The relaxation mechanism of electronically excited states of host–guest complexes between cucurbiturils (CB) and pyridinium styryl dyes is considered in detail on the basis of the recent results obtained by the up-conversion fluorescence technique. The addition of CB to aqueous dye solutions increases the longest fluorescence decay times from about 50 ps for the free dyes to 100–150 ps for the bound ones. This is attributed to the braking of intramolecular rotations around the single bonds of the styryl moiety that is provided by guest’s displacement inside the cavity, whose driving force is a Coulombic interaction of the styryl dye cation and negatively charged CB portals. This displacement, a translational movement along the CB axis, is associated with the observed decay time of about 1 ps. There is also a characteristic time of about 100 fs, attributed to vibrational relaxation. In fact, such complexes can operate as a molecular machine, the molecular switch.
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