High Tg, ambipolar, and near-infrared electrochromic anthraquinone-based aramids with intervalence charge-transfer behavior

Article abstract of DOI:10.1002/pola.24980

Two novel series of ambipolar and near-infrared electrochromic aromatic polyamides with electroactive anthraquinone group were synthesized from new aromatic diamines, 2-(bis(4-aminophenyl)amino)anthracene-9,10-dione and 2-(4-(bis(4-aminophenyl)amino)pheno

Full text of DOI:10.1002/pola.24980

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ARTICLE
High Tg, Ambipolar, and Near-Infrared Electrochromic Anthraquinone-
Based Aramids with Intervalence Charge-Transfer Behavior
Hung-Ju Yen, Kun-Ying Lin, Guey-Sheng Liou
Functional Polymeric Materials Laboratory, Institute of Polymer Science and Engineering, National Taiwan University,
1 Roosevelt Road, 4th Sec., Taipei 10617, Taiwan
Correspondence to: G.-S. Liou (E-mail: gsliou@ntu.edu.tw)
Received 1 August 2011; accepted 25 August 2011; published online 19 September 2011
DOI: 10.1002/pola.24980
ABSTRACT: Two novel series of ambipolar and near-infrared
electrochromic aromatic polyamides with electroactive anthra-
quinone group were synthesized from new aromatic diamines,
2-(bis(4-aminophenyl)amino)anthracene-9,10-dione and 2-(4-(bis(4-
aminophenyl)amino)phenoxy)anthracene-9,10-dione, respectively,
via low-temperature solution polycondensation reaction. These
polymers were readily soluble in many polar solvents and
showed useful levels of thermal stability associated with high
glass-transition temperatures (Tg) (285–360 ꢀC). Electrochemical
studies of these electrochromic polyamides revealed ambipolar
behavior with reversible redox couples and high contrast ratio
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both in the visible range and near-infrared region.
2011 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 61–69, 2012
KEYWORDS: electrochemistry; functionalization of polymers;
high performance polymers; polyamides
INTRODUCTION Electrochromic materials exhibit a reversible
optical change in absorption or transmittance upon electro-
chemically oxidized or reduced, such as transition-metal
oxides, inorganic coordination complexes, conjugated poly-
mers, and organic molecules.1 Despite the fact that the ear-
liest electrochromic devices are mostly based on inorganic
oxides, nevertheless, the organic compounds have several
advantages over the former ones, such as processability, high
coloration efficiency, fast switching ability, and multiple
colors within the same material. Initially, investigation of
electrochromic materials has been directed towards optical
changes in the visible region (e.g., 400–800 nm), demon-
strated useful and variable applications such as E-paper, op-
tical switching devices, smart window, and camouflage mate-
rials.2 Increasingly, attention of the optical changes has been
focused extending from the near infrared (NIR; e.g., 800–
2000 nm) to the microwave regions of the spectrum, which
could be exploitable for optical communication, data storage,
and thermal control (heat gain or loss) in buildings and
spacecraft.3 Therefore, NIR electrochromic materials includ-
ing organic metal complex (ruthenium dendrimer) and or-
ganic materials have been investigated in recent years.4
Wang and Wan4(c–i) made efforts on the design of anthraqui-
none-containing cathodically electrochromic materials, which
revealed high absorption in the NIR region upon electro-
chemical reduction. Reynolds5(ab) and Wudl5(c,d) reported dif-
ferent approach of color-to-transmissive NIR electrochromic
conjugated polymers, and their devices exhibited colors in
neutral state and near trasmissive in the visible-light region
at the oxidized state.
Anthraquinone (AQ)-containing molecules were used in bio-
chemical applications because of the anti-cancer behaviors
and conjugated polymers as an electron acceptor unit due to
the strong electron withdrawing characteristic.6 Recently,
anthraquinone derivatives were studied to be cathodically
electrochemical materials because of the anthraquinone moi-
ety offers a high reversibility during electrochemical reduc-
tion.7 So far, the electrochromic properties of anthraquinone
have not received much attention except a few researches
about polyanthraquinones and anthraquinone imides.4(c–i),8
Triarylamine derivatives are well known as its photo- and
electroactive properties that have potentials for optoelec-
tronic applications, such as photoconductors, hole-transport-
ers, light-emitters, and memory devices.9 Electron-rich triar-
ylamines can be easily oxidized to form stable radical
cations, and the oxidation process is always associated with
a noticeable change of coloration. Thus, studies of the syn-
thesis and electrochromism of triarylamine-based polymers
have been reported in the literature.10 Since 2005, our
groups have initiated some high-performance polymers (e.g.,
aromatic polyamides and polyimides) bearing the triaryl-
amine unit as an electrochromic functional moiety.11 It has
also been reported in our previous publications12 that the
incorporation of electron-donating substituents at the para-
position of phenyl groups on the electrochemically active
Additional Supporting Information may be found in the online version of this article.
