New electroactive and electrochromic aromatic polyamides with ether-linked bis(triphenylamine) units

Article abstract of DOI:10.1002/pola.27465

A new class of electroactive polyamides with etherlinked bis(triphenylamine) [O(TPA)₂] units were prepared through the direct phosphorylation polycondensation from N,N-di(4-aminophenyl)-N′,N′-diphenyl-4,4′-oxydianiline and aromatic dicarboxylic acids. These polyamides were amorphous with good solubility in many organic solvents, such as NMP and DMAc, and could be solution-cast into strong and flexible polymer films. Their decomposition temperatures (T_d) at a 10% weight-loss in nitrogen and air were recorded at 556-568°C and 537-555°C, respectively. The glass-transition temperatures (Tg) of all the polyamides were observed in the range of 218-253°C by DSC. Cyclic voltammograms of the polyamide films cast onto an indium-tin oxide (ITO)-coated glass substrate exhibited two reversible oxidation redox couples at 0.80-0.82 V and 0.96-0.98 V versus Ag/AgCl in an electrolyte containing acetonitrile solution. The polyamide films showed excellent electrochemical and electrochromic stability, with a color change from a colorless or pale yellowish neutral form to green and purple oxidized forms at applied potentials ranging from 0 to 1.2 V. These polymers can also be used to fabricate electrochromic devices, and they showed high coloration efficiency, high redox stability, and fast response time.

Full text of DOI:10.1002/pola.27465

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New Electroactive and Electrochromic Aromatic Polyamides with
Ether-Linked Bis(triphenylamine) Units
Sheng-Huei Hsiao, Shou-Lun Cheng
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
Received 27 September 2014; accepted 2 November 2014; published online 00 Month 2014
DOI: 10.1002/pola.27465
ABSTRACT: A new class of electroactive polyamides with ether-
linked bis(triphenylamine) [O(TPA)2] units were prepared
through the direct phosphorylation polycondensation from
N,N-di(4-aminophenyl)-N0,N0-diphenyl-4,40-oxydianiline and aro-
matic dicarboxylic acids. These polyamides were amorphous
with good solubility in many organic solvents, such as NMP
and DMAc, and could be solution-cast into strong and flexible
polymer films. Their decomposition temperatures (Td) at a 10%
weight-loss in nitrogen and air were recorded at 556–568 ꢀC
and 537–555 ꢀC, respectively. The glass-transition temperatures
(Tg) of all the polyamides were observed in the range of
2182253 ꢀC by DSC. Cyclic voltammograms of the polyamide
films cast onto an indium-tin oxide (ITO)-coated glass substrate
exhibited two reversible oxidation redox couples at 0.80–0.82 V
and 0.96–0.98 V versus Ag/AgCl in an electrolyte containing
acetonitrile solution. The polyamide films showed excellent
electrochemical and electrochromic stability, with
a color
change from a colorless or pale yellowish neutral form to
green and purple oxidized forms at applied potentials ranging
from 0 to 1.2 V. These polymers can also be used to fabricate
electrochromic devices, and they showed high coloration effi-
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ciency, high redox stability, and fast response time.
2014
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2014, 00, 000–000
KEYWORDS: electrochemistry; electrochromism; polyamides;
redox polymers; triphenylamine
electrochromic films, new materials, and components for the
devices.9
INTRODUCTION Electrochromic materials consist of redox-
active species that exhibit significant, lasting, and reversible
changes in color upon electrochemical reduction or oxida-
tion.1 Research in electrochromic devices has received a
great deal of attention due to the importance of this interest-
ing phenomenon in applications such as electrochromic win-
dows,2 displays,3 memory devices,4 electronic papers,5 and
adaptive camouflages.6 There is a vast amount of chemical
species that exhibited electrochromic properties, including
metal coordination complexes, metal oxide (especially tung-
sten oxide), viologens (4,40-bipyridium salts), and conducting
polymers (such as polyanilines, polypyrroles, polythiophenes,
and polyselenophenes).7 In recent years, the use of conju-
gated polymers as active layers in electrochromic devices
became popular due to the advantageous properties such as
fast switching time, ease of synthesis, and wide range of col-
ors.8 For efficient operation of an electrochromic device, it is
necessary to take a number of properties into consideration:
Electrochromic efficiency, optical contrast, response time,
stability, and durability. The difficulty in achieving satisfac-
tory values for all these parameters at the same time stimu-
lates the development of new methods of preparation of
Triarylamine derivatives are well known for photo- and elec-
troactive properties that may find optoelectronic applications
as photoconductors, hole-transporters, and light-emitters.10
Triarylamines can be easily oxidized to form stable radical
cations, and the oxidation process is always associated with
a noticeable change of coloration. Thus, many triarylamine-
based electrochromic polymers have been reported in litera-
ture.11 In recent years, Liou and coworkers12 has performed
extensive studies on the design and synthesis of
triarylamine-based high-performance polymers such as aro-
matic polyamides and polyimides for potential electrochro-
mic applications. In general, these polymers exhibit good
solubility to organic solvents due to the introduction of
bulky, packing-disruptive triarylamine moieties. In addition,
by incorporating the triarylamine units of different electronic
nature into the polymer backbone, different oxidation states
of the polymers could be accessed, which make them to be a
multi-electrochromic material. For example, it has been dem-
onstrated that aromatic polyamides containing N,N,N0,N0-tet-
raphenylphenylenediamine (TPPA) segments show attractive
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthetic route to the target diamine monomer 6. (a) 4-iodoanisole, Cu, K2CO3, TEGDME, 180 ꢀC; (b) BBr3, CHCl3, rt,
then quenched by MeOH; (c) p-fluoronitrobenzene, CsF, DMSO, 140 ꢀC; (d) Pd/C, hydrazine, EtOH, reflux; (e) p-fluoronitrobenzene,
CsF, DMSO, 140 ꢀC; (f) Pd/C, hydrazine, EtOH, reflux.
