English

• 1. Introduction

  Fluorescence refers to the radiation nature from singlet exciton decay in luminophores, which was first observed in fluoride compounds (fluorite) in 1852. In contrast, phosphorescence is a phenomenon of slow luminescence attenuation caused by a long-lived triplet exciton. If the extended lifetime of phosphorescence is long enough under the naked eye, one can call it the long-lived afterglow [1–14]. The population and decay of long-lived triplet exciton are associated with the spin-forbidden transition of intersystem crossing (ISC) and rigorous singlet-triplet energy gradient, leading to insufficient phosphorescence development in pure organic phosphors [1,6]. Thus, the research schedule of phosphorescence is significantly slower than that of fluorescence. Due to the requirements of organic light emitting diode (OLED) manufacturers for perfect exciton utilization ratios, the research priority has been transferred to the topic of phosphorescence or thermally activated delayed fluorescence (TADF) in recent years [7,15]. Generally, the photons could carry specific polarization states (namely circular polarization), which could be output from the chiral excited states in fluorescence even phosphorescence molecules at room temperature [16,17]. This polarized phosphorescence can provide extra chiral optoelectronic and biochemical applications [18].

  In this background, the sequential and facile regulation between polarized fluorescence and phosphorescence is highly desired [19,20]. Theoretically, phosphorus compounds have two valences but three electronic statuses, including bonding unsaturated phosphines (^ⅢP), bonding saturated phosphoniums (^ⅢP⁺), and phosphine oxides (^VP), which display distinct electronic signatures [21]. The electron-rich characteristics could be gradually changed into the electron-deficient states from phosphines to phosphine oxides and final phosphoniums, enabling a crucial role alteration from donor to accepter even mutative coordination capacity. Importantly, the phosphorus compounds can provide two types of accessible chirality, including tetrahedral or pyramidal point chirality from the sp³ P-center and other chirality for aromatic fragments [22,23]. This electronic feature is different from other elements (⁶C, 7 N, ⁸O, etc.) and thereby enables innovation for circularly polarized fluorescence and phosphorescence. In addition, the P-atom also possesses a larger nuclear charge (Z = 15) and lone pair electrons (^ⅢP compounds) with p-π* character. By integrating different donor-acceptor skeletons, the extra transformation for chemical structures and tuneable circularly polarized fluorescence (CPF)/circularly polarized room-temperature phosphorescence (CP-RTP) properties could be concisely realized via changing valance.

  For the key emission energy/efficiency regulation from ultraviolet (UV) to near infrared (NIR) region, the facile chemical tailoring protocol is important. In sharp contrast to other hetero atoms inserted emitters, organophosphorus compounds have extra advantages on structural switchover. In traditional emitters, their emission could be tuned by introducing extra functional groups, i.e., chemical post-modification. In this way, the tedious synthetic sequences are inescapable, and molecular volume/mass is also expanded, which might cause a high sublimation temperature or thermostability issues. In phosphorus compounds, the emission energy and efficiency could be easily switched by oxidation [O], sulphuration [S], selenylation [Se], and nucleophilic substitution, in which the change of molecular weight is negligible. In fact, the stability and quantum efficiency will be significantly boosted after oxidation because of the saturated bonding environment and rigid skeleton [21]. Similarly, the post-modification strategy is still efficient in phosphorus-based emitters.

  To date, some strategies for constructing organic phosphorus-based luminophores have been attempted by providing an effective charge transfer (CT) and radiative decay environment via skillful structural design and assembly conception, including chemical structure tailoring, host-guest assembly, polymerization, etc. (Scheme 1a) [24–30]. These methods are performed to push the limit of quantum efficiencies, stability, triplet populations, and asymmetry factors (g_lum). In principle, the construction of circularly polarized luminescence (CPL) emitters also follows those above-mentioned methods in chiral molecules (Schemes 1b and c). In phosphorus-based systems, three-stage development could be determined in previous works. (1) Achiral triphenylphosphine derivatives (including phosphine oxides) first emerge as research targets for traditional fluorescence and phosphorescence materials, owing to their simple molecular structures, concise syntheses, and exceptional structural modifications [25–27,29]. (2) Their phosphonium salts serve as the sequential investigation objects for ameliorating chemical stability, quantum efficiencies, and RTP performances [31,32]. (3) The CPL and CP-RTP have been initiated and promoted in chiral phosphorus-based emitters. This review article mainly focuses on the last chiral-involved subjects.

  Scheme 1

  

  Scheme 1.  (a) Design strategies for typical chiral luminescence systems. (b) Exciton generation and decay pathways. (c) Balanced strategies and key indexes for CPL and CP-RTP efficiency enhancement.

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  2. Conceptional design and synthesis strategy for triplet populations in phosphorus-based emitters

  2.1 Optimization criterion for CPL and triplet populations

  Circularly polarized light is a special kind of polarized light. During its propagation, the end of the electric field vector traces a circular trajectory in a plane perpendicular to the direction of propagation [33,34]. To quantify their polarization merits, two key performance indexes of CPL have been established, i.e., g_lum and photoluminescence quantum efficiency (PLQY). In this case, the g_lum value is the primary metric, which determines the usability of practical optics. Thus, the pursuit of the enlarged g_lum values is a crucial task. Physically, the CPL emission process involves three dominant physical parameters, including electric (μ_e), magnetic dipole moment (μ_m), and the vector angle (θ_e-m) at corresponding excited states. This theoretical quantification could be formalized by g_lum = 4|μ_e| × |μ_m| × cosθ_e-m/(|μ_e|² + |μ_m|²), and the equation could be further simplified as g_lum = 4|μ_m| × cosθ_e-m/|μ_e| because of negligible |μ_m|² (|μ_m| is 2−3 orders of magnitude smaller than |μ_e|, Scheme 1c). According to the experimental formula of g_lum = 2(I_L − I_R)/(I_L + I_R), the limit mathematic value of theoretical g_lum value is ±2. Distinctly, the ideal g_lum value is produced from perfect θ_e-m (0° and 180°) and considerable |μ_m|/|μ_e| values. However, the excessive weakening of |μ_e| would reduce luminous efficiencies. This is a paradox of CPL optimization. Hence, the ideal optimization requires a horizontal orientation of θ_e-m and appropriate |μ_m|/|μ_e| level at the molecular perspective, which depends on the tunable CT states. Fortunately, the tunable valences of the P-atom accord with this demand. On the other side, the hierarchical supramolecular structures could facilitate better g_lum levels via chiral coupling, selective polarization reflection, energy transmission, and so on [35,36].

