New Class of Supramolecular Bowl-Shaped Columnar Mesogens Derived from Thiacalix[4]arene Exhibiting Gelation and Organic Light-Emitting Diodes Applications

A new class of blue light-emitting bowl-shaped mesogens with the thiacalix[4]arene core appended with 1,3,4-thiadiazole derivatives having peripheral alkoxy side chains have been synthesized and well characterized. The liquid crystalline behavior of present synthesized derivatives was examined by optical polarizing microscopy, differential scanning calorimetry, and X-ray diffraction studies. It was observed that these thiacalix[4]arene derivatives were capable of stabilizing the observed Colh phase with a higher temperature range. The cone-shaped thiacalix[4]arene-based liquid crystals with peripheral alkoxy side chains able to pack into the columns with enriched intermolecular interactions and thermal behavior. All derivatives showed blue luminescence in solution, solid thin-film, and gelation state. The hexagonal columnar phase and emissive nature of thiadiazole-based thiacalixarene compounds having xerogel behavior make them favorable in the application of emissive electronic display devices. The electrochemical properties of these thiacalixarene-based compounds demonstrate the effect of alkyl side chain on the highest occupied molecular orbital–lowest unoccupied molecular orbital energy levels and also exhibited lower electron band gaps. The electroluminescence behavior of the compound 10c was examined as emissive layers in the fabrication of organic light-emitting diodes.


■ INTRODUCTION
In recent years, the liquid crystal of self-organizing molecular systems with multifunctional properties is one of the most attractive and active fields of current research. Liquid crystals with inherent properties like fluidity and stability have been already used in several commercial applications. 1 There are several types of liquid crystalline compounds, one of the most fruitful compound is the supramolecular columnar liquid crystal. Supramolecular assembly composed of disc-shaped aromatic molecules has gained significant attention, as it is potentially viable to the formation of columnar-type structures with the presence of Π−Π stacking interactions. 2 Presently, the demand of organic semiconductors with high mobility and strong luminescence has grown manifolds as they facilitate in the development of various electronic display applications such as organic light-emitting diodes (OLEDs), organic lightemitting transistors (OLETs), and organic lasers. 3,4 OLEDs are monolithic solid-state devices that typically consist a series of organic thin films sandwiched between two thin-film conductive electrodes which display a substantial part in the growth of new flat-panel displays. 5,6 Small OLED panels are used for the displays of mobile phones, while large OLED panels are popular in displays of television, mirror display, transparent display, and signage. 7−9 Nowadays, it is accepted that OLEDs will become the governing technology in the display market. For the fabrication of OLED products, three phosphorescent emitters are required, red, green, and blue emitters. 10−12 Literature reports several materials that emit red and green light; however, those with blue light emission are rare. Additionally, the lifetime of green and red phosphorescent devices is higher as compared to the blue fluorescent device. Nowadays, among these three light-emitting phosphorescent compounds, blue light-emitting compounds are getting much more attention because of their indispensable requirement for the fabrication of white light-emitting diodes and other devices, respectively. 13−17 In addition, columnar hexagonal liquid crystals that are formed by the packing of central benzene cores one above the other with substitution of the peripheral side chain which behave as molecular wires that help to stabilize the Col h phase in one-dimension. 18 −21 From the previous study, there are various kinds of liquid crystalline compounds based on 1,3,4-oxadiazole which were reported in the literature. Basically, 1,3,4-oxadiazoles are one of the class of heterocyclic compounds which are known for their many advantages like high-fluorescence quantum yield, thermal stability, luminescence efficiency, hydrolytic stability, electrontransporting behavior. Therefore, they found more applications in the fabrication of n-type electroluminescent layers in OLEDs. 22−24 In addition to the number of advantages of oxadiazole core-based liquid crystals, there are certain drawbacks like narrow temperature range, lower solubility, and higher melting and clearing temperatures which limit their applications in various fields. 25 In present investigation, we have focus on 1,3,4-thiadizole derivatives which have similarities with 1,3,4-oxadiazole expect hetero atoms in their core structure. The oxygen atom in oxadiazole is interchanged by sulfur atoms which enhance its properties like higher melting and clearing temperatures, dipole moments, higher viscosity, and mesophase temperature range. 26 In the literature, different shapes of fluorescent liquid crystals based on 1,3,4thiadiazole are reported till date but very few with polycatenar LCs, star-shaped LCs, and supramolecular LCs. 27−33 In the present study, we report the OLED device performance by using newly synthesized supramolecular columnar hexagonal mesogens based on thiacalix [4]arene derivatives substituted with 1,3,4-thidiazoles derivatives. Its photophysical behavior suggests that the grating of thiadiazole derivatives on two sides of the thiacalix [4]arene core, which having a sulfur-rich environment can help to disperse the emitting nature of thiadiazole derivatives in the thiacalixarene core, respectively.
