Effect of Magnetic and Electric Field on the Orientation of Rare-Earth-Containing Nematics

Article abstract of DOI:10.1021/acs.inorgchem.0c02500

We report rare-earth-containing metallomesogens with newly synthesized ligands represented by the β-diketone 1-(4-(4-propylcyclohexyl)phenyl)octane-1,3-dione (CPDk3-5) and the Lewis base 5,5′-bis(heptadecyl)-2,2′-bipyridine (bpy17-17). The stoichiometry o

Full text of DOI:10.1021/acs.inorgchem.0c02500

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Effect of Magnetic and Electric Field on the Orientation of Rare-
Earth-Containing Nematics
Andrey A. Knyazev,* Aleksandr S. Krupin, Aleksandr P. Kovshik, and Yuriy G. Galyametdinov
Cite This: Inorg. Chem. 2021, 60, 660−670
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ABSTRACT: We report rare-earth-containing metallomesogens with
newly synthesized ligands represented by the β-diketone 1-(4-(4-
propylcyclohexyl)phenyl)octane-1,3-dione (CPDk3‑5) and the Lewis
base 5,5′-bis(heptadecyl)-2,2′-bipyridine (bpy17‑17). The stoichiometry
of the complexes is [Ln(CPDk3‑5)3bpy17‑17], where Ln is a trivalent rare-
earth ion (La, Sm, Eu, Gd, Tb, Dy, Ho, Tm, and Yb). Although the
ligands themselves do not form any mesophase, the respective metal
complexes produce nematic and smectic A phases. The mesogenic rare-
earth complexes were characterized by NMR, MS, POM, DSC, X-ray
diffraction, magnetic susceptibility measurements, and dielectric
spectroscopy. The metal complexes display a remarkably large magnetic
anisotropy in the mesophase. These nematic liquid crystals can, therefore, be easily aligned by an external low-threshold magnetic
field.
the sign of Δχ is positive, LC molecules align their long axes
parallel to a magnetic field. If the sign of Δχ is negative, LC
molecules align their long axes perpendicular to a magnetic
field. An LC director can, however, have any alignment in the
plane perpendicular to a magnetic field. A combination of
these effects creates a polydomain where microdomains are
aligned arbitrarily.10
The alignment behavior of classical organic liquid crystals
has been characterized in detail. Their magnetic susceptibility
anisotropy is generally provided by diamagnetic phenyl groups.
The maximum susceptibility axis of such groups is parallel to
an LC director. The only possible alignment direction of the
respective LC molecules is, therefore, parallel to the magnetic
field force lines. For organic liquid crystals, the value of
magnetic susceptibility anisotropy does not exceed 100 × 10−6
cm3/mol.11
This value can be increased by introducing a stable
radical12−14 or paramagnetic metal ions into molecules of
liquid crystals. The examples of paramagnetic-metal-containing
liquid crystals are mesogenic complexes of copper(II),15−17
nickel(II),18 oxovanadyl,15,19,20 and trivalent ions of rare-earth
metals.21−25 The magnetic anisotropy of metallomesogens is a
sum of contributions from anisotropies provided by
INTRODUCTION
■
Liquid crystals (LCs) are anisotropic media that have acquired
the status of essential components in a broad variety of
technologies developed for processing and displaying informa-
tion. Liquid-crystalline (LC) molecules obtain an orientation
order through anisotropic intermolecular interactions.1
A
common approach to aligning LC molecules in various devices
is to apply an electric field.2−4 This method has certain
disadvantages, as an electric field initiates electrochemical
reactions at the boundary between an electrode and LC
molecules.5 In the mesophase, liquid-crystalline molecules can
be also aligned by an external magnetic field.6,7 Magnetic
alignment offers more advantages over using directors:
magnetic field force lines do not end at the surface layers of
a material; they can penetrate inside and perform complete
alignment in the bulk. Magnetic field alignment is suitable for
LC specimens with larger volumes in comparison to alignment
by an electric field. A director or an electric field generally
provides parallel or perpendicular alignment options. Alter-
natively, a magnetic field can be applied from different
directions that are easily controlled. This approach allows for
alignment that is tilted to an LC specimen surface at a required
angle.
In a magnetic field, the alignment behavior of liquid crystals
depends on their magnetic susceptibility anisotropy Δχ. The
value of this parameter can be found from the difference Δχ =
χ∥ − χ⊥, where χ∥and χ⊥ are the magnetic susceptibility tensor
components that are parallel and perpendicular to the long
molecular axis of LCs, respectively.8,9 The sign of the Δχ
parameter defines the way the long molecular axis and,
therefore, an LC director are averaged in a magnetic field. If
Received: August 21, 2020
Published: December 29, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c02500
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diamagnetic ligands (phenyl groups) and paramagnetic metal
ions. Paramagnetic anisotropy values of copper, nickel, and
oxovanadyl ions are comparable to those of the ligands, but
their signs are opposite. Total magnetic anisotropy values of
the respective compounds are in the range of (20−130) × 10−6
cm/mol. Lanthanide ions, therefore, have attracted consid-
erable attention, as their paramagnetic anisotropy is 1−2
orders of magnitude higher than that of organic liquid crystals.
