A novel water-soluble multicolor halo- And photochromic switching system based on a nitrile-rich acceptor

Article abstract of DOI:10.1039/d1nj01046h

A directed synthesis of the first nitrile-rich dye showing visible-light-induced negative photochromism in aqueous medium was implemented. The possibility to change the color of an aqueous solution from yellow to magenta by varying the pH from 5 to 8 was

Full text of DOI:10.1039/d1nj01046h

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A novel water-soluble multicolor halo- and
photochromic switching system based on a
Cite this: New J. Chem., 2021,
45, 10287
nitrile-rich acceptor†
Mikhail Yu. Belikov,
Mikhail Yu. Ievlev * and Ivan N. Bardasov
A directed synthesis of the first nitrile-rich dye showing visible-light-induced negative photochromism in
aqueous medium was implemented. The possibility to change the color of an aqueous solution from
yellow to magenta by varying the pH from 5 to 8 was shown. The synthesized dye was found to be a
thermally unstable photochrome (T-type) whose photoinduced form returned to the initial state
following the first order kinetic equation in the dark at room temperature. It was possible to change the
dark reaction rate in a wide range and to block the photochromic process by pH regulation. The
1
1S
a
calculated pK
a
of the initial form was more than 2.5 units higher than the pK
of the photoinduced
Received 2nd March 2021,
Accepted 2nd May 2021
form, which indicated the photoacidity of the dye in aqueous medium. This allowed the pH of the
solution on 1.2 pH units to be reversibly changed under visible light irradiation, followed by dark
relaxation. The obtained results revealed a new direction in the investigation of nitrile-rich negative
photochromes, namely, the synthesis of water-soluble multicolor molecular switching systems sensitive
to several factors.
DOI: 10.1039/d1nj01046h
rsc.li/njc
among the well-studied groups of water-soluble photochromes.
1
. Introduction
1
0–13
Representatives of this group are known as photoacids,
1
4
Photochromic compounds are bistable structures whose physico- photoswitchable ligands of biomolecules, molecular devices
1
5
chemical properties can be regulated by irradiation with UV or that mimic triode action, compounds with photoresponsive
1
6
17
visible light. The possibilities for the practical use of photo- electrostatic or light-controlled self-assembly, water-soluble
18
19
chromes are extremely diverse. The following recent reviews polymers, molecules showing light-induced cytotoxicity, light-
describe the practical application of photochromes in nano- driven hydrogels and light-responsive metal–organic frameworks
systems, as optically functionalized organic field-effect transistors with high ON/OFF photoswitchable proton conductivity.
20
1
21
2
3
(OFETs), solar thermal fuels, multiaddressable photochromic
Spiropyran photochromes are well studied due to their
4
architectures, fluorophores at the molecular and nanoparticle universality for imparting a wide variety of physicochemical
5
6
22
levels and modern all-visible-light-triggered compounds. The properties to them. Other representatives of water-soluble
following reviews on the use of photochromic dyes for photo- photochromic molecules are also known. Thus, azobenzenes
7
23
control in biomedicine, photoswitches in smart materials and are promising as photoactive ultrathin films, light-responsive
8
24
25
biological systems and photoswitchable molecules in long- surfactants and photoswitchable drugs. Thioindigo derivatives
9
wavelength light-responsive drug delivery should also be men- are reported for reversible spatial control in aqueous media by
26
tioned as they indicate the importance of photochromes for visible light, diarylethene molecules are shown as photoswitch-
27
biological research.