Calixarenes are the class of cyclic oligomers in supramolecular chemistry formed via condensation of phenol and aldehydes which currently belongs to the part of third generation after crown ethers and cyclodextrins. 34 The first liquid crystalline compounds based on the calixarene core were reported by Dalcanale et al. 35 In recent years, calixarenes have been successfully introduced as a central tetra cyclic core with hydrophobic and hydrophilic sides to design different types of mesogens. 36,37 Menon et al. introduced an aliphatic side chain inbuilt with different linking groups on the calixarene core. 38,39 Yang and his co-workers reported various calixarene-based liquid crystalline materials with different temperature range and thermal stability. 40−44 Among them, our research group also reported various calix [4]arene-based hexagonal columnar LCs with good photophysical behavior. 45,46 Marcos et al. reported calix [4]arene-based mesogens inbuilt with the Schiff base linking unit. 47 In contrast, new functions remain to be developed through the substitution of bridge methylene groups by hetero atoms. 48 Thiacalix [4]arene, the new member of the calixarene family has now become a robust scaffold in supramolecular chemistry and material science. 49 They possess many fascinating features such as large cavity size, binding ability toward anion, cation and transition metals with the presence of larger cavity, and also the possibility of multiple chemical modifications on their morphology. Different functionalizations on the thiacalixarene core have been extensively used in different applications like mimicking of molecular logic gates, separating biological important citations, display, and photoactive emissive compounds. 50−52 Supramolecular gels from Π-conjugated low-molecular weight gelators have attracted intense interest because of their advantages to create various superstructures, diversity, and forming self-assembly. 53 Specifically, low-molecular Πconjugated gelators based on macrocyclic structures into the oligomers like crown ethers, cyclodextrin, and calixarene effectively prevents the close packing of the molecules. 54 Among various macrocyclic compounds, calix [4]arenes are of more interest because of their ability to be functionalized on lower and upper rim, Π−Π interactions, and formation of selfassembly. From the literature, it can be noted that thiacalix [4]arene-based compounds which can act as a gelators are rarely reported. 55 Formation of gels via H-bonding consisting of the functional groups like amide, aroyl hydrozone, peptide, and sugars is well reported. 56,57 An extensive literature survey that revealed no supramolecular thiacalix [4]arene-based columnar mesogens with blue luminescence and gelation properties have been reported. In present investigation, we report four new cone-or bowl-shaped luminescent supramolecular LCs based on thiacalix [4]arene and 1,3,4-thiadiazole Schiff base group inbuilt with a variable peripheral alkyl chain to determine their structure−property-relationship. Furthermore, their electrochemical, density functional theory (DFT), and photophysical behaviors were implemented to shed light on their electronic molecular structure. Further, the bowl-shaped di-substituted thiadiazole thiacalix [4]arene-based columnar liquid crystalline derivatives (10c, 10d) also showed gelation properties in nonpolar solvents which are formed by Π−Π interactions and also the presence of H-bonding.

