Molecular design of nonchiral bent-core liquid crystals with antiferroelectric properties

Article abstract of DOI:10.1021/ja993572w

Novel bent-core (banana-shaped) liquid crystals without Schiff-base units have been synthesized and investigated by polarized light optical microscopy, differential scanning calorimetry, X-ray scattering, and electrooptical investigations. These molecules are 4-(4- alkylbenzoyloxy)benzoates and 4-(4-alkoxybenzoyloxy)benzoates of resorcinol, 3,4'-dihydroxybiphenyl, and 4,4'-dihydroxy- 1,1':3', 1'-terphenyl. Three different mesophases were found depending on the molecular structure and the length of the terminal alkyl chains: a rectangular columnar phase, a highly ordered low-temperature mesophase, and an antiferroelectric switchable fluid smectic mesophase designated as SmCP(A). The influence of the molecular structure on the occurrence of the SmCP(A) phase was investigated. The spontaneous polarization of these molecules is quite high (P(s) = 500700 nC cm^-2) and specially those molecules with long alkyl chains and short bent- core structures have low melting points and broad regions of this switchable mesophase. Furthermore, first examples of antiferroelectric switchable bent- core molecules with semifluorinated terminal chains will be described.

Full text of DOI:10.1021/ja993572w

J. Am. Chem. Soc. 2000, 122, 1593-1601
1593
Molecular Design of Nonchiral Bent-Core Liquid Crystals with
Antiferroelectric Properties
Dong Shen,† Annegret Pegenau,† Siegmar Diele,‡ Ina Wirth,‡ and Carsten Tschierske*,†
Contribution from the Institute of Organic Chemistry, Martin-Luther UniVersity Halle-Wittenberg,
Kurt-Mothes Strasse 2, D-06120 Halle, Germany, and Institute of Physical Chemistry,
Martin-Luther UniVersity Halle-Wittenberg, Mu¨hlpforte 1, D-06108 Halle (Saale), Germany
ReceiVed October 4, 1999
Abstract: Novel bent-core (banana-shaped) liquid crystals without Schiff-base units have been synthesized
and investigated by polarized light optical microscopy, differential scanning calorimetry, X-ray scattering,
and electrooptical investigations. These molecules are 4-(4-alkylbenzoyloxy)benzoates and 4-(4-alkoxyben-
zoyloxy)benzoates of resorcinol, 3,4′-dihydroxybiphenyl, and 4,4′′-dihydroxy-1,1′:3′,1′′-terphenyl. Three different
mesophases were found depending on the molecular structure and the length of the terminal alkyl chains: a
rectangular columnar phase, a highly ordered low-temperature mesophase, and an antiferroelectric switchable
fluid smectic mesophase designated as SmCPA. The influence of the molecular structure on the occurrence of
the SmCPA phase was investigated. The spontaneous polarization of these molecules is quite high (PS ) 500-
700 nC cm-2) and specially those molecules with long alkyl chains and short bent-core structures have low
melting points and broad regions of this switchable mesophase. Furthermore, first examples of antiferroelectric
switchable bent-core molecules with semifluorinated terminal chains will be described.
Introduction
ecules6 and bowl-shaped molecules7,8 have been designed to
obtain nonchiral ferroelectric fluids.
Ferroelectrics are materials with a macroscopic electric
polarization, switchable between two polarized states. They have
a variety of useful properties, such as piezo- and pyroelectricity
and second-order nonlinear optical activity (NLO). This mac-
roscopic polarization can occur when permanent electric dipole
moments of molecules are aligned parallel or when ions are
organized in a noncentrosymmetric structure. Ferroelectricity
was first detected in noncentrosymmetric crystals.1 Meyer
reported that ferroelectricity can also occur in fluid, liquid
crystalline phases which are formed by a tilted arrangement of
enantiomerically enriched chiral molecules in layers (e.g. SmC*
phases).2 Because of the fluidity of these ferroelectrics novel
technical applications are possible, as for example, in fast-
switching electrooptical devices. Therefore, ferroelectric and also
antiferroelectric liquid crystals have attracted considerable
interest.3,4 Although it was already predicted by theory that
ferroelectricity in liquid crystals is also possible without
molecular chirality,5 for a long time ferroelectric properties were
only found in SmC* phases of chiral molecules in practice. In
recent years great efforts were made to obtain ferroelectric liquid
crystals with achiral molecules. For example, polyphilic mol-
In 1996 Niori et al. reported on ferroelectricity in a smectic
phase formed by banana-shaped Schiff-base derivatives (see
Figure 1),9 first synthesized by Matsunaga et al.10 The distinct
molecular structure, the unconventional mesomorphic properties,
the occurrence of a spontaneous symmetry breaking in some
cases, and the special switching behavior of some of their
mesophases immediately attracted great interest.11,12 One of the
liquid crystalline phases, which was first designated as B2,13
revealed an antiferroelectric switching process. It was shown
that the liquidlike smectic layers of these molecules have a polar
structure provided by the dense packing of the bent-core
molecules.14,15,16 For these antiferroelectric switchable phases
(6) Tournilhac, F.; Blinov, L. M.; Simon J.; Yablonski, S. V. Nature
1992, 359, 621.
(7) Lei, L. Mol. Cryst. Liq. Cryst. 1983, 91, 77.
(8) Swager, T. M.; Serrette, A. G.; Knawby, D. B.; Zeng, H. Int. Liq.
Cryst. Conf. 15th 1994, 2, 771.
