Synthesis and mesomorphic behavior of some novel compounds containing 1,3,4-Thiadiazole and 1,2,4-Triazole rings

Article abstract of DOI:10.1080/15421400802697160

Two new mesogenic homologous series of liquid crystalline compounds containing 1,2,4-triazole and 1,3,4-thiadiazole in the molecule viz. 1,4-bis{3-[2-(4-alkoxybenzelideneamino)-1,3,4-thiadiazole-5-ethylenethio]1,2, 4-triazole-5-yl}phenyl and 5-(4-methoxyp

Full text of DOI:10.1080/15421400802697160

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Synthesis and Mesomorphic
Behavior of Some Novel
Compounds Containing 1,3,4-
Thiadiazole and 1,2,4-Triazole
Rings
Jumbed H. Tomma a , Ivan H. Rou'il b & Amar H. Al-
Dujaili a
a Department of Chemistry , College of Education,
Ibn Al-Haitham, University of Baghdad , Baghdad,
Iraq
b Department of Chemistry , College of Science,
University of Al-Mustanseryia , Baghdad, Iraq
Published online: 18 Mar 2009.
To cite this article: Jumbed H. Tomma , Ivan H. Rou'il & Amar H. Al-Dujaili (2009)
Synthesis and Mesomorphic Behavior of Some Novel Compounds Containing 1,3,4-
Thiadiazole and 1,2,4-Triazole Rings, Molecular Crystals and Liquid Crystals, 501:1,
3-19, DOI: 10.1080/15421400802697160
To link to this article: http://dx.doi.org/10.1080/15421400802697160
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Mol. Cryst. Liq. Cryst., Vol. 501, pp. 3–19, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802697160
Synthesis and Mesomorphic Behavior of Some
Novel Compounds Containing 1,3,4-Thiadiazole
and 1,2,4-Triazole Rings
Jumbed H. Tomma1, Ivan H. Rou’il2,
and Amar H. Al-Dujaili1
1Department of Chemistry, College of Education, Ibn Al-Haitham,
University of Baghdad, Baghdad, Iraq
2Department of Chemistry, College of Science, University of
Al-Mustanseryia, Baghdad, Iraq
Two new mesogenic homologous series of liquid crystalline compounds containing
1,2,4-triazole and 1,3,4-thiadiazole in the molecule viz. 1,4-bis{3-[2-(4-alkoxy-
benzelideneamino)-1,3,4-thiadiazole-5-ethylenethio]1,2,4-triazole-5-yl}phenyl and
5-(4-methoxyphenyl)-3-[2-(4-n-xybenzelidenamino)-1,3,4-thiadiazole-5-methyle-
nethio]-1,2,4-triazole were synthesized by many step procedures. Both series
have been characterized by elemental analysis, FT-IR, mass spectrometry, 1H-
NMR, and 13C-NMR. Phase transition temperature and the thermal parameters
were obtained from differential scanning calorimetery (DSC). The texture obser-
vations were performed under hot-stage polarizing optical microscopy (POM).
All compounds of the first series exhibited an enantiotropic nematic mesophase.
The first compound of the second series exhibited an enantiotropic nematic
mesophase, and that with propyloxy terminal substituent shows one enantiotro-
pic nematic phase on heating and a monotropic smectic A (SmA) phase on cool-
ing, while the last two homologues exhibit a monotropic SmA phase. In contrast,
the compound with an ethoxy terminal substituent does not show any liquid
crystalline properties. The mesomorphic behavior has been analyzed in terms
of structural property relationship.
Keywords: 1,3,4-thiadiazole; 1,2,4-triazole; mesomorphic; nematic phase; Schiff base;
smectic phase
Address correspondence to Amar H. Al-Dujaili, Department of Chemistry, College of
Education, Ibn Al-Haitham, University of Baghdad, Baghdad, Iraq. E-mail: ahdujaili@
yahoo.com
3

