Molecular design for a cybotactic nematic phase

Article abstract of DOI:10.1039/c4tc00001c

The cybotactic nematic phase (Ncyb) has attracted much attention. However, almost all molecules in which it has been investigated have had a bent-shaped structure. We designed a U-shaped compound, and some rod-like compounds which possessed a terminal hyd

Full text of DOI:10.1039/c4tc00001c

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Molecular design for a cybotactic nematic phase
Wataru Nishiya,a Yoichi Takanishi,b Jun Yamamotob and Atsushi Yoshizawa*, a
aDepartment of Frontier Materials Chemistry, Graduate School of Science and
Technology, Hirosaki University, 3 Bunkyoꢀcho, Hirosaki, Aomori 036ꢀ8561, Japan
Eꢀmail: ayoshiza@cc.hirosakiꢀu.ac.jp
bDepartment of Physics, Graduate School of Science, Kyoto University,
Oiwakeꢀcho, Kitashirakawa, Sakyoꢀku, Kyoto, 606ꢀ8562, Japan
Abstract
Cybotactic nematic phase (Ncyb) has attracted much attention, however, almost all
molecules under investigation have had a bentꢀshaped structure. We designed a
Uꢀshaped compound and some rodꢀlike compounds possessing a terminal hydroxyl
group. Then we investigated their phase transition behaviour using polarized optical
microscopy, differential scanning calorimetry, and Xꢀray diffraction. The Uꢀshaped
compound possessing two 1,4ꢀdiphenylꢀ2,3ꢀdifluorobenzene units was found to exhibit
an Ncyb phase with a temperature range of 20 K and an semctic C (SmC) phase. The
layer spacing in these phases decreases continuously with decreasing temperature. The
rodꢀlike
compound,
was
4ꢀ[4ꢀ(6ꢀhydroxyhexyloxy)phenyl]ꢀ1ꢀ(4ꢀhexyloxyphenyl)ꢀ2,3ꢀdifluorobenzene,
found to exhibit an Ncyb with a temperature range of 25 K and an SmC phase. The
layer spacing shows discontinuous decrease at the Ncyb–SmC transition. This report
presents two approaches for producing an Ncyb phase, i.e., (1) the introduction of the
lateral correlation in the Uꢀshaped compounds and (2) the introduction of the different
terminal groups in the rodꢀshaped compounds.
1

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Introduction
Nematic liquid crystals are described as differing from normal isotropic liquids only in
the spontaneous orientation of the molecules with their long axes parallel. However,
nematics of different types have been reported. De Vries presented the existence of
more than one type of nematic based on the observation of four offꢀmeridional Xꢀray
scattering peaks.1,2 The shortꢀrange structure in nematics using Xꢀray diffraction was
also reported by Chystyakov and Chailkowsky.3 Nematic phase with smecticꢀlike
ordering is ascribed to transient, local, and positional order in the nematic in the form of
smectic fluctuations having the smectic layer either normal or tilted relative to nematic
director n (Fig. 1).4 Such a nematic phase, called cybotactic nematic, is denoted as Ncyb.
