“Dependence of Mesomorphism in Combination with Fluoro and Chloro Groups”

Article abstract of DOI:10.1080/15421406.2019.1624034

A novel azoester homologous series with chloro lateral group and a fluoro terminal end group was synthesized and characterized for mesomorphic properties. It consists of thirteen azo ester derivatives from trans-4-n-alkoxy cinnamic acid with 4-hydroxy-3-chloro phenyl azo-3-chloro-4-fluoro benzene. Mesomorphic property commences from C₄homologue as smectic.C₁ to C₃ member of the series are non-mesomorphic. C_{4 to} C₁₈ homologous are enantiotropic smectic A phase. The nematic mesophase is totally absent. Texture and transition temperatures of the homologues are determined by an optical polarizing microscopy equipped with heating stage. ¹H NMR, IR spectra, mass spectra, of some member is included. The spectroscopic analysis supports the molecular structure. A phase diagram is used to illustrate the mesomorphic behaviour across the series. Mesophaselength for smectic is varying from18.0^°C to 69.0⁰C. Mesomorphic properties of the novel azoester series are compared with structurally similar series.

Full text of DOI:10.1080/15421406.2019.1624034

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
“Dependence of Mesomorphism in Combination
with Fluoro and Chloro Groups”
Avani R. Ranchchh, Bhavna R. Tharesha & U.C. Bhoya
To cite this article: Avani R. Ranchchh, Bhavna R. Tharesha & U.C. Bhoya (2018) “Dependence
of Mesomorphism in Combination with Fluoro and Chloro Groups”, Molecular Crystals and Liquid
Crystals, 675:1, 49-58, DOI: 10.1080/15421406.2019.1624034
To link to this article: https://doi.org/10.1080/15421406.2019.1624034
Published online: 27 Jun 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
018, VOL. 675, NO. 1, 49–58
https://doi.org/10.1080/15421406.2019.1624034
2
“
Dependence of Mesomorphism in Combination
with Fluoro and Chloro Groups”
Avani R. Ranchchh, Bhavna R. Tharesha, and U.C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot-360005, Gujarat, India.
ABSTRACT
KEY WORD
Liquid Crystal;
Mesomorphism; Smectic
A; Nematic
A novel azoester homologous series with chloro lateral group and a
fluoro terminal end group was synthesized and characterized for
mesomorphic properties. It consists of thirteen azo ester derivatives
from trans-4-n-alkoxy cinnamic acid with 4-hydroxy-3-chloro phenyl
azo-3-chloro-4-fluoro benzene. Mesomorphic property commences
4 1 3
from C homologue as smectic.C to C member of the series are
non-mesomorphic. C4 to C18 homologous are enantiotropic smectic A
phase. The nematic mesophase is totally absent. Texture and transi-
tion temperatures of the homologues are determined by an optical
1
polarizing microscopy equipped with heating stage. H NMR, IR spec-
tra, mass spectra, of some member is included. The spectroscopic
analysis supports the molecular structure. A phase diagram is used
to illustrate the mesomorphic behaviour acrossꢀ the series.
0
Mesophaselength for smectic is varying from18.0 C to 69.0 C.
Mesomorphic properties of the novel azoester series are compared
with structurally similar series.
Introduction
Liquid crystal materials are useful in technological applications, such as display devices,
optical imaging, light emitting diodes, anisotropic network, semiconductor materials,
etc. [1]. Such a novel physical state has dual character of flow as a liquid and optical
properties like a crystalline solid chemists have designed and synthesized novel LC com-
pounds by varying the geometrical shape, size, aromaticity, terminal end group or
groups, central and/or lateral groups, and their position of substitution as ortho, meta,
and para, and studied or evaluated their effects on LC behaviour and molecular struc-
ture [2–5] as a consequence of molecular rigidity and/or flexibility. In continuation of
the same, the present investigation is planned to synthesize a novel homologous series
of LC compounds with azo (AN@NA) linkages, which are thermally very stable and
undergo a trans-cis isomerism in presence of UV light, and are useful for electro optical
devices and photochromism [6–9]. The liquid crystalline compound widely used in
varying fields. Therefore, it is interesting to study the relationship between molecular
structure and the liquid crystal state of a substance through homologous series, whose
molecular rigidity and flexibility [10–12] dictate the formation of mesomorphism. The
present investigation will cover only the synthesis of novel azoester compounds with
CONTACT Avani R. Ranchchh
ß 2019 Taylor & Francis Group, LLC
avaniranchchh@gmail.com; drucbhoya@gmail.com

