Stabilization of the blue phases of simple rodlike monoester compounds by addition of their achiral homologues

Article abstract of DOI:10.1039/c2jm16359d

As novel rodlike liquid crystalline ester compounds exhibiting blue phases (BPs), chiral 4-(4′-alkoxyphenyl)phenyl 4-(4′-alkoxyphenyl)benzoate derivatives (R-C₆H₄-C₆H₄-OCO-C ₆H₄-C₆

Full text of DOI:10.1039/c2jm16359d

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 8484
www.rsc.org/materials
PAPER
Stabilization of the blue phases of simple rodlike monoester compounds by
addition of their achiral homologues†
*
Keiki Kishikawa, Hiroyuki Itoh, Seiji Akiyama, Takahiro Kobayashi and Shigeo Kohmoto
Received 5th December 2011, Accepted 22nd February 2012
DOI: 10.1039/c2jm16359d
As novel rodlike liquid crystalline ester compounds exhibiting blue phases (BPs), chiral 4-(40-
alkoxyphenyl)phenyl 4-(40-alkoxyphenyl)benzoate derivatives (R–C6H4–C6H4–OCO–C6H4–C6H4–
R0), 1 (R ¼ R0 ¼ (R)-1-methylheptyloxy, BP range: 1.6 K on heating), 2 (R ¼ (R)-1-methylheptyloxy,
R0 ¼ n-octyloxy, BP range: 0.8 K on heating), 4 (R ¼ (R)-1-methylheptyloxy, R0 ¼ H, BP range: 1.0 K
on heating), were synthesized and their phase behaviors were investigated. Further, the BP of 1 was
stabilized by addition of its achiral homologue ester compound 3 (R ¼ R0 ¼ n-octyloxy). In the range of
the composite ratio of 1 : 3 from 10 : 0 to 0 : 10, the BP temperature range became wider with increase
of the content of 3 in the mixture. At the ratio of 6 : 4, the BP indicated the widest temperature range in
the mixing experiment (5.0 K on heating and 6.0 K on cooling).
are useful for photonic devices such as tunable selective
Introduction
reflective devices,2,3 and also for laser emitting devices.4
However, the temperature ranges of BPs are so narrow (less
than 1 K in most cases)5 and expansion of them is so difficult
that utilization of the phases for those optical devices is limited.
Many scientists in the fields of chemistry, physics, and materials
science have been challenging for realization of BPs with a wide
temperature range. The reported methods are classified into (1)
polymer-approaches and (2) low-molecular weight molecule
approaches. As polymer-approaches, fixation of BPs by pho-
topolymerization of doped monomers6 and confining BPs in 3D
polymer network structures7 were reported. As low-molecular
weight molecule approaches, liquid crystalline dimers possessing
large flexoelectricity were used for stabilization of a BP-I, which
realized the wide-temperature range of the BP only on cooling.8
Further, chiral binaphthyl derivatives (BP range: 1.5 K on the
first heating (two types of crystals exist in the pure sample) and
about 30 K on the second heating–cooling cycle),9 rodlike
molecules connecting with cholesterol (BP range: 2.5 K),10 and
rodlike molecules possessing two asymmetric carbons (BP
range: 2.5 K)11 were reported, respectively. Recently, an enan-
tiotropic cubic BP of a supramolecular compound with a wide
temperature range (23 K) was reported by Yang’s group,12 and
a cubic BP of an asymmetric dimer compound (BP ranges as
the total range of BPI and BPII: about 9 K) was reported by
Yelamaggad et al.13 However, the mechanisms for the stabili-
zation of those BP have not been clarified. Thus, the method-
ology for stabilization of cubic BPs in low-molecular weight
molecule approaches has not been established yet, and expan-
sion of the temperature range to more than 3 K on heating is
still difficult.14 In this paper, we describe synthesis of simple
rodlike liquid crystalline monoester compounds exhibiting BPs
(BP range: 0.8–1.6 K on heating) and stabilization of the BPs by
Cubic blue phases have unique superstructures in which rodlike
molecules are organized into cylinder-like molecular aggregates
with an internal helical alignment, ‘‘double twist cylinders’’
(Fig. 1a), and the cylinders are assembled into superstructures
with cubic lattices (blue phases I and II (BP-Is and BP-IIs),
Fig. 1b and c).1 Their three-dimensionally periodical structures
Fig. 1 Models of (a) ‘‘double twist cylinder’’ of BPs (a cylinder-like
superstructure generated by helically aligned rodlike molecules), (b) unit
cell of BP-I, and (c) unit cell of BP-II. The lattice units are indicated by
red lines.
