Synthesis and characterization of symmetrical liquid crystalline compounds based on oxazole and thaizole rings

Article abstract of DOI:10.1080/15421406.2020.1824152

Twelve compounds containing a sulphur- or oxygen-based heterocyclic core, 1,3- oxazole or 1,3-thiazole ring with hydroxy, methoxy and methyl terminal substituent, were synthesized and characterized. The molecular structures of these compounds were perform

Full text of DOI:10.1080/15421406.2020.1824152

Molecular Crystals and Liquid Crystals
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis and characterization of symmetrical
liquid crystalline compounds based on oxazole
and thaizole rings
Tareq K. Ibraheem, Nisreen H. Karam & Ammar H. Al-Dujaili
To cite this article: Tareq K. Ibraheem, Nisreen H. Karam & Ammar H. Al-Dujaili (2020) Synthesis
and characterization of symmetrical liquid crystalline compounds based on oxazole and thaizole
rings, Molecular Crystals and Liquid Crystals, 710:1, 1-12, DOI: 10.1080/15421406.2020.1824152
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
020, VOL. 710, NO. 1, 1–12
2
https://doi.org/10.1080/15421406.2020.1824152
Synthesis and characterization of symmetrical liquid
crystalline compounds based on oxazole and thaizole rings
a
a
b
Tareq K. Ibraheem , Nisreen H. Karam , and Ammar H. Al-Dujaili
a
Department of Chemistry, College of Education for Pure Science, Ibn Al-Haitham, University of
b
Baghdad, Baghdad, Iraq; Hamdi Mango Center for Scientific Research, University of Jordan, Amman
1
1942, Jordan
ABSTRACT
KEYWORDS
Heterocyclic; liquid crystal;
oxazole; thiazole
Twelve compounds containing a sulphur- or oxygen-based hetero-
cyclic core, 1,3- oxazole or 1,3-thiazole ring with hydroxy, methoxy
and methyl terminal substituent, were synthesized and characterized.
The molecular structures of these compounds were performed by
elemental analysis and different spectroscopic tequniques. The liquid
crystalline behaviors were studied by using hot-stage optical polariz-
ing microscopy and differential scanning calorimetry. All compounds
of 1,4- disubstituted benzene core with oxazole ring display liquid
crystalline smectic A (SmA) mesophase. The compounds of 1,3- and
1
,4-disubstituted benzene core with thiazole ring exhibit exclusively
enantiotropic nematic liquid crystal phases.
Introduction
Liquid crystals are unique in their uses due to their excellent optical properties [1–3].
Heterocycles are of great importance as units in thermotropic liquid crystals owing to their
ability to impart lateral and/or longitudinal dipoles combined with changes in the molecu-
lar shape [4–6]. Generally, the five membered ring heterocyclics are involved and they
form part of the core in rod-shaped, bent-shaped or disk-shaped molecules. They have
been studied as structural units to liquid crystals contributing to the study of the relation
between molecular structure and mesomorphic properties [7,8]. Many compounds contain-
ing heterocyclic units with liquid-crystalline properties have been synthesized [9–13].
Furthermore, the incorporation of heteroatoms can result in considerable changes in
the corresponding liquid-crystalline phases and in the physical properties of the
observed phases [14] because that the presence of a heteroatom can cause profound
changes in mesomorphic behavior, due to the electro negativity difference presented in
relation to carbon or through changes in molecular geometry [15,16].
The 1,3-oxazole and 1,3-thiazole rings have been incorporated into a variety of meso-
genic structures and imparts some favorable characteristics for applications in electro-
optical devices, such as low viscosity (anticipated due to the more compact nature of
the ring compared with benzene), high birefringence and a significant dipole moments
CONTACT Ammar H. Al-Dujaili
ah.aldujaili@gmail.com
Hamdi Mango Center for Scientific Research, University of
Jordan, Amman, Jordan.
ß 2020 Taylor & Francis Group, LLC

2
T. K. IBRAHEEM ET AL.
(1.6D for thiazole and 1.3D for oxazole). Besides, they also confer a curvature to the
molecule, leading to bent-shaped liquid crystals [17].
