Fluorescent and electrochromic polyamides with pyrenylamine chromophore

Article abstract of DOI:10.1039/c0jm00495b

A series of novel polyamides with a pyrenylamine chromophore in the backbone were prepared from a newly synthesized diamine monomer, N,N-di(4-aminophenyl)-1-aminopyrene, and various dicarboxylic acids via the phosphorylation polyamidation technique. These polyamides were readily soluble in many organic solvents and could be solution-cast into tough and amorphous films. They had useful levels of thermal stability with glass-transition temperatures in the range of 246-326 °C and 10% weight loss temperatures in excess of 500 °C. The dilute NMP solutions of these polyamides exhibited fluorescence maxima around 522-544 nm with quantum yields up to 30.2%. These polyamides also showed remarkable fluorescence solvatochromism in various solvents. The polymer films showed reversible electrochemical oxidation and reduction accompanied by strong color changes from the yellow neutral state to a purple oxidized state and to an orange reduced state. The anodically electrochromic films had high coloration efficiency (up to 172 cm² C^-1 at 834 nm) and good redox stability, which still retained a high electroactivity after long-term redox cycles.

Full text of DOI:10.1039/c0jm00495b

View Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Fluorescent and electrochromic polyamides with pyrenylamine chromophore†
a
Yi-Chun Kung and Sheng-Huei Hsiao
b
*
Received 24th February 2010, Accepted 7th April 2010
First published as an Advance Article on the web 26th May 2010
DOI: 10.1039/c0jm00495b
A series of novel polyamides with a pyrenylamine chromophore in the backbone were prepared from
a newly synthesized diamine monomer, N,N-di(4-aminophenyl)-1-aminopyrene, and various
dicarboxylic acids via the phosphorylation polyamidation technique. These polyamides were readily
soluble in many organic solvents and could be solution-cast into tough and amorphous films. They had
useful levels of thermal stability with glass-transition temperatures in the range of 246–326 ꢀC and 10%
weight loss temperatures in excess of 500 ꢀC. The dilute NMP solutions of these polyamides exhibited
fluorescence maxima around 522–544 nm with quantum yields up to 30.2%. These polyamides also
showed remarkable fluorescence solvatochromism in various solvents. The polymer films showed
reversible electrochemical oxidation and reduction accompanied by strong color changes from the
yellow neutral state to a purple oxidized state and to an orange reduced state. The anodically
electrochromic films had high coloration efficiency (up to 172 cm2 Cꢁ1 at 834 nm) and good redox
stability, which still retained a high electroactivity after long-term redox cycles.
cyclic phosphazene cores12 have also been exploited for applica-
Introduction
tion in OLEDs.
Pyrene is a prototypical fluorescent molecule with desirable
photophysical properties that make it useful as a fluorescence
probe.1 Among the attractive properties of pyrene is its ready
functionalization, appearance of delayed fluorescence, distinct
solvatochromic phenomena, and its high propensity for forming
excimers. Pyrene and its derivatives have been widely used as
fluorescence probes in many applications.2 In recent years, pyrene
derivatives have attracted increasing interest in the fabrication of
various optoelectronic devices such as organic light-emitting
devices (OLEDs) and organic field-effect transistors (OFETs),
due to their emissive properties combined with high charge
carrier mobility. There are a vast number of pyrene derivatives
that have been reported in the literature, including tetraphe-
nylpyrene derivatives,3 oligothiophenes with pyrenyl side or
end groups,4 ethynylene-conjugated pyrene derivatives,5 hexa-
pyrenylbenzene and dipyrenylbenzene derivatives,6 as well as
pyrene-carbazole,7 pyrene-fluorene and pyrene-fluorene-carba-
zole systems.8 Pyrene-cored starbursts and dendrimers have
recently been reported in the context of organic electronic
applications.9 Furthermore, oligopyrenes, polypyrenes, and poly-
pyrene dendrimers have also been investigated for light-emitting
materials.10 Organic-inorganic hybrids based on pyrene func-
tionalized silsesquioxane cores11 or pyrenylamine-functionalized
Due to the large planar conjugated aromatic characteristic,
pyrene has strong p electron delocalization energy and efficient
fluorescence property, and the emission is pure blue to permit
ready exploitation as a blue-light emitting material in OLEDs.
However, the use of pyrene as emitting material in OLED
applications has been limited, due to aggregation between planar
pyrene molecules. The high tendency towards p-stacking of the
pyrene moieties generally lends the pyrene-containing emitters
strong intermolecular interactions in the solid state, which leads
to a substantial red shift of their fluorescence emission and
a decrease of the fluorescence quantum yields. Through mole-
cular structure design, the close packing/fluorescence quenching
effect in pyrene-type materials can be reduced or controlled. A
successful effort in the prevention of p-stacking in small mole-
cules was achieved with 1,3,6,8-tetrasubstituted highly sterically
hindered pyrenes,3e which can emit blue light in solution as well
as in the solid state and with high quantum yield. In addition,
diarylamino functionalized pyrene derivatives have been found
to perform efficiently as emitters and charge transport materials
in OLEDs.13 The emission color of the diarylamino pyrenes can
also be tuned by changing the electron-withdrawing and elec-
tron-donating nature of the diarylamine substituents.14 The
pyrene-containing triarylamines generally exhibit both hole-
transporting and emitting properties; therefore, they can be used
as hole-transporting emitters in the fabrication of OLEDs.13b,13c
Triarylamine derivatives are well known for photo and elec-
troactive properties that find optoelectronic applications as
photoconductors, hole-transporters, and light-emitters.15 Elec-
tron-rich 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 the litera-
ture.16 In recent years, Liou’s and our groups have developed
aDepartment of Chemical Engineering, Tatung University, 40 Chungshan
North Road, 3rd Section, Taipei, 10452, Taiwan
bDepartment of Chemical Engineering and Biotechnology, National Taipei
University of Technology, 1 Chunghsiao East Road, 3rd Section, Taipei,
10608, Taiwan. E-mail: shhsiao@ntut.edu.tw
† Electronic supplementary information (ESI) available: Synthetic details
and characterization data of compounds 1, 2 and M1, inherent viscosity
and solubility behavior of all polyamides, IR spectra of all the
synthesized compounds 1–4 and M1, typical IR spectrum of polymer
6a, NMR spectra of compounds 1, 2, 3 and M1, WAXD patterns of all
polyamides. See DOI: 10.1039/c0jm00495b
a
number of high-performance polymers (e.g., aromatic
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 5481–5492 | 5481

