Mesomorphism Dependence on Terminal/Lateral Substitution of Nonlinear Azoester Molecules

Article abstract of DOI:10.1080/15421406.2014.990198

A novel azo ester homologous series of liquid crystals (LCs) 2(4′-n-alkoxy benzoyloxy) napthyl 1 azo-3″ nitrobenzenes consists of eleven members of a series with enantiotropic nematogenic mesophase formation and without exhibition of smectogenic property by all the eleven homologs. The nematogenic mesophase textures are threaded or Schlieren. Solid-nematic and nematic-isotropic transition curves of the phase diagram behave in normal manner except for the decyloxy, dodecyloxy, and tetradecyloxy homologs. Analytical and spectral data confirm the molecular structures of homologues. Thermal stability for the nematic phase is 132.6°C, and the degree of mesomorphism varies between 34°C and 79°C and the series if of a middle-ordered melting type. An odd-even effect is observed for the N-I transition curve with alternation of transition temperatures. The LC behavior of the series is compared with other structurally similar known homologous series to establish the relation between mesomorphism and molecular structure.

Full text of DOI:10.1080/15421406.2014.990198

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism Dependence on Terminal/Lateral
Substitution of Nonlinear Azoester Molecules
H. R. Prajapati & A. V. Doshi
To cite this article: H. R. Prajapati & A. V. Doshi (2015) Mesomorphism Dependence on
Terminal/Lateral Substitution of Nonlinear Azoester Molecules, Molecular Crystals and Liquid
Crystals, 616:1, 28-36, DOI: 10.1080/15421406.2014.990198
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Date: 09 February 2016, At: 03:09

Mol. Cryst. Liq. Cryst., Vol. 616: pp. 28–36, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.990198
Mesomorphism Dependence on Terminal/Lateral
Substitution of Nonlinear Azoester Molecules
1,∗
1,2
H. R. PRAJAPATI AND A. V. DOSHI
1
C.U. Shah Science College, Ahmedabad, Gujarat, India
Matushri Virbaima Mahila Science and Home Science College, Saurashtra
2
University, Rajkot, Gujarat, India
ꢀ
A novel azo ester homologous series of liquid crystals (LCs) 2(4 -n-alkoxy benzoyloxy)
ꢀꢀ
napthyl 1 azo-3 nitrobenzenes consists of eleven members of a series with enantiotropic
nematogenic mesophase formation and without exhibition of smectogenic property by
all the eleven homologs. The nematogenic mesophase textures are threaded or Schlieren.
Solid-nematic and nematic-isotropic transition curves of the phase diagram behave in
normal manner except for the decyloxy, dodecyloxy, and tetradecyloxy homologs. An-
alytical and spectral data confirm the molecular structures of homologues. Thermal
◦
stability for the nematic phase is 132.6 C, and the degree of mesomorphism varies
◦
◦
between 34 C and 79 C and the series if of a middle-ordered melting type. An odd-even
effect is observed for the N-I transition curve with alternation of transition tempera-
tures. The LC behavior of the series is compared with other structurally similar known
homologous series to establish the relation between mesomorphism and molecular
structure.
Keywords Enantiotropic; liquid crystals; mesomorphism; nematic; smectic
Introduction
Liquid crystalline (LC) state of matter has molecular ordering between that of the ordered
crystalline solid and the disordered isotropic liquid, and was discovered in 1888 [1]. The
molecules of an LC state can be oriented under the influence of electric and magnetic field
[2]. Many LC materials with conventional and unconventional structures and textures have
been reported [3–5] due to their vital applications in various fields from manufacture of
various articles to biological systems [6–8]. The science and technology oriented commu-
nity of researchers have made massive contributions to thermotropic and lyotropic liquid
crystals [9,10]. A chemist has to synthesize novel LC materials through binary systems
and/or homologous series by changing molecular shape, size, polarity, polarizability, termi-
nal end group, lateral group, central group, or groups etc [11,12]. The present investigation
is planned to synthesize novel azoester homologous series of thermotropic type. Then it
1
is aimed to characterize novel LC materials by elemental analysis, infrared spectra, H
NMR spectra, mass spectra, and characterization of type or types of LC state and type of
texture of LC state through an optical polarization microscope equipped with a heating
∗
Address correspondence to H. R. Prajapati, C.U.Shah Science College, Ashram Road, Income
Tax, Ahmedabad, Gujarat, India. E-mail: drhrprajapati@yahoo.co.in
28

