Liquid crystals based on calix[4]arene Schiff bases

Article abstract of DOI:10.1007/s10847-009-9620-z

New liquid crystals based on calix[4]arene Schiff base were prepared by the reaction of tetraamino-calix[4]arene with aldehydes (4-hydroxy benzaldehyde, 2-vanillin, 4-vanillin and 2-hydroxy naphthaldehyde). Dielectric investigations on a magnetically oriented sample forming N, SmA, and SmC phases were carried out. The dielectric constant (ε' and ε) and dielectric loss (tan δ) have been determined as a function of frequency (20 Hz-2 MHz). The synthesized derivatives were purified and characterized by FT-IR, ¹H-NMR, ¹³C-NMR and MALDI-TOF MS. All the synthesized compounds were investigated for liquid crystalline properties using DSC (Differential Scanning Calorimetry), DTA (Differential Thermal Analysis) and POM (Polarizing Optical Microscopy) attached with a hot stage. They generally exhibited nematic and typical fanlike or mosaic texture, which suggest the ordered smectic mesophases. Compounds were found to adopt a specific molecular structure due to the rigid bowl like calix[4]arene core, i.e., a cone-like structure with mesogenic units aligned within the molecule.

Full text of DOI:10.1007/s10847-009-9620-z

J Incl Phenom Macrocycl Chem (2010) 66:285–295
DOI 10.1007/s10847-009-9620-z
ORIGINAL ARTICLE
Liquid crystals based on calix[4]arene Schiff bases
Ravindra V. Patel Æ Jayesh G. Panchal Æ
V. A. Rana Æ Shobhana K. Menon
Received: 17 March 2009 / Accepted: 22 June 2009 / Published online: 7 July 2009
Ó Springer Science+Business Media B.V. 2009
Abstract New liquid crystals based on calix[4]arene
Schiff base were prepared by the reaction of tetraamino-
calix[4]arene with aldehydes (4-hydroxy benzaldehyde,
2-vanillin, 4-vanillin and 2-hydroxy naphthaldehyde).
Dielectric investigations on a magnetically oriented sample
forming N, SmA, and SmC phases were carried out. The
dielectric constant (e’ and e’’) and dielectric loss (tan d)
have been determined as a function of frequency (20 Hz–
2 MHz). The synthesized derivatives were purified and
characterized by FT-IR, 1H-NMR, 13C-NMR and MALDI-
TOF MS. All the synthesized compounds were investigated
for liquid crystalline properties using DSC (Differential
Scanning Calorimetry), DTA (Differential Thermal Anal-
ysis) and POM (Polarizing Optical Microscopy) attached
with a hot stage. They generally exhibited nematic and
typical fanlike or mosaic texture, which suggest the ordered
smectic mesophases. Compounds were found to adopt a
specific molecular structure due to the rigid bowl like
calix[4]arene core, i.e., a cone-like structure with meso-
genic units aligned within the molecule.
Introduction
Substances with dielectric constant are widely used in
electronic industry [1]. As dielectric interlayers in elec-
tronic chips, they strongly increase the speed of propaga-
tion of electric impulses and reduce dielectric losses.
Electrical properties ferrites are derived from much phys-
ical and chemical characterization. The effect of frequency
on dielectric offers much valuable information about the
localized charge carrier which in turn helps to explain the
mechanisms responsible for charge transport phenomena
and dielectric behavior. Thermotropic liquid crystals are of
great technological importance [2].Terminal groups are one
of the important molecular fragments to form liquid crys-
tals. There are different terminal substituent’s affianced
that are either a small polar substituent (i.e., –CN or –X) or
alkyl or alkoxy chains. It is well known that at least one
alkyl or alkoxy chain is necessary for the molecular ori-
entation necessary for liquid-crystal phase generation [3].
The interest in synthesis and structural investigation of
liquid-crystalline alkyl or alkoxy with calix[4]arene Schiff
bases groups in the mesogenic units has considerably
increased due to their potential application in the field of
nonlinear optics, in optical storage devices and in electro-
optical displays. More recently, their applications in light
emitting diodes [4] and field effect transistors [5] have also
been explored.