The 1,3-oxazole and 1,3-thiazole cores may be either 2,5- or 2,4-disubstituted; the 2,5-
disubstituted arrangement is the more linear substitution pattern (the angles defined by
extrapolating the C–H bonds are 153 for 2,5-disubstitution and 133 for 2,4-disubstitution
when the substituent are hydrogen) which leads to enhanced mesophase thermal stability.
There are many examples in literature including the preparation of 2,5-disubstituted oxa-
zole and thiazole mesogenic molecules [13,17–30]. But there are limited number of meso-
genic 2,4-disubstituted oxazole and thiazole derivatives have been reported [31–39]. For
instance, Murza et al., [31–33] have been synthesized 4-(arylamino)-2-benzylideneamino-
4-(p-nitrophenoxy) thiazolees derivatives, which possess monotropic mesomorphism of
the nematic type over a wide range of temperature. Also, Murza et al., [34,35] have been
synthesized liquid crystalline azomethines 2-benzylideneamino-4-(p-nitrophenoxy) thia-
zoles derivatives displaying nematic mesomorphism and studied a correlation between
0
the geometric parameters and liquid crystalline properties. Several non-symmetrically 4,4 -
0
diaryl-2,2 -bithiazoles containing alkyl and alkoxy substituents at the 4-positions were syn-
thesized that exhibit wide-range high-temperature smectic C phases [36]. A two series of
liquid crystalline Schiff bases containing thiazole moiety with different length of alkoxy
spacer were synthesized. Both two-series compounds exhibit liquid crystalline mesophase
nematics. The liquid crystalline behavior was analyzed in terms of the relation of struc-
tural property [37]. A new series of 2,4-disubstituted thiazole esters was synthesized and
introduced and discussed their mesomorphic properties. Some of these compounds dem-
onstrate stable mesophase smectic A [38]. Recently, Seed and Sampson [39] highlighted
both recent and well-established synthetic approaches to 2,5- and 2,4-disubstituted 1,3-
thiazoles systems using ring closure (Gabriel- and Hantzsch-type approaches) method-
ology and modifications of an intact 1,3- thiazole ring.
As part of our study of liquid crystal derivatives containing heterocyclic units
[37,40–46] the purpose of this research was to synthesize and characterize the recent
symmetrical bent and linear-core 2,4-disubstituted 1,3-oxazole and 1,3-thiazole-based
liquid crystals and the phase behavior of these compounds.
Experimental
General methods
All chemicals were acquired from Sigma-Aldrich Company in Milwaukee, Wisconsin
(USA) and used without further purification. FT-IR spectra were recorded by KBr disk
in a Shimadzu (IR Affinity-1). Elemental analyses were performed by Euro EA 3000/
1
Italy. HNMR spectra were obtained using a Bruker 300 MHz origin: Switzerl and were
reported in ppm (d), the compounds were dissolved in DMSO-d6 solution with the tet-
ramethyl silane as internal standard. Mass spectrum was determined by scan110
(1.257 min): direct prob-013777.d. The optical behavior observations were made using
Olympus BX40 microscope equipped with a Leitz Laborlux 12 Pols hot stage and PR600
controller. The textures of the mesophases shown by the compounds were observed
using optical polarized light with crossed polarizer with the sample in a thin film sand-
wiched between a glass slide and cover slip. Photographs of texture were obtained using

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
a camera Olympus PM-10AD model. Differential scanning calorimetry (DSC) measure-
ments were performed using a STAPT-1000LINSIS Instruments at heating and cooling
ꢀ
ꢀ
rates of 10 C/min with indium as the internal standard (156.6 C, 28.45 J/g).
Synthesis
The detailed synthesis of derivatives is depicted in Scheme 1.
Synthesis of N,N’-(1,3-Phenylene)bis(2-chloroacetamide) [I] and N,N’-(1,4-
a
Phenylene)bis(2-chloroacetamide) [I]b
A mixture of 1,3-phenylenediamine or 1,4-phenylenediamine (1.08 g, 10 mmol), in DMF
(15 mL) was mixed with triethylamine (TEA) (18 mL) and chloroacetyl chloride (2.26 g,
2
0 mmol) was stirred in ice bath for 3 h. The mixture was added to an ice water bath
until precipitation occurred; the separated solid was filtered, washed with water, dried
and recrystallized from ethanol.
