Pre-organization effects on chirality, polarity and biaxiality in liquid crystals: Novel laterally connected mesogens showing anomalous properties

Article abstract of DOI:10.1080/15421406.2011.581528

Pre-organization effects are investigated for the laterally-connected liquid crystals possessing a biphenyl connecting group. The homeotropic texture of the N phase of a free-standing film sample was found to change into the other Schlieren texture on coo

Full text of DOI:10.1080/15421406.2011.581528

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Pre-Organization Effects on Chirality,
Polarity and Biaxiality in Liquid Crystals:
Novel Laterally Connected Mesogens
showing Anomalous Properties
Isa Nishiyama a , Yuka Tabe b , Jun Yamamoto c & Hiroshi Yokoyama d
a DIC Corporation, Fine Synthesis Technical Department , Komuro,
Ina-machi, Saitama 362-8577, Japan
b Department of Physics and Applied Physics , Waseda University ,
3-4-1 Okubo, 169-8555, Tokyo, Japan
c Department of Physics , Graduate School of Science, Kyoto
University, Kita-Shirakawa, Sakyo-ku , Kyoto, 606-8502, Japan
d Kent State University, Liquid Crystal Institute, Kent , Ohio,
44242-0001, USA
Published online: 07 Oct 2011.
To cite this article: Isa Nishiyama , Yuka Tabe , Jun Yamamoto & Hiroshi Yokoyama (2011) Pre-
Organization Effects on Chirality, Polarity and Biaxiality in Liquid Crystals: Novel Laterally Connected
Mesogens showing Anomalous Properties, Molecular Crystals and Liquid Crystals, 549:1, 174-183, DOI:
10.1080/15421406.2011.581528
To link to this article: http://dx.doi.org/10.1080/15421406.2011.581528
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Mol. Cryst. Liq. Cryst., Vol. 549: pp. 174–183, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.581528
Pre-Organization Effects on Chirality, Polarity
and Biaxiality in Liquid Crystals: Novel Laterally
Connected Mesogens showing Anomalous Properties
ISA NISHIYAMA,1,∗ YUKA TABE,2 JUN YAMAMOTO,3
AND HIROSHI YOKOYAMA4
1DIC Corporation, Fine Synthesis Technical Department, Komuro, Ina-machi,
Saitama 362-8577, Japan
2Department of Physics and Applied Physics, Waseda University,
3-4-1 Okubo, 169-8555 Tokyo, Japan
3Department of Physics, Graduate School of Science, Kyoto University,
Kita-Shirakawa, Sakyo-ku, Kyoto 606-8502, Japan
4Kent State University, Liquid Crystal Institute, Kent, Ohio 44242-0001, USA
Pre-organization effects are investigated for the laterally-connected liquid crystals pos-
sessing a biphenyl connecting group. The homeotropic texture of the N phase of a
free-standing film sample was found to change into the other “Schlieren” texture on
cooling even in the N phase, which then changes into the “Schlieren” texture of the
SmC phase on further cooling. The possibility of the emergence of biaxiality in the N
phase is discussed.
1. Introduction
Organization in liquid crystal phases is constructed by the molecular recognition among
1
the constituting molecules. “Supermolecular liquid crystals” are therefore the products
based on the sophisticated soft materials design. “Pre-organization” is one of the most
important concepts for producing supermolecular liquid crystals2,3. So far, the effects of
pre-organization on the chirality- or polarity-dependant structural and electrical2,4, and
optical5 properties have been intensively investigated. For example, the introduction of
chiral moieties at both ends of the twin molecular configuration (see Fig. 1(a)) is a simple
pre-organization but is a promising molecular design for producing special chiral effects.
If such molecules form a smectic layered structure as shown in Fig. 1 (b), then the chiral
moieties aggregate between the layer surfaces, producing a strong chiral interaction among
the molecules.
The structure and position of chiral centres are important for the design of the pre-
organization based on the concept shown in Fig. 1. Fig. 2 shows some examples highlighting
the effect. Introduction of 1-methylheptyl chiral groups at the peripheral ends (Fig. 2 (a))
produces a complex molecular assembly of the ferrielectric phase6, which is not observed
for the other materials possessing related structures (Fig. 2 (b)7 and (c)8).
∗Corresponding author. E-mail: isa-nishiyama@ma.dic.co.jp or isanishi@db3.so-net.ne.jp
174

