Azo-containing asymmetric bent-core liquid crystals with modulated smectic phases

Article abstract of DOI:10.1039/c5ra09764a

A new series of azo-containing bent-core liquid crystals derived from 3-hydroxybenzoic acid has been synthesized. Their mesomorphic properties have been characterized by polarizing optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction and electro-optic studies. Almost all the compounds form an enantiotropic modulated smectic (B₇ type) phase over relatively broad temperature ranges. Structural modifications, such as the type and length of the terminal chains, the rigidity of wings, and the presence of a Cl-substituent in different positions of the bent core, affect the appearance and temperature range, but not the type of the mesophase of the investigated compounds. Light-induced changes in the texture and phase transition of the mesophase, attributed to the decrease of the order parameter due to trans-cis isomerization, have also been observed.

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Textual abstract for the Table of contents entry
We present the synthesis and characterization of ten asymmetric bent-core liquid crystals with
enantiotropic modulated smectic (B7 type) phases. Their relatively low and wide photosensitive
mesomorphic temperature range offers promising potential applications.

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Azo-containing asymmetric bent-core liquid crystals with
modulated smectic phases
Nemanja Trišović,*a Jelena Antanasijević,a Tibor Tóth-Katona,b Michal Kohout,c Miroslaw
Salamonczyk,d,e Samuel Sprunt,e Antal Jáklid and Katalin Fodor-Csorbab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A new series of azo-containing bent-core liquid crystals derived from 3-hydroxybenzoic acid has been synthesized. Their
mesomorphic properties have been characterized by polarizing optical microscopy, differential scanning calorimetry,
small-angle x-ray diffraction and electro-optic studies. Almost all the compounds form an enantiotropic modulated smectic
(B7 type)phase over relatively broad temperature ranges. Structural modifications, such as the type and length of the
terminal chains, the rigidity of wings, and the presence of a Cl-substituent in different positions of the bent core, affect the
appearance and temperature range, but not the type of the mesophase of the investigated compounds. Light-induced
changes in the texture and phase transition of the mesophase, attributed to the decrease of the order parameter due to
the trans-cis isomerization, have also been observed.
www.rsc.org/
orientation of the linking group (ester, azomethine) between
aromatic rings in the bent-core. Interestingly, the position of the
Introduction
Due to their ability to form polar order and phase chirality without
chiral molecules, bent-core liquid crystals (BCLCs) offer exceptional
properties (ferro-, antiferro- piezo-, and flexoelectricity)1–2, which
have high potential in both materials science and supramolecular
chemistry. In recent years, significant attention has been devoted to
azo group in the bent-core does not seem to affect the
mesomorphic behaviour of BCLCs, whereas it has a profound effect
on their photo-induced properties12. On the other hand, the
clearing temperature and spontaneous polarization are more
sensitive to light when the azo group is situated in the wings
compared to cases where it is directly attached to the central
azo-containing symmetric and asymmetric BCLCs3–16
, dimers
combining rod-like and bent-core units17,18, molecules with short
core units containing four aromatic rings19,20, and their side-chain
polymers21,22. Additionally, photoisomerizable chromophores that
allow a change in the molecular conformation by a photoreaction
may open up possibilities for polar responses modulated by light.
phenyl ring. This can be attributed to
a greater structural
incompatibility in packing of the cis and trans isomers of the
molecules in the layers of the former type of systems.
In BCLCs with resorcinol as the central ring and azobenzene with
different alkoxy chain length as wings, the introduction of a large
substituent in position 4 of the resorcinol moiety (CH3, Br and I)
together with an additional peripheral F-substituent favours the
development of a new type of mesophase composed of chiral
domains with opposite handedness (called dark conglomerate
phases)7–9. The corresponding BCLCs with 4-chlororesorcinol form
nematic and B6 mesophases depending on the chain length4, while
the ones based on 4,6-dichlororesorcinol exhibit only a nematic
mesophase3. When resorcinol is replaced by the pyrimidine ring,
Asymmetric BCLCs with an azobenzene wing exhibit a variety of
mesophases (N, B1, B2, B7, SmA, SmAdPAetc) over
a wide
temperature range6,11–15. It has been shown that in a homologous
series of asymmetric BCLCs, the lower homologues usually exhibit a
B1 and the higher ones a B2 mesophase11. Even a small change in
the molecular structure produces drastic changes in their
mesomorphic properties, i.e. the appearance, temperature range
and nature of mesophase are governed by the types and the
the formation of a B6 mesophase is observed23
.
