1-Benzyl-3,3,5′,6′-tetramethylspiro[indoline-2,2′-[2H] pyrano[3,2-b]- pyridinium] iodide, its hydrate, and a neutral precursor of the salts: Synthesis, crystal structure, photochromic transformations in solutions and in crystals

Article abstract of DOI:10.1007/s11172-011-0210-z

A new 1-benzyl-3,3,5′,6′-tetramethylspiro[indoline-2,2′- [2H]pyrano[3,2-b]pyridinium] iodide, photochromic in the crystal state, was obtained. X-ray diffraction analysis was used for determination of crystal structures of the salt, its hydrate, and a neutral precursor. It was found that a replacement of the substituent in the indoline fragment leads to a considerable change in the crystal structure of both the neutral spiropyran and the salts as compared to the analogs studied earlier. Effects of crystal structure on photochromic properties were studied.

Full text of DOI:10.1007/s11172-011-0210-z

Russian Chemical Bulletin, International Edition, Vol. 60, No. 7, pp. 1401—1408, July, 2011
1401
1ꢀBenzylꢀ3,3,5´,6´ꢀtetramethylspiro[indolineꢀ2,2´ꢀ[2H]pyrano[3,2ꢀb]ꢀ
pyridinium] iodide, its hydrate, and a neutral precursor of the salts:
synthesis, crystal structure, photochromic transformations
in solutions and in crystals
E. A. Yurieva, S. M. Aldoshin, L. A. Nikonova, G. V. Shilov, and V. A. Nadtochenko
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Eꢀmail: yurieva@icp.ac.ru
A new 1ꢀbenzylꢀ3,3,5´,6´ꢀtetramethylspiro[indolineꢀ2,2´ꢀ[2H]pyrano[3,2ꢀb]pyridinium]
iodide, photochromic in the crystal state, was obtained. Xꢀray diffraction analysis was used for
determination of crystal structures of the salt, its hydrate, and a neutral precursor. It was found
that a replacement of the substituent in the indoline fragment leads to a considerable change in
the crystal structure of both the neutral spiropyran and the salts as compared to the analogs
studied earlier. Effects of crystal structure on photochromic properties were studied.
Key words: spiropyrans, crystal structure, photochemistry in the crystal state.
Scheme 1
Synthesis of photomagnets occupies an important place
in the rapidly developing field of studies on creation
of modern polyfunctional materials for electronics.1—3
One of the approaches to the preparation of photomagꢀ
netic materials consists in the incorporation of photoꢀ
chromic organic molecules into the interlayer space
of layered magnets, for example, layered copper(II)4
and cobalt(II)5 bihydroxides, bimetallic compounds with
oxalate6—8 or dithiooxalate9—11 bridged bonds, etc.12,13
In fact, the structural rearrangement and redistribution
of electron density during isomerization of a photoꢀ
chromic molecule placed between magnetic layers open
the way to the photocontrol of magnetic properties of
a hybrid compound.
Representatives of three major classes of photochroms
have been already used as photosensors in the hybrids based
on layered magnets, they are azobenzene4 and diarylꢀ
ethene5,13 derivatives, as well as spiropyrans.6—12 Among
the photochromes that studied, cations of spiropyrans of
the indoline series still remain the best photoswitches of
magnetic properties for such hybrids.
In 2000, the spiro[indolineꢀpyranopyridinium] salts
(R—SP)+I– (Scheme 1) were described,14 which exhibited
photochromic properties in both solutions and crystals, and
soon they were used for the synthesis of polyfunctional
compounds.
SP is the cyclic form, MC is the open (merocyanine) form
Earlier, while crystal structures of various salts
(R—SP)+I– were studied it was shown that photochroꢀ
mic transformations of (R—SP)+ are due to the speꢀ
cific structure of the salt15—18 and a dependence between
the crystal packing density and the rate of the dark
bleaching of the cation MCꢀform in these salts was esꢀ
tablished.17
The prospects in the use of spiropyran salts for prepaꢀ
ration of polyfunctional compounds, as well as their unique
photochemical properties in crystals stimulated further
studies of these compounds.15—19
In continuation of these systematic studies, we synꢀ
thesized 1ꢀbenzylꢀ3,3,5´,6´ꢀtetramethylspiro[indolineꢀ
2,2´ꢀ[2H]pyrano[3,2ꢀb]pyridinium] iodide (1), studied
crystal structures of the salt, its hydrate 1•H2O, and their
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1379—1385, July, 2011.
