Self-assembling behaviour of chiral calamitic monoacrylates targeted for polymer stabilisation of polar smectic phases in chiral liquid crystals

Article abstract of DOI:10.1016/j.molliq.2021.115723

The stabilisation and control of synclinic and anticlinic chiral liquid crystalline phases remain an actual and highlighted task. This work presents the design and mesomorphic properties of new chiral calamitic reactive mesogens, monoacrylates, based on biphenyl benzoate and phenyl biphenyl-4-carboxylate cores with reactive terminal groups placed far from the chiral chain. The compound's structures, compatible with the components of modern ferroelectric and antiferroelectric liquid crystalline mixtures, are confirmed by mass spectrometry (electron ionization) analysis and proton/carbon nuclear magnetic resonance. All the reactive mesogens possess the self-assembling, i.e. liquid crystalline behaviour with high tendency to create smectic phases in a broad temperature range, which was confirmed by a polarizing optical microscopy, differential scanning calorimetry and broad-band dielectric spectroscopy. Doping of advanced multicomponent mixtures possessing the chiral smectic phases by the designed monoacrylates with further cross-linking procedure, can be an effective and functional tool for stabilising ferroelectric and antiferroelectric phases in various states and hence, can allow the symmetrisation of the switching times in the modes employed surface stabilised geometry; this is a very highlighted task for optoelectronics. Moreover, an evident considerable tendency for thermal polymerisation of specific reactive mesogens can reduce the drawbacks of polymer stabilisation.

Full text of DOI:10.1016/j.molliq.2021.115723

Journal of Molecular Liquids 331 (2021) 115723
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Self-assembling behaviour of chiral calamitic monoacrylates targeted for
polymer stabilisation of polar smectic phases in chiral liquid crystals
a
a
b
Ewelina Dmochowska a, , Jakub Herman , Michał Czerwiński , Sergei Stulov ,
⁎
Alexej Bubnov b, Przemysław Kulaa
a
Faculty of Advanced Technologies and Chemistry, Military University of Technology, 00-908 Warsaw, Poland
Institute of Physics of the Czech Academy of Science, 182 21 Prague, Czech Republic
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The stabilisation and control of synclinic and anticlinic chiral liquid crystalline phases remain an actual and
highlighted task. This work presents the design and mesomorphic properties of new chiral calamitic reactive
mesogens, monoacrylates, based on biphenyl benzoate and phenyl biphenyl-4-carboxylate cores with reactive
terminal groups placed far from the chiral chain. The compound's structures, compatible with the components
of modern ferroelectric and antiferroelectric liquid crystalline mixtures, are confirmed by mass spectrometry
(electron ionization) analysis and proton/carbon nuclear magnetic resonance. All the reactive mesogens possess
the self-assembling, i.e. liquid crystalline behaviour with high tendency to create smectic phases in a broad tem-
perature range, which was confirmed by a polarizing optical microscopy, differential scanning calorimetry and
broad-band dielectric spectroscopy. Doping of advanced multicomponent mixtures possessing the chiral smectic
phases by the designed monoacrylates with further cross-linking procedure, can be an effective and functional
tool for stabilising ferroelectric and antiferroelectric phases in various states and hence, can allow the
symmetrisation of the switching times in the modes employed surface stabilised geometry; this is a very
highlighted task for optoelectronics. Moreover, an evident considerable tendency for thermal polymerisation
of specific reactive mesogens can reduce the drawbacks of polymer stabilisation.
Received 9 December 2020
Received in revised form 10 February 2021
Accepted 18 February 2021
Available online 23 February 2021
Keywords:
Self-assembling organic materials
Reactive mesogens
Monoacrylates
Polymer-stabilised liquid crystals
Ferroelectric phase
Antiferroelectric liquid crystals
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
chirality in oblique phases is the occurrence of spontaneous polarisation
Ps. A spontaneous polarisation enables the control of molecules by
Among self-assembling organic materials [1,2], chiral calamitic liq-
uid crystalline materials [3] with diverse self-organised superstructures
have found a broad potential application in a variety of optical and
electro-optical devices such as thermography [4], reflective displays
[5], tunable mirrorless lasers [6] and biomedical [7] as well as photonic
sensors [8] owing to their remarkable optical properties and high sensi-
tivity to different external stimuli. The self-organising structures formed
by these type of smart materials can be effectively controlled, tuned or
changed by an applied external stimulus, such as external electric/mag-
netic field, mechanical stress or irradiation with light of different wave-
length [9,10].
