Mesomorphism dependence on molecular flexibility in an azoester series

Article abstract of DOI:10.1080/15421406.2014.953740

A novel azoester homologous series with a flouro terminal end group was synthesized and evaluated for mesomorphic properties. The novel homologous series consists of 12 members. The first two-third members of the series are nonmesogenic. The butyloxy to decyloxy homologues are enantiotropic nematogenic and the rest of the mesogenic homologoues are enantiotropic smectogenic only. Transition temperatures and textures of the mesophases were observed through an optical polarizing microscope equipped with a heating stage. The textures of the nematic phase are threaded or Schlieren as judged by a miscibility method and those of the smectic phase are of the type A or C. A phase diagram is used to illustrate the mesomorphic behavior across the series. Analytical and spectral data confirm the molecular structures of the homologues. Thermal stabilities of smectic and nematic mesophases are 114.3°C and 137.0°C respectively. The mesogenic phase lengths for smectic and nematic are vary from 8°C to 26°C and 26°C to 43°C respectively.

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Mesomorphism Dependence on Molecular
Flexibility in an Azoester Series
G. N. Bholaa, Kiran Nakuma, Kaushal Kariaa & U. C. Bhoyaa
a Department of Chemistry, Saurashtra University, Rajkot, Gujarat,
India
Published online: 03 Mar 2015.
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To cite this article: G. N. Bhola, Kiran Nakum, Kaushal Karia & U. C. Bhoya (2015) Mesomorphism
Dependence on Molecular Flexibility in an Azoester Series, Molecular Crystals and Liquid Crystals,
608:1, 125-134, DOI: 10.1080/15421406.2014.953740
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Mol. Cryst. Liq. Cryst., Vol. 608: pp. 125–134, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.953740
Mesomorphism Dependence on Molecular
Flexibility in an Azoester Series
G. N. BHOLA, KIRAN NAKUM, KAUSHAL KARIA,
AND U. C. BHOYA∗
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
A novel azoester homologous series with a flouro terminal end group was synthesized
and evaluated for mesomorphic properties. The novel homologous series consists of
12 members. The first two-third members of the series are nonmesogenic. The butyloxy
to decyloxy homologues are enantiotropic nematogenic and the rest of the mesogenic
homologoues are enantiotropic smectogenic only. Transition temperatures and textures
of the mesophases were observed through an optical polarizing microscope equipped
with a heating stage. The textures of the nematic phase are threaded or Schlieren as
judged by a miscibility method and those of the smectic phase are of the type A or C.
A phase diagram is used to illustrate the mesomorphic behavior across the series. Ana-
lytical and spectral data confirm the molecular structures of the homologues. Thermal
stabilities of smectic and nematic mesophases are 114.3◦C and 137.0◦C respectively.
The mesogenic phase lengths for smectic and nematic are vary from 8◦C to 26◦C and
26◦C to 43◦C respectively.
Keywords Liquid crystals; mesomorphism; nematic; smectic
Introduction
Liquid crystal material are useful in technological applications, such as display devices,
optical imaging, light emitting diodes, anisotropic network, semiconductor materials, etc.
[1]. Biomaterials in combination with thermotropic liquid crystals improves the perfor-
mance of liquid crystal displays using LC physical gel consisting of acid-base gelators [2].
Benzoyloxy and cinnamoyloxy derivatives are useful in agricultural and pharmaceutical
fields. Chalcones derivatives are useful as anti-inflammatory anticancer and antioxidant
[3, 4]. Thus, conventional and unconventional [5, 6] liquid crystal materials of different
moieties containing ether, ester, azoeter, ethylene, etc. derivatives have been reported ear-
lier [7–9]. The present investigation is planned to synthesize novel liquid crystal materials
consisting of benzoyloxy naphthyl derivatives with n-alkoxy and fluorine as terminal end
groups and COO and N N central linking groups to understand and establish the
effects of molecular flexibility[10–13] of the molecules on liquid crystal behaviors. The
present investigation will include synthesis of novel compounds, their characterization by
∗Address correspondence to U. C. Bhoya, Department of Chemistry, Saurashtra University,
Rajkot-360005, Gujarat, India. Tel: +91 281 2583984, Fax: +91 281 2583984. E-mail: drucbhoya@
sauuni.ernet.in
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
125