4
J. H. Tomma et al.
INTRODUCTION
A relatively large number of liquid crystalline compounds containing
heterocyclic units have been synthesized, and interest in such com-
pounds is growing [1–5]. Several liquid crystalline compounds contain-
ing a heterocyclic ring have been reported [5–10]. Many series of these
compounds containing 1,3,4-oxadiazole or 1,3,4-thiadiazole have been
synthesized and studied [11–14]. The mesomorphic studies of triazole
derivatives as potential materials are very limited, and little known
examples of triazole based compounds exhibiting mesomorphic proper-
ties have been reported. A new type of five-member heterocyclic tria-
zole (1) utilized as core group to generate room temperature disorder
hexagonal columnar (Colhd) mesophases is reported compared to other
heterocyclic molecules like thiadiazole, oxadiazole and others [15].
R0¼benzyl; R¼(CH2)nH, n ¼ 6-8 and 10, 12; n ¼ 14, (Colhd 61.6ꢀC I);
n ¼ 7, (Colhd 66.5ꢀC I); n ¼ 8, (Colhd 69.1ꢀC I); n ¼ 10, (Colhd 62.7ꢀC
I); n ¼ 12, (Colhd 67.1ꢀC I); n ¼ 6, (Colhd 74.8ꢀC I).
Cristiano et al. [16] have reported the synthesis and mesomorphic
properties of 1,4-diaryl and Schiff’s base [1,2,3]-triazole derivatives
(2). They found that all compounds exhibited the SmC phase.
CrI 138.2ꢀC CrII 144.2ꢀC SmC 164.1ꢀC SmA 167.2ꢀC I
CrI 136.3ꢀC CrII 184.9ꢀC SmC 246.1 I

Thidiazole and Triazole Liquid Crystal
5
Mesogenic compounds containing 1,2,4-triazole and isonicotinic
acid ring at the terminus of the molecule (3) have recently been
reported [17].
where R¼CnH2nþ1, n ¼ 1-8, and 10, 12, 14, 16; n ¼ 1, (Cr 226ꢀC N
243ꢀC I); n ¼ 2, (Cr 213ꢀC N 239ꢀC I); n ¼ 3, (Cr 209ꢀC N 238ꢀC I);
n ¼ 4, (Cr 204ꢀC N 232ꢀC I); n ¼ 5, (Cr 196ꢀC N 229ꢀC I); n ¼ 6, (Cr
189ꢀC N 226ꢀC I); n ¼ 7, (Cr 177ꢀC N 219ꢀC I); n ¼ 8, (Cr 162ꢀC N
215ꢀC I); n ¼ 10, (Cr 164ꢀC N 208ꢀC I); n ¼ 12, (Cr 76ꢀC SmC 166ꢀC
N 207ꢀC I); n ¼ 14, (Cr 63ꢀC SmA 142ꢀC N 205ꢀC I); n ¼ 16, (Cr 30ꢀC
SmC 128ꢀC N 191)
where R¼CnH2nþ1, n ¼ 1-8, and 10, 12, 14, 16; n ¼ 1, (Cr 201ꢀC I);
n ¼ 2, (Cr 195ꢀC I); n ¼ 3, (Cr 192ꢀC I); n ¼ 4, (Cr 82ꢀC N 190ꢀC I);
n ¼ 5, (Cr 98ꢀC N 184ꢀC I); n ¼ 6, (Cr 121ꢀC N 177ꢀC I); n ¼ 7, (Cr
129ꢀC N 176ꢀC I); n ¼ 8, (Cr 142ꢀC N 169ꢀC I); n ¼ 10, (Cr 129ꢀC
SmC 159ꢀC N 167ꢀC I); n ¼ 12, (Cr 113ꢀC SmA 132ꢀC N 161ꢀC I);
n ¼ 14, (Cr 101ꢀC SmC 144ꢀC N 157ꢀC I); n ¼ 16, (Cr 87ꢀC SmC
139ꢀC N 152ꢀC I).
A few mesomorphic homologous series containing two types of
heterocyclic units in the same molecule (4) have been studied by Parra
et al. [17,18].
where x¼N¼CH, R¼CnH2nþ1, n ¼ 5-10; n ¼ 5, (Cr 149.5ꢀC I (136.1ꢀC)
SmA); n ¼ 6, (Cr 149.3ꢀC I (129.1ꢀC) SmA); n ¼ 7, (Cr 152.4ꢀC I
(129.1ꢀC) SmA); n ¼ 8 (Cr 147.5ꢀC SmA 160.8ꢀC I); n ¼ 9 (Cr 141.9ꢀC
SmA 163.3ꢀC I); n ¼ 10 (Cr 142.1ꢀC SmA 165.6ꢀC I); where x¼N¼N,
R¼CnH2nþ1ꢁ, n ¼ 5-10; n ¼ 5, (Cr 155.2ꢀC I); n ¼ 6, (Cr 155.3ꢀC I);

