Switchable columnar phase formed from bent-shaped molecules with low bent-angle naphthalene central core and alkylthio tail

Article abstract of DOI:10.1080/15421406.2011.581531

Novel low-angle bent molecules with 1,7-naphthalene central core and alkylthio tails form the hexagonal columnar phase as well as the well-known B4 phase. The columnar phase has a large two-dimensional hexagonal lattice of 65-70 and exhibits the antiferro

Full text of DOI:10.1080/15421406.2011.581531

This article was downloaded by: [University of Cambridge]
On: 09 October 2014, At: 15:37
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
Switchable Columnar Phase Formed from
Bent-Shaped Molecules with Low Bent-
Angle Naphthalene Central Core and
Alkylthio Tail
Sungmin Kang a , Xiaodong Li a , Susumu Kawauchi a , Masatoshi
Tokita a & Junji Watanabe a
a Department of Organic and Polymeric Materials , Tokyo Institute of
Technology , 2-12-1 O-okayama, Meguro-ku, Tokyo , 152-8552
Published online: 07 Oct 2011.
To cite this article: Sungmin Kang , Xiaodong Li , Susumu Kawauchi , Masatoshi Tokita & Junji
Watanabe (2011) Switchable Columnar Phase Formed from Bent-Shaped Molecules with Low Bent-
Angle Naphthalene Central Core and Alkylthio Tail, Molecular Crystals and Liquid Crystals, 549:1,
184-193, DOI: 10.1080/15421406.2011.581531
To link to this article: http://dx.doi.org/10.1080/15421406.2011.581531
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Mol. Cryst. Liq. Cryst., Vol. 549: pp. 184–193, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.581531
Switchable Columnar Phase Formed from
Bent-Shaped Molecules with Low Bent-Angle
Naphthalene Central Core and Alkylthio Tail
SUNGMIN KANG, XIAODONG LI, SUSUMU KAWAUCHI,
MASATOSHI TOKITA, AND JUNJI WATANABE∗
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
Novel low-angle bent molecules with 1,7-naphthalene central core and alkylthio tails
form the hexagonal columnar phase as well as the well-known B4 phase. The columnar
phase has a large two-dimensional hexagonal lattice of 65–70 Å and exhibits the
antiferroelectric switching with the spontaneous polarization along the column axis.
The column is considered to be constructed by the cylindrically enclosed layer.
Keywords Banana molecule; Columnar phase; Hexagonal lattice; Layer deformation;
tube-like assembly; antiferroelectric switching
1. Introduction
Since the discovery of spontaneous formation of liquid crystal phases with unique polarity
and supermolecular chirality in bent-shaped (or banana) molecules,1,2 a number of these
banana molecules have been synthesized and characterized3–5 to clarify the structure-
property relationships. The most important factor for the design of banana molecules is to
find the central bent core with an appropriate bent angle. A 1,3-disubstituted benzene ring,
2,6-disubstituted pyridine ring, 2,7-disubstituted naphthalene unit and 1,3ꢀ-disubstituted
biphenyl unit have been used. In these molecules, the bent angle is approximately 120◦
although some angular variation is allowed through the rotation of the linkage group to the
side wings. It is assumed that the bent angles in a range of 100◦ to 140◦ are favorable to
form the typical banana phases.6
When molecules have bent angle much smaller than 120◦, calamitic mesophases
with a U-shape packing are mostly formed. Matsuzaki et al.7,8 studied the molecules
based on 1,2-phenylene and 2,3-naphthalene units with a bent angle of about 60◦, and
found that only conventional nematic and smectic phases are formed. However, Watanabe
et al., recently synthesizing six kinds of bent-shaped molecules with typical Schiff-base
side wings substituted at various positions of a central naphthalene core,9–12 found that the
molecules composed of a 1,7-naphthalene central core form typical banana phases such as
B4 and SmAPA irrespective of its low bent angle of approximately 60◦. These studies have
opened a new area to exploit the new type of banana phases.
∗Corresponding author. E-mail: jwatanab@polymer.titech.ac.jp
184