1066ꢀ5285/11/6007ꢀ1401 © 2011 Springer Science+Business Media, Inc.

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Yurieva et al.
the unit cell were determined and refined on 35 reflections found
in the range of angles θ from 5 to 10°.
neutral precursor 2, as well as photochemistry of these
compounds in solutions and crystals.
The structures were solved by the direct method. Positions
and thermal parameters of nonhydrogen atoms were refined first
in isotropic and then in anisotropic approximation by the fullꢀ
matrix least squares method. Positions of hydrogen atoms in the
molecules were found from the differential Fourier syntheses
and refined using the riding model. When the oxygen atom of the
water molecules in the crystal structure of compound 1•H2O
were refined, the population was also refined along with posiꢀ
tional and thermal parameters. Positions of the hydrogen atoms
of the water molecules were not found.
The principal crystallographic data and parameters of reꢀ
finement are given in Table 1. All the calculations were perꢀ
formed using the SHELXTL program package.23
Photochemical studies in the crystal state were performed for
the thinꢀlayer samples (~10 μm), which were prepared from the
mulled single crystals dispersed in Nujol (Aldrich).
A PLꢀS 9W gasꢀdischarge low pressure mercury lamp proꢀ
ducing the UV light in the range 340—390 nm with the maxiꢀ
mum at 355 nm was used for the irradiation, as well as a LED
with the irradiation 530 nm (60 W) producing a light spot 1.5 cm
in diameter.
Absorption spectra were recorded on a Perkin—Elmer Lambda
EZ 210 spectrometer. Placement of the samples in the instruꢀ
ment was strictly fixed and the same in each measurement in
order to exclude changes in the base line, which depend on the
dispersal of the powder when samples are placed differently with
respect to the optical axis of the instrument. Changes in the
absorption spectra of solutions were recorded on an OceanOpꢀ
tics HR2000 multichannel spectrometer in the automatic scanꢀ
ning mode through the given periods of time. The experimental
data that obtained were processed using the Igor Pro 4.0 program.
Experimental
2,3,3ꢀTrimethylindolenine, benzyl bromide, and methyl
iodide from Aldrich were used for the syntheses. 3ꢀHydroxyꢀ6ꢀ
methylꢀ2ꢀpyridinecarbaldehyde (3) was obtained from furfurylꢀ
amine (Aldrich) using the known procedures.20,21 1H NMR
spectra were recorded on a Bruker Avance 200 (200 MHz)
spectrometer in CDCl3 using Me4Si as an internal standard.
1ꢀBenzylꢀ3,3ꢀdimethylꢀ2ꢀmethyleneindoline (4) was syntheꢀ
sized according to the modified method described earlier,22 with
the difference that the reaction of benzyl bromide with 2,3,3ꢀtriꢀ
methylindolenine was carried out in a homogeneous system usꢀ
ing acetonitrile as a solvent instead of toluene. The residue after
evaporation of acetonitrile was dissolved in diethyl ether, treated
with 30% aqueous KOH, and compound 4 was isolated from the
ethereal solution by distillation as a rapidly crystallized oil (b.p.
130—132 °C (0.2 Torr)).22 Colorless crystals 4 (m.p. 63 °C) turn
pink in air. Found (%): C, 86.62; H, 7.81; N, 5.77. C18H19N.
Calculated (%): C, 86.70; H, 7.68; N, 5.62. 1H NMR, δ: 1.43
(s, 6 H, Me); 3.89 (s, 2 H, Csp3=CH2); 4.74 (s, 2 H, NCH2);
6.53—7.36 (m, 9 H, Ar).
1ꢀBenzylꢀ3,3,6´ꢀtrimethylspiro[indolineꢀ2,2´ꢀ[2H]pyranoꢀ
[3,2ꢀb]pyridine] (2). A solution of compound 4 (2 g, 8 mmol)
and aldehyde 3 (1.1 g, 8 mmol) in anhydrous ethanol (45 mL)
was refluxed for 5 h. The colorless crystals that formed after
cooling of the reaction solution were filtered off, the filtrate after
concentration till ~1/3 of volume gave additionally 0.4 g of the
product. The yield of compound 2 was 2.5 g (85%). After crystalꢀ
lization from ethanol and then from heptane, m.p. 132—133 °C.
