A lyotropic chiral smectic c liquid crystal with polar electrooptic switching

Article abstract of DOI:10.1002/anie.201303344

A lyotropic analogue of the ferroelectric smectic C* phase has been found. The lyotropic smectic C* phase shows macroscopic chirality effects, such as a helical ground state and polarity-dependent electrooptic switching, thus indicating the presence of a spontaneous electric polarization. The helicity implies communication of the chiral director twist across the achiral solvent layers separating adjacent layers of the chiral mesogens. Copyright

Full text of DOI:10.1002/anie.201303344

Angewandte
Chemie
DOI: 10.1002/anie.201303344
Liquid Crystals
A Lyotropic Chiral Smectic C Liquid Crystal with Polar Electrooptic
Switching**
Johanna R. Bruckner, Jan H. Porada, Clarissa F. Dietrich, Ingo Dierking, and
Frank Giesselmann*
Dedicated to Professor Helmut Bertagnolli on the occasion of his 70th birthday
Thermotropic and lyotropic liquid crystals are often treated
separately, in research as well as in applications. While
thermotropic phases are formed by mesogenic molecules in
a certain temperature range, lyotropic phases are built up by
aggregates of nonmesogenic amphiphilic molecules in the
presence of a solvent. Even though the building blocks of
thermotropics and lyotropics are rather different, they form
a common state of matter with liquid-crystalline phases of
similar structure and symmetry. For example, the thermo-
tropic smectic A phase (SmA),[1] which is shown in Figure 1a,
has a well-known lyotropic analogue, the so called lamellar
La phase[2] (Figure 1b). On account of this, it is astonishing
that some thermotropic phases have not been observed in
lyotopics and vice versa. A significant example of this is the
smectic C phase (SmC),[3] which consists of 1D stacks of 2D
Figure 1. Schematic picture of a) the well-known thermotropic SmA
fluid layers (Figure 1c). In these layers the molecules are
ordered along the common direction of the director n, which
is tilted with respect to the layer normal k by the tilt angle q.
This tilt direction is long-range correlated over macroscopic
distances. In thermotropic liquid crystals this is one of the
most common and best investigated phases, as the chiral
variant, SmC*, which is formed by chiral molecules, is of great
phase composed of calamitic mesogens with indicated directions of
director n and layer normal k, b) the lyotropic analogue of the SmA
phase, La, composed of bilayers of amphiphilic molecules separated by
*
solvent molecules ( ), c) the thermotropic SmC phase, and d) the very
uncommon lyotropic SmC phase.
example, by X-ray diffraction.[8] Furthermore, there has
never been a report of a lyotropic chiral SmC* phase. Thus,
many important questions arise concerning the constraints in
the formation of a lyotropic SmC phase and about possible
chirality effects in a lyotropic SmC* phase. For example, it is
unclear if and how there can be a long-range transmittance of
the tilt direction through the solvent layers, as there is no
direct contact between the tilted molecules. Moreover, it is
questionable if the chirality of the individual smectic layers
and also the helical correlation could be communicated
through the intermediatory achiral solvent. To answer these
questions, molecules were devised which possess a relatively
bulky polar head group containing ethylene glycol units, an
element known from a lyotropic SmC liquid crystal inves-
tigated by Schafheutle and Finkelmann,[8] and which at the
same time carry an element of chirality. Now, we report the
observation of the first chiral SmC* phase which forms in the
presence of a solvent.
The chemical structure of the investigated mesogen 1 is
shown in Figure 2 (the synthesis is described in the Supporting
Information). The molecule combines typical structural
elements of thermotropic and lyotropic liquid crystals. On
the one hand it possesses an ethylene glycol unit and a chiral
diol head group which allow the molecule to mix with polar
solvents,[9] and on the other hand it contains an SmC-
promoting phenylpyrimidine core.[10]
scientific and technological interest[4] because it shows
[5,6]
a spontaneous polarization PS
and macroscopic chirality
in the form of a helical twist of the tilt direction with pitch p.[5]
In contrast, the lyotropic analogue to the SmC phase
(Figure 1d) is almost unknown. There are very few reports of
this phase[7] and even fewer in which the existence of this
lamellar tilted and fluid phase is definitely proven, for
[*] J. R. Bruckner, C. F. Dietrich, Prof. Dr. F. Giesselmann
Institute of Physical Chemistry, University of Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
E-mail: f.giesselmann@ipc.uni-stuttgart.de
Dr. J. H. Porada
Department of Chemistry and Biochemistry
University of Colorado, Boulder, CO 80309 (USA)
Dr. I. Dierking
School of Physics and Astronomy
University of Manchester (UK)
[**] Financial support by the Deutsche Forschungsgemeinschaft (DFG
Gi 243/4 and Materials World Network DFG Gi 243/6) is gratefully
acknowledged. J.R.B. thanks the Landesgraduiertenfçrderung
Baden-Wꢀrttemberg for a PhD scholarship. J.H.P. thanks the
Deutsche Forschungsgemeinschaft for a research fellowship.
