Site-Specific Excited-State Solute-Solvent Interactions Probed by Femtosecond Vibrational Spectroscopy

Solute-solvent interactions in liquids are studied by femtosecond vibrational spectroscopy. Upon electronic excitation, the $\mathrm{C}=\mathrm{O}$ stretching band of an organic probe molecule, a coumarin dye, dissolved in ${\mathrm{CHCl}}_3$ shows a strong blueshift on a 200 fs time scale. This demonstrates for the first time the cleavage of a site-specific intermolecular hydrogen bond with a polar solvent. After this fast process, the picosecond reorganization of the solvent due to dielectric relaxation is directly monitored via changes of the $\mathrm{C}=\mathrm{O}$ vibrational frequency.