Fluoride Ion Receptors Based on Dipyrrolyl Derivatives Bearing Electron-Withdrawing Groups: Synthesis, Optical and Electrochemical Sensing, and Computational Studies

Abstract

Two dipyrrolyl derivatives, 2,3-di(1H-2-pyrrolyl)-7,12-dihydronaphtho[2,3-f]quinoxaline-7,12-dione (1) and 5,6-di(1H-2-pyrrolyl)-2,3-pyrazine-dicarbonitrile (2), bearing electron-withdrawing quinone or dicyano subunits, have been synthesized and fully characterized by various spectroscopic and electrochemical methods. Both 1 and 2 are specific binders of F- in organic solvents and show dramatic, binding-induced changes in their color (observable in the naked-eye experiments) and also optical and electrochemical signatures. These F--induced color changes remain the same even in the presence of a large excess of Cl-, Br-, I-, or ClO4-, thus rendering 1 and 2 to be efficient fluoride ion sensors. While Ka for F- binding by receptor 1 is ∼1.6 × 104 M-1, that for receptor 2 is an order of magnitude higher. 1H NMR titrations were carried out to monitor the binding of 1/2 with F-. These experiments not only provide evidence for the hydrogen-bonding interaction between the pyrrolic NH groups of these receptors an...