Nematic and smectic a phases in ortho-hydroxy-para- hexadecanoyloxbenzylidene-para-substituted anilines

Article abstract of DOI:10.1080/15421400500377750

Four Schiff base esters containing the different substituents of R=CN, OH, SH, and NO₂ were synthesized, and the physical characterization was carried out along with spectroscopic techniques (FT-IR and NMR). Thermal properties were analyzed by using differential scanning calometry (DSC), and their liquid crystal textures were observed under polarized optical microscope. All compounds thus obtained exhibit mesomorphic properties of which the alignment of molecules are inclined to either nematic, N (R=SH), or smectic, SmA (for R=CN, OH, and NO₂). The unidentifiable subphases in crystal region were recorded in all compounds except that with R=OH. The presence of a lateral hydroxyl group has been claimed to contribute to the enhanced molecular ordering, leading to a higher clearing temperature.

Full text of DOI:10.1080/15421400500377750

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Nematic and Smectic a Phases
in Ortho-Hydroxy-Para-
Hexadecanoyloxbenzylidene-
Para-Substituted Anilines
a
a
Guan-Yeow Yeap , Sie-Tiong Ha , Phaik-Leng
a
a
b
Lim , Peng-Lim Boey , Masato M. Ito , Shigeki
b
c
Sanehisa & Volkmar Vill
a
Liquid Crystal Research Laboratory, School of
Chemical Sciences, Universiti Sains Malaysia,
Penang, Malaysia
b
Faculty of Engineering, Soka University, Tokyo,
Japan
c
University of Hamburg, Institute for Organic
Chemistry, Hamburg, Germany
Version of record first published: 31 Aug 2006.
To cite this article: Guan-Yeow Yeap, Sie-Tiong Ha, Phaik-Leng Lim, Peng-Lim Boey,
Masato M. Ito, Shigeki Sanehisa & Volkmar Vill (2006): Nematic and Smectic a Phases
in Ortho-Hydroxy-Para-Hexadecanoyloxbenzylidene-Para-Substituted Anilines,
Molecular Crystals and Liquid Crystals, 452:1, 63-72
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Mol. Cryst. Liq. Cryst., Vol. 452, pp. 63–72, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500377750
Nematic and Smectic a Phases in Ortho-Hydroxy-Para-
Hexadecanoyloxbenzylidene-Para-Substituted Anilines
Guan-Yeow Yeap
Sie-Tiong Ha
Phaik-Leng Lim
Peng-Lim Boey
Liquid Crystal Research Laboratory, School of Chemical Sciences,
Universiti Sains Malaysia, Penang, Malaysia
Masato M. Ito
Shigeki Sanehisa
Faculty of Engineering, Soka University, Tokyo, Japan
Volkmar Vill
University of Hamburg, Institute for Organic Chemistry,
Hamburg, Germany
Four Schiff base esters containing the different substituents of R¼CN, OH, SH,
2
and NO were synthesized, and the physical characterization was carried out
along with spectroscopic techniques (FT-IR and NMR). Thermal properties were
analyzed by using differential scanning calometry (DSC), and their liquid crystal
textures were observed under polarized optical microscope. All compounds thus
obtained exhibit mesomorphic properties of which the alignment of molecules
are inclined to either nematic, N (R¼SH), or smectic, SmA (for R¼CN, OH, and
2
NO ). The unidentifiable subphases in crystal region were recorded in all
compounds except that with R¼OH. The presence of a lateral hydroxyl group
has been claimed to contribute to the enhanced molecular ordering, leading to a
higher clearing temperature.
Keywords: clearing temperature; molecular ordering; ortho-hydroxy-para-hexadeca-
noyloxybenzylidene-para-substituted anilines; Schiff base esters; thermal stability
Address correspondence to Guan-Yeow Yeap, Liquid Crystal Research Laboratory,
School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang,
Malaysia. E-mail: gyyeap@usm.my or gyyeap_liqcryst_usm@yahoo.com
63