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sites of the triphenylamine (TPA) unit resulted in stable TPA
cationic radicals and decreased oxidation potential, leading
to a significantly enhanced redox and electrochromic stability
of the prepared polymers.
8.21 (m, 2H), 8.26 (m, 4H) 13C NMR (125 MHz, DMSO-d6, d,
ppm): 121.4, 124.6, 125.7, 126.7, 129.2, 129.3, 129.7, 132.8,
132.9, 134.4, 134.7, 143.4, 150.0, 150.7, 181.1, 181.9. Anal.
C26H15N3O6 (465.41): C, 67.10 %; H, 3.25 %; N, 9.03 %.
Found: C, 67.11 %; H, 3.41 %; N, 8.94 %. ESI-MS: calcd for
(C26H15N3O6)þ: m/z 465.4 found: m/z 465.1.
In this article, we therefore synthesized the novel AQ-based
monomers, 2-(bis(4-aminophenyl)amino)anthracene-9,10-dione
and 2-(4-(bis(4-aminophenyl)amino)phenoxy)anthracene-9,10-
dione, and their derived polyamides containing electroactive
TPA and AQ groups which could not only exhibit ambipolar
character but also reveal high absorbance in visible and NIR
regions during reduction procedure. It is well known that
the aromatic polyamides as microelectronic materials have
attracted great interests due to their outstanding thermal
and mechanical resistance.13 Incorporation of packing-dis-
ruptive TPA and AQ units into polyamides not only preserves
high thermal stability, glass transition temperature, and the
solubility for enhancing the film-forming ability which is bene-
ficial for their fabrication of large-area, thin-film electrochromic
devices but also provides the electroactive center to facilitate
both processing and electrochromic applications. We antici-
pated that the prepared polyamides should be stable for multi-
ple electrochromic switching, improving optical response
times, and enhancing CE with high NIR contrast.
2-(bis(4-aminophenyl)amino)anthracene-9,10-dione (2)
In a 100-mL three-neck round-bottomed flask equipped with
a stirring bar under nitrogen atmosphere, 2.33 g (5.00
mmol) of dinitro compound 1 and 0.20 g of 10 % Pd/C
were dispersed in 5 mL of ethanol and 20 mL THF. The sus-
pension solution was heated to reflux, and 1.5 mL of hydra-
zine monohydrate was added slowly to the mixture. After a
further 24 h of reflux, the solution was filtered to remove
Pd/C, and the filtrate was cooled under nitrogen atmosphere.
The precipitated product was collected by filtration and
EXPERIMENTAL
ꢀ
dried in vacuo at 80 C to give 1.51ꢀg (75 % in yield) of pur-
Materials
ple powder with a mp of 296–297 C (by melting point sys-
4-(Bis(4-nitrophenyl)amino)phenol14 was prepared according
to the reported literature. Commercially available diacid
chlorides such as isophthaloyl dichloride (5I) and terephtha-
loyl dichloride (5T) were purchased from Tokyo Chemical
Industry (TCI) Co. and used as received. Tetrabutylammo-
nium perchlorate (TBAP) (ACROS) were recrystallized twice
by ethyl acetate under nitrogen atmosphere and then dried
in vacuo prior to use. All other reagents were used as
received from commercial sources.
ꢀ
tem at a scan rate of 1 C/min).
FTIR (KBr): 3470, 3380 cmꢁ1 (NAH stretch). 1H NMR (500
MHz, DMSO-d6, d, ppm): 5.23 (s, 4H, -NH2), 6.63 (d, J ¼ 8.5
Hz, 4H, Hi), 6.88 (dd, 1H, Hb), 6.98 (d, J ¼ 8.5 Hz, 4H, Hh),
7.21 (d, J ¼ 2.5 Hz, 1H, Ha), 7.82 (t, 1H, He), 7.88 (t, 1H, Hf),
7.92 (d, J ¼ 8.8 Hz, 1H, Hc), 8.07 (d, J ¼ 7.5 Hz, 1H, Hd),
8.14 (d, J ¼ 7.5 Hz, 1H, Hg). 13C NMR (125 MHz, DMSO-d6, d,
ppm): 110.6, 114.9, 118.6, 122.0, 126.4, 126.5, 128.0, 129.1,
133.0, 133.3, 133.5, 133.6, 134.2, 134.5, 180.1, 183.1. Anal.
