Quenching of singlet molecular oxygen ( 1Δ gO 2) in silica gel-solvent heterogeneous system II. A direct time-resolved study

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Chemistry., 1993-03, Vol.71 (1), p.55-60
• Main Authors: Iu, Kai-Kong, Kerry Thomas, J.
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Exact sciences and technology; Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion); Molecular properties and interactions with photons; Physics
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/1010-6030(93)87009-C

Direct time-resolved studies of singlet molecular oxygen ( 1Δ gO 2) phosphorescence ( 3Σ g −O 2, ν=0← 1Δ gO 2 1270 nm) in heterogeneous silica gel-solvent systems are presented. The experimental results show that the singlet molecular oxygen ( 1Δ gO 2) 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 SiO 2 surfaces, show a shorter 1Δ gO 2 lifetime in silica of smaller pore size. This indicates a higher rebound frequency of 1Δ gO 2 between SiO 2 surfaces in the smaller pore size material. This effect enhances quenching of 1Δ gO 2 by the surface hydroxy groups. The lifetime of 1Δ gO 2 in benzene-SiO 2 produced by adsorbed 2-acetonaphthone is shorter when pyrene is used as the 1Δ gO 2 sensitizer. Co-adsorbed DABCO (1,4 diazabicyclo [2.2.2] octane) quenches singlet molecular oxygen ( 1Δ gO 2) 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 ( aΔ gO 2). 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 1Δ gO 2 quenching is observed under this latter condition. DABCO quenching of 1Δ gO 2 in bulk benzene is very efficient with a quenching rate ( k q) of (2.93±0.13)x10 8 M −1 s −1.