Photoinduced transformations in single crystals of 1,2-bis(2-[(E)-3-oxo-3- phenylprop-1-enyl]phenoxy)ethane and 1,5-bis(2-[(E)-3-oxo-3-phenylprop-1-enyl] phenoxy)-3-oxapentane

Article abstract of DOI:10.1007/s11172-009-0320-z

Differences in the parameters of pre-organization of chalcone podands to intermolecular photoinduced [2+2] cycloaddition (PCA) in crystals were discovered. As a result of PCA, the chalcone podand with two oxyethylene units forms stereoregular cyclobutane-containing polymer chains over the whole volume of the single crystal. In the case of the chalcone podand with one oxyethylene unit, the PCA reaction in single crystal becomes impossible; however, its mechanical destruction results in the formation of surface layers with stacking dimers. In these layers, photodimerization occurs due to a decrease in topochemical control from the molecular lattice.

Full text of DOI:10.1007/s11172-009-0320-z

Russian Chemical Bulletin, International Edition, Vol. 58, No. 11, pp. 2288—2298, November, 2009
2288
Photoinduced transformations in single crystals
of 1,2ꢀbis(2ꢀ[(Е)ꢀ3ꢀoxoꢀ3ꢀphenylpropꢀ1ꢀenyl]phenoxy)ethane
and 1,5ꢀbis(2ꢀ[(Е)ꢀ3ꢀoxoꢀ3ꢀphenylpropꢀ1ꢀenyl]phenoxy)ꢀ3ꢀoxapentane
I. G. Ovchinnikova,a O. V. Fedorova,a L. G. Kuz´mina,b P. A. Slepukhin,a A. A. Tumashov,a
G. L. Rusinov,a and V. N. Charushina
aI. Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences,
22/20 ul. S. Kovalevskoi/Akademicheskaya, 620041 Ekaterinburg, Russian Federation.
Fax: +7 (343) 374 1189. Eꢀmail: iov@ios.uran.ru
bN. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky props., 119991 Moscow, Russian Federation.
Fax: +7 (495) 954 1279. Еꢀmail: kuzmina@igic.ras.ru
Differences in the parameters of preꢀorganization of chalcone podands to intermolecular
photoinduced [2+2] cycloaddition (PCA) in crystals were discovered. As a result of PCA, the
chalcone podand with two oxyethylene units forms stereoregular cyclobutaneꢀcontaining polyꢀ
mer chains over the whole volume of the single crystal. In the case of the chalcone podand with
one oxyethylene unit, the PCA reaction in single crystal becomes impossible; however, its
mechanical destruction results in the formation of surface layers with stacking dimers. In these
layers, photodimerization occurs due to a decrease in topochemical control from the molecular
lattice.
Key words: chalcone podand, [2+2] photocycloaddition, topochemical control, photoꢀ
polymerization, αꢀtruxillic structure.
The use in polymer synthesis of methods and prinꢀ
It is rather difficult to attain crystal structure retention
during the PCA, i.e., to retain topochemical control of
the reaction course until its completion. The solution
of this problem was proposed for particular classes of
olefins, e.g., styryl dyes.15—18 The main approach is the
creation of loose region in the crystal structure due to
the inclusion into the structural units (molecules or
molecular ions) of conformationally mobile groups,
solvate molecules, or rotationally mobile counterions.
Combination of photosensitive groups with the conꢀ
formationally mobile polyoxyethylene spacer in one
molecule also could result in the appearance of loose
regions in crystals. The loose regions surround the dimeric
πꢀstacking fragments preorganized to the PCA. However,
attempts to carry out solidꢀphase PCA reactions in the
series of olefinic compounds, such as podands in which
pairs of ditopic ligands are separated by polyether chains,
were unsuccessful.19,20 In our Xꢀray diffraction study of
the arylꢀcontaining chalcone podands with the propenone
group in the orthoꢀposition to the bridging oxyethylene
units, we have earlier21 found two compounds capable
of topochemical PCA reactions: 1,2ꢀbis(2ꢀ[(Е)ꢀ3ꢀoxoꢀ
3ꢀphenylpropꢀ1ꢀenyl]phenoxy)ethane (1a) and 1,5ꢀbisꢀ
(2ꢀ[(Е)ꢀ3ꢀoxoꢀ3ꢀphenylpropꢀ1ꢀenyl]phenoxy)ꢀ3ꢀoxaꢀ
pentane (1b).
ciples of supramolecular chemistry, such as selfꢀassemꢀ
bling of highly ordered structures in single crystals, is one
of the promising trends in the production of novel mateꢀ
rials capable of demonstrating unique physicochemical
properties.1—4 For instance, polyfunctional compounds
with photocontrolled groups (vinyl, styryl, cinnamoyl,
anthryl, etc.) capable of reacting in stereospecific topoꢀ
chemical reactions of [2π+2π] and [4π+4π] photocycloꢀ
addition (PCA) and others are used in the synthesis
of linear polymers of stereoregular structure.5—9 These
processes are directly controlled by the crystal lattice
providing the maximum approach of parallel photosensiꢀ
tive double bonds to the distance that does not exceed
4.2 Å.5,10—12 The studies of the photochemical transforꢀ
mations of olefins in crystals showed that photopolyꢀ
merization is most characteristic of conjugated dienes with
the СН=СН groups in the paraꢀposition of the (hetero)ꢀ
aromatic ring.5 Photopolymerization in single crystals
depends to a considerable extent on the nature of photoꢀ
sensitive terminal groups of the reacting molecules. On
going from conjugated dienes with styryl or acrylate terꢀ
minal groups to cinnamates, the polymerization direction
shifts to photodimerization with the side formation of
small amounts of oligomers (n = 3, 4).13,14
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2219—2229, November, 2009.
