Molecular probes of solvation phenomena

The properties of the molecules present in any chemical or biological system are dependent on interactions with the environment, and a quantitative understanding of solvation phenomena remains a major challenge. Molecular recognition probes provide a new approach to quantitatively measure the properties of solvents. Traditionally, solvent polarity scales have been based on spectroscopic probes that provide insight into the nature of solvent-solute interactions. This review compares the solvent polarity parameters obtained from the wavelengths of UV/Visible absorption maxima with solute H-bond parameters obtained from the free energies of solution equilibria. The similarity of the solvent and solute H-bond scales leads to a general H-bond scale that uses the same parameters to describe both solvent and solute. The general H-bond scale provides a framework for understanding the relationship between local intermolecular interactions and the properties of the bulk medium. Intermolecular interactions are sensitive to solvation equilibria, so molecular recognition probes provide fundamentally different information from spectroscopic probes that are sensitive to the populations of different solvation states of the solute. Studies of mixed solvents demonstrate the potential of molecular recognition probes for providing new insights into solvation phenomena.