An Unusual Stacking Transformation in Liquid-Crystalline Columnar Assemblies of Clicked Molecular Propellers with Tunable Light Emissions

Thermo-Induced Bathochromic Emission in Columnar Discotic Liquid Crystals Realized by Intramolecular Planarization

Most organic thermochromic fluorescent materials exhibit thermo-induced hypsochromic emission due to the formation of excimers in ordered molecular solids; however, it is still a challenge to endow them with bathochromic emission despite its significance in making up the field of thermochromism. Here, a thermo-induced bathochromic emission in columnar discotic liquid crystals is reported realized by intramolecular planarization of the mesogenic fluorophores. A three-armed discotic molecule of dialkylamino-tricyanotristyrylbenzene was synthesized, which preferred to twist out of the core plane to accommodate ordered molecular stacking in hexagonal columnar mesophases, giving rise to bright green monomer emission. However, intramolecular planarization of the mesogenic fluorophores occurred in isotropic liquid increasing the conjugation length, and as a result led to thermo-induced bathochromic emission from green to yellow light. This work reports a new concept in the thermochromic field and provides a novel strategy to achieve fluorescence tuning from intramolecular actions.

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%.

Poly(l-lactide)s with tetraphenylethylene: role of polymer chain packing in aggregation-induced emission behavior of tetraphenylethylene

The AIE behavior of tetraphenylethylene in biocompatible poly(l-lactide)s is found to be sensitive to the polymer chain packing, polymer crystal structure, solvent, and temperature.

Room-temperature AIE ionic liquid crystals based on diphenylacrylonitrile-imidazole salts

Although various AIE liquid crystals have been studied, AIE ionic liquid crystals (ILCs) are almost unknown to date. In this work, a series of novel AIE ILCs based on diphenylacrylonitrile-imidazole salts bridged by soft spacers with different anions were prepared in yields of 63-80%. The mesomorphic, photophysical and electrochemical properties were investigated systematically to elucidate the relationship between structures and properties. The results implied that they were the first room-temperature AIE ILCs with wide ranges of mesomorphic temperature, good fluorescence emission in both the solid state and mesophase, and stable electrochemical behaviour. The samples with one alkyl chain possessed the SmA₂ mesophase while the samples with two alkyl chains prefered the Col_h mesophase. The larger anions resulted in the bigger layer spacing length for the SmA₂ mesophase and smaller values of n_cell in each slice of Col_h mesophase. The fluorescence quantum yields in the mesophase maintained reasonable values (0.15-0.22), which decreased a little in comparison with that in solid films (0.18-0.26) due to the orderly molecular stacking in the mesophase. The cyclic voltammetry experiments confirmed that all of them possessed similar and stable electrochemical behaviour. This research not only presented the first room-temperature AIE ILCs with excellent mesomorphic, photophysical and electrochemical properties, but also elucidated the relationship between structures and properties to a certain degree, contributing to the further construction of novel AIE ILCs with excellent mesomorphic, photophysical and electrochemical properties.

Luminescent Metallacycle-Cored Liquid Crystals Induced by Metal Coordination

Two rhomboidal metallacycles based on metal-coordination-driven self-assembly are presented. Because metal-coordination interactions restrict the rotation of phenyl groups on tetraphenylethene units, these metallacycles were emissive both in solution and in solid state, and their aggregation-induced emission properties were well-retained. Moreover, the rhomboidal metallacyclic structures offer a platform for intermolecular packing beneficial for the formation of liquid crystalline phases. Therefore, although neither of building blocks shows mesogenic properties, both thermotropic and lyotropic (in DMF) mesophases were observed in one of metallacycles, indicating that mesophases could be induced by metal-coordination interactions. This study not only reveals the mechanism for the formation of cavity-cored liquid crystals, but also provides a convenient approach to preparing supramolecular luminescent liquid crystals, which will serve as good candidates for chemo sensors and liquid crystal displays.

