Stable ferroelectric liquid crystals derived from salicylaldimine-core

Article abstract of DOI:10.1021/acs.jpcb.5b00209

Five pairs of enantiomers derived from salicylaldimine-core have been prepared by condensing (R)- or (S)-4-(octan-2-yloxy)anilines with 4-formyl-3-hydroxyphenyl 4-(n-alkoxy)benzoates. They have been designed to probe the correlation between molecular structure and mesomorphism, and especially to provide stable mesogens having potential for applications in ferroelectric liquid crystal devices. Thus, they have been substituted with a chiral tail at one end and by n-alkoxy chains of varying length at the other terminal. A detailed study confirms an indistinguishable behavior of all ten mesogens exhibiting an enantiotropic chiral smectic C (SmC) phase besides blue phase (BP) and chiral nematic (N) phase. The SmC phase occurring over a 50-70 °C thermal width shows ferroelectric switching with spontaneous polarization (P_s) value crossing over 100 nC/cm². Circular dichroism spectroscopic study of the mesophases confirms the chromophores of the molecules being in the macroscopic chiral (helical) environment.

Full text of DOI:10.1021/acs.jpcb.5b00209

Article
pubs.acs.org/JPCB
Stable Ferroelectric Liquid Crystals Derived from Salicylaldimine-
Core
Bhyranalyar N. Veerabhadraswamy, D. S. Shankar Rao, and Channabasaveshwar V. Yelamaggad*
Centre for Nano and Soft Matter Sciences, Prof. U. R. Rao Road, Post Box No. 1329, Jalahalli, Bangalore 560 013, India
S
* Supporting Information
ABSTRACT: Five pairs of enantiomers derived from salicylaldimine-core have been
prepared by condensing (R)- or (S)-4-(octan-2-yloxy)anilines with 4-formyl-3-
hydroxyphenyl 4-(n-alkoxy)benzoates. They have been designed to probe the
correlation between molecular structure and mesomorphism, and especially to provide
stable mesogens having potential for applications in ferroelectric liquid crystal devices.
Thus, they have been substituted with a chiral tail at one end and by n-alkoxy chains of
varying length at the other terminal. A detailed study confirms an indistinguishable
behavior of all ten mesogens exhibiting an enantiotropic chiral smectic C (SmC*)
phase besides blue phase (BP) and chiral nematic (N*) phase. The SmC* phase
occurring over a 50−70 °C thermal width shows ferroelectric switching with
spontaneous polarization (Ps) value crossing over 100 nC/cm2. Circular dichroism
spectroscopic study of the mesophases confirms the chromophores of the molecules
being in the macroscopic chiral (helical) environment.
phases. While the TGB phases, being the LC analogues of the
Abrikosov phases, have attracted intense scientific interest, the
BPs hold potential for next-generation display and photonics
device applications; consequently, there have been some
successful attempts lately to widen the thermal range of such
phases.11−13 On the other hand, the N* and SmC* phases are
the classical examples of defect-free helical LC structures
exhibiting a wide range of unique and fascinating phenomenon.
For example, the N* phase shows temperature-dependent
selective reflection (thermochromism), and thus, it is used in
thermochromic devices.2,3,35,36 The SmC* phase shows
ferroelectric switching, which is being well-used in realizing
advanced display devices featuring high image quality, high
resolution, wide-viewing angles, and fast response time.2,3,35−37
In essence, chiral LCs exhibit a dizzying array of special
properties and structures that have great significance in the
fields of both fundamental research and applied science.
In view of such a marked thermal behavior of chiral LCs, the
authors have been engaged for more than a decade in the
rational design, synthesis, and characterization of a vast array of
optically active LCs,30−54 including polymers.55 The exper-
imental results of these investigations have clearly demon-
strated the inherent capability of such chiral LCs in stabilizing a
variety of novel phase sequences and frustrated/technologically
important LC phases over a wide thermal range.
1. INTRODUCTION
One of the most fascinating themes of the contemporary
research involves bestowing molecular chirality and investigat-
ing its effect on the self-organization property of materials.1
Such interest stems from the fact that the chirality, which
enables the spontaneous mirror symmetry-breaking of mole-
cules, generally stimulates molecular self-assembly into highly
ordered mesoscopic- /or macroscopic-structures that hold great
promise for many areas of basic science and applied research.
The design and facile synthesis of optically active (enantiopure)
liquid crystals (LCs) bearing asymmetric carbon atom/s, called
chiral (mesogens) LCs, represent one such innovative area;2−29
notably, here the imbued molecular chirality generally favors a
macroscopic twist in the organization of the constituent
mesogens leading to complex ordered LC phases, which
includes frustrated2−17 and helical phases.2,3,18−29 Indeed, chiral
LCs also stabilize layered phases.20 In fact, these exotic and
highly organized LC phases can also be realized effectively
following an alternative protocol where chiral guest molecules
(dopants, which need not necessarily be LC) are dissolved in
the host achiral LC phases.2,3,11,30−34
The examples of frustrated LC phases, which are also called
defect phases, include blue phases (BPs)2−13 and twist grain
boundary (TGB)14−17 phases where the structural frustration,
originating from the competition between the chiral forces and
packing topology, is eventually relived by the formation of
lattice of defects. They generally exist over a narrow thermal
regime. Blue phases (BPI, BPII, and BPIII) occur between the
N*/smectic (Sm) phase and the isotropic liquid (I) phase,
whereas the TGB (TGBA, TGBC, and TGBC*) phases are
seen during the phase transition from the isotropic liquid/chiral
nematic (N*) to smectic A (SmA)/chiral smectic (SmC*)
The present work described herein relates to the design,
preparation, and evaluation of LC and chiroptical properties of
five enantiomeric pairs of three-ring calamatics derived from the
Received: January 8, 2015
Revised: March 3, 2015
Published: March 3, 2015
© 2015 American Chemical Society
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a
Scheme 1. Synthesis of Three-Ring Rodlike LCs Derived from Salicylaldimine-Core
a
Reagents & conditions: (i) (S)-(+)-2-octanol, Ph3P, DIAD, THF, rt, 18 h; (ii) H2, (1 atm, ballon), 10% Pd−C, THF, rt, 4 h. (iii) (R)-(−)-2-
Octanol, Ph3P, DIAD, THF, rt, 18 h; (iv) n-bromodecane, anhydrous K2CO3, DMF, 80 °C, 12 h; (v) Jone’s reagent, acetone, rt, 1 h; (vi) 2,4-
Dihydroxybenzaldehyde, DCC, DMAP, THF, rt, 4 h; (vii) EtOH, AcOH, ref lux, 2 h.
salicylaldimine-core. The target molecular architecture with
three benzene rings linked in an end-to-end manner has been
especially chosen, owing to its inherent ability to support the
stabilization of smectic phase(s).2,3,20−29,56−65 The mesogens
derived from an electron rich salicylaldimine-core have
attracted an enormous amount of research attention owing to
their sturdiness to heat and moisture, ease of derivatization and
synthesis, and remarkable thermal properties. Specifically, this
core has been incorporated in the fabrication of mesomorphic
compounds ranging from conventional LCs viz., calam-
itics,66−71 discotics,72,73 and polymers74 to nonconvetional
LCs such as dimers,74 hexacatenars,67,68 and banana-shaped
LCs.78−82 Notably, their appealing LC behavior has been
attributed to the presence of salicylaldimine segment. Thus, in
the present work, a salicylaldimine-core in the three-ring
molecular architecture has been incorporated with the rationale
that the resultant LCs ought to be displaying interesting LC
behavior with the thermal stability and hydrolytic resistance.
