The formation of effective and precise linkages in bottom-up or top-down processes is important for the development of self-assembled materials. Self-assembly through molecular recognition events is a powerful tool for producing functionalized materials. Photoresponsive molecular recognition systems can permit the creation of photoregulated self-assembled macroscopic objects. Here we demonstrate that macroscopic gel assembly can be highly regulated through photoisomerization of an azobenzene moiety that interacts differently with two host molecules. A photoregulated gel assembly system is developed using polyacrylamide-based hydrogels functionalized with azobenzene (guest) or cyclodextrin (host) moieties. Reversible adhesion and dissociation of the host gel from the guest gel may be controlled by photoirradiation. The differential affinities of ?-cyclodextrinor ?-cyclodextrinfor the trans-azobenzeneand cis-azobenzeneare employed in the construction of a photoswitchable gel assembly system.
Naturally occurring systems build functional assemblies and organs based on a bottom-up strategy through selective molecular recognition by macromolecules, such as DNAs and proteins. Selective molecular recognition among macromolecules is achieved through noncovalent bonds and involves a large number of weak bonding interactions. In recent years, much attention has focused on supramolecular science, the science of noncovalent assembly, in recognition of the importance of specific noncovalent interactions in biological systems and chemical processes. Although many supramolecular architectures have been synthesized, the self-assembly of macroscopic materials through molecular recognition poses one of the biggest challenges. Introduction of a stimulus-responsive guest molecule and its corresponding host molecule to a macroscopic material could provide a means for highly regulated macroscopic assembly through structural changes in the guest moiety triggered by a stimulus. In this way, the interactions between a host and guest can be controllably switched.
Here we describe a photoregulated macroscopic gel assembly system using polyacrylamide-based hydrogels functionalized with photoresponsive guest and host molecules. Simple agitation of the photoresponsive guest gel in the presence of the host gel leads to gel adhesion (assembly) through host?guest interactions, and the assembled gels separate into separate pure gels on irradiation with ultraviolet light. The dissociated gels reassemble on visible light irradiation. Placement of a second host gel, which binds the azobenzene (Azo) guest in an alternate conformation favoured by excitation with ultraviolet light, adjacent to the assembly of the first host?guest gels, followed by agitation in waterwith exposure to ultraviolet light, results in the pairing exchange in the gel assembly, indicating that the gel assembly is photoswitchable. A hydrogel synthesized by copolymerization of monomers bearing host and guest moieties and bis-acrylamidealso forms a homogeneous gel assembly and shows the same photoresponsive properties.
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