Synthesis and characterization of novel electrochromic poly(amide-imide)s with N,N′-di(4-methoxyphenyl)-N,N′-diphenyl-p-phenylenediamine units

Article abstract of DOI:10.1039/c5ra17520h

We developed a new efficient procedure for the synthesis of a bistriphenylamine diamine monomer, N,N′-bis(4-aminophenyl)-N,N′-bis(4-methoxyphenyl)-1,4-phenylenediamine (3). A new dicarboxylic acid monomer bearing two built-in imide rings, namely N,N′-bis(

Full text of DOI:10.1039/c5ra17520h

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Synthesis and characterization of novel
electrochromic poly(amide-imide)s with N,N0-di(4-
Cite this: RSC Adv., 2015, 5, 93591
methoxyphenyl)-N,N0-diphenyl-p-
phenylenediamine units†
a
Sheng-Huei Hsiao, Chia-Yin Tenga and Yu-Ruei Kungb
*
We developed a new efficient procedure for the synthesis of a bistriphenylamine diamine monomer, N,N0-
bis(4-aminophenyl)-N,N0-bis(4-methoxyphenyl)-1,4-phenylenediamine (3).
A new dicarboxylic acid
monomer bearing two built-in imide rings, namely N,N0-bis(trimellitimidophenyl)-N,N0-bis(4-
methoxyphenyl)-1,4-phenylenediamine (4), was synthesized from the condensation of diamine 3 with
two equivalent amounts of trimellitic anhydride. Several novel electroactive poly(amide-imide)s (PAIs)
containing N,N0-di(4-methoxyphenyl)-N,N0-diphenyl-p-phenylenediamine [TPPA(OMe)2] units have been
prepared by the phosphorylation polyamidation reactions from diamine 3 with four imide ring-
preformed dicarboxylic acids or from diimide-diacid 4 with 4,40-oxydianiline and diamine 3, respectively.
All the PAIs were readily soluble in many organic solvents and could be solution-cast into tough and
flexible polymer films. These PAIs exhibited glass-transition temperatures (Tgs) in the range 206–292 ꢀC,
and most of them did not show significant weight-loss before 450 ꢀC. The PAI films revealed reversible
electrochemical oxidation processes accompanied with strong color changes from the pale yellow
neutral state to yellowish green and deep blue oxidized. Incorporating the TPPA(OMe)2 unit on the
amide side of PAIs led to lower oxidation potentials and higher redox and electrochromic stability.
Received 29th August 2015
Accepted 25th October 2015
DOI: 10.1039/c5ra17520h
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(especially tungsten oxide), inorganic coordination complexes,
small organic molecules, and conjugated polymers (polyani-
lines, polypyrroles, and polythiophenes).1–4 Electrochromic
1. Introduction
Electrochromism is dened as a reversible change in optical
absorption (or transmittance) and color change resulting from
the redox of the material in response to an externally applied
potential by electrochemical means which has stimulated the
interest of scientists over the past few decades.1 Traditionally,
interest in electrochromic materials has been directed towards
optical changes in the visible region, leading to many techno-
logical applications such as variable reectance mirrors, smart
windows, and electrochromic displays.2–4 One of the main use of
electrochromic materials is in smart windows for car and
buildings5 and in automatic-dimming rear-view mirrors.6
Recent high-prole commercialization of electrochromic
materials includes the Boeing 787 Dreamliner windows manu-
factured by Gentex.7 Potential applications in information
storage,8 electrochromic displays,9 and adaptive camouages10
can also be envisioned. There are several chemical systems that
are intrinsically electrochromic, such as transition metal oxides
applications based on p-conjugated polymers become popular
owing to their ease of colour-tuning, fast switching time, and
high contrast ratios.11–15 Studies from the Reynolds research
group have shown a wide variety of conjugated polymers with
colour changes that cover the entire visible spectrum.16–20
Triarylamine derivatives are well known for photo- and
electroactive properties that nd optoelectronic applications as
photoconductors, hole-transporters, and light-emitters.21–23
During the last decade, Liou's and our research groups have
developed a number of high-performance polymers, such as
aromatic polyamides and polyimides, carrying the triarylamine
unit as an electrochromic functional moiety.24–32 Our strategy
was to synthesize the triarylamine-containing monomers such
as diamines and dicarboxylic acids that were then reacted with
the corresponding co-monomers through conventional poly-
condensation techniques. The obtained polymers possessed
characteristically well-dened structures, high molecular
weights, and high thermal stability. Because of the incorpora-
tion of packing-disruptive, propeller-shaped triarylamine units
along the polymer backbone, almost all the polyamides and
some of the polyimides exhibited good solubility in polar
organic solvents. They could form uniform, transparent
aDepartment of Chemical Engineering and Biotechnology, National Taipei University of
Technology, Taipei 10608, Taiwan. E-mail: shhsiao@ntut.edu.tw
bDiv. of Organic Opto-electronic Materials and Applications, Material and Chemical
Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra17520h
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amorphous thin lms by solution casting and spin-coating monomer N,N0-bis(4-aminophenyl)-N,N0-bis(4-methoxyphenyl)-
methods. This is advantageous for their ready fabrication of 1,4-phenylenediamine (compound 3 as shown in Scheme 1)
large-area, thin-lm devices.
