Columnar self-assembly, electrochemical and luminescence properties of basket-shaped liquid crystalline derivatives of Schiff-base-moulded: P-tert -butyl-calix[4]arene

Article abstract of DOI:10.1039/d0nj04148c

A new family of supramolecular liquid crystalline compounds with a calix[4]arene core and two linking groups appended on four sides, exhibiting a wide range of hexagonal columnar phases, has been synthesised and well characterised. The liquid crystal beha

Full text of DOI:10.1039/d0nj04148c

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Columnar self-assembly, electrochemical and
luminescence properties of basket-shaped liquid
crystalline derivatives of Schiff-base-moulded
p-tert-butyl-calix[4]arene†
Cite this:DOI: 10.1039/d0nj04148c
Vinay S. Sharma, *a Anuj S. Sharma,b Sheena J. B. Worthington,c
b
b
Priyanka A. Shah
and Pranav S. Shrivastav
A new family of supramolecular liquid crystalline compounds with a calix[4]arene core and two linking
groups appended on four sides, exhibiting a wide range of hexagonal columnar phases, has been
synthesised and well characterised. The liquid crystal behaviour of the novel materials was studied
by using polarising optical microscopy (POM), differential scanning calorimetry (DSC) and thermal
gravimetric analysis (TGA), and was further confirmed using high-temperature powder X-ray diffraction
(XRD). An electrochemical study was carried out using cyclic voltammetry (CV) and the results were
compared with theoretical values obtained from a DFT study. These compounds showed light sky-blue
luminescence in solution and as a thin film under long-wavelength UV light. It was observed that all
the prepared target compounds exhibited enantiotropic mesophases with good thermal stability. The
structural and conformational characterisation of these new compounds was achieved using various
1
spectroscopic techniques, including FT-IR, H NMR, 13C NMR and MALDI-TOF. The relationship between
Received 17th August 2020,
Accepted 1st November 2020
the structure and the mesomorphic behaviour of the compounds containing an ethyl bridge with linked
ester groups (5a–5d) is discussed. This new class of supramolecular derivatives is promising, especially
considering the emissive nature and stabilisation of their columnar hexagonal mesophases, and their
high range of thermal stability.
DOI: 10.1039/d0nj04148c
rsc.li/njc
acts as an insulating mantle.6,7 The self-assembly of relatively
simple building blocks into complex ordered structures is a
1. Introduction
Liquid crystals (LCs) are fascinating soft materials which can subject of increasing research towards gaining a fundamental
build highly complex chemical structures through supramole- understanding of soft-matter self-organisation.8 This self-
cular self-assembly.1 The development of functional soft assembly is driven by shape anisotropy of the molecules, strong
materials has gained a huge momentum in recent years.2 The p–p interaction and nano-segregation of incompatible molecu-
columnar phase is a class of mesophase in which molecules lar subunits, such as a central rigid core and peripheral flexible
assemble into cylindrical structures to act as mesogens.3–5 The side chains.9 Luminescent liquid crystalline materials have
columnar structures formed by the one-dimensional stacking gained much interest recently owing to the inevitable need
of disc-like mesogens are often termed as molecular wires and for white-light-emitting diodes in self-organisation, lighting
have the potential to be exploited as active layers in organic and display applications.10
electronic devices. In the case of disc-shaped and bowl-shaped
Calixarenes are cyclic oligomers obtained by the condensa-
columnar LCs, the central core, which can facilitate pathways tion reaction of 4-tert-butylphenol and formaldehyde.11 Calix-
for charge-transfer carriers and the substitution of alkyl chains, arenes can be obtained with different sizes and in good yields,
by the judicious choice of base, reaction time, and temperature.
