Electrosynthesis and electrochromic properties of poly(amide-triarylamine)s containing triptycene units

Article abstract of DOI:10.1039/c5ra18757e

Four bis(amide-triarylamine) derivatives featuring a triptycene as an interior core and terminal electroactive triphenylamine (TPA) or N-phenylcarbazole (NPC) groups were prepared by the condensation reactions from 1,4-bis(4-aminophenoxy)triptycene with 4-carboxytriphenylamine and N-(4-carboxyphenyl)carbazole, respectively, and from 1,4-bis(4-carboxyphenoxy)triptycene with 4-aminotriphenylamine and N-(4-aminophenyl)carbazole, respectively. The electrochemistry and electropolymerization of these bis(amide-triarylamine) derivatives were investigated. The stability of the oxidized forms of the bis(amide-triarylamine)s is affected by the orientation of amide linkage and the structure of terminal triarylamine unit. Two of the bis(amide-triarylamine)s could be electropolymerized into robust films on the electrode surface in an electrolyte solution. The electrogenerated polymer films exhibited reversible electrochemical oxidation processes and strong color changes upon electro-oxidation, which can be switched by potential modulation. The electrochromic properties of the films were evaluated by the spectroelectrochemical and electrochromic switching studies. The TPA-based film showed better electrochromic performance than the NPC-based one.

Full text of DOI:10.1039/c5ra18757e

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Page 1 of 23
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DOI: 10.1039/C5RA18757E
Electrosynthesis and electrochromic properties of
poly(amide-triarylamine)s containing triptycene units
†
Sheng-Huei Hsiao* and Yu-Ting Chiu
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei
10608, Taiwan. E-mail: shhsiao@ntut.edu.tw
†
Electronic supplementary information (ESI) available: See DOI: 10.1039/…
(Abstract)
Four bis(amide-triarylamine) derivatives featuring a triptycene as an interior core and terminal
electroactive triphenylamine (TPA) or N-phenylcarbazole (NPC) groups were prepared by the
condensation reactions from 1,4-bis(4-aminophenoxy)triptycene with 4-carboxytriphenylamine and
N-(4-carboxyphenyl)carbazole, respectively, and from 1,4-bis(4-carboxyphenoxy)triptycene with
4
-aminotriphenylamine and N-(4-aminophenyl)carbazole, respectively. The electrochemistry and
electropolymerization of these bis(amide-triarylamine) derivatives were investigated. The stability of
the oxidized forms of the bis(amide-triarylamine)s is affected by the orientation of amide linkage and
the structure of terminal triarylamine unit. Two of the bis(amide-triarylamine)s could be
electropolymerized into robust films on the electrode surface in an electrolyte solution. The
electrogenerated polymer films exhibited reversible electrochemical oxidation processes and strong
color changes upon electro-oxidation, which can be switched by potential modulation. The
electrochromic properties of the films were evaluated by the spectroelectrochemical and electrochromic
switching studies. The TPA-based film showed better electrochromic performance than the NPC-based
one.
1
. Introduction
Electrochromism is defined as a reversible and visible change in the transmittance and/or reflectance of
1
a material as the result of electrochemical oxidation or reduction. This functionality is of great interest
for numerous applications, such as optical switching devices, color displays, smart windows, sensors,
2
memory elements, and camouflage materials. Examples of commercially available electrochromic
3
technology includes the Boeing 787 Dreamliner dimmable windows manufactured by Gentex. The
property of electrochromism is not unique to conducting polymers, but is found in a variety of organic
4
and inorganic materials. Earlier effort has been put forth on electrochromic devices based on inorganic
1

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5
electrochromic systems such as tungsten oxide (WO ) and nickel oxide (NiO or Ni O ). π-Conjugated
3
2
3
organic polymers present improved processability and better color tuning ability than their inorganic and
6
molecular counterparts. Considerable effort in the Reynolds research group has been focused on the
understanding and the tailoring of electrochromic properties in conjugated polymers such as
7
8
poly(3,4-alkylenedioxythiophene)s (PEDOT) and poly(3,4-alkylenedioxypyrrole)s (PEDOP). Many other
conjugated polymer systems exhibiting attractive electrochromic performance have also been explored
9
by other research teams. Electrochromic materials made from π-conjugated polymers have been
gaining in popularity because they combine several advantages such as fast response time, high optical
contrast ratios, color variability, and good long-term stability as well as mechanical deformability.
