Synthesis and mesomorphic properties of novel crown ether Schiff bases

Article abstract of DOI:10.1080/15421400701832074

Different crown ether Schiff bases were prepared by condensing a different aldehyde with trans-diamino dibenzo 18-crown-6. The physical properties as well as the chemical formulations of these compounds were derived from microanalysis and spectroscopic me

Full text of DOI:10.1080/15421400701832074

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Synthesis and Mesomorphic
Properties of Novel Crown
Ether Schiff Bases
Vrajesh B. Parikh a & Shobhana K. Menon a
a Department of Chemistry, School of Sciences,
Gujarat University, Ahmedabad, India
Published online: 22 Sep 2010.
To cite this article: Vrajesh B. Parikh & Shobhana K. Menon (2008) Synthesis and
Mesomorphic Properties of Novel Crown Ether Schiff Bases, Molecular Crystals and
Liquid Crystals, 482:1, 71-83
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Mol. Cryst. Liq. Cryst., Vol. 482, pp. 71–83, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701832074
Synthesis and Mesomorphic Properties of Novel
Crown Ether Schiff Bases
Vrajesh B. Parikh and Shobhana K. Menon
Department of Chemistry, School of Sciences, Gujarat University,
Ahmedabad, India
Different crown ether Schiff bases were prepared by condensing a different
aldehyde with trans-diamino dibenzo 18-crown-6. The physical properties as well
as the chemical formulations of these compounds were derived from microanalysis
and spectroscopic methods. Phase transition temperatures and the thermal para-
meters were obtained from differential scanning calorimetry (DSC). The texture
observation was performed under polarizing optical microscopy (POM) attached
with a hot stage. Most of the compounds are thermotropic liquid crystals. Different
phases such as smectic, (SmC, SmA) and nematic (N) were shown by crown ether
Schiff bases containing azomethine and hydroxy group which are characteristic of
rodlike liquid crystals.
Keywords: dibenzo 18-crown-6; liquid crystal crown ether (LCCE) Schiff base; nematic;
smectic; thermotropic liquid crystal
INTRODUCTION
Recently, much interest has been generated in the application of new
liquid crystalline materials in the field of chiral recognization, ion
transportation, function membranes, transferring signal, and infor-
mation at the molecular level [1,2]. Liquid crystals are also applied
for photonics applications [3], as surfactants [4], elastomers [5], in
electro-optic devices [6], video projectors, optical computing, nonlinear
optics [7], thermographic measurements, temperature and pressure
sensors, spectroscopy and chromatography [8, 9], batteries and sensor
devices [10, 11], and building blocks [12].
Received August 24, 2007; accepted September 19, 2007.
Address correspondence to Shobhana K. Menon, Department of Chemistry, School of
Science, Gujarat University, Ahmedabad 380009, India. E-mail: sobhanamenon07@
gmail.com
71

72
V. B. Parikh and S. K. Menon
Percec et al. [13] have demonstrated that macrocyclization over-
rides the well-established polymer effect in the formation and stabili-
zation of liquid crystalline (LC) phases. Therefore, contrary to what
has been considered for over 100 years, it was proved that the cyclic
and not the linear architecture is the most powerful in the design of
molecular and macromolecular LCs. Liquid crystal crown ethers
(LCCE) have potential applications because of their physicochemical
properties and the utilization of crown ethers as selective ionophoric
units in other functionalized compounds.
Predominantly nematic phases and also smectic layer structures
were obtained from calamitic compounds with a crown ether moiety
at one of the termini of an extended, rod-like, rigid core [14–18]. It
was found that the isotropization temperatures of low molecular mass
liquid crystals containing crown ethers decrease upon complexation
with alkali metal salts [19], whereas smectic mesophases of polymeric
liquid crystal crown ethers can be stabilized by complexation [17, 20].
On the other hand, columnar mesophases were induced by the interac-
tion of heavy metal cations with multiarmed azacrowns [21].
Only a few LCCE Schiff bases were reported [22–26] which showed
the smectic and nematic phases. The threaded texture and schlieren
texture were observed in Schiff base type polymer LCCEs [27–28].
In this article, we report some crown ether Schiff bases prepared
from various aldehydes (salicylaldehyde, 5-bromo salicylaldehyde,
2,3-dihydroxy benzaldehyde, 2-hydroxy naphthaldehyde, Thiophene
2-carbaldehyde, Pyrrole 2-carbaldehyde, o-vanillin, 3,4,5-trimethoxy
benzaldehyde ) condensed with trans-diamino dibenzo-18-crown-6.
All compounds thus isolated were purified and characterized by
1
Fourier Transform Infrared Spectroscopy (FTIR), H NMR, FAB-MS
along with thermal analysis and texture observation using Differential
Scanning Calorimetry (DSC) and Polarizing Optical Microscopy (POM),
respectively. Effects of the changes in the polar substituents in aldehyde
on the mesophases are also discussed.
EXPERIMENTAL
Reagents and Chemicals
All the chemicals used were of A.R. grades of E. Merck, or Aldrich
unless otherwise specified.
Apparatus
The melting points were taken in a sealed capillary tube using
Toshniwal India melting point apparatus and are uncorrected. The