1066ꢀ5285/09/5811ꢀ2288 © 2009 Springer Science+Business Media, Inc.

[2+2] Cycloaddition in single crystals
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2289
using Me4Si as the internal standard. Melting points were
determined on a Boetius microheating stage. TLC was carried
out on Silufolꢀ254 plates eluting with ethyl acetate or benzene.
TLC spots were developed with iodine vapors.
The purpose of the present work is to study photoꢀ
initiated transformations in single crystals of the above
indicated compounds and to determine the degree of
influencing of the oxyethylene substituents in molecules
and topochemical control of the molecular lattice on the
depth of the PCA occurrence.
Photochemical transformations in chalcone podand 1a single
crystals and solidꢀphase synthesis of 1r,3tꢀdibenzoylꢀ2c,4tꢀbisꢀ
{2ꢀ[2ꢀ(2ꢀ[(Е)ꢀ3ꢀoxoꢀ3ꢀphenylpropꢀ1ꢀenyl]phenoxy)ethoxy]ꢀ
phenyl}cyclobutane (2a). А. Single crystals of compound 1а
(see Ref. 21) were irradiated with the nonꢀfiltered light of an
incandescent lamp with a power of 150 W from a distance
of 25—30 cm at 25 °С or in hexane with stirring and cooling to
–25 °С for 48 h, or with the nonꢀfiltered light from a mercury
lamp (500 W) from a distance of 35—40 cm at 25—30 °С for
12 h. The reaction course was monitored by IR and UV spectroꢀ
Experimental
The IR spectra were recorded on a Spectrum One FTꢀIR
spectrometer (Perkin—Elmer) with a diffuse reflectance sampling
accessory (DRA). The Raman spectrum was obtained on a
Nicolet 6700 FTIR spectrometer with an NXR Raman module
(λ = 976 nm). The UV spectra were measured on a UVꢀ2401 PC
spectrometer (Shimadzu) with an integrating sphere attachment.
1
scopy (DRA), as well as by Н NMR spectroscopy. The ratio
of E,Е/Е,Zꢀisomers was estimated by 1Н NMR spectroscopy
and chromatography using the reverseꢀphase variant of HPLC
(RP HPLC) on an Agilent 1100 analytical liquid chromatograph.
The column was Lichrosorb RPꢀ18, LKB, 4.0×250 mm, and
1
The Н NMR spectra in DMSOꢀd6 were recorded on a Bruker
DRXꢀ400 instrument with the working frequency 400 MHz
Table 1. Parameters of crystals and characteristics of Xꢀray diffraction experiments for structures 1а, 1b, and 2а
Parameter
1а
1b
2а
Molecular formula
Molecular weight/g mol–1
Crystal system
Z
Space group
a/Å
b/Å
c/Å
α/deg
β/deg
С32Н26О4
474.53
Monoclinic
4
С34Н30О5
518.58
Monoclinic
4
С64Н52О8
949.06
Triclinic
1
P21/c
С2/c
Р–1
7.8756(6)
14.7565(12)
20.9574(17)
90
91.5210(10)
90
2434.7(3)
1.295
1000
15.6193(10)
11.6107(7)
15.0945(10)
90
105.9180(10)
90
2632.4(3)
1.308
1096
11.2128(14)
11.5318(16)
11.834(2)
115.245(16)
98.443(13)
105.961(12)
1248.0(3)
1.263
γ/deg
V/Å3
dcalc/g cm–3
F(000)
500
μ(MoꢀKα)/mm–1
Crystal size/mm
Temperature/K
Radiation (λ/Å)
Scan mode
θ range/deg
hkl ranges
0.084
0.087
0.082
0.45×0.28×0.11
295
MoꢀKα (0.71073)
ω
2.74—26.42
–14 ≤ h ≤ 12,
–14 ≤ k ≤14,
–14 ≤ l ≤ 14
14790
0.22×0.22×0.18
120.0(2)
MoꢀKα (0.71073)
ω
1.69—29.50
–10 ≤ h ≤ 10,
–20 ≤ k ≤ 20,
–29 ≤ l ≤ 29
25558
0.38×0.22×0.18
120.0(2)
MoꢀKα (0.71073)
ω
2.22—29.98
–21 ≤ h ≤ 21,
–16 ≤ k ≤16,
–21 ≤ l ≤ 20
13407
Number of measured reflections
Number of independent reflections
6776
3825
4910
(Rint = 0.0399)
5042
(Rint = 0.0349)
2848
(Rint = 0.0377)
1973
Number of reflections with I > 2σ(I)
Number refinement variables
R Factors on I > 2σ(I)
430
237
325
R1 = 0.0449,
wR2 = 0.1049
R1 = 0.0662,
wR2 = 0.1126
1.049
R1 = 0.0462,
wR2 = 0.1113
R1 = 0.0673,
wR2 = 0.1198
1.043
R1 = 0.0370,
wR2 = 0.0389
R1 = 0.1322,
wR2 = 0.0427
1.002
R Factors on all reflections
Goodnessꢀofꢀfit on F2
Absorption coefficient
Residual electron
0.0004(4)
–0.244/0.292
—
—
–0.200/0.377
–0.116/0.119
density/e•Å–3, ρmin/ρ
max

2290
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009
Ovchinnikova et al.
