Fluorescent Lower Rim 1,3-Dibenzooxadiazole Conjugate of Calix[4]arene in Selective Sensing of Fluoride in Solution and in Biological Cells Using Confocal Microscopy.

p- tert-Butyl-calix[4]arene was derivatized by integrating a benzooxadiazole fluorescent tag into its 1,3-arms at the lower rim to result in L and was characterized. L was titrated with 17 anions in THF and found selective for F- ions with lowest detection limit of 109 ppb. L and F- form a 1:1 complex. L self-assembles in THF to result in sheet like structures which converts into smaller spherical particles upon addition of F-. The site of interaction of F- was deduced based on 1H NMR spectroscopy and the coordination features by density functional theory (DFT) computations wherein six noncovalent interactions of the type X-H···F (where X = O, N, or C) were noticed. The sensing of F- is reversible when titrated with Ca2+, and the reversibility was demonstrated for 10 cycles without losing sensitivity. The study has been extended to the biological cells using fluorescence and confocal microscopy. While L shows strong fluorescence in HeLa cells, increasing concentrations of F- exhibited greater fluorescence quenching. Thus, L acts as a good sensor for F- in solution as well as in biological cells, a rare and unique combination for a calixarene conjugate to exhibit such sensing behavior in dual media.