Switchable green and white luminescence in terbium-based ionic liquid crystals

Article abstract of DOI:10.1002/ejic.201100168

[C₁₂mim]Br (C₁₂mim = 1-dodecyl-3-methyl) and [C ₁₂mpyr]Br (C₁₂mpyr = N-dodecyl-N-methylpyrrolidinium), both doped with TbBr₃, as well as the neat compounds [C ₁₂mim]₃[TbBr₆] and [C₁₂mpyr] ₃[TbBr₆] show mesomorphic behavior and are able to form smectic liquid crystalline phases. The thermal phase behavior of all compounds was investigated by hot-stage polarizing optical microscopy and differential scanning calorimetry. Whilst the doped compounds crystallize around room temperature, the neat compounds solidify as glasses around-5 °C and qualify also as ionic liquids. The crystal structure of the acetonitrile solvate of the imidazolium compound, [C₁₂mim]₃[TbBr₆] ·2CH₃CN, could be determined by single X-ray diffraction. The structure features alternating double layers of [C₁₂mim]⁺ cations and [TbBr₆]^3- octahedra with hydrogen bonded acetonitrile. [C₁₂mim]₃[TbBr₆]·2CH ₃CN can serve as a structure model for the smectic mesophase of the solvate-free [C₁₂mim]₃[TbBr₆]. All materials show strong green luminescence from the ⁵D₄-level of Tb³⁺ after excitation into the 4f⁸→ 4f ⁷5d¹ transition. In case of the imidazolium compounds, the color of this emission can be switched between green and blue-white depending on the excitation energy. After excitation with λ_ex = 254 nm, strong green emission mainly from the ⁵D₄-level of Tb ³⁺ is observed. With λ_ex = 366 nm, only the blue-white luminescence from the imidazolium cation itself is detected.

Full text of DOI:10.1002/ejic.201100168

FULL PAPER
DOI: 10.1002/ejic.201100168
Switchable Green and White Luminescence in Terbium-Based Ionic Liquid
Crystals
[a]
[a]
Anna Getsis and Anja-Verena Mudring*
Dedicated to John D. Corbett on the occasion of his 85th birthday
Keywords: Ionic liquids / Ionic liquid crystals / Liquid crystals / Lanthanides / Luminescence
[
C
12mim]Br (C12mim = 1-dodecyl-3-methyl) and [C12mpyr]Br
double layers of [C12mim]+ cations and [TbBr
with hydrogen bonded acetonitrile. [C12mim]
2CH CN can serve as a structure model for the smectic meso-
phase of the solvate-free [C12mim] [TbBr ]. All materials
show strong green luminescence from the D
after excitation into the 4f Ǟ 4f 5d transition. In case of the
imidazolium compounds, the color of this emission can be
switched between green and blue-white depending on the
6
]3– octahedra
(
C
12mpyr = N-dodecyl-N-methylpyrrolidinium), both doped
3
[TbBr ]·
6
with TbBr
and [C12mpyr]
3
, as well as the neat compounds [C12mim]
3
[TbBr
6
]
3
3
[TbBr ] show mesomorphic behavior and are
6
3
6
5
3+
able to form smectic liquid crystalline phases. The thermal
phase behavior of all compounds was investigated by hot-
stage polarizing optical microscopy and differential scanning
calorimetry. Whilst the doped compounds crystallize around
room temperature, the neat compounds solidify as glasses
around –5 °C and qualify also as ionic liquids. The crystal
4
-level of Tb
8
7
1
excitation energy. After excitation with λex = 254 nm, strong
5
-level of Tb3+ is observed.
green emission mainly from the D
4
structure of the acetonitrile solvate of the imidazolium com- With λex = 366 nm, only the blue-white luminescence from
pound, [C12mim]
3
[TbBr
6
]·2CH
3
CN, could be determined by
the imidazolium cation itself is detected.
