Novel non-symmetric chiral twin liquid crystals possessing two identical chiral moieties at both peripheral ends: Highly chirality-sensitive phases appeared between smectic and isotropic liquid phases

Article abstract of DOI:10.1039/b201061e

A novel strategy for preparing non-symmetric chiral twin liquid crystals was proposed, where two identical chiral moieties were designed to be located at both peripheral ends and the non-symmetric character was introduced by the non-symmetric central conn

Full text of DOI:10.1039/b201061e

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Novel non-symmetric chiral twin liquid crystals possessing two
identical chiral moieties at both peripheral ends: highly chirality-
sensitive phases appeared between smectic and isotropic liquid phases
Isa Nishiyama,*a Jun Yamamoto,a John W. Goodbyb and Hiroshi Yokoyamaa
aYokoyama Nano-structured Liquid Crystal Project, JST, TRC 5-9-9 Tokodai, Tsukuba
300-2635, Japan. E-mail: isanishi@nanolc.jst.go.jp
bDepartment of Chemistry, The University of Hull, Hull, UK HU6 7RX
Received 29th January 2002, Accepted 22nd March 2002
First published as an Advance Article on the web 23rd April 2002
A novel strategy for preparing non-symmetric chiral twin liquid crystals was proposed, where two identical
chiral moieties were designed to be located at both peripheral ends and the non-symmetric character was
introduced by the non-symmetric central connecting structure. Thus, optically active chiral non-symmetric twin
homologues, [4’-(1-methylheptyloxycarbonyl)-1,1’-biphenyl-4-yl] 4@@-[v-[{4@-(1-methylheptyloxycarbonyl)-1@,1@’-
biphenyl-4@’-yl}oxycarbonyl]alkyloxy]benzoate, were prepared, which were found to exhibit the antiferro-, ferri-,
and ferroelectric phases. Furthermore, two more phases were observed between the ferroelectric and isotropic
liquid phases. The lower-temperature phase was optically isotropic under the polarized light microscopy for
both of the homogeneously and pseudo-homeotropically aligned thin samples, whereas the high-temperature
phase showed a mosaic texture. Both of these anomalous phases were found to have a layered structure with a
smaller layer spacing than the smectic phase. The stability of these phases was quite sensitive to the optical
activity. With reduction of the optical purity, these phases disappeared but the Twist Grain Boundary phase
was observed. A 1 : 1 mixture of (S,S)- and (R,R)-isomers just showed the smectic A phase instead of these
anomalous phases.
possesses two chiral moieties with different structures at both
1. Introduction
peripheral ends of the rigid core part.9 Recently, the intro-
duction of chiral moieties at both ends of the central aromatic
core of a molecular twin has been reported to have an impor-
tant effect on the stabilization of the ferrielectric structure.13
Since this molecular design will allow for the chiral groups
to interact with each other at the interfaces between the planes
of the smectic layers, stronger chiral interactions between
the neighbouring smectic layers have been proposed to be
obtained, and thus, the chirality-induced superstructure of the
ferrielectric ordering has been stabilized. The chiral compounds
reported there possess a symmetric twin structure, however, the
introduction of the non-symmetric structure in twin systems
has so far been reported to have a significant effect on the liquid
crystal properties.14 The non-symmetric structure is usually
introduced into a twin molecular structure by connecting two
different mesogenic parts. In this report, however, a novel
preparative strategy for non-symmetric chiral twin liquid cry-
stals is proposed, where a twin compound was designed in such
a way that two identical chiral moieties are located at both
peripheral ends, however, the non-symmetric character was
generated by introducing a non-symmetric central connecting
group. This molecular design allows for the molecules to
possess two chiral centres at both sides of the peripheral ends,
keeping strong peripheral chiral interactions, and at the same
time, any special effects produced by the non-symmetric
structure can be investigated.
Chiral molecules generally favour formation of a helical
superstructure in a liquid crystal system, which sometimes
competes with the desire for molecules to pack in such a way
that they fill space uniformly. Thus, introduction of chirality in
liquid-crystalline systems produces a variety of frustrated
phases. The experimental discovery of Twist Grain Boundary
(TGB) phase1 is a recent typical example for the appearance of
the frustrated phase. The TGB phase is generated as a result of
the competition between the stabilization of the smectic layered
structure and the desire for the molecules to form a helical
structure. Furthermore, the blue phase with a smectic layered
structure, i.e., smectic blue phase, has so far been obtained,2
which has been considered to be an example that the TGB
phase possesses the blue phase like superstructure.3 These
smectic blue and TGB phases have recently been investigated in
detail by systematic preparation of a family of fluoro-
substituted chiral compounds.4–6 The TGB phase also shows
an interesting frustrated structure if the distance between grain
boundary becomes short enough to produce a ‘‘crystal of
defects’’.7 For example, the SmQ phase8 has been reported to
be generated as a result of the formation of a 3D crystalline
network of the defect walls of the TGB phase with a local
antiferroelectric structure.7 Not only a family of the TGB
phases, but also an interesting example of the effect of chirality
can be seen in some dichiral mesogens,9 where the chirality was
found to play a dominant role in the appearance of the cubic
superstructure.10
Thus, in this study, novel non-symmetric chiral twin
compounds, [4’-(1-methylheptyloxycarbonyl)-1,1’-biphenyl-4-
yl] 4@@-[v-[{4@-(1-methylheptyloxycarbonyl)-1@,1@’-biphenyl-
4@’-yl}oxycarbonyl]alkyloxy]benzoate (I, II, III and IV, see
Scheme 1), were prepared and the liquid-crystalline properties
were investigated. Here we report that the non-symmetric twins
show anomalous phases between smectic and isotropic liquid
phases, and that the stability of these phases is quite sensitive to
the optical purity of the system.