the particle size was 5 μm. The temperature of the column was
35+1 °С. Water was used as the mobile phase, and 60%
acetonitrile served as the mobile phase B. The elution was
performed as follows. At first the gradient regime was carried out
from 0 to 100% B within 20 min, and then the isocratic regime
with the mobile phase B occurred within 40 min, the solvent
flow rate being 0.8 mL min–1. A sample dissolved in DMF was
introduced into the injector (10 μL). Detection was carried out
using the diode matrix detector in the UV range at the wavelength
254 nm. The retention time of the Е,Zꢀisomer was 17.1 min, and
that of the Е,Eꢀisomer was 34.9 min.
calculations. The crystallographic parameters and the main
characteristics of experiment and structure determination and
refinement are given in Table 1.
The crystals of 2а were obtained by the slow evaporation of
the acetonitrile solution. Xꢀray diffraction analysis was carried
out on an Xcalibur 3 automated diffractometer with a CCD
detector (МоꢀKα radiation, λ = 0.71073 Å, graphite monoꢀ
chromator, T = 295 K). The structure was solved by a direct
method and refined by fullꢀmatrix least squares in the anisotropic
approximation on F2 for all nonꢀhydrogen atoms using the
SHELXS97 and SHELXL97 programs.25,26 All hydrogen atoms
were revealed in the difference synthesis and refined in the
isotropic approximation, The crystallographic parameters
and the main characteristics of experiment and structure
determination and refinement are given in Table 1.
B. Single crystals of compound 1а were triturated uniformly
distributing on the subsupport (ashless filter) and irradiated with
the nonꢀfiltered light from the incandescent lamp with a power
of 150 W from the distance 30 cm for 48 h or with the nonꢀ
filtered light of the mercury lamp (500 W) from the distance
35—40 cm at 25—30 °С for 12 h. The completeness of the
reaction was monitored by IR and UV spectroscopy (DRA) and
by 1Н NMR spectroscopy. The ratio of the PCA products,
including 2a, in the reaction mixture was estimated by the
composition of integral intensities of signals from the α,βꢀprotons
of the E,Zꢀisomers and the protons of the cyclobutane groups in
the 1H NMR spectra. The photolysis product of 2а was purified
by column chromatography (SiO2, eluent benzene—ethyl acetate
(50 : 1)).
Results and Discussion
Molecular and crystal structures of 1а and 1b. The
structure of molecule 1а is shown in Fig. 1. The molecule
has the nonꢀcrystallographic symmetry С2. This symꢀ
metry is not rigid, because the molecule occupies the
general position in crystal, which appears as the differꢀ
ence in conformation parameters of two chemically
equivalent halves of the molecules.
In molecule 1а the chemically equivalent fragments
С(2)—С(1)—С(8)=С(9)—С(10) and С(2´)—С(1´)—
С(8´)=С(9´)—С(10´) are almost planar, the benzene rings
С(2)...С(7)/С(10)...С(15) and С(2´)...С(7´)/С(10´)...С(15´)
are turned from the plane of the corresponding fragꢀ
1
The yield was 12%, m.p. 194—196 °С. Н NMR, δ: 3.96,
4.15, 4.33, 4.50 (m, 8 H, О—CH2—СН2—О); 4.68, 4.96
(both dd, 4 Н, —СН—СН—С(О)—, 3J = 10.8 Hz, 3J = 7.6 Hz);
6.69 (m, 4 Н, Ar); 7.00 (ddd, 2 H, Ar, 3J = 8.0 Hz, 3J = 7.2 Hz,
3
4J = 1.6 Hz); 7.04 (d, 2 H, Ar, J = 7.2 Hz); 7.13 (m, 10 Н,
C(O)Ar´, Ar); 7.28 (d, 2 H, Ar, 3J = 8.0 Hz); 7.34 (m,
2 Н, C(O)Ar´); 7.41 (m, 6 Н, C(O)Ar´); 7.54 (ddd, 2 H, Ar,
3
4
3J = 7.6 Hz, J = 7.2 Hz, J = 1.6 Hz); 7.77 (d, 4 H, C(O)Ar´,
3J = 7.6 Hz); 7.94 (dd, 2 H, Ar, J = 8.0 Hz, J = 1.6 Hz);
3
4
C(16)
7.94 (d, 2 H, СН=СН—С(О), 3J = 16.0 Hz); 8.01 (d, 2 H,
C(16´)
C(13´)
СН=СН—С(О), J = 16.0 Hz). IR (DRA), ν/cm–1: 595, 617;
C(14´)
3
C(14)
C(15)
C(1´)
C(8)
C(13)
O(2)
O(2´)
C(12´)
C(11´)
693, 729, 753 (arom.); 809, 866, 899, 939, 991; 1015, 1057,
C(10´)
C(15´)
1122, 1164, 1178, 1214, 1236 (ν , ν , Сarom—О—Сalk
,
s
as
C(12)
C(11)
C(10)
Сalk—О—Сalk); 1274, 1305, 1332, 1448, 1490; 1568, 1597
(С=С); 1651, 1672 (С=О); 2837, 2884, 2920, 2927 (Сalk—Н);
3037, 3060 (Сarom—Н). Found (%): С, 81.03; Н, 5.44. С64Н52О8.