single X-ray diffraction. The structure features alternating
Introduction
tals can be addressed as ionic liquid crystals (ILCs). They
combine the properties of typical ionic liquids with those
of liquid crystals. However, ILCs feature some properties,
such as ionic conductivity, which are not encountered in
classical LCs consisting of neutral molecules. Recently, it
was shown that double alkyl-substituted imidazolium ILCs
show strong non-Newtonian fluidic behavior in the LC
state, while they are Newtonian liquids in the liquid (IL)
Ionic liquids (ILs) have attracted substantial interest in
[
1]
recent years. They may feature properties such as negligi-
ble vapor pressure, wide liquid range, good thermal sta-
bility, considerable electric conductivity, and a wide electro-
chemical window. These properties and especially the
unique property combinations have shown to be advan-
tageous for a large number of applications. Today, ILs are
predominantly used in separation, various electrochemical
applications, organic synthesis, and catalysis.[ As salts,
ionic liquids are composed of distinct cations and anions,
which make them widely tunable. The variety of cation/
[3]
state. Derivatized N-methylimidazolium cations are by far
[3,4,5]
the most popular cations used for both ILs and ILCs.
2]
It is known that certain 1-alkyl-3-methylimidazolium salts
with long alkyl side chains can form lamellar mesophases.
The mesophase stability increases with increasing length of
anion combinations has led to the denomination of ILs as
[3,4]
the alkyl chain.
However, aside from changing the sub-
designer solvents”.[ Indeed, they may be designed for spe-
1]
“
stituents on the imidazolium rings or coupling imidazolium
rings with use of different spacers, the charged head group
can be varied. Recently, pyrrolidinium ionic liquids at-
tracted attention because of their good thermal and electro-
cific applications by choosing the appropriate cation and
anion combination.
Ionic liquids that contain one or two anisotropically
shaped ions (rodlike or disklike) are likely to form meso-
phases. Such compounds that are able to form liquid crys-
[6]
chemical stability.
Metal-containing liquid crystals (metallomesogens) form
a special class of ILCs. By choosing a suitable metal ion as
constituent of the ILC, additional functionalities that origi-
nate from the metals, such as redox activity, magnetism, or
luminescence, can be introduced to the material. In conse-
quence, multifunctional materials are obtained. Several re-
views survey the intensive studies on neutral, uncharged
[
a] Anorganische Chemie I, Ruhr-Universität Bochum,
Universitätsstrasse 150, 44801 Bochum, Germany
Fax: +49-234-27408
E-mail: anja.mudring@rub.de
http://www.anjamudring.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100168.
Eur. J. Inorg. Chem. 2011, 3207–3213
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3207

A. Getsis, A.-V. Mudring
FULL PAPER
metallomesogens.[7] In contrast, studies of ionic metallome- porting Information). [C mim] [TbBr ]·2CH CN is iso-
1
2
3
6
3
[
8,9]
sogens have been neglected until recently.
However, typic with the dysprosium analogue, [C mim] [DyBr ]·
12 3 6
[
15]
metal-containing ionic liquids (ILs) have received substan- 2CH CN.
As expected from the lanthanide contraction,
3
tial attention and combining the materials properties of the molar volume of [C mim] [TbBr ]·2CH CN (1c) is
1
2
3
6
3
metal-containing ILs[ and liquid crystals appears to open larger than that of the dysprosium compound (1736 Å vs.
10]
3
3
up perspectives for new materials. A special interest in the 1677 Å ). The asymmetric unit of 1c contains three crystal-
+
3–
field of metal-containing liquid crystals focuses on light- lographically independent [C mim] cations, one [TbBr ]
1
2
6
emitting liquid crystals for potential applications in emis- unit, and two acetonitrile molecules. Tb3 is coordinated in
+
[
11]
sive LC displays.
By incorporating cations of the 4f ele- an almost ideally octahedral manner by six bromide ions.
ments, such as Tm (blue), Eu (red), and Tb (green), The Tb–Br interatomic distances range from 2.801 to
3
+
3+
3+
[
10,12]
LCs emitting in the three basic colors can be obtained.
2.868 Å (see Supporting Information), which are slightly
In addition, aligned luminescent LCs can emit polarized longer than typical inorganic salts with complex [TbBr6]3–
[
13]
light. A high magnetic anisotropy of the lanthanide ion, anions. For example, in Cs [Tb Br ], values of 2.697–
3
2
9
3
+
3+
3+
[16]
as found for Tb , Dy , and Tm , makes the alignment 2.902 Å are found.
A similar observation was made in
[
14]
of the mesophase in an external magnetic field possible.
