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 steady-state and time-resolved fluorescence spectroscopy and the fluorophor/quencher reaction pairs are chosen to vary the reactions from diffusion-controlled to kinetically-controlled. In particular, the fluorescence quenching of anthracene by CBr₄, 1,2-benzanthracene by CBr₄, and anthracene by C₂H₅Br in supercritical CO₂ at 35 °C has been reported. Experimental rate constants for the first two reaction pairs, anthracene/CBr₄ and 1,2-benzanthracene/CBr₄, follow the predicted diffusion control limit at all pressures from 77.9 to 160.6 bar, indicating that local solvation does not enhance the reaction rate nor substantially impede the diffusion process in supercritical CO₂. The rate constants for the third reaction, the quenching of anthracene by C₂H₅Br, are several orders of magnitude below the diffusion control limit, indicating that the reaction is kinetically controlled, as it is in liquids. In supercritical CO₂ the apparent rate constants (i.e., those based on bulk concentrations of the reactants) for the anthracene/C₂H₅Br reaction decrease dramatically with increasing pressure. We believe that this apparently large pressure effect on the reaction rate is primarily due to the local composition enhancement of the quencher molecules around the dilute anthracene solute. This analysis is supported by fluorescence spectra and solvatochromic shift data of anthracene in pure CO₂ and in mixtures of CO₂ with C₂H₅Br at 35 °C that indicate both local density augmentation and local composition increases around the anthracene.