Synthesis and properties of new electrochromic derivatives of 3-aryl-4,5-bis(pyridin-4-yl)oxazole

Article abstract of DOI:10.1007/s11172-019-2593-1

A series of new electrochromic derivatives of 3-aryl-4,5-bis(pyridin-4-yl)isoxazole was synthesized. Their electrochemical characteristics were studied by cyclic voltammetry, and a low eff ect of the substituents in the isoxazole and pyridinium cycles on the positions of the oxidation and reduction peaks was shown. The electrochromic cells prepared on the basis of the synthesized compounds were reversibly colored into brown upon the application of a voltage of 1.5 V. The spectral properties of the synthesized compounds under the electric field application and their stability for cycling from 0 to 2 V were studied.

Full text of DOI:10.1007/s11172-019-2593-1

Russian Chemical Bulletin, International Edition, Vol. 68, No. 8, pp. 1565—1569, August, 2019
1565
Synthesis and properties of new electrochromic derivatives
of 3-aryl-4,5-bis(pyridin-4-yl)oxazole
K. A. Chudov,a K. S. Levchenko, N. O. Poroshin, A. V. Shchegol´kov, P. S. Shmelin, and E. P. Grebennikov
a
a
b
a
a
aCentral Research Technological Institute "Tekhnomash" (JSC "Tekhnomash"),
4
ul. Ivana Franko, 121108 Moscow, Russian Federation.
E-mail: k4udov@gmail.ru
b
Tambov State Technical University,
1
06 ul. Sovetskaya, 392000 Tambov, Russian Federation
A series of new electrochromic derivatives of 3-aryl-4,5-bis(pyridin-4-yl)isoxazole was
synthesized. Their electrochemical characteristics were studied by cyclic voltammetry, and a low
effect of the substituents in the isoxazole and pyridinium cycles on the positions of the oxidation
and reduction peaks was shown. The electrochromic cells prepared on the basis of the synthe-
sized compounds were reversibly colored into brown upon the application of a voltage of 1.5 V.
The spectral properties of the synthesized compounds under the electric field application and
their stability for cycling from 0 to 2 V were studied.
Key words: electrochromism, isoxazoles, nitrile oxides, 1,3-dipolar cycloaddition.
Electrochromic materials are interesting for research-
ers in different fields because of the possibility to construct
from them a wide range of devices with low power con-
obtained by refluxing mesitonitrile oxide 4а with com-
pounds 3a,b under argon exhibits two doublets at  6.71
and 5.34 with spin-spin coupling constants of 3.9 Hz,
1
—4
sumption: units of information imaging,
"smart" win-
which correspond to the protons in positions 4 and 5 of
5
—9
10,11
14
21—23
dows,
antiglare mirrors for automobiles,
"chame-
the isoxazoline cycle of derivatives 5.
Oxidative
1
2,13
leon" type materials,
chemical sensors, molecular
aromatization with the formation of the target isoxazoles
6a—c occurred when this reaction was carried out in air.
Unsubstituted 1,2-bis(4-pyridinyl)ethylene 2 turned out
to be less reactive toward nitrile oxides 4 than N,N-alkyl-
pyridinium salts 3a,b with the electron-deficient double
bond, which was shown for its reaction with compound
4а. The 1,2-bis(4-pyridinyl)acetylene derivatives behave
machines, and organic current sources.15
The conjugated pyridine derivatives, among which are
primarily viologens, are characterized by high cyclicity, low
working potentials, and other valuable properties. Full-
color RGB (abbreviation RGB comes from the initials of
the three main colors (red, green, and blue) used together
to reproduce a broad array of colors) devices were fabri-
2
4
similarly in the Diels—Alder reaction.
16—19
cated
on the basis of these compounds. However, the
We studied the cycloaddition of nitrile oxides 4a,b to
–
–
range of these devices is rather narrow. The solution of a num-
ber of problems requires a broad variety of colors, which
causes, in turn, search for new functional compounds.
This work is aimed at preparing earlier unknown de-
rivatives of 3-aryl-4,5-bis(pyridin-4-yl)isoxazoles and at
studying their electrochromic properties.
compounds 3 containing I and PF as counterions (see
6
Scheme 1). We failed to isolate products 6 because of their
poor crystallization when hexafluorophosphates of com-
–
–
pounds 3 (X = PF ) with excess of nitrile oxides 4 were
6
refluxed in MeCN in air. The method based on the reac-
–
–
tion of diiodides 3 (X = I ) with excess of nitrile oxides
4 in MeCN containing 10% of water for the dissolution
of the starting salts turned out to be more convenient.
Under these conditions, isoxazoles 6a—c (in the form
of hexafluorophosphates) were isolated in good yields
(45—61%). The following method was used to remove
excess of nitrile oxides 4 and to isolate products 6: the
residue obtained after acetonitrile evaporation was dis-
solved in water, excess of nitrile oxides 4 was extracted
with ethyl acetate, and products 6 were precipitated by the
addition of the aqueous phase to a solution of ammonium
hexafluorophosphate.
1
,2-Bis(4-pyridinyl)ethylene (2) was synthesized by the
reaction of 4-picoline with 4-pyridinecarbaldehyde (1) in
acetic anhydride20 (Scheme 1). Compounds 3a,b were ob-
tained in high yields (86—90%) by the N-alkylation of bis-
(
pyridine)ethylene 2 with an alkylating agent excess with
stirring in MeCN, while in CH Cl the reaction terminated
2
2
at the stage of formation of a mixture in which the mono-
alkylation product prevailed ( H NMR spectroscopy data).
1
The reactivity of the 1,2-bis(4-pyridinyl)ethylene
derivatives 3a,b toward nitrile oxides 4а,b (Scheme 1) was
studied. The H NMR spectrum of the reaction mixture
1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1565—1569, August, 2019.
066-5285/19/6808-1565 © 2019 Springer Science+Business Media, Inc.
1

