SINGLET OXYGEN GENERATION BY TRIPLET CHARGE-TRANSFER COMPLEXES

Quantum yields of singlet oxygen generation by complexes of triplet p-chloranil and various benzene derivatives with partial or complete charge transfer were measured by time-resolved phosphorescence. The probability of 1O2 generation (S-DELTA) decreases in the series triplet p-chloranil greater-than-or-equal-to nonpolar triplet exciplex > triplet ion-radical pair (TIRP). For the TIRPs with anisole, durene, hexamethylbenzene, and 1,2,3-trimethoxybenzene, S-DELTA = 0.05-0.20. The quantum yields of p-chloranil semiquinone radical production were obtained by nanosecond laser photolysis. The H-atom abstraction reaction proceeds by a two-step mechanism of electron transfer followed by proton transfer inside thermalized TIRPs; the contribution of "fast" H-atom transfer is approximately 10% in the cases of durene and hexamethylbenzene and negligible with the other donors.