Synthesis and mesomorphic investigations of liquid crystalline compounds having a benzothiazole ring

Article abstract of DOI:10.1080/15421406.2013.766947

Two homologous series of calamitic liquid crystals containing a benzothiazole ring and two different linkages have been prepared, and their liquid crystalline properties are studied and compared with each other and those of similar structure. The mesogens with only the cinnamate linking group showed better thermal properties than those with an ester. Nematic and smectic phases were observed. All the compounds of both the series were characterized by elemental analysis, FT-IR, mass spectrometry, ¹H-NMR, and ¹³C-NMR. Phase transition temperatures and the thermal parameters were obtained from differential scanning calorimetery (DSC). The textural observations were performed using hot-stage Polarizing Optical Microscopy (POM).

Full text of DOI:10.1080/15421406.2013.766947

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Synthesis and Mesomorphic
Investigations of Liquid Crystalline
Compounds Having a Benzothiazole Ring
B. T. Thaker a , N. J. Chothani a , B. S. Patel a , Y. T. Dhimmar a , D.
B. Solanki a , Neeraj Patel a , K. B. Patel a & U. Makawana a
a Department of Chemistry , Veer Narmad South Gujarat University ,
Surat , Gujarat , India
Published online: 22 Apr 2013.
To cite this article: B. T. Thaker , N. J. Chothani , B. S. Patel , Y. T. Dhimmar , D. B. Solanki ,
Neeraj Patel , K. B. Patel & U. Makawana (2013) Synthesis and Mesomorphic Investigations of Liquid
Crystalline Compounds Having a Benzothiazole Ring, Molecular Crystals and Liquid Crystals, 575:1,
64-76, DOI: 10.1080/15421406.2013.766947
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Mol. Cryst. Liq. Cryst., Vol. 575: pp. 64–76, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.766947
Synthesis and Mesomorphic Investigations of Liquid
Crystalline Compounds Having
a Benzothiazole Ring
B. T. THAKER,∗ N. J. CHOTHANI, B. S. PATEL,
Y. T. DHIMMAR, D. B. SOLANKI, NEERAJ PATEL,
K. B. PATEL, AND U. MAKAWANA
Department of Chemistry, Veer Narmad South Gujarat University, Surat,
Gujarat, India
Two homologous series of calamitic liquid crystals containing a benzothiazole ring and
two different linkages have been prepared, and their liquid crystalline properties are
studied and compared with each other and those of similar structure. The mesogens with
only the cinnamate linking group showed better thermal properties than those with an
ester. Nematic and smectic phases were observed. All the compounds of both the series
were characterized by elemental analysis, FT-IR, mass spectrometry, 1H-NMR, and 13C-
NMR. Phase transition temperatures and the thermal parameters were obtained from
differential scanning calorimetery (DSC). The textural observations were performed
using hot-stage Polarizing Optical Microscopy (POM).
Keywords 6-Fluorobenzothiazole; azomesogens; cinnamate; enantiotropic; nematic;
SmA
1. Introduction
We recently reported the synthesis and characterization of a three-ring heterocyclic mesogen
incorporating the thiophene ring, and the bent nature of the molecule gave fairly low
melting points and an extensive nematic phase [1]. The introduction of a fluoro substituent
to mesogens often affords liquid crystals with modified physical properties [2]. Many liquid
crystals have fluorinated aromatic rings [3–6], and fluorinated ferroelectric liquid crystals
have a large spontaneous polarization and a low viscosity, which can improve the response
time of the liquid crystal for switching [7–11].
The study of heterocyclic liquid crystal compounds has dramatically increased in
recent years due to their wider range of optical and photochemical properties. The grow-
ing scientific interest in the synthesis of heterocyclic-based liquid crystals has prompted
us to synthesize a series of 6-fluorobenzothiazole liquid crystals. Compounds containing
benzothiazole and benzoxazole rings have been studied as they exhibit interesting fluores-
cent and photophysical properties [12,13]. A nematic benzothiazole was first reported by
Vorlander and Weisswange [14]. Ha et al. reported similar compounds based on benzothia-
zole units containing ester and Schiff base as linking groups [15]. Nematic and/or smectic
∗Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad South Gujarat
University, Surat- 395007, Gujarat, India. Tel.: +91-9228377618. E-mail: btthaker1@yahoo.co.in
64

