Bowl-shaped fluorescent liquid crystals derived from 4-: Tert butyl calix[4]arene and trans cinnamic acid derivatives

Article abstract of DOI:10.1039/c9nj03097b

A new family of bowl-shaped molecules with a calix[4]arene rigid core appended by four-side and two-side substitution with trans 4-n-alkoxy cinnamic acid has been synthesized and well characterized. The LC behaviours of the novel materials were studied by using optical polarising microscopy, differential scanning calorimetry and high-temperature powder X-ray diffraction methods to confirm the smectic organization. It is found that all of the prepared materials display enantiotropic LC phases in the present series-1 (1d₄-1, 1d₅-l, 1d₈-1, 1d₁₀-l, 1d₁₂-l, 1d₁₄-l) and series-2 (1d₄-1l, 1d₅-ll, 1d₈-1l, 1d₁₀-ll, 1d₁₂-ll, 1d₁₄-ll). The electrochemical behaviour shows the higher reactivity of the proposed derivatives, which is in good agreement with the DFT values. The relationship between the structure and the mesomorphic behaviour of the compounds in both series is discussed. These synthesized supramolecular compounds show blue luminescence in solution as well as in solid thin films under long wavelength UV light. To get more insights, HOMO and LUMO studies were carried out, which support intramolecular charge transfer interactions in this class of mesogens.

Full text of DOI:10.1039/c9nj03097b

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Bowl-shaped Fluorescent Liquid Crystals derived from 4-tert butyl
calix[4]arene and trans cinnamic acid derivatives
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Vinay S. Sharmaa*, Hemant Kumar Singhb, Anuj S. Sharmac, Akshara P.Shahd, Priyanka
A. Shahc,
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a:Department of Chemistry, Faculty of Basic and Applied Science, Madhav University, Sirohi,
Rajasthan, India.
b:Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039, India
c:Department of Chemistry, School of Science, Gujarat University, Ahmedabad, Gujarat.
d:Department of Chemistry, Mumbai University, Santacruz, Mumbai.
Email address of corresponding author*: vinaysharma3836@gmail.com
Abstract
A new family of bowlic-shaped molecules with a calix[4]arene rigid core appended four-side and
two-side substitution with trans 4-n-alkoxy cinnamic acid have been synthesized and well
characterized. The LCs behaviour of novel materials was studied by using optical polarising
microscopy, differential scanning calorimetry and high-temperature powder X-ray diffraction
methods to confirm the smectic organization. It is found that all the prepared materials display
enantiotropic LCs phases in present series-1 (1d4-1, 1d5-l, 1d8-1, 1d10-l, 1d12-l, 1d14-l) and
series-2 (1d4-1l, 1d5-ll, 1d8-1l, 1d10-ll, 1d12-ll, 1d14-ll). The electrochemical behaviour shows the
higher reactivity of the proposed derivatives which is further good agreement with DFT values.
The relationship between the structure and the mesomorphic behaviour of the compounds in both the
series are discussed. These synthesized supramolecular compounds showed blue luminescence in
solution as well as solid thin film under long wavelength UV light. To get more insights, the
HOMO, LUMO studies are carried out which supports intramolecular charge transfer
interactions in this class of mesogens.
Key words:- Calix[4]arene, cinnamate, liquid crystals, nematic, smectic.
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Graphical Abstract
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1. Introduction
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Calixarenes are an important class of macrocycles that have been extensively used as a
core in supramolecules due to their unique ‘bowl-like’ structure, high functionality, persistent
shape and size and their easy chemical modification.1 Calixarenes creates the important
supramolecular platform after crown ether and cyclodextrin.2 Various calixarene derivatives
with special properties were designed and synthesized up to now.3-8 In 1990, Dalcanale et al.
