Three trinuclear Ru(II) complexes containing 4,5-diazafluorene and 2,2′-bipyridine: synthesis, absorption spectrum, luminescence, and redox behavior

Three heterotopic ligands L1, L2, and L3 have been prepared by the reaction of 4,4′-bis(bromomethyl)-2,2′-bipyridine with 4,5-diazafluoren-9-oxime, 9-(2-hydroxy)phenylimino-4,5-diazafluorene, and 9-(4-hydroxy)phenylimino-4,5-diazafluorene, respectively, in DMF. The three ligands consist of two 4,5-diazafluorene units and one 2,2′-bipyridine unit. Ru(II) complexes [{Ru(bpy)2}33-L1−3)](PF6)6 (bpy = 2,2′-bipyridine) were prepared by refluxing Ru(bpy)2Cl2·2H2O and the ligands in 2-methoxyethanol. The three Ru(II) complexes display metal-to-ligand charge-transfer absorption at 445–450 nm and one Ru(II)-centered oxidation at 1.32 V in CH3CN solution at room temperature. Upon excitation into the metal-to-ligand charge-transfer band, the emission intensities of [{Ru(bpy)2}33-L2)]6+ and [{Ru(bpy)2}33-L3)]6+ are almost equal to that of [{Ru(bpy)2}33-L1)]6+ in CH3CN solution at room temperature, but weaker than that of [{Ru(bpy)2}33-L1)]6+ in EtOH–MeOH (4 : 1, v/v) glassy matrix at 77 K.