Synthesis, characterization and hydrolysis of aromatic polyazomethines containing non-coplanar biphenyl structures

Article abstract of DOI:10.1016/j.polymer.2011.01.011

New polyazomethines containing electron-withdrawing trifluoromethyl group and non-coplanar biphenyl structures were prepared at room temperature under reduced pressure. It was found that these polyazomethines would undergo hydrolysis in DMSO solution at temperature higher than 50 °C. The hydrolysis, evidenced by ¹H NMR spectra and GPC chromatograms, was resulted from the reverse reaction of azomethine formation and was facilitated at higher temperature. The GPC results also suggested that post-polymerization would be possible if polyazomethine films were heated at elevated temperature (200 °C) under reduced pressure (0.27 torr). The HOMO (-5.69 to -5.96 eV) and LUMO (-3.04 to -3.18 eV) energy levels of the new polyazomethines are much lower than those of other polyazomethines. Combined with the excellent solubility and good thermal stability, non-coplanar biphenyl structure containing electron-withdrawing trifluoromethyl group could be a new candidate as electron acceptor for the structure design of new conjugated polymers.

Full text of DOI:10.1016/j.polymer.2011.01.011

Polymer 52 (2011) 954e964
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis, characterization and hydrolysis of aromatic polyazomethines
containing non-coplanar biphenyl structures
Jyh-Chien Chen a, , Yen-Chun Liu , Jyh-Jong Ju , Chi-Jui Chiang , Yaw-Tern Chern
a
a
a
b
*
a
Department of Polymer Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Taipei 10607, Taiwan
Department of Chemical Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Taipei 10607, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 October 2010
Received in revised form
New polyazomethines containing electron-withdrawing trifluoromethyl group and non-coplanar bip-
henyl structures were prepared at room temperature under reduced pressure. It was found that these
ꢀ
polyazomethines would undergo hydrolysis in DMSO solution at temperature higher than 50 C. The
24 December 2010
hydrolysis, evidenced by 1H NMR spectra and GPC chromatograms, was resulted from the reverse
reaction of azomethine formation and was facilitated at higher temperature. The GPC results also sug-
gested that post-polymerization would be possible if polyazomethine films were heated at elevated
Accepted 6 January 2011
Available online 12 January 2011
ꢀ
temperature (200 C) under reduced pressure (0.27 torr). The HOMO (ꢁ5.69 to ꢁ5.96 eV) and LUMO
Keywords:
(
ꢁ3.04 to ꢁ3.18 eV) energy levels of the new polyazomethines are much lower than those of other
Polyazomethines
Conjugated polymers
Hydrolysis
polyazomethines. Combined with the excellent solubility and good thermal stability, non-coplanar
biphenyl structure containing electron-withdrawing trifluoromethyl group could be a new candidate as
electron acceptor for the structure design of new conjugated polymers.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
group and therefore can be used to tune the HOMO and LOMO
energy levels of the electron-rich conjugated polymers to fit the
Aromatic polyazomethines, containing C]N linkage, exhibit
excellent thermal stability, good mechanical properties and, in
some cases, liquid crystalline morphology [1]. In addition to high
performance fiber and metal-chelating ability [2,3], they have been
also investigated for the opto-electrical applications such as hole-
transport layer for OLED, semiconductor, non-linear optics (NLO)
and photovoltaic cell [4e7]. However, these applications have been
limited by their poor solubility in common organic solvents and
low molecular weights. Besides, the relatively rare availability of
new dialdehyde monomers also hindered the chemical structure
modifications of polyazomethines. Many strategies have been
reported to improve the solubility of polyazomethines. The intro-
duction of flexible alkyl and alkoxy groups as the substituents has
been proved to be effective, although at the expense of their
thermal stability [8e10]. The incorporations of bulky substituents
such as triphenylamine, tetraphenylethylene and diphenylfluorene,
have been investigated [11e13]. The copolymerization of electron-
rich, solubility-enhancing aromatic or heterocyclic units such as
thiophene, carbazole and fluorene [14e17], has also been explored.
The C]N linkages of polyazomethines are electron-withdrawing
best combination of energy levels and charge mobility in a multi-
layer device. However, in most cases, the enhancements in solu-
bility of these conjugated polymers are resulted from the long alkyl
or alkoxy side chains attached on the electron-rich units, leading to
lower thermal stability.
The chemical modifications of polyazomethines are mainly
achieved by synthesizing new diamines and then polymerizing
them with commercially available dialdehydes. These attempts
targeting at either solubility enhancement or investigating their
optical and electrical properties are limited by the rare availability
of new dialdehyde monomers. Most dialdehyde monomers are
prepared by electrophilic substitution (Vilsmeier reaction) on an
electron-rich aromatic or heterocyclic ring [18]. The preparation of
new dialdehydes by the reduction of cyano group has rarely been
reported [19].
Conventionally, polyazomethines are prepared by solution
polymerization from diamines and dialdehydes either at room
temperature or at higher temperature [20]. The premature
precipitation during polymerization, as a result of poor solubility,
often leads to low molecular weight products. Even for soluble
polyazomethines, it is often difficult to achieve high molecular
weight. Several polymerization conditions have been applied. In
some cases, LiCl was used as the water absorption agents [21]. It has
also been demonstrated that polymerization at room temperature
*
Corresponding author. Fax: þ886 2 27376544.
E-mail address: jcchen@mail.ntust.edu.tw (J.-C. Chen).
0
032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.01.011

J.-C. Chen et al. / Polymer 52 (2011) 954e964
955
under vacuum in weakly acidic m-cresol is the best method to
obtain the highest molecular weight [13]. However, the reason has
not been fully explained. In addition, the formation of azomethine
linkage is, like esterification, actually reversible. In the presence of
water, hydrolysis might predominate under suitable conditions
quinine sulfate in 1 N H
2
SO
4
as reference standard (
F
PL ¼ 0.546).
Cyclic voltammetric (CV) measurements were carried out on a CH
Instrument 611C electrochemical analyzer at room temperature in
a three-electrode electrochemical cell with a working electrode
(polymer film coated on ITO glass), a reference electrode (Ag/Ag ,
referenced against ferrocene/ferrocium (Fc/Fc ), 0.09 V), and
a counter electrode (Pt gauze) at a scan rate of 100 mV s . CV
measurements for polymer films were performed in an electrolyte
solution of 0.1 M tetrabutylammonium perchlorate (TBAP) in
acetonitrile. The potential window at oxidative scan was 0e2.5 V
and reductive scan was 0 to ꢁ2.5 V, respectively. The wide-angle X-
ray diffraction (WXRD) data were collected on a PANalytical X’Pert
PRO X-ray powder diffraction.
þ
þ
[22]. However, this issue has never been addressed in poly-
ꢁ1
azomethine study.
In this study, a new aromatic dialdehyde containing electron-
0
withdrawing trifluoromethyl groups at the 2 and 2 positions of
biphenyl was synthesized. The aldehyde functionality was achieved
by the reduction of cyano groups. Organosoluble polyazomethines
0
0
were prepared from this dialdehyde with 2,2 -disubstituted-4,4 -
0
benzidines, 4,4 -oxydianiline and p-phenylenediamine by solution
polycondensation at room temperature under reduced pressure.
