Silylated bent-core molecules: The influence of the direction of the carboxyl connecting groups on the mesophase behaviour

Article abstract of DOI:10.1039/c0sm00202j

Novel bent-core molecules based on phenyl benzoate rigid cores and heptamethyltrisiloxane units at one or at both ends of the aliphatic side chains were synthesized and investigated by polarized light microscopy, differential scanning calorimetry, X-ray diffraction and electrooptical methods and the effect of the direction of the carboxyl groups on the mesomorphic self assembly was investigated. A series of distinct liquid crystalline phases was found, among them ferroelectric and antiferroelectric switching and also non-switchable smectic, undulated smectic (USmC) and modulated smectic phases (columnar phases, Col_ob). All polar phases show field-induced inversion of chirality at least in a certain temperature range. Moreover, two non-polar and uniaxial smectic phases were observed, one with a hollow cone-like orientational director distribution (SmC_R) and the other one with a maximum of the director distribution at zero tilt (SmA). These two phases were discussed in relation to the distinct models of de Vries type smectic phases.

Full text of DOI:10.1039/c0sm00202j

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Silylated bent-core molecules: the influence of the direction of the carboxyl
connecting groups on the mesophase behaviour†
Ramaiahgari Amaranatha Reddy,‡a Ute Baumeister,b Jessie Lorenzo Chao,b Horst Kresseb
a
*
and Carsten Tschierske
Received 5th April 2010, Accepted 3rd June 2010
DOI: 10.1039/c0sm00202j
Novel bent-core molecules based on phenyl benzoate rigid cores and heptamethyltrisiloxane units at
one or at both ends of the aliphatic side chains were synthesized and investigated by polarized light
microscopy, differential scanning calorimetry, X-ray diffraction and electrooptical methods and the
effect of the direction of the carboxyl groups on the mesomorphic self assembly was investigated. A
series of distinct liquid crystalline phases was found, among them ferroelectric and antiferroelectric
switching and also non-switchable smectic, undulated smectic (USmC) and modulated smectic phases
(columnar phases, Colob). All polar phases show field-induced inversion of chirality at least in a certain
temperature range. Moreover, two non-polar and uniaxial smectic phases were observed, one with
a hollow cone-like orientational director distribution (SmCR) and the other one with a maximum of the
director distribution at zero tilt (SmA). These two phases were discussed in relation to the distinct
models of de Vries type smectic phases.
Oligo(siloxane) containing bent-core compounds appear to be
1. Introduction
one of the most successful classes of compounds showing
macroscopic polar order and superstructural chirality.7–11 This is
mainly due to the nano-segregation of the oligo(siloxane) units
into distinct sublayers located at the inter-layer interfaces. These
sublayers influence the coupling between the layers which
modifies the self-assembly of the bent-core molecules. In addi-
tion, the significant space required by these units leads to
a frustration of flat layers and induces undulated and modulated
morphologies. For this reason a large variety of bent-core
molecules based on oligosiloxane and carbosilane segments have
been synthesized in recent years,7–11 among them also dimeso-
gens,8a,d,g oligomesogens,8b,e dendrimers8b,d,f and side chain
polymers.8c
Liquid crystals (LCs) are ordered fluids in which the segregation
of incompatible units and parallel alignment of rigid units
combine to give rise to positional and orientational long-range
order.1 One of the most exciting areas in this field is provided by
the so-called banana-shaped or bent-core liquid crystals, devel-
oped since 1996.2,3 The exciting feature of the molecules is that
the bent shape of the aromatic core restricts the rotation around
the long axis which leads to a preferred organization of these
molecules with uniform bent direction. This gives rise to
a macroscopic polar order within these LC phases providing
ferroelectric (FE) and antiferroelectric (AF) switching LC
materials being of interest for numerous potential applications in
electrooptical switches, as optical phase modulators, nonlinear
optical materials, etc.3d,4 In addition, in most mesophases of
bent-core compounds the molecules are tilted with respect to the
layer normal. The unique combination of tilted organization in
layers and polar order reduces the symmetry of these structures.
The resulting C2 symmetry leads to an inherent superstructural
chirality5 in which layer normal, tilt direction and polar direction
describe either a right- or left-handed system and hence these
structures are chiral.5 This superstructural chirality and the
possibility of switching of chirality by external fields6 immedi-
ately arose broad and general scientific attention.3c–e
The majority of these oligosiloxane and carbosilane
substituted bent core molecules incorporate a resorcinol or
3,40-disubstituted biphenyl moiety as central bent unit, coupled
with phenyl benzoate based rod-like wings derived from
4-hydroxy benzoic acid. In these molecules the COO linking
groups have a uniform direction with all Ar–O bonds directed
towards the bent unit in the middle of these molecules (see
Chart 1, compounds of type 1). In previous work with non-
silylated bent-core molecules it has been shown that reversing
the direction of one, two, three or even all four of the COO
groups has a strong impact on mesophase stability and meso-
phase types.12
aInstitute of Chemistry, Organic Chemistry, Martin-Luther-University
Halle-Wittenberg, Kurt-Mothes-Str. 2, D-06120 Halle, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: +49 345 552 7346;
Tel: +49 345 552 5664
bInstitute of Chemistry, Physical Chemistry, Martin-Luther-University
Halle-Wittenberg, von-Danckelmann-Platz 4, D-06120 Halle, Germany
† Electronic supplementary information (ESI) available: Synthesis,
analytical data, additional XRD data, DSC curves, optical
micrographs, switching current curves and molecular models. See DOI:
10.1039/c0sm00202j
In this contribution attention is focussed on the effects of
changing the direction of the carboxyl groups in siloxane
substituted bent core molecules (see Chart 1) which are
compared with the effects of the same structural variation
previously observed in non-silylated molecules.12a Besides
different types of ferroelectric or antiferroelectric switching
smectic, undulated smectic (USmC) and modulated smectic
(columnar, Colob) phases also two non-polar and uniaxial
smectic phases were observed, one with a hollow cone-like
‡ Present address: VVi Bright inc., Boulder, Colorado 80301, USA.
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3. Results and discussion
3.1. Mesomorphic properties of bent-core compounds with
olefinic end groups
3.1.1 Compounds with double bonds at each end (En-1–En-4).
The transition temperatures and associated enthalpies of the
diolefins En-1–En-4 are recorded in Table 1. All compounds
consist of five phenyl rings interconnected by exclusively
carboxyl groups. Compound En-1 with all COO groups directed
towards the central 1,3-disubstituted phenyl ring, i.e. with the
Ar–O bonds closer to the bent unit than the Ar–C bonds, shows
[
a monotropic SmCPA *] phase which has already been investi-
gated by Jakli et al.14 All other diolefins represent new
compounds; compounds En-2 and En-4 show only monotropic
LC phases which rapidly crystallize so that they were only
detected by polarizing microscopy and a more precise phase
assignment was not possible, whereas compound En-3 has an
enantiotropic (thermodynamically stable) LC phase. In the series
En-n the melting temperatures rise with increasing number of
COO groups reversed compared to those in En-1, i.e. En-1 z
En-2 < En-3 < En-4, and also the mesophase stability is
increasing in this order.
