Synthesis and characterization of mesogenic 1,3,4-oxadiazole derivatives

Article abstract of DOI:10.1080/15421400903054550

A synthesis of homologous series of mesogenic 2-amino-5-alkkloxyphenyl-1,3, 4-oxadiazoles were prepared by the oxidative cyclisation of alkyloxybenzaldehyde semicarbozones (4) using chloramines-T (CAT) as osidate via intramolecular cycloaddition reaction. The reaction proceeds in better yield (75-85%) giving the product as solid with low melting points. The optical texture studies have been carried out using Leitz polarizing microscope to identify the liquid crystalline phases at different temperatures and it is found that the synthesized compounds exhibit nematic (N), smectic C (Sm C), smectic I (Sm I) and smectic G (Sm G) phases, sequentially at different temperatures. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421400903054550

Mol. Cryst. Liq. Cryst., Vol. 509, pp. 274=[1016]–282=[1024], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903054550
Synthesis and Characterization of Mesogenic
1,3,4-Oxadiazole Derivatives
J. Mahadeva1, K. B. Umesha2,
K. M. L. Rai3, and Nagappa4
1Department of Physics, PES College of Science, Mandya, India
2Department of Chemistry, Yuvaraja’s College, University of Mysore,
Mysore, India
3Department of Studies in Chemistry, University of Mysore,
Mysore, India
4Department of Studies in Physics, University of Mysore,
Mysore, India
A synthesis of homologous series of mesogenic 2-amino-5-alkkloxyphenyl-1,3,4-
oxadiazoles were prepared by the oxidative cyclisation of alkyloxybenzaldehyde
semicarbozones (4) using chloramines-T (CAT) as osidate via intramolecular
cycloaddition reaction. The reaction proceeds in better yield (75–85%) giving the
product as solid with low melting points. The optical texture studies have been car-
ried out using Leitz polarizing microscope to identify the liquid crystalline phases
at different temperatures and it is found that the synthesized compounds exhibit
nematic (N), smectic C (Sm C), smectic I (Sm I) and smectic G (Sm G) phases,
sequentially at different temperatures.
Keywords: homologous series; mesogenic; synthesis
INTRODUCTION
Girdziunaite et al. [1] synthesized 2-(4-alkylcyclohexyl) and
2-alkylthio-5-(4-hydroxyphenyl)-1,3,4-oxadiazoles with wide meso-
morphic ranges. Recently Semmler and Co-workers [2] have reported
non-linear mesomorphic liquid crystalline ester of 2,5-bis
(4-hydroxyphenyl)-1,3,4-oxadiazoles. Hetzheim have investigated [3]
the dielectric properties of three homologous series of 2-alkylthio-5-
phenyl-1,3,4-oxadiazole derivatives and observed the nematic, smectic
A and smectic C liquid crystalline phases depending on the chain
Address correspondence to J. Mahadeva, Department of Physics, PES College of
Science, Mandya-571 401, India. E-mail: jmahadeva@yahoo.com
274=[1016]

Characterization of Mesogenic 1,3,4-Oxadiazole Derivatives
275/[1017]
length. During the last twenty years, a large number of mesomorphic
compounds containing heterocyclic units have been synthesized
[1,4,5]. Gallardo and Begnini [6] stated that the compounds having
1,3,4-thiadiazole rings show enantiotropic phases over intervals of
about 100ꢀC. However, the corresponding compounds with 1,3,4-
oxadiazole rings are not mesomorphic and this behavior is related to
the larger molecules curvature caused by the 1,3,4-oxadiazole
rings. For the derivatives with 1,3,4-oxadiazole ring, where only
the position of the oxygen heteroatom changes, mesomorphism
returns although over smaller temperature intervals than with the
derivatives of 1,3,4-thiadiazole. Again this behavior can be attributed
to the molecular curvature produced by distribution of these hetero-
cyclic rings.
Recently Xu et al. [7] synthesized some new liquid crystalline
2,5-disubstituted 1,3,4-thiadiazole derivatives as mesogenic unit. They
observed that all 12 compounds are enantiotropic liquid crystals, and
most of them exhibit nematic mesomorphism and broad smectic C
mesomorphic ranges.
2-Amino and 2,5-diamino-1,3,4-oxadiazoles are formed by heating
1-acyl semicarbozones and substituted hydrazocarboxamides in
excess of phosphorous oxychloride. Widely applied method for the
synthesis of amino oxadiazoles involve the oxidative cyclisation of
thiosemicarbozones or semicarbozones of aldehydes [8] in presence
of an oxidizing agent such as lead oxide, bromine in acetic acid,
bromine or iodine in aqueous sodium carbonate, hypohalites,
Chloramine-T etc.
Karamyshava and Co-workers [9] showed that the substitution of
polar groups like alkyloxy part in oxadiazole causes properties to the
compounds. Recently Dingemans et al. [10] synthesized boomerang
shaped liquid crystals based on 2,5-bis (p-hydroxyphenyl)-1,3,4-
oxadiazole (ODBP) having a bond angle ꢁ134ꢀ bend in the mesogenic
cores. They found that substituting the nonlinear ODBP mesogenic
core with P-dodecyloxyphenyl tails resulted in a non-linear meso-
gens, which exhibited five distinct mesophases and a clearing tem-
perature of 204ꢀC. The highest temperature phase appears to be
nematic followed by a smectic C phase. The latter phase in freely
suspended films does not appear to exhibit polar order. The highest
temperature phase of 4,40(1,3,4-oxadiazole-2,5-diaryl)-di-p-heptyl-
benzoate shows a schlieren texture with a 2-brush pattern exclu-
sively. This and the above results prompted us to synthesize a new
compound possessing 2-amino-1,3,4-oxadiazole ring containing alky-
loxyphenyl group at position 5 (5a–k) and to study their mesogenic
properties.

