Synthesis and mesomorphic behavior of novel liquid crystalline: 2-[4′-n-alkoxy benzoyloxy]-naphthylazo-2″-methoxy benzenes

Article abstract of DOI:10.1080/15421406.2012.693010

Eleven members of a novel azoester homologous series entitled 2-[4-n-alkoxy benzoyloxy]-naphthylazo-2-methoxybenzenes are reported. The methyl to n-butyl homologues are nonmesomorphic, but the other homologues are mesomorphic. The pentyl, hexyl, and hexadecyl homologues exhibit only the nematic phase. The octyl, decyl, and dodecyl homologues exhibit a smectic phase in addition to the nematic phase, however, the tetradecyl homologue exhibits only a smectic phase. An odd-even effect with alternation of transition temperatures is observed for thenematic-isotropic transition curve. The average thermal stability and liquid crystal behavior for the smectic and nematicmesophases are 99.5 and 132.0, respectively, and the mesomorphic properties are compared with structurally isomeric homologous series. Analytical data confirms the molecular structures of the novel materials. The textures from optical polarizing microscopy of the nematic and smectic phase show the expected Schlieren and focal conic appearances, respectively. Copyright Sharp Laboratories Europe Ltd.

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Synthesis and Mesomorphic Behavior of
Novel Liquid Crystalline: 2-[4′-n-Alkoxy
Benzoyloxy]-Naphthylazo-2″-Methoxy
Benzenes
V. R. Patel a , N. N. Vyas a & A. V. Doshi b
a Chemistry Department., Sheth P. T. Arts & Science College, Gujarat
University, Godhra, Gujarat, India
b Matushri Virbaima Mahila Science & Home Science College,
Saurashtra University, Kalavad Road, Rajkot, Gujarat, India
Published online: 16 Nov 2012.
To cite this article: V. R. Patel , N. N. Vyas & A. V. Doshi (2012) Synthesis and Mesomorphic Behavior
of Novel Liquid Crystalline: 2-[4′-n-Alkoxy Benzoyloxy]-Naphthylazo-2″-Methoxy Benzenes, Molecular
Crystals and Liquid Crystals, 569:1, 15-23, DOI: 10.1080/15421406.2012.693010
To link to this article: http://dx.doi.org/10.1080/15421406.2012.693010
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Mol. Cryst. Liq. Cryst., Vol. 569: pp. 15–23, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.693010
Synthesis and Mesomorphic Behavior of Novel
Liquid Crystalline: 2-[4ꢀ-n-Alkoxy
Benzoyloxy]-Naphthylazo-2ꢀꢀ-Methoxy Benzenes
V. R. PATEL,1 N. N. VYAS,1 AND A. V. DOSHI2,∗
1Chemistry Department., Sheth P. T. Arts & Science College, Gujarat University,
Godhra, Gujarat, India
2Matushri Virbaima Mahila Science & Home Science College, Saurashtra
University, Kalavad Road, Rajkot, Gujarat, India
Eleven members of a novel azoester homologous series entitled 2-[4ꢀ-n-alkoxy
benzoyloxy]-naphthylazo-2ꢀꢀ-methoxybenzenes are reported. The methyl to n-butyl ho-
mologues are nonmesomorphic, but the other homologues are mesomorphic. The pentyl,
hexyl, and hexadecyl homologues exhibit only the nematic phase. The octyl, decyl, and
dodecyl homologues exhibit a smectic phase in addition to the nematic phase, how-
ever, the tetradecyl homologue exhibits only a smectic phase. An odd–even effect with
alternation of transition temperatures is observed for thenematic-isotropic transition
curve. The average thermal stability and liquid crystal behavior for the smectic and
nematicmesophases are 99.5◦ and 132.0◦, respectively, and the mesomorphic properties
are compared with structurally isomeric homologous series. Analytical data confirms
the molecular structures of the novel materials. The textures from optical polarizing
microscopy of the nematic and smectic phase show the expected Schlieren and focal
conic appearances, respectively.
Keywords Azoester; Mesomorphism; mesophase; nematic; smectic
Introduction
A combination of molecular rigidity and flexibility, and anisotropic forces of attractions
serve to dictate the occurrence of mesomorphism. The variation in molecular rigidity or
flexibility is brought about by varying the terminal end groups on either side or the laterally
substituted functional groups on the rigid core, which usually consists of phenyl rings. The
present work concerns the evaluation and synthesis of a homologous series of azoesters
with –OCH3 as a laterally substituted functional group on the phenyl ring bridged through
a –N N- linkage in combination with n-alkoxy left terminal end group bridged through a
–COO- central group as the flexible part of a molecule, and a naphthyl unit linked between
two phenyl rings on either sides by central bridges, viz. –COO- and –N N- as the rigid
core of the molecule. Thus, the naphthyl moiety was selected to vary molecular rigidity, and
the –OCH3 lateral group and the n-alkoxy terminal groups were selected to vary molecular
flexibility of the novel homologous series. This study will discuss the structural property
∗Address correspondence to A. V. Doshi, Matushri Virbaima Mahila Science & Home Science
College, Saurashtra University, Kalavad Road, Rajkot 360-007, Gujarat, India. E-mail: arvindbhai-
doshi@yahoo.in
15

