Laser flash photolysis investigation of the triplet-triplet annihilation of anthracene in supercritical water

The effects of the supercritical water environment on the triplet-triplet a nnihilation of anthracene, a simple, well-characterized reaction that is kn own to be diffusion controlled in normal liquids, was investigated at tempe ratures from 375 to 450 degrees C and pressures from 50 to 350 bar. The rea ction was found to occur just slightly above the diffusion-control limit, w hich was estimated from the Stokes-Einstein based Debye equation, when spin statistical factors are taken into account. This is in qualitative agreeme nt with previous studies of diffusion-controlled reactions in lower tempera ture supercritical fluids (SCFs). Thus, the supercritical water environment is similar to that of lower temperature SCFs for diffusion-controlled reac tions of nonpolar hydrocarbon species. There is no evidence of solvent clus tering and any influence of solute/solute interactions is small. Finally, t he mechanism of the reaction appears to be the same as in nonpolar liquid s olvents.