Unusual smectic phases organized by novel λ-shaped mesogenic molecules

Article abstract of DOI:10.1039/b406716a

A homologous series of novel λ-shaped mesogenic compounds in which three mesogenic groups are connected to 3, 4-dihydroxybenzoic acid has been prepared and their physical properties investigated by means of optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. The λ-shaped compounds studied showed unusual smectic phases. 4-Cyanobiphenyl-4′-yl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl) phenyloxy]hexyloxy}benzoate was found to realize an incommensurate SmA (SmAinc) phase. A "uniform planarly" aligned sample of 2-(4-octylphenyl) pyrimidin-5-yl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy} benzoate showed a Schlieren-like texture in the smectic phase. We discuss structural effects of the λ-shaped compounds on molecular organization in liquid-crystalline phases.

Full text of DOI:10.1039/b406716a

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www.rsc.org/materials | Journal of Materials Chemistry
Unusual smectic phases organized by novel l-shaped mesogenic molecules{
Akihisa Yamaguchi,a Isa Nishiyama,b Jun Yamamoto,b Hiroshi Yokoyamab and Atsushi Yoshizawa*a
Received 4th May 2004, Accepted 9th September 2004
First published as an Advance Article on the web 23rd September 2004
DOI: 10.1039/b406716a
A homologous series of novel l-shaped mesogenic compounds in which three mesogenic groups
are connected to 3, 4-dihydroxybenzoic acid has been prepared and their physical properties
investigated by means of optical microscopy, differential scanning calorimetry and X-ray
diffraction measurements. The l-shaped compounds studied showed unusual smectic phases.
4-Cyanobiphenyl-49-yl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy}benzoate was
found to realize an incommensurate SmA (SmAinc) phase. A ‘‘uniform planarly’’ aligned sample
of 2-(4-octylphenyl)pyrimidin-5-yl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy}
benzoate showed a Schlieren-like texture in the smectic phase. We discuss structural effects of the
l-shaped compounds on molecular organization in liquid-crystalline phases.
The driving force of mesophase formation is a fundamental
topic in the investigation of molecular assembly systems.
Recently, molecular topology1 and microsegregation2 have
attracted much attention as the origins for producing novel
self-organizing systems. For example, ‘‘Janus-like’’ super-
molecular liquid crystals, with molecular weights in excess of
4000 D, have been introduced as a new style of molecular
topology, and have been found to exhibit chiral nematic and
chiral smectic C (SmC*) phases.3 Novel layered structures
have also been realized by the microsegregation effect
produced by newly designed mesogenic block molecules
carrying a semiperfluorinated chain.4 In addition to these
static features of the molecular structures, molecular motion is
also important to understand the microscopic behaviour of
mesogenic molecules. Our 13C NMR studies of smectic liquid
crystals give a new concept for molecular design of liquid
crystals.5 Coupling or competition between intra-layer core–
core interaction and inter-layer interaction can produce a new
class of molecular aggregation. We have reported an unusual
endothermic SmC* to cubic phase transition in a dichiral
compound,6 layered structure in the nematic (N) phase
consisting of U-shaped molecules7 and a novel frustrated
phase produced by a binary system of non-symmetric liquid
crystals.8 In these previous studies, it was found that (1) the
endothermic transition is organized by chiral recognition as a
result of frustration between inter-layer and intra-layer
interactions, (2) the layered ordering in the N phase is
attributed to strong correlation of core parts between the
mesogenic groups of each U-shaped molecule, and (3) the
novel frustrated phase observed in the binary system results
from frustration between core–core interactions and molecular
packing. We report here the design and properties of novel
l-shaped mesogenic molecules. Fig. 1 shows the design
concept of the l-shaped system. The l-shaped molecular
topology (Fig. 1 (b)) is expected to produce a characteristic
effect since two different non-symmetric dimeric molecular
configurations (i.e., one is a straight-shaped configuration
(Fig. 1 (a)) and the other a bent structure (Fig. 1 (c))) co-exist
in a single mesogenic molecule.
