Liquid Crystal Assembly for Ultra-dissymmetric Circularly Polarized Luminescence and Beyond

A Halogen-Bonded Fluorescent Molecular Photoswitch: Transition from 3D Cubic Lattice to 1D Helical Superstructure for Polarization Inversion of Circularly Polarized Luminescence

The fabrication of materials that can switch between circularly polarized luminescence (CPL) signals is both essential and challenging. Here, two new halogen-bonded fluorescent molecular photoswitches, namely, HB-switch 1 and HB-switch 2, containing α-cyano-substituted diarylethene compounds with different end groups were developed. Upon exposure to specific UV or visible light wavelengths, they exhibited controllable and reversible Z/E photoisomerization. When these switches were integrated into blue-phase liquid crystals (BPLCs), the temperature range of BP significantly expanded. Notably, the BP system incorporating HB-switch 1 exclusively achieved reversible polarization inversion of CPL signals under irradiation with specific UV/Visible light and during cooling/heating. The photo/thermal dual-response behavior of the CPL signals can be attributed to the phase transition from a high-symmetry 3D BP Icubic lattice to a low-symmetry 1D helical superstructure induced by the Z/E photoisomerization of HB-switch 1 and temperature changes. This study underscores the significance of employing halogen-bond assembly strategies to design materials with switchable CPL signals, opening new possibilities for CPL-active systems.

Chiral Co-Assembly with Narrowband Multi-Resonance Characteristics for High-Performance Circularly Polarized Organic Light-Emitting Diodes

A promising kind of ternary chiral co-assemblies with high PLQY, large dissymmetry factor (glum), and narrowband multi-resonance characteristics are achieved by codoped-thermal annealing treatments of achiral luminescent polymer F8BT, chiral inducers R/S-5011, and achiral FRET acceptor DBN-ICZ. The optimized co-assemblies (F8BT)0.9-(R/S-5011)0.1-(DBN-ICZ)0.005 display narrowband yellow emission with full-width half maximum (FWHM) of 37 nm, PLQY of 79%, and intense CPL signals with |glum| of up to 0.26. Meaningfully, solution-processed CP-OLEDs by using those ternary chiral co-assemblies as emitting layer are successfully fabricated, which display yellow circularly polarized electroluminescence (CPEL) with EQEmax of 4.6% and gEL of up to 0.16. The corresponding Q-factor could reach up to 7.36 × 10-3, which is the highest of all the reported CP-OLEDs. Moreover, the devices also exhibit excellent comprehensive device performance with low Von of 7.0 V, high Lmax of about 25 000 cd m-2, extremely low efficiency roll-off with EQE of 4.3% at 10 000 cd m-2, as well as narrowband EL with FWHM of only 39 nm. The proposed ternary co-assembly strategy in fabricating CP-OLED provides the possibility to achieve high comprehensive device performance such as balancing high EQE and large gEL value, as well as narrowband emission, high brightness and low efficiency roll-off simultaneously.

Clustering triggered emissive liquid crystalline template for dual mode upconverted and downconverted circularly polarized luminescence

Liquid crystalline materials have attracted significant attention in chiroptical research due to their ability to form long range ordered helical superstructures. Research focus has been on exploiting the unique properties of liquid crystalline materials to demonstrate highly dissymmetric circularly polarised luminescent (CPL) systems. In this study, we present a thermally driven, facile approach to fabricate CPL-active materials utilizing cholesteryl benzoate as the active substrate. Cholesteryl benzoate, a well-known thermotropic liquid crystal, has been found to manifest intriguing optical characteristics upon subjecting to repeated heating-cooling cycles. Despite the absence of conventional fluorescent moieties, the material exhibited luminescence through aggregation induced clustering triggered emission mechanism. Systematic investigations revealed excitation-dependent CPL for solid cholesteryl benzoate films when subjected to multiple thermal cycles. The excited state chiroptical investigation performed after multiple thermal cycles showed a luminescence anisotropy (glum) of 8 × 10-2, which is a high value for simple organic molecules. Moreover, upon co-assembly with lanthanide-based upconversion nanophosphors (UCNPs), the hybrid system demonstrated upconverted circularly polarised luminescence (UC-CPL). Benefiting from the ability to endow upconversion nanoparticles of various sizes, fabrication of UCNP-ChB hybrid nanocomposites exhibiting multicoloured upconversion CPL was demonstrated. These findings highlight the potential of liquid crystalline materials for diverse applications, including 3D optical displays and anticounterfeiting technologies.

