Simple approach to regulate the spectra of novel kinds of polyazomethines containing bulky triphenylamine: Electrochemistry, electrochromism and photophysical responsive to environment

Article abstract of DOI:10.1016/j.dyepig.2012.08.003

Two series of novel polyazomethines (PAMs) PIa-PIe, PIIa-PIIe were prepared via the direct polycondensation from N,N′-bis(4-aminophenyl)-N,N′- diphenyl-1,4-phenylenediamine, and N,N′-bis(4-aminophenyl)-N,N′- diphenyl-4,4′-biphenyldiamine with various dica

Full text of DOI:10.1016/j.dyepig.2012.08.003

Dyes and Pigments 96 (2013) 158e169
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Simple approach to regulate the spectra of novel kinds of polyazomethines
containing bulky triphenylamine: Electrochemistry, electrochromism and
photophysical responsive to environment
a
,
b
,
, Jiwei Cai , Ping Zhao , Cheng Wang , Xuduo Bai ,*, Wen Wang c
a
b
a
a
*
Haijun Niu
a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Department of Macromolecular Science and Engineering, Heilongjiang University,
Harbin 150086, PR China
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
b
c
School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150080, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 January 2012
Received in revised form
Two series of novel polyazomethines (PAMs) PIaePIe, PIIaePIIe were prepared via the direct poly-
0
0
0
condensation from N,N -bis(4-aminophenyl)-N,N -diphenyl-1,4-phenylenediamine, and N,N -bis(4-
0
0
aminophenyl)-N,N -diphenyl-4,4 -biphenyldiamine with various dicarboxaldehyde such as phthaldial-
30 April 2012
0
dehyde, 1,3-isophthalaldehyde, terephthalaldehyde, 4,4 -diformyltriphenylamine, 5-thiophene-
Accepted 1 August 2012
Available online 11 August 2012
dialdehyde. The variation of the backbone structure significantly affected the dihedral angles and
changed the electronic properties of PAMs. The obtained PAMs exhibited stable and reversible electro-
chromic properties and reversible color change. The color of PAMs could be varied from initial yellowish
to red, violet or blue via electrooxidation. And multiple reversible colors states were also observed with
pH and chemical doping. Considering the sensitive chromic properties of PAMs, they will have potential
applications for display device, chemical sensor, et al.
Keywords:
Polyazomethine
Electrochromic
Electrochemical device
Chemical sensor
Functional dyes
Triphenylamine
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
catalysts, expensive monomers, and stringent conditions. Addi-
tionally, PAM has high thermal stability. Furthermore, the end
Polyazomethine (PAM) is a kind of conductive polymer, which
contains C]N bridges in the main chain. CH]N is similar to
carbonecarbon double bonds with isoelectronic characteristics,
planar configurations and optical behaviors [1]. With the proton-
capture ability and the coordination ability, PAM would be the
suitable alterative to conjugated polymers for amount of electronic
applications, including pH and ion sensors [2], molecular wires [3],
organic light-emitting diodes [4], non-linear optical devices [5],
and polymer photovoltaics [6].
It is convenient to synthesize PAM via the polycondensation of
amine and aldehyde with high yields. The polymers can be easily
purified with Soxhlet apparatus extraction and vacuum drying. In
comparison, the synthesis of poly(1,4-phenylene vinylene)s (PPV),
polythiophene (PTh), polyfluorenes (PF) should involve the noble
capped active groups at the ends of PAM, such as aldehyde and
amine, can be functionalized easily and purposefully. Thus, the PAM
seems superior to other conductive polymer [7].
However, PAM also suffers from some drawback, especially poor
solubility in common organic solvents. This makes the characteris-
tics, processing and applications much difficult. At present, there are
three methodsmainly used toimprove theprocessabilityofPAM. The
first one is to introduce long alkyls, or aryloxy groups on the main-
chain aromatic benzene ring [8]. But the flexible chains often influ-
ence the conjugation, and reduce the thermal stability, crystallinity
and interchain conduction. The second one is based on the reversible
Lewis acid complexation [9] or polymerization in the cyclodextrin
cavity [10]. The third one is to insert bulky, packing disruptive groups
into the polymer backbone without sacrificing high thermal stability
[
11]. In this method, triphenylamine derivatives with the bulky
*
Corresponding authors. Key Laboratory of Functional Inorganic Material
propeller-shaped structure are used to modify the solubility and
decrease intermolecular aggregation dramatically [12].
The triphenylamine (TPA) unit is easy to be oxidized in the nitrogen
center and has the excellent ability to transport carriers via the radical
cation species [13]. The compounds containing triarylamine are widely
Chemistry, Ministry of Education, Department of Macromolecular Science and
Engineering, Heilongjiang University, Nangang District, Xuefu Road 74, Harbin
50086, PR China. Tel.: þ86 13684501571; fax: þ86 0451 86608131.
