Phototransformation of stilbene in van der Waals nanocapsules

Article abstract of DOI:10.1002/chem.200501026

We have utilized para-hexanoylcalix[4]arene nanocapsulcs as hosts to carry out phototransformations of cis- and trans-stilbene. Single-crystal X-ray diffraction studies were performed to define precisely the location of encapsulated stilbenes inside the capsule and to analyze possible pathways of pholotransformation. cis-Stilbene stacks as a π-π dimer located at the center of the capsule, whereas trans-stilbene does not form such a dimer. Irradiation of the crystalline inclusion complexes of each isomer of stilbene in the solid state leads to the appearance of the second isomer, and after prolonged photolysis, photodimerization also occurs. syn-Tetraphenylcyclobutane is formed as the major product of dimerization and its yield depends on the time and intensity of irradiation. In most cases, the single crystals of the complexes remain intact during irradiation; hence, the nanocapsules have the potential to serve as robust nanoreactors in the solid state. The confinement in the nanocapsules is sufficient to keep the reacting molecules together, although this is less restrictive than for trans-stilbene crystals, in which the molecules cannot achieve a favorable orientation for dimerization.

Full text of DOI:10.1002/chem.200501026

FULL PAPER
DOI: 10.1002/chem.200501026
Phototransformation of Stilbene in van der Waals Nanocapsules
Gennady S. Ananchenko,*[a] Konstantin A. Udachin,[a] John A. Ripmeester,[a]
Thomas Perrier,[b] and Anthony W. Coleman[b]
Abstract: We have utilized para-hexa-
noylcalix[4]arene nanocapsules as hosts
to carry out phototransformations of
cis- and trans-stilbene. Single-crystal X-
ray diffraction studies were performed
to define precisely the location of en-
capsulated stilbenes inside the capsule
and to analyze possible pathways of
complexes of each isomer of stilbene in
the solid state leads to the appearance
of the second isomer, and after pro-
longed photolysis, photodimerization
also occurs. syn-Tetraphenylcyclobu-
tane is formed as the major product of
dimerization and its yield depends on
the time and intensity of irradiation. In
most cases, the single crystals of the
complexes remain intact during irradia-
tion; hence, the nanocapsules have the
potential to serve as robust nanoreac-
tors in the solid state. The confinement
in the nanocapsules is sufficient to
keep the reacting molecules together,
although this is less restrictive than for
trans-stilbene crystals, in which the
molecules cannot achieve a favorable
orientation for dimerization.
phototransformation.
cis-Stilbene
stacks as a p–p dimer located at the
center of the capsule, whereas trans-
stilbene does not form such a dimer. Ir-
radiation of the crystalline inclusion
Keywords: calixarenes · isomeriza-
tion · nanocapsules · photochemis-
try · supramolecular chemistry
Introduction
stable structures that also display considerable flexibility.
We have shown recently that amphiphilic para-hexanoylca-
lix[4]arene (Figure 1) forms container-type[17] or capsule-
The rational design of host–guest structures, with particular
attention given to the robustness of the materials, enables
one, in principle, to achieve not only unique physical proper-
ties, such as controlled capture and release of guest materi-
als, but also the controlled chemical transformations of cap-
tured guest molecules within the host.[1–5] To carry out chem-
ical reactions in such environments, the construction of
large capsules that can accommodate two or more substrates
in the cavity is particularly important. The frameworks can
be constructed by using covalent or metal–ligand bonds,[2–9]
hydrogen bonds,[9–13] or even van der Waals interactions.[14–16]
The last case is particularly interesting, as even these weak
interactions collectively can potentially create relatively
Figure 1. para-Hexanoylcalix[4]arene C6OH.
type[16] inclusion complexes, depending on whether they
crystallize from highly or weakly polar solvents in the pres-
ence of an excess of guest material. The essential feature of
such nanocapsules is a hydrophobic nanoenvironment con-
structed from only weakvan der Waals interactions. As a
result, the calixarene crystal structures exhibit well-defined
cavities of near-constant size and shape that can be occupied
by a variety of organic guest molecules, and are, thus, utiliz-
able as robust hydrophobic microvessels for thermal as well
as photochemical reactions.