(9) (a) Niori, T.; Sekine, F.; Watanabe, J.; Furukawa, T.; Takezoe, H. J.
Mater. Chem. 1996, 6, 1231. (b) Niori, T.; Sekine, F.; Watanabe, J.;
Furukawa T.; Takezoe, H. Mol. Cryst. Liq. Cryst. 1997, 301, 337. (c) Sekine,
F.; Takanashi, Y.; Niori, T.; Watanabe, J.; Takezoe, H. Jpn. J. Appl. Phys.
1997, 36, L 1201. (d) Sekine, F.; Niori, T.; Watanabe, J.; Furukawa, T.;
Chol, S. W.; Takezoe, H. J. Mater. Chem. 1997, 7, 1307.
(10) (a) Matsunaga, Y.; Miyamoto, S. Mol. Cryst. Liq. Cryst. 1993, 237,
311. (b) Akutagawa, T.; Matsunaga, Y.; Yashahura, K. Liq. Cryst. 1994,
17, 659.
* Address correspondence to this author. Fax: +49 (0) 345 55 27030.
E-mail: Tschierske@Chemie.Uni-Halle.de.
† Institute of Organic Chemistry.
‡ Institute of Physical Chemistry.
(1) Valasek, J. Phys. ReV. 1920, 15, 537.
(2) Meyer, R. B.; Liebert, L.; Strzelecki L.; Keller, P. J. Phys. 1975, 36,
L-69.
(11) Heppke, G.; Moro, D. Science 1998, 279, 1872.
(12) Recent review: Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999,
11, 707.
(3) (a) Goodby, J. W. J. Mater. Chem. 1991, 1, 307. (b) Lagerwall, S.
T. Handbook of Liquid Crystals; Demus, D., Goodby, J., Spiess, H.-W.,
Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B, p 515.
(4) Miyachi, K.; Fukuda, A. Handbook of Liquid Crystals; Demus, D.,
Goodby, J., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998;
Vol. 2B, p 665.
(5) (a) Saupe, A. Mol. Cryst. Liq. Cryst. 1969, 7, 59. (b) Prost J.; Barois,
P. J. Chim. Phys. 1983, 80, 65. (c) Petschek R. G.; Wiefling, K. M. Phys.
ReV. Lett. 1987, 59, 343. (d) Tredgold, R. H. J. Phys. D 1990, 23, 119. (e)
Blinov, L. M. Liq. Cryst. 1998, 24, 143.
(13) The nomenclature of the mesophases of the banana-shaped molecules
as B1, B2, etc. was recommended at the Workshop “Banana-Shaped Liquid
Crystals: Chirality by Achiral Molecules”, Dec. 1997, Berlin, see also ref
12. Their use is recommended as long as the precise structure of these
mesophases is uncertain.
(14) (a) Jakli, A.; Rauch, S.; Lo¨tzsch, D.; Heppke, G. Phys. ReV. E 1998,
57, 6737. (b) Diele, S.; Grande, S.; Kruth, H.; Lischka, C.; Pelzl, G.;
Weissflog W.; Wirth, I. Ferroelectrics 1998, 212, 169.
(15) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Ko¨rblova, N.
A.; Walba, D. M. Science 1997, 278, 1924.
10.1021/ja993572w CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/08/2000

1594 J. Am. Chem. Soc., Vol. 122, No. 8, 2000
Shen et al.
Scheme 1. Syntheses of the Compounds 2/n and 3/n
rigid cores M, and two terminal chains. The central unit Z is in
most cases an 1,3-substituted benzene ring or an aromatic ring
system, but it can also be an odd-numbered alkylene chain.18
In previous work we have described several different bent-core
molecules incorporating non-Schiff-base rigid cores, such as
2-phenylpyrimidines, 2-phenyl-1,3,4-thiadiazoles, biphenyls, and
phenylbenzoates.19 However, none of these compounds exhib-
ited an antiferroelectric switchable mesophase as known from
the Schiff-base derivatives. At this stage it seemed that the
Schiff-base unit was necessary to obtain these special meso-
phases.
A significant break-through has now been achieved by
combination of the angular 3,4′-disubstituted biphenyl central
unit with two phenyl benzoate rigid cores via ester linkages
which is described in this paper (compounds 2 and 3).20
Additionally, the sizes of the bent central unit and of the terminal
chains were changed systematically (compounds 1, 4-6), which
allows us to tailor the mesomorphic properties and to provide
some general rules for the design of SmCPA materials.
Furthermore, first examples of bent-core molecules with semi-
fluorinated terminal chains and broad regions of SmCPA phases
will be reported.21
Figure 1. Structure of a typical bent-core molecule and organization
of the molecules in the antiferroelectric switchable mesophase. Owing
to the bent-core each molecule possesses a dipole moment in the
molecular plane and perpendicular to the long axis of the molecules.
