Polar Order, Mirror Symmetry Breaking, and Photoswitching of Chirality and Polarity in Functional Bent-Core Mesogens

Article abstract of DOI:10.1002/chem.201806180

In recent years, liquid crystals (LCs) responding to light or electrical fields have gained significant importance as multifunctional materials. Herein, two new series of photoswitchable bent-core liquid crystals (BCLCs) derived from 4-cyanoresorcinol as the central core connected to an azobenzene based wing and a phenyl benzoate wing are reported. The self-assembly of these molecules was characterized by differential scanning calorimetry (DSC), polarizing light microscopy (POM), electro-optical, dielectric, second harmonic generation (SHG) studies, and XRD. Depending on the direction of the COO group in the phenyl benzoate wing, core-fluorination, temperature, and the terminal alkyl chain length, cybotactic nematic and lamellar (smectic) LC phases were observed. The coherence length of the ferroelectric fluctuations increases continuously with decreasing temperature and adopts antipolar correlation upon the condensation into superparaelectric states of the paraelectric smectic phases. Finally, long-range polar order develops at distinct phase transitions; first leading to polarization modulated and then to nonmodulated antiferroelectric smectic phases. Conglomerates of chiral domains were observed in the high permittivity ranges of the synclinic tilted paraelectric smectic phases of these achiral molecules, indicating mirror symmetry breaking. Fine-tuning of the molecular structure leads to photoresponsive bent-core (BC)LCs exhibiting a fast and reversible photoinduced change of the mode of the switching between ferroelectric- and antiferroelectric-like as well as a light-induced switching between an achiral and a spontaneous mirror-symmetry-broken LC phase.

Full text of DOI:10.1002/chem.201806180

A Journal of
Accepted Article
Title: Polar Order, Mirror Symmetry Breaking and Photoswitching of
Chirality and Polarity in Functional Bent-core Mesogens
Authors: Carsten Tschierske, Mohamed Alaasar, Marko Prehm,
Sebastian Belau, Nerea Sebastian, Marharyta Kurachkina,
Alexey Eremin, Changlong Chen, and Feng Liu
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To be cited as: Chem. Eur. J. 10.1002/chem.201806180
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10.1002/chem.201806180
Chemistry - A European Journal
Polar Order, Mirror Symmetry Breaking and Photoswitching of Chirality
and Polarity in Functional Bent-core Mesogens
Mohamed Alaasar,[a,b]* Marko Prehm,[a] Sebastian Belau, [c] Nerea Sebastián, [c] Marharyta
Kurachkina, [c] Alexey Eremin,[c]* Changlong Chen,[d] Feng Liu,[d] Carsten Tschierske[a]*
[a]
Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2, D-06120 Halle
(Saale), Germany; carsten.tschierske@chemie.uni-halle.de
[b]
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt; malaasar@sci.cu.edu.eg
[c] Department of Nonlinear Phenomena, Institute of Physics, Otto von Guericke University Magdeburg,
Magdeburg, Germany
[d] State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049,
P. R. China
Abstract:
In recent years liquid crystals (LCs) responding to light or electrical fields gained significant
importance as multifunctional materials. Here we report on two new series of photoswitchable
bent-core liquid crystals (BCLCs) derived from 4-cyanoresorcinol as the central core connected
to an azobenzene based wing and a phenyl benzoate wing. The self-assembly of these molecules
was characterized by differential scanning calorimetry (DSC), polarizing light microscopy
(POM), electro-optical, dielectric, second harmonic generation (SHG) studies and X-ray
scattering (XRD). Depending on the direction of the COO group in the phenyl benzoate wing,
core-fluorination, temperature and the terminal alkyl chain length, series of cybotactic nematic,
paraelectric and polar lamellar, as well as modulated lamellar LC phases were observed. With
decreasing temperature the coherence length of ferroelectric fluctuations appears to inrease
continuously and adopts antipolar correlation upon the condensation into superparaelectric states
of the paraelectric smectic phases. Long range polar order develops at distinct phase transitions;
first leading to polarization modulated and finally to non-modulated antiferroelectric smectic
phases. Conglomerates of chiral domains were observed in the high permittivity ranges of the
synclinic tilted paraelectric smectic phases of these achiral molecules, indicating mirror
symmetry breaking. Fine tuning of the molecular structure leads to first photoresponsive BCLC
materials exhibiting a fast and reversible photo-induced change of mode of switching between
ferroelectric-like and antiferroelectric-like as well as a light-induced switching between an
achiral and a spontaneous mirror symmetry broken LC phase.
1
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1. Introduction
Liquid crystals (LC) represent an important class of functional materials for numerous
applications, ranging from materials for displays,1,2 photovoltaics,3 photonics,4 soft
nanolithography,5 for ion carriers,6 biosensors,7 and as membrane-forming materials8 to mention
only few.9,10,11 Therefore, the design of new compounds and especially understanding of the
relations between the molecular structure and the liquid crystallinity as well as application
properties are of fundamental interest.12 One of the relevant application related properties is polar
order. Polar order in the fluid state requires the reduction of the phase symmetry, which is
usually achieved by the permanent chirality of the involved molecules.13,14,15 The discovery of
polar order and mirror symmetry broken chiral mesophases formed by bent-core liquid crystals
BCLCs16,17,18,19,20,21,22 opened the door to polar LCs formed by achiral molecules. In lamellar LC
phases the reduction of the phase symmetry results from the combination of tilt and restricted
rotation around the molecular long axis.16,23 As a result of polar order and transient molecular
chirality spontaneous mirror symmetry breaking and chiral conglomerate formation21,24,25,26 was
observed in different types of mesophases formed by BCLCs such as helical nanofilament (B4)
phases27,28,29,30 and related soft crystalline phases,31 sponge-like dark conglomerate phases (DC
phases),21,32,33,34,35 and more recently the twist bend nematic (NTB) phases.36 Chiral conglomerate
formation was also observed in paraelectric SmC phases37,38 and nematic phases of bent-core
mesogens,39,40 which initiated the search for spontaneous mirror symmetry breaking in other LC
systems.24,24,26,41,42 Overall, BCLCs lead to new types of molecular self-assembly and have
significantly contributed to the field of supramolecular stereochemistry. One of the contemporary
challenges concerns the switching of their polar order and the chirality by light, and for this
purpose photoswitchable BCLCs are required.
Azobenzene-based liquid crystalline materials, exhibiting reversible trans-cis
isomerization upon irradiation with light represent an important class of photoswitchable
materials which can be used for many applications in diverse photodriven devices43,44,45,46,47,48,49
and in photonics.50 Therefore, combining the ferroelectric and spontaneously chiral LC phases
formed by BCLCs and the photoswitchable nature of the azobenzene units could lead to potential
new multifunctional materials in which the polar response can be modified with light.51 For this
2
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reason, azobenzene-based BCLCs attracted the attention of several researchers in recent
years.52,53,54,55,56,57,58,59,60,61
Compounds having molecular shapes at the cross-over between linear (rod-like) and bent
shapes (BCLCs), such as mesogenic dimers with odd spacers,62,63,64,65,66,67,68,69 hockey stick
molecules70,71,72 and molecules with reduced length of the bent core73 or with a reduced bending
angle74 have received significant attention, because they provide new LC phase structures. 4-
Cyanoresorcinol is a very useful bent-core unit with reduced bending angle which has lead to a
number of new and unique mesophases,37,38,75,76,77,78,79,80,81,82 as for example the non-tilted
antiferroelectric SmAPA phase83 and a short pitch heliconical smectic phase designated as
hel 84,85,86
SmCsPF
.
