The influence of the thioalkyl terminal group on the mesomorphic behavior of some 6-alkoxy-2-naphthoates derived from 1,3,4-oxadiazole

Article abstract of DOI:10.1080/15421406.2016.1147329

A new series of mesogenic compounds having a naphthalene moiety has been synthesized by esterification of 4-(5-(alkyllthio)-1,3,4-oxadiazol-2-yl)phenol and 6-alkoxy-2-naphthoic acid and their liquid crystalline properties have been studied. All the members of the series are enantiotropic and exhibit smectic as well as nematic mesophase. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exhibits no odd even effect and falling tendency for isotropic transition temperatures. High anisotropy, linearity confers rich mesomorphic properties on the system.

Full text of DOI:10.1080/15421406.2016.1147329

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
The influence of the thioalkyl terminal group on
the mesomorphic behavior of some 6-alkoxy-2-
naphthoates derived from 1,3,4-oxadiazole
N. J. Chothani, V. K. Akbari, P. S. Patel & K. C. Patel
To cite this article: N. J. Chothani, V. K. Akbari, P. S. Patel & K. C. Patel (2016) The influence of
the thioalkyl terminal group on the mesomorphic behavior of some 6-alkoxy-2-naphthoates
derived from 1,3,4-oxadiazole, Molecular Crystals and Liquid Crystals, 631:1, 31-46, DOI:
10.1080/15421406.2016.1147329
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1147329
Published online: 12 Jul 2016.
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Date: 14 July 2016, At: 07:17

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
The influence of the thioalkyl terminal group on the
mesomorphic behavior of some -alkoxy--naphthoates
derived from ,,-oxadiazole
N. J. Chothania, V. K. Akbarib, P. S. Patelc, and K. C. Patelb
aArts, Science and Commerce College, Kholwad, Surat, Gujarat, India; bDepartment of Chemistry, Veer Narmad
South Gujarat University, Surat, Gujarat, India; cDepartment of Chemistry, Narmada College of Science and
Commerce, Zareshwar, Bharuch, India
ABSTRACT
KEYWORDS
Calamitic liquid crystal;
nematic N phase; smectic A
phase; smectic C phase;
thioalkyl
A new series of mesogenic compounds having a naphthalene moiety
hasbeensynthesizedbyesterificationof4-(5-(alkyllthio)-1,3,4-oxadiazol-
2-yl)phenol and 6-alkoxy-2-naphthoic acid and their liquid crystalline
properties have been studied. All the members of the series are enan-
tiotropic and exhibit smectic as well as nematic mesophase. The plot
of transition temperatures versus number of carbon atoms in the alkoxy
chain exhibits no odd even effect and falling tendency for isotropic tran-
sition temperatures. High anisotropy, linearity confers rich mesomor-
phic properties on the system.
1. Introduction
Liquid crystal (LC) science is one the most important topic in current years, even though to
establish correlation between chemical structure and mesomorphic properties in important
problems. It is an important and challenging task for the chemists to synthesize mesomor-
phic organic compounds with required physical-chemical characteristics and allows under-
standing of the influence of different structural elements of the molecule architecture. The
thermotropic mesomorphism of calamitic LC is largely influenced by molecular structure in
which a slight change brings about considerable change in its properties [1,2]. It is well estab-
lished that the linearity is prime and foremost criteria for the rod-like calamitic LC’s molecular
geometry, which is generally achieved by using aromatic, heteroaromatic, or aliphatic rings
as mesogenic units. On the other hand, some functional central bridging groups have proven
to be very useful to promote mesomorphic properties. During the last decades, heterocycles
are considerably used in the molecular architectures of novel organic materials, especially
in the development of thermotropic LCs. Mesomorphic compounds containing heterocyclic
units have been synthesized and interest has dramatically increased due to their diversified
molecular structure and distinct mesomorphic properties [3–12]. The incorporation of het-
erocyclic moieties for the design of new mesogenic molecules can result in large changes in
their mesophases and physical properties. In addition, introduction of heteroatoms can cause
CONTACT K. C. Patel
 , Gujarat, India.
drkcpatel@gmail.com
Department of Chemistry, Veer Narmad South Gujarat University, Surat–
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

