Spiropyrans and spirooxazines: 11. Complexation of photochromic 5-(1,3-benzothiazol-2-yl)-substituted 1,3-dihydrospiro[benzo[f]chromene-3,2-indole] with metal ions

Article abstract of DOI:10.1007/s11172-015-0918-2

5-(1,3-Benzothiazol-2-yl)substituted 1,3-dihydrospiro[benzo[f]chromene-3,2-indole] forms intensely colored complexes with metal ions in acetone solution in the absence of irradiation. The composition and stability of the complexes were shown to depend on the nature of the metal ion. The molecular structure of the zinc complex was determined by X-ray diffraction. The complexes with diamagnetic ions display negative photochromism associated with their reversible dissociation induced by visible light.

Full text of DOI:10.1007/s11172-015-0918-2

Russian Chemical Bulletin, International Edition, Vol. 64, No. 3, pp. 677—682, March, 2015
677
Spiropyrans and spirooxazines
11.* Complexation of photochromic 5ꢀ(1,3ꢀbenzothiazolꢀ2ꢀyl)ꢀsubstituted
1´,3´ꢀdihydrospiro[benzo[f]chromeneꢀ3,2´ꢀindole] with metal ions
I. A. Rostovtseva,a A. V. Chernyshev,a V. V. Tkachev,b S. M. Aldoshin,b N. A. Voloshin,a
A. V. Metelitsa,a N. I. Makarova,a and V. I. Minkina
aInstitute of Physical and Organic Chemistry, Southern Federal University,
194/2 prosp. Stachki, 344090 RostovꢀonꢀDon, Russian Federation.
Fax: +7 (863 2) 43 4667. Eꢀmail: photo@ipoc.sfedu.ru
bInstitute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Eꢀmail: sma@icp.ac.ru
5ꢀ(1,3ꢀBenzothiazolꢀ2ꢀyl)substituted 1´,3´ꢀdihydrospiro[benzo[f]chromeneꢀ3,2´ꢀindole]
forms intensely colored complexes with metal ions in acetone solution in the absence of irradiꢀ
ation. The composition and stability of the complexes were shown to depend on the nature of
the metal ion. The molecular structure of the zinc complex was determined by Xꢀray diffracꢀ
tion. The complexes with diamagnetic ions display negative photochromism associated with
their reversible dissociation induced by visible light.
Key words: benzothiazole, spiropyrans, merocyanines, photochromism, complexation.
An alteration of the physicochemical properties of photoꢀ
chromic organic compounds due to reversible switching
between two isomeric states induced by irradiation with
UV or visible light is a rapidly developing area due to
diverse fields of application, for example, as optical switches,
materials for data storage, photochemically controlled
chemical sensors, and so on.2—4 Spiropyrans belong to one
of the most wellꢀstudied classes of photochromic compounds
due to wide possibilities of their structural modifications.5,6
Polyfunctional molecular systems exhibiting, along with
photochromism, lightꢀswitchable fluorescence,7—11 magꢀ
netic,12 and complexation13—16 properties can be prepared
by introducing various functional moieties into spiropyran
molecules. Examples of such systems are azoleꢀsubstitutꢀ
ed spirochromenes17—19 and spirobenzochromenes,20—23
which exhibit luminescence properties in the spirocyclic
form and are also capable of efficiently chelating transiꢀ
tion metal ions. As a continuation of these studies, in the
present work we investigated the complexation of spirobenꢀ
zochromene containing the 1,3ꢀbenzothiazole substituent
at position 5 of the benzochromene moiety.
to the presence of a small amount of the merocyanine
isomer В in equilibrium with the spirocyclic isomer А
(Scheme 1). The spirocyclic form is characterized by
a longꢀwavelength absorption band with maxima at
386 and 405 nm (386 = 5.41•103 L mol–1 cm–1 and
405 = 4.93•103 L mol–1 cm–1).22 The longꢀwavelength
absorption band of the merocyanine isomer of SBC has
a maximum at 613 nm.
The addition of equivalent amounts of Zn2+, Cu2+
,
Co2+, and Ni2+ salts to acetone solutions of SBC in the
absence of irradiation leads to the immediate intense colꢀ
oration of the solutions due to the formation of the
complexes MBi of the merocyanine isomer of SBC (see
Scheme 1). The visible spectral region shows intense abꢀ
sorption bands at 500—650 nm, the maxima of which are
hypsochromically shifted with respect to the longꢀwaveꢀ
length absorption band of the merocyanine form of SBC.
A similar ionochromic effect was observed for SBC anaꢀ
logs, viz., 8´ꢀbenzothiazolylꢀsubstituted spirochromenes17
and 5ꢀbenzoxazolylꢀsubstituted SBC.23 The addition of
Mn2+ and Cd2+ salts results in the coloration of a solution
of SBC if the metal salts are present in a 5—10ꢀfold excess.
Alkali and alkalineꢀearth metals do not cause the coloraꢀ
tion even if they are present in a large excess.
Results and Discussion
The composition of the complexes was studied spectroꢀ
photometrically using the method of continuous variation
(Job´s method),24 which has been employed earlier to deꢀ
termine the compositions of similar complexes.17,20,23,25
An acetone solution of 5ꢀbenzothiazolylꢀsubstituted
spirobenzochromene (SBC) is substantially colored due
* For Part 10, see Ref. 1.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0677—0682, March, 2015.
1066ꢀ5285/15/6403ꢀ0677 © 2015 Springer Science+Business Media, Inc.

