Synthesis, liquid crystalline behaviour and structure-property relationships of 1,3-bis(5-substituted-1,3,4-oxadiazol-2-yl)benzenes

Article abstract of DOI:10.3762/bjoc.16.17

Two series containing 1,3-bis(1,3,4-oxadiazol-2-yl)benzene as a rigid core (RC) and alkyl or perfluoroalkyl as terminal chains were synthesized and characterized. Liquid crystal properties of the synthesized compounds have been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction techniques. Conformation effects of the synthesized products on the dipole moments were also investigated.

Full text of DOI:10.3762/bjoc.16.17

Synthesis, liquid crystalline behaviour and
structure–property relationships of 1,3-bis(5-substituted-
1,3,4-oxadiazol-2-yl)benzenes
Afef Mabrouki1, Malek Fouzai2, Armand Soldera3, Abdelkader Kriaa1
and Ahmed Hedhli*1
Full Research Paper
Open Access
Address:
Laboratory of Molecular Organic Chemistry, National Higher
Engineering School of Tunis, 5 avenue Taha Hussein, Montfleury,
089, Tunis, Tunisia, 2LR99ES16 Physics Laboratory of Soft Matter
and Electromagnetic Modelling, University of Tunis El Manar, 2092,
Tunis, Tunisia and 3Department of Chemistry, Quebec Center for
Functional Materials, University of Sherbrooke, Sherbrooke, Québec,
J1K 2R1, Canada
Beilstein J. Org. Chem. 2020, 16, 149–158.
doi:10.3762/bjoc.16.17
1
1
Received: 02 September 2019
Accepted: 21 January 2020
Published: 31 January 2020
Associate Editor: J. A. Murphy
Email:
© 2020 Mabrouki et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Ahmed Hedhli* - ahmed.hedhli@esstt.rnu.tn
*
Corresponding author
Keywords:
dipole moment; fluorine; liquid crystal; oxadiazole
Abstract
Two series containing 1,3-bis(1,3,4-oxadiazol-2-yl)benzene as a rigid core (RC) and alkyl or perfluoroalkyl as terminal chains were
synthesized and characterized. Liquid crystal properties of the synthesized compounds have been investigated by polarizing optical
microscopy, differential scanning calorimetry and X-ray diffraction techniques. Conformation effects of the synthesized products
on the dipole moments were also investigated.
Introduction
Liquid-crystalline (LC) materials have been known for over a chemistry [3]. The C–F bond is highly polarized and this
century [1]. It is clear that architecture and functionalization are polarity inhibits the lone pair donation from fluorine, making
essential aspects in molecular engineering of liquid crystals [2]. this element a weak coordinator. These properties are the basis
The introduction of fluorine atoms in the molecular structure for the unique properties of perfluoroalkylated compounds such
presents a successful strategy to control the liquid crystal as high viscosity, high density, high chemical stability, low sur-
proprieties. The element Fluorine presents the highest electro- face tension, low dielectric constants and low refractive index
negativity, the lowest polarizability and a small radius. When [4]. The usefulness of these properties makes fluorine an ele-
bonded to carbon, it forms the strongest single bond in organic ment of choice for the enhancement of promising properties,
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Beilstein J. Org. Chem. 2020, 16, 149–158.
remained inaccessible otherwise. However, the combination of Herein we describe the synthesis and characterization of two
small size and high polarity of the fluorine atom leads to a series of hydro- and fluorocarbonated 1,3-bis(1,3,4-oxadiazol-
subtle modification of properties such as melting point, 2-yl)benzenes. Structure–property relationships of the obtained
mesophase morphology, transition temperatures, optical compounds were investigated.
anisotropy, dielectric anisotropy, and visco-elasticity [5-10].
Therefore, many fluorinated liquid crystals have been prepared,
Results and Discussion
and the fluoro-substitution effect has been well studied, espe- Synthesis
cially in the fluoroaromatic derivatives [11-13].
