Intense near-infrared luminescence of anhydrous lanthanide(III) iodides in an imidazolium ionic liquid

Article abstract of DOI:10.1016/j.cplett.2004.12.008

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, [C₁₂mim][Tf₂N]. 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 lifetimes 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.

Full text of DOI:10.1016/j.cplett.2004.12.008

Chemical Physics Letters 402 (2005) 75–79
www.elsevier.com/locate/cplett
Intense near-infrared luminescence of anhydrous lanthanide(III)
iodides in an imidazolium ionic liquid
a
b
c,
c
a,
*
Sven Arenz , Arash Babai , Koen Binnemans *, Kris Driesen , Ralf Giernoth
,
b,
c
Anja-Verena Mudring *, Peter Nockemann
a
Institut fu¨r Organische Chemie, Universita¨t zu Ko¨ln, Greinstrasse 4, D-50939 Ko¨ln, Germany
Institut fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln, Greinstrasse 6, D-50939 Ko¨ln, Germany
Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
b
c
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, [C12mim][Tf2N]. 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
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
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 Clꢀ; Brꢀ; BF4ꢀ; PF6ꢀ or CF3SOꢀ3 is used. The
properties of ionic liquids (miscibility with water and
other solvents, dissolving ability, polarity, viscosity,
*
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).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.12.008

76
S. Arenz et al. / Chemical Physics Letters 402 (2005) 75–79
can be excellent solvents to study the near-infrared lumi-
nescence of lanthanide complexes in solution, as they
not only dissolve organic, but also inorganic lanthanide
compounds. So far, only a limited number of papers de-
scribe the lanthanide-containing ionic liquids [13–22].
In this Letter, we describe the spectroscopic proper-
ties of lanthanide(III) iodide complexes (Ln = Nd, Er)
in an anhydrous imidazolium ionic liquid [C12mim]-
[Tf2N]. Special emphasis is paid to the near-infrared
luminescence shown by these complexes and to the
importance of using carefully dried ionic liquids.
cence decay curve. All photoluminescence spectra were
recorded at room temperature. The quantum yield of
the neodymium sample was determined using an inte-
grating sphere (150 mm diameter, BaSO4 coating) of
Edinburgh Instruments. The spectra were corrected for
variations in the output of the excitation source and
for variations in the detector response. The quantum
yield can be defined as: the integrated intensity of lumi-
nescence signal divided by the integrated intensity of the
absorption signal. Only the intense luminescence band
around 1054 nm measured by the integrating sphere,
but this intensity value was corrected by taking into ac-
count the relative intensity of the other transitions (as
determined from the steady-state luminescence spec-
trum). In this way, an intensity value that corresponds
to the total luminescence output was obtained. The
absorption intensity was calculated by subtracting the
integrated intensity of the light source (with the sample
in the integrating sphere) from the integrated intensity
of the light source (with a blanco sample in the integrat-
ing sphere). The blanco sample was a silica measuring
cell, filled with the pure ionic liquid [C12mim][Tf2N].
2. Experimental
All preparations were carried out under an argon
atmosphere using a glove-box and standard Schlenk-
techniques.
NdI3 and ErI3 were synthesized from the elements
(30 h/200 °C) in a silica tube and purified by sublimation
of the crude product at 800 °C under high vacuum
[23,24]. The ionic liquid 1-dodecyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide, [C12mim][Tf2N], was
synthesized following a literature procedure [25]. First,
[C12mim]Br was obtained by 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. One
equivalent of lithium bis(trifluoromethylsulfonyl)imide
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
addition of activated charcoal and filtration through
aluminium oxide. It was washed with small aliquots of
water until no halide residues could be detected in the
extract (AgNO3 test).
The ionic liquid was dried for 120 h in a Schlenk tube
at 150 °C under reduced pressure and rigorous stirring.
NdI3 (81.5 mg, 0.155 mmol) and ErI3 (81.4 mg,
0.148 mmol) were prepared into a silica tube (11 mm
in diameter) and [C12mim][Tf2N] (3.1198 g, 5.86 mmol)
was added. The tube was sealed off under dynamic
vacuum and put in an ultrasonic bath until dissolution
of the rare-earth salts was complete.
The steady-state luminescence spectra and the life-
time measurements were measured on an Edinburgh
Instruments FS920P near-infrared spectrometer, with a
450 W Xenon lamp as the steady-state excitation source,
a double excitation monochromator (1800 lines mmꢀ1),
an emission monochromator (600 lines mmꢀ1) and a
liquid nitrogen cooled Hamamatsu R5509-72 near
infrared photomultiplier tube. For the lifetime measure-
ments we combined the FS-920P spectrofluorimeter with
the 532 nm (frequency doubled) line of a Continuum
Minilite II Nd:YAG laser. The luminescence lifetime
has been determined by measurement of the lumines-
3. Results and discussion
For this Letter, we have synthesized neodymium(III)
iodide (NdI3) and erbium(III) iodide (ErI3), and dis-
solved these compounds in the ionic liquid 1-dodecyl-
3-methylimidazolium bis(trifluoromethylsulfonyl)imide,
[C12MIM][Tf2N]. We chose this ionic liquid because imi-
dazolium is one of the most commonly used cation for
ionic liquids, and because the use of a long side chain
(dodecyl) results in a good balance between high polar-
ity of the solvent and substantial lipophilicity at the
same time. Additionally, the bis(trifluoromethylsulfo-
nyl)imide anion results in very hydrophobic liquids with
relatively low viscosity, and it is known to be weakly
coordinating. In addition, lanthanide iodides were cho-
sen as they can be reliably obtained as strictly anhydrous
samples and iodide is the weakest coordinating ligand
among the halides. Efforts have been made to make
the ionic liquid as dry as possible, in order to avoid
water molecules that can quench the near-infrared
luminescence.
Excitation and emission spectra were recorded at
room temperature, and the luminescence decay times
were measured. Both the neodymium(III) and erbiu-
m(III) sample exhibit an intense photoluminescence.
The intensity of the near-infrared luminescence strongly
depends on the water content of the ionic liquid, in the
sense that luminescence could only be observed for neo-
dymium(III) iodide and erbium(III) iodide dissolved in a
carefully dried ionic liquid sample. In order to exclude
water from the sample, sample preparation had to be
done in a glovebox and the ionic liquid solution was