electrochromic properties such as high coloration efficiency,
high optical contrast, fast response time, long-term cycling
stability, and multicolored electrochromism.13 The radical cati-
ons of the TPPA-bearing polyamides generally showed rather
strong bands in the near infrared (NIR) region due to a
mixed-valence-type transition (charge transfer transition
between pairs of chromophores in different oxidation
states).14 Inserting the ether linkage between two triphenyl-
amine (TPA) units has been shown to interrupt the electronic
coupling between the amino centers as evidenced by electro-
chemical and spectroelectrochemical results.15 Thus, the poly-
amides containing O(TPA)2 in the main chain showed
significantly reduced intervalence charge-transfer (IV-CT)
absorptions, which may lead to an enhanced optical contrast
and color saturation in the visible region because the polaron
absorption was constrained mainly in that region. Inspired by
these interesting results, this work describes the synthesis
and characterization of a new O(TPA)2-containing diamine,
N,N-di(4-aminophenyl)-N0,N0-diphenyl-4,40-oxydianiline (struc-
ture 6 as shown in Sch. 1), and its derived electroactive aro-
matic polyamides. The polyamides are expected to exhibit
high thermal stability due to their aryl amide backbones,
together with good solubility because of the laterally attached
TPA groups. Due to the different electronic nature of the
main-chain TPA unit and the pendent TPA unit, these
O(TPA)2-containing polyamides are also expected to show a
multi-colored electrochromic behavior.
resulting from the Ullmann reaction of diphenylamine with
4-iodoanisole by using copper powder according to
a
reported procedure.16 Diphenylamine, iodobenzene, 4-
iodoanisole, potassium carbonate (K2CO3), triethylene glycol
dimethyl ether (TEGDME), boron tribromide (BBr3), sodium
hydrogen carbonate (NaHCO3), p-fluoronitrobenzene, cesium
fluoride (CsF), 10% palladium on charcoal (Pd/C), hydrazine
monohydrate, and triphenyl phosphite (TPP) were used as
received from commercial sources. DMSO, DMF, N,N-dimethy-
lacetamide (DMAc), pyridine, and NMP were dried over cal-
cium hydride, distilled under reduced pressure, and stored
over 4 Å molecular sieves in a sealed bottle. The aromatic
dicarboxylic acids such as terephthalic acid (7a, Wako), 4,40-
dicarboxydiphenyl ether (7b, TCI), bis(4-carboxyphenyl) sul-
fone (7c, New Japan Chemicals), and 2,2-bis(4-carboxyphe-
nyl)hexafluoropropane (7d, TCI) were used as received.
Commercially obtained calcium chloride (CaCl2) was dried
ꢀ
under vacuum at 180 C for 8 h before use. Tetrabutylammo-
nium perchlorate (TBAP), Bu4NClO4, was recrystallized from
ethyl acetate under nitrogen atmosphere and then dried
in vacuo before use.
Monomer Synthesis
4-Methoxytriphenylamine (1)
A mixture of the diphenylamine (82.6 g, 0.3 mol), 4-
iodoanisole (70.2 g, 0.3 mol), K2CO3 (82.9 g, 0.6 mol), Cu
(19.2 g, 0.3 mol), and TEGDME (60 mL) was heated and
stirred at 180 ꢀC for 24 h. Then, the reaction mixture was
poured into 1 L of water to precipitate the product. The
crude product was collected by filtration and then recrystal-
lized from methanol to give brown crystals (30.0 g, 60%
yield) with a melting point of 1042106 ꢀC. IR (KBr):
283022920 cm21 (methoxy CAH stretch).
EXPERIMENTAL
Materials
ꢀ
4-Hydroxytriphenylamine (2) (mp 5 1042106 C) was syn-
thesized by demethylation of 4-methoxytriphenylamine (1)
2
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FIGURE 1 (a) 1H NMR (b) 13C NMR (c) HAH COSY and (d) CAH HMQC spectra of the target diamine monomer 6 in DMSO-d6.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
4-Hydroxytriphenylamine (2)
using an ice bath. The solvent was then removed under
reduced pressure, and the residue was re-dissolved in ethyl
acetate (440 mL). The resulting solution was washed with a
saturated aqueous solution of NaHCO3 (2 3 220 mL) and
water (2 3 220 mL), dried over MgSO4, and evaporated to
give green powder (22.8 g, 80% yield). IR (KBr): 3324 cm21
(broad absorption band; OAH stretch).
A solution of compound 1 (30.0 g; 0.11 mol) in 220 mL of
chloroform was cooled to 0 ꢀC, and BBr3 (55.1 g; 0.22 mol,
220 mL 1M solution in CH2Cl2) was added slowly with stir-
ring. After stirring for 1 h at 0 ꢀC, the reaction mixture was
ꢀ
allowed to warm to room temperature (ꢁ25 C) and stirred
for 18 h. The reaction was carefully quenched with methanol
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4-(4-Nitrophenoxy)triphenylamine (3)
12.1 g (0.086 mol) of p-fluoronitrobenzene, 13.1 g (0.086
mol) ofꢀCsF, in 70 mL of dry DMSO was heated with stirring
at 140 C for 12 h under nitrogen atmosphere. After cooling,
the reaction mixture was poured into 700 mL of stirred
methanol slowly, and the crude product was collected by fil-
tration and then re-crystallized from acetonitrile twice to
give orange crystals (16.0 g, 70% yield) with a melting point
of 1642167 C. IR (KBr): 1573, 1344 cm (ANO2 stretch).
1H NMR (500 MHz, DMSO-d6, d, ppm): 7.00 (d, J 5 7.8 Hz,
4H, Hc), 7.02 (t, J 5 7.8 Hz, 2H, Ha), 7.06 (s, 4H, Hd 1 He),
7.13 (d, J 5 8.8 Hz, 2H, Hg), 7.21 (d, J 5 9.1 Hz, 4H, Hh),
7.27 (d, J 5 8.8 Hz, 2H, Hf), 7.29 (t, J 5 7.8 Hz, 4H, Hb), 8.18
(d, J 5 9.1 Hz, 4H, Hi). 13C NMR (125 MHz, DMSO-d6, d,
ppm): 119.75 (C11), 120.64 (C6), 122.12 (C14), 122.77 (C1),
123.31 (C3), 125.59 (C15), 125.98 (C7), 129.45 (C10), 129.54
(C2), 139.18 (C12), 141.95 (C16), 143.42 (C5), 147.34 (C4),
151.45 (C9), 151.65 (C13), 156.15 (C8).
