Synthesis, characterization of new carboxylic acid derivatives bearing 1,3,4-thiadiazole moiety and study their liquid crystalline behaviors

Article abstract of DOI:10.1080/15421406.2016.1266881

Thirteen carboxylic acid derivatives containing 1,3,4-thiadiazole ring in their core and swinging alkoxy terminal were synthesized. They were characterized by ¹H, ¹³C NMR spectroscopy, FTIR, and mass spectrometry. Their liquid crystalline behaviors have been studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The 1,3,4-thiadiazole compounds in this study were 2,5 di-substituted asymmetrical, alkoxy, and carboxy linkages. The compounds with alkoxy of long carbon chains (n > 7) displayed Smectic C phase. The liquid crystal properties were found to be affected by the length of alkoxy chain attached to the phenyl moiety and the two types of the dimeric form were resulted from the hydrogen bonding interaction between carboxylic acid molecules.

Full text of DOI:10.1080/15421406.2016.1266881

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis, characterization of new carboxylic acid
derivatives bearing 1,3,4-thiadiazole moiety and
study their liquid crystalline behaviors
Hamed J. Jaffer, Yasmeen A. Aldhaif & Ivan H. R. Tomi
To cite this article: Hamed J. Jaffer, Yasmeen A. Aldhaif & Ivan H. R. Tomi (2017) Synthesis,
characterization of new carboxylic acid derivatives bearing 1,3,4-thiadiazole moiety and study
their liquid crystalline behaviors, Molecular Crystals and Liquid Crystals, 643:1, 199-215, DOI:
10.1080/15421406.2016.1266881
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1266881
Published online: 23 Feb 2017.
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Date: 23 February 2017, At: 22:51

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Synthesis, characterization of new carboxylic acid derivatives
bearing ,,-thiadiazole moiety and study their liquid
crystalline behaviors
Hamed J. Jaffer, Yasmeen A. Aldhaif, and Ivan H. R. Tomi
College of Science, Department of Chemistry, University of Al-Mustansiriyah, Baghdad, Iraq
ABSTRACT
KEYWORDS
,,-Thiadiazole mesogens;
carboxylic acids; dimers;
liquid crystals
Thirteen carboxylic acid derivatives containing 1,3,4-thiadiazole ring in
their core and swinging alkoxy terminal were synthesized. They were
1
characterized by H, 13C NMR spectroscopy, FTIR, and mass spectrom-
etry. Their liquid crystalline behaviors have been studied by differen-
tial scanning calorimetry (DSC) and polarized optical microscopy (POM).
The 1,3,4-thiadiazole compounds in this study were 2,5 di-substituted
asymmetrical, alkoxy, and carboxy linkages. The compounds with alkoxy
of long carbon chains (n > 7) displayed Smectic C phase. The liquid
crystal properties were found to be affected by the length of alkoxy
chain attached to the phenyl moiety and the two types of the dimeric
form were resulted from the hydrogen bonding interaction between
carboxylic acid molecules.
Introduction
Mesogenic compounds derived from heterocyclic rings have been prepared by many scientific
groups due to their wide range in optical, medicinal, and biological applications [1]. 1,3,4-
Thiadiazole core is one of heterocycle materials that has a great importance in designing the
molecules that may possess liquid crystals property, especially in expansion the thermotropic
calamitic materials [2]. The chemical geometry of 1,3,4-thiadiazole derivatives makes them
suitable in different material applications because of their chemical and thermal stabilities [3].
The presence of three heteroatoms (two nitrogen and one sulfur atoms) in 1,3,4-thiadiazole
moiety can influence the polarity and polarizibility of the molecules [4], therefore, 1,3,4-
Thiadiazole ring is regarded as one of the sulfur–nitrogen heterocycles use as core in vari-
ety of mesogenic materials [5–7], this has opened an interesting possibility for designing new
mesogens containing thiadiazole ring. Many published results demonstrated on calamitic,
hydrogen-bonded, polycatenars, bent and star-shaped mesogens bearing 1,3,4-thiadiazole
unit [8–12].
On the other hand, intermolecular interactions are essential factor in the liquid crystal field
responsible for arranging the order of mesogenic superstructures. These forces participate in
making rigid cores in rode-like molecules [13]. The hydrogen-bonding network is one of the
important noncovalent interaction essential to design super-molecules that are constructed
CONTACT Ivan H. R. Tomi
ivanhrtomy@uomustansiriyah.edu.iq
sity of Al-Mustansiriyah, Baghdad, Iraq.
Department of Chemistry, College of Science, Univer-
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