(a) The molecules are arranged in layers whereby the polar direction
changes from layer to layer (antiferroelectric order). The side views
show that the molecules can be tilted either (b) with synclinic (SmCSPA)
or (c) with anticlinic interlayer correlation (SmCAPA). (d) Schematic
representation of one polar chiral layer consisting of achiral bent-core
molecules (only one molecule is shown). In the picture at the left-
hand side the layer normal, tilt direction, and the polar axis define a
right-handed coordinate system (+), whereas in the mirror image these
vectors define a left-handed system (-).
it was proposed that the molecules arrange in layers whereby
the layer polarization alternates from layer to layer (antiferro-
electric structure). The resulting smectic phase is biaxial
whereby one optical axis is tilted relative to the layer normal
(see Figure 1).15 Therefore, this mesophase can be more
precisely described as SmCPA, a tilted smectic phase (SmC)
with a polar order of the molecules (P) within the layers and an
antiferroelectric interlayer correlation (A).15 By applying an
electric field the molecules can be switched into a ferroelectric,
i.e. noncentrosymmetric structure (SmCPF). Recently, very large
second-order NLO effects have been measured for these
ferroelectric states.17 Hence, an interesting application of such
materials could be in switchable nonlinear optical devices.
Typically, bent-core molecules with antiferroelectric switchable
SmCPA phases incorporate at least one Schiff-base unit.
Therefore, a major drawback of these compounds is their limited
thermal, hydrolytic, and photochemical stability. Furthermore,
in most cases, these special mesophases occur at rather high
temperatures. Therefore, the design of novel stable and low-
melting bent-core liquid crystals without sensitive Schiff-base
units is a topical subject in liquid crystal research.
Results and Discussion
A. Syntheses. The compounds were obtained by esterification
of 4-[4-alkyl(oxy)benzoyloxy]benzoic acids or semifluorinated
4-(4-alkyloxybenzoyloxy)benzoic acids with appropriate bent-
core dihydroxy compounds using the carbodiimide method.22
3,4′-Dihydroxybiphenyl and compounds 2 and 3 were obtained
according to the synthetic route shown in Scheme 1. The first
step was the Pd°-catalyzed Suzuki cross-coupling23 of 3-bro-
moanisole with 4-octyloxyphenylboronic acid. The obtained
(18) Watanabe, J.; Niori, T.; Choi, S.-W.; Takanishi, Y.; Takezoe, H.
Jpn. J. Appl. Phys. 1998, 37, L401.
(19) Shen, D.; Diele, S.; Pelzl, G.; Wirth, I.; Tschierske, C. J. Mater.
Chem. 1999, 9, 661.
(20) Preliminary communication: Shen, D.; Diele, S.; Wirth, I.;
Tschierske, C. J. Chem. Soc., Chem. Commun. 1998, 2573.
(21) Recently, some bent core molecules with perfluorinated chains have
been reported, but exclusively SmA phases have been found for these
compounds: Kovalenko, L.; Lose, D.; Pelzl, G.; Weissflog, W. Proc. 26.
Freiburger Arbeitstagung Flu¨ssigkristalle 1999, P40.
For simplicity, these special molecules can be considered as
built up by three distinct parts, a bent central unit Z, two linear
(16) There are several other possibilities for the packing arrangement of
such molecules in fluid phases, but most of them are unexplored: Brand,
H. R.; Cladis, P. E.; Pleiner, H. Eur. J. Phys. B 1998, 6, 347.
(17) (a) Kentischer, F.; MacDonald, R.; Warnick, P.; Heppke, G. Liq.
Cryst. 1998, 25, 341. (b) Araoka, F.; Park, B.; Kinoshita, Y.; Ishikawa, K.;
Takezoe, H.; Thisayukta, J.; Watanabe, J. Jpn. J. Appl. Phys. 1999, 38,
3526.
(22) (a) Ho¨fle, G.; Steglich, W.; Vorbru¨ggen, H. Angew. Chem. 1978,
90, 602. (b) Tschierske C.; Zaschke, H. J. Prakt. Chem. 1989, 331, 365.

Nonchiral Bent-Core Liquid Crystals
J. Am. Chem. Soc., Vol. 122, No. 8, 2000 1595
diether was then cleaved with BBr3 in benzene.24 Resorcinol is
commercially available and the synthesis of the other aromatic
dihydroxy compounds has recently been described in detail.19
The 4-[4-alkyl(oxy)benzoyloxy]benzoic acids were obtained by
esterification of 4-alkyl(oxy)benzoyl chlorides with 4-hydroxy-
benzaldehyde25 followed by CrO3 oxidation in aqueous acetic
acid.26 The related semifluorinated 4-(4-alkyloxybenzoyloxy)-
benzoic acids were obtained in the same way starting with
semifluorinated 4-alkoxybenzoic acids.27 The synthesis of
compound 7 will be reported in a separate paper together with
other molecules incorporating different mesogenic units M.
The purification of the final compounds was done by means
of column chromatography followed by recrystallization from
ethyl acetate. The purity and structure was confirmed by thin-
Table 1. Transition Temperatures (T/°C) and Corresponding
Enthalpy Values (lower lines in italics), ∆H/kJ mol-1, of
Compounds 1-7 As Obtained by DSC (peak temperatures in the
first heating cycle, heating rate: 10 K min-1
)
a
1
layer chromatography, elemental analysis, and H NMR, 19F
NMR, and 13C NMR spectroscopy.
B. Mesomorphic Properties of the Biphenyl Derivatives
2. The mesomorphic properties of the obtained compounds were
investigated by polarized light optical microscopy on a hot stage
(Mettler FP 82 HT) and by differential scanning calorimetry
(DSC, Perkin-Elmer DSC-7, heating and cooling scans with 10
deg K per min). The transition temperatures and corresponding
enthalpy values of all synthesized compounds are summarized
in Table 1. In Figure 3 the dependence of the mesomorphic
properties of the homologous series of the alkyl substituted
compounds 2/n (n ) 4, 6-14) on the chain length is shown
schematically. Three different mesophases were found. The
short-chain molecules 2/4 to 2/7 exhibit only one mesophase.
