Transformation from a rod-like to a hockey-stick-like and bent-shaped molecule in 3,4′-disubstituted azobenzene-based mesogens

Article abstract of DOI:10.1039/c3tc31593b

New 3,4′-disubstituted azobenzene-based mesogens with an azo group joined directly to the central benzene core have been synthesised and their liquid crystalline properties studied. Both molecular arms are gradually elongated by the insertion of a number

Full text of DOI:10.1039/c3tc31593b

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Materials Chemistry C
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PAPER
Transformation from a rod-like to a hockey-stick-like
and bent-shaped molecule in 3,40-disubstituted
azobenzene-based mesogens†
Cite this: J. Mater. Chem. C, 2013, 1,
7560
a
a
a
b
*
ˇ
´
´
ˇ´
´
´
Martin Horcic, Vaclav Kozmık, Jirı Svoboda, Vladimıra Novotna
and Damian Pociechac
New 3,40-disubstituted azobenzene-based mesogens with an azo group joined directly to the central
benzene core have been synthesised and their liquid crystalline properties studied. Both molecular arms
are gradually elongated by the insertion of a number of benzoate units between the central core and a
terminal dodecyloxy chain. Then the lengths of the molecular arms and the overall shape are varied
from a rod-like to a hockey-stick-like and bent-shape. Typical bent-shaped mesophases and electro-
optical switching can be observed upon reaching a sufficient length of the molecular arms. We
demonstrated how the mesoscale organization strictly depends on the molecular shape.
Received 13th August 2013
Accepted 20th September 2013
DOI: 10.1039/c3tc31593b
www.rsc.org/MaterialsC
It has been documented that the mesomorphic properties of
bent-shaped mesogens can be tuned by the type of the central
1. Introduction
Liquid crystalline (LC) materials combine order and mobility on
the molecular level and show unique electro-optic properties
due to their birefringence and dielectric anisotropy. In the past
few years, great effort has been devoted to the recognition of
their structure–property relationship. Bent-shaped mesogens
aromatic core, the overall number of aromatic units in the
molecular structure, the type and orientation of the linkage
groups (ester, imino, azo), the lateral substitution and the
length of the terminal alkyl chains.5,8–14 Very recently it has been
shown15,16 that an ester linking group can be successfully
replaced by a more rigid amide unit. Furthermore, the azo
linking group has also been utilized13,14,17–20 for the design of the
bent-shaped compounds. In most cases the azo group was
placed in the outer position of the lengthened arm(s). Recently,
Nagaveni et al.21 have presented azo-functionalized bent-shaped
compounds and found that the presence of the –N]N– linkage
at different locations in the molecular architecture does not
seem to have much effect on the mesomorphic behaviour. On
the other hand, the molecular shape and/or length of the
ending chain are essential in the design of bent-shaped
mesogens.
Recently, many non-symmetrically bent mesogens have also
been investigated. In contrast to the bent-shaped molecules
with two arms roughly equal, in hockey-stick-like mesogens the
two arms are substantially different in the length. In the rst
type of hockey-stick-like molecules the bend is introduced by an
alkyl or alkoxy chain attached in the meta position of the
terminal ring. For such a molecule, tilted smectic phases have
been found22,23 with both possibilities of tilt orientation in
adjacent layers: synclinic (SmCS) and anticlinic (SmCA). In the
second group of hockey-stick-like mesogens the molecules are
formed by a bent-shaped central core with arms containing
different numbers of aromatic rings. The lack of symmetry in
the molecular structure can be also introduced by different
lengths and/or types of terminal chain. Depending on the
represent a new subclass of a large family of liquid crystals.1–5
A
large variety of bent-shaped mesophases have been recognized
and described as Bn phases. In contrast to rod-like systems, for
which ferro- and/or antiferroelectricity requires molecular
chirality, the close packing of bent-shaped molecules in the
smectic layers results in polar properties of the phase and in the
structural layer chirality, even though the molecules are non-
chiral. Switchable structures with preferable antiferroelectric
(AF) ordering can be formed. Among bent-shaped mesophases
the most frequently studied is the B2 phase with the tilted bent-
shaped molecules organized in layers, giving rise to the in-layer
polar axis and, therefore, to ferro- or antiferroelectric proper-