E-mail addresses: haijunniu@hotmail.com (H. Niu), xuduobai@hotmail.com
X. Bai).
1
(
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.08.003

H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
159
0
investigated and applied as various opto-electronic materials such as
organic light-emitting diodes (OLEDs) [14], organic field-effect tran-
sistors [15], non-linear optical materials [16], and sensors [17], dye-
sensitized solar cells (DSSCs) [18]. From 2005, Liou et al. have
investigated deeply on a series of TPA-based high-performance poly-
imides, polyamides, polyazomethines with attractive electrochromic
C) were purchased from Acros, Aldrich. 4,4 -Diformyltriphenyl-
0
amine (4,4 -(phenylimino) bisbenzaldehyde) was prepared by the
Vilsmeier reaction according to the method described in the liter-
ature we had reported [21,26]. The resulting yellow solid was
a silica gel column (ethyl acetate/n-
hexane 1/3). N,N-diethylformamide (DMAc), 1-methyl-2-
chromatographed on
¼
0
properties bearing TPA derivatives such as N,N -bis(4-aminophenyl)-
pyrrolidone (NMP), hexamethylene phosphoramide (HMPA) were
used after standard drying method. 4-fluoronitrobenzene, triphe-
0
0
0
N,N -diphenyl-1,4-phenylenediamine [19], N,N -bis(4-aminophenyl)-
0
N,N -diphenyl-1,4-phenylenediamine, and other substituted TPAs
nylamine (C18
zenediamine (C18
H
15N) (Wujin Corporation), N,N -diphenyl-1,4-ben-
0
0
[12]. All poly(amino imide)s exhibited excellent electrochromic
16
H N
2
), N,N -diphenyl-4,4 -biphenyldiamine
0
stability. When scanning potentials from 0 V to 1.35 V, their color was
changed from the initial pale-yellowish at neutral form, to green, then
to final blue at oxidized form gradually. It was found that the polymer
with TPA cores exhibited stable electrochromic characteristic, and the
incorporation of electron-donating substituents at the para positions
of triarylamines afforded stable radical cations.
(C24
H
20
N
2
) (TCI) were used as received. N,N -bis(4-aminophenyl)-
0
0
N,N -diphenyl-1,4-phenylenediamine,
N,N -bis(4-nitrophenyl)-
0
0
0
0
N,N -diphenyl-1,1 -biphenyl-4,4 -diamine (C36
H
26
N
4
O
4
) and N,N -
0
0
bis(4-aminophenyl)-N,N -diphenyl-4,4 -biphenyldiamine
) were synthesized according to the reference [19].
30 4
(C36H N
The nitrogen atom in imine group (C]N) in the backbone of
PAM, has a lone pair electrons. It can form intra or inter molecular
interactions, conduct protonation and complexation with metal
ions, acid and iodine [1,20]. Therefore, PAM possesses novel optical
properties. The optical absorption of PAM can be modulated by
exerting potential or chemical environment.
2.2. Synthesis of monomers
0
0
Synthesis of N,N -bis(4-aminophenyl)-N,N -diphenyl-1,4-phe-
0
0
nylenediamine (denoted as 3), N,N -bis(4-aminophenyl)-N,N -
0
0
0
diphenyl-4,4 -biphenyldiamine (denoted as 3 ) and 4,4 -diformyl-
triphenylamine according to the literature [19,21,26] was con-
ducted as Scheme 1.
Recently, we have reported the synthesis of soluble aromatic
PAMs bearing TPA units in the main chain through condensation of
diamine and dialdehyde substituted TPA [21]. At the same time,
Agnieszka Iwan et al. have reported a series of similar oligomers
2.3. Synthesis of polymers
0
and polymers based on 4,4 -diformyltriphenylamine with inter-
The synthesis of PId was used as an example to illustrate the
general synthetic procedure. To a three necked 50 ml glass reactor
fitted with a magnetic stirrer, Dean-stark trap and reflux condenser
esting properties, such as liquid crystal behavior and photo-
response [22]. Distinct hypsochromic shift of UVevis absorption
was observed in comparison to the pristine ones. This result is
contrary to that obtained by Jenekhe [23] and us [21].
ꢁ3
0
was charged with 0.221
g
(0.5 ꢀ 10
mol) N,N -bis(4-
0
aminophenyl)-N,N -diphenyl-1,4-phenylenediamine, and 10 ml
DMAc, 2 ml of toluene. To the stirred solution was added 0.1505 g
Owing to the outstanding stability and optical properties, the
thiophene derivatives have attracted intensive attention. Silvia
Destri’s group has designed and studied a great deal of electro-
chromic materials made from thiophene derivatives [24]. Skene has
also synthesized a series of polythiophene azomethines with low
optical band gaps (1.3e2.0 eV), and all polymers were claimed to be
chemically and photochemically stable [25]. And the introduction
of thiophene into polymer increased the overall conjugation and
improved the opto-electronic properties. Azomethines are suitable
for functional materials due to reversible acid doping, reoxidation
and electrochromic properties.
Biphenyl is a better block in the backbone with more conjugated
degree in comparison with benzene. Thus, we designed two series
of PAMs with the aim to investigate the influence of different
diamines on the properties of PAMs. It is well known that the 1,3-
isophthalaldehyde, terephthalaldehyde, phthaldialdehyde with
the different substitution positions on the benzene ring have the
various conjugation degree. Herein, we present a new, facile, low-
cast and noble metal catalyst-free approach to synthesize TPA-
based alternating copolyazomethine massively. By changing the
substitution patterns of dialdehyde units, the color of the resulting
PAMs was controlled precisely. In order to investigate the influence
of the monomer unit and azomethine linkage on the photo-
ꢁ
3
0
(0.5 ꢀ 10 mol) 4,4 -diformyltriphenylamine three times every
ꢂ
20 min, and the solution was heated at 160 C for 17 h to remove
water under nitrogen reflux. After the reaction, the obtained
polymer solution was poured slowly into 400 ml of ice frozen
methanol. The precipitate was collected by filtration, washed
thoroughly with hot methanol in a Soxhlet apparatus for 48 h, then
was dried under vacuum at 70 C overnight. Reprecipitations of the
polymer by NMP/methanol were carried out twice for further
ꢂ
purification. Yield: 0.2771 g, 82.6%, yellow.
ꢁ1
IR (KBr, cm ): 3434 (NeH stretching), 1694 (HeC]O), 1629
(CH]N), 1592, 1502, 1490 (aromatic ring of benzene), 1313, 1267
(CeN), 1167 (CeC), 829 (p-substituted benzene), 695 (single
substituted benzene).
1
3
H NMR (400 MHz, CDCl , ppm): 8.42 (d, eCH]Ne), 7.82e7.72
(m, aromatic ring of benzeneeCH]Ne), 7.33e7.0 (m, aromatic ring
of triphenylamine).