The hydrolysis of azomethine linkages at the presence of trace
water was investigated by H NMR spectroscopy and GPC chro-
1
2.3. Synthesis of monomers
matography. The thermal, optical and electrochemical properties of
these new polyazomethines were also investigated. The effects of
strong electron-withdrawing trifluoromethyl groups and non-
coplanar biphenyl structures on the HOMO and LUMO energy levels
were also discussed.
Monomers (1)e(4) were synthesized by procedures in our
previous work [33].
2
.3.1. 2,2-Bis(trifluoromethyl)-4,4-biphenyldicarbaldehyde (5)
To a 100 mL, three-neck, round-bottom flask equipped with
condenser and mechanical stirrer were added 0.30 g (0.88 mmol) of
2
2
. Experimental
0 0
,2 -bis(trifluoromethyl)-4,4 -biphenyldicarbonitrile (4) and 10 mL
of toluene. After 9 mL (10.09 mmol) of diisobutylaluminium
hydride (DIBAL-H) (20% in hexane) was slowly added with a syringe
under nitrogen atmosphere, the reaction mixture was further
2
.1. Materials
Tetrabutylammonium perchlorate (TBAP) used in cyclic vol-
ꢀ
stirred at 16 C for 18 h under nitrogen atmosphere. The reaction
tammetric measurements was recrystallized twice with ethyl
acetate and dried at 120 C under reduced pressure overnight. All of
other reagents were purchased from commercial companies and
ꢀ
mixture was then poured into a vigorously stirred solution of
ꢀ
methanol/water (1:1, v/v) and kept at 0 C with ice bath for 3 h.
After 200 mL of 2M HCl solution was added, the mixture was
extracted with 200 mL of ether twice. The combined organic phase
was collected and washed many times with water and dried with
anhydrous magnesium sulfate. The organic phase was evaporated
used as received. All of the solvents used in this study were purified
0
0
according to standard methods prior to use. 2,2 -Dimethyl-4,4 -
0
0
benzidine (DMB 6) and 2,2 -bis(trifluoromethyl)-4,4 -benzidine
TFMB 7) were synthesized according to reported procedures with
some modifications [23,24].
(
ꢀ
and dried under reduced pressure at 60 C overnight. The crude
ꢀ
product was then sublimated under reduced pressure at 70 C to
ꢀ
afford 0.18 g (59.0% yield) of white crystals: mp 118e120 C; FTIR
2.2. Measurements
ꢁ1
ꢁ1
(
KBr) : 2757 cm (aldehyde CeH stretching), 1705 cm (aldehyde
C]O stretching). H NMR (500 MHz, DMSO-d , d, ppm): 10.17 (s,
6
1
All melting points were determined on a Mel-Temp capillary
1
13
2H, aldehyde-H ), 8.41 (d, J ¼ 0.8 Hz, 2H, AreH ), 8.26 (dd, J ¼ 0.8
melting point apparatus. Proton ( H NMR) and carbon ( C NMR)
nuclear magnetic resonance spectra were measured at 500 and
d
a
1
3
and 8.0 Hz, 2H, AreH
b
) and 7.70 (d, J ¼ 8.0 Hz, 2H, AreH
c
). C NMR
(
1
1
(
[
5
125 MHz, DMSO-d
26.54, CeCF
31.80 (C2), 132.36 (C3), 136.41 (C4), 141.00 (C1), 191.87 (C8). EIMS
6 3
, d, ppm): CF (q) (C7) 119.99, 122.17, 124.35,
125 MHz on a Bruker Avance-500 spectrometer, respectively.
3
(q) (C6) 127.63, 127.87, 128.12, 128.37, 127.24 (C5),
Infrared spectra were obtained with a Digilab-FTS1000 FTIR. Mass
spectroscopy was conducted on a Finnigan TSQ 700 mass spec-
trometer. Elemental analyses were performed on a Heraeus Vario
analyzer. High performance liquid chromatography was performed
on a JASCO HPLC system equipped with a UV (254 nm) detector
using a Thermo Hypersil column (250 mm ꢂ 4.6 mm, particle size
þ
m/z): Calcd. for C16
H
8
6
F O
2
: 346.0428; Found: 346.0 [M] ; 344.9
: C: 55.50, H: 2.33%; Found : C:
þ
M ꢁ 1] . Anal. Calcd for C16
8 6 2
H F O
5.51%, H: 2.45%.
7
5
mm) with an 80/20 (v/v) acetonitrile/water mixture as the
CF
3
5
6
c
b
a
solvent. Inherent viscosities were determined with
a
Can-
H
8
O
O
4
ꢀ
noneUbbelohde No.100 viscometer at 30.0 ꢃ 0.1 C in N-methyl-2-
pyrrolidinone (NMP). Molecular weights were measured on
a JASCO GPC system (PU-980) equipped with an RI detector (RI-
1
F C
3
H
d
3
2
(
5)
9
30), a Jordi Gel DVB Mixed Bed column (250 mm ꢂ 10 mm)
column, using dimethylacetamide (DMAc) as the eluent and cali-
brated with polystyrene standards. Thermal gravimetric analyses
0
2.3.2. 3,3 -Dibromoazoxybenzene
(
TGA) were performed in nitrogen with a TA TGA Q500 thermog-
To a 250 mL, three-neck, round-bottom flask equipped with
a mechanical stirrer, a condenser, and a stopper were added 60 mL
of ethanol and 21.20 g (530.00 mmol) of sodium hydroxide. The
ꢀ
ꢁ1
ravimetric analyzer using a heating rate of 10 C min . Differential
scanning calorimeter (DSC Pyris 1) was used to measure the glass
transition temperatures under nitrogen at a heating rate of 10
min . UV-visible measurements were carried out on a Cary-100
UV-Visible spectrometer at room temperature. Photoluminescence
ꢀ
ꢀ
C
reaction mixture was heated at 70 C until most of sodium
ꢁ
1
hydroxide was dissolved. After 66.4 mL of ethylene glycol and
10.00 g (49.50 mmol) of 1-bromo-3nitrobenzene were added, the
reaction mixture was heated at reflux for 2 h. The warm mixture
was poured into 300 mL of ice water and stirred for 30 min. The
brown solid that formed was collected by filtration and washed
(
PL) measurements were carried out on a PerkineElmer F4500
photoluminescence spectrometer. Fluorescence quantum yield
(
ꢁ
5
FPL) of the polymer in chloroform was measured by 10
M

956
J.-C. Chen et al. / Polymer 52 (2011) 954e964
several times with water. After drying under reduced pressure at
ethanol was added. The reaction solution was heated at reflux for
24 h under nitrogen. After the reaction mixture was allowed to cool
to room temperature, it was filtered to remove insoluble compo-
nents. The filtrate was collected and washed several times with
a saturated NaCl aqueous solution. After dried with anhydrous
magnesium sulfate, the combined organic phase was evaporated to
dryness under reduced pressure. The crude product was recrys-
tallized from a mixture of water and methanol to afford 2.10 g
ꢀ
9
0 C overnight, 7.71 g (80.0% yield) of crude product was obtained.
The crude product was used in the next procedure without further
purification. A small portion of crude product was decolorized with
charcoal and recrystallized from acetone to afford a yellow powder:
ꢀ
1
6
mp 114e115 C; H NMR (500 MHz, DMSO-d , d, ppm): 8.42e7.25
(
m, 8H, AreHaed).
