A bifunctional acridine-based fluorescent sensor: ratiometric sensing of image and obvious fluorescence quenching towards image 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 image and image via fluorescent bathochromic-shift and fluorescence quenching, respectively. The unique selectivity of sensor 1 towards image and image could be attributed to the synergistic binding effect of benzimidazolium and urea moieties. In addition, the additional hydrogen bond between –OH in image and the N on acridine of sensor 1 also played an important role in improving its anion binding affinities.