Synthesis, luminescence and electrochromism of aromatic poly(amine-amide)s with pendent triphenylamine moietiest

Article abstract of DOI:10.1039/b419183h

A new triphenylamine-containing aromatic dicarboxylic acid monomer, N,N-bis(4-carboxyphenyl)-N′,N′-diphenyl-1,4-phenylenediamine (2), was synthesized from the amination reaction between 4-aminotriphenylamine and 4-fluorobenzonitrile and subsequent alkaline hydrolysis of the dinitrile intermediate. A series of novel aromatic poly(amine-amide)s with triphenylamine units in the main chain and as the pendent group were prepared from the newly synthesized dicarboxylic acid and various aromatic diamines. These poly(amine-amide)s were amorphous and readily soluble in many organic solvents. All the polymers could be solution-cast into flexible films with good mechanical properties. They had excellent levels of thermal stability associated with high glass-transition temperatures (226-261°C). These polymers exhibited strong UV-vis absorption bands at 350-365 nm in NMP solution. Their photoluminescence spectra showed maximum bands around 512-543 nm in the green region. The hole-transporting and electrochromic properties are examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the poly(amine-amide) 5a prepared from the dicarboxylic acid monomer 2 with a structurally similar diamine monomer N,N-bis(4-aminophenyl)-N′,N′- diphenyl-1,4-phenylenediamine (4a) exhibited four reversible oxidation redox couples in acetonitrile solution at E_1/2 = 0.60, 0.80, 0.97 and 1.13V, respectively. All the poly(amine-amide)s exhibited excellent reversibility of electrochromic characteristics by continuous five cyclic scans between 0.0 to 1.30 V, with a color change from the original pale yellowish neutral form to the green and then to blue oxidized forms. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b419183h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis, luminescence and electrochromism of aromatic
poly(amine–amide)s with pendent triphenylamine moieties{
Guey-Sheng Liou,*a Sheng-Huei Hsiaob and Tzy-Hsiang Sua
Received 22nd December 2004, Accepted 14th February 2005
First published as an Advance Article on the web 25th February 2005
DOI: 10.1039/b419183h
A new triphenylamine-containing aromatic dicarboxylic acid monomer, N,N-bis(4-
carboxyphenyl)-N9,N9-diphenyl-1,4-phenylenediamine (2), was synthesized from the amination
reaction between 4-aminotriphenylamine and 4-fluorobenzonitrile and subsequent alkaline
hydrolysis of the dinitrile intermediate. A series of novel aromatic poly(amine–amide)s with
triphenylamine units in the main chain and as the pendent group were prepared from the newly
synthesized dicarboxylic acid and various aromatic diamines. These poly(amine–amide)s were
amorphous and readily soluble in many organic solvents. All the polymers could be solution-cast
into flexible films with good mechanical properties. They had excellent levels of thermal stability
associated with high glass-transition temperatures (226–261 uC). These polymers exhibited strong
UV–vis absorption bands at 350–365 nm in NMP solution. Their photoluminescence spectra
showed maximum bands around 512–543 nm in the green region. The hole-transporting and
electrochromic properties are examined by electrochemical and spectroelectrochemical methods.
Cyclic voltammograms of the poly(amine–amide) 5a prepared from the dicarboxylic acid
monomer 2 with a structurally similar diamine monomer N,N-bis(4-aminophenyl)-
N9,N9-diphenyl-1,4-phenylenediamine (4a) exhibited four reversible oxidation redox couples in
acetonitrile solution at E1/2 5 0.60, 0.80, 0.97 and 1.13 V, respectively. All the poly(amine–amide)s
exhibited excellent reversibility of electrochromic characteristics by continuous five cyclic scans
between 0.0 to 1.30 V, with a color change from the original pale yellowish neutral form to the
green and then to blue oxidized forms.
films can be more easily deposited over a larger area and they
Introduction
are often flexible. Furthermore, prevention of crystallization
and phase separation may improve the device performance.
The uses of polymeric materials also provide the possibility for
various chemical design and modifications.25
Triarylamines are an important class of compounds because
they are easily oxidized to form stable aminium radical cations.