When benzene-1,3-dicarbohydrazide [28] (1) was allowed to
react with hydro- or perfluorocarboxylic acids in the presence
Additionally, the mesomorphic properties of liquid crystals of phosphorus oxychloride according to standard methods [29],
depend strongly on the nature of the terminal chains that are oxadiazole derivatives 2 were obtained (Scheme 1). On
present. A terminal perfluorocarbon chain present in a LC mol- the other hand, we converted compound 1 into sulfanyloxadia-
ecule causes stiffening, which generates a lamellar packing and zole derivatives 4 by treatment with carbon disulfide and
thus contributes to smectic phase stability [14]. It was reported subsequent alkylation of the obtained intermediate 3
that even simple n-alkanes containing a fluorocarbon block (Scheme 1).
produces smectic phases [15-19]. Some molecules having only
a single aromatic ring and fluorinated tail show smectic phases, Table 1 shows the yields and melting points of the synthesized
while their hydrocarbon counterparts are non-mesomorphic compounds 2 and 4.
[
20].
Table 1: Obtained oxadiazole derivatives 2 and 4.
On the other hand, many heterocyclic-based liquid crystals have
been designed and then synthesised due to their large scope of
applications [21]. In this context, the 1,3,4-oxadiazole group
was of a particular interest from a synthetic viewpoint, consid-
ering the numerous ways to introduce this group into a mole-
cule [22,23]. So, it was established that the introduction of a
Compound R / R’
X
Yield (%)
Mp (°C)
2
4
a
b
c
d
a
b
c
d
C6F13
C7F15
–
–
–
–
I
I
Br
Br
65
61
75
72
74
66
70
72
106
120
83
n-C9H19
C6H5
C6F13C2H4
C8F17C2H4
n-C4H9
74
1
,3,4-oxadiazole ring as terminal group on a biphenyl provides
128
138
69
liquid crystalline materials [24,25]. However, when the oxadia-
zole unit is incorporated in the aromatic core, the obtained com-
pounds do not exhibit mesomorphism, only crystal–isotropic
transitions were observed [26]. The absence of mesophases is
mainly due to the strong bend (134°) between the aromatic
n-C12H25
88
rings, which disturbs the linear shape of the whole molecule The tautomeric equilibrium of compound 3 is illustrated in
[
27].
Scheme 2. On the basis of FTIR data, it has been concluded that
Scheme 1: Synthesis of oxadiazole derivatives 2 and 4.
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Beilstein J. Org. Chem. 2020, 16, 149–158.
Scheme 2: Tautomeric equilibrium of compound 3.
in solution, the equilibrium is shifted to the thione form 3a only one peak between the crystalline and the isotropic phase in
rather than the thiol one 3b. The observed IR absorptions at cooling. However, it exhibits two peaks in heating which indi-
3
387 cm−1 (νN-H) and 1263 cm−1 (νC=S) and the absence of cates the presence of monotropic intermediate phase between
absorptions in the 2600–2550 cm−1 region (νS-H) support the T = 115.8 °C and T = 125.1 °C [30]. As it can be seen in the
preference for the thione form in solution. This latter is obvi- thermogram of compound 4a, the monotropic liquid crystal
ously more solvated than thiol.
mesophase is observed in cooling between T = 91.6 °C and
8.9 °C.
9
Liquid crystal properties
The structure of compounds 2 and 4 is constituted by a rigid Concerning compound 4b, the thermogram shows an enan-
core (three aromatic rings) to which are attached the terminal tiotropic intermediate phase between 118.0 °C and 138.4 °C in
chains. Based on this structure, some liquid crystalline heating and between 96.5 °C and 121.1 °C in cooling, indicat-
mesophases were expected. Differential scanning calorimetry ing a remarkable stabilization in a temperature range of 20 °C.
(
DSC), polarized optical microscopy (POM) and X-ray diffrac-
tion pattern analysis were used to investigate this behaviour.
Polarized optical microscopy (POM)
In order to achieve a further illustration of the liquid crystal be-
havior, POM observations were realized in cooling and heating
DSC measurements
Phase transition temperature (Tt), melting temperatures (Ti) and cycles for compounds 2b, 4a and 4b (Figure 2). POM tech-
enthalpy changes (∆Ht) of compounds 2 and 4 are summarized nique has illustrated smectic phases for all compounds. Identifi-
in Table 2.
cation of the phase textures was accomplished by comparing
with those reported in literature [24].
Based on the data given in Table 2, we found that fluorinated
compounds present supplementary transition temperatures com- The texture of 2b (Figure 2a) seems to be a variant of focal
pared with the hydrocarbonated analogues.
conic texture with unusually narrow ellipses of a SmA phase.