1566
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 8, August, 2019
Chudov et al.
Scheme 1
X– = I , PF6–
–
3
4
5
: R1 = Bu (a), Me (b)
: R = R = Me (a); R = Cl, R3 = H (b)
2
3
2
, 6: R1 = Bu, R = R = Me (a), R = Cl, R = H (b); R1 = Me,
2
3
2
3
2
= Cl, R3 = H (c); R = R = R = Me (d)
1
2
3
R
It should be mentioned that the complete conversion
tion. The two-electron reduction with an increase in the
voltage results in the formation of structure 8. This is
confirmed by two reversible peaks in the CV curves of the
obtained compounds (Fig. 1, Table 1). The redox poten-
tials of derivatives 6а—с differ by 20—40 mV (see Table 1),
indicating a weak effect of the alkyl groups at the nitrogen
atom and aryl substituents in the isoxazole cycle on the
electrochemical processes.
The changes in the electronic absorption spectra of
compounds 6a—c at a constant applied voltage of 1.5 V
are shown in Fig. 2. It is seen that the absorption intensity
of compounds 6а and 6b in a range of 350—600 nm and
that of compound 6c at 420—500 nm increase in time.
The stability of compounds 6a—c in the cells for cycling
from 0 to 2 V was studied. Compound 6а (Fig. 3) with the
of the starting salt 3b was not achieved even after refluxing
1
with nitrile oxide 4а for 80 h. According to the Н NMR
spectroscopy data, the ratio of the starting salt of bis-
(
pyridine)ethylene 3b and target product 6d in the reaction
mixture was 1 : 1. We failed to isolate compound 6d.
The electrochemical properties of isoxazoles 6a—c
were studied by cyclic voltammetry (CV) using the standard
three-electrode scheme at room temperature under argon.
Upon the application of a voltage of 1.5 V, the electro-
chromic devices based on synthesized compounds 6a—c
1
5
reversibly turned brown. By analogy to viologens, we
assume that the formation of resonance-stabilized radical
cation 7, as shown in Scheme 2, is a reason for the ap-
pearance of coloration of the cells upon voltage applica-
Scheme 2