S. Arenz et al. / Chemical Physics Letters 402 (2005) 75–79
77
wavenumber / cm-1
sealed in a silica ampoule filled with argon gas. As soon
as the seal of the ampoule was broken and the sample
was exposed to atmospheric humidity, the luminescence
intensity decreased rapidly and substantially. At the
same time, the ionic liquid solution turned turbid,
although it turned clear again after addition of a drop
of water. Finally, no near-infrared luminescence could
be measured any longer. These results can be interpreted
by the fact that water enters the first coordination sphere
of the lanthanide ion, followed by quenching of the ex-
cited state of the lanthanide ion by vibronic coupling of
the lanthanide electronic states with the vibrational
states of the water molecules (multiphonon relaxation).
It is well known that water molecules can effectively
quench near-infrared luminescence of lanthanide ions.
The fact that trivalent lanthanide cations prefer water
in their first coordination sphere instead of iodide or
Tf2N anions can be explained by the HSAB-theory of
Pearson [26]. The trivalent lanthanide ion is a hard Le-
wis acid and it prefers to bind to the hard Lewis base
water, rather than to the soft Lewis base iodide. The
cloudiness of the solution was most probably due to
the formation of mixed iodide-hydroxy species or hy-
drated iodo complexes that subsequently dissolve in
the ionic liquid. Since the Tf2N ions are weakly coordi-
nating anions, they cannot compete successfully with
water molecules or even with iodo ligands for the lan-
thanide ion. The strong influence of water molecules
on the properties of lanthanide(III) ions dissolved in
ionic liquids can also be illustrated by recent literature
data. A recent theoretical study by Chaumont and Wipff
[20] shows that water molecules in ionic liquids contain-
ing europium(III) chloro complexes coordinate directly
to the europium(III) ion. In the absence of chloro li-
gands, an europium(III) aquo complex [Eu(H2O)9]3+ is
formed. Because the iodo ligands bind weaker to the
lanthanide(III) ion than chloro ligands, it can be ex-
pected that addition water molecules present in the ionic
liquid can easier replace iodo ligands than chloro ligands
from the first coordination sphere of the lanthanide ion.
An experimental spectroscopic study of europium(III)
triflate in the ionic liquid [BMIM][Tf2N] revealed that
in the presence of small amounts of water the Eu3+
ion is heavily hydrated and that the luminescence prop-
erties of Eu3+ are strongly influenced [13].
22500
21000
19500
18000
440
460
480
500
520
540
560
580
600
wavelength / nm
Fig. 1. Excitation spectrum of NdI3 in [C12MIM][Tf2N] at room
temperature. The luminescence has been monitored at 1054 nm.
cence spectrum, three transitions can be observed be-
4
4
tween 750 and 1450 nm (Fig. 2): F3/2 ! I9/2
4
4
4
4
(877 nm), F3/2 ! I11/2 (1054 nm) and F3/2 ! I13/2
4
(1330 nm). The most intense transition is the F3/2
!
4I11/2 transition. The luminescence decay curves were
measured and a single exponentional function could be
fitted to the data. The experimental luminescence life-
time was 15.3 ls. Due to the high intensity of a neodym-
ium emission, the luminescence quantum yield could be
determined. By averaging six measurements a value of
1.5 0.2% was obtained for the quantum yield. This
value is quite high for a neodymium(III) compound
wavenumber / cm-1
13000 12000 11000
10000
9000
8000
7000
4I11/2
The excitation spectrum of NdI3 in [C12MIM][Tf2N]
is shown in Fig. 1. To record the excitation spectrum,
4I9/2
4
4
the emission intensity of the F3/2 ! I11/2 transition at
1054 nm was monitored while the excitation wavelength
was continuously changed between 200 and 600 nm. In
the excitation spectrum, the 4f–4f transitions of the
Nd3+ ion are clearly visible. These transitions are intra-
configurational transitions within the 4f3 shell of the
Nd3+ ion. To record the photoluminescence spectrum,
4I13/2
800
900
1000
1100
1200
1300
1400
wavelength / nm
Fig. 2. Emission spectrum of NdI3 in [C12MIM][Tf2N] at room
temperature. The excitation wavelength was 570 nm. All transitions
4
direct excitation in the I9/2 4G5/2 absorption band
of Nd3+ at 570 nm was chosen. In the photolumines-
start from the F3/2 level.
4

