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Axial chirality

Axial chirality is a type of in which a lacks a traditional center but exhibits due to restricted about a stereogenic , leading to non-superimposable mirror images known as enantiomers. This phenomenon occurs when two pairs of substituents are arranged in a non-planar around the , preventing free and conferring optical activity to the . Unlike central chirality, which stems from a tetrahedral atom with four different substituents, axial chirality arises from geometric constraints in the molecular framework. Prominent examples of axially chiral molecules include , where a cumulative system (C=C=C) creates perpendicular planes of substituents around the central sp-hybridized carbon, and biaryl atropisomers such as substituted biphenyls, in which bulky substituents hinder rotation about the inter-ring bond. A classic case is (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl), a binaphthyl derivative with axial chirality that serves as a foundational ligand in asymmetric catalysis. Other structures, like spiranes and certain alkylidene-cyclic compounds, also display axial chirality through similar rotational barriers. Axial chirality holds significant importance in and , as it underpins the design of chiral ligands and catalysts that enable enantioselective reactions, enhancing the efficiency of producing optically pure compounds. Many bioactive natural products and pharmaceuticals incorporate axial chirality, influencing their and selectivity. Advances in catalytic methods continue to expand access to these motifs, facilitating applications in and .

Fundamentals

Definition

Axial chirality is a form of in which a exhibits due to a non-superimposable , stemming from the spatial arrangement of substituents around a rather than a stereogenic center. This type of arises when about the —often a single bond between adjacent atoms—is sufficiently hindered, preventing the interconversion of conformers at ordinary temperatures and rendering the enantiomers stable and isolable. Unlike point chirality, which involves a tetrahedral atom bound to four different groups, axial chirality depends on the overall lacking a of orthogonal to the . To understand axial chirality, it is essential to recall the broader concept of : a is one that is not superimposable on its , leading to pairs of enantiomers that share identical physical properties except for their interaction with plane-polarized light. If two stereoisomers are non-mirror images, they are diastereomers, which can differ in chemical and physical properties. In axial chirality, the basic mechanism involves steric or electronic barriers that restrict rotation, stabilizing atropisomeric conformers that function as distinct isomers with potential biological and chemical implications. The recognition of axial chirality dates to the early , when optical activity in certain was attributed to hindered around a , distinct from traditional centers. This discovery laid the foundation for understanding in systems without point stereocenters, with atropisomerism emerging as a prominent manifestation driven by steric bulk.

Molecular Requirements

Axial chirality requires sufficient steric hindrance to impede free rotation around a designated chiral axis, typically achieved through the presence of bulky substituents that create significant spatial congestion. In systems like -substituted biaryls, these substituents, such as tert-butyl or isopropyl groups at the positions, prevent planar alignment and enforce a twisted conformation. This steric barrier is the primary structural prerequisite, distinguishing axial chirality from other stereogenic elements reliant on point or planar . For atropisomers to be isolable and configurationally stable at (approximately 298 ), the free energy barrier to (ΔG‡) must generally exceed 23 kcal/, corresponding to a of interconversion longer than about 1000 seconds. This threshold ensures that occurs on timescales impractical for laboratory isolation, allowing enantiopure forms to be handled without rapid equilibration. The rotational energy barrier is quantified using the from : \Delta G^\ddagger = -RT \ln \left( \frac{k h}{k_B T} \right) where k is the first-order rate constant for rotation, h is Planck's constant, k_B is the Boltzmann constant, R is the gas constant, and T is the absolute temperature; for stable chiral cases, ΔG‡ values often range from 24 to 35 kcal/mol depending on the system. Axial chirality can arise around various bond types, including single bonds (e.g., C–C bonds in biaryl systems exhibiting atropisomerism as a classic case), cumulative double bonds (e.g., in allenes where orthogonal π-bonds enforce perpendicular planes), or helical arrangements (e.g., in helicenes where extended polycyclic ortho-fusion creates an intrinsic twist). The stability of these chiral axes is modulated by several factors, including the size and electronic nature of substituents—which can enhance steric bulk or alter conjugation to raise the barrier—as well as external conditions like temperature, which exponentially affects the rotation rate per the Arrhenius equation, and solvent polarity, which may slightly lower barriers in polar media through solvation of transition states. Larger substituents generally increase ΔG‡ by amplifying steric repulsion in the transition state, while electron-withdrawing groups can stabilize twisted geometries via altered π-overlap.