The other polymers named as Scheme 2 which were prepared
and characterized by an analogous procedure. For comparison, we
also prepared the series of polymers with HPMA using as solvent
catalyst according to the method of the literature described in the
Supporting Information.
electrochemical properties of PAMs, 2,5-thiophenedialdehyde and
2.4. Measurements
0
4
,4 -diformyltriphenylamine were also used as pristine materials.
The results indicated that PAMs have potential application in
chemical sensor and electrochromic devices.
Fourier transform infrared (FT-IR) spectra were recorded on
a PerkinElmer Spectrum 100 Model FT-IR spectrometer. H NMR
1
spectra were measured on a Bruker AC-400 MHz spectrometer in
2
. Experimental section
3
CDCl , using tetramethylsilane as an internal reference. Gel
permeation chromatographic (GPC) analysis was performed on
a Waters instrument connected with one refractive index detector.
All GPC analyses were performed using a polymer/THF solution at
2
.1. Materials
ꢂ
Terephthalaldehyde, 1,3-isophthalaldehyde, phthaldialdehyde,
a flow rate of 1 ml/min at 70 C and calibrated with polystyrene
2
,5-thiophenedialdehyde, 5% palladium on activated carbon (Pde
standards. Thermogravimetric analysis (TGA) was conducted with

160
H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
N
N
H
N
NH
NaH
NO2
NO2
+
F
NO2
1
DMSO
2
H
N
H
N
N
N
1
'
NO2
NO2
2
'
N
N
H 2
,
C
/
d
P
F
M
NH2
NH2
D
3
N
N
NH2
NH2
3
'
Scheme 1. Synthesis of two monomers.
a PerkinElmer Pyris 6 TGA. Experiments were carried out on
geometry with Cu K
a
radiation (
l
¼ 0.154 nm). UVevis absorption
approximately 6e8 mg powder samples heated in flowing nitrogen
spectra were recorded using an SHIMADZU UV-3600 spectropho-
tometer whereas the photoluminescence solution spectra were
registered on a Jasco FP-6200 spectrometer with 450 W xenon
lamp as the light source. The emission spectra of polyazomethines
3
ꢂ
(
flow rate ¼ 20 cm /min) at a heating rate of 20 C/min. DSC
analyses were performed on a PerkinElmer Pyris diamond DSC at
a scan rate of 10 C/min in flowing nitrogen (20 cm /min). X-ray
ꢂ
3
diffraction patterns were recorded using powder samples on
were taken at
lexc being equal to about the wavelength of the
a wide-angle D/max-gB diffractometer working in typical Bragg
absorption maximum.
Scheme 2. Synthesis of two series of polymers.

H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
161
Cyclic voltammetry (CV) measurements were conducted on
a CH Instruments 660 A electrochemical analyzer at a scan rate of
2 3
K CO [28]. But it needed high reaction temperature and long
reaction time over 10 h, and the reaction yields were low. So we
ꢁ1
5
0 mV s with a 0.1 M solution of LiClO
4
as an electrolyte under
CN). The oxidation and
therefore modified the procedure by using the strong base, e.g.
0 0
NaH, which gave higher yields of N,N -bis(4-nitrophenyl)-N,N -
nitrogen atmosphere in dry acetonitrile (CH
3
0
0
0
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 onsets of oxidation and reduction potentials of the
polymer thin films against the Ag/AgCl quasi reference electrode
diphenyl-1,1 -biphenyl-4,4 -diamine (C36
H
26
N
4
O
4
) and N,N -bis(4-
0
0
aminophenyl)-N,N -diphenyl-4,4 -biphenyldiamine and less
mono-substituted side product under moderate temperature.
0
Reduction of the dinitro intermediate 2 and 2 was carried out using
10% Pd/C in DMF at 200 rpm under 5 MPa of hydrogen pressure and
0
0
were measured and calibrated against the ferrocene/ferrocenium
resulted in a higher yield of N,N -bis(4-aminophenyl)-N,N -
þ
0
(
Fc/Fc ) redox couple. The HOMO and LUMO levels of the polymers
diphenyl-1,4-phenylenediamine (3), and N,N -bis(4-aminophenyl)-
N,N -diphenyl-4,4 -biphenyldiamine (C36
terization of the dinitro intermediate 2 and 2 and target diamine 3
and 3 with FT-IR and H NMR spectra were presented in
Supporting Information. The diamine 3 has been reported in Ref.
þ
0
0
0
were calculated by assuming the absolute energy level of Fc/Fc
as ꢁ4.80 eV to vacuum.
H N ) (3 ). The charac-
30 4
0
0
1
The morphology observation of the samples was carried out on
atom force microscopy (AFM, Nanoscope IIIa digital instrument,
VECCO Co.) equipped with a silicon cantilever (typical spring
0
[29], but it was obtained by the condensation of 4-
ꢁ
1
0
constant 40 N m ) in tapping mode under ambient conditions.
nitrodiphenylamine
and
4,4 -dibromobiphenyl with
the
Photographs of the polymer films were taken by using a Panisonic
palladium-catalyzed amination following by catalytic reduction of
the dinitro compound to the diamino compound by using hydra-
zine monohydrate and Pd/C catalyst. In order to avoid using the
toxic raw materials to the environment, we chose the common
approach to synthesize with high purity and yield.
(
DMC-LZ2) digital camera.
Density-functional theory (DFT) calculations were performed on
a computer. Geometry optimizations were carried out using the
B3LYP functional as implemented in Gaussian 98 program [27].
In FT-IR spectrum of monomer 3 (Fig. S1), NeH stretching
ꢁ
1
ꢁ1
2
.4.1. Synthesis of iodine-doped PAMs
The powders of the polymers were kept in a closed chamber
(w3400 cm ), aromatic CeC stretching (1624/1591 cm ), and
ꢁ
1
aromatic CeNeC vibration (1259 cm ) appeared, while NO (1550,
2
ꢁ
1
1
filled with iodine crystals at ambient temperature for 36 h. The
original orange or red film all turned to dark red.
