Influence of the core structure on the development of polar order and superstructural chirality in liquid crystalline phases formed by silylated bent-core molecules: Naphthalene derivatives

Article abstract of DOI:10.1039/b614089k

Bent-core molecules based on a bent 2,7-disubstituted naphthalene unit and heptamethyltrisiloxane units at one or both ends of the aliphatic side chains were synthesized and investigated by polarized light microscopy, differential scanning calorimetry, X-

Full text of DOI:10.1039/b614089k

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www.rsc.org/materials | Journal of Materials Chemistry
Influence of the core structure on the development of polar order and
superstructural chirality in liquid crystalline phases formed by silylated
bent-core molecules: naphthalene derivatives{{
R. Amaranatha Reddy,§a U. Baumeister,b C. Keitha and C. Tschierske*a
Received 27th September 2006, Accepted 14th November 2006
First published as an Advance Article on the web 24th November 2006
DOI: 10.1039/b614089k
Bent-core molecules based on a bent 2,7-disubstituted naphthalene unit and
heptamethyltrisiloxane units at one or both ends of the aliphatic side chains were synthesized and
investigated by polarized light microscopy, differential scanning calorimetry, X-ray scattering and
electrooptical investigations. The effect of fluorine substitution at the periphery of the rigid core
and of the replacement of ether oxygens by carboxylate groups as linking units between the
aromatic core and the flexible terminal chains was also investigated. All compounds form broad
regions of liquid crystalline phases. Different types of non-polar and polar, i.e. antiferroelectric
and ferroelectric switching smectic phases, and columnar phases were observed, depending on the
molecular structure. Most interestingly, a temperature dependent transition from an
antiferroelectric switching to a ferroelectric switching oblique columnar phase was observed. All
double-silylated compounds show switching by a collective rotation around the molecular long
axis, a process which switches the superstructural chirality in these LC systems.
switching behavior of some mesophases attracted special
interest in this field.6–8
1. Introduction
Materials with macroscopic polar order are of significant
These unique properties are due to the restricted rotation of
interest for piezoelectric, pyroelectric and nonlinear optical
the bent molecules, giving rise to a polar direction parallel to
applications. Fluid polar ordered materials with liquid crystal-
the layers (see Fig. S1 in ESI{). In order to escape from the
macroscopic polar order the bent direction usually alternates
line properties are of special interest for fast switching
electrooptical devices (displays) and liquid crystal over silicon
in adjacent layers, leading to antipolar (antiferroelectric = AF)
(LCOS) devices, used for spatial light modulators (SLMs) and
ground state structures in most cases. These SmCPA phases
optical correlators.1
show AF switching (see Fig. S1a{). Beside these smectic phases
there are also columnar phases composed of ribbon-like
Meyer et al. first reported that ferroelectricity can occur in
liquid crystalline phases with reduced symmetry.2 This is
segments of these layers which adopt a long range order in a
achieved when enantiomerically enriched chiral molecules are
2D lattice. Rectangular (Colr) and oblique (Colob) lattices can
be distinguished in most cases.9 Most of these ribbon phases
arranged in layers and have an average tilt with respect to the
layer normal (e.g. SmC* phases).3,4 In 1996 Niori et al.
are non-switchable (Colr or Colob) or show AF switching
(ColrPA or ColobPA).10,11 Several attempts have been made to
reported that (anti)ferroelectricity in smectic phases can also
be achieved by achiral molecules with a chevron shape, also
known as banana molecules or bent-core molecules.5 The high
absence of an electric field, leading to ferroelectric (FE)
design bent-core molecules with stable polar order in the
values of the polarization (Ps = 600–1000 nC cm22), the
switching materials (see Fig. S1a{).12 Some FE switching
occurrence of new LC phase types, unknown for other LC
smectic phases and also FE switching columnar phases have
been reported (ColrPFE or ColobPFE) occasionally.13 The
materials, the observation of chirality and spontaneous
symmetry breaking in some cases, as well as the special
possible modes of AF and FE organization are summarized
in Fig. S1b.{
Beside the polar order, chirality in LC phases of non-chiral
aInstitute of Organic Chemistry, Martin-Luther-University Halle-
Wittenberg, Kurt-Mothes-Str. 2, D-06120, Halle, Germany.
molecules and the observation of switching of the chirality
sense by external fields gave rise to broad and general scientific
interest in bent-core mesogens.7c–e,14 Chirality in the meso-
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: +49 345 552 7346;
Tel: +49 345 552 5664
bInstitute of Physical Chemistry, Martin-Luther-University Halle-
Wittenberg, Mu¨hlpforte, D-06108, Halle, Germany
phases of non-chiral bent-core molecules is due to the tilted
organization of the molecules in the polar layers or ribbons.
{ Electronic supplementary information (ESI) available: additional
figures and tables; experimental methods; analytical data. See DOI:
This combination of tilt and polar direction leads to the loss of
mirror symmetry. Hence, layer normal, polar direction and tilt
direction define either a right-handed or a left-handed system.
Any change of polarization direction or tilt direction changes
the chirality sense of the systems, whereas simultaneously
10.1039/b614089k
{ The HTML version of this article has been enhanced with colour
images.
§ Present address: Dept. of Chemistry & Biochemistry, University of
Colorado at Boulder, Boulder, CO 80309-0215, USA.
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changing both polar direction and tilt direction retains the
chirality sense (see Fig. S2{).6
direction, whereas the tilt direction is not changed. In this
switching process the chirality of the layers and hence the
mesophase chirality is changed (Fig. S2b{).14,17 These effects
of the silyl groups are mainly due to the nano-segregation of
the oligo(siloxane) units into distinct sublayers located at the
inter-layer interfaces, and due to the size of these units which
influences the packing density between the aromatic cores and
in addition can lead to a distortion of an organization in flat
layers.