  In pure organic emitters, although the CPL light can be output from singlet or triplet excited states, the CP-RTP component in the whole spectral band is relatively few in contrast to CPF, resulting in a weak signal of CP-RTP [37]. Therefore, it is essential to increase the triplet populations and stability through structure tailoring by using the heavy/heteroatom effect, n−π* transition, donor-acceptor (D−A) combination, and supramolecular rigidification (Schemes 1c and 2a) [38–47]. The reasonable utilization of these methods could enable effective ISC (k_ISC) and triplet stability. Significantly, the P-atom could easily afford a relatively larger Z value (15) and shallow lone pair electrons, as well as switchable coordination capacity with metals (Scheme 2a). The liberal oxidation states not only influence the D−A behaviors but also decide the metal ion chelating type, e.g., the ^ⅢP (phosphines) tends to coordinate with platinum and copper group metals but their oxides (^ⅢP=O) favor Mn, and rare earth metals. This is dependent on the predilections of different metals. In addition, the distinct metal will allow different emission mechanisms and CPL performance, e.g., the rare earth and Mn complexes possess characteristic sensitized luminescence from f-f or d-d orbital transition with large g_lum levels; the Cu and Pt group complexes display metal to ligand charge transfer (MLCT), ligand to ligand charge transfer (LLCT), halogen to ligand charge transfer (XLCT), etc. Hence, the adjustment abilities of the optical band gaps and CPL are very different.

  Scheme 2

  

  Scheme 2.  (a) Design strategies for chiral phosphorus-based luminescence systems. (b) Some synthetic methodologies for chiral phosphorus compounds. (c) Comparison between P-based emitters and other emitters.

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  2.2 Methodology for design and synthesis

  At present, the mainstream synthesis methods contain de novo synthesis and enantioselective catalysis (Scheme 2b). The former involves multiple reaction stages and chiral resolution via chiral column chromatography or chemical reagents [48,49]. This tactic has significant flexibility. Mostly, the phosphines are produced from functionalized aryl bromides (R-Br) and sulphonates (R-OTf) via [Li]-mediated or [Pd]/[Ni]-catalytic C−P bond coupling. The subsequent treatment with oxidizing agents (H₂O₂/S₈/Se reagents), R-Br/I, or transition metals produces corresponding pentavalent phosphine chalcogenides, phosphoniums, and organometallics, respectively [21,50]. The recent development of asymmetric catalysis demonstrates the new candidate protocols, which show direct derivatization from cheap commercial phosphines. In this case, the transformation is triggered by [Pd] and [Mn]-directed C−P bond activation [22,51–56]. These methods could serve as effective ways for material library searching in phosphorus-based luminescence families. Besides, simple bromination and the following functionalization are also desirable (Scheme 2c) [49].

  3. CPF from phosphorus-based emitters

  Early research focused mainly on non-chiral fluorescence and phosphorescence, which primarily continued with triphenylphosphine compounds [25–27,29]. Up to now, an increasing number of studies have begun to focus on their chiral characteristics and the improvement of luminescent performance, particularly with regard to the improvement of the triplet state. In phosphorus-based emitters, there are two design approaches, namely single-molecule and multi-component assembly systems. The former aims at the corresponding chiral structure-activity relationships and the rule from the chemical structures, while the latter prefers to further optimize luminescent performance. Following the advantages of diverse chirality and valances, we summarize recent research findings and photophysical data in the following chapters and Table S1 (Supporting information).

  3.1 Unimolecular system

  To achieve effective emission in the visible light region. By integrating a variety of sp³-P point and peripheral helical chiral centers, many expanded polycyclic aromatic hydrocarbons (PAHs) have been obtained via photo-induced and transition metal-catalyzed annulation (Scheme 3, A1−A10). In 2012, Tanaka's group first revealed the [Ru]-catalysed enantioselective double [2 + 2 + 2] cycloaddition of biaryl-linked tetraynes, producing helically chiral 1,1′-triphenylene compounds [57,58]. Using the same synthesis protocol from phosphorus-linked diyne and binaphthyl-linked tetrayne precursors, the enantiomeric phospha[7]/[9]helicenes (A7 and A8) have been achieved with high selectivity. The prepared phospha[9]helicenes showed red-shifted CPF with g_lum = 4.8 × 10⁻⁴ at 503 nm [58]. Contrastively, the short phospha[7]helicene has cyan emission and slightly large g_lum (8.1 × 10⁻⁴) because of lower conjugation. Another point to note is that these CPL metrics were significantly lower than traditional carbon[n]helicene analogues (10⁻⁴−10⁻² level) [57]. Furthermore, Crassous's group has developed a series of novel phosphahelicenes by inserting phosphorus atoms at both the inner and outer positions of the [6]helicene terminal, creating compounds that exhibited double chiral canters and helical chirality [59]. The detailed experiments and calculated surveys unveiled low PLQY and g_lum (8 × 10⁻⁴ level) for compounds A1 and A2, but no CPL signal was detected for other compounds (A3−A6). These cases implied that the key indicator of θ_e-m was inferior by direct π-extension design. Moreover, the OLED performance revealed low EQE (0.94%). In fact, these systems still obeyed π−π* transitions throughout the molecule, and the CT component was negligible, where the D−A tendency was weak. Hence, the OLED devices possessed singlet exciton harvest, and deep optimization on the electron transfer channel was necessary. For example, Karasawa's reported that the CPL can be improved through oxidation for the internal-edge-substituted phospha[5]helicene (A9) [60]. The driving force behind this was to optimize the transition dipole moment angle and oscillator strength. As a result, in contrast to phosphine, phosphine oxide showed a better θ_e-m value (99.1° vs. 90.5°) and g_lum values (1.3–1.9 × 10⁻³). It was worth noting that their chemical stability and PLQY were improved in most kinds of solvents (up to 11.6%). Thus, the bonding saturated oxides should be firstly considered for designing stable and efficient phosphorus-based emitters because of the highly stable π feedback bond (d-pπ bond). However, the stimulus-responsive phosphorus-based emitters might be accessible in phosphines due to their photosensitivity and oxygen sensitivity (vide infra) [31].

