Photoisomerization of 2-(2-pyrrol-2-ylethenyl)benzoxazole and 2-(2-pyrrol-2-ylethenyl)benzothiazole

Abstract

Hydrogen bonded compounds 2-(2-pyrrol-2-ylethenyl)benzoxazole (4) and 2-(2-pyrrol-2-ylethenyl)benzothiazole (5) were synthesized and their photochemical behavior was studied. The (Z) isomers of 4 and 5 form intramolecular hydrogen bonds as revealed by the ¹H NMR studies. The trans isomers are pale yellow in solution, but on photoirradiation, the color changes slightly to a different yellow due to the change of the absorption spectra. The quantum yield of isomerization of 4 and 5 is quite high in the range 0.4–0.6 and is somewhat dependent on the solvent properties. On triplet sensitization 4 and 5 gave T–T absorption spectra with a lifetime of ca. 1 μs in benzene. Since a transient absorption spectrum was not observed on direct irradiation of 4 and 5 with a 308 nm laser pulse, the cis–trans isomerization on direct irradiation should take place in the excited singlet state. On triplet sensitization 4 and 5 underwent cis–trans isomerization. These results indicate that the intramolecular hydrogen bonding is broken in the singlet excited state as well as in the triplet excited state of (Z)-4 and (Z)-5.