In previous work the 4-cyanoresorcinol core was combined with two identical
wings, either being azobenzenes or phenyl benzoates.37,38,76-86
Scheme 1. Chemical structures and designation of the bent-core molecules under investigation.
Herein we report new BCLCs derived from a 4-cyanoresorcinol central core connected to
two different rod-like wings; one is azobenzene-based (A) and the other is either a benzoylated
4-hydroxybenzoate wing (B) in compounds ABn or a phenyl terephthalate wing (T) in
compounds ATn, respectively, being different in the direction of the COO group between the
two outer benzene rings (see Scheme 1). The terminal alkoxy chain at the ester-based wing in
both types of compounds was fixed to m = 14, whereas that connected to the azobenzene
containing side arm has chain lengths n = 8, 12, 16 and 20. The effect of a peripheral fluorine
substitution at the outer ring of the azobenzene-based wing (AF) was investigated for one
example from each series (AFB16 and AFT16). In Section 3.1 we will focus on the series of
terephthalate compounds AT/n and in Sections 3.2 and 3.3 the focus is on the series AB/n and
compound AFB/16, respectively, both involving a 4-hydoxybenzoate cores. We will show that on
3
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decreasing temperature for all compounds a continuous evolution of polar order from apolar or
low permittivity paraelectric (NCybC, SmA, SmC) via high permittivity paraelectric (SmC[*],
[
SmAPR, SmCsPR *]) to superparaelectric (SmCsPAR, SmCaPAR) states is followed by distinct
̃
phase transitions to polarization modulated (SmCsPA) and non-modulated macroscopic polar
synclinic smectic phases (SmCsPA, SmCs’PA). Although in the SmC range, the structure of the
phase, established by X-ray scattering, does not show any significant difference, the electro-
optical switching behaviour and observation of chiral structures suggest a variety of states
distinguished in this temperature region. We designate them tentively as SmCs[*], SmCsPR *] and
[
SmCsPAR ranges and consider them as temperature ranges of one paraelectric SmCs phase,
having different polar cluster size and hence distinct properties. The ranges of the paraelectric
SmCs phases are shown in bold in Tables 1 and 2 and indicated by red color in the graphics,
whereas green color is used for all polar phases. Due to the involved azobenzene unit all
compounds are photoisomerizable, and in Section 3.4 it is shown that they provide new modes of
photoinduced switching of BCLCs, namely a fast and reversible photoinduced change of the
mode of polar switching, and a photoswitching between spontaneous symmetry broken chiral
and achiral LC states.
2. Experimental
Synthesis. - The target BCLCs molecules ABn, AFB16, ATn and AFT16 were synthesized as
shown in Scheme 2. Details of the synthesis of the intermediates and final compounds as well as
their analytical data are reported in the Supporting Information (SI, Section S1).
Methods. - The thermal behaviour of all synthesized materials was studied by polarizing optical
microscopy (POM), differential scanning calorimetry (DSC) and by electrooptical investigation
under a triangular wave field. All compounds with n = 16 were investigated by X-ray diffraction
(XRD) using a synchrotron source; dielectric studies and second harmonic generation
experiments were performed with compounds AT16 and AB16. The used methods are described
in Section S2 of the SI. The error of the d-values in XRD is about 0.02 nm and ~3° for the tilt
angle; for current response and SHG the accuracy is about 5%.
4
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Scheme 2. Synthetic route to the bent-core mesogens ABn, AFB16, ATn and AFT16. Reagents
and conditions: i. SOCl2, DMF, reflux 1 h,77 ii. Et3N, pyridine, CH2Cl2, reflux 6 h, 31a37,38 iii. H2,
10 % Pd/C, THF, stirring 48 h at 25 °C; iv. 1. H2NOH, 2.Ac2O, 3. NaOH.
3. Results and Discussion
3.1. Compounds ATn with a terephthalate wing
Table 1. Transitions of compounds ATn and AFT16 on heating (H) and cooling (C).a
T /ºC
Comp.
X
H
n
[ΔH/kJ·mol-1]
8
AT8
̃
H: Cr 80 SmCsPA 110 SmCaPAR 115 SmAPR ~155 SmA 165 Iso
[42.7] [0.2] [-] [-] [6.2]
C: Iso 164 SmA ~ 154 SmAPR 114 SmCaPAR 109 SmCsPA 82 SmCsPA 58 Cr
[7.0] [-] [-] [0.2] [-] [14.7]
H: Cr 82 SmCsPA 109 SmCsPAR 115 SmCs[*] 145 SmA 165 Iso
[30.0] [0.8] [-] [-] [7.4]
C: Iso 163 SmA 144 SmCs[*] 114 SmCsPAR 109 SmCsPA 82 SmCsPA 39 Cr
̃
H
H
H
F
12
16
20
16
AT12
AT16
AT20
AFT16
̃
̃
[7.6]
[-]
[-]
[1.0]
[-]
[24.3]
[
H: Cr 90 SmCsPA 109 SmCsPAR 112 SmCsPR *] 127 SmCs[*] 157 SmA 165 Iso
̃
[60.9]
C: Iso 163 SmA 156 SmCs[*] ~ 126 SmCsPR *] 111 SmCsPAR 109 SmCsPA 78 SmCsPA 62 Cr
[7.4] [-] [-] [-] [0.8] [-] [45.1]
H: Cr 93 SmCsPA 103 SmCsPAR 110 SmCsPR *] 154 SmA 162 Iso
[69.2] [0.6] [-] [0.2] [7.4]
C: Iso 160 SmA 154 SmCsPR *] 109 SmCsPAR 102 SmCsPA 85 SmCsPA 62 Cr
[6.3] [-] [-] [0.6] [-] [32.5]
H: Cr 114 SmCsPA 121 SmCsPAR 127 SmCsPR *] 152 SmA 166 Iso
[27.5] [1.3] [-] [-] [7.9]
C: Iso 165 SmA 152 SmCsPR *] 127 SmCsPAR 120 SmCsPA 107 SmCsPA 100 Cr
[7.4] [-] [-] [1.2] [-] [26.0]
[0.8]
[-]
[-]
[-]
[7.4]
[
̃
[
̃
[
̃
[
̃
[
̃
5
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aTransition temperatures and enthalpy values (in square brackets) were taken from the second DSC scans (10 K
min-1); continuous transitions without detectable DSC peak were determined by polarizing microscopy or switching
experiments under triangular wave fields; abbreviations: Cr = crystalline solid; Iso = isotropic liquid; SmA =
̃
uniaxial smectic phase; SmCs = synclinic tilted smectic phase (identical tilt direction in adjacent layers); SmCs
modulated SmCs phase; SmCa = anticlinic tilted smectic phase (tilt direction is opposite in adjacent layers); PR = high
permittivity paraelectric range showing one broad polarization current peak per half period of an applied E-field;
PAR = superparaelectric range showing two polarization current peaks per half period of an applied E-field; PA =
antiferroelectric switching polar phase; phases without extension do not show a measurable polarization current
peak; [*] = LC phase showing a conglomerate of chiral domains in homeotropic alignment; see Figs. 8 and S19b for
[
models of the phase structures. Note, that SmCs[*], SmCsPR *] and SmCsPAR, shown in bold, do not represent separate
phases, butare considered as ranges of one paraelectric SmCs phase with continuously growing polar cluster size.
a)
~
SmC P
Cr
s
A
[
]
*
SmC P
s
R
SmA
SmC P
s
AR
Heating
[
*
]
SmC
Iso
s
~
Iso
SmC P
s
A
Cr
[
*
]
SmC
SmC P
s
SmC P
s
s
AR
A
[
*
]
SmA
SmC P
Cooling
s
R
40
60
80
100 120 140 160
T / °C
b)
~
SmC P
s
A
Cr
SmC P
s
AR
Iso
Iso
SmA
Heating
[
]
SmC P
s
*
R
[
]
SmC P
*
s
R
~
SmC P
s
SmA
160
A
Cr
SmC P
s
AR
SmC P
s
A
Cooling
80
100
120
140
T / °C
Figure 1. DSC heating and cooling traces at a rate of 10 K min -1 for: a) compound AT16 and b)
compound AFT16; dotted lines indicate continuous transitions without visible DSC peak; in this
and the following figures green indicates macroscopic polar phases whereas red color stands for
paraelectric phase ranges.
