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Induction-Driven Stabilization of the Anion–π Interaction in Electron-Rich Aromatics as the Key to Fluoride Inclusion in Imidazolium-Cage Receptors

Authors

  • Dr. Zhaochao Xu,

    1. Department of Chemistry and Nanoscience and Department of Bioinspired Science (WCU), Ewha Womans University, Seoul, 120-750 (Korea), Fax: (+82) 232772384
    2. Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW (UK), Fax: (+44) 1223336362
    3. Contributed equally to this work.
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  • Dr. N. Jiten Singh,

    1. Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology (China), Fax: (+82) 542798137
    2. Contributed equally to this work.
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  • Dr. Sook Kyung Kim,

    1. Department of Chemistry and Nanoscience and Department of Bioinspired Science (WCU), Ewha Womans University, Seoul, 120-750 (Korea), Fax: (+82) 232772384
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  • Dr. David R. Spring,

    Corresponding author
    1. Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW (UK), Fax: (+44) 1223336362
    • Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW (UK), Fax: (+44) 1223336362
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  • Prof. Kwang S. Kim,

    Corresponding author
    1. Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology (China), Fax: (+82) 542798137
    • Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology (China), Fax: (+82) 542798137
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  • Prof. Juyoung Yoon

    Corresponding author
    1. Department of Chemistry and Nanoscience and Department of Bioinspired Science (WCU), Ewha Womans University, Seoul, 120-750 (Korea), Fax: (+82) 232772384
    • Department of Chemistry and Nanoscience and Department of Bioinspired Science (WCU), Ewha Womans University, Seoul, 120-750 (Korea), Fax: (+82) 232772384
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Abstract

Intermolecular interactions that involve aromatic rings are key processes in both chemical and biological recognition. It is common knowledge that the existence of anion–π interactions between anions and electron-deficient (π-acidic) aromatics indicates that electron-rich (π-basic) aromatics are expected to be repulsive to anions due to their electron-donating character. Here we report the first concrete theoretical and experimental evidence of the anion–π interaction between electron-rich alkylbenzene rings and a fluoride ion in CH3CN. The cyclophane cavity bridged with three naphthoimidazolium groups selectively complexes a fluoride ion by means of a combination of anion–π interactions and (C[BOND]H)+⋅⋅⋅F-type ionic hydrogen bonds. 1H NMR, 19F NMR, and fluorescence spectra of 1 and 2 with fluoride ions are examined to show that only 2 can host a fluoride ion in the cavity between two alkylbenzene rings to form a sandwich complex. In addition, the cage compounds can serve as highly selective and ratiometric fluorescent sensors for a fluoride ion. With the addition of 1 equiv of F, a strongly increased fluorescence emission centered at 385 nm appears at the expense of the fluorescence emission of 2 centered at 474 nm. Finally, isothermal titration calorimetry (ITC) experiments were performed to obtain the binding constants of the compounds 1 and 2 with F as well as Gibbs free energy. The 2-F complex is more stable than the 1-F complex by 1.87 kcal mol−1, which is attributable to the stronger anion–π interaction between F and triethylbenzene.

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