4-cyanoresorcinol-based bent-core mesogens with azobenzene wings: Emergence of sterically stabilized polar order in liquid crystalline phases

Article abstract of DOI:10.1002/adfm.201302295

A new series of azobenzene containing bent-core molecules incorporating 4-cyanoresorcinol as the central core unit exhibiting cybotactic nematic, rectangular, columnar, and different types of tilted smectic (SmC) phases are synthesized. The mesophase behavior and phase structures are characterized in bulk and freely suspended films using a variety of experimental techniques. Depending on the chain length and temperature a series of different mesophases is observed in these compounds, ranging from cybotactic nematic via paraelectric SmC phases, polarization randomized SmC_sP_R phases to ferroelectric and antiferroelectric SmC phases, associated with increasing size and correlation length of the polar domains. Spontaneous formation of chiral domains is observed in the paraelectric SmC and the SmC_sP_R phases and discussed in relation with superstructural chirality, bend elastic constants, and surface effects. Development of polar order in tilted smectic liquid crystalline phases (SmC) of bent-core mesogens is found to take place with increasing polar domain size in a series of distinct phases via a paraelectric and a new randomized polar SmC phase (SmC_sP_R) to a ferroelectric phase, which then becomes modulated and finally changes to an antiferroelectric phase.

Full text of DOI:10.1002/adfm.201302295

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4-Cyanoresorcinol-Based Bent-Core Mesogens with
Azobenzene Wings: Emergence of Sterically Stabilized
Polar Order in Liquid Crystalline Phases
Mohammed Alaasar,* Marko Prehm, Kathrin May, Alexey Eremin,* and Carsten Tschierske*
this field and brought new expectations:
A new series of azobenzene containing bent-core molecules incorporating
4-cyanoresorcinol as the central core unit exhibiting cybotactic nematic,
rectangular, columnar, and different types of tilted smectic (SmC) phases are
synthesized. The mesophase behavior and phase structures are characterized
in bulk and freely suspended films using a variety of experimental techniques.
Depending on the chain length and temperature a series of different meso-
phases is observed in these compounds, ranging from cybotactic nematic via
paraelectric SmC phases, polarization randomized SmCsPR phases to fer-
roelectric and antiferroelectric SmC phases, associated with increasing size
and correlation length of the polar domains. Spontaneous formation of chiral
domains is observed in the paraelectric SmC and the SmCsPR phases and
discussed in relation with superstructural chirality, bend elastic constants,
and surface effects.
the so-called banana-shaped LCs or bent-
core LCs. Since the pioneering results by
Niori et al.,[4] extensive research has been
carried out on these LCs, and the results
have allowed to appreciate new kind of
truly fascinating materials. The occur-
rence of novel and intriguing polar meso-
phases, the induction of supramolecular
chirality using achiral molecules and the
noticeable optical, ferroelectric and anti-
ferroelelectric responses of these mate-
rials are, among others, aspects that are
now well documented.[5]
Recently, the research became focused
on materials at the borderline between
rod-shaped and bent-core mesogens: such
as hockey stick molecules, mesogenic
dimers combining rod-like and bent-core units, molecules with
exceptionally short core units with only four or less aromatic
rings and molecules with a reduced bend.[6] Varying the chem-
ical structure of these compounds made it possible to design
materials exhibiting a whole spectrum of phases from paraelec-
tric to ferro- and antiferroelectric phases.[7,8] 4-Cyanoresorcinols
represent a typical structure of this kind providing a reduced
bend due to the effects of the CN group on the conformation
of the adjacent ester group.[9–11] It was demonstrated in pre-
vious studies that this structural unit can be used to generate
bent-core mesogens with broad regions of cybotactic nematic
phases,[11] transitions between different types of orthogonal
phases, non-polar SmA phases,[10] SmAPR phases with rand-
omized polarization[1,2] and antiferroelectric SmAPA phases[10,13]
have been observed. A new SmAP phase, in which the polari-
zation rotates uniformly from layer to layer with a fixed angle
between adjacent layers, was also suggested recently.[14]
In this work new bent-core mesogens combining the bent
4-cyanoresorcinol core with two rod-like azobenzene wings
were synthesized and investigated. The azobenzene units could
lead to materials with enhanced nonlinear optical (NLO) prop-
erties.[15] Another important attractive feature of these com-
pounds is the transformation between its trans and cis con-
figurations by light absorption (photoisomerism) which can
be used to modify the mesophase structure.[16] Such materials
could be used for holographic media, optical storage, reversible
optical waveguides, and other applications.[17–20] Azobenzene
cores have previously been used for photoisomerizable bent-
core mesogens[21,22] and in attempts to produce biaxial nematic
phases.[23,24] Combining photoswitchable azobenzenes with
1. Introduction
Materials with spontaneous or induced polar order, which
can be switched between polar and apolar states, are of poten-
tial interest for applications in sensors, memory and display
devices. Most ferroelectrics and antiferroelectrics represent
inorganic and organic solid state materials.[1] Liquid crystalline
(LC) ferroelectrics are of special interest as they can be easily
processed into devices and their configuration can be switched
by external stimuli.[1] LC phases of chiral molecules in tilted
smectic phases (SmC* phases) provided the first examples of
LC ferroelectric and antiferroelectric materials.[3] In the mid
90’s a novel type of mesogenic material breathed new life into
Dr. M. Alaasar, Dr. M. Prehm, Prof. C. Tschierske
Institute of Chemistry, Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Str. 2, 06120, Halle, Germany
E-mail: M.Alaasar@yahoo.com;
Carsten.tschierske@chemie.uni-halle.de
Dr. M. Alaasar
Department of Chemistry
Faculty of Science
Cairo University
Giza, Egypt
K. May, Dr. A. Eremin
Institute for Experimental Physics
Otto-von-Guericke University Magdeburg
Universitätsplatz 2, 39106, Magdeburg, Germany
E-mail: alexey.eremin@ovgu.de
DOI: 10.1002/adfm.201302295
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liquid crystalline ferroelectrics also provides access to poten-
tial new multifunctional materials where polar response can
be modulated by light.[25–29] Recently we have reported a first
example of a 4-cyanoresorcinol with two azobenzene wings
(compound A14).[8] An interesting feature of this compound is
the formation of synclinic tilted SmC phases with an enhanced
polarizability, which show surface supported achiral symmetry
breaking.