6
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3.1.1 Investigation of compound AT16
a) SmA (160 °C)
80
a) 160 °C
g)
0.2
0.0
-0.2
160 °C
40
0
-40
-80
-0.02 0.00 0.02 0.04 0.06 0.08
t /s
0.2
0.1
0.0
-0.1
-0.2
b) SmC[*] (150 °C)
80
40
0
b) 150 °C
150 °C
h)
-40
-80
-0.02 0.00 0.02 0.04 0.06 0.08
t /s
[
c) SmCsPR *] (125 °C)
0.15
80
40
0
1c2)512°5C°C
i)
0.10
0.05
0.00
-0.05
-0.10
-0.15
-40
-80
-0.02 0.00 0.02 0.04 0.06 0.08
t /s
0.3
d) SmCsPAR (110 °C)
1d1)211°5C°C
80
0.2
j)
40
0
0.1
0.0
-0.1
-0.2
-0.3
-40
-80
-0.02 0.00 0.02 0.04 0.06 0.08
t /s
1.5
e) SmCsPA (100 °C)
̃
80
40
0
e) 100 °C
100 °C
k)
1.0
0.5
0.0
-0.5
-1.0
-1.5
-40
-80
-0.02 0.00 0.02 0.04 0.06 0.08
t /s
f) SmCsPA (73 °C)
80
40
0
1.0
0.5
0.0
-0.5
-1.0
f) 70 °C
70 °C
l)
-40
-80
-0.02 0.00 0.02 0.04 0.06 0.08
t /s
Figure 2. Investigation of compound AT16: a-f) Optical textures as observed by polarizing
microscopy between crossed polarizers (polarizer and analyzer are horizontal, vertical,
7
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respectively) in a planar cell (6 m PI coated ITO cell) in the different phases and phase ranges
at the given temperatures; the insets shows the corresponding textures in a homeotropic cell (for
enlarged textures, see Fig. S14). g-l) Switching current response curves on applying a triangular
wave field (27 Vppm-1, 10 Hz, 6 m PI coated ITO cell) in the distinct phases at the given
temperatures (for additional switching current curves, see Fig. S15).
The phase sequences, transition temperatures and the associated transition enthalpies of
compounds ATn and the fluorinated compound AFT16 (X = F) are given in Table 1. The DSC
plots for compounds AT16 and AFT16, as examples, are shown in Fig. 1. Compound AT16, as
representative example for compounds with n ≥ 12, is discussed in the following in more detail.
DSC and optical investigations. – As shown in Fig. 2a in planar alignment a fan texture is
observed for AT16 and at 157 °C the extinction crosses change their orientation from being
parallel to slightly inclined with respect to the directions of polarizer and analyzer (Fig. 2b).
Simultaneously, in homeotropic samples the optical isotropic texture becomes birefringent with
development of a Schlieren texture, indicating a transition from SmA to a synclinic tilted SmCs
phase (see insets in Fig. 2a,b), which is not visible in the DSC traces (Fig. 1a), and therefore
assumed to be second order. On further cooling the birefringence of the planar samples increases
and the Schlieren texture in the homeotropic samples is also changing (see Figs. 2b-f and S13).
The increase of birefringence in the planar samples at the SmA-SmC transition (Fig. 2a,b)
suggests a de Vries-type of the SmA phase composed of tilt direction randomized SmCs
clusters.87
X-ray scattering. - Laboratory XRD show a diffuse scattering in the wide-angle region and only
one sharp Bragg reflection in the small angle region, corresponding to d = 5.1-5.3 nm (Fig. 3) in
the whole mesomorphic temperature range (Fig. S12). This indicates that all phases represent
smectic phases without any in-plane positional order. The measured d-value is a bit smaller than
the single molecule length (d/Lmol = 0.76-0.79, with Lmol = 6.7 nm determined for a -shaped
conformation with 120° bending angle with all-trans stretched alkyl chains, see Fig. S11), in line
with a tilted organization of the molecules in all smectic phases (uniform tilt in SmCs or
randomized tilt in SmA) with single-layer structure and conformationally disordered alkyl
chains. The tilted organization of the molecules is confirmed by the 2D diffraction patterns of
aligned samples. In the SmA phase range, the maxima of the diffuse scattering in the wide-angle
8
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region are perpendicular to the position of the layer reflection (see Fig. 3a), whereas after the
transition to the synclinic SmCs phase at T = 157 °C a clear deviation from the orthogonal
position of about 20° can be observed (Fig. 3b), although the change in the d-value is small (Fig.
3c).
b)
a)
5.40
5.35
5.30
5.25
5.20
5.15
5.10
5.05
5.00
c)
SmC P
s
AR
Iso
[
]
*
SmC P
s
R
[
*
]
SmC
s
SmC P
s
A
SmA
~
SmC P
s
A
60 70 80 90 100 110 120 130 140 150 160 170
T / °C
26
24
22
20
18
16
d)
[
SmCsPR *]
SmCs[*]
SmCsPA
SmCsPAR
SmC̃sPA
SmA
70 80 90 100 110 120 130 140 150 160
T / °C
Figure 3. XRD of an aligned sample of compound AT16: a) 2D pattern in the SmA phase at 160
°C and b) in the SmCs phase at 150 °C, after subtraction of the scattering in the isotropic liquid
state at 170 °C (for additional and the original patterns, see Fig. S12); c) shows the temperature
dependence of the d-spacing (see also Table S1) and d) dependence of tilt on temperature
(estimated from the position of the WAXS maxima whit respect to the layer reflection).
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a)
5.85 nm
b)
SmCsPA
̃
a
5.37
nm
b
c)
d)
b
a
a
b
B1Rev
B1
80 °C
95 °C
0.48 nm
e)
0.47 nm
0.46 nm
2800
2600
2400
2200
2000
1800
1600
1400
1200
108 °C
112 °C
120 °C
140 °C
160 °C
0.45 nm
14
16
18
20
22
24
2
/ °
̃
Figure 4. a) SAXS diffractogram (synchrotron source) of compound AT16 in the SmCsPA phase
at 90 °C (see Fig. S13a for a complete temperature scan) with b) reconstructed electron density
map based on the assumed c2mm lattice (purple/blue = high ED, aromatics; red = low ED, alkyl
chains) showing the proposed organization of the molecules; the gray rectangles indicate the
position of the densely packed aromatic cores (dark gray) and regions with reduced core packing
density (light gray), in the rectangles the direction of tilt could be perpendicular to the lattice or
in the plane of the lattice, in the latter case the symmetry would be reduced to p2 (dashed
rhomb); the alkyl chains are organized in wavy deformed layers (red); c,d) related models of the
B1 type phases of BCLCs89 and e) WAXS patterns depending on temperature.
