Synthesis and mesomorphic properties of schiff base esters ortwo-hydroxy-para-alkyloxybenzylidene-para-substituted anilines

Article abstract of DOI:10.1080/15421400490494508

A series of new compounds o-n-hydroxy-p-n-hexadecanoyloxybenzylideneaniline and its substituted derivatives was synthesized from the esterafication of palmitic acid with intermediary Schiff bases, which possses various terminal substitutes X (X = H, F, Cl, Br, OCH₃, CH₃, and C ₂H₅. All compounds possess mesomorphic properties within compounds with X = H, F, Cl, Br, and OCH₃ exhibits smectic A (SmA) phases and those with terminal substituents CH₃ and Ch₃ and C₂H₅ are monotropic liquid crystals showing smectic C phases (SmC). Correlation studies of the clearing temperature (T_c) of the title compounds and polarizability related to mesophase stability and molecular anisotrophy are reported.

Full text of DOI:10.1080/15421400490494508

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SYNTHESIS AND MESOMORPHIC
PROPERTIES OF SCHIFF BASE
ESTERS ORTHO-HYDROXY-PARA-
ALKYLOXYBENZYLIDENE-PARA-
SUBSTITUTED ANILINES
Guan-Yeow Yeap* a , Sie-Tiong Ha a , Phaik-Leng Lim
a , Peng-Lim Boey a , Wan Ahmad Kamil Mahmood a ,
Masato M. Ito b & Shigeki Sanehisa b
a School of Chemical Sciences, Universiti Sains
Malaysia, 11800 Minden, Penang, Malaysia
b Soka University, Hachioji, Tokyo, Japan, 192-8577
Version of record first published: 02 Sep 2010.
To cite this article: Guan-Yeow Yeap* , Sie-Tiong Ha , Phaik-Leng Lim , Peng-Lim
Boey , Wan Ahmad Kamil Mahmood , Masato M. Ito & Shigeki Sanehisa (2004):
SYNTHESIS AND MESOMORPHIC PROPERTIES OF SCHIFF BASE ESTERS ORTHO-HYDROXY-
PARA-ALKYLOXYBENZYLIDENE-PARA-SUBSTITUTED ANILINES, Molecular Crystals and
Liquid Crystals, 423:1, 73-84
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Mol. Cryst. Liq. Cryst., Vol. 423, pp. 73–84, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490494508
SYNTHESIS AND MESOMORPHIC PROPERTIES
OF SCHIFF BASE ESTERS ORTHO-HYDROXY-
PARA-ALKYLOXYBENZYLIDENE-PARA-
SUBSTITUTED ANILINES
Guan-Yeow Yeapꢀ, Sie-Tiong Ha, Phaik-Leng Lim, Peng-Lim Boey,
and Wan Ahmad Kamil Mahmood
School of Chemical Sciences, Universiti Sains Malaysia,
11800 Minden, Penang, Malaysia
Masato M. Ito and Shigeki Sanehisa
Faculty of Engineering, Soka University, Hachioji,
Tokyo 192-8577, Japan
A series of new compounds o-n-hydroxy-p-n-hexadecanoyloxybenzylideneani-
line and its substituted derivatives was synthesized from the esterification of
palmitic acid with intermediary Schiff bases, which posssess various terminal
substituents X (X ¼ H, F, Cl, Br, OCH3, CH3, and C2H5). All compounds possess
mesomorphic properties wherein compounds with X ¼ H, F, Cl, Br, and OCH3
exhibit smectic A (SmA) phases and those with terminal substituents CH3 and
C2H5 are monotropic liquid crystals showing smectic C phases (SmC). Corre-
lation studies of the clearing temperature (Tc) of the title compounds and polar-
izability related to mesophase stability and molecular anisotropy are reported.
Keywords: clearing temperature; mesophase stability; molecular anisotropy; o-n-hydroxy-p-n-
hexadecanoyloxybenzylideneaniline; palmitic acid; Schiff bases
INTRODUCTION
The pericarp of palm fruit is comprised of three layers, the exocarp (skin),
mesocarp (outer pulp containing palm oil) and endocarp (a hard shell
enclosing the kernel, which contains oil and carbohydrate reserves for
the embryo) [1]. The major saturated fatty acid obtained from mesocarp
We are grateful to the Universiti Sains Malaysia and Malaysian Government for financial
support through FRGS Grant No. 304=PKIMIA=670032 and IRPA (EA) Grant No. 305=PKIMIA=
612923.
ꢀCorresponding author. E-mail: gyyeap@usm.my
73

