5-Substituted isophthalate-based organic electrochromic materials

Article abstract of DOI:10.1039/b808638a

A new series of isophthalate-based electrochromic materials were prepared. The functional groups at the 5-position of isophthalate have a significant influence on the observed color. In particular, an electrochemically active nitro group induced a multi-color display, indicating that further electrochromic properties could be manipulated using more diverse 5-substituted isophthalate derivatives. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b808638a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
5-Substituted isophthalate-based organic electrochromic materials
W. Sharmoukh,a Kyoung Chul Ko,a Ju Hong Ko,a Hye Jin Nam,a Duk-Young Jung,a Changho Noh,b
a
Jin Yong Lee and Seung Uk Son
a
*
*
Received 21st May 2008, Accepted 7th July 2008
First published as an Advance Article on the web 4th August 2008
DOI: 10.1039/b808638a
A new series of isophthalate-based electrochromic materials were prepared. The functional groups at
the 5-position of isophthalate have a significant influence on the observed color. In particular, an
electrochemically active nitro group induced a multi-color display, indicating that further
electrochromic properties could be manipulated using more diverse 5-substituted isophthalate
derivatives.
Introduction
Electrochromism is an interesting phenomenon defined as
color display dependence on applied voltage.1 Academic, as
well as industrial research interests have been focused on
electrochromic materials for their broad application in such
commercial products as smart windows, diverse displays, and
mirror devices.2 In the case of application for electrochromic
Scheme 1 Electrochromism of 5-substituted isophthalate derivatives.
paper, organic materials are very promising candidates where
reduction or oxidation of the neutral organic compound to its
radical species induces the desired color change.3 There have
been extensive studies on the electrochromism of conducting
organic polymers.3c The polymer-based devices have many
advantages in processing. However, a drawback of these is
their relatively slow response time, which is limited by
ion transport or electron-migration rate within polymeric
materials.3d
Experimental
Materials and apparatus
Isophthalic acid, pyridine and boronic acids were purchased
from Aldrich Chemical Company, Inc., and used as supplied.
Palladium acetate was purchased from Strem Chemicals.
5-Hexene-1-ol, tetrabutylammonium hexafluorophosphate and
tritolylphosphine were purchased from TCI (Japan). Thionyl
chloride was purchased from Samchun Chemical Company
(Korea). g-Butyrolactone (Aldrich), methylene chloride
(Samchun) and DMF (Samchun) were freshly distilled before
use. 1H NMR and 13C NMR spectra were recorded on a Varian
300 MHz spectrometer. High resolution mass spectra of new
compounds were obtained using a Jeol JMS 700 spectrometer at
Korea Basic Science Institute (Daegu).
Several privileged organic molecules such as viologen and
phenothiazine derivatives are known to show electrochromism.4
Recently, the new structured devices have been developed by
attaching these materials onto the surface of nanostructured
semiconductor films, which showed very fast response times.5 In
this regard, more organic electrochromic compounds should be
developed for display of diverse colors.
Compared to viologen and phenothiazines, less attention has
been paid to the 1,4-terephthalate derivatives.6 Moreover, as
far as we are aware, there is no report on the electrochromism
of 1,3-isophthalates, which attracted our attention because
functional groups can be easily introduced at the 5-position of
these. Our research group has prepared the diverse 5-substituted
isophthalate compounds and investigated their electrochromic
behavior (Scheme 1). In this paper, we report new isophthalate-
based electrochromic materials and the facile control of their
electrochromic properties by changing the functional group at
the 5-position.
General synthetic procedure for M1, 7–10
Isophthalic acid (6.0 mmol) was dissolved in 3 mL (41 mmol) of
thionyl chloride in a 50 mL Schlenk flask. The reaction mixture
was heated for 3 hours under reflux. Then, the solution was
cooled to room temperature and the excess thionyl chloride was
removed under vacuum. The oily product was dried under
vacuum for 1 hour. Methylene chloride (20 mL) was added to
dissolve the oily product. 5-Hexen-1-ol (2.16 mL, 18 mmol) and
ꢀ
pyridine (1.94 mL, 24 mmol) were added at 0 C. The reaction
mixture was stirred for 3 hours at 0 ꢀC. After reaction, the
reaction mixture was extracted using methylene chloride and
aqueous ammonium chloride solution. The filtered solution was
dried using MgSO4 and evaporated. The product was separated
via column chromatography using a mixture of hexane and ether
(12 : 2) as eluent.
