Fundamentals, Advances, and Artifacts in Circularly Polarized Luminescence (CPL) Spectroscopy

Helically Chiral Mixed-Valence Systems Comprising N-Heterotriangulenes: Stabilization of the Cationic Species by π-Expansion

We report the efficient stabilization of nitrogen-centered radical cations in a dimeric N-heterotriangulene bridged by a rigid π-conjugated [5]helicene backbone (NTH). The rigid scaffold exhibits helical chirality, allowing the enantiomers to be separated and their chiroptical properties studied, including circular dichroism (CD) and circularly polarized luminescence (CPL). Successive oxidation towards the radical cation NTH⋅+ and dication NTH2+ reveals strong electronic communication between the two nitrogen-centers, as demonstrated by X-ray crystallography, electrochemistry, UV/Vis/nIR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. CD measurements of the enantiomerically pure cations reveal strong Cotton effects in the nIR region extending up to 2000 nm. Density functional theory confirms the observed enhanced electronic communication, classifying NTH⋅+ as a Robin-Day Class III mixed-valence charge-transfer (MV-CT) system. NTH2+ exhibits a quinoidal structure with partial diradical character and open-shell singlet ground state, as shown by variable temperature EPR measurements. Time-resolved transient absorption spectroscopy shows the photo-induced generation of NTH⋅+ in the presence of a suitable electron acceptor.

Chiral co-assembly of a polyoxometalate complex with an achiral pyrene derivative enables redox-modulated circularly polarized luminescence

We report the fabrication of helical structures with responsive circularly polarized lumunescence (CPL) via the chiral co-assembly of a cholesterol-modified Lindqvist type polyoxometalate (POM) and an achiral pyrenyl derivative. The chiral surfactant encapsulated POM (CSEP) complex was synthesized by combining (TBA)2[Mo6O19] with cholesterol-containing organic surfactants through ion exchange. It was found that the CSEP complex self-assembled into left-handed helical structures in mixed organic solvents, which could serve as a chiral template that enables achiral pyrenyl fluorophores (Py) to exhibit chiroptical properties. When doping Py at a ratio of 5 wt% into the system, the chiral co-assembly with CSEP in the mixed organic solvent results in the formation of helical nanofibers, which emit blue CPL signals. Furthermore, the chiral helical structures can be dynamically transformed to spherical aggregates upon UV illumination, accompanied by photochromism. The disappearance of CPL signals corresponded to the disruption of the chiral morphology in the co-assembled nanostructures. More importantly, the morphology transformation is reversible. The nanospheres transform into helical nanofibers under the oxidation of H2O2, which could trigger the regeneration of CPL signals. This work contributes to the understanding and development of chiral supramolecular systems featuring stimulus-responsive CPL switches.

Circularly Polarized Luminescence Inversion Induced by Achiral Dyes in Organogels

Supramolecular assembly strategy is widely applied to develop circularly polarized luminescence (CPL) materials, especially for centrally chiral systems. The CPL sign of supramolecular gels based on a centrally chiral compound is generally regulated by employing the opposite chiral enantiomers. However, controlling CPL signals by regulating the interaction model between the achiral fluorophore and chiral gelator, accompanied by the adjustment of the emission wavelength, remains challenging. Herein, we have developed binary supramolecular gels based on achiral cyanostilbene derivatives and a chiral gelator. Two binary supramolecular gels have displayed achiral fluorophore-induced CPL sign inversion, attributed to the hydrogen bonding interaction between the carboxyl group of two fluorophores and the amino group of chiral gelator. This work reveals a rational approach to the design of CPL material with tunable CPL sign and CPL emission wavelength.

Chiral light-matter interactions in solution-processable semiconductors

Chirality is a fundamental property widely observed in nature, arising in objects without a proper rotation axis, therefore existing as forms with distinct handedness. This characteristic can profoundly impact the properties of materials and can enable new functionality, especially for spin-optoelectronics. Chirality enables asymmetric light and spin interactions in materials, with widespread potential applications ranging from energy-efficient displays, holography, imaging, and spin-selective and enantio-selective chemistry to quantum information technologies. This Review focuses on the emerging material class of solution-processable chiral semiconductors, a broad material class comprising organic, inorganic and hybrid materials. These exciting materials offer the opportunity to design desirable light-matter interactions based on symmetry rules, potentially enabling the simultaneous control of light, charge and spin. We briefly discuss the various types of solution-processible chiral semiconductors, including small molecules, polymers, supramolecular self-assemblies and halide perovskites. We then examine the interplay between chirality and spin in these materials, the various mechanisms of chiral light-matter interactions, and techniques utilized to characterize them. We conclude with current and future applications of chiral semiconductors that take advantage of their chiral light-matter interactions.