Calculated (%): С, 80.92; Н, 5.47.
Solidꢀphase synthesis of oligomer containing the 1r,3tꢀdiꢀ
benzoylꢀ2c,4tꢀdi(2ꢀphenoxy)cyclobutane fragment (2b). Crystal
1b (see Ref. 22) was irradiated with the nonꢀfiltered light from
the incandescent lamp with a power of 150 W from a distance of
25—30 cm for 60 h or with the solar light (June) for 1 week. The
completeness of the reaction was monitored by IR and UV
spectroscopy (DRA) and 1Н NMR spectroscopy.
Xꢀray diffraction study. Crystals of chalcone podands 1a,b
were obtained by the slow evaporation of the solution in
acetonitrile. Xꢀray diffraction analysis was carried out on a
Bruker SMARTꢀAPEXꢀII automated diffractometer with a CCD
detector (МоꢀKα radiation, λ = 0.71073 Å, graphite monoꢀ
chromator, T = 120(2) K). Experimental reflections were
processed using the SAINT program.23 Structures were solved
by direct methods and refined by fullꢀmatrix least squares in the
anisotropic approximation on F2 for all nonꢀhydrogen atoms
using the SHELXTLꢀPlus program package.24 All hydrogen
atoms were revealed in the difference synthesis; however,
their geometrically calculated positions were used for further
C(8´)
C(9´)
C(4´)
C(9)
C(3´)
C(7´)
C(2´)
C(3)
C(2)
C(4)
C(5´)
C(6´)
C(5)
C(6)
C(1)
O(1)
C(7)
O(1´)
C(11)
C(12)
O(1)
C(8)
C(7)
C(9)
C(10)
C(2)
C(1)
C(3)
C(16´)
C(15´)
C(6)
C(13)
C(5)
C(4)
C(14´)
C(13´)
O(2)
C(14)
C(15)
C(16)
O(2´)
C(3´)
C(1´)
C(4´)
C(10´)
C(8´)
C(5´)
C(6´)
C(12´)
C(9´)
C(11´)
C(7´)
C(2´)
O(1´)
Fig. 1. Structure of molecule 1а in two projections.

[2+2] Cycloaddition in single crystals
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2291
a
0
c
b
Fig. 2. Fragment of the crystal packing of 1а.
ment by the angles 23.7(5)/3.0(5)° and 3.0(5)/4.9(5)°.
The torsion angles С(16)—О(2)—С(15)—С(10) and
С(16´)—О(2´)—С(15´)—С(10´) are 176.9(6) and 175.2(6)°,
respectively, and the torsion angle О(2)—С(16)—
С(16´)—О(2´) about the central bond equal to 62.2(5)°
corresponds to the gaucheꢀconformation. The dihedral
angle between the planes of ethylene groups —С=С—
is 77.0(5)°. The bond length distribution in the fragments
С(2)—С(1)—С(8)=С(9)—С(10) and С(2´)—С(1´)—
С(8´)=С(9´)—С(10´) 1.494(2), 1.479(2), 1.339(2),
1.455(2) Å and 1.498(2), 1.478(2), 1.336(2), 1.462(2) Å
indicate an almost full localization of the πꢀelectron denꢀ
sity on the ethylene bond, which is important for the PCA
reaction to occur.
The general position of the molecule in crystal implies
that two ethylene groups in it are crystallographically
nonequivalent and must not have an identical crystalline
environment. Therefore, only one of the ethylene fragꢀ
ments of the molecule in crystal, namely, С(8)=С(9), has
rather short contact with the like fragment of the adjacent
molecule. The second ethylene group С(8´)=C(9´) forms
no short contacts with the analogous group of the
adjacent molecule.
The crystal packing of the molecules is built of
centrosymmetric sandwich dimers (Fig. 2).
The structure of one of the dimers is shown in Fig. 3.
The distance between the carbon atoms of the ethylene
moieties С(8)...С(9А) and С(9)...С(8А) is 4.155(2) Å,
C(16)
b
a
C(10A)
C(16´)
O(1A)
C(9A)
C(1A)
C(1´A)
C(16)
C(1A)
C(10A)
O(1A)
C(11A)
C(12A)
C(14)
C(13)
O(1´A)
O(2)
O(2´)
O(2)
C(8)
C(13A)
C(15)
C(9A)
C(8A)
C(8´A)
C(9´A)
O(2´A)
C(9´)
C(8´)
O(1´)
C(2A)
C(8)
C(8A)
C(1)
C(2)
C(9)
C(12)
C(11)
C(10B)
C(1)
O(1)
C(1´)
O(2A)
C(10)
O(1)
C(10)
C(16A)
C(16B)
O(2A)
C(14A)
C(16A)
C(9)
C(15A)
Fig. 3. Structure of the centrosymmetric sandwich dimer 1а (a) and the view of π...πꢀoverlapping of the conjugated fragments of the
dimer (b, nonꢀoverlapped halves of the dimer molecules are omitted).

2292
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009
Ovchinnikova et al.