In a 250 mL three-neck round-bottom flask equipped with a
stirring bar, a mixture of 22.8 g (0.087 mol) of compound 2,
12.3 g (0.087 mol) of p-fluoronitrobenzene, and 13.2 g (0.087
mol) of CsF in 90 mL of dry DMSO was heated with stirring at
140 ꢀC for 12 h under nitrogen atmosphere. After cooling, the
reaction mixture was poured into 450 mL of stirred methanol
slowly, and the precipitated yellow crystals (23.3 g, 70% yield;
mp 5 1292131 ꢀC) were collected by filtration. IR (KBr):
21
ꢀ
1
1588, 1330 cm21 (ANO2 stretch). H NMR (500 MHz, DMSO-
d6, d, ppm): 7.04 (d, J 5 7.8 Hz, 4H, Hc), 7.05 (t, J 5 7.8 Hz, 2H,
Ha), 7.08 (d, J 5 8.9 Hz, 2H, Hd), 7.13 (d, J 5 8.9 Hz, 2H, He),
7.18 (d, J 5 9.0 Hz, 2H, Hf), 7.32 (t, J 5 7.8 Hz, 4H, Hb), 8.26 (d,
J 5 9.0 Hz, 2H. Hg). 13C NMR (125 MHz, DMSO-d6, d, ppm):
117.09 (C10), 121.55 (C7), 122.97 (C1), 123.61 (C3), 125.30
(C6), 126.10 (C11), 129.51 (C2), 142.08 (C12), 144.50 (C5),
147.09 (C4), 149.26 (C8), 163.08 (C9).
N,N-Di(4-aminophenyl)-N0,N0-Diphenyl-4,40-Oxydianiline
(6)
4-(4-Aminophenoxy)triphenylamine (4)
In a 500 mL three-neck round-bottom flask equipped with a
stirring bar, 23.3 g (0.061 mol) of the nitro compound 3 and
0.15 g of 10% Pd/C were dissolved/suspended in 350 mL of
ethanol under nitrogen atmosphere. The suspension solution
was heated to reflux, and 8 mL of hydrazine monohydrate was
added slowly to the mixture, then the solution was stirred at
reflux temperature. After a further 24 h of reflux, the solution
was filtered hot to remove Pd/C, and the filtrate was then
cooled to precipitate white product. The product was collected
by filtration and dried in vacuo at 80 ꢀC to give 15ꢀ.0 g (70% in
yield) of white powders with an mp of 1232124 C. IR (KBr):
In 250 mL three-neck round-bottom flask equipped with a
reflux condenser and a stirring bar were placed 16.0 g (0.027
mol) of the dinitro compound 5, 0.15 g of 10% Pd/C, 4 mL of
hydrazine monohydrate, and 200 mL of ethanol. The reaction
mixture was heated at a reflux temperature under nitrogen
atmosphere for 24 h. After that, the solution was filtered hot to
remove Pd/C, and the filtrate was then cooled to precipitate
white product. The product was collected by filtration and
ꢀ
dried in vacuo at 80 C to give 10.1 g (70%ꢀin yield) of purple
powders with a melting point of 198–199 C. IR (KBr): 3423,
3348 cm21 (ANH2 stretch). 1H NMR (500 MHz, DMSO-d6, d,
ppm): 4.94 (s, 2H, ANH2), 6.54 (d, J 5 8.6 Hz, 4H, Hi), 6.65 (d, J
5 9.0 Hz, 2H, Hg), 6.79 (d, J 5 8.6 Hz, 4H, Hh), 6.83 (d, J 5 9.0
Hz, 2H, Hf), 6.88 (d, J 5 8.9 Hz, 2H, Hd), 6.95 (d, J 5 7.8 Hz, 4H,
Hc), 6.97 (d, J 5 7.8 Hz, 2H, Ha), 7.01 (d, J 5 8.9 Hz, 2H, He),
7.26 (t, J 5 7.8 Hz, 4H, Hb). 13C NMR (125 MHz, DMSO-d6, d,
ppm): 114.79 (C15), 118.30 (C6), 118.49 (C11), 119.93 (C10),
122.16 (C1), 122.61 (C3), 126.62 (C7), 126.73 (C14), 129.30 (C2),
136.32 (C13), 141.53 (C12), 145.14 (C16), 145.93 (C5), 147.37
(C4), 147.79 (C9), 154.26 (C8). The assignments of all NMR sig-
nals are shown in Figure 1.
1
3441, 3361 cm21 (ANH2 stretch). H NMR (500 MHz, DMSO-
d6, d, ppm): 4.93 (s, 2H, ANH2), 6.58 (d, J 5 8.7 Hz, 2H, Hg),
6.78 (d, J 5 8.7 Hz, 2H, Hf), 6.83 (d, J 5 8.9 Hz, 2H, Hd), 6.94 (d,
J 5 7.8 Hz, 4H, Hc), 6.96 (t, J 5 7.8 Hz, 2H, Ha), 6.99 (d, J 5 8.9
Hz, 2H, He), 7.25 (t, J 5 7.8 Hz, 4H, Hb). 13C NMR (125 MHz,
DMSO-d6, d, ppm): 114.86 (C11), 117.61 (C6), 120.78 (C10),
122.10 (C1), 122.53 (C3), 126.91 (C7), 129.34 (C2), 141.05 (C12),
145.42 (C5), 145.67 (C9), 147.51 (C4), 155.32 (C8).
