Synthesis and mesomorphism of novel liquid crystalline: 4-(4-n-alkoxy benzoyloxy)-naphthylazo-4-chlorobenzenes

Article abstract of DOI:10.1080/15421406.2011.591667

A new azoester homologous series 4-(4-n-alkoxy benzoyloxy)-naphthylazo-4- chlorobenzenes is synthesized. Its first and second homologues are nonmesomorphic, while the rest of the homologues are mesomorphic, showing enantiotropic nematic behavior. Smectic character is totally absent. An odd-even effect is observed in the nematic-isotropic transition curve with alternation in transition temperatures. The average thermal stability and liquid crystal behavior for the nematic mesophase are compared with structurally similar homologous series. Analytical data support the structures of the molecules. The texture of the nematic mesophase is of threaded type. Transition temperatures are observed through hot-stage polarizing microscope. Analytical data support the structures.

Full text of DOI:10.1080/15421406.2011.591667

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Synthesis and Mesomorphism of
Novel Liquid Crystalline: 4-(4′-n-
Alkoxy Benzoyloxy)-Naphthylazo-4″-
Chlorobenzenes
B. C. Chauhan a & A. V. Doshi b
a Shri. P.H.G. Muni Arts & Science College , Kalol, Gandhinagar ,
Gujarat , India
b M.V.M. Science & Home Science College , Rajkot , Gujarat , India
Published online: 27 Dec 2011.
To cite this article: B. C. Chauhan & A. V. Doshi (2012) Synthesis and Mesomorphism of Novel Liquid
Crystalline: 4-(4′-n-Alkoxy Benzoyloxy)-Naphthylazo-4″-Chlorobenzenes, Molecular Crystals and Liquid
Crystals, 552:1, 16-23, DOI: 10.1080/15421406.2011.591667
To link to this article: http://dx.doi.org/10.1080/15421406.2011.591667
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Mol. Cryst. Liq. Cryst., Vol. 552: pp. 16–23, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.591667
Synthesis and Mesomorphism of Novel Liquid
Crystalline: 4-(4ꢀ-n-Alkoxy
Benzoyloxy)-Naphthylazo-4ꢀꢀ-Chlorobenzenes
B. C. CHAUHAN1,∗ AND A. V. DOSHI2
1Shri. P.H.G. Muni Arts & Science College, Kalol, Gandhinagar, Gujarat, India
2M.V.M. Science & Home Science College, Rajkot, Gujarat, India
A new azoester homologous series 4-(4ꢀ-n-alkoxy benzoyloxy)-naphthylazo-4ꢀꢀ-
chlorobenzenes is synthesized. Its first and second homologues are nonmesomorphic,
while the rest of the homologues are mesomorphic, showing enantiotropic nematic
behavior. Smectic character is totally absent. An odd–even effect is observed in the
nematic–isotropic transition curve with alternation in transition temperatures. The av-
erage thermal stability and liquid crystal behavior for the nematic mesophase are
compared with structurally similar homologous series. Analytical data support the
structures of the molecules. The texture of the nematic mesophase is of threaded type.
Transition temperatures are observed through hot-stage polarizing microscope. Analyt-
ical data support the structures.
Keywords Azomesogens; mesomorphism; nematic; smectic
Introduction
The present work involves the synthesis and evaluation of an azoester homologous series
with –Cl as the right terminal end group and a naphthyl unit bridged through COO
and N N linking groups as the rigid core. The mesomorphic behavior of a substance
depends on the magnitude of its molecular rigidity and flexibility, conducive to intermolec-
ular forces of attraction. Variation in molecular rigidity is carried out by changing the
rigid core of a molecule and by varying the central bridge or bridges and the number of
phenyl rings in the molecular structure. The flexible part of the molecule is varied through
the terminal groups on either side of the molecule or by lateral substitution at the phenyl
ring. Thus, for this work, a naphthyl moiety was selected to vary the rigidity of the core
structure by broadening the molecule. Azo and ester groups linking to the phenyl rings and
a chloro ( Cl) unit as the terminal end group were also included. This study will discuss
the structure–property relationships in terms of the influence of molecular breadth, and
molecular polarity and polarizability on mesomorphism.
∗Address correspondence to B. C. Chauhan, Shri. P.H.G. Muni Arts & Science College, Kalol
382721, Gandhinagar, Gujarat, India. E-mail: bakul.chauhan2010@gmail.com
16