Supporting information for this article (experimental details) is
available on the WWW under http://dx.doi.org/10.1002/anie.
201303344.
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Figure 2. Chemical structure of the investigated diol 1.
In the absence of solvent, diol 1 exhibits no enantiotropic
liquid-crystalline phases, only a monotropic chiral nematic
phase (N*) on cooling. In mixtures with water, it shows
several different lyotropic mesophases (Figure 3). Very small
Figure 4. Texture images between crossed polarizers (P, A) of the
lyotropic SmC* phase showing a) a schlieren texture (42 wt% water,
608C), b) a broken fan texture with zigzag defect lines (65 wt% water,
468C), c) and d) domains with opposite tilt directions in the surface-
stabilized state of a 1.6 mm cell (17 wt% formamide, 288C).
layer spacing of the SmA* phase shows negative thermal
expansion, with a maximum d value around the SmA*–SmC*
transition. Below this temperature the layer spacing greatly
decreases. This behavior is characteristic of an SmC or SmC*
phase, due to the increasing tilt angle q with decreasing
temperatures.[12] Perhaps the most direct proof of a lyotropic
SmC-analogue structure is given by the 2D diffraction pattern
from a monodomain sample[13] (Figure 5b). The sharp funda-
mental and higher-order pseudo-Bragg peaks in the inner
Figure 3. Phase diagrams of the diol 1/water systems.
amounts of the solvent turn the N* phase into an enantio-
tropic but still quite narrow phase. Furthermore, two colum-
nar phases, Col1 and Col2, can be observed. The dominating
phase of the diol 1/water system is a lyotropic SmA* phase. At
water concentrations above 20 wt%, an enantiotropic phase
occurs, which will later be shown to be a lyotropic SmC*
phase. The fact that this phase appears only in the presence of
a solvent has to be emphasized, because this demonstrates
that the phase is not an artifact of a thermotropic SmC* phase
but a real lyotropic phase. The phase diagram for mixtures
with formamide (see the Supporting Information) shows
a similar behavior, with the same phases occurring. However,
the two columnar phases and the SmC* phase are now
monotropic.
The first evidence to confirm the existence of the lyotropic
SmC* phase is the observation of SmC*-characteristic
textures by polarized optical microscopy. The representative
texture images in Figure 4 show schlieren and broken-fan
textures as well as zigzag defects,[11] which are all very typical
of an SmC or SmC* phase. Furthermore, the spontaneous tilt
domains in Figure 4c,d suggest that this is a surface-stabilized
ferroelectric SmC* phase.[6]
Further evidence of the existence of a lyotropic SmC*
phase can be obtained by means of X-ray diffraction. In
Figure 5a the temperature dependence of the smectic layer
spacing d of a sample with 64 wt% water is depicted. The
Figure 5. a) Temperature-dependent layer spacing d across the lyo-
tropic SmA*–SmC* transition (64 wt% water), b) 2D diffraction pat-
tern of a monodomain SmC* sample (42 wt% water, 388C), and
c) schematic image of the lyotropic SmC phase.
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small-angle regime originate from the long-range lamellar
order along a single direction of k. The pair of diffuse outer
wide-angle scattering maxima indicates the fluid intralayer
order of the orientationally ordered diol molecules. In SmA
and La phases, the diffuse maxima are perpendicular to the
layer peaks and rotate in the same direction by q after cooling
into the SmC phase, thus indicating the non-zero average tilt
of the long molecular axes with respect to the layer normal k
(q ꢀ 378 in Figure 5b).
communicated between adjacent molecular double layers
across the solvent layer in between. The reason behind the
different temporal evolution of this interlayer interaction with
different solvents is quite remarkable and still has to be
understood.
Another important aspect of this lyotropic SmC* phase is
the occurrence of a spontaneous polarization in analogy to the
thermotropic phase and thus the possibility of polarity-
dependent switching between the two surface-stabilized
states of the tilt direction (see Figure 4c,d). Such investiga-
tions are very challenging with lyotropic liquid crystals,
because of the high electric conductivity of the solvents and
the unconventional preparation process of the samples.