Types

Atropisomerism

Atropisomerism refers to a form of axial chirality arising from the restricted rotation about a , typically between two sp²-hybridized carbon atoms, due to steric hindrance that creates a sufficiently high energy barrier to allow isolation of stable enantiomers. The term "atropisomer" was coined by in 1933, derived from the Greek words "a-" (not) and "tropos" (turning), emphasizing the inability of the to rotate freely at . This phenomenon is distinct from other axial chiral systems, such as , which rely on cumulative double bonds rather than single-bond rotation. The archetypal examples of atropisomers are found in biaryl systems, particularly 2,2'-disubstituted biphenyls where bulky substituents, such as tert-butyl or iodo groups, prevent free rotation around the central C-C bond, leading to stable enantiomers. A prominent case is 1,1'-bi-2-naphthol (BINOL), a binaphthyl derivative with hydroxyl groups at the 2,2' positions, whose axial chirality stems from the steric bulk of the fused rings and enables its use as a or . These biaryls exhibit only when the substituents are sufficiently large to raise the rotational barrier above approximately 23-25 kcal/mol, ensuring half-lives for on the order of hours to years at ambient conditions. Resolution of atropisomers has historically relied on classical methods, such as diastereomeric salt formation followed by fractional using chiral resolving agents like or cinchonine, as demonstrated in the separation of 6,6'-dinitro-2,2'-diphenic acid. Modern approaches favor preparative chiral (HPLC) with stationary phases like polysaccharide-based columns, which efficiently separate enantiomers without requiring derivatization and are scalable for pharmaceutical applications. Interconversion between atropisomeric enantiomers occurs through thermal via rotation across the hindered bond, with the rate governed by the of activation (ΔG‡). The (t_{1/2}) for racemization can be calculated using the : k = (k_B T / ) exp(-ΔG‡ / RT), where k is the rate constant, and t_{1/2} = ln(2)/k, allowing prediction of stability from computed or measured barriers—typically 30-40 kcal/mol for persistent atropisomers like those in candidates. A key application of atropisomerism is in the ligand (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl), whose axial chirality imparts stereocontrol in - and ruthenium-catalyzed asymmetric hydrogenations, achieving enantioselectivities exceeding 99% in the synthesis of chiral amines and alcohols, as pioneered by Noyori and Takaya.

Allenes

Allenes possess a distinctive characterized by cumulative double bonds, represented by the general formula \ce{R^1R^2C=C=CR^3R^4}, where the central carbon is sp-hybridized and bonded linearly to the terminal sp²-hybridized carbons. The two π-bonds in this system are orthogonal, lying in perpendicular planes, which positions the substituents on the terminal carbons in twisted orientations relative to each other. This geometry arises from the incompatibility of the p-orbitals on the central carbon with coplanar arrangements, enforcing a 90° between the terminal planes. Axial in allenes emerges when each terminal carbon bears two dissimilar substituents (\ce{R^1 \neq R^2} and \ce{R^3 \neq R^4}), eliminating any plane of symmetry and rendering the enantiomers non-superimposable. The origin of this stems from the inherent perpendicularity of the π-systems, which prevents free about the axis while maintaining rigidity through the double bonds; the mirror images cannot be superimposed without breaking bonds. Unlike central at tetrahedral centers, this axial form relies on the overall molecular , analogous to a helical twist but localized to the allene unit. In contrast to atropisomerism, which depends on steric hindrance to around a , allene is intrinsic to the cumulene framework without requiring restricted . Synthesis of chiral commonly employs methods such as the Wittig olefination, where a stabilized derived from a reacts with an or to generate the cumulene, often achieving high enantioselectivity with chiral auxiliaries or catalysts. Another prevalent route is the , involving the syn-elimination of β-hydroxysilanes or similar precursors under acidic or basic conditions, which has been used to prepare 1,3-disubstituted in yields up to 95% with good stereocontrol. These approaches allow access to enantioenriched from prochiral starting materials, enabling their use in asymmetric . The stability of chiral allenes is governed by the energy barrier to inversion along the chiral axis, typically around 42-46 kcal/mol (176-192 kJ/mol) for most substituted examples, comparable to barriers in many atropisomeric systems (often >40 kcal/mol). Chiral are generally configurationally stable at . For example, the (P)- of 1,3-bis(4-(tetrathiafulvalenenyl)phenyl)allene exhibits a of [\alpha]_D^{25} = +726^\circ (c 0.853, \ce{CH2Cl2}), demonstrating significant optical activity.