reported the preparation of the first calixarene liquid crystal, which self-organized to give a
new class of mesophase, namely the bowl-shaped columnar liquid crystals.9 Since then,
several families of liquid crystals based on calixarene derivatives have been reported by
different groups.10-12 Calixarene-based liquid crystals were also studied by introducing the
long alkyl chains on calixarene skeleton.13–14 In 2015, Yang and his group reported
supramolecular two side and four side substituted calix[4]arene-cholesterol LCs and
described the described that the long spacers and more cholesterol units are favorable for
producing excellent mesomorphic properties.15 Then after, Yang et al. reported triphenylene-
based calixarene liquid crystals with different structures and showed their mesomorphic
properties.16 Fang et al. synthesized calix[4]arene based dimeric cholesteryl compounds
having naphthalene linking unit.17 Yang et al. reported calix[4]resorcinarene cholesterol
based columnar liquid crystals having schiff-base linking group.18 Later on, his group
synthesized gallic-calixarene liquid crystalline compounds.19 Recently, Yang and his group
reported bowl liquid crystal based on cyclotriveratrylene derivatives with multiple
triphenylene units.20 Menon et al. reported LCs based on schiff base calix[4]arene and azo-
ester calix[4]arene and these compounds displayed smectic and nematic mesophase.21-22
Menon et al. reported a review on lower rim substituted calixarenes derivatives and their
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application.23
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Marcos et al. reported the mesomorphic property of calixarene schiff base compounds which
have binding recognition and selectivity towards Zn+2 ion.24 Figure-1 indicates the latest 4-
tert butyl calix[4]arene based bowl-shaped liquid crystals reported by various groups.
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Figure 1: Different type of substitution on lower rim of tert butyl calix [4]arene.
Our group reported two homologous series based on schiff base-ester calix[4]arene and
also studied the biological properties.25 We have also reported some homologous series based on
cinnamate linkage group to exhibits smectic and nematic phase.26 Recently, our group reported
various bowl-shaped fluorescent columnar liquid crystals based on calix[4]arene skeleton.27
The comprehensive literature studied reveals that the supramolecular liquid crystalline
compounds with blue luminescence are very rarely reported. Our study can accordingly provide
an experimental basic for preparing and using these designs to target molecules. The goal of our
work is to synthesize bowl-shaped supramolecular LCs based on cinnamate linking unit
substituted on the lower rim of calix[4]arene skeleton with presence of polar alkoxy side chain.
All the target molecules exhibited wide range of mesophase with good thermal stability. In
addition, their electrochemical, photophysical and theoretical studies were performed to shed
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light on their electronic structure. In this paper, we have prepared cinnamate calix[4]arene based
luminescent supramolecular LCs. Here, we have also compared the mesogenic and thermal
properties of both the four side (series-1) and two side (series-2) substituted derivatives to their
correlation between molecular structure and liquid crystalline properties.
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2. Result and discussion
2.1 Synthesis and characterisation
In order to study the effects of various type of mesogens on liquid crystalline properties, p-tert-
butyl calix[4]arene with four side (series-1) and two side (series-2) cinnamate derivatives with
variable alkoxy side chain were designed as target compounds. The synthetic routes of novel
cinnamate calix[4]arene derivatives (series-1 & series-2) were illustrated in scheme-1. 4-n-
alkoxy benzaldehyde (1a) were synthesised by refluxing the reaction mixture of 4-hydroxy
benzaldehyde with various alkyl bromides (R-Br) and anhydrous K2CO3 in dry acetone.28 FT-IR
study confirms the absence of –OH peak in comp.1a. Trans 4-n-alkoxy cinnamic acids (1b) were
prepared by reported method in literature.29 IR and H-NMR studies confirm the structures of
1
final target compounds. The synthesis of p-tert butyl calix[4]arene (1c) was carried out by
microwave assisted method reported in literature.30 By reacting p-tert butyl calix[4]arene (1c)
with trans 4-n-alkoxy cinnamic acid (1b) in DCM using N,N-dicyclohexyl carbodimide (DCC)
and 4-dimethyl amino pyridine as a catalyst at room temperature for 48 hr.31, the novel 4-n-
alkoxy cinnamate-tetra-substituted calix[4]arene (1d4-l-1d5-l, 1d8-l, 1d10-l, 1d12-l, series-1) and
4-n-alkoxy cinnamate-di-substituted calix[4]arene (1d4-ll, 1d5-ll, 1d8-ll, 1d10-ll, 1d12-ll, series-2)
were purified by using column chromatography on silica gel eluting with chloroform: methanol
as eluent (4:1). The structures of newly synthesised compounds of series-1 and series-2 were
confirmed by FT-IR, 1H NMR and 13C NMR study. In the 1H NMR spectra of compounds 1dn-l
from the series-1 showed singlet for tert-butyl groups, pair of doublets for methylene bridges and
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singlet of ArH, signifying the cone conformation for calixarene core whereas no any specific
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peak of –OH group obtained in FT-IR spectra. H NMR spectra of compounds 1dn-ll from the
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series-2 showed two singlet for tert-butyl groups, pair of doublet for methylene bridges which
confirm the cone confirmation of final target compounds, singlet of ArH and presence of peak at
9.02 ppm for –OH group and also appeared in FT-IR spectra to confirm two-side substitution on
lower rim of p-tert-butyl calix[4]arene. The trans form of 4-n-alkoxy cinnamic acids were
confirmed by the observed vicinal coupling (J = 15.5 Hz) occur for trans isomer of cinnamic acid
in 1H NMR and also trans –CH=CH- stretching observed at nearly 966 cm-1 in IR spectroscopy.