Here we describe the chemical synthesis and physical
characterizations of a new series of azo-containing asymmetric
BCLCs (Fig. 1). The mesomorphic properties are varied by the type
and length of the terminal chains, rigidity of wings, and by
introducing a Cl-substituent into different positions on the bent-
core. Almost all the compounds are found to be liquid crystalline,
exhibiting an enantiotropic modulated smectic (B7 type) phase over
relatively broad temperature ranges, and offering promise for
potential applications.
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Scheme 1 Synthesis of wings with the ester linking group (14–19)
Fig. 1 Chemical structures of the investigated BCLCs
Results and discussion
Scheme 2 Synthesis of wings with the azo linking group (25, 26)
Synthesis
The studied asymmetric BCLCs were derived from 3-hydroxybenzoic
acid, whereby their wings were obtained by the already known
methodologies5,24. The wing with the ester linking group between
the phenyl rings (14–19) were prepared by an acylation of alkyl 3-
substituted-4-hydroxybenzoates (3–7) with 3-substituted-4-
benzyloxybenzoyl chloride (1, 2) and a subsequent deprotection of
the intermediate esters 8–13 according to a literature procedure24
(Scheme 1). In the synthesis of 4-alkoxyphenylazobenzoic acids
(Scheme 2), ethyl 4-aminobenzoate (20) was firstly diazotizated and
the resulting diazonium salt (21) was coupled with phenol yielding
ethyl 4-(4-hydroxyphenylazo)benzoate (22). The obtained
compound 22 was alkylated with 1-bromoundecane or 11-bromo-1-
undecene in the presence of potassium carbonate to give ester
compounds (23, 24), which were further hydrolyzed under basic
conditions to yield corresponding acids (25, 26). Similarly, (4-(11-
undecenoyl)oxyphenyl)azobenzoic acid (30) was prepared by
diazotization of 4-aminobenzoic acid (27) and coupling of the
resulting diazonium salt (28) with phenol yielding 4-
hydroxyphenylazobenzoic acid (29) which was further acylated with
11-undecenoyl chloride (Scheme 3).
Scheme 3 Synthesis of the wing with the azo linking group (30)
The synthesis of the target BCLCs was accomplished in three steps
(Scheme 4). First, hydroxy esters 14–19 and dodecyl 4’-
hydroxybiphenyl-4-carboxylate (32) were acylated by acid chloride
of the protected central core 31. Subsequently, the benzylic
protecting group was removed by transfer hydrogenation with
cyclohexene and palladium on charcoal as a catalyst. In the last
step, the hydroxy esters 40–46 were acylated by the acid chlorides
of the second side wings 25, 26 and 30 affording the target
materials Ia–j. BCLCs synthesized with the acids 26 and 30 possess a
double bond in the terminal chain and they serve as model
monomers for polymeric liquid crystals. The complete synthetic
procedures and analytical data are reported in the electronic
supporting information.
Scheme 4 Synthesis of the target materials Ia–j
sequences are presented in Table 1. All compounds are thermally
stable as confirmed by the reproducibility of the thermographs in
several heating and cooling cycles. Typical DSC thermographs taken
on the second cooling run are shown in Fig. 2 for four selected
compounds; the others are presented in Fig. S1.