Helicenes

Helicenes are polycyclic aromatic hydrocarbons consisting of six or more ortho-fused rings arranged in a non-planar, helical conformation, representing a subtype of axial where the chirality axis is extended along the molecular screw axis. This helical architecture arises in helicenes with n ≥ 5, as shorter variants like helicene are planar and achiral. The two enantiomers are distinguished by their , denoted as P for the right-handed (plus) form and M for the left-handed (minus) form, based on the Cahn-Ingold-Prelog priority rules adapted for helical . The source of chirality in helicenes is the overall screw-like distortion of the polycyclic framework, driven by severe steric overcrowding between the terminal rings, which forces the ends to overlap and prevents the molecule from possessing a plane of . Unlike simpler axial chiral systems, this is intrinsic and stable, with the extended conjugation enhancing the dissymmetry. For instance, in helicene and higher homologs, the twist angle between the first and last rings can exceed 30°, ensuring non-superimposability of mirror images. Synthesis of helicenes typically employs oxidative photocyclization of stilbene-like precursors under irradiation in the presence of an oxidant like iodine or air, leading to dehydrogenative ring closure and formation of the helical scaffold. Alternatively, intramolecular oxidative coupling, such as via hypervalent iodine reagents or metal-catalyzed methods, enables construction of the fused rings from open-chain polyynes or biaryl substrates. The first synthesis and optical resolution of chiral helicene, a benchmark carbohelicene, was reported in 1956 by Newman and Lednicer through a multi-step sequence involving Diels-Alder cycloadditions followed by dehydrogenation and fractional crystallization of diastereomeric salts for enantioseparation. Larger helicenes, such as helicene and beyond, possess high racemization barriers exceeding 40 kcal/mol due to the substantial activation energy needed to flatten the helix during interconversion, rendering them configurationally stable at room temperature. These compounds also exhibit strong fluorescence with quantum yields often above 0.2 in solution and significant chiroptical activity, including large molar ellipticities in circular dichroism spectra (up to 10^5 deg cm² dm⁻¹ mol⁻¹) and high dissymmetry factors in circularly polarized luminescence. For example, helicene demonstrates these properties prominently, with its absolute configuration—correlating the P-helicity to positive optical rotation—unequivocally established by single-crystal X-ray diffraction analysis of an enantiopure derivative. Other types of axial chirality include spiranes and certain alkylidene-cyclic compounds, where chirality arises from restricted rotation or orthogonal planes of substituents.

Nomenclature

Stereodescriptors

Stereodescriptors for axial chirality are essential for specifying the of molecules where chirality arises from restricted rotation around a or from helical arrangements, extending the Cahn-Ingold-Prelog () priority rules to non-tetrahedral stereogenic units. These descriptors ensure unambiguous , distinguishing enantiomers in systems like atropisomers, , and helicenes. The P/M descriptors are used primarily for helical chirality, where the molecule adopts a screw-like or shape. The 'P' (plus) denotes a right-handed , analogous to a positive screw sense, while 'M' (minus) indicates a left-handed , determined by viewing along the helical and assigning priorities to the substituents according to rules—higher priority groups on the near side are traced in a manner for P if they form the right-handed twist. This system applies when the chirality is best described by rather than a strict , such as in extended helical structures. For example, in helicenes and with cumulene units, the P/M notation captures the overall twist of the framework. For axial chirality in systems like biaryl atropisomers, the descriptors and extend the R/S convention to the chirality axis, treating it as a pseudo-asymmetric unit. Priorities are assigned to the four substituents attached to the axis ends using rules, with the molecule oriented such that the axis is eclipsed and the lowest priority groups are placed behind; a clockwise sequence of decreasing priority (1 to 2 to 3) from one end yields , while counterclockwise gives . This method prioritizes substituents based on and branching, ensuring consistent assignment regardless of the viewing direction along the axis. In practice, both P/M and Ra/Sa notations are applied to specific types of axial chirality, often interchangeably for certain molecules. For instance, the axially chiral ligand 1,1'-bi-2-naphthol (BINOL) is commonly designated as (P)-BINOL for the right-handed enantiomer and (M)-BINOL for the left-handed, where M corresponds to and P to based on the screw sense of the naphthyl units; alternatively, it can be specified as ()-BINOL or ()-BINOL using the axial rules. To illustrate assignment for BINOL, the oxygen atoms at the 2-positions receive the highest (1) on each naphthyl ring, the fused ring carbons at position 3 are next (2), and the hydrogens or lower substituents are lowest (3); tracing from the higher end of the clockwise assigns the descriptor. Challenges arise in molecules with multiple chirality axes, where ambiguity can occur if descriptors conflict or overlap; this is resolved by assigning locants to each axis and designating the lowest-numbered axis as principal for overall specification, following hierarchical rules to avoid redundancy. Such systems require careful enumeration of all stereogenic units to ensure complete and non-ambiguous naming.

