Dysprosium room-temperature ionic liquids with strong luminescence and response to magnetic fields

Article abstract of DOI:10.1002/anie.200802390

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

Full text of DOI:10.1002/anie.200802390

Angewandte
Chemie
DOI: 10.1002/anie.200802390
Ionic Liquids (2)
Dysprosium Room-Temperature Ionic Liquids with Strong
Luminescence and Response to Magnetic Fields**
Bert Mallick, Benjamin Balke, Claudia Felser, and Anja-Verena Mudring*
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 materi-
als 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 spec-
troscopy.[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 mB to 11 mB. 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 mag-
netic 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 cg of
40.6 ” 10ꢀ6 emugꢀ1 corresponding to a molar susceptibility
cmol of 0.0137(7) emumolꢀ1 was reported. The effective
magnetic moment of meff = 5.8 mB 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-piperidi-
nyloxyl-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 meff = 10.48 mB,
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]5ꢀx[Dy(SCN)8ꢀx(H2O)x]
(x = 0–2, C6mim = 1-hexyl-3-methylimidazolium) were syn-
thesized from [C6mim]SCN, KSCN, and Dy(ClO4)3·6H2O
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 com-
pounds [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
[*] Dipl.-Chem. B. Mallick, Prof. Dr. A.-V. Mudring
Anorganische Chemie I – Festkörperchemie und Materialien
Ruhr-Universität Bochum, 44780 Bochum (Germany)
Fax: (+49)234-32-14951
E-mail: anja.mudring@rub.de
Homepage: http://www.anjamudring.de
Dipl.-Chem. B. Balke, Prof. Dr. C. Felser
Anorganische Chemie, Gutenberg Universität Mainz
Staudinger Weg 9, 55128 Mainz (Germany)
[**] Support bythe DFG priorityprogram SPP 1166 “Lanthanoid
Specific Functionalities” and the Fonds der Chemischen Industrie
through a Dozentenstipendium for AVM is gratefullyacknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802390.
Figure 1. Response of the orange-colored ionic liquid 1 to a neo-
dymium magnet.
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Table 1: Effective magnetic moments, and gram and molar susceptibil-
ities for 1–3 at 298 K.
298 K are given in Table 1, and fit very well to the expected
value for dysprosium(III). Compounds 1–3 are (super)par-
amagnetic liquids at room temperature as can be seen from
the field dependency of the magnetic moment. Figure 2a
gives the respective plot for 2 as an example (for 1 and 3, see
the Supporting Information). No interaction between the
meff
[mB]
cg
cmol
[emumolꢀ1
]
[emugꢀ1
]
1
2
3
10.4
10.6
10.4
43”10ꢀ6
38”10ꢀ6
32”10ꢀ6
0.045
0.047
0.047
Figure 2. Field dependence of the mass susceptibility cg at a) 300 K and b) 5 K. Temperature dependence of c) the static molar susceptibilityand
d) the reciprocal molar susceptibilityat a field of 10000 Oe. Temperature dependence of e) the product of the temperature and the static molar
susceptibilityand f) the effective magnetic moment at a field of 10000 Oe for 2.
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Chemie
magnetic moments can be observed at this temperature. The
product of the static molar susceptibility and the temperature
(cmolT) and the effective magnetic moment (meff) are inde-
pendent, from room temperature down to about 150 K
(Figure 2e,f)) at which 1–3 are still liquids. Below 150 K, the
values start to drop, presumably owing to weak antiferro-
magnetic interaction in the solid state.
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.
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 Experimental Section
[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)3·6H2O 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 Han) in water (40 mL) and subse-
quent removal of the excess water under vacuum (72 h, 3 mbar).
Special caution should be taken when handling perchlorates. How-
ever, for the hydrates (!) the risk of explosion is minor.
[C6mim]5ꢀx[Dy(SCN)8ꢀx(H2O)x]; x = 0–2: Molar fractions of
[C6mim]SCN and KSCN (> 99%, AppliChem) and Dy(ClO4)3·6H2O
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 48C, 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.
Figure 3. Emission spectra with transition assignment of 1 (green), 2
(red), and 3 (black) at room temperature under excitation at
lex =453 nm.
6
4F9/2! H13/2 transition is the most intense for all the samples,
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.
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 ms. For 2 and 3, the respective lifetimes are 40.34 ms and
48.4 ms. In aqueous solutions, in which the luminescence of the
Received: May 22, 2008
Published online: August 29, 2008
ꢀ
photoexcited state is quenched predominantly by O H
vibrations of water molecules in the inner sphere of LnIII,
decay times between 9 ms and 11 ms are typically found; in
D2O, 43 ms to 139 ms.[15] In anhydrous ionic liquids, lifetimes up
to 63 ms 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 dysprosi-
um(III) ligand sphere compared to 3 with two H2O ligands,
Keywords: dysprosium · ionic liquids · lanthanides ·
luminescence · magnetic properties
.
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