Photochemistry of pyrene on unactivated and activated silica surfaces

Photolysis of pyrene at the solid/air interface of unactivated and activated silica gel proceeds slowly to give mainly oxidized pyrene products. We have identified 1-hydroxypyrene, 1,6-pyrenedione, and 1,8-pyrenedione among the main reaction products. The remaining minor products show molecular weights and spectral properties consistent with oxygenated pyrenes. Furthermore, small amounts of 1,1'bipyrene dimer are also formed at higher surface coverages (2 x 10^-5 tool/g). When photolysis is carried out at 5 x 10^-5 mol/g pyrene, photodegradation rate drops sharply and pyrene loss becomes insignificant. No significant change in the product distribution is observed when the photolysis is carried out on unactivated or activated silica, Photodegradation rate is slightly faster on activated silica compared to unactivated silica. Mechanistic studies indicate that the precursor to photoproduct formation is pyrene cation radical which is postulated to be formed by electron transfer from pyrene excited state to oxygen (type I) or by photoionization of pyrene. The cation radical reacts with physisorbed water on silica to give the observed oxidation products.