Analytical data
N,N’-(1,3-Phenylene)bis(2-chloroacetamide) [I]a
ꢀ
ꢁ1
White powder; yield: 76%; m.p.: 213 C; FTIR (ꢀ/cm ): 3259 (NH), 1681 (C ¼ O
amide). Anal. Calcd. for C H Cl N O : C, 45.98; H, 3.83; N, 10.73. Found: C, 46.11;
1
0
10
2 2 2
H, 3.73; N, 10.93.
N,N’-(1,4-Phenylene)bis(2-chloroacetamide) [I]b
ꢀ
ꢁ1
Gray powder; yield: 72%; m.p.: 316 C; FTIR (ꢀ/cm ): 3223 (NH), 1673 (C ¼ O amide).
Anal. Calcd. for C H Cl N O : C, 45.98; H, 3.83; N, 10.73. Found: C, 46.23; H, 3.64;
1
0
10
2 2 2
N, 10.94.
Synthesis of oxazole compounds [II]a,b and thiazole compounds [III]a,b
To a solution of compound [I] or [I] (2.61 g, 10 mmol) in ethanol 150 mL, urea
a
b
(1.20 g, 20 mmol) or thiourea (1.52 g, 20 mmol) was added. This reaction mixture was
heated under reflux for 12–15 h with occasional stirring. Then, the reaction mixture was
concentrated, and the residue obtained was poured over crushed ice, the separated solid
was filtered, washed with water, dried and recrystallized from ethanol.
N4,N4’-(1,3-phenylene)bis(oxazole-2,4-diamine) [II]a
ꢀ
ꢁ1
1
White powder; yield: 67%; m.p.: 199 C: FTIR (ꢀ/cm ): 3462-3190 (NH , NH), 3080
2
(
(
C-H arom), 1649 (C ¼ N endo cyclic), 1608 (C ¼ C). HNMR (300 MHz, DMSO-d ) d
6
ppm): 9.89 (s, 2H, 2NH), 7.53 (s, 2H, oxazole ring), 7.48–7.00 (m, 4H –Ar) , 5.39 (s,
4
3
H, 2NH ). MS: m/z þ 1 ¼ 273. Anal. Calcd. for C H N O : C, 52.94; H, 4.41; N,
2
12 12 6 2
0.88. Found: C, 52.73; H, 4.65; N, 31.04.

4
T. K. IBRAHEEM ET AL.
Scheme 1. The synthetic route of compounds [IV]a,b,c, [V]a,b,c, [VI]a,b,c and [VII]a,b,c
.
N4,N4’-(1,4-phenylene)bis(oxazole-2,4-diamine) [II]b
ꢀ
ꢁ1
1
White powder; yield: 73%; m.p.: 212 C: FTIR (ꢀ/cm ): 3244–3174 (NH ,NH), 3093
2
(
(
C-H arom.), 1634 (C ¼ N endo cyclic), 1593 (C ¼ C), HNMR (300 MHz, DMSO-d ) d
6
ppm): 9.89 (s, 2H, 2NH), 7.53 (s, 2H, oxazole ring), 7.48-7.00 (m, 4H –Ar), 5.39

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
(
3
s, 4H, 2NH ). MS: m/z þ 1 ¼ 273. Anal. Calcd. for C H N O : C, 52.94; H, 4.41; N,
2
12 12 6 2
0.88. Found: C, 53.10; H, 4.25; N, 30.64.
N4,N4’-(1,3-phenylene)bis(thiazole-2,4-diamine) [III]a
ꢀ
ꢁ1
1
Dark yellow powder; yield: 69%; m.p.: 254 C: FTIR (ꢀ/cm ): 3271–3154 (NH , NH),
3
2
099 (C-H arom.), 1656 (C ¼ N endo cyclic), 1596 (C ¼ C). HNMR (300 MHz, DMSO-
d ) d (ppm): 8.97 (s, 2H, 2NH), 8.93–8.71 (m, 4H, –Ar), 8.67(s, 2H), 3.26 (s, 4H,
6
2
4
NH ). Anal. Calcd. for C H N S : C, 47.37; H, 3.95; N, 27.63; S, 21.05. Found: C,
2 12 12 6 2
7.51; H, 4.15; N, 27.74; S, 20.92.
