Fluorescent unsymmetrical four-ring bent-core mesogens: 2D modulated phases

Article abstract of DOI:10.1039/c3ce41360h

Based on the chemical incompatibility between the aromatic mesogenic cores and the flexible end aliphatic (methylene units) chains, which can lead to the formation of polarization modulated layered undulated phases, four-ring bent-core compounds have been designed and synthesized. The designed compounds are thermally and hydrolytically stable due to intramolecular hydrogen bonding, highly fluorescent and exhibit 2D-polarization modulated layer undulated (PMLU) smectic phases. The phase transition temperatures have been confirmed by differential scanning calorimetry and the phases are confirmed by polarizing optical microscopy and X-ray diffraction studies. For compounds 1-10-11 and 1-15-11 the peaks are indexed to tilted columnar lattice (Col_ob), while for 2-12-11 the peaks are indexed to the rectangular lattice belonging to the PMLU family The influence of the direction of ester linking group played an important role in the mesophase as well as the evaluated parameters of the dipole moment, the bending angle and the asymmetry parameter by spin-restricted DFT calculations using the B3LYP exchange-correlation functional and the 6-311G(d,p) basis set.

Full text of DOI:10.1039/c3ce41360h

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Fluorescent unsymmetrical four-ring bent-core
mesogens: 2D modulated phases†
Cite this: CrystEngComm, 2013, 15,
0510
1
a
a
a
a
Rahul Deb, Atiqur Rahman Laskar, Dipika Debnath Sarkar, Golam Mohiuddin,
a
b
c
Nirmalangshu Chakraborty, Sharmistha Ghosh, D. S. Shankar Rao
a
and Nandiraju V. S. Rao*
Based on the chemical incompatibility between the aromatic mesogenic cores and the flexible end aliphatic
methylene units) chains, which can lead to the formation of polarization modulated layered undulated phases,
(
four-ring bent-core compounds have been designed and synthesized. The designed compounds are thermally and
hydrolytically stable due to intramolecular hydrogen bonding, highly fluorescent and exhibit 2D-polarization modu-
lated layer undulated (PMLU) smectic phases. The phase transition temperatures have been confirmed by differen-
tial scanning calorimetry and the phases are confirmed by polarizing optical microscopy and X-ray diffraction
studies. For compounds 1-10-11 and 1-15-11 the peaks are indexed to tilted columnar lattice (Colob), while for
Received 11th July 2013,
Accepted 3rd October 2013
2
-12-11 the peaks are indexed to the rectangular lattice belonging to the PMLU family The influence of the direc-
tion of ester linking group played an important role in the mesophase as well as the evaluated parameters of the
dipole moment, the bending angle and the asymmetry parameter by spin-restricted DFT calculations using the
B3LYP exchange-correlation functional and the 6-311G(d,p) basis set.
DOI: 10.1039/c3ce41360h
www.rsc.org/crystengcomm
1
. Introduction
chiral molecular assembly because chirality breaks inversion
1
2
symmetry. However based on symmetry considerations the
ferroelectricity, ascribed to the polar C2v symmetry, can be
obtained from the efficient packing of bent-core (banana-
shaped or bow-shaped or V-shaped) molecules into layers. In
these bent-core molecules exhibiting the homogeneous
The discovery of polar and chiral order in the bent-core com-
1
,2
pounds promoted the importance of the bent or banana or
V-shaped liquid crystals (BLC) as a new sub-field of liquid
crystals. Because of the sterically induced packing of these
3
–9
bent-core molecules several new smectic modifications,
in
1,4,5
2,4,5
layered banana phases, unique polar
and chiral
cha-
addition to lamellar structures with one-dimensional density
modulation (B2 phases), can be realized which have no coun-
terpart in the field of calamitic liquid crystals. On the other
hand, molecular structural modifications of these new
mesophases can lead to unusual physical properties which
are not only interesting from the theoretical point of view but
also in practical applications. Some of the unusual properties
include chiral phases (nematic, B2, B4, B7 etc.) from achiral
molecular assemblies with segregation into domains of oppo-
racteristic properties are realized due to their anisotropic
molecular shapes and distinct tilt ordering of the packed
bent-core molecules present in the layers. In such layered
structures the antiparallel arrangement of molecular dipoles
in adjacent layers compensates spontaneous electric polariza-
tion. Theoretically the bent molecular shape of π-bridged
systems with electron push–pull moieties exhibiting polar
order and mesomorphism is appropriate for the correct
design of non-linear optical materials. The use of topographic
patterning by the confinement of helical nano filaments
13
1
0
11
site handedness, and the formation of cybotactic clusters
in nematic phases etc. The much needed characteristic prop-
erty for the polar order of smectic C phase, possessing the
dual properties viz., molecular chirality and director tilt
(
HNFs) to control effects such as shear flow and temperature
gradients may prove to be a powerful combination for possi-
14
ble applications in organic electronics. Similarly uniformly
(together responsible for polar order), is generated from
aligned HNF materials over millimetre range are intriguing
1
5
in functional material science. As a consequence of the spe-
cial molecular packing of these bent molecules, smectic
a
Chemistry Department, Assam University, Silchar 788011, India.