678
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 3, March, 2015
Rostovtseva et al.
Scheme 1
A (arb. units)
1.2
1.0
0.8
0.6
0.4
0.2
0.2
0.4
C(Ni2+)/[C(Ni2+) + C(SBC)]
Fig. 2. Absorbance versus the composition of the isomolar series for
SBC with nickel in acetone, C(SBC) + C(Ni2+) = 4•10–5 mol L–1
app = 607 nm.
,

case of Co2+, Ni2+, and Cu2+ ions, the isomolar diagrams
show the formation of 1 : 2 complexes. Figure 2 depicts
the isomolar diagram for SBC and Ni(ClO4)2 in acetone.
The maximum in the diagram is shifted toward the lower
metal content compared to the equimolar amount due to
the contribution of both 1 : 1 and 1 : 2 (metal : SBC) comꢀ
plexes to the absorption at the observation wavelength.
We obtained single crystals of the open form of SBC
with zinc suitable for Xꢀray diffraction by the slow evapoꢀ
ration of the solvent from an acetone solution containing
equimolar amounts of SBC and zinc chloride, which alꢀ
lowed us to study the molecular structure of this comꢀ
pound in detail. The composition of the complex in the
solid state was found to be the same as in solution (1 : 1,
zinc: SBC). The geometry of the complex in the crystal
(Xꢀray diffraction study was performed at 150 K) is shown
in Fig. 3 (atoms are represented as thermal ellipsoids drawn
at 30% probability level).
A comparison of the structural details of the open forms
of spiropyrans described earlier26,27 and of the complex
under study, which are related to the polymethine chain,
including the indoline moiety and the phenolate oxygen
atom, demonstrates that there are no changes in the strucꢀ
ture of the open form of spiropyran upon the complexꢀ
ation. This refers both to the bipolar structure as a whole
and the C(2´)—O(9´) bond lengths (1.282(5) Å). A simiꢀ
lar situation was observed for the complex of antimony
trichloride with the merocyanine form of 1´ꢀisopropylꢀ
8ꢀmethoxyꢀ3´,3´ꢀdimethylꢀ6ꢀnitroꢀ1´,3´ꢀdihydrospiroꢀ
[chromeneꢀ2,2´ꢀindole],28 in which the coordination polyꢀ
hedron can be described as a square pyramid.
Note. hv1 and hv2 refer to irradiation with visible and UV light,
respectively.
Figure 1 displays the isomolar diagram for SBC and
Zn(ClO4)2 in acetone. A distinct extreme in the diagram
corresponds to the equimolar composition (50 mol.% Zn
and 50 mol.% SBC). The form of the diagram is indepenꢀ
dent of the observation wavelength. This attests to the
formation of a 1 : 1 (metal : SBC) complex in solution.
Similar data were obtained for Mn2+ and Cd2+ ions. In the
A (arb. units)
0.25
0.20
0.15
0.10
0.05
The coordination polyhedron of Zn in the complex
under study is a tetrahedron, with all angles at the zinc
atom being in the range of 107.14(9)—118.06(9), except
for the O(9´)—Zn(1)—N(3´) angle (91.6(1)). This is apꢀ
parently attributed to the fact that the distance between
the O(9´) and N(3´) atoms in the molecule is 2.833(5) Å
due to the necessity of the sixꢀmembered metallocycle
0
0.2
Fig. 1. Absorbance versus the composition of the isomolar series for
SBC with zinc in acetone, C(SBC) + C(Zn2+) = 4•10–5 mol L–1
app = 567 nm.
0.4
C(Zn2+)/[C(Zn2+) + C(SBC)]
,