The highest values of magnetic susceptibility anisotropy have
been reported for Tb(III), Dy(III), Ho(III), Er(III), and
Tm(III) ions.26
stable nematic complexes with acceptable temperature
parameters of the mesophase.
Table 1. Examples of Nematogenic Lanthanidomesogens
Complexes of lanthanides(III) are particularly important for
the production of light-emitting materials and are highly
attractive for such applications as the production of organic
light-emitting diodes, thin flexible displays, optical waveguides,
luminescent biological probes, adaptive lasers, and solar
cells.27−31 Over the past years, coordination compounds of
lanthanides have attracted considerable attention as promising
materials for luminescent thermometers. The narrow charac-
teristic emission bands of lanthanidomesogens enable perform-
ing high-precision temperature measurements.32−35 Liquid-
crystalline compounds of lanthanides are advantageous for
these applications, as they offer an opportunity to combine the
alignment behavior of their LC mesophases with a
monochromatic luminescence of certain Ln3+ ions, and,
therefore, develop novel multifunctional materials with highly
demanded performance properties.36−38 A combination of
luminescent and liquid crystal properties allows the creation of
luminescent media with controlled polarization for applications
that involve linearly polarized emission.39,40
Rare-earth ions have high coordination numbers (from 6 to
12), which do not favor the formation of an anisometric
(calamitic) geometry of a complex required to generate a
liquid-crystal state. Specific demands are, therefore, placed on
the structure of a coordination polyhedron and the ligands that
form the geometry of such a complex.41,42 In our previous
works, we characterized coordination compounds that
included Schiff bases,19,43 β-enamine ketones,44 β-dike-
tones,45−47 bis(benzimidazolyl)pyridines,48,49 phthalocya-
nines,50,51 porphyrins,52,53 and crown ethers.54,55 All of these
complexes, however, exhibit only a smectic mesomorphism.
Introducing lanthanide(III) ions into an LC molecule results in
a high viscosity of smectic or discotic mesophases due to
strong intermolecular interactions. Their high viscosity reduces
control of their alignment in a magnetic field. For lanthanide
complexes with organic ligands, we should also consider the
spatial positioning that depends on the coordination site
geometry. This positioning may influence the sign and value of
the magnetic susceptibility of a metal ion.56−58 The synthesis
of complexes that are capable of forming low-viscosity nematic
phases, therefore, faces a set of additional challenges, and the
reports of successful experiments are limited to a few
papers.59−65 Today, there are two approaches to the synthesis
of nematic lanthanidomesogens. The first approach is
coordination of the metal by organic ligands that possess
nematic properties. The disadvantage of such an approach is
the complex structure of nematogenic ligands and low thermal
stability of the resulting complexes. The second approach
proposed by the authors is to use simpler ligands that do not
exhibit liquid-crystalline properties to synthesize nematic
lanthanidomesogens (Table 1).65 The structures of these
non-liquid-crystalline ligands allows the synthesis of thermo-
Previous works have characterized the magnetic properties
of lanthanidomesogens21 where the ligands are represented by
Schiff bases and NO3, Cl, or DOS (dodecyl sulfate)
counterions.67−70 All of these complexes exhibit a smectic
mesomorphism. Magnetic fields with a strength of 10000−
12000 Oe are required to perform their alignment.21 Charged
counterions in the structure of lanthanidomesogens allow these
complexes to associate in solution and form aggregates. For
such systems, it is impossible to determine the sign of
molecular anisotropy from the results of magnetic birefrin-
gence.21
This work describes the authors’ approach to developing
lanthanidomesogens with high magnetic susceptibility aniso-
tropy by using originally synthesized β-diketones and a
substituted Lewis base as the ligands.59 While these ligands
are not liquid crystals, the respective lanthanide complexes
exhibit liquid-crystalline properties and demonstrate the
nematic type of mesomorphism. The unique structure of
such ligands allows the synthesis of thermostable nematic
coordination compounds33,71 that are capable of maintaining
their original mesophase morphology upon overcooling.40,72
The authors have proven that the compounds they synthesized
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are stable under applied electric fields. This stability allows a
study of their alignment behavior59 and a determination of the
value of the Kerr constant.73,74 As opposed to the previously
reported structure of the complexes with Schiff bases, these
new compounds have a saturated coordination sphere that
contains no counterions and includes ligands without any
active functional groups. Such complexes have, therefore,
limited capabilities for association in solution. Multiple alkyl
and aryl fragments in the ligand molecules shield a lanthanide
ion and impose an additional steric hindrance on the formation
of intermolecular associates. These structural features provide
an opportunity for assessing the value and sign of molecular
magnetic susceptibility anisotropy by measuring magnetic
birefringence (the Cotton−Mouton effect) in solution. Due to
the low-viscosity nematic mesophase, it is possible to
characterize the magnetic properties of lanthanidomesogens
aligned with weak magnetic fields using dielectric spectrom-
etry.75 There are numerous publications that have investigated
the dielectric permittivity behavior of organic liquid-crystalline
compounds. Reports on lanthanidomesogens are, however,
quite scarce due to the reasons given above. This work
proposes theoretical and practical approaches to synthesize a
series of liquid-crystalline complexes that comprise lanthanide-
(III) tris(β-diketonates) and 5,5′-bis(heptadecyl)-2,2′-bipyr-
idine and can form a nematic mesophase. These compounds
possess unique properties such as a high chemical stability, a
constant molecular structure at phase transitions, and a
permanent supramolecular organization in mesophases over-
cooled to room temperature. These properties make it possible
to study the effect of external fields on such metallomesogens.
