Influence of oxygen on phase behavior of N(p-n-Octyloxy Benzylidene) p-n-Alkoxy anilines: A comparative study

Article abstract of DOI:10.1080/15421400802697640

The N (p-n-octyloxy benzylidene) p-n-alkoxy anilines (8O.Om) compounds were synthesized, and the mesomorphic properties were characterized by using thermal microscopy (TM) and differential scanning calorimetry (DSC) for m=3 to 7. The nematic (N), smectic-

Full text of DOI:10.1080/15421400802697640

Mol. Cryst. Liq. Cryst., Vol. 501, pp. 86–93, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802697640
Influence of Oxygen on Phase Behavior of
N (p-n-Octyloxy Benzylidene) p- n-Alkoxy
Anilines: A Comparative Study
N. Ajeetha and Durga P. Ojha
Liquid Crystal Research Laboratory, Post-Graduate Department
of Physics, Andhra Loyola College, Vijayawada, India
The N (p-n-octyloxy benzylidene) p-n-alkoxy anilines (8O.Om) compounds were
synthesized, and the mesomorphic properties were characterized by using thermal
microscopy (TM) and differential scanning calorimetry (DSC) for m ¼ 3 to 7. The
nematic (N), smectic-C (SC), and smectic-G (SG) phases are the more commonly
observed in these compounds. The smectic-A (SA) and smectic-B (SB) phases are
observed only in the 8O.O5 compound. The influence of oxygen on phase behavior
of the 8O.Om compounds is compared with the data available on nO.m
compounds.
Keywords: benzylidene aniline compounds; mesomorphic properties; phase behavior
INTRODUCTION
Liquid crystals (LC) have become synonymous with high technology.
They provided for displays important to the current computer and
communication revolutions. Liquid crystals are also at the center of
enormous advances in our understanding of the fundamental science
of condensed systems [1,2]. The phase transitions in liquid crystals
are often accompanied by interesting changes in their properties.
The experimental evidence shows that the influence of molecular
structure on the incidence and temperature range of liquid crystal
phases is an important, complex, and subtle topic [3].
The synthesis of liquid crystals containing terminal, linking, and
lateral alkoxy groups play an important role in the realization of LC
phases for display applications. Further, the terminal groups in a
molecule have their own importance because of their polarity and
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
86

Influence of Oxygen on Phase Behavior
87
direction. It has been found that terminally substituted compounds
exhibit more stable mesophases than their unsubstituted mesogenic
counterparts [4]. One of the most common types of recent investigation
into structure–property relationships has been the synthesis of groups
of molecules differing from one another by the number of methylene
groups as a terminal substituent. There appears to be two principal
motives for this type of study. The first involves the search for techno-
logically useful liquid crystals, i.e., materials of specific nematic ther-
mal ranges. The other reason for studying homologous series is more
fundamental in nature. In proceeding along a homologous series, only
very small changes are made at each step. It is thus hoped that more
specific conclusions would be possible as to the mesophase behavior
with molecular size and shape [5].
The benzylidene anilines are popularly known, as nO.m compounds,
where n and m represent the number of carbon atoms in alkoxy and
alkyl chains respectively, which not only exhibit rich and subtle liquid
crystal polymorphism at ambient temperatures, but also useful as
model systems in phase transition phenomena. In the present investi-
gation, homologous series of benzylidene aniline compounds containing
terminal alkoxy chain on both sides of the rigid core is taken as
prime object. The values of n ¼ 8 and m vary from 3 to 7. The results
are compared with the data available on nO.m compounds [6,7].
EXPERIMENTAL
The compounds are prepared by condensation of the corresponding
benzaldehyde (0.1 mole) and aniline (0.1 mole) on refluxing with
absolute ethanol in the presence of few drops of glacial acetic acid
[8]. After refluxing the reactants for four hours, solvent is removed
by distillation under reduced pressure, and the pure compound is
recrystalized from absolute ethanol at low temperature. The optical
textural observations [9] are 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 are carried out using a Perkin-Elmer
DSC-7 instrument. The general molecular structure of benzylidene
aniline compounds is shown in Fig. 1.
FIGURE 1 The general molecular structure of the 8O.Om compound.

