High Electrical Conductivity and Hole Transport in an Insightfully Engineered Columnar Liquid Crystal for Solution-Processable Nanoelectronics.

Discotic liquid crystals (DLCs) are widely acknowledged as a class of organic semiconductors that can harmonize charge carrier mobility and device processability through supramolecular self-assembly. In spite of circumventing such a major challenge in fabricating low-cost charge transport layers, DLC-based hole transport layers (HTLs) have remained elusive in modern organo-electronics. In this work, a minimalistic design strategy is envisioned to effectuate a cyanovinylene-integrated pyrene-based discotic liquid crystal (PY-DLC) with a room-temperature columnar hexagonal mesophase and narrow bandgap for efficient semiconducting behavior. Adequately combined photophysical, electrochemical, and theoretical studies investigate the structure-property relations, logically correlating them with efficient hole transport. With a low reorganization energy of 0.2 eV, PY-DLC exhibits superior charge extraction ability from the contact electrodes at low values of applied voltage, achieving an electrical conductivity of 3.22 × 10-4 S m-1, the highest reported value for any pristine DLC film in a vertical charge transport device. The columnar self-assembly, in conjunction with solution-processable self-healed films, results in commendably elevated values of hole mobility (≈10-3 cm2 V-1s-1). This study provides an unprecedented constructive outlook toward the development of DLC semiconductors as practical HTLs in organic electronics.

Light Modulated Reversible "On-Off" Transformation of Arylazoheteroarene Based Discotics in Nematic Organization.

Three benzene-1,3,5-tricarboxamide (BTA) core-based molecular systems appended with phenylazo-3,5-dimethylisoxazole photoswitches at the peripheral position through variable-length alkoxy chains have been designed and synthesized. The supramolecular interactions of the mesogens provided discotic nematic liquid crystalline assembly as confirmed by polarized optical microscopy (POM) and X-ray diffraction (XRD) studies. Spectroscopic studies confirmed the reversible photoswitching and excellent thermal stability of the photoswitched states in solution phase and thin film. Also, atomic force microscopic (AFM) and POM investigations demonstrated the morphological changes in the self-assembly induced by the photoirradiation as monitored by the changes in the height profiles and optical appearance of the textures, respectively. Remarkably, the liquid crystalline discotic molecules showed reversible "on and off states" controlled by light at ambient temperature.

Discs to a 'Bright' Future: Exploring Discotic Liquid Crystals in Organic Light Emitting Diodes in the Era of New-Age Smart Materials.

With the advent of a new decade and the paradigm shift of every sphere of urban life to virtual platforms, it has become imperative for the global researcher community to channelize efforts into upgradation of the existing display-technology. In this context, discotic liquid crystals (DLCs) are a class of self-assembling organic materials that are recently being explored in fabricating the emissive layers of organic light emitting diodes (OLEDs). With their unique inherent structural and functional properties, they have the potential to challenge the currently prevailing OLED-emitters. Yet the applications of this promising class of materials in OLEDs have not been comprehensively reviewed in literature till now. In this account, we present an overview of the developments in the field of luminescent DLC-based emitters, supported by their associated photophysical phenomena and their performance parameters as emitters in fabricated OLED devices.

Unveiling the Role of Hydrogen Bonds in Luminescent N-Annulated Perylene Liquid Crystals.

We report the liquid-crystalline (LC) and luminescent properties of a series of N-annulated perylenes (1-4) in whose molecular structures amide and ester groups alternate. We found that the LC properties of these compounds not only depend on the number of hydrogen-bonding units, but also on the relative position of the amide linkers in the molecule. The absence of amide groups in compound 1 leads to no LC properties, whereas four amide groups induce the formation of a wide temperature range columnar hexagonal phase in compound 4. Remarkably, compound 3, with two amide groups in the inner part of the structure, stabilizes the columnar LC phases better than its structural isomer 2, with the amide groups in the outer part of the molecule. Similarly, we found that only compounds 1 and 2, which have no hydrogen bonding units in the inner part of the molecule, exhibit luminescence vapochromism upon exposure to organic solvent vapors.

Charge Mobility in Discotic Liquid Crystals.

Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals.

Impact of Substitution Pattern and Chain Length on the Thermotropic Properties of Alkoxy-Substituted Triphenyl-Tristriazolotriazines.

