Controlling the formation of heliconical smectic phases by molecular design of achiral bent-core molecules

Article abstract of DOI:10.1039/c9tc06456g

Fluids with spontaneous helical structures formed by achiral low molecular mass molecules is a newly emerging field with great application potential. Here, we explore the chemical mechanisms of the helix formation by systematically modifying the structure of a bent 4-cyanoresorcinol unit functionalized with two different phenyl benzoate based aromatic rods and terminated with two alkyl chains of variable length. The majority of these achiral compounds self-assemble, forming a short-pitch heliconical liquid crystalline phase in broad temperature ranges. In some cases, it occurs without any competing low-temperature phase. We demonstrate that the mirror symmetry broken mesophase occurs at the paraelectric-(anti)ferroelectric transition if the tilt angle of the molecules in the smectic layers is around 18-20° and if this transition coincides with a change of the tilt correlation between the layers. In the close vicinity of this transition, a field-induced heliconical phase develops as well as a new heliconical phase with polarization-randomized structure. These investigations provide a blueprint for the future design of achiral molecules capable of spontaneous mirror symmetry breaking by the formation of heliconical liquid crystalline phases.

Full text of DOI:10.1039/c9tc06456g

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C. Tschierske, J. Mater. Chem. C, 2020, DOI: 10.1039/C9TC06456G.
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DOI: 10.1039/C9TC06456G
Controlling the formation of heliconical smectic phases by molecular design of achiral
bent-core molecules
Marco Poppe,a Mohamed Alaasar, a,b Anne Lehmann, a Silvio Poppe, a Maria-Gabriela
Tamba, c Marharyta Kurachkina, c Alexey Eremin, c Mamatha Nagaraj, d,e Jagdish K.Vij, d
Xiaoqian Cai, f Feng Liu, f Carsten Tschierske a
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 Nonlinear Phenomena, Institute for Physics Otto von Guericke University
Magdeburg, Magdeburg, Germany.
d Department of Electronic and Electrical Engineering, Trinity College, Dublin, The
University of Dublin, Dublin 2, Ireland
e Present address: School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT,
United Kingdom
f State Key Laboratory for Mechanical Behaviour of Materials, Shaanxi International
Research Center for Soft Matter, Xi’an Jiaotong University, Xi’an 710049, P. R. China
Abstract
Fluids with spontaneous helical structures formed by achiral low molecular mass molecules is
a newly emerging field with great application potential. Here, we explore the chemical
mechanisms of the helix formation by systematically modifying the structure of a bent 4-
cyanoresorcinol unit functionalized with two different phenyl benzoate based aromatic rods
and terminated with two alkyl chains of variable length. The majority of these achiral
compounds self-assemble, forming a short-pitch heliconical liquid crystalline phase in broad
temperature ranges. In some cases, it occurs without any competing low-temperature phase.
We demonstrate that the mirror symmetry broken mesophase occurs at the paraelectric-
(anti)ferroelectric transition if the tilt angle of the molecules in the smectic layers is around
18-20° and if this transition coincides with a change of the tilt correlation between the layers.
In the close vicinity of this transition, a field-induced heliconical phase develops as well as a
new heliconical phase with polarization-randomized structure. These investigations provide a
blueprint for the future design of achiral molecules capable of spontaneous mirror symmetry
breaking by the formation of heliconical liquid crystalline phases.
1

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DOI: 10.1039/C9TC06456G
1. Introduction
Formation of chiral superstructures from achiral molecules by spontaneous mirror symmetry
breaking is a contemporary research field1,2,3 with great impact. It provides technologically
important chiral superstructures and materials for application in chiral photonics, for circular
polarized emission, optoelectronic devices and many other fields,4 thus circumventing the
often expensive chiral-pool approach in materials synthesis. It also contributes to the
understanding of the spontaneous chiral self-assembly, being one of the prerequisites for the
emergence of life.5 Especially the investigation of spontaneous mirror symmetry breaking
phenomena in fluids and soft matter6 and particularly in achiral isotropic liquids7 is a newly
emerging field of scientific interest.8,9 The spontaneous formation of helical superstructures in
bicontinuous cubic phases of polycatenar molecules,8,10,11 the twist-bend nematic phases
(NTB)12-14 of dimesogens and oligomesogens,15-18 as well as the so-called “dark conglomerate”
phases of bent-core mesogens15,19,20 are just a few examples.
An important feature of bent-core liquid crystals with a bending angle of around 120 °
(BCLCs) is the formation of lamellar (smectic) phases with long range polar order due to the
restriction of the molecular rotation around their long axes (Fig. 1a), leading to ferroelectric
and antiferroelectric polar smectic phases.15 Typically, the molecules in the polar smectic
phases of BCLCs are tilted (SmC phases) whereas orthogonal smectic phases (SmA phases)
are rare.15,21,22 Four different sub-structures result in these polar SmC phases, which can be
divided based on the polar direction in adjacent layers into synpolar (“ferroelectric”, PF) and
antipolar (“antiferroelectric”, PA) and based on the tilt correlation into synclinic (Cs) and
anticlinic (Cs, see Fig. 1a).9 The combination of polar direction and tilt direction reduces the
symmetry of the layers to C2v which is chiral (see Scheme S3 for explanation). Accordingly,
the four structures can be divided into racemic, with alternating chirality of the layers, and
chiral with identical layer chirality (Fig. 1b). A special application-relevant feature of these
liquid crystalline (LC) systems is the possibility to switch the polar direction of the layers, the
optical axis of the phase and the superstructural layer chirality by external electric fields. Two
fundamental switching mechanisms can be distinguished, the bistable (ferroelectric) switching
between two synpolar states and the tristable antiferroelectric switching with intermediate
relaxation to an antipolar ground state. These switching processes can take place either by
precession of the molecules on a cone which changes the orientation of the optical axis and
retains the layer chirality, or by rotation around the molecular long axis which retains the
orientation of the optical axis and reverses the layer chirality (Fig. 1e).
2

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Another source of chirality arises from the tendency of the BCLCs to escape from the
DOI: 10.1039/C9TC06456G
developing macroscopic polarization by a splay or twist of the polar directions in the layers. If
the helical twist develops preferentially parallel to the layer planes it leads to a layer
deformation. The examples are helical nano-filament phases (HNF, previously known as B4
phases)23-27 and sponge-like phases (dark conglomerate phases, DC).6,15,16,28-31 However, there
is also the possibility of a helical twist developing parallel to the layer normal, not requiring
steric layer frustration and leading to heliconical LC phases (Fig. 1c,d),32 which are the
subject of this contribution. Heliconical phases were previously known for smectic phases of
permanently chiral molecules,33-36 but here we report on related phases formed by non-chiral
molecules.
Figure 1. Structures, chirality, helix formation and switching in the LC phases of bent-core
molecules. a,b) The four structures of polar and tilted smectic phases. a) Side views, showing
synpolar (“ferroelectric” = PF) and antipolar (PA) configurations in adjacent layers, and b)
front views showing the tilt correlation of the bent-core molecules in adjacent layers (polar
direction is indicated by dots and crosses) and the superstructural chirality of the layers
(blue/red color indicates opposite chirality sense; for an explanation of the origin of layer
chirality, see Scheme S3).9 Ca and Cs refer to the anticlinic and synclinic tilt in adjacent
layers; reproduced from Ref. 83 with permission from The Royal Society of Chemistry; c, d)
The heliconical phase. c) Model (d = layer distance and P = helical pitch) of the
incommensurate Sm(CP)hel phase; d) like combinations of layer chirality and helix sense of
the heliconical structure, tentatively assumed to be energetically favoured; double-headed
arrows are used to indicate that the real existing helically twisted structure with
incommensurate 3-layer pitch is in between the shown SmCsPF and SmCaPA states. e) The
two switching modes, the precession on a cone, flipping the optical axis (rotation of the dark
extinction crosses in planar textures) and retaining the chirality of the layer, and the rotation
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around the molecular long axis, inverting the chirality sense (blue ↔ red) and retaining the
optical axis; shown for the example of the ferroelectric switching between SmCsPDFOsIt:a10t.e10.39/C9TC06456G
The investigated compounds, the 4-cyanoresorcinol bisbenzoates B-E, shown in
Scheme 1, having an angle  of about 140°,37,38,39 are less bent than ordinary BCLCs.
Therefore, the rotation around the molecular long axis is easier and this reduces the coherence
length of polar order, leading to paraelectric and superparaelectric phases with only short to
medium-range polar order in ferroelectric grains instead of polar layers.40,41 These
polarization randomized smectic phases can be divided into tilted (SmCsPR, SmCsPAR)42-44,
and non-tilted (SmAPR,40,41 SmAPAR41,45) subtypes showing either ferroelectric-like (PR) or
antiferroelectric like (PAR) switching. As the packing density increases with lowering
temperature a transition from these paraelectric to the long range polar smectic phases can
occasionally be observed for this kind of molecules.40,46 4-Cyanoresorcinol-based bent-core
molecules,21,37,39,42-50 especially compounds A with two identical phenyl terephthalate wings
(see Scheme 1)51-54 are known to provide a variety of these intermediate phases. Recently, a
new short pitch heliconical LC phase was discovered in the series An.32,55-59 This heliconical
phase with incommensurate three layer pitch in the 15 nm range (Fig. 1c) was identified and
confirmed by atomic force microscopy (AFM)32 and by the observation of field induced helix
deformation (HDF effect) for compounds A16 and A18 (n = 16,18),32,59 and by soft resonant
X-ray scattering (RSoXS) at the carbon K-edge for compound A14 (n = 14).60 It was

