Reentrant SmCPA phases: Unusual polymorphism variant SmA-SmCSPA-Colob-SmCSPA observed in new bent-core mesogens

Article abstract of DOI:10.1039/b516189d

A new homologous series of achiral five-ring bent-core mesogens is presented. The mesophase behaviour has been studied by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical measurements. The homologues with shorter terminal chains (C₈, C ₁₂) form an SmCP_A phase, homologues with longer chains (C₁₄, C₁₆, C₁₈) show liquid-crystalline tetramorphism with a sequence SmA-SmC_SP_A-Col _ob-SmC_SP_A. The SmCP_A phases are structurally identical and differ only in the mechanism of polar switching. The switching of the high-temperature SmC_SP_A phase takes place through the collective rotation of the molecules around their long axes whereas in the reentrant SmC_SP_A phase the switching is based on the director rotation around the tilt cone. By the application of an electric field the Col_ob phase can be irreversibly transformed to an SmCP _A phase. In a limited temperature range the SmA phase shows a reversible field-induced transition into the SmC_SP_F phase. The electro-optical response of the mesophases is discussed on the basis of the structural features of these phases. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516189d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Reentrant SmCPA phases: unusual polymorphism variant SmA–SmCSPA–
Colob–SmCSPA observed in new bent-core mesogens{
H. N. Shreenivasa Murthy, M. Bodyagin, S. Diele, U. Baumeister, G. Pelzl and W. Weissflog*
Received 15th November 2005, Accepted 9th January 2006
First published as an Advance Article on the web 7th February 2006
DOI: 10.1039/b516189d
A new homologous series of achiral five-ring bent-core mesogens is presented. The mesophase
behaviour has been studied by polarizing microscopy, differential scanning calorimetry, X-ray
diffraction and electro-optical measurements. The homologues with shorter terminal chains (C8,
C12) form an SmCPA phase, homologues with longer chains (C14, C16, C18) show liquid-crystalline
tetramorphism with a sequence SmA–SmCSPA–Colob–SmCSPA. The SmCPA phases are
structurally identical and differ only in the mechanism of polar switching. The switching of the
high-temperature SmCSPA phase takes place through the collective rotation of the molecules
around their long axes whereas in the reentrant SmCSPA phase the switching is based on the
director rotation around the tilt cone. By the application of an electric field the Colob phase can be
irreversibly transformed to an SmCPA phase. In a limited temperature range the SmA phase
shows a reversible field-induced transition into the SmCSPF phase. The electro-optical response of
the mesophases is discussed on the basis of the structural features of these phases.
layer in the so-called racemic states (SmCSPA, SmCAPF),
1. Introduction
whereas the layer chirality is uniform within the macroscopic
Bent-core mesogens have been the subject of extensive
domains in the homochiral states (SmCAPA, SmCSPF). It
investigations in the last years because they can form new
should be noted that another way of avoiding bulk polariza-
mesophases with unusual properties which are different from
tion in layered structures of bent-core molecules is to break the
those of calamitic mesogens. Due to their intrinsic shape the
layers into a 2D modulated structure as described in columnar
B1 phases3–8 or by splay polarization in the smectic layers.9
molecules can be packed in a polar fashion, which leads to a
long-range correlation of the lateral dipoles. The polar packing
In this paper we report new results, emphasizing the com-
of the bent molecules leads to ferroelectric or antiferroelectric
plexity of the mesophase behaviour in a homologous series of
properties.1,2 The most frequently studied phase derived from
bent-core mesogens. We found that on decreasing the tempera-
bent molecules is the SmCP phase where P stands for ‘‘polar’’.
ture the member with dodecyloxy terminal chains shows a
As first shown by Link et al.2 the structure of the SmCP phase
reversible field-induced transition from the homochiral
depends on the stacking of the bent molecules in adjacent
smectic layers. The molecules can have a synclinic or an anti-
clinic interlayer correlation and are denoted by the subscripts S
and A after the C. With respect to the direction of the polar
vector in adjacent layers the synclinic and anticlinic packing
can form ferro- or antiferroelectric structures which are
denoted by the suffixes F or A after the P. In this way we
can distinguish four possible structures which are presented in
Fig. 1: SmCSPA, SmCAPA, SmCSPF and SmCAPF. In order to
avoid bulk polarization, antiferroelectric ground states exist
in most cases and can be switched to the corresponding
ferroelectric states.2 Another aspect of fundamental interest is
the combination of director tilt and polar order in the smectic
layers of these bent-core compounds. This combination leads
to chirality of the smectic layers although the constituent
molecules are achiral.2 The chirality alternates from layer to
SmCAPA to the racemic SmCSPA phase. The homologues
with longer terminal chains exhibit a new polymorphism
with an unusual sequence of phases—SmA–SmCSPA–Colob
–
SmCSPA—and an interesting electro-optical behaviour. The
Martin-Luther-Universitaet Halle-Wittenberg, Institut fuer
Physikalische Chemie, Muehlpforte 1, 06108 Halle (Saale), Germany.
E-mail: wolfgang.weissflog@chemie.uni-halle.de; Fax: +49 345 5527157;
Tel: +49 345 5525256
{ Electronic supplementary information (ESI) available: Experimental
procedure for compound 18 and analytical data of all final products: 8,
12, 14, 16, 18 and 16-Br. See DOI: 10.1039/b516189d
Fig. 1 Possible orientations of the direction of the tilt and the polar
axes of bent-core molecules in adjacent layers leading to chiral or
racemic ferroelectric and antiferroelectric SmCP phases. Full and open
molecule symbols designate layers of opposite chirality; the signs fl [
indicate opposite bend direction (opposite polar axes).