Keywords Calix[4]arene Schiff base Á
Dielectric constant Á Mesophases Á Thermotropic Á
Dielectric loss tangent
Calixarene and their derivatives are attractive com-
pounds for host-guest chemistry and supramolecular
chemistry [6] and in the construction of novel molecular
architectures [7] because of their tunable and unique three-
dimensional structures together with the ease of function-
alization. Recently, it has also been used as a building block
for liquid crystalline materials [8]. Mesogenic materials
with a calix[4]arene central unit were first reported in 1990
R. V. Patel Á J. G. Panchal Á S. K. Menon (&)
Department of Chemistry, School of Sciences, Gujarat
University, Ahmedabad 380009, India
e-mail: shobhanamenon07@gmail.com
V. A. Rana
Department of Physics, School of Sciences, Gujarat University,
Ahmedabad 380009, India
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[9]. These compounds exhibits columnar mesophases if 12
sufficiently long aliphatic chains are connected to an exo-
calix[4]arene core [10–12]. Liquid crystalline derivatives,
based on endo-calix[4]arene has also been reported [13–
16]. Recently, liquid crystalline materials of calixarenes
have been prepared by the attachment of cyanobiphenyl
mesogenic groups to hydroxyl groups on p-tert-butylca-
lix[8]arene and calix[4]resorcinarene, via ester groups
having alkyl spacers, which exhibited smectic phases [17].
Similar liquid crystalline phases were showed by octade-
cyloxy derivative of calix[4]arene [18] and end-function-
alized tetra-oligophenylene substituted calix[4]arenes [19].
Our earlier report [20] on the synthesis of crown ether
Schiff bases and their promising liquid crystalline proper-
ties prompted us to investigate similar properties on calix-
arene Schiff bases.
purging at 30 mL min-1. The textures of the mesophases
were studied with Leica DMLP polarizing microscope,
equipped with a hot stage, Mavotherm 32 temperature
display unit and Leica MPS 32 Data back (display unit).
Agilent 4980A 005 precision LCR meter (20 Hz–2 MHz
over 8610 selectable frequencies) was used for dielectric
constant measurements. X-ray diffraction (XRD) experi-
ments were performed on SEIFERT-FPM (XRD7), using
˚
Cu Ka 1.5406 A as the radiation source with 40 kV,
30 mA power and the XRD patterns were recorded at room
temperature (25 °C).
Synthesis
p-tert-Butylcalix[4]arene (1) was synthesized by micro-
wave technique developed in our laboratory. The synthesis
of the intermediates, debutylated calix[4]arene (2) [21],
azocalix[4]arene (3) [22] and tetraaminocalix[4]arene (4)
[23] were according to the literature methods.
In this article we report six new functionalized calix[4]-
arene Schiff bases and their liquid crystalline property.
Calix[4]arene Schiff bases prepared from various aldehydes
(4-hydroxybenzaldehye, 2-vanillin, 4-vanillin, and 2-hy-
droxynaphthaldehyde) condensed with amino calix[4]are-
nes substituted with alkyl chains(n-butyl, n-pentyl and
n-octyl) at the para position through substitution of the
hydroxyl group of the lower rim of calix[4]arene.All the
compounds thus isolated were purified and characterized by
FTIR, 1H-NMR, 13C-NMR, and MALDI. Further, com-
pounds were extensively analyzed by using differential
scanning calorimetry (DSC) and polarising optical micros-
copy (POM). Effects of changes in the polar substituents in
aldehydes and effect of alkyl chain length on the mesophase
were discussed. We have also studied the dielectric prop-
erties of calix[4]arene Schiff bases and their electro-optical
effect.
Microwave-assisted synthesis of 5,11,17,23-tetra-butyl-
25,26,27,28 tetrahydroxycalix[4] arene 1
For the microwave synthesis of 1, 10 g of p-tert-butyl-
phenol was mixed with NaOH (0.05 g, 1.2 mmol) dis-
solved 0.5 mL of water and 6.2 mL 37% formaldehyde
solution and was heated in a Discover BenchMate system-
240 V(CEM Corporation) microwave at 300 watt output
power for 5 min to give a yellow solid. To this was added
7 mL of diphenyl ether and 1 mL toluene and the mixture
was heated at 300 watt output power for 15 min to obtain
corresponding calix[4]arene. The purity was checked by
TLC and the results of mp, FT-IR, 1H-NMR, 13C-NMR and
MS were compared with standard sample.
Experimental
General synthesis of calix[4]arene Schiff bases, 5
Apparatus
Tetraaminocalix[4]arene (10 mmol) and aldehydes (40 mmol)
(Scheme 1) in anhydrous ethanol (100 mL) was stirred for
2 h under N2 atmosphere. The yellow precipitate was filtered
and recrystallized from ethanol and chloroform.
