Photochemistry and photophysics of osazones II: direct and sensitized photoisomerization of d-“arabino”-hexulose phenylosazone

Abstract

The reversible direct and sensitized N-chelate → O-chelate ( A → B) isomerization of D-“arabino”-hexulose phenylosazone (PH) was studied. The quantum yield of isomerization φ A → B was slightly infuenced by the solvent properties but was not influenced by the concentration, the intensity of radiation or by external heavy atoms. With decreasing temperature φ A → B decreased significantly whereas the intensity of fluorescence of the A isomer increased. On the basis of the quantum yields of intersystem crossing, of sensitization and quenching of A isomer fluorescence of A → B photoisomerization, the reaction was shown to occur through the excited singlet states 1 A * and 1p *. φ A → B was limited by the efficiency of radiationless transitions to the perpendicular state 1p * in the 1 A * → 1p * process and hence was relatively low. The low value of the rate constant of this process may result from the presence of the intramolecular NH⋯N hydrogen bond and the quasiaromatic ring in the A isomer. Photosensitized A → B isomerization was also found to take place following triplet-triplet energy transfer, with the reaction probably occurring through the 3p * state.