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.

Chiral diphenylacrylonitrile–perylene liquid crystal with circularly polarized luminescence in the aggregated state

Novel chiral diphenylacrylonitrile–perylene liquid crystal with excellent CPL propertyin aggregated state was reported.

Photoinduced E to Z isomerization of tetraphenylethylene derivatives within organometallic supramolecular assemblies

Isolation of E-1,2-bis(4-bromophenyl)-1,2-diphenyl-ethylene from the E/Z isomer mixture obtained by a McMurry coupling reaction and reaction of this isomer with imidazole followed by N-alkylation with nBuBr and anion exchange yielded the bisimidazolium tetraphenylethylene (TPE) derivative H2-E-1(PF6)2. The reaction of H2-E-1(PF6)2 with Ag2O yielded the di-nuclear metallarectangle [Ag2(E-1)2](PF6)2 where the two bis-NHC donors E-1 bridge two silver atoms. Irradiation of [Ag2(E-1)2](PF6)2 leads to E/Z isomerization of the di-NHC ligand and formation of Z-1 in the mononuclear complex [Ag(Z-1)]PF6. Demetallation of the di-NHC ligand with NH4Cl/NH4PF6 yielded bisimidazolium salt H2-Z-1(PF6)2. The unique isomerization of the E-TPE derivative into its Z-isomer via metal complex formation/irradiation/demetallation cannot be achieved by irradiation of the individual imidazolium salt. The emissive properties of the TPE complexes [Ag2(E-1)2](PF6)2 and [Ag(Z-1)]PF6 have been investigated.

Nonlinear Nonacenes with a Dithienothiophene Substructure: Multifunctional Compounds that Act as Columnar Mesogens, Luminophores, π Gelators, and p-Type Semiconductors

Board-like liquid-crystalline semiconductors based on the dithieno[3,2-b;2',3'-d]thiophene (DTT) substructure were synthesized and their thermal, self-assembly, optical, and semiconducting properties investigated. These sanidic compounds, bearing eight peripheral chains, are mesomorphic and spontaneously self-assemble into columnar hexagonal mesophases (Col_hex ) over broad temperature ranges, as confirmed by polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering. Strong blue photoluminescence with absolute quantum yields up to 33 % were measured. These compounds also form blue-light emitting gels in various organic solvents. The fibrillar-like morphology of these gels, reminiscent of the columnar structure, and stabilized by efficient intermolecular interactions, was characterized by scanning electron microscopy. The charge carrier mobility of these π-extended mesogens was measured by time-of-flight transient photocurrent technique in the Col_hex mesophase, and showed good hole-transport performance with charge carrier mobility of 10^-3  cm²  V^-1  s^-1 .

Amorphous Ag2S Micro-rods-Enhanced Fluorescence on Liquid Crystals: Cation-π Interaction-Triggered Aggregation-Induced Emission Effect

Aggregation-induced emission (AIE) system has long been regarded as a promising substitute to overcome the aggregation-caused quenching in traditional luminescent liquid crystals, which could further enhance its efficiency and application. However, due to the intrinsic weak interaction between hybrid components, heterogeneous inorganic materials-induced AIE process was rarely reported. In this study, trace amounts of amorphous Ag2S microrods and an AIE-active liquid crystalline compound tetraphenylethylene-propylbenzene (TPE-PPE) were proposed to construct additional intense interaction to trigger AIE effect. The enhanced concentration of unsaturated Ag ions and excess positive charge on Ag2S surface promote a cation-π interaction with TPE-PPE, leading to a 36-fold increase in fluorescence, which is predominately high in luminescent liquid crystal system. To the best of our knowledge, this is the first report of the AIE process activated by cation-π interaction. This novel approach would provide guidance to fabricate high-luminescence meso phases for future luminescent display device.

Synthesis and Characterization of Liquid-Crystalline Tetraoxapentacene Derivatives Exhibiting Aggregation-Induced Emission

A series of new tetrakis(dialkoxyphenyl) dicyanotetraoxapentacene derivatives (1 a-c) were prepared by reaction of the appropriate terphenyl diols with tetrafluoroterephthalonitrile in good yields. Compounds 1 b and 1 c, which bear hexyloxy and decyloxy side chains, exhibited columnar hexagonal mesophases, as shown by polarized optical microscopy, variable-temperature powder X-ray diffraction, and differential scanning calorimetry. Single-crystal X-ray diffraction of methoxy-substituted 1 a revealed that the dicyanotetraoxapentacene core is highly planar, consistent with the notion that these molecules are able to stack in columnar mesophases. A detailed photophysical characterization showed that these compounds exhibit aggregation-induced emission in solution, emission in nonpolar solvents, weak emission in polar solvents, and strong emission in the solid state both as powder and in thin films. These observations are consistent with a weakly emissive charge-transfer state in polar solvents and a more highly emissive locally excited state in nonpolar solvents.

