Development of nematic and orthogonal smectic phases in short-core fluorinated hockey-stick shaped liquid crystal compounds

Article abstract of DOI:10.1016/j.molliq.2019.111989

Here we report synthesis and investigations on novel short-core hockey-stick shaped molecules bearing a 4-n-alkyloxy-2-hydroxybenzylidene moiety at one end and polar fluoro-biphenyl moiety at the other end. The polar chloro and fluoro substituent at the lateral position of the core confers nematic and orthogonal smectic mesomorphism. The mesophase morphology is characterized by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), electro-optical study and dielectric spectroscopy. The fluorinated compounds 1b (n = 16) and 1c (n = 18) have long terminal hydrocarbon tail and show orthogonal smectic phases with antiferroelectric switching behaviour. The chlorinated compound 2e (n = 8) with relatively short terminal chain length exhibits only a monotropic nematic phase on cooling. The nematic phase is comprised of cybotactic clusters as confirmed by XRD and exhibits electro-optic switching. The compound 2i (n = 12) manifests a short range enantiotropic nematic and additional orthogonal smectic phase. Dielectric spectroscopy reveals polar correlations in the nematic and smectic phases.

Full text of DOI:10.1016/j.molliq.2019.111989

Journal Pre-proof
Development of nematic and orthogonal smectic phases in short-core fluorinated
hockey-stick shaped liquid crystal compounds
Kalpana Upadhyaya, Sharmistha Ghosh, Raj Kumar Khan, R. Pratibha, Nandiraju
V.S. Rao
PII:
S0167-7322(17)35002-X
DOI:
https://doi.org/10.1016/j.molliq.2019.111989
MOLLIQ 111989
Reference:
To appear in: Journal of Molecular Liquids
Received Date: 20 October 2017
Revised Date: 29 July 2019
Accepted Date: 20 October 2019
Please cite this article as: K. Upadhyaya, S. Ghosh, R.K. Khan, R. Pratibha, N.V.S. Rao, Development
of nematic and orthogonal smectic phases in short-core fluorinated hockey-stick shaped liquid crystal
compounds, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.111989.
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Development of nematic and orthogonal smectic phases in short-core
fluorinated hockey-stick shaped liquid crystal compounds
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Kalpana Upadhyayaa, Sharmistha Ghosh*b, Raj Kumar Khan b, R. Pratibhac and
Nandiraju V. S. Rao*a
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aChemistry Department, Assam University, Silchar 788011, India,
bDepartment of Physics, University of Calcutta, 92 A PC Road, Kolkata 700 009, India,
cRaman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore 560 080, India
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Abstract:
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Here we report synthesis and investigations on novel short-core hockey-stick shaped molecules
bearing a 4-n-alkyloxy-2-hydroxybenzylidene moiety at one end and polar fluoro-biphenyl
moiety at the other end. The polar chloro and fluoro substituent at the lateral position of the core
confers nematic and orthogonal smectic mesomorphism. The mesophase morphology is
characterized by polarizing optical microscopy (POM), differential scanning calorimetry
(DSC), X-ray diffraction (XRD), electro-optical study and dielectric spectroscopy. The
fluorinated compounds 1b (n=16) and 1c (n=18) have long terminal hydrocarbon tail and show
orthogonal smectic phases with antiferroelectric switching behavior. The chlorinated compound
2e (n=8) with relatively short terminal chain length exhibits only a monotropic nematic phase on
cooling. The nematic phase is comprised of cybotactic clusters as confirmed by XRD and
exhibits electro-optic switching. The compound 2i (n=12) manifests a short range enantiotropic
nematic and additional orthogonal smectic phase. Dielectric spectroscopy reveals polar
correlations in the nematic and smectic phases.
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1. Introduction:
Dipolar intermolecular interactions stabilize the macroscopic polarization density, characteristic
of ferroelectric phases, in crystals. When the crystal is transformed into anisotropic layered
fluid the dipolar interactions become weak and hence the realization of ferroelectricity is
dependent on parallel alignment of the molecular dipoles within each layer. The most attractive
features of achiral bent-core compounds1-4 are their unusual mesomorphic behaviour, polar
order,5-10 and macroscopic chirality11. Recently ferroelectric properties have been
unambiguously demonstrated in the orthogonal fluid smectic (SmAPF and SmAPFmod) phases
exhibited by bent-core molecules possessing carbosilane moiety at one end and polar cyano or
trifluoromethyl moieties at other end of the molecule as terminal substituents.1-10 Further
orthogonal polar smectic phase with alternating polar order in adjacent layers (SmAPA) is
reported in compounds of unsymmetrical achiral banana shaped molecules.12-18 Very recently,
achiral unsymmetrical bent-shaped molecules possessing a layered, optically uniaxial,
orthogonal polar ordered structure with random direction of the layer polarization exhibiting
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SmAPR phase19-24 (SmA- stands for orthogonal smectic phase, P for polar layers and subscript R
stands for random orientation of polarization in adjacent layers) is reported in few compounds.
