Synthesis, characterization, and mesomorphic properties of new liquid-crystalline compounds involving ester-azomethine central linkages, lateral substitution, and a thiazole ring

Article abstract of DOI:10.1080/15421400601150379

In continuation with our work on liquid crystals with unconventional molecular structures, two homologous series that have 1,4-disubstitution and 1,3,4-trisubstitution in the central phenyl ring, such as 2[(4-n-alkoxy- benzyloxy) phenyl azomethine]-5-methyl thiazole and 2-[(4-n-alkoxy-benzyloxy)-2- hydroxy salicyladimine]-5-methyl thiazole, have been synthesized. They have been characterized by elemental analysis, FT-IR, 1HNMR, 13CNMR, and mass spectrometry. The liquid-crystalline behavior of these compounds was observed by differential scanning calorimetry (DSC) and polarizing microscopy. All the compounds of the 1,4-disubstituted series show an enantiotropic nematic phase only, whereas the compounds of the 1,3,4-trisubstituted series show smectic and nematic phases. Some mesogens with a lateral hydroxy group were also synthesized to evaluate the effect of this group on melting point, transition temperatures, and mesophase morphology.

Full text of DOI:10.1080/15421400601150379

This article was downloaded by: [University of Chicago Library]
On: 01 October 2014, At: 03:08
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis, Characterization,
and Mesomorphic Properties
of New Liquid-Crystalline
Compounds involving
Ester–Azomethine Central
Linkages, Lateral Substitution,
and a Thiazole Ring
B. T. Thaker a , Pranay Patel a , A. D. Vansadia a & H.
G. Patel a
a Department of Chemistry , Veer Narmad South
Gujarat University , Surat, Gujarat, India
Published online: 22 Sep 2010.
To cite this article: B. T. Thaker , Pranay Patel , A. D. Vansadia & H. G. Patel (2007)
Synthesis, Characterization, and Mesomorphic Properties of New Liquid-Crystalline
Compounds involving Ester–Azomethine Central Linkages, Lateral Substitution,
and a Thiazole Ring, Molecular Crystals and Liquid Crystals, 466:1, 13-22, DOI:
10.1080/15421400601150379
To link to this article: http://dx.doi.org/10.1080/15421400601150379
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views

expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 466, pp. 13–22, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601150379
Synthesis, Characterization, and Mesomorphic
Properties of New Liquid-Crystalline Compounds
involving Ester–Azomethine Central Linkages,
Lateral Substitution, and a Thiazole Ring
B. T. Thaker
Pranay Patel
A. D. Vansadia
H. G. Patel
Department of Chemistry, Veer Narmad South Gujarat University,
Surat, Gujarat, India
In continuation with our work on liquid crystals with unconventional
molecular structures, two homologous series that have 1,4-disubstitution and
1,3,4-trisubstitution in the central phenyl ring, such as 2[(4-n-alkoxy-benzyloxy)
phenyl azomethine]-5-methyl thiazole and 2-[(4-n-alkoxy-benzyloxy)-2-hydroxy sal-
icyladimine]-5-methyl thiazole, have been synthesized. They have been character-
ized by elemental analysis, FT-IR, 1HNMR, 13CNMR, and mass spectrometry. The
liquid-crystalline behavior of these compounds was observed by differential scan-
ning calorimetry (DSC) and polarizing microscopy. All the compounds of the 1,4-
disubstituted series show an enantiotropic nematic phase only, whereas the com-
pounds of the 1,3,4-trisubstituted series show smectic and nematic phases. Some
mesogens with a lateral hydroxy group were also synthesized to evaluate the effect
of this group on melting point, transition temperatures, and mesophase
morphology.
Keywords: ester; lateral group; mesophase; nematic phase; Schiff base; smectic phase;
thiazole
INTRODUCTION
Conventional thermotropic liquid crystals consist of anisotropic
molecules (mesogens) that are either rod shaped (calamitic) or disc
shaped (discotic) [1,2]. The thermal behavior of these two types of
liquid crystals is generally well understood, and therefore they have
Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad
South Gujarat University, Surat 395 007, Gujarat, India. E-mail: btthaker1@yahoo.co.in
13

