SILVER(I) PERTURBATION OF (E)-(Z) PHOTOISOMERIZATION OF STILBENE AND AZOBENZENE

The influence of AgClO4 on the photochemical behaviour of stilbene and azobenzene in acetonitrile and methanol has been compared with that of Nal, a well known perturber of the excited-state properties via the heavy-atom effect. Both the salts quench the fluorescence and the geometrical photoisomerization yields of stilbene, the effect being greater in CH3OH. I- has no effect on the photoisomerization of azobenzene whatever the solvent, while Ag+ increases the isomerization yield in CH3OH but has no effect in CH3CN. I- interacts with the singlet excited state of stilbene and I- via a charge-transfer complex which favours (owing to the heavy-atom effect) ISC to the isomerizable triplet state of the alkene. The lack of effect of I- on the azobenzene photoisomerization probably results from its exceedingly short lifetime. Ag+ forms a ground-state complex with stilbene and azobenzene in methanol solution, as is apparent from the variations of the absorption spectrum of the two aromatics. The light excitation of the complex causes (E) --> (Z) isomerization of the bond chromophore.