“Synthesis, design and characterization of supramolecular self-assembly of calix[4]resorcinare substituted LCs”

Article abstract of DOI:10.1080/15421406.2018.1562616

A new class of supramolecular calix[4]resorcinarene substituted alkoxy side chain (-OC₅H₁₁, -OC₈H₁₇, -OC₁₀H₂₁) has been synthesized and well characterized. These supramolecular compounds were investigated by polarizing optical microscope (POM), differential scanning calorimetry (DSC), thermogravimmetric analysis (TGA) and high-temperature X-ray diffraction studies (XRD). The present synthesized supramolecular derivatives are promising to stabilize the hexagonal columnar phase over a broad thermal range. All the synthesized derivatives showed hexagonal columnar phase at lower temperature and showed enantiotropical nature. Compound 2a with small alkyl spacer on eight side showed higher thermal stability as compare to higher alkyl spacer substituted compounds 2b and compound 2c. These research results suggest that calix[4]resorcinarene was a good platform to construct bowl-shaped derivatives to exhibits the columnar liquid crystal phase and the observed liquid crystalline properties were greatly effected by the substituted alkoxy side chain on eight side of calix[4]resorcinarene skeletone. All the synthesized compounds were characterized by FT-IR, ¹H NMR and ¹³C NMR spectroscopy.