This work aims at applying newly gained data to formulate
original concepts that establish correlations between the
structure of an unusual class of liquid crystals (nematic
lanthanidomesogens) and their alignment behavior in mag-
netic and electric fields.
determined by analyzing the microscope images of the
mesophases), differential scanning calorimetry (changes in
phase transition enthalpy and entropy), and X-ray phase
analysis (interlayer distances and packing parameters).
The room-temperature luminescence spectra of the
synthesized compounds in hexane contain characteristic
bands of metal ion transitions that agree with the literature
data (Figure 2).76−78
Figure 2. Luminescence of Eu(III) (a), Tb(III) (b), Sm(III) (c), and
Yb(III) (d) complexes in hexane. The concentration of the complexes
is 10−3 M.
These luminescence spectra are similar to the spectra of
analogous compounds that contain similar ligands without
long chains. These spectra, therefore, confirm coordination of
the metal ion.79,80
Upon heating, the complexes exhibit smectic A and nematic
polymorphism (Figure 3). Upon cooling from an isotropic
RESULTS AND DISCUSSION
■
In this work, we synthesized a series of liquid crystal complexes
that comprise tris(β-diketonates) of lanthanides(III) and 5,5′-
bis(heptadecyl)-2,2′-bipyridine (Figure 1).
Figure 3. Phase diagram of β-diketonate lanthanide(III) complexes.
The insets give optical microscopy images of nematic (a), transition
(b), and smectic A (c) mesophases of the Eu(III) complex at 500×
magnification.
Figure 1. Synthesis of lanthanide complexes, where Ln = La(III),
Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Tm(III),
Yb(III).
The composition and structure of the complexes were
confirmed by elemental analysis, X-ray fluorescence (XRF),
NMR spectroscopy, mass spectroscopy and IR spectroscopy
(see the Supporting Information). The liquid-crystalline
properties were identified by polarized light microscopy (the
types of mesophases and phase transition temperatures were
state, the complexes vitrify while maintaining their original
smectic A mesophase texture that can be observed by a
polarized microscope. The DSC thermograms also confirm
that the texture does not change during vitrification, as they
contain no crystallization peaks (Figure 4). There is no definite
correlation between the phase transition temperature of a
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lanthanide(III) complex and the type of lanthanide(III) ion
(Figure 3).
The presence of a large number of long alkyl substituents in
the structures of complexes does not allow one to grow a single
crystal, since amorphous products are formed in the synthesis,
as has been shown above. Therefore, we have tried
preliminarily to estimate the sizes of molecules by quantum
chemistry methods. The geometries of eight isomers of the
studied complexes with different arrangements of β-diketones
relative to the plane formed by the 5,5′-bis(heptadecyl)-2,2′-
bipyridine ligand and lanthanum ion were considered.
Probable isomers of the lanthanum complex are represented
in Table S2 in the Supporting Information. On the basis of the
calculations of the equilibrium geometries for the isomer, 5
with the energetically most advantageous arrangement of the
ligands in the complex was chosen for further calculations of
the geometric anisotropy values. As the numerical value of the
molecule is 51.4 Å (Table S2), we propose the model of
packing in layers shown in Figure 6 with interpenetration of
alkyl fragments.
Figure 4. DSC thermogram of the Tb(III) complex. The rates of
heating and cooling are both 20 °C/min.
Figure 4 shows the thermogram of the terbium(III)
complex, which is a typical DSC thermogram with heating
and cooling. As these complexes vitrify upon cooling, only the
transition from the isotropic melt to the smectic phase could
be identified. Table S1 summarizes the thermodynamic
characteristics of the phase transitions (enthalpies) calculated
for the substances studied in this work. The values of phase
transition enthalpies are typical for LC complexes of
lanthanides(III).40 Smaller Cr → SmA transition enthalpies
are observed for lanthanides with lower atomic numbers.
The synthesized adducts tend to vitrify upon cooling
without changing their molecular packing. Therefore, it is
possible to characterize their glassy states by small-angle X-ray
diffraction at room temperature (Figure 5).
Figure 6. Molecular packing presumed for the smectic mesophase.
