Columnar self-assembly of bowl shaped fluorescent liquid crystals based on calix[4]arene with Schiff base units

Article abstract of DOI:10.1039/c8nj01721b

A new family of bowl-shaped molecules with a calix[4]arene rigid core and appended on four-sides, that display a wide range of hexagonal columnar phases, has been synthesised and well characterized. The thermal behaviours of the present compounds were established using a combination of polarising optical microscopy (POM), differential scanning calorimetry (DSC) and a high-temperature powder X-ray diffraction method (XRD). It is found that all of the synthesised materials show an enantiotropic hexagonal columnar liquid crystal phase. The structural and conformation characterization of these newly synthesised compounds was achieved by FT-IR, ¹H NMR, and ¹³C NMR spectroscopy. All of the synthesised compounds exhibited good blue luminescence in solution under long wavelength UV light. To explore the structure property correlations, the alkoxy side chain group was varied from a lower alkyl spacer to a higher alkyl spacer on the lower rim of the calix[4]arene. The present research specified that the introduction of linking groups on the lower rim with an n-alkoxy side group is an influential approach to obtaining a supramolecular bowl shape liquid crystal which has good thermal and photophysical behaviour.

Full text of DOI:10.1039/c8nj01721b

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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“
Columnar selfꢀassembly of bowl shaped fluorescent liquid crystals based on
calix[4]arene with schiff base units”
a*
b
c
Vinay S. Sharma , Anuj S.Sharma , Rajesh H.Vekariya
a*
:
Department of Chemistry, Faculty of Basic and Applied Science, Madhav University, Sirohi,
Rajasthan, India.
b
:
Department of Chemistry, School of Science, Gujarat University, Ahmedabad, Gujarat, India.
C
:
Blue ORB Solution LLC, 1100 Corporate Square Drive Suite 123, Creve Coeur, Saint Lois,
MO 63132, USA.
*
E mail address of corresponding author : vinaysharma3836@gmail.com
Abstract
A new family of bowlicꢀshaped molecules with a calix[4]arene rigid core appended fourꢀside
displaying wide range of hexagonal columnar phase have been synthesised and well
characterized. The thermal behaviour of the present compounds was established using a
combination of optical polarising microscopy (POM), differential scanning calorimetry (DSC)
and highꢀtemperature powder Xꢀray diffraction method (XRD). It is found that all the
synthesised materials show enantiotropic hexagonal columnar liquid crystal phase. The structural
and conformation characterization of these newly synthesised compounds had been achieved by FTꢀ
1
13
IR, H NMR, and C NMR spectroscopy. All synthesised compounds exhibited good blue
luminescence in solution under long wavelength UV light. To explore the structure property
correlations, the alkoxy side chain group has been varied from lower alkyl spacer to higher alkyl
spacer on the lower rim of calix[4]arene. The present research specified that the introduction of
linking groups on the lower rim with nꢀalkoxy side group is an influence approach to obtain
supramolecular bowl shape liquid crystal with contains good thermal and photophysical behaviour.
Key words: ꢀ Calix[4]arene, Enantiotropic, Liquid crystal, Schiffꢀbase
1

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Graphical Abstract:
2

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1
. Introduction
Study of the liquid crystalline state is a multidisciplinary subject and useful to mankind
and numerous industrial applications through scientist and technologists due to its exceptional
1
property to flow as liquid and optical properties as crystals . Emissive organic semiconductors
are getting more attention nowadays in the development of electronic devices such as, in
fabricating white organic lightꢀemitting diodes (WOLEDs), field effect transistors and other
[
2]
display applications . The literature reports indicate that the red and green emissive materials
are broadly synthesised. However, blue emitting materials are rarely reported. Blue light emitting
materials are received an increasing consideration nowadays because of their inevitable need in
3
producing white light emitting diodes used for lighting and other display applications . The
invention of the efficient blue LED based on inorganic semiconductors has contributed for
creating white light but most of the inorganic semiconductors suffer from high cost and low
4
efficiency . Later, this problem can be resolved by using polymers which shows luminescent
property but due to its low solubility, high purity and stability. In this regard, the molecule which
shows columnar selfꢀassembly form which enhance the applicability of emissive molecules in
5
ꢀ6
OLED applications . Calixarenes, possessing tunable 3Dꢀshaped cavities are belonging to the
important building blocks in supramolecular chemistry after the discovery of crown ether and
7
cyclodextrin . The calixarenes broaden functionalization at the upper and lower rim affords them
8
with suitable binding sites for guest encapsulation and molecular assembly . Current studies on
calixarene have shown to dominate specific selfꢀassembling ability resulting from intermolecular
9
ꢀ15
forces, including coordination bonds, hydrogen bonds and ionic bonds
.
Liquid crystalline materials based on calix[4]arene rigid core was first reported in
1
6
1
990 . Calixarene based liquid crystalline compounds which are the significant part of
3

