Luminescent lanthanide liquids

Low-melting-point salts of europium and dysprosium compounds are luminescent — and magnetic — liquids

Credit: © 2008 Wiley

Ionic liquids are salts with relatively low melting points (below 100 °C) and their useful and tunable properties — such as negligible vapour pressure and thermal stability — mean they are widely investigated for a variety of applications. Those incorporating metal ions offer the possibility of these properties acting alongside the metal's magnetic, optical or catalytic properties. Compounds of f-block elements have already been combined with ionic liquids to show promise as light-emitting diodes, but ionic liquids made directly from f-elements have so far always contained water, quenching the useful emissions.

Now, a team led by Anja-Verena Mudring from the Ruhr University in Bochum, Germany, has succeeded in making ionic liquids that contain europium¹ or dysprosium² but no water. These combine the luminescent properties of the lanthanide metals with the low melting points of ionic liquids. Mudring and colleagues prepared three europium-containing salts¹ where the metal is coordinated to varying numbers of fluorinated sulfonyl amide ligands. The emission spectra of the compounds are similar, but show differences between the solid and liquid forms. A suggested explanation is that the europium ion increases its coordination from nine oxygen atoms in the solid state to ten in the liquid state.

Working in collaboration with inorganic chemists at the Gutenberg University in Mainz, Mudring and co-workers have also made a series of dysprosium-containing ionic liquids². In addition to the luminescent properties shown by the europium-containing compounds, these examples have magnetic properties, showing a strong response to a neodymium magnet. The three compounds are superparamagnetic liquids at room temperature, and the superior magnetic moment of lanthanide ions over transition metal ions, such as iron, offers the prospect of enhanced magnetic liquids. Furthermore, the combination of properties could lead to applications with luminescent ionic liquids that can be controlled by external magnetic fields.