Laterally substituted symmetric and nonsymmetric salicylideneimine-based bent-core mesogens

Article abstract of DOI:10.3762/bjoc.8.15

Bent-core mesogens have gained considerable importance due to their ability to form new mesophases with unusual properties. Relationships between the chemical structure of bent-core molecules and the type and physical properties of the formed mesophases a

Full text of DOI:10.3762/bjoc.8.15

Laterally substituted symmetric and nonsymmetric
salicylideneimine-based bent-core mesogens
Sonja Findeisen-Tandel1, Wolfgang Weissflog*1, Ute Baumeister1,
Gerhard Pelzl1, H. N. Shreenivasa Murthy1
and Channabasaveshwar V. Yelamaggad2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 129–154.
1Martin-Luther-Universität Halle-Wittenberg, Institut für Chemie,
Physikalische Chemie, von-Danckelmann-Platz 4, 06120 Halle,
Germany and 2Centre for Soft Matter Research, Jalahalli, Bangalore
560 013, India
doi:10.3762/bjoc.8.15
Received: 14 September 2011
Accepted: 29 November 2011
Published: 24 January 2012
Email:
Wolfgang Weissflog* - weissflog@chemie.uni-halle.de
This article is part of the Thematic Series "Progress in liquid crystal
chemistry II".
* Corresponding author
Guest Editor: S. Laschat
Keywords:
antiferroelectric smectic phases; bent-core mesogens; electro-optical
behaviour; liquid crystals; salicylideneanilines
© 2012 Findeisen-Tandel et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Bent-core mesogens have gained considerable importance due to their ability to form new mesophases with unusual properties.
Relationships between the chemical structure of bent-core molecules and the type and physical properties of the formed mesophases
are relatively unknown in detail and differ strongly from those known for calamitic liquid crystals. In this paper symmetric and
nonsymmetric five-ring salicylideneaniline-based bent-core mesogens are presented, and the effect of lateral substituents attached at
the outer phenyl rings (F, Cl, Br) or the central phenyl ring (CH3) on the liquid-crystalline behaviour and on the physical properties
is studied. Corresponding benzylideneaniline-based compounds were additionally prepared in order to study the influence of the
intramolecular hydrogen bond. The occurring mesophases were investigated by differential scanning calorimetry, polarising
microscopy, X-ray diffraction and dielectric and electro-optical measurements. The paper reports on new findings with respect to
the structure–property relationships of bent-core mesogens. On one hand, the disruptive effect of laterally substituted halogen
atoms, F, Cl and Br, on the mesophase behaviour of three isomeric series was much lower than expected. On the other hand, an
increase of the clearing temperature by 34 K was observed, caused by small lateral substituents. The electro-optical behaviour,
especially the type of polar switching and corresponding molecular movements, is sensitive to variations in the molecular structure.
Introduction
For a long time there was a general perception that molecules commonly referred to as “calamitic phases”. However, the
capable of exhibiting liquid crystal (LC) phases have to be inherent fallacy of this notion was pointed out in the middle of
rodlike, so-called calamitic LCs. As is well known, such com- the seventies when columnar (Col) phases were discovered in
pounds exhibit nematic and smectic phases, which are disklike (discotic) mesogens; such mesogens also exhibit
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Beilstein J. Org. Chem. 2012, 8, 129–154.
nematic behaviour like that of calamitics. In fact, a survey of B7 phases is a splay of polarization, which gives rise to an
the literature shows clearly that molecules with a bent (banana) undulation of smectic layers or two-dimensionally ordered layer
shape prepared earlier than discotics by Vorlaender (in 1929) fragments [5].
possess the ability to exhibit mesomorphism. Interest in such
nonconventional mesogens increased remarkably in the mid- Due to such unusual behaviour banana-shaped liquid crystals
1990s following the report by Niori and co-workers [1] that the have emerged as a special topic in the field of liquid crystals. It
mesophases (banana phases) formed by 1,3-disubstituted should be pointed out here, that polar banana phases have only
benzenes hold unusual properties. As a result, a large number of been formed by bent molecules up until now. Although a bent
banana-shaped compounds have been synthesized with various conformation is also exhibited by dimesogens due to the pres-
combinations of structural fragments, and thus, they represent a ence of odd-parity central spacer, they do not spontaneously
new subfield of liquid crystals. Generally speaking, these meso- form polar banana phases [6,7].
gens prefer to pack in layers so as to yield smectic phases. Due
to their bent shape there is a lateral correlation of molecular In general, banana-shaped molecules consist of five or more
dipoles yielding a polar order within the layers, which can be aromatic rings, in which two wings (rodlike arms) are attached
switched on application of an electric field. The molecular to the 1,3-positions of a central phenyl ring. The vast majority
arrangement of bent-core molecules, with respect to the tilt and of the banana-shaped mesogens contain ester and/or azo-
the polarity in adjacent layers of a SmCP phase, is shown in methine linking groups. It is remarkable that derivatives
Supporting Information File 1, Figure S1. An explanation of the containing an azomethine group (Schiff bases) often show
suffixes P, A, F, s, and a, used for the characterization of interesting polymorphism. Relationships between molecular
banana phases, is also given there. In the SmCP phase the mole- structure and the formation of banana phases have been summa-
cular long axis is tilted with respect to the layer normal, which rized, together with the structure and the behaviour of these
corresponds to a C2 symmetry of the smectic layers. The combi- phases, in several reviews, see e.g., [8-11].
nation of director tilt and polar order leads to a chirality of the
smectic layers, although the constituent molecules are achiral It is well-known that a hydroxy group in ortho-position to the
[2]. In this context it should be noted that there is a tilted CH=N (azomethine) unit not only enhances the photochemical
smectic “banana phase” designated as SmCG, which exhibits and hydrolytic stability due to intramolecular H-bonding, but
chiral smectic layers with C1 symmetry [3,4].
also increases the clearing temperature of liquid crystals. There-
fore, the introduction of salicylideneimine fragment(s) in the
In order to avoid a macroscopic polar order, in most cases the LC molecular architecture has attracted significant attention.
polar axes in adjacent layers are antiparallel such that, in Several examples demonstrate this, as many salicylideneani-
general, antiferroelectric SmCPA phases occur, which can be lines exhibiting mesophases are listed, see e.g., [12,13]. These
switched into the corresponding ferroelectric state. This salicylideneanilines, but also salicylideneimines, can form
switching, and also the switching between opposite ferroelec- metallomesogens by complexation with d- and f-block
tric states, usually takes place by the collective rotation of the elements, [14-17]. Tris(salicylideneanilines) having a discotic
molecules around the tilt cone. It is seen from Supporting Infor- shape exhibit a room-temperature columnar mesophase [18,19].
mation File 1, Figure S1 that during this switching, the chirality Dendromesogens containing up to 64 peripheral mesogenic sali-
of the layers does not change. However, there are SmCP phases cylideneimine fragments have also been reported [20]. Antifer-
where the switching is based on a collective rotation of the roelectric behaviour has been claimed by Yablonski et al. to be
molecules around their long molecular axes. In this case the exhibited by achiral liquid-crystalline monomer–polymer
chirality of the smectic layers is changed by the switching mixtures containing salicylideneaniline moieties [21,22].
process, which is a prominent feature observed in the present
investigation.
Interesting examples of bent molecules are given by the
following: Two salicylideneimine moieties can be connected to
The formation of simple layer structures is disrupted if the each other to form twin molecules (Scheme 1A). In the case
amount of space required by the rigid cores and the flexible of an odd-numbered spacer these dimers exhibit a bent shape
terminal chains is too different. Together with certain polar and form nematic, columnar nematic, smectic and columnar
forces, such connection can result in undulating tilted smectic phases. Columnar phases were not found for the dimers having
phases designated as USmCP, or columnar phases of different even-numbered spacers, because these molecules are more
structure. The columns are represented by polar-layer frag- linear [23-27]. Interestingly, for a nonsymmetric chiral dimer
ments. Two examples are given: In the B1 phase, the columns the reentrant-phase behaviour SmA–SmAb–SmA was reported
form a 2D rectangular cell. However, the structural feature of [28].
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Scheme 1: Examples for bent mesogens containing salicylideneimine moieties.
Rao et al. postulated a new chiral smectic phase formed by can have a symmetric or nonsymmetric structure. If the
W-shaped achiral molecules that contain two salicylideneani- aromatic core consists of five phenyl rings containing one and
line units (Scheme 1B) [29,30]. Recently, new achiral four-ring two salicylideneaniline moieties we speak about mono- or disal-
molecules were reported by Deb et al., in which two substituted icylideneaniline compounds.
salicylideneaniline moieties are linked by an ester group
(Scheme 1C). These compounds, which show a strong photolu- Monosalicylideneaniline compounds with different chain
minescence, exhibit typical banana phases B1 and B7 [31]. lengths in the terminal positions have been reported. Addition-
Yelamaggad et al. reported V-shaped five-ring mesogens: Two ally, the effect of a nitro group neighbouring the hydroxy group
salicylideneaniline moieties are linked by ester groups to the has been studied. A mixture of one of these derivatives with a
1,2-positions of a phenyl ring. Due to the low bending angle of chiral twin compound was investigated by dielectric methods
about 70°, the molecules form a partially intercalated SmA [33,34]. Yelamaggad et al. [35] reported nonsymmetric mole-
phase instead of banana phases [32].
cules having two fluorine atoms in the 2,3-position of one outer
phenyl ring. SmCP phases with an unusual electro-optical
Banana phases can be expected when the bending angle is switching behaviour were observed. The SmCP phase of biden-
opened to about 120°. Already several bent-core mesogens with date bent-core ligands disappears in the corresponding Cu(II)
salicylideneaniline moieties have been reported. The molecules and Pd(II) metal complexes (Scheme 1D) [36].
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Beilstein J. Org. Chem. 2012, 8, 129–154.
In 2001, homologues of a series of symmetric compounds bearing a lateral methyl group at the central phenyl ring, in
bearing both azomethine groups between the outer and the next search of further materials that may also show the unusual
neighbouring inner phenyl rings were reported for the first time multistage switching as mentioned above. To assess the effect
by Yelamaggad et al. [37-39] who also investigated the phase of the intramolecular hydrogen bond on the mesophase behav-
behaviour under pressure and the effect of light on the polariza- iour, corresponding compounds without hydroxy groups were
tion of a mixture of these salicylideneanilines with a photoac- additionally prepared. In order to focus the study on the effect
tive azo compound. The observation of a B7 mesophase on the of lateral substituents, the hydrocarbon chains at both terminal
hexadecyloxy compound is surprising, because the central phenyl rings were fixed with dodecyloxy groups.
phenyl ring was not substituted by a nitro or cyano group as is
the case for most of the other banana-shaped liquid crystals that
Results and Discussion
form this phase type [40]. The nonyloxy homologue was
Monosalicylideneaniline compounds OH 1
studied in detail by Walba et al. For this compound, also abbre-
bearing the azomethine group between an
viated as NORABOW, the existence of the very rare SmCG
mesophase was claimed [41]. Achten et al. compared the phase outer phenyl ring and its neighbour
behaviour of the complete homologous series with that of the Synthesis of the compounds OH 1
corresponding 1,5-pentylene connected dimers [27]. In 2007, The synthesis of the compounds OH 1, which contain the azo-
we reported systematic investigations on the influence on the methine connection group between an outer phenyl ring and the
mesophase behaviour of lateral groups attached to the central adjacent ring of the five-ring bent-core mesogens, is sketched in
phenyl ring of several homologous series [42].
Scheme 2. The 4-(4-dodecyloxy-3-halogenobenzoyl-
oxy)benzoic acids 2 were prepared by acylation of 4-hydroxy-
Isomeric compounds, symmetric in the aromatic core, result benzaldehyde with the corresponding 4-dodecyloxy-3-
when both azomethine groups are linked to the central phenyl halogenobenzoic acids following oxidation of the formyl com-
ring. The influence on the mesomorphism of a fluoro pounds 1. The phenolic intermediates 3 were obtained by reac-
substituent at the central phenyl ring was studied by Rao et al. tion of 3-benzyloxyphenol with the substituted benzoic acids 2
[43]. Three years ago, we reported the synthesis and a remark- and hydrogenolytic deprotection of the hydroxy group
able electro-optical behaviour of three bent-core compounds according to [47]. The final products OH 1a–c were obtained
derived from 2-methyl-1,3-phenylenediamine. The unusual by esterification of the phenols 3 with 4-(4-dodecyloxy-2-
current response showed five peaks per half period. Such multi- hydroxybenzylideneamino)benzoic acid (4), which exhibits a
stage switching was measured for the first time on banana- liquid-crystalline phase itself (Cr 204 N 269 I) [42]. It should be
shaped liquid crystals and has been proved by different electro- mentioned that Yelamaggad et al. [36] preferred a condensation
optical methods and optically observed by a high-speed camera step between the 4-n-alkyloxy-2-hydroxybenzaldehyde and the
[44,45].
related four-ring amino compound for the synthesis of short-
chain homologues of the laterally unsubstituted compound OH
It is not surprising that bent-core compounds having six phenyl 1a to study the complexation with Cu(II) and Pd(II).
rings can also form banana phases. Recently, the central phenyl
ring of a salicylideneaniline based five-ring bent-core com- Mesophase behaviour of the compounds OH 1
