Multicolored near-infrared electrochromic polyimides: Synthesis, electrochemical, and electrochromic properties

Article abstract of DOI:10.1016/j.dyepig.2013.08.019

A novel series of multicolored near-infrared electrochromic aromatic polyimides have been synthesized by a conventional two-step polymerization process. All polymers displayed outstanding thermal stability, i.e. 20 wt% loss in excess of 593 C in nitrogen and high char yields (higher than 68% at 750 C). In addition, the polymer films showed reversible electrochemical oxidation, high coloration efficiency, low switching time, and anodic green electrochromic behaviors. The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energy levels of these polymers were determined in the range of -4.759 to -4.801 and -1.758 to -2.002 eV (vs the vacuum level) by cyclic voltammetry method consisting with the results of quantum chemical calculation well, respectively. The polymer films revealed excellent stability of electrochromic characteristics for the radical cations generated, changing color from original yellowish to green and blue, with the absorption changing from ultraviolet to near-infrared.

Full text of DOI:10.1016/j.dyepig.2013.08.019

Accepted Manuscript
Multicoloured Near-Infrared Electrochromic Polyimides: Synthesis, Electrochemical,
and Electrochromic Properties
Jiwei Cai, Haijun Niu, Ping Zhao, Yan Ji, Lina Ma, Cheng Wang, Xuduo Bai, Wen
Wang
PII:
S0143-7208(13)00317-3
10.1016/j.dyepig.2013.08.019
DYPI 4066
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 7 July 2013
Revised Date: 18 August 2013
Accepted Date: 19 August 2013
Please cite this article as: Cai J, Niu H, Zhao P, Ji Y, Ma L, Wang C, Bai X, Wang W, Multicoloured
Near-Infrared Electrochromic Polyimides: Synthesis, Electrochemical, and Electrochromic Properties,
Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.08.019.
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ACCEPTED MANUSCRIPT
Electrochromic behavior of PI thin film. The inset shows the photographic images of the film at
indicated applied voltages.

ACCEPTED MANUSCRIPT
Multicoloured Near-Infrared Electrochromic Polyimides: Synthesis,
Electrochemical, and Electrochromic Properties
Jiwei Caia, Haijun Niua*, Ping Zhaob, Yan Jia, Lina Ma a, Cheng Wanga, Xuduo Baia, Wen Wangc
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Department
of Macromolecular Materials and Engineering, Heilongjiang University, Harbin 150086, P R
China
bKey Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
of Science and Technology, Shanghai 200237, PR China
cSchool of Material Science and Engineering, Harbin Institute of Technology, Harbin 150080, P R
China
*Corresponding author: Tel: 086-13684501571; Fax: 086-0451-86608131;
email: haijunniu@hotmail.com
Abstract
A novel series of multicoloured near-infrared electrochromic aromatic polyimides have been
synthesized by a conventional two-step polymerization process. All polymers displayed
outstanding thermal stability, i.e. 20 % wt loss in excess of 593 °C in nitrogen and high char yields
o
(higher than 68% at 750 C). In addition, the polymer films showed reversible electrochemical
oxidation, high coloration efficiency, low switching time, and anodic green electrochromic
behaviors. The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied
Molecular Orbital (LUMO) energy levels of these polymers were determined in the range of
-4.759 – -4.801 and -1.758 – -2.002 eV (vs the vacuum level) by cyclic voltammetry method
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consisting with the results of quantum chemical calculation well, respectively. The polymer films
revealed excellent stability of electrochromic characteristics for the radical cations generated,
changing color from original yellowish to green and blue, with the absorption changing from
ultraviolet to near-infrared.
Key words: triphenylamine, electrochromism, polyimides, near-infrared, functional dyes,
coloration efficiency
1.Introduction
Aromatic polyimides are well accepted as advanced materials for thin-film applications in
microelectronic devices and liquid crystal displays due to their outstanding mechanical, chemical,
thermal, and physical properties [1]. However, the technological applications of most polyimides
are limited by processing difficulties because of high melting or glass transition temperatures and
limited solubility in most organic solvents due to their rigid backbones. A general approach to
overcome this drawback was to introduce bulky lateral substituents, flexible linkages, and
noncoplanar biphenylene moieties into the polymer [2-9]. The introduction of propeller-shaped
triarylamine units into the polyimide backbone as a structural modification to the rigid polyimides
has the potential to form an amorphous structure exhibiting excellent solubility and film-forming
capabilities.