An Opto-Iontronic Cholesteric Liquid Crystalline Retina for Multimodal Circularly Polarized Neuromorphic Vision

Circularly polarized light (CPL) is fundamental to phase-controlled imaging, quantum optics, and optical computing. Conventional CPL detection, relying on polarizers and quarter-wave plates, complicates device design and reduces sensitivity. Among emerging CPL detectors, organic field-effect transistors (OFET) with helical organic semiconductors are highly promising due to their compact structures but suffer tedious synthesis, low dissymmetric factors (gph < 0.1), and high operating voltages (> 50 V). To address these issues, an opto-iontronic cholesteric liquid crystalline (i-CLC) film is developed that is both electrically and photonically active, serving as the dielectric in phototransistors. The well-defined cholesteric structure and broadly tunable pitches of the i-CLC film enable it to detect CPL with an excellent "handedness" selectivity across a broad spectrum. Moreover, its ionic nature provides a high capacitance (up to 580 nF cm- 2 @20 Hz). The resulting flexible CPL detectors achieve an unprecedentedly high dissymmetry factor (gph = 1.33) at low operating voltages (< 5 V), showcasing significant potential in optical communication and data encryption. Furthermore, leveraging high gph, they can perform in-sensor computing for highly accurate semantic segmentation using fused multimodal visual inputs (e.g., circularly polarized and ordinary light), achieving an accuracy of 75.73% and a mean intersection over the union of 0.3982, surpassing the performance of non-CPL photodetectors. Additionally, it optimizes power consumption by a factor of 102 compared to most conventional visual processing systems, offering a groundbreaking hardware solution for high-performance neuromorphic CPL vision.

Supergelation via Weak Interactions: An Efficient Method for the Fabrication of Hydrophobic Circularly Polarized Luminescent Materials

The hydroxylated derivative of stearic acid, 12-hydroxy stearic acid (12-HSA), is derived from castor oil and possesses a unique chemical structure that renders it noteworthy in the field of cosmetics and medicine. Being derived from renewable sources, it aligns with the growing demand for sustainable and eco-friendly material. While the gelation properties of 12-HSA are known, its chiroptical properties remain unexplored. Herein, a host-guest approach is adopted for the induction of optical activity in achiral chromophores using 12-HSA as a chiral template. The induction of optical activity from the host gelator to the guest chromophores resulted in the generation of circularly polarized luminescence (CPL) from hybrid nanocomposites. This strategy was effective in generation of chiral luminescence in non-polar chromophores, a class of molecules unexplored in CPL. The use of red- and blue-emitting chromophores led to the fabrication of chiral light-emitting materials in the respective spectral range. The structural attributes of the gelator imparted hydrophobic character to the co-assembled nanostructures. The robustness of the nanocomposites helped derive self-standing chiral luminescent soft materials of desired sizes and shapes. The readily available raw materials and ease of operation make this an effective approach for the fabrication of CPL active materials for diverse applications.

Circularly polarized luminescence with high dissymmetry factors for achiral organic molecules in solutions

Circularly polarized luminescence (CPL) in solution offers several advantages. However, it remains challenging for organic molecules to achieve circularly polarized luminescence with high dissymmetry factor (glum) in solution. Herein, a general strategy is developed by placing chiral nematic liquid crystals (N*-LCs) behind the solution of achiral organic molecules. The selective reflection-transmission mechanism of solution-N*-LC composite system enables the generation of full-color and white circularly polarized light with |glum| even reaching 2.0. This strategy demonstrates versatility, being applicable to both aqueous and organic solutions, and effectively achieving the circularly polarized luminescence of multiple molecules with high glum values. Additionally, CPL switching and logic gate applications are successfully realized by leveraging the selective reflection-transmission mechanism of N*-LCs and the reversible acid-base responsiveness of the solution systems. This work provides a general and robust strategy for achiral organic molecules to achieve circularly polarized luminescence with high |glum| values in solutions.