Antiferroelectric (AFLC) and ferroelectric liquid crystals (FLCs) are
smectic materials formed of chiral calamitic molecules that may exhibit
a variety of smectic tilted structures, anticlinic (SmCA*) and synclinic
(SmC*) for AFLCs and FLCs respectively. Molecules in both of these
phases spontaneously form a helicoid structure. A consequence of
means of an external electric field, which makes it possible to control
optical properties of the liquid crystal. In the ferroelectric phase, the
molecules are arranged in layers, and they are tilted in one direction
with respect to the layer normal by an certain angle. In turn, the mole-
cules in the layers are also tilted by the same angle in the antiferr-
oelectric phase, but the layers are arranged alternately. A schematic
diagram of the arrangement of molecules in the ferroelectric and
antiferroelectric materials is shown in Fig. 1 (a) and (b) respectively.
The FLCs are bistable, they create two synclinic states that maintain
the arrangement without an electric field. It also means that, depending
on the applied electric field, all the molecules in the layers reorientate
one way or the other. While the AFLCs are used as monostable, but tri-
state materials. The stable state is the state with no applied voltage
(E = 0), with anticlinical ordering. Depending on the sign of the applied
field, two ferroelectric states are obtained with opposite direction of the
vector of spontaneous polarisation.
Antiferroelectric smectic liquid crystals could provide an efficient al-
ternative to nematic materials, commonly used in displays and other
devices; due to their shorter response times and their specific properties
promising for passive addressing [11–13]. AFLCs are extremely
⁎
Corresponding author at: Faculty of Advanced Technologies and Chemistry, Military
University of Technology, Gen. Sylwestra Kaliskiego Str. 2, 00-908 Warsaw, Poland.
E-mail address: ewelina.dmochowska@wat.edu.pl (E. Dmochowska).
https://doi.org/10.1016/j.molliq.2021.115723
0167-7322/© 2021 Elsevier B.V. All rights reserved.

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
Fig. 1. Schematic arrangement of molecules in a) the ferroelectric phase (two stable states for E = 0) with marked turns of the spontaneous polarisation vector; b) the antiferroelectric
phase (one stable state for E = 0, and two synclinic states depending on the applied voltage on the right and left) with marked turns of the spontaneous polarisation vector.
interesting for use in large-size high-resolution display applications
[14]. Unfortunately, the main problem of these materials relays in asym-
metrical switching times. Nevertheless, the idea is to design structurally
compatible chiral reactive mesogens and add them to the base mixtures.
This will allow the polymer-stabilisation of antiferroelectric and ferro-
electric systems with a much better symmetry of the switching time
[15–17], under the condition that most of the targeted properties re-
main unchanged.
phase [27], antiferroelectric phase [46], ferroelectric phase [34] and for
the modification of the resulting electro-optical properties [44,47,48].
The basic idea of creating a polymer network in the presented examples
is stabilising a liquid crystal director configuration in which it was cre-
ated [34]. The use of neat FLCs has some limitations such as homoge-
neous orientation problems when placed in thin cells and their low
resistivity to mechanical shock [49]. The stabilisation of these systems
with a polymer network can minimise these technical problems
[34,49,50]. AFLCs without a helicoid structure beyond a synclinic state,
when molecules in all layers are tilted in the same direction, are
characterised by an anticlinic state in which molecules in adjacent
layers are tilted in opposite directions [34,51]. The main problem of
AFLCs is the asymmetric switching times between these two states.
