Solvent Dynamical Control of Electron-Transfer Rates in Mixed-Valence Complexes Observed by Infrared Spectral Line Shape Coalescence

Rate constants for intramolecular electron transfer within the intervalence charge transfer (-1) states of the complexes [{Ru3O(OAc)6(L)(CO)}2(mu-pz)] (where L= 4-(dimethylamino)pyridine (1), pyridine (2), 3-cyanopyridine (3), or 4-cyanopyridine (4) and pz = pyrazine) were determined by coalescence of infrared (IR) vibrational spectral line shapes in seven solvents. The electron-transfer times (kET-1) show a strong correlation with solvent relaxation times determined in separate ultrafast time-resolved fluorescence experiments. The best comparison is found with the parameter t1e, which is ascribed to inertial solvent relaxation. The IR spectra of these mixed-valence complexes are thus a steady-state spectral probe of ultrafast, dynamic solvent relaxation processes which are otherwise only accessible using laser-pumped, ultrafast time-resolved measurements.