Lanthanide containing ionic liquid crystals: EuBr2, SmBr 3, TbBr3 and DyBr3 in C12mimBr

Article abstract of DOI:10.1002/zaac.201000070

Doping the ionic liquid crystal C₁₂mimBr with various lanthanide halides yields interesting novel liquid crystalline and luminescent materials. The thermal phase behavior of all compounds was investigated by hot-stage polarizing optical microscopy and differential scanning calorimetry and the photophysical properties were determined by luminescence spectroscopy. C ₁₂mimBr itself is an ionic liquid crystal that shows bluish-white emission upon excitation with UV light due to transitions in the imidazolium p-system. Doping lanthanide bromides into C₁₂mimBr with concentrations of about 1 mol-% does not affect the liquid crystalline behavior of the host materials to a great extent and room temperature liquid crystals are obtained. All materials show an appreciable luminescence. EuBr₂ in C₁₂mimBr yields a material, which shows a blue emission originating from 4f-5d-transitions. SmBr₃-doped samples show a red and TbBr ₃ samples a green luminescence. Upon doping C₁₂mimBr with DyBr₃ an orange luminescent liquid crystalline material is obtained. Most interestingly the emission color for the TbBr₃ and DyBr ₃ containing materials can be tuned from bluish-white (mainly C ₁₂mimBr emission) to green (for TbBr₃) and orange-yellow (for DyBr₃) depending on the wavelength of the excitation light used.

Full text of DOI:10.1002/zaac.201000070

ARTICLE
DOI: 10.1002/zaac.201000070
Lanthanide Containing Ionic Liquid Crystals: EuBr , SmBr , TbBr and
2
3
3
DyBr in C mimBr
3
12
[a]
[a]
Anna Getsis and Anja-Verena Mudring*
Dedicated to Professor Bernd Harbrecht on the Occasion of His 60th Birthday
Keywords: Dysprosium; Europium; Ionic liquids; Liquid crystals; Luminescence; Samarium; Terbium
Abstract. Doping the ionic liquid crystal C12mimBr with various lan- extent and room temperature liquid crystals are obtained. All materials
thanide halides yields interesting novel liquid crystalline and lumines- show an appreciable luminescence. EuBr in C12mimBr yields a mate-
cent materials. The thermal phase behavior of all compounds was in- rial, which shows a blue emission originating from 4f–5d-transitions.
vestigated by hot-stage polarizing optical microscopy and differential SmBr -doped samples show a red and TbBr samples a green lumines-
scanning calorimetry and the photophysical properties were deter- cence. Upon doping C12mimBr with DyBr an orange luminescent liq-
mined by luminescence spectroscopy. C12mimBr itself is an ionic liq- uid crystalline material is obtained. Most interestingly the emission
uid crystal that shows bluish-white emission upon excitation with UV color for the TbBr and DyBr containing materials can be tuned from
light due to transitions in the imidazolium π-system. Doping lanthanide bluish-white (mainly C12mimBr emission) to green (for TbBr ) and
) depending on the wavelength of the excita-
2
3
3
3
3
3
3
bromides into C12mimBr with concentrations of about 1 mol-% does orange-yellow (for DyBr
not affect the liquid crystalline behavior of the host materials to a great tion light used.
3
Introduction
physicochemical properties of liquid crystals (LCs) such as re-
fractive index, electric permittivity, magnetic susceptibility,
and mechanical properties like viscosities are highly dependant
on the direction in which these properties are measured. This
has shown to be useful for a wide variety of applications and,
in consequence, molecular liquid crystalline materials have
found widespread applications in display technology in lap-
tops, mobile phones, or flat screen television sets [3].
Although ionic liquids (ILs) have received an exceptionally
high attention as a new class of solvents and materials for vari-
ous applications, the understanding of interactions within these
liquids is still rather limited. It has been noted from early on
that ILs are highly structured liquids [1]. Ionic liquid crystals
(
ILCs) represent a special class of ILs where weak hydropho-
bic interactions of alkyl groups, dipole–dipole, cation–π inter-
actions as well as π –π stacking of the commonly used aro-
matic head groups of the cation such as imidazolium or
pyridinium as well as ionic interactions are tremendously im-
portant and are the driving force for the formation of ordered
liquids, hence, liquid crystalline phases.