pound was replaced by a biphenyl moiety, leading to SmCP The transition temperatures and associated enthalpies of the
phases [46].
compounds OH 1 are summarized in Table 1. The introduction
of the hydroxy group causes an increase of the clearing
In continuation of our work on salicylideneaniline-based temperature by approximately 30 K (compare, e.g., compounds
banana-shaped mesogens, we herein report a study of the effect H 1a and OH 1a). This stabilization effect is already well-
of lateral substituents on the mesophase behaviour of symmetric known from calamitic Schiff bases and is caused by the
and nonsymmetric five-ring bent-core mesogens. Up to now intramolecular hydrogen bond, which could also reduce the
there have only been systematic investigations on the effect of flexibility of this molecular leg. Halogen substituents in pos-
atoms and groups introduced at the central phenyl ring. In our ition X have only a modest effect on the mesophase stability. A
work, we now attach halogen atoms at the outer phenyl rings. slight decrease in the clearing temperature with increasing size
When nonsymmetric compounds are laterally substituted with of the substituent is observed. We had expected a stronger
one halogen atom, this can be introduced in either one of both effect caused by a lateral chlorine or bromine atom.
of the molecular legs, giving isomeric compounds, which will
allow a detailed comparison. A further goal of the present work By cooling the isotropic liquid of H 1a and OH 1a a birefrin-
is the synthesis of bent-core salicylideneaniline compounds gent grainy texture together with areas showing a Schlieren
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Scheme 2: Synthetic pathway to prepare compounds OH 1.
Table 1: Mesophase behaviour, transition temperatures (°C) and enthalpies [kJ/mol], and PS values (nC/cm2) of compounds OH 1 and H 1a.
No.
R
X
Cr
•
SmCPA
I
•
•
•
•
PS
H 1a
H
H
92
[62.3]
•
•
•
•
111
[19.8]
750
900
390
250
OH 1a
OH 1b
OH 1c
OH
OH
OH
H
•
80
[44.7]
142
[22.4]
Cl
Br
•
82
[22.5]
140
[19.2]
•
85
134
[22.1]
[16.9]
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 1: Polarising optical microscopy images. a) Compound OH 1a: Grainy texture with smectic Schlieren areas at 130 °C; b), c) compound OH 1c:
Chiral domains of opposite handedness at 125 °C; polarisers decrossed from 90° by about +/-5°.
texture were observed by polarising optical microscopy
(Figure 1a), which is a hint for a layer structure. In contrast, the
mesophases of the halogen-substituted compounds OH 1b and
OH 1c appeared optically isotropic with crossed polarisers. By
rotation of one of the polarisers clockwise from the crossed
(90°) position, bright and dark domains became visible
(Figure 1b). Rotating the polariser anticlockwise by the same
angle made the dark domains bright, and vice versa (Figure 1c).
Obviously domains of opposite handedness occur. One explan-
ation is the bookshelf geometry of smectic layers in which an
anticlinic arrangement with a tilt angle of 45° leads to an ortho-
conic structure. This state is nonbirefringent [48,49].
X-ray diffraction experiments on powderlike samples of OH 1a
and OH 1b showed first and second order layer reflections and
a diffuse outer scattering, revealing a simple layer structure
having a liquidlike state within the layers (Figure 2). The layer
thicknesses amount to d = 4.0 nm (OH 1a) and d = 4.2 (OH
Figure 2: X-ray diffraction pattern of a powderlike sample of OH 1b at
131 °C (inset: Small angle region; lower part of the patterns shadowed
by the heating stage).
1b). Based on a calculated molecular length of L ~ 5.4 nm, the field the molecules are arranged with the polar axes in the field
molecular long axes are tilted with respect to the layer normal direction such that a synclinic and ferroelectric state results.
by about 42°, excluding an intercalation of the terminal chains. When the field is switched off, the molecules rotate back to the
antiferroelectric ground state. The arrangement of the molec-
EHC cells consisting of two thin glass plates with a conductive ular long axes in two adjacent layers is sketched below the
surface (ITO) were used for electro-optical investigations. After textures in Figure 3. This behaviour was found for all com-
the cell is filled with the material under study, the application of pounds OH 1a–c. The values of the spontaneous polarization
an electric field gives information on the dielectric properties are given in Table 1.
and polar switching processes. The current response curve of a
low-frequency triangular wave voltage shown in Figure 3a The switching behaviour of the compound H 1a without the
gives evidence that the tilted smectic phase exhibits an antifer- hydroxy group is more complicated (Figure 4). When an elec-
roelectric switching behaviour. Cooling the isotropic liquid in tric field is applied, circular domains are formed (Figure 4b). In
the presence of a D.C. field leads to the formation of circular the circular domains the smectic layers are cylindrically
domains, in which the layers are arranged cylindrically. In the arranged around the centre of the domains. This arrangement is
ground state the extinction brushes are parallel to polariser and indicated by the occurrence of extinction crosses. For tilted
analyser, see Figure 3c. From this observation it follows that the smectic phases the angle between the extinction crosses and the
antiferroelectric SmCP phase exhibits an anticlinic arrangement crossed polarisers corresponds to the tilt angle of the smectic
(SmCaPA). By application of an electric field the extinction phase. As shown in Figure 4b, circular domains appear under
brushes rotate clockwise or anticlockwise depending on the application of a sufficiently high electric field. After removal of
polarity of the field, see Figure 3b and Figure 3d. Under the the electric field, the relaxation of the field-induced circular
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 3: Electro-optical behaviour of compound OH 1a: a) current response curve (U = 182 Vpp, f = 30 Hz, T = 112 °C, dcell = 6 μm, PS = 900 nC/
cm2); b–d) tristable switching of circular domains by a D.C. voltage. T = 112 °C, dcell = 6 μm; c) 0 V, extinction crosses parallel to the crossed
polariser; b) +30 V, clockwise rotation of the extinction crosses (see yellow arrow); d) −30 V, anticlockwise rotation of extinction crosses (see yellow
arrow).
Figure 4: Switching behaviour of compound H 1a, which depends on how quickly the electric field is switched off.
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Beilstein J. Org. Chem. 2012, 8, 129–154.
domains strongly depends on the experimental conditions. quently, their esterification with 4-(4-n-dodecyloxy-3-substi-
When the electric field is removed quickly, the extinction tuted-benzoyloxy)benzoic acids 2 was the final step needed to
crosses rotate back to the crossed polariser position. The texture afford the compounds OH 2b, 2e, 2g. The selection of route A
becomes dark and the crosses are difficult to recognize, as or B was determined by the better purity of the final products,
shown in Figure 4a. The reason for this is that for smectic layers but short reaction paths for the expensive intermediates, e.g.,
perpendicular to the substrate an antiferroelectric arrangement the fluoro-containing compounds, were also an important
with a tilt angle of about 45° leads to an orthoconic structure. consideration.
This state is nonbirefringent in the direction perpendicular to
the substrate and therefore dark between crossed polarisers [49]. Mesophase behaviour of the compounds OH 2
Upon reapplication of the electric field, the crosses rotate back Transition temperatures and corresponding enthalpies from
to the 45° positions (Figure 4a and Figure 4b). When the field is calorimetric measurements are summarized in Table 2. For
removed very slowly (about 2 V/s) the circular domains do not comparison, the compound H 2a without the hydroxy group is
change, and only a slight decrease in the birefringence is seen added.
(Figure 4c). When the electric field is applied again, the extinc-
tion crosses do not rotate. Different movements of the molec- All compounds OH 2 form a SmCP phase. The clearing
ular long axis are responsible for such distinct behaviour: In temperature of the benzylideneaniline H 2a is increased by
most cases the molecular long axes rotate on a cone, as sketched 29 K by introduction of the hydroxy group in the ortho position
in Figure 4 below the texture. In the other case, the crosses do of the azomethine linking group, to give OH 2a. The effect of a
not change when the molecules of each second layer rotate by methyl group on the mesophase stability depends on the pos-
180° about their long axes (sketch on the right below the ition on the central phenyl ring. The lowering of the
textures in Figure 4).
smectic–isotropic phase-transition temperature by introduction
of the lateral methyl group in the 6-position, OH 2b, amounts to
In this case, of rotation about the long axes, the chirality 31 K. The influence of a methyl group in the obtuse angle of
changes from the chiral SmCsPF phase to the racemic SmCsPA OH 2c, that is in position 2, can be virtually ignored, and
phase. This means that the chirality is changed in every second surprisingly also the combination with a chlorine atom laterally
layer. Such flipping of chirality has been reported by Schröder attached to one of the outer rings in OH 2d has little effect.
et al. in 2004 [48] for the SmCP phases and by Szydlowska et Comparing the different sizes and polarities of the halogen
al. in 2003 [50] for the B1 phase. Both processes of molecular atoms in the compounds OH 2e–j, a fluorine atom increases the
movement require different energies, which correspond to clearing temperatures by about 6 K (2e, 2h in comparison to the
different coefficients of viscosity for both molecular move- laterally nonsubstituted compound OH 2a). The chlorine- and
ments [51].
bromine-substituted compounds have transition temperatures
that are only a little lower. Although the bromine atom is a rela-
tively large substituent, the clearing temperatures are, surpris-
ingly, depressed by only few degrees. Furthermore, the
disrupting effect is comparable for the both series OH 2e–g and
OH 2h–j, which are isomeric to each other. Here, the halogen
atoms are attached to the “left” or “right” arms, which are
chemically different. It should also be mentioned that by intro-
duction of one halogen atom in these nonsymmetric molecules
the melting temperatures can strongly decrease. Therefore, rela-
tively broad enantiotropic mesophase ranges result, for
example, 77 K for compound OH 2j.
Monosalicylideneaniline compounds OH 2
bearing the azomethine group between the
central phenyl ring and one neighbouring ring
Synthesis of the compounds OH 2
The compounds OH 2 have one azomethine linking group
between the central phenyl ring and the adjacent ring bearing a
hydroxy group. Two reaction pathways to prepare these com-
pounds are sketched in Scheme 3. The basic intermediates were
the 4-formyl-3-hydroxyphenyl 4-n-dodecyloxy-3-substituted-
benzoates 5a–d, which were synthesized by acylation of 2,4-
dihydroxybenzaldehyde with the corresponding 4-n-dodecyl-
oxy-3-substituted-benzoyl chlorides prepared by use of oxalyl
chloride. Following pathway A the condensation of the salicyl-
aldehydes 5a–d with 3-aminophenyl 4-(4-n-dodecyloxy-3-
substituted-benzoyloxy)benzoates 6 yielded the final com-
pounds according to [33].
Observation by polarising optical microscopy shows textures
typical for SmCP phases, for example fan-shaped, ribbonlike
and Schlieren textures (Figure 5).
X-ray diffraction studies on OH 2a give evidence for a layer
structure with a liquidlike order within the layers. The layer dis-
Following pathway B, the salicylidene intermediates 5a–d were tance, with d = 3.8 nm, is significantly smaller than the calcu-
reacted with 3-aminophenols to give the phenols 7a,b. Subse- lated molecular length (L ~ 5.4 nm). According to cos θ = d/L, a
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Scheme 3: Reaction pathways to prepare the monosalicylideneaniline compounds OH 2a–j.
tilt angle of the molecular long axis with respect to the layer diffuse scattering is a hint to a synclinic arrangement of the
normal of 45° results at a temperature of 130 °C. A similar tilt molecules in adjacent layers.
angle of 42° was determined from the mutual positions of the
maxima of the outer diffuse scattering and of the small-angle Electro-optical measurements on OH 2a by using a triangular
reflections in the 2D diffraction patterns (Figure 6). The signifi- wave voltage show a current response consisting of two repolar-
cant difference in the intensities of the two maxima of the outer ization peaks, which proves an antiferroelectric ground state.
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Table 2: Mesophase behaviour, transition temperatures (°C), transition enthalpies [kJ/mol] and PS [nC/cm2] values of the compounds OH 2 and H 2a.
No.
R
X
Z
Y
Cr
•
SmCPA
I
PS
H 2a
H
H
H
H
H
Cl
F
H
H
H
H
H
H
H
H
H
F
111
[65.5]
•
•
•
•
•
•
•
•
•
•
•
(107)
[16.9]
•
•
•
•
•
•
•
•
•
•
•
920
670
630
—a
OH 2a
[33]
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
•
105
[63.1]
136
[22.4]
OH 2b
OH 2c
OH 2d
OH 2e
OH 2f
OH 2g
OH 2h
OH 2i
OH 2j
6-CH3
•
83
[32.0]
105
[16.3]
2-CH3
•
125
[27.4]
132
[19.5]
2-CH3
•
125
[11.7]
133
[18.9]
420
730
730
470
660
640
590
H
H
H
H
H
H
•
107
[47.4]
142
[21.2]
Cl
Br
H
H
H
•
68
[22.1]
138
[20.7]
•
66
[24.8]
132
[18.9]
•
107
[30.4]
143
[21.8]
Cl
Br
•
79
[56.9]
135
[21.7]
•
56
133
[24.3]
[20.1]
aDespite many purification procedures, PS could not be reproducibly measured due to the high conductivity of the compound.
tion was calculated from the area under the repolarization peaks
to be 660 nC/cm2. The switching behaviour of OH 2a is
representative for all compounds OH 2a–j. The PS values are
given in Table 2.
Disalicylideneaniline compounds OH 3 and
OH 4 bearing the azomethine groups
between the outer and neighbouring phenyl
rings
Synthesis of the compounds OH 3 and OH 4
Compounds OH 3 and OH 4 consist of two 4-n-dodecyloxysal-
icylideneaniline moieties, which are connected to the central
Figure 5: Compound OH 2a: Texture of the SmCP phase at 126 °C.
Figure 7 shows the switching behaviour and corresponding phenyl ring by ester groups. These linking groups have different
textures. The extinction crosses rotate to the right or left in directions in the compounds OH 3 and OH 4. Esterification of
response to the polarity of the electric field. This behaviour is the carboxylic acid 4 or of the phenolic intermediate 8 with the
characteristic of a tristable switching process between the tilted corresponding resorcinols and isophthalic acids, yielded the
anticlinic antiferroelectric (SmCaPA) and the synclinic ferro- compounds OH 3 and OH 4, respectively, as sketched in
electric (SmCsPF) states. The value of the spontaneous polariza- Scheme 4.
138

Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 6: a) 2D X-ray pattern of a surface-aligned sample of compound OH 2a at 128 °C (lower part of the pattern shadowed by the heating stage); b)
χ scan for the outer diffuse scattering with maxima at χ ≈ 125 and 220° [Irel = I(128 °C)/I(140 °C, isotropic liquid], and the position of the layer reflec-
tions for comparison (gray line).
Figure 7: Electro-optical switching behaviour of compound OH 2a: a) current response (U = 230 Vpp, f = 25 Hz, T = 106 °C, dcell = 6 μm; PS = 660
nC/cm2); b–d) tristable switching on homogeneous chiral domains; starting from texture c) where the extinction crosses are parallel to the crossed
polariser, the extinction crosses rotate anticlockwise in b) and clockwise in d) in dependence on the polarity of the D.C. electric field.
Mesophase behaviour of the compounds OH 3
mesophase. The clearing temperature of the bromine-substi-
Table 3 shows the transition temperatures of the isophthalic tuted compound OH 3b increases due to the presence of
acid derivatives OH 3a–c together with those of the corres- hydroxy groups, in comparison to H 3b, by 34 K. Liquid-crys-
ponding esters H 3a–c, which do not exhibit an ortho-hydroxy talline behaviour was not observed for either of the nitro deriva-
group, for comparison.
tives H 3c or OH 3c, probably because the 5-position of the
nitro group, which is at the top of the bent molecules, is steri-
Liquid-crystalline phases were not observed for the benzyli- cally unfavourable in most cases. Generally, melting tempera-
dene derivative H 3a. The introduction of the hydroxy groups tures and phase-transition temperatures of isophthalic acid
enables the salicylideneaniline compound OH 3a to form a derivatives are much higher in comparison to those of the
139