Freiser predicted nematic of a new type having three directors termed as a biaxial
nematic (Nb).5 The Nb phase was detected in a lyotropic liquid crystal6 and in side
polymers.7 The Nb phase is expected to be of fundamental interest for general soft
matter physics and for use in display applications with an order of magnitude faster
expected switching times than the existing uniaxial nematics. According to theoretical
predictions,9–10 numerous attempts have been undertaken,11 such as with boardꢀlike
molecules or by combining rodꢀlike and diskꢀlike units.12–14 However, an Nb phase was
not confirmed in the systems. Bentꢀcore mesogens, which are far away from the
theoretically predicted structures, were reported to exhibit an Nb phase.15,16 However,
controversy persists about the correct interpretation of the characteristics of the namtic
phase formed by those bentꢀcore molecules. Recently, Francescangeli and Samulski
reported that the nematic phase of the bentꢀcore mesogen (ODBPꢀPhꢀOꢀC4H9)
comprises inherently biaxial clusters of mesogens.17 It was proposed that local biaxiality
can arise in the Ncyb phases composed of small clusters with smectic structure.18,19
Cybotactic clusters have been investigated intensively for consideration in the field of
biaxial nematic liquid crystals.11,20 Detailed investigations on nematic phases of
bentꢀcore mesogens have been performed.21 Furthermore, bentꢀcore molecules
possessing a cybotactic structure in the N phase are reported to stabilize blue phases.22,23
Therefore, Ncyb phase has attracted much attention. Nevertheless, almost all molecules
under investigation have had a bentꢀshaped structure.24–34 With respect to the formation
of cybotactic clusters, they can be formed by intermolecular interactions even in
liquids.20 Supramolecular interactions such as dipole–dipole interactions and hydrogen
bonding, which are strongly directional, can assist the formation of clusters in liquid
crystalline phases.11,35
2

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Fig. 1 Sketch of a nematic phase exhibiting skewed (left) and normal (right) smectic
fluctuations.
Yoshizawa and Yamaguchi reported that a Uꢀshaped compound exhibits a nematic phase
possessing smecticꢀlike layer structures and that the nematic phase presents the
possibility of a phase biaxiality.36 Later, a relation between cluster formation in a
nematic phase and occurrence of the phase biaxiality was noted more clearly.37 Not only
the Uꢀshaped system but also a hydrogenꢀbonding system can produce Ncyb phase.
Recently, we reported a simple rodꢀlike molecule possessing a terminal hydroxyl group,
4ꢀ[4ꢀ(7ꢀhydroxyheptyloxy)phenyl]ꢀ1ꢀ(4ꢀhexylphenyl)ꢀ2,3ꢀdifluorobenzene
(nꢀhexyl
derivative) exhibiting a nematic phase possessing cybotactic clusters and smectic C
(SmC) phases consisting of three states:38 conventional SmC with a monolayer structure,
SmC’ with a modulated bilayer structure possessing an inꢀplane modulation of
molecular arrangement 39 and SmC’’ with a bilayer structure. The structure–property
relation in the rodꢀlike compounds indicates that the phase transition behaviour is
explainable in terms of competition between microꢀsegregation and antiꢀparallel
molecular alignment in each layer.40 Although the Uꢀshaped compound and the rodꢀlike
compound showed Ncyb phases, the temperature ranges are insufficiently wide for
investigation of temperature dependent properties. This report presents two approaches
for producing an Ncyb phase, i.e., (1) the introduction of the lateral correlation in the
Uꢀshaped compounds and (2) the introduction of the different terminal groups in the
rodꢀshaped compounds.
Experimental
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Materials
Spectroscopic analysis. The purity of the final compound was cofirmed using
elemental analysis (EA 1110; CE Instruments Ltd.). Infrared (IR) spectroscopy
(FTSꢀ30; BioꢀRad Laboratories Inc.) and proton nuclear magnetic resonance (1H NMR)
spectroscopy (JNMꢀECA500; JEOL) elucidated the structure of the final product.
Preparation
of
materials.
For
use
in
this
study,
was
4ꢀhydroxylphenylꢀ1ꢀ[4ꢀ(2ꢀtetrahydropyranyloxy)phenyl]ꢀ2,3ꢀdifluorobenzene
purchased from Midori Kagaku Co. Ltd.
1,2-Bis{6-[4-(4-(4-hexyloxyphenyl)-2,3-difluorophenyl)phenyloxy]hexyloxy}ben-
-zene (1). Compound 1 was prepared by a synthetic method as depicted in Scheme 1.
Potassium carbonate (293 mg, 2.12 mmol) was added to a solution of
4ꢀhydroxylphenylꢀ1ꢀ[4ꢀ(2ꢀtetrahydropyranyloxy)phenyl]ꢀ2,3ꢀdifluorobenzene (500 mg,
1.31 mmol) and 1ꢀbromohexane (110 mg, 0.66 mmol) in cyclohexanone (10 mL). The
reaction mixture was stirred at 110 °C for 9 h. After filtration of the precipitate, the
solvent was removed by evaporation. The residue was purified on silica gel using
column chromatography with a dichloromethane and hexane (1:1) mixture as the eluent.
