Novel azoester compounds with a lateral methyl substituent

Article abstract of DOI:10.1080/15421406.2013.857576

Twelve novel homologues with an azo linkage of the series 4-carbethoxy-[3′methyl-4′ (4″-n-alkoxy benzoyloxy)] azobenzenes have been synthesized. The methyl to n-hexyl homologues exhibit only nematic mesophases, while the n-heptyl to n-tetradecyl homologues exhibit both smectic and nematic mesophases. The n-hexadecyl homologue exhibits only a smectic mesophase. The plot of transition temperatures versus number of carbon atoms in alkoxy chain exhibits an odd-even effect for the nematic-isotropic transitions. The mesogenic behavior of present series is explained by comparing each homolog of the related mesogenic series. The synthesized compounds were characterized by a combination of elemental analysis and standard spectroscopic methods. For the exhibition of mesomorphic property the role of ester and azo linkages has been discussed. The impact of the lateral methyl group on mesomorphism is also discussed. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2013.857576

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Novel Azoester Compounds with a
Lateral Methyl Substituent
Sandhya Dixita & R. A. Voraa
a Applied Chemistry Department, Faculty of Technology and
Engineering, The M.S. University of Baroda, Kalabhavan, Vadodara,
Gujarat, India
Published online: 28 Apr 2014.
To cite this article: Sandhya Dixit & R. A. Vora (2014) Novel Azoester Compounds with a
Lateral Methyl Substituent, Molecular Crystals and Liquid Crystals, 592:1, 133-140, DOI:
10.1080/15421406.2013.857576
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Mol. Cryst. Liq. Cryst., Vol. 592: pp. 133–140, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.857576
Novel Azoester Compounds with a Lateral Methyl
Substituent
SANDHYA DIXIT∗ AND R. A. VORA
Applied Chemistry Department, Faculty of Technology and Engineering, The
M.S. University of Baroda, Kalabhavan, Vadodara, Gujarat, India
Twelve novel homologues with an azo linkage of the series 4-carbethoxy-[3ꢀmethyl-
4ꢀ (4ꢀꢀ-n-alkoxy benzoyloxy)] azobenzenes have been synthesized. The methyl to
n-hexyl homologues exhibit only nematic mesophases, while the n-heptyl to n-tetradecyl
homologues exhibit both smectic and nematic mesophases. The n-hexadecyl homologue
exhibits only a smectic mesophase. The plot of transition temperatures versus number
of carbon atoms in alkoxy chain exhibits an odd-even effect for the nematic-isotropic
transitions. The mesogenic behavior of present series is explained by comparing each
homolog of the related mesogenic series. The synthesized compounds were character-
ized by a combination of elemental analysis and standard spectroscopic methods. For
the exhibition of mesomorphic property the role of ester and azo linkages has been
discussed. The impact of the lateral methyl group on mesomorphism is also discussed.
Keywords Liquid crystals; mesomorphic; monotropic; nematic; smectic
1. Introduction
Thermotropic liquid crystals possess a number of unique properties that have received
considerable research attention. Significant efforts have been focused on the synthesis of
new compounds [1, 2], and it has been found that molecules having an extended rod
like shape often exhibit a thermotropic liquid crystalline phase [3,4]. The design of novel
thermotropic liquid crystals as advanced functional materials involves the suitable selection
of a core fragment, linking group, and terminal functionality. The aromatic esters are known
for their thermal stability, easy synthesis, and relative resistance to hydrolysis. Azo dye
liquid crystals have attracted much attention because such photochromic molecules can
undergo a reversible photochemical reaction between the trans and cis forms through
photoisomerization. The trans form of the azobenzene, with rod-like shape, stabilizes the
phase structure of the LC, while the bent cis form tends to stabilize the phase structure.
These properties have enabled LCs containing azobenzene chromophores to be widely used
in display technologies and various photonic applications, including optical storage, optical
switching, optical computers, and integrated optical device for communication [5–10].
It is well known that the mesomorphic properties of calamitic liquid crystals can largely
be influenced by the structural variations in terminal and lateral substituents [11–14]. Lateral
substitution broadens the molecules, and hence plays an effective role in the mesogenic
∗Address correspondence to S. Dixit, Applied Chemistry Department, Faculty of Technology
and Engineering, The M.S. University of Baroda, Kalabhavan, Vadodara, Gujarat, India. E-mail:
snddxt 20@yahoo.com
133