0
0
ꢀ
2
.3.3. 2,2 -Dibromo-4,4 -diaminobiphenyl (8)
(71.0% yield) of light-brown crystals. mp 152e153 C (lit [26]. mp
ꢀ
1
To a 250 mL, three-neck, round-bottom flask equipped with
151e152 C); H NMR (500 MHz, DMSO-d , d, ppm): 7.07e7.03 (m,
6
a mechanical stirrer, a condenser, and a thermometer were added
6H, AreHf.g), 6.83 (d, J ¼ 7.7 Hz, 2H, AreH ), 6.47 (d, J ¼ 6.9 Hz, 4H,
c
0
18.50 g (12.45 mmol) of 3,3 -dibromoazoxybenzene, 115 mL of
AreH
e
), 6.66 (d, J ¼ 7.7 Hz, 2H, AreH
b
a
), 6.34 (s, 2H, AreH ) and 5.00
tetrahydrofuran and 161 mL of glacial acetic acid. Zinc dust
(s, 4H, NH
2
eH ).
d
(
12.90 g) was added slowly so the temperature of the reaction
ꢀ
mixture was maintained below 40 C. After the addition of zinc
dust, 23 mL of 85% phosphoric acid was added slowly so the
temperature of the reaction mixture was maintained below 55 C.
ꢀ
c
f
b
After stirring for 30 min, the reaction mixture was filtered to
remove zinc dust. The filtrate was poured into 300 mL of water. The
aqueous phase was extracted twice with 100 mL of methylene
H N
NH
2
2
d
a
chloride. The combined organic phase was added dropwise to
e
ꢀ
6
0 mL of conc. hydrochloric acid at 0 C and stirred for 30 min. The
g
precipitate that formed was collected by filtration and washed with
methylene chloride and then dissolved in 1 L of hot water. After the
solution was filtered, the clear filtrate was neutralized with
ammonium hydroxide. The precipitate that formed was collected
and recrystallized from toluene to afford 9.02 g (56.0% yield) of
(
9)
2.4. Synthesis of polyazomethines
ꢀ
ꢀ
1
white crystal: mp 151e153 C (lit [25]. mp 152e156 C); H NMR
500 MHz, DMSO-d , ppm): 6.85e6.81 (m, 4H, AreHa.c), 6.54 (dd,
J ¼ 2.0 and 8.2 Hz, 2H, AreH
Polyazomethines PAM-1e7 were prepared by solution poly-
merization using equimolar dialdehyde and diamine at room
temperature under reduced pressure (0.27 torr). The synthesis of
PAM-2 was used as an example to illustrate the general synthetic
procedure. To a 100 mL, three-neck, round-bottom flask were
added 0.50 g (1.40 mmol) of 2,2 -bis(trifluoromethyl)-4,4 -biphe-
nyldicarbaldehyde (5), 0.45 g (1.40 mmol) of diamine (7) and 10 mL
of m-cresol. After the reaction mixture was stirred at room
temperature for 48 h under reduced pressure, it was poured into
a mixture of methanol and water (1:1, v/v). The precipitate that
formed was collected by filtration and washed with hot ethanol
(Soxhlet apparatus) for 12 h and dried at 100 C overnight under
reduced pressure to afford 0.85 g (96.0% yield) of powder. IR (KBr):
3402 cm (NH2 end group), 3057 cm (aromatic), 2735 cm
(aldehyde end group O]CeH stretching), 1702 cm (aldehyde end
group O]C stretching), and 1625 cm
stretching). H NMR (500 MHz, DMSO-d6,
J ¼ 7.0 Hz, 1H, aldehyde end group), 9.02 (d, J ¼ 4.5 Hz, 2H, azo-
methine HC]N), 8.51 (d, J ¼ 6.0 Hz, 2H, AreH), 8.36e8.34 (m, 2H,
AreH), 7.84 (d, J ¼ 6.0 Hz, 2H, AreH), 7.72e7.63 (m, 4H, AreH) and
(
6
, d
b
2 d
) and 5.35 (s, 4H, NH eH ).
Br
c
b
a
0
0
H
2
N
NH
2
d
Br
8)
(
0
0
ꢀ
2
.3.4. 2,2 -Diphenyl-4,4 -diaminobiphenyl (9)
To a 250 mL, three-neck, round-bottom flask equipped with
a mechanical stirrer, a condenser, and a nitrogen inlet were added
ꢁ
1
ꢁ1
ꢁ1
0
0
ꢁ1
3
5
.00 g (8.76 mmol) of 2,2 -dibromo-4,4 -diaminobiphenyl (8) and
4 mL of toluene. After the solution was heated at 50 C under
ꢀ
ꢁ1
(azomethine C]N
, ppm): 10.19 (d,
1
nitrogen and became clear, 4.26 g (40.00 mmol) of sodium
carbonate in 18 mL of water and 0.66 g (0.57 mmol) of Pd(P(Ph ))
d
3
4
were added. After the solution was vigorously stirred for 30 min,
a solution of 2.70 g (22.14 mmol) of phenylboronic acid in 18 mL of
0
0
Scheme 1. Synthesis of 2,2 -bis(trifluoromethyl)-4,4 -biphenyldicarbaldehyde (5).

J.-C. Chen et al. / Polymer 52 (2011) 954e964
957
7
.54 (d, J ¼ 7.5 Hz, 2H, AreH) and 5.74 (br, 2H, NH
2
end group). The
10.17 ppm was attributed to the aldehyde hydrogen. The other
absorptionpeaks in the aromatic region (7e9 ppm) were assigned to
the corresponding hydrogen as shown in Fig. 1(a). IR spectrum of
inherent viscosity of the obtained PAM-2 was 0.50 dL/g (measured
at a concentration of 0.5 g/dL in NMP at 30 C). The other poly-
azomethines were prepared by the same procedure.
ꢀ
ꢁ1
dialdehyde (5) is shown in Fig. 2. The absorption peaks at 2757 cm
ꢁ
1
(
aldehyde CeH stretching) and 1705 cm (aldehyde C]O stretch-
1
ing) also showed the formation of aldehyde group. The spectra of H
3
. Results and discussion
13
NMR, C NMR and IR, together with the result of elemental analysis,
confirmed the structure of the new dialdehyde (5). Several app-
roachestosimplify thesyntheticroute have been carried out without
3.1. Synthesis of monomers
0
0
success. For example, the attempt to direct formylation (electrophilic
The new dialdehyde, 2,2 -bis(trifluoromethyl)-4,4 -biphenyldi-
0
reaction) of 2,2 -bis(trifluoromethyl)biphenyl by POCl
3
and DMF was
carbaldehyde (5), containing electron-withdrawing trifluoromethyl
groups at the 2 and 2 positions of biphenyl, was synthesized by
0
unsuccessful. It might be resulted from the electron deficiency of the
biphenyl moiety due to the presence of strong electron-withdrawing
trifluoromethyl group.