Synthesis of Model Compounds
4-Phenoxytriphenylamine (M1)
A mixture of the compound 2 (6.00 g, 0.023 mol), iodoben-
zene (4.80 g, 0.023 mol), K2CO3 (6.50 g, 0.047 mol), Cu
(1.47 g, 0.023 mol), and TEGDME (10 mL) was heated and
stirred at 180 ꢀC for 24 h. Then, the reaction mixture was
poured into methanol. The product was collected by
N,N-Di(4-nitrophenyl)-N0,N0-Diphenyl-4,40-Oxydianiline (5)
In a 250 mL three-neck round-bottom flask equipped with a
stirring bar, a mixture of 15.0 g (0.043 mol) of compound 4,
4
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SCHEME 2 Synthesis of model compounds M1–M3.
filtration, washed by hexane, and dried to give brown pow-
der (2.15 g, 28% yield) with a melting point of 1162119 C.
mL of pyridine, and 0.85 mL of NMP was heated with stir-
ring at 120 ꢀC for 3 h. The resulting viscous solution was
poured slowly with stirring into 150 mL of methanol, giving
rise to a tough, fibrous precipitate. The precipitated product
was collected by filtration, washed repeatedly with methanol
and hot water, and dried to give a quantitative yield of poly-
amide 8a. The inherent viscosity of the polymer was 0.44
dL/g, measured in DMAc (containing 5 wt % LiCl) at a con-
ꢀ
4,40-Dibenzamido-400-Phenoxytriphenylamine (M2)
A
mixture of the 4,40-diamino-400-phenoxytriphenylamine
(0.30 g, 0.82 mmol), benzoic acid (0.20 g, 1.65 mmol), 0.80
mL of TPP, 0.40 mL of pyridine, and 0.80 mL of NMP was
ꢀ
heated and stirred at 120 C for 3 h. Then, the reaction mix-
ꢀ
centration of 0.5 g/dL at 30 C. The IR spectrum of 8a (film)
ture was poured into methanol. The product was collected
by filtration, washed by water, and dried to give pale green
exhibited characteristic amide absorption bands at 3300
cm21 (amide NAH stretch) and 1650 cm21 (amide carbonyl
stretch).
crystals (0.30 g, 60% yield) with
1132115 C.
a melting point of
ꢀ
N,N-Di(4-benzamido)-N0,N0-Diphenyl-4,40-Oxydianiline (M3)
A mixture of the diamine compound 6 (0.34 g, 0.64
mmol), benzoic acid (0.16 g, 1.28 mmol), 0.65 mL of TPP,
0.30 mL of pyridine, and 0.65 mL of NMP was heated and
stirred at 120 ꢀC for 3 h. After that, the reaction mixture
was poured into methanol. The product was collected by
filtration, washed by water, and dried to give pale green
powder (0.13 g, 28% yield) with a melting point of
1082111 ꢀC.
Preparation of the Polyamide Films
A solution of the polymer was made by dissolving about
0.6 g of the polyamide sample in 8 mL of DMAc. The homo-
geneous solution was poured into a 7-cm glass Petri dish,
which was placed in a 90 ꢀC 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
ꢀC for 8 h. The obtained films were about 50–60 lm in
thick and were used for XRD measurements and thermal
analyses.
Polymer Synthesis
The synthesis of polyamide 8a was used as an example to
illustrate the general synthetic route used to produce the
polyamides. A mixture of 0.45 g (0.85 mmol) of diamine
monomer 6, 0.14 g (0.85 mmol) of terephthalic acid (7a),
0.15 g of anhydrous calcium chloride, 0.85 mL of TPP, 0.25
Fabrication of the Electrochromic Devices
Electrochromic polymer films were prepared by dropping
solution of the polyamides (3 mg/mL in DMAc) onto the
ITO-coated glass substrate (20 3 30 3 0.7 mm, 502100 X/
cm2). A gel electrolyte based on PMMA (Mw 5 120,000) and
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SCHEME 3 Synthesis of polyamides 8a–8d.
LiClO4 was plasticized with propylene carbonate to form a
highly transparent and conductive gel. PMMA (1 g) was dis-
solved in dry acetonitrile (4 mL), and LiClO4 (0.1 g) was
added to the polymer solution as supporting electrolyte.
Then propylene carbonate (1.5 g) was added as plasticizer.
The mixture was then gently heated until gelation. The gel
electrolyte was spread on the polymer-coated side of the
electrode, and the electrodes were sandwiched. Finally, an
epoxy resin was used to seal the device.
graphite monochromator (operating at 40 kV and 30 mA),
using nickel-filtered Cu-Ka radiation (k 5 1.54 Å). The scan-
ning rate was 2ꢀ/min over a range of 2h 5 10–40ꢀ. TGA
was performed with a PerkinElmer Pyris 1 TGA. Experi-
ments were performed on approximately 4–6 mg of samples
heated in flowing nitrogen or air (flow rate 5 40 cm3/min)
at a heating rate of 20 ꢀC/min. DSC analyses were per-
formed on a PerkinElmer Pyris 1 DSC at a scan rate of 20
ꢀC/min in flowing nitrogen. Electrochemistry was performed
with a CHI 750A electrochemical analyzer. Cyclic voltamme-
try was conducted with the use of a three-electrode cell, in
which ITO (polymer films area about 0.8 3 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 elec-
trode. Ferrocene was used as an external reference for cali-
Instrumentation and Measurements
IR spectra were recorded on a Horiba FT-720 FT-IR spec-
trometer. 1H and 13C NMR spectra were measured on a
Bruker Avance 500 FT-NMR system with tetramethylsilane
as an internal standard. The inherent viscosities were deter-
mined with a Cannon-Fenske viscometer at 30 ꢀC. WAXD
measurements were performed at room temperature (ca. 25
ꢀC) on a Shimadzu XRD-6000 X-ray diffractometer with a
bration (10.48
V
vs. Ag/AgCl). Voltammograms are
presented with the positive potential pointing to the left and
with increasing anodic currents pointing downwards.
TABLE 1 Inherent Viscosity and Solubility Behavior of
Polyamides
Solubility in Various Solventsb
Polymer
a
Code
ginh (dL/g) NMP DMAc DMF DMSO m-Cresol THF
8a
8b
8c
8d
0.44
0.33
0.38
0.31
11 11
11 11
11 11
11 11
11 11
11 1h
11 11
11 11
1h
1-
--
1-
1-
11
1h
1h
a
Inherent viscosity measured at a concentration of 0.5 dL/g in DMAc–5
wt % LiCl at 30 ꢀC.
b
The qualitative solubility was tested with 10 mg of a sample in 1 mL
of stirred solvent. Notation of solubility behavior: 11, soluble at room
temperature; 1h, soluble on heating; --, insoluble even on heating; 1-,
partially soluble.
