Effects of chain branching and chirality on liquid crystalline phases of bent-core molecules: Blue phases, de Vries transitions and switching of diastereomeric states

Article abstract of DOI:10.1039/c1sm05826f

Bent-core molecules based on a resorcinol bisbenzoate core with a series of distinct substituents in different positions at the central resorcinol core have been synthesized and characterized. The focus is on the effect of branched terminal groups in the racemic and chiral forms on the mesomorphic properties. These were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction, electro-optic and dielectric methods. Only bent-core mesogens derived from 4-cyanoresorcinol exhibit liquid crystalline phases and the mesophases of these compounds are strongly influenced by the branching and enantiomeric composition of the terminal chains. Depending on the structure of the rod-like wings and the enantiomeric composition, cybotactic nematic phases (N_cybC), BPIII-like isotropic mesophases (BPIII _cybC*) and various polar and apolar smectic phases (SmA, SmC, SmC*, SmC_sP_A, SmC_sP_A*) are formed. For one compound, a de Vries type smectic phase is observed and it appears that with decreasing temperature, order develops in two steps. First, at the SmA to SmC transition, the tilt direction becomes long range ordered and in a second step a long range ordering in bend direction takes place. Hence, for the optically active compound a transition from chirality induced polar switching to bend induced (shape induced) antiferroelectricity takes place. In this SmC_sP_A* phase a homogeneous layer chirality is induced under an applied electric field which interacts with the fixed molecular chirality leading to the energetically favoured diastereomeric state and giving rise to a field direction dependent uniform tilt director orientation. Field reversal induces a flipping of the layer chirality, which first leads to the less favorable diastereomeric state, and then this slowly relaxes to the more stable one by a spontaneous reversal of the tilt direction.

Full text of DOI:10.1039/c1sm05826f

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PAPER
Effects of chain branching and chirality on liquid crystalline phases of
bent-core molecules: blue phases, de Vries transitions and switching of
diastereomeric states†
Hale Ocak, Belkız Bilgin-Eran,a Marko Prehm,bc Stefan Schymura,b Jan P. F. Lagerwallbd
a
*
c
*
and Carsten Tschierske
Received 5th May 2011, Accepted 20th June 2011
DOI: 10.1039/c1sm05826f
Bent-core molecules based on a resorcinol bisbenzoate core with a series of distinct substituents in
different positions at the central resorcinol core have been synthesized and characterized. The focus is
on the effect of branched terminal groups in the racemic and chiral forms on the mesomorphic
properties. These were investigated by differential scanning calorimetry, optical polarizing microscopy,
X-ray diffraction, electro-optic and dielectric methods. Only bent-core mesogens derived from 4-
cyanoresorcinol exhibit liquid crystalline phases and the mesophases of these compounds are strongly
influenced by the branching and enantiomeric composition of the terminal chains. Depending on the
structure of the rod-like wings and the enantiomeric composition, cybotactic nematic phases (NcybC),
BPIII-like isotropic mesophases (BPIIIcybC*) and various polar and apolar smectic phases (SmA, SmC,
SmC*, SmCsPA, SmCsPA*) are formed. For one compound, a de Vries type smectic phase is observed
and it appears that with decreasing temperature, order develops in two steps. First, at the SmA to SmC
transition, the tilt direction becomes long range ordered and in a second step a long range ordering in
bend direction takes place. Hence, for the optically active compound a transition from chirality induced
polar switching to bend induced (shape induced) antiferroelectricity takes place. In this SmCsPA* phase
a homogeneous layer chirality is induced under an applied electric field which interacts with the fixed
molecular chirality leading to the energetically favoured diastereomeric state and giving rise to a field
direction dependent uniform tilt director orientation. Field reversal induces a flipping of the layer
chirality, which first leads to the less favorable diastereomeric state, and then this slowly relaxes to the
more stable one by a spontaneous reversal of the tilt direction.
phases, and to spontaneous achiral symmetry breaking on
1. Introduction
a mesoscopic scale in tilted polar smectic phases.3 Chirality in LC
phases of achiral bent-core molecules also arises due to frustration
phenomena as for example reported for the so-called dark
conglomerate phases.4 Moreover, helical conformers of bent-core
mesogens5 seem to give rise to amplification of chirality and this
could be of importance for chirality-related phenomena observed
in some of their smectic and nematic phases.6,7 Recently, the focus
in this field has shifted to nematic phases. Nematic phases of bent-
core compoundsare quite distinctfromthose formed byrod-likeor
disc-like mesogens. This is due to a special cluster structure of these
nematic phases8–10 and the distinct shape of the molecules enabling
restricted rotation and polar packing, which makes them candi-
dates for biaxial11 and ferroelectric nematic phases.12
Liquid crystal research on bent-core compounds has been given
much attention over the past decade due to the discovery of
outstanding properties of the new mesophases which differ signif-
icantly from mesophases of calamitic and disc-like compounds.1,2
The remarkable behaviour of liquid crystalline (LC) materials
possessing a bent molecular shape is the spontaneous formation of
polar order even without molecular chirality. This leads to anti-
ferroelectric and ferroelectric switching smectic and columnar
aDepartment of Chemistry, Yildiz Technical University, Davutpaxsa
Yerlexsim Birimi, TR-34210, Esenler, Istanbul, Turkey
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
dSeoul National University, Graduate School of Convergence Science and
Technology, Suwon-si, Gyeonggi-do, Korea 443-270
Bent-core mesogens usually require at least five aromatic rings in
order to show the specific properties described above.13 There are
several studies on five-ring bent-core mesogens in which the central
phenyl ringwas substituted atdifferent positions bypolar groups.14
Especially substituents in the 4-position of the bent unit play an
important role. It is known that five-ring bent-core mesogens with
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1sm05826f
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4-cyanoresorcinol as the central core and ester connecting groups
between the aromatic rings are able to form polar and apolar
smectic phases aswellas nematic phases depending on the structure
of the rod-like wing groups.8,15 As shown recently, the effect of
a cyano substituent on the mesomorphic behaviour also depends
on the length and type of the terminal groups; for example, alkyl
substituted 4-cyanoresorcinol bisbenzoates show broader nematic
phase ranges than related alkoxy substituted analogues.8
racemic and chiral form on the LC properties of bent-core
compounds derived from 4-cyanoresorcinol. For this purpose,
bent-core mesogens derived from 4-cyanoresorcinol with an (S)-
2-methylbutoxy or (S)-3,7-dimethyloctyloxy group as a terminal
substituent and their racemic mixtures were studied and the
influence of molecular chirality on the mesophases was investi-
gated.30 As major results, it was shown that nematic phases are
replaced by optically isotropic mesophases related to BPIII
phases and a temperature dependent transition between chirality
induced polar response and molecular-shape induced anti-
ferroelectricity was observed in the smectic phases. For one
compound, a de Vries type smectic phase is observed and it
appears that with decreasing temperature, order develops in two
steps. First, the tilt direction becomes long range ordered (SmA(*)
to SmC(*) transition), and in a second step a long range ordering
Chirality is well known to have a major influence on the self-
assembly and properties of liquid crystals,16–18 thus chiral liquid
crystals are widely studied. In particular, liquid crystals (LCs) of
chiral molecules self-assemble into macroscopic chiral (in most
cases helical) superstructures, as found in chiral nematic phases
(N*), blue phases (BPs),19 ferroelectric switching chiral smectic C
phases (SmC*), twist grain boundary phases (TGB) and other
chirality induced 3D phases and isotropic phases.20
(*)
in bend direction takes place, leading to a SmCsPA phase.