32
N. J. CHOTHANI ET AL.
considerable changes in polarity, polarizability, and geometry of the molecules because most
of the heteroatoms (S, O, and N) are more polarizable than carbon. These factors lead to
dramatic changes in the corresponding mesophase type, the phase transition temperatures
[13,63]. Heterocycle cores are thus able to impart lateral and/or longitudinal dipoles com-
bined with changes in the molecular shape [14,15]. Dipole-dipole interactions and structural
shapes are fundamental elements in the design of LCs [16]. Among these derivatives, the five-
membered heterocyclic rings, such as 1,3,4-oxadiazoles, have also proven to be highly efficient
in promoting mesomorphic properties [17–22]. The introduction of oxadiazole rings not only
provide lateral dipole from the oxygen and nitrogen atoms, but also the bent shape of the rigid
cores. This large lateral dipole leads to the increase possibility may leads to very interesting
mesomorphic properties, when oxadiazole rings are used as central cores [23].
In recent years, mesogenic 1,3,4-oxadiazole derivatives have been extensively investigated,
by many research group [24–42]. Recently, Han has also been reported a review on 1,3,4-
oxadiazole derivatives, in which a great attention has been paid on the structure-property
relationship [43]. Since this type of compound may exhibit tunable liquid crystalline prop-
erties with fluorescence with high quantum yields, together with excellent thermal and
chemical stability and they have strong electron affinity and often possess electron trans-
port properties [44–47]. Compounds involving the 1,3,4-oxadiazole ring have been used
as emitter in organic electroluminescent devices [48–50] and also be used as a mesogenic
core to construct organic light-emitting diodes among the heterocyclic compounds [51,52].
The materials containing the 1,3,4-oxadiazole unit hold great potential for use to synthesize
different fluorescent chemosensors for transition metal ions [53], biologically active agents
[54], and scintillators [55].
In 1989, Chudgar et al. first reported a series of liquid crystalline 1,3,4 oxadiazoles,
which exhibited nematic phase with high melting points [56]. Mahadeva et al. synthesized
another homologous series of 2-amino-5-alkoxyphenyl-1,3,4-oxadiazoles, which showed rel-
atively lower melting points [57]. Parra group [58,59] synthesized a series of 1,3,4-oxadiazole
derivatives and the effect of the 1,3,4-oxadiazole moiety on liquid crystalline properties
was investigated systematically. In investigations of liquid crystalline esters of 2,5-bis-(4-
hydroxyphenyl)-1,3,4-oxadiazoles and 2-alkylthio-5-(4-hydroxyphenyl)-1,3,4-oxadiazoles, it
was established that the esters of 2-alkylthio-5-(4-hydroxyphenyl)-1,3,4-oxadiazoles showed
the widest mesomorphic ranges and these compounds exhibited smectic A (SmA) and
nematic (N) phases [60].
The main aim of this work is to obtain calamitic LCs with different phases and to study
the effect of length of the thioalkyl chain and the effect of the heterocyclic moiety (1,3,4-
oxadiazoles) on the mesomorphic properties. To the best of our knowledge, there are reports
on the 2-alkylthio-5-(4-hydroxyphenyl)-1,3,4-oxadiazoles which are interesting systems for
the design and synthesis of liquid crystalline compounds with a classical rod-like structure.
The introduction of a 1,3,4-oxadiazole ring within core of calamitic molecules strongly influ-
ences their mesomorphic behavior due (though not only) to the dipolar moment associated
with the heterocyclic ring.
Experimental
Methods and materials
For the synthesis of compounds of the homologous series, following materials were
used. 4-Hydroxybenzoic acid was purchased from Spectrochem, alkyl bromide (Lancaster),

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
33
N
N
O
H17C8S
O
O
OR
4-(5-(octylthio)-1,3,4-oxadiazol-2-yl)phenyl-6-alkoxy-2-naphthoate
Series I
N
N
O
O
H25C12S
O
OR
4-(5-(dodecylthio)-1,3,4-oxadiazol-2-yl)phenyl-6-alkoxy-2-naphthoate
Series II
Where, R=CnH2n+1, n= 1to 8, 10,12,14,16
4-(dimethylamino)pyridine (DMAP), and N,N’-dicyclohexylcarbodiimide (DCC) were
purchased from Aldruch. The solvents were used after purification using the standard meth-
ods described in the literature.
Elemental analysis (C, H, and N) were performed on Thermo Finnigan EA 1112 Flash Ele-
mental Analyzer. Fourier transform infrared spectra were recorded with Shimadzu DRS 8400-
S Spectrophotometer (Shimadzu IR Solution 1.30 software) in the frequency range 4000–400
1
cm−1 with samples embedded in KBr discs. H NMR (300 MHz) and 13C NMR (75MHz)
spectra of the compound were recorded with BRUKER AVANCE II Spectrometer (Bruker
BioSpin AG, Fallanden, Switzerland; 300 MHz) using CDCl3 as a solvent and TMS as an inter-
nal reference. Mass spectra (EI) of the compounds were recorded with Advion Expression
CMS, USA (Compact Mass Spectrometer) using ESI as ionization source Thin-layer plates
(Merck 60 F524) are examined under short-wave UV light. The product was purified by col-
umn chromatography over silica gel (60–120 mesh, Sigma-Aldrich Co.). UV-visible spectra
were recorded with Thermo Scientific Evolution 300 UV-VIS. Thermal analyses, differential
scanning calorimetry (DSC) of the liquid crystalline compounds were performed on Perkin
Elmer, Model: DSC7 Series using Pyris Software, heating rate of 10°C/min in N2 atmosphere.
The thermobalance and temperature calibration (with Pt-100) was checked by standard sam-
ple. The optical microscopy studies were carried out with NICON ECLIPSE 50i POL (Japan)
with hot stage and the temperature controller Linkam Analysa-LTS420. The textures of the
compounds were observed using polarized light with crossed with sample in thin film sand-
wiched between a glass slide and cover slip.