was synthesized and used for the preparation of highly orga-
Aromatic poly(amide-imide)s (PAIs) possess balanced char- nosoluble and stable visible/near-infrared electrochromic
acteristics between polyamides and polyimides such as high aromatic polyamides.45 However, the polyimides based on
thermal stability and good mechanical properties together with diamine 3 were less soluble in organic solvents as compared to
ease of processability.33 Since we successfully applied the their polyamide counterparts.46 Unless bridged dianhydrides
Yamazaki–Higashi phosphorylation reaction34 to the direct such as ODPA, DSDA, and 6FDA were used, the polyimides
synthesis of high-molecular-weight PAIs from the imide ring- derived from rigid dianhydrides such as PMDA were sparingly
preformed dicarboxylic acids and aromatic diamines using tri- soluble in organic solvents. As a continuation of our efforts in
phenyl phosphite (TPP) and pyridine as condensing agents, this developing easily processable high-performance functional
efficient synthetic route has proved to exhibit signicant polymers with the TPPA segment, the present study describes
advantages in preparing operations as compared with conven-
a new convenient and efficient synthetic route of the
tional acid chloride or isocyanate methods.35 Thus, many novel TPPA(OMe)2-diamine 3 and the synthesis and characterization
PAIs have been readily prepared by this convenient technique in of its derived novel TPPA(OMe)2-containing PAIs. The PAIs are
our and other laboratories.36 Furthermore, this synthetic expected to exhibit good solubility while preserve high thermal
procedure can offer us the option of the incorporation of stability and good redox activity. The effects of the location of
specic functionalities between amide or imide groups in the TPPA(OMe)2 (i.e., on the imide side or the amide side) on the
PAI backbone. The incorporation of such functional groups may electrochemical and electrochromic properties of the PAIs are
provide a method of controlling certain physical properties or also investigated.
special functions of the resulting PAIs.
The anodic oxidation pathways of triphenylamine (TPA)
derivatives were well studied.37 The electrogenerated cation
radical of TPA is not stable and could dimerize to form tet-
raphenylbenzidine by tail to tail coupling with loss of two
protons per dimer. When the phenyl groups were incorporated
by electron-donating substituents such as tert-butyl and
methoxy groups at the para-position of TPA, the coupling
reactions were greatly prevented by affording stable cationic
radicals and lowering the oxidation potentials.38 The redox
property, electron-transfer process, and multi-colour electro-
chromic behaviour of the polymers bearing N,N,N0,N0-
tetraphenyl-1,4-phenylenediamine (TPPA) segments are inter-
esting for electro-optical applications.39–44 As reported by Yen
and Liou in 2009, a methoxy-blocked TPPA-based diamine
2. Experimental section
2.1. Materials
The diimide-diacid monomers 5a and 5b and monoimide-
diacid 5c and 5d were prepared according to the previously re-
ported procedures.35 4,40-Oxydianiline (ODA) was used as
received from Tokyo Chemical Industry (TCI). N-Methyl-2-
pyrrolidone (NMP) (Tedia) was dried over calcium hydride for
˚
24 h, distilled under reduced pressure, and stored over 4 A
molecular sieves in a sealed bottle. Triphenyl phosphite (TPP)
(TCI) was used as received from the supplier. Commercially
obtaꢀined calcium chloride (CaCl2) was dried under vacuum at
180 C for 3 h prior to use. Tetrabutylammonium perchlorate
(Bu4NClO4) (TCI) was recrystallized twice from ethyl acetate
Scheme 1 Synthetic route to the TPPA(OMe)2-diamine monomer 3.
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under nitrogen atmosphere and then dried in vacuo prior to use. h of reux, the solution was ltered to remove Pd/C, and the
All other reagents and solvents were used as received from ltrate was cooled under nitrogen atmosphere. Yellowish green
commercial sources.
crystals grew during the cooling process of the ltrate. The
precipitated crystals were collected by ltration and dried in
vacuo at 80 ꢀC to give 1.70 g (82% in yield) of pure diamino
2.2. Monomer synthesis
ꢀ
compound 3; mp ¼ 199–201 C (by DSC). IR (KBr): 3450, 3365
2.2.1. N,N0-Bis(4-methoxyphenyl)-1,4-phenylenediamine (1).
Hydroquinone (22.02 g, 0.2 mol), p-anisidine (54.2 g, 0.44 mol),
iodine (5.08 g, 0.04 mol), and 50 mL of xylene were placed in
a 100 mL round-bottomed ask equipped with a short path
distillation head. The mixture was heated to reux for 20 h
under nitrogen with removal of water. Aer that, the mixture
was poured into 120 mL of ethanol, and the precipitated
product was ltered and washed thoroughly with ethanol.
Recrystallization from toluene afforded 34.3 g (57% yield) of
pale purple crystals with a melting point of 196–199 ꢀC (by DSC
at a heating rate of 10 ꢀC minꢁ1). IR (KBr): 3386 cmꢁ1 (N–H
stretch). 1H NMR (600 MHz, DMSO-d6, d, ppm): 3.68 (s, 6H,
–OCH3), 6.80 (s, J ¼ 9.0 Hz, 4H, Hc), 6.88 (s, 4H, Ha), 6.91 (d, J ¼
9.0 Hz, 4H, Hb), 7.46 (s, 2H, N–H).
cmꢁ1 (–NH2 stretch). 1H NMR (600 MHz, DMSO-d6, d, ppm):
3.68 (s, 6H, –OCH3), 4.92 (s, 4H, –NH2), 6.53 (d, J ¼ 8.4 Hz, 4H,
He), 6.68 (s, 4H, Ha), 6.76 (d, J ¼ 8.4 Hz, 4H, Hd), 6.80 (d, J ¼ 9.0
Hz, 4H, Hc), 6.87 (d, J ¼ 9.0 Hz, 4H, Hb). 13C NMR (150 MHz,
DMSO-d6, d, ppm): 55.1 (–OCH3), 114.5 (C5), 114.9 (C9), 121.7
(C1), 123.8 (C4), 126.7 (C8), 136.4 (C7), 141.6 (C3), 142.0 (C2),
145.1 (C10), 154.1 (C6).
