Synthesis, Characterization, and Photochemical/Photophysical Properties of Ruthenium(II) Complexes with Hexadentate Bipyridine and Phenanthroline Ligands

Three hexadentate, podand-type, polypyridyl ligands, (5-bpy-2C)3Bz, (4-bpy-2C-Ph)3Et, and (4-phen-2C-Ph)3Et, and their Ru(II) and Fe(II) complexes have been prepared. Reaction of these ligands with Fe(II) produces only the monometallic hemicage species, while monometallic, bimetallic, and polymetallic Ru(II) complexes are formed. These species are separable by column chromatography, and NMR and ESI mass spectrometry demonstrate that with each ligand the first band to elute corresponds to the monometallic species, [RuL]2+. The ESI mass spectra show peaks for [RuL]2+ and [RuL](PF6)+ with expected m/z values and isotope peak spacings. 1H NMR spectroscopy shows that [Ru(5-bpy-2C)3Bz]2+ is trigonally symmetric and contains a rigid methylene bridge between the capping group and the bipyridines. The excited-state lifetimes and emission quantum yields for the hemicage complexes, [Ru(5-bpy-2C)3Bz]2+, [Ru(4-bpy-2C-Ph)3Et]2+, and [Ru(4-phen-2C-Ph)3Et]2+, are significantly enhanced (τ = 2800, 1470, and 3860 ns, and φem = 0.271, 0.104, 0.202. respectively) relative to the model compounds and to the polymetallic complexes with the same ligand. An Arrhenius fit of temperature-dependent lifetime data for [Ru(5-bpy-2C)3Bz]2+ indicates a high activation energy for crossover to the dd state (ΔE = 4960 cm-1) as well as the existence of an additional pathway for deactivation via a "4th MLCT" state. Only after extensive photolysis of [Ru(5-bpy-2C)3Bz]2+ is any decrease in emission intensity observed; this is accompanied by the formation of a bimetallic photoproduct, [Ru2L2]4+, with a quantum yield of 7.4 × 10-6. Quenching studies with a variety of quenchers show that the useful excited-state redox and energy-transfer properties characteristic of Ru(II) polypyridyls are retained, but with improved photoinertness and photophysical properties arising from the rigidity of the hemicage complex.

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Author Michael C. Fitzgerald Chemistry