Switchable Columnar Phase
185
In this study, we report a homologous series of bent-shaped molecules with 1,7-
naphthalene central core and alkylthio tail, namely, 1,7-naphthalene-bis[4-(4-alkylthiophe
nyliminomethyl)]benzoate shown below.
The compounds are designated here as N(1,7)-Sn where n indicates the number of
carbons in alkylthio tails. Of interest is that the introduction of alkylthio tail in place of the
alkoxy tail induces a switchable hexagonal columnar phase (Colh) as a new type of self
assembly. Switchable Colh phase is rather rare, there are only a few molecular systems up to
now. One is exhibited in polycatenar bent–shaped molecules discovered by Gorecka et al,13
and the other appears in urea derivatives synthesized by Kishikawa et al.14 Both of these two
systems contain branched alkoxy tails which play an important role in forming columnar
phase with a discotic-like assembly of molecules. In contrast, the column in this study
is constructed by a tube-like assembly of molecules, i.e. the cylindrically enclosed layer.
This is a new type of columnar assembly of banana molecules exhibiting antiferroelectric
switching.
2. Experiments
Synthesis
N(1,7)-Sn molecules were synthesized according to the routes illustrated in Scheme 1.12 All
reagents including bis(p-hydroxyphenyl)methane were purchased from TCI (Tokyo Kasei
kogyo Co, Ltd) and used without further purification. Solvents were purified by normal
procedures and handled under a moisture-free atmosphere.
Methods
Optical textures were observed under a crossed polarizer using an Olympus BX50 polarizing
optical microscopy (POM) system equipped with a temperature-controlled Mettler Toledo
FP 82 hot stage. Transition temperatures and corresponding enthalpies were determined by
differential scanning calorimetry (DSC) using a Perkin-Elmer DSC 7 calorimeter.
Wide-angle X-ray diffraction (WAXD) measurements were performed using a Rigaku-
Denki RINT-2500 X-ray generator with monochromic Cu-Kα radiation from the graphite
crystal of the monochromator and a flat-plate-type imaging plate. Powder X-ray inves-
tigations were carried out with samples kept in glass capillary tubes with a diame-
ter of 1.5 mm. The aligned sample for X-ray measurements was prepared on a glass

186
S. Kang et al.
Scheme 1. Synthetic route of N(1,7)-Sn.
substrate coated with commercially available organosilane N,N-dimethyl-n-octadecyl-3-
aminotropyltrimethoxysilyl chloride (DMOAP).
Electro-optic switching was observed by using a high-speed voltage amplifier
(FLC Electronics, F20A) connected to a function generator (NF Electronic Instruments,
WF1945A). The sample was sandwiched between two glass plates with a transparent in-
dium tin oxide (ITO) electrode. Neither polymer coating nor rubbing was made on the
substrate surface.
3. Results and Discussion
All the compounds, N(1,7)-Sn with n = 8, 12, 14 and 16, show enantiotropic transitions
as shown in Figure 1. Their transition temperatures and associated enthalpy changes are
listed in Table 1. While the shortest homologue N(1,7)-S8 exhibits direct transition from

Switchable Columnar Phase
187
Figure 1. Representative DSC thermograms of N(1,7)-Sn materials. (a) n = 14 and (b) n = 16. (Heat
flow of endotherm is upper direction.)
the isotropic liquid to B4 phase, N(1,7)-S12 and N(1,7)-S16 show an additional Colh phase
with a sequence of Iso-Colh-B4 on decreasing the temperature.
When cooling from the isotropic liquid between untreated glass plates, Colh phase
appears as a smooth fan textures with extinction brushes aligned along the polarizers
(Figure 2b). In some part, textures include the optically isotropic zone. On further cooling
to B4 phase, all parts change dark blue texture including two optically active domains
with opposite chirality which was recognized by clockwise and anticlockwise rotations of
polarizer from the cross polarization position (Figure 2a).
The columnar nature in Colh phase is firmly identified by the chraracteristic X-ray
pattern with the outer broad diffuse scattering of 4.6 Å corresponding to the liquid-like
lateral packing of molecules, and inner three sharp reflections (see Figure 3). As listed in
Table 1, the later three reflections have the spacings in the ratio of 1, 31/2 and 2, and they are
indexed to (100), (110) and (200) of the two dimensional hexagonal lattice. The hexagonal
lattice edge, a, is 65.0 Å for N(17)-S12, 67.3 Å for N(17)-S14 and 70.0 Å for N(17)-
S16. The lower-temperature B4 phase shows only a layer reflection, the spacings of which
are 28.3, 33.6, 36.1 and 39.2 Å for N(1,7)-S8, N(1,7)-S12, N(17)-S14 and N(1,7)-S16,
respectively. These approximate to the molecular lengths calculated for the most extended
conformation, as listed in Table 2.
To elucidate the more detailed structure of the Colh phase, we examined the X-ray
pattern for a droplet of the material deposited onto a silane coupling glass plate. It was
initially heated up to the isotropic melt and then cooled to the Colh phase at a rate of
1◦C/min. By this surface treatment, the highly birefringent fan-like texture is observed,
showing that the colums take a planar orientation with their long axes parallel to the glass
surface. Figure 4a shows the X-ray photographs taken for this sample by the irradiations
Table 1. Transition temperatures and enthalpies of N(1,7)-Sn compounds
Compounds
Transition temperature/◦Ca (enthalpy/kJmol−1
)
N(1,7)-S8
N(1,7)-S12
N(1,7)-S14
N(1,7)-S16
B4 113.1 (29.1) Iso
B4 119.6 (30.0) Colh 138.5 (5.3) Iso
B4 119.9 (28.1) Colh 143.0 (5.7) Iso
B4 118.1 (30.9) Colh 146.0 (7.0) Iso
aBased on cooling DSC curves.

