Investigation of solute-fluid interactions in supercritical CF₃H: A multifrequency phase and modulation fluorescence study

Multifrequency phase-modulation fluorescence spectroscopy is used to study the photophysics of a polar, environmentally-sensitive fluorescent probe, 6-propiony1-2-dimethylaminonaphthalene (PRODAN), in near- and supercritical CF₃H. The results show evidence for the simultaneous existence of an array of molecular environments about the PRODAN probe. In turn this indicates that the CF₃H clusters or aggregates, that form in the highly compressible region near the critical point, do not exist in a discrete form, that is, there may be an ensemble of cluster sizes being investigated by the PRODAN probe. These results demonstrate for the first time: 1) the use of time-resolved fluorescence spectroscopy as a tool to determine the dynamics of solute-fluid interactions in supercritical fluids and 2) evidence of lifetime distributions for a simple fluorophore dissolved in a neat solvent system.