Ferrocene-based multichannel molecular chemosensors with high selectivity and sensitivity for Pb(ii) and Hg(ii) metal cations

Abstract

The synthesis, electrochemical, optical and cation-sensing properties of ferrocene-imidazoquinoxaline dyads 6, are presented. Dyad 6a behaves as a highly selective redox, chromogenic and fluorescent chemosensor molecule for Pb²⁺ cations in CH₃CN solutions; the oxidation redox peak is anodically shifted (ΔE_1/2 = 110 mV); in the absorption spectrum a new low-energy band appeared at λ = 463 nm, and the emission band is red-shifted (Δλ = 31 nm) along with an important chelation-enhanced fluorescence factor (CHEF = 276), upon complexation with this metal cation. The dyad 6b, bearing two additional pyridine rings as substituents, has shown its ability for sensing Hg²⁺ cations through three different channels: the oxidation peak is anodically higher shifted (ΔE_1/2 = 300 mV), a new low-energy band appears in the absorption spectrum at λ = 483 nm, and the emission band was also red-shifted (Δλ = 28 nm) and underwent an important chelation-enhanced fluorescent factor (CHEF = 227). The changes in their absorption spectra are accompanied by color changes from yellow to orange which allow their potential use for the “naked eye” detection of these metal cations. Linear sweep voltammetry revealed that Cu²⁺ cations induced oxidation of the ferrocene unit in both dyads, which is accompanied by an important increase of the emission band.