Full text of DOI:10.1080/15421406.2018.1562616

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
“Synthesis, design and characterization
of supramolecular self-assembly of
calix[4]resorcinare substituted LCs”
Jinal Gajjar, Rajesh H. Vekariya, Vinay S. Sharma & Hitesh Parekh
To cite this article: Jinal Gajjar, Rajesh H. Vekariya, Vinay S. Sharma & Hitesh Parekh
(
2018) “Synthesis, design and characterization of supramolecular self-assembly of
calix[4]resorcinare substituted LCs”, Molecular Crystals and Liquid Crystals, 668:1, 48-58, DOI:
0.1080/15421406.2018.1562616
1
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
018, VOL. 668, NO. 1, 48–58
https://doi.org/10.1080/15421406.2018.1562616
2
“
Synthesis, design and characterization of supramolecular
self-assembly of calix[4]resorcinare substituted LCs”
a
a
b
a
Jinal Gajjar , Rajesh H. Vekariya , Vinay S. Sharma
, and Hitesh Parekh
a
b
Department of Chemistry, School of Science, Gujarat University, Ahmedabad, Gujarat; Department of
Chemistry, Faculty of Basic and Applied Science, Madhav University, Sirohi, Rajasthan
ABSTRACT
KEY WORDS
Calix[4]resorcinarene;
enantiotropic; columnar
phase; supramolecular LCs
A new class of supramolecular calix[4]resorcinarene substituted alkoxy
side chain (-OC H11, -OC H17, -OC10H21) has been synthesized and well
5 8
characterized. These supramolecular compounds were investigated by
polarizing optical microscope (POM), differential scanning calorimetry
(DSC), thermogravimmetric analysis (TGA) and high-temperature X-ray
diffraction studies (XRD). The present synthesized supramolecular
derivatives are promising to stabilize the hexagonal columnar phase
over a broad thermal range. All the synthesized derivatives showed
hexagonal columnar phase at lower temperature and showed enantio-
tropical nature. Compound 2a with small alkyl spacer on eight side
showed higher thermal stability as compare to higher alkyl spacer sub-
stituted compounds 2b and compound 2c. These research results sug-
gest that calix[4]resorcinarene was a good platform to construct bowl-
shaped derivatives to exhibits the columnar liquid crystal phase and
the observed liquid crystalline properties were greatly effected by the
substituted alkoxy side chain on eight side of calix[4]resorcinarene
skeletone. All the synthesized compounds were characterized by FT-IR,
1
13
H NMR and C NMR spectroscopy.
GRAPHICAL ABSTRACT
1. Introduction
Study of the liquid crystalline state is a multidisciplinary subject and useful to mankind
and various industrial applications through scientist, technologists [1–4] due to its
CONTACT Vinay S. Sharma
vinaysharma3836@gmail.com and Hitesh Parekh
keya714@gmail.com
ß 2018 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
49
unique property to flow as liquid and optical properties as crystals. Study on LC state is
planned with a view to understand and establish the effect of molecular structure on LC
properties [5–6] as a consequence of molecular rigidity and flexibility [7–8]. Liquid
crystals can be categorized into thermotropic, lyotropic and metallotropic phases.
Thermotropic and lyotropic liquid crystals consist mostly of organic molecules although
few minerals are also known [9]. Liquid crystals in the nematic group are most widely
used in the production of liquid crystal displays (LCD) mainly due to their unique
physical properties as well as wide range of temperature. In addition, the nematic phase
in which liquid crystal molecules are oriented on average along to a particular direction.
By applying an electric or magnetic field, the orientation of the molecules can be
derived in a probable approach [10]. Literature survey revealed that the schiff base has
been widely employed as a linking group for the synthesis of different forms of liquid
crystals [11].
Calixarenes belong to the third generation of supramolecular platforms after crown
.
ether and cyclodextrin [12–13] Calixarenes, possessing tunable 3D-shaped hydrophobic
cavities belong to the important building blocks in supramolecular chemistry [14]. The
calixarenes broaden functionalize at the upper and lower rim affords them with suitable
binding sites for guest encapsulation and molecular assembly[15]. The four hydroxyl
groups present in calix[4]arene interact by hydrogen bonding and also stabilize the cone
9
conformation . In recent years, the calixarene core was also successfully used as a rigid
core to prepare various kinds of liquid crystalline compounds [16–18].
Liquid crystalline materials based on calix[4]arene rigid core was first reported in
1990 [19]. Calixarene based liquid crystalline compounds which are the most significant
part of macrocycles based liquid crystals were investigated in literature [19]. Xu and
Swager investigated the host-guest mesomorphism of calixarene compounds. They dem-
onstrated how host-guest principles could be used to stabilize a columnar bowlic phase
of a conformationally flexible calix[4]arene was found to display transient columnar
bowlic mesophase [20]. For instance, the calixarene liquid crystals with substitution of
long aliphatic side chains were prepared and displayed interesting liquid crystalline
behaviour [21–22]. Yonetake et al. successfully synthesized two liquid crystal based on
tert-butyl calix[8]arene to displayed smectic as well as nematic mesophase [23]. Yang
et al. reported mesomorphic property in calix[4]arene cholesterol derivatives. The fol-
lowing group studied the mesophase appeared with the presence of columnar type
molecular arrangement of calixarene with cholesterol derivatives [24]. In another inves-
tigation, the similar group has synthesised and studied the liquid crystalline behavior of
calixarene-linked triphenylene derivatives [25]. Later, his group reported the meso-
morphic properties of gallic-calixarene derivatives with stable cone conformation [26].
Recently, Yang et al. reported calix[4]resorcinarene cholesterol based columnar liquid
crystals having schiff-base linking group [27]. Marcos and his coworkers synthesized
and studied the mesomorphic property of calixarene schiff base derivatives, which have
þ2
binding recognition and selectivity towards Zn ion [28]. Yang et al. reported bowl-
shaped liquid crystal based on cyclotriveratrylene derivatives with multiple triphenylene
units. They demonstrated more triphenylene units linked with CTV derivatives to
exhibit higher ordered hexagonal columnar phase [29]. Sharma et al. reported the two
liquid crystalline series of calix[4]arene based on schiff-base and ester linking unit to

