Ten isomeric five-ring bent-core mesogens: The influence of the direction of the carboxyl connecting groups on the mesophase behaviour

Article abstract of DOI:10.1039/b508488a

In order to study the role of the direction of the connecting groups in bent-core mesogens we synthesized two series of ten possible achiral isomeric five-ring bent-core compounds in which all aromatic rings are connected by ester groups and each of which possesses the same length of the terminal chains (octyloxy or dodecyloxy, respectively). The structure of the isomers is distinguished by the direction of at least one ester group, only. The mesophase behaviour of the compounds has been studied by polarizing microscopy, differential scanning calorimetry, X-ray experiments and electro-optical measurements. We have found that in spite of the minor structural differences a variety of mesophases occur (SmCP_A, Col_rec, Col _ob) whereby the clearing temperatures vary from 121 to 193 °C (octyloxy isomers) and 112 to 189 °C (dodecyloxy isomers). Depending on the direction of the ester groups some of these isomers show interesting properties, such as field-induced inversion of chirality in SmCP_A and columnar phases, the field-induced enhancement of the clearing temperature, a second-order phase transition Col_ob → SmCP_A or the reversible field-induced phase transition Col_ob → SmCP _P. The unexpectedly strong influence of the direction of the connecting groups is discussed on the base of theoretical calculations and molecular dynamics simulation on isolated molecules. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b508488a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Ten isomeric five-ring bent-core mesogens: The influence of the direction of
the carboxyl connecting groups on the mesophase behaviour{
Wolfgang Weissflog,*a Gisela Naumann,a Bedrich Kosata,a Martin W. Schro¨der,a Alexey Eremin,a
Siegmar Diele,a Zinaida Vakhovskaya,a Horst Kresse,a Rudolf Friedemann,b S. Ananda Rama Krishnanb and
Gerhard Pelzla
Received 15th June 2005, Accepted 9th August 2005
First published as an Advance Article on the web 25th August 2005
DOI: 10.1039/b508488a
In order to study the role of the direction of the connecting groups in bent-core mesogens we
synthesized two series of ten possible achiral isomeric five-ring bent-core compounds in which all
aromatic rings are connected by ester groups and each of which possesses the same length of
the terminal chains (octyloxy or dodecyloxy, respectively). The structure of the isomers is
distinguished by the direction of at least one ester group, only. The mesophase behaviour of the
compounds has been studied by polarizing microscopy, differential scanning calorimetry, X-ray
experiments and electro-optical measurements. We have found that in spite of the minor
structural differences a variety of mesophases occur (SmCPA, Colrec, Colob) whereby the clearing
temperatures vary from 121 to 193 uC (octyloxy isomers) and 112 to 189 uC (dodecyloxy isomers).
Depending on the direction of the ester groups some of these isomers show interesting properties,
such as field-induced inversion of chirality in SmCPA and columnar phases, the field-induced
enhancement of the clearing temperature, a second-order phase transition Colob A SmCPA or the
reversible field-induced phase transition Colob A SmCPF. The unexpectedly strong influence of
the direction of the connecting groups is discussed on the base of theoretical calculations and
molecular dynamics simulation on isolated molecules.
length of the terminal chains (octyloxy and dodecyloxy, resp.)
Introduction
which contain only carboxyl linkage groups. All possible 10
Bent-core mesogens have gained considerable importance
isomers are presented which are distinguished from each other
because they are able to form new mesophases with unusual
by the direction of at least one linkage group. The mesophases
properties.1 In smectic mesophases the bent molecules are
were assigned and characterized by polarizing microscopy,
polar packed in bent direction. In this way a macroscopic
X-ray diffraction and electro-optical measurements. The meso-
polarization of the smectic layers arises which gives rise to
phase behaviour of the isomers is compared and discussed on
ferroelectric or antiferroelectric properties. In tilted smectic
the basis of theoretical calculations and molecular dynamics
phases with C2 or C1 symmetry of the layers chiral structures
simulation on isolated molecules.
can occur despite the achiral nature of the molecules.2 Until
now a lot of bent-core mesogens have been synthesized and
characterized.3–12 It is obvious that the mesophase behaviour
much more depends on structural variations of the compounds
in comparison with calamitic compounds. Generally the meso-
phase behaviour is influenced by the number and structure of
the rings, the structure, position and direction of the linkage
groups, the existence of lateral groups and by the length of the
terminal chains.