Furthermore, it was deliberated to join the salicylaldimine-core
to a (n-alkoxy)benzene ring via an ester (−COO−) linkage as it
induces, similar to an imine (−CHN−) group, lateral
polarity quite efficiently;2,3,20−29,56−65 here, it was planned to
vary the length of n-alkoxy tail from n-octyloxy to n-dodecyloxy
to explore its effect on the phase transitional behavior of the
compounds. Importantly, the insertion of chiral tails, such as
(R)-2-octyloxy and (S)-2-octyloxy chains, in the molecular
design not only accounts for the molecular chirality but helps in
placing asymmetric carbon atom (stereocenter) closer to the
rigid core; such a substitution improves the magnitude of
polarization due to hindered rotation of the chiral moiety. The
general molecular structure of the target mesogens, incorporat-
ing the aforesaid features, is shown in Scheme 1.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Molecular Structural Character-
ization. The proposed mesogenic compounds were prepared
in excellent yields and fully characterized with the help of UV−
vis, FTIR, NMR, MS, and elemental analyses (see Experimental
Section for details). Scheme 1 illustrates the stepwise synthesis
of intermediates and target salicylaldimine-based mesogens.
The essential amines, (R)- and (S)-4-(octan-2-yloxy)anilines (2
and 4) were prepared starting from commercially available 4-
nitrophenol; it was subjected to Mitsunobu O-alkylatation83
with (S)- and (R)-2-octanol to get (R)- and (S)-4-(octan-2-
yloxy)nitrobenzenes (1 and 3),84,85 which upon reduction by
catalytic hydrogenation using H2 balloon (1 atm) and Pd/C
(10%) in THF afforded the corresponding anilines 2 and 4.84,85
4-(n-Alkoxy)benzaldehydes (5a−5e),45,46,52,54 readily prepared
by Williamson ether synthesis with 4-hydroxybenzaldehyde and
n-bromoalkanes, were subjected to Jones oxidation giving the
required 4-(n-alkoxy)benzoic acids (6a−6e).86 These acids 6a−
6e were subsequently reacted with 2, 4-dihydroxybenzaldehyde
in the presence of N,N′-dicyclohexylcarbodiimide (DCC) and
4-(dimethylamino)pyridine (DMAP)87 to obtain 4-formyl-3-
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hydroxyphenyl 4-(n-alkoxy)benzoates (7a−e). In the conclud-
ing step, the salicylaldehydes 7a−7e and the amines 2 and 4
were refluxed together in the presence of a solvent, absolute
ethanol, and a catalytic amount of acetic acid to get the final
mesogens. The characterization data obtained for all the
intermediates and target mesogens were found to be in
complete agreement with their proposed structure (see
Experimental Section and Supporting Information for details).
The UV−Vis spectra of all ten salicylaldimines, recorded for
their dilute solutions (∼3 × 10−3 M; cell length = 1 mm) in
spectroscopic grade dichloromethane, were found to be nearly
the same comprising two broad but intense peaks at ∼277 and
∼355 nm due to π−π* and n-π* transitions, respectively. FTIR
(KBr) spectra of the mesogens showed characteristic CO
and CN stretching vibration bands in the regions of 1719−
Table 1. Phase Sequences, Phase Transition Temperatures
b
(°C) and Associated Enthalpies (kJ/mol−1) of Transitions
a
of Three-Ring Rodlike SA(S)-n and SA(R)-n Compounds
phase sequence
compounds
heating
cooling
SA(S)-8
Cr 81.2 (24.6) SmC*
107.4 (2.1) N* 148.6 (1.2) I
I 148.2 BP 147.9 (1.0) N*
106.5 (2) SmC* 52.6 (20.3) Cr
SA(R)-8
SA (S)-9
SA (R)-9
Cr 83.2 (23.5) SmC*
I 148.3 BP 148.0 (1.0) N*
107.6 (2.0) N* 149.0 (1.3) I
106.5 (1.9) SmC* 54.1 (19.5) Cr
Cr 86.3 (33.4) SmC*
113.4 (2.5) N* 145.4 (1.0) I
Cr 85.5 (25) SmC*
113.2 (2.4) N* 145.2 (0.9) I
I 144.9 BP 144.5 (1.1) N*
112.4 (2.5) SmC* 59.1 (25.5) Cr
I 144.8 BP 144.3 (1.0) N*
112.3 (2.3) SmC* 59.9 (14.7) Cr
SA (S)-10 Cr 90.7 (24.8) SmC*
117.1 (2.0) N* 144.4 (0.8) I
SA (R)-10 Cr 92.1 (31.0) SmC*
117.9 (2.1) N* 144.5 (0.9) I
SA (S)-11 Cr 80.3 (29.5) SmC*
120.4 (2.7) N* 141 (1.1) I
SA (R)-11 Cr 80.9 (28.3) SmC*
120.2 (2.7) N* 141.1 (1) I
SA (S)-12 Cr 68.4 (24.4) SmC*
122.7 (2.6) N* 139.6 (1.1) I
SA (R)-12 Cr 66.9 (23.3) SmC*
121.8 (2.5) N* 138.8 (0.9) I
I 143.1 BP 142.6 (0.8) N*
116.2 (2.3) SmC* 62.2 (20.1) Cr
I 143.4 BP 142.4 (0.8) N*
116.8 (2.2) SmC* 61.5 (23.5) Cr
I 140.4 BP 140.0 (1.1) N*
119.4 (2.8) SmC* 48.1 (22.1) Cr
I 140.6 BP 140.3 (1.1) N*
119.5 (2.8) SmC* 49.9 (21.9) Cr
I 138.4 BP 137.9 (1.1) N*
120.6 (2.7) SmC* 44.3 (20.1) Cr
1
1727 and 1622−1624 cm−1, respectively. H NMR spectra
measured in dilute CDCl3 solutions at room temperature
exhibited well-resolved signals of aromatic and other non-
aromatic protons. It is worth mentioning that the spectra
showed two typical singlets at δ 8.6 and δ 13.8 resonating due
to imine and phenolic protons, respectively. It may be noted
here that the signal of phenolic-proton appears in the same
downfield-position (δ 13.8) regardless of the variation made in
the concentration of the mesogens. This is expected given the
fact that the chemical shift of protons participating in the strong
intramolecular H-bonding is nearly independent of the
concentration88 and such protons are deshielded, relative to
TMS, as they become more positive in their electronic nature
causing their signal to shift to a downfield.88 The 13C NMR
spectra of the mesogens recorded in CDCl3 confirmed the
structure of the basic carbon skeleton. Each spectrum,
downfield from the CDCl3 peaks (a 1:1:1 triplet at δ = 77),
showed the expected 16 signals, at ca δ = 164.4, 163.7, 162.5,
159.4, 157.7, 154.5, 140.9, 132.7, 132.4, 122.3, 121.3, 117.4,
116.6, 114.4, 113, and 110.6, belonging to sp2-hybridized
carbon atoms. Among these, the farthest downfield peak with
the chemical shift of 164.4 ppm can be assigned to the most
deshielded carbonyl-carbon. While the oxymethylene
(−OCH2−) and oxymethine (−OCH−) carbons resonated at
74.4 and 68.4 ppm, respectively, the sp3-hybridized carbons of
methylene and methyl groups of paraffinic tails appeared in the
region of 37 to 14 ppm. Thus, the spectral data coupled with
elemental analyses established molecular structure of the target
mesogenic compounds indubitably.