Experimental
Spectroscopic analysis
Purification of final products was carried out using column
chromatography over silica gel (63–210 mm) (Kanto Chemical
Co., Inc.) using dichloromethane or a dichloromethane–ethyl
acetate mixture as the eluent, followed by the recrystallization
from ethanol. The purities of all of the final compounds were
checked by normal phase HPLC (Intersil SIL 150A-5 column).
A dichloromethane–isopropylalcohol (85 : 15) mixture was
used as eluent. Detection of products was achieved by UV
irradiation (l 5 254 nm). The structures of the final products
were elucidated by infrared (IR) spectroscopy (BIO RAD
FTS-30) and proton nuclear magnetic resonance (1H NMR)
spectroscopy (JEOL JNM-GX270).
Preparation of materials
Ethyl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl)phenyoxy]hexylox-
y}benzoate. 5-Octyl-2-(4-hydroxyphenyl)pyrimidine (0.57 g,
2.0 mmol) purchased from Midori Kagaku Co., Ltd. and 1,
6-dibromohexane (0.73 g, 3.0 mmol) were dissolved in
cyclohexanone (10 mL). Potassium carbonate (0.28 g,
2.0 mmol) was then added and the resulting mixture was
stirred at 80 uC for 6 h. The reaction mixture was filtered and
the solvent was removed by evaporation under reduced pressure.
The product was purified by column chromatography using
{ Electronic supplementary information (ESI) available: Details of
material preparation. See http://www.rsc.org/suppdata/jm/b4/
b406716a/
Fig. 1 Design concept of the l-shaped mesogenic molecule.
This journal is ß The Royal Society of Chemistry 2005
*ayoshiza@cc.hirosaki-u.ac.jp
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dichloromethane as the eluent. 2-[4-(6-Bromohexyloxy)-
phenyl]-5-octylpyrimidine was obtained. Yield: 0.56 g (63%).
2-[4-(6-Bromohexyloxy)phenyl]-5-octylpyrimidine (1.1 g,
2.5 mmol) and ethyl 3,4-dihydroxybenzoate (0.23 g, 1.25 mmol)
were dissolved in cyclohexanone (15 mL). K2CO3 (0.69 g,
5.0 mmol) and KI (0.0.42 g, 0.25 mmol) were then added and
the resulting mixture was stirred at 145 uC for 14 h. The
reaction mixture was filtered and the solvent was removed by
evaporation under reduced pressure. The product was purified
by column chromatography using a dichloromethane–ethyl
acetate (20 : 1) mixture as the eluent, and recrystallized from
an ethanol–chloroform (4 : 1) mixture (10 mL). Yield: 0.83 g
(73%).
Ar–H, J 5 8.9 Hz), 6.99–6.94 (m, 5 H, Ar–H), 4.11 (t, 2 H,
–OCH2–, J 5 6.5 Hz), 4.10 (t, 2 H, –OCH2–, J 5 6.5 Hz), 4.03
(t, 2 H, –OCH2–, J 5 6.5 Hz), 4.02 (t, 2 H, –OCH2–,
J 5 6.5 Hz), 2.58 (t, 4 H, Ar–CH2–, J 5 7.7 Hz), 1.90–1.27 (m,
40 H, aliphatic-H), 0.88 (t, 6 H, –CH3, J 5 6.6 Hz).
n/cm21(KBr): 2925, 2852 (C–H str.), 1608, 1583 (CLC str.).
Purity: 100%.
Details of material preparation for the other compounds are
listed in the electronic supplementary information (ESI){.