5
0
A. R. RANCHCHH ET AL.
three phenyl rings, two laterally substituted ACl groups at the middle and right phenyl
rings, and left and right terminal end groups of AOR and AF respectively. The novel
compounds were characterized by elemental analysis, spectral data, and LC behaviour
by an optical polarizing microscope equipped with a heating stage. Numbers of homolo-
gous series have been reported till the date [13–19].
Synthesis
4
-n-Alkoxybenzaldehydes were synthesized by refluxing 4-hydroxybenzaldehyde (1
equiv.) with corresponding n-alkyl bromides (1 equiv.) in the presence of potassium
carbonate (1 equiv.) and acetone as a solvent [20]. The resulting 4-n-alkoxybenzalde-
hydes were reacted with malonic acid (1.2 equiv.) in the presence of 1-2 drops piperi-
dine as catalyst and pyridine as solvent to yield corresponding trans-4-n-alkoxy
cinnamic acids [A] [21]. Azo p-hydroxy-m-chloro phenyl azo-m-chloro-p-fluoro ben-
zene [B] was prepared by the usual established method of diazotization [22, 23] Yield-
70.5%. Coupling of compound [A] and compound [B] is done by steglich esterification
to yield [24].
The synthetic route to the novel homologous series is under mentioned in scheme 1.
Characterization
Some of members of a novel series as the representative member of a series were char-
1
acterized by elemental analysis (Table 1), Infrared spectroscopy, H NMR spectra and
mass spectroscopy. Microanalysis was performed on EuroEA Elemental Analyzer. IR
1
spectra were recorded on Shimadzu FTIR Model-IRAffinity-1S (MIRacle 10), HNMR
spectra were recorded on Bruker spectrometer using CDCl as a solvent and mass spec-
3
tra were recorded on Shimadzu GC-MS Model No.QP-2010.
Spectral Data
1
H NMR in ppm for Octyloxy Derivative
0
.90-0.95 (t, 3H, ACH3 of AOC H group), 0.99-1.84 (m, 12H, of AC H of
8
17
6
12
AOC H ), 3.49-4.04 (t, 2H, ACH AOA), 4.99(singlet,2H nearer to phenyl ring),6.45-
8
17
2
6.49(d, 1H, ACH@CHACOOA), 7.42-7.44(d, 1H, ACH@CHACOOA),7.2-7.9(m,
Aromatic region) 7.9 (1H, aromatic proton in between ACl and AN@NA), NMR data
are reliable with the molecular structure.
1
H NMR in ppm for Ethyloxy Derivative
0
4
7
.90-0.95 (t, 3H, ACH3 of AOCH3 group), 3.49-4.09 (t, 2H, ACH AOA),
.89(singlet,2H nearer to phenyl ring),6.45-6.49(d, 1H, ACH@CHACOOA), 7.42-
.56(d, 1H, ACH@CHACOOA),7.2-7.9(m, Aromatic region) 7.9 (1H, aromatic proton
2
in between ACl and AN@NA), NMR data are reliable with the molecular structure.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
51
Scheme 1. Synthetic route to the novel series
Table 1. Elemental analysis for (1) Ethyloxy (2) Butyloxy (3) Tetradecyloxy derivatives
Elements
Elements
%
Found
%Calculated
Sr. No.
Molecular formula
C
H
O
C
H
O
1
2
3
C
C
C
23
H
25
H
23
H
17Cl
21Cl
17Cl
2
FN
2
FN
2
FN
2
O
2
O
2
O
3
3
3
60.20
61.65
66.93
3.69
4.30
6.53
10.41
9.82
7.61
60.15
61.60
66.98
3.73
4.34
6.58
10.45
9.85
7.65