It
was
then
recrystallized
from
hexane,
giving
yield
4ꢀhexyloxylphenylꢀ1ꢀ[4ꢀ(2ꢀtetrahydropyranyloxy)phenyl]ꢀ2,3ꢀdifluorobenzene;
495 mg (81%); mp 108.5–109.2 °C.
Consequently, the obtained compound (490 mg, 1.05 mmol) was added to a
solution of EtOH (20 mL) and 20% aq. HCL (4 mL). The resulting solution was stirred
at 25 °C for 2 h. 40 mL of distilled water was added to the reaction mixture. The
solution was extracted using dichloromethane (40 mL × 3). The organic layers were
combined, dried over magnesium sulphate, filtered, and evaporated to give
4ꢀ(4ꢀhexyloxyphenyl)ꢀ1ꢀ(4ꢀhydroxyphenyl)ꢀ2,3ꢀdifluorobenzene; yield 323 mg (80%);
mp 182.1–183.5 °C.
Potassium carbonate (987 mg, 7.14 mmol) was added to a solution of
2ꢀhydroxyphenol (513 mg, 4.66 mmol) and 1,6ꢀdibromohexane (2.5 g, 10.2 mmol) in
cyclohexanone (10 mL). The reaction mixture was stirred at 140 °C for 7 h. After
filtration of the precipitate, the solvent was removed by evaporation. The residue was
purified on silica gel using column chromatography with a toluene and ethyl acetate
(4:1) mixture as the eluent, giving 1,2ꢀbis(6ꢀdibromohexyloxy)benzene as a colourless
liquid; yield 600 mg (30%).
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Potassium carbonate (82 mg, 0.59 mmol) was added to a solution of
4ꢀ(4ꢀhexyloxyphenyl)ꢀ1ꢀ(4ꢀhydroxyphenyl)ꢀ2,3ꢀdifluorobenzene (115 mg, 0.30 mmol)
and 1,2ꢀbis(6ꢀdibromohexyloxy)benzene (230 mg, 0.53 mmol) in cyclohexanone (10
mL). The reaction mixture was stirred at 120 °C for 10 h. After filtration of the
precipitate, the solvent was removed by evaporation. The residue was washed with
EtOH and purified on silica gel using column chromatography with a toluene and
hexane (4:1) mixture as the eluent. It was then washed with EtOH, giving the desired
compound; yield 31 mg (20%); mp 131.5ꢀ131.9 °C.
1HꢀNMR (500 MHz, CDCl3, TMS) δH/ppm: 7.49 (dd, 8H, ArꢀH, J = 8.6, 2.9 Hz), 6.98
(d, 4H, ArꢀH, J = 6.3 Hz), 6.98 (d, 4H, ArꢀH, J = 6.3 Hz), 6.96 (d, 4H, ArꢀH, J = 6.3
Hz), 6.91 (s, 4H, ArꢀH), 4.04–3.99 (m, 12H, ꢀOCH2ꢀ), 1.83–1.36 (m, 32H, aliphaticꢀH),
0.92(t, 6H, ꢀCH3, J = 6.9 Hz). IR(KBr) ν cm–1: 3422, 1609, 1526, 1462, 1078, 812.
Elemental analysis: Found: C% 74.59; H% 7.66, Calc. for C30H36O3F2: C% 74.66; H%
7.52.
The other compounds presented in this paper were obtained using a similar
method to that for compound 1. Analytical data for the other compounds are listed
below.