a five-step synthetic route as shown in Scheme 1. 2-Amino-
benzotrifluoride was first brominated at the para position by
N-bromosuccinimide (NBS) in DMF. The aromatic bromide (1) that
formed was converted to cyanide compound (2) by copper cyanide
in NMP. The amino group of compound (2) was then transformed to
aromatic iodide (3) by diazotization then Sandmeyer reaction. The
iodide compound (3) was coupled by Ullmann coupling reaction in
0
0
0
Diamines, 2,2 -dibromo-4,4 -benzidine (8) and 2,2 -diphenyl-
0
4
,4 -benzidine (9) were prepared according to the synthetic route
shown in Scheme 2. First, 1-bromo-3-nitrobezene was reduced to
azoxy compound, which then underwent benzidine rearrangement
0
0
in the presence of zinc dust to form 2,2 -dibromo-4,4 -benzidine
0
0
0
(8). 2,2 -Diphenyl-4,4 -benzidine (9) was then obtained by Suzuki
coupling reaction between phenylboronic acid and diamine (8).
the presence of activated copper powder in DMF to form 2,2 -bis
trifluoromethyl)-4,4 -biphenyldicarbonitrile (4). The new dia-
0
(
ldehyde (5) was then obtained by the reduction of cyano compound
4) using diisobutylaluminium hydride (DIBAL-H) as reducing
agent. The as-prepared dialdehyde was purified by sublimation at
(
3.2. Synthesis of polyazomethines
ꢀ
1
7
0
C under reduced pressure before polymerization. H NMR
Polyazomethines PAM-1e7 were prepared in quantitative yields
by reacting equimolar diamines and dialdehydes in m-cresol at
spectrum of dialdehyde (5) is shown in Fig. 1(a). The singlet peak at
Fig. 1. 1H NMR spectra in DMSO-d
4 h in nitrogen.
ꢀ ꢀ
of (a) monomer (5), (b) as-prepared PAM-2 (soluble part) and hydrolysis of PAM-2 with various condition, (c) 50 C for 12 h in air, (d) 150 C for
6
2

958
J.-C. Chen et al. / Polymer 52 (2011) 954e964
Fig. 2. IR spectra of monomers (5), (6) and PAM-1 (KBr).
room temperature under reduced pressure as shown in Scheme 3.
Polyazomethines PAM-1e4 were prepared by using the new tri-
fluoromethyl-containing dialdehyde (5) and four substituted
benzidines (6e9). The substituents of these benzidines included
methyl (PAM-1), trifluoromethyl (PAM-2), bromide (PAM-3) and
quantitative yields (Table 1). A representative IR spectrum of PAM-
1 is shown in Fig. 2. The characteristic peak appearing at 1615 cm
ꢁ1
(C]N stretching) confirmed the formation of azomethine linkage.
1
A representative H NMR spectrum of as-prepared PAM-2 (soluble
part in DMSO-d
solubility of PAM-2 in DMSO-d
dissolving 0.1% (w/v) as-prepared PAM-2 in DMSO-d
6
6
) is shown in Fig. 1(b). Because of the limited
0
1
phenyl (PAM-4). 4,4 -Oxydianiline (ODA) and p-phenylenediamine
6
, H NMR sample was prepared by
at room
(PDA) were also used to prepare polyazomethines PAM-5 and
PAM-6, respectively. In comparison to PAM-6, PAM-7 was prepared
temperature and then removing the insoluble part by filtration. The
peaks appearing at 9.02 ppm were assigned to the hydrogen of the
azomethine linkage. These peaks suggested the existence of syn
and anti configurations in the polymer backbone as reported earlier
0
by reacting terephthalic dicarbaldehyde (TPDCA) with 2,2 -bis(tri-
0
fluoromethyl)-4,4 -benzidine (TFMB 7). All the polyazomethines
PAM-1e7 remained in solution without any premature precipita-
tion throughout the polymerization and were prepared in
1
[27,28]. It was also found from the H NMR spectrum of PAM-2

J.-C. Chen et al. / Polymer 52 (2011) 954e964
959
Table 1
n w
Inherent viscosity, M , M and yield of polyazomethines.
a
b
(104 g/mol)
b
(104 g/mol)
h
inh (dL/g)
M
n
M
w
Yield
%)
(
PAM-1
PAM-2
PAM-3
PAM-4
PAM-5
PAM-6
PAM-7
0.79
0.50
0.38
0.32
0.33
0.79
0.44
2.4
1.2
1.3
1.3
1.4
2.0
1.6
7.5
2.5
3.0
3.0
3.5
5.1
3.7
96
96
98
97
96
96
96
a
ꢀ
Measured at a polymer concentration of 0.5 g/dL in NMP at 30 C.
By GPC in DMAc (relative to polystyrene standards).
b
protons on nitrogen and the electrical quadrupole moment of 14N
nucleus. The protons on the nitrogen might respond by giving
a broad peak that disappears in the baseline.
3.3. Characterizations of polyazomethines
The inherent viscosity and molecular weights (determined by
0
0
Scheme 2. Synthesis of 2,2 -diphenyl-4,4 -diaminobiphenyl (9).
GPC in DMAc and calibrated by polystyrene standards) are reported
in Table 1. The as-prepared polyazomethines had inherent viscosity
in the range of 0.32e0.79 dL/g (measured at a polymer concen-
ꢀ
soluble part that an appreciated amount of chain-end aldehyde
10.19 ppm) remained. However, it was surprising that the
absorption peak of amine end groups (5.74 ppm) was almost
invisible. It might be resulted from the slow exchange rate of
tration of 0.5 g/dL in NMP at 30 C). The number-averaged molec-
(
ular weights ranged from 12,000 to 24,000 g/mole. These values are
high when compared with those of conventional aromatic poly-
azomethines. The molecular weights estimated from the intensity
Scheme 3. Synthesis of polyazomethines.

960
J.-C. Chen et al. / Polymer 52 (2011) 954e964
of chain-end aldehyde peak (10.19 ppm) and that of azomethine
hydrogen (9.02 ppm) from the H NMR spectra were much smaller
than those measured by GPC. It was attributed to the limited
Table 3
1
Solubility of polyazomethines.
Solubilitya
solubility of these polyazomethines in DMSO-d
6
solvent. The
molecular weights estimated by H NMR spectra represented only
the soluble part of polyazomethines in DMSO-d . These poly-
m-cresol NMP DMF DMAc DMSO THF Chloroform Acetone
1
PAM-1 þþ
PAM-2 þþ
PAM-3 þþ
PAM-4 þþ
PAM-5 þþ
PAM-6 þþ
PAM-7 þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þꢁ
þꢁ
þꢁ
þꢁ
þꢁ
þꢁ
þꢁ
þꢁ þꢁ
þꢁ þꢁ
þꢁ þꢁ
þꢁ þꢁ
þꢁ þꢁ
ꢁ
6
þꢁ
þꢁ
þꢁ
þꢁ
ꢁ
azomethines were, however, totally soluble in DMAc, the solvent
for GPC. Among these polyazomethines, PAM-1 and PAM-6,
0
0
prepared from the new dialdehyde (5) and 2,2 -dimethyl-4,4 -
benzidine (6) and p-phenylenediamine (PDA), respectively, had the
highest molecular weights. It might be attributed to the high
reactivity of the new aldehyde (5) and these two diamines.