5
2
A. R. RANCHCHH ET AL.
1
3
C NMR in ppm for Methyloxy Derivative
5
5.8 ACH AO,128.5-130.2- Aromatic region,164.3- COOA,117.2- epso carbon nearer
3
to the AF in third phenyl ring,149.7 - epso carbon nearer to the azo group in third
phenyl ring.
Mass spectra of Hexyloxy Derivative
þ,
m/z (rel.int%):514(M) 247,266,10,413,357,156.
Mass spectra of Octyloxy Derivative
m/z (rel.int%): 542(M)+, 275,266,385,156.
21
IR in cm for Methyloxy Derivative
2
3
852, 2920 (CAH Str. of (ACH )n group of AOCH ), 1473, 1645 (aromatic CAC Str.),
068, (CAH Str. Of aromatic ring), 1085, 1138, 1226 (CAO Str. of ACOO group), 1732
2
3
(CAO, Str. of ACOO group), 1139 (CAO AC), 825, 688 (pera, ortho, and meta substi-
tuted phenyl ring) 1654 (AN@NA Str.). The IR data are consistent with the molecu-
lar structure.
21
IR in cm for Ethyloxy Derivative
2
800, 2925 (CAHStr. of (ACH ) n group of –OCH ), 1483, 1650 (aromatic CAC Str.),
2
3
3070, (CAH Str. Of aromatic ring), 1090, 1115, 1226 (CAO Str. of –COO group), 1756
(CAO, Str. of ACOO group), 1139 (CAO AC), 830, 688 (pera, ortho, and meta substi-
tuted phenyl ring) 1654 (AN@NA Str.). The IR data are consistent with the molecu-
lar structure.
Result and Discussion
p-n-Alkoxy cinnamic acids [A] are dimeric because their two molecules are bonded
through hydrogen bonding. 4-Hydroxy-3-chloro phenyl azo-3-chloro-4-fluoro benzene
[
B] is nonmesogenic. However, smectic type mesogenic character is induced by linking
it with 4-n-alkoxy cinnamic acid by steglich esterification. Transition temperatures
Table 2) of homologues were plotted for the number of carbon atoms present in n-
(
alky chain ‘R’ of –OR terminal end group. Transition curves CrAI/SmA and SmAAI
are drawn smoothly linking like or related points as shown in a phase diagram (Figure
1). CrAI/SmA transition curve follows partly zigzag path of rising and falling in usual
established manner with overall descending manner as series is ascended. SmA-I transi-
tion curve behaved normal manner with exhibition of odd-even effect. Sm-I transitions
curves for odd and even numbers merges into each other at the decyloxy (C ) homo-
10
logue of a series and then it prolong as a single curve till C18 homologue. Thermal sta-
ꢀ
ꢀ
bility for smectic A is 133.8 C and the mesophase length ranges from 18.0 C (C ) to
6
10
ꢀ
9 C at the C member of a series. Smectic A mesophase commences from C to the
4
4

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
53
Table 2. Transition temperatures
ꢀ
Transition temperatures in
C
R ¼ n-alkyl group
Smectic A
Isotropic
1
2
3
4
5
6
7
8
–
–
–
182
152
177
187
150
160
119
129
114
128
120
118
113
118
87
98
92
100
96
89
84
97
92
10
12
14
16
18
Figure 1. Phase behaviours of series
last C18 member of a series in enantiotropic manner without exhibition of nematic
property. C to C members are non-mesomorphic. Thus, present novel series is fully
1
3
smectogenic with middle ordered melting type.
Linking of dimeric p-n-alkoxy cinnamic acid by steglich esterification with 4-
hydroxy-3-chloro-phenyl azo-3-chloro-4-fluoro benzene increased molecular length,
polarity and polarizibility, rigidity and flexibility and suitable magnitudes of anisotropic
forces of intermolecular lateral and end to end attractions to such an extent that, lamel-
lar packing of molecules occur in solid crystalline state. The exhibition of only smecto-
genic character by the mesogenic (C to C ) homologues is attributed to the lamellar
4
18
packing of molecules in their crystal lattices enantiotropically during heating or cooling

