Synthesis of liquid crystals with lateral methyl group and study of their mesomorphic properties

Article abstract of DOI:10.1080/15421406.2012.657129

Two new homologous series consisting of central azo and ester linkage and methyl groups at the central as well as terminal benzene rings are synthesized and their mesomorphic properties are studied. Between these two series; Series I has lateral methyl group on terminal benzene ring where as series II has lateral methyl groups at central as well as terminal benzene cores. All the homologues of series I and II are mesogenic in nature; series I exhibits nematic phase from C1 to C14 members whereas C16 member shows smectic C mesophase alongwith nematic mesophase on the other hand all the members of series II are nematogenic in nature. The nematic phase of the series I and II shows marble/schlieren texture and the smectic phase shows schlieren texture of smectic C variety. The mesomorphic properties of both the series are compared with each other and also with other structurally related series to evaluate the effect of lateral methyl group at central as well as terminal benzene core on mesomorphism. Taylor and Francis Group, LLC.

Full text of DOI:10.1080/15421406.2012.657129

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Synthesis of Liquid Crystals with Lateral
Methyl Group and Study of Their
Mesomorphic Properties
a
a
Jayrang S. Dave & Himanshu S. Bhatt
a
Department of Applied Chemistry, Faculty of Technology and
Engineering, The M. S. University of Baroda, Vadodara, India
To cite this article: Jayrang S. Dave & Himanshu S. Bhatt (2012) Synthesis of Liquid Crystals with
Lateral Methyl Group and Study of Their Mesomorphic Properties, Molecular Crystals and Liquid
Crystals, 562:1, 1-9, DOI: 10.1080/15421406.2012.657129
To link to this article: http://dx.doi.org/10.1080/15421406.2012.657129
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Mol. Cryst. Liq. Cryst., Vol. 562: pp. 1–9, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.657129
Synthesis of Liquid Crystals with Lateral Methyl
Group and Study of Their Mesomorphic Properties
∗
JAYRANG S. DAVE AND HIMANSHU S. BHATT
Department of Applied Chemistry, Faculty of Technology and Engineering,
The M. S. University of Baroda, Vadodara, India
Two new homologous series consisting of central azo and ester linkage and methyl
groups at the central as well as terminal benzene rings are synthesized and their
mesomorphic properties are studied. Between these two series; Series I has lateral
methyl group on terminal benzene ring where as series II has lateral methyl groups at
central as well as terminal benzene cores. All the homologues of series I and II are
mesogenic in nature; series I exhibits nematic phase from C1 to C14 members whereas
C16 member shows smectic C mesophase alongwith nematic mesophase on the other
hand all the members of series II are nematogenic in nature. The nematic phase of the
series I and II shows marble/schlieren texture and the smectic phase shows schlieren
texture of smectic C variety. The mesomorphic properties of both the series are compared
with each other and also with other structurally related series to evaluate the effect of
lateral methyl group at central as well as terminal benzene core on mesomorphism.
Keywords Azo-ester mesogen; homologous series; lateral methyl group; mesomor-
phism; nematic; smectic C
Introduction
Mesomorphic behavior of mesogens depends on the molecular structures of molecules
and molecular forces arising, therefore, a slight change in the molecular geometry brings
about considerable change in its mesomorphic properties [1–15]. Lateral substitution makes
molecules broad and plays an effective role in mesogenic properties of a mesogenic com-
pound. Literature survey indicates that the study of lateral substitution at central and terminal
benzene ring is rare however [3,6,12–15]. Recently our group has reported the effect of
lateral substitution on mesogenic properties of Fluoro aniline derivatives [14]. Keeping this
in view, we have synthesized two new homologous series consisting of azo and ester central
linkages and lateral methyl groups at central as well as terminal benzene core in order to
study the effect of lateral methyl group on mesomorphism.
Experimental
4
2
-Hydroxy benzoic acid, the appropriate n-alkyl halides, 2,4-dimethylaniline, phenol, and
-methyl phenol were used as received. Solvents were dried and distilled prior to use.
∗
Address correspondence to Jayrang S. Dave, Applied Chemistry Department, Faculty of
Technology and Engineering, The M. S. University of Baroda, Vadodara-390 001, India. E-mail:
jayrangdave@yahoo.com
1