188
S. Kang et al.
Table 2. X-ray data and elucidated lattice parameter of N(1,7)-Sn compounds
Molecular
Lengthb
Compounds
Phase
d
obs/Å
hkl
dcalc/Å
aa/Å
N(l,7)-S8
N(1,7)-S12
B4
Colh
28.3
56.3
32.6
28.2
33.6
58.3
33.4
29.2
36.1
60.6
34.8
30.4
39.2
001
100
110
200
001
100
110
200
001
100
110
200
001
—
—
31.5
56.3
32.5
28.2
65.0
67.3
B4
Colh
38.2
41.5
44.9
N(l,7)-S14
N(l,7)-S16
58.3
33.6
29.2
B4
Colh
60.6
34.9
30.3
70.0
B4
aa: hexagonal lattice parameter, bBased on the most extended chains.
Figure 2. Microscopic photographs of (a) the dark texture of B4 phase at 100◦C (left) showing
the chiral domains under uncrossed polarizers (right), and (b) the fan-like texture of Colh phase (at
130◦C) in N(1,7)-S14.
Figure 3. X-ray diffraction profiles taken for the Colh and B4 phases of powder sample in N(1,7)-S14.

Switchable Columnar Phase
189
Figure 4. X-ray diffraction patterns of (a) Colh phase and (b) B4 phase, which is prepared on
the homeotoropically surface-treated glass. Using a Rigaku-Denki RINT-2500 X-ray generator with
monochromic Cu-Kα radiation from the graphite crystal of the monochromator, the X-ray beam is
irradiated along the direction parallel to the glass surface and the diffraction patterns are corrected on
a flat-plate-type imaging plate. The arrow indicates the glass surface direction.
parallel to the glass surface. All of (100), (110) and (200) reflections appear as six spots. For
example, a pair of (100) reflection spots are observed strongly on the equatorial direction
(i.e. the direction perpendicular to the glass surface), and other pairs of spots are located
every 60◦ along the azimuthal direction. On the other hand, no orientation is detected in the
X-ray photograph taken with a beam perpendicular to the glass surface. Such characteristic
patterns can be explained by the planar orientation of columns in which the hexagonal
(100) planes are parallel to the glass surface, but the column axes are randomly displaced.
On cooling to the B4 phase, the orientation disappears completely as observed in the B4
phase of usual banana molecular systems (see Figure 4b).15,16
Electro-optic switching was observed for the sample which was sandwiched between
two glass plates with a transparent indium tin oxide (ITO) electrode. Neither polymer
coating nor rubbing was made on the substrate surface. By applying an electric field on
the columnar phase, the texture with fan-shaped domains immediately disappears and is
rebuilt to give a zero birefringence (Figure 5a). In response to the applied triangular field, a
dark homeotropic texture is maintained (Figures 5b and 5c) and a clear current is detected
as a single peak (Figure 5d). Thus, we suggest that the Colh phase may be ferroelectric;
the polarization exists along the columns, and on reversing the field the molecules within
a column rotate their polar axis to the opposite direction to maintain the columnar axis
along the field. On the other, it should be noted that the SHG signal observed in field-on
state disappears on field-off state. This indicates that it is not ferroelectric, but actually
antiferroelectric.
Figure 6 shows the temperature dependence of the spontaneous polarization (Ps)
of N(1,7)-S12 against the temperature measured at 25.0 Hz. A maximum Ps value of
160 nC/cm2 is attained immediately after the transformation of B4 to the columnar phase.
It decreases with an increase in temperature and becomes zero on the transformation to the