The creation of a polymer network in the anticlinic state causes the re-
laxation time from synclinic to anticlinic state to be shorter when com-
pared to neat materials [34]. Therefore, it has a significant impact on the
electro-optical and physicochemical parameters of these materials. Be-
sides shortening the response time, there is also an improvement in
grayscale and greater mechanical resistance [34,49]. However, the pres-
ence of a polymer network in (A)FLCS brings also sever disadvantages,
such as an increase in operating voltage and a decrease in optical con-
trast [48]. For this reason, it is very important to develop reactive
mesogens with structures compatible with (A)FLCs and smectic phases.
It should allow the use of such reactive mesogens in smaller amounts
and allow the realisation of polymer-stabilisation of antiferroelectric
and ferroelectric systems, provided that most of the targeted properties
of AFLCs or FLCs remain unchanged.
Reactive mesogens (RMs) are liquid crystalline materials with an ad-
ditional functionality induced by the presence of the polymerisable ter-
minal chain or lateral groups [18]. For this purpose, acrylate [19–21],
methacrylate [22–24], vinyl [25–28] or cinnamoyl groups [29] can be
used. The reactive terminal groups can be placed on one side (bifunc-
tional monomers) or both sides (tetrafunctional monomers) of the mol-
ecule [30,31]. Acrylate esters belong to very important industrial
chemicals which are widely used as precursors in the fabrication of
technically substantial special type polymers due to the presence of a
double bond as well as a carbonyl group [32,33]. They are used in the
large-scale preparation of useful polymers [33]. The reactive mesogens
used to stabilise the liquid crystal phases may themselves be a liquid
crystal material, but this is not a prerequisite as long as they have a sim-
ilar shape and anisotropy of the liquid crystal host material [34]. Poly-
merisation of RMs leads to creation of cross-linked networks in which
the liquid crystalline order is permanently fixed [20,35] (see the exam-
ple of polymer stabilised antiferroelectric liquid crystals in Fig. 2). This
process usually occurs under UV light illumination under the presence
of specific photoinitiators [36]. The most important advantages of
photopolymerisation over the other techniques are reaction speed,
energy savings and light control [37,38]. The speed of photopolymer-
isation is particularly important if one wants to create a polymer net-
work in a given liquid crystal phase of a specific liquid crystal
material. Polymerisation of RMs in liquid crystal phases has been widely
explored for the stabilisation of particular optical states in blue phases
[39–41], nematic/cholesteric phases [42–45], paraelectric smectic
The design synthesis and characterisation of two homologues series
of new chiral monoacrylates based on biphenyl benzoate and phenyl
biphenyl-4-carboxylate molecular cores (see Fig. 3) aimed for
stabilisation of anticlinic or synclinic structures are presented in this
work. These cores, bridge bonds, flexible spencer's or chiral centre, are
commonly used in stable ferroelectric and antiferroelectric systems
[52–54]. This structural compatibility should ensure good miscibility
Fig. 2. A simplified scheme for the formation of a polymer-stabilised antiferroelectric liquid crystal structure with a hypothetical simplified arrangement of molecules.
2

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
of reactive mesogens with the (A)FLC materials. Similar low molar mass
materials with molecular core based on biphenyl benzoate [55] and
phenyl biphenyl-4-carboxylate [27,56,57] are quite common. However,
the reactive mesogens with similar molecular core are also described in
literature; nevertheless, most designed reactive mesogens possess only
one ester linkage in the structure; moreover, mostly the single com-
pounds (not the homologues series) with specific molecular structure
are described [58–63]. In the literature, there are examples of stab-
ilisation by a polymer network of antiferroelectric materials with reac-
tive mesogens with a nematic phase [64,65], which were initially
intended to stabilise the nematic and cholesteric phases [34] or reactive
mesogens based on bent-core [66]. To the best of our knowledge there
Fig. 3. General molecular structures of synthesised chiral monoacrylates.
Fig. 4. Scheme of the synthesis of the series based on a biphenyl benzoate core.
3

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
Fig. 5. Scheme of the synthesis of the series based on a phenyl biphenyl-4-carboxylate core.