a Department of Chemistry, Faculty of Basic and Applied Science, Madhav
University, Sirohi, Rajasthan, India. E-mail: vinaysharma3836@gmail.com
b Department of Chemistry, School of Science, Gujarat University, Ahmedabad,
Gujarat, India
Calixarenes are conformationally mobile; the four isomers of
calix[4]arene are termed as the cone, partial cone, 1,3-alternate,
and 1,2-alternate conformations. Calixarenes that are further
functionalised at the upper and lower rim are useful for the
encapsulation of metal ions and the formation of molecular
assemblies.12 Recent studies on calixarenes have shown that they
possess specific self-assembly ability resulting from intermolecular
c Department of Chemical Engineering, University of Chester, Thornton Science
Park, Pool Lane, Chester, UK
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04148c
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forces, including coordination bonds,13 hydrogen bonds,14 spectrometer using tetramethylsilane as the internal standard.
ionic bonds and gelation.15
The synthesis of p-tert-butyl-calix[4]arene was carried out using a
In 1990, Dalcanale and his group made the first liquid Discover Benchmate System 240 V microwave synthesiser (CEM
crystalline materials based on a calixarene core.16 Calixarene- corporation). The mesophase type was identified by visual compar-
based liquid crystalline compounds represent a significant ison with known phase standards using a Nikon Eclipse LV-100
subgroup of macrocycle-based liquid crystals and have been POL polarising optical microscope (POM) fitted with a hot-stage
investigated in several studies.17,18 Yang et al. reported columnar temperature-controlled LTSE 420 heating stage (Linkam Scientific
liquid crystals based on a calix[4]arene core with inbuilt Instruments, Tadworth, Surrey, UK) with a temperature control
cholesterol moieties, which showed excellent mesomorphic accuracy of 0.1 K. The phase transition temperatures were mea-
properties with a good mesophase temperature range.19 They sured using a Shimadzu DSC-50 at heating and cooling rates of
also studied the mesophase, which exhibited a columnar-type 10 1C minꢁ1. The X-ray diffraction (XRD) measurements were
molecular arrangement of calixarene and cholesterol derivatives.20,21 performed on a Rigaku-Ultima IV powder diffractometer equipped
Yang et al. synthesised and studied the liquid crystalline with a Cu Ka source (l = 1.5418 Å and 1.6 kW, X-ray tube with
behaviour of calixarene-linked triphenylene derivatives.22 applied voltage and current values of 40 kV and 30 mA power) and a
Later on, his group reported the mesomorphic properties of Philips X’PERT MPD. The absorption spectra were obtained using a
gallic-calixarene derivatives, which exhibit a stable cone Jasco V-570 UV-Vis recording spectrophotometer with a variable
conformation.23 Some calixarene derivatives based on Schiff- wavelength between 200 and 800 nm. The fluorescence spectra
bases showed smectic and nematic phase behaviour.24 Later were recorded on a Jasco FP-6500 spectrofluorometer.
on, two groups reported calix[4]resorcinarene-based columnar
liquid crystals containing a Schiff-base linking group.25,26 In
our previous study, we reported various luminescent liquid
crystals based on calixarene with inbuilt thiadiazole- and
3. Results and discussion
3.1 Synthesis and characterisation
oxadiazole-based derivatives, which exhibited excellent mesophase
properties.27–29 Very recently, our research group reported a blue-
Scheme 1 shows the synthetic routes for the newly synthesised
calixarene derivatives. The synthesis of p-tert-butyl-calix[4]arene
light-emitting thiacalix[4]arene core based on supramolecular
(1a) was carried out using a microwave-assisted method.32
liquid crystals which exhibited gelation and OLED properties.30
p-tert-Butyl-calix[4]arene tetra-propionic acid (2a) was prepared
Further, we also reported bowl-shaped fluorescent liquid crystals
according to a reported method.33 3,4-Di-n-alkoxybenzaldehyde
based on calix[4]arene and trans-cinnamic-acid derivatives, which
showed smectic- and nematic-type mesophases.31
derivatives (3a–3d) were prepared by refluxing a reaction mix-
ture of 3,4-hydroxybenzaldehyde with various alkyl halides
As part of our research program that focuses on developing
(R-Br) in the presence of anhydrous K2CO3 and dry acetone
new luminescent liquid crystals, we were interested to synthe-
sise new basket-shaped supramolecular columnar liquid crys-
tals to understand their structure–property relationship.