2
J. S. Dave and H. S. Bhatt
Microanalyses of some of the representative compounds were performed on a Perkin Elmer
Series II 2400-CHN analyzer; infrared (IR) spectra were recorded on a Shimadzu IR-408,
and nuclear magnetic resonance (NMR) spectra were measured on a Varian mercury-300
spectrometer. Liquid crystalline properties were investigated on a Leitz Laborlux 12 POL
optical polarizing microscope provided with a Kofler heating stage. Differential scanning
calorimetry (DSC) was performed on a Shimadzu DSC-60.
1
2
3
. 4-n-Alkoxy benzoic acids and 4-n-alkoxy benzoyl chlorides were synthesized by
the reported method [16].
ꢀ
ꢀ
ꢀ
. 4-Hydroxyphenylazo-2 ,4 -dimethylbenzene, 3-methyl-4-hydroxyphenylazo-2 ,
ꢀ
and 4 -dimethylbenzene were prepared by a known method [17].
ꢀ
ꢀꢀ
ꢀꢀ
. The series I, namely 4-(4 -n-alkoxybenzoyloxy)-phenylazo-2 , 4 -dimethyl
ꢀ
ꢀꢀ
benzenes; and series II viz. 4-(4 -n-alkoxybenzoyloxy)-3-methylphenylazo-2 ,
ꢀꢀ
4
-dimethylbenzenes were synthesized by adding drop wise a cold solution
ꢀꢀ
ꢀꢀ
of 4-hydroxy phenylazo-2 , 4 -dimethylbenzene (for series I); and 3-methyl-4
ꢀꢀ
ꢀꢀ
hydroxyphenylazo-2 , 4 -dimethylbenzene (for series II), in dry pyridine to a
cold solution of 4-n-alkoxy benzoyl chlorides. The mixture was allowed to stand
overnight at room temperature. It was acidified with 1:1 cold HCl and the separated
solid was filtered and column chromatographed on silica gel (60–120 mesh) with
◦
1
% ethyl acetate – petroleum ether (60–80 C) (1:99) mixture as eluent. Removal of
solvent from the eluate afforded a solid material, which was crystallized repeatedly
from ethyl acetate – petroleum ether until constant transition temperatures were
obtained. These are recorded in Tables 1 and 2 respectively for series I and II. The
elemental analyses of C8 member (series I) and C6 member (Series II) were found
to be satisfactory (Table 3). The synthetic route of the series is shown in Scheme 1.
−1
FTIR (Nujol, KBr pellets, cm ) were recorded on a Shimadzu IR-408.
◦
ꢀ
Table 1. Transition temperature ( C) of Series I. Series I: 4-(4 -n-alkyloxybenzoyloxy)-
ꢀꢀ
ꢀꢀ
phenylazo-2 , 4 -dimethylbenzenes
◦
Transition temperature ( C)
Nematic
R = n-Alkyl group
Smectic C
Isotropic
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
–
–
–
–
–
–
–
–
–
136
127
125
111
91
92
94
80
93
234
214
202
167
161
143
144
164
152
128
83
–
–
54
73
55
62
75

Synthesis of LCs with Lateral Methyl Group
3
◦
ꢀ
Table 2. Transition temperature ( C) of Series II. Series II: 4-(4 -n-alkyloxybenzoyloxy)
ꢀꢀ ꢀꢀ
3
-methylphenyl azo-2 , 4 - dimethylbenzenes
◦
Transition temperature ( C)
R = n-Alkyl group
Smectic C
Nematic
Isotropic
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
–
–
–
–
–
–
–
–
–
–
–
–
135
115
92
90
93
95
78
56
44
45
52
55
162
188
138
155
134
145
94
93
84
68
70
76
ꢀ
ꢀꢀ ꢀꢀ
Series I: 4-(4 -n-octyloxybenzoyloxy)-phenylazo-2 , 4 -dimethylbenzene.
2
3
925–2858 (C H St, Alkyl CH3, SP hybridization), 1734 ( C O St. of ester),
1606 ( N N St.), 1517–1479 ( C C St. of Aromatic ring), 1469 ( C H bending
of CH2 ), 1262 (Aromatic ether St.), 1075( C O St. of ester), 878 (strong C H
bending for 1:2:4 tri substituted benzene ring), 759 (weak C H bending for (CH2)7
)
−1
cm .
Series II: - 4-(4 -n-hexyloxybenzoyloxy) 3-methylphenyl azo-2 , 4 - dimethylbenzene.
ꢀ
ꢀꢀ ꢀꢀ
3
2
954–2857 (C H St, Alkyl CH3, SP hybridization), 1734 ( C O St. of ester),
1601 ( N N St.), 1577–1503 ( C C St. of Aromatic ring), 1477 ( C H bending
Scheme 1. Synthetic route for Series I and II.

