Development of Polar Order by Liquid-Crystal Self-Assembly of Weakly Bent Molecules

Article abstract of DOI:10.1002/chem.201606035

Organic ferroelectrics are of growing importance for multifunctional materials. Here we provide an understanding of the distinct stages of the development of sterically induced polar order in liquid-crystalline (LC) soft matter. Three series of weakly bent molecules derived from 4-cyanoresorcinol as the bent core unit with laterally fluorinated azobenzene wings have been synthesized, and the effects of the position of fluorine substitution, alkyl-chain length, and temperature on the LC self-assembly and polar order were studied. In the LC phases a paraelectric–ferroelectric transition took place as the size of the polar domains gradually increased, thereby crossing a permittivity maximum, similar to inorganic solid-state ferroelectrics. An increase in polar coherence length simultaneously led to a transition from synpolar to antipolar domain correlation in the high-permittivity paraelectric range. Associated with the emergence of polar order was the development of a tilted organization of the molecules and a growing coherence of tilt. This led to a transition from non-tilted via tilt-randomized uniaxial to long-range-tilted biaxial smectic phases, and to surface-stabilized symmetry breaking with the formation of chiral conglomerates and field-induced tilt. Moreover, there is a remarkably strong effect of the position of fluorination; polar order is favored by peripheral core substitution and is suppressed by inside-directed fluorination.

Full text of DOI:10.1002/chem.201606035

A Journal of
Accepted Article
Title: Development of Polar Order by Liquid Crystal Self-assembly of
Weakly Bent Molecules
Authors: Mohamed Alaasar, Marko Prehm, Silvio Poppe, and Carsten
Tschierske
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10.1002/chem.201606035
Chemistry - A European Journal
Development of Polar Order by Liquid Crystal Self-assembly of
Weakly Bent Molecules
Mohamed Alaasar,[a,b]* Marko Prehm,[a] Silvio Poppe,[a] Carsten Tschierske[a]*
[a] Dr. Mohamed Alaasar, Dr. Marko Prehm, Silvio Poppe and Prof. Dr. Carsten Tschierske
Institute of Chemistry,
Martin Luther University Halle-Wittenberg,
Kurt Mothes Str. 2, D-06120 Halle (Saale), Germany;
carsten.tschierske@chemie.uni-halle.de
[b] Dr. Mohamed Alaasar,
Department of Chemistry,
Faculty of Science,
Cairo University, Giza, Egypt;
malaasar@sci.cu.edu.eg
Abstract: Organic ferroelectrics are of growing importance for multifunctional materials. Here
we provide an understanding of the distinct stages of the development of sterically induced polar
order in liquid crystalline (LC) soft matter. Three series of weakly bent molecules derived from
4-cyanoresorcinol as the bent core unit with laterally fluorinated azobenzene wings have been
synthesized and the effects of the position of fluorine substitution, alkyl chain length and
temperature on the LC self-assembly and polar order were studied. In their LC phases a
paraelectric-ferroelectric transition takes place by gradual increase of the size of polar domains
and by crossing a permittivity maximum, similar to inorganic solid state ferroelectrics. Increasing
polar coherence length simultaneously leads to a transition from synpolar to antipolar domain
correlation in the high permittivity paraelectric range. Associated with the emergence of polar
order is the development of a tilted organization of the molecules and a growing coherence of tilt.
This leads to a transition from non-tilted via tilt randomized uniaxial to long range tilted biaxial
smectic phases, to surface-stabilized symmetry breaking with formation of chiral conglomerates
and field-induced tilt. Moreover, there is a remarkably strong effect of the position of
fluorination; polar order is favored by peripheral core substitution and it is suppressed by inside
directed fluorination.
1
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1. Introduction
Organic compounds with extended aromatic building blocks receive growing importance
as charge carrier and optical materials. The majority of these materials are composed of a
functional -conjugated core unit and flexible chains providing processibility due to reduced
melting points and increased solubilities, while retaining the organization of the functional
units.[1] However, the alkyl chains also have an active function as they can modify the mode of
self assembly and thus can lead to the emergence of new properties and functionalities. In the
ideal case this gives rise to the liquid crystalline (LC) state, intrinsically combining long range
order with mobility. In these LC phases the organization of the aromatic cores can be addressed,
by different kinds of external stimuli.[2] Liquid crystals (LCs) are nowadays considered as a
fourth state of matter providing a wide range of soft functional materials[3] for numerous
application in our daily life, such as liquid-crystal displays (LCDs), adjustable optical elements,
tunable lasers, semiconducting layers in organic field effect transistors,[4] light emitting diodes,
photovoltaic cells and photo-recording devices,[5,6] thus making them to that supramolecular
systems with the largest impact on the global economies.[7] In the wider field of materials, the
organic ferroelectrics,[8] especially those with LC properties[9,10] arose significant interest and the
discovery of ferroelectricity, antiferroelectricity and spontaneous mirror symmetry breaking in
bent-core liquid crystals (BCLCs) breathed a new area in the field of LCs.[11-13] In these
ferroelectric and antiferroelectric materials polar order is due to the restricted rotation of BCLCs
around their long axis (Scheme. 1a). However, as in these materials the polar order is often
combined with a tilt of the molecules with respect to the layer normal, the symmetry is reduced to
C2 leading to superstructural layer chirality (Scheme 1b).[13,14] Previous research in this field was
predominately focused on high polarization ferroelectric and antiferroelectric bent-core
molecules with a bending angle  ~ 120° (Scheme 1c).[12,13] However, also high permittivity
paraelectrics and related materials with medium range polar order are of significant interest for
super-capacitors and other applications.15 Therefore, molecules at the cross over between rod-like
( ~ 180°) and bent shapes ( ~ 120°),[16-20] hockey stick molecules, having the bend shifted to
one end[21-24] and molecules with comparatively short bent-core units.[25,26] receive growing
interest.
The aim of this article is to provide an understanding of the development of polar order in
lamellar liquid crystalline phases at the cross-over from rod-like to bent molecular shapes
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depending on alkyl chain length, temperature and position of core fluorination. To this end we
have synthesized and investigated a series of new bent-core molecules based on a central 4-
cyanoresorcinol core with fluorinated rod-like azobenzene side groups (Scheme 1c).