As mentioned above, bent-core molecules can show sponta-
neous achiral symmetry breaking which is of general interest for
the development of chirality in self-assembled systems.21 One
open question concerns the interaction of molecular configura-
tional and conformational chirality with chirality provided by
the superstructures.22 Hence, it is of interest to introduce
molecular chirality into bent-core molecules.22–24 This can be
achieved by asymmetric centres in the tails22,24,25 or within the
bent-core structure itself, either as linking group26 or as a cho-
lesteryl core unit in bent dimesogens.23a,27 Chirality was also
induced by mixing achiral bent-core mesogens with chiral rod-
like molecules.28 Bent-core molecules with chiral tails can exhibit
ordinary chiral smectic phases (SmC*) and polar banana phases
such as SmCPA*,9,24–26 but also blue phases (BPs)28 and chiral
nematic (N*) phases have been reported.9,23 Special attention is
presently paid to blue phases, because of their application
potential in new generations of fast switching displays.29
Moreover, a unique switching and reorientation process based
on superstructural diastereomerism is observed in the SmCsPA*
phase of the chiral compound.
2. Results and discussion
2.1 Synthesis
The synthesis of the bent-core compounds derived from 4-cya-
noresorcinol, 3, 4 and 5 is shown in Scheme 1 and the different
combinations of core and tails produced are summarized in
Table 2. As reported previously,15b,31 2,4-dihydroxybenzonitrile
was prepared from commercially available 2,4-dihydroxy-
benzaldehyde by the formation of the oxime, followed by
dehydration. Other starting materials for the synthesis of the 4-
substituted 4-benzoyloxybenzoic acids rac-1a, (S)-1a, rac-1b and
(S)-1b and the 4-substituted biphenylcarboxylic acids rac-2b and
(S)-2b were racemic 2-methylbutyl-1-tosylate, (S)-2-methylbutyl-
1-tosylate, racemic 3,7-dimethyloctyl-1-bromide and (S)-3,7-
dimethyloctyl-1-bromide. The tosylates were prepared from
the corresponding alcohols.32 Racemic and (S)-3,7-dimethyloc-
tyl-1-bromide were prepared from racemic and (S)-3,7-dimethyl-
Herein, the synthesis and characterization of new resorcinol
based bent-core molecules with polar substituents in different
positions at the central core and having branched terminal
groups are reported (see Fig. 1). Many of the compounds show
only crystalline phases, but the 4-cyanoresorcinol core turned out
to be especially successful, as LC phases are obtained even for
strongly branched molecules. Therefore, the focus of this
contribution is on the effects of branched terminal groups in the
1-octanol,
respectively,
as
described
previously.33
(S)-3,7-Dimethyl-1-octanol was obtained from (S)-(ꢀ)-b-citro-
nellol by catalytic hydrogenation (H2, Pd/C in MeOH).
Fig. 1 Structures of the bent-core compounds under investigation.
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Scheme 1 Structures of the bent-core compounds 3–5 and their synthesis. Reagents: (i) (1) NH2OH$HCl, (2) Ac2O, (3) KOH; (ii) DCC, DMAP,
CH2Cl2, r.t.
Specific optical rotations of the alcohols were measured using
a Perkin-Elmer polarimeter in order to confirm enantiomeric
purity or the absence of optical activity in the case of racemic
chiral alcohols (see the ESI†). As the stereogenic centres of the
chiral alcohols were not touched throughout the synthesis it can
be assumed that the final products have approximately equal
optical purity as the starting materials.
2-nitroresorcinol and 3,5-dihydroxybenzonitrile, respectively.
All bent-core compounds were characterized by the common
spectroscopic methods (1H-, 13C-NMR and MS). The spectro-
scopic data of the materials are consistent with the proposed
structures (see the ESI†).
2.2. Liquid crystalline properties—general trends
4-(4-Alkyloxybenzoyloxy)benzoic acids were synthesized
according to the procedures described in the literature.34,35 In
a first step ethyl 4-hydroxybenzoate was alkylated with the
appropriate alkyl bromides or tosylates, followed by hydrolysis of
the ester group. Esterification of the obtained 4-alkyloxybenzoic
acids with 4-hydroxybenzaldehyde, followed by oxidation using
sodium chlorite as oxidizing agent leads to the benzoyloxybenzoic
acids rac-1a,36 (S)-1a,37 (S)-1b38 and the new compound rac-1b.
The synthesis of the 40-alkyloxy-4-biphenylcarboxylic acids rac-
2b and (S)-2b was carried out by using known procedures.39 The
experimental details and spectroscopic data (1H-, 13C-NMR and
MS) of the substituted benzoic acids 1 and 2 are given in the ESI†
and their phase transitions are shown in Table 1. For compound
(S)-1b no phase transitions were provided in the previous report.38
For this compound a SmC*–N* dimorphism was recorded in our
investigations and the biphenyl based carboxylic acids rac-2b and
(S)-2b, which represent new compounds, show a SmI(*) phase (see
Fig. S5 and S6†) below a very small nematic or cholesteric phase
region, respectively.