1
6
E-mail: nandirajuv@gmail.com; Fax: +91 3842 270802; Tel: +91 3842 270943
layers with C , C and also C or C symmetry are realized
2
v
2
1h
1
b
Department of Spectroscopy, Indian Association for the Cultivation of Science,
giving rise to ferro-, ferri- or antiferro-electric properties.
Schematic representations of two important layered phases
Kolkata 700032, India
c
Centre for Soft Matter Research, Jalahalli, Bangalore 560013, India
(
of monoclinic C symmetry and triclinic C symmetry) have
2 1
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3ce41360h
been outlined in Fig. 1. The chemical incompatibility
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buckling of layers and the formation of sinusoidal or
triangular-type phases which can be described as columnar
structures of layer fragments. Hence to restore the system sta-
bility the uniform 1-D layers can break and transform into 2-D
density modulated systems due to the splay of the molecular
dipole organization to compensate macroscopic polarization.
In particular, bent-core systems possessing polar organiza-
tion tend to form spontaneously polar and chiral layers lead-
2
ing to the B phase with four possible local layer structures
depending upon the relative polarity and chirality of adjacent
layers. Additionally closely related phases with a myriad of
2
2–24
2-D modulated structures
due to the manifestation of an
energetically favourable polarization splay are also reported.
Prominent among them are the B7 and B1 phases and the
schematic representations are shown in Fig. 2. The B
7
phase
Fig. 1 (a) Possible single tilted bent-core molecular structure in the layered phase.
In-plane polar order of the layer polarization in fluid phase. (XY, layer plane,
XZ polar plane, and YZ tilt plane). (b) The tilt of the molecule is indicated by the bar
at the end of the molecule, which is closer to the observer. (c) Top view of the polar
order in the XY plane. Two dimensional and three dimensional molecular arrange-
ments. Two important polar smectic structures formed by bent-shape molecules.
possesses a structure of continuous periodically undulated
2
2
smectic layers resulting from the local polarization splay.
However the B related phases consist of a structure of blocks
of smectic layering organized into two-dimensional (2D)
1
2
2,23
columnar
like lattices which is also understood on the
Vectors indicate the layer polarizations. (d) In the SmCP structure (monoclinic C
symmetry), the long axis and the molecular plane tilt (“clinic”) with respect to the
layer normal. (e) The SmC (SmCTP) phase (triclinic C symmetry) has a double tilted
2
basis of polarization splay. Most importantly the majority of
these modifications are confined to five, six and seven ring
systems with different central cores, linking groups, substi-
tuents, end alkyl(oxy) or siloxane moieties to name a few. In
the empirical observations to realise molecules that exhibit
mesophases characteristic of bent-core materials, five rings
are required. Studies on the influence of the core structure
G
1
structure (G stands for general), i.e., the molecular plane and the long axis are tilted
in different directions (T stands for the tilt along the molecular plane).
between the aromatic mesogenic cores and the flexible end
aliphatic (methylene units) chains leads to micro-segregation.
Such micro-segregation has been supported by the presence of
the molten state of alkyl chains in liquid crystalline (LC) states
2
3m
in the formation of modulated phases revealed
that an
1
7
18
viz., cubic LC phase, SmE phase (which is the closest to an
1
9
ordered crystal) as well as the induction of the SmA phase in
binary cyanobiphenyl-n-heptane systems. In calamitic liquid
crystals such incompatible units lead to the formation of one
dimensional (1-D) uniform smectic layers of mesogenic aro-
matic cores and aliphatic regions and with the interlayer
boundary regions prominently occupied by the alkyl chains.
Furthermore, in mesomorphic rod like molecules possessing
2
0
strong polar terminal moieties and in polycatenar com-
2
1
pounds, columnar 2D modulated phases made of broken
layers are well known. However in some BLCs multiple peaks
in the x-ray scattering in the small angle region indicated addi-
2
2,23
tional order due to layer undulations (ripples)
beyond the
one-dimensional stack of smectic layers. In these BLCs,
decoupling between the polarization and tilt order makes the
homogeneous layered structure unstable allowing linear polar-
ization splay term in free energy density followed by nucle-
ation of defects favoured by the polarization splay term. The
splay is due to two factors a) steric contribution due to the effi-
cient packing of the rigid cores and b) entropic contribution
due to fluctuating end aliphatic chains. The mismatch or
incompatibility between the cross sections of the aromatic
Fig. 2 (a) The schematic representations of aligned bent-core molecules. The
arrow represents the dipole-moment. (b) The splay possibility due to the central
rigid core efficient packing of molecules, (c) the splay possibility due to flexible end
chain fluctuations. The splay polarization modulated and layer undulated structures,
9
,23f
core and aliphatic chain regions of the bent-core molecules
the layer structure of the molecules in the (d) B
1
phase (e) B1Rev and (f) B1Revtilt (g) of
in the bulk can possibly lead to spontaneous curvature with
spatial modulation of the layers. One possible escape from the
B
7
phase. The arrows indicate the P vector (polarization) in the column, “a and b”
are the dimensions of the crystallographic unit cell. S abbreviates splay and m
abbreviates modulation.