Whereas most known liquid crystals are neutral molecular
organic compounds of strong anisotropic shape, ionic liquid
crystals (ILCs) are constituted of anions and cations. Meso-
phasic behavior can be introduced into such an ionic material
via either one of the ions. It is known that 1-alkyl-3-methylim-
idazolium salts with twelve and more carbon atoms in the side
chain adopt mesophases and that the mesophase stability rap-
idly increases with increase in the alkyl chain length [4]. Most
times, a lamellar mesophase, a smectic A phase or other highly
ordered smectic phases are observed. It has been realized that
ILCs show unique characteristics that cannot be observed in
conventional molecular LCs. In ILCs a tetragonal smectic
The liquid crystalline state of matter shows characteristics
which are intermediate between that of a three-dimensionally
well ordered crystalline solid and an isotropic liquid where the
constituents are distributed randomly through space [2].
Generally, in the liquid crystalline state a random distribution
of particles – like in a classical liquid – is observed at least in
one direction of space whereas in another direction a substan-
tially higher degree of order is observed which comes close to
the periodicity of crystalline materials. Typically, many of the
phase S was observed for the first time [5]. Recently, it was
T
shown that double alkyl-substituted imidazolium ILCs show
strong non-Newtonian fluidic behavior in the LC state whereas
they are Newtonian liquids in the liquid IL state [6]. ILCs also
show a strong tendency for spontaneous homeotropic align-
ment [7]. Applications of ionic liquid crystals are, so far, as
ion-conductive materials, organized reaction media or self-as-
sembled nanostructured materials [8].
*
Prof. Dr. A.-V. Mudring
E-Mail: anja.mudring@ruhr-uni-bochum.de
[
a] Fakultät für Chemie und Biochemie
Anorganische Chemie I, Festkörperchemie und Materialien
Ruhr-Universität Bochum
Universitätsstrasse 150
Metal-containing liquid crystals (metallomesogens) form a
special class of ILCs. By choosing a suitable metal ion as the
4
4780 Bochum, Germany
www.anjamudring.de
1
726
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 636, 1726–1734

Lanthanide Containing Ionic Liquid Crystals
ILC constituent additional functionalities that origin from the under dynamic vacuum. Yield: ca. 70 %. For atom assignment, see
1
Scheme 1. H NMR (298 K, 300 MHz, [D
6
]DMSO): δ = 0.868 (t, 3
metals such as redoxactivity, magnetism or luminescence can
be introduced into the material. In consequence, multifunc-
tional materials are obtained. Several reviews survey the inten-
sive studies on neutral, uncharged metallomesogens [9]. In
contrast studies of ionic metallomesogens have been neglected
until recently [10, 11]. Metal based ionic liquid crystals are
known for various transition metals. The majority of the metal-
H, H-1), 1.234 (br. s., 18 H, H-2), 1.766 (quint., 2 H, H-3), 3.848 (s,
H, H-5), 4.149 (t, 2 H, H-4), 7.708 (m, 1 H, H-6), 7.777 (m, 1 H,
H-7), 9.151 (br. s., 1 H, H-8). MIR (KBr pellet): ν˜ = 3226.5 (w),
149.4 (m), 3083.8 (s), 3062.6 (s), 2950.7 (s), 2916.0 (s), 2869.7 (s),
852.4 (s), 1772.4 (w), 1668.2 (w), 1629.7 (m), 1571.8 (m), 1473.4
3
3
2
(
m), 1427.2 (w), 1384.7 (w), 1344.2 (w), 1317.2 (w), 1292.2 (w),
1
178.4 (s), 1126.3 (w), 1089.7 (w), 1054.9 (w), 1043.4 (w), 1022.2
containing ionic liquids have the [MX4]2– structural building (w), 1004.8 (w), 943.1 (w), 887.2 (w, shoulder), 862.1 (m), 792.6 (m),
–1
block with M = Co, Ni, Cu, Zn, Pd, Cd and X = Cl, Br and 763.7 (w), 742.5 (w), 729.0 (w), 715.5 (m), 661.5 (m), 622.9 (m) cm .