IUPAC Conventions

The nomenclature for axial chirality has evolved from early ad-hoc systems relying on optical rotation signs, such as dextro (d) or levo (l) prefixes, to systematic methods based on the Cahn-Ingold-Prelog (CIP) priority rules. These informal descriptors, common in initial studies of biaryls and allenes during the early 20th century, lacked structural specificity and were gradually replaced by extensions of the CIP sequence rules. In the 1974 IUPAC recommendations, the CIP rules were formally extended to axial chirality by treating the chirality axis as an elongated tetrahedral arrangement, allowing assignment of R and S descriptors through ligand prioritization and viewing along the axis. The 1996 IUPAC recommendations refined this approach by adopting specific stereodescriptors for axial chirality: uppercase and for absolute configurations, with lowercase and denoting relative configurations when the absolute stereochemistry is unknown or unspecified. These descriptors are assigned by applying the rules to the substituents on either side of the , duplicating atoms in bonds with atoms of zero to determine priority. Alternatively, P (plus, right-handed screw sense) and M (minus, left-handed screw sense) descriptors may be used, particularly for systems where is emphasized, though / is preferred for precision in substitutive . In special cases like binaphthalenes, like 1,1'-binaphthyl derivatives, the descriptors (aR) and (aS) are employed to explicitly indicate the axial nature and avoid conflicts with potential point chirality descriptors in multifunctional molecules. This notation ensures clarity when multiple stereogenic elements are present, preventing in naming. Post-2000 updates in the 2013 IUPAC integrated helical systems more comprehensively into axial nomenclature, defining a to include helical arrangements and recommending P/M descriptors for their screw-sense while retaining Ra/Sa for CIP-based .
MoleculeR/S DescriptorP/M DescriptorNotes
(R)-BINAP(R) or (aR)MThe (R) configuration exhibits left-handed (M).
(S)-BINAP(S) or (aS)PThe (S) configuration exhibits right-handed (P).

Examples and Applications

Natural Products

Axial chirality manifests in various natural products, primarily through atropisomerism, where restricted rotation around a creates stable stereoisomers essential for biological function. These compounds, often secondary metabolites, are produced by , microbes, and fungi, showcasing how nature exploits stereochemical control for potent bioactivities such as defense and . A prominent example is , a isolated from the soil bacterium Amycolatopsis orientalis in 1956, featuring atropisomeric biaryl linkages between its aromatic rings that contribute to its rigid three-dimensional structure for binding to bacterial cell wall precursors. This axial chirality is crucial for its high antibacterial potency against Gram-positive pathogens, with minimum inhibitory concentrations (MICs) as low as 1 μg/mL for . Its biosynthesis involves non-ribosomal peptide synthetases (NRPS) in the producing microbe, which enzymatically couple precursors and enforce the specific (P)-atropisomeric configuration during macrocyclization, ensuring the molecule's effectiveness. Mastigophorene A, an axially chiral diaryl dimer, was first isolated in the late 1980s from the Borneo liverwort Mastigophora diclados through bioactivity-guided of extracts, yielding low quantities (typically <1% from plant material) due to its rarity. This compound exhibits neurotrophic activity, promoting nerve cell growth and differentiation in mesencephalic dopaminergic cultures at concentrations around 10 μM, highlighting its potential in models. Enzymatic control during its in the liverwort likely involves oxidative coupling of monomeric sesquiterpenes, selectively forming the stable axial to enhance receptor interactions. The late 1980s marked a key period for identifying such natural atropisomers, expanding recognition beyond earlier examples. Gossypol, a binaphthyl with axial chirality from the plant ( spp.), is biosynthesized via the phenylpropanoid pathway in glands, where enzymatic dimerization restricts rotation around the central bond, producing a mixture but favoring the (−) in some species at ratios up to 78:22. The (−) demonstrates potent male antifertility effects by inhibiting and activity in reproductive tissues, with oral doses of 20 mg/day achieving contraception in animal models without the toxicity seen in the racemate. This underscores axial chirality's evolutionary role in secondary metabolites, enabling precise receptor binding for ecological advantages like pest deterrence while minimizing self-toxicity.

