Stereochemical Rules Govern the Soft Self-Assembly of Achiral Compounds: Understanding the Heliconical Liquid-Crystalline Phases of Bent-Core Mesogens

Article abstract of DOI:10.1002/chem.201904871

A series of bent-shaped 4-cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short-pitch heliconical lamellar liquid-crystalline phase with incommensurate 3-layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric-(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field-induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non-tilted and ferreoelectric switching lamellar phase, providing an alternative non-chiral way for the transition from anticlinic to synclinic tilt.

Full text of DOI:10.1002/chem.201904871

A Journal of
Accepted Article
Title: Stereochemical rules govern the soft self-assembly of achiral
compounds: Understanding the heliconical liquid crystalline
phases of bent-core mesogens
Authors: Anne Lehmann, Mohamed Alaasar, Marco Poppe, Silvio
Poppe, Marko Prehm, Mamatha Nagaraj, Sithara P.
Sreenilayam, Yuri. P. Panarin, Jagdish K. Vij, and Carsten
Tschierske
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10.1002/chem.201904871
Chemistry - A European Journal
Stereochemical rules govern the soft self-assembly of achiral compounds:
Understanding the heliconical liquid crystalline phases of bent-core mesogens
Anne Lehmanna, Mohamed Alaasara,b, Marco Poppea, Silvio Poppea, Marko Prehma,
Mamatha Nagarajc, Sithara P. Sreenilayamc, Yuri. P. Panarinc, Jagdish K. Vijc, Carsten
Tschierskea
a Department of Chemistry, Martin-Luther University Halle-Wittenberg, Kurt Mothes Str. 2,
06120 Halle (Saale), Germany
b Department of Chemistry, Cairo University, Giza, Egypt
c Department of Electronic and Electrical Engineering, Trinity College, Dublin, The
University of Dublin, Dublin 2, Ireland
Abstract
Spontaneous mirror symmetry breaking in achiral fluids is of actual interest for technological
applications as well as for the understanding of the spontaneous formation of chirality and
complexity in soft matter self-assembly. Here we report a series of achiral bent-shaped 4-
cyanoresorcinol bisterephthalates terminated by alkyl chains having a length n ranging from 2
to 22. Some of these achiral compounds with a specific chain length (n = 12-16)
spontaneously form a short-pitch heliconical lamellar liquid crystalline phase with
incommensurate 3-layer pitch and the helix axis parallel to the layer normal. The formation of
this helical superstructure is observed at the paraelectric-(anti)ferroelectric transition, but only
if it coincides with the transition from random to uniform tilt and with the transition from
anticlinic to synclinic tilt correlation of the molecules in adjacent layers. For compounds with
a bit longer chains (n = 18-22) the heliconical phase is only field-induced, but once formed it
is stable in a distinct temperature range, even after switching off the field. The presence of the
helix changes the phase properties and the switching mechanism in the LC phases from the
naturally preferred rotation around the molecular long axis, which reverses the chirality, to a
precession on a cone, which retains the chirality once established. These observations are
explained by diastereomeric relations between two different and coexisting modes of
superstructural chirality. One is the layer chirality, resulting from the combination of tilt and
polar order, and the other one is provided by the helical twist evolving between the layers. At
lower temperature, as the tilt is reduced, the helical structure is lost and replaced by a non-
tilted, nonchiral and ferreoelectric switching lamellar phase.
1
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1. Introduction
In biological systems chirality of proteins, carbohydrates and DNA has developed into stable
systems from initially achiral sources. 1 Formation of helical superstructures2 - 6 chirality
amplification7,8 and spontaneous mirror symmetry breaking, are nowadays well documented
for solid state and crystalline assembles.2 In contrast, for the fluid state spontaneous mirror
symmetry breaking was assumed to be impossible due to the significant contribution of the
mixing entropy; nevertheless, it was experimentally observed recently. 9 As biological
chirality has presumably developed in the fluid state an understanding of the symmetry
breaking in fluids is of significant importance.
b)
a)
Helical
molecules
Longitudinal
twist
Transversal
twist
e)
c)
d)
f)
g)
Sponges
Helix
axis
(no uniform
helix axis)
Helical
ribbons
Heliconical
smectics
NTB
N*
Figure 1. Chirality and mirror symmetry breaking in LC soft matter formed by permanently
and transiently chiral rod-like or bent molecules; a) longitudinal coupling of the helical twist
leads to: c) heliconical smectic phases (SmC, SmCTB, etc.) or d) the twist-bend nematic
phases (NTB), whereas b) transversal helix coupling leads to e) chiral nematic phases (N*), f)
helical nanofilament phases (HNF) or g) the sponge-like dark conglomerate phases.
Liquid crystals (LCs) represent fluid systems with long range orientational or
positional order, thus representing simple model systems for studying the fundamental
concepts of symmetry breaking at the cross-over from long-range order in crystalline solids to
short-range order in the isotropic liquids.10-12 Moreover, LCs are of significant technological
importance for displays and numerous non-display applications.11 Especially, helical
2
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superstructures formed by LCs arose interest as switchable optical and photonic materials for
tunable lasers, circular polarized emitters and numerous other technological applications.13 It
is known that chirality in LC systems can result from a permanent molecular chirality,
providing a helical twist of molecular conformation with a twist sense determined by the
sense of permanent chirality, and thus leading to helical LC phases with fixed helix sense (Fig.
1). In the nematic phases, involving only orientational order, permanent chirality of the
involved molecules provides a helical twist between the molecular long axes (transversal twist
correlation, Fig. 1b) leading to the chiral nematic phases (N*, see Fig. 1e).14 However, if there
is additional positional order by formation of layers in the smectic phases, then these layers
compete with this mode of twist, which distorts the layers and leads to a series of chirality
frustrated LC phases (Fig. 1f,g).14-16 If the helical twist develops longitudinally, i.e. between
the short molecular axes (Fig. 1a), then undistorted layers can be retained and heliconical
lamellar phases, having the helix parallel to the layer normal, arise (Fig. 1c). These
heliconical smectic phases are known for chiral rod-like molecules for which long pitch
heliconical phases were typically observed in the synclinic SmCs* and the anticlinic SmCa*
phases. However, the formation of short pitch heliconical LC phases (SmC*, SmCFI*)
requires a “strong” permanent chirality and a high enantiomeric purity of the involved
molecules.14,17,18
The more surprising is the recent observation that similar short pitch heliconical
phases can even be observed for achiral molecules.9,12,19,20 The use of achiral compounds
reduces the costs, because it is independent on the chiral pool, and more importantly, both
senses of handedness are easily accessible in a process of mirror symmetry breaking and
chirality amplification.2,3,7,9,12 Moreover, this transient chirality of achiral molecules has the
potential to be switched by external stimuli either on-off21,22 or between the enantiomeric
states. 23 , 24 The mirror symmetry breaking in bent molecular systems is of significant
interest25 - 30 because there are different sources of superstructural chirality.12,39,30 One is
provided by the tilted organization of the molecules with uniform polar direction, providing a
chiral C2v symmetry of the layers (layer chirality, Fig. 2a).26 The combination of tilt direction
and polar direction determines the chirality sense which is inverted either by reversing tilt
direction or polar direction and retained by the simultaneous inversion of both. As shown in
Figure 2b,c, the chiral layers can pack in stacks with uniform or opposite tilt direction (s =
synclinic or a = anticlinic) and polar direction (F = ferroelectric = synpolar or A =
antiferroelectric = antipolar) thus leading to four stereoisomeric superstructures (see Fig. 2b,c).