<Scheme 1>
The 4-cyanoresorcinol bisbenzoate core unit[27,28] was chosen because of its comparatively
weak and temperature dependent molecular bend, ( ~ 130-145°) arising from the effects of the
CN group on the conformation of the adjacent ester group.[16,27a,29,32a,b] Azobenzenes were used as
peripheral units for their capability of light-induced trans–cis isomerisation which is of interest
for potential application of the compounds as multifunctional photochromic and
photoisomerizable materials.[30-32] Due to the unique combination of high polarity and low
polarizability, as well as steric and conformational effects33 fluorination of the aromatic core is a
powerful tool for tailoring rod-like LC materials for modern LC technologies[34-35] and bent-core
mesogens for new applications.[13,16,36,37] Fluorine substitution of the weakly bent mesogens Hn32
in the peripheral 3-positions, adjacent to the terminal alkoxy chains, leads to the new series of
compounds 3Fn (n = 4-20, even numbered chains) reported herein (Scheme 1c). Only compound
3F20 with the longest chains has been described previously.[37] The investigation of the chain
length and temperature dependent development of distinct LC phase structures and polar order in
the complete series 3Fn is reported herein, which provides new phases and phase sequences and
leads to the universal phase diagram shown in Figure 1a, revealing a gradual development of
polar order via low and high permittivity paraelectric phases with increasing size of ferroelectric
domains (Figure 1b). Shifting the position of fluorine from the peripheral 3-positions to the inner
2-positions (compounds 3Fn with n = 8-16) or adding F-substituents to the 2-positions
(compounds 23Fn with n = 8-16) removes polar order. Replacing the fluorine in 3F18 by the
larger bromine reduces polar order, which is completely removed if it is replaced by the non-
polar methyl group. These investigations allow a general understanding of the development of
polar order in soft matter and the unusual phenomena associated with the formation and growth
of polar domains, as for example, spontaneous mirror symmetry breaking and electric field
induced tilt by a non-classical electroclinic effect, and the relations of these phenomena to the
molecular structure.
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2. Results and Discussion
2.1 Synthesis
<Scheme 2>
The synthesis of the new fluorinated compounds 3Fn (n = 4-18), 2Fn and 23Fn and the
related brominated and methylated compounds Br18 and Me18, respectively, was performed as
shown in Scheme 2. The target materials were obtained by acylation of 4-cyanoresorcinol (5)
with two equivalents of the appropriate azobenzene-based benzoyl chloride 4an-4en.[38] All
acylation reactions were carried out in the presence of triethylamine as base and pyridine as
acylation catalyst.[37] The benzoic acids 3an-3en were synthesized in a coupling reaction between
substituted phenols and the diazonium salt of ethyl 4-aminobenzoate affording the azobenzenes
(1a-e) in a first step. This was followed by etherification of 1a-e with different 1-bromoalkanes to
give the ester compounds 2an-2en, which were then hydrolysed under basic conditions yielding
the desired acids 3an-3en.[38]
2.2 Development of tilt and polar order in the LC phases of 3-fluorinated compounds (3Fn)
<Table 1, Figure 1>
The phase sequences, transition temperatures and the associated transition enthalpies of the series
of even numbered homologous of compounds 3Fn (n = 4 - 20) are collated in Table 1 and shown
graphically in Figure 1a. The phase abbreviations are explained in Table 1 and Figure 1b and will
be elaborated in the course of this section. In brief, SmA and SmC designate lamellar phases with
non-tiled or tilted organization of the molecules in the layers, respectively, the suffixes PR and
PAR indicate high permittivity paraelectric phases composed of ferroelectric domains, PA
indicates antiferroelectric LC phases with macroscopic polar order and [*] indicates the formation
of chiral conglomerates in the corresponding phases (see Section 3.1.5).
As schematically shown in Figure 1 all compounds 3Fn with n = 12-20 exhibit a uniaxial
SmA phase range at high temperature followed on cooling by biaxial LC phases representing
[
tilted paraelectric (SmCs[*], SmCsPR *], SmCsPAR) and polar (SmCsPA, SmCaPA) smectic phases
(Table 1). In the LC temperature range only the SmA-SmC and the paraelectric-antiferroelectric
transitions are associated with enthalpy changes, whereas the other transitions are continuous. For
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the short chain compounds (n = 4-10) other phase sequences were observed. Thus, for chain
lengths n = 6-10 uniaxial high permittivity paraelectric smectic phases (SmAPR and SmAPAR)
develop, while compounds with n = 4 and 6 form cybotactic nematic phases (NCybA). In the
following, the development of the distinct LC phases will be discussed step-by-step, starting with
the nematic phases. In Sections 2.3 and 2.4 the series 3Fn will be compared with related
compounds and the effects of changing the position of fluorination will be discussed.
2.2.1 Compounds 3F4 and 3F6 and the non-skewed cybotactic nematic phases
The shortest homologue of the series 3Fn with n = 4 forms a monotropic (metastable) nematic
phase at T = 136 °C which rapidly crystallizes at T = 97 °C, whereas the nematic phase of the
next higher homologue 3F6 is enantiotropic. The XRD pattern of an oriented sample of the
nematic phase of 3F6 in a magnetic field (B ≈ 1 T, see Figure 2a,b) shows a diffuse scattering
with a maximum at d = 0.45 nm located on the equator and a diffuse scattering with a single
maximum at d = 3.88 nm on the meridian, being slightly shorter than the molecular length Lmol
=
4.4 nm (determined for an assumed -shaped conformation with 120° bending angle and
stretched all-trans alkyl chains, see Figure S9). This diffraction pattern indicates a nematic phase
[39,40]
composed of cybotactic clusters with local SmA structure (NCybA
)
which is rarely observed
for bent-core mesogens, usually preferring tilted arrangements.[28]
<Figure 2>
2.2.2 Uniaxial SmA and SmAPR phases
At T = 110 °C the diffuse scattering in the small angle region of the diffraction pattern of 3F6
transforms into a sharp Bragg peak with d = 4.0 nm as typical for a transition to a smectic phase
(Figure 2c, d). The maximum of the diffuse scattering in the wide angle region remains
perpendicular to the small angle reflection, indicating an on average non-tilted organization of the
molecules in the layers. The d-spacing in this SmA phase is only a bit smaller than the molecular
length (d/Lmol = 0.91 nm), in line with a monolayer SmA structure. This phase assignment is in
agreement with optical investigations between crossed polarizers, indicating typical fan textures
in planar alignment (molecules parallel to the surfaces) with dark extinction crosses being parallel
to the directions of polarizer and analyzer (see inset in Figure 2c and Figure S7b). In homeotropic
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cells (layers parallel to the surfaces) the textures appear completely dark between crossed
polarizers, i.e. this phase is optically uniaxial as typical for the SmA phase. SmA phases are
formed by all compounds 3Fn with n ≥8, being the first LC phase observed on cooling from the
isotropic liquid state (Table 1, Figure 1a).