Found (%): C, 81.40; H, 6.64; N, 7.45. C25H24N2O. Calculatꢀ
ed (%): C, 81.49; H, 6.56; N, 7.60. 1H NMR, δ: 1.29, 1.35 (both s,
3 H each, Me); 2.47 (s, 3 H, Me); 4.23, 4.58 (both d, 1 H each,
CH2Ph, J = 16.4 Hz); 5.97—7.33 (m, 13 H, Ar). Single crystals
were obtained by recrystallization from ethanol.
1ꢀBenzylꢀ3,3,5´,6´ꢀtetramethylspiro[indolineꢀ2,2´ꢀ[2H]ꢀ
pyrano[3,2ꢀb]pyridinium] iodide (1) was obtained by the Nꢀmeꢀ
thylation of the pyridine ring of compound 2 with methyl iodide
in THF according to the method described earlier.14 Found (%):
C, 61.19; H, 5.37; I, 25.13; N, 5.42. C26H27IN2O. Calculatꢀ
ed (%): C, 61.18; H, 5.33; I, 24.86; N, 5.49. Anhydrous single
crystals of 1 were grown from ethanol, the hydrate 1•H2O was
obtained by crystallization from aq. methanol.
Xꢀray diffraction studies of the salt 1 were performed on an
Enraf—Nonius CADꢀ4 automatic fourꢀcircle diffractometer
(ω/2θꢀscanning, MoꢀKα radiation, 300 K). A yellow single crysꢀ
tal in the shape of parallelepiped sized 0.4×0.4×0.3 mm was used
in the experiment. Parameters of the unit cell were determined
and refined on 30 reflections found in the range of angles θ
from 5 to 10°.
Results and Discussion
Salts of pyridospiropyran are produced by a twoꢀstep
synthesis. The condensation of Nꢀsubstituted indoline 4
with pyridinecarbaldehyde 3 leads to the neutral spiropyrꢀ
an 2, whose Nꢀmethylation furnishes pyridinium iodide 1
(Scheme 2).
Molecular and crystal structure of salts 1 and 1•H2O.
In the structures of the salts, general configuration of the
cation SP+ (Fig. 1) is the same and similar to the structure
of spiropyrans studied earlier.15—18 The indoline and benꢀ
zopyran fragments are orthogonal to each other, the angle
between the planes {O(1´), C(2´2), C(3´)} and {C(3),
C(2´2), N(1)} is 88.7° (89.6°). The nitrogen atom N(1)
has a pyramidal configuration and comes out of the plane
of the surrounding carbon atoms by 0.28 Å (0.21 Å). As in
all the compounds studied earlier, such a molecular geꢀ
ometry is favorable for the nN(1)—σ*[C(2´2)—O(1´)] orꢀ
bital interactions, which lead to the elongation of the
C(2´2)—O(1´) bond to 1.474(4) Å (1.486(5) Å) as comꢀ
pared to the common bond distances, 1.41—1.43 Å, in the
The Xꢀray diffraction experiments for the single crystals of
compounds 2 and 1•H2O were performed on a Bruker Pꢀ4 autoꢀ
matic fourꢀcircle diffractometer (θ/2θꢀscanning, MoꢀKα radiaꢀ
tion, 300 K). For the study of compound 2, we used a colorless
single crystal in the shape of parallelepiped sized 0.15×0.2×0.35 mm,
and for 1•H2O, a yellow single crystal sized 0.20×0.25×0.45 mm.
It should be noted that the crystals of 1•H2O were of low quality
and underwent slow decomposition on standing. Parameters of
Here and further parameters are given for compound 1 and in
parentheses, for 1•H2O.

Solidꢀstate photochromic spiropyran
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
1403
Table 1. The crystallographic data and parameters of the Xꢀray diffraction experiment
Parameter
1
1•H2O
2
Molecular formula
M/g mol–1
C26H27N2OI
510.40
C26H29N2O2I
542.41
C25H24N2O
368.46
Crystal system
Space group
Monoclinic
P21/c
Triclinic
Pꢀ1
Monoclinic
P21/n
a/Å
b/Å
c/Å
α/deg
15.564(3)
12.738(3)
12.631(3)
90.00
106.78(3)
90.00
8.652(2)
11.151(2)
13.886(3)
103.95(3)
106.48(3)
94.75(3)
1230.2(4)
1.427
12.890(3)
9.027(2)
17.771(4)
90.00
98.93(3)
90.00
β/deg
γ/deg
V/Å3
2397.5(8)
1.414
2042.7(7)
1.198
dcalc/g cm–3
Z
4
2
4
μ/mm–1
1.354
1.325
0.073
Region of scanning/deg
Number of measured reflections (Rint
Number of reflections with I > 2σ(I)
Number of refined parameters
GOOF
1.37—25.01
4433 (0.0186)
4220
1.91—25.00
4977 (0.0301)
4181
1.82—24.98
3983 (0.0227)
2992
)
275
1.027
285
0.930
256
1.071
R1 (I > 2σ(I))
0.0385
0.0550
0.0466
wR2 (on all ther reflections)
0.1239
0.1547
0.1400
Scheme 2
distances of these bonds in cations with other substituents
(Me and Pri) in the indoline fragment.15,17. Thus, the elonꢀ
gation and weakening of the Cspiro—O bond makes its
photoexcitation cleavage easier.