Location of the chiral moieties in the overall molecular
structure seems to have a significant effect on the appearance of
these chirality-induced frustrated superstructures. For exam-
ple, compounds showing the SmQ phase possess an identical
chiral moiety at both sides of the rigid core,7,8,11,12 and the
dichiral mesogen showing a chirality-induced cubic phase also
DOI: 10.1039/b201061e
J. Mater. Chem., 2002, 12, 1709–1716
This journal is # The Royal Society of Chemistry 2002
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nuclear magnetic resonance (1H NMR) spectrometry (Bruker
DRX500 (500 MHz), JEOL GSX400 (400 MHz) or Varian
INOVA UNITY-300 (300 MHz) nuclear magnetic resonance
spectrometer), and mass (MS) spectrometry (JEOL, JMS-700
using a FD/MS method). The analyses of the structures of the
products and intermediates by spectroscopic methods were
found to be consistent with the predicted structures.
Preparation of 4-(5-carboxypentyloxy)benzoic acid. Hydro-
xybenzoic acid (2.76 g, 0.020 mol) was dissolved in a mixture
of ethanol (30 ml) and sodium hydroxide (2.64 g, 0.066 mol)
in water (15 ml). The solution was heated and stirred while
6-bromohexanoic acid (3.90 g, 0.020 mol) was added slowly.
The reaction mixture was heated under reflux for 5 hours, then
a part of the solvent (25 ml) was removed by distillation. Water
(100 ml) was added and the mixture was made strongly acidic
with the addition of hydrochloric acid. The precipitate was
isolated, dried, and recrystallized from acetic acid (42 ml),
giving a colourless solid. Yield ~ 2.84 g, (56%). dH (500 MHz,
DMSO, TMS) 12.25 (br, 2H, COOH), 7.88 (m, 2H, Ar-H), 7.00
Scheme 1 The preparative route for the homologous series of non-
symmetric chiral twins: (i) NaOH, EtOH–H2O, then H1, and (ii) DCC,
DMAP, CH2Cl2.
2. Experimental
General preparative procedures
The non-symmetric chiral twin compounds, [4’-(1-methylheptyl-
oxycarbonyl)-1,1’-biphenyl-4-yl]4@@-[v-[{4@-(1-methylheptyloxy-
carbonyl)-1@,1@’-biphenyl-4@’-yl}oxycarbonyl]alkyloxy]benzoate
(I, II, III and IV), were prepared according to Scheme 1. Final
chiral twin compounds were obtained by the esterification
between optically active (S) or (R)-1-methylheptyl 4-hydro-
xybiphenyl-4’-carboxylate and the respective 4-(v-carboxy-
alkoxy)benzoic acid in the presence of dicyclohexylcarbodiimide
(DCC) and N,N-dimethylaminopyridine (DMAP). 4-(v-Car-
boxyalkoxy)benzoic acids were prepared by the reaction
between 4-hydroxybenzoic acid and the respective v-bromo-
alkanoic acids. The (R) and (S) optically active 1-methyl-
heptyl 4-hydroxybiphenyl-4’-carboxylates were prepared
according to standard procedures.15 In the synthesis the chiral
alcohols, (S)-octan-2-ol (Azmax. Co. Ltd., 99% ee) and (R)-
octan-2-ol (Azmax. Co. Ltd., 99% ee), were used without
further purification.
3
(m, 2H, Ar-H), 4.04 (t, 2H, -CH2-CH2O-, J ~ 6.5 Hz), 2.24
3
(t, 2H, HOCO-CH2-CH2-, J ~ 7.4 Hz), 1.73, 1.57, 1.43 (m,
2H, HOCO-CH2-CH2-CH2-CH2-CH2O-). n/cm21 (KBr) 2953–
2556 (OH str.), 1696 (CLO str.), 1605 (C-C str.). m/z 252 (M1).