34
N. J. CHOTHANI ET AL.
Synthesis
Synthesis of 6-Alkoxy-2-naphthoic Acid (2)
6-Hydroxy-2-naphthoic acid (1) (2.44 g, 13 mmol, 1 equiv.) and KOH (1.46 g, 26 mmol, 2
equiv.) were dissolved in ethanol/water (25 mL, 9/1) and the solution was stirred for 20 min;
corresponding n-alkyl bromide (32.50 mmol, 2.5 equiv.) was then added and the mixture is
heated under reflux for 24 hr. When the reaction was complete, KOH (0.73 g, 13 mmol, 1
equiv.) was added and the mixture is heated under reflux for further 4 hr. The ethanol was
evaporated, and the mixture is poured into water and acidified to approximately pH = 2∼3
with acetic acid. The precipitate was filtered and washed with water and ether, and then recrys-
tallized twice from glacial acetic acid, then from ethanol to give 71%∼80% yield. (IR) (KBr):
υmax/cm−1: 3061, 2930, 2828, 1680, 1603, 1311, 1472, 1450, 1383, 1216, 1060, and 721.
Synthesis of Ethyl 4-hydroxybenzoate (4)
A solution of 4-hydroxybenzoic acid (3) (25 g, 181 mmol), ethanol (30 mL) were mixed, then
H2SO4 was added (catalytic amount), resultant mixture was heated at 80–85°C for 3 hr. After
the completion of the reaction, concentrated and solid residue was poured in 400 mL of ice-
cooled water. The solid was removed by filtration, wash with water, and dried. The crude solid
was twice recrystallized from ethanol (yield: 90%).
Synthesis of 4-Hydroxybenzohydrazide (5)
A solution of methyl ethyl 4-hydroxybenzoate (4) (14.9 g, 90 mmol) and excess of a solution
of hydrazine hydrate (80%) in 20 mL of ethanol was heated to reflux during 8 hr. The mixture
was then cooled to room temperature and the solid obtained was filtered, washed with water,
and recrystallized from ethanol (yield: 87).
Synthesis of 5-(4-Hydroxyphenyl)-1,3,4-oxadiazole-2(3H)-thione (6)
A suspension of 4-hydroxybenzohydrazide (5) (8.5 g, 56 mmol, 1 equiv.) in ethanol (20 mL)
was stirred, then a solution of KOH (3.13 g, 56 mmol, 1 equiv.) in water (20 mL) was added
dropwise to this stirred suspension at 25°C. After all of the hydrazide has dissolved, car-
bondisulphide (4.2 mL, 70 mmol, 1.25 equiv.) was added at the same temperature and stir-
ring was continuing for 2 hr. After the completion of the reaction, solution was evaporated
and crude was poured into a mixture of 300 g ice-cooled water, then the solution was carefully
acidify with concentrated hydrochloric acid. The light brown solid formed was filtered off and
recrystallized from water:ethanol (1:9) (yield: 67%) [59].
Synthesis of 4-(5-(Alkylthio)-1,3,4-oxadiazol-2-yl)phenol (7)
A solution of 5-(4-hydroxyphenyl)-1,3,4-oxadiazole-2(3H)-thione (6) (7 g, 36 mmol, 1 equiv.)
in absolute ethanol (50 mL) was stirred, to this solution triethylamine (5 mL, 36 mmol, 1
equiv.) and corresponding n-alkylbromide (36 mmol, 1 equiv.) were successively added drop-
wise. Then, reaction mixture was reflux for 6 hr, after completion of the reaction solvent was

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
evaporated. The residue was poured into 300 mL of water, the resulting precipitate was col-
lected. The compound was purified by column chromatography over silica gel (60–120 mesh),
using 7%–8% dichloromethane:methanol as eluent (yield:92%) [59] (Scheme 1).
RO
HO
(i)
COOH
COOH
(1)
(2)
Where, R=CnH2n+1, n= 1to 8, 10,12,14,16
O
O
O
(iii)
(ii)
HO
HO
HO
OH
O
NHNH2
C2H5
(3)
(4)
(5)
(iv)
H
N
N
N
N
(v)
HO
HO
SR'
S
O
O
(6)
(7)
HO
(vi)
O
OR
(2)
N
N
O
R'S
O
O
OR
(8)
Where, R=CnH2n+1, n= 1to 8, 10,12,14,16
Where, R'= C8H17 (Series I)
= C12H25 (Series II)
Scheme . Reagents and Reaction conditions: (i) R-Br, KOH, ethanol, reflux  hr, (ii) ethanol, HSO,
(iii) NHNH (%), (iv) CS, KOH, (v) EtN, RBr, and (vi) DCC, DMAP, dry CHCl, stirring  hr.
Synthesis of 4-(5-(Alkylthio)-1,3,4-oxadiazol-2-yl)phenyl-6-alkoxy-2-naphthoate
(Series I and II)
A compound 4-(5-(alkylthio)-1,3,4-oxadiazol-2-yl)phenol (7) (1.8 mmol, 1 equiv.), corre-
sponding 6-alkoxy-2-naphthoic acid (1.8 mmol, 1 equiv.) and DMAP (0.008 g, 0.08 mmol,
0.05 equiv.) were stirred in anhydrous CH2Cl2 (10 mL). To this solution, DCC (0.56 g,

