Photoinduced Reactions of Pyrene with Carbon Tetrachloride on Porous Silica Gel

Polyaromatic compounds such as pyrene are adsorbed to the porous silica gel surfaces, and the photoinduced reactions of these species on these surfaces are studied. The pyrene singlet excited state and triplet state are quenched on the surface with coadsorbed carbon tetrachloride (CCl4). Temperature studies with evacuated samples indicate that quenching occurs by CCl4 adsorbed on the surface and not by bombardment from the gas phase. The dynamic quenching rate constant of the pyrene singlet excited state increases after high-temperature pretreatment of the silica. It is suggested that this results from an increase in the rate of diffusion of carbon tetrachloride on the surface after the high-temperature removal of surface silanol groups. The dynamic quenching rate constant of the singlet excited state increases with the silica gel pore size, and the highest rate constant is observed on the 150 Å silica surface, which has been pretreated at 600 °C. A large pore size (smaller surface area) and a dehydroxylated surface both contribute to the increased dynamic rate constant, which results from increased movement of carbon tetrachloride over the surface. Carbon tetrachloride does not adsorb strongly on the porous silica gel surface, and evacuation of the CCl4−SiO2 sample removes CCl4 from the surface. Desorption occurs only after several minutes of evacuation and is monitored by the pyrene singlet excited state lifetime. The desorption time depends on the pore size and surface silanol group concentration. Photoinduced products are generated on the silica surface after extended UV irradiation of the sample. Product composition is dependent on the reaction conditions, but typically chloropyrene, dichloropyrene, and polychlorinated species are among the products. The overall picture is that for reactions of polyaromatics such as pyrene on SiO2, coadsorbed CCl4 only quenches the pyrene fluorescence via surface diffusion of CCl4. Surface diffusion can be increased by decreasing the surface silanol group concentration from preheating the silica at high temperatures. The photoinduced reaction is electron transfer to give the pyrene cation and electron attachment to CCl4. Subsequent reaction gives rise to chlorinated products of pyrene.