Amino Acid-Based Fluorescent Chiral Ionic Liquid for Enantiomeric Recognition

We report on the synthesis and characterization of a new fluorescent chiral ionic liquid (FCIL), L-phenylalanine ethyl ester bis(trifluoromethane) sulfonimide (L-PheC(2)-NTf2), capable of serving simultaneously as solvent, chiral selector, and fluorescent reporter in chiral analytical measurements. Enantiomers of different analytes, including fluorescent and nonfluorescent compounds, with a variety of structures were shown to induce wavelength- and analyte-dependent changes in the fluorescence intensity of this FCIL. This system may provide both chemo- and enantioselectivity toward multiple analytes simultaneously. The newly synthesized FCIL, derived from commercially available L-phenylalanine ethyl ester chloride and lithium bis(trifluoromethane) sulfonamide, was obtained as liquid at room temperature and is stable to thermal decomposition up to 270 degrees C. Absorption and fluorescence properties of neat L-PheC(2)NTf(2) were complex. While the absorption properties were similar to phenylalanine with a weakly absorbing tail extending beyond 400 nm, multiple excitation and emission bands were observed in its Excitation-Emission Matrix (EEM). A prominent excimer emission displayed the greatest intensity of all emission bands, and a long-wavelength emission shifted toward the red with increasing excitation wavelength. These different spectral regions were shown to respond differently toward several analytes, including sugars such as glucose and mannose, making this an ideal system to exploit the multidimensional properties of fluorescence. The unique properties of L-PheC2NTf2 combined with EEMs resulted in reliable identification of different enantiomers and measurement of enantiomeric composition. Importantly, the choice of excitation and emission wavelength regions was an important variable shown to improve prediction of enantiomeric composition.