Department of Applied Chemistry and Biotechnology, Faculty of
Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba
263-8522, Japan. E-mail: kishikawa@faculty.chiba-u.jp; Fax: +81-43-
290-3422; Tel: +81-43-290-3238
† Electronic supplementary information (ESI) available: Synthesis and
spectral data of esters 1, 2, 3, 4, and 7, and micro-photographs of 1, 2,
a mixture of 1 and 3, a mixture of 1 and 4, and a mixture of 1 and 6.
Table of the temperature ranges of the BP in the mixtures of 1 and 3.
See DOI: 10.1039/c2jm16359d
8484 | J. Mater. Chem., 2012, 22, 8484–8491
This journal is ª The Royal Society of Chemistry 2012

View Article Online
addition of achiral homologues (the maximum BP range: 5.0 K
on heating).
chiral molecules. However, our recent results showed that the
addition of achiral ester compound 6 to a chiral nematic liquid
crystalline compound enhanced the helicity in the chiral nematic
phase.17 It is explained that one chiral conformer of the achiral
ester molecule is predominant over its mirror image under the
chiral atmosphere, and the chiral ester conformer has larger
twisting power than that of the original chiral liquid crystalline
molecule. From these results, we prospected that (1) simple
rodlike monoesters possessing chiral alkyl chains would generate
strong helicity and exhibit BPs, and (2) addition of achiral
rodlike esters to the chiral esters would increase the intermolec-
ular attractive interactions without reducing their helicity, which
stabilizes the BP.
It is known that most BPs exist above chiral nematic phases
ꢀ
exhibiting very short helical pitches (<5000 A) and large helical
twisting power is necessary to generate the helical superstructure
from both theoretical and experimental studies.15 Accordingly,
addition of achiral molecules destabilizes BPs. However, as an
exception, stabilization of BPs by addition of achiral bent rodlike
compounds (1,3-phenylene bis[4-(4-octylphenyliminomethyl)
benzoates] (P8PIMB) and racemic 40-octyloxy-biphenyl-4-
carboxylic acid 4-(1-methylheptyloxycarbonyl)phenyl ester
(MHPOBC)) to a chiral nematic liquid crystal material (a 79 : 21
mixture of ZLI-2293 and MLC-6248) was reported by Takezoe’s
group (BP ranges as the total range of BPI, BPII, and amor-
phous BP (BPIII)) for P8PIMB and MHPOBC: about 5 K and
4 K, respectively.16 ZLI-2293 is a mixture of several nematic
liquid crystal compounds and MLC-6248 is a chiral dopant
compound, which are manufactured by Merck and the infor-
mation on those molecular structures are not released. They
concluded that the phenomena originated in the bent-type
molecular shapes of P8PIMB and MHPOBC, because analogous
sized straight rod molecules (terephthalylidene-bis-4-n-butylani-
line (TBBA)) did not induce the BP under similar conditions. It is
well known that achiral bent-core molecules show spontaneous
chiral induction in the liquid crystal phases, and their explana-
tion is understandable. However, stabilization of the BPs by
addition of achiral rodlike molecules is hard to understand. To
the best of our knowledge, it has not been reported yet.
Results and discussion
Synthesis
We synthesized simple rodlike esters 1–5 (Scheme 1). These
molecules were designed based on the molecular structure of 6.
