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

ABSTRACT

The 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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Colorimetric sensing
• fluoride receptor
• imidazole
• indole
• naked eye sensing

Acknowledgments

Authors are thankful to Materials Research Centre, Malaviya National Institute of Technology, Jaipur for providing the spectral facilities. Financial support from Department of Science and Technology-Water Technology Initiative (DST-WTI), New Delhi is deeply acknowledged. One of the authors, A. Jain is also grateful to Council of Scientific and Industrial Research (CSIR), New Delhi for awarding senior research fellowship (SRF).