Thus, triarylamines can be building blocks for high-spin
polyradicals that have shown ferromagnetic coupling,1,2 as
well as for conductive polymers.3 Perhaps most commonly,
triarylamines have been used as the hole-transport layer in
electroluminscent devices.4–7 A new material with longer life,
higher efficiency and an appropriate HOMO energy level is in
increasing demand and most of the charge transporting
materials are low molecular weight compounds; of these,
N,N,N9,N9-tetraryl-1,4-phenylenediamine, N,N,N9,N9-tetraryl-
benzidine and triphenylamine (TPA) derivatives have been
extensively investigated because of their high charge mobility
and electrochromism.8–14 The main disadvantage, which often
makes application difficult, is their insufficient morphological
stability and susceptibility to crystallization or phase separa-
tion. To solve this problem, attempts to introduce TPA units
into the main or side chain of the polymer backbone were
undertaken intensively, and some important results have been
obtained.15–24 One of the perceived advantages is that polymer
Wholly aromatic polyamides are characterized as highly
thermally stable polymers with
a favorable balance of
physical and chemical properties. However, rigidity of the
backbone and strong hydrogen bonding results in high
melting or glass-transition temperatures (Tg’s) and limited
solubility in most organic solvents.26,27 These properties
make them generally intractable or difficult to process, thus
restricting their applications. To overcome such a difficulty,
polymer structure modification becomes necessary. Recently,
we have reported the synthesis of soluble aromatic polyamides
and polyimides bearing triphenylamine units in the main chain
based on N,N9-bis(4-aminophenyl)-N,N9-diphenyl-1,4-phenyl-
enediamine,28,29
N,N9-bis(4-carboxyphenyl)-N,N9-diphenyl-
1,4-phenylenediamine,30 2,4-diaminotriphenylamine,31 and
N,N-bis(4-aminophenyl)-N9,N9-diphenyl-1,4-phenylenedi-
amine,32,33 respectively. Because of the incorporation of bulky,
three-dimensional triphenylamine units along the polymer
backbone, all the polymers were amorphous, had good
solubility in many aprotic solvents, and exhibited excellent
thin-film-forming capability. Imai et al. reported that the charge
injection and electroluminescent efficiency were improved
remarkably by the incorporation of the hole-transporting
polyimide containing a triphenylamine unit in the backbone.34
{ Electronic supplementary information (ESI) available: Synthesis and
discussion of monomer starting materials and characterization
Figs. S1–4 (IR and NMR spectra). Color Figs. 3, 6–8 showing PL
and electrochromic properties of the polymers. See http://www.rsc.org/
suppdata/jm/b4/b419183h/
*gsliou@ncnu.edu.tw
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Therefore, the introduction of triphenylamine units into
aromatic polyamide and polyimide backbones would be
expected to be a potential structural approach for not only
increasing solubility without sacrificing high thermal stability
but also allowing them to be used as potential hole-
transporting materials. Nevertheless, little is known to date
about the preparation, electrochemical and electrochromic
properties of aromatic polyamides or polyimides having
N,N,N9,N9-tetraphenyl-1,4-phenylenediamine unit in the main
backbone. The only examples are poly(amine–imide)s32 and
poly(amine–amide)s33 derived from N,N-bis(4-aminophenyl)-
N9,N9-diphenyl-1,4-phenylenediamine. As a continuation of
these studies, we seek to create redox arrays designed for
controlling the hole-transport and electrochromic properties of
aromatic polyamides by adjoining N-based redox functions of
different oxidation potential. In the present article, we
designed a novel class of N,N,N9,N9-tetraphenyl-1,4-phenyl-
enediamine-containing poly(amine–amide)s with mixed redox
pendent triphenylamine groups of varied oxidation potential
to elucidate the effect of their corresponding radical cations on
the electrochromic behavior of the poly(amine–amide)s
derived from the new dicarboxylic acid, N,N-bis(4-carboxy-
phenyl)-N9,N9-diphenyl-1,4-phenylenediamine (2). The general
properties such as solubility, crystallinity, thermal and
mechanical properties are reported. The electrochemical,
electrochromic and photoluminescence properties of these
polymers prepared by casting solution onto an indium tin
oxide (ITO)-coated glass substrate are also investigated
and compared with those of structurally related ones from
4,49-dicarboxytriphenylamine (3).
(TCI) were used as received. Commercially obtained anhy-
drous calcium chloride (CaCl2) was dried under vacuum at
180 uC for 8 h. Tetrabutylammonium perchlorate (TBAP) was
obtained from Acros and recrystallized twice from ethyl
acetate and then dried in vacuo prior to use. All other reagents
were used as received from commercial sources.
