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

Abstract

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 = 6phenyl-4-Hpip-2-2′-bipyridine, L 2 = 6-Hpip-2-2′-bipyridine, Hpip = 2-phenyl-1Himidazo[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 RuL 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 IIbased 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-lightabsorption antennas.