Different methods have been reported for the preparation of
polyazomethines. These methods involved polymerization at
different temperatures, including at toluene-water azeotropic
þꢁ
þꢁ
ꢁ
ꢁ
ꢁ
Abbreviations: THF, tetrahydrofuran; DMF, dimethylformamide; DMAc, dimethyla-
cetamide; NMP, N-methyl-2-pyrrolidinone; DMSO, dimethyl sulfoxide.
a
The solubility was determined by using 20 mg sample in 1 mL of stirred solvent.
þþ: soluble at room temperature; þꢁ: partially soluble at room temperature;
ꢁ: insoluble at room temperature.
ꢀ
temperature [29], at 160 C in m-cresol [16] or NMP/HMPA [30], and
at room temperature with or without reduced pressure [12,13]. In
order to find out the best condition to achieve the highest molec-
ular weights, PAM-1 and PAM-2 were prepared for 48 h in m-cresol
at room temperature under reduced pressure (condition A), in m-
cresol at reflux temperature under nitrogen atmosphere (condition
B) and in NMP/HMPA (1:1, v/v) at room temperature condition C.
Table 2 shows the inherent viscosity and molecular weights of
PAM-1 and PAM-2 prepared in different conditions. PAM-1
prepared in m-cresol at room temperature under reduced pressure
containing similar structures with PAM-6 and PAM-7, can be
prepared from 4,4 -benzidine with terephthalic dicarbaldehyde
(TPDCA). It was only soluble in concentrated sulfuric acid [9].
0
(
condition A) had the highest M
temperature was raised to reflux temperature (condition B), PAM-1
had the lowest M of 3000 g/mole. When PAM-1 was prepared in
NMP/HMPA (1:1, v/v) at room temperature for 48 h (condition C),
the M was 12,000 g/mole. PAM-2 experienced the same effect,
highest M was obtained by condition A, the lowest M by condi-
tion B. From these data, it can be concluded that polyazomethines
with the highest M can be obtained by room temperature solution
n
of 24,000 g/mole. When the
It has been reported that the C]N linkage of polyazomethine is
not in the same plane with the adjacent N-phenyl ring due to the
repulsion force between the hydrogen atom of the C]N linkage
and the adjacent N-phenylene ring [31]. The torsional angle has
n
ꢀ
n
been calculated to be 30.4 . However, this non-coplanar confor-
n
n
mation resulted from C]N linkage alone is not sufficient to provide
good solubility for polyazomethines. The excellent solubility of
these new polyazomethines PAM-1e7 was resulted from the
combined effects of trifluoromethyl groups of the new dialdehyde
(5) moiety and the non-coplanar biphenyl structures. The biphenyl
structures are forced to adapt a non-coplanar conformation by the
n
polymerization in m-cresol under reduced pressure. It was pointed
out that weakly acidic m-cresol had an activating effect on the
azomethine formation [13]. The dialdehyde was activated by acidic
solvents and deactivated by basic solvents. However, too strongly
acidic solvent might deactivate diamine. Therefore, weakly acidic
solvent such as m-cresol might be the best polymerization medium.
Table 2 also shows that polyazomethines prepared at room
temperature had higher M
phenomenon was to be discussed in the next section (Hydrolysis
and Post-Polymerization).
The solubility behavior of these new polyazomethines in
different organic solvents is reported in Table 3. All of these poly-
azomethines can be dissolved (20 mg in 1 mL of solvent) at room
temperature in DMF, DMAc, NMP and m-cresol and were partially
soluble in DMSO and THF. Especially, PAM-6 and PAM-7, containing
the most rigid p-phenylene structure, were still soluble in polar
organic solvents. Conventional aromatic polyazomethines are
insoluble in organic solvents. For example, polyazomethine PAM-8,
0
bulky substituents at the 2 and 2 positions of biphenyls. Both of
trifluoromethyl groups and non-coplanar biphenyls have been
incorporated into polyimides [32], polyamides [25] and poly(p-
biphenylene-1,3,4-oxidiazole) [33] to enhance the solubility with
great success. This enhanced solubility is attributed to the loose
interchain packing resulted from the bulkiness of trifluoromethyl
groups and non-coplanar biphenyl structures. Furthermore, the
same enhancements in solubility of these new polyazomethines
were also observed regardless various diamines were used.
Fig. 3 shows the wide-angle X-ray diffraction patterns of poly-
azomethines PAM-1e7. There were no crystalline peaks can be
observed, indicating that these new polyazomethines were amor-
phous in nature. This can be also attributed to their non-coplanar
biphenyl structures which might hinder polymer chain packing.
n
than at reflux temperature. This
3.4. Hydrolysis and post-polymerization
Table 2
Inherent viscosity, M
conditions.
n
, M
w
and yield of PAM-1 and PAM-2 prepared in different
Because of the limited solubility of these polyazomethines in
DMSO-d , we attempted to heat the NMR sample tubes to obtain
6
Conditiona
h
inh (dL/g)
b
M
n
c
(104 g/mol) M c
(104 g/mol) Yield (%)
a homogeneous solution. It was found that the aldehyde peaks
became more prominent and azomethine hydrogen peaks dimin-
ished when the NMR sample tubes were heated, indicating the
decrease in molecular weight. The azomethine linkage of small
organic molecules has ever been reported for its hydrolysis
phenomenon at the presence of water [22]. In some cases, the
formation of azomethine (imine) linkage was exploited to protect
aldehyde and ketone groups, which were then reclaimed by the
following hydrolysis in refluxing organic solvents [34]. However,
the hydrolysis of polyazomethines has never been mentioned
w
PAM-1 (A)
0.79
0.20
0.53
0.50
0.12
0.32
2.4
0.3
1.2
1.2
0.2
0.4
7.5
0.5
2.2
2.5
0.3
0.6
96
98
96
96
95
96
(
(
B)
C)
PAM-2 (A)
(
(
B)
C)
a
(
A) m-cresol, at room temperature under reduced pressure for 48 h; (B) m-cresol,
at reflux for 48 h; (C) NMP/HMPA (1:1, v/v), at room temperature for 48 h.
b
c
ꢀ
Measured at a polymer concentration of 0.5 g/dL in NMP at 30 C.
By GPC in DMAc (relative to polystyrene standards).

J.-C. Chen et al. / Polymer 52 (2011) 954e964
961
azomethine formation is favored at elevated temperature in the
presence of even trace amount of water. This is consistent with the
results from Table 2. In the same solvent (m-cresol), polymerization
at room temperature gave higher molecular weights than at reflux
temperature.
The hydrolysis of these new polyazomethines was further evi-
denced by the GPC measurements. Fig. 4 shows the GPC chro-
matograms of as-prepared PAM-2 after different levels of
hydrolysis and post-polymerization. The hydrolysis was carried out
ꢀ
by dissolving as-prepared PAM-2 in DMSO (0.1%, w/v) at 150 C for
different periods of time. PAM-2 can be totally dissolved in DMSO
ꢀ
at 150 C. The GPC chromatogram (Fig. 4(a)) of as-prepared PAM-2
was bimodal with number average molecular weight (M
n
) of
1
2,000 g/mole. When the 0.1% (w/v) of PAM-2 solution in DMSO
ꢀ
was heated at 150 C for 4 h in air, the GPC chromatogram (Fig. 4(b))
became monomodal and indicated a lower molecular weight (M
of 6000 g/mole. As the hydrolysis lasted to 16 h, M decreased to
2
2
n
)
n
ꢀ
000 g/mole Fig. 4(c). After PAM-2 solution was heated at 150 C for
4 h, it became difficult to determine the molecular weight from its
Fig. 3. Wide-angle X-ray diffraction patterns of polyazomethines PAM-1e7.