I 138.2 BP 137.5 (1.1) N*
120.3 (2.6) SmC* 43.9 (19.0) Cr
a
Acronyms: Cr, Crystal; SmC*, Chiral Smectic C Phase; N*, Chiral
b
Nematic Phase; BP = Blue Phase-I or Blue Phase-II. Peak
temperatures in the DSC traces obtained during the first heating−
cooling cycles at 5 °C/min rate.
including thermal range of the mesophase have been taken
from the microscopic observations. As shown in Figure 1, DSC
2.2. Evaluation of Liquid Crystallinity. 2.2.1. Optical
Microscopic and Calorimetric Studies. Five pairs of
enantiomers synthesized were examined in detail for their
mesomorphic behavior mainly with the aid of thermal polarized
light optical microscopy (POM) and differential scanning
calorimetry (DSC). Powder X-ray diffraction (XRD) measure-
ments and electrical switching studies were also carried out to
elucidate the mesophase structure of some representative
compounds. DSC and POM experimental results comple-
mented each other and not only evidenced the identical
thermal behavior but also confirmed remarkable thermal and
hydrolytic stability of the mesogens. Table 1 gives the summary
of the phase transitional behavior of the compounds
synthesized; the sequences and transitions temperatures
mentioned therein have been deduced from both optical and
calorimetric studies. The enthalpies of transitions presented
have been derived from DSC traces of the first heating−cooling
cycles at a rate of 5 °C/min. When the thermal signatures were
not observed in DSC thermograms, the transition temperatures
Figure 1. DSC thermograms recorded during the first heating−cooling
cycles at a rate of 5 °C for the ten (five pairs of enantiomers)
mesogens.
traces of all the mesogens were found to be virtually identical,
each comprising three endothermic/exothermic peaks in
heating/cooling cycles. The calorimetric results coupled with
microscopic observations led us to clearly identify the presence
of three enantiotropic LC phases in these compounds: detailed
textural observations under variable temperature POM and
analysis of the textures suggested the presence of BP, N*, and
SmC* phases as elucidated in the following.
The pristine solid samples held between an untreated glass
slide and a coverslip were heated to their isotropic phase and
cooled slowly. As shown in Figure 2a, a platelet textural pattern,
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Figure 2. Photomicrographs of the textures, seen under a polarizing
microscope, of different LC phases of SA(R)-12 placed between two
untreated glass substrates: (a) platelet texture of the BP-I/II phase (T
− 138.1 °C); (b) pseudo-focal conic texture of the N* phase (T − 135
°C); (c) schlieren texture with only four-brush disclinations (strength
S = 1) of the SmC* phase (T − 108 °C); the inset shows a uniform,
dull-greyish pattern observed in another portion of the same sample
slide. T = temperature.
Figure 3. Microphotographs of the various optical textures observed
for the SmC* phase under different conditions: (a) the texture with
broken-focal conic (left), schlieren (middle), and dull-greyish (right-
bottom) patterns observed for the sample SA(R)-10 held between
microscopic (untreated) glass slides (T − 108 °C); (b) the iridescent
(Grandjean) texture of the homeotropically oriented sample of SA(S)-
8 (T − 66 °C); (c) the texture comprising undulation lines and cross-
hatched pattern (right and left portions) seen for the sample SA(R)-10
sandwiched between microscopic (untreated) glass slides (T − 108
°C).
comprising blue and green plates, grows rapidly over black
background of the isotropic liquid (I) phase. Upon subjecting
to gentle mechanical stress, it instantly transforms into a
Grandjean planar pattern of the N* phase. These observations
signify the presence of a BP-I/BP-II phase.2−13,89 It may be
noted here that in DSC traces, the signature due to I-BP
transition could not be detected. Upon cooling the samples
further, the N* phase appears rapidly with a pseudo-focal conic
optical texture (Figure 2b), which changes to a typical
Grandjean plane texture (Figure 2c) when the top glass
substrate (coverslip) is pressed gently; in this planar texture, the
helical axis of the phase lies along the viewing direction. In
other words, it remains perpendicular to the confining (glass
plate) surfaces. The enthalpy (ΔH) values, ranging from 0.8 to
1 kJ/mol−1, shown in Table 1, represent the combined ΔH for
the I-BP and BP-N* transitions as the BP exists over a short
thermal width of 0.3 to 1 °C. On cooling these samples further,
a third phase transition occurs below the N* phase; this
transition was clearly identified based on the observation that
the Grandjean plane pattern of the N* phase sharply changes to
a schlieren texture comprising four-brush disclinations
(strength S = 1) exclusively; as shown in the inset of Figure
2c, in several regions of the same slide, a greyish uniform
pattern adjoining the schlieren texture also existed. As is well-
known,89 for long pitch SmC* phase where light propagates
along the helix axis, the textural patterns seen are analogous to
those identified for the achiral smectic C (SmC) phase. For
instance, one of the optical textures observed for the SmC
phase is the low birefringent (due to light propagation along
the direction of the optic axis) greyish pattern which normally
accompanies a schlieren texture having four-brush defects.89
Thus, these optical results hint the presence of a SmC* phase
below the N* phase.
This assignment was further substantiated by the observation
of other appealing textural patterns. For example, a texture
comprising broken-focal conic fan, schlieren, and dull-greyish
patterns was seen; as a representative case, such a pattern
observed for the mesogen SA(R)-10, held between the two
untreated glass plates, is shown in Figure 3a. Here, broken focal
conic and dull-greyish patterns stem respectively from the
planar and homeotropic orientation of the constituent
mesogens; the latter pattern obviously suggests the layered
structure of the phase. In the regions of pseudo-homeotropic
alignment of the sample SA(S)-8, for example, the Grandjean
texture expectedly appears iridescent as shown in Figure 3b.
Such textures are known to result from a helical macrostructure
with the tilting of molecules in smectic layers.2,3,63,90 That is,
these textures are reported to be seen for the SmC* phase.63,90
Notably, a texture consisting of both undulation lines and cross-
hatched pattern was seen for the sample SA(R)-10 held
between the two ordinary glass slides (Figure 3c).63,91 Thus,
these textural patterns observed under a polarizing microscope
evidently indicate the presence of a SmC* phase below the N*
phase. XRD and electro-optical measurements strongly
supported this assignment which we discuss in the sections
to follow.
2.2.2. X-Ray Powder Diffraction (XRD) Study. As is well-
known, in the SmC* phase, the constituent mesogens tilt at an
angle with respect to an axis perpendicular to the layer planes
and it is the precession of the tilt that yields the helical
macroscopic structure. Given the tilted orientation of mesogens
within the smectic layers, the measured spacing (d) value
derived from powder XRD data is expected to be smaller than
the molecular length (L) of the mesogens. To demonstrate this,
XRD measurements were carried out on unaligned specimens
of representative compounds SA(R)-8, SA(R)-11, and SA(S)-
12. The samples were heated to their isotropic phase and filled
into the Lindemann capillary (1 mm diameter) tube by
capillary action and both the ends of the tube were flame-
sealed. The XRD patterns in the region of SmC* phase were
recorded as a function of reducing temperature; all the profiles
obtained were found to be qualitatively identical. Specifically,
the 1D intensity versus diffraction (2θ) profiles obtained for
SmC* phase at different temperatures were found to be nearly
the same. Figure 4a represents such profiles obtained at 101,
114, and 112 °C for SA(R)-8, SA(R)-11, and SA(S)-12,
respectively.
Each pattern comprised a sharp reflection, with the layer
thickness d, in the small angle region (0 < 2θ < 5°) and a
diffuse peak in the wide-angle area (2θ ≈ 20°), clearly
establishing the layer structure of the phase. The diffuse peak in
the wide-angle region with the spacing ∼4.6 Å corresponds to
the intermolecular separation within the smectic layer arising
due to the liquidlike positional correlation within the layer.
