Solute−Fluid Coupling and Energy Dissipation in Supercritical Fluids: 9-Cyanoanthracene in C2H6, CO2, and CF3H

Abstract

We report on the coupling and dissipation of energy between a model fluorescent solute, 9-cyanoanthracene (9CA), and several supercritical fluid solvents. To this end, we have determined experimentally the fluorescence quantum yields and excited-state fluorescence lifetimes for dilute solutions of 9CA in supercritical C2H6, CO2, and CF3H. The 9CA quantum yield is substantially less than unity at lower fluid densities; it approaches unity only at the high-density, liquid-like region. The 9CA excited-state lifetime is also shortened significantly in the low-density region. The radiative (kr) and nonradiative (knr) decay rates for 9CA are found to be strongly density dependent. In the low-density region, the nonradiative rate dominates; however, in the high-density region the 9CA deexcitation follows the radiative pathway. The Strickler−Berg relationship (kr ∝ n2; n = solvent refractive index) holds for 9CA in many normal liquid solvents. However, in supercritical fluids in the low-density regime, the simple...