4

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
Table 1
Phase sequences and phase transition temperatures T [°C] of the studied reactive mesogens obtained by DSC (first row in bold). Enthalpies ΔH [kJ/mol] of the phase transitions are given in
italics (second row). $- monoacrylate that shows an evident tendency for thermal polymerisation just above the clearing temperature.
Compound
Cr
T [oC]
SmC*
T [oC]
SmA*
T [oC]
Iso
ΔH [kJ/mol]
ΔH [kJ/mol]
ΔH [kJ/mol]
16
17
18
19
20
*
*
*
*
*
84.9
27.85
84.7
24.51
51.2
19.26
56.0
24.01
51.9
32.16
–
–
–
–
*
–
–
–
–
*
*
*
*
*
90.0
3.12
104.6
4.64
99.4
3.92
101.4
2.93
94.6
2.69
*
*$
*$
*
69.1
0.07
*
30
31
32
33
34
*
*
*
*
*
93.3
29.14
47.8
21.10
73.4
24.87
97.4
–
–
*
–
–
*
*
*
*
111.9
2.31
119.6
4.63
117.9
4.63
118.8
4.36
*
*$
*$
*
89.9
0.06
–
–
34.17
64.9
Polymerisation above 65.0 in smectic phase
24.86
are nostudies on stabilisingAFLC materials with reactive mesogens hav-
ing ferroelectric and/or antiferroelectric phases.
oriented parallel to the substrates) made from glasses with transparent
electrodes of indium‑tin-oxide (ITO) (5 × 5 mm2) 5 μm thick were used
for texture observations and electro-optical measurement of the spon-
taneous polarisation.
The planar cells (Military University of Technology, Warsaw,
Poland) in bookshelf geometry with gold electrodes 5 μm thick were
used for broad-band dielectric spectroscopy. The sample cells were
filled with the studied LC materials in the isotropic phase by the capil-
lary action.
The sequence of phase transitions and temperature of phase transi-
tions were determined by polarizing optical microscopy (POM)
using”Olympus” BX51 polarizing microscope (Shinjuku, Tokyo, Japan)
equipped with a Linkam heating/cooling stage THMS-600 (Linkam Sci-
entific Instruments Ltd., Tadworth, United Kingdom). The differential
scanning calorimetry DSC 204 F1 Phoenix instrument (Netzsch, Selb,
Germany) was used to estimate enthalpy of phase transitions and as a
precise technique for determining temperature of phase transitions. In
both cases the scanning rate was 2 K min−1 on both the heating and
cooling cycles.
The frequency dispersion of complex permittivity (ε* = ε'-iε″) has
been measured within the temperature range of the SmA* and SmC*
phases on cooling, using a Schlumberger 1260 Impedance/Gain-Phase
Analyzer (Solartron Group Ltd., Hampshire, United Kingdom) in the fre-
quency range of 1 Hz ÷1 MHz keeping the temperature stable during
the frequency sweep within 0.1 K. The measurements have been per-
formed under zero d.c. bias voltage.
This work presents the systematic design of the new reactive
mesogens that are structurally compatible with the components of ferro-
electric and antiferroelectric liquid crystals mixtures [52–54,64,66–69].
This should help stabilise these phases and minimise the negative influ-
ence of these monoacrylates on destabilisation of synclinic and anticlinic
smectic phases and their target properties responding to the demands of
various electro-optical modes [67–76]. The effect of the addition of some
synthesised here monoacrylates on the properties of the newly devel-
oped high-tilted AFLC mixtures with different phase sequence, before
and after polymer stabilisation is described in [77].
2. Experimental
The chemical purity of synthesised compounds was determined
by thin layer chromatography, Shimadzu GCMS-QP2010S series gas
chromatograph (Shimadzu, Kyoto, Japan) equipped with a quadru-
pole mass analyzer (MS), high-performance liquid chromatography
HPLC-PDA-MS (APCI-ESI dual source) Shimadzu LCMS 2010 EV
(Shimadzu, Kyoto, Japan) equipped with a polychromatic UV–VIS
detector (Shimadzu, Kyoto, Japan). The optical purity was deter-
mined by HPLC analysis using Lux® 5 μm Cellulose-1 column
(Phenomenx, Torrance, USA) (hexane–i-PrOH 90:10, 1 mL/min).