On cooling from the isotropic liquid state the mesophase can
be detected by the formation of small batonnets which coalesce
into a mosaic-like texture. Sometimes also spherulitic domains
can be observed (see Figure 4a). The same texture was found
for 2/9 with nonyl side chains and for the high-temperature
mesophases of 2/8 and 2/10. This texture is identical with those
of B1-phases13 of short-chain Schiff-base bent-core molecules
for which a rectangular columnar structure was proposed.28
The X-ray diagrams of compounds 2/4 to 2/9 show two
reflections in the small-angle region in addition to a diffuse
wide-angle scattering (Table 2). The pattern of an oriented
sample of compound 2/4 displays Bragg-spots out of the
meridian as well as Bragg-spots at the meridian, which point
to the existence of a two-dimensional rectangular cell (Figure
5). It supposes to index the first reflection of the powder-like
pattern by (11) and the second one by (02). The calculated lattice
constants are given in Table 3. The outer diffuse scattering is
splitted off, too, and is arranged symmetrically to the equator.
It yields an angle of R ) 126°, which can be attributed to the
bending angle of the two halves of the molecules.
Using this bending angle the molecular lengths of compounds
2
2/n were calculated according to L ) (L12 + L2 -2L1L2 cosR)0.5
whereby L1 and L2 correspond to the lengths of the two different
halves determined using CPK models and assuming all-trans
conformations of the alkyl chains. In the case of compound 2/4,
the calculated length L agrees very well with the periodicity c.
Obviously, this long axis is within the a-c plane and is directed
a Abbreviations: Cr1, Cr2, Cr3 ) different crystalline modifications;
Colr ) rectangular columnar mesophase (B1-phase),13 SmCPA ) tilted
smectic phase with polar antiferroelectric order of the molecules (B2-
phase),13 SmX ) highly ordered smectic mesophase of unknown precise
structure, Iso ) isotropic liquid state; phase assignments in parentheses
refer to monotropic (metastable) mesophases, their transitions were
determined in a second heating scan. b Phase sequence obtained on
cooling; these transition temperatures were determined by polarized
light microscopy. c Determined from the cooling scan.
(23) (a) Miyaura, N.; Yanagi T.; Suzuki, A. Synth. Commun. 1981, 11,
513. (b) Hird, M.; Gray, G. W.; Toyne, K. J. Mol. Cryst. Liq. Cryst. 1991,
206, 187.
(24) McOmie, J. F. W.; West, D. E. Organic Syntheses; Wiley: New
York, 1973; Collect. Vol. V, p 412.
(25) Tschierske, C.; Zaschke, H. J. Prakt. Chem. 1988, 330, 1.
(26) Wiberg, K. B.; Mill, T. J. Am. Chem. Soc. 1958, 80, 3022.
(27) Percec, V.; Johansson, G.; Ungar, G.; Zhou, J. J. Am. Chem. Soc.
1996, 118, 9855.
(28) Watanabe, J.; Niori, T.; Sekine, F.; Furukawa T.; Takezoe, H. Jpn.
J. Appl. Phys. 1998, 37, L 139.
parallel to the c-axis, i.e. the molecules are nontilted. With
increasing chain length the difference between L and c increases.
The differences to the c parameter can be explained by the

1596 J. Am. Chem. Soc., Vol. 122, No. 8, 2000
Shen et al.
Figure 2. General structure of bent-core liquid crystals and structures
of the molecules under investigation.
Figure 4. Optical photomicrographs (crossed polarizers): (a) Colr
phase of compound 2/8 as obtained by cooling from the isotropic liquid
at 152 °C; (b) schlieren texture of the SmCPA phase at 120 °C; and (c)
SmCPA phase of compound 2/12 as obtained by cooling from the
isotropic liquid at 156 °C.
Figure 3. Transition temperatures of the homologous series of
compounds 2/n; for the abbreviations see Table 1; black parts of the
columns represent crystalline phases (Cr); phase assignments in
parentheses refer to monotropic (metastable) mesophases, which can
only be observed on cooling from the isotropic liquid state; the
crystalline phases of compounds 2/11 and 2/12 are not shown.
Table 2. Scattering Vectors of Compounds 2/4 to 2/12'
Colr
SmCPA
compd
d1/nm
d2/nm
d/nm
2/4
2/6
2/7
2/8
2.19
2.49
2.64
3.18
3.04
1.93
2.10
2.17
2.34
2.32
deviation of the alkyl chains from the all-trans conformation,
which becomes more pronounced on elongation of the chains.
Thus, this mesophase is a rectangular columnar mesophase
(Colr) built up of ribbons of parallel aligned bent-core molecules.
The bending direction of the molecules of neighboring ribbons
is antiparallel. In this way the dipoles cancel out from ribbon
to ribbon, and the system can escape from a macroscopic polar
order (see Figure 6a).
The parameter a is related to the number of molecules
arranged side by side in the lateral diameter of each ribbon.
Unexpectedly, this parameter has an especially strong increase
on elongation of the alkyl chains, which means that the number
of molecules also depends on the chain length. It rises from a
value between two and three molecules for compound 2/4 to a
value of about four molecules in the case of compound 2/8.
