Benzothiazolyl substituted spiropyrans with ion-driven photochromic transformation

Article abstract of DOI:10.1016/j.dyepig.2020.108337

The present work is devoted to the synthesis and detailed investigation of new 8-benzothiazole containing spiropyrans. The investigated spiropyrans exist under colorless spirocyclic isomers exhibiting positive photochromism with thermo- and photoinduced b

Full text of DOI:10.1016/j.dyepig.2020.108337

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Benzothiazolyl substituted spiropyrans with ion-driven photochromic transformation
Anatoly V. Chernyshev, Nikolai A. Voloshin, Irina A. Rostovtseva, Oleg P. Demidov,
Konstantin E. Shepelenko, Ekaterina V. Solov'eva, Elena B. Gaeva, Anatoly V.
Metelitsa
PII:
S0143-7208(20)30211-4
DOI:
https://doi.org/10.1016/j.dyepig.2020.108337
DYPI 108337
Reference:
To appear in: Dyes and Pigments
Received Date: 22 January 2020
Revised Date: 5 March 2020
Accepted Date: 6 March 2020
Please cite this article as: Chernyshev AV, Voloshin NA, Rostovtseva IA, Demidov OP, Shepelenko
KE, Solov'eva EV, Gaeva EB, Metelitsa AV, Benzothiazolyl substituted spiropyrans with ion-
driven photochromic transformation, Dyes and Pigments (2020), doi: https://doi.org/10.1016/
j.dyepig.2020.108337.
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Benzothiazolyl substituted spiropyrans with ion-driven photochromic transformation
Anatoly V. Chernysheva*, Nikolai A. Voloshina, Irina A. Rostovtsevaa, Oleg P. Demidovb,
Konstantin E. Shepelenkoa, Ekaterina V. Solov’evaa, Elena B. Gaevaa, Anatoly V. Metelitsaa
a
Institute of Physical and Organic Chemistry, Southern Federal University, Stachki Av., 194/2,
Rostov on Don, 344090 Russia
b North-Caucasus Federal University, Dzerzhinsky str. 120/3, Stavropol, 35503 5Russia
Abstract
The present work is devoted to the synthesis and detailed investigation of new 8-benzothiazole
containing spiropyrans. The investigated spiropyrans exist under colorless spirocyclic isomers
exhibiting positive photochromism with thermo- and photoinduced back reaction. UV light
irradiation leads to their effective coloration with quantum yields up to 44 %. The efficiency of
the back photocyclization induced by visible light does not exceed 0.1%. This feature
distinguishes them from the previously studied benzoxazole analogs. The enhancement of
electron-withdrawing properties of the indoline fragment substituents promotes an increase in
photo-coloration efficiency. The lifetime of the colored form varies from 17 to 200.8 s (acetone,
293 K) depending on substituents. Due to the presence of the benzothiazole moiety,
merocyanines can effectively bind Zn2+, Co2+, Ni2+, Cu2+, Cd2+, Mn2+ ions. The addition of salts
of these ions to spiropyran solutions results in the stabilization of the merocyanine in the form of
intensely colored complexes. Composition, stability and spectroscopic characteristics of
complexes have been investigated depending on the metal ion nature and substituents in ligand
* Corresponding author.
e-mail address: anatoly@ipoc.sfedu.ru (A. Chernyshev).

molecules. The colored complexes possess negative photochromism. The visible light irradiation
causes their bleaching with quantum yields up to 48%. The benzothiazole substituted spiropyrans
demonstrate ion driving photochromic transformation.
Keywords
photochromism, spiropyrans, ionochromism, complexes, quantum yield
1.
Introduction
Due to the variety of external stimuli capable of initiating the chromogenic
isomerization reaction of spiropyran-indolines, this class of organic compounds was the subject
of continuous studies for more than 60 years [1]. Under the influence of temperature, the
colorless spirocyclic form of the spiropyran-indolines is capable of isomerizing into a colored
merocyanine form, causing thermochromic properties [2]. Following thermochromism, the
photochromic properties of the spiropyran-indolines were soon described [3-5]. Subsequently,
changes in the redox potential [6–8], as well as mechanical stress [9–11], were added to the
number of external stimuli affecting the isomerization of spiropyran-indolines. All this leads to a
wide range of applications of spiropyrans, such as optical data recording and storage [12-14],
photostimulated control of biological functions [15] and many other application [16]. Separately,
it is possible to single out the ionochromism of spiropyrans associated with the stabilization of
the colored form upon interaction with charged particles. The simplest example of ionochromism
is acidochromism. It is the protonation of the merocyanine form of spiropyran at the phenolate
oxygen atom with the formation of a colored styryl salt [17-20], which is typical for almost all
spiropyrans. However, ionochromism associated with the interaction of spiropyrans with metal

ions seems to be more interesting, since the resulting complexes of the merocyanine form are
more diverse in their photophysical and photochemical properties than protonated merocyanines.
The latter is due to the fact that their properties depend not only on the structure of the
spiropyran but also on the nature of the metal ion [21,22]. In addition, an outstanding feature of
the ionochromic transformation of spiropyrans is a possibility to drive complex formation with
the help of light irradiation. The light can produce a coordinationally active form of the
spiropyran or lead to a decomplexation reaction [23]. This phenomenon finds its application in
different fields such as photostimulated metal ion sensors for real-time environmental monitoring
and clinical diagnostics [24-27], visualization [28-32] and tracking [33] of mobile forms of a
metal ion in biological objects, photoswitchable magnetics [34] and adsorbents [35,36]. For
effective ionochromic transformation of spiropyrans, it is desirable to have an additional ligand
group in its structure, which, along with the phenolate oxygen atom of merocyanine, will
participate in the binding of the metal ion. In addition to increasing the binding ability, such a
modification of the structure of spiropyran significantly affects its photochromic characteristics.
Among various ligand groups [37], azole substituents are effective modifiers of the structure of
spiropyrans to enhance the ionochromic effect. Earlier, we have described in detail the photo-
and ionochromism of benzoxazole- [38] and oxadiazole-substituted [39] spirobenzopyran-
indolines. In order to gain deeper insight into the influence of the nature of the azole substituent
on both the properties of spiropyran and its complexes this work is aimed at obtaining and
studying the photo- and ionochromic properties of new 8-benzothiazole substituted
spirobenzopyran-indolines (SPP).
2.
Experimental part
2.1. Materials and methods