FIGURE 2 TGA curves of polyamide 8d with a heating rate of
20 ꢀC/min. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Solvents: NMP, N-methyl-2-pyrrolidone; DMAc, N,N-dimethylacetamide;
DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; THF,
tetrahydrofuran.
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TABLE 2 Thermal Properties of Polyamides
groups (ANO2 asymmetric and symmetric stretching) of
compound 3 and 5 gave two characteristic bands at around
1573–1588 and 1330–1344 cm21. After reduction, the char-
acteristic absorptions of the nitro group disappeared and the
amino group showed the typical NAH stretching absorption
pair in the region of 330023500 cm21 as shown in the IR
spectra of compounds 4 and 6. The 1H NMR and 13C NMR
spectra of the intermediate compounds 3–5 are included in
Supporting Information Figures S2–S4. The NMR spectra of
the target diamine monomer 6 are shown in Figure 1. The
assignments of all proton and carbon signals were assisted
by the 2D NMR spectra. These spectra are in good agree-
ment with the proposed molecular structures of these syn-
thesized compounds. The model compounds M1–M3 were
also prepared according to Scheme 2. The FT-IR spectra of
these model compounds are shown in Supporting Informa-
tion Figure S5. The characteristic absorptions are in good
agreement with their molecular structures.
Td at 5%
Weight Loss
(ꢀC)b
Td at 10%
Polymer
Code
Weight Loss
(ꢀC)b
Char Yield
(wt %)c
Tg (ꢀC)a
In N2 In Air In N2 In Air
8a
8b
8c
8d
241
218
253
233
510
511
500
519
472
466
485
475
565
567
556
568
555
538
537
545
76
65
66
69
a
Midpoint temperature of the baseline shift on the second DSC heating
trace (heating rate 5 20 ꢀC/min) of the sample after quenching from 400
to 50 ꢀC (cooling rate 5 200 ꢀC/min) in nitrogen.
b
Decomposition temperature at which a 5 or 10% weight loss was
recorded by TGA at a heating rate of 20 ꢀC/min and a gas flow rate of
20 cm3/min.
c
Residual weight % at 800 ꢀC at a scan rate 20 ꢀC/min in nitrogen.
Spectroelectrochemistry analyses were performed 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. Absorption
spectra in the spectroelectrochemical experiments were
measured with an Agilent 8453 UV-Visible diode array
spectrophotometer.
Polymer Synthesis
According to the phosphorylation technique described by
Yamazaki et al.,17 a series of novel aromatic polyamides (8a–
RESULTS AND DISCUSSION
Monomer Synthesis
The target aromatic diamine monomer, N,N-di(4-amino-
phenyl)-N0,N0-diphenyl- 4,40-oxydianiline (6), was synthesized
starting from diphenylamine by a six-step reaction sequence
as depicted in Scheme 1. In first step, the intermediate com-
pound 4-methoxytriphenylamine (1) was synthesized
through Ullmann reaction between diphenylamine and 4-
iodoanisole by using copper powder. Demethylation of com-
pound
1
with boron tribromide gave the 4-
hydroxytriphenylamine (2).16 Then, 4-(4-nitrophenoxy)tri-
phenylamine (3) was prepared by the nucleophilic aromatic
fluoro-displacement reaction of p-fluoronitrobenzene with
compound 2 in the presence of cesium fluoride (CsF). Reduc-
tion of the nitro group of compound 3 by means of hydra-
zine and Pd/C gave 4-(4-aminophenoxy)triphenylamine (4).
The target diamine monomer 6 was prepared by hydrazine
Pd/C-catalyzed reduction of dinitro compound 5, which was
prepared from the CsF-assisted N,N-diarylation reaction of
compound
4
with two equivalent amount of p-
fluoronitrobenzene.
The chemical structures of all the intermediate compounds
and the target compound 6 were verified by FTIR and NMR
spectroscopies. The FT-IR spectra of intermediate com-
pounds 1–5 and diamine monomer 6 are compiled in Figure
S1 of Supporting Information. The methoxy groups of com-
pound 1 shows characteristic peaks (AOCH3, CAH stretch-
ing) at 2831 and 2910 cm21. After demethylation, the
characteristic absorptions of the phenol group (AOH stretch-
ing) appeared in the region of 310023500 cm21. The nitro
FIGURE 3 (a) Cyclic voltammograms and (b) differential pulse
voltammograms of the cast films of polyamides 8a–8d on the
ITO-coated glass slide in 0.1M Bu4NClO4/CH3CN at scan rate of
50 mV/s. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
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TABLE 3 Redox Potentials and Energy Levels of Polyamides
Absorption
Wavelength
of Thin
Polymer
Films (nm)
Oxidation Potential (V)a
Eg (eV)b
Energy Level (eV)
HOMOc LUMOd
abs
max
abs
max
ox1
E
ox2
k
k
Eonset
DPV
E
1=2
1=2
CV
CV
DPV
CV
DPV
CV
DPV
CV
DPV
8a
8b
8c
305
309
305
303
306
458
411
500
409
419
0.60
0.61
0.70
0.63
0.45
0.61
0.61
0.58
0.62
0.30
0.81
0.82
0.81
0.82
0.57
0.79
0.82
0.80
0.80
0.60
0.94
0.94
0.96
0.95
0.95
0.96
0.97
0.95
0.96
0.97
2.70
3.02
2.48
3.03
2.96
5.17
5.18
5.17
5.18
4.95
5.15
5.18
5.16
5.16
4.96
2.47
2.16
2.69
2.15
1.99
2.45
2.16
2.68
2.13
2.00
8d
8’a
a
c
From cyclic voltammograms versus Ag/AgCl in CH3CN. E1/2: Average
The HOMO energy levels were calculated from cyclic voltammetry and
potential of the redox couple peaks.
were referenced to ferrocene (4.8 eV).
b
d
The data were calculated from polymer films by the equation: Eg
1240/konset (energy gap between HOMO and LUMO).