GPC chromatogram due to the low signal intensity (Fig. 4(d)). It
indicated that PAM-2 was mostly hydrolyzed to oligomers whose
molecular weights were beyond the lower limit of the GPC
measurement. Even though water was not added, the trace amount
of water in DMSO solvent is enough for the hydrolysis of poly-
before. In order to study the hydrolysis of these polyazomethines in
solution state, PAM-2 was used as the example. Fig. 1(b) is the H
1
6
NMR spectrum of as-prepared PAM-2 soluble part in DMSO-d at
1
room temperature. Fig. 1(c) shows the H NMR spectrum of as-
ꢀ
ꢀ
prepared PAM-2 heated at 50 C for 12 h in air to form a homoge-
azomethines at 150 C. Aromatic polyazomethines have been
neous DMSO-d
6
solution. It was found that the intensity of the
known for their high thermal stability. This is true only for poly-
azomethines in their solid states. From the H NMR spectra (Fig. 1)
1
aldehyde peaks (10.19 ppm) increased and that of azomethine
hydrogen peaks (9.02 ppm) diminished as temperature increased.
From the increased intensity of aldehyde peak, it is obvious that
and GPC chromatograms (Fig. 4), it suggests that in solution state
the polyazomethine could be easily hydrolyzed in the presence of
ꢀ
polyazomethines could undergo hydrolysis in DMSO-d
6
solvent
a trace amount of water at temperature even as low as 50 C.
even only trace of water (inherently exists in DMSO-d ) is present
6
It has been well know that the molecular weights of poly-
azomethines are generally lower than those of aromatic conden-
sation polymers such as polyamides and polyimides. In addition to
the effects of the chemical structures and the reactivity of mono-
mers, this phenomenon might possibly be resulted from the
hydrolysis during sample preparation for GPC measurement. In
some cases, heating might be necessary to obtain a homogeneous
solution, leading to hydrolysis and thus lower molecular weights.
It has also been reported that polyazomethine can undergo
as depicted in Scheme 4. The amine peak at 5.74 ppm in Fig. 1(c)
was broad and not prominent. This might be attributed to the slow
exchange rate of protons on nitrogen and the electrical quadrupole
moment of 14N nucleus. Fig.1(d) shows the H NMR spectrum of the
1
ꢀ
same as-prepared PAM-2 heated at 150 C for 24 h in DMSO-d
6
under nitrogen atmosphere to form a homogeneous solution. The
intensity of the aldehyde and amine peaks was more intense and
that of the azomethine hydrogen peaks was decreased. Under
nitrogen atmosphere, the amine peaks can be observed at 5.53 and
post-polymerization at high temperature [12]. A thin film (ca. 2 mm)
5
.74 ppm. These peaks were attributed to the hydrolyzed diamine
monomers and the amino groups of polyazomethine chain-ends,
1
respectively. From these H NMR spectra (Fig. 1), it indicates that
the hydrolysis of polyazomethine PAM-2 is more pronounced at
elevated temperature. It also implies that the reversed reaction of
ꢀ
Fig. 4. GPC chromatograms of PAM-2 (a) as-prepared, (b) heated at 150 C for 4 h in
ꢀ
DMSO, (c) for 16 h in DMSO, (d) for 24 h in DMSO, and (e) thin film heated at 200
C
Scheme 4. Hydrolysis of polyazomethines in solution state.
under reduced pressure for 24 h.

962
J.-C. Chen et al. / Polymer 52 (2011) 954e964
was prepared by spin-coating 2% (w/v) of PAM-2 in DMF solution
ꢀ
ꢀ
on glass and heating at 50 C for 1 h then 200 C for 24 h under
reduced pressure. The GPC chromatogram of this post-polymerized
n
PAM-2 thin film is shown in Fig. 4(d). The M increased from 12,000
to 24,000 g/mole. It was attributed to the post-polymerization from
amine and aldehyde groups of as-prepared PAM-2 chain-ends. It
suggests that in solid state polyazomethines can undergo post-
polymerization at elevated temperature. The GPC results combined
1
with H NMR spectra suggested that even though hydrolysis can
occur in DMSO solution, leading to low molecular weight, poly-
azomethine films with high molecular weights can be still obtained
if the films were prepared at elevated temperature under reduced
pressure. In summary, polyazomethines prepared in m-cresol at
room temperature under reduced pressure had the highest
molecular weights. Post-polymerization at elevated temperature
ꢀ
(
200 C) in solid film can further increase their molecular weights.
3.5. Thermal properties
ꢀ
Fig. 5. TGA thermograms of PAM-2 (a) as-prepared and (b) isothermal at 200 C for
0 min.
The thermal properties of polyazomethines were evaluated by
DSC and TGA. The results, including glass transition temperatures
9
(
Tg), decomposition temperatures at 5% (T5%), 10% (T10%) weight loss,
ꢀ
10% weight loss of PAM-1e7 were in the range of 416e487 C and
4
nitrogen ranged from 40 to 63%. These polyazomethines exhibited
ꢀ
and residual weight percent (Rw800) at 800 C in nitrogen atmo-
sphere, are summarized in Table 4. The new polyazomethines PAM-
1
3
had the lowest glass transition temperature at 265 C. It might be due
to the flexibility of ether linkage on PAM-5 backbone. On the other
hand, PAM-4 derived from the new dialdehyde (5) and diamine (9)
ꢀ ꢀ
63e506 C, respectively. Their residual weights at 800 C in
e7 exhibited high glass transitiontemperaturesrangingfrom265to
ꢀ
0
high thermal stability without any significant weight loss up to
24 C. PAM-5 derived from the new dialdehyde (5) and 4,4 -ODA
ꢀ
ꢀ
400 C. This further confirmed that the enhancement in solubility
by non-coplanar biphenyl structures can be achieved without
sacrificing the thermal stability.
ꢀ
containing bulky phenyl substituents had the highest Tg (324 C).
This might be resulted from the fact that PAM-4 containing phenyl
substituents had higher chain rigidity than other polyazomethines.
PAM-2 containing bulky trifluoromethyl groups on both dialdehyde
and diamine moieties also exhibited high glass transition tempera-
3.6. Optical properties
The optical properties of PAM-1e7 were investigated by UVevis
and photoluminescence (PL) spectroscopy by using their 10
ꢁ
5
M
ꢀ
ture at 287 C. PAM-6 and PAM-7, which had the same chemical
chloroform solution. The results are summarized in Table 5. All the
polyazomethines showed azomethine characteristic absorption at
326e360 nm (Fig. 6). Among these polyazomethines, PAM-2 con-
taining electron-withdrawing trifluoromethyl substituents on both
diamine and dialdehyde moieties showed the most hypsochromic
shift with maximum absorption wavelength at 326 nm. On the
contrary, PAM-4 containing electron-rich phenyl substituents on its
diamine moiety showed the most bathochromic shift with
maximum absorption wavelength at 360 nm. PAM-5 containing
ether linkage on its backbone had maximum absorption wave-
length at 344 nm. This value is close to those of other poly-
azomethines in this study if electronic effect is excluded (i.e. PAM-2
and PAM-4). It indicates that the ether linkage and the non-
coplanar biphenyl structures might have the similar effect on
interrupting the conjugation along the polymer backbones.
structures except the azomethine arrangements, showed similar
ꢀ
glass transition temperatures at 272 and 271 C, respectively. It
suggested that the azomethine arrangements of polyazomethines
might have little effect on their glass transition temperatures.
