Pronounced Hydrogen Bonding Giving Rise to Apparent Probe Hyperpolarity in Ionic Liquid Mixtures with 2,2,2-Trifluoroethanol

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Abstract

The fascinating and attractive features of ionic liquids (ILs) can be considerably expanded by mixing with suitable cosolvents, opening their versatility beyond the pure materials. We show here that mixtures of the IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and 2,2,2-trifluoroethanol (TFE) display the intriguing phenomenon of hyperpolarity, examples of which are notably sparse in the literature. From the perspective of the E(T)(N) polarity scale and Kamlet-Taft parameters for hydrogen bond acidity (α) and basicity (β), the polarity of this mixture exceeds that of either neat component. Fluorescent molecular probes capable of engaging in hydrogen bonds (e.g., 2-(p-toluidino)naphthalene-6-sulfonate, TNS; 6-propionyl-2-(dimethylamino)naphthalene, PRODAN) also exhibit this curious behavior. The choice of IL anion appears to be essential as hyperpolarity is not observed for mixtures of TFE with ILs containing anions other than hexafluorophosphate. The complex solute-solvent and solvent-solvent interactions present in the [bmim][PF₆] + TFE mixture were further elucidated using infrared absorbance, dynamic viscometry, and density measurements. These results are discussed in terms of Coulombic interactions, disruption of TFE multimers, formation of hyperanion preference aggregates, and "free" [bmim]⁺. It is our intent that these results open the door for computational exploration of related solvent mixtures while inspiring practical questions, such as whether such systems might offer the potential for stabilization of highly charged transition states or ionic clusters during (nano)synthesis.

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