Chirality and macroscopic polar order in a ferroelectric smectic liquid-crystalline phase formed by achiral polyphilic bent-core molecules

Article abstract of DOI:10.1002/1521-3773(20020703)41:13<2408::AID-ANIE2408>3.0.CO;2-M

In conventional fluids the molecular dipole moments of the individual molecules cancel out, which leads to a macroscopic apolar structure. Directed molecular design using microsegregation and tailoring the molecular shape of compounds such as 1, can lead

Full text of DOI:10.1002/1521-3773(20020703)41:13<2408::AID-ANIE2408>3.0.CO;2-M

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with response times of less than one minute. The average
response ratio at 3 ppm NO2 was 1.16.
Chirality and Macroscopic Polar Order in a
Ferroelectric Smectic Liquid-Crystalline Phase
Formed by Achiral Polyphilic Bent-Core
Molecules**
Individual SnO2 nanoribbons are small, fast and sensitive
devices for detecting ppm-level NO2 at room temperature
under UV light. These nanodevices can be operated under
laboratory conditions over many cycles without loss of
sensitivity. The advantages of low-temperature, potentially
drift-free operation make SnO2 nanoribbons good candidates
for miniaturized, ultrasensitive gas sensors in many applica-
tions. Further sensitivity increases should be achievable by
using thinner nanoribbons, developing ohmic SnO2 metal
contacts and decorating these structures with catalysts. With
such innovations, the chemical detection of single molecules
on nanowires may soon be within reach.
Gert Dantlgraber, Alexei Eremin, Siegmar Diele,
Anton Hauser, Horst Kresse, Gerhard Pelzl, and
Carsten Tschierske*
Materials with a macroscopic polar order have a variety of
useful properties, such as piezo- and pyroelectricity and
second-order nonlinear optical activity[1, 2] Especially ferro-
electric (FE) and antiferroelectric (AF) liquid crystalli-
ne (LC) materials are of great interest, because they can be
rapidly switched between different states by means of external
electrical fields.[3, 4] These properties makes them useful for
numerous applications, such as electrooptic devices, informa-
tion storage, switchable NLO (nonlinear optic) devices and
light modulators, which may be of interest for optical
computing and other future technologies. At first, smectic
LC phases with tilted arrangements of nonracemic chiral
rodlike and disclike molecules have been used for this
purpose and for a long time molecular chirality appeared to
be essential for obtaining such materials.[3] However, the
discovery by Niori et al. that bent-core mesogenic compounds
(banana-shaped molecules) without molecular chirality, can
also organize in fluid smectic phases with a polar order
opened a new area in the field of LC research.[5, 6] The polar
structure of the smectic layers of such molecules is provided
by the dense directed packing of their bent aromatic cores.
However, to escape from a macroscopic polar order the bent
direction in adjacent layers is antiparallel, so that the layer
polarization alternates from layer to layer, which leads to a
macroscopic apolar AF structure.[7] In most cases of such
mesophases the molecules are additionally tilted relative to
the layer normal.[8] Therefore these phases (also known as
™B2∫-phases) can be described as tilted smectic phases (SmC)
with a polar order of the molecules (P) within the layers, and
an antiparallel polarization in adjacent layers (A), which leads
to the notation SmCPA. Because the molecules in adjacent
layers can have either a synclinic (molecules in adjacent layers
are tilted in the same direction, CS) or an anticlinic (molecules
in adjacent layers are tilted in opposite directions, CA)
interlayer correlation, the four different phase structures
shown in Figure 1 may result for such mesophases.[7] Usually,
the AF phases represent the ground states, whereas the FE
states (SmCSPF and SmCAPF) can only be achieved after
Received: February 22, 2002 [Z18752]
[1] a) J. Kong, N. Franklin, C. Wu, S. Pan, K. J. Cho, H. Dai, Science 2000,
287, 622; b) R. J. Chen, N. R. Franklin, J. Kong, J. Cao, T. W. Tombler,
Y. Zhang, H. Dai, Appl. Phys. Lett. 2001, 79, 2258.
[2] P. G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 2000, 287, 1801.