3.70
2/9
2/10
2/11
2/12
3.72
3.82
3.96
This observation can be explained on the basis of the ribbon
model of this columnar phase. In the borderline regions between
the ribbons, the aromatic cores and the aliphatic chains have to
overlap (see Figure 6). These unfavorable chain-core interac-
tions become increasingly more important with longer terminal
alkyl chains. It seems that on increasing the chain length the
system tries to reduce the number of these borderline regions
by increasing the size of the individual ribbons (increase in the

Nonchiral Bent-Core Liquid Crystals
J. Am. Chem. Soc., Vol. 122, No. 8, 2000 1597
Figure 5. (a) X-ray pattern of a monodomain of the Colr phase of compound 2/4 at 120 °C (supercooled state); (b) schematic sketch of the small
angle pattern, together with the Millers indices.
Table 3. Comparison of the Molecular Length (L) the Lattice
Parameter of the Colr Phase (a, c) and the Layer Distance in the
SmCPA Phases (d)
of the viscosity. This mesophase is designated as SmX.
Compound 2/8, which forms all three mesophases, was inves-
tigated in more detail by X-ray diffraction. Here the changes
of the diffraction pattern depending on the phase type can be
studied. The X-ray diagram (Figure 7) is changed on cooling
the sample from the columnar phase. Now the first and second
order of a layer reflection can be seen beside the diffuse wide
angle scattering, which is characteristic for fluid smectic phases.
Such a pattern is also obtained for compounds 2/8 and 2/10 to
2/12. The layer periods are listed in Table 3. Compound 2/8
also offers the possibility of comparing the molecular length
with the periods in both phases. As mentioned above, the c
parameter in the columnar phase corresponds with the length
of the molecule. The period in the fluid smectic phase however
is significantly lower. With both values a tilt of the molecular
long axis with respect to the layer normal of 38° can be
estimated. This means that in this smectic phase, the banana-
shaped molecules are inclined with respect to the layer normal
(see Figure 1). As the electrooptical investigations (see below)
reveal an antiferroelectric switching behavior, we can assign
this fluid smectic phase as the SmCPA phase.
Colr
SmCPA
compd
L/nm
a/nm
c/nm
d/nm
2/4
2/6
2/7
2/8
3.9
4.3
4.6
4.9
5.1
5.3
5.5
5.7
5.3
5.7
2.66
3.09
3.33
4.33
4.02
3.86
4.20
4.34
4.68
4.64
3.70
2/9
2/10
2/11
2/12
3/9
3.72
3.82
3.96
3.46
6.38
4.75
4.78
3/11
parameter a). On further elongation of the chains, however, the
ribbon phase becomes unstable. Hence, the columnar phase is
lost or it can only be observed over a small temperature range
(compounds 2/8 and 2/10).
The lattice parameters a and c of compounds with odd-
numbered chains are unusually small in comparison to those of
their neighboring homologues with even-numbered chains which
is especially evident if one compares compounds 2/8 and 2/9.
This should be attributed to an odd-even effect. Interestingly,
compounds 2/7 and 2/9 which exclusively form the columnar
phase have reduced diameters of their ribbons in comparison
to 2/8, which has only a small existence region of the columnar
phase. Hence, the odd-even effect of parameter a is in line
with the alternation of the mesophase type.
The homologues 2/8 and 2/10 show a rather complicated
polymorphism (see Table 1 and Figure 3). On cooling from the
isotropic liquid state the typical mosaic texture of the columnar
phase occurs (see Figure 4a), but on further cooling a transition
to another phase which has a schlieren texture can be found
(see Figure 4b). This mesophase always shows a distinct
birefringence and no pseudoisotropic regions can be obtained
by shearing, indicating a biaxial structure. Furthermore, it has
a rather low viscosity, comparable to conventional SmA and
SmC phases. On heating the samples, this mesophase directly
turns into the isotropic liquid phase without passing the columnar
mesophase. However, the columnar phase is obtained again by
cooling. It seems that the 2D lattice of the columnar phase can
only be formed by cooling from the isotropic liquid state.29
On further cooling of compound 2/8, at 86 °C the schlieren
texture turns into a mosaic-like texture with a significant increase
Cooling down the SmCPA phases of compounds 2/8 and 2/10
to 2/12 the transition into the SmX phase takes place. The X-ray
pattern of the nonoriented sample is only scarcely changed in
the small angle region but the wide angle region contains several
reflections, which resemble a highly ordered layer structure, such
as G or H. This pattern is maintained down to room temperature.
Remarkably no crystallization can be observed after the first
melting of compounds 2/8 and 2/10 to 2/12 even over prolonged
storage at room temperature.
Exclusively the fluid smectic phase and the SmX phases were
found for compounds 2/10 to 2/12. The typical texture of the
SmCPA phase as obtained on cooling the isotropic liquid state
of 2/12 is shown in Figure 4c. The enthalpies of the transitions
from the liquid crystalline to the isotropic phase increase in this
homologous series with the chain length ranging from 11.7 kJ
mol-1 for compound 2/4 to 21.6 kJ mol-1 for compound 2/11
(see Table 1). The enthalpy of the transition between the Colr
phase and the SmCPA phase is significantly lower and amounts
to 3.4 kJ mol-1 (compound 2/8). For the transitions between
(29) The Colr-SmCPA dimorphism of 1/10 is related to that of compounds
2/8 and 2/10 as a small region of the columnar phase can be observed in
the cooling cycle, whereas on heating only the direct transition to the SmCPA
phase is found, see Table 1.

1598 J. Am. Chem. Soc., Vol. 122, No. 8, 2000
Shen et al.
