An alternative approach: a highly selective dual responding fluoride sensor having active methylene group as binding site

A newly designed phosphonium derivative (L) having active methylene functionality, shows unusual preference towards F- over all other anions. The binding process through C-H center dot center dot center dot F- hydrogen bond formation was probed by monitoring the changes in either electronic or luminescence spectra. Changes in both cases are significant enough to allow visual detection. The loss of molecular flexibility of L on forming L center dot F- effectively interrupts the non-radiative deactivation pathway and accounts for the observed switch on fluorescence response. The results of the time-resolved emission studies for L and L center dot F- using a time-correlated single photon counting technique further corroborate this presumption. The excellent preference of L towards F- is attributed to an efficient hydrogen bonding interaction between the strongly polarized methylene protons and F-, which delineates the subtle difference in the affinity among other competing anionic analytes like CN-, H2PO4-, CH3CO2-, etc. The relative affinities of various anions and the preferential binding of F- to reagent L are also rationalized using computational studies.