Materials
In the isomeric compounds under study IVa–j all five phenyl
rings are linked by carboxylic groups (Scheme 1). Therefore,
standard procedures of esterification were used for their
preparation. In most cases reactions of the phenolic fragments
with the corresponding substituted benzoic acids were realized
with DCC in dichloromethane catalysed by DMAP. For the
final steps, especially for the combination of a three-ring
The goal of this paper is to study what role the direction of
the linkage groups plays. For this reason we synthesized two
series of isomeric five-ring bent core mesogens with equal
aInstitut fu¨r Physikalische Chemie, Martin-Luther-Universita¨t Halle-
Wittenberg, Mu¨hlpforte 1, 06108 Halle (Saale), Germany.
E-mail: wolfgang.weissflog@chemie.uni-halle.de
bInstitut fu¨r Organische Chemie, Martin-Luther-Universita¨t Halle-
Wittenberg, Kurt-Mothes-Straße 2, 06120 Halle (Saale), Germany
{ Electronic supplementary information (ESI) available: The experi-
mental procedure to prepare compound IVi/12 via IIIb/12-PG and
IIIb/12 is given. Furthermore, analytical data of the intermediates
IIId/12, IIIe/12 and of the final compounds IVb/12, IVd–j/12 are
summarized. See http://dx.doi.org/10.1039/b508488a
Scheme 1 Molecular structure of the isomeric compounds IVIa–VIj.
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Scheme 2 Two-ring compounds IIa/8–IId/8 (R: C8H17); IIa/12–IId/12
(R: C12H25).
fragment with a two-ring moiety, the acylation of the phenolic
compound with the acid chloride of the related benzoic acid
was preferred to obtain pure materials in higher yields.
Generally, there are a lot of possibilities to prepare each
compound using divergent or convergent pathways. We decide
in favour of a modular approach to synthesize the 10 isomers
with terminal octyloxy chains as well as dodecyloxy chains.
In this way we could apply some intermediates for the
preparation of several final compounds.
The synthesis of all compounds were started with the
reaction of the two-ring compounds IIa–IId (Scheme 2) with
the corresponding 1,3-disubstituted benzene derivatives Ia–If
(Scheme 3), which introduce the bending angle into the
molecules.
To get the non-symmetric compounds the preparation of
three-ring intermediates IIIa–IIIe (Scheme 4) was necessary.
To avoid unwanted side products the corresponding benzyl
ethers (IIIa-PG, IIIb-PG) and benzyl esters (IIIc-PG to IIIe-
PG) were prepared and deprotected by hydrogenolysis.
Scheme 4 Three-ring compounds IIIa/8–IIIe/8 (R: C8H17); IIIa/12–
IIIe/12 (R: C12H25).
IIIb. 4-(4-n-Alkyloxybenzoyloxy)phenyl 3-hydroxybenzoate
3-Benzyloxybenzoic acid Id was esterificated with the
4-hydroxyphenyl 4-alkyloxybenzoate IIa to give IIIb-PG.
Hydrogenation results in IIIb.
IIIa. 3-Hydroxyphenyl 4-(4-n-alkyloxybenzoyloxy)benzoate6
Reaction of the 4-(4-n-alkyloxybenzoyloxy)benzoic acid IIc
with resorcinol monobenzyl ether Ib results in IIIa-PG, which
was deprotected to obtain IIIa. Achten et al.13 reported the
direct esterification of resorcinol with the two-ring acid IIIa
with following separation of the monoester and diester. It
should be mentioned that the phenolic compound IIIa exhibits
nematic behaviour [Cr 125 (N 118) I].
IIIc. 3-[4-(4-n-Alkyloxybenzoyloxy)benzoyloxy]benzoic acid
Reaction of benzyl 3-hydroxybenzoate Ic with the 4-(4-
n-alkyloxybenzoyloxy)benzoic acid IIc results in the benzyl-
ester IIIc-PG, which was deprotected to give IIIc. Recently,
Prasad et al.14 reported the preparation of this acid by
Scheme 3 One-ring starting materials Ia–If.