The mesomorphic properties of the obtained resorcinol based
bent-core compounds 3–15 (see Tables 2 and 4) were investi-
gated by polarised light optical microscopy (PM), differential
Table 1 Mesophases and phase transition temperatures of the benzoic
acids 1 and 2a
Compound
X
R
T/ꢁC
rac-1a
(S)-1a
rac-1b
(S)-1b
rac-2b
(S)-2b
COO
COO
COO
COO
—
a
a
b
b
b
Cr 168 N 183 Iso
Cr 168 N* 184 Iso
Cr 99 SmC 169 N 197 Iso
Cr 98 SmC* 167 N* 194 Iso
Cr 163 SmI 223 N 225 Iso
Cr 163 SmI* 224 N* 226 Iso
As shown in Scheme 1, the esterification of the appropriate 4-
(4-alkyloxybenzoyloxy)benzoic acids or 40-alkyloxy-4-biphe-
nylcarboxylic acids with 4-cyanoresorcinol was performed by
means of dicyclohexylcarbodiimide (DCC) in dichloromethane
using 4-dimethylaminopyridine (DMAP) as catalyst.40 The other
bent-core compounds (6–15) derived from other differently
substituted resorcinols (see Fig. 1) were prepared in a similar way
from commercially available resorcinol, 4-chlororesorcinol, 4,6-
dichlororesorcinol, 4-bromoresorcinol, 2-methylresorcinol,
—
a
Abbreviations: Cr ¼ crystalline solid; SmC ¼ tilted smectic phase,
SmC* ¼ tilted chiral smectic phase with helical superstructure, N ¼
nematic phase; N* ¼ chiral nematic phase; SmI ¼ SmI-type hexatic
phase; SmI* ¼ chiral SmI phase; Iso ¼ isotropic liquid phase.
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scanning calorimetry (DSC), some of them by X-ray diffraction,
electro-optic measurement and dielectric studies. The bent-core
compounds 6–15 with two branched terminal chains are non-
mesomorphic if the resorcinol core is non-substituted (X]H) or
the substituent is halogen, NO2 or CH3 (see Table 2). This
indicates a relatively strong mesophase destabilizing effect of
chain branching, as the related non-branched compounds 16–19
are known to form a variety of different mesophases (see Table
3).9a,b,14,15,41 Also CN substitution in 5-position14,42 leads to non-
mesomorphic material (compound rac-15), whereas compounds
3 and 4 possessing cyano substitution at the 4-position of the
central phenyl ring exhibit LC behaviour. Therefore, further
attention was focussed on investigation of the racemic and
chiral forms of the 4-cyanoresorcinol based mesogens 3, 4
(Table 2) and 5 (Table 4). Depending on the nature of branched
terminal groups, bent-core compounds with phenylbenzoate
rod-like wings (3, 4) show nematic and optically isotropic
mesophases whereas bent-core compounds 5 with biphenyl
based wings exhibit various smectic phases.
transition enthalpies are relatively small and have comparable
values for the racemic and enantiomerically pure compound
around 0.42–0.43 kJ molꢀ1 (see Fig. 2d). On cooling the racemic
compound rac-3 from the isotropic liquid Schlieren textures and
marbled textures were observed under the polarizing microscope,
as typical for nematic phases (Fig. 2a).
The homogeneously chiral compound (S)-3 behaves differ-
ently. With decreasing temperature, at 63 ꢁC, a diffuse bluish
colour starts appearing, which becomes brighter as the
temperature is reduced further (Fig. 2b). The optical appearance
of this mesophase is very similar to that of BPIII phases (blue fog
phases) of calamitic compounds that represent a special type of
double twisted nematic phase without long-range periodicity of
the lattice of defects.16,28,43 However, this bluish appearance
seems to occur only in relatively thick cells, whereas thin layers
appear nearly isotropic in most cases. On further cooling,
another monotropic mesophase M with unknown structure
ꢁ
appears at 45 C (Fig. 2c). A comparison of the mesomorphic
properties of the enantiopure compound (S)-3 with the optically
inactive mixture rac-3 shows that the nematic phase of the
racemate is replaced by a kind of BPIII phase in (S)-3. In addi-
tion, the mesophase stability of the LC phase is about 8 K higher
than that of the nematic phase of rac-3. Moreover, an additional
mesophase occurs in the phase sequence of (S)-3 at 45 ꢁC that is
not observed for rac-3. However, as the mesophases of
compounds rac-3 and (S)-3 are only monotropic and rapidly
crystallize, more detailed investigation of these LC phases by
XRD was not possible.
2.3 4-Cyanoresorcinols with 2-methylbutoxy substituted
phenylbenzoate based rod-like wings (compounds 3)
The homogeneously chiral compound (S)-3 with (S)-configura-
tion of the stereogenic centres in both 2-methylbutoxy chains as
well as the racemic mixture of diastereomers assigned as rac-3
shows monotropic LC phases, as indicated by small exotherms in
the DSC cooling curves at 63 and 55 ꢁC, respectively. The
Table 2 Phase transitions of the resorcinol derivatives 3–15a
Compound
X
R
T/ꢁC [DH/kJ molꢀ1
]
rac-3
(S)-3
(S)-6
(S)-7
(S)-8
4-CN
4-CN
H
4-Cl
4-Br
4-CN
4-CN
H
4-Cl
4,6-Cl
4-Br
a
a
a
a
a
b
b
b
b
b
b
b
b
b
Cr 84 [21.6] (NcybC 55 [0.3]) Iso
Cr 87 [25.0] (M 45 [0.5] BPIIIcybC* 63 [0.3]) Iso
Cr 145 [30.6] Iso
Cr 120 [33.7] Iso
Cr 109 [32.5] Iso
Cr 84 [42.2] (NcybC 60 [0.4]) Iso
Cr1 54 [13.4] Cr2 80 [5.8] (M1 42 [2.4] BPIIIcybC* 58 [0.3]) Iso
Cr 100 [38.6] Iso
Cr 70 [29.8] Iso
Cr 106 [42.9] Iso
rac-4
(S)-4
rac-9
rac-10
rac-11
rac-12
rac-13
rac-14
rac-15
Cr 65 [14.6] Iso
Cr 103 [31.0] Iso
Cr 107 [14.3] Iso
Cr 71 [13.9] Iso
2-CH3
2-NO2
5-CN
a
Enthalpy values in italics in brackets, obtained from first DSC heating scans (10 K minꢀ1, peak temperatures); abbreviations: NcybC ¼ nematic phase
composed of cybotactic groups of the SmC type; M, M1 ¼ mesophases with unknown structures (M1 phase of rac-4 can only be detected in the cooling
scans, see Fig. 4d, due to crystallization); BPIIIcybC* ¼ blue phase III-like phase composed of SmC*-type cybotactic clusters; for the other abbreviations,
see Table 1; monotropic transitions are in parentheses.
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Table 3 Phase transition temperatures of representative bent core
mesogens with linear n-octyloxy chains (compounds 16–19), related to
compounds (4, rac-9, rac-10 and rac-12) with branched terminal chains
and the same number of C-atomsa
Compound
X
T/ꢁC
Ref.
16
17
18
19
4-CN
H
4-Cl
4-Br
Cr 99 NcybC 132 Iso
15a
41c
14a
41d
Cr 124.5 (B1 123) Iso
Cr 85 (SmC 75) N 97 Iso
Cr 94 (Col 75 N 90) Iso
a
Abbreviations: B1 and Col represent modulated smectic phases (ribbon
phases), for other abbreviations, see Tables 1 and 2.