In UV study, all the transition observed at above 290 nm which is generally confirmed the
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presence of trans isomerism of final target compounds. Furthermore, the C NMR results also
well supported to the corresponding structures and cone conformations of target compounds
mention in ESI.
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Scheme 1: Reagents and conditions: (a) R-Br, Anhy.K2CO3, dry Acetone, Reflux; (b) Malonic
acid, Pyridine, Piperidine, Reflux; (c) HCHO, NaOH, Microwave, 15 min; (d) DCC, DMAP,
DCM, 48 hr. stirring at RT.
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2.2 DSC analysis
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The mesomorphic behaviour of synthesized compounds were preliminarily investigated
by differential scanning calorimetry (DSC). The peak temperatures obtained in DSC traces due
to phase transitions were found to be in accordance with POM observations mention in Figure-2
Figure-S1 and Figure-S2. The transition temperatures, mesophase sequence and the
corresponding enthalpy changes of present series-1 and series-2 are summarized in Table-1. It
can be seen that lower and middle members of compounds from both the series possessed three
endothermic peaks except comp.1d4-ll which showed two endothermic peaks on heating and
cooling sequence. As shown in Figure-2, comp.1d4-1exhibited three endothermic peaks at 112.2
°C, 148.4 °C and 166.3 °C corresponding to crystal to smectic C, smectic C to smectic A and
smectic A to isotropic phase transition on heating condition, and three exothermic peaks at 107.1
°C, 142.1 °C and 162.4 °C on cooling condition. Comp.1d10-l displayed three endothermic peaks
at 95.1 °C, 110.9 °C and 128.7 °C corresponding to crystal to smectic C, smectic C to nematic
and nematic to isotropic phase transition on heating condition while again three exothermic
peaks appeared on cooling condition which were also confirmed by polarizing optical
microscopy (POM). Comp.1d4-ll displayed two endothermic peaks at 124.1 °C and 168.4 °C on
heating condition and two exothermic peaks obtained at 120.6 °C and 164.3 °C on cooling
condition, while comp.1d8-ll showed three endothermic peaks at 114.6 °C, 130.1 °C and 134.8
°C on heating condition and three exothermic peaks again traced at 131.5 °C, 127.4 °C and 110.3
°C on cooling condition respectively. One can see that the mesophase transition temperature of
series-2 is higher as compare to series-1 due to the substitution of cinnamate group inbuilt with
alkoxy chain substituted phenyl ring on two sides of calix[4]arene core which creates its less
flexible and also form hydrogen bonding between two inner hydroxyl groups respectively.
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Figure 2: The DSC traces of compounds 1d4-l (a), 1d5-l (b), 1d10-l (c), 1d12-l (d) on first
heating and cooling (scan rate 10˚C/min).
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Table 1. Thermal Transitions and Corresponding Thermodynamic Parameters of 1dn-l and 1dn-l1
Comp.