1
NMR spectra were recorded using a "Varian Unity-300" (300 MHz, Н) and Bruker
"AVANCE-600" (600 MHz, 1H; 150 MHz, 13С; 60 MHz, 15N) spectrometer at 20 °C, the signals
were referred with respect to the signal of residual protons of deutero-solvent CDCl3 (1H:7.24
ppm, 13С: 77.0 ppm). All chemical shifts are reported in the standard δ notation of parts per
million. FTIR spectra were collected on a Varian Excalibur 3100 spectrometer. High-resolution
mass spectra were obtained from a TOF mass spectrometer with an ESI source (Bruker maXis).
The instrument was operated in positive mode using an m/z range of 50–3000. The capillary
voltage of the ion source was set at 4500 V. The nebulizer gas pressure was 0.4 bar, the drying
gas flow was set to 4.0 L/min. Elemental analysis was carried out using a KOVO CHN analyzer.
Melting points were determined on a Boetius hot stage apparatus. The electronic absorption
spectra and kinetic curves of thermal and photochemical reactions of the studied compounds
have been recorded on an "Agilent 8453" spectrophotometer equipped with a temperature-
controlled cell. Irradiation light was brought into the thermostated cell compartment at 90° from
a 200W high-pressure mercury lamp "Newport" equipped with glass filters for the allocation of
mercury lines. The irradiation intensity has been determined by the help of a "Newport 2935"
optical power meter. The irradiation intensity at 365 and 546 nm has been found to be (0.7-
2.6)·10-5 and (0.6-1.4)·10-4 Einstein·L−1s−1 correspondingly. In experiments of determination of
photo-discoloration quantum yields of complexes 546 nm light flux has been attenuated by iris
diaphragm up to 3·10-6 Einstein·L-1s-1. Numerical simulations were carried out with the
simulation-adjustment program SA 3.3 (non-commercial software, Laboratoire des IMRCP,
Université Paul Sabatier, Toulouse, France) [40]. In order to prepare solutions toluene, acetone
of the spectroscopic grade and metal perchlorates have been used. Fluorescence emission and
excitation spectra were collected using an Eclipse Varian spectrofluorimeter. Fluorescence
quantum yields have been determined by the relative Parker-Rees method using cresyl violet in
ethanol (φ = 0.51) [41] as a reference with optically matched samples having absorbance of 0.1-
0.2 at λex = 540 nm; the experimental error 20%.

2.2. Synthesis
3Н-Indolinium salts 2a-f were obtained by previously described procedures [42-44].
5-(Benzo[d]thiazol-2-yl)-4-hydroxyisophthalaldehyde
(3).
A
mixture
of
benzo[d]thiazolylphenol 4 (2.28 g, 10 mmol), hexamethylenetetramine (8.40 g, 60 mmol) and
trifluoroacetic acid (30 mL) was refluxed for 8 h, cooled, and a mixture of conc. HCl (14 mL)
and H2O (28 mL) was added. The reaction mixture was poured into H2O (130 mL), the solid was
filtered off, washed with water, dried and recrystalized from chlorobenzene. Yield 72%, m.p.
о
1
196-197 C. H NMR (300 MHz, CDCl3, δ): 7.50 (1Н, dt, J = 7.7 Hz, J = 1.2 Hz, H-5'); 7.58
(1Н, dt, J = 7.8 Hz, J = 1.2 Hz, H-6'); 7.98 (1Н, dd, J = 7.9 Hz, J = 0.6 Hz, H-4'); 8.05 (1Н, dd, J
= 7.8 Hz, J = 0.9 Hz, H-7'); 8.41 (1Н, d, J = 1.9 Hz, H-6); 8.55 (1Н, d, J = 2.1 Hz, H-4); 9.99
(1Н, s, 3-CНO); 10.60 (1Н, s, 1-CНO); 14.21 (1Н, s, 4-ОН). Elemental analyses: Found: C,
63.66; H, 3.11; N, 4.81%; C15H9NO3S; requires: C, 63.59; H, 3.20; N, 4.94%.
8-(Benzo[d]thiazol-2-yl)-3',3'-dimethylspiro[chromene-2,2'-indoline]-6-carbaldehydes
(1a-f) (general procedure). A mixture of 3Н-indolium salt 2a-f (1 mmol), triethylamine (0.14
mL, 1 mmol), and aldehyde 3 (0.28 g, 1 mmol) in toluene (10 mL) and 2-propanol (4 mL) were
refluxed for 12 h and evaporated. The residue was purified by column chromatography on Al2O3
with benzene as eluent and recrystallized and recrystallized from a mixture of heptane-toluene (1:1)
.
8-(Benzo[d]thiazol-2-yl)-1'-benzyl-3',3'-dimethylspiro[chromene-2,2'-indoline]-6-
1
carbaldehyde (1a). Yield 42%, m.p. 207-208 °C. H NMR (300 MHz, CDCl3, δ): 1.37 (3H, s,
3'-Me); 1.42 (3H, s, 3'-Me); 4.16 (1H, d, J = 16.3 Hz, 1'-CH2Ph); 4.55 (1H, d, J = 16.3 Hz, 1'-
CH2Ph); 6.08 (1H, d, J = 10.4 Hz, H-3); 6.43 (1H, d, J = 7.7 Hz, H-7'); 6.98 (1H, dt, J = 7.4 Hz,
J = 1.0 Hz, H-5'); 7.14 (1H, dd, J = 7.7 Hz, J = 1.3 Hz, H-4'); 7.58 (1H, ddd, J = 7.9 Hz, J = 1.3
Hz, J = 0.7 Hz, H-7"); 7.03 (1H, d, J = 10.4 Hz, H-4); 7.16-7.35 (7H, m, H-6', H-5", PhH); 7.45

(1H, ddd, J = 8.3 Hz, J = 7.2 Hz, J = 1.3 Hz, H-6''); 8.04 (1H, ddd, J = 8.2, J = 1.0, J = 0.7, H-
4"); 7.78 (1H, d, J = 2.0, H-5); 8.95 (1H, d, J = 2.0, H-7); 10.02 (1H, s, 6-CHO). IR (cm-1): 1694
(С=О), 1645 (С=N), 1597 (С=С), 928 (Cspiro-O). Elemental analyses: Found: C, 77.11; H, 5.15;
N, 5.15%; C33H26N2O2S; requires: C, 77.02; H, 5.09; N, 5.44%. MS (ESI-TOF) m/z: found:
515.1788 [M+H]+. C33H26N2O2S. Calcd: 515.1787.
8-(Benzo[d]thiazol-2-yl)-5'-bromo-1',3',3'-trimethylspiro[chromene-2,2'-indoline]-6-
1
carbaldehyde (1b). Yield 45%, m.p. 239-240 °C. H NMR (300 MHz, CDCl3, δ): 1.28 (3H, s,
3'-Me); 1.31 (3H, s, 3'-Me); 2.70 (3H, s, 1'-Me); 5.96 (1H, d, J = 10.4 Hz, H-3); 6.48 (1H, d, J =
8.2 Hz, H-7'); 7.06 (1H, d, J = 10.4 Hz, H-4); 7.25 (1H, d, J = 2.0 Hz, H-4'); 7.33 (1H, ddd, J =
8.1 Hz, J = 7.3 Hz, J = 1.1 Hz, H-5''); 7.38 (1H, dd, J = 8.2 Hz, J = 2.0 Hz, H-6'); 7.45 (1H, ddd,
J = 8.3 Hz, J = 7.2 Hz, J = 1.3 Hz, H-6''); 7.65 (1H, ddd, J = 7.9 Hz, J = 1.3 Hz, J = 0.7 Hz, H-
7''); 7.78 (1H, d, J = 2.1 Hz, H-5); 8.01 (1H, ddd, J = 8.2 Hz, J = 1.1 Hz, J = 0.7 Hz, H-4''); 8.92
(1H, d, J = 2.1 Hz, H-7), 9.99 (1H, d, 6-CHO). IR (cm-1): 1691 (С=О), 1600 (С=N), 1580 (С=С),
931 (Cspiro-O). Elemental analyses: Found: C, 62.54; H, 4.01; N, 5.32%; C27H21BrN2O2S;
requires: C, 62.67; H, 4.09; N, 5.41%. MS (ESI-TOF) m/z: found 517.0580 [M+H]+.
C27H21BrN2O2S. Calcd: 517.0579 (79Br).
8-(Benzo[d]thiazol-2-yl)-5'-chloro-1',3',3'-trimethylspiro[chromene-2,2'-indoline]-6-
1
carbaldehyde (1c). Yield 46%, m.p. 243-244 °C. H NMR (300 MHz, CDCl3, δ): 1.28 (3H, s,
3'-Me); 1.31 (3H, s, 3'-Me); 2.71 (3H, s, 1'-Me); 5.96 (1H, d, J = 10.4 Hz, H-3); 6.52 (1H, d, J =
8.2 Hz, H-7'); 7.06 (1H, d, J = 10.4 Hz, H-4); 7.12 (1H, d, J = 2.1 Hz, H-4'); 7.23 (1H, dd, J =
8.2 Hz, J = 2.1 Hz, H-6'); 7.32 (1H, ddd, J = 8.2 Hz, J = 7.2 Hz, J = 1.2 Hz, H-5''); 7.43 (1H,
ddd, J = 8.3 Hz, J = 7.2 Hz, J = 1.3 Hz, H-6''); 7.64 (1H, ddd, J = 7.9 Hz, J = 1.3 Hz, J = 0.7 Hz,
H-7''); 7.78 (1H, d, J = 2.1 Hz, H-5); 8.01 (1H, ddd, J = 8.2 Hz, J = 1.0 Hz, J = 0.7 Hz, H-4'');
8.92 (1H, d, J = 2.1 Hz, H-7), 9.99 (1H, s, 6-CHO). IR (cm-1): 1691 (С=О), 1599 (С=N), 1581
(С=С), 933 (Cspiro-O). Elemental analyses: Found: C, 68.43; H, 4.40; N, 6.01%; C27H21ClN2O2S;

