A blue phase observed for a novel chiral compound possessing molecular biaxiality

Article abstract of DOI:10.1039/b506167a

Novel chiral compounds possessing molecular biaxiality, T-shaped and bend-shaped compounds, have been prepared and the physical properties investigated. The T-shaped chiral compound was found to show a blue phase with a wide temperature range on cooling.

Full text of DOI:10.1039/b506167a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A blue phase observed for a novel chiral compound possessing molecular
biaxiality{
Atsushi Yoshizawa,* Masatada Sato and Juri Rokunohe
Received 3rd May 2005, Accepted 23rd June 2005
First published as an Advance Article on the web 11th July 2005
DOI: 10.1039/b506167a
Novel chiral compounds possessing molecular biaxiality, T-shaped and bend-shaped compounds,
have been prepared and the physical properties investigated. The T-shaped chiral compound was
found to show a blue phase with a wide temperature range on cooling. A double twist structure
formed by the T-shaped compound can stabilize the blue phase. Furthermore, the compound induced
a twist grain boundary phase in the binary mixture with its corresponding monomeric compound.
polymer-stabilized blue phases in which the temperature range
Introduction
is extended to more than 60 uC and they demonstrated fast
electro-optical switching for the stabilized blue phase.13 Coles
Investigation of chirality in liquid crystals is one of the most
exciting areas of liquid crystal science. Frustrated phenomena
et al. reported that eutectic mixtures of two symmetric liquid
induced by chirality have been reported. Twist grain boundary
crystal dimers doped with a small percentage of a highly
(TGB) phases, blue phases, and smectic Q phases have been
twisting chiral additive shows the BPI of a very wide tempera-
observed and structure–property correlations investigated.1
ture range.14 On the other hand, many kinds of liquid-
Blue phases are of particular interest because they have a
crystalline materials possessing a centre of chirality have
fluid lattice whose structure is stabilized by lattice defects.
been investigated. Recently compounds possessing an axis or
Appearance of blue phases results from the competition
a plane of chirality have been reported.15–18 A single chiral
between the chiral twisting force and the desire for molecules
compound with a wide temperature range of a blue phase
to pack in ways such that they fill space uniformly. Theoretical
has never been obtained. Theoretical work suggests that
and experimental works have demonstrated that chiral nematic
biaxiality plays an important role in the blue phases.19
liquid crystals with short pitch can form up to three distinct
However, the biaxiality in most chiral nematic liquid crystals
blue phases, i.e. blue phase I (BPI), blue phase II (BPII) and
is slight, thus the double twist structure can not exist in a wide
blue phase III (BPIII).2–4 Blue phases are normally found in a
temperature range.
very narrow (y1 uC) temperature range between the isotropic
We reported a U-shaped molecule that induces a smectic-
liquid and a chiral nematic (N*) phase of sufficiently short
like layer ordering in the nematic phase and noted the
pitch, except in a few cases where a SmA to BPI transition has
possibility that the nematic (N) phase has biaxiality.20 Thus
been observed.5–7 Double twist cylinders are believed to exist
we designed coupling between the axial chirality and the
U-shaped system.21,22 We reported a homologous series of
in BPI and BPII. Meiboom et al. discovered that the local free
energy of most chiral nematic liquid crystals could be reduced
dimeric liquid-crystalline compounds possessing a binaphthyl
by a double twist structure.8 Such a structure requires
group ((R)-n) and found an unusual phase sequence of I–BP–
energetically unfavourable disclinations between separate
SmA.21 Furthermore, a new type of helical twist inversion was
double twist regions. As the liquid crystal approaches the
observed for the chiral system.21 The helical twist sense for
isotropic transition, the energy cost of disclinations lowers and
compounds (R)-6, (R)-7, (R)-8 and (R)-9 was right-handed,
a network of the double twist structures with disclinations
however, that for compounds (R)-10, (R)-11 and (R)-12 was
might be more stable than the helical N* phase. Li et al.
left-handed. In this paper, we have designed novel T-shaped
reported the observation of a different phase sequence I–BPs–
and bent chiral compounds possessing molecular biaxiality,
TGBA–TGBC–SmC* in a fluoro-substituted chiral tolane
derivative.9,10 Recently, there has also been experimental
and investigated the chirality-dependent properties. The
mesogens with lateral aromatic substituents were synthesized
evidence for a smectic blue phase, which is observed between
in Vorla¨nder’s group.23 T-Shaped compounds in which the
lateral substituents are attached via spacers have been
reported.23–25 We report novel chiral effects in the chiral
T-shaped molecule on appearance of frustrated phases.
