Substitution Effect on Near Infrared Absorbance Based Selective Fluoride Sensing of Indole Functionalized Thiourea Molecules

Abstract

A series of six new symmetrically functionalized thiourea based indole conjugated ligands (S1–S6) are synthesized in good yields and their anion recognition/sensing ability in the solution as well as in solid state is explored. Chemosensors S1–S6 show selective colorimetric sensing of fluoride by stable NIR signatures at 942 nm, 956 nm, 825 nm, 890 nm, 948 nm, and 1160 nm with corresponding spectral shifts (Δλ) of 592 nm, 588 nm, 603 nm, 485 nm, 610 nm, and 800 nm by virtue of position, conjugation and electronic effect of different functional groups attached to indole unit. To the best of our knowledge the peak shift of a chemosensor by magnitude of 800 nm (from UV to NIR region) in the presence of fluoride represents the highest of its kind. The changes in absorption spectra in the presence of various anions follow the Hofmeister series of anions. All the chemosensors show a bathochromic shift in the absorption spectra of the ligands in the presence of acetate, benzoate, dihydrogen phosphate and hydrogen pyrophosphate due to hydrogen bonding interactions with the indole and thiourea‐NH groups. The single‐crystal X‐ray structures evidence in favour of planar structures of the free sensors (S1, S4, and S5) and strong hydrogen bonding interactions between phosphate anion and indole–thiourea NH groups.