Singlet‐Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water

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Abstract
In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (kq) of the metal‐to‐ligand charge‐transfer‐excited state of a series of homoleptic [RuL3] complexes (where L stands for 2,2′‐bipyridine (bpy), 1,10‐phenanthroline (phen), 2,2′‐bipyrazine (bpz), 4,7‐diphenyl‐1,10‐phenanthroline (dip), diphenyl‐1,10‐phenanthroline‐4,7‐disulfonate (dpds), and 1, 10‐phenanthroline‐5‐octadecanamide (poda)) in H2O and in MeOH. These compounds are singlet‐oxygen (O2(1Δg)) sensitizers, and quantum yields of singlet‐oxygen production (ΨΔ) in both solvents are also reported. Values of kq and ΨΔ depend on the nature of the ligand L and on the solvent, ΨΔ values showing a large range of variation (0.2 to 1.0). In MeOH, the only pathway for quenching of the excited [RuL3] complexes by molecular oxygen is energy transfer: the fraction of quenched excited states yielding singlet oxygen (ƒ) is unity for all compounds in the series investigated. Changing from MeOH to H2O has several remarkable effects: higher kq and lower ΨΔ values are observed; ƒ drops to ca. 0.5 except for [Ru(bpz)3]2+. In fact, [Ru(bpz)3]2+ is by far the weakest reductant in the series and behaves differently from the other complexes, with lowest kq and ΨΔ values and a ƒ equal to 1 in both solvents. Results are interpreted on the basis of the role played by charge‐transfer interactions between the sensitizer excited state and molecular oxygen in the quenching mechanism. RuII Complexes based on the 4,7‐diphenyl‐1, 10‐phenanthroline (dip) ligand are very efficient and stable singlet‐oxygen sensitizers with ΨΔ values close to unity in air‐saturated MeOH.

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