Preparation of 4-(7-carboxyheptyloxy)benzoic acid. This acid
was prepared from hydroxybenzoic acid (2.76 g, 0.020 mol) and
8-bromooctanoic acid (4.43 g, 0.020 mol), using the procedure
given for 4-(5-carboxypentyloxy)benzoic acid, giving the pro-
duct as a colourless solid. Yield ~ 3.56 g (64%). dH (400 MHz,
DMSO, TMS) 12.25 (br, 2H, COOH), 7.87 (m, 2H, Ar-H), 7.00
(m, 2H, Ar-H), 4.03 (t, 2H, -CH2-CH2O-, 3J ~ 6.6 Hz), 2.20 (t,
2H, HOCO-CH2-CH2-, 3J ~ 7.3 Hz), 1.72 (m, 2H, -CH2-CH2-
CH2O-), 1.50–1.31 (m, 8H, HOCO-CH2-CH2-CH2-CH2-CH2-
CH2-CH2O-). n/cm21 (KBr) 2946–2562 (OH str.), 1690 (CLO
str.), 1603 (C-C str.). m/z 280 (M1).
Preparationof[4’-((S)-1-methylheptyloxycarbonyl)-1,1’-biphenyl-
4-yl] 4@@-[5-[{4@-((S)-1-methylheptyloxycarbonyl)-1@,1@’-biphenyl-
4@’-yl}oxycarbonyl]pentyloxy]benzoate (I). (S)-1-Methylheptyl
4-hydroxybiphenyl-4’-carboxylate (1.30 g, 4.0 mmol), 4-(5-
carboxypentyloxy)benzoic acid (0.50 g, 2.0 mmol), and
DMAP (0.05 g, 0.4 mmol) were added in dry dichloromethane
(10 ml). DCC (1.23 g, 6.0 mmol) was then added and the
resulting mixture was stirred at room temperature for one
day. Precipitated materials were removed by filtration. After
removal of the solvent by evaporation under reduced pressure,
the product was purified by column chromatography using a
dichloromethane–hexane (5 : 1) mixture as the eluent, and
recrystallized from ethanol (35 ml), giving a colourless solid.
Yield ~ 0.66 g (38%). Elemental analysis found: C% 76.0, H%
7.3, N% 0, calculated for C55H64O9 C% 76.0, H% 7.4, N% 0. dH
(500 MHz, CDCl3, TMS) 8.17 (m, 2H, Ar-H), 8.10 (m, 4H, Ar-
H), 7.66–7.63 (m, 8H, Ar-H), 7.31 (m, 2H, Ar-H), 7.19 (m, 2H,
Ar-H), 6.99 (m, 2H, Ar-H), 5.17 (m, 2H, -C*H(CH3)-), 4.10
Characterisation of materials
The purification of final products was carried out using column
chromatography over silica gel (70–230 mesh) (Sigma-Aldrich
Co.) using a dichloromethane–hexane mixture as the eluent,
followed by the recrystallization from an ethanol–ethyl acetate
mixture. The purities of all of the final compounds were
checked by reverse-phase and normal-phase high-performance
liquid chromatography (HPLC). Reverse-phase HPLC was
carried out using ZORBAX, Eclipse XDB-C8 column
(diameter 4.6 mm 6 150 mm) which was kept at 40 uC in an
oven. Samples were dissolved in a 1 : 1 mixture of dichloro-
methane and methanol (or acetonitrile) and the gradient
eluent system was used for elution (from water (20%) 1
acetonitrile (80%) to acetonitrile (100%) in 7 minutes) with a
flow rate of 1.0 ml min21. Detection of products was achieved
by UV irradiation (l ~ 280 nm). Similarly, normal-phase
HPLC was carried out using Shimadzu CLC-SIL(M) column
(diameter 4.6 mm 6 250 mm) which was kept at room
temperature. Samples were dissolved in dichloromethane and
dichloromethane was also used as an eluent. The flow rate of
the eluent was 1.0 ml min21. Detection of products was
achieved by UV irradiation (l ~ 254 nm). Slight differences in
purities obtained between reverse- and normal-phase HPLCs
were observed, however, it was rather difficult to identify such a
small amount of impurity. Since the obtained transition
temperatures were almost identical between each (S,S)-
isomer and (R,R)-isomer, which is reported later in this
paper, it is expected that the impurity has little effect on the
phase transition behaviour. Elemental analyses were performed
using an ELEMENTAR varioEL system. The structures of the
materials were elucidated by infrared (IR) spectroscopy
(Shimadzu FTIR-8100A infrared spectrophotometer), proton
3
(t, 2H, -CH2-CH2O-, J ~ 6.3 Hz), 2.65 (t, 2H, HOCO-CH2-
CH2-, 3J ~ 7.4 Hz), 1.90–1.29 (m, 32H, aliphatic-H), 0.88 (two
overlapping triplets, 6H, -CH2-CH3). n/cm21 (KBr) 2932, 2857
(C–H str.), 1755, 1709 (CLO str.), 1609 (C–C str.), 845 (1,4-
disub. C-H out of plane deformation (o.o.p.d.). m/z 868 (M1),
434, 325. HPLC purity Normal phase Si column: 100%,
Reverse Phase C8 column: 99.1%.
