Role of position of oxygen on benzylidene anilines: The effect of end chain

Article abstract of DOI:10.1080/15421400802305780

A family of benzylidene aniline compounds with variable end chains (for n=1, 2, 4, 8 and m=16) are synthesized by placing the electro-negative oxygen atom on either side of the rigid core (nO.m, n.Om), on both sides of the rigid core (nO.Om) and with the

Full text of DOI:10.1080/15421400802305780

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Role of Position of Oxygen
on Benzylidene Anilines: The
Effect of End Chain
N. Ajeetha a & Durga P. Ojha a
a Liquid Crystal Research Laboratory, Post-Graduate
Department of Physics, Andhra Loyola College ,
Vijayawada, India
Published online: 22 Sep 2010.
To cite this article: N. Ajeetha & Durga P. Ojha (2008) Role of Position of Oxygen on
Benzylidene Anilines: The Effect of End Chain, Molecular Crystals and Liquid Crystals,
490:1, 16-26, DOI: 10.1080/15421400802305780
To link to this article: http://dx.doi.org/10.1080/15421400802305780
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Mol. Cryst. Liq. Cryst., Vol. 490, pp. 16–26, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802305780
Role of Position of Oxygen on Benzylidene Anilines:
The Effect of End Chain
N. Ajeetha and Durga P. Ojha
Liquid Crystal Research Laboratory, Post-Graduate Department
of Physics, Andhra Loyola College, Vijayawada, India
A family of benzylidene aniline compounds with variable end chains (for n ¼ 1, 2,
4, 8 and m ¼ 16) are synthesized by placing the electro-negative oxygen atom on
either side of the rigid core (nO.m, n.Om), on both sides of the rigid core (nO.Om)
and with the absence of oxygen on rigid core moiety (n.m). The mesomorphic
properties are studied by using Thermal Microscopy (TM) and Differential Scan-
ning Calorimetry (DSC). The depression=elevation of melting=clearing tempera-
ture and the nature of liquid crystallinity are presented as a function of position
of oxygen with end chains.
Keywords: benzylidene anilines; mesomorphic properties
INTRODUCTION
Liquid crystals (LCs) are versatile systems in the field of soft condensed
matter physics. The special emphasis on their scientific research gives
an insight into the potential applications [1]. The literature abounds in
experimental and theoretical studies of phase transitions and many
unifying concepts have emerged out in recent years, which provide a
better understanding of the nature of phase transitions. The phase
transitions in liquid crystals are often accompanied by interesting
changes in their properties [2]. The investigation of physical para-
meter in homologous series of compounds is particularly useful in
order to understand the relation between the molecular structure
and polymorphic properties of liquid crystals [3,4].
The thermotropic LCs constituted by a variety of terminal, linking,
and lateral chemical groups; especially have gained importance in the
Address correspondence to Durga P. Ojha, Liquid Crystal Research Laboratory, Post-
Graduate Department of Physics, Andhra Loyola College, Vijayawada 520-008, A.P.,
India. E-mail: durga_ojha@hotmail.com
16

Benzylidene Anilines
17
development of commercial liquid crystalline materials for display
applications [5]. The benzylidene aniline compounds are popularly
known as nO.m compounds and are composed of two rigid aromatic
nuclei attached with flexible end chains of n and m. The primary role
of alkyl chain is to enhance the LC thermal range by lowering the
melting point. The liquid crystalline properties such as the clearing
temperature and the enthalpy of transitions are influenced by the
end chains. It is well known that physical and chemical properties of
the LC molecules change with the length of alkyl chains [6].
The alkoxy chain length that is in conjugation with the bridging
group plays a major role in deciding the occurrence of an LC phase,
transition, and melting=clearing temperature. The position of oxygen
with respect to the bridging site and the influence of alkyl chain length
in confirming the liquid crystalline nature at ambient temperature are
the important aspects of work to be investigated. In the present inves-
tigation, the value of n ¼ 1, 2, 4, 8 and m ¼ 16, the position of oxygen is
varied from aldehyde side to aniline side, the oxygen is introduced on
both sides of the rigid core and in the absence of oxygen on rigid core
moiety the phase transitions are studied. The results are compared
with the data available on nO.m compounds [7].
EXPERIMENTAL
The compounds were prepared by condensation of the corresponding
benzaldehyde (0.1 mole) and aniline (0.1 mole) on refluxing with absol-
ute ethanol in the presence of few drops of glacial acetic acid [8]. After
refluxing the reactants for four hours, the solvent was removed by
distillation under reduced pressure and the pure compound was
recrystallized from absolute ethanol at low temperature. The optical
textural observations [9] were made with an OLYMPUS DX 50 polar-
izing microscope equipped with DP 10 CCD display, in conjunction
with an INSTEC temperature controller of accuracy ꢀ0.1ꢁC. The
calorimetric investigations were carried out using a Perkin-Elmer
DSC-7 instrument. The general molecular structure of benzylidene
aniline compounds is shown in Fig. 1.
RESULTS AND DISCUSSION
The transition temperatures from Thermal Microscopy (TM) and
Differential Scanning Calorimeter (DSC) along with the heats of
transition involved at the phase transformation are given in Table 1.
The data pertinent to the nO.16 compounds are also given in the same
table for comparison. The DSC thermograms of n.O16, nO.O16, and

