Design and synthesis of new salicylhydrazone tagged indole derivative for fluorometric sensing of Zn2+ ion and colorimetric sensing of F ion: Applications in live cell imaging

A novel, bifunctional, colorimetric, highly selective and sensitive "turn-on" fluorescent probe, 2-((E)-(((E)-(5-bromo-1H-indol-3-yl) methylene) hydrazono) methyl) phenol (IHP) has been designed and synthesized for the detection of Zn2+/F ions in DMSO: H2O (9:1, v/v) medium. The probe IHP exposed an excellent fluorescence enhancement towards Zn2+ ion owing to the inhibition of ESIPT with concurrent enhancement of LMCT operated on IHP. Upon addition of F ion, the probe IHP displayed an obvious colorimetric change and it can be simply observed by naked eye. Further, 1:1 binding stoichiometric ratio of probe with Zn2+/F- ions was confirmed via Job's plot and highly supported by H-1 NMR titration and F-19 NMR analysis for F- ion. The plausible mechanistic pathway and the fluorescence enhancement of IHP with Zn2+ ions were highly gauged by density functional theoretical (DFT) calculations. Moreover, the probe IHP has been successfully employed for monitoring Zn2+ ion in HeLa cells and live cell imaging analysis in Zebrafish embryos.