The development of the tilt angle depending on temperature, as estimated from the 2D-
XRD pattern (position of the diffuse wide-angle scattering maxima with respect to the layer
reflections) shows an almost continuous growth of the tilt with decreasing temperature from 16°
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at T = 150 °C in the SmCs phase to 24 ° at 80 °C, just before the transition to the SmCsPA phase
(Fig. 3d). Remarkably, the d-values rises with decreasing temperature (Fig. 3c), which is the
opposite tendency as would be expected for a growing tilt with lowering temperature (Fig. 3d).
Therefore, we conclude that the effect of growing packing density (growing polar order), leading
to stretching of the alkyl chains is larger than the inverse effect of growing tilt on d. The growing
packing density is in line with the decreasing d-value of the WAXS from 0.48 nm in the SmA
̃
phase to 0.45 nm in the SmCsPA phase (Fig. 4c). This means that for BCLCs the d-values of the
layer reflection cannot be used as a reliable indication of tilt.
Investigation of AT16 by high resolution synchrotron X-ray powder diffraction
experiments show a splitting of the small angle scattering into two close reflections with
different intensity, indicating a layer modulation. Although this is only observed between 93 °C
and 80 °C (Figs. 4a and S13a) we assume that there is layer modulation in the whole temperature
range between the transitions at 109 and 75 °C, albeit with a shorter correlation length. This is in
line with textural observations of homeotropic samples on cooling, where the Schlieren texture
becomes mosaic-like at 109 °C and turns back to a Schlieren texture around the transition at T =
78 °C (see Figs. 2d→e→f and S14f→g); in planar samples no clear textural changes can be
observed. Though assignment to a specific lattice is not possible with only two reflections,88
XRD patterns with only the (11) and (20) reflections of a rectangular lattice are typical for the B1
type columnar phases of BCLCs composed of ribbons sifted along direction a (Fig. 4c).89
Though the textures are quite distinct from the typical textures of the B1 phases,17,21 a tentative
assignment of the diffraction pattern to the (11) and (20) reflections of a c2mm lattice leads to
parameters a = 10.74 nm and b = 6.98 nm. The parameter a corresponds well to about twice the
layer distance and b corresponds to about 7-8 molecules organized in the lateral cross section of
each ribbon, which is a reasonable number, too. As shown in the electron density (ED) map in
Fig. 4b a weakly modulated lamellar phase composed of layers formed by ribbons with high ED
(blue/purple) and connected by defects with reduced ED (green), both formed by the aromatic
cores having different packing density, is separated by wavy deformed low ED layers of the
alkyl chains (red). This structure is distinct from the classical models of B1 type columnar phases
of BCLCs (Fig. 4c),17 because the shift of the ribbons is along b instead along a. In this respect it
is similar to the B1rev/B1rev,tilt phases having modulated layers (Fig. 4d).89 However, a different
diffraction pattern with more reflections and different positions would be expected for this
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structure.17,19,21,22,89 Based on the diffraction pattern and textures and considering the position of
these columnar phases at the transition from paraelectric to polar smectic phases, this phase is
̃
not designated as B1, but more generally as a modulated SmCs phase. The modulation is most
likely caused by a splay of the developing polarization (see next Sections), leading to a periodic
polarisation-splay state comparable to the B7 phases.88,90,91,92
Electrooptical studies.- Under an applied triangular wave voltage with 27 Vpp m-1 (Fig. 2g-l) a
broad polarization peak starts to appear at 127 °C (Figs. 2i and S15a) and continuously increases
[
in intensity with cooling down in the SmCsPR *] range. The current peak asymptotically rises just
before the transition to the SmCsPAR range at 112 °C, indicating polar clusters increasing in size
with increasing E-field strength and lowering temperature. No transition enthalpies can be
[
[
detected for the SmC[*]↔SmCsPR *] and SmCsPR *]↔SmCsPAR transitions neither on heating nor
[
on cooling, indicating continuous changes (Fig. 1a). The continuous SmC[*]-SmCsPR *] transition
can only be deduced from second harmonic generation (SHG) investigations (see below)
[
whereas the SmCsPR *]→SmCsPAR transition at T = 112 °C is evident from the change of the
shape of the polarization current response curves. At this temperature two broad polarization
peaks start to develop (Figs. 2j and S15b) which upon further lowering the temperature increase
in size, replace the single peak and come closer together (Fig. 2i→j, see also Fig. S15c).37,38
The phase transition at T = 109 °C is associated with a small transition enthalpy of ~ 0.6-
0.8 kJ/mol (Table 1) and leads to two strong and relatively sharp polarization current peaks in the
̃
SmCsPA phase range, as typical for antiferroelectrically switchable smectic phases of bent-core
molecules (Figs. 2k and S15c-e).37,38 On further cooling a transition to another
antiferroelectrically switchable LC phase (SmCsPA) is observed but without any detectable
transition enthalpy. At this transition the shape of the two polarization peaks changes, associated
with a broadening and separation of the peaks (Fig. 2k→l). The polarization values, plotted in
[
Fig. 5 (black triangles) depending on temperature are very small in the SmC[*] and SmCPR *]
ranges and difficult to separate from contributions of conductivity. This temperature range is
considered as paraelectric with almost continuously growing correlation length of the polar
fluctuations. A steep increase of the polarization is observed upon approaching the SmCsPAR
-2
̃
range, continuing in the SmCsPAR range, reaching 500 nC cm at the transition to the SmCsPA
phase and even 750 nC cm-2 at the next transition to SmCsPA. The high polarization is retained in
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the SmCsPA phase (Fig. S17); the breakdown observed in Fig. 5 is attributed to crystallization
under these conditions.
Superparaelectric
SmC
Nonpolar
SmA
Paraelectric
SmC
Modulated
Polar SmC
Polar
SmC
SmCsPA
Cr
[
SmCsPR *]
SmA
SmCs[*]
Iso
SmC̃sPA
SmCsPAR
Figure 5. Temperature dependence of the SHG signal (blue squares) compared with the
switching polarisation (black triangles) of compound AT16 as function of temperature as
measured in 6 m PI coated ITO cell under a triangular wave filed (27 Vppm-1 at 10 Hz); the
break down of Ps in the SmCsPA range is attributed to crystallization, compare with Fig. S17.
Dielectric spectroscopy. – The dielectric spectra in the range from 10 MHz to 1 Hz exhibit a
dielectric relaxation process at the temperatures below 140 °C (Figs. 6a and S18). This process
can be attributed to a collective relaxation of the transversal dipole moments of the asymmetric
mesogens. The relaxation frequency of the process is decreasing nearly linear with temperature
[
in the SmCSPR *] range. In the SmCSPAR range the decrease becomes faster and non-linear. The
dielectric strength of this mode exhibits a nearly critical behaviour in the vicinity of the
̃
SmCSPAR → SmCsPA phase transition at 109 °C. The relaxation frequency slightly increases
below the transition and continues to decrease in a linear fashion with decreasing temperature. At
the same time, the dielectric strength of this process reduces significantly in the following
̃
SmCsPA phase and only slightly increases with decreasing temperature. Electric bias field in the
̃
range up to 12V in the SmCsPA phase enhances the dielectric losses ´´ (Fig. 6b). On further
cooling, the dielectric strength starts decreasing again below 78 °C, which may be attributed to
the phase transition to SmCSPA (Fig. 6a).
