Liquid crystalline behaviours of the complexes based on 1,3,4-oxadiazole derivative and benzoic acid

Article abstract of DOI:10.1002/cjoc.201100276

A non-liquid crystal compound (4-POXD-6) containing the 1,3,4-oxadiazole group and pyridyl (py) group was synthesized. And a series of supramolecular mesogenic complexes were obtained by mixing 4-POXD-6 and 4-alkoxybenzoic acids (Cn, n=6, 8, 12). The liquid crystalline properties of 4-POXD-6/Cn (n=6, 8, 12) were investigated by means of differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction. It was found that 4-POXD-6/Cn (6, 8) display monotropic nematic and smectic A phases, while 4-POXD-6/C12 exhibits a SmX phase in the heating process and a SmA phase in the cooling run. Variable-temperature FTIR spectroscopic studies revealed hydrogen bonds existed in both crystalline and liquid crystalline phases. Copyright

Full text of DOI:10.1002/cjoc.201100276

FULL PAPER
DOI: 10.1002/cjoc.201100276
Liquid Crystalline Behaviours of the Complexes Based on
1,3,4-Oxadiazole Derivative and Benzoic Acid
Zhao, Chengxiao (赵呈孝)
Bai, Binglian(白炳莲)
Wang, Haitao(王海涛)
Zhang, Yan(张艳)
Ran, Xia(冉霞)
Li, Min*(李敏)
Key Laboratory for Automobile Materials, Ministry of Education, Institute of Materials Science and Engineering,
Jilin University, Changchun, Jilin 130012, China
A non-liquid crystal compound (4-POXD-6) containing the 1,3,4-oxadiazole group and pyridyl (py) group was
synthesized. And a series of supramolecular mesogenic complexes were obtained by mixing 4-POXD-6 and
4-alkoxybenzoic acids (Cn, n＝6, 8, 12). The liquid crystalline properties of 4-POXD-6/Cn (n＝6, 8, 12) were in-
vestigated by means of differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray dif-
fraction. It was found that 4-POXD-6/Cn (6, 8) display monotropic nematic and smectic A phases, while
4-POXD-6/C12 exhibits a SmX phase in the heating process and a SmA phase in the cooling run. Vari-
able-temperature FTIR spectroscopic studies revealed hydrogen bonds existed in both crystalline and liquid crystal-
line phases.
Keywords 1,3,4-oxadiazole, nematic phase, smectic A, hydrogen bonding, pyridyl
Introduction
dines in detail and the results showed that the com-
plexes mainly exhibit nematic and smectic phases.[6]
Bent-core supramolecular mesogenic complexes built
through hydrogen bonding have attracted more atten-
tions in recenct years[7,8] due to their unique shapes as
well as some new mesophases.[9]
Liquid crystals are a series of ordered soft materials
that play an important role in material science field.[1,2]
Usually, chemical synthesis is used to prepare these soft
materials.[1,3] In recent decades, however, supramolecu-
lar approach has been used to obtain the mesogenic ma-
terials. The supramolecular mesogenic complexes can
be built by specific intermolecular interactions, such as
hydrogen bonding, ionic bonds, and charge-transfer in-
teraction.[4] Hydrogen bonding interaction plays a key
role in supramolecular assembly of liquid crystals due to
its stability, directionality and dynamics. Generally, in-
termolecular hydrogen bonds in calamitic mesogens
could be categorized into two types: axial hydrogen
bonding (the hydrogen bonding is parallel to the mo-
lecular long axis), and lateral hydrogen bonding (the
hydrogen bonding is perpendicular to the molecular
long axis).[4a,5] Much effort has been focussed on the
former, in which axial hydrogen bonds were applied to
generate new and elongated mesogenic units to stabilize
the mesophase. For example, a wide variety of supra-
molecular mesogenic complexes were obtained through
the formation of axial hydrogen bond between carbox-
ylic acids and pyridines.