sometimes I. Typical counter cations are long-chain quaternary FIR (PE pellet): ν˜ = 562.2 (m, shoulder), 509.1 (w), 498.9 (w), 472.5
(
w), 453.2 (m), 417.5 (m), 396.3 (m), 349.1 (w), 284.5 (w), 230.5 (w),
ammonium cations, [12–15], N-alkylpyrrolidinium [16], N-al-
kylpyridinium cations, [17–22], and 1-alkyl-3-methylimida-
zolium cations [23] Lanthanide-based ILCs such as
–1
1
3
2
54.3 (m, shoulder), 93.5 (s), 64.6 (s, shoulder) cm . Raman: ν˜ =
134.2 (w), 3082.9 (w), 2956.7 (w), 2933.6 (s), 2883.5 (s), 2848.7 (s),
727.2 (w), 1560.4 (vw), 1452.4 (w, shoulder), 1432.1 (w), 1421.6 (w,
[
C mpyr] [TbBr ] (C mpyr = N-dodecyl-N-methylimida-
12 3 3 12
zolium) [24], [C mim] LnBr (C mim = 1-dodecyl-3-methy-
shoulder). 1375.3 (vw), 1334.8 (vw), 1296.2 (w), 1128.4 (w), 1105.3
vw), 1087.9 (vw), 1062.9 (w), 1014.6 (w), 887.4 (vw), 733.1 (vw),
692.6 (vw), 654.0 (vw), 605.8 (vw), 414.9 (vw), 303.0 (vw), 231.6
vw), 181.5 (vw, shoulder), 150.6 (w, shoulder), 114.0 (w), 85.1 (w,
1
2
3
6
12
(
limidazolium; Ln = Dy, Tb)[25] or [C mim] [EuBr ]Br [26].
12
4
6
Special interest in the field of such metal-containing liquid
crystals focuses on light-emitting liquid crystals for potential shoulder) cm . [C mim]Br: calcd. C 58.00, H 9.43, N 8.45; found C
(
–1
12
applications in emissive LC displays [27]. By incorporating 4f- 57.63, H 9.74, N 8.42.
2
+
3+
3+
3+
element cations such as Eu or Tm for blue, Eu or Sm
3
+
for red and Tb for green emission LCs emitting in the three
basic colors can be obtained [28]. In addition, aligned lumines-
cent LCs can emit polarized light [29]. A high magnetic anisot-
3+
3+
3+
ropy of the lanthanide ion as found for Tb , Dy and Tm
makes the alignment of the mesophase in an external magnetic
field possible [30]. Thus, such compounds might be of interest
for the use of electric and magnetic switchable devices [31].
However, aside from lanthanide-based ILCs another viable
way to ionic liquid crystalline material which shows lanthanide
emission is to dope ILCs with suitable lanthanide salts.
Scheme 1. Atom assignment in [C12mim]Br (1).
EuBr
(2.8 g, 8 mmol, 99.97 %, Fluka Chemie, Buchs, CH) was dissolved in
tal [32], doped with EuBr , SmBr , TbBr and DyBr , which conc. HBr (80 mL, 48 %, Aldrich, Steinheim, D) and ammonium bro-
2
: According to a literature procedure [33] europium(III) oxide
Here we report on C mimBr, a well known ionic liquid crys-
12
2
3
3
3
allows to obtain ILC materials that show luminescence in the mide (11 g, 0.112 mol, 99.995 %, Merck, Darmstadt, D) was added.
three basic colors. The luminescence properties of the obtained After all starting materials were dissolved, the solvent was removed.
The residual yellow solid was heated under vacuum at 120 °C
hybrid materials are investigated by luminescence spectro-
–
1
–1
(
20 °C·h ) for 6 h, afterwards at 400 °C (20 °C·h ) for 12 h before
scopy. The thermal behavior and mesophase formation is in-
vestigated by combined thermal (differential scanning calorim-
etry, DSC) and temperature dependent polarized optical
microscopy (POM).
–1
it was cooled to room temperature (20 °C·h ). The obtained EuBr
was checked for identity and purity by PXRD (powder X-ray diffrac-
tion).