Calcd (%) for C26H19N3O2 (405.45): C, 77.02; H, 4.72; N,
10.36. Found: C, 76.51; H, 4.76; N, 10.36. ESI-MS: calcd for
(C26H19N3O2)þ: m/z 405.4; found: m/z 405.1.
Monomer Synthesis
2-(bis(4-nitrophenyl)amino)anthracene-9,10-dione (1)
A mixture of 7.86 g (51.75 mmol) of cesium fluoride in 39
mL dimethyl sulfoxide (DMSO) was stirred at room tempera-
ture. To the mixture 5.63 g (25.24 mmol) of 2-aminoanthra-
quinone and 7.30 g (51.75 mmol) of 4-fluoronitrobenzene
2-(4-(bis(4-nitrophenyl)amino)phenoxy)
anthracene-9,10-dione (3)
A mixture of 5.07 g (36.7 mmol) of potassium carbonate in 40
mL dimethyl sulfoxide (DMSO) was stirred at room temperature.
To the mixture 4.73 g (19.5 mmol) of 2-chloroanthracene-9,10-
dione and 6.79 g (19.3 mmol) of 4-(bis(4-nitrophenyl)amino)-
phenol were added in sequence. The mixture was heated with
stirring at 120 ꢀC for 20 h and slowly poured into 300 mL metha-
nol/water (2:1). The product was purified by THF to afford 9.33
were added in sequence. The mixture was heated with stir-
ꢀ
ring at 120 C for 24 h and slowly poured into 150 mL of
stirred methanol. The product was filtered to afford 10.69ꢀ g
(91% in yield) of yellow powders with a mp of 247–248 C
(by Melting Point System at a scan rate of 1 ꢀC/min). IR
(KBr): 1,520, 1,340 cmꢁ1 (ANO2 stretch).
1H NMR (500 MHz, DMSO-d6, d, ppm): 7.38 (m, 4H), 7.66
(m, 1H), 7.81 (d, J ¼ 2.4 Hz, 1H), 7.93 (m, 2H), 8.15 (m, 1H),
g (86 % in yield) of yellow powders with a mp of 131–133 C
(by Melting Point System at a scan rate of 1 ꢀC /min).
ꢀ
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1
IR (KBr): 1,575, 1,310 cmꢁ1 (-NO2 stretch). H NMR (DMSO-
d6, d, ppm): 7.26 (m, 4H), 7.33 (d, J ¼ 9.0 Hz, 2H), 7.40 (d, J
¼ 9.0 Hz, 2H), 7.58 (m, 1H), 7.68 (d, J ¼ 2.6 Hz, 1H), 7.93
(m, 2H), 8.21 (m, 6H), 8.27 (d, J ¼ 8.5 Hz, 1H). ESI-MS: calcd
for (C32H19N3O7)þ: m/z 557.5 found: m/z 557.3.
Preparation of the Films
A solution of the polymer was made by dissolving about 0.3
g of the polyamide sample in 9 mL of DMAc. The homogene-
ous solution was poured into a 9-cm glass Petri dish, which
ꢀ
was placed in a 90 C qweoven for 5h to remove most of the
solvent; then the semi-dried film was further dried in vacuo
at 170 ꢀC for 8 h. The obtained films were about 40-60 lm
thick and were used for solubility tests, and thermal
analyses.
Fabrication of the Electrochromic Device
Electrochromic polymer films were prepared by dropping
solutions of the polyamide OAQ-T (4 mg/mL in DMAc) onto
a ITO coated glass substrate (20 ꢂ 30 ꢂ 0.7 mm, 50–100
X/square). The polymers were drop-coated onto an active
area (about 20 mm ꢂ 20 mm) then dried in vacuum. A gel
electrolyte based on PMMA (Mw: 350000) and LiClO4 was
plasticized with propylene carbonate to form a highly trans-
parent and conductive gel. PMMA (3 g) was dissolved in dry
acetonitrile (15 g), and LiClO4 (0.3 g) was added to the poly-
mer solution as supporting electrolyte. Then, propylene car-
bonate (5 g) was added as plasticizer. 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.