Preparation of poly(amine–amide)s
The synthesis of poly(amine–amide) 5a is used as an example
to illustrate the general synthetic route. The typical procedure
is as follows. A mixture of 0.626 g (1.25 mmol) of the
triphenylamine-based dicarboxylic acid monomer 2, 0.553 g
(1.25 mmol) of N,N-bis(4-aminophenyl)-N9,N9-diphenyl-1,4-
phenylenediamine (4a), 0.15 g of calcium chloride, 0.9 mL of
TPP, 0.6 mL of pyridine, and 2.5 mL of NMP was heated with
stirring at 105 uC for 3 h. The polymer solution was poured
slowly into 300 mL of stirring methanol giving rise to a stringy,
fiber-like precipitate that was collected by filtration, washed
thoroughly with hot water and methanol, and dried at 150 uC
for 15 h in vacuo. Precipitations from NMP into methanol were
carried out twice for further purification. The inherent viscosity
of the obtained poly(amine–amide) 5a was 0.80 dL g21
,
measured at a concentration of 0.5 g dL21 in NMP at 30 uC.
The IR spectrum of 5a (film) exhibited characteristic amide
absorption bands at 3316 (N–H stretching), 1668 cm21 (amide
carbonyl). Anal. Calcd For (C62H46N6O2)n (907.07)n: C,
82.10%; H, 5.11%; N, 9.27%. Found: C, 81.40%; H, 5.27%;
N, 9.16%. Other poly(amine–amide)s were prepared by an
analogous procedure.
Film preparation
Experimental
A solution of polymer was made by dissolving about 0.6 g
of the poly(amine–amide) sample in 10 mL of NMP. The
homogeneous solution was poured into a 9 cm glass Petri dish,
which was placed in a 90 uC oven overnight to remove most of
the solvent. The cast film was then released from the glass
substrate and was further dried in vacuo at 160 uC for 8 h.
The obtained films were about 50–60 mm in thickness and
were used for X-ray diffraction measurements, tensile tests,
solubility tests and thermal analyses.
Materials
According to well-known chemistry,35–37 4-aminotriphenyl-
amine (mp 5 148–149 uC) was prepared by the aromatic
nucleophilic amination of 4-nitrofluorobenzene and diphenyl-
amine in N,N-dimethylformamide (DMF) in the presence of
sodium hydride, followed by reduction by means of hydrazine
and Pd/C in refluxing ethanol. 4,49-Dicarboxytriphenylamine
(3) (mp 5 313–315 uC; lit.38 308–310 uC) was synthesized by
the caesium fluoride-assisted condensation of aniline with
4-fluorobenzonitrile, followed by alkaline hydrolysis of the
intermediate dinitrile compound according to a reported
Measurements
procedure.38
N,N-Bis(4-aminophenyl)-N9,N9-diphenyl-1,4-
Infrared spectra were recorded on a Perkin Elmer RXI FT-IR
spectrometer. 1H and 13C NMR spectra were measured on
a Bruker Avance 500 MHz FT-NMR system. The X-ray
crystallographic data were collected on an Enraf-Norius FR
590 CAD-4 diffractometer. Elemental analyses were run in an
Elementar VarioEL-III. The inherent viscosities were deter-
mined at 0.5 g dL21 concentration using a Tamson TV-2000
viscometer at 30 uC. Wide-angle X-ray diffraction (WAXD)
measurements of the polymer films were performed at room
temperature (ca. 25 uC) on a Shimadzu XRD-7000 X-ray
diffractometer (40 kV, 20 mA) with a graphite monochro-
mator, using nickel-filtered CuKa radiation. Ultraviolet–
visible (UV–vis) spectra of the polymer films were recorded
on a Varian Cary 50 Probe spectrometer. An Instron universal
tester model 4400R with a load cell of 5 kg was used to study
phenylenediamine32 (4a; mp 5 245–247 uC) and 4,49-diamino-
triphenylamine39 (4b; mp 5 186–187 uC) were synthesized by
the nucleophilic fluoro-displacement reaction of 4-fluoroni-
trobenzene with 4-aminotriphenylamine and aniline, respec-
tively, followed by palladium-catalyzed hydrazine reduction.
