Effects of molecular chirality on superstructural chirality in liquid crystalline dark conglomerate phases

Article abstract of DOI:10.1039/c2sm25763g

The first examples of polyphilic bent-core compounds consisting of a biphenyl central core, a chiral terminal chain and an oligo(dimethylsiloxane) end group at the other terminus have been synthesized and characterized. The mesomorphic behaviour of the compounds was investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction and electro-optic methods. The olefinic precursors show monotropic or enantiotropic B₁ type mesophases. The siloxane substituted analogues containing a racemic chain exhibit dark conglomerate phases (DC^[*^] phases) which are composed of chiral domains with opposite chirality, whereas the siloxane derivatives with a homogeneously chiral moiety show "dark enantiomer" phases (DE* phases) with uniform chirality. Under electric fields, different types of ferroelectric switching (FE) smectic and modulated smectic phases were induced. The effects of chain branching, spacer length and molecular chirality on the stability of the DC_FE ^[*^] and DE_FE* phases and on the field-induced smectic and modulated smectic phases were investigated. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2sm25763g

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Volume 8 | Number 30 | 14 August 2012 | Pages 7729–7990
ISSN 1744-683X
PAPER
Hale Ocak et al.
Effects of molecular chirality on superstructural chirality in liquid crystalline dark
conglomerate phases
1744-683X(2012)8:30;1-B

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PAPER
Effects of molecular chirality on superstructural chirality in liquid crystalline
dark conglomerate phases†
a
c
Hale Ocak, Belkız Bilgin-Eran,a Marko Prehmbc and Carsten Tschierske
*
*
Received 1st April 2012, Accepted 1st May 2012
DOI: 10.1039/c2sm25763g
The first examples of polyphilic bent-core compounds consisting of a biphenyl central core, a chiral
terminal chain and an oligo(dimethylsiloxane) end group at the other terminus have been synthesized
and characterized. The mesomorphic behaviour of the compounds was investigated by differential
scanning calorimetry, optical polarizing microscopy, X-ray diffraction and electro-optic methods. The
olefinic precursors show monotropic or enantiotropic B1 type mesophases. The siloxane substituted
analogues containing a racemic chain exhibit dark conglomerate phases (DC[*] phases) which are
composed of chiral domains with opposite chirality, whereas the siloxane derivatives with
a homogeneously chiral moiety show ‘‘dark enantiomer’’ phases (DE* phases) with uniform chirality.
Under electric fields, different types of ferroelectric switching (FE) smectic and modulated smectic
phases were induced. The effects of chain branching, spacer length and molecular chirality on the
[
stability of the DCFE *] and DEFE* phases and on the field-induced smectic and modulated smectic
phases were investigated.
chirality and molecular chirality have recently been observed in
1. Introduction
switching experiments,32 the effects of the combination of
molecular chirality with superstructural chirality in LC phases of
bent-core molecules are only partially explored to date.
Spontaneous achiral symmetry breaking, i.e. the spontaneous
formation of chiral superstructures with uniform chirality over
macroscopic dimensions, achieved by self-assembly of achiral or
racemic molecules, has been a core research topic since it was
discovered by Pasteur.1,2 Recently, work on the phenomenon of
spontaneous reflection symmetry breaking in supramolecular
ensembles3–5 has moved into fluid liquid crystalline (LC) pha-
ses,6–12 namely LC phases formed by bent-core molecules.13–19
Superstructural chirality in the LC phases of these materials
result from the polar order in their lamellar (smectic) LC phases
(SmCPA, SmCPF, see Fig. 1a).13,15 The combination of tilt and
polar order leads to an intrinsic chirality of the layers formed by
these achiral molecules (Fig. 1b).13,15
For this reason, we have synthesized and investigated chiral
polyphilic bent-core mesogens. The introduction of poly-
philicity33 into bent-core LC molecules has been used to
stabilize smectic phases and to modify the interlayer correla-
tions. In particular, oligo(dimethylsiloxane) units represent
useful structural units capable of segregation into their own
domains (Fig. 1c).13a,34–39 Attached to bent-core units, these
moieties lead to ferroelectric switching LC phases (SmCPF)34
instead of the usually observed antiferroelectric phases
(SmCPA), and often the so-called ‘‘dark conglomerate phases’’
(DC[*] phases) can be observed.34,40–45 In these non-birefringent
or low-birefringent mesophases, which appear dark between
crossed polarizers, macroscopic chirality can be identified by
the occurrence of a conglomerate of chiral domains with
opposite handedness.13a,34,38,39 The low birefringence of these
mesophases is thought to result from an intrinsic layer frus-
tration due to the packing of the bent-cores in a flat layer,
leading to a saddle splay deformation. In this way the
3-dimensionally deformed layers become organized in
a sponge-like super-structure (Fig. 1d).46 Due to this special
spatial organization of the layers, these LC phases appear
optically isotropic. In these soft matter systems the layer
chirality couples with the molecular conformational chirality
that provides significant optical activity.13a,47,48 Because there is
no birefringence in these DC phases, chirality can easily be
detected by optical investigations.