Thus, such compounds might be of interest for applications and angles for the 1-dodecyl-3-methylimidazolium cations
in electrically and magnetically switchable devices. in 1c are in the expected range and can be compared to
case of the [C mim] [DyBr ]·2CH CN. Bonding distances
12
3
6
3
Here we report on the synthesis as well as on the struc- those in [C mim] [DyBr ]·2CH CN or to similar short-
1
2
3
6
3
tural, thermal, and photophysical behavior of 1-dodecyl-3- chain N-n-alkyl-N-methylimidazolium compounds.[ Two
methylimidazolium bromide and N-methyldodecylpyrrolid- of the dodecyl chains adopt all-trans conformation, while
inium bromide doped with terbium(III) bromide, the third reveals a gauche conformation around the C7–C8
17]
6
3
[C mim]Br·[C mim] [TbBr ] (1a) (C mim = 1-dodecyl- bond. Such crank-handle-like arrangements have also been
12 12 3 6 12
-methyl) and 6[C mpyr]Br·[C mpyr] [TbBr ] (2a) observed in related hexafluorophosphate salts, [C mim]-
1
2
12
3
6
n
[
18]
(C mpyr = N-dodecyl-N-methylpyrrolidinium), as well as [PF ] (n = 12, 14, 16). In 1c, the cations themselves form
1
2
6
the neat terbium ionic liquid crystals [C mim] [TbBr ] (1b) rows that are stacked along the a axis in such a way that
1
2
3
6
and [C mpyr] [TbBr ] (2b), and the structure of the related their alkyl tails alternate in orientation in neighboring lay-
1
2
3
6
acetonitrile solvate [C mim] [TbBr ]·2CH CN (1c) ers, and thereby a double layer is formed (Figure 1). Within
1
2
3
6
3
(
Table 1).
the layers, structural segregation into hydrophilic and
hydrophobic domains occurs. The hydrophobic parts are
built up by interdigitated n-alkyl tails of the imidazolium
cation. The charged imidazolium head groups, together
with the hexabromoterbate octahedra, form the hydrophilic
Table 1. Compounds studied.
Compound number Formula
1
1
1
2
2
a
b
c
a
b
6[C12mim]Br · [C12mim]
3
[TbBr
6
]
]
3–
[C12mim]
[C12mim]
6[C12mpyr]Br·[C12mpyr]
3
[TbBr
[TbBr
6
]
part of the structure. Within a layer, each [TbBr
6
]
anion
3
6
]·2CH
3
CN
[TbBr
is surrounded by six [C mim] cations and two acetonitrile
+
12
3
6
molecules. Nonclassical hydrogen bonds formed by (NN)-
3 6
[C12mpyr] [TbBr ]
Results and Discussion
Crystal Structure of [C mim] [TbBr ]·2CH CN
1
2
3
6
3
Several attempts employing various techniques to grow
crystals of neat hexabromoterbate complexes {[C mim] -
12
3
[
TbBr ] (1b) and [C mpyr] [TbBr ] (2b)} with sufficient
6 12 3 6
quality for X-ray structure analysis failed. However, needle-
shaped, colorless single crystals of the acetonitrile mono-
solvate of 1b, [C mim] [TbBr ]·2CH CN (1c), could be
1
2
3
6
3
grown from an acetonitrile solution of 1b at 5 °C. Single-
crystal X-ray diffraction structure analysis reveals that the
compound crystallizes in the orthorhombic space group
Figure 1. Crystal structure of [C12mim]
along the crystallographic a axis (top) and along the crystallo-
3 6 3
[TbBr ]·2CH CN (1c): view
Pbca (no. 61) [a = 14.979(4) Å, b = 18.500(5) Å, c =
3
5
0.11(2) Å, Z = 8, V = 13885(7) Å ] (Table S1 in the Sup- graphic b axis (bottom).
