HEXANUCLEAR HOMOBRIDGED AND HETEROBRIDGED RUTHENIUM(II) POLYPYRIDINE COMPLEXES - SYNTHESES, ABSORPTION-SPECTRA, LUMINESCENCE PROPERTIES, AND ELECTROCHEMICAL-BEHAVIOR

By use of the "complexes as ligands" strategy, the trinuclear complexes {Ru[(BL(a))Ru(bpy)2]2Cl2}(PF6)4 (5, BL(a) = 2,3-dpp; 6, BL(a) = 2,5-dpp) have been synthesized from the reaction of RuCl3.3H2O with [Ru(bpy)2(2,3-dpp)](PF6)2 and [Ru(bpy)2(2,5-dpp)](PF6)2, respectively (bpy = 2,2'-bipyridine; dpp = bis(2-pyridyl)pyrazine). Reaction of two units of 5 or 6 with the 2,3-dpp or 2,5-dpp bridging ligands has led to the formation of four hexanuclear complexes (1-4) of general formula {[(bpy)2Ru-(BL(a))]2Ru(BL(b))Ru[(BL(a))Ru(bppy)2]2}(PF6)12, where BL(a) = BL(b) = 2,3-dpp (1), BL(a) = BL(b) = 2,5-dpp (2), BL(a) = 2,3-dpp and BL(b) = 2,5-dpp (3), and BL(a) = 2,5-dpp and BL(b) = 2,3-dpp (4). Such hexanuclear compounds exhibit extremely intense absorption bands in the UV region (lambda-max = 283 nm, epsilon-max of the order of 2 x 10(5) M-1 cm-1), which can be assigned to ligand-centered transitions, and intense bands in the visible region (lambda-max between 540 and 582 nm, epsilon-max of the order of 5 x 10(4) M-1 cm-1), which can be assigned to metal-to-ligand charge-transfer transitions. Complexes 1-4 are luminescent both in rigid matrix at 77 K and in fluid solution at 298 K. Luminescence originates from the lowest (formally triplet) metal-to-ligand charge-transfer excited state. The emission spectra and lifetimes are approximately the same for 1 and 3 and for 2 and 4. For 1 and 3 emission occurs at higher energies and exhibits a longer lifetime compared to 2 and 4 (770 vs 810 nm and 54 vs 42 ns, in acetonitrile solution at 298 K). These results, as well as a comparison with the behavior of analogous di- and trinuclear compounds, indicate that in all cases luminescence takes place from the peripheral (bpy)2Ru(BL(a))2+ chromophoric units. Corrected excitation spectra have shown that energy transfer from the central chromophoric units to the peripheral ones takes place with approximately 100% efficiency. In electrochemical experiments, the four peripheral metal ions are oxidized at about the same potential (approximately + 1.4 V, vs SCE), whereas oxidation of the two inner metal ions cannot be observed in the potential window examined (< + 2.0 V). Several reduction overlapping waves are present beginning at about -0.5 V.