Chirality in LC phases, either molecular or superstructural in
origin, leads to chirality-modified and chirality-frustrated LC
phases and ferroelectricity.16,20–23 Though few bent-core mole-
cules containing chiral centers in the terminal chains have been
reported so far,24–32 and diastereomeric relations between layer
aDepartment of Chemistry, Yildiz Technical University, Davutpaxsa
Yerlexsim Birimi, TR-34210 Esenler Istanbul, Turkey. E-mail:
ocak_hale@hotmail.com
bInstitute of Chemistry, Physical Chemistry, Martin Luther University
Halle-Wittenberg, Von-Danckelmann-Platz 4, D-06120 Halle, Germany
cInstitute 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
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2sm25763g
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 7773–7783 | 7773

Fig. 1 The fundamental modes of molecular organization in the polar smectic phases of bent-core mesogens: (a) ferroelectric and antiferroelectric polar
order in tilted smectic (SmCPA/F) phases (the tilt occurs perpendicular to the bend directions), (b) superstructural chirality arising from the combination
of tilt and polar order, (c) formation of triply segregated layers by polyphilic bent-core mesogens with terminal silyl groups and (d) sponge-like
deformation of the layers in the dark conglomerate phases.13a,16,39a,d
Herein, we present the first examples of bent-core mesogens
combining chiral chains with siloxane units, known to force the
formation of dark conglomerate phases, thereby combining the
effects of molecular chirality with superstructural chirality. The
influence of the branched chiral chains on the self-assembly of
siloxane substituted bent-core molecules is studied and the
enantiomers are compared with the racemic mixtures.
using Karstedt’s catalyst to yield the siloxanes Six-1 to Six-3.
From the synthetic procedure it is clear that the stereogenic
centers of the starting alcohols were not touched and hence, it
can be assumed that the final products have equal optical purity
to the used starting material. Purification of all final compounds
was carried out by column chromatography on silica gel and
crystallization from ethanol. The experimental details, spectro-
scopic (1H-NMR, 13C-NMR and MS) and analytic data of the
materials are given in the ESI†.
2. Results and discussion
2.1 Synthesis
2.2 Investigation of the liquid crystalline properties
The bent-core compounds 1–3 with terminal double bonds were
synthesized as shown in Scheme 1 by stepwise esterification49 of
40-benzyloxybiphenyl-3-ol (A),50 using N,N’-dicyclohexyl-
carbodiimide (DCC) at first with chiral 4-(4-alkyloxy-
benzoyloxy)benzoic acids (B),32,51 followed by hydrogenolytic
debenzylation (H2, Pd/C in THF)39a and DCC esterification49 of
the biphenylols C with the appropriate 4-[4-(alkenyloxy)-
benzoyloxy]benzoic acids (D/n)52,53 according to known proce-
dures.37,39a,c The chiral alkyloxy groups of the benzoic acids B
were introduced by an etherification reaction of ethyl
4-hydroxybenzoate with the appropriate tosylates which were
prepared from the corresponding commercially available chiral
alcohols by tosylation.54 After hydrolysis, the chiral
4-alkyloxybenzoic acids were used for DCC esterification with
4-hydroxybenzaldehyde. The resulting 4-(4-alkyloxybenzoyloxy)
benzaldehydes were oxidized using sodium chlorite as the
oxidizing agent to yield the acids B.52 For the synthesis of
compound (S)-3b, (S)-(ꢀ)-b-citronellol was used as the starting
material and the double bond was hydrogenated to the (S)-3,7-
dimethyloctyloxy unit during the hydrogenolytic deprotection of
the benzyl group. In the final step 1H-heptamethyltrisiloxane H–
Si3 or 3H-heptamethyltrisiloxane H–Si3i, respectively, were
attached to the olefins 1–3 by a hydrosilylation reaction39a,55
The mesomorphic properties of the synthesized olefins 1–3 and
silylated compounds Six-1–Six-3 were investigated by polarised
light optical microscopy (PM), differential scanning calorimetry
(DSC), and electro-optical measurement, some of them also by
X-ray diffraction (XRD).
