A novel family of salicylaldimine-based five-ring symmetric and non-symmetric banana-shaped mesogens derived from laterally substituted resorcinol: Synthesis and characterization

Article abstract of DOI:10.1039/b610758c

We present the results of our extensive investigations into the phase behavior of several "robust" five-ring banana-shaped compounds. Six homologous series of either symmetric or non-symmetric bent-core molecules, comprising a laterally substituted 1,3-phenylene ring as the central unit covalently linked to thermally and hydrolytically stable rod-like salicylaldimine segments have been synthesized and evaluated for their mesomorphism. The mesophases have been characterized by optical, calorimetric, X-ray diffraction and electro-optical studies. With a methyl group, polarizable chlorine atom or the more polarizable nitro group as the lateral substituent(s) and terminal alkoxy tails of varying length, the structure-property correlation has been determined. Our study clearly demonstrates that these materials, as predicted, display a different behavior compared to the corresponding compounds without the lateral substituents. The phase behavior shows a critical dependence on the nature and position of the lateral substituent(s). The study includes a comparison with the phase behavior of some of the other types of banana-shaped compounds. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b610758c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A novel family of salicylaldimine-based five-ring symmetric and non-
symmetric banana-shaped mesogens derived from laterally substituted
resorcinol: synthesis and characterization{
Channabasaveshwar V. Yelamaggad,*a Manoj Mathews,a S. Anitha Nagamani,a Doddamane S. Shankar Rao,a
Subbarao Krishna Prasad,a Sonja Findeisenb and Wolfgang Weissflogb
Received 26th July 2006, Accepted 3rd October 2006
First published as an Advance Article on the web 30th October 2006
DOI: 10.1039/b610758c
We present the results of our extensive investigations into the phase behavior of several
‘‘robust’’ five-ring banana-shaped compounds. Six homologous series of either symmetric or
non-symmetric bent-core molecules, comprising a laterally substituted 1,3-phenylene ring as the
central unit covalently linked to thermally and hydrolytically stable rod-like salicylaldimine
segments have been synthesized and evaluated for their mesomorphism. The mesophases have
been characterized by optical, calorimetric, X-ray diffraction and electro-optical studies. With a
methyl group, polarizable chlorine atom or the more polarizable nitro group as the lateral
substituent(s) and terminal alkoxy tails of varying length, the structure–property correlation has
been determined. Our study clearly demonstrates that these materials, as predicted, display a
different behavior compared to the corresponding compounds without the lateral substituents.
The phase behavior shows a critical dependence on the nature and position of the lateral
substituent(s). The study includes a comparison with the phase behavior of some of the other
types of banana-shaped compounds.
these compounds are generally termed as ‘‘banana’’ (Bn)
Introduction
phases and have been classified into eight categories designated
as B1–B8 (B stands for bent-core, banana or bow)1a,7 based on
Bent-core mesogens, popularly known as banana-shaped
liquid crystals (LCs), that are formed by covalently linking
their textural appearance, X-ray diffraction pattern, switching
two rod-like anisometric segments to a central angular
behavior and miscibility studies. In addition, a number of new
aromatic core represent a distinct and novel class of thermo-
banana phases have been discovered recently and are not
tropic liquid crystals as they form new types of mesophases.1
Although the earliest report on bent-core molecules goes back
to 1929 by Vorlander,2 the current immense interest in these
materials was triggered by investigations of Niori et al. in 1996
on a banana-shaped Schiff’s base derivative (Chart 1A)3
synthesized by Matsunaga et al.4 Their study revealed the
unique feature of banana-shaped compounds forming elec-
trically switchable polar smectic phases although the con-
stituent mesogens are achiral. Since then numerous bent-core
molecules have been synthesized and evidenced to form ferro-
and/or antiferro-electric smectic or columnar mesophases and
chiral superstructures.1 The origin of the electrical switching
(ferroelectric) behavior in these unique systems is believed to
be a consequence of combination of three important factors:
(i) the highly polar character of the molecules, (ii) the packing
arrangement of the molecules in smectic layers (with the
dipoles facing a common direction within the layers) and (iii)
the tilt of the molecules in smectic layers with respect to the
smectic layer normal.1,5,6 The different mesophases formed by
aCentre for Liquid Crystal Research, Jalahalli, Bangalore 560 013, India
Chart 1 (A) The general molecular structure of the first Schiff’s base
bInstitut fu¨r Physikalische Chemie, Martin-Luther-Universita¨t,
bent-core compounds shown to exhibit electrical switching behavior,
Halle-Wittenberg, Muhlpforte 1, 06108, Halle (Saale), Germany
based on which a vast number of similar compounds have been
{ Electronic supplementary information (ESI) available: DSC thermo-
gram; experimental and characterization data. See DOI: 10.1039/
b610758c
reported; (B) the general template for the molecular structures of
banana-shaped mesogens.
284 | J. Mater. Chem., 2007, 17, 284–298
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covered by the nomenclature system mentioned above owing
to the fine variations in their mesophase structures. For
example, some of the Bn mesophases such as B1, B2 and B7
have several sub-phases with slight structural changes, in
which case assigning code letters becomes difficult.
Nevertheless, other nomenclature types like SmCP (polar
tilted smectic) for B2, Colr (rectangular columnar) for B1 and
SmCintercal (intercalated smectic C) for B6 etc. have been
used.1e,7,8 In the present work, both types are adopted for
naming the banana fluid phases.
The remarkable thermal behavior of such bent-core com-
pounds, which is attractive from theoretical and fundamental
research points of views, truly originates from the position,
nature, size and the number of lateral substituents on the
central ring. For example, substitution of one chlorine atom
into position 4 of the central phenyl ring of the Schiff’s base
results in materials having rectangular columnar (B1) and
electrically switchable tilted smectic (SmCP) phases with lower
transition temperatures compared to their non-substituted
analogs.14c,d Introduction of one more chlorine atom into
position 6 leads to the formation of nematic (N) and smectic C
(SmC) mesophases instead of banana phases.14e A sequence of
switchable SmCP-B5 phases was obtained for substances
having a methyl group in position 2 of the central core14f
while the corresponding nitro derivatives exhibit the B7
mesophase.14g In such series of compounds having a cyano
group in the central ring at position 4, banana phases are seen
accompanied by calamitic phases with the phase sequences:
isotropic (Iso)–smectic A (SmA)–SmC–SmCP and Iso–N–
SmA–SmC–SmCP. Notably, in one of the sequences men-
tioned above, the SmCP phase occurs below the SmC phase.14h
Derivatives with groups like cyano, methyl or methoxy at
position 5 do not support the formation of mesophases.14i
Apart from their influence on phase sequences, lateral
substitutions can also influence the physical properties of
mesophases. The SmCP phase obtained for a Schiff’s base
derivative with three lateral fluoro substitutions (at position 5
on the central unit and positions 30 of both terminal rings)
shows an unusually high value of spontaneous polarization
(Ps) of about 1000 nC cm22. In contrast, for a compound that
has only one fluoro substitution at the central core (at position
5), the Ps is reduced (670 nC cm22) while the lowest value
(370 nC cm22) was found for a compound with fluoro
substitution present only at position 30.14a Thus, introduction
of lateral substituents on the Schiff’s base banana-shaped
compounds significantly influences their LC behavior.