designated as SmCP
or Sm(CP)60 due to its similarity with the heliconical SmC*
phases formed by chiral rod-like molecules34 and as SmCsPFhel, because it represents a helical
stacking of polar SmCsPF layers. Herein we use Sm(CP)hel as a general descriptor for this
kind of short pitch heliconical phases, irrespectively of their precise structure (commensurate
or incommensurate; pitch length).62 It was also shown that this heliconical phase and its
properties are affected by diastereomeric relations between layer chirality and helix sense,
thus allowing only the chiral SmCaPA and SmCsPF phases to become heliconical.62 In this
heliconical phase the twist of the projections of the tilt direction on the layer planes is twisted
by around 120° between adjacent layers, meaning that this heliconical phase with almost three
layer pitch represents an intermediate structure between SmCaPA (180° twist) and SmCsPF (0°
twist), as shown in Fig. 1d. The heliconical phases can be formed either spontaneously from
the SmCaPA ground state structure or from a field-induced SmCsPF structure.32,62 To date
heliconical phases were exclusively observed in the series of the symmetrical alkyl substituted
bisterephthalates of 4-cyanoresorcinol A (Scheme 1)32,55,58,60,62 where they occur for a chain
length range of n = 12-18 in small temperature ranges. It is noted that also apolar versions of
4

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heliconical phase (designated as SmCTB) have recently been reported for mesogenic dimers
DOI: 10.1039/C9TC06456G
with odd-numbered spacers.63
N
O
O
O
O
O
O
O
A
O
O
O
H2n+1Cn
CnH2n+1

N
O
O
O
O
O
O
B-D
O
O
H2m+1Cm(O)
(O)CnH2n+1
N
O
O
O
O
O
E
O
H2n+1Cn(O)
(O)CmH2m+1
Scheme 1. Chemical structures of the 4-cyanoresorcinol bisterephthatalates A62 and the
compounds under investigation herein; compounds B and C differ from A in the direction of
one peripheral COO group at the side pointing away from the CN group; the compounds of
series D are related to B and C, but have one or both alkyl chains replaced by alkoxy chains;
in the series E the rod-like wings are exchanged compared to B-D;  is the bending angle, red
indicates the more and blue the less electron deficit benzene ring.
The main goal of this contribution is to generalize this concept of emerging chirality
and to establish general rules for the design of compounds forming broad ranges of
heliconical LC phases. A general understanding of the formation of heliconical phases
depending on molecular structure, tilt-correlation and polar order is established, which
provides a blueprint for future molecular design. In addition to the polar Sm(CP)hel phase,
new polarization randomized versions of the heliconical phases were observed. The polar
heliconical phases allow ultrafast switching in different device configurations, being of
significant interest for new generations of optoelectronic devices.32,55,58,59,64-66 For this reason,
the reported compounds with broad Sm(CP)hel phase ranges at low temperature and without
additional low temperature phases are of significant practical importance.
2. Experimental
The synthesis of all compounds was conducted according to Scheme 2 by stepwise acylation
of 4-cyanoresorcinol with the properly substituted benzoylchlorides. The synthetic procedures
and analytical data are collated in the supporting information together with the description of
the investigation methods (polarizing microscopy, DSC, XRD, electro-optical methods,
dielectric investigations and second harmonic generation (SHG) studies.
5