aDepartment of Chemistry, Sungkyunkwan University, Suwon 440-746,
Korea. E-mail: sson@skku.edu; Fax: +82 031-290-4572; Tel: +82
031-299-5932
bSamsung Advanced Institute of Technology (SAIT), Suwon 440-600,
Korea
4408 | J. Mater. Chem., 2008, 18, 4408–4413
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General synthetic procedure for M2–6
13C NMR (75 MHz, CDCl3) d 165.7, 142.1, 138.3, 135.2, 131.6,
130.7, 129.1, 128.4, 126.2, 124.5, 115.1, 65.5, 33.4, 28.1, 25.3 ppm.
HRMS (EI) calc. for C24H28O4S1 412.1708, found 412.1707.
M7; 43% isolated yield, 1H NMR (300 MHz, CDCl3) d 8.47 (s,
1H), 8.02 (s, 2H), 5.81 (m, 2H), 5.01 (d, J ¼ 15 Hz, 2H), 4.96
(d, J ¼ 6.9 Hz, 2H), 4.32 (t, J ¼ 6.9 Hz, 4H), 2.45 (s, 3H), 2.14
(q, J ¼ 6.9 Hz, 4H), 1.80 (q, J ¼ 6.9 Hz, 4H), 1.53 (q, J ¼ 6.9 Hz,
4H) ppm. 13C NMR (75 MHz, CDCl3) d 166.2, 138.7, 138.4,
134.4, 130.9, 128.0, 115.1, 65.3, 33.4, 28.3, 25.4, 21.3 ppm.
HRMS (EI) calc. for C21H28O4 344.1987, found 344.1991.
M8; 61% isolated yield, 1H NMR (300 MHz, CDCl3) d 8.26 (s,
1H), 7.73 (s, 2H), 5.82 (m, 2H), 5.03 (d, J ¼ 14 Hz, 2H), 4.97
(d, J ¼ 6.9 Hz, 2H), 4.36 (t, J ¼ 6.6 Hz, 4H), 3.88 (s, 3H), 2.12
(q, J ¼ 6.9 Hz, 4H), 1.79 (q, J ¼ 6.9 Hz, 4H), 1.54 (q, J ¼ 6.9 Hz,
4H) ppm. 13C NMR (75 MHz, CDCl3) d 165.9, 159.7, 138.4,
132.2, 122.9, 119.3, 115.1, 65.5, 55.9, 33.4, 28.2, 25.4 ppm.
HRMS (EI) calc. for C21H28O5 360.1937, found 360.1936.
5-Iodo-isophthalate M10 (0.22 mmol), aryl boronic acid (29 mg,
0.24 mmol), palladium acetate (5.0 mg, 0.022 mmol), potassium
carbonate (0.15 g, 1.1 mmol) and tritolylphosphine (13.3 mg,
0.044 mmol) were added to 20 mL of DMF. The reaction mixture
was heated at 90 ꢀC for 4 hours. After reaction, the reaction
mixture was extracted using methylene chloride and aqueous
solution of sodium chloride and then, dried using MgSO4. The
product was separated via column chromatography using
mixture of hexane and ether (12 : 1) as eluent.
Characterization data of new compounds
1
M1; 56% isolated yield, H NMR (300 MHz, CDCl3) d 8.67 (s,
1H), 8.21 (d, J ¼ 7.8 Hz, 2H), 7.52 (t, J ¼ 7.5 Hz, 1H), 5.80 (m, 2H),
5.03 (d, J ¼ 14 Hz, 2H), 4.96 (d, J ¼ 6.9 Hz, 2H), 4.34 (t, J ¼ 6.6 Hz,
4H), 2.12 (q, J ¼ 6.9 Hz, 4H), 1.79 (q, J ¼ 6.9 Hz, 4H), 1.54 (q, J ¼
6.9 Hz, 4H) ppm. 13C NMR (75 MHz, CDCl3) d 165.8, 138.3,
133.7, 130.9, 130.7, 128.6, 114.9, 65.3, 33.3, 28.2, 25.3 ppm.
