Di- and tetranuclear ruthenium(II) and/or osmium(II) complexes of polypyridyl ligands bridged by a fully conjugated aromatic spacer: synthesis,characterization, and electrochemical and photophysical studies

A series of mono-, di-, and tetranuclear homo/heterometallic complexes of Ru(II) and Os(II) based on the bridging ligand dppz(11-11')dppz (where dppz = dipyrido[3,2-a:2',3'-c]phenazine) (BL) have been synthesized and characterized. This bridging ligand is a long rigid rod with only one rotational degree of freedom and provides complete conjugation between the chromophores. The complexes synthesized are of general formula [(bpy)(2)Ru-BL](2+), [(phen)(2)/(bpy)(2)M-BL-M(bpy)(2)/(phen)(2)](4+) (M = Ru(II) and Os(II)), [(bpy)(2)Ru-BL-Os(bpy)(2)](4+), and [((bpy)(2)Ru-BL)(3)M](8+). Detailed (1)H NMR studies of these complexes revealed that each chiral center does not influence its neighbor because of the long distance between the metal centers and the superimposed resonances of the diastereoisomers, which allowed the unambiguous assignment of the signals, particularly for homonuclear complexes. Concentration-dependent (1)H NMR studies show molecular aggregation of the mono- and dinuclear complexes in solution by pi-pi stacking. Electrospray mass spectrometry data are consistent with dimerization of mono- and dinuclear complexes in solution. Electrochemical studies show oxidations of Ru(II) and Os(II) in the potential ranges +1.38 to +1.40 and +0.92 to +1.01 V, respectively. The bridging ligand exhibits two one-electron reductions, and it appears that the added electrons are localized on the phenazene moieties of the spacer. All of these complexes show strong metal-to-ligand charge-transfer (MLCT) absorption and (3)MLCT luminescence at room temperature. Quantum yields have been calculated, and the emission lifetimes of all complexes have been measured by laser flash photolysis experiments. The luminescence intensity and lifetime data suggest that the emission due to the Ru center of the heteronuclear complexes is strongly quenched (>90%) compared to that of the corresponding model complexes. This quenching is attributed to intramolecular energy transfer from the Ru(II) center to the Os(II) center (k = (3-5) x 10(7) s(-1)) across the bridging ligand.