N4,N4’-(1,4-phenylene)bis(thiazole-2,4-diamine) [III]b
ꢀ
ꢁ1
Yellow powder; yield: 76%; m.p.: 237 C: FTIR (ꢀ/cm ): 3253–3170 (NH , NH), 3089
2
(
C-H arom.), 1662 (C ¼ N endo cyclic), 1587 (C ¼ C). MS: m/z þ 1 ¼ 305. Anal. Calcd.
for C H N S : C, 47.36; H, 3.94; N, 27.63; S, 21.05. Found: C, 47.19; H, 3.67; N, 27.44;
1
2
12 6 2
S, 21.30.
Synthesis of Schiff bases compounds [IV-VII]a-c
To a solution (20 mmol) different aromatic aldehyde and (4 mL) of THF added a few
drops of piperidine, then, added (10 mmol) of compound [II]a, b or [III]a, b and reflux
for 4 h. The solvent was evaporated and the residue was extracted by diethyl ether and
the obtained solid was filtered, washed with water and recrystallized from an appropri-
ate solvent.
4
,4’-((((1,3-Phenylenebis(azanediyl))bis(oxazole-4,2-diyl))bis(azaneylylidene) )
bis(methaneylylidene))diphenol [IV]a
ꢀ
ꢁ1
White powder; yield: 74%; m.p.: 239 C: FTIR (ꢀ/cm ): 3289 (NH), 3253 (O-H), 3048
(
4
C-H arom.), 1648 (C ¼ N), 1591 (C ¼ C). Anal. Calcd. for C H N O : C, 65.00; H,
26
20 6 4
.17; N, 17.50. Found: C, 65.24; H, 3.96; N, 17.65.
N1,N3’-bis(2-((4-methoxybenzylidene)amino)oxazol-4-yl)benzene-1,3-dia- mine[IV]b
ꢀ
ꢁ1
White powder; yield: 81%; m.p.: 161 C: FTIR (ꢀ/cm ): 3200 (NH), 3093 (C-H arom.),
956–2885 (C-H aliph.), 1650 (C ¼ N), 1601 (C ¼ C). MS: m/z þ 1 ¼ 509. Anal. Calcd.
for C H N O : C, 66.14; H, 4.72; N, 16.53. Found: C, 66.30; H, 4.83; N, 16.32.
2
2
8
24 6 4
N1,N3’-bis(2-((4-methylbenzylidene)amino)oxazol-4-yl)benzene-1,3-diam- ine [IV]c
ꢀ
ꢁ1
Brown powder; yield: 76%; m.p.: 191 C: FTIR (ꢀ/cm ): 3184 (NH), 3089 (C-H arom.),
2
7
974–2812 (C-H aliph.), 1622 (C ¼ N), 1598 (C ¼ C). Anal. Calcd. for C H N O : C,
28
24 6 2
0.59; H, 5.04; N, 17.65. Found: C, 70.30; H, 4.84; N, 17.43.

6
T. K. IBRAHEEM ET AL.
4,4’-((((1,4-Phenylenebis(azanediyl)) bis (oxazole-4,2-diyl)) bis(azaneyly-
lidene))bis(methaneylylidene))diphenol [V]a
ꢀ
ꢁ1
White powder; yield: 69%; m.p.: 313 C: FTIR (ꢀ/cm ): 3110 (NH), 3253 (O-H), 3021
(
4
C-H arom.), 1616 (C ¼ N), 1600 (C ¼ C). Anal. Calcd. for C H N O : C, 65.00; H,
26
20 6 4
.16; N, 17.50. Found: C, 64.84; H, 4.30; N, 17.66.
N1,N4’-bis(2-((4-methoxybenzylidene)amino) oxazol-4-yl)benzene-1,4-dia-mine[V]b
ꢀ
ꢁ1
Brown powder; yield: 63%; m.p.: 224 C: FTIR (ꢀ/cm ): 3172 (NH), 3048 (C-H arom.),
2
6
940-2876 (C-H aliph.), 1656 (C ¼ N), 1610 (C ¼ C). Anal. Calcd. for C H N O : C,
28
24 6 4
6.14; H, 4.72; N, 16.53. Found: C, 66.02; H, 4.59; N, 16.72.