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17ꢀ18
macrocycles based liquid crystals were investigated in some literature
. For instance, the
calixarene liquid crystals with substitution of long aliphatic side chains were prepared and
19ꢀ20
displayed interesting liquid crystalline behavior
. Yonetake et al. synthesized two liquid
2
1
crystal based on tertꢀbutyl calix[8]arene to display smectic and nematic mesophases . Yang et
al. reported mesomorphic property in calix[4]areneꢀlinked cholesterol derivatives. They studied
the mesophase appeared with the presence of columnar type molecular arrangement of calixarene
[
22]
with cholesterol derivatives . In another report, Yang et al. Synthesized and studied the liquid
2
3
crystalline behavior of calixareneꢀlinked triphenylene derivatives . Later, his group reported the
2
4
mesomorphic properties of gallicꢀcalixarene derivatives with stable cone conformation . Menon
2
5
et al. reported a review on lower rim substituted calixarenes and their various applications .
Later, her group reported calix [4]arene schiff base and azoester based liquid crystalline
2
6ꢀ28
materials and studied their dielectric properties at various temperatures
. Fang et al.
synthesised calix[4]arene based dimeric cholesteryl derivative with the presence of naphthalene
2
9
linker . Recently, Yang et al. reported calix[4]resorcinarene cholesterol based columnar liquid
3
0
crystals having schiffꢀbase linking group . Marcos and his coꢀworkers synthesized and studied
the mesomorphic property of calixarene Schiff base derivatives, which have binding recognition
+
2
31
and selectivity towards Zn ions . In our recent report, we have reported the two liquid
crystalline series of calix[4]arene based on schiffꢀbase and ester linking group and also studied
3
2
their biological behaviour .
A comprehensive literature search reveals that the supramolecular columnar liquid
crystalline compounds with blue luminescence properties are rarely reported. In this present
work, we report the newly functioned bowl shaped calix[4]arene derivatives based on schiff base
4

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and ester linking group with a variable alkoxy (ꢀOR) side chain and studied their mesomorphic
and photophysical behavior.
2
2
4
. Experimental and Instrumentation
.1 Materials
ꢀtertꢀbutyl phenol, 4ꢀhydroxy benzaldehyde, 4ꢀhydroxy acetanilide, anhydrous K CO , alkyl
2
3
bromides (RꢀBr) were purchased from S.R.L., Mumbai. 1ꢀbromo ethanoic acid, N, N'ꢀ
dicyclohexyl carbodimide (DCC), 4ꢀdimethyl amino pyridine (DMAP), formaldehyde were
purchased from Sigma Aldrich. Solvents were dried and purified in the usual manner.
2
.2 Measurements
ꢀ1
The FTꢀIR spectra were recorded as KBr pellet on Shimadzu in the range of 3800ꢀ600 cm . The
1
13
H and C NMR spectra were obtained on a Bruker Advance 300 MHz spectrometer using
tetramethylsilane as the internal standard. The synthesis of pꢀtertꢀbutyl calix[4]arene was carried
out by using a Discover Benchmate System 240 V microwave synthesizer (CEM corporation).
The mesophase type was identified by visual comparison with known phase standards using a
Nikon Eclipse LVꢀ100 POL polarizing optical microscope (POM) fitted with a hot stage
temperatureꢀcontrolled LTSE 420 heating stage (Linkam Scientific Instruments, Tadworth,
Surrey, UK) with temperature controlling accuracy of 0.1K. The phase transition temperatures
ꢀ1
were measured using Shimadzu DSCꢀ50 at heating and cooling rates of 10 °C min . The Xꢀray
diffraction (XRD) measurements were performed on a RigakuꢀUltima IV powder diffractometer
equipped with a Cu kα source (λ = 1.5418 A˚ and 1.6 kW, Xꢀray tube with applied voltage and
current values as 40 kV and 30 mA power) and also Philips X'PERT MPD. The absorption
spectra were studied by using Jasco Vꢀ570 UVꢀVis recording spectrophotometer with a variable
5