Calorimetric investigations
The thermal behaviour of the compounds has been studied by
differential scanning calorimetry (DSC). Their transition
temperatures (°C) measured at 5 °C min–1 heating/cooling rates, the
associated phase transition enthalpies (kJ mol–1), and the phase
2 | J. Name., 2012, 00, 1-3
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Fig. 2 DSC temperature plots for compounds Ia–Id detected at 5 °C
min-1 heating/cooling rates. The upper curves correspond to the
second cooling and the corresponding lower curves to the second
heating run.
With exception of compound Ih, all compounds form the
enantiotropic mesophase (M) over relatively broad temperature
ranges. The transition temperatures of compounds Ia–d are nearly
identical regardless of the length of the terminal chain R2. A double
bond in the terminal chain R1 of compound Ie causes a slight
decrease in the clearing temperature compared with Ic, whereas an
ester group leads to an additional reduction of the mesophase
temperature range (from 50 °C for Ic to 25 °C for Ig). When a Cl-
substituent is introduced in the peripheral phenyl ring in the ester-
containing wing (compound Ii), both melting and clearing
temperatures decrease for ca. 35 °C and the temperature range of
the mesophase is narrowed. On the other side, introduction of a Cl-
substituent in the phenyl ring located closer to the central ring
destabilizes the mesophase and results in a crystalline compound
Ih.
Fig. 3 (a
)
X-ray intensities vs. q for Ii recorded at T=100 °C (red
line) and at T=91 °C (black line) with corresponding Miller
indices. (b) Suggested molecular packing. Note, the polar
direction cannot be determined from x-ray measurements.
X-ray results
2M detector) and at Structural Research Laboratory of University of
The structures of mesophases were identified by small-angle x-ray
diffraction (SAXS) measurements carried out on beamline 7.3.3 of Warsaw (BrukerNanoStar system with Vantec2000 detector, Cu Ka
the Advanced Light Source of Lawrence Berkeley National Lab
(10keV incident beam energy (1.24 Å wavelength), utilizing a Pilatus
radiation (1.54 Å), equipped with heating stage).
Table 1 Transition temperatures (°C) and the enthalpies of transitions (kJ mol–1) for the synthesized BCLCs
Compound
R1
C11H23
C11H23
C11H23
C11H23
R2
X
H
H
H
H
H
H
H
Y
Z
2nd Heating
2nd Cooling
Ia
Ib
Ic
Id
Ie
If
Ig
Ih
Ii
C8H17
C10H21
C12H25
C14H29
C12H25
C14H29
COO
COO
COO
COO
COO
COO
COO
H
H
H
H
H
H
H
H
Cr 122.9 (–55.3) M 151.0 (–18.3) I I 149.2 (19.1) M 81.4 (49.5) Cr
Cr 120.8 (–58.7) M 148.5 (–19.9) I I 145.8 (20.6) M 81.0 (53.1) Cr
Cr 121.0 (–64.8) M 147.6 (–20.0) I I 146.0 (21.8) M 94.0 (66.0) Cr
Cr 121.1 (–72.9) M 146.1 (–18.5) I I 144.3 (21.3) M 86.4 (72.1) Cr
Cr 113.2 (–57.1) M 141.7 (–18.9) I I 139.6 (19.8) M 77.2 (52.1) Cr
Cr 113.0 (–59.7) M 141.1 (–17.8) I I 139.2 (18.5) M 78.0 (68.9) Cr
Cr 127.0 (–46.0) M 138.9 (–13.8) I I 136.9 (15.8) M 115.1 (50.1) Cr
Cr 125.5 (–64.9) I
Cr 97.9 (–42.2) M 105.0(–14.4) I
Cr 130.1 (–40.5) M 143.4 (–18.9) I I 141.5 (20.4) M 101.4 (41.0) Cr
CH2=CH(CH2)9
CH2=CH(CH2)9
CH2=CH(CH2)8C(=O) C12H25
C11H23
C11H23
C11H23
C12H25 Cl COO
C12H25
C12H25
I 120.8 (63.0) Cr
I 103.8 (13.0) M 79.9 (41.4) Cr
H
H
COO Cl
–
Ij
H
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Table 2 The crystallographic unit cell parameters measured by x-ray
Fig. 4 Variation of q value of (01) and (10) signals vs. temperature
for compound Id
in the B7-type phase at different temperatures.