Herein, we report the synthesis and investigation of the
complete series of these azobenzene bent-core mesogens An
having even numbered alkyl chains with n ranging from 8 to
18. With these compounds additional investigations, especially
Second Harmonic Generation (SHG) experiments and inves-
tigations of freely suspended thin films were performed and
the whole sequence of different phases formed by these com-
pounds was investigated and gained important new insights
into the development of polar order in tilted smectic phases. It
turned out to be more complex than in the orthogonal case due
to the effects of tilt on packing density, layer modulation, elastic
constants and superstructural chirality. The sequence observed
Scheme 1. Synthetic route to the bent core molecules An; reagents and
conditions: i) 1. NaNO2, HCl, H2O, 0–5 °C, 2. phenol, NaOH, 3. NaHCO3;
ii) CnH2n+1Br, KI, K2CO3, 2-butanone, reflux 6 h; iii) 1. EtOH, KOH, reflux,
12 h, 2. HCl/H2O, 20 °C; iv) H2NOH.HCl, Ac2O, v) NaOH, H2O;[30]
vi) N,N′-dicyclohexylcarbodiimide, 4-dimethylaminopyridine, 20 °C, 24 h.
dehydration.[30] All the homologues acids IIIn were synthesized
according to the method described before in four steps.[22] First
the azobenzene was prepared by diazotization of ethyl 4-amin-
obenzoate and coupling of the resulting diazonium salt with
phenol yielding ethyl 4-(4-hydroxyphenylazo)benzoate (I). Com-
pound I was alkylated with 1-bromoalkane in the presence of
potassium carbonate to give ester compounds IIn. Then the
ester compounds IIn were hydrolyzed under basic conditions
to yield the benzoic acids IIIn. In the final step, the benzoic
acids IIIn were esterified with 4-cyanoresorcinol (4) in the pres-
ence of N,N′-dicyclohexylcarbodiimide (DCC) and 4-dimethyl-
aminopyridine (DMAP) as catalyst using dry dichloromethane
as solvent to achieve the target compounds A8–A18. The final
bent-core compounds were first purified by column chroma-
tography and then by crystallization using ethanol/chloroform
mixture. Complete synthetic procedures and analytical data are
reported in the electronic supporting information.
on decreasing temperature is Iso – NCybC – SmC – SmCsPR
–
SmCsPF – M1 – SmCsPA associated with increasing correlation
length of polar domains. Already the isotropic and the nematic
phases show field-induced polar order indicated by SHG
activity as also found for all other tilted smectic phases of these
compounds. This behavior is attributed to polar domains occur-
ring well before the conventional antiferroelectric switchable
SmCPA phase is reached. In the paraelectric SmC phase the
domains are small. The SmCsPR phase indicates a temperature
range of the SmC phase where the size of the polar domains
reaches an appreciable size and which can be regarded as the
tilted analog to the previously reported SmAPR phase.[12,13]
Further growth of the ferroelectric domains leads to a SmCsPF
phase which becomes modulated in the temperature range of
the phase assigned as M1, before the antiferroelectric switching
SmCsPA phase with alternating polar direction in adjacent
layers is formed. Spontaneous formation of chiral domains was
observed for all compounds under certain conditions in the
SmC and SmCsPR phases and it is assumed to be the result
of the combined effects of superstructural chirality and surface
anchoring.
2.2. Investigation Methods
The synthesized compounds A8–A18 were investigated by polar-
ised light optical microscopy (Optiphot 2, Nikon) in conjunc-
tion with a heating stage (FP82HT, Mettler) and by differential
scanning calorimetry (DSC-7 Perkin Elmer). The assignment
of the mesophases was made on the basis of combined results
of optical textures and X-ray diffraction (XRD). The measure-
ments were 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 (HI-Star, Sie-
mens AG). Uniform orientation was achieved by alignment in
a magnetic field (B ≈ 1 T) 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
2. Results
2.1. Synthesis
The synthesis of the bent-core compounds A8–A18 is shown
in Scheme 1. As reported previously 2,4-dihydroxybenzoni-
trile (4) was prepared from commercially available 2,4-dihy-
droxybenzaldehyde by the formation of the oxime, followed by
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have been carried out using a home built electro-optical setup
in commercially available ITO coated glass cells (E.H.C., Japan,
polyimide coated for planar alignment, antiparallel rubbing,
thickness 6 μm and measuring area 1 cm2).
Measurements of optical second harmonic generation
(SHG) have been performed using a Nd:YAG laser operating
at λ = 1064 nm (10 ns pulse width and 10 Hz repetition rate).
The primary beam was incident at an angle of 30° to the cell
normal. The SHG signal was detected in transmission by a
photomultiplier tube (Hamamatsu). The acquired signal was
calibrated using a 50 μm reference quartz plate.
Freely-suspended films were drawn over a custom-made rec-
tangular glass frame with a 10 mm × 3 mm slit and two brass
electrodes. The frame has been mounted into a Linkam LTS 350
heating stage. The temperature controller provided an accuracy
of 0.1 K. Optical observations have been made with a polarizing
microscope AxioImager Pol (Carl Zeiss GmbH) equipped with
a high-resolution cooled charge-coupled device (CCD) camera
AxioCam HR (Carl Zeiss GmbH). We carried out the observa-
tions in reflected light between crossed polarizers as well as
between slightly uncrossed polarizers.
Figure 1. Plot of the transition temperatures of compounds A8–A18 as
a function of the alkyl chain length. The connections between the data
points serve as guides for the eyes and indicate the upper temperature
limits for the distinct mesophases; stars (connected by a dashed line)
indicate the melting temperatures observed on heating, ᭤ (bottom solid
line) the crystallization temperature on cooling.
2.3. Mesophases and Transition Temperatures Depending
on Alkyl Chain Length and Temperature
(compound A10) induces additional tilted smectic phases (SmC
and SmCsPR phases) between the NCybC and the Colrec phase
and modifies the structure of the Colrec-phase. A typical feature
of the SmC phases is SHG activity under an applied electric
field and that under certain conditions a conglomerate of chiral
domains is observed (see below).
