Excited State Reactivity of Aza Aromatics. 9. Fluorescence and Photoisomerization of Planar and Hindered Styrylpyridines

Article abstract of DOI:10.1021/j100445a010

The effect of methyl substitution at the ethylenic bridge on the fluorescence and trans -> cis photoisomerization of styrylpyridines (StP's) has been investigated.As compared with the unsubstituted planar olefins, steric hindrance modifies the shape of the potential energy barrier for photoisomerization.The hindered compounds show a marked decrease in the fluorescence quantum yield.A parallel decrease in the photoreaction quantum yield has been observed for the α-methyl derivatives of stilbene and 3-StP.Temperature and solvent effects on planar and hindered StP's give interesting information on the intramolecular rotation in excited azastilbenes and its competition with the other deactivation pathways.While intersystem crossing is negligible in fluid solvent, it becomes competitive with fluorescence in high viscosity media.The photoisomerization mechanism is discussed on the basis of the model available for stilbene.