3208
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Eur. J. Inorg. Chem. 2011, 3207–3213

Switchable Luminescence in Terbium-Based Ionic Liquid Crystals
2
2
Csp ···Br and (NC)Csp ···Br interactions (see Supporting leads to the formation of the complex hexabromoterbate
Information), which are shorter than the commonly ac- anion and justifies the formulation of the compounds as
cepted limiting values of 3.72 Å and 3.75 Å,[ exist not 6[C mim]Br·[C mim] [TbBr ] (1a) and 6[C mpyr]Br·
19]
12
12
3
6
12
only within the hydrophilic parts of each layer but also be- [C mpyr] [TbBr ] (2a). The unsymmetrical stretching
12
3
6
tween them. The structural features of 1c already suggest vibration (F ) can be detected in the far infrared spectrum
1
u
the possibility of the formation of a smectic mesophase. at 159 cm–1 for 1a and 1b and 167 cm for 2a and 2b. The
–1
Unfortunately, the compound looses acetonitrile before symmetrical stretching vibration (A ) can be observed in
1g
such a phase transition would occur. However, the structure the Raman spectrum. For 1b and 2b, the A1g vibration is
[
20]
–1
of 1c together with the structures of [C mim]Br·H O
found at 154 and 157 cm , respectively. The Raman spec-
are valuable structural models for the trum of 2a looks like a superposition of the Raman spectra
mesophases adopted by 1-dodecyl-3-methylimidazolium of [C mpyr] [TbBr ] and [C mpyr]Br, which is another
1
2
2
[
6b]
and [C mpyr]Br
1
2
12
3
6
12
bromide and N-methyl-dodecylpyrrolidinium bromide piece of evidence that 6[C mpyr]Br·[C mpyr] [TbBr ] (2a)
12
12
3
6
doped with terbium bromide (1a and 2a) as well as for 1- is a good formula representation for the C mpyr system
12
dodecyl-3-methylimidazolium hexabromoterbate(III) (1b) doped with TbBr3.
N-methyl-N-dodecylpyrrolidinium hexabromoterbate(III)
Through Raman spectroscopy it is also possible to distin-
(2b).
guish crystalline, mesomorphous, and amorphous order-
[
21]
ing.
As the phase behavior of alkyl chains in polymers
has been well studied, this knowledge can be used to inter-
Vibrational Spectroscopy
pret the spectra of 1 and 2. Comparing the Raman spec-
–1
trum of [C mim] [TbBr ] in the region 1400–1500 cm
12
3
6
Far-infrared (FIR) and Raman spectra recorded for all
samples show the expected vibrations for the respective cat-
ion and anion (Figure 2 and Supporting Information). The
characteristic vibrations of the [TbBr6]3 octahedron prove
(Figure 3) with the typical spectrum of a partially crystal-
line polymer (polyethylene, PE; Figure 3, inset) reveals great
similarities and confirms the low degree of ordering in the
alkyl chains in the liquid crystalline state. For a crystalline
material, distinctly split lines of significantly lower line
width would be expected. In addition, the conformation of
the alkyl side chains can be elucidated. Combined experi-
–
that reaction of TbBr with the respective bromide ILCs
3
Figure 2. FIR spectra of the Tb-containing ILCs 1b (top), 2b, and
2
a (bottom) in comparison with those of neat [C12mim]Br and
[
C12mpyr]Br.
Figure 3. Details of the Raman spectrum of 1b.
Eur. J. Inorg. Chem. 2011, 3207–3213
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Getsis, A.-V. Mudring
FULL PAPER
mental and theoretical Raman investigations of various 1-
butyl-3-methylimidazolium salts show that all-trans con-
formers should give a broad signal at about 625 cm–1 while
the gauche conformer should show a band at about
–
1
6
03 cm . Both features can be detected in the Raman spec-
trum of [C mim] [TbBr ] (Figure 3, bottom.).
1
2
3
6
Thermal Analysis
The thermal properties of 1a, 1b, 2a, and 2b were exam-
ined by polarizing optical microscopy (POM) and differen-
tial scanning calorimetry (DSC). The DSC traces of the
cooling and heating cycles and the respective POM pictures
are shown in Figures 4 and 5. Exact transition temperatures
and corresponding enthalpy changes are given in the Sup-
porting Information. The highest thermal invent always
corresponds to the clearing point. In case of the imid-
azolium compounds, the clearing point is only marginally
higher than that of neat C mimBr (Figure 4). Whilst 1b
1
2
maintains its liquid crystalline state to low temperatures
and only undergoes a glass transition at about –6 °C upon
cooling, for 1a a liquid-crystal-to-solid (LC–S) transition is corresponding POM micrographs in comparison to the thermo-
Figure 5. DSC-thermogram of 2a (in red) and 2b (in blue) with
gram of neat [C12mpyr]Br.
