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 containing various quantities of modifier and solute has been constructed, enabling laser flash photolysis studies to be made in a purpose-built high-pressure thermostatted optical cell. Rate constants for triplet–triplet energy transfer have been measured for two donor–accepter systems in modified supercritical carbon dioxide. The rate of triplet–triplet energy transfer has been used as a probe for the diffusion-controlled 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 those in pure modifier solution. When the mole fraction of CO2 is greater than 0.8 the diffusion rate rapidly increases towards the maximum determined in pure supercritical carbon dioxide. Triplet–triplet energy-transfer rates approach more closely the diffusion-controlled rate constant calculated from the Debye equation as the modifier concentration is decreased, suggestive of local concentration enhancements at lower modifier concentrations.