The SmCsPF and SmCaPA phases are homogeneously chiral because they are formed by layers
3
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with uniform chirality sense (shown in blue or red). The other two, SmCsPA and SmCaPF, are
formed by alternating layers with opposite chirality (blue and red) and therefore are achiral
(racemic).26 Another source of chirality is provided by the significant transient chirality of
energy minimum helical molecular conformations of these bent-core molecules.12,31,32 This
transient molecular helicity provides a twist and the synchronization of the twist sense could
lead to mirror symmetry breaking.12 In the cases of bent-core self assembly, known so far, the
twist occurs transversal between the molecular long axes (Fig. 1b) and the helix axis develops
along the layers, leading to layer distortion. Stiff layers break into helical nano-ribbons
(helical nano-filament phases, HNF, B4 phases, Fig. 1f) 33 -38 and soft layers form more
disordered sponge-like structures, the so-called dark conglomerate phases (DC-phases, Fig.
1g).12,25,30, 39-44
Figure 2. Layer chirality and the structures of the polar SmC phases of bent-core mesogens.
a) The orthogonal combination of tilt and polar order leads to reduced C2v symmetry and
superstructural chirality of the layers (reproduced with permission from ref.25, copyright 2006,
RSC); blue/red color indicates the chirality sense. b, c) shows the four diastereomorphic
structures of tilted polar smectic phases of bent-core mesogens: b) side views, showing
synpolar (“ferroelectric”: PF) and antipolar (PA) order in adjacent layers and c) front views
showing the tilt correlation (Ca and Cs refer to the anticlinic and synclinic tilt, respectively) of
the bent-core molecules in adjacent layers; here the polar direction is indicated by dots
(pointing out from back to front) and crosses (pointing to the back); reproduced with
permission from ref. 51, copyright 2019, RSC.
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Herein we focus on helicity occurring along the layer normal (longitudinal twist
between the short, secondary, molecular axes, Fig. 1a) and its effects on the LC self-assembly
of bent molecules. The investigated compounds 1/n (Scheme 1) are based on the 140° bent 4-
cyanoresorcinol core ( = 140°) with a reduced molecular bent compared to the 120° angle of
ordinary bent-core molecules.45-51 In these molecules the 4-cyanoresorcinol core is connected
with two electron deficit terephthalate wings, 52 - 54 providing a dense packing.45, 55 , 56
A
summary of previous work on selected individual members of the 4-cyanoresorcinol based
LCs and the development of the distinct models of their phase structures is given in
reference57 as well as in Section S4 and Table S2 in the SI. Compounds 1/n with n = 14 and
16 are of special interest because for 1/16 the existence of a heliconical smectic LC phase was
observed by optical investigations19,58-61 and AFM19 and was corroborated for 1/14 by soft
resonant X-ray scattering (RSoXR).20 Herein we report the complete homologous series of
compounds 1/n from n = 2 to 22 to provide a fundamental understanding of spontaneous
mirror symmetry breaking by helix formation and its effect on the phase sequence and the
phase structure in the context of general stereochemical rules and based on diasteromeric
relations between layer chirality26 and helix chirality. It is shown that helix formation takes
place at the paraelectric-(anti)ferroelectric transition if there is uniform layer chirality and a
weak layer coupling at the SmA-SmC transition coinciding with the transition from anticlinic
to synclinic tilt. This work also answers the question, why in some cases the heliconical phase
represents a ground state structure, whereas in others it is only field induced. The relations
with other heliconical structures, as for example found in the twist-bent nematic (NTB, see Fig.
1d),62 - 66 related smectic phases (SmCTB) of bent mesogenic dimers67 and especially the
heliconical smectic phases of permanently chiral rod-like mesogens (SmC*, SmCFI*
phases),17 will be discussed briefly. Interestingly, with lowering temperature the helix and the
chirality are removed57 and it is shown that under these conditions a non-tilted and
ferroelectric switching SmA’PF phase represents an alternative non-chiral way for the
transition from anticlinic to synclinic tilt.
2. Experimental
Synthesis. - The synthesis of the compounds was conducted as shown in Scheme 1 by
acylation of 4-cyanoresorcinol with the properly substituted benzoic acid chlorides in CH2Cl2
in the presence of pyridine.19 The synthetic procedures and analytical data of the new
compounds are collated in the supporting information, together with the structural analysis
5
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data. The synthesis and analytical data of compounds 1/6, 1/12, 1/16 and 1/18 have been
reported in previous work.19,56

Scheme 1. Synthesis of compounds 1/n; Reagents and conditions: i: 1. 4-substituted benzoic
acid, SOCl2, 80 °C, 2 hrs, 2. phenol, DCM, pyridine, 50 °C, 2 hrs; ii: NaClO3, KH2PO4,
resorcinol, t-BuOH, 20 °C, 1 h.
Methods. – The investigation of the synthesized compounds was performed by polarizing
optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD)
and by switching experiments, electro-optical and dielectric investigations as described in the
Methods section in the SI.
3. Results and Discussions
The LC phases, transition temperatures and transition enthalpy values of all compounds, as
recorded on heating, are collated in Table 1, whereas Figure 3 shows graphically the phase
sequences on cooling (see Table S1 for the corresponding numerical values).
3.1. Nematic and uniaxial smectic phases of the short chain compounds 1/2 – 1/6
Only a nematic phase (N) was observed for the shortest compound 1/2. From the XRD
patterns of magnetically aligned samples it can be deduced that this nematic phase is of the
cybotactic type, characterized by a relatively high intensity of the small angle scattering. This
scattering is perpendicular to the maxima of the diffuse wide angle scattering, indicating an on
average non-tilted organization of the molecules in the cybotactic clusters (NCybA, see Fig.
6
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4a,b for the diffraction patterns of 1/4). This type of non-tilted cybotactic nematic phases is
rare for bent-core mesogens68 which usually form the skewed cybotactic nematic phases
(NCybC).55,68 This NCybA phase is observed for 1/4 as a high temperature phase only in a small
temperature range above a uniaxial smectic phase (SmA) and it is removed for 1/6 and all
following homologues. Upon cooling, at 126 °C, the diffuse small angle XRD of 1/4
condenses into a layer reflection (d = 3.4 nm; Lmol = 3.9 nm, d/Lmol = 0.87) with its second
order on the meridian (Fig. 4d,e) and perpendicular to the maximum of the diffuse wide angle
scattering on the equator, thus indicating the transition to an orthogonal smectic phase (SmA).
In optical investigations the typical fan texture with extinction brushes parallel to the
polarizers in planar alignment and the isotropic appearance in homeotropic alignment confirm
the SmA phase.
