Strong luminescence of rare earth compounds in ionic liquids: Luminescent properties of lanthanide(III) iodides in the ionic liquid 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide

Article abstract of DOI:10.1016/j.jallcom.2005.10.069

Purposely designed ionic liquids can be excellent solvents for spectroscopic studies of rare earth compounds. Absorption, excitation and emission spectra of LnI₃ (Ln = Nd, Dy and Tb) in the ionic liquid 1-dodecyl-3-methylimidazolium bis(trifluo

Full text of DOI:10.1016/j.jallcom.2005.10.069

Journal of Alloys and Compounds 418 (2006) 204–208
Strong luminescence of rare earth compounds in ionic liquids:
Luminescent properties of lanthanide(III) iodides in
the ionic liquid 1-dodecyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide
a,∗
a
b
b
Anja-Verena Mudring , Arash Babai , Sven Arenz , Ralf Giernoth ,
c
c
c
K. Binnemans , Kris Driesen , Peter Nockemann
Universit a¨ t zu K o¨ ln, Institut f u¨ r Anorganische Chemie, Greinstrasse 6, D-50939 K o¨ ln, Germany
Universit a¨ t zu K o¨ ln, Institut f u¨ r Organische Chemie, Greinstrasse 4, D-50939 K o¨ ln, Germany
Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F, B-3001 Leuven, Belgium
a
b
c
Received 30 May 2005; received in revised form 9 August 2005; accepted 17 October 2005
Available online 27 December 2005
Abstract
Purposely designed ionic liquids can be excellent solvents for spectroscopic studies of rare earth compounds. Absorption, excitation and
emission spectra of LnI (Ln = Nd, Dy and Tb) in the ionic liquid 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide are presented.
3
Electronic transitions were assigned from the energy level diagrams for Ln(III). Emission lifetimes for DyI
Traces of water dramatically reduce the otherwise long lifetimes and comparatively high quantum yields.
3 2
in [C12mim][Tf N] are discussed.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Ionic liquids; Lanthanides; Luminescence; Rare earths
1
. Introduction
Ionic liquids typically consist of a large organic cation, such
as imidazolium, pyridinium or quaternary ammonium ions, and
−
−
−
Trivalent lanthanide ions show intense luminescence in the
anionic counter ions like halides or BF4 , PF6 , or CF3SO3 .
◦
visible and near-infrared regions with many favourable prop-
erties like narrow emission bands, long decay times, and a
large Stokes’ shift. Thus, lanthanide compounds have gained
widespread attention as potential materials for light-emitting
diodes [1–3], medical [4–6], and biological applications [7–10].
Unfortunately, coordination of polar, protic solvent molecules to
the luminescent lanthanide centres lead to a dramatic reduction
of luminescence lifetimes and quantum yields, because efficient
nonradiative depopulation of the excited state takes place via
vibronic coupling with the vibrational states of X–H oscillators
Byconvention, themeltingpointofionicliquidsisbelow100 C,
although many ionic liquids are even liquid at room temperature
(room temperature ionic liquids or RTILs) and below [12–14].
Appropriate choices of the anion/cation combination allow to
tune the properties of ionic liquids such as miscibility with water
and other solvents, dissolving ability, polarity, viscosity, density,
and among others. Ionic liquids are therefore often considered
as designer solvents.
The solvent properties of ionic liquids can be as designed in
such a way that a good solubility of metal-organic and inorganic
lanthanide compounds is obtained and, that, at the same time,
no C H, N H, and O H oscillators get introduced that may
quench the luminescence by vibronic coupling into the inner
coordination sphere of the lanthanide ion. Furthermore, the sol-
vent properties of ionic liquids can be tuned in such a way that a
weakly coordinating solvent is gained. In this respect, ionic liq-
uids become interesting solvents to investigate the spectroscopic
behaviour of lanthanide complexes in solution, especially of
(
X = C, N, O) [4]. We were recently able to show that replace-
mentofconventionalsolventsbycarefullydesignedionicliquids
leads to a dramatic increase of the lifetimes of the excited states
and quantum yields of emission [11].
∗
Corresponding author. Fax: +49 221 470 5083.
