Modification of the photochemical reactivity of α,β-diacetoxystilbene by adsorption onto a Fe3+-doped sepiolite: Comparison with the direct and 2,4,6-triphenylpyrylium-sensitized photolyses

Article abstract of DOI:10.1016/0040-4020(95)00426-9

Photolysis of α,β-diacetoxystilbene (1) adsorbed onto a Fe³⁺-doped sepiolite (Fe-Sp) leads to benzoic acid (3), 9,10-diacetoxyphenanthrene (5), diphenic anhydride (6), and benzil (7). While compounds 3, 5 and 6 are also observed in the direct irradiation of 1 in hexane solution, diketone 7 is formed under electron transfer conditions using 2,4,6-triphenylpyrylium tetrafluoroborate as photosensitizer. These results strongly suggest that formation of benzil (7) in the photolysis of 1 on Fe-Sp involves the intermediacy of 1⁺·, generated by single electron transfer from excited 1 to the iron active sites of the clay. In contrast to the unsubstituted stilbene, reactivity of 1⁺· with ground state molecular oxygen has been established by cyclic voltammetry through the current intensity decrease of the oxidation peak of 1 in the presence of oxygen. Finally, the thermodynamic feasibility of a direct electron transfer pathway between excited 1* and oxygen leading to the formation of a dioxetane intermediate is discussed.