1350 cm ) vibrations were recognized. It was also confirmed by H
NMR spectra. Five peaks were successfully assigned according to
the electron-donating effect of triphenylamine and amines units
(Fig. S2). Element analysis provided well matched chemical
composition to the theoretical values, while sharp melting behavior
of the monomers also demonstrated their high purity. The mono-
mer 3 was also synthesized via the same synthesis route of the
monomer 3, except for the diamine reactant of N,N -diphenyl-4,4 -
biphenyldiamine (C18
crystals with overall yield of 48% (mp: 245e248 C) and was
characterized by FT-IR and H NMR spectra (shown in Fig. S3eS5).
The resulting compound is in agreement with Ref. [29].
The FT-IR and H NMR spectra (seen from Fig. S6) verified that
4,4 -diformyl-triphenylamine was synthesized successfully. There
is one new peak appeared at chemical shift of 10.1 which assigned
to H in formyl.
2.4.2. Synthesis of HCl-doped PAMs
The powders of the polymers were kept in a closed chamber
filled with HCl vapor at ambient temperature for 36 h. The original
orange or red film all turned to black.
0
0
0
0
2
3
.4.3. Synthesis of FeCl -doped PAMs
H
16
N
2
). 3 was grayish white needle type
ꢂ
Some FeCl powders were put into the solution (CHCl
3
3
) of
1
polymers and the mixtures were shaken for several minutes to
make the polymers change colors.
1
0
2
.4.4. Synthesis of UV doped PAMs
The solutions (in CHCl ) of PAMs were exposed under the UV
lamp with the power of 15 W at the excitation wavelength of
3
2
57 nm.
2.5. Device fabrication and test
3.2. Synthesis of polymers
The solutions of polymers in CHCl
3
were spin-coated onto
The TPA-based PAMs were prepared from dialdehyde and
diamines using a simple one-step catalyst-free and solution poly-
condensation procedure under N atmosphere conditions in high-
2
boiling point and high temperature. No attempts were under-
taken to end-cap the polymer chains or optimize the polymeriza-
indium tin oxide (ITO) electrodes laminated on ITO-coated glass
substrate (15e25 /square, ShenZhen Nanbo Technologies Ltd.) to
form electrochromic layers. The electrolyte was prepared by adding
LiClO in dry acetonitrile. Electrochromic devices with a sandwich
U
4
structure of ITO/electrochromic polymer/electrolyte/ITO were then
fabricated under atmospheric conditions.
2
tion conditions with anhydrous LiCl or CaCl as a water scavenger.
The bulk TPA reduced the rigid-rod conformation to improve the
processability of PAMs. The yield of the polymers was in the range
of 45e87%. All the PAMs could afford transparent films with good
optical quality via solution casting. Structures of the PAMs were
3
. Results and discussion
1
3.1. Synthesis of monomers
confirmed by FTIR and H NMR (shown in Fig. 1, Fig. 2, Fig. S7, and
ꢁ1
Fig. S8). The strong absorption peaks at about 1611e1648 cm are
attributed to the vibration of bond of C]N, while the chemical
shifts in the range of 7.49e8.65 are ascribed to the proton at the He
C]N. When the PIa was doped with electrochemical oxidation or
All monomers were synthesized using modified literature
0
0
procedures. We tried to synthesize N,N -bis(4-nitrophenyl)-N,N -
0
0
0
diphenyl-1,1 -biphenyl-4,4 -diamine (C36
H
26
N
4
O
4
) and N,N -bis(4-
by the
0
0
ꢁ1
ꢁ1
aminophenyl)-N,N -diphenyl-4,4 -biphenyldiamine
I
2
, C]N vibration absorption shifts from 1593 cm to 1566 cm
,
0
ꢁ1
aromatic nucleophilic amination of N,N -diphenyl-1,4-benzenedi-
and the absorption at 1593 cm became stronger in the case of
doping HCl. The lower wavenumber of the imine group absorption
indicates the better conjugation which suggested the formation of
the quinoid structure. When the PAM was doped with HCl, the
0
0
amine (C18H N ), N,N -diphenyl-4,4 -biphenyldiamine and 4-
16 2
fluoro-1-nitrobenzene or 4-chloride-1-1nitrobenzene under
nucleophilic displacement conditions using a base such as CsF or

162
H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
In order to have an insight into the packing of the polymers, all
polymers were investigated by WAXD. The WAXD (Fig. 3) patterns
of the PAMs powder indicated that there were some crystalline in
PIa and PIb due to partially orderly aggregation, but all the other
TPA-based PAMs were amorphous which displayed wide-angle
ꢂ
reflections with a maximum around 2
q
¼ 19.45 (Fig. 3 and
Fig. S10), corresponding to a lateral intermolecular spacing of
.456 nm. The amorphous nature can be attributed to the intro-
0
duction of bulky, twisted and three dimensional TPA units into the
polymer backbone.
Fig. 4 shows the tapping-mode AFM topographical images
(
500 ꢀ 500 nm) of the polyimines films. The films were casted from
ꢂ
CHCl
3
and annealed at 100 C to make the solvent evaporate. The
coatings were homogenous and porous with distinct phase sepa-
ration into islands. All the samples showed ordered linear and
terraces-like structures that the polymeric chains oriented with
different angles to each other. Roughness and root-mean-square
roughness (Ra, Rms) are determined to be 0.747 and 0.946 (PId),
2.809 and 2.400 (PIa), 1.047 and 0.841 (PIId), 0.777 and 0.969 (PIIa)
nm, respectively. The rough films of PI and PII could make ion of
electrolyte penetrate readily, and would improve the current in the
device (OLED or photovoltaic cell) in which the PAMs were applied.