O(1)
a
b
C(7)
C(2)
C(6)
C(8)
C(17)
C(1)
C(15)
C(17A)
C(10)
C(3)
C(4)
C(9)
C(5)
C(14)
C(13)
C(11)
O(2)
C(8)
O(2A)
C(1)
O(2)
C(12)
C(16)
C(8A)
C(16)
O(3)
C(17)
O(3)
C(17A)
O(2A)
O(1)
C(9)
C(16A)
C(10A)
C(16A)
C(12A)
C(13A)
O(1A)
C(11A)
C(10A)
C(4A)
C(5A)
C(10)
C(9A)
C(3A)
C(1A)
C(14A)
C(15A)
C(9A)
C(8A)
C(6A)
C(1A)
C(2A)
C(7A)
O(1A)
Fig. 4. Structure of molecule 1b in two projections.
which corresponds to the upper limit of the values (4.2 Å)
at which the PCA reaction can occur. The noticeable shift
of the ethylene bonds in parallel planes observed in the
sandwich dimer does not prevent the PCA reaction to
occur.15—18 In the case when the PCA reaction occurs in
this crystal, the centrosymmetric isomer of substituted
cyclobutane should be formed.
The structure of molecule 1b is shown in Fig. 4. Unlike
the previous case, in this crystal the molecule occupies
the partial position on the 2ꢀfold axis passing through the
central oxygen atom O(3) perpendicularly to the plane of
Fig. 4, a. Therefore, the both ethylene moieties of the
molecule are crystallographically equivalent.
The torsion angle О(3)—С(17)—С(16)—О(2) is 66.0(5)°.
The С(2)—С(1)—С(8)=С(9)—С(10) fragment is planar,
and the benzene rings С(2)...С(7) and С(10)...С(15) are
turned toward this plane by angles of 10.7(5) and 5.5(5)°,
respectively. The dihedral angle between the ethylene
groups is 72.2(5)°. The bond length distribution in the
С(2)—С(1)—С(8)=С(9)—С(10) fragment of molecule 1b
(1.500(2), 1.471(2), 1.339(2), 1.460(2) Å) coincides in
fact with that found for molecule 1а.
The crystal packing of 1b is rather complicated. One
can distinguish the infinite ribbon motive with short conꢀ
tacts of the С(8)...С(9) type between all ethylene moieties
(Fig. 5). These short contacts occur between the molꢀ
ecules joined by the symmetry centers, being 3.629(2) Å.
The PCA reaction in such a crystal should be accompaꢀ
nied by the formation of a zigzag ribbon polymer.
swered after the fact of occurrence of this reaction in the
single crystal would experimentally be established. This
problem can be solved by a comparison of the structure of
the initial single crystal and the structure of the same
single crystal after the PCA reaction has completed in it.
Photochemical behavior of compounds 1a,b. The Xꢀray
diffraction data for structures 1а and 1b indicate that both
photodimerization (1а) and photopolymerization (1b)
processes are possible as a result of topotactic control.
According to the 1Н NMR and IR spectroscopy (DRA)
data, no PCA processes were observed upon the irradiaꢀ
tion of single crystals 1а with the visible (or solar) light for
48 h at 25—30 °С. Obviously, the oxyethylene moiety in
photoexcited molecules have no enough conformational
mobility for the compensation of internal strains in the
a
C(8AA)
C(9AA)
C(9AC)
C(8AC)
C(9A)
C(8A)
b
C(8B)
C(9B)
c
0
The question whether loose regions, which can comꢀ
pensate (due to conformational tuning) internal strains in
crystal that appear as a consequence of considerable
atomic shifts of the ethylene fragments during the PCA
reaction, exist in these packings or not should be anꢀ
Fig. 5. Ribbon fragment of the crystal packing of compound 1b
with short intermolecular contacts of the ethylene groups.

[2+2] Cycloaddition in single crystals
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2293
crystal, which should appear due to large atomic shifts
between two ethylene moieties in the intermolecular
sandwich upon excimer formation.27 The principle of
"dense packing" shows that this restriction of motion
of reacting molecules is related to the influence of the
nearest environment (adjacent molecules) through interꢀ
molecular interactions.5
a
8
7
6
5
δ
However, the comparison of the 1Н NMR spectra
of the initial and irradiated samples of compound 1а
revealed the fact of photoinduced Е—Zꢀisomerization.
The interpretation of the 1Н NMR spectra data, including
the comparison of the spectra of the synthesized chalcone
podands,20,21 crownꢀcontaining styryl dyes,28 and comꢀ
pound 2а (Fig. 6, b), made it possible to establish that
only one of the terminal chalcone groups of podand 1а
is isomerized during photochemical transformations
(Scheme 1). For example, along with the signals of proꢀ
tons of the СН=СН moiety of enone in the Zꢀconfiguraꢀ
tion at δ 6.66 and 7.35 as two doublets with the spin—spin
coupling constant (SSCC) 12.7 Hz, the spectra contain
signals at δ 8.01 and 8.05 of analogous protons of the
second moiety in the Еꢀconfiguration with the SSCC
15.6 Hz (Fig. 7).
b
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5
δ
Fig. 6. 1Н NMR spectra of the reaction mixture after the irradiaꢀ
tion of powdered compound 1а (a) and PCA product 2а (b) with
the nonꢀfiltered light from a mercury lamp.
