ELECTRONIC COHERENCE, VIBRATIONAL COHERENCE, AND SOLVENT DEGREES OF FREEDOM IN THE FEMTOSECOND SPECTROSCOPY OF MIXED-VALENCE METAL DIMERS IN H2O AND D2O

We report the first < 20-fs time-resolved pump-probe study on photoinduced intramolecular electron transfer in aqueous solution. The metal-metal, charge-transfer (MMCT) absorption bands of the mixed-valence compounds (NH3)(5)(RuCNRuII)-C-III(CN)(5)(-)(RuRu) and (NH3)(5)(RuCNFeII)-C-III(CN)(5)(-) (RuFe) are studied with sufficient time resolution to measure the back-electron-transfer (b-ET) time. In RuRu, the b-ET occurs in 85 +/- 10 fs in H2O and increases to 122 +/-, 20 fs in D2O. Similar b-ET rates in these solvents are also observed for RuFe. The deuterium isotope effect is shown to originate from the solvent, demonstrating that hydrogenic solvent motions are directly coupled to the electron transfer event. The pump-probe spectroscopy on the MMCT band also provides information on the dynamics of the nuclear degrees of freedom (vibrational and solvent) that are coupled to the MMCT absorption band and the b-ET. An oscillatory vibrational response is observed and assigned to resonance impulsive stimulated Raman scattering. Analysis of these oscillations demonstrates that the average vibrational dephasing time for the observed modes is similar to 300 fs. The early-time behavior of the pump-probe transient absorption indicates that the optical dephasing time for MMCT compounds is extraordinarily short (20 fs) due to strong solvent-solute coupling. Evidence for fast optical dephasing is provided by the instrument-response limited coherence coupling signal and the absence of a pump-probe signal corresponding to transient hole burning in the solvent coordinate. The combined results indicate that this fast b-ET is an electronically incoherent process; however, vibrational coherence is maintained for some of the degrees of freedom during the b-ET.