QUENCHING OF SINGLET MOLECULAR-OXYGEN (1-DELTA-GO2) IN SILICA-GEL CYCLOHEXANE HETEROGENEOUS SYSTEMS - A DIRECT TIME-RESOLVED STUDY

Direct time-resolved studies of singlet molecular oxygen (1ΔgO2) phosphorescence (3∑g -O2 (v = 0) ← 1ΔgO2(v = 0); 1270 nm) in heterogeneous silica gel/cyclohexane systems are presented. Singlet molecular oxygen (1ΔgO2) is created through a photosensitization process on silica gel surfaces. The experimental results show that the lifetimes of singlet molecular oxygen (1ΔgO2) in both porous and compressed fumed silica/gel cyclohexane systems are significantly less than that in liquid cyclohexane. The shortened singlet molecular oxygen lifetime is due mainly to quenching by adsorbed water and silanol groups on the silica gel surface. In addition, monoamines coadsorbed on the silica gel surface do not quench singlet molecular oxygen (1ΔgO2); however, diamines such as DABCO or piperazine maintain their quenching activity, but the quenching kinetics are not of the Stern-Volmer type. The singlet molecular oxygen lifetime increases on loading the porous silica gel/cyclohexane system with monoamine. Coadsorption of piperazine increases quenching of 1ΔgO2 by DABCO. These data suggest that at least two adsorption sites exist for diamine adsorption on the porous silica gel surface. Low monoamines and diamines adsorption, where all nitrogen lone pairs are bound to silica gel surface (“double” adsorption), leads to a low efficiency for quenching of 1ΔgO2. Increasing the loading of diamine, or coadsorption of other nonreactive diamines, leads to saturation of the double adsorption sites, giving rise to adsorption at sites where only one nitrogen of the diamine is attached to the silica gel surface; increased 1ΔgO2 quenching is observed under these conditions. © 1990, American Chemical Society. All rights reserved.