HRMS (EI) calc. for C20H26O4 330.1831, found 330.1827.
1
M9; 58% isolated yield, H NMR (300 MHz, CDCl3) d 8.99
(s, 2H), 8.93 (s, 1H), 5.79 (m, 2H), 5.04 (d, J ¼ 14 Hz, 2H), 4.95
(d, J ¼ 6.9 Hz, 2H), 4.38 (t, J ¼ 6.9 Hz, 4H), 2.14 (q, J ¼ 7.0 Hz,
4H), 1.79 (q, J ¼ 6.9 Hz, 4H), 1.54 (q, J ¼ 6.9 Hz, 4H) ppm. 13
C
1
M2; 90% isolated yield, H NMR (300 MHz, CDCl3) d 8.65
NMR (75 MHz, CDCl3) d 163.9, 148.5, 138.1, 135.8, 132.8,
128.1, 115.2, 66.3, 33.3, 28.1, 25.2 ppm. HRMS (EI) calc. for
C20H25O6N1 375.1682, found 375.1677.
(s, 1H), 8.45 (s, 2H), 7.66 (d, J ¼ 6.9 Hz, 2H), 7.47 (m, 3H), 5.82
(m, 2H), 5.06 (d, J ¼ 14 Hz, 2H), 4.97 (d, J ¼ 6.9 Hz, 2H), 4.41
(t, J ¼ 6.6 Hz, 4H), 2.14 (q, J ¼ 6.9 Hz, 4H), 1.83 (q, J ¼ 6.9 Hz,
4H), 1.55 (q, J ¼ 6.9 Hz, 4H) ppm. 13C NMR (75 MHz, CDCl3)
d 166.0, 142.0, 139.3, 138.4, 132.4, 131.6, 129.4, 129.2, 128.3,
127.4, 115.1, 65.5, 33.4, 28.3, 25. ppm. HRMS (EI) calc. for
C26H30O4 406.2144, found 406.2144.
1
M10; 67% isolated yield, H NMR (300 MHz, CDCl3) d 8.59
(s, 1H), 8.51 (s, 2H), 5.78 (m, 2H), 5.00 (d, J ¼ 13 Hz, 2H), 4.96
(d, J ¼ 6.9 Hz, 2H), 4.36 (t, J ¼ 6.6 Hz, 4H), 2.10 (q, J ¼ 6.9 Hz,
4H), 1.78 (q, J ¼ 6.9 Hz, 4H), 1.50 (q, J ¼ 7.0 Hz, 4H) ppm. 13
C
NMR (75 MHz, CDCl3) d 164.4, 142.4, 138.3, 132.6, 129.9,
115.1, 93.5, 65.7, 33.3, 28.1, 25.3 ppm. HRMS (EI) calc. for
C20H25O4I 456.0797, found 456.0797.
M3; 76% isolated yield,1H NMR (300 MHz, CDCl3) d 8.64
(s, 1H), 8.39 (s, 2H), 7.62 (d, J ¼ 8.7 Hz, 2H), 7.17 (t, J ¼ 8.7 Hz,
2H), 5.83 (m, 2H), 5.04 (d, J ¼ 14 Hz, 2H), 4.98 (d, J ¼ 6.9 Hz,
2H), 4.39 (t, J ¼ 6.6 Hz, 4H), 2.14 (q, J ¼ 7.2 Hz, 4H), 1.84
(q, J ¼ 6.9 Hz, 4H), 1.58 (q, J ¼ 6.9 Hz, 4H) ppm. 13C NMR
(75 MHz, CDCl3) d 165.8, 140.9, 138.3, 135.4, 132.1, 131.6,
129.3, 129.0, 116.2, 115.9, 115.1, 65.5, 33.4, 28.2, 25.4 ppm.
HRMS (EI) calc. for C26H29O4F 424.2050, found 424.2047.
