Photo-Responsive Behavior of Azobenzene Based Polar Hockey-Stick-Shaped Liquid Crystals

Article abstract of DOI:10.1002/cphc.202100215

A study on the photoswitching behavior of azobenzene-based polar hockey-stick-shaped liquid crystals (HSLCs) has been presented. Two new series of five phenyl rings based polar HSLCs have been designed and synthesized. Solution state photoisomerization of

Full text of DOI:10.1002/cphc.202100215

Accepted Article
Title: Photo-Responsive Behavior of Azobenzene Based Polar Hockey-
Stick-Shaped Liquid Crystals
Authors: Santanu Kumar Pal, Supreet Kaur, Nazma Begum, and
Golam Mohiuddin
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ChemPhysChem
10.1002/cphc.202100215
ARTICLE
Photo-Responsive Behavior of Azobenzene Based Polar Hockey-
Stick-Shaped Liquid Crystals
Supreet Kaur,[a]† Nazma Begum,[a]† Golam Mohiuddin, Santanu Kumar Pal
[a]
[a]*
†
†
[
a]
Ms. S. Kaur, Dr. N. Begum, Dr. G. Mohiuddin, Dr. S. K. Pal*
Department of Chemical Sciences
Indian Institute of Science Education and Research (IISER) Mohali
Sector-81, Knowledge City, Manauli 140306, India.
E-mail: skpal@iisermohali.ac.in
[†]
Equally contributed authors.
Supporting information for this article is given via a link at the end of the document.
Abstract: A study on the photoswitching behavior of azobenzene-
based polar hockey-stick-shaped liquid crystals (HSLCs) has been
presented. Two new series of five phenyl rings based polar HSLCs
have been designed and synthesized. Solution state photo-
isomerization of the synthesized materials was investigated
azobenzene substituted bent-core liquid crystalline materials
exhibiting banana-type mesophases.[22,23] Since then, a library of
compounds incorporating five, six, and/or seven aromatic rings in
the molecular structure have been prepared to understand the
relationship between the structure and the nature of the
mesophases.[24-36] The reports on azobenzene-based bent-core
LCs have been reviewed by M. Alaasar.[ An interesting photo-
orientation phenomenon has been explored in films of
azobenzene containing bent-shaped compounds.[38,39] Owing to
the isomerization, light-induced phase transitions i.e. from solid
to liquid,[40-42] and gel to sol[43] have been enormously discussed
in azobenzene based materials. The surface topographies of
amorphous, nematic, as well as crystalline films of an
azobenzene-containing bent-core compound has been
investigated via polarizing optical microscopy and atomic force
microscopy.[ Recently, we have also reported azobenzene-
based four-ring hockey-stick-shaped LCs (HSLCs) and their
photoswitching characteristics. The HSLCs with chlorinated (–Cl)
azobenzene wings exhibited photo-induced phase transitions in
solid-state, LC state, and solution-state upon irradiation with UV
as well as intense visible light.[16] Another class of HSLCs with
1
thoroughly via UV-visible and H NMR spectroscopic techniques,
37]
whereas solid-state photochromic behavior was elucidated via
physical color change of the materials, solid-state UV-visible study,
powder XRD, and FE-SEM techniques. The materials exhibited
decent photochromic behavior for different potential applications. The
thermal phase behavior of the superstructural assembly has been
characterized via polarizing optical microscopy (POM), differential
scanning calorimetry (DSC), and temperature-dependent small and
wide-angle X-ray scattering (SAXS/WAXS) studies. Depending upon
the length of the terminal alkyl chain, nematic (N) and partially bilayer
44]
smectic A (SmA
d
) phases were observed. The DFT calculation
revealed the favorable anti-parallel arrangement of the polar
d
molecules that substantiate the formation of SmA phase.
Introduction
3
methylated (–CH ) azobenzene wings showed a photo masking
effect (nematic to isotropic transition) in the LC state upon UV
illumination. An optically enhanced Frꢀedericksz transition has
been observed in one of the materials, based on which a
prototype of phase grating has been devised for photonic
devices.[17]
Here, we synthesized and investigated two new series of
azobenzene-based hockey-stick-shaped compounds comprised
of azo-ester-imine-ester linkers (Figure 1). The molecules are
Photo-responsive functional materials have gained tremendous
attention from the scientific community due to their intrinsic
physio-chemical behavior and have an immense prospect in
device applications.[1-3] A great deal of effort, in the contemporary
era, has been deployed to develop suitable photo-regulated
materials or molecular motors. In this context, azobenzene, a
versatile photo-functional moiety, has been studied for decades
with equal importance as a remarkable photoisomerizable and
photochromic unit. Azobenzene exhibits a prominent, facile, and
clean geometric change between its E/Z- (trans/cis) isomers upon
irradiation with light, and hence finds potential applications in
sensors, actuators, non-linear optic devices, optical data storage,
polarization holography, solar energy harvesting, etc.[4-8] Despite
being well-known for over a century, azobenzene-based materials
are still being explored in the field of material chemistry,[9-11]
medicinal chemistry,[12,13] biochemical applications,[14,15] etc.
The molecular structure of liquid crystal (LC) systems
incorporated with azobenzene units widens the range of their
application due to its photoresponsive behavior.[16-21] The
materials realized from the combination of photosensitivity and LC
properties have immense perspective in various optical and
optoelectronic devices. In 2001, Prasad et al. reported the first
terminated with a highly electronegative –NO
and the other end is attached with alkyl chains (n = 4, 6, 8, 12,
8). A methyl (–CH ) moiety is placed at the lateral positions (X
2
group at one end
1
3
and Y of Figure 1) i.e. on the ring adjacent to the –N=N– linkage.
Figure 1. Molecular structure of the hockey-stick-shaped compounds under
research.
1
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Scheme 1. Synthetic scheme of the compounds, Reagents and conditions: (i) HCl, NaNO
2
, 0-5 °C, NaOH (75%); (ii) 4-Formylbenzoic acid, DCC, DMAP, Dry
CH
2
Cl
2
, 24 h (85%); (iii) DCC, DMAP, Dry CH
2
Cl
2
, 24 h (85%); (iv) CH
3
OH, CH
2
Cl
2
, glacial AcOH, reflux, 4h (78%).