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HO
O
View Article Online
DOI: 10.1039/C9TC06456G
O
6/(O)n
O
OH
(O)CnH2n+1
H
OH
H2m+1Cm(O)
2/(O)m
i,ii
O
O
i,ii
H
HO
O
HO
O
O
OH
8/(O)n
CN
O
(O)CnH2n+1
H2m+1Cm(O)
O
4/(O)m
i,iii
BnO
HO
OH
B-D
N
i
O
O
10/(O)n
O
O
(O)CnH2n+1
Scheme 2. Synthesis of the investigated compounds B-D; Reagents and conditions: i: 1. 4-
subst. benzoic acid + SOCl2, 80 °C, 2 hrs, 2. 4-subst. phenol, DCM, Py, 50 °C, 2 hrs; ii:
NaClO3, KH2PO4, resorcinol, t-BuOH, 20 °C, 1 h; iii: H2, Pd/C (10%), 1,4-dioxane, 20 °C,
1.5 bar, 24 hrs; the synthesis of compounds E is conducted in an analogous way (see Scheme
S2).
3. Results and Discussion
3.1 Phase sequence of compounds Bm/6 depending on chain length
Compounds B12/6-B22/6. - At first the discussion will be focussed on compounds Bm/6
with one short hexyl chain and a second much longer chain whose transition temperatures and
associated enthalpy values are collated in Table 1 and graphically shown in Fig. 2. The phase
sequence of all compounds Bm/6 with m = 12-22 is very similar and is described in the
following for compound B12/6 as representative example. In the DSC traces of this
compound there is only one phase transition at 82-83 °C with small enthalpy in the whole LC
temperature range between the melting point at 69 °C and the transition to the isotropic liquid
(Iso) at 134 °C. For all compounds B10/6 – B22/6 this reversible transition takes place
between 80 and 86 °C (Fig. 3) with a transition enthalpy decreasing upon chain elongation
from 0.6 kJ mol-1 for B10/6 to only 0.1 kJ mol-1 for B22/6 (Table 1). Upon cooling a planar
aligned samples of B12/6 (layers are perpendicular to the substrate surfaces) between crossed
polarizers from the isotropic liquid the typical fan texture with smooth fans and dark
extinction brushes aligned parallel to the polarizer and analyzer is observed (Fig. 4e), being
typical for non-tilted smectic phases (SmA). This texture is retained across the phase
transition at 82-83 °C down to 54 °C where a stripe pattern develops perpendicular to the fans
(Fig. 4f→g). In a homeotropic aligned cell with the layer organized parallel to the surfaces,
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the appearance of the SmA phase between crossed polarizers is completely daDrkOI:(1F0.i1g03.9/4Ca9T)C. 06456G
This means, that the LC phase is optical uniaxial and uniaxiality is retained on cooling down
to the onset of the phase transition at 82 °C, when a low birefringent schlieren texture appears
(Fig. 4b), indicating the transition to a biaxial phase. The birefringence is retained for only
about 2 K and then fades at the transition to a second optically uniaxial phase (Fig. 4c), which
is discussed below in more detail.
Table 1. Phase transitions of compounds Bm/6 on heating (H) and cooling (C). a
N
O
O
O
O
O
O
O
Bm/6
O
H2m+1Cm
C6H13
Comp.
m
Phase transitions T/°C [ΔH/kJ·mol-1]
m+n
H: Cr 101 [31.3] SmAPR/SmA 123 [3.6] Iso
14
B8/6
8
C: Cr 48 [-18.0] SmCaPA 69 [-2.3] SmAPR/SmA 122 [-3.7] Iso
H: Cr 90 [26.9] SmAPR/SmA 130 [4.8] Iso
10 C: Cr 34 [-6.8] SmCaPA ~60 [-] SmCaPA/Sm(CP)hel ~78 [-] SmCaPA 81 [-0.6] SmAPR/SmA 129 [-
B10/6
16
4.9] Iso
H: Cr 69 [18.1] Sm(CP)α 81 [-] SmCaPA 83 [0.5] SmAPR/SmA 134 [5.3] Iso
B12/6
B14/6
B16/6
B18/6
B20/6
B22/6
12
18
20
22
24
C: Cr < 20 SmCaPA 54 [-] Sm(CP)hel 80 [-] SmCaPA 82 [-0.4] SmAPR/SmA 131 [-5.4] Iso
H: Cr 83 [24.0] SmAPR/SmA 137 [6.4] Iso
C: Cr < 20 SmCaPA 63 [-] Sm(CP)hel 82 [-] SmCaPA 84 [-0.3] SmAPR/SmA 135 [-6.5] Iso
14
H: Cr 85 [17.8] SmAPR/SmA 139 [6.8] Iso
C: Cr < 20 SmCaPA 65 [-] Sm(CP)hel 84 [-] SmCaPA 86 [-0.2] SmAPR/SmA 137 [-6.9] Iso
16
H: Cr 86 [33.7] SmAPR/SmA 140 [7.1] Iso
C: Cr 34 [-7.8] SmCaPA 69 [-] Sm(CP)hel 82 [-] SmCaPA 84 [-0.3] SmAPR/SmA 138 [-7.3] Iso
18
H: Cr 87 [49.9] SmAPR/SmA 140 [7.3] Iso
C: Cr 35 [-14.3] SmCaPA 72 [-] Sm(CP)hel 84 [-] SmCaPA 85 [-0.1] SmAPR/SmA 138 [-7.4] Iso
20
26
28
H: Cr 91 [60.6] SmAPR/SmA 139 [7.5] Iso
C: Cr 44 [-28.4] SmCaPA 76 [-] Sm(CP)hel 84 [-] SmCaPA 85 [-0.1] SmAPR/SmA 138 [-7.6] Iso
22
a
abbreviations: Cr = crystalline solid, Iso = isotropic liquid, SmAPR/SmA = de Vries-like uniaxial lamellar
paraelectric LC phase involving a high permittivity (SmAPR) range at lower temperature, SmCaPA, = anticlinic
tilted and antiferroelectric switching lamellar phase, Sm(CP)hel = heliconical smectic phase; for DSC traces, see
Figs. 3, S2, S4, S12, S14, S17, S21, S23.
In XRD investigations only a layer reflection with its 2nd order (Fig. S9a) is observed in the
whole LC temperature range of compound 12/6. The layer distance is slightly smaller than the
molecular length (Lmol = 5.2 nm67) and increases from d = 4.3 nm at 130 °C to 4.6 nm at 60
°C, as typical for lamellar LC phases with monolayer structure (Fig. 5). The ratio d/Lmol
=
0.83-0.88 is in line with a weakly tilted organization of the molecules. A step in the
development of the d-spacing at 80-82 °C (see Fig, 5) supports the onset of a tilted
organization of the molecules in the biaxial range around this temperature. However, the d-
value does not decrease at this transition and for all following homologues of the series Bm/6
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there is even a continuous increase of d with decreasing temperature (see DFOiI:g10..10S319/5Cb9T)C, 06456G
indicating the presence of a randomized tilt already in the SmAPR/SmA phases and suggesting
a de Vries-like structure of these SmA phases.68 The average birefringence of the planar
sample does not significantly change at the SmA to SmCaPA transition (Fig. 4e→f→g),
though an increase of birefringence is usually observed at the de Vries SmA to SmCs
transition of rod-like molecules.69 In contrast, the de Vries SmA to anticlinic SmCa transition
is associated with a decrease of birefringence.70 In our case this decrease could be
compensated by the increase of birefringence due to the developing polar order.
N
O
O
O
O
O
O
O
O
Bm/6
H2m+1Cm
Cr
C6H13
SmCaPA
SmCaPA
SmCaPA
SmCaPA
SmCaPA
SmCaPA
SmCaPA
SmCaPA
60
SmAPR/SmA
SmAPR/SmA
SmAPR/SmA
8
10
12
14
16
18
20
22
Cr
Sm(CP)hel
Sm(CP)hel
SmAPR/SmA
SmAPR/SmA
SmAPR/SmA
SmAPR/SmA
SmAPR/SmA
m
Sm(CP)hel
Sm(CP)hel
Cr
Cr
Sm(CP)hel
Cr
20
40
80
T (°C)
100
120
140
Figure 2. LC phases and phase transition temperatures of compounds Bm/6 as observed on
cooling; the hashed area of compound B10/6 indicates the coexistence range of SmCaPA and
Sm(CP)hel; for more details, see Figs. S2-S25.
1st heating
1st cooling
Sm(CP)hel
SmAPR/SmA
SmAPR/SmA
Cr
Iso
Sm(CP)hel
SmCaPA
Iso
SmCaPA
20
40
60
80
T (°C)
100
120
140
Figure 3. DSC heating and cooling traces of compound B12/6 recorded at rates of 10 K min-1;
the transition between SmA and SmAPR, both representing paraelectric ranges of the same
SmA phase with different correlation length of the polar clusters, is continuous and therefore
no transition temperature can be given.71
The diffuse wide angle scattering (Figs. S9d,e, S15c,d) confirms the LC state of all
mesophases and the shift of its maximum to smaller values at lower temperature (Fig. 5)
corroborates a growing packing density. Hence, optical investigations of B12/6 combined
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with XRD indicate a uniaxial smectic high temperature phase (SmAPR/SmA) between 132
DOI: 10.1039/C9TC06456G
and 82 °C transforming into a biaxial smectic phase below 82 °C.
Figure 4. a-g) Textures and h-k) polarization current curves of compound B12/6 depending
on temperature; the left column shows the homeotropic textures (the bright spots in a) and c)
are defects with planar alignment) and the middle column the planar fan-texture at the given
temperatures in the indicated LC phases; the directions of polarizer and analyzer are indicated
by arrows in a) and are the same for all textures; the right column shows the polarization
current curves (blue lines) as observed under a triangular wave voltage (black lines); h) at 200
Vpp, i-k) at 150 Vpp in a 6 m PI-coated ITO cell; see also Figs. S8 and S10.
4.50
4.45
4.40
4.35
4.30
4.25
0.465
0.460
0.455
0.450
0.445
SmCaPA
SmCaPA
Sm(CP)hel
Iso
SmAPR/SmA
100 120
40
60
80
140
T/°C
Figure 5. Layer spacing (black) and position of the diffuse wide angle scattering maximum
(blue) of compound B12/6 depending on temperature; see Fig. S15a for related data of 16/6.
The optical biaxiality is observed only over a small temperature range and then the
homeotropic sample becomes uniaxial again at the end-set of the transition peak at 80 °C.
This uniaxial smectic phase in the temperature range between 80 and 54 °C shows the typical
features of the Sm(CP)hel phase, known from the series A.32,60,62 The uniaxiality results from
the short pitch heliconical structure and the fan texture in planar alignment is smooth without
any stripes across the fans (Fig. 4f), because the helical pitch is far below the optical wave
length range. A stripe pattern across the fans, faintly occurring in the biaxial range between 80
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and 82 °C, is missing in the uniaxial range between 80 and 54 °C and clearly dDeOvIe: 1l0o.1p0i3n9/gC9iTnC06456G
the biaxial phase below 54 °C (Fig, 4g) as a typical feature of anticlinic tilted biaxial SmCaPA
phases (see also Fig. S8 and discussion further below).62 This is explained by distinct modes
of anchoring of the molecules, either with the bow planes parallel or perpendicular to the
surfaces, which is suppressed in the presence of a helix.
In order to distinguish non-polar, paraelectric, ferroelectric and antiferroelectric phases
investigations under a triangular wave field were conducted with B12/6 (Fig. 4h-k) and all
other compounds B (Figs. S3, S6, S10, S13a, S16, S19, S22 and S24). In the uniaxial high
temperature phase a weak and relatively broad feature can be observed in each half period of
an applied electric AC field, (Figs. 4h, S10) which grows with decreasing temperature and
increasing applied voltage. This is a first indication of ferroelectric-like Langevin switching as
typical for the high permittivity paraelectric SmAPR phase. Hence, the SmAPR/SmA phases of
the investigated compounds are considered as composed of ferroelectric SmCsPF grains, being
polarization- and tilt- randomized.40,41,72 Under an electric field these grains grow in size and
polarization and then can be switched. On further cooling a second broad polarization peak
develops, which splits into two, while the previously existing single peak fades upon
approaching the short biaxial range at 81-83 °C (Figs. 4h→i and S10). In this short biaxial
smectic phase range two well-developed polarization current peaks can be observed (Fig. 4i)
indicating an antiferroelectric switching. From the step in the layer distance d at this transition
we conclude that it is a tilted phase. However, the extinctions in planar alignment remain
parallel to the polarizers, thus indicating an anticlinic tilted SmCaPA phase (see Fig. 5). On
further cooling, below T = 80 °C one of the peaks of the double peak splits into two close
peaks (Figs. 4j, S10, S13 and S27b). This three peak pattern of the polarization current
response is typical for a ferrielectric switching as proposed previously for the Sm(CP)hel phase
of A14,60 thus confirming the heliconical three-layer structure of this mesophase.73 The
biaxial smectic phase below 54 °C shows two polarization current peaks as typical for
antiferroelectric switching SmCaPA phases (Fig. 4k), being in line with the evolving phase
biaxiality and the development of a stripe pattern across the fans in the planar texture (Figs.
4g and S8e,f).
The development of the polarization current values of compound B12/6 upon cooling
is shown in Fig. 6a (see Figs S3d, S6d and S19f for additional examples). An increase of the
polarization current starts in the SmA range around 100 °C and the slope increases upon
approaching the phase transition at 82 °C; the increase of Ps continues in the Sm(CP)hel and
SmCaPA ranges, reaching about 850 nC cm-2 at 50 °C, which is a typical value for polar
10