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Article Further, compound 2 is dissolved in pyridine and reacted with 3,4,5-trimethoxy benzoyl chloride to achieve compound 3. The obtained compound 3 is further reacted with phosphorous pentasulfide (P 2 S 5 ) to give the thiadiazole derivative (4). 46 On further heating to compound 4 in the presence of anhy. AlCl 3 in benzene resulted in compound 5. 46 Compound 5 was coupled with different alkyl bromides to get compound 6a− 6c. 46 Further, the oxidation of compound (6a−6d) was carried out with chromyl chloride in the presence of pyridine and dichloromethane (DCM) to form compounds (7a−7d). 46 The Schiff-base derivatives (8a−8d) were formed by the reaction of compound (7a−7d) with 2,4-dihydroxy aniline. 47 Basecatalyzed condensation of 4-tert-butyl phenol and sulphur powder (S 8 ) in the presence of tetraethylene glycol dimethyl ether in a single step to obtain 4-tert-butyl thiacalixarene (9) is depicted in Scheme 2. 58 Finally, the target bi-substituted supramolecular derivatives (10a−10d) were synthesized by the reaction of Schiff-base thiadiazole intermediates (8a−8d) with bis-alkyl bromide thiacalix [4]arene (9) in dimethylformamide (DMF). 59 The obtained final solid derivatives were further purified through column chromatography with chloroform and methanol as eluent in the ratio of 4:1. Molecular structural characterizations of intermediates and final target compounds were carried out by using 1 H NMR, 13 C NMR, IR, and ESI-HRMS which are presented in the Supporting Information (see in Supporting Information). From the 1 H NMR, compounds 10a−10d exhibited a pair of singlets for the −C(CH 3 ) 3 group on the thiacalixarene core with the presence of two singlets for aromatic hydrogen indicates the substitution of thiadiazole Schiff-base derivatives on lower rim to form stable cone confirmation that displays columnar hexagonal liquid crystalline properties. 48,60 Literature reports confirm the substitution of thiadiazole on two sides of thiacalixarenefavored cone confirmation. 48 Additionally, the difference of the observed two singlets of the aromatic proton in final target compounds is nearly 0.37 ppm, which again confirms the cone confirmation of synthesized supramolecular derivatives. 49−52 TGA Analysis. Thermogravimetric analyses (TGA) of the compounds (10a−10d) were performed to verify thermal stability under the nitrogen atmosphere. The TGA curves for all four derivatives show similar decomposition behavior ( Figure S1, see in Supporting Information). All the compounds (10a−10d) displayed thermal stability up to ≈291°C with decomposition temperatures above 480°C. Additionally, there was no loss of mass up to 180°C suggesting the absence of water or any additional solvent molecules trapped during the exposition of the mesophase. Moreover, the degradation of the prepared thiacalixarene-based derivatives is in the range of 240−310°C, which indicates its good thermal stability behavior.
DSC Analysis. The phase transition temperature of synthesized compounds 10a−10d was preliminarily studied by the differential scanning calorimetry (DSC) technique. The observed transition temperature range of target compounds was investigated by DSC on first heating and cooling rates of 10°C ( Figure 1). The corresponding data of transition temperature and enthalpy change were summarized in Table 1. Upon heating and cooling conditions, all four derivatives exhibited two endothermic peaks on heating and cooling conditions corresponding to crystal to columnar hexagonal and columnar hexagonal to isotropic phase ( Figure S2). Compound 10a with lower alkyl chain showed two endothermic peaks at 101.2 and 197.6°C on heating conditions. On cooling, they traced two endothermic peaks at 198. 8    which is nearer to the observed phase transition on heating condition. Upon heating, compound 10d exhibited first transition peak at 65.2°C and second endothermic peak at 141.2°C, and on cooling condition the two endothermic peaks located at 66.8 and 143.6°C, respectively. It is observed that compound 10a displays higher temperature range of the mesophase as compared to other derivatives. The changes observed in phase transition temperature of the mesophase due to the substitution of the variable peripheral alkyl chain in which the methylene group is linked to each other leads to change in its properties like flexibility, molecular length, dipole moment, adhesive forces, and polarity, respectively. 53,54 POM Study. The phase transition behaviors of supramolecular LCs based on the thiacalixarene core appended with thiadiazole Schiff base derivatives were investigated by polarizing optical microscopy (POM) upon heating and cooling conditions. From Figure S3, it is apparent that there was no significant difference in the observed texture characteristic of compound (10a−10d). The defect texture characteristic in all four derivatives confirms the presence of the columnar hexagonal mesophase. All the thiadiazole-based thiacalix [4]arene compounds (10a−10d) showed lower transition and clearing temperatures. The mesophase temperature ranges for supramolecular compounds (10a−10d) were observed at 101.3, 77.8, 73.1, and 76.9°C, respectively. Thus, it is clear that the presence of the sulfur atom in the thiadiazole and thiacalix [4]arene core increases the lateral dipole moment that causes S···S interactions and is responsible for the higher mesophase temperature range. 48 The substitution of short peripheral alkyl chain length exhibited a higher mesophase range as compared to other synthesized derivatives. The phase transition temperatures for all of the compounds (10a−10d) were observed at 102.8, 94.8, 75.2, and 61.6°C upon heating and on cooling the similar textural pattern obtained for the columnar phase was noticed at 101.6, 93.6, 73.2, and 62.7°C, respectively. One can see that, with an increase in the alkoxy side chain, the temperature range of the phase transition decreased and the probability of stereo heterogeneity increased, which affect its flexibility and weakens the core− core interaction of the self-assembly of thiacalix [4]arene-based thiadiazole derivatives. Consequently, an enantiotropic columnar LCs perceived a good temperature range for the prepared thiacalix [4]arene derivatives.