Preparation of[4’-((R)-1-methylheptyloxycarbonyl)-1,1’-biphenyl-
4-yl] 4@@-[5-[{4@-((R)-1-methylheptyloxycarbonyl)-1@,1@’-biphe-
nyl-4@’-yl}oxycarbonyl]pentyloxy]benzoate (II). This homologue
was prepared from (R)-1-methylheptyl 4-hydroxybiphenyl-4’-
carboxylate (1.30 g, 4.0 mmol) and 4-(5-carboxypentyloxy)-
benzoic acid (0.50 g, 2.0 mmol) using the procedure given for I,
giving the product as a colourless solid. Yield ~ 0.88 g (51%).
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Elemental analysis found: C% 76.1, H% 7.4, N% 0, calculated
for C55H64O9 C% 76.0, H% 7.4, N% 0. dH (400 MHz, CDCl3,
TMS) 8.17 (m, 2H, Ar-H), 8.11 (m, 4H, Ar-H), 7.67–7.64 (m,
8H, Ar-H), 7.31 (m, 2H, Ar-H), 7.19 (m, 2H, Ar-H), 6.99 (m,
2H, Ar-H), 5.18 (m, 2H, -C*H(CH3)-), 4.10 (t, 2H, -CH2-
CH2O-, 3J ~ 6.1 Hz), 2.66 (t, 2H, HOCO-CH2-CH2-, 3J ~
7.3 Hz), 1.91–1.29 (m, 32H, aliphatic-H), 0.88 (two overlapping
triplets, 6H, -CH2-CH3). n/cm21 (KBr) 2938, 2855 (C–H str.),
1757, 1707 (CLO str.), 1609 (C–C str.), 845 (1,4-disub. C-H
o.o.p.d). m/z 868 (M1), 434. HPLC purity Normal phase Si
column: 99.9%, Reverse Phase C8 column: 99.4%.
microscope equipped with a Mettler FP82 microfurnace
and FP80 control unit. The heating and cooling rates were
1 uC min21. Temperatures and enthalpies of transition were
investigated by differential scanning calorimetry (DSC) using a
MAC Science MTC1000S calorimeter. The materials were
studied at a scanning rate of 1 or 5 uC min21, after being
encapsulated in aluminium pans. The X-ray scattering experi-
ment was performed by real-time X-ray diffractometer (Bruker
AXS D8 Discover). The monochromatic X-ray beam (Cu-
Kaline) was generated by 1.6 kW X-ray tube and Go¨bel mirror
optics. The 2D position sensitive detector has 1024 6 1024
pixcels in a 5 cm 6 5 cm beryllium window. A sample was
introduced in a thin glass capillary (diameter 1.0 mm), which
was placed in a custom-made temperature stabilized holder
(stability within ¡0.1 uC). The X-ray diffraction measurement
and the textural observation by the polarized light microscopy
using a CCD camera were performed simultaneously on the
sample in the glass capillary tube.
Preparation of [4’-((S)-1-methylheptyloxycarbonyl)-1,1’-biphenyl-
4-yl] 4@@-[7-[{4@-((S)-1-methylheptyloxycarbonyl)-1@,1@’-biphenyl-
4@’-yl}oxycarbonyl]heptyloxy]benzoate (III). This homologue
was prepared from (S)-1-methylheptyl 4-hydroxybiphenyl-4’-
carboxylate (1.30 g, 4.0 mmol) and 4-(7-carboxyheptyloxy)-
benzoic acid (0.56 g, 2.0 mmol) using the procedure given for I,
giving the product as a colourless solid. Yield ~ 1.18 g (66%).
Elemental analysis found: C% 76.4, H% 7.6, N% 0, calculated
for C57H68O9 C% 76.3, H% 7.6, N% 0. dH (300 MHz, CDCl3,
TMS) 8.17 (m, 2H, Ar-H), 8.11 (m, 4H, Ar-H), 7.68–7.62 (m,
8H, Ar-H), 7.31 (m, 2H, Ar-H), 7.18 (m, 2H, Ar-H), 6.99 (m,
2H, Ar-H), 5.17 (m, 2H, -C*H(CH3)-), 4.07 (t, 2H, -CH2-
3. Results
Phase transition behaviour
Phase transition temperatures, phase sequences, and transition
enthalpies for the homologous series of non-symmetric chiral
twins (I, II, III and IV) are summarized in Table 1. All of the
homologues studied showed the antiferro-, ferri- and ferro-
electric phases, and also showed a strong tendency to exhibit
pseudo-homeotropic textures in the smectic phases, when the
sample was sandwiched between clean slide and cover glasses.