The FT-IR spectra were recorded as KBr pellet on Bruker
TENSOR-27 in the range of 4,000–400 cm-1. Discover
BenchMate system-240 V (CEM Corporation) microwave
synthesizer was used for synthesis. The MALDI-TOF MS
was run on a Micromass TofSpece 2E instrument, using a
nitrogen 337 nm laser (4 ns pulse).1H-NMR spectra was
scanned on 400 MHz FT-NMR Bruker Avance-400 in the
range of 0.5–15 ppm and 13C-NMR spectra was recorded
on a Bruker DPX-300 spectrometer using internal standard
tetramethylsilane (TMS) and deuterated DMSO as a sol-
vent in the range of 0.5–250 ppm. Transition temperature
and enthalpies were scanned in Shimadzu DSC 60, dif-
ferential scanning calorimeter with a heating rate of
10.0 °C min-1 in air and it was calibrated with nitrogen
Calix[4]arene Schiff base, 5a
Yield, 2.5 g (83%); m.p. 190 °C. Anal. calc. for
C56H44N4O8: C,74.65; H, 4.92; N, 6.22% Found: C, 74.60;
H, 4.93; N, 6.24%. FT-IR(KBr)t: 3251 cm-1(–OH),
2950 cm-1, 1594 cm-1(CH = N), 1H-NMR (CDCl3) d
9.5(s, 4H, Ar OH), 9.6 (s, 4H, Ar OH), 8.54 (s, 4H, Ar
CH = N), 2.52(s, 8H, CH2), 7.5(d, 16H, ArH), 7.1(m, 8H,
ArH). 13C-NMR(CDCl3) 161(CH = N) d 162, 148, 141,
135, 132, 128, 127, 118 and 116 (Ar C), 13(–CH2–).
MALDI-TOF MS(m/z) 901(M) 902(M ? 1).
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9.5(s, 4H, Ar OH) 13.9 (s, 4H, Ar OH) 8.52 (s, 4H, Ar
CH = N) 2.52(s, 8H, CH2) 7.15 (m, 12H, Ar H)7(m, 8H,
ArH)3.83(s, 12H, CH3). 13C-NMR(CDCl3) d 160(CH = N),
159, 151, 149, 144.7, 128.9, 127.8, 124.1, 119.6, 118.1
and 113(ArC), 56.2(–OCH3), 13.0(–CH2–). MALDI-TOF
MS(m/z) 1022(M ? 1).
Calix[4]arene Schiff base, 5d
Yield, 2.5 g (83%); m.p. 205 °C. Anal. calc. for C72
H52N4O8: C, 78.53; H, 4.76; N, 5.09%. Found: C, 78.40;
H, 4.79; N, 5.19%. FT-IR (KBr)t: 3166 cm-1(–OH),
1
1904 cm-1, 1671(CH = N) cm-1. H-NMR (CDCl3) 9.85
(s, 4H, Ar OH) 12.65 (s, 4H, Ar OH) 9.33 (s, 4H, Ar
CH = N) 2.58(s, 8H, CH2) 7.83 (m, 24H, ArH) d 7.02
(m, 8H, ArH). 13C-NMR(CDCl3) d 153.8(CH = N), 172,
157, 145.1, 135.8, 129.1, 128.0, 127.8, 125.2, 124.9, 122,
118.6 and 116.4(Ar C),13 (–CH2–). MALDI-TOF MS(m/z)
1103(M?).
General procedure for the substitution of the upper-rim
and lower-rim Calix[4]arene compounds 5a and 5b [7]
Compound, 6a1
Taken sodium hydride 3.9 g (98.0 mmol) (Scheme 1) and
removed paraffin by washing with petroleum ether. Added
80 mL of dry DMF, compound a 4.6 gm (9.49 mmol) and
8.17 mL (75.92 mmol) of n-butyl bromide and stirred at
room temperature for 12 h. Filtered the solid compound.