2-(4-(bis(4-aminophenyl)amino)phenoxy)
anthracene-9,10-dione (4)
In a 100-mL three-neck round-bottomed flask equipped with
a stirring bar, 3.92 g (7.00 mmol) of dinitro compound 3
was dissolved in 43mL DMF at room temperature, and 0.35
g of 10 % Pd/C were added in solution. The suspension so-
lution was stirred under hydrogen atmosphere at room tem-
perature. After a further 24 h of reaction, the solution was
filtered to remove Pd/C, and the filtrate was slowly poured
into 300 mL methanol/water (1:1). The pꢀroduct was col-
lected by filtration and dried in vacuo at 80 C to give 2.75 g
(79 % in yield) of brown powder with mp of 243–244 ꢀC
ꢀ
(by Melting Point System at a scan rate of 1 C /min).
Measurements
FT-IR (KBr): 3,400, 3,200 cmꢁ1 (N-H stretch). 1H NMR (500
MHz, DMSO-d6, d, ppm): 5.00 (s, 4H, -NH2), 6.58 (d, J ¼ 8.5
Hz, 4H, Hk), 6.69 (d, J ¼ 9.0 Hz, 2H, Hi), 6.89 (d, J ¼ 8.5 Hz,
4H, Hj), 6.95 (d, J ¼ 9.0 Hz, 2H, Hh), 7.42 (dd, 1H, Hb), 7.46
(d, J ¼ 2.6 Hz, 1H, Ha), 7.90 (m, 2H, Heþf), 8.14 (dd, 1H, Hc),
8.17 (m, 2H, Hgþd). 13C NMR (125 MHz, DMSO-d6, d, ppm):
112.1, 114.8, 117.3, 121.1, 122.1, 126.6, 126.7, 127.2, 127.3,
129.8, 132.9, 134.1, 134.5, 135.0, 135.7, 144.9, 145.7, 147.4,
181.1, 182.0. Anal. Calcd (%) for C32H23N3O3 (497.54): C,
77.25; H, 4.66; N, 8.45. Found: C, 77.08; H, 4.64; N, 8.35.
ESI-MS: calcd for (C26H19N3O2)þ: m/z 497.5; found: m/z
497.2.
Infrared spectra were recorded on a PerkinElmer RXI FT-IR
spectrometer. Elemental analyses were run in an Elementar
Vario EL-III. 1H and 13C NMR spectra were measured on a
Bruker AVANCE-500 FT-NMR using tetramethylsilane as the
internal standard, and peak multiplicity was reported as fol-
lows: s, singlet; d, doublet; t, triplet; m, multiplet. The inher-
ent viscosities were determined at 0.5 g/dL concentration
using Tamson TV-2000 viscometer at 30 ꢀC. Gel permeation
chromatographic (GPC) analysis was performed on a Lab
Alliance RI2000 instrument (one column, MIXED-D from
Polymer Laboratories) connected with one refractive index
detector from Schambeck SFD Gmbh. All GPC analyses were
performed using a polymer/DMF solution at a flow rate of 1
Polymer Synthesis
ꢀ
The synthesis of polyamide AQ-T was used as an example to
illustrate the general synthetic route used to produce the
polyamides. A solution of 0.20 g (0.5 mmol) of 2 in 1.5 mL
of NMP was cooled to -25 to -30 ꢀC on a ice/acetone bath.
After 0.16 mL of propylene oxide was added to the mixture,
0.10 g (0.5 mmol) of 5T was added in to the mixture. The
mixture was then stirred at -10 ꢀC for 1 h then room tem-
perature for 2 h. The solution was poured slowly into 200
mL of methanol. The precipitated polymer was collected by
filtration, and dried at 100 ꢀC. Reprecipitations of the poly-
mer by N,N-dimethylacetamide (DMAc)/methanol were car-
ried out twice for further purification. The yield of the poly-
mer was 0.28 g (98%). The inherent viscosity and weight-
average molecular weights (Mw) of the obtained polyamide
AQ-T were 0.62 dL/g (measured at a concentration of 0.5 g/
mL/min at 70 C and calibrated with polystyrene standards.
Thermogravimetric analysis (TGA) was conducted with a
PerkinElmer Pyris 1 TGA. Experiments were carried out on
approximately 6–8 mg film samples heated in flowing nitro-
gen or air (flow rate ¼ 20 cm3/min) at a heating rate of 20
ꢀC /min. DSC analyses were performed on a Perkin-Elmer
ꢀ
Pyris 1 DSC at a scan rate of 20 C /min in flowing nitrogen
(20 cm3/min). Electrochemistry was performed with a CH
Instruments 612C electrochemical analyzer. Voltammograms
are presented with the positive potential pointing to the left
and with increasing anodic currents pointing downwards.