ties. Bent-shaped materials have also been found to show
nematic and columnar phases. The nematic phase created by
bent-shaped mesogens attracts attention due to the potential
biaxiality.6,7
aDepartment of Organic Chemistry, Institute of Chemical Technology, CZ-166 28
Prague 6, Czech Republic. E-mail: Jiri.Svoboda@vscht.cz
bInstitute of Physics, Academy of Science of the Czech Republic, Na Slovance 2, CZ-182
21 Prague 8, Czech Republic. E-mail: novotna@fzu.cz
cLaboratory of Dielectrics and Magnetics, Chemistry Department, Warsaw University,
Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc31593b
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structural character, hockey-stick-like mesogens can exhibit
The synthesis of compounds 1, and 2, and target compounds
standard calamitic (rod-like) mesophases, such as nematic (N), A/m/n, including their detailed molecular structure, all inter-
smectic SmA and SmC phases and/or mesophases typical for mediates, and their physico-chemical characterization, is
bent-shaped mesogens. Being on the border-line between rod- summarized and available in the ESI.† The synthesis of acids 3,
like and bent-shaped mesogens, hockey-stick-like mesogens 4 and 5 was reported elsewhere.8,9
represent the most developing eld in liquid crystal research
and are a subject of considerable interest.21–27
3. Measurements
Herein, we present a synthesis and mesomorphic study of a
series of materials with an azo group joined directly to the Phase transition temperatures were determined by differential
central benzene, creating a 3,40-dihydroxyazobenzene core. We scanning calorimetry (DSC) using a Perkin-Elmer 7 (Pyris Dia-
have chosen this type of molecular core as being rigid enough to mond) calorimeter. The samples of about 2–5 mg were
avoid conformational changes at the central part. We have hermetically sealed in aluminium pans and placed into the
varied the length of both molecular arms by inserting an calorimeter chamber, which was lled with nitrogen during the
additional number of benzene rings. We have prepared a series measurement. Temperature and enthalpy values were cali-
of materials, the design of which enabled us to change the brated using the extrapolated onset temperatures and enthalpy
overall molecular shape from rod-like to bent-core. The types of changes for water, indium and zinc. Calorimetric measure-
mesophases and their physical properties were monitored by ments were performed on cooling and heating runs at a rate of
optical microscopy studies, X-ray diffraction analysis, DSC and 5 K minꢀ1
.
electro-optical measurements.
The sequence of phases and phase transition temperatures
were determined from the optical textures and their changes,
observed under a polarizing optical microscope (POM) Nikon
Eclipse E600Pol. The planar cells for texture observation were
prepared from glasses with ITO transparent electrodes (5 ꢁ
5 mm2), without any surface treatment. The thickness was
dened by Mylar sheets as 12 mm. A commercial cell (AWAT
Comp., Warsaw) with a surfactant and thickness of 6 mm was
applied for the electro-optical studies. The cells were lled in
the isotropic phase with studied materials by capillary action.
The Linkam LTS E350 heating/cooling stage with a TMS
93 temperature regulator was used for the temperature control
and stabilization within ꢂ0.1 K.
2. Synthesis
The synthesis of the target compounds A/m/n is depicted in
Scheme 1. It is based on acylation of the phenols 1 or 2, with
acid chlorides, which were prepared in situ by the reaction with
oxalyl chloride with acids 3, 4 or 5 under standard conditions.
The length of the terminal alkyl chains was adjusted to dodecyl
(C12H25). While the central core 1 served for the synthesis
of compounds A/m/n, both with identical lengths of the arms
(m ¼ n) and non-symmetrical materials (m s n), the central core
2 was utilized for preparation of materials A/0/n. The number m
(the le arm) varies from 0 to 3, the number n (the right arm)
can be 1, 2 or 3. The overall number of benzene units grows
from 3 (A/0/1) to 8 (A/3/3).
Switching studies were performed using a driving voltage
from a Phillips generator PM 5191, accompanied by a linear
amplier, providing a maximum amplitude of 120 V. The
switching current vs. time prole was recorded with a Tektronix
memory oscilloscope DPO4034.
X-ray diffraction (XRD) studies were performed using a
Bruker Nanostar system (CuKa radiation, Vantec 2000 area
detector, MRI TCPU H heating stage) working in transmission
mode and a Bruker D8 Discover system (CuKa radiation, scin-
tillation counter, DCS350 heating stage) working in reection
mode. In both systems the temperature stability was 0.1 K.