Figure 6. Structure model of the Colr phase: (a) stable Colr phase of molecules with short chains or with large bent rigid cores; (b) the same
arrangement of molecules with long chains and small bent cores is unstable.
Figure 7. Scattering diagrams of a nonoriented sample of compound
2/8 in the Colr phase (150 °C), in the SmCPA phase (120 °C), and in
the SmX phase (room temperature 25 °C).
the SmCPA phases and the SmX phases enthalpies of about 6.5
kJ mol-1 were determined.
Figure 8. Transition temperatures of the homologous series of
compounds 3/n.
C. Mesomorphic Properties of the Alkoxy-Substituted
Compounds 3. The dependence of the mesomorphic properties
substituted compounds 2 whereas compound 3/11 has an unusual
of the homologous series of 4-(4-alkoxybenzoyloxy)benzoates
large value (a ) 6.38 nm). It corresponds to approximately six
3/8 to 3/14 on the chain length is shown in Figure 8. Again,
molecules in the diameter of the ribbons. It seems that the critical
the homologues with short chains have the columnar phase
diameter of the ribbons depends not only on the chain length
whereas the homologues with long chains display the SmCPA
but also on the precise molecular structure. Obviously, in the
case of alkoxy-substituted bent rigid cores an increased stability
of the ribbons is provided. This effect of the oxygen atoms could
be due to their polarity or to changes of the molecular
phase. However, the length of the alkyl chains which is
necessary to obtain the SmCPA phase (n ) 12) is significantly
larger than in the series of the alkyl-substituted compounds 2
(n ) 8). Another interesting difference between these two
conformation.30 Additionally, the ether oxygen atoms could
homologous series is the following. For the molecules with
increase the effective length of the rigid parts of the bent-cores
because of the enhanced rotational barrier around the C(ar)-O
bond due to the conjugation of the oxygen lone pairs with the
aromatic π-system.
D. Influence of the Core Structure. We asked if the stability
alkyloxy chains the columnar phases suddenly disappear when
the terminal chains are longer than C11 whereas in the series of
alkyl-substituted compounds this occurs over a certain chain
length range (C7-C11) with a pronounced odd-even effect.
Additionally the alkoxy-substituted compounds do more readily
of the ribbon arrangement could be related to the size of the
crystallize and no additional low-temperature mesophase could
bent rigid cores. Therefore, we have investigated the molecules
be detected.
1/n to 6/n with n ) 10, 12, and 14, incorporating bent-core
units Z with the same bending angle, but with a different size.
This comparison (Figure 9) shows that the alkyl chain length,
X-ray investigations of the columnar mesophases of 3/9 and
3/11 indicate again centered rectangular cells. The lattice
parameter are included in Table 3. The c values are in good
agreement with the molecular lengths. Also the parameter a of
compound 3/9 is in the same order as those of the alkyl-
(30) Kleinpeter, E.; Ko¨hler, H.; Lunow, A.; Tschierske, C.; Zaschke, H.
Tetrahedron 1988, 44, 1609.

Nonchiral Bent-Core Liquid Crystals
J. Am. Chem. Soc., Vol. 122, No. 8, 2000 1599
Figure 10. Comparison of mesophases and transition temperatures of
semifluorinated and related hydrocarbon compounds. The crystalline
phase of compound 1/10 is not shown.29
Figure 9. Comparison of mesophases and transition temperatures of
the compounds 1/n, 3/n, 4/n, 5/n, and 6/n depending on the alkyl chain
length (n ) 10, 12, 14); the mesophases of compound 1/10 are
monotropic,29 but its crystalline phase (see Table 1) is not shown for
clarity.
necessary to replace the columnar phase by the SmCPA phase,
increases on enlargement of the bent rigid core. For molecules
with very large central units Z (e.g. 5/n and 6/n) even a
tetradecyloxy chain (n ) 14) is not sufficient to replace the
columnar phase by the SmCPA phase. Interestingly, the columnar
phase disappears if the chain length reaches the length of the
rigid aromatic halves attached to the central 1,3-disubstituted
benzene ring (respectively the average length for compounds
2, 3, and 5 with different halves). Therefore, it seems that the
columnar phase is only stable if a partial overlap of the aromatic
cores of the antiparallel arranged molecules at the ribbon
interfaces is possible. Hence, the ribbon structure is stabilized
by core-core interaction in the borderline regions between
neighboring ribbons (see Figure 6a). These core-core interac-
tions should be especially effective for molecules with extended
rigid cores and they can be reduced or lost on increasing the
chain length (see Figure 6b). In conclusion, SmCPA phases can
only be obtained if sufficiently long and flexible chains are
chosen, which can suppress the formation of frustrated layer
structures. Thereby, the critical chain length necessary to obtain
SmCPA phases strongly rises with the size of the rigid aromatic
molecular parts (Z + M).
Figure 11. Switching current response in the SmCPA phase of
compound 2/12 obtained by applying a triangular voltage (Vpp ) 192
V, f ) 9 Hz, T ) 100 °C).
shown in Figure 10, the mesophases of banana-shaped molecules
can be efficiently stabilized by replacing the alkyl chains by
semifluorinated chains and, even more interestingly, SmCPA
phases can be induced for molecules which have Colr phases
as hydrocarbon analogues. The stability of the mesophases rises
with increasing degree of fluorination, which can be achieved
by elongation of the fluorinated segments, or alternatively, by
(33) Smart, B. E. Organofluorine Chemistry- Principles and Commercial
Applications; Banks, R. E., Smart B. E., Tatlow J. C., Eds.; Plenum Press,
New York, 1994; p 57.