2.4 4-Cyanoresorcinols with 3,7-dimethyloctyloxy substituted
phenylbenzoate type rod-like wings (compounds 4)
Fig. 2 Textures (between crossed polarizers) as seen for: (a) the NcybC
phase of rac-3 at T ¼ 53 ꢁC; (b) the amorphous blue phase BPIIIcybC* of
(S)-3 at T ¼ 61 ꢁC and (c) the low temperature mesophase (M) at T ¼ 42
ꢁC; (d) DSC cooling traces (10 K minꢀ1).
Compounds (S)-4 and rac-4 with longer and more branched
terminal chains behave similar to (S)-3 and rac-3. The racemic
compound rac-4 forms a nematic phase and for the chiral
compound (S)-4 a phase very similar to the BPIII-like phase of
(S)-3 is observed (see Fig. 3). It initially appears optically
isotropic but develops a bluish color upon shearing the sample.
In addition, in the case of the chiral compound (S)-4 the tran-
sition from the isotropic liquid to the LC phase is broadened.
Broad peaks were previously observed for other chirality induced
isotropic mesophases, such as BPIII phases and chirality induced
disordered smectic phases.44–46
scattering pattern is characterized by two diffuse scattering
regimes. The diffuse scattering in the wide angle range at d ¼ 0.47
nm is centred on the equator, confirming the liquid crystalline
state and indicating an alignment of the molecules with their
average long axes parallel to the magnetic field direction. The
diffuse scattering in the small angle region forms four maxima at
d ¼ 3.8–3.9 nm between equator and meridian, indicating
a nematic phase composed of SmC type cybotactic clusters and
an average tilt of about 28ꢁ. The correlation lengths in
parallel and longitudinal direction were estimated according to
Though the mesophases of rac-4 and (S)-4 are also mono-
tropic, they can be supercooled more easily, and hence, investi-
gation by XRD was possible in these cases. The 2D X-ray
diffraction pattern of a magnetically aligned sample (B ¼ 1 T) of
rac-4 in the nematic phase range is shown in Fig. 4a and b. The
Table 4 Phase transition temperatures and transition enthalpies of compounds 5a
Comp.
rac-5
T/ꢁC [DH/kJ molꢀ1
]
Heating: Cr 91 [32.8] SmA 99 [3.4] Iso
Cooling: Iso 97 [3.4] SmA 74b [—] SmC 50b [—] SmCsPA 38 [0.2] M2 35 [1.8] M2
Heating: Cr 85 [29.2] SmA* 99 [3.4] Iso
0
(S)-5
Cooling: Iso 98 [3.4] SmA* 73b [—] SmC* 50b [—] SmCsPA* 38 [1.8] M2
a
Values were determined from the first heating and cooling DSC traces (10 K minꢀ1, peak temperatures); for abbreviations, see Table 2; additional
0
abbreviations: SmA/SmA* ¼ de Vries type smectic phase; SmCsPA ¼ synclinic tilted antiferroelectric switching smectic phase; M2 M2
¼
0
mesophases with unknown structures (it appears that on cooling the racemic compound the transition to M2 takes place in two steps via M2 (see
Fig. 6), but on heating only the direct M2-SmCsPA transition is observed); SmCsPA* ¼ antiferroelectric switching synclinic tilted chiral smectic
b
phase. Transitions were observed by polarizing microscopy and electro-optical experiments; detailed XRD data of rac-5 and (S)-5 are shown in
the ESI (Fig. S3 and S4†).
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Fig. 3 Textures as seen in transmission between crossed polarizers for: (a) the NcybC phase of rac-4 at T ¼ 53 ꢁC; (b and c) the BPIIIcybC* phase of (S)-4
at T ¼ 45 ꢁC after shearing; (d) DSC cooling traces (10 K minꢀ1).
same two regimes (d ¼ 0.47 and 3.8 nm, respectively), but in
contrast to the nematic phase of rac-4 no alignment was possible
under the magnetic field (same strength as above) as indicated by
the perfectly circular diffuse rings (Fig. 4c and d). This is in line
with the proposed phase structure where the cybotactic clusters
of the NCybC phase adopt a chirality induced disordered BPIII-
like organization. The average cluster size, estimated as described
above from the FWHM of the diffuse small angle scattering, is
L ¼ 12 nm.49 This is in the same range as for rac-4 and indicates
that the cybotactic clusters have approximately the same size as
in the NcybC phase of the racemic mixture. Due to the cybotactic
cluster structure of the nematic phases of bent-core mesogens the
formation of double twist cylinders seems to be distorted and
a regular 3D lattice, similar to that of the type I and II blue
phases (BPI, BPII), may not be possible.28 Instead, a disordered
BPIII-like defect structure is formed, assigned here as
BPIIIcybC*. The proposed phase structure is the same as reported
previously for blue phases obtained by adding a chiral dopant to
a nematic bent-core liquid crystal.28
In the contact region between the nematic phase of the racemic
compound rac-4 and its enantiomer (S)-4 the induction of
a cholesteric phase is indicated by the typical oily streaks texture
Fig. 4 2D-XRD patterns of magnetically aligned samples (B ¼ 1 T, field
and the appearance of selective reflection (Fig. 5). With
direction is indicated by an arrow) of compound rac-4 (a and b) and (S)-4
increasing concentration of (S)-4 the colour of the reflected light
(c and d); left complete diffraction patterns, right small angle regions at T
¼ 50 ꢁC.
changes from red via yellow, green and blue to violet, indicating
a decrease of the helical pitch with increasing concentration of
(S)-4. The induced cholesteric phase appears optically isotropic
xkt ¼ 2/Dq from the full width at half maximum (FWHM, Dq) of
in regions with highest (S)-4 concentration, as the pitch is in the
range of the wavelength of UV-light. This observation is in line
with the proposed phase structure (see above). There seems to be
a strong helical twisting power of (S)-4 which leads to double
twist for the pure enantiomer. By reducing the enantiomeric
purity the twisting power is reduced and helix formation occurs
in only one direction, leading to a cholesteric phase. As in the
the fitted curves.47 The dimensions of the cybotactic clusters in
longitudinal (L ) and transversal (Lt) direction can be approx-
k
imated to three times the correlation lengths xk and xt respec-
tively.48 The obtained values of L ¼ 14 nm and Lt ¼ 4 nm at
k
T ¼ 50 ꢁC indicate clusters incorporating on average 8 molecules
organized side-by-side, assuming an average lateral molecular
distance of approximately 0.47 nm (¼ maximum of the diffuse
wide angle scattering).