276/[1018]
J. Mahadeva et al.
EXPERIMENTAL
General Synthetic Procedures
2-amino-5-alkyloxyphenyl-1,3,4-oxadiazoles (5a–k) was prepared by
the oxidative cyclisation of alkyloxybenzaldehyde semicarbazones (4)
using Chloramine-T as oxidant via intramolecular cycloaddition
reaction. Smicarbazones were in turn prepared by the reaction of alky-
loxybenzaldehydes (3) with smicarbazide hydrochloride. The starting
material alkyloxybenzaldehydes were prepared from p-hydroxy ben-
zaldehyde (1) with n-alkylbromide (2) in dry tetrahydrofuran (THF)
in the presence of tetrabutylammoniumbromide as phase transfer cat-
alyst (PTC) followed by treatment with powdered potassium hydroxide
(Scheme 1).
1H NMR, 13C-NMR and mass spectral studies gave structure
1
proof for the isolated products. In H NMR spectra, the signal due to
-CH3 protons appears as broad singlet in the region d 0.89–0.92 ppm.,
–(CH2)n-protons appear as multiplet in the region d 1.30–1.49 ppm.,
SCHEME 1 2-amino-5-alkaloxyphenyl- 1,3,4,-oxadiazoles.

Characterization of Mesogenic 1,3,4-Oxadiazole Derivatives
277/[1019]
the –NH2 peak appear as a singlet at d 3.35–3.44 ppm., OCH2 at d
3.98–4.14 ppm. and aromatic protons at d 7.04 and at 8.07 ppm. These
observations support the formation of the cycloadducts. In 13C NMR
spectra, the –C2 and C5-carbon appear as singlet in the region d
165.46–165.89 ppm and
d 162.39–162.59 ppm. respectively. The
remaining 13C carbon appeared in the expected region.
The mass spectrum of 1,3,4-oxadiazole 5e did not show molecular
ion peak, instead showed an ion corresponding to M þ 1, at m=e 318
as base peak. The (M þ 1), ion at mass 318 should have formed by
the abstraction of a hydrogen atom by the molecular ion from the neu-
tral molecule. The radical ion at mass m=e 289 with relative abun-
dance 75% could have formed from the base molecular ion m=e 318
by the loss of –N2 molecule. The ion at m=e 275 was of 43% relative
abundance formed by the loss of –NꢂC–NH2 (m=e 42) from the mole-
cular ion. The molecular radical ion at m=e 259 with relative abun-
dance 60% produced by the loss of molecular ion at m=e 58 with
relative abundance 18%. The ion at m=e 233 with relative abundance
52% formed by the loss of ion at m=e 84 with relative abundance 24%.
The base molecular ion m=e 318 could have yielded an ion at m=e 176
with relative abundance 80% by the loss of alkyl group CH3–(CH2)8–
CH2– m=e 141 have relative abundance 15%.
Typical procedure for the synthesis of 2-amino-5-
(4-decyloxyphenyl)-1,3,4-oxadiazole (5e)
A mixture of 4-n-decyloxy benzaldehyde semicarbazone (4e, 1.2 g,
3.76 mmol) and CAT (1.08 g, 3.85 mmol) in THF (20 ml) was heated
to reflux with stirring for 3 hours. Sodium chloride formed in the reac-
tion was filtered off and washed with ethanol. The combined filtrate
and washing were evaporated in vacuum. The residue was extracted
into ether, washed with 10% sodium hydroxide (3 ꢃ 20 ml), brine solu-
tion and finally with water. The ethereal layer after drying over anhy-
drous sodium sulphate was evaporated in vacuum. The resultant
residue was dissolved in chloroform (2 ml) and poured into a flask con-
taining petroleum ether and the precipitate was collected and dried.
Recrystallisation from ethanol gave oxadiazole as white crystalline
solid in 83% (0.99 g) yield, m.p. 98–99ꢀC.
Elemental analysis: C18H27N3O2 requires C 68.11, H 8.57, N
13.24; found C 68.01, H 8.47, N 13.20%.
Characterization of Materials
The purity of all the synthesized compounds was checked by thin layer
chromatography using silica gel G. The final compounds were purified

