Solvatochromism in Mixtures of Hydrogen Bond Acceptor (HBA) Solvents with Water.

The UV/Vis absorption energies νmax of Reichardt's dye B30 with respect to ET (30) and 4-nitroaniline (NA) are investigated as a function of the solvent composition Nav,z. in co-solvent/water mixtures. Nav,z . is the average molar concentration of the solvent mixture at a given solvent fraction z. The z can be the mole, the volume or the mass fraction. The co-solvents considered were acetonitrile, acetone, tetrahydrofuran, pyridine, piperidine and 2-(diethylamino)-ethanol. Acetone and acetonitrile can be expected to slightly enhance the water structure at low co-solvent concentrations. This interpretation is supported by the analysis of the refractive index as a function of the solvent composition. In general, it can be stated that the structural complexity of the binary solvent mixtures is mainly responsible for the evolution of the absorption energies ET (30) or νmax (NA) as a function of the mixture composition. In particular, the endothermic solvation of NA in co-solvent/water mixtures and its effect on the νmax (NA) is highlighted.