Hockey stick liquid crystals based on a 2,5-asymmetric disubstituted [1,3,4]oxadiazole core

Article abstract of DOI:10.1080/15421406.2011.556444

The article describes the liquid-crystalline properties of some 2,5-asymmetric disubstituted [1,3,4]oxadiazole derivatives containing an azo and an ester linkage obtained through esterification of 2-(4-methoxyphenyl)-5-(4- hydroxyphenyl)-[1,3,4]oxadiazole with a series of 4-(4-alkoxyphenyl)-benzoic acids containing 6-10 and 18 aliphatic carbon atoms. All reported compounds present liquid-crystalline properties, evidenced by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), with nematic and smectic C type structures, with very large range of stability of mesophases (between 130°C and 198°C on heating and 134°C and 214°C on cooling). Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.556444

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Hockey Stick Liquid Crystals Based
on a 2,5-Asymmetric Disubstituted
[1,3,4]Oxadiazole Core
Elena-Raluca Cioanca a , Elena Luiza Epure a , Irina Carlescu a
,
Gabriela Lisa a , Daniela Wilson a b , Nicolae Hurduc a & Dan Scutaru
a
a Department of Natural and Synthetic Polymers , Gheorghe Asachi
Technical University of Iaşi , Iaşi , Romania
b Roy & Diana Vagelos Laboratories, Department of Chemistry ,
University of Pennsylvania , Philadelphia , Pennsylvania , USA
Published online: 08 Apr 2011.
To cite this article: Elena-Raluca Cioanca , Elena Luiza Epure , Irina Carlescu , Gabriela Lisa , Daniela
Wilson , Nicolae Hurduc & Dan Scutaru (2011) Hockey Stick Liquid Crystals Based on a 2,5-Asymmetric
Disubstituted [1,3,4]Oxadiazole Core, Molecular Crystals and Liquid Crystals, 537:1, 51-63, DOI:
10.1080/15421406.2011.556444
To link to this article: http://dx.doi.org/10.1080/15421406.2011.556444
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Mol. Cryst. Liq. Cryst., Vol. 537: pp. 51–63, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.556444
Hockey Stick Liquid Crystals Based on a 2,5-
Asymmetric Disubstituted [1,3,4]Oxadiazole Core
ELENA-RALUCA CIOANCA,1 ELENA LUIZA
EPURE,1 IRINA CARLESCU,1 GABRIELA LISA,1
DANIELA WILSON,1,2 NICOLAE HURDUC,1 AND
DAN SCUTARU1
1Department of Natural and Synthetic Polymers, Gheorghe Asachi
Technical University of Ias¸i, Ias¸i, Romania
2Roy & Diana Vagelos Laboratories, Department of Chemistry,
University of Pennsylvania, Philadelphia, Pennsylvania, USA
The article describes the liquid-crystalline properties of some 2,5-asymmetric disub-
stituted [1,3,4]oxadiazole derivatives containing an azo and an ester linkage
obtained through esterification of 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-
[1,3,4]oxadiazole with a series of 4-(4-alkoxyphenyl)-benzoic acids containing
6–10 and 18 aliphatic carbon atoms. All reported compounds present liquid-
crystalline properties, evidenced by differential scanning calorimetry (DSC) and
polarizing optical microscopy (POM), with nematic and smectic C type structures,
with very large range of stability of mesophases (between 130ꢀC and 198ꢀC on heat-
ing and 134ꢀC and 214ꢀC on cooling).
Keywords asymmetric [1,3,4]oxadiazole; bent-core; hockey stick liquid crystals
Introduction
Discovery of bent-shaped (banana, boomerang, bow) liquid crystals (LCs) opened a
very promising area of new mesophases. Despite nonlinear molecules being con-
sidered as poor liquid crystals, the synthesis of 1,3-phenylene bis[4-(4-n-octyloxyphe-
nyliminomethyl)benzoate] by Matsunaga and coworkers opened a new era in the
field of bent core LCs [1,2]. Since then, many banana-shaped molecules have been
synthesized and several reviews have been written [3–7]. As a consequence, the term
banana-shaped mesogens is used as a synonym for a new field of liquid crystals [8].
The first research in the field of bent-core liquid crystals focused on synthesis
and characterization of symmetrical molecules. Their mesomorphic properties were
significantly modified by varying the number of aromatic units, lateral substituents
of the benzene rings, and types of terminal groups [9–12].
Significant attention was focused on 2,5-disubstituted-[1,3,4]oxadiazole deriva-
tives that contain a symmetric five-membered heterocycle as a central unit. The
Address correspondence to Dan Scutaru, Department of Natural and Synthetic Polymers,
Faculty ofChemicalEngineering andEnvironmental Protection, GheorgheAsachiTechnical Uni-
versity of Ias¸i, 73 Dimitrie Mangeron Street, Ias¸i 700050, Romania. E-mail: dscutaru@ch.tuiasi.ro
51