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experimental results are discussed on the basis of intermole-
cular interactions.
ITO cells (E. H. C. Corp.; spacing: 5 mm or 6 mm). The
switching polarization was measured by means of the
triangular wave voltage method.10
2. Experimental
3. Materials
The thermal behaviour (transition temperatures and transition
enthalpies) was determined using a differential scanning
calorimeter (Perkin-Elmer Pyris 1). The optical textures and
the field-induced texture changes were examined using a
polarizing microscope (Leitz Laborlux) equipped with a
Linkam hotstage (THM 600/S). X-Ray diffraction measure-
ments of non-oriented samples were performed with a Guinier
film camera (Huber, Germany) or a small angle camera with a
linear position sensitive detector (M. Braun GmbH, Germany)
(samples in glass capillaries with a diameter of 1 mm on a
temperature controlled heating stage). X-Ray studies on
oriented samples were carried out with a 2D detector (HI-
Star, Siemens AG). Oriented samples were obtained by a long
annealing of a drop of the liquid crystal on a glass plate after
very slow cooling of the isotropic liquid. In this case the
smectic layers could be oriented parallel to the substrates and
the incident X-ray beam was parallel to the smectic layers.
Electro-optical measurements were carried out in commercial
The homologous five-ring bent-core mesogens contain
4-chloro- or 4-bromoresorcinol as the central core and ester
moieties as connecting groups between the aromatic rings
(see Table 1). The synthetic approach to prepare the desired
bent-core compounds is shown in Scheme 1. 4-Chloro- and
4-bromoresorcinol are commercial products. Monobenzyl
terephthalate A was synthesized by esterification of 4-carboxy-
benzaldehyde followed by oxidation using sodium chlorite.
4-n-Alkoxyphenols B were prepared by the alkylation of
4-benzyloxyphenol with the corresponding alkyl bromides
and deprotection by means of hydrogen in the presence of
5% Pd–C. Monobenzyl terephthalate A was esterificated with
the corresponding 4-n-alkoxyphenols B in the presence of
DCC to give the intermediates C, and their deprotection by
means of ammonium formate and 5% Pd–C resulted in acids
D. The final compounds 8, 12, 14, 16, 18 and 16-Br were
prepared by the condensation of the substituted resorcinols E
Scheme 1 Synthetic route.
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Table 1 Transition temperatures (uC) and associated enthalpy values (italic) (kJ mol21) for compounds 8, 12, 14, 16 and 18
Compound
n
Cr
SmCSPA
Colob
SmCSPA
SmCAPA
—
SmA
I
N
8
12
14
16
18
a
8
N
N
N
N
N
113.0
20.7
110.0
50.0
105.0
60.3
108.0
74.9
N
145.0
16.0
—
—
N
—
—
—
—
N
12
14
16
18
N
—
140.0a
N
147.0
18.2
N
N
—
N
—
N
—
134.0a
130.0a
123.0a
138.0
0.8
135.0
1.5
128.0
1.4
N
N
N
139.5
—
—
—
144.0
8.8
145.5
8.6
142.5
8.7
N
N
N
2.2
138.5
2.7
134.0
3.1
N
N
107.0
71.9
N
N
The enthalpy for this transition could not be determined.
with compounds D. The experimental procedure to prepare
compound 18 is given together with the analytical data of all
final products in the ESI.{
The transition temperatures and the associated enthalpies
for the five compounds derived from 4-chlororesorcinol are
given in Table 1.
clearly smaller than the molecular length L assuming a bending
˚
˚
angle of 120u (L is 47 A for compound 8 and 54 A for
compound 12). This indicates the tilted arrangement of the
bent-core molecules in the smectic layers. Using the relation
d/L = cos a the tilt angle a could be estimated and was found to
be 25u for compound 8 and 28u for compound 12.
In order to prove the polar character of the tilted smectic
phase, detailed electro-optical investigations have been per-
formed. The smectic phase of compound 8 shows two
polarization current peaks per half period of a triangular
voltage indicating an antiferroelectric ground state. No change
in the extinction direction was observed when the polarity of
the applied field was interchanged. This indicates that the
ground state of the SmCPA phase is synclinic (racemic), i.e.
on applying an electric field the transition from SmCSPA to
SmCAPF could be achieved. The switching polarization current
measured by integrating the area under the current curve is
430 nC cm22. The smectic phase of compound 12 could also
be identified as an SmCPA phase and the polarization was
found to be 355 nC cm22. The polarization value did not
change markedly as the temperature was lowered. In contrast
In addition, we have also prepared one compound with a
bromine substituent in the 4-position of the central core and
hexadecyloxy terminal chains (compound 16-Br). The meso-
phase sequence is the same when compared with the analogous
4-chloro substituted compound (16) but with a reduction of all
transition temperatures by about 10–15 K which is obviously
due to the increase in the size of the lateral substituent from
chlorine to bromine.
4. Results
Compounds 8 and 12
On cooling the isotropic phase of compound 8 a fan-shaped
texture with irregular stripes was observed—a photomicro-
graph is given in Fig. 2. The same textural features have also
been observed in compound 12.
X-Ray investigations on powder-like samples of both
homologues gave two reflections, with d values in the ratio
of 1 : 1/2 in the small angle region and a diffuse scattering in
the wide-angle region. This indicates that the mesophase
possesses a simple layer structure without in-plane order. The
˚
layer spacings (d) are 42.5 A at 130 uC for compound 8 and
Fig. 2 Texture of the SmCP phase of compound 8 obtained on
˚
48.0 A at 140 uC for compound 12, respectively. They are
cooling from the isotropic phase.