2
3h
unequal surface area imbalance
(local layer dilation) is the
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increase in the size of core structure (rings 5 to 7) has to be
compensated by an increase in the chain length to realize
columnar structures. There are now thousands of bent-core
to spontaneous layer bending and polarization splay modula-
tions but also a reduction from C2 to C1 symmetry of 2D
modulated phases. To examine the influence of the direction
of the ester linkage on the phase behaviour, the transition
temperatures and the phase structure of these four ring sys-
tems, few homologues have been synthesized and their prop-
erties have been characterized by different techniques.
Unlike the bent mesogenic dimers the reported compounds
represent a novel class of bent shaped molecules comprised
of four phenyl rings. In this paper we describe the synthesis
and characterization of these materials in order to study the
effect of the terminal chain length and in turn the impor-
tance of the interlayer interaction in bent-core liquid crystals
containing four aromatic rings with ester and imine linkages
as well as the direction of the ester linkage.
3–5
molecules reported, but only a few systems of four ring bent-
core mesogens. Hence the possible changes, in the design of
the molecular structure that can lead to an unsymmetrical
molecular structure, are a reduction in the number of phenyl
rings in one of the wings, with unequal and long alkyl chain
lengths. The other possible change is the introduction of two
different functionalities like amino and carboxylic groups or
hydroxyl and carboxylic groups with or without a substituent at
the bay position in the central phenyl ring followed by its
extension to yield unsymmetrical molecules. The four-ring
achiral unsymmetrical compounds are at the boundary
2
5
between banana shaped and rod-like liquid crystals.
In spite of the initial attempts to realize the banana
mesomorphism in four-ring compounds not being encourag-
2. Experimental
2
5
ing,
the proper design with pro-mesomorphic linking
groups and the synthesis of four-ring bent-core systems
All the chemicals were procured from M/s Alfa Aesar, Aldrich
or Tokyo Kasei Kogyo Co. Ltd. The solvents and reagents are of
AR grade and were distilled and dried before use. A micro
analysis of the C and H elements was determined on a Perkin-
Elmer 2400 elemental analyzer. IR spectra were recorded on a
2
6
exhibiting banana mesomorphism has been successfully
demonstrated recently. The designed bent-core unsymmetri-
cal molecule possesses four phenyl rings with three in one
wing including the central ring and one ring in the other
wing joined through imine–ester–imine linkages to make it
an unsymmetrical molecule to exhibit polarization modu-
lated layer undulated smectic phases. The reduction in the
number of aromatic rings aided by unequal chain lengths in
the two wings can modify the packing constraints of five ring
BLCs. As a consequence of the lesser rigidity and a change in
the packing mode one can expect lower transition tempera-
tures and different mesomorphic properties in comparison to
five or six ring systems. Furthermore, the imine linkage is
stabilised by the ortho-hydroxyl group through H-bonding.
One of the end alkoxy groups is fixed and the other end was
varied. The modification of the angular 3,4′-disubstituted
biphenyl central unit with the introduction of an ester
linkage and its reversal between the two phenyl rings led to
two novel polar central cores with structural variation for the
possible extension of the bent-core molecular architecture.
The ester linkage with benzene (1,3-substituted) or naphtha-
lene (2,7-substituted) as the central core in BLCs has been
established for its stability and resistance to degradation. It
has been demonstrated earlier that the direction of the ester
−1
Shimadzu IR Prestige-21, FTIR-8400S (νmax in cm ) on KBr
1
disks. The H nuclear magnetic resonance spectra were
recorded on a Bruker DPX-400 spectrometer in a CDCl (chem-
3
ical shift δ in parts per million) solution with TMS as the inter-
nal standard. The UV–visible absorption spectra of the
compounds in CHCl3 at different concentrations were
recorded on a Shimadzu UV-1601PC spectrophotometer (λmax
in nm). The fluorescence spectra of the compounds in chloro-
form were recorded with a Shimadzu RF-5301PC Spec-
troflouorometer with a 150 W xenon lamp as the excitation
source. The liquid crystalline properties of the compounds
were characterized using a polarizing microscope (Nikon
optiphot-2-pol attached with hot and cold stage HCS302, with
STC200 temperature controller configured for HCS302 from
INSTEC Inc. USA). The phase transition temperatures and
associated enthalpies were recorded using differential scan-
ning calorimetry (Perkin-Elmer Pyris-1 system) with a heating/
−1
cooling rate of 5 °C min . The details of the X-ray diffraction
3
1
(XRD) studies have been reported earlier. The apparatus
essentially involves a high-resolution X-ray powder diffracto-
2
7
linkage shows a strong influence on the mesophase behav-
iour and the physical properties in bent-core compounds.
The nonplanarity in the central phenyl benzoate core, which
is due to the ester linkage with a dihedral angle of 50–70°,
between the phenyl carbon and the oxygen atom of the ester
meter (PANalytical X'Pert PRO) equipped with a high-
resolution fast detector PIXCEL.