<Figure 3>
As shown in the plots in Figure 3, in the high temperature range of all SmA phases (including
those designated as SmAPR) an increase of the layer spacing d is observed on cooling, which is
due to an increasing packing density of the aromatic cores with decreasing temperature. The
denser packing leads to an alkyl chain stretching and this increases the layer spacing d. However,
the slope of the d = f(T) function decreases until a maximum is reached and then d decreases on
further cooling. The decrease of d starts already in the temperature range of the uniaxial SmA
phase and therefore is attributed to a developing tilt with formation of randomly aligned tilt
domains (de Vries-like smectic phase[41]) and partially also to a reduction of the effective
molecular length by decrease of the angle  between the two rod-like azobenzene units (Figure
1c), thus favouring the development of polar domains. On further cooling the formation of tilted
smectic phases (SmC) is observed for all compounds with n > 4, which is in line with the
proposed contribution of tilt to the layer shrinkage. With growing alkyl chain length (n) the SmC
range broadens and the temperature range of the SmA phase becomes smaller and it is shifted to
higher temperatures till n = 16, but even for the longest homologues (n = 18, 20) a small range of
the SmA phase is retained (Figure 1a). The response of the SmA phases to an applied electric
field changes with chain length, applied field strength and temperature. The remarkable feature of
the short chain compounds 3F6, 3F8 and 3F10 is, that in the whole SmA range a relatively broad
single current peak is reproducibly observed in each half period of an applied triangular wave
field (Figures 4a and S13). This peak is thought to be the result of a Langevin-type growth and
switching of ferroelectric domains under the applied E-field, as known for SmAPR phases.[42,43,44]
The current peak occurs above a certain threshold field which increases with growing chain
length. Similarely, under a fixed field strength (E = 27 Vppm-1) the temperature of emergence of
this peak is continuously shifted to lower temperature with growing chain length (Table 1 and
Figure 1a). For compound 3F12 there is no current peak in the SmA range and the emergence of
[
the current peak coincides with the SmA to SmC phase transition (SmA - SmCsPR *]). For the
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following homologues (n > 12) the current peak appears only after the SmA-SmC transition
[
within the SmC range, leading to SmCs[*] - SmCsPR *] transitions. The single peak switching in
[
the SmCsPR *] ranges is associated with an increase of the birefringence of the planar sample
under the applied E-field (Figure S16i-k), confirming that this peak is indeed due to a
polarization. The polarization calculated from the peak area is the largest for the SmAPR phase of
3F6 (P = ~160 nC cm-2) and decreases with growing chain length to only ~50 nC cm-2 in the
[
SmCsPR *] ranges of compounds 3F12-3F18. No polarization peak can be observed in the SmA
phases of the long chain compounds 3F10-3F20, considered as low permittivity paraelectric SmA
phases, in contrast to the SmAPR (and SmAPAR) phases which are considered as high permittivity
paraelectric phases.
2.2.3 The uniaxial SmAPAR range of compounds 3F6 and 3F8
<Figure 4>
Within the uniaxial smectic phases of compounds 3F6 and 3F8 with shortest chains the mode of
switching changes below a certain temperature (Figures 4 and S11). For 3F8, for example, the
broad single peak in the SmAPR range (Figure 4a) is replaced by two sharp current peaks per half
period of the applied triangular wave voltage at T = 108 °C and 20 Vppm-1 (Figure 4b). These
current peaks are positioned at the maximum voltages and merge closer to each other on further
cooling (Figure 4c,d). At the transition to this uniaxial SmA phase, designated as SmAPAR, (for
textures see Figure 4e with inset) the polarization jumps from 150 to ~700 nC cm-2 and then
further grows to ~800 nC cm-2, indicating the development of an almost macroscopic polarization
under the applied field (Figure 5a). The absence of an enthalpy change at this transition (see
Table 1 and Figure S6a) means that despite of the high polarization values this phase range is still
paraelectric. Moreover, there is a strong field dependence of the temperature of occurrence of the
two switching peaks, being shifted to higher temperature with growing strength of the applied E-
field (T = 107 °C for 16 Vppm-1, 110 °C for 27 Vppm-1 and 112 °C for 33 Vppm-1, see Figure
S12). For 3F6 the same behaviour as for 3F8 is found, but the two widely separated current peaks
emerge at slightly lower temperature (T = 101 °C and 27 Vppm-1, see Figure S11).
<Figure 5>
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Optical investigations of the SmAPAR range indicates a planar fan-texture with dark extinctions
parallel to the directions of the polarizers (Figure 6a) which is changed under the applied E-field
to a texture composed of dark and bright (orange) areas (Figure 6c). In Figure 6a,c and e the
alignment of the layers is slightly inclined with the orientation of the crossed polarizers and
rotation of the sample to the opposite direction of inclination exchanges the brightness of the
domains. This indicates a field induced uniform (synclinic) tilt, having opposite tilt direction in
the distinct areas (Figure 6c,d).
< Figure 6>
Upon removal of the field the direction of tilt is retained (Figure 6e,f), indicating that the
synclinic tilted state is preserved at E = 0; thus, we conclude on a uniaxial smectic phase
composed of ferroelectric SmCsPS domains being tilt-randomized in the pristine state (Figure
6a,b). Under an applied field the domains grow and adopt uniform tilt correlation, leading to a
field-induced biaxial and synpolar SmCsPS state (Figure 6c,d) which relaxes to an antipolar
SmCsPA state at E = 0 (Figure 6e,f). This field induced formation of a tilted organization could be
considered as a non-classical kind of electroclinic effect,[45,22] or alternatively, as a transition to a
field induced and surface stabilized state.[16b,46] During relaxation and also after field reversal the
tilt direction is retained, indicating an antiferroelectric switching by collective rotation around the
molecular long axis (Figure 6d,f). This range of the uniaxial SmA phase, representing a lamellar
phase composed of tilt randomized SmCsPS domains and having a dominating antipolar
correlation between them in the ground state (Figure 6b), is designated as SmAPAR.[47-49] At the
SmAPR-SmAPAR transition the preferred mode of correlation between the ferroelectric domains
changes from synpolar in SmAPR (single current peak) to antipolar in SmAPAR (double current
peak).
2.2.4 The biaxial SmCsPAR phase of compound 3F10
For the next homologue 3F10 a birefringent schlieren texture spontaneously occurs at T = 112 °C
on cooling the homeotropically aligned uniaxial SmAPR phase (Figure S8a). In XRD patterns a
decrease of the layer spacing from d = 4.6 nm to 4.3 nm starts at ~125 °C in the SmAPR phase
8
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and continues in the biaxial phases (Figure 3b), in line with a growing tilt with decreasing
temperature. Optical investigations of planar samples confirm a synclinic tilted organization
(Figure S15h). At the transition to this biaxial phase two widely separated polarization peaks
appear in each half-period of an applied triangular wave field, which rapidly merge closer and
increase in size (Figure S14), just as found for the SmAPAR phase of 3F8 (Figure 4a-d). In
electrooptical investigations the removal of the applied field and inversion of the field direction
does not change the tilt direction (see Figure S15g-i), indicating that also in this phase the
switching process takes place by rotation around the molecular long axis (Figure S15j-l). This
biaxial smectic phase, designated as SmCsPAR, behaves just like the SmAPAR phase of 3F8. Only
the tilt correlation between the SmCsPS domains is synclinic already before application of an E-
field, meaning that elongated alkyl chains favor the long range tilt correlation between the
SmCsPS domains. Similar to the SmAPAR range, also in the SmCsPAR phase the polarization
strongly rises from ~50 to ~800 nC cm-2 (Figure 5b).
2.2.5 Development of polar order in the SmC phases of compounds 3F12-2F20: SmCs[*],
[
SmCsPR *] and SmCsPAR ranges
<Figure 7>
Whereas for compound 3F10 the phase biaxiality appears to develops simultaneously with the
double peak switching at the SmAPR-SmCsPAR transition, for the next homologue 3F12 phase
biaxiality already emerges together with the appearance of the single current peak (peak A in
Figure 7a, see also Figure S13b) at T = 135 °C (E = 27 Vm-1), i. e. the SmAPR phase is replaced
by a SmCsPR phase with long range tilt.