74
G.-Y. Yeap et al.
of palm oil is palmitic acid (44%), and this is balanced by 39% monounsa-
turated oleic acid and 11% polyunsaturated linoleic acid. The remainder
consists of stearic (5%) and myristic (1%) acids. The composition is sig-
nificantly different from palm kernel oil (obtained as coproduct during
the processing of oil palm fruits), which is almost 85% saturated [1].
It has recently been found that palm oil is useful in fish diets, without
negatively affecting growth and feed utilization efficiency [2,3]. In the field
of liquid crystals, fatty acids and cholesterol from vegetable oils other than
oil palm had also been reported well [4–15]. The synthesis of Schiff base
esters, p-n-octadecanoyloxybenzylidene-p-substituted-anilines involving
esterification of stearic acid and various substituted 4-hydroxybenzalide-
neanilines is further evidence of diversification of palm oil leading to the
formation of liquid crystals [16,17].
In order to enhance further the rigidity of the core system in these Schiff
base esters, we introduced the lateral hydroxy group into the aldehyde
fragment, wherein the esterification occurred between the palmitic acid
and 2,4-dihydroxybenzaldehyde prior to the condensation of aldehyde with
nonsubstituted and various substituted anilines (Scheme 1). It has been
claimed that the introduction of lateral polar hydroxyl group increased
the molecular polarizability as well as higher clearing temperature. As such,
a systematic study on the effect of substituent (H, F, Cl, Br, OCH3, CH3, and
C2H5) in the aniline fragment of compounds 9–11 and their connectivities
with the intramolecular interaction, lateral intermolecular interaction, and
clearing temperatures were reported.
EXPERIMENTAL
4-Bromoaniline, 4-chloroaniline, 4-ethylaniline, and 4-dimethylaminopyridine
(DMAP) were obtained from Merck (Germany). 2,4-Dihydroxybenzaldehyde
and 4-fluoroaniline were purchased from Acros Organics (USA). While
4-methoxyaniline and dicyclohexylcarbodiimide (DCC) were purchased from
Fluka Chemie (Switzerland), 4-methylaniline was obtained from Riedel-de
Haen (Germany). Aniline was obtained from BDH (England). Palmitic acid
used in this research was provided by Acidchem International Sdn. Bhd.
(Malaysia). The purity of the palmitic acid is approximately 99%.
Melting points of the synthesized compounds were measured by Gallen-
kamp melting-point apparatus. Microanalyses were carried out on 2400 LS
Series CHNS=O analyzers in The School of Chemical Sciences, Universiti
Sains Malaysia, Penang, Malaysia. IR data were recorded using a Perkin
Elmer 2000-FTIR spectrophotometer in the frequency range 4000–
400 cmꢁ1 with samples embedded in KBr discs. 1H NMR and 13C NMR
spectra were recorded on a Bruker 400 MHz ultrashield spectrometer.

Synthesis and Mesomorphic Properties of Schiff Base Esters
75
SCHEME 1 Synthetic routes towards the formation of intermediates and target
compounds 9–15.
Deuterated chloroform was used as solvent and tetramethylsilane (TMS) as
internal standard. Thin layer chromatography analyses were perfomed
using aluminium-backed silica gel plates (Merck 60 F254) and were exam-
ined under short-wave UV light.
The phase transition temperatures were measured by Shimadzu DSC-50
differential scanning calorimetry at heating and cooling rates of 5ꢂC=min
and ꢁ5ꢂC=min, respectively. The optical microscopy studies were carried
out with a Carl Zeiss polarizing microscope equipped with a Mettler
FP52 hot stage. The textures of the compounds were observed by using
polarized light with crossed polarizers, with the sample in thin film sand-
wiched between glass slide and cover.
Molecular modeling study was performed using the program ACD=
Chemsketch Version 4.5. Geometrical optimization or energy minimization