For the next homologue A12, the Colrec phase completely
disappears. The nematic range becomes smaller and the SmC
phase is stabilized. Additional phase transitions with very small
enthalpy values can be observed in the smectic phase region on
cooling before crystallization takes place. For all compounds
A12–A18 there is a sequence of five distinct smectic phases,
assigned as SmC, SmCsPR, SmCsPF, M1 and SmCsPA. For
Figure 1 shows the mesophases and the transition tempera-
tures for the synthesized homologous compounds with dif-
ferent terminal alkoxy chain lengths (for numerical data, see
Table 1, DSC traces are shown in Figure S1). As can be seen
from Figure 1, for the shortest homologue A8 a nematic phase
(NCybC) is observed in a relatively wide temperature range, which
on cooling to 83 °C is replaced by a mesophase with spherulitic
texture which was identified as a rectangular columnar phase
by XRD (Colrec, see Figure 2c–e and Section 2.5) also known
as B1rev phase. Increasing the alkoxy chain length to n = 10
Table 1. Phase transition temperatures (T in °C) and associated enthalpy values (in square brackets, ΔH in kJ mol− 1) of compounds A8-A18.a)
Compd.
n
Phase transitions
A8
A10
A12
A14
A16[8]
A18
8
Cr 102 [60.7] (Colrec 83 [6.6]) NCybC 143 [1.0] Iso
10
12
14
16
18
Cr 68 [22.7] Colrec2 69 [3.0]b) Colrec1 73 [0.5] SmCsPR 87 [-] SmC 105 [<0.01] NCybC 136 [1.1] Iso
Cr 71 [17.4] (SmCsPA 70 [0.3]) M1 75 [0.2] SmCsPF 83 [-] SmCsPR 96 [-] SmC 127 [0.15] NCybC 137 [2.0] Iso
Cr 72 [23.0] SmCsPA 76 [0.4] M1 78 [0.1] SmCsPF 90 [-] SmCsPR [-] 102 SmC 136 [0.5] NCybC 139 [3.0] Iso
Cr 75 [41.8] (SmCsPA 76 [0.4]) M1 79 [0.15] SmCsPF [-] 95 SmCsPR [-] 109 SmC 139.5 NCybC 140 [5.8]c) Iso
Cr 80 [40.7] (SmCsPA 76 [0.4] M1 79 [0.1]) SmCsPF 98 [-] SmCsPR [-] 116 SmC 142 [6.3] Iso
a)Transition temperatures and enthalpy values were taken from the second DSC heating scans (10 K min−1, see also Figure S1); transition without detectable DSC peak
were determined by polarizing microscopy; values between brackets are monotropic phase transitions and in this case the enthalpy values are taken from the first DSC
cooling scans; abbreviations: Cr = crystalline solid; Iso = isotropic liquid; NCybC = nematic phase composed of SmC-type cybotactic clusters; SmC = synclinic tilted parale-
lectric SmC phase composed of small SmCsPF domains; SmCsPR = SmC phase with significantly increased domain size; SmCsPF = ferroelectric SmC phase; M1 = highly
viscous LC phase; SmCsPA = synclinic tilted antiferroelectric SmC phase (B2 phase); Colrec and Colrec1 = centered rectangular columnar phases with c2mm lattice (B1rev
phase); Colrec2 = non-centered rectangular columnar phase (undulated smectic phase); b)this transition shows significant hysteresis, on cooling the transition takes place at
62 °C; c)transitions not resolved, enthalpy value is the total of the SmC – NCybC – Iso transitions.
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Figure 2. XRD investigation of an aligned sample of compound A8:
a) wide angle and b) small angle scattering of the NCybC phase at T =
120 °C, c) wide angle and d) small angle scattering with reciprocal lattice
and indexation as a centered Colrec phase at T = 80 °C; e) model of the
Colrec (B1rev) phase.
longer chains, the nematic range becomes smaller and it com-
pletely disappears for A18 with the longest alkoxy chains. In
the following, the distinct LC phases will be discussed in more
detail.
Figure 3. Temperature dependence a) of the splitting (Δχ/2) of the small
angle scattering and b) of the d-value of the maxima of the small angle
scattering in the NCybC, the SmC and the SmCSPR phases of compound A10.
2.4. Nematic Phases
On cooling from the isotropic liquid state, all compounds
with the exception of A18 form a nematic phase. The Iso-N
transitions are associated with transition enthalpies between
1.0 (A8) and 3.0 kJ mol−1 (A14), increasing with growing chain
length. For the longer homologue A16, the Iso-NCybC transition
is close to the NCybC-SmC transition and therefore, the contri-
butions of the enthalpies of the Iso-NCybC and NCybC-SmC tran-
sitions cannot be separated. In the temperature range of the
nematic phases the XRD patterns of magnetically aligned sam-
ples (B ∼ 1T) show a diffuse scattering in the wide angle region
centered at the equator (Figure 2a, see also Figure S3 and S4
for the other homologues) with a maximum at d = 0.46 nm
(T = 140 °C). The diffuse scattering in the small angle region
has a dumbbell-like shape, indicating the existence of cybotactic
vation is in line with the increase of the d-value on decreasing
temperature (Figure 3b).
For compounds A12 and A14 with longer chains and short
nematic ranges Δχ/2 is only around 12° (see Figure S5 and
Figure 4a). As the Δχ/2 splitting can in a first approximation be
related to the tilt of the molecules in the SmC clusters, this indi-
cates a tilted organization of the molecules in all NCybC phases
with a tendency of decreasing tilt with increasing chain length.
The transition enthalpies NCybC to SmC (<0.01–0.5 kJ mol−1) are
much smaller than the Iso – NCybC transition enthalpy values
(1.0–3.0 kJ mol−1), in line with the cybotactic cluster structure
of the nematic phases.
clusters of the SmC type; hence these nematic phases are
[10,31]
assigned as NCybC
.