190
S. Kang et al.
Figure 5. Electro-optic (EO) switching behavior observed for N(1,7)-S12 in the Colh phase.
(a) Optical microscopic virgin texture of fan-like domain and optically isotropic domain cooled
from isotropic liquid prepared within a cell, (b) Optically isotropic texture transformed under applied
electric field. (c) No substantial textural change was accepted between Eoff and Eon. (d) Polarization
reversal current in the Colh phase under the application of triangular wave gives a single reversal
current peak (Temperature: 135◦C, Voltage: 340 Vpp, Frequency: 25 Hz and Cell gap: 4.5 µm).
Figure 6. Temperature dependence of the spontaneous polarization observed in N(1,7)-S12 (Voltage:
340 Vpp, Frequency: 25 Hz and Cell gap: 4.5 µm).

Switchable Columnar Phase
191
isotropic melt. Currently, there exists no explanation for this strong temperature dependence
of Ps.
How is each column constructed by the bent molecules? Four key results help us figure
up the molecular assembly into the column forming the Colh phase. First, as observed
above in X-ray pattern of Figure 4a, there is no preferential orientation of molecules on
the projection along the column. This point is also supported by zero birefringence in the
homeotropic sample with the column axis aligned perpendicularly to the film sample (see
Figure 5b). Second, alkyl silane coupling surface treatment, promoting the homeotropic
alignment of molecules, results in the planar orientation of columns. This suggests that
the alkyl tails of molecules are protruding out of the side wall of the column. Third, the
hexagonal lattice edge, a, is approximately two times the layer thickness of the B4 phase.
Fourth, the antiferroelectric switching takes place along the column axis; probably, the
bent direction of molecules is perpendicular to the column axis at the electric field-off state
whereas it becomes perpendicular on the field-on. These results are substantiated if the
column is constructed by a tube-like assembly of molecules, as illustrated in Figure 7a.
The construction is similar to a cylindrically enclosed layer, whereby the alkyl tails are
protruding outside and inside of the enclosed mesogenic layer.
Cylindrically symmetric deformation of layers can be caused by the packing asym-
metry in two side rigid arms (refer to Figures 7a–7c). The present molecules have the
asymmetric shape because of the use of the 1,7-substituted naphthalene core. Hence, when
the asymmetric molecules are packed into a layer with a same directionality (Figure 7a),
Figure 7. Schematic illustration of (a)–(c) column formation by the winding of layer and (d) packing
of banana molecules within a column. Two rigid side-arms of asymmetric banana molecule take a
different orientation to the layer normal which produces the significant difference in the density if the
layer is flat. Hence, a dilation occurs in one half-layer (upper part divided by the dotted line in (a))
while a compression in the other half (lower part). This frustration can cause cylindrically enclosing
deformation of layer as in (b) and (c). By assuming a density of ρ = 1 g·cm−3 and a height of the
columnar slice of 4.6 Å (corresponding to the average distance between molecules), the number of
molecules included in each column slice is calculated to be ∼11.