2.2.4. N,N0-Bis(trimellitimidophenyl)-N,N0-bis(4-methoxy-
phenyl)-1,4-phenylenediamine (4). A ask was charged with
1.92 g (10 mmol) of trimellitic anhydride, 2.51 g (5 mmol) of
2.2.2. N,N0-Bis(4-methoxyphenyl)-N,N0-bis(4-nitrophenyl)-
1,4-phenylenediamine (2). In a 250 mL round-bottom ask
equipped with a stirring bar, a mixture of 9.6 g (0.03 mol) of
N,N0-bis(4-methoxyphenyl)-1,4-phenylenediamine (1), 9.3
g
(0.066 mol) of p-uoronitrobenzene, and 10.1 g of cesium
uoride (CsF) in 100 mL of dimethyl sulfoxide (DMSO) was
heated at 150 ꢀC for about 18 h. Aer cooling, the mixture was
poured into 500 mL of methanol, and the precipitated product
was collected by ltration. Recrystallization from DMF/
methanol yielded 10.5 g (62%) of red crystals with a melting
point of 228–232 ꢀC (by DSC). IR (KBr): 1585, 1311 cmꢁ1 (–NO2
stretch). 1H NMR (600 MHz, CDCl3, d, ppm): 3.84 (s, 6H,
–OCH3), 6.88 (d, J ¼ 9.0 Hz, 4H, Hd), 6.95 (d, J ¼ 9.0 Hz, 4H, Hc),
7.14 (s, 4H, Ha), 7.15 (d, J ¼ 9.0 Hz, 4H, Hb), 8.03 (d, J ¼ 9.0 Hz,
4H, He).
2.2.3. N,N0-Bis(4-aminophenyl)-N,N0-bis(4-methoxyphenyl)-
1,4-phenylenediamine (3). In a 500 mL three-neck round-
bottomed ask equipped with a stirring bar under nitrogen
atmosphere, 2.32 g (4.12 mmol) of dinitro compound 2 and 0.1
g of 10% Pd/C were dispersed in 250 mL of ethanol. The
suspension solution was heated to reux, and 2 mL of hydrazine
monohydrate was added slowly to the mixture. Aer a further 18 Fig. 1 (a) 1H and (b) H–H COSY NMR spectra of diamine 3 in DMSO-d6.
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diamine 3, and 60 mL of glacial acetic acid. The heteroge- subsequent polymerization reaction without any further
neous mixture was stirred for 1 h at room temperature and purication. IR (KBr): 2700–3500 cmꢁ1 (carboxyl O–H
then reuxed at 120 ꢀC for 12 h. Then, the mixture was poured stretch), 1776, 1725 cmꢁ1 (imide and carboxyl C]O stretch).
into 200 mL of methanol, and the precipitate was collected by 1H NMR (600 MHz, DMSO-d6, d, ppm): 13.57 (a broad hump,
ltration and washed thoroughly with methanol to remove 2H, –COOH), 3.76 (s, 6H, –OCH3), 6.96–6.98 (two overlapped
acetic acid. The product was washed several times with hot doublets, 8H, Hc+d), 7.05 (s, 4H, Ha), 7.14 (d, J ¼ 9.0 Hz,
water and then dried in vacuum to afford 3.49 g (82% yield) 4H, Hb), 7.27 (d, J ¼ 9.0 Hz, 4H, He), 8.05 (d, J ¼ 7.8 Hz, 2H,
of the diimide-diacid monomer 4 as red-brown powder; mp ¼ Hf), 8.28 (d, J ¼ 1.2 Hz, 2H, Hh), 8.39 (dd, J ¼ 7.8, 1.2 Hz,
332–336 ꢀC. The product could be directly used for the 2H, Hg).
Fig. 2 (a) 13C and (b) C–H HMQC NMR spectra of diamine 3 in DMSO-d6.
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approximately 4–6 mg of lm samples heated in owing
nitrogen or air (ow rate ¼ 40 cm3 minꢁ1) at a heating rate of
20 C minꢁ1. DSC analyses were performed on a Perkin-Elmer
ꢀ
DSC 4000 differential scanning calorimeter at a scan rate of
20 ꢀC minꢁ1 in owing nitrogen. Electrochemistry was per-
formed with a CHI 750A electrochemical analyzer. Cyclic vol-
tammetry was conducted with the use of a three-electrode cell in
which ITO (polymer lm area about 0.8 cm ꢂ 1.25 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 home-
made Ag/AgCl, KCl (sat.) reference electrode. Ferrocene was
used as an external reference for calibration (+0.48 V vs. Ag/
AgCl). Voltammograms are presented with the positive poten-
tial pointing to the le and with increasing anodic currents
pointing downwards. Spectroelectrochemistry analyses were
carried out with an electrolytic cell, which 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. Absorp-
tion spectra in the spectroelectrochemical experiments were
measured with an Agilent 8453 UV-Visible diode array
spectrophotometer.
2.3. Synthesis of PAIs
The synthesis of PAI 7a was used as an example to illustrate the
general synthetic route to produce the PAIs. A mixture of 0.5105
g (0.6 mmol) of diimide-diacid monomer 4, 0.1201 g (0.6 mmol)
of ODA, 0.1 g of anhydrous calcium chloride, 0.6 mL of triphenyl
phosphite (TPP), 0.15 mL of pyridine, and 0.6 mL of NMP was
heated with stirring at 120 ꢀC for 3 h. The resulting polymer
solution was poured slowly into 150 mL of methanol giving rise
to a tough, brous precipitate. The precipitated product was
collected by ltration, washed thoroughly with hot water and
methanol, and then dried to give PAI 7a. The inherent viscosity
of the polymer was 0.76 dL gꢁ1, measured in DMAc at
a concentration of 0.5 g dLꢁ1 at 30 C. The IR spectrum of 7a
ꢀ
(lm) exhibited characteristic amide absorption bands at 3280
cmꢁ1 (amide N–H stretch) and 1668 cmꢁ1 (amide carbonyl
stretch), together with the imide absorption bands at 1778 cmꢁ1
(asymmetrical C]O stretch) and 1720 cmꢁ1 (symmetrical C]O
stretch).