View Online
polyamides and polyimides) carrying the triarylamine unit as an
electrochromic functional moiety.17 Our strategy was to synthe-
size the triarylamine-containing monomers such as diamines and
dicarboxylic acids that were then reacted with the corresponding
comonomers through conventional polycondensation tech-
niques. The obtained polymers possessed characteristically well-
defined structure, high molecular weights, and high thermal
stability. Because of the incorporation of packing-disruptive,
three-dimensional triarylamine units along the polymer back-
bone, most of these polymers exhibited good solubility in polar
organic solvents. They can form uniform, transparent, and
amorphous films by solution processes such as spin-coating,
inkjet-printing, or spray-coating. This is advantageous for the
production of large-area, thin-film devices at low cost. To our
knowledge, there has been no report of pyrenylamine-containing
condensation polymers. In view of the attractive properties
associated with the pyrene and triarylamine units, herein we
report the synthesis of N,N-di(4-aminophenyl)-1-aminopyrene,
as a new diamine monomer, and its derived polyamides con-
taining pyrenylamine chromophores. The introduction of the
pyrenylamine moiety into the polyamide backbone leads to
interesting photophysical, redox, and electrochromic properties
useful for optoelectronic applications.
N,N-Di(4-aminophenyl)-1-aminopyrene (4). In a 250-mL three-
neck round-bottomed flask equipped with a stirring bar under
nitrogen atmosphere, 4.0 g (8.7 mmol) of dinitro compound 3
and 0.15 g of 10% Pd/C were dissolved/suspended in 60 mL of
ethanol. The suspension solution was heated to reflux, and
2.5 mL of hydrazine monohydrate was added slowly to the
mixture, then the solution was stirred at reflux temperature.
After a further 16 h of reflux, the solution was filtered to remove
the Pd/C, and the filtrate was cooled under a nitrogen flow to
grow yellow crystals. The products were collected by filtration
ꢀ
and dried in vacuo at 80 C; yield ¼ 2.61 g (75%), mp ¼ 227–
229 C (by DSC at 2 C minꢁ1). FT-IR (KBr): 3200–3400 cmꢁ1
(N–H stretch). 1H NMR (500 MHz, DMSO-d6, d, ppm) (for the
peak assignments, see Fig. 1a): 4.79 (s, 4H, -NH2), 6.48 (d, J ¼
8.7 Hz, 4H, Hk), 6.66 (d, J ¼ 8.7 Hz, 4H, Hj), 7.63 (d, J ¼ 8.3 Hz,
1H, Ha), 7.94 (d, J ¼ 9.3 Hz, 1H, Hd), 7.99 (t, J ¼ 7.6 Hz, 1H, Hf),
8.03 (d, J ¼ 8.9 Hz, 1H, Hi), 8.06 (d, J ¼ 8.9 Hz, 1H, Hh), 8.12
ꢀ
ꢀ
Experimental
Synthesis of monomers
1-Nitropyrene (1) was prepared from the nitration of pyrene
using copper nitrate trihydrate, according to a reported proce-
dure.18 Catalytic reduction of nitro compound 1 by means of
hydrazine and Pd/C in refluxing ethanol gave 1-aminopyrene (2).
The synthetic details and characterization data of compounds 1
and 2 are included in the ESI.†
N,N-Di(4-nitrophenyl)-1-aminopyrene (3). In a 250-mL one-
neck round-bottomed flask equipped with a stirring bar under air
atmosphere, to a solution of 10.2 g (47 mmol) of 1-aminopyrene
and 13.4 g (95 mmol) of 4-fluoronitrobenzene in 100 mL of dried
dimethyl sulfoxide (DMSO), 14.5 g (95 mmol) of dried caesium
fluoride (CsF) was added with stirring all at once, and the
mixture was heated at 140 ꢀC for 18 h under nitrogen atmo-
sphere. The mixture was poured into 800 mL of stirred methanol
slowly, and the precipitated compound was collected by filtration
and washed thoroughly by methanol and hot water. The crude
product was filtered and recrystallized from DMF–methanol to
afford 18.3 g (85% in yield) of brown needles with a mp of 271–
273 ꢀC (by DSC at a scan rate of 2 ꢀC minꢁ1). FT-IR (KBr): 1302,
1
1579 cmꢁ1 (NO2 stretch). H NMR (500 MHz, DMSO-d6, d,
ppm): 7.24 (d, J ¼ 9.2 Hz, 4H, Hj), 7.96 (d, J ¼ 9.2 Hz, 1H, Ha),
8.04 (d, J ¼ 8.1 Hz, 1H, Hd), 8.13 (t, J ¼ 7.7 Hz, 1H, Hf), 8.15 (d,
J ¼ 9.3 Hz, 4H, Hk), 8.20 (d, J ¼ 9.3 Hz, 1H, Hb), 8.28 (d, J ¼ 9.0
Hz, 1H, Hi), 8.32 (d, J ¼ 9.0 Hz, 1H, Hh), 8.32 (d, J ¼ 7.4 Hz, 1H,
He), 8.40 (d, J ¼ 7.3 Hz, 1H, Hg), 8.47 (d, J ¼ 8.2 Hz, 1H, Hc). 13
C
NMR (125 MHz, DMSO-d6, d, ppm): 121.36 (C2), 121.43 (C18),
123.68 (C4), 125.41 (C14), 125.61 (C19), 125.96 (C8), 126.25 (C10),
126.78 (C5), 126.97 (C9), 127.12 (C13), 127.67 (C15), 127.84 (C6),
128.27 (C12), 129.46 (C3), 130.25 (C16), 130.55 (C11), 130.81 (C7),
136.95 (C1), 141.81 (C20), 151.86 (C17).
Fig. 1 (a) 1H and (b) 13C NMR spectra of target diamine monomer 4 in
DMSO-d6.
5482 | J. Mater. Chem., 2010, 20, 5481–5492
This journal is ª The Royal Society of Chemistry 2010

View Online
(two overlapped doublets, J ¼ 9.0 Hz, 2H, Hc + He), 8.16 (d, J ¼
8.3 Hz, 1H, Hb), 8.19 (d, J ¼ 7.6 Hz, 1H, Hg). 13C NMR (125
MHz, DMSO-d6, d, ppm) (for the peak assignments, see Fig. 1b):
114.88 (C19), 123.70 (C4), 123.81 (C18), 124.19 (C8 + C5), 124.43
(C14), 124.68 (C10), 125.54 (C2), 125.66 (C15), 125.68 (C13), 125.85
(C3), 126.28 (C9), 126.38 (C6), 127.18 (C16), 127.26 (C12), 130.57
(C11), 130.97 (C7), 139.44 (C17), 143.20 (C1), 143.67 (C20). Anal.
Calcd (%) for C28H21N3 (399.49): C, 84.18; H, 5.30; N, 10.52.
Found: C, 84.02; H, 5.29; N, 10.50.
d6, d, ppm) (for the proton assignments, see Fig. 2): 7.00 (d, J ¼
8.5 Hz, 4H, Hj), 7.69 (d, J ¼ 8.5 Hz, 4H, Hk), 7.86 (d, J ¼ 7.9 Hz,
1H, Ha), 8.05 (s, 4H, Hl), 8.06–8.12 (m, 3H, Hf + Hd + Hc), 8.15–
8.18 (m, 2H, Hh + Hi), 8.22 (d, J ¼ 6.7 Hz, 1H, He), 8.30 (d, J ¼
7.2 Hz, 1H, Hg), 8.34 (d, J ¼ 7.7 Hz, 1H, Hb), 10.32 (s, 2H, amide
N–H).
Preparation of the polyamide films
A solution of polymer was made by dissolving about 0.5 g of the
polyamide sample in 10 mL of hot DMAc. The homogeneous
solution was poured into a 9-cm glass Petri dish, which was
placed in a 90 ꢀC oven for 5 h to remove most of the solvent; then
Synthesis of polyamides
The synthesis of polyamide 6a was used as an example to illus-
trate the general synthetic route used to produce the polyamides.
A mixture of 0.399 g (1.0 mmol) of diamine 4, 0.166 g (1.0 mmol)
of terephthalic acid (5a), 0.1 g of dried calcium chloride, 1.0 mL
of triphenyl phosphite (TPP), 0.3 mL of pyridine, and 1.0 mL of
N-methyl-2-pyrrolidinone (NMP) was heated with stirring at
120 ꢀC for 3 h. The obtained polymer solution was poured slowly
into 300 mL of methanol with stirring giving rise to a stringy,
fiber-like precipitate that was collected by filtration, washed
thoroughly with hot water and methanol, and dried under
vacuum at 100 ꢀC. Reprecipitations of the polymer by N,N-
dimethylacetamide (DMAc)/methanol were carried out twice for
further purification. The inherent viscosity of the obtained
polyamide 6a was 1.08 dL/g, measured at a concentration of
0.5 g/dL in DMAc at 30 ꢀC. IR (KBr): 3290 (amide N–H stretch),
ꢀ
the semidried film was further dried in vacuo at 180 C for 8 h.
The obtained films were about 30–50 mm in thickness and were
used for X-ray diffraction measurements, solubility tests, and
thermal analyses.
Fabrication of the electrochromic device
Electrochromic polymer films were prepared by dropping solu-
tions of the polyamides (5 mg mLꢁ1 in DMAc) onto a ITO-
coated glass substrate (20 ꢂ 30 ꢂ 0.7 mm, 50–100 U/square). The
polymers were drop-coated onto an active area of letters TTU
using a mask. A gel electrolyte based on PMMA (Mw: 120000)
and LiClO4 was plasticized with propylene carbonate to form
a highly transparent and conductive gel. PMMA (3 g) was dis-
solved in dry acetonitrile (15 g), and LiClO4 (0.3 g) was added to
the polymer solution as supporting electrolyte. Then, propylene
carbonate (5 g) was added as plasticizer. The mixture was then
slowly 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.
1653 cmꢁ1 (amide C]O stretch). H NMR (500 MHz, DMSO-
1
Measurements
Infrared spectra were recorded on a Horiba FT-720 FT-IR
spectrometer. Elemental analyses were run in a Heraeus Vari-
1
oEL-III CHNS elemental analyzer. H and 13C NMR spectra
were measured on a Bruker AVANCE-500 FT-NMR using tet-
ramethylsilane as the internal standard. The inherent viscosities
were determined at 0.5 g/dL concentration using Cannon-Fenske
viscometer at 30 ꢀC. Weight-average molecular weights (Mw)
and number-average molecular weights (Mn) were obtained via
gel permeation chromatography (GPC) on the basis of poly-
styrene calibration using Waters 2410 as an apparatus and THF
as the eluent. Wide-angle X-ray diffraction (WAXD) measure-
ments were performed at room temperature (ca. 25 ꢀC) on
a Shimadzu XRD-6000 X-ray diffractometer (40 kV, 20 mA),
using graphite-monochromatized Cu-Ka radiation. Thermo-
gravimetric analysis (TGA) was conducted with a PerkinElmer
Pyris 1 TGA. Experiments were carried out on approximately
3ꢃ5 mg film samples heated in flowing nitrogen or air (flow
rate ¼ 20 cm3 minꢁ1) at a heating rate of 20 ꢀC minꢁ1. DSC
analyses were performed on a PerkinElmer Pyris 1 DSC at a scan
rate of 20 ꢀC minꢁ1 in flowing nitrogen (20 cm3 minꢁ1). Ther-
momechanical analysis (TMA) was conducted with a Perki-
nElmer TMA 7 instrument. The TMA experiments were
conducted from 50 to 350 ꢀC at a scan rate of 10 ꢀC minꢁ1 with
a penetration probe 1.0 mm in diameter under an applied
Fig. 2 (a) 1H and (b) H–H COSY NMR spectra of the polyamide 6a in
DMSO-d6.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 5481–5492 | 5483