Beilstein J. Org. Chem. 2012, 8, 129–154.
Scheme 4: Synthetic steps followed to prepare the compounds OH 3 and OH 4.
Table 3: Mesophase behaviour, transition temperatures (°C) and enthalpies [kJ/mol] of compounds OH 3 (on the right) and compounds H 3 (on the
left).
Mesophase behaviour
R = H
Z
R = OH
Mesophase behaviour
Cr 190 [98.0] I
H 3a
H 3b
H 3c
H
OH 3a
OH 3b
OH 3c
Cr 179 [63.5] SmCPA 223 [22.5] I
Cr 136 [40.8] SmCPA 189 [21.6] I
Cr 224 [61.9] I
Cr 163 [84.2] (SmCPA 155 [28.2]) I
Cr 193 [57.5] I
4-Br
5-NO2
isomeric resorcinol derivatives (see compounds OH 4), as also Helical filaments and circular domains grow upon cooling of
proved for two other series of isomeric five-ring mesogens the isotropic liquid of compound OH 3a (Figure 8a), which
containing only ester linking groups [47].
coalesces into a nonspecific texture. Such spiral germs could be
140

Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 8: Optical photomicrographs of compound OH 3a: a) On cooling of the isotropic liquid; c) U = 0 V; b) UD.C. = ±67 V, T = 172 °C, dcell = 6 μm.
a hint for undulated layer structures or a B7 phase. X-ray axes does not change. Therefore, the switching process can be
patterns on nonoriented samples display two orders of layer explained by a collective rotation of the molecules about their
reflections (d = 4.7 nm), however do not provide any evidence long axis.
for a modulation or undulation of the layers.
Substitution of the central phenyl ring with a bromine atom
Upon application of an electric field, the birefringence slightly (Z = 4-Br) results in a broader mesophase range at lower
changes, as shown in Figure 8b and Figure 8c for a D.C. field. temperatures for OH 3b. Upon cooling of the isotropic liquid,
In Figure 9b, the parallel arrangement of extinction crosses with lancetlike filaments grow into a fan-shaped texture exhibiting
respect to the position of the crossed polarisers indicates an some cross-stripes (Figure 10).
anticlinic arrangement of the molecules in adjacent layers. With
high fields, two repolarization peaks per half period were sep- X-ray patterns from partially aligned samples of the 4-bromo-
arated, which is typical for an antiferroelectric switching of a isophthalic acid derivative OH 3b display up to five orders of
SmCaPA phase (Figure 9a). The extinction crosses do not rotate the layer reflections on the meridian of the 2D-pattern
and the electro-optical behaviour is independent of the polarity (Figure 11a), which is evidence for a layer structure with well
of the electric field. This behaviour suggests that the position of defined layers (d = 4.8 nm at 160 °C). From the position of the
the optical axis and hence the direction of the molecular long maxima of the outer diffuse scattering at approximately 115 and
Figure 9: Electro-optical behaviour of compound OH 3a: a) Current response curve (U = 308 Vpp, f = 35 Hz, T = 172 °C, dcell = 6 μm, PS = 370 nC/
cm2); b) texture of the SmCPA phase depending on the polarity of the applied field.
141

Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 10: Growth of a fan-shaped texture from lancetlike filaments upon cooling of the isotropic liquid of compound OH 3b.
Figure 11: a) 2D X-ray pattern of a surface-aligned sample of compound OH 3b at 160 °C (lower part of the pattern shadowed by the heating stage);
b) χ-scan for the diffuse scattering (black line), compared to the position of the layer reflections on the meridian (grey line).
245° in the χ scan (Figure 11b), an average tilt angle of about contradiction to the optical behaviour shown in Figure 12. A
25° towards the layer normal was found.
tristable switching process is observed, and after the electric
field is switched off the texture is clearly changed (Figure 12b).
Initial electro-optical investigations employing an A.C. field In the case of a ferroelectric response, the compound would
(U = 260 Vpp, f = 20 Hz, T = 106 °C, dcell = 6 μm, switch between the states (Figure 12a and Figure 12c), and after
PS = 180 nC/cm2) yielded a current response with one repolar- the field is switched off one of these textures would be remain.
ization peak per half period only. This would normally be a hint By extensive electro-optical studies employing lower frequen-
for ferroelectric properties of compound OH 3b, but it is in cies and a modified triangular wave voltage, the current
Figure 12: Compound OH 3b: Texture of the SmCPA phase in dependence on the polarity of the applied D.C. field.
142

Beilstein J. Org. Chem. 2012, 8, 129–154.
response peaks split. Thus, the results of the electro-optical compounds H 4a–d with OH 4a–d. The increase amounts to
studies are consistent with each other, and the mesophase of 69 K for the laterally nonsubstituted compounds H 4a/OH 4a,
compound OH 3b can be assigned as SmCPA phase.
but is reduced to 33 K for 4,6-dichloro-substituted compounds
(see Supporting Information File 1, Table S4).
Mesophase behaviour of the compounds
OH 4
The 4-bromoresorcinol derivative OH 4b (Z = 4-Br) is isomeric
The carboxylic groups attached to the central phenyl ring of the to the 4-bromoisophthalic acid ester OH 3b. The texture of the
molecules OH 4 have an inverse direction in comparison to mesophase of compound OH 4b is quite different from that of
those of the compounds OH 3. The basic series OH 4, i.e., OH 3b. It shows spherolitic, lancetlike filaments and regions of
without lateral substituents, has been an object of interest for a growing Schlieren texture upon cooling the isotropic liquid
several research groups [27,37-41]. Some time ago, the influ- (Figure 13).
ence of lateral substituents on six complete homologous series,
in which the aromatic five-ring core corresponds to formula
OH 4, was investigated by us. The effect of atoms and groups
laterally attached to different positions on the central phenyl
ring was studied [42]. Therefore, Table 4 shows only a few
examples of interest for the present paper.
Compounds H 4a–d and OH 4b and 4d are new materials. As
seen from Table 4, the nature of the mesophase changes upon
introduction of the hydroxy groups, from a crystal-like BX
phase to a SmCP phase (H 4c to OH 4c) and from a nematic
phase to an undulated SmCP phase (H 4b to OH 4b). In the
case of the 5-methoxy-substituted compound OH 4d, liquid-
crystalline behaviour was induced by introduction of the
hydroxy groups. The increase of the clearing temperatures by
Figure 13: Compound OH 4b, exhibiting a fan-shaped texture together
with a Schlieren texture upon cooling of the isotropic liquid, T = 145 °C.
the introduction of two ortho-hydroxy groups in compounds
OH 4 depends strongly on the pattern of substitution; compare
Table 4: Mesophase behaviour, transition temperatures (°C) and -enthalpies [kJ/mol] of compounds OH 4 (on the right) and corresponding com-
pounds H 4 (on the left).
Mesophase behaviour
R = H
Z
R = OH
Mesophase behaviour
PSc
Cr
Cr
Cr
Cr
124
(SmCPA
110)
I
I
I
H 4a
H
OH 4a [27]
Cr
Cr
Cr
Cr
114
SmCPA
USmCPA
SmCPA
179
[21.6]
146
I
I
I
I
500
[73.4]
88
[—a]
95
[21.1]
112
N
(BX
I
H 4b
H 4c
H 4d
4-Br
2-Me
OH 4b
OH 4c
OH 4d
600
640
590
[37.3]
125
[21.5]
116)b
[—a]
[18.0]
148
[12.2]
171
[80.9]
98
[36.3]
134
[22.1]
127)
[20.1]
5-OMe
(SmCPA
[45.7]
[73.2]
acould not be found by calorimetric measurements due to crystallization.
bBX phase: Preliminary assignment as crystal-like phase.
cin [nC/cm²]
143

Beilstein J. Org. Chem. 2012, 8, 129–154.
In addition, the X-ray patterns show significant differences in
the structures of the mesophases for both compounds. Partially
surface-aligned samples of OH 4b show additional very weak
X-ray reflections next to the first-order layer reflections
(Figure 14), which suggest a two-dimensionally modulated
electron density, i.e., an undulated or modulated layer structure
(layer spacing d = 4.5 nm). The position of the reflections
implies an oblique 2D lattice for this modulation, but only
partial alignment was achieved, and the very strong ringlike-
layer reflection affects the measurements of the other very weak
ones. Therefore, further quantitative conclusions are rather
speculative. One plausible interpretation of the data is given in
Supporting Information File 1, Figure S2.
Figure 14: 2D X-ray diffraction pattern of a partially surface-aligned
sample of OH 4b at 115 °C (inset: Small angle region, the arrow points
to one of the additional non-layer reflections).
The designation as a polar phase results from electro-optical
investigations. Two repolarization peaks per half period of an
applied triangular wave voltage are generated (Figure 15a). The
spontaneous polarization amounts to 600 nC/cm2. The extinc- However, electro-optical studies on OH 4c show two peaks in
tion crosses rotate clockwise (Figure 15b) or anticlockwise the current response, as usually found for a SmCPA phase.
(Figure 15d) depending upon the polarity of the electric field,
by an angle of about 20°. According to the electro-optical In compound OH 4d a methoxy group is attached at the top of
switching behaviour and the results of the X-ray measurements, the central phenyl ring; this 5-position is often sterically
the mesophase was assigned as a USmCPA phase
unfavourable for the formation of banana phases. Nevertheless,
a metastable SmCP phase was observed and investigated. The
The 2-methyl-substituted compound OH 4c is isomeric with growth of a fringe pattern typical for a SmCP phase can be seen
OH 5b, which exhibits an interesting multistage switching [44]. in Figure 16a.
Figure 15: Switching behaviour of compound OH 4b at 120 °C: a) Current response curve (U = 116 Vpp, f = 40 Hz, T = 120 °C, dcell = 6 μm, PS = 600
nC/cm2); b) tristable switching observed on circular domains applying a D.C. voltage (U = ± 40 V, T = 122 °C, dcell = 6 μm).
144

Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 16: Compound OH 4d: a) Microphotograph of a growing fringe pattern of a SmCP phase upon cooling of the isotropic liquid; b) current
response curve showing antiferroelectric behaviour (U = 19 V/µm, 20 Hz, dcell = 6 µm, PS = 500 nC/cm2).
The current response shows two peaks per half period, which is the long molecular axes with respect to the layer normal
typical for an antiferroelectric switching. The spontaneous po- (Figure 18), from which an effective molecular length of Leff =
larization amounts to 500 nC/cm2 (Figure 16b). Action of an d/cos θ = 5.0 nm results, which is significantly shorter than the
electric D.C. field shows a transition from a striped, fan-shaped molecular length Lcalc = 5.5 nm estimated by molecular models.
texture to a smooth one (Figure 17).
The difference may be caused by a deviation of the molecular
configuration from that assumed in the model calculations (e.g.,
The X-ray diffraction measurements on the monotropic liquid- by another bending angle or a high proportion of gauche
crystalline phase of OH 4d show the patterns of a tilted smectic conformers), or by an interdigitation of the terminal chains (see
layer structure with a layer spacing of 4.3 nm and a 30° tilt of discussion for OH 4b in Supporting Information File 1).
Figure 17: Electro-optical behaviour observed on the fan-shaped texture of compound OH 4d; UD.C. = 47 V; T = 123 °C; a) −47 V; b) 0 V; c) +47 V.
Figure 18: a) 2D X-ray diffraction pattern for a surface-aligned sample of OH 4d at 122 °C on cooling; b) χ-scan for the outer diffuse scattering with
maxima at about 120 and 240° (Irel = I(122 °C)/I(140 °C, isotropic liquid).
145