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Second harmonic generation studies. - SHG studies of AT16 confirm a continuous increase of
[
the size of the fluctuating polar clusters in the paraelectric SmC range (SmCs[*], SmCsPR *]),
reaching a maximum in the SmCsPAR range just before the transition to the antiferroelectric
̃
SmCsPA phase (Fig. 5, blue squares), similar to the previously reported paraelectric-ferroelectric
transitions of 4-cyanoresorcinol BCLCs derived compounds with two terephthalate wings.80
a)
[
SmCsPR *]
SmCsPA
SmC̃sPA
SmCs[*]
SmCsPAR
b)
Figure 6. Dielectric response in compound AT16: a) Temperature dependence of the frequency
and the dielectric strength of a low-frequency mode (for 3D plot of dielectric losses as a function
of the frequency and temperature, see Fig. S18). b) Enhancement of the dielectric losses of the
̃
mode from a) in a bias field at T = 90 °C (SmCsPA).
Neither the isotropic nor the SmA phase shows any appreciable SHG activity. Field-induced
SHG first develops in the SmC[*] phase below 157 °C and grows continuously on cooling (Fig.
5). The SHG intensity increases continuously with increasing applied field without any
detectable threshold (Langevin-like, see Fig. 7a, 145 and 135 °C). Such a behaviour can be
attributed to the growing polar correlations in the paraelectric SmCs phase and an increase of the
amplitude of the fluctuating dipole moment. The SHG response increases drastically upon
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approaching the SmCsPAR range at 112 °C (Fig. 7a, 119 °C). This occurs concurrently with the
softening of the polar mode in the dielectric spectra, nearly critical behaviour of the dielectric
permittivity ´ (Fig. 6a) and a drastic increase of the switching polarisation Ps (Fig. 5). The field-
dependence of the SHG response has a rather steplike shape with a plateau at high fields and a
steep but smooth slope at a low field (Fig. 7a). This is in agreement with the fact that the current
response during the switching has broad peaks (Fig. 2j). One interpretation could be that the
polar clusters with a fluctuating size and anitpolar order develop. The threshold behaviour is only
weakly pronounced and the phase appears superparaelectric.93 The SHG response changes again
upon the transition into the antiferroelectric phase. The signal has a sharper threshold behaviour
and a plateau at high fields (Fig. 7b). Sharper current response peaks (Fig. 2k,l) accompany the
changes of the SHG activity.
a)
119 °C
124 °C
135 °C
145 °C
b)
100 °C
Figure 7. a) Field-induced SHG in the paraelectric SmCs[*] (black, red) and the randomised
[
SmCsPR *] phases (magenta, blue) of AT16; b) the step-like field dependence of the SHG in the
antiferroelectric SmCPA phase at T = 100 °C.
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[
Chiral domains and mirror symmetry beaking. - In the SmCs[*]/SmCsPR *] range of the
paraelectric SmCs phase the formation of conglomerates of chiral domains was observed in thin
cells with homeotropic alignment, as detected by rotating one of the polarizers from the crossed
position with a small angle ~ 5°-10° in clockwise direction where dark and bright domains were
observed. Rotating the polarizer with the same angle in anti-clockwise direction reverses the dark
and bright domains. Moreover, rotating the sample between crossed polarizers does not lead to
any change in the optical textures, thus confirming that chirality and not a simple director
alignment is responsible for the observed effect (see Fig. S16 and Fig. 15a,b in Section 3.5). On
the other hand, chirality is not visible in planar alignment and in freely suspended films, meaning
that the chiral conglomerate structure is stabilized in the homeotropic cells, similar to the case of
conglomerate formation in nematic phases of BCLCs.39,94 We assume that the basis of chirality
in the investigated smectic phases is the formation of a heliconical structure with the helix axis
parallel to the layer normal, due to the escape from a growing polarization in the clusters.95,84,96
This might be combined with a synchronization of helical molecular conformations of the
transiently chiral 4-cyanoresorcinol based bent-core mesogens, allowing a denser packing.25,26
The developing chirality can in addition couple via diastereomeric relations with the layer
chirality of the SmCsPF clusters, defined by tilt direction and polar direction.23 With growing
polarization and cluster size the energy gain of this cooperative coupling between transient
molecular and superstructural chirality increasingly compensates unfavourable entropic
effects.25,26 If a uniform tilt direction is stabilized by the surface anchoring of the molecules, then
the preferred tilt and chirality of the SmCsPF clusters can become biased over macroscopic
areas.25,37,38 Therefore, the formation of conglomerates of chiral domains is considered as an
indication of the appreciable correlation length of the fluctuating polar SmCsPF clusters in the
[
paraelectric SmCs range (SmCs[*], SmCsPR *]). If the polar coherence length falls below a critical
value, then only the achiral SmCs phase is observed (see compound AB12 in Table 2). Because
the chirality is not permanent, the spontaneously developing handedness is stochastic and on a
macroscopic length scale a conglomerate of the chiral domains is observed. The optical activity
can result from a long pitch heliconical structure acting as an optical wave guide, or from the
layer optical chirality of the local SmCsPF structure,97 or a combination of both. On cooling, at
[
the transition from SmCsPR *] to SmCsPAR, the chiral conglomerate structure disappears due to
the macroscopic racemic structure of this phase. This on-off switching of spontaneous
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macroscopic chirality depending on the phase symmetry is similar to that observed in cubic
phases at the Im3¯m (I432) - Ia3¯d transition.98 Though chirality changes the phase symmetry, we
[
still consider SmCsPR *] and SmCsPAR as distinct states of the same paraelectric SmCs phase,
because chirality is not completely spontaneous and requires surface support.
3.1.2 Homologous series of compounds ATn
All of the synthesized ATn derivatives with n = 12-20 exhibit essentially the same sequence of a
non-polar SmA high temperature phase, a paraelectric SmCs phase, involving the
SmCs[*]/SmCsPR *] and SmCsPAR ranges, the polar modulated SmCsPA phase and the non-
[
̃
modulated SmCsPA phases at lowest temperature (Figs. S22 and S23). In this sequence the polar
̃
coherence length continuously increases with a distinct jump at the SmCsPAR - SmCsPA
transition, the only one with H (see Fig. 1a). Already in the SmA range there are SmCs clusters
with short range correlation of tilt (de Vries type SmA phase). At the SmA-SmCs[*] transition the
tilt alignment becomes long range synclinic and polar (ferroelectric) fluctuations grow with
decreasing temperature. After reaching a critical size the polar fluctuations condense into
SmCsPF clusters which assume an antipolar correlation in the superparaelectric SmCsPAR range
and then the clusters grow further with formation of ribbons in the polarization splay modulated
̃
SmCsPA phase and finally these ribbons fuse to quasi infinite polar layers with antipolar
correlation in the SmCsPA phase, which is considered to represent a classical B2-type phase of
BCLCs (see Fig. 8).16,17,23
Compound AT8 with the shortest alkyl chain (for DSC, see Fig. S19a and POM in Fig.
S20a-e) has a reduced tilt. For this compound the Langevin type switching can already be
observed in the SmA phase before uniform tilt is established (SmAPR range below T ~ 155 °C,
see Fig. S20g).79,99 An anticlinic tilt develops together with the double peak switching, as
indicated in the planar textures by extinction crosses coinciding with the directions of the
polarizers (Fig. S20c). This provides a new SmCaPAR phase (see Fig. S19b), completing the
100
presently known series of non-tilted SmAPAR and synclinic tilted SmCsPAR phase types.38,101
Obviously the reduced tilt of the mesogens leads to a reduced layer coupling which favours the
anticlinic tilt instead of the synclinic. The anticlinic tilt is replaced by a synclinic on further
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̃
cooling in the modulated SmCsPA phase and the synclinic tilt is retained on further cooling in the
polar SmCsPA phase (Fig. S20d,e,i,j, for more details see Section S3.2 and Figs. S19-S21).