Cyclic voltammetry (CV) was conducted with the use of a
three-electrode cell in which ITO (polymer films area about
0.5 cm x 1.2 cm) was used as a working electrode and a
platinum wire as an auxiliary electrode at a scan rate of 50
mV/s against a Ag/AgCl reference electrode in anhydrous
acetonitrile (CH3CN) and dimethylformamide (DMF), using
0.1 M of tetrabutylammonium perchlorate (TBAP) as a sup-
porting electrolyte under a nitrogen atmosphere. All cell
ꢀ
dL in DMAc at 30 C) and 117,700 daltons, respectively. The
FT-IR spectrum of AQ-T (film) exhibited characteristic amide
absorption bands at 3,315 cmꢁ1 (N-H stretch), 3,065 cmꢁ1
(aromatic C-H stretch), 1,670 cmꢁ1 (amide carbonyl).
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SCHEME 1 Synthesis route to diamine monomers 2 and 4.
potentials were taken by using a homemade Ag/AgCl, KCl
(sat.) reference electrode. Spectroelectrochemical experi-
ments were carried out in a cell built from a 1 cm commer-
cial UV-visible cuvette using Hewlett-Packard 8453 UV-Visi-
ble diode array spectrophotometer. The ITO-coated glass
slide was used as the working electrode, a platinum wire as
the counter electrode, and a Ag/AgCl cell as the reference
electrode. CE (g) determines the amount of optical density
change (dOD) at a specific absorption wavelength induced as
a function of the injected/ejected charge (Q) which is deter-
mined from the in situ experiments. CE is given by the equa-
tion: g ¼ dOD/Q ¼ log[Tb/Tc]/Q, where g (cm2/C) is the col-
oration efficiency at a given wavelength, and Tb and Tc are
the bleached and colored transmittance values, respectively.
The thickness of the polyamide thin films was measured by
alpha-step profilometer (Kosaka Lab., Surfcorder ET3000, Ja-
pan). Colorimetric measurements were obtained using JASCO
V-650 spectrophotometer and the results are expressed in
terms of lightness (L*) and color coordinates (a*, b*).
showed characteristic bands at around 1520 and 1340 cmꢁ1
due to NO2 asymmetric and symmetric stretching. After
reduction to diamine monomers, the characteristic absorp-
tion bands of the nitro group disappeared and the primary
amino group revealed the typical absorption pair at 3470
and 3380 cmꢁ1 (N-H stretching). The 1H, 13C, and two-
dimensional (2D) HMQC NMR spectra of the diamine mono-
mers 2 and 4 are illustrated in Supporting Information (Figs.
S1–S3) and agree well with the proposed molecular struc-
tures. Thus, the results of all the spectroscopic and elemental
analyses suggest the successful preparation of the target dia-
mine monomer.
Two series of novel polyamides were obtained by the low-
temperature solution polycondensation15 from the diamines
and commercial diacid chlorides in NMP in the presence of
propylene oxide as an acid acceptor as shown in Scheme 2.
All polymerization reactions proceeded homogenously and
gave highly viscous polymer solutions. The obtained polya-
mides had inherent viscosities in the range of 0.57–0.64 dL/
g with weight-average molecular weights (Mw) and polydis-
persity (PDI) of 79,800–117,800 daltons and 1.25–2.17,
respectively, relative to polystyrene standards (Supporting
Information Table S1). All the high molecular weights poly-
mers could be cast into transparent and tough films via solu-
tion casting. The appearance and quality of these films are
also shown in Scheme 2. The structure of the polyamides
was confirmed by IR spectroscopy. A typical IR spectrum for
polyamide AQ-T shown in Supporting Information Figure S4
exhibits characteristic IR absorption bands of the amide
group at around 3315 (N-H stretching) and 1670 cmꢁ1
(amide carbonyl).
RESULTS AND DISCUSSION
Monomer and Polymer Synthesis
The new AQ-based diamine monomer 2-(bis(4-aminopheny-
l)amino)anthracene-9,10-dione (2) was prepared by the CsF-
mediated nucleophilic displacement reaction of 2-amino-
anthraquinone with 4-fluoronitrobenzene followed by hydra-
zine Pd/C-catalytic reduction (Scheme 1). The diamine 2-(4-
(bis(4-aminophenyl)amino)phenoxy)anthracene-9,10-dione
(4) with a bulky pendent anthraquinone unit was success-
fully synthesized by Pd/C-catalyzed reduction of the aro-
matic dinitro compound 2-(4-(bis(4-nitrophenyl)amino)phe-
noxy)anthracene-9,10-dione (3) as shown in Scheme 1.
