An Insight into the complexation of pyrazine-functionalized calix[4]arenes with Am³⁺ and Eu³⁺ - Solvent extraction and luminescence studies in room-temperature ionic liquids

The complexation behaviour of six pyrazine-functionalized calix[4]arenes, containing substituents such as carbamoyl dioctyl (L-1, L-2), diisopropyl phosphonate (L-3, L-4) and diphenyl phosphoryl (L-5, L-6), with Am³⁺ and Eu³⁺ ions was investigated by solvent extraction as well as by luminescence spectroscopy (only for Eu³⁺) in room-temperature ionic liquids (RTILs), [C_nmim][NTf₂] (1-alkyl-3-methylimidazolium, n = 4, 6, and 8). The spectacular enhancement in the extraction values of the trivalent metal ions in RTILs, as compared to those reported in a molecular diluent from 3 M HNO₃, was rather unusual and was further investigated by luminescence studies. Luminescence studies involving Eu³⁺ and performed with L-4 and L-6 confirmed the 1:1 stoichiometry of the extracted species (Eu³⁺ L) with log ß values of 4.42'0.51 and 5.18'0.38 for L-4 and L-6, respectively. Judd-Ofelt parameters, which give information about the nature of bonding as well as the coordinating environment, were computed from the emission spectra of the Eu³⁺ L-4 and Eu³⁺L-6 complexes.