Nevertheless, it was possible to make some preliminary
measurements to demonstrate that the lyotropic SmC* phase
exhibits a polar electrooptic effect in an alternating electric
field. The change in the texture of the lyotropic SmC* phase,
following the reversal in the direction of an applied electric
field, is shown in Figures 7a,b. The observed polarity-
dependent electrooptic effect is analogous to the polar
electrooptic effect in ferroelectric SmC* liquid crystals,
where the field-induced reversal of the spontaneous polar-
ization reverses the tilt direction. Saturated switching as
occurs in thermotropic SmC* phases is not observed here,
because of the large conductivity and threat of dielectric
breakdown. This polar switching process can be followed
dynamically by using a photomultiplier to record the change
Considering the molecular length l = 2.5 nm[14] of 1 and
a layer spacing of d = 7.0 nm (Figure 5a), the solvent layer at
the onset of the SmA*–SmC* transition is at least 2.0 nm
thick and becomes even thicker in the case of partial double
layers. A schematic image of the lyotropic SmC structure
which is consistent with the X-ray data is shown in Figure 5c.
The important question now is, does this phase, being
composed of chiral molecules, also exhibit the macroscopic
chirality effects of the thermotropic SmC* phase? Such
behavior can be demonstrated explicitly by the observation of
periodic pitch lines arising from a spiraling director on the tilt
cone (Figure 6d). Indeed, pitch lines appear in mixtures of the
Figure 6. a) Pitch lines of a sample with 59 wt% water at 368C and
b) of a sample with 32 wt% formamide at 308C. c) Pitch values of the
latter sample derived directly from the texture images plotted against
temperature and d) model of the SmC* phase illustrating the helical
distortion of the tilt direction.
diol 1/water system (Figure 6a) when a sealed sample is
equilibrated for several weeks. The pitch of the SmC* phase
for the pictured sample with 59 wt% water at 368C is p =
4.7 mm. The pitch lines appear much more clearly in mixtures
with formamide (Figure 6b), in which the pitch develops
within seconds. The temperature dependence of the pitch
measured directly in a sample with 32 wt% formamide is
plotted in Figure 6c and shows a rapid increase as the SmC*–
SmA* transition is approached. The helicity provides clear
evidence that the lyotropic SmC* phase exhibits macroscopic
chirality and that the helical twist of the tilt direction is
Figure 7. The lyotropic SmC* phase under the influence of an electric
field. a) and b) show texture images with reversed directions of an
applied electric field with U=5 Vmmꢁ1 and f=0.5 Hz (18 wt% form-
amide, 258C). c) Temperature dependence of the change of trans-
mitted light intensity of a sample with 64 wt% water and d) exemplary
*
transmission change ( ) at 448C together with the applied square-
wave voltage (c).
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of transmitted light intensity as a function of the applied
voltage (Figure 7d). The plot demonstrates clearly that the
switching is of polar and not of dielectric nature, in which the
transmission—and thus the tilt direction—does not depend
on the voltage polarity. The temperature dependence of the
change of transmitted intensity is depicted in Figure 7c, thus
illustrating that switching ceases above TC = 558C, which
matches the transition to the lyotropic SmA* phase. This
finding indicates that the observed effect is not due to possible
solvent-related processes such as electroconvection, but is an
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of discotic mesophases.[15] Moreover, the related piezoelectric
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In conclusion, we demonstrate the first lyotropic surface-
stabilized ferroelectric SmC* phase and present evidence for
its existence through texture studies, X-ray diffraction, pitch
measurements, and polarity-dependent director switching.
Furthermore, we demonstrate that this phase possesses
macroscopic chirality effects, that is, helical pitch lines and
a polar electrooptic effect, which implies communication of
chirality between the molecular double layers across the
intermediatory layers of solvent molecules. Following the
widespread notation of lyotropic phases by Luzzati and co-
workers,[17] we suggest this new lamellar phase should be
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*
denoted La0 , where the index a’ indicates a tilted fluid phase
and—according to IUPAC recommendations[18]—the super-
script * indicates the chirality of the molecules.
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Received: April 20, 2013
Published online: && &&, &&&&
[14] Derived from molecular modeling studies at the AM1 level.
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Keywords: chirality · ferroelectricity · helical structures ·
liquid crystals · lyotropics
.
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Communications
Liquid Crystals
A lyotropic analogue of the ferroelectric
smectic C* phase has been found. The
lyotropic smectic C* phase shows mac-
roscopic chirality effects, such as a helical
ground state and polarity-dependent
electrooptic switching, thus indicating
the presence of a spontaneous electric
polarization. The helicity implies com-
munication of the chiral director twist
across the achiral solvent layers separat-
ing adjacent layers of the chiral meso-
gens.
J. R. Bruckner, J. H. Porada, C. F. Dietrich,
I. Dierking,
F. Giesselmann*
&&&& — &&&&
A Lyotropic Chiral Smectic C Liquid
Crystal with Polar Electrooptic Switching
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