Herein, we report the synthesis, characterisation, luminescence
and thermal behaviour of calix[4]arene as a rigid core in
supramolecular LCs, as well as discotic LC compounds. In this
paper, we have prepared calix[4]arene-appended Schiff-base
using a reported method.34 The 4-(30,40-di-n-alkoxybenzylidene)
aminophenol (4a–4d) was prepared by treating of 3,4-di-n-
alkoxybenzaldehyde (3a–3d) and 4-aminophenol in ethanol in
the presence of a few drops of glacial acetic acid.35 The 1H NMR
spectrum shows a singlet peak of –CHQN (d = 8.18 ppm,
–CHQN). In the FT-IR spectrum, the peak found at 1626 cmꢁ1
signifies the presence of an imine linking group (CHQN
stretch). The final target compounds (5a–5d) were prepared
ester derivatives and studied their mesomorphic, electroche-
mical and photophysical properties.
by using EDCꢀHCl and DMAP as catalysts in DCM solvent for
24 hours.36 The resultant crude residue was purified using
column chromatography on silica gel with methanol : chloro-
form (1 : 4) as the eluent. The tetra substitution on the lower rim
2. Experimental and instrumentation
2.1 Materials
of p-tert-butyl-calix[4]arene is evidenced by the absence of –OH
4-tert-Butylphenol, 3,4-hydroxybenzaldehyde, 4-aminophenol, groups in the 1H NMR and FT-IR studies. In the ESI,† we show
anhydrous K2CO3, and alkyl bromides (R-Br) were purchased the synthesis and characterisation part of the final target
1
from S. R. L., Mumbai. 1-Bromopropionic acid, 1-ethyl-3- derivatives: 5a, 5b, 5c and 5d. From the H NMR spectra, the
(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCꢀHCl), cone shapes of the compounds were confirmed by the presence
4-dimethylaminopyridine (DMAP) and formaldehyde were pur- of two singlets at 3.28 ppm and 4.23 ppm, respectively. All the
chased from Merck. Solvents were dried and purified in the analytical results are well supported by the corresponding
usual manner.
structures of the final target compounds (5a–5d) and can be
found in the ESI.†
2.2 Measurements
3.2 DSC analysis
The FT-IR spectra were recorded using KBr pellets on a Shimadzu
FT-IR spectrophotometer in the range of 3800–600 cmꢁ1. The H The mesomorphic behaviours of compounds 5a, 5b, 5c and 5d
1
and 13C NMR spectra were obtained on a Bruker Advance 400 MHz were preliminarily investigated by differential scanning calorimetry
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Scheme 1 (i) HCHO, NaOH, microwave, 15 min; (ii) Br–(CH2)n–COOH, (n = 2), anhydrous K2CO3, dry acetone; (iii) R-Br, anhydrous K2CO3, dry acetone
(R = CnH2n+1, n = 4, 8, 12, 14); (iv) 4-aminophenol, EtOH, few drops of glacial acetic acid (as a catalyst); (v) EDCꢀHCl, DMAP, DCM, 24 h stirring at room
temperature.
(DSC). The transition temperatures and the associated enthalpy the thermal behaviour of these synthesised samples in the heating
changes obtained from the DSC scans of the second heating– cycle. All of the synthesised compounds exhibit three endothermic
cooling cycles are shown in Table 1 and Fig. 1. Fig. 2 summarises peaks on heating and two exothermic peaks on cooling.
