Light-triggered crystallization of a molecular host-guest complex

Article abstract of DOI:10.1021/ja103620z

The control of structural changes in supramolecular assemblies is a key point in the development of molecular machines. The reversible photoisomerization of organic compounds such as azobenzene using light as an external input is especially suited because no waste products are generated. Based on our previous studies on the quantitative encapsulation of suitably sized bis-sulfonate guests by a self-assembled, metal-organic cage consisting of four rigid, bent bis-monodentate pyridyl ligands and two Pd(II) ions, we show here how the light-switchable guest cis-4,4'-azobenzene bis-sulfonate can be expelled from its 1:1 host-guest complex triggered by its photoi-somerization to the trans-isomer. Using a highly soluble, PEGylated cage derivative, the full reversibility of this light-driven encapsulation/release process is demonstrated. In contrast, a sample of the less soluble, unsubstituted cages including 1 equiv of the cis-guest was shown to result in immediate crystallization upon photoisomerization of the guest. X-ray structure analysis confirmed the guest molecules having left the cavity of the host and on the contrary joining the cages into a polymeric material by binding to their Pd(II) centers from outside.

Full text of DOI:10.1021/ja103620z

Published on Web 07/06/2010
Light-Triggered Crystallization of a Molecular Host-Guest Complex
Guido H. Clever, Shohei Tashiro, and Mitsuhiko Shionoya*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
Received April 28, 2010; E-mail: shionoya@chem.s.u-tokyo.ac.jp
Abstract: The control of structural changes in supramolecular
assemblies is a key point in the development of molecular
machines. The reversible photoisomerization of organic com-
pounds such as azobenzene using light as an external input is
especially suited because no waste products are generated.
Based on our previous studies on the quantitative encapsulation
of suitably sized bis-sulfonate guests by a self-assembled,
metal-organic cage consisting of four rigid, bent bis-monodentate
pyridyl ligands and two Pd(II) ions, we show here how the light-
switchable guest cis-4,4′-azobenzene bis-sulfonate can be ex-
pelled from its 1:1 host-guest complex triggered by its photoi-
somerization to the trans-isomer. Using a highly soluble, PEGylated
cage derivative, the full reversibility of this light-driven encapsula-
tion/release process is demonstrated. In contrast, a sample of
the less soluble, unsubstituted cages including 1 equiv of the cis-
guest was shown to result in immediate crystallization upon
photoisomerization of the guest. X-ray structure analysis con-
firmed the guest molecules having left the cavity of the host and
on the contrary joining the cages into a polymeric material by
binding to their Pd(II) centers from outside.
Figure 1. (a) Quantitative encapsulation of cis-2 by PEGylated cage 1a
and reversible switching between host-guest complex cis-2@1a and the
The self-assembly of polymeric supramolecular nanostructures
is the key feature of a number of processes forming biological
materials such as the cellular cytoskeleton1 and virus capsids.2 On
the way to mimic nature’s sophisticated self-assembled architectures
by artificial compounds, a great number of elaborate examples of
discrete structures (e.g., molecular cages)3 and infinite constructs
such as hydrogen-bonded coordination polymers4 have been
reported.
Beyond the static artificial systems of molecular self-assembly,
the possibility to control the formation process and functions of a
supramolecular architecture by external stimuli can be seen as the
next step in the development of new intelligent materials.5
Light as a reagent offers the advantages of being easy to apply
for a chosen time and intensity and does not result in the
accumulation of waste products even during a reversible switching
sequence using light of various wavelengths.6
In the context of host-guest chemistry, the control of encapsula-
tion of light-switchable compounds such as stilbene or azobenzene
derivates by molecular cages has been reported before by Fujita,7
Rebek,8 and Jiang9 et al.
Recently we reported the ability of a new type of molecular cage
1 formed from four rigid, bent bis-monodentate pyridyl ligands and
two Pd(II) or Pt(II) ions favoring a square-planar geometry to
encapsulate suitably sized bis-sulfonate guests by a specific anion
recognition process.10,11
mixture of trans-2 + 1a by photoisomerization of the guest; (b) Quantitative
encapsulation of cis-2 by cage 1b; (c) crystallization of [(1b)(trans-2)2] by
photoisomerization; (d) crystallization of [(1b)(cis-2)2] by addition of a
second equivalent of cis-2; (e) structures of cage 1 and guest 2.
material,10 we examined bis-sulfonate guest 2 whose size (S-S
distance) can be switched reversibly by photoisomerization.
The tetrabutylammonium salt of trans-4,4′-azobenzene bis-
sulfonate trans-2 was prepared according to reported procedures
and converted into its isomer cis-2 by irradiation of its solution in
acetonitrile with UV light of 365 nm wavelength.12 First, we
examined molecular cage 1a comprising eight polyethylene glycol
(PEG) chains rendering the cage highly soluble in organic solvents
regardless of the presence of a cross-linker in the solution. cis-
4,4′-Azobenzene bis-sulfonate (cis-2) has an ideal size for being
quantitatively incorporated into molecular cage 1 in the previously
reported fashion.10 The complete uptake of 1 equiv of cis-2 inside
the cage 1a could be unambiguously shown by 1H NMR spectros-
copy (Figure 2). In good accordance with our previous observation
for the structurally related guest compound 1,1′-ferrocene bis-
sulfonate, the cage’s NMR spectrum shows characteristic changes
upon addition of the guest.10 Most notably, the 1H NMR signal of
the cage’s inward pointing hydrogen atom Hb (blue in Figures 1
and 2) undergoes a downfield shift by ∼0.5 ppm upon encapsulation
of the guest cis-2. Additionally, the NMR signals of guest cis-2
show a characteristic upfield shift of approximately 1.1 and 0.7