the isotropic and smectic phases.11,12 From the viewpoint
of applications, blue phases are interesting for fast light
modulators or tunable photonic crystals. However, the narrow
temperature range is a critical problem. Kikuchi et al. reported
Department of Materials Science and Technology, Faculty of Science
and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561,
Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp; Fax: +81-172-39-3558;
Tel: +81-172-39-3558
{ Electronic supplementary information (ESI) available: Synthetic
procedures for T-1 and the bent compound (B-1). See http://dx.doi.org/
10.1039/b506167a
Experimental
The T-shaped chiral compound (T-1) was prepared by the
synthesis outlined in Scheme 1. Synthetic procedures for T-1
and the bend-shaped compound (B-1) are listed in electronic
supplementary information (ESI).{
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Liquid-crystalline and physical properties
and FP80 control unit. The heating and cooling rates were
5 uC min21. Temperatures and enthalpies of transition were
investigated by differential scanning calorimetry (DSC) using a
Seiko DSC 6200 calorimeter. The material was studied at a
scanning rate of 5 uC min21, for both heating and cooling
cycles, after being encapsulated in aluminium pans. The helical
The initial assignments and corresponding transition tempera-
tures for the final products were determined by thermal optical
microscopy using
a Nikkon Optiphoto POL polarizing
microscope equipped with a Mettler FP82 microfurnace
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Scheme 1 Synthesis of compound T-1.
pitch in the N* phase was measured by the Cano wedge
Results and discussion
method for a chiral nematic mixture consisting of nematic
liquid-crystalline material, 4-hexyl-49-cyanobiphenyl (6CB)
(purchased from BDH) and each chiral additive. The helical
pitch in the SmC* phase was measured by observing the
length between the dechiralization lines corresponding to
the full pitch band for a mixture consisting of 5-octyloxy-2-(4-
hexyloxyphenyl)pyrimidine (8-PYP-6O) and each chiral
additive. The helical pitches were measured at room tempera-
ture. The mixtures were studied using the contact method and
their chiral helical twist senses were established. The definition
of the helical twist senses used in this article is the same as that
of Goodby.26
Liquid-crystalline properties
Transition temperatures and enthalpies of transition for T-1
and B-1 are shown in Table 1. Phase transition behaviour of
T-1 was investigated by optical microscopy and DSC. On
cooling (5 uC min21) the isotropic liquid exhibited a blue
colour of low birefringence (Fig. 1(a)). The blue colour was
clearly observed in uncovered regions of T-1. The blue
coloured phase showed fluidity and did not appear as platelets
which is usually observed in BPI. These observations indicate
that the phase is BPIII. On further cooling the BP changed to a
chiral nematic phase (Fig. 1(b)). The temperature range of the
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are normally found in a very narrow (y1 uC) temperature
Table 1 Transition temperatures (uC) on cooling and enthalpies
(kJ mol21) of transition (in brackets) for compounds T-1 and B-1
range. T-1 showed the BP with a wide temperature range.
Nakata et al. reported that a mixture consisting of chiral
nematic material and achiral bend compound induces a blue
phase.27 Furthermore, Blatch et al. reported that blue phase I
behaviour was observed in a very narrow temperature range
(0.6 uC) between isotropic liquid and N* phases for the odd
non-symmetric dimers, whereas a direct I–N* transition was
observed for the even non-symmetric dimers.28 The conforma-
tion of T-1 in the liquid-crystalline phases is not clear. MM2
calculations suggest that T-1 exists as a l-shaped structure
rather than a T-shaped structure. T-1 has two possible
molecular long axes, i.e. an axis through the biphenylbenzoate
unit and the other through the phenylpyrimidine unit. We can
Recryst.a
N*
BP
I
Mpb
T-1
B-1
a
N 15c
N 28 (1.4)
N
N
63 (66)
81 (30)
N 80 (31)
T-1 was not crystallized on cooling at a rate of 5 uC min21
.
The
b
c
melting points were measured by DSC. The enthalpy of the BP to
N* transition for T-1 was too small to be detected.
BP was about 13 uC. On the other hand, B-1 did not show any
liquid-crystalline phase.
T-1 exhibited BP and N* phases. Some achiral T-shaped
compounds were reported to show a N phase.23 Blue phases
Fig. 1 Photomicrographs of (a) the blue phase at 25 uC and (b) the N* phase at 10 uC of T-1 in an uncovered region on a glass slide. Magnification
6100.