134
S. Dixit and R. A. Vora
properties of the material. Studies on the effect of lateral substitution have been carried out
by several researchers. A survey of the literature indicates that generally the mesophase
range of a mesogen with lateral substituent is less than that of a laterally unsubstituted
mesogen [15–20]. Thus, in order to study the correlation between chemical constitution and
mesomorphism, a new mesogenic homologous series consisting of three phenyl rings joined
through ester and azo linkages and a lateral methyl group with n-alkoxy and carbethoxy as
terminal end groups was synthesized and its mesomorphic properties were studied.
2. Experimental
2.1 Synthesis
4-Hydroxy benzoic acid, n-alkyl halides, thionyl chloride, pyridine, methanol, potassium
hydroxide, orthocresol, ethyl 4-amino benzoate, and sodium nitrite were chemically pure
of lab grade. Solvents were dried and distilled prior to use.
1. 4-n-Alkoxy benzoic acids and 4-n-alkoxy benzoyl chlorides were synthesized by
the modified method of Dave and Vora [21].
2. 4-Carbethoxy-3ꢀ-methyl-4-hydroxy azobenzene (DA-1) was synthesized by known
method [22].
3. The series namely 4-Carbethoxy-[3ꢀmethyl-4ꢀ(4ꢀꢀ-n- alkoxy benzoyloxy)] azoben-
zene was synthesized by adding dropwise the solution of 4-Carbethoxy-3ꢀ-methyl-
4-hydroxy azobenzene (DA-1) in dry pyridine to the respective solution of 4-n-
alkoxy benzoyl chloride at 0–5◦C. The mixture was allowed to stand overnight at
room temperature. It was acidified with cold 1:1 aqueous hydrochloric acid and
the separated solid was washed successively with dilute sodium hydroxide solution
and water. The final product obtained was recrystallized several times from glacial
acetic acid until constant transition temperatures were obtained which are recorded
in Table 1. The elemental analysis of each compound was found to be satisfactory
(Table 2). The route of synthesis of series 1 is illustrated in Scheme 1.
2.2 Characterization
Micro analysis of the compounds was performed on a Coleman carbon, hydrogen analyzer.
FTIR spectra were determined on Nicolet impact 400 FTIR using Nujol. 1H NMR spectra
was performed on Perkin-Elmer (90 MHz) spectrometer using tetra methylsilane (TMS)
as internal reference substance and CDCl3 as solvent. Liquid crystalline properties were
determined on a Leitz Laborlux 12 POL microscope equipped with a heating stage. Ther-
modynamic quantities enthalpies (ꢀH) and entropies (ꢀS = ꢀH\T) of transitions were
determined using differential scanning calorimeter (DSC) via a Mettler TA-4000 system
and reported in Jg−1 at a scanning rate of 5◦C min−1. The calorimeter was calibrated using
pure indium as standard.
2.3 Analytical Data
FTIR (Nujol) Spectra cm−1. n-Butoxy Derivative : 3000 (C-H Str. Aromatic), 2900, 2850
(C-H Str.Aliphatic), 1735 (C O Str. Ester), 1600 (N N Str.Azo), 1380, 1280, 1150 (aryl
ether), 1050, 870, and 760.

Novel Azoester Compounds with a Lateral Methyl Substituent
135
RBr, KOH
EtOH
SOCl2
∆
HO
COOH
RO
COOH
RO
COCl
(A)
NaNO2, HCl
0-5oC
C2H5OOC
NH2
C2H5OOC
N2Cl
OH
CH3
NaOH
0-5oC
C2H5OOC
N
N
OH
DA1 (B)
CH3
DA1
(B)
RO
COCl
+
(A)
Dry Pyridine
5-10oC, HCl
RO
C
O
O
N
N
COOC2H5
H3C
R = CnH2n+1 : n = 1 to 8, 10, 12, 14 & 16
Scheme 1. The route of synthesis to series 1 compounds.
n-Hexadecyloxy derivative : 3000 (C-H Str. Aromatic), 2900, 2850(C-H Str. Aliphatic),
1730 (C O Str.Ester), 1600 (N N Str. Azo),1370, 1250, 1160 (aryl ether), 1050, 850,
and 770.
Proton NMR spectra (Solvent CDCl3, 90 MHz, δ PPM, standard TMS). n-Butoxy Deriva-
tive : δ 0.9 (t,3H,CH3), 1.3 (t,3H, CH3), 1.4–1.9 (m, 4H, Ar-O-(CH2)2 -), 2.3 (s, 3H, Ar-
CH3), 3.9 (t, 2H, Ar-OCH2), 4.2 (q, 2H, Ar-COOCH2), 6.8–8.1 (m, 11H, Ar-H).
2.4 Calorimetry
Calorimetry is a valuable method for the detection of phase transitions. Conclusions may
be drawn concerning the nature of the phases that participate in the transition with help of
calorimetric studies. In the present study, enthalpies of tetradecyl homologue of series 1
are determined by DSC and data is recorded in Table 3.