Electrochromism is known as the alternation of color by the application of a potential [10-12].
Color changes are commonly between a transparent state, where the chromophore only absorbs in
the UV region, and a colored state or between two colored states in a given electrolyte solution.
The electrochromic material may exhibit several colors and can be termed as polyelectrochromic.
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Therefore remarkable effort has been performed for achieving new polymers which meet the
criteria for commercial applications. Some key issues such as long-term stability, rapid redox
switching, high coloration efficiency (CE), and high optical transmittance change (ꢀ%T) during
operation played important roles and are required to be achieved.
As an electrochromic functional moiety, the TPA unit has two basic properties: (1) the easy
oxidizability of the nitrogen center and (2) its hole-transporting ability via the radical cation
species. Triarylamine derivatives are well known as their photo- and electro-active properties that
have potentials for optoelectronic applications, such as photoconductors, hole transporters, light
emitters, and memory devices [13].
It also can be easily oxidized to form radical cations, and the oxidation process is always
associated with a noticeable change of coloration. In recent years, Liou and coworkers have
developed several high-performance polymers such as polyimides[14] and polyamides[15]
carrying the triarylamine unit as a redox-chromophore. They found that the polymer based on TPA
as core exhibited stable, excellent, multi-color, electrochromic characteristic, furthermore the
corporation of electron-donating substituents at the para positions of triarylamines afforded stable
radical cations.
On the other hand, near-infrared (NIR)-absorbing electrochromic materials are receiving
great attention due to their potential applications in optical communications [16], biomedicine [17],
camouflage materials in warfare [18], electro-optic switching in devices [19], thermal control and
thermal emission detectors for spacecrafts [20]. However, compared with Uv-visible absorbing
materials, few NIR electrochromic materials have been developed to date [21].
In our ongoing effort to develop electrochromic polymeric materials, we have synthesized
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and investigated a series of high-performance polymers containing triphenylamine (TPA) [22],
which showed good electrochromic reversibility in the visible region. In this contribution, we
therefore synthesized the diamine monomer N,N′-bis(4-aminophenyl)-N,N′-di-2- naphthalenyl
-1,4-benzenediamine. PIs as thermally stable microelectronic materials have attracted great
interest. Thus, we anticipated that the prepared electroactive PIs would reveal multielectrochrmic
behaviors, high-electrochemical stability, improved optical response times, and enhanced contrast
of optical transmittance in NIR region. Lastly, investigation of the oxidation mechanism for the
conjugated polymer was done using theoretical analysis based on molecular orbital theory.
2 Experimental
2.1 Materials
N, N′-di(2-naphthyl)-1,4-benzenediamine were purchased from TCI Co., 4,4'-oxydiphthalic
anhydride, 4,4'-biphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride,
pyromellitic dianhydride, 4,4'-(hexafluoroisopropylidene) diphthalic anhydride were purchased
from TCI Co.; Pd/C(10%) were purchased from Acros;. 4-fluoronitrobenzene,
N,N-dimethylformamide (DMF) were supplied from Sinopharm Chemical Reagent Co., Ltd.,
China and used as received.
2.2 Synthesis of Monomers
N,N′-bis(4-aminophenyl)-N,N′-di-2-naphthalenyl-1,4-benzenediamine (3) were synthesized
according to the procedure reported [22].
2.3 Synthesis of Polymers
Preparation of PIs P1-P5 by Two-Step Method via Thermal Imidization Reaction. The synthesis of
PI P1 was used as an example to illustrate the general synthetic route. To a solution of 0.2704 g
(0.5 mmol) of diamine 3 in 3.8 mL of N,N-dimethylformamide (DMF), 0.1098 g (0.5 mmol) of
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dianhydride (pyromellitic dianhydride a) was added in one portion. Thus, the solid content of the
solution is approximately 10 wt%. The mixture was stirred at room temperature for about 6 h to
afford a viscous poly (amic acid) solution. The poly (amic acid) film was obtained by casting from
the reaction polymer solution onto a glass plate and drying at 90 °C overnight under vacuum. The
inherent viscosity of polyamic acid of P1 was 0.74 dL/g in DMF at a concentration of 0.5 g/dL at
o
30 C. The polyamic acid in the form of film was converted to PI by successive heating under
vacuum at 100 °C for 1 h, 200 °C for 1 h, and then 300 °C for 30 min. All other PIs were
synthesized by an analogous procedure as described.