The turn of the ring in the benzyl substituent with
respect to the indoline fragment due to the rotation around the
N(1)—C(10) and C(10)—C(11) bonds in the salts is someꢀ
what different, that, obviously, is caused by the packing
effects. The torsional angle N(1)—C(10)—C(11)—C(12)
is 166.8° (169.4°).
The salt 1, like the (R—SP)+I– salts (R = Me and
Pri),15,17 crystallizes in the monoclinic crystal system (the
C(5)
C(4)
C(6)
C(9)
C(3)
C(8A´)
O(1´)
C(8´)
C(7´)
C(7)
C(8)
C(6´)
C(7A)
N(1)
C(4A´)
C(4´)
C(3´)
C(9´)
N(5´)
C(16)
C(15)
C(10)
C(11)
C(5´)
C(12)
C(13)
sixꢀmembered heterocycles and the shortening of the
C(2´2)—N(1) bond to 1.435(5) Å (1.429(6) Å) as comꢀ
pared to the normal C(sp3)—N(sp3) bond distances in the
fiveꢀmembered heterocycles 1.47—1.48 Å.24 The values
that obtained coincide within the error limits with the
C(14)
Fig. 1. The cation in the crystal structure 1, the dashed line
shows the cation in the structure of 1•H2O.

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Yurieva et al.
space group is P21/c). An increase in the size of substituꢀ
ent in the order Me, Pri, and CH2Ph leads to a natural
increase in the parameter b of the unit cell (10.959(2),
11.051(3) and 12.738(3) Å, respectively), as well as to
a decrease in the calculated density (1.482, 1.439, and
1.414 g cm–3, respectively), that indicate a decrease in the
density of packing. The same principle of packing operꢀ
ates for the salts (R—SP)+I– (R = Me and Pri):15,17 the
cations in the structure are packed in stacks with the indoꢀ
line fragments around the axis of symmetry 21, whereas
the pyranopyridinium fragments are turned into the interꢀ
stack space to the iodine anions; the neighboring stacks
are bound between each other by the center of inversion.
Unlike the structures studied earlier, whose planes of the
indoline fragments of the cation are placed one over anꢀ
other and are almost perpendicular to the axis 21 (the
angles between the benzene ring of the indoline fragments
of the molecules and the axis 21 are equal to 71 and 73°),
in the crystal structure 1, the angle between the mean
plane of the benzene ring of the indoline fragment and the
axis is 29.1° (Fig. 2).
Compound 1 is an ionic one, however, the energy of
interactions can be a quality criterion of the characteristic
properties of molecular packing in the crystal structure.
Calculations of the energy of intermolecular interactions
(IMI) in the structure were performed without allowance
for the anions in the framework of the atomꢀatom approxꢀ
imation with the "6ꢀexp" potential.25 According to the
data that obtained, the strongest interactions exist between
the molecules bound by the center of inversion: –8.1 and
–9.4 kcal mol–1, respectively, for the pairs C—E and G—E
(see Fig. 2), whereas in the structures (R—SP)+I– (R = Me
and Pri) studied earlier,15,17 the direction along the b axis
between pairs of the molecules bound by the screw axis
was more pronounced. In this case, the energy of IMI
between molecules E—H and E—B (see Fig. 2) is considꢀ
erably lower: –4.2 and –3.3 kcal mol–1, respectively. The
energies of IMI (kcal mol–1) with other molecules are of
the same order: D and E (–4.0), F and E (–3.8), I and E
(–5.4, is somewhat higher due to the overlap of the benꢀ
zene rings in the substituent). It can be concluded that no
pronounced stacks are observed in this salt.
