Europium-based ionic liquids as luminescent soft materials

Article abstract of DOI:10.1002/anie.200801159

Low melting, highly luminescent: [C₃mim] [Eu(Tf ₂N)₄] (1), [C₄mim][Eu(Tf₂N) ₄], and [C₄mpyr]₂[Eu(Tf₂N) ₅] are the first lanthanide ionic liquids that do not need stabilization of the liquid state by neutral coligands. They show excellent photophysical properties, such as long lifetimes of luminescence at large Eu^III concentration, small line width, and high color purity (see emission spectrum of 1 and photograph of a sample under UV light).

Full text of DOI:10.1002/anie.200801159

Angewandte
Chemie
DOI: 10.1002/anie.200801159
Ionic Liquids (1)
Europium-Based Ionic Liquids as Luminescent Soft Materials**
Sifu Tang, Arash Babai, and Anja-Verena Mudring*
As versatile materials, ionic liquids have received substantial
attention in recent years. Their inherent properties, such as
generally negligible vapor pressures, wide liquid ranges, good
thermal stabilities, considerable electric conductivities, and
wide electrochemical windows, are advantageous for a large
optical applications, water efficiently absorbs the energy of
the excited state through excitation of OꢀH vibrations,
leading to high radiationless decay rates. In the case of f-
element luminescence in the NIR region, this can result in the
[3d]
complete quenching of any emission. However, water-free
compounds could overcome this obstacle.
[
1]
number of applications. Many of these properties are
[
2]
related to the principles of “green”chemistry.
As salts,
Herein, we report an unprecedented series of low-melting
europium-containing ionic liquids that avoid the use of any
neutral coligand such as water and consequently can be
regarded as the first true f-element-based ionic liquids. They
have the general composition [R] [Eu(Tf N) ] (Tf N = bis-
ionic liquids are composed of distinct cations and anions,
which renders them tunable. They may be designed for
specific applications by incorporation of functionalities in one
or both ions as well as by the choice of cation–anion
combination.
x
2
3+x
2
(trifluoromethanesulfonyl)amide; x = 1 for R = 1-propyl-3-
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
f-element compounds in ionic liquids are promising novel soft
luminescent materials for use in photochemistry and spec-
methylimidazolium (C mim) and 1-butyl-3-methylimidazo-
3
lium (C mim); x = 2 for R = 1-butyl-1-methylpyrrolidinium
4
(C mpyr)). These compounds are synthesized by metathesis
4
reactions from stoichiometric amounts of europium(III)
bis(trifluoromethanesulfonyl)amide salts and the correspond-
ing Tf N-based ionic liquids. The melting points (onset of
2
melting) of crystalline material were determined by differ-
[3]
troscopy. Complex ionic f-element compounds may be
advantageous as active materials in organic light-emitting
ential scanning calorimetry (DSC) to be 81.08C for [C mim]
3
[Eu(Tf N) ] (1) and 67.98C for [C mim][Eu(Tf N) ] (2).
2
4
4
2
4
[4]
diodes (OLEDS). Special interest for f elements in ionic
liquids comes from actinide and lanthanide separation and
extraction as well as nuclear fuel reprocessing and waste
[C mpyr] [Eu(Tf N) ] (3) melts at 92.18C; however, before
4 2 2 5
melting to an isotropic liquid, an endothermic thermal event
is observed, which could be assigned to a phase transition at
73.98C. The lower melting point of 2 than 1 can be understood
[
5]
treatment. Furthermore, ionic liquids offer the possibility of
electrodeposition of the electropositive f elements them-
to originate from a combination of two effects. As C mim is
4
[
6]
selves. Thus, the investigation of f-element chemistry in
larger than C mim from the Kapustinskii equation, a lower
3
[
7]
[12]
ionic liquids is of relevance for various fields.
lattice energy can be estimated for 2. Furthermore, a C4
Examples of ionic liquids containing f elements are, to
side chain offers more conformational flexibility than a C
3
[
13]
date, Na [Ln(TiW O ) ]·xH O (x = 27–44) with f-element
chain, leading to higher packing frustrations.