  Scheme 3

  

  Scheme 3.  Representative helical chirality-based fluorophores from phosphorus-embedded PAHs.

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  In contrast to helical phosphines and their oxides, cationic phosphoniums are quite strong electron acceptors, which could serve as excellent acceptor blocks for designing robust D−A emitters. However, chiral phosphoniums were rare, especially for sp³-P chirality, which is mainly caused by the limited methods of asymmetric syntheses and separation methods for polar phosphoniums (Scheme 2a). In recent advances, some phosphonium-based quasi-chiral PAHs were prepared by [Pd]-catalyzed intramolecular cyclization and intermolecular functional group exchange. It should be noted that the maintenance of enantiomer purity is unsuccessful due to high reaction temperature (> 150 ℃) and low racemic barrier (< 20 kcal/mol). Nevertheless, these quasi-chiral products could be facilely used in the next structure transformation via enantioselective coupling (Scheme 2b) [22,51]. In our recent discovery, we reported Mn(Ⅲ)-mediated C−P bond activation in the synthesis of phospha[5]helicene cations (A10) [54,55]. After the annulation of nonluminous diarylphosphines, the fluorescence of the cyclization product was increased significantly. Their crystals of the corresponding salts displayed moderate g_lum values (3.3 × 10⁻³) and PLQY (41%), while their chiral liquid crystal assemblies displayed extremely high g_lum values (0.51) [54]. The strong electron-withdrawing character endowed obvious highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) separation, thereby enabling significant optical bandgap tunability from deep blue to near-infrared region [55]. These outcomes differed from trivalent phosphines and pentavalent oxides. Nevertheless, although the achiral phosphoniums were successful units for RTP exploitation [32], the development of chiral phosphoniums was behind the other phosphorus-based emitters. This direction deserves for developing ultra-long CP-RTP materials, especially the state of a single molecular CP-RTP.

  In addition to helically chiral molecules, point chiral molecules have also been used to develop possible CPL materials via chiral perturbation (B1−B10, Scheme 4a). In 2020, Ishii's group reported an intramolecular [4 + 2] cycloaddition strategy to build dibenzobarrelene-based 1,4-diaryl-1,3-butadiene derivatives (B1) [61]. These compounds showed blue fluorescence in solution (PLQYs = 49%−86%) and the solid state (PLQYs = 2%−72%) in a narrow range (442−463 nm) except the phenylphosphine selenide with a low PLQYs. The optically resolved phosphole derivatives could exhibit weak CPL of g_lum = 2.8−8.8 × 10⁻⁴. Using similar chiral perturbation tactics, Hissler, and his collaborators have synthesized phosphetene-based polyaromatics with moderate CPF efficiency (g_lum = ~1.0 × 10⁻⁴), where the sp³-P center was placed at the periphery via a nonconjugated 4-membered ring (B2) [62]. In general, the chiral perturbation effect on CPL was too low due to the fact that the chiral units did not participate well in the molecular frontier orbitals. By designing more complex topologies and [2 + 2] cyclization, the enhanced CPF activity was realized for a P-stereogenic macrocycle (B3), but the flexible conformation of the macrocycle might cause excessive non-radiative dissipation [63]. In those above-mentioned emitters, the triplet population was seriously scarce at room temperature. To overcome this drawback, Zheng's group illustrated that the desired triplet excitons could be induced from ramified B-N fragments in the chiral phosphole (B5 and B6), which displayed reserved multiple-resonance thermally activated delayed fluorescence (MR-TADF) from the B-N nucleus [64]. The two compounds showed narrow-band CP-TADF emission with g_lum factors of 1.2 × 10⁻³ and 4.3 × 10⁻³ in doped films, respectively. Thanks to the introduction of TADF functional groups in the phosphorus backbone, their PLQYs far exceeded those of conventional phospholes. This nonplanar terminal phosphole group could also restrain molecular aggregates for better OLED performances. At further, the same research group reported that the reduced benzene bridge between B−N chromophore and chiral phosphine oxides (B9 and B10) can realize the highest EQE_max (37.0%) and narrowest full-width at half-maximum (FWHM) of 20 nm in the OLEDs (Scheme 4b). Unfortunately, their g_lum values (up to 6.1 × 10⁻⁴) were still very low. Thus, the balanced g_lum and PLQY values should be firstly considered through optimizing |μ_e|, |μ_m|, and cosθ_e-m [65]. Then, this group further manipulated the electron-transfer pathway of diphenylphosphine oxides to realize the CP-TADF in the axially chiral skeletons (B7) [66]. Notably, the diastereoisomers have similar emission energy but distinct g_lum indexes, implying that θ_e-m is sensitive to geometrical configurations. For diastereoisomeric systems or those with polychiral centers, more theoretical studies need to be examined to reveal the metrics of the CPL emission environment.

  Scheme 4

  

  Scheme 4.  (a) Representative point and axial chirality-based fluorophores from phosphorus-embedded backbones. (b) EL spectra of B9. Copied with permission [65]. Copyright 2024, Chinese Chemical Society. (c) Emission image of B8 analogues. Copied with permission [68]. Copyright 2022, Elsevier Publishing Group.

