Supercritical fluids are interesting both fundamentally and practically [1]. The most important properties of a supercritical fluid are the low densities, which are between those; of a gas and a liquid and are easily tunable with changes in pressure isothermally, and the clustering effects. Solute-solvent clustering and solute-solute clustering in supercritical fluids have been studied extensively, and their effects on unimolecular and bimolecular chemical processes have been evaluated. Supercritical fluids are also investigated as alternatives to normal liquid solvents for a variety of physical and chemical processes. Particularly, supercritical CO2 has attracted much attention, due largely to the ambient critical temperature and the environmentally benign characteristics of the fluid. For example, supercritical CO2 has been used in the preparation and processing of perfluorinated polymers [2]. Supercritical CO2 - surfactant systems have also been examined for the handling of biological materials [3]. In our laboratory, we have been using solvent-sensitive luminescent molecular probes and well-established photochemical reactions in the study of supercritical fluids aimed toward a clear understanding of solute-solvent and solute-solute interactions. For example, our study of the photodimerization reaction of anthracene shows that the bimolecular reaction rate in supercritical CO2 is enhanced by an order of magnitude in comparison with the rates in normal liquid solvents such as benzene [4], Unlike in normal liquid solvents, the photodimerization reaction of anthracene in supercritical CO2 is significant even at anthracene concentrations as low as a few micro-molar. The dramatic increases in the photodimerization reaction efficiency in supercritical CO2 may be explained by a mechanism in which the reaction is diffusion-controlled even when the diffusion rate constants are on the order of 10~(11)M~(-1)S~(-1).
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