Novel Liquid Crystalline Naphthylazo Chlorobenzenes
17
Figure 1.
Experimental
Characterization
Some representative homologues of the series were selected for characterization of the
structures by elemental analysis, infrared (IR), and 1H NMR (nuclear magnetic resonance)
techniques. Microanalysis was performed on a Perkin Elmer PE 2400 CHN analyzer. IR
spectra were recorded on a Perkin Elmer Spectrum GX, and 1H NMR was recorded on a
Bruker using CDCl3 as a solvent. Liquid crystalline properties were investigated using a
LeitzLabourlux 12 POL hot-stage polarizing microscope. Enthalpy (ꢀH) and entropy (ꢀS)
changes are discussed qualitatively instead of through differential scanning calorimeter
(DSC) scan.
Synthesis
4-chloroaniline, sodium nitrite, α-naphthol, pyridine, thionylchloride (GSC), 4-hydroxy
benzoic acid, and n-alkyl halides (SRL, Lancaster) were used as received. The synthetic
route to the present homologous series is outlined in Scheme 1. 4-n-alkoxy benzoic acid
and corresponding 4-n-alkoxy benzoyl chloride [A] were synthesized by the modified
method of Dave and Vora [7]. 4-hydroxy-naphthyl azo-4ꢀ-chlorobenzene [B] was prepared
by the known method of Furnis et al. [8]. 4-(4ꢀ-n-alkoxy benzoyloxy)-naphthylazo-4ꢀꢀ-
chlorobenzenes were synthesized by condensing 4-n-alkoxy benzoyl chloride [A] with 4-
hydroxy-naphthyl azo-4ꢀ-chlorobenzene [B] in pyridine [9]. The azoesters were crystallized
from alcohol until constant transition temperatures were obtained. The elemental analysis
of the compounds was found to be satisfactory and is presented in Table 1.

18
B. C. Chauhan and A. V. Doshi
Scheme 1. Synthetic route to series 1.
Results and Discussion
Transition temperatures and melting temperatures of the homologues of the titled homolo-
gous series are relatively lower compared with the corresponding alkoxy benzoic acid, and
the molecular length and width are increased by fusing a phenyl ring, i.e., a naphthyl unit
linked between two phenyl rings. The azo-dye, 4-hydroxy naphthyl azo-4ꢀ-chlorobenzene,
is nonmesomorphic, but on linking it with n-alkoxy benzoyl chloride, the resulting novel
products are mesomorphic from the propyl to the hexadecyl derivative. The methyl and
ethyl homologues of the series are nonmesomorphic in behavior. The propyl-to-hexadecyl
derivatives are enantiotropicnematic, without exhibiting any smectic property.
Transition temperatures as recorded in Table 2 are plotted versus the number of carbon
atoms in the n-alkyl chain of the left n-alkoxy terminal of the homologues. Smooth curves
are drawn through the like or related transition points and the phase diagram thus obtained
is shown in Fig. 2. From the phase diagram, it can be seen that the solid–isotropic or the
solid–nematic transition curve gradually falls from the methyl to the octyl derivative, with-
out following a zigzag path, up to the octyl homologue. However, a zigzag path is followed
from and beyond the octyl homologue. The nematic–isotropic transition curve shows an
overall descending tendency as the series is ascended and behaves with negligible abnor-
mality from the octyl to the hexadecyl homologue. An odd–even effect is observed in the
nematic–isotropic transition curve, wherein both the nematic and the isotropic curve merge
Table 1. Elemental analysis for the hexyloxy and octyloxy derivatives
Element % found (compared with % calculated)
Molecular formula
C
H
N
C27 H23 N2 O3Cl
C29 H27 N2 O3Cl
70.45 (70.66)
71.38 (71.53)
4.89 (5.01)
5.36 (5,54)
6.18 (6.10)
5.87 (5.75)