Electrochromic isoxazole derivatives
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 8, August, 2019
b
1567
a
c
E/mV–1700
–1000
–300
E/mV–1700 –1000
–300
E/mV–1700
I/A
–1000
–300
I/A
I/A
–
–
50
–50
–50
100
–100
–100
Fig. 1. CV curves of compounds 6а (a), 6b (b), and 6c (c) in 0.1 М LiClO /MeCN, glassy carbon. The potentials are indicated rela-
4
tive to the silver chloride electrode (Ag/AgCl/(sat. KCl).
Table 1. Potentials of the oxidation (Eox) and reduction (Ered
)
mesityl moiety showed the best result among derivatives
6a—c. For compound 6а, the absorption intensity at 450
and 500 nm after 40 cycles of voltage application decreased
by 11 and 19%, respectively, whereas the decrease in the
absorption intensity at the same wavelengths by 20 and
peaks for compounds 6a—c in 0.1 М LiClO /MeCN*
4
Compound
–Ered1
–Ered2
–Eox1
–Eox2
mV
2
5%, respectively, was observed for compound 6b. For
compound 6c, after 80 cycles, the absorption intensity at
50 and 500 nm increased in both the colored (by 7—8%)
6
6
6
а
b
c
725
699
681
863
849
854
574
578
595
784
754
776
4
and decolorized states (by 20—24%). This was visualized
as yellowing of the electrochromic cell. The decrease in
the absorption intensity about 550 nm did not exceed 7%
*
A glassy carbon electrode as the working electrode and a silver
chloride electrode (Ag/AgCl/(sat. KCl) as the reference electrode.
D (arb. units)
.6
a
D (arb. units)
0.6
b
D (arb. units)
.0
c
0
0
0
1
0
0
0
0
.8
.6
.4
.2
.4
.2
0.4
0.2
4
00
500
600 /nm
400
500
600 /nm
450
550
650 /nm
Fig. 2. Changes in the optical density of the electrochromic cells based on compounds 6a (a), 6b (b), and 6c (c) in time at a permanent
applied voltage of 1.5 V. The spectra were recorded in a range of 0—3 s with the interval between measurements 0.33 s.
Note. Figures 2 and 3 are available in full color on the web page of the journal (https://link.springer.com/journal/volumesAnd
Issues/11172).
D (arb. units)
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
4
50 nm
500 nm
550 nm
2
00
400
600
800
t/s
Fig. 3. Cyclic changes in the optical density of compound 6а at 450 (1), 500 (2), and 550 nm (3) in time.