6
J. H. Tomma et al.
n ¼ 7, (Cr 157.4ꢀC I); n ¼ 8 (Cr 150.5ꢀC I); n ¼ 9 (Cr 144ꢀC I (136ꢀC) N);
n ¼ 10, (Cr 140ꢀC I (136ꢀC) N).
To our knowledge, there is no study for the preparation and meso-
morphic properties of Schiff bases containing thiadiazole and triazole
units in the same molecule. In light of this, it seems of interest to
design new molecules having the above structure and providing
further information concerning the relative importance of rigidity,
linearity, polarizability, and enhancement of luminescent by the
heterocyclic rings in mesophase stability.
EXPERIMENTAL
Reagents and Technique
All chemicals and solvents were of reagent grade (Aldrich Chemicals
Co.) and used without further purification. Elemental analysis (C,
H, N) were carried out using a Perkin-Elmer model 2400 instrument.
Infrared spectra were recorded with a Shimadzu 8000 FT-IR spectro-
photometer in the wave number range 4000–400 cmꢁ1 with samples
1
embedded in KBr disc. H-NMR spectra were obtained with a Varian
Mercury plus spectrometer 400 MHz instrument using suitable deut-
erated solvent and TMS as internal standard. 13C-NMR spectra of
the compound were recorded on a Varian Mercury plus 100 MHz spec-
trometer. Mass spectra were recorded on IEOL JMS-7 high resolution
instrument. The optical behavior observations were made using Olym-
pus BX40 microscope equipped with a Leitz Laborlux 12 Pols hot stage
and PR600 controller. The textures shown by the compounds were
observed using polarized light with crossed polarizers with the sample
in a thin film sandwiched between a glass slide and cover slip. Differ-
ential scanning calorimetry (DSC) measurements were conducted
with TA instruments Q1000 DSC, Ramp rate: 10 degree centigrade
per minute under nitrogen atmosphere. Temperature and heat flow
calibrated with standard indium of purity < 99.99%.
Synthesis
The reaction sequences leading to the formation of compounds of the
two series [V]n and [X]n are outlined in Schemes 1 and 2, respectively.
Synthesis of 1,4-Terphthalic Acid Thiosemicarbazide [I]
To a stirring solution of thiosemicarbazide (0.91 g, 0.01 mol) in dry pyr-
idine 15 mL, at ꢁ5ꢀC, a solution of terphthaloyl chloride (1.020 g, 5
mmol) in dry benzene 15 mL was added dropwise. The stirring was

Thidiazole and Triazole Liquid Crystal
7
SCHEME 1 The preparation route of series [V]n.
continued for half an hour at ꢁ5ꢀC and then overnight at room tem-
perature. The solvent was evaporated. To the residue, water 30 mL
was added, the precipitate was filtered and recrystallized from ethanol
[20] to yield 70%, m.p. ¼ 262–265ꢀC. FT-IR (KBr, cmꢁ1 1182 (nC¼S);
1674 (nC¼O, amide); three bands at 3440, 3284, and 3182 (nN-H of
amide and amine groups, respectively).
Synthesis of 1,4-Bis(3-mercapto-1,2,4-triazole-5-yl)phenyl [II]
A stirring solution of compound [I] (0.40 g, 0.01 mol) and 10 mL 4%
aqueous sodium hydroxide solution was refluxed for 3–4 hours. After
that, the mixture was filtered, and the filterate was acidified with
dilute hydrochloric acid. The precipitate was filtered and washed sev-
eral times with water [21], yield 85%. m.p. > 300ꢀC. FT-IR (KBr,

