The influence of oxygen on mesomorphic behavior of benzylidene anilines-The effect of end chain

Article abstract of DOI:10.1080/15421406.2010.504619

A variety of terminal chain modifications were made by placing/removing the electro-negative oxygen atom to either or both sides of the rigid core of the benzylidene aniline compounds, the chosen alkyl chain lengths were n=5 and m=16. The mesomorphic properties were determined by polarizing thermal microscopy (TM) and differential scanning calorimetry (DSC). They exhibit two mono-variant and two di-variant phase sequences. The results are discussed in light of the depression in melting temperature, elevation of clearing temperature, and quenching of liquid-crystalline phases as a function of influence of oxygen with the end alkyl chain length. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2010.504619

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The Influence of Oxygen on Mesomorphic
Behavior of 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, A.P., India
Published online: 20 Oct 2010.
To cite this article: N. Ajeetha & Durga P. Ojha (2010) The Influence of Oxygen on Mesomorphic
Behavior of Benzylidene Anilines—The Effect of End Chain, Molecular Crystals and Liquid Crystals,
528:1, 96-102, DOI: 10.1080/15421406.2010.504619
To link to this article: http://dx.doi.org/10.1080/15421406.2010.504619
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Mol. Cryst. Liq. Cryst., Vol. 528: pp. 96–102, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.504619
The Influence of Oxygen on Mesomorphic
Behavior of 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, A.P., India
A variety of terminal chain modifications were made by placing=removing the
electro-negative oxygen atom to either or both sides of the rigid core of the benzy-
lidene aniline compounds, the chosen alkyl chain lengths were n ¼ 5 and m ¼ 16. The
mesomorphic properties were determined by polarizing thermal microscopy (TM)
and differential scanning calorimetry (DSC). They exhibit two mono-variant and
two di-variant phase sequences. The results are discussed in light of the depression
in melting temperature, elevation of clearing temperature, and quenching of
liquid-crystalline phases as a function of influence of oxygen with the end alkyl chain
length.
Keywords Benzylidene anilines; liquid-crystalline phase; terminal chain
modification
Introduction
Liquid crystals (LCs) have become quintessential self-assembled materials of the
modern era. The LC science and applications now permeate almost all segments
of society from large industrial displays to individual homes and offices [1]. The
scientific research devoted to liquid-crystalline materials has expanded greatly during
the last several years. Of particular importance is the organic synthesis aspect to
study the chemical, structural, and mesomorphic properties of compounds [2–4].
The mesomorphic behavior of liquid crystals is basically dependent on their molecu-
lar structures in which a slight change in their molecular geometry may bring about
considerable change in their mesomorphic properties [5].
The liquid-crystal properties of the clearing temperature and enthalpy of transi-
tions are influenced by the end chains. It is well known that physical and chemical
properties of liquid-crystal molecules change with the length of end chains. In gen-
eral, trans configurations have been used because they result in a more linear,
rod-like molecular shape, a general requisite for the mesophase to occur. The Schiff’s
base compounds are trans and acoplanar. The existence of mesomorphic properties
Address correspondence to Durga P. Ojha, Liquid Crystal Research Laboratory,
Post-Graduate Department of Physics, Andhra Loyola College, Vijayawada-8, A.P., India.
E-mail: durga_ojha@hotmail.com
96