[3] Y. Cui, Q. Wei, H. Park, C. M. Lieber, Science 2001, 293, 1289.
[4] F. Favier, E. C. Walter, M. P. Zach, T. Benter, R. M. Penner, Science
2001, 293, 2227.
[5] C. G. Founstadt, R. H. Rediker, J. Appl. Phys. 1971, 42, 2911.
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Delabouglise, A. M. Gaskov, Mater. Sci. Eng. B 2001, 85, 43.
[7] See for example: a) V. E. Henrich, P. A. Cox, The Surface Science of
Metal Oxides, Cambridge University Press, Cambridge, 1994; b) J.
Tamaki, M. Nagaishi, Y. Teraoka, N. Miura, N. Yamazoe, L. Moriya, Y.
Nakamura, Surf. Sci. 1989, 221, 183.
[8] N. Barsan, M. S. Berberich, W. Goepel, Fresenius J. Anal. Chem. 1999,
365, 287.
[9] E. Comini, A. Cristalli, G. Faglia, G. Sberveglieri, Sens. Actuators B
2000, 65, 260.
[10] Z. R. Dai, Z. W. Pan, Z. L. Wang, Solid State Commun. 2001, 118, 351.
[11] M. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater.
2001, 13, 113.
[12] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R.
Russo, P. Yang, Science 2001, 292, 1897.
[13] Resistivity estimates were made difficult by uncertainties in the cross-
sectional area of a nanoribbon dispersed across the electrodes of a
device. Dark values in air fell from 1 to 500 Wcm. The resistivity
values of the nanoribbons are highly dependent on ambient light
levels, the chemical state of the surface and the quality of the electrical
contacts; they cannot be directly compared with bulk values. We
therefore use only conductance values here.
[14] N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Surf. Sci. 1978,
86, 335.
[15] H. Kind, H. Yan, B. Messer, M. Law, P. Yang, Adv. Mater. 2002, 14,
158.
[*] Prof. Dr. C. Tschierske, G. Dantlgraber
Institute of Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Strasse 2, 06120 Halle (Germany)
Fax : (‡49)345-55-27223
E-mail: tschierske@chemie.uni-halle.de
A. Eremin, Dr. S. Diele, Dr. A. Hauser, Prof. Dr. H. Kresse,
Prof. Dr. G. Pelzl
Institute of Physical Chemistry
Martin-Luther-University Halle-Wittenberg
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
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Figure 1. The four possible states of SmCP phases that are distinguished by
the relative tilt sense and the polar order in adjacent layers. The notations
CS and CA refer to synclinic and anticlinic tilt while PF and PA refer to
ferroelectric (FE) and antiferroelectric (AF) polar order in adjacent
layers.[7]
applying a sufficiently strong external electric field. They are
not stable and relax back to the AF state after the electric field
is switched off. Only recently first examples of ferroelectric
switchable mesophases were reported for a few special bent-
core molecules. Remarkably, the materials showing this
special switching behavior in fluid smectic phases (SmCP
phases) are chiral molecules or their mixtures.[9] Ferroelectric
switching was also observed for higher-ordered ™banana
phases∫, for a ™B7∫ phase formed by a racemic mixture[10] of
bent molecules and for a ™B5∫ phase formed by achiral
molecules.[11] Here we report first examples of nonchiral
molecules forming a ferroelectrical switchable fluid smectic
phase (SmCPF phase). They are a novel type of polyphilic
liquid crystal[2, 12] composed of three incompatible units: A
bent rigid aromatic core,[13] two flexible alkyl chains, and a
bulky oligosiloxane unit[14] at one end.