Chart 1 Structures of compounds under investigation.
The clearing temperature of the SmA phase of En-4 with all
COO linking groups pointing away from the central 1,3-disub-
stituted benzene ring (isophthalate based bent core) is the highest
and it is more than 80 K more stable than the SmCPA phase of
En-1, in which all COO groups point towards the central 1,3-
disubstituted benzene ring (resorcinol based bent core). This
indicates that the orientation of the COO groups has a large
effect on the mesophase stability. A similar effect has been
reported for related bent-core molecules with simple alkyl chains,
without olefinic end groups and was attributed to changes of the
molecular flexibility, the torsional and bending angles, and the
magnitude and distribution of the dipole/quadrupole
moments.12b As the rotational barrier of the Ar–COO bond is
higher than the Ar–OOC bond, compounds with the COO
groups directed towards the bent core-unit have a higher flexi-
bility, providing lower transition temperatures.12b Beside
conformational effects also the effects of the COO direction upon
the electrostatic surface potential have to be considered as elec-
tron deficient aromatics and also an alternation of electron rich
and electron deficient aromatics favors intermolecular interac-
tions, hence stabilizing smectic phases.15 This should also
contribute to the mesophase stability of compounds En-3 and
En-4 incorporating electron deficient terephthalate and iso-
phthalate units, respectively.16 Together with the effects provided
by the terminal chains (segregation, intercalation, space filling,
flexibility, etc.) these factors influence the packing mode of the
molecules in a complex and hardly predictable way.
orientational director distribution (SmCR) and a SmA phase
with a maximum of the director orientation at zero tilt. These
two phases are of interest with respect to the actual discussion
around the distinct models of de Vries type smectic phases.
2. Synthesis and methods of investigation
The synthesis of the olefinic compounds En-1–En-6 was carried
out by esterification reactions as outlined in the ESI† (Schemes
S1–S5). The silylated compounds Si-1–Si-6 were obtained from
these olefins and 1,1,1,3,3,5,5-heptamethyltrisiloxane by hydro-
silylation reaction using Karstedts catalyst.13 All the silylated
compounds were purified by column chromatography with
a short pad of silica gel using chloroform and methanol mixture
as eluent, followed by crystallization/precipitation using chlo-
roform/acetonitrile solvent mixtures. The experimental proce-
dures and analytical data are also reported in the ESI†.
Phase transitions were determined by polarizing optical
microscopy (Optiphot 2, Nikon) in conjunction with a heating
stage (FP 82 HT, Mettler) and by differential scanning calo-
rimetry (DSC-7, Perkin Elmer). The assignment of the meso-
phases was made on the basis of optical textures and by X-ray
diffraction (XRD). Investigations of surface aligned samples
were performed using a 2D detector (HI-Star, Siemens). Uniform
orientation was achieved by slowly cooling a drop of the
compound on a glass surface. The X-ray beam was applied
parallel to the substrate (exposure time: between 30 min and 3 h).
Electrooptical experiments have been carried out using a home
built setup in commercially available ITO coated glass cells
(E.H.C., Japan) with a measuring area of 1 cm2. Dielectric
investigations of compound Si-3 were performed in the range
from 1 Hz to 10 MHz using a Solartron-Schlumberger
Impendance Analyzer Si 1260 and a Chelesa Interface. A brass
cell coated with gold (distance ¼ 0.05 mm) was used as capacitor.
Compound En-3 is the most interesting compound in this
series. This molecule is symmetric with respect to the central
1,3-disubstituted benzene ring and the direction of the COO
groups in the inner and outer positions of the rod-like wing
groups is opposite (terephthalate based rod-like wings). This
compound shows spherulitic textures (Fig. 1a), the same texture
as observed for the related compounds with simple alkoxy
chains, assigned to an undulated smectic phase (USmCPFE
phase).6c However, only the layer reflections (01 and 02) at
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Table 1 Phase transition temperatures (T/ꢀC) and enthalpy values (DH/kJ molꢁ1, in square brackets) obtained for the olefinic bent-core compounds En-
1 to En-4a
No
X1
Y1
Y2
X2
Phase transitions
En-1
En-2
En-3
En-4
COO
COO
OOC
OOC
COO
OOC
COO
OOC
OOC
OOC
OOC
COO
OOC
OOC
COO
COO
Cr 101 [52] (SmCPA[*] 95 [18]) Iso10,14
Cr 107 [66] (SmC 92 [14])b Iso
Cr 147 [88] USmCPFE 153 [22] Iso
Cr 182 [80] (SmA 177)b Iso
a
COO groups which are reversed compared to En-1 are highlighted in bold; peak temperatures (temperatures at maximum heat flow) of the first DSC
heating curves (10 K minꢁ1) are given; monotropic transitions (shown in parenthesis) were taken from the second heating run; abbreviations:
Cr ¼ crystalline solid; Iso ¼ isotropic liquid; SmC ¼ polar or non-polar tilted smectic phase with unknown structure (Schlieren texture with
[
]
relatively low viscosity and spherulitic domains were observed by polarizing microscopy); SmA ¼ nontilted smectic phase; SmCPA
SmC phase. b Mesophase crystallizes immediately.
*
¼
antiferroelectric switching tilted smectic phase with dark conglomerate texture in the ground state; USmCPFE ¼ ferroelectric switching undulated
Fig. 1 Investigation of compound En-3: (a) optical photomicrograph (crossed polarizers) showing the growth of the mesophase from the isotropic
liquid at 153 ꢀC (dark areas are residues of the liquid phase in equilibrium); (b) FE switching current response obtained under a triangular wave field
(320 Vpp, 20 Hz, 5 mm, Ps ¼ 220 nC cmꢁ2); (c–e) bistable switching as seen under a DC electric field at 145 ꢀC; (c) texture under an applied DC field of +50
V; (d) texture as observed after removing the field and (e) texture at 0 V after application of an field with opposite sign; arrows indicate the direction of
polarizer and analyzer.
d ¼ 4.5 nm could be measured with sufficient accuracy by XRD
for the mesophase of compound En-3 (see Fig. S1, Table S1†).
The additional hk reflections expected for an undulated smectic
phase are very weak or coincide with the strong 01 reflection,
therefore exact 2D lattice parameters cannot be calculated.