By varying the position of –CH
3
group (X and Y) and the terminal
chromophore (–N=N–) accompanied by a forbidden n-π*
flexible alkyl chains, we intend to understand the photoswitching
behavior, self-assembly, and materialistic properties of this class
of HSLCs.
transition band at 455 nm as shown in Figure 2a. Upon irradiation
with UV light of 365 nm (UV365), the intensity of π–π* band
decreased (λ ~ 333 nm) whereas the intensity of n–π* band
increased (λ ~ 442 nm) indicative of trans-cis photoisomerization.
A similar trend was observed in the case of compound 2-4 (Figure
2
b). The photostationary state (PSS) was found to be 20s and 30s
Results and Discussion
for compounds 1-4 and 2-4, respectively (Figure S46). An
additional peak was observed at ~270 nm due to π-π* transition
in Schiff’s base (–CH=N–) moiety.
To study the effect of positional variance of the methyl group on
the stability of the cis-isomer, a thermal reverse isomerization
kinetics study was carried out using UV-visible spectroscopy (in
Synthesis and characterization: A general representation of
two series (1-n and 2-n) of azo-based hockey-stick-shaped
compounds is presented in Figure 1. 5-Amino-2-chlorophenol
was taken as the central core to provide the bent-molecular
architecture. The desired compounds were synthesized following
the schematic procedure as outlined in Scheme 1. All the details
2 2
CH Cl at 25±1 °C). Both compounds 1-4 and 2-4 were switched
1
from their respective trans to cis-isomers using UV365 light (i.e. UV
light with λ=365 nm) before the thermal reverse isomerization
kinetics experiment. The plots of the exponential rate of formation
of the trans-isomer at an absorbance λmax corresponding to the
of the spectroscopic data, molecular characterization viz. H NMR,
13C NMR, HRMS, ATR (Figures S1-S45, Table S1),
photoswitching characterization (Figures S46-S49), and material
characterization such as POM, DSC, temperature-dependent
SAXS/WAXS studies and DFT calculations (Figures S50-S54,
Tables S2-S4) are sequentially elaborated in the Supporting
Information.
Table 1. Absorption spectral properties and formation rate constants in solution
phase using UV-visible spectroscopy.
Analysis of photoswitching behavior via UV-visible and NMR
techniques: UV-visible spectroscopy is an important technique
to understand the trans-cis photoisomerization of the azobenzene
unit. The spectral features of both trans and cis-isomers can be
well differentiated due to strong changes in the electronic
structures. The photoswitching and kinetics studies were carried
Compound
λ
max/(ε)
λ
π-π*, λn-π*
(cis)
Rate
Half-life
(trans)
constant
-
3
k (10 )
1-4
346 (10629)
351 (9611)
333, 442
336, 443
9.23
3.93
75.05
2-4
176.34
2 2
out in CH Cl at room temperature (~ 25 °C). The UV-visible
spectrum of compound 1-4 showed a significant absorption
maximum (λmax) at 346 nm (10-4 M, 3.5 eV, ε ~ 10629 L mol-1 cm-
[The values of λmax (trans) and λπ-π*, λn-π* (cis) are given in nm and ε value in L
-
1
-1
-1
mol cm (in parenthesis). Rate constants (min ) and the half-lives (min) have
been measured in CH Cl at 25±1 °C.]
1)
2
2
corresponding to symmetry allowed π-π* transition of the
2
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ChemPhysChem
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ARTICLE
(
a)
(b)
(
c)
(d)
Figure 2. Photoswitching studies of compounds (a) 1-4, (b) 2-4 in CH
2
Cl
2
before UV irradiation, after UV irradiation, and after keeping in dark. First-order formation
kinetics for the reverse switching (cis-trans) in 10-4 M solution in CH
for compounds (c) 1-4 and (d) 2-4.
2
Cl
2
π-π* transition of the respective trans-isomer (λmax = 346 nm and
51 nm for compounds 1-4 and 2-4, respectively) are presented
isomerization (cis-trans) was attained after exposure to normal
visible light (Figure 3c), and the nature of the peaks obtained was
3
in Figures 2c,d. Following the first-order kinetics, the rate constant, exactly similar to the all trans-state spectrum (Figure 3a).
k, and the half-life of the cis-isomer were calculated (Table 1). The
net rate of formation of the trans-isomer of compound 2-4 was
observed to be greater than 1-4. The difference in the half-lives
and rate constants for both representative compounds arise due
Similar results were obtained for compound 1-4 upon UV365
exposure (Figure S47). 25% conversion to cis-isomer could be
achieved upon UV365 irradiation for ~120 min. The results for
compound 1-4 are elaborated in the Supporting Information.
It may be noted that % conversion to cis-isomer upon irradiation
with UV365 was approximately the same for both the
representative compounds i.e. 25 % and 27 % for 1-4 and 2-4,
respectively for approximately similar UV exposure time.
to the variation in the lateral substitution of the –CH
3
group
adjacent to the –N=N– bond.
1H NMR spectroscopic studies also supported the photoswitching
behavior. The photoswitching and reverse thermal isomerization
for two representative compounds 1-4 and 2-4 were studied in
CDCl at room temperature (25 °C). For compound 2-4, upon
3
UV365 light irradiation, an additional significant peak at 8.58 ppm
Solid-state photochromism: Apart from the solution-phase
photoswitching (as discussed in the above section), the
azobenzene-based compounds were also considered solid-state
photochromic materials. A representative compound from both
the series (compounds 1-6 and 2-4) was investigated in detail for
the photoswitching studies. The compound 1-6 was dissolved in
emerged in the upfield region, adjacent to the –CH=N– singlet at
8.60 ppm as illustrated in Figure 3b. Upon trans-cis isomerization,
spectral changes i.e. multiple peaks were observed both in the
upfield as well as downfield region (highlighted in grey). Also, a
signal at 6.23 ppm appeared after UV exposure (Figure 3b). 27%
conversion to cis-isomer could be achieved upon UV365 exposure
for 60 min (7.5 mM) as shown in the zoomed region of Figure 3b.
The chemical shift (훿) values did not show any difference before
and after UV365 light irradiation (Figure 3a,b). The reverse thermal
3
chloroform (CHCl ) and irradiated with UV365 light for ∼5 h
followed by solvent evaporation and drying under vacuum. A
preliminary observation was the color change from yellowish-
orange to reddish-orange after UV365 light irradiation (Figure 4c).
