Negative Photochromism of Solutions of Functionalized Spiropyrans in a Water—Acetonitrile Mixture

Article abstract of DOI:10.1134/S1070363219090093

A spectral-kinetic study of photochromic functionalized nitro-substituted indoline spiropyrans in water-acetonitrile mixtures has allowed to observe for the first time the effect of negative photochromism due to the self-assembly of photochromic molecules in a mixed solvent with the formation of hydrogen bonds and proton complexes.

Full text of DOI:10.1134/S1070363219090093

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 9, pp. 1783–1786. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Obshchei Khimii, 2019, Vol. 89, No. 9, pp. 1375–1378.
Negative Photochromism of Solutions
of Functionalized Spiropyrans in a Water–Acetonitrile Mixture
T. M. Valovaa, V. A. Barachevskya, b*, A. A. Khuzinc, and A. R. Tuktarovc
a Photochemistry Center of the Federal Research Center “Crystallography and Photonics”
of the Russian Academy of Sciences, ul. Novatorov 7a/1, Moscow, 119421 Russia
*e-mail: barva@photonics.ru
b Interdepartmental Center for Analytical Research, the Presidium of the Russian Academy of Sciences, Moscow, Russia
c Institute of Petrochemistry and Catalysis of the Russian Academy of Sciences, Ufa, Russia
Received April 4, 2019; revised April 4, 2019; accepted April 12, 2019
Abstract—A spectral-kinetic study of photochromic functionalized nitro-substituted indoline spiropyrans in
water-acetonitrile mixtures has allowed to observe for the first time the effect of negative photochromism due to
the self-assembly of photochromic molecules in a mixed solvent with the formation of hydrogen bonds and proton
complexes.
Keywords: negative photochromism, absorption spectra, spiropyrans, complexes, hydrogen bond
DOI: 10.1134/S1070363219090093
Special attention in the search for possible application
of the photochromism phenomenon has been paid to the
systems exhibiting inverse photochromism, which is of
interest for the design of diverse photochromic materials,
for example, for elaboration of multifunctional clothes
and camouflage coatings with dynamic photoinduced
color changing [1, 2]. In continuation of our previous
studies [3–5], herein we report on the results of the
spectral-kinetic study of inverse photochromism of func-
tionalized nitro-substituted spiropyrans.
In this study, we investigated the indoline nitro-
substituted spiropyrans 1 and 2 (Scheme 2).
The experimental data collected in the table showed
that compound 1 in acetonitrile demonstrated positive
photochromism, the colorless cyclic form (Fig. 1, curve
1) being converted into the colored merocyanine form
(curve 2) under the UV irradiation and recovered into
the initial state after turning off the light (curves 3–6).
Asolution of compound 1 in a water–acetonitrile mix-
ture (1 : 1) initially was colorless as well and its spectrum
was similar to that of the initial solution in acetonitrile.
However, after 20 h of storage in the dark, the solution
turned colored (Fig. 2, curve 1) and the absorption band of
the merocyanine form of the spiropyran appeared, which
was shifted by 50 nm to the shortwave region with respect
to the absorption band in acetonitrile (see the table). The
formed colored solution demonstrated inverse photochro-
Positive photochromism is typical of nitro-substituted
spiropyrans [6]: the starting colorless form A exhibits
reversible photodissociation of the С–О bond under the
action of UV irradiation, followed by cis/trans isomeriza-
tion with the formation of colored merocyanine form B.
The latter is spontaneously relaxed into the initial cyclic
form A, the process being accelerated by visible light or
heating.
Scheme 1.
R3
R3
R4
R4
h
Q1
R1
R1
Gꢁ
R5
hȞ2 or ǻ
N
N
R2
O
R5
R2
Gꢀ
O
ꢀꢀȺꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ%
1783