6
4
G.-Y. Yeap et al.
INTRODUCTION
A considerable number of Schiff bases derived from salicyaldehyde
have been reported in the past few decades because they can give
rise to either thermochromism or photochromism in the solid state
[
1,2]. Studies on these photochromic compounds have increased ever
since the potential applications of photochromic materials emerged in
various areas such as the control and measurement of radiation
intensity and in optical computer and display systems. In view of
the importance and the usefulness of these compounds, chemists
were prompted to generate the derivatives by introducing different
substituents into the existing molecular system. The presence of an
ortho hydroxyl group, for instance, has been regarded as one of the
important elements favoring the existence of intramolecular hydro-
.
. .
. . .
gen bonding (OꢀH N and O HꢀN) and also the tautomerism
that accounts for the formation of either enol-imino or keto-amino
tautomer [3].
In our recent study, a series of p-n-octadecanoyloxybenzylidene-
p-substituted anilines has been isolated from the reactions of stearic
acid and various p-n-hydroxybenzylidene-p-substituted anilines [4,5].
The attempt to form this series of Schiff base esters has been initiated
by using the anilines containing pꢀCN, pꢀOH, pꢀNO , and pꢀCOOH.
2
Subsequent to this study, the Schiff base esters obtained from the
reaction of palmitic acid with a series of the analogous anilines posses-
sing substituents R¼H, F, Cl, Br, OCH , CH , and C H have been
3
3
2
5
carried out, of which the reactions led to the formation of mesogens
showing SmA (where X¼H, F, Cl, Br and OCH ) and SmC phases
3
(
where R¼CH and C H ) [6].
3
2
5
To accomplish the investigation of the mesomorphism of the series
consisting of palmitic acid as the elongated tail, we continued with
the other substituents (pꢀCN, pꢀOH, pꢀNO , and p-SH) positioned
2
along the molecular long axis. The molecular structure of the title
compounds were characterized by elemental analysis, mass spec-
trometry, Fourier transformed infrared spectroscopy (FT-IR), and
NMR techniques. Their liquid crystal properties were investigated by
using differential scanning calorimetry (DSC) and polarized optical
microscopy (POM).
EXPERIMENTAL
4
-Aminophenol, 4-aminobenzonitrile, 4-aminothiophenol, and 4-
dimethylaminopyridine (DMAP) were obtained from Merck
Germany). Dicyclohexylcarbodiimide (DCC) was purchased from
(

Nematic and Smectic A Phases
65
TABLE 1 Phase Transitions and Transition Enthalpy Changes for
Compounds 6–9 upon Heating and Cooling
Phase transitions, C
(corresponding enthalpy changes, kJmol
Heating
Cooling
ꢀ1
Compounds
)
6
7
8
9
Cr 59 (21) Cr 81 (29) SmA 140 (7) I
I 141 (6) SmA 57 (28) Cr
Cr 149 (66) I
1
2
I 135 (6) SmA 118 (49) Cr
Cr
I 111 (2) N98 (24) Cr
Cr 52 (4) Cr 88 (38) SmA 140 (4) I
I 139 (4) SmA 49 (26) Cr 29 (3) Cr
1
95 (2) Cr
2
111 (60) I
2
82 (2) Cr
1
1
2
2
1
1 2
Note: Cr, Cr , Cr , crystal; SmA, smectic A; N, nematic; I, isotropic.
Fluka Chemie (Switzerland). 2,4-Dihydroxybenzaldehyde was pur-
chased from Acros Organics (USA); 4-nitroaniline was obtained from
Riedel-de Haen (Germany). Palmitic acid used in this research was
provided by Acidchem International Sdn. Bhd. (Malaysia). The purity
of the palmitic acid is approximately 99%.
The melting points of the synthesized compounds were measured on
Gallenkamp melting-point apparatus. Analytical data were performed
on 2400 LS series CHNS=O analyzers. Electron impact mass spectra
(
EI-MS) were recorded by using Hewlett Packard 5989 A mass spec-
trometer operating at 70-eV ionizing energy. Samples were introduced
ꢁ
by using a direct inlet system with a source temperature of 200 C. IR
data were recorded on a Perkin-Elmer 2000 FT-IR spectrophotometer
ꢀ1
in the frequency range 4000–400 cm with samples embedded in KBr
discs. NMR spectra were recorded in CDCl at 298 K on a Bruker 400-
3
MHz Ultrashield spectrometer equipped with a 5-mm broad band
inverse (BBI) inverse gradient probe. Chemical shifts were referenced
to internal TMS. Thin-layer chromatography analyses were performed
by using aluminium-backed silica-gel plates (Merck 60 F524) and
examined under short-wave UV light.
The phase-transition temperatures were measured using
a
ꢁ
ꢀ1
Shimadzu DSC-50 at heating and cooling rates of 5 C min and
ꢁ
ꢀ1
ꢀ
5 C min , respectively. The DSC data are shown in Table 1. The
optical microscopy studies were carried out with a Carl Zeiss polariz-
ing microscope equipped with a Mettler FP52 hot stage. The textures
of the compounds (Fig. 1) were observed using polarized light with
crossed polarizers with the sample in thin film sandwiched between
a glass slide and cover.