FTIR spectrum ( KBr pellet, polyimide of P1): 1775 cm-1 (asymmetrical C=O), 1721 cm-1
(symmetrical C=O), 1650 cm-1 (amide carbonyl),1377 cm-1 (C-N), and 739 cm-1 (imide ring
deformation). 1H NMR (400 MHz, DMSO-d6, ppm, polyamic acid solid of Pa): 10.44-10.52 (m,
the terminal groups of polyamide), 8.30-8.35 (the amide group), 6.99-7.99 (aromatic ring of
benzene).
2.3.1 Polymer P2. Monomer 3 feed 0.2705 g (0.5×10-3mol); dianhydride (b) 0.1472g
1
(0.5×10-3mol); FTIR spectrum (KBr pellet, polyimide): 1777, 1715, 1665, 1596, 1499, 1385. H
NMR (400 MHz, DMSO-d6, ppm, polyamic acid solid of Pa): 10.37-10.42 (m, the terminal groups
of polyamide), 8.30-8.35 (the amide group), 6.86-8.13 (aromatic ring of benzene).
2.3.2 Polymer P3. monomer 3 feed 0.1370 g (2.5×10-4mol); dianhydride (c) 0.0725g
(2.5×10-4mol); FTIR spectrum (KBr pellet, polyimide):1778, 1718, 1659, 1595, 1500, 1383. 1H
NMR (400 MHz, DMSO-d6, ppm, polyamic acid solid of Pa): 10.37-10.40 (m, the terminal groups
of polyamide), 8.31-8.35 (the amide group), 6.89-8.00 (aromatic ring of benzene).
2.3.3 Polymer P4. monomer 3 feed 0.1367 g (2.5×10-4mol); dianhydride (d) 0.0835 g
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1
(2.5×10-4mol); FTIR spectrum (KBr pellet, polyimide):1777, 1718, 1658, 1595, 1492, 1382. H
NMR (400 MHz, DMSO-d6, ppm, polyamic acid solid of Pa): 10.37-10.44 (m, the terminal groups
of polyamide), 8.31-8.35 (the amide group), 6.88-8.02 (aromatic ring of benzene).
2.3.4 Polymer P5. monomer 3 feed 0.1370 g (2.5×10-4 mol); dianhydride (e) 0.1117 g
1
(2.5×10-4mol); FTIR spectrum (KBr pellet, polyimide):1776, 1718, 1659, 1596, 1500, 1385. H
NMR (400 MHz, DMSO, ppm, polyamic acid solid of Pa): 10.49-10.55 (m, the terminal groups of
polyamide), 8.30-8.32 (the amide group), 7.00-7.98 (aromatic ring of benzene).
Scheme 1 Synthesis of polymers
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2.4 Preparation of the PIs Films
A solution of polymer was made by dissolving about 0.5 g of the PIs sample in 10 mL of
DMF. The homogeneous solution was poured into a 9 cm glass Petri dish, which was placed in a
90 °C oven for 6 h to remove most of the solvent; then the semidried film was further dried in
vacuum at 190 °C for 10 h. The obtained films were about 40-60 ꢁm in thickness and were used
for solubility tests, and thermal analyses.
2.5 Measurements
FT-IR spectra were recorded on a PerkinElmer Spectrum 100 Model FT-IR spectrometer. 1H
NMR spectra were measured on a Bruker AC-400 MHz spectrometer in DMSO, using
tetramethylsilane as an internal reference. SEM measurement was carried out on a S4800
instrument with an accelerating voltage of 2 kV, and the samples were sputtered with Pt prior to
observation. Thermogravimetric analysis (TGA) was conducted with a PerkinElmer Pyris 6 TGA.
Experiments were carried out on approximately 6~8 mg powder samples heated in flowing
nitrogen or air (flow rate = 20 cm3 / min) at a heating rate of 10 °C / min. DSC analyses were
performed on a PerkinElmer Pyris diamond DSC at a scan rate of 10 °C / min in flowing nitrogen
(20 cm3 / min). UV-vis-NIR absorption spectra were recorded using a SHIMADZU UV-3600
spectrophotometer whereas the photoluminescence solution spectra were registered on a Jasco
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FP-6200 spectrometer with 450 W xenon lamp as the light source. The emission spectra of PIs
were taken at λexc being equal to about the wavelength of the absorption maximum.