76
G.-Y. Yeap et al.
of the molecules was performed in order to gain an appreciation of molecu-
lar shape and geometry. The structural conformation obtained was subse-
quently used in calculating the polarizability of each compound.
Synthesis
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzaldehyde, 1
This compound was synthesized via modified reaction described by
Sudhakar et al. [18]. 2,4-Dihydroxybenzaldehyde (20 mole), palmitic acid
(20 mmole), and DMAP (2 mmole) were dissolved in 60 ml mixture of
dichloromethane (DCM) and dimethylformamide (DMF) and stirred at
0ꢂC. To this solution 20 mmole of DCC dissolved in 20 ml of DCM was added
dropwise and stirred at 0ꢂC for 1 h and then stirred at room temperature for
3 h. Finally, the reaction mixture was filtered and DCM was removed from
the filtrate by evaporation. The grey solid thus obtained was recrystallized
with n-hexane, whereupon pure compound was formed. IR (KBr),
3441 cmꢁ1 (OH), 1760 cmꢁ1 (C¼O of ester), 1690 cmꢁ1 (CH¼O).
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzylidene-p-aniline, 9
In a round-bottom flask, a mixture of 1 (1.0 mole), aniline (1.0 mole),
and absolute ethanol or DCM (50 ml) was refluxed for 3 h with stirring.
The reaction mixture was filtered and the solvent removed from the filtrate
by evaporation. The yellowish crystalline was obtained after being recrys-
tallized with absolute ethanol. Yield 79%. Elemental analysis found:
C, 77.20; H, 9.25; N, 3.07. Calculated (for C29H41NO3): C, 77.12; H, 9.15;
N, 3.10. IR (KBr), 3436 cmꢁ1 (OH), 2953, 2916, 2847 cmꢁ1 (CꢁH
aliphatic), 1756 cmꢁ1 (C¼O of ester), 1627 cmꢁ1 (CH¼N), 1594 cmꢁ1
1
(C¼C aromatic). H NMR (CDCl3), 0.91–0.94 ppm (CH3), 1.25–2.57 ppm
(CH2), 6.71–7.46 ppm (ArꢁH), 8.62 ppm (CH¼N), 13.64 ppm (OH).
13C NMR (CDCl3), 14.56 ppm (CH3), 23.14–34.86 ppm (CH2), 110.85–
162.99 ppm (ArꢁC), 162.15 ppm (C¼N), 172.11 (C¼O of ester).
The preparation and characterization with the same procedures were
carried out for compounds 10–15. The analytical data were summarized
as follows:
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzylidene-p-fluroaniline, 10
Yield 75%. Elemental analysis found: C, 74.03; H, 8.42; N, 2.93. Calculated
(for C29H40FNO3): C, 74.17; H, 8.55; N, 2.98. IR (KBr), 3443 cmꢁ1 (OH), 2953,
2917, 2848 cmꢁ1 (CꢁH aliphatic), 1758 cmꢁ1 (C¼O of ester), 1626 cmꢁ1
(CH¼N), 1608 cmꢁ1 (C¼C aromatic). 1H NMR (CDCl3), 0.88–0.92 ppm

