Model study of the abrupt excess‐energy dependence of radiationless decay in benzene and azabenzenes

Model studies are reported aimed at accounting for the abrupt dependence of radiationless decay‐rate constants on excess energy (known as channel‐three decay in the case of S 1 benzene) in singlet and triplet manifolds of benzene and azabenzenes. The favored model involves L a (ππ*) state, strongly distorted along an out‐of‐plane coordinate, crossing the ground‐state potential at an energy close to the minimum of the lowest excited state. The results are compared with experimental observations on benzene and three azabenzenes. Implications for photochemical reactions are also discussed.