136
S. Dixit and R. A. Vora
Table 1. Transition temperature (◦C) for Series 1
Transition temperatures (◦C)s
n-Alkyl CnH2n+1 group
Compound No.
Series I
SmC
SmA
N
I
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
150
145
134
132
114
105
96
78
85
82
96
230
235
210
224
192
200
173
164
153
140
130
127
—
(55)∗
(60)
(70)
55
10
11
12
81
82
105
—
()∗ indicate monotropy, Sm, smectic; N, nematic; I, isotropic.
3. Results and Discussion
The homologous series 4-Carbethoxy [3ꢀmethyl-4ꢀ-(4ꢀꢀ-n-alkoxybenzoyloxy)] azobenzenes
consisting of 12 mesomorphic homologues have been synthesized. The methyl to n-hexyl
derivatives exhibit enantiotropic nematic mesophases. The n-heptyl, n-octyl, and n-decyl
derivatives exhibit monotropic smectic mesophases in addition to enantiotropic nematic
mesophases. The n-dodecyl and n-tetradecyl derivatives are enantiotropically smectics and
nematics. The n-hexadecyl derivative exhibits only an enantiotropic smectic mesophase.
The transition temperatures (Table 1) are plotted versus the number of carbon atoms in
alkyl chain of n-alkoxy terminal end group. The phase diagram showing phase behavior
is depicted in Figure 1. The smectic-nematic transition curve shows rising tendency. The
nematic-isotropic transition curve shows falling tendency as series is ascended with exhibi-
tion of odd-even effect. The nematic phase lengths vary from 15◦ to 95◦C and the smectic
phase lengths vary from18◦C to 42◦C.
There is a close relationship between mesomorphism and molecular constitution of
organic compounds. Hence thermal stability, a measure of mesomorphism, can be cor-
related with the molecular constitution of the compounds. Figure 2 shows the transition
Table 2. Elemental analysis for ethyloxy, pentyloxy, octyloxy, and decyloxy derivative
Element% found (compared with% calculated)
Molecular formula
C
H
N
C25H24O5N2
C28 H30O5N2
C31H36O5N2
C33H40O5N2
68.88(68.57)
69.79(70.12)
71.84(71.42)
71.78(72.18)
5.93(5.71)
6.17(6.49)
7.07(7.14)
7.05(7.51)
6.42(6.66)
5.65(6.06)
5.82(5.55)
5.45(5.26)

Novel Azoester Compounds with a Lateral Methyl Substituent
137
Table 3. DSC data
Peak temperature
Series
1
Member
Transition (microscopic reading) ꢀ (H/Jg−1) ꢀ (S/Jg−1 K−1)
Teteradecyl
Cr–Sm
Sm–N
N-Iso
80.9(81)
92.4(96)
124.5(130)
5.791
44.345
5.316
0.01636
0.12136
0.01337
temperatures and molecular structures of the n-decyloxy derivative (compound 10) of the
present series 1 and the structurally related compound A [23], B [24], and C [25] reported
in literature. Reference to Figure 2 indicates that the smectic and nematic thermal stabilities
of compound 10 is lower by 47◦C and 46◦C, respectively, than that of compound A. The
molecules of compound 10 and A differ only at lateral substituent, other structural features
are being the same. Compound 10 has a lateral methyl group ortho to ester linkage whereas
compound A has a hydroxyl group ortho to azo linkage. Due to intramolecular association
of lateral hydroxyl group with an azo central linkage it is less effective in broadening of
the molecules of compound A whereas the effect of increase in breadth is prominent in
compound 10 because of lateral methyl group. Therefore, the nematic and smectic thermal
stabilities of compound 10 are lower than those of compound A.
SmC-N
SmC-SmA
Cr-Meso
240
N-Iso
220
SmA-Iso
200
180
160
140
120
100
80
60
40
20
0
0
2
4
6
8
10
12
14
16
18
No of C atoms in alkoxy chain
Figure 1. The phase behavior of series 1.