5
0
J. GAJJAR ET AL.
show smectic and nematic phase and also studied their biological behaviour [30].
Recently, Sharma et al. reported calix[4]arene core based LCs which showed stable
mesomorphism over long temperature range and good photophysical behaviour in solu-
tion as well as thin film [31–32].
The comprehensive literature studied reveals that the supramolecular columnar liquid
crystalline compounds are very rarely reported. In this present study, we wish to report
first newly functionalized bowl-shaped supramolecular resorcinarene derivatives based
on resorcinarene as a central core substituted by the variable peripheral alkoxy side
chain on eight side and studied their liquid crystalline properties. In present study, we
have introduced total three novel compounds having pentyloxy, octyloxy and decyloxy
side group.
2. Experimental and Instrumentation
2.1. Materials
Resorcinol, cinnamaldehyde, various alkyl bromides (R-Br) were purchased from Sigma
Aldrich. All the other chemical reagents were purchase from Sigma Aldrich. Solvents
were dried and purified in the usual manner. Column chromatography was performed
using silica gel (60-120 mesh).
2.2. Measurements
The FT-IR spectra were recorded as KBr pellet on Shimadzu in the range of 3800-
ꢀ
1
1
13
6
00 cm . The H and C NMR spectra were obtained on a Bruker Advance 400 MHz
spectrometer using tetramethylsilane as the internal standard. The mesophase type was
identified by visual comparison with known phase standards using a Nikon Eclipse LV-
100 POL polarizing optical microscope (POM) fitted with a hot stage temperature-con-
trolled LTSE 420 heating stage (Linkam Scientific Instruments, Tadworth, Surrey, UK)
with temperature controlling accuracy of 0.1K. The transition temperatures and associ-
ated enthalpy changes were determined by differential scanning calorimeter under nitro-
gen atmosphere. Thermo gravimetric analysis (TGA) was performed using a Perkin
Elmer-STA 6000 apparatus under high purity nitrogen. The X-ray diffraction (XRD)
measurements were performed on a Rigaku-Ultima IV powder diffractometer equipped
with a Cu ka source (k ¼ 1.5418 A˚ and 1.6 kW, X-ray tube with applied voltage and
current values as 40 kV and 30 mA power) and also Philips X’PERT MPD.
3. Result and discussion
3.1. Synthesis and characterization
The synthetic approach for the preparation of the derivatives of oxadiazole is presented
in below Scheme 1. Calix[4]resorcinarene (1a) was prepared by the reaction of resor-
cinol with cinnamaldehyde in presence of sodium hydroxide [33]. The final target com-
pounds (2a-2c) were prepared by the reaction of compound (1a) with various alkyl
bromides in the presence of anhydrous K CO and dry acetone [34]. The resultant
2
3

