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 C₂H₄follows 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.