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

Emerging Aspects of Triazole in Organic Synthesis: Exploring its Potential as a Gelator

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) - commonly known as the "click reaction" - serves as the most effective and highly reliable tool for facile construction of simple to complex designs at the molecular level. It relates to the formation of carbon heteroatomic systems by joining or clicking small molecular pieces together with the help of various organic reactions such as cycloaddition, conjugate addition, ring-opening, etc. Such dynamic strategy results in the generation of triazole and its derivatives from azides and alkynes with three nitrogen atoms in the five-membered aromatic azole ring that often forms gel-assembled structures having gelating properties. These scaffolds have led to prominent applications in designing advanced soft materials, 3D printing, ion sensing, drug delivery, photonics, separation, and purification. In this review, we mainly emphasize the different mechanistic aspects of triazole formation, which includes the synthesis of sugar-based and non-sugar-based triazoles, and their gel applications reported in the literature for the past ten years, as well as the upcoming scope in different branches of applied sciences.

Bimetallic liquid crystal blends based on structurally related 3d-metal coordination complexes

Hetero-bimetallic liquid crystalline materials, exhibiting a single Colhex mesophase, were obtained by simple chemical blending between two structurally-related Cu(II) and Zn(II) metallomesogens based on 1,10-phenanthroline and two chelating gallate ligands. Mesomorphous and optical properties were modified upon their relative respective proportions. This study highlights the numerous possibilities for the fabrication of new multifunctional polymetallic materials, with the possibility of tuning the properties and controlling supramolecular interactions between metal centres and corresponding synergistic effects.

Self-assembled discotics as molecular semiconductors

With the advent of a new era of smart-technology, the demand for more economic optoelectronic materials that do not compromise with efficiency is gradually on the rise. Organic semiconductors provide greener alternatives to the conventional inorganic ones, but encounter the challenge of balancing charge carrier mobility with processability in devices. Discotic liquid crystals (DLCs), a class of self-assembling soft organic materials, possess the perfect degree of order and dynamics to address this challenge. Providing unidimensional charge carrier pathways through their nanoscale columnar architecture, DLCs can behave as efficient charge transport systems across a wide range of optoelectronic devices. Moreover, DLCs are solution-processable, thus reducing the fabrication cost. In this article, we have discussed the approaches towards developing DLCs as semiconductors, focusing on their molecular design concepts, supramolecular structures and electronic properties in the context of their charge carrier mobilities.

Tris(boranil) Columnar Liquid Crystalline Fluorophores: pseudo-Triphenylene Boron(III) Complexes with Peripheral N-B-O Linkages

The borate complexes derived from salicylaldimine ligands, called boranils, possess a wide range of photophysical and electronic characteristics intrinsically. The unique combination of molecular rigidity, rendered by four-coordinate boron bridges, and extended π-conjugation enable them to serve as technically feasible fluorescent materials (dyes). The incorporation of liquid crystallinity in these boron(III) complexes, especially the columnar (Col) mesomorphism, which is overlooked hitherto, would provide a new dimension to these complexes. Herein, we report the first examples of tris(boranil) discotic liquid crystal (LC) dyes that have been readily synthesized by treating tris(N-salicylideneaniline)s, (TSAN)s, with BF₃ .Et₂ O in the presence of an acid quencher. These C₃ -symmetric borate complexes self-assemble into the Col phase, existing over a wide thermal span including room temperature. The 2D periodic order of the Col phases shows dependence on the length of the peripheral tails. The photophysical measurements reveal the fluorescence emission in their two condensed states viz., solid and Col phase, and in solution. Their electrochemical, two-step oxidation process coupled with the aforesaid features upholds their significance in applied research.