Effects on Chirality, Polarity and Biaxiality in Liquid Crystals
175
Figure 1. (a) Twin structure as an example of the pre-organization, and (b) expected molecular
assembly of the pre-organized molecules.
The importance of the style and structure of the linking part has also been emphasized
by the systematic investigation, in which the identical chiral centres are introduced in the
same position of the molecular structure (i.e., peripheral ends), on the correlation between
the pre-organization styles and the resulting liquid crystal phases (Fig. 3).
In this study, a novel strategy of the pre-organization is introduced, which is related to
the lateral connection of the mesogenic parts, instead of the previously mentioned linear
connection. This pre-organization is expected to enhance the molecular biaxiallity and
sometimes to produce the molecular chirality. Among the molecular designs for the pre-
organization, a special attention has been paid in the structure of the connecting group. Three
related molecular architectures of the laterally-connected systems, in which the connecting
group is systematically varied, are compared in Fig. 4. A “binaphthyl” connecting group
(Fig. 4 (a)) is famous as the origin of axis-chirality, as such the resulting laterally-connected
compound can be optically active. However, a “phenyl” connecting group (Fig. 4 (c))
possesses no axis in the connection structure, and thus no axis-chirality appears. As such,
the binaphthyl derivatives (Fig. 4 (a)) could possess both molecular biaxility and chirality
but the phenyl derivatives (Fig. 4 (c)) can exhibit only the molecular biaxiality. A “biphenyl”
connecting group (Fig. 4 (b)) possesses an intermediate character among them. The biphenyl
Figure 2. Effect of the structure and position of chiral centres in three related chiral liquid crystal
compounds on the liquid crystal phase: (a) 1-methylheptyl chiral groups at the peripheral ends6, (b)
1-methyl (or 1-trifluoromethyl) heptyl groups located in central region7, and (c) 2-methybutyl chiral
groups at the peripheral ends8.

176
I. Nishiyama et al.
Figure 3. A variety of chiral liquid crystal phases observed in systematically pre-organized systems:
(a) chiral monomeric compound as a component of pre-organization, (b) symmetric twin6, (c) non-
symmetric twin9, (d) symmetric twin showing a thermotropic sponge phase10. (e) trimer11, and (f)
dichiral compound possessing a rigid core12.
group itself does not show the axis-chirality in most cases, which can however be induced
if the rotational barrier around the biphenyl axis is enhanced by any means, e.g., by the
intra- and/or intermolecular interactions of the mesogenic parts attached to the phenyl
ring.
Figure 4. Three related laterally-connected mesogenic compounds with different connecting groups,
i.e., (a) binaphthyl, (b) biphenyl, and (c) phenyl groups.

Effects on Chirality, Polarity and Biaxiality in Liquid Crystals
177
Figure 5. Laterally-connected mesogenic compounds studied based on the new pre-organization
strategy.
This delicate nature with respect to the axis-chirality, as well as expected large molec-
ular biaxial nature than the phenyl analogue, is attractive as the design for the pre-
organization. Therefore, in this study, the biphenyl connecting group has been selected
as the target and the effect of the pre-organization investigated. The structures studied are
shown in Fig. 5.
2. Experimental
Characterisation of Materials
The purification of the final compound (I) was carried out using column chromatography
over silica gel (70-230 mesh) (Sigma-Aldrich Co.) using dichloromethane-hexane mixture
as the eluent, followed by the recrystallization from ethylacetate. The purities of all of
the final compounds were checked by reverse-phase (ZORBAX, Eclipse XDB-C8) and
normal-phase (CLC-SIL(M), Shimadzu) high-performance liquid chromatography. Ele-
mental 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 nuclear magnetic resonance (1H NMR) spectrometry
(Bruker DRX-500 (500 MHz) nuclear magnetic resonance spectrometer), and mass (MS)
spectrometry using an FD-MS (JEOL, JMS-700) 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 5-{4-(6-bromohexyl)}phenyl-2-(4-hexyloxyphenyl)pyrimidine (I-a)
Potassium carbonate (0.59 g, 4.3 mmol) was added to a solution of 5-(4-hydroxy)phenyl-
2-(4-hexyloxyphenyl)pyrimidine (1.50 g, 4.3 mmol) (purchased from Midori Kagaku Co.
Ltd., Japan) and 1,6-dibromohexane (1.58 g, 4.0 mmol) in cyclohexanone (30 ml). The reac-
tion mixture was stirred at 95◦C overnight. Precipitated materials were removed by filtration.
After removal of the solvent by evaporation under reduced pressure, the product was puri-
fied by column chromatography over silica gel using a mixture of dichloromethane/hexane
(1:1) as the eluent, to give an intermediate bromo-compound. Yield = 1.26 g, (57%).
Preparation of 2,2ꢀ-Bis{6-[4-(2-(4-hexyloxyphenyl)pyrimidine-5-yl)
phenyloxy]hexyloxy}biphenyl (I)
Potassium carbonate (0.25 g, 1.8 mmol) was added to a solution of the obtained bromo-
intermediate, 5-{4-(6-bromohexyl)}phenyl-2-(4-hexyloxyphenyl)pyrimidine (I-a), (0.95 g,

