HEXANUCLEAR POLYPYRIDINE COMPLEXES CONTAINING DIFFERENT METALS, BRIDGING LIGANDS AND OR TERMINAL LIGANDS - ABSORPTION-SPECTRA, ELECTROCHEMICAL OXIDATION, LUMINESCENCE PROPERTIES AND INTERCOMPONENT ENERGY-TRANSFER

Four novel hexanuclear complexes of general formula [(L)2M(mu-BL)]2M(mu-BL)M[(mu-BL)M(L)2]212+, where the metal ions M are Ru2+ and/or Os2+, the bridging ligands BL are 2,3-dpp and/or 2,5-dpp, and the terminal ligands L are bpy and/or biq, have been investigated (dpp = bis(2-pyridyl)pyrazine; bpy = 2,2'-bipyridine; biq = 2,2'-biquinoline). These polymetallic complexes can be considered as supramolecular species made of six distinct metal-containing units. They display very intense ligand centered absorption bands in the UV region (epsilon(max) in the order of 2.5 X 10(5) M-1 cm-1) and broad and intense bands in the visible region (epsilon(max) in the order of 5 X 10(4) M-1 cm-1). On electrochemical oxidation, the metal centers are oxidized at the same or different potentials depending on the nature of the metal ions (Ru2+ or Os2+) and on their positions (inner or outer) in the supramolecular structure. For all the novel compounds, luminescence can be observed in the red or near-IR spectral region. The luminescence properties, which are characteristic of specific metal-containing units, show that exoergonic electronic energy transfer between adjacent units is 100% efficient, whereas it is much lower when higher energy units are interposed. Various energy migration patterns can be obtained by placing different units in suitable sites of the supramolecular array.