It is well known that 1,3,4-oxadiazole has a bent an-
gle of ca. 134°. So the 1,3,4-oxadiazole could be used to
build the bent-core supramolecules. Scheme 1 shows
some reported bent-core supramolecular complexes
containing 1,3,4-oxadiazole which were built by hydro-
gen bonding between carboxylic acids and pyri-
dines.[10-12] Madsen and coworkers reported that a series
of symmetric 1,3,4-oxadiazole derivatives with ca. 140°
apex angle showed biaxial nematic phase,[11] which may
originate from a delicate combination of excluded vol-
ume considerations coupled with electrostatic attraction
originating from the large electric dipole moment (ca.
4D) of the 1,3,4-oxadiazole ring. However, complexes
1a—1b showed only smectic A phase and complexes 2a
—2b exhibited nematic and smectic A phases.[8] These
suggest that the formation of hydrogen bonding induces
the bent angle increasing and the angle of 1,3,4-oxa-
diazole compounds are actually rather close to the point
at which the mesomorphism would change from that of
a bent-core to a calamitic mesogen.
Besides the specific intermolecular interactions, the
rigid core shapes of the supramolecules are interesting
to investigate. Kato and coworkers have investigated the
rod-like mesogenic complexes based on hydrogen
bonding interaction between carboxylic acids and pyri-
Herein, we synthesized a bent-core 1,3,4-oxadiazole
derivative (4-POXD-6) with a short wing and mixed it
with 4-alkoxybenzoic acids (Cn, n＝6, 8, 12) to ob-
*
E-mail: minli@jlu.edu.cn; Tel.: 0086-0431-8516 8254
Received August 25, 2011; accepted October 25, 2011.
Chin. J. Chem. 2012, 30, 785—790
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Zhao et al.
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Scheme 1 Structures of the symmetric and unsymmetric com-
Scheme 2 Synthetic route of 4-POXD-6
plexes based on 1,3,4-oxadiazole
O
O
C
Cl
N
N
N
C
O
NHNH2
O
O
N
N
N
H
H
O
H
R
R
NMP
stiring for 12 h
O
O
1
R
N N
O
O
O
N
N
O
O
R
2
C
N
O
a: R = OC8H17
b: R = OC12H25
N
O
O
C
POCl3
reflux for 40 h
tained the unsymmetric bent-core supramolecules of
4-POXD-6/Cn. In the supramolecular complexes, the
unsymmetric bent-core of 4-POXD-6 has no mesophase
and it acts as the proton acceptor, while Cn act as the
proton donor and show smectic C and nematic phases.[13]
After introducing the hydrogen bonding interactions
between 4-POXD-6 and Cn, the complexes 4-POXD-
6/Cn show the homeotropic smectic A and nematic
phase. In addition, the liquid crystalline phases depend
on the length of the alkoxy chain of alkoxybenzoic acid.
O
O
4-PHP-6
4-POXD-6
1179, 1127, 1107, 1077, 1065, 1023, 988, 840, 742, 703
cm . Anal. calcd for C19H21N3O2: C 70.57, H 6.55, N
－
1
12.99; found C 70.76, H 6.87, N 12.72.
Experimental
Preparation of hydrogen bonding complexes
Synthesis
4-POXD-6 and Cn (n＝6, 8, 12) were weighed
equimolar and dissolved separately in THF. The two
solutions were mixed and stirred regularly at room
temperature for 10 h. The complexes were obtained by
removing the THF and dried at 70 ℃ in vacuum.[8]
The synthesis of 2-(4-hexyloxy)phenyl-5-(pyridine-
4-yl)-1,3,4-oxadiazole (4-POXD-6) is outlined in
Scheme 2. 4-(Alkoxy)benzoic acid and isonicotinohy-
drazide were purchased without further purification.