N
cybC phase of the racemic mixture the cholesteric phase should
be composed of NcybC clusters and therefore it is assigned as
NcybC*. The pitch length of the induced cholesteric phase
The XRD pattern of the chiral compound (S)-4 at the same
temperature is characterized by diffuse scattering maxima in the
increases with further decreasing (S)-4 concentration. As is often
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appearance in homeotropic alignment (layers parallel to the
substrate surfaces), as typical for SmA phases. On further cool-
ing, at T ¼ 74 ꢁC, a phase transition takes place at which
a schlieren texture appears in the homeotropically aligned
regions (see Fig. 7a and b), indicating a transition to an optically
birefringent LC phase. However, this phase transition is not
associated with a peak in the DSC traces, indicating a second
order phase transition (Fig. 6).
As the temperature is lowered further, at T ¼ 50 ꢁC additional
textural changes take place (compare textures in Fig. 7b and c),
but again no transition enthalpy could be detected for this
ꢁ
transition (Fig. 6). Upon further cooling, at T ¼ 38 C another
textural transformation is observed, indicated by a slight change
of the birefringent color (Fig. 7d), which is associated with
a small enthalpy change (DH ¼ 0.5 kJ molꢀ1), before crystalli-
zation occurs at T ¼ 35 ꢁC.
Fig. 5 Induced cholesteric phase (NcybC*) as seen between crossed
polarizers in the contact region between the achiral NcybC phase of rac-4
(bottom right) and the BPIIIcybC* phase of (S)-4 (top left) (a) at T ¼
54 ꢁC and (b) at T ¼ 45 ꢁC (same region as indicated in (a) by the dotted
rectangle).
XRD investigations were carried out with surface aligned
samples in the temperature range between 95 and 60 ꢁC; below 60
ꢁC crystallization sets in. In the whole investigated temperature
range the XRD patterns (Fig. 8a–d) are characterized by two
diffuse scattering maxima located on the equator (d ¼ 0.47 nm)
and two sharp layer reflections (together with a weak second order
reflection) on the meridian, a pattern that at first sight might
suggest that the smectic structure is orthogonal at all tempera-
tures. However, another possibility is that the molecules are in fact
tilted already in the SmA phase but in random directions, and the
transition to SmC is characterized by the development of long-
range order of the tilt directions. Thistype of ‘de Vries’ transition50
from SmA to SmC would have no impact on the XRD pattern
(even in the SmC phase we will have domains with different tilt
directions within the beam, hence the experiment still averages
over all tilt directions) and is thus essentially compatible with the
data in Fig. 8. Below we will provide further evidence from the
chiral compound that this phenomenon in fact takes place in rac-
and (S)-5. It is however not the only process taking place at the
phase transition: a slight broadening of the WAXS maxima in the
low-temperature phase suggests that some additional molecule tilt
does occur in the SmC phase.
the case in chiral nematic the pitch increases with decreasing
temperature as seen by the rainbow coloured band being shifted
towards higher (S)-4 concentration (Fig. 5b).
2.5 4-Cyanoresorcinols with biphenyl wings (compounds 5)
Exchange of the phenylbenzoate units of compounds 4 by
biphenyls leads to compounds 5. This structural variation causes
a change of the LC phase type from nematic to smectic and the
clearing as well as the melting point are raised by some 40 K.
Both effects are reasonable considering that the biphenyl moie-
ties in compounds 5 render the core stiffer (albeit somewhat
shorter) than that of compounds 4, in which all phenyl rings are
linked by semiflexible COO groups. The enantiopure compound
(S)-5 as well as the mixture rac-5 form enantiotropic LC phases.
2.5.1 Racemic mixture rac-5—transition from nonpolar SmA/
SmC phases to a polar SmCsPA phase. Compound rac-5 exhibits
three smectic phases and an additional low temperature phase M
with unknown structure (see Table 4). On cooling from the
isotropic liquid, the high temperature smectic phase develops
a focal conic texture in planar alignment or an optically isotropic
On cooling, in the high-temperature SmA phase the d-value of
the layer reflection (see Fig. 8e) at first slightly increases from d ¼
ꢁ
ꢁ
3.82 nm at T ¼ 95 C to d ¼ 3.84 nm at T ¼ 80 C and then
decreases to d ¼ 3.75 nm in the SmC phase at T ¼ 60 ꢁC. The first
increase of the layer thickness could be explained by the
increasing degree of orientational order on cooling from the
isotropic phase, possibly also with a contribution from an
improved packing of the aromatic cores which reduces chain
folding. The decrease below 80 ꢁC reflects the small additional tilt
of the molecules that must also be the origin of the broadening of
the wide-angle maxima. The additional tilt can be calculated
through the layer shrinkage as qadd ¼ arcos (3.75/3.82) z 11
degrees. The optical tilt was estimated, as described below, to be
about 20 degrees, substantially more than the additional mole-
cule tilt detected by X-ray investigations. The transition thus
indeed seems to be in part of ‘de Vries’ type, partly connected to
regular increased molecule tilt.
Optical investigations of planar aligned samples (Fig. 7e–m)
indicate the onset of an optic axis tilt of about 20ꢁ at T ¼ 74 ꢁC. As
shown in Fig. 7e, above 74 ꢁꢁC the extinction is parallel to the
polarizer, whereas below 74 C it is generally impossible to get
Fig. 6 DSC cooling traces (10 K minꢀ1) of compounds (S)-5 and rac-5.
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Fig. 7 Textures as observed on cooling between crossed polarizer and analyzer (vertical and horizontal) for the mesophases of compound rac-5: (a–d)
homeotropic alignment; (e–h) planar alignment, rubbing direction vertical; (i–m) planar alignment, rubbing direction 20ꢁ inclined anticlockwise to the
direction of the polarizer; (a, e and i) SmA phase at 80 ꢁC; (b, f and k) SmC phase at 62 ꢁC; (c, g and l) SmCsPA phase at 45 ꢁC; (d, h and m) M2 phase at
35 ꢁC.
extinction, indicating a twist of the optic axis from substrate to
substrate. In a few regions the twist seems to be negligible,
however, such that extinction is possible here and it is then found
upon rotating the sample by 20ꢁ (Fig. 7f and k). This indicates
a transition from a SmA high temperature phase to a slightly tilted
SmC phase with synclinic tilt. The optical tilt is retained through
both low temperature phases (see Fig. 7g–m).