Proton (1H) and carbon (13C) nuclear magnetic resonance (NMR)
spectra in CDCl3 was collected using a Bruker, model Avance III spec-
trometer (Bruker, Billerica, MA, USA).
The values of spontaneous polarisation, PS, were determined using
the switching current profile detected with Tektronix oscilloscope
(Tektronix, Beaverton, OR, USA); a specific software for automation of
The planar cells (Military University of Technology, Warsaw,
Poland) in bookshelf geometry (where the long axes of molecules are
5

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
Fig. 6. Phase diagram for monoacrylates based on a biphenyl benzoate core (DSC measurements on the heating cycle).
the spontaneous polarisation measurements was used. An electric field
of triangular modulation at frequency of 50 Hz with amplitude of 10 V/
μm was applied during the Ps measurements.
second series is based on phenyl biphenyl-4-carboxylate core (see the
synthesis in Fig. 5).
The synthesis scheme of the first series is presented in Fig. 4. The
synthesis was started from n-(benzyloxy)alkan-1-ol (1), where the
number of carbons in the chain ranged from n = 2 to n = 6. The hy-
droxyl group of (1) was esterified with mesyl chloride, receiving com-
pound (2) with a good leaving group, which was useful in the reaction
of the nucleophilic substitution with ethyl 4-hydroxybenzoate (3).
Then carboxylic acid derivative (5) was obtained by reaction of 4-
[(benzyloxy)alkyl]-benzoate (4) with KOH, next with HCl and used in
the esterification reaction with 4-(1,3,2-dioxaborinan-2-yl) phenol
(9). The synthesis of 4-(1,3,2-dioxaborinan-2-yl) phenol (9) was started
from 4-bromophenol (6) in which the hydroxyl group was protected by
the benzyl group. The next step was the Grignard reaction through
which a boronic acid derivative (7) was obtained. Subsequently, the bo-
ronic ester derivative was obtained by reaction with 1,3-propanediol.
The hydroxyl group was then deprotected by the hydrogenolysis (9).
The boronic acid derivative (10) obtained as a result of the esterification
reaction was used in the Suzuki-Miyaura cross-coupling reaction with
2.1. Materials
(S)-2-octanol, acryloyl chloride, methanesulfonyl chloride, benzyl
chloride, were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sol-
vents were purchased from Avantor Performance Materials Poland S.A
(Gliwice, Poland). and used without further purification unless other-
wise noted. Tetrahydrofuran (THF) was distilled from sodium benzo-
phenone ketyl and stored under nitrogen and toluene was distilled
from P2O5.
2.2. Synthesis
Two series of reactive mesogens were synthesised; the first series is
based on biphenyl benzoate core (see the synthesis in Fig. 4) and the
Fig. 7. Phase diagram for monoacrylates based on a phenyl biphenyl-4-carboxylate core (DSC measurements on the heating cycle).
6

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
of the hydroxyl group (28) and the esterification of the hydroxyl deriv-
ative (29) with acryloyl chloride respectively. Five final products were
obtained (30 for n = 2, 31 for n = 3, 32 for n = 4, 33 for n = 5, 34 for
n = 6).
The final reactive mesogens were isolated and purified using a mul-
tiple crystallisation sequences and liquid column chromatography
techniques.
A detailed description of the synthesis and related basic characteri-
sation are given in the Appendix (see Supplementary materials).
2.3. Mesomorphic properties and discussion
The sequence of phases and phase transition temperatures of the
designed reactive mesogens determined by POM and DSC on heating
cycles are presented and summarised in Table 1. For a better visualisa-
tion of the mesomorphic behaviour, Figs. 6 and 7 show the phase tem-
perature ranges for all obtained monoacrylates from the first and the
second series, respectively.
