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

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 (k_r) and nonradiative (k_nr) 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 (k_r ∝ n²; n = solvent refractive index) holds for 9CA in many normal liquid solvents. However, in supercritical fluids in the low-density regime, the simple Strickler-Berg expression cannot account fully for the observed k_r results. Additional corrections, accounting for the shifts in the 9CA absorbance spectra, also cannot compensate completely for deviations from the predicted Strickler-Berg behavior. To yield agreement between the experimental k_r data and the Strickler-Berg predictions, we require there to be changes in the total 9CA molar absorptivity with density. Recent experiments on anthracene and pyrene in supercritical CO₂ (Rice, J. K.; Niemeyer, E. D.; Bright, F. V. Anal. Chem. 1995, 67, 4354) demonstrate that the average solute molar absorptivity is indeed a function of fluid density. The strong density dependence of the nonradiative decay rate is interpreted in terms of an increase in fluid density leading to an increase in the energy gap (ΔE) between T₂ and S₁ states. Specifically, at the lower fluid densities the S₁-T₂ intersystem crossing (ISC) rate increases because (1) ΔE is small and the fraction of 9CA molecules that occupy states within the S₁ manifold above the lowest vibrational level of the T₂ envelope is increased and (2) the number of effective ISC crossing pathways from S₁ to T₂ is increased because the Franck-Condon factor depends strongly on ΔE. Finally, our data demonstrate that the extent of solute-fluid coupling to/with the fluid bath (i.e., the 9CA radiative or nonradiative decay rates) can be tuned over more than an order of magnitude by simply adjusting the density of the supercritical fluid.