View Online
constant load of 10 mN. Softening temperatures (Ts) were taken
as the onset temperatures of probe displacement on the TMA
traces. Absorption spectra were measured with an Agilent 8453
UV-Visible diode array spectrophotometer. Photoluminescence
(PL) spectra were measured with a Varian Cary Eclipse fluo-
rescence spectrophotometer. Fluorescent quantum yield was
determined using solutions in NMP and was calculated by
comparing emission with that of a standard solution of 9,10-
diphenylanthracene in cyclohexane (FPL ¼ 90%) at room
temperature. Electrochemistry was performed with a CH
Instruments 611c electrochemical analyzer. Cyclic voltammetry
was conducted with the use of a three-electrode cell in which ITO
(polymer film area about 1 cm2, 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 (+0.44 V vs. Ag/
AgCl). Voltammograms are presented with the positive/negative
potential pointing to the right/left with increasing anodic/
decreasing cathodic current pointing upward/downward. Spec-
troelectrochemistry 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. Absorption spectra in the spec-
troelectrochemical experiments were also measured with an
Agilent 8453 UV-Visible diode array spectrophotometer. The
DC (direct current) power source was using a model HCPS-03-
0310 laboratory Pulse Power Supply to fix the potential between
the two ITO electrodes of the device.
1-aminopyrene (2). According to a well-established procedure,19
the target diamine monomer N,N-di(4-aminophenyl)-1-amino-
pyrene (4) was prepared by hydrazine Pd/C-catalyzed reduction
of N,N-di(4-nitrophenyl)-1-aminopyrene (3) resulting from N,N-
diarylation of 1-aminopyrene with 4-fluoronitrobenzene in the
presence of caesium fluoride (CsF). The structures of all the
synthesized compounds were confirmed by elemental, IR, and
NMR analyses. The FT-IR spectra of compounds 1 to 4 can be
seen in the ESI (see Fig. S1†). The nitro groups of compounds 1
and 3 gave two characteristic bands at around 1580–1590 and
1300–1330 cmꢁ1, respectively (-NO2 asymmetric and symmetric
stretching). After reduction to 2 and 4, the characteristic
absorptions of the nitro group disappeared and the primary
amino group showed the typical absorption pair at 3400–
3200 cmꢁ1 due to N–H stretching. The 1H and 13C NMR spectra
of the intermediate compounds 1–3 are also shown in the ESI
(Fig. S2 to S7†). H NMR and 13C NMR spectra of the target
1
diamine monomer 4 are illustrated in Fig. 1. The complete
conversion of the nitro groups to the amino groups was
confirmed by the high-field shift of the phenylene protons and by
the resonance signals at around 4.81 ppm corresponding to the
amino protons. Because of the substitution at the 1-position of
the pyrene unit, the NMR spectra of 4 are very complicated.
Nevertheless, full assignments of all peaks can be done with the
Results and discussion
Synthesis and characterization
The pyrene-containing triarylamine-based diamine monomer 4
was synthesized by the synthetic route outlined in Scheme 1.
1-Nitropyrene (1) was synthesized by nitration of pyrene with
copper nitrate.18 Catalytic reduction of the nitro group
of compound 1 by means of hydrazine and Pd/C gave
Fig. 3 (a) H–H COSY and (b) C–H HMQC NMR spectra of the pyrenyl
unit of diamine 4 in DMSO-d6.
Scheme 1 Synthetic route to the target diamine monomer 4.
5484 | J. Mater. Chem., 2010, 20, 5481–5492
This journal is ª The Royal Society of Chemistry 2010

View Online
1
The desired structures of all polymers were confirmed by H
aid of two-dimensional (2D) COSY and HMQC NMR spectra,
as shown in Fig. 3. Thus, the results of all the spectroscopic and
elemental analyses suggest the successful preparation of the
target diamine monomer. The model compound M1 was
prepared from the condensation of diamine 4 with two equiva-
lent amounts of benzoic acid as shown in Scheme 2. Its spec-
troscopic data can be found in the ESI (Fig. S8–S10†).
NMR spectra, as shown in Fig. 2 for polyamide 6a. The reso-
nance peak appearing at 10.3 ppm clearly indicates the formation
of amide linkage. As it was mentioned earlier, all the polyamides
could be solution-cast into transparent, flexible, and strong films.
The WAXD studies of these film samples indicated that all the
polymers were essentially amorphous (Fig. S13†). All the poly-
amides were readily soluble in common organic solvents such as
NMP, DMAc, DMF, and m-cresol. Polyamide 6f showed good
solubility in less polar solvents like THF because of the addi-
tional contribution of the hexafluoroisopropylidene (-C(CF3)2-)
fragment in the polymer backbone. The high solubility of these
polyamides can be attributed in part to the introduction of bulky,
packing-disruptive diphenylpyrenylamine units in the polymer
structure. Thus, the excellent solubility makes these polymers
potential candidates for practical applications by simple spin- or
dip-coating processes to afford high performance thin films for
optoelectronic devices.
According to the phosphorylation technique described by
Yamazaki and coworkers,20 a series of novel polyamides 6a–6j
with main-chain pyrenylamine units were synthesized from the
diamine monomer 4 with various aromatic or aliphatic dicar-
boxylic acids (5a–5j) (Scheme 3). The polymerization was carried
out via solution polycondensation using triphenyl phosphite and
pyridine as condensing agents. All polymerization reactions
proceeded homogeneously through the reaction and gave clear,
highly viscous polymer solutions. All the polyamides precipitated
in a tough fiber-like form when slowly pouring the resulting
polymer solutions into stirred methanol (see Fig. S11†). The
as-prepared samples, their solutions, and the cast films showed
a strong fluorescence upon UV irradiation. The obtained poly-
amides had inherent viscosities in the range of 0.54–1.23 dL/g
(Table S1).† All the polymers could afford transparent and tough
films via solution casting, indicating high molecular weights. The
GPC measurement of a THF-soluble polyamide 6f showed
weight-average molecular weight (Mw) of 49 500 and poly-
dispersity index (Mw/Mn) of 1.87. The structures of the poly-
amides could be confirmed by IR and NMR spectroscopy. The
typical IR spectrum for a representative polyamide 6a is included
in the ESI Fig. S12†, which shows the characteristic absorption
Thermal properties
The thermal stability and phase transition temperatures of these
polyamides were recorded by thermogravimetric analysis (TGA),
differential scanning calorimetry (DSC), and thermomechanical
analysis (TMA), and the thermal behavior data of are summa-
rized in Table 1. A typical set of TGA and DSC curves of
polyamide 6a are shown in Fig. S14†. Except for the aliphatic-
aromatic polyamides 6i and 6j, these polyamides did not show
significant weight loss before 500 ꢀC in air or nitrogen atmo-
sphere. The decomposition temperatures (Td) at a 10% weight-
loss of the aromatic polyamides (6a to 6h) in nitrogen and air
were recorded in the range of 570–593 ꢀC and 543–573 ꢀC,
respectively. The amount of carbonized resꢀidue (char yield) of
these polymers was more than 72% at 800 C in nitrogen. The
high char yields of these polymers can be ascribed to their high
aromatic content. The polyamides 6i and 6j in these series
exhibited a lower Td value as compared with 6a–6h obtained
from aromatic dicarboxylic acids. This is reasonable when
considering the less stable aliphatic segments. Due to the
bands of the amide group at around 1650 cmꢁ1 and 3300 cmꢁ1
.
Scheme 2 Synthesis of model compound M1.
Scheme 3 Synthesis of pyrenylamine-based polyamides.
This journal is ª The Royal Society of Chemistry 2010 J. Mater. Chem., 2010, 20, 5481–5492 | 5485