Liquid-crystalline and physical properties
The initial phase assignments and corresponding transition
temperatures for the final products were determined by optical
polarized light microscopy using a Nikon Optiphoto POL
polarizing microscope equipped with a Mettler FP82 hot stage
and FP80 control processor. The heating and cooling rates
were 5 uC min21. The photomicrographs were taken using a
camera (Olympus Digital Camera C-5050 ZOOM) in conjunc-
tion with a Nikon Optiphoto POL polarizing microscope.
Temperatures and enthalpies of transition were investigated by
dH (270 MHz, CDCl3, TMS): 8.55 (s, 4 H, Ar–H), 8.34 (d,
4 H, Ar–H, J 5 9.2 Hz), 7.64 (dd, 1 H, Ar–H, J 5 8.6 Hz,
1.9 Hz), 7.55 (d, 1 H, Ar–H, J 5 1.9 Hz), 6.96 (d, 4 H, Ar–H,
J 5 8.6 Hz), 6.87 (d, 1 H, Ar–H, J 5 8.4 Hz), 4.34 (q, 2 H,
–COOCH2–, J 5 7.3 Hz), 4.06 (t, 4 H, –OCH2–, J 5 6.5 Hz),
4.02 (t, 4 H, –OCH2–, J 5 6.5 Hz), 2.58 (t, 4 H, Ar–CH2–,
J 5 7.6 Hz), 1.87–1,27 (m, 43 H, aliphatic-H), 0.88 (t, 6 H,
–CH3, J 5 6.8 Hz). n/cm21(KBr): 2925, 2852 (C–H str.), 1717
(–CO–O–C– str.), 1611, 1597 (CLC str.).
differential scanning calorimetry (DSC) using
a Seiko
Instruments Inc. DSC6200. The materials were studied at a
scanning rate of 2–10 uC min21 after being encapsulated in
aluminum pans.
3,4-Bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy}ben-
zoic acid. Ethyl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]-
hexyloxy}benzoate (0.83 g, 0.91 mmol) was added to a
solution of KOH (0.50 g, 8.9 mmol) in ethanol (95%, 20 mL).
The resulting mixture was stirred under reflux for 2 h. The
solution was acidified with aq. HCl. Water (20 mL) was added
to the mixture and the aqueous phase was extracted with
dichloromethane (3 6 20 mL). The organic extracts were
combined, dried over Na2SO4, filtered and evaporated. Yield:
0.72 g (89%).
The X-ray scattering experiments were performed using a
real-time X-ray diffractometer (Bruker AXS D8 Discover).
The monochromatic X-ray beam (Cu Ka line) was generated
by a 1.6 kW X-ray tube and Go¨bel mirror optics. The 2D
position sensitive detector has 1024 6 1024 pixels in a 5 cm 6
5 cm beryllium window. A sample was introduced into a thin
glass capillary tube (diameter 1.0 mm), which was placed in a
custom-made temperature stabilized holder (stability within
¡0.1 uC). The X-ray diffraction measurements and the
textural observations by polarized light microscopy using a
CCD camera were performed simultaneously on the sample in
the glass capillary tube.
dH (270MHz, CDCl3, TMS): 8.57 (s, 4 H, Ar–H), 8.33 (dd,
4 H, Ar–H, J 5 8.4 Hz, 2.6 Hz), 7.72 (dd, 1 H, Ar–H,
J 5 8.4 Hz, 2.0 Hz), 7.59 (d, 1 H, Ar–H, J 5 2.2Hz), 6.96 (dd,
4 H, Ar–H, J 5 8.9 Hz,1.4 Hz), 6.89 (d, 1 H, Ar–H, J 5 8.4 Hz),
4.10–4.00 (m, 8 H, –OCH2–), 2.58 (t, 4 H, Ar–CH2–,
J 5 7.7 Hz), 1.87–1.27 (m, 40 H, aliphatic-H), 0.88 (t, 6H,
–CH3, J 5 6.8 Hz). n/cm21(KBr): 2927, 2854 (C–H str.), 1680
(CLO str. –COOH),1607, 1585 (CLC str.).