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
51
Scheme 1. Reaction and condition: (i) Cinnamaldehyde, NaOH, Reflux; (ii) R-Br, Anhydrous.
crude residue was purified by using column chromatography on silica gel eluting with
1
13
methanol: chloroform as eluent (1:4). The H NMR, C NMR and IR results also well
supported to the corresponding structures of derivatives and final target compounds
.
ꢀ
1
Comp.2a: Yield 74%, FT-IR (KBr) in cm : 2910 (-C-H- Str in aromatic), 1260 (-C-
1
O str), 891 polymethylene (-CH -)n of –OC H , 930 (alkene group). H NMR: 0.88-
2
5
11
0
.90 (t, 16H, -OC H ), 1.31 (m, 32H, -OC H ), 1.77 (p, 16H, -OC H ), 2.63 (16H,
5 11 5 11 5 11
-
ArCH Ar-), 4.08 (16H, -OC H ), 4.06 (t, 16H, -OC H ), 3.63 (s, 4H, -CH-), 6.55 (s,
2 5 11 5 11
4
H, Ar-H), 7.10 (s, 4H, Ar-H), 7.19 (d, 8H, Ar-H), 7.24 (d, 8H, Ar-H), 6.87 (s, 1H, Ar-
1
3
H); C NMR: 160.4, 152.7, 147.3, 146.3, 128.8, 128.1, 96.7, 69.8, 69.0, 37.2, 34.7, 29.3,
8.1, 22.7, 14.1.
Comp.2b: Yield 71%, FT-IR (KBr) in cm : 2930 (-C-H- Str in aromatic), 1250 (-C-
2
ꢀ1
1
1
O str), 748 polymethylene (-CH -)n of –OC H , 930 (alkene group). H NMR: H
2
8
17
NMR: 0.88-0.90 (t, 16H, -OC H ), 1.28-1.31 (m, 64H, -OC H ), 1.47 (sext, 16H,
8
17
8
17
-
OC H ), 1.77 (p, 16H, -OC H ), 2.63 (16H, -ArCH Ar-), 4.08 (16H, -OC H ), 4.06
8 17 8 17 2 8 17
(
8
1
t, 16H, -OC H ), 3.63 (s, 4H, -CH-), 6.55 (s, 4H, Ar-H), 7.10 (s, 4H, Ar-H), 7.19 (d,
8 17
13
H, Ar-H), 7.24 (d, 8H, Ar-H), 6.87 (s, 1H, Ar-H); C NMR: 160.4, 152.7, 147.3, 146.3,
28.8, 128.1, 96.7, 69.8, 69.0, 37.2, 34.7, 29.3, 28.1, 22.7, 22.1, 21.6, 14.1.
ꢀ
1
Comp.2c: Yield 76%, FT-IR (KBr) in cm : 2910 (-C-H- Str in aromatic), 1260 (-C-
1
O str), 624 polymethylene (-CH -)n of –OC H , 930 (alkene group). H NMR: 0.88-
2
10 21
0
1
.90 (t, 16H, -OC H ), 1.28-1.31 (m, 84H, -OC H ), 1.47 (sext, 16H, -OC H ),
10 21 10 21 10 21
.77 (p, 16H, -OC H ), 2.61 (16H, -ArCH Ar-), 4.06 (16H, -OC H ), 4.06 (t, 16H,
1
0
21
2
10 21
-
OC H ), 3.63 (s, 4H, -CH-), 6.55 (s, 4H, Ar-H), 7.10 (s, 4H, Ar-H), 7.19 (d, 8H, Ar-
10 21
1
3
H), 7.24 (d, 8H, Ar-H), 6.87 (s, 1H, Ar-H); C NMR: 160.4, 152.7, 147.3, 146.3, 128.8,
28.1, 96.7, 69.8, 69.0, 37.2, 34.7, 29.3, 28.1, 22.7, 14.1.
1

5
2
J. GAJJAR ET AL.
Table 1. Phase transition temperatures (˚C) and corresponding enthalpies (kJ/mol) of synthe-
sized compounds.
Phase sequence
Compound
Heating
Cooling
2
2
2
a
a
b
c
Cr 183.60 (1.7) Col
Cr 179.58 (1.2) Col
Cr 164.41 (1.1) Col
h
h
h
218.92 (7.4) I
210.98 (9.5) I
191.01 (6.6) I
I 216.72 (4.2) Col
I 208.67 (3.1) Col
I 190.82 (2.6) Col
h
h
h
181.32 (9.3)
178.41 (7.6)
163.37 (9.4)
Peak temperatures in the DSC thermograms obtained during the first heating and first cooling cycles at 10˚C/min; Col
columnar hexagonal phase.
h
¼
Figure 1. Bargraph showing the thermal behaviour of compounds 2a-2c (heating condition).
3.2. Reaction and Scheme
K CO , dry Acetone, Reflux, (R ¼ C H2nþ1, n ¼ 5,8,10).
2
3
n
3
.3. DSC investigation
The mesomorphic behavior of present synthesized resorcinarene core based derivatives
a, 2b and 2c were preliminarily studied by using differential scanning calorimetry
DSC) represented in Figure 3. The transition temperatures and the associated enthalpy
2
(
changes obtained from the DSC scans of first heating and cooling cycles are mentions
in Table 1 respectively. Figure 1 and Figure 2 indicates the thermal behavior of these
synthesized compounds in first heating and first cooling cycle. In DSC scans, compound
2a-2c exhibited two endothermic peaks corresponding to crystal to mesophase (Cr-
Colh) and mesophase to isotropic (Colh-I) on heating and cooling condition. From
these results, it can be noted that all the synthesized derivatives showed enantiotropical
liquid crystalline properties. Compound 2a with a pentyloxy tail group exhibited two
endothermic peaks at 183.60 ˚C and 218.92 ˚C on heating cycle while on cooling cycle,
two endothermic peaks traced at 181.32 ˚C and 216.72 ˚C. Similarly, Compound 2b
with octyloxy tail group exhibited two endothermic peaks at 179.58 ˚C and 210.98 ˚C
on heating cycle while on cooling condition it appeared at 178.41 ˚C and 208.67 ˚C. In