178
I. Nishiyama et al.
1.9 mmol) and 2,2ꢀ-dihydroxybipheny (0.17 g, 0.91 mmol) in cyclohexanone (10 ml).
The reaction mixture was stirred at 130◦C overnight. Precipitated materials were re-
moved by filtration. After removal of the solvent by evaporation under reduced pressure,
the product was purified by column chromatography over silica gel using a mixture of
dichloromethane/hexane (2:1) as the eluent, and the resulting product was recrystallized
from ethylacetate (80 ml), to give a colorless solid. Yield = 0.55 g, (57%). Elemental
analysis found; C% 77.9, H% 7.6, N% 5.2, calculated for C68H78O6N4; C% 78.0, H% 7.5,
N% 5.3. δH (500MHz, CDCl3, TMS); 8.91 (s, 4H, Ar-H), 8.40 (m, 4H, Ar-H), 7.52 (m,
4H, Ar-H), 7.26 ((m, 4H, Ar-H), 7.00 (m, 12H, Ar-H), 4.03 (t, 4H, Ar-O-CH2-, J = 6.6
Hz), 3.93 (two overlapping triplets, 8H, Ar-O-CH2-), 1.82-1.36 (m, 32H, aliphatic-H), 0.92
(t, 6H, -CH2-CH3, J = 5.0 Hz). ν/cm-1 (KBr); 2941, 2869 (C-H str.), 1607 (C-C str.), 830
(1, 4-disub. C-H o.o.p.d). m/z; 1046 (M+). HPLC purity; Normal phase Si column: 99.8%,
Reverse Phase C8 column: 100%.
Preparation of 5-(12-bromododecyl)-2-(4-dodecyloxyphenyl)pyrimidine (II-a)
Potassium carbonate (0.81 g, 5.9 mmol) was added to a solution of 5-dodecyl-2-(4-
hydroxyphenyl)pyrimidine (2.00 g, 5.9 mmol) (purchased from Midori Kagaku Co. Ltd.,
Japan) and 1,12-dibromododecane (2.89 g, 8.82 mmol) in cyclohexanone (15 ml). The reac-
tion mixture was stirred at 80◦C overnight. Precipitated materials were removed by filtration.
After removal of the solvent by evaporation under reduced pressure, the product was puri-
fied by column chromatography over silica gel using a mixture of dichloromethane/hexane
(1:1) as the eluent, to give an intermediate bromo-compound. Yield = 2.21 g, (64%).
Preparation of 2,2ꢀ-Bis{12-[4-(5-dodecylpyrimidin-2-yl)phenyloxy]
dodecyloxy}biphenyl (II)
Potassium carbonate (0.24 g, 1.7 mmol) was added to a solution of the obtained bromo-
intermediate, 5-(12-bromododecyl)-2-(4-dodecyloxyphenyl)pyrimidine (II-a), (1.01 g, 1.7
mmol) and 2,2ꢀ-dihydroxybipheny (0.16 g, 0.9 mmol) in cyclohexanone (8 ml). The reaction
mixture was stirred at 130◦C overnight. Precipitated materials were removed by filtration.
After removal of the solvent by evaporation under reduced pressure, the product was purified
by column chromatography over silica gel using dichloromethane as the eluent, and the
resulting product was recrystallized from a mixture of ethanol/ethylacetate (2:3) (25 ml),
to give a colorless solid. Yield = 0.47 g, (46%). Elemental analysis found; C% 79.9, H%
10.4, N% 4.5, calculated for C78H118O4N4; C% 79.7, H% 10.1, N% 4.8. δH (500MHz,
CDCl3, TMS); 8.57 (s, 4H, Ar-H), 8.34 (m, 4H, Ar-H), 7.25 ((m, 4H, Ar-H), 6.99 (m, 12H,
Ar-H), 4.02 (t, 4H, Ar-O-CH2-), 3.89 (t, 4H, Ar-O-CH2-), 2.59 (t, 4H, Ar- CH2-), 1.82-1.26
(m, 80H, aliphatic-H), 0.89 (t, 6H, -CH2-CH3). ν/cm-1 (KBr); 2918, 2851 (C-H str.), 1609
(C-C str.), 843 (1, 4-disub. C-H o.o.p.d). m/z; 1198 (M+). HPLC purity; Normal phase Si
column: 99.9%, Reverse Phase C8 column: 99.6%.
Preparation of 2,2ꢀ-Bis{4-oxtyloxy-4ꢀ-carbonyloxybipheny}biphenyl (III)
4-(4-Octyloxyphenyl) beozoicacid (0.84 g, 2.5 mmol), 2,2ꢀ-dihydroxybipheny (0.24 g,
1.3 mmol), and DMAP (0.03 g, 0.3 mmol) were added to dry dichloromethane (13 ml).
DCC (0.80 g, 3.9 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