The I– anions are placed between the pyran (charged)
fragments of four cations (see Fig. 2). The shortest disꢀ
tances N(5´)D...I(1A) 3.61 Å and N(5´)A...I(1A) 3.71 Å
indicate a relatively weak interaction of the anion with the
positively charged nitrogen atoms of these cations. The
anion also forms shortened contacts with two other catꢀ
ions C(4´)B...I(1A) 3.88 Å and C(7´)E...I(1A) 3.85 Å (the
angle I...H—C is 154.9 and 171.4°, respectively).
Crystal structure of the hydrate 1•H2O (Fig. 3) qualiꢀ
tatively resembles the structure of the salt (Pri—SP)I studiꢀ
ed earlier:17 the indoline fragments of cations are perpenꢀ
dicular to the b axis and are also packed in stacks, whereas
the pyranopyridinium cations are turned into the interꢀ
stack space, being parallel to each other. The major differꢀ
ence is that in this case cations in stacks are bound by the
center of inversion, rather than the screw axis 21.
The presence of stacks in this structure was confirmed
by calculations of energy of IMI (without allowance for
the iodine atoms and water molecules). The energy of IMI
between the pairs of cations C—D and D—C´ is –8.7 and
–10.8 kcal mol–1 (see Fig. 3), i.e., the stack forms along
the axis. The crystal 1•H2O significantly differs from the
structure of (Pri—SP)I obtained earlier17 by the presence
of interactions not only in stacks, but also between them.
Thus, the energy of interaction between the pair B and C is
B
C
A
A
a
a
I(1A)
E
I(1A)
F
D
B
E
O(WA)
I(1)
O(W)
C
0
c
b
D
I
G
H
I(I)
0
b
c
Fig. 3. The projection of the crystal structure 1•H2O on the
plane of the cell ac. The letters A—E show cations. Cations A´
and C´ are bound with cations A and C by translation along the
axis b and are not shown in the Figure.
Fig. 2. The projection of the crystal structure 1 on the plane of
the cell ac. The letters A—I show cations.

Solidꢀstate photochromic spiropyran
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
1405
–7.5 (the short contact C(9)B...C(9)C is 3.38 Å), the interꢀ
actions B—D with the energy –6.6 kcal mol–1 (the short
contact C(5´)B...C(13)D is 3.42 Å) and C—E with the
energy –7.2 kcal mol–1 (due to the partial overlap of the
pyridine rings) should be additionally mentioned. The rest
of the interactions are weaker than –3.0 kcal mol–1. The
I– anions in the structure are located between the pyranꢀ
opyridinium fragments in the interstack space (see Fig. 3),
the closest contacts with the cations are: I–...C(7´) 4.02 Å
and I–...C(4´) 4.14 Å. The shortest distance to the posiꢀ
tively charged atom N(5´) of the cation is equal to 3.82 Å.
Since compounds 1 and 1•H2O are salts, an ionic type
of interaction is mainly responsible for the formation of
their crystal structure. Therefore, for the comparison of
the crystal structures of these compounds, we selected
those structural fragments, where these interactions are
the strongest. In the structure 1, we found the infinite
chains ...I–...N+...I–... in the direction of the unit cell axis b
(Fig. 4). In this chain, the organic cations are arranged
according to the "tailꢀtoꢀtail" principle and are bound beꢀ
tween each other by the screw axis. In the structure 1•H2O,
the chains ....I–....N+....I–.... are also observed (Fig. 5),
but in this case cations in the chain are bound between
each other by translations along the a axis and the disꢀ
tance N+—I–—N+ is longer than in the first case.
I(1AA)
N(5´C)
I(1B)
OW(1B)
C(8´)
N(5´B)
N(5´)
3.82 Å
C(8´B)
3.38 Å
OW(1)
I(1)
4.91 Å
I(1A)
OW(1A)
Fig. 5. The fragment of the crystal structure 1•H2O. The dashed
lines show the intermolecular contacts.
The water in the structure 1•H2O has weak van der
Waals interactions with I– anion O(W)...I(1) 3.64 Å (the
radius for O is 1.40 Å, for I 2.15 Å).26 The water molecule
also has a weak contact with the pyridine part of the catꢀ
ion O(W)...C(8´) 3.37 Å. The water in the structure is
weakly bound, that, by all accounts, is the reason for the
high thermal parameters, and its positions are populated
by 80%. Probably, the loss of water is responsible for deꢀ
composition of the crystals on storage. Despite that water
in the structure is weakly bound, it significantly affected
formation of the crystal 1•H2O. The crystal structure of
anhydrous compound 1 differs from the structure of the
hydrate, but in this case, a single formal unit in the crysꢀ
tals has virtually the same volume (599.4 and 615.1 Å3 for
1 and 1•H2O, respectively).