Novel Liquid Crystalline Naphthylazo Chlorobenzenes
IR for Butoxy Derivative
19
Vmax/cm−1 2929, 2871, 1429 ( C H, aliphatic), 1722, 1253 (ester group), 1423
( N N group), 1540, 1470 ( C C aromatic stretch), 1061 (ether group), 844
(p-substituted benzene ring), 1120.5 (C Cl aromatic).
IR for Decyloxy Derivative
Vmax/cm−12922, 2852, ( C H aliphatic), 1679, 1296, (ester group), 1429 ( N N
group), 1583.4 ( C C aromatic stretch), 1066 (ether group), 848 (p-substituted benzene
ring), 1115.6 (C Cl aromatic).
Analytical data.
into each other at the eighth homologue. The nematic mesophase exhibits a threaded type
of texture, as determined by the miscibility method using optical polarizing microscopy.
The nematic–isotropic transition temperatures are between 141.0◦C and 101.0◦C, with the
nematogenic range varying from a minimum of 31.0◦C at the 14th derivative to a maximum
of 50.0◦C at the 12th derivative of the series. Thus, the mesomorphic temperature range is
fairly wide. Thus, the series under discussion is entirely nematogenic, without exhibiting
smectic character. The statement that liquid crystals containing a naphthyl unit of moderate-
chain length yield only nematogenic homologues [10,11] is well supported by the present
investigation. The lowering of transition and melting temperatures of dimeric 4-n-alkoxy
benzoic acids is attributed to the elimination of dimerization by esterification due to the
removal of hydrogen bonding. The nonmesomorphic behavior of the azo-dye and methyl
and ethyl homologues of the homologous series is due to their high crystallizing tendency,
arising because the net intermolecular forces of attractions are neither anisotropic nor of
suitable magnitude to maintain molecular alignment in molecules as a consequence of
molecular rigidity [1–6]. Thus, the azo-dye and methyl and ethyl homologues of the titled
Table 2. Transition temperatures of the series
Compound
no.
n (for R =
Nematic
(◦C)
Isotropic
(◦C)
−CnH2n+1
)
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
166.0
154.0
141.0
139.0
130.0
128.0
118.0
116.0
119.0
115.0
112.0
101.0
101.0
91.0
85.0
82.0
76.0
69.0
84.0
65.0
78.0
70.0
10
11
12

20
B. C. Chauhan and A. V. Doshi
Figure 2. Phase behavior of the series.
homologous series show nonmesomorphic behavior. The exhibition of nematogenic be-
havior by propyl-to-hexadecyl homologues is attributed to the properly aligned molecules,
in which intermolecular adhesion forces pertaining to molecular polarizability and end-
to-end and lateral attractions are anisotropic and of suitable magnitude as a consequence
of their molecular rigidity, flexibility, and bending, which can withstand or resist thermal
vibrations exposed upon them. Thus, statistically parallel orientations of the molecules in
floating condition are not restricted, which causes the nematic mesophase. The absence of
a smectic mesophase is due to the lack of lamellar packing of the molecules caused by the
broad core. Thus, only the nematic mesophase is exhibited by the homologous series under
discussion.
The appearance of the odd–even effect in the nematic–isotropic transition curve and
the alternation in transition temperatures are attributed to the sequentially added number