2.3 Mesophases of the olefins 1–3
All compounds combining an olefinic group at one end and
a branched chain at the other exhibit monotropic or enantio-
tropic B1 mesophases (Table 1), as indicated by the XRD of the
aligned samples. The small angle diffraction pattern of the B1
phase of compound (S)-1a can be indexed to a rectangular lattice
(c2mm) with the lattice parameters a ¼ 2.6 nm and b ¼ 4.1 nm (at
ꢁ
150 C, see Fig. 2a and b). The diffuse wide angle scattering is
perpendicular to b, which means that the molecules are aligned
parallel to b and a/2 corresponds to the diameter of the ribbon
forming the rectangular lattice (Fig. 2c). The parameter b is in
good agreement with the molecular length (Lmol ¼ 4.3 nm for
120ꢁ bent molecules with the most stretched wing groups deter-
mined by space filling molecular models) and the number of
molecules arranged in the cross-section of the ribbons is about
only three. Slightly larger ribbons comprising about 5 molecules
7774 | Soft Matter, 2012, 8, 7773–7783
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Scheme 1 Synthesis of the chiral oligo(dimethylsiloxane) substituted bent-core compounds. Reagents and conditions: (i) DCC, DMAP, CH2Cl2, r.t.;49
(ii) H2, Pd/C, THF, 40 ꢁC, 18 h;39a (iii) H–Six, Karstedt’s catalyst, toluene, r.t., 48 h;39a,55 for preparation of C/3 the benzoic acid B/4 with R1 ¼ (S)-(ꢀ)-b-
citronellyl is used; the double bond is hydrogenated in step (ii).
ꢁ
organized side-by-side (a ¼ 3.7 nm, b ¼ 4.8 nm at 110 C) were
observed for the B1 phase of rac-3b (see Fig. S1†). Surprisingly,
the fan-like texture of the mesophase of compound (S)-1a
(Fig. 3a) is more similar to a B6-type intercalated smectic phase,
whereas for all other synthesized compounds the typical textures
of B1 phases (Fig. 3b) were observed. This is in line with the
model of the B1 phases, which are related to the B6 phases,
mainly differing in the correlation length of the two-dimensional
(2d) lattice, which is long range for the B1 phases and only short
range for the B6 phases.56 Compound (S)-1a appears to be an
intermediate case showing a B6-like fan-texture though a 2d
lattice is already observable by XRD. No polar switching could
be observed for these mesophases in the electro-optical studies,
which is also in line with other B1 phases.13
stability and mesophase type remain the same, indicating the
absence of a significant chirality effect on the formation of the B1
phases.28
2.4 Mesophases of the siloxanes
The phase transition temperatures of the silylated compounds
Six-1–Six-3 are collated in Table 2. All compounds exhibit
mesophases which appear dark (optically isotropic) between
crossed polarizers, but for some racemic compounds
(compounds rac-Si3-3a and rac-Si3i-3b) small birefringent defects
can be observed. The transition from the isotropic liquid state to
these dark mesophases is indicated in the DSC scans by relatively
high transition enthalpy values between 15 and 21 kJ molꢀ1 as
typical for the transition to polar mesophases for bent-core
molecules.13 Optically there is no change, only a strong increase
of the viscosity is observed at these temperatures. On further
coolingꢁthe viscosity increases further at temperatures between 70
and 85 C, but the dark appearance of the mesophase does not
change and no peak or distinct step is observed in the DSC curves
(see Fig. 4 for an example). However, in the heating curves
a small hump starts around this temperature. It appears that at
As shown in Table 1, the stability of the B1 phases decreases
with growing effective diameter of the branched chain (2-
methylbutyl > 3,7-dimethyloctyl > 2-ethylhexyl), determined by
the position of the chain branching and the length of the branch
located closest to the core. Stability is also affected by the alkenyl
chain length and decreases in the order n ¼ 4 (a) > n ¼ 9 (b).
Comparing the mesomorphic properties of the enantiomer (S)-
3b with the racemic compound rac-3b shows that the mesophase
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 7773–7783 | 7775

Table 1 Mesophases, phase transition temperatures and transition enthalpies of the olefins 1–3a
Compound
n
R1
T/ꢁC [DH/kJ molꢀ1
]
(S)-1a
rac-2a
rac-2b
rac-3a
rac-3b
(S)-3b
4
4
9
4
9
9
(S)-2-Methylbutyl
rac-2-Ethylhexyl
rac-2-Ethylhexyl
rac-3,7-Dimethyloctyl
rac-3,7-Dimethyloctyl
(S)-3,7-Dimethyloctyl
Cr 143 [22.3] B1 166 [11.1] Iso
Cr 125 [35.7] (B1 108 [6.4]) Iso
Cr 111 [22.9] (B1 99 [4.0]) Iso
Cr1 80 [8.9] Cr2 104 [4.6] B1 139 [9.2] Iso
Cr 105 [33.3] B1 123 [12.8] Iso
Cr 61 [4.1]b B1 124 [11.7] Iso
a
Perkin-Elmer DSC-7; peak temperatures of the first heating scans at a rate of 10 K minꢀ1; enthalpy values are given in square parentheses;
abbreviations: Cr ¼ crystalline; B1 ¼ rectangular columnar phase with a c2mm lattice, the molecules are non-tilted and the polarization is
antiparallel and along the column long axes (B1rev phases, see model in Fig. 2c) and Iso ¼ isotropic liquid phase; monotropic transitions are
indicated by parentheses. b Partial crystallization.
