FLUORESCENCE STUDIES OF SOLVATION AND SOLVATION DYNAMICS IN IONIC-SOLUTIONS

We have employed steady-state and time-resolved emission spectroscopy to study static and dynamic aspects of the solvation of polar aromatic solutes in ionic solution. Several common solvatochromic probe molecules (Cu102, Cu153, Prodan, 4-AP) were examined in a wide range of ionic solutions consisting of a variety of salts (mainly Li+, Na+, Mg2+, Ca2+, Sr2+, Ba2+ perchlorates) in a number of nonaqueous solvents (tetrahydrofuran, acetone, propylene carbonate, acetonitrile, dimethylformamide, dimethyl sulfoxide, methanol, 1-propanol, and formamide). The presence of ions causes shifts in the spectra of these probe solutes similar to those observed in pure solvents of varying polarity. As a function of increasing salt concentration or time, the primary spectral change is a frequency shift with little accompanying change in the spectral shape or width. Ion-induced frequency shifts in steady-state spectra are typically in the range of several hundred cm-1 in 1 M salt solutions. The magnitudes of these shifts decrease as the strength of solvent-solute interactions increases. They depend little on the identity of the anion but are approximately proportional to the charge-to-size ratio of the cation of the salt considered. The ionic solvation dynamics measured by time-resolved fluorescence take place on a 1-10-ns time scale. The kinetics is significantly dependent on excitation wavelength, especially for excitation on the red edge of the absorption spectrum. The rate of spectral shift is proportional to salt concentration and inversely proportional to viscosity for concentrations of 1 M or less. When these two factors are accounted for, the observed solvation rates are found to decrease with solvent polarity and decrease with the charge-to-size ratio of the cation. A number of the above results are inconsistent with the commonly used description of solute-ion interactions in terms of a diffuse ion atmosphere. To rationalize our observations we propose a model based on equilibrium among a limited set of solvates distinguished by the number of cations in the first solvation shell of the probe.