Mesomorphism Dependence on Terminal/Lateral Substitution
29
stage. Thus, the present study of LC materials is aimed with a view to understand and
establish the effects of molecular structure on the LC state of the matter on the basis of
molecular rigidity and flexibility [13–17] or to establish and corelate the relation between
LC properties and the molecular structure.
Synthesis
4-Hydroxybenzoic acid was alkylated by suitable alkylating agents (R-X) to convert it
into corresponding n-alkoxybenzoic acids by the usual established method of Dave and
ꢀ
Vora [18]. Azo dye 2-Hydroxy-1-napthyl azo 3 -nitro benzene was prepared by a reported
◦
method (M.P. is 201.0 C and Yield is 78%) [19]. 4-n-Alkoxybenzoic acids and azo dye
were individually condensed in ice-cooled pyridine to obtain a series of final azo ester
products [20]. Final products were decomposed, filtered, washed, dried, and purified until
constant transition temperatures were obtained.
The chemicals 4-hydroxybenzoic acid, alkyl halides [R-X], methanol, ethanol, KOH,
thionyl chloride, pyridine, β–napthol, meta nitro 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 shown below in scheme 1.
Characterization
Some selected members of the novel series were characterized by elemental analysis,
1
polarizing microscopy, mass spectra, infrared spectra, H-NMR spectra, and texture deter-
mination by a miscibility method. Microanalysis was performed using a Perkin Elmer PE
2
400 CHN analyzer, IR spectra were performed using a Perkin Elmer spectrometer, and
1
H-NMR spectra were performed using a Bruker spectrometer with CDCl3 as solvent.
Analytical Data
−1
IR Spectra in cm : for Methoxy and Propyloxy derivatives:
Methoxy: 2918,2873,1447(alkyl group),1733,1684,1200(ester group),1528,1347(Ar
NO2), 1156 (C N Str. due to N N-), 668,761(m-sub.benzene) 840(p-sub benzene).
Propyloxy: 2966, 2848,1457(alkyl group),1733(ester group), 1528,1350 (Ar NO2), 1167
(
C N Str. due to N N ), 642,773 (m-sub. benzene), 848 (p-sub benzene).
1
H- NMR Spectra in CDCl3 in δ ppm: for Butyloxy and Octyloxy derivatives
Butyloxy: 0.98(t, 3H CH3),1.25(m,4H, CH2),4.052 (t, 2H, OCH2),7.0–8.40(m,14H,
Ar H).
Octyloxy:0.89(t,
3H,-CH3),1.80(m,12H, CH2),4.01(t,2H, OCH2),6.90-8.36(m,14H,
Ar H).
Mass spectra (molecular weight) propyloxy homolog:
Calculated-455
Experimental-455
Textures by miscibility method
Nematic:
Propyloxy homolog—threaded texture
Heptyloxy homolog—threaded texture
Decyloxy homolog—Schlieren texture

30
H.R. Prajapati and A.V. Doshi
Scheme 1. Synthetic route to the series.
Results and Discussion
Transition and melting temperatures of the novel homologues (Table 2) were plotted versus
the number of carbon atoms present in the n-alkoxy terminal end group. Like or related
points are linked to draw the solid-nematic and nematic-isotropic transition curves forming
a phase diagram (Fig. 1) showing phase the behavior of the series.
The solid-nematic transition curve adopts a zigzag path of rising and falling values with
an overall descending nature and behaves in a normal manner. The N-I transition curve for
odd members (C1–C7) initially rises and then descends and for even members of the series
it descends up to the octyloxy homolog and then it ascends to the dodecyloxy homolog
through the decyloxy homologue derivative and finally descends to the tetradecyloxy ho-
mologue. Thus, the N-I transition curve behaves in a normal manner up to the octyloxy
◦
◦
homologue, but it deviates from normal behavior by about 5 C–10 C from and beyond the