Compound 4a presents a mosaic SmB phase where the mole-
DSC thermograms of fluorinated compounds 2b, 4a and 4b are cules are organized in a hexagonal network as shown in
shown in Figure 1. The thermogram of derivative 2b presents Figure 2b. The low average of enthalpy value (1.89 J/g) given
Table 2: DSC thermograms data of compounds 2 and 4.
Compound
Heating
Cooling
Ti (°C)
Tt (°C)
∆Ht (J/g)
Tt (°C)
∆Ht (J/g)
2
2
a
b
115.50
115.80
−48.49
−2.15
85.42
103.85
41.61
48.13
116.91
126.13
1
25.11
−50.55
−63.76
4
4
4
a
b
d
129.85
91.63
1.89
48.33
7.21
36.28
95.09
130.81
139.92
78.31
9
8.92
118.00
−8.39
−41.66
−133.43
96.51
121.13
49.24
138.41
77.10
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Figure 1: DSC thermograms of fluorinated compounds 2b, 4a and 4b recorded at 5 °C/mn at heating (down traces) and cooling (top traces) cycles.
in Table 2 is due to the first order phase transition marked by Taking into account these observations, the inability of 2d to
the coexistence of the crystalline phase and the liquid crystal exhibit mesophases becomes axiomatic, since this compound is
phase at least 5 °C as it is shown in Figure 2c. Figure 2d shows devoid of any terminal alkyl chain.
the POM observation for compound 4b in heating cycles at
1
26 °C. The texture in cooling is very similar to that in heating; X-ray patterns analysis
this indicates a remarkable thermodynamic stability of the com- In order to correlate the obtained results from POM and DSC,
pound. Based on the texture in Figure 2d, we can note that the we have investigated the X-ray diffraction at the mesophases in
mesophase is hexagonal SmB with strongly double refracting cooling and heating cycle of 2b, 4a and 4b. Figure 3 illustrates
lancets and regions [31].
typical diffraction spectra for compound 2b. A typical X-ray
pattern recorded for the SmA phase is obtained, it showed two
Under crossed polarizes, compound 2d is not capable to induce small reflections at 23.69 Å and 15.71 Å at low angle region,
any liquid crystalline behaviour, they present only crystal and a broad reflection at 5.64 Å. These features are character-
phases. Gallardo et al. investigated on the liquid crystalline be- istic of SmA phase and are close to data described in the litera-
haviour of some bis(phenyl-1,3,4-oxadiazolyl)benzene deriva- ture [33]. Figure 4 shows a typical X-ray pattern for the hexago-
tives with varied number and length of terminal alkoxy chains nal SmB of 4a (anyway, we must emphasize that both com-
[
32]. The authors established that the mesogenicity is strongly pounds 4a and 4b exhibit an identical X-ray pattern). As we can
enhanced in materials with four long terminal alkoxy substitu- observe, three peaks of diffraction are recorded. The diffraction
ents, compared to two-chain and shorter-chain homologues. pattern shows oriented reflections in the small angle region.
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Beilstein J. Org. Chem. 2020, 16, 149–158.
Figure 2: Optical texture (×10) of liquid crystal phase for fluorinated compounds, (a): SmA phase observed in heating at 120 °C for compound 2b;
(
b): Coexistence between cristalline phase and liquid crystal phase observed in cooling at 91 °C for compound 4a; (c): Mosaic SmB phase observed
at 96 °C for compound 4a and (d): SmB phase observed in heating cycle at 126 °C for compound 4b.
Figure 3: Typical diffractogram observed for compound 2b at 398 K.
Figure 4: Typical diffractogram observed for compound 4a at 411 K.
Bragg peaks at 33.04 Å, and approximately its second- and However, in the high 2θ-region of Figure 4, slightly different
third order multiples at d = 16.09 Å and 10.69 Å, indicate a values of the d-spacings calculated from these peaks suggest a
highly condensed layered structure [34].
weakly distorted hexagonal lattice [34].
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Beilstein J. Org. Chem. 2020, 16, 149–158.
Structure–conformation relationships
Molecular dipole moment
Calculated electric dipole moments of compounds 2 and 4 are
reported in Table 3.
Table 3: Calculated dipole moments of 2 and 4.
