SINGLET OXYGEN ((1)DELTA(G)) SENSITIZATION AND QUENCHING BY VITAMIN-B-12 DERIVATIVES

Two Co(III) corrins, cobester and pyrocobester, and a Co(II) corrinate , derived from vitamin B-12, have been investigated for their efficien cies of singlet oxygen (O-2((1) Delta(g))) production and quenching in polar and non-polar media. The quantum yields of singlet oxygen produ ction have been determined by measurements of the singlet oxygen lumin escence in the near-infrared region upon continuous excitation of the sensitizer. The results show that, among the three derivatives investi gated, only pyrocobester produces singlet oxygen in significant amount s, with a quantum yield (Phi(Delta)) of 0.2(+/-0.02) in benzene and 0. 3( +/-0.03) in methanol. Combining luminescence experiments and kineti c measurements of the vitamin B-12 derivatives disappearance, the rate constants of singlet oxygen physical quenching and of the chemical re action of these compounds with singlet oxygen have been determined, in polar and non-polar media. The results show that the three vitamin Bi t derivatives are more efficient physical quenchers than chemical acce pters of singlet oxygen. Among them, the Co(II) corrinate is a particu larly efficient physical quencher with a rate constant greater than 10 (9) l mol(-1) s(-1) in methanol, most probably implying a charge trans fer between the Co(II) metal center and singlet oxygen. The chemical r eactivity, higher for pyrocobester, depends on the structure of the co rrin ligand, as well as on the degree of oxidation of the metal.