Figure 4b illustrates the temperature dependence of the layer
spacings derived from the low-angle Bragg reflections obtained
in the smectic region of compounds SA(R)-8, SA(R)-11, and
SA(S)-12. The spacings d of the low-angle reflections, wide-
angle peak positions, the molecular length (L) of the three
mesogens in their all-trans conformation, the d/L ratio and the
tilt angle (θ), estimated using the expression θ = cos−1(d/L),
are given in Table S1 of the Supporting Information. It
apparent from Figure 4b and Table S1 of the Supporting
Information that the layer spacing values ranging between 26.04
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Figure 4. (a) 1D intensity vs 2θ profiles obtained for the SmC* phase of mesogens SA(R)-8, SA(R)-11, and SA(S)-12; note that in each trace, a
diffuse peak in the wide-angle region (indicating liquidlike order), and a sharp reflection in the low-angle region are commonly seen. (b) The layer
spacing (d) recorded as a function of temperature in the SmC* phase of SA(R)-8, SA(R)-11 and SA(S)-12.
Figure 5. Current response peaks obtained by applying a triangular-wave filed for the SmC* phase of compounds (a) SA(R)-8 (90 °C; 46.8 Vrms,
20 Hz; 9 kΩ; Ps = 121.1 nC cm−2) and (b) SA(S)-12 (90 °C; 46.8 Vrms, 20 Hz; 9 kΩ; Ps = 111.1 nC cm−2). (c) Profile showing the temperature
dependence of the Ps in the SmC* phase of mesogens SA(R)-8 (red trace) and SA(S)-12 (blue trace).
and 26.4 Å, 28.6−28.9 Å, and 29.7−29.8 Å are much less than
the estimated all-trans molecular length (L) 37.5, 41.2, and 42.5
Å of the respective compounds SA(R)-8, SA(R)-11, and
SA(S)-12 and thus, the d/L ratio are less than one. These
results point to tilted monolayer organization of the mesogens
within the smectic layers. The tilt angle, θ, of the molecules
with respect to the layer normal was found to be in the range of
45.3−46.1. Thus, the XRD study evidence the tilted
organization of the molecules within the smectic layers.
It is clear from Figure 4b that for mesogens SA(R)-8 and
SA(R)-11, the layer spacing decreases on cooling the phase, as
expected, suggesting that the tilt angle of the mesogens within
the smectic layers increases upon reducing the temperature. But
for mesogen SA(S)-12, the layer spacing decreases initially and
then increases marginally upon cooling (Figure 4b, blue trace).
Although such a trend of reversing the layer spacing (or tilt
angle) is rather unusual but can be interpreted in terms of the
negative thermal expansion of the layer structure, which stems
from the stretching of the paraffinic tails, counterbalances the
decrease in the layer spacing due to the tilted nature of the
phase. Thus, the foregoing XRD measurement data and the
results of the optical textural pattern observation unequivocally
support the presence of a SmC* phase. As discussed below,
electrical switching studies confirmed ferroelectric behavior of
the phase.
2.2.3. Electro-Optic Measurements. The SmC* phase is a
helical superstructure that results from the self-assembly of the
constituent chiral mesogens, as discussed previously. In
particular, such a novel structure originates from a gradual
change in molecular tilt direction from layer-to-layer, about an
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axis perpendicular to the layer planes.2,3,20−29 Notably, in this
phase, which lacks mirror symmetry due to the presence of
chiral mesogens, the tilting of mesogens in each smectic layer
induces a bias in their rotation around their long axes. This
leads to spontaneous polarization (Ps) in the plane of each layer
when the mesogens possess a transverse dipole moment parallel
to the layer planes. Due to the helix, the tilt direction is rotated
about the normal to the layer planes, and accordingly, the layer
Ps averages out to zero. However, the macroscopic Ps can be
realized by unwinding the helix by surface action or by an
external stimulus such as the electrical field.2,3,20−29 In fact,
experimental results have well-demonstrated that the SmC*
phase possesses distinct polarization and optical properties that
are practically apt for various technological applications.2,3,20−29
Thus, the SmC* phase formed by the compounds SA(R)-8 and
SA(S)-12, as representative cases, was subjected to a detailed
electrical switching study.
For this purpose, cells were fabricated using a pair of
electrically conducting indium tin oxide (ITO) glass plates
treated with polyimide solution and rubbed unidirectionally to
promote planar alignment. Samples were carefully filled into
cells in their isotropic phase, cooled slowly to the SmC* phase,
and a triangular wave field was applied. The current response
profiles of the samples SA(R)-8 (Figure 5a) and SA(S)-12
(Figure 5b), obtained upon applying a voltage of 46.8 Vrms and
a frequency of 20 Hz, showed a single polarization current peak
for each half period confirming the intrinsic ferroelectric
switching characteristics of the SmC* phase. The switching
current peaks were found to be existing without any significant
change over the whole thermal range of the SmC* phase.
Importantly, such switching peaks were not seen in the
isotropic liquid phase, suggesting that the current profiles are
real and not stemming, owing to the presence of ionic
impurities. In the Supporting Information, the optical textural
patterns seen under these experimental conditions are given
(Figures S1 of the Supporting Information). The macroscopic
Ps values obtained by integrating the area under the peaks of
the respective profiles of SA(R)-8 and SA(S)-12 were found to
be 121.1 and 111.1 nC cm−2. The temperature dependence of
Ps was measured, and the corresponding profiles obtained are
shown in Figure 5c. It is immediately apparent that the Ps
increases progressively with decreasing temperature and
vanishes across the SmC*-N* transition, as expected. Thus,
the preceding experimental data clearly point to the ferro-
electric nature of the SmC* phase.
The helical sense (left or right) of the N* phase illustrates
the direction in which rotation of the director occurs from
layer-to-layer. Fundamentally, the molecular chirality imparts
the form chirality (handedness) to the structure; that is, the
helical twist sense is determined by the molecule’s chirality. As
is well-known, two enantiomeric forms of a chiral compound
display optical activities of equal magnitudes but opposite signs.
Thus, CD study facilitates in ascertaining the handedness as the
circularly polarized light (CPL), being a chiral system, interacts
differently with the N*/SmC* phase formed by the two
enantiomers of a chiral mesogen. That is to say, the N* phase
shows CD at a given well-defined wavelength band where the
incident unpolarized light beam (comprising equal amounts of
left and right handed circularly polarized components) will be
split into its right and left circularly polarized (r-CPL and l-
CPL) components. When white light propagates through a
right-handed (RH) N* phase, the RH circularly polarized
component with wavelength of the N* pitch is selectively
reflected while the left-handed (LH) component is completely
transmitted. Therefore, the helical screw sense of the N*/
SmC* phase of a pair of mirror-image isomers, the
enantiomers, ought to be opposite to each other. Of particular
interest is to know what happens to the helix sense of the N*
and SmC* phases when their phase transition occurs in
succession, which is the case with all compounds synthesized in
the present study.
In order to examine these characteristics, the CD experi-
ments were carried out in the entire thermal range of SmC*
and N* phases formed by two pairs of enantiomers, namely,
SA(S)-10 and SA(R)-10 and SA(S)-12 and SA(R)-12. Initially,
with the aim of quantifying the measurements, prefabricated
cells of known thickness were used. However, the stronger CD
signals obtained were found to be out of the measurable range
of the instrument. Thus, to work within the upper (saturation)
limits of measurement, the usage of very thin films of the
samples was found be essential, and thus, our CD measure-
ments presented herein, needless to say, are qualitative. The
above-mentioned representative samples, held between two
quartz plates, were heated slightly above their isotropization
temperatures and pressed-hard to obtain evenly spread very
thin films. This process was repeated until the films showed the
genuine CD signals without saturation of the detector.
However, despite several attempts, we could not circumvent
the saturation problem for the samples SA(S)-12 and SA(R)-
12, especially in the SmC* phase (see Figure S2 of the
Supporting Information) the intensity of the bands was found
to exceed the range of the CD instrument.
