Dependence of mesomorphism on terminal polar group in novel azoester series

Article abstract of DOI:10.1080/15421406.2019.1655985

Novel homologous series 4-(4′-n-alkoxy benzoyloxy) napthyl azo 4″–bromo benzenes, consisted of 11 members of a series. All the 11 members (ethoxy to hexadecyloxy) except hexadecyloxy are only enantiotropically nematogenic without exhibition of any smectogenic character. Transition temperatures and the textures are determined by an optical polarizing microscopy equipped with a heating stage. Textures of a nematic phase are threaded or schlieren. Analytical and spectral data supported the molecular structure of homologs. Transition curves viz., solid-nematic and nematic-isotropic showing phase behavior of the mesophase in a phase diagram behave in normal manner. Alternation of transition temperatures is exhibited by N–I transition curve. Thus, novel series is entirely nematogenic and high ordered melting type. Thus, synthesis of a novel azoester homologous series is carried out with a view to understand and establish the effect of molecular structure on Liquid crystal (LC) behaviors of a substance.

Full text of DOI:10.1080/15421406.2019.1655985

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Dependence of mesomorphism on terminal polar
group in novel azoester series
Seema N. Kher, H. R. Prajapati, N. G. Makwana & Raviprakash S. Chandra
To cite this article: Seema N. Kher, H. R. Prajapati, N. G. Makwana & Raviprakash S. Chandra
(2019) Dependence of mesomorphism on terminal polar group in novel azoester series, Molecular
Crystals and Liquid Crystals, 682:1, 44-53, DOI: 10.1080/15421406.2019.1655985
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2019, VOL. 682, NO. 1, 44–53
https://doi.org/10.1080/15421406.2019.1655985
Dependence of mesomorphism on terminal polar group in
novel azoester series
Seema N. Khera, H. R. Prajapatia, N. G. Makwanab, and Raviprakash S. Chandrac
b
aDepartment of Chemistry, School of Sciences, Gujarat University, Ahmedabad, Gujarat, India; Science
c
& Humanities Department, R.C. Technical Institute, Ahmedabad, Gujarat, India; Department of Physical
Sciences, Institute of Advanced Research, Gandhinagar, Gujarat, India
KEYWORDS
ABSTRACT
Novel homologous series 4-(40-n-alkoxy benzoyloxy) napthyl azo
400–bromo benzenes, consisted of 11 members of a series. All the 11
members (ethoxy to hexadecyloxy) except hexadecyloxy are only
enantiotropically nematogenic without exhibition of any smecto-
genic character. Transition temperatures and the textures are deter-
mined by an optical polarizing microscopy equipped with a heating
stage. Textures of a nematic phase are threaded or schlieren.
Analytical and spectral data supported the molecular structure of
homologs. Transition curves viz., solid-nematic and nematic-isotropic
showing phase behavior of the mesophase in a phase diagram
behave in normal manner. Alternation of transition temperatures is
exhibited by N–I transition curve. Thus, novel series is entirely nema-
togenic and high ordered melting type. Thus, synthesis of a novel
azoester homologous series is carried out with a view to understand
and establish the effect of molecular structure on Liquid crystal (LC)
behaviors of a substance.
Liquid crystal; mesogen;
mesomorphism; nem-
atic; smectic
Introduction
The Liquid crystals (mesogens) are fascinating functional materials and are important in
the field of advanced materials such as electro-optical devices, laser addressed devices,
passive block filters, and devices based on thermochromism such as fever indicators,
gadgets, reflective type LCDs, field effect transistors and sensors [1]. The synthesis of
azoester molecules was carried out with a view to understanding and establishing the
effect of molecular structure on liquid crystal behavior of a substance. Their meso-
morphic properties were investigated by differential scanning calorimetry and polarized
optical microscopy. The textures of nematic mesophase are schlieren or threaded.
