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

The solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RE), 1-methyl-8-oxy-quinolinium 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, E-T (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 (THE 1-butanol, or water). Although the non-aqueous solvent mixtures behave ideally plots of E-T 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 RE 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 RE in binary aqueous mixtures, plots of E-T 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.