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 (Delta mu approximate to 0) and quinoxaline (Delta mu approximate to 1.3 D) in n-propanol and other glass-forming solvents. The Stokes shift for naphthalene (approximate to 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 alpha- 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 alpha-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.