These results are in accordance to UVevis spectra and XRD
measurements. The results suggest that the morphologies are
dependent on the molecule structure, and convolution of mole-
cules assembly during the vaporization of solvents. The molecular
configuration was also investigated by method of molecular
simulation which will be depicted in the following section.
Fig. 1. FT-IR spectra of PIa to PIe.
ꢁ
1
stretching vibration of 1593 cm indicated to the formation of C]
þ
NH (see the Fig. 10).
Two typical sets of polymerization conditions were investi-
gated: (1) condensation in refluxing toluene/DMAc, and (2) in the
mixed solvent of 1:3 HMPA/NMP. The LiCl was used as a drying
agent according to Morgan’s procedure [30]. Polymerization in
toluene/DMAc afforded homogeneous solution, which can be
precipitated when poured into the ice frozen methanol. However,
the polymer prepared in HMPA/NMP can not precipitate when
poured into the ice frozen methanol. This phenomenon indicated
different polymers were synthesized in two mixed solvents, as
3.4. Molecular weight and thermal properties
The molecular weight of PAMs was measured by GPC, using
1
proved by FT-IR, H NMR and UVevis spectra. Furthermore, the
polystyrenes as standard and THF as eluent. The weight-average
molecular weight (M ) and polydispersity (PDI) were determined
as 2797e16,828 and 1.42e2.39, respectively (Table S1). In general,
polymer obtained in HMPA/NMP did not show typical electro-
chromic and acidchromic phenomena. The reason was probably
attributed to the tight interaction between TPA and HMPA [31].
w
0
0
the
M
w
s
of PAMs containing N,N -bis(4-aminophenyl)-N,N -
0
diphenyl-4,4 -biphenyldiamine are larger than those of PAMs
3
.3. Polymer solubility and morphology
0
0
containing N,N -bis (4-aminophenyl)-N,N -diphenyl-1,4-phenylene
diamine which may be due to steric hindrance. Thus, the latter with
higher molecular weight is more insoluble than the former.
The rigidity, symmetry, and regularity of the backbones influ-
ence the chain packing ability and interchain interactions, and
therefore determine the solubility of polymers. The introduction of
TPA group increased solubility of the PAMs which were dissolved
well in common organic solvents, such as CHCl , DMSO, NMP,
3
DMAC, DMF, THF. It is facile for device fabrication by spin coating.
n
According to M data, the polymers are composed of about 5 repeat
units.
From the thermal properties of PAMs (see Table S1 and Fig. S9),
the series of PAMs did not show clear T , but possessed an enhanced
g
thermal stability. Owing to the decreased conformational flexibility
or free volume caused by the introduction of biphenyl groups, PII
Fig. 2. 1H NMR spectra of PIa to PIe.
Fig. 3. XRD curves of PIa to PIe.

H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
163
Fig. 4. AFM images of Id, IId (upper), Ia, IIa (below) in tapping mode of polyimines coated on silicon substrates at room temperature.
series are more thermally stable than PI series. All the polymers
polymerization, but PIc shows interesting property that its
l
is even
indicated no clear melting endotherms up to the decomposition
temperatures.
The weight loss below 400 C could be attributed to out-gassing
2
of residual monomer species and H O. The latter is considered to be
the result of a post-polycondensation process, for which the poly-
mer amine and aldehyde end groups react to give out water.
longer than that of PIa, owing to the prolonged conjugated
structure.
The maximum peak also depends on the polarity of solvent. As
shown in Fig. 5 and Fig. S11, the PI series of polymers exhibit
maximum UVevis absorption at 406e477 nm in NMP solutions but
ꢂ
show maximum UVevis absorption at 415e481 nm in CHCl
3
solutions. It indicated obvious negative solvatochromism. With
increasing the solvents polarity from chloroform to NMP, as well as
3.5. UVevis and photoluminescence
dipole moment (
m
¼ 1.01D for CHCl
3
,
m
¼ 4.09D for NMP,
m
¼ 1.61D
for m-Cresol), the peak located at the long wavelength shift blue in
NMP solution. The result is similar to the report by Sek [22], but
contrary to the positive solvatochromism result by Jenekhe [23],
this indicated the charge-transfer nature of the absorption band
and the ground state is more stable than the excited state of PAM
[32]. We suppose that the interesting phenomena may be also
induced by complexion of the proton in the certain solvent such as
CHCl , m-Cresol (see the curves in Fig S11) and phenol. Phenol is
3
much more acidic than aliphatic alcohols (pKa < 10) and in some
cases is capable of protonating polymer with basic centers [33].
The optical properties of the PAMs were investigated by UVevis
and photoluminescence spectroscopy. The results were summa-
rized in Table 1. These polymers exhibited maximum UVevis
absorption at 400e600 nm in CHCl
3
solutions as shown in Fig. 5
and Fig. 6, ascribed to the * transitions of the aromatic chro-
pep
mophores, C]N bonds and triarylamine units. The UVevis
absorption in film revealed little red shift by about 3 nm
compared to that in solution samples, which implied the slight
aggregation due to the propeller structure of TPA.