3. Results and discussion
2
8
[4-(N-4′-alkyloxysalicylidenemino)benzoate]-
linkage,
can influence and lower the mesomorphic-
[
[
3-(N-4′-alkyloxysalicylideneamino) phenyl] [1-n-m] and
3-(N-4′-alkyloxysalicylideneamino)benzoate]-[4-(N-4′-alkyloxy-
isotropic clearing temperatures with a greater effect than the
other linking groups. Furthermore, the longer terminal end
alkyl chains govern the interlayer molecular order and hence
the interlayer steric interactions play an important role to
salicylidene-amino) phenyl]: (2-n-m)
The important requirement for achieving polar order and
chirality in any of the materials is that the individual constit-
uent molecule should possess a bent shape in the central core,
and the non-coplanarity of the phenyl rings in the molecule
2
9
realize a variety of phase structures. The electrostatic or ste-
ric interactions between the neighbouring molecules due to
3
0
the special shape of the bent-core molecules not only lead
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results in a noncentrosymmetric and non-coplanar archi-
tecture. In attempting to devise a strategy for the synthesis
of bent-shaped molecules, the phenyl rings in the angular
in the ESI.† The analytical data is in good agreement with
their chemical structures.
2
6
3
,4′ and 4,3′-disubstituted biphenyl central unit are sepa-
Mesomorphic properties: polarizing optical microscopy
rated by an ester linkage to yield two different central
cores 3-aminophenyl-4-aminobenzoate and 4-aminophenyl-
(
POM) and differential scanning calorimetry (DSC) studies
3
-aminobenzoate. These model central moieties, which can
The four ring molecule possesses two OH groups in both
the wings and through intramolecular H-bonding stabilize
the imine linkages of the molecule. The modification of the
angular 3,4′-disubstituted biphenyl unit with the introduc-
tion of an ester linkage viz., COO or OOC moiety between the
two phenyl units leads to structural variation of the polar
groups in the bent-core molecules and hence can promote a
broad range of interesting variations in their mesogenic
properties. Any variation in the length of the end alkyl
chains changes the molecular interactions. As a part of our
work on four-ring bent-core systems we modified the central
core design with opposite directions of carbonyl moiety of
the ester linkage. Both the wings are unsymmetrical and
unequal in length and deviate significantly from the typical
symmetrical and/or V-shape of other bent-core molecules.
The transition temperatures, enthalpies and entropies
associated with the phase transitions of all of the compounds
of the homologous series 1-n-m (m = 11, n = 10, 12, 13, 14, 15
and 16; m = 15, n = 11) and 2-n-m (m = 11, n = 10, 12, 14 and
16; m = 12, 13 and 15, n = 11) obtained from DSC at a scan
be extended on both sides, possess sufficient bent shapes
and are unsymmetrical in nature, which yields unsymmet-
rical molecules with the required perturbation in the
form of a shape factor and changes in the dipole moment
direction. However the two parts of the unsymmetrical
bent-core molecule were derived from the reaction
between 4-n-alkoxysalicylaldehyde with 3-aminophenol or
4
-aminobenzoic acid. The synthesis of individual segments
can lead to desired unsymmetrical end alkyl chain lengths.
A further esterification reaction between the phenol and
acid moieties yielded the proposed compounds. The synthe-
sis of the designed compounds 1-n-m (m = 11, n = 10, 12,
1
1
3, 14, 15 and 16; m = 15, n = 11) and 2-n-m (m = 11, n = 10,
2, 14 and 16; m = 12, 13 and 15, n = 11) are presented in
Scheme 1 and the details of the representative compounds
are presented in the ESI.† The chemical structures of the
final compounds are confirmed by the elemental analysis
and spectral techniques ( HNMR and FTIR spectroscopy)
and the data of the representative compounds are presented
1
−
1
rate of 5 °C min in the second heating and cooling scans
are presented in Table 1. The DSC spectra of two representa-
tive compounds 1-11-15 and 2-12-11 are presented in Fig. S1.†
All of the compounds exhibit an enantiotropic mesomorphic
behaviour except for 2-10-11. The molecule is made up of two
parts with one wing shorter than the other wing. The shorter
wing is fixed with 4-n-undecyloxy salicylaldimine, while the
length of the n-alkyloxy chain is varied in the longer wing of
the molecule. All of the compounds were found to exhibit
one mesophase only, exhibiting similar textures. 2-10-11
exhibits a monotropic LC phase. The mesomorphic proper-
ties of the materials in series 1 and series 2 which differ in
the orientation of the ester linkage are found to differ in their
transition temperatures as well as their mesomorphic range.
The high stabilities of the compounds were demonstrated
by the absence of any significant perturbation of the DSC
patterns following several heating–cooling cycles (Fig. S2
and S3 (ESI†)).