An Alloy Engineering Strategy toward Helical Microstructures of Achiral π-Conjugated Molecules for Circularly Polarized Luminescence

Helical assembly has been demonstrated to efficiently facilitate the circularly polarized luminescence (CPL) performances, but the synthesis of micro- and nanohelices from rigid achiral π-conjugated compounds remains challenging due to the absence of bilayer structures or complementary hydrogen-bonding interactions. Here, we develop an alloying strategy for the realization of helical microstructures from achiral anthracene/anthracene derivatives with x-/x-axis modification or anthracene/tetracene derivatives with x-/y-axis modification via solution coassembly. Interestingly, two anthracene derivatives bearing asymmetric phenyl/phenylethynyl groups and symmetric phenylethynyl groups can assemble into spiral microribbons with a fractal branching pattern. Using such an alloying strategy, color-tailorable ternary spiral microtubules/microribbons referring to high-efficiency energy transfer processes are achievable. Molecular dynamics simulations reveal that the Von Mises stress produced by symmetry differences of two components induces symmetry breaking of alloy structures associated with twisting. Additionally, the contents of the guest and H2O also play a vital role in the formation of intricate helical microstructures. Single binary and ternary spiral microribbons present considerable CPL properties with a dissymmetric factor ('glum') of more than 0.01. The present work provides new insights into the formation of helical microcrystals with complex topologies and new optoelectronic functions.

Circularly polarized electroluminescence from chiral supramolecular semiconductor thin films

Current organic light-emitting diode (OLED) technology uses light-emitting molecules in a molecular host. We report green circularly polarized luminescence (CPL) in a chirally ordered supramolecular assembly, with 24% dissymmetry in a triazatruxene (TAT) system. We found that TAT assembled into helices with a pitch of six molecules, associating angular momentum to the valence and conduction bands and obtaining the observed CPL. Cosublimation of TAT as the "guest" in a structurally mismatched "host" enabled fabrication of thin films in which chiral crystallization was achieved in situ by thermally triggered nanophase segregation of dopant and host while preserving film integrity. The OLEDs showed external quantum efficiencies of up to 16% and electroluminescence dissymmetries ≥10%. Vacuum deposition of chiral superstructures opens new opportunities to explore chiral-driven optical and transport phenomena.

Amino-Acid-Induced Circularly Polarized Luminescence of Octahedral Lanthanide Cage

Chiral metal organic cage compounds with excellent circularly polarized luminescent performance have broad application prospects in many fields. Herein, two lanthanide complexes with luminescent properties in the form of racemic hexagonal octahedral cages were synthesized using a tri (β-diketone) ligand. Eu6(C21H6F15O6)8(H2O)6 exhibited red light emission with high quantum yields of 61 %. Then, we successfully synthesized lanthanide compounds with circularly polarized luminescent activity by chiral induction of the racemic mixture with arginine. The luminescence asymmetry factor was 0.53, and the circularly polarized luminescent merit figure reached 0.323. High-performance circularly polarized luminescent of racemic cage like compound systems have achieved using cheap and readily available chiral amino acid molecules through chiral induction.

Inversion of Circularly Polarized Luminescence in the Left-Handed Chitosan-Templated Co-assemblies

Circularly polarized luminescence (CPL) materials are attractive due to their unique applications in fields such as 3D displays, information encryption, and chiroptical switches. Natural biomolecules-based CPL materials are gaining plenty of attention due to their chiral diversity and sustainability. However, it is still challenging to construct CPL materials with opposite CPL signs from a single natural biomolecule due to its inherent chirality. Here, chiral assemblies with opposite CPL signs using chitosan oligosaccharide (COS) and achiral luminescent dyes are successfully prepared. It is found that COS can serve as a chiral template to induce the ordered assembly of the dyes along the polymer chain through electrostatic attraction interaction. It is demonstrated experimentally that the structural planarity of the dye molecules is crucial for the formation of chiral co-assemblies. Interestingly, the left-handed COS-templated co-assemblies can emit CPL with opposite handedness, which is controlled by the helicity degree of the co-assemblies. This study not only deepens the understanding of the complex assembly of natural biomacromolecules but also provides new insights into the design and construction of CPL materials.