Oligo(siloxane) units at one or both ends of bent-core
compounds have a large impact upon mesophase type, polar
order and chirality.15–17 Several of these materials exhibit
surface stabilized FE switching (SmCPFE phases), and in
addition show spontaneous achiral symmetry breaking, seen as
a spontaneous formation of regions with opposite chirality
sense in the so-called dark-conglomerate texture, designated
with ‘‘[*]’’ in the phase assignment.7c,8 For this reason a large
variety of bent-core molecules based on oligosiloxane and car-
bosilane segments have been synthesized in recent years.15–17
As exemplified in the series of compounds in Scheme 1, the AF
switching smectic phase (SmCPA) of the dialkyl-substituted
molecule Bp-H18 is replaced by a smectic phase showing
ferroelectric switching (SmCPFE[*]) after introduction of the
first heptamethyltrisiloxane unit.15a,c This smectic phase is
replaced by a non-polar columnar phase (Colob), an AF
switching columnar (ColobPA) and AF as well as FE switching
smectic phases (SmCPA, SmCPFE) if a second silyl group is
attached to the other end (compound Bp-Si2).17 In addition
the mechanism of the switching process is changed by the
introduction of silyl units. The switching in the mesophases of
Bp-H and Bp-Si1 takes place by a collective rotation of the
bent-core molecules on a cone. This process, which is usually
observed for bent-core molecules, reverses the polar direction
as well as the tilt direction and hence retains the chirality of the
layers as well as the macroscopic chirality (Fig. S2a{). In
contrast, the switching in all mesophases of compounds
Bp-Si2, silylated at both ends, takes place by a collective
rotation around the long axis which changes only the polar
In nearly all silylated materials the 3,49-disubstituted
biphenyl unit18 was used as the central bent unit (BU) within
the bent aromatic core.15,16a–c,f,g,17 In order to improve our
understanding of the general structure–property relations of
bent-core mesogens, to further modify the mesomorphic
properties and to find new LC phases with interesting
application properties we have modified the bent aromatic
core. Herein we report the effects of replacing the 3,49-biphenyl
core used in compounds Bp-Si2 and Bp-Si1 by a 2,7-
disubstituted naphthyl unit,19 leading to compounds 1-Si and
4-Si, respectively. In addition, the effect of lateral substituents
at the outer phenyl rings (Y = F, 2-Si) and the type of linking
unit X connecting the terminal chains to the bent rigid cores
(ether unit in 1-Si vs. ester unit in 3-Si) was investigated for the
double silylated bent-core molecule 1-Si.
2. Results and discussion
2.1. Synthesis
The general synthetic pathway used to obtain compounds 1-Si
to 3-Si is outlined in Scheme 2. As shown, the 4-(benzoyloxy)
benzoic acids B were obtained by the esterification reaction of
Scheme 1 Comparison of the mesomorphic properties of a compound consisting of conventional bent-core molecules (compound Bp-H18) with a
related one having one heptamethyltrisiloxane-1-yl unit at one end (compound Bp-Si115a,c) and compound Bp-Si2 having heptamethyltrisiloxane
units at both ends.17 In this compound there is an additional p-substituted benzene ring in the bent aromatic core which gives rise to significantly
enhanced mesophase stabilities in comparison to compounds Bp-H and Bp-Si1; abbreviations: Cr = crystalline state; Iso = isotropic liquid state;
SmCPA = antiferroelectric switching birefringent SmC phase; SmCPFE[*] = ferroelectric switching SmC phase which is optically isotropic and
composed of a conglomerate of optically active domains of opposite handedness (dark conglomerate phase); SmCPFE = ferroelectric switching
birefringent SmC phase; ColobPA = antiferroelectric switching oblique columnar phase; Colob = non-switchable oblique columnar phase.
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4-(10-undecen-1-yloxy)benzoic acid, of 3-fluoro-4-(10-unde-
cen-1-yloxy)benzoic acid or 4-(9-decen-1-yloxycarbonyl)ben-
zoic acid with 4-hydroxybenzaldehyde to give A, followed by
the oxidation of the aldehyde to the corresponding benzoic
acid B.20 These substituted benzoic acids were further esterified
with naphthalene-2,7-diol to yield the olefinic precursors 1-En
to 3-En.19d The hydrosilylation of these olefinic compounds
using 1,1,1,3,3,5,5-heptamethyltrisiloxane resulted in the cor-
responding siloxane substituted compounds 1-Si to 3-Si.17
The unsymmetrical olefin 4-En with a double bond at only
one end was synthesized from the precursor C, similar to
earlier reports,19d by esterification with 4-(10-undecen-1-
yloxy)benzoic acid. Hydrosilylation with 1,1,1,3,3,5,5-hepta-
methyltrisiloxane yielded compound 4-Si with only one silyl
unit (see Scheme 3). All target compounds were purified using
centrifugal thin layer chromatography followed by several
crystallizations. The purity was checked by thin-layer chro-
matography. The chemical structure of all these compounds
was confirmed by 1H-, 13C-, 19F-, 29Si-NMR and elemental
analysis (see ESI{ for details). The mesophase behaviour was
investigated by polarized light microscopy, differential scan-
ning calorimetry, electro-optical investigations and X-ray
scattering using powder investigations (Guinier film camera)
and 2D patterns of aligned samples.
2.2. Mesomorphic properties of the olefinic precursors
Transition temperatures and associated enthalpies for the
intermediate compounds n-En with olefinic double bonds on
both the terminals are collated in Table 1. Compound 1-En
exhibits an enantiotropic AF switching smectic phase, which is
optically isotropic and shows chiral domains of opposite
handedness, as seen by slightly uncrossing the polarizers.19d
Hence, it belongs to the dark conglomerate phases and this
phase is assigned as SmCPA *]. Though the earlier reports
[
indicate that this mesophase is a non-polar BX phase,19d our
observations clearly demonstrate two polarization current
peaks indicating an AF switching which is shown in Fig. S3.{
Compound 2-En with fluorine substituents at the two outer
phenyl rings, in ortho positions to the terminal alkoxy chains,
also shows a smectic phase with chiral domains. However for
this compound no electro-optical response can be observed up
to a voltage of about 400 Vpp (peak-to-peak voltage) for a 5 mm
thick cell, the maximum voltage attainable from our experi-
mental set-up. Though no switching is found experimentally,
the appearance of a dark conglomerate texture suggests the
presence of polar order, but the voltage required for switching
is obviously not reached. Therefore this mesophase is assigned
as a SmCP[*] phase.
Scheme 2 Synthesis of compounds 1-Si to 3-Si (X = CH2O, OOC;
Y = H, F; R3SiH = Me3Si(OMe2Si)2H,). Reagents and conditions:
i: 4-hydroxybenzaldehyde, DCC, DMAP, dry CH2Cl2, ii: NaClO2,
NaH2PO4, resorcinol, t-BuOH, iii: DCC, DMAP, dry CH2Cl2, iv:
Karstedt’s cat., dry toluene, 25 uC, 48 h.
Scheme 3 Synthesis of monofunctionalized compounds 4-En and 4-Si
(R3SiH
= Me3Si(OMe2Si)2H). Reagents and conditions: i: DCC,
DMAP, CH2Cl2, ii: Karstedt’s cat., toluene, 25 uC, 48 h.
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Table 1 Phase transition temperatures (T/uC) and enthalpy values (DH/kJ mol21, values in square brackets) obtained for the olefin-terminated
bent-core compounds n-En derived from a 2,7-disubstituted naphthalene unita
Comp.