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oxidation of 3-[4-(4-n-alkyloxybenzoyloxy)benzoyloxy]benz-
corresponding two-ring benzoic acids and by acylation of
the two-ring phenols with the acid chloride of isophthalic
acid Ie, respectively. The non-symmetric final products
(IVe–IVj) were prepared by combination of the three-ring
intermediates IIIa–IIIe and the two-ring compounds
IIa–IId. The reaction pathways to synthesize the compounds
IVa–IVj are summarized in Scheme 5. The reaction of the
phenolic components with the acid chlorides of the corre-
sponding carboxylic acid was performed in toluene using
triethylamine as base and 4-dimethylaminopyridine as co-
catalyst. The compounds were purified by flash chromato-
graphy and following recrystallization from DMF–ethanol
mixtures.
aldehyde by means of Jones reagent.
IIId. 3-[4-(4-n-Alkyloxybenzoyloxy)phenoxycarbonyl]benzoic
acid
Monobenzyl isophthalate If was reacted with 4-hydroxyphenyl
4-alkyloxybenzoate IIa to give IIId-PG. Hydrogenation results
in the three-ring acid IIId.
IIIe. 3-[4-(4-n-Alkyloxyphenoxycarbonyl)benzoyloxy]benzoic
acid
Esterification of 4-(4-n-alkyloxyphenoxycarbonyl)benzoic acid
IId with benzyl 3-hydroxybenzoate Ic gives the benzyl pro-
tected three-ring intermediate IIIe-PG, which was deprotected
by means of hydrogen to give IIIe.
The experimental procedure to prepare compound IVi/12
via IIIb/12-PG and IIIb/12 is given in the electronic supple-
mentary information (ESI){. Furthermore, analytical data
of the intermediates III and of the final compounds IV are
summarized there.
Final compounds IVa–IVj (Scheme 5)
Table 1 and Table 2 show the transition temperatures (uC),
the transition enthalpies (kJ/mol) and the mesophase beha-
viour of the two series of the ten isomers IVa–IVj with terminal
The preparation of the symmetric molecules (IVa–IVd)
was realized by esterification of resorcinol (Ia) with the
Scheme 5 Synthetic pathways to prepare the final compounds IVa–IVj.
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Table 1 Phase transition temperatures (uC) and transition enthalpies (kJ mol21; in square brackets after the temperatures) of the isomers with
terminal dodecyloxy chains. Monotropic phases are indicated by brackets. COO groups having an inverse direction in comparison to those of the
first compound IVa/12 are marked by bold type
W
X
Y
Z
Cr
SmCP
Col
I
IVa/12
IVb/12
IVc/12
IVd/12
IVe/12
IVf/12
IVg/12
IVh/12
IVi/12
IVj/12
a
COO
COO
OOC
OOC
COO
COO
COO
COO
COO
OOC
COO
OOC
COO
OOC
COO
COO
COO
OOC
OOC
COO
OOC
COO
OOC
COO
COO
COO
OOC
COO
OOC
COO
OOC
OOC
COO
COO
COO
OOC
COO
COO
COO
COO
N
N
N
N
N
N
N
N
N
N
109 [41.9]
157 [56.7]
152 [79.7]
191 [92.2]
105 [28.5]
120 [73.2]
110 [25.0]
142 [75.2]
146
Na
119 [22.5]
—
—
Nd
N
N
N
N
N
N
N
N
N
N
—
—
—
—
(Na
Na
162 [23.7]
189) [c]
140 [20.2]
(N
Nb
112) [15.8]
133 [23.0]
158 [c]
—
—
Nb
Na
168 [20.6]
144)
168 [23.0]
—
—
d
(N
138 [57.7]
Nb
b
c
Antiferroelectric Oblique This transition could not be detected by calorimetry. This phase has been designated earlier on the base of
powder-like X-ray patterns as an undulated SmCP phase.18
Table 2 Phase transition temperatures (uC) and transition enthalpies (kJ mol21; in square brackets after the temperatures) of the isomers with
terminal octyloxy chains. Monotropic phases are indicated by brackets. COO groups having an inverse direction in comparison to those of the first
compound IVa/8 are marked by bold type
W
X
Y
Z
Cr
SmCP
Col
I
IVa/8
IVb/8
IVc/8
IVd/8
IVe/8
IVf/8
IVg/8
IVh/8
IVi/8
IVj/8
a
COO
COO
OOC
OOC
COO
COO
COO
COO
COO
OOC
COO
OOC
COO
OOC
COO
COO
COO
OOC
OOC
COO
OOC
COO
OOC
COO
COO
COO
OOC
COO
OOC
COO
OOC
OOC
COO
COO
COO
OOC
COO
COO
COO
COO
N
N
N
N
N
N
N
N
N
N
125 [68.8]
163 [49.2]
160 [68.7]
201
126 [27.8]
131 [53.3]
111 [22.5]
123 [17.0]
151 [58.2]
133 [29.4]
—
—
—
—
—
—
—
—
—
Na
(Nb
—
Ne
121) [17.6]
N
N
N
N
N
N
N
N
N
N
168 [21.5]
193)
(N
Nb
138 [17.0]
124) [17.1]
131 [18.6]
161 [15.8]
134) [15.5]
163 [15.4]
(Nc
Nc
Nb
(N
146 [2.0]
e
Nb
b
c
d
Antiferroelectric Oblique Rectangular This transition could not be detected by calorimetry. This phase has been designated earlier on
the base of powder-like X-ray patterns as an undulated SmCP phase.18
dodecyloxy chains IVa/12–IVj/12 and with terminal octyloxy
chains IVa/8–IVj/8, respectively.
