MODIFICATION OF PHOTOCHEMICAL REACTIVITY BY ZEOLITES - GENERATION, ENTRAPMENT, AND SPECTROSCOPIC CHARACTERIZATION OF RADICAL CATIONS OF ALPHA-OMEGA-DIPHENYL POLYENES WITHIN THE CHANNELS OF PENTASIL ZEOLITES

Inclusion of all-trans-alpha,omega-diphenyl polyenes (trans-stilbene, 1,4-diphenyl-1,3-butadiene, 1,6-diphenyl-1,3,5-hexatriene, 1,8-diphenyl-1,3,5,7-octatetraene) in thermally activated Na-ZSM-5 zeolite leads to the rapid formation of cation radicals of the polyenes. These cation radicals are stabilized within the zeolite host and are conveniently studied by conventional spectroscopic techniques (ESR, UV-vis-NIR diffuse reflectance, emission) enabling us to determine for the first time the steady state fluorescence spectrum of the 1,6-diphenyl-1,3,5-hexatriene and 1,8-diphenyl-1,3,5,7-octatetraene cation radicals. Our method of generation and stabilization of organic cation radicals appears to be both general in scope and simple in practice. While the mechanism of oxidation remains unclear, it appears that the presence of aluminum in the zeolite is a requirement; as the Si/Al ratio decreases the extent of oxidation increases. Using a series of substituted stillbenes of known redox potential leads to an estimated redox potential of 1.65 +/- 0.1 eV vs SCE for the zeolite-based oxidant.