Bent molecules with a 60° central core angle that form B7 and B2 phases

Article abstract of DOI:10.1002/anie.201403762

Small-angle bent-core liquid-crystalline (LC) molecules based on a 1,2-bis(phenylethynyl)benzene central core have been synthesized that form banana smectic phases with a ferroelectric B7-antiferroelectric B2 phase sequence upon cooling. The formation of

Full text of DOI:10.1002/anie.201403762

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DOI: 10.1002/anie.201403762
Liquid Crystals
Bent Molecules with a 608 Central Core Angle that Form B7 and B2
Phases**
Eun-Woo Lee, Koji Takimoto, Masatoshi Tokita, Junji Watanabe, and Sungmin Kang*
Abstract: Small-angle bent-core liquid-crystalline (LC) mol-
ecules based on a 1,2-bis(phenylethynyl)benzene central core
have been synthesized that form banana smectic phases with
a ferroelectric B7–antiferroelectric B2 phase sequence upon
cooling. The formation of polar, switchable ferro-/antiferro-
electric banana phases indicates that, despite the low core bend
angle of approximately 608, banana smectic phases are still
formed with the bend direction parallel to the layer. This study
offers significant evidence that shows bent-core molecules with
a 608 bend angle can form the well-known B2 and B7 banana
phases. Consequently, it may lead to the preparation of a wide
variety of novel bent molecules with low bend angles that
spontaneously form an LC phase with both polarization and
chirality.
imately 608 showed that only conventional nematic and
smectic phases were formed.[4] These molecules are assumed
to have U-shaped conformations for which bent direction
packing along the layer may not take place; therefore, they
behave in a manner similar to that of conventional calamitic
molecules, aligning their bent directions up and down in the
layer.[5] However, by synthesizing six different types of bent-
shaped molecules with typical Schiff-base side wings on the
various branching positions of the central naphthalene core,[6]
we have discovered that molecules composed of 1,7-naph-
thalene derivatives form typical banana phases, such as B4
and antiferroelectric smectic A (SmAPA) phases, despite their
small bend angles of approximately 608. Also, for the same
homologous series based on a 2,3-naphthalene central core,
Ros et al. and Choi et al. reported different investigation
results in which Choi et al. claimed a polar SmA phase,[7]
whereas Ros et al. interpreted it to be a non-polar SmA
phase.[8] Therefore, to observe not only polarity but also
a direction of molecular director is still controversial.
Furthermore, similar homologues based on a small-bend-
angle 1,7-naphthalene core were found to form both a switch-
able hexagonal columnar (Colh)[9] and cubic (Cub) phases[10]
that are constructed of enclosed smectic layers. It is thus
believed that the proper selection of central cores with a small
bend angle can induce the promotion of not only bend
direction molecular packing, which gives rise to the formation
of polar banana phases, but also the formation of a diverse
array of deformed layering structures, such as Colh and Cub
phases.
Herein, novel low-angle bent-core LC molecules based on
a 1,2-bis(phenylethynyl)benzene central core, P(1,2)-On,
containing seven aromatic rings and alkoxy tails with carbon
numbers of 12, 16, and 18 (Scheme 1) were synthesized. This
ortho-bistolane central core offers a 608 bend angle without
conformational freedom.
Figure 1 shows a representative differential scanning
calorimetry (DSC) thermogram of P(1,2)-O18 upon heating
and cooling at a scanning rate of 108CminÀ1. As observed in
cooling DSC data, two endothermic peaks at 249 and 2328C
were detected before crystallization at Tc = 1238C that
correspond to the isotropic (Iso)–X and X–B7 phase tran-
sitions, respectively. Furthermore, a small peak due to the B7–
B2 phase transition was observed at 1598C, as shown in the
inset at enlarged scale (scanning rate: 108CminÀ1).