5
4
A. R. RANCHCHH ET AL.
a specimen. Which manages their molecular arrangement as sliding layered organization
under floating condition, after disalignment at an angle ninety degree with the plane of
a surface, as a consequence of more rigidity and low molecular flexibility which match-
ing to suitable magnitudes of anisotropic forces of only intermolecular lateral attraction
and not at all matching to end to end attraction and restricting formation of statistically
parallel orientational order of molecules under floating condition on the surface. Thus,
only smectogenic mesophase formations occur without exhibition of nematogenic char-
acter for definite range of temperature. The appearance of odd-even effect in SmA-I
transition curve is attributed to stratified configuration of n-alkyl chain AR bonded to
first phenyl ring through oxygen atom by even and odd ACH A unit or units. But, the
2
disappearance of odd-even effect from and beyond C10 members of longer n-alkyl chain
AR is attributed to the unexpected status of chain AR by coiling or bending or flexing
or coupling with core structure of a molecule. Thus unexpected status of n-alkyl chain
AR may alter the magnitudes of molecular flexibility and induce more or less abnor-
mality in the behaviours of a transition curve. Early commencement of mesophase is
attributed to the favourable extent of molecular noncoplanarity. The changing trend in
LC properties from homologue to homologue in the same series is attributed to the
changing number of methylene unit or units, combine effect of partly changing molecu-
lar rigidity and predominantly changing with molecular flexibility. Nonmesomorphic
behaviours of C to C may be due to the low magnitudes of dispersion forces and low
1
3
dipole-dipole interactions. The mesogenic properties of present novel series can be com-
pared with other structurally similar isomeric or nonisomeric analogous series to deter-
mine and evaluate difference of mesogenic properties with changing molecular structure
in terms of graph efficiency order or polarity and polarizability for the same homo-
logues from series to series. The liquid crystal properties of present homologous series 1
are compound with the structurally similar known homologue series X [25] and series
Y [26] as shown in figure 2.
Homologous series 1 and series X and Y selected for comparative study are identical
with respect to three phenyl rings, a central bridge AN@NA linking middle and third
phenyl ring, left n-alkoxy terminal end group and lateral chloro group middle ring for
the same homologues. However, they differ with respect to central bridge, lateral group
Figure 2. Structurally similar series

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
55
o
Table 3. Relative thermal stabilities in C
Series ⟶
1
X
Y
Smectic-Isotropic
133.8 (C
4
AC18) C
4
103.66 (C12AC16) C12
- -
Or Smectic-Nematic
Commencement of
Smectic phase
Nematic phase Commencement
of Smectic phase
- -
143.80 (C
1
AC12) C
1
4 4
100.33 (C AC16) C
or terminally situated tail group. Homologous series 1 and X differ by central bridges
ACH@CHACOOA and ACOOA and lateral group ACl & AH respectively. Whereas
series 1 and Y differ by central bridges ACH@CHACOOA and ACOOA and lateral
group AF and ACl respectively, therefore, molecular rigidity and flexibility differs for
series 1, X and Y. However, suitable or unsuitable magnitudes of anisotropic forces of
intermolecular attraction to stabilize or destabilize liquid crystalline state depend upon
the combine effect of molecular rigidity and flexibility. The combined effect of molecu-
lar rigidity and flexibility varies from homologues to homologues in the same series or
for the same homologue from series to series. Thus, variation in mesomorphic behav-
iours and the degree of mesomorphism depend upon the changing part of molecules,
operating molecular rigidity or flexibility responsible for suitable magnitudes of aniso-
tropic forces of intermolecular end to end and/or lateral attractions. Following table 3
represents some LC properties or series 1, X and Y in comparative manner.
On comparing the mesomorphic behaviours of homologous series 1 are present
investigation and the series X and Y chosen for comparison.
1.
Series 1 is only smectogenic, series Y is only nematogenic and series X is smecto-
genic in addition to nematogenic.
2
3
4
.
.
.
Smectic thermal stability of series 1 is higher than a series X and Y.
Nematic thermal stability of series X is greater than the series Y and 1.
Smectic mesophase commences earlier for the series 1(C ) and later from C of
4
12
the series X, whereas it does not commences till the last member of a series Y.
Nematic mesophase commences from C and C homologues for the series X and
5.
1
4
Y respectively, but it does not commence till the last member of a series 1.
The absence of nematic property in recent novel series is attributed to the relative
rigidity offered by a central groups ACH@CHACOOA and ACOOA which links the
phenyl rings through at least one multiple bond. But ACH@CHACOOA is longer than
ACOOA which increases molecular polarity, polarizibility and the permanent dipole
moment across the long molecular axis. The stereochemistry of the units through pre-
serve from this comparision, it is clear that, due to increased length of uncommon cen-
tral bridges, uncommon lateral group and common tail group AF, the homologues of
series-1 resist the forces of thermal break down well enough to yield smectic A phase
from forth member of a series 1 and from dodecyl(C ) of a series X. Thus, C AC
12
4
18
homologues of series 1 and the higher homologues of series X (from and beyond C )
12
do get exhibited smectic mesophase formation earlier (series 1) and later (series X)