UV-visible spectroscopy and cyclic voltammetry
The UV-visible spectra were recorded using Jasco V630 and
Ocean Optics USB4000 spectrometer. Cyclic voltammetry
measurements were carried out using a CH instruments model
CHI600 potentiostat. Conventional three electrodes assembly
was used under nitrogen to record cyclic voltammograms. The
working electrode was an ITO-coated glass electrode. The
counter-electrode was a platinum wire, and Ag/AgNO3 (Ag/Ag+)
was used as the reference electrode. The scan rate was 100 mV
1
M4; 91% isolated yield, H NMR (300 MHz, CDCl3) d 8.60
(s, 1H), 8.41 (s, 2H), 7.61 (d, J ¼ 6.9 Hz, 2H), 7.00 (d, J ¼ 6.9 Hz,
2H), 5.81 (m, 2H), 5.04 (d, J ¼ 15 Hz, 2H), 4.99 (d, J ¼ 7.0 Hz,
2H), 4.40 (t, J ¼ 6.9 Hz, 4H), 3.86 (s, 3H), 2.15 (q, J ¼ 6.9
Hz, 4H), 1.84 (q, J ¼ 6.9 Hz, 4H), 1.59 (q, J ¼ 7.2 Hz, 4H) ppm.
13C NMR (75 MHz, CDCl3) d 166.0, 159.9, 141.5, 138.4, 131.8,
131.4, 128.7, 128.4, 115.1, 114.5, 65.4, 55.4, 33.4, 28.2, 25.4 ppm.
HRMS (EI) calc. for C27H32O5 436.2250, found 436.2249.
sꢁ1
. The 0.20 M anhydrous tetrabutylammonium hexa-
fluorophophate (TBAPF6) solution in distilled g-butyrolactone
was used as a supporting electrolyte. The 5 mM solutions of each
compound were used for measurement.
1
M5; 70% isolated yield, H NMR (300 MHz, CDCl3) d 8.66
(s, 1H), 8.51 (s, 2H), 7.74 (s, 4H), 7.65 (d, J ¼ 7.5 Hz, 2H), 7.48
(t, J ¼ 7.2 Hz, 2H), 7.40 (t, J ¼ 7.2 Hz, 1H), 5.84 (m, 2H), 5.05 (d,
J ¼ 14 Hz, 2H), 4.98 (d, J ¼ 6.9 Hz, 2H), 4.40 (t, J ¼ 6.6 Hz, 4H),
2.15 (q, J ¼ 7.2 Hz, 4H), 1.84 (q, J ¼ 7.2 Hz, 4H), 1.57 (q, J ¼ 7.2
Hz, 4H) ppm. 13C NMR (75 MHz, CDCl3) d 166.1, 141.6, 141.3,
140.6, 138.5, 138.2, 132.3, 131.7, 129.5, 129.1, 127.9, 127.8, 127.3,
115.3, 65.6, 33.5, 28.4, 25.5 ppm. HRMS (EI) calc. for C32H34O4
482.2457, found 482.2452.
Preparation of sandwich-type two ITO electrodes cells
M6; 89% isolated yield 1H NMR (300 MHz, CDCl3) d 8.56 (s,
1H), 8.43 (s, 2H), 7.46 (d, J ¼ 3.6 Hz, 1H), 7.36 (d, J ¼ 5.1 Hz, 1H),
7.12 (dd, J ¼ 3.6, 5.0 Hz, 1H), 5.84 (m, 2H), 5.07 (d, J ¼ 14 Hz,
2H), 4.98 (d, J ¼ 6.9 Hz, 2H), 4.40 (t, J ¼ 6.6 Hz, 4H), 2.14 (q, J ¼
6.9 Hz, 4H), 1.84 (q, J ¼ 6.9 Hz, 4H), 1.57 (q, J ¼ 6.9 Hz, 4H) ppm.
The electrochromic cells were prepared using two ITO-coated
glass electrodes, which were sandwiched by Surlyn tape. The
solution was injected into the cell using a syringe.
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Scheme 2 Synthesis of 5-substituted isophthalate derivatives.
Results and discussion
Diverse 5-substituted isophthalate derivatives used in this study
were prepared via esterification of 5-substituted isophthalic acids
or Suzuki coupling between 5-iodo-isophthalate and diverse
boronic acids (Scheme 2). All compounds were fully character-
ized by 1H and 13C NMR and high resolution mass spectroscopy.
5-(2-Thienyl)-isophthalate has a pale yellow color and other
compounds are colorless in their neutral state.