Results and discussion
Liquid-crystalline properties
Molecular structures and transition properties of the l-shaped
mesogenic compounds are shown in Fig. 2. Compound 1
showed the phase sequence of Iso–N–SmA–SmC, on the other
hand, compound 2 with a biphenyl as X showed only a N
phase, suggesting that lack of inter-layer permeation of tails
for compound 2 destabilizes the layer ordering. Compound 3
having a chiral centre showed only a SmA phase. Texture at
the Iso–SmA transition for compound 3 seems to be different
from a typical fan-shaped texture of a SmA phase (Fig. 3). On
cooling from the istropic liquid, coloured curved stripes
appeared. Then they grew and curled up. A similar texture
has been reported in the chiral non-symmetric dimeric liquid
crystal (10B5T8*),9 and also in the achiral liquid crystal
compound.10 The racemic mixture between (R)- and
(S)-isomers of compound 3 also showed the unusual texture
at the Iso–SmA transition. Thus, the unusual texture of
compound 3 (see Fig. 3) is thought not to be produced by the
chirality-dependent effect but is attributable to the layer
4-Cyanobiphenyl-49-yl 3,4-bis{6-[4-(5-octylpyrimidin-2-
yl)phenyloxy]hexyloxy}benzoate, 4. To a solution of 3,4-
bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy}benzoic
acid (0.20 g, 0.23 mmol) in dichloromethane (15 mL), 4-cyano-
49-hydroxybiphenyl (0.044 g, 0.23 mmol), dicyclohexylcarbo-
diimide
(0.047
g,
0.23
mmol),
and
4-(N,
N-dimethylamino)pyridine (0.003g, 0.023 mmol) were added.
The resulting solution was stirred at room temperature
overnight. Precipitated materials were removed by filtration.
After removal of the solvent by evaporation, the residue was
purified by column chromatography on silica gel (using a
dichloromethane–ethyl acetate (20 : 1) mixture as the eluent).
Recrystallization from an ethanol–chloroform (5 : 1) mixture
(6 mL) gave the desired product. Yield: 0.16 g (68%).
dH (270MHz, CDCl3, TMS): 8.56 (d, 4 H, Ar–H, J 5 1.4 Hz),
8.34 (dd, 4 H, Ar–H, J 5 8.9 Hz, 3.0 Hz), 7.83 (dd, 1 H, Ar–H,
J 5 8.5 Hz, 2.2 Hz), 7.74–7.60 (m, 7 H, Ar–H), 7.33 (d, 2 H,
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Fig. 2 Molecular structures, transition temperatures/uC and DS/R (in
parenthesis) of the l-shaped mesogenic compounds. Square brackets
indicate a monotropic transition.
Fig. 4 Photomicrographs of (a) the SmA phase at 170 uC and (b) the
SmA9 phase at 150 uC of a planar unidirectionally aligned sample of
compound 4.
Fig. 3 A photomicrograph of a texture of compound 2 on cooling
from the isotropic liquid phase. Magnification 1006.
deformation induced by the l-shaped structure of compound 3
at the direct transition from isotropic liquid to SmA phase.
Compound 4 showed two smectic A phases, designated as
SmA and SmA9. Both of the SmA and SmA9 phases showed a
homeotropic texture as usually observed for common smectic
A phases, which was completely dark under the polarized light
microscopy. However, the planar unidirectionally aligned
SmA9 phase exhibited an anomalous texture with many defects
(Fig. 4). The SmA–SmA9 phase transition was also confirmed
by means of differential scanning calorimetry (Fig. 5). A small
but clear peak was obtained at the transition, indicating that
the SmA and SmA9 phases are two thermodynamically
different phases.
Fig. 5 DSC thermogram for compound 4. The rate of cooling and
heating was 5 uC min21
.
