Enhancement of visual chiral sensing via an anion-binding approach: Novel ionic liquids as the chiral selectors

• Novel chiral ionic liquids were used as the chiral sensor for the first time.
• Color changes and precipitate formation were visually determined.
• Responsive mechanism and the structures of the precipitate were both discussed.
• Other racemates were also successfully examined by this method.

Novel task-specific ionic liquids derived from (S)-phenyl amino acid were prepared as a chiral sensor for the first time. It was found that the ionic liquids exhibited different chiral visual responses to the enantiomers. When (S)-tryptophan was treated with the sensor and Cu(II) in water, the clear solution would change to a sapphire suspension. Differently, another enantiomer could not produce the precipitate. Furthermore, the enantiomers could generate different colors in DMSO under the same conditions. The substantial difference contributes to the easy discrimination with visual determination. Four racemates were successfully tested with this strategy. The precipitation was isolated in the range of 90–94% yields after complete formation of the coordinative bonds, which could be as a simple way for enantioseparation. The responsive mechanism of the sensor was studied by NMR spectroscopy, FTIR and SEM. UV-vis spectrophotometer was used to monitor the colorimetric signal of the racemate with different e.e. values.

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