The solid-state photochromism was supported by UV-visible
spectroscopic studies in KBr medium. The change in spectral
3
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ChemPhysChem
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ARTICLE
(
c)
(
b)
(
a)
Figure 3. 1H NMR spectra of compound 2-4 (7.5 mM in CDCl
3
): (a) before irradiation, (b) after irradiating with 365 nm UV light for 60 min, (c) after 24 h in normal
visible light corresponding to the reverse isomerization. The zoomed regions correspond to the aromatic protons with trans-cis conversion, and the changes in the
nature and intensity of peaks are highlighted in grey.
(
a)
(b)
π-π*
n-π*
(
c)
(d)
(e)
UV
λ=365 nm
Figure 4. For compound 1-6: (a) UV-visible spectroscopy in solid-state before (0 min) and after (1, 2, 5, 10, and 15 min) UV365 irradiation; (b) X-ray diffraction
patterns of the solid sample before (magenta line) and after (blue line) UV irradiation (λ=365 nm) along with 2D images (inset), at room temperature; (c)
photochromism of the solid sample: color switching from yellowish-orange to reddish-orange upon UV365 irradiation; FE-SEM images (d) in the pristine state, (e)
after irradiation with UV365 light (Scale bars: 100 nm, Magnification: X35,000).
4
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ChemPhysChem
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ARTICLE
behavior was observed upon UV light irradiation (UV365). The
comparative intensity of the peak at 346 nm (π-π* band)
decreased whereas the intensity of peak at 465 nm (n-π* band)
increased as UV365 was irradiated for 1, 2, 5, 10, 15 min (Figure
Table 2. Thermal properties of the final compounds of series 1-n and 2-n
recorded from DSC studies (heating and cooling rates: 10 °C min ).
-
1
-
1
Compound Phase transition temperatures in °C (ΔH in kJ mol )
4
a). The XRD patterns (powder XRD) revealed completely
1
2
1
2
-4
-4
-6
-6
Heating: Cr 167.7 (45.27) Iso
Cooling: Iso 153.0 Cr
different peaks in the small and the wide-angle region of before
[a]
(
(
magenta line) and after (blue line) UV light exposed samples
Figure 4b). This experiment confirmed the structural changes in
Heating: Cr 148.9 (46.62) N 154.1 (0.27) Iso
Cooling: Iso 134.8 (-0.02) N 108.3 (-5.85) Cr
the solid material upon exposure to UV light. The surface
morphology of the samples in two states (pristine and after UV
irradiation) was studied using FE-SEM (field emission scanning
electron microscopy) technique. The SEM micrograph of the
pristine sample (before UV) of compound 1-6 showed thick
bundles of clumsy and entangled fibers with a diameter in the
range of ~230-280 nm (Figure 4d). After UV irradiation,
comparatively thin and straight fibers with a reduced diameter of
1 2
Heating: Cr 98.1 (-27.96) Cr 150.6 (25.96) Iso
Cooling: Iso 123.8 (-0.04) N 58.2 Cr
[
a]
Heating: Cr 130.9 (46.80) Iso
Cooling: Iso 123.6 (-0.03) N 97.4 Cr[
a]
Heating: Cr[ 163.4 N[a] 167.0 Iso
a]
[a]
1-8
Cooling: Iso[ 128.1 N 100.9 Cr
a]
[a]
[a]
~
100-130 nm (Figure 4e) were observed. The variation in FE-
Heating: Cr[a] 120.1 N[a] 129.5 Iso
Cooling: Iso 130.1 N 97.0 Cr
[a]
2
-8
SEM images before and after UV irradiation, is attributed to the
structural changes i.e. trans and cis isomers, respectively.
Therefore, photoisomerization of the azobenzene unit reflected in
the physical color change, UV-visible spectroscopy, powder XRD
and FE-SEM results. Similar experiments were carried out in the
case of compound 2-4 to prove the photochromic nature and are
detailed in Supporting Information (Figures S48, S49).
[a]
[a]
[a]
1-12
2-12
1-18
Heating: Cr 127.1 (27.20) SmA 136.0 (1.07) Iso
Cooling: Iso 132.7 (-1.25) SmA 92.1 (-16.40) Cr
Heating: Cr 118.1 (38.43) SmA 128.6 (0.97) Iso
Cooling: Iso 123.9 (-0.53) SmA 95.8 (-29.64) Cr
Heating: Cr
1.88) Iso
Cooling: Iso 141.8 ˚C (-1.59) SmA 112.8 ˚C (-24.57) Cr
1 2
93.2 (26.53) Cr 129.1 (23.94) SmA 143.4
(
Thermal behavior: The thermal behavior of all the compounds
was investigated using a combination of polarized optical
microscopy (POM) and differential scanning calorimetry (DSC)
studies. All the compounds exhibited mesomorphic behavior
except 1-4. The compounds with shorter alkyl chains in both the
series (n = 4, 6, 8) displayed nematic mesomorphism (1-4 was
non-LC) whereas the ones with longer alkyl chains (n = 12, 18)
showed SmA phase. The textures were observed under POM with
the sample sandwiched between a normal glass slide and
2-18
Heating: Cr
0.64) Iso
Cooling: Iso 141.7 (-0.19) SmA 90.6 Cr
1 2
105.2 (34.77) Cr 119.2 (32.48) SmA 148.5
(
[
a]
[a]Transition temperatures observed from POM and SAXS/WAXS studies.
Abbreviations: Cr = crystal, N = nematic, SmA= smectic A, Iso = isotropic liquid.
phase range of 26.2 °C. Similarly, compound 1-6 also showed an
enantiotropic nematic phase (Figure S50b). Homologues with
longer alkyl chains, for example, compound 1-18 exhibited focal-
conic textures characteristic of the SmA phase (Figure 5b) from
-1
coverslip. Upon cooling from the isotropic melt (rate: 5 °C min ),
the compound 2-6 displayed schlieren texture of the nematic
phase (Figure 5a) and crystallized at 97.4 °C with a nematic
141.8 °C up to crystallization at 112.8 °C and likewise, compound
2
-18 displayed SmA phase (Figure S50i). The textures of the
(
a)
(c)
remaining mesomorphic compounds are presented in Figure S50
in the Supporting Information. The mesophases observed under
POM were further confirmed by SAXS/WAXS studies. The
differential scanning calorimetry (DSC) studies provided the
phase transition temperatures and the associated enthalpies of all
the homologues of series 1-n and 2-n as mentioned in Table 2.
The DSC spectra of two representative compounds are shown in
Figure 5c,d and the rest are presented in the Supporting
Information (Figure S51).