The alignment of a metal-containing liquid crystal in a
magnetic field was characterized by the dependence of
magnetic susceptibility on temperature. The generally
applicable techniques are the Faraday method,81 EPR spec-
troscopy,23,44,82 and determination of the anisotropy sign by
the Cotton−Mouton effect.21,26 The magnetic susceptibility
anisotropy of the mesophase formed by the nematic Ln-
(CPDk3‑5)3bpy17‑17 adducts (Tb, Dy, Gd, Eu, Ho, Er, Tm) was
evaluated by the Faraday method (Figure 7). Cooling the
isotropic melt of this sample leads to molecular ordering under
an external magnetic field and increases the total magnetic
moment (Figure 7a). The values of magnetic susceptibility
anisotropy of complexes were calculated from the difference
between the effective magnetic moments of the ordered and
disordered states of this system. The sign of magnetic
susceptibility anisotropy was determined from the data for
their molecular values.66 Table S3 compares the magnetic
susceptibility anisotropy values calculated for the mesophases
and the molecular values of magnetic susceptibility anisotropy
calculated for the Ln(CPDk3‑5)3bpy17‑17 adducts. It is known
that the magnetic susceptibility anisotropy of lanthanide
complexes is defined not only by the metal but also by
distortions in the coordination sphere. According to ref 58, the
values of magnetic susceptibility anisotropy for, e.g., complexes
of Dy(III), can vary from −4 × 10−2 to 8 × 10−2 cm3/mol,
depending on the geometry of the coordination center. In
comparison to organic paramagnetic liquid crystals containing
stable radicals,83 the orientation of which is stable in magnetic
fields of 1.5−2 T intensity, the magnetic susceptibility
anisotropy of lanthanide complexes is larger by 2 orders of
magnitude, which should allow us to orient a layer of a
lanthanide mesogen by weak external magnetic fields.
Figure 5. Diffractogram of the La(CPDk3‑5)3bpy17‑17 complex.
The diffraction patterns of the polymorphous states of the
complexes are represented in the diffractogram of the
lanthanum complex (Figure 5). This diffractogram shows the
peaks (2θ = 2.84, 5.7°) at room temperature. These peaks
correspond to the first and second reflexes of the interlayer
distance d that is equal to 31 Å and confirm the existence of an
ordered smectic mesophase. A wide diffusion peak at 2θ =
19.56° corresponds to the intermolecular distance dw = 4.5 Å.
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Figure 8. Absolute values of magnetic susceptibility anisotropy for
various types of Ln(III) ions.
In this work, we implemented an original approach to
characterize the alignment behavior of the synthesized LC
complexes in magnetic and electric fields by analyzing the data
for both the sign and value of magnetic susceptibility
anisotropy (Δχ). While constant magnetic and varying electric
fields were simultaneously applied to the samples of
lanthanidomesogens, the directions of these fields varied.
The behavior of samples was controlled by the magnetic field.
The mesophase alignment degree and direction were assessed
from the values of dielectric permittivity (Figure 9).
Figure 7. Effective magnetic moments of the complexes Dy-
(CPDk3‑5)3bpy17‑17 (a) and Dy(DK12‑16)3Phen (b) at different
temperatures.
The temperature dependence of the effective magnetic
moment plotted for the complex Dy(CPDk3‑5)3bpy17‑17
confirms that it maintains its supramolecular organization
upon further cooling and vitrification of the system. This
temperature dependence is compared to the temperature
behavior of the effective magnetic moment demonstrated by
the smectic Dy(DK12‑16)3Phen complex,47 which partially
crystallizes upon cooling (Figure 7b). The respective plot
shows a sharp fall of the effective magnetic moment caused by
crystallization.
According to Figure 8 and Table S3, we observe a similar
magnetic susceptibility anisotropy behavior of the Ln-
(CPDk3‑5)3bpy17‑17 complexes in solution and in the
mesophase. Absolute molecular values of magnetic suscepti-
bility anisotropy are always higher than the values of magnetic
susceptibility anisotropy due to cooperative effects in the layer
of liquid crystals and incomplete alignment of molecules in the
mesophase (the nematic phase order parameter values are
usually in the range of 0.3−0.7).1 There is an adequate
correlation between these values. The results of magnetic
birefringence in solutions are thus applicable to qualitative and
quantitative characterization of mesophase alignment in a
magnetic field.
Figure 9. Alignment of lanthanidomesogens in a magnetic field: (a, b)
lanthanidomesogens with a positive anisotropy of magnetic
susceptibility (for example, Eu(CPDk3‑5)3bpy17‑17); (c, d) lanthani-
domesogens with a negative anisotropy of magnetic susceptibility (for
example, Tb(CPDk3‑5)3bpy17‑17).
Dielectric permittivity was measured at parallel (εpar)
(Figure 9a,c) and normal (εnor) (Figure 9b,d) directions of
magnetic and electric fields; the orientation of LC molecules
between the electrodes was not considered. For complexes
with Δχ > 0, a parallel overlap of the magnetic and electric
fields was set to determine the longitudinal component of
dielectric permittivity (εpar = ε∥) (Figure 9a), and a normal
overlap was set to determine its transverse component (εnor
=
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ε⊥) (Figure 9b). For complexes with Δχ < 0, a parallel overlap
of magnetic and electric fields was set to determine the
transverse component of dielectric permittivity (εpar = ε⊥)
(Figure 9c) and a normal overlap was set to determine the sum
of the ε∥ and ε⊥ contributions (εnor = ε, Figure 9d).
For all the samples of the lanthanide complexes, it was found
that applied electric voltages below 2.5 V do not make a
substantial contribution to their alignment (Figure S1).
