Photochemistry of pyrene on unactivated and activated silica surfaces

Photolysis of pyrene at the solid/air interface of unactivated and activate d silica gel proceeds slowly to give mainly oxidized pyrene products. We ha ve identified 1-hydroxypyrene, 1,6-pyrenedione, and 1,8-pyrenedione among t he main reaction products. The remaining minor products show molecular weig hts and spectral properties consistent with oxygenated pyrenes. Furthermore , small amounts of 1,1'-bipyrene dimer are also formed at higher surface co verages (2 x 10(-5) mol/g). When photolysis is carried out at 5 x 10(-5) mo l/g pyrene, photodegradation rate drops sharply and pyrene loss becomes ins ignificant. No significant change in the product distribution is observed w hen the photolysis is carried out on unactivated or activated silica. Photo degradation rate is slightly faster on activated silica comp a red to unact ivated silica. Mechanistic studies indicate that the precursor to photoprod uct formation is pyrene cation radical which is postulated to be formed by electron transfer from pyrene excited state to oxygen (type I) or by photoi onization of pyrene. The cation radical reacts with physisorbed water on si lica to give the observed oxidation products.