Fig. 1 Representative cyclic voltammorgrams of M1 (a), M2 (b), M7 (c)
and M8 (d).
First, cyclic voltammeric analysis was conducted for each
sample (5 mM) to understand the electrochemical behavior.
0.2 M Tetrabutylammonium hexafluorophosphate (TBAPF6)
was used as the supporting electrolyte in g-butyrolactone. As
summarized in Table 1, the compounds showed reversible
reduction peaks in range of ꢁ2.5 to ꢁ2.90 V (vs. Ag/Ag+) with
a scan in the negative direction (100 mV sꢁ1). At these reduction
potentials, we could observe a color on the surface of the working
electrode (ITO glass) (Fig. 1). In comparison, there was no
change of color with a scan in the positive direction (0–3 V vs.
Ag/Ag+). This means that isophthalate derivatives in this study
generate colored species via reduction. Similarly, it has been
reported that electrochromism of 1,4-terephthalate derivatives
resulted from the formation of the anionic radical via electro-
chemical reduction.6
Table 1 Redox potentials of compounds in cyclic voltammetry and
applied voltage for color display
Redox potentiala
vs. Ag/Ag+/V
Applied DC
voltageb/V
We prepared solution-based simple electrochromic devices
using sandwich-type indium tin oxide (ITO)-coated glass
electrodes.6b Each sample (0.225 M) was dissolved in
g-butyrolactone, containing 0.2 M TBAPF6 as the electrolyte.
The DC voltage was slowly provided to the cells (from 0 to 6 V,
100 mV sꢁ1). The displayed colors of isophthalate derivatives via
supply of DC voltage are summarized in Fig. 2 and Fig. 3.
As expected, isophthalate derivatives (M1–M8) displayed
diverse colors. It is evident that a functional group in the
Compounds R0 ¼
H, M1
Ph, M2
ꢁ2.68 (rev)c
ꢁ2.55 (rev)
5.05
5.14
5.20
5.54
4.38
5.15
5.32
5.21
4-Fluorophenyl, M3
4-Methoxyphenyl, M4
4,4-Biphenyl, M5
2-Thienyl, M6
CH3, M7
OMe, M8
NO2, M9
Nitrobenzene
ꢁ2.58 (rev)
ꢁ2.62 (rev)
ꢁ2.57 (rev)
ꢁ2.63 (rev)
ꢁ2.72 (rev)
ꢁ2.61 (rev)
ꢁ1.88, ꢁ2.71, ꢁ2.81 (rev)
4.17, 5.19
4.38, 5.27
5-position has
a significant influence on the electronic
ꢁ2.04, ꢁ3.08
surroundings of the reduced species. In case of M1 (5-H), M7
(5-Me) and M8 (5-OMe), the colored species showed maximum
absorption peak at 450, 449 and 445 nm, respectively. When the
functional groups were monoarenes, the maximum absorption
peaks were red-shifted to 509 (M2), 512 (M3) and 530 nm (M6).
The colored species by M5 (5-4,40-biphenyl) showed the
maximum absorption peak at 568 nm (Fig. 3). Considering the
a
Redox potential vs. Ag/Ag+(reference electrode) determined in
a conventional three electrodes cell by using 0.2 M tetrabutylammonium
hexafluorophosphate as the supporting electrolyte in g-butyrolactone,
ITO glass as the working electrode, and platinum as the counter
electrode.b Applied DC voltage for color displays from sandwich-type
ITO electrode cells, 0.2 M tetrabutylammonium hexafluorophosphate as
supporting electrolyte in g-butyrolactone.c Reversible peak.
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Table 2 The comparison of calculated absorption peak based on time-
dependent density functional theory (TDDFT) with the experimentally
observed ones
Compounds
Calculated absa/nm
Observed absb/nm
M1
M3
M5
476
537
562
450
510
568
a
The calculation was conducted based on B3LYP/6-31+G*
method.b Experimentally observed UV-visible absorption peaks.
for the reduced species of M1, M3 and M5 as the representatives
were calculated to be 723.2, 893.4 and 1265.6 nm, respectively.
As the conjugated system was longer by introducing a substituent
at 5-position, the HOMO–LUMO energy gap became smaller.