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wavelength between 200 and 800 nm. The fluorescence spectra were recorded on a Jasco FPꢀ
500 spectrofluorometer. The mixtures in square quartz cuvettes were excited at 320 nm, and the
emission spectra were collected from 320 to 600 nm.
6
3
3
. Result and discussion
.1 Synthesis and characterization
Scheme 1 shows the synthetic routes of the present bowl shape calixarene derivatives. The
3
3
synthesis of pꢀtertꢀbutyl calix[4]arene (1d) was carried out by microwave assisted method . Pꢀ
3
4
tertꢀbutyl calix [4] arene tetra Bromo ethanoic acid (1e) was carried out by reporting method .
ꢀnꢀalkoxy acetanilide (1a) was prepared by refluxing the reaction mixture of 4ꢀhydroxy
acetanilide with RꢀBr in the presence of anhydrous K CO and dry acetone by using reported
4
2
3
3
5
method . 4ꢀnꢀalkoxy aniline (1b) was prepared by hydrolysis of 4ꢀnꢀalkoxy acetanilide (1a) in
3
5
HCl with the presence of water . 4ꢀ(4ꢀnꢀalkoxy phenyl) imino methyl phenol (1c) was prepared
by the reaction of comp. 1b with 4ꢀhydroxy benzaldehyde in the presence of a few drops of
36
1
glacial acetic acid in ethanol . The H NMR of shows singlet of 1H (δ = 8.43 ppm, ꢀN=CHꢀ).
ꢀ1
From FTꢀIR, the peak found at 1630 cm signifying the presence of ꢀN=CHꢀ group. The final
target comp. (1f) was prepared by the reaction of comp.(1c) with comp.1e in presence of
3
7
anhydrous K CO and dry acetone . The resultant crude residue was purified by using column
2
3
chromatography on silica gel eluting with methanol: chloroform as eluent (1:4). The tetra
substitution on lower rim of pꢀtert butyl calix[4]arene is evidenced by the absence of –OH group
1
in H NMR and FTꢀIR sturdy. In ESI, we have shown the synthesis and characterisation part of
1
final target derivatives comp.1f , 1f , 1f , 1f , 1f and 1f . From the H NMR, the cone shapes
4
6
8
10
12
14
of final compounds were confirmed by presence of bridging methylene protons appear two
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1
13
doublets at 3.2 ppm and 4.2 ppm respectively. The H and C NMR results also well supported
to the corresponding structures of comp.1f , 1f , 1f , 1f , 1f and 1f are mention in ESI.
4
6
8
10
12
14
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Scheme 1: Reagents and conditions: (a) RꢀBr, Anhy.K CO , dry Acetone, Reflux; (b) HCl, H O,
2
3
2
2
ꢀ3 h., Reflux; (c) 4ꢀhydroxy benzaldehyde, EtOH, glacial acetic acid, 2 h., Reflux; (d) HCHO,
NaOH, Microwave, 15 min; (e) bromo acetic acid, DCC, DMAP, DCM, 24 h. stirring at RT; (f)
c, Anhy.K CO , dry Acetone, Reflux.
1
2
3
8