Yield, 90.2%; m.p. 180 °C. Anal. calc. for C96H124N4O8:
C, 78.86; H, 8.55; N, 3.83. Found: C 78.88; H, 8.57; N,
3.81. FT-IR(KBr)t: 2958 cm-1, 2869 cm-1, 1594 cm-1
(CH = N), 1454 cm-1, 1360 cm-1, 1249 cm-1(C–O),
Scheme 1
1
1080 cm-1. H-NMR(CDCl3) d 0.88 (t, 24H, CH3) d 1.42
Calix[4]arene Schiff base, 5b
(m, 16H, CH2) d 8.58(s, 4H, ArCH = N) d 2.52(s, 8H,
CH2) d 7.5 (m, 8H, ArH) d 7.0(m, 16H, ArH) d 4.12(t, 16H,
OCH2) d 1.34(m, 32H, CH2). 13C-NMR (CDCl3) d 160
(CH = N), 161.7, 158, 144.19, 129, 128,117.3, 114.9 and
113.1(ArC), 68.4(–OCH2–), 35.8, 29, 27, 25.1 and
14.0(alkyl), 13.0(–CH2–). MALDI-TOF MS(m/z) 1462(M)
1463(M ? 1).
Yield, 2.5 g (83%); m.p. 185 °C. Anal. calc. for C60
H52N4O12: C, 70.58; H, 5.13; N, 5.49%. Found: C 70.61: H,
5.12; N, 5.47%. FT-IR(KBr)t: 3434 cm-1(–OH),
2932 cm-1, 1598 cm-1 (CH = N). 1H-NMR (CDCl3) d
9.3(s, 4H,Ar,OH) 9.55 (s, 4H,Ar OH) 8.54(s, 4H, Ar
CH = N) 2.52 (s, 8H, CH2) 7.15 (m, 12H, ArH) 7(m, 8H,
ArH)3.83(s, 12H, CH3). 13C-NMR(CDCl3) d 160(CH = N),
157, 151, 149, 144.5, 135, 124.8, 118.3, 114.9, 112.1 and
111(Ar C), 56.2(–OCH3), 13.0(–CH2–). MALDI-TOF MS
(m/z) 1021(M?).
In the same way substitution of the hydroxyl group on
both upper and lower rim of the Schiff bases 6a2, 6a3, 6b1,
6b2 and 6b3 were carried out using n-pentyl bromide, n-
hexyl bromide and, n-heptyl bromide.
Calix[4]arene Schiff base, 5c
Compound, 6a2
Yield, 2.5 g (83%); m.p. 120 °C. Anal. calc. for C60
H52N4O12: C, 70.58; H, 5.13; N, 5.49%. Found: C, 70.61;
H, 5.12; N, 5.47%. FT-IR (KBr)t: 3221 cm-1(–OH),
2958 cm-1, 1603 cm-1(CH = N). 1H-NMR (CDCl3) d
Yield, 90.2%; m.p. 175 °C. Anal. calc. for C104H140N4O8:
C, 78.86; H, 8.55; N, 3.83%. Found: C, 78.88; H, 8.57; N,
3.81. FT-IR(KBr)t: 2958 cm-1, 2869 cm-1, 1594 cm-1
(CH = N), 1454 cm-1, 1360 cm-1, 1249 cm-1 (C–O),
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1082 cm-1. 1H-NMR(CDCl3) d 0.91(t, 24H, CH3) d 1.42(m,
32H, CH2) d 8.58(s, 4H, ArCH = N) d 2.58 (s, 8H, CH2) d
7.5(m, 8H, ArH) d 7.1(m, 16H, ArH) d 4.16(t, 16H, OCH2) d
1.34(m, 32H, CH2). 13C-NMR(CDCl3) d 160(CH = N),
161.7, 158, 144.19, 129, 128, 117.3, 114.9 and 113.1(ArC),
68.4(–OCH2–), 35.8, 29, 27, 25.1 and 14.0 (alkyl), 13.0
(–CH2–). MALDI-TOF MS(m/z) 1574(M)1575(M ? 1).
Compound, 6b3
Yield, 94.2%; m.p. 176 °C. Anal. calc. for C116H164N4O12:
C, 76.44; H, 9.95; N, 3.07. Found: C, 77.60; H, 9.47; N, 2.93.