8
J. H. Tomma et al.
SCHEME 2 The preparation route of series [X]n.
cmꢀ1): 1224 (nC¼S); 1639 (nC¼N of triazole moiety); 2630 (nS-H); 3446
1
(nN-H of triazole moiety). H NMR (DMSO), ppm: d ¼ 3.19–3.59 broad
(s, 2H, NH); 8.05–8.11 (s, 4H, arom. H); 13.66–14.12 (s, broad, 2H,
SH). Ms: m=z þ 1 ¼ 277.
Synthesis of 1,4-Bis(3-thioaceticacid-1,2,4-triazole-
5-yl)phenyl [III]
To a stirring of a-chloroacetic acid (1.64 g, 0.02 mol) in 10% aqueous
sodium hydroxide 10 mL, a solution of compound [II] (0.01 mol) in
10% aqueous solution of sodium hydroxide, 10 mL was added, the mix-
ture was refluxed for three hours. After cooling, the solution was acid-
ified with concentrated hydrochloric acid. The precipitate was filtered
and recrystallized from ethanol [22] to yield 75%, m.p. ¼ 277ꢁC. FT-IR,

Thidiazole and Triazole Liquid Crystal
9
(KBr, cmꢁ1): 1693 (nC¼O, carboxylic moiety); 2927 (nC-H aliphatic);
2750–3400 (nO-H broad of carboxylic acid). 1H NMR (DMSO), ppm:
d ¼ 4.15 (s, 4H, SCH2); 4.99–5.19 (s broad, 2H, NH); 8.05–8.19 (s,
4H, arom. H); 10.69 (s, 2H, COOH). MS: m=z þ 1 ¼ 393.
Synthesis of 1,4-Bis[3-(2-amino-1,3,4-thiadiazole-5-
methylenethio)1,2,4-triazole-5-yl]phenyl [IV]
A mixture of compound [III] (1.66 g, 0.01 mol), thiosemicarbazide
(1.82 g, 0.02 mol), phosphorusoxychloride 10 mL was refluxed gently
for (48) hours. After cooling, 50 mL of water was added in portions with
stirring. The precipitate was filtered, washed with hot water, recrystal-
lized from DMF-water [20], yield 65%, m.p. ¼ 250ꢀC. FT-IR (KBr,
cmꢁ1): 1631 (nC¼N of thiadiazole moiety); 1639 (nC¼N of triazole moiety);
1
two peaks at 3168, 3300 (ns and nas of NH2 group). H NMR (DMSO),
ppm: d ¼ 4.02–4.10 (broad s, 2H, NH); 4.65 (s, 4H, SCH2); 7.25–7.56
(broad s, 4H, NH2); 8.15–8.20 (s, 4H, arom. H). MS: m=z þ 1 ¼ 503.
Synthesis of 1,4-Bis{3-[2-(4-alkoxybenzelideneamino)-1,3,4-
thiadiazole-5-methylenethio]1,2,4-triazole-5-yl}phenyl [V]n
A mixture of [IV] (0.01 mmol), excess of 4-substitutedbenzaldehyde,
and three drops of glacial acetic acid was heated in an oil bath at
150ꢀC for 2 hours. The residue was cooled and crystallized from petro-
leum ether (40–60ꢀC), giving yellow leaflets.
Data Compound [V]3
Yield (60–80%). M.P. ¼ 105ꢀC. Calc. for C36H38O2N12S4;, C, 54.41; H,
4.28; N, 21.16; Found: C, 54.33; H, 4.17; N, 21.06. MS: m=z þ 2 ¼ 796.
FT-IR (KBr, cmꢁ1): 1665 (nC¼N, imine); 2875–2964 (nC-H aliphatic). 1H
NMR (DMSO): d ¼ 0.91–1.02 ppm (t, 6H, CH3); 1.62–1.79 (m, 4H, CH2);
3.78–4.15 (t, 4H, Ph-OCH2); 4.65 (s, 4H, SCH2); 4.23 (weak s, 2H,
N-H); 6.98–7.85 (d-d, 8H, Ar-H of phenyl ring attached OR); 8.18
(s, 4H, Ar-H of phenyl ring between triazole rings); 8.58 (s weak,
2H, imine group). 13C NMR (DMSO), 14.72 ppm (CH3), 23.11–
35.61 ppm (CH2), 30.31–32.41 ppm (C-S), 123.50–166.21 ppm (Ar-C),
159.11 ppm (C¼N), 139.21–161.79 ppm (Triazole-C).
Synthesis of N-(4-Methoxybenzoyl)-thiosemicarbazide [VI]
This compound was prepared by using the same procedure given for
compound [I] to yield 95%, m.p. ¼ 226ꢀC. FT-IR (KBr, cmꢁ1) 1184