Novel Liquid Crystalline Naphthylazo Chlorobenzenes
21
Table 3. Average thermal stability in (◦C)
Series
1
A
B
Nematic–isotropic
Commencement of nematic
mesophase
129.9 (C3 C16) 140.45 (C1 C14) 128.63 (C1 C14)
C3 C1 C1
of methylene units linked through the oxygen atom in the n-alkyl chain of the left n-alkoxy
terminal end group. This effect diminishes as the series is ascended from and beyond the
eighth homologue, because longer n-alkyl chains coil and bend. Thus, the nematic–isotropic
transition curve follows a smooth trend for both odd and even homologues of longer
chains. Negligible abnormality observed for higher members of the series is attributed to
the presence of a polarizable Cl end group positioned at 4ꢀꢀ carbon of the third phenyl
ring linked through the N N central bridge of the homologues.
The liquid crystal properties of the titled homologous series 1 are compared with struc-
turally similar homologous series (A) and (B) (Fig. 2) in terms of molecular characteristics
and average thermal stability of the nematic mesophase, as shown in Table 3.
Homologous series (1), (A), and (B) [12] are geometrically identical and consist
of a left n-alkoxy terminal linked at para position to the first phenyl ring. The central
naphthyl unit is bridged through COO and N N central groups. Another phenyl
ring is linked through the N N group to the naphthyl unit. This phenyl ring possesses
a varying functional group substituted at para position, i.e., terminal end groups Cl,
H, and CH3 (without any lateral substitution) in series (1), (A), and (B), respectively.
Thus, the uncommon part of the series under comparison is the right-side terminal end
group. Therefore, variation in liquid crystal properties, thermal stabilities, and degree of
mesomorphism exhibited from series to series depends upon variation in polarity, size,
polarizability, arrangement of atom in space, etc., of the right-side terminal end groups
[17] Cl, H, and CH3. Similarly, variation in liquid crystal properties and degree of
mesomorphism exhibited in the same series depends upon the number of methylene units
linked in the varying n-alkyl chain of the n-alkoxy terminal end group, keeping the right-
side terminal intact. Thus, variation in liquid crystal properties or degree of mesomorphism
arises due to uncommon features of the molecules from series to series or in the same
series. The uncommon part of the molecule causes varying effects on shape, size, polarity,
polarizability, aromaticity, π-electron density, electron–electron interactions, planarity of
molecule, etc., and the resulting effect contributes to intermolecular forces of attractions of
desired or undesired magnitude affecting mesogenic characteristics of the molecule.
All the homologous series under comparison are only nematogenic in character. How-
ever, the nematogenic character of the titled homologous series starts from the third member
of series (1), while it starts from the very first member of series (A) and (B). This variation
may be attributed to the difference in polarity, presence of electron clouds, and steric hin-
drance associated with end groups Cl, H, and CH3. Moreover, the length-to-breadth
ratio and extent of non-coplanarity caused by the molecules differ more or less for series
(1), (A), and (B). Hence, the polarity of the end groups affecting the end-to-end inter-
molecular forces of attractions undergoes variation, which in turn causes variation in the
commencement of the nematic mesophase, degree of nematogenic character, and average
thermal stability of the smectic (zero) and nematic mesophase.

22
B. C. Chauhan and A. V. Doshi
The average thermal stabilities of the nematic mesophase for series (1) and (B) are
almost equal, with negligible difference, because the Cl and CH3 functional groups are
equipolar [13]. A considerable difference in average thermal stability is observed for series
(A), which has the highest value of thermal stability among the series under comparison.
This may be attributed to the resultant force of intermolecular attractions arising from
two opposing effects operating at a time due to the broadening of the molecule by the
fusion of the other phenyl ring with the middle phenyl ring (i.e., naphthyl ring). Two
opposing effects operate: one that reduces the intermolecular forces of attractions due to
the broadening of the molecule, and thus increasing the intermolecular distance, and one
that increases the intermolecular forces of attractions due to the increase in molecular
polarizability and molecular dipolarity caused by the broadening of the molecule. Thus,
the resultant intermolecular forces of attractions [14–16] are a direct result of the two
opposing effects, which operate equally, but the predominating resultant effect is due to
the polarity difference and other related differences in the terminal end groups Cl, H,
and CH3. In the case of homologous series (A) without end group ( H), the second
effect operates as the predominating effect, increasing intermolecular forces of attractions,
as compared with series (1) and (B). The additional phenyl ring fused with the middle
phenyl ring (i.e., naphthyl ring) in series (1), (A), and (B) (Fig. 1) not only increases the
breadth of the molecules but also increases the extent of non-coplanarity in the system due
to electron–electron interaction and steric interactions. Both these factors would eliminate
the smectogenic property of all series under comparison, including the titled homologous
series. According to Gray’s [14,16] view, an increase in molecular breadth or the broadening
of a molecule reduces both the nematic and the smectic mesophase thermal stability, and it
is well supported by the present investigation.
Thus, from the above discussion, the nematic group efficiency order derived on the
basis of thermal stability with respect to the terminal end group is:
Nematic group efficiency:
H
>
Cl
>
CH3
Order:
(series-A) (series-1) (series-B)
Conclusion
In conclusion, the study highlights that:
(i) Broadening of a molecule by fusing a phenyl ring and extending the molecular
length by adding a third phenyl ring bridged through the N N group are
enough to induce mesophase formation in the absence of a terminal end group.
(ii) Introduction of a flexible Cl moiety as the terminal end group by replacement
of H in series (1) reduces the nematic mesophase thermal stability.
(iii) Widening of the molecular breadth in combination with the presence of the terminal
end group Cl reduces the nematic and smectic mesophase thermal stability, and
a homologous series having a naphthyl unit as part of the molecule is generally
nematogenic in character, without exhibiting any smectogenic character.
(iv) The presence of a naphthyl unit in a mesogenic molecule restricts lamellar packing
of molecules.
(v) Intermolecular anisotropic forces of attractions strongly favor statistically parallel
orientational order in mesogenic molecules in floating condition in the presence
of a naphthyl unit and a polarizable Cl terminal.