scattering maxima in the 2D pattern have been measured by
the scans I(h), Q 5 const. and I(Q), h 5 const. where h is the
Bragg angle and Q is the angle between the scattering vector
and the equator of the pattern. The last scan is used to measure
the tilt-angle of the molecules with respect to an axis.
Experimental
The thermal behaviour of the compounds was investigated
by a differential scanning calorimeter DSC Pyris 1 (Perkin
Elmer). The microscopic textures and the field-induced
texture changes were examined using a polarizing microscope
(Leitz Orthoplan) equipped with a Linkam THM 600/S
hot stage.
Electro-optical measurements were carried out using the
usual experimental set-up where the cells (EHC Corp.) are
heated on the hot stage of the polarizing microscope and a
power supply (Keithley 3910) generates the voltage signals.
Results
X-Ray diffraction measurements on non-oriented samples
were performed using a Guinier film camera or a Guinier
goniometer. X-Ray investigations on oriented samples were
done with a 2D detector (Siemens AG). The positions of the
The four compounds IVa–d, listed in the upper part of Table 2
and Table 3, are symmetric with respect to the central phenyl
ring. In the non-symmetric isomers IVe–j the dipole moments
Table 3 Layer spacings d of the SmCP phases and lattice parameters a, b, and c of the columnar phases. The angle q is the tilt angle in SmCP
phases or the angle between the b-axis and the director in columnar phases
W
X
Y
Z
8
12
d 5 36.1 A
˚
˚
˚
˚
˚
˚
˚
˚
˚
IVa
IVe
IVf
IVg
IVh
COO
COO
COO
COO
COO
COO
COO
COO
COO
OOC
OOC
COO
COO
OOC
COO
OOC
COO
OOC
COO
COO
a 5 35.0 A, b 5 41.4 A, c 5 90u, q 5 6u
˚
˚
a 5 74.1 A, b 5 43.1 A, c 5 116u, q 5 6u
a 5 128.1 A, b 5 47.2 A, c 5 106u, q 5 8u
˚
a 5 34.0 A, b 5 43.4 A, c 5 90u, q 5 25u
d 5 38.4 A
˚
a 5 46.9 A, b 5 42.5 A, c 5 90u, q 5 0u
d 5 42.2 A, q 5 24u
˚
˚
˚
˚
a 5 90.4 A, b 5 43.9 A, c 5 102u, q 5 0u
a 5 116.8 A, b 5 55.6 A, c 5 67u, q 5 11u,
˚
d 5 50.1 A, q 5 34u
˚
˚
˚
˚
IVj
OOC
COO
COO
COO
a 5 212.4 A, b 5 42.8 A, c 5 95.5u, q 5 10u,
a 5 167.9 A, b 5 55.5 A, c 5 93u
˚
d 5 43 A, q 5 18u
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of the linking groups of the left and right leg of the molecules
are not compensated to each other.