For series with the same monomer amine, the
maximum absorption peaks were presented in the order:
Photoluminescence (PL) quantum yields (
CHCl were measured by using quinine sulfate dissolved in 1 N
sulfuric acid as a reference standard (
std ¼ 0.546). Fig. 6 also shows
the PL spectra of PAMs measured in CHCl
funk) of the samples in
3
l
P
>
l
P
>
l
P
>
l
P
> , as the author expected, which indicating
l
f
e
a
c
d
P
b
ꢁ
6
P
b
had the shortest conjugated structure and P
e
with S donor atom
3
(Conc.: 10 mol/L). The
owed the best conjugated form. It is worth to be noted that
phthaldialdehyde is not usually used as monomer in
quantum yields of these polymers can be calculated according to
the following Equation [21]:

164
H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
Table 1
Optical and electrochemical properties for polyazomethines.
max
abs
max
abs
max
abs
max
PL
onset
abs
onset
peak
electro
HOMO
electro
LUMO
op
g
quantum
HOMO
quantum
LUMO
quantum
E
g
l
Sol
l
NMP
l
m-Cresol
l
(excited
l)
l
Sol
f
(%)
E
E
peak
E
E
E
E
E
Ia
Ib
Ic
Id
453.0
415.0
444.8
422.0
481.1
444.2
400.9
441.2
419.1
474.8
447
406
448
421
477
440
400
467
419
470
466
427
455
440
486
571
430
489
542
474
535, 570 (421)
462, 595 (395)
470, 504 (439)
476, 589 (410)
594 (495)
568 (355)
483 (390)
461, 490 (446)
481 (393)
603 (477)
529.5
481.5
535.6
488.6
572.9
515.1
463.2
484.2
479.9
554.5
0.357
0.149
0.04
0.118
0.083
0.135
0.0056
0.0494
0.0567
0.102
0.751
0.810
0.500
0.650
0.256
0.667
0.892
0.863
0.711
0.123
0.926
1.600
0.961
0.840
0.379
1.134
1.271
1.476
1.078
0.440
ꢁ5.125
ꢁ5.184
ꢁ4.874
ꢁ5.024
ꢁ4.630
ꢁ5.041
ꢁ5.266
ꢁ5.237
ꢁ5.085
ꢁ4.497
ꢁ2.784
ꢁ2.609
ꢁ2.559
ꢁ2.486
ꢁ2.466
ꢁ2.634
ꢁ2.589
ꢁ2.676
ꢁ2.501
ꢁ2.261
2.341
2.575
2.315
2.538
2.164
2.407
2.677
2.561
2.584
2.236
ꢁ4.871
ꢁ4.836
ꢁ4.863
ꢁ4.801
ꢁ4.869
ꢁ4.886
ꢁ4.860
ꢁ4.891
ꢁ4.845
ꢁ4.916
ꢁ2.220
ꢁ1.955
ꢁ2.256
ꢁ1.910
ꢁ2.508
ꢁ2.227
ꢁ2.149
ꢁ2.447
ꢁ2.066
ꢁ2.640
2.650
2.836
2.607
2.890
2.360
2.659
2.711
2.444
2.779
2.276
Ie
IIa
IIb
IIc
IId
IIe
628
ꢀI ꢁꢀA ꢁꢀ
ꢁ
2
3
.6. Electrochemistry properties
h
unk
h
std
unk
std
f
¼
f
std
unk
I
A
std
unk
To further understand the electronic structures of the polymers,
and further figure out key parameters for the design of electro-
chromic cells devices, it is necessary to determine the energy levels
of the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of the conjugated polymers.
Thus, the electrochemical characteristics of the polymers films on
a Pt electrode were investigated by cyclic voltammetry (CV) with
unk
where f , fstd, Iunk, Istd, Aunk, Astd, hunk, and hstd are the fluorescent
quantum yield, integration of the emission intensity, absorbance at
the excitation wavelength, and the refractive indices of the corre-
sponding solvents for the samples and the standard, respectively.
Here, we use the refractive indices of the pure solvents as those of
the solutions. The aromatic PI series exhibited PL emission maxima
4
LiClO (0.1 M in acetonitrile) as the electrolyte and Ag/AgCl elec-
3
around 461e610 nm in CHCl solution with PL quantum yield below
ꢁ1
trode as reference at a scanning rate of 50 mV s (see Fig. 7 and
Table 1). Both polymers exhibited one or two obvious oxidation
peaks: both of them are attributed to the oxidation of N atoms in
the triphenylamine groups. To obtain accurate redox potentials, the
1
%. This phenomenon can possibly be attributed to non-planar
configurations of the whole PAM molecules, a higher rate of non-
radiative decay, the strong acceptor characteristic of C]N struc-
ture, and the intramolecular photoinduced electron transfer (PET)
from triphenylamine to C]N bond in both PI and PII [34].
Fig. 5. UVevis spectra of polyazomethines in CHCl3.
Fig. 6. PL spectra of polyazomethines in CHCl3.

H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
165
þ
ꢁ1
Fig. 7. Cyclic voltammograms for polymers in CH
3
CN/0.1 M LiClO
4
with ferrocene/ferrocenium (Fc/Fc ) as an internal standard, at 50 mV s . Black ones are presenting PId, PIc, PIb,
0
PIa, PIe, monomer 3; red ones are presenting PIId, PIIc, PIIb, PIIa, PIIe monomer 3 . (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
reference electrode was calibrated by the ferrocene/ferrocenium
Meanwhile, the PIa film color changed from yellow to red. When
the applied potential was higher than 1.5 V, the film color changed
from red to purple. This was different to the change from green to
dark blue [22]. The purple film was not stable and easily dissolved
in the organic solvents. Stepwise oxidation of the polymer films by
þ
(
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 [35]
EHOMO ¼ ꢁeðEox þ 4:347ÞeV
applied external potentials also resulted in a decrease in the
transitions and their lower energy transitions were intensified at
88 for PIa and 622 nm for PIe, respectively. A similar spectral
pep*
where Eox is the onset oxidation potential vs. Ag/AgCl electrode. The
HOMO energy values of PIIe and PIIb were calculated to be
about ꢁ4.497 to ꢁ5.266 eV, respectively. The LUMO energy levels of
5
change was observed for other PAMs of PIaePIe series and the
corresponding PIIaePIIe series. The dynamic optical behavior of the
device was also measured.