FT-IR(KBr)t: 2959 cm-1, 2869 cm-1, 1593 cm-1 (CH =
N), 1454 cm-1, 1380 cm-1, 1249 cm-1 (C–O), 1053 cm-1
,
1
961 cm-1. H-NMR (CDCl3) 0.88(t, 24H, CH3) 1.45 (m,
16H, CH2) 8.58(s, 4H, ArCH = N) 2.52(s, 8H, CH2)
7.48(m, 12H, ArH) 7.0(m, 8H, ArH) 4.12(t, 16H, OCH2), d
148(m, 32H, CH2) 1.79(q, 32H, CH2) d 3.75 (s, 12H, CH3);
13C-NMR(CDCl3) d 160(CH = N), 159, 150, 144, 129.6,
124.1, 118.3,115.7, 113.1 and 112.2(Ar C), 69.4(–OCH2–),
56.2(–OCH3), 29.6, 29.1, 25.3 and 14.1(alkyl), 13.0
(–CH2–). MALDI-TOF MS(m/z) 1806(M ?).
Compound, 6a3
Yield, 93.2%; m.p. 173 °C. Anal. calc. for C112H156N4O8:
C, 79.01; H, 10.18; N, 3.29%. Found: C, 80.15; H, 9.62%; N,
3.11. FT-IR(KBr)t: 2958 cm-1, 2927 cm-1, 2868 cm-1
,
1590 cm-1(CH = N), 1454 cm-1, 1380 cm-1, 1250 cm-1
1
(C–O), 1120 cm-1, 1082 cm-1. H-NMR (CDCl3) d 0.88
(t, 24H, CH3) d 1.42 (m, 16H, CH2) 8.58 (s, 4H, Ar
CH = N) d 2.52 (s, 8H, CH2) 7.5 (m, 8H, ArH) d 7.0 (m,
16H, ArH) 1.30 (m, 32H, CH2) 4.12 (t, 16H, OCH2) d 1.68
(q, 32H, CH2). 13C-NMR(CDCl3) d 160(CH = N), 161.7,
158, 145, 129, 128, 127.3, 119.9 and 114.1(ArC), 68.4
(–OCH2–), 31.8, 29, 25.3, 22.1 and 14.1(alkyl), 13.0
(–CH2–). MALDI-TOF MS(m/z) 1685 (M), 1686(M ? 1).
Results and discussion
Synthesis
Microwave synthesis of p-tert-butylcalix[4]arene was car-
ried out for the first time by base-catalysed condensation of
p-substituted phenol and formaldehyde with an improve-
ment of yield to 90–95%.
Compound, 6b1
The synthesis run under microwave irradiation, require
only 5–10 mL of solvent as compared to 250–300 mL
required for conventional heating method and are radically
fast with reaction accomplished within 15–20 min or less
with significant increase in the yield. The calix[4]arene
derivative (Scheme 2) was prepared by irradiating phenol
and formaldehyde mixture by microwave. The preparation
of azo calixarenes by three different multi step routes was
reported by Taniguchi and coworkers in 1989. The most
convenient route involved azo coupling of calixarenes with
p-carboxybenzenediazonium chloride and reduction of the
azo derivatives (aqueous alkaline medium) with sodium
dithionite. This compound can also be prepared by first
nitrating calixarene and then carrying out reduction by
Raney-Ni and hydrazine hydrate but by this route yields
were low. On the other hand, product obtained by reduction
of dye was in good yield and reaction was accomplished in
a rather short span of time. Condensation of teraamino
derivative with various aldehydes such as 4-hydroxy
benzaldehyde, 2-vanillin, 4-vanillin and 2-hydroxy napth-
aldehyde gave respective Schiff bases. Substitution of the
hydroxyl group on both upper and lower rim of the Schiff
bases 6a1–6a3, and 6b1–6b3 were carried out using
n-pentyl bromide, n-hexyl bromide and, n-heptyl bromide.
The introductions of four long alkoxyl group on the lower
rim of calix[4]arenes locks the macrocycle in cone con-
formation. This conformation was exclusively achieved by
alkylating lower rim hydroxyl groups in DMF at room
temperature using NaH to impart template effect. In the
Yield, 94.8%; m.p. 185 °C. Anal. calc. for C100H132N4O12:
C, 75.91; H, 8.41; N, 3.54. Found: C, 75.88; H, 8.45; N, 3.54.
FT-IR(KBr)t: 2957 cm-1, 2871 cm-1, 1594 cm-1(CH =
N), 1464 cm-1, 1381 cm-1, 1250 cm-1 (C–O), 1211 cm-1
,
1086 cm-1, 958 cm-1. 1H-NMR (CDCl3) 0.89(t, 24H, CH3)
8.58 (s, 4H, ArCH = N) 2.52 (s, 8H, CH2) 7.48(m, 12H,
ArH) 7.0(m, 8H, ArH) 1.68(q, 16H, CH2) 4.22(t, 16H,
OCH2) 1.34(m, 32H, CH2) d 3.75 (s, 12H, CH3); 13C-
NMR(CDCl3) d 160(CH = N), 152.9, 150.6, 144.7, 129.9,
124.8, 117.3, 114.9, 111.1 and 112(ArC), 69(–OCH2–),
56.2(–OCH3), 29.3, 24.1 and 14.1(alkyl), 13.0(–CH2–).