[a] Dr. G. S. Ananchenko, Dr. K. A. Udachin, Dr. J. A. Ripmeester
Steacie Institute for Molecular Sciences
National Research Council Canada
100 Sussex Drive, Ottawa, Ontario, K1A 0R6 (Canada)
Fax : (+1)613-998-7833
E-mail: Gennady.Ananchenko@nrc-crnc.gc.ca
[b] T. Perrier, Dr. A. W. Coleman
Institut de Chimie et Biologie des Proteins
7 Passage du Vercors, 69367 Lyon Cedex 07 (France)
In this context, we have examined the influence of hydro-
phobic nanocapsules based on amphiphilic para-hexanoylca-
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2006, 12, 2441 – 2447
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lix[4]arene (Figure 1) on the photoreactions of stilbenes.
Stilbenes exhibit diverse photochemical behavior in solu-
tion,[18] for example, reversible cis/trans isomerization,[19] cyc-
lization of cis-stilbene[20] 1 to dihydrophenanthrene and fur-
ther oxidation to phenanthrene 3, and dimerization[21,22] of
trans-stilbene to yield tetraphenylcyclobutane products 4
and 5 (Scheme 1). Hence, they are good candidates for dem-
Figure 2. Cut-away view of C6OH capsules entrapping a) two molecules
of 1 (complex A); b) one molecule of 1 (complex B). The two molecules
of cis-stilbene in complex A are marked in different colors. For simplicity,
only one of several disordered positions of each alkyl arm of the calixar-
ene is shown.
plex (complex A) has tetragonal P4/nnc symmetry with
unit-cell parameters 15.52; 15.52; 22.71 Š and is quite simi-
lar to the structure of the complex of C6OH with chloro-
form.[16] Two molecules of cis-stilbene are located in the
center of the capsule, and these are intermeshed in an at-
tempt to achieve p–p interactions of the benzene rings. The
shortest distance between benzene-ring planes is around
3.3 Š, which is somewhat less than the sum of the van der
Waals radii of carbon atoms (ca. 3.4 Š), and also less than
the typical distance of benzene rings that interact through
p–p stacking (3.5–4.2 Š).[28–30] As a result, the repulsion be-
tween rings may well exceed possible stabilizing interac-
tions.[28–30] The torsion angle along the ethylenic double
bond is 20.98, which is larger than the optimum for cis-stil-
bene obtained from ab initio calculations[31] or gas-phase
electron diffraction experiments[32] (6 and 58, respectively).
On the other hand, the average phenyl rotation angle of ap-
proximately 298 out-of-plane is smaller than those given in
the literature (40 and 438, respectively),[31,32] and this is
probably due to the moleculeꢁs attempt to achieve a more
planar conformation for p–p stacking with a neighbor. The
entire “dimer” appears to be a compromise between p-elec-
tron stabilization of a single cis-stilbene molecule and p–p
interaction of the two molecules. Notably, our attempts to
fix the aforementioned angles according to literature
values[31,32] did not improve the quality of refinement of the
structure.
Scheme 1.
onstrating the modulation of photochemical reactivity by
solid-state host–guest interactions. So far, stilbenes have
been utilized to study their phototransformation on flat sur-
faces,[23a] in zeolites,[23b] cyclodextrins[24–27] in solution and in
the solid state, as well as in micelles in aqueous solutions.[25]
The essential feature of unsubstituted stilbenes is that nei-
ther liquid cis- nor crystalline trans-isomers undergo photo-
dimerization.[18] Here, inclusion complexes of cis- and trans-
stilbene with para-hexanoylcalix[4]arene were prepared and
characterized by single-crystal X-ray diffraction, and these
were irradiated by using steady-state techniques to explore
their phototransformation pathways.