Powder samples (for Nanostar) were prepared in thin-walled
glass capillaries (1.5 mm diameter), aligned samples for
experiments in reection mode were prepared as thin lms on a
heated silicon wafer.
4. Results
DSC studies were performed for all prepared compounds. The
phase transition temperatures and associated enthalpy changes
are summarized in Table 1. The DSC thermographs are shown
in Fig. 1a–c for representative materials A/0/2, A/1/3 and A/2/3,
respectively. Phases and phase sequences have been identied
by texture observation under a polarizing optical microscope
and conrmed by X-ray measurements (see below). With the
exception of non-mesogenic A/0/1, A/1/1 and A/1/2, all other
Scheme 1 Synthesis route of materials A/m/n.
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Table 1 Phase transition temperatures, Ttr, in ꢃC and corresponding enthalpies, DH, were recorded for the second cooling. Melting points, m.p., were recorded for the
second heating. All experiments were performed at a rate of 5 K minꢀ1. The enthalpies are presented in brackets in kJ molꢀ1. The letter i denotes the overall number of
benzene rings in the molecule
A/m/n
i
M.p. (DH)
Ttr (DH)
M3
Ttr (DH)
M2
Ttr (DH)
M1
Ttr (DH)
Iso
A/0/1
A/0/2
A/0/3
A/1/1
A/1/2
A/1/3
A/2/1
A/2/2
A/2/3
A/3/1
A/3/2
A/3/3
3
4
5
4
5
6
5
6
7
6
7
8
62 (+71.0)
86 (+94.5)
104 (+34.4)
84 (+49.4)
121 (+63.8)
131 (+40.9)
115 (+50.2)
111 (+49.3)
132 (+32.8)
118 (+27.9)
131 (+50.2)
151 (+31.6)
51 (ꢀ65.7)
63 (ꢀ68.5)
99 (ꢀ29.7)
66 (ꢀ47.0)
116 (ꢀ42.9)
123 (ꢀ39.5)
91 (ꢀ30.1)
69 (ꢀ15.4)
113 (ꢀ29.7)
102 (ꢀ25.8)
113 (ꢀ29.5)
136 (ꢀ31.4)
—
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
SmCA
108 (ꢀ0.02)
SmC
SmC
139 (ꢀ0.35)
223 (ꢀ0.25)
SmA
SmA
ꢀ
145 (ꢀ6.0)
234 (ꢀ3.04)
ꢀ
SmCA
158 (ꢀ0.08)
SmC
189 (ꢀ0.74)
138 (ꢀ0.69)
N
191 (ꢀ1.35)
112 (ꢀ20.1)
156 (ꢀ19.1)
193 (ꢀ18.5)
154 (ꢀ19.1)
217 (ꢀ21.3)
245 (ꢀ12.3)
SmCAPA
B1
B1
B1
B1
SmCAPA
B1
which the molecules can be considered as rod-like or hockey-
stick-like type, form calamitic mesophases. For all compounds
with m $ 2 bent-shaped mesophases are observed.
Mesogens A/0/2, A/0/3 and A/1/3 exhibit the nematic (N),
smectic A (SmA), synclinic smectic C (SmC) and anticlinic
smectic C (SmCA) phases (Table 1). Marble-like and fan-shaped
textures are observed in the nematic and smectic phases,
respectively. For A/1/3 a planar texture in the nematic, SmC and
SmCA phase, revealed in a commercial cell (6 mm) upon cooling
from the isotropic phase, is shown in Fig. 2a–c, respectively.
Surprisingly, rather complicated fan-shaped textures for the
tilted SmC phase were observed when we increased the thick-
ness of the cell. We can identify a twist in the broken fan-shaped
texture (see Fig. 3 for A/0/3). Domains with twisted structures
have been found, which provides evidence for the separation of
molecular conformers differing in axial chirality. The cell
thickness is very important, for thin enough samples the
uniform organization of the molecules through the cell prole
is more advantageous, whereas for thicker samples the twisted
structure is preferred because the twist elastic energy dominates
over the surface energy.