(34) Examples for the stabilization of liquid crystalline phases by
perfluorinated chains: (a) Viney, C.; Twieg, R, J.; Russell, T. P.; Depero,
L. E. Liq. Cryst. 1989, 5, 1783. (b) Doi, T.; Sakurai, Y.; Tamatani, A.;
Takenaka, S.; Kusabashi, S.; Nishihata, Y.; Terauchi, H. J. Mater. Chem.
1991, 1, 169. (c) Nguyen, H. T.; Sigaud, G.; Achard, M. F.; Hardouin, F.;
Twieg, R. J.; Betterton, K. Liq. Cryst. 1991, 10, 389. (d) Pensec, S.;
Tournilhac, F.-G.; Bassoul, P. J. Phys. II Fr. 1996, 6, 1597. (e) Arehart, S.
V.; Pugh, C. J. Am. Chem. Soc. 1997, 119, 3027. (f) Takenaka, S. J. Chem.
Soc., Chem. Commun. 1992, 1748. (g) Johansson, G.; Percec, V.; Ungar,
G.; Smith, K. Chem. Mater. 1997, 9, 164. (h) Dahn, U.; Erdelen, C.;
Ringsdorf, H.; Festag, R.; Wendorff, J. H.; Heiney, P. A.: Maliszewskyj,
N. C. Liq. Cryst. 1995, 19, 759. (i) Percec, V.; Schlueter, D.; Kwon, Y. K.;
Blackwell, J.; Mo¨ller, M.; Slangen, P. J. Macromolecules 1995, 28, 8807.
(j) Reference 27. (k) Pegenau, A.; Cheng, X. H.; Tschierske, C.; Go¨ring,
P.; Diele, S. New J. Chem. 1999, 23, 465. (l) Cheng, X. H.; Diele, S.;
Tschierske, C. Angew. Chem., Int. Ed. Engl. 2000, 39, 592.
E. Semifluorination as Designing Principle. As shown
above, the separation of aliphatic and aromatic regions is an
important factor determining the mesophase structure of such
molecules.31,32 We asked if it would be possible to stabilize the
SmCPA phases by using the fluorophobic effect, i.e. the inherent
incompatibility of perfluorinated alkyl chains with aliphatic
chains, aromatic hydrocarbons, and polar groups.33,34 Indeed as
(31) Review on the importance of segregation effects in liquid crystals:
Tschierske, C. J. Mater. Chem. 1998, 8, 1485.
(32) Segregation of aliphatic chains and rigid cores is also important for
alkylene-bridged dimers (ref 18) and main chain polymers: Watanabe, J.;
Nakata, Y.; Simizu, K. J. Phys. II Fr. 1994, 4, 581.

1600 J. Am. Chem. Soc., Vol. 122, No. 8, 2000
Shen et al.
Figure 12. Rotation of an extinction cross in the SmCPA phase of compound 2/12 on applying an electric field (polyimide coated ITO cell, sample
thickness 6 µm, T ) 125 °C) and proposed organization of the molecules in the different states; H+, H- ) homochiral states of opposite handedness.
The brightness of each image was separately scaled, which means that in reality the birefringence of part b is significantly lower than those of parts
a and c.
increasing the number of fluorinated chains (see compound 7).35
The mesophase stabilization amounts to ca. 50 °C per C6F13
segment.
voltage. Only the mesophases of compounds 3/4F6 and 4/4F6
could not be investigated because of their high clearing
temperatures which did not allow the filling of the ITO cells
without damage. Figure 11 shows the electric response of
compound 2/12 under a triangular wave voltage with a peak-
to-peak voltage of 192 V and a frequency of 9 Hz at T ) 100
°C. Two sharp peaks were recorded during a half period
indicating an antiferroelectric switching behavior.36 The spon-
taneous polarization determined by integration of the switching
current peaks amounts to 700 nC cm-2
.
The switching process was also observed between crossed
polarizers. On slowly cooling a cell filled with 2/12 from the
isotrop liquid state (-0.5 deg K per min), without applied
voltage, exclusively a schlieren texture was obtained, which
means that the smectic layers are predominately oriented parallel
F. Electrooptical Investigations. The electrooptical behavior
of the SmCPA phases of all compounds with the exception of
3/4F6 and 4/4F6 was studied in 6 µm thick polyimide-coated
ITO cells (measuring area 100 mm2) by applying a triangular
(35) X-ray investigations indicate a layer distance corresponding to a
single-layer arrangement, i.e. no hint on the segregation of the fluorinated
and the nonfluorinated chains could be observed, which should lead to a
bilayer arrangement.
(36) Only one sharp current peak can be detected by applying a peak to
peak voltage below 92 V which would suggest a ferroelectric switching
(see ref 20). On increasing the peak-to peak voltage a broad second peak
occurs which becomes sharper on increasing the triangular voltage.

Nonchiral Bent-Core Liquid Crystals
J. Am. Chem. Soc., Vol. 122, No. 8, 2000 1601
alternation at zero field to give uniformly tilted chiral domains
(ferroelectric order) with synclinic interlayer correlation
(SmCSPF, see Figure 12a,c). Hence, in the predominate ground
state the polar axes of the molecules alternate from layer to
layer and the interlayer correlation is anticlinic (SmCAPA phase,
see Figure 12b).
