Cyclodextrins and Stilbenes as Components of Supramolecular Structures

Abstract

Background Cyclodextrin-based supramolecules have assumed importance since such assemblies may be either prototypes for, or components of, molecular devices with potential uses in nanotechnology. Cyclodextrins (cyclic oligomers comprising α-1,4-linked D-(+)-glucopyranose units) form host guest complexes with a wide range of hydrophobic molecules in an aqueous environment. [ 1 ] Stilbenes are known to form stable complexes with cydodextrins. [ 2 , 3 ] Their ability to interconvert between the cis and trans forms has made them the focus of our research. Mechanically interlocked structures called rotaxanes are formed when bulky end-capping groups are introduced on the stilbene so that the complex can no longer dissociate (Fig. 1). [ 3 ] The guest stilbene is free to rotate within the cyclodextrin; however, this free motion may be restricted by introduction of substituents on the stilbene and the cyclodextrin (Fig. 2a). [ 4 ] Fig. 2 also shows other examples of cyclodextrin- and stilbene-based structures being investigated. Cyclodextrins where a single hydroxy group is substituted with a stilbene are called hermaphrodites as they have both host and guest components (Fig. 2b). In concentrated solutions, trans-stilbene hermaphrodites form dimeric complexes, which can be characterized when end-capping groups are introduced to form rotaxanes. [ 5 a ] Alternatively, the hermaphrodites may assemble into linear daisy chain type structures. In molecular shuttles (Fig. 2c), the cyclodextrin moves from one binding site to another upon application of an external stimulus. [ 6 ] .