6
6
G.-Y. Yeap et al.
FIGURE 1 (a) Optical photomicrograph of compound 7 exhibiting smectic A
with focal conic fan and homeotropic (dark area) textures; (b) optical photo-
micrograph of compound 8 exhibiting nematic phase with schlieren texture;
(
c) optical photomicrograph of 9 exhibiting smectic A with fan-shaped textures.

Nematic and Smectic A Phases
67
Synthesis
Synthesis of o-n-Hydroxy-p-n-hexadecanoyloxy-
benzaldehyde, 1
This compound was synthesized via modified reaction described by
Sudhakar et. al. [7]. 2,4-Dihydroxybenzaldehyde (20 mmol), palmitic
acid (20 mmol), and DMAP (2 mmol) were dissolved in a 60-mL mixture
of dichloromethane (CH Cl ) and dimethylformamide (DMF) and stir-
2
2
ꢁ
red at 0 C. To this solution, 20 mmol of DCC dissolved in 20 mL of
CH Cl was added dropwise, stirred at 0 C for 1 h, and then stirred
ꢁ
2
2
at room temperature for 3 h. Finally, the reaction mixture was filtered,
and CH Cl was removed from the filtrate by evaporation. The grey
2
2
solid thus obtained was recrystallized with n-hexane, whereupon the
ꢀ1
pure compound was formed. IR (KBr), 3441 cm (OH), 2954, 2918,
850 cm (CH aliphatic), 1760 cm (C¼O ester), 1690 cmꢀ1 (CHO).
ꢀ
1
ꢀ1
2
Synthesis of Schiff Base Esters, 6–9
The Schiff bases were synthesized by mixing equimolar amounts of
compound 1 with the appropriate aniline, both dissolved in absolute
ethanol. The reaction mixture was refluxed for 3 h with stirring before
it was filtered. The solvent was removed from the filtrate by evaporation.
The yellowish crystals were obtained after being recrystallized with
absolute ethanol. The entire synthetic route is outlined in Scheme 1.
The title compounds were characterized by spectroscopic and elemental
analysis data. The elemental analysis, selected mass spectrometric, IR,
1
13
and NMR ( H and C) data are summarized as follows.
Compound 6: Yield 22%. Found: C, 75.62; H, 8.45; N, 5.87. Cal. for
C H N O ; C, 75.59; H, 8.46; N, 5.88%. EI-MS m=z (rel. int. %):
3
0
40
2
3
þ
4
2
7
76(1) (M) , 359(1), 280(1), 258(1), 240(3), 239(25), 238(100),
37(18), 228(1), 154(2), 136(6), 129(4), 127(1), 115(1), 111(2), 77(1),
ꢀ1
6(3), 69(12), 57(40), 56(8), 55(36), 43(81). IR (KBr), 3426 cm
ꢀ1
ꢀ1
(
OH), 2954, 2914, 2850 cm (CꢀH aliphatic), 1765 cm (C¼O of
ꢀ ꢀ1 ꢀ1
1
ester), 1624 cm
(CH¼N), 1599 cm
(C¼C aromatic), 1279 cm
1
(
6
CꢀO). H NMR (CDCl ), 0.88–0.92 ppm (CH ), 1.28–2.61 ppm (CH ),
3
3
2
13
.75–7.75 ppm (ArꢀH), 8.61 ppm (CH¼N), 12.92 (OH).
C NMR
(
CDCl ), 14.52 ppm (CH ), 23.09–34.84 ppm (CH ), 110.64–163.04 ppm
3 3 2
(
ArꢀC), 164.47 ppm (C¼N), 172.01 (C¼O ester).
Compound 7: Yield 45%. Found: C, 74.50; H, 8.81; N, 2.98. Cal. for
C H NO ; C, 74.48; H, 8.84; N, 3.00%. EI-MS m=z (rel. int. %):
2
9
41
4
þ
4
1
5
67(2) (M) , 271(1), 240(1), 239(1), 231(3), 230(21), 229(100), 228(27),
54(1), 136(3), 129(1), 127(1), 115(1), 111(1), 77(1), 69(7), 57(18), 56(5),
ꢀ1
ꢀ1
5(19), 43(38). IR (KBr), 3460 cm (OH), 2948, 2917, 2847 cm (CꢀH
aliphatic), 1739cm (C¼O of ester), 1623 cmꢀ1 (CH¼N), 1600cm (C¼C
ꢀ
1
ꢀ1

