Photoisomerization and photoinduced hydrogen-atom transfer in 1-chloro-4-(2-pyrrol-2-ylethenyl)phthalazine

An olefin having a phthalazine ring and a pyrrole ring (2) exhibited a fluorescence spectrum with a large Stokes shift in the cis-isomer. The cis-isomer composition at the photostationary state was considerably high, so as give an isomer ratio at the photostationary state ([cis]/ [trans]) pss of 97/3 upon irradiation at 366 nm. These results indicate the occurrence of intramolecular hydrogen-atom transfer in the singlet excited state. The produced tautomer scarcely gave the trans isomer by a reverse hydrogen-atom transfer, and cis-to-trans isomerization was prevented by the existence of intramolecular hydrogen bonding in the cis isomer. Both cis-2 and trans-2 underwent intersystem crossing to give the same triplet excited state with a lifetime of 10 mu s in benzene. Based on these results, the photochemical isomerization of 2 in the singlet excited state and in the triplet excited state is discussed.