Heating
Cooling
I 162.4 (20.6) SmA 142.1 (38.3) SmC 107.1
(24.2) Cr
1d4-l Cr 112.2 (14.5) SmC 148.4 (41.6) SmA
166.3 (27.6) I
1d5-l Cr 107.6 (16.7) SmC 128.5 (32.6) SmA
142.8 (24.9) I
I 141.6 (17.4) SmA 124.8 (31.4) SmC 101.5
(24.1) Cr
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1d8-l Cr 106.7 (28.5) SmC 117.9 (36.5) SmA
126.3 (26.8) I
1d10-l Cr 95.1 (19.1) SmC 110.9 (32.5) N 128.7 I 126.5 (29.3) N 106.4 (32.6) SmC 92.3 (27.3)
(34.6) I Cr
1d12-l Cr 86.8 (18.4) SmC 107.6 (37.6) N 121.6 I 118.4 (20.1) N 105.6 (42.6) SmC 83.4 (15.6)
I 123.4 (38.6) SmA 114.6 (49.4) SmC 101.3
(26.4) Cr
(31.5) I
Cr
1d14-l Cr 79.8 (12.5) N 114.6 (32.6) I
I 113.6 (23.3) N 76.4 (19.5) Cr
1d4-ll Cr 124.1 (18.6) SmC 168.4 (46.8) I
1d5-ll Cr 116.5 (17.4) SmC 145.1 (30.8) SmA I 150.4 (13.5) SmA 141.4 (42.5) SmC 112.3
151.8 (36.8) I
1d8-ll Cr 114.6 (15.9) SmC 130.1 (34.6) SmA
134.8 (21.7) I
1d10-ll Cr 108.5 (14.3) SmC 128.9 (40.6) N
141.2 (28.9) I
1d12-ll Cr 102.4 (15.8) SmC 118.6 (36.5) SmA
130.6 (21.9) I
1d14-ll Cr 91.5 (24.3) SmC 116.4 (30.6) N 132.2 I 131.6 (27.1) N 108.3 (14.3) SmC 88.6 (17.3)
(21.9) I Cr
Phase transition temperature (°C) of the compounds 1dn-l/1dn-ll (n = 4, 5, 8, 10, 12, 14) recorded for first heating
and cooling cycles at 10 °C min-1 from differential scanning calorimetry (DSC). The enthalpies (ΔH in kJmol-1)
during the transition are presented in parentheses.
I 164.3 (48.6) SmC 120.6 (10.6) Cr
(3.2) Cr
I 131.5 (49.5) SmA 127.4 (26.8) SmC 110.3
(10.8) Cr
I 138.6 (17.5) N 125.7 (22.5) SmC 106.3 (7.9)
Cr
I 128.9 (38.4) SmA 110.2 (40.5) SmC 98.6
(6.5) Cr
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2.3 POM investigation
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The transition temperatures of newly synthesized compounds of tetra-substituted (series-
1) and di-substituted (series-2) were carried out on the basis of the polarizing optical microscope
(POM) by observing the textures when sample was sandwiched between a glass slide and a cover
slip and heated solid state to isotropic state is mention in Table-S1 and Table-S2 in ESI. All the
synthesized compounds show enantiotropic mesophase with good thermal stability. In series-1,
comp.1d4-l, 1d5-l and 1d8-l showed enantiotropical smectic C and smectic A mesophase while
comp.1d10-l, 1d12-l exhibited smectic C and nematic mesophase. However, comp.1d14-l
exhibited only nematic mesophase. In second series-2, comp. 1d4-ll showed only smectic C
mesophase, while comp.1d5-ll to 1d10-ll exhibited smectic C and smectic A mesophase.
Additionally, comp.1d12-ll and 1d14-ll showed smectic C and nematic mesophase. The phase
diagram of present series-1 and series-2 are shown in Figure-3 and Figure-4 shows descending
tendency of phase transition appeared in both the series. The transition temperature of
synthesized homologous monomers (1d4-1d14) are mentioned in Table-S3 (see in ESI).
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Figure 3: Phase behaviour of series-1 (1dn-1).
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Figure 4: Phase behaviour of series-2 (1dn-ll).
2.4 Texture study
Furthermore, based on the DSC results, the mesophase texture images of comp.1d6-1,
1d8-1, 1d8-ll, 1d5-ll, 1d5-l and 1d6-ll from series-1 & series-2 is shown in Figure-5 and Figure-S3
respectively. The phase transition temperatures were nearly in agreement with the peaks traced in
DSC analysis on heating and cooling condition. The texture patterns of smectic A and smectic C
phase are fan, broken fan, needle and threaded line shape type while the texture images of
nematic phase are schlieren type in both series. Comp.1d6-l showed fan type texture pattern of
smectic A phase at 128 °C on heating condition. Comp.1d8-l exhibited marble type texture image
of smectic A phase at 116 °C on heating condition. Comp.1d10-ll displayed threaded line type
texture image of smectic A phase at 127 °C on heating condition. Comp.1d5-ll displayed typical
texture pattern of smectic C phase at 124 °C on heating condition. From Figure-S3, it can be seen
that, comp.1d5-l displayed fan type texture image of smectic A phase at 134 °C on heating
condition while comp.1d8-ll displayed threaded line type texture image of SmA phase at 130 °C
on cooling condition.