able and water-soluble fluorescent nano-aggregates, as well as
7
–9
28
Review studies
show that the water-solubility of photo- biomarkers for super-resolution optical imaging, and fulgide
chromes is a highly desirable characteristic for their use in the derivatives are described in the ultrafast reactions of the water-
2
9
control of biological processes. In this regard, it should be noted soluble photoswitches. Fig. 1 shows examples of known water-
that the recent research trend is the intensification of studies on soluble photochromes. It should be noted that a possibility to
the creation of photochromic materials reversibly transformed use visible light for switching is crucial, as it is a harmless
by irradiation in aqueous medium. Spiropyran derivatives are and selective external trigger to regulate the properties of
6
various media and materials. In addition, most of the studies
describe the investigation of direct photochromes, compounds
Ulyanov Chuvash State University, Moskovsky pr., 15, Cheboksary, 428015, Russia.
whose color appears or deepens upon irradiation. Reverse
photochromes, which lose color under irradiation, are much
E-mail: hiliam@bk.ru
Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nj01046h
†
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34
one was also shown. It should be noted that, among the described
examples of NRP, there were no data on water-soluble species.
Thus, the purpose of this study was to synthesize the first
water-soluble representative of nitrile-rich negative photochromes
and to study the dependence of its color and photochromic
properties on the acidity of aqueous medium.
2
. Results and discussion
2.1 Synthesis and structure determination
The preparation of NRP was based on the reactions of salicylic
3
2–38
aldehyde derivatives with a cyano-substituted acceptor.
A
sulfonate moiety was one of the common fragments used to
impart water solubility to an organic molecule. For example,
the introduction of this moiety allowed the synthesis of a
number of water-soluble photochromes of the spiropyran
1
0,16
26
series
and thioindigo derivatives (Fig. 1).
3
9
At the initial stage, according to the known procedure we
synthesized azomethine 2 starting from salicylic aldehyde and
aniline. Then, sulfonation of compound 2 by concentrated
sulfuric acid to obtain an intermediate product 3 was carried
out. Further, the azomethine protection was removed by heating
compound 3 in the presence of sodium carbonate, which led to the
formation of the desired sodium salt of 5-sulfosalicylic aldehyde 4.
The tricyanofuran (TCF) acceptor was also prepared according to
the literature, starting from 3-hydroxy-3-methylbutan-2-one and
40
malononitrile. The target product 1 was synthesized in 87% yield
by the reaction of TCF with aldehyde 4 in the presence of a catalytic
amount of sodium acetate in ethanol under an argon atmosphere
under moderate heating (Scheme 1).
Fig. 1 Examples of known water-soluble photochromes, known nitrile-
The structure of compound 1 was considered with the data
rich photochromes and the object of the present study.
1
13
of H, C NMR and IR spectroscopy, mass spectrometry and
1
elemental analysis. Thus, the H NMR spectrum included a
characteristic pair of doublets at 7.21 and 8.12 ppm with a
coupling constant of 16.5 Hz. These signals corresponded to
protons appeared after the double bond formation. The proton
of the phenolic hydroxyl required for the photochromic reaction
appeared at 11.08 ppm.
3
0,31
less common.
This indicates a requirement of the intensi-
fication of research in this area.
The data described above demonstrate the importance of the
search for novel groups of water-soluble visible-light-switchable
photochromic dyes with wide possibilities for the regulation of
their color and other physicochemical characteristics.
After the analysis of the literature data, we started to study
the possibility of preparation of water-soluble photochromes
based on the poorly studied group of reverse photochromes.
Their essential structural part was the combination of a nitrile-
rich acceptor and a 2-vinylphenol fragment in the molecule
2.2 Study of absorption spectra in water at different pH values
As we expected, compound 1 was found to be water-soluble,
thus allowing us to carry out the investigation of its optical
absorption and photochromic behavior in aqueous medium,
3
2–38
(Fig. 1).
The first representative of this group of photo-
chromes was described in 2014; it included a tricyanofuran
3
2
moiety with a salicylic aldehyde fragment in the structure.
Later, the synthetic possibilities were expanded both for changing
34
the structure of the nitrile-rich acceptor and for introducing
35
various substituents into the aromatic moiety. These several
works showed the practical significance of photochromes of this
3
2,33
group, because the possibility for photoregulation of acidic
3
6
and fluorescence properties had been reported. An example
of a high-contrast photochromic transformation with 100%
Scheme 1 Directed synthesis of water-soluble nitrile-rich dye 1 and
conversion of the initial intensely colored form to a colorless intermediates.