4-Fluorobenzonitrile (Tokyo Chemical Industries), sodium
hydride (95%, dry, Aldrich), potassium hydroxide (Tedia) were
used as received. N,N-Dimethylacetamide (DMAc) (Tedia),
N,N-dimethylformamide (DMF) (Acros), dimethyl sulfoxide
(DMSO) (Tedia), N-methyl-2-pyrrolidinone (NMP) (Tedia),
pyridine (Py) (Tedia), triphenyl phosphite (TPP) (Acros) were
used without further purification. Commercially available
aromatic diamines such as p-phenylenediamine (4c) (TCI),
m-phenylenediamine (4d) (TCI), and 4,49-oxydianiline (4e)
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the stress–strain behavior of the samples. A gauge length of
2 cm and a crosshead speed of 5 mm min21 were used for this
study. Measurements were performed at room temperature
with film specimens (0.5 cm width, 6 cm length), and an
average of at least three replicates was used. Thermogravi-
metric analysis (TGA) was conducted with a Perkin Elmer
Pyris 1 TGA. Experiments were carried out on approximately
6–8 mg film samples heated in flowing nitrogen or air (flow
rate 5 40 cm3 min21) at a heating rate of 20 uC min21. DSC
analyses were performed on a Perkin Elmer Pyris 1 DSC at a
scan rate of 20 uC min21 in flowing nitrogen (20 cm3 min21).
Thermomechanical analysis (TMA) was conducted with a
Perkin Elmer TMA 7 instrument. The TMA experiments were
conducted from 50 to 350 uC at a scan rate of 10 uC min21 with
a penetration probe 1.0 mm in diameter under an applied
constant load of 10 mN. Softening temperatures (Ts) were
taken as the onset temperatures of probe displacement on the
TMA traces. Electrochemistry was performed with a CHI
611B electrochemical analyzer. Voltammograms are presented
with the positive potential pointing to the left and with
increasing anodic currents pointing downwards. Cyclic
voltammetry was performed with the use of a three-electrode
cell in which ITO (polymer films area about 0.7 cm 6 0.5 cm)
was used as a working electrode. A platinum wire was used
as an auxiliary electrode. All cell potentials were taken with
the use of a homemade Ag/AgCl, KCl (sat.) reference
electrode. The spectroelectrochemical cell was composed of a
1 cm cuvette, ITO as a working electrode, a platinum wire as
an auxiliary electrode and a Ag/AgCl reference electrode.
Absorption spectra were measured with a HP 8453 UV–visible
spectrophotometer. Photoluminescence spectra were measured
with a Jasco FP-6300 spectrofluorometer.
Fig. 1 Molecular structure of dicarboxylic acid 2 by single crystal
X-ray analysis.
Results and discussion
Polymer synthesis
A series of novel aromatic poly(amine–amide)s 5a–e having the
triphenylamine unit in the main chain and as the pendent
group were prepared from the newly synthesized dicarboxylic
acid 2 (Fig. 1 and Scheme 1) and various aromatic diamines
4a–e by the phosphorylation polycondensation reaction40
using TPP and pyridine as condensing agents (Scheme 2). All
the polymerizations proceeded homogeneously throughout
the reaction and afforded clear, highly viscous polymer
solutions. All the polymers precipitated in a tough, fiber-like
form when the resulting polymer solutions were slowly poured
with stirring into methanol. These poly(amine–amide)s were
obtained in almost quantitative yields, with inherent viscosity
values in the range of 0.59–1.04 dL g21. The formation of
poly(amine–amide)s was confirmed by elemental analysis
and IR spectroscopy. The elemental analysis values of these
polymers are listed in Table 1. Fig. 2 shows a typical IR
spectrum of poly(amine–amide) 5a. They exhibited charac-
teristic IR absorption bands of the amide group around 3316
(N–H stretching) and 1668 cm21 (amide carbonyl). For a
comparative study, a series of referenced poly(amine–amide)s
6a–e were also synthesized from 4,49-dicarboxytriphenylamine
(3) and corresponding diamines 4a–e.
Scheme 1
Scheme 2
1814 | J. Mater. Chem., 2005, 15, 1812–1820
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Table 1 Inherent viscosity and elemental analysis of poly(amine–amide)s
Table 2 Solubilitya of aromatic poly(amine–amide)s
Poly(amine–
amide)s
Solvent
Elemental analysis (%) of poly(amine–amide)s
Formula
Polymer NMP DMAc DMF DMSO m-Cresol THF Chloroform
a
/
ginh
Code dL g21 (molecuar weight)
C
H
N
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
a
+
+
+
+
+h
+
+h
+h
+
—
¡
+h
+h
¡
+h
+h
+h
+h
+
¡
¡
+h
+
¡
+h
¡
—
—
—
—
—
¡
—
—
—
—
—
—
—
—
—
—
—
—
—
—
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
a
0.80
0.39
0.70
0.96b
1.04
0.57
0.53
0.76
0.45
1.75
(C62H46N6O2)n
(907.07)n
(C50H37N5O2)n
(739.86)n
(C38H28N4O2)n
(572.65)n
(C38H28N4O2)n
(572.65)n
(C44H32N4O3)n
(664.75)n
(C50H37N5O2)n
(739.86)n
(C38H28N4O2)n
(572.65)n
(C26H19N3O2)n
(405.45)n
Calcd
Found 81.40 5.27
Calcd 81.17 5.04
Found 79.71 5.13
Calcd 79.70 4.93
Found 77.71 5.13
Calcd 79.70 4.93
Found 76.94 5.07
Calcd 79.50 4.85
Found 77.55 5.03
Calcd 81.17 5.04
Found 79.04 5.23
Calcd 79.70 4.93
Found 77.85 5.11
Calcd
Found 74.42 5.02 10.06
Calcd 77.02 4.72 10.36
Found 73.55 5.07 10.29
82.10 5.11
9.27
+
+
+
9.16
9.47
9.07
9.78
9.51
9.78
9.52
8.43
8.54
9.47
9.13
9.78
9.29
¡
+
—
¡
+
—
¡
+
+h
¡
+h
¡
+
+
+
+
¡
+
+
+
¡
Solubility: +, soluble at room temperature; +h, soluble on heating;
¡, partially soluble or swelling; 2, insoluble even on heating.
