Purely Mechanical Solvation Dynamics in Supercooled Liquids:  The S₀← T₁(0−0) Transition of Naphthalene

We have measured the Stokes shift and its dynamics for the probe molecules naphthalene (Δμ ≈ 0) and quinoxaline (Δμ ≈ 1.3 D) in n -propanol and other glass-forming solvents. The Stokes shift for naphthalene (≈63 cm -1 ) turns out to be independent of the solvent polarity over a wide range of the static dielectric constants. Its time dependence is governed by the structural (or α- or shear stress) relaxation time of n -propanol, without any signature of the strong dielectric relaxation. For this solvent, structural and dipolar contributions can be distinguished because the time scale for dipole reorientation is a factor of 25 slower than the α-relaxation time. We conclude that the solvation of naphthalene reflects excitation-induced changes in the van der Waals interactions, which makes it an ideal probe for assessing shear stress or mechanical relaxations on microscopic spatial scales near T g .