52
E.-R. Cioanca et al.
2,5-disubstitution provided a convenient bent shape useful for studying the depen-
dence of the LC properties on the molecular design [13], and several symmetric or
asymmetric 2,5-disubstituted-[1,3,4]oxadiazole derivatives have been synthesized
[14–18]. The compounds exhibited different types of monotropic or enatiotropic
mesophases such as nematic, smectic, or biaxial nematic.
In the last period, a special kind of nonsymmetrical bent-shaped liquid crystal
of hockey-stick form was developed, and these compounds are considered at the
borderline between bent core and rod-like mesogens [19–22].
The article presents the synthesis, structural characterization, and mesomorphic
behavior of a series of hockey-stick-like mesogens based on an [1,3,4]oxadiazole cen-
tral core and with a strongly nonsymmetrical molecular shape. The new compounds
contain five aromatic rings, connected via ester and azo groups. One arm of the new
mesogens presents three benzene units, with different alkoxy terminal groups,
whereas the other one consists of only one 4-methoxy-benzene ring. All synthesized
compounds show liquid-crystalline properties. Unexpectedly, taking into account
the presence of the azo linking group, the range of stability of mesophases is extremely
large (up to 198ꢀC on heating and up to 214ꢀC on cooling). For a better understand-
ing of the liquid-crystalline behavior of the synthesized compounds, molecular simu-
lations were performed using Materials Studio software. The presence of azobenzen
moieties can induce photoresponsive properties in the system (due to the capacity to
generate trans-cis isomerization under ultraviolet-visible [UV–VIS] irradiation) [23].
Experimental
Materials
All reagents, solvents, and starting materials were purchased from Aldrich (Munich,
Germany) and Merck (Darmstadt, Germany) and were used without further purifi-
cation unless otherwise noted. The 4-(4-alkoxyphenylazo)-benzoic acids (5a-f)
were obtained by adapting literature data [18]. All reactions involving N,N0-
dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) were per-
formed in anhydrous dichloromethane, under a dry atmosphere of nitrogen. Silicagel
60 (Merck (Darmstadt, Germany)) was used for column chromatography.
Instrumentation
Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance
DRX 400 MHz spectrometer (Rheinstein, Germany). Chemical shifts were reported
in ppm relative to tetramethylsilane (TMS) as internal standard. Infrared (IR) spec-
tra were recorded using a Nicolet (Connecticut, USA) Magna 550 Fourier transform
infrared (FTIR) spectrometer (NaCl crystal window). Mass spectra were recorded
on a quadrupole time-of-flight mass spectrometer equipped with an electrospray
ion source (Agilent 6520 Accurate Mass Q-TOF LC=MS (Santa Clara, CA,
USA)). Transition temperatures were determined using a Linkam heating stage
and Linksys 32 temperature control unit (Surrey, BC, Canada) in conjunction with
a Axioscop 40 Zeiss polarizing optical microscope (Jena, Germany) and Qimaging=
Retiga-1000 R camera (Surrey, BC, Canada) for image capture, and the transitions
were confirmed by analysis (Mettler-Toledo (Toledo, OH, USA) differential scan-
ning calorimetry (DSC) TGA-SDTA 851e). Heating and cooling cycles were run
at rates of 10ꢀC=min under a nitrogen atmosphere, with sample measured in