16
V. R. Patel et al.
Table 1. Elemental analysis for the hexyloxy and octyloxy derivatives
Elemental% found (compared with% calculated)
Molecular formula
C
H
N
C30H30N2O4
C32H34N2O4
74.69 (74.70)
75.29 (75.55)
6.22 (6.29)
6.66 (6.51)
5.81 (5.75)
5.49 (5.39)
relationships across the homologous series in terms of the molecular flexibility, molecular
breadth, and polarizability on mesomorphic behavior.
Experimental
Characterization
1
Selected representative homologues of the series were characterized by infrared (IR), H
NMR, and elemental analysis. IR spectra were recorded on a Perkin Elmer spectrum
1
GX, H NMR spectra were recorded on Bruker spectrometer using CDCl3 as a solvent,
and microanalysis was performed on a Perkin Elmer PE 2400 CHN analyzer. Mesogenic
properties were investigated using LeitzLaborlux 12 POL hot stage polarizing microscope
(Ernst Leitz, Ltd., Midland, Ontario, Canada).
Synthesis
4-n-alkoxy benzoic acid and 4-n-alkoxy benzoyl chlorides (Z) were synthesized by a
modified method of Dave and Vora [1]. 2-hydroxy-naphthyl azo–2ꢀ-methoxy benzene (Y)
was prepared by known method of Furnis et al. [2]. 2-(4ꢀ-n-alkoxy benzoyloxy)-naphthyl
azo-2”-methoxybenzenes were synthesized by condensing 4-n-alkoxy benzoyl chloride (Z)
with 2-hydroxy-naphthyl-azo-2ꢀ-methoxybenzene (Y) in ice cold pyridine [3–5]. Azoester
formed were filtered off, washed, and dried before crystallization. Crystallization continued
until the final product gave constant transition temperatures. Analytical data confirms the
molecular structures, as presented in Table 1. Ortho-anisidine, β-naphthol, sodium nitrate,
pyridine, thionyl chloride, 4-hydroxy benzoic acid, n-alkyl halide, and other chemicals
required for synthesis are used as received. The synthetic route to the present homologous
series is outlined in Scheme 1.
Analytical Data
NMR in ppm. Decyl, 0.910 (alkyl group of –OC10H21), 1.27 (long –CH2- chain), 1.656
(-O-CH2-CH2 of –OC9H21), 4.024 (-O-CH2-CH2-), 6.904 & 6.947 & 8.026 & 8.069
(p- sub. Benzene)
Dodecyl, 0.88 (alkyl group of –OC12H25), 1.26 (long –CH2- chain), 1.801.656 (-O-
CH2-CH2 of –OC9H21), 4.024 (-O-CH2-CH2-), 6.904 & 6.947 & 8.026 & 8.069 (p-sub.
Benzene)
IR in Cm−1. Hexyl, Vmax/cm−1 772 (o-sub. Benzene ring), 847 (p-sub. Benzene ring),
1024, 1108, 1678 (–COO- group), 1428 (-N N- group), 2866 & 2939 (alkyl group), 1204
(-C-O of ether group)

