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The redox and photophysical properties of a series of bis(bipyridine)-bridged bimetallic complexes of the type [(dmb)2Ru(BL)](PF6)2 and {[(dmb)2Ru]2(BL)}(PF6)4 (dmb = 4,4'-dimethyl-2,2'-bipyridme; BL = l,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 l,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, [(dmbLRuKPFéh• The butadienyl complexes exhibit very weak luminescence and transient absorbance spectra which are indicative of the presence of another excited state. For [(dmbLRu-(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 *123MLCT state. A comparison of the luminescence behavior of several different bridged complexes with the triplet energies of related aromatic hydrocarbons suggests that the triplet energy of the bridging ligand is relatively unaffected by coordination.

The photophysical behavior of the transition metal complexes {[(bpy)2Ru]2(bphb)}(PF6)4, {[(tpy)(CN)Ru]2(bphb)}(PF6)2, and [(bpy)2Ru(bphb)Ru(tpy)(CN)](PF6)4 (bpy = 2,2‘-bipyridine, tpy = 2,2‘,6‘,2‘‘-terpyridine, bphb = 1,4-bis(2,2‘-bipyrid-4-yl)benzene) was investigated in acetonitrile solution and low-temperature glasses. Luminescence spectra, excitation spectra, and transient absorption decays of the three complexes serve to show that intramolecular electronic energy transfer from the MLCT excited state of the [(bpy)2Ru(bphb) chromophore to the MLCT state of the tpy-containing chromophore occurs in the unsymmetric bimetallic complex. Nearly complete energy transfer from the [(bpy)2Ru(bphb)] chromophore to the tpy-containing chromophore was observed even in 4:1 ethanol:methanol glasses at 20 K. A semiclassical exchange energy transfer mechanism was used to treat the available data; the Franck−Condon weighted density of states (FCWD) was obtained using parameters determined from fits of luminescence spectra. Given the FCWD at room temperature and the experimental rate constant, an electronic coupling matrix element of approximately 60 cm-1 was determined for this system.

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