3. Results and discussion
3.1. Monomer synthesis
The TPPA(OMe)2-diamine monomer 3 was synthesized by
a three-step reaction sequence as shown in Scheme 1. In the
rst step, N,N0-bis(4-methoxyphenyl)-1,4-phenylenediamine (1)
was synthesized by condensation of hydroquinone with p-ani-
sidine in the presence of small amounts of iodine according to
the Knoevenagel synthesis of aromatic secondary amines.47 In
the second step, the intermediate dinitro compound, N,N0-bis(4-
methoxyphenyl)-N,N0-bis(4-nitrophenyl)-1,4-phenylenediamine
(2) was obtained by the nucleophilic aromatic uoro-
displacement reaction of p-uoronitrobenzene with the
secondary amine 1 in the presence of cesium uoride (CsF).
Finally, the target diamine monomer 3 was prepared by
2.4. Preparation of the PAI lms
A solution of the polymer was made by dissolving about 0.6 g of
the PAI sample in 9 mL of hot DMAc. The homogeneous solu-
tion was poured into a 7 cm glass Petri dish, which was placed
in a 90 ꢀC oven overnight for the slow release of the solvent. The
cast lm was then released from the glass substrate and was
further dried in vacuo at 160 ꢀC for 8 h. The obtained lms were
about 75 mm in thickness and were used for thermal analyses.
2.5. Instrumentation and measurements
Infrared (IR) spectra were recorded on a Horiba FT-720 FT-IR hydrazine Pd/C-catalyzed reduction of dinitro compound 2. The
spectrometer. 1H and 13C NMR spectra were measured on yield of crude product in each step was generally higher than
a Bruker Avance III HD-600 MHz NMR spectrometer with tet- 80%. This strategy for the preparation of diamine 3 seems to be
ramethylsilane as an internal standard. The inherent viscosities more convenient and more efficient than that using Ullmann
were determined with a Cannon-Fenske viscometer at 30 ꢀC. reaction reported by Yen and Liou.45
Thermogravimetric analysis (TGA) was performed with a Perkin-
Fig. S1 (ESI†) illustrates FT-IR spectra of compounds 1 to 3.
Elmer Pyris 1 TGA instrument. Experiments were carried out on The IR spectrum of compound 1 gives rise to a sharp, medium
Scheme 2 Synthesis of diimide-diacid monomer 4.
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absorption at about 3386 cmꢁ1 ascribed to the secondary amine NH groups appear at 7.46 ppm as a sharp singlet, and the tall
N–H stretching vibration. The dinitro compound 2 shows the singlet at 3.68 ppm is attributed to the methoxy groups. The
characteristic absorption of nitro groups at around 1585 and hydrogens on the benzene rings appear in the range of 6.79–
1
1311 cmꢁ1 (–NO2 asymmetric and symmetric stretching). Aer 6.93 ppm. The H and H–H COSY NMR spectra of the dinitro
reduction, the characteristic absorptions of the nitro group monomer 2 are compiled in Fig. S3 (ESI†). The 1H, H–H COSY,
disappeared and the amino group shows the typical –NH2 13C and C–H HMQC NMR spectra of the diamine monomer 3
stretching absorption pair at 3450 and 3365 cmꢁ1 as shown in are compiled in Fig. 1 and 2, respectively. These 1H and 13C
the IR spectrum of diamine 3. The 1H NMR spectrum of the NMR spectra are essentially identical to those reported in
secondary amine 1 is illustrated in Fig. S2 (ESI†). The aromatic literature,45 where the intermediate dinitro compound 2 has
Fig. 3 (a) 1H and (b) H–H COSY NMR spectra of diimide-diacid monomer 4 in DMSO-d6.
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Scheme 3 Synthesis of PAIs 6a–6d.
been prepared by a different synthetic route. Only a slight
difference in chemical shis of the resonance peaks was
observed possibly due to the use of different test solvents. The
1H NMR spectra conrm that the nitro groups have been
completely transformed into amino groups by the high eld
shi of the aromatic protons, especially for protons He ortho to
the amino group, and the resonance signal at about 4.92 ppm
corresponding to the aryl primary amino protons. The assign-
ments of all hydrogen and carbon atoms are assisted by two-
dimensional (2-D) NMR spectra and are consistent with the
proposed structures of compounds 2 and 3.
3.2. Synthesis of PAIs
According to the phosphorylation polyamidation technique
described by Yamazaki and co-workers,34 a series of novel PAIs
6a–6d were synthesized from various combinations of diamine
3 with imide ring-preformed dicarboxylic acids shown in
Scheme 3 by using triphenyl phosphite (TPP) and pyridine (Py)
as condensing agents. Another type of PAIs 7a and 7b were
prepared from ODA and diamine 3, respectively, with the newly
synthesized diimide-diacid 4 by a similar synthetic technique
(Scheme 4). All the polymerization reactions proceeded homo-
geneously throughout the reaction and afforded clear, highly
viscous polymer solutions. All the polymers precipitated in
a tough, ber-like form when the resulting polymer solutions
were slowly poured under stirring into methanol. The obtained
PAIs had inherent viscosities in the range of 0.72–0.84 dL gꢁ1, as
listed in Table 1. All the polymers can be solution-cast into
exible and transparent lms, indicating the formation of high
molecular weight polymers.
The structures of the PAIs were conrmed by IR and NMR
spectroscopy. Fig. S5 (ESI†) shows the IR spectra of PAIs 6a–6d,
and Fig. S6† illustrates those of PAIs 7a and 7b. All the PAIs
showed the characteristic absorption bands of the amide group
at around 3288 cmꢁ1 (N–H stretching) and 1668 cmꢁ1 (amide
C]O), and the characteristic imide absorption bands at around
1774 cmꢁ1 (asymmetric C]O) and 1722 cmꢁ1 (symmetric C]O).