The understanding of the relationship between the chemical
structure and LC properties, viz., polarity and chirality of
the mesophases formed by banana-shaped materials is not
revealed clearly, although
a large number of different
homologous series of compounds have already been studied.1
Chart 1B shows a general template that can be used to describe
the molecular structure of banana-shaped LCs. From the
viewpoint of chemical structure, bent-core molecules with five,
six and seven aromatic rings have been found to form LC
phases. However, it appears that the existence of banana
phases depends not only on the shape of the molecules but
also on other parameters such as local dipolar moments and
extent of conjugation etc.,1 which in turn depend on various
structural fragments like the angular central unit (CU),1,9–12,
linking groups (X and Y),13 lateral substituents (Z),14–17 linear
mesogenic rigid cores (RC1 and RC2, also called arms or
wings)1,18 and terminal moieties (R1 and R2).5,19–21 Since 1996
a large number of banana-shaped compounds have been
synthesized with various combinations of the structural
fragments mentioned above. It appears from these synthetic
investigations that a minor change in the molecular structure
can lead to drastic changes in the mesomorphic behavior and
therefore any prediction is extremely difficult. In particular,
the introduction of lateral substituent(s) either to the central
core or to the inner and outer rings of the banana-shaped
compounds is an important structural modification that can
alter the thermal behavior. Studies in this direction indicate the
dependence of mesomorphic behavior on the position, volume,
and electronic (polarizable) properties of the substituents.
Specifically, in laterally substituted bent-core compounds the
dipolar effects perhaps govern over the steric factors. In most
cases, the various substituents in the bent-core molecular
fragments induce lower melting and isotropization tempera-
tures. Further, such substitutions either widen the thermal
range of the mesophase or destabilize mesomorphism.
Many of the banana-shaped LCs exhibiting ferroelectric or
antiferroelectric mesophases are Schiff’s base derivatives
showing an unusually broad variety of polymorphism. It is
well known that the major drawback of these bent-core
compounds is their limited thermal, hydrolytic and photo-
chemical stability owing to the presence of imine linkages. In
this direction our group22 and other research teams have been
engaged23,24 in design, synthesis, and evaluation of meso-
morphic properties as well as electro-optic investigations of
salicylaldimine-based achiral banana-shaped LCs, which we
hereafter refer to as BCP-n series (Chart 2). The novelty of the
molecules originates from the fact that the molecular structure
consists of a relatively thermally and hydrolytically stable
salicylaldimine unit as a linear rigid segment attached to an
angular central 1,3-disubstituted benzene nucleus. The results
obtained are quite promising with regard to their stability to
heat and moisture owing to the presence of intramolecular
hydrogen bonding and stabilization of electrically switchable
banana mesophases over a wide thermal range. A systematic
investigation focusing on molecular design and synthesis
leading to such stable banana-shaped mesogens is very much
essential. In particular, lateral substitutions at the central unit
of these bent-core molecules would furnish a large number of
Of all the molecular fragments in the bent-core system, the
central unit is the key entity as it governs the bending angle
(see Chart 1B) between the two rod-like rigid cores attached to
it and hence the bent conformation of the molecules. A large
number of the banana-shaped LCs reported are five-ring
symmetrical Schiff’s bases (Chart 1A) derived from resorcinol
possessing a 1,3-phenylene core as the central unit connected
to two Schiff’s base linear mesogenic rigid cores. The structural
modifications of these Schiff’s bases achieved by introducing
various lateral substituents such as fluoro (F), chloro (Cl),
cyano (CN), nitro (NO2) and methyl (CH3) groups into the
central 1,3-phenylene ring at the 2-, 4- and 6-position(s) proved
to be very useful for obtaining mesogens with novel phase
sequences and lowering the phase transition temperatures.14
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Aldrich company and were used as received. Solvents were
purified and dried following standard procedures. The
intermediates (1a–k) were purified by column chromatography
techniques on silica gel (Acme, 60–120 mesh), whereas the
acid derivatives (2a–k) were purified by repeated recrystalliza-
tions using appropriate solvents to constant isotropic phase
transition temperature. Thin layer chromatography (TLC) was
performed on aluminium sheets pre-coated with silica gel
(Merck, Kieselgel10, F254). IR spectra were recorded using a
Perkin Elmer Spectrum 1000 FT-IR spectrometer. 1H NMR
and 13C NMR spectra were recorded using a Bruker Aveance
series DPX-200 (200 MHz) or a BrukerAMX-400 (400 MHz)
spectrometer. For 1H NMR spectra, the chemical shifts are
reported in ppm relative to tetramethylsilane as an internal
standard. Mass spectra were recorded on a Jeol-JMS-600H
spectromter in FAB+ mode using 3-nitrobenzyl alcohol as a
liquid matrix. Elemental analyses were carried out using a
Eurovector EA3000 series CHNOS analyzer. The final
compounds as well as acid derivatives were initially evaluated
for their LC behavior by the observation of their textural
patterns with the help of an optical polarizing microscope
(Leitz DMRXP) equipped with a programmable hot stage
(Mettler FP90). The transition temperatures and associated
enthalpies were obtained from thermograms recorded on a
differential scanning calorimeter (Perkin Elmer DSC7) that
was calibrated previously using pure indium as a standard.
X-Ray diffraction studies of the unoriented samples were
carried out using an Image Plate Detector (MAC Science,
Japan) equipped with double mirror focusing optics and the
sample contained in a Lindemann capillary tube. To examine
the electrical switching characteristics of the mesophases,
samples in the isotropic phase were placed in the cell
(constructed using two indium tin oxide (ITO) coated glass
plates pre-treated with a polyimide solution, which enables
the molecules to align homogeneously) by capillary action
and cooled slowly. Then the electro-optical switching
features were observed under the microscope. In order to
ascertain the nature (ferro- or antiferro-electric) of the
mesophase as well as to determine the Ps, the triangular wave
method was followed.
Chart 2 The general molecular structure of salicylaldimine-based
banana-shaped mesogens pertaining to the preceding (BCP-n) and
present (BC1–6-n) investigations.
such novel materials that in turn may help in understanding
the relation between the molecular structures and thermal
behavior, which, to the best of our knowledge, has not been
undertaken hitherto. Herein we report the synthesis and
characterization of six different series of bent-core compounds,
abbreviated as BC1-n to BC6-n (see Chart 2), in which the
central 1,3-phenylene core is laterally substituted with methyl,
nitro or chloro substituents. These substituents, which essen-
tially differ in their steric as well as electronic properties, are
expected to exert their influence on the bending angle as well as
on the steric packing of the molecules depending on their
position on the central unit. The length of the terminal alkoxy
tail has been varied to realize several homologues in each series.
Synthesis and molecular structural characterization
The target banana-shaped molecules BC1-n to BC6-n series
were synthesized as shown in Scheme 1. Derivatives of
substituted 1,3-phenylenes (BC1-n to BC6-n series) were
prepared by reaction of variously substituted resorcinols with
two equivalents of the respective 4-(2-hydroxy-4-n-alkoxy-
benzylidineamino)benzoic acids 2a–k using DCC and
a
catalytic amount of DMAP in dichloromethane. The
required 4-(2-hydroxy-4-n-alkoxybenzylideneamino)benzoic
acids (2a–k) were prepared by acid catalyzed condensation of
4-aminobenzoic acid with 2-hydroxy-4-n-alkoxy-benzalde-
hydes 1a–k in absolute ethanol. It may be pointed out here
these intermediates, 2a–k, in general, were found to exhibit
nematic and/or smectic C phase(s), the details of which are
given in the electronic supplementary information (ESI{). The
crude materials obtained were recrystallized repeatedly with
suitable solvents so that the target compounds obtained
were pure and showed a constant isotropic phase transition
temperature. Molecular structures of all the target compounds
and intermediates were confirmed by spectroscopic methods of
analyses (see Experimental part of the ESI{ for the details)
Results and discussion
Four homologous series of symmetrical and two series of non-
symmetrical five-ring banana-shaped molecules differing in
structure, especially at the central 1,3-phenylene unit, display
LC behavior with the exception of two materials. Owing to the
presence of lateral substituents like CH3, NO2 and Cl at
different positions of central unit, these six series of new
banana-shaped compounds have been evidenced to exhibit
diverse thermal behavior. In particular, the substituents near
the linking groups of the central unit enforce changes in the
bending angle between the two wings of the bent-core
molecules and thus monomorphic to polymorphic behavior
has been observed in these six series of compounds. In the
following, detailed accounts of the mesomorphic behavior of
all the series of mesogens are presented and compared with
their parent series and also with other varieties of banana-
shaped compounds.25–34
General information
The requisite chemicals, namely resorcinols, 2,4-dihydroxy-
benzaldehyde and 1-n-bromoalkanes were obtained from
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Scheme 1 Reagents and conditions: (i) 4-aminobenzoic acid, EtOH, AcOH (catalytic), reflux, 2 h; (ii) 2a–k, DCC, DMAP, CH2Cl2, rt, 48 h.
