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

<div><p>Three heterotopic ligands L<sup>1</sup>, L<sup>2</sup>, and L<sup>3</sup> 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)<sub>2</sub>}<sub>3</sub>(μ<sub>3</sub>-L<sup>1−3</sup>)](PF<sub>6</sub>)<sub>6</sub> (bpy = 2,2′-bipyridine) were prepared by refluxing Ru(bpy)<sub>2</sub>Cl<sub>2</sub>·2H<sub>2</sub>O 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 CH<sub>3</sub>CN solution at room temperature. Upon excitation into the metal-to-ligand charge-transfer band, the emission intensities of [{Ru(bpy)<sub>2</sub>}<sub>3</sub>(μ<sub>3</sub>-L<sup>2</sup>)]<sup>6+</sup> and [{Ru(bpy)<sub>2</sub>}<sub>3</sub>(μ<sub>3</sub>-L<sup>3</sup>)]<sup>6+</sup> are almost equal to that of [{Ru(bpy)<sub>2</sub>}<sub>3</sub>(μ<sub>3</sub>-L<sup>1</sup>)]<sup>6+</sup> in CH<sub>3</sub>CN solution at room temperature, but weaker than that of [{Ru(bpy)<sub>2</sub>}<sub>3</sub>(μ<sub>3</sub>-L<sup>1</sup>)]<sup>6+</sup> in EtOH–MeOH (4 : 1, v/v) glassy matrix at 77 K.</p></div>