SmX, SmY and SmZ phases of a uniform planarly aligned
region on cooling from the N phase (Fig. 7 (b), (c) and (d),
respectively). A Schlieren texture in smectic phases, which
results from deformation of the c-director, is usually observed
in a pseudo-homeotropically aligned sample of tilted smectic
phases; however, the Schlieren-like texture this time was
observed in the smectic phases of the uniform planarly aligned
sample. The phase transition behaviour of compound 5 was
also examined by DSC measurement. Fig. 8 shows a cooling
DSC thermogram for compound 5, where a small peak at the
N to SmX transition was observed, on the other hand,
enthalpy changes associated with the SmX to SmY and SmY
to SmZ transitions could not be detected.
Compound 5 in which a phenylpyrimidine moiety is
introduced as X showed three unusual smectic phases, i.e.,
SmX, SmY and SmZ phases. Microscopic observation on a
homeotropically aligned sample revealed that the N and SmX
phases are optically uniaxial phases. However, the SmY and
SmZ phases did not show the homeotropic texture but showed
some patterns due to the increase of the birefringence,
indicating that they are biaxial phases (Fig. 6 (a) and (b)),
where clearly different textures were observed between the
SmY and SmZ phases. A Schlieren texture was observed in the
uniform planarly aligned N phase as usual (Fig. 7 (a)).
Furthermore, a Schlieren-like texture was also observed in the
Let us discuss the nematic–isotropic transition entropies.
The transition entropy is made up of two contributions, one
resulting from the onset of long range orientational order and
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Fig. 8 Cooling DSC thermogram for compound 5. The rate of
cooling was 5 uC min21
.
dimeric molecules, which is thought to reflect the design
concept as shown in Fig. 1.
Phase structures
Fig. 6 Photomicrographs of (a) the SmY phase at 75 uC and (b) the
SmZ phase at 60 uC of a homeotropically aligned sample of compound
5. Magnification 1006.
Compounds 1 and 3
˚
˚
Layer spacings in the SmA phase of about 40 A and 41 A were
obtained for compounds 1 and 3, respectively, from X-ray
diffraction (XRD) measurements. Molecular lengths were
the other from the change in the conformational composi-
tion.11 For rigid and rod-like molecules, DS/R should be about
0.3. On the other hand, the N–I transition for dimeric
compounds is considerably larger than for the monomeric
compounds. Furthermore, the transition entropies show a
pronounced alternation as parity of the spacer is varied. The
values for the N–I transition entropy for even members are
typically several times larger than those for the odd members.
The observed transition entropies for the l-shaped molecules,
for example, 1.94 for compound 1 and 2.72 for compound 5,
are much larger than those for rod-like molecules. These large
values suggest that the l-shaped molecules are behaving as
˚
˚
estimated, from MM2 models, to be about 46 A and 49 A
for compounds 1 and 3, respectively. Thus, compounds 1 and 3
are considered to form a monolayer SmA phase in which inter-
layer permeation of the tails is thought to occur. The smectic
layer spacing calculated form the X-ray reflection of com-
˚
pound 1 changed from 40 A at 73.2 uC in the SmA phase to
˚
39 A at 68.5 uC in the SmC phase.
Compound 4
Fig. 9 shows X-ray diffraction profiles in the small angle
region of compound 4 in the SmA and SmA9 phases. The layer
Fig. 7 Photomicrographs of (a) the N phase at 105 uC, (b) the SmX phase at 90 uC, (c) the SmY phase at 75 uC and (d) the SmZ phase at 60 uC of
a uniform planarly aligned sample of compound 5. Magnification 1006.
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co-exist along the layer normal.12–14 In the X-ray pattern of the
SmA phase (Fig. 9(a)), two small peaks are recognized to
locate at the same positions as observed in the SmA9 phase
(Fig. 9(b)). These results suggest that weak density waves with
the same periodicity as the SmA9 phase may co-exist in the
SmA phase.
