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

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 (Delta E-1/2 = 110 mV); in the absorption spectrum a new low-energy band appeared at lambda = 463 nm, and the emission band is red-shifted (Delta lambda = 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 (Delta E-1/2 = 300 mV), a new low-energy band appears in the absorption spectrum at lambda = 483 nm, and the emission band was also red-shifted (Delta lambda = 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.