200
H. J. JAFFER ET AL.
in the direction of molecular long axes [14]. The first examples concerning the effect of the
hydrogen-bonding interaction on calamitic mesogens were investigated earlier by Jones [15].
Recently, there are many reports for synthesis of various liquid crystals compounds that are
depends on noncovalent interactions [16–19].
The dimerization of carboxylic acids through hydrogen bonding creates supermolecular
structures which frequently used as mesogenic materials [20]. As we did not found any stud-
ies reported on the liquid crystal properties of carboxylic acid compounds incorporating with
1,3,4-thiadiazole derivatives. Therefore and as a part of our interests in the mesomorphic
properties of heterocyclic liquid crystals, a series of substituted 1,3,4-thiadiazole compounds
with alkoxy and carboxy linkages were synthesized and characterized in this study, their liquid
crystalline properties were studied and the relationship between the structure and mesomor-
phic property, the hydrogen bonding interaction was discussed.
Experimental
General
All starting materials in this study were received from Sigma–Aldrich and used as supplied.
Compounds (A, B, E, Fn and Gn) in this study were prepared according to the procedures in
our previous reference with a few simple modifications [21]. FTIR spectra of all samples in
ATR technique were recorded on Shimadzu 8400S spectrometer. 1H and 13C (NMR) spectra
were measured on Bruker ultra-shield at (300 MHz for proton and 100 MHz for carbon) with
chemical shifts (ppm scale) relative to tetramethylsilane as an internal standard (δ = 0 ppm)
using CDCl3 and DMSO-d6 as solvents. (SCI-MS) of some compounds were recorded on Shi-
madzu QP1000EX. Mesophase textures of series (Fn and Gn) were studied using a polarizing
optical microscope (POM) (PW-BK 5000 PR) equipped with a hot-stage (HS-400). Transition
temperatures, enthalpies, and entropies were determined from differential scanning calorime-
try (DSC) (Linseis instrument STA PT-1000), in 5°C/min heating and cooling rates.
Methods
Synthesis of -methoxy-N’-(-methylbenzoyl)benzohydrazide (C)
The titled compound was prepared according to the previously reported method [22] with
some modifications. To a round bottomed flask (250 mL) containing (0.12 mol, 18 g) of 4-
toluic hydrazide (B) dissolved in (100 mL) of anhydrous pyridine, 4-methoxybenzoyl chlo-
ride (0.12 mol, 20.5 g) was added dropwise at −5°C. The reaction mixture was left overnight
at room temperature. The product was poured into a cold (250 mL) of 10% HCl to give a light
yellow solid. The crude product was boiled in water for 15 min—to dissolve any unreacted
materials (compound B and 4-methoxybenzoyl chloride). The residue was purified by recrys-
tallization from ethanol to give the desired product. Yield (91%); mp: 210–212°C; FTIR (ATR,
1
cm−1), νmax: (3207, 3151, 2NH), (2943, 2845, C–H aliph.), (1614, C O); HNMR (DMSO-
=
d6), δ, ppm: 10.20, 10.17 (2×s, 2H, 2NH), 7.75–7.78 (d, 2H, Ar–H, -ph–CH3, J = 8.76 Hz),
7.60–7.63 (d, 2H, Ar–H, -ph–OCH3, J = 8.03 Hz), 7.15–7.18 (d, 2H, Ar–H, -ph–OCH3, J =
8.00 Hz), 6.88–6.91 (d, 2H, Ar–H, -ph–CH3, J = 8.80 Hz), 3.68 (s, 3H, OCH3), 2.35 (s, 3H,
CH3); 13CNMR (DMSO-d6), δ, ppm: 166.24, 165.80, 162.47, 142.25, 130.32, 129.81, 129.48,
127.93, 125.22, 114.19, 55.86, 53.16, 21.49, 20.09; MS (SCI) (relative intensity %): m/z = 284
(M+, 100%).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
201
Synthesis of -(-methoxyphenyl)--(p-tolyl)-,,-thiadiazole (D=F)
This compound was prepared by the procedure reported by [23]. Compound C (0.087 mol,
25 g) and phosphorouspenta sulfide (0.087 mol, 24.9) were dissolved in pyridine (150 mL) in a
(250 mL) round bottomed flask. The mixture was refluxed for 24 hr and the crude was poured
in (500 mL) cold water. The resulted precipitate was filtered and washed with excess of water
and recrystallized from ethyl acetate to obtain the product D = F1. Yield (87%); FTIR (ATR,
1
cm−1), νmax: (3066, C–H arom.), (2964, 2837, C–H aliph.), (1602, C N); HNMR (CDCl3),
=
δ, ppm: 7.91–7.94 (d, 2H, Ar–H, -ph–CH3, J = 8.71 Hz), 7.85–7.88 (d, 2H, Ar–H, -ph–OCH3,
J = 7.97 Hz), 7.26–7.28 (d, 2H, Ar–H, -ph–OCH3, J = 7.88 Hz), 6.96–6.99 (d, 2H, Ar–H, -ph–
CH3, J = 8.71 Hz), 3.86 (s, 3H, OCH3), 2.40 (s, 3H, CH3); 13CNMR (CDCl3), δ, ppm: 167.47,
167.45, 163.27, 161.80, 141.33, 129.79, 129.40, 127.73, 127.62, 127.57, 122.94, 114.49, 98.11,
96.77, 63.80, 21.50; MS (SCI) (relative intensity %): m/z = 282 (M+, 97%).
Synthesis of -(-(p-tolyl)-,,-thiadiazol--yl)phenol (E)
Compound (D=F1) (0.0024 mol, 0.68 g) was dissolved in dry benzene (25 mL) and
(0.0075 mol, 1.00 g) of anhydrous aluminum chloride was added. The reaction mixture was
refluxed for 24 hr. The solvent was evaporated and the residue was poured into water. The
resulted solid was filtered then purified by dissolving it in (30 mL) of 10% potassium hydrox-
ide solution. The solution was filtered and the filtrate was neutralized with 10% hydrochloride
acid. The formed precipitate was washed with water, dried, and recrystallized from ethanol to
give compound (E). Yield (81%); mp: 221–223°C; FTIR (ATR, cm−1), νmax: (3111, broad O–
1
=
H), (3045, C–H arom.), (2966, 2850, C–H aliph.), (1608, C N); HNMR (DMSO-d6), δ, ppm:
10.09 (s, 1H, OH), 7.64–7.66 (d, 4H, Ar–H, -ph–CH3 and –ph–OH, J = 8.43 Hz), 7.13–7.16
(d, 2H, Ar–H, -ph–CH3, J = 8.17 Hz), 6.79–6.77 (d, 2H, Ar–H, –ph–OH, J = 8.65 Hz), 2.18
(s, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 167.84, 166.88, 160.85, 141.57, 130.37, 130.11,
129.84, 127.81, 127.46, 121.04, 116.65, 98.22, 96.34, 63.99, 21.44; MS (SCI) (relative intensity
%): m/z = 268 (M+, 100%).
Synthesis of -(-alkoxyphenyl)--(p-tolyl)-,,-thiadiazole (Fn, n = –,, and )
To a solution of compound (E) (0.01 mol, 2.68 g) in acetone (50 mL), anhydrous potassium
carbonate (0.01 mol, 1.38 g) was added. The n-alkyl bromide (0.011 mol) was added to the
mixture dropwise followed by refluxing the reaction mixture for 12 hr. After cooling, the crude
product was added to cold water. The resulted solid was filtered, washed with 5% aqueous
potassium hydroxide and with distilled water several times. The solid products were dried
and recrystallized from ethyl acetate.
-(-Ethoxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (85%); FTIR (ATR, cm−1), νmax: (3080, C–H arom.), (2982, 2866, C–H aliph.), (1605,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.92–7.95 (d, 2H, Ar–H, -ph–CH3, J = 8.75 Hz), 7.88–7.90
(d, 2H, Ar–H, -ph–OR2, J = 7.95 Hz), 7.25–7.27 (d, 2H, Ar–H, -ph–OR2, J = 7.91 Hz), 6.97–
7.00 (d, 2H, Ar–H, -ph–CH3, J = 8.73 Hz), 4.06–4.02 (q, 2H, OCH2), 2.42 (s, 3H, CH3), 1.45–
1.41 (t, 3H, -OCH2-CH3); 13CNMR (CDCl3), δ, ppm: 167.57, 167.41, 163.02, 161.26, 141.34,
129.81, 129.43, 127.77, 127.65, 127.61, 122.80, 114.99, 98.00, 96.69, 63.73, 21.16, 14.73; MS
(SCI) (relative intensity %): m/z = 296 (M+, 94%).
-(-Propoxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (84%); FTIR (ATR, cm−1), νmax: (3066, C–H arom.), (2964, 2866, C–H aliph.),
1
=
(1602, C N); HNMR (CDCl3), δ, ppm: 7.89–7.91 (d, 2H, Ar–H, -ph-CH3, J = 8.75 Hz),