XRD Analysis. The liquid crystalline properties of compounds (10a−10d) were further explored by using X-ray diffraction (XRD) examination. All supramolecular derivatives were examined for their liquid crystalline properties at their transition temperature observed in POM and DSC analysis. The sample was melted and filled in the Lindemann capillary tube as an isotropic state and cooled to their mesomorphic state and scanned for X-ray studies. The X-ray profile of oxadiazole-and thiadiazole-based thiacalix [4]arene showed three reflection peaks in the lower angle region and two other reflections peaks in the higher angle region shown in Figure  S4a−d. The X-ray traces of compound 10a at 104.0°C displayed reflections at 2.62°, 6.20°, 7.62°, 19.92°, and 22.61°i n small angle and wide angle regions. In the lower angle region, compound 10b showed three reflections at 2.46°, 6.26°, and 7.64°, compound 10c at 2.18°, 6.13°, and 7.69°, and compound 10d at 2.11°, 5.62°, and 7.71°while in the wide angle region compound 10b showed two reflections at 19 planes. In the wide angle region, two small diffuse peaks are found which are corresponding to the packing of alkoxy side chain shaking of core−core interactions within the column in the self-assembly. From the literature, similar XRD data were reported for various columnar liquid crystals based the on calixarene core. 40−44 The estimated length of the target molecules is closer to the observed length of target compounds. Noticeably, the presence of a broad peak wide angle region indicates definite order in the peripheral alkoxy-trisubstituted alkyl chain-substituted thiadiazole derivatives inbuilt with the thiacalix [4]arene core. The possible arrangement of supramolecular compounds to form the columnar hexagonal shape is depicted in Figure 2. Based on the POM, DSC, and XRD analysis, it can be noted that all the supramolecular two side-substituted thiadiazole derivatives adopted a stable hexagonal columnar phase.
Photophysical Behaviors. We have investigated the photophysical behavior of compounds (10a−10d) in solution and solid thin films. The absorption and fluorescence spectra of compounds 10a and 10d were measured in different solvents as given in Figures S5 and S6, respectively. It can be noted that all derivatives showed the intensities of absorbance and fluorescence higher in tetrahydrofuran (THF) as compared to other organic solvents. The moderate behavior of tetrahydro furan can dissolve a wide range of polar and nonpolar compounds. Thus, the solutions of all bowl-shaped

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Article thiacalix [4]arene derivatives were made in THF to measure the absorption and fluorescent properties. Compounds 10a− 10d revealed the same absorption band at 448, 451, 454, and 451 nm in THF, respectively. The corresponding maximum emission was observed at 459, 461, 464, and 478 nm as shown in Figure 3a,b. Changes in fluorescence intensity and absorption peaks were dependent on the solvent polarity. 61 The highly delocalized systems with the presence of Π−Π* transitions and also the presence of sulfur environment of the thiacalixarene core showed higher absorption coefficient (ε = 24.9 to 26.3 × 10 6 L mol −1 cm −1 ) ( Table 2). The solid thin film of compounds 10a−10d presented higher absorption at 451, 454, 459, and 462 nm and higher emission at 467, 464, 476, and 498 nm as mentioned in Figure 3c,d. The intramolecular charge-transfer efficiency of synthesized compounds 10c−10d was higher as compared to compounds 10a− 10b suggesting that the higher alkyl chain substitution with the electron-donating oxygen atom on the phenyl ring and conjugation with aromatic rings inbuilt with thiadiazole derivatives on the lower rim of thiacalixarene. All the synthesized derivatives (10a−10d) showed dark blue fluorescence in long UV (Figure 3b). The material emitting blue light are not only limited in blue color but also their energy levels become very high and they transport an effective fine-tuning emissive wavelength by combining with an additional dopant emitter to fabricate OLEDs. 62−64 All the compounds showed a stable emission band-centered maxima at 459−478 nm. The present supramolecular thiadiazole Schiff-base thiacalix [4]arene compounds demonstrated red shift in the absorption and emission maxima. It has been observed that the variable alkyl chain on the terminal phenyl ring had no effect on the influence of absorption and emission spectra. The quantum yields of synthesized bowl-shaped columnar liquid crystalline compounds in solution and solid thin-film state were observed in the range of 0.28−0.57 (Table  2). Furthermore, all the compounds in the solid thin-film state showed lower quantum yield as compared to the solution state, this is due to the presence of aggregation in the solid state. The presence of trisubstituted alkyl chain length decreased the aggregation-caused quenching effect and enhanced the fluorescence intensity. 