Photochemistry in porous materials .4. Oxygen quenching of the fluorescence of fluoranthenes adsorbed on silica gel differing in pore size

Oxygen quenching of fluoranthene and benzo [b] fluoranthene adsorbed on silica gels (pore size between 2.2 and 100 nm) was investigated via time-resolved and steady-state measurements. In contrast with fluorescence spectra and lifetimes, excitation spectra of adsorbed fluoranthenes were strongly altered compared with solutions. Quenching efficiencies and rate constants were found to coincide for the two fluorescers but to depend on pore size, on the surface area covered, on the presence of inert gases and in a time-dependent manner on the series order of gas addition. Since saturation of quenching occurs, the results support a Langmuir-Hinshelwood mechanism of the quenching reaction. Data evaluation yielded heats of adsorption of oxygen between 24 and 31.5 kJ mol(-1), higher values being found in material of small pore size.