Effects on Chirality, Polarity and Biaxiality in Liquid Crystals
179
chromatography over silica gel using a mixture of dichloromethane/hexane (1:1) as the
eluent, and the resulting product was recrystallized from a mixture of ethanol/ethylacetate
(3:1) (40 ml), to give a colorless solid. Yield = 0.62 g, (60%). Elemental analysis found;
C% 80.7, H% 7.4, calculated for C54H58O6; C% 80.8, H% 7.2. δH (500MHz, CDCl3,
TMS); 8.01 (m, 4H, Ar-H), 7.57 (m, 8H, Ar-H), 7.40-7.28 (m, 4H, Ar-H), 6.98 (m, 4H,
Ar-H), 4.00 (t, 4H, Ar-O-CH2-, J = 6.5 Hz), 1.81-1.30 (m, 24H, aliphatic-H), 0.89 (t, 6H,
-CH2-CH3, J = 5.9 Hz). ν/cm-1 (KBr); 2926, 2855 (C-H str.), 1740 (C = O str.), 1605
(C-C str.), 830 (1, 4-disub. C-H o.o.p.d). m/z; 802 (M+). HPLC purity; Normal phase Si
column: 99.9%, Reverse Phase C8 column: 99.4%.
Liquid-Crystalline and Physical Property Measurements
The initial phase assignments and corresponding transition temperatures for the final prod-
ucts were determined by thermal optical microscopy using an Nikon Optiphot-pol polarizing
microscope equipped with a Mettler FP82 microfurnace and FP80 control unit. The heating
and cooling rates were 2◦C min−1. Free-standing films were made for textural observation
by spreading the sample across a small hole (1∼2 mm diameter) in a metal plate. Temper-
atures and enthalpies of transition were investigated by differential scanning calorimetry
(DSC) using a MAC Science MTC1000S calorimeter. The materials were studied at a scan-
ning rate of 5◦C min−1, after being encapsulated in aluminium pans. The X-ray scattering
experiment was performed by real-time X-ray diffractometer (Bruker AXS D8 Discover).
The monochromatic X-ray beam (CuKαline) was generated by 1.6 kW X-ray tube and
Go¨bel mirror optics. The 2D position sensitive detector has 1024 × 1024 pixels in a 5 cm ×
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◦C). 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.
3. Results and Discussion
Phase transition temperatures of the compounds studied are as follows:
Compound (I) Isotropic liquid 173.8oC N 162.2 SmC 90 recryst. (mp = 143oC),
Compound (II) Isotropic liquid 71.4 SmA 65 recryst. (mp = 97), and
Compound (III) Isotropic liquid 72 recryst. (mp = 120).
Compound (II) shows the usual homeotropic and fan-shaped textures in the vertically and
homogenously aligned regions, respectively, which is typical for the SmA phase. However,
Compound (I) shows rather an anomalous textural change at the SmC-N phase transition
as shown in Fig. 6. Fig. 6 (a) shows the Schlieren texture observed for the SmC phase of
Compound (I). On heating, the texture changes at the SmC-N phase transition (Fig. 6 (c)),
resulting in the other Schlieren texture of the N phase (Fig. 6 (e)). Since the inner surfaces
of the glass plates had been treated as vertical alignment, the appearance of the Schlieren
texture in the SmC is reasonable. However, the appearance of the Schlieren texture in the
N phase is anomalous, because the N phase of the vertical alignment sample is expected to
show the homeotropic texture; the Schlieren texture is usually observed for the N phase with
the planar alignment. There are two possibilities for this anomalous Schlieren-Schlieren
textural change at the SmC-N transition: (1) the occurrence of the director change at the
SmC-N transition, e.g. change in the molecular axis direction from pseudo-homeotropic