(
b)
(d)
Temperature-dependent SAXS/WAXS studies: The intrinsic
nature of the self-assembled superstructures of the LC materials
was investigated by the temperature-dependent SAXS/WAXS
experiments. The XRD profiles of representative compounds from
both 1-n and 2-n series have been presented in Figure 6. In
compound 2-18, SmA phase was confirmed by the presence of
two peaks with layer spacings as d = 66.40 Å and d = 33.22 Å in
the small-angle region at 115 °C (Figure 6a). The wide-angle
region showed a diffused halo at d ~ 4.93 Å due to the liquid-like
correlation of the alkyl chains (Figure S52d). The first and second
peaks were assigned reflections of the (100) and (200) planes due
Figure 5. Optical micrographs of two representative compounds on a normal
glass slide with coverslip: (a) schlieren texture of the N phase of 2-6 at 123.3 °C
and (b) fan-shaped texture of the SmA phase of 1-18 at 126.0 °C (Crossed
polarizers, Magnification: X100). DSC thermograms (scan rate: 10 °C min ) of
compounds (c) 2-6 and (d) 1-18.
-
1
5
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ChemPhysChem
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ARTICLE
(a)
Wide-angleview
Small-angleview
(
100)
(c)
(
200)
(
b)
Wide-angleview
Small-angleview
(d)
(
100)
(
200)
Figure 6. Intensity vs 2θ profile in the SAXS region displaying (100) peak (the (200) peak is zoomed in the inset) along with 2D diffraction images of the small as
well as the wide-angle region (inset) in SmA phase of compounds: (a) 2-18 at 115 °C and (b) 1-18 at 140 °C. Variation of correlation length (ξ) and d-spacing
corresponding to the first peak (100) as a function of temperature of compounds: (c) 2-18 and (d) 1-18.
to their ratio of layer spacings (66.40/33.22 ~ 2:1). The calculated
d/L (where d = layer spacing, L = molecular length from DFT
optimized structure) value for compound 2-18 at 115 °C was
found to be 1.34 (d = 66.40 Å, L = 49.60 Å; d/L = 1.34 i.e. d > L)
and indicated the formation of partially bilayer Smectic A phase
monomeric form supports the anti-parallel arrangement of the
molecules with a highly electronegative –NO end group.
Likewise, compound 1-18 displayed three peaks with layer
2
i.e. SmA
d
.[45] Due to the presence of a polar terminal group, an
antiparallel arrangement of the molecules is expected to minimize
the overall dipole moment of the bulk material.[16,17,46,47] The layer
spacing d could be assumed as the length of the antiparallel
dimeric arrangement of neighboring molecules with overlapping
of the rigid cores. To understand this, we proposed dimers of
two representative compounds 1-8 and 2-8 (Figure 7) with the
help of Density Functional Theory (DFT) calculations (by
employing B3LYP hybrid functional and the 6-311G (d,p) basis
set). In the case of compound 1-8, the layer spacing (d ~ 48.36-
1-8
[48]
53.00 Å) calculated from the SAXS/WAXS studies was
approximately equivalent to the length of the antiparallel dimeric
arrangement of molecules (d ~ 47.28 Å) calculated from the DFT
studies. Similar results were observed in compound 2-8 (where d
2
-8
~
48.03-53.00 Å from SAXS/WAXS studies and probable d ~
47.58 Å from DFT studies).
Table 3 represents the calculated binding energy and resultant
dipole moment of the respective dimers. There was not much
difference observed in the binding energy of both the dimers
probably due to the similarity in the composition of the structures.
The decrease in the resultant dipole moment of the dimer from its
Figure 7. Proposed dimers of compounds 1-8 and 2-8 in a Cartesian coordinate
frame, optimized by DFT calculations (B3LYP and 6-311G (d,p)).
6
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The instrumental details for structural characterization are similar to as
mentioned in our previous papers[46,47,49] and briefed here for the reader’s
convenience. Structural characterization of the compounds was carried
out with a combination of Attenuated total reflectance (ATR; FTIR Bruker-
ALPHA Bruker-1227-3513), UV-Vis-NIR spectrophotometer (LABINDIA
UV-Vis Spectrophotometer 3000+), High Resolution Mass Spectrometry
Table 3. The resultant dipole moments of the monomers and the dimers and
the binding energy in the dimer form.
Compound
µ
resultant of
µ
resultant
Binding energy
-
1
monomer
(
of dimer
(Debye)
(kJ mol )
Debye)
(
Waters synapt g2s), 1H NMR and 13C NMR (Bruker Biospin Switzerland
Avance III 400 MHz and 100 MHz spectrometers, respectively). NMR
spectra were recorded using deuterated chloroform (CDCl ) as solvent and
1
-8
-8
8.02
8.07
1.27
0.76
|34.25|
|35.07|
3
2
tetramethylsilane (TMS) as an internal standard. FESEM was performed
by using a JEOL JSM- 7600F instrument. Polarising Optical Microscopy
(
POM) textural observations of the mesophases were performed with
spacings as d = 65.98 Å (100 peak), d = 33.06 Å (200 peak) and
d = 5.08 Å at 140 °C (Figure 6b). The d/L was calculated to be
Nikon Eclipse LV100POL polarising microscope provided with a Linkam
heating stage (LTS 420). Differential Scanning Calorimetry (DSC)
measurements were performed on Perkin Elmer DSC 8000 coupled to a
Controlled Nitrogen Accessory (CLN2) with a scan rate of 10 ˚C min-1 both
1.34 (= 65.98/49.40) and therefore, the phase was designated as
partially bilayer Smectic A phase (SmA ) phase. The temperature-
d
dependent SAXS/WAXS profiles of compound 1-18 are
presented in the Supporting Information (Figure S52c). The
correlation length (ξ) and d-spacing increased on decreasing the
temperature in the entire smectic region for both compounds 2-18
and 1-18 as shown in their respective Figures 6c,d (Table S2 in
the Supporting Information). In addition to this, upon lowering the
temperature from the isotropic liquid, the intensity of the (100) and
on heating and cooling. X-ray Diffraction (XRD) studies were carried out
on samples using CuKα (λ = 1.54 Å) radiation from GeniX3D microsource
operating at 50kV and 0.6 mA, using Pilatus 200K detector in Xeuss 2.0
SAXS/WAXS system.
General procedure for the synthesis of 1-n and 2-n
The synthetic route for the preparation of the asymmetrical azo-based
bent-shaped compounds is presented in Scheme 1 in the manuscript. The
final compounds were prepared by performing a Schiff’s base reaction.
