Polymerization-with-assembly enables homogeneous circularly polarized luminescence structures

Research on circularly polarized luminescent (CPL) materials has evolved into a hot research topic because of their potential application prospects in the optoelectronics and chiroptical fields. Achieving a high glum value and high quantum efficiency is essential and challenging in CPL research. To date, various material design strategies, such as chiral organic small molecules, CPL polymers, chiral lanthanide complexes, chiral liquid crystals and supramolecular self-assembly, have been proposed to achieve a CPL emitter with a high glum value. Among them, chiral luminescent liquid crystals (CLLCs) are recognized as a key approach for achieving CPL materials with a high glum factor owing to their exceptional optical properties and flexibility. In this review, we focused on the various synthesis methods employed for developing CLLCs, their properties and their potential applications. The synthesis section discusses various approaches employed to design chiral luminescent liquid crystals, including (i) doping systems for incorporating chiral dopants into achiral liquid crystalline hosts and (ii) nondoping methods for preparing AIE active chiral luminescent liquid crystalline materials. The section on properties highlights how chirality influences the optical, electronic and structural characteristics of CLLCs. Finally, we discuss the diverse applications of CLLCs from photonics and chiral switching to optoelectronic devices and beyond. This review provides new insights into recent research developments and future opportunities in this booming research field. We anticipate that this review could offer a clear picture of the interesting properties of chiral luminescent liquid crystal materials and inspire more researchers to work in this potential area.

Deliberately targeted emission wavelengths, high quantum yields (QYs), and high luminescence dissymmetry factors (glum) are essential for chiral materials exhibiting circularly polarized luminescence (CPL). The addition of achiral luminophores to chiral nematic liquid crystals (N*-LCs) is a promising way to generate CPLs. We aimed to generate red-green-blue (RGB)- and white-colored CPLs by adding each of three types of fluorene-based luminescent molecules and their mixtures to N*-LCs. Herein, achiral fluorene-based derivatives with RGB emission colors were synthesized and added to N*-LCs containing alkylcyanobiphenyl- and alkylcyanoterphenyl-based N-LCs and an axially chiral binaphthyl dopant. A mixture of the RGB fluorene derivatives was subsequently prepared at an optimum mixing ratio and added to the N*-LCs. The fluorene-based derivatives dissolved in the N*-LCs exhibited induced CPLs of RGB and white colors with QY values of 60-90% and glum values of 0.5-1.0. The glum value of 1.0 is the highest among white CPLs reported thus far, and there are no examples that simultaneously exhibit both high QY and high glum. Furthermore, a mixture of the RGB fluorene derivatives was added to a temperature-responsive N*-LC containing a thermally invertible chiral compound, resulting in reversible thermal inversion of white CPL.

Chiral liquid crystal (LC) materials are indispensable functional components of advanced optoelectronic devices. In this study, we fabricated chiral nematic liquid crystal (N*‐LC) devices by introducing perylene and its N,N ‐di‐ n ‐octyl‐3,4,9,10‐perylenetetracarboxylic diimide (OPDI) derivative, as achiral luminophores, into an N*‐LC host composed of the achiral 4'‐pentyl‐4‐cyanobiphenyl (5CB) LC and the planar‐chiral 4,12‐dihydroxy[2.2]paracyclophane ([2.2]‐PCP) dopant. Remarkably, both perylene and OPDI exhibited pronounced circularly polarized luminescence (CPL) within the planar‐chiral‐dopant‐induced N*‐LC environment, despite the use of achiral luminophores. Furthermore, the handedness of the CPL can be controlled by the planar chirality of the [2.2]‐PCP dopant. In addition, the resulting N*‐LC materials exhibited rapid and reversible inversion of CPL handedness when a direct‐current (DC) electric field was applied and then removed. This reversible behavior is ascribable to the electric‐field‐induced phase transition between the helical N*‐LC structure of 5CB and another ordered phase. These findings provide a new material design principle in which a planar‐chiral dopant induces the N*‐LC phase in 5CB, thereby enabling high‐speed, reversible CPL modulation based on the application of an electric field.

Chiral side-chain liquid crystal polymer is synthesized from polysiloxanes and liqud crystal monomer 4-(Undecenoic-1- yloxybenzoyloxy)-4’-benzonitrile and 6-[4-(4- Undecenoic -1-yloxybenzoyloxy)- hydroxyphenyl] cholesteryl hexanedioate. The optical and thermal property of the monomer and polymer are shown by POM and DSC. As the unique optical property of the polymer, the bandgaps are shifted for heating temperature. The reflection bandgaps is shifted from 546nm to 429nm with temperature increase. As a photonic material, the chiral polymer which sensitive responses under the outfield is widely studied for reflection display, smart switchable reflective windows and defect model CLC laser etc.