<Figure 8>
The texture of the homeotropically aligned sample of the SmCsPR phase of 3F12 before
application of an E-field is shown in Figure 8. Between crossed polarizers there are areas with
uniformly weak birefringence, being separated by dark domain walls (Figure 8b). After
decrossing the polarizers by a small angle of about 5-10° the distinct areas become dark and
bright and decrossing the polarizer in the opposite direction reverses the brightness (Figure 8a,c).
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Rotation of the sample between crossed polarizers does not change the brightness, meaning that
simple tilt domains can be excluded and formation of a conglomerate of optical active domains is
observed. This indicates chiral conglomerate formation in these SmCsPR phases, which leads to
[
the designation SmCsPR *], where the star in brackets stands for the conglomerate structure. On
cooling, the chiral domains disappear and simultaneously two broad and widely separated
polarization peaks develop at T = 111 °C (peaks B in Figure 7b-d), just as observed for the
SmCsPAR phase of 3F10. However, the broad shape of the two emerging polarization peaks in the
SmCsPAR range of compound 3F12 is distinct from the sharp peaks in the SmAPAR and SmCsPAR
phases of the shorter homologues 3F8 and 3F10. Peak broadening increases with growing chain
length and is associated with a smoother increase of the polarization in the SmCsPAR range to
only ~400 nC cm-2 (see Figure 5c for 3F12 and Figure 5d for 3F14). It appears that the lateral
expansion of the conformationally disordered alkyl chains reduces the polar packing of the
aromatic cores, thus reducing the overall polarity of the domains and requiring larger domain size
to achieve the same polarization. Apparently, high polarization density leads to small domains
with narrow size distribution, yielding sharp polarization peaks, whereas low polarization density
requires larger domains which have a broader size distribution, leading to broader polarization
peaks.
For compounds 3Fn with n > 12 chiral domain formation appears together with the onset of
tilt, whereas the emergence of a current peak is delayed and takes place significantly after tilt
correlation, i.e. the paraelectric SmC phase is divided into a low permittivity chiral SmCs[*] range
at highest temperature, followed at lower temperature by chiral and achiral high permittivity
[
[
SmCsPR *] and SmCsPAR ranges, respectively. In the planar textures of the SmCsPR *] and
SmCsPAR ranges of all compounds 3Fn the orientation of the extinction crosses is inclined with
the directions of polarizer and analyzer under the E-field as well as at E = 0 (Figs. 7f-h and j-l and
S16i-k and l-n). This confirms a synclinic tilt-correlation and a switching by reorganization of the
molecules by a rotation around the molecular long axis (Figure 7i and m). This switching requires
a threshold field which decreases with decreasing temperature due to growing polarization and
domain size. For a fixed field strength (27 Vm-1) the temperature of appearance of the single
[
polarization peak (SmCs[*]-SmCsPR *] transition temperature) decreases with growing chain
[
length and for compound 3F20 with the longest chains the SmCsPR *] range appears to be
[
completely removed and a direct SmCs[*] - SmCsPAR transition, without intermediate SmCsPR *]
phase, is observed (Table 1 and Figure 1a).37 Dielectric studies of this compound[37] have
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confirmed a continuous increase of the dielectric permittivity in the paraelectric SmC range,
reaching a maximum in the SmCsPAR range just before the transition to the antiferroelectric
SmCsPA phase (Figure 9b),[50] in line with previously reported paraelectric-ferroelectric
transitions.[51] Furthermore, chunks of SmCsPS domains, being about 10-20 layers thick, have
been observed by polarizing microscopy of freely suspended films in the SmCsPAR range of
3F20, thus supporting the proposed polar domain structure of the - SmCsPAR phase.[37]
The unique feature of the series of compounds 3Fn is, that four different types of
polarization randomized smectic phases, the uniaxial (SmAPR, SmAPAR)[42,47] and the biaxial
[
(SmCsPR *] and SmCsPAR)[32a,b,37] were for the first time observed side by side in a single series of
compounds which provides insights into the actual structure of these high permittivity
paraelectric phases and the relations between them. The broad single peak in the SmAPR phases
[
is attributed to a Langevin-type switching.[42] However, in the synclinic tilted SmCsPR *] phases
of the higher homologues[32a,b,37] tilt and polar order are intrinsically coupled due to the restricted
rotation around the molecular long axis and therefore, the polar direction cannot be completely
randomized. Due to the uniform tilt there is an Ising-like synpolar coupling between the SmCsPS
domains which supports the formation of synpolar order under the applied field, even if the local
polarization is relatively weak. With growing domain size and polarization at reduced
temperature a growing preference of antipolar correlation between the SmCsPS domains develops,
leading to a transition to the tristable switching in the SmCsPAR ranges.
<Figure 9>
Even in the uniaxial SmAPR and SmAPAR phases with randomized tilt the local tilt could
provide cooperativity by Ising-like polar coupling and this could contribute to the polar response
in these uniaxial phases.
For all compounds 3Fn chiral conglomerates appear (Figure 8) just at the onset of phase
[
biaxiality (tilt) at the SmA-SmCs[*] or SmA-SmCsPR *] transition and fade upon approaching the
SmCsPAR region which is achiral. Mirror symmetry breaking in synclinic SmC phases of achiral
bent-core molecules requires the presence of an intrinsically chiral SmCsPS structure with C2
[
symmetry.[14] In the SmC[*] and SmCsPR *] ranges the Ising-like synpolar coupling between the
local SmCsPS domains and the further stabilization of this coupling by polar surface interactions
in the homeotropic cells leads to long range uniform chirality and the emergence of
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conglomerates of macroscopically chiral domains (see Figure 8).[32] Moreover, the 4-CN group
supports twisted minimum energy conformations, thus compounds 3Fn are considered as
transiently chiral molecules favoring chirality synchronization in their smectic phases.[52,27a,53]
The SmCsPAR range with synclinic tilt correlation, but antipolar correlation between adjacent
SmCsPS domains is on a macroscopic scale racemic and thus achiral. This is in line with a fading
of the chiral domains at the transition to SmCsPAR.