Mesomorphic Behavior of Benzylidene Anilines
97
=
in these compounds is due to a twist in the aniline ring out of CꢀN CꢀC plane and
the calculations have indicated this twist to be as much as 55 degrees [6,7]. When two
aromatic nuclei are not coplanar, it is possible for the terminal of one molecule to
interact strongly with one of the rings of another molecule, causing additional attrac-
tions and a more thermally stable mesophase as the terminus grows in length [8].
The synthesis of liquid crystals containing terminal, linking, and lateral alkoxy
groups plays an important role in display applications because of their promising
physicochemical and electro-optical properties. The benzylidene aniline compounds
are popularly known as nO.m compounds where n and m are flexible end chains.
They constitute a series of compounds exhibiting rich polymesomorphism. In the
present investigation, an attempt is made to address the influence of the position
of oxygen on either side (nO.m or n.Om), on both sides (nO.Om), or in the absence
of oxygen (n.m) to the essential rigid core where n ¼ 5 and m ¼ 16.
Experimental
The compounds were prepared by condensation of the corresponding benzaldehyde
(0.1 mol) and aniline (0.1 mol) on refluxing with absolute ethanol in the presence of a
few drops of glacial acetic acid [9]. After refluxing the reactants for 4 h, the solvent
was removed by distillation under reduced pressure and the pure compound was
recrystallized from absolute ethanol at low temperature. The schematic diagram
explaining the synthesis of the respective compound is shown in Fig. 1.
The Textural Identification
The identification of polymesomorphic class is decided from evidence gleaned from
several experimental sources, but microscopic textures of different smectic and nematic
forms still remain and will remain a powerful and economically practical experimental
means of classifying the smectic as well as nematic phase [10]. The changes in texture
that occur at a phase transition are frequently important factor in phase identification.
The optical textural observations [11] are made with an OLYMPUS DX 50 (USA)
polarizing microscope equipped with a DP 10 CCD display, in conjunction with an
INSTEC temperature controller (USA) of accuracy ꢁ0.1ꢂC.
Calorimetric Investigation
Calorimetry is a valuable method for the detection of phase transitions. It yields
quantitative results; therefore, conclusions may be drawn concerning the nature of
Figure 1. Schematic diagram explaining the synthesis of the n.Om compound where n ¼ 5 and
m ¼ 16.

98
N. Ajeetha and D. P. Ojha
Figure 2. General molecular structure of benzylidene anilines with n ¼ 5 and m ¼ 16.
the phase that participates in the transition. 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. 2.
Results and Discussion
The phase characterization results of three compounds that differ in the presence (or
absence) of an oxygen atom to the left and=or right of the rigid core of the benzy-
lidene aniline are presented here. The phase variants exhibited by these compounds
along with the phase transition temperatures and the heats of transition involved at
the phase transformation are given in Table 1. The phase transition temperatures
observed by thermal microscopy are found to be in agreement with those from the
corresponding differential scanning calorimetry (DSC) thermograms shown in Fig. 3.
Although the optical textural observations reveal that these compounds exhibits
traditional textures, they are found to exhibit two mono-variant and two di-variant
sequenced systems. On cooling from the isotropic phase, the compound 5.O16 is
found to grow a nematic phase exhibiting a threaded marble texture at 79.2ꢂC,
whereas in the case of 5.16 a smectic phase in the form of batonnets is seen at
52.9ꢂC. This phase also exhibits a pseudo-isotropic texture in the homeotropic
region, revealing it as an orthogonal phase (smectic-A). Further, with lowering of
temperature, 5.O16 exhibits a smectic-G mosaic texture at 72.3ꢂC (see Fig. 4).
This smectic-G phase is found to transform into a solid crystal phase on further
cooling of the sample at 51.3ꢂC. Similarly, the mono-variant 5.16 compound is
found to transform the smectic-A phase to the solid crystal at 40.2ꢂC on further
cooling. The compound 5.16 exhibiting two peaks at 37.60ꢂC and 34.02ꢂC indicating
the solid–solid transitions as evident from Fig. 3.
Role of Oxygen
Table 1 reveals that the position of oxygen plays a prominent role in determining the
liquid crystallinity as well as the polymorphism of the compounds. The position of
oxygen on either side of the rigid core favors the liquid-crystalline nature, and the
absence causes a depression in melting temperature and a reduction of the liquid-
crystal thermal range. But, an oxygen on both sides of the rigid core causes an

Mesomorphic Behavior of Benzylidene Anilines
99
ꢂ
Table 1. Transition temperatures in C along with the enthalpy values of N(4-n-
pentyloxy benzylidene) 40-n-hexa decyl aniline (5O.16), N(4-n-pentyl benzylidene)
40-n-hexa decyloxy aniline (5.O16), N(4-n-pentyloxy benzylidene) 40-n-hexa
decycloxy aniline (5O.O16), and N(4-n-pentyl benzylidene) 40-n-hexa decyl aniline
(5.16) are shown
Compound Phase
name
variant Method
I-N=A N-A N=A-G I=A=B-K Ref.
5O.16
5.O16
NA
NG
TM
DSC Heating
Cooling 68.8 67.4
55.1
76.11a
[13]
78.25a
DH.J=g
53.25
69.98
4.43
Cooling 74.49
DH.J=g 5.84
Cooling 79.20
60.33
34.51
51.3
TM
72.30
5O.O16
5.16
N
A
DSC Heating
DH.J=g
Cooling
97.66
66.77
85.63
65.89
87.00
50.80
28.69
DH.J=g
Cooling 87.40
TM
DSC Heating
DH.J=g
Cooling 48.27
DH.J=g 11.91
Cooling 52.90
37.60 34.02
9.38
5.19
40.20
TM
aPeaks are not well resolved.
Figure 3. Comparative DSC heating and cooling thermograms of 5O.16, 5.O16, 5O.O16, and
5.16 compounds.