1
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Fig. 2 shows a significant difference in absorption spectra
for different pH values. This indicates the sensitivity of com-
pound 1 to the changes of acid–base characteristics of the
medium. Thus, the absorption spectra were almost similar in
acidic media at pH 2–4. They were characterized with an
intensive band in the visible region of the spectrum centered
at 420 nm. The second maximum in the visible region (535 nm)
appeared starting from pH 6 and became dominant after pH 7,
accompanied by a decrease of the band at 420 nm. At higher pH
values, there was no significant change in the spectra. The
intensity of the band centered at 535 nm was almost constant
in the range of pH 8–10. Fig. 2 also included photos of solutions
of compound 1, showing significant color changes in the range
of pH 5–8.
We associated the presence of two absorption bands in the
visible region of the spectrum with phenol–phenolate equilibrium
ꢀ
1
" 1 (Scheme 2) which was strongly affected by pH. Thus, in
acidic media, phenolic form 1 prevailed with a maximum at
ꢀ
420 nm, while at neutral or basic pH values phenoxide form 1
with a maximum at 535 nm became dominant. According to the
constant absorbance at pH 8–10, we could conclude that com-
ꢀ
pound 1 was completely in the anionic form 1 at these pH values.
ꢀ
Relatively high molar absorptivity of both forms 1 and 1
ꢀ1
ꢀ1
Fig. 2 UV-Vis spectra and photos of compound 1 at different pH values was found to be 33 500 and 35 400 M ꢁ cm , respectively.
C = 2.5 ꢁ 10ꢀ M, phosphate buffer, 20 1C).
5
(
ꢀ
The red-shifted absorption band of the phenolate form 1 was
probably associated with the stabilization of an anion by
ꢀ
resonance structures, namely, quinoid form 1 Q and resonance
which was the first time for the compounds of the NRP series
Fig. 1). We registered the absorption spectra for the solutions
of compound 1 in phosphate buffers at constant pH from 2.08
ꢀ
hybrid 1 H (Scheme 2). The presence of an isosbestic point at
72 nm shown in Fig. 2 supported the existence of a reversible
equilibrium between two forms. It should be noted that forms
similar to 1 and 1 Q were described in the literature for
pH-sensitive analogues of compound 1 containing the hydroxyl
(
4
to 9.94 (Fig. 2, Table 1).
ꢀ
ꢀ
4
1
Table 1 Photochromic parameters of compound 1 in aqueous media at group attached to the para-position of the phenyl ring.
different pH values
2
.3 Determination of the acidity constant of the initial form
l
abs
l
PSS
absa(nm)
Rate constant, Half-life, PC c
b
ꢀ1
The absorption spectra of dye 1 (Fig. 2) registered at different
pH (nm)
A
max
A
max
k (s
)
t1/2 (s)
(%)
ꢀ
ꢀ
ꢀ
4
3
pH values allowed us to estimate the logarithmic acidity con-
2
3
4
.08 422
.08 420
.08 420
0.819 413
0.828 414
0.836 310
0.451 3.2 ꢁ 10
0.451 2.9 ꢁ 10
0.304 2.06 ꢁ 10
0.377
2166
239
33.6
0.45
0.46
1
stant pK
a
. Fig. 3 shows absorbances at 420 nm and 535 nm,
2
ꢀ
0.55 corresponding to forms 1 and 1 , respectively, plotted against
4
15
0.818 310
15
0.783 310
15
0.439 3.71 ꢁ 10ꢀ2
0.577 5.01 ꢁ 10ꢀ