[1,3,4]Oxadiazole Liquid Crystals
53
closed-lid aluminum pans. Mesophase type was assigned by visual comparison
(under the microscope) with known phase standards.
All thermal analyses were performed on 2.5–4.5 mg samples on a Mettler-Toledo
TGA SDTA851e derivatograph in an N2 atmosphere, with a flow rate of 20 mL=min
and a heating rate of 10ꢀC=min from 25ꢀC to 900ꢀC. In order to obtain comparable
data, constant operational parameters were kept for all samples.
The molecular simulations were performed using Materials Studio 4.0. software
[24]. The minimum energy conformation was obtained using a molecular mechanics
procedure, Forcite module (Dreiding and Compass force fields, alternatively with
molecular dynamics, in order to identify the global minimum of the energy value).
The dipole moment values corresponding to the oxadiazole derivatives were calcu-
lated using the DMol3 module, AM1 method.
Synthesis
4-Methoxybenzoyl Chloride (1). In a round-bottom flask 4-methoxybenzoic acid
(10 g, 65.72 mmol) and 200 mL thionyl chloride were refluxed for 1.5 h, after which
the thionyl chloride in excess was distillated under vacuum. The acyl chloride was
used immediately without purification.
N,N0-bis-(4-Methoxybenzoyl-hydrazine (2). Acid chloride (1) was dissolved in
100 mL of anhydrous CH2Cl2 and hydrazine hydrate (9.6 mL, 190 mmol) was added
using a dropping funnel. After 10h of refluxing, the reaction mixture was rotary
evaporated and the product was purified by recrystallization from methanol. A
yellow-white product, yield 5.89 g (60%), was obtained. 1H-NMR dH (DMSO): 10.29
(s, 2H, -NH-), 7.90 (d, 4H, Ar), 7.05 (d, 4H, Ar), 3.82 (s, 6H, -OCH3).13C-NMR dC
(DMSO): 165.39, 161.99, 129.30, 124.78, 113.7, 55.4.
2,5-bis-(4-Methoxyphenyl-[1,3,4]oxadiazole (3). Compound
2
(3 g, 5 mmol)
dissolved in 120 mL of anhydrous toluene was heated and stirred at 80ꢀC, POCl3
(10.94 mL, 50 mmol) was added and the mixture was refluxed for 2 h. Reactants
conversion was checked by thin-layer chromatography (TLC) (silica gel plates=CH2Cl2,
the reaction product being strongly fluorescent). After cooling and concentration, the
compound was purified by column chromatography (CH2Cl2). A white product, yield
1
2.59 g (74%), was obtained. H-NMR dH (DMSO): 8.04 (d, 4H, Ar), 7.16 (d, 4H, Ar),
3.85 (s, 6H, -OCH3). 13C-NMR dC (DMSO): 163.46, 161.96, 128.40, 115.80, 114.83, 55.52.
2-(4-Methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4]oxadiazole (4). Compound
3
(1g, 3.54 mmol) and 20mL HBr (66%) were stirred at 120ꢀC for 30 min under TLC
monitoring. The reaction mixture was cooled down and poured into 50 mL H2O,
filtered off, and purified by column chromatography (CH2Cl2: ethyl acetate 2:1.2,
1
v=v). A white product, yield 0.52g (58%), was obtained. H-NMR dH (DMSO):
10.28 (s, 1H, -OH), 7.97 (d, 4H, Ar), 7.07 (d, 4H, Ar), 3.86 (s, 3H, -OCH3).
13C-NMR dC (DMSO): 163.74, 163.49, 161.89, 160.63, 128.54, 128.45, 128.32, 116.13,
114.82, 114.29 (10 C aromatic), 55.51 (-OCH3).
General Method of Esterification
A mixture of 1 equiv. of compound (4), 1 equiv. of aromatic type acid 5, and a cata-
lytic amount of DMAP, dissolved in dry CH2Cl2, was stirred for a few minutes and

