Applicable properties of cucurbiturils

This account highlights three significant fields of applications for cucurbiturils (CBs). The first relates to cucurbituril-based devices and sensors involving spectral effects associated with the formation and dissociation of inclusion complexes. The second refers to the use of CBs as catalysts that control the rate and selectivity of chemical reactions. The third deals with the potential use of CBs for the assembly of mechanical devices and molecular motors. An example of the sensor application is based on the discovery that 4-aminobipyridine derivatives form strong inclusion complexes with CB6, exhibiting remarkably large enhancements of fluorescence intensity and quantum yields. This reversible binding phenomenon can be used for the design of switchable beacons that can be incorporated into cascades of binding networks for various applications. The second application is exemplified by selective, enhanced photoaddition reactions as well as [3+2]cycloaddition and other reactions mediated by the CB cavity. The third application is based on the idea that low-barrier molecular rotary motors having rotaxane architecture can be constructed using a cucurbituril host and a polyyne guest serving as stator and rotator, respectively. The "low friction" between these components is supported by molecular mechanics calculations with model systems and experimentally verified by X-ray crystallography with several synthetic host-guest complexes, all suggesting that the diyne rod floats at the center of the macrocyclic host with no apparent van der Waals contacts between them.