A STEADY-STATE AND TIME-RESOLVED FLUORESCENCE STUDY OF QUENCHING REACTIONS OF ANTHRACENE AND 1,2-BENZANTHRACENE BY CARBON TETRABROMIDE AND BROMOETHANE IN SUPERCRITICAL CARBON-DIOXIDE

This paper reports on the solvent effect on energy transfer reactions in supercritical CO2. The energy transfer reactions are studied by ste ady-state and time-resolved fluorescence spectroscopy and the fluoroph or/quencher reaction pairs are chosen to vary the reactions from diffu sion-controlled to kinetically-controlled. In particular, the fluoresc ence quenching of anthracene by CBr4, 1,2-benzanthracene by CBr4, and anthracene by C2H5Br in supercritical CO2 at 35 degrees C has been rep orted. Experimental rate constants for the first two reaction pairs, a nthracene/CBr4 and 1,2-benzanthracene/CBr4, follow the predicted diffu sion control Limit at all pressures from 77.9 to 160.6 bar, indicating that local solvation does not enhance the reaction rate nor substanti ally impede the diffusion process in supercritical CO2. The rate const ants for the third reaction, the quenching of anthracene by C2H5Br, ar e several orders of magnitude below the diffusion control limit, indic ating that the reaction is kinetically controlled, as it is in liquids . Ln supercritical CO2 the apparent rate constants (i.e., those based on bulk concentrations of the reactants) for the anthracene/C2H5Br rea ction decrease dramatically with increasing pressure. We believe that this apparently large pressure effect on the reaction rate is primaril y due to the local composition enhancement of the quencher molecules a round the dilute anthracene solute. This analysis is supported by fluo rescence spectra and solvatochromic shift data of anthracene in pure C O2, and in mixtures of CO2 with C2H5Br at 35 degrees C that indicate b oth local density augmentation and local composition increases around the anthracene.