5
LUMO 5 HOMO – Eg.
8d) with O(TPA)2 units were synthesized from diamine
monomer 6 and aromatic dicarboxylic acids 7a27d using
TPP and pyridine as condensing agents (Sch. 3). All the poly-
merizations proceeded homogeneously throughout the reac-
tion and afforded clear and highly viscous polymer solutions.
The products precipitated in a tough, fiber-like form when
the resulting polymer solutions were slowly poured into
methanol. As shown in Table 1, the obtained polyamides had
inherent viscosities in the range of 0.3120.44 dL/g and
could be solution-cast into flexible and strong films (Sup-
porting Information Fig. S6), indicating the formation of high
molecular weight polymers. This mechanical property is
important for the further development of flexible optoelec-
tronic devices. The formation of polyamides was also con-
firmed by IR and NMR spectroscopy. The representative IR
spectrum for polyamide 8a is included in Supporting Infor-
mation Figure S7, which shows the characteristic absorption
bands of the amide group at around 1650 cm21 (amide car-
bonyl) and 3300 cm21 (NAH stretching). Supporting Infor-
mation Figure S8 shows the 1H NMR and COSY spectra of
polyamide 8a in DMSO-d6. All the peaks could be readily
assigned to the hydrogen atoms in the repeating unit. The
1
resonance peak appearing at 10.35 ppm in the H NMR spec-
trum also supports the formation of amide linkages.
Solubility of Polyamides
The WAXD patterns of the polyamide films are shown in Sup-
porting Information Figure S9. These polymers exhibited an
amorphous nature because of the bulky, packing-disruptive
O(TPA)2 unit along the polymer backbone, which does not
favor their close chain packing. The solubility behavior of pol-
yamides 8a28d was tested qualitatively, and the results are
summarized in Table 1. All the polyamides were highly solu-
ble in polar solvents such as NMP, DMAc, and DMF, and the
FIGURE 4 Cyclic voltammetric diagrams of model compounds M1, M2, and M3 (1023M) in 0.1M Bu4NClO4/CH3CN solution at a
scan rate of 50 mV/s. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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SCHEME 4 Anodic oxidation pathways of polyamides (a) 8a and (b) 80a. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
good solubility could be attributed in part to the introduction
of propeller-shaped TPA moiety in the repeat unit, which
decreases interchain interactions and increases the free vol-
ume. The polyamide 8d showed a better solubility as com-
pared with the other polymers due to the effect of the large
volume of CF3 groups. Thus, the excellent solubility makes
these polymers potential candidates for practical applications
by spin-coating or inkjet-printing processes to afford high per-
formance thin films for optoelectronic devices.
therms up to the Td on the DSC thermograms. The thermal
analysis results showed that these polyamides exhibited excel-
lent thermal stability, which in turn is beneficial to increase
the service time in device application and enhance the mor-
phological stability to the spin-coated film.
Electrochemical Properties
The electrochemical behavior of the polyamides was investi-
gated by CV and differential pulse voltammetry (DPV) con-
ducted for the cast films on an ITO-coated glass substrate as
working electrode immersed in dry acetonitrile (CH3CN) con-
taining 0.1M of Bu4NClO4 as an electrolyte under nitrogen
atmosphere. In comparison to CV, DPV offers a higher sensi-
tivity and sharper redox onsets throughout the
Thermal Properties
The thermal properties of all the polyamides were investi-
gated by TGA and DSC techniques, and their thermal behavior
data are summarized in Table 2. Typical TGA curves of a rep-
resentative polyamide 8d in both air and nitrogen atmos-
pheres are illustrated in Figure 2. All the polymers exhibited
good thermal stability with insignificant weight loss up to 450
ꢀC in both air and nitrogen atmospheres. The decomposition
temperatures (Td) at a 10% weight-loss of the polyamides in
nitrogen and air were recorded in the range of 5562568 and
5372555 ꢀC, respectively. The mount of carbonized residue
(char yield) of these pꢀolymers in nitrogen atmosphere was
more than 65% at 800 C. The high char yields of these poly-
mers can be ascribed to their high aromatic content. The Tg of
all the polymers were measured to be in the range of
TABLE 4 Electrochemical Properties of Model Compounds and
Polyamide 8a
Oxidation Potentials (V) vs. Ag/
Code
AgCl in CH3CN
ox1
1=2
ox2
1=2
E
E
M1
M2
M3
8a
0.93
0.73
0.74
0.81
–
–
0.96
0.94
ꢀ
2182253 C by DSC as shown in Supporting Information Fig-
ure S10. All the polymers indicated no clear melting endo-
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energy levels obtained, the polyamides in this study appear
to be appropriate as hole injection and transport materials.
For a clear understanding the order of the oxidation proc-
esses occurring in these polyamides, we studied the CV
behaviors of the model compounds M1–M3. The results are
shown in Figure 4, and the relevant oxidation potential data
are summarized in Table 4. Judging from the CV curves of
these model compounds, we believe that the first oxidation
peak appears to involve one electron loss from the main-
chain TPA unit of the polyamides 8a–8d [see Sch. 4(a)]. This
can be rationalized because this TPA segment is more
electron-rich and becomes more easily oxidized. The second
oxidation peak is related to the electron losses from the
side-chain TPA unit.
Furthermore, the CV curves of polyamides 8a and 80a are
shown in Figure 5 for comparison. The oxidation peaks in
the CV curve of the referenced polyamide 80a are well sepa-
rated (DEpa 5 0.40 V), indicating a strong electronic cou-
pling between the amino centers. The radical cation species
of 80a belongs to the Robin/Day class III [delocalized redox
centers, see Sch. 4(b)]14(b) where both TPA centers share a
common phenylene spacer. In contrast, the radical cation
species of 8a displayed a class I like transition (localized
redox centers) with a smaller redox potential splitting DEpa
of the two redox waves. This implies that the separated
redox centers in the O(TPA)2 unit exhibited an independent
electrochemical behavior, and two electrons within the
O(TPA)2 segment are removed in sequence to form a dica-
tionic structure without intramolecular electron transfer.
