Solvation dynamics versus inhomogeneity of decay rates as the origin of spectral shifts in supercooled liquids

The recent discussion on the origin of spectral shifts observed for the luminescence of sensor molecules in fluid solution has guided us to reinvestigate solvation dynamics data in glassy matrices. It is demonstrated that the Stokes shift of a phosphorescent probe in a supercooled liquid is of pure relaxation origin and, thus, directly reflects the dynamics of solvation. At lower temperatures, anti-Stokes shifts are observed due to site specific S_o ← T₁ rates relating to the emission energy by means of external heavy atoms which increase both the decay rate and local polarizability. This inhomogeneity differs phenomenologically from solvation effects and is found to be effective only at temperatures below the onset of solvation.