1
1,4-Bis(4-hexyloxyphenyl)-2,3-difluorobenzene (2). mp 107.9–108.1 °C. HꢀNMR
(500 MHz, CDCl3, TMS) δH/ppm: 7.52 (d, 4H, ArꢀH, J = 8.6 Hz), 7.18–7.23 (m, 2H,
ArꢀH), 6.99 (d, 4H, ArꢀH, J = 8.6 Hz), 4.02 (t, 4H, ꢀOCH2ꢀ, J = 6.6 Hz), 1.36–1.82 (m,
16H, aliphaticꢀH), 0.92(t, 6H, ꢀCH3, J = 7.2 Hz). IR(KBr) ν cm–1: 3434, 1609, 1526,
1458, 1110, 815. Elemental analysis: Found: C% 77.10; H% 7.67, Calc. for C30H36O2F2:
C% 77.22; H% 7.78
4-[4-(6-Hydroxyhexyloxy)phenyl]-1-(4-hexyloxyphenyl)-2,3-difluorobenzene
(3).
mp 120.7–121.0 °C. 1HꢀNMR (500 MHz, CDCl3, TMS) δH/ppm: 7.52 (d, 4H, ArꢀH, J =
8.6 Hz), 7.21 (d, 2H, ArꢀH, J = 4.0 Hz), 6.99 (d, 4H, ArꢀH, J = 8.6 Hz), 4.02 (t, 4H,
ꢀOCH2ꢀ, J = 6.3 Hz), 3.68 (q, 2H, ꢀCH2ꢀOH, J = 6.3 Hz), 1.83–1.36 (m, 16H,
aliphaticꢀH), 1.23 (s, 1H, ꢀCH2ꢀOH), 0.92(t, 3H, ꢀCH3, J = 7.5 Hz). IR(KBr) ν cm–1:
3365, 1609, 1525, 1458, 1091, 813. Elemental Analysis: Found: C% 74.66; H% 7.52,
Calc. for C30H36O3F2: C% 74.77; H% 7.30.
1,4-Bis[4-(6-hydroxyhexyloxy)phenyl]-2,3-difluorobenzene (4). mp 145.6–145.9 °C.
1HꢀNMR (500 MHz, CDCl3, TMS) δH/ppm: 7.52 (d, 4H, ArꢀH, J = 8.6 Hz), 7.20 (d, 2H,
ArꢀH, J = 2.9 Hz), 6.99 (d, 4H, ArꢀH, J = 8.6 Hz), 4.02 (t, 4H, ꢀOCH2ꢀ, J = 6.3 Hz),
5

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3.66–3.68 (m, 4H, ꢀCH2ꢀOH), 1.84–1.47 (m, 16H, aliphaticꢀH), 1.23 (t, 2H, ꢀCH2ꢀOH,
J = 5.4 Hz). IR(KBr) ν cm–1: 3429, 1609, 1526, 1459, 1077, 813. Elemental analysis:
Found: C% 72.58; H% 7.09, Calc. for C30H36F2O4: C% 72.27; H% 7.28.
4-[4-(4-Methylpentyloxy)phenyl]-1-[4-(6-hydroxyhexyloxy)phenyl]-2,3-difluoro-
1
-benzene (5). mp 123.6–124.1 °C. HꢀNMR (500 MHz, CDCl3, TMS) δH/ppm: 7.52 (d,
4H, ArꢀH, J = 8.6 Hz), 7.20–7.21 (m, 2H, ArꢀH, J = 4.0 Hz) , 6.99 (d, 4H, ArꢀH, J = 8.6
Hz) , 4.04ꢀ3.98 (m, 4H, ꢀOCH2ꢀ, J = 6.3 Hz) , 3.70ꢀ3.66 (m, 2H, ꢀCH2ꢀOH, J = 6.3 Hz),
1.83ꢀ1.37 (m, 16H, aliphaticꢀH) , 1.24 (t, 1H, ꢀCH2ꢀOH, J = 5.2 Hz), 0.94(d, 6H, ꢀCH3,
J = 6.3 Hz). IR(KBr) ν cmꢀ1
：3422, 1609, 1526, 1462, 1078, 812. Elemental analysis:
Found: C% 74.59; H% 7.66, Calc. for C30H36O3F2: C% 74.66; H% 7.52.