E-mail address: a.mudring@uni-koeln.de (A.-V. Mudring).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.10.069

A.-V. Mudring et al. / Journal of Alloys and Compounds 418 (2006) 204–208
205
complexes with weakly binding ligands, which otherwise would
be unable to compete with the solvent molecules for a binding
site on the lanthanide ion [15]. A prerequisite is that the ionic
liquid itself is optically transparent in the spectral region under
investigation.
Here we report on the optical properties of lanthanide(III)
iodides LnI3 (Ln = Nd, Tb and Dy) in [C12mim][Tf2N] (1-
dodecyl- 3-methylimidazolium-bis( trifluoromethanesulfonyl )-
imide).
is known to be weakly coordinating. Any complexes formed
in solution are expected to be of low symmetry (as shown for
[Yb(Tf2N)4]2 ) [15]. The low site symmetry of the complexes
promotes intense f–f transitions, as it is less likely in an asym-
metric environment that some of the odd crystal field parameters
(necessary for mixing states of opposite parity into the 4f states)
will have a very small value or will vanish [21]. Furthermore,
as iodine is also an only weakly coordinating ligand we chose
the lanthanide triiodides for our studies in order to come close
to the “free ion”.
−
2
. Experimental
3.1. Neodymium(III)
All preparations were carried out under an argon atmosphere using a glove-
box and standard Schlenk techniques.
To synthesize LnI3 (Ln = Nd, Dy, Tb), the respective amounts of the ele-
ments with 10% excess of iodine were placed in a silica tube which was then
sealed off under vacuum and subsequently heated for 30 h at 200 C. The crude
product was purified by sublimation under high vacuum at 800 C [16,17]. For
The solution of NdI3 in [C12mim][Tf2N] is transparent and of
light green color. Photoluminescence spectra and the determina-
tion of lifetime and quantum yield have been reported previously
11]. The absorption spectrum of NdI3 in [C12mim][Tf2N] is
shown in Fig. 1.
◦
◦
[
[
C12mim][Tf2N], [C12mim]Br was synthesized first via solvent-free alkylation
◦
of N-methylimidazole with 1-bromododecane at 80 C. The crude product was
recrystallized from acetonitrile/toluene, dissolved in water and heated to 70 C.
◦
In the absorption spectrum several intraconfigurational tran-
3
3+
One equivalent of lithium bis(trifluoromethanesulfonyl)-amide in water was
added dropwise and the mixture was stirred for another 24 h at room temperature.
The product, which formed a second phase to water, was purified by stirring with
activated charcoal for several days and subsequent filtration through aluminium
oxide. It was washed with small aliquots of water until no halide residues could
be detected in the extract (AgNO3 test) [18]. The ionic liquid [C12mim][Tf2N]
sitions within the 4f shell of the Nd ion are visible in the
−
1
region 11,000–20,000 cm (Fig. 1). The most intense tran-
4
4
sition is the I9/2 → G transition. This transition shows a
5/2
pronounced fine structure. Being a hypersensitive transition, the
form of the absorption is strongly dependent on the local sur-
rounding of the lanthanide ion [22,23]. Thus, by comparison
with spectra of similar compounds conclusions about the local
coordination of the rare earth cation in solution can be drawn.
Recently, Binnemans and co-workers reported absorption spec-
tra of NdBr3·6H2O in 1-n-hexyl-3-methylimidazliumbromide
[C6mim][Br]andNd(OTf)3 in1-ethyl-3-methylimidazoliumtri-
◦
was finally dried for 120 h in a Schlenk tube at 150 C under reduced pressure
and rigorous stirring.
LnI3 (∼0.15 mmol) was put in a silica tube (11 mm in diameter), and
[
C12mim][Tf2N] (∼6 mmol) was added. The tube was sealed off under dynamic
vacuum and put in an ultrasonic bath until the lanthanide(III) salt was completely
dissolved.
Absorption spectra were measured with a Cary 05E (Varian, Palo Alto,
USA) with a quartz-iodine radiation source (visible) and a deuterium lamp
flate [C mim][OTf] [24].
2
(
UV). Excitation and emission spectra were recorded at room temperature with
Comparison of the absorption spectrum of NdI3 in
a SPEX-Fluorolog DM3000F (Jobin Yvon GmbH, M u¨ nchen, D) with a xenon
lamp as the excitation source and a photomultiplier tube for detection. Electronic
transitions were assigned according to the energy level diagrams of trivalent rare
earths ions [19,20].
