Imidazole-Annelated Ferrocene Derivatives as Highly Selective and Sensitive Multichannel Chemical Probes for Pb(II) Cations

Abstract

New ferrocenyl-containing imidazopyridine and imidazophenazine receptors 2−5 show high selective affinity for Pb(II) ions over a range of other metal ions examined through different channels. Imidazopyridine−ferrocene dyad 2 behaves as a highly selective redox, chromogenic, and fluorescent chemosensor molecule for Pb2+ cations: the oxidation redox peak is anodically shifted (ΔE1/2 = 150 mV), and the low energy band of the absorption spectrum is red-shifted (Δλ = 44 nm) upon complexation with this metal cation. This change in the absorption spectrum is accompanied by a color change from colorless to orange, which allows the potential for “naked eye” detection. The emission spectrum undergoes an important chelation-enhanced fluorescence (CHEF) effect (CHEF = 620), with an unprecedented detection limit of 2.7 μg L−1. The presence of Zn2+ cations also induced a perturbation of the redox potential, absorption, and emission spectra although in less extension than those found with Pb2+ cations. Imidazophenazine−ferrocene dyad 3 has also shown its ability for sensing Pb2+ cations through redox (ΔE1/2 = 120 mV), absorption (Δλ = 23 nm), and emission (CHEF = 133) channels, whereas the presence of Zn2+ only has a little effect on the emission spectrum (CHEF = 74). The electrochemical changes observed in the two-armed ferrocenes 4 and 5 upon complexation show that the potential shift is higher for Zn2+ ions (ΔE1/2 = 190−170 mV) than for Pb2+ ions (ΔE1/2 = 180−110 mV), which is in clear contrast to those observed for the monoarmed ferrocenes 2 and 3. The recognition properties of the two-armed imidazopyridine−ferrocene triad 4 are quite similar to those exhibited by the parent monosubstituted receptor 2, and the most salient features are a strong perturbation of the redox wave (ΔE1/2 = 180 mV), a dramatic increasing of the fluorescent quantum yield (Φcomplex/Φligand = 890) in the presence of Pb2+, while the optical responses toward Zn2+ cations were silent. The two-armed imidazophenazine−ferrocene triad 5 senses Pb2+ cations through perturbation of the oxidation potential of the Fe(II)/Fe(III) redox couple (ΔE1/2 = 110 mV), important blue shift (Δλ = 160 nm) of the high energy band in the absorption spectrum, and a remarkable increase of the emission band (CHEF = 220), whereas smaller changes were observed in the presence of Zn2+ cations. 1H NMR studies as well as DFT calculations have been carried out to get information about which molecular sites are involved in the binding event.