4-(Hexyloxy)benzoyl chloride was obtained from
4-(hexyloxy)benzoic acid (2.2 g) according to the lit-
erature.[13] 4-(Hexyloxy)benzoyl chloride was regularly
rejected into N-methylpyrrolidone (NMP) solution (100
mL) of isonicotinohydrazide (1.37 g) and stirred vigor-
ously for 10 h at room temperature. Then solution was
poured into 500 mL distilled water then filtered to ob-
Characterization
1H NMR spectra were recorded with a Mercury-
300BB 300 MHz spectrometer, using tetramethylsilane
(TMS) as an internal standard. Phase transitional prop-
erties were investigated with a Netzsch DSC 204. The
－
1
rate of heating and cooling was 5 ℃•min ; the weights
of the sample of 4-POXD-6/C6, 4-POXD-6/C8 and
4-POXD-6/C12 were 7.4, 7.3, and 6.4 mg, respectively.
Zinc was used for calibration. The peak maximum was
taken as the phase transition temperature. Texture ob-
servation was conducted on a Leica DMLP polarizing
optical microscope (POM) equipped with a Leitz 350
microscope heating stage. X-ray diffraction was carried
out with a Bruker Avance D8 X-ray diffractometer.
Fourier transform infrared (FTIR) spectra were recorded
with a Perkin-Elmer spectrometer (Spectrum One B).
The sample was pressed into tablet with KBr.
tain
N'-(4-(hexyloxy)benzoyl)isonicotinohydrazide
(4-PHP-6). The dried 4-PHP-6 without any purification
was dissolved in phosphorous oxychloride (POCl3) and
refluxed for about 40 h. The reactants were poured into
the ice-water slowly and white precipitation separated
out. The solid was filtered off, washed with water and
then dissolved in tetrahydrofuran (THF). The solution
was filtered and evaporated. The pure 4-POXD-6 was
obtained through recrystallization from ethanol for the
NMR, FT-IR spectroscopy and elemental analysis.
1H NMR (DMSO, 300 MHz) δ: 8.84—8.86 (d, J＝
5.42 Hz, 2H), 8.04—8.10 (m, 4H), 7.16—7.19 (d, J＝
8.68 Hz, 2H), 4.06—4.11 (t, J＝6.43 Hz, 2H), 1.73—
1.78 (m, 2H), 1.32—1.45 (m, 6H), 0.86—0.91 (t, J＝
6.67 Hz, 3H); FT-IR (KBr, pellet) ν: 2958, 2930, 2868,
2848, 1610, 1585, 1570, 1537, 1497, 1478, 1426, 1415,
1398, 1327, 1302, 1260,
Results and Discussion
Hydrogen bonding in 4-POXD-6/Cn
The complexes formed between 4-POXD-6 and Cn
via hydrogen bonding which were investigated by FTIR
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Chin. J. Chem. 2012, 30, 785—790

Liquid Crystalline Behaviours of the Complexes Based on 1,3,4-Oxadiazole Derivative and Benzoic Acid
spectroscopy (Figure 1). The formation of the hydrogen
bonds in 4-POXD-6/Cn is detected through the modifi-
cation of the O—H and C＝O stretching vibrations of
the acid. The associated O—H stretching vibration in
the complex of 4-POXD-6/C8 located at around 2495
cm－ and 1918 cm－1, which are broad due to the
1
OH…N (py) interaction, whereas carboxylic acid
dimers (Cn) showed the O—H stretching vibration at
－
1
around 2660 and 2550 cm . Meanwhile, the C＝O
stretching vibration of the complex appeared at 1693
－
－
1
1
cm , in contrast it centered at 1678 cm in Cn due to
double hydrogen bonding between COOH groups.[7,8,16]
Figure 2 Temperature-dependent FTIR spectra of 4-POXD-6/
－
1
C8 in the range of 1550—1800 cm .
and the same liquid crystalline phases were also de-
tected in the cooling with the crystalline points of 97, 95
and 89 ℃. No mesophase was detected for 4-POXD-6
in the heating or in cooling run. However, The DSC
curves of 4-POXD-6/Cn (n＝6, 8, 12) in the first cool-
ing and second heating cycles show the isotropic points
at 113, 112, and 108 ℃ (Figure 4), respectively.
Moreover, nematic and smectic A phase were only de-
tected in 4-POXD-6/Cn (n＝6, 8) in cooling runs. In
contrast, 4-POXD-6/C12 showed smectic A phase in
cooling run and SmX phase during heating.