Synthesis and Mesomorphic Properties of Schiff Base Esters
77
(CH3), 1.29–2.60 ppm (CH2), 6.71–7.42 ppm (ArꢁH), 8.59 ppm (CH¼N),
13.42 ppm (OH). 13C NMR (CDCl3), 14.52 ppm (CH3), 23.09–34.85 ppm
(CH2), 110.86–163.32 ppm (ArꢁC), 161.96 ppm (C¼N), 172.13 (C¼O of
ester).
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzylidene-p-cloroaniline, 11
Yield 79%. Elemental analysis found: C, 71.58; H, 8.40; N, 2.87. Calcu-
lated (for C29H40ClNO3): C, 71.66; H, 8.29; N, 2.88. IR (KBr), 3435 cmꢁ1
(OH), 2951, 2917, 2847 cmꢁ1 (CꢁH aliphatic), 1759 cmꢁ1 (C¼O of ester),
1
1625 cmꢁ1 (CH¼N), 1604 cmꢁ1 (C¼C aromatic). H NMR (CDCl3), 0.91–
0.94 ppm (CH3), 1.27–2.60 ppm (CH2), 6.72–7.42 ppm (ArꢁH), 8.58 ppm
(CH¼N), 13.15 ppm (OH). 13C NMR (CDCl3), 14.52 ppm (CH3), 23.10–
34.85 ppm (CH2), 110.89–162.87 ppm (ArꢁC), 162.47 ppm (C¼N),
172.08 (C¼O of ester).
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzylidene-p-bromoaniline, 12
Yield 78%. Elemental analysis found: C, 65.45; H, 7.56; N, 2.60. Calcu-
lated (for C29H40BrNO3): C, 65.65; H, 7.60; N, 2.64. IR (KBr), 3433 cmꢁ1
(OH), 2952, 2917, 2847 cmꢁ1 (CꢁH aliphatic), 1750 cmꢁ1 (C¼O of ester),
1
1615 cmꢁ1 (CH¼N), 1586 cmꢁ1 (C¼C aromatic). H NMR (CDCl3), 0.90–
0.93 ppm (CH3), 1.27–2.59 ppm (CH2), 6.72–7.58 ppm (ArꢁH), 8.56 ppm
(CH¼N), 13.14 ppm (OH). 13C NMR (CDCl3), 14.28 ppm (CH3), 22.95–
34.84 ppm (CH2), 110.84–162.97 ppm (ArꢁC), 162.47 ppm (C¼N), 171.69
(C¼O of ester).
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzylidene-p-methoxyaniline, 13
Yield 74%. Elemental analysis found: C, 74.90; H, 9.12; N, 2.90. Calcu-
lated (for C30H43NO4): C, 74.81; H, 9.00; N, 2.91. IR (KBr), 3444 cmꢁ1
(OH), 2953, 2916, 2848 cmꢁ1 (CꢁH aliphatic), 1758 cmꢁ1 (C¼O of ester),
1
1634 cmꢁ1 (CH¼N), 1605 cmꢁ1 (C¼C aromatic). H NMR (CDCl3), 0.91–
0.95 ppm (CH3), 1.32–2.59 ppm (CH2), 3.85 ppm (OCH3), 6.70–7.36 ppm
(ArꢁH), 8.58 ppm (CH¼N), 13.59 ppm (OH). 13C NMR (CDCl3),
14.28 ppm (CH3), 22.96–34.84 ppm (CH2), 55.89 ppm (OCH3), 110.69–
162.87 ppm (ArꢁC), 159.96 ppm (C¼N), 171.78 (C¼O of ester).
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzylidene-p-methylaniline, 14
Yield 76%. Elemental analysis found: C, 77.46; H, 9.45; N, 2.99. Calcu-
lated (for C30H43NO3): C, 77.38; H, 9.31; N, 3.01. IR (KBr), 3435 cmꢁ1
(OH), 2952, 2917, 2848 cmꢁ1 (CꢁH aliphatic), 1757 cmꢁ1 (C¼O of ester),