3.61 Å
3.71 Å
c
b
0
Molecular and crystal structure of neutral spiropyran 2.
Molecular structure of spiropyran 2 differs little from the
structures of the salt forms. In the neutral compound,
likewise in the salts, the C(2´2)—O(1´) bond is elongated
to 1.471(2) Å, whereas the C(2´2)—N(1) bond is shortꢀ
ened to 1.449(3) Å. The nN(1)—σ*[C(2´2)—O(1´)] interꢀ
actions tend to strengthen in the order of compounds 2, 1,
and 1•H2O.
Like the neutral spiropyran15 Me—SP, compound 2
crystallizes in the monoclinic crystal system, the space
group is P21/n. An increase in the size of the molecule 2
leads to the natural growth in the size of the unit cell
(1568(1) and 2042.7(7) Å3 for Me—SP and 2, respectiveꢀ
ly). Unlike the salts, molecules in the crystal structure of
a
Fig. 4. The fragment of the crystal structure 1. The dashed lines
show the intermolecular contacts.

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Yurieva et al.
Table 2. Spectral characteristics and constants of the dark
bleaching of the salt 1 in solutions
A
a
Solvent
λ
max/nm (εmax/L mol–1 cm–1
)
k•105/s–1
D
SP
MC
B
CHCl3
EtOH
355 (17950)
350 (13200)
565 (sh), 593
545 (sh), 580
11.6(2)
3.1(1)
C
0
c
b
The packing of molecules in the crystals of salts 1 and
1•H2O provides a free volume enough for the photoꢀ
chromic transformations. The longꢀlived MCꢀforms are
formed in the irradiated crystals of iodides, like in soluꢀ
tions, whose stability is secured by the electronꢀwithꢀ
drawing pyridinium ring.
Fig. 6. The projection of the crystal structure 2 on the plane of
the cell ac. The letters A—D show the molecules.
the neutral compound 2 are packed along the axis 21 with
the pyranopyridine fragments (Fig. 6). According to the
calculations of the energy of IMI by the known method,25
the packing has no clearly cut stacks. Thus, the energies of
IMI between the molecules bound by the screw axis are
–9.2 and –7.0 kcal mol–1 (for the pairs A—B and B—C,
respectively, see Fig. 6). For the molecules bound by the
center of inversion, the energies of IMI are –5.1 (B and D)
and –11.3 kcal mol–1 (D and B´). The structure has no
shortened interactions, the shortest intermolecular conꢀ
tacts exceed 3.46 Å.
Photochemistry of neutral and salt forms of spiropyrꢀ
ans. Photochromic properties of the neutral compound 2
do not differ from the properties of the earlier studied
R—SP (R = Me and Pri). The absorption spectrum of 2 in
ethanol has a band with λmax = 310 nm. Steady irradiation
of the solution with the UV light (330 nm) leads to its
coloring and formation of a shortꢀlived open form with
the absorption maximum at 580 nm and the rate constant
of dark cyclization k = 6.16(2)•10–2 s–1 (18 °C).
The changes in the absorption spectra of polycrystals
of the salt 1 upon irradiation with the UV light (355 nm)
are shown in Fig. 7. A band in the region 550—650 nm
characteristic of absorption of the MCꢀform appears in
the spectra, with the simultaneous decrease in the intensiꢀ
ty of the band of the cyclic form. Similar behavior is obꢀ
served for the crystals 1•H2O and the spectra that recordꢀ
ed for both the starting and the colored forms do not differ
from the corresponding spectra in the case of compound
1. Like in solutions, a shift of the absorption maximum of
the open form to the longꢀwave region is observed in the
series of (R—SP)+I– (R = Me, Pri, CH2Ph) (Table 3).