Transition curves of a phase diagram behave in a normal manner. The authors have
synthesized novel LC compounds, evaluated their LC properties and characterized
respective molecular structure of novel LC material. Thus, dependence of mesomor-
phism and the degree of mesomorphism on the molecular structure are mainly studied
by Gray et al. [2, 3]. Dave et al. [4, 5] studied homologous series of novel Schiff ’s bases,
carboxy ester, and their binary systems, Lohar et al. studied binary systems and LC ester
CONTACT H. R. Prajapati
drhrprajapati@yahoo.co.in
Department of Chemistry, School of Sciences, Gujarat
University, Navrangpura, Ahmedabad 380009, Gujarat, India.
ß 2019 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
45
Table 1. Transition Temperatures of Homologous series
Transition temperatures in ꢀC
Number of carbon
atoms in n-alkyl chain
Nematic
Isotropic
2
3
4
5
6
7
8
10
12
14
16
185.0
130.0
133.0
104.0
148.0
129.0
138.0
56.0
194.0
143.0
171.0
125.0
198.0
138.0
140.0
61.0
91.3
196.0
–
169.0
200.0
144.0
homologous series of two phenyl rings [6, 7]. Vora et al. [8–10] studied cholesteric LC
and conventional and unconventional LC, N. H. Shah and Jadav studied azoester [11]
compounds, Doshi et al. studied ester azoester, chalcones, Schiff ’s bases [12–14] in
homologous series and binary systems, Prajapati et al. studied dimers of novel varieties
[15]. The chemists have synthesized novel LC compounds, evaluated their LC properties
and characterized respective molecular structure of novel LC material. Thus, depend-
ence of mesomorphism and the degree of mesomorphism on the molecular structure
are mainly studied by Gray et al. [2, 3, 16]. Physicists and biologist studied physical
properties and biological activity of LC compounds respectively. Technocrats and tech-
nologists applied their knowledge in the scientific pharmaceutical, medicinal and other
allied fields of applications [17–19]. Present investigation is planned to synthesize novel
azoester homologous series consisting of napthyl unit linking a phenyl ring through
–COO- and other phenyl ring through –N ¼ N- central bridges to which –OR/n-alkoxy
terminal end group and at the other end para substituted bromo polar group with
respect to –N ¼ N- group to be synthesized and characterized by analytical data. Then
mesomorphic characteristics will be evaluated with bromo group substituted at para
position of right side phenyl ring. Thus study is aimed to understand and establish the
effect of molecular structure on mesomorphism as a consequence of molecular rigidity
and flexibility [20–22].
Synthesis
4-hydroxy benzoic acid was alkylated by suitable alkylating agents (R-X) to convert it
into corresponding n-alkoxy benzoic acids by the modified method of Dave and Vora
[23]. Azodye 4-hydroxy napthyl-1-azo-40-bromo benzene was prepared by usual estab-
lished method [24]. 4-n-alkoxy benzoic acids and azodye were individually condensed
in ice cooled pyridine to get a series of final azoester products [25]. Final products were
decomposed, filtered, washed, dried, and purified till constant transition tempera-
tures obtained.
The chemicals 4-hydroxy benzoic acid, alkyl halides [R-X], Methanol, Ethanol, KOH,
Thionyl chloride, pyridine, 1–Napthol, para bromo aniline, HCl, NaNO2, etc. required
for synthesis were used as received excepts solvents which were dried and distilled prior
to use. The synthetic route to the series is mentioned below in Scheme 1.

46
S. N. KHER ET AL.
Scheme 1. Synthetic route of homologous series I.
Characterization
Some selected members of a novel series were characterized by elemental analysis,
1
polarizing microscopy with heating stage, mass spectra, Infrared spectra, H-NMR spec-
tra, texture determination by miscibility method and DSC scan. Microanalysis of the
compound was performed on Perkin Elmer PE 2400 CHN analyzer, IR spectra were
1
performed on Perkin Elmer spectrum, and H-NMR spectra were performed on Bruker
spectrometer using CDCl3 as solvent. Transition temperatures of synthesized homolo-
gous series are recorded in Table 1. Elemental analysis is shown in Table 2. IR spectra
analysis is also shown.