Table 3 Mechanical properties of poly(amine–amide) films
77.02 4.72 10.36
Tensile
strength/MPa
Elongation
at break (%)
Initial
modulus/GPa
Polymer
(C26H19N3O2)n
(405.45)n
5a
5b
5c
5e
6a
6b
6c
6d
6e
74
68
78
110
73
72
73
96
66
22
11
23
8
24
8
26
10
9
1.7
1.6
2.0
2.3
1.8
1.8
1.7
2.0
1.8
(C32H23N3O3)n
(497.54)n
Calcd 77.25 4.66
Found 74.25 4.87
8.45
8.43
Measured at a polymer concentration of 0.5 g dL21 in NMP at
b
30 uC Measured at polymer concentration of 0.5
NMP contain 5% LiCl.
g
dL21 in
summarized Table 4. All the polymers exhibited good thermal
stability with insignificant weight loss up to 400 uC in nitrogen.
The 10% weight-loss temperatures of the polyamides in
nitrogen and air were recorded in the range of 454–568 and
468–578 uC, respectively. The amount of carbonized residue
(char yield) of these polymers in nitrogen atmosphere was
more than 55% at 800 uC. The high char yields of these
polymers can be ascribed to their high aromatic content. All
the polymers indicated no clear melting endotherms up to the
decomposition temperatures on the DSC thermograms. This
Fig. 2 IR spectrum (film) of poly(amine–amide) 5a
Basic characterization
Table 4 Thermal properties of aromatic poly(amine–amide)s
The wide-angle X-ray diffraction studies of the poly(amine–
amide)s indicated that these polymers were essentially
amorphous. The qualitative solubility properties of the
poly(amine–amide)s are presented in Table 2, and most of
these polymers exhibited good solubility to highly polar
organic solvents due to the introduction of the three-
dimensional triphenylamine core in the repeat unit. Thus, the
excellent solubility makes these polymers potential candidates
for practical applications by spin- or dip-coating processes. It
is worth noting that polymer 5d formed an opaque film via
solution casting from NMP and showed less solubility. The
reason is not apparent to us at this time. Except for 5d, these
polymers could afford transparent, flexible and tough films
and they were subjected to tensile testing. As shown in Table 3,
the tensile strengths, elongations to break, and initial moduli
of these films were in the ranges of 66–110 MPa, 9–26% and
1.6–2.3 GPa, respectively.
Td at 5%
Td at 10%
weight loss/uCc weight lossuCc
Char
yield
Tg/
uCa
Ts/
uCb
Polymer
N2
Air
N2
Air
(wt.%)d
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
a
257
261
257
260
226
268
277
294
265
256
257
261
250
255
215
268
274
295
262
251
510
505
464
505
464
471
463
450
428
442
518
488
458
476
445
475
453
457
423
445
557
568
526
545
522
534
530
496
454
495
578
570
534
511
514
552
528
513
468
483
59
66
69
55
68
68
69
70
69
67
Midpoint temperature of baseline shift on the DSC heating trace
(rate 20 uC min21) of the sample after heating at 300 uC for 1 h.
Softening temperature measured by TMA with a constant applied
b
load of 10 mN at a heating rate of 10 uC min21
.
Decomposition
c
temperature, recorded via TGA at a heating rate of 20 uC min21 and
a gas-flow rate of 40 cm3 min21
.
Residual weight percentage at
d
The thermal properties of all the poly(amine–amide)s
were investigated by TGA, DSC and TMA. The results are
800 uC in nitrogen.