In respect to the stripe pattern these novel materials behave
different from some Schiff-base materials for which SmA-like
fan textures without stripes were obtained above a certain
saturation voltage.37 In our case the stripes do not disappear on
further increase of the voltage. It seems that this stripe pattern
results from the interruption of the predominating homogeneous
chiral regions by small regions with opposite handedness.
Antiferroelectric switching behavior was also proven for the
SmCA phases of all other investigated compounds. Also the
switching process observed between crossed polarizers is
identical with that of 2/12. However, in the Colr phases and in
the SmX phases no antiferroelectric switching can be detected.
As an example for a semifluorinated molecule, the electric
response of 1/6F4 is shown in Figure 13. Here, the spontaneous
polarization amounts to 500 nC cm-2 at 145 °C.
Figure 13. Switching current response in the SmCPA phase of
compound 1/6F4 by applying a triangular voltage (6 µm polyimide
coated ITO-cell, Vpp ) 212 V, f ) 10 Hz, T ) 140 °C).
to the surfaces. A focal conic texture with parallel stripes is
obtained by slow cooling (-0.3 deg K per min) in an electric
field of at least 30 V. On increasing the voltage the number of
stripes increases and at a voltage of 46 V the stripes suddenly
become significantly brighter and colored, indicating that the
birefringence increases. On further rising the voltage no further
changes of the stripe pattern take place.
Conclusions
A wide variety of novel bent-core molecules without the
unstable Schiff-base structural unit have been synthesized.
Depending on the precise molecular structure they can form
antiferroelectric switchable SmCPA phases or rectangular co-
lumnar phases (and a highly ordered smectic low-temperature
mesophase). The rectangular columnar mesophase obviously
enables the most efficient escape from the macroscopic polar-
ization of the molecular dipole moments provided by the special
shape of these molecules. However, this arrangement is ac-
companied by a partial loss of the segregation of incompatible
molecular parts. Therefore, structural variation which increases
the segregation of rigid bent cores and flexible terminal chains
destabilizes the columnar phase leading to the SmCPA phase in
which the polarization cancels out from layer to layer. Therefore,
short rigid core units, long alkyl chains, and especially semi-
fluorinated chains suppress columnar phases and favor the
occurrence of the SmCPA phases. In some cases especially broad
regions of SmCPA phases with remarkably low melting points
(some of the compounds do not crystallize at all) and high values
of spontaneous polarization have been obtained. Under our
experimental conditions the SmCPA phases of the investigated
compounds have a predominately homogeneous chiral ground
state (SmCAPA). In this respect they are different from the
classical Schiff-base molecules for which the racemic ground
state (SmCSPA) is dominating.15 It seems that the SmCPA phases
of these compounds have an enhanced tendency for the achiral
symmetry breaking. Hence, these novel and stable materials
should be of interest for various future applications.
The textures of a sample of compound 2/12 are shown in
Figure 12, after switching off the voltage (b) and after applying
a voltage of -110 (a) and +110 V (c). The images of the two
switched states are nearly identical, independent of the sign of
the applied field. Furthermore, the orthogonal extinction brushes
rotate clockwise, respectively anticlockwise with inversion of
the polarization of the electric field. Brushes of different domains
can rotate in the same direction or in opposite directions.
The field-induced rotation of extinction brushes was first
analyzed by Link et al.15 The tilt direction of the molecules
and the direction of the polar order of the bent molecules are
two independent symmetry breaking factors which cause the
chirality of the smectic layers (see Figure 1d).15 Changing one
of them changes their handedness. Accordingly, there are two
possible equilibrium structures which both have an antiferro-
electric switching behavior: (a) In the racemic state (R) the tilt
of the molecules is uniform (synclinic interlayer correlation,
see Figure 1b) whereas the polar direction alternates in sign
from layer to layer (antiferroelectric order). Thus the handedness
changes from layer to layer. The texture of the ground state is
characterized by parallel stripes. On applying an electric field
the stripes disappear and the extinction brushes do not rotate
when the sign of the field is reversed. (b) In the homogeneous
chiral ground state (H) the tilt direction and also the polar axis
alternate from layer to layer (anticlinic and antiferroelectric, see
Figure 1c). Here the handedness of the layers is uniform. In
this case the extinction crosses of flowerlike domains rotate in
opposite directions depending on the polarity of the field.
Under our experimental conditions the extinction brushes
rotate in different directions on reversing the sign of the field,
which points to a predominately homogeneous chiral ground
state. As the rotation directions can be different in different
domains, separate regions of opposite handedness (H+ and H-)
should coexist in the samples. The angle between the extinction
crosses amounts to ca. 86°. Provided that the smectic layers
are arranged perpendicular to the substrate (book-shelf geom-
etry) the average optical axis should be tilted about 43° with
respect to the layer normal. The increase in birefringence upon
field application should be the result of the loss of the tilt
Acknowledgment. This work was supported by the Kul-
tusministerium des Landes Sachsen-Anhalt and the Fonds der
Chemischen Industrie.
Supporting Information Available: Experimental proce-
dures, 1H NMR, 13C NMR, and 19F NMR spectra, a table with
the elemental analyses data, and schemes describing the
synthesis of the aromatic dihydroxy compounds (PDF). This
material is available free of charge via Internet at http://pubs.
acs.org.
JA993572W
(37) Pelzl, G.; Diele, S.; Grande, S.; Jakli, A.; Lischka, C.; Kresse, H.;
Schmalfuss, H.; Wirth, I. Liq. Cryst. 1999, 26, 401.