Conjugated polymers can be synthesized by either chemical or electrochemical polymerization.
Compared with the chemical routes, electrochemical polymerization can obtain conjugated polymer
films on conductive substrates directly. This not only enlarges the scope of candidate polymers, but also
avoids the procedure of the film coating.
Triarylamine and carbazole derivatives are well-known for their electroactive and photoactive
properties that may find optoelectronic applications as photoconductors, hole-transporters, and
1
0
light-emitters.
As
a
class of promising electrochromic materials, triarylamine-containing
high-performance polymers such as aromatic polyamides and polyimides have various useful properties
such as easily forming relatively stable polarons (radical cations), high carrier mobility, high thermal
1
1
stability, and good mechanical properties. In general, this kind of electrochromic polymers have been
prepared by conventional polycondensation technique based on oligoaniline or triarylamine-containing
1
2-14
diamine or dicarboxylic acid monomers.
On the other hand, electroactive and electrochromic
polymer films with triarylamine units could be directly fabricated on the electrode surface via
triphenylamine or carbazole-based electrochemical oxidative coupling reactions from various
1
5
triarylamine-terminated electropolymerizable monomors in an electrolyte solution.
Triptycene is a rigid molecular unit with three blades each composed of a benzene ring. The use of
triptycene moiety as a rigid and shape-persistent component is a method to introduce molecular-scale
free volume into a polymer film. Polymers with high triptycene content were reported to be interesting
1
6
low-k dielectric materials due to the high degree of internal free volume. It has also been
demonstrated that the polyimide incorporating a three-dimensional, rigid triptycene framework has a
1
7
high internal free volume to allow fast molecular diffusion, thus leading to high gas permeability. To
our best knowledge, there are still no reports about the electrosynthesis and electrochromic properties
of electroactive poly(amide-triarylamine)s bearing the triptycene moiety. In this study, four diamide
derivatives featuring a triptycene as an interior core and terminal electroactive triphenylamine (TPA) or
N-phenylcarbazole (NPC) groups were prepared, and their electrochemistry and electropolymerization
were investigated. The electrochromic properties of the electrogenerated films were evaluated by the
spectroelectrochemical and electrochromic switching studies. Incorporation of the three-dimensional,
2

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rigid triptycene unit into the polymer backbones was expected to increase the internal free volume in
the polymer film to allow better charge transfer at doping and thus leading to fast switch speeds.
2
. Experimental
2
.1. Materials
,4-Bis(4-aminophenoxy)triptycene (3) (mp = 254-255 C) was synthesized by the aromatic nucleophilic
o
1
substitution reaction of p-chloronitrobenzene with 1,4-dihydroxytriptycene (1) in the presence of
potassium carbonate, followed by a palladium-catalyzed hydrogen reduction of the intermediate dinitro
compound 2 (Scheme 1). The synthetic details and characterization data of triptycene-bis(ether amine) 3
1
8
19
have
been
described
previously.
According
to
the
reported
procedures,
o
1
1
,4-bis(4-carboxyphenoxy)triptycene (5) (mp = 386-388 C) was prepared by the alkaline hydrolysis of
,4-bis(4-cyanophenoxy)triptycene (4) obtained from the fluoro-displacement reaction of
p-fluorobenzonitrile with 1,4-dihydroxytriptycene in the presence of potassium carbonate.
-Carboxytriphenylamine (6) and N-(4-carboxyphenyl)carbazole (7) were synthesized by the cesium
4
fluoride (CsF)-mediated N-arylation reaction of diphenylamine and carbazole, respectively, with
p-fluorobenzonitrile, followed by the alkaline hydrolysis of the intermediate dintrile compounds (Scheme
2
0,21
2
).