Semiconductor quantum dots (QDs) are attractive materials for light-emitting devices, and the photoluminescence (PL) from QDs can be enhanced near a metal surface due to surface plasmon (SP) resonance. To integrate QDs into metal structures, QD/poly(methyl methacrylate) (PMMA) composite thin films are generally used. However, it has been reported that QDs tend to aggregate in the PMMA matrix. In this study, we fabricated two types of QD/polymer composite thin films with different degrees of QD aggregation by additionally using poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MA), which is known to prevent QD aggregation. Furthermore, these two types of films were fabricated on Ag films, with the distance between the Ag films and the QDs controlled by Al2O3 spacer layers, and the PL enhancement was compared between the two film types. Finally, we reveal that QD aggregation in the polymer matrix significantly affects the PL enhancement. Although the aggregation trends differed between PMMA and PMMA-co-MA, the results suggest a possible increase in the internal quantum efficiency (IQE) in both film types.

Polymer-based circularly polarized luminescence (CPL) materials with the advantage of diversified structure, easy fabrication, high thermal stability, and tunable properties have garnered considerable attention. However, adequate and precise tuning over CPL in polymer-based materials remains challenging due to the difficulty in regulating chiral structures. Herein, visualized full-color CPL is achieved by doping red, green, and blue quantum dots (QDs) into reconfigurable blue phase liquid crystal elastomers (BPLCEs). In contrast to the CPL signal observed in cholesteric liquid crystal elastomers (CLCEs), the chiral 3D cubic superstructure of BPLCEs induces an opposite CPL signal. Notably, this effect is entirely independent of photonic bandgaps (PBGs) and results in a high glum value, even without matching between PBGs and the emission bands of QDs. Meanwhile, the lattice structure of the BPLCEs can be reversibly switched via mechanical stretching force, inducing on-off switching of the CPL signals, and these variations can be further fixed using dynamic disulfide bonds in the BPLCEs. Moreover, the smart polymer-based CPL systems using the BPLCEs for anti-counterfeiting and information encryption have been demonstrated, suggesting the great potential of the BPLCEs-based CPL active materials.

Co-assembling nanostructures with achiral luminescent moieties represents a significant approach for developing circularly polarized luminescence (CPL) materials. However, for chiral nanotube structures, it remains unclear whether their inner and outer surfaces exhibit equivalent induction effects. Here, enantiomeric C3-symmetric chiral methylphenylalanine derivatives were designed, and uniform supramolecular helical nanotubes were successfully fabricated. These helical nanotubes serve as host materials for the selective loading of achiral luminescent organic dye or inorganic quantum dots (QDs) into their inner channels or onto their outer surfaces via co-assembly. It was revealed that through chiral transfer, the achiral luminescent units acquired CPL activity upon integration with the chiral nanotubes, achieving dissymmetry factors (glum) on the order of 10-2 to 10-3, respectively. Interestingly, the CPL signals induced by the inner and outer surfaces exhibited opposite signs, with the signal from the inner surface being ten times stronger than that from the outer surface. Further structural analysis indicated that the concave geometry and nanoconfinement effects of the inner surface contributed to the CPL sign inversion and the enhanced luminescence dissymmetry. This work provides a novel and versatile platform for customizing the fluorescence and CPL properties of achiral luminescent units.

Circularly polarized luminescent (CPL) materials featuring high luminescence dissymmetry factors (glum), high photoluminescence quantum yields (ΦPL), and tunable emission properties are essential for practical applications. Herein, three chiral aggregation-induced emission (AIE) tetraphenylbutadiene (TPB) derivatives (TPB-Chol*-H, TPB-Chol*-CHO, TPB-Chol*-OC6) with high ΦPL were synthesized, each bearing symmetric chiral bis-cholesteryl moieties and different electron-donating/accepting groups. When employed as chiral dopants, minute quantities of each derivative were individually doped into the achiral nematic liquid crystal (LC) 5CB, affording three series of luminescent chiral nematic LC composite systems (TPB-N*-LCs). These composites exhibit remarkable CPL activity with high glum values, attributed to host-guest interactions that facilitate co-assembly into ordered supramolecular helices and chiral amplification, despite the chiral AIE dopant itself lacking CPL emission. CPL signals and glum values enhance with increasing chiral dopant loading, peaking at +0.31, +0.42, and +0.58 for TPB-Chol*-H/5CB, TPB-Chol*-CHO/5CB, and TPB-Chol*-OC6/5CB, respectively, at 3 wt% doping. Notably, the introduction of electron-donating hexyloxy groups (TPB-Chol*-OC6) or electron-accepting aldehyde groups (TPB-Chol*-CHO) modulates molecular packing, compatibility with 5CB host, and intermolecular interactions. Doping TPB-Chol*-OC6 into 5CB achieves the optimal performance, with glum = +0.58 and ΦPL = 84.5%. This work offers an effective strategy for developing CPL materials with simultaneously high glum and ΦPL.