The splitting of the small angle scat-
2.5. Colrec Phases of Compounds A8 and A10
tering maxima of compound A8 is Δχ/2 = 28° (T = 140 °C)
and it only slightly decreases to Δχ/2 = 25° at T = 90 °C (see
Figure S2). In case of compound A10, the maximum close to
the clearing temperature is Δχ/2 = 18° and a strong temperature
dependence is observed, reaching a minimum of Δχ/2 = 4–5°
close to the transition to the SmC phase (Figure 3a). This obser-
Upon cooling the nematic phase of A8 the XRD pattern
changes significantly at T = 83 °C, at the transition to the low
temperature phase with spherulitic texture (Figure 5b). This
transition is associated with a significant enthalpy change (see
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Figure 5. Optical micrographs of textures observed between crossed
polarizers in a homeotropic cell for compound A8: a) nematic phase at
T = 130 °C and b) Colrec phase (B1rev phase) at T = 80 °C. For a colour
version of the textures, see Figure S8 in the Supporting Information.
Figure 4. Temperature dependence a) of the tilt angle ß (calculated from
the splitting Δχ/2) and b) of the d-value of the XRD patterns in the LC
phases of compound A14.
[32]
assigned as B1rev
.
In this phase the molecules are polar
Figure 6a). At this transition the wide angle scattering remains
diffuse, indicating a fluid LC phase, and in the small angle
region several sharp Bragg peaks appear (Figure 2c,d). These
peaks can be indexed on a centered rectangular lattice with
the parameters a = 3.7 nm and b = 4.6 nm (T = 70 °C, see
Table S1). The position of the wide-angle scattering maxima
remain on the equator, indicating the alignment of the mol-
ecules with their long axes parallel to b. The parameter b is in
good agreement with the molecular length (Lmol = 4.6 nm for
a 120° bent molecule with stretched wing groups and the alkyl
chains in all-trans conformation as determined using space
filling models). Both observations are in line with a nontilted
organization of the molecules. The 2D lattice results from
the formation of ribbons; the lateral diameter of these rib-
bons can be calculated from a/2 to be approximately 4–5 mol-
ecules organized side-by side. These XRD data, together with
the typical texture indicate a rectangular columnar LC ribbon
phase (Colrec), typical for short chain bent core mesogens also
ordered in the ribbons with antiparallel organization of the
polar direction between adjacent ribbons (see Figure 2e).[33]
At the transition from NCybC to Colrec the relatively strong
tilt of the molecules in the NCybC phase is suddenly removed
completely.
For compound A10 additional SmC phases appear between
the NCybC phase and the Colrec phase. The parameters a =
5.43 nm and b = 4.95 nm (at T = 70 °C; see Figure S3e and
Table S1) indicate a Colrec phase composed of broader rib-
bons, formed by about 6 molecules in the cross section (Colrec1
phase; B1rev phase). On further cooling a transition to a second
mesophase with 2D lattice takes place, associated with a sig-
nificant enthalpy change. For this non-centered rectangular
columnar phase with the parameters a = 10.10 nm and b =
4.93 nm (T = 60 °C, see Figure S3f and Table S1) an undulated
smectic structure is assumed (Colrec2 phase, USm phase). In
both low temperature phases the parameter b corresponds well
with the molecular length (Lmol = 5.1 nm), which is in line with
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2.6. Smectic Phases
2.6.1. DSC and Optical Investigations
A different kind of behavior is observed for compounds
A12–A18. All these compounds show essentially the same
sequence of five distinct smectic phases occurring below the
nematic phase (compounds A12–A16), only for A18 the nematic
phase is missing (see Figure 1 and Table 1). The smectic phase
sequence for all these compounds is SmC – SmCsPR – SmCsPF
– M1 – SmCsPA, the assignment of the distinct smectic phases
is based on the experimental results described below.
All enthalpies of the phase transitions between the smectic
phases are very small (only 0.1–0.4 kJ mol−1) and increase
slightly with growing chain length (Table 1, Figure 6b and
S1). No enthalpy values could be determined for the transition
between the phases assigned as SmC, SmCsPR and SmCsPF in
any case. The enthalpy of the phase transition SmCsPF – M1 is
very small (0.1–0.2 kJ mol−1). The highest transition enthalpy in
the smectic LC range is observed for the M1 – SmCsPA transi-
tion which is for all compounds around 0.4 kJ mol−1. Hence, it
appears that this is the major transition in the smectic region,
associated with the most significant changes of the phase
structure.
In planar cells the smectic phases develop with a typical fan
texture. With decreasing temperature the smooth fans become
sharper and the birefringence significantly increases, indi-
cated by the change of the interference color from red to dark
green, due to the increase of the order parameter of the aro-
matic cores with decreasing temperature (see Figure 7f,g, 6 μm
cell, any bluish interference color appears green due to orange
color of the compound). A tiny, but distinct jump in interfer-
ence color is observed at a specific temperature in the SmC
region of all compounds (see Figure S10). At this temperature
also other effects were observed, like a step in the dielectric
strength, as previously reported in ref. [8] for compound A16
(see Figure S15), and stripe pattern formation in free standing
films of compound A14 (see below). Therefore, the region
[8]
previously assigned as SmCsPR obviously includes two very
similar phases, a high temperature paraelectric SmC phase
and a low temperature phase assigned as SmCsPR (Figure 1,
hexagonal dots connected by a dotted line). Additional phase
transitions to the following phases are associated with only
slight textural changes. The major changes are a further jump
of the birefringence color from green to yellowish green at the
SmCsPR-SmCsPF transition (Figure 7g,h) and an increased vis-
cosity occurring at the transition to the phase assigned as M1
(Figure 7i) which becomes fluid again at the next transition to
SmCsPA (Figure 7j).
Under homeotropic anchoring conditions some of the tex-
tural changes are better pronounced. Especially remarkable are
the SmC and SmCsPR phase regions for which a low birefrin-
gent texture composed of distinct chiral domains is reproduc-
ibly observed for all compounds An by uncrossing the polarizers
by a small angle (Figure 8a–c). This leads to the appearance
of dark and bright domains; uncrossing the polarizer in the
opposite direction reverses the brightness of the domains
(Figure 8a,b). That the domains are due to chirality and not the
result of a different tilt-alignment is shown by rotation of the
Figure 6. a,b) DSC heating (top) and cooling (bottom) traces at a rate
of 10 K min−1: a) compound A8 and b) compound A14; c) shows the
DSC cooling trace of compound A14 for the temperature range of the
SmCsPR-M1-SmCsPF transition between 80 °C and 65 °C.
a non-tilted organization of the molecules. These 2D-modu-
lated low temperature LC phases of compounds A8 and A10
do not exhibit any response to an applied electric field.[32]
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to the phases assigned as M1 and SmCsPA
are associated with minor textural changes
(Figure 7c–e,h–j).