Investigations of Liquid Crystalline Compounds
65
C mesophases formed by these rod-like compounds were observed. In addition, a number
of benzothiazole-derived compounds were found to exhibit interesting photophysical and
fluorescent properties by Iwata et al. [16] and Gallas et al. [17]. Pavluchenko et al. [18] re-
ported liquid crystals containing benzothizole and benzoxazole rings with different central
linkages and different lateral substitution at different positions so as to evaluate the effect
of structural changes on mesomorphic properties. They observed that the substitution at
sixth position was thermally more stable as compared to that of the fifth position on the
benzothiazole ring.
As part of our continuing studies of heterocyclic mesogens, in this paper, we describe
the synthesis, characterization, and mesomorphic properties of a two series of calamitic
benzothiazole derivatives. The present work describes a comparative study of the influence
of the linkage group on the mesomorphism of the materials. The influence of the terminal
fluoro substituent on the mesomorphic properties is discussed through the comparison with
known liquid crystals of similar structure. The structures of the two novel series of liquid
crystals (A and B) are shown below.
Series-A
N
RO
C
O
O
CH
N
S
F
Where R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16, 18
Series-B
N
RO
CH
CH
C
O
CH
N
S
F
O
Where R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16, 18
2. Experimental Details
2.1 Materials
4-hydroxybenzoic acid and 4-hydroxy benzaldehyde were obtained from Merck, Germany,
alkyl bromides were purchased from Lancaster, England; malonic acid was purchased from
Fluka Chemie, Switzerland; N,N’-dicyclohexylcarbodiimide (DCC) and dimethyl amino
pyridine (DMAP) were purchased from Acros Organics, USA; 4-fluoroaniline was provided
by Aarti Chemical [P] Ltd., Vapi and used without further purification. Bromine, potas-
sium thiocyanate, pyridine, piperidine, anhydrous potassium carbonate, acetone, ethanol,
methanol, acetic acid, ethyl acetate, HCl, KOH, NaOH, NH4OH, etc. were used as re-
ceived. Column chromatography employed Acme’s Silica Gel (100–200 mesh). Solvents
were dried and distilled prior to use.
2.2 Measurements
Elemental analysis (C, H, N) was performed using a thermo Scientific FLASH 2000 in-
strument at G.N.F.C. (Gujarat Narmada Valley Fertilizer Company Ltd., Bharuch). Infrared

