Synthesis and study of a novel azoester mesogenic homologous series: 4-[4′-n-alkoxy benzoyloxy]-naphthylazo-4″-acetyl benzenes

Article abstract of DOI:10.1080/15421406.2012.703029

The synthesis and mesomorphic properties of a novel azoester homologous series consisting of 12 members is reported. The methyl to butyl derivatives of the series are non-mesogenic, and the others are enantiotropically smectogenic and/or nematogenic. The nematic mesophase shows a threaded or Schlieren texture and smectic mesophase shows a focal conic fan-shaped texture of the SmA or the SmC. The smectic and nematic thermal stabilities are 137.4 and 166.6 respectively. Analytical data support the molecular structures. The transition temperatures of the homologues are determined using optical polarizing microscopy. An odd-even effect is observed for the nematic-isotropic transition curve and the series is equally smectogenic and nematogenic over a wide temperature range, and the melting points are in the middle order. The mesogenic properties of the present series are compared with a structurally similar homologous series. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2012.703029

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Synthesis and Study of a Novel Azoester
Mesogenic Homologous Series: 4-[4′-
n-Alkoxy Benzoyloxy]-Naphthylazo-4″-
Acetyl Benzenes
V. R. Patel a , M. L. Chauhan a & A. V. Doshi b
a Department of Chemistry, Sheth P. T. Arts & Science College,
Gujarat University, Godhra, 389 001, Gujarat, India
b Matushri Virbaima Mahila Science & Home Science College,
Kalavad Road, Saurashtra University, Rajkot, 360 007, Gujarat, India
Published online: 16 Nov 2012.
To cite this article: V. R. Patel , M. L. Chauhan & A. V. Doshi (2012) Synthesis and Study of a Novel
Azoester Mesogenic Homologous Series: 4-[4′-n-Alkoxy Benzoyloxy]-Naphthylazo-4″-Acetyl Benzenes,
Molecular Crystals and Liquid Crystals, 569:1, 57-63, DOI: 10.1080/15421406.2012.703029
To link to this article: http://dx.doi.org/10.1080/15421406.2012.703029
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Mol. Cryst. Liq. Cryst., Vol. 569: pp. 57–63, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.703029
Synthesis and Study of a Novel Azoester Mesogenic
Homologous Series: 4-[4ꢀ-n-Alkoxy
Benzoyloxy]-Naphthylazo-4ꢀꢀ-Acetyl Benzenes
V. R. PATEL,1,∗ M. L. CHAUHAN,1 AND A. V. DOSHI2
1Department of Chemistry, Sheth P. T. Arts & Science College, Gujarat
University, Godhra 389 001, Gujarat, India
2Matushri Virbaima Mahila Science & Home Science College, Kalavad Road,
Saurashtra University, Rajkot 360 007, Gujarat, India
The synthesis and mesomorphic properties of a novel azoester homologous series con-
sisting of 12 members is reported. The methyl to butyl derivatives of the series are
non-mesogenic, and the others are enantiotropically smectogenic and/or nematogenic.
The nematic mesophase shows a threaded or Schlieren texture and smectic mesophase
shows a focal conic fan-shaped texture of the SmA or the SmC. The smectic and ne-
matic thermal stabilities are 137.4◦ and 166.6◦ respectively. Analytical data support the
molecular structures. The transition temperatures of the homologues are determined
using optical polarizing microscopy. An odd–even effect is observed for the nematic-
isotropic transition curve and the series is equally smectogenic and nematogenic over a
wide temperature range, and the melting points are in the middle order. The mesogenic
properties of the present series are compared with a structurally similar homologous
series.
Keywords Azoester; liquid crystal; mesogen; mesomorphism; nematic; smectic
Introduction
The synthesis of this novel azoester homologous series enables a better understanding
of structure–property relationships in respect of the influence of flexibility and rigidity
changes on melting point, transition temperatures, and mesophase morphology. The novel
molecular structure consists of a naphthyl unit at the center and two phenyl rings linked to
it by linking groups (–COO– and –N N–) with n- alkoxy and –COCH3 terminal groups.
The core units provide molecular rigidity, the linking groups can maintain rigidity, but
also facilitate some flexibility, and the terminal groups provide some flexible nature to
the molecular structure. Thus, mesogenic characteristics and the degree of mesomorphism
are varied by changing molecular moieties, which affect flexibility and rigidity, molecular
breadth, and hence polarizability. The mesomorphic properties of the novel homologous
series are compared with a known homologous series where a lateral chloro substituent on
a phenyl ring features in place of the naphthyl core [1–5].
∗Address correspondence to V. R. Patel, Department of Chemistry, Sheth P. T. Arts & Science
College, Gujarat University, Godhra 389 001, Gujarat, India. E-mail: vrp181968@yahoo.co.in
57