Cyclic voltammetry (CV) measurements were conducted on a CH Instruments 660A
electrochemical analyzer at a scan rate of 50 mV s-1 with a 0.1 M solution of LiClO4 as an
electrolyte under nitrogen atmosphere in dry acetonitrile (CH3CN). The oxidation and reduction
potentials of polymer film coated on an ITO disk were measured using a Pt wire and an Ag / AgCl
electrode as a counter electrode and a quasi reference electrode, respectively. Under these
conditions, the onset of oxidation and reduction potentials of the polymer thin films against the
Ag/AgCl quasi reference electrode were measured and calibrated against the
ferrocene/ferrocenium (Fc/Fc+) redox couple. The HOMO and LUMO levels of the polymers were
calculated by assuming the absolute energy level of Fc / Fc+ asꢀ4.80 eV to vacuum.
3 Results and discussion
3.1 Synthesis of monomers
All monomers were synthesized using modified literature procedures. In general, many series
of dinitrotriphenylamines are synthesized by the nucleophilic amination of aromatic amine and
4-fluoro-1-nitrobenzene under nucleophilic displacement conditions using NaH as base. Reduction
of the dinitro intermediates was carried using 10 % Pd / C in DMF in autoclave with high pressure
H2 and resulted in a near quantitative yield of diaminotriphenylamine derivatives without toxicity
to atmosphere by using hydrazine and tedious process by using SnCl2. Especially, the H2 can
convert the little raw materials to be soluble in the solution to be got rid of. The monomer
structure was verified to be consistent with the object by the NMR and FT-IR characterization.
3.2 Synthesis of polymers
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The PIs were synthesized from diamine 3 and five commercially available dianhydrides a-e
via a typical two step polymerization method. The structures and codes of the prepared polymers
are shown in Scheme 1. In the first step, the viscosities of the reaction mixtures became very high
as poly(amic acid)s formed, indicating the formation of high-molecular-weight polymers. The
poly(amic acid) precursors could be thermal imine dehydrated to PIs by extended heating at
elevated temperatures. The chemical structures of the PIs were characterized with IR analysis, and
the formation of poly(amic acid)s was confirmed with NMR spectroscopy. The IR spectra of these
PIs exhibited characteristic imide absorption bands at around 1776 (asymmetrical C=O), 1720
(symmetrical C=O), 1360 (C-N), and 745 cm -1 (imide ring deformation). The microstructures of
these poly(amic acid)s were also verified by the NMR spectra (Fig. S1).
3.3 Inherent viscosity and thermal properties
Table 1 Thermal properties and inherent viscosity of the PIs
These PIs were obtained in almost quantitative yields, with inherent viscosities in the range
of 0.74–0.82 dL/g, as shown in Table 1. As shown in Table 1, compared with the analogous PI, the
series of PI did not show clear Tg, but showed an enhanced thermal stability (Fig. S2) due to the
decreased conformational flexibility or free volume caused by the introduction of planar
napthalene groups in the repeat unit. All the polymers indicated no clear melting endotherms up to
the decomposition temperatures on the DSC thermograms.
The thermal stabilities of PIs were evaluated by TGA in nitrogen atmospheres. The Fig. S2
inset depicts a typical set of TGA curves for Pa-Pe in nitrogen. The decomposition temperatures
(Tds ) at 5 % and 20 % weight losses in nitrogen were taken from the original TGA thermograms
and are given in Table 1. All the prepared PIs exhibited good thermal stability with insignificant
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o
weight loss up to 470 C under nitrogen atmosphere. The 5% weight loss temperatures of these
o
polymers in nitrogen were recorded in the range of 479 – 580 C respectively. The amount of
carbonized residues (char yield) at 750 oC in nitrogen for all PIs was in the range of 66 – 78 wt %.
The high char yields of these polyamides can be attributed to their high aromatic content. The
lowest Tds value of Pc could be explained in terms of the ether bond segment in its backbone. Thus,
the thermal analysis results revealed that these PIs exhibited excellent thermal stability, which in
turn is beneficial to increase the service time in device application and enhance the morphological
stability to the spin-coated film.