Figure 1 FT-IR spectra of 4-POXD-6, C8, and 4-POXD-6/C8.
The phase transitional properties of 4-POXD-6/Cn
are summarized in Table 1. Compared to the complexes
2a — 2b (Scheme 1), although the rigid cores of
4-POXD-6/Cn are shorter, they exhibit the same liquid
crystalline phases as 2a—2b. it should be noted that the
enthalpic values for this nematic-smectic A transition of
4-POXD-6/Cn (n＝6, 8) are 0.57 and 1.86 kJ/mol,
which are lower than the conventional nematic-smectic
A transition enthalpies (4—6 kJ/mol), indicating that
these transitions are second-order in nature.[17] Usually,
nematic phase gives a little enthalpy (ΔH＜1 kJ/mol) of
transition to the isotropic liquid in rod-like and
bent-core molecules without intermolecular hydrogen
bonding interactions,[18] while the enthalpies for
nematic-isotropic liquid phase of 4-POXD-6/Cn (n＝6,
8) are 1.38 and 3.57 kJ/mol which is consistent with the
hydrogen-bonded complexes,[8,19] indicating first-order
transition.[20] In addition, the isotropic point of the com-
plexes exhibited nearly the same temperature (111 ℃)
in the heating runs which may be due to 4-POXD-6, and
the crystallization points of the complexes are close to
that of Cn,[13] which suggest Cn determined the crystal-
lization points of the complexes.
Hydrogen bonding in the complexes was further
confirmed by temperature-dependent FTIR spectros-
copy (Figure 2). The C＝O stretching vibration band in
4-POXD-6/C8 was observed at 1687 and 1711 cm－ at
1
temperatures of 160 ℃. The band at 1711 cm－ shifted
1
to 1703 cm－ with temperature decreasing to 111 ℃
1
and further shifted to 1693 cm－1 in the liquid crystalline
phases (111—96 ℃). It was reported that hydrogen
bonding formed between benzoic acid and pyridine with
pKa ca. 4 was non-ionic, which showed a double mini-
mum potential.[14,15] The two carbonyl bands observed
in liquid crystalline phases may be ascribed to the dou-
ble minimum potential state of the hydrogen bonds.[15]
In the crystalline state, the carbonyl band centered at
1693 cm－ was ascribed to the free carbonyl group of
1
the complex from the carboxylic acid and the pyridyl
group.[16] These results suggest that the hydrogen bonds
(OH…N (py)) were stable in the mesophases of the
complexes.
Phase behaviours of 4-POXD-6/Cn
To further confirm the complexes, the phase transi-
tional properties of Cn, 4-POXD-6, and 4-POXD-6/Cn
were investigated by differential scanning calorimetry
(DSC). Figure 3 shows the DSC curves of Cn (n＝6, 8,
12) and 4-POXD-6. C6, C8 and C12 showed nematic
phase and smectic C in the heating runs with the iso-
tropic points of 154, 148 and 140 ℃ respectively,[13]
POM photographs of 4-POXD-6/C8 at different
temperatures are shown in Figure 5. Cooling from iso-
tropic phase, 4-POXD-6/C8 showed schlieren texture
with four brushes and the dark areas (Figure 5a), which
were homeotropic domains with the molecules aligned
perpendicular to the glass substrates. The characteristic
Chin. J. Chem. 2012, 30, 785—790
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Figure 3 DSC curves of Cn (n＝6, 8, 12) and 4-POXD-6 for the first cooling and the second heating (5 ℃/min).
Figure 4 DSC curves of the complexes of 4-POXD-6/Cn (n＝6, 8, 12) for (a) the first cooling and (b) the second heating (5 ℃/min).