The phenyl groups of 6 were replaced with biphenyl groups to
make the intermolecular core–core interactions stronger.
Investigation of the phase transition behaviors of 1–5
The phase behaviors of 1–5 are shown in Table 1. Chiral
compound 1 (R ¼ R0 ¼ (R)-1-methylheptyloxy) exhibited a BP
with chiral smectic C and chiral nematic phases. The BP was
observed in a narrow temperature range (1.6 K on heating).
From the textures in POM (Fig. 3), the BPs observed in this
study were identified as BP-Is.18 Compound 2 (R ¼ (R)-1-
methylheptyloxy, R0 ¼ n-octyloxy) exhibited a BP (Fig. 4) with
0.8 K on heating. Almost the same temperature range as that of 1
suggested that the effect of one chiral group in the molecule was
the same as that of two chiral groups on temperature range of the
BP. To investigate the additive effect of achiral rodlike ester
molecules, the achiral homologue 3 (R ¼ R0 ¼ n-octyloxy) was
In the double twist cylinders (Fig. 2), one molecule and each of
the adjacent molecules are arranged twisted parallel. Though the
chiral alkyl chains are effective for generation of the molecular
arrangement, their steric repulsions suppress simultaneously the
intermolecular attractive core–core interactions which are
necessary for maintaining the superstructure in BPs. Accord-
ingly, reduction of the intermolecular steric repulsion between
their alkyl chains is effective for stabilization of BPs. Usually,
partial replacement of the chiral molecules with their achiral
homologues possessing less-bulky normal alkyl chains increases
the intermolecular core–core attractive interactions. This also
destabilizes the BPs because of decrease of the proportion of
Fig. 2 Schematic representation of the arrangement of chiral molecules
in the double twist cylinder. Each molecule interacts with the molecules
surrounding it (the intermolecular attractive interactions and steric
repulsions are indicated).
Scheme 1 Structures of 1–7.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8484–8491 | 8485

View Article Online
Table 1 Phase behaviours of 1–5 and 7a
a
SmC*, SmC, N*, N, BP, SmA, TGBA, and Iso indicate chiral smectic C, smectic C, chiral nematic, nematic, blue, smectic A, twist grain boundary A,
and isotropic liquid phases, respectively. Cr, Cr1, Cr2, and Cr3 indicate crystal phases. The heating and cooling rates are 5 ꢀC minꢁ1. The transition
b
temperature (ꢀC) is indicated above and below the arrows, and each transition enthalpy (kcal molꢁ1) is written in the parenthesis. The transition
temperature was measured by POM. c h1 + h2 ¼ 2.1 and h3 + h4 ¼ ꢁ1.9.
prepared. Compound 3 showed smectic C and nematic phases,
whose transition temperatures (SmC–N and N–Iso) were higher
than those of 1 and 2, respectively. It was assumed that the
replacement of the chiral alkyl chains with the normal alkyl
chains reduced the intermolecular steric repulsions and increased
the intermolecular attractive interactions, which caused the
significant increase in the melting and clearing points.
Mixing experiments of 1 with 3–6
The phase diagram of the mixture of 1 and 3 against the mole
fraction of 3 in 1 (¼ x3) is shown in Fig. 5a and b. With an
increase of x3, the transition temperatures are increased except
Fig. 3 Microphotograph of 1 on cooling (blue phase, 600ꢂ, 150.5 ꢀC).
that of SmC–N phases. At x3 ¼ 0.4 and 0.5, the transition
temperatures of SmC–N are higher than those at x3 ¼ 0.6 and
0.7. Temperature ranges of BP of the mixtures are shown in
Fig. 6. At x3 ¼ 0.0–0.4, the temperature range of the BP
becomes wider with an increase of the content of 3 in the
mixture. At x3 ¼ 0.4, the BP indicated the widest temperature
range (5.0 and 6.0 K on heating and cooling, respectively) in this
mixing experiment and the textures in POM at x3 ¼ 0.4–0.5
were observed as clear and large mosaic patterns (Fig. 7).