8.0 δ
Analogous processes of Е—Zꢀizomerization were also
observed upon the irradiation of the single crystals with
the visible light at –25 °С and with the nonꢀfiltered light
from the mercury lamp at 25—30 °С for 48 and 12 h,
respectively. According to the HPLC data, the yield of
the isomerization product increased from 5 to 12% in the
latter case. No isomerization usually occurs in crystals,
because it should be accompanied by such a global rearꢀ
8.0
7.5
7.0
6.5 6.0
5.5 5.0
4.5 δ
1
Fig. 7. Н NMR spectrum of compound E,Eꢀ1а after the irꢀ
radiation of the single crystals with the nonꢀfiltered light from
a mercury light (fraction enriched with isomer E,Zꢀ1а, see
Scheme 1).
Scheme 1

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009
Ovchinnikova et al.
A
rangement of the molecular skeleton, which is impossible
in the rigid crystalline environment of the molecules.
However, there are examples of this type of solidꢀphase
transformations (Z—Eꢀisomerization) proceeding presumꢀ
ably through the metastable excimer complex for some
orthoꢀ and αꢀsubstituted cinnamic acids.10—12 Another
example is the soꢀcalled "pedal" isomerization that
occurs in crystals of stilbenes and azobenzenes. Its
essence is that the С=С or N=N double bond is twisted
relative to the aromatic sixꢀmembered rings that experiꢀ
ence only small migrations in their own planes.29 This
isomerization in the heterostyryl dyes15—18 results in the
appearance and accumulation in crystal of the second
conformer, which is related to the initial one by the nonꢀ
crystallographic 2ꢀfold axis running along the main axis
of the molecule, i.e., the appearance of the disordering
corresponding to the turn of the molecule by 180° about
this axis. This isomerization is temperatureꢀdependent,
whereas the PCA reaction is temperatureꢀindependent.
In the case of "pedal" isomerization, the process occurs
within the "van der Walls contours" of the molecules,
i.e., the region of the crystal that is assigned to each
molecule in the crystalline packing and, hence, the proꢀ
cess in the crystal becomes possible.
1.0
0.5
0
a
1
2
200
300
400
500
λ/nm
A
The results of our studies suggest that the photochemiꢀ
cal processes of Е—Zꢀisomerization occur, most likely,
on defects and/or faces of crystals involving the terminal
chalcone group, which is not related by stacking interacꢀ
tions and partially liberated from the influence of the
neighboring environment (see Fig. 3), and accompanied
by the destruction of the crystal. The presence of the
benzoyl substituent at the СН=СН bond of chalcone
podand 1а capable of increasing the single—triplet interꢀ
system crossing constant (sensibilizing ability) increases
the probability of photoinduced Е—Zꢀisomerization.30
To shift the reaction toward isomerization, the single
crystals of chalcone podand 1а were triturated with the
uniform distribution of the powder over the whole supꢀ
port surface (ashless filter), which was further irradiated
for 12 h with the nonꢀfiltered light from the mercury
lamp. The reaction course was monitored by UV spectra
(DRA), which demonstrated the exponential decrease in
the light absorption of the initial compound in the range
from 200 to 400 nm up to the establishment of the
photostationary state after 6 h after (Fig. 8).
b
1
2
2800
2000
1600
1200
800
ν/cm–1
Fig. 8. (a) Change in the UV absorption spectra (DRA) of comꢀ
pound 1а upon the irradiation with the nonꢀfiltered light from a
mercury lamp: the spectrum of the starting compound (1) and
the spectrum after irradiation for 12 h (2); (b) IR absorption
spectra (DRA) of chalcone podand 1а (1) and cyclobutaneꢀ
containing dipodand 2а (2).
formed due to the stereospecific photodimerization of
chalcone podand 1а according to the synꢀ"headꢀtoꢀtail"
type.28,31 In the IR spectra (DRA), the appearance of
cyclobutaneꢀcontaining photoadduct 2а is accompanied
by the appearance of the new, higherꢀfrequency absorpꢀ
tion band in the region of stretching vibrations of the
carbonyl group at 1672 cm–1 along with 1651 cm–1
(stretching vibrations of the С=О group of the conjugated
system of the chalcone), the absorption band at 2927 cm–1
(region of stretching vibrations of the С—Н bonds,) and
the redistribution of absorption band intensities in the
region of bending vibrations of the С—О—С bonds in the
oxyethylene fragment at 1238—1214 and 1057—1015 cm–1
(see Fig. 8).
The exhaustive explanation of the changes observed
was obtained by the analysis of the 1Н NMR spectra of the
reaction mixture, which contained, along with the signals
of protons of the initial podand, the signals of protons of
the E—Zꢀisomerization and PCA products in a ratio
of ~83 : 5 : 12, respectively (see Fig. 6, Scheme 1). The
signals of protons of the cyclobutane fragment of dipodand
1
2а in the Н NMR spectra represent the symmetric spin
system of the АА´ВВ´ type as two doublets of doublets
with the SSCC 3J1,2 = 10.8 and 3J2,3 = 7.6 Hz at δ 4.68 and
4.96, which is characteristic of the rcttꢀisomer (see Fig. 6)

[2+2] Cycloaddition in single crystals
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2295
C(2A)
C(1A)
C(19A)
C(20A)
C(21A)
C(17A)
C(3A)
C(4A)
C(16A)
C(14A)
C(13A)
O(1A)
O(4A)
O(3A)
C(6A)
C(5A)
C(18A)
C(23A)
C(25A)
C(32A)
C(31A)
C(8)
C(22A)
C(24A)
C(29)
C(28)
C(15A)
C(12A)
C(10A)
C(7A)
C(11A)
C(8A)
O(2)
C(9)
C(26A)
O(2A)
C(30)
C(27)
C(27A)
C(28A)
C(29A)
C(4)
C(26)
C(11)
C(30A)
C(7)
C(5)
C(31)
C(9A)
C(10)
C(12)
C(15)
O(3)
C(32)
C(25)
C(23)
C(18)
C(24)
C(22)
C(21)
O(4)
C(17)
C(6)
O(1)
C(13)
C(14)
C(16)
C(1)
C(3)
C(2)
C(20)
C(19)
Fig. 9. Geometry of a molecule of cyclobutaneꢀcontaining dipodand 2а according to the Xꢀray diffraction data.