Recent advances in circularly polarized luminescence of planar chiral organic compounds

Circularly polarized luminescence (CPL), as an important chiroptical phenomenon, can not only directly characterize excited-state structural information about chiroptical materials but also has great application prospects in 3D optical displays, information storage, biological probes, CPL lasers and so forth. Recently, chiral organic small molecules with CPL have attracted a lot of research interest because of their excellent luminescence efficiency, clear molecular structures, unique flexibility and easy functionalization. Planar chiral organic compounds make up an important class of chiral organic small molecular materials and often have rigid macrocyclic skeletons, which have important research value in the field of chiral supramolecular chemistry (e.g., chiral self-assembly and chiral host-guest chemistry). Therefore, research into planar chiral organic compounds has become a hotspot for CPL. It is time to summarize the recent developments in CPL-active compounds based on planar chirality. In this feature article, we summarize various types of CPL-active compounds based on planar chirality. Meanwhile, we overview recent research in the field of planar chiral CPL-active compounds in terms of optoelectronic devices, asymmetric catalysis, and chiroptical sensing. Finally, we discuss their future research prospects in the field of CPL-active materials. We hope that this review will be helpful to research work related to planar chiral luminescent materials and promote the development of chiral macrocyclic chemistry.

Near-Infrared Circularly Polarized Luminescent Physical Unclonable Functions

Distinguished from traditional physical unclonable functions (PUFs), optical PUFs derive their encoded information from the optical properties of materials, offering distinct advantages, including solution processability, material versatility, and tunable luminescence performance. However, existing research on optical PUFs has predominantly centered on visible photoluminescence, while advanced optical PUFs based on higher-level covert light remain unexplored. In this study, we present optical PUFs based on the utilization of the covert light of near-infrared circularly polarized luminescence (NIR-CPL). This interesting property is achieved by incorporating Yb-doped metal halide perovskite nanocrystals (Yb-PeNCs) possessing NIR emission property into chiral imprinted photonic (CIP) films. By employing a solvent immersion method, we successfully integrated Yb-PeNCs into these CIP films, thereby creating an optically unclonable surface. The resulting NIR-CPL emission adds a layer of advanced security to the optical PUF systems. These findings underscore the potential of solution-processable chiral films to play a pivotal role in advancing the next generation of PUFs.

Boost the Circularly Polarized Phosphorescence of Chiral Organometallic Platinum Complexes by Hierarchical Assembly into Fibrillar Networks

Circularly polarized luminescence (CPL)-active molecular materials have drawn increasing attention due to their promising applications for next-generation display and optoelectronic technologies. Currently, it is challenging to obtain CPL materials with both large luminescence dissymmetry factor (glum) and high quantum yield (Φ). A pair of enantiomeric N N C-type Pt(II) complexes (L/D)-1 modified with chiral Leucine methyl ester are presented herein. Though the solutions of these complexes are CPL-inactive, the spin-coated thin films of (L/D)-1 exhibit giantly-amplified circularly polarized phosphorescences with |glum| of 0.53 at 560 nm and Φair of ~50 %, as well as appealing circular dichroism (CD) signals with the maximum absorption dissymmetry factor |gabs| of 0.37-0.43 at 480 nm. This superior CPL performance benefits from the hierarchical formation of crystalline fibrillar networks upon spin coating. Comparative studies of another pair of chiral Pt(II) complexes (L/D)-2 with a symmetric N C N coordination mode suggest that the asymmetric N N C coordination of (L/D)-1 are favorable for the efficient exciton delocalization to amplify the CPL performance. Optical applications of the thin films of (L/D)-1 in CPL-contrast imaging and inducing CP light generation from achiral emitters and common light-emitting diode lamps have been successfully realized.

Chiral Liquid Crystalline Metal-Organic Framework Thin Films for Highly Circularly Polarized Luminescence

Combining metal-organic frameworks (MOFs) with liquid crystals to construct liquid crystalline MOFs (LCMOF) offers the advantage of endowing and enhancing their functionality, yet it remains a challenging task. Herein, we report chiral liquid crystalline MOF (CLCMOF) thin films by cross-linking the chiral liquid crystals (CLC) with MOF thin films to realize highly circular polarization luminescence (CPL) performance with photo and thermal switching. By layer by layer cross-linking stilbene-containing CLC with stilbene-based MOF (CLC/MOF) thin film, the CLCMOF thin films were successfully obtained after UV irradiation due to the abundant [2 + 2] photocycloaddition. The resulted CLCMOF thin films have strong chirality, obvious photochromic fluorescent, and strong CPL performance (the asymmetry factor reaches to 0.4). Furthermore, due to the photochromic fluorescent MOF and thermotropic CLC, the CPL can be reversed and red-shifted after heating and UV irradiation treatment, showing photo- and thermal CPL switching. Such MOF-based CPL thin films with photo/thermal CPL switching were prepared to patterns and codes for the demonstration of potential application in advanced information anticounterfeit and encryption. This study not only opens a strategy for developing chiral thin films combining MOFs and liquid crystals but also offers a new route to achieve CPL switching in optical applications.

