Solvatochromism in binary solvent mixtures: Effects of the molecular structure of the probe

Abstract

AbstractThe solvatochromic 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 spectrophotometrically in mixtures of cyclohexane‐THF and cyclohexane‐1‐butanol. It was also studied in mixtures of water with acetone, acetonitrile, 1,2‐dimethoxyethane (DME), 1,4‐dioxane, and THF. Empirical parameters of solvent polarity, ET (in kcal/mol), were calculated from the position of the longest‐wavelength absorption band of the respective solvatochromic probe, and they were then correlated with the mole fraction of the more polar component of the binary solvent mixture (THF, 1‐butanol, or water). Although the non‐aqueous solvent mixtures behave ideally, plots of ET versus the mole fraction of THF, or 1‐butanol, are not linear for all probes, due to nonspecific (e.g., dipolar) interactions and specific solute‐solvent interactions (e.g., H‐bonding). The contribution of each type of interaction has been quantified and correlated with the structure of the solvatochromic probe molecule. For RB in cyclohexane‐THF mixtures, the deviation from linearity of the above mentioned plot is due to nonspecific probe dipole/solvent dipole interactions. For cyclohexane‐1‐butanol binary mixtures, both nonspecific and specific interactions contribute to the observed solvatochromism. For RB in binary aqueous mixtures, plots of ET versus the mole fraction of water are generally sigmoidal, and follow closely the changes of solvent structure, particularly in the microheterogeneous domain. The same plots for PB show a positive deviation from linearity, because this probe forms its own water‐rich solvation shell, whose composition is relatively insensitive to the nature of the organic co‐solvent. In all binary solvent mixtures, the solvatochromic behavior of QB is more or less similar to that of PB.