ELECTRON-TRANSFER, ENERGY-TRANSFER, AND EXCITED-STATE ANNIHILATION IN BINUCLEAR COMPOUNDS OF RUTHENIUM(II)

Electronic excited states of binuclear compounds of ruthenium(II) bridged by bis-2,2'-(2"-pyridyl)bibenzimidazole (bpbimH2) were studied by means of laser photolysis kinetic spectroscopy. Excitation of RuL2(bpbimH2)RuL2(4+) [L is 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (dmbpy), or 1,10-phenanthroline (phen)] into the metal-to-ligand charge-transfer (MLCT) triplet state gives rise to a transient absorption spectrum revealing electron occupation on both L and bpbimH2 in CH3CN. In an asymmetric binuclear compound, excitation-energy transfer takes place from the higher energy site to the other site. Production of the MLCT triplet excited state in a symmetric binuclear compound, Ru(bpy)2(bpbimH2)Ru(bpy)2(4+), is compared with that of the corresponding mononuclear compound, Ru(bpy)2(bpbimH2)2+. Smaller production of the excited triplet state in the binuclear compound is ascribed to a rapid triplet-triplet annihilation process. A decay rate of the excited Ru(dmbpy)2-(bpbimH2)2+ linked to Rh(phen)2(3+) in butyronitrile was obtained by extrapolation of rates measured at lower temperatures. Mechanisms of the intramolecular reaction are discussed.