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 fl uorescent solute, 9-cyanoanthracene (9CA), and several supercritical f luid solvents. To this end, we have determined experimentally the fluo rescence quantum yields and excited-state fluorescence lifetimes for d ilute solutions of 9CA. in supercritical C2H6, CO2, and CF3H. The 9CA quantum yield is substantially less than unity at lower fluid densitie s; it approaches unity only at the high-density, liquid-like region. T he 9CA excited-state lifetime is also shortened significantly in the l ow-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) proportional to n(2); n = solvent r efractive index) holds for 9CA in many normal liquid solvents. However , in supercritical fluids in the low-density regime, the simple Strick ler-Berg expression cannot account fully for the observed k(r) results . Additional corrections, accounting for the shifts in the 9CA absorba nce spectra, also cannot compensate completely for deviations from the predicted Strickler-Berg behavior. To yield agreement between the exp erimental k, data and the Strickler-Berg predictions, we require there to be changes in the total 9CA molar absorptivity with density. Recen t experiments on anthracene and pyrene in supercritical CO2 (Rice, J. K.; Niemeyer, E. D.; Bright, F. V. Anal. Chern. 1995, 67, 4354) demons trate that the average solute molar absorptivity is indeed a function of fluid density. The strong density dependence of the nonradiative de cay rate is interpreted in terms of an increase in fluid density leadi ng to an increase in the energy gap (Delta E) between T-2 and S-1 stat es. Specifically, at the lower fluid densities the S-1-T-2 intersystem crossing (ISC) rate increases because (1) Delta E is small and the fr action of 9CA molecules that occupy states within the S-1 manifold abo ve the lowest vibrational level of the T-2 envelope is increased and ( 2) the number of effective ISC crossing pathways from S-1 to T-2 is in creased because the Franck-Condon factor depends strongly on Delta E. 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 ad justing the density of the supercritical fluid.