DIRECT AND SENSITIZED SYN-ANTI PHOTOISOMERIZATION OF BENZOYLPYRIDINE-4-NITROPHENYLHYDRAZONES

syn-anti Photoisomerization of 2-benzoylpyridine-4-nitrophenylhydrazone(I) and 3-benzoylpyridine-4-nitrophenylhydrazone(II) by direct excitation and by benzophenone sensitization has been investigated. Under direct irradiation the syn form of I undergoes irreversible isomerization to the anti form, whereas compound II attains a photo-equilibrium state of the syn-anti isomers. The different behaviour of the two compounds has been attributed to a hydrogen bond between the NH group and the pyridine nitrogen in the singlet excited state of anti I, this latter state being responsible for the direct photoisomerization. As the sensitization involves the triplet states, which are not affected by the hydrogen bond, it brings about a photo-equilibrium state of syn-anti isomers for both compounds studied. © 1979.