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Compounds 14, 16, 18 and 16-Br
The long-chain members of the 4-chloro- or 4-bromo-
substituted compounds (14, 16, 18 and 16-Br) exhibit
tetramorphism in the liquid-crystalline state. First the phase
transitions have been studied by observing the optical textures,
and are described for compound 18 as a representative
example. On cooling the sample from the isotropic phase, a
homeotropic texture as well as a smooth focal-conic texture
was observed. This is a typical feature of an SmA phase. On
lowering the temperature (below 134 uC) the homeotropic
regions transform to
a weakly birefringent fluctuating
schlieren texture, which is characteristic for a biaxial phase.
At the next phase transition observed on cooling the sample
below 128 uC, the fluctuations disappear and in the
paramorphotic texture fine fan-like domains are visible. By
further cooling the sample there is a minor change of the
texture at 123 uC where the schlieren texture again becomes
fluctuating. The texture changes between the polymorphic
mesophases are more pronounced when we start from a fan-
shaped texture of an SmA phase in a cell with planar alignment
conditions (Fig. 4a). On cooling the fan-shaped texture a
broken fan-shaped texture appears (Fig. 4b). This transition is
coupled with an enthalpy change of 2.7 kJ mol21. At 128 uC a
clear change of the texture occurs which can be recognized by a
clear change of the birefringence (Fig. 4c). The corresponding
transition enthalpy is rather low (1.5 kJ mol21). On a further
decrease of the temperature to 123 uC a nearly identical
paramorphotic texture was obtained. No enthalpy change was
observed for this phase transition in the DSC thermogram
indicating a second order phase transition.
Fig. 3 Photomicrographs of the textures obtained for the SmCAPA
phase of compound 12, (a) +4 V mm21; (b) 0 V; (c) 24 V mm21
temperature: 144 uC; photomicrographs of textures obtained for the
SmCSPA phase, (d) +4 V mm21; (e) 0 V; (f) 24 V mm21; temperature:
136 uC; cell thickness: 6 mm.
;
to compound 8, the optical appearance of the polar switching
of compound 12 clearly depends on the temperature. On
cooling the isotropic liquid of compound 12 in an EHC cell of
6 mm thickness under a DC field of 4 V mm21, two types of
domains (green and dark) were observed (Fig. 3a,c). On
reversing the polarity of the applied field the colour of the
textures is interchanged. On terminating the field the molecules
relax to a ground state, which shows a different texture with a
pink colour (Fig. 3b). The two inter-convertible green and
dark regions represent the opposite ferroelectric states
(SmCSPF) and are distinguished by the opposite tilt sense.
On terminating the field the molecules relax to the antiferro-
electric ground state (SmCAPA). This switching mechanism
confirms the anticlinic interlayer correlation in the ground
state of the mesophase. On lowering the temperature with the
same applied field below 140 uC, the entire texture adopts a
blue colour and changing the polarity of the applied field has
no effect on the textural pattern (Fig. 3d,f). If the field is
removed, the colour of the texture changes to pink and stripes
appear perpendicular to the fans (Fig. 3e). The stripes
represent synclinic regions with anticlinic ferroelectric bound-
aries.11 This indicates that below 140 uC the ground state has
a synclinic structure (racemic state; SmCSPA), which switches
to the anticlinic ferroelectric phase (SmCAPF), similar to
compound 8. Thus we have established that at 140 uC there
is a field-induced transition from a chiral antiferroelectric
(SmCAPA) phase to a racemic antiferroelectric (SmCSPA)
phase. It should be noted that the smooth fan-shaped structure
of the chiral SmCAPA phase obtained on cooling the isotropic
liquid in the absence of an electric field does not change below
140 uC. This means that the ground state structure remains as
SmCAPA on lowering the temperature. On the other hand,
the fan-shaped texture with irregular stripes of the racemic
SmCSPA phase obtained below 140 uC after removal of the
applied field, does not change on heating up to the clearing
temperature. That means that in this case the SmCSPA
ground state is stable in the whole temperature range of the
SmCPA phase.
X-Ray studies on the mesophases of compounds 14, 16, 18
and 16-Br confirm the polymorphism in the liquid-crystalline
state. As a representative example we present the results
obtained for compound 18. A well-aligned sample of the high-
temperature phase shows one strong small angle reflection on
the meridian of the pattern (Fig. 5a) from which a layer
Fig. 4 Optical textures of compound 18 in the (a) SmA phase; (b)
SmCSPA phase; (c) Colob phase; and (d) reentrant SmCSPA phase.
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Fig. 5 The small-angle X-ray diffraction pattern of compound 18 in the (a) SmA phase; (b) SmCSPA phase; (c) Colob phase; and (d) reentrant
SmCSPA phase; (e) indexing of the Colob phase.
˚
spacing of 54 A could be determined. On lowering the
can be seen from Fig. 6b–d that the maximum located in the
upper left quarter of the pattern is much stronger than the
corresponding one in the upper right one. Such an asymmetry
can only be caused by a combination of a synclinic tilt of
the molecules within the Colob and the SmCPA phases and
a non-equal distribution of domains around the axis of the
fibre-like disordered sample. The calculated tilt angle is about
25u. The lattice parameter b is comparable with twice the
molecular length considering the tilt of the molecules. An
temperature to 132 uC, two reflections were obtained with d
values in the ratio of 1 : 1/2 (Fig. 5b). The layer spacing was
˚
found to be 55 A. The presence of second order reflections
indicates that the smectic layer structure is more pronounced
which means that the distribution of the electron density along
the layer normal cannot be approximated by only one sinus
wave. It can be understood that because of the polar packing
of the bent-core molecules the smectic layer structure has
become more pronounced.