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  To evade chiral separation, Zhou's group reported the enantioselective synthesis of P-chiral tertiary phosphine oxides via Cu(Ⅰ)-catalyzed azide-alkyne cycloaddition (B4) [67]. Unfortunately, these compounds and their CPL qualities were poor due to the low chiral induction (g_lum = ~10⁻⁴). Subsequently, Shi et al. reported asymmetrical palladium-catalyzed C−P bond coupling and P−N cyclization synchronously. The scalable functionalization and high ee values enabled the desired CPL (1.3 × 10⁻³) in one of P, N-heterocycles (B8) [68]. Their emission color was tuned from blue to orange (Scheme 4c). Therefore, the development of novel C−P bond activation will facilitate the study of chiral luminescent materials and lower the threshold of asymmetric synthesis.

  3.2 Multicomponent systems

  Typical chiral fluorescent systems do not require additional doping modulation to achieve CPF expression except for some unsuccessful emitters with poor θ_e-m angle (90°), but significantly higher g_lum values can be reached through supramolecular self-assembly [36]. In these cases, the induced supramolecular chiral structures (micro-nano helices) were presented. In our previous discovery, the reported serial racemic phosphoniums could act as bright emission guests in the helical cholesteric phases [54]. This ternary-assembled system rendered remarkable g_lum of (+0.51, −0.48) via partial Förster resonance energy transfer (FRET). Many similar themes demonstrate that the enlarged CPL is driven by chiral/optical coupling and selective light reflection [69–71]. For some extremely weak CPL emitters, polymer doping can promote chiral expression by using a rigid chiral transition environment at excited states [33]. This strategy was frequently applied in systems with triplet-involved emission, owing to the necessity of employing more inert and rigid configurations to quell triplet deactivation caused by excited-state distortion. This will be discussed in the following chapters.

  4. Ultra-long CP-RTP from phosphorus-based emitters

  Unlike chiral fluorescent materials, ultra-long CP-RTP materials are still rare, especially for single molecular CP-RTP emitters [37,72], which is attributed to the differing parity selection rules that govern the electric and magnetic dipole moments in chiral organics, as well as poor triplet populations [73]. In chiral phosphorus-based emitters, only a few pure RTP molecules have been discovered and studied. Nevertheless, the development of this area has become a popular direction, which is stepwise pushed by Xu's [74–77], Zhao's [32,78,79], other groups, and our team [80,81]. In the CP-RTP active phosphorus-based emitters, their developments have transitioned from accidental discoveries to a mechanistic examination of the system, and some design guidelines and verification have been established. However, the phosphorescence performance and device manufacture threshold need further optimization. Because most circularly polarized phosphorescence (CPP) and CP-RTP characters are only active in stubborn and fragile crystals. On the other hand, heavy metal coordination is a fairly well-established tactic for the design of CPP. They are characterized by high efficiency but short lifetimes of phosphorescence.

  4.1 Unimolecular organic systems

  The initial discovery of RTP-active diphosphines began with Zhu's group. In their work, they found racemism enhanced organic room temperature phosphorescence in (Rac)-BINAP crystals (C1, Scheme 5) [82]. In comparison to its chiral counterparts of (R)/(S)-BINAP, racemic BINAP crystals exhibited denser packing density, leading to the generation of enhanced non-polarized RTP emission at 680 nm (18 ms). Thus, the design of CP-RTP was highly strict for triplet stabilization and chiral conservation. Until recently, we reported dynamic luminescence properties from chiral SEGPHOS via inset additional O-atoms (C2−C4) in phosphines, which consisted of unprecedented doublet and triplet CPL in single crystals [80]. Although the doublet and triplet exciton were unstable theoretically, the dense crystal packing provided rigid and inert conditions to resist molecular oxygen and moisture. Moreover, the molecular and stacking symmetry influenced the photo-induced electron transfer (PET) and following radicalization. The noncentral arrangement facilitated a charge separation via intermolecular D−A avenue, bringing up triplet CP-RTP and doublet CPF from inherent SEGPHOS and its radical species. This mixed feature endowed polarized optical encryption applications.

  Scheme 5

  

  Scheme 5.  Representative CPP and CP-RTP materials from phosphines and their oxides.

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  However, the luminescence disappeared in solutions for traditional or commercial phosphine ligands. For instance, Jena et al. prepared propeller-like phosphine oxides (C5), which consisted of the donor (phenothiazine)-acceptor (P=O, P=S, and P=Se) architectures [83]. These compounds exclusively showed weak fluorescence in solutions but both fluorescence and RTP in the crystals. Importantly, the heavy S/Se atom effects can adjust red-shifted emission and more phosphorescent components, where the Se-inserted emitters have improved CPL (3.4 × 10⁻³) in crystals. To overcome the obstruction of luminescent quenching in solutions, our team has further designed and prepared P, N embedded BINAPs by integrating aryl phosphines and aryl amines/carbazoles (C6−C10) [81]. The aryl phosphines and aryl amines/carbazoles fragments of emitters had analogous electronic structures, endowing a weak charge transfer bias between aryl phosphines and aryl amines/carbazoles, thereby producing a hybridized local and charge transfer (HLCT) governing CPF and ultra-long afterglow (> 6 s) with high g_lum up to 6.2 × 10⁻³ as well as near-unity PLQY in solution, polymer, and solid states. Importantly, the law and mechanism of the HLCT-modulated triplet population have been elucidated. Consequently, the synergistic implantation of the lone pair electrons (n−π*) from nitrogen and heavy phosphorus atoms, along with the HLCT from weak D−A structures, could effectively regulate the ISC and internal conversion (IC) processes, thereby achieving the desired optimized up-regulated triplet population for organic ultra-long RTP (OURTP). Conversely, excessive enhancement of the CT characteristics will impede the formation of triplet decay, leading to higher fluorescence PLQY but LE-governing triplet populations (CP-RTP). This landscape is still under exploration in other chiral diphosphines and their derivatives.