N-(4-aminophenyl)carbazole (9) were prepared by the hydrazine Pd/C-catalyzed reduction of
-nitrotriphenylamine and N-(4-nitrophenyl)carbazole, respectively. These two nitro precursors were
According
to
the
reported
procedures,
4-aminotriphenylamine
(8)
and
4
synthesized from the N-arylation of diphenylamine and carbazole, respectively, with
p-fluoronitrobenzene in the presence of a base. The bis(amide-triarylamine) derivatives T-TPA-C,
T-TPA-N, T-CBZ-C and T-CBZ-N were prepared by the condensation reactions from
1
,4-bis(4-aminophenoxy)triptycene with 4-carboxytriphenylamine and N-(4-carboxyphenyl)carbazole,
respectively, and from 1,4-bis(4-carboxyphenoxy)triptycene with 4-aminotriphenylamine and
N-(4-aminophenyl)carbazole, respectively (Scheme 3). The synthetic details and characterization data of
these compounds are included in the Supporting Information. Triphenyl phosphite (TPP, Acros) and
dichloromethane (Tedia) were used as received. N-Methyl-2-pyrrolidone (NMP, Tedia) and pyridine (Py,
Wako) were dried over calcium hydride for 24 h, distilled under reduced pressure, and stored over 4 Å
molecular sieves in a sealed bottle. Commercially obtained calcium chloride (CaCl
2
, Wako) was dried
NClO (TCI) was
o
under vacuum at 200 C for 3 h prior to use. Tetrabutylammonium perchloate, Bu
4
4
,
recrystallized from ethyl acetate under nitrogen atmosphere and then dried in vacuo prior to use.
2
.2. Electrochemical polymerization
Electrochemical polymerization was performed with a CH Instruments 750A electrochemical analyzer.
−4
4 4 2 2
The polymers were synthesized from 10 M monomers and 0.1 M Bu NClO in dichloromethane (CH Cl )
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−1
solution via cyclic voltammetry repetitive cycling at a scan rate of 50 mV s . The polymer was deposited
onto the surface of the working electrode (ITO/glass surface, polymer films area about 0.7 cm × 3 cm),
and the film was rinsed with plenty of acetone for the removal of un-reacted monomer, inorganic salts
and other organic impurities formed during the process.
2
.3. Instrumentation and measurements
1
13
Infrared (IR) spectra were recorded on a Horiba FT-720 FT-IR spectrometer. H and C NMR spectra were
measured on a Bruker AVANCE 500 FT-NMR system with tetramethylsilane as an internal standard.
Electrochemical measurements were performed with a CH Instruments 750A electrochemical analyzer.
Cyclic voltammetry was conducted with the use of a three-electrode cell in which ITO (polymer film area
about 0.8 cm × 1.25 cm) was used as a working electrode. A platinum wire was used as an auxiliary
electrode. All cell potentials were taken with the use of a home-made Ag/AgCl, KCl (sat.) reference
electrode. Ferrocene was used as an external reference for calibration (+0.48 V vs. Ag/AgCl).
Spectroelectrochemistry analyses were carried out with an electrolytic cell, which was composed of a 1
cm cuvette, ITO as a working electrode, a platinum wire as an auxiliary electrode, and a Ag/AgCl
reference electrode. Absorption spectra in the spectroelectrochemical experiments were measured with
an Agilent 8453 UV-Visible spectrophotometer.
3
3
1
.
Results and discussion
.1. Monomer synthesis
18
19
,4-Bis(4-aminophenoxy)triptycene (3) and 1,4-bis(4-carboxyphenoxy)triptycene (5) were synthesized
according to the synthetic routes outlined in Scheme 1. The triarylamine derivatives 6−9 used for the
end-capping reactions could be easily prepared on the basis of a well-known chemistry (Scheme 2). Four
triptycene-cored diamide derivatives end-capped with electroactive TPA or NPC groups were prepared
by the condensation reactions from triptycene-bis(ether amine) 3 with 4-carboxytriphenylamine (6) and
N-(4-carboxyphenyl)carbazole (7), respectively, and from bis(ether-carboxylic acid)
5
with
4
-aminotriphenylamine (8) and N-(4-aminophenyl)carbazole (9), respectively, using TPP and pyridine as
2
2
condensing agents. As shown in Scheme 3, these four compounds were coded as T-TPA-C, T-CBZ-C,
T-TPA-N, T-CBZ-N. The former two compounds are isomeric to the latter two ones with different
orientations of the amide linkage. The triphenylamine units of compounds T-TPA-C and the
N-phenylcarbazole units of T-CBZ-C are attached to the carbonyl side of the amide group, and those of
T-TPA-N and T-CBZ-N are linked to the amide NH side.