36
N. J. CHOTHANI ET AL.
2.7 mmol, 1.5 equiv.) in CH2Cl2 (2 mL) added slowly at room temperature and the mixture
was stirred for 24 hr at room temperature. After the completion of the reaction, the white
solid N,N’-dicyclohexylure was filtered off; the filtrate was diluted with CH2Cl2 (20 mL), then
was washed thoroughly with 4% HCl, saturated NaHCO3 and twice with water. The organic
layer was separated, dried over Na2SO4, and evaporated. The product was purified by column
chromatography over silica gel (60–120 mesh), using 40% hexane:ethyl acetate as eluent and
recrystallized from the solvent mixture dichloromethane:ethanol (1:9) obtain slight brown
solid (yield: 83–91%) [61–63].
Compound I7: Yield: 89%; transitions (°C): Cr 131 N 170 Iso; UV-Vis (CHCl3) λmax(nm):
260, 293; 1HNMR (CDCl3): δ (ppm) = 0.87–0.90 (t, J = 7.6 Hz, 6H, 2CH3), 1.25–1.89 (m,
22H, 11CH2), 3.23–3.28 (t, J = 6.1 Hz, 2H, SCH2), 4.06–4.11 (t, J = 6.2 Hz, 2H, OCH2), 7.19
(d, J = 8.2 Hz, 2H, Ar-H), 7.31 (dd, J = 2.2, 9.0 Hz, 1H, naphthalene), 7.57 (dd, J = 1.8,
8.5 Hz, 1H, naphthalene), 7.86 (d, J = 1.6 Hz, 1H, naphthalene), 7.92 (d, J = 8.4 Hz, 1H,
naphthalene), 8.01 (d, J = 8.8 Hz, 1H, naphthalene), 8.10 (s, 1H, naphthalene), and 8.14 (d,
J = 8.8 Hz, 2H, Ar-H); 13C NMR (CDCl3): δ (ppm) = 14.12 (CH3 aliphatic), 22.62–32.60
(CH2 aliphatic), 32.71 (SCH2), 68.26 (OCH2), 106.47–159.63 (aromatic C), 164.78, 165.20
−1
=
–
(oxadizole ring), and 165.79 (C O); IR (KBr) νmax (cm ): 3030, 3064 (C H str. aromatic),
–
=
=
2921, 2851 (C H str. aliphatic), 1735 (C O str. ester), 1627 (C N str. oxadizole), 1575, 1534
=
–
– –
– –
(C C str. aromatic), 1278 (C O str. ester), 1218 (C O C asym. str. alkoxy), and 1067 (C O C
sym. str. alkoxy); MS m/z (rel. int.%): 575.3 (M+1)+; elemental analysis for C34H42N2O4S:
=
=
Calc.: C 71.05%, H = 7.37%, and N = 4.87%; Found: C 70.86%, H = 7.32%, and N =
4.97%.
Compound I16: Yield: 91%; transitions (°C): Cr 64 smectic C (SmC) 88 SmA 155 Iso; UV-
Vis (CHCl3) λmax (nm): 261, 293; 1HNMR (CDCl3): δ (ppm) = 0.85–0.87 (t, J = 7.6 Hz, 6H,
2CH3), 1.25–1.89 (m, 40H, 20CH2), 3.23–3.28 (t, J = 6.2 Hz, 2H, SCH2), 4.07–4.011 (t, J =
6.2 Hz, 2H, OCH2), 7.06–8.15 (m, 10H, aromatic H), 7.12 (d, J = 8.1 Hz, 2H, Ar-H), 7.30
(dd, J = 1.8, 9.0 Hz, 1H, naphthalene), 7.55 (dd, J = 2.1, 8.7 Hz, 1H, naphthalene), 7.80 (d,
J = 1.8 Hz, 1H, naphthalene), 7.88 (d, J = 8.4 Hz, 1H, naphthalene), 7.92 (d, J = 8.8 Hz,
1H, naphthalene), 8.09 (s, 1H, naphthalene), and 8.11 (d, J = 8.7 Hz, 2H, Ar-H); 13C NMR
(CDCl3): δ (ppm) = 14.12 (CH3 aliphatic), 22.69–32.57 (CH2 aliphatic), 32.69 (SCH2), 68.29
=
(OCH2), 106.43–163.58 (aromatic C), 164.85, 165.13 (oxadizole ring), and 165.82 (C O); IR
(KBr) νmax (cm ): 3034, 3064, 3119 (C H str. aromatic), 2929, 2852 (C H str. aliphatic), 1734
−1
–
–
=
=
=
–
(C O str. ester), 1627 (C N str. oxadizole), 1574, 1523 (C C str. aromatic), 1272 (C O str.
– –
– –
ester), 1215 (C O C asym. str. alkoxy), and 1064 (C O C sym. str. alkoxy); MS m/z (rel. int.
%): 701.2 (M+1) ; elemental analysis for C43H60N2O4S: Calc.: C 73.67%, H = 8.63%, and
+
=
=
N = 4.00%; Found: C 73.60%, H = 8.47%, and N = 3.97%.
Compound II7: Yield: 89%; transitions (°C): Cr 110 SmA 159 Iso; UV-Vis (CHCl3) λmax
(nm): 261, 291; 1HNMR (CDCl3): δ (ppm) = 0.87–0.90 (t, J = 7.4 Hz, 6H, 2CH3), 1.25–1.89
(m, 30H, 15CH2), 3.23–3.28 (t, J = 6.3 Hz, 2H, SCH2), 4.09–4.13 (t, J = 6.1 Hz, 2H, OCH2),
7.12 (d, J = 8.5 Hz, 2H, Ar-H), 7.40 (dd, J = 1.9, 8.8 Hz, 1H, naphthalene), 7.55 (dd, J =
1.8, 8.7 Hz, 1H, naphthalene), 7.80 (d, J = 1.5 Hz, 1H, naphthalene), 7.87 (d, J = 8.4 Hz, 1H,
naphthalene), 7.96 (d, J = 8.8 Hz, 1H, naphthalene), 8.12 (s, 1H, naphthalene), and 8.14 (d,
J = 8.6 Hz, 2H, Ar-H); 13C NMR (CDCl3): δ (ppm) = 14.12 (CH3 aliphatic), 22.62–32.21
(CH2 aliphatic), 32.79 (SCH2), 68.27 (OCH2), 106.49–159.64 (aromatic C), 164.78, 165.20
−1
=
–
(oxadizole ring), and 165.80 (C O); IR (KBr) νmax (cm ): 3034, 3060, 3120 (C H str. aro-
matic), 2923, 2851 (C H str. aliphatic), 1733 (C O str. ester), 1627 (C N str. oxadizole),
1574, 1535 (C C str. aromatic), 1283 (C O str. ester), 1217 (C O C asym. str. alkoxy), and
1066 (C O C sym. str. alkoxy); MS m/z (rel. int.%): 631.4 (M+1) ; elemental analysis for
–
=
=
=
–
– –
+
– –

