Rotational Dynamics of Coumarin 153 in Supercritical Fluoroform

Subpicosecond fluorescence anisotropy decay curves have been measured using the fluorescence up-conversion technique to examine the rotational dynamics of coumarin 153 (C153) in supercritical fluoroform ( T = 302 and 310 K). For reduced densities (ρ r = ρ/ρ c ) above 0.9, the rotation times of C153 increase with density. For reduced densities lower than 0.9, the rotation times increase with decreasing density and has a maximum at a density near ρ r = 0.5. The comparison with the extrapolation of the data in polar aprotic solvents indicates that the local solvent density around the solute exceeds about 4 times the bulk density at a density near ρ r = 0.5, which is consistent with those obtained from the steady-state electronic spectral shifts.