Electrochromism properties of polyimides possessing triphenylamine moieties with different substituents

Article abstract of DOI:10.1080/15421406.2014.932674

Two diamine monomers with methyl (CH3TPA) and methoxy (OCH3TPA) group substituents in the para position of the triphenylamine (TPA) moiety were synthesized by nucleophilic aromatic substitution using 4-fluoronitrobenzene with p-toluidine and p-anisidine,

Full text of DOI:10.1080/15421406.2014.932674

Mol. Cryst. Liq. Cryst., Vol. 598: pp. 6–15, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.932674
Electrochromism Properties of Polyimides
Possessing Triphenylamine Moieties with Different
Substituents
SEUNG HUN LEE, JUN BAE, HYE MI SEO, JI HYUK IM,
HYUNG SUB SHIN, EUN JOO YOO, AND SEUNG WOO LEE
∗
School of Chemical Engineering, Yeungnam Universuty, Gyeongsan, Korea
Two diamine monomers with methyl (CH3TPA) and methoxy (OCH3TPA) group sub-
stituents in the para position of the triphenylamine (TPA) moiety were synthesized
by nucleophilic aromatic substitution using 4-fluoronitrobenzene with p-toluidine and
p-anisidine, respectively. From these diamine monomers, electroactive polyimides (PI)s
were prepared by the chemical imidization of poly(amic acid) solutions, which were
ꢀ
1
prepared with 4,4 -oxydiphthalic anhydride (ODPA). H nuclear magnetic resonance
and Fourier transform infrared spectroscopy confirmed that both PIs had been synthe-
sized successfully. The measured inherent viscosity for ODPA-CH3TPA PI and ODPA-
OCH3TPA PI was 0.42 and 0.49 dL/g, respectively. The glass transition temperatures
◦
◦
for ODPA-CH3TPA PI and ODPA-OCH3PDA PI were 236 C and 238 C, respectively,
and the thermal degradation temperature of both PIs was above 390 C. The band gaps
◦
of ODPA-CH3PDA and ODPA-OCH3PDA were 3.00 and 2.72 eV, respectively. The ox-
idation onset potential Eox of ODPA-CH3PDA PI and ODPA-OCH3PDA PI from cyclic
voltammetry was 0.73 V and 0.65 V, respectively. The highest occupied molecular orbital
and lowest unoccupied molecular orbital values differed according to the groups sub-
stituted into the triphenylamine moiety. After applying a voltage, the absorption peak
intensity at 723 nm and 752 nm for ODPA-CH3TPA and ODPA-OCH3TPA, respec-
tively, increased. The electrical properties varied according to the chemical structure
of synthesized diamines.
Keywords Electroactive polyimide; HOMO; LUMO; triphenyl amine moiety; spec-
troelectrochemical behaviors
Introduction
Organic charge transporting materials play import roles in many electrical and optical ap-
plications, such as organic photoconductors, electroluminescence (EL) and electrochromic
devices [1–5]. In particular, electrochromic devices exhibit a reversible optical change in
absorption or transmittance after being oxidized or reduced electrochemically by apply-
ing an external voltage using electrochromic materials, such as transition-metal oxides,
inorganic coordination complexes, organic molecules, and conjugated polymers [6–11]. In
general, studies of electrochromic materials focusing on the optical changes in the visible
∗
Address correspondence to Prof. Seung Woo Lee, School Chemical Engineering, Yeung-
nam University, 280 Daehak-ro, Gyeongsan 712-749, Korea (ROK). Tel.: (+82)53-810-3972;
Fax: (+82)53-810-4631. E-mail: leesw1212@ynu.ac.kr
[
172]/6