requires: C, 68.56; H, 4.48; N, 5.92%. MS (ESI-TOF) m/z: found 473.1085 [M+H]+.
C27H21ClN2O2S. Calcd: 473.1085.
8-(Benzo[d]thiazol-2-yl)-1',3',3'-trimethyl-5'-(trifluoromethyl)spiro[chromene-2,2'-
1
indoline]-6-carbaldehyde (1d). Yield 48%, m.p. 212-213 °C. H NMR (300 MHz, CDCl3, δ):
1.30 (3H, s, 3'-Me); 1.37 (3H, s, 3'-Me); 2.77 (3H, s, 1'-Me); 5.99 (1H, d, J = 10.4 Hz, H-3); 6.65
(1H, d, J = 8.1 Hz, H-7'); 7.10 (1H, d, J = 10.4 Hz, H-4); 7.30 (1H, ddd, J = 8.1 Hz, J = 7.2 Hz, J
= 1.0 Hz, H-5''); 7.39 (1H, d, J = 1.8 Hz, H-4'); 7.43 (1H, ddd, J = 8.3 Hz, J = 7.2 Hz, J = 1.2 Hz,
H-6''); 7.50-7.59 (2H, m, H-6, H-7''); 7.79 (1H, d, J = 2.0 Hz, H-5); 8.00 (1H, ddd, J = 8.2 Hz, J
= 1.7 Hz, J = 0.9 Hz, H-4''); 8.93 (1H, d, J = 2.0 Hz, H-7); 10.00 (1H, s, 6-CHO). 13C NMR (150
MHz, CDCl3, δ): 19.37 (3'-Me), 25.22 (3'-Me), 29.18 (N'-Me), 51.97 (C-3'), 107.42 (C-7'),
108.76 (C-2/2'), 119.00 (C-4', q, J = 3.7), 119.56 (C-3), 119.98 (C-4a), 120.32 (C-7''), 121.16 (C-
5''), 122.20 (5'-CF3, к, J = 32.1), 122.74 (C-4''), 124.16 (C-5'), 125.03 (C-4a''), 126.15 (C-6''),
125.80 (C-6', q, J = 4.0), 127.64 (C-5), 129.85 (C-6), 130.10 (C-4), 133.68 (C-7), 135.97 (C-8),
137.39 (C-3a'), 151.03 (C-7a'), 151.73 (C-7a''), 156.08 (C-1a), 160.52 (C-2''), 190.49 (6-CHO).
15N NMR (60 MHz, CDCl3, δ): 97.30 (N-1'), 303.81 (N-3''). IR (cm-1): 1695 (С=О), 1600
(С=N), 1581 (С=С), 1096 (C-F), 920 (Cspiro-O). Elemental analyses: Found: C, 66.45; H, 4.11;
N, 5.42%; C28H21F3N2O2S; requires: C, 66.39; H, 4.18; N, 5.53%. MS (ESI-TOF) m/z: found
507.1349 [M+H]+. C28H21F3N2O2S. Calcd: 507.1348.
8-(Benzo[d]thiazol-2-yl)-1',3',3'-trimethyl-5'-nitrospiro[chromene-2,2'-indoline]-6-
1
carbaldehyde (1e). Yield 39%, m.p. 282-283 °C. H NMR (300 MHz, CDCl3, δ): 1.33 (3H, s,
3'-Me); 1.41 (3H, s, 3'-Me); 2.90 (3H, s, 1'-Me); 5.99 (1H, d, J = 10.4 Hz, H-3); 6.66 (1H, d, J =
8.7 Hz, H-7'); 7.14 (1H, d, J = 10.4 Hz, H-4); 7.32 (1H, ddd, J = 8.2 Hz, J = 7.3 Hz, J = 1.1 Hz,
H-5''); 7.47 (1H, ddd, J = 8.4 Hz, J = 7.2 Hz, J = 1.3 Hz, H-6''); 7.57 (1H, ddd, J = 8.0 Hz, J =
1.3 Hz, J = 0.7 Hz, H-7''); 7.83 (1H, d, J = 2.0 Hz, H-5); 8.04 (1H, ddd, J = 8.2 Hz, J = 1.7 Hz, J
= 0.9 Hz, H-4''); 8.07 (1H, d, J = 2.3 Hz, H-4'); 8.31 (1H, dd, J = 8.7 Hz, J = 2.3 Hz, H-6'); 8.97
(1H, d, J = 2.0 Hz, H-7); 10.03 (1H, s, 6'-CHO). IR (cm-1): 1692 (С=О), 1600 (С=N), 1581

