Synthesis, photophysical, and electrochemical properties of six Ru(II) complexes: asymmetric ligands composed of 6-phenyl substituted 2,2′-bipyridine and 4,5-diazafluorene derivatives

Six asymmetric ligands, 6-[(4,5-diazafluoren-9-ylimino)methyoxyphenyl]-2,2′-bipyridine (L¹), 4-[(4,5-diazafluoren-9-ylimino)methyoxyphenyl]-6-phenyl-2,2′-bipyridine (L²), 6-[4-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-2,2′-bipyridine (L³), 4-[4-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-6-phenyl-2,2′-bipyridine (L⁴), 6-[2-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-2,2′-bipyridine (L⁵) and 4-[2-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-6-phenyl-2,2′-bipyridine (L⁶), and corresponding Ru(II) complexes [(bpy)₂Ru(L^1–6)](PF₆)₂ (bpy = 2,2′-bipyridine) were synthesized. All six ligands have two kinds of nonequivalent chelating sites: one involving the 6-phenyl substituted 2,2′-bipyridine moiety, and the other involving the 4,5-diazafluorene moiety. All six Ru(II) complexes have metal-to-ligand charge transfer absorptions around 447 nm and one Ru(II)-centered oxidation around 1.35 V versus SCE in CH₃CN solution at room temperature. These complexes are non-emissive in CH₃CN solution at room temperature. The emission intensities of [(bpy)₂Ru(L¹)]²⁺ and [(bpy)₂Ru(L²)]²⁺ are stronger than that of [(bpy)₂Ru(L^3–6)]²⁺ in EtOH-MeOH (4 : 1, v/v) glassy matrix at 77 K.