138
S. Dixit and R. A. Vora
C12H25
O
COO
N
N
COOC2H5
H3C
Compound 10 :Cr 55 oC SmC 82 o C N 140 oC Iso
C12H25
O
COO
N
N
COOC2H5
OH
Compound A: Cr 102 oC SmC128 oC N128 oC Iso
C12H25O
COO
N
N
F
H3C
Compound B: Cr 78 oC N 96 oC Iso
C12H25O
COO
N
N
COOC2H4OCH3
Compound C: Cr 97 oC SmA 159 oC N 226o C Iso
Figure 2. The Comparative molecular structures of Compounds 1, A, B, and C.
Figure 2 further shows that the compounds 10 and B consists of three phenyl rings,
two central groups –COO– and –N N–, one n-alkoxy terminal end group and laterally
substituted methyl –CH3 group as common identical features. The only one terminal end
groups of compounds 10 and B viz. –COOC2H5 and –F are uncommon features. Fig-
ure 2 indicates that the compound 10 exhibits smectic and nematic mesophases whereas
compound B exhibits only nematic mesophase. The nematic phase length and thermal sta-
bility of compound 10 is higher by 40◦C and 44◦C, respectively, than those of compound
B. Both the compounds under comparison possess laterally substituted methyl groups at
the middle phenyl ring which will affect the length to breadth ratio and polarizability to
a different extent for compounds 10 and B due to the presence of different end groups
–COOC2H5 and –F. The molecules of compound 10 are longer and highly polarizable due
to the presence of longer and polar –COOC2H5 terminal group at one end, as compared
to short polar –F group in compound B at that position. The longer terminal –COOC2H5
group increases molecular length of compound 10, which interact with one another and

Novel Azoester Compounds with a Lateral Methyl Substituent
139
stabilizes the molecular packing required for nematic phase. Gray [26] has also explained
that increased length and polarizability enhances both mesogenic properties and thermal
stabilities. Hence, nematic thermal stability of compound 10 should be higher than that
of compound B, which is the case. The exhibition of smectic phase in compound 10 as
compared to none in compound B can be explained on the basis of long polar –COOC2H5
terminal group, which aids the formation of smectic mesophase due to enhanced lateral
attraction, as compared to terminal attraction, which facilitates lamellar packing required
for the formation of smectic mesophase [26].
The comparison of compound 10 and compound C shows that the smectic and nematic
thermal stabilities of compound 10 are lower by 42◦C and 86◦C, respectively, than those
of compound C. This can be explained on the basis of their structural variations. The
molecular structure of compound 10 differs from the molecules of compound C in the
terminal as well as in the central part of the benzene ring. The molecules of the compound
10 has –COOC2H5 group at the one terminal and –CH3 lateral group at the central benzene
ring, whereas molecules of compound C has –COOC2H4OCH3 group at the same terminal
position without any lateral substituent. The presence of lateral methyl group increases the
breadth of the molecules of compound 10 and the length is also shorter than the molecules
of compound C because of absence of additional –OCH3 unit. Gray [26] has explained that
increase in breadth of molecules reduces both nematic and smectic thermal stabilities. Thus,
both reduced length and enhanced breadth factors resulted in lower smectic and nematic
thermal stabilities of compound 10 reported in present investigation.
An interesting aspect of the present series is that it exhibits polymorphism of the
smectic phase. Five members of the series exhibit the smectic C phase and one member
exhibits a smectic A (SmA) phase. In the present series the SmA (focal conic texture)
and SmC (Schlieren texture) and nematic (marble texture) are inferred from the textures
observed under a polarizing microscope.
4. Conclusion
In this paper, we have presented the synthesis and characterization of a homologous series
with central ester and azo linkages and lateral methyl substituent. Compounds of the present
series exhibit a nematic mesophase along with the SmC and SmA mesophases. The study
revealed that enhanced length of the molecules of the present series due to the presence of
the long –COOC2H5 terminal substituent resulted in higher mesophase thermal stabilities
while, increased breadth due to presence of the lateral methyl group lowered its mesophase
thermal stabilities. The interesting aspect of the present series is that due to the lateral
methyl substituent their solid to mesomorphic transitions (melting points) are lower.
Acknowledgment
The authors thank the Head of the Applied Chemistry Department, Faculty of Technology
and Engineering, The M.S. University of Baroda, Vadodara for providing research facilities.
References
[1] Marin, L., Damaceanu, M. D., & Timpu, D. S. (2009). Soft Mate., 7, 1–20.
[2] Bhowmik, P. K., Han, H., Nedeltchev, A. K., & Mandal, H. D. (2009). Liq. Cryst., 36(1),
1389–1399.