5
6
A. R. RANCHCHH ET AL.
relatively at higher and lower temperature respectively, resulting into high and low ther-
mal stability for smectic. Thus lamellar packing of molecules facilitated smectic meso-
phase formation as a consequence of molecular rigidity difference between series 1 and
X. Thus, display of only smectic property in absence of nematic property in case of ser-
ies 1 indicates that, the forces holding the plane of the layers together in the rigid crys-
tal are weaken relatively to the attraction between the molecules within a given layer at
solid-mesomorphic transition temperature and the layers may become free to slide and
rotate over one another giving smectic phase for series 1 and X. However on heating
the smectic phase to a higher temperature, the intermolecular end to end forces between
the sides of the molecules weaken to such an extent for the homologues of series 1 and
X, that resultant intermolecular cohesive forces do not permit the molecules to slide out
of the layers and statistically parallel orientation are not permitted for homologues of
series 1 (C AC ) but permitted for maintenance of nematic phase of homologue C of
4
18
12
series X and its lower homologues C to C in absence of smectic phase because C14
1
10
and C16 are only smectogenic. Thus, only smectic property exhibited by series 1 and
partly only nematic (C - C ), partly only smectic (C - C ) and partly smectic plus
1
12
12
16
nematic (C ) which exhibited by series X. Absence of lamellar packing of molecules in
1
2
the crystal lattice of series Y, in which AF is replaced by ACl as flexible tail group and
ACH@CHACOOA is replaced by ACOOA as rigid central linking group, the changing
molecular flexibility and rigidity as series 1 permit only statistically parallel orientational
order of molecules (C - C ) and hinders lamellar packing of molecules through the
4
16
series Y. Therefore, size of linking group (ACH@CHACOOA > ACOOA) and tail
group (ACl > AF) play an important role in mesomorphic behaviours of structurally
dissimilar unit of a molecules. The variation in the mesomorphic properties from series
to series for the same homologue arises from uncommon molecular part of the differing
entity of the corresponding molecular structure. Differing entity may be terminal end
group, lateral group, position of same group or groups on same or different phenyl ring
or rings, central group or groups etc.
Conclusions
A novel homologues series consisted of three phenyl rings and two central bridges -
CH@CHACOOA and AN@NA with para substituted AOR and AF tail groups and meta
substituted ACl groups is predominantly smectogenic with middle ordered melting type
without exhibition of nematic property.
ꢁ
ꢁ
Molecular rigidity and flexibility are very sensitive and susceptible to molecu-
lar structure.
The group efficiency order derived on the basis of (i) thermal stabilities and (ii)
the early commencement of smectic and nematic phase are as under.
i. Smectic:
Series 1 > Series X > Series Y
Nematic:
Series X > Series Y > Series 1

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
57
i. Smectic:
Series 1 > Series X > Series Y
Nematic:
Series X > Series Y > Series 1
Acknowledgements
Authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded & UGC-SAP
Sponsored), Saurashtra University, Rajkot, for research work. Authors are also thankful to Dr.
A.V. Doshi, Ex. Principal M.V.M. Science and Home Science College Rajkot, for his valuable co-
operation during present investigation as and when needed. Also thanks to the National Facility
for Drug Discovery through New Chemical Entities (NCE’s) for analysis of samples.
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