this temperature the mesophase enters into a kind of glassy
state.39d With the exception of compound (S)-Si3-1a, apparently
none of the compounds crystallize.
smaller than in other cases. Therefore these mesophases are
[
assigned as DC phases (without *]) here.
The mesophases of the enantiomers (S)-Si3-1a and (S)-Si3-3b
also appear optically isotropic between crossed polarizers and no
chiral domains could be observed by decrossing the polarizers
Fig. 5d–f. However, for these compounds the brightness depends
on the direction of inclination of the polarizers with respect to the
90ꢁ position. A comparison of the mesophase stability of the
compounds rac-Si3-3b and (S)-Si3-3b shows that the phase
transition temperatures remain the same by replacing the racemic
chain by a homogeneously chiral chain. From contact prepara-
tions between the enantiomer (S)-Si3-3b and the related racemic
compound rac-Si3-3b it is evident that the chirality sense of the
domains is biased by the molecular chirality (see Fig. 5g and h).
Only one type of chiral domain is formed by the enantiomerically
pure compound (dark enantiomer phases, DE* phases), whereas
the racemic compound shows domains with opposite chirality in
the ratio 1 : 1 (DC[*] phases). In the contact region between the
racemic mixture and the enantiomer the dominance of one type
of domain at first increases and then the other type of domain is
completely removed. Though it was shown previously that chiral
additives can change the dark conglomerate phases to
a uniformly chiral texture,42 to the best of our knowledge the
2.4.1 Optical investigations. For the racemic compounds rac-
Si3-2a and rac-Six-3a/b a mixture of dark and bright domains
become visible by rotating the polarizer by a small angle (2–3ꢁ)
either clockwise or anti clockwise. If the polarizer is rotated in the
opposite direction, the brightness of the domains is reversed as
shown in Fig. 5a and c. No domains were observed if the sample
was rotated between the crossed polarizers and the brightness of
the chiral domains was not changed if the sample was rotated
between slightly decrossed polarizers. This indicates a conglom-
erate of domains with opposite chirality as typical for dark
conglomerate (DC[*]) phases.34,35,39 For the racemic compounds
rac-Six-2b no chiral domains can be detected and the brightness
increases equally by decrossing the polarizers in either direction.
For these compounds the chiral domains are either too small to
be detected by optical microscopy or the phase itself is achiral if
layers with opposite chirality alternate.57 Based on electro-
optical investigations (see below) it appears that the sponge-like
layer deformation is especially strong for these compounds and
hence it is likely that also the chiral domains are indeed much
Fig. 2 X-ray diffraction patterns of compound (S)-1a in the B1 mesophase (a ¼ 2.6 nm; b ¼ 4.1 nm) at T ¼ 150 ꢁC; (a) original wide angle pattern; (b)
the same pattern after subtraction of the scattering in the isotropic liquid state with the reciprocal lattice and indexation and (c) model of the molecular
organization in the B1 phase.
7776 | Soft Matter, 2012, 8, 7773–7783
This journal is ª The Royal Society of Chemistry 2012

2.4.2 XRD investigations. In the XRD patterns of the
mesophase of the compound rac-Si3-2a, shown in Fig. 6, there
are two diffuse scatterings in the wide-angle region. One has
a maximum at d ¼ 0.47 nm, corresponding to the mean distance
between the aromatic cores and the alkyl chains and a second
one, which is much weaker and has its maximum at d ¼ 0.70 nm.