The structures of the target compounds were confirmed by means of FTIR spectroscopy and
high-resolution NMR analysis. The FTIR spectra of these compounds are included in Fig. S1, where we
4

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−
1
can find the characteristic absorption bands of amide group at near 3300 (N–H str.) and 1650 cm
−
1
(
amide C=O str.), and the characteristic ether absorption bands at near 1220 cm (C−O str.). Figs. S2 to
1
13
S5 compile the H NMR, C NMR, H-H COSY, and C-H HSQC spectra of T-TPA-C, T-TPA-N, T-CBZ-C and
T-CBZ-N, respectively. The assignments of all proton and carbon resonance peaks are also shown in the
spectra and they are in good agreement with the proposed molecular structures of these synthesized
1
compounds. The resonance peaks appearing in the downfield regions of the H NMR spectra (10−11
1
3
ppm) and C NMR spectra (164−165 ppm) support the presence of amide groups in the synthesized
compounds.
3
.2. Electropolymerization
All electrochemical polymerization processes of monomers were performed on the ITO glass slides in a
−
4
reaction medium containing 10 M monomer in 0.1 M Bu NClO /dichloromethane (CH Cl ) solution via
4
4
2
2
−
1
repetitive cycling at a potential scan rate of 50 mV s . Fig. 1(a) displays the successive cyclic
voltammograms (CV) of the T-TPA-C solution between 0 and 1.30 V. As the CV scan continued, the
P(T-TPA-C) polymer film was formed on the working electrode surface. The increase in the redox wave
current densities implied that the amount of conducting polymers deposited on the electrode was
increasing. The polymerization of T-TPA-C showed two oxidation peaks (E1/2 = 0.88 and 1.01 V), which
are attributed to their polaronic and bipolaronic states, respectively. The inset in Fig. 1(a) displays the
first three consecutive CV scans of T-TPA-C. In the first scan, only one oxidation peak (E ) at 1.21 V is
pa
observed, which is attributed to the oxidation of TPA. A new oxidation wave appeared as a shoulder at
0
.92 V during the second scan, and the wave shifted to a more positive potential with increased current
during the third scan. This is a typical oxidation wave of the benzidine group, indicating the occurrence
of the oxidative coupling between the TPA units. It has been proposed that the radical cation of TPA
dimerizes (couples) or reacts with the parent TPA unit rapidly to form a tetraphenylbenzidine (TPB)
2
3
segment.
The formed dimer TPB is more easily oxidized than the starting TPA unit and undergoes
•+
further oxidations in two discrete one-electron steps to the TPB monocation and finally the quinoidal
2
+
TPB .
The electrochemical properties of the isomeric T-TPA-N are completely different. Fig. 1(b) displays
its reversible electrochemical behavior, which indicates that the intermediate produced in the oxidation
process of T-TPA-N is stable on the time scale of the CV scan. Repetitive scans between 0 and 1.30 V
produced almost the same patterns as those observed in the first scan, and no new peaks were detected
under these experimental conditions. This reversible process suggests that the radical cation of T-TPA-N
is more stable than that produced from the oxidation of its T-TPA-C isomer. Therefore, no polymer films
were built on the electrode surface. The less stability of the T-TPA-C radical cation can be explained by
the fact that its TPA units are attached to the electron-withdrawing carbonyl side of the amide linkage.
5

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Thus, the orientation of the amide linkage in these compounds significantly affected their
electrochemical behaviors and electropolymerization reactions.