Bicontinuous Cubic and Hexagonal Columnar Liquid Crystalline Ion-Conductors at Room Temperature in Ion-Doped Dendritic Amphiphiles

A bicontinuous cubic (Cubbi) liquid crystalline (LC) phase consisting of three dimensional (3D) conducting networks is a promising structural platform for ion-conductors. For practical applications using this fascinating LC structure, it is necessary to suppress crystallization at room temperature (RT). Herein, we report the Cubbi structure at RT and the morphology–dependent conduction behavior in ionic samples of a non-crystallizable dendritic amphiphile. In the molecular design, branched alkyl chains were used as an ionophobic part instead of crystallizable linear alkyl chains. Two ionic samples with Cubbi and hexagonal columnar (Colhex) LC phases at RT were prepared by adding different amounts of lithium salt to the amphiphile. Impedance analysis demonstrated that the Cubbi phase contributed to the faster ion-conduction to a larger extent than the Colhex phase due to the 3D ionic networks of the Cubbi phase. In addition, the temperature–dependent impedance and electric modulus data provided information regarding the phase transition from microphase-separated phase to molecularly mixed liquid phase.

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.

Highly Efficient Luminescent Liquid Crystal with Aggregation-Induced Energy Transfer

A luminescent liquid crystal molecule (TPEMes) with efficient solid-state emission is rationally constructed via the chemical conjugation of blue-emitting tetraphenylethene cores and luminescent mesogenic tolane moieties, which are both featured with aggregation-induced emission properties. As for this fluorophore, aggregation-induced energy transfer from the emissive tolane mesogens to the lighting-up tetraphenylethene units endows the molecule pure blue emission in the suspension and bulk state. Combining differential scanning calorimetry, polarized optical microscope, and one-dimensional X-ray diffraction (1D XRD) experiments, the compound TPEMes is deduced to adapt thermodynamically more stable layered crystalline phase and can be "frozen" into a monotropic smectic mesophase due to kinetic reasons. As a result of more dense packing of TPEMes in the crystalline phase indicated by 1D XRD, the luminescence of TPEMes in crystalline phase blue-shifted by 17 nm relative to the metastable mesophase.

Synthesis, properties and photo-responsive behavior of luminescent side chain polymers containing D–π-A α-cyanostilbene units

A series of luminescent side chain polymers with aggregation-induced emission enhancement characteristics, high solid fluorescence quantum yield, and reversible fluorescence photo-responsive properties were prepared.

Design, synthesis and properties of hemiphasmidic luminescent liquid crystal polymers with the AIEE effect

Novel hemiphasmidic side-chain luminescent liquid crystal polymers with different self-assembly structures and fluorescence properties have been prepared and investigated.

Sequential phase transformation of propeller-like C3-symmetric liquid crystals from a helical to ordered to disordered hexagonal columnar structure

In this paper, we report thermally induced intercolumnar phase transitions of C3-symmetric liquid crystals (LCs) bearing a triazole-based propeller-like aromatic mesogen. Since the constituting aromatic rings are conjugated through rotatable single bonds, the mesogenic shape is tuneable depending on the degree of conformational motion. Molecule 1 with ninefold octyl peripheries shows a hexagonal columnar liquid crystalline phase transition from ordered mesogenic stacking to disordered mesogenic stacking upon heating. On the other hand, molecule 2 with sixfold octyl peripheries displays a helical hexagonal columnar phase with the P6/mmm space group at ambient temperature as well as the ordered and disordered hexagonal columnar phases at higher temperatures. The intracolumnar helical order can be understood by an interdigitated stacking of the propeller-like mesogens along the columnar axis and the optimized space-filling. Notably, all the intercolumnar phase transformations in this study are revealed as second-order transitions. The thermodynamic nature agrees well with the fact that the conformational motions of the C3-symmetric aromatic mesogen change abruptly with each columnar transition.

Chain-dependent emission color codes of extended tetraphenylethylene derivatives: discrimination between water and methanol

Despite the identical extended tetraphenylethylene chromophore, the degree of solubility between the peripheral chains and the solvent guides the aggregate morphology. The distinct emission color codes of the two luminogens can distinguish between water and methanol.

Di(naphthalen-2-yl)-1,2-diphenylethene-based conjugated polymers: aggregation-enhanced emission and explosive detection

Conjugated polymers with rotatable naphthalen-2-yl pendants show aggregation-enhanced emission characteristics and explosive detection potential.