Preferential solvation in aqueous-organic mixed solvents using solvatochromic indicators

Solvatochromism of the twisted intramolecular charge-transfer (TICT) fluorescence of 4-(N,N-dimethylamino)benzonitrile (DMABN) in pure water, methanol, ethanol, 1-propanol, 2-propanol, acetone, acetonitrile, and in the corresponding aqueous-organic binary mixed solvents was systematically studied and an empirical solvent polarity scale (F-B) based on the DMABN solvatochromism was defined. The F-B parameters of the explored binary mixed solvents as a function of solvent composition were analyzed by a stepwise solvent-exchange (SSE) model to clarify the preferential solvation (PS) of the probe dye in these binary mixed solvents. Solvation diagrams toward DMABN in the mixed solvents, i.e., the local solvent composition in the solvation shell of DMABN molecules was depicted as a function of bulk solvent composition to visualize the PS in these mixed solvents. For comparison, a similar PS analysis was applied to the solvatochromism of 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1- yl)phenolate (ET-30) and pyrene (Py) in these mixed solvents; the responsive PS pattern of the mixed solvents toward the specific indicator dye of DMABN, ET-30, and Py was then discussed in terms of the chemical properties of the probe dye, the properties of the mixed solvents, and the solute-solvent and solvent-solvent interactions.