N1,N4’-bis(2-((4-methylbenzylidene)amino) oxazol-4-yl)benzene-1,4-dia- mine[V]c
ꢀ
ꢁ1
1
Brown powder; yield: 82%; m.p.: 245 C: FTIR (ꢀ/cm ): 3259 (NH), 3091 (C-H arom.),
2
956-2884 (C-H aliph.), 1619 (C ¼ N), 1593 (C ¼ C). HNMR (300 MHz, DMSO-d ) d
6
(
ppm): 9.60 (s, 2H, 2NH), 7.77 (s, 2H, two CH ¼ N), 7.58(s, 2H, oxazole ring), (7.53-
6
.96) (m, 12H-Ar), 2.54(s, 6H, 2CH ). MS: m/z þ 1 ¼ 477. Anal. Calcd. for C H N O :
3
28 24 6 2
C, 70.58; H, 5.04; N, 17.64. Found: C, 70.73; H, 5.18; N, 17.83.
4
,4’-((((1,3-Phenylenebis(azanediyl))bis(thiazole-4,2-diyl))bis(azaneylyli-
dene))bis(methaneylylidene))diphenol [VI]a
ꢀ
ꢁ1
White powder; yield: 66%; m.p.: 221 C: FTIR (ꢀ/cm ): 3280 (NH), 3283 (O-H), 3048
(
C-H arom.), 1651 (C ¼ N), 1599 (C ¼ C). Anal. Calcd. for C H N O S : C, 60.94;
26
20 6 2 2
H,3.91; N, 16.41. Found: C, 70.09; H, 3.78; N, 16.69.
N1,N3’-bis(2-((4-methoxybenzylidene)amino)thiazole-4-yl)benzene-1,3-
di- amine[VI]b
ꢀ
ꢁ1
Yellow powder; yield: 78%; m.p.: 167 C: FTIR (ꢀ/cm ): 3323 (NH), 3050 (C-H arom.),
946–2875 (C-H aliph.), 1660 (C ¼ N), 1600 (C ¼ C). MS: m/z þ 1 ¼ 541. Anal. Calcd.
for C H N O S : C, 62.22; H, 4.44; N, 15.56. Found: C, 62.43; H, 4.68; N, 15.69.
2
2
8
24 6 2 2
N1,N3’-bis(2-((4-methylbenzylidene) amino)thiazole-4-yl)benzene-1,3-
diam- ine [VI]c
ꢀ
ꢁ1
White powder; yield: 63%; m.p.: 221 C: FTIR (ꢀ/cm ): 3174 (NH), 3089 (C-H arom.),
2
6
935-2864 (C-H aliph.), 1622 (C ¼ N), 1595 (C ¼ C) .Anal. Calcd. for C H N S : C,
28
24 6 2
6.14; H, 4.72; N, 16.54. Found: C, 65.93; H, 4.68; N, 16.71.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
4,4-((((1,4-Phenylenebis(azanediyl)) bis (thiazole-4,2-diyl)) bis(azaneylyli- dene))
bis(methaneylylidene))diphenol[VII]a
ꢀ
ꢁ1
Yellow powder; yield: 60%; m.p.: 192 C: FTIR (ꢀ/cm ): 3292 (NH), 3311 (O-H), 3041
(
C-H arom.), 1658 (C ¼ N), 1595 (C ¼ C). Anal. Calcd. for C H N O S : C, 60.94;
26
20 6 2 2
H,3.91; N, 16.41. Found: C, 61.10; H, 3.81; N, 16.26.
N1,N4’-bis(2-((4-methoxybenzylidene)amino) thiazole-4-yl)benzene-1,4-
di- amine[VII]b
ꢀ
ꢁ1
White powder; yield: 71%; m.p.: 203 C: FTIR (ꢀ/cm ): 3232 (NH), 3011 (C-H arom.),
2
6
954-2865 (C-H aliph.), 1653 (C ¼ N), 1597 (C ¼ C). Anal. Calcd. for C H N O S : C,
28
24 6 2 2
2.22; H, 4.44; N, 15.56. Found: C, 62.02; H, 4.53; N, 15.48.