278/[1020]
J. Mahadeva et al.
by column chromatography on silica gel (70–230 mesh, Merck) using
chloroform-acetone (7:1) as the eluent. The yield of the compounds
was in the range 75–85%.
The chemical structure of all the 2-amino-5-alkyloxyphenyl-
1,3,4-oxadiazoles were confirmed by 1H NMR spectra (Bruker
300 MHz spectrometer) as well as 13C NMR spectra (Jeol GSX 400
spectrometer) with 1% tetramethylsilane in deuteriochloroform as
an internal standard, Mass spectra were obtained on an electron
impact Maspec MSW 9629 spectrometer and important fragments
are given with the relative intensities in brackets.
In order to investigate the different properties of liquid crystals, the
optical texture studies, DSC thermal analysis, X-ray and optical aniso-
tropic studies have been carried out. The optical textures exhibited by
the liquid crystalline compounds are studied using the polarizing
microscope. The Leitz orthoplan-polarizing microscope is one of the
important variants equipment for the observation of isotropic and
anisotropic substances in polarized transmitted light.
The homogeneous specimen can be prepared by using a very clean
glass slide and cover slip. Clean surfaces are obtained by soaking
the glass in concentrated nitric acid for an hour, washing with distilled
water, and finally rinsing with acetone which is then allowed to dry off
and evaporate. For the optical texture studies, the liquid crystalline
specimen has taken in the form of thin film in the order of 25 to 30
microns thickness between slide and cover glass. The textural chan-
ging with temperature is observed by using specially designed hot
stage. The transition temperatures are measured by using platinum
resistant thermometer or thermometer.
RESULTS AND DISCUSSION
Determination of Phase Transition Temperatures
The phase transition temperatures of the liquid crystalline compounds
were determined using a polarizing microscope in conjunction with a
conventional hot stage [11]. The transition temperatures of the speci-
men are varied across the transition points at the rate of 0.2ꢀC per
minute. Measurements were made using a calibrated thermometer
graduated at the interval of 0.1ꢀC or platinum resistance thermo-
meter. The bulb of the thermometer was kept close to the specimen
in order to ensure that there was no lag between the observed and
actual temperatures. The phase transition temperatures of different
compounds were shown in Table 1.

Characterization of Mesogenic 1,3,4-Oxadiazole Derivatives
279/[1021]
TABLE 1 The Phases Obtained at Different Temperature for the Series
of Prepared Compounds
Compounds
Iso
N
Sm C
Sm I
Sm G
5a
5b
5c
5d
5e
5f
5g
5h
5i
136.0
119.1
104.5
94.2
86.2
82.3
79.8
81.1
83.0
83.6
84.9
59.6
66.0
73.2
79.5
85.8
82.1
79.5
80.6
81.7
82.3
83.5
56.2
62.8
70.1
77.4
84.5
81.6
77.3
78.3
79.1
81.2
83.0
—
—
—
—
—
—
69.2
75.7
69.4
63.2
65.8
—
66.4
73.1
65.9
—
—
—
5j
5k
—
—
—
—
Optical texture studies of 2-amino-5-alkyloxyphenyl-
1,3,4-oxadiazoles
The homologous series of 2-amino-5-alkyloxyphenyl-1,3,4-oxadia-
zoles (5a–k) have been synthesized and these compounds exhibit
nematic, smectic C, smectic I and smectic G phases sequentially shown
below. The nematic mesophase for all compounds in this series, exhibit
Schlieren texture and the typical nematic droplets on cooling the
specimen from isotropic liquid.
86:2ꢀC
85:8ꢀC
84:5ꢀC
75:7ꢀC
73:1ꢀC
ISO
N
Sm C
Sm I
Sm G
Cry
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ƒƒƒƒ!
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When the specimen is cooled from its isotropic melt at 86.2ꢀC. It
exhibits isogirs of nematic (N) drops with four brushes (Fig. 1). The
FIGURE 1 Isogirs of nematic (N) drops with four brushes (165ꢃ).