66
B. T. Thaker et al.
spectra were recorded using a Thermo Scientific Nicolet iS-10 spectrophotometer in the
frequency range 4000–400 cm−1 with samples embedded in KBr discs. High resolution
(400 MHz) NMR Spectra of the mesogenic compounds were recorded at room temperature
as a 15%–20% solution in CDCl3 using TMS as internal standard on a Bruker Avance II
400 NMR spectrometer at SAIF (Sophisticated Analytical Instrument Facilities), Panjab
University, Chandigarh. Mass spectra (TOF MS ES+) of the compounds were recorded
using a Finnigan MAT-8230 Mass Spectrometer at SAIF (Sophisticated Analytical In-
strument Facilities), Panjab University, Chandigarh. Thin-layer chromatography (TLC)
analyses were performed using aluminium-backed silica-gel plates (Merck60 F524) and
examined under shortwave UV light. Thermal (DSC) analyses of the liquid crystalline com-
pounds were carried out by Atul industries Ltd. P.P site Atul. DSC analyses were performed
on a Mettler M-3 thermo balance (Switzerland) with microprocessor TA-300 instrument
at a heating rate of 10◦C/min in a N2 atmosphere. The optical microscopy studies were
determined by using polarizing microscope Nikon Eclipse 50i POL (Japan) equipped with
Linkam Analysa-LTS420 hot stage (London) at our department as standard procedure.
2.3 Synthesis of Series-A and -B Compounds
The synthetic route to Series-A and B is illustrated in Scheme 1.
2.3.1 2-Amino-6-fluorobenzothiazole (2). 2-Amino-6-fluorobenzothiazole (2) was pre-
pared from 4-fluoroaniline (1) by the reported method [19]. The resulting compound was
crystallized from 1:1 aqueous ethanol to afford pure pale yellow needles of (2). Yield: 87%.
Clearing Point: 184◦C.
2.3.2 4-n-Alkoxybenzoic Acids (4). 4-n-Alkoxybenzoic acids (4) were synthesized by the
modified method of Dave and Vora [20]. The melting point and clearing point of these
compounds were compared with those reported, and they are similar to reported values
[21].
2.3.3 4-(4ꢀ-n-Alkoxy benzoyloxy) Benzaldehydes (5) [22,23]. These compounds were pre-
pared by esterification of a stirred mixture of the appropriate compound 1 (2.22 m mol)
and 4-hydroxybenzaldehyde (0.271 g, 2.22 mmol), N,Nꢀ-dicyclohexylcarbodiimide (DCC)
(0.455 g, 2.22 mmol), dimethyl amino pyridine (0.024 g, 0.2 mmol), and dry CH2Cl2
(20 mL) at room temperature overnight under an inert N2 atmosphere. The ensuing white
precipitate was isolated by Buchner filtration and discarded, while the filtrate was evaporated
to dryness in vacuum. The resultant crude residue was purified by column chromatography
on silica gel eluting with dichloromethane, followed by repeated recrystallization from
ethanol until constant transition temperatures were achieved.
2.3.4 6-Fluoro 2-[4-(4ꢀ-n-Alkoxy benzoyloxy) Benzylidenamino] Benzothiazole (Series-
A). 2-Amino-6-fluorobenzothiazole (2) (0.721 g, 4 mmol) and 4-(4ꢀ-n-alkoxybenzoyloxy)
benzaldehydes (5) (4 mmol) were refluxed for 3 h in an ethanol solution (60 mL) upon the
addition of two drops of acetic acid as catalyst. The reaction mixture was then filtered and
the filtrate was left to evaporate at room temperature in the fume hood. The yellow solid
was obtained and it was recrystallized from ethanol and used for further reaction. The crude
product was purified by column chromatography (silica gel, CHCl3/EtOAc 9/1).
2.3.5 General Synthesis of 4-n-Alkoxy benzaldehydes (8), 4-n-Alkoxy Cinnamic Acids (9)
and 4-(4-n-alkoxycinnamoyloxy) Benzaldehydes (10). 4-n-Alkoxy benzaldehydes (8) and

Investigations of Liquid Crystalline Compounds
67
N
(i)
H2N
F
H2N
S
F
1
2
(ii)
(iii)
HO
COOH
RO
COOH
RO
C
O
5
O
CHO
2
+
3
4
(iv)
N
S
RO
C
O
O
CH
N
F
6
(Series-A)
(vi)
(v)
RO
CH
CH
COOH
HO
CHO
RO
CHO
9
7
8
(vii)
RO
CH CH
C
O
CHO
2
+
O
10
(viii)
N
S
RO
CH CH
C
O
O
CH
N
F
11
(Series-B)
Where R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16, 18
Scheme 1. Synthetic route to Series-I and -II. Reagents and conditions: (i) KSCN, Br2, AcOH; (ii)
RBr, KOH, Ethanol; (iii) DCC, DMAP, CH2Cl2, 4-hydroxy benzaldehyde stirred at 0◦C for 1 h,
stirred at room temperature for 24 h; (iv) Ethanol, 2–3 drop AcOH reflux for 4 h; (v) RBr, K2CO3,
Dry acetone; (vi) Malonic acid, pyridine, pipyridine, reflux; (vii) DCC, DMAP, CH2Cl2, 4-hydroxy
benzaldehyde stirred at 0◦C for 1 h, stirred at room temperature for 24 h; (viii) Ethanol, 2–3 drop
AcOH reflux for 4 h.
4-n-alkoxycinnamic acids (9) were prepared by the method of Gray and Jones [21]. Also,
4-(4-n-alkoxy cinnamoyloxy) benzaldehydes (10) was prepared by following the procedure
described for the compounds of Series-A (2.3.3). Here, 4-n-alkoxy cinnamic acids were
used instead of 4-n-alkoxy acid as in Series-A.
2.3.6 6-Fluoro 2-[4-(4ꢀ- n-Alkoxy Cinnamoyloxy) benzylidenamino] Benzothiazole (Series-
B). The Schiff’s bases of Series-B were synthesized by condensing equimolar quantities