It can be seen at the first view that the melting behaviour
of these isomeric compounds is very different. Thus, the
melting temperatures of the dodecyloxy substituted com-
pounds IVa–j/12, e.g., vary between 105 uC and 191 uC and
between 111 uC and 201 uC in the case of the octyloxy com-
pounds IVa–j/8. This is also true for the clearing temperatures
which vary between 112 uC and 189 uC for the dodecyloxy
compounds and between 121 uC and 193 uC for the octyloxy
compounds, that means, the mesophase stability strongly
depends on the direction of the carboxylic linking groups.
Most intensive studies of the physical properties were
realized using the dodecyloxy substituted isomers IVa–j/12.
Therefore, the presentation of the results will start with these
compounds. After that we will discuss the properties of the
octyloxy substituted isomers IVa–j/8 searching for differences
caused by the different length of the terminal chains, only.
Two isomers with a terminal dodecyloxy chain (IVa/12,
IVc/12) have been reported in earlier papers. Compound
IVa/12 in which the connecting groups in each leg of the bent
molecule have the same direction with respect to the central
phenyl ring was already described in refs. 15–17. This com-
pound forms an antiferroelectric SmCP phase. In our earlier
paper17 we were able to show that the mechanism of the polar
switching clearly depends on the experimental conditions. At
very low frequency (,0.2 Hz) the polar switching is based on
the collective rotation of the molecules around their long axes.
At higher frequencies the polar switching takes place in the
usual way by the director rotation around the tilt cone. A polar
switching by rotation of the molecules around the long axes is
unusual, but was already reported for SmAPA phases,20,21 B1
phases22 and SmCP phases.17,18,23–26 It should be emphasized
that in the case of tilted phases this mechanism of polar
switching is accompanied by an inversion of layer chirality.
Compound IVc/12 we have already reported in ref. 18. In
this compound the direction of the linkage groups between the
outer phenyl rings are opposite (in each leg) in comparison to
IVa/12. This leads to a clear enhancement of the clearing
temperature in comparison with the isomer IVa/12. The
structure of the isomer IVc/12 has been described in ref. 18
as consisting of undulated smectic layers (see footnote of
Table 1). In addition, the ground state was found to be
ferroelectric. In this ferroelectric phase of compound IVc/12 it
was found that the mechanism of the polar switching depends
on the frequency of the field and on the temperature. At lower
temperature and higher frequency the switching takes place in
the usual way, by the rotation of the director around the tilt
cone. At higher temperature and sufficiently low frequency the
switching mechanism is based on the collective rotation of the
molecules around their long axes.18
Fig. 1 Current response to a triangular voltage in the SmCPA phase
of compound IVf/12 (temperature: 100 uC; cell gap: 6 mm; applied
voltage: 360 VPP; frequency: 20 Hz).
compound IVf/12 or compound IVg/12 is cooled in the
presence of an alternating field of about 15 V mm21 a texture
with some well-developed circular domains arises. In the
circular domains the extinction crosses are parallel to the
crossed polarizers. By application of an electric field the crosses
rotate clockwise or anticlockwise depending on the polarity of
the field. After removing the field the crosses go back in the
original position and simultaneously the birefringence strongly
decreases. The rotation angle for compound IVf/12 is about
¡40u and ¡25u for compound IVg/12 which indicates a tilt
angle q of 40u and 25u, respectively. For both isomers the
current response to a triangular field shows two peaks per half
cycle indicating an antiferroelectric ground state (Fig. 1). For
compound IVf/12 the switching polarization PS was found
to be 320 nC cm22, for the isomeric compound IVg/12 PS is
560 nC cm22. On the basis of these experimental findings this
smectic phase can be assigned as a SmCAPA phase which
is switched into the SmCSPF state. In the case of compound
IVg/12 we were able to induce the SmCPA phase by a
sufficiently high electric field above the clearing temperature.
The maximum enhancement of the clearing temperature was
found to be 2.5 K at a field of #40 V mm21. Such behaviour
was already reported in refs. 27 and 28 for other bent-core
mesogens. It can be explained by the field-induced alignment
of ferroelectric clusters which obviously exist already in the
short range order region in the isotropic liquid.
