Dipolar solvation as a signature of dielectric responses in supercooled liquids

Time-resolved Stokes shifts of phosphorescence bands are investigated by monitoring the solvation dynamics of electronically excited probe molecules embedded at low concentrations in organic supercooled liquids of varying polarities. The long-lived TI states of the solutes give access to dipolar equilibration of the glassy medium near T_G on a millisecond to second time scale. Coincidence of solvation and glass transition temperatures over a large T_G range relates the spectral shift to the a process of the glass with only minor sensitivities to the specific probe molecule. Emission energy data for the Franck-Condon and equilibrated solvent configurations allow determination of the change in dipole moment upon T₁ ← S₀ excitation in agreement with theoretical predictions. Relative polarities of the glasses as quantified by the positive solvatochromism of quinoxaline are highly correlated to the corresponding values for the liquid state at 20°C. The observed solvation dynamics map the dielectric response to a motion of charge and can be rationalized in terms of dielectric dispersion, importance of microscopic solvent behavior, and an excess cooperativity of longitudinal relaxations.