It is assigned to the mean distance between the segregated oli-
go(dimethylsiloxane) units.39a,c One intense reflection is observed
in the small angle region together with very weak 2nd and 3rd
order reflections as typical for lamellar phases. The d-value is
smaller than the molecular length and would be in line with
a tilted organization of the molecules in layers. It is now accepted
that DC[*] phases represent smectic phases with sponge-like
deformed layers.46 For this reason uniform alignment of the
samples is impossible and therefore only a ring with uniform
brightness is observed for the layer reflection in the XRD
patterns (after attempted surface alignment). This kind of
diffraction pattern is very typical for DC[*] phases39a (for addi-
tional diffraction patterns, see Fig. S2–S4†), but it prevents the
precise determination of the phase structure by 2d-XRD. If
a sponge-like structure formed by lamellae is assumed, then from
the d-values (d ¼ 3.8 nm for rac-Si3-2a at 110 ꢁC and d ¼ 4.5 nm
for compound rac-Si3-3b at 100 ꢁC (Fig. S2†)) tilt angles around
50ꢁ and 42ꢁ, respectively, can be estimated according to cosb ¼ d/
Lmol (rac-Si3-2a: Lmol ¼ 5.9 nm; rac-Si3-3b: Lmol ¼ 6.1 nm). Such
high tilt angles between 35ꢁ and 50ꢁ are reasonable values for
SmCP phases formed by bent-core mesogens.13
In the temperature range between 70 and 80 ꢁC the small angle
scattering becomes more diffuse and the viscosity increases. This
supports the idea of a transition to a glassy state at this
temperature.39d
Fig. 3 The textures obtained between crossed polarizers as observed for
the B1 phases of (a) (S)-1a at 156 ꢁC and (b) rac-3b at 116 ꢁC.
2.4.3 Electro-optical investigation of the siloxanes rac-Six-3.
Electro-optical investigations were carried out in 6 mm indium tin
oxide (ITO) coated cells without additional polyimide (PI)
alignment layer. The textural changes observed under the electric
field and the type of response on an applied triangular wave field
strongly depend on the molecular structure, applied voltage and
temperature.
compounds reported here represent the first dark phases formed
by chiral bent-core LC molecules. As these mesophases are
uniformly chiral and do not represent conglomerates they are
assigned as ‘‘dark enantiomer phases’’ (DE* phases) in analogy
to the DC[*] phases formed by the racemates. The textural
features are dark (optically isotropic) uniform textures, where the
maximum darkness is achieved by slightly decrossing the polar-
izers in a specific direction (clockwise or counterclockwise). The
maximum contrast is obtained for approximately the same
angles as observed for the DC[*] phases (2–3ꢁ) of the corre-
sponding racemate, suggesting that it is mainly determined by the
chiral superstructure than by the molecular chirality. Therefore,
the major effect of molecular configurational chirality seems to
be the bias of the conformational chirality of the molecules.
Though the DE* phases of the investigated enantiomerically
pure compounds always appear uniformly dark without bire-
fringent defects, for the DC[*] phases of the racemic compounds
in some cases small birefringent defects were observed, especially
for compounds rac-Si3-3a and rac-Si3i-3b. This might be due to
some distortion of the sponge-like structure. The racemic
compounds seem to be more sensitive to this kind of distortion,
probably due to the presence of the conglomerate structure with
grain boundaries between domains with opposite chirality. These
boundaries might induce some distortion to the sponge-like
structure, i.e. there are regions with a not completely random and
locally lamellar structure that appear birefringent.
The dark conglomerate phases of the racemic compounds
rac-Si3i-3b and rac-Si3-3a become strongly birefringent under an
applied electric field of 22 V mmꢀ1 and 26 V mmꢀ1, respectively.
These threshold fields are required for the removal of layer
deformation and induction of strongly birefringent smectic
phases, which are then further stabilized by surface interaction
(Fig. 7b).13a These field-induced smectic phases show only one
very sharp current peak in the half period of a triangular wave
voltage, indicating a ferroelectric switching process. The position
of the peak maximum is at relatively low voltage (2 V mmꢀ1 and
3 V mmꢀ1, respectively, see Fig. 7a and c) but at slightly higher
voltage compared to related silylated bent-core mesogens with
linear alkyl chains (ꢂ1 V mmꢀ1).39a The calculated polarization
values are 570 and 680 nC cmꢀ2 and these values are in the typical
range observed for polar SmCP phases of bent-core mesogens.39a
However, it seems that the field-induced smectic layers still retain
some layer undulation or layer modulation, as indicated by the
distinct textures (Fig. 7b and d) and by the position of the current