View Online
Table 1 Thermal properties of polyamides
d
Td /ꢀC
b
c
Polymera Tg /ꢀC
Ts /ꢀC in N2
in air
Char yielde (%)
6a
6b
6c
6d
6e
6f
6g
6h
6i
321 (295)f 318
593 (546) f 573 (526) f 81
570 (554) 559 (513) 80
580 (573) 554 (528) 80
582 (571) 559 (523) 78
576 (515) 537 (504) 75
577 (544) 543 (525) 72
572 (564) 556 (532) 79
582 (569) 557 (535) 80
486 (431) 470 (436) 52
442 (393) 448 (413) 52
298 (290) 294
326 (302) 309
296 (273) 292
317 (296) 314
319 (295) 315
307 (288) 306
313 (307) 311
277 (259) 267
246 (208) 233
6j
a
The polymer film samples were heated at 300 ꢀC for 1 h prior to all the
thermal analyses. b The sample were heated from 50 to 400 ꢀC at a scan
rate of 20 ꢀC minꢁ1 followed by rapid cooling to 50 ꢀC at ꢁ200 ꢀC minꢁ1
in nitrogen. The midpoint temperature of baseline shift on the subsequent
DSC trace (from 50 to 400 ꢀC at heating rate 20 ꢀC minꢁ1) was defined as
c
Tg. Softening temperature measured by TMA using a penetration
method. Decomposition temperature at which a 10% weight loss was
d
e
recorded by TGA at a heating rate of 20 ꢀC minꢁ1
.
Residual weight
f
percentages at 800 ꢀC under nitrogen flow. Values in parentheses are
data of analogous polyamides 60 having the corresponding diacid
residue as in the 6 series.
Fig. 4 Absorption and emission spectra of selected polyamides in NMP
solution (1 ꢂ 10ꢁ5 M). 9,10-Diphenylanthracene (DPA) dissolved in
cyclohexane (1 ꢂ 10ꢁ5 M) was used as the standard (FF ¼ 90%).
Photographs were taken under illumination of a 365 nm UV light.
incorporation of thermally stable pyrene unit, all the polymers
exhibited higher Td values compared to their corresponding 60
series counterparts derived from 4,40-diaminotriphenylamine.
These polyamides not only showed good thermal stability but
also possessed high glass-transition temperatures (Tg) of 246–
326 ꢀC. The lowest Tg value of 6j is expected and can be explained
in terms of the flexible polymethylene segments in its backbone.
As compared to the 60 series analogs, the present series poly-
amides exhibit a remarkably increased Tg as a result of the
presence of rigid pyrene segments. The softening temperatures
(Ts) (may be referred as apparent Tg) of the polyamide films
determined by the TMA method using a loaded penetration
probe are also listed in Table 1. They were read from the onset
temperature of the probe displacement on the TMA trace. In
most cases, the Ts values of the polyamides obtained by TMA are
comparable to the Tg values measured by the DSC experiments.
Thus, the thermal analysis results revealed that these polyamides,
especially for the aromatic ones, exhibited excellent thermal
stability, which in turn is beneficial to increase the service time in
device application and enhance the morphological stability to the
spin-coated film.
absorption and emission profiles of the selected polyamides in
NMP, together with their PL images on exposure to an UV light
in both solution and solid states. The relevant absorption and PL
data are collected in Table 2. In the absorption spectra all the
polyamides display two to three prominent bands with unsym-
metrical shape and adjoining shoulders. The most bath-
ochromically shifted transitions may be ascribed to that
originating from the amine to pyrene and amine to amide
intramolecular charge-transfer (ICT) states. The strong high-
energy absorption bands at ca. 325 nm are assigned to that
arising from pyrene based p–p* transitions. These absorption
bands are relatively insensitive to the solvent compared with the
corresponding emission bands. Solid film absorption spectra of
these polyamides were similar to those in solution, with a very
slight red-shift of ca. 1–6 nm. All the polyamides exhibit similar
emission profiles in NMP solutions with emission maxima in the
range 522–544 nm. This clearly indicates that the emission
originates from a similar excited state in this class of polymers.
We believe that the emission mainly comes from the diphe-
nylpyrenylamine core because it has a lower energy gap. As
shown in Fig. 4, the emission color of the NMP solutions of these
polyamides is greenish yellow, whereas that of the solid films is
green or yellowish green. As shown in Table 2, the fluorescence
quantum yields (FF) of these polyamides in NMP solution
were found to vary from 0.7% to 30.2% relative to the
Optical properties
All the polyamides were examined by UV-Vis absorption and
photoluminescence (PL) spectroscopy in both solution and the
solid state. Fig. 4 shows representative examples of the
5486 | J. Mater. Chem., 2010, 20, 5481–5492
This journal is ª The Royal Society of Chemistry 2010

View Online
Table 2 Photophysical properties of polyamides
instance polyamide 6f displays a blue-shifted emission profile in
solid film (lmax ¼ 513 nm) when compared to that observed for
NMP solution (lmax ¼ 531 nm), as shown in Fig. 5. This latter
observation suggests that the polarity in the solid film is less than
that in NMP solution. This may be accounted for by the
planarity violation of the diphenylpyrenylamine segment in the
solid state due to the more restricted bond rotation. As can be
seen from Fig. 5, the dilute solution of polyamide 6f in less polar
THF exhibited an emission profile (lmax ¼ 511 nm) similar to
that in the solid film. Therefore, the emission maximum or color
of these pyrenylamine-based polyamides is dependent upon the
solvent polarity, i.e., they display a solvatochromic behavior.
The expressed solvatochromism is typical to pyrene derivatives,
as well as to triarylamine-containing compounds22 and poly-
mers23 with donor–acceptor architecture.
In solutiona
As solid film
abs
lonset lmax
PL
Polymer lmaxabs/nm
lmaxPL/nmb FF (%)c lmaxabs/nm /nme
/nmb
f
6a
6b
6c
6d
6e
6f
335, 385
335, 412
525 (425)d 1.2 (0.2)d 335, 388 469
533 (429) 25.0 (0.5) 336
—
465
509
—
302, 335, 371 533 (433) 1.3 (0.2) 308, 384 470
336, 417 536 (416) 26.1 (0.3) 336 467
320, 334, 392 538 (454) 0.7 (0.2) 323, 403 474
335, 412 531 (452) 8.8 (0.4) 334 461
509
—
513
—
—
516
519
6g
6h
6i
320, 334, 387 524 (438) 1.1 (0.2) 321, 384 465
302, 334, 387 522 (430) 0.8 (0.3) 308, 394 476
321, 331, 421 544 (387) 29.7 (5.7) 321, 411 466
321, 331, 421 543 (385) 30.2 (5.6) 324, 412 469
6j
a
The polymer concentration was 1 ꢂ 10ꢁ5 mol Lꢁ1 in NMP. b Excited at
the absorption maximum for both solution and the solid film states.
Fluorescent quantum yield estimated using 9,10-diphenylanthracene
It is well known that pyrene derivatives exhibit environment
sensitive solvatochromic behavior in which the relative intensity
of emission bands is dependent on the solvent polarity.24 In order
to further investigate the solvatochromic properties, we investi-
gated the absorption and fluorescence of model compound M1 in
solvents with different polarity. The model compound was
chosen for study because it exhibits better solubility than the
polyamides; it can dissolve in less polar solvents like toluene,
chloroform, and dichloromethane. Fig. 6 showed the normalized
PL spectra of M1 in dilute solution in various solvents, together
with fluorescence images of its solutions. The absorption and PL
emission data are collected in Table 3. The solution absorption
spectra of M1 are similar, with little shift in the peak maximum
(absorption lmax ꢃ 334 nm). This clearly indicates that the
solvent polarity exerts little effect on its ground-state electronic
transition. In contrast, the PL emission spectra of M1 show
strong solvent-polarity dependence, revealing a dominant broad
emission band that undergoes remarkable bathochromic shifts
with an increase of the solvent polarity. The emission color
c
d
in cyclohexane (1 ꢂ 10ꢁ5 M) as standard (FF ¼ 90%). Values in
parentheses are data of analogous polyamides 60 having the
corresponding diacid residue (see footnote in Table 1) as in the 6 series.
Wavelength of the low energy absorption band edge. Difficult to be
e
f
defined due to low PL intensity.
9,10-diphenylanthracene standard (in cyclohexane solution,
FF ¼ 90%).21 The low FF of most aromatic polyamides such as
6a, 6c, 6e, 6g, and 6h may be attributed to the quenching effect
arising from intra- and interchain charge transfer (CT) com-
plexing between the triarylamine donor and the aryl amide (or
aroyl) acceptor. Polyamides derived from aromatic dicarboxylic
acids such as isophthalic acid (5b), 4,40-dicarboxydiphenyl
ether (5d), and 2,2-bis(4-carboxyphenyl)hexafluoropropane (5f)
seemed to have reduced CT interactions and showed a higher FF.
It is worth noting that CT-inhibited polyamides 6i and 6j derived
from aliphatic dicarboxylic acids also showed a relatively higher
FF of ca. 30%. When compared with the corresponding 60 series
counterparts, the higher quantum efficiency of the present series
polyamides could be attributable to the presence of rigid, highly
fluorescent pyrene chromophore.
In contrast to absorption spectra, a strong medium effect is
observed in the fluorescence spectra of these polyamides. For
Fig. 5 The normalized UV-Vis absorption and PL spectra of polyamide
6f in dilute NMP and THF solutions (1 ꢂ 10ꢁ5 M), as well as in the solid
film.
Fig. 6 Normalized PL spectra the dilute solutions of M1 (ca. 1 ꢂ
10ꢁ5 M) in solvents of various polarity. Photographs were taken under
illumination of a 365 nm UV light.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 5481–5492 | 5487