54
E.-R. Cioanca et al.
then 1.1 equiv. of DCC dissolved in dry dichloromethane was added dropwise. The
reaction mixture was left under stirring for 24 h at room temperature under an inert
atmosphere of nitrogen. After removal by filtration of the dicyclohexylurea (DCU),
the solvent was evaporated and the product was purified by column chromatography
on silica gel using a mixture of CH2Cl2:ethyl acetate ¼ 15:1 as eluent.
5-(4-Methoxyphenyl)-2-(4-hexyloxyphenylazo-4-phenylcarbonyloxy)phenyl-[1,3,4]
oxadiazole (6a). Quantities: 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4]oxad-
iazole (4) (0.2 g, 0.746 mmol), 4-(4-hexyloxyphenylazo)-benzoic acid (5a) (0.243 g,
0.745 mmol), DCC (0.169 g, 0.819 mmol), DMAP (catalytic), 100 mL dichloro-
methane. Orange fluffy product, g ¼ 28% (0.12 g), liquid crystal: 148ꢀC (K=LC),
1
346ꢀC (LC=I), 331ꢀC (I=LC), 117ꢀC (LC=K), H-NMR dH ppm (CDCl3): 8.32 (d,
2H, Ar), 8.20 (d, 2H, -Ar), 8.07 (d, 2H, Ar), 7.96 (m, 4H, -Ar), 7.43 (d, 2H, Ar),
7.02 (m, 4H, Ar), 4.05 (t, 2H, -O-CH2), 3.88 (s, 3H, -OCH3), 1.82 (cv, 2H, -CH2),
1.49 (cv, 2H, -CH2), 1.36 (m, 4H, -CH2), 0.92 (t, 3H, -CH3). 13C-NMR dC ppm
=
(CDCl3): 164.68 (-C O esteric), 164.23, 163.58, 162.65, 162.51, 156.12, 153.50,
147.00, 131.31, 129.99, 128.77, 128.30, 125.37, 122.64, 122.56, 121.95, 116.52,
114.94, 114.36, (18 C aromatic), 68.56 (-O-CH2), 55.49 (OCH3), 31.58, 29.18,
25.71, 22.60, 14.00 (5 C aliphatic). FTIR (KBr, cmꢁ1): 2929.86, 2866.21 (n CH,
þ
=
aliphatic), 1734 (n > C O), 1604.77 (n O-C). m=z (CHCl3): 576 [M] .
5-(4-Methoxyphenyl)-2-(4-heptyloxyphenylazo-4-phenylcarbonyloxy)phenyl-[1,3,4]
oxadiazole (6b). Quantities: 2-(4-methoxyphenyl)-5-(4-hidroxyphenyl)-[1,3,4] oxad-
iazole (4) (0.2 g, 0.746 mmol), 4-(4-heptyloxyphenylazo) benzoic-acid (5b) (0.253 g,
0.744 mmol), DCC (0.169 g, 0.819 mmol), DMAP (catalytic), 100 mL dichloro-
methane. Orange fluffy product, g ¼ 34% (0.12 g), liquid crystal: 138ꢀC (K=LC),
1
293ꢀC (LC=I), 284ꢀC (I=LC), 105ꢀC (LC=K), H-NMR dH ppm (CDCl3): 8.37 (d,
2H, Ar), 8.24 (d, 2H, -Ar), 8.11 (d, 2H, Ar), 8.01 (m, 4H, -Ar), 7.47 (d, 2H, Ar),
7.06 (m, 4H, Ar), 4.09 (t, 2H, -O-CH2), 3.92 (s, 3H, -OCH3), 1.86 (cv, 2H, -CH2),
1.51 (cv, 2H, -CH2), 1.36 (m, 6H, -CH2), 0.93 (t, 3H, -CH3). 13C-NMR dC ppm
=
(CDCl3): 164.65 (-C O esteric), 164.26, 163.54, 162.58, 162.43, 156.03, 153.42,
146.88, 131.32, 129.89, 128.75, 128.29, 125.36, 122.62, 122.58, 121.88, 116.40,
114.87, 114.56 (18 C aromatic), 68.49 (-O-CH2), 55.87 (OCH3), 31.77, 29.70,
29.17, 25.97, 22.61, 14.08 (6 C aliphatic). FTIR (KBr, cmꢁ1): 2926.01, 2856.57 (n
þ
=
CH, aliphatic), 1734 (n > C O), 1604.77 (n O-C). m=z (CHCl3): 590 [M] .
5-(4-Methoxyphenyl)-2-(4-octyloxyphenylazo-4-phenylcarbonyloxy)phenyl-[1,3,4]
oxadiazole (6c). Quantities: 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4] oxad-
iazole (4) (0.2 g, 0.746 mmol), 4-(4-octyloxyphenylazo)-benzoic acid (5c) (0.264 g,
0.745 mmol), DCC (0.169 g, 0.819 mmol), DMAP (catalytic), 100 mL dichloro-
methane. Orange nacreous product, g ¼ 24% (0.11 g), liquid crystal: 145ꢀC
1
(K=LC), 324ꢀC (LC=I), 321ꢀC (I=LC), 120ꢀC (LC=K), H-NMR dH ppm (CDCl3):
8.37 (d, 2H, Ar), 8.24 (d, 2H, -Ar), 8.11 (d, 2H, Ar), 8.01 (m, 4H, -Ar), 7.47 (d, 2H,
Ar), 7.06 (m, 4H, Ar), 4.09 (t, 2H, -O-CH2), 3.92 (s, 3H, -OCH3), 1.86 (cv, 2H, -
CH2), 1.51 (cv, 2H, -CH2), 1.33 (m, 8H, -CH2), 0.92 (t, 3H, -CH3). 13C-NMR dC
=
ppm (CDCl3): 164.66 (-C O esteric), 164.26, 163.54, 162.59, 162.43, 156.03, 153.42,
146.88, 131.32, 129.89, 128.75, 128.29, 125.36, 122.62, 122.57, 121.88, 116.40, 114.87,
114.56 (18 C aromatic), 68.50 (-O-CH2), 55.49 (OCH3), 31.81, 29.34, 29.33, 29.17,
26.02, 22.66, 14.09 (7 C aliphatic). FTIR (KBr, cmꢁ1): 2926.01,þ2854.64 (n CH,
=
aliphatic), 1734 (n > C O), 1612.49 (n O-C). m=z (CHCl3): 604 [M] .