IR absorptions of 4-(40-n-ethoxy benzoyloxy)-napthyl-azo-400-bromo benzene
818 cmꢁ1 para substituted benzene ring,
1,020 cmꢁ1 C ¼ O bend,
1,156 cmꢁ1 C-O str. 1,249 cmꢁ1 C-O str. (ester),

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
47
Table 2. Elemental analysis for butyloxy, hexyloxy, octyloxy and dodecyloxy derivatives.
Elements % found (Calculated % )
Sr. No.
Molecular formula
C
H
N
1
2
3
4
C25H19N2O3Br
C27H23N2O3Br
63.25(63.15)
64.71(64.41)
66.69 (66.54)
68.53(68.29)
4.22(4.00)
4.68(4.57)
5.69(5.54)
6.64(6.34)
5.99 (5.89)
5.79 (5.56)
5.28 (5.00)
4.73 (4.55)
C31H31 N2O3Br
C35H39 N2O3Br
1,354 cmꢁ1 and 1,419 cmꢁ1 sat. C-H str.,
1,489 cmꢁ1 1,507 cmꢁ1 1,621 cmꢁ1 aromatic -C ¼ C- str.,
1,684 cmꢁ1 1,733 cmꢁ1 -C ¼ O str. (ketone) -C ¼ O str. (ester)
2841 cmꢁ1 2,916 cmꢁ1
sat. C-H str.
IR confirms the structure
Characteristic IR absorptions of 4-(40-n-propyloxy benzoyloxy)-napthyl-azo-400-
bromo benzene
849 cmꢁ1ꢁp1 ara substituted benzene ring
1,064 cmꢁ1 C ¼ O bend
1,168 cmꢁ1 C-O str.
1,249 cm C-O str. (ester)
1,399 cmꢁ1 1,419 cmꢁ1 sat. C-H str.
1,489 cmꢁ1 1,507 cmꢁ1 1,602 cmꢁ1 aromatic -C ¼ C- str.
1,733 cmꢁ1 C ¼ O str. (ester)
2,848 cmꢁ1 2,965 cmꢁ1 sat. C-H str.
IR confirms the structure
1H NMR of 4-(40-n- pentyloxy benzoyloxy)-napthyl-azo-400-bromo benzene in
CDCl3, d ppm.
0.94 (t, 3H, -CH3) 1.42 (m, 6H, -CH2) 4.02 (t, 2H, -OCH2) 6.8–9.02 (m, 14H, Ar-H)
1H NMR of 4-(40-n-decyloxy benzoyloxy)-napthyl-1-azo-400-bromo benzene in
CDCl3, d ppm
0.88 (t, 3H, -CH3) 1.27 (m, 16H, -CH2) 4.013 (t, 2H, -OCH2) 6.9–8.2 (m, 14H, Ar-H)
Mass spectra (molecular weight) tetradecyloxy homolog (Gram/Mole)
Calculated – 643
Experimental – 643
Textures by miscibility method (nematic)
Propyloxy homolog – Threaded texture
Heptyloxy homolog – Threaded texture
Decyloxy homolog – Schlieren texture

48
S. N. KHER ET AL.
Figure 1. Phase behavior of homologous series.
Results and Discussion
Novel homologous series 4-(40-n-alkoxy benzoyloxy) naphthyl azo-400-bromobenzenes
consisted of 11 homologs. All the homologs except hexadecyloxy are enantiotropically
nematogenic without exhibition of any smectogenic property. Transition and melting
points of homologs as determined by an optical polarizing microscopy, equipped with a
heating stage were plotted versus the number of carbon atoms present in n-alkyl chain
of left n-alkoxy terminal end group of the molecules of each homolog (Fig. 1). Some
selected members of a novel series were characterized by elemental analysis, DSC (Fig.
