Photochemical cis ⇔trans isomerization reaction of 1,2-bispyrazylethylene

Abstract

A quantitative and comparative study was carried out on the direct and the sensitized $\underline{\mbox{cis} \rightleftarrows \mbox{trans}}$ photoisomerization reaction of 1,2-bispyrazylethylene (BPE). Various conditions (solvent effect, salt effect, pH effect, etc.) were applied on the photoisomerization reaction in order to investigate the character of the reactive state, HPLC techniques were used for all quantitative analyses. Photoisomerization in direct isomerization or sensitized reaction was little affected by changing solvents. The salt effect and pH effect indicated the intersystem crossing very important. By azulene quenching studies, the intersystem crossing is proved to occur very efficiently on the direct isomerization of BPE in contrast to bispyridylethylene. These results are very different from those of the stilbenes which have only ($\pi,\;\pi^\star$) excited states. In benzophenone triplet sensitized reactions, triplet-triplet annihilation of benzophenone is observed. In direct excitation of BPE, a triplet mechanism is proposed for the $\underline{\mbox{cis} \rightleftarrows \mbox{trans}}$ photoisomerization from the experimental results and comparison of these results of diazastilbenes.