Complexes of spiro[benzo[f]chromeneꢀ3,2´ꢀindole]
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 3, March, 2015
679
C(6´)
C(7´)
C(5´)
C(8´)
C(4A´)
C(9)
C(8)
C(4´)
C(8A´)
C(1´)
C(3)
C(11)
C(3´)
C(4)
S(1)
C(2)
C(2´)
C(10)
C(3A)
C(2)
C(7A)
N(11)
O(9´)
C(5)
C(7A)
C(131)
C(7)
C(6)
N(3)
Cl(2´)
C(6)
C(7)
C(3A)
C(121)
Zn(1´)
C(141)
C(4)
C(142)
C(5)
Cl(1´)
Fig. 3. Molecular structure of the complex of SBC with zinc.
B + MB
MB2.
closure because all angles at the carbon, nitrogen, and
oxygen atoms of the metallocycle are in the range of
122.0(3)—127.4(3). The C(8)—C(9)—C(1´) (129.4(4))
and N(3)—C(2)—C(10) (127.4(3)) angles are also larger
than 120 due apparently to steric factors as well. Thus,
the distance between the H(8) and O(9´) atoms is 2.03 Å,
which is undoubtedly associated with the coordination of
zinc by the O(9´) and N(3) atoms. A similar distortion of
the tetrahedral environment of the zinc atom involving
two chlorine, one oxygen, and one nitrogen atoms was
observed in the zinc complex with the oxazoleꢀcontaining
ligand,29 in which the O...N distance is 3.04 Å, and the
O—Zn—N and Cl—Zn—Cl angles are 95.6 and 123.0,
respectively; the other angles in the tetrahedron are in the
range of 102—113. For comparison, it may be noted that
the coordination polyhedron of BF2 in the complex of
4ꢀ(4ꢀchlorophenyl)ꢀ1,1ꢀdifluoroꢀ3ꢀmethylꢀ1Hꢀpyridoꢀ
[1,2ꢀc][1,3,2]oxazaborininꢀ9,1ꢀylide is also a tetraꢀ
hedron,30 but the N...O distance is 2.477(6) Å and the
angles at the boron atom are in the range of 106.6—110.6.
The terminal atom of the propyl group in the complex
under study is disordered over two positions with an occuꢀ
pancy ratio of 2 : 1 (the less occupied position is not
dashed).
These equilibria are characterized by the following conꢀ
stants:
KT = [B]/[A],
(1)
(2)
(3)
K1 = [MB]/([B][M]),
K2 = [MB2]/([MB][M]).
Spirobenzochromene in acetone is characterized by
a low equilibrium content of the merocyanine isomer,
which cannot be quantified by NMR spectroscopy.31 Conꢀ
sequently, the constant KT cannot be calculated. For this
reason, the evaluation of the true stability constants of the
merocyanine form with metal ions (see Eqs (2) and (3)) is
hindered. Besides, the equilibrium constants (2) and (3)
cannot be determined by the method proposed in the
study32 because of the very short (<1 s)22 relaxation time
of the merocyanine isomer at Т = 293. Therefore, we calꢀ
culated the effective stability constants, which account for
the tautomeric equilibrium of the ligand:20
Kieff = [MBi]/([MBi–1][L]i),
(4)
The important characteristics of complexes in solution
are their stability constants. The formation of SBC comꢀ
plexes with metal ions in solution (see Scheme 1) can be
represented as a combination of coupled equilibria:
where [L] = [B] + [A] = CL – [MBi] is the equilibrium
concentration of the SBC forms, which are not involved
in the complex, and CL is the total concentration of
SBC. It is the values of Kieff that are required for pracꢀ
tical calculations of the degree of complexation and
the prediction of the strength of the binding of metal
ions in complexes under real conditions, which is imporꢀ
A
B,
B + M
MB,

