INFLUENCE OF SOLUTE FLUID CLUSTERING ON THE PHOTOPHYSICS OF PYRENE EMISSION IN SUPERCRITICAL C2H4 AND CF3H

In this paper, we continue our investigations of the photophysics of pyrene emission in supercritical fluids using steady-state and time-resolved fluorescence spectroscopy. Specifically, we study pyrene photophysics in sub- and supercritical C2H4 and CF3H. Steady-state fluorescence is utilized to determine how the local fluid density affects the ground and excited states of pyrene and to probe any potential ground-state dimerization (preassociation). Time-resolved fluorescence provides details of the mechanism of pyrene excimer formation and the kinetics of solute-solute interactions. The temperature and density dependence of the recovered bimolecular rate constant for pyrene in supercritical C2H4 follows that which had been reported in supercritical CO2 and liquid solvents, i.e., the excimer formation is diffusion controlled. In supercritical CF3H, this rate constant is much slower than predicted based on diffusion control arguments. These results indicate that enhanced solute-solvent interactions (clustering) strongly affect the pyrene excimer reaction in CF3H.