0.717 5.88 ꢁ 10ꢀ
0.736 6.00 ꢁ 10ꢀ
pH. An interception of the curves at pH 6.81 indicated the same
4
5
6
6
.54 420
.03 421
.03 419
35
.50 415
35
18.7
13.8
11.8
11.5
0.66
concentration of both forms. This pH value corresponded to an
4
0.277
2
1
1
0.79 approximate pK
a a
of compound 1. To refine the obtained pK
4
0.162
2
value, we used eqn (1):
0.679 310
0.143 415
0.567 310
0.301 415
0.90
0.87
ꢀ
ꢁ
5
0.070
1
A ꢀ Ai
2
1
lg
¼ pH ꢀ pKa
(1)
ꢀ
i
1
Ai ꢀ A
5
0.072
0.038
530
1
1ꢀ
0.657 6.97 ꢁ 10ꢀ
2
10.0
0.78
where A is the absorbance of phenolic form 1, A is the
absorbance of phenoxide 1 and A
solution in respective buffers with pH
7
.02 410
36
0.370 310
0.509 410
ꢀ
5
0.079
0.134
is the absorbance of the
. For the best results,
i
5
35
0.684 310
35
0.341 6.91 ꢁ 10ꢀ2
i
7
.99 536
10.0
0.26
l = 535 nm was chosen, because of the greatest difference
between the absorbance curves of the acidic and basic solutions.
The obtained linear regression is shown in Fig. 3B. The inter-
5
—
—
0.500
—
—
8
9
.95 537
.94 537
0.705
0.702
—
—
—
—
—
—
a
b
Spectral data registered at photostationary state (PSS). Half-life of ception of the fitted line with the x-axis at pH 6.6 corresponds to
c
the reverse thermal reaction was found to be 0.693/k. Photoconversion
PC) was estimated as a percentage of the decrease of the main long-
wavelength absorption band (at 420 nm or 535 nm) in the photo-
stationary state.
1
the pK
a
value of compound 1.
(
The analysis of the phenol–phenolate equilibrium, as well as
1
the determined pK
a
value, confirmed that the investigated dye
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Scheme 2 Acid–base equilibrium and resonance structures of compound 1.
ꢀ
Fig. 3 Plot of absorbance at wavelengths of 420 nm and 535 nm vs. pH (A), linear regression of logarithmic absorbance ratio of 1 and 1 forms vs. pH (B).
1
could be used as an aqueous pH indicator with maximum (PC) of compound 1 into the photoinduced spiro-form 1S was
sensitivity in the range of pH 5–8.
observed depending on pH (Scheme 3, Table 1).
Thus, for the solutions at pH 2–4 a 45–55% decrease of the
maximum at 420 nm was observed under irradiation. When
increasing the pH value, the conversion degree also increased,
2
.4 Investigation of the photochromic properties in water at
different pH values
An investigation of the photochromic properties of compound reaching a maximum of about 90% at pH 6–6.5. It should be noted
1
in an aqueous medium as well as study of the medium acidity that at almost neutral media of pH 6–7, where comparable amounts
ꢀ
effect on the photochromic behavior is of considerable scientific of phenolic 1 and quinoid 1 forms were observed (420 nm and
interest. The results are promising for the creation of a multi- 535 nm maxima), a similar decrease of both visible bands was
color dye 1 whose color can be adjusted not only by changing registered under irradiation. In the pH range of 7–8, a slight
the pH, but also by irradiation.
The prepared solutions of compound 1 at different pH media (pH 9–10) photochromism was not practically registered.
