A hexagonal columnar phase formed in lateral fluorinated bent-shaped molecules based on a 1,7-naphthalene central core

Article abstract of DOI:10.1039/c2nj21072j

Lateral fluorinated bent-shaped molecules based on a 1,7-naphthalene central core and alkoxy tails can form a switchable hexagonal columnar phase and B₄ phase. The columnar phase has a large two-dimensional hexagonal lattice of 65-70 A and exhibits ferroelectric switching with spontaneous polarization along the column axis. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj21072j

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LETTER
A hexagonal columnar phase formed in lateral fluorinated bent-shaped
molecules based on a 1,7-naphthalene central corew
Xiaodong Li, Mao-sheng Zhan* and Kai Wang*
Received (in Montpellier, France) 29th December 2011, Accepted 23rd February 2012
DOI: 10.1039/c2nj21072j
Lateral fluorinated bent-shaped molecules based on a 1,7-naphthalene
central core and alkoxy tails can form a switchable hexagonal
columnar phase and B4 phase. The columnar phase has a large
˚
two-dimensional hexagonal lattice of 65–70 A and exhibits
ferroelectric switching with spontaneous polarization along the
column axis.
Hexagonal columnar liquid crystals have attracted considerable
interest owing to their distinctive structure and potential applica-
tion in the fields such as photovoltaics,1–3 light-emitting diodes4 and
field effect transistors.5,6 Compared with nematic and smectic liquid
crystals, fewer structural skeletons have been found to exhibit
Scheme 1 The molecular structure of N(1,7)-Fn.
Table 1 Transition temperatures and enthalpies of N(1,7)-Fn compounds
a hexagonal columnar phase. Disk-like aromatic compounds
Transition temperaturea/1C (enthalpy/kJ molÀ1
)
such as triphenylene, porphyrin and phathalocyanine with
mutiple tails have been known to exhibit hexagonal columnar
liquid crystal properties.7
N(1,7)-F12
N(1,7)-F14
N(1,7)-F16
B4 87.6 (11.8); Colh 147.5 (2.8); iso
B4 84.9 (14.1); Colh 142.3 (2.7); iso
B4 82.3 (15.9); Colh 137.1 (2.2); iso
Recently not only simple disklike molecules but also a variety
of molecular shapes such as rings, fans, cones or bows are utilized
to form hexagonal columnar self-assemblies.8 In particular, some
bent-shaped molecular systems exhibit hexagonal columnar
phases. Kishikawa et al. prepared urea derivatives based on an
abuse-angle central core and three alkoxy tails on each terminal
benzene ring. The molecules are connected linearly through
hydrogen bonds to form columns.9 Gorecka et al. synthesized
polycatenar bent-shaped molecules with 2,6-pyridine central
units and three alkoxy tails on each side. And the molecules
can be assembled into a conical-shaped unit that forms
columns.10 Both of these cases contain mutiple alkoxy tails,
which is assumed to be a necessary structural characteristic to
induce a hexagonal columnar stacking nature.
a
Based on cooling DSC data.
lateral fluorinated bent-shaped molecules based on the same
central core but with a single alkoxy tail in each side wing. And
the fluorine substituent is introduced at the position 3 of the
terminal phenyl ring. The compound was designated as
N(1,7)-Fn, where n indicates the number of carbon atoms in
the alkoxy tails (Scheme 1). Interestingly, the introduction of
the fluorine substituent has also induced a specific hexagonal
columnar mesophase, which exhibits ferroelectric switching
under electric field.
The synthetic route to N(1,7)-Fn is described in Scheme S1
(ESIw). All compounds exhibited a hexagonal columnar phase
(Colh) with a phase sequence of Iso–Colh–B4 upon decreasing
temperature (Table 1). Their transition temperatures and
associated enthalpy changes are listed in Table 1. When slowly
cooling the isotropic liquid of N(1,7)-F14 on an untreated
glass slide under POM, large domains of dendritic texture
were observed (Fig. 1a), as is characteristic of the columnar
mesophase.10 While on homogeneously rubbing glass, a uniform
birefringent texture was detected while the rubbing direction is
tilted from the polarizer’s direction (Fig. 1b), which additionally
supports the one-dimensional organization of the Colh phase.