100
N. Ajeetha and D. P. Ojha
Figure 4. Smectic-G domain texture exhibited by 5.O16 compound on cooling at 72.3ꢂC.
elevation in clearing temperature as well as the quenching of liquid-crystalline nat-
ure. A comparative picture of phase variants exhibited by the homologous series
of N(4-n-alkoxy=alkyl benzylidene) 40-n-hexa decyl=decyloxy anilines is presented
in Table 2. Evidently, the fourth row (nO.O16) of the table infers that four com-
pounds (except 5O.O16) exhibit no liquid-crystalline nature, accompanied by a
direct transformation of isotropic liquid to solid crystal on cooling.
However, a narrow thermal range of about 0.4ꢂC is observed in compound
5O.O16 that exhibits nematic phase through the exhibition of Schlieren texture on
cooling (see Figure 5). In Figure 5, one may observe the simultaneous growth of
solid and nematic Schlieren (four brushes) texture for which no traces are found even
in the DSC thermograms at slow scan rates. It may be argued that the narrow range
of the liquid-crystalline phase is dominated by the growth of the solid crystal phase
from isotropic melt. Moreover, the oxygen on the aldehyde side is the most favorable
condition for the mesophase.
The Effect of End Chain
Although oxygen plays a major role, the effect of the alkyl chain cannot be ruled out.
Close observation of Table 2 reveals that even though the position of oxygen on
Table 2. Phase variants exhibited by nO.16, n.O16, nO.O16, and n.16 compounds
with n ¼ 1, 2, 4, 5, and 8
n ¼
1
2
4
5
8
nO.16
n.O16
nO.O16
n.16
N
A
NA
NA
NG
N
AB
Non-LC
Non-LC
Non-LC
Non-LC
Non-LC
Non-LC
Non-LC
Non-LC
Non-LC
Non-LC
A
A
The blank space indicates the compound yet to be reported.

Mesomorphic Behavior of Benzylidene Anilines
101
Figure 5. Simultaneous growth of nematic Schlieren texture and the solid crystalline phase
exhibited by compound 5O.O16.
aldehyde is favorable, the lengths of the alkyl chain are also important in tuning the
mesomorphism. In such a case, the middle homologous where n ¼ 4 and 5, especially
n ¼ 5, is the most favorable series for the occurrence of the mesomorphism irrespec-
tive of oxygen.
Comparison with Other Homologous Series
The first row of Table 2 reveals that irrespective of chain length all the compounds
exhibit mesomorphism because an alkoxy group is expected to pronounce the meso-
phase stability by its oxygen (with unshared-pair) overlap with the adjacent aromatic
ring. While moving along each column, except the middle homologous (n ¼ 4 and 5)
members, all other lower and higher homologous series show a non-liquid-crystalline
nature irrespective of the position of oxygen. It may be claimed that the repulsion
between neighboring molecules with increasing intermolecular separation may
responsible for these suppressed mesomorphic properties [12].
Conclusion
It may be concluded from the above discussion that the oxygen atom along with the
longer alkyl chain length is less probable for the mesomorphism, whereas the oxygen
atom in the aldehyde side is the most favorable condition for the occurrence of meso-
morphism. However, the effect is prominently observed in the lower and the higher
homologous series compared to the middle members.
Acknowledgment
The financial support rendered by the Council of Scientific & Industrial Research
(CSIR), Department of Sciences & Technology (DST), University Grants Com-
mission (UGC), New Delhi, and Department of Atomic Energy-Board of Research
in Nuclear Sciences (DAE-BRNS), Mumbai, India, is gratefully acknowledged.

102
N. Ajeetha and D. P. Ojha
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