2.2.6 Macroscpic polar SmCsPA and SmCaPA phases
For all compounds with n > 6 the further reduction of temperature leads to an increase of
the size and polarization of the ferroelectric domains, giving rise to phase transitions to antipolar
SmC phases composed of polar layers, associated with a transition enthalpy decreasing with
growing alkyl chain length (Table 1). The short chain compounds 3F8 and 3F10 form exclusively
anticlinic tilted SmCaPA phases on cooling the paraelectric SmAPAR and SmCsPAR phases,
respectively. In these SmCaPA phases antiferroelectric switching takes place with high
polarization values and by rotation on the tilt cone as typical for the B2 phases of bent-core
molecules.[13] This mode of switching is indicated by the flipping of the dark extinction crosses
from being parallel to inclined with the direction of the polarizers either clockwise or
anticlockwise, depending on the field direction (Figures 7r-u and S16r-t). The polarization values
are high (~900 nC cm-2) for the short homologues and decrease with growing alkyl chain length
to about 450 nC cm-2 (Figure 9a). For the homologues with n ≥12 an additional synclinic SmCsPA
range is introduced between the SmCsPAR and the SmCaPA phases. The SmCsPA-SmCaPA
transition temperature, which is not associated with any transition enthalpy, only slightly
decreases with chain elongation from T = 98 °C for 3F12 to T = 92 °C for 3F20 (Table 1). The
absence of a transition enthalpy confirms that both phases belong to the polar smectic phases with
long range polar order in the layers. However, for compounds 3F14-3F20 the switching in the
SmCsPA phases takes place by rotation around the long axis, as indicated by the fixed position of
the dark extinctions with and without applied field (see Figure S16o-q). Only in the SmCsPA
phase of 3F12, being the shortest homologue with this phase, the position of the extinctions is
retained after switching off the applied field and changes after field reversal (Figure 7n-p). Since
there are two polarization peaks (Figure 7d) a bistable switching can be excluded and the actual
switching process is interpreted as a combination of two switching mechanisms (see Figure 7q).
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A relaxation after switching off the field at 0V taking place by rotation around the long axis, thus
retaining the position of the extinctions (Figure 7n,o) and after field reversal a switching by
rotation on the tilt cone which rotates the position of the extinctions to the opposite direction
(Figure 7o,p), indicating an inversion of the tilt direction.[54] In general, high packing density in
highly polarized SmC phases of strongly bent molecules suppresses the switching around the
long axis whereas low packing density and reduced polar order of weakly bent molecules are
favorable for the rotation around the molecular long axis.[55] Therefore, it can be deduced that in
the SmCsPA phase the packing density still grows and that a critical limit of packing density and
bending angle, changing the tilt correlation and switching mechanism, is achieved at the SmCsPA
to SmCaPA transition. The SmCsPA phase of 3F12, combining both switching mechanisms
represents an intermediate case with intermediate packing density. In the SmCsPA ranges the
barrier for the rotation around the molecular long axis is still growing with decreasing
temperature, leading to the strong growth of the polarization in the SmCsPA ranges (Figure 5c,d).
2.2.7 Overall phase sequence depending on temperature and chain length
Because the shape of bent molecules based on 4-cyanoresorcinol can change from being almost
rod-like to increasingly bent on decreasing temperature,[27a] restriction of the rotation around the
long axis, increasing packing density and growing coherence length of polar order are observed
by reducing the temperature. For only weakly bent molecules there are entropically favored
interlayer fluctuations along the molecular long axes, transferring polar order predominately
between the layers, whereas the rotation around the long axis reduces polar order in the layers
(see Figure 10, left). The weak synpolar coupling in all directions leads to the polarization
[
randomized paraelectric SmAPR and SmCsPR *] phases with a single switching current peak and
small polarization values.
<Figure 10>
[
The transition from a preferred synpolar correlation in SmAPR/SmCsPR *] to the antipolar
correlation in the SmAPAR/SmCsPAR phases is thought to be mainly the result of the stronger
molecular bend, disfavoring the fluctuations parallel the molecular long axes, but favoring
fluctuations by intercalations of the two individual rod-like wings (see Figure 10, right). This
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leads to a preference of antipolar coupling, whereas the simultaneous reduction of the ability of
rotation around the long axis strengthens the polar coupling in the layers. Thus, two polarization
peaks are observed in the paraelectric SmAPAR and SmCsPAR phases and the polarization values
grow. Further growing in-layer polarization finally leads to polar layers in the antipolar SmCsPA
and SmCaPA phases, resembling the B2 type polar smectic phase of typical (strongly bent) bent-
core mesogens. In the series of compounds 3Fn growing coherence length of polar order
unavoidably leads to antipolar correlation, even before macroscopic polarization is achieved.
However, as previously shown, the interlayer fluctuations of the rod-like wings can be suppressed
by introduction of nano-seggregating silyl groups to the ends of the alky chains[56,57] and this
shifts the synpolar-antipolar transition into the region of the macroscopic polar smectic phases,
thus leading to the ferroelectric switching SmCsPF[58,59] and SmAPF phases.[60 ]
There is also a significant effect of chain length; short chains (n = 4-6) have a small
segregation tendency from the aromatic cores[61] and this leads to cybotactic nematic phases and
SmA phases (SmAPR, SmAPAR). For medium chain length (n = 8-10) core-chain nano-
segregation is increased, favouring polar packing in the layers and tilting. Therefore, the medium
chain compounds have high polarization values and an increased tendency for antipolar packing
in adjacent layers, and this leads to a dominance of the high polarization SmCaPA phase. For long
chains (n = 12-20) the growing mismatch between the cross sectional areas of the rigid aromatic
cores and the conformationally disordered flexible chains gives rise to further growing tilt and
reduces the packing density, indicated by increasing tilt, reduced polarization and a decreasing
[
temperature range of the SmCsPR *] phase. Simultaneously, with growing chain length the
temperature dependent development of the coherence length of polar order becomes more
continuous, leading to a growing temperature range of the high permittivity paraelectric SmCsPAR
region above the paraelectric-antiferroelectric transition and the emergence of an additional
synclinic SmCsPA range in the antiferroelectric phase region before the transition to the B2-like
anticlinic SmCaPA phase (Figure 1a). In the SmCsPA range there is still a growing packing
density, which leads to growing polarization; tilt correlation and switching process change at a
critical coherence length of polarization at the SmCsPA-SmCaPA transition.
2.3 Effects of the replacement of F by H or larger groups in the 3-positions
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Comparison of compounds 3Fn (Figure 1a) with the related non-fluorinated analogues Hn (X, Y
= H, see Figure 11)[32a,b] shows that core fluorination at the periphery leads to an increased
stability of smectic phases and shifts the nematic range to much shorter chain length. Moreover,
for the fluorinated compounds, there is a reduced tendency to form uniformly tilted phases. Thus,
the NCybC phase of Hn is replaced by the NCybA phase and uniaxial smectic phases (SmA, SmAPR,
SmAPAR) are introduced which are completely missing in the series of the non-fluorinated
compounds Hn. The stronger tilt in the series Hn might also be responsible for the presence of an
additional, possibly modulated phase M1, separating the paraelectric and antiferroelectric smectic
[
phases. In both series there is mirror symmetry breaking in the SmCs[*] and SmCsPR *] ranges as
indicated by surface stabilized conglomerate formation. However, only compounds 3Fn form the
[
B2-like SmCaPA phase with long range polar layers, whereas the paraelectric SmCs[*], SmCsPR *]
and SmCsPAR phases and the SmCsPA phase with limited coherence length of polar order were
found in both series. Thus, in the series of the fluorinated compound 3Fn an increased polar order
is achieved. These effects of core fluorination appear to be the result of increased core-core
interactions between the electron deficit fluorinated aromatic units.[62]
<Figure 11>
Moreover, in the series of compounds 3Fn and Hn there is an opposite effect of increasing
[
chain length on polar order. For compounds 3Fn the SmCsPR *] and SmCsPAR phases are
destabilized with growing chain length whereas for compounds Hn these are stabilized (compare
Figures 1a and 11). It appears that for compounds Hn the weaker core-core interactions are
strengthened by increased core-chain nano-segregation, growing with rising chain length. For the
fluorinated compounds 3Fn with stronger core-core interactions core chain segregation is not
required. In this case it even disturbs the development of polar order as longer fluid alkyl chains
could reduce the packing density of the aromatic cores due to the growing lateral expansion of
the disordered alkyl chains.