Synthetic Compounds

Synthetic compounds exhibiting axial chirality are pivotal in asymmetric catalysis and , where designed molecules leverage restricted rotation around a chiral axis to induce in reactions. One prominent class includes atropisomeric biaryls such as 1,1'-bi-2-naphthol (BINOL) and its derivatives, which serve as chiral ligands in asymmetric synthesis. BINOL's axial chirality arises from the hindered rotation between the naphthyl rings, enabling its use in metal complexes for enantioselective transformations like epoxidations and reductions. Preparation of enantioenriched BINOL often involves regioselective sulfonation of its dialkyl ether with concentrated at 50°C, yielding the 6,6'-disulfonic acid derivative in 75% yield, which facilitates through diastereomeric salt formation or further derivatization for chiral separation. Axially chiral phosphine ligands, such as (5,5'-bis(diphenylphosphino)-4,4'-bi-1,3-benzodioxole), extend this utility by coordinating to metals for high enantioselectivity. SEGPHOS and its derivatives, like DTBM-SEGPHOS, are synthesized via lithiation and phosphination of the biaryl core, followed by using chiral auxiliaries. These ligands achieve enantiomeric excesses exceeding 99% in palladium-catalyzed reactions, including allylic alkylations and cross-couplings, due to their wide bite angle and electronic tuning that stabilizes reactive intermediates. Key synthesis strategies for enantioenriched axially chiral compounds include dynamic kinetic resolution (DKR) of racemic mixtures and metal-catalyzed couplings. DKR exploits rapid of biaryl atropisomers under acidic or basic conditions, allowing both enantiomers to be converted to a single enantiopure product through selective acylation or alkylation; for instance, chiral catalysts enable DKR of 2-hydroxybiaryls with up to 99% ee and near-quantitative yields. Complementarily, metal-catalyzed couplings like the asymmetric Suzuki-Miyaura reaction construct the biaryl axis directly from aryl halides and boronic acids using with chiral BINAP-type ligands, affording atropisomers with 90-99% ee by controlling the and steps. Recent advances in the and have utilized rhodium-catalyzed enantioselective azide-alkyne cycloadditions to access axially chiral heterobiaryls.

Pharmaceutical Relevance

Axial chirality, particularly through atropisomerism, plays a critical role in pharmaceutical design due to its influence on molecular recognition and binding affinity to biological targets. In drugs like , an featuring multiple atropisomeric biaryl bonds, the specific axial configuration enables enantioselective interactions with bacterial precursors, enhancing potency against Gram-positive infections. This stereospecific binding underscores how axial chirality can dictate therapeutic efficacy by allowing precise fit into chiral pockets of enzymes or receptors. A major challenge in developing atropisomeric drugs is ensuring configurational stability to prevent racemization in vivo, where physiological temperatures and pH can accelerate bond rotation, leading to loss of activity or emergence of off-target effects. For instance, atropisomers with rotational barriers below 25 kcal/mol may interconvert rapidly, complicating pharmacokinetics and requiring careful assessment of half-lives under biological conditions. Regulatory guidelines, such as the FDA's 1992 policy on stereoisomeric drugs, mandate evaluation of individual enantiomers or atropisomers for safety and efficacy, emphasizing the need to develop single isomers when racemization poses risks. In therapeutic applications, axially chiral biaryls have proven valuable in anticancer agents, particularly kinase inhibitors. , an FDA-approved G12C inhibitor featuring a stable atropisomeric axis, demonstrates potent covalent binding with a kinact/KI of 9,900 M⁻¹ s⁻¹, leading to robust pharmacodynamic effects like p-ERK inhibition ( ~100 nM in MIA PaCa-2 cells) and favorable , including oral and a supporting once-daily dosing in non-small cell patients. These properties highlight how axial chirality can optimize selectivity and exposure, reducing off-target liabilities compared to racemic mixtures. Looking ahead, the development of libraries via modular synthetic strategies, such as nucleophilic aromatic substitutions, enables for novel drug candidates, expanding access to diverse axial chiral space for targeted therapies. Additionally, ligands like facilitate enantioselective in the synthesis of chiral pharmaceuticals, indirectly supporting atropisomer production.