2
LnBr
3
(Ln = Sm, Tb, Dy): LnBr
3
was synthesized according to a
and DyBr , the respec-
modified literature procedure [34]. For SmBr
3
3
Experimental Section
tive lanthanide(III) oxide (0.005 mol, 99.9 % Fluka Chemie AG,
Buchs, CH) and ammonium bromide (0.07 mol, suprapur® 99.995 %
Merck, Darmstadt, D); for TbBr Tb O (0.004 mol, 99.9 % Fluka
3 4 7
Chemie AG, Buchs, CH) and ammonium bromide (0.102 mol) were
dissolved in conc. HBr (48 %, Aldrich, Steinheim, D). After removal
Handling of C12mimBr and EuBr
Dy) containing samples was carried out under strict inert conditions
vacuum, argon) using advanced Schlenk and glove-box techniques.
2 3
as well as LnBr (Ln = Pr, Sm, Tb,
(
of the solvent, the formed (NH
4 3 6 2
) LnBr ·yH O was decomposed to
1
-Dodecyl-3-methylimidazolium Bromide (C12mimBr): For
LnBr
3
at elevated temperatures where also any excess of ammonium
C
12mimBr methylimidazole (0.1 mol, 99 %, ABCR GmbH&Co, Karl-
bromide sublimed off. The crude LnBr
3
was afterwards purified by
sruhe, D) was dissolved in dry acetonitrile (70 mL). Afterwards, n-
dodecyl bromide (0.1 mol, 99 % Acros organics, Geel, B) was added
dropwise and the reaction mixture was heated under reflux for 18 h.
–5
sublimation under vacuum (< 10 mbar). The purity of the obtained
3
LnBr was checked by PXRD.
After cooling to room temperature most of the solvent was removed LnX
2 3
/LnX in C12mimBr: To obtain lanthanide halide containing
under vacuum. 1-Dodecyl-3-methylimidazolium bromide precipitated 12mimBr samples the respective amounts of the lanthanide bromide
C
as a white powder upon addition of the concentrated acetonitrile solu- and 1-dodecyl-3-methylimidazolium bromide were heated in a sealed
tion to cold (–30 °C) and dry toluene (300 mL). The reaction product silica ampoule under vacuum at 120 °C until a homogeneous mixture
was filtered off and re-crystallized two times from dry acetonitrile/ was obtained. After cooling to room temperature waxy solids were
toluene. The product was dried from any remaining solvent for 48 h obtained, which were subjected to further analysis. Alternatively the
Z. Anorg. Allg. Chem. 2010, 1726–1734
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1727

A. Getsis, A.-V. Mudring
ARTICLE
lanthanide halide and C12mimBr could be dissolved in CH
2
Cl
2
at room UV/Vis spectra were obtained with a Cary 5000 spectrometer (Bruker,
temperature. After removal of the solvent under vacuum, again, waxy Bremen, Germany) at room temperature. Excitation and emission spec-
samples of the desired composition were obtained.
tra and also luminescence decays were recorded on a Jobin Yvon Hor-
iba Fluorolog FL3–22 (HORIBA Jobin Yvon Inc., Unterhaching, D)
with a xenon lamp as the excitation source and a photomultiplier tube
for detection. Interconfigurational f–f-transitions are assigned accord-
ing to the Dieke diagram [35].
Instrumentation
Elemental analyses were obtained with an Euro Vektor elemental ana-
lyzer EuroEA 3000 (HEKAtech GmbH, Wegberg, D) for C12mimBr.
Results and Discussion
1
H-Nuclear magnetic resonance (NMR) spectra were recorded with a
1
Bruker AC-300 spectrometer (operating at 300 MHz for H) (Bruker
Germany GmbH, D).
C mimBr
12
Dry C mimBr was chosen as a host matrix with liquid-crys-
12
IR spectra of the solid were recorded with a IFS-66V-S Fourier Trans-
form IR spectrometer (Bruker Optik GmbH, Ettlingen, D); samples
were measured pressed in a polyethylene matrix for far IR range (50–
talline properties and doped with various amounts of EuBr2,
SmBr , TbBr and DyBr
3
3
3.
DSC measurements show that upon cooling liquid samples
–
1
6
4
00 cm ) and in a KBr matrix for middle IR (MIR) range (600–
000 cm ).
of C mimBr from the isotropic melt a liquid to liquid crystal-
–
1
12
line (L↔LC) phase transition occurs at 99.2 °C and a liquid
crystalline to crystalline phase transition at 14.2 °C (LC↔S).