[1,3,4]Oxadiazole Liquid Crystals
55
5-(4-Methoxyphenyl)-2-(4-nonyloxyphenylazo-4-phenylcarbonyloxy)phenyl-[1,3,4]
oxadiazole (6d). Quantities: 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4]
oxadiazole (4) (0.2 g, 0.746 mmol), 4-(4-nonyloxyphenylazo) benzoic acid (5d)
(0.274 g, 0.746 mmol), DCC (0.169 g, 0.819 mmol), DMAP (catalytic), 100 mL
dichloromethane. Orange fluffy product, g ¼ 26% (0.12 g), liquid crystal: 105ꢀC
(K=LC), 142ꢀC (LC=LC), 275ꢀC (LC=I), 274ꢀC (I=LC), 112ꢀC (LC=LC), 100ꢀC
(LC=K), 1H-NMR dH ppm (CDCl3): 8.34 (d, 2H, Ar), 8.22 (d, 2H, -Ar), 8.09
(d, 2H, Ar), 7.98 (m, 4H, -Ar), 7.45 (d, 2H, Ar), 7.04 (m, 4H, Ar), 4.06 (t, 2H,
-O-CH2), 3.90 (s, 3H, -OCH3), 1.83 (cv, 2H, -CH2), 1.49 (cv, 2H, -CH2), 1.33 (m,
10H, -CH2), 0.89 (t, 3H, -CH3). 13C-NMR dC ppm (CDCl3): 164.66 (-C O
=
esteric), 164.27, 163.55, 162.60, 162.43, 156.04, 153.43, 146.89, 131.32, 129.90,
128.76, 128.30, 125.37, 122.62, 122.58, 121.88, 116.40, 114.88, 114.57 (18 C
aromatic), 68.51 (-O-CH2), 55.49 (OCH3), 31.89, 29.70, 29.53, 29.39, 29.18, 26.01,
22.68, 14.11 (8 C aliphatic). FTIR (KBr, cmꢁ1): 2924.08, 285þ4.64 (n CH,
=
aliphatic), 1735.93 (n > C O), 1606.7 (n O-C). m=z (CHCl3): 618 [M] .
5-(4-Methoxyphenyl)-2-(4-decyloxyphenylazo-4-phenylcarbonyloxy)phenyl-[1,3,4]
oxadiazole (6e). Quantities: 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4] oxad-
iazole (4) (0.2 g, 0.746 mmol), acid 4-(4-decyloxyphenylazo) benzoic (5d) (0.285 g,
0.746 mmol), DCC (0.169 g, 0.819 mmol), DMAP (catalytic), 100 mL dichloro-
methane. Orange fluffy product, g ¼ 42% (0.2 g), liquid crystal: 141ꢀC (K=LC),
1
313ꢀC (LC=I), 309ꢀC (I=LC), 122ꢀC (LC=K), H-NMR dH ppm (CDCl3): 8.35 (d,
2H, Ar), 8.22 (d, 2H, -Ar), 8.09 (d, 2H, Ar), 7.98 (m, 4H, -Ar), 7.45 (d, 2H, Ar),
7.04 (m, 4H, Ar), 4.06 (t, 2H, -O-CH2), 3.90 (s, 3H, -OCH3), 1.83 (cv, 2H, -CH2),
1.49 (cv, 2H, -CH2), 1.28 (m, 12H, -CH2), 0.89 (t, 3H, -CH3). 13C-NMR dC ppm
=
(CDCl3): 164.65 (-C O esteric), 164.26, 163.54, 162.57, 162.41, 156.01, 153.39,
146.86, 131.31, 129.87, 128.74, 128.29, 125.36, 122.61, 122.57, 121.86, 116.39,
114.86, 114.54 (18 C aromatic), 68.48 (-O-CH2), 55.49 (OCH3), 31.90, 29.70,
29.55, 29.38, 29.32, 29.17, 26.01, 22.68, 14.12 (9C aliphatic). FTIR (KBr, cmꢁ1):
=
2920.22, 2850.78 (n CH, aliphatic), 1735.93 (n > C O), 1606.7 (n O-C). m=z
(CHCl3): 632 [M]þ.
5-(4-Methoxyphenyl)-2-(4-octadecyloxyphenylazo-4-phenylcarbonyloxy)phenyl-[1,3,
4]oxadiazole (6f). Quantities: 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4]
oxadiazole (4) (0.2 g, 0.746 mmol), acid 4-(4-octadecyloxyphenylazo) benzoic (5d)
(0.365 g, 0.745 mmol), DCC (0.169 g, 0.819 mmol), DMAP (catalytic), 100 mL
dichloromethane. Light orange nacreous product, g ¼ 20% (0.11 g), liquid crystal:
95ꢀC (K=LC), 137ꢀC (LC=LC), 260ꢀC (LC=I), 258ꢀC (I=LC), 123ꢀC (LC=LC),
1
77ꢀC (LC=K), H-NMR dH ppm (CDCl3): 8.34 (d, 2H, Ar), 8.21 (d, 2H, Ar), 8.08
(d, 2H, Ar), 7.87 (m, 4H, Ar), 7.45 (d, 2H, Ar), 7.03 (m, 4H, Ar), 4.06 (t, 2H,
-O-CH2), 3.90 (s, 3H, -OCH3), 1.83 (cv, 2H, -CH2), 1.52 (cv, 2H, -CH2), 1.27 (m,
26H, -CH2), 0.88 (t, 3H, -CH3). 13C-NMR dC ppm (CDCl3): 164.72 (-C O
=
esteric), 164.28, 163.62, 162.68, 162.55, 156.18, 153.55, 147.05, 131.34, 130.03,
128.81, 128.34, 125.38, 122.66, 122.58, 122.00, 116.58, 114.98, 114.65 (18 C
aromatic), 68.60 (-O-CH2), 55.51 (OCH3), 31.96, 29.72, 29.68, 29.61, 29.59, 29.53,
29.49, 29.47, 29.40, 29.37, 29.28, 29.24, 29.19, 29.16, 26.05, 22.70, 14.08 (17 C
aliphatic). FTIR (KBr, cmꢁ1): 2918.29, 2848.85 (n CH, aliphatic), 1735.93
þ
=
(n > C O), 1604.77 (n O-C). m=z (CHCl3): 742 [M] .