14
B. T. Thaker et al.
been recognized as materials for many practical applications,
especially in liquid-crystalline display devices [3]. A considerable num-
ber of Schiff bases derived from salicyaldehyde have been reported in
the past few decades because they can give rise to either thermochro-
mism or photochromism in the solid state [4,5]. The synthesis of
liquid-crystalline compounds incorporating thiazole is of considerable
interest [6,7]. Thiazoles represent a very interesting class of compound
because of their wide applications in pharmaceutical, phytosanitary,
analytical, and industrial aspects (e.g., as antibacterial [7], fungicide
[8], anti-inflammatory [9], and anti-HIV [10] substances). It is known
that 2-amino thiazole is a biologically active compound with a broad
range of activity that also is an intermediate in the synthesis of
antibiotics and dyes.
The lateral hydroxyl substitution of the 1,4-phenylene fragment of
5,6,7,8-tetrahydroxyquinoline cyano derivatives results in increases
in melting points compared to those of the corresponding unsubsti-
tuted analogues [11]. This may be due to the enhanced axial molecular
polarizability resulting from delocalization of electrons caused by
intramolecular hydrogen bonding, which is possibly responsible for
the monomeric density wave observed from the X-ray diffraction study
of the structure of the nematic phase [11]. Similar effects caused
by intramolecular hydrogen bonding have been found for other lat-
erally hydroxyl-substituted derivatives [12]. The presence of an ortho
hydroxyl group, for instance, has been regarded as one of the impor-
tant elements favoring the existence of intramolecular hydrogen
bonding (OꢀH . . . N and O . . . HꢀN) and also the tautomerism that
accounts for the formation of the either enol-imine or the keto-amine
tautomer [12]. In our present work, series B contains an ꢀOH group
as a lateral substitution and because of the hydroxyl group, the melt-
ing point of the compounds increases. All compounds in series B show
mesomorphic properties. In each case, an enantiotropic nematic meso-
phase is observed in the lower members, whereas the higher members
of series B (10, 12, 14, 16) show an enantiotropic smectic C phase and
nematic phase. In series A, there is no lateral substitution but only the
nematic phase. There is a big difference in enthalpies for both the ser-
ies, shown in Table 1.
EXPERIMENTAL
Reagents and Technique
The solvents were used after purification using the standard methods
described in the literature [13]. 4-Hydroxy benzoic acid, p-hydroxy

Mesomorphic properties of Liquid Crystalline Compounds
15
TABLE 1 DSC Data for Series A and B Compounds
Peak temp.
(Microscopic Temp.) (ꢁC)
DH
DS
Series
A
Compound
Transition
(Jgꢀ1
)
(Jgꢀ1Kꢀ1
)
A8
CrꢀN
NꢀI
75.72 (76)
92.03 (92)
57.19 (57)
81.23 (81)
92
164
67
101
160
13.94
6.59
36.00
6.47
14.95
7.69
32.9
12.3
18.6
0.040
0.018
0.109
0.018
0.162
0.046
0.491
0.121
0.116
A16
B10
B16
CrꢀN
NꢀI
B
CrꢀN
NꢀI
CrꢀSm C
Sm CꢀN
NꢀI
Note. Cr, crystal; SmC, smectic C; N, nematic; I, isotropic.
benzaldehyde, 2,4-dihydroxy benzaldehyde, and DMAP (4-dimethyla-
minopyridine) were obtain from Merck (Germany). 2-Amino-4-methyl
thiazole and DCC (dicyclohexylcarbodiimide) were purchased from
Fluka Chemie (Switzerland). Elemental analyses (C, H, N) were per-
formed at Central Drug Research Institute (CDRI), Lucknow. Infrared
spectra were recorded with a Perkin-Elmer 2000 FT-IR spectrophot-
ometer in the frequency range 4000–400 cmꢀ1 with samples embedded
1
in KBr discs. HNMR spectra of the compounds were recorded with a
Jeol-GSX-400 instrument using CDCl3 as a solvent and TMS as an
internal reference at SAIF, IIT Madras, Chennai. 13C NMR spectra
of the compound were recorded with Bruker Avance II 400 NMR spec-
trometer, SAIF, Chandigarh. Mass spectra (EI) of the compounds were
recorded at Sophisticated Analytical Instrumentation Facility (SAIF),
IIT Madras, Chennai. Thin-layer chromatography (TLC) analyses
were performed using aluminium-backed silica-gel plates (Merck 60
F524) and examined under shortwave UV light.
The phase-transition temperatures wꢁere measured using a Shimadzu
DSC-50 at heating and cooling rates of 5 C minꢀ1, respectively. The DSC
data are shown in Table 1. The optical microscopy studies were carried
out with a Carl Zeiss polarizing microscope equipped with a Mettler
FP52 hot stage. The textures shown by the compounds (Fig. 1) were
observed using polarized light with crossed polarizers with the sample
in a thin film sandwiched between a glass slide and cover slip.
Synthesis
Synthesis of 4-n-Alkoxy Benzoic Acid [14]
This was prepared by the reported method. A mixture of
4-hydroxybenzoic acid (1 mmol) and KOH (1.5 mmol) in 250 ml of

