Electron-transfer photochemistry of cis- and trans-1,2-diphenylcyclopropane with singlet acceptors: Recombination of radical ion pairs of singlet and triplet multiplicity

Radical ion pairs generated by photoinduced electron transfer from cis- and trans-1,2-diphenylcyclopropane to various singlet or triplet acceptors may undergo back electron transfer in pairs of singlet as well as triplet multiplicity. The pair energy relative to the reactant ground states and an accessible triplet state, respectively, determines whether this process is competitive. For the cis isomer, cis-1̇⁺, an additional reaction, o,o′ coupling, generating 9-methylanthracene, 4, competes with singlet and triplet back electron transfer. This reaction is not accessible in the chloranil reaction for energetic reasons or from the trans isomer on steric grounds.