(С=С), 931 (Cspiro-O). ). Elemental analyses: Found: C, 66.95; H, 4.27; N, 8.77%; C27H21N3O4S;
requires: C, 67.07; H, 4.38; N, 8.69%. MS (ESI-TOF) m/z: found 484.1326 [M+H]+.
C27H21N3O4S. Calcd: 484.1325.
8-(Benzo[d]thiazol-2-yl)-5'-bromo-3',3'-dimethyl-1'-propylspiro[chromene-2,2'-
1
indoline]-6-carbaldehyde (1f). Yield 44%, m.p. 195-196 °C. H NMR (300 MHz, CDCl3, δ):
0.87 (3H, t, J = 7.4, 1'-CH2CH2CH3); 1.27 (3H, s, 3'-Me); 1.29 (3H, s, 3'-Me); 1.53-1.70 (2H, m,
1'-CH2CH2CH3); 2.98-3.15 (2H, m, 1'-CH2CH2CH3); 5.96 (1H, d, J = 10.4 Hz, H-3); 6.51 (1H, d,
J = 8.2 Hz, H-7'); 7.04 (1H, d, J = 10.4 Hz, H-4); 7.24 (1H, d, J = 2.0 Hz, H-4'); 7.32 (1H, ddd, J
= 8.2 Hz, J = 7.2 Hz, J = 1.2 Hz, H-5''); 7.36 (1H, dd, J = 8.2 Hz, J = 2.0 Hz, H-6'); 7.44 (1H,
ddd, J = 8.3 Hz, J = 7.2 Hz, J = 1.3 Hz, H-6''); 7.65 (1H, ddd, J = 7.9 Hz, J = 1.3 Hz, J = 0.7 Hz,
H-7''); 7.77 (1H, d, J = 2.0 Hz, H-5); 8.01 (1H, ddd, J = 8.2 Hz, J = 1.1 Hz, J = 0.7 Hz, H-4'');
8.92 (1H, d, J = 2.0 Hz, H-7); 9.99 (1H, s, 6-CHO). IR (cm-1): 1690 (С=О), 1599 (С=N), 1580
(С=С), 943 (Cspiro-O). Elemental analyses: Found: C, 63.92; H, 4.49; N, 5.04%; C29H25BrN2O2S;
requires: C, 63.85; H, 4.62; N, 5.14%. MS (ESI-TOF) m/z: found 545.0893 [M+H]+.
C29H25BrN2O2S. Calcd: 545.0892 (79Br).
2.3 Single crystal X-ray diffraction studies
The single crystals of complex 1f-Zn have been grown from acetone solution. The data
collection have been performed on an Agilent SuperNova diffractometer using microfocus X-ray
source with copper anode (λ = 1.54184) and Atlas S2 CCD detector. The diffraction data of 1f-
Zn were obtained at 103(4) K. Single crystals of C29H25BrN4O8SZn are triclinic, a =
10.8733(2) Å, b = 14.7522(3) Å, c = 20.6289(5) Å, α = 71.798(2)°, β = 76.023(2)°, γ =
73.251(2)°, V = 2968.00(12) Å3, Z = 4, T = 103(4) K, ꢀ(Cu Kα) = 3.882 mm-1, Dcalc = 1.645 g,
space group P-1 (no. 2). 12404 reflections measured (6.888° ≤ 2Θ ≤ 152.554°), 12404 unique
(Rsigma = 0.0394) which were used in all calculations. Twinned data refinement Scales: 0.753(2);
0.247(2). The final R1 was 0.0572 (I > 2σ(I)) and wR2 was 0.1634 (all data). The collection of
reflexes, determination and refinement of unit cell parameters were performed by using the

specialized CrysAlisPro 1.171.38.41 software suite [45]. The structures were solved by using
ShelXT program [46], structure refinement was also performed with ShelXL program [47].
Molecular graphics were rendered and prepared for publication with the Olex2 software suite
[48]. The complete X-ray diffraction datasets were deposited at the Cambridge Crystallographic
Data Center (CCDC 1977817).
3.
Results and discussion
3.1 Synthesis
8-Benzothiazolyl-6-formyl substituted spirobenzopyrans 1a-f have been obtained by the
interaction of 3Н-indolium salts 2a-f in the presence of triethylamine as a base with 5-
benzothiazolyl substituted 4-hydroxy-isophthalaldehyde 3 obtained by Duff-formylation of the
phenol 4 (Scheme 1).
Scheme 1. Synthesis of 8-Benzothiazolyl-6-formyl substituted spirobenzopyrans

1
The structure of the compounds 1a-f has been established using H NMR spectroscopy
13
15
1
(1Н NMR, С NMR, N NMR), heteronuclear correlation spectroscopy (COSY Н-1Н, HSQC
1Н-13С, HMBC 1Н-13С, HMBC 1Н-15N), mass spectrometry and elemental analysis.
1
The H NMR spectra of spiropyrans 1a-f contain two signals of geminal methyl groups,
which are prochiral due to the presence of a stereogenic tetrahedral carbon atom; signals of
protons of the N-alkyl group (Me, Pr, CH2Ph) of the indoline moiety in the high field, as well as
the signal of the formyl proton and several groups of interrelated proton signals in the low field
region of the spectrum related to the indoline, benzopyran, and benzothiazole fragments.
Prochirality of the protons of the methylene group of N-benzyl substituent in the indoline
fragment of spiropyran 1e leads to diastereotopic splitting of proton signals of this group which
appear as two one proton doublets at 4.16 and 4.55 ppm, respectively.
Full assignment of signals of hydrogen, carbon and nitrogen atoms of the spiropyran 1d
has been carried out based on the data of two-dimensional heteronuclear correlation
spectroscopy COSY 1H-1H, HSQC 1H-13C, HMBC 1H-13C, HMBC 1H-15N.
The signal of the carbon atom of the 5'-trifluoromethyl group of the indoline fragment of
the spiropyran 1d appears at δ 122.20 ppm as a quartet with J = 32.1 Hz, and the signals of
carbon atoms 4-C and 6-C appear as quartets at δ 119.00 (J = 3.7 Hz) and 125.80 ppm. (J = 4.0
Hz) respectively.
All of the mentioned above NMR spectroscopy data unambiguously confirm the structure
of the obtained compounds 1a-f. The absence of proton signals of the indoline and benzopyran
moieties in the spectral regions typical for the open merocyanine form [49,50] indicates that the
obtained spiropyrans are in the spirocyclic form in CDCl3 solution.
3.2 Spectroscopic properties of spiropyrans

In toluene, the obtained spiropyrans 1a-f are completely in cyclic form, which is
evidenced by the absence of absorption in the visible spectral region at λ> 400 nm. The
electronic absorption spectra of the cyclic forms A (Scheme 2) of spiropyrans 1a-f are
characterized by two absorption bands: a structured long-wavelength band with maxima in the
range 344–346 and 361–364 nm (ε =7300–19600 and 6400–20100 M-1·cm-1) and a more intense
short-wavelength band possessing maxima at 290 - 299 nm (ε = 21500 – 33000 M-1·cm-1) (Table
1, Fig. S1).
Table 1
Data of absorption spectra (λmax, ε) and kinetic properties (τ(B), ФАВ, ФВА) of SPP 1a-f, T=293K
(* - shoulder).
Toluene
λmax, nm
Acetone
τ(B),
s
SPP isomer
λmax, nm
τ(B),
ФАВ
ФВА
(ε·10-3, M-1·cm-1)
290 (21.5),
s
(ε·10-3, M-1·cm-1)
346 (10.6),
363 (9.3)
321*(13.1), 346
(7.3), 364 (6.4)
А
23.1
101.8
0.16
0.0008
1a
404 (12.8),
600 (38.9)
444, 647
В
294 (25.5),
345 (8.2),
362 (8.0)
321*(16.1), 345
(8.5), 363 (8.3)
А
25.4
23.6
177.7
200.8
0.20
0.17
0.0005
0.0004
1b
403 (16.2),
595 (49.1)
448, 648
В
299 (24.8), 321*
(16.8), 345 (9.0),
363 (8.5)
345 (8.7),
362 (8.3)
1c
А