ppm, respectively, upon encapsulation by the cage. A comparison
of molecular sizes of the free anion cis-2 and the encapsulated guest
Based on our observation that, depending on its size, a bis-anionic
compound can act either as a guest being quantitatively encapsulated
or as a cross-linking agent connecting the cages to form a crystalline
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C O M M U N I C A T I O N S
cis-2 in the host-guest complex cis-2@1a by a DOSY-NMR
experiment confirmed the full uptake of the relatively small guest
inside the much bigger host compound. Finally, we observed the
occurrence of a peak at m/z ) 2586.7 for the molecular ion of the
host-guest complex [cis-2@1a]2+ in the ESI-TOF mass spectrum
(Supporting Information).
When the solution containing the host-guest compound cis-
2@1a was irradiated for ∼3 h using a conventional white-light
compact fluorescent lamp, guest compound cis-2 was quantitatively
converted back to its isomer trans-2 accompanied by a characteristic
change in the 1H NMR spectrum of the sample. First, the
characteristic downfield shift of Hb is lost with the formation of a
new, broad signal at 8.5 ppm, and second, the signals of the
encapsulated guest at approximately 5.7 and 6.9 ppm vanish, which
indicates the formation of a loosely associated system composed
of cage 1a and compound trans-2 undergoing a fast exchange
process on the NMR time scale. A DOSY experiment indicates a
slight increase in the hydrodynamic radius of the cage upon release
of the guest which is explainable by electrostatic binding of trans-2
to the outside of the cage rather than to the inside.
chains [(1b)(trans-2)]n and the other “dipping” into the interior space
of two cages of neighboring chains, each with both its sulfonate groups
in the fashion of a cross-link (Supporting Information). The composi-
1
tion [(1b)(trans-2)2] was additionally confirmed by H NMR spec-
troscopy after dissolution of the crystalline material in hot DMSO-d6.
In contrast, when a solution of cage 1b was treated with 2 equiv
of guest cis-2, immediate crystallization was observed even under
the exclusion of white-light (Figure 3f-j; separating the sample
from the white-light source by a yellow filter did prevent the
formation of the trans-isomer but allowed monitoring the sample
chamber by light microscopy). In this case, however, the crystals
were of a totally different morphology from that of the crystals
formed in the previous experiments, being rather oval than square-
shaped (compare Figure 3e and j). Since 2 equiv of guest cis-2
were needed to afford the formation of crystals and no white-light
was administered to the sample, the composition most likely can
be explained by the formula [(1b)(cis-2)2]. After this crystalline
sample was exposed to white-light, the oval crystals dissolved and
gave place to concomitantly forming square-shaped crystals (Figure
3j to k). Although the new crystals formed at the same spots where
the oval crystals were located before, this process most probably
is not a topotactic “crystal-to-crystal” phase transition13 but rather
the remains of the dissolving oval crystals functioning as seeds for
the growth of the new square-shaped crystals.
Most interesting, however, is the observation that this process
is fully reversible. Again irradiating the sample with UV light of
365 nm wavelength results in a full regeneration of the spectrum
assignable to the host-guest complex cis-2@1a (Figures 1a and
2). Subsequent exposure to white-light again leads to release of
the guest from the cage, and we were able to repeat this process
four times without notable major changes in the NMR spectra
(Supporting Information).
Figure 3. (a) 1b + 1 equiv of cis-2 under yellow-light; (b-e) crystallization
of [(1b)(trans-2)2] after changing to white-light at t ) 11 min; (f-j) 1b +
2 equiv of cis-2 leads to crystallization of [(1b)(cis-2)2] even under yellow-
light; (k) transformation of the crystals upon changing to white-light at t )
16 min.
The soluble host-guest system that we present here may be seen
as a supramolecular building block, which contains a kind of
noncovalent glue as a guest hidden in its cavity. Irradiation with
light of an appropriate wavelength triggers the release of this glue
and leads to aggregation of the cage units into a higher structure.
This principle of a light-induced phase change from soluble
host-guest complexes into insoluble materials may be useful for
the development of new strategies for nanoconstruction and the
spatially controlled lithographic deposition of supramolecular
networks on surfaces.
Figure 2. 1H NMR titration (500 MHz, CD3CN, 293 K) of cage 1a with
cis-2 to form the host-guest complex cis-2@1a followed by reversible
photoswitching between cis-2@1a and trans-2 + 1a. Signals of encapsulated
cis-2 shown in red, free cis-2 in green, inward pointing hydrogen atom of
cage in blue.
Acknowledgment. This work was funded by the Global COE
Program for Chemistry Innovation and a JSPS postdoc and start-
up grant for G.C.
Next, we examined the interaction of guest cis-2 with cage 1b
which differs from cage 1a in the way that it has no solubility-
enhancing PEG chains. Likewise to cage 1a, the addition of up to
1 equiv of guest cis-2 results in the quantitative formation of a
soluble host-guest complex cis-2@1b. A DOSY-NMR experiment
and ESI-TOF mass spectrum again support the formation of the
anticipated host-guest complex (Supporting Information).
Surprisingly, when the clear, yellow solution of cis-2@1b was
exposed to white-light, yellow crystals of [(1b)(trans-2)2] formed
immediately (Figure 3a-e). An X-ray structure, albeit of low quality
due to extensive solvent disordering, showed the presence of 2 equiv
of trans-2 inside the crystal: one linking individual cages by binding
to the outer faces of the coordinated Pd(II) centers to form infinite
Supporting Information Available: Synthetic procedures, NMR,
mass spectra, X-ray structure, and movies of the light-initiated
crystallization. This material is available free of charge via the Internet
at http://pubs.acs.org.
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