Compounds IVd/12, IVe/12, IVh/12, IVi/12 and IVj/12
form a mesophase with a mosaic or a more fan-like texture
(see Fig. 2) which point to a columnar phase. In the case of
compounds IVd/12 and IVi/12 this mesophase is monotropic
and crystallizes immediately after formation and therefore
X-ray investigations could not be performed. For the com-
pounds IVe/12, IVh/12 and IVj/12 well-developed mono-
domains of the mesophase could be obtained by very slow
cooling of a drop of the isotropic liquid on a glass plate.
Fig. 3a shows the X-ray pattern of the mesophase of
compound IVh/12 obtained from a well-oriented sample. The
small-angle reflections can be described by a 2D oblique cell
(see Table 3).
As seen from Table 1 the isomer IVb/12 does not exhibit a
mesophase.
As found by X-ray investigations the isomers IVf/12 and
IVg/12 form tilted smectic phases like compound IVa/12. It is
˚
interesting that the layer spacing in compound IVf/12 (38.4 A)
is clearly different from that found in compound IVg/12
˚
(42.2 A). The polar character of these smectic phases could
It follows from the pattern that the molecules are tilted by
11u with respect to the b-axis. Surprisingly, cooling down the
sample to about 10 uC below the clearing temperature a
be detected by electro-optical studies. If the isotropic liquid of
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Fig. 2 Mosaic texture of the Colob phase (a) of compound IVe/12
(135 uC) and (b) of compound IVh/12 (160 uC).
continuous change of the X-ray pattern takes place (Fig. 3b),
which corresponds to the transition into a simple layer
˚
structure with a layer spacing of 50.1 A which is clearly higher
Fig. 3 X-Ray pattern of a monodomain in the small angle region and
a schematic sketch of the pattern with the indexing of the reflections
and the resulting reciprocal lattice (a) in the Colob phase (165 uC) and
(b) in the SmCPA phase (155 uC) of compound IVh/12.
than the d-values in the SmCPA phase of the isomers IVa/12,
IVf/12 and IVg/12 (Table 3).
From the relative position of the layer reflections with
respect to the diffuse wide angle maxima a tilt angle q of 34u
could be determined. The structural change at this transition is
schematically sketched in Fig. 4.
It should be noted that this transition is not indicated by a
change of the optical texture or by a calorimetric signal which
points to a phase transition of second order. On the other
hand, this transition is accompanied by a change of the electro-
optical response. In the columnar phase a small change of the
birefringence is observed which is independent of the polarity
of the applied field but measurements of the current response
give no hint to a polar switching. In contrast, in the smectic
phase the textures are different for an opposite sign of the field.
The current response to a triangular voltage shows two peaks
per half period which gives evidence for an antiferroelectric
ground state. According to our experimental findings the
smectic phase can be assigned as SmCAPA phase
Fig. 4 Structure models of the Colob phase and of the SmCPA phase
of compound IVh/12. The electro-optical measurements proved
an antiferroelectric packing. According to these an up-and-down
orientation of the polar axes (perpendicular to the a–b-plane) is
sketched, which demands—in principle—a doubling of the structure
parameter b and d.
columnar structure remains unchanged up to the crystal-
lization. Below #130 uC the columnar phase shows a clear
electro-optical response. By application of a sufficiently high
alternating electric field (15…12 V mm21 at 10 Hz) the original
Fig. 5 displays the X-ray pattern of the mesophase of
compound IVj/12 which can be also described by an oblique
2D cell (Table 3). In contrast to compound IVh/12 the
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Fig. 5 X-Ray pattern of a monodomain in the small angle region of
the Colob phase of compound IVj/12 (164 uC) and a schematic sketch
of the pattern with the indexing of the reflections and the resulting
reciprocal lattice.
mosaic texture with large domains transforms in a texture with
relatively small domains. In this state a polar switching is
observed where the texture of the switched state depends on
the polarity of the field. By means of the triangular wave
voltage method one current peak was recorded per half cycle
which points to a ferroelectric phase (PS # 300 nC cm22). On
the other hand, if the voltage is removed the switched-off state
exhibits a clearly different texture. We assume that the applied
field causes a transformation of the Colob phase into the
switchable SmCPF phase. Once this phase is formed it remains
in this state even when a triangular field is applied provided
that the field is higher than the threshold field. Note that such
field-induced transition of a columnar phase into a SmCP
phase was described in refs. 29–31.