View Online
Table 3 Photophysical properties of M1 in different solvents
Solvent
3a
lmaxabs/nmb
lmaxPL/nmc
F
F (%)d
Toluene
CHCl3
THF
CH2Cl2
NMP
2.4
4.8
7.5
334
334
334
334
335
334
336
480
492
507
510
531
533
544
45.1
42.9
42.0
41.6
30.2
28.6
24.1
9.1
32.2
37.0
47.0
MeCN
DMSO
a
Dielectric constant of the solvent. b Concentration of the solution ¼ 1 ꢂ
c
d
10ꢁ5 M. Excited at the absorption maximum. Fluorescent quantum
yield estimated using 9,10-diphenylanthracene in cyclohexane (1 ꢂ
10ꢁ5 M) as standard (FF ¼ 90%).
changes from blue in toluene (PL lmax ¼ 480 nm) to greenish
yellow in DMSO (lmax ¼ 544 nm). A decreased fluorescence
quantum yield and an increased band width were also observed
in the solvent of higher polarity. The solvatochromism could be
attributed to the fast intramolecular charge-transfer process
resulting in a large change of dipole moment in the excited
state.25 Such solvatochromic behavior is associated with the
stabilization of the polar emissive excited states by the polar
solvents.
Fig. 7 Cyclic voltammograms of the polyamide 6d film on an ITO-
coated glass substrate in 0.1 M Bu4NClO4 acetonitrile (for the oxidation
process) and DMF (for the reduction process) solutions at a scan rate of
50 and 100 mV sꢁ1, respectively. The arrows indicate the film color change
during CV scan. E1/2 values are indicated by dashed lines.
from yellow to pale orange due to electrochemical reduction of
the polymers. The onset and half-wave potentials (Eox1/2) of the
oxidation process were found to be around 0.66–0.70 V and
0.81–0.83 V, respectively. The oxidation potentials are relatively
insensitive to the structure of the dicarboxylic acid component in
these polyamides. Variations of less than 0.04 V were observed in
the 6a–6j series, and this indicates that the diacid residue does not
significantly disturb the electronic nature of the oxidizable tri-
arylamine core. The color of the polyamide films turned from
yellow to purple upon oxidation. In comparison to the corre-
sponding 60 analogs, the 6 series polyamides displayed a slightly
lower oxidation potential. This suggests that the presence of
electron-rich pyrene moiety in the latter case is responsible for
the decrease in the oxidation potential.
Electrochemical properties
Cyclic voltammetry (CV) experiments were conducted to inves-
tigate the electrochemical properties of the polyamides. The
oxidation and reduction cycles of the film samples were measured
in acetonitrile and DMF, respectively, using tetrabutylammo-
nium perchlorate, Bu4NClO4, as the electrolyte. Samples were
coated on the surface of the ITO-glass by a solution-casting
process. Typical CV curves of polyamide 6d are shown in Fig. 7.
All the polyamides displayed reversible oxidation and reduction
processes, indicating their high electrochemical stability for both
p- and n-doping. These reversible waves also represent the
formation of stable radical cation and radical anion originating
from electrochemical redox reactions of the diphenylpyrenyl-
amine core (Scheme 4). The onset potential of the reduction
process was recorded in the range from ꢁ1.86 to ꢁ1.77 V. From
the positions of the redox couple peaks of the reduction process,
the n-doping half-wave potentials, Ered1/2, for polyamides 6a–6j
were determined, and these values range from ꢁ2.05 to ꢁ1.96 V
(Table 4). These potentials are similar to those found in related
pyrene derivatives.26 The color of the polyamide films changed
On the basis of the oxidation and reduction onset potentials,
the bandgaps for polyamides 6a–6j were calculated, and were
found to vary from 2.45 to 2.56 eV. The bandgaps calculated
from the CV measurements are somewhat lower (by 0.11–
0.22 eV) than those obtained from the absorption spectra. By
using the oxidation and reduction onset potentials, the highest
occupied molecular orbital (HOMO) levels for these polyamides
were calculated and found to be between 5.02 and 5.06 eV
(relative to the vacuum energy level), whereas the values for the
lowest unoccupied molecular orbital (LUMO) levels lay between
2.50 and 2.59 eV (Table 4). When the calculations are based on
Scheme 4 Postulated redox chemistry of polyamides.
This journal is ª The Royal Society of Chemistry 2010
5488 | J. Mater. Chem., 2010, 20, 5481–5492

View Online
Table 4 Redox potentials and energy levels of polyamides
Reduction
potential/Vb
Oxidation potential/Va
EgCV/eVc
HOMO/LUMO/eVe
Egopt/eVd
from Eonset
from E1/2
ꢀ
nset
1/2
onset
1/2
Polymer
Eox
Eox
Ered
Ered
from Eonset
from E1/2
6a
6b
6c
6d
6e
6f
6g
6h
6i
0.69
0.68
0.68
0.68
0.70
0.70
0.70
0.68
0.66
0.67
0.82 (0.84)f
0.82 (0.86)
0.82 (0.84)
0.82 (0.85)
0.83 (0.86)
0.83 (0.86)
0.82 (0.85)
0.82 (0.84)
0.81 (0.84)
0.81 (0.84)
ꢁ1.82
ꢁ1.80
ꢁ1.77
ꢁ1.84
ꢁ1.77
ꢁ1.77
ꢁ1.86
ꢁ1.80
ꢁ1.84
ꢁ1.85
ꢁ2.02
ꢁ1.98
ꢁ2.05
ꢁ1.96
ꢁ2.02
ꢁ2.04
ꢁ2.04
ꢁ2.04
ꢁ1.96
ꢁ1.96
2.51
2.48
2.45
2.52
2.47
2.47
2.56
2.48
2.50
2.52
2.84
2.80
2.87
2.78
2.85
2.87
2.86
2.86
2.77
2.77
2.64
2.67
2.64
2.66
2.62
2.69
2.67
2.61
2.66
2.64
5.05/2.54
5.04/2.56
5.04/2.59
5.04/2.52
5.06/2.59
5.06/2.59
5.06/2.50
5.04/2.56
5.02/2.52
5.03/2.51
5.18/2.34
5.18/2.38
5.18/2.31
5.18/2.40
5.19/2.34
5.19/2.32
5.18/2.32
5.18/2.32
5.17/2.40
5.17/2.40
6j
a
vs. Ag/AgCl in CH3CN. E1/2 ¼ average potential of the redox couple peaks. b vs. Ag/AgCl in DMF. c Bandgaps calculated from CV measurements.
d
e
opt
Bandgaps calculated from absorption edge of the polymer films: Eg ¼ 1240/lonset
.
The HOMO energy levels were calculated from Eonset or E1/2
values of CV curves and were referenced to ferrocene (4.8 eV relative to the vacuum energy level). Values in parentheses are data of analogous
f
polyamide 60 having the corresponding R as that of the 6 series.
the E1/2 values, the HOMO and LUMO energy levels are esti-
mated to be in the range of 5.17–5.19 eV and 2.31–2.40 eV,
respectively, which are similar to those reported for the pyrene-
containing triarylamines.13c The closeness of the HOMO of these
polyamides to the work function of ITO (Ef ¼ 4.7 eV) indicates
that hole injection into the molecular layer of these polymers will
be effective if they are employed as hole-transporters in OLEDs.
Spectroelectrochemical and electrochromic properties
The electro-optical properties of the polymer films were investi-
gated using the changes in electronic absorption spectra at
various applied voltages. For these investigations, the polyamide
film was cast on an ITO-coated glass slide (a piece that fit in the
commercial UV-visible cuvette), and a homemade electro-
chemical cell was built from a commercial UV-visible cuvette.
The cell was placed in the optical path of the sample light beam in
a commercial diode array spectrophotometer. This procedure
allowed us to obtain electronic absorption spectra under poten-
tial control in a 0.1 M Bu4NClO4/acetonitrile solution. The result
of the 6d film upon oxidation is presented in Fig. 8a as a series of
UV-vis absorbance curves correlated to electrode potentials.
Fig. 9 shows the surface plot diagram for the % transmittance-
wavelength-applied potential correlations of this sample. In the
neutral form, at 0 V, the film exhibited strong absorption at
wavelength around 336 nm, characteristic for triarylamine, but it
was almost transparent at wavelengths > 450 nm. Upon oxida-
Fig. 8 Spectral changes of (a) polyamide 6d and (b) 6d0 thin film on an
tion of the 6d film (increasing applied voltage from 0 to 1.00 V),
ITO-coated glass substrate (in CH3CN with 0.1 M Bu4NClO4 as the
the intensity of the absorption peak at 336 nm gradually
supporting electrolyte) along with increasing of the applied voltage up to
decreased while new peaks at 403, 550, and 597 nm and
1.00 V (vs. Ag/AgCl couple as reference). The inset shows the color
a broadband having its maximum absorption wavelength at
834 nm gradually increased in intensity. Meanwhile, the color of
the film changed from yellow to purple. We attribute this spectral
change to the formation of a stable radical cation of the diphe-
nylpyrenylamine moiety. For a comparative study, the spectral
changes of the analogous triphenylamine-based polyamide 6d0
are presented in Fig. 8b. The spectral changes in the region of
500–600 nm of polyamide 6d0 upon oxidation are slightly
different from those of 6d, and the film of 6d0 appeared a greenish
changes of the polymer films between neutral and oxidized states.
blue color at its fully oxidized state. The observed UV-vis
absorption changes in the film of 6d at various potentials are fully
reversible and are associated with strong color changes; indeed,
they even can be seen readily by the naked eye. From the
photographs shown in Fig. 9, it can be seen that the film of 6d
switches from a transmissive neutral state (yellow) to a highly
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 5481–5492 | 5489