126
G. N. Bhola et al.
1
polarizing microscopy, mass spectrometry, IR, HNMR spectra, elemental analysis, their
comparative study of mesogenic behaviors with known series based on effects of molecular
structure on liquid crystal behaviors [14, 15].
Experimental
Synthesis
4-Hydroxybenzoic acid was alkylated by suitable alkylating agents to give the required
4-n-alkoxy benzoic acids, which on further treatment with thionyl chloride gave the cor-
responding acid chlorides [A] by the modified method of Dave and Vora [16] and these
were used directly for further reaction without purification. Azo dye p- fluoro-p’-hydroxy
azonaphthalene [B] M.P. 80◦C was prepared by the usual established method of diazotiza-
tion [17, 18]. Components [A], i.e., 4-n-alkoxy benzoyl chloride and component [B], i.e.,
azo dye were condensed in dry cold pyridine to prepare the final azoester molecules by the
usual etherification method [19]. Final products were individually decomposed, filtered,
washed, dried, and purified until constant transition temperatures were obtained. Transi-
tion temperatures of homologues were determined on an optical polarizing microscope
equipped with a heating stage. The chemicals viz. 4-hydroxy benzoic acids, alkyl halides
(R–X), methanol, ethanol, KOH, thionyl chloride, α- naphthol, p-fluoro aniline, pyridine,
NaOH, etc. required for synthesis were used as received excepted solvents, which were
dried and purified prior to use. The synthetic route to the novel azoester series is shown in
scheme 1.
Characterization
Some of members of the novel series as the representative members were characterized by
elemental analysis (Table 1), infrared spectroscopy, 1H NMR spectra, mass spectrometry,
and textural identification. Microanalysis was performed on EuroEA Elemental Analyzer.
1
IR spectra were recorded on Shimadzu FTIR-8400 spectrometer, HNMR spectra were
recorded on Bruker spectrometer using DMSO-d6 as a solvent and mass spectra were
recorded on Shimadzu GC-MS Model No.QP-2010 spectrometer. The liquid crystal be-
havior and the type of textures were determined by a miscibility method on microscopic
observations.
Analytical Data
Elemental Analysis (Table 1)
Spectral Data
1HNMR in ppm for Hexyloxy Derivative
0.87–091 (t, 3H, CH3 of OC6H13 group), 1.28–1.33 (4H, CH3 CH2 CH2 ), 1.74–1.77
(2H, CH2 CH2 CH2 O ), 4.06–4.08 (t, 2H, CH2 O ), 6.85–8.71 (12H, Aromatic
Hydrogen), 8.21–8.74 (d, 1H, in phenyl ring ortho to N = N-group),The NMR data are
reliable with the molecular structure.

Mesomorphism Dependence on Molecular Flexibility in an Azoester Series 127
Scheme 1. Synthetic route to the novel series.