Thermal behavior of BC1-n series
clockwise or anticlockwise direction from the crossed position
dark domains in the texture become bright and vice versa
indicating the occurrence of chiral domains with opposite
handedness. This texture is quite similar to the ones reported
for the SmCP phase.1 The homologue BC1-22 exhibits an
enantiotropic but different mesophase having the following
features. Upon cooling the sample slowly from the isotropic
phase, beautiful twisted filaments appear (Fig. 1b) indicating
the presence of a helical structure. In the same sample
preparation a myelinic texture was also observed. On cooling
the sample further, the different textures coalesced into a non-
specific pattern. These textural patterns, in particular the
occurrence of both single and double wound helices during
slow cooling from the isotropic phase, essentially show the
features of the two- or three-dimensional antiferroelectric
The transition temperatures and the associated enthalpies of
BC1-n series of compounds, in which a methyl group is
substituted at position 2 of the 1,3-phenylene core, are
summarized in Table 1. All the compounds were found to
exhibit monomesomorphic behavior and the enthalpy values
obtained in the DSC thermograms for the mesophase to
isotropic phase transition were in the range of about 10 to
25 J g21. On cooling from the isotropic liquid, all the samples,
except for the last compound, displayed a mesophase which
initially grows as spherulitic domains (Fig. 1a) with a sub-
sequent superposition of a fringe pattern. On further cooling,
new layers aggregate outside the initially formed spherulites. If
one polarizer is rotated by a small angle (5 to 10 uC) in a
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Table 1 Phase transition temperaturesa (uC) and corresponding enthalpies [J g21] of 2-methyl-1,3-phenylenebis[4-(2-hydroxy-4-n-alkyloxybenzyl-
idene)-49-aminobenzoates]: BC1-n series. Cr = crystal; BXA = two- or three-dimensional antiferroelectric banana (like B7) mesophase; SmCPA (B2)
= lamellar antiferoelectric banana mesophase; Iso = isotropic phase. N = Phase exists; — = phase does not exist
Heating
Cooling
Heating
Cooling
Heating
Cooling
Compound
BC1-7
BC1-8
BC1-9
BC1-10
BC1-11
BC1-12
BC1-16
BC1-18
BC1-22
a
R
Cr
BXA
—
—
—
—
—
—
—
—
N
SmCPA
I
N
N
N
N
N
N
N
N
N
C7H15
C8H17
C9H19
C10H21
C11H23
C12H25
C16H33
C18H37
C22H45
N
N
N
N
N
N
N
N
N
167.4 [17.5]
143.6 [17.8]
155.5 [11.8]b
142.4 [11.9]
152.1 [12.5]b
144.6 [8.7]
151.3 [16.5]
145.3 [13.4]
149.3 [15.8]b
142.3 [13.4]
147.7 [16.1]b
140.5 [14.2]
139.7 [62.3]
133.6 [13.7]
138.9 [64.3]
130.3 [12.8]
135.8 [11.7]b
127.9 [12.6]
N
173.8 [19.0]
169.8 [20.5]
173.3 [24.4]
168.8 [18.8]
172.5 [24.1]
167.2 [18.0]
174.5 [21.5]
171.7 [22.5]
171.8 [21.1]
167.6 [16.1]
171.0 [21.9]
167.7 [18.5]
166.1 [19.1]
163.4 [16.2]
162.8 [18.4]
160.1 [16.8]
N
N
N
N
N
N
N
153.4 [10.8]
151.1 [10.6]
—
Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at a rate of 5 uC min21
.
Additional crystal
b
to crystal transitions were observed for the compounds BC1-8 (130.9 [8.2], 147.3 [24.4] and 152.0 [4.1]), BC1-9 (131.6 [8.7] and 143.2 [27.3]),
BC1-11 (95.6 [4.1]), BC1-12 (109.3 [7.9]) and BC1-22 (127.0 [13.4]).
In order to ascertain the structures of the two different
mesophases observed, X-ray diffraction (XRD) experiments
were carried out on non-oriented samples. The diffraction
patterns were recorded on cooling the sample into the
mesophase from the isotropic phase. A representative and
typical diffraction pattern obtained for compound BC1-10 at
165 uC is shown in Fig. 2a (see also Table 2). The diffuse
˚
character of the wide angle peak at 4.7 A indicates a liquid like
in-plane order. In the small angle region two sharp reflections
Fig. 1 Photomicrographs of the optical textures observed during
cooling just below the isotropic phase for the SmCP (a) and BXA (b)
phases of the bent-core molecules BC1-8 and BC1-22 respectively.
˚
˚
were seen with spacings (d) 42.9 A and 21.2 A. These
reflections are in the ratio 1 : 2 indicating a lamellar ordering
in the mesophase. The layer spacing is lower than the
˚
calculated molecular length of 44.7 A obtained from the
banana (similar to B7) phase, and since, as we shall see later,
antiferroelectric switching is seen in this sample, we label this
1,22b
energy minimized conformation with the methylene units in
phase as BXA
.
the fully extended form in the all-trans conformation,
Fig. 2 X-Ray angular intensity profiles obtained in (a) the SmCP phase of BC1-10 at 165 uC and (b) the BXA phase of BC1-22 at 145 uC.
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Table 2 The spacings (d) corresponding to the low angle reflections obtained from XRD studies on different mesophases with the Miller indices
(shown in brackets). Also shown are spontaneous polarization (Ps) values obtained in the switchable mesophase. SmA = smectic A phase; SmC =
smectic C phase; B7 = two- or three-dimensional non-switchable mesophase
˚
Low angle ‘d’ values/A
Compound (T/uC)
d1
d2
d3
d4
d5
Mesophase assigned
Ps/nC cm22
BC1-10 (165)
BC1-22 (145)
BC2-12 (145)
BC3-11 (140)
BC3-18 (155)
BC3-18 (130)
BC3-22 (140)
BC3-22 (120)
BC5-12 (150)
BC5-18 (120)
BC6-7 (112)
BC6-18 (150)
BC6-18 (110)
42.9 (01)
59.0
43.4 (01)
44.9 (01)
41.7
53.2
56.7
59.9
41.6
21.2 (02)
39.9
21.5 (02)
21.6 (02)
—
—
20.3
—
—
15.1
—
SmCPA
BXA
SmCPA
SmCPA
SmC
SmCPA
SmC
SmCPA
B7
B7
436
404
338
129
—
208
—
74
—
29.1
14.3 (03)
—
—
—
20.1
—
19.5 (02)
—
29.3 (02)
—
—
12.8 (03)
—
—
—
—
—
—
50.0
—
484
—
39.7 (01)
52.7
60 (01)
SmCPA
SmA
SmCPA
379
Fig. 3 Electrical switching current response peak obtained by applying a triangular wave field in (a) the SmCP phase of compound BC1-10 at
155 uC (1.04 V mm21, 11 Hz) and (b) the BXA phase of compound BC1-22 at 136 uC (6.4 V mm21, 10 Hz).
suggesting that this is a tilted smectic phase.1,22a On applica-
tion of a triangular wave electric field (1.04 V mm21, 11 Hz),
the current passing through the sample BC1-10 showed two
peaks per half period of the applied field as shown in Fig. 3a.