The high
1
3
11 39
2
2
[
8]
polyoxometallates of the Keggin type and complex lantha-
nide thiocyanates [C mim] [Ln(NCS) (H O) ] (x = 6, 7, 8
for La), y = 0–2, x + y < 10). However, all these compounds
melting point of [C mpyr] [Eu(Tf N) ] (3) compared to the
4
2
2
5
C mim (n = 3,4) compounds 1 and 2 is expected, as pyrroli-
4
xꢀ3
9]
x
2
y
n
[
(
dinium salts generally show higher melting points than
imidazolium salts. Additionally, the higher charge of the
complex anion also contributes to a higher melting point.
However, all three compounds show a strong tendency to
form supercooled liquids. Compound 1 crystallizes from the
melt at about 118C, 2 at 108C, and 3 at 538C (for DSC traces
and further information, see the Supporting Information).
The crystallization of imidazolium salts is often inhibited
because of the high degree of charge delocalization and the
large geometrical aniosotropy of the substituted, nearly
contain water either as crystal water or even as a coligand.
They are similar to compounds like the Glauber salt
[10]
Na SO ·10H O that melts in its own crystal water at 328C
2
4
2
or NaOAc·3H O at 588C, which is used commercially as a
2
[
11]
latent heat storage material. Their liquid state is more like a
solution than a true melt. Unfortunately, when it comes to
[
*] Dr. S. Tang, Dr. A. Babai, 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
[14]
planar imidazolium cation.
A liquid of the composition
“
[C mim] [Eu(Tf N) ]”showed no tendency to crystallize at
4
2
2
5
all. Instead, a glass transition around ꢀ508C was found. By
applying careful cooling procedures, [C mim][Eu(Tf N) ]
4
2
4
[
**] Support from the DFG within the priority program “Lanthanoid
specific functionalities” SPP 1191 and from the Fonds der
Chemischen Industrie is gratefully acknowledged. A.V.M. thanks the
Fonds der Chemischen Industrie for a Chemiedozenten-Stipen-
dium.
crystallized from the melt, leaving [C mim][Tf N] behind. In
4
2
fact, a solution of 1 mol [C mim][Eu(Tf N) ] in 1 mol
4
2
4
[
C mim][Tf N] represents the eutectic in the phase diagram.
4 2
In contrast, 1–3 are congruently melting compounds.
Crystal structure analyses of compounds 1–3 reveal nine-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801159.
3
+
fold oxygen coordination for the Eu center, which can best
Angew. Chem. Int. Ed. 2008, 47, 7631 –7634
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7631

Communications
be described as
a
distorted tricapped trigonal prism
conformation. On the contrary, in 2, four of the five Tf N
2
(
2
Figure 1). The mean oxygen–europium separations of 244–
45 pm are similar to values found for typical organometallic
ligands adopt the cisoid conformation. For all compounds 1–3,
the SꢀO interatomic distances on the binding site of the
ligand are on average about 0.03 Š larger than those on the
non-coordinating site (1.43–1.47 vs. 1.42–1.44 Š). The SꢀN
interatomic distances (d (SꢀN) = 1.56–1.57 Š) and the S-
mean
N-S bonding angles (/mean(S-N-S) = 125–1278) are compara-
[
17]
ble to the values found for similar compounds. Further-
[
18]
more, they match the values found for the free ligand.
Raman spectroscopy has proven to be a valuable tool to
monitor the complexation of metal cations by the Tf N
2
[
19]
ligand. The n (SNS) bands of 1 and 2 are found at 754 and
52 cm , respectively. For uncomplexed Tf N in the ionic
s
ꢀ
1
ꢀ
7
2
3
+
liquids [C mim][Tf N] and [C mim][Tf N], the n (SNS) band
3 2 4 2 s
is found at much lower wavenumber (741 cm , see the
Figure 1. Local environment of Eu in a) [C mpyr] [Eu(Tf N) ] and
4
2
2
5
ꢀ
1
b) [C mim][Eu(Tf N) ]. CF groups are omitted for clarity. S yellow,
3
2
4
3
O red, N blue, Eu green.
