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

A highly fluorescent receptor, BDTU, featuring a photoresponsive benzothiadiazole unit and HB donor thiourea motif, displays anion selectivity toward Cl⁻ over other halides and oxyanions, while a preliminary solid–liquid extraction experiment suggests its ability to extract Cl⁻ in CH₃CN. Moreover, it can also be used as a reversible fluorescence “off–-on” switch for F⁻ detection.

Abstract

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.

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