Nano-achiral complex composites for extreme polarization optics

Composites from 2D nanomaterials show uniquely high electrical, thermal and mechanical properties1,2. Pairing their robustness with polarization rotation is needed for hyperspectral optics in extreme conditions3,4. However, the rigid nanoplatelets have randomized achiral shapes, which scramble the circular polarization of photons with comparable wavelengths. Here we show that multilayer nanocomposites from 2D nanomaterials with complex textured surfaces strongly and controllably rotate light polarization, despite being nano-achiral and partially disordered. The intense circular dichroism (CD) in nanocomposite films originates from the diagonal patterns of wrinkles, grooves or ridges, leading to an angular offset between axes of linear birefringence (LB) and linear dichroism (LD). Stratification of the layer-by-layer (LBL) assembled nanocomposites affords precise engineering of the polarization-active materials from imprecise nanoplatelets with an optical asymmetry g-factor of 1.0, exceeding those of typical nanomaterials by about 500 times. High thermal resilience of the composite optics enables operating temperature as high as 250 °C and imaging of hot emitters in the near-infrared (NIR) part of the spectrum. Combining LBL engineered nanocomposites with achiral dyes results in anisotropic factors for circularly polarized emission approaching the theoretical limit. The generality of the observed phenomena is demonstrated by nanocomposite polarizers from molybdenum sulfide (MoS2), MXene and graphene oxide (GO) and by two manufacturing methods. A large family of LBL optical nanocomponents can be computationally designed and additively engineered for ruggedized optics.

Dual Enhancement of Phosphorescence and Circularly Polarized Luminescence through Entropically Driven Self-Assembly of a Platinum(II) Complex

Addressing the dual enhancement of circular polarization (glum) and luminescence quantum yield (QY) in circularly polarized luminescence (CPL) systems poses a significant challenge. In this study, we present an innovative strategy utilizing the entropically driven self-assembly of amphiphilic phosphorescent platinum(II) complexes (L-Pt) with tetraethylene glycol chains, resulting in unique temperature dependencies. The entropically driven self-assembly of L-Pt leads to a synergistic improvement in phosphorescence emission efficiency (QY was amplified from 15 % at 25 °C to 53 % at 60 °C) and chirality, both in the ground state and the excited state (glum value has been magnified from 0.04×10-2 to 0.06) with increasing temperature. Notably, we observed reversible modulation of phosphorescence and chirality observed over at least 10 cycles through successive heating and cooling, highlighting the intelligent control of luminescence and chiroptical properties by regulating intermolecular interactions among neighboring L-Pt molecules. Importantly, the QY and glum of the L-Pt assembly in solid state were measured as 69 % and 0.16 respectively, representing relatively high values compared to most self-assembled CPL systems. This study marks the pioneering demonstration of dual thermo-enhancement of phosphorescence and CPL and provides valuable insights into the thermal effects on high-temperature and switchable CPL materials.

Mechanically Tunable Circularly Polarized Luminescence of Liquid Crystal-Templated Chiral Perovskite 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.

Endowing Metal-Organic Coordination Materials with Chiroptical Activity by a Chiral Anion Strategy

Recently, chiral metal-organic coordination materials have emerged as promising candidates for a wide range of applications in chiroptoelectronics, chiral catalysis, and information encryption, etc. Notably, the chiroptical effect of coordination chromophores makes them appealing for applications such as photodetectors, OLEDs, 3D displays, and bioimaging. The direct synthesis of chiral coordination materials using chiral organic ligands or complexes with metal-centered chirality is very often tedious and costly. In the case of ionic coordination materials, the combination of chiral anions with cationic, achiral coordination compounds through noncovalent interactions may endow molecular materials with desirable chiroptical properties. The use of such a simple chiral strategy has been proven effective in inducing promising circular dichroism and/or circularly polarized luminescence signals. This concept article mainly delves into the latest advances in exploring the efficacy of such a chiral anion strategy for transforming achiral coordination materials into chromophores with superb photo- or electro-chiroptical properties. In particular, ionic small-molecular metal complexes, metal clusters, coordination supramolecular assemblies, and metal-organic frameworks containing chiral anions are discussed. A perspective on the future opportunities on the preparation of chiroptical materials with the chiral anion strategy is also presented.