2.6.2. Behavior in Freely Suspended Films
The polar character of the smectic phases
was studied in freely suspended films in
compound A14. The films where prepared
in the smectic phase near the transition into
the nematic. The SmC phase exhibited a typ-
ical schlieren texture in thin (<200 nm) and
in thick films (>5 μm). Observing the films
under inclined incidence, we could confirm
that the phase has a synclinic character. No
polar response to an in-plane DC electric
field was found. On the contrary, dielectric
re-orientation was observed in an AC field.
The slow (optical) axis is aligned perpen-
dicular to the electric field. Distinct changes
occur at the SmC – SmCsPR transition at
about 102 °C, which is accompanied by the
formation of a network of strings, similar
to that described previously[34] (Figure 7k,l).
The width of the stripes increases with
decreasing temperature over an interval of
3–4 K (Figure 7m). This transition coincides
with the inflection points in the SHG signal
intensity vs. T (see Section 2.6.5) and polari-
zation vs. T curves (see Section 2.6.4). There
is also a distinct jump in dielectric strength
Δε at this transition, as reported previously
for A16 (see Figure S15).[8] So the SmC –
SmCsPR transition appears to be associated
with a distinct increase of the size of the
polar domains.
At about 90 °C, corresponding to the
SmCsPR – SmCsPF transition, the stripe pat-
tern has disappeared and a smooth schlieren
texture is observed (Figure 7m). This phase
shows a clear polar response to an electric
field exhibiting 2π director inversion walls
typical for polar films (see Section 2.6.4).
Figure 7. Optical micrographs of the textures observed for the different LC phases of com-
pound A14 in a homeotropic cell (left column), in a planar cell (middle column, arrows indicate
a region where textural changes can be identified) and in free standing films (right column) at
the indicated temperatures for the indicated LC phase types. In a) the homeotropic alignment
(dark areas) is developing from the initially formed homogeneous alignment (green areas).
sample between crossed polarizers, which gives no significant
change of brightness, independent on the angle. (Figure 8d,e).
This corroborates that the distinct regions indeed represent
chiral domains with opposite handedness.[8] The chiral domains
are clearly visible for the SmC and SmCsPR phases of all com-
pounds without any change at the SmC – SmCsPR transition,
but they disappear at the transition to the NCybC phases. The
transition of the SmCsPR phase to the SmCsPF phase is asso-
ciated with an anchoring transition to a birefringent defect
texture (Figure 7b,c). At this transition birefringence strongly
increases and it becomes difficult to detect the chiral domains
anymore. However, it appears that chirality is conserved in the
few remaining low birefringent regions and reheating into the
SmCsPR phase completely restores the chiral domains. Possible
reasons for achiral symmetry breaking in the SmC and SmCsPR
phases are discussed in Section 3.3. The next phase transitions
These findings indicate that the phase has a residual polari-
zation. That is why we designated it as SmCsPF. The polar
director aligns along the electric field and the c-director (tilt) is
perpendicular to it. We found this behavior in thick and in very
thin films as well.
The transition into the M1 phase is marked by development
of a mosaic-like schlieren texture (Figure 7n). No response to
an electric field could by found in this phase.
During the next transition, the texture becomes more dis-
torted in thick films. On the other hand, thin films exhibit a
typical schlieren texture (Figure 7o). The texture response to an
electric field was found only in thin films. The switching char-
acter showed an odd-even effect as typical for antiferroelectrics.
There is a reorganization of the c-director only in films with
odd number of layers; for films with an even number of layers
either no response is observed or the c-director aligns parallel
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of the diffuse scattering in the wide angle
region remains on the equator, indicating a
synclinic arrangement of the molecules in
the smectic phase (Figure 9c,d). No funda-
mental changes in the XRD patterns are vis-
ible at the following phase transitions until
crystallization. A more detailed analysis of
the XRD data indicates an increase of the
tilt angle with decreasing temperature in
all SmC and SmCsPR phases. In agreement
with this, the d-values become smaller
with decreasing temperature (Figure 4,
S5 and S7). These trends continue in the
SmCsPF phase. As shown in Figure 4a for
compound A14, in the SmCsPA phases
at lower temperature the tilt angles (β =
29–31°) are larger than in the SmCsPR
and SmCsPF phases and the d-values are
also increased (Figure 4b). This is due to
a chain stretching induced by the denser
packing of the molecules in the SmCsPA
phases. An increase of the packing den-
sity in the SmCsPA phases is also found for
compounds A12 (Figure S5) and compound
A16 (Figure S7). The M1-phases represent
a discontinuity in the development of the
d-values and tilt-angles. There is a jump
to smaller tilt angles at the SmCsPF – M1
transition and an increase of the d-values in
the M1-phase. This confirms that a denser
packing of the molecules starts in the M1
phase region. Overall the transition SmC
– SmCsPR – SmCsPF is nearly continuous,
as also indicated by the absence of any
transition enthalpy. Major changes with
significant increase of the packing density,
associated with distinct jumps in d-values
and tilt angles, take place at the SmCsPF
M1 and M1 – SmCsPA transitions.
–
Denser packing in the M1 and SmCsPA
phases is also evident from a slight shift of the
position of the wide angle scattering maximum
from 0.48 to 0.46 nm (Figure 10b and S7c).
Also the layer reflection becomes more intense
in the M1 and SmCsPA phases which is in line
with the development of sharper interlayer
interfaces resulting from a denser packing of
the aromatic cores (Figure 10a and S7d).