The non-polar nature of the SmA and SmC phases was verified
also by dielectric spectroscopy, cf. Fig. 10. Below 50 ꢁC the sample
crystallized, hence the SmCsPA phase could not be investigated in
this experiment. No collective polar mode was seen at any
temperature, only a non-collective mode following an Arrhenius
dependence on temperature. This is what one generally expects to
see in a non-polar smectic phase, although the mode is unusually
strong considering that the sample is essentially planar aligned.
The strength of the mode within the SmA phase shows that the
molecules are substantially tilted within the layers even in this
phase, in line with the view of a de Vries type material. In the
temperature range of the SmC phase we can understand the
presence of the mode as a result of the twisted director geometry,
leading to true planar alignment only at the two sample bound-
aries. In the sample bulk the molecules tilt out of the sample plane
due to the twist and thereby they contribute to the mode.
Electro-optic investigations were carried out in 6 mm polyimide
coated-ITO cells. No switching current response could be observed
downtoT¼ 50ꢁC (voltage up to 300 Vpp),indicatingtheabsenceof
polar order in the two high temperature smectic phases (SmA,
SmC). Only upon further cooling, at the transition to the low
temperature phase at T ¼ 50 ꢁC two polarization current peaks per
half cycle of the applied voltage waveform appeared (see Fig. 9a).
This twin peak response clearly evidences antiferroelectric switch-
ing and quantitative analysis reveals a very large polarization value
of 700–800 nC cmꢀ2. Hence, this mesophase is assigned as SmCsPA
(synclinic tilt is confirmed by optical investigation described above,
see Fig. 7g and l). By optical investigation of the switching process
no difference between the texture of the two switched states could
be seen (Fig. 9b–d). This observation, together with the confirmed
synclinic tilt, indicates antiferroelectric switching by a collective
rotation of the molecules around their long axis.
2.5.2 The enantiomer (S)-5—transition from chirality induced
polar switching on a cone to bent-shape induced antiferroelectric
switching around the long axis. The unichiral compound (S)-5
exhibits a series of smectic phases with the same transition
temperatures as observed for the racemic mixture (see Table 4).
Also for (S)-5 the phase transitions at 74 and 50 ꢁC are not
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Fig. 9 (a) Switching current response on applying a triangular wave field
(49 ꢁC, 280 Vpp, 9 Hz, 5 kU, 6 mm polyimide coated ITO cells, planar
alignment) in the SmCsPA phase of compound rac-5 and textures
observed at T ¼ 49.0 ꢁC: (b) +20 V, (c) 0 V, (d) ꢀ20 V, respectively.
homeotropic sample not to reflect light, but not so long that the
homeotropic sample starts displaying birefringence and optical
activity. The helix is obviously distorted under shear forces,
resulting in the shear-induced visible birefringence in the home-
otropic samples. The time required for the shear induced bire-
fringence to disappear increases with decreasing temperature
without a distinct jump at the SmC*–SmCsPA* transition
temperature. This means that also in the SmCsPA* phase the
helical superstructure is retained. Only after transition to the low
temperature phase M2 at T ¼ 38 ꢁC does birefringence occur
spontaneously in homeotropic alignment (Fig. 12c).
Fig. 8 X-Ray diffraction data from a surface aligned sample of
compound rac-5: (a and c) original patterns; (b and d) patterns after
subtraction of the diffraction pattern of the isotropic liquid at T ¼
105 ꢁC; (a and b) SmA phase at T ¼ 90 ꢁC, and (c and d) SmC phase at
T ¼ 70 ꢁC; (e) d ¼ f(T) plot; for c-scan over the small angle and wide
angle scattering at different temperatures, see Fig. S3g†.
The de Vries nature of (S)-5 leaves a very clear fingerprint on
the planar textures, as shown in Fig. 11. First we note that the
color within the SmA* phase changes upon cooling from yellow-
green to orange (Fig. 11b and c), reflecting an increase in bire-
fringence and thus an increased degree of orientational order.
This is what is expected upon approaching a de Vries transition
to a tilted phase and it also fits well with the increasing smectic
layer thickness upon cooling seen in X-ray scattering, as
mentioned above. As the sample is cooled into the SmC* phase
the color again changes, now suddenly, from orange to green
(Fig. 11d). This reflects a decreased effective birefringence which
is to be expected as we in this chiral sample have a helical
modulation of the tilt direction. By applying an electric field we
can unwind the helix and then the color changes drastically to
pink (Fig. 11e and h), reflecting a birefringence that is not just
higher than the one of the helical state but also higher than that
of the SmA* phase prior to the transition (Fig. 11c). Again, this is
exactly what is expected in the low temperature phase following
a de Vries transition. The ease in distorting the helix, the slow
reformation of the helix through relaxation, and the many
disturbances in the striped pattern in Fig. 11d, g and j–l are also
associated with any enthalpy change (Fig. 6). On cooling from
the isotropic liquid, the SmA phase first occurs with a fan texture
in planar alignment (Fig. 11a–c) and homeotropically aligned
regions appearing completely dark, i.e. optically isotropic. In
contrast to the achiral sample, the homeotropic sample remains
dark also below the transition to the SmC phase at T ¼ 74 ꢁC. A
birefringent schlieren texture can however be induced in the
homeotropically aligned regions upon shearing the sample (see
Fig. 12a) and in the planar-aligned sample a stripe pattern can be
detected in some regions (Fig. 11d). The shear-induced bire-
fringence of the homeotropic sample disappears soon after the
shear force is released (Fig. 12b). These observations can be
explained through the formation of a helical superstructure in the
chiral SmC phase (SmC*) with a pitch length somewhat longer
than the wavelength of visible light. It is long enough for
a modulation in planar sample to be visible and for the
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Fig. 10 (a and c) Dielectric absorption spectra obtained on cooling showing the imaginary part of the dielectric permittivity 300 as a function of
temperature and frequency for the racemic and unichiral versions of 5. Color coding: green-Iso; magenta-SmA/A*; yellow-SmC/C*; cyan-SmCsPA/
CsPA*/crystal. (b and d) The temperature dependence of the frequency f of the main absorption peak, as obtained by fitting a Cole–Cole equation to the
absorption data. By plotting ln f vs. 1/T linear curves result, indicating an Arrhenius dependence on temperature with activation energies varying from
phase to phase, as indicated in the figure.
all well known from de Vries materials,51 indicating unusually
weak interlayer correlations in tilt direction. The XRD patterns
of a surface aligned sample were recorded in the temperature
range between 95 ꢁC and 60 ꢁC (onset of crystallization) and the
results are close to identical with those recorded for the racemic
mixture (see Fig. S4†).
a change of the tilt director orientation which is parallel to one of
the polarizers. After field reversal the dark texture is retained
(Fig. 14h) but after a while turns into the initial birefringent
bright texture (Fig. 14a and h–j).