202
H. J. JAFFER ET AL.
7.85–7.87 (d, 2H, Ar–H, -ph–OR3, J = 7.93 Hz), 7.26–7.27 (d, 2H, Ar–H, -ph–OR3, J =
7.91 Hz), 6.95–6.97 (d, 2H, Ar–H, -ph–CH3, J = 8.74 Hz), 4.01–3.97 (t, 2H, OCH2), 2.42
(s, 3H, CH3), 1.90–1.88 (m, 2H, CH2) 1.09–1.04 (t, 3H, -O(CH2)2–CH3); 13CNMR (CDCl3),
δ, ppm: 167.59, 167.39, 161.47, 161.27, 141.33, 129.81, 129.42, 127.76, 127.65, 127.60, 122.75,
115.02, 99.99, 88.69, 67.32, 22.50, 21.52, 10.49.
-(-Butoxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (88%); FTIR (ATR, cm−1), νmax: (3068, C–H arom.), (2953, 2868, C–H aliph.), (1600,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.91–7.93 (d, 2H, Ar–H, -ph–CH3, J = 8.76 Hz), 7.86–
7.88 (d, 2H, Ar–H, -ph–OR4, J = 7.90 Hz), 7.27–7.30 (d, 2H, Ar–H, -ph–OR4, J = 7.92 Hz),
6.97–6.99 (d, 2H, Ar–H, -ph–CH3, J = 8.77 Hz), 3.97–4.05 (t, 2H, OCH2), 2.41 (s, 3H,
CH3), 1.82–1.85 (m, 2H, CH2), 1.48–1.61 (m, 2H, CH2), 1.02–0.96 (t, 3H, -O(CH2)3-CH3);
13CNMR (CDCl3), δ, ppm: 167.56, 167.40, 161.47, 161.31, 141.34, 129.82, 129.77, 129.42,
127.77, 127.64, 122.72, 115.00, 100.00, 87.98, 67.95, 31.20, 21.54, 19.23, 13.86; MS (SCI) (rel-
ative intensity %): m/z = 324 (M+, 100%).
-(-Pentyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (89%); FTIR (ATR, cm−1), νmax: (3067, C–H arom.), (2958, 2870, C–H aliph.), (1602,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.92–7.94 (d, 2H, Ar–H, -ph–CH3, J = 8.73 Hz), 7.84–
7.87 (d, 2H, Ar–H, -ph-OR5, J = 7.96 Hz), 7.26–7.28 (d, 2H, Ar–H, -ph–OR5, J = 7.94 Hz),
6.95–6.98 (d, 2H, Ar–H, -ph–CH3, J = 8.74 Hz), 3.98–4.03 (t, 2H, OCH2), 2.42 (s, 3H, CH3),
1.81–1.84 (m, 2H, CH2), 1.42–1.57 (m, 4H, 2×CH2), 0.88–0.96 (t, 3H, -O(CH2)4–CH3);
13CNMR (CDCl3), δ, ppm: 167.61, 167.40, 161.48, 161.33, 141.33, 129.82, 129.42, 129.37,
128.03, 127.77, 122.72, 114.96, 99.98, 87.00, 68.28, 31.58, 29.13, 22.57, 21.53, 14.05; MS (SCI)
(relative intensity %): m/z = 338 (M+, 91%).
-(-Hexyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (80%); FTIR (ATR, cm−1), νmax: (3064, C–H arom.), (2961, 2862, C–H aliph.), (1601,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.95–7.97 (d, 2H, Ar–H, -ph–CH3, J = 8.76 Hz), 7.84–
7.86 (d, 2H, Ar–H, -ph–OR6, J = 7.92 Hz), 7.23–7.25 (d, 2H, Ar–H, -ph–OR6, J = 7.96 Hz),
6.95–6.97 (d, 2H, Ar–H, -ph–CH3, J = 8.79 Hz), 3.98–4.02 (t, 2H, OCH2), 2.40 (s, 3H, CH3),
1.78–1.82 (m, 2H, CH2), 1.40–1.59 (m, 6H, 3×CH2), 0.95–0.89 (t, 3H, -O(CH2)5–CH3);
13CNMR (CDCl3), δ, ppm: 167.62, 167.44, 161.46, 161.30, 141.30, 129.87, 129.46, 129.41,
128.13, 127.81, 122.76, 114.99, 99.96, 87.05, 68.31, 31.62, 29.13, 28.84, 22.62, 21.55, 14.08.
-(-Heptyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (83%); FTIR (ATR, cm−1), νmax: (3060, C–H arom.), (2962, 2852, C–H aliph.), (1603,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.93–7.95 (d, 2H, Ar–H, -ph–CH3, J = 8.79 Hz), 7.85–
7.88 (d, 2H, Ar–H, -ph–OR7, J = 7.94 Hz), 7.26–7.29 (d, 2H, Ar–H, -ph–OR7, J = 7.99 Hz),
6.96–6.99 (d, 2H, Ar–H, -ph–CH3, J = 8.76 Hz), 3.99–4.05 (t, 2H, OCH2), 2.41 (s, 3H, CH3),
1.76–1.79 (m, 2H, CH2), 1.45–1.62 (m, 8H, 4×CH2), 0.96–0.87 (t, 3H, -O(CH2)6–CH3);
13CNMR (CDCl3), δ, ppm: 167.67, 167.56, 161.43, 161.37, 141.35, 129.81, 129.41, 129.38,
128.10, 127.90, 122.83, 114.83, 99.91, 87.09, 68.37, 31.65, 29.17, 29.77, 28.89, 22.57, 21.54,
14.03; MS (SCI) (relative intensity %): m/z = 366 (M+, 100%).
-(-Octyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (90%); FTIR (ATR, cm−1), νmax: (3072, C–H arom.), (2972, 2853, C–H aliph.), (1604,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.91–7.94 (d, 2H, Ar–H, -ph–CH3, J = 8.75 Hz), 7.87–
7.90 (d, 2H, Ar–H, -ph–OR8, J = 7.97 Hz), 7.28–7.30 (d, 2H, Ar–H, -ph–OR8, J = 7.81 Hz),