Fig. 6 Texture of the field-induced SmCPF phase of compound
IVe/12: (a) + 190 V and (b) 2190 V (sample thickness: 6 mm, tempera-
ture 100 uC). Depending on the polarity of the applied field the extinc-
tion crosses rotate clockwise or anti-clockwise which is indicated by
arrows.
According to X-ray investigations on an oriented sample
also the mesophase of compound IVe/12 forms an oblique
˚
columnar phase the cell parameter of which are a 5 128.1 A,
˚
ferroelectric states. The texture with the circular domains
relaxes into a non-specific texture after removal of the field.
We assume that the switchable phase is also a field-induced
phase which only exists in presence of the field and has
probably a simple layer structure (SmCAPF). At temperatures
below 95 uC the texture with circular domains does not
disappear if the field is removed. In this case the higher
viscosity prevents obviously the relaxation into the columnar
phase. In this temperature region we observed an interesting
effect in some circular domains in which the outer and inner
parts possess opposite handedness which is indicated by an
opposite direction of the field-induced rotation of the
extinction crosses (clockwise or anticlockwise). If an alternat-
ing field of 35 V mm21 (50 Hz) is applied for some minutes the
inner ring grows at the cost of the outer one which has
opposite chirality. In this way the irreversible field-induced
inversion of chirality could be visualized. We observed the
same effect for the first time in the columnar phase of
compound IVc/12.18
b 5 47.2 A, c 5 106u. In this phase the molecules are tilted by
q 5 8u with respect to the b-axis (Table 3). An oblique cell of
the columnar phase of compound IVe/12 was also reported by
Murthy and Sadashiva,19 recently, on the base of X-ray studies
on a partially oriented sample. In contrast to ref. 19 we
found a clear electro-optical response of the Colob phase.
The application of an electric field at higher temperatures
leads to a slight change of the texture but a current response
indicating a polar switching was not detectable. Below 120 uC
the optical appearance of the switching changes. By applica-
tion of an alternating electric field (30–35 V mm21; 10–50 Hz)
the original fan-like texture transforms in a texture with
many well-developed circular domains in which the extinction
crosses enclose an angle of about 20u with respect to the
crossed polarizers. The extinction crosses rotate clockwise
or anticlockwise in dependence on the polarity of the field
(Fig. 6).
The characteristic current response (one peak per half
period) is an indication for the switching between two opposite
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Fig. 7 X-Ray pattern of a monodomain in the Colrec phase (127 uC)
of compound IVg/8 and a schematic sketch of the pattern with the
indexing of the reflections and the resulting reciprocal lattice.
For comparison, the analogous isomers (IVa/8–IVj/8) with
terminal octyloxy chains are presented in Table 2. It is seen
that for some homologues the mesophase behaviour is the
same as in the corresponding dodecyloxy isomers, see for
example compounds IVb/8, IVd/8, IVe/8, and IVi/8. In the
case of the homologue IVc the columnar phase of the octyloxy
compound IVc/8 is antiferroelectric, whereas that of the
dodecyloxy compound IVc/12 is ferroelectric.18 Furthermore,
compounds IVa/8, IVf/8, and IVg/8 exhibit a columnar phase,
whereas the corresponding dodecyloxy homologues form a
SmCPA phase. It was proved by X-ray diffraction on oriented
samples that the columnar phase of these three compounds
Fig. 9 Texture (a) of the Colob phase (160 uC) and (b) of the SmCPA
phase (130 uC) of compound IVj/8.
exhibits a 2D rectangular cell; as an example see the X-ray
pattern (Fig. 7) and the resulting structural model (Fig. 8) of
the Colrec phase of compound IVg/8.
The Colob phase of compound IVj/8 transforms on cooling
at 135 uC into a SmCPA phase which shows a relatively small
switching polarization (PS # 25 nC cm22). The phase
transition from columnar to smectic can be recognized by a
clear change of the texture (Fig. 9) but also by a peak in the
DSC curve. This smectic phase possesses a layer spacing of
˚
43 A and a tilt angle of q 5 18u.
Discussion
Two series of five-ring bent-core compounds presented here
possess terminal octyloxy chains and dodecyloxy chains,
respectively, and only ester connecting groups. The structure
of these isomers is distinguished by the direction of at least
one ester group. We found that in spite of the minor struc-
tural differences a variety of mesophases occur. It is also
remarkable that the clearing temperatures of the isomers are
distinguished by 72 K (IVa–j/8) or by 77 K (IVa–j/12) depend-
ing on the direction of the ester groups (see Tables 1 and 2).
