Tuning the Liquid Crystallinity and Electroluminescence via Sulfonation of S-Annulated Perylene Tetraester.

This study presents a comparative analysis of S-annulated perylene tetraester (PTE-S) and its sulfone (PTE-SO2) analogue. This sulfone modification reduced melting point and stabilized a room temperature columnar rectangular (Colr) phase in contrast to its parent PTE-S which showed a crystalline behaviour at room temperature. This molecular design also leads to red-shifted absorbance and emission in comparison to PTE-S, along with a tuning of photoluminescence from sky blue to green, achieving an impressive quantum yield of 85 %. OLED devices fabricated using PTE-SO2 as emitter material at concentrations of 0.2, 0.5, and 1 wt.% in CBP as host material. A maximum external quantum efficiency (EQE) of 2.9 % was observed with the 0.5 wt.% PTE-SO2 in CBP with CIE coordinates of (0.45, 0.35), accompanied by an orange luminance of 848 cd/m2. Notably, a device with a 0.5 wt% doping concentration of PTE-S demonstrates an EQE of 3.5 %, and cyan luminance of 2,598 cd/m2.

Structure-property relationships of quinoxaline-based liquid crystals.

Quinoxaline derivatives with different molecular structures stabilizing liquid crystalline self-assembly are discussed in this review. This class of molecules can be systematically modified with careful molecular engineering to achieve different molecular shapes, directing them to self-assemble into various types of mesophases. The structure-property relationships of such molecules and the resultant self-assembled functional structures are of utmost importance in organic electronic devices and related applications.

Functional Ionic Liquid Crystals.

Ionic liquid crystals have emerged as a new class of functional soft materials in the last two decades, and they exhibit synergistic characteristics of ionic liquids and liquid crystals such as macroscopic orientability, miscibility with various species, phase stability, nanostructural tunability, and polar nanochannel formation. Owing to these characteristics, the structures, properties, and functions of ionic liquid crystals have been a hot topic in materials chemistry, finding various applications including host frameworks for guest binding, separation membranes, ion-/proton-conducting membranes, reaction media, and optoelectronic materials. Although several excellent review articles of ionic liquid crystals have been published recently, they mainly focused on the fundamental aspects, structures, and specific properties of ionic liquid crystals, while these applications of ionic liquid crystals have not yet been discussed at one time. The aim of this feature article is to provide an overview of the applications of ionic liquid crystals in a comprehensive manner.

Perylene-Based Liquid Crystals as Materials for Organic Electronics Applications.

Columnar phases formed by the stacking of disclike molecules with an intimate π-π overlap forms a 1D pathway for the anisotropic charge migration along the columns. Columnar phases have great potential in organic electronic devices to be utilized as active semiconducting layers in comparison to organic single crystals or amorphous polymers in terms of processability, ease of handling, and high charge carrier mobility. Intelligent molecular engineering of perylene and its derivatives provided access to tune the physical properties and self-assembly behavior. The columnar phase formed by perylene derivatives has great potential in the fabrication of organic electronic devices. There are several positions on the perylene molecule, which can be functionalized to tune its self-assembly, as well as optoelectronic properties. Thus, many liquid-crystalline molecules stabilizing the columnar phase, which are based on perylene tetraesters, perylene diester imides, and perylene bisimides, have been synthesized over the years. Their longitudinal and laterally extended derivatives, bay-substituted derivatives exhibiting a columnar phase, are reported. In addition, several liquid-crystalline oligomers and polymers based on perylene derivatives were also reported. All such modifications provide an option to tune the energy levels of frontier molecular orbitals with respect to the work function of the electrodes in devices and also the processability of such materials. In this feature article, we attempt to provide an overview of the molecular design developed to tune the applicable properties and self-assembly of perylene derivatives as well as recent developments related to their application in the fabrication of organic solar cells, organic light-emitting diodes, and organic field-effect transistors.

Effect of Atomic-Scale Differences on the Self-Assembly of Thiophene-based Polycatenars in Liquid Crystalline and Organogel States.

Two series of polycatenars are reported that contain a central thiophene moiety connected to two substituted oxadiazole or thiadiazole units. The number, position, and length of the peripheral chains connected to these molecules were varied. The oxadiazole-based polycatenars exhibited columnar phases with rectangular and hexagonal or oblique symmetry, whereas the thiadiazole-based polycatenars exhibited columnar phases with rectangular and/or hexagonal symmetry. All of the compounds exhibited bright emission in the solution and thin-film states. Two oxadiazole-based molecules and one thiadiazole-based molecule exhibited supergelation ability in hydrocarbon solvents, which is mainly supported by attractive π-π interactions. These gels showed aggregation-induced enhanced emission, which is of high technological importance for applications in solid-state emissive displays. X-ray diffraction studies of the xerogel fibers of oxadiazole-based polycatenars revealed a columnar rectangular organization, whereas a hexagonal columnar arrangement was observed for thiadiazole-based polycatenars. Rheological measurements carried out on the samples quantitatively confirmed the formation of gels and showed that these gels are mechanically robust. The impact of an atomic-scale difference (oxygen to sulfur, <2 % of the molecular weight) on the self-assembly and the macroscopic properties of those self-assembled structures are clearly visualized.

Bay-Annulated Perylene Tetraesters: A New Class of Discotic Liquid Crystals.