Upon the very slow cooling of a thin film of the isotropic
liquid of different homologues of the homologous series 1-n-m
different types of textures were observed as displayed in
Fig. 3a–h, which are characteristic of 2D modulated layer
undulated smectic phases (B7/B1RevTilt phases). The common
textures observed are the spiral domains of the telephone-
wire structures resembling the B7 phase (Fig. 3a–c) on very
slow cooling, the pseudo broken focal conic-fan or circular
domains texture with dark extinction crosses (Fig. 3d) coin-
ciding with the direction of the polarizer and analyser and
long leaf like textures. Some representative optical textures
of the compounds 1-12-11 in a polyimide coated ITO cell and
Scheme 1 Reagents and conditions. (i) Ethanol, a few drops of glacial AcOH,
Δ 4 h; (ii) DCC, DMAP, DCM, 48 h.
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Table 1 The phase transition temperatures (°C) and mesomorphic thermal range of compounds 1-n-m (m = 11, n = 10, 12, 13, 14, 15 and 16; m = 15, n = 11)
and 2-n-m (m = 11, n = 10, 12, 14 and 16; m = 12, 13 and 15, n = 11), recorded for second heating (first row) and second cooling (second row) cycles at
−
1
−1
−1 −1
5
°C min from DSC and confirmed by polarized optical microscopy. The enthalpies (ΔH in kJ mol ) and entropies (ΔS in J mol
K ) respectively are
a
presented in the parentheses
Compound
Phase transition temperatures in °C (enthalpy, entropy)
Cr 104.5 (22.5, 59.5) B 110.4 (7.20, 18.7) Iso
Mesomorphic range in °C
1-10-11
1-12-11
1-13-11
1-14-11
1-15-11
1-16-11
1-11-15
2-10-11
2-12-11
2-14-11
2-16-11
2-11-12
2-11-13
2-11-15
7
6.2
Cr 86.7 (27.6, 76.7) B 108.3 (7.88, 20.6) Iso
Cr 104.2 (19.2, 51.0) B
Cr 85.3 (26.2, 73.2) B
7
21.6
6.4
23.5
6.8
23.1
10.0
24.8
13.9
25.5
13.3
23.1
11.2
22.4
—
7
110.6 (7.77, 20.2) Iso
108.8 (9.08, 23.7) Iso
7
Cr 105.6 (30.5, 80.7) B
Cr 86.9 (44.3, 123.3) B
Cr 102.0 (26.8, 71.5) B
Cr 85.6 (43.7, 122.1) B
Cr 104.1 (26.8, 72.5) B
Cr 90.6 (48.3, 135.0) B
7
112.4 (14.5, 37,8) Iso
110.0(13.8, 36.2) Iso
112.0(12.5, 32.5) Iso
110.4(11.3, 29.4) Iso
114.9 (14.9, 38.8) Iso
113.2 (12.9, 33.6) Iso
7
7
7
7
7
Cr 93.6 (17.3, 47.3) B
Cr 80.8 (25.4, 71.9) B
7
106.9 (7.92, 20.8) Iso
103.9 (4.18, 11.1) Iso
7
Cr 103.3 (31.5, 83.9) B
Cr 90.6 (56.5, 155.6) B
7
114.5 (14.7, 38.0) Iso
113.0 (13.7, 35.6) Iso
7
Cr 136.9 (59.2, 144.5) Cr1 147.2 (24.5, 58.3) Iso
Cr 117.3 (43.2, 110.9) B
Cr 134.6 (53.1, 130.5) B
Cr 114.8 (51.1, 132.1) B
7
1
1
138.0 (16.6, 40.4) Iso
139.9 (18.9, 45.8) Iso
138.6 (19.2, 46.7) Iso
134.4 (11.0, 27.1) Iso
131.9 (9.06, 22.3) Iso
134.1 (11.5, 28.4) Iso
131.0 (10.4, 25.9) Iso
140.0 (19.3,46.8) Iso
138.5 (19.3,47.0) Iso
138.9 (20.2,49.2) Iso
20.7
5.7
24.5
11.9
29.8
11.1
29.7
6.5
22.8
7.2
23.5
10.3
24.9
Cr 122.5 (29.9, 75.7) B
Cr 102.1 (32.3, 86.1) B
Cr 123.0 (32.5, 82.1) B
Cr 101.3 (32.3, 86.3) B
1
1
1
1
Cr 133.5 (50.6,124.5) B
Cr 115.7 (49.1,126.5) B
Cr 131.7 (57.0,141.0) B
1
1
1
Cr 114.2 (57.7, 149.0) B
1
137.7 (22.2,54.1) Iso
138.6 (19.5,47.5) Iso
137.5(19.9,48.5) Iso
Cr 128.3 (49.5,123.3) B
Cr 112.6 (50.0,129.6) B
1
1
a
All compounds exhibited enantiotropic crystal–crystal transitions and are not tabulated.
on an untreated glass plate and coverslip are presented in
Fig. 3d and e.
extinction brushes directed along the polarizer directions. On
further cooling low birefringent stripes appeared across the
blades of the columnar fan and finally became uniform with-
out any changes in the columnar texture before it crystallizes
as shown in Fig. 4. The developable domain-like textures
exhibited by the different homologues of the 2-n-11 series
resemble the textures exhibited by the B1 phase and the
The observed representative optical textures exhibited by
compounds of 2-n-m homologues are presented in Fig. 4.
