Femtosecond to nanosecond solvation dynamics in pure water and inside the γ-cyclodextrin cavity
The dynamics of solvation of an excited chromophore in pure water and in a restricted space with a limited number of water molecules have been studied. The time-dependent Stokes shift of Coumarin 480 (C480) and Coumarin 460 (C460) were measured using femtosecond fluorescence upconversion and time-correlated single-photon-counting techniques. The system with a limited number of water molecules was an inclusion complex of Coumarin dyes with γ-cyclodextrin (γCD). The results of molecular dynamics simulations are compared with the observed solvent response in pure water and in the γCD cavity. The observed relaxation times range from <100 fs to 1.2 ns. Solvation of C480 in pure water is observed to occur with time constants of <50 and 310 fs. In sharp contrast with the solvation response in pure water, in the case of the C480/γCD inclusion complex, additional long solvation time constants of 13, 109 and 1200 ps are observed. The stoichiometry, structure and dynamics of the Coumarin/γCD complexes are also discussed.