Fig. 2(a) presents three consecutive CV scans of T-CBZ-C. For the first scan, one oxidation peak (Epa
)
at 1.82 V due to the oxidation of the CBZ moieties is detected. During subsequent scans, the oxidation
peak in the CV is shifted positively with the increasing number of cycles and no polymer films were built
on the electrode surface upon repetitive scanning. The electron-withdrawing effect of the carbonyl (C=O)
groups led to a relatively higher oxidation potential of the carbazole groups. This might result in an
over-oxidation of the compound and undesired side reactions; thus, we could not observe any polymer
films were built on the ITO surface. In contrast, the electropolymerization process of the isomeric
compound T-CBZ-N is feasible. Repeated scans for several cycles at 0-1.60 V led to a thin film built on the
electrode, and the resulting P(T-CBZ-N) exhibited two reversible redox couples at E1/2 values of 0.98 V
and 1.30 V. As shown in inset of Fig. 2(b), only one oxidation peak at 1.46 V is observed in the first CV
scan of T-CBZ-N. After the second scan, a new oxidation peak appeared at 1.03 V that corresponded to
the first oxidation of the biscarbazole dimer generated through oxidative coupling of carbazole radical
2
4
cations. The increase in the redox couples current densities suggests that the amount of electroactive
polymers deposited on the electrode is increasing. In comparison to the reversible redox behavior
observed for T-TPA-N, T-CBZ-N revealed a feasible electropolymerization property. This can be attributed
to the fact that the planarity of the carbazole system and the attendant high electron density at the
2
4
reactive sites, the coupling rate of NPCs has been found to be much faster than for the TPAs.
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O
O
toluene
AcOH
HBr
O
OH
O
HO
1
O N
2
O
O
NO2
H N
2
O
O
NH2
Pd/C
DMF
K CO3
2
2
Cl
NO2
DMF
3
2
1
NC
O
O
CN
HOOC
O
O
COOH
KOH
K CO3
2
2
F
CN
H O/ EtOH
2
toluene, DMF
5
4
Scheme 1 Synthetic route to triptycene-bis(ether amine) 3 and bis(ether-carboxylic acid) 5.
7

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HOOC
N
NC
N
HN
KOH
CsF
6
+
F
CN
H O/ EtOH
2
DMSO
HOOC
N
NC
N
HN
7
8
NaH
H N
2
N
HN
O N
2
N
+
F
NO2
DMAc
Pd/C
hydrazine
EtOH
CsF
HN
+
F
NO2
H
2
N
N
O N
2
N
DMSO
9
Scheme 2 Synthetic routes to end-capping compounds 4-carboxytriphenylamine (6), N-(4-carboxyphenyl)carbazole (7),
-aminotriphenylamine (8), and N-(4-aminophenyl)carbazole (9).
4
8

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O
C
O
C
H
N
H
N
HOOC
N
N
O
O
N
H
2
N
O
O
NH
2
6
7
TPP, Py
NMP, CaCl
2
+
T-TPA-C
O
C
O
C
H
N
H
N
N
O
O
N
3
HOOC
N
T-CBZ-C
O
O
H
H
N
N
C
O
O
C
N
N
H
2
N
N
HOOC
O
O
COOH
8
TPP, Py
NMP, CaCl
+
2
T-TPA-N
O
C
O
C
H
N
H
N
N
O
O
N
H
2
N
N
5
9
T-CBZ-N
Scheme 3 Synthesis of monomers T-TPA-C, T-CBZ-C, T-TPA-N, and T-CBZ-N.
9

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(
a)
(
b)
Fig. 1. Repeated potential scanning of (a) T-TPA-C and (b) T-TPA-N between 0 and 1.30
−1
V in 0.1 M Bu
4 4 2 2
NClO /CH Cl with a scan rate of 50 mV s . The inset displays the first,
second, and third CV scans of T-TPA-C.
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(
a)
(
b)
Fig. 2 Repeated potential scanning of (a) T-CBZ-C and (b) T-CBZ-N monomer in 0.1 M
−1
Bu NClO /CH Cl with a scan rate of 50 mV s . The inset displays the first, second, and
4
4
2
2
third CV scans of T-CBZ-N.