For the pseudo-homeotropic preparation of microscope speci-
mens of the homologues, apparent textural changes were
observed at the phase transitions to and from the smectic
phases. The ferrielectric phase was clearly distinguished from
the antiferro- or ferroelectric phase under the polarized light
microscope because the characteristic texture with constant
motion as domains form, coalesce, and disappear, which had
been reported for analogous monomeric materials,16 was
observed (see Fig. 1).
3
3
CH2O-, J ~ 6.4 Hz), 2.61 (t, 2H, HOCO-CH2-CH2-, J ~
7.5 Hz), 1.85–1.29 (m, 36H, aliphatic-H), 0.88 (two overlapping
triplets, 6H, -CH2-CH3). n/cm21 (KBr) 2928, 2857 (C–H str.),
1752, 1713 (CLO str.), 1609 (C–C str.), 849 (1,4-disub. C-H
o.o.p.d). m/z 896 (M1), 450. HPLC purity Normal phase Si
column: 99.7%, Reverse Phase C8 column: 98.9%.
Preparation of [4’-((R)-1-methylheptyloxycarbonyl)-1,1’-biphenyl-
4-yl] 4@@-[7-[{4@-((R)-1-methylheptyloxycarbonyl)-1@,1@’-biphenyl-
4@’-yl}oxycarbonyl]heptyloxy]benzoate (IV). This homologue
was prepared from (R)-1-methylheptyl 4-hydroxybiphenyl-4’-
carboxylate (1.30 g, 4.0 mmol) and 4-(7-carboxyheptyloxy)-
benzoic acid (0.56 g, 2.0 mmol) using the procedure given for I,
giving the product as a colourless solid. Yield ~ 1.10 g (61%).
Elemental analysis found: C% 76.3, H% 7.6, N% 0, calculated
for C57H68O9 C% 76.3, H% 7.6, N% 0. dH (300 MHz, CDCl3,
TMS) 8.17 (m, 2H, Ar-H), 8.10 (m, 4H, Ar-H), 7.66–7.63 (m,
8H, Ar-H), 7.31 (m, 2H, Ar-H), 7.19 (m, 2H, Ar-H), 6.99 (m,
2H, Ar-H), 5.18 (m, 2H, -C*H(CH3)-), 4.07 (t, 2H, -CH2-
3
3
CH2O-, J ~ 6.4 Hz), 2.61 (t, 2H, HOCO-CH2-CH2-, J ~
7.5 Hz), 1.83–1.29 (m, 36H, aliphatic-H), 0.88 (two overlapping
triplets, 6H, -CH2-CH3). n/cm21 (KBr) 2928, 2857 (C–H str.),
1752, 1717 (CLO str.), 1609 (C–C str.), 849 (1,4-disub. C-H
o.o.p.d). m/z 896 (M1), 450. HPLC purity Normal phase Si
column: 99.9%, Reverse Phase C8 column: 100%.
Liquid-crystalline and physical properties
The initial phase assignments and corresponding transition
temperatures for the final products were determined by thermal
optical microscopy using an Nikon Optiphot-pol polarizing
Fig. 1 The texture obtained for the ferrielectric phase of compound I
(145 uC).
Table 1 Phase transition temperaturesa/uC and transition enthalpiesb /kJ mol21 (in square brackets) for the non-symmetric chiral twins
cryst
antiferro
ferri
ferro
Unidentified isoc
Mosaicd
Iso
I
II
III
IV
$
$
$
$
65.9 [37.19]
64.1 [32.91]
56.4 [24.08]
56.4 [24.87]
$
$
$
$
144.4 [0.10]
144.5 [0.08]
120.4 [0.17]
120.6 [0.15]
$
$
$
$
146.7 [0.06]
146.6 [0.07]
122.0 [0.07]
122.2 [0.05]
$
$
$
$
148.8 [0.75]
148.7 [0.66]
125.7 [0.50]f
125.1 [0.62]f
$
$
$
$
151.0 [0.15]
150.7 [0.12]
128.7e [-]f
$
$
$
$
151.5 [7.50]
151.2 [7.31]
128.7e [7.10]
128.3e [7.61]
$
$
$
$
128.3e [-]f
aPhase transition temperatures were determined by the polarized light microscopy on heating (scan rate ~ 11 uC min21) except melting tem-
peratures which were measured by DSC (scan rate ~ 15 uC min21). bTransition enthalpies were determined by DSC measurements on heating
(scan rate ~ 15 uC min21) for the melting, and the antiferro–ferri and ferri–ferro transitions, and on heating (scan rate ~ 11 uC min21) for
all of the other transitions. cThe unidentified isotropic phase looked isotropic under the polarized light microscope for thin samples. dThe
mosaic phase showed mosaic texture under the polarized light microscope. eThe mosaic texture definitely appeared but the temperature range
f
of the mosaic phase was quite narrow (ca. 0.1 uC). Peaks corresponding to the ferro–unidentified isotropic and unidentified isotropic–mosaic
transitions were not separated clearly, and therefore, only the total enthalpy values for the two transitions were measured.