  4.2 Multicomponent systems

  Likewise, phosphoniums were electron-deficient and useful subunits in the design of D−A architectures. The created D−A architectures can rationalize the singlet-triplet split energy. In 2024, Zhao et al. reported a dynamically modulating the CP-RTP from soft helical superstructures (D1, Scheme 6a), which was realized by the photoirradiation-driven chirality control in bilayer structure films [84]. This system consisted the multiple photosensitive components, chiral liquid crystal, achiral phosphonium emitters, and a polymer matrix. The optimized helical polymer films allowed selective light reflection, endowing CP-RTP with an emission lifetime of 805 ms and a g_lum value of up to 1.38. Their further extended project demonstrated that chiral liquid crystal technology was a propagable method to promote CPL and CP-RTP [79]. However, the brightness loss was undesired. In addition, the structural complexity needs to be further reduced and the emission brightness should be enhanced simultaneously.

  Scheme 6

  

  Scheme 6.  (a) Representative CP-RTP materials from phosphorus-based emitters via polymer assembly (D1). (b) Representative Cu(Ⅰ) cluster doped system (D2). (c) Representative halide hybrid bulk glass with photochromic ultra-long phosphorescence (D3). Copied with permission [86]. Copyright 2018, Springer Publishing Group.

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  Recently, organic-inorganic hybrid assemblies have also been designed to construct CP-RTP. Using a triphenylphosphine (TPP) host with trace BINAP-CuX (X = Cl, Br, I) complexes as co-melting assemblies (D2, Scheme 6b), three chiral ionic solids were prepared to investigate the intensification of CP-RTP [85]. This ionic solid showed a perceivable afterglow than 3 h to human eyes, which was driven by the electron-trapping mechanism for this unusual long-persistent luminescence. However, the PLQYs of chiral phosphine Cu(Ⅰ) halides were low in current advances, and their emission was also easily lost in solutions or amorphous states. Unlike classical crystal materials, the glass solid provided a nice fabrication method by thermal treatment. Yan's group illustrated a colorful 0-D halide hybrid bulk glass (D3) with three-component grinding-melting-quenching (Scheme 6c) [86]. Their emission could be easily switched from blue to red by changing the metal salts. Interestingly, the mixed triplet and doublet emission was formed by photostimulation. Therefore, this nature enabled dynamic ultra-long CP-RTP (10⁻²) for anti-counterfeiting. Actually, the trivalent phosphines were also photoresponsive matters for dynamic luminescence. In this case, the PET process was pivotal for radical cation generation [80].

  4.3 Organic-inorganic hybrid materials

  Phosphorus-based backbones allow two valence states and d-orbital with distinct chelating preference to metal ions. For example, tricoordination phosphorus compounds have a sp³ tetragonal pyramidal configuration, and phosphorus atoms have lone pair electrons, which display electrophilic, amphiphilic, and dienophilic reaction properties. Thus, there is a strong affinity between platinum and copper group metals to form stable complexes. Many complexes could emit remarkable phosphorescence with short lifetimes because of the giant spin-orbit coupling parameters ζ_soc (e.g., Pt 4481 cm⁻¹, Cu 857 cm⁻¹). In addition, phosphorus tends to form tetracoordinate compounds containing P=O bonds, which have extremely strong bond energies and are related to the formation of d-pπ feedback coordination bonds. This property affects the bonding mode and properties of many P=O containing complexes. The coordination compounds for oxyphilic metals (Mn, Eu) are diverse and stable due to the extremely strong bond energy of the coordination bonds. This stability makes phosphorus compounds exhibit a strong tendency to form 4/5/6-coordinated Mn(Ⅱ) and 8-coordinated Eu(Ⅲ) compounds.

  4.3.1   Manganese(Ⅱ) complexes

  At present, the Mn(Ⅱ) coordination complexes are a topical research field in photoluminescence and radioluminescence because of their unique d-orbital transition involved phosphorescence [87]. Exploitation chiral Mn(Ⅱ) complexes are still in the initial stage (E1−E6, Scheme 7). The reported complexes have merely been presented since 2023. For instance, Artem'ev et al. have designed and reported a pair of point-chiral alkylphosphine oxides linked to Mn(Ⅱ) polymers (E1) [88]. The electroneutral structures featured good stability for versatile applications in magnetically responsive CPP (g_lum = 0.02, PLQY = 89%) and X-ray scintillation (174 μGy_air/s). The results indicated that the design of electroneutral Mn(Ⅱ) complexes was the effective platform for obtaining both robust moisture resistance and CPP. In 2024, Wong et al. reported many neutral manganese(Ⅱ) complexes bearing achiral bidentate phosphine oxide ligands (E2) [89]. These complexes enabled green color-tunable phosphorescence by modulating the crystal field strength through steric hindrance. Interestingly, the achiral crystals can emit obvious CPP, which was attributed to the anisotropic optical activity of achiral crystals. The classical crystal/ligand-field theory suggested a weak strength of [MnX₄] in contrast to [MnX₅]/[MnX₆] [87]. The stronger field ensured low-energy red phosphorescence. However, the very recent work reported a [MnX₄] displayed red CPP (633 nm, 5.1 × 10⁻³) in 2,2′-bis(diphenylphosphino)−1,1′-binaphthyl oxide (BINAPO) coordinated MnBr₂ complexes (E3), while the CP-OLED investigation revealed an impressive g_lum ± 8.5 × 10⁻³ and moderate EQE (4%) [90].

  Scheme 7

  

  Scheme 7.  Representative CPP and CP-RTP materials from Mn(Ⅱ) complexes.

