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 tetrame thoxysilane (TMOS)/water/methanol mixtures. N2 and CO2 adsorption isot herms were determined for the xerogels. The pyrene in acid-catalyzed l ow TMOS/water ratio xerogels was found to be nonleachable. Yet the exc ited-state dopant was observed to be completely reactive to O2 quenchi ng in all samples. This finding is of general importance in relation t o applications of sol-gel materials as reactive transparent supports f or organic molecules. Penetration rates of oxygen into samples were de termined by following the drop in pyrene fluorescence over time, after exposure of a precooled sample to the gas. From this data the activat ion energy for the penetration of oxygen into a xerogel was assessed a nd found to be 18 +/- 1 kJ/mol. This long-range diffusion value is sig nificantly higher than the activation energy for short-range diffusion of oxygen found previously on mesoporous silica.