Photoisomerization and Photoinduced Hydrogen-Atom Transfer in l-Chloro-4-(2-pyrrol-2-ylethenyI)phthalazine

摘要

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 iso-mer 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 mus in benzene.Based on these results,the photochemical isomerization of 2 in the singlet excited state and in the triplet excited state is discussed.