DOI: 10.1002/asia.201500696
Communication
Cyclodextrins
A Cyclodextrin-Based Photoresponsive Molecular Gate that
Functions Independently of Either Solvent or Potentially
Competitive Guests
Ka-Heng Loh,[a] Nicole M. Smith,[a] Hideki Onagi,[a] Stephen F. Lincoln,[b] and
Christopher J. Easton*[a]
Abstract: The photoinduced interconversion between cin-
namido-substituted cyclodextrins constitutes
a gating
switch through which the substituent moves to open or
block access to the cyclodextrin cavity. Most unusually for
a cyclodextrin-based device, the operation of this gate is
solvent-independent and unaffected by potentially com-
petitive guests. It occurs in MeOH and DMSO, as well as in
water. This contrasts with other cyclodextrin inclusion
phenomena that are usually driven by hydrophobic effects
and limited to aqueous media.
Scheme 1. Photoisomerization of the cinnamides 1 and 2. (The truncated
cone represents a b-cyclodextrin moiety which has been modified by substi-
tution of either a C3A or C6A hydroxy group).
Cyclodextrins have been known for more than a century, and
the continuing interest in these compounds since their discov-
ery has largely arisen from their ability to form host–guest in-
clusion complexes with hydrophobic guests in aqueous solu-
tions.[1,2] Such complexes have found widespread applications
in the pharmaceutical, cosmetic, and food industries.[2,3] They
have also been used to facilitate chemical transformations,[4] as
well as in separation science[5] and the development of new
materials.[6] In supramolecular chemistry, they have provided
the foundation for a diverse range of molecular devices, in-
cluding rachets,[7] switches,[8,9] muscles,[10] and machines,[11,12]
that perform specific functions in response to external stimuli.
In aqueous media, inclusion of a guest by a cyclodextrin is
typically driven by the change in hydration of both the hydro-
phobic cyclodextrin cavity and the guest.[1] Relatively rarely, cy-
clodextrins also form inclusion complexes in non-aqueous sol-
vents[13] but, even in those cases, the inclusion is still controlled
by solvent effects. In any solvent, complexation is characteristi-
cally determined by the balance of the cyclodextrin–solvent,
Figure 1. Structure of cinnamide isomers 3 and 4.
guest–solvent, cyclodextrin–guest and solvent–solvent interac-
tions. By contrast, we now report the solvent-independent, in-
tramolecular inclusion of the substituent of cyclodextrin 1,
which is instead driven only by photoinduced cis–trans isomer-
ization of the cinnamido group (Scheme 1). Previously, we
found that the 6A-trans-cinnamido-b-cyclodextrin 3 (Figure 1)
formed a self-inclusion complex in water, that was disrupted
by organic solvents and competitive guests, whereas the corre-
sponding cis-isomer 4 did not.[11] The cis--trans photoisomeriza-
tion therefore enabled the water-dependent substituent com-
plexation. With the 3A-trans-cinnamidocyclodextrin 1, the sub-
stituent is included in the cyclodextrin annulus irrespective of
the solvent or the presence of potentially competitive guests.
The cis-isomer 2 does not form a complex, so in this case the
interconversion between the cinnamides 1 and 2 constitutes
a solvent- and guest-independent photoresponsive gating
switch.
[a] K.-H. Loh, Dr. N. M. Smith, Dr. H. Onagi, Prof. Dr. C. J. Easton
Research School of Chemistry
The Australian National University
Canberra ACT 2601 (Australia)
[b] Prof. Dr. S. F. Lincoln
Department of Chemistry
The University of Adelaide
Adelaide SA 5005 (Australia)
In this study we used the substituted b-cyclodextrins 1 and
13 having an N-methylcinnamide unit covalently attached at
the C3A position. In the latter case (13), the substituent is
linked to an altropyranose unit, whereas in the other example
(1) it is bonded to a glucopyranose. This is a consequence of
Supporting information and ORCID(s) from the author(s) for this article are
Chem. Asian J. 2015, 10, 2328 – 2332
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