65 Electrochemical Behavior. Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement generally used to study the electrochemical properties of the molecule that adsorbed onto the electrode. All the synthesized thiacalixarene-based compounds were examined for their electrochemical behavior by carrying out CV studies in anhydrous micromolar THF. Cyclic voltammograms of compounds 10a and 10b with computationally observed highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are mentioned in Figures 4 and S7, respectively. The calculated band gaps, energy level with oxidation, and reduction potential are mentioned in Table S1.
The oxidation and reduction waves of all the synthesized compounds become similar in cyclic voltammograms. The auxiliary electrode used in the present study is 0.1 M solution of tetrabutylammonium perchlorate in deoxygenated THF equipped with Ag/AgNO3 (0.1 M) reference, platinum, and

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Article carbon working electrode. 66 From Figures 4 and S7, it is clearly observed that the compounds 10a and 10b exhibited definite irreversible waves of oxidation and reduction. The synthesized thiacalix [4]arene derivatives displayed a lower band gap, which clearly indicates the higher reactivity of the compounds. This band gap observed for thiacalix [4]arene derivatives is much less compared to previously reported compounds based on the 4-tert-butyl calix [4]arene core. 45,46 The energy levels of LUMO were calculated by E LUMO = −(4.8 − E 1/2 , Fc,Fc + + E red , onset) eV, whereas the energy levels of HOMO were calculated by E HOMO = −(4.8 − E 1/2 , Fc,Fc + + E oxd , onset) eV. Compounds (10a, 10b) with lower alkyl side spacer showed LUMO and HOMO levels as −2.71, −2.73, −5.82, and −5.80 eV, while higher alkyl chain-substituted thiacalixarene-based compounds (10c,10d) revealed the LUMO and HOMO energy levels as −2.74, −2.69, −5.81, and −5.84 eV ( Table  S1). The energy level stabilization of HOMO and LUMO has little influence by the trisubstituted peripheral alkoxy side chain on the lower rim of thiacalixarene, respectively. Among all four synthesized bowl-shaped thiacalix [4]arene liquid crystals, compound 10d becomes more stable because of the existence of a greater band gap. The calculated values of band gaps by CV were nearly supported by the theoretical values obtained from DFT. Further, the cyclic voltammograms of compounds 10c and 10d are presented in Figures S8 and S9 (Supporting Information).
Cone Conformation Study. It is identified that thiacalix [4]arene derivatives have four main analogues, namely, cone, partial cone, 1,2-alternate, and 1,3-alternate. Compared to normal calix [4]arene, the four sulfur bridges replacing methylene groups provide various new characteristics like large cavity inside the core, more flexibility, greater scope for chemical modifications, and sensor capability. 67 Thiacalixarene always adopt the cone shape because of the formation of the hydrogen bonding between the phenolic groups on the lower rim and further the crystal structure data are well supported by XRD studies. 68,69 The 1 H NMR spectra of thiacalix [4]arene in CDCl 3 suggests weaker intramolecular hydrogen bonding compared to normal 4-tert-butyl calixarene because of its enlarged framework. Further, it is difficult to distinguish exact confirmation of thiacalix [4]arene from cone and 1,3-alternate caused by the absence of methylene bridges in the thiacalix [4]arene core, respectively. All synthesized thiacalix [4]arene-based compounds showed cone conformation, which is further confirmed by the 1 H NMR technique. All derivatives exhibited two singlets (1:1) for the −C(CH 3 ) 3 group of thiacalix [4]arene and presence of two singlets (1:1) for the aromatic protons suggesting the cone conformation adopted by all derivatives. However, the 1 H NMR exhibited two other singlets (1:1) for −OCH 2 -and −CH 2 Odisubstituted groups on the lower rim nearly at 4.21 and 4.83 ppm that confirms the existence of the cone shape by all newly synthesized derivatives. 69 The geometries of 1,3,4thiadiazole thiacalix [4]arene compounds were evaluated at the DFT level (B3LYP/3-21G*), as given in Figures S10−S16 (see in Supporting Information). This evidently confirms that all the synthesized supramolecular derivatives are in cone confirmation. The calculated energies, dipole moments, and band gaps of compounds 10a−10d are given in Table 3. One can see that, the dipole moment and band gap of compound 10d is lower, which indicates its higher reactivity as compared to other derivatives (10a−10c).
