Submicromolar fluorescence 'turn-on' detection of fluoride anions using meso-(tetra-aryl) calix[4]pyrrole

Meso-(tetra-aryl) picket calix[4]pyrrole 1 featuring p-fluorophenyl groups at all four meso-positions in a cis-configuration has been synthesized and characterized unambiguously using single-crystal X-ray diffraction analysis. This pre-organized system possesses a deep binding pocket created by the four aryl groups so that anions can be accommodated through anion-π interactions and four-point N-H⋯anion hydrogen bonds. Single-crystal X-ray diffraction analysis of the CsF and TEAF (TEA = tetraethylammonium) complexes of receptor 1 unequivocally confirms the formation of 1 : 1 complexes, revealing the binding modes in the solid state. The anion binding studies conducted in acetonitrile confirmed that the receptor forms stable 1/1 complexes with fluoride anions and coumarin anion 2- in solution. The host-coumarin ensemble 3 was then applied for the detection of fluoride anions through a fluorescent dye displacement assay (FDDA). The highly fluorescent coumarin anion becomes completely non-fluorescent upon complexation with receptor 1, and its fluorescence is recovered in a concentration-dependent manner upon the addition of fluoride anions. This system proves to be an excellent 'turn-on' sensor for detecting fluoride anions with high selectivity and sensitivity, with a detection limit for fluoride anions estimated at 177 nM.