Novel Liquid Crystalline Naphthylazo Chlorobenzenes
23
Hence, the present investigation supports the early views of Gray [10,13] very well and
raises the credibility of conclusions drawn in past.
Acknowledgments
The authors acknowledge and thank the head and teaching staff of Applied Chemistry
Department, Faculty of Technology and Engineering, M. S. University, Vadodara, Gujarat,
India, for their valuable cooperation in the work. They are also thankful to CSMCRI,
Bhavnagar, Gujarat, India, for the analysis of sample.
References
[1] Marcos, M., Omenat, A., Serrano, J. L., and Ezcurra, A. (1992). Adv. Mater., 4, 285.
[2] Hird, M., Toyne, K. J., and Gray, G. W. (1993). Liq. Cryst., 14, 741.
[3] Hird, M., Toyne, K. J., Gray, G. W., Day, S. E., & Mc Donnell, D. G. (1993). Liq. Cryst., 15,
123.
[4] Gonzalez, Y., Ros, M. B., Serrano, J. L., and Perezjubind, A. (1998). Liq. Cryst., 18, 751.
[5] Gray, G. W. and Jones, B. (1954) J. Chem. Soc., 683; Gray, G. W. and Jones, B. (1955). J. Chem.
Soc., 236.
[6] Prajapati, A. K. and Pandya, H. M. (2003). Mol. Cryst. Liq. Cryst., 393, 31–39.
[7] Dave, J. S. and Vora, R. A. (1970). Liquid Crystals and Ordered Fluids, Plenum Press: New
York.
[8] Furnis, B. S., Hannford, A. J., Smith, P. G. W., and Tatchell, A. R. (1989). Vogel’s Text Book of
Practical Organic Chemistry, 4th ed., Longman: Singapore.
[9] Doshi, A. V. and Ganatra, K. J. (1999). Proc. Ind. Acad. Sci. (Chem. Sci.), 4, 562.
[10] Gray, G. W. (1962). Molecular Structure and the Properties of Liquid Crystals, Academic Press:
London.
[11] Bhoya, U. C. and Doshi, A. V. (2005). J. Indian Chem. Sci., 82, 143.
[12] Prajapati, H. R. (2005). Study of Mesomorphic State of matter through new homologous series
and Binary mixtures, PhD thesis, Saurashtrauni University, Rajkot, Gujarat, India.
[13] Gray, G. W. and Windsor, P. A. (1974). Liquid Crystals and Plastic Crystals, Ellis Hardwood:
Chichester.
[14] Patel, V. R. and Doshi, A. V. (2010). DerpharmaChemica, 2, 429.
[15] Doshi, A. V. and Chauhan, B. C. (2003). ActaCiencia Indica, 21C, 107.
[16] Prajapati, H. R. and Doshi, A. V. (2009). J. Indian Chem. Soc., 86, 470.
[17] Chauhan, B. C., Shah, R. R., and Doshi, A.V. (2011). DerpharmaChemica, 4.