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
37
=
=
C38H50N2O4S: Calc.: C 72.35%, H = 7.99%, and N = 4.44%; Found: C 72.26%, H = 7.87%,
and N = 4.38%.
Compound II16: Yield: 91%; transitions (°C): Cr 68 SmC 82 SmA 146 Iso; UV-Vis (CHCl3)
λmax (nm): 261, 291; 1HNMR (CDCl3): δ (ppm) = 0.85–0.89 (t, J = 7.2 Hz, 6H, 2CH3), 1.25–
1.89 (m, 48H, 24CH2), 3.24–3.28 (t, J = 6.2 Hz, 2H, SCH2), 4.11–4.13 (t, J = 6.3 Hz, 2H,
OCH2), 7.21 (d, J = 8.5 Hz, 2H, Ar-H), 7.32 (dd, J = 2.1, 9.0 Hz, 1H, naphthalene), 7.68 (dd,
J = 1.7, 8.4 Hz, 1H, naphthalene), 7.80 (d, J = 1.8 Hz, 1H, naphthalene), 7.88 (d, J = 8.2 Hz,
1H, naphthalene), 7.92 (d, J = 8.8 Hz, 1H, naphthalene), 8.05 (s, 1H, naphthalene), and 8.19
(d, J = 9.0 Hz, 2H, Ar-H); 13C NMR (CDCl3): δ (ppm) = 14.12 (CH3 aliphatic), 22.69–32.21
(CH2 aliphatic), 32.80 (SCH2), 68.28 (OCH2), 106.50–159.65 (aromatic C), 163.86, 165.22
−1
=
–
(oxadizole ring), and 165.82 (C O); IR (KBr) νmax (cm ): 3035, 3061, 3119 (C H str. aro-
–
=
=
matic), 2920, 2850 (C H str. aliphatic), 1734 (C O str. ester), 1626 (C N str. oxadizole),
=
–
– –
1573, 1536 (C C str. aromatic), 1284 (C O str. ester), 1220 (C O C asym. str. alkoxy), and
+
– –
1066 (C O C sym. str. alkoxy); MS m/z (rel. int.%): 757.2 (M+1) ; elemental analysis for
=
=
C47H68N2O4S: Calc.: C 74.56%, H = 9.05%, and N = 3.70%; Found: C 74.49%, H = 9.09%,
and N = 3.68%.
Result and discussion
In the this article, we report the synthesis and phase characterization of a homologous series
of 1,3,4-oxadiazole achiral asymmetrical four-ring calamitic liquid crystalline compounds, as
a feature of their structure-property relationships. The molecular structures of these newly
synthesized compounds were characterized by spectroscopic methods. The thermotropic liq-
uid crystalline behavior of the achiral rod-like 1,3,4-oxadiazole compounds, series I and II,
was studied using polarizing optical microscopy (POM) and DSC.
Thermal and phase behavior
Mesomorphic properties of series I and II were determined by DSC and POM. The phase
transition temperatures, reported in this article, were the peak values of the transition on the
DSC curves. Phase identification was made by comparing the observed textures with those
reported in the literature. The phase transition temperatures, the associate enthalpy changes,
and the mesophase textures of the series I and II are summarized in Tables 1 and 2, respec-
tively. Clearcut transition temperatures and textures were obtained from the DSC and POM
Table . Transition temperature data of the series I.
Transition temperature (°C)
Compounds
R = n alkoxy
Cr
SmC
SmA
N
I
I
I
I
I
I
I
I
I
I
I
I
I
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
•
•
•
•
•
•
•
•
•
•
•
•
—
—
—
—
—
—
—

