Chiroptical Study of Chiral Discrimination by Amino Acid Based Ionic Liquids

The chiral discrimination ability of amino acid based chiral ionic liquids is studied using chiroptical luminescence techniques. A racemic mixture of dissymmetric europium tris(2,6-pyridinedicarboxylate) complexes are dissolved in five chiral ionic liquids, including L- and D-alanine methyl ester bis(trifluoromethanesulfonimide), L-leucine methyl ester bis(trifluoromethanesulfonimide), L-proline methyl ester bis(trifluoromethanesulfonimide), and tetrabutylammonium L-alanate. Circularly polarized luminescence spectra are measured for the samples over the 283-323 K temperature range. Analysis of the spectroscopic results shows that the amino acid methyl ester chiral ionic liquids show discrimination with a preference (handedness) that corresponds to the stereoisomer (L- vs D-). Most of the chiral ionic liquids show enthalpically dominated discrimination, but L-leucine methyl ester bis(trifluoromethanesulfonimide) shows entropically dominated chiral discrimination.