128
G. N. Bhola et al.
Table 1. Elemental analysis for (1) Ethyloxy (2) Butyloxy (3) Tetradecyloxy derivatives
Elements % Found Elements % calculated
Sr. no.
Molecular formula
C
H
N
C
H
N
1
2
3
C25H19FN2O3
C27H23FN2O3
C37H43FN2O3
72.49
73.24
76.23
4.60
5.27
7.42
6.73
6.35
4.86
72.45
73.29
76.26
4.62
5.24
7.44
6.76
6.33
4.81
1HNMR in ppm for Dodecyloxy Derivative
0.83–0.89 (t, 3H, CH3 of OC12H25 group), 1.26–1.30 (m, 16H, CH3 CH2 ( CH2
)
6
CH2–), 1.43 ( O CH2 CH2 CH2 ), 1.74–1.77 (m, 2H, CH2 CH2 O ), 4.07
(t, 2H, CH2 O ), 6.84–8.12 (12H, Aromatic Hydrogen), 8.22–8.72 (d, 1H, in phenyl
ring ortho to N N group),The NMR data are reliable with the molecular structure.
IR in cm−1 for Butyloxy Derivative
2852, 2920 (C H Str. of ( CH2 )n group of OC7H15), 1440, 1604 (aromatic C C Str.),
3068, (C H Str. of aromatic ring), 1010, 1068, 1168, 1267 (C O Str. of COO-group),
1728 (C O, Str. of COO group), 1139 (C O C ), 750, 848, 698 (pera, ortho, and
meta substituted phenyl ring) 1604 ( N N Str.). The IR data are consistent with the
molecular structure.
IR in cm−1 for Octyloxy Derivative
2852, 2933 (C H Str. of ( CH2–)n group of OC12H25), 1465, 1506, 1579 (aromatic
C C Str.), 3070 (C H Str. of aromatic ring), 1066, 1170 (C O Str. of COO-group),
1732 (C O, Str. of COO-group), 1143 (C O C ), 750, 842, 692 (pera, ortho, and
meta substituted phenyl ring).1604 ( N N Str.). The IR data are consistent with the
molecular structure.
Mass Spectra of Heptyloxy Derivative
m/z (rel. int%): 485 (M)+, 249, 235, 115, 99
Mass Spectra of Decyloxy Derivative
m/z (rel. int%): 527(M)+, 385, 277, 249, 141
Results and Discussion
The novel homologous series 4-[4ꢀ-n-alkoxy benzoyloxy]nephthylazo 4ꢀ-fluoro benzenes
generates mesomorphism from the butyloxy homologue to the hexadecyloxy homologue,
but the shorter chain homologues are not liquid crystalline. The butyloxy to decyloxy
homologues are enantiotropic nematogenic, the dodecyloxy, tetradecyloxy, and hexadecy-
loxy homologues are enantiotropic smecogenic. The transition temperatures of the series,
as determined by optical polarizing microscopy equipped with a hot stage (Table 2), are

Mesomorphism Dependence on Molecular Flexibility in an Azoester Series 129
Table 2. Transition temperatures in ◦C
Compound no.
n-alkyl chain CnH2n+1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
130.0
122.0
109.0
99.0
83.0
85.0
—
160.0
175.0
145.0
156.0
143.0
140.0
131.0
124.0
128.0
118.0
115.0
110.0
10
11
12
92.0
94.0
102.0
—
—
Sm = Smectic, N = Nematic.
plotted against the number of carbon atoms in the terminal chain. The like or related points
are linked to give smectic–isotropic, nematic-isotropic and solid-isotropic/mesomorphic
curves to represent a phase diagram (Fig. 1) showing the phase behaviors of the novel
series. The solid-isotropic/mesomorphic transition curve follows a zigzag path of rising
and falling values up to the third member of the series, then it alternatively descends con-
tinuously to the octyloxy homologue and then it ascends to the hexadecyloxy homologue
with overall descending tendency as series is ascended. A smectic-isotropic transition curve
Figure 1. Phase behavior of series.