Table 1. Phase transitions of compounds 1/n depending on chain length, measured upon
heating.[a]
T/ °C [∆H/ kJ·mol-1]
1/n
Cr 117 [32.2] (NcybA 117 [0.3]) Iso
1/2
Cr 153 [53.0] (SmCaPA 90 [1.5] SmA 126 [1.7] NCybA 129 [0.8]) Iso
Cr 128 [39.6] (SmCaPA 113 [1.1] SmAPAR ~118 [-]) SmA 150 [5.3] Iso
Cr 125 [43.9] (SmCaPA 116 [0.6] SmAPAR ~118 [-]) SmA 161 [6.8] Iso
Cr 112 [38.2] SmCaPA 114 [0.5]) SmAPR / SmA 165 [7.8] Iso
1/4
1/656
1/8
1/10
1/1256
1/14
1/1619
1/1819
Cr 114 [42.6] (SmCaPA 101 [-] Sm(CP)hel 112 SmCaPA 113 [0.8]) SmAPR/SmA[b] 167 [8.0] Iso
Cr 113 [48.3] (SmCaPA[c] 83 [-] SmCaPA
99 [-] Sm(CP)hel 112 [1.0]) SmAPR/SmA 165 [8.0] Iso
(hel)
Cr 110 [47.9] (SmA’PF[d] 83 [-] SmCaPA 92 [-] Sm(CP)hel 110 [0.8]) SmCxPR 127 [-] SmAPR/SmA 162 [7.7] Iso
[d]
[
Cr 114 [60.8] (SmA’PF 85 [-] SmCsPA 110 [1.1]) SmCsPR *] 135 [-] SmAPR/SmA 161 [7.4] Iso
After field treatment: [e] Cr 71 SmA’PF 85 Sm(CP)hel 110 SmCsPR *] 135 SmAPR/SmA 160 Iso
[
[d]
Cr 106 [79.5] (SmA’PF 87 [-]) SmCsPA 108 [1.2] SmCsPAR 113 [-] SmCs 130 [-] SmA 156 [6.5] Iso
1/20
1/22
After field treatment: [e] Cr’ 82 SmA’PF 86 SmCsPA 95 Sm(CP)hel 106 SmCsPAR 111 SmCs 129 SmA 155 Iso
Cr 108 [91.7] (SmCsPA 105 [1.3] SmCsPAR 108 [-]) SmCs 130 [-] SmA 153 [5.8] Iso
After field treatment: [e] Cr’ 87 SmCsPA 100 Sm(CP)hel 102 SmCsPAR 106 SmCs 128 SmA 152 Iso
[a] Phase transitions on heating, for data on cooling, see Fig. 3 and Table S1. The references refer to the
first reported synthesis of the compounds, for a complete list of references, see Table S2. All transition
temperatures with enthalpies represent peak maxima in the DSC traces, recorded on heating with a rate
of 10 K min-1; transitions without enthalpies were determined under similar conditions by optical or
electro-optical methods in thin cells. Values in parenthesis refer to monotropic transitions, observed in
second heating scans if crystallization could be suppressed. There could be a slight deviation of the
DSC values given in Tables 1 and S1 from those observed by other investigations, usually performed in
thin films (1-2 K, larger deviations can be found for the crystallization temperatures). Abbreviations: Cr,
Cr’ = crystalline solids (Cr’ indicates an optically uniaxial crystalline phase, appearing with the same
fan-like optical texture as SmA and Sm(CP)hel phases, see Fig. S51); Iso = isotropic liquid, NcybA
=
cybotactic nematic phase; SmA = de Vries type uniaxial lamellar phase; SmAPR = high permittivity
7
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paraelectric range of the SmA phase showing a single broad polarization peak per half period of an
applied triangular E-field; SmAPAR = high permittivity paraelectric SmA range showing two broad
polarization peaks; SmA’PF = non-tilted and ferroelectric switching lamellar phase; SmCaPA, anticlinic
(hel)
tilted and antiferroelectric switching lamellar phase; SmCaPA
= SmCaPA phase with short range
helical structure; SmCsPA, synclinic tilted and antiferroelectric switching lamellar phase; Sm(CP)hel
=
heliconical lamellar phase; SmCs = synclinic tilted paraelectric lamellar phase; SmCsPR *] = high
permittivity paraelectric SmCs phase showing a single broad polarization peak and domains with
opposite optical rotation in homeotropic alignment ([*]); SmCxPR = high permittivity paraelectric SmC
phase showing a single broad polarization peak; depending on the conditions the extinctions in the
planar texture are either parallel or inclined to the direction of the polarizers. [b] There is no transition
enthalpy for the SmA-SmAPR transition as mistakenly stated in ref 56, the corrected DSC is shown in
[
Fig. 6b. This phase was designated as SmCaPF in ref.20, the reasons for using SmCaPA are given in
[c]
Section 3.6. [d] This phase was designated as SmAPA in ref.57, the reasons for using SmA’PF are given in
[e]
Section 3.6. Phase transitions were determined by optical observations (crystallization temperatures
were observed on cooling).
Figure 3. Bar diagram of compounds 1/n showing the development of the LC phases on
cooling and depending on chain length; for 1/18-1/22 the phase structures depend on pre-
a
b
treatment; shows the phase transitions before applying an external electric field and after
application of a few cycles of an AC field (see Table 1 for abbreviations and Table S1 for
numerical values).
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NCybA
b)
c)
a)
B
SmA
f)
e)
d)
Figure 4. 2D-XRD patterns of a magnetically aligned sample and textures of compound 1/4,
a-c) in the nematic phase at T = 133 °C (the double headed arrow indicates the direction of the
magnetic field) and d-f) in the SmA phase at T = 125 °C; a,d) wide angle scans b,e) SAXS
patterns and c,f) textures as observed between crossed polarizers; the arrows in c) indicate the
directions of polarizer and analyzer.
3.2 Transition from randomized to anticlinic tilt in the smectic phases of compounds 1/4-
1/10
On further cooling, at T = 90 °C, an additional phase transition is observed in the smectic
range of compound 1/4, associated with a small peak in the thermograms (H = 1.5 kJ mol-1,
Table 1). Optical investigation of homeotropically aligned samples (layer planes organized
parallel to the substrate surfaces) between crossed polarizers show that at this phase transition
the isotropic appearance of the homeotropic SmA phase is replaced by a gray schlieren
texture as typical for the transition to a biaxial smectic phase (Fig. 5a→d). The developing
periodic stripe pattern (yellow arrows in Fig. 5d) is typical for biaxial SmAb phases as well as
for anticlinic tilted SmCa phases.69 - 71 The presence of 4-brush disclinations (red arrow)
between the stripes is in favour for an anticlinic SmCa structure of this biaxial smectic phase;
in a non-tilted biaxial SmAb phase only two-brush disclinations would be expected.69 In
planar alignment, where the layers are organized perpendicular to the substrate surfaces, the
dark extinctions retain their position parallel to the directions of polarizer and analyzer,
confirming the absence of a uniform (synclinic) tilt in the low temperature smectic phase
(Figs. S1, S2 and S7). Associated with this phase transition a speckled texture develops (Fig.
5b→e) which is unique for the anticlinic SmCa phases (SmCaPA) of the compounds under
investigation. It is assumed to be due to two different modes of planar alignments of the bent
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molecules, either with the bow plane parallel (low n, green) or perpendicular to the substrate
surfaces (high n, yellow), see Fig. 5f, views B and A, respectively.20
SmA
c)
b)
a)
(de Vries-like)
homeotropic
R
planar
SmCaPA
f)
d)
e)
homeotropic
R
A
planar
B
Figure 5. Optical investigation of compound 1/4 a, b) in the SmA phase at T = 118 °C and
d,e) in the SmCaPA phase at T = 80 °C. a, d) show the homeotropic textures between plane
microscopic glass plates and b, e) the planar textures (PI coated ITO cell 6 m, R is the
rubbing direction of the substrates); b) show the smooth texture in the SmA phase and e) the
“speckled” texture in the SmCaPA phase; the inset in e) shows the polar directions in the
distinct domains. c, f) show the models of the molecular organization in the two phases (the
bent molecules are simplified and shown as rods; dots and crosses indicate the polar direction,
corresponding to the bow direction, pointing out of and into the plane of projection,
respectively; molecules in gray are tilted out of the plane and their polar direction is
arbitrary); c, f) show the directions of view in homeotropic (perpendicular to the layer planes)
and planar alignment (parallel to the layers); f) in the speckled texture the view in planar
samples is along B in the green speckles and along direction A in the yellow continuum (for
related textures of compounds 1/6 and 1/8, see Figs S1, S2, S7 and S11).
