Synthesis and structural, electrochemical, and optical properties of Ru(II) complexes with azobis(2,2′-bipyridine)s

Two new ditopic ligands, 5,5″-azobis(2,2′-bipyridine) (5,5″-azo) and 5,5″-azoxybis(2,2′-bipyridine) (5,5″-azoxy), were prepared by the reduction of nitro precursors. Mononuclear and dinuclear RU(II) complexes having one of these bridging ligands and 2,2′-bipyridine terminal ligands were also prepared, and their properties were compared with previously reported RU(II) complexes having 4,4″-azobis(2,2′-bipyridine) (4,4″-azo). The X-ray crystal structure showed that 5,5″-azo adopts the trans conformation and a planar rodlike shape. The X-ray crystal structure of [(bpy)₂Ru(5,5″-azo)Ru(bpy)₂] (PF₆)₄ (Ru(5,5″-azo)Ru) showed that the bridging ligand is in the trans conformation and nearly planar also in the complex and the metal-to-metal distance is 10.0 Å. The azo or azoxy ligand in these complexes exhibits reduction processes at less negative potentials than the terminal bpy's due to the low-lying π* level. The electronic absorption spectra for the complexes having 5,5″-azo or 5,5″-azoxy exhibit an extended low-energy metal-to-ligand charge-transfer absorption. The ligands, 5,5″-azo and 5,5″-azoxy, and the mononuclear complex, [(bpy)₂Ru(5,5″-azo)]²⁺, isomerize reversibly upon light irradiation. The low-energy MLCT state sensitizes the isomerization of the azo moiety in this complex. While [(bpy)₂Ru(4,4″-azo)Ru(bpy)₂] (PF₆)₄ exhibits light switch properties, namely, significant electrochromism and a large luminescence enhancement, upon reduction, Ru(5,5″-azo)Ru does not show these properties. The radical anion formation upon reduction of these complexes has been confirmed by ESR spectroscopy.