Singlet oxygen ( 1Δg) sensitization and quenching by vitamin B 12 derivatives

Abstract

Two Co(III) corrins, cobester and pyrocobester, and a Co(II) corrinate, derived from vitamin B 12, have been investigated for their efficiencies of singlet oxygen ( O 2( 1Δ g ) ) production and quenching in polar and non-polar media. The quantum yields of singlet oxygen production have been determined by measurements of the singlet oxygen luminescence in the near-infrared region upon continuous excitation of the sensitizer. The results show that, among the three derivatives investigated, only pyrocobester produces singlet oxygen in significant amounts, with a quantum yield ( Φ Δ ) of 0.2(±0.02) in benzene and 0.3(±0.03) in methanol. Combining luminescence experiments and kinetic measurements of the vitamin B 12 derivatives disappearance, the rate constants of singlet oxygen physical quenching and of the chemical reaction of these compounds with singlet oxygen have been determined, in polar and non-polar media. The results show that the three vitamin B 12 derivatives are more efficient physical quenchers than chemical acceptors of singlet oxygen. Among them, the Co(II) corrinate is a particularly efficient physical quencher with a rate constant greater than 10 91 mol −1s −1 in methanol, most probably implying a charge transfer between the Co(II) metal center and singlet oxygen. The chemical reactivity, higher for pyrocobester, depends on the structure of the corrin ligand, as well as on the degree of oxidation of the metal.