Preparation, photophysical, and electrochemical properties of three trinuclear Ru(II) complexes: Bridging ligands composed of 2,2′-bipyridine and 4,5-diazafluorene fragments

Three bridging ligands 5,5′-bis[(4,5-diazafluoren-9-ylimino)methyl]-2,2′-bipyridine (L1), 5,5′-bis[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-2,2′-bipyridine (L2), 5,5′-bis[4-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-2,2′-bipyridine (L3), and corresponding Ru(II) complexes [(bpy)6Ru3(L1–3)](PF6)6 (bpy = 2,2′-bipyridine) have been synthesized. The three ligands have two kinds of nonequivalent chelating sites: one involving the 2,2′-bipyridine moiety, and the other involving the 4,5-diazafluorene moiety. The emission intensities of complexes [(bpy)6Ru3(L2)]6 + and [(bpy)6Ru3(L3)]6 + are almost equal to that of complex [(bpy)6Ru3(L1)]6 + in CH3CN solution at room temperature, but weaker than that of complex [(bpy)6Ru3(L1)]6 + in EtOH–MeOH (4:1, v/v) glassy matrix at 77 K. Cyclic voltammetry and differential pulse voltammetry studies of the three complexes show one Ru(II)-centered oxidation at around 1.33 V for the RuII/III couple.