This transition with small enthalpy is observed for all investigated compounds and it is the
only transition in the whole LC range which is associated with an enthalpy change, and for all
compounds 1/6-1/20 it takes place around 112 ±4°C, as shown in Fig. 6 for the DSC traces of
compounds 1/6, 1/12 and 1/18 as representative examples (see also Tables 1, S1 and Fig. 3).
For compounds 1/4 to 1/14 this transition coincides with the onset of phase biaxiality,
whereas for all following compounds with longer chains phase biaxiality already develops
before this transition. In the powder XRD patterns there is an almost continuous increase of
the layer spacing with decreasing temperature, but neither the expected decrease of d, nor any
step in the d-values or change of the inclination of the d=f(T) curve takes place at this
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transition (Fig. 7a). The absence of a d-value discontinuity at the transition from the SmA
phase to the biaxial phase suggests that tilt is already present in the SmA phase, but it is
randomized as typical for de Vries phases.72,73 The continuous increase of d with lowering
temperature is attributed to a growing packing density of the molecules, leading to a
stretching of the alkyl chains, compensating and exceeding the shrinkage of d due to the tilt.
a)
n = 6
SmCaPA
SmA
SmA
Iso
Iso
Cr
SmA
SmCaPA
Iso
80
100
120
140
160
T/ °C
b)
n = 12
Cr2
SmA
Sm(CP)hel
Iso
Cr1
Iso
SmA
SmA
SmCaPA
M
Sm(CP)hel
Iso
20
40
60
80 100 120 140 160 180
T / °C
c)
n = 18
[ ]
SmCsPR *
SmCsPA
(Sm(CP)hel

SmA
Iso
Iso
SmCs
Cr2
Cr1
SmA
PR *]
[
Iso
SmA
Cr
SmCsPA
[ ]
SmA’PF
SmCsPR *
(Sm(CP) hel
40
60
80
100 120 140 160
T / °C
Figure 6. Representative DSC traces (10 K min-1) of compounds a) 1/6, b) 1/12 and c) 1/18
recorded on first heating (dashed magenta line) following cooling (blue) and on second
heating after cooling to 100 °C (red). The insets show enlargements of the paraelectric-
(anti)ferroelectric transition peak in second heating and on cooling (for other DSCs, see SI).
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a)
1/22
1/20
1/18
1/16
1/14
1/12
1/10
1/8
6.5
6.0
5.5
5.0
4.5
4.0
1/6
60
80
100 120 140 160 180
T/°C
b)
8
7
6
5
4
8
7
L
mol
d
1/18
1/16
1/22 6
1/20
1/14
1/12
5
4
1/10
1/8
1/6
12 16 20 24 28 32 36 40 44
n
Figure 7: a) Dependence of the layer distance d from chain length and temperature; the gray
bar indicates the temperature range of the paraelectric-(anti)ferroelectric transition; b) d-
values measured at 100 °C and molecular length (Lmol, determined with CPK space filling
molecular models in the most stretched conformation with 140° bending angle and all-trans
conformation of the alkyl chains) of compounds 1/n; the values of 1/14 were taken from
ref.20; for individual plots of d-values, see Figs. S3, S8, S12, S16, S27, S33, S44 and S53.
The ratio d/Lmol is in the range between 0.85-0.89 for all compounds 1/4-1/16 (Fig. 7b),
which in principle would allow tilted as well as non-tilted arrangements (see Section S4 in the
SI). XRD of aligned samples also provides no clear indication of tilt (see Figs. S4, S15 and
S28). Only in few cases of compounds 1/12-1/16, electro-optical investigations uncover the
actually tilted organization with an optical tilt (tilt of the aromatic cores) of 15-20°.20,57,59 For
compounds 1/18-1/22 with longer chains the d/Lmol ratio becomes smaller (0.80, Fig. 7b), in
line with an enhanced tendency to form synclinic tilted SmCs phases (Table 1), though the tilt
angle itself appears to be not increased. In the polar smectic phases of compounds 1/4-1/10
with short chains the switching takes place exclusively by rotation around the long axis and in
this case a SmCaPA phase cannot be distinguished from a non-tilted SmAPA phase by optical
and electro-optical investigations. However, the linear increase of d from 1/6 to 1/14 (Fig. 7b)
suggests that the tilt should be almost identical in these smectic phases. Moreover, in the
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speckled texture below the phase transition around 112 ± 4°C there are areas with increased
birefringence (yellow) due to the developing polar order (see below) as well as areas with
slightly reduced birefringence (green) compared to the SmA texture (Fig. 5b→e), the latter
might indicate the emerging tilt and thus supports the SmCaPA structure.
3.3 Development of polar order and the paraelectric-(anti)ferroelectric transition
Figure 8. Polarization current response curves depending on alkyl chain length. a) 1/4, b) 1/6,
c) 1/8 and d) 1/10 as measured in a 6 m PI coated ITO cell at the indicated temperatures in
the SmCaPA range about 10 K below the paraelectric-(anti)ferroelectric transition temperature
under an applied peak-to-peak voltage of 160 Vpp; for data at other temperatures, see Figs. 9,
S5, S9 and S14.
Electro-optial investigations. - For the short chain compound 1/4 there is no response on an
electric field in the SmA phase region, meaning that this SmA phase is considered as apolar.
Also in the biaxial smectic phase occurring below the transition peak at 90 °C no polarization
peaks can be observed under a triangular wave AC field (Fig. 8a), even at an applied peak-to
peak voltage of 200 Vpp. However, the shape of the polarization current curve shows that at
higher voltage an antiferroelectric switching could be expected. For compound 1/6 two widely
separated very broad peaks appear at 118 °C in the SmA range close to the DSC transition
peak which become sharp at the phase transition at T = 113 °C (see Figs. 8b and S5). This
means that in this SmA phase polar cluster (ferroelectric grains) with antipolar correlation
grow under the applied field, leading to a high permittivity paraelectric SmAPAR range74 upon
approaching the uniaxial-biaxial transition (Fig. 9a).48,49,53 Below the phase transition two
sharp polarization peaks indicate antiferroelectric switching which reaches polarization values
of about 500-700 nC cm-2. Considering the anticlinic tilted organization of the molecules this
phase is designated as SmCaPA. The two polarization current peaks come closer by alkyl chain
elongation (Fig. 8c,d), indicating an increasing correlation length of polar order with growing
chain length.
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Figure 9. Distinct modes of development of the polarization current curves: a-c) via SmAPAR
for compound 1/8 and d-i) via SmAPR for compound 1/10 (6 m PI coated ITO cell, 160 Vpp,
10 Hz) measured with decreasing temperature, the encircled peak is due to conductivity; see
Fig. S9 and S14 for additional temperatures.
No field
SmCaPA
DC field
SmCaPF
a)
b)
Field
direction
Sm(CP)hel
SmCsPF
hel
Figure 10. a) Switching in the helix-free SmCaPA phase by rotation around the long axis
reverses the polar direction (dot vs. cross) and chirality in every second layer (indicated by
red/blue color), the tilt direction does not change and therefore the position of the extinctions
in the textures is retained (1/10 at 100 °C); b) In the Sm(CP)hel phase switching takes place
by precession on a cone which reverses the polar direction and the tilt direction
simultaneously, while the sign of layer chirality is retained; this switching is associated with a
rotation of the optical axis (optical switching), often associated with tilt domain formation
(1/14 at 110 °C).