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Figure 5: Photomicrographs of the texture obtained for series-1 and series-2: (a) fan type texture
image of SmA phase at 128 °C for comp. 1d6-l; (b) marble type texture image of SmA phase at
116 °C for comp. 1d8-l; (c) threaded line type texture image of SmA phase at 127 °C for comp.
1d10-ll; (d) typical pattern texture image of SmC phase observed at 124 °C for comp. 1d5-ll.
2.5 TGA analysis
Thermal stability of comp.1d4-l, 1d4-ll was studied by thermogravimetric analysis
(TGA). Figure-6 specifies the TGA of compound 1dn-I (series-1) and compound 1dn-II. The
TGA of compound 1dn-I shows only 5% weight loss from 150 °C to 300 °C. The TGA of
compound 1dn-ll shows 3% weight loss from 200 °C to 300 °C, which indicates good thermal
stability of the mesophase. As there is no loss seen up to 150 °C that means no water or other
solvent entrapped in mesophase formation. Both the compounds show good thermal stability.
The thermal stability of all the phase transition present in series-2 is higher as compared to
series-1 respectively. Smectic mesophase commences equally at comp.1d4-l (series-1) and
comp.1d4-ll (series-2) while nematic mesophase commences early in series-1 as compare to
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series-2. In series-2, the presence of two polar hydroxyl groups with two side substitution of
cinnamate linking groups showed higher stability of mesophase due to the presence of hydrogen
bonding between hydroxyl group as compared to four side substituted compounds (series-1) on
lower rim of calix[4]arene. The flexibility of series-1 is more as compare to series-2 due to the
presence of four side substitution of variable 4-n-alkoxy side chain at the end terminal groups.
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Figure 6: Thermal gravimetric analysis of 1dn-I (series-1) and 1dn-II (series-2).
2.6 XRD investigation
To confirm the mesophase observed through polarising microscope (POM) and
differential scanning calorimetry (DSC), XRD studies have been performed to confirm the
structure of smectic phases in both series respectively. Four samples have been studied for their
LC phase by variable temperature XRD as representatives. 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 compound 1d5-l, 1d5-ll, 1d8-l, 1d8-ll, 1d10-l and 1d10-ll showed two
sharp reflections and a single diffuse scattering peak in the wide angle region as mention in
Figure-7, Figure-8 and Figure-S4 respectively. Compound 1d5-1 (series-1) scanned at 126.3 °C
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and 103.2 °C to confirm the molecular organization in the present mesophase, three reflections
were appeared as d1=21.52 Å, d2=12.32 Å, d3=4.02 Å which indicates the presence of SmA
mesophase characteristics. That kind of arrangement presence the characteristic for liquid
crystalline compounds with two dimensional density modulations and moving liquid type
molecules. The diffractograms found in the Smectic A mesophase for compound 1d5-l can be
well indexed with d-spacing values. However, for the SmC phase, the d-spacings of compound
1d5-1 as d1=19.14 Å, d2=10.73 Å and d3=4.21 Å appear at 103.2 °C. Careful microscopic
textural observations and along with these reflections patterns confirm the phase type is smectic
C. The d-spacing values of smectic A phase are slightly higher as compare to smectic C phase to
confirm the molecules become titled in the smectic C mesophase. For compound 1d5-ll, three
reflections were appeared as d1=20.98 Å, d2=10.23 Å and d3=4.04 Å at 142.6 °C and again on
decreasing temperature at 114.8 °C, three reflections appeared as d1=18.12 Å, d2=9.34 Å and
d3=3.98 Å. Compound 1d8-1 shows three reflection as d1=26.32 Å, d2=14.23 Å and d3=4.05 Å at
116.7 °C while decreasing temperature at 102.1°C, it again showed three reflections at d1=23.21
Å, d2=13.21 Å and d3=3.84 Å. From the careful observation, it can be seen that the d-spacing
values decreasing form higher temperature to lower temperature which mean the presence of
smectic A to smectic C phase respectively. Furthermore, compound 1d8-ll shows three
reflections as d1=24.27 Å, d2=13.32 Å and d3=3.91 Å at 130 °C while on 125.6 °C, its shows
reflections at d1=23.98 Å, d2=11.45 Å and d3=3.54 Å. Compound 1d10-1 showed only one
reflections as d = 4.01 Å at 109.5 ˚C while decreasing temperature upto 90.32 ˚ it exhibited three
reflections as d1 = 26.86 Å, d2 = 12.54Å and d3 = 4.01 Å. Similarly compound 1d10-ll showed the
reflections pattern to confirm the nematic and SmC mesophase which is further carefully
checked by POM analysis. The predicted length of the synthesized derivatives is nearly equal to
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the observed molecular length which confirmed the present of highly layered type arrangement
of molecules in smectic phase. The proposed molecular packing of SmC phase in present
synthesized four side (series-1) and two side (series-2) appended calix[4]arene derivatives which
transforms into SmA and nematic phase are mentioned in Figure-S5 and Figure-S6.