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  In addition to neutral complexes, the ionic emitters (E4–E6) could be designed by coordinating unsaturated tetragonal cones or inherent phosphonium counter ions. The pentacoordinated crystal field has stronger strength to produce red CPP (636 nm) for E4 crystals, featuring a CPP g_lum value of 1.56 × 10⁻² and a lifetime of 5.76 ms [91]. The red chiral Mn(Ⅱ) crystals were rare up to now, and the high PLQY (76.3%) allowed white LED fabrication and flexible X-ray imaging capacity (spatial resolution up to 11.9 lp/mm). Besides, the chiral salt (E5) consisted of a [MnX₄]²⁻ anion, which exhibited green CPP with near-unity PLQY and high g_lum of 2.0 × 10⁻³ in Quan's work [92]. Importantly, these crystals displayed mechanoluminescence (ML) under mechanical stimulation, which was caused by the breaking of hydrogen bonds generating opposite charges at crystal fracture sites in (R)-isomer, causing electron bombardment and subsequent energy transfer to inorganic luminescent centers, thereby allowing quick ML emission. In addition to small Mn(Ⅱ) complexes, the polymeric phosphoniums could also serve as counter cations in Mn(Ⅱ) complexes. This design will improve the film-forming ability of scintillators. In Guo's work, two chiral phosphoniums have been synthesized by introducing a chiral side chain (E6) [93]. The prepared organic-inorganic hybrid polymers exhibited CPP g_lum of 5.823 × 10⁻² and −2.877 × 10⁻², as well as excellent X-ray scintillation performances (14.84 lp/mm).

  4.3.2   Copper group complexes

  For a simple small Cu(Ⅰ) molecular design (F1–F12, Scheme 8), Yao's group reported intense CPP from the chiral phosphine-copper iodide hybrid cluster (F3) [94]. By controlling crystallographic morphology, including hexagonal platelet-shaped microcrystals and oriented crystalline films, the enlarged CPP was realized with high g_lum ≈ 9.5/5.0 × 10⁻³. Time-dependent density functional theory (TD-DFT) results manifested a triplet metal-to-ligand charge transfer (³MLCT) mixed with the halide-to-ligand charge transfer (³XLCT) mechanism in the lowest excited states, which induced a HOMO−LUMO distribution in whole chiral motifs to boost CPP. Later, Steffen et al. reported mechano-stimulus and environment-dependent CP-TADF in carbazole-connected BINAP-Cu complexes (F1 and F2) [95]. In comparison to the F3 cluster, the new mononuclear structures expelled the halogen atom of the precursor by carbazoles, facilitating additional CP-TADF in solution with modest quantum efficiencies (22%). Their g_lum values were increased from 6.0 × 10⁻³ (in THF) to 2.1 × 10⁻² (ground powder). Their large values were befitted from near-perfect θ_e-m indexes (178° and 180°). Besides, when nitrogen-based ligands and phosphines were used, ionic copper complexes can also express some CPP activity (F4–F6) in the 10⁻⁴ level [96].

  Scheme 8

  

  Scheme 8.  Representative CPP and CP-RTP materials from copper group complexes and clusters.

  DownLoad: Full-Size Img PowerPoint

  The hierarchical higher-order chiral superstructures were interesting blocks for the design of CPP materials. In recent years, several research groups have reported the famous polynuclear clusters. The reactions contained a common BINAP ligand but slightly different metal precursors, generating distinct chiral topology construction (F7–F9). In 2017, Liu's group reported a trinuclear gold cluster (Au₃) for aggregation-induced emission (AIE) trigged CPP, showing a strong 7.0 × 10⁻³ in cubic nano assemblies (F8) [97]. With the same BINAP but a different Cu(Ⅰ) salt and reaction conditions, Zang's group disclosed a trinuclear cluster that has a similar triangle geometry but a different connection, where the Cu₃ was isolated by a CO₃ linker (F7) [98]. This Cu₃ cluster also exhibited AIE nature (a_AIE = 17.3) by cluster-centered transition and triplet ³MLCT processes. The chiral aggregates endowed remarkable CPP (2.0 × 10⁻²) at 610 nm, and photo-responsive emission in solvents. Photochromism was supported by green emissions originating from BINAP's oxidized products. Until recently, they introduced an additional 4-ethynylpyridine ligand into the synthesized sequence to produce a 1-D cluster (F9), by controlling the base type [99]. These clusters revealed photo-activated CPP enhancement (9.0 × 10⁻³) by photo-induced deoxygenation process.

  In addition to phosphine ligand-involved chiral induction in Cu clusters, Liu's group reported the Au₆ cluster synthesis via alkynyl alcohol enantiomers-assisted assembly (F12) [100]. The chiral activity and luminescence of the cluster could be induced at aggregate states, displaying high PLQY (70%) and g_lum value (6.2 × 10⁻³). Moreover, the cluster with a higher nuclearity of 42 has been reported by Zang's group, which consisted of an enantiomeric pair of bidentate sulfur-based ligands, namely (R/S)-binaphthyldithiophosphoric acid [101]. The CPP-silent homosilver (F10, Ag₁₂Ag₃₂) cluster can be transformed into a heterometal (F11, Au₁₂Ag₃₂) cluster via the treatment with S₂−@M₁₂@S₈, the F11 cluster activated the red CPP at 626 nm with a maximum g_lum value up to 6.0 × 10⁻³. This work provided novel structural models for enhancing the CPP of Ag/Au superclusters. On the whole, the emission color of the many reported clusters was located in the red region because of low band gaps, and the lighting-emitting applications should be considered in the future.