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
203
6.97–6.99 (d, 2H, Ar–H, -ph–CH3, J = 8.69 Hz), 4.00–4.04 (t, 2H, OCH2), 2.43 (s, 3H, CH3),
1.78–1.81 (m, 2H, CH2), 1.37–1.59 (m, 10H, 5×CH2), 0.87–0.91 (t, 3H, -O(CH2)7–CH3);
13CNMR (CDCl3), δ, ppm: 167.73, 167.59, 161.39, 161.55, 149.72, 129.99, 129.55, 129.43,
128.23, 127.95, 122.73, 114.75, 99.96, 87.22, 68.41, 31.44, 29.62, 29.42, 28.69, 26.28, 22.34,
21.66, 14.29; MS (SCI) (relative intensity %): m/z = 380 (M+, 99%).
-(-Nonyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (86%); FTIR (ATR, cm−1), νmax: (3022, C–H arom.), (2955, 2852, C–H aliph.), (1603,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.90–7.92 (d, 2H, Ar–H, -ph–CH3, J = 8.77 Hz), 7.87–
7.89 (d, 2H, Ar–H, -ph–OR9, J = 7.95 Hz), 7.29–7.31 (d, 2H, Ar–H, -ph–OR9, J = 7.84 Hz),
6.96–6.98 (d, 2H, Ar–H, -ph–CH3, J = 8.73 Hz), 3.96–4.03 (t, 2H, OCH2), 2.42 (s, 3H, CH3),
1.79–1.82 (m, 2H, CH2), 1.34–1.60 (m, 12H, 6×CH2), 0.85–0.90 (t, 3H, -O(CH2)8–CH3);
13CNMR (CDCl3), δ, ppm: 168.01, 167.76, 161.65, 161.42, 149.55, 129.89, 129.72, 129.52,
128.53, 127.99, 122.82, 114.55, 99.86, 87.34, 68.54, 31.78, 29.79, 29.62, 28.58, 26.41, 25.89,
22.51, 21.45, 14.67.
-(-Decyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (91%); FTIR (ATR, cm−1), νmax: (3072, C-H arom.), (2964, 2849, C-H aliph.), (1602,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.91–7.93 (d, 2H, Ar–H, -ph–CH3, J = 8.74 Hz), 7.84–7.87
(d, 2H, Ar–H, -ph–OR10, J = 7.93 Hz), 7.27—7.30 (d, 2H, Ar–H, -ph–OR10, J = 7.83 Hz),
6.98–7.00 (d, 2H, Ar–H, -ph–CH3, J = 8.77 Hz), 3.97–4.04 (t, 2H, OCH2), 2.41 (s, 3H, CH3),
1.80–1.83 (m, 2H, CH2), 1.36–1.61 (m, 14H, 7×CH2), 0.83–0.89 (t, 3H, -O(CH2)9-CH3);
13CNMR (CDCl3), δ, ppm: 167.84, 167.72, 161.66, 161.54, 149.34, 129.91, 129.84, 129.61,
128.66, 128.00, 122.65, 114.67, 99.71, 87.32, 68.66, 31.54, 29.91, 29.81, 28.67, 26.71, 25.76,
25.41, 22.63, 21.54, 14.71; MS (SCI) (relative intensity %): m/z = 408 (M+, 89%).
-(-Dodecyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (87%); FTIR (ATR, cm−1), νmax: (3057, C–H arom.), (2961, 2848, C–H aliph.), (1604,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.90–7.93 (d, 2H, Ar–H, -ph–CH3, J = 8.73 Hz), 7.86–7.89
(d, 2H, Ar–H, -ph–OR12, J = 8.14 Hz), 7.26–7.29 (d, 2H, Ar–H, -ph–OR12, J = 8.29 Hz),
6.95–6.98 (d, 2H, Ar–H, -ph–CH3, J = 8.76 Hz), 3.98–4,03 (t, 2H, OCH2), 2.41 (s, 3H, CH3),
1.78–1.85 (m, 2H, CH2), 1.26–1.46 (m, 18H, 9×CH2), 0.85–0.90 (t, 3H, -O(CH2)11-CH3);
13CNMR (CDCl3), δ, ppm: 167.08, 166.87, 160.97, 160.39, 140.81, 129.31, 128.90, 127.25,
127.15, 122.22, 114.50, 98.90, 67.77, 31.43, 29.67, 29.17, 29.16, 29.11, 29.09, 28.90, 28.87, 28.66,
25.51, 22.21, 21.02, 15.50, 13.40.
-(-Hexadecyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (88%); FTIR (ATR, cm−1), νmax: (3061, C–H arom.), (2955, 2849, C-H aliph.), (1602,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.90–7.93 (d, 2H, Ar–H, -ph–CH3, J = 8.74 Hz), 7.87–7.90
(d, 2H, Ar–H, -ph–OR16, J = 7.93 Hz), 7.27–7.30 (d, 2H, Ar–H, -ph–OR16, J = 7.83 Hz),
6.96–6.99 (d, 2H, Ar–H, -ph–CH3, J = 8.77 Hz), 3.99–4.03 (t, 2H, OCH2), 2.42 (s, 3H, CH3),
1.79–1.83 (m, 2H, CH2), 1.25–1.47 (m, 26H, 13×CH2), 0.84–0.96 (t, 3H, -O(CH2)15–CH3);
13CNMR (CDCl3), δ, ppm: 167.10, 166.89, 160.97, 140.83, 129.31, 128.91, 128.74, 127.26,
127.15, 122.22, 114.51, 99.11, 67.77, 31.44, 30.95, 29.69, 29.21, 29.19, 29.17, 29.11, 29.08, 28.95,
28.88, 28.66, 28.29, 27.70, 25.51, 22.21, 21.02, 15.50, 13.40; MS (SCI) (relative intensity %):
m/z = 492 (M+, 93%).