J. Mater. Chem., 2002, 12, 1709–1716
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Fig. 2 The textural change obtained for the transition from the
ferroelectric phase to the unidentified isotropic phase on heating for the
homogeneously aligned sample of compound I, where the texture of
the unidentified isotropic phase appeared as lots of round shaped dark
domains.
Fig. 4 The texture of the mosaic phase for compound I (151.3 uC).
further heating, a whole region of the texture became dark
which looks quite similar to an isotropic liquid phase.
Microscopic observation on the pseudo-homeotropically
aligned sample at the ferroelectric–unidentified isotropic transi-
tion was also performed using a slightly de-crossed polarized
light microscope. The textural changes of compound I are
shown in Fig. 3. Fig. 3 (a) shows a pseudo-homeotropic texture
of the ferroelectric phase. With increasing the temperature, the
round shaped domains of the unidentified isotropic phase
appeared (Fig. 3 (b)), and merged into each other (Fig. 3(c)).
On further heating, both the homogeneously and pseudo-
homeotropically aligned samples of compound I showed a
mosaic texture (see Fig. 4) for the mosaic phase. With further
increase of temperature, the mosaic texture of the mosaic phase
disappeared and the totally dark texture was again obtained
under the polarized light microscopy. Compounds II, III and
IV also showed similar textures for the unidentified isotropic
and mosaic phases.
Textures for the unidentified isotropic and mosaic phases
All of the homologues showed unidentified isotropic and
mosaic phases between the ferroelectric and the isotropic
liquid phases. The unidentified isotropic phase appeared at
a lower temperature region than the mosaic phase. The uniden-
tified isotropic phase looked isotropic when the sample was
sandwiched between two glass plates. Fig. 2 shows a textural
change of compound I observed at the transition from the
ferroelectric phase to the unidentified isotropic phase on heat-
ing for the sample which had been placed into a commercially
available evaluation cell (polyimide-coated, unidirectionally
buffed, 3 mm spacing, purchased from E.H.C. Co., Ltd
(Japan)). The texture of the unidentified isotropic phase
appeared as lots of round shaped dark domains, and on
Differential scanning calorimetry thermogram
Fig. 5 shows the differential scanning calorimetry (DSC) ther-
mogram for compound I, which shows small but definite peaks
corresponding to the antiferro–ferri, ferri–ferro and unidenti-
fied isotropic–mosaic transitions, respectively. Transitions
between mosaic and isotropic liquid phases produced a sharp
DSC peak with a large transition enthalpy of 7.50 kJ mol21
.
Compound II showed a similar DSC pattern to that obtained
for compound I, of which transition enthalpies are listed in
Table 1. Fig. 6 shows the DSC thermogram for compound III,
Fig. 3 The textural change obtained at the transition from the
ferroelectric phase to the unidentified isotropic phase: (a) a pseudo-
homeotropic texture of the ferroelectric phase, (b) the round shaped
domains of the unidentified isotropic phase appeared, and (c) most
regions changed into the unidentified isotropic phase.
Fig. 5 The differential scanning calorimetry (DSC) thermogram
for compound I obtained on a heating process (heating rate ~
1 uC min21).
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Fig. 6 The differential scanning calorimetry (DSC) thermogram
for compound III obtained on a heating process (heating rate ~
1uC min21).
which shows small but definite peaks corresponding to the
antiferro–ferri and ferri–ferro transitions, respectively. How-
ever, transitions between ferro–unidentified isotropic and
unidentified isotropic–mosaic this time were not separated
clearly, producing a small peak with a shoulder at a higher
temperature region. The transition between mosaic and
isotropic liquid phases again produced a large sharp DSC
peak with a transition enthalpy of 7.10 kJ mol21. Compound
IV showed a similar DSC profile to that obtained for
compound III, of which transition enthalpies are also listed
in Table 1.
Fig. 8 (a) The inter-layer spacing and the intensity as a function of
temperature for compound II obtained from X-ray diffraction
measurements, and (b) the intra-layer molecular distance and the
intensity as a function of temperature.
region, even though the peak intensities were smaller than those
obtained for the smectic phases, indicating that the layered
structures are more or less formed in the unidentified isotropic
and mosaic phases.
Fig. 8(a) shows an inter-layer spacing obtained for com-
pound II as a function of temperature. The inter-layer spacing
was calculated from the peak which appeared at the low angle
region of the X-ray diffraction profile. At the transition from
the ferroelectric phase to unidentified isotropic phase, the
intensity of the peak corresponding to the smectic layer spacing
suddenly decreased, however, it remained at a certain value,
indicating that the layered structure of the unidentified
isotropic or mosaic phase is weaker than that of the ferro-
electric phase but both of the phases have a definite layered
structure. Interestingly, a layered structure was still observed
ca. 2 uC above the mosaic–isotropic liquid transition tempera-
ture. At approximately 153 uC, the intensity of the peak
corresponding to the layer spacing was again decreased
suddenly, indicating the disappearance of the layered structure.