192
S. Kang et al.
the two side rigid arms orient differently and then two parts of layer have the different
densities if the layer is flat. This frustration, a dilation in one half-layer and a compression
in the other half, can be reduced by cylindrical enclosing of layers (Figures 7b and 7c). By
assuming a density of ρ = 1 gcm−3 and a height of the columnar slice of h (corresponding
to the average distance between molecules), the number of molecules, n, included in each
column slice is calculated to be n = 2.4 h (Figure 7d). If h is assumed to be 4.6 Å (the
spacing of the outer broad reflection), n is 11 as an average. It should be noted here that by
such a large number of molecules in a unit slice we cannot build up the assembly model
of bent molecules into a disk-shaped object reported by Gorecka et al.13 and Kishikawa
et al.14
However, it has been never reported that the cylindrically enclosed column is stably
isolated to form the hexagonal columnar phase in the banana molecular system. This is
due to the difficult accommodation of alkyl tail groups in a space between the columns. At
this aspect, it is important to state that the rotational freedom of thio-ether linkage in the
alkylthio tail is considerably higher than that of ether linkage in the alkoky tail. Figure 8
shows the relative energy diagram as a function of dihedral angle around the C-S bond in
phenyl-S and C-O bond in phenyl-O. Here, dodecylthiobenzene and dodecyloxybenzene
were used for simple calculation. Two stable conformations are found in both compounds
(dihedral angle: 0 or 180◦). But the path from one stable conformer to the other is different.
The path for C-O is obstructed by high energy barrier (13.2 kJ mol−1, in position of
dihedral angle: 90◦). In contrast, the potential barrier for C-S is lower (1.88 kJ mol−1, in
position of dihedral angle: 50 and 130◦) although metastable conformer exists at 90◦. These
particularly low barriers of the alkylthio terminal chain enable the compound to change the
conformation more easily and make their accommodation into the tube-like layer possible.
In summary, we found that novel low-angle bent molecules with 1,7-naphthalene cen-
tral core and alkylthio tails are able to form a columnar phase as a new type of self-assembly
of bent molecules. This columnar phase is switchable; the spontaneous polarization exists
along the column axis and the antiferroelectric switching takes place without changing the
long column axis. The combination of asymmetric low-bent-angle 1,7-naphathalen core
Figure 8. Internal rotation energy against the dihedral angles around C-S bond in pheny-S (open
circles) and C-O bond in phenyl-O (closed circles).

Switchable Columnar Phase
193
and flexible alkylthio tails is considered to be responsible for the formation of switchable
columnar phase.
Acknowledgement
This research was supported by a Grant-in-Aid for Creative Scientific Research and a
Grant-in-Aid for Young Scientists B(21750138) from the Ministry of Education, Culture,
Sports, Science and Technology in Japan.
References
[1] T. Niori, T. Sekine, J. Watanabe, and H. Takezoe, J. Mater. Chem. 6, 1231 (1996).
[2] D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark, E. Korblova, and D. M. Walba,
Science 278, 1924 (1997).
[3] G. Pelzl, S. Diele, and W. Weissflog, Adv. Mater. 11, 707 (1999).
[4] H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys. 45, 597 (2006).
[5] R. A. Reddy, and C. Tschierske, J. Mater. Chem. 16, 907 (2006).
[6] I. Wirth, S. Diele, A. Eremin, G. Pelzl, S. Grande, L. Kovalenko, N. Pancenko, and W. Weissflog,
J. Mater. Chem. 11, 1642 (2001).
[7] H. Matsuzaki and Y. Matsunaga, Liq. Cryst. 14, 105 (1993).
[8] M. Kuboshita, Y. Matsunaga, and H. Matsuzaki, Mol. Cryst. Liq. Cryst. 199, 319 (1991).
[9] S. K. Lee, Y. Naito, L. Shi, M. Tokita, H. Takezoe, and J. Watanabe, Liq. Cryst. 34, 935 (2007).
[10] S. K. Lee, M. Tokita, H. Takezoe, and J. Watanabe, Ferroelectrics 365, 1 (2008).
[11] S. K. Lee, L. Shi, R. Ishige, S. Kang, M. Tokita, and J. Watanabe, Chem. Lett. 37(12), 1230
(2008).
[12] S. K. Lee, X. Li, S. Kang, M. Tokita, and J. Watanabe, J. Mater. Chem. 19(26), 4517 (2009).
[13] E. Gorecka, D. Pociecha, J. Mieczkowski, J. Matraszek, D. Guillon, and B. Donnio, J. Am.
Chem. Soc. 126, 15946 (2004).
[14] K. Kishikawa, S. Nakahara, Y. Nishikawa, S. Kohmoto, and M. Yamamoto, J. Am. Chem. Soc.
127, 2565 (2005).
[15] J.Thisayukta, Y. Nakayama, S. Kawauchi, H. Takezoe, and J. Watanabe, J. Am. Chem. Soc. 122,
7441 (2000).
[16] J. Thisayukta, H. Takezoe, and J. Watanabe, Jpn. J. Appl. Phys. 40, 3277 (2001).