MALDI-TOF MS(m/z) 1582(M ? 1).
Compound, 6b2
Yield, 94.8%; m.p. 180 °C. Anal. calc. for C108H148N4O12:
C, 75.84; H, 9.66; N, 3.28. Found: C, 75.88; H, 8.45; N, 3.5.
FT-IR(KBr)t: 2957 cm-1, 2871 cm-1, 1597 cm-1(CH =
N), 1465 cm-1, 1381 cm-1, 1249 cm-1 (C–O), 1211 cm-1
,
1086 cm-1, cm-1, 958 cm-1 1H-NMR (CDCl3) 0.89(t,
.
24H, CH3) 8.58 (s, 4H, Ar CH = N) 2.52(s, 8H, CH2) 7.48
(m, 8H, ArH) d 7.0(m, 12H, ArH), 1.45 (m, 16H, CH2), 1.68
(q, 16H, CH2) d 4.22 (t, 16H, OCH2) 1.34 (m, 32H, CH2) d
3.7 (s, 12H, CH3). 13C-NMR(CDCl3) d 160(CH = N),
152.9, 150.6, 144.7, 129.9, 124.8, 117.3, 114.9, 111.1 and
112(ArC) 69(–OCH2–), 56.2(–OCH2–),29.3, 24.1, 14.1,
13.0(–OCH2–). MALDI-TOF MS(m/z) 1693(M ? 1).
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289
NMR and MALDI-TOF-MS) have been employed to elu-
cidate the structures of the compounds (5a–5d, 6a1–6a3 and
6b1–6b3). FTIR data show that the investigated bonds,
which can be assigned to the stretching of imine (C = N) of
all compounds were observed at the frequencies of 1,590 to
1,671 cm-1. The bands appearing at 3,166 to 3,434 cm-1 in
the spectra of compounds except 5a, 5b, 5c and 5d can be
ascribed to the presence of hydroxy (–OH) group. Fur-
thermore this bands fully disappears in the IR spectra of
6a1–3 and 6b1–3 that indicate absence of hydroxyl (–OH)
groups in the compound. Ether bands corresponding to C–O
Microwave 15-20 min
6.2 ml HCHO
0.05gNaOH
7 ml diphenylEther
1 ml toluene
4
OH
OH
tert-butylcalix[4]arene
p-tert butyl phenol
Scheme 2
stretching appears at 1,080 to 1,381 cm-1
.
present investigation, calix[4]arene based Schiff bases
showing liquid crystalline property is reported for the first
time as compared to different liquid crystalline compounds
of calixarene reported earlier.
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 = 8.52 ppm to 9.33 ppm.
The appearance of a peak within the range of d = 4.12 ppm
to 4.22 ppm is due to the presence of the proton of ether
(–OCH2–) groups.
Physical characterization
13C-NMR data of all compounds showed a peak due to
All the calix[4]arene Schiff bases thus obtained are found to
be thermally stable. Spectroscopic methods (FTIR, 1H-
the presence of azomethine carbon (CH = N) observable at
Fig. 1 XRD traces of 6a1–6a3
measured at room temperature
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the chemical shift d = 160 ppm; only compound 5d
showed a peak at d = 153.8 ppm. The aromatic carbon
showed peaks observable at d = 171–111.3 ppm; aliphatic
carbon chain at d = 69.4–14.1 ppm and methylene bridge
carbon showed peak at d = 13.0 ppm.
chains. The reflections for the small angle area correspond
to a smectic layer structure. The d-spacings of the reflec-
tions observed for 6a3 at 2 h = 5.56, 6.39 and 7.4 are 1.4,
1.38 and 1.19 nm while for 6b3 2h = 5.30, 6.39 and 11.99
The formulations and molecular structure of compounds
given in Scheme 2 (5a–5d, 6a1–6a3 and 6b1–6b3) are also
supported by the data obtained from MALDI-TOF MS and
microanalysis, where in the percentage of C, H and N from
the analysis conform the calculated values.