Light-Driven Sign Inversion of Circularly Polarized Luminescence Enabled by Dichroism Modulation in Cholesteric Liquid Crystals

Stimuli-responsive circularly polarized luminescence (CPL) materials show great promise in applying information encryption and anticounterfeiting. Herein, light-driven CPL sign inversion is achieved by combining a photoresponsive achiral negative dichroic dye (KG) and a static achiral positive dichroic dye (NR) as dopants at the 0.5:0.5 weight ratio into the cholesteric liquid crystal (CLC) host. The side chains of KG undergo trans/cis isomerization after 365 nm UV light irradiation, leading to the dichroism (SF) decrease. The |glum| value of CLC doping with KG (CLC-KG) weakens from 0.67 to 0.28 in response to the order degree change. Taking advantage of its unique CPL response property, the light-driven CPL sign inversion is achieved (from -0.20/0.14 to 0.02/-0.04) by incorporating NR (0.5:0.5) into the CLC-KG with helical superstructure static. Based on the synergistic use of circular polarization and responsiveness state as cryptographic primitives, the multidimensional information encryption CLC system can be realized.

Dynamically Modulating the Dissymmetry Factor of Circularly Polarized Organic Ultralong Room-Temperature Phosphorescence from Soft Helical Superstructures

Achieving circularly polarized organic ultralong room-temperature phosphorescence (CP-OURTP) with a high luminescent dissymmetry factor (glum ) is crucial for diverse optoelectronic applications. In particular, dynamically controlling the dissymmetry factor of CP-OURTP can profoundly advance these applications, but it is still unprecedented. This study introduces an effective strategy to achieve photoirradiation-driven chirality regulation in a bilayered structure film, which consists of a layer of soft helical superstructure incorporated with a light-driven molecular motor and a layer of room-temperature phosphorescent (RTP) polymer. The prepared bilayered film exhibits CP-OURTP with an emission lifetime of 805 ms and a glum value up to 1.38. Remarkably, the glum value of the resulting CP-OURTP film can be reversibly controlled between 0.6 and 1.38 over 20 cycles by light irradiation, representing the first example of dynamically controlling the glum in CP-OURTP.

Biobased and biodegradable films exhibiting circularly polarized room temperature phosphorescence

There is interest in developing sustainable materials displaying circularly polarized room-temperature phosphorescence, which have been scarcely reported. Here, we introduce biobased thin films exhibiting circularly polarized luminescence with simultaneous room-temperature phosphorescence. For this purpose, phosphorescence-active lignosulfonate biomolecules are co-assembled with cellulose nanocrystals in a chiral construct. The lignosulfonate is shown to capture the chirality generated by cellulose nanocrystals within the films, emitting circularly polarized phosphorescence with a 0.21 dissymmetry factor and 103 ms phosphorescence lifetime. By contrast with most organic phosphorescence materials, this chiral-phosphorescent system possesses phosphorescence stability, with no significant recession under extreme chemical environments. Meanwhile, the luminescent films resist water and humid environments but are fully biodegradable (16 days) in soil conditions. The introduced bio-based, environmentally-friendly circularly polarized phosphorescence system is expected to open many opportunities, as demonstrated here for information processing and anti-counterfeiting.

General Strategy to Prepare Nondoped Circularly Polarized Luminescent Liquid Crystal Materials with Tunable Performance

Chiral luminescent liquid crystals have attracted widespread attention from researchers due to their unique advantages in constructing circularly polarized luminescent (CPL) materials with large luminescent asymmetry factor (glum) values. However, how to effectively prepare nondoped CPL chiral liquid crystals remains a challenge. In this article, we developed an effective and universal method to prepare nondoped CPL chiral liquid crystal materials. To achieve our strategy, we copolymerized chiral monomer M0Mt with α-cyanostilbene-based luminescent monomers MmPVPCN (m = 6, 8, 10) bearing different flexible spacer lengths to obtain a series of CPL chiral liquid crystal copolymers poly(MmPVPCN(x)-co-M0Mt(y)). Under the induction of the chiral component, the α-cyanostilbene component assembles to form chiral liquid crystals. Meanwhile, α-cyanostilbene also exhibits aggregation-induced emission enhancement characteristics. Therefore, with the help of the selective reflection effect of chiral liquid crystals, the copolymer films can emit efficient CPL. For poly(M8PVPCN(0.85)-co-M0Mt(0.15)), the glum and solid luminescence quantum yield can achieve -2.61 × 10-2 and 25.04%, respectively. In addition, by altering the chemical structure of the copolymers, the phase structure of the copolymers can be effectively controlled, thereby regulating their CPL properties.

