Ru(II)–Ru(II) and Ru(II)–Os(II) Homo-/Heterodinuclear Complexes and Ru3(II)–Ru(II) Homotetranuclear Complexes Based on Heteroditopic Bridging Ligands: Synthesis, Photophysics, and Effective Energy Transfer

In this paper, the synthesis, photophysics, electrochemistry, and intramolecular energy transfer of two series of dinuclear and tetranuclear metallic complexes [(bpy)2M1 L x M2(bpy)2]4+ (x = 1, 2; M1 = Ru, M2 = Ru/Os; M1 = Os, M2 = Ru) and {[Ru­(bpy)2(L x )]3Ru}8+ based on new heteroditopic bridging ligands (L 1 = 6-phenyl-4-Hpip-2-2′-bipyridine, L 2 = 6-Hpip-2-2′-bipyridine, Hpip = 2-phenyl-1H-imidazo­[4,5-f]­[1,10]­phenanthroline) are reported. The dimetallic and tetrametallic complexes exhibit rich redox properties with successive reversible metal-centered oxidation and ligand-centered reduction couples. All complexes display intense absorption in the entire ultraviolet–visible spectral regions. The mononuclear [L x Ru­(bpy)2]2+ and homodinuclear [(bpy)2RuL x Ru­(bpy)2]4+ complexes display strong Ru-based characteristic emission at room temperature. Interestingly, the optical studies of heterodinuclear complexes reveal almost complete quenching of the RuII-based emission and efficient photoinduced energy transfer, resulting in an OsII-based emission in the near-infrared region. As a result of the intramolecular energy transfer from the center to the periphery and steric hindrance quenching of the peripheral RuII-centered emissive triplet metal-to-ligand charge transfer states, the tetranuclear complexes exhibit weak RuII-based emission with a short lifetime. Since the light absorbed by several chromophores is efficiently directed to the subunit with the lowest-energy excited state, the present multinuclear complexes can be used as well-visible-light-absorption antennas.