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3
.2 DSC analysis
The mesomorphic behaviours of compounds 1f , 1f , 1f , 1f , 1f , 1f and 1f were
4
6
8
10
12
14
16
preliminarily studied by differential scanning calorimetry (DSC). The transition temperatures
and the associated enthalpy changes obtained from the DSC scans of second heatingꢀcooling
cycles are shown in Figure1 and Table 1. Figure S summarizes the thermal behaviour of these
1
synthesised samples in heating and cooling cycle. All the synthesised compounds (1f to 1f )
4
16
display three thermal peaks on heating and cooling cycles. Comp.1f with shortest alkyl chain,
4
exhibited enantiotropic columnar mesophase spanning a thermal range of 18 degrees in the
heating cycle after passing through a crystal to crystal phase transition at 124.6 ˚C to 142.8 ˚C
with an enthalpy change of 18.2 kJ/mol respectively as evidenced by DSC thermograms which
was further confirmed by using POM study. Comp.1f showed three endothermic peaks at 119.1
6
˚
C, 139.6 ˚C and 174.6 ˚C corresponding to CrꢀCr', Cr'ꢀCol and Col ꢀI phase transition. Also,
h h
three thermal peaks appeared at 180.1˚C, 138.3˚C and 117.3˚C on cooling process which
indicated the reversible phase transition process of IꢀColh, ColhꢀCr' and Cr'ꢀCr phase upon
second heating and cooling conditions. The observed temperature range of comp.1f was
6
spanning over a good thermal range of 35.0 degrees with the isotropic temperature of 174.6 ˚C.
Comp.1f with dodecyloxy chain group traced three endothermic peaks at 98.7 ˚C, 122.3 ˚C and
12
1
1
51.6 ˚C on heating cycles, on applying cooling, these exothermic peaks were appeared at
56.1˚, 124.6˚ and 102.1˚C on cooling cycle and also it showed lower mesophase temperature
range 29.3 degrees on heating as compare to other compounds. It could be seen that, compound
f4 with a butyloxy chain group showed broad temperature range of mesophase as compare to
1
other compounds which contain higher alkyl side spacer chain group. The result obtained from
DSC study is further confirmed by POM and XRD analysis, respectively. The DSC thermograms
of compounds 1f , 1f and 1f are shown in Figure S ꢀS (ESI).
1
2
14
16
2
3
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a
Table1. Phase transition temperatures (˚C) and corresponding enthalpies (kJ/mol) of synthesised
compounds
Compound
Phase sequence
Heating
Cr 124.6 (9.2) Cr' 142.8 (18.2) Colh
82.1 (24.2) I
Cr 119.1 (8.3) Cr' 139.6 (12.6) Colh
74.6 (23.1) I
Cr 112.7 (6.2) Cr' 134.5 (10.4) Colh
69.9 (20.5) I
Cr 102.6 (5.1) Cr' 124.2 (14.6) Colh
58.8 (18.2) I
Cr 98.7 (8.2) Cr' 122.3 (16.3) Colh
51.6 (21.8) I
Cr 86.8 (7.5) Cr' 118.4 (23.2) Colh
48.6 (21.5) I
Cr 78.1 (5.2) Cr' 102.5 (17.3) Colh
36.4 (15.6) I
Cooling
1
1
1
f4
f6
f8
I 187.3 (7.3) Colh 144.8 (15.1)
Cr'121.8 (6.5) Cr
1
I 180.1 (9.7) Col 138.3 (13.5)
h
1
Cr' 117.3 (8.4) Cr
I 167.4 (5.2) Col 137.5 (26.4) Cr'
h
1
116.0 (9.4) Cr
1
1
1
1
f10
f12
f14
f16
I 161.5 (9.4) Col 128.7 (18.6) Cr'
h
1
108.5 (6.2) Cr
I 156.1 (8.4) Col 124.6 (24.5) Cr'
h
1
102.1(7.8) Cr
I 151.4 (6.7) Col 120.1 (19.3) Cr'
h
1
90.4 (10.6) Cr
I 139.8 (4.9) Col 104.1 (11.8) Cr'
h
1
82.5 (8.6) Cr
a
Peak temperatures in the DSC thermograms obtained during the second heating and second
cooling cycles at 10˚C/min; Col = columnar hexagonal phase.
h
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Figure 1: The DSC traces of compounds 1f (a), 1f (b), 1f (c), 1f (d) on second heating and
4
6
8
10
cooling (scan rate 10˚C/min).
.3 POM investigation
The transition temperatures of newly synthesised compounds were further carried out by
3
using of polarizing optical microscope (POM) by observing the textures when the sample was
sandwiched between a clear glass slide and a cover slip. The all synthesised bowlꢀshaped
supramolecular derivatives displayed a texture pattern of columnar phase. It is interesting to note
that the observed defect texture images pattern of hexagonal columnar phase which is commonly
observed in Col phase. The observed texture pattern of compounds 1f , 1f , 1f and 1f are
4
8
10
14
shown in Figure 2. Compound 1f and 1f showed a similar texture pattern of hexagonal
4
10
columnar phase at 143.0˚C and 124.0˚C on heating condition (Fig.2a, 2c), but having a different
isotropic temperature range. Similarly, compounds 1f and 1f showed same texture pattern of
8
14
1
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columnar mesophase at 134.0˚ and 118.0˚C with different temperature ranges of mesophase on
heating condition (Fig.2b, 2d). On cooling condition, same textural pattern of Col mesophase
was observed before the solid crystallization. Thus, an enantiotropic mesomorphic behavior over
a broad thermal range of mesophase was seen for the present synthesized bowlꢀshaped
Supermolecules based on calixarene core.
Figure 2: Photomicrographs of compound 1f at 145.0 ˚C (a) 1f at 136.0 ˚C (b) 1f at 119.0
4
8
10
˚
C (c) and 1f at 120.0 ˚C (d) on heating from the solid crystalline state as seen under cross
14
polarizers.
.4 XRD analysis
To confirm the mesophase observed through a polarizing microscope (POM) and
3
differential scanning calorimetry (DSC), XRD studies have been performed to confirm the liquid
crystalline phase. The liquid crystalline behaviour of supramolecular calixarene based LCs (1f4,
1
2