On further lowering of the temperature to 126 uC, we can see
additional reflections out of the meridian (Fig. 5c) which can
be indexed to an oblique lattice with the lattice parameters a =
˚
˚
106 A, b9 = 58 A, c = 106u (Fig. 5e). In the following this
parameter is doubled (2b9 = b) because of the assumed model
(see Discussion). Below 123 uC the X-ray pattern is quite
similar to that of Fig. 5b which points to a transition of the
Colob phase to the SmCPA phase (Fig. 5d). On heating the
SmCPA phase (above 123 uC) the X-ray pattern of the Colob
phase reappears; i.e. without field this transition is reversible.
As seen from Table 1 this transition cannot be detected by
calorimetric measurements. The corresponding wide-angle
images in different mesophases are given in Fig. 6. In all the
phases a diffuse scattering maximum is observed indicating the
liquid-like arrangement of the molecules within the layers. For
the highest temperature phase the diffuse scattering maxima
are located at the equator (Fig. 6a). This corresponds to an
orthogonal arrangement of the molecules within the layers and
confirms the assignment as an SmA phase. In the smectic
phase below the SmA phase the diffuse wide angle maxima are
out of the equator which is a clear indication for a tilt of the
molecules in the layers (Fig. 6b). On a further lowering of
the temperature to 126 uC and 121 uC (the phase range of the
oblique columnar and the lower temperature SmCPA phase), it
Fig. 6 The wide-angle X-ray diffraction pattern of compound 18
in the (a) SmA phase; (b) SmCSPA phase; (c) Colob phase; and (d)
reentrant SmCSPA phase.
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Detailed electro-optical investigations described here for
compound 18 should give additional information about the
structural features of the polymorphic mesophases. The SmA
phase does not show a polar electro-optical response. For the
tilted smectic phase below the SmA phase two polarization
current peaks per half period were observed on applying a
triangular-wave voltage which indicates that the ground state
structure is antiferroelectric (Fig. 8a). The measured polariza-
tion value is 250 nC cm22. The Colob phase shows a similar
current response (Fig. 8b); importantly the position of the
current peaks is different from that of the higher temperature
SmCPA phase. From the current response
a switching
polarization of 320 nC cm22 could be determined. The low-
temperature SmCP phase is also an antiferroelectric smectic
phase with a slightly higher polarization (340 nC cm22).
To get more information about the organization of the
molecules in the field-on and field-off states we have carried
out DC field experiments. On cooling the sample of compound
18 from the isotropic phase, a focal-conic texture was observed
which is typical for an SmA phase. It can be seen from Fig. 9a
that the extinction directions of half circular domains are
parallel to the crossed polarizers. This indicates that the
director (optical axis) is perpendicular to the smectic layers. On
lowering the temperature to 133 uC, the extinction directions
enclose an angle of y22u with respect to the directions of
the crossed polarizers (see Fig. 9b). This tilt of the extinction
directions indicates that the smectic phase has a tilted
organization which rules out that the mesophase is a polar
smectic A (SmAP) phase. If the antiferroelectric ground state is
switched to the ferroelectric state or vice versa, the extinction
directions do not change and also the birefringence remains
unchanged (Fig. 9c). Furthermore, this effect is independent
of the polarity of the applied field. This finding suggests
that the antiferroelectric ground state as well as the switched
ferroelectric states have a synclinic tilt and the switching
corresponds to the transition from an SmCSPA state to an
Fig. 7 Plot of the layer spacing d as a function of the temperature
obtained for compound 16.
estimation of the number of molecules per unit cell yields
about 32 molecules.
Interestingly, the layer spacings gradually increase in the
sequence SmA–SmCSPA–Colob–SmCSPA (Fig. 7) although a
transition from a non-tilted to a tilted phase takes place. This
unusual result has been observed in earlier studies of laterally
substituted bent-shaped molecules, too.12 There, on the basis
of detailed studies of the temperature dependence of the
bending angle as well as the order parameter the unusual
increase of the d-values was explained by a different degree of
intercalation of the molecules of adjacent layers in the different
phases. The increase in the layer spacings on lowering the
temperature in the present set of compounds can be explained
in a similar way.
We have also performed small-angle X-ray studies for
compounds 14, 16, and 16-Br and we obtained results
similar to those for compound 18. The d-values and the lattice
parameters of the columnar phase of all compounds are
summarized in Table 2.