Properties of Novel Crown Ether Schiff Bases
73
FT-IR spectra were recorded on JASCO-410 FT-IR Spectrometer as
KBr pellets. Electronic spectra were recorded on JASCO V-530 double
beam UV-Vis spectrophotometer in chloroform.
Transition temperatures and enthalpies were scanned in Shimadzu
DSC 60, differential scanning calorimeter with a heating rate of 10.0ꢀC
minꢁ1 in air and it was calibrated with indium (156.6ꢀC, 28.45 Jgꢁ1).
The temperatures were read as the maximum of the endothermic
peaks.
The textures of the mesophases were studied with Leica DMLP
polarizing microscope, equipped with a hot stage, Mavotherm 32 tem-
perature display unit, and Leica MPS 32 Data back (display unit). The
samples were investigated using cross polarizers at room temperature
with a magnification of 2.5X and a magnification 10X or 20X was at
elevated temperature. The samples were mounted on quartz slides
of 26 ꢂ 26 mm with 1 mm thickness. The film used was Kodak Max
400, 24 ꢂ 36 mm, 35 mm color print.
Procedure: The substance to be examined is heated up to the
isotropic liquid point on a microscopic slide and a cover slip is pushed
into the melt in such a way that thin layer is formed between the slide
and cover slip. The slide was cooled thoroughly and placed on the
heating stage of the polarizing microscope. The temperature initially
was raised rapidly (5ꢀC minꢁ1) and the slide was then recooled. After-
wards the heating was then continued at a rate of 1ꢀC minꢁ1 and cool-
ing too was carried out at the same rate. The mesomorphic transition
temperatures and the textures under the microscope were recorded
for the compounds.
Synthesis
Dibenzo 18-crown-6 was synthesized by earlier reported method [29].
Nitration of dibenzo 18-crown-6 was also done by reported method [30].
Preparation of trans-diamino Dibenzo 18-Crown-6
(t-DADBC)
A mixture of 12.0 g (0.026 mol) trans-dinitro dibenzo 18-crown-6, 4 g
Raney Nickel catalyst, and 300 mL ethanol was refluxed in a liter flask
and added 20 mL hydrazine hydrate with stirring in about 20 min at a
rate so as to maintain a vigorous reaction. The reaction mixture was
refluxed until the effervescence ceased and continued for further
15 min. The catalyst was filtered hot with suction and the filtrate on
cooling in ice-bath deposited t-DADBC (Fig. 1). Yield 6.5 g (63.5%),
mp 199–201ꢀC, reported [30] 201–203ꢀC.

74
V. B. Parikh and S. K. Menon
FIGURE 1 Preparation of trans-diamino dibenzo 18-crown-6.
General Synthesis of Crown Ether Schiff Bases
A solution of trans-diamino dibenzo 18-crown-6 (10 mmol) and alde-
hyde (20 mmol) (Fig. 2) in anhydrous ethanol (100 mL) in the presence
of anhydrous MgSO4 were stirred for 4 h under N2 atmosphere at 80ꢀC,
FIGURE 2 Synthesis of crown ether Schiff base containing DB18C6.