View Online
Fig. 11 Optical transmittance changes at l ¼ 834 nm for a solution-cast
film of polyamide 6d (with an optical density of 0.44 a.u.) in 0.1 M
Bu4NClO4/CH3CN between the potentials of 0 and 1.0 V.
Fig. 9 Electronic absorption spectra of the cast film of polyamide 6d on
ITO in 0.1 M Bu4NClO4/CH3CN at various applied potentials between
0.0 and 1.0 V (vs. Ag/AgCl couple as reference). The photo shows the
color change of the film on an ITO electrode at indicated potentials.
a function of time with UV-vis-NIR spectroscopy. The switching
time was calculated at 90% of the full switch because it is difficult
to perceive any further color change with the naked eye beyond
this point. The polyamides switch rapidly (within three seconds)
between the highly transmissive neutral state and the colored
oxidized state. As depicted in Fig. S16†, the thin film of poly-
amide 6d required only 2.9 s at 1.00 V for coloring and 1.0 s for
bleaching. As shown in Fig. 10, the absorbance changes at
834 nm reflect the switch in current, and the kinetics of the charge
transport process can be referenced to the coloration response
time. Fig. 11 shows the change in transmittance measured as the
rate of switching the polymer film of 6d, between oxidized and
neutralized, is increased. As the polymer is switched between
0 and 1.0 V with a 10 s hold at each potential, the percent
transmittance (DT) contrast for 834 nm is 65.0%. As the time the
oxidizing and reducing potentials are held is decreased to 5 s, the
optical contrast decreases only slightly to 61.4%, and subse-
quently to 46.6% for a 2 s hold. The electrochromic coloration
efficiencies (h ¼ DOD834/Q)27 after various switching steps of the
film of polyamide 6d are summarized in Table 5. The
absorbing oxidized state (purple). The film colorations are
distributed homogeneously across the polymer film and survive
for more than hundreds of redox cycles. Moreover, coloration
changes were also observed in these polyamides upon reduction.
However, the color change is not strong as that observed in the
anodic scanning. In general, the polyamide films changed from
yellow neutral form to a pale orange reduced form. Typical
electronic absorption changes of the 6d film upon reduction are
illustrated in Fig. S15†.
Upon oxidation, the polyamide films exhibited a strong
coloration change between redox states. The stability, response
time, and color efficiency are the key parameters for an electro-
active polymer film to be amenable for usage in optical and
electrochromic devices. For optical switching studies, polymer
films were cast on ITO-coated glass slides in the same manner as
described above, and each film was potential stepped between its
bleaching (0.0 V) and coloring (+1.00 V) state. While the films
were switched, the absorbance at 834 nm was monitored as
Table 5 Coloration efficiency of polyamide 6d
Cyclesa
DOD834
Qc/mC cmꢁ2
hd/cm2 Cꢁ1
Decaye (%)
b
1
50
0.400
0.395
0.389
0.383
0.374
0.367
0.358
0.352
0.345
0.337
0.328
2.322
2.348
2.351
2.357
2.348
2.358
2.356
2.344
2.339
2.321
2.319
172
168
165
162
159
156
152
150
147
145
141
0.0
2.3
4.1
5.9
7.6
9.3
11.6
12.8
14.5
15.7
18.0
100
150
200
250
300
350
400
450
500
a
Number of cyclic switching by applying potential steps between 0 and
1.00 V (vs. Ag/AgCl), with a pulse width of 10 s. Optical density
Fig. 10 Dynamic changes of (a) the transmittance and (b) current upon
switching the potential between 0 and 1.0 V (vs. Ag/AgCl) with a pulse
width of 10 s applied to the cast film of polyamide 6d on the ITO-glass
(active area ꢃ 1 cm2) in CH3CN containing 0.1 M Bu4NClO4. The
absorption was recorded at 597 and 834 nm.
b
c
change at 834 nm. Ejected charge, determined from the in situ
d
experiments. Coloration efficiency is calculated by the equation: h ¼
DOD834/Q. e Decay of coloration efficiency after cyclic scans.
5490 | J. Mater. Chem., 2010, 20, 5481–5492
This journal is ª The Royal Society of Chemistry 2010

