Photoinduced electron transfer reactions within zeolites: detection of radical cations and dimerization of arylalkenes

Abstract

Photosensitized electron transfer reactions between excited singlet acceptors and arylalkenes included within NaX zeolites have been studied using a combination of product studies, fluorescence spectroscopy, and diffuse reflectance laser flash photolysis. Steady-state and time-resolved fluorescence quenching of cyanoaromatic and ionic sensitizers by arylalkenes demonstrates that singlet quenching occurs predominantly by a static process. Diffuse reflectance flash photolysis studies indicate that quenching of singlet cyanoaromatic sensitizers by trans-anethole and 4-vinylanisole occurs via electron transfer and yields relatively long-lived radical cations. Signals due to trapped electrons (Na₄³⁺) are also observed, suggesting that photoionization of the cyanoaromatic sensitizer occurs in competition with electron transfer quenching of the excited singlet by the alkene. The long lifetimes of the radical cations indicate the utility of the zeolite environment for controlling the energy-wasting back electron transfer step. Photosensitized electron transfer reactions of five alkenes (trans-anethole, 4-vinylanisole, phenyl vinyl ether, and two indenes), using both ionic and cyanoaromatic sensitizers, lead to predominantly dimeric cyclobutane products as in solution. However, the dimer ratios are substantially different with the cis/syn cyclobutanes formed preferentially in the zeolite reactions, presumably as a result of constraints imposed by the restricted space of the zeolite supercage. In fact the zeolite environment is more important in determining the geometry of the dimeric products than is the method (direct or sensitized photocycloaddition vs radical ion initiation) for their generation.