Properties of Novel Crown Ether Schiff Bases
75
and then the mixture was cooled. The different yellow colored precipi-
tate was filtered and washed with ethanol. After recrystallization from
ethanol, yellow crystals were obtained.
Synthesis of N,N’-bis(2-hydroxy-3-methoxybenzidene)-4,4’-
diamino-2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacycloocta
deca-2,11-diene (1)
A solution of trans-diamino dibenzo 18-crown-6 (3.90 g, 10 mmol)
and 2-hydroxy 3-methoxy benzaldehyde (3.04 g, 20 mmol) in anhy-
drous EtOH (100 mL) in the presence of anhydrous MgSO4 were stir-
red for 4 h under N2 atmosphere at 80ꢀC, and then the mixture was
cooled. The yellow precipitate was filtered and washed with EtOH.
After recrystallization from EtOH, yellow crystals (yield 91%) were
ꢀ
1
obtained. mp 122 C. H-NMR d (ppm): 10.20 (s, 1H, OH), 9.25 (s, 1H,
=
N CH), 8.16 6.75(m, 14H, ArH), 4.38 4.15 (m, 14H, OCH2, NCH2Ar);
FT-IR (KBr,cmꢁ1)tmax: 3445, 2930, 1588, 1506, 1235,1129; FAB MS
(m=z): 660 (Mþ1) 682 (MþNa); (Found: C 65.64, H 5.81, N 4.25.
C36H38N2O10 calcd.: C 65.67, H 5.78, N 4.25%).
Synthesis of N,N’-bis(3,4,5-trimethoxybenzidene)-4,4’-
diamino-2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacycloocta
deca-2,11-diene (2)
Compound 2 was prepared by following the general procedure as
above by taking a solution of trans-diamino dibenzo 18-crown-6
(3.90 g, 10 mmol) and 3,4,5-trimethoxy benzaldehyde (3.92 g, 20 mmol).
After recrystallizationꢀfrom, EtOH, yellow crystals (yield 82%) were
1
=
obtained. mp 125–126 C. H-NMR d (ppm): 8.14 (s, 1H, N CH), 7.70
6.48 (m, 14H, ArH), 3.84 3.65 (m, 14H, OCH2, NCH2Ar); FT-IR
(KBr,cmꢁ1)tmax: 2884, 1588, 1260, 1130; FAB MS (m=z): 770 (MþNa);
(Found: C 64.33, H 6.21, N 3.75. C40H46N2O12 calcd.: C 64.30, H 6.20,
N 3.80%).
Synthesis of N,N’-bis-thiophene-2-carbalidiene-4,4’-
diamino(2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxa cycloocta
deca-2,11-diene (3)
Compound 3 was prepared by taking a solution of trans-diamino
dibenzo 18-crown-6 (3.90 g, 10 mmol) and thiophene-2-carbaldehyde
(2.24 g, 20 mmol). After recrystallization from EtOH, yellow crystals
(yield 84%) were obtained. Mp 187–189ꢀC. 1H-NMR d (ppm): 8.34
=
=
(s,1H, N CH), 8.43(s, 1H, N CH), 7.39 6.42 (m, 14H, ArH), 4.23 4.04
(m, 14H, OCH2, NCH2Ar); FT-IR (KBr,cmꢁ1)tmax: 2928, 1588, 1246,
1134; FAB MS (m=z): 579 (Mþ); (Found: C 62.28, H 5.88, N 4.84.
C30H30N2O6S2 calcd.: C 62.30, H 5.85, N 4.79%).

