Benzimidazole scaffold as dipodal molecular cleft for swift and efficient naked eye fluoride ion recognition via preorganized N-H and aromatic C-H in aqueous media

Abstract

A series of elegantly designed cleft-like dipodal receptors, N,N'-bis-(5-(un)substituted-1H-benzoimidazol-2-ylalkyl)-isophthal-amides (RA-RD) has been synthesized and characterized for colorimetric detection of fluoride ion in 9:1 DMSO-water. The phenyl ring in the molecular framework of receptors is symmetrically armed with two benzimidazole moieties using amide groups as linkers to yield dipodal receptors, with multiple hydrogen bond donor sites for anion sensing. Anion binding studies, conducted qualitatively and spectroscopically in 9:1 DMSO-water, show that the receptor RC binds fluoride ion exclusively with a detection limit of 1.5 ppm over other anions. UV-visible spectra of receptor RC shows a considerable bathochromic shift of 117 nm from 348 nm to 465 nm upon addition of varying concentrations of fluoride ion (tetrabutylammonium salt). Jobs plot and mass spectroscopic data confirm 1:1 stoichiometric ratio between receptor RC and fluoride ion. ¹H NMR titration reveals the presence of hydrogen binding interactions between receptor RC and fluoride ion responsible for naked eye colour change. ¹⁹F NMR titration further supports the binding interaction between receptor RC and fluoride ion. The binding constant of receptor RC for fluoride ion is calculated to be 5.59×10³ M^-1.