130
G. N. Bhola et al.
is descended from the dodecyloxy to hexadecyloxy homologues without the exhibition of
an odd–even effect. The smectic-isotropic transition curve is extrapolated [20, 21] from the
even members, decyloxy, and octyloxy, which are actually smectogenic in character, and
the extrapolated transition curve suggests a monotropic N-Sm transition temperatures of
113◦C and 99◦C. Re-examination of the decyloxy and octyloxy samples on polarizing mi-
croscopy still did not show monotropic nematic behavior. The nematic-isotropic transition
curve shows a descending trend from the butyloxy to decyloxy derivatives with exhibition of
an odd–even effect and alternation of transition temperatures. Thereafter the nematogenic
mesophase ceased to appear for the higher homologues. The nematic-isotropic transition
curves for odd homologues (7 and 5) were extrapolated [22, 23] up to the third member
of the series to magnify odd–even effect. All three transition curves of the phase diagram
show phase behavior in a normal manner.
The odd–even effect disappears from and beyond the heptyloxy homologue for the
longer n-alkoxy chain. The mesomorphic behavior varies from homologue to homologue
in the present series. The present novel series is predominantly nematogenic and partly
smectogenic with middle ordered melting type, whose mesomorphic phase lengths vary
between 08◦C to 43◦C at the hexadecyloxy and decyloxy homologues respectively.
The dimerization of n-alkoxy benzoic acids disappeared by esterification process due
to breaking of hydrogen bonding between two molecules of 4-n-alkoxy benzoic acids,
azodye 4-hydroxy naphthylazo 4ꢀ-fluoro benzene is a nonmesogenic component of a novel
azoester molecule. However, linking the azo section with the 4-n-alkoxy benzoic acid
through corresponding acid chloride increases molecular length and breadth which causes
an increase in molecular polarity and polarizibility to withstand against the external force
operating by thermal vibrations. The laterally substituted phenyl ring of naphthalene unit
increases molecular rigidity which increases resistivity against exposed thermal vibrations,
through molecular polarity and polarizibility. However, induced polarity and polarizability
causing end to end and lateral attractions are of unsuitable magnitudes for methyloxy
to propyloxy homologues to resist exposed thermal vibrations. Therefore, they sharply
transform into isotropic liquid without passing through an intermediate state of existence
between crystalline solid and isotropic liquid; as a consequence of unfavorable magnitudes
of molecular rigidity and flexibility. But molecular rigidity and flexibility of the molecules
from and beyond butyloxy homologue of a present novel series become favorable to
induce mesogenic mesophase within definite range of temperature. The resultant molecular
rigidity and flexibility generates suitable magnitudes of anisotropic forces of end to end and
lateral intermolecular attractions of appropriate intermolecular closeness for mesomorphic
homologues, which causes the disalignment of molecules at an angle less than 90 degree
with the plane of a floating surface. The molecules of butyloxy homologue to decyloxy
homologue under the influence of end to end attractions floats with statistically parallel
orientational two dimensional orders within some degree temperature range, giving rise to
appearance of nematogenic mesophase. The molecules of dodecyloxy, tetradecyloxy, and
hexadecyloxy homologues with relatively more favorable magnitudes of intermolecular
attractions bear lamellar packing of molecules in their crystal lattices which form focal
conic molecular networking which float on the plane of a surface as sliding layers within
definite temperature range with appearance of smectic mesophase formation. The variation
in mesomorphic properties from homologue to homologue in the present series is attributed
to the sequential addition of methylene unit or units. The disappearance of odd-even effect
from and beyond heptyloxy homologue for relatively longer n-alkyl chain is attributed
to the coiling or bending or flexing or coupling of n-alkyl chain with major axis of the
core structure. Thus, uncertainty in the status of n-alkyl chain of higher homologues

Mesomorphism Dependence on Molecular Flexibility in an Azoester Series 131
Figure 2. Structurally similar series.
plays the role for presence or absence of prolongation of odd–even effect through n-alkyl
chain of n-alkoxy terminal end group. The mesomorphic behavior of presently investigated
homologous series-1 is compared with a structurally similar other known homologous
series X [23] and Y [24]; as shown in following Fig. 2.
Homologous series 1 and series X and Y chosen for comparison are identical with
respect to two phenyl rings bonding two fused phenyl rings (naphthyl) through COO
and N N central bridges which builds up molecular rigidity. i.e., the effects due to
molecular rigidity for all the three series 1, X, and Y play an equivalent role in inducing
mesomorphism. But, they differ with respect to their molecular flexibility which immerges
from changing n-alkyl chain length from homologue to homologue in the same series
and changing terminal end group F, Cl and lateral OCH3 group for the same homo-
logue from series to series. Thus, flexibility contribution to the mesomorphism varies from
homologue to homologue in the same series and for the same homologue from series to se-
ries. Therefore, the mesomorphic behavior and the degree of mesomorphism depend upon
the differing features of each series under comparison and discussion. Following table 3
represents some mesomorphic behavior like, thermal stabilities for smectic and nematic
mesophase, commencement of mesophases, mesomorphism exhibition by homologues, etc.
Table 3 indicates the following.
• Homologous series-1 exhibited smectogenic and nematogenic mesomorphism;
whereas series X and Y selected for comparison exhibited only nematogenic meso-
morphism.
Table 3. Average thermal stabilities in ◦C
Series →
1
X
Y
Smectic-isotropic
Commencement of smectic mesophase
Nematic-isotropic
114.3
(C12–C16) C12
137.0
—
—
129.9
—
—
149.2
Commencement of nematic phase
(C4–C10) C4
(C3–C16) C3
(C6–C14) C6