Assigning the indices 10 or 20 to the weak inner reflection at
d ¼ 9.0 nm an undulation period of about 9 or 18 nm would
result,17 a reasonable value if compared with those for En-5 and
En-6 (see next Section 3.1.2). The splitting of the diffuse wide
angle scattering indicates a tilted organization of the molecules in
the layers with a tilt angle in the range of 25ꢀ (see Fig. S1 and
S2†). Though tilt is proven by XRD, the extinction crosses in the
circular domains of the textures (Fig. 1a) are parallel to polarizer
and analyzer. This can be explained by a polarization modulated
and undulated B7-like structure of this mesophase as described
by Clark et al.18
Triangular-wave electric field experiments evidence a single
polarization peak for each half period indicating a ferroelectric
switching for this mesophase (see Fig. 1b). The spontaneous
polarization is 220 nC cmꢁ2, which is also comparable to the
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However, at lower temperature (T < 145 ꢀC) or at higher
frequency (f > 10 Hz) switching around a tilt cone is observed. In
this switching process polar direction and tilt direction are
changed simultaneously (see Fig. 2a), i.e. this switching process is
associated with a rotation of the extinction crosses and it retains
the layer chirality. These extinction crosses do not change their
position after switching off the electric field, and only rotate after
reversal of the applied field, which confirms bistable, i.e. FE
switching on a cone. This indicates that under an electric field the
layer modulation is largely removed and an SmC phase is
induced.20
3.1.2 Compounds with only one double bond (En-5 and En-6).
The mesophase of compound En-5 (Table 2) with only one
terminal double bond is very similar to En-3, having the same
direction of the COO linking groups. Not only the texture but
also the XRD patterns and the electrooptical switching of both
compounds are very similar (Fig. S10–S12†). As in the case of
En-3 beside the layer reflections (layer spacing 4.9 nm) weak
additional reflections were observed in the XRD pattern (see
Fig. S1, Table S1†), the inner reflection (d ¼ 8.9 nm) pointing to
a similar period of about 9 or 18 nm as in the case of En-3,
confirming an undulated or modulated SmC structure of this
mesophase.17 This compound also shows FE switching (Ps ¼ 240
nC cmꢁ2) with a temperature and frequency dependent trꢀansition
from rotation on a cone at low temperature (T < 142 C) and
high frequency (>10 Hz) to collective rotation around the long
axis at high temperature and low frequency (Fig. S11 and S12†).
Compound En-6 which differs in the direction of the two COO
groups Y1 and X2 from En-5 shows a distinct, more fan-like
texture (Fig. S13†). Since we got a well aligned sample of this
compound for XRD measurements (Fig. 3), the 2D lattice
could be determined as an oblique one with lattiꢀce parameters
a ¼ 15.6 nm, b ¼ 4.8 nm, and g ¼ 103ꢀ at 110 C, indicating
a similar layer spacing as observed for En-3 and En-5, and
an undulation period of 15.6 nm (see Table S2†). At 120 ꢀC,
a ¼ 13.9 nm, b ¼ 4.8 nm, and g ¼ 102ꢀ have been calculated that
means the layer distance remains almost constant with rising
temperature and the undulation period decreases. The wide angle
XRD pattern proves the tilted organization of the molecules with
respect to the normal to a by 26ꢀ (see Fig. 3 and S2†). Based on
Fig. 2 Mechanisms of polar switching in SmCP phases and their
modulated and undulated variants. (a) Switching by collective rotation of
the molecules on the tilt cone and (b) switching by collective rotation of
the molecules around their long axes; mechanism (b) is accompanied by
an inversion of the layer chirality.
values obtained for the related alkyloxy substituted analogues
without double bonds.6c Moreover, bistable switching is clearly
confirmed by electro-optical investigations as follows. As seen in
Fig. 1c–e, under an electric field the dark extinction crosses are
inclined with the positions of polarizer and analyzer which
indicates a field-induced homogeneously chiral SmCsPF structure
(actually a (+)-SmCsPF and a (ꢁ)-SmCsPF domain can be seen in
Fig. 1c–e). The extinction crosses are inclined by 30ꢀ, indicating
an optical tilt of 30ꢀ.19 This mesophase evidences a temperature
and frequency dependent change of the switching process. Under
an applied voltage of 320 Vpp (peak-to-peak voltage of the
applied triangular wave AC voltage) in a 5 mm cell at high
temperature (T > 145 ꢀC) or at low frequency (f < 10 Hz) no
rotation of the extinction crosses is observed. This indicates that
under these conditions a switching around the long axes takes
place, which changes only the polar direction. Hence, the layer
chirality and also the phase chirality are reversed in this switching
process, i.e. the (+)-SmCsPF domains become (ꢁ)-SmCsPF and
the (ꢁ)-SmCsPF domains become (+)-SmCsPF (see Fig. 2b).
Table 2 Phase transition temperatures (T/ꢀC), enthalpy values (DH/kJ molꢁ1, in square brackets) and lattice parameters obtained for the bent-core
compounds En-5 and En-6 with one terminal double bonda
No
X1
Y1
Y2
X2
Phase transitions
Lattice parameters/nm (T/ꢀC)
En-5
En-6
OOC
OOC
COO
OOC
OOC
OOC
COO
OOC
Cr 144 [82] USmCPFE 156 [24] Iso
Cr 103 [55] ColobPFE 123 [19] Iso
d ¼ 4.9
a ¼ 15.6, b ¼ 4.8, g ¼ 103ꢀ (110)
a ¼ 13.9, b ¼ 4.8, g ¼ 102ꢀ (120)
a
Peak temperatures in the first DSC heating curves (10 K minꢁ1), for a representative DSC curve see Fig. S9†; abbreviations: ColobPFE ¼ ferroelectric
switching Colob phase; for further explanations see Table 1.
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Fig. 4 Schematic sketches comparing the molecular assembly in smectic
C phases with those in modulated (Colob) and undulated (USmC) smectic
C phases. Only non-symmetric undulation and non-polar variants of the
phases are shown, in which the bent-core molecules are rotationally
disordered around their long axes (therefore simplified to straight lines);
restricted rotation leads to polar order and hence to ferroelectric or
antiferroelectric variants; the transition between these three phase types is
continuous and is often influenced by different effects; the most often
observed effects of temperature (T) and electric fields (E) are shown with
arrows.
Fig. 3 XRD patterns obtained in the USmCPFE phase of compound
En-6: (a) complete XRD pattern at 100 ꢀC; the diffuse character of the
wide angle scattering indicates the absence of positional long range order
as typical for LC phases; the maxima of the wide angle scattering are
located between equator and meridian, indicating a tilted arrangement of
the molecules (XRD tilt ¼ 26ꢀ); (b) small angle region at 110 ꢀC, and (c)
same pattern with indexation to an oblique lattice.
rotation around the long axis, making this process at lower T
slower than the switching on a cone. Therefore, the switching
frequency must be decreased to give the system sufficient time for
switching around the long axis, which explains the frequency
dependence of the switching process. Though for all three
compounds En-3, En-5 and En-6 an undulated/modulated layer
structure was confirmed, only the USmCPFE phases of
compounds En-3 and En-5 seem to be similar to polarization
undulated/modulated B7 phases whereas the ColobPFE phase of
En-6 is a mesophase with oblique lattice and non-symmetric layer
modulation, more similar to B1-type phases.
this XRD evidence this mesophase is assigned as ColobPFE. The
same temperature and frequency dependence of the FE switching
process as reported for the USmCPFE phases of En-3 and En-5
are observed with a transition between switching on a cone and
switching around the long axis at T ¼ 110 ꢀC and 10 Hz (300 Vpp
in a 5 mm cell), but the spontaneous polarization is larger
(Ps ¼ 800 nC cmꢁ2, see Fig. S13–S15†)21 and all transition
temperatures are shifted to lower values.