peak maxima at increased voltage. For this reason these dark
conglomerate phases, showing ferroelectric switching, are
This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 7773–7783 | 7777

Table 2 Mesophases, phase transitions, transition enthalpies and spontaneous polarization values (Ps) of the silylated compounds Six-1–Six-3a
Compound
(S)-Si3-1a
rac-Si3-2a
rac-Si3i-2b
rac-Si3-2b
rac-Si3-3a
rac-Si3i-3b
rac-Si3-3b
(S)-Si3-3b
R3Si–
n
4
4
9
9
4
9
9
9
R1
T/ꢁC [DH/kJ molꢀ1
]
Ps/nC cmꢀ2 (T/ꢁC)
330 (114)
3.5 (108)
—
Me3Si(OMe2Si)2–
Me3Si(OMe2Si)2–
(Me3SiO)2MeSi–
Me3Si(OMe2Si)2–
Me3Si(OMe2Si)2–
(Me3SiO)2MeSi–
Me3Si(OMe2Si)2–
Me3Si(OMe2Si)2–
(S)-1
rac-2
rac-2
rac-2
rac-3
rac-3
rac-3
(S)-3
Cr1 123 [10.4] Cr2 142 [13.6] Iso
*
Iso 140 [18.7] DEFE* 75 DE(g)
DCFE[*] 119 [16.9] Iso
Iso 116 [17.1] DCFE[*] 85 DC(g)
DC 126 [17.8] Iso
*
[
]
Iso 126 [17.6] DC 70 DC(g)
DC127 [17.8] Iso
—
Iso 125 [17.5] DC 70 DC(g)
DCFE[*] 140 [14.6] Iso
680 (131)
570 (136)
720 (120)
650 (120)
Iso 141 [15.0] DCFE[*] 72 DC(g)
DCFE[*] 150 [18.2] Iso
*
*
*
[
[
[
]
]
]
Iso 146 [17.3] DCFE[*] 70 DC(g)
DCFE[*] 149 [20.7] Iso
Iso 148 [20.5] DCFE[*] 75 DC(g)
DEFE* 152 [19.9] Iso
Iso 149 [19.9] DEFE* 75 DE(g)
*
a
Perkin-Elmer DSC-7; heating (top lines)/cooling (bottom lines) rates were 10 K minꢀ1; enthalpy values are given in square parentheses; abbreviations:
DCFE[*] ¼ dark conglomerate phase showing ferroelectric switching, DEFE* ¼ dark mesophase with uniform chirality sense showing ferroelectric
switching; DC ¼ non-switching dark mesophase without visible chiral domains; DC(g)[*], DE(g)* and DC(g) indicate the non-switching glassy states
of the DCFE[*], DEFE* and DC phases, respectively.
been investigated and compared. Both compounds exhibit
a sharp single polarization current peak per half period of the
applied triangular wave voltage (see Fig. 8 and 9) with high
values of the spontaneous polarization of 720 and 650 nC cmꢀ2
,
respectively. However, the textures observed for the field-induced
states at identical field strength (6 V mmꢀ1) are different for the
racemic compound and the enantiomer.
The birefringence of the texture of the racemic compound rac-
Si3-3b in the field-induced state (see Fig. 8b and d) is lower than
that observed for the enantiomer (S)-Si3-3b (Fig. 9b and d) which
indicates an increased stability of the sponge-like DC[*] structure
of the racemic compound with respect to distortion by external
electric fields. Reducing the temperature increases the contribu-
tion of birefringent areas (see Fig. 8b and d) and as the bire-
fringence increases the polarization peak grows (Fig. 8a and c).
Because the current peak occurs at relatively high voltage
(ca. 4–5 V mmꢀ1, see Fig. 8a and c) it is likely that in the bire-
fringent texture significant layer modulation is retained.
Fig. 4 DSC heating and cooling curve of compound rac-Si3-3b
(10 K minꢀ1).
assigned as DCFE[*] phases instead of SmCPFE[*], which was used
previously for DC[*] phases forming non-modulated field-
induced smectic phases.13a,34a,39 Below about 75 ꢁC, when the
viscosity of the mesophase increases, the switching peak disap-
pears completely, which is in line with the proposed glass tran-
The field-induced texture of the enantiomer (S)-Si3-3b
(Fig. 9b) has a much higher birefringence, indicating that the
sponge-like layer deformation is completely removed in this case.
However, the texture is untypical for a smectic phase, but
appears more similar to the textures of modulated smectic pha-
ses. This means that in the chiral compound the sponge structure
is much more easily removed, but layer distortion is retained,
leading to a modulated smectic phase. Also the position of the
current peak at relatively high voltage (4 V mmꢀ1) is in line with
[
sition at this temperature (DC(g) *] phase).39d
2.4.4 Comparative electro-optical investigation of racemic
compounds and enantiomers. In order to study the effect of
chirality on the electro-optical behaviour, the racemic compound
rac-Si3-3b and the corresponding enantiomer (S)-Si3-3b have
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Fig. 5 Comparison of the textures of DC[*] and DE* phases between crossed polarizers and after decrossing the polarizers. (a–c) The chiral domains of
rac-Si3-3b in the DCFE[*] phase at 130 ꢁC; (a and c) between slightly uncrossed polarizers (ꢃ10ꢁ) (b) between crossed polarizers; (d–f) the homogeneously
chiral DE* phase of the (S)-Si3-1a phase at 137 ꢁC; (d and f) between slightly uncrossed polarizers (ꢃ3ꢁ) and (e) between crossed polarizers; (g and h) the
[
contact region between the DCFE *] phase of the rac-Si3-3b (bottom left) and the DEFE* phase of the (S)-Si3-3b (top right) at 132 ꢁC between uncrossed
polarizers (ꢃ10ꢁ). The contrast in (a–c) and (g and h) was increased to improve visibility of the domains.
