Mesomorphism dependence on terminal polar group in the nonlinear novel azoester series

Article abstract of DOI:10.1080/15421406.2018.1507076

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. Novel series consists of ten LC substances. All the homologs are enantiotropically nematogenic without exhibition of smectic property. Transition and melting temperatures, textures of LC are determined by an optical polarizing microscopy equipped with a heating stage. Textures of nematic phase are threaded or schlieren. Transition curves of a phase diagram behaved in normal manner. Nematic–isotropic transition curve exhibited odd–even effect. Analytical and spectral data supported and confirmed the structures of homologues.

Full text of DOI:10.1080/15421406.2018.1507076

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on terminal polar
group in the nonlinear novel azoester series
Seema N. Kher, H. R. Prajapati, Raviprakash S. Chandra & N. G. Makwana
To cite this article: Seema N. Kher, H. R. Prajapati, Raviprakash S. Chandra & N. G. Makwana
(2018) Mesomorphism dependence on terminal polar group in the nonlinear novel azoester series,
Molecular Crystals and Liquid Crystals, 665:1, 1-9, DOI: 10.1080/15421406.2018.1507076
To link to this article: https://doi.org/10.1080/15421406.2018.1507076
Published online: 13 Feb 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
018, VOL. 665, NO. 1, 1–9
2
https://doi.org/10.1080/15421406.2018.1507076
Mesomorphism dependence on terminal polar group in the
nonlinear novel azoester series
Seema N. Kher, H. R. Prajapati, Raviprakash S. Chandra, and N. G. Makwana
Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad, Gujarat, India
ABSTRACT
KEYWORDS
Azoester; liquid crystal;
mesogen; mesomorphism;
nematic; smectic
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. Novel series consists
of ten LC substances. All the homologs are enantiotropically nemato-
genic without exhibition of smectic property. Transition and melting
temperatures, textures of LC are determined by an optical polarizing
microscopy equipped with a heating stage. Textures of nematic
phase are threaded or schlieren. Transition curves of a phase dia-
gram behaved in normal manner. Nematic–isotropic transition curve
exhibited odd–even effect. Analytical and spectral data supported
and confirmed the structures of homologues.
Introduction
The mesomorphic state of matter intermediate to crystalline solid and isotropic liquid
was discovered in 1888 [1]. Chemists design, synthesize and evaluate liquid crystal (LC)
materials by carrying out changes or minor major alternations in molecular shape, size,
aromaticity, terminal and/or lateral functional groups, and their positional substitution
etc. On changing the above variables, the molecular rigidity and/or flexibility [2–5]
changes and novel LC material are generated. The present investigation is planned with
a view to understanding and establishing the relation between LC behavior of a sub-
stance and the molecular structure [6–8], as a consequence of molecular rigidity and
flexibility. The novel homologous series of azoester molecules have potential applica-
tions in the photochromism and electro optical devices [9, 10].
Synthesis
4
-Hydroxy benzoic acid was alkylated by suitable alkylating agents (R-X) to convert it
into corresponding n-alkoxy benzoic acids by usual established method of Dave and
0
Vora [11]. Azo dye 2-Hydroxy naphthyl azo 4 -bromobenzene was prepared by
reported method [12]. 4-n-alkoxy benzoic acids and Azo dye were condensed in ice
cooled pyridine to get a series of final azo ester products [13]. Final products were
CONTACT H. R. Prajapati
drhrprajapati@yahoo.co.in
Department of Chemistry, School of Sciences, Gujarat
University, Ahmedabad, Gujarat 380009, India.
ß 2018 Taylor & Francis Group, LLC

2
S. N. KHER ET AL.
Scheme 1. Synthetic route for homologous series I.
decomposed, filtered, washed, dried, and purified till their constant transition tem-
perature, obtained.
The chemicals 4-hydroxy benzoic acid, alkyl halides [R–X], methanol, ethanol, KOH,
thionyl chloride, pyridine, b-naphthol, para bromoaniline, 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.
Characterization
Some selected members of a novel series were characterized by elemental analysis,
1
polarizing microscopy, mass spectra, infrared spectra, H-NMR spectra, texture deter-
mination by miscibility method and DSC scan. Microanalysis of the compound was per-
formed on Perkin Elmer PE 2400 CHN analyzer, IR spectra were performed on Perkin
1
Elmer spectrum, and H-NMR spectra were performed on Bruker spectrometer using
CDCl3 as solvent. Transition temperatures of synthesized homologous series are
recorded in Table 1. Elemental analysis and IR spectra analysis are shown in Tables 2
and 3, respectively.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
ꢀ
0
Table 1. Transition temperatures in C for 2-(4 -n-alkoxy benzoyloxy) naphthyl -1-azo-
0
0
-bromobenzenes.
4
ꢀ
Transition temperatures in
C
Number of carbon atoms in n-alkyl chain
Nematic
Isotropic
2
3
4
5
6
7
8
1
1
1
78.7
69.4
82.4
109.2
57.5
63.9
74.5
47.2
51.1
–
94.9
114.0
130.5
125.5
100.0
93.0
96.2
72.8
74.6
113.7
0
2
4
1
0
00
H NMR of 2-(4 -n-hexyloxy benzoyloxy)-naphthyl-1-azo-4 -bromobenzene in CDCl3,
d ppm.
0
1
4
6
.927 (t, 3H, –CH3)
.251-1.356 (m, 8H, –CH2)
.004 (t, 2H, –OCH2)
.846–8.529 (m, 14H, Ar–H)
NMR confirms the above structure.
H NMR of 2-(4 -n-heptyloxy benzoyloxy)-naphthyl-1-azo-4 -bromobenzene in CDCl ,
1
0
00
3
d ppm.
0
1
3
7
.88 (t, 3H, –CH3)
.256–1.315 (m, 10H, –CH2)
.428 (t, 2H, –OCH2)
.266–8.383 (m, 14H, Ar–H)
NMR confirms the above structure.
Mass spectra (molecular weight) octyloxy homolog (gram/mole)
Calculated-559
Experimental-559
Textures by miscibility method
Nematic
Propyloxy homolog – threaded texture
Heptyloxy homolog – threaded texture
Decyloxy homolog – schlieren texture
Results and discussion
0
00
Novel homologous series 2-(4 -n-alkoxy benzoyloxy) naphthyl azo -4 -bromobenzenes
consisted of ten homologs. All the members 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

