Ia
IIa
IIIa
Ia
IIa
IIIa
Ia
IIa
IIIa
1
3
0.331
0.323
0.309
0.298
0.288
0.267
0.256
0.245
0.234
0.223
0.201
0.649
0.639
0.628
0.623
0.613
0.604
0.582
0.569
0.548
0.521
0.490
0.898
0.898
0.897
0.897
0.897
0.896
0.894
0.894
0.894
0.893
0.893
6.703
6.703
6.701
6.701
6.698
6.698
6.701
6.698
6.699
6.696
6.687
7.645
7.644
7.644
7.641
7.641
7.641
7.637
7.636
7.633
7.632
7.632
5.383
5.383
5.382
5.382
5.382
5.383
5.383
5.382
5.382
5.383
5.381
49
48
46
44
43
40
38
37
35
33
30
85
84
82
82
80
79
76
75
72
68
64
168
168
167
167
167
166
166
166
166
166
166
0
0
0
2.04
6.12
1.18
3.53
3.53
5.88
7.06
10.6
11.8
15.3
20.0
24.7
0
6
0.59
0.59
0.59
1.19
1.19
1.19
1.19
1.19
1.19
9
10.2
12
15
18
21
24
27
30
12.2
18.4
22.5
24.5
28.6
32.7
38.8
a Switching between 0 and 0.83 V for Ia, 0 and 1.10 V for IIa, and 0 and 0.94 V for IIIa (V vs Ag/AgCl). b Optical density change at the given wavelength.
c Ejected charge, determined from the in situ experiments. d Coloration efficiency is derived from the equation η = δOD/Q. Decay of coloration
e
efficiency after cyclic scans.
respectively. The thickness of the polyimide thin films was measured
by alpha-step profilometer (Kosaka Lab., Surfcorder ET3000, Japan).
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Corresponding Author
*E-mail: gsliou@ntu.edu.tw.
9609
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