78
S. Arenz et al. / Chemical Physics Letters 402 (2005) 75–79
wavenumber / cm-1
to obtain a very dry ionic liquid. Near-infrared lumines-
cence could only be observed for anhydrous samples,
and the luminescence intensity fell back to zero when
atmospheric moisture was allowed to enter the solution.
Ionic liquids provide a unique environment to study the
luminescence properties of complexes of lanthanide
ions, because ionic liquids are polar, but weakly coordi-
nating solvents.
6800 6700
6600
6500
6400
6300
6200
6100
Acknowledgements
K. Binnemans and K. Driesen thank the FWO-Flan-
ders (Belgium) for a postdoctoral fellowship. Financial
support by the F.W.O.-Flanders (G.0117.03), by the
K.U. Leuven (GOA 03/03) and by the EU (GROWTH
No. GRD2-2000-30346 OPAMD) is gratefully acknowl-
edged. A.-V. Mudring acknowledges the BMBF and
Fonds der Chemischen Industrie for support through a
Liebig-fellowship. A.-V. Mudring and R. Giernoth
thank the Deutsche Forschungsgemeinschaft for grants
(SPP 1166 – Lanthanoidspezifische Funktionalita¨ten),
Prof. Dr. G. Meyer and Prof. Dr. A. Berkessel for their
continuous support. R. Giernoth acknowledges the
Deutsche Forschungsgemeinschaft for financial support
to establish an Emmy Noether Research Group.
1450
1500
1550
1600
1650
wavelength / nm
Fig. 3. Emission spectrum of ErI3 in [C12MIM][Tf2N] at room
temperature. The excitation wavelength was 520 nm. The emission
4
4
band corresponds to the I13/2 ! I15/2 transition.
dissolved in an organic solvent. In fact, it is even higher
than the values obtained for many neodymium(III)
complexes in deuterated solvents [27,28].
Except for the differences in the positions and intensi-
ties of the 4f–4f transitions, the excitation spectrum of
ErI3 in [C12MIM][Tf2N] is similar to that observed for
NdI3. The 4f–4f transitions are intraconfigurational tran-
sitions within the 4f11 electronic configuration of the Er3+
ion. The luminescence spectrum was measured by excit-
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