Upon very slow cooling from the isotropic liquid, compounds
of 2-n-m (m = 11, n = 10, 12, 14 and 16; m = 12, 13 and 15,
n = 11) exhibited a pseudo-broken fan like texture with
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Fig. 3 Typical textures observed in the B
7
phase of a 1-n-11 homologue (a) the growth of a dendritic texture with spiral filaments of 1-6-11; (b) the helical spiral texture
formation from the isotropic phase of 1-10-11; (c) the full growth of the texture shown in b; (d) the 1-12-11 fan-shaped texture in a 5 μm cell at 103 °C; (e) 1-12-11 in a
normal glass plate at 108 °C; (f) and (g) the fan-shaped texture of compounds 1-13-11 and 1-14-11; (h) the optical texture of an uncovered free drop of the B7 phase
equally spaced striped pattern of 1-11-15 at 112.5 °C.
optical textures resemble the 2D polarization splay modu-
than the 1-n-11 homologues by almost 30 °C. The mesomor-
phic thermal range of 1-n-11 and 2-n-11 as a function of the
number of carbons in the end alkyl chain length is depicted
in Fig. S4a and S4b (ESI†). All of the compounds were
found to exhibit one mesophase only, exhibiting similar tex-
tures. On increasing the length of the end alkoxy chain the
mesomorphic range slightly increases from 21 °C (n = 10)
to 25 °C (n = 14) and further decreases. A decreasing trend
of the melting and clearing transition temperatures with
lated layer undulated smectic phases of the B7/B1
RevTilt
2
2,23
family.
On further cooling, the compound exhibited a
structured columnar fan like texture.
All the homologues (1-n-m and 2-n-m) exhibited banana
liquid crystalline behaviour over a moderate temperature
range of ~20–30 °C in the cooling cycle. The influence of the
ester linkage direction is reflected in the clearing transition
temperatures. The clearing temperatures of 2-n-11 are greater
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Fig. 4 Typical textures observed of the 2-n-11 homologue (a) at 133.3 °C of 2-10-11, developable domains; (b) 2-12-11 in a normal glass plate; (c) and (d) focal conic B
1
of 2-12-11 in different cell thickness 3.5 and 9 μm respectively; (e) at 130.6 °C of compound 2-14-11; (f) the fully developed columnar texture of the 2D modulated smectic
phases of compound 2-16-11.
the increasing chain length of the terminal alkoxy chains
is observed.
versus 2θ profile) in the mesophase at 108 °C and 95 °C for the
compounds 1-10-11 and 1-15-11, respectively. The broad diffuse
peak in the wide angle region corresponding to spacing of
4.5 Å confirms the liquid like ordering of the phases. In the
small angle region the compound 1-10-11 exhibits four sharp
reflections whereas the compound 1-15-11 shows an additional
sharp reflection. The indexing carried out for the reflections
with incommensurable scattering vectors in the small angle
On application of the triangular-wave electrical field of
1
00 Vpp the switching current response (Ps), varying the
frequency from 0.1 Hz, 0.5 Hz and 1.0 Hz, was investigated
in the LC phase at 100 °C. A large broad peak was observed
up to 0.1 Hz (Fig. S5a–c†), which decreased with an increase
in the frequency, and disappeared at a frequency larger than
2
1
.0 Hz (Fig. S5d†). The diminishing broad peak at very low
region and they fitted very well (with low χ value) for the tilted
frequencies can be attributed to ionic relaxation. Hence the
absence of polarization in the current peak above 1 Hz indi-
cated the absence of switching in this phase.
columnar phase (Col ) with the lattice parameters and tilt
ob
angle being tabulated in Table 2. The textural patterns
described in the earlier section for the 1-n-11 series corrobo-
rates the X-ray investigations to assign the phase as B7. Such
kind of indexing for B7 mesophase to the Colob phase is well
4. X-ray studies
2
2,23
known in the literature.
The spacing number of the pri-
To confirm the mesophase structures XRD measurements
were performed on a few representative compounds 1-10-11,
mary peak is smaller than the length of the molecule in both
compounds. Perhaps this could be due to the partial overlap of
aromatic cores of the antiparallel arranged molecules and
interdigitation of the terminal alkyl chains as depicted in the
proposed model (Fig. 6).
1
-15-11 and 2-12-11 to get information on the structural
aspects, and the diffractograms are displayed in Fig. 5a–c.