3. CIRCULAR DICHROISM SPECTROSCOPIC STUDY
The samples were cooled from their isotropic liquid state
using a programmable hot stage having accuracy of about 0.5
°C. CD spectra were recorded as a function of decreasing
temperature from isotropic state to the temperature range
covering both the N* and SmC* phases. It is important to
mention here that none of the samples displayed the CD effect
in the isotropic state, whereas both the mesophases showed
very strong characteristic CD signals under the identical
experimental conditions. The presence of CD signals is a
direct proof for chiral orientation of chromophores. This
implies that the chromophores are not influenced by the
molecular chirality. In other words, these results evidently
suggest the origin of optical activity of these phases from the
chiral induction in the LC structure but not from the molecular
chirality. The CD spectra obtained for SA(S)-10 and SA(R)-
10, as a representative case, are shown in Figure 6. As shown in
The N* and SmC* phases, being helical structures, exhibit
special optical properties such as circular dichroism (CD) and
Bragglike light reflection. The CD measurement has been
proven to be one of the best techniques to probe the chiroptical
properties of both N* and SmC* phases.45,46,53,92−104 In a
planarly well aligned N* phase, the constituent mesogens adopt
a macroscopic helical order (perpendicular to the substrate)
with a pitch p; that is, the director rotates in a helical way
around a perpendicular axis. Iridescent colors due to selective
reflection of an incident light beam, remaining parallel to the
helix axis, can be observed. Bragg reflections occur when the
helix pitch is on the order of the wavelength of the incident
light beam passing along the helix axis according to the
equation p = nλ (where n is the refractive index and λ is the
wavelength of incident light).
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Figure 6. Temperature-dependent CD spectra recorded in the N* (a and c) and SmC* (b and d) phases formed by the mirror-image isomers
SA(S)-10 and SA(R)-10.
Figure S2 of the Supporting Information, the CD spectral
patterns obtained for the samples SA(R)-12 and SA(S)-12
were found to be virtually indistinguishable when compared to
those of the samples SA(S)-10 and SA(R)-10
counterpart enantiomer SA(R)-10 shows CD signals of
opposite sign with some variations in their magnitude (Figure
6c); the notable differences in the peak positions and intensity
of the CD signals, when compared to that of enantiomer
SA(S)-10, can be ascribed to the thickness of the sample. It
may be recalled here that the UV−Vis (solution) spectra of
these enantiomers show an intense broad band at 355 nm due
to the n−π* transition, and thus, among the aforesaid CD
signals, the intense maximum (430 nm) can be ascribed to the
n−π* transition. The weak signals at 707 and 540 nm of
samples SA(S)-10 and SA(R)-10, respectively, must be arising
from a very weak n−π* state embedded in the lowest energy
absorption system and therefore are not seen in the UV−Vis
spectra. This means that intrinsically weak n−π* transitions are
reasonably sensed in CD spectral profiles than in the solution
UV−Vis spectra. The CD bands appearing below 300 nm are
apparently due to electronic transitions from a bonding π
orbital to an antibonding π* orbital.
Thus, the presence of CD peaks in the LC phase corresponds
to chromophores that are organized in a specific chiral
orientation. In other words, CD activity originates because of
intermolecular interactions between two or more electronic
transition dipoles arranged in a helical order. Indeed, when
compared with the absorption behavior of the solution, the
films of the neat N* phase exhibit a strong bathochromic effect
(red shift) that can be attributed to the presence of
auxochrome, hydroxyl (−OH) group, and/or strong cofacial
proximity of chiral aggregates. It must be remarked here that
the CD spectra of both the N* phases of the samples were
found to be highly reproducible with only minor (negligible)
variations in position and intensity (mdeg) of the CD signals.
The broadening of the CD-band must be related to the
distribution (inconsistently or homogeneously)97 of chiral
aggregates. Another important feature apparent from Figure 6
(panels a and c) is that the CD intensity increases with
In fact, in the absorption regime, the visible CD band
combines the genuine CD signal, linear dichroism (LD), and
birefringence. LD effect was completely ruled out by employing
a well-known simple protocol that involves rotating the cell
through 360o perpendicular to the incident light.105−110
Specifically, the CD activity observed in the mesophases was
ascertained by recording the spectra at different temperatures
by rotating the sample cells (in the plane normal to the incident
light beam). The spectra were found to be practically the same
to those obtained in the original position of the samples. Given
the fact that the CD spectrometer used in our study also
measures LD, the samples were subjected to such a
measurement directly. As expected, no LD activity was
apparent, clearly signifying that the peaks seen in the spectra
are due to genuine CD activity. The peak position (λmax/nm),
sign (+ve/−ve) and intensity (mdeg) of the CD signals
obtained as a function of temperature for the two LC phases of
the samples SA(S)-10 and SA(R)-10 are shown in Table 2.
Similar data obtained for the mesophases of SA(R)-12 and
SA(S)-12 are given in Table S2 of Supporting Information.
As can be seen in Figure 6 (panels a and c), the CD spectra
of the N* phase obtained at different temperatures for
enantiomers SA(S)-10 and SA(R)-10 show nearly mirror-
image CD spectral patterns wherein multiple CD signals in
each spectrum can be seen. The origin of such multiple bands
in each CD spectrum points to different excitonically coupled
chromophores capable of exhibiting several electronic tran-
sitions. For instance, the N* of an enantiomer SA(S)-10 shows
three negative peaks at ∼266, 430, and 707 nm; besides, it
shows two very weak positive bands at ca. 265 and 295 nm (see
Figure 6a and Table 2). As expected, the N* phase of the
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SA(R)-10, respectively, can be attributed to π−π* transitions.
It is well-known that a CD couplet results from the through-
space coupling of two or more chromophores in chiral
substrates, leading to an exciton-coupled CD. In fact, as
mentioned earlier, coupling also happens when more than two
identical or different chromophores are present. It is instantly
apparent from Figure 6b of enantiomer SA(S)-10 that within
each bisignate CD couplet (with zero crossing at ca. 450) nm),
a −ve Cotton-effect at higher wavelength (in the range of 588−
632 nm; see Table 2) and a +ve Cotton-effect at lower
wavelength (∼400 nm) exist, whereas the CD spectral features
of the antipode SA(R)-10 were nearly equal in magnitude but
opposite in sign, as expected. Thus, the SmC* phase of
enantiomers SA(S)-10 and SA(R)-10 show a positive-couplet
and a negative-couplet, respectively (see Table 2). The intense
sharp peak observed in the 395−400 nm regions of the CD
spectra of both the samples should be arising from the n−π*
transition as the UV absorption spectra show a peak in the
corresponding region assigned to the n−π* transition. The
broad and relatively strong peak seen in the range of 600−720
nm indicates the presence of a weak n−π* transition and thus
supports a similar occurrence in the N* phase.
As noted earlier, the CD spectra of the N* phase belonging
to enantiomers SA(S)-10 and SA(R)-10 show intense CD
bands in the wavelength (λ) range of 424−430 nm that
correspond to the absorption wavelength of n−π* transition of
the chiral chromophore. Likewise, the intense but sharp CD
bands corresponding to the absorption wavelength of the n−π*
transition also exist for the SmC* phase of the chiral
compounds SA(S)-10 and SA(R)-10. However, for the sample
SA(S)-10, the CD band (λ ∼ 400 nm) obtained in the SmC*
phase is positive (Figure 6b) whereas it is a −ve CD peak (λ ∼
425 nm) for the N* phase (Figure 6a). It can therefore be
assumed that the sense of the helix in the SmC* phase is right-
handed, while it is left-handed in the case of the N* phase. The
counterpart CD bands are positive (Figure 6c) and negative
(Figure 6d) for the N* and SmC* phases, respectively, formed
by SA(R)-10. These observations imply that reversal of helical
handedness from left-to-right and vice versa occurs during the
N* to SmC* phase transition. Aptly, the screw sense of the
helical array of the N* and SmC* phases of an enantiomer is
opposite.
