(1 5. I. 96) In the context of our studies on ruthenium(I1) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (k,) of the metal-to-ligand charge-transfer-excited state of a series of homoleptic [RuL,] complexes (where L stands for 2,2'-bipyridine (bpy), I,l0-phenanthroline (phen), 2,2'-bipyrazine (bpz), 4,7-diphenyI-l,lO-phenanthroline (dip), diphenyl-l,IO-phenanthroline-4,7-disulfonate (dpds), and l,l0-phenanthroline-5-octadecanamide (poda)) in H20 and in MeOH. These compounds are singlet-oxygen ( 0 2 ( ' A g ) ) sensitizers, and quantum yields of singlet-oxygen production (Gd) in both solvents are also Ag)), the lowest electronically excited state of the oxygen molecule, is a highly reactive species and a key intermediate in chemical and biological processes, such as photooxidation reactions, DNA damage, and photodynamic therapy of cancer [l-51. Most often, singlet oxygen is produced by photosensitization involving electronic-energy transfer from the triplet excited state of a sensitizer (generally a dye) to ground-state molecular oxygen. Investigations on sensitizers able to produce efficiently singlet oxygen, as well as studies aimed at elucidating the role of the solvent or the surrounding medium, are of great interest because of the importance of singlet oxygen in both photochemical and photobiological applications.Coordination compounds of Ru" with polyazaheteroaromatic chelating ligdnds have attracted considerable attention during the last fifteen years. This is due to the unique combination of their spectroscopic, redox, and photochemical features, together with the possibility of 'fine tuning' their properties by a judicious choice of the type, number, and substituents of the heteroaromatic chelating ligands of the metal coordination sphere [6]. The excitation of Ru" chelate complexes in the 180-550-nm spectral region is followed by a fast intersystem crossing leading to the formation of the metal-to-ligand charge-transfer