X-ray measurements conrmed the mesophase identica-
tion of A/0/2, A/0/3 and A/1/3. In the nematic phase of the A/1/3
compound a weak diffused signal in a small angle range was
recorded, which can be attributed to a mean intermolecular
distance along the long molecular axis (molecular length). In
the smectic phases a sharp peak was detected in a small-angle
diffraction range and a diffuse scattering maximum in the wide
angle region. This provides evidence for a lamellar structure
with liquid-like molecular packing (without in-plane order)
within the layers. From the small angle reections the layer
spacing, d, was evaluated for all calamitic mesogens A/0/2, A/0/3
and A/1/3 (Fig. 4). A slight increase of d was found in the SmA
phase upon cooling. Such behaviour is oen observed in the
Fig. 1 Temperature dependencies of the heat flow measured by DSC for
selected compounds (a) A/0/2, (b) A/1/3 and (c) A/2/3. The phase transitions
which are not properly recognizable are marked by arrows and presented in
enlarged views in the insets.
compounds exhibited the formation of at least one mesophase. SmA phase and can be explained by an increase of the long
All observed mesophases have enantiotropic character except molecular axis ordering and/or by stretching of the aliphatic
for A/0/1 (where the SmCP phase is monotropic, i.e. is observed chains. An abrupt decrease in d(T) at the N–SmC and SmA–SmC
only on cooling). As follows from Table 1, the studied materials phase transitions reects the tilt of the molecules. The SmC–
can be divided into two groups. Compounds with m ¼ 0 or 1, in SmCA phase transition is accompanied by a slight jump of d(T)
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Fig. 3 Photo of the planar texture for A/0/3 in the SmC phase at T ¼ 150 ꢃC (the
cell thickness is 12 mm).
Fig. 4 Temperature dependencies of the layer spacing, d, recorded upon cooling
for denoted compounds. Phases are indicated.
the application of an electric eld with an intensity of about
20 V mmꢀ1 a fan-shaped texture with a low birefringence could
be observed under crossed polarizers. Under the electric eld
the birefringence of the former fan-shaped pattern signicantly
increases. Additionally, under the applied electric eld the
extinction position rotates from the orientation parallel to the
polarizers by an angle of about 45ꢃ. The change of the extinction
position is characteristic of the transformation from an anti-
clinic to synclinic tilt structure under the electric eld. Switch-
ing properties and dielectric measurements conrm the
antiferroelectric character. Two distinct peaks per half-period of
the applied electric eld within the polarization current prole
Fig. 2 Planar textures for compound A/1/3 in the (a) nematic, (b) SmC and (c)
anticlinic SmCA phase. Observations were made using a commercial planar cell of
6 mm.
and an increase of the X-ray peak intensity. In spite of the were measured (see Fig. 5), which is typical for the antiferro-
continuous tilting of molecules, d values saturate and then start electric phase. Therefore the mesophase can be attributed to the
to grow on further cooling. It is probable that the stretching of SmCAPA phase. The spontaneous polarization value Ps ¼ 380 nC
the aliphatic chains becomes predominant. When comparing cmꢀ2 detected at T ¼ 110 C slightly grows within the temper-
ꢃ
the studied mesogens, one can see that d values are propor- ature interval of the SmCAPA phase. Unfortunately, the studied
tional to the molecular length (Fig. 4).
mesophase has a monotropic character and the crystallization
Starting from the compound A/2/1, the mesophases typical starts when the electric eld is applied. From the small angle
for bent-core molecules appear (Table 1) when lengthening X-ray measurements the layer spacing, d, was evaluated, which
˚
molecular arms. A switchable lamellar SmCP phase has been was found (d z 35.8 A) to be almost temperature independent
observed for compound A/2/1. A complicated texture with tiny within the whole temperature range of the SmCAPA phase.
features appeared upon cooling from the isotropic phase. Aer When comparing the approximate length of the molecule and
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Table 2. For all compounds the a parameter, which reects the
block cross-section length, slightly decreases upon cooling,
while the b parameter, which can be attributed to the molecular
length, slightly increases. As for the calamitic molecules, this
can be explained by a stretching of the aliphatic chains.
An uncommon phase sequence (with a lamellar below a
columnar phase upon cooling) was found for compound A/2/2.