88
N. Ajeetha and D. P. Ojha
RESULTS AND DISCUSSION
The transition temperatures from thermal microscopy and differential
scanning calorimetry along with the heats of transition involved at the
phase transformation are given in Table 1. For comparison, the data
pertinent to the 8O.m compounds are also given in the same table.
The DSC thermograms of 8O.O3 and 8O.O4 compounds are presented
in Figs. 2 and 3.
The preliminary polarizing microscopic studies on 8O.Om com-
pounds (for m ¼ 3 to 7) reveal that all compounds exhibit mesomorphic
phases. These compounds show three different types of phase variants
TABLE 1 The Transition Temperatures in ꢁC Along with the Enthalpy Values
of N (p-n-Octyloxy Benzylidene) p-n-Alkoxy Aniline (8O.Om) are Shown
Compound Phase Method
I-N=A N=A- N=A- C-B C=
N=A= Ref.
name
variant
A=B C=B B-G B=G-K
8O.3
8O.O3
NABG TM
Cooling
Heating 106.39
DH J=gm
Cooling 103.96
DH J=gm 1.80
Cooling 105.2
83.6 80.7
68.4
64.3
64.8
52.7
[17]
99.23
62.58
84.33
51.41
N
DSC
1.47
TM
85.3
8O.4
8O.O4
ABG
NC
TM
DSC
Cooling
Heating 112.05
81.2 68.0
39.5 [18]
105.92
74.43
82.97
57.36
84.2
42.2 [19]
89.79
DH J=gm
Cooling 109.45
DH J=gm 2.25
Cooling 110.0
Cooling 85.7
Heating 108.87 99.65
DH J=gm 4.00 4.24
Cooling 105.85 96.44
DH J=gm 3.78 3.71
Cooling 104.6 95.3
Cooling 86.2
Heating 111.8
2.35
93.7
1.55
95.0
70.8
TM
8O.5
8O.O5
ABG
ABG
TM
DSC
77.86
83.26 74.74
4.23 49.80
81.9
TM
ACBG TM
73.6
54.7 [19]
93.85
8O.6
8O.O6
75.8 75.0 65.0
104.23
NCG
DSC
D J=gm
4.93
4.09
73.12
Cooling 108.75
100.77
3.73
102.2
75.6 74.4 58.2
104.09
2.86
100.63
2.45
100.8
84.35 79.53
2.83 56.58
D J=gm
Cooling 110.3
Cooling 87.0
Heating 108.62
D J=gm 3.43
Cooling 105.54
D J=gm 2.56
Cooling 105.6
5.43
TM
85.6
80.4
8O.7
ACBG TM
29.6 [19]
95.49
83.84
87.13
60.68
87.6
8O.O7
NC
DSC
TM

Influence of Oxygen on Phase Behavior
89
FIGURE 2 The DSC heating and cooling thermogram of the 8O.O3 compound
at the scan rate of 10ꢁC=min.
viz. monovariant, divariant, and trivariant phase sequences. On cool-
ing the isotropic melt, the compounds for m ¼ 3, 4, 6, and 7 (except 5)
show a characteristic threaded marble texture of nematic phase at
FIGURE 3 The DSC heating and cooling thermogram of the 8O.O4 compound
at the scan rate of 10ꢁC=min.

90
N. Ajeetha and D. P. Ojha
105.2, 110.3, 110.0, and 105.6ꢁC, respectively. On further cooling, the
nematic phase transformed to other liquid crystalline phase with the
exhibition of schlieren texture characterizing the phase as smectic-C
in all the compounds (except 8O.O3 and 8O.O5). The nematic phase
in compound 8O.O3 transforms to solid crystalline phase at 85.3ꢁC.
Other compounds with n ¼ 4, 6, and 7 exhibit the nematic to smec-
tic–C transition at 95.0, 102.2, and 100.8ꢁC, respectively. Compounds
8O.O4 and 8O.O7 do not exhibit other liquid crystalline phases on
further cooling. Instead, they transform to solid crystalline phase at
84.2 and 87.6ꢁC. However, compound 8O.O6 is found to exhibit mosaic
texture below smectic-C phase at 85.6ꢁC characterizing the phase as
smectic-G as shown in Plate 1. This compound is found to crystalize
at 80.4ꢁC. Compound 8O.O5 shows a smectic phase in the form of
batonnets at 104.6ꢁC by cooling from the isotropic melt. These baton-
nets float in the isotropic phase are observed to coalesce and form focal
conic fan texture with the decrease of temperature. This phase also
exhibits pseudoisotropic texture in the homeotropic region, revealing
the phase as an orthogonal phase indicating the phase in the present
case as of smectic-A.
Further, lowering of temperature results in the appearance of
transient transition bars across the fans, and these bars slowly
disappear and transform to smooth focal conic fans at 95.3ꢁC. The
appearance of transient transition bars and the formation of smooth
focal conic fan texture with reduced discontinuities at the focal conic
PLATE 1 The mosaic texture of smectic G (SG) phase exhibited by the 8O.O6
compound at 85.6ꢁC on cooling.