Elemental analysis, FT-IR, and NMR spectroscopy were used
to identify structures of the intermediate dinitro compound
1 and the target diamine monomer 2. The FT-IR spectra of
these two synthesized compounds are illustrated in Support-
ing Information Figure S1. The nitro groups of compounds
Solubility and Film Property
The solubility behavior of polymers was investigated qualita-
tively, and the results are listed in Supporting Information
Table S2. Most of the polyamides were readily soluble in po-
lar aprotic organic solvents such as N-methyl-2-pyrrolidinone
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SCHEME 2 Synthesis of polyamides by low-temperature solution polycondensation.
(NMP), N,N-dimethylacetamide (DMAc), and m-cresol. The
polyamides OAQ had higher solubility than AQ can be attrib-
uted to the introduction of the soft segment of ether linkage
with bulky pendent anthraquinone-substituted TPA moiety
into the repeat unit. The excellent solubility in polar organic
solvents makes these polymers as potential candidates for
practical applications by spin-coating or inkjet-printing pro-
cesses to afford high performance thin films for optoelec-
tronic devices.
pared to the TPA-based polyamides11(b) can be ascribed to
their planar molecular structure and intra-/inter-molecular
hydrogen bonding even with the introduction of pendant AQ
groups. The lower Tg value of polymers OAQ in this series
polymers can be explained in terms of the flexible ether link-
age in its pendent group.
Electrochemical Properties
The redox behavior of the polyamides were investigated by
cyclic voltammetry (CV) conducted for the cast film on an
The thermal properties of polyamides were examined by TGA
and DSC, and the thermal behavior data are also summarized
in Supporting Information Table S3. Typical TGA curves of
representative polyamides AQ-T and OAQ-T in both air and
nitrogen atmospheres are shown in Supporting Information
Figure S5. All the prepared polyamides had good thermal sta-
ꢀ
bility with insignificant weight loss up to 440 C under nitro-
gen or air atmosphere. The 10 % weight loss temperatures of
these polymers in nitrogen and air were recorded in the range
ꢀ
of 470–495 and 440–490 C, respectively. The carbonized resi-
due (char yield) of these polymers in a nitrogen atmosphere
ꢀ
was more than 60 % at 800 C. The high char yields of these
polymers can be ascribed to their high aromatic content. The
glass-transition temperatures (Tg) of polyamides were meas-
ured in the DSC thermograms; they were observed in the
range of 283–360 ꢀC (as shown in Supporting Information
Fig. S6), depending upon the stiffness of the polymer chain.
Higher glass transition temperature of these polymers com-
FIGURE 1 Cyclic voltammetric diagrams of polyamide AQ-T
and OAQ-T films on an ITO-coated glass substrate over cyclic
scans in 0.1 M TBAP/CH3CN at a scan rate of 50 mV/s.
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TABLE 1 Redox Potentials and Energy Levels of Polyamides
Thin films
(nm)
Oxidationa
Reductionb
Eg
Eg
HOMOEC
(eV)c
LUMOOpt
(eV)
LUMOEC
(eV)c
EC
Opt
Code
kmax
konset
E1/2 (V)
E1/2 (V)
Ere p,ca
(eV)
(eV)
AQ-I
AQ-T
OAQ-I
OAQ-T
EC
325
323
335
340
613
610
438
447
1.11
1.11
0.84
0.89
ꢁ0.87
ꢁ0.82
ꢁ0.84
ꢁ0.78
ꢁ1.79
ꢁ1.75
ꢁ1.73
ꢁ1.69
a
1.98
1.93
1.68
1.67
2.02
2.03
2.83
2.77
ꢁ5.47
ꢁ5.47
ꢁ5.20
ꢁ5.25
ꢁ3.45
ꢁ3.44
ꢁ2.37
ꢁ2.43
ꢁ3.49
ꢁ3.54
ꢁ3.52
ꢁ3.58
Eg
(Electrochemical band gap): Difference between HOMOEC and
Versus Ag/AgCl in CH3CN.
Versus Ag/AgCl in DMF.