Upon heating the samples undergo two solid-solid (S↔SI)
Raman spectra were recorded with a FRA 106-S Fourier Transform
Raman spectrometer (Bruker Optik GmbH, Ettlingen, D) at 150 mW.
Raman samples were measured in glass capillaries with an inner diam- phase transitions at 2.0 and 7.2 °C which probably belong to
eter of 0.1 cm and 0.15 mm wall thickness.
order-disorder transitions of the imidazolium alkyl chains.
Then the reversible crystalline solid ↔ liquid crystalline
Differential scanning calorimetry (DSC) was performed with a com-
puter-controlled Phoenix DSC 204 F1 thermal analyzer (Netzsch, Selb,
D) with argon as protection gas. The samples were placed in aluminum
pans which were cold-sealed under argon. Experimental data are dis-
played in such a way that exothermic peaks occur at negative heat
flow and endothermic peaks at positive heat flow. Heating and cooling
(
S↔LC) and liquid crystalline ↔ isotropic liquid (LC↔L)
transitions are observed at 42.0 °C and 100.2 °C, respectively
(
Figure 1).
The enantiotropic liquid-crystalline mesophase can be identi-
fied as a smectic A phase by polarizing optical microscopy
(POM). The POM micrographs show focal conic fan textures
with dark homeotropic areas (Figure 2). The dark homeotropic
areas where no birefringence are observed when the long axis
–
1
rates were 5 K·min . Given temperatures correspond to the onset of
the respective thermal process.
Optical polarizing microscopy was carried out with an Axio Imager of the imidazolium cations is aligned parallel to the incident
A1 microscope (Carl Zeiss MicroImaging GmbH, Göttingen, D)
polarized light so that the light wavevector can not be rotated.
equipped with a hot stage THMS600 (Linkam Scientific Instruments
The focal conic fan textures origin when a layered structure
Ltd, Surrey, UK) and a temperature controller Linkam TMS 94
forms conical domains.
(
Linkam Scientific Instruments Ltd, Surrey, UK) and crossed polari-
C mimBr alone shows a bluish photoluminescence, which
12
zers. Images were recorded at a magnification of 100× as movies with
a digital camera after initial heating during the cooling stage. Heating
and cooling rates were 5 K·min . For the measurement the samples
were placed under argon between two cover slips which were sealed
is due to electronic transitions within the imidazolium head
group. It is well known that the imidazolium π electron system
can be excited with UV radiation [36]. The energy of the ex-
–
1
with a two-component adhesive (UHU plus 300, UHU GmbH & Co. cited state cannot be easily converted into vibrational energy
KG, Bühl, D).
as mixing between π and σ orbitals of imidazolium is negligi-
st
nd
Figure 1. DSC thermograms of neat [C12mim]Br. 1 cooling (left) and 2 heating (right).
1
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Z. Anorg. Allg. Chem. 2010, 1726–1734

Lanthanide Containing Ionic Liquid Crystals
found at 37.6 °C (0.5 mol-% EuBr ) and 27.5 °C (1.0 mol-%
2
EuBr ) compared to 38.8 °C for neat C mimBr. The LC↔L
2
12
transitions are detected at 98.8 °C (0.5 mol-% EuBr ) and
2
1
01.4 °C (1.0 mol-% EuBr ) compared to 98.6 °C for neat
2
C mimBr. Larger concentrations of EuBr favor the LC state.
12
2
In samples containing 1.9 and more mol-% EuBr the meso-
2
phasic region is extended drastically and the liquid crystalline
state is even stable at room temperature and below. The LC↔L
transitions occur at 0.7 °C (0.9 mol-% EuBr ), 0.5 °C
2
(
2.2 mol-% EuBr ) and 0.3 °C (1.0 mol-% EuBr ). The clear-
2
2
ing points are detected at temperatures similar to that for neat
C mimBr (100 °C for 0.9 mol-% EuBr , 99.5 °C for 2.2 mol-
12
2
%
EuBr and 100.3 °C for 2.9 mol-% EuBr ).
2 2
Figure 2. POM micrographs of the LC state of C12mimBr.
ble and, in consequence, photoluminescence can be observed.
