PHOTOCHEMISTRY OF STILBENE ADSORBED ON SILICA-GEL AND NAX ZEOLITE - A DIFFUSE-REFLECTANCE LASER FLASH-PHOTOLYSIS STUDY

Diffuse reflectance laser flash photolysis (266, 308, or 355 nm) of either cis- or trans-stilbene (St) adsorbed on silica gel or included in NaX zeolite leads to the formation of the trans-St radical cation with lambda(max) at 475 nm at high laser powers. At low laser intensities trans-St also yields radical cation while cis-St photocyclizes to give dihydrophenanthrene with lambda(max) at 450 nm. In contrast to the results for irradiation of stilbene alone on solid supports, irradiation of the cis-St/TNM charge transfer complex on silica or zeolite leads to a mixture of both trans- and cis-St(.+) (lambda(max) at 510 nm), demonstrating that the cis radical cation is stable with respect to isomerization on these two solids. This result, in combination with product studies which demonstrate that there is substantial cis-trans isomerization within a single laser pulse, leads to the conclusion that the formation of trans-St(.+) following laser irradiation of cis-St occurs via cis-trans isomerization followed by photoionization of trans-St. Laser irradiation of St or pyrene on NaX zeolite results in strong transient signals in the 500-600 nm region due to trapped electrons, in addition to the signals due to radical cations. The effects of both water and oxygen on the trapped electron and radical cation have been examined. The trapped electron can be photobleached with a second 532 nm laser pulse. The bleaching does not lead either to trapping of the electron by ground state aromatic to give its radical anion or to recombination with the radical cation to regenerate the starting material. This suggests that irradiation leads to a redistribution of the electron to other zeolite sites.