This slow transformation is not observed for the racemic
compound rac-5 and therefore should be chirality related. In
order to explain this unusual observation it must be considered
that during the switching process the antiferroelectric SmCsPA*
ground state is changed to a field induced polar SmCsPF* state.
The field induced SmCsPF* state is unichiral due to layer chirality
(see Fig. 14k) as well as due to the fixed molecular chirality.
Hence, there are two energetically different diastereomeric
structures: (+)-SmCsPF-(S)-5 (blue models) and (ꢀ)-SmCsPF-
(S)-5 (red models). One of them (let us assume (+)-SmCsPF-(S)-
5) is favoured under a positive voltage and therefore a nearly
uniform director orientation (tilt direction) is observed for the
field aligned sample (Fig. 14a). Reversing the field direction
switches the molecules around their long axis. This rotation does
not change the director tilt direction (and hence not the optic
axis) and therefore the new switched state (Fig. 14c) cannot be
distinguished optically from the initial field induced state
(Fig. 14a). However, rotating the molecules around the long axis,
as in this switching process, reverses layer chirality. The resulting
new diastereomeric structure (ꢀ)-SmCsPF-(S)-5 does not corre-
spond to a global energy minimum under the applied electric field
Similar to the racemic mixture rac-5 no polarization current
response could be detected for the enantiomer (S)-5 under an
applied triangular-wave field in the temperature range of the
SmA* phase and in the SmC* phase down to T ¼ 50 ꢁC. Below
this temperature antiferroelectric switching sets in with polari-
zation values in the same range (700–800 nC cmꢀ2) as observed
for the racemic compound (Fig. 13). The antiferroelectric
switching in the SmCsPA* phase again takes place by rotation
around the long axis as was observed for the racemic mixture (see
Fig. 14a–c). The reduced birefringence at 0 V (Fig. 14b) should
result from the antiferroelectric structure and a reduction of the
order parameter after removal of the alignment field, which is
usually observed for polar switching phases of bent-core meso-
gens. However, there is another interesting and completely
unexpected observation. After reversal of the DC field a slow
change of the texture is observed, as shown in Fig. 14c–f; within
about one minute the bright birefringent texture turns dark
(Fig. 14f). Rotation of the sample between crossed polarizers
leads to birefringence and indicates that the extinction is due to
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Fig. 13 (a) Switching current response on applying a triangular wave
field (49 ꢁC, 202 Vpp, 9 Hz, 5 kU, cell thickness: 6 mm) in the SmCsPA*
phase of compound (S)-5.
and it is thus metastable and slowly relaxes (in about one minute,
see Fig. 14c–f) by direct tilt reversal back to the more stable
diastereomer (+)-SmCsPF-(S)-5. Note that this is not regular
director rotation around the SmC tilt cone, as such rotation
would also reverse the direction of the spontaneous polarization.
Through the tilt reversal the direction of the optic axis changes
such that it now coincides with the direction of the polarizer,
resulting in the texture now appearing dark (Fig. 14f). Removing
the field relaxes the synpolar state via rotation around the long
axis in every second layer to the antipolar (+/ꢀ)-SmCsPA-(S)-5
state, which is racemic regarding layer chirality. Because the
director tilt is unaffected the texture remains dark. If we now
apply a positive +100 V field the system readopts a synpolar state
via molecular long axis rotation in the layers that did not switch
upon field removal. The optic axis is not affected and the texture
thus remains dark (Fig. 14f–h), but the layer chirality is again
changed to (ꢀ)-SmCsPF-(S)-5. Being in the metastable diaste-
reomeric state the system again slowly transforms to the original
field induced birefringent (+)-SmCsPF-(S)-5 state by tilt flipping
(Fig. 14a and h–j).
Fig. 11 Textures of a planar-aligned 6 mm sample with (S)-5 upon
cooling through the SmA* phase ((a) and (b): T ¼ 96 ꢁC; (c): T ¼ 78 ꢁC)
to SmC* ((d)–(l): T ¼ 70 ꢁC). The polarizers are crossed horizontal/
vertical except in (j) and (l), where the analyzer has been turned 20ꢁ in
each direction, and the rubbing direction is aligned along the polarizer in
(a), (d), (e), (g), (h) and (j–l), while it is rotated +20ꢁ in (b) and +15ꢁ in (c).
In (e) and (f) a +10 V voltage is applied and the sample is rotated +20ꢁ to
the new extinction orientation in (f). The corresponding images for ꢀ10 V
are shown in (h) and (i) the latter with the sample rotated ꢀ20ꢁ. In (j)–(l)
the defect-rich helical modulation slowly appearing after removing the
electric field is shown, and the presence of a director twist between the
substrates is demonstrated by decrossing the polarizers in (j) and (l),
opposite decrossing directions shifting the color towards green and pink,
respectively.
To the best of our knowledge this is the first example of a field
induced chirality flipping between two diastereomeric configu-
rations resulting from the coupling of molecular chirality and
layer chirality. As the molecular chirality is fixed a relaxation of
the energetically less favorable diastereomer to the more stable
one requires a spontaneous reversal of the tilt direction.
The absence of measurable spontaneous polarization in the
SmC* phase of (S)-5 is truly bewildering. This absence is further
Fig. 12 Polarised light optical photomicrographs of the mesophases of (S)-5 as observed on cooling in homeotropic alignment; (a and b) SmC* phase at
T ¼ 71 ꢁC after shearing and then 2 seconds of relaxation (a), and after 20 seconds of relaxation (b); (c) spontaneously formed schlieren texture of the M2
phase at T ¼ 38 ꢁC.
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Fig. 14 (a–j) Textures as observed for the SmCsPA* phase of compound (S)-5 under an applied DC field at T ¼ 43 ꢁC; (a) and (h–j) +100 V; (c-f) ꢀ100 V
and (b) and (g) after switching off the applied field (6 mm polyimide coated ITO cell, planar alignment); (c) and (h) were taken immediately after field
reversal, whereas (d–f), (i) and (j) were taken with a time difference of 20 s between each photo; (k) shows the origin of chirality in tilted polar smectic
phases due to the configuration provided by the specific combination of tilt direction and polar direction with respect to the layer normal. The models
show a projection of the molecules along the polar axis (C means that the apex is pointing outwards and x means that the apex is pointing backwards),
red color means (ꢀ) layer chirality, blue color (+)-layer chirality. The field induced SmCsPF structures are homogeneously chiral and have uniform
chirality in all layers, whereas in the racemic SmCsPA structures at 0 V the layer chirality changes between each layer. In the phase assignment ‘‘s’’ means
synclinic tilt correlation, ‘‘A’’ indicates an ‘‘antiferroelectric’’ (antipolar) structure with opposite polar direction in adjacent layers, ‘‘F’’ indicates
a ‘‘ferroelectric’’ (synpolar) structure with uniform polar direction in adjacent layers. The orientations of the layers and of the molecules in the layers are
indicated by the models overlaying the textures.
corroborated by the dielectric spectroscopy data (Fig. 10) which
reveal only a non-collective mode, any mode being related to
a spontaneous polarization being too weak to be detected.
