A bifunctional acridine-based fluorescent sensor: Ratiometric sensing of H₂PO₄^- and obvious fluorescence quenching towards HSO₄^- through a synergistic binding effect of benzimidazolium and urea moieties

Three tweezer-like fluorescent sensors 1-3 based on acridine fluorophore were designed and synthesized. Anion binding properties of sensors 1-3 were studied by fluorescence, UV-vis, ¹H NMR, and HRMS. Results showed that the sensor 1, which bears both benzimidazolium and urea groups as binding sites exhibited better anion sensing performance compared with sensors 2 and 3 containing only one kind of binding sites. Especially, it could be used as a dual-responsive selective fluorescent sensor for both H₂PO ₄^- and HSO₄^- via fluorescent bathochromic-shift and fluorescence quenching, respectively. The unique selectivity of sensor 1 towards H₂PO₄^- and HSO₄^- could be attributed to the synergistic binding effect of benzimidazolium and urea moieties. In addition, the additional hydrogen bond between -OH in H₂PO₄^-/HSO ₄^- and the N on acridine of sensor 1 also played an important role in improving its anion binding affinities.