The intra-layer molecular distance, i.e., the distance between
the molecules within each smectic layer, was estimated by the
broad scattering which appeared in the wide angle region of the
X-ray diffraction profile. With increasing temperature, the
intra-layer molecular distance also showed a sudden change at
153 uC (Fig. 8(b)). These results strongly indicate that even in
the isotropic liquid phase just above the clearing temperature,
the pre-transitional molecular assemblies already occurred.
X-Ray scattering profile
Fig. 7 shows wide angle X-ray diffraction scattering patterns
obtained at the antiferro, unidentified isotropic, mosaic and
isotropic liquid phases of compound II. In the antiferroelectric
phase, a sharp peak corresponding to the smectic layer spacing
was observed at the small angle region, however, only diffuse
scattering was observed at the wide angle region, indicating
that there is no positional order within the layered structure.
Similar X-ray scattering profiles were also obtained for the
ferrielectric and ferroelectric phases, thus the smectic phases
obtained were found to be sub-phases of the smectic C phase.
Interestingly, even in the unidentified isotropic or mosaic
phase, a clear peak was observed in the small angle scattering
Effect of optical activity
Fig. 9 shows a phase diagram between (R,R)-isomer (com-
pound II) and a 1 : 1 mixture of (S,S)- and (R,R)-isomers
(I and II). The 1 : 1 mixture just showed a smectic A phase
instead of showing the unidentified isotropic and mosaic
phases. With increasing the optical purity, the smectic A phase
was found to change into the TGB phase, which showed
a typical filament texture (see Fig. 10), even though pure
(S,S)- and (R,R)-isomers (I and II) themselves did not exhibit
Fig. 7 Wide angle X-ray diffraction patterns obtained at the antiferro,
unidentified isotropic, mosaic and isotropic liquid phases of compound
II.
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Fig. 9 A miscibility phase diagram between (R,R)-isomer (compound
II) and the 1 : 1 mixture of (S,S)- and (R,R)-isomers (compounds I
and II).
Fig. 11 The textural change of the homogeneously aligned sample of
the mixture (98%-II): (a) the broken fan texture of the ferroelectric
phase, (b) at the transition between the TGB phase (the Grandjean
texture) and the unidentified isotropic phase (the dark domains), and
(c) the mosaic texture of the mosaic phase.
Fig. 10 A typical filament texture obtained at the transition between
the smectic A and TGB phases of the mixture (87%-II).
measurements, and are shown in Fig. 12 as a function of
reduced temperature (T 2 Tc; Tc ~ the clearing point). One
mixture (II (84%) 1 I (16%)) showed a ferro–smectic A phase
sequence, and the other mixture (II (95%) 1 I (5%)) showed a
the TGB phase. The clearing temperatures were found to
decrease with increasing optical purity, in particular, it
decreased substantially near to the pure (R,R)-isomer (II) in
the miscibility phase diagram (Fig. 9). The stability of the
ferrielectric phase also decreased when the optical purity of the
system was reduced which is a similar tendency observed for
the first ferrielectric compound, MHPOBC.17
The mixture containing 98% of compound II showed a phase
sequence of mosaic–unidentified isotropic–TGB–ferro–ferri–
antiferro. Fig. 11 shows the textural change of the mixture
(98%-II) observed for the homogeneously aligned sample in a
cell (3 mm thickness). Fig. 11(a) shows a broken fan-shaped
texture for the ferroelectric phase which was changed into the
Grandjean texture of the TGB phase with increasing tempera-
ture. Fig. 11(b) shows a phase transition from the TGB phase
to the unidentified isotropic phase, where the TGB phase
showed a Grandjean texture (blue and green in colour) and the
unidentified isotropic phase appeared as lots of dark round-
shaped domains. On further heating, the completely dark
texture of the unidentified isotropic phase changed into mosaic
texture (see Fig. 11(c)), as a result of the appearance of the
mosaic phase.
Fig. 12 The inter-layer spacing and the intensity as a function of
temperature obtained from X-ray diffraction measurements: (') the
mixture with 84% of compound II, (#) the mixture with 95% of
compound II.
Smectic layer spacings for two of the mixtures between
compounds I and II were obtained from X-ray diffraction
1714
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ferro–TGB transition due to the increase of the optical activity.