times larger than of the (10) peaks (see Table 2). This type of
pattern is similar to the so-called B7 phase25,26, in which the
undulated layers are continuous through the polarization splay
defects. However, when the layer undulation is so strong that the
layers break into small ribbons, i.e., the layer displacement steps at
the defects27, it can be considered as a columnar (B1) structure28. A
model structure of the molecular packing corresponding to this
latter situation is shown in Fig. 3(b). The distinctions between the
B1 and B7 structures is discussed in a recent review29. For recent
reviews discussing modulated and undulated smectic phases, as
well as columnar phases made of bent-core mesogens30,31. The
angle β between the (10) and (01) peaks was found to be very close
to 90° with the exception of Ic where β ~ 65°. The cell parameters at
the high and low temperature ranges of the B7-type phase for all
compounds are summarized in Table 2.
T (°C)
151
103
145
110
148
112
143
91
141
107
134
117
100
91
a (Å)
c (Å)
β (deg)
90
90
Ia
Ib
Ic
Id
If
47.14
48.43
48.11
48.27
54.52
52.63
49.37
50.15
50.61
51.67
50.11
50.15
49.75
49.91
48.09
49.14
162.42
122.98
141.33
196.22
158.34
208.75
137.12
233.21
135.3
180.39
161.99
218.98
105.73
112.02
113.48
145.79
87.6
88.1
65.2
71.7
89.2
88.0
89.2
89.6
89.7
89.6
88.7
89.4
85.9
88.4
Ig
Ii
Ij
143
113
Typical temperature dependencies of q(10) and q(01) are shown for
compound Id in Fig. 4. One can see that while q(10) is only slightly
increasing on cooling (for example a(140 °C) = 4.95 nm and a(90 °C)
= 5.07 nm), q(01)varies strongly ranging between 0.045 and 0.025 Å–1
in cooling, corresponding to variation of the periodicity the layer
modulation in the range of c ~12–22 nm.
The x-ray patterns recorded for all studied compounds show 2-D
lattice structure corresponding either a columnar (B1) phase, or a
modulated smectic (B7) mesophase. The typical q dependence of
the diffracted intensities in the peaks, and their corresponding
Miller indices, are shown in Fig. 3.
Polarizing optical microscopy studies
The strongest signal indexed as (10) corresponds to electron density
Typical POM textures are shown in Fig. 5. On cooling from the
isotropic phase flower–like patterns grow and fill the space
eventually. This texture is not typical for the classical B7 phases with
characteristic helical superstructures25, but rather similar to that of
another asymmetric bent-core liquid crystal that possesses two
ferroelectric mesophases32. This dissimilarity might be related to
the absence of large steric inclusions in the molecular cores, which
are present in classical B7 materials33, or indicates that the phase is
rather a columnar phase.
modulations parallel to the smectic layers with spacing a = 2π / q(10)
,
that was found to be about 5.0 nm for all studied materials (see
Table 2 and S1). The (01) peaks are an order of magnitude weaker
than the (10) peaks and the corresponding c = 2π / q(01) periodicities
are 2–4
We also tested the effect of light on the textures and phase
transition. The result of the simple test when illuminated the
sample by the light of a 100 W halogen lamp of the polarizing
microscope (I=0.14 W cm–2) is shown in Fig. 5(c) and (d) zero and six
minutes after the light was turned on. One clearly sees more
isotropic areas after 6 minutes illumination, which shows the
decrease of the order parameter due to the trans–cis isomerization
of the molecules. More quantitative measurements to measure the
optical effects are under way.