404 (15.2),
595 (49.2)
447, 647
В
А
В
298 (24.9), 320*
(18.6), 344 (10.9),
362 (10.6)
344 (8.5),
361 (8.6)
22.7
33.0
17.0
90.1
0.15
0.0007
1d
407 (14.2),
600 (39.8)
449, 644
297 (33.0), 305
(22.6), 322 (22.7),
344 (19.6), 361
344 (21.6),
360 (23.6),
381* (14.0)
А
4.7
0.44 <0.0001
1e
(20.1), 372* (11.7)
415 (19.4),
617 (54.6)
466, 655
В
А
В
297 (33.0), 322*
(21.3), 346 (11.4),
364 (11.0)
346, 363
1f
41.7
0.27
0.0010
410,
444, 647
601 (44.7)
Except for the nitro group, the substituents R1 and R2 in the indoline part of the
spiropyran molecule have practically no effect on the position of the absorption bands of the
spirocyclic form. The presence of a nitro group at position 5 of the indoline fragment
significantly enhances the absorptivity of the cyclic form in the visible spectroscopic region due
to the appearance of an additional charge transfer band [51].
Solutions of SPP 1a-f in the more polar acetone are slightly colored (Fig. 1) due to the
equilibrium between the cyclic and merocyanine forms (Scheme 2), which is significantly shifted

towards the cyclic form. The approximate values of the equilibrium constant estimated based on
the data of the molar coefficients of the merocyanine forms (vide infra), did not exceed 10-2. In
the case of nitro-substituted SPP 1e, passing to a more polar solvent caused a bathochromic shift
of the long-wavelength absorption band of the cyclic form by 9 nm, which is characteristic of the
charge transfer band, while for the other SPPs, an increase in the polarity of the medium did not
affect the position of the absorption bands of cyclic forms.
Fig. 1. Absorption spectra of SPP 1a-f in acetone, C(SPP)=3·10-5 М, Т = 293 К.
The merocyanine isomers B of spiropyrans 1a-f in toluene are characterized by two
absorption bands located in the region of 444 - 466 and 644 - 655 nm. The longest wavelength
absorption has been demonstrated by the merocyanine of 5-nitro-substituted SPP 1e. When
increased the polarity of the solvent, a hypsochromic shift of the absorption bands of
merocyanines occurs. Based on the previously obtained data [38, 39], the influence of the nature
of the azole substituent at position 8 of the benzopyran part of the molecule consists of a
bathochromic shift of the absorption bands of the merocyanine in the range oxadiazole -
benzoxazole - benzothiazole.

Me
Me
CHO
S
R2
Me
Me
R2
N
R1
O
N
CHO
+
N
R1
-
O
h (UV),
h (Vis),
A
N
S
B
1aꢀf
+ M2+
- M2+
CHO
S
Me
Me
R2
+
N
O
R1
N
M
n
MB
n = 1, 2
M2+: Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Mn2+
Scheme 2. Transformation of studed SPP
3.3
Kinetics of thermal and photochemical transformations
Irradiation of SPP 1a-f solutions with UV light in the absorption region of their
spirocyclic isomers leads to their coloration as a result of the formation of the merocyanine
isomer B (Fig. 2a, Scheme 2).

Fig.2a. Evolution of SPP 1b spectra in acetone
upon UV light irradiation, T = 293 K, λirr=365
nm, time between spectra 10 s.
Fig. 2b. A relaxation kinetic curve of SPP 1b
in acetone, T = 293 K.
After switching off the irradiation source, discoloration of the solutions occurs due to
the thermal recyclization reaction B → A. The kinetics of the dark relaxation process B → A in
solutions of compounds 1a-e are satisfactorily described by the monoexponential function (Fig.
2b). In the general case, the time constant of the relaxation process is τ = (kBA + kAB)-1. However,
taking into account that the equilibrium constant K = kAB/kBA << 1, kAB<< kBA and, therefore, τ(B)
= kBA-1. The time constant of the relaxation process τ(B) for spiropyrans 1a-e is in the range 4.7–
200.8 s (Table 1) and is determined by both the structure of the molecule and the parameters of
the medium. An increase in the kinetic stability of the merocyanine forms of SPP is observed
with an increase in the polarity of the medium upon passing from toluene to acetone. At the same
time, strong electron-withdrawing groups in the indoline fragment (CF3, NO2), on the contrary,
reduce its stability.
Irradiating the preliminary colored solutions of SPP 1a-f with visible light in the
absorption region of the merocyanine forms accelerates the bleaching process (Fig.2b, for
instance, irradiation of a colored solution of 1b reduces relaxation time from 177.7 s to 123.4 s).
This is a consequence of the presence of a reverse photochemical reaction of cyclization of the
merocyainine form. Therefore, the investigated spiropyrans exhibit positive photochromism with
thermal and photochemical back reactions. Thus, the general kinetic scheme characterizing the
photochemical and thermal processes will have the following form (scheme 3):
hν (ΦAB
)
hν (ΦBA
)
A
B
)
∆ (k_BA
Scheme 3.
The scheme 3 includes the forward photoinduced reaction of the formation of the

merocyanine from the spirocyclic form; spontaneous thermal process of the recyclization of the
merocyanine into the initial spiroform; as well as the back photochemical pathway of the
recyclization of the merocyanine form into the spirocyclic one. Taking into account that kAB<<
kBA the contribution of the forward thermal reaction of the merocyanine formation can be
neglected. It means that kAB can be supposed to equal to 0.
In order to evaluate the efficiency of the photo-coloration and photo-bleaching, the
quantum yields ΦAB and ΦBA of the corresponding photoreactions have been calculated. For this
purpose, the photokinetic method has been applied [52]. In accordance with Scheme 3, kinetic
curves describing the photo-coloration of an SPP solution under UV light irradiation can be
described by equation (1):
d[B]
irr
A
irr
B
= Φ
ε
I0irr Firr lirr (C −[B]) − (Φ
ε
BA
I0irr Firrlirr + kBA )[B]
(1)
AB
dt
When the colored solution is irradiated with visible light, ΦAB = 0 and equation (1) can be
represented as:
d[B]
irr
B
−
= (Φ
ε
I0irr F irrlirr + kBA )[B]
(2)
BA
dt
where F = (1–10-D’ )/D’ – photokinetic factor;
D’ – total absorbance at the irradiation wavelength;
(‘) – indicates a certain irradiation wavelength;
I’0 – the intensity of monochromatic light;
ε’A and ε’B – molar absorption coefficients of isomeric forms А and В;
ΦAB and ΦBA – quantum yields of photoreactions А→В and В→А;
C – the total concentration of SPP;
[A], [B] – the concentrations of the spirocyclic and the merocyanine form of SPP at a
given time;
kBA – rate constant of thermal reaction В→А.