Supporting Information). The n (SO ) bands should be even
s
2
more sensitive to complex formation, and indeed for 1 and 2,
ꢀ1
the bands are split into two peaks at 1115 and 1150 cm ,
whereas in the ionic liquids [C mim][Tf N] and [C mim]-
[
15]
complexes with the same coordination number. The oxygen
3
2
4
3
+
ꢀ1
donors surrounding the Eu cation in [C mpyr] [Eu(Tf N) ]
come from four bidentate bis(trifluoromethanesulfonyl)-
amide ligands (bonding mode shown in Scheme 1a) and one
[Tf N] the vibrations occur at 1113 and 1136 cm and 1115
2
4
2
2
5
ꢀ
1
and 1136 cm , respectively.
The excitation and emission spectra of 1–3 show the
3
+
monodentate ligand (Scheme 1c). The monodentate Tf N
characteristic transitions for Eu ions (Figure 2). Interest-
ingly, the emission spectra of compounds 1–3 look alike.
However, the spectra of solid samples are distinctly different
from those of the liquid ones. Whereas in the emission spectra
2
Scheme 1. Coordination modes of Tf N ligand in compounds 1–3.
2
ligand adopts a transoid conformation (relative orientation of
the CF groups with respect to the S-N-S plane), which is also
3
[
16]
the more stable conformation for the free Tf N anion. For
2
the bidentate ligands, both conformations are observed. One
of the four ligands adopts a cisoid conformation and another
one a transoid conformation, whilst the other two ligand
positions show a mixed occupation by ligands in cisoid and
transoid conformations. Whereas in [C mpyr] [Eu(Tf N) ] (3)
4
2
2
5
2
ꢀ
isolated [Eu(Tf N) ] units are present, for [C mim][Eu-
2
5
3
(
Tf N) ] (1) and [C mim][Eu(Tf N) ] (2), dimeric europium
2 4 4 2 4
2
ꢀ
units of the composition [Eu (Tf N) ] are formed. In 1 and
2
2
8
2
, one of the Tf N ligands coordinates not only in a bidentate
2
+
3
chelating fashion to one Eu center but also forms a bridge to
a neighboring Eu ion, where it coordinates in a mono-
3+
dentate fashion (Scheme 1b). Although 1 and 2 have very
Figure 2. Emission spectrum of [C mim][Eu(Tf N) ] in the a) solid and
b) metastable liquid state. Insets: Sample under UVradiation.
3
2
4
similar structures, the Tf N ligands adopt different conforma-
2
tions. In 1, only one Tf N ligand does not adopt a transoid
2
7632
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 7631 –7634

Angewandte
Chemie
5
7
of the solid samples all D ! F (J = 0–4) transitions are
quite high and seem not to be significantly influenced by the
0
J
5
7
visible, in the liquid state the D ! F transition is absent.
nature of the cation, regardless of the state of mater. The
lifetimes are astonishing, especially when the high europium
concentrations (100% Eu-doped) are taken into account. At
such concentrations, radiationless decay by concentration
quenching usually becomes important.
0
0
3
+
According to the crystal structure, in the solid state the Eu
center in 1–3 is surrounded by oxygen atoms in a slightly
distorted tricapped triangular prism of approximate D
3
h
5
symmetry. For ideal symmetry, it is expected that the D !
0
7
F transition would not be observed. However, in the solid,
With [C mim][Eu(Tf N) ], [C mim][Eu(Tf N) ], and
0
3
2
4
4
2
4
the transition occurs with weak intensity owing to deviations
from the ideal point symmetry. In the liquid, the transition
becomes even less intense, indicating a change of the
coordination environment. The most obvious difference
between the emission spectra measured for solid and liquid
[C mpyr] [Eu(Tf N) ] we present the first f-element ionic
4 2 2 5
liquids that do not need stabilization of the liquid state by
neutral coligands. Their crystallization is strongly inhibited
such that 1 and 2 form metastable liquids even below room
temperature. Compounds 1–3 illustrate how the cation of an
ionic liquid determines its structure and the overall compo-
sition. All compounds possess excellent photophysical prop-
5
7
samples is observed for the D ! F transition. In the liquid
0
4
state, the signal consists of two overlapping peaks around 690
and 698 nm. In the solid state, the peak splits into three
transitions at 687, 690, and 694 nm. One possible explanation
of this difference could be that a higher coordination number
of 10 is adopted in the liquid state. In the case of 3, this
situation could be easily achieved if the monodentate ligand
III
erties (high lifetimes at high Eu concentration, small
linewidth, and high color purity), which might render them
extremely valuable for various optical applications.
flips to the metal center and coordinates in a chelating mode. Experimental Section
For 1 and 2, one possibility is that the “free”oxygen atom in
Experimental details can be found in the Supporting Information.
