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

Two previously developed spectrofluorometric probe methods are critica lly examined regarding their ability to model preferential solvation a round probe molecules dissolved in binary solvents. The first method a ssumes that each fluorophore is solvated by only one type of solvent m olecule and that the observed spectral wavelength shift results from a dditivity of emission spectrum for the two solvational fluorophore spe cies. The second method treats the solvational sphere as a binary solv ent microphase, with the fluorophore's energy in both the ground and e xcited 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'-bia nthracene and 9,9'-bianthracene-10-carboxaldehyde dissolved in binary toluene + acetonitrile and dibutyl ether + acetonitrile mixtures, whic h were measured as part of the present study. Also discussed is the co mpatibility of the inherent assumptions upon which the various spectro fluorometric probe methods are based, versus the thermodynamic princip les that govern the conformational equilibria responsible for the obse rved solvatochromic behavior.