Trifluoromethyl group containing C3 symmetric coumarin-triazole based fluorometric tripodal receptors for selective fluoride ion recognition: A theoretical and experimental approach

A series of unique C3 symmetric tripodal receptors, A–E containing hexasubstituted aryl core decorated with an alternate array of coumarin-triazole moieties and methyl groups have been synthesized, characterized and investigated for fluoride ion recognition (tetrabutylammonium salt) in dimethyl sulphoxide (DMSO). A pleothora of spectroscopic studies (fluorescence, UV–vis, 1H NMR and 19F NMR titrations) exhibits that out of the five derivatives synthesized, only one, receptor D displays selective five fold fluorescence enhancement with fluoride ion (TBA salt) at 480 nm wavelength over other anions at a detection limit of 2.6 × 10−5 M. Selectivity may be ascribed to strategical incorporation of trifluoromethyl group in the scaffold of receptor D which modulated the binding ability of the receptor, so that it is able to bind fluoride ion exclusively and selectively in preorganized C3 symmetric cavity formed by its three arms. 1H NMR and 19F NMR titration validate C H⋯F- hydrogen binding interactions via downfield shift of C H peak at δ 8.47 ppm by δ 0.5 ppm and upfield shift of peak at δ −102 ppm due to free fluoride ion 2.98 ppm, respectively. Further, HRMS study of receptor-fluoride complex gave evidence for the formation of 1:1 stoichiometric ratio between receptor and fluoride ion. Computations using density functional theory have shown that in 1:1 complex of receptor and fluoride ion, the three arms are directed in same orientation forming a cavity where it fits fluoride ion by C H⋯F- hydrogen binding interactions.