Differences of Eu(iii) and Cm(iii) chemistry in ionic liquids: investigations by TRLFS

Abstract

In this study the coordination structure and chemistry of Eu(III) and Cm(III) in the ionic liquid C₄mimTf₂N (1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide) was investigated by time-resolved laser fluorescence spectroscopy (TRLFS). The dissolution of 1 × 10⁻² M Eu(CF₃SO₃)₃ and 1 × 10⁻⁷ M Cm(ClO₄)₃ in C₄mimTf₂N leads to the formation of two species for each cation with fluorescence emission lifetimes of 2.5 ± 0.2 ms and 1.0 ± 0.3 ms for the Eu-species and 1.0 ± 0.3 ms and 300.0 ± 50 µs for the Cm-species. The interpretation of the TRLFS data indicates a comparable coordination for both the lanthanide and actinide cation in this ionic liquid. The quenching influence of Cu(II) on the fluorescence emission of Eu(III) and Cm(III) was also measured by TRLFS. While Cu(II) does not quench the Cm(III) fluorescence emission in C₄mimTf₂N the Eu(III) fluorescence emission lifetime for both Eu-species in C₄Tf₂N decreases with increasing Cu(II) concentration. Stern–Volmer constants were calculated (k_SV = 1.54 × 10⁶ M⁻¹ s⁻¹ and k_SV = 2.70 × 10⁶ M⁻¹). By contrast, the interaction of Cu(II) with Eu(III) and Cm(III) in water leads to a quenching of both the lanthanide and actinide fluorescence. The calculated Stern–Volmer constants are 1.20 × 10⁴ M⁻¹ s⁻¹ for Eu(III) and 1.27 × 10⁴ M⁻¹ s⁻¹ for Cm(III). The investigations show, while the chemistry of trivalent lanthanides and actinides is similar in an aqueous system it is dramatically different in ionic liquids. This difference in chemical behavior may provide the opportunity for a separation of lanthanides and actinides with regard to the reprocessing of nuclear fuel.