Three-Pulse Echo Peak Shift Studies of Polar Solvation Dynamics

The three-pulse photon echo peak shift technique is used to study room temperature solvation dynamics in acetonitrile, benzonitrile, methanol, ethanol, ethylene glycol, and chloroform utilizing the tricarbocyanine dye, IR144 as the probe molecule. In all cases, an ultrafast component, ascribed to inertial solvent motion, was evident after the very rapid dephasing of the intramolecular vibrational modes. All the solvents except acetonitrile show biexponential behavior on the picosecond (diffusive) time scale. In the case of acetonitrile, the spectral density governing the solute-solvent interaction is recovered by numerical fitting of peak shift data. The inertial component of the spectral density is compared with the solvation spectral density obtained from an instantaneous normal mode calculation. The two spectra agree well. The fitting parameters allow the absorption spectrum to be well reproduced. For ethylene glycol, data are also presented for 397 K; the ultrafast component of solvation is quite independent of temperature, while the diffusive portion speeds up dramatically at high temperature.