Beilstein J. Org. Chem. 2012, 8, 129–154.
The results of electro-optical, microscopic and X-ray diffrac- phase type of the compounds OH 5 and OH 6 can be seen from
tion studies prove the existence of a SmCPA phase.
Table 5. For comparison, corresponding compounds having the
same aromatic core but without the hydroxy groups are given.
Disalicylideneimine compounds OH 5 and
OH 6 bearing the azomethine groups
between the central phenyl ring and the
neighbouring rings
Synthesis of the compounds OH 5 and OH 6
The phase-transition behaviour shown in Table 5 demonstrates
the strong influence of the ortho-hydroxy groups on the
mesophase type and the mesophase stability. The comparison of
the compounds H 5a–g with OH 5a–g shows that in all cases
the clearing temperatures are increased by about 60 K or liquid-
The 1,3-phenylenediamine derivatives OH 5 and OH 6 were crystalline behaviour is induced.
prepared by condensation of the corresponding 1,3-phenylene-
diamines with 4-formyl-3-hydroxyphenyl 4-n-dodecyloxy-3- Compound OH 5a exhibits a very high melting point; the
substituted-benzoates 5 and 4-formyl-3-hydroxyphenyl 4-n- occurrence of the metastable columnar phase is established by
dodecyloxycinnamate (9), respectively, as sketched in means of typical textures only. It should be mentioned that the
Scheme 5. Compound 9 was obtained by esterification of 2,4- B6 banana phase was first reported for short-chain members of
dihydroxybenzaldehyde with 4-n-dodecyloxycinnamic acid, the homologous series of H 5a [52]. The effect of lateral
under the same conditions as reported for the salicylaldehydes substituents X and Z is remarkable. Introduction of a methyl
5.
group into the 2-position of the central phenyl ring of OH 5a
not only decreases the melting temperature by 54 K but also
increases the clearing point by 21 K for OH 5b. Additional
attachment of fluorine atoms at the outer phenyl rings increases
Mesophase behaviour of the compounds OH 5 and
OH 6
The influence of the pattern of substitution at the outer and/or the smectic–isotropic phase-transition temperature again by
the central phenyl ring on the phase-transition temperatures and 13 K for OH 5c. That means that the laterally substituted com-
Scheme 5: Reaction steps employed for the preparation of the compounds OH 5 and OH 6.
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Table 5: Mesophase behaviour, transition temperatures (°C) and enthalpies [kJ/mol] of compounds OH 5 and OH 6 (on the right) and corresponding
compounds H 5 and H 6 (on the left).
Mesophase behaviour
R = H
V
X
H
Z
R = OH
Mesophase behaviour
Cr
Cr
120
[48.9]
128
(SmCP 114)
I
H 5a [52]
—
H
OH 5a
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
177
[45.2]
123
(Col
172)
[—a]
I
I
I
I
I
I
I
I
I
I
[17.1]
I
H 5b
—
H
F
2-Me OH 5b [44]
2-Me OH 5c
SmCPA 193
[19.5]
[26.2]
[71.2]
105
SmCPA 206
[20.4]
[63.2]
97
Cr
Cr
Cr
Cr
Cr
Cr
Cr
98
I
H 5d
H 5e
H 5f
—
H
F
4-Me OH 5d
BX
126
[32.1]
186
[58.0]
127
[8.5]
179
(SmCP 124)
[8.6]
I
I
I
—
H
H
H
H
OH 5e
OH 5f
OH 5g
OH 6a
USmC
USmC
USmC
[75.1]
116
[25.9]
165
[26.7]
173
(SmCP 108)
[13.5]
—
Cl
Br
H
H
H
[74.1]
79
25.1
143
[22.2]
161
SmCP
92
H 5g
H 6a
H 6b
H 6c
—
[14.1]
146
[6.2]
[17.6]
189
[13.5]
I
N
I
HC=CH
HC=CH
HC=CH
(SmCPA 183)
[—a]
[56.8]
136
[54.4]
131
140
I
2-Me OH 6b
4-Me OH 6c
SmCPA 202
[16.9]
[67.6]
107
[0.5]
[74.1]
114
SmCPA 134
[14.4]
[40.8]
[23.4]
aCould not be found by calorimetric measurements due to crystallization.
pound OH 5c exhibits a transition temperature to the isotropic The methyl-substituted compound OH 5b is of special interest.
phase that is 34 K higher than the laterally unsubstituted parent The X-ray patterns of nonoriented samples show a strong
compound OH 5a. This is a really exciting result.
reflection in the small-angle region with a very weak second
order reflection and a diffuse scattering in the wide-angle region
Upon shifting of the methyl groups of OH 5b from the 2-pos- indicating a simple layer structure without in-plane ordering.
ition into the 4-position, the clearing temperature decreases by The layer spacing of 3.8 nm is clearly smaller than the molec-
67 K for the isomeric compound OH 5d. By cooling the ular length L = 5.4 nm as estimated by commercial molecular
isotropic liquid of OH 5d, an intensive blue colour arises. modelling software (Cerius 2) and thus from cosθ = d/L = 3.8
Furthermore, chiral domains of different handedness are visible nm/5.4 nm a tilt angle of about 45° was estimated. Such a tilt
in the texture (Figure 19a and Figure 19b). Both appearances angle explains the optical behaviour. Upon cooling of the
could be a hint for a B4 banana phase [8,53,54]. X-ray patterns isotropic liquid, the SmCP phase occurs as nonbirefringent
show slightly broadened layer reflections (d = 4.0 nm) and addi- black texture. After slight decrossing of the polarisers, however,
tional weak and sharp reflections together with broadened ones optically active domains become visible, as shown in
over the entire diffraction range (Figure 19c), which are also Figure 20a and Figure 20b. Under the action of an A.C. electric
arguments for a B4 or another soft-crystal-like BX phase of field, nucleation of a birefringent texture starts, which coalesces
OH 5d.
to a non-specific or broken fan-shaped texture (Figure 20c).
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Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 19: Compound OH 5d: Optical photomicrographs of chiral domains at 122 °C; polariser and analyser are uncrossed by about a) −8° and b)
+8° from the 90° position; c) X-ray pattern of a powderlike sample at 110 °C.
This state of compound OH 5b exhibits an interesting current phase. However, a two-peak current response typical of an anti-
response consisting of five or more peaks per half period ferroelectric switching is found. As mentioned above, com-
(Figure 20d).
pound OH 5d isomeric with OH 5b, shows a crystal-like phase
only. That means that simple changes of the molecular struc-
Such a multistage switching is unique for bent-core compounds ture can prevent the unusual multistage switching found for
and was reported by us some time ago [44]. This switching OH 5b.
cannot be explained by using a simple layer structure of the
SmCP phase. Therefore, a structure model for an intermediate Substitution by fluorine, chlorine, and bromine atoms at both
field-induced ferrielectric state was proposed [45]. In the outer phenyl rings of compound OH 5a results in OH 5e–g.
present work it is helpful to search for further bent-core com- The following tendency of clearing temperatures is found:
pounds exhibiting a multistage switching. Hence, the molecular X = H (172 °C), F (186 °C), Cl (173 °C), Br (161 °C). It is very
structure was changed in a stepwise manner. Starting from com- surprising that compound OH 5f, bearing two chlorine atoms in
pound OH 5b, the attachment of fluoro atoms on the outer rings lateral positions, exhibits mesophase stability similar to the
results in compound OH 5c, which also exhibits a SmCPA laterally unsubstituted parent compound OH 5a.
Figure 20: Compound OH 5b: a), b) Chiral domains at 175 °C, 0 V, polarisers uncrossed by about ±8° from the 90° position; c) birefringent texture
formed under an electric field (U = 100 VA.C.); d) current response (U = 158 Vpp, f = 20 Hz, T = 174 °C, dcell = 6 μm).
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Beilstein J. Org. Chem. 2012, 8, 129–154.
X-ray diffraction patterns for the halogen-substituted com- on our X-ray results [5,9]. Since no sufficient alignment of the
pounds OH 5e–g show several sharp small-angle reflections samples was achieved, the tilt angle of the molecules with
and a diffuse outer scattering (Figure 21). The sample of OH 5f respect to the axes of the 2D lattice could not be determined.
was partially surface-aligned and the diffraction angles of four
reflections were determined. Thus the reflections were indexed
Table 6: Structural parameters for the modulated-layer phases of com-
to an oblique 2D lattice (for parameters see Table 6). In the
same way the small-angle reflections of the other two com-
pounds can be interpreted. Comparing the molecular volume
with the volume of a hypothetical 3D unit cell with a thickness
of 0.52 nm, corresponding to the assumed stacking distance of
the molecules in bending direction, the number of molecules in
the cross section of the unit cell, i.e., in the blocks of the modu-
lated structure, was calculated (see Table 6 and Supporting
Information File, Table S3). These structural parameters for the
liquid-crystalline phases of the three compounds closely
resemble each other. There are several packing models for the
molecules that fit these data, namely the undulated-layer model
(USmC phase), the broken-layer (columnar) model (Colob
phase), and the polarization-modulation model, and several
modes within these models depending on the direction of the
polarization between the layers, which cannot be decided based
pounds OH 5e, 5f and 5g: Temperature, T (°C) on cooling, 2D lattice
parameters a, b (nm), and γ (°), molecules in the cross section of the
unit cell nlc.
Compound
T
a
b
γ
nlc
OH 5e
OH 5f
OH 5g
177
158
150
14.0
14.8
17.0
4.9
4.5
4.4
116
112
112
22
21
24
In principle the appearance of spiral germs upon slow cooling
of the isotropic liquid of the compounds OH 5 hints at an undu-
lated smectic phase or a B7 banana phase, see Figure 22a.
However, in the presence of an electric D.C. field, a texture
with chiral domains is observed, with a change in contrast seen
upon rotation of one polariser from the 90° position by several
degrees (Figure 22). Surprisingly, it was not possible to find a
Figure 21: X-ray diffraction patterns of compounds OH 5: a) Pattern of a powderlike sample of compound OH 5g at 150 °C on cooling (inset: small-
angle region); b) small-angle pattern of OH 5g showing the reciprocal 2D lattice and the indexing of the observed reflections; c) small-angle pattern of
a partially surface-aligned sample of compound OH 5f at 158 °C on cooling, showing the two preferred orientations of the reciprocal 2D lattice in the
sample and the indexing of the observed reflections.
Figure 22: Photomicrographs of compound OH 5f: a) Appearance of spiral filaments on slow cooling of the isotropic liquid at 171 °C; b), c) cooling
from the isotropic phase in the presence of an electric field of 15 V/μm results in a texture with chiral domains, which are visible by rotation of one
polariser clockwise or anticlockwise by several degrees from the crossed position; T = 165 °C.
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Beilstein J. Org. Chem. 2012, 8, 129–154.
current response typical for a polar banana phase. The birefrin- By shifting the methyl group from the 2-position in OH 6b to
gence of the sample changed depending on the power of the the 4-position of the central phenyl ring of compound OH 6c,
electric field. Although the chemical structure and the molec- the clearing temperature decreases by 68 K. Nevertheless, an
ular shape suggest that the compounds might be capable of enantiotropic mesophase exists and can be investigated. Micro-
forming banana phase(s), the polar character of the mesophases scopic pictures show textures typical for SmCP phases
could not be unambiguously determined. Therefore, the (Figure 24).
mesophases of the compounds OH 5e–g are designated as
USmC phases.
A further slight modification of the chemical structure of the
series OH 5 is the insertion of C=C groups between the outer
and the neighbouring phenyl rings, giving OH 6a–c. Of all
three hydroxy group-free compounds, H 6a–c, only the
2-methyl-substituted compound shows liquid-crystalline behav-
iour. A nematic phase is observed, which is very surprising for
this mesogen having a molecular shape characteristic of banana-
shaped liquid crystals. Although both legs of the bent-core
molecules are extended by the C=C units, the clearing tempera-
tures increase only by about 10 K. Introduction of a lateral
methyl group on the central phenyl ring drastically decreases
the melting temperatures. Therefore, broad SmCP phase ranges
exist for OH 6b and OH 6c in comparison to the laterally
unsubstituted derivative OH 6a. That also means that the
nematic phase of H 6b changes to a polar smectic phase in
OH 6b by insertion of the C=C units.
Figure 24: On cooling the isotropic liquid phase of compound OH 6c;
a Schlieren texture together with fringe pattern appears characteristic
for SmCPA phases.
The mesophase behaviour of compound OH 6b is character- Guinier X-ray patterns of a powderlike sample of OH 6c show
istic for a SmCPA phase. On cooling, the isotropic liquid fringe first and second order layer reflections and a diffuse outer scat-
textures as well as Schlieren textures are observed. The current tering indicating a smectic layer structure with a layer spacing
response shows two peaks per half period (Figure 23). The of 4.0 nm. Electro-optical studies of the mesophase of com-
spontaneous polarization amounts to 220 nC/cm2. However, pound OH 6c support the assignment as a SmCPA phase.
notably, the multistage switching found for the corresponding Figure 25 shows the current response curve and microscopic
compound OH 5b disappears.
pictures, which demonstrate the switching between SmCaPA to
SmCsPF upon application of a D.C. electric field.
Conclusion
The use of salicylideneaniline moieties in five-ring bent-core
mesogens is shown to be helpful in deriving the relationship
between the chemical structure and the mesophase behaviour.
Here, the effect of fluorine, chlorine, and bromine atoms later-
ally attached to the outer phenyl rings, the influence of the
hydroxy group in ortho-position to azomethine units, and the
effect of a methyl group situated at the central phenyl ring on
the liquid-crystalline behaviour and physical properties were
studied.
For most of the salicylideneaniline compounds synthesized, the
corresponding benzylideneaniline derivatives were prepared for
a comparative study. As expected, the clearing temperatures
generally increase due to the presence of the ortho-hydroxy
group. The increase amounts to about 30 K for mono-salicylide-
Figure 23: Current response of compound OH 6b exhibiting two repo-
larisation peaks proving an antiferroelectric switching (UA.C. = 15 V/
µm, f = 50 Hz, T = 174 °C, dcell = 6 μm).
150