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1
f , 1f , 1f , 1f , 1f ) were further confirmed by using Xꢀray scattering studies of unoriented
6 8 10 12 14
samples filled in Lindemann capillaries. All the samples have been studied for their LC phase at
their transition temperature. The sample was filled by melting as isotropic state and cooled down
to their LC state and scanned for Xꢀray studies. The Xꢀray patterns of LC phases of all the
supramolecular calix[4]arene derivatives showed three reflections at the small angle region and
two diffuse scattering peaks in the wide angle region as shown in Figure 3 (a, b) and Figure S ꢀS
4
5
(
a,b) respectively. Compound 1f showed three reflections at 5.08˚, 9.17˚ and 10.21˚ in the small
4
angle region. Similarly, Compound 1f at 4.97˚, 8.78˚ and 10.63˚, compound 1f at 4.98˚, 9.32˚
6
8
and 10.18˚, compound 1f at 4.08˚, 8.08˚ and 10.01˚, compound 1f at 4.97˚, 8.03˚ and 10.04˚,
1
2
14
compound 1f at 4.93˚, 8.97˚ and 10.21˚ in the small angle region and a single small reflection
1
0
appeared at 17.38˚, 16.58˚, 17.12˚, 16.01˚, 19.38˚ and 19.91˚ for compound 1f , 1f , 1f , 1f 1f
12
4
6
8
10,
and 1f in wide angle region. The dꢀspacing value of compound 1f is 17.41Å, 9.67Å, 8.69Å in
1
4
4
small angle region. In the same way, 17.80Å, 10.10Å and 8.36Å for compound 1f , 17.83Å,
6
9
.52Å and 8.72Å for compound 1f , 17.94Å, 9.88Å and 8.70Å for compound 1f , 21.68Å,
8 10
1
0.97Å and 8.87Å for compound 1f , 17.80Å, 11.04Å and 8.84Å for compound 1f . The ratio
1
2
14
of these present reflections agreed with 1:1/√3:1/√4, signifying the [100], [110] and [200]
plane of hexagonal lattice. The presence of small diffuse peak at wide angle region at 21.24˚,
1
9.01˚, 19.97˚, 19.89˚, 21.12˚ and 21.08˚ with dꢀspacing 4.25 ~ 4.73 Å for compounds 1f to 1f14
4
due to the coreꢀcore interactions of ordered hexagonal phase in addition of selfꢀassembly
organizations. The literature survey indicated that the similar XRD traces were reported for
25
supramolecular calix[4]arene liquid crystals with different substituted derivatives on lower rim .
The estimated molecular length of present synthesised compounds is nearer to the observed
molecular length. It can be noted that the presence of broad halos at 2Ɵ = 15~20˚ indicating the
1
3

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ordering of the peripheral alkyl chain group at the lower region of calix[4]arene. The proposed
mechanism of present synthesised compounds mentions in Figure 4 which indicated the
arrangement of the bowlic shape calixarene column with substitution on lower rim by schiffꢀbase
ester linking group and also aliphatic alkyl chain at the terminal and lateral side group. Based on
all the data from DSC, POM and XRD, it is confirmed that all the synthesised schiff baseꢀester
calix[4]arene derivatives display hexagonal columnar liquid crystalline mesophase respectively.
Figure 3: XRD profiles depicting the intensity against the 2Ɵ obtained from the Colh phase of
compound 1f at 141.0 ˚C (a); Colh phase of compound 1f at136.0 ˚C (b) on cooling from
4
6
isotropic temperature.
1
4

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Figure 4: Schematic representations of the columnar hexagonal molecular arrangements in
compound 1f4.
3
.5 Photophysical behaviors
The photophysical properties of these supramolecular calix[4]arene compounds 1f , 1f , 1f , 1f ,
4
6
8
10
1
f , 1f and 1f were studied in solution. Figure 5 represents the absorption and emission
12 14 16
spectra of compound 1f in various organic solvents. One can see that the absorbance and the
4
emission intensities of compound 1f are higher in organic solvents as compared to polar
4
solvents. The absorbance and emission intensities of compound 1f in THF is higher as compare
4
to other organic solvents. Micromolar THF solutions of all supramolecular compounds were
used to measure the absorption and fluorescence spectra mention in Figure 6 and Figure 7. The
corresponding spectral data were collected in Table 2. As can be seen, compound 1f , 1f and 1f
8
4
6
exhibited an absorption maximum centered at 318 nm, 316 nm and 310 nm and maximum
1
5