3
+
the chelating bridging ligand binds to a neighboring Eu
Received: March 10, 2008
Revised: July 7, 2008
Published online: August 29, 2008
center, turning the ligand from tridentate to tetradentate. A
coordination number of 10 would be in agreement with X-ray
absorption fine structure (XAFS) studies of similar solutions
3
+
[20]
5
7
of Eu in Tf N-based ionic liquids. The D ! F transition
2
0
2
Keywords: europium · ionic liquids · lanthanoids ·
is the most intense for all samples; its lineshape is extremely
sharp for solid and liquid samples, and the intensity ratios of
.
luminescence · X-ray diffraction
5
7
5
7
the D ! F and D ! F transitions are high. Both facts lead
0
2
0
1
to a good color purity of the emission. The intensity ratios of
5
7
5
7
the D ! F and D ! F transitions in the emission spectra
0
2
0
1
[1] N. V. Plechkova, K. R. Seddon, Chem.Soc.Rev. 2008, 37, 123.
[2] P. T. Anastas, J. C. Warner, Green Chemistry: Theory and
Practice, Oxford University Press, New York, 1998.
3
+
can be taken as a measure of the asymmetry of the Eu
[
21]
site.
[
3] a) K. Driesen, P. Nockemann, K. Binnemans, Chem.Phys.Lett.
004, 395, 306; b) E. Guillet, D. Imbert, R. Scopelliti, J.-C.
It is generally accepted that the asymmetry parameter
becomes larger as the interaction of europium with its
2
Bünzli, Chem.Mater. 2004, 16, 4063; c) A. Babai, A.-V. Mudring,
Chem.Mater. 2005, 17, 6230; d) S. Arenz, A. Babai, K. Binne-
mans, K. Driesen, R. Giernoth, A.-V. Mudring, P. Nockemann,
Chem.Phys.Lett. 2005, 402, 75; e) P. Nockemann, E. Beurer, K.
Driesen, R. Van Deun, K Van Hecke, L. Van Meervelt, K.
Binnemans, Chem.Commun. 2005, 4345; f) A.-V. Mudring, A.
Babai, S. Arenz, R. Giernoth, K. Binnemans, K. Driesen, P.
Nockemann, J.Alloys Compd. 2006, 418, 204.
3
+
neighbors becomes stronger and the Eu site symmetry
becomes lower. The values of 9.94 and 6.56 for solid and
liquid 1 and 8.36 and 3.49 for solid and liquid 2 can be
3
+
compared to values found for Eu in different coordination
[
22]
environments and suggest a lower site symmetry and larger
europium–ligand interaction in the solid compounds than in
3
+
the liquids. For a hydrated Eu species, an even lower
intensity ratio and an asymmetry factor of approximately 0.63
[
4] J. Kido, Y. Okamoto, Chem.Rev. 2002, 102, 2357.
[
5] “Ionic liquid technologies for utilization in nuclear-based
separations”: K. E. Gutowski, N. Bridges, V. Cocalia, S. K.
Spear, A. E. Visser, J. D. Holbrey, J. H. Davis, R. D. Rogers, ACS
Symp.Ser. 2005, 902, 33 – 48 (Ionic Liquids IIIB: Fundamentals,
Progress, Challenges, and Opportunities).
[23]
would be expected. Water entering the coordination sphere
of the oxophilic lanthanide center has long been noted to have
a strong influence on the luminescent properties of lantha-
nides in ionic liquids. It typically leads to reduced lifetimes for
the excited state and eventually to total quenching of any
[
6] A. I. Bhatt, I. May, V. A. Volkovich, D. Collison, M. Helliwell,
I. B. Polovov, R. G. Lewin, Inorg.Chem. 2005, 44, 4934.
7] For a recent overview, see: K. Binnemans, Chem.Rev. 2007, 107,
[
24,3c,d]
emission.