Upon further cooling to the B4 phase, dark blue textures were
exhibited (Fig. 1c). Two optically active domains with opposite
chirality were observed, in which the brightness was interchanged
In the previous research, we prepared bent-shaped molecules
with a 1,7-dioxynaphthalene central core and a single alkylthio tail
in each side wing.11 Surprisingly, the material exhibits a hexagonal
columnar phase although neither flat aromatic backbones nor
multiple tails are involved in the molecular structure. The relatively
flexible alkylthio tail and the low bent-angle central core are
considered to play a significant role. In this study, we prepared
Beijing University of Aeronautics and Astronautics, Beijing,
P.R.China. E-mail: zhanms@buaa.edu.cn, wangkai@buaa.edu.cn;
Fax: +86 1082317120; Tel: +86 1082317120, +86 1082338557
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj21072j
c
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Table 2 X-Ray spacing data and calculated lattice parameter of
N(1,7)-Fn
Compound
N(1,7)-F12
Phase
dobs/A
dcal/A
hkl
aa/A
57.9
33.6
28.8
35.2
58.6
34.0
29.3
38.4
59.2
34.0
29.6
41.3
57.9
33.4
28.9
—
58.6
33.8
29.3
—
59.2
34.1
29.6
—
100
110
200
100
100
110
200
100
100
110
200
100
Colh
B4
66.9
N(1,7)-F14
Colh
B4
67.6
70.1
N(1,7)-F16
Colh
B4
Fig. 1 Microphotographs observed for N(1,7)-F14 (a) dendritic
texture of the Colh phase on an untreated glass substrate (at 120 1C),
(b) texture of the Colh phase on a homogenous rubbing cell, while the
rubbing direction is 301 tilted from the polarizer’s direction. (c) The B4
phase on an untreated glass substrate (at 60 1C), (d) chiral domains in
the B4 phase under uncrossed polarizers.
a
Hexagonal lattice parameter.
by positive and negative rotations of the polarizer from the
cross polarization position (Fig. 1d). These characteristics of
the B4 phase have been explained by the twisted grain boundary
(TGB) alignment12 or helical nanofilaments of the layers.13
The columnar nature in the Colh phase is firmly identified by
a characteristic X-ray pattern (Fig. 2). Powder X-ray investigations
were carried out with samples kept in glass capillary tubes. By
slowly cooling the compound from isotropic liquid to the Colh
phase, a broad halo scattering is detected in the wide angle
region, corresponding to the liquid-like disorder of molecules
with a mean distance of 4.5 A. In the small-angle region, three
sharp reflections corresponding to layer spacings in the ratio of
1, (1/3)1/2 and 2 were identified (Table 2). They were further
indexed to (100), (110) and (200) in the two-dimensional
hexagonal lattice with p6mm symmetry.10 The hexagonal
lattice constant, a, is calculated to be 66.9 A, 67.6 A, 70.1 A
for N(1,7)-F12, N(1,7)-F14 and N(1,7)-F16 respectively,
showing steady increment with the growth of n. Upon further
cooling to the B4 phase, a few equidistant smectic layer
reflections and outer reflections of 4.5 A were exhibited, as
generally observed in B4 phases.14
Fig. 3 X-Ray diffraction patterns of the Colh phase (at 120 1C) and the
B4 phase (at 60 1C) observed in N(1,7)-F14 on a homogeneously rubbing
substrate. The X-ray beam is irradiated parallel to the rubbing direction.
well-aligned Colh could be observed (Fig. 3), which clearly
proves the existence of a hexagonal columnar phase. Fig. 3
illustrates the X-ray photographs taken upon irradiation
parallel to the rubbing direction of the glass substrate. In the
small angle region, well-aligned sharp spots of (100), (110),
and (200) reflections clearly appeared. For instance, a pair of
(100) reflection spots was observed in the meridional direction,
and the additional pairs of spots are located every 601 along
the azimuthal direction, confirming the hexagonal packing of
molecules. However, in contrast to the high orientation of the
inner reflections, the outer broad reflection appeared as a
diffuse ring pattern similar to that of the non-oriented sample.