Electrochromism Properties of Polyimides
[173]/7
region have been useful for variable reflectance mirrors, sunglasses, controllable aircraft
canopies, smart windows, and electrochromic displays [11].
Triphenylamine (TPA) derivatives have attracted considerable interest over the past
few decades owing to their excellent charge transporting properties as hole transporting
materials in thin layer electro-optical devices, such as organic light-emitting diodes, solar
cells, organic field effect transistors and photorefractive holography [1–4, 12–16]. The
basic electrical properties of the TPA derivatives are the easy oxidizability of the amino
center and its ability to transport the positive charge centers via cationic radical species
[17]. As a result, significant research effort is currently being invested in the development
of electrochromic polymer materials with the properties and chemical structure of TPA
derivatives. Several studies on such polymer materials have been reported [18–23].
The electrochemical properties of TPA derivatives are affected by the substitution
∗
group at the para position of the TPA moiety. The oxidation potential and π-π band gap
of the product are different from that of the chemical structures of TPA derivatives [24]. For
example, unsubstituted TPA undergoes coupling deprotonation to form tetraphenylbenzi-
dine after the formation of the initial monocationic radical [25, 26]. When TPA derivatives
with electron-donating substituents, such as methoxy groups at the para position of TPA,
prevents coupling reactions and affords stable cationic radicals [25–27].
Aromatic polyimides (PIs) have excellent thermal stability, dimensional stability, and
mechanical properties [28–31]. Commonly, PIs are prepared using dianhydride and di-
amine monomers in a polar aprotic solvent, such as N-methylpyrrolidinone (NMP), N,N-
dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc) to synthesize poly(amic
acid), which is a precursor of PI. The PAA precursor obtained is then converted to PI by
thermal imidization or chemical imidization.
In this study, PAAs were prepared using different TPA derivatives with electron-
donating methyl groups and methoxy groups in the para position of the TPA moiety, with a
dianhydride monomer in NMP. The PIs were then synthesized by the chemical imidization
of PAAs solutions. The chemical structures of the synthesized monomers were character-
1
ized by H nuclear magnetic resonance (NMR) spectroscopy. The chemical structures and
thermal properties of the synthesized PIs were determined using spectroscopic methods
1
including H NMR and Fourier transform infrared (FTIR) spectroscopy and differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. In ad-
dition, the electrical properties and spectroelectrochemical behavior was investigated in
terms of the chemical structure of the TPA derivatives.
Experimental
Materials
The materials, p-toluidine (TCI), potassium permanganate (Aldrich), 4-fluoronitrobenzene
(from Alfa Aesar), cesium fluoride (Aldrich), hydrazine monohydrate (Aldrich), tetrabuty-
lammonium perchlorate (TPBA) and 10% palladium on activated carbon (Aldrich) were
used as received. The solvents, dimethyl sulfoxide (DMSO), acetonitrile and pyridine (Py),
were purchased from Aldrich, and used as received. The solvent for polymerization, N-
methyl-2-pyrrolidinone (NMP), was purchased from Aldrich, purified by distillation over
calcium hydride and stored over 4 Å molecular sieves.
Measurements
1
The H NMR spectra were obtained using a Bruker AM 300 spectrometer. The FTIR
(BOMEM DA8 FTIR) spectra were obtained on a spectrometer equipped with a Diamond