Fig. 2 A schematic representation for the double twist arrangement by T-1.
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say that T-1 has molecular biaxiality. Thus coupling between
the molecular biaxiality and chirality is thought to induce the
blue phase. Fig. 2 shows a schematic model for the double
twist arrangement by T-1.
Pitch measurements
We investigated the twisting power of each chiral compound.
The helical pitch values and their helical twist senses in the N*
and SmC* phases are listed in Table 2.
Both compounds were found to have the same magnitude of
twisting power in the N* and SmC* phases.
Miscibility studies
We investigated the transition behaviour of binary mixtures
between 8-PYP-6O and T-1. The phase diagram on cooling
is shown in Fig. 3(a). The transition temperatures were
determined by optical microscopy. The N phase of both
materials proved to be miscible across the full composition
range. The BP was observed for the mixtures containing more
than 50 wt% of T-1. Furthermore, a twist grain boundary
phase was found to appear for a mixture containing 10 wt% of
T-1. An optical photomicrograph of the TGBA phase of the
mixture is shown in Fig. 4. Fig. 3(b) shows the phase diagram
between 8-PYP-6O and B-1. A SmA phase was found to be
induced for mixtures containing more than 60 wt% of B-1. A
SmC* phase was also induced for mixtures containing more
than 75 wt% of B-1. However, no frustrated phase was
observed in the binary phase diagram.
Fig. 3 (a) Binary phase diagram on cooling for mixtures of
8-PYP-6O and T-1, and (b) binary phase diagram on cooling for
mixtures of 8-PYP-6O and B-1.
Pitch measurements reveal that T-1 and B-1 have the same
magnitude of twisting power in the N* and SmC* phases. T-1
induced frustrated phases, i.e. TGBA and blue phases, in the
binary mixture with 8-PYP-6O, however, B-1 did not induce
any. The appearance of the TGBA phase is attributed to the
difference in molecular structure between T-1 and B-1.
Possible molecular organization models in the SmA phase of
the mixture with each chiral compound are shown in Fig. 5
and Fig. 6.
recently reported that a l-shaped molecule induces layer
deformation.29 Fig. 5(b) shows another possibility. T-1 adopts
a l-shaped structure, with the two rods arranged nearly
parallel, in each layer. The strong frustration could be induced
by splaying of the two rods. Coupling between the twist owing
to the molecular chirality and the splay of the two core
units may induce the TGBA phase. Further investigation is
necessary to understand the appearance of the TGBA phase in
The phenylpyrimidine group of T-1 can happily exist in
layer structures composed of the host molecules, and the chiral
part of T-1 is thought to localize in the inter-phase region
(Fig. 5(a)). Thus, the appearance of the TGBA phase in the
mixture between 8-PYP-6O and T-1 may be a result of the
competition between twist deformation owing to the molecular
chirality localized in the inter-phase region and the desire for
the molecules to form a layered structure. However, it is not
realistic that the long rods should lie between the alkyl chains
at the interlayer interfaces parallel to these interfaces. We
Table 2 Helical pitch (mm) in the N* and SmC* phases for a mixture
consisting of 2 wt% of each chiral compound and the helical twist
senses (in brackets), i.e. right-hand (RH) or left-hand (LH)a
Compound
SmC*
N*
T-1
B-1
a
20 (RH)
20 (RH)
7 (RH)
8 (RH)
Helical pitch (mm) in the N* phase for a chiral nematic mixture of
6CB and 2 wt% of each chiral compound and that in the SmC*
phase for a chiral mixture of 8-PYP-6O and 2 wt% of each chiral
compound were observed at room temperature.
Fig. 4 Filament texture of the TGBA phase of a mixture containing
10 wt% of T-1 between untreated glass plates at the transition to the
SmA phase.
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the T-shaped compound induced a TGBA phase in the mixture
with its corresponding monomeric compound.
Acknowledgements
We thank Dr Isa Nishiyama for helpful discussions. This work
was partly supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Science, Sports, Culture, and
Technology in Japan (No. 15550156). We also acknowledge
the input of a referee.
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Conclusion
We have designed novel chiral compounds possessing mole-
cular biaxiality. The T-shaped compound was found to show a
blue phase with a wide temperature range. Coupling between
the molecular biaxiality and chirality is thought to produce a
stable double twist structure of the blue phase. Furthermore,
3290 | J. Mater. Chem., 2005, 15, 3285–3290
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