Novel Liquid Crystalline Cinnamoyloxy Cinnamates
17
Tetradecyl, Vmax/cm−1 748 (o-sub. Benzene ring), 844 (p-sub. Benzene ring), 1064,
1128 & 1716(–COO- group), 1431 (-N N- group), 1107 cm−1 (-C-O of ether group), 719
(polymethylene of alkyl group), 694 & 748 (mono-sub. Benzene ring)
Texture:
Nematic
Texture
Hexyloxy
Octyloxy
Threaded type
Schlieren type
Smectic
Dodecyloxy
Tetradecyloxy
Smectic A
Smectic C
NH2
COOH
OCH3
+ R-X
KOH
MeOH
RO
0-5°C
NaNO2, HCl
diazotization
COOH
N=N-Cl
OCH3
O
RO
CO
-SO2 Freshly
HO
N=N
-HCl
+ β-Naphthol inNaOH
distilled
-HCl
OCH3
SOCl2
RO
COCl
[Z]
[Z]
+
[Y]
HO
Cooled
Pyridine
N=N
[Y]
OCH3
where R = CnH2n+1, n = 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16.
Scheme 1. Structurally similar isomeric series.
Results and Discussion
An azo dye 2-hydroxynaphthylazo 2ꢀ-methoxybenzene is not a liquid crystal but, on linking
it with 4-n-alkoxybenzoyl chloride, the resultant majority of novel products are liquid
crystalline, i.e., the pentyl to hexadecyl derivatives are monotropically or enantiotropically
exhibited, as either smectic and/or nematic mesomorphism. The methyl to butyl homologues
of the series are not liquid crystalline in character. A phase diagram (Fig. 1) of the novel
series is drawn for the number of carbon atoms in the n-alkyl chain of the n-alkoxy terminal
end group versus the transition temperatures of the series as recorded in Table 2, by joining
related points. Solid-isotropic or solid-mesomorphic transition curve adopts a zigzag path of
rising and falling values, and behaves in a normal manner. The transition curve initially rises,

18
V. R. Patel et al.
Table 2. Transition temperatures in ◦C
Compound no.
n (for R = -CnH2n+1
)
Smectic
Nematic
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
8
10
12
14
16
–
–
–
–
–
–
–
–
150.0
180.0
176.0
132.0
166.0
141.0
133.0
135.0
122.0
96.0
–
(155.0)
108.0
104.0
(125.0)
103.0
–
–
(86.0)
(107.0)
85.0
83.0
–
10
11
(101.0)
151.0
Note: Values in ( ) indicate monotropic.
Series 1: 2-[4'-n-alkoxy benzoyloxy]-naphthylazo-2"-methoxybenzenes.
O
CO
RO
N=N
OCH3
smoothly passes through a maxima, and then gradually falls from the octyl to tetradecyl
homologues without showing an odd-even effect. The curve is smoothly extrapolated to the
hexadecyl derivative of the series to predict and to determine its latent transition temperature
(LTT) for the probable occurrence of smectogenic mesophase formation. Thus, predicted
LTT for smectic phase formation of hexadecyl derivative is 88.0◦C. The nematic-isotropic
transition curve shows a descending tendency as the series is ascended with the exhibition
of an odd-even effect and alternation of transition temperatures, which appears from the
pentyl to the nonyl derivatives. Both curves for the odd and even homologues merge into
each other at nonyl to decyl derivatives, thus the nematic-isotropic curve behaves in the
usual expected manner. The nematic mesophase exhibits a threaded or Schlieren texture and
the smectic mesophase exhibits a focal conic fan-shaped texture of smectic A or smectic C,
as determined by a miscibility method using optical polarizing microscopy. The nematic-
isotropic transition temperatures are between 141.0◦C and 122.0◦C with the nematogenic
range varying from a minimum of 19.0◦C at the dodecyl homologue to a maximum of
33.0◦C at the hexyl homologue, and the smectogenic phase range is between 13.0◦C and
18.0◦C for tetradecyl and dodecyl homologues, respectively. Thus, mesomorphic range
of the smectic phase is shorter than that of the nematic phase. Hence, the series under
discussion is predominantly nematogenic and partly smectogenic. The statement that liquid
crystallinity involving a naphthyl unit of moderate chain length yields only or predominantly
nematogenic character in the homologues [6,7] of the series is supported by the present
investigation. The lack of mesomorphism of the methyl to butyl homologues is due to their
high crystallizing tendency arising from the relatively high level of forces of attractions and

