Rotaxane formation by cucurbit[7]uril in water and DMSO solutions.

The cucurbit[7]uril (CB7) host forms rotaxane-type complexes with dumbbell-shaped, cationic guests bis(3,5-dimethoxybenzyl)-4,4'-bipyridinium (1(2+)) and bis(3,5-diethoxybenzyl)-4,4'-bipyridinium (2(2+)). The kinetics of complex formation is slower with the latter guest because of its bulkier end groups. Rotaxane formation was found to be thermodynamically more favorable and kinetically faster in D2O than in DMSO-d6 solution, which highlights the importance of hydrophobic interactions in the assembly of cucurbituril complexes.