Two electroactive ferrocenyl chalcones as original optical chemosensors for Ca2+ and Ba2+ cations in CH3CN

The optical study of ferrocenyl ligands 1–2 is presented, and reveals several interesting points. Contrary to their monosubstituted counterparts, these ligands exhibit fluorescence properties in acetonitrile. They can detect calcium, and also barium, by four different techniques: NMR, electrochemistry, UV–Vis absorption spectroscopy and fluorimetry in the same solvent. Each ligand detects both salts in the same manner by UV–Vis absorption and by fluorimetry. The response depends on the nature of the N terminal groups of the ligand. In each case, the ligand–calcium interaction is complex and involves 3–5 species in equilibrium in solution. Their association constants have been determined by fitting the UV–Vis data. Remarkably, for 2 and the calcium salts, nearly the same set of association constants can be used to fit not only the UV–Vis data obtained with calcium triflate (in a restricted range of concentration) or with calcium perchlorate, but also the NMR data obtained with calcium triflate. Interestingly, these results strengthen the fact that, in this family of compounds, the azacrown derivatives are less sensitive to high calcium triflate concentrations than their simple N-alkyl homologues. It is noteworthy that the complex non-monotonous fluorescence behaviour of compounds 1 and 2 upon Ca2+ or Ba2+ addition is quite original for ferrocenyl chalcones. These ligands constitute scarce examples of multi-signalling fluorescent ferrocenyl chemosensors for Ca2+ and Ba2+ cations in CH3CN.