10
J. H. Tomma et al.
(nC¼S); 1664 (nC¼O, amide); three bands at 3402, 3253, and 3145 (nN-H
of amide and amine groups, respectively).
Synthesis of 5-(4-Methoxyphenyl)-1,2,4-triazole-3-thiol [VII]
This compound was prepared by using the same procedure given for
compound [II] to yield 80%, m.p. ¼ 259–262ꢀC. FT-IR (KBr, cmꢁ1):
1612 (nC¼N of triazole moiety); 1224 (nC¼S); 2682 (nS-H); 3242 (nN-H
of triazole moiety). Ms: m=z ¼ 207.
Synthesis of 5-(4-Methoxyphenyl)-3-thioaceticacid-1,2,4-
triazole [VIII]
This compound was prepared by using the same procedure givenꢁf1or
compound [III] to yield 82%, m.p. ¼ 204–206ꢀC. FT-IR (KBr, cm
:
1679 (nC¼O, carboxylic moiety); 2837–2939 (nC-H aliphatic); 2777–
1
3205 (nO-H broad of carboxylic acid). H NMR (DMSO), ppm: d ¼ 3.85
(s, 3H, OCH3); 4.09 (s, 2H, SCH2); 5.05 (weak s, 1H, NH); 7.05-7.92
(d-d, 4H, arom. H); 10.29 (weak s, 1H, COOH). Ms: m=z ¼ 265.
Synthesis of 5-(4-Methoxyphenyl)-3-(2-amino-1,3,4-
thiadiazole-5-methylenethio)-1,2,4-triazole [IX]
This compound was prepared by using the same procedure given for
compound [IV] to yield 63%, m.p. ¼ 145ꢀC. FTIR(KBr, cmꢁ1): 1614
(nC¼N of thiadiazole moiety); 1630 (nC¼N of triazole moiety); two peaks
1
in the region 3100–3380 (ns and nas of NH2 group). H NMR (DMSO),
ppm: d ¼ 3.88 (s, 3H, OCH3); 4.02 (s, 2H, SCH2); 4.62 (s, 1H, NH); 6.15
(weaks, 2H, NH2); 6.95–8.12 (d-d, 4H, arom. H). 13C NMR (DMSO),
14.69 ppm (OCH3), 30.11–36.71 ppm (C-S), 40.38 ppm (C-N),
55.92 ppm (C-O), 115.10–129.41 ppm (Ar-C), 161.31 ppm (C¼N).
Synthesis of 5-(4-Methoxyphenyl)-3-[2-(4-n-
alkoxybenzelidenamino)-1,3,4-thiadiazole-5-
methylenethio]-1,2,4-triazole [X]n
The compounds of this series were prepared by using the same proce-
dure given for compounds [V]n.
Data Compound [X]1
Yield 58–67%. M.P. 117ꢀC. Calc. for C20H18O2N6S2;, C, 54.79; H, 4.11;
N, 19.18; Found: C, 54.65; H, 4.01; N, 20.75. MS: m=z þ 2 ¼ 796. FTIR