68
B. T. Thaker et al.
of appropriate 4-(4-n-alkoxy cinnamoyloxy) benzaldehydes (10) (4 mmol) with 2-amino-
6-fluorobenzothiazole (2) (0.721 g, 4 mmol) in boiling ethanol. The reaction mixture
was refluxed for 1–2 h with 2–3 drops of acetic acid. The reaction mixture was kept
overnight. The yellow solid product was purified by column chromatography (silica gel,
CHCl3/EtOAc 9/1). The Schiff’s bases of the Series-B were crystallized from ethanol until
constant transition temperatures were obtained.
2.4 Data
2.4.1 6-Fluoro 2-[4-(4ꢀ-tetradecanoyloxybenzoyloxy) benzylidenamino] benzothiazole.
Yield 73%. Clearing Point: 144◦C, UV(CHCl3) λmax: 556 nm, Found C, 71.55; H,
6.83; N, 4.51; Calc. for C35H41N2SO3F (588 g/mol); C, 71.42; H, 6.97; N, 4.76;%. IR
(KBr) υmax cm−1 3065 (C–H Str. aromatic), 2925, 2853 (C–H Str. aliphatic), 1734 (C O
Str. ester), 1642 (CH N, Str. azomethine) 1475, 1362, 1065 (benzothiazole), 882, 848, 638
1
(C–S–C). H NMR (400 MHz, CDCl3): δ/ppm 0.86–0.89 (t, CH3), 1.26–1.91 (m, CH2),
4.03–4.06 (t, OCH2), 6.97–8.12 (m, Ar–H), 8.54 (s, CH N). 13C NMR (CDCl3): δ/ppm
14.03 (CH3), 24.83–33.51 (CH2) 68.42 (OCH2), 115.20–163.91 (Ar–C), 164.29 (–CH N),
171.37 (–C O–), TOF MS ES+ m/z (rel.int%): 588 (M)+ m/z.
2.4.2 6-Fluoro 2-[4-(4ꢀ-tetradecanoyloxycinnamoyloxy) benzylidenamino] benzothiazole.
Yield 75%. Clearing Point: 195◦C, UV (CHCl3) λmax: 566 nm, Found C, 72.55; H, 6.83;
N, 4.43; Calc. for C37H43N2SO3F (614 gm/mole); C, 72.31; H, 7.00; N, 4.56;%. IR (KBr)
υmax cm−1 3091 (C–H Str. aromatic), 2973, 2880 (C–H Str. aliphatic), 1731 (C O Str.
ester), 1635 (CH N, Str. azomethine) 1478, 1364, 1069 (Benzothiazole), 881, 850, 636
(C–S–C). 1H NMR (400 MHz, CDCl3): δ/ppm 0.86–0.90(t, CH3), 1.22–1.91 (m, CH2),
3.98–4.08(t, OCH2), 6.94–7.97 (m, Ar–H), 8.53 (s, CH N). 13C NMR (CDCl3): δ/ppm
14.13 (CH3), 24.76–33.30 (CH2) 68.26 (OCH2), 114.04–161.67 (Ar–C), 165.08 (–CH N),
169.91 (–C O–), 115.01; 147.40 (–CH CH–). TOF MS ES+ m/z (rel.int%): 615 (M+1)+
m/z.
3. Results and Discussion
The synthesis of Series-A and -B is shown in Scheme 1. All compounds were characterized
1
by elemental analysis, UV/Visible, FT-IR, H-NMR, 13C-NMR and mass spectroscopy.
The mesomorphic properties of all the synthesized compounds have been characterized by
differential scanning calorimetry (DSC) and polarizing optical microscope (POM) attached
with a Linkam hot stage. The thin-film samples were obtained by sandwiching them a glass
slide and cover slip.
3.1 Thermal and Phase Behavior
All the compounds synthesized in Series-A exhibit an enantiotropic nematic phase. The
SmC mesophase commences from the n-octyloxy derivative. The n-octyloxy and n-
decyloxy compounds are monotropic and the n-dodecyloxy to n-octadecyloxy(C12, C14, C16,
C18,) members exhibit an enantiotropic SmC phase. The enantiotropic SmC mesophase is
obtained from the n-dodecyloxy derivative and persists up to the n-octadecyloxy derivative.
The transition temperatures are recorded in Table 1 and a plot of transition temperatures
against the number of carbon atoms in the alkoxy chain in Fig. 1 shows a steady fall in the
temperature of nematic isotropic transitions. The trend exhibits a marked odd-even effect in