The polar orthogonal smectic phases are found to possess excellent potential applications to
develop fast electro-optic devices. However an orthogonal smectic phase with overall polar
order9-24 is rare and is an experimental challenge which needs special molecular design.
Bent-core nematic (BCN) phase has been potentially attractive from both scientific
understanding and application perspective owing to remarkable features like biaxiality25,
ferronematic phase26-29, giant flexoelectricity (which is yet to be confirmed)30 and twist-bend
nematic phase31-33. Polar dynamics of electro-optic switching in biaxial bent-core nematic liquid
crystal was first observed by Lee et.al, in an 1,3,4-oxadiazole based bent core LC27(a). However
the interpretation of the current peaks was based on biaxiality of the system which was not
proved unambiguously and also was not in line with the symmetry arguments of LC27(b). Later
similar bent-core molecules based on 1,2,4-oxadiazole core were reported exhibiting
ferroelectric switching in the nematic phase and the effect was attributed to reorganization of
the polar cybotactic groups.28-29 The existence of the cybotactic clusters was proved explicitly
on the basis of X-ray diffraction, computer simulation, dielectric measurements etc. Dielectric
spectroscopy in the cybotactic nematic phase revealed a peak with very large dielectric strength
appearing at low frequency region corresponding to relaxation of cybotactic clusters. However
achieving stable BCN was difficult as most of the BC molecules prefer stacking into layers
owing to steric interaction and form banana phases. To prohibit the tendency of the bent-core
molecules to lock into layers and promote stable nematic phases several tailoring in the
molecular structure were required such as substitution of bulky moiety at the core or asymmetry
in the terminal chains etc.34 Another promising approach was to reduce the number of benzene
rings in the core or/and reduce the bend angle. Wide range of nematic phases has been
successfully developed in four ring bent-core LCs with reduced bend angle (~145o).35-37 Further,
ferroelectric-like switching in the nematic phase in a four-ring compound has been reported
recently35.
Molecular architecture at the interface of conventional rod-like and bent-core molecules is
utmost important owing to their distinct structure-property relationship. One such structure is
hockey-stick shaped molecule where one arm of a bent-core is significantly larger than the
other. These molecules have attracted growing attention since they help us understand how
self-assembly of asymmetric molecules leads to development of varieties of mesophases38-41
.
Here we report two homologous series of hockey-stick liquid crystals with the same core
structure but having either chloro- or fluoro- lateral substituents at one of its rings. The
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molecular architecture is such that there is reduction in rotational disorder as well as a strong
dipole lies in the bent direction, facilitating orthogonal polar phases. The mesophase
morphology is characterized by polarizing optical microscopy (POM), differential scanning
calorimetry (DSC), X-ray diffraction (XRD), electro-optical and dielectric spectroscopy. It is
found that the chlorinated compound with short terminal chain length 2e (n=8) exhibits only a
monotropic nematic phase on cooling. The nematic phase is comprised of cybotactic clusters as
confirmed by XRD and dielectric spectroscopy. The higher homologue 2i (n=12) manifests a
short range enantiotropic nematic and an additional orthogonal smectic phase whereas the other
two compounds 1b (n=16) and 1c (n=18) show only polar orthogonal smectic phases with
antiferroelectric switching behavior. Dielectric spectroscopy reveals polar correlations in the
nematic and smectic phases.