Thidiazole and Triazole Liquid Crystal
11
1
(KBr, cmꢁ1): 1680 (nC¼N, imine); 2848–2920 (nC-H aliphatic). H NMR
(DMSO): d ¼ 3.57 ppm (s, 1H, N-H); (two singlet near together at 3.71
and 3.76 ppm, 6H, OCH3); 4.55 ppm (s, 2H, SCH2); 6.91–7.19 ppm (d-d,
4H, Ar-H of phenyl ring attached triazole ring); 7.81–8.09 ppm (d-d,
4H, arom. H); 9.82 ppm (s, 1H, imine group). 13C NMR (DMSO),
14.36 ppm (OCH3), 32.41 ppm (C-S), 41.14 ppm (C-N), 124.51–
166.93 ppm (Ar-C), 161.26 ppm (C¼N), 142.11–163.11 ppm (Triazole-C).
RESULTS AND DISCUSSION
Schemes 1 and 2 outline the synthesis of the ten compounds of the two
series [V]n and [X]n. The elemental analyses of series [V]n and [X]n
compounds synthesized above are shown in Table 1. The observed
values are in agreement with theoretical values indicating structure
of respective compounds. All compounds were also characterized by
1
FT-IR, H, 13C NMR, and mass spectroscopy.
Mesomorphic Behavior
Many examples of nonlinear mesogens were prepared, starting with
Vorlander and Apel [23] who considered nonlinear mesogens pre-
pared. Dingemans and Samulski [24] used the term boomerang-
shaped mesogen to characterize the more rigid and better defined
nonlinearity with mesogens derived from 2,5-bis-p-hydroxyphenyl-
1,3,4-oxadiazole having 134ꢀ bend in the mesogenic core (5).
TABLE 1 Elemental Analysis Data for the Series [V]n and [X]n
Analysis % found (calcd.)
Molecular
Molecular
Compound
formula
weight (g=mole)
C
H
N
[V]1
[V]2
[V]3
[V]4
[V]5
[X]1
[X]2
[X]3
[X]4
[X]5
C32H26O2N12S4
C34H30O2N12S4
C36H34O2N12S4
C38H38O2N12S4
C40H42O2N12S4
C20H18O2N6S2
C22H22O2N6S2
C24H26O2N6S2
C26H30O2N6S2
C28H34O2N6S2
738
766
794
822
850
438
466
494
522
550
51.79 (52.03) 3.47 (3.52) 22.71 (22.76)
53.19 (53.26) 3.85 (3.92) 21.80 (21.93)
54.33 (54.41 4.17 (4.28) 21.06 (21.16)
55.41 (55.47) 4.54 (4.62) 20.36 (20.44)
56.38 (56.47) 4.99 (4.94) 19.65 (19.76)
54.65 (54.79) 4.01 (4.11) 20.75 (21.18)
56.50 (56.65) 4.51 (4.72) 17.99 (18.03)
58.17 (58.30) 5.29 (5.26) 17.00 (16.89)
59.69 (59.77) 5.67 (5.75) 16.02 (16.09)
61.02 (61.09) 6.11 (6.18) 15.20 (15.72)

12
J. H. Tomma et al.
In this work, the nonlinear molecules series [V]n are prepared from
1,4-bis(3-mercapto-1,2,4-triazole-5-yl)phenyl [II]. These new com-
pounds [V]n are examples of the continuing effort to explore the limits
of molecular shapes compatible with liquid crystallinity, and differ
from the above reported nonlinear mesogens by the directional sense
of the 2,5-substituted 1,2,4-triazole linkage in addition to 1,3,4-
thiadiazole heterocyclic ring.
The mesomorphic behavior of all compounds of the two series was
investigated using differential scanning calorimetry (DSC) and hot-
stage polarizing optical microscopy (POM).
Table 2 collects the transition temperatures obtained by (DSC) and
verified by (POM) for all ten compounds of series [V]n and [X]n.
Figure 1 shows the DSC thermogram of compound [V]4 as representa-
tive example of these compounds. All five compounds of series were
they examined under POM showed anisotropic fluid after melting.
On cooling from the isotropic melt, schlieren texture characteristic of
the nematic phase was observed as shown in Figure 2.
The optical and thermal data of the compounds of series [X]n are
gathered in Table 2. All compounds show mesomorphic properties.
Contrary to this, the compounds with two carbon atoms of terminal
alkoxy group do not show mesomorphic behavior, and only crystal to
isotropic liquid transition was observed. Probably, this is due to a loss
of linearity in the molecules by the presence of two different heterocyc-
lic rings which are not compatible with the even number of carbon
atoms of the terminal group, for it is unable to make co-linear distribu-
tion bonding. However, the conformational caugch structure arises
from the two carbon atoms of the ethoxy terminal group could be the
only reason for diminishing the mesomorphity. Compounds [X]1 and