6
8
G.-Y. Yeap et al.
SCHEME 1 Preparation of Schiff base esters 6–9.
ꢀ
1
1
aromatic), 1272cm (CꢀO). H NMR (CDCl ), 0.90–0.93 ppm (CH ),
3
3
1
.31–2.60 ppm (CH ), 6.69–7.34 ppm (ArꢀH), 8.56 ppm (CH¼N), 13.58
2
13
(
OH). C NMR (CDCl ), 14.26 ppm (CH ), 22.94–34.85 ppm (CH ),
3 3 2
1
09.99–162.90 ppm (Ar-C), 159.96 ppm (C¼N), 172.05 (C¼O ester).
Compound 8: Yield 37%. Found: C, 71.97; H, 8.57; N, 2.92. Cal. for
C H NO S; C, 72.01; H, 8.54; N, 2.90%. EI-MS m=z (rel. int. %):
2
9
41
3
þ
4
1
5
83(3) (M) , 257(1), 256(2), 247(7), 246(24), 245(100), 244(29), 228(13),
54(4), 136(14), 129(3), 127(3), 115(4), 111(5), 77(5), 69(17), 57(49),
6(15), 55(47), 43(99). IR (KBr), 3429 cm (OH), 2951, 2917, 2848 cm
ꢀ1
ꢀ1