However, at x3 ¼ 0.5–0.7, the temperature range becomes
narrow with increase of the content of 3. At x3 ¼ 0.8 and 0.9,
the BP is not observed. The phase transition behavior at x3 ¼
0.4 (1 : 3 ¼ 6 : 4) is shown in entry 1 of Table 2. The transition
temperatures of the N–BP and BP–Iso were much higher than
those of pure 1, which meant that the intermolecular interac-
tions became larger by addition of 3. It strongly suggests that
Fig. 4 Microphotograph of 2 on cooling (blue phase, 600ꢂ, 239.0 ꢀC).
8486 | J. Mater. Chem., 2012, 22, 8484–8491
the intermolecular interaction is important to keep the wide
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Fig. 7 Microphotograph of the mixture of 1 and 3 at the ratio of 5 : 5
(x3 ¼ 0.5) on cooling (600ꢂ, 227.5 ꢀC).
In order to reduce the intermolecular steric repulsions,
compound 1 was mixed with 4 and 5, respectively. Compound 4
(R ¼ (R)-1-methylheptyloxy, R0 ¼ H) possesses the chiral
chain at one terminal and no chain at the other terminal, and 5
(R ¼ R0 ¼ H) does not possess any terminal chain. The
temperature ranges of the BP of the mixture of 1 and 4 (¼ 5 : 5)
were 1.9 and 2.2 K on heating and cooling, respectively (entry 2
of Table 2). The temperature range of the BP increased slightly.
In the mixture of 1 and 5 with the ratio of 5 : 5, crystals of 5 were
segregated partially from the mixture in the liquid crystal phase.
To avoid this, the mixture of 1 and 5 with the ratio of 8 : 2 was
prepared. It was homogeneous in both liquid crystal and
isotropic liquid phases (entry 3 of Table 2). Its temperature
ranges of the BP on heating and cooling were 2.3 and 2.9 K,
respectively. The effects of 4 and 5 on the expansion of the BP
range were smaller than that of 3.
Fig. 5 Phase diagram (a) and its magnified one (b) in the mixtures of 1
and 3. The mole fraction of 3 in 1 is indicated by x3. Cr, SmC, N, BP and
Iso indicate crystal, smectic C, nematic, blue and isotropic liquid phases,
respectively. The transition temperatures were measured by POM with
In order to investigate matching in the core lengths of chiral
and achiral molecules, compound 1 was mixed with 6 in the ratio
of 5 : 5 (entry 4 of Table 2). The temperature ranges of the BP on
heating and cooling were 1.0 and 1.5 K, respectively. The mixing
of achiral and chiral molecules with different core lengths was
not effective for stabilization of the BP. Further, the phase
diagram of the mixture of 1 and 6 against the mole fraction of 6
in 1 (¼ x6) was investigated (Fig. 8a and b). With increase of x6,
each of the transition temperatures, BP–Iso, N–BP, and SmC–N,
decreased. Temperature ranges of the BP are shown in Fig. 9. At
x6 ¼ 0.0, the BP indicated the widest temperature range (1.7 K on
heating) in this mixing experiment. At x6 ¼ 0.0–0.6, the
temperature range of the BP becomes narrower with increase of
the content of 6 in the mixture, and at x6 > 0.7, the BP is not
observed. The textures at x6 ¼ 0.5 were observed as small mosaic
patterns (Fig. 10). The phase diagram also suggests that mixing
of the achiral and chiral molecules with different core lengths
decreases the transition temperatures and narrows the BP
temperature range.
a temperature rate of 0.1 ꢀC minꢁ1
.
Fig. 6 Temperature ranges of the BP of the mixtures of 1 : 3. The mole
fraction of 3 in 1 is indicated by x3.
Investigation of the selective reflection and helical twisting
powers of the mixtures of 1 with 3 and 6
temperature range of the BP. This experiment also indicates
that addition of more than one equivalent of 3 to 1 destabilizes
the BP.