For all samples, a monoexponential intensity
[
3
+
decay was detected, indicating that just one Eu species is
present. The Eu ( D ) lifetime of 1 for l
2592.
5
= 394, 613 nm in
0
ex,em
[8] L. Dai, S. Y. Yu, Y. K. Shan, M. Y. He, Eur.J.Inorg.Chem. 2004,
237.
[9] P. Nockemann, B. Thijs, N. Postelmans, K. Van Hecke, L. Van
the solid state is about 2.15 ms; in the (metastable) liquid state
it is 1.66 ms (for lex,em = 393, 611 nm). For 2 the respective
lifetimes are about 2.21 (for lex,em = 394, 613 nm) and 1.91 ms
Meervelt, K. Binnemans, J.Am.Chem.Soc.
2006, 128, 13658.
[
10] N. Wiberg, E. Wiberg, A. F. Holleman, Lehrbuch der Anorga-
nischen Chemie, deGruyter, 2007.
(
for lex,em = 393, 611 nm). The shorter lifetimes for the liquid
state can be well understood, as it is easier to excite vibrations
in the liquid than in the solid state. Lifetimes of 0.4 ms have
[
[
11] E. Imhof, K. Roth, G. Schmid, German patent DE3108760, 1982.
12] A. F. Kapustinskii, Q.Rev.Chem.Soc. 1956, 10, 283.
3
+
been reported for water-free solutions of Eu in Tf N ionic
2
[13] J. D. Holbrey, W. M. Reichert, M. Nieuwenhuyzen, S. Johnston,
[
25]
liquids. However, the lifetimes of 1–3 in the liquid state are
K. R. Seddon, R. D. Rogers, Chem.Commun. 2003, 1636.
Angew. Chem. Int. Ed. 2008, 47, 7631 –7634
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7633

Communications
[
14] T. Timofte, S. Pitula, A.-V. Mudring, Inorg.Chem. 2007, 46,
0938.
15] M. Kawamura, S. Shimada, Inorg.Chim.Acta 2007, 360, 2162.
16] A.-V. Mudring, A. Babai, S. Arenz, R. Giernoth, Angew.Chem.
[21] R. Reisfeld, Struct.Bonding (Berlin) 1973, 13, 53.
1
[22] For comparison: Compound (point symmetry) asymmetry
factor: Y O (C ) 9, YPF (C ) 7.8, YVO (D ) 6.2, LOCl (C )
[
[
2
3
2
3v
4
2d
4v
3.2. From: N. P. Efryushina, N. S. Poluektov, N. I. Smirdova,
E. A. Zhikhareva, J.Appl.Spectrosc. 1977, 26, 52.
[23] I. V. Barsukov, E. N. Kabaeva, V. V. Sytꢀko, S. D. Umreiko, J.
Appl.Spectrosc. 2003, 70, 470.
[24] I. Billard, S. Mekki, C. Gaillard, R. Hesemann, G. Moutiers, C.
Mariet, A. Labet, J. C. Bünzli, Eur.J.Inorg.Chem. 2004, 1190.
[25] R. Nagaishi, M. Arisaka, T. Kimura, Y. Kitatsuji, J.Alloys
Compd. 2007, 431, 221.
2005, 117, 5621; Angew.Chem.Int.Ed. 2005, 44, 5485.
[
[
17] A. Babai, A.-V. Mudring, Dalton Trans. 2006, 1826.
18] A. Haas, C. Klare, P. Betz, J. Bruckmann, C. Kruger, Y.-H. Tsay,
F. Aubke, Inorg.Chem. 1996, 35, 1918.
19] A. Babai, A.-V. Mudring, Inorg.Chem. 2006, 45, 3249.
20] C. Gaillard, I. Billard, A. Chaumont, S. Mekki, A. Ouadi, M. A.
Deneke, G. Moutiers, G. Wipff, Inorg.Chem. 2005, 44, 8355.
[
[
7634
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 7631 –7634