76
V. B. Parikh and S. K. Menon
Synthesis of N,N’-bis-pyrrole-2-carbalidiene-4,4’-
diamino(2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxa cycloocta
deca-2,11-diene (4)
By taking a solution of trans-diamino dibenzo 18-crown-6 (3.90 g,
10 mmol) and pyrrole-2-carbaldehyde (1.90 g, 20 mmol) compound 4
was prepared. After recrystallization from, EtOH, yellow crystals
(yield 76%) were obtained. Mp 170–172ꢀC. 1H-NMR d (ppm): 8.95
=
(s,1H,N CH), 7.94 6.90(m, 14H, ArH), 4.23 4.04 (m, 14H, OCH2,
NCH2Ar); FT-IR (KBr,cmꢁ1)tmax: 2932, 2861, 1612, 1508, 1254,1127;
FAB MS (m=z): 568 (MþNa); (Found: C 66.16, H 5.92, N 10.29.
C30H32N4O6 calcd.: C 66.05, H 6.00, N 10.35%).
Synthesis of N,N’-bis-salicylidiene-4,4’-diamino(2,3,11,12-
dibenzo-1,4,7,10,13,16-hexaoxa cycloocta deca-2,11-diene (5)
In the case of compound 5 a solution of trans-diamino dibenzo
18-crown-6 (3.90 g, 10 mmol) and salicylaldehyde (2.44 g, 20 mmol) were
reacted as above method. After recrystallization from EtOH, yellow
crystals (yield 92%) were obtained. Mp 212–214ꢀC. H-NMR d (ppm):
=
10.97 (s, 1H, OH), 9.12 (s, 1H, N CH), 8.02 6.93 (m, 14H, ArH), 4.23
4.06 (m, 14H, OCH2, NCH2Ar); FT-IR (KBr,cmꢁ1)tmax: 3410, 1595,
1513, 1227, 1049; FAB MS (m=z): 600 (Mþ); (Found: C 68.29, H
5.65, N 4.64. C34H32N2O8 calcd.: C 68.26, H 5.67, N 4.62%).
Synthesis of N,N’-bis-2-hydroxy naphthalidiene-4,4’-
diamino(2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxa cycloocta
deca-2,11-diene (6)
Compound 6 was prepared by taking a solution of trans diamino
dibenzo 18-crown-6 (3.90 g, 10 mmol) and 2-hydroxy naphthaldehyde
(3.44 g, 20 mmol). After recrystallization from EtOH, yellow crystals
(yield 83%) were obtained. Mp 226–228ꢀC. 1H-NMR d (ppm): 10.03
=
(s, 1H, OH), 8.55 (s, 1H, N CH), 8.06 6.90 (m, 14H, ArH), 4.23 4.06
(m, 14H, OCH2, NCH2Ar); FT-IR (KBr,cmꢁ1)tmax: 3414, 1591, 1508,
1279, 1132, 1060; FAB MS (m=z): 698 (M); (Found: C 72.22, H 5.41,
N 4.03. C42H38N2O8 calcd.: C 72.20, H 5.45, N 4.01%).
Synthesis of N,N’-bis-5-bromosalicylidiene-4,4’-
diamino(2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxa cycloocta
deca-2,11-diene (7)
Compound 7 was prepared by the same procedure as given above by
taking a solution of trans-diamino dibenzo 18-crown-6 (3.90 g,10 mmol)
and 5-bromo salicylaldehyde (4.02 g, 20 mmol) After recrystallization
from EtOH, yellow crystals (yield 79%) were obtained. Mp 198–
200ꢀC. 1H-NMR d (ppm): 11.97 (s, 1H, OH), 9.89 (s, 1H, OH), 8.60

