Ionic liquids containing fluorinated β-diketonate anions: synthesis, characterization and potential applications

Abstract

Ionic liquids (ILs) sporting anions comprising the β-diketonate functionality were prepared, fully characterized via ESI-MS, FTIR, and ¹H/¹³C NMR, and tested in multiple scenarios. We present eight new salts based on four different β-diketonate anions, each coupled with the choline or tetrabutylphosphonium cation. The thermal stabilities and transitions of these β-diketonate salts were explored using DSC and TGA. Seven of the compounds displayed melting points at or below 100 °C and hence formally qualify as ILs. The inherent binding capability of the β-diketonate moiety made possible a task-specific IL amenable to lanthanide recognition. For example, coordination with Eu³⁺ was accompanied by a striking three order-of-magnitude intensification of luminescence (‘turn-on’). Additionally, these ILs display prominent acidochromism. That is, the intense visible color of β-diketonate ILs is modulated in the presence of an acid source, permitting the visual transduction of local pH changes. Utility for carbon capture was also considered, however, these ILs were essentially incapable of binding CO₂. Computational studies were better able to elucidate this behavior, revealing that the association of CO₂ with the β-diketonate anion is thermodynamically unfavored and sterically hindered. Despite this negative result for CO₂ capture, these newly introduced β-diketonate ILs show interesting and useful physicochemical properties applicable to a number of future applications.