16
B. T. Thaker et al.
FIGURE 1. (a) Optical photomicrograph of compound B7 exhibiting nematic
Phase. (b) Optical photomicrograph of compound B14 exhibiting nematic
Phase.
methanol or ethanol was stirred and heated at 70ꢁC while 1-bromo
alkane (1.02 mmol) was added slowly (1–2 h), after which the mixture
was refluxed for 11–13 h. The reaction mixture was cooled to room
temperature and poured into acidified crushed ice. The product
obtained was filtered off and purified as reported. The molecular
structures of these acids were confirmed by spectroscopic analysis
and found to be in agreement with the reported data.

Mesomorphic properties of Liquid Crystalline Compounds
17
Synthesis of 2-[(4-n-Alkoxy-benzyloxy)-2-
X-Benzaldehyde [15,16]
To a mixture of 4-n-alkoxy benzoic acid (1 mmol) and p-hydroxy
benzaldehyde (1 mmol) in 150 ml of distilled dichloromethane,
0.1 mmol of dimethyl aminopyridine (DMAP) was added under an
argon atmosphere. The mixture was cooled in an ice-water bath, and
after 10 min, 1 mmol of dicyclohexylcarbodiimide (DCC) was added
under an argon atmosphere. The mixture was stirred overnight at
room temperature, the salts were filtered off, and the solvent was eva-
porated. The crude product was purified by flash chromatography
using a mixture of hexanes=ethyl acetate (7=1) as eluent.
Data
Compound 10p. Yield 80%. Found: C, 75.00; H, 7.60; calc. for
C24H30O4; C, 75.39; H, 7.85%. IR (KBr): Vmax=cmꢀ1 2954, 2917,
2849 (CꢀH aliphatic), 1727 cmꢀ1 (C O of ester), 1701 (C O of
=
=
aldehyde).
Compound 10q. Yield 79%. Found: C, 75.12; H, 7.68; calc. for
C24H30O4; C, 75.39; H, 7.85%. IR (KBr): Vmax=cmꢀ1 2953, 2919,
2850 cmꢀ1 (CꢀH aliphatic), 1729 cmꢀ1 (C O of ester), 1702 (C O of
=
=
aldehyde), 3441cmꢀ1 (OH).
Synthesis of 2-[(4-n-Alkoxy-benzyloxy)-2-
X-benzaldemine]-5-methyl Thiazole [15,17]
A mixture of 1 mmol of 2-[(4-n-alkoxy-benzyloxy)-2-X-benzaldehyde
and 1 mmol of 2-amino 5-methyl thiazole, and three drops of acetic
acid in absolute ethanol, were heated at reflux for 4 h. The reaction
mixture was allowed to cool and was stirred at room temperature
overnight. The solid was collected and recrystallized from technical
ethanol.
Data
Compound A10. Yield 90%. mp 90ꢁC. Found: C, 70.29; H, 7.11; N,
5.87. Calc. for C28H34O3; N2S; C, 70.10; H, 7.25; N, 5.55%. EI-MS
m=z (rel. int. %): 478(1) (M)þ. IR (KBr): Vmax=cmꢀ1 2950,
2850 cmꢀ1 (CꢀH aliphatic), 1725 cmꢀ1ꢀ(1C O of ester), 1612 (CH N),
=
=
1602 cmꢀ1 (C C aromatic), 1279 cm
(CꢀO), 1570 cmꢀ1 (C N,
=
=
thiazole), 1034 cmꢀ1 (CꢀSꢀC, thiazole).1H NMR (CDCl3):
d
0.90–0.93 ppm (CH3), 1.24–1.47 ppm (CH2), 4.03–4.07 ppm (OCH2),
=
6.90–8.07 ppm (Ar-H), 9.00–9.2 ppm (CH N), 6.89 ppm (thiazole-H).
13C NMR (CDCl3), 14.59 ppm (CH3), 23.07–34.84 ppm (CH2),
=
=
110.64–163.25 ppm (Ar-C), 164.47 ppm (C N), 172.01 ppm (C O ester).