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Figure 7: XRD patterns of (a) Compound 1d5-I (series-1) at 126.3 °C and 103.2 °C; (b)
Compound 1d5-II (series-2) at 142.6 °C and 114.8 °C.
Figure 8: XRD patterns of (a) Compound 1d8-I (series-1) at 116.7 °C and 102.1 °C; (b)
Compound 1d8-II (series-2) at 128.6 °C and 108.5 °C.
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2.7 Photophysical Behaviour
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Photophysical properties of these supramolecular calix[4]arene derivatives are
tabulated in Table-2. Micromolar THF solutions of compounds 1d18-ll, 1d16-ll, 1d14-ll, 1d10-l,
1d8-l and 1d4-l were used to measure an absorption and fluorescence spectra mention in Figure-9.
As can be seen, compound 1d18-ll and 1d16-ll exhibited an absorption maximum centered at 288
nm and 289 nm, with two small shoulders at 325 nm and 302 nm while compounds 1d4-l and
1d8-l showed first absorption maxima at 298 nm and second absorption maxima at 324 nm. The
highly delocalized electronic systems and Π-Π* transitions with presence of cinnamate linking
part inbuilt with lower rim of calixarene with high molar absorption coefficient (ε = 22.2-34.2 ×
106 Lmol-1cm-1) recognized to the maximum absorption in the present synthesized
supramolecular compounds. The absorption and the intensity of the peak in the absorption
spectrum is depends on the molecules which absorbs the light of a given wavelength. The
fluorescence intensity of tetra substituted derivatives of compounds 1d10-ll, 1d8-ll and 1d6-ll
were higher than compounds 1d10-l, 1d8-l and 1d6-l suggesting that the alkyl substitution inbuilt
with ester linking group on lower rim increased the fluorescence intensity. All the compounds
showed bluish fluorescence in THF solution. Emission spectra of all the compounds showed the
one stable emission maxima bands at centered around 300-330 nm which attributed to the
electronic transition from lowest unoccupied molecular orbital (LUMO) to the highest occupied
molecular orbitals (HOMO) respectively. The absorbance and emission spctra of the solid thin
films of the synthesized derivatives are mention in Figure 10. Supramolecular calix[4]arene
cinnamate-ester derivatives exhibited hypsochromic shift in the absorption and emission
maxima. It can be noted that the effect of alkyl substitution chain had no effect on the absorption
and emission peaks, but the intensity is fluctuated as changing side alkyl spacer from lower
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number to higher number. It can be noted that the emission spectra of tetra substituted
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derivatives in solid film showed red shift by compared with that of solution while two side
substituted derivatives showed blue shift, it is generally due to the presence of greater
intermolecular interactions and their effect on energy states in solid thin film. The fluorescence
in liquid crystalline phase by heating the solid films was also investigated, but little emission was
observed due to the strong aggregation The quantum yields of present synthesized compounds
are in the range of 0.28-0.41 in solution and solid state.
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Table 2: Photophysical properties of the supramolecular mesogens recorded in the THF (10-6 M)
solution.
Comp.
Absorptiona
(nm)
Emissiona Stoke Quantum
(ε/106
L.mol-1
cm-1)
Absorptionb Emissionb
Quantum
Yieldb
(nm)
shift
(nm)
Yielda
(nm)
(nm)
1d10-ll
1d8-ll
1d6-ll
1d10-l
1d8-l
302
308
311
312
314
312
328
327
329
323
326
324
26
19
18
11
12
12
0.41
0.39
0.35
0.38
0.43
0.28
32.1
34.2
28.3
22.2
24.6
32.3
309
307
301
298
299
288
327
324
327
328
329
326
0.36
0.41
0.33
0.34
0.39
0.31
1d6-l
a Micromolar solution in THF; aExcited at the respective absorption maxima; bEmission obtained by exciting
the thin films at their respective absorption maxima in solution; a: Relative quantum yield calculated with
respect to quinine sulphite solution in 0.1 M H2SO4 with a quantum yield of 0.54.