Efficient Spin-Light-Emitting Diodes With Tunable Red to Near-Infrared Emission at Room Temperature

Spin light-emitting diodes (spin-LEDs) are important for spin-based electronic circuits as they convert the carrier spin information to optical polarization. Recently, chiral-induced spin selectivity (CISS) has emerged as a new paradigm to enable spin-LED as it does not require any magnetic components and operates at room temperature. However, CISS-enabled spin-LED with tunable wavelengths ranging from red to near-infrared (NIR) has yet to be demonstrated. Here, chiral quasi-2D perovskites are developed to fabricate efficient spin-LEDs with tunable wavelengths from red to NIR region by tuning the halide composition. The optimized chiral perovskite films exhibit efficient circularly polarized luminescence from 675 to 788 nm, with a photoluminescence quantum yield (PLQY) exceeding 86% and a dissymmetry factor (glum) ranging from 8.5 × 10-3 to 2.6 × 10-2. More importantly, direct circularly polarized electroluminescence (CPEL) is achieved at room temperature in spin-LEDs. This work demonstrated efficient red and NIR spin-LEDs with the highest external quantum efficiency (EQE) reaching 12.4% and the electroluminescence (EL) dissymmetry factors (gEL) ranging from 3.7 × 10-3 to 1.48 × 10-2 at room temperature. The composition-dependent CPEL performance is further attributed to the prolonged spin lifetime as revealed by ultrafast transient absorption spectroscopy.

Hyperfluorescence circularly polarized OLEDs consisting of chiral TADF sensitizers and achiral multi-resonance emitters

Developing circularly polarized organic light-emitting diodes (CP-OLEDs) that simultaneously achieve narrow-spectrum emission and high electroluminescence (EL) efficiency remains a formidable challenge. This work prepares two pairs of efficient circularly polarized thermally activated delayed fluorescence (CP-TADF) materials, featuring high photoluminescence quantum yields, short delayed fluorescence lifetimes, good luminescence dissymmetry factors and large horizontal dipole ratios. They can function as emitters for efficient sky-blue CP-OLEDs, providing high maximum external quantum efficiencies (ηext,maxs) (33.8%) and good EL dissymmetry factors (gELs) (-2.64 × 10-3). More importantly, they can work as sensitizers for achiral multi-resonance (MR) TADF emitters, furnishing high-performance blue and green hyperfluorescence (HF) CP-OLEDs with intense narrow-spectrum CP-EL and good ηext,maxs (31.4%). Moreover, tandem HF CP-OLEDs are fabricated for the first time by employing CP-TADF sensitizers and achiral MR-TADF emitters, which radiate narrow-spectrum CP-EL with an extraordinary ηext,maxs (51.3%) and good gELs (4.87 × 10-3). The circularly polarized energy transfer as well as chirality-induced spin selectivity effect of CP-TADF sensitizers are considered to contribute greatly to the generation of efficient CP-EL from achiral MR-TADF emitters. This work not only explores efficient CP-TADF materials but also provides a facile approach to construct HF CP-OLEDs with achiral MR-TADF emitters.

New Concept on the Generation and Regulation of Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) has attracted tremendous attention because of its significant application prospect across multiple fields of three-dimensional display, data storage, and information encryption. Chirality and luminescence are two necessary prerequisites for the generation of CPL. However, controlling these two factors simultaneously in a rational manner remains a challenge. Herein, we highlight the recent advances on the rational generation and regulation of CPL through the new concept, named matching rules, mainly including fluorescence-selective absorption, circularly polarized fluorescence energy transfer, and chiral communication between excited state and ground state. An important commonality among these strategies is that they need a good overlap of the corresponding spectra between the luminescent part (fluorescence, phosphorescence, or CPL) and the chiral part (organic, inorganic, or organic-inorganic hybrid materials), which can be contactless and separate. Different from most previous studies, no covalent or noncovalent interactions between the two parts are required, which makes them more facile and convenient. Finally, we summarize the main advantages of these strategies and the current challenges. We expect this concept to offer insightful and novel understanding and inspiration on CPL generation and regulation.