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smectic phases of bent-core molecules. Dielectric investigations (Fig. 6b) additionally support
DOI: 10.1039/C9TC06456G
the phase assignments. Two relaxation processes, P1 and P2 were observed in the measured
frequency range (Fig. S11). The low frequency process P1 is attributed to conductivity. The
high frequency relaxation process P2 was observed in all of the LC phases. The dielectric
strength δε and the relaxation frequency fR of P2 are obtained by fitting the relaxation spectra
to the Havriliak-Negami equation. The temperature dependence of these parameters is given
in Fig. 6b. On cooling,  starts growing already at the Iso-SmA transition and rises steeply
upon approaching the paraelectric-(anti)ferroelectric transition at 82 °C. This is in line with a
growth of polar grains. There is a steep decrease of  in the biaxial range between 82 and 80
°C, indicating a further growth of the polar cluster with formation of polar layers. The values
of  are still high in the Sm(CP)hel phase and decrease smoothly on further cooling. The
transition to the antiferroelectric SmCaPA phase at 54 °C is indicated by a stronger decrease of
.
Figure 6. a) Polarization values (Ps) of B12/6 depending on temperature (see also Figs. S3d,
S6d and S19g for additional Ps = f(T) curves) and b) temperature dependence of the ε and
relaxation frequency; c) shows the temperature dependence of the SHG response I2 of
compound B18/6; the measurements were made on cooling in an AC electric field (Epp=25
V/µm) in a 10 µm thick IPS cell with ITO electrodes. d) The field dependence of the SHG
signal in the range of the Sm(CP)hel phase of B18/6 (for more data see Figs. S17-S20).
11

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DOI: 10.1039/C9TC06456G
Second harmonic generation (SHG, see Fig. 6c, d) was investigated for compound
B18/6 which shows essentially the same phase sequence as B12/6. No SHG signal was
observed in the absence of an applied field, indicating the absence of a polar (ferroelectric)
ground state of all LC phases. A weak field-induced SHG signal occurs in the temperature
range of the SmAPR/SmA phase at 115 °C and increases upon approaching the transition to
the first biaxial range. This confirms the polar domain structure of the uniaxial SmAPR/SmA
phase. The field-dependence of the I2(T) is continuously increasing in the whole range of
accessible field strength. This suggest a Langevin-like behaviour of the polarisation in the
random phase. There is a steep increase of the SHG signal around the paraelectric-
(anti)ferroelectric transition around 82-83°C. This suggests increasing polar correlations upon
approaching the antiferroelectric SmCaPA phase. The character of the field dependence
changes to a more pronounced step-like form, which cannot be fit by the Langevin model
(SHG, see Fig. 6, d). There is a switching threshold characteristic for this phase. However, the
material remains easily polarisable and the further polarisation occurs in a continuous fashion
in an in-plane switching (IPS) cell. The continuous growth of the SHG response slows down
in the uniaxial Sm(CP)hel range. This can be interpreted by assuming that the helical structure
suppresses the polarisation alignment in the bulk of the phase. The transition to the biaxial
SmCaPA phase is marked by a slight jump of the I2(T), meaning that the removal of the
helical constraint improves the field-induced alignment of the layer polarisation.
Hence, below 82 °C a long range polar order develops in the layers of the LC phases
of B18/6 and B12/6, and the antipolar correlations between the layers as well as the long
range anticlinic tilt correlation strengthen with decreasing temperature. In a certain
temperature range a developing twist between the layers compensates the growing polar order
and simultaneously satisfies the desire of the transiently helical molecules to assume a helical
packing.6,29,74,75,76 This (partial) synchronization of the helix sense of molecular helical
conformers provides a denser packing and thus an energetic advantage over the non-
synchronized organization. There is some hysteresis of helix formation leading to a short
biaxial range at the paraelectric-(anti)ferroelectric transition due to some delay of the
development of the long range helical order. Therefore, the width of this short biaxial range
depends on the conditions, like heating and cooling rates and surface alignment effects and it
is in some cases only found on cooling. In the biaxial SmCaPA phase below the heliconical
phase the antipolar and anticlinic organization becomes energetically favourable and removes
the heliconical structure.
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DOI: 10.1039/C9TC06456G
Electro-optical investigation. – As shown for compound B14/6 in Figure 7a,b there is
no effect of the applied electric field on the texture of the SmAPR/SmA phase, only the
birefringence is marginally increased (green to yellowish green). In the complete temperature
range of the heliconical Sm(CP)hel phase of compound B14/6 and all compounds Bm/6 the
birefringence of planar aligned samples is significantly increased under the applied E-field
and relaxes back to the initial value after switching off the field (Fig. 7c-f). This indicates an
unwinding of the helix and relaxation of the polar field induced synclinic states back to an
apolar helical ground state (Fig. 7i↔j↔k). In addition, there is a field strength and
temperature dependence of the textural changes. At low E-field or high temperature there is
no effect of the field, whereas with increasing field strength and decreasing temperature at
first a periodic stripe pattern develops perpendicular to the fans (“tiger stripes”,60 see Figs. 7d,
and S24), which changes into a tilt domain texture upon further increasing the field strength
or lowering the temperature (Fig. 7f). This shows that the short pitch helix is unwounded and
becomes a long pitch helix (tiger stripes, Fig. 7i→j) which is finally removed, thus leading to
the field-induced tilt domains with opposite polarity and chirality (Fig. 7k); the threshold
voltage for tilt domain formation decreases with growing chain length. In the temperature
range of the SmCaPA phase below Sm(CP)hel the optical axis is parallel to the layer normal as
in the heliconical phase, but the fans adopt a speckled appearance (Fig. 7g), indicating the
change to a SmCaPA phase at 0V. No change of the position of the extinction crosses is
observed under the field, while the fans become smooth and the birefringence is increased
under the applied field (Fig. 7g,h). This indicates a tristable (antiferroelectric) switching
around the molecular long axis (SmCaPA↔SmCaPF).
So, overall, it appears that the fundamental mode of switching in the polar smectic
phases (SmCaPA) of this series of compounds is the rotation around the long axis, which is
caused by the relatively small bent (and tilt) of these molecules. However, as soon as a
heliconical structure is involved, the switching process changes and takes place by precession
on a tilt cone, because a rotation around the long axis, leading to an inversion of layer
chirality, would lead to a racemic structure which is incompatible with the helix formation.
62,72 ,77
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DOI: 10.1039/C9TC06456G
Figure 7. Textures of the distinct mesophases of compound B14/6 at the given temperatures
in a planar cell as observed between crossed polarizers in the ground state without applied E-
field (left) and as observed under an applied E-field at the indicated voltages (right) with
corresponding models of molecular organization; see Figs. S7 and S20 for the field-induced
textures of the related compounds B10/6 and B18/6, respectively.
Compounds B8/6 and B10/6 and the effect of alkyl chain length.- The longer compounds
B14/6 to B22/6 all behave as described for B12/6 (see Table 1 and Figs. 2, S12-S25), whereas
for the shorter homologues the capability of formation of heliconical phases is reduced.
Compound B10/6 (Figs. S4-S7) shows only a decrease of the birefringence of the
homeotropic sample in the SmCaPA phase with a minimum around 71 °C, but without
becoming optically uniaxial, indicating the coexistence of the optically uniaxial Sm(CP)hel
phase with the (probably surface stabilized) biaxial SmCaPA phase (Fig. S5). On further
cooling the birefringence continuously increases due to a growing contribution of the non-
helical SmCaPA structure. A direct SmAPR-SmCaPA transition without indication of any
intermediate Sm(CP)hel phase is observed for compound B8/6 with the shortest chains (Figs.
S2,S3).
3.2 Compounds C with two long alkyl chains - increasing contribution of synclinic tilt
The compounds C with two alkyl chains with almost identical chain length are collated in
Table 2 and shown graphically in Fig. 8. The mesomorphic range is not significantly affected
by the more equal distribution of the chain lengths, but the paraelectric-(anti)ferroelectric
transition is shifted to slightly higher values around 92±4 °C and an increasing contribution of
synclinic tilted phases is recognized.
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DOI: 10.1039/C9TC06456G
Table 2. Phase transition of compounds Cm/n on heating (H) and cooling (C).
N
O
O
O
O
O
O
O
O
Cm/n
H2m+1Cm
CnH2n+1
Comp.
m
n
Phase transitions T/°C [ΔH/kJ·mol-1]
m+n
H: Cr 89 [23.8] SmAPR/SmA 142 [6.8] Iso
C: Cr 62 [-43.7] Sm(CP)hel 89 [-0.6] SmAPR/SmA 140 [-7.1] Iso
C12/12 12 12
C12/14 12 14
C14/14 14 14
C16/14 16 14
C18/14 18 14
C22/12 22 12
C18/18 18 18
C22/18 22 18
24
H: Cr 91 [23.2] SmAPR/SmA 142 [6.9] Iso
C: Cr 67 [-43.9] Sm(CP)hel 88 [-0.5] SmAPR/SmA 140 [-7.0] Iso
H: Cr 84 [57.1] Sm(CP)hel 98 [0.8] SmCsPAR ~ 102 [-] SmCsPR *]118 [-] SmAPR 136 [7.3] Iso
C: Cr 63 [-55.6] Sm(CP)hel 94 [-0.8] SmCsPAR ~ 100 [-] SmCsPR *] 118 [-] SmAPR 134 [-7.1] Iso
H: Cr 86 [6.1] Sm(CP)hel 96 [0.6] SmCsPAR ~ 103 [-] SmCsPR *]127 [-] SmAPR 144 [7.5] Iso
C: Cr 67 [-59.3] Sm(CP)hel 95 [-0.7] SmCsPAR ~ 102 [-] SmCsPR *] 127 [-] SmAPR 142 [-7.4] Iso
H: Cr 88 [61.1] SmCsPA{Sm(CP)hel} 97 [0.5] SmCsPAR ~ 105 [-] SmCsPR *] 131 [-] SmAPR 143 [7.7] Iso
C: Cr 70 [-67.3] SmCsPA{Sm(CP)hel} 95 [-0.7] SmCsPAR ~ 103 [-] SmCsPR *] 131 [-] SmAPR 142 [-7.7] Iso
26
[
28
30
32
34
36
40
[
[
[
[
[
H: Cr 83 [63.2] SmCsPA{Sm(CP)hel} 95 [0.5] SmCsPAR ~ 103 [-] SmCsPR *] 133 [-] SmAPR 142 [7.5] Iso
C: Cr 63 [-15.5] SmCsPA{Sm(CP)hel} 92 [-0.4] SmCsPAR ~102 [-] SmCsPR *] 132 [-] SmAPR 140 [-7.8] Iso
[
[
[
H: Cr 102 [26.7] SmCsPR *] 132 [-] SmAPR 142 [7.0] Iso
C: Cr 81 [-69.2] SmCsPA{Sm(CP)hel}94 [-0.8] SmCsPAR ~102 [-] SmCsPR *] 131 [-] SmAPR 140 [-7.1] Iso
[
[
H: Cr 95 [77.3] SmCsPAR ~ 103 [-] SmCsPR *] 130 [-] SmAPR 140 [6.9] Iso
C: Cr 82 [-79.6] SmCsPA{Sm(CP)hel} 95 [-1.0] SmCsPAR ~102 [-] SmCsPR *] 130 [-] SmAPR 138 [-7.0] Iso
[
[
Abbreviations: SmCsPR *] = high permittivity paraelectric SmCs phase showing ferroelectric-like switching and
chiral domains ([*]) in homeotropic alignment; SmCsPAR = high permittivity paraelectric SmCs phase showing
antiferroelectric-like switching; SmCsPA = antiferroelectric switching anticlinic tilted SmC phase; {Sm(CP)hel}
indicates a field-induced Sm(CP)hel phase; for the other abbreviations, see Table 1. For DSCs see Figs. 9a, S26,
S30, S32, S40, S43, S47 and S50.
Compounds C12/12 and C12/14 with dominating Sm(CP)hel phases. – The LC phases of
compounds C12/12 (Figs. S26-S29) and C12/14 (Figs. S30, S31) are optically uniaxial
(completely dark in homeotropic alignment) and show a smooth SmA-like fan texture of the
planar samples in the whole mesomorphic temperature range. Only the small DSC peaks at 88
and 89 °C, respectively, indicate the paraelectric-(anti)ferroelectric transition (Table 2), where
in the polarization current curves the broad single peak in the SmAPR range changes to the
typical three-peak pattern of the Sm(CP)hel phase (see Figs. S27, S29 and S31). Thus, for these
compounds the phase sequence on cooling is SmAPR → Sm(CP)hel, i.e. the SmCaPA phase
ranges above and also below the Sm(CP)hel phase of compounds Bm/6 are completely
removed, and for C12/12 and C14/12 exclusively the heliconical phase is observed in a broad
temperature range below the tilt-randomized paraelectric SmAPR phase. The stabilizing effect
of symmetric chain distribution on the heliconical phase is especially evident if the isomeric
15