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H. J. JAFFER ET AL.
-(-Octadecyloxyphenyl)--(p-tolyl)-,,-thiadiazole (F)
Yield (91%); FTIR (ATR, cm−1), νmax: (3059, C–H arom.), (2954, 2848, C–H aliph.), (1603,
1
=
C N); HNMR (CDCl3), δ, ppm: 7.89–7.92 (d, 2H, Ar–H, -ph–CH3, J = 8.73 Hz), 7.87–7.89
(d, 2H, Ar–H, -ph–OR18, J = 7.91 Hz), 7.27–7.29 (d, 2H, Ar–H, -ph–OR18, J = 7.86 Hz),
6.95–6.98 (d, 2H, Ar–H, -ph–CH3, J = 8.73 Hz), 3.98–4.03 (t, 2H, OCH2), 2.41 (s, 3H, CH3),
1.77–1.84 (m, 2H, CH2), 1.26–1.48 (m, 30H, 15×CH2), 0.93–0.98 (t, 2H, -O(CH2)17–CH3);
13CNMR (CDCl3), δ, ppm: 167.08, 166.91, 160.90, 140.79, 129.32, 128.88, 128.69, 127.24,
127.15, 122.21, 114.50, 99.22, 67.78, 31.41, 30.97, 29.66, 29.20, 29.17, 29.16, 29.12, 29.05, 28.97,
28.90, 28.63, 28.22, 27.71, 25.52, 22.24, 21.04, 15.54, 13.55.
Synthesis of -(-(-alkoxyphenyl)-,,-thiadiazol--yl)benzoic acid (Gn, n = –,,
and )
A solution of potassium permanganate, (0.02 mol, 3.33g) in (10 mL) distilled water was added
dropwise to a solution of compound D = F1 (as example of series Fn) (0.0025 mol, 0.77 g)
in (20 mL) pyridine at about 70°C with continuous stirring. The resulted violet solution was
heated under reflux for 24 h. After cooling, water (25 mL) was added to the crude product
which was slightly warmed and filtered. The resulted solution was acidified with (2 M HCl)
and the precipitated carboxylic acid (Gn) was filtered, washed with water and dried. Note: use
a minimum amount of water in case (n > 6).
-(-(-Methoxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (93%); mp: > 350°C; FTIR (ATR, cm−1), νmax: (3112–2546, broad O–H), (2947, 2845,
1
=
=
C–H aliph.), (1683, C O), (1604, C N); HNMR (DMSO-d6), δ, ppm: 13.32–13.43 (s, 1H,
br., COOH), 7.91–7.97 (2×d, br., 4H, Ar–H, -ph-COOH), 7.81–7.84, J = 7.95 and 6.97–6.99,
J = 7.81 (2×d, 4H, Ar–H, -ph-OCH3), 3.94 (s, 3H, CH3O); 13CNMR (DMSO-d6), δ, ppm:
196.51, 168.18, 166.81, 165.99, 161.33, 154.44, 133.71, 130.41, 130.26, 130.09, 129.55, 127.99,
127.67, 121.71, 115.42, 69.44; MS (SCI) (relative intensity %): m/z = 312 (M+, 100%).
-(-(-Ethoxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (83%); mp: > 350°C; FTIR (ATR, cm−1), νmax: (3091–2548, broad O–H), (2980, 2841,
1
=
=
C–H aliph.), (1685, C O), (1607, C N); HNMR (DMSO-d6), δ, ppm: 13.25–13.46 (s, 1H,
br., COOH), 8.09–8.13 (2×d, br., 4H, Ar–H, -ph–COOH), 7.96–7.99, J = 7.17 and 7.11–
7.14, J = 6.96 (2×d, 4H, Ar–H, -ph-OR2), 4.12–4.15 (q, 2H, CH2O), 1.37–1.39 (t, 3H, CH3);
13CNMR (DMSO-d6), δ, ppm: 196.21, 167.98, 167.31, 165.91, 161.72, 149.94, 133.69, 130.41,
130.29, 130.23, 129.43, 127.81, 127.60, 121.87, 115.34, 63.53, 21.18; MS (SCI) (relative inten-
sity %): m/z = 315 (M+, 89%).
-(-(-Propoxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (83%); mp: > 350°C; FTIR (ATR, cm−1), νmax: (3102–2558, broad O–H), (2964, 2872,
1
=
=
C–H aliph.), (1685, C O), (1606, C N); HNMR (DMSO-d6), δ, ppm: 13.29–13.44 (s, 1H,
br., COOH), 8.14–8.12 (2×d, br., 4H, Ar–H, -ph–COOH), 7.97–8.00, J = 6.91 and 7.13–
7.16, J = 6.67 (2×d, 4H, Ar–H, -ph–OR3), 4.03–4.05 (t, 2H, CH2O), 1.75–1.79 (m, 2H, CH2),
0.99–1.03 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 195.87, 168.19, 166.88, 165.91, 161.30,
154.57, 133.65, 130.28, 130.24, 130.13, 129.42, 127.96, 127.61, 121.69, 115.36, 69.32, 21.89,
10.29.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
205
-(-(-Butoxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (79%); mp: > 350°C; FTIR (ATR, cm−1), νmax: (3099–2544, broad O–H), (2956, 2872,
1
=
=
C–H aliph.), (1684, C O), (1605, C N); HNMR (DMSO-d6), δ, ppm: 13.29–13.37 (s, 1H,
br., COOH), 8.09–8.12 (2×d, br., 4H, Ar–H, -ph-COOH), 7.96–7.99, J = 8.63 and 7.12–7.15,
J = 8.72 (2×d, 4H, Ar–H, -ph–OR4), 4.06–4.10 (t, 2H, CH2O), 1.71–1.81 (m, 2H, CH2), 1.42–
152 (m, 2H, CH2), 0.93–0.98 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 196.41, 168.23,
166.60, 165.89, 161.33, 154.65, 133.14, 130.28, 130.21, 130.13, 129.66, 129.22, 127.81, 127.54,
121.68, 115.37, 71.74, 30.56, 18.65, 13.64.
-(-(-Pentyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (81%); mp: > 350°C; FTIR (ATR, cm−1), νmax: (3101–2557, broad O–H), (2961, 2857,
1
=
=
C–H aliph.), (1683, C O), (1604, C N); HNMR (DMSO-d6), δ, ppm: 13.32–13.47 (s, 1H,
br., COOH), 8.08–8.11 (2×d, br., 4H, Ar–H, -ph-COOH), 7.97–7.99, J = 6.90 and 7.13–7.16,
J = 7.14 (2×d, 4H, Ar–H, -ph–OR5), 4.05–4.09 (t, 2H, CH2O), 1.73–1.83 (m, 2H, CH2), 1.22–
143 (m, 4H, 2×CH2), 0.87–0.93 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 195.77, 168.67,
166.55, 165.71, 161.33, 155.11, 133.21, 130.33, 130.19, 130.02, 129.45, 129.32, 127.69, 127.44,
121.64, 115.37, 67.86, 30.94, 28.47, 22.03, 16.56.
-(-(-Hexyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (77%); mp: > 350°C; FTIR (ATR, cm−1), νmax: (3098–2555, broad O-H), (2953, 2854,
1
=
=
C-H aliph.), (1684, C O), (1604, C N); HNMR (DMSO-d6), δ, ppm: 13.35–13.42 (s, 1H,
br., COOH), 8.11–8.14 (2×d, br., 4H, Ar-H, -ph-COOH), 8.01–8.04, J = 6.97 and 7.10–7.13,
J = 7.01 (2×d, 4H, Ar-H, -ph-OR6), 4.02–4.07 (t, 2H, CH2O), 1.75–1.84 (m, 2H, CH2), 1.21–
141 (m, 6H, 3×CH2), 0.89–0.95 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 194.61, 168.91,
166.81, 165.77, 161.54, 154.22, 133.26, 130.31, 130.25, 130.11, 129.43, 129.21, 127.69, 127.41,
117.26, 115.36, 67.85, 30.87, 28.65, 28.20, 23.01, 15.87; MS (SCI) (relative intensity %): m/z =
382 (M+, 71%).
-(-(-Heptyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (86%); mp: > 350°C; FTIR (ATR, cm−1), νmax: (3101–2556, broad O–H), (2953, 2855,
1
=
=
C–H aliph.), (1683, C O), (1605, C N); HNMR (DMSO-d6), δ, ppm: 13.32–13.39 (s, 1H,
br., COOH), 8.10–8.13 (2×d, br., 4H, Ar–H, -ph–COOH), 7.97–7.99, J = 8.84 and 7.13–7.15,
J = 8.84 (2×d, 4H, Ar–H, -ph–OR7), 4.05–4.10 (t, 2H, CH2O), 1.73–1.78 (m, 2H, CH2), 1.24–
143 (m, 8H, 4×CH2), 0.86–0.90 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 193.91, 169.01,
167.11, 165.88, 161.29, 154.19, 133.21, 130.45, 130.31, 130.09, 129.66, 129.34, 127.73, 127.43,
127.19, 115.42, 67.88, 30.78, 28.61, 28.43, 28.26, 23.02, 15.91; MS (SCI) (relative intensity %):
m/z = 396 (M+, 94%).
-(-(-Octyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (81%); FTIR (ATR, cm−1), νmax: (3101–2556, broad O–H), (2953, 2855, C–H aliph.),
1
=
=
(1699, 1683, C O), (1605, C N); HNMR (DMSO-d6), δ, ppm: 13.43–13.55 (s, 1H, br.,
COOH), 8.10–8.13 (2×d, br., 4H, Ar–H, -ph–COOH), 7.96–7.99, J = 8.75 and 7.12–7.15, J
= 8.83 (2×d, 4H, Ar–H, -ph–OR8), 3.99–4.05 (t, 2H, CH2O), 1.74–1.79 (m, 2H, CH2), 1.25–
1.41 (m, 10H, 5×CH2), 0.84–0.89 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 192.00, 168.76,
166.57, 165.71, 161.45, 154.27, 133.27, 130.42, 130.32, 130.11, 129.45, 129.22, 127.81, 127.65,
127.22, 114.99, 67.43, 30.81, 28.76, 28.59, 28.39, 28.19, 23.12, 15.78.