It is worth noting that equations (1)-(2) are acceptable under the premise that ΦBA values
546
365
do not depend on the irradiation wavelength. In other words, ΦBA
= ΦBA
.
The numerical solution of the system of equations (1) - (2) for two experiments: photocoloration
(Fig. 3a) and photobleaching (Fig. 3b) (taking into account C = [A] + [B] - mass balance
equation and D(λ) = ε(λ)A[A] + ε(λ)B[B] – Beer’s law for absorbance at selected wavelength)
has allowed to obtain the ΦAB and ΦBA (Table 2).
Fig. 3a. Dependence of absorbance at the
selected wavelength of SPP 1b in acetone
(3.1·10-5 M) at 293 K upon irradiation with
UV (λ= 365 nm, I = 1.4·10-5 Einstein·L-1·cm-
1) light. λobs= 365 (□), 546 (○) and 595 (∆) nm.
Points are experimental data, lines are results
of kinetic modeling.
Fig. 3b. Dependence of absorbance at the
selected wavelength of SPP 1b in acetone
(3.1·10-5 M) at 293 K upon irradiation with
Vis (λ= 546 nm, I = 1.1·10-4 Einstein·L-1·cm-
1) light. λobs= 365 (□), 546 (○) and 595 (∆)
nm. Points are experimental data, lines are
results of kinetic modeling.
As can be seen from the data presented, SPP 1a-f are effective photochromic compounds
characterized by the photo-coloration quantum yields as high as 0.44 for 5-nitro substituted SPP
1e. At the same time, the efficiency of the reverse photobleaching reaction is more than two
orders of magnitude lower, which markedly distinguishes benzothiazole substituted SPP 1a-f
from the previously described analogs bearing other azole moieties at position 8 of the

benzopyran fragment (Fig. S2).
3.4
Ionochromic transformation of spiropyrans
Spiropyrans 1a-f exhibited an ionochromic effect. The addition of Zn2+, Co2+, Ni2+, Cu2+,
(in the form of well soluble perchlorates or nitrates) to solutions of SPP in acetone caused their
slow staining. This is accompanied by the appearance of intense absorption bands in the visible
region of the spectrum (Fig. 4), the position of the maxima of which depended both on the nature
of the added ion and, in the case of Ni2+, Cu2+, on the concentration of the metal perchlorates.
Upon addition of Cd2+, Mn2+ perchlorates the coloration has been observed only for SPP 1a-c, f,
while SPP 1d, e bearing strong acceptor groups in the indoline fragment have not exhibited an
ionochromic effect in the presence of these ions. The appearance of the coloration of the SPP
solutions is caused by the formation of complex compounds of the merocyanine form, whose
stability, due to the bidentate site, including the phenolate oxygen atom and the azole
heteroatom, becomes higher compared to unbound merocyanine (Scheme 3). The structure of the
complex compound SPP 1f with zinc ions in the crystal has been studied by the XRD method.
Single crystals of compound 1f –Zn(NO3)2 have been obtained in the form of orange plates by
slow evaporation of acetone solution of SPP 1f with 1 equivalent of Zn(NO3)2 at room
temperature. We have used nitrate instead of zinc perchlorate in experiments on crystal growth
since in the case of perchlorate, the complexes have greater solubility and therefore it was not
possible to obtain acceptable crystals. The complex 1f- Zn(NO3)2 crystallizes in the triclinic
space group P-1 with two non-equivalent molecules in a centrosymmetric unit cell, the
difference in the side chain conformation (Fig. S8 Suppl. Inform.). The coordination sphere of
the Zn atom (refer to Figure 5) is described as a distorted octahedral. The zinc ion is coordinated
by the nitrogen atom of benzothiazole and the phenol oxygen atom of merocyanine molecules
and by the oxygen atoms of two chelating nitrate ions. The merocyanine ligand is in the TTC

conformation. The bond distances and angles are within expected values.
Fig. 4. Absorption spectra of pure SPP 1a and absorption spectra obtained after addition of the
excess of the metal perchlorates in acetone, C(1a)=3·10-5 М, C(M2+)=1.5·10-4 М, Т=293 К.
Fig. 5. Molecular structure of compound 1f- Zn(NO3)2 in the solid-state. The second molecule
difference in the side chain conformation is omitted for clarity. Selected bond lengths [Å]: Zn1–
O1 1.943 (4), Zn1–O2 2.083 (4), Zn1–O3 2.344 (5), Zn1–O4 2.213 (6), Zn1–O5 2.115 (5), Zn1–
N1 2.040 (5). Showing displacement ellipsoids drawn at the 50% probability level.

The composition and stability of SPP complexes in acetone solutions have been
determined by means of the spectrophotometric method, which we had described in detail earlier
[38,53]. It has been revealed that, depending on the metal and substituents in the spiropyran
molecule, both the formation of 1:1 complexes (metal-merocyanine) MB and stepwise formation
of 1:1 (MB) and 1:2 (MB2) complexes are possible. In the latter case, the long-wavelength
absorption band observed in the spectrum of the SPP solution showed a slight hypochromic shift
with an increasing concentration of added metal perchlorates (Fig.6b, S7). In the case of the
formation of only 1:1 complexes, an increase in the metal ion concentration has caused only an
increase in the intensity of the long-wavelength absorption band, but not its shift (Fig.6a, S3, S4,
S6). The experimental dependences of the absorbance of SPP solutions on the concentration of a
metal ion have been satisfactorily described within the framework of the selected complexation
models. The calculated values of the logarithms of the stability constants, as well as the
spectroscopic properties of the complex compounds, are presented in Table 2.
Fig. 6a. Alteration of the absorption spectrum
of SPP 1b upon the addition of different
amounts of the cobalt perchlorate in acetone.
C(1b) = 3.88·10-5 M, Т = 293 К. Insert –
Dependence of the absorbance at the maximum
Fig. 6b. Alteration of the absorption
spectrum of SPP 1b upon the addition of
different amounts of the nickel perchlorate in
acetone. C(1b) = 3.45·10-5 M, Т = 293 К.
Insert – dependence of the absorbance at 520

absorption on the cobalt concentration. Points
are experimental data; the line is a result of the
theoretical calculation.
and 570 nm on the nickel concentration.
Points are experimental data; the line is a
result of the theoretical calculation.
As can be seen from the data presented, SPP 1a-f form moderately stable complexes with the
studied ions. Their stability mainly increases in the range Cd2+, Mn2+, Zn2+, Co2+, Ni2+, Cu2+ that
is typical for the ligands with N, O chelating cites. The enhancement of the electron-withdrawing
properties of the substituent at position 5 of the indoline fragment suppresses the ability of
spiropyrans to form complexes. It should also be noted that, in comparison with benzoxazolyl-
substituted [38] spirobenzopyran-indolines, the benzothiazolyl-substituted SPPs under study
form less stable complexes. Previously, a similar effect was observed for spironaphthopyran
analogs and was associated with a decrease in the basicity of the phenolate oxygen atom of the
merocyanine form upon passing from benzoxazole to benzothiazole [53]. Complexation causes a
hypsochromic shift of the long-wavelength absorption band of the ligand by more than 100 nm.
The complexes of the 1:2 composition exhibit longer wavelength absorption in comparison with
the 1:1 complexes.
3.5
Photoinitiated processes in the complexes
Solutions of SPP complexes are sensitive to light. This is especially true for complexes
with diamagnetic ions such as Cd2+, Zn2+. Their solutions are not stable even at room lighting.
As a result, the reaction of their formation must be carried out in the dark. Under the action of an
intense source of visible radiation, solutions of the Cd2+, Zn2+ complexes completely decolorize
in a short period of time. Solutions of paramagnetic ion complexes are more stable. An exception
is the Mn complexes, whose solutions undergo rapid discoloration, like solutions of the Cd2+,