Table 1 Phase transition temperatures (1C)a and corresponding enthalpies (kJ molꢁ1) of synthesised compounds
Phase sequence
Compound
Heating
Cooling
5a
5b
5c
5d
Cr 25.3 (62.2) Cr1 89.8 (73.2) Colh 146.4 (34.2) I
Cr 23.2 (51.2) Cr1 86.2 (73.4) Colh 138.9 (30.6) I
Cr 18.9 (67.2) Cr1 70.6 (86.9) Colh 126.2 (24.3) I
Cr 13.6 (40.1) Cr1 62.5 (68.2) Colh 120.7 (44.8) I
I 143.6 (31.3) Colh 90.6 (57.1) Cr
I 141.1 (19.7) Colh 81.4 (53.5) Cr
I 127.8 (34.2) Colh 76.1 (46.4) Cr
I 122.9 (30.6) Colh 60.9 (41.6) Cr
a
Peak temperatures in the DSC thermograms obtained during the second heating and cooling cycles at 10 1C minꢁ1
.
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Fig. 1 The DSC traces of compounds 5a (a), 5b (b), 5c (c) and 5d (d), on second heating and cooling (scan rate 10 1C minꢁ1).
Fig. 2 Bar graphs showing the thermal behaviour of compounds 5a–5d on (a) heating cycle and (b) cooling cycle.
Compound 5a with a butyloxy group shows three endothermic suggest that the lower alkyl chain substitution on the lower
peaks corresponding to crystal-to-crystal (Cr–Cr1), crystal-to- rim of calix[4]arene is more favourable for increasing the phase
mesophase (Cr1–Colh) and mesophase-to-isotropic liquid transition temperature and further enhancing the scope of the
(Colh–I) transitions. The observed mesophase spans over a mesophase temperature. The results obtained from the DSC
good thermal range of 64.5 degrees with an isotropic tempera- study were further confirmed by POM and XRD analysis.
ture of 146.4 1C. The isotropic liquid on cooling showed the
3.3 POM investigation
commencement of a columnar mesophase at 90.6 1C, which
was further confirmed by POM analysis. Further, three The transition temperatures of the newly synthesised compounds
endothermic peaks appear at 23.2 1C, 86.2 1C and 138.9 1C were further investigated using polarising optical microscopy
for compound 5b, 18.9 1C, 70.6 1C and 126.2 1C for compound (POM) by observing the texture when a sample was sandwiched
5c, and 13.6 1C, 62.5 1C and 120.7 1C for compound 5d on between a glass slide and a cover slip. All of the synthesised
heating. Two exothermic peaks appear at 81.4 1C and 141.1 1C materials under crossed polarisers display a texture pattern of a
for compound 5b, 76.1 1C and 127.8 1C for compound 5c, and columnar-type mesophase. It is interesting to note that the
60.9 1C and 122.9 1C for compound 5d upon cooling. It can be texture pattern images show a hexagonal columnar phase
seen that the compounds 5a–5c show a higher temperature (Colh), which is commonly observed for columnar-type meso-
range for the mesophase compared to compound 5d, which is phase materials. The observed texture patterns for compounds
substituted by a dodecyloxy tail group. These phenomena 5a, 5b, 5c and 5d are shown in Fig. 3. Compounds 5a, 5b, 5c
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Fig. 4 XRD patterns recorded for compound 5a at 88.0 1C (a) and 5c at
74 1C (b) on cooling from isotropic temperature.
the presence of [100], [110] and [200] planes of a hexagonal
lattice.20 There are two diffuse peaks at 19.81 and 23.71 for
compound 5a, 18.91 and 22.61 for compound 5b, 19.41 and
21.11 for compound 5c, and 19.81 and 21.31 for compound 5d,
with observed d-spacing values in the range of 3.81–4.63 Å. In
the wide-angle region, the first diffuse reflection appears,
which corresponds to the ordering of bridging alkyl chains in
alkoxy side groups (–OR), while the second diffused reflection
favours the ordered packing arrangement of the hexagonal
phase. A literature survey indicated that similar XRD traces
have been reported for calixarene-based LCs on the lower and
upper rim side.21–23 The estimated molecular lengths of
compounds 5a and 5c are approximately 29.32 Å and 32.17 Å,
which almost correspond to the observed molecular lengths.