Table 2 The d-spacings, the lattice parameters and the polarization values for compounds 8, 12, 14, 16, 18 and 16-Br. In column 2 (d-spacing) the
evaluation of the X-ray pattern is given. In column 3 the lattice parameters are listed in agreement with the assumed model. Here the values of the
b parameters are doubled in comparison to those resulting from the X-ray data (see Discussion)
Lattice parameter
Polarization
No.
a/A
b/A
c/u
Phase type
value/nC cm22
˚
d-Spacing (order/hk)/A
˚
˚
8
12
14
42.5 (1), 21.2 (2)
48.0 (1), 24.0 (2)
49.7 (1)
50.1 (1), 25.0 (2)
121.8 (10), 54.5 (211), 51.0 (01), 42.0 (11), 25.4 (02)
51.5 (1), 25.7 (2)
52.8 (1)
53.5 (1), 26.8 (2)
114.1 (10), 58.3 (20), 54.0 (01), 54.0 (211), 44.2 (11)
55.0 (1), 27.5 (2)
54.0 (1)
55.0 (1), 27.4 (2)
102.2 (10), 55.8 (211), 55.3 (01), 27.9 (02), 44.1 (11)
57.1 (1), 28.5 (2), 19.0 (3)
52.0 (1)
52.5 (1), 26.3 (2)
120.8 (10), 53.4 (211), 53.5 (01), 26.8 (02), 45.3 (11)
54.7 (1), 27.4 (2)
—
—
—
—
130
—
—
—
119
—
—
—
106
—
—
—
—
—
—
—
109
—
—
—
113
—
—
—
116
—
—
—
—
—
—
—
111
—
—
—
106.5
—
—
—
106
—
SmCSPA
SmCAPA
SmA
SmCSPA
Colob/SmCAPA
SmCSPA
SmA
SmCSPA
Colob/SmCAPA
SmCSPA
SmA
SmCSPA
Colob/SmCAPA
SmCSPA
SmA
460
335
—
310
350
400
—
330
480
410
—
250
320
340
—
a
a
a
a
16
18
16-Br
a
—
—
102.5
—
SmCSPA
Colob/SmCAPA
SmCSPA
190
250
275
123.5
—
110
—
The SmCAPA phase was obtained from the Colob phase by application of an electric field.
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Fig. 8 Switching current response traces obtained for compound 18 on applying a triangular-wave electric field; (a) SmCSPA phase, polarization
250 nC cm22 (132 uC); (b) SmCAPA phase, obtained from the Colob phase by application of an electric field, polarization 320 nC cm22 (126 uC).
that the ground state has an anticlinic structure (SmCAPA). It
is important to note that the relaxed state is completely
different from that obtained for the SmCSPA phase without a
field. Since the anticlinic arrangement of the molecules in the
layer fragments of the Colob phase is unfavourable, we assume
that under the electric field the modulated structure of the
columnar phase transforms irreversibly to the smooth layer
structure of the SmCSPF state, which relaxes to the SmCAPA
Fig. 9 (a) Texture obtained in the smectic A phase under DC field;
(b) and (c): texture obtained in the SmCSPA phase with (¡4 V mm21
on the removal of the field. This means that the transition
)
from SmCAPA to SmCSPF and vice versa takes place in a usual
way, by the collective rotation of the molecules around the
tilt cone. During this phase transition the layer chirality does
not change.
and without an electric field, respectively; cell thickness: 5 mm;
temperature: 131 uC (compound 18).
SmCSPF state. The switching mechanism is based on the
collective rotation of the molecules around their long axes.
Such a polar switching is unusual for SmCP phases and was
observed in only a few bent-core compounds.13–16 It should
As seen from Fig. 11a,c the extinction directions rotate in
the direction of the crossed polarizers on cooling to the low-
temperature phase in the presence of an electric field and
simultaneously the colour of the texture changes from red to
yellow-green (Fig. 10a–c, Fig. 11a–c). Since the texture of the
switched ferroelectric state is the same for a field of opposite
polarity, this state corresponds to the anticlinic SmCAPF
phase. On terminating the field the extinction directions
remain parallel to the crossed polarizers but irregular stripes
appear (Fig. 11b). The tilt within the striped domain is
synclinic and the tilt sense is different in neighbouring
stripes.2,11 From the optical features under the field, we can
confirm the ground state as SmCSPA, which is switched to the
SmCAPF phase.2,11,17 On heating the sample under the same
be emphasized that such
a field-induced transition is
accompanied by an inversion of the layer chirality in each
second layer.
If the synclinic SmCSPF (Fig. 9b) phase is cooled down to
the Colob phase in the presence of the electric field, the
extinction directions remain unchanged but the change in
the colour of the domains is clearly visible (Fig. 10a). If the
polarity of the field is reversed, the extinction directions
rotate in a counterclockwise direction (Fig. 10c). However on
terminating the field the extinction directions rotate in the
direction of the crossed polarizers (Fig. 10b), which indicates
Fig. 10 (a) Texture of the field-induced ferroelectric structure
(SmCSPF: +4 V mm21); (b) texture of the antiferroelectric ground state
at zero field (SmCAPA); (c) texture of the field-induced ferroelectric
structure (SmCSPF: 24 V mm21); cell thickness: 5 mm; temperature:
126 uC (compound 18).
Fig. 11 (a) Texture of the field-induced ferroelectric structure
(SmCAPF: +4 V mm21); (b) texture of the ground state antiferroelectric
structure at zero field (SmCSPA); (c) texture of the field-induced
ferroelectric structure (SmCAPF: 24 V mm21); cell thickness: 5 mm;
temperature: 118 uC (compound 18).
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experimental conditions the lower temperature SmCSPA phase
is directly transformed to the higher temperature SmCSPA
phase without passing through the Colob phase. From these
results it is clear that the transition of the field-induced
SmCAPA phase into the reentrant SmCSPA phase is irrever-
sible. Furthermore, the Colob phase does not exist above a
critical electric field.
bending angle of the molecules with decreasing temperature.
According to this it can be assumed that at higher tem-
peratures the molecules adopt a more rod-like shape con-
formation, which enables an easier rotation of the molecules
around their long axes, which is characteristic for SmA (or
SmC) phases. It is important to note that in the present case
only the compounds with longer terminal chains (n ¢ 14)
showed the SmA phase. This points to a situation, wherein the
terminal alkyl chains too have a strong influence on the
rotational freedom of the molecules about their long axes.