Novel Liquid Crystalline Cinnamoyloxy Cinnamates
19
the lack of flexibility to cause mesophase formation [6,8–10]. The exhibition of the smectic
mesophase from the octyl to tetradecyl derivatives of the series is attributed to the presence
of lamellar packing of the molecules caused by broadening of the nonlinear shape of
molecules and laterally substituted highly polar methoxy group at ortho position to –N N-
central group, which increases molecular polarity and polarizability and raises suitable
magnitude of anisotropic forces of attractions to cause sliding layered two-dimensional
array of molecules. The exhibition of the nematic mesophase by the pentyl to hexadecyl
homologues except tetradecyl homologue is attributed to the occurrence of molecular
disalignment at an angle less than 90◦ with the plane of the surface under applied heat during
floating condition which is due to the emergence of suitable magnitudes of anisotropic forces
of intermolecular attractions related to molecular rigidity and flexibility [8–10] which resist
exposed thermal vibrations, resulting to cause statistically parallel orientational order of
molecules. Thus, the smectic phase in addition to the nematic phase or only the smectic
or only the nematic mesophase appears, either monotropically or enantiotropically, for
the homologous series under discussion. The alternation of transition temperatures and
appearance of an odd-even effect in the nematic-isotropic transition is attributed to the
progressively and sequentially added methylene unit in the n-alkyl chain of n-alkoxy
flexible terminal end group. Such odd-even effect diminishes as the series is ascended
because the longer n-alkyl chain may coil, bend, or flex in irregular manner. Thus, the
nematic-isotropic transition curve follows a smooth trend from the longer chain lengths
without any odd-even effect.
The liquid crystal properties of the titled homologous series 1 are compared with a
structurally isomeric series X [3b] (Fig. 2) in terms of molecular characteristics and average
thermal stabilities of the smectic and nematic mesophases which are shown in Table 3.
Both homologous series 1 and X are isomeric from point of view of their molecular
formula, but they differ in their molecular arrangement. Series1 is polymesomorphic in-
volving both the smectic and nematic mesophase; while series X under comparison gives
rise to only nematic mesomorphism. The smectic-isotropic average thermal stability of
series 1 is 99.5◦ while that of series X is zero. That is series X is nonsmectogenic. Likewise,
the nematic-isotropic average thermal stability of series 1 is 132.0◦ while that of series X
is 149.2◦. Thus, nematic-isotropic thermal stability of series X is higher than series 1 of
present series under discussion. Overall, mesophase length of series X is also higher than
that of series 1. Hence, though series 1 shows lower phase length than series X, it exhibits
both smectic and nematic types of mesophases while series X exhibits only the nematic
Table 3. Average thermal stability in ◦C
Series
1
X
–
Smectic–nematic or
smectic-isotropic
Commencement of smectic
phase 99.5
(C8–C14)
C8
–
Nematic-isotropic
132.0
(C5–C12)
C5
149.2
(C6–C14)
C6
Commencement of nematic
phase

20
V. R. Patel et al.
O
CO
RO
Series 1
N=N
OCH3
2-[4'-n-alkoxy benzoyloxy]-naphthylazo-2"-methoxybenzenes
COO
RO
N=N
Series X
OCH3
4-[4'-n-alkoxy benzoyloxy]-naphthylazo-2"-methoxybenzenes
Figure 1. Structurally similar isomeric series.
mesophase without exhibition of smectic mesophase. Both series 1 and X under comparison
contain the same two phenyl rings in addition to a naphthyl unit at the center, the same
central groups [viz., –COO- and –N N-] and same n-alkoxy terminal end group as well
as laterally ortho substituted –OCH3 group at the same identical position. Thus, molecules
of series 1 and X are identical with respect to their rigid part, flexible parts in addition to
molecular aromaticity but, they differ in geometrical shape, size, molecular length, mag-
nitude of breadth, length to breadth ratio, ratio of the polarity to polarizability, and many
other parameters which have direct relation with suitable magnitudes of anisotropic forces
of intermolecular attractions required for emergence of smectic and/or nematic mesophase
or mesophase formation under the influence of applied heat from surrounding to a ther-
modynamic system. Such above molecular variations causes difference in mesomorphic
properties from series to series for the same homologue and homologue to homologue
Figure 2. Structurally similar isomeric series.

