EFFECT OF MULTIPLICITY ON REGIOCHEMISTRY OF [2+2] PHOTOCYCLOADDITION OF 3,9-DICYANOPHENANTHRENE TO TRANS-BETA-METHYLSTYRENE

The title reaction is known to afford two isomeric [2 + 2] cycloadducts. It is shown that the major adduct isolated under preparative conditions, the head-to-head (HH) isomer, is the exclusive product in the triplet reaction. The singlet reaction affords both HH and its head-to-tail isomer. Rationales are presented based on synchronous (four-center) collapses to adducts from the (emissive) exciplex in the singlet process and non-synchronous collapse in the triplet. A simple method of estimating the exciplex lifetime and the equilibrium constant for its formation from the lifetime of the monomer and the aerated and deaerated Stern-Volmer fluorescence quenching slopes is presented. © 1979.