The molecular structure and type of stereoisomerism
of the cyclobutane moiety of dipodand 2а assigned to the
point symmetry group Сi were unambiguously confirmed
by the Xꢀray diffraction analysis for the crystal grown
from an acetonitrile solution (Fig. 9). The molecule is
situated in the symmetry center of the crystal and, thereꢀ
fore, its fourꢀmembered cycle is rigidly planar. In cycloꢀ
butane the С(8)—С(9) (1.546(1) Å) and С(8)—С(9А)
(1.574(1) Å) bond lengths differ insignificantly, as well
as the conjugated angles С(9А)—С(8А)—С(9) and
С(8)—С(9)—С(8А) (91.33(1)° and 88.67(1)°, respecꢀ
tively). The torsion angles С(10А)—С(9А)—С(8)—С(7)
and С(10)—С(9)—С(8)—С(7) are 102.7(2)° and 1.2(2)°,
respectively.
Thus, the mechanical destruction of crystal 1а allows
the PCA reaction to occur in the solid phase. It is most
likely that this treatment increases the surface of the polyꢀ
crystalline sample, which contains stacking dimers preꢀ
organized to PCA and partially withdrawn from the rigid
environment of the crystalline lattice.
On the contrary, as mentioned above, on going to 1b
an elongation of the oxyethylene fragment in molecules
favor the formation of a system of parallel ethylene dimers
with better (than in 1а) initial geometric characteristics of
preorganization to the PCA reaction (see Fig. 5). In turn,
an increase in the reactivity of molecules 1b results in the
occurrence of the PCA process in the whole volume of
the sample irradiated with the visible (or solar) light for
50—60 h at 25—30 °С. The crystal decomposes during
the reaction retaining its shape and remaining almost
transparent. This is indicated by both the disappearance
of the diffraction pattern from the irradiated single crysꢀ
tals and the fact that the former single crystals stop quenchꢀ
ing during their rotation under a microscope in the polarꢀ
ized light. In addition to the loss of crystallinity of the
initial sample, the polymerization process in the single
crystals of podand 1b due to photolysis is accompanied
by the appearance of many small cracks on the surface
(Fig. 10). Similar morphological changes occur in single
crystals of 2,5ꢀdistyrylpyrazine.5 Analogous processes
b
a
Fig. 10. Crystals of compound 1b before the irradiation with the visible light (a) and after irradiation for 24 h (b).

2296
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009
Ovchinnikova et al.
A
a
0.007
A
b
1
1
0.005
2
2
3600
2800
2000
1600
1200
800 ν/cm–1
0.000
300
450
λ/nm
Fig. 11. (a) UV absorption spectra (DRA) of compound 1b upon the visible light irradiation: the starting compound (1) and the product
of the PCA reaction (2); (b) IR absorption spectra (DRA) of chalcone podand 1b (1) and cyclobutaneꢀcontaining oligomer 2b (2).
caused by the accumulation of disordering regions in the
crystal along with cracking of the initial samples have
been observed earlier for the styryl dyes.15,18
The formation of the [2+2] cycloaddition product was
detected in the Н NMR spectra by the disappearance of
1
signals of protons of the ethylene moiety of chalcone at
δ 7.95 and 8.01 in the region of signals of aromatic groups
and by the appearance of the symmetric spin system
(АА´BB´) of protons at δ 5 characteristic of rcttꢀcycloꢀ
butane (Fig. 12). Only one set of signals of protons in the
spectrum indicated that the process is stereospecific. The
multiplicity of signals of protons of the fourꢀmembered
cycle has a shape of doublets of doublets with the SSCC
3J1,2 = 10.8 and 3J2,3 = 7.6 Hz analogous for the signals of
protons of the cyclobutane moiety of dipodand 2а (see
Fig. 6). Thus, the formation of the fourꢀmembered cycle
of αꢀtruxillic structure is confirmed by both the type of
the spin system of protons of the cyclobutane moiety28,31
and the structural preorganization "headꢀtoꢀtail" of the
chalcone groups of the adjacent molecules in the crystal
packing of compound 1b (see Fig. 5).
In the UV spectra (DRA) of the chalcone podand,
the disappearance of conjugation of the chromophoric
moieties is accompanied by a decrease in the light absorpꢀ
tion maximum (λ = 370 nm) with the simultaneous hypsoꢀ
chromic shift of the absorption band in the wavelength
region 288—460 nm (Fig. 11). Similarly, the IR spectra
(DRA) (see Fig. 11) exhibit the disappearance of the IR
absorption band at 1567 cm–1 responsible for stretching
vibrations of the С=С bond of the propenone moiety. On
the contrary, the formation of the cyclobutane ring is
observed as the appearance of a new absorption band
at 2926 cm–1 (the region of stretching vibrations of the
С—Н bonds) and the highꢀfrequency shift by 22 cm–1 of
the absorption maximum of stretching vibrations of the
carbonyl group (1672 cm–1 (С=О)).