Scheme 1. Synthesis of compounds 1 4. a) cat. [Pd(PPh3)4], NaHCO3,
H2O, glyme, reflux, 8 h;[15] b) 4-(4-dodecyloxybenzoyloxy)benzoic acid,[13]
CMC, DMAP, CH2Cl2, 208C, 24 h;[16] c) H2, Pd/C, AcOEt, 208C, 24 h; d) 4-
(10-undecene-1-yloxy)benzoic acid,[17] CMC, DMAP, CH2Cl2, 208C,
24 h;[16] e) R3SiH (R is shown in Table 1), Karstedt×s catalyst, toluene,
208C, 30 min;[14d] CMC ˆ N-cyclohexyl-N'-(2-morpholinoethyl)carbodi-
imide methyl p-toluenesulfonate; DMAP ˆ 4-dimethylaminopyridine
Table 1. Transition temperatures (T) and corresponding enthalpy values
[in square brackets] of the compounds 2 4.[a]
The synthesis of these molecules is shown in Scheme 1 and
the phase-transition temperatures obtained by polarized-light
optical microscopy and differential scanning calorimetry
(DSC) are summarized in Table 1.[18] The parent compound
1 of this series, composed only of the rigid bent-core unit and
two terminal hydrocarbon chains shows a conventional
SmCPA-phase with a typical AF switching behavior, which is
characterized by the occurrence of two polarization peaks in
the switching current response. This phase is only monotropic
[metastable, phase sequence: Cr 108 (SmCPA 98) Iso], but
Compound
R3Si-
T [8C] [DH/kJmolÀ1
Cr 77 SmCPA 118
[17.0] [23.5]
Cr 70 SmCPF 115
[29.3] [24.3]
Cr 63 SmCPF 116
[8.9] [21.1]
]
2
Me3SiOSiMe2-
Iso
Iso
Iso
3
4
Me3Si(OSiMe2)2-
(Me3SiO)2SiMe-
[a] Abbreviations: Crˆ crystalline solid state; SmCPA ˆ antiferroelectri-
cally switchable smectic mesophase in which the molecules are tilted with
respect to the layer normal and have a polar order within the layers;
SmCPF ˆ ferroelectrically switchable smectic mesophase in which the
molecules are tilted with respect to the layer normal and have a polar
order within the layers; Iso ˆ isotropic liquid phase. Transition temper-
atures and enthalpies were determined by DSC (Perkin-Elmer DSC-7, first
heating scan, rate: 10 KminÀ1) and confirmed by polarized-light optical
microscopy.
hydrosilylation[14d] leads to the ternary block molecules 2
4
which show significantly increased stabilities of their meso-
phases, despite that the oligosiloxane units are very bulky and
therefore are expected to reduce the mesophase stability.
Instead, the mesophase stability is nearly independent of the
size of the siloxane units. Even for molecule 4, with a
branched trisiloxane unit, nearly the same transition temper-
ature is found. All siloxane derivatives 2 4 form mesophases
which are quite distinct from that of the parent compound 1
and all other known liquid-crystalline phases. Typically, they
appear from the isotropic liquid state as fractal nuclei which
coalesce to a grainy unspecific texture which is completely
dark (optically isotropic) between crossed polarizers, showing
only very small irregularly distributed bright spots. The most
remarkable feature is that in these mesophases domains of
opposite handedness can be distinguished. Rotating the
analyzer by a small angle (5 108) dark and more bright
domains become visible. If the analyzer is rotated in the
opposite direction the brightness of the domains is reversed
(Figure 2). The light transmission does not change if the
sample is rotated. This effect has already been reported for
the ™B4∫ phase,[19] which actually represents a soft crystal, and
the ™Sm1∫-phase which also seems to have a three-dimen-
sional superstructure though only a simple layer structure is
found by X-ray diffraction.[20] However, in contrast to these
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rization current in response to an applied triangular-wave
field[21] shows two peaks for the disiloxane derivative 2 (AF
switching) and only one peak for compounds 3 and 4, as
typical for FE switching (Figure 4). Compound 4 was inves-
Figure 2. Texture of the mesophase of compound 4 (Tˆ 1058C) obtained
by cooling the isotropic liquid without applied field. The photographs were
taken with polarizer and analyzer slightly uncrossed to distinguish domains
of opposite chirality. Changing the direction of the analyzer as indicated by
the arrows [compare (a) and (b)] reverses the brightness of the domains.