The photoluminescence of C mimBr at λ(em) = 450 nm after
12
1
1
excitation with λ(ex) = 366 nm is due to the allowed ( π← π*)
transition (Figure 3). The fluorescence decay time is too short
to be measured (ns regime). Phosphorescence measurements
reveal a second, weaker emission which is detected at λ(em) =
5
15 nm for C mim that can be assigned to the spin forbidden
12
1
3
(
π← π*) transition (Figure 3). Measurements of the lumines-
cence intensity decay reveals a lifetime of ca. 200 μs for the
1
3
(
π← π*) transition.
Figure 4. DSC thermograms of C12mimBr (top) and C12mimBr doped
with different 0.5, 1.0, 1.9, 2.2 and 2.9 mol-% EuBr
2
(from top to
nd
bottom, 2 heating cycle).
Figure 3. Photoluminescence of C12mimBr at 77 K, λ(ex) = 366 nm.
EuBr in C mimBr
2
12
2
Figure 4 shows the thermal behavior of C mimBr doped Figure 5. Typical POM micrographs of a EuBr -doped C12mimBr
1
2
sample in the LC state.
with different amounts of europium dibromide. All samples
show the formation of a liquid crystalline state of matter. By
POM measurements the mesophase can be identified as a
All EuBr containing samples show an intense yellow color
2
smectic A phase (Figure 5) similar to the mesophase formed in agreement with the UV/Vis spectrum where an absorption
by neat C mimBr. Samples with 0.5 and 1.0 mol-% EuBr
around 420 nm can be detected (Figure 6, left). This absorp-
1
2
2
show only small deviations in their thermal behavior when tion is due to the f→d-transition of Eu2+ and lies in the ex-
compared to that of neat C mimBr. The S↔LC transitions are pected region.
12
Z. Anorg. Allg. Chem. 2010, 1726–1734
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1729

A. Getsis, A.-V. Mudring
ARTICLE
Figure 6. Left: UV/Vis absorption spectrum of C12mimBr (bottom) and C12mimBr doped with 0.5, 1.9 and 2.9 mol-% EuBr
nescence spectrum of C12mimBr with 2.9 mol-% EuBr [T = 77 K, λ(ex) = 350 nm].
2
. Right: photolumi-
2
At low temperatures (77 K) a weak blue emission of Eu2+
can be detected (Figure 6, right). The emission band shows
2+
the typical quite symmetric bell shape expected for Eu . The
emission maximum upon excitation with λ(ex) = 350 nm is
observed at 473 nm. This value lies between that found for
2
+
2+
KMgBr :Eu (470 nm)[37] and CsCdBr :Eu (476 nm)[38].
3
3
SmBr in C mimBr
3
12
Similar to the investigations for EuBr 1.1 mol-% SmBr3
2
was doped into C mimBr. The thermal behavior of this sam-
1
2
ple is similar to that of C mimBr with 1.0 mol-% EuBr (Fig-
12
2
ure 7). In this case, the smectic mesophase could be stabilized
from about –9 °C up to 105 °C.
Figure 8. Emission spectrum of C12mimBr with 1.1 mol-% SmBr
ex = 310 nm, 77k).
3
(λ
TbBr in C mimBr
3
12
Doping C mimBr with 1.1 mol-% TbBr yielded a liquid
12
3
crystalline phase which showed a thermal behavior similar to
3
+
C mimBr:Sm . Upon cooling liquid-crystalline mesophase
12
formation is observed from about 100 °C down to about
–
10 °C (Figure 9). Polarizing optical microscopy suggests that
a smectic A phase is formed (Figure 10).
Strong greenish luminescence is observed (Figure 11). The
5
7
transitions occur at 488, 494 nm ( D → F ), 543, 548 nm
4
6
4
7
4
7
4
7
(
D → F ), 583, 590 nm ( D → F ), 620 nm ( D → F ) and
4 5 4 4 4 3
Figure 7. DSC thermogram of C12mimBr with 1.1 mol-% SmBr
3
[1st
4
7
6
50 nm ( D → F ).
nd
4 2
cooling (top) and 2 heating cycle (bottom)].
Most interestingly the emission color of the system can be
tuned by the choice of the respective excitation wavelength.
