PHOTOCHEMICAL-REACTIONS OF PYRENE ON SURFACES OF GAMMA-ALUMINA AND SILICA-ALUMINA

Photochemical reactions of pyrene on surfaces of gamma-alumina and silica-alumina were investigated. Pyrene molecules adsorb on these surfaces and form charge-transfer (CT) complexes at activated sites of gamma-alumina and silica-alumina which are generated by preheating the solids to T(a) > 350-degrees-C. The CT complexes exhibit absorption bands near the red edge of the characteristic absorption band of pyrene molecules. Pyrene excimer formation becomes significant on activated gamma-alumina (T(a) < 300-degrees-C). Excited states of pyrene are quenched by surface states, leading to the formation of CT complexes. The radical cations formed on photoexcitation adsorb strongly on the surfaces. These species exhibit a multilined EPR signal, while diffuse reflectance studies show that the radical cations also interact with parent molecules forming monopositive dimeric radical cations. The most interesting reaction of the radical cations on the surfaces is hydrolysis. The photochemical products on both activated and nonactivated surfaces were separated by HPLC, and the collected fractions were studied spectroscopically. The pH dependence of the absorption and fluorescence spectra taken together with results of mass spectrometry indicates that the main products are hydroxypyrenes. Water reacts with the radical cation forming the radical Pyr(OH)., which is a key intermediate in subsequent reactions.