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H. J. JAFFER ET AL.
-(-(-Nonyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (88%); FTIR (ATR, cm−1), νmax: (3088–2551, broad O–H), (2955, 2852, C–H aliph.),
1
=
=
(1701, 1683, C O), (1605, C N); HNMR (DMSO-d6), δ, ppm: 13.01–13.12 (s, 1H, br.,
COOH), 8.09–8.12 (2×d, br., 4H, Ar–H, -ph–COOH), 7.96–7.99, J = 7.59 and 7.12–7.15, J
= 7.75 (2×d, 4H, Ar–H, -ph–OR9), 4.01–4.07 (t, 2H, CH2O), 1.72–1.77 (m, 2H, CH2), 1.23–
1.40 (m, 12H, 6×CH2), 0.82–0.87 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 194.82, 168.79,
166.66, 165.61, 161.39, 154.02, 133.67, 130.39, 130.31, 130.03, 129.38, 129.21, 127.82, 127.71,
127.32, 115.01, 67.44, 30.74, 28.83, 28.66, 28.53, 28.32, 28.11, 24.11, 14.98.
-(-(-Decyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (90%); FTIR (ATR, cm−1), νmax: (3092–2549, broad O–H), (2955, 2853, C–H aliph.),
1
=
=
(1701, 1684, C O), (1605, C N); HNMR (DMSO-d6), δ, ppm: 13.28–13.39 (s, 1H, br.,
COOH), 8.09–8.12 (2×d, br., 4H, Ar–H, -ph–COOH), 7.97–8.00, J = 7.63 and 7.10–7.13,
J = 7.66 (2×d, 4H, Ar–H, -ph–OR10), 4.00–4.06 (t, 2H, CH2O), 1.74–1.79 (m, 2H, CH2),
1.26–1.38 (m, 14H, 7×CH2), 0.83–0.89 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 194.71,
168.88, 166.67, 165.43, 161.43, 154.12, 133.56, 130.41, 130.33, 130.11, 129.41, 129.23, 127.99,
127.86, 127.43, 115.12, 67.34, 30.73, 28.83, 28.65, 28.63, 28.31, 28.22, 27.88, 24.11, 14.98.
-(-(-Dodecyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (85%); FTIR (ATR, cm−1), νmax: (3103–2561, broad O–H), (2948, 2851, C–H aliph.),
1
=
=
(1698, 1683, C O), (1604, C N); HNMR (DMSO-d6), δ, ppm: 13.35–13.44 (s, 1H, br.,
COOH), 8.11–8.14 (2×d, br., 4H, Ar–H, -ph–COOH), 7.98–8.01, J = 7.66 and 7.08–7.11, J =
7.69 (2×d, 4H, Ar–H, -ph–OR12), 3.98–4.03 (t, 2H, CH2O), 1.77–1.81 (m, 2H, CH2), 1.25–
1.40 (m, 18H, 9×CH2), 0.80–0.86 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 192.92, 168.75,
166.71, 165.33, 161.56, 154.23, 133.61, 130.55, 130.29, 130.12, 129.40, 129.24, 127.87, 127.72,
127.34, 115.42, 67.66, 30.67, 28.91, 28.88, 28.65, 28.31, 28.22, 28.00, 27.88, 27.83, 24.21, 14.67;
MS (SCI) (relative intensity %): m/z = 466 (M+, 83%).
-(-(-Hexadecyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (83%); FTIR (ATR, cm−1), νmax: (3098–2544, broad O–H), (2953, 2848, C–H aliph.),
1
=
=
(1699, 1685, C O), (1605, C N); HNMR (DMSO-d6), δ, ppm: 13.43–13.51 (s, 1H, br.,
COOH), 8.06–8.08 (2×d, br., 4H, Ar-H, -ph-COOH), 7.99–8.02, J = 7.53 and 7.08–7.10, J =
7.62 (2×d, 4H, Ar-H, -ph-OR16), 3.96–4.00 (t, 2H, CH2O), 1.76–1.80 (m, 2H, CH2), 1.23–1.39
(m, 26H, 13×CH2), 0.83–0.88 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 193.12, 168.82,
166.76, 165.53, 161.88, 154.32, 133.60, 130.41, 130.33, 130.23, 129.37, 129.19, 127.65, 127.54,
127.32, 115.32, 67.12, 30.54, 28.95, 28.81, 28.78, 28.63, 28.41, 28.32, 28.20, 27.81, 27.65, 27.43,
26.12, 24.33, 15.71; MS (SCI) (relative intensity %): m/z = 522 (M+, 76%).
-(-(-Octadecyloxyphenyl)-,,-thiadiazol--yl)benzoic acid (G)
Yield (78%); FTIR (ATR, cm−1), νmax: (3112–2557, broad O–H), (2955, 2849, C–H aliph.),
1
=
=
(1697, 1687, C O), (1604, C N); HNMR (DMSO-d6), δ, ppm: 13.42–13.53 (s, 1H, br.,
COOH), 8.10–8.13 (2×d, br., 4H, Ar–H, -ph–COOH), 8.00–8.03, J = 7.79 and 7.10–7.13,
J = 7.83 (2×d, 4H, Ar–H, -ph–OR18), 3.94–3.99 (t, 2H, CH2O), 1.72–1.77 (m, 2H, CH2),
1.26–1.41 (m, 30H, 15×CH2), 0.85–0.89 (t, 3H, CH3); 13CNMR (DMSO-d6), δ, ppm: 194.43,
168.71, 166.69, 165.61, 161.88, 155.12, 133.63, 130.45, 130.23, 130.19, 129.57, 129.37, 127.63,
127.51, 127.41, 114.97, 67.52, 30.42, 28.95, 28.81, 28.71, 28.69, 28.54, 28.40, 28.32, 28.18, 27.79,
27.60, 27.42, 26.32, 25.97, 24.46, 14.99.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
207
Results and discussion
Synthesis
The mesophase compounds, 4-(5-(4-alkoxyphenyl)-1,3,4-thiadiazole-2-yl)benzoic acid (Gn),
and their corresponding precursors were prepared by the synthetic route described in
Scheme 1.
Scheme . Reactions and reagents: (i) methanol, HSO; (ii) ethanol, NHNH.HO; (iii) pyridine, °C; (iv) PS,
pyridine; (v) AlCl, benzene; (vi) RBr, acetone; (vii) KMnO, pyridine, HO.
The chemical structures of the synthesized compounds in this study were confirmed on
the basis of their FTIR, 1HNMR, 13CNMR and some of them by mass spectroscopy.
Methyl p-toluate (A) was prepared in good yield according to the standard esterification
procedure of 4-methylbenzoic acid. The hydrazide compound (B) was prepared by the reac-
tion of compound (A) with hydrazine hydrate in ethanol. This compound was condensed
with anisoyl chloride in pyridine to form 4-methoxy-N’-(4-methylbenzoyl)benzohydrazide
(C). The two singlet signals in 1HNMR spectrum of compound (C) at 10.17 and 10.20 ppm
were corresponded for two protons of two NH groups in addition to amidic carbonyl in FTIR
are a good indicator for the formation of this compound. The 1,3,4-thiadiazole derivative (D)
was prepared by the sulfuration reaction of compound (C) using P2S5 in anhydrous pyridine,
the disappearance of amidic carbonyl groups in FTIR and two NH signals in 1HNMR spectra
of compound C are a good proof on cyclization reaction success. The compound (D) was con-
verted, with anhydrous aluminum chloride in dry benzene, to desired 2-(4-hydroxyphenyl)-
5-(4-methylphenyl)-1,3,4-thiadiazole (E). The 1HNMR spectrum of compound (E) showed
the appearance phenolic hydroxyl group in FTIR and at 10.09 ppm in 1HNMR is good proof