6

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Scheme 1 Synthesis of compound 1.
Liquid-crystalline and physical properties
The initial phase assignments and corresponding transition temperatures for the
products were determined using thermal optical microscopy with a polarizing
microscope (POL, Optiphoto; Nikon Corp.) equipped with a microfurnace (FP82;
Mettler Inst. Corp.) and a control unit (FP80). Temperatures and enthalpies of
transitions were investigated using differential scanning calorimetry (DSC, DSC 6200
calorimeter; Seiko Corp.).
The XRD patterns of the sample during cooling processes were obtained using a
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realꢀtime Xꢀray diffractometer (D8 Discover; Bruker AXS GmbH) or MicroMaxꢀ007HF
(Rigaku Corp.). A sample was put on a convex lens, which was then placed in a
customꢀmade temperature stabilized holder (stability within ±1 °C). The phase
transition of the sample under the Xꢀray beam was monitored by observing the texture
simultaneously using polarised light microscopy with a chargeꢀcoupled device (CCD)
camera. The Xꢀray apparatus was equipped with a crossꢀcoupled Göbel mirror on a
platform system with a twoꢀdimensional positionꢀsensitive proportional counter (PSPC)
detector (HIꢀStar; Bruker AXS GmbH). Then Xꢀrays were generated at 45 kV and 20
mA; a parallel Cu Kα Xꢀray beam was used to irradiate the sample.
The correlation length along the layer normal (ξ) was determined using the
Ornstein–Zernicke expression as follows. First the Xꢀray profile as a function of 2θ is
converted to a scattering function of q according to:
q = (4π/λ)sin θ.
(1)
By fitting the Xꢀray profiles using the following Lorentzian equation, the correlation
length ξ is determined.
(2)
Therein, I0 and q0 respectively signify the peak height and the peak position of q.
Results and discussion
U-shaped system
The first approach for stabilizing an Ncyb phase is inducing lateral correlation in each
layer using a Uꢀshaped system. The molecular structure and the phase transition
temperatures determined using POM and DSC are presented in Fig. 2. The compound
exhibited nematic and smectic C phases. Although the N–SmC transition did not
accompany the enthalpy change, a typical fan texture was observed in the SmC phase.
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Fig. 2 Molecular structure, transition temperatures (°C) and enthalpies (kJ mol–1, in
parentheses) of transition for compound 1. aToo small to be detected.
Fig. 3 shows Xꢀray diffraction patterns in the small angle region of compound 1 in the
N and SmC phases. Alignment was performed by slow cooling of a small drop of a
sample on a glass plate. On cooling the sample, a diffuse scattering appeared beside the
meridian in the nematic phase (Fig. 3(b)), indicating that molecules are organized in a
cybotactic cluster that has an SmCꢀlike skewed structure. However the molecular
alignment was insufficient, we cannot determine a detailed structure of the cluster based
on the diffraction pattern. The scattering became sharp in the SmC phase (Fig. 3(c)).
(a)
(b)
(c)
Fig. 3 Xꢀray diffraction patterns in the small angle region of compound 1: (a) N phase at
T–TNC = +24.2 K; (b) N phase at T–TNC = +4.2 K; (c) SmC phase at T–TNC = –0.8 K.
Fig. 4 shows the Xꢀray diffraction profile of compound 1 in the N and SmC phases. Fig.
5(a) exhibits the temperature dependence of the layer spacing of compound 1. The Ncyb
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phase appeared at T–TNC = +20 K. The molecular length of compound 1 using MOPAC
is estimated as 33 Å. Both Ncyb and SmC phases have a monolayer structure. The layer
spacing decreases continuously in the Ncyb phase and the SmC phase with decreasing
temperature, indicating that the Ncyb phase has an SmCꢀlike layer structure. Fig. 5(b)
shows temperature dependence of the correlation length of compound 1 in the Ncyb and
SmC phases. The correlation length increases discontinuously at the Ncyb–SmC
transition.