Results and Discussion
cis-Stilbene/para-hexanoylcalix[4]arene
inclusion
com-
plexes: Slow evaporation of chloroform from a solution of
the calixarene in chloroform/cis-stilbene at 808C results in
the crystallization of para-hexanoylcalix[4]arene (C6OH)
(Figure 1) in large, square, colorless crystals of up to 3”
3 mm in size. NMR analysis of the solution of the complex
in CDCl3 shows host/guest ratios from 2/1.6 to 2/1.9 for dif-
ferent batches. Single-crystal X-ray diffraction reveals that
the calixarene molecules are arranged in tail-to-tail pairs to
form a hydrophobic capsular complex with a cavity approxi-
mately 8 Š wide and 16 Š long, and with two molecules of
cis-stilbene within the capsule (Figure 2). The inclusion com-
The inclusion complex A is not expected to be stable at
elevated temperatures and it would probably prefer to eject
a molecule of cis-stilbene. Indeed, the thermogravimetric
analysis (TGA) trace (Figure 3) clearly shows that one cis-
stilbene molecule leaves the capsule within the temperature
interval 60–1208C. We monitored the release of 1 within the
range 25–1008C by hyperpolarized (HP)-129Xe NMR spec-
troscopy, which provides an in situ probe of the tempera-
ture-dependent access to the guest sites in the complex.[33,34]
Figure 4 shows a set of 129Xe NMR spectra for complex A in
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Phototransformation of Stilbene
FULL PAPER
After the complex has been purged by Xe at 1008C, it
contains just a single molecule of cis-stilbene per capsule
(complex B), as confirmed by results of solution NMR stud-
ies and TGA. The single crystal of complex A survives the
thermal treatment of three hours at 1208C, therefore, XRD
analysis of the resulting complex B was performed. The
unit-cell parameters obtained are almost the same as those
of complex A, that is, tetragonal P4/nnc symmetry with
15.52; 15.52; 22.32 Š. Figure 2b shows the structure of com-
plex B. One can see that cis-stilbene has moved deeper into
the cavity. The electron-density map in XRD data for en-
capsulated guests was very complicated because it contained
residual densities from p–p-stacked cis-stilbene molecules
from the original complex, as well as those from cis-stilbene
in complex B. Hence, we were unable to obtain precise data
for bond lengths and angles in the stilbene molecule. How-
ever, it is clear that, because p–p stacking with the partner
stilbene is not possible, the remaining guest now can interact
through p–p interactions with the benzene rings of the calix-
arene rim, analogous with other calixarene-based sys-
tems.[14–16] The approximate distances between host and
guest aromatic carbon atoms are approximately 3.7–4.0 Š.
Comparison of the 13C cross-polarization magic-angle
spinning (CP-MAS) NMR spectrum of complex A with the
spectrum of liquid cis-stilbene recorded in the CP-MAS
probe without spinning reveals an upfield shift of approxi-
mately 1 ppm of all signals of the encapsulated stilbene
(Figure 5). It has been reported[16] that the extra shielding of
Figure 3. TGA traces of complexes A and C.
Figure 4. In situ study of the transformation (from bottom to top) of com-
plex A to complex B by exposing it to a Xe/He/N2 gas mixture at elevat-
ed temperatures, as monitored by continuous-flow HP-129Xe NMR spec-
troscopy.
contact with a flowing gas mixture (98% He, 1% N2, 1%
Xe) containing HP Xe (partial pressure of 7 Torr) as a func-
tion of temperature under static conditions. The room-tem-
perature spectrum shows a resonance at 0 ppm only, which
can be assigned to free xenon gas. In the spectrum recorded
at 508C, another line appears as a very broad signal at
around 90 ppm and this can be attributed to xenon interact-
ing with host cavities.[15,35] The relatively high chemical shift
reflects only the start of the interaction with the solid before
Xe actually enters the deep cavity. As the temperature in-
creases, the line becomes sharper and a small signal (seen as
a right shoulder) appears, indicating that Xe starts to dis-
place stilbene in the capsule, and, hence, more space for the
gas becomes available in the cavity. At around 1008C, Xe fi-
nally occupies a position deep within the capsule, and a rela-
tively sharp line at 92 ppm indicates fast exchange between
the gas interacting with the outside surface, and that inter-
acting with the host cavities.
