Concentration‐dependent deuterium isotope effects on the quantum yield of arene–alkene meta photocycloaddition: The role of excimers

Abstract A deuterium isotope effect on the arene–alkene meta photocycloaddition, reported earlier for toluene/toluene‐d8 photoaddition to cyclopentene, was found to depend on the concentration of both arene and alkene. This phenomenon is explained in terms of an unbalanced dissociation of ‘mixed’ excimers, which play a role in the intermolecular competition experiments.