The layer spacing of the TGB phase is similar to that of the
smectic A phase, indicating that the obtained TGB phase has a
local structure of the smectic A type molecular ordering.
homologues showed a sharp DSC peak at the mosaic–isotropic
liquid transition. Thus, the mosaic phase may not be a BPIII-
type phase. Also the enthalpy corresponding to the mosaic–
isotropic liquid transition of the chiral twins, i.e., a range from
7.1 to 7.6 kJ mol21, is larger than the reported values4 corres-
ponding to the transition between the BPIII-type fog phase and
the isotropic liquid phase (0.8 to 2.9 kJ mol21),4 also suggesting
that the mosaic phase has a different character from the BPIII-
type fog phase. The SmQ phase has also been reported to show
smaller enthalpies (0.2 to 0.7 kJ mol21) at the SmQ–isotropic
liquid transition,12 which again suggests that the mosaic is not
the SmQ phase. It should be noted that the unidentified
isotropic and mosaic phases were quite viscous under the
shearing of the samples which had been placed between two
glass plates. This result also indicates that the unidentified
isotropic phase has, to some extent, an organized structure. The
X-ray diffraction measurement showed a clear peak in the
small angle region so that the layered structure was proposed
for the unidentified isotropic and mosaic phases. However, a
cubic or columnar type structure cannot be totally ruled out for
these new phases at this stage of work. Detailed studies on the
viscoelastic properties are now in progress for investigating the
structures of these phases.
4. Discussion
Two anomalous phases, i.e., unidentified isotropic and mosaic
phases, produced by the non-symmetric chiral twin homo-
logues were found to be thermodynamically stable phases
because DSC peaks were obtained associated with respective
transitions. Since these phases turned into the TGB phase when
the optical purity of the system was reduced, the unidentified
isotropic and mosaic phases are considered to possess more
strongly twisted molecular assemblies than the TGB structure.
Thus, one possible assignment of these phases is the smectic
blue phase. This assumption is consistent with the X-ray
diffraction scattering results which clearly showed that the
unidentified isotropic and mosaic phases possess a layered
structure. Interestingly, the layer spacing obtained for the
˚
unidentified isotropic and mosaic phases (ca. 45 A, see Fig. 8)
was found to be smaller than that obtained for the TGB phase
˚
(ca. 49 A, see Fig. 12), whereas the TGB phase showed a similar
smectic layer spacing to the smectic A phase of the 1 : 1
mixture of the (S,S)- and (R,R)-isomers. These results indicate
that the TGB phase possesses a local smectic structure similar
to the original smectic A phase but the unidentified isotropic
and mosaic phases have a different local smectic structure.
If molecules align in such a way that they form a more disor-
dered or locally twisted structure in the smectic layer in the
unidentified isotropic and mosaic phases, then the shorter
smectic layer spacing can be obtained. The unidentified iso-
tropic and mosaic phases are considered to produce a highly
twisted structure so that the locally twisted molecular assembly
could be one possible reason for the shortening of the layer
spacing. The gradual decrease of the layer spacing during the
temperature range of the smectic blue phase has so far been
reported,5,6 however, compound II studied here showed almost
a constant layer spacing value in the region of the unidentified
isotropic and mosaic phases.
5. Conclusions
Optically active non-symmetric twin homologues possessing
two identical chiral moieties at both peripheral ends exhibited
an ‘‘antiferro–ferri–ferro–unidentified isotropic–mosaic–Iso’’
phase sequence. The unidentified isotropic and mosaic phases
were found to possess a layered structure, however, the layer
spacing of these phases was smaller than that of the smectic
phase. These phases were quite sensitive to the optical purity of
the compound. With a reduction in the optical purity, the
unidentified isotropic and mosaic phases changed into the TGB
phase, and eventually, the smectic A was obtained for the 1 : 1
mixture of the (S,S)- and (R,R)-isomers, suggesting that the
unidentified isotropic and mosaic phases possess more strongly
twisted molecular assemblies than the TGB structure. Thus, the
unidentified isotropic and mosaic phases are considered to have
a smectic blue phase like molecular ordering.
So far the smectic blue phases have only been obtained
between the isotropic liquid phase and the TGBA or TGBC
phase.5 The non-symmetric chiral twin homologues studied
here however exhibited the unidentified isotropic and mosaic
phases between the ferroelectric smectic C phase and the
isotropic liquid phase. If the unidentified isotropic and mosaic
phases are kinds of the smectic blue phases, the homologues are
the first examples showing the smectic blue phase without the
TGB phases. When the optical purity of the (R,R)-isomer was
decreased, a phase sequence of ‘‘TGB–unidentified isotropic–
mosaic–isotropic liquid’’ was obtained, thus the (S,S)- and
(R,R)-isomers themselves are considered to favour a strong
helical structure which makes molecules twist in not only one
direction as in the TGB phases but also in the two radial
directions forming the smectic blue structures.
The mosaic texture obtained for the mosaic phase is some-
what similar to the SmQ phase,12 however, no indication of the
appearance of the SmQ phase was obtained in the X-ray
diffraction scattering. Moreover, the SmQ phase has been
reported to be a strongly twisted TGB phase with a local
antiferroelectric structure and always appears between the
antiferroelectric and isotropic liquid phases, so that the phase
sequence obtained here, i.e., antiferro–ferri–ferro–unidentified
isotropic–mosaic–isotropic liquid, is not consistent with the
emergence of the SmQ phase.
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