Paraelectric
SmCs
a)
b)
SmCs
[ ]
SmCs *
growing
coherence
length of polar
fluctuations
[ ]
SmCsPR *
SmCsPAR
fluctuating
polar clusters
T
c)
Modulated
SmCs
(polar
ribbons)
SmCsPA
̃
d)
Polar
layers
SmCsPA
Figure 8. Schematic presentation of the proposed development of polar order in the ground state
structures of the SmCs phases of compounds ATn; dotted lines indicate phase transitions; color
indicates chirality and dots and crosses indicate the preferred polar direction in the fluctuating
clusters; the gray molecules have a randomized polar direction.
3.1.3 The core-fluorinated compound AFT16.
Due to a higher melting point the LC range of AFT16 is narrower than that of AT16, but with
essentially the same phase sequence; i.e. the SmA phase is followed by a paraelectric SmCs
[
phase with continuous transition from SmCsPR *] to SmCsPAR, followed by phase transitions to
̃
the polar SmCsPA and SmCsPA phases. Also the textures and polarization current curves are very
similar to AT16 and indicate the occurrence of layer modulation in the temperature range
between ~110 and 121 °C (see Figs. S24 and S25), though it is not evident from the SAXS
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pattern in this case, even by using synchrotron radiation (Fig. S26). Obviously the ED
modulation along the modulated layers is smaller (reducing the peak intensity) or the correlation
length of the layer modulation is shorter (broadening the small scatterings) than for AT16.
Overall, there seems to be no significant effect of the monofluorination of a single benzene ring
on the sequence and structures of the LC phases.
3.2. Compounds Bn with a 4-hydroxybenzoate wing
All ABn compounds (n = 8, 12, 16 and 20) involving a 4-hydroxybenzoate based wing have
~30-50 K higher melting points and about 40 K lower LC-Iso transition temperatures compared
with compounds ATn (Table 2). Therefore, these compounds exhibit only monotropic LC phases
except the longest derivative AB20. The SmA phase of compounds ATn (n = 8-16) is replaced
by a skewed cybotactic nematic phase (NCybC)77 in the series of compounds ABn. The nematic
phase range decreases with growing chain length and is completely removed and replaced by a
[
paraelectric SmCsPR *] phase for the longest homologue AB20. With lowering temperature the
[
paraelectric SmCsPR *] and SmCsPAR ranges are replaced by a series of polar SmC phases
̃
(SmCsPA, SmCs’PA, SmCsPA), which have almost identical textures (Fig. S27c-e), but all are
separated by distinct phase transitions with small transition enthalpy values (0.3-0.5 kJ mol-1, see
Fig. 9 and Table 2).102
The X-ray diffraction pattern of a magnetically aligned sample of the nematic phase of
AB16 (Fig. 11a) indicates a tilt angle of ~25° (Fig. S28). This means that already in the nematic
phase the tilt is relatively large, comparable with that found for the SmCsPA phase of AT16 at
[
lowest temperature. At the transition from the NCybC to the SmCsPR *] phase at T = 121 °C the
alignment is lost and therefore no direct determination of the tilt angle was possible. However,
the d-value of the layer reflection in the smectic phases (Fig. 11b, d = 4.3-4.6 nm) is significantly
smaller than the related values recorded for compound AT16 (Fig. 3c, d = 5.1-5.3 nm) though
having the same molecular length (Lmol = 6.7 nm). This is in line with a much larger tilt of
compound AB16 in the smectic phases. The layer distance d decreases with decreasing
temperature, indicating a further growing tilt of the molecules on cooling (Fig. 11b).
Table 2. Transitions of compounds ABn and AFB16 on heating (H) and cooling (C). a
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T /ºC
Comp.
X
H
n
[ΔH/kJ·mol-1]
8
H: Cr 128 Iso
AB8
[66.0]
C: Iso 126 NCybC 126 SmCs 98 Cr
[0.9]
[1.1]
[46.9]
AB12
AB16
AB20
AFB16
H
H
H
F
12 H: Cr 126 Iso
[46.1]
̃
C: Iso 117 NCybC 89 SmCs 77 SmCsPAR 67 SmCsPA
[0.8]
16 H: Cr 123 Iso
[46.5]
[1.2]
[0.9]
[0.4]
[
C: Iso 123 NCybC 121 SmCsPR *] 96 SmCsPAR 80 SmCsPA 73 SmCs’PA 66 SmCsPA 48 B5 36 M
̃
[1.7]
20 H: Cr 123 SmCsPR *] 132 Iso
[49.5] [5.6]
C: Iso 130 SmCsPR *] 98 SmCsPAR 75 SmCsPA 36 Cr
[5.5] [-] [0.3] [15.0]
16 H: Cr 88 SmCsPAR 105 SmCsPR *] 132 Iso
[19.6] [-] [5.6]
C: Iso 131 SmCsPR *] 104 SmCsPAR 87 SmCsPA 84 SmCs’PA 60 Cr
[5.5] [-] [0.3] [0.4] [48.0]
aTransition temperatures and enthalpy values were measured as mentioned in Table 1; Abbreviations: NCybC
[1.5]
[-]
[0.5]
[0.4]
[0.4]
[3.4] [5.8]
[
[
̃
[
[
̃
=
nematic phase with cybotactic clusters of the SmC-type; SmCsPA, SmCs’PA = different types of antiferroelectric
switching polar and synclinic tilted SmCs phase; B5 = hexatic SmCsPA phase and M = unknown (soft) crystalline phase;102
for other phase assignments see Table 1.
Synchrotron SAXS confirms a layer modulation in the temperature range between 73-80
̃
°C (SmCsPA phase, Fig. S29). However, as in the case of compound AT16 there is only a limited
number of very weak additional reflections besides the layer reflection (Fig. S29b).88 This
̃
indicates the presence of a layer modulation (SmCs phase) though the precise molecular
organization is still unknown.88 Hence, it appears that layer modulation is a general feature of the
transition from (super)paraelectric to macroscopically polar phases of bent-core mesogens.
Electro-optical investigations confirm that only non-polar SmCs phases are formed below
the NCybC phases of compounds AB8 and AB12 with relatively short chains (Table 2). The
absence of a polarization peak indicates a short coherence length of polar order in the SmC
phases of AB8 and AB12 which is in line with the absence of mirror symmetry breaking in these
SmCs phases. For compounds AB16 and AB20 with longer chains the achiral SmCs phase is
[
replaced by a chiral paraelectric SmCsPR *] phase (see Fig. S30), in line with an increased
coherence length of the polar order. Two widely separated and relatively sharp polarization
current peaks located at the voltage maxima start to develop in the SmCsPAR range (Fig. 10b). In
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̃
the following polar SmC phases (SmCsPA to SmCsPA) the polarization values rise further to
almost 1000 nC cm-2 (see Figs. 10 and 12, for more details, see Section S3.6).
SmC P
A
s
Iso
Iso
Cr
M
B5
Heating
[
R
]
SmC P
*
s
B5
SmC P
M
A
s
SmC P
s
AR
SmC 'P
s
N
CybC
A
Cooling
~
SmC P
A
s
20
40
60
80
T / °C
100
120
140
Figure 9. DSC heating and cooling traces of compound AB16 at a rate of 10 K min -1; dashed
line indicates continuous transition.
