PHOTOCHEMICAL trans →cis ISOMERIZATION OF 6‐STYRYLQUINOXALINE*

Abstract The trans → cis photoisomerization of fran.s‐6‐styrylquinoxaline (6‐StQx) has been investigated. Fluorescence quantum yields increase with increasing solvent polarity, whereas internal conversion and intersystem crossing decrease because of the weakening of vibronic coupling of the lowest 1 (π,π*) state and 1 (n,π*) state in polar solvents. Laser flash photolysis studies show an efficient singlet‐triplet intersystem crossing on direct irradiation at room temperature. Azulene quenching of fluorescence, and direct and sensitized photoisomerization indicate that the direct photoisomerization proceeds through both the singlet and triplet manifolds, mainly via the triplet state (75‐80%) at room temperature, in contrast to corresponding hydrocarbons.