New luminescent and redox-active mono- and polynuclear ruthenium(ii) and osmium(ii) polypyridine complexes

Abstract

A series of mono and polynuclear Ru(II) and Os(II) polypyridine complexes based on the bpy–O–bpy ligand {bpy–O–bpy = bis[4-(2,2′-bipyridinyl)]ether} has been prepared. The redox, absorption and luminescence properties of these species have been measured and compared with those of the [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ parent compounds. Electrochemical oxidation involves the metal centers, and occurs reversibly in acetonitrile at room temperature at about +1.30 and +0.85 V vs. SCE, respectively, for the Ru- and Os-based units. Reduction is ligand-centered and features a first irreversible wave followed by several reversible processes. Absorption spectra are essentially the sum of the spectra of the component monometallic species. Luminescence emission is observed both in acetonitrile solution (298 K) and in frozen matrix (77 K), originating from ³MLCT states. Homometallic complexes display luminescence properties which are close to that featured by the parent [M(bpy)₃]²⁺ species. In heterometallic species luminescence is observed only from the Os-based unit, indicating that efficient energy transfer takes place from the Ru-based to the Os-based moiety. The results indicate that the electronic communication through the bpy–O–bpy bridging ligand is so small that it doesn't substantially modify the properties of the metal units, which are those of the corresponding isolated [M(bpy)₃]²⁺ units, but large enough to allow efficient energy transfer through the bridge. The bpy–O–bpy bridging ligand appears thus a promising component for the synthesis of multimetallic antenna systems.