Spectroscopic line shapes in polar supercooled liquids

We have measured the inhomogeneously broadened T₁ → S₀ (0-0) emission spectra of quinoxaline in a series of organic glass-forming solvents, which are characterized in terms of their optical and static dielectric constants. The experimental results regarding Stokes shift and inhomogeneous line width are compared with the predictions of a statistical mechanical line shape theory (Loring, R. F. J. Phys. Chem. 1990, 94, 513), whose relevant input parameters are known. Using the MSA approach for delineating the solvation free energy, a good agreement between prediction and observation is achieved, indicating of a clear improvement over Onsager's continuum calculation. As a conclusion, spectroscopic line shapes in these systems are completely determined by electrostatic interaction and, therefore, can be rationalized quantitatively in terms of the solvent dielectric constant. The relation to empirical polarity scales is briefly discussed.