Intramolecular Exchange Energy Transfer in a Bridged Bimetallic Transition Metal Complex: Calculation of Rate Constants Using Emission Spectral Fitting Parameters

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.