The proposed mechanism of the synthesised compounds
indicates the prearrangement of the self-assembly process for
the novel basket-shaped supramolecular Schiff-base calixarene
derivatives, which arrange to form columnar hexagonal (Colh)
mesophases in compounds 5a–5d, as shown in Fig. 5.
Fig. 3 Photomicrographs of 5a at 93.1 1C (a), 5b at 84.2 1C (b), 5c at
76.4 1C (c) and 5d at 66.6 1C (d) on heating from the solid crystalline state
as seen under cross polarisers.
and 5d show similar texture patterns of their hexagonal colum-
nar phase (Colh) at 88.4 1C, 84.2 1C, 72.4 1C and 66.6 1C and
become isotropic liquids at 148.7 1C, 144.2 1C, 129.6 1C and
124.1 1C on heating. Moreover, one can see that compound 5b
shows a higher mesophase temperature range compared to the
other synthesised derivatives. This result indicates that the meso-
phase temperature range is largely dependent on the substitution
of lower alkyl chains in the synthesised compounds. On cooling,
the same textural pattern of the Colh phases was observed before
the formation of solid crystals at lower temperature. Thus, an
enantiotropic mesomorphic behaviour over a broad thermal range
of the mesophase was seen for the synthesised basket-shaped
supramolecular calixarene derivatives.
3.5 Photophysical behaviour
p-tert-Butyl-calix[4]arene, with various groups and possible
p-conjugation between the long alkoxy peripheral side chains
and central rigid core, exhibits promising photophysical prop-
3.4 XRD analysis
The liquid crystalline behaviour of two supramolecular erties, which are tabulated in Table 2. The absorption and
calixarene-based LCs (5a and 5c) was further confirmed by fluorescence spectra of the compounds 5a–5d were recorded in
X-ray scattering studies of unoriented samples filled in Linde-
mann capillaries. Powder XRD measurements were performed
at the transition temperatures observed using POM to confirm
the mesophases exhibited by the compounds 5a–5d. One can
see that all four derivatives showed three reflections in the
small-angle region and another two diffuse peaks at the high-
angle region (Fig. 4 and Fig. S1, ESI†). The XRD pattern of
compound 5a at 88.0 1C shows three reflections at 3.21, 5.31 and
6.71 in the small-angle region and the observed d-spacing
values are 27.59 Å, 16.67 Å and 13.20 Å, respectively. Similarly,
compound 5c exhibits three reflections at 2.81, 5.81 and 7.11
and the observed d-spacing values are 31.53 Å, 15.24 Å and
12.45 Å, respectively. There are three reflections at 3.11, 5.11 and
6.51 for compound 5b, and 2.41, 5.011 and 6.31 for compound
5d in the small-angle region, and the observed d-spacing values
are 28.48 Å, 17.34 Å and 13.60 Å for compound 5b and 36.78 Å,
17.63 Å and 14.03 Å for compound 5d, respectively. The ratio of
these present reflections agrees with 1 : 1/2 : 1/O3, suggesting
Fig. 5 Schematic representations of the columnar hexagonal molecular
arrangements (5a).
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Table 2 Photophysical properties of the supramolecular mesogens recorded in THF (5 mM) solution
Absorptiona
(nm)
Emissiona
(nm)
Stokes shift
(nm)
Quantum
yieldc
Absorptionb
(nm)
Emissionb
(nm)
Quantum
yieldb
Comp.
(e/106 L molꢁ1 cmꢁ1
)
5a
5b
5c
5d
356
354
350
348
446
441
438
432
90
87
88
84
0.57
0.67
0.58
0.71
19.6
22.8
23.4
25.6
358
361
366
368
441
443
447
450
0.51
0.62
0.49
0.63
a
Micromolar solution in THF, excited at the respective absorption maxima. b Emission obtained by exciting the thin films at their respective absorption
c
maxima in solution. Relative quantum yield calculated with respect to quinine sulphite solution in 0.1 M H2SO4 with a quantum yield of 0.54.