—
—
—
—
—
—
—
•
•
•
•
•
—
—
—
—
—
—
—





—
—
—
—
—
—
—
•
•
•
•
•







—
—
—
—
—
•
•
•
•
•












•
•
•
•
•
•
•
•
•
•
•
•
•
•
—
—
—
—
–S

Dodecyl
Tetradecyl
Hexadecyl




38
N. J. CHOTHANI ET AL.
Table . Transition temperature data of the series II.
Transition temperature (°C)
Compounds R = n alkoxy Cr
SmC
SmA
N∗
I
II
II
II
II
II
II
II
II
II
II
II
II
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
•
•
•
•
•
•
•
•
•
•
•
•
—
—
—
—
—
—
—

—
—
—
—
—
—
—
•
•
•
•
•
—
—
—
—
—





—
—
—
—
—
•
•
•
•
•






—
—
—
—
—
—
•
•
•
•
•









•
•
•
•
•
•
•
•
•
•
•
•
•
—
—
—
—
—
—



Dodecyl
Tetradecyl
Hexadecyl





•
•

observations for the representative compounds, and were in good agreement with each other
for heating cycles.
All the compounds synthesized in series I are mesomorphic in nature. The smectic
mesophase commences from the n-octyloxy derivative. n-Octyloxy to n-hexadecyloxy (C8,
C10, C12, C14, and C16) members exhibit an enantiotropic SmC coexist with SmA phase. Tran-
sition temperature shows that compounds I1 to I7 displayed enantiotropic nematic phase,
while no smectic phase was observed. After sufficient length increment, smectic mesophase
is exhibit in case of higher homologous. It is well known that the stability of the mesophase
would be augmented by an increase in the polarity or polarizability of the mesogenic part
of the molecule. The relative strength of the lateral and terminal cohesive forces between
molecules determines the type of the mesophase formed [64]. The thermal stability of a smec-
tic mesophase is largely determined by the low ratio of terminal to lateral intermolecular cohe-
sive forces while a high ratio of these forces is important in determining nematic mesophase.
Since the dipolar (alkoxy) and polarizable (ester) centers of the molecule have become further
separated from one another as a result of the lengthened alkyl chain, the terminal intermolec-
ular attraction has decreased while the residual lateral attractions are essentially unchanged.
This increases the ratio of lateral to terminal cohesive forces, makes the probability greater
that the layer arrangement, which is characteristic of the smectic mesophase, will persist after
melting occurs. Changes in this ratio are, therefore, quite important in determining the type of
the mesomorphism being exhibited by certain molecule in a series of liquid crystalline com-
pounds as well as the temperature at which mesomorphic transition occur [65]. Strong lateral
and weak terminal intermolecular cohesions will give rise to a smectic mesophase, which, if
the lateral cohesions are high enough, may persist until the isotropic liquid is formed [64].
Therefore, in series I higher homologues exhibit only enantiotropic smectic mesophase. This
is because the long chains become attracted and intertwined, which facilitates the lamellar
packing required for smectic phase generation. As a result, the smectic tendency increases
and eventually eliminates the nematic phase. A plot of transition temperatures against the
number of carbon atoms in the alkoxy chain given in Fig. 1. It shows a steady fall in the tem-
perature of smectic, nematic, and isotropic transitions.
Similarly, all the 12 members of series II exhibit either enantiotropic nematic or
smectic mesophase. The SmA and SmC phases commence from the n-hexyloxy and
n-octyloxyderivative, respectively, as enantiotropic mesophase and remain up to the n-
hexadecyloxy derivative. In series II, C1 to C6 members show enantiotropic nematic phase,
among them C6 member also coexist with SmA phase. The melting clearing temperatures of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
39
220
200
180
160
140
120
100
80
Tm (Cr to SmC)
Tm (SmC to SmA)
Tm (Cr to N)
Tm (SmA/N to I)
60
0
2
4
6
8
10
12
14
16
18
Number of Carbon atom
Figure . Transition temperature curve for series I.
series II are recorded in Table 2 and a plot of transition temperatures against the number of
carbon atoms in the alkoxy chain given in Fig. 2.
It can be noticed that mesophase transition temperatures decrease with the increase in the
length of terminal alkoxy tail. The plot of transition temperature against the number of carbon
atom in the alkoxy chain shows a smooth falling tendency for mesomorphic-isotropic transi-
tion temperature throughout the series. Series II exhibits falling tendency of TCr-N and TCr-SmA
and exhibits falling tendency of TSmC-SmA transition temperatures for higher homologous.
DSC is a valuable method for the detection of phase transitions. It yields quantitative
results; therefore, we may draw conclusions concerning the nature of the phases that occur
during the transition. In the present study, DSC measured enthalpy of two derivatives of series
I and II. DSC data of series I and II are recorded in Table 3, which further help to confirm the
mesophase. Table 3 shows the phase transition temperatures, associated enthalpy (ꢀH), and
molar entropy (ꢀS) for compounds of series I (I7 and I16), series II (II7 and II16). The DSC
curves of representative compounds of the series I and II are shown in Figs. 3(a,b) and 4(a,b).
Microscopic transition temperature values are almost similar to DSC data.
Table 4 shows average thermal stabilities and mesophase range of compounds of series I
and II. The TN-I thermal stabilities of compounds of series I and II are 180.42°C and 168.57°C
220
Tm (Cr to SmC)
Tm (Cr/SmC to SmA)
Tm (Cr/SmA to N)
200
Tm (SmA/N to I)
180
160
140
120
100
80
60
0
2
4
6
8
10
12
14
16
18
Number of Carbon atom
Figure . Transition temperature curve for series II.