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result also supports the amorphous nature of these triphenyl-
amine-containing polymers. The Tg’s of all the polymers were
measured to be in the range of 226–294 uC by DSC. The values
are substantially higher than that of 4,49-bis[N,N-(m-tolyl)-
phenylamino]biphenyl (TPD) (63 uC), indicating that the use
of these polymers may greatly improve the device durability,
which relates to the Tg’s of the materials as reported by Tokito
et al.41,42 The softening temperatures (Ts) of the polymer film
samples were determined by the TMA method with a loaded
penetration probe. They were obtained from the onset tem-
perature of the probe displacement on the TMA trace. In all
cases, the Ts values obtained by TMA are comparable to the
Tg values measured by the DSC experiments. When compared
with the analogous poly(amine–amide)s 6, the 5 series of
poly(amine–amide)s showed a slightly decreased Tg and Ts
that may be attributed to the increased conformational
flexibility or free volume caused by the introduction of the
bulky pendent triphenylamine group in the repeat unit.
Optical and electrochemical properties
The optical and electrochemical behavior of the poly(amine–
amide)s were investigated by UV–vis, photoluminescence
spectroscopy, and cyclic voltammetry. The results are sum-
marized in Table 5. The UV–vis absorption spectra of these
polymers exhibited strong absorption bands at 350–365 nm in
NMP solution, which are assignable to the p–p* transition
resulting from the conjugation between the aromatic rings
and nitrogen atoms. Fig. 3 shows the UV–vis absorption and
photoluminescence spectra of poly(amine–amide)s 5c–e, 6d
and 6e for comparison. In the solution photoluminescence
spectra, 5c–e emitted in the green region (512–519 nm), while
6d and 6e emitted in the blue region (451–469 nm). Compared
to the polymer 5d exhibiting an emission peak at 513 nm,
the corresponding polymer 6d showed blue-shifted emission
spectrum with the maxima peak at around 451 nm. Apparently
the emission in long wavelength region for the 5 series was due
to the existence of the pendent triphenylamine groups, which
resulted in a lower HOMO energy level and decreased the
energy gap between HOMO and LOMO. Comparing with the
corresponding PL intensity of poly(amine–amide)s, the 6 series
is relatively greater than that of the 5 series. This phenomenon
Fig. 3 Absorptions and PL spectra of poly(amine–amide)s with a
concentration of NMP (5 mg mL21). Quinine sulfate dissolved in 1 N
H2SO4 (aq.) with a concentration of (7.8 mg mL21), assuming wPL of
0.55.
possibly could be attributed to the Stokes shift being clearly
greater for the 5 series and this results in the lower emission
quantum yield than the 6 series due to the greater number of
conformations and a higher rate of radiationless decay. The
polymer films were measured for optical transparency using
UV–vis spectroscopy, and the cutoff wavelengths (absorption
edge; l0) were in the range of 412–462 nm. The electrochemical
behavior of the 5 and 6 series poly(amine–amide)s was
investigated by cyclic voltammetry performed for the cast film
on an ITO-coated glass substrate as working electrode in
dry acetonitrile (CH3CN) containing 0.1 M of TBAP as an
electrolyte under nitrogen atmosphere. The typical cyclic
voltammograms for poly(amine–amide) 5a and 5c are shown
in Fig. 4 and Fig. 5, respectively. There are four reversible
Table 5 Optical and electrochemical properties for the aromatic poly(amine–amide)s
Oxidation/V (vs. Ag/AgCl)
labs,max
nma
/
labs,onset
nma
/
lPL
/
l0/
nmc
HOMO–LUMO
gape/eV
HOMOf/
eV
Code
nmb
First
Second
Third
Fourth
LUMOgeV
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
a
358 (353)
356 (355)
361 (358)
350 (357)
350 (355)
363 (359)
363 (359)
365 (355)
354 (361)
354 (355)
417 (428)
412 (421)
406 (421)
403 (416)
398 (414)
407 (417)
404 (413)
392 (403)
386 (401)
385 (404)
543 (526)
525 (516)
519 (515)
513 (513)
512 (509)
537 (514)
518 (497)
469 (454)
451 (453)
458 (452)
464
458
454
—
448
462
446
422
429
412
0.60
0.80
0.77
0.78
0.77
0.59
0.83
0.80
1.14
1.13
1.14
1.14
0.96
(1.19)d
—
0.97
—
—
—
—
—
—
—
—
—
1.13
—
—
—
—
—
—
—
—
—
2.90
2.95
2.95
2.98
3.00
2.97
3.00
3.08
3.09
3.07
4.96
5.16
5.13
5.14
5.13
4.95
5.19
5.58
5.50
5.56
2.06
2.21
2.18
2.16
2.13
1.98
2.19
2.50
2.41
2.49
b
(1.22)d
(1.14)d
(1.20)d
—
—
UV–vis absorption measurements in NMP (0.02 mg mL21) at room temperature, values in parentheses are polymer thin films. PL spectra
measurements in NMP (5 mg mL21) at room temperature, values in parentheses are polymer thin films. The cutoff wavelengths (l0) from the
transmission UV–vis absorption spectra of polymer films. Irreversible peak potential (Ep,a). The data were calculated in film state by the
c
d
e
f
equation: gap 5 1240/lonset
LUMO 5 HOMO 2 gap.