In all three compounds En-3, En-5 and En-6 the ground state
texture is transformed under an electric field into a spherulitic
texture (see Fig. 1c–e, S12 and S15†) which can be interpreted as
a field induced transition from the undulated/modulated ground
state structures into a field induced smectic phase with uniform
polar order (SmCPF), i.e. as a partial suppression or even
a complete removal of layer undulations/modulations under the
influence of the electric field (Fig. 4).18 As the degree of layer
undulation/modulation influences the switching process, it
strongly depends on the experimental conditions, such as the
strength of applied voltage, cell thickness, applied frequency,
temperature, etc. In general, switching by rotation on a cone is
the faster process in flat layers as obtained at low temperature
and high voltage, but it is inhibited by layer modulations. As the
modulation/undulation wave length is temperature (and voltage)
dependent and for the compounds under discussion decreases
(i.e. layer modulation increases) with rising temperature, sharp
layer modulation remaining at higher temperature (or at lower
voltage) inhibits the switching by rotation on a cone. In this case
switching around the long axis becomes the dominating switch-
ing process. On the other hand, the increase of the viscosity with
decreasing temperature has a stronger retarding effect on the
3.2. Mesomorphic properties of bent-core compounds with two
oligosiloxane groups (Si-1 to Si-4)
Phase transition temperatures and associated enthalpies for the
Si-containing compounds Si-n consisting of five phenyl rings
interconnected exclusively by COO groups and heptamethyl-
trisiloxane units at both the terminals are collated in Table 3.
As observed in the series of compounds En-1–En-4, also in
the series Si-1–Si-4 there is a strong impact of the direction of
the COO groups upon the mesophase type, the melting temper-
atures and the mesophase stability. Also in this series compound
Si-1 (ref. 10) with all COO groups directed towards the central
1,3-disubstituted benzene ring has a significantly lower meso-
phase stability than compound Si-4 with all COO groups
pointing away from the centre (DT is about 70 K). Hence,
the general trends concerning the mesophase stability are the
same as seen for the olefin terminated molecules. However,
the silyl groups reduce the melting points and in most cases
stabilize the LC phases and increase the mesophase region
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Table 3 Phase transition temperatures (T/ꢀC), enthalpy values (DH/kJ molꢁ1, in square brackets) and lattice parameters obtained for the silicon
containing bent-core compounds Si-1 to Si-4a
No
X1
Y1
Y2
X2
Phase transitions
Lattice parameters/nm (T/ꢀC)
Si-1
Si-2
COO
COO
COO
OOC
OOC
OOC
OOC
OOC
Cr 83 [13] ColobPA 116 [22] Iso10
Cr 70 [6.4] Colob 90 [—] ColobPA
107 [—] Colob 115 [17.5] Iso
Cr 69 [6.6] Colob 125 [3.8] SmCR
140 [3.5] Iso
Cr 161 [59] SmCaPA 174 [7.1] SmA
185 [15.6] Iso
a ¼ 2.0, b ¼ 4.3, g ¼ 97ꢀ (105)
d ¼ 4.7 (100)
d ¼ 5.2 (132), a ¼ 7.9, b ¼ 5.5, g ¼ 112ꢀ (86)
0
Si-3
Si-4
OOC
OOC
COO
OOC
COO
COO
OOC
COO
d ¼ 5.3 (180), d ¼ 5.3 (165)
a
COO groups which are reversed compared to Si-1 are highlighted in bold; peak temperatures in the first DSC heating curves (10 K minꢁ1), for DSC
0
scans see Fig. S16–S18†; abbreviations: Colob, Colob ¼ oblique columnar phases showing no polar switching; ColobPA ¼ antiferroelectric switching
Colob phase; SmCR ¼ optical uniaxial smectic phase with randomly tilted organization of the molecules; SmA ¼ nontilted smectic phase;
SmCaPA ¼ antiferroelectric switching SmC phase with anticlinic layer corrrelation; for further explanations, see Table 1.
compared to related non-silylated compounds. Therefore, in
contrast to the diolefins En-n, all silylated molecules Si-n show
enantiotropic LC phases and there is a larger diversity of
different mesophases.
In electric field experiments two repolarisation current peaks
per half period of an applied triangular wave voltageꢀ were found
in the temperature range between T ¼ 90 and 107 C (Fig. 5d),
indicating an AF switching (ColobPA) with a spontaneous
polarization Ps ¼ 165 nC cmꢁ2 at T ¼ 100 ꢀC. No switching
current peak is observed below 90 ꢀC and above T ¼ 107 ꢀC
(however, these temperatures can be slightly shifted to higher or
lower depending on the experimental conditions, such as applied
voltage and frequency). The changes of the switching behaviour
are not associated with any enthalpy change in the DSC curves
(see Fig. S16†) and there is also no distinct change of the textures
at these temperatures. The extinction crosses of circular domains,
grown under a triangular wave electric field upon cooling from
the isotropic state, are inclined with the directions of polarizer
and analyzer and this indicates a synclinic tilted organization
(Fig. 5e) with an optical tilt of about 30ꢀ. The position of the
extinction crosses does not change at the transition to the polar
switching phase at T ¼ 107 ꢀC, but below this temperature
a reduction of the birefringence is observed (probably due to
a change of the mode of anchoring of the ribbons with respect to
the surface22). The extinction crosses do not rotate upon polar-
ization reversal and hence, switching takes place by a collective
rotation around the long axis of the molecules. This mode of
switching and the strong splitting of the polarization peaks
associated with a significant threshold voltage are similar to
observations made for the ColobPA phases of other double sily-
lated bent-core compounds9b as for example compound Si-1.10
Hence, it seems that compound Si-2 behaves similar to Si-1,
forming a modulated and AF switching tilted smectic phase
(ColobPA) in the temperature range between 90 and 107 ꢀC. The
3.2.1 Compounds Si-1 and Si-2: modulated smectic phases. As
reported earlier, compound Si-1 exhibits an AF switching obli-
que columnar phase (ColobPA, a ¼ 2.0 nm, b ¼ 4.3 nm and
g ¼ 97ꢀ).10 As typical for the AF switching in ColobPA phases
also for this compound the switching takes place (at all
temperatures) by rotation around the molecular long axis which
reverses the chirality.
In compound Si-2 (with a 3-hydroxybenzoate bent unit) only
one of the inner ester groups (Y1) is reversed in comparison to
compound Si-1. Upon cooling from the isotropic liquid state
a rather low birefringent texture develops (see Fig. 5a). This
mesophase cannot be aligned homeotropically; instead by
shearing a more birefringent texture is induced, as often observed
for columnar LC phases. However, X-ray investigations indicate
only a layer reflection corresponding to d ¼ 4.7 nm with the
second order (Fig. 5b and c) for the whole temperature region.