Dipole moment (D)
Components
Compound
X
Y
Z
Total
The prepared compounds perform three kinds of conformations,
A, B and C. The hydrocarbon derivatives exhibit conformation
A (Figure 5a–a”), whereas the fluorocarbon analogues
adopt, depending on whether they carry linking segment
C2H4S or not, the conformation B (Figure 5b–b”) or C
2
2
a
b
0.08
1.54
1.95
−3.34
2.59
4.35
5.13
0.85
0.77
-4.48
−4.30
0.71
4.74
4.72
3.61
2.78
4.58
5.63
1.74
1.70
−0.04
−1.20
−0.93
−1.25
1.40
2c
2d
0.38
4
4
4
4
a
b
c
d
−0.75
−0.67
−0.22
−0.67
(
Figure 5c–c”).
−1.51
−1.35
In Figure 6 the vectors of the dipole moments of compounds 4c,
b and 2b are shown.
4
Figure 5: Conformer of lowest energy of compounds: 4c, conformation A, (a) front view, (a’) top view, (a”) side view; 4b, conformation B, (b) front
view, (b’) top view, (b”) side view; 2b, conformation C, (c) front view, (c’) top view, (c”) side view. Carbon atoms are shown in gray, hydrogen atoms in
white, sulfur atoms in yellow, nitrogen atoms in blue, oxygen atoms in red and fluorine atoms in orange.
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Beilstein J. Org. Chem. 2020, 16, 149–158.
Figure 6: Vector of dipole moment of compounds 4c, 4b and 2b.
In Figure 7 we plotted the calculated dipole moments. To quan- side compared to the horizontal green line (standard µ0), the
tify the effect of terminal chains, we calculated the dipole blue plot of the hydrocarbon derivatives being in the bottom
moment µ0 of 1,3-bis(1,3,4-oxadiazol-2-yl)benzene (RC) as it side.
contains no terminal tails and can serve as standard.
On the other hand, we can note from Figure 5 that the confor-
mation adopted by the hydrocarbon compounds is different
from that of the fluorinated homologues, moreover these latter
do not adopt the same conformation, according to whether they
carry a sulfur atom or not.
Obviously, the inductive effects alone are inconsistent with the
observed results. Hence, we considered the rigid-core (RC) and
the terminal chains separately in order to identify a possible
interaction between them.
Rigid-core
With three electronegative heteroatoms and only two carbons,
the 1,3,4-oxadiazole core has a great electron deficient char-
acter. Nevertheless, when incorporated in the rigid-core, the two
oxadiazole rings exhibit a slight difference in their electron
deficiency.
Figure 7: Plot of molecular dipole moments. Orange, fluorocarbon
compounds; blue, hydrocarbon compounds; green (horizontal line),
dipole moment (µ0) of RC.
We depicted in Table 4 the electric charge of heteroatoms in the
obtained compounds, as well as those of RC. As shown in
The oxadiazole moieties are thought to be responsible for the Table 4, with the exception of O13, all the other heteroatoms
majority of the dipole moment in the prepared compounds. Due have almost the same electrical charge. The mean value of elec-
to the electron-donating effect of alkyl groups in combination tric charge for O13 is −0.27. Hence, O13 is highly charged rela-
with the electron-accepting oxadiazole [35], the prepared tive to the other heteroatoms, suggesting that Oxd 1 is more
hydrocarbon compounds are expected to perform dipole polar than Oxd 2.
moment magnitudes higher than µ0. In the fluorinated counter-
parts, the electron-withdrawing effect of perfluoroalkyl groups Such a difference in polarity was corroborated by the direction
would produce a reverse effect leading to a dipole lower than of the dipole moment of RC (Table 5). Therefore, it is not
µ0. Figure 7 shows an inverse result: the orange plot corre- surprising to find the molecular electron-deficient center so far
sponding to the fluorocarbon derivatives is entirely in the upper from the axis of the molecule in the prepared compounds.
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Beilstein J. Org. Chem. 2020, 16, 149–158.
ized. In (CF2)m segment, Hasegawa et al. noted that the local
dipoles Cδ+–Fδ− cannot be cancelled out and the surface of fluo-
rocarbon chain remains polar [38].
Table 4: Electric charge of heteroatoms in compounds 2 and 4.
Interaction rigid core-terminal chains
The typical model for aromatic electron donor–acceptor (D–A)
interactions was established by Hunter and Sanders in 1990
[
39]. According to this model, benzene and hexafluorobenzene
Cpd.
O13
N15
N16
O1
N4
N3
form a complex where benzene is the donor (electron-rich) and
hexafluorobenzene is the acceptor (electron-deficient). Experi-
mental evidence for this complex was first reported in 1960
[40]. The electron D–A concept may be regarded as Lewis
base–Lewis acid type or charge-transfer.