4
S. N. KHER ET AL.
Table 2. Elemental analysis for ethyloxy, hexyloxy, octyloxy and dodecyloxy derivatives.
Elements % found (calculated %)
Sr. No.
Molecular formula
C
H
N
1
2
3
4
C
25
C
29
C
31
C
35
H
19
H
27
H
31
H
39
N
2
O
3
Br
63.25(63.15)
65.83(65.53)
66.69 (66.54)
68.53(68.29)
4.22(4.00)
5.58(5.08)
5.69(5.54)
6.64(6.34)
5.99 (5.89)
5.39 (5.27)
5.28 (5.00)
4.73 (4.55)
N
N
N
2
O
2
O
2
O
3
3
3
Br
Br
Br
0
00
Table 3. IR absorptions of 2-(4 -n-heptyloxy benzoyloxy)-naphthyl-1-azo-4 -
bromobenzene.
ꢁ
1
7
8
1
1
1
1
1
1
1
1
2
2
1
59 cm
86 cm
Ortho substituted benzene ring
Para substituted benzene ring
–C ¼ O bend
ꢁ1
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
1
1
1
1
1
1
1
1
1
1
1
010 cm
155 cm
202 cm
260 cm
471 cm
503 cm
600 cm
736 cm
854 cm
924 cm
052 cm
C–N str. Due to –N ¼ N–
C–O str.(ester)
C–O str. (ether)
Aromatic –C ¼ C–
C ¼ O str.(ester)
Sat. C–H str.
Ar–Br
IR confirms the above structure
Figure 1. Phase behaviour of Series.
(Table 1) 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. 2a–b), polarizing microscopy, IR spectra, texture determination by miscibility
method. Textures observed by optical polarizing microscope (POM) are shown in
Figure 3. 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

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
0
00
0
Figure 2. (a) 2-(4 -n-hexyloxy benzoyloxy)-naphthyl-1-azo-4 -bromobenzene. (b) 2-(4 -n-octyloxy ben-
00
zoyloxy)-naphthyl-1-azo-4 -bromobenzene.
Figure 3. Textures observed by POM.
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 [2–5].
The present investigation has been inspired with the desire to search for new LCs or
nonamphiphile 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 strength-
ened from the present investigation, as the results obtained supports the generally
accepted criteria and some new trends of transition curves and characteristics. The fol-
lowing homologous series have been synthesized and their characteristics have
been determined.
LC characteristics of individual series
0
00
Homologous series: 2-(4 -n-alkoxy benzoyloxy)-naphthyl-1-azo-4 -bromobenzenes
Ten compounds have been synthesized and their mesogenic properties are determined.
The phase transition temperatures of these compounds are recorded in Table 1. All the
members exhibit mesomorphic character, except last member of present homologous
series. Because mesomorphism is displayed by all the members of the series except last
homolog of this series enantiotropically with nematic phase. Appearance of nematic

