Solvatochromism in Alcohol‐Water Mixtures: Effects of the Molecular Structure of the Probe

Abstract

AbstractSolvatochromic behavior of 2,6‐diphenyl‐4‐(2,4,6‐triphenyl‐1‐pyridinio)‐1‐phenolate (RB), 1‐methyl‐8‐oxyquinolinium betaine (QB), sodium 1‐methyl‐8‐oxyquinolinium betaine‐5‐sulphonate (QBS), and 1‐methyl‐3‐oxypyridinium betaine (PB) was studied UV/Vis spectrophotometrically in binary mixtures of water with methanol, ethanol, 1‐propanol, 2‐methyl‐2‐propanol, ethane‐1,2‐diol, 2‐methoxyethanol, and 2‐n‐butoxyethanoI. The solvent polarity parameter, ET (in kcal/mole), was calculated from the position of the longest‐wavelength absorption band of the solvatochromic probe, and was then correlated with χw, the mole fraction of water in the mixture. Except for PB in aqueous methanol, aqueous ethanol and aqueous 1‐propanol, plots of ET versus χw are not linear because of preferential solvation of the probe by one component of the mixed solvent and, when applicable, because of solvent microheterogeneity. In the same aqueous alcohol mixtures, deviation from linearity in the ET versus χw plots increases as a function of the increasing hydrophobic character of the probe. For the same solvatochromic probe in different aqueous mixtures, the above mentioned deviation increases as a function of increasing the chain length of the alcohol. Preferential solvation of the probe is described in terms of the so called “solvent fractionation factor, ϕs”, whose value indicates the relationship between composition of the solvation shell of the probe, and composition of the bulk solvent. In the absence of solvent microheterogeneity, all values of ϕs are less than unity, which indicates preferential solvation of the probe molecule by the alcohol. An attempt to evaluate contributions from preferential solvation and solvent microheterogeneity to solvatochromism in aqueous methanol, and aqueous THF showed that the approach used by Park et al. [J. Phys. Org. Chem. 7, 757 (1994)] is not applicable.