Investigations of Liquid Crystalline Compounds
69
Table 1. Transition temperature (◦C) data of 6-fluoro 2-[4-(4ꢀ-n-alkoxy benzoy-
loxy)benzylidenamino]benzothiazole (Series-A)
Transition temperature (◦C)
Compounds
R = n alkoxy
Cr
SmC
N
I
A1
A2
A3
A4
A5
A6
A7
A8
A10
A12
A14
A16
A18
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
•
•
•
•
•
•
•
•
•
•
•
•
•
–
–
–
–
–
–
–
–
–
–
–
–
•
•
•
•
•
•
165
157
160
148
142
130
119
102
91
78
76
79
86
•
•
•
•
•
•
•
•
•
•
•
•
•
208
203
197
194
192
183
176
165
158
152
144
139
145
•
•
•
•
•
•
•
•
•
•
•
•
•
–
–
(57)a
(61)a
64
66
70
77
Decyl
Dodecyl
Tetradecyl
Hexadecyl
Octadecyl
aMonotropic.
crystal-mesophase transition temperatures for the lower member of homologues series, and
as the number of carbon atoms increases the odd-even effect is not so pronounced. There is
also a tendency for rising smectic-nematic transition temperatures in the ascending series.
The graph of phase transition temperatures against the number of carbons in the
alkanoyloxy chain enables the effect of the terminal chain on the mesomorphic properties
to be established (Fig. 1). The adequate geometric anisotropy (ratio between length and
240
200
160
Cr-SmC/N
120
80
40
0
SMC-N
N-I
0
2
4
6
8
10 12 14 16 18 20
Number of carbon atom in alkoxy chain
Figure 1. Transition temperature curve of Series-A.

70
B. T. Thaker et al.
Table 2. Transition temperature (◦C) data of 6-fluoro 2-[4-(4ꢀ- n-alkoxy cinnamoyloxy)
benzylidenamino] benzothiazole (Series-B)
Transition temperature (◦C)
Compounds
R = n alkoxy
Cr
SmC
N
I
B1
B2
B3
B4
B5
B6
B7
B8
B10
B12
B14
B16
B18
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
•
•
•
•
•
•
•
•
•
•
•
•
•
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
•
•
•
•
•
179
176
165
153
142
135
129
124
126
133
122
119
115
•
•
•
•
•
•
•
•
•
•
•
•
•
223
217
212
209
205
203
201
199
198
196
195
189
182
•
•
•
•
•
•
•
•
•
•
•
•
•
Decyl
(117)a
108
101
103
106
Dodecyl
Tetradecyl
Hexadecyl
Octadecyl
aMonotropic.
width of the molecule) is an important point. Based on Fig. 1, in Series-A, as the length of
the carbon chain increased, a monotropic (metastable) SmC phase was observed for the C8
derivative. When the carbon chain reached C12, the enantiotropic (stable) SmC phase was
observed, which accompanied the nematic phase at a higher temperature was induced. The
nematic transition temperature shows a descending trend from the C1 to C2 members and
then gradually increases from the C3 and higher members. As the alkyl chain gets longer, the
temperature continued to decrease from C3 to C14 members. Due to the excessive van der
Waals attractive forces between the long alkyl chains, the melting temperatures increased
obviously from C14 to C18 members [24]. Likewise, clearing points then descended with the
increase in length of the carbon chain due to the dilution of the mesogenic core, resulting
from the flexibility provided by the terminal alkanoyloxy chain [25]. A clear increment was
observed when moving to the longest chain member (C18). By referring to the graph, the
nematic phase range.
All the thirteen members of Series-B exhibit an enantiotropic nematic phase. The
SmC phase commences from the n-decyloxy derivative as a monotiotropic mesophase,
the enantiotropic mesophase is seen from the n-dodecyloxy derivative and remains up to
the n-octadecyloxy derivative. The transition temperatures are recorded in Table 2 and a
plot of transition temperatures against the number of carbon atoms in the alkoxy chain is
given in Fig. 2. It can be seen that the crystal to mesophase transition temperatures decrease
with the increase in the length of terminal alkoxy tail. The nematic-isotropic transition
temperatures also decrease with an increase in the number of carbon atoms in the alkoxy
chain, whereas it exhibits a tendency for rising SmC-nematic transition temperature in
ascending series, which levels off slightly in the higher homologues. In Series-B, as the
length of the carbon chain increased, a monotropic (metastable) SmC phase was observed
for the C10 derivative. Once the carbon chain reached C12, the enantiotropic (stable) SmC
phase is observed, which accompanies the nematic phase at a higher temperature. The