Mesomorphism Dependence on Terminal/Lateral Substitution
31
Table 1. Elemental analysis for propyloxy, pentyloxy, heptyloxy, and dodecyloxy
derivatives
Elements% found (calculated%)
Sr. no.
Molecular formula
C
H
N
1
2
3
4
C26H21N3O5
C28H25N3O5
C30H29N3O5
C35H39N3O5
68.67(68.57)
68.90(69.56)
70.35(70.45)
72.18(72.28)
4.51(4.61)
5.01(5.17)
5.49(5.67)
6.51(6.71)
9.23(9.33)
8.59(8.69)
8.10(8.21)
7.10(7.22)
octyloxy homolog. An odd-even effect is exhibited by the N I transition curve. The N I
transition curves for odd and even members merges into each other between the octyloxy
and the nonyloxy derivatives of the series and the N I curve propagates as a single curve.
Smectogenic property is totally absent against the nematogenic property, which is totally
present for all the members of the series. LC behavior of the series varies from homolog to
◦
homolog. The average thermal stability for nematic is 132.6 C. The mesomorphic phase
◦
◦
length varies from a minimum of 34 C at the propyloxy homolog to a maximum of 79 C at
◦
◦
the dodecyloxy homolog. The isotropic temperatures range from 125 C to 146 C. Thermal
data confirmed on an optical polarizing microscope. Analytical and spectral data confirms
the molecular structures of novel homologs. Thus novel homologous series is entirely
nematogenic with a rich degree of mesomorphism.
The odd-even effect observed for the N I transition curve is due to the sequentially
added methylene unit to the n-alkyl chain. Exhibition of LC state by the members of the
◦
series is attributed to the misalignment of the molecules at an angle of less than 90 with the
plane of a surface resisting against the exposed thermal vibrations for some temperatures
◦
ꢀ
ꢀꢀ
Table 2. Transition temperatures in C for 2(4 -n-alkoxy benzoyloxy)-napthyl-1-azo-3 -
nitro Benzenes
◦
Transition temperatures in C
Compound no.
n-alkyl group Cn H2n+1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
—
—
—
—
—
—
—
—
—
—
—
94.0
109.0
99.0
94.0
89.0
86.0
81.0
72.0
83.0
67.0
69.0
129.0
140.0
133.0
132.0
125.0
128.0
125.0
128.0
133.0
146.0
140.0
10
12
14
10
1
1
Sm, smectic; N, nematic.

32
H.R. Prajapati and A.V. Doshi
Figure 1. Phase behaviours of series.
limit; beyond which withstanding of exposed thermal vibrations ceases to resist. The net
magnitude of intermolecular attractions actually generated is sufficient to maintain statis-
tically parallel orientational order of molecules to facilitate nematic mesophase formation
only. But it is insufficient to maintain two- or three-dimensional molecular network in a
floating condition to slide out the layers of molecules on the surface, causing elimination
of the smectic mesophase for all the novel homologs of presently investigated series. The
normal behavior of the N I transition curve is attributed to normal status of n-alkyl chain

Mesomorphism Dependence on Terminal/Lateral Substitution
33
Figure 2. Structurally similar series.
ineffective to molecular rigidity and flexibility. The diminishing of the odd-even effect in
the N I transition curve from and beyond the seventh or eighth homolog of the series
is due to coiling or bending or flexing or coupling of n-alkyl chain with the major axis
of a core structure of the homolog molecules. The variations in mesomorphic properties
from homolog to homolog in the same series is due to the changing number of carbon
atoms present in n-alkyl chain of left n-alkoxy terminal end group; which causes gradual
variations in molecular flexibility in combination of molecular rigidity. The low melting
◦ ◦
67 C–109 C) is due to the nonlinear shape of the molecules, which widens the homologue
(
molecules to increase the intermolecular closeness avoiding effect of polarizability factor.
However, molecular rigidity and flexibility responsible to induce a rich mesomorphism
◦ ◦
34 C–79 C) is enough for nematogenic mesophase but it is not favorable to fulfill the
(
requirements to induce and stabilize smectic mesophase. The mesomorphic (LC) behaviors
of the presently investigated novel homologous series 1 are compared with the structurally
similar known homologous series X [21] and Y [22] as shown in Fig. 2.
The presently investigated azoester homologous series 1 and the structurally similar
known homologous series X and Y chosen for comparison are identical with respect to