Fluorine-containing bistolanes as light-emitting liquid crystalline molecules

We synthesised a series of dissymmetric bistolane derivatives and evaluated their liquid-crystalline (LC) and photoluminescence properties in detail. In measuring LC behaviours, rational structural design based on the dissymmetric molecular structure and electron-density distribution facilitated the production of the LC phase with a wide temperature range (up to 97 °C). In addition, dissymmetric bistolane derivatives were shown to strongly emit blue-photoluminescence in dilute solution and in crystalline states. It was found that dissymmetric bistolanes possess emissive features in even the LC phase and photoluminescence behaviours such as emission intensity and colour were sensitively switched depending on the molecular aggregate structure caused by applying a thermal stimulus.

Crystal Crosslinked Gels with Aggregation-Induced Emissive Crosslinker Exhibiting Swelling Degree-Dependent Photoluminescence

The synthesis and photoluminescence properties of crystal crosslinked gels (CCGs) with an aggregation-induced emission (AIE) active crosslinker derived from tetraphenylethene (TPE) is discussed in this article. The CCG was prepared from a metal organic framework (MOF) with large pore aperture to allow the penetration of TPE crosslinker. The obtained CCG possessed a rectangular shape originated from the parent MOF, KUMOF. The CCG showed stimuli-responsive photoluminescence behavior depending on the swelling degree, thus the photoluminescence intensity was higher at higher swelling degree. By changing the solvent, water content, or ionic strength, the photoluminescence intensity was controllable, accompanying the change of swelling degree. Moreover, emission color tuning was also achieved by the introduction of luminescent rare earth ions to form a coordination bonding with residual carboxylate inside the CCG.

Novel AIE columnar liquid crystals: the influence of the number of diphenylacrylonitrile groups on the mesomorphic and fluorescence properties

Triphenylene derivatives with one or two diphenylacrylonitrile groups exhibited not only an ordered hexagonal columnar mesophase but also good AIE fluorescence properties in both solution and solid state.

Phenylacrylonitrile-bridging triphenylene dimers: the columnar liquid crystals with high fluorescence in both solid state and solution

The phenylacrylonitrile-bridging triphenylene dimers were prepared and exhibited the ordered hexagonal columnar mesophase and high fluorescence in both the solid state and solution.

[12]aneN3 Modified Tetraphenylethene Molecules as High-Performance Sensing, Condensing, and Delivering Agents toward DNAs

Four [12]aneN3 modified tetraphenylethene (TPE) compounds with different numbers of polyamine units and structure configurations, namely 1, 2, 3, and 4, were designed and synthesized. All compounds showed strong aggregation-induced emission (AIE) features. Compounds 2 and 4 showed significant emission enhancement after the addition of ssDNAs and dsDNAs of different lengths as well as calf thymus DNA (ctDNA). Compounds 1 and 3 showed very poor fluorescent responses toward DNAs. Gel electrophoresis demonstrated the abilities of 1-4 to condense DNA effectively. Complete retardation of plasmid DNA can be achieved at a concentration of 25 μM (1), 8 μM (for 2 and 3) and 4 μM (4). Experiments including fluorescent contrastive titrations, scanning electron microscopy, dynamic laser scattering, EB displacement, and gel electrophoresis demonstrated that the four compounds were able to integrate with DNA through electrostatic interactions and supramolecular stacking. A vicinal configuration around TPE (2) and more triazole-[12]aneN3 recognition sites (4) evidently enhanced the sensing capability toward oligonucleotides, and the TPE unit played an important role in the plasmid DNA condensation process because of its strong binding. With the advantages of low cytotoxicity, effective DNA sensing, and DNA condensing properties, compound 4 was successfully applied as a nonviral DNA vector and fluorescent tracer for label-free gene delivery, which is the first example of a nonviral gene vector with AIE activity.