Nevertheless, the textural response upon switching (Fig. 11e–f
and h–i) is clearly polar in nature, with the optic axis rotating in
opposite sense for opposite signs of the applied voltage, as
expected for a chiral SmC* phase. Obviously, there is a very
small spontaneous polarization present that couples to the
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applied field, inducing a rotation of the director around the
smectic C tilt cone until the polarization is aligned with the field.
This type of switching of a SmC* phase with very small value of
the spontaneous polarization was previously reported for an
achiral phenylpyrimidine mixture doped with a small quantity of
naproxen as a rather weak chiral dopant.52 In the present case the
behaviour is however much more surprising because this is
a unichiral sample, which at lower temperatures, in the SmCsPA*
phase, demonstrates very large spontaneous polarization values.
One reason could be that the molecular chirality is ‘‘weak’’ as
there are three very similar alkyl groups at the stereogenic centers
and that two CH2 groups decouple the stereogenic centers from
the core structure.53 Though the weak molecular chirality cannot
induce significant polarization in the SmC* phase it can effi-
ciently couple with the layer chirality.
with cubic defect lattice. This indicates the strong helical twisting
power of the bent-core mesogens and the strong distortion of
a cubic lattice, probably due to the cybotactic cluster structure of
the nematic phase which distorts formation of double twist
cylinders.
The related bent-core compounds 5 with a biphenyl core form
a series of smectic phases: SmA(*)–SmC(*)–SmCsPA(*). It appears
that the uniaxial smectic phase (SmA(*)) is of the de Vries type.
This is likely to be the result of the strong segregation of the
rather rigid and extended aromatic cores from the branched and
flexible terminal chains. Hence, layer formation is mainly due to
segregation. In the layers order develops in two separate steps
with decreasing temperature. First, at the SmA to SmC transi-
tion, the tilt direction becomes long range ordered and in
a second step a long range ordering in bend direction takes place.
Hence, this can be regarded as a new type of de Vries material
bridging the classical rod-like de Vries materials and the
randomly tilted phases of bent-core molecules.55
In order to explain the unexpected absence of measurable
spontaneous polarization in the SmC* phase we tentatively
propose that the de Vries nature of compounds 5 is in fact of
a new type, where the transition from disorder to order occurs in
two steps. First, at the SmA*–SmC* transition the tilt directions
become long-range ordered, as in previously studied de Vries
materials. But as the molecule in this question has a bent core, we
also have a freedom regarding the bend direction. We believe that
the long-range ordering in bend direction occurs only at yet
lower temperatures, at the transition to the SmCsPA* phase. In
the SmC* phase the packing density of the aromatic cores is
increased compared to the SmA* phase. A change of the
molecular configuration from more rod-like to more bent-core-
like takes place by reduction of the temperature,15b which is
combined with a further increase of the packing density at
reduced temperature. The combination of these two contribu-
tions leads to the restriction of the molecular rotation around the
long axis and hence to the appearance of order of the bend
direction at the SmC*-to-SmCsPA* transition. Because the
molecular dipole in bent-core molecules such as 5 is essentially
along the bend, a lack of long-range order in bend direction leads
to a loss of any substantial spontaneous polarization in the SmC*
phase, although the symmetry of the phase allows the appearance
of spontaneous polarization. Electrooptic and dielectric
measurements thus reveal no sign of spontaneous polarization
although a tiny residual spontaneous polarization in fact exists
which is sufficient to allow the polar switching revealed in the
texture studies.
For the optically active compound (S)-5 this leads to a unique
transition from chirality induced polar switching to bent-core-type
antiferroelectricity. A related transition between bent-core-like
and rod-like behaviour was observed for a rod–rod-dimesogen,27
between a chiral nematic and a polar columnar or smectic phase.
Herein this transition was for the first time observed within a series
of smectic phases.
Moreover, as the switching in the SmCsPA* phase takes place
by collective rotation around the long axis application of an
electric field leads to the chiral SmCsPF* structure, and hence,
induces layer chirality. The emerging layer chirality couples with
the molecular chirality and gives rise to the more stable diaste-
reomeric configuration. This provides a uniform tilt director
orientation in the whole sample with the tilt director depending
on the direction of the applied field. Field reversal switches the
layer chirality by fast rotation around the long axis. As the
molecular chirality is fixed this process leads to the other, less
stable diastereomeric configuration, which is only metastable and
then slowly relaxes to the more stable one by spontaneous
director reorientation. This unique switching and reorientation
process based on superstructural diastereomerism is observed for
(S)-5 for the first time.
Both observed phase types, polar smectic phases as well as blue
phases are of significant interest for the development of fast
switching display.29 Especially the blue phases are of contem-
porary interest as new LC (with switching times <10ꢀ3 s) for
sequential color and 3D-displays. For these purposes bent-core
based blue phases might become especially useful due to their
high tendency for blue phase formation over broad temperature
ranges, also in mixtures with rod-like molecules,28 combined with
the shape induced polarization which could allow fast switching
under relatively weak electric fields.
3. Summary and conclusions
Bent-core molecules based on resorcinol derivatives with
branched terminal chains in the racemic and enantiomeric forms
and substituents in different positions at the central aromatic
core have been investigated. With exception of cyano substitu-
tion in the 4-position, all other substituents and substitution
patterns give no liquid crystalline properties. This confirms the
exceptional properties of these 4-cyanoresorcinol based bent-
core mesogens.8,54
Acknowledgements
H. Ocak is grateful to The Scientific and Technological Research
Council of Turkey (TUBITAK) and the Deutsche For-
schungsgemeinschaft (Grand TS 39/23-1) for a research fellow-
ship at Martin Luther University, Halle, Germany. B. Bilgin-
Eran is grateful to the Alexander von Humboldt Foundation, for
financial support toward liquid crystal research.
The benzoyloxybenzoates 3 and 4 exhibit isotropic blue phases
composed of tilted smectic clusters (BPIIIcybC*) over wide
temperature ranges. To the best of our knowledge, these are the
first bent-core compounds showing BPIII phases without an
accompanying cholesteric phase (N*) or any other blue phase
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498, 19–28; I. Nishiyama, J. Yamamoto, J. W. Goodby and
H. Yokoyama, Mol. Cryst. Liq. Cryst., 2005, 443, 25–41.
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