8/[174]
S. H. Lee et al.
ATR system (Harrick Scientific). The glass transition temperature (Tg) and degradation
temperature (Td) of each film were measured by DSC (model DSC-7, Perkin-Elmer, USA)
and TGA (model TGA-7, Perkin-Elmer, USA), respectively. In the DSC and TGA mea-
surements, dry nitrogen gas was purged at a flow rate of 100 cc/min and a ramping rate
◦
of 10.0 C/min was used. The UV-visible absorption spectra were obtained as a function of
the exposure dose using a Hewlett-Packard 8453 spectrophotometer. The inherent viscosity
◦
of each PI product in NMP with a concentration of 0.50 g/dL was measured at 25.0 C
using an Ubbelohde suspended-level capillary viscometer. Cyclic voltammetry (Epsilon)
was performed using a three-electrode cell, in which indium tin oxide (ITO, the polymer
2
film area was ca. 0.5 × 1.1 cm ) was used as the working electrode. The electrochemical
cell was composed of a 1 cm cuvette, ITO as the working electrode, platinum wire as the
auxiliary electrode, and Ag/AgCl as the reference electrode.
ꢀ
ꢀꢀ
1
Synthesis of 4,4 -dinitro-4 -methyltriphenylamine
p-Toluidine (2.5 g, 23.3 mmol), 4- fluoronitrobenzene (7.24 g, 51.3 mmol) and cesium
fluoride (7.79 g, 51.3 mmol) were dissolved in dried 100 mL DMSO, and the reactant
◦
mixture was stirred at 120 C for 48 h (Scheme 1). The reaction mixture was cooling to
room temperature, poured into 500 ml of cold water and the precipitated orange solid
was then collected by filtration and washed with methanol. The yield of the product was
1
5
.91 g (66%); H NMR (DMSO-d6, ppm): 2.35 (s, Ar-CH3, 3H), 7.13–7.16 (d, Ar-H, 2H),
7.16–7.19 (d, Ar-H, 4H), 7.30–7.33 (d, Ar-H, 2H), 8.15–8.18 (d, Ar-H, 4H).
Scheme 1. Synthesis route of diamines with methyl and methoxy substituents into the para position
of triphenylamine derivatives and chemical structures of polyimides.
ꢀ
32
Synthesis of 4,4 -dinitro-methoxytriphenylamine
p-Anisidine (2.5 g, 20.3 mmol), 4-fluoronitrobenzene (6.30 g, 44.6 mmol) and cesium
◦
fluoride (6.78 g, 44.6 mmol) were dissolved in dried 100 mL DMSO, and stirred at 150 C

Electrochromism Properties of Polyimides
[175]/9
for 24 h (Scheme 1). The reaction mixture was cooled to room temperature, and poured
into methanol. The precipitated orange solid was collected by filtration and washed with
1
methanol. The product yield was 5.71 g (70%); H NMR (DMSO-d6, ppm): 3.67 (s, Ar-
OCH3, 3H), 7.03–7.06 (d, Ar-H, 2H), 7.13–7.16 (d, Ar-H, 4H), 7.18–7.21 (d, Ar-H, 2H),
8.12–8.15 (d, Ar-H, 4H).
ꢀ
ꢀꢀ
1
Synthesis of 4,4 -diamino-4 -methyltriphenylamine (CH3TPA)
ꢀ
ꢀꢀ
A mixture of 5.0 g (14.3 mmol) of 4,4 -dinitro-4 -methyltriphenylamine, 2.52 g (40.0 mmol)
◦
of hydrazine monohydrate, 0.2 g of 10% Pd–C and 100 ml of ethanol was stirred at 120 C
for 8 h (Scheme 1). The reaction mixture was filtered through Celite to remove Pd–C, and
the filtrate was added to 100 ml of water to obtain precipitates that were filtered and dried.
1
The crude product was recrystallized from ethanol. The product yield was 3.93 g (95%); H
NMR (DMSO-d6, ppm): 2.16 (s, Ar-CH3, 3H), 4.84 (s, Ar-NH2, 4H), 6.50–6.53 (d, Ar-H,
2H), 6.56–6.59 (d, Ar-H, 4H), 6.72–6.75 (d, Ar-H, 2H), 6.87–6.90 (d, Ar-H, 4H).
ꢀ
Synthesis of 4,4 -diamio-methoxytriphenylamine (OCH3TPA)
The reaction of OCH3TPA was conducted in a similar manner of CH3TPA except that 5.0 g
ꢀ
(
13.7 mmol) of 4,4 -dinitro-methoxytriphenylamine was used (Scheme 1). The product
yield was 3.80 g (91%); 3.67 (s, Ar-OCH3, 3H), 4.72 (s, Ar-NH2, 4H), 6.47–6.50 (d, Ar-H,
H), 6.67–6.70 (d, Ar-H, 4H), 6.68–6.71 (d, Ar-H, 6H).
2
Synthesis of Polyimide
Polymerization was conducted by the conventional two-step solution imidization process.
ꢀ
Poly(amic acid)s (PAA) was prepared by adding 1 equivalent of 4,4 -oxydiphthalic anhy-
dride (ODPA) in an anhydrous NMP solution and 1 equivalent of a diamine, which was
pre-dissolved in NMP to form a 10.0 wt.% solution, under nitrogen with vigorous stirring
(Scheme 1). Once addition was complete, the reaction flask was sealed tightly and stirring
was continued for 48 h until the polymerization mixture was homogeneous and viscous.
A small excess of acetic anhydride and pyridine was added to the PAA solutions at room
◦
temperature and allowed to stir for 12 h at 150 C. After dilution with 5 mL of NMP, the
solutions were poured into 1000 mL of methanol with vigorous stirring. The solids formed
◦
were filtered, washed repeatedly with hot methanol, and dried at 150 C for 12 h in vacuo.
The yields were almost quantitative in polymerizations using the two different diamines.
Results and Discussion
ꢀ
ꢀ
ꢀ
ꢀ
4
,4 -Dinitro-4 -methyltriphenylamine and 4,4 -dinitro-methoxytriphenylamine were syn-
thesized via a nucleophilic aromatic substitution reaction in the presence of cesium fluoride
between 4-fluornitrobenzene and p-toluidine and p-anisidine, respectively. The obtained
1
dinitro group-containing molecules were characterized by H NMR spectroscopy. The
spectra were coincident with the expected chemical structure. A characteristic chemical
1
ꢀ
shift at 2.35 ppm for the methyl group was observed on the H NMR spectrum for 4,4 -
ꢀꢀ
dinitro-4 -methyltriphenylamine. A chemical shift at 3.67 ppm for the methoxy group in
ꢀ
4
,4 -dinitro-methoxytriphenylamine was observed.
The monomers with diamino groups for polymerization were prepared in the presence
of hydrazine monohydrate with a catalytic amount of a Pd/C catalyzed reduction reaction of
dinitro group containing compounds. The prepared diamine compounds were characterized