Obviously, the higher conformational disorder of the long
aliphatic chains in the highest temperature range enables a
rotation of the molecules (or the aromatic middle parts) and as
a result the SmA phase is obtained.
We found that on application of a sufficiently high electric
field above the transition temperature for SmCSPA to SmA
(i.e. in the existence region of the SmA phase) the SmA fan-
shaped texture can be transformed to another texture, which
is identical to that of the field-induced SmCSPF phase. If the
field is switched off the fan-shaped texture of the SmA
phase reappears. Furthermore, the switching current response
obtained in the field-induced SmCPA phase is identical to that
of the lower temperature SmCPA phase. That means, by the
application of a sufficiently high electric field the transition
temperature for SmCPA to SmA can be enhanced. The critical
field which is necessary for this field-induced transition
from SmA to SmCSPF increases with the temperature.
Applying a field of 35 V mm21 the enhancement of the
transition temperature was found to be 2 K. This effect is in a
sense similar to the field-induced enhancement of the transi-
tion temperature for SmCPA to isotropic which was reported
for a few bent-core mesogens.18,19
On lowering the temperature this rotation is hindered and
the molecules are packed in a specific way that results in
the polar order of the SmCSPA phase. Since the texture of the
field-on and field-off states remained the same in the SmCSPA
phase, the switching mechanism is based on a collective
rotation of the molecules around their long axes (see Fig. 12).
5. Discussion
It was shown that all members of the series form an
antiferroelectric SmCPA phase. With the exception of com-
pound 8, the SmCPA phase can occur in different structural
variants. In the case of compound 12 we found a field-induced
transition from the anticlinic (chiral) SmCAPA phase to the
synclinic (racemic) SmCSPA phase below a definite tempera-
ture. This field-induced transition is reversible. But in the
absence of an electric field either the chiral SmCAPA (formed
on cooling of the isotropic liquid) or the racemic SmCSPA
(formed by relaxation of the field-induced SmCAPF state) is
stable in the whole temperature range of the SmCPA phase. We
have no explanation of why the anticlinic SmCAPA phase is the
high-temperature phase with respect to the synclinic SmCSPA
phase. Actually the synclinic structure should be more stable
and should occur at a higher temperature because a synclinic
stacking of the layers favours out-of layer fluctuations and
therefore enhances the entropy. Heppke et al.20 reported that
the transition from the racemic SmCSPA phase into the chiral
SmCAPA phase can be achieved by the application of a high
rectangular or DC field whereas the reverse transition is
possible by a high triangular field.
The longer-chain homologues 14, 16, 18 and the bromine-
substituted compound 16-Br show a rich polymorphism.
On the basis of experimental observations of texture, X-ray
diffraction and electro-optical measurements, four mesophases
in the sequence SmA–SmCSPA–Colob–SmCSPA could be
identified. Phase sequences with conventional smectic phases
as well as ‘‘banana phases’’ are not unknown but are hitherto
observed in only a few homologous series (see for example
references 12,21,22). As shown by NMR measurements such
polymorphism variants are mainly due to the decrease of the
Fig. 12 (a) Possible molecular arrangements of the mesophases in the
ground state and under the applied electric field (compounds 14, 16, 18
and 16-Br); (b) block diagram explaining the transitions of the
mesophases with temperature and electric field.
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of the molecules around their long axes (SmCSPA A SmCSPF).
In the latter case the polar switching is accompanied by the
inversion of the chirality in each second layer. Interestingly,
there are a few examples reported in the literature where in the
existence range of the SmCPA phase the mechanism of the
polar switching depends on the temperature. At a higher
temperature the switching takes place by rotation of the
molecules around their long axes and at a lower temperature
by rotation around the tilt cone.13–16 This dependency is
identical in the high- and low-temperature SmCSPA phases of
the present set of compounds.
Fig. 13 Structure model for the Colob phase (considering the number
of molecules in the unit cell).
Another finding of fundamental interest is the field-induced
transition of the non-polar SmA phase to the polar SmCPA
phase above the transition temperature of SmCPA A SmA.
This field-induced enhancement of this transition can be up to
2 K depending on the strength of the applied electric field. This
effect is reminiscent of the field-induced enhancement of the
SmCPA A isotropic temperature by application of the electric
field.18,19 This effect may be connected with the existence of
polar clusters in a short-range order region already in the
non-polar phases (SmA, isotropic).
On further lowering of the temperature we have observed
an interesting structural change in the arrangements of the
molecules. The molecules, which are arranged in smectic
layers, are fragmented into the broken-layer structure of a
columnar phase with an oblique lattice. It is known from the
electro-optical measurements that the Colob phase has an
antiferroelectric packing of bent-core molecules. It is plausible
because an intercalation of molecules of the adjacent layers
favours a synclinic tilt (Fig. 13). From the X-ray studies a
period in one direction is derived which corresponds to the
length of the molecules if the tilt is considered. But the
knowledge of the antiferroelectric packing demands a doubling
of the translational period. Under an electric field this Colob
phase can be transformed to a simple layered structure
(SmCSPF state). On terminating the field this SmCSPF state
does not relax to the SmCSPA state (as found for the high-
temperature SmCSPA phase) but instead to the homochiral
SmCAPA state by rotation of the molecules around the tilt
cone (see the schematic representation in Fig. 12). On cooling
of the field-induced homochiral SmCSPF phase below 123 uC,
a transition to the racemic SmCAPF phase takes place (like in
compound 12), which relaxes to the SmCSPA ground state on
termination of the field. In both of the mesophases the
relaxation takes place by a collective rotation of the molecules
around the tilt cone (Fig. 12a). Interestingly the racemic
SmCAPF or SmCSPA state remains stable on heating up to
134 uC where the homochiral SmCSPF phase reappears (see
block diagram in Fig. 12b).