132
G. N. Bhola et al.
• Nematogenic mesomorphism commences from C4, C3, and late from C6 carbon
atom for series 1, X, and Y respectively.
• Smectogenic property commences from C12 carbon in case of series 1 but it does
not commence till the last member of the series X and Y.
• Mesomorphic phase length vary from 08◦C to 43◦C for series 1, while it ranges from
31◦C to 50◦C for series X and 28◦C to 65◦C for series Y.
• Smectic thermal stability of series 1 is 114.3◦C while smectic mesophase formation
did not stabilize and facilitate for series X and Y.
• Nematic mesophase formation facilitated and stabilized for certain degree of meso-
morphism.
The electron attracting and electron accepting tendency of terminally/laterally situated
fluorine, chlorine, and oxygen atom are in the order of F > O > Cl. Therefore, the bond
polarity of a bond linking carbon atom of a third phenyl ring and terminal end group (i.e.,
C+δ F−δ
<
C+δ O−δ
<
C+δ Cl−δ) decreases from F to Cl. Therefore, end to end
intermolecular attractions and the degree of molecular flexibility from homologue to homo-
logue in the same series and for the same homologue from series to series as well as extent
of molecular co-planarity undergo variations accordingly. However, the lateral substitution
of OCH3 group and the varying atomic size of F and Cl contribute to the molecu-
lar polarizability toward suitable magnitudes of intermolecular attractions, intermolecular
closeness positively, excluding the negative effect to intermolecular attractions by steric
hindrance due to ortho substituted OCH3 to N N central bridge, etc. Thus, the next
effects of the suitable magnitudes operating intermolecular attractions by changing molec-
ular flexibility, inducing anisotropic forces to cause mesophase formation either smectic
or/and nematic. The net intermolecular anisotropic forces of intermolecular attractions are
sufficient to cause lamellar packing of molecular in the crystal lattices of dodecyloxy,
tetradecyloxy, and hexadecyloxy homologues of series 1 which, forms sliding layers to
show only smectic mesophase formation and then the rest of the mesomorphic homologues
of series 1 due to absence of lamellar packing, floated with only statistically parallel ori-
entational ordered and exhibited only nematic mesophase formation. However, under the
same condition, the net anisotropic forces operating amongst the molecules of series X
and Y chosen for comparison is insufficient to facilitate lamellar packing of molecules in
their crystal lattices but sufficient to float with only statistically parallel orientational order
from C3 and C6 homologues with relatively higher degree of nematogenic mesomorphism
only. The highest value of nematic thermal stability is attributed to the highest polarizibility
contributed by laterally substituted OCH3 polar group, which exceeded the magnitudes
of end to end attractions. Absence of lamellar packing in the crystal lattices as well as
unsuitable extent of noncoplanarity caused by the molecules of series X and Y, fails to
facilitate interlinking of molecules to form focal conic three dimensional arrangement of
layered structure. Hence, smectogenic mesophase formation is missing for the series X and
Y. Thus, smectogenic mesophase commences though late from twelfth member of a series
1 but it does not commence till the last member of a series X and Y. Early statement that
“naphthalene derivatives of LC are generally nematogenic” is partly supported by present
investigation, but it is not universally true. Thus, variations in mesomorphic behaviors from
homologue to homologue in the same series and for the same homologue from series to
series depend upon changing n-alkyl chain by methylene unit or units for same series but
it vary with changing right handed terminal or lateral end group for the same homologue
from series to series respectively.

Mesomorphism Dependence on Molecular Flexibility in an Azoester Series 133
Conclusions
• Presently investigated naphthylazoester series is predominantly nematogenic and
partly smectogenic with middle ordered melting type whose degree of mesomor-
phism between 8◦C to 43◦C.
• Fluorine is corrosive to the health; therefore, it should be carefully handled while
using it.
• The group efficiency order derived for smectic and nematic mesophase formation
on the basis of
(a). Thermal stability and
(b). The early commencement of mesophase is as under.
a). Smectic: –F > –Cl = –OCH3(lateral)
Nematic: lateral –OCH3 > –F > –Cl
b). Smectic: –F > –Cl = –OCH3 (lateral)
Nematic: –Cl > –F > –OCH3 (lateral)
• Molecular flexibility difference can cause variations in mesomorphic behaviors of a
substance.
• Mesomorphic properties of a substance are sensitive and susceptible to molecular
flexibility.
Acknowledgment
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 are
due to the National Facility for Drug Discovery through New Chemical Entities (NCE’s)
for analysis of samples.
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