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2. Experimental:
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2.1 General methods:
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We have synthesized two homologous series 1a-1c and 2a-2i. Among the 1a-1c homologous
series the n-heptyloxy homologue did not exhibit liquid crystalline phase while the n-
octadecyloxy exhibited monotropic phase. The n-hexadecyloxy homologue, 1b, exhibited
enantiotropic orthogonal smectic A phase with polar characteristics and will be described in the
next section. In the 2a-2i homologous series the lower homologues of n-butyloxy to n-octyloxy
homologues exhibited monotropic nematic phase, while the n-nonyloxy to n-undecyloxy
homologues exhibited monotropic nematic and orthogonal smectic A phases. However the n-
dodecyloxy homologue 2i exhibited enantiotropic nematic and smectic A phases. In this
manuscript we have presented the results on four hockey-stick shaped samples viz., 1b and 1c
of the homologous series I and 2e and 2i of the series II. Both homologous series possess
essentially the same core except the lateral fluorine atom in series I is substituted by chlorine
atom in series II. The samples were filled in planar aligned cells (E.H.C Japan), at temperatures
well above their isotropic transition temperatures. The cell is consisted of two parallel glass
plates whose inner surfaces are first coated with ITO and then a homogeneous aligning material
(Polyvinyl alcohol). Finally the two plates are rubbed in antiparallel directions and mylar
spacers are used to maintain uniform thickness of the cell. Dielectric measurements are done in
10µm thick cells whereas 4µm cells are used to study the electro-optical properties of the
samples. The temperature of the cells was controlled by a Mettler FP82 hot stage attached to a
FP90 temperature controller. For electro-optic measurement, HP33120A signal generator, F10A
voltage amplifier and DL1620 oscilloscope were used. Current response was studied by
polarization reversal method using a triangular wave field. Dielectric data were recorded in the
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frequency range from 10Hz to 13MHz using a HP4192A Impedance Analyzer. Thermal
behaviour of the samples was studied using differential scanning calorimetry (DSC) and texture
was observed by polarizing microscope (DMLP, Leica).X-ray diffraction studies were carried
on the compound 2e on powder samples using CuKα radiation and the DY 1042-Empyrean
Diffractometer System.
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2.2 Design and General Synthetic procedures of the hockey-stick compounds:
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The hockey-stick liquid crystals bearing a 4-n-alkyloxy-2-hydroxybenzylidene moiety at one
end and the replacement of an azobenzene or 2-hydroxybenzylidene or phenyl benzoate unit in
the longer arm of the bent molecule by a polar fluoro-biphenyl moiety, which lends stiffness to
the molecule with a strong dipole moment in the lateral direction, were designed. Further the
introduction of a 2-chloro moiety or 2,6-difluoro substituents in the 1,3-disubstituted phenyl
ring of a hockey-stick molecular architecture can generate an obtuse bend angle of ~145°,
which gives rise to an increase in bend angle as well as a strong dipole in the bending direction.
The reduced bend from ~120 to ~145° of the 1,3-phenyl moiety by the introduction of a chloro
moiety in the 2-position or fluoro substituents in 2- and 6- positions and with the decrease in the
number of rings from five or more to four in the molecular unit, places these compounds at the
borderline between classical rod-like LCs and bent-core mesogens. A combination of the lateral
dipole in the form of a fluoro-biphenyl moiety in one of the arms of the bent-core molecule,
while the central bent core possesses a chloro moiety projected at a location inside the
molecular core to represent the transverse dipole or the projected 2-, 6- difluoro substituents
contribute towards the transverse (2-fluoro) dipole as well as lateral (6-fluoro) dipole. The
other end of the molecule is linked to 4-n-alkyloxy-2-hydroxybenzaldehyde through an imine
moiety, which actually seems superior to the benzylidene aniline core and is more stable
towards hydrolysis due to intramolecular hydrogen bonding. The realization of such a
molecular architecture leads to a reduction in rotational disorder as well as a strong dipole in the
bent direction. If molecular interactions are strong enough then the molecular structure can
promote polar biaxial nematic or smectic phases.
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The starting material in the present study 4-n-alkyloxy-2-hydroxybenzaldehyde was prepared
by Williamson etherification of 2, 4-dihydroxybenzaldehyde with appropriate n-alkyl bromide.
2-chloro-3-nitrobenzoicacid and 2-, 6-difluoro-3-nitrobenzoicacid were converted into
corresponding amines by reduction under balloon pressure filled with hydrogen gas using 5%
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Pd-C. The crude products were then purified by column chromatography (silica gel 60-120
mesh, using dichloromethane/ethanol, (99.5:0.5::V/V) as eluent to give pure products 3-amino-
2-chlorobenzoicacid and 2-, 6-difluoro-3-aminobenzoicacidrespectively.
3-amino-2-
chlorobenzoicacid was condensed with 4-n-octyloxy (or n-dodecyloxy)-2-hydroxybenzaldhyde
in presence of few drops of glacial acetic acid to get 3-[N-(4-n-octyloxy (or n-dodecyloxy) 2-
hydroxybenzylidene)] amino 2-cholorobenzoicacid in good yields. 2-, 6-difluoro-3-
aminobenzoicacidwas
condensed
with
4-n-hexadecyloxy
(or
n-octadecyloxy)-2-
hydroxybenzaldhyde in presence of few drops of glacial acetic acid to get 4-[N-(4-n-
hexadecyloxy (or n-octadecyloxy) 2-hydroxybenzylidene)amino]-2,6-difluorobenzoicacid in
good yields.