Fig. 8. Microphotographs of the characteristic POM textures obtained for different
mesophases for monoacrylates 20 (top row: the SmC* at T = 64 °C and SmA* at
T = 92 °C) and 32 (bottom row: the SmC* at T = 78 °C and SmA* at T = 114 °C) on the
cooling cycle. The width all the microphotographs corresponds to ~200 μm.
The monoacrylates with a rigid biphenyl benzoate core (the first se-
ries) are characterised by the presence of the orthogonal SmA* phase. In
the case of odd homologues, an increase in the length of the terminal
chain causes a decrease in the phase transition temperatures and an in-
crease in the temperature range of the smectic phases. For homologues
with an even number of carbon atoms in the olighomethylene spacer,
the reactive mesogen 18 with the medium chain length exhibits the
lowest melting point and simultaneously has the broadest temperature
range of the smectic A* phase. The monoacrylate 20 with the longest
terminal chain exhibits additionally the tilted ferroelectric SmC*
phase. The enthalpies of melting points are the lowest for monoa-
crylates with intermediate terminal chain lengths in this series.
The monoacrylates with a rigid phenyl biphenyl-4-carboxylate core
(the second series) exhibit, similar as previous ones, only the smectic
phases within the mesophases. For the even-numbered homologues
of this series, a decrease in the melting point is observed with an in-
crease in the length of the terminal alkyl chain. The reactive mesogen
34 with the longest even chain has a very strong tendency towards ther-
mal polymerisation in the smectic phase, while the monoacrylate 32
with an intermediate length of the terminal chain additionally exhibits
the tilted ferroelectric SmC* phase. Among compounds with an odd
number of the carbon atoms in the alkyl chain, the longest one for
monoacrylate 32 has the narrowest temperature range of the smectic
(S) 2-octan-2-yl 4-iodobenzoate (13) (this compound was obtained by
the esterification reaction between 4-iodobenzoyl chloride (11) and
(S)-2-octanol (12)). Finally, the hydroxyl group of the coupling reaction
product (14) was deprotected by hydrogenolysis and the obtained hy-
droxyl derivative (15) was used for the esterification reaction with
acryloyl chloride. As a result of the synthesis, five final products were
obtained and each reactive mesogen in the series differs in the length
of the carbon chain in the structure of the molecule (16 for n = 2, 17
for n = 3, 18 for n = 4, 19 for n = 5, 20 for n = 6).
The synthetic pathway of the second series based on phenyl
biphenyl-4-carboxylate core is presented in Fig. 5. The synthesis was
started with 4-(benzyloxy)benzoic acid (21) which was reacted with
oxalyl chloride, followed by the esterification with (S)-chiral alcohol
(12) to get the compound (22). Then, during the hydrogenolysis, the
hydroxyl group was deprotected to form a chiral hydroxyl derivative
(23). After the esterification with 4-(1,3,2-dioxaborinan-2-yl)benzoyl
chloride (24); one of the main reactants (25) to the Suzuki-Miyaura
cross-coupling reaction was obtained. Then, the reaction of 2-
(benzyloxy)alkan-1-ol (1) with 4-iodophenol (26) gave iodine deriva-
tive (27)- the second main substrate for the cross-coupling reaction.
The next steps, analogous to the previous series, were the deprotection
Fig. 9. 3D plots of real ε’ (a) and imaginary ε” (b) parts of complex permittivity versus frequency and temperature for the 20th monoacrylate.
7

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
Fig. 10. Temperature dependence of the spontaneous polarisation, Ps for monoacrylates 20 and 32 possessing the ferroelectric SmC* phase.
phase. It is worth noting the abnormally high melting points and their
enthalpies for monoacrylates 30 and 33 compared to other homologues
of this series.
phenyl 4-carboxylate molecular cores. It is important to keep in mind
that these reactive mesogens are designed with a specific molecular
structure making them more compatible with the components of ferro-
electric and antiferroelectric liquid crystalline mixtures possessing well-
known properties. This can ensure good miscibility of designed reactive
mesogens with LCs bases forming the synclinic and anticlinic phases. All
the designed reactive mesogens are highly smectogenic; most of the de-
signed materials possess a relatively broad temperature range of the or-
thogonal SmA* phase; moreover, materials with longer alkyl chains
exhibit the tilted smectic SmC* phase possessing the ferroelectric
order. The existence of ferroelectric behaviour for monoacrylates 20
and 32 was demonstrated by the Ps measurements and dielectric spec-
troscopy. The latter revealed the presence of the Goldstone mode in
the SmC* phase and this is specific for the synclinic phases with a ferro-
electric order. The spontaneous polarisation for reactive mesogens 20
and 32 reaches values more than 150 nCcm−1 at lower temperatures.