680
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 3, March, 2015
Rostovtseva et al.
A (arb. units)
tant for the application of these compounds for analytiꢀ
cal purposes.
The stability constants Kieff were determined spectroꢀ
photometrically based on the dependences of the absorꢀ
bance of solutions containing a fixed amount of SBC and
a variable amount of the metal salt.
In the case when SBC forms only a 1 : 1 complex with
a metal ion, the absorption spectra show a single band of
the complex in the visible region (Fig. 4), and the concenꢀ
tration dependence of the absorbance can be described by
Eq. (5):33
A (arb. units)
13
6
1.5
1
2
1.5
1.2
0.9
0.6
0.3
13
6
5
1.2
0.9
0.6
0.3
0
4
3
0
1
С(Сo2+)•104/mol L–1
2
1
A = A0 + [(Amax – A0)/(2CL)]{CL + CM + 1/Kieff
– [(CL + CM + 1/Kieff)2 – 4CLCM]1/2}.
–
(5)
350 400
450
500
550
600
650 /nm
Figure 4 depicts the absorption spectra of SBC in the
presence of cadmium ions, which are indicative of the
formation of 1 : 1 complexes. Figure 5 presents the abꢀ
sorption spectra of SBC containing different amounts of
cobalt ions. As can be seen in Fig. 5, the intensity and
position of the longꢀwavelength absorption maximum
largely depends on the amount of the metal salt in soluꢀ
tion, resulting from the formation of both 1 : 1 and 1 : 2
complexes. Solutions 1—6 containing an excess of SBC
are characterized by the predominance of the 1 : 2 comꢀ
plex. The visible region of the absorption spectra shows
a maximum at 606 nm. An increase in the metal concenꢀ
tration (solutions 7—13) up to the 15ꢀfold excess leads to
a decrease in the percentage of the 1 : 2 complex and an
increase in the percentage of the 1 : 1 complex, which is
manifested in the spectra as a decrease in the absorption at
600 nm and the appearance of a new band with a maxiꢀ
Fig. 5. Changes in the absorption spectrum of SBC upon the
addition of different amounts of Co(ClO4)2 in acetone; C(SBC) =
= 3.66•10–5 mol L–1; C(Co2+) = 3.14•10–6 (1), 6.28•10–6 (2),
9.42•10–6 (3), 1.26•10–5 (4), 1.88•10–5 (5), 3.14•10–5 (6),
4.39•10–5 (7), 6.28•10–5 (8), 8.79•10–5 (9), 1.26•10–4 (10),
1.88•10–4 (11), 3.77•10–4 (12), and 5.65•10–4 mol L–1 (13).
The inset shows the dependence of the absorbance at 582 (1) and
603 nm (2) on the Co2+ concentration; the points correspond to
the experimental data, and the solid curves represent the calcuꢀ
lated data.
mum at 578 nm. In this case, the calculation of the stability
constants is based on the iterative numerical solution of the
system of Eqs (6)—(10) by the minimization of the function
where nS is the number of solutions and n is the number
of the observation wavelengths.
A (arb. units)
A587 (arb. units)
9
1.5
Acalc = ML[MB] + ML2[MB2] +  eff[L] +  [M]
(6)
(7)
L
M
1.2
0.9
0.6
0.3
0
1.2
0.9
0.6
0.3
[MB] = CMK1eff[L]/(1 + K1eff[L] + K1effK2eff[L]2)
9
1
[MB2] = CMK1effK2eff[L]2/(1 + K1eff[L] +
+ K1effK2eff[L]2)
0.8
2.4
4.0
С(Сd2+)•104/mol L–1
(8)
(9)
CL = [MB] + 2[MB2] + [L]
CM = [MB] + [MB2] + [M]
1
400
500
600
/nm
(10)
Fig. 4. Changes in the absorption spectrum of SBC upon the
addition of different amounts of Cd(ClO4)2 in acetone; C(SBC) =
= 2.45•10–5 mol L–1; C(Cd2+) = 2.14•10–6 (1), 8.54•10–6 (2),
1.71•10–5 (3), 2.99•10–5 (4), 5.12•10–5 (5), 8.54•10–5 (6),
1.28•10–4 (7), 2.14•10–4 (8), and 3.84•10–4 mol L–1 (9). The
inset shows the dependence of the absorbance in the absorption
maximum of the complex (at 587 nm) on the Cd2+ concentraꢀ
tions; the points correspond to the experimental data, and the
solid curves represent the calculated data.
Figure 5 also depicts the experimental dependences of
the absorbance at the chosen wavelengths on the concenꢀ
tration of the metal ion. These dependences are in good
agreements with the theoretically calculated values (solid
curves). The stability constants of the complexes and their
absorption spectroscopic characteristics are given in
Table 1. The stability of the complexes increases in the

