Electronic singlet state quenching by transition metal complexes

The excited singlet state quenching of organic hydrocarbons by Co(III) and Cr(III) βdiketonate complexes have been investigated using the technique of single photon counting. It is established that the most likely quenching mechanism is that of electronic energy transfer. The observed rate constants for the quenching of naphthalene and pyrene by Cr(acac)3 in a series of solvents are shown to be consistent with the predictions made for a dipole-dipole mechanism coupled with molecular diffusion. In addition it has been possible to demonstrate the breakdown of the Stokes-Einstein equation in the determination of the diffusion coefficients required in the calculations for hydroxylic solvent systems. The importance of exchange contributions in the less efficiently quenched pyrene systems is also discussed [continued]…