140
S. Dixit and R. A. Vora
[3] Luckhurst, G, R., & Gray, G. W. (1979). The Molecular Physics of Liquid Crystals, Academic
Press: London.
[4] Gelbart, W. M. (1982). J. Phys. Chem., 86, 4298–4307.
[5] Ruslim, C., & Ichimura, K. (1998). Chem. Lett., 789–790.
[6] Kozlovsky, M. V., Shibaev, V. P., Stakhanov, A. I., Weyrauch, T., & Iaase, W. I. (1998). Liq.
Cryst., 24, 759–767.
[7] Shibaaev, V. P., Kostomin, S. A., & Ivanov, S. A. (1996). In Polymers as Electro Optically and
Photo-Optically Active Media, Shibaev, V. P. (Ed.), Springer: Berlin, 37.
[8] Ikeda, T., & Tsutsumi, O. (1995). Science, 268, 1873.
[9] Anderle, K., & Wendorff, J. (1994). Mol. Cryst. Liq. Cryst., 243, 51–75.
[10] Yu, Y., & Ikeda, Y. (2004). J. Photochem. Photobio. C: Photochem. Rev., 5, 247–265.
[11] Johnson, J. F., & Porter, R. S. (1970). Liquid Crystals and Ordered Fluids, Plenum Press: New
York.
[12] Smith, G. (1975). Advances in Liquid Crystals, Academic Press: New York, 20.
[13] Gray, G. (1987). Thermotropic Liquid Crystals, Wiley: New York, 113.
[14] Chandrasekhar, S. (1977). Liquid Crystals, Cambridge University Press: London, 51.
[15] Prajapati, A. K., & Pandya, H. M. (2003). Mol. Cryst. Liq. Cryst., 393, 31–39.
[16] Patel, D. H., & Doshi, A. V. (2007). Acta Ciencia Indica, 33, 53–56.
[17] Giss, P., Cluzeau, P., Ravaine, V., & Nguyen H. T. (2002). Liq. Cryst., 29(1), 91–98.
[18] Kasipar, M., Hamplovaa, V., Pakhomov, S. A., Stibor, I., Bubnov, A. M., & Vaneik, P. (1997).
Liq. Cryst., 22(5), 557–561.
[19] Karahaliou, P. K., Kouwer, P. H. J., Meyer, T., Mehl, G. H., & Photino, D. J. (2008) Phys. Chem.
B, 112, 6550–6556.
[20] Bhoya, U. C., & Doshi, A. V. (2005). J. Indian Chem. Soc., 82, 143–145.
[21] Dave, J. S., & Vora, R. A. (1970). Liquid Crystals and Ordered Fluids, Johnson, J. F., & Porter,
R. S. (Eds.), Plenum Press, 477.
[22] Vogel, A. I. (1989). In Text Book of Practical Organic Chemistry, 5th edn. ELBS, and Longman
Group, 946.
[23] Dixit, S., & Vora, R. A. (2009). Mol. Cryst. Liq. Cryst., 501, 43–52
[24] Dave, J. S., Upasani, C. B., & Patel, P. D. (2010). Mol. Cryst. Liq. Cryst., 533, 73–81.
[25] Prajapati, A. K., & Pandya, H. M. (2005). J. Chem. Sci., 117, 255–261.
[26] Gray, G. W. (1962). In Molecular Structure and Properties of Liquid Crystals, Academic Press:
London.