Recognition, Sensing, and Trapping of Bicarbonate Anions with a Dicationic meso-Bis(benzimidazolium) Calix[4]pyrrole

A meso-aryl calix[4]pyrrole derivative bearing benzimidazolium groups at the two diametrical meso positions was synthesized. The receptor acts as an effective host for the bicarbonate anion at concentrations as low as 4 nM. A change in fluorescence response was observed under conditions of fluorescence dye displacement when this functionalized calix[4]pyrrole was exposed to the HCO3- anion. Tests with various carbonated drinks revealed its ability to function as an easy-to-use sensor for dissolved CO2, as monitored through the bicarbonate anion concentration. Depending on the specific choice of conditions, recrystallization of the calix[4]pyrrole receptor in the presence of Cs2CO3 and methanol yielded crystals of either the HCO3- anion complex or methyl carbonate (CH3OCO2-), a normally difficult-to-access species whose formation under non-forcing equilibrium conditions is ascribed to trapping by the calix[4]pyrrole receptor. The present system thus provides a recognition-based approach to the chemical capture of hydrated, anionic forms of CO2.