The d-spacing is significantly smaller than the molecular length L
(estimated from CPK molecular models as L ¼ 7.1 nm in
a conformation shown in Fig. S21d†), indicating a strongly tilted
arrangement of the molecules in the mesophase. In the wide angle
region there are two diffuse scatterings, one with the maximum at
d ¼ 0.7 nm forms a closed ring and is assigned to the mean
distance between the siloxane units; the second one at d ¼ 0.47
nm is assigned to the mean distances between the aliphatic and
aromatic units. It has maxima at c ¼ 134ꢀ and 226ꢀ, confirming
a tilt of ca. 44ꢀ (see Fig. 5b and S3†). Though no clear indication
of a layer modulation or undulation can be found in the XRD
pattern, the texture is more typical for a mesophase with 2D
lattice than for a lamellar LC phase (see mosaic-like features in
the texture in Fig. 5a at the left). This assumption is also in line
with the observed switching behaviour.
0
loss of polar switching below 90 ꢀC (Colob phase) might be due
to the enhanced viscosity which makes the rotation around the
long axis too slow to follow the reversal of the field direction
under the maximal available threshold voltage. A rotation on
a cone, as observed for compounds En-3, En-5 and En-6, is dis-
favoured in this case due to the presence of sharp boundaries
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reversed compared to Si-1, whereas the direction of the inner
COO groups (Y1 and Y2) was retained (resorcinol bisbenzoates
with terephthalate based rod-like wings). This compound shows
two mesophases (Table 3, Fig. S17†), the high temperature phase
exhibits a weakly birefringent fan-shaped texture (see Fig. 6a).
On shearing it becomes completely dark (optically isotropic), very
similar to SmA phases (Fig. 6c, left).25 This indicates the optical
uniaxiality of this LC phase. XRD confirms a layer structure, but
the layer distance (d ¼ 5.2 nm, Table S1†) is significantly smaller
than the length L ¼ 7.1 nm estimated for this kind of molecules
(see Fig. S21d†). This is a first indication for a tilted organization
of the molecules in this smectic phase which is confirmed by the
XRD pattern of an aligned sample in which the diffuse wide-angle
maxima make an angle of about 30ꢀ with respect to the equator
(Fig. 6d and S4†).26 The clearly proven tilted organization is in
conflict with the optical uniaxiality of this smectic phase. This can
be explained either by a tilted organization with a random
distribution of the tilt direction in adjacent layers (non-correlated
SmC phase) or by an orientationally disordered micro-domain
structure, where the differently tilted domains have a size which is
smaller than the wave length of light.
No electro-optical switching could be observed even at high
electric fields and therefore we assign this phase as SmCR, where
R stands for the random distribution of the tilt director. The
random tilt is also assumed to be responsible for the low bire-
fringence of the fan-texture in this mesophase. The same phase
type was also observed for a recently reported molecule related to
Si-3, but with an additional methyl group in 2-position of the
resorcinol unit.11 For this methyl substituted compound a tran-
sition to a columnar phase with p2gg lattice was observed at
reduced temperature.
Also for compound Si-3, on decreasing temperature, a phase
transition to a columnar phase takes place (Fig. 6b and c), which
is associated with the occurrence of a birefringent texture in the
homeotropically aligned regions (Fig. 6c, inset). This mesophase
is more viscous than the smectic phase and the reflections in the
XRD pattern of an aligned sample can be indexed to an oblique
lattice having lattice parameters a ¼ 7.9 nm, b ¼ 5.45 nm and
g ¼ 112ꢀ at 86 ꢀC (see Fig. 6e, g and h, Table S3†). Also in this
mesophase no electro-optical switching could be observed even
at high electric fields up to 400 Vpp in a 5 mm cell. Hence, the low
temperature mesophase is designated as Colob. This Colob phase
seems to be a non-polar variant of the ColobPA phase of
compound Si-110 with the additional difference that the cross-
section of the ribbons is about four times larger (Table S5†).
Based on these observations and the fact that the fluid smectic
phase at higher temperature also does not show any polar
switching, it is assumed that the absence of long-range polar order
in this ribbon phase (Colob) is due to the nearly free rotation of the
molecules around their long axes, which is retained in the ribbon
phase. This macroscopically nonpolar structure is also in agree-
ment with the comparatively small transition enthalpy values of
3.5 kJ molꢁ1 for the Iso–SmCR transition and 3.8 kJ molꢁ1 for the
SmCR–Colob transition, whereas the enthalpy values for transi-
tions to polar ordered phases are typically much higher, usually in
Fig. 5 Investigation of compound Si-2: (a) texture as observed between
crossed polarizers at T ¼ 110 ꢀC; X-ray diffraction pattern at 110 ꢀC: (b)
complete pattern showing the two diffuse wide angle scatterings and (c)
small angle region; (d) repolarization current response curves under
a triangular wave electric field (300 Vpp at 30 Hz, ITO coated 5 mm cell) in
the ColobPA phase at 100 ꢀC, PS ¼ 165 nC cmꢁ2; (e) synclinic domain as
seen under the same conditions between crossed polarizers.
within the layer modulation even under a relatively high applied
electric field.23 At higher temperature (T > 107 C, assigned as
ꢀ
Colob) the polar order might be lost due to the thermal expansion
of the bulky siloxane units, which reduces the packing density of
the aromatic cores.24
the order of 15–25 kJ molꢁ1
.
Dielectric measurements on
a
non-oriented sample of
3.2.2 Compound Si-3: randomly tilted SmCR phase. In
compound Si-3 both outer COO groups (X1 and X2) were
compound Si-3 were performed in a brass cell coated with gold,
and a distance between the capacitor plates of 0.05 mm.27 The
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Fig. 6 Investigation of compound Si-3: optical images obtained under a polarizing microscope between crossed polarizers: (a) SmCR phase at 135 ꢀC,
(b) phase transition to the Colob phase at 125 ꢀC, (c) phase transition as observed in a homeotropically aligned sample at 125 ꢀC, inset shows the texture
of the Colob phase developed from the homeotropically aligned smectic phase; XRD patterns of a surface aligned sample: (d) complete pattern obtained
in the SmCR phase at T ¼ 132 ꢀC showing two maxima of the outer diffuse scattering, (e) in the Colob phase at 86 ꢀC; small angle region (f) in the SmCR
phase, (g) in the Colob phase, and (h) same pattern with reciprocal lattices for two domains and reciprocal axes and indices for one domain.
calculated limits of the dielectric permittivity and the relaxation
times are presented in Fig. 7a and b, respectively.27 In the SmCR
phase only one well visible relaxation in the MHz range was
detected. No hints to a kHz process, typical for ferroelectric
clusters, are seen. Due to the compensation of the longitudinal
dipole components of compound Si-3 this relaxation must be
related to the fast reorientation of the molecules about the long
axis. The low frequency limit of the permittivity decreases
strongly at the transition into the Colob phase. This effect is
related to a spontaneous orientation of the long axes of the
molecules in direction of the measuring field and reduces
considerably the dielectric strength of the processes to the
experimental limit. A second, also very weak absorption, is seen
at high frequencies during crystallization where the homeotropic
order is destroyed. This process may be related to the faster
dynamics of the terminal groups in the molecules. It has to be
noted that relaxation process 1 did not show a strong decrease of
relaxation frequency at the phase transition SmCR/Colob
.
Therefore, there is no evidence for a ferroelectric order in the
columnar phase, as well. Hence, the dielectric investigations are
in line with a non-polar structure of the SmCR as well as Colob
phase (see Fig. 7).