In the SmA phases of compounds 1/10-1/14 only one broad single polarization peak
develops upon approaching the paraelectric-(anti)ferroelectric phase transition on cooling (see
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Fig. 9d). This single peak indicates a Langevin switching of ferroelectric grains with SmCsPF
structure developing a synpolar correlation under an applied E-field (SmAPR range75). As
shown in Figure 9d→e→f for compound 1/10, it splits into two broad peaks just at the
uniaxial-biaxial transition, and on further cooling the peaks become sharper in the
macroscopic polar SmCaPA phase. Hence, with alkyl chain elongation the high permittivity
paraelectric phase changes from SmAPAR to SmAPR (Fig. 3, Table1). For all compounds 1/4-
1/10 the switching in the SmCaPA phase takes place by rotation around the long axis (SmCaPA
↔ SmCaPF) under all conditions, i.e. the switching can only be detected by a change of the
birefringence, whereas the orientation of the dark extinctions, and thus the orientation of the
molecules does not change (Figs. 10a, S13).76
Sm(CP)hel
107
50

SmAPR
fR,2
106
105
104
103
102
101
40
30
20
10
0
SmCaPA
SmA

60
80
100
120
T/ oC
140
160
180
Figure 11. The plot of dielectric relaxation strength, δε2, and relaxation frequency, fR,2, of
compound 1/12, for the mid frequency process, P2, as a function of temperature.
Dielectric investigation. - Dielectric spectroscopy of 1/12, as example, was performed in the
frequency range between 1 Hz and 10 MHz and three relaxation processes, P1, P2 and P3
were observed in the measured frequency range (for details, see Fig. S20 and related
discussions in the SI). The medium frequency relaxation process P2, exists in the measured
frequency range in all LC phases and was assigned to the polar switching mechanism. Figure
11 shows the temperature dependence of dielectric strength and relaxation frequency for P2.
As the sample was cooled down from the isotropic phase, initially, δε2 does not change
significantly whereas fR,2 continuously decreases. When the sample approaches 120 °C, a
massive increase in δε2, changing from ~5 to 45 and a significant decrease in fR,2 (from 3*106
to 2*104 Hz) were observed. The increase of confirms ferroelectric grains with growing
polarization. Below T = 112 °C, in the polar phase range, δε2 decreases, still remaining high in
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magnitude, with a smooth step like change in both δε2 and fR,2 at the Sm(CP)hel to SmCaPA
phase transition (see next section). Further, δε2 continuously decreases until crystallization at
60 °C. This decrease is due to the significant growth of the ferroelectric grains, fusing to
almost infinite layers in the polar smectic phases. Thus the phase transition around 112-
113 °C can be attributed to the paraelectric-(anti)ferroelectric transition with Curie Weiss type
divergence (see Fig. 11). 77
3.4 Helix formation in the smectic phases of the medium chain length compounds 1/12-
1/16 - the heliconical smectic phase Sm(CP)hel
For compounds 1/12-1/16 additional uniaxial and polar smectic phases were observed below
the paraelectric-(anti)ferroelectric transition. Similar to the shorter homologues, on cooling in
homeotropic samples of 1/12 a birefringent schlieren texture is formed at the onset of the
transition from SmAPR to SmCaPA at 113 °C (similar to Fig. 5d). However, the birefringence
significantly decreases immediately after the phase transition and the homeotropically aligned
phase becomes almost isotropic at the end-set of the phase transition at T = 111 °C (Fig.
12d→e→f). The birefringence increases again on further cooling below ~101 °C (Fig. 12g),
but without any indication of an additional phase transition in the DSC traces (Fig. 6b). For
compounds 1/14 and 1/16 the homeotropic samples become even completely dark at the end-
set of this DSC peak, indicating the transition to a uniaxial LC phase (Figs. S23 and S29). For
these compounds the small birefringent range around the phase transition can only be
observed upon fast cooling (>10 K min-1) whereas upon slower cooling or upon heating no
intermediate birefringent state is observed. On further cooling the birefringence of the
homeotropic sample emerges again at 91 °C and 90 °C, respectively (see Table S1 and Figs. 3,
S23 and S29). This change is also not associated with a DSC peak. In planar samples the
smooth SmA-like fan texture with extinctions parallel to the polarizers is observed at all
temperatures; there is only a faint, almost invisible, stripe formation on the fans at the
paraelectric-(anti)ferroelectric phase transition (Fig. 12a→b) and a clearly visible strip pattern
across the fans develops at the transition to the biaxial phase (Fig. 12b→c), which then
changes to the speckled texture as typical for the SmCaPA phase.
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Figure 12. Textural changes observed at the SmA→Sm(CP)hel →SmCaPA transitions of 1/12
a-c) in a planar cell (6 m PI coated ITO cell) and d-g) in homeotropic alignment (ordinary,
non-treated microscopy glass plates) at the indicated temperatures (for textures of 1/14 and
1/16, see Figs. S23-S25 and S29 and S30).
Uniaxiality of a tilted smectic phase requires that either the tilt is randomized as in the
de Vries SmA phases,72,73,78 or that a short pitch helical superstructure with a helix axis
perpendicular to the layer planes is formed.78,14 A helical structure was first proposed for the
polar and uniaxial smectic phase of compound 1/14.58 It was observed that an in-plane electric
field, applied to a homeotropic aligned sample at first induces a birefringence, but at a certain
voltage a uniaxial state is formed. 58, 79,80 This was interpreted as deformation of a helical
phase, designated as SmAP.58 The uniaxial state was assumed to result from helix
deformation into a four layer structure with 90° twist between the secondary optical axes in

adjacent layers.58 Later, as the tilted organization was recognized, it was renamed to SmCP

hel 19
Sm(CP) and SmCsPF
and is herein designated as Sm(CP)hel (for more details, see
Section S4 in the SI). Helix formation was further confirmed for compound 1/16 by the
observation of the so-called deformed helical ferroelectric (DHF) effect in planar cells,
indicating a helix deformation under an electric field19 similar to that previously known for
helical SmC* and SmC* phases of permanently chiral molecules.81 In addition, the helical
superstructure with a pitch of only 14 nm was visualized by AFM of freeze fractured
samples19 and finally unambiguous proven by soft resonant X-ray scattering at the carbon K-
edge (RSoXS, see Fig 13a), confirming a pitch of 15 nm for compound 1/14.20 The pitch of
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almost three times the layer distance is not exactly commensurate with a three-layer helix (see
Figs. 3,13b,c and Table 1). Only one resonant scattering without umklapp peak confirms the
formation of a SmC*-like heliconical phase with incommensurate 2.8-layer periodicity
(Sm(CP)). Herein we prefer to use Sm(CP)hel as a general phase assignment of heliconical
smectic phases, also including possible commensurate phase types.
Figure. 13. The Sm(CP)hel phase. (a) Development of the XRD data (SAXS and RSoXS) in
the LC phases of 1/14 depending on temperature; (b) shows a computer generated model of
the helical structure with twist of 110° and 140° bent-core angle and (c) shows a simplified
models; the helical pitch P in terms of a number of smectic layers is incommensurate and
smaller than 3 times the thickness of a layer; a, b) were reprinted with permission from refs.
20 and 61, respectively, copyright (2019) by the American Physical Society
(http://dx.doi.org/10.1103/PhysRevLett.122.107801;http://dx.doi.org/10.1103/PhysRevMateri
als.3.045603).