B
anana-shaped LC molecules, or so-called bent-shaped or
bent-core LC molecules, have fascinated a great number of
researchers in the field of liquid crystal science since the
initial report of their existence by Watanabe et al.[1] Banana-
shaped LCs have attracted much attention because of their
distinct mesomorphic behavior, that is, the spontaneous
formation of chirality and polarity from achiral molecules,
which consequently induces a variety of smectic (Sm) sub-
phases comprised of B1–B7 banana phases.[2] Since the
discovery of banana-shaped LC molecules,[1] it has been
thought that a bend angle of approximately 1208 is appro-
priate for the formation of banana phases. Typical bent-core
groups used for banana-shaped LC molecules include 1,3-
disubstituted benzenes, 2,6-disubstituted pyridines, 2,7-disub-
stituted naphthalenes, and 3,4’-disubstituted biphenyls. The
angles reported to date for these compounds, which form
banana phases, range from 1108 to 1408.[3]
We have focused on smaller bend-angle molecules, and we
are interested in determining the smallest critical bend angle
at which polar packing of bent molecules occurs in the smectic
layer. Early reports based on 1,2-phenylene and 2,3-naphtha-
lene central cores with very small bend angles of approx-
[*] E.-W. Lee, K. Takimoto, Prof. Dr. M. Tokita, Prof. Dr. J. Watanabe,
Prof. Dr. S. Kang
Organic and Polymeric Materials
Tokyo Institute of Technology
2-12-1 O-okayama, Tokyo 152-8552 (Japan)
E-mail: skang@polymer.titech.ac.jp
[**] This research was supported by a Grant-in-Aid for Scientific
Research (c) (26410086) from the Ministry of Education, Culture,
Sports, Science and Technology in Japan. The synchrotron radiation
X-ray measurement was performed at the 4C beamline in the
Pohang Light Source II (PLS-II).
Figure 1 also presents the polarized optical microscopy
(POM) images for each LC phase observed during the cooling
process. The highest temperature of the X phase shows no
birefringent texture at all; however, it is distinguished from
the isotropic melt by a drastic fall in the fluidity at the Iso-X
phase transition. Upon cooling of the X phase into the B7
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403762.
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Figure 2. Phase-transition behaviors of P(1,2)-On determined by DSC
measurements with a scanning rate of 108CminÀ1
.
Scheme 1. Chemical structure of P(1,2)-On.
phase sequential properties is significant. As n decreased
from 18 to 16, the phase-transition sequence was unchanged,
but the B7 LC regime became narrower owing to an elevation
of the B7–B2 transition temperature. A further decrease in
n to 12 led to the disappearance of the X phase and the
appearance of the B7 phase directly from the isotropic melt
followed by the B2 LC regime.
Of most interest is the formation of the B2 and B7 phases.
Electro-optical measurements clarified the polar switching
nature of the B7 and B2 phases. Typical textural trans-
formations were observed in the initial circular domain for
n = 18 by applying a voltage, as shown in Figure 3. Without
the applied field, the extinction brush was lying parallel to the
cross-nicol direction. When either a plus or minus DC voltage
was applied, the birefringence increased slightly, but the
extinction direction remained unchanged. Additionally, when
the triangular wave field was reversed, a single reversal
current peak was observed within a half period of the
triangular wave voltage (Figure 3b). This switching behavior
can be explained by the presence of an anticlinic ferroelectric
layer structure; in other words, a SmCAPF layer structure
(Figure 3c).
Figure 1. Typical DSC thermogram for P(1,2)-O18 with a scanning rate
of 108CminÀ1. The inset shows enlarged peaks corresponding to the
B7–B2 phase transition. POM observation images were taken at
2348C, 2208C, and 1478C for the X, B7, and B2 phases, respectively,
upon cooling. The B7 image was taken with a slow-cooling sample
preparation (18CminÀ1). Scale bar: 100 mm.
