Calix[4]pyrroles with Shortest Possible Strap: Exclusively Selective toward Fluoride Ion

Four new calix[4]­pyrroles with the shortest possible strap so far through ortho-linking of the aromatic unit have been synthesized, including a naphthalene-derived fluorescent receptor. They show exclusive selectivity toward the fluoride ion as confirmed by ¹H NMR, isothermal titration calorimetry, and fluorescence spectroscopic study. Anion affinity could also be modulated further via functionalization at the strap. Computational analysis displays calix[4]­pyrroles binding to fluoride ion in a very unusual 1,3-alternate conformation where the anion resides on the opposite side of the strap.