After exposure to the visible light (530 nm), the specꢀ
trum returns to the starting shape. The open form slowly
returns to the cyclic one at room temperature under dark
conditions. Kinetics of the dark bleaching of the salts is
described by the biexponential dependence (Fig. 8). For
the hydrate 1•H2O, the contribution of the fast reaction
is noticeably larger than for 1, with the contribution of
the fast reaction increasing in the order (Me—SP)+I–,
(Pri—SP)+I–, 1, 1•H2O (see Table 3). If kinetics of
In the series of R—SP (R = Me, Pri, CH2Ph), the
absorption maximum of the open form is somewhat disꢀ
placed to the longꢀwave region of the spectrum, whereas
the dark bleaching constant values are close; the lifetime
of the MCꢀform is <1 min. The open form was not regisꢀ
tered upon the steady irradiation in the nonpolar solvent
(hexane). Such a difference in the stability of the open
form in polar and nonpolar solvents is explained not only
by polarity of the solvent, but also, probably, by the forꢀ
mation of hydrogen bonds of the alcohol molecules with
the nitrogen atom of the pyridine ring of the molecule.
Crystals of the spiropyran 2 are not photochromic upon
the steady irradiation.
Unlike the neutral forms, their salts form an MCꢀform
in solutions of polar solvents, whose lifetime lasts for hours.
When irradiated with the UV light (355 nm), the solution
in ethanol acquires a bright pink color (a wide maximum
at 545—560 nm), whereas a solution in chloroform beꢀ
comes dark violet (λmax = 593 nm). Kinetics of the dark
bleaching of solutions is described by a monoexponential
equation (Table 2).
A
1.4
1.2
5
4
3
2
1
1.0
0.8
400
500
600
700
λ/nm
Fig. 7. The changes in the absorption spectra of the polycrystals 1
with time upon irradiation with the UV light (355 nm): t = 0 (1),
10 (2), 30 (3), 60 (4), and 120 s (5).

Solidꢀstate photochromic spiropyran
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
1407
Table 3. Spectral and kinetic characteristics and calculated densities of the crystal salts (R—SP)+I–
Compound
dcalc/g cm–3
λ
max/nm
MC
k1•104
k2•106
A1/A2
SP
s–1
(Me—SP)I *
(Pri—SP)I *
1
1.482
1.439
1.414
1.427
355
352
355
360
560, 595
560, 597
560, 610
560, 620
—
1.6
3.3(7)
1.5(1)
2.6
11.0
6.4(5)
3.1(2)
—
0.22
0.54(4)
1.09(2)
1•Н2О
* Photochemical studies of the salts have been performed earlier.6,15
bleaching is monoexponential in (Me—SP)+I–,14,15 the
biexponential kinetics6 corresponds to the salt (Pri—SP)+I– 6
with a low contribution of the fast reaction. For the salt 1,
the ratio A1/A2 is already 0.54(4), whereas for the hydrate
1•H2O it reaches 1.09(2). The data that obtained agree
with the decrease in the density of packing in this series of
spiropyrans described by us above (see Table 3).
In conclusion, the positively charged pyridinium ring
in the salts is an important factor for the photochromic
transformations to take place not only in solutions, but
also in the crystal state: the positively charged ring stabiꢀ
lizes the open form due to its electronꢀwithdrawing effect.
Molecular geometry in both the neutral compounds
and the salts favors the nN(1)—σ*[C(2´2)—O(1´)] orbitꢀ
al interactions, which lead to the elongation of the
C(2´2)—O(1´) bond and shortening of the C(2´2)—N(1)
bond as compared to the normal bond distances in hetꢀ
erocycles. The tendency to the strengthening of the n—σ*ꢀinꢀ
teractions is observed in the series of studied compounds
2, 1, 1•H2O.
interactions, whereas the pyranopyridine fragments toꢀ
gether with the I– ions are placed between stacks and have
more loose packing. Such a structure of the salts provides
a possibility of photochromic transformations of the cation
in crystals.
This work was financially supported by the Presidium
of Russian Academy of Sciences (Program of Basic Reꢀ
search of the RAS No. 18 "Development of Methods for
Preparation of Chemical Compounds and Creation of New
Materials", Subprogram "Polyfunctional Materials for
Molecular Electronics").
References
1. O. Sato, J. Tao, Y.ꢀZ. Zhang, Angew. Chem., Int. Ed., 2007,
46, 2152.
2. Y. Einaga, J. Photochem. Photobiol. C: Photochem. Rev., 2006,
7, 69.
3. S. M. Aldoshn, Izv. Akad. Nauk, Ser. Khim., 2008, 704 [Russ.
Chem. Bull., Int. Ed.], 2008, 57, 718.
4. W. Fujita, K. Awaga, T. Yokoyama, Appl. Clay Sci., 1999,
15, 281.
