Determination of Activation Energy of Entrance into Micropores: Quenching of the Fluorescence of Pyrene-Doped SiO₂ Sol-Gel Matrices by Oxygen

Pyrene-doped xerogels were prepared by the sol-gel method from tetramethoxysilane (TMOS)/water/methanol mixtures. N₂ and CO₂ adsorption isotherms were determined for the xerogels. The pyrene in acid-catalyzed low TMOS/water ratio xerogels was found to be nonleachable. Yet the excited-state dopant was observed to be completely reactive to O₂ quenching in all samples. This finding is of general importance in relation to applications of sol-gel materials as reactive transparent supports for organic molecules. Penetration rates of oxygen into samples were determined by following the drop in pyrene fluorescence over time, after exposure of a precooled sample to the gas. From this data the activation energy for the penetration of oxygen into a xerogel was assessed and found to be 18 ± 1 kJ/mol. This long-range diffusion value is significantly higher than the activation energy for short-range diffusion of oxygen found previously on mesoporous silica.