56
E.-R. Cioanca et al.
Results and Discussion
Synthesis
The synthetic strategy involved the synthesis of the phenolic asymmetric [1,3,4]
oxadiazole core 4 and of the 4-(4-alkoxyphenylazo)-benzoic acids followed by
esterification, which provided the final compounds (Scheme 1).
The asymmetric [1,3,4]oxadiazole core was obtained by reacting 4-methoxyben-
zoyl chloride with hydrazine hydrate in anhydrous dichloromethane to form N,N0-
bis-(4-methoxybenzoyl)-hydrazine 2 followed by intramolecular cyclization to form
the symmetric 2,5-bis-(4-methoxyphenyl-[1,3,4]oxadiazole 3. The asymmetric
2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4]oxadiazole (4) was obtained via par-
tial hydrolysis of 3 with 66% HBr (120ꢀC, 30 min). The reaction was monitored by
TLC until completion; longer reaction times afforded mainly the symmetric diphenol.
The mesogenic units were obtained through a Williamson reaction by alkylating
4-(4-hydroxyphenylazo)-benzoic acid 5 with alkyl bromides containing 6–10 and 18
Scheme 1. Synthesis of the asymmetric [1,3,4]oxadiazole derivatives: 6a, n ¼ 6; 6b, n ¼ 7; 6c,
n ¼ 8; 6d, n ¼ 9; 6e, n ¼ 10; 6f, n ¼ 18.