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Figure 9: Absorption and Emission spectra of compounds in THF (10-6 M).
Figure 10: Absorption and fluorescence spectra of compounds in solid thin films.
2.8 Density functional theory studies
The nature of the electronic transition and frontier molecular orbital (FMO) distributions
are investigated by using density functional theory (DFT) at the B3LYP/6-31G (dp) method
using Gaussian 09 program package.32 The energy minimized structures and energy levels of the
frontier molecular orbitals (HOMO-LUMO) of compounds 1d5-ll, 1d5-l, 1d8-ll and 1d8-l are
shown in Figure-11 & S7. It is shown that comp.1d5-ll and comp.1d8-ll, HOMO is localized on
the two side substituted phenyl ring with cinnamate linking group and alkyl chain in pentyloxy
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side group in the opposite sides, while the LUMO is mostly concentrated on the one side
substituted phenyl ring substituted on lower rim of calix[4]arene. The HOMO-LUMO energy
levels of comp.1d5-ll and comp.1d8-ll suggested the distribution of energy level on the same side
substituted aromatic ring on the lower rim of calix[4]arene unit. It can be seen that the di-
substituted derivatives on calix[4]arene unit shows the distribution on opposite side while the
tetra substituted derivatives on lower rim on calix[4]arene unit shows the distribution on same
side. The energy difference between the HOMO and LUMO is termed as HOMO-LUMO gap
which provide the knowledge about the molecular length of conjugation and also the band gap
level. The theoretical band gaps for comp.1d5-ll, 1d5-l, 1d8-ll and 1d8-l were found to be 3.41
eV, 3.8 eV, 3.42 eV and 3.79 eV respectively. The HOMO-LUMO gap of comp.1d5-ll (series-2)
is 3.41 eV which is lower as compare to comp.1d5-l (series-1). The presence of cinnamate
linking group which affects the possibility of charge transfer of electron in the HOMO to the
LUMO. The higher value of HOMO energy contributes negatively to the activity due to presence
of side alkoxy chain as electron-donating group which increase the energy of the HOMO
orbitals. The dipole moments of the 1d5-ll, 1d5-l, 1d8-ll and 1d8-l are 5.51, 5.53. 5.37 and 5.42 D
respectively. The HOMO, LUMO and 3D molecular electrostatic potential contour map of
optimized structure of comp.1d5-l, 1d5-ll, 1d8-l and 1d8-ll are shown in Figure-12, Table-S4 and
Table-S5 (ESI) respectively. In the map electrostatic potential surface, red and blue colours are
mainly signified the electron-rich and electron-poor part while the green colour indicates the zero
electrostatic potential.
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Figure 11: Optimized geometry and HOMO and LUMO frontier molecular orbitals of the
compounds 1d5-l and 1d5-1l at the B3LYP/6-31G (dp) level (some part of alkyl chain is omitted
for clear view in HOMO and LUMO orbitals).
Figure 12: Molecular electrostatic potential (MEP) diagram of comp.1d5-ll (a) and comp.1d5-l
(series-1) (b).