The brightness of the images is strongly enhanced to make the different
regions visible.
phases, in the mesophases of 2 4 the transparent blue color
which is typical for B4 and Sm1 phases cannot be observed,
and these mesophases are highly fluid, like conventional SmA
and SmC phases.
Figure 4. Switching current response in the mesophase of compound 4 on
applying a triangular-wave voltage at 1048C in a 10 mm non-coated ITO cell
(EHC, Japan) at a frequency of 1 Hz.
X-ray investigations (non-oriented samples, Guinier cam-
era) confirm the presence of a well-defined layer structure by
the appearance of a sharp layer reflection and its higher
orders (up to the 4th order) with d ˆ 4.4 nm for compound 4.
For this compound the diffuse scattering in the wide-angle
range has a very asymmetric profile in which two maxima can
be separated, one maximum at 0.45 nm corresponds to the
mean distance between the fluid alkyl chains and the second
one with a maximum at about 0.7nm corresponding to the
mean distance between the disordered siloxane units. This is
an indication of a fluid nanosegregated organization. It can be
assumed that because of the significantly larger space
required by the siloxane units compared to the rest of the
molecule the molecules should adapt an antiparallel end-to-
end packing within the layers. In this way the siloxane units
build up their own sublayers, which leads to the triple-layer
structure shown in Figure 3. The effective molecular length L
tigated in more detail. Even at very low frequency (0.02 Hz)
we observed only one very sharp current response peak,
corresponding to a high value of the spontaneous polarization
of 700 nCcmÀ2. This one-peak response is found in every case,
independent of the sample preparation. It is a strong
indication for an FE switching behavior which is additionally
supported by optical observations of the switching process.
If the isotropic liquid is slowly cooled under an applied dc
electric field (10 VmmÀ1) a texture with many circular
domains appears (Figure 5a). In these domains the smectic
layers are circularly arranged around the center of the
domains. The characteristic feature of these domains are
extinction crosses, whereby the direction of the extinction
brushes corresponds to the direction of the optical axes of the
smectic layers. In these bright birefringent field-induced
circular domains the extinction crosses are rotated by 70
808 clockwise or anticlockwise depending on the sign of the
applied field. This angle corresponds to twice the tilt angle. It
should be emphasized that the extinction crosses remain
unchanged if the field is switched off, that means, there is no
relaxation of the switched state as observed in the AF phases.
This bistable switching clearly points to an FE ground state,
which confirms the results of the current-response measure-
ments. In this FE ground state the correlation between
adjacent smectic layers is synclinic (SmCSPF).
If however, an alternating field (ac, 10 V mmÀ1, 200 Hz) is
applied during cooling the isotropic liquid, the appearing
mesophase shows in addition to a weakly birefringent fanlike
texture also weakly birefringent circular domains (Figure 5b).
In these domains the extinction crosses coincide with the
direction of polarizer and analyzer. On applying an electric
field the extinction crosses do not rotate, although a switching
process is clearly visible. In this case the ferroelectric ground
state can be explained by an anticlinic packing of the
molecules in adjacent layers (SmCAPF).
Figure 3. Proposed triple-layer organization of the molecules 3 and 4 in the
SmCPF state.
is around 5.5 nm, which is significantly larger than the layer
spacing found in X-ray measurements. This result is in line
with the proposed monolayer structure in which the molecules
are additionally tilted by an angle of about 30 35 degrees
with respect to the layer normal.
