A novel, convenient, quinoline‐based merocyanine dye: probing solvation in pure and mixed solvents and in the interfacial region of an anionic micelle

Abstract

A novel solvatochromic probe—2,6‐dibromo‐4‐[(E)‐2‐(1‐butylquinolinium‐4‐yl)ethenyl] phenolate, BuQMBr2—has been synthesized and its properties examined. The quinoline‐based probe is soluble in more organic solvents than the parent merocyanine dye, 4‐[2‐(1‐methylpyridinium‐4‐yl)ethenyl] phenolate, and its pKa is lower by 3.7 units. Its solvatochromic data in binary mixtures of cyclohexane–n‐butanol showed that the deviation from ideal behavior is due to a combination of non‐specific and specific solvent–probe interactions. Its thermo‐solvatochromism has been studied in mixtures of water with methanol, ethanol, 1‐propanol, 2‐propanol and 2‐methyl‐2‐propanol, respectively. The data obtained were analyzed according to a recently introduced model that explicitly considers the presence of 1:1 alcohol–water hydrogen‐bonded species, ROH–W, in bulk solution, and its exchange equilibria with water and alcohol in the probe solvation microsphere. The composition of the latter is given in terms of the appropriate set of solvent fractionation factors. These indicate that the probe is more solvated by alcohol than by water. Additionally, solvation by ROH–W is favored over solvation by either W or ROH. Solvation by alcohols is more affected by probe–ROH hydrophobic interactions than by hydrogen bonding of ROH to the probe phenolate oxygen. Temperature increase results in a gradual desolvation of the probe, due to a decrease in the hydrogen bonding of all components of the binary solvent mixture. The probe has been employed to calculate the effective concentration of interfacial water of sodium dodecyl sulfate micelles, which is 38.9 mol l−1. Copyright © 2005 John Wiley & Sons, Ltd.

Solvation simplified

“…Studies of solvatochromism and thermosolvatochromism are becoming increasingly important because of the current interest in the use of green solvents, e.g., super-critical CO 2 , 30 , and ILs. 30 …”

“…4 Indeed, E T (probe) were found to be non-linear functions of c higher (er) for the ideal binary mixtures cyclohexane-THF and cyclohexane-1-butanol. 30 This interaction mechanism is nonspecific and is, therefore, independent of probe-structure. A more fundamental reason for the non ideal solvation behavior is, however, solvent micro-heterogeneity, i.e., where one component of the mixed solvent prefers a molecule of the same type.…”

Solvation in pure and mixed solvents: Some recent developments

“…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]. How can this dependence be explained?…”

“…This interaction, if it occurs, implies a positive deviation in the E T (probe) vs. χ higher (ε r ) plot, even when Onsager dielectric function [f(ε r = 2(ε r -1)/2ε r + 1] of the mixture is linear [10]. Indeed, E T (probe) were found to be nonlinear functions of χ higher (ε r ) for the ideal binary mixtures cyclohexane-THF and cyclohexane-1-butanol [37,43]. This interaction mechanism is nonspecific and is, therefore, independent of probe structure.…”

“…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.…”