The steady state luminescence spectra and the lifetime measurements
for the dysprosium sample were recorded with an Edinburgh Instruments
FS920P near-infrared spectrometer, with a 450 W xenon lamp as the steady
[
C12mim][Tf2N] (Fig. 1) with the spectra of NdBr3/[C mim]Br
6
and Nd(OTf)3/[C2mim][OTf] (Fig. 2) allows us to draw
the conclusion that two neodymium species are present
3+
in solution—one presumably Nd coordinated by iodine
3−
3+
[
NdI6] , the other is assumed to be Nd
in an oxy-
−
gen surrounding, like [Nd(Tf2N)4] (Table 1). This assump-
tion is further backed by the fact that we were recently
able to isolate compounds like [bmpyr]4[PrI6][Tf2N] [25]
and [mppyr] [Yb(Tf N) ] (mppyr, 1-methyl-1-propylpyrrolid-
−
1
state excitation source, a double excitation monochromator (1800 lines mm ),
−1
an emission monochromator (600 lines mm ) and a liquid nitrogen cooled
Hamamatsu R5509-72 near-infrared photomultiplier tube. For the lifetime
measurements we combined the FS-920P spectrofluorimeter with the 532 nm
2
2
4
(
frequency doubled) line of a Continuum Minilite II Nd:YAG laser. The
inium) from the ionic liquid [mppyr][Tf2N] [15].
luminescence lifetime has been determined by measurement of the lumines-
cence decay curve. All photoluminescence spectra were recorded at room
temperature. The quantum yield of the dysprosium sample was determined
using an integrating sphere (150 mm diameter, BaSO4 coating) of Edinburgh
Instruments.
3
. Results and discussion
C12mim][Tf2N] has many favourable properties for the
[
study of optical properties of trivalent lanthanide cations in the
liquid state. We chose this solvent in order to prevent any water
from entering the coordination sphere of the lanthanide cation
as water leads to a rapid radiationless quenching of the lumines-
cence. Both the cation [C12mim], with its dodecyl side chain,
and the perfluorated anion [Tf2N], are strongly hydrophobic.
At the same time, the bis(trifluoromethanesulfonyl)imide anion
Fig. 1. Absorption spectrum of NdI3 dissolved in [C12mim][Tf2N]. All the tran-
4
3+
sitions start from the I9/2 ground state of Nd
.

2
06
A.-V. Mudring et al. / Journal of Alloys and Compounds 418 (2006) 204–208
Fig. 4. Excitation spectrum of DyI3 in [C12mim][Tf2N] at room temperature.
6
All transitions start at the H15/2 level.
Fig. 2. Absorption spectra of NdBr3 and Nd(OTf)3 in different ionic liquids (cf.
Ref. [24]).
The luminescence decay curves were measured and a sin-
gle exponential function is fitted to the data. The experimen-
tal luminescence liftetime for Dy3 was determined to be
Table 1
4
4
3+
Absorption maxima for the I9/2 → G5/2 transition of Nd in different local
+
surroundings
3+
6
3 ␮s. This value is quite high—even for Dy . In aqueous
solutions, typically values between 9 ␮s and 11 ␮s are found
depending on the counterion) [26–29], and even in D2O the
NdBr3/[C6mim]Br
nm) [24]
Nd(OTf)3/[C2mim][OTf]
(nm) [24]
NdI3/[C12mim][Tf2N]
(nm)
(
(
5
5
5
6
82
92
99, 601
10
586
582
595
603.5
612
decay times are no longer than 43–139 ␮s. In aqueous solu-
tions, the luminescence of photo-excited trivalent lanthanide
cations is quenched dominantly by O H vibrations of water
molecules in the inner sphere of Ln(III). From this, it can be
concluded that the anions present in solution, namely iodide
and bis(trifluoromethanesulfonyl)imide prevent non-radiative
energy transfer processes. As both are only weakly coordinat-
ing ligands, any water enters the inner coordination sphere of
the lanthanide ions and leads to an immediate quenching of the
luminescence.
3
.2. Dysprosium(III)
DyI3 dissolves in [C12mim][Tf2N] under the formation of a
pale yellow solution.
Several transition bands are observed of which all except
one can be assigned to the solvent [C12mim][Tf2N]. The tran-
−
1
sition at 7710 cm
corresponds to the intraconfigurational
3.3. Terbium
6
6
6
3+
H15/2 → F11/2, H9/2 transition of Dy (Fig. 3).