(a)
12000
10000
8000
6000
4000
2000
0
1
2
3
4
5
6
2θ / °
(b)
12000
10000
8000
6000
4000
2000
0
1
2
3
4
5
6
2θ / °
(c)
25000
20000
15000
10000
5000
0
1
2
3
4
5
6
2θ / °
Fig. 4 Xꢀray diffraction profiles in the small angle region of compound 1: (a) N phase at
T–TNC = +24.2 K; (b) N phase at T–TNC = +4.2 K; (c) SmC phase at T–TNC = –0.8 K.
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(a)
(b)
3500
32
31.5
31
3000
N
N
SmC
2500
2000
SmC
1500
1000
500
0
30.5
30
29.5
-20 -15 -10
-5
0
5
10
15
20
25
30
35
-20 -15 -10
-5
0
5
10
15
20
25
30
35
T－TNC/K
T－T NC/K
Fig. 5 Temperature dependence of (a) layer spacing and that of (b) correlation length of
compound 1 in the N and SmC phases.
Fig. 6 shows optical textures of compound 1 in the N phase at T–TNC = +34.1 K and the
Ncyb phase T–TNC = +12.1 K. Both textures have homeotropic alignment regions and
planar alignment regions. In the Ncyb phase, the homeotropic region shows a
completely dark texture. Biaxiality was not detected in the Ncyb phase. Considering
XRD and POM studies, the director is perpendicular with respect to the glass plate and
the layer normal is tilted with respect to the glass plate. The Ncyb phase is thought to
have a skewed structure (Fig. 1 left). The Ncyb–SmC transition shows discontinuous
increase, however, it did not accompany enthalpy change. The interlayer translational
diffusion constant is thought to decrease continuously in the Ncyb and SmC phases, as
observed for the Ncyb–SmA transition of the nꢀhexyl derivative.41 On the other hand,
the previously reported Uꢀshaped system possessing two phenylpyrimidine units
exhibited a first order Ncyb–SmC transition.36 The intermolecular interaction due to the
C–F bond of compound 1 contributes to stabilize the layer ordering in the N phase.
(a) N phase
(b) Ncyb phase
Fig. 6 Optical textures of compound 1 in (a) the N phase at T–TNC = +34.1 K and (b) the
Ncyb phase at T–TNC = +12.1 K.
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We observed phase transition behaviour of its monomeric compound (Fig. 7). The
compound has the same phase sequence as compound 1. However, it showed no
diffraction pattern in the small angle region corresponding to the layer structure in the N
phase.
Fig. 7 Molecular structure, transition temperatures (°C) and enthalpies (kJ mol–1, in
parentheses) of transition for monomeric compound 2.
Uꢀshaped compound 1 has the following characteristics: (a) strong correlation of
rotation around the long axis of each mesogenic group and (b) lateral intermolecular
interaction because of the C–F bond. Cꢀ13 NMR studies of a smectic liquid crystal
suggest that cooperative motion for the mesogenic parts contributes to the orientational
order of the molecules in each layer.42 Therefore, the Uꢀshaped system enhances lateral
interactions producing smecticꢀlike positional order and induces a layer structure with a
short correlation length in the N phase. The smecticꢀlike structures can coexist with
nematic molecules as presented in Fig. 8.
Fig. 8 Model for the Ncyb phase of compound 1.
Hydrogen-bonding system
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The second approach for stabilizing Ncyb phase is regulation of interlayer interaction.
Our previous studies on the nꢀhexyl derivative exhibiting an Ncyb phase revealed that
the generation of cybotactic clusters affects not only the static structure but also the
dynamics in the N phase.41 To investigate the effects of interlayer interaction on
formation of a Ncyb phase, we prepared its derivatives with different terminal groups
and observed their phase transition behaviour. Molecular structures and phase transition
properties of the compounds are shown in Fig. 9. Molecular lengths of compounds 1–3
using MOPAC are estimated as 29 Å for 3, 27 Å for 4, and 27 Å for 5.