White circularly polarized luminescence from a dual-component emitter induced by FRET between tetraphenylene and PDI derivatives

A chiral agent, TPE-ASP, incorporating aspartic acid as the chiral source and tetraphenylene derivatives as chromophores, was designed and synthesized. The chiral agent was self-assembled into regular spherical nanoparticles with a maximum luminescence asymmetry factor of |2.41 × 10-2| at 460 nm which is attributed to TPE-ASP. These nanoparticles can be co-assembled with a perylenediimide (PDI) derivative through electrostatic interactions, enabling the successful construction of a chiral light-harvesting system (C-LHS). The maximum Förster resonance energy transfer (FRET) efficiency (ΦET) of 94.7% was achieved at the optimal molar ratio of TPE-ASP to PDI. Fortunately, multicolour circularly polarized luminescence (CPL), spanning from blue to red, was successfully achieved with a two-component co-assembly system, and bright white CPL with CIE coordinates of (0.33, 0.32) was also obtained. Meanwhile, the average glum is |7.1 × 10-3| in the wavelength range of 400-700 nm. This discovery demonstrates the potential for spectral regulation through the two-component co-assembly strategy. It is significant for developing CPL devices with white light emission via the FRET process and also expands the functional range of chiral light-harvesting systems.

Hollow Superhelices Based on Chiral Europium Coordination Polymers

The construction of helical nanotubes based on chiral coordination polymers (CPs) is an intriguing but challenging task, which is important for the development of functional materials that combine macroscopic chirality with tube-related properties. Here, we selected a chiral europium phosphonate system, e. g., Eu(NO3)3/R-,S-pempH2, and carried out a systematic work. By controlling the hydrothermal reaction conditions such as the pH value of the reaction mixture, the molar ratio and concentration of the reactants, we obtained block-like crystals of R/S-1 b, rod-like crystals of R/S-3 r, hollow superhelices of R/S-2 hh, and solid superhelices of R/S-4 sh. In the latter two cases, the chirality has been successfully transferred and amplificated from the molecular level to the macroscopic level. Interestingly, compounds R/S-2 hh and R/S-4 sh have the same chemical composition of Eu(R/S-pempH)3⋅2H2O and show identical PXRD patterns, thus can be considered as the same material except for different morphologies. We further investigated their circularly polarized luminescence (CPL) properties and found that the hollow superhelix of R/S-2 hh had a larger dissymmetry factor than the solid superhelix of R/S-4 sh. This study not only provides the first example of hollow superhelices of chiral CPs, but also offers the possibility of modulating the chiroptical properties of CPs through morphological control.

Solution-Processable Metal-Organic Framework Featuring Highly Tunable Dynamic Aggregation States

The limited processability of metal-organic frameworks (MOFs) is hindered flexibility in the manipulation of their aggregation state and applications. Therefore, achieving highly processable MOFs is of great significance but a challenging goal. Herein, a facile strategy is presented for achieving the construction of solution-processable Mg-based MOF, NKU-Mg-1, allowing for dynamic control of the aggregation state through dynamic self-assembly (DySA) process and reversible circularly polarized luminescence (CPL) switcher modulation. Notably, micron-sized crystals of NKU-Mg-1 can be readily dispersed in water to form nano-sized colloids, triggered by the dynamic COO-Mg coordination bonding interruption by the competitive H2O-Mg bonding. Accordingly, the aggregation state of the colloid MOF can be readily tuned from 50-80 nm up to 1000 nm, in turn enabling control of aggregation-dependent emission. Specially, the solid-phase aggregation can be controlled via structural transitions between 3D NKU-Mg-1-rec-1 and 2D NKU-Mg-1-rec-2 nano-crystals, as confirmed by 3D electron diffraction. Furthermore, benefiting from its highly dynamic tunable aggregation nature, the rational incorporation of the chiral module confers significant CPL activity (glum up to 0.01). Importantly, controllable dynamic aggregation enables reversible switching of the CPL activity by precisely regulating the aggregation states. The solution-processable and dynamic aggregation-tunable features endow it highly promising for applications.

Synthesis and Properties of Donor-Acceptor-Type Cyclobisbiphenylenecarbonyls

The scalable synthesis of figure-eight π-systems is challenging for the conventional bottom-up approach. We have recently reported that the oxidative inner-bond cleavage of commercially available dibenzo[g,p]chrysene efficiently furnishes a figure-eight π-acceptor, cyclobisbiphenylenecarbonyl (CBBC), in large quantity. Furthermore, its donor-acceptor-type derivative with four N-carbazolyl substituents at the meta-positions of the carbonyl groups exhibited thermally activated delayed fluorescence (TADF) and circularly polarized luminescence (CPL) with a high |gCPL| value of 1.0×10-2. Herein, we synthesized nine donor-acceptor-type CBBC derivatives by changing the donor substituents and their positions. Compared to previously reported carbazole-substituted CBBC, tetramethylcarbazole- and 9,10-dihydro-9,9-dimethylacridine-substituted derivatives exhibited enhanced emission quantum yields and accelerated reverse intersystem crossing. The functionalization of the para-positions of the carbonyl groups resulted in better tunability of emission colors rather than meta-functionalization, whose color variation ranges from light blue to red. The incorporation of bulky substituents at the meta-positions of the carbonyl groups induced the conformational change to a distorted ring structure. Investigation of the substituent effect on the chiroptical properties revealed that the introduction of less bulky donor units such as carbazole at the meta-positions of the carbonyl groups is effective in achieving high |gCPL| values.