Solution-processable organic light-emitting diodes utilizing electroluminescent perylene tetraester-based columnar liquid crystals

Herein, we reveal a homologous series of liquid crystals involving perylene tetraesters as the core connected to the four trialkoxyphenyl units at the periphery using the triazole moiety as the linker. A thorough analysis using differential scanning calorimetry, polarized optical microscopy, and small- and wide-angle X-ray scattering studies confirm that all the mesogens 1a-c hold a stable enantiotropic columnar mesophase. Suitable molecular orbital levels and excellent material photophysical and thermal properties encouraged the study of their electroluminescent properties. Due to this, a well designed solution-processable organic light emitting diode device structure is configured as ITO (125 nm)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (35 nm)/host: x wt% emitter (x = 0.5, 1.0, 3.0, 5.0) (20 nm)/2,2'2''-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) (40 nm)/lithium fluoride (LiF) (1 nm)/aluminium (Al) (200 nm) using compounds 1a-c as emitters. 4,4',4''-Tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) and 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) were chosen as two different host materials. The current density-voltage-luminance and current efficacy-luminance-power efficacy plots suggest that m-MTDATA is a better host than CBP. Amongst, device based on 1 wt% emitter 1c doped in the m-MTDATA host matrix displayed the best performance, with a maximum power efficacy of 17.2 lm W^-1, current efficacy of 18.5 cd A^-1, and external quantum efficiency of 6.3%.

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.

Construction and Performance of OLED Devices Prepared from Liquid-crystalline TADF Materials

The device performance is reported from three compounds which show both thermally activated delayed fluorescence and liquid crystallinity, which use the donor 3,6-bis(3,4-didodecyloxyphenyl)carbazole. Two of the compounds, whose photophysics were...

Advanced Functional Luminescent Metallomesogens: The Key Role of the Metal Center

The use of liquid crystals for the fabrication of displays incorporated in technological devices (TVs, calculators, screens of eBook's, tablets, watches) demonstrates the relevance that these materials have had in our way of living. However, society evolves, and improved devices are looked for as we create a more efficient and safe technology. In this context, metallomesogens can behave as multifunctional materials because they can combine the fluidic state of the mesophases with properties such as photo and electroluminescence, which offers new exciting possibilities in the field of optoelectronics, energy, environment, and even biomedicine. Herein, it has been established the role of the molecular geometry induced by the metal center in metallomesogens to achieve the self-assembly required in the liquid-crystalline mesophase. Likewise, the effect of the coordination environment in metallomesogens has been further analyzed because of its importance to induce mesomorphism. The structural analysis has been combined with an in-depth discussion of the properties of these materials, including their current and potential future applications. This review will provide a solid background to stimulate the development of novel and attractive metallomesogens that allow designing improved optoelectronic and microelectronic components. Additionally, nanoscience and nanotechnology could be used as a tool to approach the design of nanosystems based on luminescent metallomesogens for use in bioimaging or drug delivery.

Luminescent Liquid Crystals Based on Carbonized Polymer Dots and Their Polarized Luminescence Application

Traditional luminescent liquid crystals (LLCs) suffer from fluorescence quenching caused by aggregation, which greatly limits their further application. In this work, a kind of novel LLCs (named carbonized polymer dot liquid crystals (CPD-LCs)) are designed and successfully synthesized through grafting the rod-shaped liquid crystal (LC) molecules of 4'-cyano-4-(4″-bromohexyloxy) biphenyl on the surface of CPDs. The peripheral LC molecules not only increase the distance between different CPDs to prevent them from aggregating and reduce intermolecular energy resonance transfer but also make this LLC have an ordered arrangement. Thus, the obtained CPD-LCs show good LC property and excellent high luminous efficiency with an absolute photoluminescence quantum yield of 14.52% in the aggregated state. Furthermore, this kind of CPD-LC is used to fabricate linearly polarized devices. The resultant linearly polarized dichroic ratio (N) and polarization ratio (ρ) are 2.59 and 0.44, respectively. Clearly, this type of CPD-LC shows promising applications for optical devices.