The in detail analysis of intermolecular contacts
showed that for the salts (R—SP)+I– (R = Me, Pri), 1,
1•H2O, the same principle of packing operates: in the
crystal their indoline fragments are bound by more strong
5. H. Shimizu, M. Okubo, A. Nakamoto, M. Enomoto, N. Koꢀ
jima, Inorg. Chem., 2006, 45, 10240.
6. S. Bénard, E. Rivière, P. Yu, K. Nakatani, J. F. Delouis,
Chem. Mater., 2001, 13, 159.
7. S. M. Aldoshin, N. A. Sanina, V. I. Minkin, N. A. Voloshin,
V. N. Ikorskii, V. I. Ovcharenko, V. A. Smirnov, N. K. Naꢀ
gaeva, J. Mol. Struct., 2007, 826, 69.
A
101
8. S. M. Aldoshin, N. A. Sanina, V. A. Nadtochenko, E. A.
Yurieva, V. I. Minkin, N. A. Voloshin, V. N. Ikorskii, V. I.
Ovcharenko, Izv. Akad. Nauk, Ser. Khim., 2007, 1055 [Russ.
Chem. Bull., Int. Ed., 2007, 56, 1095].
9. I. Kashima, M. Okubo, Y. Ono, M. Itoi, N. Kida, M. Hikita,
M. Enomoto, N. Kojima, Synth. Met., 2005, 155, 703.
10. N. Kida, M. Hikita, I. Kashima, M. Okubo, M. Itoi, M. Enoꢀ
moto, K. Kato, M. Takata, N. Kojima, J. Am. Chem. Soc.,
2009, 131, 212.
2
10–1
1
11. N. Kida, M. Hikita, I. Kashima, M. Enomoto, M. Itoi,
N. Kojima, Polyhedron, 2009, 28, 1694.
0
1
2
3
4
5
t•10–5/s–1
Fig. 8. Kinetics of the dark bleaching of the polycrystals 1 (1)
and 1•H2O (2). The experimental dots are normalized on the
value of initial absorption. The solid lines are the approximation
of the biexponential function A(t) = A1exp(–k1t) + A2exp(–k2t)
fall. The ordinate axis has a logarithmic scale.
12. S. Bénard, A. Léaustic, E. Rivière, P. Yu, R. Clément, Chem.
Mater., 2001, 13, 3709.
13. M. Okubo, M. Enomoto, N. Kojima, Synth. Met., 2005,
152, 461.
14. S. Bénard, P. Yu, Adv. Mater., 2000, 12, 48.

1408
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 7, July, 2011
Yurieva et al.
15. S. M. Aldoshin, L. A. Nikonova, V. A. Smirnov, G. V. Shilov,
N. K. Nagaeva, J. Mol. Struct., 2005, 750, 158.
16. S. M. Aldoshin, L. A. Nikonova, V. A. Smirnov, G. V. Shilov,
N. K. Nagaeva, Izv. Akad. Nauk, Ser. Khim., 2005, 2050
[Russ. Chem. Bull., Int. Ed., 2005, 54, 2113].
21. C. D. Weis, J. Heterocycl. Chem., 1978, 15, 29.
22. C. H. Tilford, J. Med. Chem., 1971, 14, 1020.
23. G. M. Sheldrick, SHELXTL v. 6.14, Structure Determination
Software Suite, Bruker AXS, Madison (Wisconsin, USA),
2000.
17. S. M. Aldoshin, L. A. Nikonova, G. V. Shilov, E. A. Bikaꢀ
nina, N. K. Artemova, V. A. Smirnov, J. Mol. Struct., 2006,
794, 103.
18. V. V. Tkachev, S. M. Aldoshin, N. A. Sanina, B. S. Luk´yaꢀ
nov, V. I. Minkin, A. N. Utenyshev, K. N. Khalanskii, Yu. S.
Alekseenko, Khim. Geterotsikl. Soedin., 2007, 690 [Chem.
Heterocycl. Compd. (Engl. Transl.), 2007, 43, 576].
19. P. Naumov, P. Yu, K. Sakurai, J. Phys. Chem. A, 2008,
112, 5810.
24. F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
Orpen, R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1.
25. G. V. Timofeeva, N. Yu. Chernikova, P. M. Zorkii, Usp.
Khim., 1980, 49, 966 [Russ. Chem. Rev. (Engl. Transl.), 1980,
49, 509.
20. N. ClausonꢀKaas, M. Meister, Acta Chem. Scand., 1967,
21, 1104.
Received March 4, 2011;
in revised form June 14, 2011