78
G.-Y. Yeap et al.
1
1627 cmꢁ1 (CH¼N), 1606 cmꢁ1 (C¼C aromatic). H NMR (CDCl3), 0.90–
0.93 ppm (CH3 of ester chain), 1.27–2.61 ppm (CH2), 2.41 ppm (CH3 of
aniline fragment), 6.70–7.40 ppm (ArꢁH), 8.62 ppm (CH¼N), 13.74 ppm
(OH). 13C NMR (CDCl3), 14.29 ppm (CH3 of ester chain), 22.93–
34.85 ppm (CH2), 110.74–163.03 ppm (ArꢁC), 161.14 ppm (C¼N),
172.17 (C¼O of ester).
Synthesis of o-n-hydroxy-p-n-hexadecanoyloxy-
benzylidene-p-ethylaniline, 15
Yield 76%. Elemental analysis found: C, 77.65; H, 9.76; N, 2.91. Calcu-
lated (for C31H45NO3): C, 77.62; H, 9.46; N, 2.92. IR (KBr), 3439 cmꢁ1
(OH), 2953, 2916, 2847 cmꢁ1 (CꢁH aliphatic), 1756 cmꢁ1 (C¼O of ester),
1
1628 cmꢁ1 (CH¼N), 1606 cmꢁ1 (C¼C aromatic). H NMR (CDCl3), 0.91–
0.95 ppm (CH3 of ester chain), 1.27–1.29 (CH3 of aniline fragment),
1.32–2.61 ppm (CH2 of ester chain), 2.68–2.74 ppm (CH2 of aniline frag-
ment) 6.70–7.39 ppm (ArꢁH), 8.62 ppm (CH¼N), 13.76 ppm (OH). 13C
NMR (CDCl3), 14.56 ppm (CH3 of ester chain), 16.02 ppm (CH3 of aniline
fragment), 23.14–34.86 ppm (CH2 of ester chain), 28.89 ppm (CH2 of ani-
line fragment), 110.80–162.98 ppm (ArꢁC), 161.22 ppm (C¼N), 172.10
(C¼O of ester).
RESULTS AND DISCUSSION
Phase Transition Behavior and Liquid Crystallinity
of Compounds 9–15
The phase transition temperatures and their associated enthalpy obtained
from DSC analysis over heating and cooling cycles are tabulated in Table I.
From Table I, it is clearly shown that upon heating all compounds except
compounds 10, 14, and 15 exhibit endotherms characteristic of the
crystal–mesophase and mesophase–isotropic transitions at temperatures
greater than melting temperature (Tm). It is very clear that an endotherm
that appeared over heating cycle for compound 14 suggests a direct melt-
ing from crystal-to-isotropic phase. However, the thermograms of com-
pounds 10 and 15 have each shown an endotherm before the
isotropization at 75 and 61ꢂC, respectively. The texture observed under
the polarizing microscope is indicative of the presence of subphases within
the crystal phase (Cr1-Cr2) as those phenomena observed for isoflavone
derivatives [19]. While the clearing points for compounds 9 and 12 rep-
resent the transition temperature of SmA-I, the clearing temperatures
observed for compounds 10 and 15 at respective temperatures of 95ꢂC
and 86ꢂC are merely for the crystal-isotropic phase transition. These

Synthesis and Mesomorphic Properties of Schiff Base Esters
79
TABLE I Phase Transitions and Transition Enthalpy Changes For Compounds
9–15 upon Heating and Cooling
Phase transitions=ꢂC
(corresponding enthalpy change=Jgꢁ1
Heating=
cooling
Compound
)
9
10
11
12
13
14
15
Cr 68 (123) SmA 86 (15) I
I 64 (99) Cr
Cr1 75 (50) Cr2 95 (100) I
I 88 (11) SmA 72 (133) Cr
Cr1 85 (75) Cr2 110 (93) SmA 124 (16) I
I 124 (16) SmA 97 (92) Cr2 77 (13) Cr1
Cr1 70 (7) Cr2 117 (76) SmA 128 (14) I
I 138 (15) SmA 110 (77) Cr2 67 (6) Cr1
Cr 98 (109) SmA 111 (5) I
I 111 (5) SmA 78 (107) Cr
Cr 93 (178) I
I 88 (5) SmC 83 (151) Cr
Cr1 61 (45) Cr2 86 (101) I
I 82 (4) SmC 79 (92) Cr
Cr1 and Cr2, crystal; SmA, smectic A; SmC, smectic C; I, isotropic.
phenomena could be ascribed to the increase of the melting point, which
lies above the mesophase–isotropic liquid transition temperatures [20].
The data in Table I has also shown that upon cooling all compounds
except compound 9 exhibit smectic phase. The cooling scan upon com-
pound 11 shows the presence of mesophase (Figure 1(a)) with fan-
shaped texture. This feature is characteristic of the SmA phase owing
to the formation of batonnets that coalesce to form the fan-shaped tex-
ture [21]. The appearance of SmA phase is found to conform with
analogue p-n-octadecanoyloxybenzylidene-p-chloroanilines (18B-Cl) as
reported earlier [16]. One of the remarkable features is that the clearing
temperature of compound 11 (124ꢂC) is very much greater than com-
pound 18B-Cl (106ꢂC) during heating cycle. This information indicates
that the introduction of hydroxyl group at ortho position in the aldehyde
fragment increases the degree of anisotropy of the molecular polarizabil-
ity of compound 11 and hence increases the degree of molecular order
causing the smectic phase to have increased stability [22]. A representa-
tive DSC thermogram of compound 11 during both heating and cooling
scan is shown in Figure 2.
For compound 12, the SmA with ellipses texture (Figure 1(b)) and SmA
with fan-shaped texture (Figure 1(c)) were observed during heating and
cooling, respectively. The smectic character of compound 12 is found to
be similar to that reported in the literature [21].

