Ultrafast intramolecular proton transfer reactions and solvation dynamics of DMSO

Ultrafast intramolecular proton transfers of 1,2-dihydroxyanthraquinone (alizarin-h₂) and its deuterated product (alizarin-d₂) in dimethyl sulfoxide (DMSO) have been investigated by femtosecond stimulated Raman spectroscopy. The population dynamics in the solute vibrational mode of ν_C=O and the coherent oscillations observed in all of the skeletal vibrational modes ν_C=O and ν_C=C clearly showed the ultrafast excited-state intramolecular proton transfer dynamics of 110 and 170 fs for alizarin-h₂ and alizarin-d₂, respectively. Interestingly, we have observed that the solvent vibrational modes ν_S=O and ν_CSC may also represent ultrafast structural dynamics at the frequencies for its "free" or "aggregated" species. From the kinetic analysis of the ν_S=O and ν_CSC modes of DMSO, the ultrafast changes in the solvation or intermolecular interactions between DMSO molecules initiated by the structural changes of solute molecules have been thoroughly investigated. We propose that the solvent vibrational modes ν_S=O and ν_CSC of DMSO can be used as a "sensor" for ultrafast chemical reactions accompanying the structural changes and subsequent solute-solvent interactions.