280/[1022]
J. Mahadeva et al.
FIGURE 2 Mosaic texture of smectic I phase (135ꢃ).
brushes of the drops will rotate, when polarizer rotates along the same
axis. These nematic drops collapse at temperature 85.8ꢀC, then trans-
formed into schlieren texture, which is characteristic of Sm C phase.
On further cooling the specimen, nematic phase change over to Sm I
phase at 84.5ꢀC see Figure 2. This phase is a tilted biaxial phase,
the pseudohexogonal molecular packing identifies the molecular struc-
tures in the smectic I phase and this phase is hexatic in nature. On
further cooling smectic I phase transformed into striped pattern tex-
ture at 75.7ꢀC which is characterized by Sm G phase see Figure 3.
The molecules in this phase are packed in layers and with in the
layers, molecules have their long axes tilted with respect to the normal
FIGURE 3 Striped pattern of G phase (135ꢃ).

Characterization of Mesogenic 1,3,4-Oxadiazole Derivatives
281/[1023]
FIGURE 4 Plot of transition temperature versus the number of carbon atoms
in the alkoxy chain..
to the molecular layer [12]. Finally, at 73.1ꢀC the specimen changes
over to crystalline phase. The transition temperatures recorded in
DSC trace are in agreement with the transition temperature observed
in optical texture studies and are given in Table 1.
A plot of transition temperature as a function of the number of
carbon atoms (n) in the alkyloxy chain is shown in Figure 4. The first
five members are monotropic nematic in nature and no odd-even effect
has been observed. The nematic to isotropic curve shows the increas-
ing tendency up to pentoxy derivative and then levels off. This beha-
viour has been discussed by Gray [13] for comparatively low melting
series of liquid crystalline compounds.
REFERENCES
[1] Giridziunaite, D., Tschierske, C., Novotna, E., Kresse, H., & Hetzheim, A. (1991).
Liq. Cryst., 10, 397–407.
[2] Semmler, K. J. K., Dingemans, T. J., & Samulski, E. T. (1998). Liq. Cryst., 24, 799–
803.
[3] Hetzheim, A., Wasner, C., Werner, J., Kresse, H., & Tschierske, C. (1999).
Liq. Cryst., 26, 885–891.
[4] Deutscher, W. M., Vorbradt, H. M., & Zaschke, H. Z. (1981). Chem., 21, 9–14.
[5] Cai, R. & Samulski, E. T. (1991). Liq. Cryst., 9, 617–634.
[6] Gallardo, H. & Begnini, I. M. (1995). Mol. Cryst. liq. Cryst., 258, 85–94.
[7] Xu, Y., Lu, W., & Tai, Z. (2000). Mol. Cryst. liq. Cryst., 350, 151–159.

282/[1024]
J. Mahadeva et al.
[8] Rai, K. M. L., Linganna, N., Hassner, A., & Murthy, C. A. (1992). Org. Prep. Proc.
Int., 24, 91–94.
[9] Karamysheva, L. A., Torgova, S. I., Agafonova, I. F., & Shtikov, N. M. (1995). Mol.
Cryst. liq. Cryst., 260, 217–225.
[10] Dingemans, T. J. & Samulski, E. T. (2000). Liq. Cryst., 27, 131–136.
[11] Gray, G. W. (1976). General Structural Features Common to Many Mesogens.
In: Advances in Liquid Crystals, Brown, G. H. (Ed.), Academic Press: New York,
Vol. 2, 11.
[12] Leadbetler, A. J., Mazid, M. A., Kelly, B. A., Goodby, J. W., & Gray, G. W. (1979).
Phys. Rev. lett., 630–633.
[13] Gray, G. W. (1962). Molecular Structure and Properties of Liquid Crystals,
Academic press: New York, 162.

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