3-[N-(4-n-octyloxy (or n-dodecyloxy) 2-hydroxybenzylidene)]amino 2-
cholorobenzoicacid was further esterified with 4'-fluoro-[1,1'-biphenyl]-4-ol using DCC-DMAP
reaction to yield the designed products 2e and 2i. 4-[N-(4-n-hexadecyloxy (or n-octadecyloxy)
2-hydroxybenzylidene) amino]-2,6-difluorobenzoicacid was further esterified with 4'-fluoro-
[1,1'-biphenyl]-4-ol using DCC-DMAP reaction to yield the designed products 1b and 1c
(Scheme 1). The compounds were further recrystallized repeatedly from ethanol to get the
pure samples. Elemental analyses of all the compounds were consistent with the proposed
1
molecular formulae. All the compounds were characterized by FTIR, UV-Vis and H NMR
studies. The synthetic procedures for all the proposed compounds as detailed in scheme 1 and
characterization of the compounds 1a-1c and 2a-2i (Scheme 1) by elemental analysis and
spectroscopic data are described in electronic supporting information (ESI). The assigned νO-H—
N stretching band confirmed the presence of intermolecular H-bonding in all the compounds.
The introduction of ortho hydroxyl group in benzylidene moiety not only enhances the stability
of the imines through intramolecular H-bonding to overcome the hydrolytic instability of the
molecules towards moisture but also enhances the transverse dipole-moment. The infrared
spectra of all the compounds exhibited characteristic bands in region 1625~1631 cm-1 (νCH=N
imine), 1743~1755 cm-1 (νC=O, ester), 3045~3201 cm-1 (νO-H, H-bonding).
,
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5

CHO
OH
CHO
OH
i
Y
RO
HO
iii
3
OH
3a-3i
N
X
O
ii
Y
Y
RO
OH
X = Y = F; R = CnH2n+1; n = 7: 6a,16:6b,18:6c
X = Cl, Y = H; R = CnH2n+1; n = 4-12: 7a-7i
OH
OH
O2N
H2N
X
O
X
O
4a: X = Y = F
4b: X = Cl; Y=H.
5a: X = Y = F
5b: X = Cl; Y=H
iv
Y
O
O
N
X
RO
OH
F
Series I: X = Y = F; R = CnH2n+1; n = 7:1a,16:1b,18:1c
Series II: X = Cl, Y = H; R = CnH2n+1; n = 4-12: 2a-2i
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Scheme 1: Synthetic details of compounds 1a - 1c, 2a- 2i. Reagent and conditions: i.
Dry acetone KHCO3, CnH2n+1Br (n = 4-12, 16, 18); KI, ꢀ, 48 h; ii. 5% Pd/C, H2, EtOAc
stirring 24 h; iii. Abs EtOH, AcOH, ꢀ, 6 h; iv. DCC, DMAP, DCM, stirring 48 h.
TABLE 1: Phase transition temperatures (oC) and liquid crystalline phase thermal range of the compounds 1a-1c (n = 7,
16, 18) and 2a-2i (n = 4- 12) recorded for second heating (first row) and second cooling (second row) cycles at 5oC/min
from DSC and confirmed by polarized optical microscopy. The enthalpies (∆H in kJ/mol) and entropies (∆S in J/mol.K)
respectively are presented in parentheses.