4
J. S. Dave and H. S. Bhatt
Table 3. Elemental analysis
Theoretical
Practical
H (%)
Series
Homologue
C (%)
H (%)
N (%)
C (%)
N (%)
I
II
C8
C6
75.92
75.66
7.40
7.25
6.10
6.30
75.51
75.40
7.67
7.75
6.27
6.35
of CH2 ), 1247 (Aromatic ether St.), 1061( C O St. of ester), 893 (strong C H
bending for 1:2:4 tri substituted benzene ring), 725 (weak C H bending for (CH2)5
)
−1
cm .
1H NMR (CDCl3, 300 MHz, δ ppm, standard TMS) were recorded on Varian mercury
3
00 (CDCl3, 300 MHz, δ ppm, standard TMS).
ꢀ
ꢀꢀ ꢀꢀ
Series I: 4-(4 -n-octyloxybenzoyloxy)-phenylazo-2 , 4 -dimethylbenzene.
δ = 0.89 (t, 3H, CH3), 1.29–1.82 (m, 14H, 7( CH2 )), 1.85 (Qunt, 2H, COCH2),
.38 (s, 3H, Ar CH3), 2.69 (s, 3H, Ar CH3), 4.04 (t, 3H, OCH2), 6.95–8.15 (m, 11H,
2
Ar H).
ꢀ
ꢀꢀ ꢀꢀ
Series II: 4-(4 -n-hexyloxybenzoyloxy) 3-methylphenyl azo-2 , 4 -dimethylbenzene.
δ = 0.90 (t, 3H, aliphatic CH3), 1.35–1.82 (m, 14H, 5(-CH2-)), 1.85 (Qunt, 2H,
COCH2), 2.32 (s, 3H, Ar CH3), 2.37 (s, 3H, Ar CH3) 2.68 (s, 3H, Ar CH3), 4.04 (t,
H, OCH2), 6.95–8.15 (m, 11H, Ar H).
3
Results and Discussion
In the present investigation, twelve homologues from each of the two series;
ꢀ
ꢀꢀ
ꢀꢀ
ꢀ
viz. 4-(4 -n-alkyloxybenzoyloxy)-phenylazo-2 , 4 -dimethylbenzenes (series I); 4-(4 -n-
ꢀꢀ
ꢀꢀ
alkyloxybenzoyloxy) 3-methylphenyl azo-2 , 4 -dimethylbenzenes (series II), were syn-
thesized and their mesomorphic properties were studied.
All the twelve homologues of series I are mesogens (Table 1); the nematic phase
commences from the very first derivative and remains upto the tetradecyl derivative, whereas
the smectic C phase is observed only in the hexadecyl derivative. Figure 1 shows the plot of
transition temperatures against number of carbon atoms in the n-alkoxy chain; it indicates
that Cr M transitions show overall falling tendency from the methyl to octyl derivative
and then a slight rising tendency at the 10th homologue and then the curve shows falling
tendency upto hexadecyl derivative. The N I curve shows overall falling tendency with a
jump at the C8 derivative as the series is ascended. The nematic phase of the series shows
marble/schlieren texture whereas the smectic phase shows schlieren texture of the smectic
C variety.
All the twelve homologues of series II are mesogens (Table 2); the nematic phase
commences from the very first derivative and remains upto the last hexadecyl derivative.
Smectic phase is absent in the entire series; thus, all homologues are nematogens. Figure 2,
shows plot of transition temperatures against number of carbon atoms in the n-alkoxy chain;
it indicates that the N I curve shows an odd–even effect in the initial members upto C6
derivative, then the N I curve shows falling tendency from C7 to C12 member with slight
rise at C10 homologue; from C12 upto C16 homologue the N I curve shows slight rising
tendency as the series is ascended. The Cr M transitions show initially falling tendency
upto C3 derivative and then rising tendency upto C6 derivative, then the Cr M curve shows

Synthesis of LCs with Lateral Methyl Group
5
ꢀ
ꢀꢀ
ꢀꢀ
Figure 1. Series I: 4-(4 -n-alkyloxybenzoyloxy)-phenylazo-2 , 4 -dimethylbenzenes.
overall falling tendency with slight increase from C12 to C16 derivative. The nematic phase
of the series shows the same Marble/Schlieren texture as that of the series I.
Figure 3 shows polarizing optical microscopic images of Marble and Schlieren texture
of nematic phase of compound C8 of series I and C6 of series II respectively.
ꢀ
ꢀꢀ
ꢀꢀ
Figure 2. Series II: 4-(4 -n-alkyloxybenzoyloxy)-3-methylphenylazo-2 , 4 -dimethylbenzenes.

6
J. S. Dave and H. S. Bhatt
Figure 3. Textures of mesophase observed under polarizing optical microscope. (a) Marble texture
◦
of nematic phase of compound C8 Series I at 114 C on cooling. (b) Schlieren texture of the nematic
◦
phase of Compound C6 Series II at 121 C on cooling.
Series I and II are compared with each other and structurally related homologous series
[8,15]. Figure 4 shows molecular structures of the series in comparison and Table 4 shows
Thermal stabilities of smectic and nematic mesophase of these series under comparison.
Series II is purely nematogenic as it does not exhibit smectic mesophase till C16 derivative
studied, whereas series I shows smectic C mesophase in the C16 homologue alongwith
nematic phase; this can be due to introduction of one more lateral methyl group in the
central benzene ring in series II. Both series I and II have lower N I thermal stability than
series A; this can be due to the introduction of lateral methyl group in the aromatic core
O
RO
RO
H C
3
O
N
N
CH3
CH3
.
..……..…. (I)
O
O
H C
3
N
N
H C
3
…………. (II)
O
RO
O
N
N
CH3
CH3
…
……….. (A) [8]
O
O
RO
N
N
H C
3
….……. (B) [15]
…
Figure 4. Molecular structures of the series in comparison.