The transition of the ordered structure of single crysꢀ
tals to the "pseudomorphous" state5 during the PCA did
not allow the Xꢀray diffraction study to be carried out
and, therefore, the compound was characterized using
This assertion is indicated by substantial differences in
the IR and Raman spectra32 of compound 2b in the waveꢀ
length region from 500 to 2000 cm–1. The product of
2b dimerization is characterized by the IR absorption
bands at 1370, 1320, 1177, 1110, 982, 941, 920, 699, and
687 cm–1 and the signals in the Raman spectrum at 1208,
1160, 1028, 959, 843, and 616 cm–1. According to the rule
of "alternative inhibition" for molecules with the point
symmetry group Сi (the cyclobutane ring of the αꢀtruxillic
type is characterized by the Сi symmetry), the normal
vibration (Аg, Аu) can be active either only in IR spectra,
1
IR, Raman, and Н NMR spectroscopy. As known, the
structure of topochemical reaction products in crystals is
directly determined by the supramolecular preorganization
of the initial compounds. Based on the Xꢀray diffraction
analysis of the molecular packing of podand 1b, we proꢀ
posed the following scheme for the synthesis of comꢀ
pound 2b (Scheme 2).

[2+2] Cycloaddition in single crystals
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2297
Scheme 2
or only in Raman spectra.33 Indeed, the comparison of
the spectral data shows the absence in the Raman spectra
of a series of bands responsible for stretching and bending
vibrations of the hydrocarbon skeleton and identical to
absorption bands in the IR spectrum and vice versa.
An important evidence for polymer chain formation is
some broadening of all signals of protons in the 1Н NMR
spectrum and of the IR absorption bands in the region of
oxyethylene moiety (1238—1057 cm–1) (see Figs 11, 12).
The linear structure of compound 2b is confirmed by the
1H NMR broadened singlet signal of the geminal protons
bonded to the second carbon atom of the oxyethylene
moiety (Ar—O—CH2—CH2—O). For comparison, in the
macrocycles (for example, of cyclobutaneꢀcontaining
dibenzocrown ether22) considerable changes in the polyꢀ
ether chain geometry affect the complication of the spin
system of all protons of this fragment. The Xꢀray difꢀ
fraction analysis results for compound 1b suggest that a
stereoregular polymer in the form of a macromolecular
helix is formed during the photoinitiated selfꢀassembling
of chalcone podand molecules in crystal due to the
oxyethylene moiety (see Fig. 5, Scheme 2). Since the
1Н NMR spectrum of compound 2b contains no chemiꢀ
cal shifts of protons of the terminal chalcone groups, the
number of units (n) in the oligomer chains can approxiꢀ
mately be estimated (more than 100).
stretching vibrations of the С—Н bond (2950—2750 cm–1
and bending vibrations of the С—О—С bonds of the
)
a
Thus, the influence of the structural preorganization
of chalcone podands 1а and 1b with the propenone group
in the orthoꢀposition to the polyether fragment of the
bridging aryl substituent on the ability to photoinduced
[2+2] cycloaddition reactions under the solidꢀphase synꢀ
thesis conditions. An example of rare stereospecific
photopolymerization for compound 1b in crystals belongꢀ
ing to the space group С2/с was found. A substantial role
of both topochemical control from adjacent molecules
and the oligooxyethylene fragment length in the molꢀ
ecules in controlling of the distance between approached
double С=С bonds of the intermolecular sandwich sysꢀ
tem was revealed. An increase in the polyether chain length
8
7
6
5
δ
b
7
6
5
4
δ
Fig. 12. 1Н NMR spectra of chalcone podand 1b (1) and
cyclobutaneꢀcontaining oligomer 2b (2).

2298
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009
Ovchinnikova et al.
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Lobova, P. S. Loginov, A. V. Churakov, Yu. A. Strelenko,
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[Crystallogr. Repts (Engl. Transl.), 2009, 54].
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on going from compound 1a to 1b was found to favor the
appearance of rather loose regions in the crystal surroundꢀ
ing the dimeric πꢀstacking fragments and, as a conseꢀ
quence, the depth of occurrence of the PCA reaction
(over the whole crystal volume). However, this conforꢀ
mational tuning was insufficient to compensate internal
strains in the crystal and to retain its structure.
The authors are grateful to M. A. Uimin and A. A. Mysik
(Institute of Metal Physics, Ural Branch of the Russian
Academy of Sciences) for help in photography of the
single crystals.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 07ꢀ03ꢀ96111,
06ꢀ03ꢀ08144, 07ꢀ03ꢀ96113, and 08ꢀ03ꢀ00031), the Counꢀ
cil on Grants of the President of the Russian Federation
(Program for State Support of Leading Scientific Schools,
Grant NShꢀ3758.2008.3), and the Presidium of the Rusꢀ
sian Academy of Sciences (Projects "Design of Novel
Supramolecular Structures Containing Heterocyclic
Fragments" and "Development of Efficient Methods of
Chemical Analysis and Investigation of Substance and
Material Structure").
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Received December 30, 2008;
in revised form July 17, 2009