Quenching of singlet molecular oxygen (1ΔgO2) in silica gel-solvent heterogeneous system II. A direct time-resolved study

Direct time-resolved studies of singlet molecular oxygen (1ΔgO2) phosphorescence (3Σg−O2, ν=0←1ΔgO2 1270 nm) in heterogeneous silica gel-solvent systems are presented. The experimental results show that the singlet molecular oxygen (1ΔgO2) lifetime inside the pores of silica gel is significantly less than that in a neat solvent layer. This shortening of the singlet oxygen lifetime is mainly because of quenching by hydrogen-bonded adsorbed water and silanol groups on the silica gel surface. Studies on the sensitizer, 2-acetonaphthone, which is H-bonded on SiO2 surfaces, show a shorter 1ΔgO2 lifetime in silica of smaller pore size. This indicates a higher rebound frequency of 1ΔgO2 between SiO2 surfaces in the smaller pore size material. This effect enhances quenching of 1ΔgO2 by the surface hydroxy groups. The lifetime of 1ΔgO2 in benzene-SiO2 produced by adsorbed 2-acetonaphthone is shorter when pyrene is used as the 1ΔgO2 sensitizer. Co-adsorbed DABCO (1,4 diazabicyclo [2.2.2] octane) quenches singlet molecular oxygen (1ΔgO2) produced by the above sensitizers, but the kinetics are not of the Stern-Volmer type. The data again suggest that at least two adsorption sites exist for diamine-DABCO adsorption on porous silica gel surfaces. At low DABCO loadings, both nitrogen lone pairs of the diamine are bound to the silica gel surface (‘double’ adsorption), and this leads to a low efficiency for quenching of singlet molecular oxygen (aΔgO2). Further loading of DABCO, saturates the ‘double’ adsorption sites, giving rise to adsorption at sites where only one nitrogen is attached to the silica gel surface. Increased 1ΔgO2 quenching is observed under this latter condition. DABCO quenching of 1ΔgO2 in bulk benzene is very efficient with a quenching rate (kq) of (2.93±0.13)x108 M−1 s−1.