Y
X
X9
Phase transitions
Ref.
1-En
2-En
3-En
a
H
F
H
CH2O
CH2O
OOC
OCH2
OCH2
COO
Cr 126 [37] SmCPA[*] 158 [17] Iso
Cr 154 [65] SmCP[*] 171 [18] Iso
Cr 155 [52] ColobPFE 170 [24] Iso
19d
—
—
SmCP[*] = smectic C phase with dark conglomerate texture which does not show electrooptical switching under the used experimental
conditions (see text), ColobPFE = ferroelectric switching oblique columnar phase, for the other abbreviations, see Scheme 1; transition
temperatures were determined from DSC heating scans (10 K min21).
The most interesting LC phase was observed for compound
3-En, with ester groups linking the terminal chains to the rigid
core. The optical texture shown in Fig. 1a indicates the
columnar nature of the mesophase. The X-ray diffraction
pattern of a partially aligned sample (Fig. 1b,c) can be indexed
organization of the molecules in this mesophase. Due to the
relatively dense packing of the molecules the switching process
requires very high voltages. In the case of compound 3-En the
dense packing might be due to the presence of the electron
deficient terephthalate units which increase the attractive p–p
interactions between adjacent aromatic rings. The best packing
of the molecules in a ribbon phase with a tilted organization of
the molecules is provided in an AF structure where the polar
direction changes within the modulated smectic layers along
direction a and with a synpolar order along direction b (see
Fig. 1e). In such an arrangement all molecules are aligned
parallel at the inter-ribbon interfaces. The synpolar order
between adjacent ribbons along direction b requires a splay of
the polar direction, as shown in Fig. 1e, to reduce the
polarization within the layer stacks and to improve the
packing of the molecules at the inter-ribbon interfaces.24 Due
to the rather large number of molecules organized in the cross
section of the ribbons the occurrence of some splay of
polarization is likely.25 The fact that related bent-core
mesogens with ester bonds interconnecting the rigid core with
the alkyl chains show B7 type mesophases with FE switching
additionally supports this model.13a,b,26
˚
to an oblique cell. In this way the lattice parameters a = 56.4 A;
˚
b = 46 A and c = 114u were obtained (see Table 1 and Table
S1{). Most columnar phases of bent-core mesogens represent
modulated smectic phases built up by ribbon-like fragments of
smectic layers organized in a 2D lattice. The parameter a
provides information about the cross section of the ribbons. It
was estimated that the total number of molecules per unit cell
(ncell) amounts to 8–9, which means that on average 8–9
molecules are located in the cross section of each ribbon (see
Table S2{).21 The parameter b is smaller than the molecular
22
length (L = 55 A) which indicates that the molecules are
˚
tilted with respect to direction b. The scattering in the wide-
˚
angle region with a maximum at d = 4.7 A is diffuse and
indicates the liquid crystalline character of the mesophase. The
position of the maxima of the wide angle scattering between
meridian and equator and the unequal distribution of the
intensity with respect to the meridian reveals a synclinic tilt (tilt
angle # 27u) of the molecules in this mesophase (see Fig. 1b
and S4{).
2.3. Mesomorphic properties of Si-containing compounds
The transition enthalpy (DH = 24 kJ mol21) for the Colob to
Iso transition is rather large (Table 1). As no sharp reflections
in the wide angle region of the X-ray diffraction pattern can be
2.3.1 Influence of the bent unit on the mesomorphic properties.
Table 2 shows the influence of the central bent unit (BU) on
the mesomorphic properties of the terminally silylated bent-
core molecules. It can be seen that the highest transition
temperatures were found for the 2,7-disubstituted naphthalene
derivative 1-Si, whereas the broadest mesomorphic range
was obtained for the 3,49-disubstituted biphenyl derivative
Bp-Si2.17b The resorcinol derivative Ph-Si shows only one
oblique columnar phase (ColobPA) over the whole meso-
morphic temperature range.27 In contrast, compounds Bp-Si2
and 1-Si with larger bent rigid cores show a series of quite
different smectic and columnar mesophases. It is interesting to
note that for the biphenyl derivative Bp-Si2 the smectic phases
occur below the columnar phases, whereas for the naphthalene
derivative 1-Si a smectic phase is observed at temperatures
above the columnar phases. However, not only the position of
these smectic phases in the phase sequence, but also their
structure and switching behaviour, are different.
found (see Fig. 1b)
a highly ordered (semi-crystalline)
mesophase can be excluded. In this case the high transition
enthalpy is a hint of a polar organization in this mesophase. A
single polarization current peak was detected in electrooptical
experiments only at a very high voltage of more than 300 Vpp
in a 5 mm cell (see Fig. 1d). It disappears at the transition to the
isotropic liquid state and the appearance of this peak is
associated with a change of the texture. Hence, it could be
possible that either a FE switching smectic phase is induced at
high voltage or FE switching of the columnar phase itself is
observed under these conditions. Upon slow cooling under an
AC field partially aligned circular domains were obtained
which do not change their position upon reversal of the
applied electric field.23 In these domains the extinction crosses
are inclined with the polarizers, which confirms the synclinic
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2.3.2 Compound 1-Si—transition from AF to FE switching
columnar phases. Phase transition temperatures and associated
enthalpies for the Si-containing bent-core molecules 1-Si to
3-Si are summarized in Table 3. Silylation of the precursors
n-En (Table 1) gives rise to a stabilization of the mesophases
and in most cases the melting points are depressed. Hence, all
compounds n-Si form enantiotropic liquid crystalline phases
with enlarged mesophase regions and these mesophases are
distinct from those of the related precursors n-En.
Compound 1-Si in which the 3,49-disubstituted biphenyl
unit is replaced by a 2,7-disubstituted naphthalene central unit
shows three different mesophases. The high temperature phase
shows a typical SmC schlieren texture, but also a broken fan
texture can be observed (Fig. 2a,b). The phase transitions to
the other mesophases can be observed optically by slight
textural changes (see Fig. 2c,d). The high temperature phase is
22
˚
˚
a tilted smectic phase (d = 44.7 A, L = 69.5 A ). The other two
mesophases are columnar phases as indicated by XRD of
aligned samples (Fig. 2f,g, Fig. 3, Table 4 and Table S1{).
In all diffraction patterns the diffuse scattering in the wide
angle region indicates a synclinic tilted organization of the
molecules in all three LC phases (tilt angle about 50u in the two
phases at higher temperatures, about 45u in the low-
temperature phase) with microsegregated oligosiloxane sub-
˚
layers (diffuse ring at d = 6.9 A). Electrooptical investigations
(see below for a more detailed discussion) indicate a non-polar
SmC phase at high temperature, the columnar phase below
this smectic phase shows AF switching (ColobPA) which
changes to FE at the transition to the low temperature
columnar phase (ColobPFE).