34
H.R. Prajapati and A.V. Doshi
Table 3. Average Thermal Stabilities in C
◦
Series →
1
X
Y
Smectic-nematic
or Smectic-isotropic
commencement of smectic
mesophase
—
—
—
Nematic-isotropic
commencement of nematic
phase
132.6
(C1–C14) C1
127.27
(C1–C14) C1
162.90
(C1–C14) C1
◦
◦
◦
◦
◦ ◦
29 C–93 C
Degree of mesomorphism
34 C–79 C
10 C–41 C
◦
from t1 to t2 C
two phenyl and one napthyl ring, COO and N N central bridges and an n-alkoxy
OCnH2n + 1 terminal end group for the same homolog from series to series, but it differs
–
with respect to changing meta –NO2, para –CH3 and para –NO2 from series 1 to X to Y,
respectively. Therefore, variation in liquid crystal or mesomorphic property for the same
homolog from series to series and the degree of mesomorphism, average thermal stability,
commencement of mesophase, and other physical properties vary according to changing
molecular flexibility in combination with the molecular rigidity from series to series for the
same homolog. The variations in liquid-crystal properties from homologue to homologue
in the same series depend upon the changing molecular flexibility due to changing number
of methylene unit or units of the n-alkoxy terminal for all the homologs of the same
series. Table 3 represents changing trend of average thermal stability, commencement of
mesophase, and degree of mesomorphism as shown below.
Homologous series 1 and Y are isomeric except positions of the NO2 group substi-
tuted at the meta or para position of the same phenyl ring. Homologous series Y is identical
in all respects to series 1 except for the CH3 group substituted in para position of a phenyl
ring bonded to the N N central bridge. Therefore, variations arising in LC behaviors
will depend only on the changing molecular part; viz., meta NO2, para CH3 and para
NO2 terminal or laterally substituted group of each series under comparison. Table 3
indicates that
All the three azoester homologous series (1,X,Y) under comparison are entirely nemato-
genic without exhibition of smectogenic mesophase.
Nematogenic mesophase commences from the very first member of all the three series and
continued up to the last tetra decyloxy homologue with more or less thermal stabilities
and the degree of mesomorphism.
Nematic thermal stability of presently investigated series 1 is more than a series X and less
than a series Y under comparison.
The degree of mesomorphism of present series 1 is more (79 – 34 = 45) than a series X
(
41 – 10 = 31) and less than (93 – 29 = 64) a series Y.
All the three series contains a napthyl unit and two phenyl ring as well as two cen-
tral bridges viz., COO and N N as common identical part which contributed to
molecular rigidity almost in equal proportion but molecular flexibility due to laterally and
terminally substituted groups vary from homolog to homolog in the same series and for
the same homologue from series to series (1 to X to Y). The absence of smectogenic

Mesomorphism Dependence on Terminal/Lateral Substitution
35
mesophase formation indicates that the combine effects of molecular rigidity and flexibility
emerging unequal anisotropic forces of intermolecular attractions of almost equally spaced
nonlinear molecules and of unequal molecular polarity and polarizability of series 1, X,
and Y under comparison, hinders the lamellar packing of molecules in their crystal lattices
to form three-dimensional layered molecular network ordered arrangement which can re-
sult into sliding layered ordered arrangement while floating on the surface. Thus, smectic
mesophase formation is hindered and only nematogenic mesophase formation is favored
by suitable net magnitudes of anisotropic forces of intermolecular end to end attractions
in series 1, X, and Y. The suitable magnitudes of anisotropic forces of intermolecular end
to end attractions as a consequence of favorable molecular flexibility and rigidity facilitate
and maintain the statistically parallel orientational order of molecules only from very first
member to last member of all the series under comparison in floating condition; but it is
efficient to hinder sliding layered arrangement of molecules until the last tetra decyloxy
homolog, eliminating formation of smectic phase. The changing magnitudes of molecular
flexibility in combination with their corresponding rigidity vary for the same homolog from
series to series due to the variations of individual polar nature of less polar –CH3 and highly
polar –NO2 groups and their varying positional (-ortho, -meta, and -para) status.
Conclusions
Presently investigated novel azoester homologous series consisting of 11 nematogenic
homologues is an entirely nematogenic with absence of smectogenic character.
Nonlinearly shaped azoester series containing napthyl unit are generally nematogenic.
Group efficiency order for nematic derived on the basis of (i) thermal stability and (ii) the
early commencement of nematic are as under.
Nematic:
para NO2- > meta NO2- > para CH3
Nematic: meta NO2- para NO2- para CH3
Group efficiency of same polar group depends upon its position of substitution.
The phenomena of mesomorphism is sensitive and susceptible to rigidity and flexibility of
a molecular structure.
Degree of mesomorphism and mesomorphic behaviors of a substance are sensitive to
molecular structure.
Acknowledgments
Authors acknowledge thanks to the Principal and the Management of M.V.M. Science and
Home Science College, Rajkot, for their valuable support and cooperation, as and when
needed. Thanks are due to the CSMCRI Bhavnagar for analytical services and The M.S.
University of Baroda, Faculty of Tech and Engg., Vadodara, for providing microscopic
facility by applied chemistry department.
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