N1,N3’-bis(2-((4-methylbenzylidene)amino) thiazole-4-yl)benzene-1,4-diam-
ine [VII]c
ꢀ
ꢁ1
Yellow powder; yield: 81%; m.p.: 204 C: FTIR (ꢀ/cm ): 3245 (NH), 3016 (C-H arom.),
942-2871 (C-H aliph.), 1632 (C ¼ N), 1594 (C ¼ C). MS: m/z þ 1 ¼ 509. Anal. Calcd.
for C H N S : C, 66.14; H, 4.72; N, 16.54. Found: C, 66.33; H, 4.69; N, 16.77.
2
2
8
24 6 2
Results and discussion
Synthesis
All of the new synthesized compounds gave satisfactory analysis for the proposed struc-
tures, which were confirmed on the basis of their elemental analysis (see analytical
1
data); FTIR, HNMR and mass spectroscopy. The compounds [I] and [I] were pre-
a
b
pared by reaction of 1,3-phenylenediamine or 1,4-phenylenediamine with two moles
from chloroacetyl chloride in dimethylformamide (DMF) and triethylamine (TEA) (as a
catalyst). The FTIR spectra for compounds [I] and [I] showed the appearance of
a
b
ꢁ
1
absorption stretching bands of N-H at 3259 and 3223 cm and C═O groups of (amide
moiety) at 1681and 1673 cm , respectively, and disappearance of stretching bands of
ꢁ
1
NH groups. The compounds [II] and [III]a,b were synthesized by reaction of com-
2
a,b
pound [I] or [I] with two moles from urea or thiourea in absolute ethanol. The FTIR
a
b
spectra showed disappearance absorption bands of the carbonyl amide for starting
material with showed absorption stretching bands for C ¼ N endo cyclic groups at 1649
ꢁ
1
ꢁ1
and 1634 cm for compounds [II] and 1656 and 1662 cm for compounds [III] ,
a,b
a,b
1
respectively. The HNMR spectrum for compound [II] showed a singlet signal at d
a
9.89 ppm for two protons of NH groups, singlet signal at d 7.53 ppm for two protons of
oxazole rings, many signals at region d (7.48–7.00) ppm for four benzene protons, a
1
singlet signal at d 5.39 ppm for two protons of NH groups. The HNMR spectrum for
2
compound [III] showed a signal at d 8.97 ppm for two protons of NH groups, many
a
signals at d (8.93–8.71) ppm four benzene protons and singlet signal at d 8.67 ppm for
two protons of thiazole rings, a signal at d 3.26 ppm for two protons of NH groups.
2

8
T. K. IBRAHEEM ET AL.
The compounds [IV–VII]a,b,c are formed from the reaction of compound [II]a,b or
III]a,b with two moles from different aromatic aldehydes in THF and few drops of
[
pipyridine. The FTIR spectra for Schiff bases showed disappearance of stretching bands
for amine and aldehyde groups for starting material and showed absorption stretching
ꢁ
1
bands of imine groups (-N ¼ CH-) in the region 1656–1619 and 1660–1622 cm for
1
the compounds [IV] , [V] , [VI]
and [VII] , respectively. The HNMR spec-
a,b,c
a,b,c
a,b,c
a,b,c
trum for compound [V] showed singlet signal at d 9.90 ppm for two protons of NH
c
groups, singlet signal at d 7.77 ppm for two protons of CH ¼ N groups, singelat signal
at d 7.58 ppm for two protons of oxazole rings, many signals at d 7.53–6.96 ppm for
twelve aromatic protons, a signal at d 2.54 ppm for six protons of CH groups.
3
The mesomorphic properties
The transition temperatures and mesophase type (texture identity) were investigated by
using optical polarizing microscopy (OPM) and DSC. Phase transition temperatures
observed by OPM were found to be in reasonable agreement with the corresponding
DSC thermograms. The mesophases exhibited by the synthesized compounds were iden-
tified from their optical textures which were observed by OPM, using the classification
systems reported by Richter [47] and Gray and Goodby [48]. The thermotropic
liquid crystalline properties and thermodynamic data of [IV]a,b,c, [V]a,b,c, [VI]a,b,c and
[
VII]a,b,c are summarized in Table 1.