80

Synthesis and Mesomorphic Properties of Schiff Base Esters
81
FIGURE 2 DSC thermograms of compound 11.
The observation on compound 10 confirms the monotropic feature
wherein the SmA phase appeared with fan-shaped texture during cooling
cycle. Compounds 14 and 15, which possess methyl and ethyl group in
the respective aniline fragment, also exhibit monotropic properties. Both
of these compounds upon cooling exhibit SmC phase at 88ꢂC and 82ꢂC,
respectively. Figure 1(d) shows the optical photomicrograph of compound
14 exhibiting SmC with schlieren texture during cooling. One remarkable
feature is that compound 14 possesses a higher clearing temperature in
comparison with compound 15. This phenomenon is similar with that
reported by Artigas and coworkers [23], wherein the introduction of the
hydroxyl group in the ortho position causes the mesophase stability to

82
G.-Y. Yeap et al.
TABLE II Polarizability (calculated value)
for Compounds 9–15
Polarizability
(a, ꢃ0.5 ꢄ 10ꢁ24
Compound
)
9
10
11
12
13
14
15
54.33
54.28
56.16
57.33
56.64
56.09
57.09
favor for terminal chains of shorter length. As such, the compound with
short terminal length is expected to possess a higher degree of molecular
order, which is favored by the increase in the polarizabilty. This has been
evident from the calculated polarizability values in which the polarizability
of compound 14 is higher than compound 15 (Table II).
The influence of terminal group on mesophase stability can also be con-
sideredasoneofthefactorscontributingtowards thedifferenceoftemperature
for transition from mesophase to isotropic phase. Among compounds 10–12,
wherein each compound possesses the halogen in the aniline fragment, the
clearing temperature for compound 10 is very much lower in comparison with
12 and 13. This comparison suggests that the fluorine (F) atom, which is the
most electronegative, reduces the degree of molecular order despite the pres-
ence of hydroxyl group at ortho position. This thermal data also indicates that
the influence of steric hindrance casued by the asymmetry of the central core of
compound 10 is the least in comparison with compounds 12 and 13, which pos-
sess higher clearing temperatures. This has also been confirmed by the polariz-
ability values wherein the calculated polarizability (Table II) increases from
compound 10 [(54.28 ꢃ 0.5) ꢄ 10ꢁ24 cmꢁ3] to compound 11 [[(56.16 ꢃ 0.5)
ꢄ 10ꢁ24 cmꢁ3] and compound 12 [(57.33 ꢃ 0.5) ꢄ 10ꢁ24 cmꢁ3].
It needs to be pointed out that while the fluorine atom shows very low
polarizability because the valence electrons residing at this atom are tightly
held to the nucleus, bromine atom is considered larger than fluorine and
hence is more easily polarized because electrons on this atom are far from
the nucleus [24].
Influence of Lateral Polar Hydroxy Group on
Mesomorphic Properties
This investigation has also revealed a correlation between the influence of
lateral polar hydroxyl group on compounds 9–15 whereby a comparison