Gelation Studies. The gelation properties of synthesized compounds were examined in various organic solvents at different concentrations, respectively. From the four synthesized thiacalix [4]arene derivatives, compounds 10c and 10d were heated in a glass sample vial until the solid materials were completely dissolved and further cooled at room temperature to form gel. Compounds 10c and 10d having two side thiadiazole rings with alkyl chain inbuilt with Schiff-base thiacalixarenes formed organogels in dodecane and decane (Figure 5a,b, Table S2 in Supporting Information). This gel formation was possible because of multivalent Π−Π interactions between the bowl-shaped compounds and the organic solvent. These supramolecular thiacalix [4]arene-based derivatives were soluble in nonpolar solvents but formed precipitates in polar solvents, respectively. Compounds 10c and 10d showed critical a gelation concentration (CGC) of 1.1 and 1.6 wt % in dodecane while they showed CGC concentration of 1.5 and 1.9 wt % in decane. In the present study, it is observed that compounds with lower alkoxy peripheral side chain stabilized the gel at a lower CGC value as compared to the higher substituted alkoxy side on the terminal side of the phenyl ring. To explore the aggregation behavior, thiacalix [4]arene-based LCs in dodecane to the form network gelator. The further studies of the gel were carried out by using atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). From the AFM study, it is clearly seen that the aggregated molecules form network-type structures of the nanofiber at lower concentration in dodecane as mentioned in Figure S17a,b. The morphology of the obtained xerogels was further studied by FE-SEM. Figure  S17c,d represents the SEM images of the prepared xerogel of compounds 10c and 10d, which exhibits micro and nano thin

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Article fibers to give a network structure of nanofibers. The realignment of the gelator fibers in the supramolecular structure of the bi-substituted thiacalix [4]arene derivatives in decane and dodecane clearly indicates the formation of the network structure in the organogels because of the presence of hydrogen bonding between −CHN and −OH groups at the lower part of the thiacalixarene core. The additional interactions of the sulfur bridge with nearby hydroxyl groups may thereby induce alignment of fibers.
The presence of peripheral alkoxy side chain (−OR) creates van der Waals forces in addition to the intermolecular Π−Π interaction between aromatic phenyl rings, which is the driving force for the formation of gel. 70 The gel of compounds 10c (Figure 5a) and 10d (Figure 5b) in dodecane (a 1 ,b 1 ) solvent showed stable gelation (a 2 ,b 2 ) behavior at room temperature, which further showed fluorescence (a 3 ,b 3 ) on exposure to UV light, respectively. The formation of organogels was observed within 4.0 min during the cooling process. Compounds 10c and 10d showed higher emission intensity as temperature decreases from 70°C (sol) to 18°C (gel) (Figures S18a, S19a). The emission spectra of compounds 10c and 10d were transferred from 475 to 491 and 468 to 486 nm, respectively (Figures S18b and S19b). Upon heating and cooling conditions, compounds 10c, 10d showed the gelation process in a reversible manner for number of cycles as evident from Figures S18c,d and S19c,d.