The HOMOEC and LUMOEC energy levels were calculated from cyclic
LUMOEC
.
b
c
Opt
Eg
k
(Optical band gap): Calculated from polymer films (Eg ¼ 1240/
onset).
voltammetry and were referenced to ferrocene (ꢁ4.8 eV; E1/2 ¼ 0.44 V)
LUMOOpt (LUMO energy levels calculated from optical method): Differ-
and (E1/2 ¼ 0.52 V), respectively.12(h)
Opt
ence between HOMOEC and Eg
.
indium-tin oxide (ITO)-coated glass slide as working elec-
trode in anhydrous CH3CN and DMF, using 0.1 M of TBAP as
a supporting electrolyte under a nitrogen atmosphere. The
typical CV for polyamides AQ-T and OAQ-T are shown in Fig-
ure 1 and Supporting Information Figure S7 for comparison.
The half-wave potentials (E1/2) of the reduction process
were recorded in the range from -0.78 to -0.87 V due to the
reduction of anthraquinone unit. The color of the polyamide
OAQ-T film changed from pale yellow to red upon electro-
chemical reduction. The half-wave potentials (E1/2) of the ox-
idation process were found to be around 0.84-1.11 V
resulted from the electroactive triarylamine group. The color
of the polyamide OAQ-T film turned from pale yellow to
green upon oxidation. After 50 continuous cyclic scans, the
polyamide AQ-T (without the ether linkage in pendent
group) gradually lost its redox reversibility. This result
shows that the introduction of AQ unit on the electron-rich
nitrogen atom not only increased the oxidation potential but
also decreased electrochemical stability. The other polya-
mides showed similar CV curves to Figure 1. The redox
potentials of the polyamides as well as their respective high-
est occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO) of the corresponding poly-
mers can be determined from the oxidation and reduction
half-wave potentials (E1/2) and the onset absorption wave-
length, and the results were listed in Table 1.
exhibited strong absorption at 317 nm (triarylamine) and
500 nm (charge-transfer complex formation). Upon oxida-
tion, the intensity of the absorption peak at 317 nm gradu-
ally decreased while a new peak at 875 nm gradually
increased in intensity. We attribute the spectral change in
visible range to the formation of a stable monocation radical
of the triarylamine moiety. The polyamide AQ-T film
Spectroelectrochemistry
Spectroelectrochemical experiments were used to evaluate
the optical properties of the electrochromic films. For the
investigations, the polyamide films were cast on an ITO-
coated glass slide, and a homemade electrochemical cell was
built from a commercial ultraviolet (UV)-visible cuvette. The
cell was placed in the optical path of the sample light beam
in a UV-vis-NIR spectrophotometer, which allowed us to ac-
quire electronic absorption spectra under potential control
in a 0.1 M TBAP/CH3CN solution for oxidation and 0.1 M
TBAP/DMF solution for reduction, respectively.
The result of polyamide AQ-T film is presented in Figure
2(a) as a series of UV-vis-NIR absorbance curves correlated
to electrode potentials. In the neutral form (0 V), the film
FIGURE 2 Electrochromic behavior of polyamide AQ-T (ꢃ170
nm) and OAQ-T (ꢃ80 nm) thin film on the ITO-coated glass
substrate (coated area: 1.6 cm ꢂ 0.6 cm) in 0.1 M TBAP/CH3CN.
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Electrochromic Switching Studies
For electrochromic switching studies, polymer films were
cast on ITO-coated glass slides in the same manner as
described above, and chronoamperometric and absorbance
measurements were performed. While the films were
switched, the absorbance at the given wavelength was moni-
tored as a function of time with UV-vis-NIR spectroscopy.
Switching data for the representative cast film of polyamide
AQ-T were shown in Supporting Information Figure S8. The
switching time was calculated at 90 % of the full switch
because it is difficult to perceive any further color change
with naked eye beyond this point. As depicted in Supporting
Information Figure S8a, polyamide AQ-T thin film revealed
switching time of 5.5 s at 1.13 V for coloring process at 885
nm and 1.2 s for bleaching, polyamide OAQ-T required 5.7 s
for coloration at 820 nm at 0.95 V and 1.7 s for bleaching
(Supporting Information Fig. S8b). The polyamides switched
between the highly transmissive neutral state and the col-
ored oxidized state.
FIGURE 3 UV-vis-NIR spectra of polyamide OAQ-T thin film
(ꢃ180 nm in thickness) on the ITO-coated glass substrate
(coated area: 1.6 cm ꢂ 0.6 cm) in 0.1 M TBAP/DMF.