Thidiazole and Triazole Liquid Crystal
13
TABLE 2 Phase Behavior for Compounds of Series [V]n and [X]n
Compound
[V]1
Phase behavior
[V]2
[V]3
[V]4
[V]5
[X]1
[X]2
[X]3
[X]4
[X]5
n represents the number of carbons in the alkoxy chain; Cr ¼ crystal
phase; Gm ¼ gummy; N ¼ nematic phase; SmA ¼ smectic phase;
I ¼ isotropic liquid. The transition temperatures (ꢀC).
[X]3 to [X]5 displayed the liquid crystalline properties. In the com-
pound [X]1 only enantiotropic nematic mesophase was observed, while
compound [X]3 displayed enantiotropic nematic phase on heating and
monotropic smectic A (SmA) on cooling. Compounds [X]4 and [X]5 exhi-
bit monotropic SmA phase. Figure 3 shows the DSC thermogram of
compound [X]3, the endotherms at 152ꢀC and 255ꢀC as examined by
POM are assigned to the crystal ! nematic and nematic ! isotropic
transitions, while the exotherms at 143ꢀC are assigned to the

14
J. H. Tomma et al.
FIGURE 1 DSC thermogram for the compound [V]4.
isotropic ! smectic A transition. Figure 4 shows typical focal conic
SmA texture of [X]3. We believe that the bend imposed by the 1,2,4-
triazole ring is so large that the effective polar packing necessary to
build up a mesophase and neutralize the bending imposed by 1,3,4-
thiadiazole ring, but with odd number of alkoxy terminal groups.
FIGURE 2 Polarizing micrograph of schlieren texture of the nematic phase
(ꢂ200 magnification) of compound [V]5 at 125ꢀC on cooling.

Thidiazole and Triazole Liquid Crystal
15
FIGURE 3 DSC thermogram for the compound [X]3.
It is well known that the introduction of heteroatom causes consid-
erable changes in chemical and physical properties and influence the
type of liquid crystalline phase, also phase transition temperatures
and other properties of the mesogens [8]. In our present study, we
prepared two new series [V]n and [X]n containing two heterocyclic
rings; one is 1,2,4-triazole and the second is 1,3,4-thiazole. The iso-
tropic temperature of compounds in series [V]n are higher than the
FIGURE 4 Polarizing micrograph of the focal conic texture of the smectic A
phase (ꢂ200 magnification) for the compound [X]3 at 125ꢀC on cooling.

16
J. H. Tomma et al.
TABLE 3 Different Transition Temperatures and Mesophase Stability Range
SmA-N mesophase
stability (ꢀC)
N-I mesophase
stability (ꢀC)
Compound
SmA
(143)
N
I
[V]3
[X]3
135
152
257
255
122
103
09
compounds of series [X]n, which is probably due to the fact that com-
pounds of series [V]n represented the twin of compounds of series
[X]n. The twin compound was proved to induce boomerang shape favor
of more stable liquid crystalline phases.
Table 3 shows the comparison of mesophases transition temperatures
of compound [V]3 and structurally related compound [X]3. The nematic-
isotropic mesophase stability of compound [V]3 is higher by 19ꢀC, when
compared with compound [X]3. Moreover, compound [X]3 showed
dimorphism enantiotropic nematic and monotropic SmA phases when
compared with compound [V]3. Compound [X]3 is shorter than [V]3,
and this could increase the close packing of the former leads to more lat-
eral molecular interaction, which is favored of smectic phase formation.
In order to show the different possibilities that 1,2,4-triazole ring
can offer in the liquid crystal field, it is interesting to compare the
present homologous series [V]n and [X]n with twin Schiff basses (6)
prepared by us [25] and analogous Schiff’s basses (7) prepared by
Parra et al. [26], respectively.