Y
O
N
O
X
H
O
OCnH2n+1
F
Code
n
7
X
F
Y
F
Phase transition temperatures oC (enthalpy kJ/mol; entropy J/mol.K))
Cr 114.2 (28.3, 73.2) Iso
1a
Cr 126.4 (28.3, 70.9) Iso
Cr 114.8(22.6, 58.4) SmA 117.9(9.75, 24.9) Iso
Cr 101.2 (27.4, 73.4) SmA 116.1(8.15, 20.9)Iso
Cr 117.8 (61.9, 158.4) Iso
16
18
F
F
F
F
1b
1c
6

Cr 99.0 (54.9, 147.6) SmA 112.6 (11.2, 29.0) Iso
Cr 122.9 (35.9, 90.9) Iso
4
5
6
7
8
9
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
H
H
2a
2b
2c
2d
2e
2f
Cr 83.5 (20.7, 58.2) N 110.7 (0.085, 0.22) Iso
Cr 117.2 (26.7, 68.6) Iso
Cr 74.2 (14.3, 41.3) N 106.4 (0.13, 0.35) Iso
Cr 133.2 (28.0, 68.9) Iso
Cr 79.0 (9.80, 27.8) N 105.4 (0.15, 0.41) Iso
Cr 114.5 (34.7, 89.7) Iso
Cr 92.9 (30.2, 82.6) N 100.5 (0.15, 0. 41) Iso
Cr 112.3 (32.6, 84.6) Iso
Cr 88.8 (26.6, 73.7) N 103.7 (0.15, 0.40) Iso
Cr 112.3 (34.8, 90.5) Iso
Cr 78.0 (24.5, 70.0) SmA 85.3 (0.22, 0. 63) N 103.1 (0.18, 0.48) Iso
Cr 109.0 (27.9, 72.9) Iso
10 Cl
11 Cl
12 Cl
2g
2h
2i
Cr 85.0 (20.9, 58.6) SmA 96.9 (0.33, 0.89) N 106.1 (0.23, 0.62) Iso
Cr 104.8 (38.3, 101.4) Iso
Cr 76.3 (31.3, 89.7) SmA 101.8 (0.88, 2.35) N 105.0 (0.30, 0.79) Iso
Cr 106.2 (27.0, 71.2) SmA 108.9 (0.83, 2.18) N 109.5 (0.45, 1.18) Iso
Cr 78.0 (24.1, 68.7) SmA 108.4 (1.27, 3.33) N 109.0 (0.47, 1.24) Iso
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3. Results and discussions:
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3.1 DSC and Polarizing optical microscopy:
The phase transition temperatures are determined through differential scanning calorimetry
(DSC) at a scan rate of 5°C in both heating and cooling cycles (Supplementary). A
representative DSC plot for the compound 2i is shown in Figure 1.
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16
21.5
21.0
20.5
20.0
19.5
19.0
18.5
105
106
107
Temperature o
108
109
110
C
bpF-2Cl-12
80
90
100
110
Temperature oC
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177
Figure 1. DSC plot of the compound 2i at a scan rate of 5°C in both heating and cooling cycles.
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The phase transitions of these compounds are further confirmed by polarizing optical
microscopy (POM) and summarized in Table 1. The compounds 1b and 1c exhibit fan shaped
textures typical for smectic A phase in non-treated cell on cooling (figure 2(a) and 2(d)). In
planar cells before and after applying electric field, the extinction regions are parallel to the
direction of polarizer and analyzer (figure 2(b) and 2(e)). After the a.c. electric field is applied,
electro-optical switching along with textural changes occurred with a simultaneous change in
birefringence (figure 2(c) and 2(f)). The increase in birefringence is attributed to rotation of the
local polar domains towards the electric field direction. The lower homologue 1a of this series
was not mesogenic (table 1).
Substitution of fluorine atom in the transverse direction by another electronegative atom
chlorine and the lateral fluorine by hydrogen dramatically changes the character of the formed
mesogens. 2e shows a nematic phase on cooling. It exhibits characteristic marble textures in
cells without any aligning layer (figure 2(g)), while homogeneous alignment was observed in
planar cell (figure 2(h)). Under electric field, the change of birefringence is clearly observed
(figure 2(i)). The optical response varies quadratically with applied electric field and indicates
dielectric reorientation of the molecules. Another higher homologue of this series 2i shows
polymorphism, i.e. an additional nematic to SmA phase transition although the nematic range is
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very small (~0.5OC). The SmA phase has striped pattern in planar cells which turns to nematic-
like texture after application of electric field (20V/µm) suggesting that the width of the stripes
are reduced considerably (figure 2(k)-(l)). These changes of textures upon application of
electric field phases indicate that the molecules are actually reorienting and the phase cannot be
paraelectric in nature. The current response is confirmed further the polar properties of these
phases.
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Figure 2. Optical micrographs during cooling for (a) 1b at 108OC (SmA phase) in non-treated
glass plates, (b) 1-b at 108OC in 10µm planar cell & (c) 1b at 108OC in 10µm planar cell
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applying electric field 15V/µm, 10Hz ; (d) 1c at 104OC (SmA phase) in nonaligned glass plates,
(e)& (f) 1c at 105OC with applied electric field 0V and 15V/µm, 10Hz, (g) 2e at 98OC in N
phase in non-treated glass plate, (h)& (i) 2e in planar cell at 95OC under electric field 0V and
15V/ µm, 10Hz (j) 2i at 109OC showing isotropic to nematic transition (Inset: fan-shaped
texture ); (k) & (l) optical micrographs of 2i in planar cells at 0V and 20Vµm-1 ,10Hz at
104.5OC.