As shown in Supporting Information Figures S9 and S10, the
electrochromic stability of the polyamide AQ-T and OAQ-T
was determined by measuring the optical change as a func-
tion of the number of switching cycles. The better electro-
chromic stability of OAQ-T could be due to the lower oxida-
tion potential and the isolated ether bond between TPA and
AQ groups. The electrochromic CE (g=dOD/Q) and injected
charge (electroactivity) after various switching steps were
monitored and summarized in Supporting Information Tables
S4 and S5. The modern result of these electrochromic films
was due to the strong electron withdrawing AQ group
decreasing the reversibility and stability of the monocation
radical form.
switched from a transmissive neutral state (pale-red; L*,
80.9; a*, 29.7; b*, 22.0) to a highly absorbing oxidized state
(brown; L*, 50.5; a*, 2.9; b*, 22.0) with a high optical trans-
mittance change (D%T) of 64% at 875 nm.
On the other hand, the spectroelectrochemical behavior of
polyamide OAQ-T film shown in Figure 2(b) also exhibited
strong absorption at around 357 nm in the neutral form (0
V). Upon oxidation (increasing applied voltage from 0 to
1.10 V), the intensity of the absorption peak at 357 nm
decreased while a new peak at 400 and 815 nm gradually
increased in intensity due to the formation of a stable mono-
cation radical of the TPA unit. The polyamide OAQ-T film
switches from a transmissive neutral state (pale-yellow; L*,
91.9; a*, -3.2; b*, 50.6) to a highly absorbing oxidized state
(green; L*, 51.3; a*, -23.2; b*, 23.2) with a high optical trans-
mittance change (D%T) of 76% at 815 nm. Besides, the dis-
tribution of coloration across these polymer films was very
homogeneous with high electrochemical stability.
Moreover, coloration changes were also observed in these
polyamides during reduction procedure as shown in Figure
3. Upon reduction (decreasing applied voltage from 0 to
ꢁ1.20 V), the absorption of OAQ-T film at 545 nm and a
broad IV-CT band centered around 875 nm gradually
increased in intensity. We attribute the broad absorption
around 875 nm was the characteristic result due to IV-CT ex-
citation between states in which the negative charge is
located at different oxygen atoms, which was consistent with
the phenomenon reported by Wang and Wan.4(c–i) The color
of film changed from pale yellow neutral state to a red
reduced form (L*, 45.0; a*, 27.4; b*, 19.0), which also exhib-
ited good contrast in the visible region with a high optical
transmittance change (D%T) of 61% at 545 nm for red col-
oring at the reduction stage. This result demonstrated that
the incorporation of anthraquinone groups into TPA-based
polymers is a new approach for preparing ambipolar and
NIR electrochromic materials.
FIGURE 4 (a) Photographs of single-layer ITO-coated glass
electrochromic device, using polyamide OAQ-T (ꢃ100 nm in
thickness) as active layer. (b) Schematic diagram of polyamide
electrochromic device sandwich cell.
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wards, the gel electrolyte was spread on the polymer-coated
side of the electrode and the electrodes were sandwiched. To
prevent leakage, an epoxy resin was applied to seal the de-
vice. As a typical example, an electrochromic cell based on
polyamide OAQ-T was fabricated. The polymer film is pale
yellow in neutral form. When the voltage was increased (to
a maximum of 2.0 V), the color changed to green due to elec-
tro-oxidation, the same as that was already observed in the
solution type electrolyte system. When the potential was
subsequently set back at 0 V, the polymer film turned back
to original pale yellow. We believe that optimization could
further improve the device performance and fully explore
the potential of these electrochromic polyamides.
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A novel series of electrochromic aromatic polyamides with
TPA and AQ units were readily prepared via the low-temper-
ature solution polycondensation. In addition to high Tg, ther-
mal stability, and good solubility, all the obtained polymers
reveal valuable electrochromic characteristics such as high
contrast ratio and low switching times. Thus, these charac-
teristics suggest that the introduction of reduction active
anthraquinone group into the polymer side chain could pro-
vide the prepared polyamide as potential anodically and
cathodically coloring materials.
The authors are grateful to the National Science Council of the
Republic of China for the financial support of this work. C. W. Lu
at the Instrumentation Center, National Taiwan University, for
CHNS (EA) analysis experiments and C. H. Ho at the Instrumen-
tation Center, Department of Chemistry, National Taiwan Nor-
mal University, for the measurement of 500 MHz NMR
spectrometer are also acknowledged.
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