1568
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 8, August, 2019
Chudov et al.
for all compounds. It is most likely that the obtained results
show the influence of the nature of the aryl substituent in
the isoxazole ring.
through the cell testing radiations were detected on an S150-2-
1
2
024USB spectrometer (SOLAR TII) in the wavelength range
30—730 nm. The spectral absorbance coefficient was the stud-
ied characteristic of the cells and was determined as decimal
logarithm of the ratio of the testing radiation passed through the
cell to the incident radiation.
Thus, we developed the method for the synthesis of
electrochromic derivatives of 3-aryl-4,5-bis(pyridin-4-yl)-
isoxazole. The electrochemical properties and the changes
in the spectral properties upon the application of perma-
nent voltage were studied. The synthesized compounds
reversibly changed color to brown thus supplementing the
palette of colors known for bipyridines. The synthesized
isoxazole derivatives can be of interest for the production
from them of devices with low power consumption, for
example, units of information imaging, "smart" windows,
antiglare mirrors for automobiles, and chameleon type
materials.
4
,4-(Ethene-1,2-diyl)bis(1-butylpyridinium) iodide (3а).
1
,2-Bis(4-pyridinyl)ethylene (2) (2.1 g, 10 mmol) was dissolved
in MeCN (30 mL), iodomethane (8 mL, 18.2 g, 128 mmol) was
added, and the mixture was refluxed for 10 h. The product was
filtered off and washed with acetonitrile. The yield was 5.7 g
1
(
90%). M.p. 286 С (decomp.). H NMR, : 9.17 (d, 4 H,
J = 6.6 Hz); 8.41 (d, 4 H, J = 6.6 Hz); 8.22 (s, 2 H); 4.61 (t, 4 H,
J = 7.3 Hz); 2.04—1.79 (m, 4 H); 1.53—1.20 (m, 4 H); 0.93
1
3
(
t, 6 H, J = 7.3 Hz). C NMR, : 150.85, 145.56, 134.41, 125.93,
0.64, 32.97, 19.24, 13.80. Found (%): C, 43.71; H, 5.06.
C20H28N I . Calculated (%): C, 43.66; H, 5.13.
6
2
2
4
,4-(Ethene-1,2-diyl)bis(1-methylpyridinium) iodide (3b).
,2-Bis(4-pyridinyl)ethylene (2) (1.8 g, 10 mmol) was dissolved
1
Experimental
in MeCN (30 mL), iodomethane (8 mL, 18.2 g, 128 mmol) was
added, and the mixture was stirred at room temperature for 72 h.
The product was filtered off and washed with acetonitrile. The
1
Н NMR spectra were recorded on Bruker AC-200 (200 MHz)
1
3
1
and Bruker AM-300 (300 MHz) instruments. С NMR spectra
were run on a Bruker AM-300 (75 MHz) instrument using the
signals of the residual protons and carbon atom of the solvent as
the internal standard. Mass spectra were obtained on a Varian
MAT CH-6 instrument with the direct injection of a sample into
the radiation source with an ionization energy of 70 eV and
a control voltage of 1.75 kV. Melting points were measured on
the Boetius heating stage and were not corrected. TLC on the
Merck Silica gel 60 F254 UV-254 plates was used to analyze the
reaction mixtures and monitor purity of the isolated compounds.
,2-Bis(4-pyridinyl)ethylene (2) was synthesized according to
the patent. Mesitonitrile oxide was prepared using the known
procedure. 2,6-Dichlorobenzonitrile oxide was synthesized
similarly from 2,6-dichlorobenzaldehyde.
yield was 4 g (86%). M.p. 316 С (decomp.). H NMR, : 9.04
(d, 4 H, J = 6.6 Hz); 8.37 (d, 4 H, J = 6.6 Hz); 8.16 (s, 2 H);
1
3
4.37 (s, 6 H). C NMR, : 150.52, 146.34, 134.21, 125.54, 48.20.
Found (%): C, 35.9; H, 3.52. C14H16N I . Calculated (%):
2
2
C, 36.08; H, 3.46.
Synthesis of compounds 6a—c (general procedure). Compound
3 (0.45 mmol) was dissolved in acetonitrile containing 10% of
water, the corresponding nitrile oxide 4 (1.35 mmol) was added,
and the mixture was refluxed with stirring. The reaction course
1
was monitored by H NMR spectroscopy. The reaction mixture
1
was evaporated, distilled water was added, and the organics were
extracted with ethyl acetate. The aqueous layer was poured into
a solution containing threefold excess of ammonium hexafluoro-
phosphate. The precipitate was filtered off and recrystallized from
water containing 10% of acetone.
2
0
2
5
Electrochromic devices were assembled on the basis of the
synthesized compounds using two indium—tin oxide (ITO)
electrodes on a poly(ethylene terephthalate) (PET) flexible sup-
port (resistance 18 Ohm cm– ) glued along the perimeter with
two-sided Scotch tape. The electrodes were prewashed with
distilled water and acetone and dried. A solution of the electro-
4,4´-[3-(2,4,6-Trimethylphenyl)isoxazole-4,5-diyl]bis(1-butyl-
pyridin-1-ium) hexafluorophosphate (6а). The reaction time was
2
1
50 h. The yield was 45%. M.p. 195—197 С. H NMR, : 9.19
(d, 2 H, J = 6.3 Hz); 9.00 (d, 2 H, J = 6.2 Hz); 8.43 (d, 2 H,
J = 6.3 Hz); 7.84 (d, 2 H, J = 6.3 Hz); 7.03 (s, 2 H); 4.69 (t, 2 H,
J = 7.2 Hz); 4.55 (t, 2 H, J = 7.2 Hz); 2.30 (s, 3 H); 2.00 (br.s,
6 H); 1.99—1.79 (m, 4 H); 1.42—1.21 (m, 4 H); 0.93 (m, 6 H).
–
1
lyte contained 30 mmol L of the studied compound 6а—с,
–
1
–1
1
0 mmol L of ferrocene, and 20 mmol L of tetrabutylam-
1
monium hexafluorophosphate in anhydrous propylene carbonate.
The electrolyte was introduced into the pre-assembled device
using a syringe with a thin needle.
H NMR, : 163.18, 162.58, 146.20, 145.88, 144.97, 140.53,
140.33, 137.51, 129.12, 128.77, 127.78, 126.30, 122.55, 117.49,
61.23, 60.95, 33.22, 32.95, 21.21, 20.04, 19.23, 19.18, 13.77.
Found (%): C, 48.42; H, 5.06. C H N OP F . Calculated (%):
Cyclic voltammetry. Measurements were carried out under
argon on a P-8nano potentiostat (JSC "Elins," Russia) at 25 С
3
0
37
3
2 12
C, 48.33; H, 5.00.
in a 0.1 М solution of LiClO in acetonitrile using a glassy carbon
4,4´-[3-(2,6-Dichlorophenyl)isoxazole-4,5-diyl]bis(1-butyl-
4
electrode as the working electrode, a platinum wire as the coun-
ter electrode, and a silver chloride electrode (Ag/AgCl/KCl) as
the reference electrode.
pyridin-1-ium) hexafluorophosphate (6b). The reaction time was
1
36 h. The yield was 56%. M.p. 146—147 С. H NMR, : 9.20
(d, 2 H, J = 5.6 Hz); 9.07 (d, 2 H, J = 5.9 Hz); 8.47 (d, 2 H,
J = 6.0 Hz); 7.98 (d, 2 H, J = 5.5 Hz); 7.72 (br.s, 3 H); 4.69
(t, 2 H, J = 7.3 Hz); 4.58 (t, 2 H, J = 7.3 Hz); 1.91 (m, 2 H);
Measurement of electrochromic effect. The cell was switched
by the two-electrode scheme to a P-8nano potentiostat-galva-
nometer (JSC "Elins"). The measurements were conducted with
an applied potential of ±2 V in the programmer mode with the
potentials +2/0/–2/0 V and a duration of 3/17/3/16 s, respec-
tively. The voltage application onto the cell changed its color.
An OZK DG instrument (lighter with halogen and deuterium
lamps and mirror condenser produced by LOMO FOTONIKA)
served as a source of testing radiation. The incident and passed
1
3
1.33 (m, 2 H); 1.03—0.82 (m, 3 H). C NMR, : 163.35,
159.60, 146.24, 144.10, 139.80, 135.07, 134.51, 129.56, 128.01,
126.25, 124.52, 117.65, 61.26, 61.13, 32.98, 19.21, 19.16, 13.77.
Found (%): C, 41.95; H, 3.81. C H Cl N OP F . Calculated (%):
27
29
2
3
2 12
C, 41.99; H, 3.78.
4,4´-[3-(2,6-Dichlorophenyl)isoxazole-4,5-diyl]bis(1-meth-
ylpyridin-1-ium) hexafluorophosphate (6c). The reaction time was