The XRD pattern of the high temperature columnar phase
(Fig. 2f, 3a) can be indexed to a rectangular cell with the lattice
˚
˚
parameters a = 23.8 A and b = 87 A. However, the proven
synclinic tilted organization of the molecules in this columnar
phase (see wide angle region in Fig. 2f which shows a clear
maximum of the intensity at the right top) excludes a true
rectangular lattice. In fact this mesophase has an oblique
˚
˚
lattice with the parameters a = 23.8 A, b = 45.1 A and c = 105u
(Fig. 3b), which accidentally coincides with a larger pseudo-
rectangular cell, and therefore this mesophase is assigned as
ColobPA. In Fig. 4a the pseudo-rectangular lattice is shown in
dotted black lines together with an oblique lattice (shown in
solid lines) in a possible model of the ColobPA phase. It should
be pointed out that this phase assignment, based on non-
resonant X-ray scattering, cannot distinguish the polar
direction of the columns. The assignment to an overall AF
structure is based on electrooptical investigations which
indicate an AF switching in this mesophase. There are several
different possible modes for such an AF organization. In the
most probable model, shown in Fig. 4a, there is an overlapping
of the rod-like wings of the bent cores at the interfaces between
adjacent ribbons in the modulated layers, as also proposed for
the Colob phase of 3-En. However the actual organization is
slightly different, as there is no interaction of the aromatic
cores across adjacent layers. This avoids any unfavourable
interruption of the oligosiloxane sublayers. A change of the
polar direction between each ribbon within these modulated
layers (along direction a) provides a parallel organization of
the rod-like wings at the interfaces between the ribbons (see
Fig. 1 The mesophase of compound 3-En: (a) optical photomicro-
graph (crossed polarizers) at 168 uC during growing from the liquid
state (dark region); (b) XRD pattern obtained for a partially aligned
sample at 162 uC, intensity of the isotropic liquid is subtracted to
enhance the visibility of the anisotropic distribution of the diffuse
scattering (the slightly higher intensity at the top right suggests a
synclinic tilt of the molecules); (c) small angle pattern at 162 uC; (d)
switching current response attained under a triangular wave field
(360 Vpp, 5 Hz, 5 mm, 155 uC); (e) model of the organization in the
ribbon phase (only 5 of the 8–9 molecules located in the cross section
of the ribbons are shown).
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Table 2 Influence of the bent unit (BU) upon the liquid crystalline phases (T/uC) of silylated bent-core mesogensa
Comp.
BU
Phase transitions
Ph-Si
Cr 83 ColobPA 116 Iso27
1-Si
Cr 130 ColobPFE 148 ColobPA 170 SmC 175 Iso
Bp-Si2
Cr 89 SmCPFE 126 SmCPA 143 ColobPA 147 Colob 155 Iso17b
a
For the abbreviations, see Scheme 1.
Table 3 Phase transition temperatures (T/uC), and enthalpy values (DH/kJ mol21, values in square brackets) obtained for the silyl-terminated
bent-core compounds 1-Si to 3-Si
Comp.
X
X9
Y
Phase transitions
1-Si
2-Si
3-Si
a
CH2O
CH2O
OOC
OCH2
OCH2
COO
H
F
H
Cr 130 [7.5] ColobPFE 148 [1.6] ColobPA 170 SmC 175 [19]a Iso
Cr 134 [23] Colob 180 [11] Iso
Cr 140 [20] Colob 177 [3] USmCb 181 [2.8] Iso
b
Enthalpy includes the transitions ColobPA to SmC and SmC to Iso. USmC = undulated smectic phase.
Fig. 2 (a)–(d) Optical textures and (e)–(g) XRD patterns of aligned samples of the mesophases of compound 1-Si: (a) SmC phase at 171 uC; (b)
Iso–SmC transition at 173 uC; (c) paramorphotic texture of the ColobPA phase at 160 uC; (d) paramorphotic texture of the ColobPFE phase at
140 uC; (e) SmC phase at 171 uC; (f) ColobPA phase at 160 uC; (g) ColobPFE phase at 140 uC.
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Table 4 X-Ray data of the columnar phases of compounds 3-En and
1-Si to 3-Sia
˚
Comp. T/uC Phase type Lattice parameters/A; u
ncell
3-En
1-Si
162
160
140
151
170
ColobPFE
ColobPA
ColobPFE
Colob
a = 56.4; b = 46; c = 114u
a = 23.8, b = 45.1; c = 105u
a = 51.2, b = 47.5; c = 110.4u
a = 84.5; b = 53.2; c = 113.4u 10–11
a = 87.9, b = 55.5; c = 114.5u 11
8–9
2–3
5–6
2-Si
3-Si
a
Colob
For details see Table S1 (ESI); abbreviations: ncell = molecules per
unit cell, for the calculations see Table S2 (ESI).
Fig. 4 Proposed models for (a) the ColobPA phase of 1-Si (the dotted
rectangle indicates the pseudo-rectangular lattice, the solid lines
indicate the oblique lattice and the black arrows indicate the settings
of the layer group, the light gray areas indicate the position of the
oligosiloxane subspaces); (b) FE order within the modulated layers
leads to unfavourable inter-ribbon interfaces; the insets in (a) and (b)
show the organization of the aromatic cores at the inter-ribbon
interfaces in a side view along direction a; (c) model of the AF ground
state structure assumed for the ColobPFE phases of 1-Si.
of the silylated bent-core molecules.15d,16g,17b This organization
would correspond to the monoclinic/oblique layer group p112/
a,10b,28 indicated by arrows in Fig. 4a. A splay of polarization
is unlikely in this case as the ribbons have only a few molecules
in the cross section.
On decreasing temperature, a complete change of the XRD
small angle pattern could be observed at the transition to the
lowest temperature phase (see Fig. 3d,e) and these reflections
can be indexed to an oblique cell with the lattice parameters
Fig. 3 Indexing of the 2D X-ray patterns for a surface-aligned sample
of compound 1-Si in the ColobPA (160 uC, (a)–(c)) and in the ColobPFE
phase (140 uC, (d)–(f)); (a),(d) original patterns and (b),(e) reciprocal
lattices with hk values for the observed reflections in the small-angle
region; (c),(f) full scattering range with reciprocal lattice (gray lines),
real lattice (black lines), direction of the maximum of the outer diffuse
scattering (dashed line), and the most probable orientation of the
molecules (wedge shapes) with respect to the axes of the 2D unit cell.
˚
˚
a = 51.2 A, b = 47.5 A and c = 110.4u (Tables 4 and S1{).