Influence of Oxygen on Phase Behavior
91
domain boundaries infer that this transformation is from smectic-A to
smectic-B phase sequence. On further cooling of smectic-B phase exhi-
bits striped broken focal conic fan texture from smooth focal conic tex-
ture at 81.9ꢁC. These observations indicate that the phase is smectic-G
phase. The smectic-G phase is found to transform to solid crystal
phase on further cooling of the sample at 73.6ꢁC.
Influence of Alkoxy Chain
The phase variants of 8O.Om compounds are presented in Table 2
along with the phase variants of 7O.m and 8O.m series for compari-
son. A glance at Table 2 reveals that the general trend is observed
in case of 8O.m series, the increment of methylene group cause the
compound to form pure smectogenic nature for m ꢂ 4. The 8O.Om
series show an anomalous nature by exhibiting pure smectic character
in 8O.O5 compound, where the former and latter compounds of this
series exhibit both nematic and smectic mesomorphism. The similar
trend is observed in the case of 7O.m series, for the value of m ¼ 6 exhi-
bit a pure smectogenic character unlike other compounds that show
both nematic and smectic polymesomorphism [10].
A comparison between the 8O.m series and the 8O.Om series
reveals that the alkoxy derivatives, which have higher values of polar-
izability than those of the corresponding alkyl compounds should
always exhibit higher clearing temperature [11]. Both nematic and
smectic-C phases are more favored and the nematic phase is existing
up to m ¼ 7 (except 5) that is not observed in case of the 8O.m series
[12]. In general, the intermolecular attractive forces such as van der
Waals forces, hydrogen bonds, electron donor interactions, and inter-
molecular repulsion forces influence the situation of molecules in a
mesophase. These forces, separately or together, may be responsible
for increasing or decreasing the stability of liquid crystalline phases
or for creating new phases. However, the increase of attractive forces
involves the creation of phases [13].
TABLE 2 The Phase Variants Exhibited by 8O.Om along with the Phase
Variants of 7O.m and 8O.m Compounds
M
3
4
5
6
7
7O.m
8O.Om
8O.m
NAB
N
NAB
NACG
NC
ABG
NACBG
ABG
ABG
ACFG
NCG
ACBG
NACG
NC
ACBG

92
N. Ajeetha and D. P. Ojha
Role of Oxygen
The terminal group that increases the molecular polarizability (without
significantly increasing the molecular breadth) increases the thermal
stability of the resulting mesophase. It has been argued that increasing
the anisotropy of polarizability should enhance the nematic thermo-
stability [14]. The two terminal alkoxy groups of benzylidene anilines
may confer a stronger lateral dipole that aids the formation of tilted
phases. The alkoxy group is expected to enhance the mesophase stabi-
lity by its oxygen unshared-pair overlap with the associated benzene
ring [15,16]. The nematic and smectic C phases are more favored, and
the increase in alkoxy chain length is directly proportional to the phase
length of SC, and it shows inverse proportion to N. The close observation
on Table 1 reveals that the phase length of SC is increasing with the
decrease of N, and it reaches to maximum at m ¼ 6 and decrease in
phase length of both SC, N phases is observed for m ¼ 7.
Figure 4 shows the comparative chart of alkoxy carbon number
versus liquid crystallinity of the compounds. The gradual increasing
trend is observed from m ¼ 3 to 5, at m ¼ 6 the liquid crystallinity
reaches the maximum, and a sudden decrease is started from m ¼ 7.
A comparison between Table 2 and Fig. 4, we may drive a general con-
clusion that as the alkoxy chain goes on increasing the polymeso-
morphism is also increasing up to m ¼ 3 to 6, further increment of
FIGURE 4 A comparative chart of alkoxy carbon number versus liquid
crystalline range.

Influence of Oxygen on Phase Behavior
93
alkoxy chain cause the reduction of ploymesomorphic behavior that
observed from m ¼ 7. Further more, increment of alkoxy chain may
eliminate the mesomorphism slowly, and it is indicating a limit to
which the end alkoxy group can be extended in a mesogen.
CONCLUSION
The data presented for homologous series of Schiff base 8O.Om
compounds with alkoxy group on both sides indicate that the kind of
interaction between the end group, and the core portion of the mole-
cule plays an important role in determining the polymesomorphism
and the liquid crystalline nature.
ACKNOWLEDGMENTS
The financial support rendered by the CSIR, DST, UGC, New Delhi
and DAE–BRNS, Mumbai, India, is gratefully acknowledged.
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