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3.2 Electro-optical studies:
We have investigated the current responses in these mesogens applying a triangular wave
voltage. Distinct current peaks are observed per half cycle of the applied voltage. In 1b and 1c
two well-separated current humps are present in the SmA phase (Figure 3(a) and 3(b)). This
indicates to the antiferroelectric nature of the smectic phase. The threshold voltage at which
peaks appear prominently is 12Vµm-1 for 1b (Figure 3(a) inset). Figure 3(b) shows evolution
of the current peaks with temperature for 1c. The two peaks A and B disappear completely as
the isotropic phase is approached indicating pure polar origin of the peaks. Relatively broader
peak B suggests two overlapped processes.
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100
Input Voltage
Output current
Current response
Applied voltage
(b)
4
(a)
20
0
B
50
2
0
0
0.05
0.10
Time (S)
30
110OC (SmA)
6V/
µ
m
8V/
µ
m
10V/µm
111OC (SmA)
112OC (SmA)
112.5OC(SmA)
114OC (Iso)
20
10
0
12V/
µm
A
14V/
µ
m
m
m
16V/µ
18V/µ
-50
-100
-20
0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 0.14
115OC (Iso)
time (s)
0.08
0.12
0.16
0.040
0.045
0.050
0.055
time (s)
time (s)
12
280
80
260
240
220
200
180
160
140
120
B
A
(d)
N
(c)
60
40
20
0
8
4
Iso
8
90
95
100
105
110
115
Temperature
(
OC)
0
90OC(N)
94OC(N)
98OC(N)
102OC(N)
105OC(Iso)
110OC(Iso)
-20
-40
-60
-4
-8
4
Current response
Applied voltage
0.08
0.12
0.16
0.09
0.10
0.11
0.12
time (S)
time (S)
80
C
100OC (SmA)
102OC (SmA)
104OC (SmA)
106OC(SmA)
108OC(SmA)
108.5OC(N)
(f)
0.06
0.05
0.04
0.03
(e)
B
Iso
SmA
10
5
10
A
40
0
110OC (Iso)
120OC (Iso)
8
6
4
100
102
104
106
108
110
112
114
Temperature (OC)
0
-5
Current response
Applied voltage
-40
-80
-10
10
0.096
0.100
0.104
0.108
0.112
0.08
0.12
0.16
time (S)
time (S)

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235
236
237
238
239
240
241
242
243
Figure 3. (a) Current response curve of 1b at110OC applying triangular voltage 100VPP, 10 Hz
(Inset: voltage dependence of current response at frequency 10 Hz), (b) Current response curve
of 1c at different temperatures applying triangular voltage 90VPP, 10Hz (Inset: variation of
current response in a full cycle of applied triangular voltage at 110OC), (c) Current response
curve of 2e at 98OC applying triangular voltage 70VPP, 10Hz, (d) Current response curve of 2e
at different temperatures applying triangular voltage 70VPP, 10Hz (Inset: temperature
dependence of area under the peak of current response), (e)Current response curve of 2i at
100OC applying triangular voltage 70VPP, 10Hz (Inset: temperature dependence of area under
the peak of current response), (f) evolution of the three peaks in the current response curve with
temperature for 2i applying triangular voltage 70VPP, 10 Hz.
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245
246
247
248
249
250
251
252
253
254
255
The compound 2e exhibits two closely spaced current peaks on application of triangular electric
field and their development over temperature is studied (Figure 3(c) and 3(d)). Peak A is
almost negligible in isotropic phase and becomes prominent with reduction of temperature.
Since the molecular structure of these mesogens involve an OH group which is capable of
proton conductivity and can easily complex ions to provide a current peak, the ionic
contribution to the current response should also be considered. Since the peak at right (peak B)
is growing with temperature and present in the isotropic phase, it may have partly ionic origin.
Thus Peak A can be attributed to purely polar origin whereas peak B is a combination of polar
and ionic contribution. Area under the peak is calculated for different temperatures and seems
to increase continuously throughout the nematic phase and has high saturated value in the
isotropic phase (Figure 3(d) inset).
256
257
258
259
260
261
262
The other compound 2i shows similar current peaks in the SmA phase, however the broad
overlapped peak in the right side now can be separated. Hence we have now three peaks A, B
and C (Figure 3(e)). The peaks do not grow with temperature and suppresses appreciably in
the isotropic phase (Figure 3(f)). Here it can be noticed that the short range nematic phase also
shows polar switching and can be identified as cybotactic nematic phase. The area of the current
peak gradually diminishes with increase of temperature suggesting typical orthogonal smectic
nature of the phase.