The higher temperature LC mesophase, appearing upon cooling
from the isotropic phase, was revealed to be a columnar B1
mesophase. Textures, observed under the polarizing micro-
scope, are typical for such a phase, with colourful domain-like
and mosaic character (Fig. 6). An applied electric eld does not
affect the texture, which is a typical feature of a B1 phase. In the
B1 phase, XRD measurements conrmed a 2D modulated
structure (see Table 2). Upon further cooling, at a temperature T
¼ 138 ꢃC, a phase transition was detected by DSC measure-
ments, however, the mosaic-like texture of the B1 phase
changed only slightly. Nevertheless, under the applied electric
eld the planar texture changed to the texture typical for a
Fig. 5 Switching current profile, detected in the B2 phase (T ¼ 110 ꢃC) of A/2/1.
the d value, it is evident that molecules in layers are tilted.
Comparing the POM observation, switching properties and
X-ray data we can conclude that the observed lamellar phase can
be identied only with the SmCAPA phase.
Compounds A/2/3, A/3/1, A/3/2 and A/3/3 form a columnar
mesophase of B1 type, which is built of smectic layer fragments
(blocks). The X-ray patterns of the B1 phase were analysed
assuming a centred rectangular crystallographic unit cell. Two
signals, recorded in a small angle range, were indexed as (11)
and (02). The obtained unit cell parameters are summarized in
ꢃ
SmCP phase (Fig. 6) below T ¼ 138 C. Comparing the texture
under the eld (Fig. 6c) and the texture when the eld is
switched off (Fig. 6d), one can see that the extinction brushes
(regions, where the optical axis is either parallel or perpendic-
ular to the polarization of the incident light when observed in
crossed polarizers) rotate by an angle approaching 45ꢃ. Such
induced changes are characteristic for the transition from the
SmCAPA (without an applied electric eld) to the SmCSPF phase
(under an electric eld), similar to what we observed for A/2/1.
When the eld is switched off, the low-birefringent texture of
the SmCAPA phase is present and persists until the crystalliza-
tion upon cooling or up to the phase transition to the B1 phase,
Table 2 Cell parameters in the columnar B1 phase
T/ꢃC
˚
a/A
˚
b/A
A/2/2
150
145
142
180
130
140
120
215
140
230
200
150
41.0
41.0
40.9
41.4
40.2
41.7
41.0
43.1
41.4
44.8
44.5
43.3
52.2 which takes place at about T ¼ 139 ꢃC upon the subsequent
52.3
52.3
57.8
58.7
heating (Fig. 7). When integrating the polarization current
prole (having two peaks per half-period) we obtained the
A/2/3
A/3/1
A/3/2
A/3/3
spontaneous polarization value Ps ¼ 300 nC cmꢀ2. Structural
parameters of the studied mesophase were established from the
52.9
53.4 X-ray diffraction data (Fig. 8). Below the phase transition at T ¼
138 ꢃC only commensurate peaks were detected in the small
57.0
58.4
61.8
62.6
63.6
angle region, conrming a lamellar structure of the studied low-
˚
temperature mesophase. The layer spacing, d ¼ 38.5 A, is
almost temperature independent. By comparison with the
Fig. 6 Planar textures, observed for A/2/2 (a) at the isotropic–B1 phase transition, (b) in the B1 phase at T ¼ 150 ꢃC, (c) in the SmCAPA phase at T ¼ 128 ꢃC under an
electric field of about 15 V mmꢀ1 and (d) when the field is switched off. Arrows show the orientation of the polarizer and the analyzer.
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found in a thick planar cell (Fig. 3), which provides evidence for
the separation of conformers based on the axial chirality. The
separation of conformers arises spontaneously, but polar
surface interactions might enhance this process. The coexis-
tence of various types of uniform and twisted domains is a
result of the agreement of their energies. For non-chiral rod-like
compounds with an ester linkage group such a phenomenon
has been found and described theoretically.27 The lack of elec-
tro-optical response under the applied electric eld in the
studied compounds can be explained by the weakness of such a
polar effect. We can conclude that a deviation from the molec-
ular linearity and/or symmetry can support a chiral separation.