Electrochromic isoxazole derivatives
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 8, August, 2019
1569
1
2

2
4
1
1
4 h. The yield was 61%. M.p. 268—270 С. (decomp.). H NMR,
11. G. Sonmez, Chem. Commun., 2005, 42, 5251.
12. D. S. K. Mudigonda, D. L. Meeker, D. C. Loveday, J. M.
Osborn, J. P. Ferrariс, Polymer, 1999, 40, 3407.
: 9.08 (d, 2 H, J = 6.4 Hz), 8.98 (d, 2 H, J = 6.3 Hz), 8.37 (d,
H, J = 6.4 Hz), 7.93 (d, 2 H, J = 6.3 Hz), 7.78—7.61 (m, 3 H),
1
3
.43 (s, 3 H), 4.34 (s, 3 H). C NMR, : 163.31, 159.52,
47.12, 143.73, 139.42, 135.10, 134.43, 129.53, 127.75, 125.80,
24.55, 117.76, 48.65, 48.48. Found (%): C, 36.61; H, 2.51.
13. I. D. Brotherson, D. S. K. Mudigonda, J. M. Osborn, J. Belk,
J. Chen, D. C. Loveday, J. L. Boehme, J. P. Ferraris, D. L.
Meeker, Electrochim. Acta, 1999, 44, 2993.
C21H17Cl N OP F12. Calculated (%): C, 36.65; H, 2.49.
14. W. Xu, K. Fu, C. Ma, P. W. Bohn, Analyst, 2016, 141, 6018.
2
3
2
15. L. Striepe, T. Baumgartner, Chem. Eur. J., 2017, 23, 16924.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 18-33-01253).
16. G. Bar, N. Larina, L. Griniс, V. Lokshin, R. Gvishi,
I. Kiryuschev, A. Zaban, V. Khodorkovsky, Sol. Energy Mater.
Sol. Cells, 2012, 99, 123.
1
7. Pat. CN 104698714 A; Faming Zhuanli Shenqing, 2015.
8. R. J. Mortimer, J. Electrochem. Soc., 1991, 138, 633.
9. Y. Alesanco, A. Viñualeс, J. Palenzuela, I. Odriozola,
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