Thermosolvatochromism of Merocyanine Polarity Indicators in Pure and Aqueous Solvents:  Relevance of Solvent Lipophilicity

Abstract

The following novel solvatochromic probes were synthesized: 2,6-dibromo-4-[(E)-2-(1-alkylpyridinium-4-yl)ethenyl] phenolate, where the alkyl groups are methyl, n-butyl, n-hexyl, and n-octyl, respectively. Solvatochromism of three of these probes (C(1), C(4), and C(8)) was studied in 36 protic and aprotic solvents. A modified linear solvation energy relationship has been applied to the data obtained at 25 degrees C. Correlation of (empirical) polarities with other solvent properties showed more dependence on lipophilicity than on basicity. A similar conclusion has been reached for a series of other solvatochromic indicators. Exceptions are those that carry acidic hydrogens, being biased toward solvent basicity. Thermosolvatochromism has been studied in mixtures of water with methanol, 1-propanol, acetonitrile, and DMSO. Thermosolvatochromic data have been treated according to a model that explicitly considers the presence in bulk solution of three "species": water, organic component, and solvent-water hydrogen-bonded aggregate. Solvation by the latter is favored over solvation by either of the two precursor solvents (aqueous DMSO is an exception). Temperature increase resulted in desolvation of the probes, due to concomitant decrease of the structures of the component solvents. The above-mentioned modified solvation equation has been successfully applied to solvatochromism in aqueous methanol and aqueous 1-propanol.

Solvation in pure and mixed solvents: Some recent developments

“…4 for a total of 23 solvatochromic probes of different molecular structures, where adequate numbers of solvents have been tested. The same conclusion about the relative importance of β stat of (b) applies, except where the probes carry acidic hydrogens (e.g., nitrophenols and nitroanilines), the latter being "biased" toward solvent basicity [45].…”

“…3 [36][37][38][39][40][41][42][43][44][45]. Historically, the solvent polarity scales of RB and WB have been termed E T (30) and E T (33), respectively.…”

“…Equation 4 is general, unless the molecular structure of the probe makes it biased toward a particular solvent property. Figure 4 shows the dependence of E T (MePMBr 2 ) on χ W for mixtures of W with MeOH, 1-propanol, PrOH, MeCN, and dimethylsulfoxide (DMSO) at 25°C [45]. This dependence is not linear (i.e., it is not ideal); a large body of solvatochromic data shows that this is a general behavior [10,[13][14][15][16][37][38][39][40][41][42][43][44][45].…”

Solvation simplified

“…: 14 E T (probe) = Constant + a a S + b b S + s (p* S + dd) + p log P S (3) Where a new term (p log P S ) has been introduced because the spectroscopic response of merocyanine probes is sensitive to solvent lipophilicity, as measured by the empirical scale log P S (= partition coefficient of a substance between (mutually saturated) 1-octanol and water: log P = log ([substance] 1-octanol /[substance] water )). Figure 2 shows some solvatochromic probes that the author´s group has employed in order to study solvents and their mixtures, along with their pKa in water and log P.…”

“…Whereas the pKa of merocyanines 2 to 5 (MePMeBr 2 to OcPMeBr 2 ) is the same, their log P vary by 2.86 units (relative to water, OcPMeBr 2 is 724 times more soluble in 1-octanol than MePMeBr 2 ). This difference has been employed for evaluation of the relative importance of solute-solvent solvophobic interactions, for binary mixtures of water with both molecular solvents, 14 and ionic liquids, ILs. 15 Although RB is 3467 time more basic than WB, the susceptibility of their solvation toward solvent acidity is almost the same.…”