On the other hand, the B2 phase exhibited a grainy
circular domain when observed at the same position. Upon
application of a field, it immediately transformed into a clear
circular domain with the extinction direction parallel to the
cross-nicol direction, as seen under field. Additionally, when
a triangular wave voltage was applied, two reversal peaks
were observed within a half period (Figure 4b). Thus, the
ground state of the layer structure for B2 can be explained by
a synclinic antiferroelectric SmCSPA structure (See Fig-
ure 4c). The spontaneous polarization (Ps) value calculated
by integrating the reversal current peaks was fairly large (ca.
800 nCcmÀ2) for the B2 phase, while for the Ps value for the
B7 phase was less than 200 nCcmÀ2 (Supporting Information,
Figure S1). Furthermore, the second harmonic generation
(SHG) was measured for the two phases during application of
the field. The SH signals showed the same tendency as the Ps
values, increasing upon the B7–B2 phase transition. Based on
the previously observed switching behavior of these types of
compounds, the B7 phase has been divided into subgroups
according to its switching ability, that is, non-switchable B7
and switchable B7’ phases.[11,2g] In the present case, therefore,
the B7 phase should be classified as a B7’ phase.
phase, typical myelinic helical filaments and a striped focal
conical texture could be observed that are reminiscent of the
B7 phase. Upon further cooling to the B2 phase, the overall
shape of the texture was not significantly changed, although
the birefringence increased and typical fringed and grainy
textures were superimposed. Such a slight texture change
between the B2 and B7 phases was expected given the very
small peak in the aforementioned DSC thermogram.
The thermodynamic data and phase sequence information
for P(1,2)-On obtained by the DSC and POM analyses are
listed in Table 1 and Figure 2, respectively. At a glance, it can
be seen that the effect of the terminal chain length, n, on the
Table 1: Phase-transition behaviors of P(1,2)-On upon cooling process
with a scanning rate of 108CminÀ1
.
Compound
Transition property
[Temperature (Enthalpy): 8C (kJmolÀ1)]
P(1,2)-O12
P(1,2)-O16
P(1,2)-O18
Cr 141(27.1) B2 213(0.4) B7 234(14.4) Iso
Cr 124(38.2) B2 174(0.2) B7 232(10.5) X 245(2.7) Iso
Cr 123(49.8) B2 159(0.2) B7 230(12.4) X 247(5.8) Iso
Next, an X-ray analysis was carried out to clarify the
structure in more detail using both wide-angle X-ray diffrac-
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Figure 3. Electro-optic properties of B7 phase for P(1,2)-O18 measured
at 2008C. a) POM observations during the application of DC voltage
(cell gap: 1.8 mm, DC: 60 V), b) polarization reversal current (cell gap:
1.9 mm, 120Vpp), and c) an illustration of the polar layer structure of
the B7 phase. Scale bar: 10 mm.
Figure 4. Electro-optic properties of B2 phase for P(1,2)-O18 measured
at 1458C. a) POM observations during the application of DC voltage
(cell gap: 1.8 mm, DC: 40 V), b) polarization reversal current (cell gap:
1.9 mm, 120Vpp), and c) an illustration of the polar layer structure of
the B2 phase. Scale bar: 10 mm. The layer structure of ground state
contains two different domains (the dotted line indicates the domain
boundary).
tion (WAXD) and small-angle X-ray scattering (SAXS)
(Supporting Information, Table S1). Figure 5a shows the
one-dimensional (1D) and two-dimensional (2D) SAXS
profiles for P(1,2)-O18 during cooling. Upon entering the X
phase at 2508C, a broad reflection at q (= 2p/d) = 2p/88.3 ꢀÀ1
(where d indicates d-spacing) appeared; therefore, the
optically isotropic X phase could be distinguished again
from the isotropic melt (see also the 2D patterns). Upon
further cooling to the B7 phase at 2208C, a set of multi-
scattering peaks appeared in the small angle region that were
incommensurate. The strongest diffraction at 2p/46.5 ꢀÀ1
seemed to stem from the layering order of the B7 phase.