As a representative example, the 1H and H–H COSY NMR
The new diimide-diacid monomer
4 containing the
TPPA(OMe)2 unit was synthesized by condensation of diamine
3 with two molar equivalent amount of trimellitic anhydride
(TMA) in reuxing glacial acetic acid (Scheme 2). The structure
of diimide-diacid 4 was conrmed by IR and 1H NMR spec-
troscopies. As shown in Fig. S4 (ESI†), the IR spectrum of
diimide-diacid 4 shows absorption bands around 2700–3500
cmꢁ1 (–OH stretch, carboxylic acid), 1776 (imide C]O asym-
metrical stretching), and 1725 cmꢁ1 (imide C]O symmetrical
stretching and carboxyl C]O stretching), conrming the
presence of imide ring and carboxylic acid groups in the
structure. Fig. 3(a) shows the high-resolution 1H NMR spec-
trum of 4 in deuterated dimethyl sulfoxide (DMSO-d6).
Assignments of each proton are assisted by the H–H COSY
NMR spectrum (Fig. 3(b)), and the spectra agree well with the
proposed molecular structure of 4.
Scheme 4 Synthesis of PAIs 7a and 7b.
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Table 1 Inherent viscosity and solubility behavior of PAIs
Solubility in various solventsb,c
hinh (dL gꢁ1
)
NMP
DMAc
DMF
DMSO
m-Cresol
THF
a
Polymer code
6a
6b
6c
6d
7a
7b
0.84
0.79
0.75
0.80
0.76
0.72
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
++
ꢁꢁ
+ꢁ
ꢁꢁ
+ꢁ
+ꢁ
ꢁꢁ
+ꢁ
+ꢁ
++
++
+ꢁ
a
b
Inherent viscosity measured at a concentration of 0.5 dL gꢁ1 in DMAc at 30 ꢀC. Solvent: NMP: N-methyl-2-pyrrolidone; DMAc: N,N-
c
dimethylacetamide; DMF: N,N-dimethylformamide; DMSO: dimethyl sulfoxide; THF: tetrahydrofuran. The qualitative solubility was tested with
10 mg of a sample in 1 mL of stirred solvent. ++, soluble at room temperature; +ꢁ, partially soluble on heating; ꢁꢁ, insoluble even on heating.
spectra of PAI 7a are compiled in Fig. 4. The proton NMR spectra individual hydrogen atoms could be made with the aid of 2-D
of another two examples for PAIs 6a and 7b are respectively NMR spectra. All the spectra agree well with the proposed
depicted in Fig. S7 and S8 (ESI†). The specic assignment of repeating units in these PAIs.
1
Fig. 4 (a) H and (b) H–H COSY NMR spectra of PAI 7a in DMSO-d6.
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3.3. Properties of PAIs
3.3.1. Solubility. The qualitative solubility properties of
PAIs in several organic solvents at 10% (w/v) are summarized in
Table 1. All of the PAIs were readily dissolved in polar aprotic
solvents such as NMP, DMAc, and DMF at room temperature.
They are also easily soluble in m-cresol. However, most of them
are sparingly soluble in DMSO and less polar THF. The high
solubility of these PAIs can be attributed to the incorporation of
bulky, packing-disruptive, and three-dimensional TPPA(OMe)2
units in the polymer structure, which results in a high steric
hindrance for close packing and thus reduces their crystalliza-
tion tendency and interchain interactions. Therefore, the good
solubility to most polar organic solvents makes these polymers
potential candidates for practical applications by simple solu-
tion processing.
Fig. 5 TGA and DSC curves of PAI 7b with a heating rate 20 ꢀC minꢁ1
.
3.3.2. Thermal properties. The thermal properties of all the
PAIs were investigated by DSC and TGA techniques. The
thermal behavior data of PAIs are summarized in Table 2.
Typical TGA and DSC curves of the representative PAI 7b are
shown in Fig. 5. The glass-transition temperatures (Tg) of these
PAIs could be easily determined by the DSC method; they were
observed in the range of 206–292 ꢀC (see Fig. S9 in ESI†). Semi-
aromatic PAIs 6b and 6d exhibited relatively lower Tg values (218
and 206 ꢀC, respectively) when compared with wholly aromatic
PAIs 6a (Tg ¼ 287 ꢀC) and 6c (Tg ¼ 278 ꢀC) due to the presence of
The high char yields of the aromatic PAIs can be ascribed to
their high aromatic content. The thermal analysis results
revealed that these PAIs generally exhibited good thermal
stability, which in turn is benecial to increase the service time
in device application and enhance the morphological stability
to the spin-coated lms.
3.3.3. Electrochemical properties. The electrochemical
behavior of the PAIs was investigated by cyclic voltammetry
(CV) conducted for the cast lms on an ITO-coated glass
substrate as working electrode in dry acetonitrile (CH3CN) (for
anodic oxidation) containing 0.1 M of Bu4NClO4 as the sup-
porting electrolyte using saturated Ag/AgCl as the reference
electrode. The derived oxidation potentials are summarized in
Table 3. All the 6 series PAIs exhibit two reversible oxidation
redox couples with half-wave potentials (E1/2) in the ranges of
0.60–0.65 V and 0.95–1.00 V, respectively, corresponding to
successive one electron removal from the TPPA(OMe)2 units.
The CV diagrams of PAI 6d, representative for the other 6 series
PAIs, are shown in Fig. 6. PAI 6d exhibits very high electro-
chemical stability because of the active sites of the TPPA are
blocked with methoxy groups. It displays two reversible
oxidation processes with oxidation peaks at about 0.60 and 0.95
V, respectively. During the electrochemical oxidation of the PAI
lms, it was also found that the color of the lm changed from
pale yellow to yellowish green and then to dark blue. Because of
the electrochemistry stability and good adhesion between the
polymer thin lm and the ITO substrate, 6d still exhibited very
high electroactivity aer 100 repetitive cyclic scans between 0.0
V to 1.2 V.