Thus, it is concluded that there is no long range orientational
correlation in the adjacent molecules on the projection along
the column axis. Upon further cooling to the B4 phase, the
orientation with respect to the inner reflections disappeared
completely as observed in the B4 phase of usual bent-core
molecular systems.14
To elucidate more detailed structure of the Colh phase, an
oriented X-ray pattern of the compound was obtained. By slowly
cooling a droplet of the compound N(1,7)-F14 deposited onto a
homogeneously rubbing glass slide, the diffraction pattern of a
Electro-optic switching was observed for samples sandwiched
between two glass plates with a transparent ITO electrode.
The substrate surface was neither polymer coated nor rubbed.
Fan-shaped domains were first observed upon cooling the
isotropic liquid to the Colh phase (Fig. 4a). By applying
an electric field on the Colh phase, the fan-shaped texture
immediately disappeared and was rebuilt to give a zero
birefringence (Fig. 4b). Thus, an electric field induced the
Fig. 2 Powder X-ray diffraction patterns of the Colh phase (at 120 1C)
and the B4 phase (at 60 1C) observed in N(1,7)-F14.
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1134 New J. Chem., 2012, 36, 1133–1136
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Fig. 5 Electrostatic potential maps of the N(1,7)-Fn series (the
electrostatic potential was calculated by the B3LYP/6-31G* method
with Spartan 08 V1.12). The correlation of the colour and electron
density is on the right side bar.
Fig. 4 Switching behaviour observed for N(1,7)-F12. (a) Microscopic
fan-like texture of the Colh phase originally prepared within a cell.
(b) Dark texture observed under the applied electric field. (c) Polarization
reversal current under the application of triangular wave (135 1C, 400 Vpp,
30 Hz, cell gap: 4.0 mm). (d) The switching model.
Table 3 Values of the calculated electrostatic potential (kcal molÀ1
on the different parts of the molecule
)
Series
N(1,7)-Fn
N(1,7)-On
homeotropic alignment of columns with their axes perpendi-
cular to the ITO substrate. A single current peak in response to
the applied triangular field was observed (Fig. 4c), indicating
that the columnar phase is ferroelectric.9,10 The spontaneous
polarization was calculated to be about 180 nC cmÀ2. The
switching occurred with no texture change, and the dark
texture was maintained after the electric field switched off.
This indicates that the polarization exists along the columns
and switches along the direction vertical to the ITO plate
(Fig. 4d). At the field-on state, the molecules rotate around its
long axis and the bent direction of molecules is parallel to the
column axis. Upon reversing the field the molecules within a
column can rotate their polar axis to the opposite direction to
maintain the columnar axis along the field. And the direction
of the columns is maintained after the field is off.
Central ring
O
CQO
Intermediate ring
CHQN
Outer ring
O at the end of side wing
À67
À60
À69
À71
À185
À42
À197
À57
À137
À85
À148
À107
À130
À157
At present time, our understanding of how molecules in this
study construct hexagonal columnar assembly is still at an
early stage. The value of the lattice parameter a (the diameter
of the column) was approximately two-fold larger than the layer
thickness of the B4 phase. By assuming a density of r = 1 g cmÀ3
and a height of the columnar slice of h = 4.5 A (corresponding to
the average distance between molecules), the number of
molecules calculated in each column slice is found to be
10–11. This value is relatively large, so we believe that the
column formation involving disc-like stacking by a conical
assembly of bent-core molecules reported by Kishikawa et al.9
and Gorecka et al.10 is unlikely in our case. Based on the broad
diffuse scattering observed in the wide-angle region of the well-
aligned sample, it is concluded that there is no preferential
orientation of molecules on the projection along the column.11
One possible model is that the column is shaped with slices
containing 10–11 molecules. The groups (or ribbons) of parallel
molecules packed without any orientational correlation along
the column axis (Fig. S2a, ESIw), which is similar to the self-
assembly proposed for phasmidic molecules.17 Another possible
model is that the column is constructed from a tube-like
assembly of molecules and cylindrically symmetric deformation
of layers was assumed (Fig. S2b, ESIw). The alkoxy tails of
molecules are protruding outside of the column wall and inside
of the enclosed mesogenic layer respectively. In this case, the
column includes a very high curvature and the concentration of
the aliphatic chains in the central part is quite high. However,
such a high curvature is not unusual since it is generally
observed in the smectic layers to form the focal conic defects.13
Further clarification of the molecular assembly in this phase is
now proceeding.