found at roughly the same temperature as that of [C mim]-
12
Br. Likewise the DSC traces of 2a and [C mypr]Br bear
1
2
close resemblances (Figure 5). The three thermal changes
occurring between 120 °C and 55 °C are due to LC–LC
phase transitions. At 54.2 °C crystallization occurs. The
thermal behavior of 1b is similar to that of 2b. After conver-
sion from the isotropic liquid to the liquid crystalline state
upon cooling, no crystallization but rather a glass transition
takes place at –5 °C. The clearing points of the imidazolium
compounds, 1a and 1b, are several tens of degrees lower
than those of the pyrrolidinium analogues. Also, the glass
transition and crystallization temperatures are found at
lower values for the imidazolium compounds. In summary,
all compounds show an appreciable LC window including
the room temperature range. The phase transitions of the
imidazolium compounds take place at lower temperatures
than those of the pyrrolidinium analogues.
It not easy to observe unambiguous defect textures for
mesophases of imidazolium- and pyrrolidinium-based ILCs
by polarizing optical microscopy, as these compounds show
a strong tendency to form spontaneously single homeo-
tropic monodomains, which results in dark fields between
the crossed polarizers of the microscope. It is extremely dif-
ficult to disturb this alignment in order to get more mean-
[
22,23]
ingful textures.
The dilemma is further enhanced by
the fact that all compounds are extremely air-sensitive and
have to be handled under strictly inert conditions. However,
POM investigations confirm that the highest thermal events
measured by DSC correspond to the clearing point. The
optical textures observed on cooling from the isotropic
melt, as fan-shaped, oily-streaky forms and bâtonnetes, are
characteristic for smectic mesophases. Judging from the bi-
[
20]
layered crystal structure of [C mim]Br·H O, [C mpyr]-
12
2
[15]
12
[
6b]
Br,
and [C mim] [DyBr ]·2CH CN,
it seems quite
12
3
6
3
likely that all compounds form smectic mesophases. Indeed,
smectic phases are typical of 1-n-alkylimidazolium
[
4,24]
ILCs.
This behavior is possibly due to the fact that the
layer structure originates not only from the hydrophobic
van der Waals interactions between the long cation–alkyl
tails but is strongly stabilized by Coulomb interactions in
the hydrophilic part of the structure where the charged cat-
ion head groups and the anions are located. In conse-
quence, the alkyl tails can melt independently from the lay-
ers and several smectic phases can be adopted due to the
different space requirements of the alkyl tails with rising
freedom as well as their orientation towards the layer nor-
mal. Unfortunately, we were experimentally not able to
carry out temperature-dependent SAXS measurements un-
der strictly anhydrous conditions so far.
Figure 4. DSC thermogram of 1a (in red) and 1b (in blue) with
corresponding POM micrographs in comparison to the thermo-
gram of neat [C12mim]Br.
3210
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Eur. J. Inorg. Chem. 2011, 3207–3213

Switchable Luminescence in Terbium-Based Ionic Liquid Crystals
Luminescence Spectroscopy
Tb3+ is known to be an efficient emitter of green light
5
originating from electronic transitions from the D level to
4
7
[25]
the F multiplet of the ground state.
Important for ef-
J
3+
ficient luminescence is the effective excitation of Tb .
However, trivalent lanthanide cations generally have a low
absorption coefficient. Yet, Tb3+ can be efficiently excited
n
n–1
1
with far-UV light, as it features 4f Ǟ4f 5d transitions
at low energies with appreciable absorption. In compounds
3
–
n
n–1
1
containing [TbBr ] , the 4f Ǟ4f 5d transition is known
6
–
1
[26]
to take place around 36000 cm (ca. 278 nm).
The excitation as well as the emission spectra of 1a, 2a, Figure 7. 2D emission–excitation spectra of 1a.
3
+
1
b, and 2b show the characteristic transitions for Tb (Fig-
[
27]
ure 6). The emission spectra of the imidazolium and the
pyrrolidinium ILCs show the same transitions in the same
energy range with the same relative intensities. This applies
not only to the f–f transitions but also to the f–d transition,
which is found at about 282–289 nm. This points to the
Excitation with UV light of short wavelength leads to an
3+
efficient activation of the Tb ion, and the sample appears
5
7
green due to the high intensity of the D Ǟ F emission
4
6
around 545 nm. Excitation with longer wavelengths
Ͼ 390 nm) leads to strong, broad emission in the blue re-
(
3
+
same local coordination environment for Tb in all com-
gion of light originating from the imidazolium cation. The
3
–
pounds and confirms the presence of [TbBr ] .