The selective reflections of the pure chiral compound 1 and the
mixtures of 1 and 3 were carried out by POM at T ꢁ TBP–N ¼ 5 K
(TBP–N: transition temperature between the BP and nematic
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8484–8491 | 8487

View Article Online
Table 2 Phase transitions of the mixtures of 1 with 3, 4, 5, and 6a
a
Cr, SmC*, N*, BP, SmA*, TGBA, and Iso indicate crystal, chiral smectic C, chiral nematic, blue, chiral smectic A, twist grain boundary A, and
isotropic liquid phases, respectively. The transition temperature (ꢀC) is indicated above and below the arrows. The transition temperature was
measured by POM with a temperature rate of 0.1 ꢀC minꢁ1
.
phase) without the polarizers. Pure chiral compound 1 did not
have color. The mixtures at 9 : 1 and 8 : 2 showed orange-red and
green-yellow, which was surprising because this meant that the
helical pitch became shorter by addition of achiral compound 3.
Choi et al. also reported this chiral enhancement in the addition
of achiral compound 6 to a chiral nematic liquid crystalline
compound.17 Accordingly, it is assumed that the decrease of the
helical pitch is common to these rodlike LC ester compounds (3
and 6). However, the mixtures at 7 : 3 and 6 : 4 showed orange
and orange-red, and at 5 : 5 it exhibited pale red, which meant
that the helical pitch became longer by addition of the achiral
compound (3). It is usual that the helical pitch increases with
a decrease of the proportion of chiral molecules. From these
investigations, it is assumed that the chiral enhancement occurs
only in high-concentrated mixture of 1 in 3.
longer than that of 2. In the POM of 2, the finger print textures
were not observed because of the very short helical pitches in its
chiral nematic phase. Accordingly, it was confirmed that the ester
group played an important role in generating the strong helicity
in these chiral liquid crystal phases. The twisted molecular
conformation and/or the dipole of the ester moiety are thought to
be the important factors for generation of the short helical
pitches.
Mechanism estimated for stabilization of the BP by addition of
achiral compounds
We postulate the following mechanism for stabilization of the BP
of 1 by addition of its achiral homologues. The ester molecules
(1) in the BP cancel out their dipoles by generation of time-
averaged anti-parallel dimers, in which the two rodlike molecules
are arranged twisted parallel (Fig. 11a). It is assumed that an
introduction of the chiral terminal chain affects the handedness
(right- or left-handedness) in the arrangement of the two rodlike
molecules and the induced handedness strongly influences the
conformation of chirally twisted ester group (Fig. 12).17 In the
case of 1, the chiral bulky chains in the dimers (homodimers 1$1)
cause large steric repulsions between the two molecules
(Fig. 11a). On the other hand, in the case of the heterodimer 1$3
(Fig. 11b), the intermolecular steric repulsions are smaller than
those of 1$1, and the intermolecular attractive interactions of 1$3
are stronger than those of 1$1. In the case of 1$3, it is assumed
that both ester groups of 1 and 3 have the same chiral confor-
mation to stabilize the complexation as shown in Fig. 13, in
which attractive electrostatic interaction between the two ester
groups and p–p interactions between the benzene rings interact
effectively. However, at the ratio exceeding 1 : 3 ¼ 6 : 4, the BP
was destabilized because of the decrease in the ratio of the chiral
molecules in the environment. Accordingly, the temperature
To investigate the correlation between the helical twisting
powers (HTPs) and the width of the temperature ranges of the
BPs, the HTPs of the mixtures 1$3 and 1$6 were measured by the
Cano wedge method. The HTPs of the mixtures 1$3 (10 : 90) and
1$6 (5 : 95) were 8.1 and 12.9 mmꢁ1 at TN–Iso ꢁ T ¼ 5 K (275.5
and 87.1 ꢀC), respectively. The order of the HTPs in 1$3 and 1$6
did not agree with the widths of the temperature ranges of the
BPs. This result indicates that compounds with a higher HTP do
not necessarily give wider BP temperature ranges.