263
264
265
3.3 Dielectric spectroscopy: The dielectric spectroscopy is a useful tool to study the molecular
dynamics of liquid crystals. Dielectric investigations of the compounds 2e and 2i were carried
out in planar aligned cells with thickness of 4
ꢁ
m in the frequency range 10Hz to 13MHz, at
11

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268
269
270
various temperatures. The frequency dependence of the complex dielectric permittivity in the
specified conditions is described by the superposition of the Havriliak–Negami fit function and
conductivity contribution. Characteristic dielectric parameters such as dielectric strength and
relaxation frequency were extracted after incorporating the dielectric data into the following
extended Havriliak–Negami function equation.
271
ꢁ
ꢄ
ꢆ
∆ꢃ
ꢋ
ꢂ
ꢇ
272 ꢀ′′ = × ꢅ
+
ꢈꢉꢄ Im ꢊꢌꢄ ꢍ ꢏꢐꢑ
ꢒ
ꢋ
ꢕꢖ………………….(1)
∑
ꢓ
ꢔ
ꢋ
ꢋ
ꢃ
ꢂ
ꢎ
ꢒ
273
274
275
276
277
278
Where ∆ꢀꢈ is the dielectric strength and
involved in dielectric relaxation, ꢀꢘ is the vacuum permittivity (8.854 pF/m) and ꢙꢘ is the
conduction parameter. The exponents and are empirical fit parameters, which describes a
ꢗꢈis relaxation time of each individual process k
α
β
symmetric and non-symmetric broadening, respectively, of the relaxation peaks. The exponent s
of the angular frequency determines the nonlinearity of the dc conductivity arising from charge
accumulation at the interfacial layers. In the case of an Ohmic behaviour s=1.
279
280
281
282
283
Two compounds 2i exhibiting SmA and a very narrow range nematic phase and 2e exhibiting
monotropic nematic phase were investigated by dielectric spectroscopy with a weak applied
field (0.1 V
SmA phase of 2i. Dielectric investigations are carried out for the hockey-stick compound 2i in
SmA phase (30oC) in planar aligned cells with thickness 4
m.
ꢁ
m-1) in the frequency range 10 Hz to 13MHz and at different temperatures in the
ꢁ
130OC
120OC
110OC
106OC
104OC
103OC
102OC
100OC
(b)
(a)
0V
5V
10V
15V
25V
284
100
10
1
100
10
1
Mode I
Mode II
285
286
287
288
289
290
291
292
293
Mode III
0.01
0.1
1
10
100
1000
10000
0.01
0.1
1
10
100
1000
10000
Frequency (kHz)
Frequency (kHz)
(c)
0.060
0.055
0.050
0.045
0.040
0.035
0.030
0.025
0.020
400
(d)
Iso
0.018
0.016
0.014
0.012
∆εR1
320
350
300
250
200
150
fR1
280
240
200
Iso
12
11
10
9
SmA
Nematic
22
20
18
16
14
∆εR(1)
8
7
6
5
4
Nematic
Iso
fR(1)
∆εR2
fR1
90
95
100 105 110 115 120
Temperature(OC)
90
95
100
105
110
Temperature(OC)
90
95
100
105
110 115
120
Temperature (OC)
12

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296
297
298
Figure 4. Dielectric investigations of 2i: (a) Dielectric spectra at different temperatures, (b)
effect of bias voltage on relaxation peaks at 100°C, (c) temperature dependence of dielectric
strength (ꢀεR) and relaxation frequency (fR) of Mode I (Inset: variations of ꢀεR and fR with
temperature for mode II ); (d) temperature dependence of dielectric strength (ꢀεR) and
relaxation frequency (fR) of P1 for 2e (Inset: variations of ꢀεR and fR with temperature for P2).
299
300
301
302
303
304
305
306
307
308
309
310
311
The compound 2i exhibited three relaxation modes in the smectic A phase and also in the fluid
phase. Typical dielectric spectra variations at different temperatures are shown in Figure 4(a).
Due to narrow range of nematic phase the data was not recorded in nematic phase because of
experimental limitation. The influence of bias voltage on the relaxation peaks is shown in
Figure 4(b). The low frequency mode (Mode I) ranging between 12 Hz and 20 Hz is a
collective process as it gets suppressed totally under an applied bias voltage (Figure 4(b)). This
peak can be ascribed to the formation of ferroelectric clusters even in isotropic phase. The
dielectric strength (ꢀεR(I)) and relaxation frequency (fR(I)) decreases upon cooling (Figure
4(c)).The large dielectric permittivity in low frequency region with low relaxation frequency
indicates the presence of a strong positive dipolar correlation and dipole cooperative motions in
the SmA phase. These characteristic features reflect the ferroelectric short range order in this
phase.
