INTERSYSTEM CROSSING OF RADICAL PAIR IN SOLVENT CAGE - EXTERNAL HEAVY-ATOM EFFECT ON DUAL PHOTOREACTIONS OF PHTHALAZINE

The effects of the temperature, initial concentration, and chemical quenchers on dual photoreactions of phthalazine were investigated. The results give further supporting evidence for a previously proposed reaction mechanism, i.e., phthalazine in the lowest excited singlet and triplet states affords singlet and triplet radical pairs, respectively, in the initial hydrogen abstraction process from 2-propanol. The singlet radical pair gives a reduction product in a solvent cage, whereas the radicals produced in the triplet state escape from the solvent cage, causing dimerization. On the basis of the reaction mechanism of dual photoreactions, external heavy-atom effects on the radical pairs within a solvent cage have been studied. The results indicate that heavy-atom perturbations bring about an enhancement of the intersystem crossing efficiency from the triplet-state radical pair to the singlet one, including a spin inversion of the radical pair in the solvent cage, rather than S1 --> T1 intersystem crossing induced within a single molecule of phthalazine.