[1,3,4]Oxadiazole Liquid Crystals
57
carbon atoms in KOH alcoholic solution via a literature procedure [25]. All (5a-f)
type compounds exhibited liquid-crystalline properties, evidenced by polarizing
optical microscopy (POM) and DSC and confirming the literature data [18].
Esterification of 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4] oxadiazole 4
with the 4-(4-alkoxyphenylazo)-benzoic acids (5a-f) in the presence of DCC and
catalytic quantities of DMAP afforded the final products 6a-f in 20–42% yield.
Liquid-Crystalline Properties
Investigation of the mesomorphic properties of asymmetric [1,3,4]oxadiazole deriva-
tives 6a-6f by POM and DSC evidenced enantiotropic liquid-crystalline properties
for all compounds.
The asymmetric oxadiazol derivatives exhibited a very large range of meso-
phases (between 130ꢀC to 198ꢀC on heating and 134ꢀC to 214ꢀC on cooling; Table 1).
Generally, the mesomorphic properties of derivatives 6a-f were of nematic
and smectic C type. Table 2 summarizes the phase transition temperatures of the
asymmetric [1,3,4]oxadiazole derivatives 6a-6f.
All derivatives showed a very good thermal stability, evidenced by thermal
analysis studies. Thermogravimetric studies evidenced one or two degradation stages
for all 6a-6f derivatives, the initial temperature at which thermal degradation for the
first stage begins (Tonset) being situated between 340ꢀC and 360ꢀC (Table 2).
Derivative 6a containing the smallest number of carbon atoms in the alkyl chain
(n ¼ 6) showed the largest existence range of the mesophase, both on heating (198ꢀC)
and on cooling (214ꢀC), whereas the last compound of the class, 6f (n ¼ 18), showed
the smallest one, both on heating (122ꢀC) and on cooling (134ꢀC).
Due to thermostability reasons, in some cases isotropization temperatures were
determined only from POM investigations.
Compound 6a showed the presence of a single phase transition on both heating
and cooling cycles: K–LC, LC-I (Figure 1). On heating, compound 6a presented
characteristic nematic Schlieren textures (Figures 1a and 1b) from 148ꢀC up to
346ꢀC when the isotropization occurred.
On cooling, nematic droplets were observed that unified into an extended texture
with dark brush Schlieren defects. The nematic domain was clear to about 215ꢀC,
when a ribbon-like texture occurred (Figure 1c).
The behavior of compounds 6b, 6c, and 6e are similar to 6a (Figures 2a and 2b).
Table 1. Range of mesophases corresponding to compounds 6a–6f
Range of mesophases (ꢀC)
Compounds
On heating
On cooling
6a
6b
6c
6d
6e
6f
n ¼ 6
n ¼ 7
n ¼ 8
n ¼ 9
n ¼ 10
n ¼ 18
148–346
138–293
145–324
142–275
141–313
137–260
331–117
284–105
321–120
274–112
309–122
258–123

58

[1,3,4]Oxadiazole Liquid Crystals
59
Figure 1. Optical texture of compound 6a: (a) 292ꢀC, first heating; (b) 321ꢀC, first cooling;
and (c) 215ꢀC, first cooling.
In the case of compound 6e, DSC curves showed the presence of a single phase
transition on both heating and cooling cycles: K-LC and LC-I. Unfortunately, for
thermostability reasons, the LC-I=I-LC transitions could not be assigned on DSC
heating=cooling curves, but from the microscopy data, the isotropization was
observed at 313ꢀC; on cooling the liquid-crystalline ordering started at 309ꢀC and
was stable down to 122ꢀC when crystallization occurred.
In the case of compound 6d, the DSC curves revealed a polymorphism phenom-
enon both on heating and cooling cycles, due to the reorganization of molecules into
a solid state (Figure 3). The nematic mesophase started at 142ꢀC and was stable up
to 275ꢀC with specific Schlieren textures (Figure 4).
The mesomorphic behavior of the last compound of the series, 6f, is different
when compared with compounds 6a-e. This could be explained by the presence of
the long aliphatic segment, susceptible to inducing a slight geometric modification
in the solid state.
Although the same polymorphism has been observed, on successive heating and
cooling cycles, the mesophase texture was different when compared with the other
compounds (Figure 5, they are less birefringent and the Schlieren defects suggest a
nonpolar smectic C phase).
Figure 2. Optical texture of compound 6b: (a) 227ꢀC, first heating; and (b) 256ꢀC, first cooling.