2
2
4
4
2
2
4
4
a
b
a
b
c
d
c
d
−0.25 −0.14 −0.10 −0.16 −0.14 −0.13
−0.25 −0.14 −0.10 −0.16 −0.14 −0.13
−0.28 −0.14 −0.13 −0.15 −0.13 −0.12
−0.28 −0.14 −0.13 −0.15 −0.13 −0.12
−0.29 −0.17 −0.12 −0.18 −0.15 −0.15
−0.26 −0.16 −0.13 −0.18 −0.15 −0.15
−0.28 −0.14 −0.13 −0.15 −0.13 −0.12
−0.28 −0.14 −0.13 −0.15 −0.13 −0.12
Based upon the above considerations, we could attribute the
close proximity of fluorinated chains in conformation B to a
throw space electron D–A intramolecular interaction between
the perfluoroalkyl chains (electron-rich moieties) and the elec-
tron-deficient center of the molecule. The two fluorocarbon
chains are symmetrically arranged with respect to the origin of
the vector of dipole moment (Figure 6, compound 4b). In com-
pound 4b, which adopts the conformation B (Figure 5b–b”), the
mean −0.27 −0.15 −0.12 −0.16 −0.14 −0.13
value
RC
−0.28 −0.16 −0.11 −0.18 −0.15 −0.15
Terminal chains
The intrinsic difference between fluorocarbon and hydrocarbon fluorinated terminal chains resemble a twin [41]. The molecule
chains has to be taken into account to elucidate the conforma- is linear in the meaning that the rigid-core is in one side and the
tional arrangements of the prepared compounds.
two arms in the other. Thus arranged, the molecule is polar and
performs liquid crystal phases.
Because of electrostatic repulsions of fluorine atoms in the rela-
tive 1,3-positions in the crystalline state, the perfluorocarbon The presence of the linking group C2H4S in compounds 4a and
chain adopt a helical conformation of the carbon backbone [36]. 4b has the desired effect of increasing conformational flexi-
The cylinder-like structure of the segment (CF2)m resembles a bility, bringing the two fluoroalkyl chains closer. As a result, 4a
stiff rod in which the carbon skeleton is covered by fluorine and 4b adopt the conformation B (Figure 5b–b”). However, al-
atoms. In the hydrocarbon counterpart the segment (CH2)n though the non-fluorinated analogues 4c and 4d also carry the
adopts the typical in-plane zigzag conformation [36].
segment C2H4S, an electron D–A interaction was not observed
in these compounds, obviously because of the fundamental
Fluorine is the most electronegative element of the periodic ta- difference between fluoro- and hydrocarbon chains mentioned
ble. This high electronegativity confers to C–F bond a large above. The conformation A adopted by these compounds
dipole moment of 1.39 D while that of C–H bond is only 0.40 D (Figure 5a–a”) lowers the dipole moments and thereby
[
37]. Owing to the all-trans conformation, the local dipole precludes mesophases formation. In the same way, we can
moments Cδ−–Hδ+ of hydrocarbon chain are mutually neutral- rationalize the absence of mesomorphism in compound 2c
Table 5: Dipole moment of RC.
µ (D)
Compound
X
Y
Z
Total
4.24
RC
0.59
−4.20
−0.01
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Beilstein J. Org. Chem. 2020, 16, 149–158.
which is not only non-fluorinated, but also devoid of the seg- ORCID® iDs
ment C2H4S.
Malek Fouzai - https://orcid.org/0000-0001-9004-0433
Armand Soldera - https://orcid.org/0000-0001-5467-5714
Abdelkader Kriaa - https://orcid.org/0000-0002-9893-9416
Ahmed Hedhli - https://orcid.org/0000-0001-8541-5958
On the other hand, direct linkage of perfluoroalkyl groups to the
rigid-core makes the system more rigid. As shown in
Figure 5c–c”, the conformation C adopted by compound 2b is
quite different from conformation B described above. While Preprint
RF2 (RF linked to Oxd 2) folds to interact with the molecular
electron-deficient center according to an electron D–A interac-
tion, RF1 (RF linked to Oxd 1) remains straight. The dipole
A non-peer-reviewed version of this article has been previously published
as a preprint doi:10.3762/bxiv.2019.99.v1
moment of compound 2b (Figure 6) originates close Oxd 1. References
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2
2
2
2
2
2
2
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