18
N. Ajeetha and D. P. Ojha
FIGURE 1 The general molecular structure of nO.16, n.O16, nO.O16, and
n.16 compounds.
n.16 compounds along with nO.16 compounds for comparison are
presented in Figs. 2 to 5. In case of compounds for alkoxy=alkyl chain
number n ¼ 1, 2, 4, and 8 and m ¼ 16, the preliminary textural results
on N-(p-n-alkoxy benzylidine)p-n-hexadecyloxy anilines (nO.O16’s)
infer that four compounds do not exhibit liquid crystalline nature
and a direct transformation of isotropic liquid to solid crystal is
observed on cooling.
The similar results are also observed in N-(p-n-alkyl benzylidine)
p-n-hexadecyloxy anilines (n.O16’s) compounds. However, in the case
of N-(p-n-alkyl benzylidine)p-n-hexadecyl anilines, viz., n.16’s also
exhibit no liquid crystalline nature in the case for n ¼ 1, 2, and 8, while
the compounds with n ¼ 4 exhibit monotropic smectic-A phase. The
transition temperatures are observed in the vicinity of room
temperature and the observed phase is monotropic. These results
are found to be in contrast with the results those observed in the case
of N-(p-n-alkoxy benzylidine)-p-n-hexadecyloxy anilines (nO.16’s),
where all the compounds show liquid crystalline phases [10].
Effect of Alkyl Chain Length with Change in Position
of Oxygen
The change in position of oxygen from aldehyde to aniline side has no
effect on melting=clearing temperature of the (n.O16) compounds, but
the elimination of liquid crystallinity is observed. A glance at Table 2

Benzylidene Anilines
19
TABLE 1 The Phase Variant, Transition Temperatures along with the
Enthalpy Values are Shown
Compound
name
Phase
variant Method
I-N=A N-A A-B I=N=A=B-K Ref.
1O.16
1.O16
N
TM
DSC
Cooling
66.7
64.5
74.35
67.26
63.12
55.17
65.16
109.74
123.43
92.60
156.64
94.4
57.96
44.40
37.03
106.87
38.2
[10]
[17]
[18]
Non LC
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm
Cooling
Heating
DH ꢂ J=gm
Cooling
TM
DSC
1O.O16
1.16
Non LC
Non LC
DH ꢂ J=gm
TM
DSC
Cooling
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm
TM
TM
DSC
Cooling
2O.16
2.O16
A
Cooling
70.1
52.2
Non LC
Heating
DH ꢂ J=gm
Cooling
76.75
54.56
71.98
50.75
76.3
102.98
143.43
89.60
136.64
91.4
51.96
34.40
37.03
16.87
53.20
51.3
DH ꢂ J=gm
TM
DSC
Cooling
2O.O16
2.16
Non LC
Non LC
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm
Cooling
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm
Cooling
TM
DSC
TM
TM
4.O16
NA
Cooling
69.2
68.5
4O.O16
Non LC
DSC
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm
Cooling
101.06
37.16
88.46
37.17
90.5
TM
4.16
A
DSC
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm 26.25
Cooling 38.1
48.56
134.3
35.79
77.18
35.0
38.41
TM
(Continued)

20
N. Ajeetha and D. P. Ojha
TABLE 1 Continued
Compound
name
Phase
variant Method
I-N=A N-A A-B I=N=A=B-K Ref.
75.5 71.4 [10]
8O.16
8.O16
AB
Non LC
TM
Cooling
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm
Cooling
60.67
112.10
45.91
75.06
51.3
TM
8O.O16
8.16
Non LC
Non LC
DSC
Heating
DH ꢂ J=gm
Cooling
93.50
97.23
78.46
94.13
90.1
47.42
30.23
31.10
16.21
35.3
DH ꢂ J=gm
TM
DSC
Cooling
Heating
DH ꢂ J=gm
Cooling
DH ꢂ J=gm
Cooling
TM
FIGURE 2 A comparative DSC thermograms of 1O.16, 1.O16, 1O.O16, and
1.16 compounds.

