Mobility of spin probes in viscous cyclodextrin solutions.

TEMPO-based spin probes and beta-cyclodextrin form 1 ratio 2 guest-host complexes in viscous aqueous glycerol solutions at low temperatures. The spin probe mobility in these complexes is much higher than in the bulk glassy solution. The similar tumbling rates of the spin probe in viscous host-guest complexes and in solvent-free complexes suggest that the complexation effectively shields the nitroxide unit from the solvent molecules. This is consistent with the low activation barrier for molecular tumbling of immobilised label.