180
I. Nishiyama et al.
Figure 6. Textural changes at the SmC-N transition of Compound (I) on heating: (a) 162.0◦C (SmC),
(b) 162.1◦C (SmC), (c) 162.2◦C (SmC-N transition), (d) 162.3◦C (N), and (e) 162.5◦C (N).
to planar or molecular tilting in a certain portion of the sample, and (2) the emergence of
biaxiality in the N phase13.
The apparent biaxiality is produced even in the vertically aligned nematic sample if a
part of the molecules tilts with respect to the normal to the substrates. In order to reduce
the tendency of undesirable tilting of the molecules, free-standing films of the Compound
(I) have been made and the textural change at the SmC-N phase transition observed. Air
is known to produce a strong vertical anchoring to the liquid crystal molecules at the air-
liquid crystal interface. Figure 7 shows the textural change of the free-standing film of the
Compound (I), in which the Schlieren-Schlieren textural change, same as observed for the
sandwiched sample between glass plates, is observed (see Fig. 7 (b)).
The textural observation was also made in the cooling process for the nematic phase of
the free-standing film sample of Compound (I). The N phase shows a typical homeotropic
texture as shown in Fig. 8(a). On further cooling, the Schlieren texture gradually appears
even in the N phase (Fig. 8 (b)), which becomes clear on further cooling (Fig. 8 (c)).
The appearance of the Schlieren texture is not resulted from the phase transition to the
SmC phase, because the Schlieren texture shown in Fig. 8 (c) again changes into the other
Schlieren texture of the SmC phase on further cooling.
These results on the textural change suggest a possibility of the emergence of the biaxial
N phase in Compound (I). If this is the case, the homeotropic texture (Fig. 8 (a)) indicates
the usual uniaxial N phase, whereas the Schlieren texture (Fig. 8 (b) and (c)) is produced
Figure 7. Textural changes at the SmC-N transition observed on heating, for the free-standing film
sample of Compound (I): (a) a Schlieren texture of the SmC, (b) coexistence of two Schlieren textures
of the SmC and N at the SmC-N phase transition, and (c) a Schlieren texture of the N.

Effects on Chirality, Polarity and Biaxiality in Liquid Crystals
181
Figure 8. Textural changes in the N on cooling, for the free-standing film sample of Compound (I):
(a) a Schlieren texture of the N, (b) gradual appearance of the Schlieren textures on cooling from (a),
and (c) a Schlieren texture of the N in the lower temperature region.
by the biaxiality of the N phase. A possible uniaxial-biaxial phase transition was tried to
be detected by the thermal study. However, no additional peak appears, corresponding the
biaxial-uniaxial transition in the N phase (Fig. 9). This may be because the transition is
second order in nature or two DSC peaks with respect to the SmC-N and the uniaxial-biaxial
phase are too close to be separated.
Layer spacing of Compound (I) is calculated from the position of the X-ray diffraction
peak at the small angle region, which is shown in Fig. 10, together with the amplitude of
the diffraction.
The molecular length of Compound (I) in the cisoid, i.e., the molecular configuration
shown in Fig. 4 (b) or Fig. 5, is estimated to be c.a. 30 Å, which well corresponds to the
obtained layer spacing that is smaller value than the estimation due to the tilt in the SmC
phase. The molecules have a tendency to form cisoid also in the nematic phase, which is
suggested by the position of the weak x-ray diffraction in the small angle region. Thus,
Compound (I) are expected to possess board-like molecular configuration of cisoid in the
N phase, which makes molecular biaxiality larger.
A free-standing film of Compound (I) exhibits that a normal homeotorpic texture of
the N phase changes into the “Schlieren” texture on cooling even in the lower temperature
region of the N phase, which shows, on further cooling, the other “Schlieren-Schlieren”
textural change at the N-SmC phase transition. The lower temperature range of the N phase
Figure 9. DSC chart of Compound (I) measured on heating, in which two DSC peaks appear
corresponding to the SmC-N and the N-Iso.Liq. phase transitions, respectively.

182
I. Nishiyama et al.
Figure 10. Layer spacing calculated from the position of the X-ray diffraction peak at the small
angle region and the amplitude of the diffraction as a function of temperature, for Compound (I).
thus exhibits a “biaxial” nature. One possible reason for the appearance of the biaxiality is
the emergence of the biaxial N phase. The other possibilities, however, e.g., the tilt of the
molecules, cannot be completely ruled out at this stage of work. Further analyses of the
molecular organization and alignment of the free-standing film sample of Compound (I)
are now in progress.
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