3.2.3 Compound Si-4: SmCaPA–SmA transition. In
compound Si-4, all the ester groups are reversed with respect to
compound Si-1. This compound has the highest transition
temperatures of all reported compounds Si-n (see Table 3,
Fig. S18†). There areꢀtwo mesophases, separated by a phase
transition at T ¼ 174 C with a substantial transition enthalpy
(DH ¼ 7.1 kJ molꢁ1). The high temperature phase shows a strongly
birefringent fan-shaped texture as typical for non-polar smectic A
phases (see Fig. 8a). Shearing indicates a relatively low viscosity,
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530 nC cmꢁ2 (SmCaPA phase, Fig. 8f). In circular domains the
extinction crosses are oriented along the directions of the crossed
polarizers and no change in the position of the extinction crosses
could be observed either on field-removal or reversing the field
direction (Fig. 8g and h). Hence, there should be an anticlinic tilt
(SmCaPA phase) and polarization reversal seems to take place by
collective rotation around the long axes. This might be due to the
bulkiness of the siloxane units providing additional space for
rotation around the long axis.9 Related direct SmCPA-to-SmA
transitions (without intermediate SmC phase) are rare and have
been previously observed only for few compounds.12c,28–30
3.2.4 Relation of the SmCR and SmA phases of Si-3 and Si-4
with de Vries type smectic phases. No layer shrinkage could be
observed for the SmA–SmCaPA transition of compound Si-4
which is in line with the behaviour typically observed for SmA–
SmCPA transitions of (non-silylated) bent-core mesogens, where
at the transition to the tilted phase no layer shrinkage and some
times even an expansion of the d-value is observed.28,30 This can
be explained as a consequence of the more dense packing of bent
core molecules in smectic phases with polar order, where layer
shrinkage by tilt is compensated by stretching the alkyl chains
due to packing constraints resulting from a more dense core
packing.
Fig. 7 (a) Limits of the dielectric permittivity of Si-3 vs. temperature; 30
and 31 representing the low and high frequency limits of the first relax-
ation mechanism, 31 and 32 that of the second mechanism; (b) relaxation
frequency of the two mechanism vs. temperature.
However, in the case of rod-like molecules non-layer shrinkage
at the SmA–SmC transition is regarded as a first indication of
a de Vries like behaviour.31–33 According to the present under-
standing of de Vries type smectic phases there are two distinct
models. One assumes a diffuse orientational director distribution
having a maximum at zero tilt, and the other model suggests
a hollow-cone type director distribution with maxima at
a preferred tilt angle and a local minimum at zero tilt.32 It is
proposed that de Vries type smectic phases combine unusual low
orientational order and high smectic order, e.g. they are stabi-
lized by micro-segregation rather than orientational ordering.32
Hence, de Vries behaviour is generally found in materials with
a strong tendency for nano-segregation,34 as compounds with
fluorinated tails and siloxane units.35 Bent-core mesogens, espe-
cially those with silylated segments clearly belong to this type of
mesogens.7–9 Hence, it is interesting to compare the optically
uniaxial mesophases of compounds Si-3 and Si-4 under this
aspect.
as typical for usual SmA phases, and immediately leads to
homeotropic alignment which appears completely dark (optically
isotropic) between crossed polarizers, indicating an optically
uniaxial mesophase. The texture becomes broken at the transition
to the low temperature phase (Fig. 8b) and in the homeotropically
aligned sample a mixture of regions with Schlieren texture and
regions with non-specific birefringent texture, as often seen for
SmCPA phases, develops (Fig. 8c). The XRD patterns of the high
temperature phase show a diffuse wide-angle scattering at d ¼ 0.48
nm with maxima perpendicular to the layer reflection in the small
angle region (see Fig. 8e and S5, Table S1†), confirming a SmA
structure with on average non-tilted molecular cores and a layer
spacing of d ¼ 5.3 nm. There are only very weak second order layer
reflections indicating an almost sinusoidal electron density
distribution along the layer normal.
There are significant differences between compounds Si-3 and
Si-4 with respect to the detailed structures of their mesophases. In
both cases the high temperature phase is an optically uniaxial
smectic phase with nearly identical layer spacing (d ¼ 5.2–5.3 nm)
which is much smaller than the molecular length (L ¼ 7.1 nm, see
Fig. S21d†). However, the director distribution is distinct in these
two smectic phases as evident from the XRD patterns of aligned
samples (Fig. 6d, 8e, S4 and S5†). Compound Si-4 with an iso-
phthalate core seems to favour anticlinic tilt at lower temperature
and a randomization of the tilt with an orientational director
distribution having a maximum at zero tilt is observed in the
smectic phase at higher temperature (SmA). In the case of
compound Si-3 with a resorcinol core and terephthalate based
wings synclinic organization is preferred in the low temperature
phase (Colob) and randomization of tilt with hollow-cone type
orientational director distribution is observed for the smectic
phases at higher temperature (SmCR, see Fig. 9a). In this case the
The layer distance is the same in the low temperature phase,
but it is derived from layer reflections of first to third order
resulting from sharper defined layers, and the outer diffuse
scattering at d ¼ 0.47 nm shows a broader azimuthal distribution
with maxima between equator and meridian from which an
average tilt of the long molecular axes of about 23ꢀ can be
calculated (Fig. 8d and S5†). These features clearly hint to an
SmC structure at low temperature. In both phases there is a weak
second outer diffuse scattering at about d ¼ 0.7 nm showing
uniform intensity distribution along a ring which can be attrib-
uted to the average distance between the siloxane units.
Under an applied triangular-wave field, no polarization
ꢀ
current response could be detected at T > 174 C (SmA phase).
Upon decreasing temperature at the phase transition to the low
temperature phase two polarization current peaks appeared,
which clearly evidence AF switching with a polarization value of
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Fig. 8 Investigation of compound Si-4: optical images obtained between crossed polarizers: (a) broken focal-conic texture of the SmA phase at 180 ꢀC,
(b) weakly birefringent texture of SmCaPA phase at 165 ꢀC, (c) birefringent texture developing in the homeotropically aligned sample of the SmA phase
at the transition to the SmCaPA phase at 174 ꢀC; XRD patterns of a partially surface aligned sample on heating (see also q-scans over the wide angle
scattering, shown in Fig. S5†): (d) SmCaPA phase at 165 ꢀC and (e) SmA phase at T ¼ 180 ꢀC; (f) antiferroelectric switching current response in the
SmCaPA phase at 164 ꢀC (130 Vpp, 6 Hz, 5 mm, PS ¼ 530 nC cmꢁ2); optical images: (g) under a DC electric field, no change in the position of the extinction
crosses is observed even after changing the sign of the applied field, (h) same extinction cross after field removal.
orientational order parameter has a preferred angle (b ¼ 30ꢀ) and
a minimum at zero tilt. It appears that the SmCR phase is formed
by SmC clusters with only short range correlation of the tilt
direction in adjacent clusters. Upon cooling, these clusters
coalesce with formation of ribbons which organize to a 2D lattice,
leading to the Colob phase. For Si-4 anticlinic tilt seems to be
more preferred which inhibits formation of a ribbon structure and
leads to a SmCaPA phase at lower temperature. In the high
temperature smectic phase of Si-4 the preference for anticlinic tilt
is retained and together with the generally reduced tendency for
a uniform tilt, provided by this kind of aromatic core (also the
non-silylated compound En-4 forms a SmA phase), an orienta-
tional director distribution with a maximum at zero tilt is obtained
(see Fig. 9b).