6
S. N. KHER ET AL.
mesophase comes from the second member of the series and continues up to the
twelfth member of the series. This series is purely nematogenic. No smectic mesophase
occurs i.e. smectic mesophase is absent in all members of this homologous series.
Odd–even effect is clearly observed. Transition temperatures are plotted versus the num-
ber of carbon atoms in n-alkyl chain of left alkoxy group of the homologs as given in
Figure 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., i.e.
falling from second to third homolog and then continuously rise up to fifth member
and again falls at the sixth member of the series; with rises up to eighth homolog and
ꢀ
then falls at the tenth homolog and small rising of 3.9 C for twelfth homolog of 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
member follows smooth zigzag path of rising and falling nature up to the twelfth homo-
log and for the odd members it rises from the third homolog to the fifth and falls at
the seventh. The usual odd–even effect is observed in the nematic–isotropic transition
curve with alternation of transition temperatures.
Thus, the present homologous series under discussion can be considered as middle
ordered melting type series. Thus, present series under discussion is purely enantio-
tropic nematic. Thus, smectic mesophase is totally absent. The nematic phase has
threaded texture as judged directly by visualizing the field of view in the hot stage
POM. The series under discussion is purely nematogenic with middle ordered melt-
ing type.
Homologous series with above core structure reported is known to show meso-
morphic characteristics. It is interesting to note that all the homologous series display
mesomorphism from the second homolog of the homologous series in the enantiotropic
condition. Mesomorphic characteristics exhibited by the molecules are directly related
with the intermolecular or intramolecular forces arising out of the design of the molecu-
lar structure which is nothing but its molecular geometry. Polarity of the attached sub-
stituent 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-elec-
tron 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 homologous series (1). The left ter-
minal 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 series (1) along with the pro-
gressive increase in the number of carbon atoms of the n-alkyl chain at the left terminal
for present homologous series.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
0
-Hydroxy-napthyl azo-4 -bromo-benzene is non- mesomorphic in nature. However,
2
addition of napthyl ring linked through carboxylate –COO– central bridge and Para
substituted 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 crystallinity in the
molecule. Thus, added linking units to the different azo dyes induces mesomorphism
enantiotropically optically especially nematic type mesophase of presently reported hom-
ologous series up to fourteenth homolog. This results in the exhibition of mesomor-
phism 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 vibra-
tions at high temperature to yield oriented anisotropic liquid. The molecules set them-
selves parallel to each other in a flowing condition giving rise to a threaded texture of
nematic mesophase enantiotropically for ten 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
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. But in present study, homologous series is nonlinear so transitions
are relatively lower. Thus, the view that, the elimination of hydrogen bonding lowers
the transitions of mesomorphism 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 tempera-
tures for odd and even members of the series is due to the difference in the length and
linearity of n-alkyl group of left alkoxy group. It is directly related to magnitude of
cohesive forces arising out of length and linearity of n-alkyl chain. Generally odd num-
ber 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 intermo-
lecular cohesive forces of attraction are affected by odd and even members of the hom-
ologous series; and hence the normal behavior of nematic–isotropic transition curves is
also affected as series is ascended. It is seen that nematic–isotropic transition curve
shows alternation of transition temperatures up to seventh 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 seventh homolog is understandable because alkyl chain of alkoxy group at
the left alternates in number of –CH – group. Generally odd homologs are loosely
2
packed as compared to even homologs up to fifth member of the series but thereafter
longer n-alkyl chain is coiled and does not maintain linearity. Hence transition temper-
atures of homologs beyond fifth 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 n-alkoxy benzoic acids, nonmesomorphicity of tetradecyloxy (C )
14
derivative and smectogenic property of ethyloxy to tetradecyloxy (C –C ) derivatives
2
14

8
S. N. KHER ET AL.
disappears on esterification of n-alkoxy benzoic acids through their corresponding acid
0
chlorides. Azo dye 2-hydroxy naphthyl azo 4 -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 lowering of transition temperatures of azoester molecules and the dis-
appearance of dimerization of n-alkoxy benzoic acids are due to the breaking of hydro-
gen 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 n-alkoxy benzoic acid molecules which increases molecular length
and width and acquires the suitable magnitudes of anisotropic forces of end to end
intermolecular attractions as a consequence of appropriate molecular rigidity and flexi-
bility. The disappearance of smectogenic behavior of ethyloxy to tetra decyloxy
(
C –C ) n-alkoxy acids is due to the nonlinear geometrical shape of corresponding
2 14
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 ther-
mal 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 C –C14 adopt a statistically parallel
2
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.
Conclusions
In conclusion, this work presents a successful approach to the synthesis of azoester mol-
ecules exhibiting liquid crystalline phases. To the best of our knowledge, as stated at the
beginning, these may help to improve the technology of reflective LC devices. The for-
mation of the low temperature nematic phase in these azo esters may also be useful in
the area of ferroelectric LC displays. Azoester novel homologous series of ten homologs
is entirely nematic with absence of smectic phase.
ꢂ
ꢂ
LC with naphthyl derivatives are generally nematogenic.
Suitable magnitudes of anisotropic forces of intermolecular end to end attrac-
tions occur as a consequence of favorable rigidity and flexibility.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
9
ꢂ
ꢂ
Molecular rigidity and flexibility are sensitive and susceptible to molecu-
lar structure.
A phenomenon of mesomorphism depends upon the molecular structure of a
substance concerned i.e. molecular shape, size, molecular polarity and polariz-
ability, dispersion forces, aromaticity constituting the molecular rigidity and
flexibility.
Acknowledgment
The authors are thankful to the Prof. and Head, Dr. N. K. Shah, Department of Chemistry,
School of Sciences, Gujarat University, Ahmedabad for her valuable support and co-operation in
research work.
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