Helical-caging enables single-emitted large asymmetric full-color circularly polarized luminescence

Colorful circularly polarized luminescence materials are desired for 3D displays, information security and asymmetric synthesis, in which single-emitted materials are ideal owing to self-absorption avoidance, evenly entire-visible-spectrum-covered photon emission and facile device fabrication. However, restricted by the synthesis of chiral broad-luminescent emitters, the realization and application of high-performing single-emitted full-color circularly polarized luminescence is in its infancy. Here, we disclose a single-emitted full-color circularly polarized luminescence system (spiral full-color emission generator), composed of whole-vis-spectrum emissive quantum dots and chiral liquid crystals. The system achieves a maximum luminescence dissymmetry factor of 0.8 and remains an order of 10-1 in visible region by tuning its photonic bandgap. We then expand it to a series of desired customized-color circularly polarized luminescence, build chiral devices and further demonstrate the working scenario in the photoinduced enantioselective polymerization. This work contributes to the design and synthesis of efficient chiroptical materials, device fabrication and photoinduced asymmetric synthesis.

A Single-Enantiomer Emitter Enabled Superstructural Helix Inversion for Upconverting and Downshifting Luminescence with Bidirectional Circular Polarization

The helical twisting tendency of liquid crystals (LCs) is generally governed by the inherent configuration of the chiral emitter. Here, we introduce the multistage inversion of supramolecular chirality as well as circularly polarized luminescence (CPL) by manipulating the ratio of single enantiomeric emitters (R-PCP) to LC monomers (5CB). Increasing the content of R-PCP from 1 wt % to 3 wt % inverted the helix of LCs from left-handed to right-handed, accompanying a CPL sign changed from positive to negative. The biaxiality of chiral emitters, as well as the steric effect of chiral-chiral and chiral-achiral interaction, were identified as the reasons for helical sense inversion. Due to the strong helical twisting power, 4 wt % R-PCP drove the photonic band gap (PBG) of chiral LCs to match up with their emission range, leading to an inversion of the CPL again with a high dissymmetry factor (≈1.2). Directly adjusting the PBG using chiral emitters is seldom achieved in cholesteric LCs. On this basis, an achiral sensitizer PtTPBP was assembled into the helical superstructure. The generation of triplet-triplet annihilation-induced upconverted CPL from R-PCP and the downshifting CPL from PtTPBP with opposite rotation was achieved in a single chiral LC system by tuning the position of the PBG.

Achiral Dichroic Dyes-mediated Circularly Polarized Emission Regulated by Orientational Order Parameter through Cholesteric Liquid Crystals

It is noteworthy that cholesteric liquid crystal (CLC) platforms have been witnessed in high-performance circularly polarized luminescence (CPL) behaviors through the highly organized chiral co-assembled arrangement of achiral dyes. However, most CPL-active design strategies are closely relative to the helix co-assembly structure of CLC rather than achiral dyes. Herein, we developed an intriguing regulation strategy for CPL-active CLC materials. They were regulated using the orientational order parameter (SF ) of achiral dichroic dyes as an incisive probe for the order arrangement degree of achiral dyes in CLC media. The I-shaped phenothiazine derivative PHECN dye (SF =0.30) emitted a strong CPL signal (|glum |=0.47). In contrast, the T-shaped derivative (PHEBen) dye (SF =0.09) showed a weak circular polarization level (|glum |=0.07) at similar CLC textures. Most interestingly, this kind of dichroic PHECN dye with a higher SF could greatly improve the contrast ratio of CPL (Δglum =0.47) and emission intensity (ΔFL=46.0 %) at direct-current electric field compared with the T-shaped PHEBen (Δglum =0.07 and ΔFL=1.0 %) in CLC. This work demonstrates that an induced CPL emission can be mediated using achiral dichroic dye, which will open a new avenue for developing excellent CPL-active display materials.