1
2), polarizing microscopy (Fig. 3), IR spectra, H NMR Spectra, Texture determination
by miscibility method. Present investigation is planned to synthesize novel azoester
homologous series consisting of naphthyl unit linking a phenyl ring through –COO-
and other phenyl ring through –N ¼ N- central bridges to which –OR n-alkoxy terminal
end group and at the other end para substituted -Br with respect to –N ¼ N- group to
be synthesized and characterized by analytical data. Thus study is aimed to understand
and establish the effect of molecular structure on mesomorphism as a consequence of
molecular rigidity and flexibility [20–22].
The present investigation has been inspired with the desire to search for new liquid crys-
tals or non amphiphile which may be endowed with specific characteristics. The synthesis
had been undertaken after giving due consideration to the molecular geometry enhancing
the potential probability of the new compounds to exhibit mesomorphism preferably in the
lower range of temperature. The generalization concerning mesophase get strengthened
from the present investigation, as the results obtained supports the generally accepted crite-
ria and some new trends of transition curves and characteristics. The following homologous
series have been synthesized and their characteristics have been determined.
Liquid Crystal Characteristics of Individual Series
Homologous series: 4-(40-n-alkoxy benzoyloxy)-napthyl-1-azo-400-bromo benzenes
Eleven compounds have been synthesized and their mesogenic properties are deter-
mined. The phase transition temperatures of these compounds are recorded in Table 1.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
49
Figure 2. DSC scan.
Figure 3. Textures observed by polarizing microscope with heating stage.
All the members exhibit mesomorphic character except last member (hexadecyloxy) of
present homologous series. Because mesomorphism is displayed by all the members of
the series except last homolog (hexadecyloxy) of this series enantiotropically with nem-
atic phase. Appearance of nematic mesophase comes from the second member of the
series and continues up to the fourteenth member of the series. This series is purely
nematogenic. No smectic mesophase occurs, that is, smectic mesophase is absent in all
members of this homologous series. Odd-even effect is clearly observed. Transition tem-
peratures are plotted versus the number of carbon atoms in n-alkyl chain of left alkoxy
group of the homologs as given in Fig. 1. Smooth curves are drawn through the points
for like or related transitions.
The solid-mesomorphic curve follows a zigzag path of rising and falling nature, that
is, falling from second to third homolog by 55 ꢀC and then continuously rising-falling
up to tenth member and again rises at the twelfth and fourteenth member of the series.
All the members of the homologous series are enantiotropic nematic and smoothly
transform into isotropic liquid at definite temperature.
The usual odd even effect is observed in nematic-isotropic transition curves with
alternation of transition temperatures. The nematic-isotropic transition curve for even

50
S. N. KHER ET AL.
member follows smooth zigzag path of rising and falling nature up to the sixteenth
homolog and for the odd members it falls from the third homolog to the fifth and rises
at the seventh.
Thus the present homologous series under discussion can be considered as high
ordered melting type series. Present homologous series under discussion is purely enan-
tiotropic nematic. Smectic mesophase is totally absent in this homologous series. The
nematic phase has threaded texture as judged directly by visualizing the field of view in
the hot stage polarizing microscope. The series under discussion is purely nematogenic
with high ordered melting type.
Homologous series with above core structure reported is known to show meso-
morphic characteristics. It is interesting to note that all the homologs except hexadecyl
homolog of homologous series display mesomorphism from the second homolog of the
homologous series in the enantiotropic condition. Mesomorphic characteristics exhib-
ited by the molecules are directly related with the intermolecular or intramolecular
forces arising out of the design of the molecular structure which is nothing but its
molecular geometry. Polarity of the attached substituent groups, polarizability, length to
breadth ratio, rigidity and linearity of molecule, steric hindrance caused by certain
groups attached to the phenyl rings of the molecule, planner and non-planner nature,
electron-electron interaction or charge transfer, p-electron density are considered
important factors responsible for mesogenic characteristics of the molecules.