It should be pointed out that not only the HOMO–LUMO
transition but also the other orbital transitions have contributed
to the absorption in the visible region. As a matter of fact, from
the TDDFT calculations of the reduced species of M1, HOMO–
LUMO + 10 transition was shown to give a significant contri-
bution to the absorption at 476.4 nm. Nevertheless, the HOMO–
LUMO energy gap gives the consistent trend that shows more
red-shifted absorption peak when the conjugated system is
longer. Table 2 summarizes the calculated absorption peaks in
the visible region along with the experimentally observed
maximum visible absorption peak. The calculated absorption
maximum peak positions in the visible region are in good
agreement with the experimental results.
Fig. 2 Electrochromism of 5-substituted isophthalate derivatives (with
the computed coordinates in the CIE chromaticity diagram).
Another interesting feature is the electronic structures of the
molecular orbitals (MO). Fig. 4 shows the MOs that are involved
in the maximum absorption peak in visible region for M1. It was
found that the LUMO and LUMO + 10 show the electron
density localized at the central conjugated systems, as seen in
Fig. 4. The MOs between LUMO and LUMO + 10 were not
involved in the absorption process, which means that the tran-
sitions from HOMO to the orbitals between LUMO and LUMO
+ 10 are not involved in the absorption. It is very interesting to
find that the electron densities are mostly distributed over the
two ester moieties in the MOs between LUMO and LUMO + 10.
This may be responsible for the quite pure colors for the
5-substituted isophthalate derivatives investigated.
Next, we were curious about what would happen if an
electrochemically active group was introduced at the 5-position.
Considering the functional-group-dependent color, multicolor
displays can be expected.
Fig. 3 Representative UV-visible absorption spectra of the reduced
species of M1, M3, M5 and M6.
Anionic radicals of aromatic nitro compounds have been
a very important electrochemical research subject, in part
because of their role in the unique functions of organic aromatic
nitro compounds,8 such as the activity of commercial drugs and
length of conjugated system, the trend of the resultant color
could be rationalized; as the conjugated system is longer, the
maximum absorption peak is more red-shifted. Based on this
speculation, we believe that the color can be controlled
by properly choosing a functional group at 5-position. It is
noteworthy that more than two hundred kinds of boronic acid are
now commercially available.
The trend of the observed color of reduced species was ratio-
nalized by time-dependent density functional theory (TDDFT)
calculations.7 The excitation properties (transition energies and
oscillator strengths) of the reduced species of M1, M3 and M5 by
one electron were calculated. The HOMO–LUMO energy gaps
Fig. 4 Calculated HOMO, LUMO and LUMO + 10 of the reduced
species of M1.
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Scheme 3 Known reduction process of nitrobenzene.
pesticides, both based on the nitro aromatic radical species.9
Thus, reduction of aromatic nitro compounds has been electro-
chemically characterized.10 Until now, it has been known that
two reduced species can be generated step-by-step with two
electrons as shown in Scheme 3. The first reduced species is an
anionic radical that can undergo further reduction to form the
dianion.
Considering this multi-step reduction process of nitroben-
zene, we prepared the 5-nitro-isophthalate, M9, and investi-
gated its electrochromism behavior. As shown in Fig. 6a, the
cyclic voltammogram of 5 mM M9 in a 0.2 M TBAPF6 solution
in g-butyrolactone showed the multi-step reduction process at
ꢁ1.88, ꢁ2.71 and ꢁ2.81 V (vs. Ag/Ag+) (Table 1). As well-
documented, in the case of nitrobenzene, nearly the same
pattern was observed for the two reduction peaks with their
shift to more negative potentials of ꢁ2.04 and ꢁ3.08 V (vs. Ag/
Ag+), denoting that reduction of nitrobenzene requires higher
energy than that of M9.10 It should be noted that M9 has two
additional electron-withdrawing ester groups. Thus, the first
two reduction peaks of M9 can be attributed to the formation
of the anionic radical and dianion, respectively, as with nitro-
benzene.
Fig. 6 Cyclic voltammogram of 5-nitro-isophthalate derivative, M9
with Ag/Ag+ reference electrode and scan rate of 100 mV sꢁ1 (a), I–V
curves of sandwich-type two ITO electrodes cells of 5-nitro-isophthalate
derivatives, M9 (b), nitrobenzene (c), and isophthalate derivatives,
M1 (d).