The two-peak trace shows that the switching observed is of the
tri-state antiferroelectric type. These features are commonly
observed for the SmCP phase. The combined area under the
two peaks is a direct measure of the spontaneous polarization
(Ps). The high value of Ps (436 nC cm22) observed for this
sample is typical for the banana-shaped compounds. In view of
these observations we assign the phase under discussion as an
antiferroelectric SmCP (SmCPA) phase.
antiferroelectric type switching (Fig. 3b). The combined area
under the peak is a measure of the Ps and was found to be
404 nC cm22 which is comparable to that reported earlier for
the B7 mesophase of the parent series (BCP-n).22b Therefore, it
is reasonable to propose that the BXA phase could be a variant
(sub-phase) of the novel B7 mesophase.
It can be seen that in this homologous series of compounds
both melting and clearing temperatures decreased with
increasing chain length although the mesophase range is
not varied. In order to examine the effect of the 2-methyl
substitution, the LC behavior of the BC1-n series of bent-core
mesogens was compared with that of the BCP-n derivatives.
Seemingly, in the BC1-n series the B7 phase is stabilized only in
the case of the compound having a C22 chain length, whereas
in the parent series of compounds this phase commences from
the C16 homologue.22b In terms of the electrical switching
characteristics of both SmCP and BXA phases no significant
changes were noticed when compared with the behavior of the
BCP-n series of compounds. Furthermore, the phase behavior
of these compounds (BC1-n series) is comparable to that of the
corresponding derivatives in the original banana series:1a
SmCP phases exist up to high temperatures for most of the
homologues. In contrast, reaction of 2-methylresorcinol with
4-(4-n-dodecyloxybenzoyloxy)benzoic acid results in a com-
pound containing only ester connecting groups which is not
liquid crystalline (mp 121 uC) (unpublished). Inversion of the
direction of the outer carboxylic groups results in isomeric
The diffraction pattern obtained for the BXA phase of the
non-oriented BC1-22 sample at 145 uC is shown in Fig. 2b. In
˚
the wide-angle region, a diffuse peak with spacing (d) y4.7 A
is observed which is indicative of a liquid like order within the
smectic layer. In the small angle region five peaks are seen with
˚
˚
˚
˚
˚
59.0 A, 39.9 A, 29.1 A, 20.3 A and 15.1 A spacings. These
reflections rule out a simple layered phase but point towards a
two- or three-dimensional structure. Similar XRD patterns
have been reported for the B7 phases (the exact structural
features of which are yet to be ascertained) of other banana-
shaped compounds.1,22b The electrical switching property of
this phase was investigated by the triangular wave method. On
application of a triangular wave electric field (6.4 V mm21
,
10 Hz) for the sample BC1-22 at 136 uC, two peaks per half
cycle of the applied field were obtained indicating an
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Table 3 Phase transition temperaturesa (uC) and corresponding enthalpies [J g21] of 5-methyl-1,3-phenylenebis[4-(2-hydroxy-4-n-alkyloxybenzyl-
idene)-49-aminobenzoates]: BC2-n series
Heating
Cooling
Heating
Cooling
Heating
Cooling
Compound
BC2-7
BC2-8
BC2-9
BC2-10
BC2-11
BC2-12
BC2-16
BC2-18
BC2-22
a
R
Cr1
Cr2
SmCPA
I
N
N
N
N
N
N
N
N
N
C7H15
C8H17
C9H19
C10H21
C11H23
C12H25
C16H33
C18H37
C22H45
N
N
N
N
N
N
N
N
N
112.9 [4.6]
111.5 [5.6]
146.9 [6.5]b
110.2 [12.2]
116.2 [17.5]
117.8 [19.2]
N
149.7 [32.2]
143.3 [17.8]
149.1 [30.9]
143.1 [24.3]
150.2 [28.1]
143.7 [8.3]
143.7 [16.2]
140.0 [14.9]
140.4 [14.2]
134.4 [14.3]
136.4 [13.6]
131.3 [13.2]
—
N
—
N
N
—
146.1 [19.3]
152.9 [26.2]
147.6 [20.5]
153.4 [24.0]
148.0 [22.8]
153.2 [22.9]
148.9 [22.4]
151.0 [22.7]
145.7 [21.8]
148.5 [20.2]
142.9 [16.3]
143.3 [14.3]
138.5 [14.1]
b
N
N
N
N
N
N
126.2 [47.8]
120.3 [51.3]
126.0 [50.6]
118.1 [52.1]
130.3 [54.0]
123.3 [60.7]
—
—
—
N
N
N
Peak temperatures in the DSC thermograms obtained during the first heating and cooing cycles at a rate of 5 uC min21
crystal to crystal transition has been observed for this compound (108.1 [11.1]).
.
An additional
2-methyl-1,3-phenylenebis-4-[4-n-alkyloxyphenoxy)benzoates]
without liquid crystalline behaviour. As recently published, a
crystalline modification is a polar switchable one, which is of
high interest.26,29
conjunction with the existence of a tilted structure as
mentioned below indicates the antiferroelectric ground state
structure of the mesophase. In some regions, as shown in
Fig. 4d, circular domains with equidistant circular stripes were
seen, suggesting the helical periodicity of the mesophase.
Undoubtedly, some of these optical textures, especially the
Thermal behavior of BC2-n series
It is reported that methyl substitution at position 5 of the
central 1,3-phenylene ring is unfavorable for the stabilization
of mesophases in five-ring Schiff’s base bent-core com-
pounds.1a,14i Remarkably, the present salicylaldimine-based
(BC2-n) series of compounds with the same central angular
core (5-methylresorcinol) exhibit mesomorphism quite convin-
cingly. As illustrated in Table 3, compounds BC2-7 and BC2-8
are non-mesomorphic, all the other higher homologues
stabilize a mesophase, monotropic in BC2-9, but enantiotropic
in the rest of the compounds. From the optical textures as well
as other physical measurements, the only mesophase observed
in these compounds was characterized as SmCPA phase.
Unlike the SmCP phase of other series of compounds, these
BC2-n bent-core molecules exhibited
a remarkable and
interesting optical textural pattern. Upon very slow cooling
the isotropic liquid, the mesophase appears in the form of
straight or undulated filaments, which eventually transform
into a helical pattern (Fig. 4a) and also, in some regions,
coalesce to a highly non-specific textural pattern. On cooling
the isotropic liquid at the usual rate (3 to 5 uC min21), a
schlieren texture along with helical patterns are obtained as
shown in Fig. 4b. The schlieren texture contained two-
and four-brush disclinations (Fig. 4c). This information in
Fig. 4 Microphotographs of the optical textural patterns in different
regions of the slide for the SmCP phase of BC2-12 when cooled from
the isotropic phase: (a) straight or undulated filaments, which
transform in to a helical pattern; (b) schlieren pattern along with
filaments as well as circular domains; (c) schlieren texture having two-
and four-brush disclinations; (d) circular domains with equidistant
circular stripes developed from the undulated filaments.
290 | J. Mater. Chem., 2007, 17, 284–298
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Fig. 5 X-Ray intensity (a) and the current response (b) profiles obtained for the SmCP phase of BC2-12 at 145 uC and 135 uC respectively.
schlieren defects, are characteristic of the SmCP phase,
whereas the occurrence of other patterns suggests it could be
a modified version of the SmCP phase.16e X-Ray diffraction
study was carried out on the SmCP phase of BC2-12 at 145 uC,
as a representative case. In the XRD pattern (Fig. 5a), three
number of phenyl rings is increased to six or seven and
perfluoroalkyl chains exist in the terminal positions.16e,31
Thermal behavior of BC3-n series
All the nine compounds of the BC3-n series, possessing chloro
substitution at position 4 of the central phenyl ring, exhibit LC
behaviour as presented in Table 4. The first homologue BC3-7
with a heptyloxy terminal chain while cooling from the
isotropic phase shows the typical schlieren texture of the
nematic phase (Fig. 6) with large director fluctuations. On
further cooling, small focal conics with fine stripes across
emanate from the nematic phase and finally the field of view
fills completely with a focal-conic texture. The N–I transition
enthalpy is about 0.5 J g21 whereas it is 6.5 J g21 for the N to
the low temperature mesophase transition. The homologues
BC3-8, BC3-9, BC3-10, BC3-11 and BC3-12 show mono-
mesomorphic behaviour and the textural pattern was found to
be identical to that of the low temperature phase of BC3-7. The
striped focal-conic texture observed for BC3-11 while cooling
from the isotropic phase, as a representative case, is shown in
Fig. 7a. In a homeotropically aligned sample, schlieren texture
is observed, suggesting the optical biaxiality of the phase. In
order to ascertain the structure of this biaxial mesophase, an
XRD experiment was carried out on a representative sample.