compound 5d. A fluorescence maximum was observed at
around 446 nm for compound 5a, 441 nm for compound 5b,
438 nm for compound 5c and 432 nm for compound 5d. The
highly delocalised electronic systems exhibit p–p* and n–p*
transitions from the central core to the peripheral side chains
(–OR). The molar absorption coefficient was found to be in the
range of e = 19.6–25.6 ꢂ 106 L molꢁ1 cmꢁ1 for the synthesised
basket-shaped materials. The absorbance and fluorescence
wavelengths cannot be affected by changing the side alkoxy
chain on the lower rim of the calixarene core, but can be
affected by the intensity of the absorbance and fluorescence
of the parent compounds (5a–5d). As can be seen, all derivatives
showed sky-blue fluorescence in solution as well as in a solid
thin film. The Stokes shifts for all four derivatives are in the
desirable range of 84–90 nm. The appearance of luminescence
at a higher wavelength than the absorption wavelength is in
good agreement with the common explanations. It can be
observed that the various substitutions on the calixarene core
only at the peripheral side could slightly affect their Stokes
shift. The emission spectra showed one stable maximum band
Fig. 6 (a) Absorption and (b) fluorescence spectra of compound 5a in
different solvents. (c) Absorption and (d) fluorescence spectra of com-
in the range of 432–446 nm, which is recognised as the
electronic transition from lowest unoccupied molecular orbital
to the highest occupied molecular orbital. The quantum yield
pounds 5a–5d in THF solution (5 mM).
represents the percentage photon emission and is important
for assessing fluorescent molecules. Relative quantum yields of
anhydrous THF, and are shown in Fig. 6 and 7. Solvent screenings
the compounds (5a–5d) were measured with respect to quinine
of compound 5a were performed in various solvents, including
sulphate 0.1 N sulfuric acid solution with a quantum yield
THF, DCM, chloroform, MeOH and DMF, revealing changes in
of 0.54, and was found to be in the range of 0.57–0.71 (Table 2).
its optical properties (Fig. 6a and b) which further confirm the
proposed intramolecular charge-transfer mechanism.37 Upon
Thin films of the supramolecules were produced by drop-casting on
glass slides. The absorption and emission wavelengths of the thin
irradiating the solution, compound 5a exhibits an absorption
films are slightly higher than the solution spectra and show red-
maximum centred at 356 nm, while the values are 354 nm for
shifting, due to the presence of aggregation and intermolecular
compound 5b, 350 nm for compound 5c and 348 nm for
interactions.38
3.6 TGA analysis
The thermal stability of compounds 5a–5d was further studied
by thermogravimetric analysis (TGA), as shown in Fig. 8. The
TGA of compounds 5a and 5b shows only 6% weight loss from
160 1C to 240 1C. The TGA of compound 5c and 5d shows 8%
weight loss from 180 1C to 300 1C, which demonstrates the
good thermal stability of the Schiff-base-moulded calixarene
compounds. One can see that there is no observed weight
loss up to 150 1C, which clearly indicates that there is no water
or other solvent captured in the mesomorphic state. All the
derivatives exhibit good thermal stability. The thermal stability
Fig. 7 (a) Absorption and (b) fluorescence spectra of compounds 5a–5d
in solid thin film state.
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Table 3 Electrochemical properties of compounds 5a–5d
Comp.
Eoxd
Ered
EHOMO
ELUMO
DEg, CV
EA
PI
5a
5b
5c
5d
1.87
1.91
1.93
1.92
ꢁ1.59
ꢁ1.62
ꢁ1.64
ꢁ1.67
ꢁ6.10
ꢁ6.14
ꢁ6.16
ꢁ6.15
ꢁ2.64
ꢁ2.61
ꢁ2.59
ꢁ2.56
3.46
3.53
3.57
3.56
6.10
6.14
6.16
6.15
2.64
2.61
2.59
2.56
Experimental conditions: reference electrode (Ag/AgNO3), counter
electrode (platinum electrode), working electrode (glassy carbon), sup-
porting electrolyte (0.1 M TBAP).