Synthesis and Mesomorphic Properties of Schiff Base Esters
83
was made between the data of compounds 9–15 (Table I) and their analo-
gues p-n-octadecanoyloxybenzylideneaniline (18B-H), -p-chloroaniline
(18B-Cl), -p-methoxyaniline (18B-OCH3), and -p-methylaniline (18B-
CH3) [16]. The clearing temperature of compounds 9 (86ꢂC), 11
(124ꢂC), 13 (111ꢂC), and 14 (93ꢂC) are found to be higher than those
for compounds 18B-H (78ꢂC), 18B-Cl (106ꢂC), 18B-OCH3 (104ꢂC), and
18B-CH3 (90ꢂC), respectively.
It can therefore be assumed that the presence of an OH group in an
ortho position in compounds 9–15 confers a higher degree of stability on
the mesophase resulting from the increase in van der Waals forces, which
in turn is caused by the increased anisotropy of the molecular polarizability.
Physical Characterization
The spectroscopic methods (FTIR, 1H NMR, and 13C NMR) have been
employed to elucidate the target compounds (9–15). FTIR data show that
the diagnostic bands assignable to the stretching of aliphatic CꢁH and
carbonyl (C¼O of ester) of compounds 9–15 were observed at the respect-
ive frequency ranges of 2847–2953 cmꢁ1 and 1750–1759 cmꢁ1. The absorp-
tion bands appeared at higher frequency 3433–3444 cmꢁ1 can be attributed
to the stretching of OꢁH for compounds 9–15. The absorption bands
assignable to the stretching of C¼N bond for compounds 9–15 were
observed at frequency range of 1615–1634 cmꢁ1, and these values conform
with those reported in the IR spectra for various substituted aromatic
Schiff bases that possesses the formulation HOC6H5CH¼NC6H5 [16].
1H NMR data of compounds 9–15 show the presence of a singlet peak
aasigned to the azomethine proton (CH¼N) at the chemical shift
d ¼ 8.56–8.62 ppm. Compound 9 shows a triplet assignable to the methyl
group of ester linkage at d ¼ 0.91–0.94 ppm, and a multiplet that occurred
1
at d ¼ 1.25–1.27 ppm can be ascribed to methylene (CH2) proton. The H
NMR spectrum of compound 9 also shows the presence of aromatic protons
within the range of d ¼ 6.71–7.46 ppm. Another singlet appeared at
d ¼ 13.64 ppm can be attributed to the hydroxyl (OꢁH) proton. The peaks
with similar characteristics were also observed for compounds 10–15. An
additional peak occurring at d ¼ 3.85 ppm can be attributed to the pres-
ence of methoxy (OCH3) group in compound 13. The structural infor-
mation of compounds 9–15 can also be inferred from the 13C NMR data,
wherein a peak owing to the presence of ester carbonyl (ꢁCOOꢁ) was
observed within the range of d ¼ 171.69–172.17 ppm, which is in agree-
ment with those analogues earlier reported [16]. The appearance of a peak
within the range of d ¼ 160.0–162.5 ppm is due to the presence of the car-
bon in the imine (C¼N) group in compounds 9–15. An additional peak that
appears at d ¼ 55.9 ppm suggested the presence of the methoxy (OCH3)

84
G.-Y. Yeap et al.
group. The results as inferred from the IR and NMR spectra suggest that
the molecular structures for compounds 9–15 consist of two aromatic rings
bridged by an imine group, which in turn forms an intramolecular hydrogen
bonding (Scheme 1).
The formulations and molecular structures of compounds 9–15 are also
supported by the microanalytical data wherein the percentages of C, H, and
N thus found are in good agreement with the theoretical values, and the
deviation of each result, in general, is not more than 0.3%. As such, the
purity of each compound is considerably high, which entailed the sub-
sequent thermal analysis and texture observation as we described in the
earlier sections.
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