Several reactive mesogens show an increased tendency to thermally po-
lymerise just above the clearing temperatures. The reactive mesogen
with a rigid phenyl biphenyl-4-carboxylate core and a six‑carbon alkyl
chain already polymerises in the temperature range of the smectic
phase. Such a strong tendency for self-polymerisation is favourable as
it can considerably reduce the amount of the reactive mesogen used as
dopant for stabilisation of the ferroelectric and antiferroelectric phases;
this is very important as it reduces a very common drawback related to
the polymer network presence in the LC medium. Further study of reac-
tive mesogens presence on mesomorphic and physicochemical proper-
ties of ferroelectric and antiferroelectric liquid crystalline materials and
their multicomponent mixtures are in progress and will be presented
elsewhere.
The reactive mesogens from the first series generally have lower
melting and clearing point temperatures than the monoacrylates be-
longing to the second series. The tilted ferroelectric SmC* phase is
detected from the monoacrylate with shorter alkyl chains in the sec-
ond series based on phenyl biphenyl-4-carboxylate than their first
series counterparts. The examples of the observed microphotograph
textures of the smectic phases for monoacrylates 20 and 32 are
shown in Fig. 8.
The presence of the ferroelectric phase for the obtained monoa-
crylates was also confirmed by dielectric spectroscopy measurements.
The real, ε', and the imaginary parts, ε″, of the complex permittivity
for monoacrylate 20 versus temperature and versus frequency are pre-
sented in Fig. 9a and b as an illustrative result to confirm the ferroelec-
tric character of the polar phase. The obtained dielectric spectra within
the whole temperature range of the ferroelectric SmC* phase at zero
bias electric field reveal a strong contribution of the Goldstone mode
(this is a collective process i.e. the relaxation mode related to azimuthal
fluctuations of the molecules in the smectic layer). In the vicinity of the
SmA* – SmC* phase transition, a collective mode related to the molecu-
lar fluctuations in the direction of the tilt magnitude, the so-called soft
mode, was detected. The results of the dielectric spectroscopy fully con-
firm the polar ferroelectric character of the lower temperature smectic
phase detected for monoacrylates 20 and 32. A detailed discussion of
the specific behaviour of detected modes, revealed by the broad-band
dielectric spectroscopy, with respect to the molecular structure of the
monoacrylates is beyond the scope of the present work and will be pre-
sented elsewhere.
Declaration of Competing Interest
The temperature dependence of the spontaneous polarisation Ps
(T) was measured for monoacrylates 20 and 32 under saturating field
10 V/μm and is presented in Fig. 10. Starting from the SmA* – SmC*
phase transition there is a continuous increase of the Ps values without
saturation; the maximal amplitude for spontaneous polarisation was
170 nC/cm2.
No potential conflict of interest was reported by the authors.
Acknowledgements
This work is co-financed by the European Social Fund under the “Op-
erational Programme Knowledge Education Development 2014-2020”.
The authors Sergei Stulov & Alexej Bubnov greatly acknowledge the
financial support of the Czech Science Foundation [project number CSF
19-03564S], EU COST Action [project number CA17139] and MEYS [pro-
ject number 8J20PL008].
2.4. Summary of the results and conclusions
This work presents an advanced synthetic approach for new chiral
acrylate-based reactive mesogens build up on the phenyl benzoate and
8

E. Dmochowska, J. Herman, M. Czerwiński et al.
Journal of Molecular Liquids 331 (2021) 115723
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2021.115723.
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