PHOTOINDUCED ELECTRON-TRANSFER REACTIONS WITHIN ZEOLITES - DETECTION OF RADICAL CATIONS AND DIMERIZATION OF ARYLALKENES

Photosensitized electron transfer reactions between excited singlet ac cepters and arylalkenes included within NaX zeolites have been studied using a combination of product studies, fluorescence spectroscopy, an d diffuse reflectance laser flash photolysis. Steady-state and time-re solved fluorescence quenching of cyanoaromatic and ionic sensitizers b y arylalkenes demonstrates that singlet quenching occurs predominantly by a static process. Diffuse reflectance flash photolysis studies ind icate that quenching of singlet cyanoaromatic sensitizers by trans-ane thole and 4-vinylanisole occurs via electron transfer and yields relat ively long-lived radical cations. Signals due to trapped electrons (Na -4(3+)) are also observed, suggesting that photoionization of the cyan oaromatic sensitizer occurs in competition with electron transfer quen ching of the excited singlet by the alkene. The long lifetimes of the radical cations indicate the utility of the zeolite environment for co ntrolling the energy-wasting back electron transfer step. Photosensiti zed electron transfer reactions of five alkenes (trans-anethole, 4-vin ylanisole, phenyl vinyl ether, and two indenes), using both ionic and cyanoaromatic sensitizers, lead to predominantly dimeric cyclobutane p roducts as in solution. However, the dimer ratios are substantially di fferent with the cis/syn cyclobutanes formed preferentially in the zeo lite reactions, presumably as a result of constraints imposed by the r estricted space of the zeolite supercage. In fact the zeolite environm ent is more important in determining the geometry of the dimeric produ cts than is the method (direct or sensitized photocycloaddition vs rad ical ion initiation) for their generation.