Table 2 Optical and electrochemical properties for PIs
Fig. 1 Uv-visible absorption spectra of PIs (films)
3.4 Optical and electrochemistry properties
The optical properties of the TPA-based polymers P1 – P5 were investigated by UV-vis and
photoluminescence (PL) spectroscopy, and the relevant data are presented in Table 2. PL quantum
yields φs of the samples in different solvents were measured by using quinine sulfate dissolved in
0.5 M sulfuric acids as reference standard (φ =0.546). The φs of these PIs after refractive index
correction can be calculated according to the following eq[23]:





2
Iunk Astd ηunk





φunk= φstd

Istd Aunk η std




where φ unk
,
φ
std, Iunk, Istd, Aunk, Astd,
η
unk, and
η
are the fluorescent quantum yield,
std
integration of the emission intensity, absorbance at the excitation wavelength, and the refractive
indices of the corresponding solutions for the samples and the standard, respectively. Here, we use
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the refractive indices of the pure solvents as those of the solutions. Their PL spectra in DMF
solution showed emission maximum around 504–515 nm (in the yellow-green region), with very
low fluorescence quantum yields. These PIs exhibited strong UV–vis absorption bands in the
range of 319 – 336 nm for solid film shown in Fig. 1, assignable to the π-π* transitions resulting
from the conjugated TPA segment. Optical band gaps (Egs) determined from the absorption edge
of the solid-state spectra of polymers are found to be 2.77 – 3.00 eV.
The redox behaviors of the PIs were investigated by cyclic voltammetry (CV) conducted for
the cast film on an ITO-coated glass substrate as working electrode in anhydrous CH3CN
containing 0.1 M of LiClO4 as an electrolyte under nitrogen atmosphere for oxidation and
reduction measurements, respectively. The results are summarized in table 2. Fig. 2 shows
representative CV curves for P1 recorded at a scan rate of 50 mV/s. The CV diagram indicates two
well-defined redox waves, with half-wave potentials (E1/2) of 0.54 and 0.91 V. The first peak, at
0.80 V, can be ascribed to oxidation of the electron-rich nitrogen atom in the TPA core. The
second peak, at 1.17 V, can be attributed to the formation of the TPA2+ dication, causing radical
recombination and formation of a quinoid structure. Because of the stability of the films and good
adhesion between the polymer and ITO substrate, the P1 exhibited excellent reversibility of
electrochromic characteristics by five continuous cyclic scans between 0.0 and 1.60 V, changing
color from original colourless to yellow-green. A similar spectral change was observed for other
PIs (see the Fig. S3, supporting information).
The energy levels of the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of the investigated polymers can be determined from the
oxidation onset potentials (Eonset) or half-wave potentials (E1/2) and the onset absorption
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wavelengths, and the results are listed in Table 2. In the voltammograms, the redox peaks have
been associated with reduction–oxidation process accompanying the double injection/extraction of
-
ClO4 ions and electrons. To obtain accurate redox potentials, the reference electrode was
calibrated by the ferrocene/ ferrocenium (Fc/Fc+), whose redox potential is assumed to have an
absolute energy level of -4.80 eV to vacuum. As a result, the HOMO energy values were
calculated using the equation [24].
EHOMO= -e(Eox vs Ag/AgCl + 4.35)eV
where Eox is the onset oxidation potential vs Ag/AgCl.
The LUMO energy levels of the polymers are estimated from the HOMO energy levels and Eg
using the equation
ELUMO= EHOMO+Egopt
Eg(eV)=1240/λonset
The HOMO energy values of P2 and P5 were calculated to be in the range from -4.759 to
-4.801 eV. Due to the reduction curves could hardly be obtained, ELUMOs are at between -1.758
(Pc) to -2.00282 (Pa) eV. In the PIs containing the same amine monomer, P5 showed the highest
HOMO, and the P2 exhibited the lowest HOMO. The rule is the same as that of Uv-visible
absorption. The different dianhydrides play the key role in the electron structure and regulate the
Eg of PIs. The lower ionization potential (IP, about 4.7eV) could suggest an easier hole injection
into films from ITO electrodes (4.7eV) in electronic device applications.
Fig. 2 Repetitive CV scanning of the P1 film on the ITO/glass electrode in 0.1 M LiClO4/MeCN
solution over the potential range from 0 to 1.6 V at a scan rate of 50 mV s-1.