  4.3.3   Platinum group complexes

  It is well-known that the noble metal complexes have outstanding phosphorescence performances because of their huge SOC effect and coordination stability, including d₈ and d₆ configurational Pd(Ⅱ)/Pt(Ⅱ) and Ir(Ⅲ) complexes, respectively. In fact, these phosphorescent emitters received great prosperity in OLEDs for commercial applications. In recent years, the Pd(Ⅱ)/Pt(Ⅱ) and Ir(Ⅲ) complexes have been studied in CP-OLEDs [102–104]. The achievement of future applications of CP-OLEDs such as in 3-D displays, optical data storage, and spintronics, is the challenge and the ultimate goal. In virtue of the strong chelating driving force for phosphines, some chiral platinum group phosphors have been designed and investigated (G1–G15, Scheme 9). In 2021, Crassous's group reported a series of triphenylphosphine (TPP) decorative platina[5]helicenes (G1–G3) [105]. These compounds displayed dual luminescence, including CPF between 450 nm and 600 nm and red/NIR CPP between 700 nm and 900 nm (g_lum = 3 × 10⁻³ at 750 nm). DFT simulations revealed that this unusual behavior was attributed to the limited electron interactions between the d orbitals of the Pt-atom and the π orbitals of the organic ligand in the excited states. Importantly, CPP above 900 nm was achieved by introducing a donor in the primary ligand [106]. Using chiral perturbation, Shi et al. reported achiral phosphines bridged by chiral dinuclear Pt(Ⅱ) complexes (G4–G7), and these complexes showed enhanced CPP signal profoundly, with the g_lum up to 10⁻³ [107]. Tsubomura's group prepared CPP-active Pd(0) complexes bearing chiral diphosphines (G8–G11). Unlike divalent Pt(Ⅱ) and Pd(Ⅱ) ions, Pd(0) has an electronic configuration of d₁₀ with a tetrahedral coordination mode helping to inhibit π aggregation. Thus, their phosphorescence emission at the solid state is based upon the ³MLCT emission of the monomer rather than metal-metal to ligand charge transfer (³MMLCT) [108]. By regulating the phosphine ligands, the rare red-NIR CPP was found and the peaks ranged from 600 nm to 740 nm because of low laying triplet energy levels. In addition, the hexa-coordinated octahedral Ir(Ⅲ) complexes featured monomer CPP and circularly polarized electroluminescence (CPEL) emission with a g_lum level of ~2 × 10⁻³ and maximum external quantum efficiency (EQE) of 13.8% (G13), which afforded an efficient strategy for the design of blue CP-OLEDs with good triplet exciton harvest (EQE = 13.8%) [109].

  Scheme 9

  

  Scheme 9.  Representative CPP and CP-RTP materials from platinum group complexes.

  DownLoad: Full-Size Img PowerPoint

  To pursue the Pt(Ⅱ)-containing aryleneethynylene polymers via chiral self-assembly, Sanda's groups reported a ligand-exchange method to realize better optical activity (G12). Unexpectedly, this polymer only had some CD activity but no CPP [110]. Hence, the subtle regulation of Pt-complex-containing polymers was important and this landscape has been explored by Wu's work (G14 and G15) [111]. The refined cyclic-helical polymers showed weak CPP (g_lum = ~10⁻⁴), indicating the significance of side chain and nanostructure tailoring.

  4.3.4   Europium(Ⅲ) complexes

  Similar to the oxyphilic Mn(Ⅱ) ion, the Eu(Ⅲ) complexes have oxyphilic coordination with the P=O bond. However, their 4f orbits possess more empty positions for coordination, which will trigger the Eu(Ⅲ)-center involved D–F transition, with long-lived phosphorescence from ~560 nm to 720 nm. In fact, the study of chiral Eu(Ⅲ) has a long history in phosphine chemistry relative to the afore-mentioned organics, Mn(Ⅱ), and other metal complexes (H1–H16, Scheme 10). For example, as early as 2009, Hasegawa et al. reported CPP from the point- and axis-chiral ligands (chiral β-diketone and BINAPO) coordinated Eu(Ⅲ) emitters (H1 and H2) [112]. Until today, the investigations about Eu(Ⅲ) complexes are still a fashion direction although the more complex topological chirality has been included in very recent years. Due to the transition allowing magnetic dipole moment (μ_m) with large magnitudes, the typical Eu(Ⅲ) complexes showed larger g_lum values (0.24−0.47 in H1/H2). Hasegawa's group reported point-chiral phosphine oxide coordinated Eu(Ⅲ) complexes emitting CPP with a g_lum value of 0.08 (H5–H8) [113], suggesting that the point-chiral perturbation was weaker than that of axis chirality. In recent reports, the structures of Eu(Ⅲ) complexes were gradually iterated from 0-D small molecules to 1-D polymers and finally 3-D molecular cages (H3 and H4–H17). Significantly, the polymerization could boost photosensitized energy transfer efficiency and CD/CPL performances as well as structural rigidity. For example, Hasegawa et al. reported a helically chiral Eu(Ⅲ) coordination polymer H3 that significantly outperformed mononuclear chiral Eu(Ⅲ) complex H4 in crystals in terms of better energy transfer efficiency (η_ET = 53%) and g_lum (0.17) [114]. In 2021, Pointillart et al. reported Eu(Ⅲ) polymers with multiple stereogenic elements, which implanted chiral 1,10-bi-2-naphtol-derived bisphosphate ligands and the same β-diketone (H9–H12) [115]. The large g_lum of 0.12 was detected from three stereogenic elements driven polymer (H11), implying that ligands with a π-extended system/rigid system and three steric elements may favor the μ_m order of magnitude.

  Scheme 10

  

  Scheme 10.  Representative CPP materials from europium(Ⅲ) complexes, polymer, and cages. Reproduced with permission [116] for H15/H16. Copyright 2019, American Chemical Society.

  DownLoad: Full-Size Img PowerPoint

  The breakthrough of BINAP-decorative cages has been made in recent years (H13–H17). Yan's group designed and synthesized a pair of tetrahedral cages via a chiral ancillary BINAPO ligand induction strategy (H15) [116]. The C₃-symmetric TPA-diketone main ligands directed a tetrahedral topology. As expected, tetrahedral cages emitted strong CPL with g_lum values up to 0.20 and a remarkable PLQY of 81%. More importantly, the chiral memory effect for cages was observed, where the chiral coordination configuration and optical activity could be reserved after replacing BINAPO with achiral DPEPO (H16). Besides, this group further reported that triple-stranded Eu(Ⅲ) 4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl) diphenyl sulphide ligand facilitated [3 + 2] coordination and triple-stranded helical configuration (H13 and H14) [117]. Importantly, when the photochromic diarylethene ancillary ligand (L*) is used, the Eu(Ⅲ) photoswitch (H17) can show reversible photochromism, PL (ϕPL = 20.5%), and CPL (g_lum up to 0.29) responses [38]. Thus, the stimulus-responsive CPP materials could be realized in phosphorus-based emitters.