Low temperature photophysical investigations show aside Whereas excitation with λex = 377 nm leads to the typical yel-
3
+
from transitions in the π-system of imidazolium the character- lowish-green emission of Tb , excitation with λ = 400 nm
ex
3
+
4
6
istic f–f transitions of Sm at 567 nm ( G → H5/2), 601 nm yields the typical white emission of C mimBr (Figure 12, top
5/2
12
4
4
6
4
6
3+
(
(
G5/2→ H7/2),
648 nm
G5/2→ H11/2) (Figure 8).
( G → H9/2)
and
710 nm and middle). The lifetime of the most intense f–f Tb transi-
5/2
6
tion is with τ = 3.4 ms astonishingly high.
1
730
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Lanthanide Containing Ionic Liquid Crystals
Figure 9. DSC thermogram of C12mimBr with 1.0 mol-% TbBr
3
[1st cooling (left) and 2nd heating cycle (right)].
DyBr in C mimBr
3
12
3
+
The system C mimBr:Dy behaves both thermally as well
12
3+
as photophysically similar to the analogous C mimBr:Tb
12
system. Again, mesophase formation is observed in the range
from 100 °C down to ca. –10 °C (Figure 13). The textures ob-
served by optical polarizing microscopy point to a smectic me-
sophase (Figure 14).
C mimBr doped with DyBr shows strong luminescence
1
2
3
4
6
with transitions at 578 nm ( F → H13/2), 668 nm
9
/2
4
F9/2→ H ) and 754 nm ( F →6F , H9/2) (Figure 15).
6
4
6
(
11/2 9/2 11/2
As for TbBr in C mimBr the emission color can be tuned
3
12
by the choice of the respective excitation wavelength. Whereas
excitation with the short wavelength λe x3 = 254 nm leads to the
+
characteristic yellowish emission of Dy , excitation with λ
=
ex
3
60 nm results in a bluish white emission which originates
3
+
from the superposition of Dy and C mimBr emissions (Fig-
1
2
3+
ure 12, bottom). The lifetime of the most intense f–f Dy tran-
sition is determined to τ = 53 μs.
3
Figure 10. Typical POM micrographs of a TbBr -doped C12mimBr
sample in the LC state.
Conclusions
2+
3+
Ln - and Ln -doped C mimBr turn out to be promising
new soft optical materials. The host material C mimBr itself
12
12
is an ionic liquid crystal. Lanthanide bromide dopant concen-
trations of up to ca. 1 mol-% seem to have only little effect on
the liquid crystalline properties of the host material.
All doped materials form layered liquid crystalline meso-
phases even at room temperature. At the same time all com-
pounds show reasonably good luminescence. Whereas neat
C mimBr shows a bluish white emission, by doping
12
C mimBr with EuBr blue luminescence can be achieved.
1
2
2
3+
3+
Analogous Eu or Sm materials show a strong red lumines-
cence.
Of special interest is the observation that the emission color
of C mimBr doped with TbBr and DyBr allows for the tun-
12
3
3
ing of the emission color by a mere change of the excitation
light wavelength. In case of TbBr the color can be changed
3
3+
between green (mainly Tb ) emission and white whereas in
case of DyBr a switch between orange-yellow and bluish
3
white is possible. It turns out that doping C mimBr with vari-
Figure 11. Emission spectrum of C12mimBr with 1.1 mol-% TbBr
ex = 310 nm, 77k).
3
12
(λ
ous lanthanide bromides yields interesting new materials
Z. Anorg. Allg. Chem. 2010, 1726–1734
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1731

A. Getsis, A.-V. Mudring
ARTICLE
3+
Figure 13. DSC thermogram of C12mimBr: Dy .
3
Figure 14. Typical POM micrographs of a DyBr -doped C12mimBr
sample in the LC state.
Figure 12. Top and middle: 2D excitation-emission spectra of
3
+
C
12mimBr:Tb ; bottom: 2D excitation-emission spectra of C12mimBr:
3+
Dy . The photos show the sample under irradiation with short and
longer wavelength UV light.
Figure 15. Emission spectrum of C12mimBr with 1.1 mol-% DyBr
ex = 310 nm, 77k).
3
(λ
1
732
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 1726–1734

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[
1
734
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 1726–1734