Figure 5. Comparison of the 13C CP-MAS NMR spectra of complex A
(top) and pure cis-stilbene (bottom). Signals of calixarene are marked by
“c”.
guests in para-hexanoylcalixarene nanocapsules is not very
high and that 1 ppm represents a typical difference between
the carbon chemical shifts of free and encapsulated guests.
This is contrary to other inclusion complexes within capsules
formed by covalent or hydrogen bonds,[11,36–38] but is charac-
teristic of dynamic systems, as moieties that are fixed deep
inside the cavity can experience shifts as large as 5–6 ppm.
The C6OH-capsule provides sufficient freedom for the guest
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G. S. Ananchenko et al.
within it to move and, therefore, for the large deshielding to
be modified by chemical-shift averaging.
be more open to the environment and the cups are slightly
shifted along the equatorial plane. Considering that this
complex was obtained from ethanol, in which no capsules
are formed in the absence of appropriate guests,[17] one can
regard this structure as a transition from the capsule-type to
an open-container[17] or self-inclusion complex.[17,39]
trans-Stilbene is disordered over six symmetry-independ-
ent positions in the unit cell. This probably reflects capsules
containing one and two molecules of 2 each. Only three rel-
ative orientations of two trans-stilbene molecules are possi-
ble (Figure 7): edge-to-edge (ee), edge-to-plane (ep), and
trans-Stilbene/para-hexanoylcalix[4]arene inclusion com-
plex: The inclusion complex of the calixarene with trans-stil-
bene (complex C) was prepared by recrystallization from an
ethanol solution containing the components in an approxi-
mately 1/5 ratio of calixarene/stilbene, upon very slow cool-
ing (ca. 58C per day). Although the material was of excel-
lent quality for XRD, we were unable to collect larger
amounts of the pure complex for HP-129Xe or for solid-state
1
NMR experiments. H NMR analysis of the solution of the
crystals in acetone revealed the ratio of components to be 2-
(C6OH)/0.65(EtOH)/1.3(2). The crystals appeared under
the microscope to be uniform, so one can conclude that two
types of complex are present: in the first, the capsule con-
tains one molecule of 2 and one molecule of ethanol; in the
second, the capsule captured two molecules of 2 with proba-
bly a small amount of ethanol. This generally agrees with
TGA traces obtained for this complex (Figure 3), in which
the first weight loss of around 4% at 70–1308C corresponds
to the release of ethanol and trans-stilbene to form a com-
plex with only one molecule of 2 per capsule, and the
second loss reflects ejection of the remaining guest. At-
tempts to isolate the pure complex with two molecules of 2
per capsule by increasing the concentration of 2 in the solu-
tion yielded the simultaneous precipitation of complex C
and pure trans-stilbene. Therefore, the precise control of the
composition of complex C was not possible.
The unit-cell parameters of complex C derived from
single-crystal X-ray diffraction experiments proved to be dif-
ferent from those for complexes A and B: monoclinic P21/n
symmetry with unit-cell parameters 15.58; 22.14; 15.96 Š;
and angles 90.0; 91.0; 90.08. The two cups of the calixarene
(Figure 6) in the capsule are almost aligned, but are not
twisted, as for the tetragonal P4/nnc complexes A and B,
that is, the benzene rings of the lower cup lie almost exactly
below those of the upper cup. The entire capsule appears to
Figure 7. Possible orientations of two trans-stilbene molecules within the
capsule.
plane-to-plane (pp). By analogy with complexes A and B,
and by minimizing the van der Waals repulsion between two
guests, we can expect the structures of the capsules contain-
ing one and two molecules of 2, shown in Figure 6 (for sim-
plicity, ethanol was not taken into account). The position of
trans-stilbene in the capsule with one molecule of 2 (Fig-
ure 6b) resembles that of 1 in complex B, that is, p–p inter-
actions between guest and host benzene rings. In the capsule
with two trans-stilbene molecules (Figure 6a), we expected
p–p interactions between guests to be more preferable. The
smallest distance between two molecules is approximately
3.7 Š between the benzene rings and approximately 4.4 Š
between the olefinic carbons. Because the molecules are
shifted with respect to each other, they cannot yield a good
p–p stack, though local stabilization remains possible.