Hence, the lattice parameter a is more than doubled whereas
the parameters b and c are only slightly increased. This means
that the number of molecules in the cross section of the
ribbons is nearly doubled from 2–3 to 5–6 (see Tables 4 and
S2{) at the phase transition. As no significant change of the
wide angle scattering is observed at the transition between
the two columnar phases (see Fig. 2f,g), the tilt direction of the
molecules should not be fundamentally changed at this
transition. As seen in Fig. 2f and g there is a maximum
Fig. 4a, inset). In addition, it is assumed that the correlation
between adjacent ribbons is synclinic and synpolar, as typically
observed for the preferred organization in the smectic phases
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intensity of the outer scattering at the right side in both
mesophases. Such an asymmetric distribution arises from a
packing of molecules with synclinic tilt in a sample consisting
of polydomains that are not perfectly fibre-like disordered
around an axis perpendicular to the alignment surface, but
with a preferred orientation. In this case the orientation of the
molecules with respect to the axes of the 2D lattice can be
deduced from the position of this maximum. The actual tilt
angle of the molecules with respect to the b axis (see Fig. 3c,f)
may be estimated from the direction of the maximum of the
outer scattering with respect to the b* axis on the meridian of
the 2D X-ray patterns (= 90u 2 tilt angle) and the lattice
parameter c as c 2 (90u 2 tilt angle). This tilt is almost the
same in both phases and amounts to about 65u. But the tilt of
the molecules with respect to direction a (90u 2 tilt angle)
changes from about 40u in the high temperature phase to about
45u in the lower temperature phase, i.e. the molecules become
less inclined against the normal to the broken layers, and
simultaneously the ribbons become broader at the phase
transition.
(ColobPFE, see Fig. 5b). The polarization values are in the
range of 200–300 nC cm22 in both mesophases. On slow
cooling from the isotropic liquid under a triangular wave field,
circular domains with extinction crosses inclined with the
direction of the polarizers were obtained which indicate a
synclinic tilt of the molecules in all three phases (Fig. 6). In
addition, optical observations imply that the polarization
reversal takes place around the long axis of the molecules in
the AF as well as in the FE columnar mesophase as indicated
by the textures shown in Fig. 6, where no rotation of the
extinction crosses can be observed, either in the non-switching
SmC phase (Fig. 6a) or in the two polar switching columnar
phases (Fig. 6b,c).
The behaviour of the ColobPA phase is in line with the
observations made with other ColobPA phases of bent-core
molecules with silyl groups.15,17 For these mesophases it is
assumed that the unfavourable inter-ribbon interfaces occur-
ring in the FE organization along the modulated layers (see
Fig. 4b) are responsible for the stabilization of the AF ground
state structure, leading to high threshold voltages and
relatively low polarization values.15c,17b
A model of the ColobPFE phase is shown in Fig. 4c. As will
be explained later, the ground state structure of this mesophase
is assumed to be AF, though a FE switching is observed
experimentally. The organization of the molecules is very
similar to that proposed for the ColobPA phase observed for
1-Si at higher temperature with the difference that the ribbons
have a larger size.
The FE switching at lower temperature is more difficult to
explain. In principle it is possible that the layer distortion is
removed under the alignment field and a smectic phase is
induced. In this induced smectic phase the unfavourable inter-
ribbon interfaces (stabilizing an AF organization) are largely
As already mentioned, the high-temperature columnar
phase shows AF switching (ColobPA) which changes to FE in
the temperature range of the low temperature Colob phase
Fig. 6 Circular domains with extinction crosses as grown under a
triangular wave voltage (300 Vpp, 100 Hz, 5 mm) for compound 1-Si,
their position under the applied field (left) is compared with that after
switching off the applied field (right): (a) SmC phase at 172 uC; (b)
ColobPA phase at 160 uC; (c) ColobPFE phase at 140 uC; the positions of
the extinctions are inclined with the polarizers (indicated by arrows)
which indicates a synclinic organization in all mesophases; in (a) and
(c) the circular domains are smooth indicating a smectic phase, a
segmented structure can clearly be seen in (b), there is a slightly lower
birefringence in the 0 V states for (b) and (c) which is typically seen for
a polar switching by rotation around the long axis.
Fig. 5 Switching current response traces obtained under a triangular
wave (300 Vpp, 100 Hz, 5 mm) for the mesophases of compound 1-Si:
(a) ColobPA phase at 155 uC, (b) ColobPFE phase at 135 uC.
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removed, and hence, surface stabilized FE switching can take
place, as typical for the smectic phases (SmCPFE) of other
silylated bent-core molecules.15,16g This is in line with the
behaviour of other silylated bent core molecules where a
transition from ColobPA phases at higher temperature to
smectic and undulated smectic phases at lower temperature is
often observed.15c,17 This transition is due to the larger thermal
expansion coefficient of the oligosiloxane units in comparison
to the other parts of the molecules, giving rise to a stronger
steric frustration caused by these units at higher temperature.
This is in line with the observation that the size of the ribbons
modulated smectic phase (ribbon phase) are too weak or that
this mesophase is a smectic phase with a very long or only
weakly correlated layer undulation (wavy deformed layers).
The wide-angle pattern of a partly aligned sample (Fig. S5a{)
evidences
a tilted smectic mesophase. In electrooptical
investigations no polar switching could be observed, and
therefore, the mesophase is tentatively assigned as a USmC
phase. On further cooling a transition to an oblique columnar
phase takes place which is associated with a textural change
˚
(Fig. 8b) and a change of the XRD pattern (a = 87.9 A, b =
˚
55.5 A and c = 114.5u, synclinic tilt, see Fig. 8d,e, Fig. S5b{
is nearly doubled at the phase transition ColobPA to ColobPFE
.
Also the smooth appearance of the texture seen under an
electric field (Fig. 6c), which is similar to that of the SmC
phase (Fig 6a) but different from the ColobPA phase (Fig. 6b) is
in line with the assumption of a field induced SmCPFE phase.
However, in these SmCPFE phases the switching between the
FE states usually occurs at very low voltage, closely after zero-
voltage crossing and high values of the spontaneous polariza-
tion (600–1000 nC cm22) were measured.15,16a–c,f,g,17b This is
clearly not the case for the switching of the ColobPFE phase
which is characterized by relatively low polarization values and
a rather high threshold voltage. One possible reason could be
that in the field induced smectic phase there is still some layer
undulation or layer modulation which provides an energetic
barrier for the switching between the two field induced FE
states.