Enhanced Asymmetric Circularly Polarized Luminescence in Self-Organized Helical Superstructures Enabled by Macro-Chiral Liquid Crystal Quantum Dots

Circularly polarized luminescent (CPL) materials have garnered considerable interest for a variety of advanced optical applications including 3D imaging, data encryption, and asymmetric catalysis. However, the development of high-performance CPL has been hindered by the absence of simple synthetic methods for chiral luminescent emitters that exhibit both high quantum yields and dissymmetry factors. In this study, we present an innovative approach for the synthesis of macro-chiral liquid crystal quantum dots (Ch-QDs/LC) and their CPL performance enhancement through doping with 4-cyano-4'-pentylbiphenyl (5CB), thus yielding a CPL-emitting generator (CEG). The Ch-QDs/LCs were synthesized, and their surfaces functionalized with a chiral mesogenic ligand, specifically cholesteryl benzoate, anchored via a lipoic acid linker. Under the regulation of chiral 2S-Zn2+ coordination complexes, the chiral LC encapsulation process promotes coordinated ligand substitution, resulting in an exceptional quantum yield of 56.3%. This is accompanied by high absorption dissymmetry factor (gabs) and luminescence dissymmetry factor (glum) values ranging from 10-3 to 10-2, surpassing most reported dissymmetry factors by at least an order of magnitude. The modular Ch-QDs/LCs demonstrate the ability to transfer chirality to the surrounding medium efficiently and manifest macro-chiral characteristics within a nematic LC matrix. Utilizing Ch-QDs/LC as an effective CPL emitter within achiral 5CB matrices enabled the system to achieve a maximum glum value of 0.35. The resultant CEG device acted as a direct CPL source, initiating enantioselective photopolymerization.

Inner-Bond-Cleavage Approach to Figure-Eight Macrocycles from Planar Aromatic Hydrocarbons

Figure-eight-shaped nonplanar π-systems adopt distinctive chiral D2-symmetric structures, which are ideal for realizing efficient circularly polarized luminescence (CPL). However, the short-step and enantioselective synthesis of figure-eight π-systems represents a considerable challenge for the conventional bottom-up synthetic strategy. Herein, we report that the oxidative cleavage of the internal double bond of a commercially available polycyclic aromatic hydrocarbon, i.e., dibenzo[g,p]chrysene (DBC), catalytically affords a figure-eight electron-accepting macrocycle, i.e., cyclobisbiphenylenecarbonyl (CBBC), with high scalability (up to 3.3 g) and excellent enantioselectivity (94% ee). This inner-bond-cleavage approach also applies to larger PAHs, affording highly distorted molecular frameworks that comprise two figure-eight subunits. Furthermore, we demonstrate that the peripheral functionalization of CBBC with carbazole afforded donor-acceptor-type emitter, which shows thermally activated delayed fluorescence and emits CPL with a g value of 1.0 × 10-2. This g value is ten times higher than those of previously reported chiral TADF-active emitters for circularly polarized organic light-emitting diodes. These results demonstrate that oxidative inner-bond cleavage is a powerful synthetic strategy for creating innovative materials that incorporate molecules with figure-eight structures.

Enhanced chiroptical activity for narrow deep-blue emission in axial chiral frameworks via three-dimensional interlocking

The advancement of desirable circularly polarized luminescence (CPL) emitters is predominantly constrained by the effective regulation of magnetic and electric transition vectors, particularly within the deep-blue spectral domain. Herein, we present four pairs of novel chiral emitters with systematically varied molecular rigidity, symmetry, and chiral centers to elucidate the intrinsic coupling of key molecular parameters influencing their chiroptical properties. Notably, the incorporation of appropriate intramolecular 3D-interlocking within a natural binaphthyl chirality skeleton offers an effective approach to achieving both significantly narrowed full width at half maximum (FWHM, as low as 18 nm) and substantially enhanced chiroptical activity (luminous dissymmetry factor, g PL, up to 3.0 × 10-3). Additionally, introducing a secondary chiral center closely parallel to the primary chiral plane facilitates strong chiral-chiral interactions, further affording a 50% improvement in their g PL values. As a demonstration, vacuum-deposited circularly polarized organic light-emitting diodes incorporating these pure fluorescent emitters exhibit outstanding electroluminescent performance, with maximum external quantum efficiency exceeding 5.35%, favorable FWHM of approximately 25 nm, and extreme CIE y values below 0.03.