.
The HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV).
g
1816 | J. Mater. Chem., 2005, 15, 1812–1820
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poly(amine–amide) 6d without pendent triphenylamine groups
(1.14 V). The first electron removal for poly(amine–amide) 5d
is believed to occur at the N atom on the pendent triphenyl-
amine unit, which is more electron-rich than the N atom on the
main-chain triphenylamine group surrounded by two phenyl
groups with amide linkages. The energy of the HOMO and
LUMO levels of the investigated poly(amine–amide)s can be
determined from the oxidation half-wave potentials and the
onset absorption wavelength and the results are listed in
Table 5. For example, the oxidation half-wave potential
for poly(amine–amide) 5c has been determined as 0.77 V vs
Ag/AgCl. The external ferrocene/ferrocenium (Fc/Fc+) redox
standard E1/2 is 0.44 V vs. Ag/AgCl in CH3CN. Assuming that
the HOMO energy for the Fc/Fc+ standard is 4.80 eV with
respect to the zero vacuum level, the HOMO energy for
poly(amine–amide) 5c has been evaluated to be 5.13 eV.
Fig. 4 Cyclic voltammogram of poly(amine–amide) 5a film onto an
indium tin oxide (ITO)-coated glass substrate in CH3CN containing
0.1 M TBAP. Scan rate 5 0.1 V s21
.
Electrochromic characteristics
Electrochromism of the thin films from poly(amine–amide)s
was determined by optically transparent thin-layer electrode
(OTTLE) coupled with a UV–vis spectroscopy. The electrode
preparations and solution conditions were identical to those
used in cyclic voltammetry. The typical electrochromic
absorption spectra of poly(amine–amide) 5a and 5c are shown
as Figs. 6–8. When the applied potentials increased positively
from 0 to 0.73, 0.93, 1.09, 1.26 V, respectively, corresponding
to the first, second, third and fourth electron oxidation, the
peak of characteristic absorbance at 342 nm for neutral form
poly(amine–amide) 5a decreased gradually, while four new
bands grew up at 987, 938, 730 and 714 nm, respectively. The
new spectral patterns were assigned as those of the cationic
radical poly(amine–amide)+, poly(amine–amide)2+
,
poly-
(amine–amide)3+, and poly(amine–amide)4+, respectively, the
products obtained by electron removal from the lone pair of
nitrogen atom on different two p-phenylenediamine structures.
According to the electron density among the four nitrogen
atoms on different two p-phenylenediamine structures in each
repeating unit, the anodic oxidation pathway of poly(amine–
amide) 5a was postulated as in Scheme 3.
Fig. 5 Cyclic voltammogram of poly(amine–amide) 5c film onto an
indium tin oxide (ITO)-coated glass substrate in CH3CN containing
0.1 M TBAP. Scan rate 5 0.1 V s21
oxidation redox couples for poly(amine–amide) 5a at E1/2
5
0.60, 0.80, 0.97 and 1.13 V corresponding to successive one-
electron removal from the nitrogen atoms at both N,N,N9,N9-
tetraphenyl-1,4-phenylenediamine structures in each repeating
unit to yield two stable delocalized radical cations, poly-
(amine–amide)+ and poly(amine–amide)3+, and two stable
quinonoid-type dications, poly(amine–amide)2+ and poly-
(amine–amide)4+, respectively.43 Both triarylamine centers at
each N,N,N9,N9-tetraphenyl-1,4-phenylenediamine structures
of repeating unit are strongly coupled by the fact that the
redox couple separations are 370 mV (0.97–0.60 V) and 330 mV
(1.13–0.80 V), respectively, which are larger than the statistical
value for non-interacting centers, where the separation of the
first and the second oxidation redox couple is 35.6 mV.44
Because of the good stability of the films and excellent adhesion
between the polymer and ITO substrate, the poly(amine–
amide)s 5a exhibited great reversibility of electrochromic
characteristics by continuous five cyclic scans between 0.0 to
1.30 V, changing color from the original pale yellowish to
green, and then to blue. Comparing the oxidation potential
data, it was found that poly(amine–amide) 5d with pendent
triphenylamine groups (0.78 V) is much easier oxidized than
Fig. 6 Electrochromic behavior of poly(amine–amide) 5a thin film (in
CH3CN with 0.1 M TBAP as the supporting electrolyte) at (a) 0 (b)
0.73 (c) 0.93 (d) 1.09 (e) 1.26 V.
