Separation of dielectric nonideality from preferential solvation in binarysolvent systems: An experimental examination of the relationship between solvatochromism and local solvent composition around a dipolar solute

Dielectric nonideality of a binary solvent system refers to the deviation o f the Onsager reaction field function from linearity in the polar mole frac tion of the solvent mixture. A dipolar fluorophore dissolved in an ideal di electric mixture exhibits a solvatochromic shift that is linear in the solv ent polar mole fraction in it's solvation sphere. As a result, the "local c omposition" can be easily determined from the peak shift. Here we identify the conditions under which this linear approximation is appropriate for est imating local compositions around dipolar solutes. In a previous study (Kha jehpour, M. H.; Kauffman, J. F. J. Phys. Chem. A 2000, 104, 7151-7159), we have demonstrated the influence of dielectric nonideality on the observed e mission peak shifts of the charge-transfer excited state of ADMA [1-(9-anth ryl)-3-(4-N,N-dimethylaniline) in hexane-ethanol mixtures. The linear appro ximation fails for this binary solvent, and a more elaborate method of anal ysis such as Suppan's nonlinearity ratio method must be used to determine t he local composition from solvatochromic shifts. In this work, we examine m ixture nonideality and dielectric enrichment in hexane-tetrahydrofuran and hexane-dichloromethane mixtures. Our analysis demonstrates that the contrib ution of nonideality to the observed solvatochromic shifts cannot be neglec ted in these binary solvents. Using Suppan's theory of dielectric enrichmen t, we have calculated the local composition of ADMA's solvation sphere and find that it is enriched in the polar component by similar to 30% over the bulk composition. This calculated value agrees with experimental measures o f the local composition based on analysis of solvatochromic shifts using Su ppan's nonlinearity ratio method which accounts for dielectric nonideality. The linear approximation overpredicts this composition by as much as 50%, even though these binary solvents are more nearly ideal than the hexane-eth anol system. Following this observation, we have identified conditions unde r which the linear approximation is justified, and find that for most cases of practical importance the linear approximation will not provide accurate estimates of the local solvent composition from solvatochromic studies. Si milarly, solvatochromic shifts can only be accurately predicted from theore tical local compositions if dielectric nonideality is taken into account. T hese results along with our previous studies indicate that the charge-trans fer excited state of ADMA behaves as an ideal dipolar solute.