Properties of Novel Crown Ether Schiff Bases
77
=
=
(s, 1H, N CH), 8.43 (s, 1H, N CH), 7.78 6.56 (m, 14H, ArH), 4.16 3.73
(m, 14H, OCH2, NCH2Ar), 2.86 (t, J ¼ 5.3 Hz, 4H, NCH2); FT-IR
(KBr,cmꢁ1)tmax: 3354, 1595, 1507, 1252, 1124, 1051; FAB MS (m=z):
756(Mþ); (Found: C 54.01, H 4.20, N 3.70, Br 21.15. C34H32N2O8Br2
calcd.: C 53.96, H 4.23, N 3.70, Br 21.16%).
Synthesis of N,N’-bis-2,3-dihydroxybenzalidiene-4,4’-
diamino(2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxa cycloocta
deca-2,11-diene (8)
Compound 8 was prepared by taking a solution of trans-diamino
dibenzo 18-crown -6 (3.90 g, 10 mmol) and 2,3-dihydroxy benzaldehyde
(2.76 g, 20 mmol). After recrystallization from EtOH, yellow crystals
ꢀ
1
(yield 81%) were obtained. Mp 187–189 C. H-NMR d (ppm): 13.24 (s,
=
1H, OH), 10.92 (s, 1H, OH), 9.84 (s, 1H, N CH), 7.60 6.70 (m, 14H,
ArH), 4.23 4.04 (m, 14H, OCH2, NCH2Ar); FT-IR (KBr,cmꢁ1)tmax
:
3445, 1588, 1506, 1235, 1129; FAB MS (m=z): 631 (M þ 1); (Found:
C 64.75, H 5.4, N 4.46. C34H34N2O10 calcd.: C 64.76, H 5.39, N 4.44%).
RESULTS AND DISCUSSION
Physical Characterization
All the crown ether Schiff bases thus obtained are found to be thermally
stable. The spectroscopic methods (FTIR, 1H NMR and FAB-MS) have
been employed to elucidate the structures of the compounds (1–8).
FTIR data show that the diagnostic bonds, which can be assigned
=
to the stretching of imine (C N) of all compounds were observed at
the frequencies of 1588 to 1622 cmꢁ1. The bands appearing at 3354 to
3450 cmꢁ1 in the spectra of compounds except 2, 3, and 4 can be
ascribed to the presence of hydroxy (ꢁOH) group. A band assigned
to the stretching of CꢁOꢁC of the ether linkage is also observed in
the FTIR data of all compounds.
1H NMR data of all the compounds showed that a peak due to the
=
presence of azomethine proton (CH N) was observable at the chemical
shift d ¼ 8.2 to 9.0 ppm. While the multiplet owing to the presence of
aromatic protons was observed within the range of d ¼ 6.4 to 8.0 ppm,
the phenolic protons was observable within the range of d ¼ 10.0 to
11.0 ppm. The appearance of a peak within the range of d ¼ 3.5 to 4.3
is due to the presence of the proton of ether (ꢁOCH2) group.
The formulations and molecular structure of compounds 1–8 in
Figure 2 are also supported by the data obtained from FAB-MS and
microanalysis, wherein the percentage of C, H, and N from the
analysis conform the calculated values.

78
V. B. Parikh and S. K. Menon
FIGURE 3 ORTEP-3 drawing of compound N,N’-bis(2-hydroxy-3-methoxy-
benzidene)-4,4’-diamino-2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacycloocta
deca-2,11-diene (1).
ORTEP-3 drawing of compound 1 and 2 are given in Figures 3 and 4,
respectively.
Phase Transition Behavior and LCCE Schiff Bases
All the newly synthesized crown ether Schiff bases 1–8 were studied
for the liquid crystalline properties. Melting points of selected crown
ether Schiff bases given in Table 1 were taken in open capillaries for
approximate evaluation of the transition temperatures. The transition
FIGURE 4 ORTEP-3 drawing of compound N,N’-bis(3,4,5-trimethoxybenzi-
dene)-4,4’-diamino-2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacycloocta
deca-
2,11-diene (2).