the polymers were estimated from the HOMO energy levels and
opt
E
using the equation
g
The wavelength of 422 nm was selected as the reference
wavelength due to the maximum value of optical contrast. In
potential-step experiments as shown in Fig. 9, the switching time
interval was 5 s between neutral and oxidized states. The optical
switch was fast and fully reversible. Close observation of a single
transition more closely indicated that the switching time between
these two extreme redox states was about 1 s. After 23 cyclic
switches between 0.00 and 1.20 V, the polymeric films still
exhibited good stability of electrochromic characteristics.
opt
ELUMO ¼ EHOMO þ Eg
which is in the range from ꢁ2.261 (PIIe) to ꢁ2.784 eV (PIa),
respectively.
opt
E
ðeVÞ ¼ 1240= ðnmÞ
l
g
The different site linked aldehyde played the key role in the
electron structure and regulated the gaps of PAMs. The suitable
energy levels of the conjugated PAMs indicate their potential
applications in OLED or solar energy cell.
3.8. Solvatochromism and acidichromism properties
As shown in the spectra (Fig. 10), initially neutral PId and PIe in
3
.7. Electrochromic properties
solution had the absorption maxima at 425 and 485 nm, respec-
tively. Upon continual exposure of HCl vapor, new absorption bands
emerged at 559 and 750 nm, accompanied with the decrease of
neutral state absorptions. Neutral solutions of two kinds of poly-
mers were yellow and yellow-green. However, upon the exposure
of HCl vapor, the solution turned into blue and violet as shown as
inset pictures in Fig. 10 for PId. Nitrogen atom of the imine linkage
doped by acid resulted in the planar backbone, which led to an
The spectroelectrochemical properties of the polymer solution
were monitored by a UVevis spectrometer at different applied
potentials. The absorption spectral changes of PIa and PIe are
shown in Fig. 8, which exhibit that lmax value for the pep* tran-
sitions of pristine PIa is centered at 453 nm and that of PIe is
centered at 481 nm, respectively. When the applied external
potential was gradually raised from 0.6 to 1.2 V, the absorption
intensity at 453 nm of PIa slightly decreased, while two new peaks
at about 588 nm and 1077 nm in near-infrared area appeared. The
increased
p-electron delocalization. This also conforms the fact that
TPA is a more stronger electron-donating moiety, and protonation
of the C]N in PAMs results in a much stronger electron-accepting
þꢃ
þꢃ
new spectrum was assigned to the cationic radical PAM [22].
center (that is N ), thereby enhancing charge separation of
p
-

166
H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
solutions upon HCl vapor or acid addition. Further exposure of the
solution to ammonia vapor reversed the polymer to the initial
absorption of the pristine states. This indicates the reversibility of
acidichromism of the obtained PAMs in solution, which means the
polymers are sensitive to pH values. In organic camphorsulfonic
þ
acid solution, due to being doped with H , the lowest-energy
absorption peak shifted to longer wavelength of 547 nm
(Fig. S11). The difference of absorption peaks of PAM doped with
camphorsulfonic acid and HCl is perhaps due to the stronger
interacting force of camphorsulfonic acid with than that of inor-
ganic acid, as well as the steric or interaction induced lower chain
conformational planarity. These effects reduce electron cloud
overlap and widened the band gap.
3
.9. Doped upon oxidation with I
2
, FeCl
3
and UV exposure
to confirm
The compounds were oxidized chemically with FeCl
3
their stability. In Fig. 11, PIa underwent significant color changes
similar to changes obtained anodically. The absorbance at 452 nm,
which corresponding to the neutral state of PIa, decreases and
a new peak at 614 nm is formed. It can be concluded that the
oxidized species are stable since their colors are persistent and do
not fade. The stability of PIe and its oxidized state are further
investigated by the reversible change in absorbance intensity with
the addition of hydrazine. The same phenomena were observed in
PIaePIe series and PIIaePIIe series PAMs.
Fig. 11c shows the UVevis absorption spectra of the PId in CHCl
3
under UV irradiation. Yellow solution of pristine PId changed color
to reddish-violet under the UV irradiation of 365 nm light from 0 to
Fig. 8. (a) Spectra and color changes of the cast film of PIa recorded in anhydrous and
deaerated CH CN with 0.1 M LiClO at 1.2 V (vs. Ag/AgCl) for 0 (black color), 2 (dark
yellow color), 5 (navy blue color), 8 (purple color) min. (b) Spectroelectrochemistry of
PIe recorded in anhydrous and deaerated CH CN with 0.1 M LiClO at 1.2 V for
(orange color), 4 (dark orange color) minutes and 2 (brown color), 5 (blue color)
3
4
3
4
0
minutes. Inset: color observed for the neutral (left) and oxidized (right) PAMs during
electrooxidation. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
electrons in the conjugated polymer with donoreacceptor struc-
ture. Absorption spectra of PId and PIe reveal the existence of iso-
bestic point in solution, which confirm the coexistence of discrete
chromophores. It is easy to detect different colors for PId and PIe
Fig. 9. Double-potential-step chronoamperometry (E
and E
¼ 1.5 V (fully oxidized state)) of PId in a 0.1 mol l LiClO
during 23rd cycle. One cycle corresponds to one double-potential step.
1
¼ ꢁ0.1 V (the fully neutral state)
ꢁ1
2
4
/CH
3
CN solution
Fig. 10. UVevis spectra changes of PId and PIe doped with acid vapor after various
times.

H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
167
Fig. 11. UVevis absorbance changes of PAMs doped with FeCl
3 2
in dichloromethane: (a) PIa; (b) PIe; (c) PId doped with UV irradiation and (d) IR spectra of PIa before HCl and I
doping and after doping. Inset: the photographs of changes of PIa and PIe with the addition of FeCl
3
.