out cyclic voltammetry experiments in THF solvent. For organic
semiconductor materials, the highest occupied molecular
orbital (HOMO) indicates the energy required to extract an
electron from the molecules via oxidation and the lowest
occupied molecular orbital (LUMO) means the energy required
to inject an electron into the molecule via reduction. The cyclic
voltammograms of compounds 5a–5d look similar, with
oxidation and reduction wave potential peaks (Fig. 9). The
observed values of the HOMO, LUMO, energy levels, electron
affinity (EA), ionisation potential (PI) and calculated energy
band gaps are tabulated in Table 3. In the present experiment,
an Ag/AgNO3 (0.1 M) reference electrode, platinum rod counter
electrode and glassy carbon working electrode were used in a
single-compartment cell. The reference electrode in this experi-
ment was calibrated with a ferrocene/ferrocenium redox couple
with an absolute energy level of ꢁ4.80 eV.39,40 In this experi-
Fig. 8 Thermal gravimetric analysis of compound 5a–5d.
of the lower chain substituted materials (5a and 5b) showed
slightly lower thermal stability as compared to the higher chain
substituted materials (5c and 5d). This is due to the presence of
periphery higher alkyl side groups, which stabilised the meso-
phase more than the lower alkyl side groups. All the derivatives
degrade completely at or above 300 1C.
3.7 Electrochemical study
Cyclic voltammetry (CV) is a potentiodynamic electrochemical ment, the solution of the supporting electrolyte is THF contain-
method which provides information about the electrochemical ing a 0.1 M solution of tetra-butylammonium perchlorate
properties of molecules in solution. In CV studies, the observed (TBAP). The scanning rate for recording the cyclic voltammo-
onset of reduction and oxidation potential values estimate grams was 0.05 mV sꢁ1. The lower alkyl chain substituted
the HOMO and LUMO values and further provide information Schiff-base-moulded supramolecular derivatives (5a and 5b)
regarding electron band gap (Eg) of the molecules. The synthe- showed a lower band gap value as compared to higher alkoxy
sised Schiff-base-moulded supramolecular calixarene derivatives side chain substituted derivatives (5c and 5d), which clearly
were investigated for their electrochemical behaviour by carrying indicates the higher reactivity of the lower chain substituted
Fig. 9 Cyclic voltammograms of (a) 5a, (b) 5b, (c) 5c, and (d) 5d in anhydrous THF solution of TBAP (0.1 M) at a scanning rate of 0.5 mV sꢁ1
.
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Fig. 10 Optimised geometries, HOMO and LUMO energy levels of compound 5a (a–c), and compound 5b (d–f), obtained by DFT calculations at the
B3LYP/31G (dp) level.
calixarene materials. The LUMO and HOMO energy levels were protons in different environments, such as axial and equatorial
calculated by using the formula ELUMO = ꢁ(4.8 ꢁ E1/2, Fc, Fc+ + Ered
,
positions. In addition, the spectra show two singlets (1 : 1) for
onset) eV and EHOMO = ꢁ(4.8 ꢁ E1/2, Fc, Fc+ + Eoxd, onset) eV, aromatic hydrogen, which confirm the presence of a cone
respectively. All the synthesised materials (5a–5d) show LUMO conformation of the p-tert-butyl-calix[4]arene unit functiona-
energy levels of ꢁ2.64 eV, ꢁ2.61 eV, ꢁ2.59 eV and ꢁ2.56 eV, lised at the lower side. The symmetries of non-equivalent
while the HOMO energy levels are ꢁ6.10 eV, ꢁ6.14 eV, ꢁ6.16 eV protons of the methylene groups in the cone conformation
and ꢁ6.15 eV. All four derivatives (5a–5d) showed band gaps are called axial and equatorial by their similarity to the nomen-
(Eg) of 3.46 eV, 3.53 eV, 3.57 eV and 3.56 eV, demonstrating clature commonly used for protons. The peaks for –CQO and
their good reactivity. A lower band gap is more favourable –OCH2 at nearly 158–160 and 72–74 ppm in the 13C NMR
for optoelectronic device applications. The band gap values spectrum confirm the cone shape of the synthesised
obtained from CV are in good agreement with the results from materials.42 The optimised geometries of the target Schiff-
the DFT study. Further, out of all of the synthesised com- base-moulded calixarene derivatives (5a and 5b), along with
pounds, compound 5a was found to be more reactive because their HOMO and LUMO energy levels, were evaluated using
of the lower HOMO–LUMO energy gap, while compound 5c is Density Functional Theory (DFT) calculations at the B3LYP/
more stable with its slightly higher band gap value.
