Dysprosium Room‐Temperature Ionic Liquids with Strong Luminescence and Response to Magnetic Fields

Abstract

Attractive liquids: Dysprosium-based ionic liquids 1–3 show the highest response to external magnetic fields to date, allowing magnetic manipulation of the liquid (see pictures). At the same time, the liquids have excellent photophysical properties, such as long luminescence decay times τ and high color purity. The figure shows emission spectra of the ionic liquids. Ionic liquids have inherent properties, such as negligible vapor pressures in most cases, wide liquid ranges, good thermal stabilities, considerable electric conductivities, wide electrochemical windows, which have been shown to be advantageous for a large number of applications.1 As salts, ionic liquids are composed of distinct cations and anions, which makes them widely tuneable. They may be designed for specific applications through the choice of the respective cation and anion and the ion combination. Metal-containing ionic liquids are auspicious new materials which can favorably combine the properties of ionic liquids with magnetic, photophysical/optical or catalytic properties that originate from the metal incorporated in the complex anion. Recently, it has been shown that solutions of lanthanide compounds in ionic liquids are promising soft luminescent materials for use in photochemistry and spectroscopy.2 However, trivalent lanthanides are not only of interest as luminescent materials, but also as magnetic metal centers. The ions Gd–Tm in particular have high local magnetic moments ranging from 8 μB to 11 μB. The challenge we took is to synthesize room temperature liquids that combine the properties of a soft luminescent material with a ferrofluid. The term ferrofluid traditionally applies to fluids in which colloidal or nanoparticles of magnetic materials, for example iron oxides, are embedded in a carrier liquid.3 Usually these liquids are water or long chain hydrocarbons. However, recently we were able to show that ionic liquids also offer advantages as the liquid nanoparticle support.4 Recently a novel class of magnetic ionic liquids was discovered. In these single-component materials, the magnetic ion is no longer introduced as small particles, but through complex ions of metals which have a high magnetic moment. The first example is [C4mim][FeCl4] (C4mim=1-butyl-1-methylimidazolium). Although the compound has been known for a while,5 its magnetic properties and potential application have only recently been evaluated.6 For [C4mim][FeCl4], a magnetic mass susceptibility χg of 40.6×10−6 emu g−1 corresponding to a molar susceptibility χmol of 0.0137(7) emu mol−1 was reported. The effective magnetic moment of μeff=5.8 μB is in accordance with a high-spin S=5/2 state. Similar values are found for other iron(III)-based liquids.7 Owing to the high single-ion magnetic moments, these ionic liquids show a strong response to external magnetic fields. It has been shown that not only the liquid itself can be manipulated by a magnetic field, but also that nonmagnetic (diamagnetic) materials can be transported and separated according to their density and magnetic susceptibility in these liquids.8 In this context, purely organic magnetic ionic liquids based on radical ions, such as 2,2,6,6-tetramethyl-1-piperidinyloxyl-4-sulfate (TEMPO-OSO3), have recently become available.9 With respect to metal-based magnetic ionic liquids, to date only transition-metal-based liquids, mostly based on high-spin d5 iron(III) in the form of tetrachloro- and tetrabromoferrate(III) with various counter cations have been explored.10 Recently, other magnetic ionic liquids with other transition metal ions and gadolinium(III) were synthesized.11 The incorporation of lanthanide ions into ionic liquids offers the advantage of a metal ion that has a considerably higher effective magnetic moment than any known transition metal, and thus would give the best response to an external magnetic field. The effective magnetic moment of dysprosium(III) has been calculated to be μeff=10.48 μB, which is roughly twice the value of iron(III).12 Furthermore, the photophysical properties of dysprosium(III) would also make it possible to have a luminescent fluid. The new ionic liquids [C6mim]5−x[Dy(SCN)8−x(H2O)x] (x=0–2, C6mim=1-hexyl-3-methylimidazolium) were synthesized from [C6mim]SCN, KSCN, and Dy(ClO4)3⋅6 H2O according to a literature procedure for similar lanthanide-based ionic liquids.13 All the liquids show a strong response to conventional neodymium magnets (Figure 1). The compounds [C6mim]3[Dy(SCN)6(H2O)2] (1), [C6mim]4[Dy(SCN)7(H2O)] (2), and [C6mim]5[Dy(SCN)8] (3) all contain dysprosium(III) as the magnetically active ion with a 4f9 electron configuration. The effective magnetic moments at 298 K are given in Table 1, and fit very well to the expected value for dysprosium(III). Compounds 1–3 are (super)paramagnetic liquids at room temperature as can be seen from the field dependency of the magnetic moment. Figure 2 a gives the respective plot for 2 as an example (for 1 and 3, see the Supporting Information). No interaction between the magnetic moments can be observed at this temperature. The product of the static