The crystal structures of Imine-Enamine ‘tautomerism’ in solid state and “turn on” sensing for Zn2+: spectroscopy, DFT and cell imaging studies

The crystallographically established ligand 2-((E)-(benzyl imino)methyl)-4-bromophenol (HL) (existing keto at 296 K and enol at 120 K in the solid state), which is in equilibrium of interconversion (ΔE) = 4.39Kcal) with its tautomer, showed weak emission in the excited state, due to the fact of PET but in the presence of Zn²⁺, PET was blocked and chelation-enhanced fluorescence (CHEF) was set up. It exhibits 'turn on' sensor with the coordination of Zn²⁺. Among various metal ions, the synthesised probe HL selectively detected Zn²⁺ and LOD was about to ∼ 1.63 µM. The Zn(II) coordinated complex and the relative stabilities of the tautomers of HL were thoroughly calculated by spectroscopic and computational studies. Furthermore, cell imaging was also performed to validate the sensing property as well as cyto-toxicity of the probe.