11

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3
.3. Electrochemistry of the polymer films
The electrochemical behavior of the electrodeposited polymer films was investigated by cyclic
voltammetry in a monomer-free Bu NClO /CH CN solution. Fig. 3(a) and 3(b) respectively displays the CV
4
4
3
curves of the P(T-TPA-C) and P(T-CBZ-N) at different scanning rates between 50 and 300 mV/s. The
P(T-TPA-C) film shows two closely separated redox couples, one of them centered around 0.93 V and
another centered around 1.02 V. The first anodic peak corresponds to one-electron oxidations of the
TPB units to form radical cations, followed subsequently by the oxidation to dicationic species (see
Scheme 4a). The P(T-CBZ-N) film exhibits two reversible redox couples at E1/2 value of 1.03 and 1.27 V,
attributed to the radical cation and dication states of biscarbazole, respectively (see Scheme 4b). In the
scan rate dependence experiments for P(T-TPA-C) and P(T-CBZ-N) films, both anodic and cathodic peak
current values increase linearly with an increasing scan rate (insets of Fig. 3), indicating that the
electrochemical processes are reversible and not diffusion limited, and the electroactive polymer is well
adhered to the working electrode (ITO glass) surface.
The redox potentials of the investigated polymer films as well as their respective highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) (versus vacuum) energy
+
levels are summarized in Table 1. The external ferrocene/ferrocenium (Fc/Fc ) redox standard has E1/2
value of 0.44 V vs Ag/AgCl in acetonitrile. Under the assumption that the HOMO level or called ionization
potentials (versus vacuum) for the ferrocene standard was 4.80 eV with respect to the zero vacuum level,
Ox1
the HOMO levels for the electro-generated polymer films are estimated from the E1/2 values of from
their CV diagrams as 5.29−5.39 eV. The energy gaps esꢀmated from the absorpꢀon spectra were then
used to obtain the LUMO energy levels of 2.30−2.35 eV. According to the HOMO and LUMO energy
levels obtained, the polymers in this study seem to be appropriate as hole injection and transport
materials.
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Fig. 3 Scan-rate-dependent cyclic voltammograms of (a) P(T-TPA-C) and (b) P(T-CBZ-N)
in 0.1 M Bu NClO /CH CN solution at different scan rates between 50 and 300 mV/S. (a
50 mV s , b = 100 mV s , c = 150 mV s , d = 200 mV s , e = 250 mV s and f = 300
4
1
4
3
−
−1
−1
−1
−1
=
−1
mV s ). Inset: scan rate dependence of the anodic and cathodic peak current densities
graph.
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Scheme 4 Anodic oxidation pathways of (a) P(T-TPA-C) and (b) P(T-CBZ-N).
14

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3
.4. Spectroelectrochemistry and electrochromic switching
The electro-optical properties of the electro-generated polymer films were investigated using the
changes in electronic absorption spectra at various applied voltages. Spectroelectrochemical diagrams of
th P(T-TPA-C) and P(T-CBZ-N) polymeric films deposited on ITO were recorded in the monomer-free
electrolyte solution. The result of P(T-TPA-C) film upon electro-oxidation is presented in Fig. 4(a) as a
series of UV-vis-NIR absorption curves correlated to electrode potentials. Before electrical potential was
applied, the P(T-TPA-C) film showed a strong absorption at 357 nm, but it almost colorless in the visible
region. By setting the potential to 1.1 V, the intensity of the absorption band around 357 nm decreased
gradually, and the growth of new peaks at 494 and 711 nm was observed. As the potential increased
more positively to 1.3 V, the absorbance at 494 nm slightly decreased and the absorption at 711 nm
further intensified. Meanwhile, the film changed color from orange to deep blue. We attribute the
development of bands at 494 and 711 nm to the formation of TPB radical cations and dications,
respectively, as shown in Scheme 4a. The film showed a multicolored electrochromism from colorless
neutral state to orange semi-oxidized state and blue fully oxidized state. This phenomenon persisted for
several subsequent scans.
Fig. 4(b) shows the spectral changes of the electro-deposited film P(T-CBZ-N) upon incremental
oxidative scans from 0 to 1.6 V. In the neutral form, at 0.0 V, the P(T-CBZ-N) exhibited strong absorption
at λmax of 313 nm. As the applied voltage was stepped from 0.0 to 1.2 V, the absorbance at 313 nm
decreased, and new peak at 431 nm gradually increased in intensity. In the same time, the film turned
into yellowish green. We attribute these spectral changes to the formation of cation radicals of the
biscarbazole moieties. At higher oxidation levels (1.2-1.6 V), the absorbance at 431 nm decreased and a
new broad absorption band centered at about 859 nm started to intensify and extended into the near-IR
region, which indicates the formation of dication states of the polymer film. The inset of Fig. 4(b) shows
photographs of the P(T-CBZ-N) film in uncharged (neutral, very pale yellow), semi-oxidized (yellowish
green) and fully oxidized states (bluish green).