Replacing fluorine (crystal volume cv = 12.8 nm3)[63] in 3F18 by the more bulky and less
polar bromine (cv = 33.0 nm3) reduces the phase transition temperatures considerably (compound
3Br18, Table 2), though a phase sequence Iso-SmA- SmCs[*]-SmCsPAR is retained. However, the
polarization is smaller (P ~ 100 nC cm-2) and the polarization peaks are much broader in the
SmCsPAR range (see Figure S19), indicating a smaller polarization and a significant size
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distribution of the polar domains. In this case no transition to a macroscopically polar SmCPA
phase can be found before crystallization takes place at T = 67 °C. Replacing Br by the non-polar
CH3 group (cv = 31.7 nm3) with similar volume further reduces the LC phase stability
considerably, removes the non-tilted SmA phase and removes polar order completely, leaving
only a nematic (NCybC) and a nonpolar and nonchiral SmC phase (see Table 2). This is mainly
attributed to the unfavorable reduction of core-core interactions by the non-polar and electron
donating CH3 group.
<Table 2>
2.4 Effects of variation of the F-substitution pattern
<Table 3>
Changing the position of the lateral fluorine substituents from the ortho position (3Fn) to the
meta position with respect to the terminal alkyloxy chains (2F) reduces melting and clearing
temperatures (Tables 1 and 3). Moreover, only LC phases with tilted organization (NCybC phases,
see Figure S17 and SmCs phases) were observed. Neither chiral domain formation nor any
polarization current peaks and optical response on the applied fields can be found up to an
electric field of 33 Vppm-1. Therefore, introducing F substitution in this position favours tilt,
reduces mesophase stability and removes polar order, which is mainly attributed to the
unfavourable steric effect of fluorine in this inside-directed position. This is in line with previous
observations made with rod-like molecules.[34]
<Table 4>
Using two lateral fluorine substituents at ortho and meta positions (series 23Fn, Table 4)
leads to higher transition temperatures compared to series 2Fn, but lower if compared with the
series 3Fn. A nematic phase cannot be observed, and the SmA phase appears only as a
monotropic phase for n = 10. Compounds with n ≥ 12 form exclusively the non-polar SmCs[*]
phase, showing chiral conglomerates in homeotropic alignment (see Figure S18). Though chiral
conglomerate formation indicates a paraelectric phase with SmCsPS domain structure no
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polarization current peaks can be observed in triangular wave experiments (up to 33 Vppm-1).
Due to the relatively high melting points and increased crystallization tendency, the temperature
ranges with polar switching cannot be accessed for compounds 23Fn.
A comparison of the effect of the azobenzene wings with respect to other rod-like units is
given in the Supporting Discussion S5.1 in the SI.
3. Conclusions
Three series of bent-core molecules consisting of a 4-cyanoresorcinol-based central core
and laterally fluorinated azobenzene wings have been synthesized and investigated, which
provided fundamental insights into the development of polar order and spontaneous symmetry
breaking by self-assembly of achiral bent-core molecules in lamellar LC phases (see Figure 1). It
shows that already in the SmA phase locally tilted and polar SmCsPS domains emerge. With
growing size of these ferroelectric domains macroscopic polar order develops continuously from
the low permittivity SmA phases (SmA) via a series of uniaxial (SmAPR, SmAPAR) and biaxial
[
(SmCsPR *], SmCsPAR) high permittivity paraelectric smectic phases to the antiferroelectric polar
smectic phases (SmCsPA and SmCaPA). As known for the inorganic solid state ferroelectrics (e.g.
perovskites)[15] the paraelectric-ferroelectric transition takes place by gradual changes of the polar
domain size and by crossing a permittivity maximum temperature corresponding to the SmAPAR
-
SmCPA and /SmCsPAR - SmCPA transitions (see Figure 9b),[37] which are the only transition of
this process being associated with a small transition enthalpy (see Table 1). However, increasing
polar coherence length leads to strengthening of antipolar correlation between the ferroelectric
domains, thus giving rise to antiferroelectric switching already in the high permittivity
paraelectric SmAPAR and SmCsPAR ranges. Because the development of polar order is associated
with the development of a tilted organization in the polar domains and a growing coherence of tilt
between these domains, the paraelectric phases show unusual effects, as spontaneous symmetry
breaking, non-classical electroclinic effects and field induced phase transitions. In the paraelectric
phases and most SmCsPA phases the switching takes place by rotation around the molecular long
axis, whereas only in the high polarization (B2-like) SmCaPA phase with enhanced packing
density it occurs by rotation on the tilt cone.
Core fluorination in the periphery appears to give increased electrostatic interactions
between the aromatic cores, thus removing nematic phases and leading to enhanced smectic
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phase stability, increased polar order and increased stability of chiral conglomerate type SmC[*]
phases. It also modifies the core packing, thus leading to reduced tilt and emergence of additional
tilt randomized paraelectric SmC phases (SmAPR and SmAPAR). These polar effects are
dominating for core substitution in the peripheral 3-position, where steric effects are small. The
importance of polar interactions is also evident from the effects of replacing F by Br and CH3. In
this order of decreasing electron acceptor ability the polar phases are destabilized and finally
removed completely. Fluorination in the inside-directed 2-position strongly reduces smectic
phase stability[34a] and polar order and removes superstructural chirality, due to the dominating
steric distortion of the packing of the bent cores by F-substiution in this position.
Overall, this work provides clues for the understanding of the development of polar order
in the LC phases of achiral bent-core mesogens depending on temperature, chain length and core-
structure, and it indicates similarities to related transitions in solid state ferroelectrics. The
advantage of fluid materials with polar response is their capability of being easily processible and
addressable by a variety of external stimuli. The azo linkage in all reported compounds can
undergo light-induced trans–cis isomerisation (see Supporting Discussion S5.2 in the SI for
preliminary investigations in solution) which could in future work be used for optical addressing
polar order and chirality.