60
E.-R. Cioanca et al.
Figure 3. The DSC curves of compound 6d: 1, first heating, and 2, first cooling, 10ꢀC=min.
The thermal behavior of asymmetric derivatives 6a-6f varies, both on heating
and cooling cycles, as a function of the number of carbon atoms of the alkyl end
chain [26], respecting the odd–even effect (Figure 6).
Molecular Modeling Study
For a better understanding of the LC behavior, molecular modeling studies were
performed. The geometry of the molecules was described by different parameters,
Figure 4. Optical texture of compound 6d at 255ꢀC, first heating.

[1,3,4]Oxadiazole Liquid Crystals
61
Figure 5. Optical texture of compound 6f. (a) 192ꢀC, second heating; and (b) 140ꢀC, first
cooling.
such as the length of arms (Larm), length between arms (L), diameter (D), and
bending angle (w); Figure 7).
A first observation is that in the case of the synthesized 2,5-asymmetric disubsti-
tuted [1,3,4]oxadiazole derivatives, the molecule curvature is not as pronounced as in
the case of a classical banana-shaped LC (Figure 8a). This can explain a better order-
ing ability, reflected by relatively low values corresponding to the coefficient of
asymmetry L=D (Table 3). In the case of compound 6f one can appreciate that
the geometry can be considered almost linear (Figure 8b).
The asymmetric substitution of the oxadiazolic core induces high values of the
corresponding dipole moments, situated in the range of 8.4–8.7 D (Table 3). The
strong dipolar interactions between molecules can explain the very good stability
of the mesophases, which also follows the odd–even effect (Figure 6). We assume
that the terminal chains containing an odd number of carbon atoms may generate
more disorder in the system and, the because the dipole moment values are similar
for all of the synthesized structures, the global effect will be the decreasing of the
dipole–dipole interactions.
Figure 6. Variation of isotropization and crystallization temperatures as a function of the
even–odd number of carbon atoms in the flexible end chain.

62
E.-R. Cioanca et al.
Figure 7. Theoretical model of a banana-shaped molecule.
Figure 8. Molecular geometry: (a) compound 6a and (b) compound 6f.
Table 3. Geometric characteristics and dipole moments corresponding to
compounds 6a-f
Number Molecule
of carbon
atoms
Short Long Bending Dipole
arm
length
Diameter
˚
(A)
arm
angle
(ꢀ)
moment
(Debye)
˚
(A)
˚
L=D (A)
˚
(A)
Comp.
6a
6b
6c
6d
6e
6f
6
7
8
9
10
18
32.8
33.8
35.7
37.1
38.2
48.0
6.9
7.1
7.3
7.5
7.7
8.4
4.7
4.8
4.9
4.9
5.0
5.7
8.6
8.6
8.6
8.6
8.6
8.6
26.9
28.2
29.3
30.6
31.7
41.5
142.1
142.3
141.0
141.2
140.0
137.1
8.4
8.5
8.5
8.5
8.4
8.7

[1,3,4]Oxadiazole Liquid Crystals
63
Conclusions
The article presents synthesis and structural characterization of some 2,5-asymmetric
disubstituted [1,3,4]oxadiazole derivatives containing azo and ester linkages. All of the
reported compounds present enantiotropic liquid-crystalline properties, mainly with
nematic and smectic C type phase structures, with a very large range of mesophases.
The mesophase stability could be explained by the high values of the dipole moment
corresponding to the asymmetric substituted derivates. The transition temperatures
follow the odd–even effect of the hydrocarbon segment. All compounds show a good
thermal stability in the mesophase domain, evidenced by thermal analysis studies.
Acknowledgment
This work was supported by CNCSIS-UEFISCSU (Romania), project number
PNII-IDEI 356=2008.
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