Furthermore, the structure of xerogel in compounds 10c and 10d were studied by the XRD pattern. For the XRD analysis, the thin film of compounds 10c and 10d were made by the drop-casting method on a glass slide, respectively. The XRD pattern of xerogel of compound 10c exhibited several peaks at 2.89, 5.52, 9.08, 14.09, 19.96, 22.60, 24.05, and 27.03 ( Figure  S20a). In the same way, the xerogel compound 10d displayed peaks at 2.86, 4.64, 7.18, 11.56, 13.63, 15.98, 20.37, 25.11, and 28.82 ( Figure S20b). Thiacalixarene-based xerogels showed reflections in the XRD state due to the presence of strong intermolecular interactions between the molecules. Compound 10c showed d-spacing values at 32.76, 20.26, 10.38, 7.05, 4.49, 3.77, and 3.18 Å for compound 10d, 34.06, 20.18, 10.38, 7.05, 4.69, 3.77, and 3.18 Å. The thiacalixarene-based supramolecules in the xerogel state formed a lamellar type of arrangement confirmed from the ratio of d-spacing and clearly suggest the rectangular type of arrangement of the bowlshaped thiadiazole molecules. The lattice parameters "a" and "b" of compounds 10c and 10d in the rectangular cell were found to be 65.22, 28.97, 63.72, and 28.85 Å (Tables S3, and  S4). The columnar arrangement of supramolecules in the xerogel state is supported by the birefringent texture pattern, which represents the occurrence of hierarchical self-assembly in presently synthesized bowl-shaped mesogens ( Figure S21, see in Supporting Information). These results support the organization of thiacalix [4]arene derivatives arranged in hierarchical self-assembly given in Figure 6. Here, in the present study, we have first time demonstrated thiadiazolesubstituted thiacalix [4]arene derivatives as a gelator in the absence of any metal doping to induce gelation properties on account of the H-bonding interaction.
Electroluminescence Study. The fluorescent blue lightemitting thiacalix [4]arene-based compounds were further studied for their electroluminescence (EL) performance in the OLED device. The synthesized supramolecular derivative 10c was selected to study its EL property. The fluorescence quantum yield of compound 10c was employed as the emitter with doped and nondoped solution-processed OLED devices was fabricated with simplified making of the device shown in Table S5. The organic supramolecular compounds were doped in bipolar CBP to permit host-to-guest energy transfer. Figure  S22a represents the architecture design of the device with the energy level diagram. The EL spectra with current density− voltage−luminescence characteristics of the fabricated OLED devices were mentioned in Figure S22b,c.
OLED panels are made from carbon-based organic materials that emit light when electricity is passed through them. The emitting light from doped and nondoped emissive layers of the organic compound in OLEDs may be due to relaxation of electron transfer from the LUMO state to HUMO state in the molecules, respectively. 71−73 The device with different % of CBP and neat state showed good EL performance. The fabricated device with 1 wt % displayed a power efficiency (PE) of 0.2 lm/W, luminescence of 438 cd/m 2 , current efficiency (CE) of 0.6 cd/A, and an external quantum efficiency (EQE) of 0.8%. By increasing the dopant with the device (6 wt %) showed a maximum luminescence of 314 cd/ m 2 at 6.9 V with CE of 0.5 cd/A, PE of 0.2 lm/W, and an EQE of 0.3%. The device inbuilt with a emissive layer of compound 10c (nondoped) showed a maximum luminescence of 286 cd/ m 2 at 7.4 V, CE of 0.6 cd/A, PE of 0.2 lm/W, and EQE of 0.4%, respectively. It can be observed that the dilution effect occurs with an increasing dopant ratio with a bipolar CBP host; it means the higher concentration increases the formation of crystallinity and self-aggregation organization of bowlshaped derivatives in the devices which affect its morphology in the film. The device with 1 wt % doping concentration of dye shows higher maximum luminescence with higher power and CE as compared to other doping concentration ( Figure  S22c).
