Effective polarity of frozen solvent glasses in the vicinity of dipolar solutes

The polarity of frozen organic solvent glasses is found to be substantially larger than that of liquid solvents at room temperature. The extent of this increase depends on the dipolar character of both the solvent and solute as observed in differences in the shifts of the absorption spectra of two different classes of solvent polarity indicator dyes in a number of different frozen solvents. These findings cannot be explained solely by contributions from the contraction of the solvent at lower temperatures. Instead, the data are consistent with a model in which the local solvent organization around the solute increases as the temperature is lowered and depends on the dipolar properties of the solvent and solute. This ordering is preserved when the solvent freezes and leads to a much larger effective polarity in the vicinity of the solute that is not reflected in measurements of bulk solvent properties. Because this effective solvent polarity can affect electronic properties as well as reactivity of the solute, these results may have significant implications for the design of complex molecular systems in solid solution and models for protein function.