A comprehensive photophysical and NMR investigation on the interaction of a 4-methylumbelliferone derivative and cucurbit[7]uril

Abstract

The host-guest interaction of a 7-hydroxycoumarin (3-[2-(diethylamino)ethyl]-7-hydroxy-4-methylcoumarin, 3ED4MU) with cucurbit[7]uril, CB7, was investigated in aqueous solution. From the steady-state fluorescence Job plot and 1H NMR spectroscopy the formation of a 1:1 inclusion complex, with a binding constant of 1.75 × 105 M−1 was obtained. Time-resolved fluorescence data in water, DMSO and dioxane for 3ED4MU and for umbelliferone (UM, here taken has the model compound) show that the decays are double or triple-exponentials mirroring the presence of excited neutral (N*), anionic (A*), tautomeric (T*) and zwitterionic (T*′) species (the latter only present in 3ED4MU). In the time-resolved experiments, the interaction of 3ED4MU with CB7 is mirrored by an increase of the pre-exponential factor associated to the zwitterionic species (thus giving support for the stabilization of this species when interacting with CB7) and a dramatic change in the pKa* value (from ~0.5 to ~8), associated to a displacement of the water environment probed by 3ED4MU.