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compounds with the same total number carbon atoms in the terminal chains i.e. B18/6 and
DOI: 10.1039/C9TC06456G
C12/12 or B20/6 and C12/14 are compared (Figs. 2 and 8).
N
O
O
O
O
O
O
O
Cm/n
O
H2m+1Cm
CnH2n+1
m/n
12/12
12/14
14/14
16/14
18/14
22/12
18/18
22/18
Cr
Cr
Cr
Cr
Cr
Cr
Sm(CP)hel
Sm(CP)hel
Sm(CP)hel
Sm(CP)hel
SmAPR/SmA
SmAPR/SmA
[ ]
SmCsPR
SmAPR
[
]
SmCsPR
SmAPR
SmAPR
[
[
]
]
SmCsPA SmCsPAR SmCsPR
SmCsPA SmCsPAR SmCsPR
SmCsPA
SmCsPA
SmAPR
SmAPR
SmAPR
[
]
Cr
Cr
SmCsPR
[
]
SmCsPR
50
100
150
T (°C)
Figure 8. Chain length dependence of the LC phase and transitions of compounds C as
observed on cooling in the absence of an E-field. After application of a short AC sequence the
SmCsPA phases change to field induced Sm(CP)hel phases, for details, see Figs. S26-S52.
Compounds C14/14 and C16/14 - emergence of synclinic tilt. - The next longer homologue
[
C14/14, shows an additional synclinic tilted SmCsPR *] phase, replacing a part of the
SmAPR/SmA phase (compare Figs. 2 and 8). Similar to the shorter homologues C12/12 and
C12/14 the SmCaPA phase is completely removed and replaced by the Sm(CP)hel phase. This
leads to a transition from a uniaxial via a biaxial to another uniaxial phase with lowering
temperature (Fig. 10a,d,g,j), but in this case the biaxial range appears above the paraelectric-
(anti)ferroelectric transition and not below it and it is much broader than observed for the
isomeric compound B22/6 (see Figs. 9a and Fig. 10a,d,g,j). That the tilt sets in already in the
paraelectric range indicates a greater tendency for tilt correlation by adjusting the alkyl chain
[
length at both ends. Typical features of this polarization randomized SmCsPR *] phase are a
tilt of the extinctions being inclined with the directions of the polarizers in planar alignment
(Fig. 10e), a broad polarization current peak (Fig. 10f) and the formation of chiral domains in
[
homeotropically aligned samples (therefore the additional *]; see Figs. 12, S33 and S35).78
Just before the transition to the heliconical phase there is an additional small ca. 7-8 K wide
temperature range of a SmCsPAR phase, indicated by the vanishing of the chiral domains and
the emergence of two broad polarization current peaks instead of only one (Fig. 10g-i). The
Langevin switching in both polarization randomized ranges of the paraelectric SmC phase
takes place between two identical synclinic SmCsPF states by reorganization around the long
axis (Figs. S38 and S39).
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DOI: 10.1039/C9TC06456G
Figure 9. Investigation of compound C16/14. a) DSC traces recorded with scanning rates of
10K min -1 and b) plot of d-values of the layer spacing (black) and maxima of the diffuse wide
angle scattering (blue) depending on temperature as observed by XRD.
Whereas the Sm(CP)hel phase of compounds Bm/6 and C12/n, occurring below a tilt-
and polarization-randomized SmAPR phase, have three polarization peaks as typical for the
ferrielectric three-layer heliconical phases (see for example see Fig. S27b, S29 and S31), there
are only two peaks in the Sm(CP)hel phases of all compounds C with a longer total chain
length (C14/14, C14/16) and having an onset of tilt before helix formation (see for example
Figs. 10l, S34 and S37). This might indicate a shorter or longer helix being closer to the two-
or four-layer periodicity, for which antiferroelectric switching could be expected.34
Surprisingly, for all compounds C there is an almost linear increase of the d-spacing
with lowering temperature and without any jump either at the SmA-SmC phase transition or
at the paraelectric-(anti)ferroelectric transition temperature (Figs. 9b, S44a and S48a), even if
a synclinic tilt develops. This supports the hypothesis that the tilt is present in all phases (de
Vries like SmAPR phases) and only the coherence length of tilt changes with temperature,
either from tilt randomized to heliconical for compounds Cm/n with n+m <28, from tilt
randomized via synclinic tilted to heliconical for n+m = 28, 30 or exclusively to synclinic
(SmCsPA) for compounds with n+m >30. Moreover, the change of the tilt angle with
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temperature is small and the increasing layer spacing results mainly from the streDtcOhI:i1n0g.10o39f/Ct9hTeC06456G
alkyl chains due to the increasing packing density at lower temperature (Fig. 9b).
Figure 10. Electro-optical investigation of compound C16/14. Homeotropic (left), planar
textures (middle) and corresponding polarization current curves (right) at the given
temperatures in the indicated LC phases; c,f,i) Vpp = 240 V, l) Vpp = 80 V in a 6 m PI-coated
ITO cell; for more detail, see Figs. S35-S39.
Compounds C18/14 to C22/18: Dominating synclinic tilt and the field-induced
Sm(CP)hel phase. - Further increasing the total alkyl chain length to m+n ≥ 32 completely
removes the heliconical Sm(CP)hel phase which is replaced by a synclinic tilted and
antiferroelectric switching SmCsPA phase. The data of compound C18/18 are shown as
representative example for this case in Figs. 11-13 and S49. The typical feature of these
[
compounds is a SmAPR-SmCsPR *]–SmCsPAR – SmCsPA phase sequence (Fig. 11), i.e. the
synclinic tilt, once formed at the SmAPR-SmCsPR transition is retained in the whole LC range
and no heliconical phase is found. In this case the transition from the paraelectric SmCsPAR
phase to the antiferroelectric SmCsPA phase is accompanied by an inversion of the
birefringence (Fig. 13), which is assumed to be the result of the competition between the
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contribution of the biaxiality due to the polar order and the biaxiality due to the tilt, being
DOI: 10.1039/C9TC06456G
perpendicular to each other, i.e. the secondary optical axis can change from being along the
tilt direction to perpendicular with growing polar order.47,62 The inversion point is close to the
paraelectric-(anti)ferroelectric (SmCsPAR - SmCsPA) transition, but in most cases does not
coincide with this transition and usually takes place a few degrees lower.
Figure 11. Electro-optical investigation of compound C18/18. Homeotropic (left), planar
textures (middle) and corresponding polarization current curves (right) at the given
temperatures in the indicated LC phases at c) 320 Vpp f) 260 Vpp, i) 80 Vpp in a 6 m PI-
coated ITO cell.
Application of a short sequence of an AC field to the SmCsPA phase leads to a change
of the orientation of the dark extinction in the planar samples from being inclined to parallel
to the polarizers (Figs. 11e and Fig. 14a→d), indicating the transition to a state with the main
optical axis parallel to the layer normal. Under an applied DC field the extinctions become
inclined again, indicating the induction of a tilted state (Fig. 14d→e,f). The direction of
inclination is inverted by changing the field direction, thus showing that the switching takes
place by precession on a cone. All these observations confirm the field induced formation of a
Sm(CP)hel phase, as previously observed and confirmed by the HDF effect for compound
A18.32,62 The absence of any stripes or speckles in the fan texture of the ground state structure
(Fig. 14e) confirms the heliconical structure and excludes the possibility of an alternative
SmCsPF↔SmCaPA switching. Once formed the heliconical phase is stable after removing the
field for all investigated compounds Cm/n in the whole SmCsPA temperature range at least for
several hours, as long as crystallization is prevented.
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[
Figure 12. Chiral domains in the SmCsPR *] phase of C18/18 at T = 110 °C as observed b)
between crossed and a,c) between slightly uncrossed polarizers (white arrows) with the
analyzer twisted by +/- 10 ° out of the perpendicular direction. The inverted brightness in a,c)
indicates chiral domains; that these domains do not represent simple tilt domains is indicated
by rotation of the sample between crossed polarizers which does not give an inversion of the
domain brightness.
Figure 13. Inversion of birefringence in the SmCs phase range of C18/18 as observed
between crossed polarizers (white arrows in b) with additional -retarder plate (the direction
of the indicatrix is indicated by the blue arrow) in a homeotropic aligned sample; the position
of the bluish and yellowish/red areas exchanges with an inversion of birefringence with n ~
0 around 90 °C; in the insets (yellow) the arrow indicate the relative orientations of the polar
direction (P) to the projection of the tilt direction on the layer plane (, dotted line).
Overall, the formation of the heliconical Sm(CP)hel phase requires a distinct chain
length range and is associated with the cross-over of the mode of tilt correlation from
anticlinic to synclinic, achieved by chain elongation. The heliconical phase replaces a part of
the SmCaPA phase range (compounds Bm/6), which has uniform superstructural layer
chirality. Symmetric chain distribution widens the heliconical phase range which eventually
replaces the SmCaPA phase completely (compounds Cm/n with m+n = 24-30). Further
strengthening of the synclinic tilt correlation with respect to the anticlinic one leads to the
transition to the SmCsPA phase (compounds Cm/n with m+n = >30), which is achiral and thus
disfavours helix formation. In the SmCsPA phase the switching initially takes place by
chirality flipping around the long axis and this leads to the field induced SmCsPF state which
is chiral (Fig 14a→b). Once the SmCsPF states is formed the helix can develop (Fig. 14c→d)
and the SmCsPA phase is replaced by the Sm(CP)hel phase. In the field induced Sm(CP)hel
phase the switching takes place by precession on a cone which retains the layer chirality and
the heliconical phase is retained also after removing the field (see Figs. 14d-f, S38, S39, S46
and S52). 32,62
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DOI: 10.1039/C9TC06456G
Figure 14. Switching of C18/14 at 80 °C under a DC field in 6 m ITO cell, with the field
applied between the substrate surfaces. a-c) in the SmCsPA phase and d-f) in the field-induced
Sm(CP)hel phase; the first application of a DC voltage switches around the long axis from the
racemic SmCsPA state to the chiral SmCsPF state (a→b) and the following switching cycles
(b→c and d↔e↔f) take place by precession on a cone, between the SmCsPF states and
heliconical Sm(CP)hel phase; see also Fig. S42 for the other LC phases and Fig. S38 for
C16/14.
3.3 Effect of ether oxygens on the formation of the heliconical smectic phase and the
polarization randomized heliconical phase
The effect of replacing alkyl chains by alkoxy chains was investigated for compound
B18/6 (see Table 4 and Fig. 15). As expected, the introduction of ether oxygens between alkyl
chain and aromatic core leads to a general stabilization of LC phases79 as indicated by the
increase of the LC-Iso transition temperature. Also the melting points are enhanced with
increasing number of alkoxy chains and there is a strong effect of the position of the alkoxy
chain on the formation of heliconical LC phases. Replacing the shorter alkyl chain at the CN
substituted phenyl terephthalate side by an alkoxy chain (compound D18/O6) removes the
heliconical phase completely, only the SmCaPA phase is observed below the SmAPR phase
(see Table 3 and Figs. S53-S55). In contrast, replacing the longer C18 chain at the 4-
benzoyloxybenzoate substituted side by an alkoxy chain (compound DO18/6) retains and
even expands the heliconical phase range (see Table 3 and Fig. 15; and Figs. S57-S59).
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Table 3. Phase sequence and transition temperatures of compounds D with different chain
length at both ends, measured on heating (H) and cooling (C).a
N
O
O
O
O
O
O
O
O
R1
R2
Phase transitions T/°C [ΔH/kJ·mol-1]
m+n
Comp.
R1
R2
H: Cr 91 [27.2] SmCaPA 102 [0.1] SmAPR/SmA 145 [7.3] Iso
D18/O6
-C18H37
-OC6H13
24
C: Cr 72 [-23.2] SmCaPA 100 [-0.1] SmAPR/SmA 143 [-7.4] Iso
H: Cr 104 [48.0] SmAPR/SmA 153 [6.9] Iso
C: Cr <20 SmCaPA 57 [-] Sm(CP)hel 82 [-] SmCaPA 87 [-0.4]
SmAPR/SmA 153 [-6.8] Iso
H: Cr 102 [44.3] Sm(CP)helR 125 [-] SmAPR/SmA 157 [7.6] Iso
C: Cr 37 [-6.8] SmCaPA 64 [-] Sm(CP)hel 86 [-0.3] SmChelPR 108
[-] SmCxPR 125 [-] SmAPR/SmA 155 [-7.8] Iso
H: Cr 117 [36.9] SmAPR/SmA 161 [7.9] Iso
DO12/6
DO18/6
-OC12H25
-OC18H37
-C6H13
-C6H13
18
24
24
DO18/O6 -OC18H37 -OC6H13
DO22/O6 -OC22H45 -OC6H13
C: Cr 103 [-32.9] SmAPR/SmA 160 [-8.1] Iso
[
H: Cr 116 [36.7] Sm(CP)hel 125 [-] SmCsPR *] 140 [-] SmAPR/SmA
160 [8.9] Iso
28
[
C: Cr 107 [-34.6] Sm(CP)hel 123 [-] SmCsPR *] 139 [-]
SmAPR/SmA 158 [-8.7] Iso
a for abbreviations, see Tables 1 and 2; SmChelPR and SmCxPR represent polarization randomized
heliconical smectic phases; for DSCs, see Figs. 16, S53, S60 and S62 and for additional details, see
Figs. S53-S64.
N
O
O
O
O
O
O
O
O
H2m+1Cm(O)
SmCaPA
(O)CnH2n+1
B18/6
DO18/6
Cr
Cr
Sm(CP)hel
Sm(CP)hel
SmAPR/SmA
SmCaPA
Cr
SmChelPR
SmCxPR
SmAPR/SmA
D18/O6
SmCaPA
SmAPR/SmA
DO18/O6
Cr
SmAPR/SmA
20
40
60
80
100
120
140
160
T (°C)
Figure 15. Effect of replacing one or both alkyl chains of B18/6 by alkoxy chain(s) on the
phase transitions on cooling.
An especially interesting case is compound DO18/6 which is described here in more detail.
The typical fan texture of the SmA phase with extinctions parallel to the polarizers is for this
compound retained in planar samples in the whole mesomorphic range down to the
crystallization at T = 37 °C (see DSC traces in Fig. 16a and corresponding textures in Fig. 17).
As observed for the majority of compounds A-D there is a continuous increase of the d-value
of the layer reflection with lowering temperature (Fig. 16b). The paraelectric-
(anti)ferroelectric transition takes place at 86 °C, indicated by a small DSC peak (Fig. 16a)
22