The X-ray diffraction pattern obtained at 140 uC for the
mesophase of compound BC3-11 shows a diffuse peak in the
˚
reflections in the small angle region with spacings 43.4 A,
˚
21.5 A and 14.3 A are seen which indicate a layered structure.
˚
In addition to these reflections a broad diffuse wide-angle
˚
reflection at about 4.6 A shows the absence of in-plane order.
The layer spacing is much smaller than the molecular length,
˚
i.e. l = 56 A (calculated for the molecule in its all trans-
conformation) suggesting that this could be a tilted smectic
phase. The SmCP phase was investigated for its electrical
switching behavior. The current response curve (Fig. 5b) shows
two current peaks per half period of an applied triangular
wave voltage (3.3 V mm21, 10 Hz) confirming the antiferro-
electric nature of the phase. From the current response, a
switching polarization of 338 nC cm22 could be determined.
A dependence of the transition temperatures on the number
of carbon atoms in the terminal alkyl chain is clearly evident
from Table 3. The clearing temperature, i.e. the SmCP phase–
isotropic phase transition temperature, rises initially, reaches a
maximum and then gradually decreases as the chain length
increases. A similar trend is seen for the thermal range of the
SmCP phase. However, when compared with the isomeric
analogues comprising of 2-methylresorcinol as the central core
(BC1-n), the thermal range and Ps value of the SmCP phase
seems to have reduced for the BC2-n banana-shaped com-
pounds. In general, the introduction of a methyl group as a
lateral substituent into the central core of five-ring salicyl-
aldimine-based parent molecules (BCP-n) reduces the phase
transition temperatures, the thermal range of the mesophases
and their ability to form different banana phases.22 As already
mentioned, the attachment of substituents at the top of bent
five-ring molecules is disadvantageous for steric reasons:
5-methyl-, 5-cyano- and 5-methoxyresorcinol derivatives do
not show mesophases in the original series bearing terminal
octyloxy chains.1a,14i Therefore, the existence of enantiotropic
SmCP phases in the new series BC2-n is remarkable. Fluorine
atoms being the smallest substituents are accepted in the 5
position of five-ring bent-core mesogens.30,26 Recently,
Sadashiva et al. reported the mesophase behaviour of five-
ring compounds derived from 5-methoxyisophthalic acid.16a
Larger lateral groups in the 5 position are accepted only if the
˚
wide angle region with d = 4.5 A that can be attributed to
the liquid like in-plane order. In addition, two orders of
˚
lamellar reflections at d = 44.9 A and d = 21.6 A in the small
˚
angle region could be clearly seen. The layer spacing is lower
˚
than the theoretical molecular length (l = 55.1 A) indicating
that this could be a tilted smectic phase. The above mentioned
textural observations and X-ray diffraction results bear a
significant resemblance to the ones observed in the SmCP
phase of other banana-shaped compounds.1,22a
The characteristic feature of the SmCP phase, namely the
electrical switching behaviour was studied using BC3-11 as a
representative compound. The fan-shaped texture with stripes
running across the domains (Fig. 7a) obtained by slow cooling,
switches above a threshold voltage. Under the polarizing
microscope, a textural change was observed (stripes disappear)
with a slight increase in birefringence as shown in Fig. 7b.
Simultaneously two current peaks could be seen on the
oscilloscope screen for the half period of the triangular wave
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Table 4 Phase transition temperaturesa (uC) and the corresponding enthalpies [J g21] of 4-chloro-1,3-phenylenebis[4-(2-hydroxy-4-n-alkyloxy-
benzylidene)-4-aminobenzoates]: BC3-n series. N = nematic phase
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Compound
BC3-7
BC3-8
BC3-9
BC3-10
BC3-11
BC3-12
BC3-16
BC3-18
BC3-22
a
R
Cr
SmCPA
SmC
—
—
—
—
—
—
N
SmA
—
—
—
—
—
—
—
—
N
N
I
N
N
N
N
N
N
N
N
N
C7H15
C8H17
C9H19
C10H21
C11H23
C12H25
C16H33
C18H37
C22H45
N
N
N
N
N
N
N
N
N
119.2 [22.8]b
105.9 [24.0]
124.9 [17.3]
102.3 [19.8]
125.1 [6.8]b
100.6 [20.1]
117.6 [17.5]b
94.9 [18.4]
110.5 [78.3]
94.4 [14.6]
104.6 [30.9]
95.0 [15.8]
110.6 [65.6]
92.6 [49.7]
111.8 [73.9]
100.4 [65.9]
112.9 [76.4]
103.7 [76.5]
N
N
N
N
N
N
N
N
N
132.3 [9.8]
129.2 [6.5]
142.2 [12.1]
138.0 [8.2]
145.2 [12.1]
143.1 [10.9]
145.2 [11.8]
138.9 [7.3]
151.5 [12.3]
146.2 [7.7]
153.0 [12.9]
150.6 [12.1]
150.2 [2.9]
145.1 [2.4]
146.3 [2.2]
145.6 [2.2]
131.9 [1.3]
129.8 [1.0]
N
135.1 [0.6]
133.9 [0.5]
—
—
—
—
—
—
—
—
158.8 [7.1]
157.0 [6.3]
160.2 [7.0]
159.4 [6.9]
153.5
N
N
154.7 [3.7]c
150.7 [3.5]
b
149.0
Peak temperatures in the DSC thermograms obtained during the heating cycle at a rate of 5 uC min21
transitions have been observed for the compounds BC3-7 (100.4 [48.9]), BC3-9 (106.2 [17.8], 114.3 [6.6]) and BC3-10 (77.5 [13.2]). This is the
combined enthalpy for the SmC–SmA and SmA–I transitions as the SmC–SmA transition enthalpy could not be obtained due to the existence
of the SmA phase for a very short thermal range.
.
Additional crystal to crystal
c
homologues BC3-16 and BC3-18 exhibit a dimesomorphic
sequence involving a transition from a SmCP to a SmC phase
that was evidenced by optical and calorimetric studies. For
example, on cooling the sample BC3-18 from the isotropic
phase, the SmC phase exhibits a broken fan-shaped texture
for planar alignment and a schlieren texture for homeotropic
alignment of the mesogens (Fig. 8a). The textural pattern
obtained for the SmCP phase on subsequent cooling is shown
in Fig. 8b. The presence of an SmCP phase was confirmed by
X-ray diffraction (see Table 2) and electrical switching studies.
The tri-state switching yields a Ps value of about 208 nC cm22
.
As predicted the SmC phase did not show any response to the
applied field.
Fig. 6 Photomicrograph of the N phase appearing just below the
isotropic phase for compound BC3-7.
For the BC3-22 homologue, the occurrence of three
mesophases, viz. the SmCP, SmC and SmA phases, was
established preliminarily by microscopic and calorimetric
studies. The mesophase appearing below the isotropic phase
exhibits a focal-conic (Fig. 9a) or a pseudoisotropic texture in
voltage. The apparent saturated polarization Ps is about
129 nC cm22. These observations clearly indicate the anti-
ferroelectric nature of this phase. The longer chain length
Fig. 7 Photomicrographs of the textures observed for the SmCP
phase of BC3-11 at 136.4 uC: (a) 0 V mm21 and (b) 1.3V mm21
Fig. 8 Photomicrographs of textures observed (a) for the SmC phase
at 150 uC and (b) for the SmCP phase at 125.5 uC of BC3-18.
.