One of the basic problems, which are of interest to synthetic
chemists as well as to theoreticians, is the pronounced
influence of the direction of the linking groups (in our case
of ester groups) on the mesophase behaviour in bent-core
mesogens. Therefore we compare the homologues under
discussion with analogous isomeric compounds I, which
differ only by the direction of the outer ester groups.28,29
The clearing temperatures of compounds I are about 50 K
lower than those of compounds 8, 12, 14, 16 and 18.
On the other hand, the mesophase behaviour is also com-
pletely different. The short-chain members of the series I
(C9–C12) exhibit a nematic phase and a highly viscous and
optically isotropic ferroelectric mesophase which sponta-
neously forms domains of opposite handedness.28,29 The
hexadecyloxy homologue shows—instead of the isotropic
mesophase—a polar tilted smectic phase with unusual
switching behaviour.30
An irreversible field-induced transition of a Col phase to an
SmCPF phase was observed by Reddy et al.8,23 It is also
reported in the literature that the field-induced transition
from a columnar to a smectic phase can be reversible, i.e. on
removal of the applied field the Col phase reappears. Such
25
26
behaviour was found for Colr,24 Colob
undulated SmCSPF phases.27
,
bilayer Colob and
We can notice that there are similar differences between
analogous isomeric compounds without lateral substituents in
the 4-position of the central core. In this case also the clearing
temperatures differ by about 40 K and the mesophase
behaviour is clearly different.15,25 It could be shown that the
direction of the ester linking groups has a significant influence
on the dipole moment, on the conformation (in particular on
the bending angle) and on the conformational flexibility. For
example, isomers where the direction of the ester groups in
each leg is opposite (like in the compounds under discussion)
have a lower conformational degree of freedom, which is
responsible for higher clearing temperatures and a distinct
mesophase behaviour.25,31
The significant point to be noticed is the occurrence of two
SmCSPA phases, one above and the other one below the
existence range of the Colob phase. These phases are struc-
turally identical and exhibit a synclinic stacking of the smectic
layers. Because of their structural identity the low-temperature
SmCSPA phase can be regarded as a reentrant phase, which
reappears on cooling the Colob phase. Both SmCSPA phases
differ only in the mechanism of polar switching. In the
reentrant SmCSPA phase the switching takes place in the usual
way through a rotation of the director around the tilt cone
(SmCSPA A SmCAPF). However, in the high-temperature
SmCSPA phase the switching is based on a collective rotation
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9 (a) D. A. Coleman, J. Fernsler, N. Chattham, M. Nakata,
Y. Takanishi, E. Korblova, D. R. Link, R.-F. Shao, W. G. Jang,
J. E. Maclennan, O. Mondainn-Monval, C. Boyer, W. Weissflog,
G. Pelzl, L.-C. Chien, J. Zasadzinski, J. Watanabe, D. M. Walba,
H. Takezoe and N. A. Clark, Science, 2003, 301, 1204; (b)
V. Novotna, V. Hamplova, M. Kaspar, M. Glogorova, K. Knizek,
S. Diele, C. Jones, C. Coleman and N. A. Clark, Liq. Cryst., 2005,
32, 1115.
10 K. Miyasato, S. Abe, H. Takezoe, A. Fukuda and E. Kuze, Jpn. J.
Appl. Phys., 1983, 22, L661.
11 R. Amaranatha Reddy and C. Tschierske, J. Mater. Chem., 2006,
DOI: 10.1039/b504400f.
12 (a) I. Wirth, S. Diele, A. Eremin, G. Pelzl, S. Grande,
L. Kovalenko, N. Pancenko and W. Weissflog, J. Mater. Chem.,
2001, 11, 1642; (b) A. Eremin, H. Nadasi, G. Pelzl, H. Kresse,
W. Weissflog and S. Grande, Phys. Chem. Chem. Phys., 2004, 6,
1290.
13 M. Nakata, R.-F. Shao, W. Weissflog, H. Takezoe and N. A. Clark,
poster at the Ferroelectric Liquid Crystal Conference, Dublin,
2004.
14 W. Schro¨der, S. Diele, G. Pelzl and W. Weissflog, ChemPhysChem,
2004, 5, 99.
15 R. Amaranatha Reddy, M. W. Schro¨der, M. Bodyagin, H. Kresse,
S. Diele, G. Pelzl and W. Weissflog, Angew. Chem., Int. Ed., 2005,
117, 784.
6. Summary
We have presented a new homologous series of five-ring bent-
core mesogens having chlorine substituents in the 4-position of
the central phenyl ring. The homologues with shorter terminal
chains formed an SmCPA phase, the homologues with longer
terminal chains (14, 16 and 18) showed a new polymorphism
variant SmA–SmCSPA–Colob–SmCSPA for the first time. The
Colob phase can be irreversibly transformed to the SmCSPF
phase by the application of a sufficiently high electric field.
Both SmCSPA phases are structurally identical and differ only
by the mechanism of polar switching. The switching of the
high-temperature SmCSPA phase is based on a collective
rotation of the molecules around their long axes whereas the
switching of the reentrant SmCSPA phase takes place through
the collective rotation of the molecules around the tilt cone.
