Ru(II)-Ru(II) and Ru(II)-Os(II) Homo-/Heterodinuclear Complexes and Ru 3 (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) 2 M 1 L x M 2 (bpy) 2 ] 4+ ( x = 1, 2; M 1 = Ru, M 2 = Ru/Os; M 1 = Os, M 2 = Ru) and {[Ru(bpy) 2 ( L x )] 3 Ru} 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-1 H -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) 2 Ru L 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 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.