Synthesis of LCs with Lateral Methyl Group
7
◦
Table 4. Average thermal stabilities ( C)
◦
Smectic-isotropic ( C) or
Nematic-isotropic
Commencement of
smectic mesophase
◦
◦
Series
smectic–nematic ( C)
( C)
1
2
A
B
62 (C16)
155.6 (C1–C16)
117.2 (C1–C16)
242 (C1–C8)
C16
–
C10
–
–
106.5 (C10–C16)
–
147.5 (C8 and C16)
of series I and II, which forces apart the long molecular axes of the molecules, thereby
lowering the N I stability [2]. The introduction of the lateral methyl group in series I
as well as in series II changes the molecular conformation and broadens the molecules,
which decreases, the transition temperatures and thermal stabilities of smectic and nematic
mesophase [2,14,15]. This factor is a direct result of an increase in the thickness of the
molecule [18]. The increased dissymmetry resulting from the lateral methyl group leads
to less effective molecular packing in the crystal lattice, and therefore, lowers the crystal
to mesophase transition temperature as shown in Tables 1 and 2 [15]. Thus, the effect
of substitution, which leads to an increase in molecular breadth, is that the long narrow
molecules will be forced further apart so reducing the strength of the intermolecular lateral
attractions. Both smectic and nematic mesophases depend to a greater or lesser extent
on these cohesive forces to maintain the parallel orientation of the molecules. Therefore,
substitution leading to an increase in molecular breadth will decrease the smectic and
nematic thermal stabilities, which is actually found in the present series I and II [2].
When the two homologues of series B Viz. C8 and C16 are compared with C8 and
C16 homologues of series II, it is found that the N I stability of C8 and C16 homologues
of series B is higher than those of series II; it seems that the introduction of lateral methyl
group in the central benzene ring in series II has much pronounced effect in the reduction of
transition temperature, N I thermal stability and thereby mesophase range [2]. The N I
thermal stability of series II is lower than that of series I which can be due to increase
in breadth of the molecule of series II due to introduction of one more methyl group in
the central benzene ring [2]. These results are in agreement with our previous results of
the substitution of lateral methyl group in central benzene core [14]. The commencement
Table 5. DSC analysis
Heating rate
( C/min)
Transition temperature
H =
S =
◦
◦
Series
I
Member
Octyl
( C)
J/g
J/g.K
10
10
Cr–M 79.50
N–I 164.29
Cr–M 52.74
S–N 60.38
N–I 74.54
77.42
2.72
1.00
17.88
0.37
107.55
2.70
0.2195
0.0062
0.0031
0.0536
0.0011
0.2936
0.0065
I
Hexadecyl
Hexyl
II
10
Cr–M 93.10
N–I 139.02

8
J. S. Dave and H. S. Bhatt
4
2
0
-2
-4
-6
Series I C8 Homologue
-8
-
10
12
14
-
-
40
60
80
100
120
140
160
180
200
Temperature (ºC)
2
0
-2
-4
-6
Series I C16 Homologue
-8
-10
-12
40
60
80
100
120
Temperature (ºC)
5
0
-5
-10
Series II C6 Homologue
-15
-20
40
60
80
100
120
140
160
Temperature (ºC)
Figure 5. DSC curves.

Synthesis of LCs with Lateral Methyl Group
9
of smectic mesophase in series I is at a later C16 homologue as compared to laterally
unsubstituted series A where it is at an early C10 homologue.
Two homologues from series I (C8, C16) and one homologue from series II (C6) were
analyzed by DSC; their data are given in Table 5 whereas Figure 5 shows DSC curves of
these homologues.
Conclusion
The introduction of methyl group in lateral position at terminal and central benzene core
decreases thermal stabilities of smectic and nematic mesophases. The commencement of
smectic mesophase is delayed with introduction of lateral methyl group in terminal benzene
ring in series I whereas with introduction of one more lateral methyl group in central benzene
core, the smectic phase is altogether eliminated.
Acknowledgments
The authors are thankful to the Heads, Prof. Deota and Prof. Murthy of the Applied
Chemistry Department, Faculty of Technology and Engineering, The M.S. University of
Baroda, Vadodara, for providing the laboratory as well as analytical instruments facilities.
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London and New York.
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[
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