In addition, the RSoXS investigation indicated the persistence of a local helical
structure with a pitch between 13 and 15 nm (between two and three layers) in the
birefringent SmCaPA range of 1/14 below the Sm(CP)hel phase down to 83 °C (Fig. 13a,
encircled diffuse scattering).20 The decreasing intensity of the diffuse scattering is associated
with an increasing intensity and sharpening of the resonant scattering corresponding to the
18
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double layer periodicity (d02). This short range heliconical structure is assumed to contribute
(hel)
to the special properties of this SmCaPA
smectic phase.
phase occurring in the vicinity of the heliconical
3.5 Effects of the heliconical structure on the switching mechanism
Figure 14. Electrooptical investigation of the switching process of compound 1/12 in planar
alignment (6 m PI coated ITO cell) under an applied DC field in the Sm(CP)hel range at T =
105 °C with modles of the distinct field-induced states; see Fig. S19 for the effect of changing
the field direction.
Two distinct textures were induced in the temperature range of the Sm(CP)hel phase by
application of an electric field. For compounds 1/12-1/14 a unique periodic “tiger stripe”
texture develops in a limited temperature range of the Sm(CP)hel phase under an applied
electric field with a certain strength (Fig. 14b).20 Further increasing the applied voltage leads
to the typical tilt domain texture indicating field induced SmCsPF states with opposite tilt
direction (Fig. 14c). The transition from tiger stripes to tilt domains is shifted to higher
temperature from 1/12 to 1/14; for 1/14 tiger stripes can only be found at the transition from
SmAPR to Sm(CP)hel and for 1/16 no tiger stripes and exclusively the tilt domains can be
observed. In the same sequence the threshold fields for tiger stripe formation and tilt domain
formation decreases, meaning that the synclinic tilted states become stabilized with growing
chain length, being in line with the observation of a SmCaPA-SmCsPA transition by further
alkyl chain elongation from 1/16 to 1/18 (see Fig. 3). Thus, the field-induced tilt domain
texture with synclinic tilt correlation becomes dominating with decreasing temperature and
growing alkyl chain length. The dark and bright tiger stripes exchange upon inverting the field
19
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direction (Fig. S19), thus the stripes appear to represent regular arrays of SmCsPF layer stacks
with opposite tilt direction and chirality. The periodicity obviously results from a residual
long pitch heliconical organization in the micrometer range, after field induced deformation
and partial unwinding of the short pitch heliconical Sm(CP)hel structure (Fig. 14e). Complete
unwinding of the helix at higher voltage leads to the tilt domains, indicating a switching to a
field stabilized non-helical SmCsPF state (Fig. 14f).
Rotation around
the long axis
a)
+
E

+
b)
(hel)
SmCaPA
Precession
on a cone
Figure 15. Diastereomeric relations between layer chirality and helix chirality (blue/red) in
the SmCaPA(hel) phase with short-range helix; the blue-blue interactions are arbitrarily assumed
to be low energy whereas blue-red combinations are considered to have higher energy (the
used textures are only for illustration). a) Switching by rotation around the long axis reverses
the layer chirality in half of the layers, which becomes chirality inverted and thus
incompatible with the helical chirality; this leads to a higher energy for this diastereomeric
pair between residual helix and developing layer chirality. b) Switching by precession on a
cone retains the layer chirality and the low energy diastereomeric pair is retained during
switching.
For compound 1/14 the switching into field induced tilt domains is retained in the adjacent
SmCaPA(hel) phase with short range heliconical organization, occurring below the Sm(CP)hel
phase.20 As the naturally preferred switching for the weakly bent 4-cyanoresorcinols 1/n in
the helix-free SmCaPA phases is around the long axis, which does not lead to tilt domains and
retains the orientation of the extinctions parallel to the polarizers. This means that the residual
helix obviously hinders the rotation around the long axis, because this process would invert
the layer chirality of every second layer, thus leading to the racemic SmCaPF structure where
half of the layers assumes the thermodynamically less stable (higher energy) state of the
diastereomeric pair of layer chirality and helix sense (red layer + blue helix, Fig. 15a).
Therefore, the presence of a helix changes the switching mechanism from SmCaPA ↔
SmCaPF by rotation around the long axis (chirality flipping) to SmCaPA ↔ SmCsPF by a
20
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precession on a cone, which retains the layer chirality of all layers, but flips the optical axis
(optical switching, Fig. 15b). Hence, in this class of compounds the switching on a cone is an
indication of a long- or short-range helical organization. Upon further cooling of compound
1/14 the short range helix is completely removed at 83 °C, as indicated by the disappearance
of the diffuse resonant scattering (Fig. 13a).20 The optical axis remains parallel to the layer
normal, together with the d02 resonant scattering indicating an anticlinic tilted smectic phase.
The loss of the helix leads to a change of the mode of switching from precession on a cone to
rotation around the long axis (Fig. S25). For the shorter homologue 1/12 the mode of
switching changes already at the Sm(CP)hel to SmCaPA transition (Fig. 16a-c), meaning that
for this compound no Sm(CP)hel phase is formed and the helix is completely removed already
at the Sm(CP)hel to SmCaPA transition.
Figure 16. Electrooptical investigation of the switching process of compound 1/12 under an
applied DC field in the SmCaPA phase at a-c) at 90 °C and d-f) at 70 °C; the switching is fully
reversible at both temperatures; in d) the speckled texture is composed of coexisting areas of
the SmCaPA phase with the bow planes parallel and perpendicular to the surface, whereas in e)
all bow planes in the SmCaPF state are parallel and in f) perpendicular to the surfaces; for
more details of this switching process, see explanations associated with Fig. S20.
3.6 The non-helical low temperature phases of compounds 1/14-1/20
The helix-free low temperature SmCaPA phase range. - In the SmCaPA phases of 1/8-1/14
the two polarization current peaks become increasingly non-symmetric on cooling (Figs. 9, S9,
S14 and S17) and can even fuse to a non-symmetric broad single peak (Figs. 9g→h→i and
S17). We attribute the broadening to a growing packing density with lowering temperature
(see Fig. 17), which slows down the switching. The unsymmetrical shape could indicate the
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coexistence of two different switching mechanisms. As shown in Figure 16d-f for 1/12, below
a temperature around 80 °C the switching changes. Though the position of the extinctions
does not change, an additional low birefringent state (blue, Fig. 16e) is formed under low
applied field before the transition to the high birefringent SmCaPF state at higher voltage
(orange, Fig. 16f). This low birefringent intermediate state is explained with an orientation of
the bow-planes predominately parallel to the surfaces,82 which realigns into an orientation
perpendicular to the surfaces at further increased voltage (Fig. 16e→f).
SmAPR/SmCPR
SmCaPA or
SmCsPA
4.0
3.5
3.0
2.5
1/12
1/14
1/16
1/18
1/20
SmA’PF
60
80
100
120
T/°C
140
160
SmPR
0.48
0.47
0.46
0.45
SmPA
1/12
1/14
1/16
1/18
1/20
60
80
100
120
T/°C
140
160
Figure 17. a) FWHM and b) position of the scattering maximum of the WAXS of compounds
1/n depending on chain length and temperature, both indicating a growing packing density
with decreasing temperature; the gray line indicates the paraelectric-(anti)ferroelectric
transition range.
Obviously, with growing chain length (1/8→1/16) and decreasing temperature there is a
stabilization of the anchoring of the molecules with the bow-planes parallel to the surface
which might be due to a decreasing tilt and increasing stabilization of the SmCaPF state (see
below) at lower temperature. A surface stabilized (SS) SmCaPF state was observed for the
SmCaPA phase of 1/1657 and ferroelectric switching was also proposed for the helix-free
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SmCaPA phase of 1/14 (designated as SmCaPF in ref.20). However, because the texture returns
back to the speckled ground state texture after removal of the field we still consider the
switching as antiferroelectric, and therefore we retain the phase assignment as SmCaPA. This
is in line with a relatively high threshold voltage observed for this switching.57
Upon further chain elongation, starting with compound 1/18, the synclinic tilt becomes
dominating and the SmCaPA phase is replaced by a synclinic SmCsPA phase. Also for this
compound an asymmetric double peak develops on cooling (Fig. S36), which turns into a
symmetric single polarization current peak at 85 °C on further cooling (see Fig. 21p below).
