Azocalix[4]arene Strapped Calix[4]pyrrole: A Confirmable Fluoride Sensor

A new chromogenic fluoride sensor based on 1,3-di-<i>p</i>-nitrophenylazocalix[4]arene-calix[4]pyrrole (<b>1</b>) was designed and synthesized. The color of the solution of probe <b>1</b> changed upon the addition of any F^–, CH₃CO₂^–, PhCO₂^–, and H₂PO₄^– ions. However, from these ions the highly specific sensing of F^– is achieved by the addition of Ca²⁺ which leads to a color change from light sky blue (of <b>1·F</b>^<b>–</b>) back to the original light orange color of <b>1</b>