values from 2.08 to 9.94 were studied. The first absorption scan As an example, Fig. 4 (A and B) show the dynamics of spectral
decrease in the degree of photoconversion was recorded. In basic
was carried out in the dark conditions to obtain the initial changes in the dark after the irradiation of solutions with pH 5.03
spectrum (marked as ‘before irradiation’ in Fig. 4). The second and 7.02. Fig. 4C also shows the photos of solutions of compound
scan was carried out under light irradiation to register the 1 at pH 5.03 and 7.02, as well as the corresponding color changes
spectrum of the photo-stationary state (PSS). Then, irradiation under irradiation by visible light. Similar transitions were observed
was stopped, and scans were continued in the dark at the for solutions at other pH values. Thus, we could conclude that
maximum possible rate for fast reverse reactions (at pH 4–8) compound 1 was a hybrid multicolor water-soluble molecular
and at a medium rate for the slower ones (below pH 4). It was switch whose spectral properties were tunable both by the acid–
found that compound 1 underwent a photochromic transfor- base characteristics of the medium (halochromism) and by visible
mation in a wide pH range. A different photoconversion degree light irradiation (photochromism).
1
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Fig. 5 Cyclicity of photochromic transformation of compound 1 at pH
.03 (C = 2.5 ꢁ 10ꢀ M, 20 1C).
5
6
fatigue resistance upon sequential irradiation by visible light and
storage in the dark after ten cycles. Fig. 5 shows a plot of
absorbance changes of solution 1 at 420 nm and pH 6.03 before
and after irradiation.
As we could see from Fig. 5 the absorbance both before and
after irradiation remained unchanged during cycles. Cyclicity at
other pH values was also found to be excellent (ESI†).
2.5 Influence of pH on the rate of dark thermal reaction
Table 1, as well as Fig. 4A and B, shows that compound 1 was a
thermally unstable photochrome (T-type). Its solutions irradiated
by visible light restored the initial spectral pattern when being
stored in the dark at 20 1C. Fig. 6 and 7 show the change of
absorbance at 420 nm and the logarithm of the concentration of
the photoinduced form 1S for solutions at pH 3.08 (Fig. 6) and
Fig. 4 Absorption spectra of compound 1 at pH 5.03 (A) and pH 7.02 (B) 6.50 (Fig. 7) plotted against time.
and photos of these solutions (C) before and after irradiation (C = 2.5 ꢁ
Fig. 6 and 7 show that the reaction order was similar at
ꢀ
5
1
0
M, 20 1C).
different pH values, which might indicate the same mechanism of
the process in media with different acid/base parameters (Scheme 3).
A more thorough analysis on the kinetics of the dark thermal
An important and practically significant parameter of photo-
chromic systems is the cyclicity of phototransformations. This transformation of 1S into 1 (Scheme 3) allowed the relationship
process was investigated at different pH values from 2.08 to 7.99. between the acid/base characteristics of the medium and
Compound 1 showed good cyclicity of phototransformations and the half-life period (t1/2) of the photoinduced form 1S to be
Scheme 3 Possible photochromic transformations of compound 1.
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Fig. 6 Kinetics of the reverse thermal transformation for compound 1 in
2
H O at pH 3.08 (A), the plot of fitting the concentration of photo-induced
ꢀ5
form [1S] in a first-order kinetic equation (B) (C = 2.5 ꢁ 10 M, phosphate
buffer, 20 1C).
Fig. 8 Dependence of the negative logarithm of the dark reaction rate
constant on the pH value (C = 2.5 ꢁ 10ꢀ M, 20 1C).
5
ꢀ
existence of a similar equilibrium as 1S " 1S was described
3
3
in previous studies of NRP derivatives. The evidence for the
ꢀ
1
3
S
form of compound 1 was the absorption band centered at
3
3,43
10 nm (Fig. 4) according to previous work.
pH effect on the 1S " 1S equilibrium had not been described
prior to this study.