Table 2. CD Spectroscopic Data* Obtained in N* and SmC*
Phases As a Function of Temperature for the Enantiomers
SA(S)-10 and SA(R)-10
CD
chiral LC temperature
compounds
phase
(°C)
λmax (nm)
CD (mdeg)
SA(S)-10
N*
122
266, 297, 380,
430, 707
−5, +3, +9, −65,
−9
124
126
128
130
265, 295, 377,
428, 709
−5, +2, +7, −62,
−11
266, 295, 374,
427, 705
−4, +2, +5, −57,
−10
266, 295, 374,
428, 708
−3, +2, +3, −52,
−11
266, 295, 374,
427, 717
−2, +1, +2, −44,
−11
132
121
426, 721
−35, −9
SA(R)-10
N*
259, 294, 376,
430, 569
+18, −6, −19,
+179, +27
123
126
129
131
134
104
107
110
113
116
103
107
111
113
115
259, 294, 376,
428, 543
+16, −3, −14,
+168, +27
258, 296, 375,
426, 539
+16, −3, −13,
+145, +26
257, 293, 372,
424, 572
+17,-6, −17,
+116, +31
258, 372, 426,
537
+15, −8, +105,
+25
258, 372, 426,
539
+15, −8, +103,
+25
SA(S)-10
SmC*
265, 296, 401,
632
−20, +5, +80,
−116
265, 295, 401,
630
−20, +6, +80,
−120
265, 294, 401,
608
−20, +5, +79,
−124
264, 295, 401,
589
−20, +5, +79,
−126
264, 294, 400,
588
−20, +6, +77,
−125
SA(R)-10
SmC*
255, 291, 396,
722
+4, −7, −33,
+19
255, 291, 396,
713
+4, −7, −32,
+21
255, 293, 397,
702
+4, −7, −32.
+22
255, 293, 396,
672
+5, −6, −31,
+25
255, 292, 397,
653
+5, −5, −29,
+26
4. CONCLUSIONS
*
The significant (intense) peaks have been shown in the table for
Summarizing, the facile synthesis and characterization of ten
optically active mesogenic compounds, in the form of five pairs
of enantiomers, are reported. The salicylaldimine-core has been
especially incorporated in the molecular design to maximize the
probability of ferroelectric properties as well as to provide
resistance to heat and moisture to these new monomeric
motifs. Chiral mesogens that are nonsuperimposible mirror
images, making up a pair of enantiomers, have been derived
from (R)-octyloxy and (S)-octyloxy tails. The length of the n-
alkoxy chain substituted at the other end has been varied from
n-octyloxy to n-dodecyloxy. This variation influences the
clearing temperatures of the mesogens; that is, the N*-I
(clearing) transition temperatures decrease with the increase in
the length of the terminal tail; this behavior can be interpreted
in terms of enhanced molecular flexibility with the increase in
the length of the terminal tail. All the mesogens behave
identically, exhibiting BP, N*, and SmC* phases, meaning that
the alteration in the length of the n-alkoxy tail has no significant
effect on their LC behavior. The thermal width of the SmC*
simplicity.
decreasing temperature which can be perhaps ascribed to the
higher mobility of the terminal alkyl chains. On the basis of the
literature guidelines,97,105−110 one may assign a right-handed
screw sense to the helix of the N* phase of SA(R)-10 as it
shows a positive CD peak and a left-handed screw sense to the
helix of the N* phase of SA(S)-10 as it shows a negative CD
band.
Figure 6 (panels b and d) show the temperature-dependent
CD spectra recorded, under the similar experimental
conditions, in the thermal range of the SmC* phase of
SA(S)-10 and SA(R)-10. Both the spectra in the wavelength
range of 400−800 nm (Table 2), exhibit quasi-mirror imaged
bisignate CD curves, called CD couplets which are defined as
the through progression of two intensive Cotton-effects with
inverted signs. In addition, weak peaks at ca. 265 (−), 295 (+),
and 265 (+), 296 (−) seen in the spectra of SA(S)-10 and
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phase widens upon elongating the length of the n-alkoxy tail.
Notably, the SmC* phase exhibits ferroelectric switching with
high polarization value. The circular dichroism study, apart
from confirming the helical structure of the N* and SmC*
phases, also serve to demonstrate the reversal of the helix-sense
during the N*−SmC* phase transition.
FP90 Central Processor. To corroborate the phase transition
temperatures of microscopic study and to determine the
enthalpies of phase transitions, differential scanning calorimeter
(DSC), PerkinElmer Diamond DSC with the PC system
operating on Pyris software, was employed. Indeed, prior to
use, the DSC was calibrated using pure indium. DSC traces of
both heating and cooling cycles were obtained at a rate of 5 °C/
min under the constant flow of nitrogen gas. X-ray diffraction
studies were performed on powder samples in Lindemann
capillaries with Cu Kα (λ = 0.15418 nm) radiation using the
PANalytical X’Pert PRO MP X-ray diffractometer comprising a
focusing elliptical mirror and a fast resolution detector
(PIXCEL).
5. 2. Synthesis. 5.2.1. Synthesis and Characterization of
Intermediates (1−4, 5a−5e, 6a−6e, and 7a−7e). The
synthetic procedures and characterization data of all the
intermediates namely, (S)- and (R)-1-nitro-4-(octan-2-yloxy)-
benzenes (1 and 3), (R)- and (S)-4-(octan-2-yloxy)-anilines (2
and 4), 4-(n-alkoxy)benzaldehydes (5a−5e), 4-(n-alkoxy)-
benzoic acids (6a−6e), and 4-formyl-3-hydroxyphenyl 4-
(alkoxy)benzoates (7a−7e) have been given in the Supporting
Information.
5.2.2. General Procedure for the Synthesis of (S)-3-
Hydroxy-4-{[(4-(octan-2-yloxy)phenyl)-imino]methyl}phenyl
4-(n-alkoxy)benzoates. A mixture of 4-formyl-3-hydroxyphen-
yl-4-(alkoxy)benzoate (0.4 mmol,1eq ) and (S)-4-(octan-2-
yloxy)aniline (0.33 mmol, 1.2eq ), and a catalytic amount of
acetic acid in ethanol (10 mL) was refluxed under inert
atmosphere for 2 h. The yellow solid separated upon cooling
the reaction mixture was collected by filtration, washed with
ethanol, and air-dried. The crude product was purified by
repeated recrystallizations using absolute ethanol.
5. EXPERIMENTAL SECTION
5.1. Materials and Methods. Chemicals such as (R)-2-
octanol, (S)-2-octanol, triphenylphosphine, diisopropyl-azodi-
carboxylate, 10% Pd−C, 2,4-dihydroxy benzaldehyde, n-alkyl
bromide, N,N′-dicyclohexylcarbodiimide (DCC), 4-dimethyla-
minopyridine (DMAP), 4-nitrophenol, and acetic acid
purchased either from overseas companies or local sources
were used as received. Solvents were dried as per the standard
procedures: DMF was stored over KOH pellets for 12 h and
then distilled carefully under reduced pressure; it was then
stored over molecular sieves (4 Å) for 48 h prior to use.
Commercially available, ultra pure, dry nitrogen was generally
employed to maintain an inert atmospheric condition. The
reactions that are sensitive to air- and/or moisture were carried
out under a nitrogen atmosphere in oven-dried glassware.