  5. Applications

  Based on the current reports, one can summarize that the desired CPF and CPP/CP-RTP properties of phosphorus-based emitters have promoted their practical applications in the following fields. (1) CP-OLED materials require higher PLQYs and stability for practical device manufacture. Thus, the D–A design in small molecules should be first adopted to enhance the triplet harvest. (2) By integrating photoresponsive or chemically responsive subunits, e.g., manipulating the valence of P-atom and the CPL switch and sensing can be easily reached. (3) Heavy metal atoms in emitters empower high-energy X-ray absorption and radioluminescence for scintillation imaging. (4) Ultra-long lifetime of polarized afterglow provides anti-counterfeiting and information storage under an operational time scale. Moreover, emissive chiral phosphorus-based compounds may further advance the real-world exploitation of optical magnetic switches, chiral spin electronics, and asymmetric photocatalysis/photosynthesis in the near future [118,119].

  6. Conclusions and outlook

  In summary, the advances in phosphine complexes and phosphine-oxygen complexes are quite fast, especially for cheap Cu(Ⅰ), Mn(Ⅱ), and Eu(Ⅲ) complexes. However, even with these advancements, there are still many challenging issues that need to be resolved. For instance, PLQY still needs improvement in phosphine Cu(Ⅰ) complexes through innovative ligand engineering and cluster engineering to rival famous carbene Cu(Ⅰ) emitters (Table S1) [120]. For metal orbital-derived CPP in Mn(Ⅱ) and Eu(Ⅲ) complexes with high PLQYs, the sensitized phosphorescence wavelength is hard to change. The investigation of the CP-OLED application should be included for the Mn(Ⅱ)/Eu(Ⅲ) family in the next stage.

  Additionally, the progress of organic phosphorescence emitters is still slow. From a comprehensive overview of recent advances in chiral phosphine emitters, we can conclude that the conventional chiral phosphine ligands are based on catalytic considerations, which do not have a clear charge separation tendency and only exhibit large singlet-triplet large energy gaps and weak emission. In addition, their intrinsic molecular motion leads to undesired non-radiative quenching, thus requiring the stabilization of the triplet state by intermolecular coupling and interactions in rigid environments. By integrating reasonable donors, some pure organic phosphine derivatives have demonstrated enlarged triplet populations, suitable singlet-triplet gaps, and ultra-long CP-RTP properties with impressive afterglow and high quantum efficiencies. Appropriate oxidative integration can endow compounds with high stability and D−A features. Conversely, metastable phosphines can lead to high variable stimulus-responsive luminescence at specified states. Nevertheless, the current phosphorescence proportion and CPL g_lum values are still insufficient. The TD-DFT simulation should be adopted to guide molecular design rather than blind screening. In addition, the intrinsic CP-TADF backbone based on the chiral phosphines has not been well experimentally determined, while the optimization and utilization of triplet and double states are still in their infancy. To gain approving phosphorescence and practicable g_lum, incorporating heavy atoms and novel D−A features may promote the phosphorus-based emitters with strong CP-RTP, CP-TADF, and CP-OLED manufacturing capabilities, while the chiral assembly must be adopted to break the physical limit of g_lum parameters in the current stage, including liquid crystal assembly, supramolecular polymerization, and artificial device engineering.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Bo Yang: Writing – review & editing, Writing – original draft, Conceptualization. Suqiong Yan: Data curation. Shirong Ban: Data curation. Wei Huang: Writing – review & editing, Supervision, Conceptualization.

  Acknowledgments

  This research was supported by the National Natural Science Foundation of China (No. 21871133), the Natural Science Foundation of Jiangsu Province (No. BK20211146), and the Science, Technology, and Innovation Commission of Shenzhen Municipality (No. JCYJ20180307153251975).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110837.

  
  
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  Scheme 1  (a) Design strategies for typical chiral luminescence systems. (b) Exciton generation and decay pathways. (c) Balanced strategies and key indexes for CPL and CP-RTP efficiency enhancement.

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  Scheme 2  (a) Design strategies for chiral phosphorus-based luminescence systems. (b) Some synthetic methodologies for chiral phosphorus compounds. (c) Comparison between P-based emitters and other emitters.

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  Scheme 3  Representative helical chirality-based fluorophores from phosphorus-embedded PAHs.

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  Scheme 4  (a) Representative point and axial chirality-based fluorophores from phosphorus-embedded backbones. (b) EL spectra of B9. Copied with permission [65]. Copyright 2024, Chinese Chemical Society. (c) Emission image of B8 analogues. Copied with permission [68]. Copyright 2022, Elsevier Publishing Group.

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  Scheme 5  Representative CPP and CP-RTP materials from phosphines and their oxides.

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  Scheme 6  (a) Representative CP-RTP materials from phosphorus-based emitters via polymer assembly (D1). (b) Representative Cu(Ⅰ) cluster doped system (D2). (c) Representative halide hybrid bulk glass with photochromic ultra-long phosphorescence (D3). Copied with permission [86]. Copyright 2018, Springer Publishing Group.

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  Scheme 7  Representative CPP and CP-RTP materials from Mn(Ⅱ) complexes.

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  Scheme 8  Representative CPP and CP-RTP materials from copper group complexes and clusters.

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  Scheme 9  Representative CPP and CP-RTP materials from platinum group complexes.

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  Scheme 10  Representative CPP materials from europium(Ⅲ) complexes, polymer, and cages. Reproduced with permission [116] for H15/H16. Copyright 2019, American Chemical Society.

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