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Fig. 3 Electrochromic behavior of P4 thin film (in CH3CN with 0.1 M LiClO4 as the supporting
electrolyte) at 0.0, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3 (V vs. Ag/AgCl couple as reference).
The inset shows the photographic images of the film at indicated applied voltages.
3.5 Electrochromic properties
The spectroelectrochemistry characteristics of the polymer P4 are also shown in Figure 3. In
the neutral form, at 0 V the film exhibited strong absorption at wavelength around 335 nm,
characteristic for triarylamine, but it was almost transparent in the visible region. Upon oxidation
of the P4 film (increasing applied voltage from 0 to 1.0 V), the intensity of the absorption peak at
335 nm gradually decreased while a new peak at 425 nm and a broad band with maximum
absorption wavelength at 1086 nm in the NIR region gradually increased in intensity. We attribute
the spectral change in visible range to the formation of a stable monocation radical of the TPA
center in diamine 3 moiety. Furthermore, the broad absorption in NIR region was the
characteristic result owing to IV-CTꢁleading an intervalence charge transferꢂ excitation
associated with ETꢁintramolecular electron transferꢂfrom active neutral nitrogen atom to the
cation radical nitrogen center of diamine 3 moiety, which was consistent with the phenomenon
classified by Robin and Day [25]. As the applied potential became more anodic to 1.3 V, the
absorption bands of the cation radical decreased gradually in intensity, with the formation of a
new broadband centered at 832 nm. The disappearance of NIR absorption band can be attributable
to the formation of dication by the further oxidation of monocation radical species in the diamine
3 segments. The observed UV-vis absorption changes in the film of P4 at various potentials are
fully reversible and are associated with strong color changes. Indeed, they even can be seen
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readily by the naked eye. The other PIs showed similar spectral changes to that of P4 (see the Fig
S4, supporting information).
The stability, response time, and color efficiency are the key parameters for an electroactive
polymer film to be amenable for usage in optical and electrochromic devices, thus the
electrochromic switching studies were further measured. The color switching times were
estimated by applying a potential step, and the absorbance profiles were followed. The switching
time was calculated at 90% of the full switch because it is difficult to perceive any further color
change with naked eye beyond this point [26]. The polymer switched rapidly between the highly
transmissive neutral state and the colored oxidized state. As depicted in Fig. 4, P4 film revealed
switching time of 1.76 s at 1.1V for coloring process at 425 nm and 1.90 s for bleaching. After
continuous cyclic scans between 0.0 V and 1.10 V in 100 s, the polymer films still exhibited
excellent stability of electrochromic characteristics, indicating that the film was very stable and
had good adhesion with the ITO substrate.
After over continuous 190 cyclic switches between 0.00 and 1.10 V, the polymer films still
exhibited good stability of electrochromic characteristics (see Fig. 5 and Fig. S5). Because of the
apparent high electrochromic contrast, optical switching studies were investigated more deeply to
manifest the outstanding electrochromic characteristics of these obtained novel anodically
electrochromic materials.
The electrochromic coloration efficiency (CE; η) is also an important characteristic for the
electrochromic materials. CE can be calculated using the equations and given below [27]:
δOD =lg(Tb/Tc)
η=δOD/Q
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where Tb and Tc are the transmittances before and after coloration, respectively, δOD is the change
of the optical density, which is proportional to the amount of created color centers. η denotes the
coloration efficiency(CE) and Q (mC / cm2) is the amount of injected/ejected charge per unit
sample area. CE of P4 film was measured as 111.8 cm2/C. (at 425 nm) at full doped state, which
had reasonable coloration efficiency (the others were listed in table 3).
Table 3. Optical and Electrochemical Data Collected for Coloration Efficiency Measurements of
PIs
Fig. 4 Current consumption (upper) and potential step absorptometry (lower) of P4 (at 425 nm) (in
0.1 M LiClO4 / CH3CN as the supporting electrolyte) by applying a potential step (0.00↔1.1 V)
with a cycle time of 8 s.
Fig. 5 Current consumption between 0.00 and 1.0 V (vs. Ag/AgCl) of polymer P4 thin film on the
ITO-coated glass substrate in a 0.1 M LiClO4/CH3CN solution with a cycle time of 8 s.
3.6 Photoelectrochemical activity
To clarity the flow trend and mechanism of photo-generated electrons, the photocurrent and
photo-voltage properties of PIs were investigated.