Electrochromic switching studies for the electro-generated poly(amide-triarylamine) films were
performed to monitor the percent transmittance changes (ΔT %) as a function of time at their absorption
maximum (λmax) and to determine the response time by stepping potential repeatedly between neutral
and oxidized states. Fig. 5(a) and 5(b) depict the optical transmittance of P(T-TPA-C) film as a function of
time at 494 and 711 nm by applying square-wave potential steps between 0 and 1.1 V for a resident time
of 6 s and between 0 and 1.3 V for a resident time of 8 s. Generally, the charge passed at 90% of the full
optical switch, which was used to evaluate the response time, since the naked eye could not distinguish
the color change after this point. As shown in Fig. 5(c), P(T-TPA-C) film attained 90% of a complete
coloring and bleaching in 1.79 and 0.97 s, respectively. The optical contrast measured as ΔT% between
neutral colorless and oxidized orange states was found to be 32 % at 494 nm. Fig. 5(d) shows that
P(T-TPA-C) attained 90% of a complete coloring and bleaching in 2.92 and 0.99 s, respectively. The
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optical contrast measured as ΔT% between neutral colorless and oxidized deep blue states was found to
be 70 % at 711 nm. The electrochromic coloring efficiency (CE) is defined as the ratio between the
change in optical density units and the injected/ejected charge as a function of electrode area (η =
d
ΔOD/Q ). The CE values of P(T-TPA-C) estimated from the data shown in Fig. 5(c) and 5(d) were found to
2
2
be 348 m /C at 494 nm and 151 cm /C at 711 nm (Table 1). The fast switch speeds of the polymer film
may be attributable to the presence of rigid, three-dimensional framework in the polymer chain which
increases the internal free volume in the polymer film to allow rapid counter ion diffusion.
As shown in Fig. 6, P(T-CBZ-N) thin film was switched by stepping the potential between 0 and 1.2
V and between 0.0 and 1.6 V for a resident time of 10 s. In this case, a response time required for 90%
full-transmittance change of 2.04 s for the coloration step and 1.10 s for the bleaching step at 431 nm,
and 2.46 s for the coloration step and 1.42 s for the bleaching step at 859 nm. In addition, the optical
contrast measured as ΔT% recorded at neutral and oxidized forms was found to be 18 % at 431 nm and
2
5
7 % at 859 nm. (Fig. 6(c) and 6(d)). The CE values of P(T-CBZ-N) were calculated as 114 m /C at 431 nm
2
and 196 cm /C at 859 nm (Table 1) by chronoamperometry. The long-term stability of P(T-CBZ-N) at
yellowish green coloring (λmax = 494 nm) and bluish green coloring (λmax = 711 nm) were investigated by
monitoring the electrochromic contrast (ΔT%) of the thin film upon repeated square-wave potential
steps between 0 and 1.2 V, and between 0 and 1.6V, respectively, as shown in Fig. 6(a) and 6(b).
P(T-CBZ-N) film still exhibited good reversibility of electrochromic characteristics over 40 cyclic scans
between 0 and 1.2 V; however, it showed a remarkably decreased switching stability when stepping
between 0 and 1.6 V. The NPC-based polymer film has less long-term switching stability as compared to
TPA-based polymer film might be explained by its lower chemical stability during the potential switching.
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Fig. 4 Spectroelectrograms of (a) P(T-TPA-C) and (b) P(T-CBZ-N) thin films on the
ITO-coated glass substrate in 0.1 M Bu NClO /CH CN solution.