120
120
a) 105 o
C
b)
b) 90 °C
0.6
a) 90
90 °C
90
60
105 °C
0.2
0.0
-0.2
0.4
60
30
0.2
30


0
0
0.0
-30
-60
-90
-120
-30
-60
-90
-120
-0.2
-0.4
-0.6
-0.02
0.00
0.02
0.04
0.06
0.08
-0.02
0.00
0.02
0.04
0.06
0.08
t /s
t /s
120
d) 120
c)
77d°) C77 °C
0.8
1.2
90
60
e) 70 °C
90
70 °C
0.6
0.8
60
0.4
0.4
30
30
0.2

0
0.0

0
-30
0.0
-30
-60
-90
-120
-0.4
-0.8
-1.2
-0.2
-0.4
-0.6
-0.8
-60
-90
-120
-0.02
0.00
0.02
0.04
0.06
0.08
-0.02
0.00
0.02
0.04
0.06
0.08
t /s
t /s
Figure 10. Switching current response curves of compound AB16 as measured on applying a
triangular wave field (27 Vppm-1, 10 Hz, 6 m PI coated ITO cell) at the indicated temperatures.
The third peak in d) is also observed for the related compound with two azobenzene wings37b and
might indicate two different relaxation mechanisms of the ferroelectric to the AF states, which
might be due to the anchoring effect of the surfaces (see also Fig. S27).
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a)
b)
SmCs’PA
SmCsPAR
NCybC
SmCsPA
SmC̃sPA
[
SmCsPR *]
Figure 11. XRD investigations compound AB16: a) diffraction pattern of the NCybC phase of a
magnetically aligned sample at 123 °C (for a scan over this pattern, see Fig. S28); b) temperature
̃
dependence of the d-value and c) synchrotron SAXS pattern in the SmCsPA phase; for additional
XRD data, see Fig. S25.
Dielectric spectra of compound AB16 in the range from 10 MHz to 1 Hz are presented in Fig.
13a. A relaxation process, related to a collective relaxation of the transversal dipole moments, in
the range of 10 kHz – 100 kHz was observed. The dielectric strength continuously increases in
the range of the paraelectric SmCsPR *] range and in the SmCsPAR range exhibiting a subcritical
[
̃
behavior at the transition to the antiferroelectric SmCsPA phase at 80 °C. The critical character of
the behavior of Δr is less pronounced compared to AT16 (Fig. 6a). Especially, there is no strong
discontinuity of the dielectric permittivity across the paraelectric-antiferroelectric transition.
Plotting inverse value of Δr, we find nearly linear dependence of it on the temperature, which
can be extrapolated to the Néel temperature TN = 56 °C (Fig. S31a), the temperature of the
transition to uniformly polar layers. This temperature is significantly lower than the actually
̃
observed SmCsPAR-to-SmCsPA transition and suggests that only between 48 and 66 °C a typical
B2 type SmCsPA phase composed of alternating ferroelectric layers is formed. Comparing the
dielectric behavior with the dependence Ps(T) (Fig. 12), we find that the subcritical Δr (T) also
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corresponds to a nearly continuous increase of the switching polarization in the
[
SmCsPR *]/SmCsPAR phase range. This suggests that the SmCs’PA and SmCsPA phases of AB16
̃
are still of the cluster type and the long-range ferroeletric order in the layers is mostly suppressed
at high temperatures. Therefore, only the low temperature polar SmCs phase is designated as
SmCsPA whereas all polar smectic phases occurring at a higher temperature and having a shorter
polar coherence length can be considered as superparaelectric and are assigned as SmCs’PA and
̃
SmCsPA, respectively. The decrease in temperature from 120 °C to 70 °C leads to nearly linear
decreasing of the relaxation frequency in all SmCs phases (Fig. S31b). The activation energy of
the relaxation process is about 53 kJ/mol for the paraelectric phase and Ea = 47 kJ/mol for the
antiferroelectric phase. At the same time applied bias electric fields in the range up to 12V in the
̃
SmCSPA phase at 80 °C enhances the dielectric loses ε (Fig. 13b) due to the growing polar cluster
̃
size, but this field induced increase is less pronounced than in the modulated SmCsPA phase of
compound AT16 (Fig. 6b).
Superparaelectric
SmC (fluctuating polar
clusters)
Polar
SmCaPA
(polar layers)
Nonpolar
NCybC
Polar SmC
(large clusters)
Paraelectric SmC
(polar fluctuations)
1000
800
600
400
200
0
SmC P
s
AR
N
CybC
SmC P
s
A
Iso
[
*
]
SmC 'P
SmC P
s
s
A
R
~
SmC P
s
A
50
60
70
80
90
100 110 120 130
T / °C
Figure 12. Polarization as function of temperature as measured for compound AB16 in 6 m PI
coated ITO cell on applying a triangular wave field (27 Vppm-1 at 10 Hz).
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a)
SmC’sPA
SmCsPA
[
SmCsPR *]
SmCsPAR
SmC̃sPA
b)
Figure 13. Dielectric investigation of AB16. a) Temperature dependence of the frequency and
the dielectric strength of a low-frequency mode. b) Enhancement of the dielectric losses of the
mode from a) in a bias field at T = 80 °C in SmCSPA shortly after transition from SmCaPAR.
These observations suggest that polar order develops more smoothly in the paraelectric
SmCs phase of compound AB16 compared to AT16 (compare Figs. 6a and 13a). This may be
attributed to a reduced packing density for compound ABn with relatively electron rich
aromatics in the 4-hydroxybenzoate wing and the stronger tilt achieved in their self-assembly.
However, the polarization values measured under an applied electric field in the polar smectic
phases are high in both series of compounds, being 750 nC cm-2 for AT16 (Fig. 5) and reaching
even 900-1000 nC cm-2 for AB16 (Fig. 12). An increased flexibility of the phenylbenzoate wings
of compound ABn in the less densely packed paraelectric phases might require lower
temperatures to reach a critical coherence length of polar order (80 °C vs 110 °C) and also leads
to an expansion of the SmCaPAR phase range. Once polar order is established, the packing
density increases, giving rise to SmCaPA subtypes (compare Figs. 6a and 13a). The larger
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collective contribution of the C=O dipoles to the transversal molecular dipole moment along the
polar director might contribute to the enhanced polarization of AB16 after achieving a dense
packing in the SmCsPA phase.
A comparison of compounds ATn and ABn with related symmetric compounds is given
in Section S4 of the SI (Table S2).
3.3 Compounds AFB16 with fluorinated azobenzene side-arm
For the fluorinated compound AFB16 the melting point (88 °C) is much lower compared to that
of AB16 (123 °C), leading to additional enantiotropic LC phases. Moreover, the nematic phase
formed by AB16 is completely removed (Table 2). The LC phases and the phase sequence of
AFB16 are almost identical with those of AB20, suggesting that reducing the electron density of
the core by fluorination has the same effect as elongation of the alkyl chain. Hence, fluorination
in the case of compounds ABn is a useful tool for expansion of the LC phase region; more
details can be found in Section S3.7 (Figs. S32-S35).
3.4 Isothermal photo-switching between FE-like and AF-like, and between chiral and
achiral states
The azobenzene unit involved in the molecular structures allows photoinduced
isomerization of the azobenzene unit under UV irradiation (for solution spectra, see Fig. S36)
which in turn can be used to modify the materials properties. Previous attempts of photoinduced
switching with azobenzene based BCLCs were based on a transition of the antiferrolelectric B2
or the B7 phases into the isotropic liquid state which reduces Ps,52,53 whereas here we report a
photoswitching of the mode of switching under an E-field as well as an on/off switching of
superstructural chirality.