18
B. T. Thaker et al.
Compound B10. Yield 85%. mp 164ꢁC. Found: C, 67.67; H, 7.07; N,
5.16. Calc. for C28H34O4 N2S; C, 68.01; H, 6.88; N, 5.55%. EI-MS m=z
(rel. int. %): 494(2) (M)þ. IR (KBr): Vmax=cmꢀ1 3434 cmꢀ1 (OH),2948,
ꢀ1
2917,2847 cmꢀ1 (CꢀH aliphatic), 1739 cmꢀ1 (C O of ester), 1611 cm
=
ꢀ1
ꢀ1
(CꢀO), 1574 cmꢀ1
=
=
(CH N), 1600 cm
(C C aromatic), 1272 cm
(C N, thiazole), 1038 cm (CꢀSꢀC, thiazole). ꢀ1H NMR (CDCl3): d
ꢀ1
=
0.90–0.93 ppm (CH3), 1.31–2.60 ppm (CH2), 4.03–4.07 ppm (OCH2),
=
6.69–7.34 ppm (Ar-H), 8.50–8.90 ppm (CH N), 6.89 ppm (thiazole-H).
12.83 ppm (OH). 13C NMR (CDCl3), 14.26 ppm (CH3), 22.94–
=
34.85 ppm (CH2), 109.99–162.90 ppm (Ar-C), 159.96 ppm (C N),
=
172.05 ppm (C O ester).
RESULTS AND DISCUSSION
In the present study, 12 homologues from each of the two series, 2[(4-n-
alkoxy-benzyloxy) phenyl azomethine]-5-methyl thiazole (series A) and
2-[(4-n-alkoxy-benzyloxy)-2-hydroxy salicylamine]-5-methyl thiazole
(series B), are synthesized, and their mesomorphic properties are
studied. The general molecular structure of the series is shown below.
X ¼ H in series A,
X ¼ OH in series B, R ¼ CnH2n þ 1 and n ¼ 1–8, 10, 12, 14, 16.
As preliminary investigation, the mesophases exhibited by com-
pounds of series A and B were examined using an optical polarizing
microscope. Thin films of the samples were obtained by sandwiching
them between glass slides and cover slips. All the compounds of series
A and B show mesomorphism on cooling the isotropic liquid, on ordi-
nary slides. Thread-like textures, characteristic of the nematic phase
are observed for the compounds of series A. In series B compounds,
on cooling the isotropic liquid, small droplets appear, which coalesce
to a classical schlieren (threaded) texture characteristic of the nematic
phase. On further cooling, higher members (n > 10) show the focal
conic texture characteristic of the smectic C mesophase. It is consist-
ent with the assignment of each mesophase type using the classi-
fication systems reported by Sackmann and Demus [18] and Gray
and Goodby [19]. The textures of the nematic phase for compounds
of series B are shown in Fig. 1. DSC is a valuable method for the