The basic length due to the two phenyl rings and one napthyl ring and central link-
ing units viz. -COO- and -N ¼ N-, aromaticity of the molecule etc. are the constant
unchanging feature and display of mesomorphism due to the molecular forces arising
on account of these will remain the same for the present homologous series. The left
terminal is the n-alkyl (-R) chain of the alkoxy group in which progressively increases
the number of carbon atom. Thus the number of carbon atoms increases in the case of
the various homologs of the present homologous series, keeping the other core structure
intact. The right terminal is bromo functional group for present series along with the
progressive increase in the number of carbon atoms of the n-alkyl chain at the left ter-
minal for present homologous series.
4-hydroxy-napthyl-1-azo-40-bromo-benzene is non- mesomorphic in nature. However
addition of napthyl ring linked through carboxylate -COO- central bridge and para sub-
stituted n-alkoxy terminal increases the length and aromaticity of the molecule.
Increased length and aromaticity of the molecule increases molecular polarity, molecular
polarizability and length to breadth ratio, causing induction of liquid crystallanity in the
molecule. Thus added linking units to the different azo dyes induces mesomorphism
enantiotropically especially nematic type mesophase of presently reported homologous
series up to fourteenth homolog. This results in the exhibition of mesomorphism in the
molecules of the present homologous series. The molecules exhibiting only nematic
property is due to the capability of molecule to resist the strong thermal vibrations at
high temperature to yield oriented anisotropic liquid. The molecules set themselves par-
allel to each other in a flowing condition giving rise to a threaded texture of nematic
mesophase enantiotropically for 10 homologs under present discussion.
Intermolecular hydrogen bonding and mesomorphism: p-n-alkoxy benzoic acids are
mesomorphic in behavior with relatively high transitions, their mesogenic properties are

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
51
due to the dimerization of the acid molecules through hydrogen bonding. The elimin-
ation of hydrogen bonding through esterification should yield mesomorphism with rela-
tively lower transitions. However, coupling of unit to the napthyl ring linked through
–COO- bridge increases the transition temperatures of the “azo ester” molecules in case
of linear molecule. In present study, homologous series is linear so transitions are rela-
tively higher. Thus the view that, the napthyl ring linked through –COO- bridge
increases the transitions temperatures of present homologous series is further supported
by the present investigation.
Alternation in transition temperature due to odd-even effect is exhibited in present
homologous series in nematic-isotropic curve. The alternation of transition temperatures
for odd and even members of the series is due to the difference in the length and lin-
earity of n-alkyl group of left alkoxy group. It is directly related to magnitude of cohe-
sive forces arising out of length and linearity of n-alkyl chain. Generally, odd number
n-alkyl chain causes weaker cohesion as compared to even number n-alkyl chain. Role
of oxygen in n-alkoxy terminal should be considered. However overall intermolecular
cohesive forces of attraction are affected by odd and even members of the homologous
series and hence the normal behavior of nematic-isotropic transition curve is also
affected as series is ascended. It is seen that nematic-isotropic transition curve shows
alternation of transition temperatures up to tenth homolog but alternation diminishes
as series is ascended. This is because in case of longer n-alkyl chain, it coils and behaves
as shorter chain and hence the cohesive forces are also altered.
Showing of alternations of transition temperatures for odd and even members of the
series up to tenth homolog is understandable because alkyl chain of alkoxy group at the
left alternates in number of -CH2- group. Generally, odd homologs are loosely packed
as compared to even homologs up to seventh member of the series but thereafter longer
n-alkyl chain is coiled and does not maintain linearity. Hence transition temperatures
of homologs beyond seventh are less affected or rather irregular change of transition
temperatures is observed or normal behavior is observed for nematic-isotropic (or vice
versa) transition curve.
Dimerization of 4-n-alkoxy benzoic acids, nonmesomorphicity of tetradecyloxy (C14)
derivative and smectogenic property of ethyloxy to tetradecyloxy (C2–C14) derivatives
disappears on esterification of 4-n-alkoxy benzoic acids through their corresponding
acid chlorides. Azo dye 4-hydroxy napthyl-1-azo 40-bromobenzene is nonmesomorphic.