Hence, the uniaxial smectic phases SmA and SmCR of these
two compounds can be regarded as examples for the two distinct
models of organization in optically uniaxial smectic phases which
are under discussion for de Vries type smectic phases.31b,32 The
fact that relatively sharp layers showing a second and third order
layer reflection were observed for the SmCR phase of Si-3, but
only a sinusoidal electron density modulation is found for the
SmA phase of Si-4 is in line with these two distinct models. In the
SmA phase sinusoidal director distribution leads to a stronger
distribution of the effective molecular length which requires
a stronger interpenetration of molecules in adjacent layers and
hence the interfaces are more diffuse in this phase.
To the best of our knowledge, there is presently no direct XRD
proof for a hollow cone director distribution in a de Vries type
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preferred, then it is more likely that the maximum of the orien-
tational director distribution is along the layer normal.
Based on the shape of the diffuse wide angle scattering (see
Fig. S5†) and the relatively high birefringence of the smectic
phase (Fig. 8a) it seems that the SmA phase of compound Si-4 is
more similar to classical SmA phases and layer shrinkage at the
SmA-to-SmC transition is compensated by the more dense
packing achieved in the SmCaPA phase where rotation around
the long axis is reduced. However, the SmCR phase of compound
Si-3 has some relation to de Vries type smectic phases with
hollow cone-like director distribution.
3.3 Mesomorphic properties of bent-core compounds Si-5 and
Si-6 with only one silyl group
Fig. 9 Schematic sketches of the proposed molecular organizations and
schematic plots of the director distributions in the smectic phases of
compounds Si-3 and Si-4 as deduced from the distinct XRD patterns: (a)
SmCR phase of compound Si-3 with preferred synclinic tilt correlation
(only two tilt directions are shown, but in reality all tilt directions are
possible) and (b) SmA phase of compound Si-4; in both cases the bent
core molecules are rotationally disordered around their long axes (hence
shown as solid lines).
Attaching a siloxane unit to only one alkyl chain of a bent-core
mesogen is in the case of phenyl benzoate based resorcinols of
type 1 (see Chart 1) often associated with replacement of anti-
ferroelectric switching SmCPA phases (B2 phases) by ferroelec-
[
tric switching dark conglomerate phases (SmCPFE *] phases).7
This was attributed to the softening and decoupling of the
smectic layers by the micro-segregated sublayers formed by the
silyl groups.7 Layer distortion due to the bulkiness of the silyl
groups is in the case of monosilylated compounds minimized by
antiparallel packing of the molecules in the layers.7 Also for
compounds Si-5 and Si-6 the siloxane units modify the properties
of the non-silylated precursors En-5 and En-6 (see Tables 2
and 4).
smectic phase of rod-like mesogens. However, for bent-core
mesogens, beside the non-polar SmCR type reported herein and
also found for a related compound, similar to Si-4, but with
additional methyl group in 2-position of the resorcinol core,11
there is also anexample of a bent-core mesogen witha ferroelectric
switching smectic phase with randomization of tilt and polar
order, assigned as SmCPR.8b Hence, it appears that for bent-core
mesogens de Vries type smectic phase with hollow cone-like
director distribution are more common, or it is easier to detect
them experimentally because for these compounds the tilt angle is
sufficiently large. It appears that formation of SmCR phases
requires a sufficiently dense packing, leading to a strong tendency
for clustering as provided by bent core molecules. This has to be
combined with a preference for relatively large tilt angles and
synclinic tilt. In the case of bent-core mesogens there is a high
intrinsic tendency for strongly tilted arrangements which is mainly
due to the distinct space filling of the densely packed aromatic
cores and fluid aliphatic chains.36 This intrinsic tilt is strengthened
or weakened by dipolar, quadrupolar and electrostatic (donor–
acceptor) interactions between the aromatic cores, and hence, is
affected by the orientation of the linking groups. If the tendency
for tilt is relatively weak, as in most rod-like mesogens and also in
the case of the terephthalate Si-4 and, in addition, anticlinic tilt is
The textures of compounds Si-5 and Si-6 are very similar to
each other and indicate mesophases with 2D-lattice (Fig. 10a and
11a), which is confirmed by the XRD patterns (Fig. S7† and
11c–e). For both compounds there are two diffuse wide angle
scatterings. The one at d ¼ 0.7 nm assigned to the mean distance
between the siloxanes is very weak, as the number of siloxane
units is reduced compared to compounds Si-1–Si-4.7 This wide
angle scattering is circular without distinct maximum whereas
the second one at d ¼ 0.46 nm, assigned to the mean distance
between alkyl chains and aromatics, is more intense and in the
case of compound Si-6 has maxima at c ¼ 64, 123, 237, and 302ꢀ
indicating a tilt of about 30ꢀ with respect to the normal to the
modulated layers (Fig. 11c and S6†). For this compound the
small angle region shows a strong layer reflection with three
harmonics and additional hk and k0 reflections which can be
indexed to an oblique lattice with parameters a ¼ 17.9 nm,
Table 4 Phase transition temperatures (T/ꢀC), enthalpy values (DH/kJ molꢁ1, in square brackets) and lattice parameters obtained for the silicon
containing bent-core compounds Si-5 and Si-6a
No
X1
Y1
Y2
X2
Phase transitions
Lattice parameters/nm (T/ꢀC)
Si-5
Si-6
OOC
OOC
COO
OOC
OOC
OOC
COO
OOC
Cr 127 [19] Colob 140 [—] ColobPFE 157 [23] Iso
Cr 112 [33] Colob 130 [—] ColobPFE 136 [21] Iso
d10 ¼ 15, d01 ¼ 5.5 (130)
a ¼ 17.9, b ¼ 5.3, g ¼ 100ꢀ (130)
a
Peak temperatures in the first DSC heating curves (10 K minꢁ1); for DSC curves, see Fig. S19 and S20†; for abbreviations, see Tables 1–3.
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Like compounds En-5 and En-6 also compounds Si-5 and Si-6
show ferroelectric switching (Fig. 10c and 11b). However, for Si-
5 and Si-6 polar switching is restricted to certain temperature
ranges, which depend on the strength of the applied electric field
and the frequency. Under a voltage of 400 Vpp with a frequency
of 10 Hz in a 5 mm cell for compound Si-5 ferroelectric switching
takes place between 140 and 157 ꢀC (PS ¼ 220 nC cmꢁ2) and
under the same conditions for compound Si-6 switching is
observed between 130 and 136 ꢀC (PS ¼ 180 nC cmꢁ2). No
ꢀ
switching could be observed below 140ꢀ C and 130 C, respec-
tively. For both compounds there is no change of the position of
the extinction crosses during switching, indicating a polar
switching around the long axis at all frequencies. The loss of
polar switching below a certain temperature limit might in both
cases be due to an increased threshold voltage for this switching
process at reduced temperature, as viscosity increases; an alter-
native switching by rotation on a cone, as observed for the polar
phases of the corresponding olefins En-5 and En-6 seems to be
inhibited by the modulated (broken) layer structure of the
mesophases of compounds Si-5 and Si-6 (Colob phases) which
seems to be not removed under the applied voltage (see Fig. 10b,
this kind of mosaic-like texture is typically observed for modu-
lated smectic phases) and these modulations are unfavorable for
rotation on a cone.
