SPECTROCHEMICAL INVESTIGATIONS OF PREFERENTIAL SOLVATION .2. COMPATIBILITY OF THERMODYNAMIC MODELS VERSUS SPECTROFLUOROMETRIC PROBE METHODS FOR TAUTOMERIC SOLUTES DISSOLVED IN BINARY-MIXTURES

Two previously developed spectrofluorometric probe methods are critically examined regarding their ability to model preferential solvation around probe molecules dissolved in binary solvents. The first method assumes that each fluorophore is solvated by only one type of solvent molecule and that the observed spectral wavelength shift results from additivity of emission spectrum for the two solvational fluorophore species. The second method treats the solvational sphere as a binary solvent microphase, with the fluorophore's energy in both the ground and excited states mathematically expressed in terms of the ''nearly ideal binary solvent'' (NIBS) model. Expressions derived from each model are illustrated using observed fluorescence emission behavior of 9,9'-bianthracene and 9,9'-bianthracene-10-carboxaldehyde dissolved in binary toluene + acetonitrile and dibutyl ether + acetonitrile mixtures, which were measured as part of the present study. Also discussed is the compatibility of the inherent assumptions upon which the various spectrofluorometric probe methods are based, versus the thermodynamic principles that govern the conformational equilibria responsible for the observed solvatochromic behavior.