INFLUENCE OF BRIDGING LIGAND UNSATURATION ON EXCITED-STATE BEHAVIOR IN MONOMETALLIC AND BIMETALLIC RUTHENIUM(II) DIIMINE COMPLEXES

The redox and photophysical properties of a series of bis(bipyridine)-bridged bimetallic complexes of the type [(dmb)(2)Ru(BL)](PF6)(2) and {[(dmb)(2)Ru](2)(BL)}(PF6)(4) (dmb = 4,4'-dimethyl-2,2'-bipyridine; Fl, = 1,4-bis(4'-methyl-2,2'-bipyridin-4-yl)buta-1,3-diene (bbdb), 1,4-bis(4'-methyl-2,2'-bipyridin-4-yl)-2-cyclohexene-5,6-dicarboxylic acid diethyl ester (bchb), and 1,4-bis(4'-methyl-2,2'-bipyridin-4-yl)benzene (bphb)) are reported. Complexes having cyclohexenyl and phenyl bridging ligands have redox and photophysical properties similar to the parent complex of the series, [(dmb)(3)Ru](PF6)(2). The butadienyl complexes exhibit very weak luminescence and transient absorbance spectra which are indicative of the presence of another excited state. For [(dmb)(2)Ru(bbdb)](PF6)(2) the room temperature luminescence lifetime differs from the lifetime obtained by transient absorbance and the luminescence and transient absorbance are quenched with different rate constants by a series of triplet quenchers. The results suggest that the complexes of bbdb have a second excited state which is populated along with the (3)MLCT state: A comparison of the luminescence behavior of several different bridged complexes with the triplet energies Of related aromatic hydrocarbons suggests that the tripler energy of the bridging ligand is relatively unaffected by coordination.