Mesomorphic properties of Liquid Crystalline Compounds
19
TABLE 2 Transition Temperatures of Series B
Transition temperatures (ꢁC)
Compounds
R ¼ n alkoxy
SmC
N
I
B1
B2
B3
B4
B5
B6
B7
B8
B10
B12
B14
B16
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
—
—
—
—
—
—
—
—
92
86
84
67
134
131
124
116
110
103
109
105
113
101
97
175
171
172
168
167
165
168
170
164
162
154
160
Decyl
Dodecyl
Tetradecyl
Hexadecyl
101
Note. SmC, smectic C; N, nematic; I, isotropic.
detection of phase transition. It yields quantitative results; therefore
we may draw conclusions concerning the nature of the phases that
occur during the transition. In the present study, enthalpies of two
derivatives of series A and series B were measured by DSC. Data
are recorded in Table 1. Enthalpy values of the various transitions
agree well with the existing related literature values [20], which helps
the further confirm the mesophase type. Inspection of the clearing
temperatures also revealed a correlation between the influence of
the lateral polar hydroxy group in series B. Table 2 clearly shows that
compounds with the ortho-hydroxy group (series B) possess higher
clearing temperatures than compounds without the ortho-hydroxy
group (series A). There are also compounds in which the lateral substi-
tuents are shielded, so that they are less effective in molecular broad-
ening [21]. The compounds of series B may give rise to shielding effects
owing to the presence of intramolecular association [22], and hence the
polarizability along the long axis of molecule will be larger in such
compounds than in the unsubstituted analogue [23]. This results in
higher clearing temperatures than in the corresponding compound
without an ortho-hydroxy group. It can therefore be assumed that
the presence of an OH group in an ortho position in compounds of
series B confers a higher degree of stability on the mesophase, result-
ing from the increase in van der Waals forces, which in turn is caused
by the increased anisotropy of the molecular polarizability. This infor-
mation indicates that the introduction of hydroxyl group at the ortho
position in the aldehyde fragment increases the degree of anisotropy

20
B. T. Thaker et al.
TABLE 3 Transition Temperatures of Series A
Transition Temperatures (ꢁC)
Compounds
R ¼ n alkoxy
Sm
N
I
A1
A2
A3
A4
A5
A6
A7
A8
A10
A12
A14
A16
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
—
—
—
—
—
—
—
—
—
—
—
—
119
115
103
108
95
84
72
76
68
148
145
132
126
121
118
106
92
90
87
85
81
Decyl
Dodecyl
Tetradecyl
Hexadecyl
63
61
57
Note. Sm, smectic; N, nematic; I, isotropic.
of the molecular polarizability of the compound in series B and hence
increases the degree of molecular order, causing the smectic phase to
have increased stability [24].
In series A, 12 compounds have been synthesized, and their
mesogenic properties are evaluated. All the compounds synthesized
exhibit an enantiotropic nematic mesophase. For series A, transition
temperatures are recorded in Table 3, and a plot of transition tempera-
FIGURE 2 Mesomorphic behaviour as a function of the number of carbon
atoms (n) in the terminal alkoxy chain for series-A.

Mesomorphic properties of Liquid Crystalline Compounds
21
FIGURE 3 Mesomorphic behaviour as a function of the number of carbon
atoms (n) in the terminal alkoxy chain for series-B.
tures against the number of carbon atoms in the alkoxy chain (Fig. 2)
shows a steady fall in N–I transitions.
In series B, all 12 members synthesized exhibit an enantiotropic
nematic mesophase. The smectic C mesophase appears from the
n-dodecyloxy derivative as an enantiotropic phase and remains up to
the n-hexadecyloxy derivative. The transition temperatures are
recorded in Table 2. The entire homologous series B exhibits meso-
morphism. The plot of transition temperature against the number of
carbon atoms in the alkoxy chain ( Fig. 3) shows a smooth falling tend-
ency for nematic–isotropic transition temperatures throughout the
series. Series B also exhibits a falling tendency of smectic–nematic
transition temperatures for higher homologues.
The enthalpies of the n-hexadecyloxy derivatives of each of the
series were measured by DSC, which is recorded in Table 1. The
enantiotropic nematic phase of series B is shown in Fig. 1.
CONCLUSION
In this article, we have presented the synthesis and characterization
of two new mesogenic homologous series of ester-azomethine deriva-
tives containing a thiazole ring as a terminal group and an ꢀOH group
as a lateral substitution. Ester derivatives are more stable compounds
than Schiff base derivatives. Series A is purely nematogenic, as it is a
short (two-phenyl-ring) system, whereas compounds of series B exhibit
the nematic as well as the smectic C mesophases because of the
presence of an additional phenyl ring along with an ester linkage.