6-31G(dp) level, as depicted in Fig. 10a–f. The optimised
structures of all four derivatives clearly suggest that the geo-
metries of all supramolecular compounds are basically cone
shaped. The HOMO, LUMO, band gap (Eg), dipole moment and
total energies of compounds 5a–5d are tabulated in Table 4.
One can see that the total energy and dipole moment of the
lower alkyl chain substituted compounds (5a and 5b) is lower
compared with the higher chain substituted compounds
(5c and 5d). This is because of the substitution of the highly
polar peripheral alkyl chain group on the terminal side.
3.8 Cone confirmation and DFT study
The conformation of the parent p-tert-butyl-calix[4]arene core
and its functionalised derivatives were analysed in terms of
four different conformations, cone, partial cone, 1,2-alternate
and 1,3-alternate, arising from different modifications on the
upper and lower rims, respectively.41 In the present case, all of
the synthesised p-tert-butyl-calix[4]arene derivatives adopt a
1
cone conformation as confirmed using H NMR and 13C NMR
techniques. In the 1H NMR spectroscopy results, all derivatives
show two singlets (1 : 1) for a tert-butyl group substituted on the
upper side, and also one pair of doublets (1 : 1) for a methylene
bridge (–CH2) at around 3.66 ppm and 4.22 ppm, indicating
4. Conclusion
We herein reported a family of novel Schiff-base supramolecu-
lar luminescent liquid crystals based on a p-tert-butyl-calix[4]
arene core substituted with various peripheral alkoxy groups on
the terminal side chains. These molecules show sky-blue emis-
sion in solution as well as in the solid thin film state with good
quantum yield. The blue-light-emitting materials exhibited
columnar hexagonal (Colh) liquid crystals with good thermal
and photophysical properties. The Schiff-base-moulded calix-
arene self-assembled to form an ordered hexagonal columnar
phase with a good mesophase temperature range. The length
of the side alkoxy spacer had no effect on the absorption and
Table 4 Energetics and structural properties of the calixarenes
Dipole
moment
HOMO
Comp. (a.u.)
LUMO
(a.u.)
Gap (LUMO–
Total
HOMO) a.u. (eV) energy (a.u.) (Debye)
5a
5b
5c
5d
ꢁ0.28871 ꢁ0.15711 0.1316 (3.581)
ꢁ0.28865 ꢁ0.15693 0.1317 (3.583)
ꢁ0.29471 ꢁ0.16273 0.1319 (3.589)
ꢁ0.29440 ꢁ0.16229 0.1321 (3.594)
ꢁ6553.96
ꢁ6780.19
ꢁ7257.45
ꢁ8161.67
16.254
16.596
16.435
17.012
1 a.u. = 27.2116 eV = 627.509 kcal molꢁ1
.
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Paper
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1
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
The authors acknowledge the Department of Chemistry, Faculty of
Basic and Applied Science, Madhav University, Sirohi, Rajasthan for
providing the lab facility. The authors are thankful to NFDD Centre,
IIT Mandi for providing analytical and spectral services. The
authors are also thankful to Mr Vinod Vishwakarma, and
Dr A. S. Achalkumar (Department of Chemistry, Indian Institute
of Technology, Guwahati, Assam) for providing DFT facility.
The authors thank Mr Deepak Suthar (Department of Physics,
Mohanlal Sukhadia University) for his fruitful discussions on
the high-temperature XRD analysis.
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