3+
6
Tb cation can no longer be sensitized. Similar behavior
has recently been reported for the ionic liquid crystal com-
3+
pound [C mim]Cl when doped with Eu (red to bluish-
12
white) as well as for [C mim] [DyBr ] (yellow-orange to
12
3
6
[
28]
blue-white).
Conclusions
[
C mim] Br and [C mpyr]Br, both doped with TbBr
12 3 12 3
(
1a and 2a), as well as the neat compounds [C mim] -
12 3
[
TbBr ] (1b) and [C mpyr] [TbBr ] (2b), are interesting
6 12 3 6
new materials as they are able to form mesophases over a
wide temperature range. Whilst 1a and 2a crystallize around
room temperature and have clearing points around 100 °C
and 190 °C, respectively, the neat compounds 1b and 2b so-
lidify as liquid crystal glasses around –5 °C. The neat com-
pounds [Cat] [TbBr] (Cat = C mim, C mpyr) essentially
3
6
12
12
show the same clearing points as the doped materials 1a
and 2a. The thermal phase behavior of all compounds was
investigated not only by differential scanning calorimetry
but also by hot-stage polarizing optical microscopy in order
to get more insight into the nature of the formed mesoph-
ases. For all phases, textures indicating to smectic mesoph-
ases were found. Unfortunately, we did not succeed in ob-
taining reliable temperature-dependent SAXS data, as all
Figure 6. Normalized excitation spectra (monitored at λem
=
7
1
547 nm, left) and emission spectra under excitation of the 4f d
transition (right) for 1a and 1b (top) as well as for 2a and 2b (bot-
tom).
All compounds show an efficient green light emission compounds are highly moisture-sensitive. However, we were
5
with long lifetimes of the excited D level. The lifetimes of able to determine the crystal structure of the acetonitrile
4
the pyrrolidinium compounds are somewhat higher (4.4 ms solvate [C mim] [TbBr ]·2CH CN (1c), which might serve
12
3
6
3
for 2a and 4.0 ms for 2b) compared to those of the corre- as a structural model. Compound 1c forms a layered struc-
sponding imidazolium ILCs (3.3 ms for 1a and 3.7 ms for ture in which double layers of hydrophilic and hydrophobic
2b). However, in case of the imidazolium compounds, the structure regions alternate. The hydrophobic structure parts
light emission is more intense than that of the pyrrolidinium are formed by interdigitated dodecyl tails of imidazolium
compounds. This is due to an energy transfer from the imid- cations with different orientation. The hydrophilic structure
3
+
azolium cation to the Tb ion. Such an influence has been regions are built up by the charged imidazolium head
reported previously.[ Furthermore, in case of the imid- groups and octahedral hexabromodysprosiate(III) complex
azolium salts, it is possible to tune the emission color be- anions together with hydrogen-bonded acetonitrile. It is an-
tween white and green depending on the excitation wave- ticipated that the ion arrangement is largely retained when
9b]
length (Figure 7).
the solvate acetonitrile molecules are lost.
Eur. J. Inorg. Chem. 2011, 3207–3213
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3211

A. Getsis, A.-V. Mudring
All materials show a strong green luminescence from the Axio Imager A1 microscope (Carl Zeiss MicroImaging GmbH,
FULL PAPER
5
8
7
1
Göttingen, D) equipped with a hot stage, THMS600 (Linkam Sci-
entific Instruments Ltd, Surrey, UK), and temperature controller
Linkam TMS 94 (Linkam Scientific Instruments Ltd, Surrey, UK).
The images were recorded as movies with a digital camera after
initial heating during the cooling stage.
D level upon excitation of the 4f Ǟ4f 5d transition of
4
3
+
Tb . In case of the imidazolium compounds, the color of
this emission can be switched between green and blue-white
depending on the excitation energy used. After excitation
with λ = 254 nm, strong green emission mainly from the
ex
5
D level of Tb3+ is observed. With λex = 366 nm excitation,
Crystal Structure Analysis: A few crystals of 2c were selected at
about 0 °C under a perfluorinated polyether (viscosity 1600 cSt,
ABCR GmbH, Karlsruhe, D) with the help of an optical micro-
scope. The best specimen was chosen and adhered to a glass capil-
lary with the help of the oil and a complete intensity data set was
collected with the aid of a Bruker Smart 1000 single-crystal X-ray
diffractometer (Bruker, Karlsruhe, D) with graphite monochro-
4
only the bluish-white luminescence from the imidazolium
cation itself is detected.