58
V. R. Patel et al.
Experimental
Characterization
Some selected members of the titled series were characterized by elemental analysis,
1HNMR and IR spectra. The textures of mesophases were characterized by the miscibility
method. Microanalysis was performed on a Perkin Elmer PE 2400 analyzer (Table 1).
1HNMR spectra were obtained on a Bruker spectrometer using CDCl3 as solvent. IR
spectra were recorded on a Perkin-Elmer GX spectrometer. The transition temperatures and
liquid crystal properties were investigated using optical polarizing microscope with heating
stage. Thermodynamic quantities, viz. enthalpy (ꢀH) and entropy (ꢀS) are discussed
qualitatively.
Synthesis
The 4-n-alkoxybenzoic acids were synthesized from 4-hydroxybenzoic acid using the
corresponding suitable alkyl halide by the modified method of Dave and Vora [5].
The 4-n-alkoxybenzoic acids were converted to the corresponding 4-n-alkoxybenzoyl
chloride (X) using freshly distilled thionyl chloride. The 4-hydroxy naphthylazo-4ꢀ-acetyl
benzene (Y) was prepared by the known method of Furnis et al. [6]. Final products 4-
[4ꢀ-n-Alkoxy benzoyloxy] naphthylazo 4ꢀꢀ-acetyl benzenes were prepared by condensing
components (X) and (Y) in ice-cooled pyridine. Each product (XY) was filtered, washed,
dried, and finally recrystallized from alcohol until constant transition temperatures were
obtained. 4-Hydroxy benzoic acid, alkyl halides, thionyl chloride, pyridine, and other
chemicals required for synthesis were used as received. The synthetic route to the series is
outlined in Scheme 1.
Scheme 1. Synthetic route to series-1.
Analytical Data
Nuclear Magnetic Resonance (NMR)
Decyl, 1.28 (alkyl group of –OC10H21), 2.61 (–CH3 of –CH3CO), 1.84 (–OCH2–CH2
of –OC10H21), 4.09 (–OCH2 of –OC10H21), 6.90 & 7.03 & 8.99 & 8.31 (aromatic).

Synthesis of Azoester Mesogenic Homologous Series
59
Table 1. Elemental analysis for the hexyloxy and octyloxy derivatives
Elemental% found (Compared with % calculated)
Molecular formula
C
H
N
C33H34N2O4
C35H38N2O4
75.86 (75.01)
76.36 (76.98)
6.51 (6.18)
6.91 (6.58)
5.36 (5.53)
5.09 (5.15)
Tetradecyl, 1.26 (alkyl group of –OC10H21), 2.69 (–CH3 of –CH3CO), 4.118
(–OCH2–CH2 of –OC10H21), 6.933 & 7.024 & 8.99 & 8.26 (aromatic).
Infrared (IR)
Decyl, Vmax/cm−1 817.8 & 846.7 (aromatic), 1066, 1151, 1732 (–COO– group),
1421(–N = N–), 1359 & 2850 (alkyl), 1168(–C–O of ether group), 1678 & 1226
(>C = 0 group).
Tetradecyl, Vmax/cm−1 808 & 835 (aromatic), 1064, 1259 & 1672 (–COO– group),
1409 (–N = N– group), 1128 (–C–O of ether group), 719 (polymethylene of alkyl
group), 1357 & 2850 (alkyl group), 1668 (>C = 0 group).
Texture
Confirmed by the miscibility method.
1. Pentyloxy and hexyloxy nematic threaded or schlieren type texture.
2. Decyloxy and dodecyloxy smectic-A.
3. Hexadecyloxy homologue smectic-C.
Judged directly by microscopic observation.
Results and Discussion
The novel homologous series, 4-[4ꢀ-n-Alkoxy benzoyloxy] naphthylazo 4ꢀꢀ-acetyl ben-
zoates, consists of 12 members. The methyl to butyl members of the series are non-
mesogenic, The pentyl and hexyl members are only enantiotropic nematogenic, the heptyl
to dodecyl members are enantiotropically smectogenic in addition to enantiotropic ne-
matogenic character, while the tetradecyl and hexadecyl members of the series are only
enantiotropic smectogenic. A phase diagram (Fig. 1) plots the transition temperatures de-
termined on a hot stage optical polarizing microscope as recorded in Table 2 versus the
number of carbon atoms present in n-alkyl chain. The phase diagram indicates that the
solid to isotropic or mesophase transition curve follows a zigzag path of rising and falling
values from and beyond the butyloxy homologue. The smectic to nematic or isotropic
transition curve initially rises, passes through a maxima and then smoothly falls as series
is ascended without the exhibition of an odd-even effect. The nematic to isotropic tran-
sition curve initially rises and adopts descending tendency as the series is ascended with
the exhibition of an odd–even effect and an alternation of transition temperatures. The
nematic to isotropic transition curve is extrapolated to the tetradecyloxy homologue from
the dodecyloxy homologue and the curve merges into the smectic to isotropic point for
the tetradecyloxy homologue. These trends show that beyond the tetradecyloxy homologue
only smectogenic mesophase appears and the nematogenic mesophase formation ceases to
appear. The smectic to nematic or isotropic transition curve is extrapolated to the hexyloxy

