The photophysics of pyrene, perylene, coronene, and several other polyaromatic adsorbates on silica gel and related oxide surfaces have been examined using their excited singlet and triplet states. It is found that the fluorescence and triplet quantum yields for the arenes are lower on silica gel compared to simple solution. This is related to the adsorption of the molecules on SiO2 sites where charge-transfer (CT) complexes are formed. The CT complex is identified by its spectral absorption and emission, which are red-shifted with respect to pyrene, and by a much shorter lifetime of 20 ns compared to greater than 100 ns for pyrene. The extent of the CT state can be reduced by addition of polar coadsorbates such as nitromethane. The latter adsorb strongly at the active surface sites and replace pyrene, which then adsorbs at nonactive sites.

Oxygen quenching of the singlet state of pyrene adsorbed on amorphous silica gel is studied by pulsed laser and rapid spectrophotometric methods. The quenching rate constant depends on the pore size of the silica and the method and temperature used to remove adsorbed water and surface silanol groups. The two possible mechanisms for O 2 quenching of excited pyrene on an SiO 2 surface are quenching by direct collision encounter on the excited state (Eley-Rideal) and quenching by surface adsorbed oxygen (Langmuir-Hinshelwood). In the present system, these two mechanisms are distinguished using temperature studies. Both mechanisms are operative in the current system: the Eley-Rideal mechanism dominates at temperatures greater than 30°C, and the Langmuir-Hinshelwood mechanism dominates at lower temperatures, T < 10°C. Several other surfaces are also briefly studied in order to add insight into the surface processes which are involved. One product of the oxygen quenching of the singlet excited state is the triplet state. Quenching of the singlet state leads to an increase in the triplet yield for pyrene and coronene. However, the efficiency of the oxygeninduced intersystem crossing is much smaller on the SiO 2 surface compared to that in solution. The pyrene triplet excited state is also quenched by oxygen. Unlike the singlet state, the quenching mechanism is predominantly Eley-Rideal or direct collision of oxygen with the excited triplet state on the surface. The efficiency of the triplet quenching decreases with increasing oxygen pressure. This is explained by the formation of an intermediate O 2 -triplet state complex.

Small particles of TiO2 have been synthesized on porous silica. XRD and spectral measurements place the sizes of the particles at about 50 A. With these small particles the band gap of the semiconductor material is markedly blue shifted with respect to bulk TiO2. This enables the direct excitation of co-adsorbed arenes on the TiO2-SiO2 material. Direct excitation of TiO2 leads to Ti3+, by photoinduced extraction from the SiO2. Direct excitation of pyrene on the material leads to a shortened fluorescence lifetime and lower quantum yield. Meanwhile, the yield of pyrene cation radical increases compared to that on pure SiO2. The production of Ti3+ on the pyrene samples, by direct excitation of the TiO2, leads to a shortened pyrene fluorescence lifetime and decreased quantum yield, the quantum yield decreasing to zero at high enough Ti3+. Removal of Ti3+ by O2 causes the original properties of the pyrene fluorescence to return.

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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.