Chirality and Solvent Coassist the Structural Evolution of Hybrid Manganese Chlorides with Second-Harmonic-Generation Response

Chiral hybrid metal halides have shown great potential in optoelectronics, including for spin splitting, circularly polarized luminescence, and nonlinear-optical properties. However, despite their inherent inversion symmetry breaking, studies on second harmonic generation (SHG) of chiral hybrid manganese(II) halides remain relatively rare. Here, we report a series of structurally diverse hybrid manganese(II) chlorides: (Rac-MBA)2[MnCl4(H2O)2] (1), (S-MBA)2[MnCl4(H2O)2] (2), (S-MBA)2[Mn2Cl6(H2O)4] (3), and (S-MBA)[MnCl3(MeOH)] (4), where MBA = α-methylbenzylammonium, providing tunability of the coordination environment and structural dimensionality via fine control of the MBA cation chiral state and crystal preparation process, thereby enabling modulation of the SHG effects. Specifically, as the amount of methanol increases during the crystal preparation process, the structures of the chiral compounds vary from a 0D structure consisting of isolated octahedra to a 0D structure composed of octahedra dimers and to 1D chains of edge-sharing Mn-centered octahedra. In contrast, the structure of the racemic compound remains unchanged, independent of the crystal preparation pathway. The structural details, including the coordination environment, H-bonding, dimensionality, and lattice distortion, are described. The SHG response of the racemic compound derives only from the inorganic lattice, while the responses of the chiral compounds are attributed to the synergetic effect of the chiral cations and inorganic moieties.

Unlocking the Potential of Organic-Inorganic Hybrid Manganese Halides for Advanced Optoelectronic Applications

Organic-inorganic hybrid manganese(II) halides (OIMnHs) have garnered tremendous interest across a wide array of research fields owing to their outstanding optical properties, abundant structural diversity, low-cost solution processibility, and low toxicity, which make them extremely suitable for use as a new class of luminescent materials for various optoelectronic applications. Over the past years, a plethora of OIMnHs with different structural dimensionalities and multifunctionalities such as efficient photoluminescence (PL), radioluminescence, circularly polarized luminescence, and mechanoluminescence have been newly created by judicious screening of the organic cations and inorganic Mn(II) polyhedra. Specifically, through precise molecular and structural engineering, a series of OIMnHs with near-unity PL quantum yields, high anti-thermal quenching properties, and excellent stability in harsh conditions have been devised and explored for applications in light-emitting diodes (LEDs), X-ray scintillators, multimodal anti-counterfeiting, and fluorescent sensing. In this review, the latest advancements in the development of OIMnHs as efficient light-emitting materials are summarized, which covers from their fundamental physicochemical properties to advanced optoelectronic applications, with an emphasis on the structural and functionality design especially for LEDs and X-ray detection and imaging. Current challenges and future efforts to unlock the potentials of these promising materials are also envisioned.

Tunable circularly polarized luminescence behaviors caused by the structural symmetry of achiral pyrene-based emitters in chiral co-assembled systems

The regulation of circularly polarized luminescence (CPL) behavior is of great significance for practical applications. Herein, we deliberately designed three achiral pyrene derivatives (Py-1, Py-2, and Py-3) with different butoxy-phenyl substituents and the chiral binaphthyl-based inducer (R/S-B) with anchored dihedral angle to construct chiral co-assemblies, and explored their induced CPL behaviors. Interestingly, the resulting co-assemblies demonstrate tunable CPL emission behaviors caused by the structural symmetry effect of achiral pyrene-based emitters during the chiral co-assembly process. And in spin-coated films, the dissymmetry factor (gem) values were 9.1 × 10-3 for (R/S-B)1-(Py-1)10, 5.6 × 10-2 for (R/S-B)1-(Py-2)7, and 8.6 × 10-4 for (R/S-B)1-(Py-3)1, respectively. The strongest CPL emission (|gem| = 5.6 × 10-2, λem = 423 nm, QY = 34.8 %) was detected on (R/S-B)1-(Py-2)7 due to the formation of regular and ordered helical nanofibers through the strong π-π stacking interaction between the R/S-B and the achiral Py-2 emitter. The strategy presented here provides a creative approach for progressively regulating CPL emission behaviors in the chiral co-assembly process.