Fig. 5 shows the TGA thermograms of PAM-2 as-prepared (Fig. 5
ꢀ
(
a)) and after isothermal treatment at 200 C for 90 min before
heating scan (Fig. 5(b)). The as-prepared PAM-2 experienced an
ꢀ
initial weight loss starting at around 190 C. This initial weight loss
was attributed to water released from the post-polymerization
between chain-end amine and aldehyde groups. When PAM-2 was
ꢀ
heated under nitrogen at 200 C for 90 min, the thermogram (Fig. 5
(
4
b)) shows no trace of post-polymerization and exhibited T5% at
ꢀ
82 C. All the decomposition temperatures of polyazomethines
ꢀ
PAM-1e7 were determined after the isothermal treatment (200 C
in nitrogen for 90 min) to exclude the effect of post-polymerization.
The decomposition temperatures in nitrogen atmosphere at 5% and
0
Compared with polyazomethine PAM-8 prepared from 4,4 -
benzidine with terephthalic dicarbaldehyde (TPDCA) (maximum
Table 5
Table 4
Optical properties of polyazomethines.
Thermal properties of polyazomethines.
a
b
c
labs (nm)
l
onset (nm)
l
emi (nm)
FPL (%)
Tga ( C)
ꢀ
T
5% ( C)
b
ꢀ
T10% ( C)
b
ꢀ
Char yieldc (wt.%)
PAM-1
PAM-2
PAM-3
PAM-4
PAM-5
PAM-6
PAM-7
348
326
357
360
344
345
344
398
391
408
410
397
400
401
433
425
434
491
431
468
442
0.13
0.05
0.05
1.45
0.61
1.10
0.05
PAM-1
PAM-2
PAM-3
PAM-4
PAM-5
PAM-6
PAM-7
273
287
270
324
265
272
271
416
482
475
432
438
487
484
470
502
506
468
463
501
498
62
40
63
48
47
42
40
a
ꢁ5
Measured by UVevis spectroscopy in chloroform (10 M).
Measured by photoluminescence spectroscopy in chloroform (10 M).
Quantum efficiency relative to quinine sulfate (10 M quinine sulfate in 1 N
a
ꢀ
ꢀ
b
c
ꢁ5
Measured by DSC at a heating rate of 10 C/min in nitrogen.
b
c
ꢁ5
Measured by TGA at a heating rate of 10 C/min in nitrogen.
ꢀ
Residual weight percentage at 800 C in nitrogen.
2 4
H SO ).

J.-C. Chen et al. / Polymer 52 (2011) 954e964
963
ꢁ
5
Fig. 6. UVevis spectra of PAM-1e7 in chloroform solution (10 M).
Fig. 8. Cyclic voltammogram (CV) of PAM-6.
absorption wavelength at 491 nm in concentrated sulfuric acid) [9],
PAM-1e7 exhibited significant hypsochromic shift due to the
interruption of conjugation along the polymer backbones.
voltammogram of PAM-6 in 0.1 M n-Bu
4
NClO
4
/acetonitrile solu-
ꢁ1
tion with a scan rate of 100 mV s is shown in Fig. 8. The values of
HOMO, LUMO and energy gap were calculated by similar equa-
tions from previous publication [35]. The electrochemical prop-
erties are summarized in Table 6. All of the polyazomethines
showed an irreversible peak both at oxidation and reduction
region. However, the polymer films remained stable during
repeating scans and showed essentially the same cyclic voltam-
mograms. The irreversibility, indicating the short lifetime of the
formed radical anions and radical cations during electrochemical
processes, might be resulted from the short conjugation length of
the non-coplanar biphenyls and the electron-withdrawing nature
of trifluoromethyl group.
Photoluminescence (PL) spectra were recorded by using the
maximum absorption wavelength as the excited wavelength in
ꢁ5
chloroform solution (10 M). The representative PL spectra of PAM-
, PAM-5 and PAM-6 are shown in Fig. 7. All the polyazomethines
4
PAM-1e7 showed a broad emission peak with emission wave-
lengths ranging from 425 to 491 nm. PL quantum efficiency of pol-
ꢁ
5
yazomethines was calculated by using quinine sulfate (10 M) in
N H SO as standard (
1
2
4
F
¼ 0.546) [15]. These new polyazomethines
had PL quantum efficiency ranging from 0.05 to 1.45%. It was
reported that polyazomethines had PL quantum efficiency less than
0
.03%, which is much lower than those of the analogous poly(ary-
lenevinylene)s and polyfluorenes [15]. The low PL quantum effi-
ciency limits their applications as the luminescent layers.
The HOMO and LUMO energy levels were determined to be
ꢁ
5.69 to ꢁ5.96 and ꢁ3.04 to ꢁ3.18 eV, respectively. These values
are much lower when compared with those (EHOMO ¼ ꢁ5.19 eV,
0
ELUMO ¼ ꢁ2.46 eV) of polyazomethine PAM-8 prepared from 4,4 -
3.7. Electrochemical properties
benzidine and terephthalic dicarbaldehyde (TPDCA) [9]. The LUMO
energy level is related to the conjugation length and the electron-
The electrochemical properties of these polyazomethines were
investigated by cyclic voltammetry (CV). A representative cyclic
accepting ability of polymers. The higher the delocalization of
p
electrons, the easier the acceptance of an additional electron during
reduction process. Although these new polyazomethines had
shorter conjugation length due to the interruption of conjugation
along the polymer backbones, the presence of strong electron-
withdrawing trifluoromethyl group outweighed the effect of short
conjugation length, leading to low ELUMO [33]. The low EHOMO was
Table 6
Electrochemical properties of polyazomethines.
Reductiona (V) Oxidationa (V)
E
HOMO (eV)
b
E
LUMO (eV) Egd (eV)
c
EReonset
EOxionset
PAM-1 ꢁ1.66
PAM-2 ꢁ1.53
PAM-3 ꢁ1.65
PAM-4 ꢁ1.67
PAM-5 ꢁ1.62
PAM-6 ꢁ1.57
PAM-7 ꢁ1.55
1.10
1.27
1.05
0.98
1.09
1.22
1.21
ꢁ5.81
ꢁ5.96
ꢁ5.76
ꢁ5.69
ꢁ5.80
ꢁ5.93
ꢁ5.92
ꢁ3.05
ꢁ3.18
ꢁ3.06
ꢁ3.04
ꢁ3.09
ꢁ3.14
ꢁ3.16
2.76
2.80
2.70
2.65
2.71
2.79
2.76
a
Measured by cyclic voltammetry.
b
c
Oxi
E
HOMO ¼ ꢁ(E onset þ 4.80ꢁ0.09 eV) [35].
Re
ELUMO ¼ ꢁ(E onset þ 4.80ꢁ0.09 eV) [35].
Fig. 7. PL spectra of PAM-4, 5 and 6 in chloroform solution (10ꢁ5 M).
d
Calculated by the equation : Eg ¼ ELUMO ꢁ EHOMO [35].