ꢀ
exible polymethylene segments. PAIs 7a (Tg ¼ 285 C) and 7b
(Tg ¼ 292 ꢀC) also reveal high Tgs because of high aromatic
content. All of the aromatic PAIs exhibited a similar TGA pattern
ꢀ
with no signicant weight loss below 450 C in air or nitrogen
atmosphere. The decomposition temperatures (Td) at which
a 10% weight-loss of the PAIs in nitrogen and air were recorded
in the range of 474–548 ꢀC and 457–540 ꢀC, respectively. As
expected, PAIs 6b and 6d decompose at slightly lower temper-
atures than other PAIs because of the less stable aliphatic
segments. The amount of carbonized residue (char yield) of all
the PAIs in nitrogen atmosphere was more than 58% at 800 ꢀC.
Table 2 Thermal properties of PAIs
Td at 5%
weight
Td at 10%
weight
lossc (ꢀC)
lossc (ꢀC)
Polymer codea Tg (ꢀC) N2
Air
N2
Air
Char yieldd (wt%)
b
6a
6b
6c
6d
7a
7b
287
218
278
206
285
292
496 488 548 540 68
482 465 502 497 63
480 465 515 509 67
443 414 474 457 58
487 491 530 535 69
496 483 548 539 71
The rst anodic CV scans of PAIs 6a and 7a with an isomeric
repeat unit are compared in Fig. 7. PAI 6a revealed a lower
oxidation potential (Eonset ¼ 0.41 V, Epa ¼ 0.65 V) as compared to
its isomeric PAI 7a (Eonset ¼ 0.49 V, Epa ¼ 0.74 V) because its
a
All the polymer lm samples were heated at 300 ꢀC for 1 h before all
b
the thermal analyses. Midpoint temperature of the baseline shi on TPPA(OMe)2 unit is located in the amide side, which should be
the second DSC heating trace (rate ¼ 20 ꢀC minꢁ1) of the sample aer
more easily oxidized than that in the imide side. PAI 7b contains
the TPPA(OMe)2 segment in both amide and imide sides and is
expected to show more oxidation processes. However, as shown
in Fig. 8, PAI 7b only showed two redox couples similar to other
quenching from 400 to 50 ꢀC (rate ¼ 20 ꢀC minꢁ1) in nitrogen.
c
Decomposition temperature at which a 5% orꢁ110% weight loss was
recorded by TGA at a heating rate of 20 ꢀC min and a gas ow rate
of 20 cm3 minꢁ1
.
Residual weight% at 800 ꢀC at a scan rate 20 ꢀC
d
minꢁ1 in nitrogen.
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Table 3 Optical and electrochemical properties of PAIs
UV-vis
absorptiona (nm)
Oxidation potentialb (V)
Ox1
1/2
Ox2
1/2
c
Polymer
lmax
lonset
Eonset
E
E
Optical bandgap Eg (eV)
HOMOd (eV)
LUMOd (eV)
6a
6b
6c
6d
7a
7b
311
310
307
320
345
315
434
426
421
413
433
422
0.41
0.38
0.34
0.35
0.49
0.35
0.56
0.51
0.48
0.50
0.63
0.49
0.92
0.87
0.84
0.86
1.02
0.88
2.86
2.91
2.94
3.00
2.86
2.94
4.92
4.87
4.84
4.86
4.99
4.85
2.06
1.96
1.90
1.86
2.13
1.91
a
c
UV-vis absorption maximum and onset for the polymer lms. b Calculated from rst CV scans, versus Ag/AgCl in acetonitrile at a scan rate of 50 mV sꢁ1
.
d
Optical bandgap calculated from absorption edge of the polymer lm: Eg ¼ 1240/lonset
.
The HOMO energy levels were calculated from EO1/x21 values from
the CV curves and were referenced to ferrocene (4.8 eV relative to the vacuum energy level, E ¼ 0.44 V vs. Ag/AgCl). EHOMO ¼ EO1/x21 + 4.8 ꢁ 0.44 (eV);
Ox
1/2
ELUMO ¼ EHOMO ꢁ Eg.
PAIs. The peaks are broader than those of other PAIs due to immerged in an electrolyte solution. The electrode preparations
close overlapping of the redox waves.
and solution conditions were identical to those used in the CV
The redox potentials of the PAIs as well as their respective experiments. During the test, a three-electrode conguration
highest occupied molecular orbital (HOMO) and lowest unoc- was used for applying potential to the polymer lms in a 0.1 M
cupied molecular orbital (LUMO) (versus vacuum) energy levels Bu4NClO4/CH3CN electrolyte solution. When the lms were
are summarized in Table 3. The external ferrocene/ferrocenium electrochemically oxidized, a strong color change was observed.
(Fc/Fc+) redox standard has E1/2 value of 0.44 V vs. Ag/AgCl in
Fig. 9(a) shows the spectral changes of PAI 6a lm upon
acetonitrile. Under the assumption that the HOMO level or oxidative scans from 0 to 1.00 V. In the neutral form, PAI 6a
called ionization potentials (versus vacuum) for the ferrocene exhibited strong absorption at lmax of 311 nm due to the p–p*
standard was 4.80 eV with respect to the zero vacuum level, the transitions. As the applied voltage was stepped from 0.0 to 0.65
HOMO levels for the PAIs are estimated from the EO1/x21 values of V, the absorbance at 311 nm decreased, and new peak at 426 nm
their CV diagrams as 4.84–4.99 eV. The energy gaps estimated and a broadband from 985 nm extended into the near-infrared
from the absorption spectra were then used to obtain the LUMO (NIR) region grew up. In the same time, the lm turned into
energy levels of 1.86–2.06 eV. The low ionization potentials yellowish green. We attribute these spectral changes to the
suggest an easier hole injection into lms from ITO electrodes formation of a stable monocation radical in the TPPA(OMe)2
in electronic device applications.
moiety. The absorption band in the NIR region is assigned to an
3.3.4. Spectroelectrochemical and electrochromic proper- intervalence charge-transfer (IV-CT) between states in which the
ties. Spectroelectrochemical measurements were performed on positive charge is centered at different amino centers.48,49 As the
lms of polymers drop-coated onto ITO-coated glass slides applied voltage was raised to 1.00 V, the absorbance at 426 nm
Fig. 6 Cyclic voltammograms of the cast film of PAI 6d on the ITO- Fig. 7 Cyclic voltammograms of the cast films of PAIs 6a and 7a on the
coated glass substrate in 0.1 M Bu4NClO4/CH3CN at a scan rate of ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN at a scan rate of
50 mV sꢁ1
.