Therefore, the entire structure is a compromise between p-
electron interaction of a trans-stilbene molecule with the
host and p–p interactions of the two trans-stilbene mole-
cules. Notably, ep configuration of two stilbene molecules
(Figure 7) in the capsule cannot be ruled out completely,
due to the expected favorable edge-to-plane interaction of
benzene rings.[40] However, such configuration seems to be
less probable because of restricted space in the cavity.
Photolysis: The solid inclusion complexes of C6OH with cis-
stilbene (complexes A and B) and trans-stilbene (complex
C) were irradiated by UV light (320–390 nm, either 500 or
2500 mWcmÀ2) at 08C. Single crystals or powders of the
complexes were used. After a variable duration of photoly-
sis, the samples were dissolved in deuterated acetone and
analyzed by 1H NMR spectroscopy. The results indicated the
Figure 6. Cut-away view of complexes C with a) two molecules of trans-
stilbene; b) one molecule of trans-stilbene. For simplicity, only one of
several disordered positions of each alkyl arm of the calixarene is shown.
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Phototransformation of Stilbene
FULL PAPER
presence of at least one of the following products
(Scheme 1): cis-stilbene 1 (olefinic protons at 6.65 ppm),
trans-stilbene 2 (p- and m-aromatic protons at 7.62 and
least partly, as a scavenger of the excited stilbene providing
rapid sensitization and, therefore, isomerization.
At higher concentrations of trans-stilbene, at which two
molecules of 2 are present in one capsule of the photolyzed
complex A, photodimerization becomes possible. This re-
moves 2 irreversibly from the mixture at this wavelength,
however, the fast isomerization of 1 maintains the overall
concentration of 2 as relatively constant. The stilbene photo-
dimer, yield about 15% (Figure 8), was detected after 3 hr
of photolysis, and significant syn-stereoselectivity (ca. 5/1) is
achieved. An increase in light intensity affects the stereose-
lectivity, so that the ratio of the dimers becomes about 2/1
again in favor of the syn-dimer 4. It is expected that in the
photolyzed complex A, most of the trans-stilbene molecules
are positioned in close proximity and in fixed orientations to
each other and, thus, can readily dimerize upon photoexcita-
tion. If the orientations of two molecules of 2 in the capsule
after photolysis of complex A are about the same as those
in complex C, that is, plane-to-plane (Figure 7), then the
probability of dimerization should be sufficient, because the
distance between olefinic double bonds is already close to
the required 4.2 Š,[45] and only a small sliding of the mole-
cules is needed to give overlapping orbitals. However, due
to fast cis/trans isomerization, the probability that at a given
moment two molecules of trans-stilbene are present in one
capsule and are located at an appropriate distance is not so
high. This can explain why the yield of tetraphenylcyclobu-
tanes 4 and 5 is low even after a reasonable reaction time.
We tried to checkthe positions of the products in the photo-
lyzed complex A and to compare this with complex C. A
single crystal of complex A remains intact following irradia-
tion for 20 min (Figure 8). X-ray diffraction data were easily
obtained after photolysis of the same crystal that was used
for the structural study before irradiation. The quality of the
crystal was satisfactory; however, because the material al-
ready contains capsules with both isomers of stilbene that
are highly disordered, it was not possible to localize each
compound precisely. Nevertheless, the unit-cell parameters
proved to be accurate and equal to those obtained for the
parent complex A, that is, tetragonal and not monoclinic.
The possibility of obtaining large encapsulated molecules,
such as 4 and 5, by the photolysis of stilbene is very interest-
ing and promising. Our attempts to co-crystallize C6OH
with the independently synthesized syn-dimer 4 failed, and
both components precipitated separately from ethanol solu-
tion. Thus, the photolysis of encapsulated “building blocks”
can be a potential source of larger encapsulated mole-
cules.[38]
7.38 ppm, respectively), phenanthrene
3
(H-9,10 at
8.82 ppm), and the two photodimers of 1,2,3,4-tetraphenyl-
cyclobutane, 4 and 5 (cyclobutane protons at 4.60 and
3.71 ppm,[27,41] respectively).