40
N. J. CHOTHANI ET AL.
Table . Transition temperature and DSC data of the series I and II.
Microscopic temp. (peak
ꢀS (J g−
k−
Compound
Transition
Cr-N
temp.) (°C)
ꢀH (J g−
)
)
I
.()
.()
.()
.()
.()
.()
.()
.()
.()
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
N-I
I
Cr-SmC
SmC-SmA
SmA-I
Cr-SmA
SmA-N
N-I
Cr-SmC
SmC-SmA
SmA-I
II
II
.()
.()
.
Figure . (a) DSC thermogram of compound I (series I) and (b) DSC thermogram for compound I (series I).
Figure . (a) DSC thermogram of compound II (series II) and (b) DSC thermogram for compound II
(series II).
Table . Average thermal and mesophase stabilities of the series I and II.
Series
I
II
Diff.
N-I (°C)
SmA-I (°C)
SmA-N (°C)
SmC-SmA (°C)
N mesophase range (°C)
SmC mesophase range (°C)
SmA mesophase range (°C)
Commencement of smectic
phase
.
.
—
.
.
.
.
.
.
.
.
.
.
.
—
.
.
.
.
—
.
C
.
C

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
41
and TSmA-I thermal stabilities are 161.60°C and 149.40°C, respectively. The TSmC-SmA thermal
stabilities of compounds of series I and II are 90.8°C and 95.4°C, respectively. Series II also
shows TSmA-N thermal stability about 133.5°C. From the Table 4, it can be seen that the average
SmC mesophase range of compounds of series I and II are 19.0°C and 13.6°C and average
SmA mesophase range are 28.6°C and 31.14°C, respectively. The average nematic mesophase
range of compounds of series I and II are 29.71°C and 19.14°C, respectively.
Structure-mesomorphic property relationships
Intermolecular interactions are extremely important in understanding the mesogenic behav-
ior of LCs. The melting point (the temperature at which an ordered geometrical arrangement
collapses and gives rise to the ordered isotropic melt) depends largely on the nature of the
intermolecular interaction existing within the system. The cycle of mesomorphism (heating
or cooling) and the transition temperature are governed by the intermolecular forces that act
between the planes and ends of the molecule. It is generally agreed that the prime requirement
for the formation of a thermotropic LC is anisotropy in the molecular interaction [64].
In this article, at one end thioalkyl group is fixed, i.e., series I having -SC8H17, series II
having -SC12H25, and alkoxy chain length of 6-alkoxy-2-naphtoic acid is varied. It has been
observed that compounds of series I have higher transition temperature than that of the com-
pounds of series II. This may be attributed to the shorter alkoxy chain giving higher temper-
ature then the longer alkoxy chain. For compounds with longer terminal chains, of series II,
cooling the nematic phase resulted in an additional SmA phase is appeared form C6 mem-
ber, while C8 member in case of series I. The dependence of the transition temperatures
on the total chain length can be seen by comparing series I and II in which the two termi-
nal chains are of the different length. While both the melting and clearing temperatures fall
with increasing chain length, the reduction of the clearing temperatures is more significant.
For the shortest chain, compound of series I, the widest mesophase range, was seen from
Table 1. Increasing the chain length decreases the temperature range of the compounds of
series II, with decyloxyl chains. Further symmetry lowering, by introducing two chains of
different lengths as appears in series I and II to stabilize the mesophase range. Compounds
of series II have a longer total chain length by an extra methylene units than compounds
of series I, therefore, its transition temperatures are expected to be lower and its mesophase
range should be smaller. Interestingly, compounds of series II not only showed a higher SmA
mesophase range but also lower melting and clearing points. The most pronounced difference
between these two series is the nematic phase range. The unequal chain lengths do not appear
to disrupt the lamellar structure; indeed, a much wider smectic range is found for series I
and II.
As shown in Table 4 differences in thermal stability as well as mesophase range of the com-
pounds of series I and II. The overall thermal stability of compounds of series I is higher
than that of compounds of series II. The nematic-isotropic transition is higher in compounds
of series I by 11.85°C than compounds of series II. The compounds of series II have SmC-
SmA thermal stabilities higher by 4.6°C than those of compounds of series I. The thermal
stabilities of SmA phase of compounds of series I is also higher by 12.20°C than that of com-
pounds of series II. The SmC mesophase avarge width of compound of series I is higher by
5.4°C than that of series II, while comparing the SmA mesophase range of compounds of
series I is lower by 2.54°C than that of compounds of series II. The nematic mesophase width
of compounds of series I is higher by 10.87°C than that of compounds of series II. How-
ever, the addition of a methylene group increases the overall polarizability of the molecule