50 mV sꢁ1
.
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Fig. 8 Cyclic voltammogram of the cast film of 7b on the ITO-coated
glass substrate in 0.1 M Bu4NClO4/CH3CN at a scan rate of 50 mV sꢁ1
.
decreased and a new broad absorption band at 815 nm started
to intensify. Meanwhile, the lm changed color from yellowish
green to dark blue as shown in the inset of Fig. 9(a). The spectral
change of semi-aromatic PAI 6d upon oxidation is very similar
to that of 6a (see Fig. S10 in ESI†). For comparison, the spectral
change of the cast lm of the isomeric PAI 7a on ITO-glass upon
oxidative scans is illustrated in Fig. 9(b). In the neutral form, the
7a exhibited strong absorption at lmax of 345 nm. As the applied
voltage was stepped from 0.0 to 0.74 V, the absorbance at 345
nm decreased, and new peak at 425 nm and a broadband having
its maximum around 980 nm in the NIR region grew up because
of the IV-CT effect. In the same time, the lm turned from
nearly colorless into pale yellow due to the rst oxidation
process. At a higher applied potential of 1.15 V, the absorbance
at 425 nm decreased and a new broad absorption band at 796
nm, a slight blue shi compared with that of isomeric 6a (815
nm), started to intensify, and the lm changed color from pale
yellow to deep blue (see the inset shown in Fig. 9(b)).
Fig. 9 Spectroelectrograms and color changes of (a) PAI 6a (330 ꢃ 50
nm in thickness) and (b) PAI 7a (250 ꢃ 20 nm in thickness) thin films on
the ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN at various
applied voltages (inset: polymer films with about 1.6 mm in thickness).
A similar spectral change was observed for PAI 7b at early
stage of oxidation (Fig. 10). The absorption peak at 423 nm and
the IV-CT band at 988 nm grew up when the applied potential
was increased to 0.76 V. This result illustrates that the rst two
oxidations of the TPPA(OMe)2 unit in both the amide and imide
sides occurred almost simultaneously. Further increase of the
applied potential over 0.95 V led to the increase of the absorp-
tion intensity at about 585 and 801 nm. This absorption change
may be explained by the occurrence of the third oxidation. Upon
further increase of the potential to 1.12 V, the intensity of the
absorption band at 423 nm decreased and the bands around
585 and 801 nm further intensied. It was also found the IV-CT
band in the NIR region diminished gradually, implying the
occurrence of the fourth oxidation. When the polymer lm of 7b
was electrochemically oxidized, its color changed from pale
yellow to yellowish green, cyan, and deep blue, as can be seen
from the inset of Fig. 10.
optical and electrochromic devices, thus the electrochromic
switching studies were further measured. Electrochromic
switching studies for the PAIs were performed to monitor the
percent transmittance changes (DT%) as a function of time at
their absorption maximum (lmax) and to determine the
response time by stepping potential repeatedly between the
neutral and oxidized states. The polymer lm was cast onto an
ITO-coated glass slides in the same manner as described earlier,
and the active area of the polymer lm on ITO glass is about 1
cm2. When the lms were switched, the absorbance at the given
wavelength was monitored as a function of time with UV-vis-
NIR spectroscopy. Fig. 11 and 12 depict the optical trans-
mittance of 6a lm as a function of time at 426 and 815 nm by
applying square-wave potential steps between 0 and 0.65 V and
between 0 and 1.00 V for a switching time of 15 s. Generally, the
charge passed at 90% of the full optical switch was used to
evaluate the response time since the naked eye could not
distinguish the color change aer this point. As shown in
Fig. 11(a), the polymer lm of 6a still exhibited very good elec-
trochromic response aer over 20 cyclic scans between 0 and
3.3.5. Electrochromic switching and stability. The stability,
response time, and color efficiency are the key parameters for
an electroactive polymer lm to be amenable for usage in
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respectively. The optical contrast between neutral pale yellow
and oxidized deep blue states was found to be 73% at 815 nm
[Fig. 12(b)]. In addition, the electrochromic coloration efficiency
(CE; h) can be calculated via optical density using the following
equation:1,2 h ¼ DOD/Qd, where DOD is the optical absorbance
change, and Qd (mC cmꢁ2) is the inject/ejected charge during
a redox step. On the basis of this equation, the CE values of the
6a lm estimated from the data were found to be 318 cm2 Cꢁ1 at
426 nm and 375 cm2 Cꢁ1 at 815 nm.
The potential step absorptiometry of the 6d lm is included
in the ESI.† Fig. S11 and S12† depict the optical transmittance
of 6d lm as a function of time at 430 and 817 nm by applying
square-wave potential steps between 0.00 and 0.70 V for a resi-
dence time of 7 s and between 0.00 and 1.05 V for a residence
time of 12 s. As shown in Fig. S11(a),† the polymer lm of 6d still
exhibited very good electrochromic response aer over 50 cyclic
scans between 0 and 0.70 V. PAI 6d lm attained 90% of
a complete coloring and bleaching in 2.2 and 1.4 s, respectively.
The optical contrast measured as DT% between neutral pale
yellow and oxidized yellowish green states was found to be 36%
at 430 nm [Fig. S11(b)†]. When 6d was switched between neutral
and fully oxidized states (the applied potential switched
between 0 and 1.05 V), only a very small optical contrast loss was
observed during switching [Fig. S12(a)†]. The response times for
coloring and bleaching are similar to those switching between
neutral and rst oxidized state. The optical contrast measured
as DT% between neutral pale yellow and oxidized deep blue
states was found to be 71% at 817 nm [Fig. S12(b)†].