Dielectric permittivities of all the samples were, therefore,
measured with the applied voltage U = 1 V, which does not
exceed the Freedericksz transition threshold.84
Taking into consideration that the director of LC complexes
may be unaligned with the magnetic field,19 the angular
behavior of dielectric permittivity components was studied at
the temperature that corresponds to the existence of the
nematic mesophase to eliminate a possible experimental error
(Figure 10 and Figure S2). The samples were aligned
macroscopically by a magnetic field of 5000 Oe.
fields in the 100 Hz to 5 MHz electric field frequency range.
The results revealed a dispersion of both components over the
entire temperature range of the mesophase existence (Figure
S3). A rapid growth of all the εpar and εnor components at
frequencies below 5 kHz is caused by the contribution of a
reach-though conductivity to the effective values of dielectric
permittivity. The dielectric permittivity of all the samples was,
therefore, measured at 10 kHz to ensure that both its
components undergo dispersion.
The dependence of dielectric permittivity on the frequency
of an applied electric field was approximated by the Cole−Cole
equation. Figure 11a and Figure S4a show the resulting
temperature dependence of the quasistatic values of dielectric
permittivity.
Figure 10. Dielectric permittivity of the complexes Eu-
(CPDk3‑5)3bpy17‑17, Tb(CPDk3‑5)3bpy17‑17, and Er(CPDk3‑5)3bpy17‑17
vs magnetic field angle at 130 °C. The frequency of the electric
current is 10 kHz.
The experimental values of the dielectric permittivity
components at different angles demonstrate that the molecular
axes of the Gd(III), Sm(III), and Er(III) complexes coincide
with the direction of the aligning magnetic field. For the
Eu(III) complex, the deviation is 10°, which may be caused by
the deformation of its coordination polyhedron geometry. For
the Tb(III) and Dy(III) complexes, the respective directors are
perpendicular to the magnetic field direction. The complexes
Gd(III), Sm(III), Er(III), and Eu(III) are thus aligned
according to the scheme shown in Figure 9a,b; the alignment
behavior of the complexes Tb(III) and Dy(III) is shown in
Figure 9c,d.
The angular behavior of dielectric permittivity reveals that
these coordination compounds of lanthanides have a negative
(Δε < 0) dielectric permittivity anisotropy (Figure 10 and
Figure S2). The maximum values of dielectric permittivity
anisotropy are achieved with the alignment of the LC
molecules perpendicular to the applied electric field. Different
values of the dielectric permittivity minimums at φ = 90° and
φ = 270° (Figure 10 and Figure S2) indicate that the LC
domains are aligned differently with the direction of electrodes.
Dielectric permittivities εpar and εnor were measured at a
parallel or a perpendicular overlap of magnetic and electric
Figure 11. (a) Quasistatic values of ε0 and (b) relaxation times of the
complex Tb(CPDk3‑5)3bpy17‑17 at different temperatures.
An analysis of this temperature dependence has shown that
the dielectric permittivity anisotropies (Δε) of all the
lanthanide complexes are negative (Δε < 0). Their absolute
values are similar. We can thus conclude that the ligand
environment has a more substantial effect on the dielectric
properties of lanthanidomesogens than the type of the
lanthanide ion, as opposed to their magnetic properties. At
temperatures that provide the existence of the nematic phase,
the dielectric anisotropy Δε of the complexes varies in the
range between 0.5 and 3.0. Therefore, it is possible to
effectively control the LC director alignment by an applied
electric field.
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Dispersions of dielectric permittivities ε∥ and ε⊥ were
analyzed to determine relaxation times at various temperatures.
Figure 11b and Figure S5 show logarithmic relaxation times τ∥,
τ⊥, and τis as the functions of the inverse temperature 1/T. The
Arrhenius equation was used to calculate the activation
energies of the intermolecular mechanisms that are responsible
for the dipole polarization in the mesophase and isotropic state
in the frequency range considered in this work.
The activation energies U1 and U2 were calculated for
smectic and nematic phases. The difference between these
values does not exceed the experimental error. The activation
energy values are in the range of (100−120) 4 kJ/mol for all
of the Ln(III) complexes. They are slightly higher than the
values provided for organic nematic liquid crystals85−87 and are
comparable to the values obtained for LC polymers.88 The
activation energy calculated for the isotropic phase is
anisotropy (Figure 12a). For the complexes of Tb(III) (Figure
12b) and Dy(III) (Figure S6), which have the highest
anisotropy of magnetic susceptibility, a magnetic field as
weak as 200 Oe is sufficient to create a plateau on the curve
plotted in the dielectric permittivity vs magnetic field strength
coordinates. Organic liquid crystals, however, require at least
500−1000 Oe magnetic fields for their alignment.89 Even the
smectic derivatives of terbium and dysprosium ions that were
examined in our previous work can be aligned by magnetic
fields with a starting strength of 10000−12000 Oe.21
CONCLUSION
■
In this work, we described lanthanidomesogens with original β-
diketones and Lewis bases as the ligands. The ligands
themselves do not form liquid-crystalline phases, but their
respective complexes exhibit mesomorphism. All of the
complexes are polymorphic and produce the smectic A and
low-viscosity nematic phases. Due to their large magnetic
anisotropy, paramagnetic liquid crystals can be aligned by an
external magnetic field. This remarkable property makes it
possible to maintain the supramolecular organization in the
liquid crystal phase upon overcooling. Depending on the type
of rare-earth ion, the complexes align their long molecular axis
either parallel or perpendicular to the magnetic field.