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
The other mode at high frequency range defined as Mode II (kHz region) can be attributed to
molecular processes as it is not suppressed by bias voltage. Mode II is due to rotation of the
molecules around the short axis. ꢀεR(II) increases while fR(II) decreases upon cooling (Figure
3c inset). The increase in relaxation frequency with an increase in temperature for both Mode I
and II is due to reduction of viscosity of the material (Figure 4(c)). Similar relaxation modes
could be found in the calamitic nematic LCs in GHz regime. Here the down shift of the
relaxation frequencies of these modes can be because of high viscosity of the medium and its
large molecular size. Mode III is assigned to ITO relaxation as it is independent of both bias
voltage and temperature. Similar results are reported previously for few other bent-core
materials exhibiting smectic A phase with random polarization (SmAPR) inside the layers.
The compound 2e exhibits two peaks; one (P1) at very low frequency (12-18 Hz) region and
another (P2) at high frequency (40-80 kHz) rangeThe peaks have similar polar and molecular
origin as its higher homologue. The variations of dielectric strength and relaxation frequency
with temperature of these peaks are studied (Figure 4(d)).
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329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
3.4 XRD investigations: To get information on the conformation of the nematic phase of the
compound 2e, we carried out the XRD measurements. On cooling the sample from the
isotropic phase down to ~ 1o above the isotropic - nematic phase transition, only a diffuse
wide angle peak corresponding to the fluid-like ordering is revealed in the diffraction
pattern. But as the temperature is decreased further and the sample enters to nematic phase,
a distinct peak starts emerging in the small angle region (Figure 5), well above the nematic
to crystal transition temperature (TN-Cr~13°C). Hence this peak can be attributed to the
formation of cybotactic clusters in the nematic phase and not due to start of crystallization.
On further cooling in the nematic phase, the peak in the small angle region becomes
gradually sharper (Figure 5 inset) with corresponding layer spacing d~3.24nm at 91°C.
From
DFT
studies
we
have
found
the
length
(l) of the molecule to be 3.22nm. Thus the d to l ratio is ~1, which indicates that the clusters
are of SmA type. This is clearly sharper than diffuse small angle peak of conventional
nematics. Hence, this might indicate the cybotactic nature of the nematic phase (probably
clusters with lamellar order).
14000
114OC
5000
103OC
12000
100OC
97OC
94OC
91OC
4000
3000
2000
1000
0
10000
8000
6000
4000
2000
0
1
2
4
5
3 2
θ
5
10
15
20
25
2
θ
354
355
Figure 5. X-ray diffraction studies of 2e. Intensity profiles in the isotropic and nematic phases.
Inset: Enlarged view of the small angle peak at 2θ~2.8°.
356
4. Conclusion:
357
358
359
In conclusion we have designed, synthesized and characterized new achiral unsymmetrical four
-ring hockey-stick compounds with a biphenyl moiety with polar substituents in the transverse
and lateral directions of the molecule successfully. We further performed the measurements of
14

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363
364
365
366
367
368
369
370
the induced polarization in two homologous series of hockey-stick liquid crystals, one
exhibiting nematic as well as SmAP phase and the other exhibiting only SmAP in a cell of
thicknesses of 4 ꢁm with homogeneous alignment. The most interesting observation is the
existence of polar switching in the entire nematic and isotropic phases of the material 2e and the
SmAP phase exhibited by 2i, 1b and 1c. Dielectric investigation has complemented the
electro-optically obtained results. X-ray analysis confirms the existence of cybotactic clusters in
the nematic phase of 2e. On the basis of these investigations it is clear that the hockey-stick
molecules stick together to form clusters of cybotactic groups featuring short range SmA-like
ordering in the nematic phase and even manifest themselves in isotropic phase because of polar
interactions. We believe the present study would give significant insight to the development of
ferroelectric fluids with overall polar order.
371
372
Acknowledgements: Author SG is grateful to the Department of Science and Technology,
India for supporting this work under INSPIRE Faculty Award (IFA-13, PH-60) scheme.
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377
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18

Highlights:
•
•
•
•
Investigations on novel short-core hockey-stick molecules are reported.
The compounds exhibit nematic and orthogonal smectic mesomorphism.
Cybotactic clusters in the nematic phase are confirmed by XRD.
Polar orthogonal smectic phases with antiferroelectric type switching are observed.