Photooxygenation of aromatic alkenes in zeolite nanocavities

Abstract

Aromatic alkenes such as styrene (1), 1,1-diphenylethene (2), and cis- and trans-stilbenes (3), but not triphenylethene (4), formed their contact charge-transfer (CCT) complexes with O₂ in solutions. In zeolite NaY, the CCT absorption band was observed only for 1 and 3. Irradiation of alkenes 1–4 included in the zeolite nanocavities under O₂ produced benzaldehyde and benzophenone as the major oxygenation products. In particular, for 3 and 4, the photooxygenation competed with a photoelectrocyclic reaction, which subsequently yielded phenanthrenes as the exclusive photoproducts under O₂ in solution. It is likely that the oxygenation products were produced through the alkene cation radicals and superoxide anion radical generated by excitation of the CCT complexes and/or photoinduced electron transfer from the excited alkenes to O₂. YAG laser (266 nm) excitation of 1 included in the zeolite cavities under vacuum produced its alkene cation radical and the trapped electron, Na₄³⁺, both of which were quenched by O₂. On the basis of the optimum structure for the guest molecules obtained by semi-empirical molecular orbital calculations (AM1), it is suggested that the photooxygenation reaction was regulated by the electrostatic interaction between the guest molecules and alkali-metal cations in the nanocavities as well as by the strong electrostatic field which stabilized the ion radical pairs generated.