Experimental Section
General Remarks: Synthesis and sample handling were carried out
under strictly inert conditions by using standard Schlenk and argon
glove box techniques. All solvents were dried by using standard
procedures. [C12mim]Br and [C12mpyr]Br were prepared according
to literature procedures.[
α
mated Mo-K radiation (λ = 0.71073 Å) at –60 °C. Data reduction
[30]
was carried out with the program package SAINT, and a numer-
ical absorption correction was performed with the program SAD-
ABS.[ Crystal structure solution by direct methods with SIR92
31]
[32]
6b,20]
yielded the heavy atom positions. Subsequent difference Fourier
analyses and least-squares refinement with SHELXL-97[ allowed
for the location of the remaining atom positions. In the final step
of the crystal structure, refinement hydrogen atoms of idealized –
33]
Terbium Tribromide (TbBr
3
): TbBr
3
was synthesized according to
Terbium(III,IV) oxide (2.00 g,
a modified literature procedure.[
29]
0.0027 mol, 99.9% Fluka Chemie AG, Buchs, CH) and ammonium
2 3
CH and –CH groups were added and treated with the riding atom
bromide (6.40 g, 0.0648 mol, suprapur® 99.995% Merck, Darm-
stadt, D) were dissolved in aqueous hydrobromic acid (48%, Ald-
mode; their isotropic displacement factor was chosen as 1.2 times
the preceding carbon atom. For further information see the Sup-
porting Information. CCDC-682994 contains the supplementary
crystallographic data for 1c. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. For drawings of the crystal
structure the program Diamond was used.[
rich, Steinheim, D). The formed (NH
posed to TbBr at 900 °C. At this temperature any excess ammo-
nium bromide sublimes off. Crude TbBr is then purified by subli-
mation under vacuum (Ͻ 10–5 mbar) at 920 °C. The purity of
TbBr was checked by powder XRD (Mo-K , Huber Image Plate,
Rimsting, D).
4 3 6 2
) TbBr ·xH O was decom-
3
3
3
α
34]
Supporting Information (see footnote on the first page of this arti-
cle): Information on the crystal structure solution and refinement
of [C12mim] [TbBr ]·2CH CN, additional graphical representation
3 6 3
of vibrational spectra, and data regarding the thermal behavior of
all compounds.
1
a, 1b, 2a, and 2b: These compounds were obtained by reacting the
respective amounts of TbBr with [C12mim]Br or [C12mpyr]Br at
20 °C. After cooling to room temperature, highly viscous bulk
3
1
products were obtained. Single crystalline 1c formed after cooling
a solution of 1b in dry acetonitrile to 5 °C. The thermolabile com-
pound loses the solvate acetonitrile below room temperature.
1H NMR Spectroscopy: Nuclear magnetic resonance (NMR) spec- Acknowledgments
tra were recorded with a Bruker AC-300 spectrometer (operating
1
at 300 MHz for H, Bruker Germany GmbH, D).
This work was supported by the Deutsche Forschungsgemeinschaft
DFG) in the priority program SPP 1191 “Ionic Liquids”. A. G.
and A.-V. M. thank the Fonds der Chemischen Industrie for a
Doktoranden- and a Dozentenstipendium, respectively. Dr. Klaus
Merz is acknowledged for X-ray data collection.
(
IR-Raman Spectroscopy: IR spectra of the solid were recorded with
a IFS-66V-S Fourier Transform IR spectrometer (Bruker Optik
GmbH, Ettlingen, D); samples to be measured were pressed in a
–1
polyethylene matrix for the far IR range (50–600 cm ) and in a
–
1
KBr matrix for the middle IR (MIR) range (600–4000 cm ). Ra-
man spectra were recorded with a FRA 106-S Fourier Transform
Raman spectrometer (Bruker Optik GmbH, Ettlingen, D) at
50 mW. Raman samples were measured in glass capillaries with
an inner diameter of 0.1 cm and a wall thickness of 0.15 mm.
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Eur. J. Inorg. Chem. 2011, 3207–3213
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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