Physical gels based on supramolecular gelators, including host–guest complexes and pseudorotaxanes

Abstract

This article reviews recent studies on the gels formed by host–guest complexes, rotaxane and pseudorotaxanes. Pseudorotaxanes of crown ethers and rotaxanes of cyclic viologens with organic axle molecules form organogels through bonding of the functional end groups of the axle component. Cyclodextrins and cucurbiturils form gels in organic media and in aqueous mineral acids, respectively. The host–guest complexes and the pseudorotaxane of these macrocyclic compounds form hydrogels. These physical gels can be changed to sols by heating or addition of competing guests. Precise control of the gelation was enabled by using these supramolecules as the hydrogelator. Amphiphilic N-alkylpyridiniums and α-cyclodextrin form the hydrogel composed of their pseudorotaxanes. The mechanism of the gelation is described.