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

Abstract

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 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.

Solvatochromism in pure and binary solvent mixtures: effects of the molecular structure of the zwitterionic probe

“…Dependence of E r T on w water for RB, WB and QB, for three mixtures of water with dipolar aprotic solvents We begin our discussion of Figs 1 and 2 by considering the expected solvatochromic behaviour of a probe in a binary mixture of solvents, S1 and S2, where the latter is more polar and has a higher dielectric constant. As discussed elsewhere, E T for an ideal binary solvent mixture is given by 12,13,18 …”

“…QB and RB show a similar behavior. 12,13 Non-ideal behavior also results from solvent microheterogeneity, i.e. when one component of the mixed solvent prefers a molecule of the same kind.…”

“…10 Finally, no data are available for two important binary solvents, aqueous 1,4-dioxane and aqueous 1-propanol. 7,8,12,13 The present study remedies the problems outlined in the preceding paragraph, and sheds light on the dependence of the solvatochromic behavior of these probes on their acidity (pK a in water = 8.65, 6.80 and 4.78, for RB, QB, and WB, respectively) [4][5][6]9,14 and hydrophobicity (order RB bWB bQB; see below). Our results show that RB and WB have similar susceptibilities to solvent dipolarity/polarizability and acidity.…”

Selective glycerol oxidation over ordered mesoporous copper aluminum oxide catalysts

“…65 The E T (30) value is derived from the solvatochromism of the dye betaine 30 and is a descriptor for the hydrogen bond and electrostatic interactions of solvents. 81 Low E T (30) values correspond to low solvent polarities.…”

“…Solvatochromic parameters were taken from the literature. 65 The reaction mixture was analyzed via HPLC with a 300 × 6.5 mm Metacarb 67H column equipped with a guard cartridge. The mobile phase was 0.1% trifluoroacetic acid with a flow rate of 0.8 mL min −1 at a temperature of 323 K; peaks were detected with a refractive index detector.…”