Nematic and Smectic A Phases
69
1
ꢀ1
ꢀ1
(
(
(
8
2
1
CꢀH aliphatic), 1758 cmꢀ (C¼O of ester), 1623 cm (CH¼N), 1609 cm
ꢀ
1
1
C¼C aromatic), 1275 cm (CꢀO). H NMR (CDCl ), 0.90–0.94 ppm
3
CH ), 1.26–2.59 ppm (CH ), 3.50 ppm (SH), 6.71–7.37 ppm (ArꢀH),
3
2
1
3
.59 ppm (CH¼N), 13.32 (OH). C NMR (CDCl ), 14.29 ppm (CH ),
3
3
2.96–34.84 ppm (CH ), 110.78–162.98 ppm (ArꢀC), 161.63 ppm (C¼N),
2
71.74 (C¼O ester).
Compound 9: Yield 20%. Found: C, 70.16; H, 8.14; N, 5.63. Cal. for
C H N O ; C, 70.13; H, 8.12; N, 5.64%. EI-MS m=z (rel. int. %):
2
9
40
2
5
þ
4
1
4
96(1) (M) , 260(3), 259(23), 258(100), 257(8), 228(8), 154(4), 136(4),
29(3), 127(2), 115(2), 111(2), 77(3), 76(5), 69(13), 57(40), 56(9), 55(36),
ꢀ1
ꢀ1
3(79). IR (KBr), 3444 cm (OH), 2952, 2921, 2850 cm (CꢀH ali-
ꢀ1
ꢀ1
ꢀ1
phatic), 1751 cm (C¼O of ester), 1621 cm (CH¼N), 1604 cm (C¼C
ꢀ1
1
aromatic), 1275 cm (CꢀO). H NMR (CDCl ), 0.88–0.92 ppm (CH ),
3
3
1
.28–2.61 ppm (CH ), 6.76–8.34 ppm (ArꢀH), 8.64 ppm (CH¼N), 12.85
13
2
(
OH). C NMR (CDCl ), 14.51 ppm (CH ), 23.08–34.84 ppm (CH ),
3 3 2
1
11.10–163.11 ppm (ArꢀC), 164.85 ppm (C¼N), 171.98 (C¼O ester).
The percentages of C, H, and N from the elemental analysis agree
with the calculated values for compounds 6–9. The molecular ion
peaks obtained from the mass spectrometric analysis are also in
accordance with the theoretical values. The diagnostic bands observ-
able via the IR analysis are characteristic of the structure properties
of title compounds including the presence of intermolecular hydrogen
1
bonding between the lateral OH groups among the molecules. All H
1
3
and C NMR data further support the identification of each molecule
in CDCl of which the assignment of peaks agree with the respective
3
formulation of compounds 6–9.
RESULTS AND DISCUSSION
Phase-Transition Behavior and Liquid Crystallinity
of Compounds 6–9
Table 1 clearly shows that upon heating all compounds except
compounds 7 and 8 exhibit endotherms, which can be attributed to
the crystal–mesophase and mesophase–isotropic transitions at
temperature higher than melting temperature (T ). It is very clear
m
that an endotherm appeared over heating cycle for compound 7
suggests a direct melting from crystal to isotropic phase. However,
the thermogram of compound 8 has shown an endotherm before
ꢁ
isotropization at 95 C. The texture observed under the polarizing
microscope is indicative of the presence of subphases within
the crystal phase (Cr -Cr ) as those observed in its analogue o-n-
1
2
hydroxy-p-n-hexadecanoyloxybenzylidene-fluoroaniline [6]. Although

7
0
G.-Y. Yeap et al.
the clearing points for compounds 6 and 9 represent the transition
temperature of SmA-I, the clearing temperature observed for com-
ꢁ
pound 7 at 149 C is merely for the crystal–isotropic phase transition.
These phenomena could be ascribed to the increase of the melting
point lying above the mesophase–isotropic liquid transition tempera-
tures [8].
Table 1 also shows that upon cooling, all the compounds exhibit
smectic phase except compound 8. For compound 6, 7, and 9,
microscopy studies revealed that on cooling from the isotropic liquid,
smectic batonnets were observed during the phase transition from
the isotropic to the smectic phase. These batonnets disappeared and
changed to a pseudo-isotropic texture (homeotropic alignment). This
characteristic behavior of the microscopic textures indicates that the
smectic phase observed for compounds 6, 7, and 9 can be identified
as a smectic A [9]. As the representative illustration, the partial home-
otropic alignment for smectic A phase is shown in Fig. 1a wherein the
focal conic fan texture and homeotropic texture (dark area) were
observed.
The appearance of enantiotropic smectic A phase in compound 6 is
found to be in conformity with its analogue p-n-octadecanoyloxybenzy-
lidene-p-cyanoaniline (18B-CN) as that reported earlier [4]. The pres-
ence of thermally stable mesomorphic Schiff base esters with the CN
substituent can be ascribed to the ability of this compound to form
bilayers as reported for the compounds with OH substituent [10].
One of the remarkable features is that the clearing temperature of
ꢁ
compound 6 (140 C) is very much higher than that for the earlier
ꢁ
reported compound 18B-CN (107 C) on a heating run. This infor-
mation indicates that the introduction of a hydroxyl group at the
ortho position in the aldehyde fragment increases the degree of
anisotropy of the molecular polarizability of compound 6 and hence
increases the degree of molecular order, causing the smectic phase
to be more stable [11].
For compound 7, the monotropic smectic A observed under the
polarized optical microscope is in agreement with the DSC data
ꢀ1
wherein direct melting (Cr-I) with high enthalpy changes (66 kJmol )
was observed on the heating run. The small enthalpy changes associa-
ted with the mesophase–isotropic transition were not observed.
The appearance of smectic A also conforms with its analogue 4-(4-n-
hexadecyloxy-2-hydroxy-benzylideneamino)phenol as reported by
Sakagami and Takase [12]. Although compound 7 has not shown
any mesomorphic behavior during the heating cycle, it possesses the
highest clearing point. The presence of thermally stable mesomorphic
Schiff base esters with the OH substituent can be ascribed to the

