Time-Dependent Vibration Stokes Shift during Solvation: Experiment and Theory

Peak shift dynamics of CO stretching modes of [Re(R2-bpy)(CO)3Cl] [R = COOH and COOEt] complexes in their MLCT excited state were studied in polar solvents using femtosecond visible pump and IR probe spectroscopy. For Re complex in dimethyl formamide, peak shift magnitudes for the three CO stretching modes are different but their dynamics are identical. Comparison in different alcohols showed that peak shift magnitude increases with solvent dielectric constant and peak shift time follows the trend of solvation time. The observed peak shift is attributed to solvation-induced time-dependent vibrational Stokes shift. A general theory based on the Onsager dielectric continuum model of solute solvent interaction and time-dependent reaction field was developed to qualitatively describe time-dependent vibrational Stokes shift. The theory predicts that the peak shift correlation function is determined by solvent dielectric relaxation at the vibration frequency, and the peak shift magnitudes are solvent and normal mode dependent. Although the observed trend of solvation-induced vibrational Stokes shift is consistent with the theoretical predictions, other contributions such as solute polarizability should also be included in more complete models. Solvation induced vibrational Stokes shift may be a general phenomenon in time-resolved vibrational spectroscopy of molecules undergoing significant change in charge or charge distribution.