SUPRAMOLECULAR RU AND OR OS COMPLEXES OF TRIS(BIPYRIDINE) BRIDGING LIGANDS - SYNTHESES, ABSORPTION-SPECTRA, LUMINESCENCE PROPERTIES, ELECTROCHEMICAL-BEHAVIOR, INTERCOMPONENT ENERGY, AND ELECTRON-TRANSFER

The tris(bipyridine) tripod ligands 1,3,5-tris[4-(((2,2'-bipyridyl-5-yl)carbonyl)benzylamino)methyl]benzene (1), 1,3,5-tris[4-(((2,2'-bipyridyl-5-ylcarbonyl)benzylamino)methyl)phenyl]benzene (2), and 1,3,5-tris[4-((((2,2'-bipyridyl-5-ylcarbonyl)benzylamino)methyl)phenylyl)phenyl]benzene (3) have been synthesized and their complexes 1[Ru(bpy)2]2+, 1[Ru(bpy)2]2(4+), 1[Ru(bpy)2]3(6+), 1[Os(bpy)2]3(6+), 1[Ru(bpy)2]2[Os(bpy)2]6+, 2[Ru(bpy)2]3(6+), 2[Os-bpy)2]3(6+), 2[Ru(bpy)2]2[Os(bpy)2]6+, and 3[Ru(bpy)2]3(6+) have been prepared. All the complexes display very intense, ligand centered absorption bands in the UV region and moderately intense metal-to-ligand charge-transfer bands in the visible. Electrochemical oxidation of each Ru(II) or Os(II) metal center occurs always at the same potential (+1.30 V for Ru(II), +0.87 V for Os(II)), regardless of tripod ligand and number and type of metal-based units that are present in the supramolecular structure. The five homometallic Ru(II) species exhibit the same luminescence properties, and this is also the case for the two homometallic Os(II) species. The luminescence data obtained for the two mixed-metal species show that electronic energy transfer takes place from the Ru-based to the Os-based components. The efficiency of energy transfer decreases in going from 1[Ru(bpy)2]2[Os(bpy)2]6+ to 2[Ru(bpy)2]2[Os(bpy)2]6+, i.e., as the size of the spacer which links the three arms of the bridging ligand increases. Oxidation of 1[Ru(bpy)2]3(6+), 2[Ru(bpy)2]3(6+), 1[Os(bpy)2]3(6+), and 2[Os(bpy)2]3(6+) by Ce(IV) leads to mixed-valence species where the oxidized metal-based units quench the luminescent excited state of the units that are not oxidized. The quenching efficiency decreases as the size of the spacer increases. The mechanisms of the quenching processes are discussed. The results obtained indicate that at least in the case of the trinuclear species of 2, there are conformers in which the quenching can take place very rapidly and conformers where it does not take place at all.