However, the
ꢀ
The fact that the lowest reverse reaction rate was observed at
pH 2–3 supported our assumption, since in strongly acidic
Fig. 7 Kinetics of the reverse thermal transformation for compound 1 in
H
2
O at pH 6.50 (A), the plot of fitting the concentration of photo-induced media the dissociation of CH-acid 1S must be suppressed
ꢀ5
form [1S] in a first-order kinetic equation (B) (C = 2.5 ꢁ 10 M, phosphate (Scheme 3). As it followed from the absorption spectra of the
buffer, 20 1C).
photoinduced form at pH 2–3, there was no clear maximum at
310 nm in PSS (ESI†), which was a proof for the suppression of
the 1S dissociation.
revealed (Table 1). Table 1 shows that the half-life period of 1S
At the same time, a comparable dark reaction rate at pH 6–8
was significantly longer at pH 2–3 than that at pH 5–8. Upon might indicate the complete ionization of the CH-acid. In other
changing the pH from 2.08 to 4.08, the half-life of the dark words, the maximum dark reaction rate was achieved with the
reaction changed from 35 minutes to 0.5 minutes. At pH 4.5–5 complete dissociation of the 1S form.
the reaction rate changed insignificantly and in the range of
At the same time photochromism was almost not observed
pH 6–8 it remained almost similar (t1/2: 10–11 s). This signifi- at pH 9–10. To explain this fact, it was necessary to consider the
cant change in the dark thermal reaction rate upon pH changes pH effect on the acid–base equilibrium of the initial form 1.
1
is clearly demonstrated in Fig. 8, showing the negative loga- According to the estimated pK
a
of compound 1 at pH 9–10 the
ꢀ
rithm of the reverse reaction rate constant k plotted against pH. anionic form 1 predominated in the solution (Scheme 2).
There were two almost linear sections which could be Therefore, we could conclude that the photochromic reaction
distinguished on the curve in Fig. 8. In the first section (in occurred through the phenolic form 1, while the phenoxide
ꢀ
the range of pH 2–4), there was a sharp increase of the reverse form 1 was apparently non-photochromic.
reaction rate, while in the second one (in the range of pH 6–8)
The observed dark reaction kinetics showed that by varying
the reaction rate constant was almost similar. To explain these the pH we could significantly change the reverse reaction rate,
results, it was necessary to consider the structure of the meta- accompanied by multicolor transformations (Fig. 2). Increasing
stable photoinduced form 1S and the effect of the acid–base the pH to 9–10 also allowed the photochromic process to be
parameters of the medium in details.
blocked.
From the obtained data, we could conclude that the investi-
The 1,3,3-tricyanopropene fragment of 1S (Scheme 3) con-
ꢀ
tained a mobile proton. Anion 1S formed after deprotonation gation of NRP photochromes must be associated with the study
was efficiently stabilized by delocalization on three conjugated of the effect of the acid–base characteristics of the medium on
cyano groups. Therefore, the 1S structure exhibited enhanced the ionization of both the initial 1 and photoinduced 1S forms.
CH-acidic properties. In our opinion, the process of dark In this regard, to obtain more accurate data on the tuning
thermal transformation to the initial form 1 proceeded through of photochromic behavior of compound 1, it was necessary
ꢀ
ꢀ
1S
a
this anionic form 1S . The greater amount of the anion 1S led to estimate the acidity constant pK
of the photoinduced
to the higher rate of dark reaction. It should be noted that the form.
1
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2.6 Determination of the acidity constant of the
photoinduced form
To determine the logarithmic acidity constant of the photo-
1
S
induced form pK
a
, we considered that at pH 6–8 a comparable
rate of the dark reaction of structure 1 formation was observed.
As we assumed above, at these pH values acid 1S was completely
1
S
ionized and therefore the expected pKa value should be in the
range of 3–4.
We used the data on the change in the intensity of the band
centered at 310 nm, corresponding to the anionic spiro-form
ꢀ
1
S , from the spectra registered in the photo-stationary state
(PSS) and fitted them in eqn (1). The resulting plot (Fig. 9)
showed a linear regression of the logarithmic absorbance ratio
ꢀ
between 1S and 1S forms plotted against pH. Its intersection
with the x-axis at pH 3.9 corresponds to an apparent dissociation
1
a
S
constant of the photoinduced form pK
.