Fabrication of device
For practical use, all-solid-state devices are preferable to the
solution-based ones due to technical problems, such as leakage of
electrolyte solution. Thus, we fabricated the quasi-solid devices
using poly(vinylpyrrolidone)-based gel electrolyte.13 A mixture
of 3.33 g of 1-vinyl-2-pyrrolidinone (30 mmol), 0.031 g of N,N0-
methylenebisacrylamide (0.2 mmol), 0.99 g of 1-methyl-2-
pyrrolidinone (10 mmol), 4.6 mg of ammonium persulfate
(0.02 mmol), 1.2 mg of N,N,N0,N0-tetramethylethylenediamine
M9 and nitrobenzene (0.225M) were dissolved in g-butyr-
olactone, containing 0.2 M TBAPF6 as the electrolyte and added
to the electrochromic cells. And then, the DC voltage was slowly
provided to the cells (from 0 to 6 V, 100 mV sꢁ1). As shown in
current–voltage curves of M9 and nitrobenzene in Fig. 6b and 6c,
two redox peaks were observed. In the case of M9, vivid blue,
then red appeared at 4.17 and 5.19 V, respectively (Fig. 5).11 In
contrast, simple nitrobenzene did not show any detectable color
change in a range of 0–6 V, which reveals that two ester groups
play an important role in electrochromism. Considering Fig. 6b
and 6c, we assigned the blue and red species as the anionic radical
and dianion, respectively. The reason why two distinct colors
were generated is unknown at this stage and further work on the
computational approach is needed. However, we believe that this
discovery opens the door not only to diverse spectroelec-
trochemical approaches for the study of nitro radicals and
dianions academically, but also to the development of multi-
color display devices.12
Fig. 7 UV-visible spectra of the colored species by M1 in gel-electrolyte-
based quasi-solid device (a) and in solution-based device. (b) The inten-
sity change of UV-visible absorption peak at 450 nm by applying
a potential step (0.00–3.6 V) to the gel-electrolyte-based quasi-solid
device using M1 (c).
Fig.
5 Multicolor display of 5-nitro-isophthalate derivative with
a supply of DC voltage; before voltage application (a), colors of the first
reduced species (b, c) and the second reduced species (d–f).
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4 (a) D. Corr, U. Bach, D. Fay, M. Kinsella, C. McAtamney,
F. O’Reilly, S. N. Rao and N. Stobie, Solid State Ionics, 2003, 165,
315; (b) M. O. M. Edwards, T. Gruszecki, H. Pettersson,
G. Thraisingham and A. Hagfeldt, Electrochem. Commun., 2002, 4,
963; (c) M. Felderhoff, S. Heinen, N. Molisho, S. Webersinn and
L. Walder, Helv. Chim. Acta, 2000, 83, 181.
(0.01 mmol), 0.068 g of tetrabutylammonium hexafluorophosphate
(0.2 mmol), ferrocene (0.2 mmol) and the coloring material
(0.2 mmol, M1) was injected into the cell via syringe. Then, the
cell was gently heated at 40 ꢀC for 12 h to induce the gelation of
solution. The colored species in this device showed maximum
absorption peak at 452 nm, which is nearly same with that by M1
in solution-based device. With help of contemporary redox
reaction of ferrocene (charge compensation via oxidation), the
electrochromic performance was quite reversible (Fig. 7).
5 D. Cummins, G. Boschloo, M. Ryan, D. Corr, S. N. Rao and
D. Fitzmaurice, J. Phys. Chem. B, 2000, 104, 11449.
6 (a) K. Nakamura, Y. Oda, T. Sekikawa and M. Sugimoto, Jpn. J.
Appl. Phys., 1987, 26, 931; (b) H. Urano, S. Sunohara, H. Ohtomo
and N. Kobayashi, J. Mater. Chem., 2004, 14, 2366, also refer to
experimental details in the ESI for the preparation of devices.
7 The DFT calculations with B3LYP exchange functionals with
6-31+G* basis sets were carried out to optimize the structures of
the reduced species of M1, M3 and M5 using a suite of Gaussian 03
programs. (Frisch, et al., Gaussian 03, revision A1; Gaussian Inc.:
Pittsburgh, PA, 2003). Then, at the optimized geometries, TDDFT
calculations have been performed to obtain their excitation
properties.