4. Experimental
Synthesis. - The benzoic acid 3an-3en (2.4 mmol), synthesized according to the procedures
given in the references,[38,64] were heated under reflux with excess SOCl2 (3 mL) and a catalytic
amount DMF for one hour to give the corresponding benzoyl chloride 4an-4en. The excess of
thionyl chloride was removed under reduced pressure. The formed benzoyl chloride was then
dissolved in dry CH2Cl2 (20 mL) and 1.2 mmol of 4-cyanoresorcinol 5[28] previously dissolved in
CH2Cl2 (10 mL) was added to that solution, followed by addition of triethylamine (2.8 mmol) and
a catalytic amount of pyridine. The reaction mixture was then refluxed for 6 hrs under an argon
atmosphere. The reaction mixture was cooled to room temperature and washed with 10% HCl (2
× 50 mL) and with cold water (2 x 50 mL). The aqueous phase was extracted with CH2Cl2 (3 ×
50 mL) and the combined organic phases were dried over anhydrous Na2SO4. The crude residue
obtained after removal of solvent was purified by chromatography on silica gel using chloroform
as eluent affording an orange material which was crystallized from chloroform/ethanol mixture to
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give the final bent core compounds. The analytical data of 3F12, 2F12 and 23F12 are given as
representative examples below; the analytical data for all other reported compounds are collated
in the Supporting Information (SI).
4-Cyano-1,3-phenylene bis-[4-(3-fluoro-4-dodecyloxyphenylazo)benzoate] (3F12). Orange
powder, yield 68 %, 1H-NMR (400 MHz, CDCl3): δ 8.40 (d, J = 8.5 Hz, 2H, Ar-H), 8.33 (d, J =
8.5 Hz, 2H, Ar-H), 8.07–7.93 (m, 4H, Ar-H), 7.83 (d, J = 8.6 Hz, 3H, Ar-H), 7.79–7.70 (m, 2H,
Ar-H), 7.59 (d, J = 2.2 Hz, 1H, Ar-H), 7.36 (dd, J = 8.5, 2.2 Hz, 1H, Ar-H), 7.10 (t, J = 8.6 Hz,
2H, Ar-H), 4.14 (t, J = 6.6 Hz, 4H, OCH2CH2), 1.95–1.81 (m, 4H, OCH2CH2), 1.60–1.16 (m,
13
36H, CH2), 0.88 (t, J = 6.7 Hz, 6H, CH3). C-NMR (126 MHz, CDCl3): δ 163.44 (COOAr),
163.08 (–COOAr), 155.89 (CAr–O–CH2), 155.79 (CAr–O–CH2), 154.77, 153.87, 153.45, 151.89,
150.92, 150.87, 150.83, 150.78, 146.46, 146.43, 146.39, 133.99, 131.65, 131.41, 129.57, 129.19,
123.94, 123.93, 122.89, 122.82, 120.01, 117.34, 114.73 (CN), 113.44, 113.41, 107.96, 107.89,
107.81, 107.74, 104.28 (CAr–CN), 69.56 (CAr–O–CH2), 31.89, 29.63, 29.61, 29.56, 29.51, 29.32,
19
29.31, 29.04, 25.87, 22.67, 14.09 (CH3). F-NMR (470 MHz, CDCl3) δ -132.27 – -132.73 (m).
Elemental Analysis: Calc. for C57H67F2N5O6 C, 71.60; H, 7.06; N, 7.32. Found C, 71.63; H, 7.00;
N, 7.31 %.
4-Cyano-1,3-phenylene bis-[4-(2-fluoro-4-dodecyloxyphenylazo)benzoate] (2F12). Orange
powder, yield 66 %, 1H-NMR (400 MHz, CDCl3) δ 8.39 (d, J = 8.5 Hz, 2H, Ar-H), 8.32 (d, J =
8.5 Hz, 2H, Ar-H), 8.07 – 7.97 (m, 4H, Ar-H), 7.89 – 7.78 (m, 3H, Ar-H), 7.59 (d, J = 2.2 Hz,
1H, Ar-H), 7.36 (dd, J = 8.5, 2.2 Hz, 1H, Ar-H), 6.82 – 6.73 (m, 4H, Ar-H), 4.04 (t, J = 6.5 Hz,
4H, OCH2CH2), 1.89 – 1.76 (m, 4H, OCH2CH2), 1.64 – 1.17 (m, 36H, CH2), 0.88 (t, J = 6.8 Hz,
13
6H, CH3). C-NMR (126 MHz, CDCl3): δ 164.09 (COOAr), 164.05 (COOAr), 164.01 (CAr–O–
CH2), 163.96 (CAr–O–CH2), 163.46, 163.20, 163.11, 161.13, 156.33, 156.22, 154.78, 153.46,
134.93, 134.89, 134.87, 134.84, 133.97, 131.63, 131.39, 129.47, 129.09, 122.96, 122.89, 120.00,
118.59, 118.56, 117.36, 114.74 (CN), 111.56, 111.54, 111.52, 111.49, 104.28 (CAr–CN), 102.42,
102.23, 68.93 (CAr–O–CH2), 68.92 (CAr–O–CH2), 31.87, 29.63, 29.61, 29.55, 29.52, 29.32,
19
29.30, 28.97, 25.92, 22.66, 14.09 (CH3). F-NMR (470 MHz, CDCl3): δ -119.77 – -120.14 (m).
Elemental Analysis: Calc. for C57H67F2N5O6 C, 71.60; H, 7.06; N, 7.32. Found C, 71.60; H, 7.11;
N, 7.38 %.
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4-Cyano-1,3-phenylene bis-[4-(2,3-difluoro-4-dodecyloxyphenylazo)benzoate] (23F12). Orange
powder, yield 68 %, 1H-NMR (400 MHz, CDCl3) δ 8.40 (d, J = 8.5 Hz, 2H, Ar-H), 8.32 (d, J =
8.5 Hz, 2H, Ar-H), 8.09 – 7.97 (m, 4H, Ar-H), 7.83 (d, J = 8.6 Hz, 1H, Ar-H), 7.67 – 7.54 (m,
3H, Ar-H), 7.36 (dd, J = 8.5, 2.1 Hz, 1H, Ar-H), 6.88 – 6.76 (m, 2H, Ar-H), 4.15 (t, J = 6.6 Hz,
4H, OCH2CH2), 1.95 – 1.80 (m, 4H, OCH2CH2), 1.68 – 1.14 (m, 36H, CH2), 0.88 (t, J = 6.6 Hz,
13
6H, CH3). C-NMR (126 MHz, CDCl3): δ 163.37 (COOAr), 163.01 (COOAr), 156.02 (CAr–O–
CH2–), 155.92 (CAr–O–CH2), 154.73, 153.42, 151.95, 151.19, 151.10, 149.11, 149.02, 142.68,
142.58, 140.70, 140.60, 135.55, 135.52, 135.49, 134.00, 131.66, 131.42, 129.93, 129.55, 123.13,
123.06, 120.03, 117.33, 114.70 (CN), 111.98, 111.95, 108.46, 104.31 (CAr–CN), 70.08 (CAr–O–
19
CH2), 31.89, 29.62, 29.60, 29.54, 29.49, 29.32, 29.27, 29.02, 25.80, 22.66, 14.08 (–CH3). F-
NMR (470 MHz, CDCl3): δ -145.72 – -146.13 (m), -157.86 – -158.19 (m). Elemental Analysis:
Calc. for C57H65F4N5O6 C, 69.00; H, 6.60; N, 7.06. Found C, 68.98; H, 6.61; N, 6.99 %.