Fig. 5(a) and (b) shows representative diffractograms (intensity
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Table 2 The XRD data of the compounds studied. In all the compounds
(
1-10-11, 1-15-11 and 2-12-11) studied the columnar axis is taken as the
c-axis and the ab the lattice plane
1-10-11 T = 108 °C
dmeas (Å)
dcalc (Å)
(hk)
1
2
3
4
5
42.54
39.93
36.57
21.68
4.54
42.50
40.01
36.68
21.41
(10)
(01)
(11)
(12)
3
a = 45.75 Å; b = 43.06 Å; γ = 68.6°; V = 8952 Å
1-15-11 T = 108 °C
dmeas (Å)
dcalc (Å)
(hk)
1
2
3
4
5
6
45.48
42.15
37.97
23.11
15.46
4.54
45.67
42.11
38.13
22.84
15.01
(10)
(01)
(11)
(20)
(32)
3
a = 48.60 Å; b = 44.81 Å; γ = 70°; V = 9880 Å
T = 95 °C
1
3
4
5
6
6
45.79
42.73
39.62
23.22
15.50
4.51
45.91
(10)
42.73
39.61
22.96
15.52
(01)
(11)
(20)
(32)
3
a = 49.58 Å; b = 46.14 Å; γ = 67.6°; V = 10313 Å
2-12-11 T = 136 °C
dmeas (Å)
dcalc (Å)
(hk)
1
2
3
4
5
43.57
41.19
38.16
22.34
43.57
43.60
41.22
38.52
21.80
(10)
(02)
(11)
(20)
a = 43.60 Å; b = 82.24 Å; V = 16386 Å
Fig. 5 (a) The X-ray intensity versus 2θ profile in the low angle region (main panel)
and wide angle (inset) regions for the compound 1-10-11 at T = 108 °C, (b) 1-15-11
at 95 °C and (c) 2-12-11 at 136 °C in the mesophase.
Fig. 5(c) shows representative diffractograms in the
mesophase at 136 °C for compound 2-12-11. With the textural
pattern discussed in the earlier section for the 2-n-11 series
at background the indexing carried out for the sharp low
Fig. 6 A proposed model structure for the Colob (B7) phase and Col
r
(B1) phase
2
exhibited four-ring bent-core compounds with unequal arms of aromatic cores.
angle peaks fitted very well (with low χ value) for the rectan-
gular lattice with the lattice parameters tabulated in Table 2.
Thus the phase exhibited by the 2-n-11 series can be indexed
to mesophase of the B1 type. Again for this compound the
spacing value of the primary peak is smaller than the length
of the molecule 2-12-11 obtained from the DFT optimised
molecular structure in gaseous phase in its most extended
conformation in the gaseous phase (see Fig. 7c) although it is
likely that a somewhat more bent conformation is adopted in
the condensed phases. Perhaps this could be due to the par-
tial overlap of the aromatic cores of the antiparallel arranged
molecules and interdigitation of the terminal alkyl chains.
It is about 7 molecules of 1-10-11 or 1-15-11 compounds and
about 12 molecules of 2-12-11 compound organize to form one
slice to fit into the column. Based on the XRD and textural
studies the mesophase exhibited by the 1-n-11 series can be
identified as the B7 type and 2-n-11 series to the B1 type.
The formation spiral domains and 2D focal conic textures,
the multiple peak reflections at low angles, the tilt in the
molecular layering and the non-switchable phase structure
are indicative of a B /B1
phase structure and suggest
RevTilted
7
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studies. The strong coupling between the polarization splay
and the tilt of the molecules led to 2D modulated phases viz.,
polarization modulated and layer undulated phases or colum-
nar phases and are evidenced in the optical textures resem-
bling the similar phases reported earlier. Furthermore,
2
3m
contrary to earlier reports
on the influence of the core
structure in the formation of modulated phases, a decrease
in the size of the core structure (4 rings) with increased alkyl
chain lengths can also form columnar structures. Therefore it
appears that the columnar phase is only stable if a partial
overlap of the aromatic cores of the antiparallel arranged
molecules at the border between the two columnar blocks
is possible.
5. DFT studies
A DFT computational study based on quantum mechanical
calculations was performed to obtain the information related
to the molecular conformation, bend angle, dipole moment,
molecular polarizability and electrostatic potential distribu-
tion of the molecules 1-n-11 and 2-n-11. Full geometry opti-
mizations have been carried out without imposing any
3
2
constraints using the Gaussian 09 program package. Spin-
restricted DFT calculations were carried out in the frame-
work of the generalized gradient approximation (GGA) using
Becke3-Lee-Yang-Parr hybrid functional (B3LYP) exchange–
3
3,34
correlation functional and the 6-311G(d,p) basis set.
The
B3LYP functional with the standard basis set 6-311G(d,p)
has been used due to its successful application for larger
organic molecules, as well as hydrogen bonded systems in
3
5–37
38–41
the past,
and bent-core molecules
recently.
Among the two studied homologous compounds 1-n-11
and 2-n-11 (Table S1†), the calculated bond lengths are found
to be noticeably different. In the central phenyl benzoate
core, the dihedral angle is found to be ~49°, between the phe-
nyl carbon and the oxygen atom of the ester linkage. The
dihedral angle between the phenyl carbon and nitrogen atom
of the imine linkage is found to be ~37°. Furthermore, a sig-
nificant difference in the bend angles (123 ± 3° and 141°) as
well as the dipole moments was found. For example the
dipole moment of 1-12-11 is 2.48 D whereas 2-12-11 is 6.98 D
(
Fig. 7). The major contribution of the dipole moment comes
from the Y and Z components of 2-12-11 whereas in the case
of 1-12-11, the major role in the dipole moment is played by
the X and Y components. The polarizabilities are crucial for
an understanding of the molecular properties in molecular
optics and spectroscopy. Electrostatic intermolecular interac-
tion energy is related to this quantity, in particular for sys-
tems without a permanent dipole moment. The molecular
polarizability component αXX is comparatively larger along
the molecular longitudinal X-axis (Table S2†) for both mole-
cules (1-12-11 and 2-12-11) than the other two directions. How-
Fig. 7 The DFT calculated minimised energy conformation of the molecular structure
(a) in the fully extended all trans conformation of 1-10-11 with a bent angle 125°
and dihedral angles (37°, 49°, 37°) showing the twisted molecular structure
(
(
b) the polarizability ellipsoid and resultant dipole moment direction of 1-12-11
c) the polarizability ellipsoid and resultant dipole moment direction of 2-12-11.