In order to investigate the stability of the SmAinc phase,
some miscibility studies were performed. Fig. 11 shows
transition temperatures of binary mixtures of compound 4
(50 mol%) with each of some cyanobiphenyl derivatives. Both
1OCB and 6OCB showed the N phase, whereas 2-CN did not
exhibit liquid-crystalline phases. Addition of 4,49-dicyanobi-
phenyl (2-CN) or 4-cyano-49-methoxybiphenyl (1OCB) to
compound 4 was found to stabilize the SmAinc phase.
However, addition of 4-cyano-49-hexyloxybiphenyl (6OCB)
removed the SmAinc phase. These results indicate that the
SmAinc phase organized by compound 4 can recognize the
difference in molecular length between 1OCB and 6OCB.
Furthermore, the SmAinc was stabilized even in the mixture
containing 66 mol% of 2-CN. Transition temperatures for the
mixture were Iso Liq 180 uC N 165 uC SmAinc 81 uC recryst.
Three sharp reflections in the small angle region of the mixture
at 157 uC were detected and the obtained layer spacings were
˚
52, 39, and 23 A. The three periodic density waves in the
SmAinc phase of compound 4 were not destroyed in the
mixture containing 66 mol% of 2-CN. This system shows a new
type of inclusion phenomenon.
Let us discuss a possible model for the SmAinc phase. Fig. 12
shows a grand state conformer obtained from MM2 calcula-
tions, which shows a l-shaped structure. Although the
molecule should keep the molecular flexibility and stability
of the conformer from MM2 calculations is still questionable,
we consider the single conformer to discuss the phase structure
Fig. 9 X-Ray diffraction patterns of compound 4 in (a) the SmA
phase at 162 uC and (b) the SmA9 phase at 148 uC.
˚
spacing of the SmA phase of compound 4 was 53 A, but the
˚
molecular length was estimated to be 40 A, suggesting the
formation of an interdigitated phase.11 The SmA9 phase
showed a complicated X-ray pattern. In the SmA9 phase at
148 uC three sharp reflections, 2h 5 1.62u, 2.20u and 3.82u,
were observed in the small angle region, corresponding to the
˚
at the first approximation. A periodicity of 40 A which is
corresponding to the molecular length results from a mono-
˚
layer structure (Fig. 13 (a)). That of 23 A is attributed to a
layer spacing of an intercalated structure (Fig. 13(b)). The
intercalated structure seems to be unrealistic, because it
˚
contains many voids. A layer spacing of 55 A in the SmAinc
˚
˚
˚
spacings of about 55 A, 40 A and 23 A, respectively (Fig. 9).
The 2D pattern of the aligned sample in the SmA9 phase
showed that three reflection peaks in the small angle region are
positioned at one direction perpendicular to the diffuse
scattering in the wide angle region (Fig. 10). These results
strongly indicate that the SmA9 phase is an incommensurate
SmA (SmAinc) phase in which three periodic density waves
phase, longer than the molecular length, suggests the existence
Fig. 11 Transition temperatures of binary mixtures of compound 4
(50 mol%) with each of some cyanobiphenyl derivatives. Temperatures
were determined by polarized light microscopy on cooling.
Fig. 10 (a) X-Ray diffraction patterns of an aligned sample of
compound 4 at 148 uC and (b) its expanded one.
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Fig. 12 Conformation of compound
4 obtained from MM2
calculations.
of an interdigitated structure. Two different interdigitated
structures can be considered (Fig. 14). Interactions between
cyanobiphenyl and phenylpyrimidine moieties can be seen in
Fig. 14(a). On the other hand, there are dipole–dipole
interactions between cyanobiphenyl moieties and those
between phenylpyrimidine moieties in Fig. 14(b). Although
there are unfavorable free volumes in the interdigitated
structure (Fig. 14(b)), the structure seems to be realistic. This
is because the electron density is less localized in Fig. 14 (b)
compared with Fig. 14(a) and this is consistent with the
absence of second order layer diffraction in the X-ray pattern
of compound 1 in the SmAinc phase. Furthermore, if the other
monolayered and intercalated structures are superimposed
onto the interdigitated structure, the unfavorable free volumes
can be reduced to some extent and the interdigitated structure
(Fig. 14(b)) be stabilized. Thus, the coexistence of three
different molecular packings, corresponding to the monolayer,
intercalated and interdigitated structures, can be proposed as a
possible model of the observed SmAinc phase.