Complexes of spiro[benzo[f]chromeneꢀ3,2´ꢀindole]
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 3, March, 2015
681
Table 1. Calculated logarithms of the effective complex formation constants and absorption spectroscopic
characteristics of the SBC complexes in acetone; Т = 293 K
Metal
MB
MB2
•10–3
logK2

/nm

•10–3
max
eff
eff
logK1

/nm

max
max
max
/L mol–1 cm–1
/L mol–1 cm–1
Cd
Zn
Co
Cu
Mn
Ni
3.92 0.01
5.85 0.01
5.5 0.1
7.2 0.5
3.18 0.01
6.0 0.5
587
572
578
562
585
585
51.6
44.7
45.5
32.9
33.5
51.3
—
—
6.6 0.1
6.1 0.5
—
—
—
606
574
—
—
—
182.3
150.3
—
7.3 0.5
607
101.6
series of Mn2+, Cd2+, Zn2+, Co2+, Ni2+, Cu2+ ions. The
absorption bands of the 1 : 2 complexes are bathochromiꢀ
cally shifted with respect to the absorption bands of the
1 : 1 complexes and have a 1.5—2 times higher intensity.
The visible light irradiation of solutions of the SBC
complexes with diamagnetic cadmium and zinc ions leads
to thermal decoloration due to the photodissociation to
the initial spirocyclic isomer and free metal ions. After the
switchingꢀoff of the radiation source, the initial equilibriꢀ
um is recovered. Complexes with paramagnetic ions are
resistant to radiation. The quantum yield is a quantitative
parameter characterizing the efficiency of the photoproꢀ
cess. This parameter was calculated using the photostaꢀ
tionary state approximation. For this purpose, a solution
of SBC in the presence of an excess of a metal salt was
irradiated with filtered visible light ( = 546 or 578 nm) in
order to completely transform SBC into the correspondꢀ
ing complex, and the absorption spectra were simultaꢀ
neously recorded until the photostationary state was
achieved (Fig. 6). Based on the results of this study, the
molar fraction of the complex in the photostationary state
was determined from the relation st = Ast/A0, where Ast is
the absorbance of the solution in the absorption maximum
Table 2. Photodissociation quantum yields of the
complexes at  = 546 (I) and 578 nm (II)
Metal
•103
I
II
Cd
Zn
3.3
6.5
2.9
5.3
of the complex in the photostationary state and A0 is the
absorbance of the solution before irradiation. The quanꢀ
tum yield was calculated by the equation23
Ф = k(1 —  )C0/{[1 – exp(–2.303Amax )]I0}.
st
st
The calculated values are in the range of 0.0029—
0.0065 (Table 2) and are virtually independent of the radiꢀ
ation wavelength.
Therefore, 5ꢀ(1,3ꢀbenzothiazolꢀ2ꢀyl)ꢀsubstituted
1´,3´ꢀdihydrospiro[benzo[f]chromeneꢀ3,2´ꢀindole] exhibꢀ
its ionochromic properties due to the formation of inꢀ
tensely colored stable complexes of the merocyanine isomer
with transition metal ions. The composition and stability
of the complexes are determined by the nature of the cenꢀ
tral ion. The complexes with zinc and cadmium ions disꢀ
play negative photochromism due to photodissociation.
A (arb. units)
A0
1.5
Experimental
Ast
The synthesis of spirobenzochromene has been described
earlier.22 The electronic absorption spectra and the kinetic curves
were recorded on an Agilent 8453 spectrophotometer equipped with
a temperatureꢀcontrol accessory. The photolysis of solutions was
performed by irradiation with a Newport system based on a 200 W
mercury lamp equipped with interference filters. Solutions were
prepared using acetone of spectrophotometric grade (Aldrich).
The optical radiation intensity, which was determined with
a Newport 2935 optical power meter at the wavelengths of 546 and
578 nm, was 1.8•1017 and 1.9•1017 photon s–1, respectively.
Xꢀray diffraction study. The unit cell parameters were deterꢀ
mined and the threeꢀdimensional set of intensities was measured
on a Xcalibur, Eos automated diffractometer (MoꢀK radiation,
1.0
0.5
350
400
450
500
550
600
/nm
Fig. 6. Changes in the absorption spectrum of the SBC complex
with zinc ions upon irradiation with visible light at  = 578 nm.