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Fig. 8 Electrochromic behavior of poly(amine–amide) 5c thin
Fig. 7 Electrochromic behavior of poly(amine–amide) 5c thin
film (in CH3CN with 0.1 TBAP as the supporting elec-
film (in CH3CN with 0.1 M TBAP as the supporting electrolyte)
at (a) 1.01 (b) 1.05 (c) 1.07 (d) 1.10 (e) 1.13 (f) 1.16 (g) 1.19 (h) 1.25(i)
1.30 V.
M
trolyte) at (a) 0 (b) 0.70 (c) 0.73 (d) 0.77 (e) 0.80 (f) 0.83 (g) 0.85 (h)
0.95 V.
Scheme 3
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Conclusions
A series of new high-molecular weight poly(amine–amide)s
5a–e having triphenylamine units both in polymer main chain
and pendent group have been readily prepared from the
aromatic dicarboxylic acid, N,N-bis(4-carboxyphenyl)-N9,N9-
diphenyl-1,4-phenylenediamine and various aromatic dia-
mines. Attaching bulky and inherently electron-donating
triphenylamine units to the polymer main chain and/or as
pendent group, not only could be a good approach to facile
color tuning of the electrochromic behaviors due to the
oxidation potentials being adjusted, but also disrupts the
coplanarity of aromatic units in chain packing, which increases
the between-chains spaces or free volume, thus most of
the polymers were amorphous with good solubility in many
polar aprotic solvents and exhibited excellent thin-film-
forming ability. In addition to high Tg or Ts values, good
thermal stability, and mechanical properties, all the obtained
poly(amine–amide)s also revealed excellent stability of electro-
chromic characteristics by the electrochemical and spectro-
electrochemical methods, changing color from the original
pale yellowish to green, and then to blue. Thus, these
novel triphenylamine-containing poly(amine–amide)s may find
applications in electroluminscent devices as hole-transporting
layer and electrochromic materials due to their proper HOMO
values, excellent electrochemical and thermal stability.
Fig. 9 Potential step absorptometry of poly(amine–amide) 5c (in
CH3CN with 0.1 M TBAP as the supporting electrolyte) by applying a
potential step (0 V 0.95 V).
Meanwhile, the complementary color of the 5a film changed
from the original pale yellowish to green, and then to blue
(as shown in Fig. 6) due to different oxidation state. The
electrochromic characteristics of poly(amine–amide) 5c was
also shown in Fig. 7 and Fig. 8. Thus, this will be a good
approach for facile color tuning of the electrochromic
behaviors by attaching triphenylamine units to polymer main
chain and/or as pendent group.
Acknowledgements
The color switching times were estimated by applying a
potential step, and the absorbance profiles were followed. The
switching time was defined as the time that was required to
reach 90% of the full change in absorbance after switching
potential. Thin films from poly(amine–amide) 5c would
require 3 s at 0.95 V for switching absorbance at 416 and
885 nm and 2 s for bleaching (Fig. 9). When the potential was
set at 1.30 V, thin films from poly(amine–amide) 5c would
require almost 4 s for coloration at 612 nm and 2 s for
bleaching (Fig. 10). Continuous five cyclic scans between 0.0 to
1.40 V, the polymer films still exhibited excellent stability of
electrochromic characteristics.
The authors are grateful to the National Science Council of the
Republic of China for financial support of this work (Grant
NSC 92-2216-E-260-001).
Guey-Sheng Liou,*a Sheng-Huei Hsiaob and Tzy-Hsiang Sua
aDepartment of Applied Chemistry, National Chi Nan University,
1 University Road, Nantou Hsien 545, Taiwan, Republic of China.
E-mail: gsliou@ncnu.edu.tw
bDepartment of Chemical Engineering, Tatung University, 40
Chungshan North Rd. 3rd Sec., Taipei 104, Taiwan, Republic of China
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