Eventually, the inner angled multiple reflections disappeared
along with a slight simultaneous change in the d-spacing of the
layer reflection at 2p/47.5 ꢀÀ1 upon formation of the B2
phase. It should also be noted that during the consecutive
phase transitions of the X–B7–B2 phases, the diffuse wide
angle diffractions at 2p/4.4 ꢀÀ1 in each LC phase were
observed in the WAXD patterns (Figure 5b), indicating
a liquid-like lateral packing of molecule in each LC phase.
Moreover, the tilt angle in the B2 phase was estimated as
36.28 for n = 18 based on the most extended chain model
obtained by a Spartan 10 calculation (Supporting Informa-
tion, Table S2 and Figure S2).
Finally, the B7 phase was extensively examined using
a homeotropically aligned sample and synchrotron-radiation
SAXS measurement. Figure 6 presents the oriented SAXS
profile collected at 2208C. Along the meridional line, a pair of
the strongest layer reflections with a spacing of 46.5 ꢀ was
concentrated, indicating that smectic layering also developed
along this line direction. However, the inner satellite reflec-
tions with spacings of 55.2, 76.2, 110.4, and 157.7 ꢀ show split
patterns from this line with different split angles. These
observations suggest a frustrated structure with a two-dimen-
sional oblique lattice in which a = 401 ꢀ, c = 170 ꢀ, and b =
168, similar to the previously reported 2D oblique lattice,
which is composed of layer blocks with same slope in the B7
phase.[12] Thus, the (100) refection of (h00) and only (101),
(201) reflections of (h01) showed enough intensity to be
observed.[12b] Additionally, the value of the lattice parameter
a depended on the temperature, increasing from 400 to 600 ꢀ
as the temperature decreased from 220 to 1958C. Thus, it can
be concluded that the B7 phase can be attributed to a well-
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Figure 7. a) Illustration of possible structural transformation occurring
Figure 5. 1D and 2D profiles of X-ray investigations of powder sample
for P(1,2)-O18. One-dimensional a) SAXS and b) WAXD profiles
obtained from each phase. In the upper row, 2D SAXS patterns
measured at Iso (2558C), X (2508C), B7 (2208C), and B2 (1608C)
phases are shown.
at the B7–B2 phase transition and real lattice structure of the B7
phase. Here, the polarization splay modulation takes place, forming
a frustration explained by a 2D oblique lattice. b) Three-dimensional
representation of the B2 phase structure in which a molecular packing
in a bent direction along the layer is established; in other words, the
direction of n-director lies in the layer normal.
molecular packing along the molecular bend direction. This
result is striking because it had been believed that banana
phases can only be stabilized when the bending angle is in the
range from 110–1408, and these results thus provide addi-
tional insight into the nature of banana-shaped molecules.
Further investigation of the relationship between the ultimate
molecular shape and the formation of banana LCs is,
however, still needed.
Figure 6. a) 2D oriented SAXS profiles at 2208C for P(1,2)-O18 and
b) a reciprocal lattice structure interpreted as a 2D oblique lattice
(a=401 ꢀ, c=170 ꢀ, and b=168). Here, every diffraction spot
matched well with the indexation of (h0l).
Received: March 27, 2014
Published online: June 24, 2014
Keywords: banana phases · bent-shaped molecules · ferro/
.
antiferro-electricity · liquid crystals · X-ray diffraction
known frustrated structure in which layer undulation and/or
polarization modulation have occurred resulting in the
formation of an oblique 2D lattice (as proposed in
Figure 7). It is believed that the increase in the birefringence
upon application of a voltage may be understood as the
rearrangement of the polarization splay deformation in the
B7 phase.
In conclusion, this study revealed the formation of typical
banana phases of B7 and B2 in a novel bent-core system with
a significantly small bend angle of 608. Despite the low bend
angle, the formation of the polar switchable ferro-/antiferro-
electric banana phases is indicative of smectic layering by
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