(
200) peaks in the small-angle region gradually increased
throughout the SmA phase range in both compounds 1-18 and 2-
8 (Figure S52c,d, respectively).
It was observed that varying the position of the –CH
two series did not affect the molecular arrangement i.e. d ~ 66 Å
100 peak) and d ~ 33 Å (200 peak) in the case of both
compounds 2-18 and 1-18. The presence of a highly polar
terminal –NO group forced the antiparallel bilayer arrangement
1
Compound 4.1 or 4.2 (1 eq.) was refluxed in
a mixture of
3
group in the
methanol:dichloromethane (CH OH:CH Cl :: 50:50) with slow addition of
3
2
2
7.n (1 eq.) dissolved in CH OH with a few drops of glacial acetic acid as
3
(
catalyst for 4 hours. The crude precipitate was dissolved in hot boiling
methanol and the insoluble yellow-orange solid was collected as the pure
product. Yield ~ 78 %
2
in the smectic phase.
Compound 1-4:
The XRD profiles of compounds 1-8 and 2-8 are discussed
elaborately in the Supporting Information (Figure S52a,b, Table
S2, and Figure S53).
1H NMR (400 MHz, CDCl
H, J = 8.0 Hz, Ar-H), 8.35 (d, 2H, J = 12.0 Hz, Ar-H), 8.20 (d, 2H, J = 8.0
Hz, Ar-H), 8.08-8.03 (m, 4H, Ar-H), 7.95-7.91 (m, 2H, Ar-H), 7.53 (d, 1H, J
8.0 Hz, Ar-H), 7.38 (d, 1H, J = 8.0 Hz, Ar-H), 7.24 (d, 1H, J = 4.0 Hz, Ar-
H), 7.17 (dd, 1H, J = 8.0, 4.0 Hz, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H), 4.07
t, 2H, J = 8.0 Hz, -OCH -), 2.38 (s, 3H, -CH ), 1.86-1.79 (m, 2H, -CH -),
-), 1.00 (t, 3H, J = 8.0 Hz, -CH
). 13C NMR (100
, 훿 in ppm): 훿 = 13.83, 16.53, 19.20, 31.11, 68.05, 114.42,
, 훿 in ppm): 훿 = 8.59 (s, 1H, -CH=N-), 8.40 (d,
3
2
=
Conclusion
(
2
3
2
1.54-1.48 (m, 2H, -CH
2
3
In summary, we have synthesized and investigated two new
series of azo-based polar hockey-stick-shaped liquid crystals
MHz, CDCl
3
1
1
1
16.30, 119.97, 120.61, 122.63, 122.99, 123.44, 124.77, 125.07, 126.03,
(
HSLCs) and studied their photoswitching behavior in solution as
29.08, 130.64, 130.69, 131.37, 131.70, 132.57, 140.44, 147.80, 148.69,
50.25, 150.84, 152.56, 155.65, 159.70, 163.87, 163.92, 163.96. ATR
well as in solid-state. UV-visible and 1H NMR spectroscopic
techniques explained the solution state photo-isomerization in the
compounds. Solid-state photochromism was observed i.e. color
change from yellowish-orange to reddish-orange upon UV light
illumination. This observation was well supported by changes in
solid-state UV-visible spectroscopy, powder XRD patterns, and
FE-SEM images. The optical textures revealed the presence of N
and SmA mesophases in the synthesized compounds that was
further supported by DSC and temperature-dependent
SAXS/WAXS studies. The N phase appeared in the lower
homologues (n = 4, 6, 8) whereas SmA phase was observed in
the higher ones (n = 12, 18) of both the series. Overall, the studies
reported here would be useful for a general understanding of the
photo-isomerization process in azo-based LC materials, and the
advancement of molecular design of suitable photo-switching
materials for technological applications.
-1
(ν
max in cm ): 1733 (νC=O, ester), 1627 (νCH=N, imine), 1608 (νC=C, aromatic),
1525 (νN-O, nitro), 1259 (νC-O, ester), 754 (νC-Cl). HRMS (ESI-MS): m/z
+
calculated for C38
Compound 1-6:
1H NMR (400 MHz, CDCl
H
32ClN
4
O
7
[M+H] : 691.1959; found: 691.1985.
3
, 훿 in ppm): 훿 = 8.59 (s, 1H, -CH=N-), 8.40 (d,
2H, J = 8.0 Hz, Ar-H), 8.34 (d, 2H, J = 8.0 Hz, Ar-H), 8.20 (d, 2H, J = 8.0
Hz, Ar-H), 8.08-8.03 (m, 4H, Ar-H), 7.95-7.91 (m, 2H, Ar-H), 7.52 (d, 1H, J
=
12.0 Hz, Ar-H), 7.38 (d, 1H, J = 8.0 Hz, Ar-H), 7.24 (d, 1H, J = 4.0 Hz,
Ar-H), 7.17 (dd, 1H, J = 8.0, 4.0 Hz, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H),
.06 (t, 2H, J = 8.0 Hz, -OCH -), 2.38 (s, 3H, -CH ), 1.87-1.80 (m, 2H, -
CH -), 1.52-1.45 (m, 2H, -CH -), 1.38-1.34 (m, 4H, -(CH -), 0.92 (t, 3H, J
). 13C NMR (100 MHz, CDCl
8.0 Hz, -CH , 훿 in ppm): 훿 = 14.05, 16.53,
4
2
3
2
2
2 2
)
=
3
3
22.60, 25.66, 29.04, 31.54, 68.37, 114.43, 116.30, 119.97, 120.60, 122.63,
122.99, 123.44, 124.77, 125.07, 126.03, 129.08, 130.64, 130.69, 131.37,
1
1
1
31.70, 132.57, 140.44, 147.81, 148.69, 150.25, 150.84, 152.56, 155.65,
59.70, 163.87, 163.92, 163.96. ATR (νmax in cm ): 2936 (νC-H, alkanes),
738 (νC=O, ester), 1628 (νCH=N, imine), 1609 (νC=C, aromatic), 1528 (νN-O,
-
1
Experimental Section
nitro), 1258 (νC-O, ester), 755 (νC-Cl). HRMS (ESI-MS): m/z calculated for
C
+
H36ClN O
40 4 7
[M+H] : 719.2272; found: 719.2299.
General
7
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.202100215
ARTICLE
Compound 1-8:
1611 (νC=C, aromatic), 1528 (νN-O, nitro), 1259 (νC-O, ester), 755 (νC-Cl).
