Ultrafast measurements of the dynamics of solvation in polar and non-dipolar solvents

The dynamics of solvation of the probe solute coumarin 153 has been measured with ∼ 100 fs time resolution using the fluorescence upconversion technique. A wide variety of solvents, including polar aprotic solvents, hydrogen bonding solvents, and non-dipolar solvents have been examined. For all solvents of even moderate polarity (ε{lunate}₀>5), the solvation dynamics observed follows the predictions simple models based on the solvent's bulk dielectric response. For a number of "non-dipolar" solvents such as dioxane and benzene, for which ε{lunate}₁₀<5, we observe surprisingly large time-dependent shifts, which seem to reflect the solute interacting with the large quadrupole (and higher multipole) moments of the solvent molecules. These dynamics represent a solvation distinct from the dipolar solvation dynamics previously studied and they cannot be modeled in terms of any currently available theory.