Reactions of pyrene excited states with oxygen on the surface of porous silica gel and similar surfaces

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 SiO2 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 degrees C, and the Langmuir-Hinshelwood mechanism dominates at lower temperatures, T < 10 degrees 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 oxygen-induced intersystem crossing is much smaller on the SiO2 surface compared to that in solution. The pyrene tripler excited state is also quenched by oxygen. Unlike the singlet state, the quenching mechanism is predominantly Eley-Rideal or direct collision of oxygen with die excited triplet slate on the surface. The efficiency of the tripler quenching decreases with increasing oxygen pressure. This is explained by the formation of an intermediate O-2-triplet stale complex.