Fig. 9 Molecular structures, transition temperatures (°C) and enthalpies (kJ mol–1, in
parentheses) of transition for compounds 3, 4, and 5. aToo small to be detected.
XRD profiles of compound 3 in the N and SmC phases are presented in Fig. 10.
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Diffraction peak was observed in the N phase (Fig. 10(a)), revealing that compound 3
exhibits an Ncyb phase. The correlation length increased discontinuously at the Ncyb–
SmC transition as same as compound 1. Fig. 11 shows the temperature dependence of
(a) layer spacing and that of (b) correlation length of compound 3 in the N and SmC
phases. The Xꢀray diffraction peak appears over 25 K above the N–SmC transition point
of compound 3. Both Ncyb and SmC phases of compound 3 were found to have a
monolayer structure. The layer spacing increases and then decreases slightly as
decreasing temperature in the Ncyb phase. It shows a discontinuous decrease at the
Ncyb–SmC transition, as observed for a typical SmA–SmC transition. The temperature
dependence of layer spacing for compound 3 in the Ncyb phase differs from that for
compound 1. There might be difference between them in terms of the layer structure.
The optical texture of a homeotropic alignment sample of compound 3 appears to be
completely dark. We infer that the Ncyb phase of compound 3 has an SmAꢀlike normal
layer structure (Fig. 1 right).
(a)
25
20
15
10
5
0
1
2
3
4
5
6
2θ / °
(b)
250
200
150
100
50
0
1
2
3
4
5
6
2θ / °
Fig. 10 Xꢀray diffraction profiles in the small angle region of compound 3: (a) N phase
at T–TNC = +13.0 K; (b) SmC phase at T–TNC = –2.0 K.
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(a)
(b)
1600
27.5
27.0
26.5
26.0
25.5
25.0
1400
N
N
SmC
1200
SmC
1000
800
600
400
200
0
-20 -15 -10 -5
0
5
10 15 20 25 30 35 40 45 50
-20 -15 -10 -5
0
5
10 15 20 25 30 35 40 45 50
T － TNC / K
T － TNC / K
Fig. 11 Temperature dependence of (a) layer spacing and that of (b) correlation length of
compound 3 in the N and SmC phases.
Fig. 12 portrays the temperature dependence of the periodicity length of compound 4
possessing two terminal hydroxyl groups in the N and SmA phases. No diffraction peak
corresponding to layer spacing was detected in the N phase.
30
29.5
N
SmA
29
28.5
28
27.5
-20 -18 -16 -14 -12 -10 -8
-6
-4
-2
0
2
T－TNA/K
Fig. 12 Temperature dependence of the periodicity length in the N and SmA phases for
compound 4.
Compound 5 was found to exhibit another SmC phase, denoted as SmC’ phase. The
SmC–SmC’ transition did not accompany enthalpy change. Fig. 13 shows a
photomicrograph of the sample on the glass in the SmC’ phase. The texture of the SmC’
phase is similar to that of a modulated bilayer SmC phase observed for the nꢀhexyl
derivative.38,39
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Fig. 13 Optical texture of compound 5 on a glass slide with a cover glass in the SmC’
phase at T–TNC = –20 K.
Fig. 14 presents Xꢀray diffraction patterns in the small angle region of compound 5 in
the SmC and SmC’ phases. In the SmC phase, scattering corresponding to a layer
spacing of 25 Å was observed. The SmC phase was found to have a monolayer structure.
Cooling to the SmC’ phase, two diffraction spots at small angle appear symmetrically
with respect to the diffraction spot corresponding to the monolayer structure. Such a
pattern is observed in a modulated smectic phase.43,44 Both POM and XRD studies
reveal that the SmC’ phase is a modulated bilayer SmC phase possessing an inꢀplane
modulation of molecular arrangement.39 At present we cannot exclude a possibility that
the SmC’ phase has a bilayer structure observed for the SmC’’ phase of the
nꢀhexylderivative.33 However, no diffraction peak corresponding to layer spacing was
detected in the N phase of compound 5. Fig. 15 shows temperature dependences of the
periodicity lengths in the N, SmC, and SmC’ phases for compound 5.