Tristriazolotriazines (TTTs) with a threefold alkoxyphenyl substitution were prepared and studied by DSC, polarized optical microscopy (POM) and X-ray scattering. Six pentyloxy chains are sufficient to induce liquid-crystalline behavior in these star-shaped compounds. Thermotropic properties of TTTs with varying substitution patterns and a periphery of linear chains of different lengths, branching in the chain and swallow-tails, are compared. Generally, these disks display broad and stable thermotropic mesophases, with the tangential TTT being superior to the radial isomer. The structure-property relationships of the number of alkyl chains, their position, length and structure were studied.

Patterning of Discotic Liquid Crystals with Tunable Molecular Orientation for Electronic Applications.

The large-area formation of functional micropatterns with liquid crystals is of great significance for diversified applications in interdisciplinary fields. Meanwhile, the control of molecular alignment in the patterns is fundamental and prerequisite for the adequate exploitation of their photoelectric properties. However, it would be extremely complicated and challenging for discotic liquid crystals (DLCs) to achieve the goal, because they are insensitive to external fields and surface chemistry. Herein, a simple method of patterning and aligning DLCs on flat substrates is disclosed through precise control of the formation and dewetting of the capillary liquid bridges, within which the DLC molecules are confined. Large-area uniform alignment occurs spontaneously due to directional shearing force when the solvent is slowly evaporated and programmable patterns could be directly generated on desired substrates. Moreover, the in-plane column direction of DLCs is tunable by slightly tailoring their chemical structures which changes their self-assembly behaviors in liquid bridges. The patterned DLCs show molecular orientation-dependent charge transport properties and are promising for templating self-assembly of other materials. The study provides a facile method for manipulation of the macroscopic patterns and microscopic molecular orientation which opens up new opportunities for electronic applications of DLCs.

Electroluminescent and Optoelectronic Properties of OLEDs with Bay-Extended, Distorted Perylene Esters as Emitter Materials.

Three esters with a perylene, a unilaterally, and a bilaterally extended perylene core, respectively, were used as emitter materials for organic light-emitting diodes. The electroluminescent properties of these devices were studied. Different spectral shifts were found, which can be attributed to the formation of excited dimers (excimers) in the nanofilms of the emitter materials. Thermal treatment of the unilaterally extended derivative resulted in a red-shift of the electroluminescence owing to the formation of a denser nanofilm. The luminance and efficiency of optoelectronic devices employing the extended perylene esters exceed those of devices using an emitter layer comprised of the perylene ester. Different deposition methods, limitations in the deposition process, and the role of hole-transporting materials are compared.

The influence of intraannular templates on the liquid crystallinity of shape-persistent macrocycles.

A series of shape-persistent phenylene-ethynylene-naphthylene-butadiynylene macrocycles with different extraannular alkyl groups and intraannular bridges is synthesized by oxidative Glaser-coupling of the appropriate precursors. The intraannular bridges serve in this case as templates that reduce the oligomerization even when the reaction is not performed under pseudo high-dilution conditions. The extraannular as well as the intraannular substituents have a strong influence on the thermal behavior of the compounds. With branched alkyl chains at the periphery, the macrocycles exhibit liquid crystalline (lc) phases when the interior is empty or when the length of the alkyl bridge is just right to cross the ring. With a longer alkyl or an oligoethylene oxide bridge no lc phase is observed, most probably because the mesogene is no longer planar.

Structural properties and ring defect formation in discotic liquid crystal nanodroplets.

In this work, we performedNpTMonte Carlo simulations of a Gay-Berne discotic liquid crystal confined in a spherical droplet under face-on anchoring and fixed pressure. We find that, in contrast to the unbounded system, a plot of the order parameter as function of temperature does not show a clear evidence of a first-order isotropic-nematic transition. We also find that the impossibility of simultaneously satisfy the uniform director field requirement of a nematic phase with the radial boundary conditions, results in the appearance of a ring disclination line as a stress release mechanism in the interior of the droplet. Under further cooling, a columnar phase appears at the center of the droplet.

Nanoporous Films with Photoswitchable Absorption Kinetics Based on Polymerizable Columnar Discotic Liquid Crystals.

A photoresponsive nanoporous polymer film has been produced from the templated self-assembly of a columnar liquid crystal containing azo units. A liquid crystalline complex of polymerizable azobenzoic acid and a tris-benzimidazolyl benzene template molecule was cross-linked via thiol-ene radical copolymerization with dodecanedithiol. Subsequent removal of the template yielded nanoporous polymer films with pores of approximately 1 nm in diameter. Both trans-cis and cis-trans photoisomerizations of azobenzoic acid took place in the porous films. At room temperature, the cis isomer was sufficiently long-lived to establish a difference in dye absorption kinetics of the two isomers. The cationic dye rhodamine 6G was bound to both isomers, but the rate of binding to films enriched in the cis isomer was 8 times faster.