However on linking it with 4-n-alkoxy benzoic acids through their acid chlorides
yielded nematogenic novel azoester homolog derivatives. Transition temperatures and
melting temperatures of homologs (Table 1) are plotted versus the number of carbon
atoms in n-alkyl chain of the left n-alkoxy terminal end group. Like or related points
are linked to form solid-nematic and N–I transition curves to represent the phase
behaviors of a novel series in a phase diagrams (Fig. 1).
The relative higher of transition temperatures of azoester molecules and the dis-
appearance of dimerization of 4-n-alkoxy benzoic acids are due to the breaking of
hydrogen bonding between two molecules of 4-n-alkoxy benzoic acid on esterification
process. The induction of LC property in azoester homolog derivatives is due to the
linking of azo dye to 4-n-alkoxy benzoic acid molecules which increases molecular
length and width and acquires the suitable magnitudes of anisotropic forces of end to

52
S. N. KHER ET AL.
end intermolecular attractions as a consequence of appropriate molecular rigidity and
flexibility. The disappearance of smectogenic behavior of ethyloxy to hexa decyloxy (C2
to C16) n-alkoxy acids is due to the linear geometrical shape of corresponding novel
azoester molecules, which hinders lamellar packing of molecules in their crystal lattices
as required for smectic mesophase formation. The molecules of all the members of a
presently investigated azoester series under the influence of externally exposed thermal
vibrations disalign at an angle less than ninety degree and resisting exposed thermal
vibrations for some temperature difference called as degree of mesomorphism and then
the molecules of a sample under microscopic examination ceases to appear from and
beyond isotropic temperature. The molecules of C2 to C14 adopt a statistically parallel
orientational molecular order within definite range of temperature prior to isotropic
temperature in floating condition. However same molecules are randomly oriented in
all possible directions with high order of disorder or high entropy (DS ¼ DH/T) from
and beyond isotropic temperature. The same molecules show nematic mesophase
reversibly below isotopic temperature on cooling the melt under microscopic observa-
tion. Absence of smectic phase is attributed to the absence of lamellar packing of mole-
cules in their crystal lattices and unfavorable extent of noncoplanarity of azoester
molecules due to its odd fitting of intermolecular nonlinear shape and size.
Hexadecyloxy (C16) derivative of a series is unable to resist exposed thermal vibrations
and abruptly breaking its crystal lattices. Thus, C16 homolog sharply and smoothly
transform into isotropic liquid from solid state without passing through an intermediate
state of existence between crystalline solid and isotropic liquid, that is, liquid crystalline
state of a substance is missing. The inadequate magnitudes of end-to-end intermolecular
adhesion and closeness hinder the exhibition of mesophase by C16 homolog. The
absence of smectogenic character is attributed to the absence of lamellar packing of
molecules in the crystal lattices of the homologs C1–C16.
Conclusion
ꢂ
ꢂ
ꢂ
Presently investigated novel homologous series is entirely nematogenic with rela-
tively middle ordered melting type and of considerable rich degree of
mesomorphism.
Homolog 10 having relatively low transition temperature as compared to other
homologs of present homologous series. Therefore, definitely, this homolog may
have potential applications in the photochromism and electro optical devices.
Homologous series with naphthyl unit are generally nematogenic. A phenom-
enon of mesomorphism and the degree of mesomorphism are very sensitive and
susceptible to the favorable magnitudes of molecular rigidity and/or flexibility or
molecular structure.
ꢂ
Hence present investigation supported early views about mesomorphism and
raises the credibility to the conclusions drawn earlier [2–4].
Acknowledgements
The authors are thankful to the Prof. and Head; Dr. V. K. Jain, Department of Chemistry, School of
Sciences, Gujarat University, Ahmedabad for his valuable support and co-operation in research work.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
53
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