Zn2+ complexes. The bleaching process is caused by photo-cyclization of merocyanine, which is
accompanied by the release of a metal ion from the complex [54] and is thermally reversible.
Thus, SPP 1a-f complexes possess the property of negative photochromism.
Fig. 7a. Evolution of absorption spectra of a
1b-Zn2+ solution (5.04·10-5 M) in acetone
under continuous irradiation (λirr = 546 nm, I =
2.7·10-6 EinsteinꢁL-1ꢁs-1).
Fig. 7b. Kinetic traces from monitoring the
absorbance at 491 nm (absorption maximum
of the complex) and 546 nm (maximum of
irradiation source) during the photoreaction
under continuous 546 nm irradiation of a 1b-
Zn2+ solution in acetone at 293 K (Fig.7a).
The geometric shapes indicate experimental
data, the solid lines are the calculated results.
In order to determine complex dissociation quantum yields a photokinetic method based on
equation (2) and described in detail earlier [54] has been used. The kinetic equation describing
the photodissociation of the complex under steady-state irradiation can be represented as:
−d[MB]/dt = I0Φr(1-10-ε′[MB]l) – kꢀ (C0- [MB])
(3)
where I0 - incident photon flux (EinsteinꢁL-1ꢁs-1), ε′ - molar extinction coefficient of MB at
irradiation wavelength (M-1ꢁcm-1), l - optical path length for the incident light (cm), kꢀ – the
observed rate constant for the complex formation of pseudofirst order, C0 is MB concentration at

t = 0. The current value of the complex concentration [MB] can be obtained from the absorption
spectra of a complex solution under its irradiation according to Beer’s law [MB] = Aλ/ελl. The
solution of the MB complex has been prepared in the dark by the addition of a metal perchlorate
(taken at a 10-100 excess) to the SPP solution of the known concentration. These conditions can
be calculated using complexation constant values thus providing the full transformation of the
SPP into the complex of 1:1 composition. The experiment of the determination of the efficiency
of photobleaching consisted of the irradiation of the exact volume of the MB complex solution
with visible monochromatic light of the known intensity and simultaneously registration of the
absorption spectra (Fig.7a). Kinetic curves at the absorption maximum of the complex and
maximum of irradiation source were then fitted by the help of the equation (3) (Fig. 7b). The
simulation results are shown in Table 2. The efficiency of photobleaching of the complexes
sharply increases in the range of paramagnetic Co2+, Ni2+, and Mn2+ ions and reaches 48% in the
case of diamagnetic Zn2+ ions. The introduction of electron-withdrawing substituents into the
indoline fragment significantly reduces the quantum yields of photoreaction. Compared with the
complexes of benzoxazolyl- [38] and oxadiazolyl-substituted [39] spirobenzopyran-indolines,
the complexes of the studied benzothiazolyl substituted analogs demonstrate the highest
efficiency of the photobleaching reaction.
It should be also noted that complex compounds of SPP 1a-f with Zn2+ and Cd2+ ions
exhibit fluorescence in the red spectral region (table 2, fig.S5).
Table 2
The calculated values of the effective complexation constants (Kieff), the spectral absorption
(λmax, ε
) and spectral luminescent properties (λflu, φflu) and the quantum yields (Φ546) of
photodissociation of SPP complexes with metals in acetone. Т = 293 К.
MB
MB2
SPP
M
eff
eff
logK1
λmax, nm
λflu, nm
Φ546
logK2
λmax, nm

(ε·10-4 M-1·cm-1
)
(φflu)
(ε·10-4 M-1·cm-1)
Co 5.56±0.01
508 (2.81)
490 (3.41)
527 (3.29)
502 (3.02)
530 (3.73)
522 (2.77)
511 (2.78)
486 (2.49)
520 (3.21)
493 (3.01)
522 (3.72)
515 (2.80)
510 (2.85)
491 (2.93)
519 (3.09)
492 (3.01)
522 (3.89)
514 (2.86)
517 (2.33)
494 (2.85)
524 (3.31)
498 (2.66)
526 (4.22)
-
-
0.0007
-
-
-
Cu
Ni
6.6±0.8
5.9±0.1
-
8.1±0.4
508 (5.56)
-
0.0008 6.70±0.06
543 (5.42)
1a
1b
1c
1d
Zn 6.00±0.02
Cd 2.85±0.01
Mn 3.63±0.01
Co 5.85±0.02
Cu 6.16±0.07
Ni 5.75±0.05
Zn 5.68±0.03
Cd 2.96±0.01
Mn 3.62±0.01
626 (0.066)
0.47
0.32
0.084
0.0009
-
-
-
632 (0.067)
-
-
-
-
-
-
-
-
-
6.03±0.05
503 (6.88)
-
0.0009 6.30±0.04
537 (5.60)
622 (0.063)
0.46
0.31
0.12
0.0009
-
-
-
631 (0.075)
-
-
-
-
-
Co
Cu
8.6±0.2
8.2±0.3
-
6.7±0.1
7.2±0.2
514 (5.31)
-
506 (5.13)
Ni 6.50±0.08
Zn 5.55±0.02
Cd 2.89±0.01
Mn 3.70±0.01
Co 4.18±0.02
Cu 6.82±0.08
-
0.0011 6.15±0.05
539 (6.70)
621 (0.062)
0.48
0.28
0.14
0.0005
-
-
-
628 (0.064)
-
-
-
-
-
-
-
-
-
6.59±0.06
511 (6.17)
Ni
6.7±0.1
-
0.0004
0.37
0.13
-
7.1±0.1
546 (4.38)
Zn 5.71±0.01
Cd 1.83±0.01
630 (0.115)
-
-
634 (0.080)
-
-
Mn
-
-
-
-
Co 3.28±0.01
Cu 4.49±0.01
Ni 4.27±0.01
Zn 3.06±0.08
519 (3.02)
506 (3.14)
545 (3.03)
512 (3.07)
-
-
0.0003
-
-
-
-
-
-
-
0.0002
0.25
-
-
-
1e
1f
626 (0.097)
-
-
Cd
-
-
-
-
-
Mn
-
-
-
-
-
Co 5.89±0.03
519 (3.24)
498 (3.34)
535 (3.84)
503 (3.73)
528 (4.03)
-
0.0008
-
-
-
Cu
Ni
6.7±0.1
7.0±0.1
-
5.91±0.06
510 (8.03)
-
0.0008
0.41
0.27
6.3±0.1
545 (6.06)
Zn 5.72±0.06
Cd 3.13±0.01
621 (0.081)
626 (0.077)
-
-
-
-