2.3.3 Influence of F-substitution at the periphery of the rigid
core (compound 2-Si). Remarkably, fluorine substitution at the
outer phenyl rings (compound 2-Si) eliminates the non-polar
SmC phase, as well as the AF switching columnar phase and
˚
˚
only an oblique columnar phase (Fig. 7, a = 84.5A, b = 53.2 A
and c = 113.4u) is found throughout the whole mesomorphic
temperature range. About 10–11 molecules are organized in
the cross section of these ribbons, which means that the size of
the ribbons is nearly doubled in comparison to the FE
switching ColobPFE phase of 1-Si. However, this Colob phase
shows no polarization peak under a triangular wave electric
field. It is also important to mention that the transition
enthalpy of the Iso–Colob transition is only nearly half of the
value observed for the Iso–SmC–ColobPA transition of 1-Si.
Hence it is likely that there is no polar order within the ribbons
of this Colob phase and that the formation of the ribbon
structure is in this case only due to the steric effect of the silyl
end-groups.
2.3.4 Influence of the linking group (compound 3-Si). In
compound 3-Si the flexible chains are connected to the
aromatic core by carboxylate linking groups (X, X9) instead
of the ether groups in 1-Si. This compound shows two
mesophases with textures shown in Fig. 8a,b. The growth of
the high temperature phase suggests a columnar or undulated
smectic phase (Fig. 8a). This is confirmed by shearing
experiments, where the birefringence is retained, but no
schlieren texture or oily streaks texture as typical for smectic
phases can be observed. However, in the XRD pattern only
layer reflections can be found (Fig. 8c). In order to resolve this
contradiction it is assumed that the cross reflections of a
Fig. 7 Investigation of compound 2-Si: (a) optical texture (crossed
polarizers) obtained at the Iso–Colob transition at 178 uC, (b) natural
texture of the Colob phase at 170 uC; (c)–(e) 2D X-ray patterns for a
surface-aligned sample of compound 2-Si in the Colob phase (151 uC):
(c) small angle region; (d) indexing of the reflections in the small-angle
region; reciprocal lattices of the two mainly scattering domains of the
fibre-like disordered sample (black and gray lines, respectively) with hk
values for the observed reflections and axes given for the black lattice;
(e) complete diffraction pattern.
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texture is optically isotropic and exhibits chiral domains of
opposite handedness under slightly uncrossed polarizers as
shown in Fig. 9a–c (dark conglomerate texture). XRD
˚
indicates a smectic phase with a layer spacing of d = 45.5 A
at 130 uC which is again smaller than the calculated molecular
length (L = 66 A)22 suggesting a tilted organization. Moreover,
˚
this mesophase shows FE switching (see Fig. 9d) with a
polarization of about 440 nC cm22. Hence the mesophase is
similar to that observed for the monosilylated biphenyl
[ ]
*
derivative Bp-Si1 (SmCPFE
phase, see Table 2).15c
However, the value of the spontaneous polarization is smaller.
This is because the aromatic bent core is smaller than that of
Bp-Si1. In this case the steric effect of the silyl groups becomes
more important. As a consequence, the aromatic cores become
more separated and polar order is reduced.
It is also interesting to note that the two very different
compounds, the non-silylated compound 4-En and the
silylated compound 4-Si, both show the same dark conglom-
erate textures, though they have
a different switching
behaviour. Compounds related to 4-En (with decyl and
dodecyl chains) were reported to show no switching,19d but
in our experiments with a higher voltage (.75 Vpp mm21) we
can indicate an AF switching for 4-En (SmCPA *] phase).
[
Introduction of a heptametyltrisiloxane segment at one end
changes the interlayer interfaces in such a way that only one
polarization current peak is observed for the mesophase of
4-Si. This peak does not split under a modified triangular wave
voltage (see Fig. 9d) and indicates FE switching. This
behaviour is very similar to that observed for the related
biphenyl derivatives and suggests that this mesophase is
composed of [SmCsPF]aPA layer stacks (see Fig. 10b) as
explained earlier.15c,16g,17b For 4-Si the size of these SmCsPF
layer stacks is sufficiently large to efficiently stabilize the FE
states by polar interaction with the substrate surfaces and
hence FE switching is experimentally observed.17b In addition
the steric layer distorting effect of only a single silyl group is
not sufficient to induce a ribbon structure and therefore FE
switching is retained.
Fig. 8 Optical textures (crossed polarizers) and XRD pattern
obtained for compound 3-Si: (a) texture of the USmC phase at
178 uC; (b) texture of the Colob phase at 170 uC; (c) small angle XRD
patterns of an aligned sample in the USmC phase at 178 uC; (d) in the
Colob phase at 150 uC; (e) indexation of the Colob phase showing the
reciprocal lattices of the two mainly scattering domains of the fibre-
like disordered sample (black and gray lines, respectively) with hk
values for the observed reflections and axes given for the black lattice.
Interestingly, the birefringent textures induced under an
applied electric field are stable only under the field, on
terminating the applied field the birefringence disappears
rather quickly and the field of view becomes dark as shown in
Fig. 9f. It seems that upon field removal the layers relax to a
randomly deformed layer structure, similar to the dark
conglomerate phase in the ground state. Such a relaxation
and Table 4). The diffraction pattern and the lattice parameters
are very similar to those of the Colob phase of 2-Si. There are
about 11 molecules in the cross section of the ribbons and as for
2-Si no electro-optical current response could be detected in this
mesophase, indicating a non-polar structure of this columnar
phase (Colob). The presence of a non-polar USmC phase above
the Colob phase and also the small phase transition enthalpies
(see Table 3) are in line with this observation.
was also found for the SmCPFE *] phases of some disilylated
[
bent-core molecules with 3,49-disubstituted biphenyl based
bent-core units17b and it was attributed to a relaxation of the
field induced racemic [SmCsPF]aPS structure into a (+)/(2)-
[SmCsPF]sPA structure (see Fig. 10) which immediately forms
the optically isotropic texture, as explained in ref. 17b. As there
is an equal probability for the relaxation either into a (+)-
[SmCsPF]sPA or into a (2)-[SmCsPF]sPA structure (follow
either i or ii in Fig. 10) the chiral domains are microscopically
mixed and the optically isotropic state obtained after field
removal does not exhibit macroscopic chiral domains. This
kind of relaxation requires a rotation of the molecules around
the long axis (see Fig. 10). It seems that in the series of
monosilylated compounds this relaxation process is more easy
2.4. Functionalization of only one end—compounds 4-En and
4-Si
For comparison we have also synthesized the naphthalene
derived compound 4-Si with an oligosiloxane unit at only one
terminus. This compound shows
a completely different
mesophase than the double silylated compound 1-Si. The
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Fig. 9 Characterization of the mesophase of compound 4-Si: (a)–(c) textures at 150 uC; (a), (c) were obtained with slightly uncrossed polarizers
showing macroscopic chiral domains; (b) dark texture as seen between crossed polarizers; (d) switching current response trace obtained under a
modified triangular wave field (300 Vpp, 0.5 Hz, 5 mm) at 130 uC, indicating a surface stabilized FE switching; (e) birefringent textures obtained
under these conditions; (f) relaxation to the dark texture within a few seconds after removal of the applied field.