292 | J. Mater. Chem., 2007, 17, 284–298
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Fig. 9 Photomicrographs of (a) fan-shaped texture of the SmA phase (150 uC), (b) broken fan-shaped texture of the SmC phase (145 uC) and (c)
non-specific texture of the SmCP phase (125 uC) observed for BC3-22.
slides treated for the planar or homeotropic geometry
respectively, which is the characteristic textural behavior of
the SmA phase. On cooling, the focal-conic texture is
transformed into a broken focal-conic pattern (Fig. 9b),
whereas for the homeotropic boundary conditions a schlieren
texture is formed indicating the presence of the SmC phase.
The SmCP phase occurring below the SmC phase shows a
non-specific textural pattern (Fig. 9c) for both planar
and homeotropic alignment conditions. The electro-optical
investigations performed on these phases showed antiferro-
electric switching behavior for the low temperature SmCP
phase, while as expected the SmC phase could not be switched.
From the current response, a Ps value of about 74 nC cm22
could be determined in the SmCPA mesophase.
favorable (with tilt of the molecules) for steric reasons. In such
a situation for better packing the aliphatic chains stretch out
and thereby increasing the layer spacing. The alternating polar
direction in subsequent layers causes the antiferroelectric
character of the ground state and stabilizes the layer structure
of the SmCP phase by minimizing the Coulomb energy.
It can be generalized that in these chloro-substituted
compounds, the melting as well as clearing temperatures, and
thus the mesophase range, are decreased in comparison to the
analogous non-substituted derivatives, the parent series of
compounds (BCP-n).22–24 The mesomorphic behavior is also
significantly modified; in addition to the SmCP phase
characteristic for a bent molecular shape, the SmC and/or
the SmA phase occur for the same substance. These observa-
tions lead to the conclusion that concerning the chemical
constitution, the boundary between calamitic mesogens and
banana mesogens is very thin. This is a relatively new aspect
concerning the relationships between chemical structure and
mesophase behaviour of banana-shaped liquid crystals.
Previous NMR investigations suggest that the angle between
the two arms of the banana-shaped molecules in the
mesophase varies between 105u to 140u depending on the
chemical structure of the molecules.1a,14j If the angle is greater
or smaller than these limits, then calamitic mesophases such as
N, SmA or SmC phases occur. In a first approximation, the
bending angle is influenced by the chloro atom in position 4
and should be nearly independent of the length of the terminal
chains. Such a state can be assumed in the cases of both BC3-n
and BC6-n (see later for details) series of compounds. Thus,
the occurrence of both banana (SmCP) and calamitic (nematic
and/or smectic) phases in these bent-core compounds is not
surprising. This is in agreement with descriptions not only
in the original banana series,14c,h,j but also for bent-core
mesogens containing ester connecting groups.33 It may be
X-Ray measurements (non-oriented samples) indicate a
layer structure without in-plane order for all the three meso-
phases described above. Fig. 10 illustrates the temperature
dependence of the layer spacing (d) for the SmC and SmCP
phases. It should be pointed out here that X-ray studies were
not carried out in the SmA phase owing to its short thermal
range. As can be seen the layer spacing increases continuously
with decreasing temperature at the transition from the SmC to
the SmCP phase instead of an expected decrease with
decreasing temperature, which is in agreement with the results
reported for a similar situation.14h It can be assumed that the
bent molecules can freely rotate in the SmC phase and at the
transition from the SmC to the SmCP phase the long range
packing of the molecules in the bent direction becomes more
mentioned here that an unusual polymorphism with
a
reentrant SmCP phase has been found for derivatives of
4-chlororesorcinol recently.32 A transition between a polar and
a non-polar SmA phase was reported for the corresponding
4-cyanoresorcinol esters.26,34
Thermal behavior of BC4-n series
In this series of compounds, two chlorine atoms are substituted
at positions 4 and 6 of the central 1,3-phenylene ring. The
phase transition temperatures and the associated enthalpies
obtained for the compounds of the BC4-n series are
summarized in Table 5. Mesophase characterization by optical
Fig. 10 Temperature dependence of the layer spacing (d) in the SmC
and SmCP phases of the compound BC3-22.
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Table 5 Phase transition temperaturesa (uC) and corresponding enthalpies [J g21] of 4,6-dichloro-1,3-phenylenebis[4-(2-hydroxy-4-n-alkyloxy-
benzylidene)-49-aminobenzoates]: BC4-n series
Heating
Cooling
Heating
Cooling
Heating
Cooling
Compound
BC4-7
BC4-8
BC4-9
BC4-10
BC4-11
BC4-12
BC4-16
BC4-18
BC4-22
a
R
Cr
SmC
N
N
I
N
N
N
N
N
N
N
N
N
C7H15
C8H17
C9H19
C10H21
C11H23
C12H25
C16H33
C18H37
C22H45
N
N
N
N
N
N
N
N
N
133.0 [34.6]
94.2 [28.2]
131.5 [42.8]
82.9 [35.3]
137.2 [35.2]
88.4 [20.1]
140.2 [44.3]
99.4 [36.3]
136.5 [49.2]
93.0 [42.4]
135.2 [54.2]
74.8 [16.7]
132.7 [62.9]
106.2 [53.2]
132.0 [66.1]
101.8 [54.4]
129.3 [78.1]
110.4 [86.5]
—
158.5 [0.8]
157.3 [0.5]
152.2 [1.1]
149.8 [0.8]
159.2 [0.5]
158.6 [0.4]
164.1 [1.4]
162.6 [1.3]
156.2 [1.6]
154.2 [1.5]
159.0 [2.3]
158.1 [1.7]
—
N
—
N
N
N
N
N
N
N
—
N
130.5 [0.4]
143.9 [0.7]
139.8 [0.3]
155.4 [0.8]
152.5 [0.4]
166.9 [7.3]
165.0 [5.7]
165.0 [7.6]
162.2 [6.2]
162.0 [7.3]
160.9 [7.1]
N
N
—
—
—
Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at a rate of 5 uC min21
.
textural observation revealed the presence of the N and/or
agreement with the behaviour in other series of bent-core
compounds.14e,26,27
SmC phases. Lower homologues BC4-7, BC4-8 and BC4-9
stabilized an enantiotropic
N phase. Observation of a
Thermal behavior of BC5-n series
characteristic texture, in slides treated for planar orientation
(unrubbed), namely the schlieren texture having both four-
and two-brush defects, evidenced the presence of the N phase.
Compound BC4-10 stabilized a monotropic SmC phase in
addition to the enantiotropic N phase. The SmC phase was
identified by the observation of a characteristic broken
focal-conic texture on glass substrates treated for planar
orientation. In compounds BC4-11 and BC4-12 both N and
SmC phases are enantiotropic in nature. The higher homo-
logues BC4-16, BC4-18 and BC4-22 displayed only an
enantiotropic SmC phase, exhibiting both the broken focal-
conic texture and schlieren pattern, when untreated slides
were used.