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which is not associated with any major textural change, neither in planar nor in DhOoIm: 10e.1o0t3r9o/Cp9iTcC06456G
samples (Fig. 17g,i,h,k). In homeotropic alignment a very low birefringent and almost
invisible schlieren texture evolves at 125 °C (Fig. 17e) and is retained down to 108 °C where
the homeotropic sample of DO18/6 becomes completely optically isotropic (black) again
(Fig. 17h). In the switching current curves only one broad polarization current peak is
observed in the whole temperature range between 125 and 108 °C (Fig. 17f).
Figure 16. DSC traces of compound DO18/6 measured with scanning rates of 10K min-1 and
b) d-values of the layer spacing and of the maxima of the diffuse wide angle scattering in the
XRD patterns of this compound depending on temperature.
These observations indicate a polarization randomized smectic phase with
ferroelectric-like Langevin switching. The main optical axis is parallel to the layer normal
(Fig. 17d) which could be in line either with an anticlinic tilted SmCaPR phase or a
polarization randomized SmCsPR phase with an additional heliconical superstructure. These
two possibilities require RSoXS to be distinguished, and therefore the present phase
assignment is SmCxPR. The almost isotropic appearance in homeotropic alignment (Fig. 17e)
is in favour of an optical uniaxial heliconical structure of this polarization randomized SmC
23