for the 2,7-disubstituted naphthalene derivatives which have a
shorter rigid core than for the analogous 3,49-disubstituted
biphenyl derivatives, with a larger rigid core, where it is not
observed or significantly slower.15c
Despite the smaller size of the 2,7-naphthyl unit in
comparison to the 3,49-biphenyl unit these compounds exhibit
enhanced mesophase stabilities. This is rather unexpected and
indicates that probably stronger p-stacking interactions in the
aromatic sublayers (flat p-system of the naphthalene core in
comparison to the twisted biphenyl unit) are of significant
importance for the mesophase stability. This and also the
enhanced symmetry of the naphthalene unit leads to increased
melting temperatures and this is the major reason for the
smaller mesophase ranges observed for the naphthalene
derived compounds in most cases (see Table 2). It is also
interesting to note that all non-silylated naphthalene deriva-
tives n-En require very high voltages for electrooptical switch-
ing. This might also be caused by the relatively strong
p-stacking interactions which provide a more dense packing
of the bent cores. The bulky silyl groups can reduce the
packing density in the aromatic sublayers and this contributes
to the reduction of the threshold voltages. However, this
reduced packing density also reduces the polarization values
and influences the switching mechanism. These effects are
This indicates that an oligosiloxane unit at only one of the
wings is sufficient to induce FE switching, while retaining the
layer structure and polar order. The second siloxane unit in
compound 1-Si further increases the steric packing frustration,
giving rise to a transition to columnar ribbon phases where the
inter-ribbon interfaces destabilize the FE states and change the
switching process to AF. A complete loss of polar order is
observed in this case at higher temperature.
3. Summary and conclusions
Novel bent-core molecules based on a rigid core incorporating
a bent 2,7-disubstituted naphthalene unit with oligosiloxane
terminated alkyl chains at one or both ends have
been synthesized and their mesophase behaviour has been
investigated.
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disorder of the rigid cores at higher temperature (no polar
order) increases the effective space required by the aromatic
cores and this can partly compensate the steric frustration
provided by the silyl units. With decreasing temperature the
space occupied by the silyl groups is reduced. As
a
consequence polar order can set in and due to the more dense
packing of the aromatic cores the layer distorting effect of the
silyl groups is increased. This leads to the formation of ribbons
and in the resulting columnar phase the FE state is
destabilized, giving rise to an AF switching oblique columnar
phases (ColobPA). With further decreasing temperature the
layer frustration is further reduced and this gives rise to an
increase of the size of the ribbons, which in turns allows a
relatively easy (complete or partial) removal of the layer
frustration under an electric field and in this field induced
mesophase FE switching takes place (ColobPFE). The intro-
duction of fluorine on the outer phenyl rings (2-Si) as well as
introduction of carboxylate groups as linking units (3-Si)
removes polar order. This is surprising, as in related non-
silylated molecules (e.g. 3-En) these units favour a polar
packing and FE switching.
As another consequence of the steric frustration and the less
dense packing of the aromatic cores the switching process
within the polar smectic and columnar phases takes place by a
collective rotation around the long axis for all disilylated
compounds. This switching process is of special interest as it
switches the superstructural chirality in these LC systems.
In summary, these investigations have contributed to a
better understanding of the structure–property relationships in
bent-core LCs, especially concerning the effects of micro-
segregation and packing density upon the mesophase structure
and the mode of switching.
Fig. 10 Models showing the modes of organization of compound
4-Si. The assignment of the phases is given at the bottom, where the
structure within the layer-stacks (SmCsPF) is given in square brackets
and the following subscripts define the correlation of the interfaces
between the layer stacks (a = anticlinic, s = synclinic, A = antipolar,
S = synpolar); (a) apparently anticlinic and synpolar rac-[SmCsPF]aPS
structure as obtained by cooling under an applied AC voltage; (b) the
two enantiomeric [SmCsPF]aPA structures as obtained either by cooling
from the isotropic liquid state without applied field or in the relaxation
process after switching off an applied AC voltage, it seems that the
formation of this structure is associated with a layer deformation into a
dark texture for which a sponge-like deformation of the layers is
proposed, the chiral domains are microscopic in size (no chiral
domains can be detected) if the phase is formed by relaxation from
structure (a) and macroscopic upon cooling from the isotropic liquid
(dark conglomerate texture).17b
Acknowledgements
R. A. Reddy is grateful to the Alexander von Humboldt
Foundation for a research fellowship.
stronger for the naphthalene derivatives, because the packing
of these shorter bent cores can more easily be distorted.
Bent-core compounds having only one heptamethyltrisilox-
ane unit are characterized by smectic phases showing surface
stabilized FE switching as well as dark conglomerate textures
(SmCPFE[*] phases), irrespective of whether a 3,49-biphenyl or
a 2,7-naphthalene structure is incorporated into the bent
aromatic core. However, the polarization values are reduced
for the naphthalene derivative due to the reduced size of the
bent aromatic core.
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1998, 2573; (b) D. Shen, A. Pegenau, S. Diele, I. Wirth and
C. Tschierske, J. Am. Chem. Soc., 2000, 122, 1593.
19 2,7-Disubstituted naphthalene units as bent units: (a)