The examples shown in this paper give evidence of the subtle
relationship between the chemical structure and mesomorphic
properties of the bent-shaped liquid crystals. The introduction of
a lateral fluorine substituent at position 3 on the terminal phenyl
rings leads to a drastic change in mesomorphic behaviour since a
switchable hexagonal columnar mesophase is observed, while the
non-substituted homologous N(1,7)-On only exhibited the B4
phase.15 Also, in contrast to the typical molecular structures which
form a hexagonal columnar phase, neither flat aromatic backbones
nor multiple tails are involved in the N(1,7)-Fn compound. It
is assumed that the molecular organisation of bent-shaped
compounds results in a complex balance between the electro-static
interactions and the van der Waals interactions developed by the
aliphatic chain.16 Thus the distribution of the electrostatic potential
extrema along the molecule will strongly influence the molecular
arrangement and the nature of the mesophases. Electrostatic
potential maps of N(1,7)-Fn and N(1,7)-On were calculated by
the B3LYP/6-31G* method with Spartan 08 V1.12 software
(Fig. 5). The presence of the fluorine substituents on the outer
rings modifies the distribution of the potential extrema (Table 3).
In the N(1,7)-Fn series, the fluorine substitution slightly reduces the
electrostatic potential around the Schiff base and the ester group
but largely reinforces the negative potential between the fluorine
substituted phenylene group and the alkoxy group. This small
difference might be able to induce different molecular arrange-
ment and result in formation of different mesophases.
In summary, lateral fluorinated bent-shaped molecules based
on a 1,7-dioxynaphthalene central core have been studied.
c
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The material can form a switchable hexagonal columnar phase
and B4 phase. The columnar phase has a large two-dimensional
hexagonal lattice of 65–70 A and exhibits ferroelectric switching
with spontaneous polarization along the column axis. The
number of molecules calculated in each column slice is found
to be 10–11. It is assumed that the combination of the acute
angle central core and the lateral polar fluorine substituent
cooperates to generate a hexagonal columnar self-assembly,
although the molecule itself only has one single alkoxy tail on
each side wing. This could open up prospects for new molecular
designs in hexagonal columnar liquid crystal systems.
Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (No. 51073007). X. Li is grateful to Prof. Junji
Watanabe in Tokyo Institute of Technology for the stay in his
lab on the exchange research.
Notes and references
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Experimental
The synthetic routes are illustrated in Scheme S1 (ESIw). All
reagents including 1,7-dihydroxynaphthalene were purchased
from TCI (Tokyo Kasei kogyo Co, Ltd) and used without
further purification. Solvents were purified by normal procedures
1
and handled under a moisture-free atmosphere. H-NMR and
13C-NMR spectra were recorded on a JEOL FT-NMR AL400
(400 MHz) spectrometer using CDCl3 as an internal standard.
Elemental analysis was determined by a CHN corder MT-6.
Optical textures were observed under a crossed polarizer
using an Olympus BX51 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-Ka radiation from the graphite crystal of the monochromator
and a flat-plate-type imaging plate. Powder X-ray investigations
were carried out with samples kept in glass capillary tubes with a
diameter of 1.5 mm. 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 indium tin oxide (ITO) electrode. Neither polymer
coating nor rubbing was performed on the substrate surface.
16 J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
H. T. Nguyen and M. F. Achard, J. Mater. Chem., 2002, 12,
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c
1136 New J. Chem., 2012, 36, 1133–1136
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