2.9 Electrochemical study
Cyclic voltammetry (CV) gives valuable information and allows the estimation of
HOMO and LUMO levels of the present synthesized cinnamate group based derivatives
constructed on lower rim of calix[4]arene skeleton. Four selected bowl-shaped two side and four
side substituted calixarene derivatives were investigated for their electrochemical behaviour by
carrying out cyclic voltammetry studies in anhydrous micromolar THF solution. The energy
levels, HOMO, LUMO and calculated energy band gaps are shown in Table 3. The cyclic
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voltammograms of compound 1d5-ll, 1d5-l, 1d8-ll and 1d8-l are shown in Figure 13. All the four
derivatives showed similar oxidation and reduction wave peaks respectively. A 0.1 M solution of
tetrabutylammonium perchlorate (TBAP) was used as a supporting electrolyte in deoxygenated
THF. A single compartment cell equipped with Ag/AgNO3 (0.1M) reference electrode, platinum
rod counter electrode and glassy carbon working electrode were used for the experiments. The
reference electrode was calibrated with the ferrocene/ferrocenium (Fc/Fc+) redox couple
(absolute energy level of – 4.80 eV to vacuum).33-34 The recorded scanning rates of cyclic
voltammograms were 0.05 mVs-1. The present synthesized four side substituted cinnamate
appended calix[4]arene derivatives showed higher band gaps as compared to two side substituted
sccinnamate linked calix[4]arene derivatives which indicates the lower reactivity of compound
(1d5-l, 1d8-l) as compared to compound (1d5-ll, 1d8-ll) respectively. Energy levels LUMO were
determined by using the formula of ELUMO = – (4.8 –E1/2, Fc,Fc+ + Ered, onset) eV, while the
HOMO energy levels were determined by using the formula of EHOMO = – (4.8 –E1/2, Fc,Fc+ +
Eoxd, onset) eV. Four side appended bowl-shaped cinnamate calix[4]arene derivatives (1d5-l,
1d8-l) exhibited LUMO and HOMO levels -2.44 eV, -2.46 eV, -6.16 eV and -6.14 eV (Fig.13b-
13d), while two side substituted calix[4]arene derivatives (1d5-ll,1d8-ll) exhibited LUMO and
HOMO levels -2.66 eV, -2.61 eV, -6.04 eV and -5.99 eV (Fig.13a-13c). All the four derivatives
showed band gap (Eg) as 3.38 eV, 3.72 eV, 3.38 eV and 3.68 eV respectively. One can see that
the two sides substituted derivatives showed a lower band gap as compare to four side
substituted derivatives on lower rim of calix[4]arene. This indicates the presence of hydrogen
bonding between two hydroxyl group and cinnamate units may influence the charge transfer of
electron and also stabilized the HOMO and LUMO energy levels. The calculated band gaps
values by CV are in good agreement with DFT study as well. Interestingly, from all the
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synthesized four supramolecular derivatives, compound 1d8-ll found to be more stable because
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Figure 13: Cyclic voltammogram of (a) comp.1d5-ll; (b) comp.1d5-l; (c) 1d8-ll; (d) 1d8-l in
anhydrous THF solution of TBAP (0.1 M) at a scanning rate of 0.5 mV/s.
Table 3: Electrochemical properties of compounds 1dn-ll and 1dn-l
Comp.
Eoxd
Ered
EHOMO
ELUMO
ΔEg, CV
1d5-ll
1d5-l
1d8-ll
1d8-l
1.81
1.93
1.76
1.91
-1.57
-1.79
-1.62
-1.77
-6.04
-6.16
-5.98
-6.14
-2.66
-2.44
-2.61
-2.46
3.38
3.72
3.37
3.68
Experimental conditions: Ag/AgNO3 as a reference electrode, platinum wire as counter electrode,
glassy carbon as working electrode, tetrabutylammonium perchlorate (0.1 M) as supporting electrolyte,
room temperature
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Conclusions
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Supramolecular cinnamate-ester calix[4]arene derivatives with terminal alkoxy side chains are a
promising class of bowlic shaped liquid crystals with good thermal stability and photophysical
behaviour. The thermal behavior of all the synthesized compounds was studied with the help of
TGA, DSC, POM and XRD studies. All these synthesized supramolecular cinnamate
calix[4]arene derivatives showed blue emission in solution as well as solid state, low band gap
which is important for the fine-tuning of emission wavelength on combining with added dopant
emitter for the formation of white OLEDs. The electrochemical study revealed that the higher
reactivity of target compounds which were further good agreement with DFT study. The length
of the side spacer alkyl chain had no effect on the absorption and emission properties in the
solution state. All synthesized supramolecular compounds display enantiotropic LCs phase.
Disubstituted derivatives of Series-ll show higher thermal stability of mesophase as compared to
tetrasubstituted derivatives of Series-l. The lower (n= 4) and middle member (n= 5, 8, 10, 12)
derivatives from both the series showed smectic C and smectic A phase while the higher member
derivatives (n= 14) displayed only nematic phase without displaying any smectic property. In
essence, these supramolecular LCs molecules are promising with respect to their photophysical
properties and their liquid crystalline properties.
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Acknowledgement
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VS sincerely thanks Department of Chemistry, Faculty of Basic and Applied Science, Madhav
University for providing lab facility. VS, APS and AS are also thankful to NFDD Centre, Rajkot
for providing analytical and spectral services. Hemant Kumar Singh is grateful to DST, SERB
for providing fellowship under N-PDF (PDF/2016/001706) scheme and IIT Guwahati for
computational facilities.
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