Therefore, the direction of alignment can be changed by a
simple replacement of a rare-earth ion with another one,
without modifying the ligands. The presence of a low-viscosity
nematic mesophase makes it possible to control the orientation
of lanthanidomesogens by a low-threshold magnetic field. The
magnetic properties of lanthanidomesogens were studied by
dielectric spectrometry. The anisotropy of the dielectric
constant of lanthanidomesogens with β-diketones and Lewis
bases as the ligands was measured. While the replacement of a
Ln(III) ion does not have any significant influence on the
dielectric properties of LC complexes of lanthanides, the ligand
environment has been established to exert a significant effect
on these properties. For the first time, the possibility of
controlling alignment by weak magnetic fields of 200 Oe has
been demonstrated for nematic lanthanidomesogens contain-
ing Tb(III) and Dy(III) ions. In comparison to these
lanthanidomesogens, the other known representatives of
smectic lanthanide-containing LCs require magnetic fields
with a strength of more than 10000 Oe for their alignment.
The unique structure of the ligands makes it possible to obtain
thermostable rare-earth-containing metallomesogens with a
mesophase that can be overcooled to a vitrified state and form
an anisotropic glass. Anisotropic magnetic materials can,
therefore, be produced by performing vitrification of such
systems in combination with an applied magnetic field.
significantly lower. Its value is Uis = 80
5 kJ/mol for the
majority of the LC complexes.
The experimental data that characterize the alignment of
lanthanidomesogens in an electric field were taken into
consideration to analyze the effect of the magnetic field
strength on the Freedericksz transitions in the complex
Tb(CPDk3‑5)3bpy17‑17 (Figure 12).
The results confirm that a lower magnetic field strength is
required to initiate the Freedericksz transition in nematic
lanthanidomesogens with higher magnetic susceptibility
EXPERIMENTAL SECTION
The synthesis procedure of the europium tris[1-(4-(4-
■
propylcyclohexyl)phenyl)octane-1,3-dione 5,5′-bis(heptadecyl)-2,2′-
bipyridine complex (Eu(CPDk3‑5)3bpy17‑17
) is described
below.66,72,90,91 An alcohol solution containing 0.037 g (0.1 mmol)
of EuCl3·6H2O was carefully added dropwise to an agitated and hot
alcohol solution containing 0.103 g (0.3 mmol) of the β-diketone 1-
(4-(4-propylcyclohexyl)phenyl)octane-1,3-dione, 0.063 g (0.1 mmol)
of 5,5′-bis(heptadecyl)-2,2′-bipyridine, and 0.017 g (0.3 mmol) of
KOH. A yellow precipitate was then filtered from the agitated residual
hot solution, washed with ethanol, and dried under vacuum. Yield:
0.110 g (61%). Formula: C113H175N2O6Eu. Anal. Found: C, 74.97; H,
9.82; N, 1.48; Eu, 8.50. Calcd: C, 75.00; H, 9.75; N, 1.55; Eu, 8.40.
ESI-MS (m/z): 1832.6 (M + Na)+.
Figure 12. Dielectric permittivity of the Tb(CPDk3‑5)3bpy17‑17
complex vs magnetic field strength at 130 °C (a) and the threshold
field H vs magnetic susceptibility anisotropy for different types of
nematic lanthanidomesogens (b).
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La(CPDK3‑5)3bpy17‑17. Yield: 0.108 g (60%). Formula:
The applied magnetic field intensity was 1.5 T. To optimize the
alignment of molecules, the cooling rates did not exceed 1 °C/min.
Molar susceptibilities were corrected by applying the Pascal scheme to
consider internal diamagnetism.26 The equation μeff/μB = 2.828-
(χT)1/2 was used to calculate magnetic moments.
The degree of alignment of lanthanidomesogens was analyzed by
the capacitance method.94,95 Under applied external fields, the degree
of alignment correlates with the electric capacity of the experimental
cell. The changing capacity of liquid crystal cells under applied
magnetic fields of various strengths makes it possible to characterize
the effect of a magnetic field on the degree of alignment in a sample.
The samples were aligned macroscopically by a magnetic field of 5000
Oe.
The measurement cell was a flat titanium capacitor with 12 pF
capacity and d = 200 μm distance between electrodes. Capacities were
found with a HIOKI-3532 RLC-meter.84 Dielectric permittivity
components in the 100 Hz to 5 MHz electric field frequency range
were determined from the reference set of dielectric permittivity
values corresponding to the computer-set frequencies.
The threshold voltage was found from the response of the dielectric
permittivity of a cell filled with a lanthanide complex to the electric
field strength. Dielectric permittivities at various angles φ between
electric and magnetic fields were found by rotating the cell in relation
to the magnets at the temperature of the nematic mesophase
existence. The rotation frequency of 104 Hz provided quasistatic
values of dielectric permittivity components.
