Steady-State and Time-Resolved Fluorescence Investigations of Pyrene Excimer Formation in Supercritical CO₂

Detailed studies on the formation of pyrene excimer in supercritical CO₂ are reported. The photophysics of pyrene are investigated as a function of temperature and fluid density. Over the broad density range studied, there is no evidence for ground-state (solute-solute) interaction. Comparison is made between excimer formation in supercritical CO₂ and ground-state dimerization of pyrene in γ-cyclodextrin (γ-CD). Time-resolved fluorescence spectrocopy is used to recover the individual rate terms that describe the total emission process. The recovered density-dependent bimolecular rates for pyrene excimer formation in supercritical CO₂ follow a simple diffusion-controlled model. This result parallels reports on pyrene excimer formation in normal liquid solvents. Finally, the relative decrease in pyrene excimer formation, with increased fluid density, is easily explained from our time-resolved experiments.