Photophysics of phenanthrene in supercritical carbon dioxide. Solvent-solute and solute-solute interactions revealed by lifetime distribution analysis

The exponential series method of analysis is applied to single photon counting measurements of phenanthrene fluorescence decay in supercritical carbon dioxide at 32 and 35 degrees C and at pressures ranging from 76 to 207 bar for chromophore concentrations between 5 x 10(-5) and 5 x 10(-6) M. The analysis revealed trimodal fluorescence lifetime distributions near the critical temperature which can be explained by the presence of solvent-solute and solute-solute clustering. This local aggregation causes an increase in nonradiative relaxations and, therefore, a decrease in the observed fluorescence lifetimes. Concentration and density gradients are responsible for these three unique lifetimes (trimodal) in the supercritical fluid, as contrasted with the single lifetime observed in a typical organic solvent. An increase in temperature to 35 degrees C induces no change in lifetime distributions at high chromophore concentration, but the trimodal distribution collapses to a single mode at lower concentrations, indicating that the disappearance of solvent-solute and solute-solute clustering with an increase in temperature is dependent on local chromophore concentration. Furthermore, the pressure has little or no effect on the clustering over the pressure range studied along an isotherm.