8 (a) H. Kojima and A. J. Bard, J. Am. Chem. Soc., 1975, 97, 6317; (b)
W. R. Fawcett and A. Lasia, J. Phys. Chem., 1978, 82, 1114; (c)
R. A. Petersen and D. H. Evans, J. Electroanal. Chem., 1987, 222,
129; (d) B. G. Chauhan, W. R. Fawcett and A. Lasia, J. Phys.
Chem., 1977, 81, 1476; (e) W. R. Fawcett, M. Fedurco and
M. Opallo, J. Phys. Chem., 1992, 96, 9959; (f) B. Kwiatek and
M. K. Kalinowski, J. Electroanal. Chem., 1987, 226, 61; (g)
C. Kraiya, P. Singh and D. H. Evans, J. Electroanal. Chem., 2004,
563, 203; (h) A. Kapturkiewicz and M. Opallo, J. Electroanal.
Chem., 1985, 185, 15; (i) G. Pezzatini and R. Guidelli,
J. Electroanal. Chem., 1979, 102, 205.
Conclusion
A new series of isophthalate-based electrochromic materials were
prepared. The functional groups at the 5-position of isophthalate
have a significant influence on the observed color. In particular,
the electrochemically active nitro group induced multi-color,
indicating that further electrochromic properties could be
manipulated using more diverse 5-substituted isophthalate
derivatives.
Acknowledgements
This work was supported by the Samsung Research Fund,
Sungkyunkwan University, 2007. JHK is grateful for a grant
from the Korean Research Foundation (MOEHRD) (KRF-
2005-005-J11901). JYL acknowledges the financial support by
the Korea Science and Engineering Foundation (KOSEF) grant
funded by the Korean Government (MEST) (No. R11-2008-012-
03002-0).
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´
´
´ˇ
9 (a) P. Morkovska, M. Hromadova, L. Pospısil and S. Giannarelli,
Langmuir, 2006, 22, 1896; (b) C. R. Worthing and D. Phil, The
Pesticide Manual, British Crop Protection Council, Hampshire,
UK, 1987.
10 G. L. McIntire, J. Electrochem. Soc., 1981, 128, 427C.
11 In N-methylpyrrolidone (NMP) solution, blue, then red color
appeared at 3.11 and 4.40 V (DC voltage).
12 (a) A. A. Argun, P.-H. Aubert, B. C. Thompson, I. Schwendeman,
C. L. Gaupp, J. Hwang, N. J. Pinto, D. B. Tanner,
A. G. MacDiarmid and J. R. Reynolds, Chem. Mater., 2004, 16,
4401; (b) G. Sonmez, C. K. F. Shen, Y. Rubin and F. Wudl,
Angew. Chem., Int. Ed., 2004, 43, 1498; (c) N. Kobayashi, S. Miura,
M. Nishimura and H. Urano, Sol. Energy Mater. Sol. Cells, 2008,
References
1 M. Gra¨tzel, Nature, 2001, 409, 575.
2 D. R. Rosseinsky and R. J. Mortimer, Adv. Mater., 2001, 13, 783.
3 (a) P. Anderson, R. Forchheimer, P. Tehrani and M. Berggren,
Adv. Funct. Mater., 2007, 17, 3074; (b) R. J. Mortimer, A. L. Dyer
and J. R. Reynolds, Displays, 2006, 27, 2; (c) S. A. Sapp,
G. A. Sotzing and J. R. Reynolds, Chem. Mater., 1998, 10, 2101;
92, 136; (d) S. Hunig, M Kemmer, H. Wenner, F. Barbosa,
¨
G. Gescheidt, I. F. Perepichka, P. Ba¨uerle, A. Emge and K. Peters,
Chem.–Eur. J., 2000, 6, 2618.
13 H. Tsutsumi, Y. Nakagawa and K. Tamura, Sol. Energy Mater. Sol.
Cells, 1995, 39, 341.
´
(d) S. Beaupre, J Dumas and M. Leclerc, Chem. Mater., 2006, 18,
4011.
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 4408–4413 | 4413