Fig. 8 Structure model of the Colrec phase of compound IVg/8.
Neglecting the polar axes of the molecules in direction of the column
axis a centred rectangular lattice with the two dimensional space group
symmetry c2mm would result (projection on the a–b-plane). But the
physical properties demand—as sketched—an up-and down orienta-
tion of the axis, according to which the lattice is not longer centred but
has a layer group symmetry pmmm.38
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arrangement of the polar ester groups. In some way isomers
with higher flexibility can related to lower clearing tempera-
tures and to the preferred formation of smectic phases.
On the other hand, compounds with lower conformational
degree of freedom show a mostly higher mesophase stability
and form columnar phases. Preliminary MD simulations on
the banana-shaped molecules support the conformational
findings of the DFT studies.36
Rouillon et al.34 and Bedel et al.37 were able to show by
means of the electrostatic maps of bent-core molecules that
the rotation around the linkage groups strongly modifies the
repartition of charge density and therefore the mesophase
behaviour. Therefore we determined from the electrostatic
potential charges (ESP) a global pattern of the charge
distribution on the rings and connecting groups. The electron
density r on the central ring A and on the external rings C, C9
of the isomers correlates with mesophase properties. In such
cases where the electron density on the central ring is smaller
than that one on the external rings (r(A) , r(C,C9)) the
compounds form smectic phases . In the opposite cases
((r(A) . r(C,C9)) columnar phases result for the correspond-
ing isomers.36
For a given arrangement of the ester groups (IVa–j) the
mesophase behaviour depends also on the length of the
terminal chains. Comparing the isomers with octyloxy and
dodecyloxy chains we found in some cases the same phase
behaviour (IVb, IVd, IVe, IVi). There are three examples,
however, that the short-chain homologues IVa/8, IVf/8, and
IVg/8 exhibit a columnar phase (Col) whereas the homologues
IVa/12, IVf/12, and IVg/12 form a SmCPA phase. The
occurrence of Col phases for short-chain homologues and of
SmCP phases for long-chain members in homologous series of
bent-core mesogens is not unusual.3,6,11,16,17,32–35
It is important to stress, if the mesophase type of homo-
logues compounds is the same, it does not mean that the
physical properties are the same, especially the electrooptical
behaviour. For example, the Colob phase of the isomer IVe/8
shows no electro-optical response whereas that of the isomer
IVe/12 shows a reversible field-induced phase transition into a
ferroelectric SmCP phase (SmCPF). On the other hand, the
homologues IVc/8 and IVc/12 form a columnar phase, but that
of the short-chain derivative behaves antiferroelectric and that
of the long-chain homologue is ferroelectric.18
Another example are the homologues IVj. The compound
IVj/12 forms a Colob phase only, whereas the homologue IVj/8
shows the phase sequence Colob–SmCPA. Only the Colob phase
of compound IVj/12 exhibits a reversible field-induced transi-
tion into the ferroelectric SmCP phase. The same dimorphism
Colob–SmCPA was observed for compound IVh/12 but in this
case the phase transition is of second order.
Summarizing the results we can say that the mesophase
behaviour and the physical properties vary unexpectedly
strongly in dependence on the direction of the carboxyl
linkage groups in both series of isomeric compounds. It is
well-known that in calamitic liquid crystals the direction of
connecting groups between aromatic rings can influence the
mesophase behaviour, but not in such significant level.
Formally, we can see from the Tables 1 and 2 that the
highest clearing temperatures occur when four or three –OOC–
linkage groups are arranged with the C-atom in direction to
the central ring A. Significantly lower clearing temperatures
are observed when less (0…2) ester groups have this direction.
On the other hand, among the dodecyloxy isomers it is obvious
that the isomers with lower clearing temperatures preferably
form SmCP phases and the compounds with higher clearing
temperatures have a stronger tendency to form columnar
phases. For the octyloxy isomers the occurrence of columnar
phases is much more pronounced than for the dodecyloxy
isomers (see Tables 1 and 2).
Acknowledgements
This work was supported by the Deutsche Forschungs-
gemeinschaft (DFG, Graduiertenkolleg 894).
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