Amaranatha Reddy and B. K. Sadashiva, J. Mater. Chem., 2004,
14, 1936; (b) H. N. Shreenivasa Murthy and B. K. Sadashiva,
Liq. Cryst., 2004, 31, 1347; (c) V. Kozmik, M. Kuchar, J. Svoboda,
V. Novotna, M. Glogarova, U. Baumeister, S. Diele and G. Pelzl,
Liq. Cryst., 2005, 32, 1151; (d) V. Kozmik, A. Kovarova,
M. Kuchar, J. Svoboda, V. Novotna, M. Glogarova and
J. Kroupa, Liq. Cryst., 2006, 33, 41; (e) J. Svoboda, V. Novotna,
V. Kozmik, M. Glogarova, W. Weissflog, S. Diele and G. Pelzl,
J. Mater. Chem., 2003, 13, 2104; see also ref. 8a, 11c,13c.
20 R. Achten, A. Koudijs, M. Giesbers, A. T. M. Marcelis and
E. J. R. Sudho¨lter, Liq. Cryst., 2005, 32, 277.
9 Hexagonal columnar phases (Colh) are extremely rare in this class
of compounds and have distinct structures: (a) E. Gorecka,
D. Pociecha, J. Mieczkowski, J. Matraszek, D. Guillon and
B. Donnio, J. Am. Chem. Soc., 2004, 126, 15946; (b) S. Kang,
J. Thisayukta, H. Takezoe, J. Watanabe, K. Ogino, T. Doi and
T. Takahashi, Liq. Cryst., 2004, 31, 1323; (c) H. Takezoe,
K. Kishikawa and E. Gorecka, J. Mater. Chem., 2006, 16, 2412.
10 Examples of columnar phases formed by bent-core molecules: (a)
J. Watanabe, T. Niori, T. Sekine and H. Takezoe, Jpn. J. Appl.
Phys., 1998, 37, L139; (b) K. Pelz, W. Weissflog, U. Baumeister
and S. Diele, Liq. Cryst., 2003, 30, 1151; (c) D. Kardas, M. Prehm,
U. Baumeister, D. Pociecha, R. Amaranatha Reddy, G. H. Mehl
and C. Tschierske, J. Mater. Chem., 2005, 15, 1722; (d)
Y. Takanishi, H. Takezoe, J. Watanabe, H. Takahashi and
A. Lida, J. Mater. Chem., 2006, 16, 816; (e) C. L. Folcia,
J. Etxebarria, J. Ortega and M. B. Ros, Phys. Rev. E, 2006, 74,
031702; (f) C. L. Folcia, I. Alonso, J. Ortega, J. Etxebarria, I. Pintre
and M. B. Ros, Chem. Mater., 2006, 18, 4617.
11 Polar switching columnar phases: (a) J. Mieczkowski, K. Gomola,
J. Koseska, D. Pociecha, J. Szydlowska and E. Gorecka, J. Mater.
Chem., 2003, 13, 2132; (b) H. N. Shreenivasa Murthy and
B. K. Sadashiva, J. Mater. Chem., 2003, 13, 2863; (c)
R. Amaranatha Reddy, B. K. Sadashiva and V. A. Raghunathan,
Chem. Mater., 2004, 16, 4050; (d) J. P. Bedel, J. C. Rouillon,
J. P. Marcerou, H. T. Nguyen and M. F. Achard, Phys. Rev. E,
2004, 69, 061702; (e) H. N. Sheenivasa Murthy and B. K. Sadashiva,
J. Mater. Chem., 2004, 14, 2813; (f) R. Amaranatha Reddy,
B. K. Sadashiva and U. Baumeister, J. Mater. Chem., 2005, 15,
3303; (g) E. Gorecka, N. Vaupotic, D. Pociecha, M. Cepic and
J. Mieczkowski, ChemPhysChem, 2005, 6, 1087.
12 Examples of FE switching bent-core materials without silyl units:
(a) M. Nakata, D. R. Link, F. Araoka, J. Thisayukta, Y. Takanishi,
K. Ishikawa, J. Watanabe and H. Takezoe, Liq. Cryst., 2001, 28,
130; (b) J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
H. T. Nguyen and M. F. Achard, J. Mater. Chem., 2002, 12, 2214;
(c) R. Amaranatha Reddy and B. K. Sadashiva, J. Mater. Chem.,
2002, 12, 2627; (d) R. Amaranatha Reddy and B. K. Sadashiva,
Liq. Cryst., 2003, 30, 1031; (e) S. Rauch, P. Bault, H. Sawade,
G. Heppke, G. G. Nair and A. Jakli, Phys. Rev. E, 2002, 66,
021706; (f) K. Kumazawa, M. Nakata, F. Araoka, Y. Takanishi,
K. Ishikawa, J. Watanabe and H. Takezoe, J. Mater. Chem., 2004,
14, 157; (g) H. Nadasi, W. Weissflog, A. Eremin, G. Pelzl, S. Diele,
B. Das and S. Grande, J. Mater. Chem., 2002, 12, 1316; (h)
S. K. Lee, S. Heo, J.-G. Lee, K.-T. Kang, K. Kumazawa,
K. Nishida, Y. Shimbo, Y. Takanishi, J. Watanabe, T. Doi,
T. Takahashi and H. Takezoe, J. Am. Chem. Soc., 2005, 127,
11085; (i) K. Nishida, M. Cepic, W. J. Kim, S. K. Lee, S. Heo,
J. G. Lee, Y. Takanishi, K. Kishikawa, K.-T. Kang, J. Watanabe
and H. Takezoe, Phys. Rev. E, 2006, 74, 021704; (j) V. Novotna,
M. Kaspar, V. Hamplova, M. Glogarova, L. Lejcek, J. Kroupa
and D. Pociecha, J. Mater. Chem., 2006, 16, 2031.
21 ncell = molecules per unit cell was estimated from the crystal
volumes of the molecules according to A. Immirzi and B. Perini,
Acta Crystallogr., Sect. A, 1977, 33, 216, with a correction for the
lower packing density in the mesophase and the volume of a 3D
packing unit calculated from the 2D unit cell assuming a height of
h = 0.52 nm (for details see ESI{ Table S2).
22 In all cases the molecular length was determined using CPK models
and assuming a 120u bent V-shaped conformation with most
extended alkyl chains (all-trans-conformation) and heptamethyl-
trisiloxane units.
23 This can be explained by polarization reversal around the long axis,
which would indicate that some layer modulation is retained.
13 FE switching undulated and modulated smectic phases: (a)
D. M. Walba, E. Ko¨rblova, R. Shao, J. E. Maclennan,
D. R. Link, M. A. Glaser and N. A. Clark, Science, 2000, 288,
2181; (b) J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
H. T. Nguyen and M. F. Achard, Liq. Cryst., 2000, 27, 1411; (c)
R. Amaranatha Reddy, V. A. Raghunathan and B. K. Sadashiva,
74 | J. Mater. Chem., 2007, 17, 62–75
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24 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.
25 However, there should be only a slight contribution of splay, at
first the number of molecules in the cross section of the ribbons is
smaller than in other polarization modulated smectic phases,
secondly a splay with a ¡90u change of the polar direction is also
unlikely, as this should lead to the formation of a rectangular
ribbon phase.
26 (a) S. Umadevi and B. K. Sadashiva, Liq. Cryst., 2005, 32, 1233; (b)
S. Umadevi, A. Jakli and B. K. Sadashiva, Soft Matter, 2006, 2,
875.
27 R. Amaranatha Reddy, U. Baumeister, C. Keith, K. Hahn,
H. Lang and C. Tschierske, Soft Matter, DOI: 10.1039/b615791b.
28 V. Kopsky and D. B. Litvin, International Tables of
Crystallography, Vol. E, Subperiodic groups, Kluwer Academic
Publishers, Dordrecht, 2002.
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