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)₂M₁L^xM₂(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 (L¹ = 6-phenyl-4-Hpip-2-2'-bipyridine, L² = 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^xRu(bpy)₂]²⁺ and homodinuclear [(bpy)₂RuL^xRu(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 Ru^II-based emission and efficient photoinduced energy transfer, resulting in an Os^II-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 Ru^II-centered emissive triplet metal-to-ligand charge transfer states, the tetranuclear complexes exhibit weak Ru^II-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.