The enthalpies obtained in the range of about 0.5–1.5 J g21
and 6–7 J g21, respectively, for the N–I and SmC–I phase
transitions are in good agreement with values obtained for
such phase transitions in calamitic LCs. Not surprisingly, this
tilted smectic phase (SmC) was found to be apolar in nature
similar to the ones formed by calamitic LCs as evidenced from
the lack of any response to the applied electric field. Banana-
shaped compounds derived from 1,3-phenylene units with
lateral choloro-substitution at the 4 and 6 positions of the
central angular ring reported up to now do not form banana
mesophases.1 The introduction of two chlorine atoms into the
4,6 positions of the central core gives rise to a bending angle
of about 165u between the two arms favoring the formation
of N and/or SmC mesophases. Thus, as anticipated, such
behavior was also seen for the compounds BC4-n, a feature in
The compounds of the series BC5-n, which are substituted with
a highly polarizable nitro group at position 2 of the central
unit, exhibit identical thermal behavior (Table 6). These bent-
core materials, contained between untreated glass slides as thin
films, display several fascinating textural patterns appearing
concomitantly at different regions. In particular, upon cooling
slowly (at a rate of 0.2 uC min21) from the isotropic phase,
elongated single- and double-spiral domains appear. In
addition, elongated worms, myelinic texture and two-dimen-
sional patterns were seen. The pitch of the helix seems to be
correlated with the cooling rate as previously reported.1 Under
fast cooling (2 uC min21) rate, thin filaments appearing like
threaded beads are formed. When the transition to the liquid
crystal phase is completed filaments as well as other patterns
collapse to yield a non-specific texture. These patterns are
identical to those observed for the mesophase of different
series of banana-shaped compounds for which the symbol B7
has been assigned. Thus, based on microscopic textural
observations, the mesophase of the BC5-n series of compounds
is identified as B7. The wide mesophase range observed during
cooling mode for these BC5-n homologues can be attributed
to their tendency to impede the immediate reorganization
necessary for crystallization when compared to other series of
compounds. For example, compound BC5-10 did not crystal-
lize while cooling but showed a glass transition at about 70 uC
as evidenced by a step like variation in the DSC thermogram.
As a representative case the various textural patterns observed
294 | J. Mater. Chem., 2007, 17, 284–298
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derivatives14g and other banana-shaped compounds with a
polar substitution at the 2 position of the 1,3-phenylene
derivatives is reported to be non-switchable.25 Thus, in
contrast to the observed current response for the BXA phase
of BC1-22 compound, the B7 phase shown by this BC5-n
series of bent-core compounds did not show any current
response to the applied electric field. This was true not only in
our series BC5-n, but also in several series of five-ring bent-
core mesogens having different connecting groups.27,25a,b,28
Thus, attachment of strong polar groups like cyano or nitro
between the two legs of the bent molecules is a sure design to
prepare materials exhibiting an apolar B7 phase.
Table 6 Phase transition temperaturesa (uC) and corresponding
enthalpies [J g21] of 2-nitro-1,3-phenylenebis[4-(2-hydroxy-4-n-alkyl-
oxybenzylidene)-49-aminobenzoates]: BC5-n series
Heating
Cooling
Heating
Cooling
Compound
BC5-8
BC5-9
BC5-10
BC5-11
BC5-12
BC5-16
BC5-18
a
R
Cr
B7
I
N
N
N
N
N
N
N
C8H17
C9H19
C10H21
C11H23
C12H25
C16H33
C18H37
N
N
N
N
N
N
N
128.8 [14.0]
,40
121.3 [17.6]
,40
N
N
N
N
N
N
N
158.8 [23.3]
148.8 [12.0]
150.2 [15.6]
145.0 [13.7]
168.8 [25.0]
165.2 [18.9]
156.9 [25.6]
153.2 [20.4]
170.2 [24.1]
163.1 [11.3]
163.4 [15.5]
155.7 [7.4]
Furthermore, the B7 mesophase observed for the BC5-n
compounds did not show any dependence on the terminal
alkyl chain length for its occurrence unlike in the other series
of compounds mentioned earlier. Polarizable nitro group
substitution at the 2 position not only modifies the type of
mesophase formed but also increases the mesophase range in
comparison to the analogous 2-methyl derivatives. Further, in
the context of the electrical switching behaviour, the B7 phase
formed by these homologues is different from that observed
for the higher homologues of the parent series (BCP-n) or that
of the BXA phase of the BC1-22 derivative, i.e., the latter ones
show antiferroelectric switching behaviour. Thus the structural
features of the B7 phase seems to be highly complicated and
consequently not known yet, although some studies have been
initiated in this direction.1
115.9 [10.8]b
65.4 [0.8]c
112.3 [12.0]b
70.4 [2.2]c
117.0 [20.3]
66.7 [5.4]
113.2 [23.2]b
85.6 [24.1]
102.3 [64.3]
89.5 [12.8]
148.7 [18.7]
143.7 [12.8]
Peak temperatures in the DSC thermograms obtained during the
first heating and cooling cycle at a rate of 5 uC min21
crystal to crystal transitions have been observed for the compounds
.
Additional
b
BC5-10 (64.0 [17.7]) BC5-11 (74.8 [15.6]) and BC5-16 (92.9 [28.2]).
a glassy state as
c
At this temperature the phase freezes into
evidenced by a step like variation in DSC.
Thermal behavior of BC6-n series
for the B7 phase just below the isotropic phase of compound
BC5-12 are shown in Fig. 11.
The BC6-n series of compounds, to the best of our knowledge,
are the first examples of banana-shaped molecules in which a
highly polarizable nitro group is substituted at the 4 position
of the middle phenyl ring. The transition temperatures and
associated enthalpies of this series of compounds are given in
Table 7. Phase assignments based on complementary techni-
ques such as optical, DSC, X-ray diffraction and electrical
switching studies revealed the presence of two types of
mesophases depending on the length of the terminal alkyl
chains. As shown in Table 7, for the isotropic mesophase
(especially for the banana phase) transition, the enthalpy (DH)
value obtained for the first cooling cycle is nearly half that in
the first heating scan. A similar tendency is also seen for some
of the compounds of BC5-n series as shown in Table 6.
However, during the subsequent (second) heating/cooling
runs, the DH value obtained agrees with that of first cooling
(see ESI{) scan with a shift in the phase transition temperature.
Currently, we do not have an understanding of this feature.
On cooling from the isotropic liquid, the phase formed by
the lower homologues BC6-6, BC6-7, BC6-8, BC6-10, BC6-11
and BC6-12 emerges as baˆtonnets which coalesce to form
broken fan shaped textures as shown in Fig. 12a. In different
regions of the cell a schlieren texture is also obtained (Fig. 12b).
As a representative case, the X-ray angular intensity profile
was obtained for a non-oriented sample of BC6-7 at 112 uC.
The diffractogram showed a strong Bragg reflection in the
small angle region with two harmonics, suggesting a layer
structure. The diffuse scattering maxima in the wide angle
region indicated a liquid like order within the layer. The layer
In the X-ray pattern of non-oriented samples of BC5-12 and
BC5-18, typical diffuse scattering in the wide angle region (d =
˚
4.6 A) was observed pointing to the liquid like order within the
smectic layers. In the small angle region a sharp reflection (at
˚
˚
41.6 A for BC5-12 and 50 A for BC5-18) was detected (see
Table 2). A similar XRD pattern has been reported recently
for the B7 phase of some of the banana compounds.13a
Electrical switching studies carried out over a range of
temperatures, voltages and frequencies by the application of
a triangular wave field did not result in any observable
response from the samples. This is not surprising given the fact
that the B7 phase observed for the first time in Schiff’s base
Fig. 11 Optical photographs of the textural patterns of the non-
switchable B7 mesophase obtained for the compound BC5-12 in
different regions of the sample preparation just below the isotropic
phase.
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Table 7 Phase transition temperaturesa (uC) and corresponding enthalpies [J g21] of 4-nitro-1,3-phenylenebis[4-(2-hydroxy-4-n-alkyloxybenzyl-
idene)-49-aminobenzoates]: BC6-n series
Heating
Cooling
Heating
Cooling
Heating
Cooling
Compound
BC6-6
BC6-7
BC6-8
BC6-10
BC6-11
BC6-12
BC6-16
BC6-18
a
R
Cr
SmCPA
SmA
—
—
—
—
—
—
N
I
N
N
N
N
N
N
N
N
C6H13
C7H15
C8H17
C10H21
C11H23
C12H25
C16H33
C18H37
N
N
N
N
N
N
N
N
114.5 [17.6]
98.9 [14.1]
104.9 [10.9]b
89.9 [7.7]
N
N
N
N
N
N
N
N
174.6 [18.9]
161.7 [11.7]
178.8 [15.7]
165.2 [9.1]
181.5 [18.8]
166.7 [7.9 ]
176.4 [15.2]
158.1 [7.5]
184.6 [25.6]
177.7 [15.3]
182.5 [19.6]
171.3 [7.4]
161.5 [5.6]
116.8c
97.1 [9.9]b
80.1 [2.4]
90.9 [9.8]b
70.3 [7.8]
92.8 [10.5]b
68.0 [20.4]
94.9 [6.9]b
64.6 [15.3]
107.1 [51.5]b
68.2 [15.6]
110.8 [47.7]b
69.0 [17.7]
173.6 [5.6]
173.1 [4.2]
172.3 [5.1]
171.3 [4.0]
b
162.2 [5.9]
111.0c
N
Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycle at a rate of 5 uC min21
.