Though there is some similarity between the mesophases of the
silylated compounds Si-5 and Si-6 and the corresponding olefin
En-5 and En-6, both showing large modulation wave lengths and
ferroelectric switching, the mesophases of the non-silylated
compounds have less steric frustration and these mesophases are
more similar to undulated/modulated smectic phases for which
the contribution of polarization splay to layer modulation is
important.18,37 In these mesophases layer undulation/modulation
can be completely removed at sufficiently high threshold voltage
and switching around the cone can take place in distinct
temperature ranges and at sufficiently low frequencies after
completely suppressing the layer modulation under the applied
electric field. However, in the broken layers of the Colob phases
of the silylated compounds Si-5 and Si-6, where the steric layer
distortion by the bulky silyl units provides a significant addi-
tional contribution to layer modulation which in this case cannot
be completely removed under the applied electric field, a switch-
ing on a cone is not possible.
Fig. 10 Investigation of compound Si-5: (a) optical texture as obtained
under a polarizing microscope between crossed polarizers at T ¼ 157 ꢀC
during growing from the isotropic liquid (dark areas); (b) texture as
obtained by cooling from the isotropic liquid under a triangular wave
field (with or without applied field, T ¼ 153 ꢀC); (c) polarization current
curve as obtained under an applied triangular wave voltage (400 Vpp, 10
Hz, 5 mm ITO cell, PS ¼ 220 nC cmꢁ2) at T ¼ 153 ꢀC.
4. Summary and conclusions
The present investigation is focussed on the effects of the direc-
tion of COO linking groups on the mesomorphic behaviour of
silyl substituted bent core mesogens and the comparison with
related non-silylated compounds. It turned out that reversing the
direction of one, two, or all four COO groups in silylated bent-
core molecules leads to a change of mesophase stability and
mesophase structure, similar to the previously reported non-
silylated bent-core compounds. The mesophases formed by
molecules with oligosiloxane groups at only one end are in some
respect similar to those of the related olefins, though modulated
variants of smectic phases are more common in the series of
silylated compounds due to the steric layer frustration provided
by the bulky oligosiloxane groups. A stronger effect of the silyl
groups on mesophase structure is observed for bent core
b ¼ 5.3 nm and g ¼ 100ꢀ (Fig. 11d and e, Table S4). For Si-5 we
could not get as well aligned samples for the XRD measurements
as in the case of compound Si-6. Only h0 and 0k reflections could
be distinguished for a tentative indexing analogous to that for Si-
6 (see Fig. S7 and Table S1†) and therefore the angle g of the 2D
lattice could not be determined. From the calculated d values for
these reflections (d10 ¼ 15 nm, d01 ¼ 5.5 nm) a similar 2D lattice
as in the case of Si-6 may be considered. Because of few other
similarities between Si-5 and Si-6, as textures and switching
behaviour (see below), it is quite sure that the mesophases of both
compounds represent the same type of ColobPFE phase.
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Fig. 11 Investigation of compound Si-6: (a) optical texture obtained under a polarizing microscope between crossed polarizers at T ¼ 133 ꢀC; (b)
polarization current curve as obtained under an applied triangular wave voltage (400 Vpp, 10 Hz, 5 mm ITO cell; PS ¼ 165 nC cmꢁ2) at T ¼ 132 ꢀC; XRD
patterns of surface aligned samples at T ¼ 130ꢀ in the ColobPFE phase: (c) complete pattern, (d) small angle region, and (e) part of the small angle region
with reciprocal axes, 2D lattice and indices for the observed reflections for one domain.
molecules silylated at both ends. This is most likely due to the
stronger steric frustration resulting from the bulky siloxane units
at both ends, which in this case cannot be compensated by
antiparallel packing of the molecules.7,9 Nevertheless, it is
remarkable that for some compounds in distinct temperature
regions polar switching phases can be achieved despite this
significant steric frustration. However, if polar switching is
observed it is of the antiferroelectric type, whereas compounds
with only one silyl end group show ferroelectric switching.
Among the mesophases formed by the double silylated
compounds the modulated tilted smectic phases (Colob) are
dominating which can be non-polar or antiferroelectric. In the
modulated smectic phases of silylated compounds polar switch-
ing usually takes place by collective rotation around the long axis
which reverses the chirality, whereas in the undulated USmCPFE
and modulated ColobPFE phases of the non-silylated compounds
also rotation on a cone is possible if the rotation around the long
axis becomes slow due to enhanced viscosity. An especially
interesting phase is formed by the resorcinol bisbenzoates with
terephthalate based rod-like wings (Si-3), which represents
a randomly tilted SmCR phase. It is considered as a kind of de
Vries type smectic phase with hollow cone-like orientational
director distribution and represents a new variant of mesophases
formed by bent-core mesogens.38 It seems to have a cluster
structure composed of synclinic tilted molecules with random-
ized tilt direction.39 Moreover, in all siloxane-containing
compounds melting points are lower than for the corresponding
compounds with hydrocarbon chains or olefinic double bonds.
Therefore, the introduction of siloxane units widens the LC
phase ranges and provides enantiotropic LC properties for
compounds which otherwise would be crystalline or only mon-
otropic. Moreover, the mesophases formed by silylated bent-core
compounds are less viscous; this solves one of the major prob-
lems in investigation and application of bent-core compounds.
Acknowledgements
R. A. Reddy is grateful to the Alexander von Humboldt Foun-
dation for a research fellowship.
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17 If the small angle scattering is assigned as 10 reflection the undulation
period would correspond to d, however, as the 10 reflection could
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reflection could also correspond to 20 and in this case a period
corresponding to 2d would result.
3896 | Soft Matter, 2010, 6, 3883–3897
This journal is ª The Royal Society of Chemistry 2010

View Article Online
J. Watanabe, V. Novotna and V. Hamplova, Phys. Rev. E: Stat.
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38 In addition, also an uniaxial nontilted smectic phase with randomized
polar direction was observed for bent core molecules (SmAPR):
Y. Shimbo, Y. Takanishi, K. Ishikawa, E. Gorecka, D. Pociecha,
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39 Cluster models were recently also proposed for the development of
phase biaxiality and polar order in nematic phases:
P. K. Karahaliou, A. G. Vanakaras and D. J. Photinos, J. Chem.
Phys., 2009, 131, 124516; C. Tschierske and D. Photinos, J. Mater.
Chem., 2010, 20, 4263.
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Soft Matter, 2010, 6, 3883–3897 | 3897