208
H. J. JAFFER ET AL.
that demethylation reaction was obtained. The homologous compounds of series (Fn, n =
2–10, 12, 16, and 18) were prepared by alkylation reaction of compound (E) with n-alkyl bro-
mide in acetone with the presence of anhydrous K2CO3. The disappearance of hydroxyl peak
of compound (E) and the appearance of alkyl signals in 1HNMR and FTIR spectra for com-
pounds (Fn) are clear index of success the alkylation reaction. The acids derivatives (Gn) were
prepared by oxidation of compounds (Fn) using pyridine–water mixture, the FTIR spectra of
these compounds displayed broad O–H stretching between 3090 and 2550 cm−1 and C O
=
stretching at 1680 cm−1 for compounds G1–G7, and two peaks at 1680 and 1700 cm−1 for
compounds G8–G10, G12, G16, and G18. Also, the 1HNMR spectra of the prepared carboxylic
acids showed weak broad peaks between 13.30 and 13.50 ppm which were assigned for pro-
tons of COOH groups in compounds Gn, Fig. 1 show 1HNMR spectra of compounds G3 and
G4 as examples of series Gn.
Mesomorphic properties
The liquid crystals properties of compounds of the two series, (Fn) and (Gn), were investigated
by polarizing optical microscopy (POM) and deferential scanning calorimetry (DSC). Thin
films of the samples were obtained by sandwiching them between a glass slide and cover slip.
Mesomorphic properties of compounds Fn
We have studied the mesomorphic properties of all compounds in series (Fn) (F1–8, F10, F12,
F16, and F18) using (POM) and (DSC) techniques. The results were in agreement with those
reported by Han et al. (24–25) (F1–8 and F10). The newly synthesized compounds (F9, F12,
F16, and F18) behaved as a thermotropic liquid crystal properties. On heating scan, the (DSC)
curves of compounds (F9) and (F12) displayed three endothermic peaks which were assigned
to (Cr–SmC), (SmC–N), and (N–I) transitions, also on cooling, three exothermic peaks were
observed which were assigned to (I–N), (N–SmC), and (SmC–Cr). The SmC and N phases of
compounds (F9) and (F12) were Schlieren texture type. In compounds F16 and F18, the nematic
phase was disappeared due to the increases in number of carbon atoms in the terminal alkyl
chains. The increasing in molecule length (F16 and F18) led to the formation of SmC only; this
was due to the enhanced of Van der Waals forces and dipole–dipole interaction between alkyl
chains [26]. The (DSC) thermographs of these compounds showed only two transitions on
first heating (Cr–SmC and SmC–I), and two peaks cooling assigned to (I–SmC and SmC–Cr)
transitions. The SmC phase of compounds (F16 and F18) was Schlieren texture type under opti-
cal microscope. The mesophase assignments according to (POM) observations are in good
agreement with the corresponding (DSC) curves. Selected (POM) images of some (Fn) meso-
gens obtained by heating or cooling and (DSC) curve of compound (F18) as an example of
these compounds are collectively showed in Fig. 2. The phase transition temperature (T/°C),
enthalpies (ꢀH/ KJ mol−1) and entropies (ꢀS/J mol−1 K−1) data of compounds (F9, F12, F16,
and F18) were listed in Table 1.
Mesomorphic properties of compounds Gn
The liquid crystalline behaviors of carboxylic acid derivatives (G1-G10, G12, G16, and G18)
were investigated by (POM) and (DSC) techniques. The first seven compounds of this series
(G1–G7) exhibited no liquid crystalline phases when heated up to 350°C on (POM). Higher

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
209

Figure . HNMR spectra of compounds G (up) and G (down).
melting points of these compounds (G1–G7) caused disappearance the liquid crystal charac-
ter; this was due to shorten of alkyl terminal softening groups. The rest of the series which
possess longer terminal alkoxy tail (n = 8–10, 12, 16, and 18) exhibited mesomorphic prop-
erties, the increases in the length of alkoxy tails led to increase interaction forces between
the molecules resulted as a mesophases behavior, particularly SmC phase typical of calamitic
liquid crystals [27]. Although these compounds showed SmC phase, however, they suffer

210
H. J. JAFFER ET AL.
Figure . (a) Transition from smectic C to nematic textures of compound F on heating at °C; (b) nematic
(Schlieren) texture of compound F on heating at °C; (c) smectic C texture of compound F on heating at
°C; (d) nematic droplets from cooling compound F at °C; (e) smectic C texture of compound F on
heating at °C transition; (f) DSC thermogram of compound F.
thermal decomposition started near 220°C which was very clear in (DSC) thermographs. This
phenomenon prevents us from further study of their liquid crystalline properties on cooling
from their melts.
The (DSC) thermographs of compounds (G8–G10, G12, G16, and G18) showed two or three
endothermic transitions that are represented to Cr–Cr or Cr–SmC phases, also, the (DSC)
curves showed exothermic transitions assigned to SmC–dec. on first heating scan, a further
heating above 220°C in (DSC) technique led to decomposition of the materials accompa-
nied by the loss of their liquid crystal properties. This phenomenon was confirmed by (POM)
darkening in SmC textures near this temperature. Figure 3 shows the (DSC) thermograms of
compounds (G8) and (G10) as representative examples of the studied series (Gn). Phase details
and transition temperatures of all compounds in series (Gn) are summarized in Table 2.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
211
Table . Phase transition temperatures (T/°C), phase transition enthalpy changes (ꢀH/ KJ mol−) and
entropies (ꢀS/ J mole− K−) of the compounds F, F, F, and F in first heating and cooling cycles.
Phase transitions T°C (ꢀH/ kJ
mol−) [ꢀS/J mole− K−] on
first heating
Phase transitions T°C (ꢀH/ kJ
mol−) [ꢀS/J mole− K−] on
first cooling
Compound
F
n

Cr–SmC . (.) [.]
SmC–N . (.) [.]
I–N . (−.) [−.]
N–SmC . (−.)
[−.]
N–I . (.) [.]
SmC–Cr . (−.)
[−.]
F

Cr–SmC . (.) [.]
SmC–N . (.) [.]
I–N . (−.) [−.]
N–SmC . (−.)
[−.]
N–I . (.) [.]
SmC–Cr . (−.)
[−.]
F
F