Fig. 10 Spectroelectrograms and color changes of PAI 7b thin film
(320 ꢃ 30 nm in thickness) on the ITO-coated glass substrate in 0.1 M
Bu4NClO4/CH3CN (inset: polymer film with 1.7 mm in thickness).
0.65 V. PAI 6a lm attained 90% of a complete coloring and
bleaching in 4.8 and 2.1 s, respectively. The optical contrast
measured as DT% between neutral pale yellow and oxidized
yellowish green states was found to be 33% at 426 nm
[Fig. 11(b)]. When 6a was switched between neutral and fully
oxidized states (the applied potential switched between 0 and
1.00 V), almost no optical contrast loss was observed during
switching in the rst twenty cycles [Fig. 12(a)]. The response
times for coloring and bleaching are recorded as 3.6 and 4.1 s,
The stability and switching behavior of the 7a lm at lmax
425 nm and 796 nm were investigated by monitoring the elec-
¼
trochromic contrast (DT%) of the thin lm upon repeated
Fig. 11 Potential step absorptiometry of the cast films of 6a on the ITO-glass slide (coated area ꢄ 1 cm2) (in CH3CN with 0.1 M Bu4NClO4 as the
supporting electrolyte) by applying a potential step: (a) optical switching at potential 0.00 V 5 0.70 V (20 cycles) and a switching time of 15 s,
monitored at lmax ¼ 426 nm; (b) the 1st cycle transmittance change for the 6a thin film.
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Fig. 12 Potential step absorptiometry of the cast films of 6a on the ITO-glass slide (coated area ꢄ 1 cm2) (in CH3CN with 0.1 M Bu4NClO4 as the
supporting electrolyte) by applying a potential step: (a) optical switching at potential 0.00 V 5 1.00 V (20 cycles) and a switching time of 15 s,
monitored at lmax ¼ 815 nm; (b) the 1st cycle transmittance change for the 6a thin film.
square-wave potential steps between 0 and 0.75 V and between yellowish green coloring step and 2.2 s for the bleaching step at
0 and 1.15 V for a pulse width of 15 s. The results are shown in 425 nm and 3.5 s for the blue coloring step and 2.4 s for the
Fig. 13 and 14, respectively. In this case, a response time bleaching step at 796 nm. In addition, the optical contrast
required for 90% full-transmittance change of 3.9 s for the measured (DT%) recorded at neutral and oxidized forms was
Fig. 13 Potential step absorptiometry of the cast film of 7a on the ITO-glass slide (coated area ꢄ 1 cm2) (in CH3CN with 0.1 M Bu4NClO4 as the
supporting electrolyte) by applying a potential step: (a) optical switching at potential 0.00 V 5 0.75 V (20 cycles) and a switching time of 15 s,
monitored at lmax ¼ 425 nm; (b) the 1st cycle transmittance change for the 7a thin film.
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Fig. 14 Potential step absorptiometry of the cast film of 7a on the ITO-glass slide (coated area ꢄ 1 cm2) (in CH3CN with 0.1 M Bu4NClO4 as the
supporting electrolyte) by applying a potential step: (a) optical switching at potential 0.00 V 5 1.15 V (20 cycles) and a switching time of 15 s,
monitored at lmax ¼ 796 nm; (b) the 1st cycle transmittance change for the 7a thin film.
found to be 32% at 425 nm and 65% at 796 nm. The CE values of 0 and 1.15 V (Fig. S14†). This result also indicated that incor-
the 7a lm were calculated as 284 cm2 Cꢁ1 at 425 nm and porating the TPPA(OMe)2 unit on the amide side of PAIs led to
319 cm2 Cꢁ1 at 796 nm (Table 4) by chronoamperometry. The 7a lower oxidation potentials, higher redox reversibility, and better
lm showed a larger loss of electrochromic contrast as electrochromic performance.
compared to the 6a lm with the isomeric repeat unit in the rst
20 scans. The less stability of the 7a lm might be attributable
to the strong electron-withdrawing effect of the imide group. In
4. Conclusions
A new synthetic procedure for N,N0-bis(4-amionophenyl)-N,N0-
bis(4-methoxyphenyl)-1,4-phenylenediamine was developed.
Several novel TPPA(OMe)2-based PAIs have been easily prepared
from the imide ring-preformed dicarboxylic acids with the
synthesized diamine or from N,N0-bis(trimellitimidophenyl)-
N,N0-bis(4-methoxyphenyl)-1,4-phenylenediamine with aromatic
contrast, the 6a lm exhibited excellent electrochromic stability
because the TPPA(OMe)2 unit is located in the amide side. The
switching results of PAI 7b are included in the ESI.† As shown in
Fig. S13,† PAI 7b exhibits good stability in the rst 20 full
switches, and almost no optical contrast loss aer 20 full
switches was observed if the polymer lm was switched between
Table 4 Electrochromic properties of PAIs
Response timeb
Polymer
6d
lmaxa (nm)
DT%
tc (s)
tb (s)
DODc
Qdd (mC cmꢁ2
)
CEe (cm2 Cꢁ1
)
430
817
426
815
425
796
423
812
36
71
33
73
32
65
31
75
2.24
2.70
4.83
3.64
3.97
3.51
5.29
2.19
1.37
1.34
2.11
4.06
2.23
2.37
3.41
1.97
0.232
0.709
0.235
0.723
0.216
0.680
0.214
0.755
0.64
1.76
0.74
1.93
0.76
2.13
0.71
1.95
362
403
318
375
284
319
301
387
6a
7a
7b
a
b
c
Wavelength of absorption maximum. Time for 90% of the full-transmittance change. Optical density change (DOD) ¼ log[Tbleached/Tcolored],
d
where Tcolored and Tbleached are the maximum transmittance in the oxidized and neutral states, respectively. Qd is ejected charge, determined
from the in situ experiments. e Coloration efficiency (CE) ¼ DOD/Qd.
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