Evidence for changes in the conformation of flexible solutes dissolved in supercritical solvents

Steady-state and time-resolved fluorescence were used to study the effects of near-critical and supercritical C₂H₆ and CHF ₃ on the conformation and dynamics of flexible fluorescent probe, 1,3-bis(1-pyrenyl)propane (BPP). In liquids, BPP forms only an intramolecular excimer and the excimer emission results from an excited-state species only; there is absolutely no ground-state pre-association between the two pyrenes. In supercritical solvents, the results are different. First, in the lower density region (0.5 ≥ ρ_r ≤ 1.2; ρ_r = reduced density), there is little, if any, excimer-like emission. Second, in the higher density region (ρ_r > 1.2), there is strong excimer-like emission. Third, emission wavelength-dependent excitation scans show that there is substantial pre-association between the two BPP pyrenes at higher fluid densities. Hence much of the excimer-like emission is a result of the two pyrenes in a single BPP interacting with one another in the ground state. Finally, time-resolved fluorescence spectrometry shows that a portion of the observed excimer-like emission results from an eximer formed dynamically in the excited state; however, ground-state pre-association is the dominant cause for the observed excimer-like emission. These results demonstrate that the conformation of BPP is altered substantially in supercritical fluids. This represents the first detailed information on fluid-induced changes in the conformation of highly flexible solutes.