Novel Liquid Crystalline Cinnamoyloxy Cinnamates
21
in the same series. Geometrically, the arrangement of atoms in space of the molecules
of series 1 and X appreciably differs in respect of the extent of their molecular width.
Therefore, net molecular polarity and polarizability vary according to molecular width and
length to breadth ratio. Greater molecular width (i) increase intermolecular distance and
reduces the magnitude of intermolecular anisotropic forces of attractions on one hand,
while on the other hand, (ii) increases the magnitude of intermolecular anisotropic forces
of attractions due to increased molecular breadth or polarizability. Thus, two opposing
effects, viz., (i) and (ii) are operating at a time. Therefore, net resultant effect observed is,
that of predominating effect out of two opposing effects (i) and (ii) operating at a time.
In case of lower value of average nematic-isotropic thermal stability or total mesophase
length of series 1 as compared to series X is attributed to the predominancy of factor (i),
which resists the thermal vibrations to cause statistically parallel orientational order of
molecules in floating condition a little bit less in series 1 as compared to series X. The
absence of smectogenic character in series X and appearance of smectogenic character
in addition to nematic mesophase in series 1 is attributed to the absence and presence of
lamellar packing in the crystal lattices, respectively, depending upon net result effect, out
of two opposing effects (i) and (ii). The commencement of smectic phase occurance from
octyloxyhomologue in series 1, which does not occur until the last hexadecyl homologue
in series X. This observation is attributed to the extent of noncoplanarity caused by the
molecule due to electron–electron interactions and steric interactions [3,6]. This effect,
i.e., difference in magnitude of noncoplanarity of molecule of series 1 and series X is
responsible to cause early or late commencement of smectic phase. Thus, lamellar packing
in crystal lattices of molecules of series 1 is possible while it lacks in case of molecules of
series X. Hence, smectogenic character in series X is absent till the last hexadecyl derivative
and it appears from the octyloxy homologue of series 1. The stable state of a substance is
associated with definite quantity of energy stored by the molecule at room temperature and
one atmospheric pressure. That is, thermodynamically it is called enthalpy change (H) of
a substance at normal temperature and pressure. As heat is supplied to a sample substance
(system) from a surrounding to system; supplied heat energy from surroundings operates
against the intermolecular anisotropic forces of attractions and molecules tend from sta-
bilized state to destabilized state at a temperature T1. Under this condition, molecules of
a sample substance under investigation are disaligned at an angle less than 90◦ with the
plane of a surface resisting exposed thermal vibrations and acquire mesophase formation
at T1. Mesophase formation continues till a higher temperature T2 and then mesophase
disappears for anisotropic liquid crystal. As temperature rises beyond T2, molecules are
randomly oriented in all possible directions in an irregular manner, such randomness or
molecular disorder or entropy increases with a rise of temperature and decreases with a
fall of temperature. Similarly, for monotropic liquid crystal T2 < T1, where T1 is isotropic
temperature or higher temperature. Difference of T1 and T2 is called a phase length or a
range of liquid crystallinity. Thus, S value is calculated for any transition state from DSC
scans using H value of corresponding transition temperature value. Thus, thermal stability
values are related with a H value of a sample substance. The variation in mesomorphic
properties from homologue to homologue in the same series and from series to series for
the same homologue are due to the sequentially added methylene unit and laterally or ter-
minally present functional group of fixed polarity, respectively. Thus, according to Gray’s
view [3,6] that, presence of anaphthyl unit in a molecular moiety induces nematogenic
character relatively more and increase in molecular breadth reduces smectogenic character
and smectic–nematic thermal stability. This statement is well supported by present inves-
tigation. Hence, the smectic and nematic group efficiency order derived on the bases of

22
V. R. Patel et al.
thermal stability are as under with reference to –OCH3 laterally substituted group, which
widens or broadens the molecular shape.
Nematic group
efficiency order for
lateral –OCH3
Series X
(Semi linear)
>
Series 1
(Nonlinear)
Smectogenic group
efficiency order for
lateral –OCH3
Series 1
(Nonlinear)
>
Series X
(Semi linear)
Conclusion
The presence of a naphthyl unit in a mesogenic molecule restricts or hinders lamellar
packing of the molecules in their crystal lattices. However, broadening of a molecule favors
to induce smectic character.
A naphthyl moiety reduces thermal stabilities of mesophase.
Homologous series involving a naphthyl unit are only nematogenic or predominantly
nematogenic.
The degree of widening of a molecule containing anaphthyl unit is effective to induce
only nematogenic character or smectogenic character in addition to nematogenic character
in isomeric homologous series.
Radiation in molecular rigidity and flexibility plays an important vital role in liquid
crystal behavior.
Thus, present investigation support the early views [1,4–5,11] and raises the credibility
to the conclusion drown in past.
Acknowledgments
The authors are thankful to the management of S.P.T. Arts and Science College–Godhra
and M.V.M. Science and Home Science College–Rajkot for their valuable support in the
present investigation. Also, we thank the CSMCRI–Bhavnagar for their analytical services.
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