Dinuclear ruthenium(II) and/or osmium(II) complexes of bipyridyl ligands bridged by rigid spacers: synthesis, characterization, electrochemical behavior and luminescence properties

Abstract

New bridging ligands, 1,4-bis[4-(4′-methyl)-2,2′-bipyridyl)imine]benzene (BL 1) and 4,4′-bis[4-(4′-methyl)-2,2′-bipyridyl)imine]benzene (BL 2), and their dinuclear complexes of the type [(bpy) 2Ru-BL-Ru(bpy) 2] 4+, [(bpy) 2Os-BL-Os(bpy) 2] 4+ and [(bpy) 2Ru-BL-Os(bpy) 2] 4+ (where BL=BL 1 and BL 2) have been synthesized and characterized. Mononuclear model complexes [(bpy) 2RuL 1] 2+, [(bpy) 2OsL 1] 2+ and [(bpy) 2RuL 2] 2+, where L 1=[4-(4′-methyl)-2,2′-bipyridyl)imine]-4-aminobenzene and L 2=[4-(4′-methyl)-2,2′-bipyridyl)imine]-4-aminodiphenylene, have also been synthesized. Ligands and metal complexes were characterized on the basis of elemental analysis, FAB mass, IR and 1H NMR data. All metal complexes exhibit characteristic MLCT absorption and luminescence bands in the visible region. Cyclic and square wave voltammograms of all complexes exhibit metal-based oxidations (M(II)→M(III)) in the potential ranges +1.20 to +1.23 V for Ru(II) and +0.75 to +0.82 V for Os(II). Ligand-based reductions occurred in the potential range −1.28 to−1.90 V. Mixed-valence species of the dinuclear complexes were generated by the addition of a standard solution of ammonium cerium(IV) nitrate. The excited state life-time of all complexes were measured in acetonitrile and also in a methanol–ethanol (4:1) mixture at room temperature. The homodinuclear complexes exhibit significantly long life-times, which are attributed to delocalization of excited electron on the conjugated bridging unit. In heterodinuclear complexes the Ru-based emission life-time decreased significantly; this quenching process takes place via energy transfer from Ru(II) to Os(II) center through spacers. The rate constants for energy transfer are calculated.