Investigation of the effect of the carbonyl group in compound 2 on
the stabilization of the BP
To investigate the importance of the ester group, we prepared
compound 7 possessing a –CH2O– group instead of the ester
group (–(C]O)–O–) in 2. The phase behavior of 7 is shown in
Table 1. In POM, 7 did not exhibit any BP, and showed clear
finger print textures (5 mm at TN–Iso ꢁ T ¼ 5 K) in its chiral
nematic phase, which meant that its helical pitch was much
8488 | J. Mater. Chem., 2012, 22, 8484–8491
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Fig. 10 Microphotograph of the mixture of 1 and 6 at the ratio of 5 : 5
(x6 ¼ 0.5) on cooling (600ꢂ, 125.0 ꢀC).
Fig. 11 Schematic representation of time-averaged dimers generated by
intermolecular dipole–dipole interaction; (a) homodimer of two chiral
rodlike molecules (1$1), (b) heterodimer of chiral and achiral rodlike
molecules (1$3) (no difference in their core lengths), (c) heterodimer of
chiral rodlike molecule and achiral non-alkylated rodlike molecule (1$5),
and (d) heterodimer of chiral and achiral rodlike molecules (1$6)
(difference in their core lengths).
Fig. 8 Phase diagram (a) and its magnified one (b) in the mixtures of 1
and 6. The mole fraction of 6 in 1 is indicated with x6. Cr, SmC, N, BP
and Iso indicate crystal, smectic C, smectic A, nematic, blue and isotropic
liquid phases, respectively. The transition temperatures were measured by
POM with a temperature rate of 0.1 ꢀC minꢁ1
.
Fig. 12 The racemic pair of the chirally twisted conformations of 1
(Ar ¼ 4-((R)-1-methylheptyloxy)phenyl).
range of the BP against the content ratio of 3/(1 + 3) should have
the local maximum. The mixing experiment of 1 with 4, and 1
with 5, showed that the terminal alkyl chains were necessary for
expansion of the BP temperature range (Fig. 11c). In addition,
the mixing experiment of 1 with 6 indicated the importance of
matching in their core lengths to expand the BP temperature
Fig. 9 Temperature ranges of the BP of the mixtures of 1 : 6. The mole
fraction of 6 in 1 is indicated by x6.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8484–8491 | 8489

View Article Online
conclude that smaller K33/K11 stabilizes BPs,19,20 while the other
describes that larger K33/K11 stabilizes BPs.21 The relationship
between the ratio and stability of the BPs is not fully clarified.
Though we did not approach the BP stability from the elastic
constants in this study, we would like to measure the constants of
the mixtures of 1 and 3 as our next project.
Acknowledgements
We are grateful to Mukai Science and Technology Foundation,
the Asahi Glass Foundation for the funds, and we thank Grant-
in-Aid for Scientific Research (B) 19350090.
Fig. 13 The estimated models for the time-averaged anti-parallel het-
erodimer 1$3 (molecules 1 and 3 are indicated in blue and red, respec-
tively. Ar ¼ 4-((R)-1-methylheptyloxy)phenyl, Ar0 ¼ 4-octyloxyphenyl)
generated by p–p interactions and attractive electrostatic interactions.
Both molecules in each dimer have the same chirality in their twisted
ester-conformations.
Notes and references
€
1 H. Stegemeyer, T. Blumel, K. Hiltrop, H. Onusseit and F. Porsch,
Liq. Cryst., 1986, 1, 3; Phase Structures and Transitions in
Thermotropic Liquid Crystals, in Handbook of Liquid Crystal
Research, ed. P. J. Collings and J. S. Patel, Oxford University Press,
New York, 1997, ch. 4, pp. 106–108; Phase Transitions, Barois, in
Physical Properties of Liquid Crystals, ed. D. Demus, J. Goodby, G.
W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH, New York, 1999,
ch. 6, pp. 223–224; S. Hekimoglu and J. Conn, Liq. Cryst. Today,
2003, 12, 1; Defect Structures, in One and Two-Dimensional Fluids—
Properties of Smectic, Lamellar and Columnar Liquid Crystals, ed.
