Sensitization and quenching processes of alkylcobalt(III) compounds

Abstract

Ethyl(aquo)cobaloxime is found to be a potent quencher of a wide variety of excited states. Laser flash photolysis studies show that triplet states of organic molecules with E_T > 170 kJ mol^–1 are quenched at rates exceeding 10⁹ dm³ mol^–1 s^–1, while those of rubrene and β-carotene, with E_T < 110 kJ mol^–1, are not effectively deactivated. A variety of excited states of inorganic molecules are also quenched at rates exceeding 10⁹ dm³ mol^–1 s^–1 by ethyl(aquo)cobaloxime: the insensitivity of the quenching rate to the reduction potentials of the excited states concerned (differing by 1.8 V) indicates a mechanism of energy (rather than electron) transfer. Analogous kinetic studies have been carried out, by way of comparison, on tris(acetylacetonato)cobalt(III) and a few studies on methylcobalamin. A number of the quenchers examined have been shown to sensitize the photodecomposition of ethyl(aquo)cobaloxime, indicating the mechanism of energy transfer to be one of transfer to the photodissociative state of the alkylcobalt(III) compound: this is dissociated very efficiently by donors with energies as low as 170 kJ mol^–1(14 210 cm^–1 or 704 nm) compared with the energy of its longest wavelength transition of 262 kJ mol^–1.