Investigations of Liquid Crystalline Compounds
71
240
200
160
120
80
Cr-SmC/N
SmC-N
N-I
40
0
0
2
4
6
8
10 12 14 16 18 20
Number of carbon atom in alkoxy chain
Figure 2. Transition temperature curve of Series-B.
nematic transition temperature shows a descending trend from the C1 to C12 members and
then gradually increases from the C14 members. Clearing points then descended with the
increase in length of the carbon chain due to the dilution of the mesogenic core resulting
from the flexibility provided by the terminal alkanoyloxy chain.
DSC is a valuable method for the detection of phase transitions. It yields quantitative
results. The phase transition temperatures and corresponding enthalpy changes of com-
pounds A5, A14, B5, and B14 were determined using a DSC. The data obtained from the
DSC analysis are summarized in Table 3, which helps further to confirm the mesophase
type. Table 3 shows the phase transition temperature, associated enthalpy (ꢀH), and molar
entropy ꢀS for compounds of Series-A (A5, A14,) and Series-B (B5, B14). The DSC curves
of representative compounds are shown in Figs 3 and 4. Microscopic transition temperature
values are similar to the DSC data.
Table 3. DSC data of the Series-A and B
Series Compounds No. Transition Peak/(POM) Temp. (◦C) ꢀH Jg−1 ꢀS Jg−1K−1
A
A5
Cr-N
N-I
Cr-SmC
SmC-N
N-I
Cr-N
N-I
Cr-SmC
SmC-N
N-I
141.49 (142.8)
191.46 (192.2)
65.90 (66.6)
27.54
8.44
21.20
9.21
—
12.18
3.51
3.79
11.19
14.31
0.1946
0.0440
0.3216
0.1212
—
0.0859
0.0171
0.0375
0.0921
0.0734
A14
75.91 (76.2)
144.0 (144.1)
141.66 (142.8)
204.57 (205.8)
100.92 (101.1)
121.41 (122.4)
194.75 (195.7)
B
B5
B14

72
B. T. Thaker et al.
Figure 3. (a) DSC thermogram for compound no. 5 (Series-A) and (b) DSC thermogram for
compound no. 14 (Series-A).
3.2 Structure-Mesomorphic Property Relationships
Table 4 summarizes the average thermal stabilities and comparative geometry of the present
Series-A, -B and the structurally related Series-I [26], −II [27] and –III [28]. The smectic
as well as nematic thermal stabilities of Series-A are much lower than those of Series-
B. The average smectic mesophase range of Series-B is higher by 1.6◦C and the nematic
mesophase range is higher by 3.4◦C compared to the respective mesophase ranges of Series-
A. The molecules of Series-B have a cinnamoyloxy (–CH CH–COO–) central linkage,
while Series-A have an ester (–COO) central linkage. The mesomorphism results can be
attributed to the greater length and higher polarizability of Series-B molecules.
Reference to Table 4 indicates that the smectic and nematic thermal stabilities of Series-
A compounds are higher than those of Series-I. Compared with molecules of Series-I the
molecules of Series-A are a little longer and more polarizable. Although smectic phase
stability is enhanced from the C12 to C18 members, the increasing melting temperatures
cause the SmC phase to become significant. The smectic phase can be only observed in
both the series for the mid to long chain derivatives.
Figure 4. (a) DSC thermogram for compound no. 5 (Series-B) and (b) DSC thermogram for com-
pound no. 14 (Series-B).