A steady anodic photocurrent was obtained from the dye monolayer-modified electrode when
the PI-ITO electrode was illuminated under white light of 150 mWcm-2 intensity without any bias
voltage in 0.1 M LiClO4 electrolyte solution (Fig. 6a). A reversible rise/decay of the photocurrent
was observed when switching on and off many times, as shown in Fig. 6. For comparison, the bare
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ITO did not exhibit the phenomena suggesting that the PI was responsible for photocurrent
generation.
Fig. 6b showed typical open-circuit photovoltaic response of electrode on illumination. After
the light was turned off, the photovoltage decayed to lowest slowly. On the contrast, after the light
was turned on, the photovoltage increased instantly. A similar characteristic was observed for
other PIs of this series. Thus, these PIs can act as the photoelectric conversion material for
optoelectronic applications.
The observation result can be explained by that there is charge separation and electron
transfer in the interface of PI film and electrolyte solution under illumination. The electrons absorb
energy from light and jump onto LUMO, followed by the injection of electrons from the LUMO
(-1.85 eV) of P1 to the conduction band of ITO surface (-4.7 eV), then the interparticulate hopping
of electrons generates photocurrent and photo-voltage. It is to be noted that there is no sacrifice
-
regent in the electrolyte such as I3 /I- ion couple, which can result in the photocurrent and
photo-voltage decaying with on/off illumination cycles procedure.
Fig. 6 A typical photocurrent (A) and photovoltage (B) response for P4 immobilized ITO glass
upon exposure to light at room temperature.
3.7 Quantum chemistry calculation
To better understand the evolution of the oxidation and reduction potentials, the electronic
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structures of the polymer series (as isolated oligomers) were evaluated by using DFT at the
B3LYP/6-31G level of theory. The HOMO energy, LUMO energy and HOMO-LUMO energy
gaps (Eg) are given in Table 2. Pictorial representations of the frontier molecular orbitals of
polymer series are shown in Fig. 7, which illustrate that the electron density distribution of the
HOMO state of the basic unit (ground state) was mainly located on diamine moiety, and that of
the LUMO state of the basic unit was mainly located on the dianhydride group of PI. The
electronic density contours in Fig. 7 obviously show that the lone pair of electron on the nitrogen
atoms have little coupling with π electrons.
Fig. 7 Pictorial representations of the electron density in the frontier molecular orbitals of
repetition units for P1 to P5 in the order.
The trend of Eg for the oligomer series follows in the order: P4< P1 < P5 < P2 < P3 which
basically corresponds with the experimental data in the order: P1 ≈ P4 < P2 ≈P5 < P3 (Table 2).
The bias may be due to the adjacent ring which plays extra interaction on the energies. It should be
noted that the calculated results are based on monomer unit. In fact, the experiment data are
obtained from polymer which are influenced by the solvent and electrolyte and more complicated.
4. Conclusion
The triarylamine-based aromatic diamine monomer, N, N′- bis( 4- aminophenyl) -N, N′- di-
2- naphthalenyl-1, 4- benzenediamine was synthesized in high purity and high yields from readily
available reagents. The novel aromatic PIs containing pendent triarylamine groups were prepared
by the two- step method starting from diamine with various aromatic tetracarboxylic dianhydrides.
All of them exhibit good thermal stability. The multicolor electrochromic behavior of the PIs film
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of P1- P5 exhibit from original pale yellowish to green and then to blue color when various
potentials were applied. Thus, our novel PIs have a great potential as a new type of
hole-transporting and electrochromic materials due to their proper HOMO values and excellent
electrochemical and thermal stability.
Acknowledgement
The authors are grateful to the support of the National Science Foundation of China (Grant No.﹣
51373049ꢃ51303045ꢃ21273056ꢃ21372067), Doctoral Fund of Ministry of Education of Chinaꢃ
Foundation of Heilongjiang Education Bureau (12531504), Foundation of Heilongjiang University
Innovator Group (No. hdtd-2010-11).
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Figures Captions
Table 1 Thermal properties and inherent viscosity of the PIs
Table 2 Optical and electrochemical properties for PIs
Table 3. Optical and electrochemical data collected for coloration efficiency measurements of PIs
Fig. 1 Uv-visible absorption spectra of PIs (film)
Fig. 2 Cyclic voltammograms for P1 in CH3CN/0.1 M LiClO4 with ferrocene/ferrocenium (Fc/Fc+)
as an internal standard, at 50 mV s-1.