molar susceptibility and the temperature (χmolT) and the effective magnetic moment (μeff) are independent, from room temperature down to about 150 K (Figure 2 e,f)) at which 1–3 are still liquids. Below 150 K, the values start to drop, presumably owing to weak antiferromagnetic interaction in the solid state. Response of the orange-colored ionic liquid 1 to a neodymium magnet. Field dependence of the mass susceptibility χg at a) 300 K and b) 5 K. Temperature dependence of c) the static molar susceptibility and d) the reciprocal molar susceptibility at a field of 10 000 Oe. Temperature dependence of e) the product of the temperature and the static molar susceptibility and f) the effective magnetic moment at a field of 10 000 Oe for 2. μeff [μB] χg [emu g−1] χmol [emu mol−1] 1 10.4 43×10−6 0.045 2 10.6 38×10−6 0.047 3 10.4 32×10−6 0.047 The room-temperature ionic liquids 1–3 all have an intense yellow emission, which is characteristic for dysprosium(III). The excitation spectra (see the Supporting Information) and the emission spectra of 1–3 show the characteristic DyIII transitions (Figure 3).14 As expected, the 4F9/2→6H13/2 transition is the most intense for all the samples, and its lineshape is extremely sharp, indicating high color purity. For all the samples, a monoexponential intensity decay was detected, indicating that only one dysprosium(III) species is present. The Dy(4F9/2) lifetime of 1 at room temperature is 23.8 μs. For 2 and 3, the respective lifetimes are 40.34 μs and 48.4 μs. In aqueous solutions, in which the luminescence of the photoexcited state is quenched predominantly by OH vibrations of water molecules in the inner sphere of LnIII, decay times between 9 μs and 11 μs are typically found; in D2O, 43 μs to 139 μs.15 In anhydrous ionic liquids, lifetimes up to 63 μs have been observed.16 In this context the observed lifetimes of 1–3 are quite high. As expected, the highest decay time is found for anhydrous 1, for which the thiocyanate ligands are not prone to take up the energy of the excited state for ligand vibrations, and thus seem to provide a fairly rigid ligand environment. The compounds with water as co-ligand have lower lifetimes, also as expected. However, in 2, for which only one water molecule is present in the dysprosium(III) ligand sphere compared to 3 with two H2O ligands, excitation of OH vibrations is by far less likely, leading to a rather high luminescence decay time for 2. Emission spectra with transition assignment of 1 (green), 2 (red), and 3 (black) at room temperature under excitation at λex=453 nm. Compounds 1–3 are the first examples of room temperature ionic liquids that combine magnetic and luminescent properties. All three compounds show strong response to the magnetic field and are far superior to the known transition-metal ionic liquids because of the extremely high effective moment of dysprosium(III). Furthermore, 1–3 have excellent photophysical properties: namely, long luminescence decay times and high color purity. Thus, the compounds are not only of great interest from an academic viewpoint, but they might also be extremely valuable materials for various applications as an ionic liquid that can be manipulated by external magnetic fields and at the same time can be monitored by its luminescence. [C6mim]SCN was synthesized by reacting 35.5 g (0.144 mol) C6mimBr with 27.9 g (0.287 mol) KSCN in acetone (300 mL).17 Yield: 97 %. Elemental analysis (%) calcd for [C6mim]SCN: C 58.63, H 8.50, N 18.65, S 14.23; found: C 58.60, H 8.89, N 18.70, S 13.70. Dy(ClO4)3⋅6 H2O was obtained by dissolving Dy2O3 (4.0 g, 10.72 mmol; 99.9 %, ChemPur) in 60 % aqueous HClO4 (10.75 g, 7.02 mL, 64.2 mmol; Riedel-de Haën) in water (40 mL) and subsequent removal of the excess water under vacuum (72 h, 3 mbar). Special caution should be taken when handling perchlorates. However, for the hydrates (!) the risk of explosion is minor. [C6mim]5−x[Dy(SCN)8−x(H2O)x]; x=0–2: Molar fractions of [C6mim]SCN and KSCN (>99 %, AppliChem) and Dy(ClO4)3⋅6 H2O to give about 1 mmol of product were stirred in 20 mL dry ethanol for 24 h. During this time, the majority of KClO4 formed precipitates. To remove the remaining KClO4 after filtration, ethanol was removed under vacuum from the reaction mixture and the liquid residue was dissolved in dry dichloromethane (20 mL), left for 24 h at 4 °C, and then filtered to remove the remaining KClO4. The products are then dried for 48 h under vacuum. Finally, light orange liquids are obtained. Elemental analysis (%) calcd for [C6mim]3[Dy(SCN)6(H2O)2] (1): C 47.60, H 17.23, N 6.54; found: C 47.08, H 17.09, N 6.42; for [C6mim]4[Dy(SCN)7(H2O)] (2): C 44.94, H 16.73, N 6.26; found: C 44.63, H 16.69, N 6.20; for [C6mim]5[Dy(SCN)8] (3): C 41.23, H 5.86, N 16.03, found: C 40.07, H 5.44, N 15.52. The magnetic properties were investigated by a superconducting quantum interference device (SQUID, Quantum Design MPMS-XL-5) using approximately 40 mg to 60 mg of the sample. Vibrational Spectroscopy (MIR), thermal investigations, and further magnetic measurements are given in the Supporting Information. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z802390_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.