C113H175N2O6La. Anal. Found: C, 75.18; H, 9.97; N, 1.52; La, 7.45.
1
Calcd: C, 75.55; H, 9.82; N, 1.56; La, 7.73. H NMR (CDCl3, 400
MHz): δ −0.80 to 1.07 (m, 24H, CH3); 1.13−1.92 (m, 117H, CH2,
C6H10); 2.00−2.45 (m, 6H, RCH2CO); 2.48−2.65 (m, 3H, C6H10);
2.61−2.67 (m, 4H, CH2Pyr); 5.72−5.83(m, 1H, CHCO); 6.12 (s,
2H, CHCO); 7.23−7.37 (m, 6H, C6H4); 7.56−7.59 (m, 2H, Pyr H4);
7.79−7.84 (m, 6H, C6H4), 8.43−8.52 (m, 2H, Pyr H3); 8.23−8.26
(m, 2H, Pyr H6). ESI-MS (m/z): 1795.2 (M+). IR (ν, cm−1): 208,
412, 467 (La−N), 175, 208, 308, 403 (La−O).
Sm(CPDK3‑5)3bpy17‑17. Yield: 0.110 g (61%). Formula:
C113H175N2O6Sm. Anal. Found: C, 74.57; H, 9.97; N, 1.54; Sm,
8.16. Calcd: C, 75.07; H, 9.76; N, 1.55; Sm, 8.32. ESI-MS (m/z):
1831.1 (M + Na)+.
Gd(CPDK3‑5)3bpy17‑17. Yield: 0.118 g (65%). Formula:
C113H175N2O6Gd. Anal. Found: C, 74.37; H, 10.01; N, 1.52; Gd,
8.48. Calcd: C, 74.78; H, 9.72; N, 1.54; Gd, 8.66. ESI-MS (m/z):
1814.3 (M+).
Tb(CPDK3‑5)3bpy17‑17. Yield: 0.122 g (67%). Formula:
C113H175N2O6Tb. Anal. Found: C, 74.20; H, 10.03; N, 1.53; Tb,
8.81. Calcd: C, 74.71; H, 9.71; Tb, 8.75; N, 1.54. ESI-MS (m/z):
1838.3 (M + Na)+.
Dy(CPDK3‑5)3bpy17‑17. Yield: 0.118 g (65%). Formula:
C113H175N2O6Dy. Anal. Found: C, 74.11; H, 9.98; N, 1.53; Dy,
8.56. Calcd: C, 74.57; H, 9.69; N, 1.54; Dy, 8.93. ESI-MS (m/z):
1843.2 (M + Na)+.
Dielectric permittivity components at various electric field
frequencies or magnetic field strengths were found at the following
angles: φ = 10° for the Eu(III) complex; φ = 0° (εpar = ε∥) and then φ
= 90° in relation to the initial position (εnor = ε⊥) for the Gd(III),
Sm(III), and Er(III) complexes; φ = 0° (εpar = ε⊥) and then φ = 90°
(εnor = ε) for the Tb(III) and Dy(III) complexes.
Dielectric permittivities at various magnetic field strengths were
found by varying the electric current in magnet coils at the
temperature of the nematic mesophase existence.
Ho(CPDK3‑5)3bpy17‑17. Yield 0.113 g (62%). Formula:
C113H175N2O6Ho. Anal. Found: C, 74.29; H, 9.73; N, 1.52; Ho,
9.32. Calcd: C, 74.47; H, 9.68; N, 1.54; Ho, 9.05. ESI-MS (m/z):
1822.6 (M+).
Er(CPDK3‑5)3bpy17‑17. Yield: 0.118 g (65%). Formula:
C113H175N2O6Er. Anal. Found: C, 74.25; H, 9.76; N, 1.52; Er, 9.35.
Calcd: C, 74.37; H, 9.67; N, 1.54; Er, 9.17. ESI-MS (m/z): 1847.8 (M
+ Na)+.
Tm(CPDK3‑5)3bpy17‑17. Yield 0.113 g (62%). Formula:
C113H175N2O6Tm. Anal. Found: C, 74.19; H, 9.73; N, 1.53; Tm,
9.38. Calcd: C, 74.30; H, 9.66; N, 1.53; Tm, 9.25. ESI-MS (m/z):
1849.6 (M + Na)+.
The frequency data were approximated by the Cole−Cole equation
(1); the reach-through conductivity of the samples was taken into
consideration (solid lines in Figure S3):
Yb(CPDK3‑5)3bpy17‑17. Yield 0.123 g (67%). Formula:
C113H175N2O6Yb. Anal. Found: C, 74.02; H, 9.71; N, 1.52; Yb,
9.48. Calcd: C, 74.14; H, 9.64; N, 1.53; Yb, 9.45. ESI-MS (m/z):
1853,4 (M + Na)+.
ε0 − ε∞
B
ε* = ε∞
+
+ i
1 + (i2πfτ)1−α
f N
(1)
where ε0 is the quasistatic dielectric permittivity value, ε∞ is the high-
frequency dielectric permittivity value, τ is the average dielectric
relaxation time; α is the parameter that characterizes the relaxation
time distribution, and B and N ≤ 1 are the numerical coefficients.