Pushing Trap-Controlled Persistent Luminescence Materials toward Multi-Responsive Smart Platforms: Recent Advances, Mechanism, and Frontier Applications

Smart stimuli-responsive persistent luminescence materials, combining the various advantages and frontier applications prospects, have gained booming progress in recent years. The trap-controlled property and energy storage capability to respond to external multi-stimulations through diverse luminescence pathways make them attractive in emerging multi-responsive smart platforms. This review aims at the recent advances in trap-controlled luminescence materials for advanced multi-stimuli-responsive smart platforms. The design principles, luminescence mechanisms, and representative stimulations, i.e., thermo-, photo-, mechano-, and X-rays responsiveness, are comprehensively summarized. Various emerging multi-responsive hybrid systems containing trap-controlled luminescence materials are highlighted. Specifically, temperature dependent trapping and de-trapping performance is discussed, from extreme-low temperature to ultra-high temperature conditions. Emerging applications and future perspectives are briefly presented. It is hoped that this review would provide new insights and guidelines for the rational design and performance manipulation of multi-responsive materials for advanced smart platforms.

Enhancing Circularly Polarized Electroluminescence through Energy Transfer within a Chiral Polymer Host

Organic light-emitting diodes (OLEDs) that are able to emit high levels of circularly polarized (CP) light hold significant promise in numerous future technologies. Such devices require chiral emissive materials to enable CP electroluminescence. However, the vast majority of current OLED emitter classes, including the state-of-the-art triplet-harvesting thermally activated delayed fluorescence (TADF) materials, produce very low levels of CP electroluminescence. Here a host-guest strategy that allows for energy transfer between a chiral polymer host and a representative chiral TADF emitter is showcased. Such a mechanism results in a large amplification of the circular polarization of the emitter. As such, this study presents a promising avenue to further boost the performance of circularly polarized organic light-emitting diode devices, enabling their further development and eventual commercialization.

Helically Chiral π-Expanded Azocines Through Regioselective Beckmann Rearrangement and Their Charged States

We report a synthetic approach to π-expanded [6]helicenes incorporating tropone and azocine units in combination with a 5-membered ring, which exhibit intriguing structural, electronic, and chiroptical properties. The regioselective Beckmann rearrangement allows the isolation of helical scaffolds containing 8-membered lactam, azocine, and amine units. As shown by X-ray crystallographic analysis, the incorporation of tropone or azocine units leads to highly distorted [6]helicene moieties, with distinct packing motifs in the solid state. The compounds exhibit promising optoelectronic properties with considerable photoluminescence quantum yields and tunable emission wavelengths depending on the relative position of the nitrogen center within the polycyclic framework. Separation of the enantiomers by chiral high-performance liquid chromatography (HPLC) allowed characterization of their chiroptical properties by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy. The azocine compounds feature manifold redox chemistry, allowing for the characterization of the corresponding radical anions and cations as well as the dications and dianions, with near-infrared (NIR) absorption bands extending beyond 3000 nm. Detailed theoretical studies provided insights into the aromaticity evolution upon reduction and oxidation, suggesting that the steric strain prevents the azocine unit from undergoing aromatization, while the indene moiety dominates the observed redox chemistry.

Ab initio investigations of circularly polarised luminescence in Samarium(III)-based complexes

The present study aims to gain insight into the circularly polarised luminescence (CPL) of lanthanide complexes through the angle of one of their elements, namely Samarium. The simulation of luminescent properties of Samarium(III) complexes remains a challenge for computational chemistry, considering the multiconfigurational character of the electronic structure, the importance of the spin-orbit coupling and the fact that its emissive level is high in energy and preceded by numerous states of various multiplicity. Herein, a methodology based on CASSCF/RASSI-SO calculations is exposed and applied to simulate the CPL properties of two different Samarium(III) complexes, presenting either a rigid or a flexible architecture around the centre ion.