Photochemical Reactions of Phthalazine in 2-Propanol

Abstract Phthalazine undergoes dual photoreactions to simultaneously give two photoproducts, 1,2-dihydrophthalazine (1) and 1,1′,2,2′-tetrahydro-1,1′-biphthalazine (2), upon ultraviolet light irradiation in 2-propanol under nitrogen. There occurs neither photochemical nor thermal interconversion between 1 and 2; these compounds are formed independently from a common intermediate through different reaction pathways. The results of quenching and sensitization experiments for both the reactions and the phosphorescence emission show that the lowest excited singlet and triplet states of phthalazine participate in the formation of 1 and 2, respectively. The observed photochemical behaviors under various conditions lead us to propose a reaction mechanism: Phthalazine is photoreduced in the S1 state to form 1,2-dihydro-1-phthalazinyl radical. The resulting radical in a solvent cage undergoes a subsequent hydrogen abstraction to form 1. On the other hand, the same radical produced in the T1 state escapes from the solvent cage to cause a dimerization which affords 2.