Nematic and Smectic A Phases
71
lateral intermolecular hydrogen bonding as that reported by
Sakagami et al. [13] and Naoum et. al. [14]. The lateral intermolecular
hydrogen bonding in compound 7 resulted in the formation of hydro-
gen bonding between the hydroxyl group and the carbonyl group as
that inferred from the IR spectroscopy.
The observation on compound 8 confirms the monotropic feature
wherein the schlieren texture (Fig. 1b) appeared during cooling cycle.
This feature is characteristic of the nematic phase where the disclina-
tion points show 2-brush singularities (s ¼ ꢂ1=2) [9].
In the previous article, we had reported that the emergence of liquid
crystalline properties of p-n-octadecanoyloxybenzylidene-p-nitro-
aniline (18B-NO ) seemed to be suppressed during heating and
2
cooling cycles [4]. However, the introduction of the lateral hydroxy
group as reported for compound 9 has enhanced the mesomorphism
of compound 9, leading to a mesophase containing a fan-shaped
texture which, can be classified as SmA phase (Fig. 1c). The smectic
character of compound 9 is found to be similar to that reported in
the literature [9].
Influence of Lateral Polar Hydroxy Group on Mesomorphic
Properties
This investigation has also revealed a correlation between the influ-
ence of lateral polar hydroxyl group on compounds 6–9 wherein a com-
parison was made between the data of compounds 6–9 (Table 1) and
their analogues p-n-octadecanoyloxybenzylidene-p-cyanoaniline (18B-
CN), -p-hydroxyaniline (18B-OH) and -p-nitroaniline (18B-NO ) [4].
2
ꢁ
ꢁ
The clearing temperature of compounds 6 (140 C), 7 (149 C), and 9
135 C) are found to be higher than those for compounds 18B-CN
ꢁ
ꢁ
(
(
ꢁ
ꢁ
107 C), 18B-OH (125 C), and 18B-NO (48 C), respectively.
2
ACKNOWLEDGMENT
G.-Y. Yeap thanks the Malaysia government, especially the Ministry
of Science, Technology and Innovation (MOSTI) for funding this pro-
ject through the intensified research in priority area (IRPA) Research
Grant Nos. 09-02-05-3211 EA012 and 305=PKIMIA=612923. The
assistance given by the Universiti Sains Malaysia, especially research
creativity and management office (RCMO), is also vital for the success
of this project. This project is partly financed by university science
Malaysia (USM) Short-Term Grant No. 304=PKIMIA=634151 and
Soka University’s Open Research Project for year 2004–2005. S.-T.
Ha is grateful to the MOSTI for the National Science Fellowship.

7
2
G.-Y. Yeap et al.
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