10/[176]
S. H. Lee et al.
Figure 1. 1H NMR spectra of two triphenylamine derivatives with diamine groups dissolved in
dimethyl-d sulfoxide (DMSO-d ): (a) CH3TPA and (b) OCH3TPA.
6 6
1
by H NMR spectroscopy. After the reduction reaction, singlet peaks at 4.84 and 4.72 ppm
for CH3TPA and OCH3TPA, respectively, were observed and assigned to the amino proton
from the reduction of nitro groups (Figure 1). The proton peaks from aromatic rings were
shifted to high field after the reduction reaction. These results suggest that the two diamine
compounds, CH3TPA and OCH3TPA, were synthesized successfully.
Imidization was conducted using a two steps solution polymerization process. The
precursor PAAs were prepared via a room temperature reaction between each diamine and
the ODPA at room temperature in NMP. Imidization of the PAA precursors to PIs was
carried out by a chemical treatment with acetic anhydride. The obtained PIs were identified
by IR spectroscopy (Figure 2). The characteristic absorption bands of the imide group were
−
1
observed near 1773 and 1713 cm (asymmetry and symmetry stretch of carbonyl (C=O),
−1
imide I) and 1371 cm (C-N-C stretch, imide II). The carbonyl stretch bands of amide and
carboxylic acid in the PAA precursors were not detected in the IR spectra of the resulting
PIs, indicating that the PAA precursors had been converted completely to the corresponding
PIs [28–31].
1
The synthesized PIs were characterized by H NMR spectroscopy (Figure 3). Both PIs
showed aromatic proton peaks at 6.8–8.2 ppm. A characteristic chemical shift at 2.35 ppm
and 3.81 ppm for the methyl group and methoxy substituents of TPA moiety, respectively,
1
was observed on the H NMR spectra for ODPA-CH3PDA PI and ODPA-OCH3PDA PI.
In addition, unreacted acid, amine and amide group from PAA were not detected in the
1
H NMR spectra. These spectroscopic characterizations confirmed that the imidization
reactions had proceeded completely after treating the polymers with acetic anhydride
◦
at 150 C. For PIs, the measured inherent viscosities of ODPA-CH3TPA PI and ODPA-
OCH3TPA PI were 0.42 and 0.49 dL/g, respectively. Considering the spectroscopy results
and inherent viscosity, it can be inferred that the PIs had been synthesized successfully with
a reasonably high molecular weight.