Figure 8. Textures of the SmC phase of compound A14 at T = 130 °C between glass slides
(homeotropic alignment): c) between crossed polarizers and a,b) between slightly uncrossed
polarizers, showing dark and bright domains, indicating the presence of areas with opposite
chirality sense; d,e) show the texture between crossed polarizers, but after rotation of the
sample by 20° either clockwise or anticlockwise; the birefringence does not change which con-
firms chirality as origin of the effects seen in a–c); there is no change of the textures at the tran-
sition to the SmCsPR phase. For a colour version, see Figure S9 in the Supporting Information.
to the applied field. This effect is a direct evidence for an anti-
2.6.4. Electro-Optical Studies in ITO Cells and Free Standing Films
ferroelectric structure of the phase.[35] Therefore, we designated
this phase as SmCsPA.
Electro-optical investigations were carried out in 6 μm
indium tin oxide (ITO) coated cells with polyimide alignment
layer under an applied triangular wave voltage. Figure 11
shows the current response curves at different temperatures
for A14 and Figure 12 shows the dependence of the polari-
zation on temperature for the same compound. Switching
curves obtained in 2 μm cells are shown in Figure S12.
Upon application of a triangular wave voltage of frequency
2.6.3. XRD Studies
Samples of A14 aligned by magnetic field were studied by 2D
XRD. At the transition from the NCybC to the SmC phase the
four diffuse spots in the small angle region transform into
two peaks located beside the meridian, whereas the maxima
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Figure 9. XRD investigation of an aligned sample of compound A14: a,b)
NCybC phase at T = 138 °C; c,d) SmC phase at T = 132 °C; a,c) wide angle
patterns after subtraction of the pattern of the isotropic phase (150 °C)
and b,d) small angle scattering after subtraction of the pattern of the
isotropic phase (150 °C).
10 Hz a relatively weak and rather broad single peak per
half cycle of the applied voltage was observed in the current
curves in the SmC phase range, which increases in intensity
as temperature decreases (see Figure 11a,b). In the SmCsPR
region the slope of the I vs. T curve increases and reaches a
polarization value of PS ∼ 130 nC cm−2 close to the transition
to the SmCsPF phase occurring at T = 90 °C (see Figure 12).
Below this temperature, the shape of the current response
curves changes and two widely separated peaks appear and
grow in intensity (Figure 11c,d, PS ∼ 360 nC cm−2). Both
peaks appear to be positioned at the ends of the conduc-
tive part of the curve symmetrically with respect to the field
inversion point (Figure 11c and S12c,d). This indicates that
the switching occurs hysteresis free. With further decreasing
temperature the distance between the peaks decreases and
the polarization value rises to PS ∼ 440 nC cm−2 (Figure 11d)
A switching with two polarization peaks and similar polari-
zation values between 540 and 660 nC cm−2 is also observed
in the two LC phases at lower temperature (M1, SmCsPA,
Figure 11e,f).
Under the field applied across a homogeneously aligned cell
a birefringent fan-like texture, with dark extinctions inclined
with the directions of polarizer and analyzer, is observed
(Figure S11). This confirms a synclinic tilt in these smectic
phases as also deduced from the XRD investigation of aligned
samples (Figure 9c,d). Neither in the SmC or SmCsPR phases
nor in the SmCsPF phase or in one of the low temperature
phases the switching process is associated with a change of the
orientation of the extinctions (see Figure S11), which indicates
Figure 10. a) Small angle and b) wide angle θ−scans over the XRD pat-
terns of compound A14 at different temperatures. For a clearer iden-
tification of the distinct curves, see colour version in Figure S6 in the
Supporting Information.
that in all phases switching takes place by a collective rotation
around the molecular long axis.
Electro-optical investigation of free standing films shows
only a dielectric response to a 100 Hz AC electric field in the
SmC phase down to 102 °C. Below this temperature, in the
SmCsPR and SmCsPF regions, a polar response on an external
electric field is observed; the switching is ferroelectric and
independent on the film thickness and the number of layers
(Figure 13). In the whole temperature range of both phases
the molecular long axis is perpendicular to the field direction,
in line with the polar moment being along the bend direc-
tion and the switching taking place around the molecular long
axis. Only regions with a fine stripe pattern, occurring in the
SmCsPR phase below the transition at 102 °C do not respond
to the electric field. It appears that the modulation resulting in
stripe formation is responsible for the absence of a response in
these regions of the thin films. As the stripes are dominating
in the SmCsPR range and disappear in the SmCsPF phase, the
switching becomes uniformly ferroelectric in the temperature
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field induced states of LC phases.[36] A plot
of SHG intensity versus temperature for
compound A14 is shown in Figure 14a.
Under constant applied electric field (Epp
= 20 V μm−1) a very weak SHG signal is
already observed in the isotropic phase at
T = 148 °C. It is retained in the nematic
phase. In the temperature range of the SmC
phase it slightly grows on cooling,[37] the
slope increases in the SmCsPR region and
then it continuously and rapidly increases
in the SmCsPF phase range to reach the
saturation in the M1 phase. The SHG
response remains nearly constant in the
temperature region of the M1 phase and
also in the upper temperature range of the
SmCsPA phase and then it drops (s. Figure
S14b). There is a different field depend-
ence of the SHG signal in the temperature
range of the SmC and SmCsPF phases. In
the SmC range the SHG signal continu-
ously grows without reaching a saturation
upon increasing the applied voltage (max-
imum: 25 V μm−1, Figure 14b), whereas in
the SmCsPF phase, close to the transition to
the M1 phase a distinct plateau is reached
at a voltage of ∼17 Vμm−1 (Figure 14c) the
former typical for paraelectric switching,
the latter indicating polar switching above a
certain threshold voltage.
Figure 11. a–f) Switching current response curves for compound A14 in a 6 μm ITO cell on
applying a triangular wave field (10 Hz); the dotted line indicates the applied triangular wave
voltage; a) SmC phase at T = 120 °C; b) SmCSPR phase at T = 100 °C; c) SmCSPR – SmCSPF tran-
sition at T = 90 °C; d) SmCSPF phase at T = 85 °C; e) M1 phase at T = 74 °C and f) SmCsPA phase
at T = 65 °C; different scales are used for the current response plots; for more switching current
curves in the SmC region, see Figure S13 and for investigations in 2 μm cells, see Figure S12.
