Intense near-infrared luminescence of anhydrous lanthanide(III) iodides in an imidazolium ionic liquid Sven Arenz a , Arash Babai b , Koen Binnemans c, * , Kris Driesen c , Ralf Giernoth a, * , Anja-Verena Mudring b, * , Peter Nockemann c a Institut fu ¨ r Organische Chemie, Universita ¨t zu Ko ¨ ln, Greinstrasse 4, D-50939 Ko ¨ ln, Germany b Institut fu ¨ r Anorganische Chemie, Universita ¨t zu Ko ¨ ln, Greinstrasse 6, D-50939 Ko ¨ ln, Germany c Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium Received 19 October 2004; in final form 29 November 2004 Available online 19 December 2004 Abstract Anhydrous neodymium(III) iodide and erbium(III) iodide were dissolved in carefully dried batches of the ionic liquid 1-dodecyl- 3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C 12 mim][Tf 2 N]. Provided that the ionic liquid had a low water content, intense near-infrared emission could be observed for both the neodymium(III) ion and for the erbium(III) ion. Luminescence life- times have been measured, and the quantum yield of the neodymium(III) sample has been measured. Exposure of the hygroscopic samples to atmospheric moisture conditions caused a rapid decrease of the luminescence intensities. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Just like inorganic ionic compounds (for instance so- dium chloride), ionic liquids consist entirely of positive and negatively charged ions [1–3]. While the melting point of inorganic salts is typically several hundreds of degrees centigrade, the melting point of ionic liquids is by definition below 100 °C. Several types of ionic liquids are even at room temperature liquid (room temperature ionic liquids or RTILs). The cation of an ionic liquid is typically a large organic cation, such as imidazolium, pyridinium or a quaternary ammonium ion. As the an- ion, often Cl À ; Br À ; BF À 4 ; PF À 6 or CF 3 SO À 3 is used. The properties of ionic liquids (miscibility with water and other solvents, dissolving ability, polarity, viscosity, density, etc.) can be tuned by an appropriate choice of the anion and the cation. Thus, ionic liquids are often considered as Ôdesigner solventsÕ [2]. So far, most studies of ionic liquids have focused mainly on their use as Ôgreen solventsÕ for organic reactions including polymer- izations, as a catalyst, as an extraction solvent for the separation of metal ions or as an electrolyte in fuel cells. Only a few papers demonstrate the use of ionic liquids as a solvent for spectroscopic or photophysical studies [4–8]. For instance, the solvatochromism of selected dyes and the fine structure in the fluorescence spectra of polycyclic aromatic hydrocarbons have been used to probe the polarity of ionic liquids [9–12]. The solvent properties of ionic liquids can be designed in such a way that a weakly-coordinating solvent is gained. In this respect, ionic liquids become interesting solvents to investigate the spectroscopic behaviour of lanthanide complexes in solution, especially of complexes with weakly binding ligands, which otherwise would be un- able to compete with the solvent molecules for a binding site on the lanthanide ion [13–15]. In fact, ionic liquids 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.008 * Corresponding authors. Fax: +32 16 32 79 92 (K. Binnemans); +49 470 5102 (R. Giernoth); +49 470 5083 (A.-V. Mudring). E-mail addresses: koen.binnemans@chem.kuleuven.ac.be (K. Bin- nemans), ralf.giernoth@uni-koeln.de (R. Giernoth), a.mudring@ uni-koeln.de (A.-V. Mudring). www.elsevier.com/locate/cplett Chemical Physics Letters 402 (2005) 75–79