Applications of liquid crystals in biosensing

Liquid crystals (LCs), as a promising branch of highly-sensitive, quick-response, and low-cost materials, are widely applied to the detection of weak external stimuli and have attracted significant attention. Over the past decade, many research groups have been devoted to developing LC-based biosensors due to their self-assembly potential and functional diversity. In this paper, recent investigations on the design and application of LC-based biosensors are reviewed, based on the phenomenon that the orientation of LCs can be directly influenced by the interactions between biomolecules and LC molecules. The sensing principle of LC-based biosensors, as well as their signal detection by probing interfacial interactions, is described to convert, amplify, and quantify the information from targets into optical and electrical parameters. Furthermore, commonly-used LC biosensing targets are introduced, including glucose, proteins, enzymes, nucleic acids, cells, microorganisms, ions, and other micromolecules that are critical to human health. Due to their self-assembly potential, chemical diversity, and high sensitivity, it has been reported that tunable stimuli-responsive LC biosensors show bright perspectives and high superiorities in biological applications. Finally, challenges and future prospects are discussed for the fabrication and application of LC biosensors to both enhance their performance and to realize their promise in the biosensing industry.

Highly Luminescent Liquid Crystals in Aggregation Based on Platinum(II) Complexes

Luminescent liquid crystals (LLCs) attract considerable attention because of their broad applications in displays, chemosensors, and anti-counterfeiting. However, it remains challenging to achieve a high luminescence efficiency in LCs because of the common aggregation-caused quenching effect. Herein, we demonstrate a facile approach to designing LLCs with a high quantum yield up to 88% by deliberately tuning the aggregation behavior of platinum(II) complexes with alkoxy chains (C_nH_2n+1O-). LLCs in hexagonal columnar and rectangular columnar phases are achieved when n = 12 and 16, respectively, as revealed by one-dimensional wide-angle X-ray diffraction and small-angle X-ray scattering. These LLCs are able to not only exhibit strong emission at elevated temperatures but also show attractive reversible vapochromism upon alternative CH₂Cl₂ and EtOH fuming, which imparts added functions and promises technological utility.

Luminescent Conductive Columnar π-Gelators for Fe(II) Sensing and Bio-Imaging Applications

The high demand and scarcity of luminescent, photoconductive, and transparent gels necessitate its finding as they are potential components in photonic devices such as solar cell concentrators where optical losses via scattering and reabsorption require to be minimized. In this direction, we have reported highly transparent, blue luminescent as well as photoconductive gels exhibiting the hole mobility of 10^-3 cm²/V s at ambient temperature as investigated by the time-of-flight technique. The π-driven self-standing supergels were formed using triazole-modified phenylene-vinylene derivatives as gelators in a nonpolar solvent. Different microscopic studies revealed its entangled network of interwoven fibrilar self-assembly and anisotropic order in the gel state. Supramolecular assembly of xerogels, studied by small- and wide-angle X-ray scattering (SAXS/WAXS) suggesting their local columnar hexagonal (Col_h) superstructure, is beneficial for conducting gels. Rheological measurements direct the stiffness and robustness of the organogels. In addition, the gelators were developed as a sensing platform for the ultrasensitive detection of Fe(II) ions at ppb level. ¹H nuclear magnetic resonance (NMR) titrimetric studies revealed that the interaction of the H-atom of triazole units with Fe(II) is responsible for quenching of blue fluorescence. Also, one of the gelators was successfully applied in bio-imaging using the pollen grains of the Hibiscus rosa-sinensis plant.

Seeing the Unseen: The Role of Liquid Crystals in Gas-Sensing Technologies

Fast, real-time detection of gases and volatile organic compounds (VOCs) is an emerging research field relevant to most aspects of modern society, from households to health facilities, industrial units, and military environments. Sensor features such as high sensitivity, selectivity, fast response, and low energy consumption are essential. Liquid crystal (LC)-based sensors fulfill these requirements due to their chemical diversity, inherent self-assembly potential, and reversible molecular order, resulting in tunable stimuliresponsive soft materials. Sensing platforms utilizing thermotropic uniaxial systems-nematic and smectic-that exploit not only interfacial phenomena, but also changes in the LC bulk, are demonstrated. Special focus is given to the different interaction mechanisms and tuned selectivity toward gas and VOC analytes. Furthermore, the different experimental methods used to transduce the presence of chemical analytes into macroscopic signals are discussed and detailed examples are provided. Future perspectives and trends in the field, in particular the opportunities for LC-based advanced materials in artificial olfaction, are also discussed.