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emission at 331 nm, 333 nm and 325 nm (λmax) wavelength absorption peaks. The highly
delocalized electronic systems and ΠꢀΠ* transitions with the presence of heteroaromatic part
inbuilt with the lower rim of calixarene derivatives with high molar absorption coefficient (ε =
6
ꢀ1
ꢀ1
1
1.6ꢀ24.0 × 10 L mol cm ) recognized to the maximum absorption in the present synthesised
supramolecular compounds. The absorption and fluorescence of the compounds 1f , 1f , 1f
14
10
12
and 1f were higher than that of compounds 1f , 1f and 1f , this is due to the variation of the
1
6
4
6
8
side alkyl chain group which suggesting that the long alkyl substitution with electron donating
oxygen and the conjugation of aromatic rings with electron accepting imide group increases the
intramolecular charge transfer efficiency which increased the fluorescence intensity. As can be
seen in the Figure 7aꢀ7b, all the compounds showed sky bluish fluorescence in THF solution.
This is promising as the blue light emitting materials are not only limited blue also their energy
levels are high. They provide an efficient approach in fineꢀtuning the emission wavelength on
3
8
combining with another dopant emitter in the construction of white OLEDs . The emission
spectra of all the compounds showed the one stable emission maxima bands at center around
3
24ꢀ333 nm which attributed to the electronic transition from lowest unoccupied molecular
orbital (LUMO) to the highest occupied molecular orbital (HOMO) respectively. Supramolecular
calix[4]arene schiffꢀbase ester derivatives exhibited a bathochromic shift in the absorption and
emission maxima. It can be noted that the effect of alkyl substitution chain in the terminal side
chain had no effect on the absorption and emission peaks, but the intensity is fluctuated as
varying alkyl chain from short alkyl spacer to higher alkyl spacer. Relative quantum yields were
3
9ꢀ40
measured in solution as reported earlier
. The quantum yields of present synthesised
compounds are in the range of 0.32ꢀ0.50 respectively.
1
6

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Table 2: Photophysical properties of the supramolecular mesogens recorded in the THF (5ꢁM)
solution.
a
Compounds Absorption λabs/nm
Emission
λ /nm
em
Stoke shift
nm
Quantum yield
6
ꢀ1
ꢀ1
(ε/10 L.mol cm )
1
1
1
f4
f6
f8
318 (11.6)
331
333
325
332
330
325
324
13
17
15
13
20
17
16
0.32
0.43
0.38
0.32
0.50
0.43
0.40
316 (12.6)
310 (13.4)
319 (18.4)
310 (20.4)
308 (22.4)
308 (24.0)
1
1
1
1
f10
f12
f14
f16
a
:
Relative quantum yield calculated with respect to quinine sulphite solution in 0.1 M
H SO with a quantum yield of 0.54.
2
4
Figure 5: Absorption and fluorescence spectra of compound 1f in 5ꢁM different solvents.
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Figure 6: Absorption spectra of compounds in 5ꢁM THF solution.
Figure 7: Fluorescence spectra of compounds in 5ꢁM THF solution.
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5
. Conclusions
Supramolecular schiffꢀbase ester calix[4]arene derivatives inbuilt with terminal alkoxy side
groups are a promising class of bowlic shaped columnar liquid crystals with good thermal
stability of the mesophase. The studies on photophysical properties revealed that these
supramolecular derivatives exhibited good fluorescence properties. The selfꢀassemble to form an
ordered hexagonal columnar phase with a good thermal range of mesophase. These all
synthesised calix[4]arene derivatives show blue light emission in the solution. The length of the
side spacer alkyl chain had no effect on the absorption and emission properties in the solution
state. The increase of the flexible alkyl chain length on terminal side is affected on the
mesophase range. The compounds with lower alkyl chain group show the higher temperature
range of mesophase as comparison to higher alkyl chain substituted compounds. In essence,
these supramolecular LCs molecules are promising with respect to their photophysical properties
and selfꢀassembly behaviour to show hexagonal columnar liquid crystal phase.
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Acknowledgment
Authors are thankful to NFDD Centre for providing analytical and spectral services. Authors
thanks to Miss Akshara Shah (Department of Chemistry, Mumbai University) for helping fruitful
discussion in the high temperature XRD analysis.
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