the presence of significant layer modulation. Only after applying
a much higher voltage the texture becomes more fan-like
(Fig. 9d). In addition, a polarization current peak close to 0 V
crossing (<1 V mmꢀ1), as typical for non-distorted SmCPF states,
is observed beside the residual peak at significantly higher
voltage (4 V mmꢀ1), assigned to the switching of the remaining
modulated smectic phase (Fig. 9a and c). The coexistence of
modulated and non-modulated (or weakly modulated) field-
induced smectic states might be due to surface interactions,
which change their influence depending on the distance to the
surfaces.
unfavorable defects, arising from helical deformations, to escape
into the defect regions.
As for all other investigated compounds, the polarization peak
disappears below a distinct temperature, in this case below 75 ꢁC,
[
due to the increased viscosity in the DC(g) *] and DE(g)* phases,
respectively.
For the optical investigation of the switching process,
compound rac-Si3-3b was chosen, as for this compound the field-
induced layer structure is sufficiently stable also after removing
the alignment field. For most other compounds layer deforma-
tion arises as soon as 0 V is approached and in these cases the
rotation of extinction crosses in circular domains cannot be
followed. Circular domains of the field-induced smectic phase of
rac-Si3-3b were grown by slow cooling under a DC field (Fig. 10).
The extinction crosses are inclined with polarizer and analyzer
indicating a synclinic organization in the field-induced state
confirming a strongly tilted organization of the molecules in the
DC phases at 0 V as well as in the field-induced states under
applied voltage (see Fig. 10).13,15 A rotation of the extinction
crosses is only observed after application of the opposite voltage,
which confirms a bistable (ferroelectric) switching and indicates
a switching process by rotation on a cone.13
These investigations provide some evidence that the sponge-
like deformation of the layers is more stable in the racemic case.
It is possible that molecular chirality to some degree destabilizes
the 3d layer deformation. However, though, the racemates
appear to be more resistant against field effects it was shown in
the previous section that birefringent defects were observed in the
[
DCFE *] phases of the racemates whereas the DEFE* phases of
the enantiomers are in all cases defect free. This was attributed to
the presence of grain boundaries in the DCFE[*] phases. These
grain boundaries might also have a stabilizing effect on the
sponge-like structure of the racemic compound as they allow
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Fig. 8 (a and c) Switching current response obtained for rac-Si3-3b under
a triangular wave field (120 Vpp, 308 Hz, 2 kU, 6 mm noncoated ITO cells)
and (b and d) the textures observed under these conditions, (a and b) at 141
ꢁC (Ps ¼ 260 nC cmꢀ2) and (c and d) at 120 ꢁC (Ps ¼ 620 nC cmꢀ2).
Fig. 6 XRD patterns of the DCFE[*] phase of compound rac-Si3-2a
obtained on a glass substrate (attempted surface alignment) at T ¼ 110 ꢁC
(a) complete diffraction pattern (bottom is shadowed by the glass
substrate) and (b) q-scan and (c) small angle region showing the
completely isotropic distribution of the small angle scattering.
deformation of the layers is very strong and is retained even
under high applied electric fields. For compound (S)-Si3-1a
a single polarization current peak was detected under a relatively
high triangular-wave voltage and the spontaneous polarization
value (Ps around 330 nC cmꢀ2, see Fig. S5a†) is significantly
smaller than those observed for the compounds with the longer
chiral chains. For compound rac-Si3-2a an even smaller single
polarization current peak appeared at relatively high voltage
(>6 V mmꢀ1) and the spontaneous polarization value is only
3.5 nC cmꢀ2 for this compound (Fig. S5b†). No polarization peak
is observed for compounds rac-Si3-2b and rac-Si3i-2b under
applied electric fields of up to 50 V mmꢀ1, which for this reason
2.4.5 Electro-optical investigation of compounds with short
chains. A different behaviour is observed for all compounds with
relatively short branched chains (compounds (S)-Si3-1a and rac-
Six-2a/b). For these compounds no field-induced formation of
a uniform birefringent texture is observed, the appearance of the
mesophase remains almost dark even under high field strengths
(up to 50 V mmꢀ1), indicating that the layer deformation forces
are especially strong in these cases. Only nonspecific small bire-
fringent spots occur in the dark areas of compound (S)-Si3-1a.