Thidiazole and Triazole Liquid Crystal
17
Series [V]n is compared with related series (6). The Schiff’s bases
[V]n and (6) have the same central core and the same alkoxy chains;
however, the only differences between their central linkages is the
1,2,4-triazole ring. All the members of series (6) exhibit an enantiotro-
pic dimorphism SmA-nematic, (Table 4), except the first two homolo-
gues, whereas, in series [V]n all homologues exhibit enantiotropic
nematic phase. Thermal stabilities of series (6) is greater than com-
pounds of series [V]n. The flexibility and bending characters of 1,2,4-
triazole could be the reason of such behavior.
Comparison of series [X]n with series (7) (despite of different chain
length of alkoxy terminal substituent) give insight on the role played
by the introduction of the 1,2,4-triazole ring. The structural difference
between the series is one of the central linkages. All compounds of ser-
ies [X]n are exhibited an enantiotropic nematic phase. The smectic
SmA mesophase commences from [X]3 to [X]5 derivatives as a monotro-
pic phase (Table 2), whereas all compounds of series (7) display an
enantiotropic dimorphism SmC-nematic (Table 5). Thermal stabilities
of compounds of series (7) is greater than compounds of series [X]n.
This probably can be explained considering the flexibility and bending
properties of 1,2,4-triazole ring in the liquid crystalline phases. Conte
et al. [28] reported that introduction of the 1,2,3-triazole in central
linkages in the mesogenic molecules resulted in it showing the SmA
phase, and lowered the melting point in comparison with compounds
containing the only 1,3,4-thiadiazole ring in the central linkage of
the molecules.
TABLE 4 Phase Behavior for Compounds of Series (6)
Compound
Phase behavior
(6) n ¼ 1
(6) n ¼ 2
(6) n ¼ 3
(6) n ¼ 4
(6) n ¼ 5

18
J. H. Tomma et al.
TABLE 5 Phase Behavior for Compounds of Series (7)
Compound
Phase behavior
(7) n ¼ 5
(7) n ¼ 6
(7) n ¼ 7
(7) n ¼ 8
(7) n ¼ 9
From previous studies on crystal structure by Burgiand and Dunitz
[28] and on structural effect by Byron et al. [29] on the analogous Schiff
bases, a non-coplanar structure for the Schiff’s bases, reported by them,
which pack, together in smectic layers in a juxtaposed arrangement
with a cooperative rotational arrangement in the highly order smectic
phase (i.e., SmB), and free rotation in the nematic or low order smectic
phase (i.e., SmA). So we may assume that in the molecules of the pre-
sent Schiff’s bases [X]n the juxtaposed arrangement and the cooperative
rotation are absent and a free rotation of the molecules must exist, this
could explain the existence of the nematic and low order SmA Phases.
CONCLUSIONS
This work has described the synthesis of two new series containing two
heterocyclic rings, i.e., 1,3,4-thiadiazole and 1,2,4- triazole in the same
molecules. Almost all homologues of both series showed enanatiotropic
mesomorphic properties. The presence of more polarized nitrogen atoms
on the both heterocyclic rings might be responsible for the formation of
mesomorphic properties over other molecules containing only one hetero-
cyclic ring. It was found that the conformational molecular structure play
an important role in the stability and formation of these mesophases.
AKNOWLEDGMENT
Prof. A. H. Al-Dujaili thanks the International Institute of Education
(IIE), New York, USA, for the award of a fellowship at the University
of Jordan.

Thidiazole and Triazole Liquid Crystal
19
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