In the first case, a sample of compound AB16, placed in a 6 m ITO cell on a
temperature controlled heating stage, was illuminated at T = 101 °C, i.e. close to the SmCsPAR-
[
SmCsPR *] transition temperature, by light with 405 nm wavelength for 2 s. The antiferroelectric-
like double peak switching changes into a ferroelectric-like single peak switching and relaxes
back to the AF-like mode almost immediately after switching off the light source (< 3 s; see Fig.
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10.1002/chem.201806180
Chemistry - A European Journal
14). This allows for the first time a photoinduced change of the mode of the switching. That the
switching is not due to any thermal effect was checked by control experiment with LC
compounds without any azobenzene core as described in Section S5 in the SI (Figs S39-S41).
120
80
0,15
0,10
0,05
0,00
-0,05
-0,10
-0,15
a)
40
0
-40
-80
-120
-0,02
0,00
0,02
0,04
0,06
t /s
Thermal
relaxation
h
(405 nm)
120
80
b)
0,05
0,00
-0,05
40
0
-40
-80
-120
-0,02
0,00
0,02
0,04
0,06
t /s
Figure 14. Reversible isothermal switching between a) antiferroelectric-like and b) ferroelectric-
like switching in the paraelectric SmC phase range as recorded for compound AB16 at T = 101
°C in a 6 m ITO cell; a related switching of compound AT16 is shown in Fig. S37.
Likewise, the symmetry broken SmCs[*] phase of AT16 was successfully switched to the
achiral SmA phase under light irradiation (405 nm, 5 mW/mm2 see Fig. 15a,b→c). The transition
to a SmA phase is evident from the optical isotropic appearance in homeotropic alignment,
accompanied by the formation of birefringent defects (Maltese crosses, see Fig. 15c). Also this
process is fast and reversible (< 3s). After switching off the light, the chiral domains almost
immediately (within < 3 s) reform (Fig. 15c→d,e). However, there appears to be no chiral
memory, meaning that the positions of the chiral domains change during the switching process
(Fig. 15a,b→d,e). This is attributed to the fluidity of these smectic phases, leading to movement
of the material, but it also shows that the chirality in these phases is not based on a chiral surface
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10.1002/chem.201806180
Chemistry - A European Journal
effect. The change of molecular shape from a rather well-defined bent shape to a more non-
specific one by trans→cis photoisomerization leads to a decrease in the packing density and a
reduction of the coherence length of polar order below a critical value, thus changing the mode
of polar switching. The reduced packing density is also responsible for the observed
photoinduced on-off switching of chirality.
[
SmCs *]
a)
b)
(+)
(+)
(-)
(-)
50 m
h
(405 nm)
c)
SmA
achiral
Thermal
relaxation
SmCs[*]
e)
d)
(-)
(-)
(+)
(+)
Figure 15. Reversible isothermal photo-on-off witching of chirality as observed for AT16 at T =
152 °C in a homeotropic cell (ca. 10 m); the inset in c) shows an enlarged region, indicating
that the small spots in c) represent Maltese crosses as typical for defects in homotropic SmA
phases (see also Fig. S38).
In contrast to previous reported work on chirality modulation based on photoisomerization
in LCs, the chirality is in this case not based on a molecular chirality,46,47,103,104,105 but the
chirality is spontaneously formed by molecular self-assemly of achiral molecules in the LC state.
A related on-off switching of this kind of superstructural chirality was only reported for a
polycatenar compound48 and for the NTB-N transition in mesogenic dimers.67 In the case of
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10.1002/chem.201806180
Chemistry - A European Journal
mesogenic dimers uniform chirality has been achieved with circular polarized light during the
growth of soft crystalline HNF phases.56,106 In another report a mirror symmetry broken HNF-
like soft crystalline phase with conglomerate structure, in this case formed by a mesogenic
trimer, was switched by UV irradiation into an achiral phase and then thermally relaxes back to
the soft crystalline symmetry broken DC phase.107 However, there were no reports of
photoinduced chirality switching in any fluid polar smectic or dark conglomerate phase of
achiral BCLCs. The reason might be, that in these densely packed polar smectic phases the
molecular shape is strongly fixed in the binding sites provided by the surrounding bent and polar
ordered molecules and in this way trans-cis photoisomerization is efficiently inhibited. In the
cases reported here, only the correlation length of the polar order is modified which takes place
much easier and therefore photoisomerization can easily change the molecular shape.
4. Summary and Conclusions
In summary, we have reported the synthesis and investigation of two new series of non-
symmetric photosensitive bent-core molecules consisting of 4-cyanoresorcinol as central core
and two different wing groups, an azobenzene and a phenylbenzoate wing with two distinct
directions of the COO groups (ATn and ABn). Compounds with a terephthalate-based wing
ATn show broader LC phases, lower melting temperatures and a smaller tilt compared to the
phenylbenzoate analogues ABn, while core fluorination in both series retains the fundamental
phase sequence.
In the two series of compounds there is a transition from non-polar de-Vries-like SmA
(ATn) or from NCybC phases (ABn), both formed by small synclinic SmCs clusters via
paraelectric SmC phases to antiferroelectric SmCPA phases upon lowering temperature and thus
increasing packing density. At the transition from the paraelectric phases with short correlation
[
length of polar order (fluctuating clusters in SmCs[*] and SmCsPR *]) to the polar smectic phases
(SmCs’PA and SmCsPA) at first the polar correlation becomes antipolar already in the
superparalectric range of the SmC phase (SmCPAR) and then the layers become modulated due to
̃
the developing splay of polarization (SmCsPA), before the non-modulated antiferroelectric SmC
phases (SmCs’PA and SmCsPA) with quasi infinite polar layers but antipolar correlation between
them (B2 type polar “banana” phases) are formed (see Fig. 8).17,19,21 Only for the short-chain
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10.1002/chem.201806180
Chemistry - A European Journal
compound AT8 the tilt is relatively small, leading to a non-tilted and achiral paraelectric SmAPA
phase followed by a transition to an anticlinic SmCaPAR phase.
Mirror symmetry breaking by surface assisted formation of chiral conglomerates sets in at
a distinct coherence length of the fluctuating SmCsPF clusters in the homeotropically aligned
[
paraelectric SmCs phases (SmCs[*] and SmCsPR *] ranges) and these are considered as long pitch
heliconical SmCs phases.
For some of the compounds we report for the first time a photoinduced switching of
azobenzene derived BCLCs between a ferroelectric-like switching and an antiferroelectric-like
switching, and the reversible photo-induced on-off-switching of chirality. This is based on the
limited correlation length of the polar order in the paraelectric and superparaelectric SmC ranges
which can easily be affected by the change of the molecular shape due to the trans-cis
isomerization process and this represents a new concept for developing photoswitchable LC
materials towards specific applications.
Acknowledgements
The work was supported by the DFG (Grants Ts 39/24-2 and ER 467/8-2), the National Natural
Science Foundation of China (No. 21761132033, 21374086). Nerea Sebástian acknowledges the
financial support of Humboldt Foundation. We thank Beamline BL16B1 at SSRF (Shanghai
Synchrotron Radiation Facility, China) for providing the beamtimes.
Keywords: Bent-core liquid crystals; azobenzenes; mirror symmetry breaking; ferroelectricity;
photoswitching
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