In the next step electrooptical investigations were carried
out in a transparent sandwich-type capacitor cell consisting of
two indium-tin-oxide (ITO) coated glass plates. The repola-
In contrast to the states obtained under the influence of an
external electric field, there is obviously no birefringence if
the smectic phase is formed without such an applied field, that
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comparatively low temperatures. The appearance of siloxane
sublayers seems to have an essential influence on the
preference of a parallel alignment of the molecular bent
directions in adjacent layers. Usually, the packing of the bent
molecules in adjacent layers is antiparallel (AF), which
provides synclinic interlayer interfaces (see side views in
Figure 1). These easily allow interlayer fluctuations and are
therefore entropically favorable. In the FE state the parallel
molecular bent directions in adjacent layers lead to anticlinic
interlayer interfaces between them, which are disfavored
because they suppress the interlayer fluctuations. Decoupling
the layer interfaces by the siloxane sublayers reduces or
inhibits these interlayer penetrations and therefore their
importance for the molecular organization is reduced, which
allows a ferroelectric order more easily.[23] The importance of
the decoupling of the layers is also shown in that reducing the
number of siloxane units gives rise to AF properties
(compounds 1 and 2). This decoupling may also be respon-
sible for the possibility to control the relative tilt direction of
the molecules in adjacent layers (synclinic versus anticlinic)
by changing the type of the applied electric field (dc versus
ac). Additionally, it should be pointed out that the supra-
molecular organization of these molecules is strongly influ-
enced not only by external fields, but also by the interactions
with the substrate surfaces, which all together define the
system as a whole.
Figure 5. Switching behavior observed between crossed polarizers. a) Tex-
ture obtained by cooling the isotropic liquid under an electric dc field of
10 VmmÀ1. The extinction cross of the circular domains does not coincide
with the crossed polarizers. By switching the electric field between
‡1.5 V mmÀ1 (left-hand side) and À1.5 V mmÀ1 (right-hand side) the ex-
tinction cross rotates by an angle of 70 808 (sample thickness: 10 mm; Tˆ
708C)[22] b) Texture obtained by cooling the isotropic liquid under an
electric ac field of 10 VmmÀ1 (200 Hz). The extinction crosses coincide with
the crossed polarizers and do not rotate on applying an electric field
(sample thickness: 10 mm; Tˆ 708C).
Received: March 19, 2002 [Z18929]
[1] M. C. Petty, M. R. Bryce, D. Bloor, Introduction to Molecular
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[3] H.-S. Kitzerow, C. Bahr, Chirality in Liquid Crystals, Springer, Berlin,
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[4] S. T. Lagerwall, Ferroelectric and Antiferroelectric Liquid Crystals,
Wiley-VCH, Weinheim, 1999.
[5] T. Niori, F. Sekine, J. Watanabe, T. Furuwara, H. Takezoe, J. Mater.
Chem. 1996, 6, 1231.
[6] A related effect was observed in mixtures of side-chain polymers and
low molecular-weight mesogens: E. A. Soto Bustamante, S. V. Ya-
blonskii, B. I. Ostrovskii, L. A. Beresnev, L. M. Blinov, W. Haase, Liq.
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[8] An orthogonal phase is reported in ref. [6] and was also reported for a
rigid bent-core molecule: A. Eremin, S. Diele, G. Pelzl, H. Nadasi, W.
Weissflog, J. Salfetnikova, H. Kresse, Phys. Rev. E 2001, 64, 51707.
[9] a) E. Gorecka, D. Pociecha, F. Araoka, D. R. Link, M. Nakata, J.
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Ishikawa, J. Watanabe, H. Takezoe, Liq. Cryst. 2001, 28, 1301.
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is, this phase seems to be optically uniaxial. However, the
polar order and the tilted organization of the molecules in the
layers would require an optical biaxiality of this phase, that is,
a birefringence is expected to occur. Another characteristic
feature of this texture is the existence of domains with
opposite chirality (see above). These findings point to the
presence of a helical arrangement of the molecules with the
helix axis perpendicular to the substrate surfaces, whereby the
pitch of this helix is different from the wavelength of the
visible light and the helix-sense is different in regions with
opposite chirality. The helix axis can occur perpendicular to
the layer planes (SmC*-like) or parallel to the layer planes
(TGB-like). Both helical superstructures would lead to optical
uniaxiality of the phases and would allow a compensation of
the layer polarity on a macroscopic scale by retaining a nearly
parallel alignment of the bent directions of the molecules in
adjacent layers.
[11] H. Nadasi, W. Weissflog, A. Eremin, G. Pelzl, S. Diele, B. Das, S.