+
HRMS (ESI-MS): m/z calculated for C38
H32ClN O
4 7
[M+H] : 691.1959;
1H NMR (400 MHz, CDCl
found: 691.1991.
3
, 훿 in ppm): 훿 = 8.59 (s, 1H, -CH=N-), 8.40 (d,
2
H, J = 8.0 Hz, Ar-H), 8.34 (d, 2H, J = 8.0 Hz, Ar-H), 8.20 (d, 2H, J = 8.0
Hz, Ar-H), 8.08-8.03 (m, 4H, Ar-H), 7.95-7.91 (m, 2H, Ar-H), 7.52 (d, 1H, J
8.0 Hz, Ar-H), 7.38 (d, 1H, J = 8.0 Hz, Ar-H), 7.24 (d, 1H, J = 4.0 Hz, Ar-
H), 7.17 (d, 1H, J = 8.0 Hz, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H), 4.06 (t,
Compound 2-6:
=
1H NMR (400 MHz, CDCl
3
, 훿 in ppm): 훿 = 8.57 (s, 1H, -CH=N-), 8.39 (d,
2
1
CH
H, J = 6.0 Hz, -OCH
.45 (m, 2H, -CH -), 1.40-1.29 (m, 8H, -(CH
). 13C NMR (100 MHz, CDCl
, 훿 in ppm): 훿 = 14.12, 16.53, 22.67, 25.98,
2
-), 2.38 (s, 3H, -CH
3
), 1.87-1.80 (m, 2H, -CH
2
-), 1.52-
2
H, J = 8.0 Hz, Ar-H), 8.31 (d, 2H, J = 8.0 Hz, Ar-H), 8.19 (d, 2H, J = 8.0
Hz, Ar-H), 8.04 (t, 4H, J = 8.0 Hz, Ar-H), 7.82 (d, 1H, J = 8.0 Hz, Ar-H),
.52 (d, 1H, J = 8.0 Hz, Ar-H), 7.30 (d, 1H, J = 2.5 Hz, Ar-H), 7.23 (d, 1H,
J = 2.2 Hz, Ar-H), 7.14-7.20 (m, 2H, Ar-H), 7.00 (d, 2H, J = 12.0 Hz, Ar-H),
.05 (t, 2H, J = 6.0 Hz, -OCH -), 2.81 (s, 3H, -CH ), 1.86-1.79 (m, 2H, -
CH -), 1.52-1.45 (m, 2H, -CH -), 1.38-1.33 (m, 4H, -(CH -), 0.92 (t, 3H, J
). 13C NMR (100 MHz, CDCl
8.0 Hz, -CH , 훿 in ppm): 훿 = 14.03, 17.71,
2
2 4
) -), 0.90 (t, 3H, J = 6.0 Hz, -
3
3
7
2
1
1
1
1
9.08, 29.23, 29.33, 31.81, 68.38, 114.43, 116.30, 119.97, 120.60, 122.63,
22.99, 123.44, 124.77, 125.07, 126.03, 129.08, 130.63, 130.69, 131.37,
31.70, 132.57, 140.44, 147.81, 148.69, 150.25, 150.84, 152.56, 155.65,
59.70, 163.87, 163.92, 163.96. ATR (νmax in cm ): 2923 (νC-H, alkanes),
741, 1730 (νC=O, ester), 1625 (νCH=N, imine), 1608 (νC=C, aromatic), 1526
4
2
3
2
2
2 2
)
-
1
=
2
1
1
1
3
3
2.59, 25.66, 29.05, 31.54, 68.39, 114.44, 116.31, 116.94, 119.93, 120.02,
20.65, 123.52, 124.28, 124.77, 125.05, 129.02, 130.62, 130.66, 131.59,
32.56, 140.39, 141.61, 147.83, 148.28, 148.62, 150.86, 153.90, 155.92,
(ν
N-O, nitro), 1262 (νC-O, ester), 755 (νC-Cl). HRMS (ESI-MS): m/z calculated
+
for C42
H40ClN O
4 7
[M+H] : 747.2585; found: 747.2554.
-
1
59.72, 163.87, 163.94, 164.22. ATR (νmax in cm ): 2934 (νC-H, alkanes),
Compound 1-12:
1735 (νC=O, ester), 1624 (νCH=N, imine), 1611 (νC=C, aromatic), 1528 (νN-O
,
nitro), 1257 (νC-O, ester), 754 (νC-Cl). HRMS (ESI-MS): m/z calculated for
+
1H NMR (400 MHz, CDCl
C H36ClN O
40 4 7
[M+H] : 719.2272; found: 719.2242.