X-ray diffraction measurements
The PXRD pattern of the sample indicated some reflections
for small and wide angle areas as illustrated in Figs. 1 and
2 for compounds 6a1–6a3 and 6b1–6b3. A sharp, low angle
peak (2h = 5–6’’), a mid angle region peak (2h = 8–9’’)
and a broad region feature at high angle (2h = 19–22’’).
The X-ray data show that the position of the small angle
reflections depends linearly on the length of the aliphatic
Fig. 3 Variation of dielectric constant with log frequency of com-
pound 6a1–3
Fig. 2 XRD traces of 6b1–6b3
measured at room temperature
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291
are 1.59, 1.38 and 0.70, respectively, and are associated
with first, second and third reflections. The reflections for
the wide angle area suggest a highly ordered smectic
phases [17].
complex permittivity of dilute solution of 6a1–3 (Fig. 3)
and 6b1–3 was recorded in chloroform.
From the Fig. 3 it can be seen that at very low frequency
dielectric loss (e’ and e’’) is high and it decreases with
increase in the frequency. High value of dielectric loss at
very low frequency is due to the electrode polarization
Dielectric constant studies
Dielectric investigations were carried out in the frequency
range from 20 Hz to 2 MHz using Agilent 4980A 005
precision LCR meter. The variation of dielectric permit-
tivity at different frequencies of 20 Hz and 2 MHz is
depicted. Variation in real part and imaginary part of
Fig. 6 DSC thermograms for the heating and cooling run at scanning
of 10 °C min-1 compound 6b3
Table 2 Transition temperatures observed by means of polarized
optical microscopy, Thermal transition data in first heating run of
DSC Phase transitions (°C) and corresponding enthalpy changes
(J g-1
)
Compound Transition
Temperature (°C) DT/°C DH (J g-1
078.60 051.66
Sm C–Sm A 094.65
)
6a1
6a2
6a3
6b1
6b2
6b3
Cr–Sm C
–
Fig. 4 The dielectric loss tangents of compound 6a1–3
Table 1 Melting point (DTA)
of calix[4]arene Schiff bases,
016.05 015.23
042.15 009.43
043.23 021.44
Sm A–N
N–I
136.80
180.03
070.92
Compound name
m.p. °C
Cr–Sm C
–
047.97
5a–d
5a
5b
5c
5d
190
185
120
205
Sm C–Sm A 090.72
019.18 011.43
039.76 008.23
044.62 018.89
Sm A–N
N–I
130.48
175.10
063.18
Cr–Sm C
–
043.54
Sm C–Sm A 085.42
022.44 009.67
042.90 007.45
045.11 015.80
Sm A–N
N–I
128.32
173.43
084.54
Cr–Sm C
–
055.76
Sm C–Sm A 102.87
018.33 016.20
037.78 011.65
045.00 027.32
Sm A–N
N–I
140.65
185.65
080.34
Cr–Sm C
–
050.89
Sm C–Sm A 097.99
017.65 013.11
034.72 009.45
047.51 024.71
Sm A–N
N–I
132.71
180.22
068.03
Cr–Sm C
–
045.73
Sm C–Sm A 090.32
022.29 011.52
039.90 008.76
046.21 020.54
Sm A–N
N–I
130.22
176.43
Fig. 5 DSC thermograms for the heating and cooling run at scanning
of 10 °C min-1 compound 6a3
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J Incl Phenom Macrocycl Chem (2010) 66:285–295
effect. When an electric field is applied to the materials
then the long-range drift of ions and barrier layer formation
on the electrode surface results in [24] large values of the
relative dielectric function. As the frequency increases the
dielectric loss decreases approaching near zero value. This
suggests that at this dilution and in the considered fre-
quency range all compounds are loss less. Similarly the
dielectric constant values of all compounds remain con-
stant over the considered frequency range of 5.36, 5.49,
5.50 for compounds 6a1–6a3 and 5.38, 5.45, 5.55 for
compounds 6b1–6b3.
lags behind the altering field and is caused by the presence
of structural in homogeneities.
Phase transition behaviour and liquid crystallinity
of calix[4]arene Schiff bases
All the recently synthesized calix[4]arene Schiff bases (5a–
5d, 6a1–6a3 and 6b1–6b3) were studied for the liquid
crystalline properties. Melting points of the synthesized
calix[4]arenes Schiff bases are given in Table 1 which were
obtained from DTA. The transition from the solid state to
the first mesophase is considered as the melting point. The
thermal and liquid crystalline behaviour of these com-
pounds was investigated by the combination of differential
scanning calorimetry and polarized optical microscopy.
