SINGLET AND TRIPLET-STATE BACK ELECTRON-TRANSFER FROM PHOTOGENERATED RADICAL-ION PAIRS STUDIED BY TIME-RESOLVED CIDNP

Radical ion pairs are generated in acetontrile solution by photoinduce d electron transfer from naphthalene derivatives to cyanobenzenes and undergo reverse electron transfer to the singlet ground and to excited triplet states of the parent compounds. Both pathways lead to chemica lly induced nuclear polarization of the ground state products. They ca n be separated because the triplet contribution appears delayed by the triplet lifetime. Variation of the radical pair energy leads to a dom inance of either the singlet or triplet channel. The analysis of the a bsolute net CIDNP effects and their time dependencies yields reaction probabilities for the reverse electron transfers which depend on the a ppropriate energy gaps as described by the Marcus theory, as well as r ate constants for degenerate electron and spin exchange processes. For the donor/acceptor system naphthalene/(E)-1,2-dicyanoethene, the trip let pathway is also found dominant.