Electrochromism Properties of Polyimides
[177]/11
Figure 2. ATR-FTIR spectra of polyimide films: (a) ODPA-CH3TPA PI and (b) ODPA-OCH3TPA
PI.
The thermal properties of the PIs were determined, and the results are listed in Table 1.
◦
The Tg of ODPA-CH3TPA PI and ODPA-OCH3PDA PI measured by DSC were 236 C
◦
and 238 C, respectively. These glass transition behaviors might come from the similar
chemical structures of both PIs. The two PIs have similar Tds (394–407 C). Of these
◦
polymers, ODPA-CH3PDA PI had a higher Td.
The PI films were examined by UV-vis spectroscopy and cyclic voltammetry (CV).
From the UV-vis spectra, the band gap (which is the difference between the highest occupied
molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO)) was
Figure 3. 1H NMR spectra of polyimides dissolved in dimethyl-d
6
6
sulfoxide (DMSO-d ): (a) ODPA-
CH3TPA PI and (b) ODPA-OCH3TPA PI.

12/[178]
S. H. Lee et al.
Table 1. Inherent viscosity, glass transition temperatures (Tg), and decomposition temper-
atures (Td) of polyimides
Inherent viscositya
Tg ( C)
◦
b
Td ( C)
◦
c
Sample designation
ODPA-CH3TPA PI
ODPA-OCH3TPA PI
0.42
0.49
236
238
407
394
a
Measured in NMP at a concentration of 0.5g/dL at room temperature.
b
◦
Measured by DSC with a heating rate of 10.0 C/min.
c
◦
Determined by TGA at a heating rate of 10.0 C/min under a nitrogen atmosphere: T
d
= temperature
of 5.0% weight loss.
determined using the λonset value from the measured UV-vis spectroscopy. The band gap
was estimated to be 3.00 eV for ODPA-CH3PDA PI, and 2.72 eV for ODPA-OCH3PDA
PI [33]. The electrochemical behavior of the PIs were examined by CV of the coated
film on ITO-coated glass substrates as the working electrode in dry acetonitrile containing
0.1 M of TBAP as an electrolyte under a nitrogen atmosphere. Figure 4 shows the typical
CVs for ODPA-CH3TPA and ODPA-OCH3TPA PI films. CV analysis showed that the
oxidation onset potential Eox of ODPA-CH3PDA PI and ODPA-OCH3PDA PI were 0.73 V
and 0.65 V, respectively. The external ferrocene/ferrocenium (Fc/Fc+) redox standard half-
wave potential (E1/2) was 0.48 V versus the Ag/AgCl electrode in acetonitrile. The HOMO
of ODPA-CH3TPA PI and ODPA-OCH3PDA PI was determined to be 5.55 eV and 5.52 eV,
respectively. From these band gap and HOMO values, LUMO value of ODPA-CH3TPA
PI and ODPA-OCH3PDA PI were estimated to be 2.11 eV and 2.33, respectively. The
Figure 4. CV responses of (a) ODPA-CH3TPA PI and (b) ODPA-OCH3TPA PI films coated on
indium tin oxide glass substrates in acetonitrile containing 0.1 M tetrabutylammonium perchlorate.

