Rationally designed tri-armed imidazole–indole hybrids as naked eye receptors for fluoride ion sensing

Abstract

ABSTRACTThe present communication describes the design, synthesis, and characterization of unique tri-armed imidazole–indole hybrids RA-RE for naked eye detection of fluoride ion in 9:1 DMSO–water at concentration level of 1.5 ppm, which is recommended permissible limit of fluoride ion. Molecular structures of the receptors are so fabricated that both heterocyclic units, namely, indole and imidazole have been rationally used as chromophore and binding unit, respectively. Strategical introduction of nitro group(s) at the fifth position of indole ring remarkably enhances the binding ability of receptor RD making it highly selective toward fluoride ion (tetrabutylammonium salt) over other typical anionic species. The sensing event can be visualized by naked eyes swiftly with color change. This observation is well corroborated by a redshift of 80 nm in UV–visible spectroscopic studies. A peak at 16.1 ppm due to in 1H NMR titration validates the deprotonation of imidazolium N‒H by fluoride ion.