The use of azide-alkyne click chemistry in recent syntheses and applications of polytriazole-based nanostructured polymers

The rapid development of efficient organic click coupling reactions has significantly facilitated the construction of synthetic polymers with sophisticated branched nanostructures. This Feature Article summarizes the recent progress in the application of efficient copper-catalyzed and copper-free azide-alkyne cycloaddition (CuAAC and CuFAAC) reactions in the syntheses of dendrimers, hyperbranched polymers, star polymers, graft polymers, molecular brushes, and cyclic graft polymers. Literature reports on the interesting properties and functions of these polytriazole-based nanostructured polymers are also discussed to illustrate their potential applications as self-healing polymers, adhesives, polymer catalysts, opto-electronic polymer materials and polymer carriers for drug and imaging molecules.

High aspect ratio, processable coordination polymer gel nanotubes based on an AIE-active LMWG with tunable emission

A new TPE based low molecular weight gelator (LMWG) which displays both AIE and MCIE phenomena in gel state has been synthesized. LMWG self-assembles to form 1D nanofibers which undergo morphology transformation to coordination polymer gel (CPG) nanotubes upon metal ion coordination. CPG shows enhanced mechanical stability along with tunable emission properties.

Photoluminescent epoxy microspheres: preparation, surface functionalization via grafting polymerization and photophysical properties

Photoluminescent epoxy microspheres with the size of 1–3 μm were prepared. Their surfaces were functionalized with poly(N-vinylpyrrolidone) via RAFT polymerization.

Photomodulated fluorescence of supramolecular assemblies of sulfonatocalixarenes and tetraphenylethene

Self-assembled fluorescent nanoparticles responding to specific stimuli are highly appealing for applications such as labels, probes, memory devices, and logic gates. However, organic analogues are challenging to prepare, due to unfavorable aggregation-caused quenching. We herein report the preparation of self-assembled fluorescent organic nanoparticles in water by means of calixarene-induced aggregation of a tetraphenylethene derivative (QA-TPE) mediated by p-sulfonatocalix[4]arenes. The self-assembled nanoparticles showed interesting photoswitching behaviors, and the fluorescence output of the generated nanoparticles was opposite to that of free QA-TPE both before and after irradiation. Free QA-TPE is nonfluorescent, owing to intramolecular rotations of the phenyl rings. In contrast, the self-assembled nanoparticles that formed upon complexation of QA-TPE with p-sulfonatocalix[4]arene exhibited aggregation-induced emission fluorescence (λ(em) = 480 nm, Φ = 14%), as a result of the inhibition of rotations. Upon UV light irradiation, free QA-TPE was cyclized to the corresponding diphenylphenanthrene, which showed typical fluorescence of a π-conjugated system (λ(em) = 385 nm, Φ = 9.3%), whereas the nanoparticles were nonfluorescent upon irradiation due to the aggregation-caused quenching. In effect, this system allows programmed modulation of TPE fluorescence at two different emission wavelengths by means of host-guest complexation and irradiation. Relative to a single-mode stimulus-responsive system, our new developed system of highly integrated modes into a single molecular unit that can exhibit modulation of fluorescence by multiple stimulus is expected to be more adaptable for practical applications and to show enhanced multifunctionality.

AIE macromolecules: syntheses, structures and functionalities

A comprehensive review of macromolecules with aggregation-induced emission attributes is presented, covering the frontiers of syntheses, structures, functionalities and applications.

Triphenylene discotic liquid crystal trimers synthesized by Co2(CO)8-catalyzed terminal alkyne [2 + 2 + 2] cycloaddition

The synthesis of star-shaped discotic liquid crystal trimers using Co₂(CO)₈-catalyzed terminal alkyne [2 + 2 + 2] cycloaddition reaction is reported. The trimers consist of three triphenylene discotic units linked to a central 1,2,4-trisubstituted benzene ring via flexible spacers. The trimers were synthesized in the yields up to 70% by mixing the monomers with 10 mol % of Co₂(CO)₈ as the catalyst in refluxing 1,4-dioxane. The liquid crystalline properties were investigated by using polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Trimer 4 with an ester connecting group and a longer spacer exhibited a rectangular columnar mesophase, while 5b and 5c possessing an ether linkage and a shorter spacer display a hexagonal columnar mesophase. The connecting functional group and the length of the flexible spacer between the central benzene ring and the triphenylene units have pivotal influence on the mesomorphism.