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; F l, = 4-bis(4'-methyl-2,2'-bipyridin-4-yl)buta-1,3-diene (bbdb), ,2'-bi pyridin-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 report ed. Complexes having cyclohexenyl and phenyl bridging ligands have red ox and photophysical properties similar to the parent complex of the s eries, [(dmb)(3)Ru](PF6)(2). The butadienyl complexes exhibit very wea k luminescence and transient absorbance spectra which are indicative o f the presence of another excited state. For [(dmb)(2)Ru(bbdb)](PF6)(2 ) tile room temperature luminescence lifetime differs from the lifetim e obtained by transient absorbance and the luminescence and transient absorbance are quenched with different rate constants by a series of t riplet 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 bridg ed complexes with the triplet energies Of related aromatic hydrocarbon s suggests that the tripler energy of the bridging ligand is relativel y unaffected by coordination.