Angewandte
Chemie
DOI: 10.1002/anie.201108074
Inclusion Complexes
Selective Guest Exchange in Encapsulation Complexes Using Light of
Different Wavelenghts**
Henry Dube and Julius Rebek Jr.*
Reversible control over the state of host–guest encapsulation
complexes is a topic of current interest, and at least as many
different methods are conceivable as there are unique
hosts.[1–5] The task is to trigger the exchange of guests by an
“external” signal after the initial assembly is generated. The
use of light to accomplish this exchange has appeal, because
no additional components are introduced into the system and
application of the signal is easily done.[6] Accordingly, light
has been used as early as 1979 to manipulate the state of host–
guest complexes in cyclodextrins.[7,8] Much more recently, we
applied this method to control encapsulation in our hydrogen-
bonded capsule 1·1 and in the extended capsule 1·24·1
(Figure 1a) by using azobenzenes as light-switchable
dummy guests.[9] With this refinement, we were able to
control the guest that is encapsulated and even the type of
assembly that is present in solution. A supramolecular
fluorescence switch that is operated by light and heat was
membranes,[19,20] to manipulate the folding of peptides,[21] to
change the potency of an inhibitor by irradiation,[22] or to
control ionic currents in gramicidin channels.[23] Applications
in supramolecular chemistry are still a novelty.[24]
One additional requirement was a change in shape, from
a linear structure (a congruent guest) to a bent structure (an
inconvenient shape) upon photoisomerization. The core HTI
without any substituents does not show that feature, because
both E and Z isomers have an easily accommodated shape.
Appropriate groups on the aromatic rings result in a bent
shape of one isomer and a (more-or-less) linear shape for the
other. Methyl substitutions at both the 5-position of the
heterocycle and at the para-position of the stilbene unit (HTI
3)[25] were found to be effective: the Z isomer of 3 is
encapsulated exclusively in 1·1 in the presence of the inferior
guest n-tridecane; on irradiation at 410 nm for 80 min the
guests are completely exchanged, with n-tridecane encapsu-
lated and the E isomer of 3 present in solution. Heating that
solution to 1608C for 6 min restores the starting state. The
system can be cycled many times (Figure 1b and Figure S3 in
the Supporting Information). A similar photoregulated guest
exchange is possible in the extended assembly 1·24·1, when the
longer HTI 4, bearing an n-pentyl chain at the para-position
of the stilbene unit, is employed as the switchable guest
together with excess p-cymene as the secondary guest (see
Figure 1c and Figure S6 in the Supporting Information).
Three social isomers of p-cymene are possible in 1·24·1, but
only two are observed, as previously described[26] (only one
social isomer is depicted in Figure 2). The syntheses of HTIs 3
and 4 are given in the Supporting Information and follow
established literature procedures[19,20,25,27] starting from com-
mercially available p-toluenethiol.
constructed as
a
second-generation system.[10] For an
increased information, a corresponding escalation in “input”
structures is necessary, and a system that responds differently
to different wavelengths of light is a likely first step. Systems
already exist that show such multi-light input responses, but
their photoresponsive entities are usually covalently con-
nected.[11–14] A supramolecular system that uses different
wavelengths of light to selectively affect guest exchange has
not been available; herein, we report a wavelength-depen-
dent and sequential light-triggered guest exchange from two
different assemblies.
For this sequence we developed a second “dummy” guest,
which can be extruded from the capsular hosts by photo-
isomerization at a wavelength complementary to that of
azobenzene (365 nm). Hemithioindigo (HTI) appeared
promising, because it is readily isomerized at longer wave-
lengths (typically 410 to 430 nm) with high efficiency while
showing very little photofatigue.[15–18] The back-isomerization
can either be achieved by irradiation at wavelengths higher
than 480 nm or by simple heating. Because of these advan-
tages, HTI is increasingly applied as photoswitch: to embed in
Next, we investigated the photoswitching behavior of
a mixture of HTI 3 (a prospective guest for 1·1) and the long
4-methyl-4’-n-hexylazobenzene 5 (a prospective guest for
1·24·1) at different wavelengths. At short wavelengths
(350 nm) both dyes isomerize, which prevented selective
switching. However, at wavelengths of 410 to 430 nm it was
possible to isomerize HTI
3 leaving the azobenzene
unchanged (data are shown in Figure S7 in the Supporting
Information). These experiments augured well for the
sequential guest exchange of two different assemblies: the
HTI dummy guest was first isomerized using light at a wave-
length of ꢀ 410 nm to trigger the first exchange; then the
azobenzene dummy guest could be expelled from its assembly
by irradiation at approximately 365 nm to affect the second
guest exchange. The trans-4,4’-dimethylazobenzene (trans-6)
is a very good guest for capsule 1·1, but the longer azobenzene
trans-5 is only a mediocre guest for the extended assembly
1·24·1. Since the azobenzene-occupied assembly should be
[*] Dr. H. Dube, Prof. J. Rebek Jr.
The Skaggs Institute for Chemical Biology and Department of
Chemistry, The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
E-mail: jrebek@scripps.edu
[**] We are grateful to the NSF/CHE 1037590 and the Skaggs Institute
for Research for financial support and the Alexander von Humboldt-
Stiftung for a Feodor Lynen Fellowship for H.D.. H.D. is also Skaggs
Postdoctoral Fellow and was supported by the Swiss National
Science Foundation (SNF).
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 3207 –3210
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3207
Dube, Henry