Water-Induced Self-Assembly of Amphiphilic Discotic Molecules for Adaptive Artificial Water Channels.

Inspired by the induced-fit mechanism in nature, we developed the process of water-induced self-assembly (WISA) to make water an active substrate that regulates the self-assembly and function of amphiphilic discotic molecules (ADMs). The ADM is an isotropic liquid that self-assembles only when in contact with water. Characterization results indicate that water fits into the hydrophilic core of the ADMs and induces the formation of a hexagonal columnar phase (Colh), where each column contains a hydrated artificial water channel (AWC). The hydrated AWCs are adaptive rather than static; the dynamic incorporation/removal of water results in the reversible assembly/disassembly of the adaptive AWCs (aAWCs). Furthermore, its dynamic characteristics can enable water to act as an orientation-directional guest molecule that controls the growth direction of the aAWCs. Well-aligned aAWC arrays that showed the ability of water transport were obtained via a "directional WISA" method. In WISA, water thus governs the supramolecular chemistry and function of synthetic molecules as it does with natural materials. By making water an active component in adaptive chemistry and enabling host molecules to dynamically interact with water, this adaptive aquatic material may motivate the development of synthetic molecules further toward biomaterials.

Self-assembled ordered structures in thin films of HAT5 discotic liquid crystal.

Thin films of the discotic liquid crystal hexapentyloxytriphenylene (HAT5), prepared from solution via casting or spin-coating, were investigated by atomic force microscopy and polarizing optical microscopy, revealing large-scale ordered structures substantially different from those typically observed in standard samples of the same material. Thin and very long fibrils of planar-aligned liquid crystal were found, possibly formed as a result of an intermediate lyotropic nematic state arising during the solvent evaporation process. Moreover, in sufficiently thin films the crystallization seems to be suppressed, extending the uniform order of the liquid crystal phase down to room temperature. This should be compared to the bulk situation, where the same material crystallizes into a polymorphic structure at 68 °C.

Saddle-shaped tetraphenylenes with peripheral gallic esters displaying columnar mesophases.

Tetraphenylenes 2 with eight peripheral gallic esters were prepared in two steps from octamethoxytetraphenylene 1 in 19-72% yield. Investigation of the mesomorphic properties of 2 by DSC, POM and X-ray diffraction revealed that derivatives 2a-d with short alkoxy chain lengths (C(5)-C(8)) did not show any mesomorphic properties, whereas compounds 2e-i with C(9)-C(13) chains displayed rectangular columnar mesophases and compounds 2j-l with C(14)-C(16) chains displayed hexagonal columnar mesophases. Furthermore an anomalous odd-even effect of the clearing points of compounds 2e-l versus chain length was detected.

Room-Temperature Columnar Liquid Crystals as Efficient Pure Deep-Blue Emitters in Organic Light-Emitting Diodes with an External Quantum Efficiency of 4.0.

A novel design of aggregation-induced emission (AIE) active columnar (Col) luminomesogens is reported, and they are demonstrated to act as highly efficient deep-blue emitters in organic light-emitting diodes (OLEDs). All derivatives exhibit Col liquid crystalline (LC) behavior at room temperature over a wide temperature range and desirable alignment properties, which is very important in using them as materials for organic electronic devices. These new AIE active luminomesogens were found to act as highly efficient emitters in OLEDs and unveiled a maximum external quantum efficiency of 4.0% for the first time in Col LCs with Commission International de l'E'clairage coordinates of (0.17, 0.07), which closely matches the National Television System Committee (NTSC) standard, corresponding to pure deep blue color. The detailed supramolecular assembly of the compounds has been characterized by modeling in the mesophase derived from small- and wide-angle X-ray scattering results.

A UV-Responsive Multifunctional Photoelectric Device Based on Discotic Columnar Nanostructures and Molecular Motors.