964
J.-C. Chen et al. / Polymer 52 (2011) 954e964
also resulted from the electron-withdrawing trifluoromethyl
group. For example, PAM-2 containing electron-withdrawing tri-
fluoromethyl group on both diamine and dialdehyde moieties
exhibited exceptionally low EHOMO of ꢁ5.96 eV. It can be concluded
that trifluoromethyl-containing non-coplanar biphenyls could be
regarded as a new electron acceptor when designing a donor-
acceptor system for different applications. This electron acceptor
can be used to tune the HOMO, LUMO energy levels and charge
mobility for electron-rich conjugated polymers.
biphenyl structure could be a new choice as the electron acceptor
for the structure design of new conjugated polymers.
References
[1] Iwan A, Sek D. Prog Polym Sci 2008;33(2):289e345.
[2] Morgan PW, Kwolek SL, Pletcher TC. Macromolecules 1987;20(4):729e39.
[
[
3] Higuchi M, Tsuruta M, Chiba H, Shiki S, Yamamoto K. J Am Chem Soc 2003;125
33):9988e97.
4] Niu H, Huang Y, Bai X, Li X, Zhang G. Mater Chem Phys 2004;86(1):33e7.
(
The electrochemical energy gaps of these new polyazomethines
were in the range of 2.65e2.80 eV. It was reported that the energy
gap is resulted from the combined effects of the conjugated length
of the polymer backbone and the electronic nature of the substit-
uents (electron-rich or electron-withdrawing) [9]. The energy gaps
of PAM-6 (2.79 eV) and PAM-7 (2.76 eV) were larger than that of
PAM-8 (2.73 eV). PAM-6 and PAM-7 with bulky trifluoromethyl
group give larger twisted angles than PAM-8 along the biphenyl
structure. The larger twisted angle combined with the electron-
withdrawing nature of trifluoromethyl group gives larger energy
gaps of PAM-6 and PAM-7 compared with that of PAM-8. However,
for PAM-1e5, their energy gaps are difficult to rationalize when
compared with that of PAM-8. It is probably due to this combined
effect of planarity and electronic nature of the substituents on the
energy gap.
[5] Bourgeaux M, Skene WG. Macromolecules 2007;40(6):1792e5.
[
[
6] Dutta PK, Jain P, Sen P, Trivedi R, Sen PK, Dutta J. Eur Polym J 2003;39
5):1007e11.
7] Hindson JC, Ulgut B, Friend RH, Greenham NC, Norder B, Kotlewski A, et al.
J Mater Chem 2010;20(5):937e44.
(
[
[
8] Park SB, Kim H, Zin WC, Jung JC. Macromolecules 1993;26(7):1627e32.
9] Yang CJ, Jenekhe SA. Macromolecules 1995;28(4):1180e96.
[
10] Thomas O, Inganas O, Andersson MR. Macromolecules 1998;31(8):2676e8.
[11] Liou GS, Lin HY, Hsieh YL, Yang YL. J Polym Sci Part A Polym Chem 2007;45
21):4921e32.
(
[
[
12] Matsumoto T, Yamada F, Kurosaki T. Macromolecules 1997;30(12):3547e52.
13] Park KH, Tani T, Kakimoto M, Imai Y. Macromol Chem Phys 1998;199
(6):1029e33.
[14] Wang C, Shieh S, Legoff E, Kanatzidis MG. Macromolecules 1996;29
9):3147e56.
(
[
15] Tsai FC, Chang CC, Liu CL, Chen WC, Jenekhe SA. Macromolecules 2005;38
(5):1958e66.
[16] Kim HC, Kim JS, Kim KS, Park HK, Baek S, Ree M. J Polym Sci Part A Polym
Chem 2004;42(4):825e34.
[
[
17] Grigoras M, Antonoaia NC. Polym Int 2005;54(12):1641e6.
18] Mallegol T, Gmouh S, Meziane MAA, Blanchard-Desce M, Mongin O. Synthesis
2005;2005(11):1771e4.
4
. Conclusions
new dialdehyde containing electron-withdrawing tri-
[
19] Anuragudom P, Newaz SS, Phanichphant S, Lee TR. Macromolecules 2006;39
(10):3494e9.
A
[
20] Morgan PW, Pletcher TC, Kwolek SL. Polym Prepr 1983;24:470.
[21] Yang CJ, Jenekhe SA. Chem Mater 1991;3(5):878e87.
22] Zhao D, Moore JS. J Org Chem 2002;67(11):3548e54.
23] Everitt PM, Loh SM, Turner EE. J Chem Soc; 1960:4587.
24] Shiang WR, Woo EP. J Polym Sci Polym Chem Ed 1993;31(8):2081e91.
[25] Rogers HG, Gaudiana RA, Hollinsed WC, Kalyanaraman PS, Manello JS,
McGowan C, et al. Macromolecules 1985;18(6):1058e68.
[26] Sako S. Bull Chem Soc Jpn 1935;10(12):593e4.
0
fluoromethyl groups at the 2 and 2 positions of biphenyl was
prepared via a five-step synthetic route. The formed poly-
azomethines exhibited excellent solubility in polar organic solvents
such as DMF, DMAc, NMP and m-cresol while maintained their high
[
[
[
thermal stability. It was found that these polyazomethines would
1
undergo hydrolysis in solution. The hydrolysis, evidenced by
H
[
27] Olinga TE, Destri S, Botta C, Porzio W, Consonni R. Macromolecules 1998;31
NMR spectra and GPC chromatograms, was resulted from the
reverse reaction of azomethine formation and was favored at
higher temperature. It was concluded that polyazomethines
prepared in m-cresol at room temperature under reduced pressure
had the highest molecular weights. Post-polymerization of poly-
azomethine thin films at elevated temperature can further
increased their molecular weights. These polyazomethines also had
low electrochemical EHOMOs and ELUMOs. This is resulted from the
combined effects of electron-withdrawing trifluoromethyl group
and non-coplanar biphenyls. Combined with the excellent solu-
(4):1070e8.
[28] Destri S, KhotinaI A, Porzio W. Macromolecules 1998;31(4):1079e86.
29] Beam CF, Brown J, Hall RW, Bernhardt FC, Sides KL, Mack MH, et al. J Polym Sci
Polym Chem Ed 1978;16(10):2679e81.
30] Sek D, Iwan A, Jarzabek B, Kaczmarczyk B, Kasperczyk J, Mazurak Z, et al.
Macromolecules 2008;41(18):6653e63.
31] Liu CL, Tsai FC, Chang CC, Hsieh KH, Lin JL, Chen WC. Polymer 2005;46
[
[
[
[
(13):4950e7.
32] Li F, Fang S, Ge JJ, Honigfort PS, Chen JC, Harris FW, et al. Polymer 1999;40
(16):4571e83.
[
[
33] Chen JC, Chiang CJ, Liu YC. Synth Met 2010;160(18):1953e61.
34] Greene TW, Wuts PGM. In protective groups in organic synthesis [Chapter 4].
New York: Wiley; 1999. 359e360.
0
0
bility and high thermal stability, 2,2 -bis(trifluoromethyl)-4,4 -
[35] Yasuda T, Imase T, Yamamoto T. Macromolecules 2005;38(17):7378e85.