This means that for these compounds the sponge-like
Fig. 7 (a and c) Switching current response obtained under a triangular wave field (6 mm noncoated ITO cells) and (b and d) the corresponding texture
of the mesophase of the racemic compounds; (a and b) rac-Si3i-3b (136 ꢁC, 178 Vpp, 12 Hz, 5 kU); (c and d) rac-Si3-3a (131 ꢁC, 170 Vpp, 60 Hz, 5 kU).
7780 | Soft Matter, 2012, 8, 7773–7783
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Fig. 9 (a and c) Switching current response obtained for (S)-Si3-3b in a 6 mm noncoated ITO cell under a triangular wave field and (b and d) the textures
observed under these conditions, (a and b) at T ¼ 120 ꢁC, 120 Vpp, 180 Hz, 2 kU (the same texture is observed at 141 ꢁC, i.e. under the same conditions as
for the racemate in Fig. 8b) and (c and d) at T ¼ 125 ꢁC, 180 Vpp, 25 Hz, 5 kU.
are assigned as DC phases, for which no polar switching is
observed and no chiral domains can be identified (see Section
2.4.1). The relation between the degrees of field-induced bire-
fringence and measured polarization values confirm that ferro-
electric switching is only observed after removal of the sponge-
like structure of the dark conglomerate phases. It appears that
the relatively short branched group allows an especially strong
3d-layer deformation, giving rise to very stable DE*, DC[*] and
DC phases which can be only partly or not removed under strong
electric fields.
therefore the darkest state is obtained after decrossing the
polarizers by a small angle in a specific direction, defined by the
direction of the rotation of the plane of the polarized light. These
chiral phases with uniform chirality are assigned as DE* (‘‘dark
enantiomer’’) phases, which represent the homochiral analogues
of the dark conglomerate phases (DC[*]).
Under an applied electric field the sponge-like layer deforma-
tion in the smectic phases of the enantiomers and racemates can
in some, but not in all cases, be removed. If layer deformation is
removed, then ferroelectric switching is observed. However, the
voltage required to remove the layer deformation is strongly
dependent on the molecular structure, and this voltage is regar-
ded as a measure of the strength of layer deformation. It depends
mainly on the type of alkyl chain and increases in the order
n-alkyl < 3,7-dimethyloctyl < 2-methylbutyl < 2-ethylhexyl, i.e. it
rises the closer the branching point is located to the aromatic core
unit and the larger the branch is. Hence, layer distortion
increases with the increasing effective average diameter required
by the alkyl chains at the aliphatic–aromatic interfaces, which
means that steric layer frustration plays an important role in the
3. Conclusion
The first examples of chiral polyphilic bent-core molecules with
oligo(dimethylsiloxane) end groups have been synthesized in
racemic and enantiomerically pure forms. These compounds
show dark LC phases with a sponge-like structure similar to the
DC[*] phases of related silylated achiral bent-core molecules with
linear alkyl chains. In the enantiomers, due to the uniform
chirality, these mesophases have uniform optical activity and
Fig. 10 Optical photomicrographs of circular domains of the DCFE[*] phase of compound rac-Si3-3b at T ¼ 120 ꢁC and under a DC voltage of (a) +20 V,
(b) 0 V and (c) ꢀ20 V, respectively; dark areas represent residues of the DC phase.
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formation and stabilization of DC[*], DE* and DC phases. The
temperature dependence of the strength of layer distortion is also
in line with this assumption, as chain mobility, and hence effec-
tive chain diameter increases with rising temperature, requiring
a higher voltage to remove layer deformation at higher temper-
atures. It seems that there is also an effect of chirality on the
stability of the sponge structure in the DC[*] and DE* phases.
For the enantiomers layer deformation appears to be easier to
remove by applying an electric field. It is thought that the grain
boundaries, separating the chiral domains in the DC[*] phases,
and which are absent in the DE* phases, stabilize the DC[*]
structure. The degree of layer deformation (undulation/modu-
lation) remaining in the field-induced smectic phases is influenced
by the molecular structure, temperature and electric field
strength.
20 H.-S. Kitzerow and C. Bahr, Chirality in Liquid Crystals, Springer-
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Acknowledgements
H. Ocak is grateful to the Scientific and Technological Research
Council of Turkey (TUBITAK) for a research fellowship at
Martin Luther University, Halle, Germany. B. Bilgin-Eran is
grateful to the Alexander von Humboldt Foundation, for
financial support toward liquid crystal research.
32 H. Ocak, B. Bilgin-Eran, M. Prehm, S. Schymura, J. P. F. Lagerwall
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