The excitation spectrum of DyI3 in [C12mim][Tf2N] (Fig. 4)
Solutions of TbI in [C mim][Tf N] are of a yellowish color.
3
12
2
6
shows transitions all starting from the H
ground level of
5/2
To record the excitation spectrum, the emission intensity of
1
3
+
5
7
−1
Dy .
The emission spectrum DyI3 in [C12mim][Tf2N] (Fig. 5)
the D → F transition at 18,250 cm was monitored while
4
6
the excitation wavelength was continuously changed between
400 nm and 240 nm (Fig. 6).
3
+
−1
shows the characteristic emissions of Dy at 17,450 cm and
−
1
4
6
2
0,900 cm due to transitions from the F9/2 level to the HJ
levels.
Fig. 3. NIR absorption spectrum of DyI3 in [C12mim][Tf2N] at room tempera-
Fig. 5. Emission spectrum of DyI3 in [C12mim][Tf2N] at room temperature
(excitation into the I15/2 level).
−1
6
3+
4
ture. The transition at 7710 cm starts from the H15/2 level of Dy
.

A.-V. Mudring et al. / Journal of Alloys and Compounds 418 (2006) 204–208
207
Acknowledgements
A.-V. Mudring acknowledges the BMBF and the Fonds der
Chemischen Industrie for support through a Liebig-fellowship.
A.-V. Mudring and R. Giernoth thank the Deutsche Forschungs-
gemeinschaft for grants (SPP 1166—Lanthanoidspezifische
Funktionalit a¨ ten) and Prof. Dr. G. Meyer and Prof. Dr. A.
Berkessel for their continuous support. R. Giernoth acknowl-
edges the Deutsche Forschungsgemeinschaft for financial sup-
port through the Emmy Noether programme. K. Binnemans
and K. Driesen thank the FWO-Flanders (Belgium) for a post-
doctoral fellowship. Financial support by the FWO-Flanders
Fig. 6. Excitation spectrum of TbI3 in [C12mim][Tf2N] at room temperature.
(G.0117.03), by the K.U. Leuven (GOA 03/03) and by the
−1
The luminescence has been monitored at 18,250 cm
.
EU (GROWTH No.: GRD2-2000-30346 OPAMD) is gratefully
acknowledged.
References
[
[
1] R.J. Curry, W.P. Gillin, Curr. Opin. Solid State Mater. Sci. 5 (2001)
81–486.
2] S. Capecchi, O. Renault, D.-G. Mon, M. Halim, M. Etchells, R.J.
Dobson, O.V. Salata, V. Christou, Adv. Mater. 12 (2000) 1591–
4
1594.
[
[
[
3] J. Kido, Y. Okamoto, Chem. Rev. 102 (2002) 2357–2368.
4] L. Thunus, R. Lejeune, Coord. Chem. Rev. 184 (1999) 125–155.
5] K. Wang, R. Li, Y. Chen, B. Zhu, Coord. Chem. Rev. 190–192 (1999)
297–308.
[
[
6] D. Parker, Coord. Chem. Rev. 205 (2000) 109–130.
7] P. Caravan, J.J. Ellison, T.J. McMurry, R.B. Lauffer, Chem. Rev. 99
(
1999) 2293–2352.
Fig. 7. Emission spectrum of TbI3 in [C12mim][Tf2N] at room temperature.
[
[
8] V.W.W. Yam, K.K.W. Lo, Coord. Chem. Rev. 184 (1999) 157–240.
9] H. Tsukube, S. Shinoda, J. Tamiaki, Coord. Chem. Rev. 226 (2002)
−1
The excitation wavelength was 390 nm (25,641 cm ). All transitions start from
5
the D4 level.
2
27–234.
10] N. Sabbatini, M. Guardigli, J.-M. Lehn, Coord. Chem. Rev. 123 (1993)
01–228.
[
[
2
To record the photoluminescence spectrum, excitation in the
broad absorption band of at 390 nm was chosen. In the photo-
luminescence spectrum, two transition bands can be observed
11] S. Arenz, A. Babai, K. Binnemans, K. Driesen, R. Giernoth, A.-V.