60
V. R. Patel et al.
Figure 1. Two-phase behavior of series 4- [4ꢀ-n-Alkoxy benzoyloxy] naphthyl azo 4ꢀꢀ-acetyl ben-
zenes.
homologue following the trend of the curve to predict the probable latent transition temper-
ature (LTT) for smectic mesophase temperature in the hexyl derivative, which is 119.0◦C,
and is not realizable due to the high melting point. Thus all the transition curves behave
in the expected manner. The zigzag path of solid to isotropic or mesomorphic transition
curve is attributed to the sequential and progressive addition of a methylene unit in the
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
7
8
10
12
14
16
–
–
–
–
–
–
–
–
185.0
178.0
169.0
161.0
155.0
182.0
168.0
176.0
166.0
153.0
138.0
120.0
–
128.0
139.0
133.5
141.0
147.0
145.0
–
–
116.0
93.0
113.0
94.0
87.0
71.0
10.
11.
12.
–

Synthesis of Azoester Mesogenic Homologous Series
61
n-alkoxy chain. This causes the difference in molecular rigidity and flexibility [1–4], which
is directly related to suitable or unsuitable magnitudes of anisotropic intermolecular forces
of attraction. Thus, the overall end-to-end intermolecular anisotropic forces of cohesion are
altered from homologue to homologue from and beyond the fourth member of the series.
The absence of smectic or nematic or any sort of mesophase in methyl to butyl derivatives of
the series is due to their high crystallizing tendency, which arises from resultant net unstable
magnitude of anisotropic intermolecular forces of attraction as a consequence of molecular
rigidity. The solid to isotropic transition curve does show an irregular trend for odd and even
members up to the butyl homologue. The nematic to isotropic transition for odd and even
homologues follows a smooth gradual falling tendency from the hexyloxy to dodecyloxy
homologue of the series in expected manner. Both curves for odd and even members of the
series merge with each other and to the main stream of the nematic to isotropic transition
curve at the decyloxy homologue indicating that from and beyond decyloxy homologue
there is a single curve with descending tendency. The odd–even effect diminished for higher
homologues because the longer n-alkyl terminal chain may coil or bend, or flex to lie in
the line with major axis of core [7,8]. Thus, the end-to-end contact would be the same for
odd and even homologues of the series. The occurrence of the nematic mesophase from
the pentyloxy to decyloxy homologues is attributed to the misalignment of molecules at an
angle less than 90◦ with the plane of the surface under the influence of heat to maintain a
statistically parallel orientational order or two-dimensional array of molecules in a float-
ing condition. The absence of an odd–even effect in the smectic to nematic or isotropic
transition curve arises because the smectogenic character is exhibited by almost all even
numbered homologues except the heptyloxy homologue. The molecules of the heptyloxy
to hexadecyloxy homologues show smectogenic character because of the lamellar packing
of the molecules in a layered arrangement.
The mesogenic characteristics of the novel homologous series-1 are compared with a
structurally similar series-A [9] (Fig. 2).
The novel homologous series-1 is polymesomorphic, i.e. it exhibits smecticandnematic
mesophases, but homologous series-A selected for comparison is only nematogenic without
the exhibition of smectogenic character. Average phase thermal stability values are shown
in Table 3, with average clearing points from series 1 and A of 166.6◦C and 122.8◦C
respectively.
Figure 2. Two structurally similar homologous series for comparison.