Benzylidene Anilines
21
FIGURE 3 A comparative DSC thermograms of 2O.16, 2.O16, 2O.O16, and
2.16 compounds.
FIGURE 4 A comparative DSC thermograms of 4O.O16 and 4.16 compounds.

22
N. Ajeetha and D. P. Ojha
FIGURE 5 A comparative DSC thermograms of 8O.16, 8.O16, 8O.O16, and
8.16 compounds.
reveals a pronounced effect in case of lower (n ¼ 1, 2) and higher
homologous series (n ¼ 8) of compounds with m ¼ 16. However, no
prominent effect is observed in case of middle (n ¼ m ¼ 4, 5) homolo-
gous series with identical alkyl chain lengths [11]. Further, the
reduction of liquid crystallinity and the change in phase length are
observed. This indicates that the position of oxygen in aldehyde side
is more favourable for liquid crystallinity and polymesomorphism of
TABLE 2 The Phase Variants Observed in n.Om Compounds
n=m
4
5
16
1
2
4
5
8
No LC phase (N)
No LC phase (N)
No LC phase (N)
No LC phase (A)
NG (NABG)
NG (NACFG)
RT liquid (ABG)
No LC phase (AB)
Blank space refers the phases are to be identified.
The phases given in parentheses are the phases observed in nO.m compounds.

Benzylidene Anilines
23
compounds. It may be due to the linking group that disturbs the
conjugation of rigid core [12].
Influence of Oxygen with Long Alkyl Chain Length
In nO.O16 compounds, the quenching of liquid crystallinity is
observed in the lower, middle, and higher homologous series of com-
pounds. The clearing temperature of these are raised to above 100ꢁC
compared to nO.m and n.Om compounds. The Table 3 shows the
reduction of polymorphism in all stages of nO.Om compounds,
however, the drastic change is observed in nO.O16. Further, it is
evident from Table 3 that in nO.Om compounds the alkyl chain length
is playing a major role in nO.O16 and some nO.O5 compounds with
n ¼ 4, 5, 16, and 18 [13]. It is observed that the identical alkyl chain
length and long alkyl chain length with oxygen on both sides of the
rigid core may eliminate the liquid crystalline nature. Although, the
oxygen is playing the major role, the effect of alkyl chain cannot be
ruled out.
In case of n. m compounds, except 4.16 compounds, are exhibiting
nonliquid crystalline nature [14]. Table 4 shows the polymesomorphic
properties of compounds with various n and m values. The compounds
1.5, 2.5, 4.4 are exhibiting their liquid crystalline nature at below room
TABLE 3 The Phase Variants Observed in nO.Om Compounds
n=m
3
4
5
6
7
16
1
2
3
N (N)
N (N)
N (NA)
No LC (N)
No LC (A)
4
5
NG (NABG) N (NACG)
N (NACFG)
No LC (NAB)
6
N (NABG)
7
NCBG (NACBG)
8
9
N (NAB) NC (ABG) ABG (ABG)
ABG (ABG)
ABG (ABG)
AB (ABG)
NCG (ACBG) NC (ACBG) No LC (AB)
10
11
13
14
15
16
18
AB (ABG)
AB (ABG)
AB (ABG)
A (F)
No LC (F)
Blank space refers the phases are to be identified.
The phases reported in parentheses are the phases observed in nO.m compounds.

24

Benzylidene Anilines
25
temperature level [15]. However, in higher homologous series for n ¼ 8
and m ¼ 5, 10, 12 the effect is more pronounced, and they exhibit room
temperature liquid nature. Furthermore, the quenching of liquid crys-
tallinity is observed in both lower and higher homologous series of
compounds for the values of m ¼ 14 and 16, and the middle homolo-
gous the effect is less, only the reduction of polymesomorphism is
observed. It is argued that the alkyl chains actually take an active part
in anisotropic interaction and alkyl chain carbon atoms plays an
important role in the anisotropy of the molecule and helps the ordering
process and hinders it also [16]. It is indicating that the limit which
the end chain can be extended in a mesogen.
CONCLUSIONS
. The phase ranges of all compounds are affected, in particular short
and long chain homologous. However, the effect is more pronounced
compared to middle homologous compounds.
. The total number of enantiotropic phases is less and most of the
compounds become monotropic in nature.
. The effect of chain length in presence of oxygen on both sides of the
benzylidene aniline (nO.O16) compounds seems to be greater, the
elimination of liquid crystallinity.
. The change in position of oxygen with long terminal alkyl chain has
no effect on clearing temperature. The inclusion of oxygen on both
sides of the rigid core cause the rise in clearing temperature where
as the absence of oxygen cause a fall in it.
ACKNOWLEDGMENT
The financial support rendered by the Council of Scientific & Indus-
trial Research, New Delhi, India is gratefully acknowledged.
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