Fluorescence Response of Alkaloids and DAPI on Inclusion in Cucurbit[7]uril: Utilization for the Study of the Encapsulation of Ionic Liquid Cations

The encapsulation of biologically important compounds in the cavity of cucurbit[7]urils (CB7) brings about a variety of effects. The binding of lumichrome to CB7 promotes the tautomerization from alloxazine‐ to isoalloxazine‐type structure in the ground‐state, leading to substantial changes in the absorption and fluorescence properties. Even partial embedding in CB7 macrocycle is able to protect sanguinarine, a natural alkaloid, against nucleophilic addition and photooxidation. The inclusion of berberine in CB7 results in about 500‐fold enhancement of its fluorescence intensity, and the change in its fluorescence decay characteristics indicates ternary complex formation with cations. DAPI (4′,6‐diamidino‐2‐phenylindole dication) is an excellent fluorescent probe for the study of the CB7 complex formation with ionic liquids, surfactants, and biologically important compounds. The binding of 1‐alkyl‐3‐methylimidazolium ions to CB7 is strongly exothermic; both the enthalpy and the entropy change in the reaction go through a minimum when the 1‐alkyl group is lengthened, reaching the smallest values for the hexyl derivative.