For mesogens with a lengthened le arm (m $ 2), bent-
shaped mesophases were observed, namely the non-switchable
columnar B1 phase (for A/2/3, A/3/1, A/3/2 and A/3/3) and the
polar SmCAPA phase for A/2/1. Surprisingly, for the bent-core
mesogen A/2/2 we detected an unusual phase sequence:
columnar B1–lamellar SmCAPA phase upon cooling from the
isotropic phase. The presence of a less ordered phase below a
more ordered one upon cooling can be observed in liquid
crystals due to a large number of competing interactions. The
phase sequence of a tilted lamellar phase below the columnar
B1 phase has been reported previously.5,28 Such a phenomenon
was observed several times in different bent-core mesogens5
with ester linking groups in the bending arms as well as for a
mesogen with a cinnamonic group28a inserted between the
carboxyl group and the outer phenyl rings. A structural study of
a B1–SmCP phase transition has been published by Folcia
et al.28c for a bent-core mesogen with a biphenyl central core. It
seems that for all compounds the character and orientation of
the linking groups play an important role. One can explain such
an uncommon sequence with conformational changes in
molecular arms being under way upon cooling, probably
determined by the arrangement of the linkage groups. As was
explained by Coleman et al.29 the B1 phase is driven by a
polarization splay connected with a defect array. In the B1
phase, the layers are displaced at the defect boundaries.
Fig. 7 Planar texture of A/2/2 at the B2–B1 phase transition, observed upon
heating from the B2 phase. In the B2 phase the sample was subjected to an electric
field for several minutes and then the field was switched off. The upper right
corner is still in the SmCAPA phase and the rest is transformed into the B1 phase.
Fig. 8 For A/2/2 the X-ray intensity, Int., versus the corresponding scattering
angle, recorded at the temperature T ¼ 145 ꢃC (black line) in the B1 phase and T ¼
130 ꢃC (red line) in the SmCAPA phase. In the inset (upper right corner) the
temperature evolution of the X-ray intensity is presented, starting from
the isotropic phase. The colour scale corresponds to the X-ray intensity (blue is for
the most intense signal).
˚
estimated length of the molecule (52.3 A), the tilt angle can be
evaluated as 43ꢃ, which agrees well with the angle of the
extinction brushes rotation under the applied voltage (Fig. 6).
Comparing the X-ray data and the POM observations, we can
conclude that the low-temperature phase corresponds to the
SmCAPA phase.
5. Discussion and conclusions
We have synthesized 3,40-disubstituted azobenzene-based
liquid crystalline compounds, denoted as A/m/n. The aim is to
compare the mesomorphic properties with respect to the length
of the constituent arms, lengthening the central molecular core.
Compounds A/0/1, A/1/1 and A/1/2 do not form any mesophase.
It is probable that the self-assembling process is not possible for
the studied molecules with a short arm in combination with an
asymmetrical bent molecular core. Compounds A/0/2, A/0/3 and
A/1/3 with only partially asymmetrical bent shapes exhibit
calamitic mesophases with no tendency for switching and/or
other electro-optical effects under the applied electric eld.
Fig. 9 Schematic organization of blocks (in the cross-section view) of a columnar
B1 phase: (a) and (b) are two possibilities, which cannot be distinguished
between; (c) anticlinic and antiferroelectric organization of the tilted molecules in
Nevertheless, the domains with the twisted structure were the neighbouring layers for the SmCAPA phase.
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Nevertheless, with the conventional non-resonant XRD we are
not able to describe all the details about the molecular
arrangement in the studied columnar B1 phase.30 We can
propose a simple scheme to show how the molecules can be
organized in a columnar phase (Fig. 9a and b show two alter-
native arrangements in the B1 phase, which cannot be recog-
nized by X-ray analysis). Molecular ribbons can slide and at the
phase transition the shi between them can disappear so the
structure degenerates into a layered system (Fig. 9c). According
to the switching behaviour and electro-optical properties the
lamellar phase (conrmed by XRD results) can be unambigu-
ously attributed to the SmCAPA phase (Fig. 9c).
It is worth mentioning that a phase transition from the
undulated/modulated ground structure to a switchable SmCP
phase can be also induced by an electric eld.31 We did not
observe any textural modication under the applied electric
eld in the upper columnar phase (above T ¼ 138 ꢃC). We
observed a clear phase transition from the columnar to the
lamellar structure upon cooling by XRD methods and the clear
anomaly was also detected by DSC. We expect that in our case
the layer undulation is more sensitive to the temperature
change than to the applied electric eld.
References
1 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and
H. Takezoe, J. Mater. Chem., 1996, 6, 1231–1234.
2 R. Amaranatha Reddy and C. Tschierske, J. Mater. Chem.,
2006, 16, 907–961.