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Rostovtseva et al.
graphite monochromator) at 150 K. Black single crystals of the
complex C32H28N2OCl2SZn (M = 624.89) are monoclinic, a =
= 9.7805(5) Å, b = 29.2694(16) Å, c = 10.2480(5) Å,  = 93.375(4),
V = 2928.6(3) Å3, Z = 4, dcalc = 1.417 g cm–3, (MоꢀK) =
= 1.121 mm–1, space group P21/c. The intensities of 17452 reꢀ
flections were measured in the angle range 2  59.0 using
the ꢀscanning technique from a single crystal of dimensions
0.40×0.30×0.25 mm. The empirical absorption correction
was applied using the Multiscan technique. After rejection of
systematic absences and merging of equivalent reflections
(R(int) = 0.018), the Xꢀray data set contained 8088 unique reꢀ
flections (F2(hkl) and (F2)), of which 6749 reflections were
with F2 > 2(F2). The structure was solved by direct methods
and refined by the fullꢀmatrix leastꢀsquares method based on F2
using the SHELXTL program package with anisotropic displaceꢀ
ment parameters for nonhydrogen atoms, except for the less
occupied C(142) atom (see Fig. 3). Attempts to refine the latter
atom with an anisotropic displacement parameter revealed the
further disorder of the atoms of the propyl group. In the crystal
structure of the complex under study, most of H atoms were
located in difference Fourier maps, and then the coordinates
and isotropic thermal parameters for all H atoms were refined by
the leastꢀsquares method using a riding model.34 In the last cycle
of the fullꢀmatrix refinement, the absolute shifts of all 352 variꢀ
able parameters of the structure were 0.003. The final refineꢀ
ment parameters were R1 = 0.081, Rw = 0.15; GOOF = 1.247
based on the observed reflections. After the refinement, the maxꢀ
imum and minimum difference electron densities were 1.789
and –0.772 e А–3, respectively. The CIF file, which contains the
complete information on the structure, was deposited in the
Cambridge Crystallographic Data Centre (CCDC 1039032)
and can be obtained, free of charge, on application to
www.ccdc.cam.ac.uk/data_request/cif. In conclusion, let us note
that we treated the quite pronounced residual electron density
maximum as an indication of the presence of a water molecule.
The refinement of the occupancy of this water molecule gave
0.25. However, taking into account an insignificant decrease in
the R factor (0.003) and the absence of possible hydrogen bonds,
this assumption cannot be made with confidence.
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This study was financially supported by the Russian
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Received December 11, 2014;
in revised form February 3, 2015