Photoinduced Reactions of Pyrene with Carbon Tetrachloride on Porous Silica Gel

Abstract

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.

Physical Aspects of Radiation-Induced Processes on SiO2, γ-Al2O3, Zeolites, and Clays

“… a Data taken from ref . b Calculated rates from back extrapolation using the slope and intercept data from plots of T 1/2 versus temperature.…”

“…CCl 4 provides a quencher that may quench by bombardment from the gas phase or via surface adsorption . However, temperature studies indicate that quenching occurs by CCl 4 adsorbed on the surface and not by bombardment from the gas phase .…”

“…As carbon tetrachloride does not adsorb strongly on the porous silica gel surface, evacuation of the CCl 4 −SiO 2 sample readily removes CCl 4 from the gas phase with surface desorption occurring only after several minutes of evacuation. , Again, the desorption time depends on the pore size and surface silanol group concentration. Similar data have been also obtained for O 2 but only at low temperatures, as the desorption at room temperature 56 is too rapid.…”

“…CCl 4 provides a quencher that may quench by bombardment from the gas phase or via surface adsorption. 78 However, temperature studies indicate that quenching occurs by CCl 4 adsorbed on the surface and not by bombardment from the gas phase. 78 The dynamic quenching rate constant of the pyrene singlet excited state increases after hightemperature pretreatment of the silica and results from an increase in the rate of diffusion of CCl 4 on the surface after the high-temperature removal of surface silanol groups.…”

Behavior of organic compounds confined in monoliths of sol–gel silica glass. Effects of guest–host hydrogen bonding on uptake, release, and isomerization of the guest compounds

“…Interaction of CCl 4 with silica has just been reported. 50 These interactions are to be expected because chlorine atoms in organic molecules are known to form weak hydrogen bonds. 51 Styrene, which lacks heteroatoms and is incapable of hydrogen bonding, lacks special attraction to the silica surface and cannot displace CCl 4 from the silica surface.…”