Rare-earth elements, or lanthanides, are uniquely suited for use in a wide range of applications including magnetic devices, catalysts, and medicine. The increasing demand for lanthanides motivates the development of new molecular species capable of efficient and selective separations of these elements. While the predominant method for lanthanide separation is based on solvent extraction, missing mechanistic information regarding what governs its selectivity on the molecular scale limits the rational design of extractants with high efficiency and chemical selectivity. In the present study, we employ two-dimensional infrared spectroscopy to capture the ultrafast dynamics evolving on femtosecond timescales in lanthanide ion-ligand complexes that are ultimately responsible for stable complexation. Spectral diffusion dynamics demonstrate that ion-loading into a flexible pseudo-macrocycle ligand substantially reduces the inhomogeneity of the local environment around the extractants. This observation, combined with strong coupling between coextracted water molecules, suggests that efficient funneling of excited energy through coupled vibrational modes away from binding sites can be used to enable reverse size-selective lanthanide separations. These results help understand the molecular motions, interactions, and energy dissipation channels available to stabilize extractant-ion complexes that mediate extraction. Our work provides insight into the specific interactions between extractants and lanthanide ions as well as their hydration shells that define selectivity on the molecular level, and has implications for the design of new extractants for rare-earth separations.

Ultrafast dynamics in lanthanide ion-ligand complexes revealed by two-dimensional infrared spectroscopy: Mechanisms of rare-earth chemical recognition in separations

ACS Spring 2022

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