3
, 훿 in ppm): 훿 = 8.59 (s, 1H, -CH=N-), 8.40 (d,
2
H, J = 8.0 Hz, Ar-H), 8.34 (d, 2H, J = 8.0 Hz, Ar-H), 8.20 (d, 2H, J = 8.0
Hz, Ar-H), 8.08-8.03 (m, 4H, Ar-H), 7.95-7.91 (m, 2H, Ar-H), 7.52 (d, 1H, J
8.0 Hz, Ar-H), 7.38 (d, 1H, J = 8.0 Hz, Ar-H), 7.24 (d, 1H, J = 4.0 Hz, Ar-
H), 7.17 (dd, 1H, J = 8.0, 4.0 Hz, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H), 4.06
t, 2H, J = 6.0 Hz, -OCH -), 2.38 (s, 3H, -CH ), 1.86-1.79 (m, 2H, -CH -),
-), 1.39-1.27 (m, 16H, -(CH -), 0.88 (t, 3H, J = 6.0
, 훿 in ppm): 훿 = 14.13, 16.52, 22.70,
Compound 2-8:
=
1H NMR (400 MHz, CDCl
3
, 훿 in ppm): 훿 = 8.57 (s, 1H, -CH=N-), 8.39 (d,
(
2
3
2
2
H, J = 8.0 Hz, Ar-H), 8.31 (d, 2H, J = 8.0 Hz, Ar-H), 8.19 (d, 2H, J = 8.0
Hz, Ar-H), 8.04 (t, 4H, J = 8.0 Hz, Ar-H), 7.82 (d, 1H, J = 8.0 Hz, Ar-H),
.52 (d, 1H, J = 8.0 Hz, Ar-H), 7.30 (d, 1H, J = 1.2 Hz, Ar-H), 7.23 (d, 1H,
J = 2.4 Hz, Ar-H), 7.20-7.14 (m, 2H, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H),
.06 (t, 2H, J = 6.0 Hz, -OCH -), 2.81 (s, 3H, -CH ), 1.86-1.79 (m, 2H, -
CH -), 1.52-1.44 (m, 2H, -CH -), 1.39-1.29 (m, 8H, -(CH -), 0.90 (t, 3H, J
). 13C NMR (100 MHz, CDCl
6.0 Hz, -CH , 훿 in ppm): 훿 = 14.12, 17.72,
1.51-1.44 (m, 2H, -CH
2
2 8
)
Hz, -CH
3
). 13C NMR (100 MHz, CDCl
3
7
2
1
1
5.99, 29.09, 29.36, 29.57, 29.60, 29.64, 29.66, 31.93, 68.41, 114.46,
16.31, 119.95, 120.65, 122.62, 123.00, 123.44, 124.77, 125.08, 126.05,
29.09, 130.63, 130.69, 131.41, 131.71, 132.57, 140.48, 147.84, 148.74,
50.28, 150.86, 152.59, 155.68, 159.67, 163.89, 163.95. ATR (νmax in cm-
4
2
3
2
2
2 4
)
1
=
2
1
1
1
3
3
1
)
: 2917 (νC-H, alkanes), 1736 (νC=O, ester), 1625 (νCH=N, imine), 1608 (νC=C,
2.67, 25.98, 29.08, 29.23, 29.32, 31.81, 68.37, 114.42, 116.30, 116.91,
19.95, 120.03, 120.59, 123.53, 124.28, 124.77, 125.04, 129.02, 130.62,
30.66, 131.56, 132.56, 140.36, 141.63, 147.80, 148.25, 148.59, 150.84,
aromatic), 1528 (νN-O, nitro), 1260 (νC-O, ester), 754 (νC-Cl). HRMS (ESI-
+
MS): m/z calculated for C46H48ClN O
4 7
[M+H] : 803.3211; found: 803.3231.
-
1
53.87, 155.88, 159.74, 163.86, 163.95, 164.23. ATR (νmax in cm ): 2923
Compound 1-18:
(νC-H, alkanes), 1734 (νC=O, ester), 1623 (νCH=N, imine), 1607 (νC=C,
aromatic), 1529 (νN-O, nitro), 1256 (νC-O, ester), 755 (νC-Cl). HRMS (ESI-
+
1H NMR (400 MHz, CDCl
MS): m/z calculated for C42H40ClN O
4 7
[M+H] : 747.2585; found: 747.2612.
3
, 훿 in ppm): 훿 = 8.59 (s, 1H, -CH=N-), 8.40 (d,
2
H, J = 8.0 Hz, Ar-H), 8.34 (d, 2H, J = 8.0 Hz, Ar-H), 8.20 (d, 2H, J = 8.0
Hz, Ar-H), 8.08-8.03 (m, 4H, Ar-H), 7.95-7.91 (m, 2H, Ar-H), 7.52 (d, 1H, J
8.0 Hz, Ar-H), 7.38 (d, 1H, J = 8.0 Hz, Ar-H), 7.24 (d, 1H, J = 4.0 Hz, Ar-
H), 7.17 (d, 1H, J = 8.0 Hz, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H), 4.06 (t,
Compound 2-12:
=
1H NMR (400 MHz, CDCl
3
, 훿 in ppm): 훿 = 8.58 (s, 1H, -CH=N-), 8.40 (d,
2
1
H, J = 6.0 Hz, -OCH
.44 (m, 2H, -CH -), 1.36-1.26 (m, 28H, -(CH
). 13C NMR (100 MHz, CDCl
, 훿 in ppm): 훿 = 14.13, 16.52, 22.70,
2
-), 2.38 (s, 3H, -CH
3
), 1.87-1.79 (m, 2H, -CH
2
-), 1.51-
2
H, J = 8.0 Hz, Ar-H), 8.32 (d, 2H, J = 8.0 Hz, Ar-H), 8.20 (d, 2H, J = 12.0
Hz, Ar-H), 8.05 (t, 4H, J = 10.0 Hz, Ar-H), 7.83 (d, 1H, J = 12.0 Hz, Ar-H),
.52 (d, 1H, J = 8.0 Hz, Ar-H), 7.31 (d, 1H, J = 4.0 Hz, Ar-H), 7.23 (d, 1H,
J = 2.4 Hz, Ar-H), 7.21-7.15 (m, 2H, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H),
.06 (t, 2H, J = 6.0 Hz, -OCH -), 2.81 (s, 3H, -CH ), 1.86-1.79 (m, 2H, -
CH -), 1.51-1.44 (m, 2H, -CH -), 1.39-1.27 (m, 16H, -(CH -), 0.88 (t, 3H,
). 13C NMR (100 MHz, CDCl
J = 6.0 Hz, -CH , 훿 in ppm): 훿 = 14.17, 17.75,
2
2
)
14-), 0.88 (t, 3H, J = 6.0 Hz,
-
CH
3
3
7
2
1
1
5.99, 29.09, 29.37, 29.57, 29.60, 29.67, 29.71, 31.93, 68.40, 114.45,
16.32, 119.95, 120.64, 122.61, 122.99, 123.44, 124.76, 125.07, 126.05,
29.08, 130.63, 130.68, 131.40, 131.70, 132.56, 140.47, 147.83, 148.72,
50.27, 150.85, 152.59, 155.67, 159.67, 163.88, 163.95. ATR (νmax in cm-
4
2
3
2
2
2 8
)
1
3
3
1
)
: 2916 (νC-H, alkanes), 1735 (νC=O, ester), 1624 (νCH=N, imine), 1607 (νC=C,
2
1
1
1
2.73, 26.00, 29.10, 29.39, 29.59, 29.62, 29.67, 29.69, 31.95, 68.41,
14.46, 116.35, 116.94, 119.99, 120.05, 120.63, 123.56, 124.31, 124.80,
25.07, 129.05, 130.65, 130.69, 131.59, 132.59, 140.39, 141.66, 147.83,
48.28, 148.62, 150.87, 153.91, 155.92, 159.78, 163.89, 163.98, 164.26.
aromatic), 1526 (νN-O, nitro), 1259 (νC-O, ester), 755 (νC-Cl). HRMS (ESI-
MS): m/z calculated for C52
+
H60ClN O
4 7
[M+H] : 887.415; found: 887.4182.