It is remarkable that the SmA phase shows a reversible
field-induced transition to the SmCSPA phase. Comparing
the mesophase behavior of isomeric compounds leads to the
conclusion that the influence of the dipole moment and the
conformational flexibility is more profound in bent-core
compounds than in calamitic liquid crystals.
16 W. Weissflog, U. Dunemann, M. W. Schro¨der, S. Diele, G. Pelzl,
H. Kresse and S. Grande, J. Mater. Chem., 2005, 15, 939.
17 M. Zennyoji, Y. Takanishi, K. Ishikawa, J. Thisayukta,
J. Watanabe and H. Takezoe, J. Mater. Chem., 1999, 9, 2775.
18 W. Weissflog, M. W. Schro¨der, S. Diele and G. Pelzl, Adv. Mater.,
2003, 15, 630.
Acknowledgements
19 M. W. Schro¨der, G. Pelzl, U. Dunemann and W. Weissflog, Liq.
Cryst., 2004, 31, 633.
20 G. Heppke, A. Jakli, S. Rauch and H. Sawade, Phys. Rev. E, 1999,
60, 5575.
This work was supported by the Deutsche Forschungs-
gemeinschaft (Graduiertenkolleg 894) and by the Fonds der
Chemischen Industrie.
21 W. Weissflog, H. Nadasi, U. Dunemann, G. Pelzl, S. Diele,
A. Eremin and H. Kresse, J. Mater. Chem., 2001, 11, 2748.
22 A. Eremin, H. Nadasi, G. Pelzl, S. Diele, H. Kresse, W. Weissflog
and S. Grande, Phys. Chem. Chem. Phys., 2004, 6, 1290.
23 R. Amaranatha Reddy, B. K. Sadashiva and V. A. Raghunathan,
Chem. Mater., 2004, 16, 4050.
24 J. Ortega, M. R. de la Fuente, J. Etxebarria, C. L. Folcia, S. Diaz,
J. A. Gallastegui, N. Gimeno, M. B. Ros and M. A. Perez-Jubindo,
Phys. Rev. E, 2004, 69, 011703.
25 W. Weissflog, G. Naumann, B. Kosata, M. W. Schro¨der,
A. Eremin, S. Diele, H. Kresse, R. Friedemann, S. Ananda
Rama Krishnan and G. Pelzl, J. Mater. Chem., 2005, 15, 4328.
26 J. P. Bedel, J. C. Rouillon, J. P. Marcerou, H. T. Nguyen and
M. F. Achard, Phys. Rev. E, 2004, 69, 061702.
27 M. Nakata, D. R. Link, Y. Takanishi, J. Thisayukta, H. Niwano,
D. A. Coleman, J. Watanabe, A. Lida, N. A. Clark and
H. Takezoe, Phys. Rev. E, 2005, 71, 011705.
References
1 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe,
J. Mater. Chem., 1996, 6, 1231.
2 D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark,
E. Ko¨rblova and D. M. Walba, Science, 1997, 278, 1924.
3 G. Pelzl, S. Diele and W. Weissflog, Adv. Mater., 1999, 11, 707.
4 J. Watanabe, T. Niori, T. Sekine and H. Takezoe, Jpn. J. Appl.
Phys., 1998, 37, L139.
5 (a) K. Pelz, S. Diele, U. Baumeister and W. Weissflog, Liq. Cryst.,
2003, 30, 1151; (b) H. N. Shreenivasa Murthy and B. K. Sadashiva,
J. Mater. Chem., 2005, 15, 2056; (c) C. Keith, R. Amaranatha
Reddy, U. Baumeister and C. Tschierske, J. Am. Chem. Soc., 2004,
126, 14132; (d) D. Kardas, M. Prehm, U. Baumeister, D. Pociecha,
R. A. Reddy, G. H. Mehl and C. Tschierske, J. Mater. Chem.,
2005, 15, 1722; (e) Y. Takanishi, H. Takezoe, J. Watanabe,
Y. Takahashi and A. Iida, J. Mater. Chem., 2006, DOI: 10.1039/
b514831f.
28 G. Pelzl, A. Eremin, S. Diele, H. Kresse and W. Weissflog,
J. Mater. Chem., 2002, 12, 2591.
29 W. Weissflog, S. Sokolowski, H. Dehne, B. Das, S. Grande,
M. W. Schro¨der, A. Eremin, S. Diele, G. Pelzl and H. Kresse, Liq.
Cryst., 2004, 31, 923.
30 A. Eremin, S. Diele, G. Pelzl and W. Weissflog, Phys. Rev. E, 2003,
67, 020702.
31 S. Ananda Rama Krishnan, W. Weissflog, G. Pelzl, H. Kresse and
R. Friedemann, Phys. Chem. Chem. Phys., 2006, DOI: 10.1039/
b513934a.
6 (a) J. Szydlowska, J. Mieczkowski, J. Matraszek, D. W. Bruce,
E. Gorecka, D. Pociecha and D. Guillon, Phys. Rev. E, 2003, 67,
031702; (b) E. Gorecka, N. Vaupotic, D. Pociecha, M. Cepic and
J. Mieczkowski, ChemPhysChem, 2005, 6, 1087.
7 J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
H. T. Nguyen and M. F. Achard, J. Mater. Chem., 2002, 12, 2214.
8 R. Amaranatha Reddy, B. K. Sadashiva and V. A. Raghunathan,
Chem. Mater., 2005, 17, 274.
This journal is ß The Royal Society of Chemistry 2006
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