A Busy March Meeting Is Brewing in Seattle

Tunable AI-CPL behavior by regulation of microstructure of AIE-active isomers through chiral emissive liquid crystals

Fluorescence quenching caused by aggregation of water-soluble CdSe quantum dots

Endowing perovskite quantum dots (PQDs) with circularly polarized luminescence (CPL) offers great promise for innovative chiroptical applications, but the existing strategies are inefficient in acquiring stimuli-responsive flexible chiral perovskite films with large, tunable dissymmetry factor (glum) and long-term stability. Here, we report a strategy for the design and synthesis of luminescent cholesteric liquid crystal elastomer (Lumin-CLCE) films with mechanically tunable CPL, which is enabled by liquid crystal-templated chiral self-assembly and in situ covalent cross-linking of judiciously designed photopolymerizable CsPbX3 (X=Cl, Br, I) PQD nanomonomers into the elastic polymer networks. The resulting Lumin-CLCE films showcase circularly polarized structural color in natural light and noticeable CPL with a maximum glum value of up to 1.5 under UV light. The manipulation of CPL intensity and rotation direction is achieved by controlling the self-assembled helicoidal nanostructure and the handedness of soft helices. A significant breakthrough lies in the achievement of a reversible, mechanically tunable perovskite-based CPL switch activated by biaxial stretching, which enables flexible, dynamic anti-counterfeiting labels capable of decrypting preset information in specific polarization states. This work can provide new insights for the development of advanced chiral perovskite materials and their emerging applications in information encryption, flexible 3D displays, and beyond.

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

The development of stimuli-responsive chiral cholesteric liquid crystals (CLCs) materials holds significant potential for achieving three-dimensional (3D) anti-counterfeiting and multi-level information encryption. However, constructing phototunable CLCs systems with easy fabrication and fast response remains a great challenge. Herein, we exploit an excitation-dependent CLCs (ExD-CLCs) material by establishing dynamically photoresponsive dichroic competition between two achiral dyes: a negative dichroic dye (SP-COOH) and a positive dichroic dye (Nile Red, NR) within a CLCs medium. The ExD-CLCs exhibits a negative circularly polarized luminescence (CPL) signal (glum=-0.16) at 625 nm when excited at 365 nm. Remarkably, under excitation at 430 nm, the CPL signal is inverted, and the glum value increases to +0.26. Notably, the helical superstructure and handedness of the ExD-CLCs remain unchanged during this reversal process. The CPL signal reversal is driven by the dichroic competition between the SP-COOH dimer, which displays strong negative dichroism in its open-ring isomer form and silent negative dichroism in its closed-ring isomer form, and the NR dye, which exhibits static positive dichroism. Leveraging these excitation-dependent CPL properties, the quadruplex numerical anti-counterfeiting using ExD-CLCs is achieved.

Nanocomposite films combining organic semiconductors (OSCs) and colloidal quantum dots (QDs) are promising systems for next-generation optoelectronic technologies such as singlet-fission photon multiplication (SF-PM). Here, we show that tuning the solubilizing substituents on the high-triplet-energy SF-OSC (1E,3E,5E)-1,6-diphenylhexa-1,3,5-triene (DPH) enables precise control over film morphology and QD dispersibility. Grazing-incidence X-ray scattering reveals that PbS QDs ligated with oleic acid are poorly dispersed in all DPH derivatives, whereas hexanoic acid or DPH-carboxylic acid ligands significantly improve QD dispersibility. A clear design rule emerges: increasing solubilizing group volume relative to the DPH core enhances QD dispersibility, enabling well-dispersed QDs even in highly ordered DPH matrices. An exception arises in a derivative that forms an amorphous, nonequilibrium morphology that fully disperses QDs, but later crystallizes, resulting in QD aggregation. These findings show that OSC:QD nanocomposites require co-optimization of ligand-OSC chemistry and crystallization kinetics, providing a framework for designing efficient SF-PM and related technologies.