A Benzothiadiazole-Based Thiourea Receptor for Selective Chloride Extraction and Reversible Fluoride Sensing

In supramolecular chemistry, thiourea binding sites have gained paramount attention due to their ability to bind anions employing directional hydrogen bonding (HB) interactions. In this work, we report a new thiourea-based anion receptor, BDTU, incorporating electron-deficient benzothiadiazole (BTD) as a signaling unit. The binding affinity and signaling ability of BDTU toward various anions of different geometry and basicity are investigated by ¹H NMR, UV-vis, and fluorescence techniques in CH₃CN. As determined by NMR spectroscopic analyses, BDTU shows moderate selectivity toward chloride (Cl^-) over other halides and oxyanions, forming relatively stable 1:2 host-guest stoichiometric complex that is stabilized by thiourea NH HBs. In this regard, the single crystal X-ray structure of the Cl^- complex of BDTU has revealed short and directional intermolecular N─H⋯Cl^- HB with chloride. Importantly, this strong HB with Cl^- is successfully utilized in the solid-liquid extraction of chloride. Furthermore, the photophysical studies reveal a reversible anion sensing behavior of BDTU toward fluoride (F^-) in CH₃CN. Thus, BDTU is demonstrated for dual applications in extraction as well as sensing of anions.