Investigations of Liquid Crystalline Compounds
73
Table 4. The mesophase range and thermal stabilities of Series-A, -B and structurally
related Series I, II, and III
Mesophase range
Thermal stabilities
Commencement of
smectic phase
Series
Smectic
Nematic
Smectic
Nematic
A (C10–C18)
B (C10–C18)
I (C10–C18)
II (C8–C16)
III (C8–C16)
14.4
16.0
6.36
45
65.6
69.0
30.9
55.60
63.60
67.6
107.0
75.04
101.00
90.20
82.00
123.00
81.5
146.00
148.2
C8
C10
C10
C7
58.00
C8
Table 4 shows that both Series-A and I exhibit an enantiotropic nematic mesophase.
Mesophase length and nematic thermal stability of Series-A are higher by 34.7◦C and
0.5◦C, respectively, than that of Series-I because of the greater length of Series-A provided
by the additional aromatic ring.
N
C
H
O
N
C
O
OR
S
F
Series-A
N
S
C
H
N
O
C
O
CH CH
OR
F
Series-B
N
C
H
O
N
C
O
Cn-1H2n-1
S
H3CO
Series-I
C
H
O
N
C
O
OR
Series-II
C
N
O
C
CH CH
OR
H
O
Series-III

74
B. T. Thaker et al.
Figure 5. Microphotograph of liquid crystalline compound.
Table 4 indicates that the nematic and smectic thermal stability of Series-II are higher
by 64.0◦C and 33.4◦C, respectively, than that of present Series-A. Similarly Series-III
members are higher by 25.2◦C and 22.6◦C, respectively, than those of novel Series-B.
Both the series differ only at one terminus. Series-A and -B have a 6-fluorobenzothiazole
ring at the terminus instead of the naphthalene ring of Series-II and -III. Compared with
molecules of Series-II and -III, the molecules of Series-A and -B are more polarizable, but
have increased breadth due to the 6-fluorobezothiazole moiety.
Gray [6] has explained that the increase in the breadth of the molecules reduces both
nematic and smectic thermal stabilities. In the Series-II and -III, the effect of breadth is
not so prominent, due to the 2-substitution in the naphthalene nucleus of the molecules,
making them linear. Therefore, probably both the smectic and nematic thermal stabilities
are higher in the Series-II and -III.
All the compounds of Series-A and -B exhibit mesomorphism. On cooling the isotropic
liquid of Series-A, compounds formed small droplets that coalesce to classical Schlieren
textures characteristic of the nematic phase. On further cooling, the higher members show
a focal-conic texture characteristic of the SmC mesophase. For Series-B on cooling the
isotropic liquid, all the members exhibit the Schlieren texture of the nematic phase; on fur-
ther cooling, n-octyloxy to n-octadecyloxy members exhibit an enantiotropic SmC phase.
The results are consistent with the assignment of each mesophase type using the classi-
fication system reported by Sackmann and Demus [29] and Gray and Goodby [30]. The
textures of A5,A14,B5, and B14 are shown in Fig. 5.
4. Conclusion
In this paper, we have reported the synthesis and characterization of two new mesogenic
homologous series viz 6-fluoro 2-[4-(4ꢀ-n-alkoxybenzoyloxy) benzylidenamino] benzoth-
iazole (Series-A), which contains an ester-azomethine central linkage, and 6-fluoro 2-[4-
(4ꢀ-n-alkoxy cinnamoyloxy) benzylidenamino] benzothiazole (Series-B), which contains a

Investigations of Liquid Crystalline Compounds
75
cinnamate-azomethine central linkage. Series-A with an ester-azomethine central linkage
has lower thermal stabilities compared with Series-B with a cinnamate-azomethine central
linkage. The mesophase range of novel Series-B is higher than that of novel Series-A that
has been attributed to the high polarizability of the molecule of Series-B with a cinnamoy-
loxy central linkage, which are more stable as compared with the Series-A containing
ester-azomethine central linkage due to greater molecular length and polarizability of the
molecule resulting from additional –CH CH– units in the central linkage.
Acknowledgment
We thank Gujarat Narmada Valley Fertilizer Company Ltd. (G.N.F.C.), Bharuch for pro-
viding facilities of elemental analysis, Atul Industries Ltd. Atul. for DSC analysis, and we
1
also thank SAIF Chandigarh for providing facilities of FT-IR, H-NMR, 13C-NMR, and
Mass spectral analysis.
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