FIGURE 5 Cyclic voltammograms of the cast films of polya-
mides (a) 8a and (b) 80a on the ITO-coated glass slide in 0.1M
Bu4NClO4/CH3CN at a scan rate of 50 mV/s. [Color figure can
be viewed in the online issue, which is available at wileyonline-
library.com.]
Spectroelectrochemistry and Electrochromic Properties
Spectroelectrochemical measurements were performed on
films of polymers drop-coated onto ITO-coated glass slides
immerged in electrolyte solution. The electrode prepara-
tions and solution conditions were identical to those used
in the CV experiments. The change in absorption of polyam-
ide 8d film at various applied potentials is shown in Figure
6, which was representative for the other 8a–8c counter-
parts. In the neutral form, at 0 V, polyamide 8d exhibited
strong absorption at wavelength around 303 nm, but it
almost transparent in the visible region. When the applied
voltage was stepped from 0 to 0.8–1.0 V, the intensity of
the absorption band around 303 nm decreased gradually,
and new peaks at 400 and 790 nm gradually increased in
intensity [Fig. 6(a,b)]. We attribute these spectral changes
to the formation of a stable cation radical of the main-
chain TPA moiety. Upon further oxidation at applied vol-
tages to 1.2 V, corresponding to the second step oxidation,
the peaks of characteristic absorbance of the radical cation
decreased gradually and one new band grew up at 558 nm
[Fig. 6(c)]. The new spectrum arises from the dication for-
mation in the O(TPA)2 segment of polyamide 8d. From the
photos shown in inset Figure 6(a,c) it can be seen that the
film changed from a transmissive neutral state (nearly col-
orless) to a highly absorbing semi-oxidized state (light
green, green) and a fully oxidized state (purple). When the
electrochemical process owing to a reduced contribution of
the charging background currents. Figure 3 shows the CV
and DPV curves for all polyamides, and the relevant oxida-
tion potential data are summarized in Table 3. There are two
reversible oxidation redox couples with half-wave potentials
(E1/2) of 0.80–0.82 V and 0.96–0.98 V at scan rate of 50
mV/s. In comparison with those found in TPPA-based polya-
mides,13 the present polyamides showed a closer redox
potential splitting DE of the two waves. This implies a weak
electronic coupling between the two TPA amino redox cen-
ters mediated by the ether connector and is consistent with
the observation made by the Liou and coworkers.15 During
the CV scanning we also found these polyamides showed
interesting multi-colored electrochromic behaviors with colo-
ration changing from colorless through green to purple. The
HOMO energy levels of the investigated polyamides were cal-
culated from the oxidation onset potentials (Eonset) and by
comparison with ferrocene (4.8 eV). These data together
with absorption spectra were then used to obtain the LUMO
energy levels (Table 3). According to the HOMO and LUMO
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FIGURE 6 Spectroelectrochemistry of the polyamide 8d thin film on the ITO-coated glass substrate in 0.1M Bu4NClO4/CH3CN at (a)
0.8 V, (b) 1.0 V, and (c) 1.2 V. The photos (d) show the transmittance change (᭝T%) of the film on an ITO electrode at indicated
potentials. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
applied voltage was stepped 0–1.0 V, the polymer 8d exhib-
ited high contrast of optical transmittance change (DT%)
up to 48% at 400 nm and 87% at 790 nm for green color-
ing and when the applied voltage was stepped 0–1.2 V, the
polymer 8d exhibited DT% up to 58% at 558 nm for pur-
ple coloring [see Fig. 6(d)].
FIGURE 7 Spectroelectrochemistry of the polyamides (a) 8a and (b) 80a thin film on the ITO-coated glass substrate in 0.1M Bu4N-
ClO4/CH3CN. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 8 Potential step absorptometry of the cast film of polyamide 8d on the ITO-glass slide (coated area: 1 cm2)(in CH3CN with
0.1M TBAP as the supporting electrolyte) by applying a potential step (a) 0.0 V0.8 V and (b) 0.0 V1.2 V with a cycle time of 16 s.
Optical switching for polyamide 8d at (c) kmax 5 790 nm as the applied voltage was stepped between 0 and 0.8 V (vs. Ag/Ag/Cl)
and (d) at kmax 5 558 nm as the applied voltage was stepped between 0 and 1.2 V (vs. Ag/Ag/Cl). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
For a comparative purpose, the spectral changes of polya-
mides 8a and 80a upon electro-oxidation are illustrated in
Figure 7. Upon electro-oxidation of the 80a film (increasing
applied voltage from 0 to 0.7 V), the absorption of p-p* tran-
sition at 306 nm gradually decreased, whereas a new
absorption peak at 425 nm and a broadband having its max-
imum around 990 nm in the NIR region grew up. We attrib-
ute this spectral change to the formation of a stable
monocation radical from the TPPA moiety. The absorption
band in the NIR region is assigned to an IV-CT between
states in which the positive charge is centered at different
amino centers.13 Upon further oxidation at applied voltages
to 1.1 V, the IV-CT band gradually decreased, with a forma-
tion of a new strong absorption band centered at about 725
nm. From the photos shown in Figure 7(b) inset, it can be
seen that the film changed from a transmissive neutral state
(nearly colorless) to a highly absorbing semi-oxidized state
(yellowish green) and a fully oxidized state (blue). Figure
7(a) presents the UV-vis-NIR absorption spectra of polyam-
ide 8a film at various applied potentials. No IV-CT band
could be detected for the 8a film upon oxidation. The two
amino centers in the O(TPA)2 segment were oxidized in
sequence with a color switches from near colorless (neutral
state) to green, blue, and purple (oxidized states) of the elec-
trochromic film. The result suggests that the introduction of
ether-linkage between the TPA units can tune the UV-vis-NIR
absorption profile of the polarons of polyamides.