HPLC-grade THF was distilled from sodium and benzophe-
none under inert atmosphere. Reagent grade acetone was
stored over anhydrous potassium carbonate for 24 h and then
distilled. Absolute alcohol was distilled with magnesium
ethoxide (obtained by treating ethanol with magnesium
turnings and iodine). Silica gel (60−120 and 100−200 mesh)
and basic aluminum oxide were used as the stationary phases
used in column chromatography. Industrial grade ethyl acetate
and hexanes used for column chromatography were distilled
twice prior to use. Purity of the compounds and progress of the
reaction were examined with the help of thin layer
chromatography (TLC); for this purpose, aluminum sheets
precoated with silica gel (Merck, Kieselgel 60, F254) were used.
UV-light and iodine vapors were used for visualization. The
final compounds were dissolved in CH2Cl2 and microfiltered to
eliminate the fine particles, cotton threads, filter paper fibers,
etc. UV−vis spectra were recorded on a PerkinElmer Lambda
20 UV−vis spectrometer (1 cm path length, CH2Cl2). FTIR
spectra of the compounds were recorded on KBr pellets or in
neat form using a PerkinElmer Spectrum 1000 FTIR
spectrometer, and the spectral positions are given in a
SA(S)-8. A yellow solid; yield: 0.208 g (96%); IR (KBr
pellet): νmax in cm−1 2922, 2852, 1725, 1609, 1574, 1506, 1466,
1392, and 1169; UV−vis: λmax = 354.15 nm, mol. conc. = 6.45
× 10−3 M in CH2Cl2, ε = 2.806 × 102 L mol−1 cm−1; 1H NMR
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz,
1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H,
Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87(d, J = 2 Hz, 1H, Ar), 6.82
(dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H, Ar), 4.39 (m, 1H, −OCH),
4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.85−0.86 (m, 31H, 3 ×
CH3, 11 × CH2); 13C NMR (100 MHz): 164.44, 163.71,
162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31,
121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.40,
36.49, 31.82, 29.34, 29.30, 29.24, 29.12, 26.01, 25.53, 22.67,
22.62, 19.76, 14.11, and 14.09; anal. Calcd for C36H47NO5: C,
75.36; H, 8.26; N, 2.44. Found: C, 75.43; H, 8.36; N, 2.35.
SA(S)-9. A yellow solid; yield: 0.180g (91%); IR (KBr
pellet): νmax in cm−1 2923, 2853, 1725, 1611, 1506, 1465, 1391,
and 1165; UV−vis: λmax = 353.46 nm, mol. Conc. = 8.85 × 10−3
1
wavenumber (cm−1) unit. H and 13C NMR spectra were
recorded in a CDCl3 solvent at ambient temperature using a
Bruker AMX-400 (400 MHz) spectrometer. The coupling
constants (J) are given in Hz. The chemical shifts are reported
in ppm on a scale downfield from TMS regarded as an internal
standard. The pattern (splitting or multiplicity) of the 1H NMR
signals are presented as s = singlet, d = doublet, dd = doublet of
doublet, brs = broad singlet, and t = triplet. Elemental
microanalysis was performed using Eurovector E300 elemental
analyzer. The molecular lengths of target LCs were measured
using the energy minimized (all-trans) space filling models
deduced from the ChemBio3D Ultra 12.0 program. Space-
filling models of molecular structures were minimized using
MM2 computations. CD spectra were recorded with the help of
the Jasco J-810 spectropolorimeter.
1
M in CH2Cl2, ε = 2.565 × 102 L mol−1 cm−1; H NMR (400
MHz, CDCl3): δ 13.79(s, 1H, Ar−OH), 8.61 (s, 1H, −CH
N), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.38 (d, J = 8.4 Hz, 1H, Ar),
7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 9.2 Hz, 2H, Ar), 6.93
(d, J = 8.8 Hz, 2H Ar), 6.87(d, J = 2.4 Hz, 1H, Ar), 6.82 (dd, J1
= 8.4 Hz, J2 = 2.4 Hz, 1H, Ar), 4.38 (m, 1H, −OCH), 4.06 (t, J
= 6.6 Hz, 2H, −OCH2), and 1.85−0.86 (m, 33H, 3 × CH3, 12
× CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48,
159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32,
117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39, 36.49,
31.89, 31.82, 29.54, 29.39, 29.30, 29.27, 29.12, 26, 25.53, 22.69,
The mesophase assignments and determination of transition
temperatures of the ten new chiral compounds were carried out
with the help of an Olympus BX50 (BX50F4 model) optical
polarizing transmitted light microscope equipped with a digital
camera and a Mettler FP82HT hot stage programmed by an
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22.62, 19.76, 14.12, and 14.09; anal. Calcd for C37H49NO5: C,
75.60; H, 8.40; N, 2.38. Found: C, 75.74; H, 8.25; N, 2.15.
SA(S)-10. A yellow solid; yield: 0.194g (93%); IR (KBr
pellet): νmax in cm−1 2923, 2853, 1725, 1611, 1574, 1506, 1465,
1392, and 1166; UV−vis: λmax = 354.04 nm, mol. Conc. = 3.66
× 10−3 M in CH2Cl2, ε = 2.922 × 102 L mol−1 cm−1; 1H NMR
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz,
1H, Ar), 7.26 (d, J = 8.8 Hz Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz,
2H, Ar), 6.94 (d, J = 9.2 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz,
1H,Ar), 6.82 (dd, J1 = 8.4 Hz, J2= 2 Hz, 1H, Ar), 4.37 (m, 1H,
−OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.87 (m,
35H, 3 × CH3, 13 × CH2); 13C NMR (100 MHz): 164.44,
163.71, 162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39,
122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41,
68.39, 36.49, 31.92, 31.82, 29.57, 29.38, 29.33, 29.30, 29.12, 26,
25.53, 22.70, 22.62, 19.76, 14.12, and 14.09; anal. Calcd for
C38H51NO5: C, 75.84; H, 8.54; N, 2.33. Found: C, 75.81; H,
8.63; N, 2.25.
SA(R)-8. A yellow solid; yield: 0.202 g (93%); IR (KBr
pellet): νmax in cm−1 2926, 2855, 1726, 1607, 1573, 1506, 1465,
1393, and 1169; UV−vis: λmax = 354.05 nm, mol. Conc. = 8.89
× 10−3 M in CH2Cl2, ε = 2.205 × 102 L mol−1 cm−1; 1H NMR
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz,
1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H,
Ar), 6.94 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz, 1H,Ar),
6.82 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H,Ar), 4.40 (m, 1H,
−OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.86−0.86 (m,
31H, 3 × CH3, 11 × CH2); 13C NMR (100 MHz): 164.44,
163.71, 162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39,
122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41,
68.40, 36.49, 31.82, 29.34, 29.30, 29.24, 29.12, 26.01, 25.53,
22.67, 22.62, 19.76, 14.11 and 14.09; anal. Calcd for
C36H47NO5: C, 75.36; H, 8.26; N, 2.44. Found: C, 75.30; H,
8.03; N, 2.37.