Properties of Novel Crown Ether Schiff Bases
79
TABLE 1 Capillary Data of Crown Ether Schiff Bases
Compound no.
Compound name
MpꢀC
ꢃcpꢀC
1
2
3
4
5
6
7
8
5
6
7
8
3
4
1
2
212
227
198
187
154
170
121
125
–
–
–
–
196
208
245
248
ꢃ
cp ¼ clearing point.
from the solid state to the first mesophase is considered as the melting
point. Heating the compound in a capillary tube showed viscous turbid
melt sticking to the walls of the tube, followed by a turbid mobile state
that ultimately yielded the isotropic liquid state. The thermal and
liquid crystal behavior of these compounds were investigated by the
combination of DSC and POM.
Transition temperatures and thermal behavior of crown ether
Schiff bases recorded under the POM and DSC are shown in Table 2.
The results show that most of the compounds are mesomorphic in
TABLE 2 Thermal Transition Data of Compounds 1–8 Upon Cooling Process
and Corresponding Enthalpy Changes (J gꢁ1
)
Compound
Transition
Temperature (ꢀC)
DT=ꢀC
DH (J gꢁ1
)
1
Cr-SmC
SmC-SmA
SmA-N
N-I
Cr-SmA
SmA-N
N-I
Cr-SmC
SmC-I
Cr-N
122.62
153.07
225.05
243.99
91.89
131.51
300.17
154.7
196.4
171.2
209.6
212.3
227.2
198.6
187.5
2.11
35.90
4.07
1.63
15.20
9.64
11.78
5.04
23.80
16.46
2.72
–
30.45
71.98
18.94
2
71.14
50.76
3
4
41.7
N-I
Cr-I
Cr-I
Cr-I
38.4
5
6
7
8
–
–
–
–
–
–
–
Cr-I
Cr - crystal; SmA - smectic A; SmC - smectic C; I - isotropic.

80
V. B. Parikh and S. K. Menon
character. The mesophases were identified from their optical textures.
Focal conic fan texture of smectic phase and schlieren nematic phase
are easily distinguishable from their typical textures and features.
Both smectic and nematic mesophases were observed in most of the
compounds. It has been observed that the crystal-to-smectic and
smectic-to-nematic transitions overlapped in some cases.
Compound 1 shows SmC phase at 122ꢀC and SmA mesophase at
153ꢀC (Fig. 5a), which on further heating at 225ꢀC transforms into
nematic (Fig. 5c) and finally at 244ꢀC (Fig. 5d) into isotropic phase.
The textures are taken in cooling condition. After cooling nematic
phase gave schlieren and broken focal-conic fan texture of the SmC
phase and focal-conic fan texture of the SmA Phase.
Compound 2 shows smectic mesophase at 92ꢀC (Fig. 6a), which on
further heating at 131ꢀC transforms into nematic phase (Fig. 6b).
After cooling the Schlieren nematic phase the compound gave
focal-conic fan texture of the SmA phase.
FIGURE 5 POM study of the liquid crystals textures of compound 1.

Properties of Novel Crown Ether Schiff Bases
81
FIGURE 6 POM study of the liquid crystals textures of compound 2.
Compound 3 also shows smectic phaseat 154.7ꢀC which may be due
to the presence of the thiophene ring in a lateral position within the
structure of the compound [31].
Compound 4 shows Nematic phase at 171.2ꢀC. Compound 5 to 8
does not exhibit any mesophases. Presumably this compound is too
rigid to display mesomorphic behavior as it does not have a terminal
alkoxy or alkyl group.
CONCLUSION
By substituting the parent dibenzo-18-crown-6 with mesogenic units
(e.g., cyano, nitro, amino, alkoxy, olefin, azomethine, nitrone, azoxy,
azo, ester, etc.) newer compounds with greater mesomorphic range
can be designed and hence in the present work molecules have been

82
V. B. Parikh and S. K. Menon
synthesized by attaching azomethine and methoxy units to the crown
ether base. Most of the compounds exhibit liquid crystalline behavior
on the basis of results obtained from POM and DSC. The relatively
higher transition temperature in 1 is perhaps due to the presence of
an additional Schiff base unit and structural rigidity. Micrographs
show birefringence due to the mesomorphic nature of the compounds.
The order of the phase transition: Spherulitic texture–Smectic
texture–Nematic texture–Isotropic Texture.
Compounds 1 to 4 show liquid crystalline behavior at higher tran-
sition temperature which can be minimized by substituting long ester
chain or by branched alkanes. LCCE Schiff bases can be utilized as one
of the indicative tools for the complexation study as well as for the
applying both the properties of crown ether and liquid crystal. The com-
pounds are showing focal-conic fan texture of SmA phase, schlieren,
and broken focal-conic texture of SmC phase, mosaic texture of SmC
phase threaded texture of the nematic phase, schlieren nematic phase,
and mobile texture of the nematic phase.
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