View Online
under atmospheric condition. To prevent leakage, an epoxy resin
was applied to seal the device. As a typical example, an electro-
chromic cell based on polyamide 6d was fabricated. When
a voltage of 0.0 V is applied, the polymer is neutral and yellow.
When the voltage applied was increased (to a maximum of 2.4 V),
the color changed from yellow to purple, the same as was already
observed for the solution spectroelectrochemistry experiments.
The color change is uniform, as can be seen in Fig. 13a where the
letters TTU were written by spray-coating 6d solution over
a stencil. The neutral state is yellow, while the oxidized state is
purple. We believe that optimization could further improve the
device performance and fully explore the potential of these
electrochromic polyamides.
Conclusions
Fig. 12 The PL images (upon excitation with a 365-nm UV light) of the
cast film of polyamide 6d on ITO-coated slide before and after redox
cycling between 0 and 1.0 V.
A series of novel pyrenylamine-functionalized polyamides have
been prepared from a newly synthesized diamine monomer, N,N-
di(4-aminophenyl)-1-aminopyrene, with various aromatic and
aliphatic dicarboxylic acids via the phosphorylation poly-
amidation reaction. All the polymers could form morphologi-
cally stable and uniform amorphous films using solution-casting
techniques. In addition to high Tg, high thermal stability, notable
fluorescence and solvatochromism, the polymers also revealed
interesting electrochromic characteristics with color change from
yellow neutral state to purple oxidized state and orange reductive
state. Thus, these polyamides have great potential for use in
optoelectronic applications as new charge-transporting, light-
emitting, and electrochromic materials. Further development of
materials of this type would seem to be warranted by the very
encouraging initial results presented here.
Acknowledgements
Fig. 13 (a) Photos of sandwich-type ITO-coated glass electrochromic
cell, using polyamide 6d as active layer. (b) Schematic illustration of the
structure of the electrochromic cell.
The authors are grateful to the National Science Council of
Taiwan (Republic of China) for financial support of this work.
References
electrochromic film of 6d was found to exhibit good coloration
efficiencies up to 172 cm2 Cꢁ1 at 834 nm. It was confirmed that
the polymer film does not lose its electro-optical activity signifi-
cantly and retains about 80% of its optical response after 500
coloring/bleaching cycles. Therefore, the electrochromic switch-
ing behavior appears to be a highly reversible process. Moreover,
the film fluorescence properties after of the electrochemical
treatment were also evaluated. The PL spectra of the polyamide
5d film display almost no change after 100 repeated electro-
chemical cycling, indicating high oxidation stability and little
change in the molecular structure after redox cycling. The PL
images of the polymer film of 5d before and after redox cycling
are reproduced in Fig. 12.
1 (a) F. M. Winnik, Chem. Rev., 1993, 93, 587; (b) M. Beinhoff,
W. Weigel, M. Jurczok, W. Rettig, C. Modrakowski, I. Brudgam,
H. Hartl and A. D. Schluter, Eur. J. Org. Chem., 2001, 3819; (c)
H. Maeda, T. Maeda, K. Mizuno, K. Fujimoto, H. Shimizu and
M. Inouye, Chem. Eur. J., 2006, 12, 824; (d) P. Conion, C. J. Yang,
Y. Wu, Y. Chen, K. Martinez, Y. Kim, N. Stevens, A. A. Marti,
S. Jockusch, N. J. Torro and W. Tan, J. Am. Chem. Soc., 2008,
130, 336.
2 (a) A. Okamoto, K. Kanatani and I. Saito, J. Am. Chem. Soc., 2004,
126, 4820; (b) J.-M. Kim, S. J. Min, S. W. Lee, J. H. Bok and
J. S. Kim, Chem. Commun., 2005, 3427; (c) W. Zhou, Y. Li, H. Liu,
S. Wang, C. Li, M. Yauan, X. Liu and D. Zhu, Chem.–Asian J.,
2006, 1–2, 224; (d) J. K. Choi, S. H. Kim, J. Yoon, K.-H. Lee,
R. A. Bartsch and J. S. Kim, J. Org. Chem., 2006, 71, 8011; (e)
M. Nakamu, Y. Murakami, K. Sosa, H. Hayashi and K. Yamana,
J. Am. Chem. Soc., 2008, 130, 6904; (f) Y. W. Gao, H. Bai and
G. Q. Shi, J. Mater. Chem., 2010, 20, 2993.
3 (a) V. de Halleux, J.-P. Calbert, P. Brocorens, J. Cornil,
J.-P. Declercq, J.-J. Bredas and Y. Geerts, Adv. Funct. Mater., 2004,
14, 649; (b) H. Zhang, Y. Wang, K. Shao, Y. Liu, S. Chen, W. Qiu,
X. Sun, T. Qi, Y. Ma, G. Yu, Z. Su and D. Zhu, Chem. Commun.,
2006, 755; (c) T. Oyamada, S. Akiyama, M. Yahiro, M. Saigou,
M. Shiro, H. Sasabe and C. Adachi, Chem. Phys. Lett., 2006, 421,
295; (d) Y. Sagara, T. Mutai, I. Yoshikawa and K. Araki, J. Am.
Chem. Soc., 2007, 129, 1520; (e) J. N. Moorthy, P. Natarajan,
P. Venjatakrishnan, D.-F. Huang and T.-J. Chow, Org. Lett., 2007,
Based on the foregoing results, it can be concluded that these
polyamides can be used in the construction of electrochromic
devices and optical display due to the fast response time and the
robustness of the polymers. Therefore, we fabricated as prelim-
inary investigations single layer elecrochromic cells (Fig. 13). The
polymer films were spray-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
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 5481–5492 | 5491