Mudring, P. Nockemann, Chem. Phys. Lett. 402 (2005) 75–79.
[12] (a) P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. Engl. 39 (2000)
772–3789;
−
1
−1
5
7
3
between 22,000 cm and 17,500 cm (Fig. 7): D4 → F
5
−
1
5
7
−1
(b) P. Wasserscheid, W. Keim, Angew. Chem. 102 (2000) 3926–3945.
13] Ed.P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis,
VCH–Wiley, Weinheim, 2003.
14] T. Welton, Chem. Rev. 99 (1999) 2071–2083.
[15] (a) A.-V. Mudring, A. Babai, S. Arenz, R. Giernoth, Angew. Chem. 117
(
18,250 cm ) and D4 → F (20,250 cm ). The most intense
6
[
5 7
transition is the D4 → F transition. A strong intensity of
5
5
5
the D4 emission over the D3 emission might be attributed to
cross-relaxation between two neighbouring Tb3+ ions [30] as a
saturated solution of TbI3 in [C12mim][Tf2N] was investigated.
[
(
(
2005) 5621–5624;
b) A.-V. Mudring, A. Babai, S. Arenz, R. Giernoth, Angew. Chem. Int.
Ed. 44 (2005) 5485–5488.
[
[
16] J.D. Corbett, Inorg. Synth. 22 (1983) 31–36.
17] G. Meyer, in: G. Meyer, L.R. Morss (Eds.), Synthesis of Lanthanide
Actinide Compounds, Kluwer Acad. Publ., Dordrecht/NL, 1991, pp.
4
. Conclusion
This work shows that the ionic liquid 1-dodecyl-3-
135–144.
methylimidazolium bis(trifluoromethanesulfonyl)imide is an
excellent medium to study luminescent properties of trivalent
rare earth cations in solution. Most lanthanide cations show tran-
sitions in this ionic liquid that are most of the times quenched
in solution. Furthermore lifetimes of excited states seem to be
generally higher in this medium compared to those observed in
conventional solvents. These excellent luminescent properties
seem to be due to the absence of any C H, N H, O H bonds in
the immediate neighbourhood of the rare earth cation. In conse-
quence, radiationless decay becomes less likely when compared
to typical solvents.
[
[
[
18] R. Giernoth, M.S. Krumm, Adv. Synth. Catal. 346 (2004) 989–992 (and
references cited therein).
19] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals,
Interscience Publishers, New York, 1968.
20] W.T. Carnall, H.M. Crosswhite, H. Crosswhite, Energy level structure
and transition propabilities in the spectra of trivalent lanthanides in
LaF3, Special Report, Chemistry Division, Argonne National Labora-
tory, Argone, IL, 1977.
[
[
21] C. G o¨ ller-Walrand, K. Binnemans, in: K.A. Gschneidner Jr., L. Eyring
(Eds.), Handbook on the Physics Chemistry of Rare Earths, vol. 25,
North-Hollond, Amsterdam, 1998, p. 101 (Chapter 167).
22] H.C. Aspinall, Chemistry of the f-Block Elements, Taylor & Francis
Group, London, 2003.

2
08
A.-V. Mudring et al. / Journal of Alloys and Compounds 418 (2006) 204–208
[
[
23] J.-C. B u¨ nzli, G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical
and Earth Science, Elsevier, Amsterdam, 1989.
24] K. Driesen, P. Nockemann, K. Binnemans, Chem. Phys. Lett. 395 (2004)
[27] T. Berthoud, P. Decambox, B. Kirsch, P. Mauchien, C. Moulin, Anal.
Chim. Acta 220 (1989) 235.
[28] T. Kimura, R. Nagaishi, Y. Kato, T. Yoshida, Radiochim. Acta 89 (2001)
125.
3
06–310.
[
[
25] A. Babai, A.-V. Mudring, Chem. Mat. 17 (2005) 6230–6238.
26] S.P. Shina (Ed.), Systematics and the Properties of the Lanthanides, D.
Reidel Publishing Company, Dordrecht, pp. 451–499 (Chapter 10).
[29] R. Nagaishi, T. Kimura, S.P. Sinha, Mol. Phys. 101 (2003) 1007–10014.
[30] D. de Graff, S.J. Stelwagen, H.T. Hintzen, G. De With, J. Non-Cryst.
Solids 325 (2003) 29–33.