1
S
Fig. 10 pH Changes of the aqueous solution of photoacid 1 upon sequential
The obtained pKa value of 3.9 was also supported by the
dependence of the dark reaction rate constant k on the pH
described above (Fig. 8). As it followed from Fig. 8, a dramatic
change in the reverse reaction rate occurred near pH 4, namely,
O, C = 1.0 ꢁ 10ꢀ4 M,
irradiation with visible light and storage in the dark (H
0 1C).
2
2
when the linear section was curved. According to the literature, 5.6 to 4.4 (Fig. 10) by irradiation of 0.1 mmol solution of
this area can also be used to approximately estimate the pK
a
compound 1 in water.
4
2
value.
As it is shown in Fig. 10, this process could be repeated
The established pKa of 6.6 for the initial form 1 and pKa of several times. This valuable result made a significant contribution
.9 for the photoinduced form 1S demonstrated that dye 1 was a to the topical field of research on metastable photoacids allowing
1
1S
3
compound with fully reversible photoacidity (P) in aqueous the pH of the aqueous media to be regulated.
1
0
medium, which could be estimated according to eqn (2) as 2.7.
1
a
1S
a
3. Experimental
P ¼ pK ꢀ pK
(2)
3.1 Materials and methods
The observed difference between the pK
and photoinduced 1S forms had practical significance beside
a
values of the initial
The progress of reactions and the purity of products were monitored
by TLC on Sorbfil plates (spots were visualized under UV light, by
treatment with iodine vapor, or by heating). The IR spectra were
recorded on an FSM-2201 spectrometer with Fourier transform from
the samples dispersed in mineral oil. The NMR spectra were
1
the theoretical one. This was due to the possibility of using such
compounds for photo-regulated pH control of the aqueous
media. In this regard, we estimated the possibility to regulate
the acidity of the aqueous medium in the presence of com-
pound 1. We found that it was possible to change the pH from
6
measured in DMSO-d on Bruker DRX-500 and Varian 400 spectro-
meters using tetramethylsilane as an internal reference.
Elemental analyses were performed using a FlashEA 1112
CHN analyzer. The mass spectra were obtained on a Finnigan MAT
INCOS-50 mass spectrometer. The UV spectra were recorded on an
Agilent Cary 60 UV-Vis spectrophotometer. Melting points were
determined on an OptiMelt MPA100 device. To study the photo-
chromic behavior of compounds, the solutions were irradiated
with non-filtered visible light using LED XM-LT6 CREE (spectral
range of 400–700 nm, a luminous power density equal to
ꢀ2
2
05 mW cm ) for 0.5 min. Kinetic data of the dark thermal
reaction were recorded at 20 1C. The syntheses of the starting
3
9
40
compounds 2, 3 and 4, as well as tricyanofuran TCF, were
carried out according to the described procedures.
3
2
.2 Synthesis of sodium (E)-3-(2-(4-cyano-5-(dicyanomethylene)-
,2-dimethyl-2,5-dihydrofuran-3-yl)vinyl)-4-hydroxybenzene-
sulfonate 1
To a suspension of 0.199 g (1 mmol) of TCF in 3 ml 96%
ethanol, 0.224 g (1 mmol) of aldehyde 4 and 0.82 mg (0.01 mmol)
Fig. 9 Linear regression of the logarithmic absorbance ratio between 1S
and 1S forms vs. pH (C = 2.5 ꢁ 10ꢀ5 M, 20 1C).
ꢀ
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of sodium acetate were added under an argon atmosphere. The Notes and references
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.3 Spectral characterization of compound 1
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ꢀ
1
1
1041 (OQSQO). H NMR (500.13 MHz, DMSO-d
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CH =), 7.59 (1H, dd, J = 2.1, 8.5 Hz, C H ), 8.04 (1H, d, J = 2.1, Hz,
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3
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C
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6 3
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The research was performed with the financial support of the
Russian Science Foundation (Grant no. 18-73-10065).
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