42
N. J. CHOTHANI ET AL.
[64]. Consequently, the lateral intermolecular attractions increase, as the chain length grows.
Each methylene unit force apart polarizable centers in the molecule and decrease the resid-
ual terminal attraction. There should be a relative decrease in the strength of the terminal
intermolecular cohesive interactions [64]. This will decreased the nematic-isotropic transi-
tion temperature. So, in series II having lower nematic-isotropic transition temperature than
in series-I. In series I terminal intermolecular cohesive interactions is more due to shorter
alkoxy chain at one terminal compare to series II.
The influence on the thermal behavior of replacing one of the naphthyl units in compounds
I10 and II10, with a phenyl unit (compounds 2.5 and 2.9), is shown in the Table 5. Table 5 sum-
marizes the transition temperature of the present compounds I10 and II10 and the structurally
related compounds 2.5 and 2.9. In compound I10, SmC phase is obseved at 72°C, whereas in
compound 2.5 it is observed at 75°C. The SmA mesophase is appeared form 95°C in com-
pound 2.5, while it is observed at 95°C in compound I10. The compound II10 shows SmC
Table . The Comparison of Transition Temperatures (˚C) of representative Compound K and L of the
present Series I & II and structurally related Compound . and .
Transition temperature °C
Compound
Cr
SmC
SmA
N
I
I
•
•
•
•




•
•
•
•



•
•
-
-
-
-
-
-
-
•
-


.

•
•
•
•
II
.
.

•
N
N
O
H17C8S
O
O
OC10H21
Compound I10
N
N
O
H17C8S
O
OC10H21
O
Compound 2.5
N
N
O
H25C12
S
O
O
OC10H21
Compound II10
N
N
O
H25C12
S
O
OC10H21
O
Compound 2.9
Scheme . Comparison of existing I and II compounds with structurally related compound . & .. As
shown in Table .

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
43
Figure . Microphotograph of liquid crystalline compounds.
phase at 83°C and SmA phase at 104°C, that is, 85°C and 116°C for compound 2.9. The clear-
ing point temperature of both compounds I10 and II10 are higher than that of compounds
2.5 and 2.9. This due the fact that upper transition points for 6-n-alkoxy-2-naphthoic acids
are higher than those for p-n-alkoxybenzoic acids with the same alkyl groups, by an average
of 47°C, therefore, 6-n-alkoxy-2-naphthoic acids are more mesomorphic. Thus, despite their
˚
smaller molecular breadths (6.8 A) the benzoic acids are less mesomorphic than the naph-
˚
˚
thoic acids (7.9 A). Moreover, the molecules of the naphthoic acids are longer by 2.2 A, but
unlike the increases in a homologous series, this greater length results from the presence of

44
N. J. CHOTHANI ET AL.
the second aromatic ring of the naphthalene nucleus. This will contribute more to the inter-
molecular cohesion than a single benzene ring and so enhances the thermal stability. Due to
above reason, the compounds I10 and II10 have higher smectic thermal stability compare with
its benzene analogue compounds 2.5 and 2.9.
Texture study
The optical microscopy studies were carried out with a “NICON ECLIPSE 50i POL” (Japan)
equipped with LinkamAnalysa-LTS 420 hot stage. The textures of the compounds were
observed using polarized light with crossed polarizers with sample in thin film sandwiched
between a glass slide and cover slip. The textures of compounds of series I and II are shown in
Fig. 5. Compound I7 shows nematic phase on cooling the isotropic at 140.5°C. The nematic
phase was confirmed by the appearance of a planar Schlieren texture or a homeotropic dark
texture observed under polarized light microscopy. Compound II16 shows focal conic fan-
like texture of SmA phase on cooling the isotropic liquid at 95.6°C, on further cooling the
SmA phase displayed broken fan-shape texture of SmC phase at 72.8°C. Compound II6 shows
droplet nematic phase on cooling the isotropic at 139.5°C. Compound II16 shows simple fan-
like texture of SmA phase on cooling the isotropic at 76.4°C and also displayed characteristic
broken fan-like texture of SmC phase at 92.1°C.
Conclusion
We have synthesized and studied the thermal behavior of an altogether new set of rod-like
molecules, in which the mesogenic (6-alkoxy-2-naphthoic acid) core is combined covalently
with the 1,3,4-oxadiazole segment through a ester linkage. The length of thioalkyl chain, as
well as the length of the alkoxy tail were varied to understand the structure-property correla-
tion. It is clear that the length of the alkoxy chain had an influence, not only on the nature of
the mesophases but also on the mesomorphic temperature ranges. Tables 1 and 2 illustrate the
transition temperatures of compounds of series I and II during heating, as a function of the
number of methylenic units (n) in the alkoxy chain. In general, an increase in terminal length
resulted in enhanced dipole-dipole interaction between the terminal chains, leading to sta-
bility of the mesophasesin rod-like mesogens. The length of thioalkyl chain is varied in both
series. With lower length of thioalkyl chain, in series I, shows not only higher clearing point
with wider mesophase range, with longer the thioalkyl chain shows lower in clearing point,
but also having much considerable mesophase range. However, the presence of nitrogen, sul-
fur, or oxygen atoms in these heterocyclic rings [59], which probably introduce a transverse
dipole moment, often, result in a change in dielectric anisotropy. The formation of liquid
crystallinity in such heterocyclic compounds might be facilitated by weak π-π interaction
between these aromatic or heterocyclic rings.
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