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phase, where the residual birefringence is attributed to polar surface layers. If there would be
DOI: 10.1039/C9TC06456G
a SmCaPR phase a stronger birefringence would be expected for the homeotropic sample.
Below 108 °C the homeotropic texture becomes completely dark (Fig. 17h), though a single
broad polarization current peak is retained and shifted almost to the zero voltage crossing
(Fig. 17i).
Figure 17. Electro-optical investigation of compound DO18/6. Hometropic (left) and planar
textures (middle) and corresponding polarization current curves (right) at the given
temperatures in the indicated LC phases; c,f,i) 200 Vpp l,o) 100 Vpp, in a 6 m PI-coated ITO
cell; for additional data, see Fig. S57-S59.
This broad feature is retained down to 86 °C when the typical three peak pattern (the
two peaks at the left are overlapping) of the ferrielectric Sm(CP)hel phase develops (Fig. 17l).
This uniaxial smectic phase occurring between 108-68 °C on cooling, i.e. between the
SmCxPR and Sm(CP)hel phases must have either a randomized tilt direction (re-entrance of a
SmAPR phase), or more likely, there is a helical superstructure representing a kind of
polarization randomized heliconical SmC phase (SmChelPR). This would represent a new kind
24

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of heliconical smectic phase, expanding the family of polarization randomized high
DOI: 10.1039/C9TC06456G
permittivity smectic phases (SmAPR, SmAPAR, SmCsPR, SmCsPAR) by a new heliconical
mode. It is assumed to represent a polarization randomized phase with short pitch helical twist
of the tilt direction between the layers. The main difference to the SmCxPR range is a larger
coherence length of polar order, but also the helical pitch length and mode of correlation
between pitch and layer spacing could be different.
Figure 18. Effect of replacing one or both alkyl chains of C12/12 by alkoxy chain(s) in
compounds D; phase transitions on cooling are shown.
Table 4. Phase sequence and transition temperatures of compounds D with similar chain
length at both ends, measured on heating (H) and cooling (C). a
N
O
O
O
O
O
O
O
O
R1
R2
Phase transitions T/°C [ΔH/kJ·mol-1]
m+n
Comp.
R1
R2
H: Cr 98 [40.0] SmAPAR 110 SmAPR/SmA 158 [7.5] Iso 
C: Cr 75 [-56.1] Sm(CP)hel 91 [-0.5] SmAPAR 110 [-]
SmAPR/SmA 156 [-7.5] Iso
DO12/12
-OC12H25
-C12H25
-C14H29
24
H: Cr 100 [57.8] SmAPAR 110 [-] SmAPR/SmA 158 [7.7] Iso
C: Cr 79 [-62.5] Sm(CP)hel 92 [-0.4] SmAPAR 110 [-]
SmAPR/SmA 158 [-7.5] Iso
DO12/14
D12/O12
-OC12H25
-C12H25
26
24
H: Cr 103 [36.6] SmAPR/SmA 147 [6.9] Iso
-OC12H25 C: Cr 92 [-36.7] Sm(CP)hel 101 [-0.6] SmAPR/SmA 145 [-6.9]
Iso
Cr 119 [39.5] SmAPR/SmA 162 [8.4] Iso
DO12/O12 -OC12H25 -OC12H25
24
28
Cr 103 [-41.2] SmAPR/SmA 163 [-8.3] Iso
[
Cr 120 [39.1] SmCsPR *] 145 [-] SmAPR/SmA 165 [7.8] Iso
DO14/O14 -OC14H29 -OC14H29
[
Cr 108 [-39.5] SmCsPR *] 144 [-] SmAPR/SmA 162 [-7.9] Iso
a
Abbreviations: SmCsPR = high permittivity paraelectric SmCs phase without chiral domain structure in
homeotropic alignment;78 for other abbreviations, see Tables 1 and 2; for DSCs, see Figs. S65, S68, S70, S72,
S73 and S75, and for more details, see Figs. S65-S77.
On further cooling of DO18/6 the Sm(CP)hel phase forms at 86 °C and at 64 °C it
transforms into the SmCaPA phase (Fig. 17j-o). Thus, compound DO18/6 appears to form
even three different heliconical LC phases (Sm(CP)hel, SmChelPR and SmCxPR). On heating
only a uniaxial phase with extinction parallel to the layer normal is found in the whole
25

