Ultrafast solvation processes in polar liquids probed with large organic molecules

Several optical techniques are used to characterize the ultrafast solvation dynamics of two structurally different molecules, LD690 and LDS750. Linear absorption, fluorescence, resonance Raman, and time-resolved four-wave mixing experiments have been performed on both molecules in a variety of solvents. The combination of different experiments is necessary to characterize the femtosecond dynamics. We find that both molecules exhibit very fast dephasing, due to sub-100 fs decays of the solvation correlation function M(t). The LD690 data is analyzed in depth, using a multimode oscillator model to treat both intramolecular vibrations and the solvent coordinates. The dynamics are solvent-dependent, but this dependence cannot be explained in a straightforward manner using quantities such as inertial solvent rotational motion or a simple dielectric continuum response. Various aspects of the spectroscopy, both linear and nonlinear, provide clues that the dynamics are influenced by molecular-level interactions that are not taken into account by theories that only consider bulk solvent properties.