Compound 5
The X-ray pattern of compound 5 in the SmX phase shows
two relatively broad reflections, 2h 5 2.60u and 3.35u, in the
˚
small angle region, corresponding to spacings of about 34 A
˚
and 26 A, respectively. However, a diffuse scattering in the
Fig. 13 Possible models of monolayer and intercalated structures for
the SmAinc of compound 4.
wide-angle region was observed, which indicates the fluid state
of the mesophase (Fig. 15). The origin of the two reflections in
the SmX phase is not clear; however, at least two density waves
usually observed in a uniform planarly aligned sample of a N
phase, which results from deformation of the n-director, and in
a homeotropically aligned sample of a tilted smectic phase,
which results from that of the c-director. The Schlieren-like
texture observed in the uniform planarly aligned sample of the
SmX phase can result from bend deformation of the
n-director, which is thought to be forbidden in smectic phases.
The two relatively broad reflections in the small angle region in
the SmX phase of compound 5 indicate that: (i) two different
density waves coexist and (ii) both of the layer periodicities are
weak. The shorter layer spacing than the molecular length
˚
coexist. The molecular length was estimated to be 53 A. The
shorter layer spacings than the molecular length indicate that
an intercalated structure occurs in the SmX phase.11 Two
density waves were also observed in the SmY and SmZ phases
but a layer structure with longer periodicity is stronger than
the other.
Let us discuss the structure of the SmX phase. By means of
the optical microscopic studies, the SmX phase is an optically
uniaxial phase but the Schlieren-like texture was observed in
the uniform planarly aligned sample. A Schlieren texture is
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Fig. 15 X-Ray diffraction patterns for compound 5 in (a) the
SmX phase at 92 uC, (b) the SmY phase at 78 uC and (c) the SmZ
phase at 68 uC.
Compound 7 showed a phase sequence of Iso–SmA–SmC–Cr,
but compound 8 showed that of Iso–SmC–Cr. These results
indicate that the anti-parallel orientation of the phenylpyr-
imidine groups between adjacent molecules plays an important
role in the anomalous molecular organization observed in
compound 5. Furthermore, if the overall molecular configura-
tion of compound 5 were Y-shaped, the transition behaviour
of compound 7 should be similar to that of compound 8,
because a Y-shaped molecule should have a C2 axis along the
molecular long axis. The marked difference in the transition
behaviour between compounds 7 and 8 suggests that the
molecular shape is not Y-shaped but l-shaped. A possible
molecular arrangement responsible for the appearance of
the Schlieren-like in the SmX phase is shown in Fig. 17. This
unique molecular assembly is considered to be stabilized
by several types of anti-parallel interactions between the
phenylpyrimidine groups. Effective packing of the l-shaped
Fig. 14 Possible models of two different interdigitated structures for
the SmAinc phase of compound 4.
suggests an intercalated structure. The molecules in the SmX
phase are considered to have relatively higher mobility along
the layer normal than those in a conventional SmA phase.
Thus, the Schlieren texture produced by the deformation of the
n-director in the N phase can remain even in the layered
molecular assembly produced in the SmX phase.
In order to investigate the interactions between phenylpyr-
imidine groups, compounds 6, 7 and 8 were prepared and the
transition properties compared with those of compound 5
(Fig. 16). Compounds 6, 7 and 8 did not show the unusual
phases observed for compound 5. Compound 6 showed only a
SmA phase in which a typical fan texture was observed.