History

Early Discoveries

The establishment of Emil Fischer's tetrahedral model for central chirality in 1874 marked a pivotal shift in understanding molecular asymmetry, leading chemists to investigate alternative sources beyond atoms, including potential axial chirality in structures like . Although theoretical predictions for axial chirality in appeared shortly after in van 't Hoff's work, experimental validation for systems lagged until the early 20th century. In 1922, H. Christie and James Kenner achieved the first resolution of a derivative into stable enantiomers by separating the salts of 6,6'-dinitro-2,2'-diphenic acid, demonstrating that hindered rotation about the inter-ring bond could prevent at and thus confer optical activity. This breakthrough provided concrete evidence for axial chirality in biaryls, where bulky substituents create a significant rotational barrier exceeding 23 kcal/mol, allowing isolation of enantiopure forms. During the 1920s and early 1930s, Roger Adams advanced the field through systematic studies of ortho-substituted biphenyls, synthesizing numerous derivatives and measuring rotation barriers via kinetics, which confirmed that barriers above approximately 25 kcal/mol enable stability under ambient conditions. Adams' work on compounds like 2,2'-dicarboxy-6,6'-dinitrobiphenyl highlighted the role of steric hindrance in maintaining axial chirality, laying the groundwork for predicting isolable enantiomers. In the 1930s, formalized the concept of atropisomerism—stereoisomers arising from restricted single-bond rotation—and conducted studies using racemization kinetics to examine interconversion in and related systems, revealing barriers typically ranging from 20 to 40 kcal/mol depending on substitution patterns. These studies emphasized the thermodynamic basis of axial , distinguishing it from central .

Key Milestones

In the mid-20th century, building on early investigations of atropisomerism, significant advances in measuring rotational barriers emerged through the application of dynamic (NMR) by Kurt Mislow and collaborators in the 1950s and 1960s, enabling precise determination of energy barriers for axial rotation in biaryl systems and other atropisomers. Concurrently, the first resolutions of axially chiral were achieved in the late 1950s, with Jerome A. Berson and colleagues demonstrating the enantiomeric separation of 1,3-diphenylallene via classical resolution techniques, confirming the predicted axial chirality of cumulenes. The 1970s and early 1980s saw the extension of the priority rules to encompass chirality axes, as detailed by and Günter Helmchen in their comprehensive revision, which provided a systematic framework for assigning absolute configurations to atropisomers and other axially chiral molecules beyond tetrahedral centers. During the 1980s and 1990s, the development of axially chiral phosphine ligands revolutionized asymmetric , with Ryoji Noyori's introduction of (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) enabling highly enantioselective hydrogenations and other transformations, a breakthrough recognized by the 2001 shared with William S. Knowles for foundational work in chiral catalysis. In the and , (DFT) computations became indispensable for predicting and rationalizing rotational barriers in axially chiral compounds, as exemplified by studies on triarylamines and biaryls that correlated steric and electronic effects with barrier heights, facilitating the design of stable atropisomers. advanced chirality through interlocked structures like catenanes, where Jean-Pierre Sauvage and coworkers demonstrated mechanically induced chirality in catenanes during the 1980s and 1990s, leveraging metal templating to create topologically chiral systems with potential for . In the 2020s, advances in catalytic methods have expanded access to axially chiral motifs, including enantioselective syntheses of biaryls and for pharmaceutical and materials applications. Axially chiral molecules have found applications in organic light-emitting diodes (OLEDs), with developments in circularly polarized thermally activated delayed fluorescence (CP-TADF) emitters achieving high quantum yields (up to 74%) and dissymmetry factors (around 5 × 10^{-3}), enhancing display technologies through chiral perturbation strategies as of 2020.