Phosphorescence quenching by molecular oxygen: zinc tetraphenylporphin on solid supports

The quenching of zinc tetraphenylporphin (ZnTPP) phosphorescence by molecular oxygen is found to be markedly different for dye supported on silica gel and on NaCl crystals. With the latter substrate quenching by oxygen shortens the phosphorescence lifetime and obeys Stern-Volmer kinetics. The quenching rate constant is approximately 1100 Torr/sup -1/ s/sup -1/ at room temperature and shows no dramatic dependence on temperature. On silica gel quenching of ZnTPP phosphorescence takes place at lower oxygen pressure and is not accompanied by a change in the luminescence rate constant. The phosphorescence quenching data agree with a modified Langmuir adsorption model which assumes that only oxygen adsorbed on the silica gel surface next to a ZnTPP molecule will quench luminescence. The temperature dependence of quenching on the silica gel substrate is explained by the adsorption model if it is assumed that oxygen competes with water molecules for adsorption sites on the substrate surface. The mean difference in adsorption energy between water and oxygen, E/sub m/, is a fitting parameter in the luminescence quenching model and is found to be 11 kJ/mol.