Cr–SmC . (.) [.]
SmC–I . (.) [.]
I–SmC . (−.) [−.]
SmC–Cr . (−.)
[−.]
Cr–SmC . (.) [.]
SmC–I . (.) [.]
I–SmC . (−.) [−.]
SmC–Cr . (−.)
[−.]
Abbreviations: Cr = crystal phase; SmC = smectic C phase; N = nematic phase; I = isotropic phase.
Figure . DSC thermograms of compounds G and G.
Table . Phase transition temperatures (T/°C), phase transition enthalpy changes (ꢀH/kJ mol−) and
entropies (ꢀS/J mole− K−) of the compounds G–G, G, G, and G in first heating cycles.
Compound
G
n
Phase transitions T°C (ꢀH/kJ mol−) [ꢀS/J mole− K−] on first heating

Cr–Cr . (.) [.]
Cr–Cr . (.) [.]
Cr–SmC . (.) [.]
Cr–Cr . (.) [.]
Cr–SmC . (.) [.]
Cr+Cr–SmC . (.) [.]
Cr–Cr . (.) [.]
Cr–SmC . (.) [.]
Cr–Cr . (.) [.]
Cr–SmC . (.) [.]
Cr–Cr . (.) [.]
Cr–SmC . (.) [.]
G

G
G


G
G


Abbreviations: Cr, Cr, Cr = crystal phases; SmC = smectic C phase.
The study by polarizing microscope upon heating, a schlieren texture of SmC was observed
for the last six compounds of series (Gn) (G8-G10, G12, G16, and G18). Figure 4 shows examples
of polarizing optical photomicrographs of some compounds of this series.
The FTIR spectra for acid derivatives in (Gn) series in the solid state at room tempera-
ture revealed that the first seven compounds are present as cyclic hydrogen-bonded dimers

212
H. J. JAFFER ET AL.
Figure . Smectic C textures of (a) compound G on heating at °C, (b) compound G on heating at °C,
(c) compound G on heating at °C, and (d) compound G on heating at °C.
(Scheme 2a). The stretching frequencies data of carbonyl bands of these compounds were
appeared near 1680 cm−1 which it evidence that these compounds are in this form. The rest
compounds of this series have the longer alkoxy terminal swinging groups (G8–G10, G12, G16,
and G18) are exist in two equilibrium dimeric forms (cyclic hydrogen-bonded dimer and open
Scheme . The schematic representation of H-bonded dimers in the compounds (G)n.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
213
Figure . FTIR spectra of compounds G (up) and G (down).
hydrogen-bonded dimer, Scheme 2b). The FTIR spectra of these compounds showed two fre-
quencies for carbonyl bands near 1680 and 1700 cm−1 confirming the suggested two equilib-
rium dimeric forms. These results are consistent with those reported by Martinez-Felipe et al.
[28, 29] and Paterson et al. [30]. Figure 5 shows examples of FTIR spectra of some compounds
in (Gn) series. It is clear that increasing the length of alkoxy group in these compounds derive
some molecules to prevent formation of cyclic hydrogen bonding dimer via their relatively
easy movement or swinging.
The hydrogen bonding represents one of the important factors that affect the liquid crys-
talline properties of the compound; therefore, new properties could be arising from the influ-
ence of such interaction between the molecules [31].
In this study, our results indicated that hydrogen bonding in compounds of series (Gn) is
greatly influences the appearance of mesomorphic properties of the last compounds in this
series. These properties could be resulted from the overall effect of the types of hydrogen
bonding between the molecules. The FTIR of the first seven compounds of this series revealed

214
H. J. JAFFER ET AL.
that these compounds exist in cyclic hydrogen-bonding dimer form only; therefore, they
have high melting points (> 350°C). The (POM) analysis of these compounds proves that
no of them is mesogen probably due to their existence in this form of dimerization, which is
more stable thermodynamically and higher melting points than the open hydrogen-bonded
dimer [32].
The increase in length of alkoxy terminal chains in last six derivatives of series might pre-
vent molecules from constructing cyclic dimer form by intramolecular hydrogen bonding and
force them to be in the equilibrium state between cyclic and open forms dimers. The proposed
equilibrium is reflected directly on the melting points and appearance the liquid crystalline
properties.
The limited difference in chemical structures between compounds of the two series, (Fn
and Gn), have a large impact on their liquid crystals properties which we can summarized
as follows: (a) all compounds of series Fn are mesogens while only last six compounds of
series (Gn) have liquid crystal properties; (b) all the compounds of series (Fn) displayed enan-
tiotropic liquid crystals with nematic and smectic C phases, while only compounds of series
(Gn) have alkoxy tail of (n > 7) exhibited smectic C phase; (c) the thermal stability of com-
pounds (Fn) is more than those of compounds (Gn) which are decompose around 220°C; (d)
the mesomorphic temperature ranges of all compounds in (Fn) series are wider than the last
six compounds in (Gn) series.
The main difference in chemical structures between compounds (Fn and Gn) is related to
the one terminal group, (terminal-CH3 substituent in Fn compounds and terminal-COOH
substituent in Gn compounds). It is evident the strong relationship between the mesomor-
phism character and chemical constituents of the organic compounds (mesogens).
The electron-donating methyl group in compounds of (Fn) series tends to form the nematic
and smectic C mesophase, while the compounds bearing carboxy electron-withdrawing sub-
stituent in series (Gn) will be able to form smectic C phase only (n > 7). The free rotation
of methyl croup in compounds (Fn) drive them to be mesogens. However, the conjugation of
carboxy group with phenyl moiety in addition to the formation of cyclic hydrogen bonding
dimer may cause the disappearance of the mesomorphic properties of first seven compounds
and appeared in short extents in the rest of series (Gn).
Conclusions
Two series (Fn and Gn) of asymmetrical liquid crystalline compounds with 1,3,4-thiadiazole
central core have been synthesized and characterized by different spectral techniques. All
the compounds of series (Fn) show good mesomorphic properties, exhibiting calamitic
mesophases (nematic and smectic), while only last six compounds of series (Gn) were meso-
gens with smectic type only. We observed that the type of terminal groups substituted in the
two series and the chain length were significantly affected the mesomorphic properties of
these compounds and the thermal stability of the produced liquid crystal phases. The results
indicated that the dimeric forms resulted from the hydrogen bonding interaction between
the carboxylic acid derivatives are responsible for the formation of the liquid crystal property
in the last six compounds in series (Gn). Therefore, the replacement of the terminal (CH3)
group in compounds of series (Fn) by (COOH) group in compounds of series (Gn) led to
drastic changes in liquid crystalline behaviors of these compounds.
Acknowledgments
The authors are grateful to the staff of chemistry department in Al-Mustansiriyah University
(Baghdad, Iraq) for their help and cooperation’s with us. Also many thank to Dr. Mohanad H. Masad (Al

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
215
al-Bayt University, Jordan) and Dr. Dhafir Al-Heetimi (Baghdad University) for their help us in carry-
ing out 1HNMR and DSC analysis, respectively. We are indebted to ASTF (Arab Science of Technology
Foundation), for their generous support by providing us with optical polarizing microscope (OPM) and
hot-stage.
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