3 H. Takezoe and Y. Takanishi, Japanese Journal of Applied
Chemistry, 2006, 45, 597–625.
4 W. Weissog, H. N. Shreenivasa Murthy, S. Diele and
G. Pelzl, Philos. Trans. R. Soc., A, 2006, 364, 2657–2679.
5 W. Weissog, G. Naumann, B. Kosata, M. W. Schroder,
A. Eremin, S. Diele, Z. Vakhovskaya, H. Kresse,
R. Friedemann, S. Ananda Rama Krishnan and G. Pelzl,
J. Mater. Chem., 2005, 15, 4328–4337.
6 C. H. Keith, A. Lehmann, U. Baumeister, M. Prehm and
C. Tschierske, So Matter, 2010, 6, 1704–1721.
7 L. A. Madsen, T. J. Dingemans, M. Nakata and E. T. Samulski,
Phys. Rev. Lett., 2004, 92, 145505.
8 M. Kohout, J. Svoboda, V. Novotna, D. Pociecha,
M. Glogarova and E. Gorecka, J. Mater. Chem., 2009, 19,
ˇ
¨
´
´
3153–3160.
´
´
9 M. Kohout, J. Svoboda, V. Novotna, M. Glogarova and
D. Pociecha, Liq. Cryst., 2010, 37, 987–996.
Photosensitivity due to the presence of an azo-unit was also
found in the studied compounds. In the solution of the studied
molecules the trans–cis isomerization takes place under illu-
mination with UV-light. We have not studied light induced
changes in the mesophases systematically. We are aware of the
fact that the alignment of the molecules can be disturbed when
we induce the trans–cis isomerization process by strong UV light
in the mesophase. We intend to use the studied molecules as a
photosensitive dopant and this issue will be studied elsewhere.
All lengthened molecules, A/2/3, A/3/1, A/3/2 and A/3/3, form
the columnar B1 mesophase showing a rectangular cell with a
similar dimension in the ribbon cross-section. The compounds
differ only in the b parameter (Table 2), which corresponds to
the molecular length. For bent-shaped mesogens the lamellar
phases are preferred when increasing the length of the terminal
alkyl chain.11,28d In our case, the lengthening of the arms is
caused by adding a phenyl ring that substantially changes the
character of the molecular packing. More probably, the packing
character is then more determined by a dipole moment and its
orientation with respect to the whole molecular shape.
To summarize the results, we present how the overall
molecular shape inuences the type of self-organization.
Hockey-stick molecules with strong asymmetry between the
arms exhibit only calamitic mesophases. Lengthening the
molecular arms leads to the restriction of the molecular rota-
tion around the long axis. It results in the close packing of the
molecules and thus mesophases typical for bent-core systems
can be observed. We can conrm that the mesoscale organiza-
tion strictly depends upon the molecular shape.
´
10 M. Kohout, J. Svoboda, V. Novotna and D. Pociecha, Liq.
Cryst., 2011, 38, 1099–1110.
11 G. Pelzl, S. Diele and W. Weissog, Adv. Mater., 1999, 11,
707–724.
12 K. C. Majumdar, P. K. Shyam and S. Chakravorty, Liq. Cryst.,
2010, 37, 1237–1243.
13 V. Prasad, S.-W. Kang and S. Kumar, J. Mater. Chem., 2003,
13, 1259–1264.
´
14 V. Prasad and A. Jakli, Liq. Cryst., 2004, 31, 473–479.
15 K. Gomola, L. Guo, D. Pociecha, F. Araoke, K. Ishikawa and
H. Takezoe, J. Mater. Chem., 2010, 20, 7944–7952.
´
ˇ
´
16 V. Kozmık, M. Horcic, J. Svoboda, V. Novotna and
D. Pociecha, Liq. Cryst., 2012, 39, 943–955.
17 C. L. Folcia, I. Alonso, J. Ortega, J. Etxebarria, I. Pintre and
M. Blanca Ros, Chem. Mater., 2006, 18, 4617–4626.
18 I. C. Pintre, N. Gimeno, J. L. Serrano, M. Blanca Ros,
I. Alonso, C. L. Folcia, J. Ortega and J. Etxebarria, J. Mater.
Chem., 2007, 17, 2219–2227.