View Online
9, 5215; (f) M. J. Sienkowska, H. Monobe, P. Kaszynski and
Y. Shimizu, J. Mater. Chem., 2007, 17, 1392.
14 K.-R. Wee, H.-C. Ahn, H.-J. Son, W.-S. Han, J.-E. Kim, D. W. Cho
and S. O. Kang, J. Org. Chem., 2009, 74, 8472.
4 (a) T. Otsubo, Y. Aso and K. Takimiya, J. Mater. Chem., 2002, 12,
2565; (b) F. Moggia, C. Videlot-Ackermann, J. Ackermann,
P. Raynal, H. Brisset and F. Fages, J. Mater. Chem., 2006, 16, 2380.
5 (a) A. C. Benniston, A. Harriman, D. J. Lawrie and S. A. Rostron,
Eur. J. Org. Chem., 2004, 2272; (b) S. Leroy-Lhez and F. Fages,
Eur. J. Org. Chem., 2005, 2684; (c) G. Venkataramana and
S. Sankararaman, Eur. J. Org. Chem., 2005, 4162; (d) H. M. Kim,
Y. O. Lee, C. S. Lim, J. S. Kim and B. R. Cho, J. Org. Chem.,
2008, 73, 5127; (e) J.-W. Oh, Y. O. Lee, T. H. Kim, K. C. Ko,
J. Y. Lee, H. Kim and J. S. Kim, Angew. Chem., Int. Ed., 2009, 48,
2522.
6 (a) D. Rausch and C. Lambert, Org. Lett., 2006, 8, 5037; (b)
C.-H. Yang, T.-F. Guo and I.-W. Sun, J. Lumin., 2007, 124, 93; (c)
K.-C. Wu, P.-J. Ku, C.-S. Lin, H.-T. Shih, F.-I. Wu, M.-J. Huang,
J.-J. Lin, I-C. Chen and C.-H. Cheng, Adv. Funct. Mater., 2008, 18, 67.
7 (a) K. R. J. Thomas, J. T. Lin, Y.-T. Tao and C.-W. Ko, J. Am. Chem.
Soc., 2001, 123, 9404; (b) J.-Y. Shen, X.-L. Yang, T.-H. Huang,
J. T. Lin, T.-H. Ke, L.-Y. Chen, C.-C. Wu and M.-C. P. Yeh, Adv.
Funct. Mater., 2007, 17, 983; (c) Y. Xing, X. Xu, P. Zhang,
W. Tian, G. Yu, P. Lu, Y. Q. Liu and D. B. Zhu, Chem. Phys.
Lett., 2005, 408, 169; (d) Z. J. Zhao, X. J. Xu, F. Wang, G. Yu,
P. Lu, Y. Q. Liu and D. B. Zhu, Synth. Met., 2006, 156, 209; (e)
Z. Zhao, X. Xu, H. Wang, P. Lu, G. Yu and Y. Liu, J. Org. Chem.,
2008, 73, 594.
8 (a) S. L. Tao, Z. K. Peng, X. H. Zhang, P. F. Wang, C.-S. Lee and
S.-T. Lee, Adv. Funct. Mater., 2005, 15, 1716; (b) J. Y. Shen,
C. Y. Lee, T.-H. Huang, J. T. Lin, Y.-T. Tao, C.-H. Chien and
C. T. Tsai, J. Mater. Chem., 2005, 15, 2455; (c) C. Tang, F. Liu,
Y.-J. Xia, L.-H. Xie, A. Wei, S.-B. Li, Q.-L. Fan and W. Huang,
J. Mater. Chem., 2006, 16, 4074; (d) J. A. Mikroyannidis,
L. Fenenko and C. Adachi, J. Phys. Chem. B, 2006, 110, 20317; (e)
Z. J. Zhao, J.-H. Li, P. Lu and Y. Yang, Adv. Funct. Mater., 2007,
17, 2203; (f) Z. J. Zhao, X. J. Xu, Z. T. Jiang, P. Lu, G. Yu and
Y. Q. Liu, J. Org. Chem., 2007, 72, 8345; (g) F. Liu, C. Tang,
Q.-Q. Chen, F.-F. Shi, H.-B. Wu, L.-H. Xie, B. Peng, W. Wei,
Y. Cao and W. Huang, J. Phys. Chem. C, 2009, 113, 4641; (h)
K. M. Omer, S.-Y. Ku, K.-T. Wong and A. J. Bard, Angew. Chem.,
Int. Ed., 2009, 48, 9300.
9 (a) S. Bernhardt, M. Kastler, V. Enkelmann, M. Baumgarten and
K. Mullen, Chem.–Eur. J., 2006, 12, 6117; (b) Z. J. Zhao, J.-H. Li,
X. P. Chen, P. Lu and Y. Yang, Org. Lett., 2008, 10, 3041; (c)
M. Gingras, V. Placide, J.-M. Raimundo, G. Bergamini, P. Ceroni
and V. Balzani, Chem.–Eur. J., 2008, 14, 10357; (d) F. Liu,
W.-Y. Lai, C. Tang, H.-B. Wu, Q.-Q. Chen, B. Peng, W. Wei,
W. Huang and Y. Cao, Macromol. Rapid Commun., 2008, 29, 659;
(e) R. D. Xia, W.-Y. Lai, P. A. Levermore, W. Huang and
D. D. C. Bradley, Adv. Funct. Mater., 2009, 19, 2844.
15 (a) M. Thelakkat, Macromol. Mater. Eng., 2002, 287, 442; (b)
Y. Shirota, J. Mater. Chem., 2005, 15, 75; (c) Y. Shirota and
H. Kageyama, Chem. Rev., 2007, 107, 953.
16 (a) M.-k. Leung, M.-Y. Chou, Y. O. Su, C.-L. Chiang, H.-L. Chen,
C.-F. Yang, C.-C. Yang, C.-C. Lin and H.-T. Chen, Org. Lett.,
2003, 5, 839; (b) M.-Y. Chou, M.-k. Leung, Y. O. Su, C.-L. Chiang,
C.-C. Lin, J.-H. Liu, C.-K. Kuo and C.-Y. Mou, Chem. Mater.,
2004, 16, 654; (c) S. Beaupre, J. Dumas and M. Leclerc, Chem.
Mater., 2006, 18, 4011; (d) K. Choi, S. J. Yoo, Y.-E. Sung and
R. Zentel, Chem. Mater., 2006, 18, 5823; (e) L. Otero, L. Sereno,
F. Fungo, Y.-L. Liao, C.-Y. Lin and K.-T. Wong, Chem. Mater.,
2006, 18, 3495; (f) J. Qua, R. Kawasakia, M. Shiotsukia, F. Sanda
and T. Masuda, Polymer, 2006, 47, 6551; (g) J. Natera, L. Otero,
L. Sereno, F. Fungo, N.-S. Wang, Y.-M. Tsai, T.-Y. Hwu and
K.-T. Wong, Macromolecules, 2007, 40, 4456; (h) J. Qua,
Y. Suzukia, M. Shiotsukia, F. Sanda and T. Masuda, Polymer,
2007, 48, 4628; (i) C. C. Chiang, H.-C. Chen, C.-S. Lee, M.-
k. Leung, K.-R. Lin and K.-H. Hsieh, Chem. Mater., 2008, 20, 540.
17 (a) S.-H. Cheng, S.-H. Hsiao, T.-H. Su and G.-S. Liou,
Macromolecules, 2005, 38, 307; (b) G.-S. Liou, S.-H. Hsiao and
T.-H. Su, J. Mater. Chem., 2005, 15, 1812; (c) G.-S. Liou,
S.-H. Hsiao and H.-W. Chen, J. Mater. Chem., 2006, 16, 1831; (d)
G.-S. Liou, S.-H. Hsiao, N.-K. Huang and Y.-L. Yang,
Macromolecules, 2006, 39, 5337; (e) G.-S. Liou, S.-H. Hsiao,
W.-C. Chen and H.-J. Yen, Macromolecules, 2006, 39, 6036; (f)
C.-W. Chang, G.-S. Liou and S.-H. Hsiao, J. Mater. Chem., 2007,
17, 1007; (g) S.-H. Hsiao, G.-S. Liou, Y.-C. Kung and H.-J. Yen,
Macromolecules, 2008, 41, 2800; (h) Y.-C. Kung, G.-S. Liou and
S.-H. Hsiao, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 1740.
18 P. Babu, N. M. Sangeetha, P. Vijaykumar, U. Maitra, K. Rissanen
and A. R. Raju, Chem.–Eur. J., 2003, 9, 1922.
19 Y. Oishi, H. Takado, M. Yoneyama, M. Kakimoto and Y. Imai,
J. Polym. Sci., Part A: Polym. Chem., 1990, 28, 1763.
20 (a) N. Yamazaki, F. Higashi and J. Kawabata, J. Polym. Sci., Polym.
Chem. Ed., 1974, 12, 2149; (b) N. Yamazaki, M. Matsumoto and
F. Higashi, J. Polym. Sci., Polym. Chem. Ed., 1975, 13, 1373.
21 S. Hamai and F. Hirayama, J. Phys. Chem., 1983, 87, 83.
22 (a) K. R. J. Thomas, J. T. Lin, M. Velusamy, Y.-T. Tao and
C.-H. Chuen, Adv. Funct. Mater., 2004, 14, 83; (b) K.-T. Wong,
S.-Y. Ku, Y.-M. Cheng, X.-Y. Lin, Y.-Y. Hung, S.-C. Pu, P.-
T. Chou, G.-H. Lee and S.-M. Peng, J. Org. Chem., 2006, 71, 456;
(c) J. M. Hancock, A. P. Gifford, Y. Zhu, Y. Lou and
S. A. Jenekhe, Chem. Mater., 2006, 18, 4924; (d) M.-X. Xu,
L.-C. Chang, C.-H. Lin, J.-P. Duan, F.-I. Wu, I.-C. Chen and
C.-H. Cheng, Adv. Funct. Mater., 2007, 17, 369; (e) M. Holzapfel
and C. Lambert, J. Phys. Chem. C, 2008, 112, 1227.
23 (a) A. Qin, J. W. Y. Lam, H. Dong, W. Lu, C. K. W. Jim, Y. Dong,
€
10 (a) L. T. Qu and G. Q. Shi, Chem. Commun., 2004, 2800; (b)
S. Kawano, C. D. Yang, M. Ribas, S. Baluschev, M. Baumgarten
M. Haussler, H. H. Y. Sung, I. D. Williams, G. K. L. Wong and
B. Z. Tang, Macromolecules, 2007, 40, 4879; (b) G.-S. Liou,
C.-W. Chang, H.-M. Huang and S.-H. Hsiao, J. Polym. Sci., Part
A: Polym. Chem., 2007, 45, 2004; (c) G.-S. Liou, H.-M. Huang,
S.-H. Hsiao, C.-W. Chang and H.-J. Yen, J. Polym. Res., 2007, 14,
191; (d) G.-S. Liou, H.-M. Huang, S.-H. Hsiao and C.-W. Chang,
Polym. J., 2007, 39, 448.
€
and K. Mullen, Macromolecules, 2008, 41, 7933; (c) T. M. Figueira-
Duarte, P. G. Del Rossa, R. Trattnig, S. Sax, E. J. W. List and
€
K. Mullen, Adv. Mater., 2010, 22, 990; (d) T. M. Figueira-Duarte,
S. C. Simon, M. Wagner, S. I. Druzhinin, K. A. Zachariasse and
€
K. Mullen, Angew. Chem., Int. Ed., 2008, 47, 10175.
11 M. Y. Lo, C. G. Zhen, M. Lauters, G. E. Jabbour and A. Sellinger,
J. Am. Chem. Soc., 2007, 129, 5808.
24 (a) K. Kalyanasundaram and J. K. Thomas, J. Am. Chem. Soc., 1977,
99, 2039; (b) D. S. Karpovich and G. J. Blanchard, J. Phys. Chem.,
1995, 99, 3951.
25 C. Reichardt, Chem. Rev., 1994, 94, 2319.
26 (a) C. Westermeier, H.-C. Gallmeier, M. Komma and J. Daub, Chem.
Commun., 1999, 2427; (b) J. Straub and J. Daub, Adv. Mater., 2002,
12 (a) S. Sudhakar and A. Sellinger, Macromol. Rapid Commun., 2006,
27, 247; (b) H. J. Bolink, E. Barea, R. D. Costa, E. Coronado,
S. Sudhakar, C. Zhen and A. Sellinger, Org. Electron., 2008, 9, 155.
13 (a) W.-L. Jia, T. McCormick, Q.-D. Liu, H. Fukutani, M. Motala,
R.-Y. Wang, Y. Tao and S. Wang, J. Mater. Chem., 2004, 14, 3344;
(b) T.-H. Huang, W.-T. Whang, J. Y. Shen, J. T. Lin and
H. Zheng, J. Mater. Chem., 2005, 15, 3323; (c) K. R. J. Thomas,
M. Velusamy, J. T. Lin, C. H. Chuen and Y. T. Tao, J. Mater.
Chem., 2005, 15, 4453; (d) Y.-J. Pu, M. Higashidate, K. Nakayama
and J. Kido, J. Mater. Chem., 2008, 18, 4183.
ꢀ
14, 1652; (c) Z. Shen, R. Prochazka, J. Daub, N. Fritz, N. Acar and
S. Schneider, Phys. Chem. Chem. Phys., 2003, 5, 3257.
27 (a) P. M. S. Monk, R. J. Mortimer and D. R. Rosseinsky,
Electrochromism and Electrochromic Devices, Cambridge University
Press, Cambridge, UK, 2007; (b) P. M. Beaujuge and
J. R. Reynolds, Chem. Rev., 2010, 110, 268.
5492 | J. Mater. Chem., 2010, 20, 5481–5492
This journal is ª The Royal Society of Chemistry 2010