Table 1 Transition temperature (℃) and enthalpies (kJ/mol, in parentheses) of the complexes determined by DSC at a scan rate of 5 ℃/
min
Compound
First cooling
Second heating
4-POXD-6
I 82 ℃ (32.85) Cr
Cr 124 ℃ (35.87) I
4-POXD-6/C6
4-POXD-6/C8
I 111 ℃ (1.38) N 102 ℃ (0.57) SmA 99 ℃ (25.02) Cr
I 111 ℃ (3.57) N 106 ℃ (1.86) SmA 96 ℃ (35.70) Cr
Cr 113 ℃ (31.05) I
Cr 112 ℃ (46.14) I
4-POXD-6/C12 I 110 ℃ (6.88) SmA 87 ℃ (33.81) Cr
Cr 108 ℃ (30.13) SmX 112 ℃ (5.71) I
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Liquid Crystalline Behaviours of the Complexes Based on 1,3,4-Oxadiazole Derivative and Benzoic Acid
Figure 5 Polarizing optical photomicrograph of 4-POXD-6/C8. (a) nematic schlieren texture at 109 ℃, (×400); (b) smectic A texture
at 102 ℃ (×200 ) in surface anchoring (black cross arrows illustrated the crossed-polarizer and blue arrow illustrated direction of rub-
bing).
shimmer or flickering caused by Brownian motion of
the molecules was observed. These are the characteristic
features of nematic phase. The powder X-ray diffraction
of 4-POXD-6/C8 at 110 ℃ (Figure 6a) exhibited two
diffuse peaks centered at d-spacings of 23.5 and 4.5 Å,
respectively, strongly suggesting its nematic feature,
which fits well with the POM observation. Further
cooling caused the gradual disappearance of schlieren
texture and the dark field under POM, suggesting the
formation of another new mesophase. To identify the
mesomorphic phase of 4-POXD-6/C8, the molecules
were oriented parallel to the substrate by rubbing the
slides, then homogeneous green texture was observed
(Figure 5b), indicating smectic A phase. 4-POXD-6/C12
showed smectic A phase in cooling, while a mesomor-
phic phase in heating run, whose type cannot be con-
firmed due to its narrow temperature range. The WXRD
patterns of 4-POXD-6/Cn (n＝8, 12) in smectic A phase
consist of one sharp diffraction in the small angle region
and a broad diffraction in the high angle region (Figure
6b, 6c), suggesting their layered structure with the layer
spacings of 37.4 and 39.7 Å, respectively.
In addition, phenyl-5-(pyridine-4-yl)-1,3,4-oxadi-
azole core may possess an apex angle of ca. 135°,[10]
and the distance for O—H…N is about 2.65 Å (D).[21]
The calculated molecular lengths of the complexes of
4-POXD-6/Cn (n＝8, 12) with the bent-shaped molecu-
lar conformation (Scheme 3) were almost equal to their
layer spacings (Table 2).[22]
Table 2 Summary of X-ray diffraction results of 4-POXD-6/Cn
(n＝8, 12) in their smectic phase
Molecular length
Layer spacing
Compound
T/℃
d/l
l/Å
d/Å
37.4
39.7
4-POXD-6/C8
4-POXD-6/C12
37.1
41.9
100
100
1.01
0.95
Conclusions
A 1,3,4-oxadiazole derivative 4-POXD-6 containing
terminal pyridyl group was synthesized. The complexes
based on 4-POXD-6 and 4-alkoxybenzoic acids (Cn, n
Figure 6 X-ray diffraction patterns of the complexes in the first
cooling, 4-POXD-6/C8 at (a) 110 ℃ and (b) 100 ℃, and (c)
4-POXD-6/C12 at 100 ℃.
Chin. J. Chem. 2012, 30, 785—790
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Scheme 3 Structure of 4-POXD-6/C8
N
N
O
O
N
C
H
O
O
O
l
Mol. Struct. 2008, 891, 312.
[8] Martin, P. J.; Bruce, D. W. Liq. Cryst. 2007, 34, 767.
＝6, 8, 12) exhibited nematic and smectic A phase. Hy-
drogen bonding between 4-POXD-6 and Cn was con-
firmed by FTIR spectroscopy.
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Acknowledgement
The authors are grateful to the National Science
Foundation Committee of China (Project Nos.
51073071, 51103057, 21072076), and Project 985-
Automotive Engineering of Jilin University for their
financial support of this work.
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Chin. J. Chem. 2012, 30, 785—790