All the final compounds of 1,3-disubstituted benzene derivatives based on oxazole
ring [IV] , [IV] and [IV] , exhibited liquid crystalline phases characterized by OPM
a
b
c
and showed a smectogenic profile in special smectic A (SmA) phase which was found
to be present in all three compounds. Representative texture of SmA mesophase by
OPM shown by these final compounds are presented in Fig. 1 and shows typical focal
conic SmA texture. Contrary to the compounds [IV]a,b,c, the compounds of 1,4-disub-
stituted benzene core with oxazole ring [V] , [V] and [V] do not reveal any liquid
a
b
c
crystalline behavior, but simply changes from the solid crystalline state to the isotropic
ꢀ
liquid at 312, 223 and 244 C, respectively. The absence of mesomorphic properties of
the [V]a,b,c derivatives may be attributed to the presence of the 2,4-disubstituted oxazole
ring which causes a significant loss of linearity of 1,4-disubstituted benzene core then
disfavoring the mesophases formation [49].
All compounds of 1,3-disubstituted benzene [VI]a,b,c and of 1,4-disubstituted benzene
[
VII]a,b,c derivatives based on thiazole ring were found to be enantiotropic nematic
phase under OPM as shown in Fig. 1. The texture observed by OPM on cooling from
the isotropic liquid showed a typical schlieren texture as shown in Fig. 1 which is a
characteristic of nematic mesophase. The nematic phase of these compounds could be
attributed to the formation of intermolecular hydrogen bonding, i.e., terminal molecular
interaction in case of compounds [VI] and [VII] and to short methyl and methoxy
a
a
groups in terminal substituent in compounds [VI] , [VII] , [VI] and [VII] (the
b
b
c
c
shorter methyl and methoxy groups led to a higher ratio of terminal to lateral intermo-
lecular attraction forces) which are a favor of nematic mesophase formation [50].
It is interesting to compare compounds [V]a,b,c with compounds [VII]a,b,c
.
Compounds [V]a,b,c and [VII]a,b,c have the same central rigid core and the same

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
9
ꢀ
Table 1. Phase transition temperatures ( C) and transition enthalpies DH(kJ/mol) of compounds
[
ꢀ
IV]a,b,c, [V]a,b,c, [VI]a,b,c and [VII]a,b,c determined by DSC (10 C/min) during first heating.
Cr: Crystal; N: Nematic; SmA: smectic A; I: Isotropic transition.
Figure 1. Polarizing microscopy images: The image on the left is focal conic texture of SmA phase at
ꢀ
1
97 C on cooling of compound [IV] . The image on the right shows the schlieren texture of the
b
ꢀ
nematic phase at 269 C upon cooling of compound [VI] . (200 X magnification).
c

1
0
T. K. IBRAHEEM ET AL.
terminal substituent. The difference between those compounds is the bioxazole rings in
V]a,b,c and the bithiazole rings in [VII]a,b,c. All compounds [VII]a,b,c exhibit nematic
[
phase, while, compounds [V]a,b,c do not show mesomorphic behavior, and only crystal
to isotropic liquid transition was observed. This results show that the presence of thia-
zole ring has a profound influence on the mesogenic properties. This could be due to,
first, the bend associated with the exocyclic bonds in the 2- and 4- positions of the 1,3-
oxazole unit was too extreme to achieve the necessary ordered mesophase packing, and,
second, the increased polarizability of the thiazole ring compared to the oxazole ring.
The same results were obtained with the analogous 1,3-oxazole and 1,3-thiazole deriva-
tives containing a symmetric rigid core [51–53].
Conclusion
This work describes the synthesis and liquid crystalline properties of 1,3- and 1,4-disub-
stituted benzene core with oxazole or thiazole rings. The reported 1,3-disubstituted ben-
zene core with oxazole ring were all found to exhibit SmA phases which were in
contrast to that compounds of 1,4-disubstituted benzene that do not show any liquid
crystalline behavior. All compounds containing symmetrical bithiazole exhibit purely
ennatiotropic nematic phases. The influence of the symmetrical bioxazole and bithiazole,
the substituted position on central benzene ring and the terminal substituents on the
type and temperature range of the mesomorphous properties of these compounds has
been discussed.
ORCID
Ammar H. Al-Dujaili
http://orcid.org/0000-0002-1759-8585
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