SA(R)-9. A yellow solid; yield: 0.2 g (92%); IR (KBr pellet):
νmax in cm−1 2923, 2852, 1724, 1605, 1574, 1505, 1467, 1394
SA(S)-11. A yellow solid; yield: 0.181g (91%); IR (KBr
pellet): νmax in cm−1 2922, 2851, 1738, 1719, 1605, 1574, 1508,
1468, 1395, and 1169; UV−vis: λmax = 352.02 nm, mol. Conc. =
and 1169; UV−vis: λ
= 355.86 nm, mol. Conc. = 6.98 ×
max
1
10−3 M in CH2Cl2, ε = 2.064 × 102 L mol−1 cm−1; H NMR
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz,
1H, Ar), 7.25 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H,
Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2 Hz, 1H, Ar),
6.81 (dd, J1 = 8.4 Hz, J2 = 2 Hz, 1H, Ar), 4.40 (m, 1H, −OCH),
4.06 (t, J = 6.4 Hz, 2H, −OCH2), and 1.85−0.86 (m, 33H, 3 ×
CH3, 12 × CH2); 13C NMR (100 MHz): 164.44, 163.71,
162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39, 122.31,
121.32, 117.36, 116.60, 114.38, 112.96, 110.58, 74.41, 68.39,
36.49, 31.89, 29.53, 29.38, 29.30, 29.27, 29.12, 26, 25.53, 22.69,
22.62, 19.76, 14.12 and 14.09; anal. Calcd for C37H49NO5: C,
75.60; H, 8.40; N, 2.38. Found: C, 75.92; H, 8.43; N, 2.44.
SA(R)-10. A yellow solid; yield: 0.213 g (94%); IR (KBr
pellet): νmax in cm−1 2922, 2852, 1724, 1605, 1574, 1505, 1467,
1394, and 1169; UV−vis: λmax = 354.88 nm, mol. Conc. = 3.66
1
5.36 × 10−3 M in CH2Cl2, ε = 2.929 × 102 L mol−1 cm−1; H
NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s,
1H, −CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar); 7.40 (d, J = 8.4
Hz, 1H, Ar); 7.26 (d, J = 8.8 Hz, 2H, Ar); 6.98 (d, J = 8.8 Hz,
2H, Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2 Hz, 1H,Ar),
6.82 (dd, J1= 8.4 Hz, J2= 2 Hz, 1H, Ar), 4.40 (m, 1H, −OCH),
4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.85−0.86 (m, 37H, 3 ×
CH3, 14 × CH2); 13C NMR (100 MHz): 164.44, 163.71,
162.48, 159.42, 157.64, 154.51, 140.88, 132.75, 132.39, 122.31,
121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39,
36.49, 31.93, 31.82, 29.63, 29.61, 29.57, 29.38, 29.36, 29.30,
29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13, and 14.09; anal.
Calcd for C39H53NO5: C, 76.06; H, 8.67; N, 2.27. Found: C,
75.84; H, 8.56; N, 2.19.
SA(S)-12. A yellow solid; yield: 0.192g (90%); IR (KBr
pellet): νmax in cm−1 2922, 2852, 1720, 1605, 1574, 1507, 1467,
1394, and 1169; UV−vis: λmax = 354.71 nm, mol. Conc. = 5.72
× 10−3 M in CH2Cl2, ε = 3.691 × 102 L mol−1 cm−1; 1H NMR
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz,
1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H,
Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87(d, J = 2.4 Hz, 1H,Ar),
6.82 (dd, J1 = 8.4 Hz, J2 = 2.2 Hz,1H, Ar), 4.37 (m, 1H,
−OCH), 4.06 (t, J = 6.4 Hz, 2H, −OCH2), and 1.84−0.87 (m,
39H, 3 × CH3, 15 × CH2); 13C NMR (100 MHz): 164.43,
163.71, 162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39,
122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41,
68.39, 36.49, 31.94, 31.82, 30.93, 29.68, 29.66, 29.58, 29.38,
29.30, 29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13 and 14.09;
Anal. Calcd for C40H55NO5: C, 76.27; H, 8.80; N, 2.22. Found:
C, 76.07; H, 8.87; N, 2.14.
× 10−3 M in CH2Cl2, ε = 3.11 × 102 L mol−1 cm−1; H NMR
1
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz,
1H, Ar), 7.25 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H,
Ar), 6.93 (d, J = 9.2 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz, 1H,Ar),
6.82 (dd, J1= 8.4 Hz, J2= 2 Hz, 1H, Ar), 4.37 (m, 1H, −OCH),
4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.87 (m, 35H, 3 ×
CH3, 13 × CH2); 13C NMR (100 MHz): 164.44, 163.71,
162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39, 122.31,
121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39,
36.49, 31.92, 31.82, 29.57, 29.38, 29.33, 29.30, 29.12, 26, 25.53,
22.70, 22.62, 19.76, 14.12, and 14.09; anal. Calcd for
C38H51NO5: C, 75.84; H, 8.54; N, 2.33. Found: C, 75.88; H,
8.39; N, 2.18.
SA(R)-11. A yellow solid; yield: 0.208 g (90%); IR (KBr
pellet): νmax in cm−1 2922, 2852, 1725, 1607, 1573, 1506, 1466,
1392, and 1169; UV−vis: λmax = 354.98 nm, mol. Conc. = 4.38
× 10−3 M in CH2Cl2, ε = 1.733 × 102 L mol−1 cm−1; 1H NMR
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz,
1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H,
Ar), 6.94 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz, 1H, Ar),
6.82 (dd, J1 = 8.4 Hz, J2 = 2 Hz, 1H, Ar), 4.37 (m, 1H, −OCH),
4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.86 (m, 37H, 3 ×
CH3, 14 × CH2); 13C NMR (100 MHz): 164.44, 163.71,
162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31,
121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39,
5.2.3. General Procedure for the Synthesis of (R)-3-
Hydroxy-4-{[(4-(octan-2-yloxy)phenyl)-imino]methyl}phenyl
4-(n-Alkoxy)benzoates. A mixture of 4-formyl-3-hydroxyphen-
yl-4-(alkoxy)benzoate (0.4 mmol, 1eq ) and (R)-4-(octan-2-
yloxy)aniline (0.33 mmol, 1.2 eq), catalytic amount of acetic
acid in ethanol (10 mL) was refluxed under inert atmosphere
for 2 h. The yellow solid separated upon cooling the reaction
mixture was collected by filtration, washed with ethanol and air-
dried. The crude product was purified by repeated recrystalliza-
tions using absolute ethanol.
4548
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J. Phys. Chem. B 2015, 119, 4539−4551

The Journal of Physical Chemistry B
Article
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36.49, 31.93, 31.82, 29.63, 29.61, 29.57, 29.38, 29.36, 29.30,
29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13, and 14.09; anal.
Calcd for C39H53NO5: C, 76.06; H, 8.67; N, 2.27. Found: C,
76.01; H, 8.77; N, 2.29.
SA(R)-12. A yellow solid; yield: 0.214 g (91%); IR (KBr
pellet): νmax in cm−1 2923, 2851, 1724, 1605, 1574, 1505, 1467,
1391 and 1169; UV−vis: λmax = 354.77 nm, mol. Conc. = 8.26
× 10−3 M in CH2Cl2, ε = 2.398 × 102 L mol−1 cm−1; 1H NMR
(400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H,
−CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz,
1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 9.2 Hz, 2H,
Ar), 6.94 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2 Hz, 1H, Ar),
6.82 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H, Ar), 4.40 (m, 1H,
−OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.87 (m,
39H, 3 × CH3, 15 × CH2); 13C NMR (100 MHz): 164.44,
163.71, 162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39,
122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41,
68.39, 36.49, 31.94, 31.82, 29.68, 29.66, 29.61, 29.58, 29.37,
29.30, 29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13, and 14.09;
anal. Calcd for C40H55NO5: C, 76.27; H, 8.80; N, 2.22. Found:
C, 76.13; H, 8.84; N, 2.29.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, structural characterizations, powder
X-ray data, the microphotographs of textural changes seen
under microscope during the electric field studies and CD
spectroscopic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yelamaggad@cnsms.com and yelamaggad@gmail.com.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.V.Y. sincerely thanks the DST, New Delhi, Govt. of India, for
financial support through SERB project no. SR/S1//OC-04/
2012.
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