5
0 s at room temperature. The color change was dependent on the
irradiation time. When the irradiation time increased from 0 to
0 s, the peak of the absorption at 424 nm of PId decreased grad-
3.10. Geometric and electronic structure calculated by quantum
chemistry
5
ually, whereas one new band grew at 531 nm gradually. The
polymer doped with photoirradiation could be recovered by adding
the hydrate to the solution. However, the recovered absorption
peak was hypsochromic compared with pristine solution, sug-
gesting there was some degradation in the doping process. We also
determined the absorption of the polymers in NMP solution and
PMMA matrix under photoirradiation. However, it was strange that
Ground-state geometry optimizations and calculation of energy
levels of oligomer of PI and PII series, were carried out using
density-functional theory (DFT) at the B3LYP/6-31G level in order
to clarify the relationship between polymer structure and property.
The average CeC bond lengths (C4eC5) between the N3]C4
linkage and phenylene ring of PIaePIe are in the range of 1.452e
ꢀ
ꢀ
1.469 A and those of PIIaePIIe are in the range of 1.386e1.540 A,
respectively, while the C]N bond lengths (N3]C4) are in the
3
no color change took place. The CHCl seemed to decompose and
ꢀ
give out Cl under the UV irradiation. So the result was attributed to
2
range of 1.244e1.386 A (Table S2). The bonds in PII series (C2eC3,
the reaction of Cl atom with the PId. The dramatic color changes can
be explained by the different electronic states (proton tautom-
erism) and configuration rearrangement resulted from the photo-
irradiation [36]. The variation of the absorption spectra of the TPA-
based PAMs responsive to acidity, metal complex and Cl suggests
2
that these polymers may have potential applications for chemical
sensors.
N3]C4 and C4eC5) are more uniform than those of PI series. It
suggests that the biphenyl can take effect in shortening the CeC
bond and lengthening the N]C bond, which averages the lengths
of single bond and double bond to strengthen the conjugation. The
torsional angles (42) between the C]N linkage and adjacent phe-
nylene or thiophene ring (with aldehyde group) are in the range of
179.0e180.0, which suggest that they are nearly in one planar
conformation. The replacement of the phenylene ring (with alde-
hyde group) by thiophene and TPA or alternation of the substituted
position, has no influence on the dihedral angle 41 and 42. It is
contrary to the results in our previous work [23] that the confor-
mation was more dependent on the phenylene ring (with alde-
hyde) than the N phenylene ring. This may be explained by the
significant effect of the diamine in the conformation of PAMs.
To better understand the evolution of the oxidation and
reduction potentials, the electronic structures of the polymer series
(as isolated oligomers) were evaluated by using DFT at the B3LYP/6-
31G level of theory; HOMO, LUMO energies and HOMOeLUMO
To investigate the structure changes of the PAM during the
doping process, we determined the IR spectra of PAM after doped
with I
at 1566 and 1158 cm , which are attributed to the complex of PAM
doped with I . Interestingly, the spectrum of PAM doped with HCl is
similar to the original, but the one doped with I is different, which
means the doping mechanism is different. Oxidative reaction plays
2
and HCl (shown in Fig. 11d). There are two prominent peaks
ꢁ1
2
2
the key role in the I
imine hydrochloride C]NH does not remarkably alternate the
2
doping process. It suggests the formation of
þ
main peaks in the IR spectra of pristine PAM. The result shows that
HCl molecule is easy to escape, but the I binds to the PAM tightly
2
which is difficult to be released.
energy gaps (E
g
) are given in Table 1. Pictorial representations of

168
H. Niu et al. / Dyes and Pigments 96 (2013) 158e169
Fig. 12. Molecular orbital geometry and energy levels obtained from DFT calculations on Id, Ia, Ib, Ic, Ie based on amine 1 and 2 with a chain length n ¼ 1 at the B3LYP/6-31G level
a), (b); molecular structure of Ia (c), IIa (d).
(
the frontier molecular orbitals of PI and PII series are shown in
Fig. 12, 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 was mainly located on
dialdehyde and extended to the C]N moiety. The large conjugated
area provides an extended region across which the excited state can
be efficiently dissipated. The DFT calculation suggests that the
fluorescence can be efficiently quenched by PET which occurs from
the HOMO on amine to the LUMO on both the CH-aryl and azo-
methine moiety.
that the experimental results were in agreement with the quantum
chemical calculation. The introduction of biphenyl ring induces
narrower band gap and the wider length of conjugation than those
of 1,4-benzenediamine.
Acknowledgment
The author would like to thank Prof. H Tian for great helpful
advice and discussion, and thank Prof. Y F Lian and Prof. H Xu for
their characterization and modification of the article. The authors
are grateful to the support of the National Science Foundation of
China (Grant No.21074031), Heilongjiang University Outstanding
Youth Science Foundation, Foundation of Heilongjiang Education
Bureau (12511379), Postdoctoral Science-research Developmental
Foundation of Heilongjiang Province (LBH-Q10026), Foundation of
Heilongjiang University Innovator Group (No. hdtd-2010-11).
Mostly, the energy gaps of the PAMs containing biphenyldiamine
are narrower than the corresponding ones of PAMs containing 1,4-
benzenediamine, which indicates the former one with the more
conjugated structure. The trend of E
in the order: P (thiophene) < P
phthaldialdehyde) < P (1,3-isophthalaldehyde) < P
amine), which is basically in accord with the experimental data in
the following order: P < P z P < P z P (Table 1). It should be
g
for the oligomer series follows
(terephthalaldehyde) < P
(triphenyl-
e
a
c
(
b
d
e
c
a
d
b
Appendix A. Supplementary material
noted that the calculated results are only based on the monomer
unit arranged in cis form, but, the experiments data are obtained
from oligomer with more complicated steric conformation. The
thiophene-based PAMs revealed the narrowest gaps, best conjuga-
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.dyepig.2012.08.003.
tion,
p-delocalization and hole transport, which illustrated that
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