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
53
Figure 2. Bargraph showing the thermal behaviour of compounds 2a-2c (cooling condition).
Figure 3. The DSC traces of compounds 2a (a), 2b (b) and 2c (c) on first heating and cooling (scan
rate 10˚C/min).
addition, compound 2c showed phase transition at 164.41 ˚C and 191.01 ˚C on heating
condition and on cooling it exhibited at 163.37 ˚C and 190.82 ˚C with a lower enthalpy
change value to form Col -I phase transition peak noticed in DSC scans on heating and
h
cooling condition. It can be noted that the compounds (2b-2c) with higher alkyl chain

5
4
J. GAJJAR ET AL.
Figure 4. Phase diagram of comp.2a-2c (heating and cooling condition) by POM.
showed lower transition temperature as compare to compound (2a). The result obtained
from DSC study is further confirmed by using POM and XRD analysis.
3
.4. POM investigation
The mesophase phase transition temperatures of present resorcinarene based derivatives
2a-2c) were further investigated by polarizing optical microscope (POM) study. It can be
(
observed that the texture pattern of compounds (2a-2c) showed same textural pattern of
hexagonal columnar phase. The mesomorphic temperature ranges for the synthesized
compounds (2a-2c) are 41.0 ˚C, 39.0 ˚C, 32.0 ˚C on heating cycles while it exhibited at
36.0 ˚C, 30.0 ˚C and 26.0 ˚C on cooling cycles respectively. One can remark the meso-
phase temperature range of compound 2a is higher as compare to compound 2b and 2c
due to the presence of short alkyl chain spacers on eight side substitutions on resorcinar-
ene core. The phase transition temperatures for all compounds were noticed on heating
condition at 181.0 ˚C, 177.0 ˚C and 166.0 ˚C which is nearer to the phase transition tem-
perature obtained from the DSC analysis. On cooling condition, same textural pattern of
Colh mesophase was observed at 178.0 ˚C, 180.0 ˚C and 162.0 ˚C before the formation
of solid crystal at lower temperature. It can be noted that, the phase transition tempera-
ture is decreased with increasing alkoxy side chain on terminal position which enhance
the flexibility of molecule and weaken the core-core interaction inbuilt with resorcinarene.
Thus, an enantiotropic mesomorphic behavior over a good thermal range of mesophase
was seen for the present synthesized supramolecular calix[4]resorcinarene based LCs. The
texture pattern of hexagonal columnar phase is additionally confirmed by XRD analysis.
3
.5. XRD investigation
To confirm the mesophase observed through a polarizing optical microscope (POM) and
differential scanning calorimetry (DSC), XRD studies have been performed to confirm the
observed liquid crystalline phase in synthesized calix[4]resorcinarene based supramolecular