The fully developed mesophase in thin films of sample sandwiched
between substrates treated for planar alignment show broken fan
textures typical of smectic phases. The characteristic size of the
smectic domains increases with slower cooling rates. A typical
example for a planar texture formed under 0.1 °C min–1 cooling rate
in 5 µm film is shown in Fig. 6. Although no polarization current
peaks could be detected up to 50 V µm–1 fields, the birefringence of
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the films changed under strong applied electric fields. This is
illustrated for compound Ia in Fig. 6(b). After removal of the field,
the
Fig. 6 Polarized optical microscopy texture of Ia without electric
field (a) and with electric field (ca. 55 V µm–1). Scale bar, 150 µm.
Crossed polarizers.
Discussion
To make conclusions about structure-property relationships, we
compare our materials with related compounds from the literature.
Nagaveni et al. have reported that compound with the opposite
orientation of the ester linking groups in the bent core (see 47 in
Fig. 7a) does not have any liquid crystalline properties11. This is
because the orientation of the ester linking groups has significant
influence on the flexibility polarity and bending angle of the
molecules34. It has also been demonstrated that replacing the
terminal alkanoyloxy chain in 47 with an alkyl or alkyloxy chain
leads to BCLCs which exhibit a B2 mesophase14.
Interestingly, the mesophase textures of our materials are rather
similar to those of another asymmetric bent-core liquid crystal that
possesses a single tilted synclinic ferroelectric (SmCsPFE) phase in a
higher temperature range and an antiferroelectric (SmCsPAF) phase
at lower temperatures24,32 (see material 48 of Fig.7a). Furthermore
even the clearing temperature of 48 is similar to our corresponding
azo-compound Ig.
Fig. 5 Polarised optical microscopy textures in cooling during the
phase transition from the isotropic to the B7-type phase of Ii at
106.6 °C (a) and Ia at 150.5 °C (b). Texture of Ii at T=106 °C a few
seconds (c), and 6 min (d) after the microscope light is turned on.
Scale bar, 50 µm. Crossed polarizers are along the edges of the
picture.
There are only few examples of azo-containing BCLCs forming the
B7 mesophase. Srinivasan et al. have found that compound 49 (Fig.
7b) with a biphenyl unit exhibits an unusual B7mesophase, where
different textures, like helical filaments, striped focal conics or fan-
shaped, oval domains, myelinic and checkerboard textures have
been observed6. Compound 50 (Fig. 7b) with symmetrical wings
exhibit an enantiotropic B7 mesophase over about 15 °C the
temperature range13. A series of six-ring BCLCs (51 in Fig. 7b) is also
shown to have a B7 mesophase16. Although characterized by a wide
mesomorphic range (ca. 100 °C), these compounds possess very
high clearing temperatures. The low and wide mesomorphic ranges
of the compounds reported in this study might be useful for
potential light sensitive applications.
birefringent texture was unchanged indicating that the electric field
only caused some realignment of the layers and was not related to
ferroelectric switching.
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photochromic properties, we have presented the chemical
synthesis and physical characterization of a new series of azo-
containing asymmetric BCLCs. Almost all compounds form an
enantiotropic layer modulated (B7 type) phase. Interestingly,
structural modifications, such as the type and length of the terminal
chains, the rigidity of wings, and the presence of a Cl-substituent in
the peripheral phenyl ring in the ester-containing wing, influence
only the width of the mesomorphic temperature range, but not the
type of the mesophase formed. The relatively low and wide
photosensitive mesomorphic temperature range offers possible
applications in such areas as high-density data storage systems,
sensors, photonic switches and molecular logic gates.
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This work was supported by the Serbian Ministry of Education,
Science and Technological Development (Project no. 172013), M-
era.Net [project MACOSYS (OTKA NN110672)]. Measurements at
Berkeley and Kent were supported by NSF under grant DMR-
1307674. The authors acknowledge important technical help and
discussions from C. Zhu at the ALS, Lawrence Berkeley National Lab.
M.S. acknowledges Prof. E. Gorecka (University of Warsaw) for the
opportunity to perform an x-ray measurements.
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6 | J. Name., 2012, 00, 1-3
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26. G. Pelzl, S. Diele, A. Jákli and W. Weissflog, Liq. Cryst., 2006,
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