All three photochemical reactions depicted in Scheme 1
occurred upon irradiation of complex A (Figure 8). The cis/
trans isomerization is the fastest process with the highest
Figure 8. Kinetic traces of the photolysis of complex A at light intensities
a) 500 mWcmÀ2 and b) 2500 mWcmÀ2
.
quantum yield,[18] and this agrees with literature data for the
photolysis of stilbenes in solution[18–22] and in some organ-
ized media.[23–27] In addition, slow accumulation of the di-
meric products is observed after longer periods of photoly-
sis. An increase in light intensity (Figure 8b) increases
dimer formation, although the quality of the material wor-
sens and the solid turns brown. Because all reaction path-
ways originate from the excited-singlet state of stilbene as
the common precursor,[18,42] the predominance of the photoi-
somerization[43] is due solely to its rapid rate relative to that
of the diffusion-limited photodimerization. After fast equili-
bration, the concentration of 2 reaches its photostationary
value, so the isomer ratio [1]/[2] at equilibrium can be esti-
mated from Figure 8 to be almost 1/1. Because the cis/trans
ratio upon equilibrium is very close to that published for
sensitized photolysis of stilbenes in solution[44] in the pres-
ence of aromatic ketones and at about the same wavelength,
one can conclude that the capsule, acylphenol, serves, at
The photolysis of complex B (Figure 9a) leads to the ac-
cumulation of trans-stilbene and to slow cyclization of 1 to
phenanthrene 3. An increase in light intensity affects only
the proportion of 3, which doubles after the same duration
of photolysis. The quantum yield of reversible formation of
the intermediate dihydrophenanthrene (Scheme 1) is around
3.5-times smaller then that of isomerization (at 313 nm[18]),
so the low yield of product 3 is not surprising. The absence
of dimeric products 4 and 5 is confirmation that the capsule
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G. S. Ananchenko et al.
ient relatively freely within it. This gives these nanocapsules
some advantages over micelles,[25] for which escaping guests
must always be accounted for. The nanocapsule can be con-
sidered as a solvent cage isolated in the solid state. In
almost all our experiments, single crystals of the complexes
remained intact after irradiation and, hence, were suitable
for single-crystal XRD. This unambiguously confirms the
stability of this supramolecular structure during various
chemical transformations. Notably, amphiphilic calixarenes
can easily form solid lipid nanoparticles[35,46] that can serve
as prospective carrier systems. The combination of control-
led release of encapsulated guest with the possibility of pho-
totransformations during transportation of biologically
active guests provides the opportunity to fine tune the prop-
erties of encapsulated species according to biological targets.
Experimental Section
All chemicals were obtained from Aldrich and were used as received.
para-Hexanoylcalix[4]arene was synthesized as described in ref. [47].
cis,trans,cis-1,2,3,4-Tetraphenylcyclobutane 4 was prepared by photolysis
of trans-stilbene in benzene.[48] Thermogravimetric analysis was per-
formed by using a TA Instruments device with a temperature ramp of
38CminÀ1. Solid-state and HP-129Xe NMR spectra were recorded by
using Bruker AMX 300 and DSX 400 spectrometers, respectively. The
experimental setup for obtaining HP Xe was described in ref. [33].
Figure 9. Kinetic traces of the photolysis of a) complex B and b) complex
C at 500 mWcmÀ2
.
Photolysis was performed by using a Novacure 2100 spot curing system
within a UV range of 320–390 nm. 10 mg of powders of complexes were
distributed equally over an area of approximately 3 cm2 in the bottom of
a vial. The vial was placed in an ice/water mixture to avoid overheating
of samples during UV irradiation. The distance between the edge of the
light guide and the samples was 5 cm. Two light intensities, 500 and
2500 mWcmÀ2, were applied.
contains only one molecule of the guest. One can see from
Figure 9a that the isomerization rate and the [1]/[2] ratio at
the photostationary state are very similar to those during
the photolysis of complex A and, hence, the position of the
guest inside the capsule has no influence. Moreover, we can
conclude that the possible self-quenching[44] of excited stil-
bene by neighboring ground-state stilbene in one capsule of
complex A is insignificant.