Analytical methods. - Thin layer chromatography (TLC) was performed on aluminium sheet
precoated with silica gel. Starting materials were obtained from commercial sources and used
without further purification. The solvents were dried using the standard methods when required.
The purity and the chemical structures of all synthesized compounds were confirmed by the
spectral data. The structure characterization of the synthesized bent-core compounds is based on
1H-NMR, 13C-NMR and 19F-NMR (Varian Unity 400 spectrometer, in CDCl3 solutions, with
tetramethylsilane as internal standard). Microanalyses were performed using a Leco CHNS-932
elemental analyzer.
Investigation methods. - The phase behaviour of the synthesized compounds has been
investigated by polarizing optical microscopy (PLM), differential scanning calorimetry (DSC),
X-ray diffraction (XRD) studies and electro-optical measurements. All compounds are thermally
stable as confirmed by the reproducibility of thermograms on several heating and cooling cycles.
The assignment of the mesophases was made on the basis of combined results of optical textures,
electro optical studies and XRD. For optical microscopy an Optiphot 2, polarizing microscope
(Nikon) in conjunction with a heating stage (FP82HT, Mettler) was used. Photoisomerization of
the azobenzene group is negligible under the used experimental conditions so that formation of
trans-cis mixtures can be excluded. Differential scanning calorimetry (DSC) was performed
using a DSC-7 (Perkin Elmer) with a scanning rate of 10 K min-1. X-ray diffraction (XRD) was
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done at Cu-Kα line (λ = 1.54 Å) using standard Coolidge tube source with a Ni-filter.
Investigations of oriented samples were performed using a 2D-detector (H1-Star, Siemens AG).
Uniform orientation was achieved by alignment in a magnetic field (B ≈ 1T) using thin
capillaries. The orientation once achieved is maintained by slow cooling (0.1 K min-1) in the
presence of the magnetic field. Electro-optical experiments have been carried out using a home
built electro-optical setup under an applied triangular wave voltage in commercially available
indium tin oxide (ITO) coated glass cells (E.H.C., Japan, polyimide (PI) coated for planar
alignment, antiparallel rubbing, thickness 6 and 10 µm and measuring area 1 cm2).
Supporting Information. Synthesis and analytical data of the compounds, additional
experimental data (DSC traces, textures, XRD data, polarization current curves and electrooptical
studies) and supporting discussion (effects of core structure and photoisomerization in solution)
Acknowledgements. The work was supported by the DFG (Grant Ts 39/24-1).
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25
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SmCsPS domains. In ref. 37 the same phase was reported for 3F20 and this phase was
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Scheme 1. Development of a) polar order (PS = synpolar, PA = antipolar) and b) tilt (Cs =
synclinic, Ca = anticlinic) and chirality (red/blue color) in the lamellar phases of bent-core LCs;
c) chemical structures of the considered 4-cyanoresorcinol bases BCLCs.
Scheme 2. Synthesis of the bent-core mesogens (intermediates: a: X = F, Y= H; b: X= H, Y = F;
c: X = Y = F; d: X = Br, Y = H and e: X = CH3, Y = H; for the structures of the bent-core
mesogens see also Scheme 1c).
Figure 1. a) Plot of the transition temperatures of compounds 3Fn as a function of the alkyl chain
length (n). The solid black line indicates the crystallization temperature on cooling; the colored
lines indicate the upper temperature limits for the corresponding mesophases; colored areas
visualize the molecular organization (yellow = nematic, red = SmA ranges and green SmC
ranges); the square grid pattern indicates LC phases composed of ferroelectric domains and the
parallel line pattern (bottom) LC phases composed of polar layers; b) shows simplified models of
the fundamental phase structures under discussion and their development depending on
temperature and alkyl chain length; dots and crosses indicate opposite polar directions, red/blue
indicate opposite chirality; the abbreviations are explained in Table 1; for the tilt randomized
SmAPAR structure only one chirality is shown, though on a larger scale the structure is racemic
(see also Figure 6b).
Figure 2. XRD patterns of a magnetically aligned sample of compound 3F6: a, b) in the NCybA
phase at 122 °C and c, d) in the SmA phase at 110 °C; a, c) represent the complete patterns after
subtraction of the scattering in the isotropic liquid state at 130 °C and b, d) show the small angle
patterns, the insets in a, c) show the corresponding optical textures (between crossed polarizers,
horizontal and vertical) in planar alignment (see also Figure S7).
Figure 3. Temperature dependence of the d-value of compounds a) 3F8, (Lmol = 5.1 nm), b) 3F10
(Lmol = 5.6 nm), c) 3F12 (Lmol = 6.1 nm) and d) 3F14 (Lmol = 6.6 nm) measured by SAXS on
cooling down to the to crystallization temperature; Lmol was determined as shown in Figure S9;
note that the tilt already arises in the SmA range, so that the SmA-SmC phase transitions do not
correspond to the maxima of the d-values; data points in the Iso range refer to the maxima of the
diffuse small angle scattering.
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Figure 4. Switching current response curves and textures of compound 3F8 (6 m PI coated ITO
cell). a-d). Polarization current response curves on applying a triangular wave field (20 Vppm-1,
10 Hz,); e-g) textures as observed for planar samples (molecules parallel to the substrate surfaces)
between crossed polarizers e) in the SmAPAR phase, f) at the SmAPAR-SmCaPA transition and g)
in the SmCaPA phase; the insets show the corresponding textures in homeotropic alignment
(layers parallel to the substrate surfaces) as observed between non-treated microscopy glass
plates; the temperature dependence of the polarization is displayed in Figure 5a and DSCs are
shown in Figure S6a.
Figure 5. a,b) Development of polarization depending on temperature (26.7 Vppm-1, 10 Hz) for
a) 3F8, b) 3F10 and c) 3F12 and d) 3F14 as observed on cooling solid lines indicate phase
transitions associated with a transition enthalpy whereas dashed lines indicate continuous
transitions.
Figure 6. Textures as observed in DC field experiments in the SmAPAR phase range of 3F8 at T
= 108 °C: a) before application of an E-field, c) under the E-field and after reversal of the field
and e) after removal of the applied field (planar ITO cell between crossed polarizers see arrows in
a)); b, d, f) shows the models of the molecular organization in the distinct states; R is the rubbing
direction.
Figure 7. a-e) Switching current response curves of compound 3F12 on applying a triangular
wave field (10 Hz in 6 m ITO cell); f-h, j-l, n-p, r-t) textures as observed between crossed
polarizers in DC field experiments at +6.7 Vm-1 (left column), 0 V (middle column) and -6.7
Vm-1 (right column) and i, m, q, u) models of the molecular reorganization in the switching
processes. The green color in o, s) corresponds to the next lower order of birefringence, due to
the intense yellow/orange color of the azo compound blue and purple birefringence colors appear
green; for DSCs, see Figure S6c.
Figure 8. Optical textures of the homeotropic aligned (layers parallel to the substrate surfaces)
[
SmCsPR *] phase of 3F12 at T = 133 °C, b) between crossed polarizers and a,c) between slightly
29
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