PHOTOCHEMISTRY IN MODIFIED SUPERCRITICAL CARBON-DIOXIDE - EFFECT OF MODIFIER CONCENTRATION ON DIFFUSION PROBED BY TRIPLET-TRIPLET ENERGY-TRANSFER

A novel high-pressure system for preparing supercritical fluids contai ning various quantities of modifier and solute has been constructed, e nabling laser flash photolysis studies to be made in a purpose-built h igh-pressure thermostatted optical cell. Rate constants for triplet-tr iplet energy transfer have been measured for two donor-accepter system s in modified supercritical carbon dioxide. The rate of triplet-triple t energy transfer has been used as a probe for the diffusion-controlle d rate constants in acetonitrile- and n-hexane-modified supercritical carbon dioxide far from the compressible region. Fluids with modifier mole fractions above 0.5 show diffusion rates almost identical with th ose in pure modifier solution. When the mole fraction of CO2 is greate r than 0.8 the diffusion rate rapidly increases towards the maximum de termined in pure supercritical carbon dioxide. Triplet-triplet energy- transfer rates approach more closely the diffusion-controlled rate con stant calculated from the Debye equation as the modifier concentration is decreased, suggestive of local concentration enhancements at lower modifier concentrations.