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mesomorphic temperature range above the melting point at 102 °C, suggestDinOgI: 10t.h10a3t9/Ct9hTeC06456G
SmChelPR phase once formed replaces the SmCxPR phase on heating (Table 3).
For all compounds D, combining one alkoxy and one alkyl chain with similar length at
both ends (Table 4), the Sm(CP)hel phase occurs directly below a tilt- and polarization-
randomized SmAPA or SmAPAR phase without any tilted or heliconical paraelectric
intermediate phase and can be observed down to crystallization. As shown in Fig. 18 the ether
linkage stabilizes the polar heliconical phase with respect to the paraelectric phases, though it
simultaneously tightens their phase ranges due to the rising crystallization temperatures.
However, for the compounds with two alkoxy chains (Tables 3, 4 and Fig. 18) the Sm(CP)hel
phase cannot be observed, due to the increase of the crystallization temperature. Only for
DO22/O6 (Table 3) with very different chain length at both ends the crystallization
temperature is sufficiently low that the Sm(CP)hel phase is observed. In this case the
Sm(CP)hel phase is stabilized by ~40 K compared to B22/6 (Table 1) and thus becomes an
enantiotropic phase.
3.4 Compounds E and the effect of the direction of the cyano group
Table 5. Phase sequence and transition temperatures of compounds E with exchanged
position of phenyl terephthalate and 4-benzoyloxybenzoate wings with respect to the CN
group, as measured on heating (H) and cooling (C). a
N
O
O
O
O
O
O
O
O
R2
R1
Comp.
R1
R2
Phase transitions T/°C [ΔH/kJ·mol-1]
H: Cr 87 [24.9] SmCsPA{Sm(CP)hel} 90 [0.7] SmCsPAR 98 [-]
m+n
SmCsPR 130 [-] SmA 134 [6.1] Iso
C: Cr 48 [-18] SmCsPA {Sm(CP)hel} 88 [-0.9] SmCsPAR 96 [-]
SmCsPR 129 [-] SmA 132 [-6.7] Iso
E12/12
-C12H25
-C12H25
24
H: Cr 76 [19.8] SmCsPA 102 [1.2] SmCsPAR 105 [-] SmCs 137 [5.5]
EO12/12
-OC12H25
-C12H25 Iso
24
24
C: Cr < 20 SmCsPA 99 [-1.1] SmCsPAR 103 [-] SmCs 135 [-5.6] Iso
Cr 114 [37.5] (SmCsPA 106 [1.0]) SmCsPAR 116 [-] SmCsPR *] 148
[
[6.3] Iso
EO12/O12 -OC12H25 -OC12H25
Cr < 20 SmCsPA 104 [-1.3] SmCsPAR 115 [-] SmCsPR *] 147 [-6.6]
[
Iso
a
Values in parentheses refer to monotropic transitions recorded in the second heating cycle,
phases in braces indicate field induced phases; abbreviations: SmC = paraelectric SmC phase
without visible polarization peak; for other abbreviations, see Tables 1,2 and 4, for DSCs see
Figs. S78, S81, and S84 and for more details see Figs. S79-S83 and S85.
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DOI: 10.1039/C9TC06456G
The effect of exchange of the phenyl terephthalate and 4-benzoyloxybenzoate wing in
compounds B-D is shown for selected compounds E in Table 5. By comparison with the alkyl
substituted compounds (see C12/12 in Table 2) an increasing tendency to synclinic tilt
correlation is found for the compound E12/12, as indicated by the formation of tilted
paraelectric SmCs, SmCsPR and SmCsPAR phase ranges replacing the majority of the SmA
phase (see Figs. S78, S79).80 As well, the heliconical Sm(CP)hel phase of C12/12 is
completely removed and replaced by an antiferroelectric switching SmCsPA phase for E12/12.
In this phase a heliconical Sm(CP)hel phase is induced by application of an electric field,
associated with a reorganization by precession of the molecules on a cone (see Fig. S80). The
same phase sequence is observed for the two other compounds E with one or two alkoxy
chain, but for these compounds no helix is formed in the SmCsPA phases under the applied
AC field and the switching takes place by rotation around the long axis (SmCsPA↔SmCsPF)
confirming the absence of a field induced heliconical phase (see EO12/12 in Fig. S83). The
larger tilt of compounds E (~ 22° for E12/12 compared to 18-20° in the series An) and the
stronger tilt correlation between the layers obviously disfavours helix formation (Table 5).
The significantly smaller d-values of the layer spacing observed for compound EO12/O12 (d
= 4.34 to 4.40 nm, Fig. S85) compared to compound DO18/6 with a similar total chain length
(d = 4.84-5.35 nm, Fig. 16b) confirms the stronger tilt of compounds E.
Figure 19. d-values of the layer spacing of EO12/O12 depending on temperature.
Moreover, compound EO12/O12 is the only investigated compound which shows a
decrease of the layer spacing d with lowering temperature and a clearly visible step to larger
d-values at the paraelectric-(anti)ferroelectric transition due to the increased packing density
(Fig. 19), whereas for all other compounds A-D the development of d is characterized by a
continuously increasing d-value with lowering temperature (see for example Fig. 16b).
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Overall, the inversion of the direction of the CN group increases the tilt, thus DlOeaI: d10i.n10g39/tCo9TaC06456G
stronger layer coupling which suppresses the heliconical phases.
3.5 Effects of the direction of the COO groups
Table 6. Effect of inverting one or both peripheral COO group(s) on the phase sequence and
phase transitions.a
N
O
O
O
O
Y
X
CnH2n+1
H2m+1Cm
X
Y
m = n
Phase transitions T/°C [ΔH/kJ·mol-1]
Compd.
Cr 114 [47.2] (SmCaPA 91 [-] Sm(CP)hel 110 [0.9]) SmAPR/SmA 165 [8.0]
Iso
A1460,62 OOC COO
14
[
Cr 84 [57.1] Sm(CP)hel 98 [0.8] SmCsPR *] 118 [-] SmA 136 [7.3] Iso
C14/14 COO COO
14
14
F1447
COO OOC
Cr 91 (M 36 SmCI 73 SmCII 77) CybC 114 NCybC 115 Iso
a
Phase transitions on heating; values in parentheses indicate monotropic phases, observed on cooling; SmCI,
SmCII and CybC are different types of non-polar synclinic SmCs phase, M is an unknown mesophase;47 for the
other abbreviations, see Tables 1 and 2.
A comparison of A14, C14/14 and F14 in Table 6 shows the effect of inversion of the
direction of the two peripheral COO groups in the rod-like wings. Compound F14 with two 4-
benzoyloxybenzoate wings forms exclusively non-polar and synclinc tilted SmCs phases with
high tilt angles (~35°) besides the skewed cybotactic nematic phase (NCybC).47 Inversion of
both outer COO groups in A14 reduces the tilt, removes all synclinic tilted phases and the
transition from the paraelectric SmA/SmAPR to a polar SmCaPA phase takes place via an
intermediate heliconical phase. For C14/14, combining both directions of the COO group,
there is a slightly larger tendency to synclinic tilt correlation which leads to the insertion of an
[
intermediate SmCsPR *] phase between SmAPR and Sm(CP)hel. Simultaneously with the
decreasing tilt in the order F14 > C14 > A14 the mesophases stability rises. Both effects are
attributed to a growing core-core interaction and growing packing density due to the
increasing electron deficit character of the polyaromatic cores in this order. The surprising
similarity of the LC phases of all three series A, B and C compared to the series of
compounds F might be attributed to the strength of the core-core interaction. The two electron
deficit phenyl terepthalate wings provide strong attractive core-core interactions81 for
compounds A. Replacement of one phenyl terephthalate wings by an electron rich phenyl
benzoate wing leads to attractive donor-acceptor interactions between the electron rich and
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electron deficit wings in an antiparallel packing of the compounds B and C in the layers. The
DOI: 10.1039/C9TC06456G
electrostatic interaction become weaker for compounds F having exclusively electron rich
phenyl benzoate wings.81 The comparison of C12/12 with E12/12 (Tables 2 and 5) shows that
the position of the electron deficit phenyl terephthalate wing at the CN substituted side is
important for retaining a small tilt.
The series B and C provide access to broad heliconical smectic phase regions for
compound with a total number of C-atoms in the terminal alkyl chains of 16-30, which is
shorter than the 22-36 required for the related compounds A. This is in line with the reduced
intrinsic tilt in series A which requires longer chains to reach the anticlinic-synclinic cross-
over point. Moreover, the heliconical phase of C14/14 is stable until crystallization and no
other phase can be found below it, whereas an additional anticlinic tilted phase range is
formed below the Sm(CP)hel phase of A14.
Overall the major effects come from the core structure, determining the bending angle
 (Scheme 1) and the electrostatic intermolecular interactions between the polyaromatic
cores, whereas alkyl chain engineering is an important tool for fine-tuning of the strength and
mode of layer coupling and inter-layer tilt-correlation, growing in the order random → helical
→ anticlinic → synclinic.
4. Conclusions
This work provides new series of 4-cyanoresorcinol bisbenzoate derived bent-core
mesogens with relatively weak bend and shows that the heliconical smectic phase, designated
as Sm(CP)hel is not restricted to compounds with two phenyl terephthalate wings (compounds
An in Scheme 1), but represents a new kind of mirror symmetry broken LC phase generally
occurring if certain conditions are fulfilled. It can be expected for the smectic phases of bent-
core mesogens below the paraelectric-(anti)ferroelectric transition if the tilt angle is around
18-20° and if this transition coincides with the transition from randomized to uniform tilt and
with the anticlinic-to-synclinic cross-over point.
As graphically shown in Figure 20, the Sm(CP)hel phase appears below a polarization
[
randomized high permittivity SmAPR, SmAPAR, SmCsPAR or SmCsPR *] phase32,58 and
replaces the emerging homogeneously chiral polar and anticlinic SmCaPA phase. As the
synclinic tilt correlation becomes stronger by chain elongation, by introduction of ether
oxygens between chains and core, or by flipping the direction of the CN group (= exchange of
the positions of 4-benzoyloxy benzoate and phenyl terephthalate wings), the anticlinic
SmCaPA phase is replaced by the synclinic SmCsPA phase which is achiral and thus
29