A. Jakli and S. Saupe, Taylor and Francis, New York, 2006, ch. 6,
pp. 199–201; K. Higashiguchi, K. Yasui and H. Kikuchi, J. Am.
Chem. Soc., 2008, 130, 6326.
2 P. R. Gerber, Mol. Cryst. Liq. Cryst., 1985, 116, 197; H. J. Coles and
H. F. Gleeson, Mol. Cryst. Liq. Cryst., 1989, 167, 213;
V. E. Dmitrienko, Liq. Cryst., 1989, 5, 847; H.-S. Kitzerow, Mol.
Cryst. Liq. Cryst., 1991, 202, 51; G. Heppke, B. Jerome,
H.-S. Kitzerow and P. Pieranski, J. Phys., 1991, 50, 2291;
P. Etchegoin, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat.
Interdiscip. Top., 2000, 62, 1435.
range. Because of the difference in their core lengths, the anti-
parallel heterodimer 1$6 (Fig. 11d) cannot have enough inter-
molecular attractive interactions.
By addition of 3 to 1 up to 1 : 3 ¼ 6 : 4, an increment in the
BP-I transition temperature was larger than that in the N*–BP
transition temperature, which resulted in the expansion of
temperature range of the BP. This phenomenon is explained as
follows. The achiral rodlike molecule 3 is sterically less-hindered
than the chiral molecule 1. Accordingly, in the BP of the mixture
of 1 and 3, the molecule 3 interacts equally with each of the
laterally surrounded 1, which stabilizes the superstructure
‘‘double twist cylinder’’ of the BP. In contrast, in chiral nematic
phases molecules are aligned one-directionally in each of planes
and the planes stack in parallel with changing the directors
helically. The laterally omnidirectional interaction of molecule 3
with its adjacent molecules generates the heterodimers 1$3 even
within the same plane, which destabilizes the one-dimensionally
helical superstructure of the chiral nematic phase.
3 Y. Hisakado, H. Kikuchi, T. Nagamura and T. Kajiyama, Adv.
Mater., 2005, 17, 96; M. Sato and A. Yoshizawa, Adv. Mater.,
2007, 19, 4145.
~
4 W. Cao, A. Munoz, P. Palffy-Muhoray and B. Taheri, Nat. Mater.,
2002, 1, 111; S. Yokoyama, S. Mashiko, H. Kikuchi, K. Uchida
and T. Nagamura, Adv. Mater., 2006, 18, 48.
