Metal-coordination driven intramolecular twisting: a turn-on fluorescent-redox probe for Hg2+ ions through the interaction of ferrocene nonbonding orbitals and dibenzylidenehydrazine

Abstract

A unique C2 symmetric azine bridged bi-ferrocenyl receptor (4) has been modelled and synthesized. In this work, we are able to synthetically regulate conjugation of the dibenzylidenehydrazine fluorophore unit to unexpectedly reveal metal-coordination driven intramolecular twisting. The present probe shows a dramatic turn-on fluorescence response with 91 fold increment of quantum yield along with 17 nm blue shift upon binding with Hg²⁺ ions selectively with a limit of detection as low as 15 nM. Upon Hg²⁺ recognition, the ferrocene/ferrocinium redox peak was anodically shifted by ΔE_1/2 = 78 mV, indicating the formation of a new complex species. A plausible binding mode of Hg²⁺ ions with compound 4 has been proposed based on ¹H NMR titration, a high-resolution mass spectrometry (HRMS) study and a density functional theory (DFT) study along with the Job's plot analysis. Interestingly, DFT calculations have revealed that the reason for fluorescence enhancement after coordination to Hg²⁺ ions is not due to conventional restricted CN isomerization or interrupted N–N single bond rotation rather it is due to the increase of the π ← π* transition at the expense of the n → π* (aromatic) transition of the free ligand. Furthermore, TD-DFT calculations of the first excited singlet state of 4 and [4·Hg²⁺] revealed the involvement of the aromatic π electrons with the vacant site of Hg²⁺ ions which may be further attributed to the fluorescence enhancement phenomenon. In addition, receptor 4 was successfully applied for the detection of Hg²⁺ ions in real samples.