62
V. R. Patel et al.
Table 3. Average thermal stability in ◦C
Series
1
A
–
Smectic–nematic or smectic–isotropic
137.4
(C7–C16)
C7
166.6
(C5–C12)
C5
Commencement of smectic phase
Nematic-isotropic
–
122.8
(C6–C14)
C6
Commencement of nematic phase
Thus, smectic and nematic thermal stabilities of series-1 are higher than that of series-
A under comparison. The smectic mesophase commences from the heptyloxy homologue
in case of series-1, while it does not commence in case of series-A until the hexadecyloxy
homologue. The mesomorphic phase temperature range varies from a minimum of 49◦C for
hexadecyloxy homologue to a maximum of 83◦C for the octyloxy homologue in series-1,
while in series-A, such ranges varies from 22◦C to 48◦C for the tetradecyloxy homologue
and the decyloxy homologue respectively. The observed differences in mesogenic properties
between series 1 and A are attributed to difference in their molecular structures (Fig. 2). The
terminal units and the linking groups are identical in each case, but middle part is naphthyl
ring (series-1) and a chloro substituted phenyl ring in series-A. Thus, the molecular structure
of series-1 differs from series-A in respect of lateral substitution. Series-1 incorporates a
naphthyl unit, while series-A involves a phenyl ring with a lateral Cl group substituted at an
ortho position to –COO– linking group in a central molecular location. The molecules of
series-1 are broadened by a fused phenyl ring, while the molecules of series-A are broadened
by a larteral chloro substituent. The difference in molecular rigidity and flexibility causes
variations in aromaticity, polarity, and polarizability, and hence magnitudes of anisotropic
intermolecular forces of attraction determine the presence or absence of smectic and/or
nematic mesomorphism, and homologue at which the phases commence. Broadening of a
molecule increases intermolecular distance and hence decreases intermolecular attractions
on one hand, and it increases molecular polarizability, which increases intermolecular
attraction, on the other. Thus, two opposing effects [7,8] are operating, with the observed
net effect in case of series-1 causing end-to-end intermolecular attractions relatively more
for series-1 as compared to series-A. Therefore, degree of nematogenic character in terms
of longer nematic phase length in combination with the presence of smectogenic character
is observed for series-1, while the nematogenic phase length is relatively shorter with
absence of smectogenic character in series-A. This observation is attributed to weaker
intermolecular attractions arising by poorer aromaticity and lateral attractions of the Cl
unit as compared to lateral attractions caused by a fused phenyl ring. The commencement
of smectogenic mesophase formation takes place earlier in the case of series-1 than in
series-A because of the extent of coplanarity caused by the molecular structure [10,11].
The presence of a fused phenyl ring (series-1) induces higher magnitude of mesogenic
character as compared to chloro unit (series-A).
Thus, the group efficiency order derived for the fused phenyl ring and a Cl unit in terms
of their thermal stability is as follows.
Smectic and nematic
>
Cl
group efficiency order

Synthesis of Azoester Mesogenic Homologous Series
63
Conclusion
1. Generally, the members of the naphthyl ring containing homologous series are
nematogenic; however smectogenic character is also seen in several homologues.
2. The novel series is of middle ordered melting type with clearing points varying
between 120◦C and 182◦C.
3. The presence of the naphthyl moiety invariably induces nematogenic character in
the series.
4. Increasing molecular rigidity by fused phenyl ring at the middle part of a molecule
induces smectogenic character in the mesophase.
Acknowledgments
The authors acknowledge the management of M. V. M Science and Home Science College,
Rajkot and S. P. T. Arts and Science College, Godhra, as well as Dr. N. D. Jadav, Ex. Head,
Department of Applied Chemistry, Faculty of Technology & Engineering, M. S. University,
Baroda, for their valuable support and cooperation. The authors also thank the authorities
of CSMRI, Bhavnagar for analytical services.
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