4 4 3
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3 4 4
Fig. 5 Potential step absorptometry of the P(T-TPA-C) thin film on the ITO-glass slide (in CH CN with 0.1 M Bu NClO as the supporting
electrolyte) by applying a potential step (a) 0.0 V 1.1 V with a pulse width of 6 s (b) 0.0 V 1.3 V with a pulse width of 8 s. Optical
switching at (c) λmax = 494 nm as the applied voltage was stepped between 0 and 1.1 V (vs. Ag/Ag/Cl) and (d) at λmax = 711 nm as the applied
voltage was stepped between 0 and 1.3 V (vs. Ag/Ag/Cl).
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3 4 4
Fig. 6 Potential step absorptometry of the P(T-CBZ-N) thin film on the ITO-glass slide (in CH CN with 0.1 M Bu NClO as the supporting
electrolyte) by applying a potential step (a) 0.0 V 1.2 V with a switching time of 10 s (b) 0.0 V 1.6 V with a switching time of 10 s. Optical
switching at (c) λmax = 431 nm as the applied voltage was stepped between 0 and 1.2 V (vs. Ag/Ag/Cl) and (d) at λmax = 859 nm as the applied
voltage was stepped between 0 and 1.6 V (vs. Ag/Ag/Cl).
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Table 1 Optical, electrochemical and electrochromic data of the electro-generated polymer films
HOMO/
b
abs
max
abs
onset
a
e
d
f
Polymer
code
λ
(nm)
λ
(nm)
Potential (V )
E
( eV )
g
Color of the
polymer films
ΔT%
(%)
Q
CE
(mC cm ) (cm C )
d
LUMO
eV)
ΔOD
c
−2
2
−1
Ox1
1/2
Ox2
E
1/2
(
E
Ox1: Orange
32
0.209 0.60
0.533 3.53
0.151 1.32
0.371 1.89
348
151
114
196
P(T-TPA-C)
P(T-CBZ-N)
357
415
0.93
1.03
1.02
1.27
2.99
5.29/2.30
5.39/2.35
Ox2: Deep
blue
7
1
5
0
8
7
Ox1: Greenish
yellow
313
408
3.04
Ox2: Bluish
green
a
b
Bandgaps calculated from the polymer films by the equation: E = 1240 /λonset. The HOMO energy levels were calculated from E
Ox1
1/2
values
g
Ox
Ox1
from the CV diagrams and were referenced to ferrocene (4.8 eV relative to the vacuum energy level, E1/2 = 0.44 V vs. Ag/AgCl). EHOMO = E1/2
c
. The colors of polymer films at the partially oxidized state and the fully oxidized state.
+
4.8 − 0.44 (eV); ELUMO = EHOMO − E
Optical Density (ΔOD) = log[Tblenched/Tcolored], where Tblenched and Tcolored are the maximum transmittance in the neutral and oxidized states,
g
d
e
f
d d
respectively. Q is ejected charge, determined from the in situ experiments. Coloration efficiency (CE) =ΔOD/Q .
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4
. Conclusions
Four new triptycene-cotaining bis(amide-triarylamine) compounds T-TPA-C, T-TPA-N, T-CBZ-C, and
T-CBZ-N derived from 1,4-bis(4-carboxyphenoxy)triptycene or 1,4-bis(4-aminophenoxy)triptycene have
been successfully synthesized. Polymer films could be built on the electrode surface from the T-TPA-C
and T-CBZ-N solution by repetitive CV scanning. The electro-deposited films exhibit reversible
electrochemical oxidation accompanied by strong color changes having high coloration efficiency that
can be switched rapidly through modulation of the applied potential. The P(T-TPA-C) film changes color
from the almost colorless neutral state to orange and deep blue oxidized states when scanning from 0 to
1
.3 V and exhibited good switching stability. The color of P(T-CBZ-N) film changes from very pale
yellowish neutral form to yellowish green and bluish green oxidized forms upon potential scanning from
.0 to 1.6 V. The P(T-CBZ-N) film shows good switching stability between neutral and first oxidation
0
states; however, it is not so stable at its fully oxidized state. These characteristics suggest that these
electro-deposited polymers are promising materials for the use in optoelectronics applications.
Acknowledgement
This work was financially supported by the Ministry of Science and Technology of Taiwan, Republic of
China (Grant no. NSC 101-2221-E-027-028-MY3).
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