Electrochromism Properties of Polyimides
[179]/13
Table 2. Redox potentials and energy levels of the polyimides
Thin film
nm)
Oxidation
potential (V)
a
(
Eg
(eV)b
HOMO
(eV)c
LUMO
(eV)
Sample
λmax
λonset
E1/2
Eonset
ODPA-CH3TPA
ODPA-OCH3TPA
306
308
413
456
1.17
1.14
0.73
0.65
3.00
2.72
5.55
5.03
2.55
2.80
aMeasured by cyclic voltammograms (CV) versus Ag/AgCl in acetonitrile containing 0.1 M
tetrabutylammonium perchlorate. E1/2: Oxidation half-wave potential from CV
b
Calculated from polyimide films by the equation: Eg (energy band gap between HOMO and
LUMO) = 1240/λonset
c
Calculated from E1/2 of CV. This value were referenced to ferrocene (4.8 eV)
differences between the band gaps and HOMO/LUMO levels of the PIs might be due to
the differences in their substituent effects in the same position of the TPA moiety.
Both the oxidation of PIs occurred at the nitrogen center of the TPA unit. On the
other hand, the introduction of more electron-donating methoxy substituents into the para-
position of TPA not only stabilizes the cationic radicals but also reduces the oxidation
potentials [32]. The HOMO and LUMO levels of the PIs investigated could be determined
from the oxidation onset or half-wave potentials and the onset absorption wavelength of
polymer films (Table 2).
Spectroelectrochemical experiments were carried out to reveal the optical characteris-
tics of the electrochromic films. The electrode preparations and solution conditions were the
same as the CV experimental condition. Figure 5 shows the spectroelectrochemical results
using UV-vis spectroscopy for ODPA-OCH3TPA PI. In the neutral state, ODPA-OCH3TPA
Figure 5. Spectroelectrochemical behavior at the applied potentials of ODPA-OCH3TPA PI thin
film coated on indium tin oxide glass substrates in acetonitrile containing 0.1 M tetrabutylammonium
perchlorate.

14/[180]
S. H. Lee et al.
Figure 6. Spectroelectrochemical behavior at the applied potentials of ODPA-CH3TPA PI thin film
coated on indium tin oxide glass substrates in acetonitrile containing 0.1 M tetrabutylammonium
perchlorate.
PI has strong absorption at wavelengths around 308 nm, which is characteristic for the TPA
moiety, but is almost transparent in the visible region. The neutral ODPA-OCH3TPA PI
film showed an absorption maximum at 308 nm (λmax = 308 nm), but no absorbance in the
visible region. This UV absorption might have originated from the TPA moiety in the poly-
mer. The absorption peak intensity at λmax = 308 nm decreased drastically with increasing
applied voltage, whereas a new peak at 752 nm increased gradually. The increasing new
peak is due to the formation of a stable cationic radical of the methoxy group-substituted
TPA unit [32].
Figure 6 shows the spectroelectrochemical behavior of an ODPA-CH3TPA PI film.
The λmax = 306 nm for the ODPA-CH3TPA PI film decreased gradually with increasing
applied positive potential, whereas new bands grew at 723 nm. This new absorbance peak
was assigned to a cationic radical TPA with a methyl substituent. Furthermore, when the
λmax of ODPA-OCH3PDA and ODPA-CH3PDA in neutral state were compared, a 2 nm
blue-shift was observed for ODPA-OCH3PDA PI. This means that ODPA-OCH3PDA PI
has a shorter conjugation length than ODPA-CH3PDA PI.
Conclusions
Two diamine monomers with a methyl (CH3TPA) and methoxy (OCH3TPA) group sub-
stituent in the para- position of triphenylamine (TPA) moiety were synthesized by nu-
cleophilic aromatic substitution. From these diamine monomers, electroactive polyimides
(
PI)s were prepared via the chemical imidization of poly(amic acid) (PAA) solutions,
ꢀ
which were prepared with 4,4 -oxydiphthalic anhydride (ODPA). Both PIs were synthe-
sized successfully and showed good thermal stability. The band gaps of the PIs were 3.00
and 2.72 eV for ODPA-CH3PDA and ODPA-OCH3PDA, respectively. The HOMO and
LUMO values differed according to the substituent groups into the TPA moiety. The spec-
troelectrochemical behavior varied according to the chemical structure of the diamines and
applied voltage.

Electrochromism Properties of Polyimides
[181]/15
Acknowledgment
This study was supported by Yeungnam University Research Grants in 2012 and a Human
Resources Development Program of Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant (No. 20104010100580) funded by the Korean Ministry of
Knowledge Economy.
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