Beilstein J. Org. Chem. 2012, 8, 129–154.
Figure 25: Electro-optical behaviour of compound OH 6c: a) Current response (UA.C. = 17 V/µm, f = 39 Hz, T = 123 °C, dcell = 6 μm, PS =
520 nC/cm2); c) 0 V, ground state SmCaPA; b) and d) formation of the SmCsPF phase upon application of a D.C. field of different sign: b) +7 V/µm; d)
−7 V/µm.
neaniline compounds OH 1 and OH 2, and can be up to 60 K OH 2a 136 °C) The replacement of the chlorine atom by the
for the mesogens containing two salicylideneaniline moieties, larger bromine atom additionally decreases the clearing
namely OH 4, OH 5, OH 6. As shown in Table 1–Table 5 and temperatures by only 2–6 K.
Supporting Information File 1, Table S4, the definite increase of
the clearing temperature can be much lower depending on the Looking at the compounds OH 5e–g, which have two halogen
size of the molecules, the length of the terminal chains, and the atoms, i.e., one halogen atom at each leg, the lowering of the
presence of further lateral substituents and their positions at the clearing points found for mono-halogen derivatives is approxi-
different phenyl rings. More importantly, in many cases the mately doubled. Nevertheless, the effect of lateral halogen
introduction of the hydroxy group induces liquid-crystalline atoms on the mesophase stability is much lower for the salicyli-
behaviour, which was not observed for the corresponding deneanilines under study, as reported for comparable deriva-
benzylideneaniline compounds. There are other cases where the tives in the original banana series. There, in dependence on
type of mesophase also changed, for example from nematic to further substituents at the central phenyl ring, the clearing
SmCP, nematic to USmCP, BX to SmCP, and SmCP to temperatures of corresponding terminally dodecyloxy-substi-
columnar.
tuted derivatives decreased by the introduction of chlorine
atoms, by 18–34 K. For related octyloxy-substituted homo-
The influence of one halogen atom attached at the outer phenyl logues the effect was much stronger [55,56]. The reason for
rings was investigated in several nonsymmetric molecules. The such differences found in different series of bent-core meso-
mono-halogen derivatives of three series are isomeric to each gens is not clear up to now.
other (OH 1b–c, OH 2e–g, OH 2h–j). All these compounds ex-
hibit SmCP phases. The lowering of the clearing temperatures Comparing the compounds OH 5a–c, an unusually strong
by a lateral fluorine, chlorine, or bromine atom is unexpectedly increase of the mesophase stability caused by lateral
low and nearly independent of the different chemical structure substituents was found. In comparison to the salicylideneimine-
of the aromatic cores of the compounds OH 1 and OH 2. For based compound OH 5a (clp 172 °C), the clearing point of the
example, the clearing temperatures of the isomeric mono-chlori- 2-methyl-substituted compound OH 5b increases to 193 °C;
nated compounds (OH 1b 140 °C; OH 2f 138 °C; OH 2i compound OH 5c having additional fluorine atoms at the outer
135 °C) have nearly the same values as those of the corres- phenyl rings exhibits a SmCP–isotropic phase-transition
ponding laterally unsubstituted compounds (OH 1a 142 °C; temperature of 206 °C.
151

Beilstein J. Org. Chem. 2012, 8, 129–154.
In relation to this it should be mentioned that the cinnamic ester the central phenyl ring (OH 5d), by insertion of C=C units
H 6b, having a methyl group in the same position, exhibits a (OH 6b and OH 6c) and by preparation of a corresponding
nematic phase. The formation of nematic and nonpolar smectic compound having only one salicylideneaniline moiety together
phases by bent-core mesogens can be explained by flexible with a methyl group in the 2-position (OH 2c). Unfortunately,
molecular fragments, and especially by a bending angle clearly the creation of new compounds exhibiting a multistage
above 120° [57]. Such molecules have a more rodlike shape, switching was not successful.
and therefore “calamitic phases” can be formed. The
4-bromoresorcinol derivative H 3c also shows a nematic phase. The studies on the influence of lateral halogen atoms, and also
It is known from NMR studies of other related bent-core meso- of a lateral methyl group, on the mesophase behaviour and
gens that halogen atoms in the 4-position extend the bending further physical properties of salicylideneimine-based bent-core
angle to about 135°, and hence this would explain the nematic mesogens show that the relationships strongly differ from those
phase in compound H 3c. The influence of a methyl group in reported for calamitic liquid crystals, and that they also vary in
the 2-position of the central phenyl ring on the bending angle of comparison to other similar five-ring bent-core mesogens.
bent-core mesogens depends on the chemical structure and on
the direction of the linking groups, which connect both molec- Experimental
ular legs to the central phenyl ring, as we know from solid state The thermal transition behaviour was investigated by using a
NMR studies. For 2-methylresorcinol bisbenzoates, for Perkin Elmer DSC Pyris 1 differential scanning calorimeter.
example, the bending angle amounts to about 120° [58]. In Texture observations of liquid-crystal films sandwiched
contrast, for isomeric diphenyl 2-methylisophthalates the between two glass plates were carried out with a Leitz polar-
bending angle is extended to about 145° [59]. Transferring this izing microscope (Laborlux 12 Pol S, Germany). For the
knowledge to the methyl-substituted compound H 6b would electro-optical measurements we used commercial sandwich
mean that the existence of a nematic phase strongly hints at a cells (E.H.C., Japan) with thickness of 6 µm, having a rubbed
bending angle larger than 120°. However, the evidence can only polyimide coating for planar surface alignment. The cells were
be delivered by NMR studies of the liquid-crystalline state.
observed in transmission by using a Leica (DMRXP, Germany)
polarizing microscope equipped with a digital camera (Nikon
The electro-optical behaviour of the salicylideneaniline com- Coolpix 4500, Japan). The cells are mounted on a heating stage
pounds can change in comparison to that of the corresponding (Linkam LTS 350, UK, and Mettler FP82HT and FP90, UK) for
compounds without a hydroxy group. Upon action of an elec- temperature control. The magnitude of the spontaneous polar-
tric field on a SmCP phase, the tilted molecules can rotate on a ization was measured by integrating the polarization-reversal
cone or can rotate about their long axis. Only the latter move- current peak obtained by switching the sample with a triangular
ment changes the chirality of the layers. In a few cases both electric field.
molecular processes have been observed for the same material,
in dependence on the frequency, temperature and/or the field X-ray diffraction measurements on powderlike samples in glass
[48,50,51]. For compound H 1a both molecular movements capillaries kept in a temperature-controlled heating stage were
exist depending on how rapidly the electric field is removed. performed by using a Guinier film camera (Huber Diffraktions-
This interesting behaviour disappears in compound OH 1a by technik GmbH) and quartz-monochromatized Cu Kα radiation.
the substitution with an ortho-hydroxy group.
2D diffraction patterns were recorded on an area detector (HI-
Star, Siemens/Bruker) from Ni-filtered Cu Kα radiation.
A further motivation for the present work was the exciting Surface aligned fibrelike disordered samples were obtained by
multistage switching reported for the 2-methyl-substituted com- slowly cooling a drop of the isotropic liquid placed on a glass
pound OH 5b (and for two longer-chain homologues) bearing plate on a temperature-controlled heating stage.
two salicylidene legs [44]. Therefore, the chemical structure of
this molecule was varied step by step. Liquid-crystalline behav- Reaction pathways to prepare the final compounds OH 1–6 are
iour was not observed for the corresponding compound without sketched in Scheme 2–Scheme 5.
an ortho-hydroxy group, H 5b. In compound OH 4c only the
COO and CH=N linking groups are exchanged in comparison to Generally, all compounds H1–H6, which do not bear the ortho-
the isomeric compound OH 5b; however, the characteristic hydroxy group, were prepared by following the same reaction
electro-optical behaviour of a SmCPA phase was found. pathways sketched in Scheme 2–Scheme 5, but instead of the
Furthermore, the chemical structure of compound OH 5b was salicylidene intermediates the corresponding benzylidene inter-
varied by substitution with fluorine atoms at the outer phenyl mediates were used. Further details are given in Supporting
rings (OH 5c), by shifting the methyl group to the 4-position of Information File 1.
152

Beilstein J. Org. Chem. 2012, 8, 129–154.
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Eur. J. Inorg. Chem. 2009, 4274–4281. doi:10.1002/ejic.200900536
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Supporting Information
Supporting Information File 1
Schematic presentation of the SmCaPA phase, additional
X-ray data, mesophase behaviour of selected compounds
and synthesis of compounds (experimental procedures and
analytical data).
19.Yelamaggad, C. V.; Achalkumar, A. S.; Rao, D. S. S.; Prasad, S. K.
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The authors are indebted to the Deutsche Forschungsgemein-
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