22
B. T. Thaker et al.
ACKNOWLEDGMENT
We are thankful to I. I. T. Bombay, CDRI Lucknow, and Garda
Chemicals Ltd., Ankleshwar, Gujarat, for providing elemental
1
analysis, FT-IR, H NMR, mass, and thermal studies.
REFERENCES
[1] Gray, G. W., & Winsor, P. A. (1974). Liquid Crystals and Plastic Crystals,
Ellis Horwood: Chichester, UK, Vol. 1.
[2] Brown, G. H. & Wilfird, S. G. (1957). Chem. Rev., 57, 1049.
[3] Chigrinov, V. G. (1999). Liquid Crystal Devices, Adtech Book Co Ltd.: Boston-
London, pp. 357.
[4] Hadjoudis, E., Vittorakis, M., & Maustakali-Mavridis, I. (1987). Tetrahedron, 43,
1345.
[5] Kletskii, M. E., Millov, A. A., Metelitsa, A. V., & Knyazhansky, M. I. (1997).
J. Photochem. Photobio. A: Chem., 110, 267.
[6] Deniel Thangadurai, T., & Son-Kilhm. (2005). Synthesis and Reactivity in Inor-
ganic, Metal Organic-Naeno Metal Chemistry, 35, 499–507.
[7] Agrawal, N., Kumar, S., Srivastav, A. K., & Sarkar, K. P. C. (1997). Ind. J. Het.
Chem., 6, 291–294.
[8] Sup, R. C., Sukp, R. Y., & Bang, C. W. (1995). Korean J. Med. Chem., 5, 72–75.
[9] Hadjipavlou-Litina, J. D. & Geornikaki, A. (1996). Forsch.=Drug Res., 46, 805–808.
[10] Maass, G. I. U., Koening, B., Leser, U., Mueller, B., Good, R., & Pfatt, B. (1993).
Antimicrob. Agents. Chemother., 37, 2612–2617.
[11] Petrov, V. F., Pavluchenko, A. I., & Smirnova, N. I. (1995). Mol. Crys. Liq. Cryst.,
265, 47.
[12] Elmali, A., Kabak, M., Kavlakoglu, E., & Elerman, Y. (2000). J. Mol. Struct., 510,
207.
[13] Saleh, A. A., Pleune, B., Fettinger, J. C., & Poli, R. (1997). Polyhedron, 16, 1391.
[14] Dave, J. S. & Vora, R. A. (1970). In: Liquid Crystal and Org. Fluids, Johnson, J. F.
& Porter, R. S. (Eds.), Plenum Press: NewYork, 477.
[15] Pinol, R., Ros, M. B., Serrano, J. L., Sierra, T. (2004). Liquid Crystals, 31(9),
1293–1303.
[16] Guillevic, M.-A. & Dvancanw, B. (2000). Liquid Crystals, 27(1), 153–156.
[17] Dec, M., Pina, C., & Serrano, J. L. (2003). Liquid Crystals, 30(6), 651–661.
[18] Sackmann, H. & Demus, D. (1966). Mol. Crys. Liq. Cryst., 2, 81.
[19] Gray, G. W. & Goodby, J. W. (1984). Smectic Liquid Crystals: Texture and
Structures, Leonard Hill: Philadelphia: Heyden
[20] Marzotko, D. & Demas, D. (1975). Dramana, 1, 189.
[21] Coates, D. (1990). In: Liquid Crystals: Applications and Uses, Badahur, B. (Ed.),
World Scientific: Singapore, Vol. 1, Chap. 3.
[22] Rao, N. V. S., Singha, D., Das, M., Paul, M. K. (2002). Mol. Cryst. Liq. Cryst.,
373, 105.
[23] Yeap, G. Y., Ha, S. T., Lim, P. L. Boey, P. L., Mahmood, W. A. K., Ito, M. M., &
Sanehisa, S. (2004). Mol. Cryst. Liq. Cryst., 423, 73.
[24] Collings, P. J. & Hird, M. (1997). Introduction to Liquid Crystals, Taylor & Francis
Ltd.: London, UK.