1784
VALOVA et al.
Scheme 2.
H
H
3C
3C
CH3
O
CH3
O
N
N
NO2
NO2
O
O
O
OH
OEt
1
2
mism (Fig. 2), but the recovery of the studied compound
to the initial state occurred slower, within up to tens of
hours (Fig. 2, curves 5, 6). The concentration of the col-
ored product decreased during repeated photobleaching
and dark relaxation (cf. curves 1, 3, 6), and an absorption
band with maximum at 410 nm simultaneously appeared.
Remarkably, the shift of the equilibrium from the
cyclic form of spiropyran to the merocyanine structure
occurred during prolonged storage of the solutions in the
dark. Apparently, that process was due to self-assembly
and structurization of the mixed solution. The hypso-
chromic shift of the absorption band maximums might
be indicative of the formation of hydrogen bonding
between the molecules of spiropyran and water. The ob-
served decrease in the concentration of the merocyanine
Spiropyran 2demonstrated similar dark and photoin-
duced spectral changes, although less pronounced (see
the table).
Spectral characteristics of spiropyrans in solutiona
Compound
Solvent
CH3CN
λAmax, nm (DA
)
λBmax, nm (DBmax
)
Light filter
max
1
220 (0.48)
245 (0.45)
265(0.38)
300(0.18)
340(0.21)
560 (0.68)
UFS-1
CH3CN–H2O
220(0.32)
–
510 (0.00)
ZhS-10
(20 h in the dark)
265(0.17)
360(0.30)
510(0.38)
2
CH3CN
220(0.53)
240(0.52)
265(0.4)
568(0,58)
520(0,04)
UFS-1
335(0.24)
CH3CN–H2O
245(0.45)
270(0.37)
350(0.31)
520(0.11)
ZhS-10
(20 h in the dark)
а λA and λBmax are the maximums of adsorption bands of the initial А and longwave-photoinduced forms В of spiropyrans, respectively;
max
DАmax and DBmax are the values of optical density in the maxima of absorption bands of forms А and В, respectively. Solutions concentration
2×10–4 mol/L.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

NEGATIVE PHOTOCHROMISM OF SOLUTIONS OF FUNCTIONALIZED SPIROPYRANS
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λ, nm
λ, nm
Fig. 2. Absorption spectra of compound I in a solution in a
Fig. 1. Absorption spectra of compound 1 in acetonitrile be-
fore (1) and after UV irradiation (UFS-1 filter) (2) and during
dark relaxation (3–6).
water–acetonitrile mixture after 20 h storage in the dark (1),
after irradiation with visible light (ZhS-10 filter) (2, 4, 6),
and after subsequent dark relaxation for 72 (3) and 96 h (5).
form and the appearance of a new absorption band with
maximum at 410 nm during the repeated processes of
photobleaching under visible light and dark relaxation
could be explained by the formation of slightly light-
sensitive proton complexes.
chromatography eluting with a petroleum ether–ethyl
acetate mixture (5 : 1).
Spectral-kinetic measurements were performed in
acetonitrile (Aldrich) and its mixtures with bidistilled
water. Spectral-kinetic parameters of the solutions were
obtained using a Cary 60 bio spectrophotometer (Agilent
Technologies) in a 0.2 cm quartz cell, concentration of the
solutions being 2×10–4 М. The irradiation was performed
with an L8253 xenon lamp (LC-4 device, Hamamatsu)
equipped with UFS-1 (for UV irradiation) and ZhS-10
ZhS-11 (for visible light irradiation) filters, respectively.
In summary, in this study we observed for the first
time the phenomenon of formation of photochromic
systems with negative photochromism of spiropyrans in
water-acetonitrile mixed solvent and investigated it by
spectral-kinetic method. The revealed peculiarities were
explained by self-organization of photochromic mol-
ecules in the mixed solvent involving hydrogen bonding
and formation of proton complexes.
FUNDING
This study was financially supported by the Ministry
of Education and Science in the scope of the State Task for
Photochemistry Center “Crystallography and Photonics”
of RussianAcademy of Sciences (spectral-kinetic studies)
and RFBR (projects 18-33-20027 and MK-3058.2018.3)
(synthesis of compounds).
EXPERIMENTAL
2-[3',3'-Dimethyl-6-nitrospiro(chromene-2,2'-
indol)-1'(3'H)-yl]ethanol (1) was obtained as described
elsewhere [7]. Its spectral parameters fully coincided with
those described in the literature.
CONFLICT OF INTEREST
No conflict of interest was declared by the authors.
REFERENCES
2-[3',3'-Dimethyl-6-nitrospiro(chromene-2,2'-
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spiropyran 1 in 30 mL of dry methylene chloride in a
50 mL three-neck flask. The reaction mass was stirred at
room temperature for 2 h and then filtered. The solvent
was evaporated off, the residue was purified by column
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