The ferroelectric non-tilted low temperature SmA’PF phase of compounds 1/16-1/20. –
The development of a single peak switching for compounds 1/16-1/18 is obviously associated
with a transition from the anticlinic tilted SmCaPA phase to a non-tilted smectic phase
(SmA’PF) taking place on cooling.57 This transition is only recognizable by a slight increase
of the birefringence in the ground- and field-induced states, whereas the extinctions retain
their position parallel to the layer normal (Figs. 19k-m and p-r, S32, S39, S40, S42, S46, and
S50) and no peak can be observed in the DSC for this transition (Figs. 6c,S26,S43). This
unusual inverted temperature dependence of the SmC-SmA transition was discovered and
described for 1/16 in more detail in ref.57 and is shown to be associated with a decreasing tilt
angle with lowering temperature. The increased slope of the d = f(T) curves of the long chain
compounds 1/16-1/22 compared to the compounds 1/6-1/12 with shorter chains (see Figure
7a) might be related to this decreasing tilt, adding to the effect of packing density (Fig. 17) to
the development of the layer periodicity d. The decreasing tilt might also be the reason why
the driving force for longitudinal helical twist decreases with lowering temperature. Below a
certain critical tilt angle the secondary director might become too short for the helical
correlation between the layers (see Fig. 18a) and in a first step the helix is lost. Upon further
cooling even the tilt correlation between the layers is lost and the SmCaPA phase transforms
into the SmA’PF phase. The decreasing tilt is associated with a decrease of the full width at
half maximum (FWHM) of the WAXS in the XRD pattern (Fig. 17a), indicating that the
correlation length of in-plane order grows. Below a FWHM of 2.9° the transition to the SmA’
phase takes place (Fig. 17a). That the increase of the packing density can indeed lead to a
transition from tilted to non-tilted lamellar phases is known for SmC*-HexB* transitions.83
As the change of the line shape of the WAXS is continuous and a Lorentzian line-shape is
retained84 we still consider the SmA’PF phase as a smectic phase with enhanced in-plane
order instead of a hexatic phase with long range bond orientational order (HexB).85
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b)
a)
Tilt

T
SmA
T
SmC
(hel)
PF
PA
Tilt / longitudinal
twist correlation
Figure 18. Schematic sketches a) of the effect of tilt on the strength of interlayer tilt- and
twist-correlation and b) the effect of inter-layer fluctuations on the mode of polar order; the
double headed arrows in a) indicate the secondary director and in b) the direction of the
fluctuations.
Associated with the transition to SmA’PF the shape of the polarization current peak
changes from a non-symmetric double peak to a relatively sharp and fully symmetric single
peak, which is especially well developed for 1/16 and 1/18 (Figs. 19o→t, 21 l→p, S31 and
S36). This symmetric single peak, occurring shortly after zero-voltage crossing, is a first
indication of a ferroelectric switching (SmA’PF phase). In line with a transition to
ferroelectric switching the threshold voltage of the switching significantly decreases at the
SmCaPA-SmA’PF transition (Fig. 21).57 The reason for the growing contribution of (surface
stabilized) ferroelectric switching with lowering temperature might be the reduced importance
of interlayer fluctuations, entropically stabilizing the antiferroelectric state at higher
temperature (Fig. 18b).41 The reduced contribution of this entropic effect at lower temperature
allows easier formation of polar (ferroelectric) order, which is stabilized by polar surface
anchoring. This reduced interlayer fluctuation, and hence layer coupling, could also contribute
to the loss of long range tilt and tilt correlation (within and between the layers) at the
SmCaPA-SmA’PF transition. Though there is a clear single peak switching, the birefringence
decreases after switching off the applied field, indicating an orientation of the bow-planes
predominately parallel to the surfaces after release of the applied field (Fig. 19p-r), as
previously observed for the SmAPF phases of silylated bent-core molecules.82 In contrast to
the switching in the SmCaPA phase (Fig. 16d-f) the 0V state has the lowest birefringence (Fig.
19q-r), meaning that the state with the bow planes parallel to the surfaces presumably does
not relax to an antiferroelectric state.
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Figure 19. Electro-optical investigation of the LC phases of compound 1/16, a-e) SmCxPR
range at 115 °C (rand = randomized), f-j) Sm(CP)hel range at 105 °C, k-o) SmCaPA range at
85 °C and p-t) SmA’PF phase at 80 °C; see also Figs. S31 and S32.
For the longer homologue 1/20 there is a direct transition from the synclinic tilted
SmCsPA phase (see below) to SmA’PF. This means that the transition SmC-SmA is
independent on the actual tilt correlation between the layers in the smectic phase as long as it
is weak. On further cooling the SmA’PF phase crystallizes with formation of a crystalline
phase (Cr’). This Cr’ phase cannot be distinguished from the SmA’PF phase (and the high
temperature SmA phase) by optical investigations (see Fig. S51), though the transition
enthalpy values and the hysteresis of the phase transitions (as well as the sharp peaks in the
WAXS patterns) clearly confirm a crystalline phase. For compound 1/22 the SmA’PF phase is
completely replaced by Cr’ (Table 1 and Fig. 3).
Overall it appears that the removal of the tilt and the stabilization of the ferroelectric
state in the SmA’PF phase are associated with the anticlinic-synclinic transition taking place
by chain elongation, especially in the chain length range from n = 14 to 18, i.e. it has
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essentially the same origin as the helix formation occurring at higher temperature. This also
provides a new alternative approach to materials with non-tilted ferroelectric (SmAPF) phases
without requiring chain silylation.82
3.7 Compound 1/16 – emergence of tilt correlation in the paraelectric SmCxPR phase
On cooling compound 1/16, long range tilt develops already in the paraelectric smectic phase
range as indicated by the transition to a weakly birefringent texture in homeotropic alignment
at T = 125 °C (Fig. 19d), i.e. 15 K above the paraelectric-(anti)ferroelectric transition. This
means that starting with 1/16 tilt correlation sets in already before development of long range
polar order. In the planar texture between 125 °C and the paraelectric-(anti)ferroelectric
transition at 108/110 °C (cooling/heating) the orientation of the extinctions is parallel to the
polarizers (Fig. 19b). Application of an electric field to the planar sample leads to a tilt-
domain texture, indicating the switching into a polar state with synclinic tilt (SmCsPF) which
relaxes back to the original smooth fan texture of the polarization randomized phase upon
removing the applied field (Fig. 19a-c). This indicates a switching by precession on a cone
and thus is likely to indicate that a short- or long-range heliconical structure could already
develop in the high permittivity paraelectric range. In the polarization current curves there is
only one broad maximum, indicating a ferroelectric-like paraelectric switching between
125 °C and the transition to the antiferroelectric phase around 110 °C (Fig. 19e),46,47 being
typical for polarization randomized (PR type) smectic phases. This phase, tentative designated
as SmCxPR, requires further clarification by RSoXS investigations. It transforms to the
uniaxial Sm(CP)hel phase at around 110 °C, as indicated by the disappearance of birefringence
in the homeotropic samples (Fig. 19d→i), by the retention of a smooth fan texture with
extinctions parallel to the polarizers (Fig. 19b→g) and by the emergence of two additional
close polarization current peaks under a triangular wave voltage (Fig. 19j). The resulting three
peak switching (two sharp and one broad) is considered as a ferrielectric switching as typical
for heliconical smectic phases with three-layer periodicity.17,18,20 For this compound the long
range helical Sm(CP)hel structure is stable down to 92 °C. At this temperature the transition to
the SmCaPA phase takes place and the two close polarization peaks merge to one, leading to
the unsymmetrical double peak feature of the helix-free low-temperature SmCaPA phase (Fig.
