Emission peak shifts of a dipolar solute dissolved in nondipolar solvents:: A quantitative measure of quadrupole-dipole interactions in supercritical CO2

Solvents are often broadly categorized as polar and nonpolar, depending on solvent permittivity. However, some common nonpolar solvents such as benzene, supercritical CO2, and 1,4-dioxane exhibit much larger polarity than that predicted from their permittivities. These solvents are unusual in that their polarities are not the result of dipolar charge distributions and are therefore more appropriately categorized as "nondipolar". Our recent studies of ADMA (1-(9-anthryl)-3-(4-N,N-dimethylaniline) in liquids demonstrate that the ADMA sandwich heteroexcimer is an excellent probe for investigation of solvent-solute interaction. In this paper we present measurements of the emission spectra of ADMA in a series of nondipolar solvents and determined the excess stabilization energy experienced by ADMA in each of these solvents. These excess energies have been analyzed according to a new theory (Matyushov and Voth, J. Cham. Phys. 1999, 111, 3630), demonstrating that quadrupole-dipole interactions are responsible for the excess stability. Our measurements agree with theory, indicating the importance of the role of the length scale in quadrupolar stabilization. The excess solvent shifts observed in supercritical CO2 are primarily the result of the quadrupolar nature of the solvent, calling into question assumptions often used in the analysis of local density augmentation.