Additional crystal
to crystal transitions have been observed for the compounds BC6-7 (89.1 [16.2]), BC6-8 (87.4 [20.2]), BC6-10 (67.5 [52.7]), BC6-11 (58.7 [11.2],
84.6 [17.9]), BC6-12 (77.8 [59.2]), BC6-16 (88.2 [42.6]) and BC6-18 (95.1 [21.5]). Determined by the textural change observed in POM, and a
corresponding enthalpy change was not observed in DSC.
c
spacing was found to be relatively lower than the calculated
molecular length (in its all trans-conformation) suggesting
that this is a tilted smectic phase. The switching behavior of
this phase was investigated by applying a triangular wave
voltage (5 V mm21, 10 Hz) for a representative compound
BC6-7 at 112 uC and two current response peaks were recorded
during a half period of the applied field indicating anti-
ferroelectric behaviour. The spontaneous polarization deter-
mined by integration of the switching current peaks amounts
to 484 nC cm22. Thus, the phase under discussion is
unambiguously SmCPA.
SmA phase some striations appear (Fig. 13b) whereas the
homeotropic SmA regions fill with the schlieren texture. The
X-ray diffraction study ensured the layered structure of
the SmA phase of BC6-18: it showed a sharp reflection in
˚
the small angle region at d = 52.7 A. The wide-angle diffuse
˚
peak at 4.6 A indicates a liquid-like ordering of the molecules
in the mesophase. Similarly, an X-ray profile obtained for the
low temperature phase of BC6-18 showed a strong Bragg
˚
reflection (60 A) and its higher order reflection (29.3 A) in the
˚
small angle region, indicating a layer structure. Further, the
˚
diffuse wide angle scattering (4.5 A) indicates a liquid-like
Optical textural observations clearly showed the occurrence
of two mesophases for compounds BC6-16 and BC6-18.
On cooling the isotropic liquid, the high temperature phase
grows as baˆtonnets that coalesce to form a focal-conic texture
(Fig. 13a). Additionally, in a different region of the slide,
a dark field of view was also seen, indicating the presence
of a SmA phase. On further cooling, at the transition to
the low temperature phase, on the focal-conic texture of the
order within the smectic layer. Two current response peaks
obtained per half period on application of a triangular wave
voltage confirmed an antiferroelectric switching characteristic
of the mesophase with the saturated polarization being about
379 nC cm22. Based on these results we have identified this
mesophase as the SmCPA phase. It may be mentioned here
that the SmA phase did not show any response to the applied
electrical field.
Fig. 12 Optical photomicrographs showing the broken fan-shaped
(a) and schlieren (b) textures obtained in different regions for the
SmCP phase of compound BC6-7.
Fig. 13 Optical photomicrographs showing (a) focal-conic texture of
SmA and (b) broken focal conics pattern of SmCP mesophases at
120 uC and 110 uC respectively, obtained for the compound BC6-18.
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It can be seen that the melting temperatures of these 4-nitro
derivatives are slightly lower than those of analogous non-
substituted compounds and thereby exhibit a higher meso-
phase range. The relatively high values of the spontaneous
polarization found in these homologues can be attributed to
the enhancement of the lateral dipole moment by the polar
nitro group. The transition from the SmCP phase to the
conventional SmA phase in higher homologues obviously
indicates that the bulky NO2 substituent at the 4 position of
the central 1,3-phenylene ring modifies the mesomorphic
behaviour by exerting its influence on the bending angle
between the two arms. Stabilization of different banana
mesophases with different structural and electrical switching
properties in structurally isomeric nitro-substituted derivatives
(with substitutions at the 2 and 4 positions) clearly demon-
strates the dependence of mesomorphic behaviour on the
subtle changes in the molecular structures.
derivatives exhibit only the N and/or SmC phases, supporting
the view that the optimum bending angle required for the
occurrence of banana mesophases is in the range of 110–140u.
Compounds with 2-nitro substitution exclusively form a B7
(non-switchable) phase irrespective of the terminal alkyl chain
length they possess, in contrast to the antiferroelectric B7 phase
observed for non-substituted and methyl group containing
analogues. On the other hand the thermal behavior of bent-
core molecules derived from 4-nitroresorcinol was found to be
different from the 2-nitro derivatives. The lower and middle
homologues display an antiferroelectric SmCP phase whereas
the higher ones exhibit the SmA phase additionally. The trend
observed for the clearing temperatures in these homologues is
unusual. For a particular homologous series of compounds
with increasing number of carbon atoms in the chain, the
transition temperatures can increase or decrease drastically or
remain nearly the same. The SmCP phase stabilized by
different homologues exhibit antiferroelectric switching beha-
vior. Relatively high values of the spontaneous polarization,
i.e. about 380–400 nC cm22, were found in the SmCP phases
formed by 4-nitro derivatives. The B7 phase observed for the
bent-core molecules derived from 2-methylresorcinol deriva-
tives show an antiferroelectric switching behavior though no
such feature was observed for the B7 phase formed by the
2-nitro derivatives. This points to the complex structural
behavior of this phase and more investigations are needed to
understand the properties of this phase. Thus, understanding
the relationship between molecular architecture and liquid
crystal properties in banana-shaped molecules is not straight-
forward, a feature often emphasized in the literature.
Summary
We have reported the synthesis and characterization of more
than fifty novel banana-shaped compounds belonging to six
different homologous series, the novelty of which originates
from the fact that the molecular design comprises laterally
substituted 1,3-phenylene ring as a central unit covalently
linked to thermally and hydrolytically stable rod-like salicyl-
aldimine segments (arms). It is well demonstrated that the
nature (electro-negativity or polarizability and size) and
position of these substituents generally decide the type of
mesophase(s) and phase sequence to be formed by the banana-
shaped compounds in which they are present. All the mesogens
derived from 2-methylresorcinol exhibit an antiferroelectric
SmCP phase with the exception of a higher homologue that
displays an antiferroelectric BX phase. In contrast to this, a
methyl group present at the 5 position of the 1,3-phenylene
ring reduces the clearing temperature as well as the thermal
range of the antiferroelectric SmCP phase. Further, unlike in
the case of the parent series of non-substituted analogues,
banana-shaped compounds derived from 2- or 5-methyl-
resorcinol suppress the formation of the B7 phase irrespective
of the lengths of the terminal alkoxy chains. The chlorine atom
substituent near the linking groups, i.e. at the 4 and/or 6
position of the central 1,3-phenylene ring, is found to change
the bending angle between the two arms and hence the
mesomorphic behavior of the molecules, an observation in
agreement with the literature reports. In general, mesogens
derived form 4-chlororesorcinol exhibit an antiferroelectric
SmCP phase. Compounds having short or long chain lengths
(with one exception) display dimesomorphic SmCP–N and
SmCP–SmC sequences respectively, whereas the middle
homologues exhibit only the SmCP phase. Interestingly, a
homologue with a docosanoxy tail displays a trimesomorphic
SmCP–SmC–SmA sequence. Observation of the SmCP phase,
characteristic for a bent molecular shape, and conventional
phases like N, SmC and/or SmA occurring in the same
substance suggest that in the context of molecular structures,
the boundary between rod-like and banana-shaped mesogens
is narrow and can be altered by having appropriate lateral
substitutions at the central core. The 4,6-dichlororesorcinol
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