Spectroscopic studies of solvent effects on reactions in supercritical fluids

A number of studies have shown that the local environmental around dissolved solutes in supercritical fluids (SCFs) is significantly different than the bulk. Moreover, in SCF mixtures, there is evidence that the local composition around a dilute solute can be significantly enriched with one of the components. In this paper, we will show how we have used laser flash photolysis and time-resolved fluorescence to study the influence of these molecular scale interactions on the rates of electron transfer and energy transfer reactions in SCFs. In particular, we will present results of the reaction of triplet benzophenone with either triethylamine or 1,4-diazabicyclo[2.2.2]octane in SC CO2 and ethane. These reactions occur by way of an electron transfer mechanism in normal liquids at rates close to diffusion control. However, in SCFs, they occur over an order of magnitude below the diffusion control limit. We will compare these results with the rates of energy transfer from anthracene to CBr4 in supercritical CO2, which occurs at the diffusion control limit in both liquids and SCFs.