In concord with the current reversal
measurements, no residual SHG signal
was found after switching off the applied
field in ITO cells in the switchable phases.
It appears that the SmC phase behaves as a
paraelectric phase[7,34,37] with unusually high
range of the SmCsPF phase. This is in line with dielectric results
obtained for the SmC/SmCsPR phase region of compound A16
(Figure S15), where a high dielectric strength is observed in
the SmCsPR region close to the SmCsPF phase and it steeply
decreases on approaching the M1 phase.[8] This is indicative of
a strong increase of the polar domain size in the temperature
range of the SmCsPF phase.
The M1 phase does not show any response in freely sus-
pended films, neither to DC nor to AC fields, most probably
due to the high viscosity. In ITO cells, under higher field
strength, antiferroelectric switching is observed in both phases
M1 and SmCsPA (Figure 11e). In the SmCsPA phase antifer-
roelectric switching is observed as well in free standing films.
In the films the response depends on the film thickness and
occurs with an odd-even effect of the number of layers on the
response. This phase at lowest temperature behaves like a typ-
ical B2 phase of bent-core molecules.
2.6.5. SHG Investigations
Figure 12. Polarization as function of temperature as measured for com-
pound A14 in a 6 μm ITO cell on applying a triangular wave field (10 Hz)
Second harmonic generation (SHG) is a powerful tool to
detect macroscopic polar order in the ground state and in
at 160 Vpp
.
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Figure 14. a) SHG intensity of compound A14 vs. temperature (6 μm
ITO cell, applied voltage: 20 V/μm) and dependence of the SHG-Intensity
on the applied voltage b) in the SmC phase at T = 110 °C and c) in the
SmCsPF phase at T = 78 °C.
Figure 13. Reorientation of the polar director in a freely-suspended film in
the SmCsPR phase of A14 at T = 97 °C: a) E = –12 V/mm, b) E = 0 V/mm,
c) E = +12 V/mm; the field direction is indicated by arrows; the film is
observed under inclined incidence.
susceptibility to an electric field, in the SmCsPR phase range
the switching appears to be ferroelectric as in the SmCsPF
phase. On the contrary, all switchable phases below SmCsPR
behave as antiferroelectrics in ITO cells; even the phase desig-
nated as SmCsPF does not exhibit bistable switching. Possible
reasons are outlined in the discussion.
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3. Discussion
3.1. Phase Sequence Depending on Alkyl Chain
Length and Temperature
As expected, nematic phases dominate for the short chain com-
pounds and the nematic range decreases with chain elonga-
tion. All nematic phases represent NCybC phases as typical for
bent-core mesogens and the size of the cybotactic SmC clus-
ters increases with alkyl chain length and with decreasing tem-
perature.[10] The tilt in the SmC clusters decreases with rising
chain length, but there is no uniform temperature dependence.
For the compounds with short chains (n = 8, 10) the core-core
interactions dominate, which lead to a transition from the
Ncybc phases to Colrec phases, occurring directly (A8) or via a
SmC phase (A10). In the Colrec phases polar ribbons of non-
tilted molecules arranged on a centered rectangular lattice with
antiparallel correlation between the ribbons (B1rev phases).
In this Colrec phase of A8 there are relatively large interfaces
between the aromatic ribbons and the columns incorporating
the aliphatic chains and this is unfavorable for molecules
with longer chains. At first the ribbon diameter is increased
for compound A10 in the Colrec1 phase and at lower tempera-
ture a transition to non-centered rectangular columnar phase
(Colrec2 phase), presumably representing an undulated smectic
phase, takes place. For molecules with n > 10 columnar phases
are removed and formation of smectic phases is favored. In
the smectic phases there is a tendency to increased tilt with
rising alkyl chain length, which is the result of the increased
area required by longer conformationally disordered chains.
With decreasing temperature the packing density of the aro-
matic cores increases. This gives rise to local polar domains
in the SmC phase, the size and the correlation length of these
domains increases with decreasing temperature leading to the
SmCsPR phase. At the transition to the M1 phase the combi-
nation of a denser packing and enhanced polar order most
probably leads to a layer modulation, which is assumed to be
the reason for the relatively high viscosity of this phase. In the
SmCsPA phase with highest packing density the molecules
adopt antiferroelectric order (see Figure 15).
Figure 15. Development of polar order in the SmC phases of compounds
An depending on temperature; PE, FE and AF denote paraelectric, ferro-
electric and antiferroelectric switching, respectively.
observed in freely suspended films. In analogy to the SmAPR
phase, we designate this phase as SmCsPR, as a SmC phase
with uniform tilt, but with randomly distributed polar domains.
In the SmCsPF phase the size of the polar domains diverges
and a polar phase develops. Observation of the polar response
and inversion walls in freely-suspended films unambiguously
indicates that the phase has a residual polarization. Hence, we
designate it as SmCsPF. Antiferroelectric-type switching found
in ITO cells seems to contradict our finding. This contradiction
can be explained by a strong surface anchoring on the glass,
which prevents a bistable switching and leads to a hysteresis-
free director reorientation. Another possibility is the formation
of a modulated structure of the director as observed in freely
suspended films, which suppress the uniform ferroelectric
state in the absence of a field. As the polarization is still rela-
tively small a relatively high threshold field is required for fer-
roelectric switching, despite the switching itself appears to be
thresholdless.
Further increased polar coupling leads to the highly vis-
cous phase M1. The viscosity in this M1 phase might be due
to the layer distorting effect provided by polar order and chi-
rality to the layers, though a layer modulation is not evident
from the XRD patterns. As polar order becomes stronger the
packing density in the layers is further increased and the layers
become sufficiently robust to withstand layer distortion, leading
to the transition to the SmCsPA phase. The fact that there are
two polarization peaks in the whole temperature region from
SmCsPF via M1 to SmCsPA and that these peaks come closer by
reducing the temperature without distinct jump at the phase
transitions (Figure 11b–e) supports the suggested stabilization
of the SmCsPA structure in the ITO cells.
3.2. Development of Polar Order
The phase sequence Iso – (NcybC) – SmC – SmCsPR – SmCsPF –
M1 – SmCsPA – Cr is typically observed for compounds An with
sufficient chain length (n > 10). Figure 15 shows the observed
development of polar order in the series of smectic phases.