-
1
Compound 2-4:
ATR (νmax in cm ): 2917 (νC-H, alkanes), 1748, 1736 (νC=O, ester), 1624
CH=N, imine), 1609 (νC=C, aromatic), 1530 (νN-O, nitro), 1258 (νC-O, ester),
755 (νC-Cl). HRMS (ESI-MS): m/z calculated for C46 48ClN
[M+H]+:
03.3211; found: 803.3246.
(ν
1H NMR (400 MHz, CDCl
H
4 7
O
3
, 훿 in ppm): 훿 = 8.58 (s, 1H, -CH=N-), 8.40 (d,
8
2
H, J = 8.0 Hz, Ar-H), 8.32 (d, 2H, J = 8.0 Hz, Ar-H), 8.20 (d, 2H, J = 12.0
Hz, Ar-H), 8.05 (t, 4H, J = 10.0 Hz, Ar-H), 7.83 (d, 1H, J = 12.0 Hz, Ar-H),
.52 (d, 1H, J = 8.0 Hz, Ar-H), 7.31 (d, 1H, J = 4.0 Hz, Ar-H), 7.24 (d, 1H,
J = 4.0 Hz, Ar-H), 7.21-7.15 (m, 2H, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H),
.07 (t, 2H, J = 6.0 Hz, -OCH -), 2.81 (s, 3H, -CH ), 1.86-1.79 (m, 2H, -
CH -), 1.53-1.50 (m, 2H, -CH -), 1.00 (t, 3H, J = 8.0 Hz, -CH
). 13C NMR
100 MHz, CDCl , 훿 in ppm): 훿 = 13.83, 17.72, 19.19, 31.11, 68.05, 114.42,
7
Compound 2-18:
4
2
3
1H NMR (400 MHz, CDCl
3
, 훿 in ppm): 훿 = 8.58 (s, 1H, -CH=N-), 8.40 (d,
2
2
3
2
H, J = 8.0 Hz, Ar-H), 8.32 (d, 2H, J = 8.0 Hz, Ar-H), 8.20 (d, 2H, J = 12.0
Hz, Ar-H), 8.05 (t, 4H, J = 10.0 Hz, Ar-H), 7.83 (d, 1H, J = 8.0 Hz, Ar-H),
.52 (d, 1H, J = 8.0 Hz, Ar-H), 7.31 (d, 1H, J = 4.0 Hz, Ar-H), 7.23 (d, 1H,
J = 2.4 Hz, Ar-H), 7.21-7.15 (m, 2H, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H),
.06 (t, 2H, J = 6.0 Hz, -OCH -), 2.81 (s, 3H, -CH ), 1.86-1.79 (m, 2H, -
(
3
116.30, 116.91, 119.95, 120.03, 120.61, 123.53, 124.29, 124.77, 125.04,
129.02, 130.62, 130.67, 131.57, 132.56, 140.36, 141.63, 147.80, 148.26,
148.59, 150.85, 153.87, 155.89, 159.74, 163.86, 163.94, 164.23. ATR
7
4
2
3
-
1
(νmax in cm ): 2968 (νC-H, alkanes), 1736 (νC=O, ester), 1627 (νCH=N, imine),
8
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.202100215
ARTICLE
CH
2
-), 1.51-1.44 (m, 2H, -CH
). 13C NMR (100 MHz, CDCl
2.70, 25.99, 29.09, 29.37, 29.57, 29.60, 29.67, 29.71, 31.93, 68.39,
2
-), 1.39-1.26 (m, 28H, -(CH
2
)
14-), 0.88 (t, 3H,
[24] M. K. Paul, G. Kalita, A. R. Laskar, S. Debnath, V. Gude, D. D. Sarkar,
G. Mohiuddin, S. K. Varshney, N. V. S. Rao, J. Mol. Struct. 2013, 1049,
78-89.
J = 6.0 Hz, -CH
3
3
, 훿 in ppm): 훿 = 14.13, 17.71,
2
1
1
1
1
1
14.44, 116.32, 116.93, 119.93, 120.02, 120.64, 123.52, 124.27, 124.76,
25.05, 129.02, 129.70, 130.62, 130.66, 130.83, 131.59, 132.56, 140.38,
41.61, 147.82, 148.27, 148.62, 150.85, 153.90, 155.91, 159.71, 163.87,
63.93, 164.21. ATR (νmax in cm ): 2934 (νC-H, alkanes), 1735 (νC=O, ester),
624 (νCH=N, imine), 1611 (νC=C, aromatic), 1528 (νN-O, nitro), 1257 (νC-O
[25] K. Upadhyaya, V. Gude, G. Mohiuddin, N. V. S. Rao, Beilstein J. Org.
Chem. 2013, 9, 26-35.
[26] D. D. Sarkar, R. Deb, N. Chakraborty, N. V. S. Rao, Liq. Cryst. 2012, 39,
1003-1010.
-
1
,
[27] V. Gude, K. Upadhyaya, G. Mohiuddin, N. V. S. Rao, Liq. Cryst. 2013,
40, 120-129.
ester), 754 (νC-Cl). HRMS (ESI-MS): m/z calculated for C52
H60ClN O
4 7
+
[M+H] : 887.415; found: 887.4191.
[28] M. Alaasar, M. Prehm, C. Tschierske, Liq. Cryst. 2014, 41, 126-136.
[
29] M. Alaasar, S. Poppe, C. Tschierske, Liq. Cryst. 2017, 44, 729-737.
30] N. Sebastián, S. Belau, A. Eremin, M. Alaasar, M. Prehm, C. Tschierske,
Phys. Chem. Chem. Phys. 2017, 19, 5895-5905.
[
Acknowledgements
[
31] M. Alaasar, M. Prehm, C. Tschierske, Chem. Eur. J. 2016, 22, 6583-6597.
32] M. Alaasar, M. Prehm, C. Tschierske, RSC Adv. 2016, 6, 82890-82899.
[
SKP acknowledges SERB Project (CRG/2019/000901/OC). SK
acknowledges CSIR (09/947(0254)/2020-EMR-I) and IISER
Mohali for PhD fellowship. N.B. acknowledges DST, India for the
Women Scientist-A (SR-WOS-A-CS-26-2017) award. GM
acknowledges DST-SERB, India for NPDF (NPDF/2016/000560).
We thank IISER Mohali for all central, departmental facilities, in
particular, for access to the NMR, HRMS, FESEM, and
SAXS/WAXS system.
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New Azobenzene based polar hockey-stick-shaped liquid crystals displaying photo-switching behavior
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