Quenching of excited polyazaaromatic Ru(II) complexes by oxygen: evidence for an electron transfer process by photoelectrochemical study

Abstract The possible contribution of an electron transfer process to the quenching of excited Ru(II) polypyridyl complexes by oxygen has been the subject of controversy for many years; the existence of this process is re-examined in this work with new polyazaaromatic complexes and by a photoelectrochemical (PEC) method. The luminescence quenching rate constants k Q by oxygen in acetonitrile for a series of polyazaaromatic complexes with 1,4,5,8-tetraazaphenanthrene (tap), 1,4,5,8,9,12-hexaazatriphenylene (hat) and 2,2′-bipyridine (bpy) as ancillary ligand, can clearly be correlated with the oxidation potentials in the excited state. A PEC study of the quenching of Ru(bpy) 3 2+* by oxygen in acidic medium allows determination of the quenching rate constant via the measurements of cathodic photocurrents induced at a transparent SnO 2 electrode. These two investigations show that the electron transfer mechanism is certainly involved in the luminescence quenching of these Ru(II) complexes by oxygen and that the energy transfer contribution becomes rather weak for the more reducing excited complexes.