Optical switching studies were examined for probing changes
in transmittance with time while repeatedly stepping the
potential between neutral and oxidized states. The polymer
film was cast onto an ITO-coated glass slide in the same
manner as described earlier and each film was potential
stepped between its neutral (0 V), oxidized (10.8 V), and
fully oxidized (11.2 V) state. While the films were switched,
the absorbance at 790 and 558 nm was monitored as a func-
tion of time with UV-vis-NIR spectroscopy. Switching data for
the cast film of polyamide 8d are given in Figure 8. The
switching time was calculated at 90% of the full switch
TABLE 5 Electrochromic Properties of Polyamide 8d
Response
Timeb
a
d
kmax (nm) D%T
DODc Qd (mC/cm2) CEe (cm2/C)
tc (s) tb (s)
790
558
72
50
2.4
4.3
1.3
2.0
0.589 3.69
0.301 3.42
160
88
a
Wavelength of absorption maximum.
b
c
Time for 90% of the full-transmittance change.
Optical Density (DOD)
5 log[Tbleached/Tcolored], where Tcolored and
Tbleached are the maximum transmittance in the oxidized and neutral
states, respectively.
d
Qd is ejected charge, determined from the in situ experiments.
Coloration efficiency (CE) 5 DOD/Qd.
e
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FIGURE 9 (a) Cyclic voltammograms of the cast film in the electrochromic device based on polyamide 8d, (b) schematic illustra-
tion of the structure of the electrochromic cell, (c) photos of sandwich-type ITO-coated glass electrochromic cell, and (d) spectroe-
lectrochemistry of the device. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 10 Potential step absorptometry of the cast film of device 8d on the ITO-glass slide (in CH3CN with 0.1M TBAP as the sup-
porting electrolyte) by applying a potential step (a) 0.0 V2.2 V and (b) 0.0 V2.6 V with a cycle time of 16 s. Optical switching for
device 8d at (c) kmax 5 800 nm as the applied voltage was stepped between 0 and 2.2 V (vs. Ag/Ag/Cl), and (d) at kmax 5 558 nm
as the applied voltage was stepped between 0 and 2.6 V (vs. Ag/Ag/Cl). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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because it is difficult to perceive any further color change
with naked eye beyond this point. As shown in Figure 8(a,b)
after over 100 cyclic scans between 0 and 0.8 V and over 50
cyclic scans between 0 and 1.2 V, the polymer films still
exhibited good reversibility of electrochromic characteristics.
As shown in Figure 8(c,d) the polyamides switch rapidly
between the highly transmissive neutral state and the col-
ored oxidized state. Thin film from polyamide 8d required
2.4 s at 0.8 V for switching absorbance at 790 nm and 1.3 s
for bleaching. When the potential was set at 1.2 V, the thin
film from polyamide 8d would require 4.3 s for the coloring
process at 558 nm and 2 s for the bleaching process. The
trochemical and electrochromic stability along with multi-
electrochromic behavior. The color changes from the color-
less or pale yellowish neutral form to the green, blue, and
purple oxidized forms when scanning potentials positively
from 0.0 to 1.2 V. These polyamides did not show IV-CT
absorption during oxidation because of the presence of
ether linkage between the TPA units. The electrochemical,
electrochromic, and device performance suggest that the
present polyamides have great potential for use in optoe-
lectronic applications.
ACKNOWLEDGMENT
electrochromic coloring efficiency (CE) of green (g
5
We thank the financial support from the Ministry of Science
and Technology in Taiwan, ROC.
DOD790/Q) and purple coloring (g 5 DOD558/Q) of the poly-
amide 8d were calculated to be 160 and 88 cm2/C, respec-
tively (Table 5).
REFERENCES AND NOTES
On the basis of the foregoing results, it can be concluded that
these polyamides can be used in the construction of electro-
chromic devices and optical display due to the fast response
time and the robustness of the polymers. Therefore, we fabri-
cated as preliminary investigations single layer electrochromic
cells [Fig. 9(b)]. The polymer films were drop-coated onto ITO-
coated glass and then dried. Afterwards, the gel electrolyte
was spread on the polymer-coated side of the electrode and
the electrodes were sandwiched under atmospheric condition.
To prevent leakage, an epoxy resin was applied to seal the
device. As a typical example, an electrochromic cell based on
polyamide 8d was fabricated. When the voltage applied was
increased (to a maximum of 2.6 V), the color changed from
colorless (neutral) to green (semi-oxidized) and purple (fully
oxidized), the same as was already observed for the solution
spectroelectrochemistry experiments. The color and spectral
changes of the electrochromic device can be seen in Figure
9(c,d), which are similar to those observed in the electrochem-
ical cell. Switching data for the electrochromic layer in the
device based on polyamide 8d are given in Figure 10.
Although the response time is slightly slower than that
observed in the electrochemical cell (Table 5), the device
showed stable absorbance change in the first 40 cyclic scans
between 0 and 2.2 V at 800 nm, and in the first 10 cyclic
scans between 0 and 2.6 V at 558 nm.
1 P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky, Electro-
chromism and Electrochromic Devices; Cambridge University
Press: Cambridge, 2007.
2 (a) R. D. Rauch, Electrochim. Acta 1999, 44, 3165–3176; (b) A.
Michaelis, H. Berneth, D. Haarer, S. Kostromine, R. Neigl, R.
Schmidt, Adv. Mater. 2001, 13, 1825-1828; (c) H. W. Heuer, R.
Wehrmann, S. Kirchmeyer, Adv. Funct. Mater. 2002, 12, 89-94;
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3 (a) D. R. Rosseinsky, R. J. Mortimer, Adv. Mater. 2001, 13,
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The new TPA-based aromatic diamine monomer, N,N-di(4-
aminophenyl)-N0,N0-diphenyl-4,40-oxydianiline (6) was suc-
cessfully synthesized in high purity and good yield from
readily available reagents. Aromatic polyamides bearing
ether-linked bis(triphenylamine) [O(TPA)2] units were
readily prepared from the newly synthesized diamine
monomer and various aromatic dicarboxylic acids by the
phosphorylation polyamidation technique. Because of the
introduction of 3D TPA units in polymer backbone, all the
polymers were amorphous, had good solubility in many
polar aprotic solvents, and exhibited excellent film-forming
ability. In addition to high Tg values and good thermal sta-
bility, all the obtained polyamides also showed good elec-
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