The same products were detected following photolysis of
complex C (Figure 9b). The ratio of stilbene isomers [1]/[2]
reaches its photostationary value of around 55/45 after a
similar period of photolysis, so there is no distinct depend-
ence of product ratio on the structure of the complexes, that
is, tetragonal vs monoclinic. Only traces of dimers 4 and 5
were detected, which unambiguously confirms that primarily
one molecule of trans-stilbene occupies the capsule.
Crystals of complex A: The calixarene (500 mg) was dissolved in chloro-
form (3 mL) and cis-stilbene (2 g) was added. The solution was placed in
a furnace preheated to 808C. After 5 d, large (up to 5”5 mm), colorless,
square crystals were isolated. Crystals of complex B were obtained from
complex A by heating the latter in a furnace at 1208C for 3 hr.
Complex C: trans-Stilbene (90 mg) was dissolved in ethanol (20 mL) with
some heating. para-Hexanoylcalix[4]arene (100 mg) and ethanol (10 mL)
were then added and the mixture was stirred under slow boiling until the
calixarene had dissolved completely. The hot solution was placed in a fur-
nace that was preheated to 808C and programmed to decrease in temper-
ature by 58C every 12 hr. The solvent was allowed to evaporate slowly.
After the volume of the solution had returned to its original 20 mL, the
flaskwas closed tightly and cooling continued without evaporation until
RT was reached. The crystals were isolated by filtration and rinsed with
cold methanol.
Conclusion
The single-crystal structure studies of complexes A, B, and C were per-
formed by using a Bruker SMART diffractometer with MoKa radiation,
up to 2q=568. The structures were solved by direct methods and refined
by full-matrix least-squares analysis (SHELXL-97). All non-hydrogen
atoms (except guest molecules in complex C) were refined anisotropical-
ly, whereas all hydrogen atoms were placed at their idealized positions.
Benzene rings in guest molecules were set as planar and their bond
lengths were fixed, so that only internal rotation around double and
single bonds of the olefinic part of cis-stilbene was allowed during refine-
ment.
The photolysis of stilbenes encapsulated in van der Waals
nanocapsules in the solid state enabled us to gain a deeper
understanding of the features of host–guest interactions in
hydrophobic nanocapsules. The nanocapsule constructed of
amphiphilic calixarenes is sufficiently robust to keep re-
agents close to each other and to prevent the easy escape of
the two components. It allows reagents to interact closely
and to form some kind of sub-complexes, as in the example
of two cis-stilbene molecules p–p stacked inside complex A.
On the other hand, the capsule allows the reagents to reor-
The site occupancies were found to be 0.25(3) for guest in complex A,
0.050(5) and 0.10(1) for old and new positions of guests, respectively, in
complex B, and 0.07(1), 0.12(1), 0.17(2), 0.21(2), 0.21(2), and 0.22(2) for
six positions of trans-stilbene in complex C.
2446
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Chem. Eur. J. 2006, 12, 2441 – 2447

Phototransformation of Stilbene
FULL PAPER
Errors were measured by variations in fixed isotropic thermal parameters
from 0.1 to 0.5. Magnitudes of anisotropic thermal parameters (guest
molecules) were varied up to the maximum of 0.5, which is acceptable
for guest molecules in the cage.
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Crystal data and structure refinement parameters are reported in the
Supporting Information. CCDC-281734, -281735, and -281736 contain
the supplementary crystallographic data for complexes A, B, and C, re-
spectively. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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This workwas supported in part by the CNRS-NRC collaborative pro-
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Received: August 22, 2005
Published online: January 3, 2006
Chem. Eur. J. 2006, 12, 2441 – 2447
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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