SmC-type cybotactic clusters were already found in the nematic
phase (NCybC) which persisted also in the isotropic phase. Field-
induced SHG in the nematic and isotropic phases may be
attributed to the polar order within the clusters.[38] The polar
domains persist in the SmC phase too, which are probably
responsible for the current response observed in the ITO cells
and an enhanced field-induced SHG. On the other hand, the
response of the phase to an electric field remains paraelectric
in the SmC region. In the SmCsPR region these domains reach
appreciable size and macroscopic ferroelectric domains can be
As the largest enthalpy value is associated with the
M1-SmCsPA transition it is assumed that major changes take
place at this transition and therefore all phases above this tem-
perature are associated with a growing size of the polar SmCsPF
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Table 2. Comparison of 4-cyano substituted bent-shad mesogens with different linking groups X1 and X2.
Comp.
X1
X 2
Ref.
Phase transitions T/ °C
Cr 71 (SmCsPA 70) M1 75 SmCsPF 83 SmCsPR 96 SmC 127 NCybC 137 Iso
Cr 65 SmCPA 122 SmC 141 SmA 188 Iso
A12
B12
C12
D12
–N=N–
–N=CH–
–OOC–
–COO–
–N=N–
–CH=N–
–COO–
–OOC–
[9]
Cr 100 SmAPA 112 SmA 185 Iso
[10]
[10]
Cr 103 (SmCPA’’ 68 SmCPA’ 75 SmCPA 94) SmC 109 NCybC 129 Iso
domains and increasing polar correlation between them. Some
of the continuous transitions, especially the SmC–SmCsPR
transitions, manifest only under certain conditions.
phenylbenzoate wings. For both compounds, only tilted
smectic phases were observed and a nematic phase (NCybC) is
formed at higher temperatures. The Schiff base mesogen B12
and the terephthalate C12 behave differently; they have about
50 K higher clearing temperatures; show no nematic phases
and have a higher tendency to form non-tilted phases, the
terepthalate forming exclusively SmA phases. The absence of
N phases and the higher mesophase stability indicate a denser
molecular packing, which is most likely due to the reduced
electron density in rings B which provides stronger π-stacking
interactions between these rings.[22] The presence of electron
deficit rings seems also to favor the formation of nontilted
phases. Compounds A12 and B12 with double bonds involved
in the π-conjugation pathway between the benzene rings have
about 30–40 K lower melting points compared to the ester com-
pounds C12 and D12, being more flexible, but having a sig-
nificant component of the C = O dipole aligned perpendicular
to the molecular long axis. In all cases polar order develops in
a stepwise manner via paraelectric SmA or SmC phases on
decreasing temperature. The specific feature of the azobenzenes
An is that this transition takes place via a new SmCsPR phase,
a ferroelectric SmCsPF phase[44,45] and an additional highly vis-
cous phase M1. Moreover, among the compounds mentioned
in Table 2 only for A12 chiral domains were reported in the
SmC range. Advantageous is also the low melting point of A12
which is comparable with the Schiff base compound B12. The
azobenzenes An provide the additional option of modulation of
the phase structure and mode of polar order by photoisomeri-
zation,[25–29] which is presently under investigation.
3.3. Achiral Symmetry Breaking
Another important observation is the chirality in the SmC/
SmCsPR phase region, which is observed by the forma-
tion of conglomerates of macroscopic chiral domains for all
compounds in sufficiently thin cells. It appears at the NCybC
– SmC transition and is retained in the SmCsPR phase. The
origin of this chirality should be the locally chiral SmCsPF
structure in the polar domains. The coupling of this supras-
tructural chirality with the molecular chirality,[39] in this case
with the conformational molecular chirality[40,41] favors molec-
ular conformations with a helical shape and a distinct helix
sense. As helical molecules cannot pack exactly parallel[42] the
emergence of chirality leads to a reduction or even possibly
to an inversion of the bend elastic constant k33.[43] Formation
of the chiral domains is supported by surface anchoring, as
indicated by the thickness dependence of this phenomenon[8]
and the absence of chiral domains in freely suspended films.
However also the above mentioned specific molecular and
superstructural factors provided by the compounds and their
distinct modes of self-assembly are favorable for the occur-
rence of chiral segregation for the compounds under discus-
sion. Hence, the combined effects of superstructural chirality
and surface interaction should contribute to achiral symmetry
breaking.
4. Summary and Conclusions
3.4. Comparison with Related 4-Cyanoresorcinol-Based Bent-
Core Mesogens
We have reported the synthesis and phase behavior of a series
of photosensitive azo functionalized bent-core molecules
consisting of 4-cyanoresorcinol as central core. This series
allowed the investigation of the development of polar order
in tilted smectic phases depending on chain length and tem-
perature. It appears that with increasing polar domain size a
ferroelectric phase is formed first which becomes modulated
and then changes to the antiferroelectric phase. It turned out
that this transition from nonpolar to polar phases is more com-
A comparison of compound A12 with the liquid crystalline
properties of previously reported related compounds with the
same chain length, where only the azo group is replaced by
other linking groups, like –CH=N–, –COO– or –OOC–
(compounds B12-D12), is shown in Table 2.
[9,10]
Considering the mesophase stability (clearing tempera-
tures) the azobenzene A12 is similar to compound D12 with
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probably due to the restrictions provided by tilt and chirality in
the SmCsPF clusters. All observed LC phases, with exception
of the SmCsPA phase at lowest temperature are composed of
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Supporting Information is available from the Wiley Online Library or
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Acknowledgements
M. Alaasar is grateful to the Alexander von Humboldt Foundation for
the research fellowship at the Martin-Luther University Halle-Wittenberg;
A. Eremin acknowledges German Research Foundstion DFG (project ER
467/2) for the financial support.
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highly viscous intermediate states in free standing films was recently
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indicates that these states might be general features of the transition
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[45] Considering the antiferroelectric response found in all smectic
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[38] This is interesting if one compares these compounds with the
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wileyonlinelibrary.com
Adv. Funct. Mater. 2014, 24, 1703–1717
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1717