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reported tuning-fork shaped molecules which show undulated
smectic structures16 and Attard et al. reported Y-shaped
molecules in which three mesogenic groups are symmetrically
attached to a phenyl ring.17 As discussed above, the transition
behaviour and the phase structures shown by the present
l-shaped molecules are different from those of the liquid-
crystalline materials so far reported. Thus, in the final part of
this article, we consider intermolecular interactions of the
l-shaped structures and highlight the unique features pro-
duced by the l-shaped molecules.
Phase transition behaviour depends on the structure of
X. Compounds 1 and 3 showed a monolayer SmA phase. The
most important factor for the organization in the SmA phase
of both compounds is steric interactions due to the particular
shape of the molecules. In the case of compound 4, dipole–
dipole interactions through cyanobiphenyl groups and anti-
parallel interactions through phenylpyrimidine groups is
responsible for producing the incommensurate SmA phase
with interdigitated structure. On the other hand, on introduc-
tion of a phenylpyrimidine group as X (compound 5), several
types of anti-parallel interactions between phenylpyrimidine
groups can produce the unusual phases with intercalated
structure in which the layer ordering and the deformation of
the n-director coexist. This nature surely increases the
penetration length18 of the n-director perturbation when
the bending deformation is imposed in a system. Thus, the
l-shaped configuration is one possible molecular design
realizing a dislocation-driven ‘‘bend grain boundary’’ phase19
which was predicted18 but has not yet been observed. The
proper arrangement of the mesogenic parts in a particular
l-shaped structure regulates the inter-layer and intra-layer
interactions, and thus can produce a variety of novel molecular
aggregation styles.
Fig. 16 Molecular structure, phase transition temperatures/uC and
DS/R (in parenthesis) of compounds 6, 7 and 8.
Conclusions
The newly designed l-shaped mesogenic molecules were found
to realize a novel class of exciting molecular organization in
liquid crystalline phases, i.e., a stable incommensurate SmA
phase and a liquid-crystalline phase in which the layer ordering
and the deformation of the n-director coexist. The SmAinc
phase can include a large amount of 4, 49-dicyanobiphenyl
which does not show a liquid-crystalline phase. Furthermore,
the SmAinc phase was found to recognize difference in
molecular length between 1OCB and 6OCB. Modification of
the l-shaped structure can produce various organizations of
molecules.
Fig. 17 A possible model for the molecular orderings producing a
Schlieren texture of the SmX phase of compound 5.
Acknowledgements
We thank Dr Lianhui Qi of Yokoyama Nano-structured
Liquid Crystal Project for XRD measurements. 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).
molecules and the electrostatic forces, both polar and
quadrupolar, between the phenylpyrimidine groups make it
possible to produce bend deformation of the layer structure.
Characteristic molecular assembly organized by l-shaped
Akihisa Yamaguchi,a Isa Nishiyama,b Jun Yamamoto,b
Hiroshi Yokoyamab and Atsushi Yoshizawa*a
mesogenic molecules
aDepartment of Materials Science and Technology, Faculty of Science
and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561,
Many types of liquid crystals with unconventional molecular
shapes have been reported.15 For example, Go¨ring et al.
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Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp
bYokoyama Nano-structured Liquid Crystal Project, ERATO, JST,
TRC 5-9-9 Tokodai, Tsukuba 300-2635, Japan
10 I. Nishiyama, T. Yamamoto, J. Yamamoto, J. W. Goodby and
H. Yokoyama, J. Mater. Chem., 2003, 13, 1868.
11 C. T. Imrie and G. R. Luckhurst, in the Handbook of Liquid
Crystals, ed. D. Demus, J. W. Goodby, G. W. Gray, H. -W. Spiess
and V. Vill, Wiley-VCH, Weinheim, 1998, vol 2B, ch. X, pp. 801–
833.
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