Effect of Environment on the Unimolecular Decay of the Triplet State

Abstract

Lifetimes and relative quantum yields of phosphorescence were measured over the temperature range −10° to −196°C for solutions of palladium (II) mesoporphyrin IX, dimethyl ester, and ρ‐phenylbenzophenone in methylphthalylethylglycolate. The observed variation in the rate constants for the unimolecular decay of the triplet state coupled with the changes in the relative quantum yields of phosphorescence provide conclusive evidence in favor of a viscosity influence on the unimolecular decay processes. Employing values of the activation energy for internal degradation of the triplet state in the range of those reported in the literature, and other information, the total rate constants are divided into the individual ones. The changes in the individual rate constants for the radiative and nonradiative processes are presented and discussed. In the cases studied, the probability of internal degradation from the triplet state decreases with increasing viscosity. However, the probability for the radiative process decreases for palladium porphyrin but increases for ρ‐phenylbenzophenone with increasing viscosity.