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
55
Figure 5. XRD profiles depicting the intensity against the 2Ɵ obtained for the Colh phase of com-
pound 2a at 182.0 ˚C (a); Colh phase of compound 2b at 177.0 ˚C (b) on cooling from isotropic
temperature.
derivatives. All the samples have been studied for their LC phase in their transition tem-
perature by using XRD study. The sample was filled by melting as isotropic state and
cooled down to their LC state and scanned for X-ray studies. The X-ray patterns of LC
phases of all the supramolecular calix[4]resorcinarene derivatives exhibited three reflections
in small angle region and two other diffuse scattering peak in the wide angle region as
shown in Figure 5 (a-b). Compound 2a showed three reflections at 1.89˚, 2.78˚ and 4.13˚
in the small angle region and 19.01˚ and 20.08˚ in the wide angle region. Similarly,
Compound 2b at 1.76˚, 2.81˚ and 5.23˚ in small angle region while 19.61˚ and 20.12˚ in
wide angle region. The d-spacing value of compound 2a is 38.37Å, 28.51Å and 18.42Å in
the small angle region. Similarly, 42.77Å, 25.24Å and 17.51Å for compound 2b, The ratio
of these present reflections agreed with 1:1=ͱ3: 1=ͱ7, signifying the [100], [110] and [200]
plane of hexagonal lattice [35]. The presence of small diffuse peak at wide angle region at
19.01˚, 20.08˚ in compound 2a and 19.61˚ and 20.12˚ in compound 2b, this is due to the
presence of core-core interactions of ordered hexagonal phase in addition of formation of
self-assembly of bowlic compounds. The literature survey indicated that the similar XRD
traces were reported for supramolecular calixarene based liquid crystalline compounds with
different substituted derivatives on lower and upper rim [35]. The estimated molecular
length of present synthesized compounds is nearer to the observed molecular length. One
can see that the presence of broad halos at 2Ɵ ¼ 19ꢁ 20˚ indicating the ordering alkoxy
side chain group on the terminal side of octa-substituted side alkoxy chain on calix[4]resor-
cinarene derivatives. The possible schematic diagram of self-assembly of bowl-shaped LCs
derivatives arranged into the form hexagonal columnar arrangement in compounds 2a-2c
are mention in Figure 6. Based on all the data from DSC, POM, and XRD, it is confirmed
that all the synthesized resorcinarene derivatives exhibited stable hexagonal columnar liquid
crystalline mesophase with good thermal stability.
3
.6. Thermogravimetric analysis
Thermal stability of compounds 2a, 2b, and 2c were studied by thermogravimetric ana-
lysis (TGA) under nitrogen atmosphere shown in Figure 7. All the synthesized

5
6
J. GAJJAR ET AL.
Figure 6. Proposed mechanism of hexagonal columnar arrangement in comp.2a-2c
Figure 7. TGA curves of the compounds 2a-2c carried out at a rate of 10 ˚C/min.
calix[4]resorcinarene based supramolecular derivatives were stable at least up to ꢂ
250˚C and complete degradation occurs at around 400˚C which showed the good stabil-
ity of this synthesized compounds respectively. As there is no loss seen up to tempera-
ꢃ
ture at around 150 C that means no water or other any solvent entrapped in
mesophase formation. All the synthesized compounds show good thermal stability as
also confirmed by POM temperature range as well. From the POM study, compound
2a-2c showed mesophase temperature range 41 degree, 39 degree and 32 degree which
clearly indicates that the increasing side alkyl chain inbuilt with alkoxy group on the
core of resorcinarene as a result the mesophase temperature range is decreased
respectively.
4. Conclusion
Supramolecular n-alkoxy calix[4]resorcinarene derivatives are a promising class of
bowlic shaped columnar liquid crystals. The self-assembly of synthesized supramolecular

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
57
derivatives to form an ordered hexagonal columnar phase. The calix[4]resorcinarene
based LCs is prepared by the single step process by the reaction of calix[4]resorcinarene
and n-alkyl bromides. The length of the side spacer alkyl chain had no effect on the
absorption and emission properties in the solution state. The increase of flexible alkyl
chain length on terminal side is effect on the mesophase range. The compounds with
lower alkyl chain group shows higher temperature range of mesophase as compare to
higher alkyl chain substituted compounds. All the synthesized derivatives showed higher
thermal stability and stabilize columnar mesophase.
Acknowledgment
Authors acknowledge thanks to Department of Chemistry, School of Science, Gujarat University
for providing lab facility. Authors are also thankful to NFDD Centre for providing analytical and
spectral services. Authors thanks to Dr. Anuj Sharma (Department of Chemistry, Gujarat
University) for helping fruitful discussion in high temperature XRD analysis.
ORCID
Vinay S. Sharma
http://orcid.org/0000-0003-4970-0676
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