For complexes with a negative anisotropy of magnetic suscepti-
bility, the longitudinal component of dielectric permittivity was
estimated using the Maier−Meier theory for dielectric polarization of
liquid crystals.96 According to this theory, the average dielectric
permittivity (2)
1H NMR spectra were recorded with Bruker Avance 400
spectrometers. The mass spectra were obtained using a Bruker
Esquire LC-Ion Trap mass spectrometer. CHN elemental micro-
analysis was performed on a Delta V Plus isotope mass spectrometer
(Thermo Fisher Scientific). Lanthanide elemental microanalysis was
performed on a Bruker S8 TIGER X-ray fluorescence instrument. FT-
IR spectra were recorded on a Bruker IFS-66v/s IR Fourier
spectrometer. Luminescence spectra were measured with a Varian
Cary Eclipse spectrofluorimeter.
The liquid-crystal properties of the complexes were examined on an
Olympus BX51 polarizing optical microscope with a Linkam precision
temperature control system. Thermodynamic parameters of phase
transitions were characterized on a Mettler Toledo Star System DSC
1 differential scanning calorimeter. The scan rate was 10 K/min.
The presence of a large number of long alkyl substituents in the
structure of the studied anisometric Eu(III) complexes does not allow
one to prepare a single crystal for X-ray diffraction analyses and obtain
a crystal structure. Therefore, quantum-chemical simulation is the
only technique for the study of the structure and certain properties of
anisometric Eu(III) compounds.
Quantum-chemical calculations of the equilibrium geometry of
Eu(III) complexes in the ground state were performed in the gas
phase using the Priroda 06 software by the DFT method with the PBE
exchange correlation functional as described in ref 36.
The temperature dependence of magnetic susceptibilities was
analyzed on a Faraday magnetometer with a custom heating system.92
The measurements were performed using a Cahn RG computer
microbalance and a Bruker B-MN 200/40 power supply system.93
ε = (ε∥ + 2ε )/3
(2)
̅
⊥
is equal to εis or differs insignificantly from it (by several percent) at
the temperature of the transition from the isotropic phase to the
mesomorphous phase. Extrapolation of the temperature curve εis to
the mesophase range makes it possible to characterize the
temperature behavior of ε and calculate the values of ε∥ at various
temperatures (Figure 9a).
̅
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02500.
■
sı
Thermodynamic characteristics of phase transitions,
magnetic susceptibility anisotropy, dielectric permittivity
of the Ln(III) complexes, and the optimized structures
of Eu(III) complex isomers (PDF)
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Article
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AUTHOR INFORMATION
■
Corresponding Author
Andrey A. Knyazev − Physical and Colloid Chemistry
Department, Kazan National Research Technological
University, 420015 Kazan, Russia; orcid.org/0000-0001-
6697-1473; Email: knjazev2001@mail.ru
Authors
(17) Alonso, P. J.; Martínez, J. I. Relationship between the Magnetic
Field Induced Orientation in the Mesophases of Metallomesogens
Derived from Schiff’s Bases and Their Mesophase Structure. Liq.
Cryst. 1996, 21, 597.
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Ovchinnikov, I.; Haase, W. Paramagnetic Liquid Crystalline Nickel-
(II) Compounds. Adv. Mater. 1994, 6, 381.
(19) Galyametdinov, Y. G.; Haase, W.; Goderis, B.; Moors, D.;
Driesen, K.; Van Deun, R.; Binnemans, K. Magnetic Alignment Study
of Rare-Earth-Containing Liquid Crystals. J. Phys. Chem. B 2007, 111,
13881.
(20) Marin, L.; Destri, S.; Porzio, W.; Bertini, F. Synthesis and
Characterization of New Azomethine Derivatives Exhibiting Liquid
Crystalline Properties. Liq. Cryst. 2009, 36, 21.
(21) Binnemans, K.; Galyametdinov, Y. G.; Van Deun, R.; Bruce, D.
W.; Collinson, S. R.; Polishchuk, A. P.; Bikchantaev, I.; Haase, W.;
Prosvirin, A. V.; Tinchurina, L.; Litvinov, I.; Gubajdullin, A.;
Rakhmatullin, A.; Uytterhoeven, K.; Van Meervelt, L. Rare-Earth-
Containing Magnetic Liquid Crystals. J. Am. Chem. Soc. 2000, 122,
4335.
Aleksandr S. Krupin − Physical and Colloid Chemistry
Department, Kazan National Research Technological
University, 420015 Kazan, Russia
Aleksandr P. Kovshik − Department of Molecular Biophysics
and Polymer Physics, Saint Petersburg State University,
199034 Saint Petersburg, Russia
Yuriy G. Galyametdinov − Physical and Colloid Chemistry
Department, Kazan National Research Technological
University, 420015 Kazan, Russia
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02500
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Russian Science Foundation,
grant No 18-13-00112.
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Liquid Crystals and Surfactants. Chem. Rev. 2002, 102, 2303.
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