Grande, J. Mater. Chem. 2002, 12, 1316.
[12] C. Tschierske J. Mater. Chem. 2001, 11, 2646.
[13] D. Shen, A. Pegenau, S. Diele, I. Wirth, C. Tschierske, J. Am. Chem.
Soc. 2000, 122, 1593.
[14] Siloxane units were used for several calamitic polyphilic mesogens, for
example, a) M. Ibn-Elhaj, H. Mˆhwald, M. Z. Cherkaoui, R. Zniber,
In summary, the mesophase presented here is the first FE
switchable SmCP phase which is formed by achiral bent-core
mesogens. Additionally, the materials have interesting prop-
erties which could be of use in applications; they are stable,
have a low conductivity, and their mesophases occur at
Angew. Chem. Int. Ed. 2002, 41, No. 13
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COMMUNICATIONS
Langmuir 1998, 14, 504; b) P. Sebastiao, S. Mery, M. Sieffert, J. F.
Nicoud, Y. Galerne, D. Guillon, Ferroelectrics 1998, 212, 133; c) H. J.
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129, 521.
163.75, 163.51, 155.46, 151.28, 150.81, 142.08, 137.66, 132.34, 132.23,
131.74, 129.75, 128.95, 128.15, 126.82, 124.58, 122.11, 122.04, 121.44,
120.94, 120.48, 120.35, 114.27, 68.39, 68.32, 33.21, 31.92, 29.65, 29.63,
29.62, 29.57, 29.37, 29.36, 29.12, 29.09, 26.00, 25.98, 23.08, 22.69, 17,65,
14.12, 1.88, À0.23 ppm; elemental analysis calcd (%) for C63H88O10Si3:
C 69.44%, H 8.14; found C 69.33%, H 8.12.
[15] a) N. Miyaura, T. Yanagi, A. Suzuki, Synth. Commun. 1981, 11, 513;
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Takezoe, J. Watanabe, J. Mater. Chem. 2001, 11, 2717.
[21] Determined by the three angular wave method as described in ref. [4].
[22] Two distinct enantiomeric configurations, which differ in the tilt-
direction and the direction of the polar axes of the molecules in the
smectic layers (‡/ ‡ and À/ À ), are possible for the SmCSPF state.[7]
They occur in different regions within the sample, in which the
extinction crosses rotate either clockwise or anticlockwise.
[23] Quite similarly, decreasing the temperature reduces the out-of-layer
fluctuations and could lead to a transition from an AF to FE order as
recently shown for a B5-phase.[11]
[18] All analytical data are in accordance with the proposed structure, for
example, compound 4: 1H NMR (400 MHz, CDCl3) d ˆ 8.30 (d, J ˆ
8.8 Hz, 2H, Ar-H), 8.15 (d, J ˆ 8.8 Hz, 4H, Ar-H), 7.64 (d, J ˆ 8.6 Hz,
2H, Ar-H), 7.49 (d, J ˆ 5.3 Hz, 2H, Ar-H), 7.46 7.45 (m, 1H, Ar-H),
7.38 (d, J ˆ 8.6 Hz, 2H, Ar-H), 7.29 (d, J ˆ 8.6 Hz, 2H, Ar-H), 7.23
7.20 (m, 1H, Ar-H), 7.00 (d,J ˆ 9.0 Hz, 2H, Ar-H), 6.97(d, J ˆ 9.0 Hz,
2H, Ar-H), 4.04 (t, J ˆ 6.6 Hz, 4H, OCH2), 1.82 (m, 4H, CH2), 1.45 (m,
4H, CH2), 1.36 1.28 (m, 30H, CH2), 0.89 (t, J ˆ 6.6 Hz, 3H, CH3),
0.46 (t, J ˆ 7.4 Hz, 2H, Si-CH ), 0.10 (s, 18H, Si-CH3), 0.06 ppm (s, 3H,
2
Si-CH3); 13C NMR (100 MHz, CDCl3) d ˆ 164.78, 164.34, 164.18,
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