Light-sensitive gel based on cyclic 'pumpkins'

Macrocyclic cavitands such as cyclodextrins or calixarenes having hydrophilic differences between inside and outside the cavities often show a binding or inclusion ability toward a wide variety of molecules to give a supramolecular construct called inclusion complex, provided the sizes of the guests match the cavities of the hosts. The inclusion complex is formed non-covalently and thus can be dissociated reversibly, under certain condition. This reversible feature has enable scientists to develop various stimuli-responsive supramolecular-structured gels. Cyclodextrins, for example, have been employed with well defined block or graft copolymers to form hydrogels with sensitivities to a number of environmental stimuli^†, which is the direction of my graduate thesis. Host-guest interaction is thought to be the driving force of the physical cross-linking responsible for the gelation. Cucurbituril, named by its pumpkin-like morphology, is another family of macrocyclic molecules. Comprising n glycoluril units, cucurbit[n]urils (n=5-10) have a variable-sized cavity that is accessible through two identical carbonyl-fringed portals.

Cucurbit[7]urils (CB[7]), whose cavity size is comparable to that of β-cyclodextrin, has modest solubility in water. Kimoon Kim, who is one of the most active supramolecular chemists, and co-workers recently reported a hydrogel based on only CB[7], a host molecules, via self-assembly in acidic conditions (Angew. Chem. Int. Ed. 2006. DOI: 10.1002/anie.200603149). The CB[7] gel shows a slow gel-to-sol transition over a temperature range of 43-57°C and a sharp sol-to-gel transition at around 42°C. Characterized by AFM and small-angle X-ray scattering (SAXS), it is found that the CB[7] molecules self-assembly into unit 1D fibrils, which further align into thick bundles of fiber with lengths up to several tens of micrometers and form a network (figure on the right^‡, click to enlarge). Addition of proper guest molecules, however, may induce further stimuli-responsive behavior into the CB[7] gel. 4,4'-diaminostilbene dihydrochoride, a light sensitive molecule that shows photoisomerization, is known to form stable 1:1 inclusion complexes with CB[7] at both trans and cis conformations. The guest-including CB[7] molecules can thus self-assembly into a gel under certain condition. When irradiated by UV light (365nm), the gel turns into yellow sol; heating the sol follwed by cooling induce the gelation again. Therefore, by choosing the appropriate guest molecules, one may be able to design various stilmuli-responsive gel systems based on the intriguing host-guest chemistry of CB[7] and the properties of the included guest molecules.

† For example, Biomaterials 2006 27 4132-4140; Macromolecules 2005 38 5223-5227; Macromol. Rapid Commun. 2006 27 238-241; Macromolecules 2006 39 2614-2620.

‡ Adapted from the original paper. Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim