New ruthenium(II) and osmium(II) trinuclear dendrons. Synthesis, redox behavior, absorption spectra, and luminescence properties

The new trinuclear dendrons [Cl2Os{(mu -2,3-dpp)Ru(bpy)(2)}(2)](4+) 3, Cl2O sRu2 (bpy=2,2'-bipyridine; 2,3-dpp= 2,3-bis(2-pyridyl)pyrazine), [Cl2Os{(mu -2,3-dpp)Os(bpy)(2)}(2)](4+) 4, Cl2OsOs2, and [(bpy)Ru{(mu -2,3-dpp)Os(bpy )(2)}(2)}(6+) 6, bpyRuOs(2) have been synthesized, and their redox properti es, absorption spectra, and luminescence properties studied, together with the same properties of the species [Cl2Ru{(mu -2,3-dpp)Ru(bpy)(2)}(2)](4+) 1, Cl2RuRu2, [Cl2Ru{(mu -2,3-dpp)Os(bpy)(2)}(2)](4+) 2, Cl2RuOs2, and [(bpy )Ru{(mu -2,3-dpp)Ru(bpy)(2)}(2)}(6+) 5, bpyRuRu(2). All the compounds under go several reversible metal-centered oxidation and ligand-centered reductio n processes which are assigned to specific subunits of the trinuclear struc ture. Analysis of the reduction patterns of the various compounds allows on e to obtain information on the extent of ligand-ligand interactions mediate d by the central metals. The compounds show very intense ligand-centered ab sorption bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the visible region, which can be assigned to transitions i nvolving specific subunits of the dendron structure. All except 3 and 4 exh ibit MLCT luminescence both at room temperature in acetonitrile fluid solut ion and in a MeOH-EtOH 4:1 (v/v) rigid matrix at 77 K. The luminescence pro perties are dominated by the photophysical properties of the subunit(s) in which the lowest-lying excited state of any structure is localized, suggest ing that fast intercomponent energy transfer takes place in all the compoun ds.