4
2
that the phase represents the 2D polarization splay modu-
lated layer undulated molecular structure of the SmCPU
1
1,12
iso
family.
The 2D structure of columnar phases is a result
ever, the asymmetry parameter η = [(αYY − α )/(α − α )],
ZZ
XX
of chiral symmetry breaking, which occurs in these bent-core
molecules if they are tilted with respect to the smectic layer
normal. The tilt of the molecules is evidenced from the X-ray
which is dependent on the bending angle and polarizability
components is found to be distinctly different for the two
homologous series (0.4–0.5 and 0.03–0.05) respectively for the
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molecules 1-n-11 and 2-n-11. The principle components of the
a large Stokes shift (~180 nm), which reflects the structural
relaxation of the excited molecule, and is significantly larger
than that reported in push–pull systems exhibiting liquid
polarizability tensor (αXX, αYY, α ) are different but the isotro-
ZZ
iso
pic polarizability α = (αXX + αYY + αZZ)/3 is almost same for
4
3–47
both homologues. Though the molecular formula and number
of electrons of the two studied molecules are the same, the
direction of the ester linkage must be the major contributor to
the significant differences in the principal components of the
polarizabilities and dipole moment, the magnitude of dipole
moment, the bend angle and asymmetry parameter. The DFT
crystal behaviour
reflecting changes in the molecular con-
formation upon excitation. These results are in good agree-
4
8
ment with the reported results of molecular J-aggregates in
which the excitonic energy is delocalized as a result of the
intermolecular coupling within the head-to-tail arrangement
of the molecules in the solution.
calculated principal polarizability components (αXX, αYY, α ),
ZZ
iso
the isotropic polarizability component α , the polarizability
7. Conclusions
anisotropy Δα = [αXX − (αYY + αZZ)/2], and the asymmetry
iso
Unsymmetrical four-ring bent-core compounds with unequal
chain lengths and reversing the ester linkage in the molecule
have been designed, successfully synthesized and character-
ized. It is interesting to note that the 1-series exhibit the B7
phase with an oblique columnar lattice, while the 2-series
exhibits a B1 phase family with a rectangular lattice. Bent-
core molecules of uneven aromatic–aliphatic moieties are
ideal candidates to exhibit 2D polarization splay modulated
parameter, η = [(αYY − αZZ)/(αXX − α )], which are parame-
ters relative to the molecular polarizability tensor, α, in the
Cartesian reference frame are displayed in Fig. 7.
6
. UV–Visible and fluorescence studies
The emission properties of organic LCs are currently an
active area of research due to their potential applications in
opto-electronics. Emissive liquid crystals are intriguing sys-
tems for materials research because they couple molecular
self-assembly with intrinsic light generation capability. The
unsymmetrical bent-core plays an important role in the mod-
ification of the electronic characteristics of the molecules.
Hence the absorption and emission properties of a represen-
tative compound were studied, and the results of the UV–
visible absorption and fluorescence spectroscopic properties
and layer undulated phases of B /B
structures. The
1RevTilted
7
interaction of molecules within the layers and/or between the
layers can lead to layer modulations. The hydroxyl group in
the wings of these molecules participate in inter and/or
intra-molecular H-bonding which adds to the strength and
molecular interactions in the formation of undulated and
modulated smectic phases. The influence of the carbonyl
direction reflected in a significant change in the magnitude
of dipole moment of the molecules which will contribute to
the physical properties of these compounds as well as the
phase structure.
−
5
of 2-12-11 in chloroform (conc. 1.0 × 10 M) were studied in
solution, to obtain information regarding the absorption and
emission maxima and the Stokes shift of fluorescence are
presented. The UV–visible spectra in these homologous com-
pounds exhibited strong absorption peaks at ~350 nm with a
large molar extinction coefficient (Fig. 8, 3.54 eV for 2-12-11,
Acknowledgements
Financial assistance provided by the Department of Atomic
Energy, the Department of Science and Technology, the Naval
Research Board and University Grants commission India is
gratefully acknowledged.
−
1
−1
−5
ε ~ 264 000 L mol cm concentration = 10 M solution in
CHCl ). The strong absorption band reflects the π–π* transi-
3
tion of the highly π-conjugated system having the substituted
phenyl benzoate unit as the core. 2-12-11 is found to exhibit
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