MODIFICATION OF THE PHOTOCHEMICAL REACTIVITY OF ALPHA,BETA-DIACETOXYSTILBENE BY ADSORPTION ONTO A FE3-DOPED SEPIOLITE - COMPARISON WITH THEDIRECT AND 2,4,6-TRIPHENYLPYRYLIUM-SENSITIZED PHOTOLYSES()

Photolysis of alpha,beta-diacetoxystilbene (1)adsorbed onto a Fe3+-dop ed sepiolite (Fe-Sp) leads to benzoic acid (3), 9,10-diacetoxyphenanth rene (5), diphenic anhydride (6), and benzil (7). While compounds 3, 5 and 6 are also observed in the direct irradiation of 1 in hexane solu tion, diketone 7 is formed under electron transfer conditions using 2, 4,6-triphenylpyrylium tetrafluoroborate as photosensitizer. These resu lts strongly suggest that formation of benzil (7) in the photolysis of 1 on Fe-Sp involves the intermediacy of 1(+.), generated by single el ectron transfer from excited 1 to the iron active sites of the day. In contrast to the unsubstituted stilbene, reactivity of 1(+.) with grou nd sate molecular oxygen has been established by cyclic voltammetry th rough the current intensity decrease of the oxidation peak of 1 in the presence of oxygen. Finally, the thermodynamic feasibility of a direc t electron transfer pathway between excited 1 and oxygen leading to t he formation of a dioxetane intermediate is discussed.