5 H.-S. Kitzerow, ChemPhysChem, 2006, 7, 63.
6 H.-S. Kitzerow, H. Schmid, A. Ranft, G. Heppke, R. A. M. Hikmet
and J. Lub, Liq. Cryst., 1993, 14, 911; H. Kikuchi, M. Yokota,
Y. Hisakado, H. Yang and T. Kajiyama, Nat. Mater., 2002, 1, 64.
7 T. Noma, M. Ojima, H. Asagi, Y. Kawahira, A. Fujii, M. Ozaki and
H. Kikuchi, e-J. Surf. Sci. Nanotechnol., 2008, 6, 17.
8 H. J. Coles and M. N. Pivnenko, Nature, 2005, 436, 997.
9 A. Yoshizawa, Y. Kogawa, K. Kobayashi, Y. Takanishi and
J. Yamamoto, J. Mater. Chem., 2009, 19, 5759.
10 C. V. Yelamaggad, N. L. Bonde, A. S. Achalkumar, D. S. S. Rao,
S. K. Prasad and A. K. Prajapati, Chem. Mater., 2007, 19, 2463.
11 S.-L. Wu and W.-J. Hsieh, Chem. Mater., 2003, 15, 4515.
12 W. He, G. Pan, Z. Yang, D. Zhao, G. Niu, W. Huang, X. Yuan,
J. Guo, H. Cao and H. Yang, Adv. Mater., 2009, 21, 2050.
13 C. V. Yelamaggad, V. P. Tamilenthi, D. Shankar Rao, G. G. Nair and
S. Krishna Prasad, J. Mater. Chem., 2009, 19, 2906.
Conclusions
We realized a BP by using simple rodlike monoester compounds
possessing asymmetric carbons and demonstrated that cubic BPs
of a chiral ester compound could be stabilized by addition of its
achiral homologue ester compound. This study suggested that
increase of the intermolecular attractive forces is important for
stabilization of BPs. We believe that this methodology is also
useful to stabilize BPs of other rodlike liquid crystalline ester
compounds. Further, in the mixing of the chiral molecules and
the several achiral homologues, we also found that existence of
the terminal alkyl chains and matching in the core lengths are
important for stabilization of the BP. The information obtained
in these results should also be important for designing the
molecules exhibiting stable BPs.
14 Examples of cubic BPs recently reported: T. Seshadri and
H.-J. Haupt, Chem. Commun., 1998, 735; J. Buey, P. Espinet,
H.-S. Kitzerow and J. Strauss, Chem. Commun., 1999, 441;
ꢁ
R. Bayon, S. Coco and P. Espinet, Chem. Mater., 2002, 14, 3515;
W.-R. Chen and J.-C. Hwang, Liq. Cryst., 2004, 31, 1539;
J. Rokunohe and A. Yoshizawa, J. Mater. Chem., 2005, 15, 275;
M. L. Parra, P. I. Hidalgo and E. Y. Elgueta, Liq. Cryst., 2008, 35,
823; J.-S. Hu, K.-Q. Wei, B.-Y. Zhang and L.-Q. Yang, Liq. Cryst.,
2008, 35, 925; M. L. Parra, P. I. Hidalgo, E. A. Soto-Bustamante,
ꢁ
J. Barbera, E. Y. Elgueta and V. H. Trujillo-Rojo, Liq. Cryst.,
Recently, it is reported that the stability of BPs is related to
their elastic constants, especially the ratio K33/K11 is known as
one of the important factors (K33, K11: elastic constants for bend
and splay transformations, respectively). However, some reports
2008, 35, 1251; Y. Kogawa and A. Yoshizawa, Liq. Cryst., 2011,
38, 303.
15 H. Grebel, R. M. Hornreich and S. Shtrikman, Phys. Rev. A: At.,
Mol., Opt. Phys., 1983, 28, 1114; M. B. Bowling, P. J. Collings,
8490 | J. Mater. Chem., 2012, 22, 8484–8491
This journal is ª The Royal Society of Chemistry 2012

View Article Online
C. J. Booth and J. W. Goodby, Phys. Rev. E: Stat. Phys., Plasmas,
Fluids, Relat. Interdiscip. Top., 1993, 48, 4113.
18 The colors and shapes of the platelets were compared with BP-Is in
the previous papers (for example; ref. 6, 8 and 16).
19 G. P. Alexander and J. M. Yeomans, Phys. Rev. E: Stat., Nonlinear,
Soft Matter Phys., 2006, 74, 061706.
20 S.-T. Hur, M.-J. Gima, H.-J. Yoo, S.-W. Choi and H. Takezoe, Soft
Matter, 2011, 7, 8800.
16 M. Nakata, Y. Takanishi, J. Watanabe and H. Takezoe, Phys. Rev. E:
Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 2003, 68, 041710.
17 S.-W. Choi, K. Fukuda, S. Nakahara, K. Kishikawa, Y. Takanishi,
K. Ishikawa, J. Watanabe and H. Takezoe, Chem. Lett., 2006, 35,
896; S. Kawauchi, S.-W. Choi, K. Fukuda, K. Kishikawa,
J. Watanabe and H. Takezoe, Chem. Lett., 2007, 36, 750.
21 S.-K. Hong, H. S. Choi, S. Shibayama, H. Higuchi and H. Kikuchi,
Mol. Cryst. Liq. Cryst., 2010, 528, 32.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8484–8491 | 8491