Mn 3.80±0.01
523 (2.72)
-
0.10
-
-
4.
Conclusion
Synthetic procedures have been developed and new benzothiazolyl substituted
spiropyrans have been obtained. The structure of new compounds has been established by means
of NMR spectroscopy methods, as well as mass spectroscopy and elemental analysis. The
obtained spiropyrans demonstrate positive photochromism with thermal and photochemical back
reactions. The quantum efficiency of photo-coloration attains a value of 44%. In contrast to
photo-coloration, the quantum yields of photobleaching reactions take extremely low values in
the range 0.1–0.01%, which is the smallest indicator in the series of spiropyrans with known
azole substituent fragments. The thermal bleaching reaction is characterized by a time constant
that varies over a wide range from 4.7 to 200.8 s depending on the substituent mainly in the
indoline part of the molecule and the polarity of the solvents. The thermal reaction is noticeably
accelerated with the introduction of electron-withdrawing substituents at position 5 of the
indoline fragment and upon passing from polar acetone to weakly polar toluene. All spiropyrans
obtained exhibit ionochromic properties. When interacting with Cd2+, Mn2+, Zn2+, Co2+, Ni2+,
Cu2+ metal ions, they form complexes of 1:1 and 1:2 compositions, the structure of which has
been established in the case of SPP 1 by the help of different methods including X-ray
diffraction. It has been established that electron-withdrawing substituents in the indoline part of
SPP molecules inhibit the complexation reaction. Photoinitiated processes in complexes of
merocyanine isomers with metal cations include both thermally reversible negative
photochromism and fluorescence in the case of Zn2+ and Cd2+ ions. Thus, the obtained
spiropyrans are a polychromogenic molecular system with ion-controlled photochromic
properties, manifested as positive photochromism in the case of free SPP molecules and negative
photochromism of complexes of SPP merocyanine isomers with metal ions.

Acknowledgments
This work was financially supported by the Ministry of Education and Science of the
Russian Federation within the framework of the State Assignment for Research (project No.
4.9645.2017/8.9). I.A. Rostovtseva, E.V. Solov’eva, and A.V. Metelitsa thanks the Russian
Foundation for Basic Research for supporting the study of the photochromism of spiropyrans
(project no. 19–03–01029 A).
Appendix A. Supplementary data
Supplementary data related to this article can be found online at
References
[1] Kortekaas L, Browne WR. The evolution of spiropyran: fundamentals and progress of an
extraordinarily versatile photochrome. Chem Soc Rev 2019; 48:3406—3424.
https://doi.org/10.1039/c9cs00203k
[2] Day JH. Thermochromism . Chem Rev 1963; 63(1):65-80.
https://doi.org/10.1021/cr60221a005
[3] Fischer E, Hirshberg J. Formation of coloured forms of spiropyran by low-temperature
irradiation. J Chem Soc 1952:4522-4524. https://doi.org/10.1039/JR9540000297
[4] Chaude O, Rumpf P. Sur la double phototropic certains spiranes. Compt Rend 1953;
236:697-699.

[5] Bertelson RC. Spiropyrans. In: Crano JC, Guglielmetti R, editors. Organic Photochromic
and Thermochromic Compounds New York: Kluwer Academic Publishers; 2002; 1:11-83.
https://doi.org/10.1007/b115590
[6] Zhi JF, Baba R, Hashimoto K, Fujishima A. Photoelectrochromic properties of a
spirobenzopyran derivative. J Photochem Photobiol A 1995; 92:91–97.
https://doi.org/10.1016/1010-6030(95)04167-0
[7] Zhi JF, Baba R, Hashimoto K and Fujishima A. A Multi-Functional Electro-Optical
Molecular Device. The Photoelectrochemical Behavior of Spirobenzopyrans in
Dimethylformamide. Ber Bunsen-Ges 1995; 99:32–39.
https://doi.org/10.1002/bbpc.19950990106
[8] Preigh MJ, Stauffer MT, Lin FT, Weber SG. Anodic oxidation mechanism of a spiropyran.
J Chem Soc, Faraday Trans 1996; 92(20):3991-3996.
https://doi.org/10.1039/FT9969203991
[9] Tipikin DS. Mechanochromism of Organic Compounds as Exemplified by Spiropyran.
Russ J Phys Chem 2001; 75:1720– 1722.
[10] Davis DA, Hamilton A, Yang J, Cremar LD, Gough DV, Potisek SL, Ong MT, Braun
PV, Martinez TJ, White SR, Moore JS, Sottos NR. Force-induced activation of covalent
bonds in mechanoresponsive polymeric materials. Nature 2009; 459:68– 72.
https://doi.org/10.1038/nature07970
[11] O’Bryan G, Wong BM, McElhanon JR. Stress Sensing in Polycaprolactone Films via an
Embedded Photochromic Compound. ACS Appl Mater Interfaces 2010; 2(6):1594-1600.
https://doi.org/10.1021/am100050v
[12] Berkovic G, Krongauz V, Weiss V. Spiropyrans and Spirooxazines for Memories and
Switches. Chem Rev 2000; 100(5):1741-1754. https://doi.org/10.1021/cr9800715

[13] Seok WC, Son SH, An TK, Kim SH, Lee SW. Photo-enhanced polymer memory device
based on polyimide containing spiropyran. Electronic Materials Letters 2016; 12(4):537-
544. https://doi.org/10.1007/s13391-016-4019-7
[14] Liu Q, Jiang K, Wen Y, Wang J, Luo J, Song Y. High-performance optoelectrical dual-
mode memory based on spiropyran-containing polyimide. Applied Physics
Letters 2010; 97(25) :253304. https://doi.org/10.1063/1.3529453.
[15] Albert L, Vaґzquez O. Photoswitchable peptides for spatiotemporal control of biological
functions. Chem Commun 2019; 55:10192-10213. https://doi.org/10.1039/c9cc03346g
[16] Klajn R. Spiropyran-based dynamic materials. Chem Soc Rev 2014; 43:148-184.
https://doi.org/10.1039/C3CS60181A
[17] Drumond CJ, Furlog DN. Photochromism of a surface-active spirobenzopyran moiety in
dioxane–water mixtures and self-assembled surfactant aggregates. J Chem Soc, Faraday
Trans 1990; 86:3613-36211. https://doi.org/10.1039/FT9908603613
[18] KeumSR, Lee KB, Kazmaier PM, Buncel E. A novel method for measurement of the
merocyanine-spiropyran interconversion in non-activated 1, 3, 3-trimethylspiro- (2H-1-
benzopyran-2, 2′-indoline) derivatives. Tetrahedron Letters 1994; 35(7):1015-1018.
https://doi.org/10.1016/S0040-4039(00)79953-9
[19] Zhou J, Li Y, Tang Y, Zhao F, Song X, Li E. Detailed investigation on a negative
photochromic spiropyran. J Photochem Photobiol A Chem 1995; 90( 2–3):117-123.
https://doi.org/ 10.1016/1010-6030(95)04082-Q
[20] Sumaru K, Takagi T, Satoh T, Kanamori T. Photo-induced reversible proton dissociation
of spirobenzopyran in aqueous systems. J Photochem Photobiol A 2013; 261: 46– 52.
https://doi.org /10.1016/j.jphotochem.2013.04.017
[21] Görner H, Chibisov AK. Complexes of spiropyran-derived merocyanines with metal ions.
Thermally activated and light-induced processes. J Chem Soc, Faraday Trans 1998;
94:2557-2564. https://doi.org/10.1039/A803330G