Selenium-annulated perylene tetraesters that stabilize the hexagonal columnar phase have been synthesized and characterized, and their thermal and photophysical behavior has been determined. The mesophase range decreased with an increase in chain length. A comparative account of the structure-property relationships of this series of compounds with respect to parent perylene tetraesters, N- and S-annulated perylene tetraesters, in terms of their thermal, photophysical and electrochemical behavior is provided. The bay-annulation of perylene tetraesters is a good option to modify the thermal and photophysical properties of perylene derivatives and it can provide a new avenue for the synthesis of several technologically important self-assembling perylene derivatives.

Liquid-Crystalline Star-Shaped Supergelator Exhibiting Aggregation-Induced Blue Light Emission.

A family of closely related star-shaped stilbene-based molecules containing an amide linkage are synthesized, and their self-assembly in liquid-crystalline and gel states was investigated. The number and position of the peripheral alkyl tails were systematically varied to understand the structure-property relation. Interestingly, one of the molecules with seven peripheral chains was bimesomorphic, exhibiting columnar hexagonal and columnar rectangular phases, whereas the rest of them stabilized the room-temperature columnar hexagonal phase. The self-assembly of these molecules in liquid-crystalline and organogel states is extremely sensitive to the position and number of alkoxy tails in the periphery. Two of the compounds with six and seven peripheral tails exhibited supergelation behavior in long-chain hydrocarbon solvents. One of these compounds with seven alkyl chains was investigated further, and it has shown higher stability and moldability in the gel state. The xerogel of the same compound was characterized with the help of extensive microscopic and X-ray diffraction studies. The nanofibers in the xerogel are found to consist of molecules arranged in a lamellar fashion. Furthermore, this compound shows very weak emission in solution but an aggregation-induced emission property in the gel state. Considering the dearth of solid-state blue-light-emitting organic materials, this molecular design is promising where the self-assembly and emission in the aggregated state can be preserved. The nonsymmetric design lowers the phase-transition temperatures.The presence of an amide bond helps to stabilize columnar packing over a long range because of its polarity and intermolecular hydrogen bonding in addition to promoting organogelation.

Perylo[1,12-b,c,d] Thiophene Tetraesters: A New Class of Luminescent Columnar Liquid Crystals.

Perylo[1,12-b,c,d] thiophene tetraesters exhibiting wide-range hexagonal columnar phase have been synthesized. These compounds also exhibit good homeotropic alignment in the liquid-crystalline phase which is very important for the device fabrication. These compounds showed sky-blue luminescence in solution under the long-wavelength UV light. With high solubility and high quantum yield these compounds can serve as standards to measure quantum yields of unknown samples. This new class of materials is promising, considering the emissive nature and stabilization of hexagonal columnar mesophase over a wide thermal range and ease of synthesis.

Ambipolar Columnar Self-assembled Organic Semiconductors based on Heteroatom bay-Annulated Perylene Bisimides.

Highly electron-deficient heteroatom (N, S, Se) bay-annulated PBIs exhibiting ordered columnar phase over a wide mesomorphic range including ambient temperature are reported in this manuscript. These compounds with six peripheral n-decyloxy chains exhibited absorption spectra with high molar extinction coefficients, electron-deficient nature and self-assembling behaviour. A detailed comparison with the PBIs bearing six peripheral n-decyl chains was also carried out to get the valuable insights on the structure-property relations in this important class of organic semiconductors. Both of the PBI series were tested for their charge carrier mobility by space charge limited current method and found that they exhibit ambipolar conductivity. This is in contrary to the vast body of literature, where most of the PBI based semiconductors exhibit electron transport behaviour. In general, PBIs derived from tri-n-alkyl anilines exhibit higher mobility values than the PBIs derived from tri-n-alkoxy anilines. Especially, the ambipolar S-annulated PBI derived from tri-n-alkyl aniline exhibited highest hole (8.39×10-3  cm2 /V.s) and electron (1.5×10-2  cm2 /V.s) mobility values and promising for the application in organic electronics.

Room-Temperature, Deep-Red/NIR-Emissive, C 3-Symmetric (n,π-conjugated) Columnar Liquid Crystals: C 3h -Tris(keto-hydrazone)s.

The first examples of deep-red/near-infrared (NIR) photoluminescent, (n,π-conjugated) discotics, namely, C 3h -tris(keto-hydrazone)s, which are the tautomers of tris(azo-enol)s, have been synthesized via a facile one-step triple azo-coupling and characterized. The n,π-resonance-assisted intramolecular H-bonding, rendering planarity and shape persistence to the central core, facilitates their self-assembly into either a hexagonal columnar (Colh) phase (p6mm lattice) or a columnar rectangular (Colr) phase (p2mm lattice), over an extended thermal range including room temperature, fluorescing in the deep-red/NIR-I region. The low band gap with deep-red/NIR emission makes them ideal candidates for NIR-organic light-emitting diodes (OLEDs) and bioimaging.

Helical supramolecular polymers with rationally designed binding sites for chiral guest recognition.

Since various helical supramolecular polymers became available, their application to molecular chirality recognition have been anticipated but not extensively studied. So far, only a few examples of chiral reactions have been reported, but none for chiral separation. Here, we report the application of a helical supramolecular polymer to the enantio-separation of chiral guest molecules. The monomer of this supramolecular polymer is the salt-pair of a dendritic carboxylic acid with an enantiopure amino alcohol. In an apolar solvent, this salt-pair stacks via hydrogen bonds to form a helical polymer. In conjunction with this carboxylic acid, various amino alcohols afford supramolecular polymers, whose helical handedness is determined by the stereochemistry of the amino alcohols. When two salts with the same chirality are mixed, they undergo copolymerization, while those with opposite chirality do not. Owing to this stereoselective copolymerizability, the helical supramolecular polymer could bias the enantiomeric composition of chiral amino alcohols.