Consistently with their mesophase behavior, compounds
exhibit two endothermic peaks in the heating and cooling
The value of the loss tangent obtained in the present
work in the range of 10-1–10-2 at frequencies from a Hz to
MHz. These values of the loss tangent are about three
compounds 6a1–6a3 (Fig. 4) orders of magnitude lower
than those obtained in ferrites prepared by the conventional
method [25].The dielectric loss arises, if the polarization
Fig. 7 POM study of the liquid
crystals textures of compound
6a3
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J Incl Phenom Macrocycl Chem (2010) 66:285–295
293
solid to smectic C and smectic C–smectic A phase.
Repeated observations, however, gave the same results.
Also, during cooling the phase change from smectic C to
solid phase takes place at a lower temperature of 57 °C
indicating a super cooled state.
cycles: a small peak at low temperature assigned to the solid
to mesophase transition and a large peak at high tempera-
ture assigned to the mesophase to liquid transition. The
transition from the mesophase to isotropic liquid is not
detected in DSC thermograms but is clearly observed in the
polarizing optical microscope.
Figures 5 and 6 show DSC study of 6a3 and 6b3. The
DSC thermograms of 6a3 and 6b3 are lower compared to
other members. The thermograms show first endothermic
peak at 63.18 and 68.03 °C, respectively, while its enthalpy
change (DH) was approximately 43.54 and 45.73 Jg-1. The
second endothermic peak was at 128.32 °C(6a3) and
130.22 °C(6b3) and its enthalpy change (DH) was 7.45 and
8.76 J g-1. During cooling process two endothermic peaks
were observed at 99.92 and 145.67 °C (Fig. 5) and 102.44
and 136.81 °C (Fig. 6).There was also a small shoulder
observed in Fig. 6. From POM investigation it was found
that the sample began melting near 190 °C became iso-
tropic at 200 °C. The cooling process showed typical lined
texture of the nematic phase after melting. The nematic
phase transferred to ordered smectic mesophases.
Texture studies
The transition temperatures of the compound 6a3 was
determined using a polarizing microscope with a hot-stage.
The sequence of phase transitions is given below:
63ꢀC
90ꢀC
140ꢀC
200ꢀC
Solid ꢀ Smectic C ꢀ Smectic A ꢀ Nematic ꢀ Isotropic
88ꢀC
139ꢀC
57ꢀC
185ꢀC
The transition temperatures from solid to smectic C,
smectic C to smectic A, smectic A to nematic and nematic
to isotropic liquid phase are obtained. However, there
exists a discrepancy between the experimentally observed
and quoted values of the transition temperature between the
Fig. 8 POM study of the liquid
crystal texture of compound 6b3
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J Incl Phenom Macrocycl Chem (2010) 66:285–295
Transition temperatures and thermal behaviour of
both the upper and lower rim. Micrographs show birefrin-
gence due to the mesomorphic nature of the compounds.
The order of the phase transition: like SmecticA texture-
Nematic texture-Isotropic Texture. The compounds are
showing mosaic and lancet texture like Smectic A phase,
schlieren and broken focal-conic fan texture like Sm C
phase, grass like and fan like texture Smectic A phase,
broken focal-conic fan texture like Smectic C phase and
Nematic phase.
calix[4]arene Schiff bases recorded under the POM and
DSC are shown in Table 2. The results show that most of
the compounds are mesomorphic in character. The meso-
phases were identified from their optical textures. Com-
pounds 5c and 5d did not show liquid crystal phases which
may be due to absence of alkoxy or alkyl chain in the
structure. In contrast, a long alkoxy or alkyl group com-
pound 6a1–6a3 and 6b1–6b3 showed nematic and mosaic
and lancet and focal-conic fanlike texture which suggest
the ordered smectic mesophases. The transition tempera-
ture decreases with the increase in the chain length of the
alkoxy or alkyl substitution. The order of the change with
the transition temperature is heptyl \ hexyl \ pentyl in
both 6a1–a3 and 6b1–b3 and there was not much pro-
nounced effect of polarity in these two series.
Acknowledgments Financial assistance from UGC, New Delhi is
gratefully acknowledged. The authors wish to thank Lucknow for
spectral data.
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