Orientation control of ordered materials would not only produce new physical phenomenon but also facilitate the development of fancy devices. Discotic liquid crystals (DLCs) form 1D charge transport pathway by self-organizing into columnar nanostructures via π-π stacking. However, controlling the electrical properties in such nanostructures with some direct and instant way is a formidable task for their high viscosity and insensitivity to external stimuli. Herein, the arbitrary control over electrical conductivity of such columnar nanostructures is achieved with UV light by incorporating DLCs with molecular motors. Highly ordered DLC microstripe arrays are generated on desired substrate through a capillary bridge dewetting strategy. The conductivity of the microstripes could be continuously modulated by 365 nm light due to the influence of molecular motion under UV irradiation on the electron orbital overlap of columnar nanostructures. This is so because the disorder degree of the DLC molecules is associated with the intensity of UV light and the doping concentration of molecular motors. Moreover, the device shows memory effect and reversible conductivity change. The DLC microstripe arrays are very promising for the applications in UV detectors, memory devices, optical switches, and so on.

Supramolecular Organization and Photovoltaics of Triangle-shaped Discotic Graphenes with Swallow-tailed Alkyl Substituents.

Two novel triangle-shaped discotic graphenes with swallow-like alkyl tails are synthesized; these discotic graphenes allow facile purification, control over thermotropic properties, and solution fabrication into efficient photovoltaic devices. The unique molecular design results in an extremely broad liquid-crystalline range and the ability to self-heal at low processing temperatures, which improves the performance of photovoltaic cells.

Easily Accessible Thermotropic Hydrogen-Bonded Columnar Discotic Liquid Crystals from Fatty Acid- Tris-Benzoimidazolyl Benzene Complexes.

We report the formation of easily accessible hydrogen-bonded columnar discotic liquid crystals (LCs) based on tris-benzoimidazolyl benzene (TBIB) and commercially available fatty acids. By increasing the length of the fatty acid, the temperature range of liquid crystallinity was tuned. Introducing double bonds in octadecanoic acid lowered the crystallization temperature and increased the temperature range of the mesophase. Surprisingly, dimerized linoleic acid also forms an LC phase. When using branched aliphatic acids with the branching point close to the acid moiety, the mesophase was lost, whereas phosphonic acid or benzenesulfonic acid derivatives did have a mesophase, showing that the generality of this approach extends beyond carboxylic acids as the hydrogen-bond donor. Furthermore, a polymerizable LC phase was obtained from mixtures of TBIB with a methacrylate-bearing fatty acid, providing an approach for the fabrication of nanoporous polymer films if the methacrylate groups are polymerized. Finally, the higher solubility of methyl-TBIB was used to suppress phase separation in stoichiometric mixtures of the template molecule with fatty acids.

Mobility versus Alignment of a Semiconducting π-Extended Discotic Liquid-Crystalline Triindole.

The p-type semiconducting properties of a triphenylene-fused triindole mesogen, have been studied by applying two complementary methods which have different alignment requirements. The attachment of only three flexible alkyl chains to the nitrogen atoms of this π-extended core is sufficient to induce columnar mesomorphism. High hole mobility values (0.65 cm2 V-1 s-1) have been estimated by space-charge limited current (SCLC) measurements in a diode-like structure which are easily prepared from the melt, rendering this material a good candidate for OPVs and OLEDs devices. The mobility predicted theoretically via a hole-hopping mechanism is in very good agreement with the experimental values determined at the SCLC regime. On the other hand the hole mobility determined on solution processed thin film transistors (OFETs) is significantly lower, which can be rationalized by the high tendency of these large molecules to align on surfaces with their extended π-conjugated core parallel to the substrate as demonstrated by SERS. Despite the differences obtained with the two methods, the acceptable performance found on OFETs fabricated by simple drop-casting processing of such an enlarged aromatic core is remarkable and suggests facile hopping between neighboring molecular columns owing to the large conducting/isolating ratio found in this discotic compound.

Extended-Core Discotic Liquid Crystals Based on the Intramolecular H-Bonding in N-Acylated 2,2'-Bipyridine-3,3'-diamine Moieties.

A new type of disc-shaped molecule (1 a-c) has been synthesised and characterised. The molecules were built up by linking three lipophilic, N-monoacylated 2,2'-bipyridine-3,3'-diamine wedges to a central 1,3,5-benzenetricarbonyl unit. They show liquid crystalline behaviour, as shown by DSC, polarisation microscopy and X-ray diffraction. In all cases the mesophase was characterised as a Dho phase. From (1) H NMR results it was shown that the interior of compounds 1 a-c preferentially adopts a C3 symmetrical conformation owing to strong intramolecular H-bonding, which gives rise to an extended core. This large core induces strong interactions between molecules, leading to mesophases of enhanced thermal stability.