19 N. G. Nagaveni, A. Roy and V. Prasad, J. Mater. Chem., 2012,
22, 8948–8959.
20 (a) M. Vijaysrinivasan, P. Kannan and A. Roy, Liq. Cryst.,
2012, 39, 1465–1475; (b) N. G. Nagaveni, V. Prasad and
A. Roy, Liq. Cryst., 2013, 40, 1001–1015.
21 N. G. Nagaveni, P. Raghuvanshi, A. Roy and V. Prasad, Liq.
Cryst., 2013, 40, 1001–1005.
22 (a) R. Stannarius, J. Li and W. Weissog, Phys. Rev. E: Stat.,
Nonlinear, So Matter Phys., 2003, 90, 025502; (b) B. Das,
¨
S. Grande, W. Weissog, A. Eremin, M. W. Schroder,
G. Pelzl, S. Diele and H. Kresse, Liq. Cryst., 2003, 30, 529–
539.
23 A. Chakraborty, B. Das, S. Findeisen-Tandel, M.-G. Tamba,
U. Baumeister, H. Kresse and W. Weissog, Liq. Cryst.,
2011, 38, 1085–1097.
Acknowledgements
This work was supported by the Czech Science Foundation
(projects P204/11/0723 and 13-14133S) and the Academy of
Sciences of the Czech Republic (project M100101211).
24 F. C. Yu and L. J. Yu, Chem. Mater., 2006, 18, 5410–5420.
7566 | J. Mater. Chem. C, 2013, 1, 7560–7567
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
ˇ
´
´
25 V. Novotna, J. Zurek, V. Kozmık, J. Svoboda and 29 D. A. Coleman, C. D. Jones, M. Nakata, N. A. Clark,
´
M. Glogarova, Liq. Cryst., 2008, 35, 1023–1036.
D. M. Walba, W. Weissog, K. Fodor-Csorba,
J. Watanabe, V. Novotna and V. Hamplova, Phys. Rev. E:
Stat., Nonlinear, So Matter Phys., 2008, 77,
021703.
26 (a) S. Radhika, H. T. Srinivasa and B. K. Sadashiva, Liq.
Cryst., 2011, 38, 785–792; (b) P. Sathyanarayana,
S. Radhika, B. K. Sadashiva and S. Dhara, So Matter,
2012, 8, 2322–2327.
27 M. Glogarova, F. Hampl, L. Lejcek, V. Novotna, J. Svoboda
and M. Cigl, J. Chem. Phys., 2010, 133, 221102.
30 C. L. Folcia, J. Ortega, J. Etxebarria, L. Dong Pan, S. Wang,
C. C. Huang, V. Ponsinet, P. Barois, R. Pindak and
N. Gimeno, Phys. Rev. E: Stat., Nonlinear, So Matter Phys.,
2011, 84, 010701.
28 (a) K. Gomola, L. Guo, S. Dhara, Y. Shimbo, E. Gorecka,
D. Pociecha, J. Mieczkowski and H. Takezoe, J. Mater. 31 (a) R. Amaranatha Reddy, U. Baumeister, J. Lorenzo Chao,
Chem., 2009, 19, 4240–4247; (b) G. Pelzl, M. G. Tamba,
S. Findeisen-Tandel, M. W. Schroeder, U. Baumeister,
S. Diele and W. Weissog, J. Mater. Chem., 2008, 18, 3017–
3031; (c) C. L. Folcia, J. Etxebaria, J. Ortega and M. B. Ros,
Phys. Rev. E: Stat., Nonlinear, So Matter Phys., 2006, 74,
031702; (d) H. N. Shreenivasa Murthy and B. K. Sadashiva,
Liq. Cryst., 2004, 31, 1347.
H. Kresse and C. Tschierske, So Matter, 2010, 6,
3883–3897; (b) J. Ortega, C. L. Folcia, J. Etxebarria,
J. Martinez-Perdiguero, J. A. Gallastegui, P. Ferrer,
N. Gimeno and M. Blanca Ros, Phys. Rev. E: Stat.,
Nonlinear, So Matter Phys., 2011, 84, 021707; (c)
J. Kirchhoff and L. S. Hirst, Appl. Phys. Lett., 2007, 90,
161905.
This journal is ª The Royal Society of Chemistry 2013
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