Colorimetric fluoride ion sensors based on dipyrrolic and bipyrrolic compounds: Synthesis and anion recognition

The synthesis and spectroscopic studies of four sensors for fluoride chromogenic sensing are described. The new compounds were prepared by the Knoevenagel condensation of diformyl-substituted bipyrrolic and dipyrrolic synthons ([2,2′-bipyrrole]-5,5′-dicarbaldehyde and dipyrromethane-1,9-dicarbaldehyde moieties) with malononitrile or indane-1,3-dione. They strongly absorb in the visible region and significant color changes occur in the presence of fluoride anions. Acetate and dihydrogenphosphate anions also induce observable colorimetric changes, albeit to a lesser extent. These changes, which are visible to the unaided eye, are associated with NH-bonding interactions that are unique to each anion. Non-linear regression analysis of the ground- and excited-state changes revealed anion recognition in a 2:1 stoichiometry (Host:Guest), where the electronegative character of the substituents (malononitrile or indane-1,3-dione residues) controls the sensitivity of the binding. The proposed systems all feature exceptional anion receptors that display an impressive chromogenic response through NH-bonding. Among these, compound 3 stands out with exceptionally high affinity constants of up to 7.39x10⁹ M⁻², as well as an extremely low limit of detection at 92 ppm. NMR spectroscopy and mass spectrometry confirmed the structures of the synthesized compounds, with increased complexity in the NMR spectra due to the presence of malononitrile and indane-1,3-dione moieties. These findings highlight the potential of incorporating highly conjugated push-pull chromophores into bipyrrolic and dipyrrolic synthons for improved fluoride sensing performance in terms of both binding and signaling.