Infrared spectroelectrochemical and spectroscopic studies of the picosecond dynamics of intramolecular reactions of ruthenium complexes

Ultrafast intramolecular electron transfers and isomerizations are investigated by variable temperature infrared spectroscopy (IR) and spectroelectrochemistry (IR-SEC). Picosecond rates are estimated for these processes via lineshape analysis of infrared bands. Arrhenius behavior is observed in the isomerization of square pyramidal ruthenium dithiolene complexes. Ruthenium cluster dimers of the type, [Ru3O(muOAc)6](CO)(L)(mu-BL)[Ru 3O(mu-OAc)6](CO)(L), where L = pyridyl ligand; BL = bridging ligand (pyrazine or substituted pyrazine) are synthetically "fine tuned" to vary the electronic coupling (and therefore electron transfer rates) between the cluster moieties in the mixed valent state. Mixed valence isomers are observable in complexes that contain asymmetric bridging ligands and isotopically labeled CO so that the two possible charge distributions in the mixed valence state become energetically nondegenerate and spectroscopically distinguishable by IR. Solvent modes play a critical role in determining electron transfer rates in that the extent of IR line broadening depends directly on dipolar relaxation parameters of the solvents. These electron transfers can be viewed as being nearly barrierless, and hence the "rate limiting step" becomes the pre-exponential term in a normal Arrhenius rate equation, which is dominated by how fast the solvent dipole can reorient after a charge transfer. Cryogenic studies show that near the freezing point of methylene chloride measured rates of intramolecular electron transfer are faster than in fluid solution, suggesting that solvent motions, not internal modes, limit rates of charge transfer. Evidence for the importance of vibronic coupling in the ground state electronic structures of these complexes is discussed. Normally "IR forbidden" totally symmetric vibrational modes that are vibronically coupled to strongly allowed intervalence charge transfer transitions show very substantial intensity enhancements for this reason.