(a) SmC
(b) SmC’
Fig. 14 Xꢀray diffraction patterns in the small angle region of compound 5: (a) SmC
phase at T–TNC = –13.4 K; (b) SmC’ phase at T–TNC = –25.4 K.
16

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55
50
45
40
35
30
25
20
N
SmC
SmC'
-30
-25
-20
-15
-10
-5
0
5
10
15
T－ TNC / K
Fig. 15 Temperature dependence of periodicity lengths in the N, SmC, and SmC’ phases
for compound 5.
We propose a possible model for appearance of the Ncyb phase of compound 3 as
depicted in Fig. 16. Antiꢀparallel molecular alignment because of microꢀsegregation
prefers a bilayer structure, whereas parallel molecular alignment because of packing
entropy prefers a monolayer structure. For a bilayer structure, interꢀlayer interaction
between adjacent alkyl chains is much weaker than intraꢀlayer interaction because of
hydrogen bonding, which might destabilize a smecticꢀlike layer structure possessing a
longꢀrange periodicity (right hand in Fig. 16). However, in the case of the monolayer
structure, it has no difference in interꢀlayer interaction, which might stabilize a layer
structure in the N phase (left hand in Fig. 16). Competition between monolayer and
bilayer characters can induce the generation and decomposition of a smecticꢀlike layer
structure in the N phase.
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Fig. 16 Possible model for formation of the Ncyb phase of compound 3.
With respect to compound 4 possessing two terminal hydroxyl groups, every interꢀlayer
interaction caused by hydrogen bonding is so strong that the smectic phases are greatly
stabilized. Regarding compound 5, the modulated structure was observed in the SmC’
phase as observed for the nꢀhexyl derivative possessing an Ncyb phase. However, the N
phase of compound 5 has no cybotactic structure. The phase transition behaviour of an
antiꢀclinic smectic liquid crystal possessing a terminal branched group reveals that the
interꢀlayer interaction between adjacent branched chains is stronger than that between
linear chains.45 The interꢀlayer interactions between adjacent branched chains for
compound 5 might disturb the layer structure decomposition. The smectic phases of
compound 5 are stable, and the generation and decomposition of a layer ordering occurs
only with difficulty in the N phase.
Conclusions
We propose two molecular designs for a cybotactic nematic phase. The first approach
for producing an Ncyb phase is inducing lateral correlation in each layer using a
Uꢀshaped system. The Uꢀshaped compound might enhance intraꢀlayer interaction,
producing a stable smecticꢀlike layer ordering with a short correlation length in the N
phase. The second one is regulation of interꢀlayer interaction using a hydrogenꢀbonding
system. Competition between monolayer and bilayer characters can induce the
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generation and decomposition of a smecticꢀlike layer in the N phase.
Acknowledgement
This work was supported by a GrantꢀinꢀAid for Scientific Research (no. 25107702) on
the Innovation Areas: “Fusion Materials” (Area no 22006) from MEXT.
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Graphical abstract
O
F
F
OH
F
F
O
O
O
O
O
O
F
F
O
27.5
32
N
27.0
31.5
SmC
N
SmC
26.5
26.0
25.5
25.0
31
30.5
30
29.5
-20 -15 -10 -5
0
5
10 15 20 25 30 35 40 45 50
-20 -15 -10
-5
0
5
10
20
25
30
35
T－TNC/K 15
T － TNC / K
We propose two molecular designs for a cybotactic nematic (Ncyb) phase. The
U-shaped compound exhibits a skewed Ncyb phase, although the rod-like compound
possessing a terminal hydroxyl group shows a normal Ncyb phase.

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We propose two molecular designs for a cybotactic nematic (Ncyb) phase. The
U-shaped compound exhibits a skewed Ncyb phase, although the rod-like compound
possessing a terminal hydroxyl group shows a normal Ncyb phase.