Fig. 3 Electrochromic behavior of P1 thin film (in CH3CN with 0.1 M LiClO4 as the supporting
electrolyte) at 0.0, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 (V vs. Ag/AgCl couple as
reference). The inset shows the photographic images of the film at indicated applied voltages.
Fig. 4 Current consumption (upper) and potential step absorptometry (lower) of P4 (at 425 nm) (in
0.1 M LiClO4 / CH3CN as the supporting electrolyte) by applying a potential step (0.00↔1.1 V)
with a cycle time of 8 s.
Fig. 5 Current consumption between 0.00 and 1.0 V (vs. Ag/AgCl) of polymer P4 thin film on the
ITO-coated glass substrate in a 0.1 M LiClO4/CH3CN solution with a cycle time of 8 s.
Fig. 6 A typical photocurrent (A) and photovoltage (B) response for P3 immobilized ITO glass
upon exposure to light at room temperature.
Fig. 7 Pictorial representations of the electron density in the frontier molecular orbitals of
repetition units for P1 to P5 in the order.
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Table 1 Thermal properties and inherent viscosity of the PIs
Char
Yield(% )b
Poly(amic acid)
c
c
c
T5%
T20%
T50%
η(dL/g)a
P1
P2
P3
P4
P5
580
576
479
528
549
632
628
593
688
637
>800
>800
>800
>800
>800
68
67
66
78
74
0.74
0.78
0.82
0.73
0.76
a Measured at a polymer concentration of 0.5 g/dL in DMF at 30 oC.
b Measured at 750 oC in nitrogen.
c The decomposition temperatures (Tds ) at 5 %, 20 % and 50 % weight losses in nitrogen.

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Table 2 Optical and electrochemical properties for PIs
abs
film
PL
solution
abs
onset
peak
onset
electro
HOMO
electro
LUMO
quntum
HOMO
quntum
λ
φ c
E peak
Egfilm
Egquntum
λ
λ
E
E
E
E
E
LUMOH
(DMF)
vs
d
(nm)a
319.6
343.9
336.1
335.6
330
(nm)a
504
506
518
511
(nm)b
447.3
427.7
412.5
424.8
421.0
%
(ev) f
(ev) f
(ev) e
2.772
2.899
3.006
2.919
2.945
(ev) g
-5.109
-5.031
-5.039
-5.042
-5.047
(ev)g
(ev) g
1.387
1.688
1.986
1.328
1.630
Ag/AgCl
vs Ag/AgCl
0.424
0.409
0.413
0.422
0.451
P1
P2
P3
P4
P5
0.0294
0.0546
0.1985
0.1683
0.3566
0.547
0.534
0.552
0.540
0.528
-4.774
-4.759
-4.763
-4.772
-4.801
-2.002
-1.860
-1.758
-1.853
-1.856
-3.721
-3.343
-3.053
-3.714
-3.417
515
a UV-vis absorption and PL spectra measurements in DMF at room temperature.
b λonset of the polymer film.
c The quantum yield was calculated with quinine sulfate as the standard (φ=54.6 %).
d E1/2: average potential of the redox couple peaks
e Egap = 1240 /λonset
f The HOMO energy levels were calculated from cyclic voltammetry and were referenced to
ferrocene (4.8 eV).
ELUMO =EHOMO - Eg; Eox/onset (Fc/Fc vs. Ag/AgCl) = 0.45 V.
g Theoretical calculation of the polymers

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Table 3 Optical and Electrochemical Data Collected for Coloration Efficiency Measurements of
PIs
c
Polymer codea
λ (nm)b
δOD
Q (mC/ cm2)d
η(cm2/ C)e
P1
P2
P3
P4
P5
428
430
428
425
425
0.7907
0.7914
0.9193
0.7990
1.0538
15.24
2.037
6.9
51.83
388.2
133.23
111.8
150.9
8.6
6.89
a Switching between 0.0 and 1.10 V for P1–P5 (V vs. Ag/AgCl).
b The given wavelength where the data were determined.
c Optical density change at the given wavelength.
d Injected charge, determined from the in situ experiments.
e Coloration efficiency is derived from the equation η=δOD/Q.

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