19o→t).
26
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10.1002/chem.201904871
Chemistry - A European Journal
3.8 The long chain compounds 1/18-1/22: Developing synclinic tilt and the field induced
heliconical phase
For the next homologue 1/18 phase biaxiality sets in already at 134 °C, in the paraelectric
range (Fig. 20a→b). In this case the dark extinction brushes incline with the directions of the
polarizers in the planar samples (Figs. 20e→f, 21f). This confirms a uniform (synclinic) tilt
emerging at 134 °C which is on further cooling retained across the paraelectric-
(anti)ferroelectric transition at 107/110 °C (cooling/heating) down to the transition at 85 °C
without formation of a heliconical phase. In the paraelectric SmCsPR phase the inclined
extinction crosses do not rotate by inverting the field direction (Figs. 21e-g and S42),
indicating a switching by rotation around the long axis and confirming the absence of a helix
in the planar samples.86
Homeotropic textures:
d)
b)
c)
a)
SmCsPR
(115 °C)
SmA’PF
(80 °C)
SmCSPA
(100 °C)
SmA
(140 °C)
Planar textures:
g)
h)
f)
e)
Figure 20. Textures of the LC phases of 1/18 at the indicated temperatures in the ground
state: a-d) in homeotropic alignment and e-h) in planar alignment.
However, the paraelectric and biaxial smectic phase between ~110 and 135 °C shows
the formation of a conglomerate of chiral domains in homeotropic alignment (Fig. S34) which,
together with the single broad polarization peak under a triangular wave field (Fig. 21h),
[
confirms a high permittivity paraelectric SmCsPR *] phase. The optical rotation in this
[
SmCsPR *] phase is assumed to be caused by surface assisted alignment of the ferroelectric
SmCsPF grains with a certain size to enable the formation of macroscopic SmCsPF domains,
then assuming a long-pitch heliconical organization between the cell surfaces.46,47,48,49 Thus,
[
the SmCsPR *] phase is considered as a long pitch analogue of the short pitch Sm(CP)hel phase
27
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10.1002/chem.201904871
Chemistry - A European Journal
with some similarity to the partly unwounded helical state, being responsible for the tiger
stripe texture. This is a first indication that heliconical phases can also appear if the phase
structure is SmCsPF. It also indicates that helix formation can be supported or suppressed by
surface anchoring.
100
0.4
0.2
0.0
-0.2
-0.4
d)
SmA
b)
c)
a)
50 145 °C
0
-50
0 V/μm
+32 V/μm
-32 V/μm
-100
-0.02 -0.01 0.00 0.01 0.02
SmCsPF
SmCsPF
SmCsPR
100
h)
f)
g)
e)
0.1
0.0
-0.1
115 °C
50
0
-50
-100
-10 V/μm
0 V/μm
+10 V/μm
-0.02 -0.01 0.00 0.01 0.02
Sm(CP)hel
100
SmCsPF
SmCsPF
l)
j)
k)
i)
0.5
0.0
-0.5
100 °C
50
0
-50
-100
-6 V/μm
+6 V/μm
0 V/μm
-0.02 -0.01 0.00 0.01 0.02
t /s
SmA’PF
SmA’PF
SmA’PF
m)
n)
o)
100
p)
1.0
0.5
0.0
-0.5
-1.0
78 °C
50
0
-50
-100
+2 V/μm
-2 V/μm
0 V/μm
-0.02 -0.01 0.00 0.01 0.02
t /s
Figure 21. Electro-optical investigation of the smectic phases of compound 1/18 at the
indicated temperatures (6 m non-coated ITO cell, 160 Vpp); a-d) SmA phase, e-h)
[
hel
SmCsPR *] phase, i-l) SmCaPF phase and m-p) SmA’PF phase, with corresponding
polarization current curves (right columns), see Figs. S36-S42 for more details.
At the small DSC peak around 110 °C the two broad polarization peaks emerge and
develop into a pair of close peaks while the synclinic tilt is retained (Figs. 20g and 21l),
indicating that the polar phase is no more an anticlinic SmCaPA phase as that of the shorter
homologues, but has changed into a synclinic tilted SmCsPA phase for 1/18 and the following
homologues. 59 In this SmCsPA phase application of an AC-field, followed by switching off
the field leads to a decrease of birefringence (blue to red, see Fig. 21i-k) and the orientation of
the extinction crosses at 0V becomes parallel to the polarizers, indicating a field-induced
transition to a state with the main optical axis parallel to the layer normal (Fig. 21j) as known
for the Sm(CP)hel phases. That a heliconical Sm(CP)hel phase is indeed induced by the electric
field is confirmed by the helix deformation (DHF effect, V-shaped switching) and a helix
unwinding to a field-induced polar SmCsPF state upon application of a DC field to a planar
sample (Fig. 21i-k), being identical to that observed for the Sm(CP)hel phase of 1/16.19,60,61
Field-induced and field modified helical structures are known for LC systems formed by
28
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10.1002/chem.201904871
Chemistry - A European Journal
permanently chiral molecules,13 but to the best of our knowledge there is only one recent
report about field induced helix formation in a (nematic) LC phase of achiral molecules.87 The
SmCsPA phase capable of assuming a field induced helix is found for all compounds 1/18-
1/22 with long chains, though the temperature range of the field-induced helical state
decreases with chain elongation. The heliconical Sm(CP)hel phase is induced in the whole
SmCsPA range of all three compounds, but only for 1/18 it is retained in the whole SmCsPA
range also after removing the field. For 1/20 and 1/22 it is only stable in the upper
temperature range of the SmCsPA phase and at lower temperature it relaxes back to SmCsPA.
The Sm(CP)hel-SmCsPA transition temperature, which is easily determined by the change of
the extinction direction under the polarizing microscope, rises with growing chain length,
meaning that with growing synclinic tilt correlation the field-induced Sm(CP)hel range
tightens (see Fig. 3).
3.9 The long chain compounds 1/18-1/22 - paraelectric SmCsPAR phase and inversion of
birefringence
For compounds 1/20 and 1/22 the paraelectric SmCs phase above the SmCsPA phase does not
show chiral domains under any conditions. However, two widely separated broad current
peaks emerge about 5 K above the transition to the polar smectic phase (Figs. S48 and S55),
indicating a short SmCsPAR range. The antipolar coupling between the ferroelectric grains
leads to a racemic structure and thus might inhibit the development of a surface stabilized
helix in the whole paraelectric SmCs range.
As another typical feature of the synclinic tilted phases of compounds 1/18-1/22 there
is an inversion of birefringence in the vicinity of the paraelectric-(anti)ferroelectric transition
(see Fig. S35 and S47). This is assumed to be due to the changing orientation of the secondary
optical axis, which depends on polarization and tilt which are perpendicular and compete with
each other in the polar SmC phases of bent-core molecules.52 This inversion takes place
around or a few degrees below the paraelectric-(anti)ferroelectric transition, at 110 °C for
1/18, at 98 °C for 1/20 and at 88 °C for 1/22. The shift to lower temperatures becomes larger
with growing alkyl chain length, i.e. with growing synclinic tilt correlation. Thus, the
direction of the main optical axis is likely to be mainly determined by the tilt at higher
temperature and at lower temperature by the direction of polar order, becoming stronger with
growing packing density.
29
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