It is observed that the higher dopant concentration of blue dye shows relative low turn-on voltage and higher current densities for the device. However, the lower luminance value could be because of the unbalanced charge-transport ratio which results in the leakage of charge at the interface of electrodes without recombining inside the emitting layer. The current density versus voltage and luminance versus current density of all fabricated OLED devices are mentioned in Figure  S23 (see in Supporting Information). All the CBP-based doped

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Article (1, 2, 5 and 6 wt %) devices show a bluish-green EL emission with peaks at 438, 407, 353, and 314 nm. Consequently, the Commission International de 1'E clairage coordinates (CIE) color coordinates were (0.24, 0.33), (0.24, 0.33), (0.24, 0.33), and (0.24, 0.32) while nondoped CBP-based device (neat) also showed the EL peak at 286 nm and CIE coordinates of (0.24, 0.31). From this, it can be noted that as the dopant ratio varies from 1 to 6 wt %, the dye did not display any variations in EL maxima and CIE coordinates probably suggest that the bowlshaped molecules become aggregate to repel uniformly at higher concentrations. 74

■ CONCLUSIONS
In conclusion, we have designed, synthesized, and characterized bowl-shaped thiacalixarene-based supramolecular LCs that self-assembles to form a columnar hexagonal phase with a good temperature range. All the compounds displayed blue fluorescence in the solution and solid thin-film state. The selfassembly of supramolecular thiacalixarene-based mesogens stabilize columnar hexagonal phase with broad mesophase range and good thermal stability. These supramolecular compounds (10c, 10d) are the first examples of liquid crystals based on the thiacalix[4]arene core to form columnar selfassembly, respectively. Also, they possess emissive nature in the gelation state which makes this research applicable in the fabrication of emissive displays. The selected supramolecular compound 10c with trisubstituted octyloxy side chain was used in the fabrication of OLEDs as doped and nondoped with emitter bipolar CBP. Therefore, these supramolecular thiacalixarenes derivatives can serve as a good platform for the development of bowl-shaped LCs used to fabricate OLEDs. Such multifunctional supramolecular materials should have great potentials in the search for new thermally stable luminescent columnar LCs and its applications in displays, photochemical molecular switches, and gelation, and as chemosensors.

■ EXPERIMENTAL SECTION
Materials and Methods. All the starting raw materials and chemical reagents were obtained as analytical grade and used without purification. The required solvents were distilled and purified prior to use in the synthesis work. 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were collected on the Bruker ADVANCE spectrometer (400 MHz). The IR spectra were collected on Shimadzu in the range of 3600−500 cm −1 . Elemental analyses (C,H,N) were carried out with the PE (CHN) 2400 analyzer. The CV experiment data were carried out on CH Instruments electrochemical workstation. The reference electrode used was calibrated with the ferrocene/ ferrocenium redox couple. The POM images were performed on a polarized pptical microscope equipped under heating and cooling conditions. The mass spectra of target compounds were carried out by using high-resolution mass spectrometer. The phase-transition temperatures were observed on Shimadzu DSC-50. TGA were performed on a PerkinElmer-STA 6000 apparatus under the nitrogen atmosphere. The samples were heated at room temperature to 550°C at 10°C/min. XRD studies were recorded on a Rigaku-Ultima IV powder diffractometer equipped with a Cu kα source (λ = 1.54 Å, 40 kV). The absorption and fluorescence spectra were studied by using Jasco V-570 UV−vis and Jasco FP-6500 spectrofluorometer at different wavelengths.
General Method for the Preparation of Compound 8a. Compound (8a) is prepared by the reaction of compound (7a) and 2,4-dihydroxy aniline with the presence of few drops of glacial acetic acid in ethanol. 4 The reaction mixture is refluxed for 3 h and monitored by using thin-layer chromatography (TLC) followed by the methanol/chloroform system (1:4 General Method for the Preparation of Compound 8b. Compound (8b) is prepared by the reaction of compound (7b) and 2,4-dihydroxy aniline with the presence of few drops of glacial acetic acid in ethanol. 4 The reaction mixture is refluxed for 3 h and monitored by using TLC followed by methanol: chloroform system (1:4 General Method for the Preparation of Compound 8c. Compound (8c) is prepared by the reaction of compound (7c) and 2,4-dihydroxy aniline with the presence of few drops of glacial acetic acid in ethanol. 4 The reaction mixture is refluxed for 3 h and monitored by using TLC followed by the methanol/chloroform system (1:4 General Method for the Preparation of Compound 8d. Compound (8d) is prepared by the reaction of compound (7c) and 2,4-dihydroxy aniline with the presence of few drops of glacial acetic acid in ethanol. 4 The reaction mixture is refluxed for 3 h and monitored by using TLC followed by the methanol/chloroform system (1:4 General Method of the Preparation of p-tert-Butyl thiacalix [4]arene (9). Parent p-tert-butyl thiacalix[4]arene (9) was synthesized by involving base-catalyzed cyclo-

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