Small organic molecules for circularly polarized luminescence: from design to applications

Abstract

Circularly polarized luminescent-active small organic molecules (CPL-SOMs) exhibit broad application prospects in 3D displays, information encryption, biosensing, and circularly polarized organic light-emitting diodes (CP-OLEDs), owing to their simple and readily modifiable molecular structures, high luminescence efficiency and well-defined structure–property relationships. Nevertheless, the luminescence dissymmetry factor (g_lum) of most CPL-SOMs remains in the range of 10⁻³–10⁻⁵, significantly below the theoretical maximum. Therefore, enhancing the g_lum value remains a top priority. Achieving effective regulation of CPL signals is also an important research direction, thereby enabling further expansion of their applications into emerging fields such as smart sensing and security technologies. This review focuses on recent research advances in high-performance CPL-SOMs, covering design strategies for central chiral, axially chiral, helical chiral, and planar chiral systems, as well as their photophysical and chiroptical properties and device performance. Finally, the current challenges and future research directions for these materials are discussed, with the aim of providing valuable references for the development of high-performance chiral optoelectronic organic molecules.

This review summarizes recent advances in circularly polarized luminescent-active small organic molecules, including design strategies, photophysical and chiroptical properties, device performance, and current challenges and future directions.

1. Introduction

Circularly polarized luminescence (CPL) refers to the phenomenon in which a nonracemic chiral luminescent system, or an achiral luminescent system under a chiral environment, emits left- and right-handed circularly polarized light with differential intensity after photoexcitation with nonpolarized light.^1,2 Owing to its broad application prospects in fields such as 3D display,³ information anti-counterfeiting and encryption,^4–7 CPL lasers,^8–10 circularly polarized organic light-emitting diodes (CP-OLEDs),^11–14 biosensing,^15,16 and asymmetric photochemical synthesis,^17,18 CPL has garnered extensive attention in recent years. Correspondingly, circular dichroism (CD) describes the differential absorption of left- and right-handed circularly polarized light by a nonracemic chiral system, which can be quantified by the absorption dissymmetry factor (g_abs),

1

where ε_L and ε_R represent the molar absorption coefficient for left- and right-handed circularly polarized light, respectively. While CD provides structural information of chiral systems in the ground state, CPL enables the investigation of structural information in the excited state of chiral luminescent systems.

The CPL performance of chiral luminescent systems is characterized by the luminescence dissymmetry factor (g_lum) and the fluorescence quantum yield (Φ_F), which reflect the degree of polarization and the luminescence efficiency, respectively. The g_lum is quantified analogously to g_abs in CD, and is experimentally determined as eqn (2),

2

where I_L and I_R denote the emission intensities of left- and right-handed circularly polarized light, respectively. Theoretically, g_lum can also be predicted by eqn (3),^19,20

3

R = µ·m 4

D = |µ|² + |m|² 5

where R represent the rotatory or rotational strength, D represent the dipolar strength, m and µ represent the magnetic and electric transition dipole moments, respectively, and θ is the angle between m and µ. For most organic molecular systems, |m| is substantially smaller than |µ|, rendering |m|² negligible; thus, the expression simplifies to eqn (6).

6

Consequently, g_lum is proportional to |m| and inversely proportional to |µ|. In contrast, the Φ_F is proportional to |µ|². This reveals an inherent trade-off between g_lum and Φ_F,²¹ which underlies the significant challenge in developing chiral luminescent materials that simultaneously exhibit high g_lum and high Φ_F. To comprehensively evaluate the CPL performance of chiral materials, Zinna et al. introduced the concept of CPL brightness, B_CPL, defined as eq (7).²²

7

Since Emeis et al.²³ first reported a CPL-active small organic molecule (CPL-SOM), chiral indanone, in 1967, the field has experienced rapid development. In 1997, Meijer et al. reported the first CP-OLED.²⁴ To date, a wide variety of CPL-active materials have been reported, including small organic molecules (SOMs),^25,26 transition metal complexes,^27,28 polymers,^29,30 supramolecular assemblies,^31–33 nanomaterials,^34–36 and liquid crystals^37,38et al. Among these, SOMs have emerged as a research hotspot for the development of chiral optoelectronic functional materials, owing to their advantageous characteristics such as simple and readily modifiable molecular structures, high luminescence efficiency, well-defined structure–property relationships, and easy fabrication.^39,40 Current CPL-SOMs are mainly realized through covalent bonding between chiral moieties and luminescent units. Furthermore, to enhance the optical performance of the materials, molecular design strategies for multifunctional CPL-SOMs have been proposed, such as CPL-active systems exhibiting thermally activated delayed fluorescence (CP-TADF)^41–43 and mechanochromic (CP-MCL).^44–46 These designs not only significantly improve the comprehensive performance of the materials but also further broaden their application scope.

In this paper, SOMs broadly refers to low-molecular-weight organic compounds that exhibit CPL signals either in the single-molecule state and/or in the aggregated state. This review focuses on the recent development of CPL-SOMs across four distinct chirality categories, central chirality, axial chirality, helical chirality, and planar chirality, and provides a systematic account of advances in their molecular design and synthesis, photophysical properties, chiroptical properties, and device applications. This review summarizes representative high-performance chiral CPL-SOMs reported in recent years, encompassing multifunctional systems that combine CPL with TADF or MCL. These examples offer new design strategies for multifunctional CPL materials. Furthermore, the review analyzes the strategies for modulating chiroptical properties, providing a theoretical basis for understanding structure–property relationships. In summary, it presents frontier advances in the field. The highlighted high-performance cases and modulation strategies enable researchers to rapidly screen promising molecular scaffolds and modification sites, thereby accelerating the rational design of next-generation CPL-SOMs.

2. Central chirality

Central chirality offers notable advantages, including configurational stability, resistance to racemization, high design flexibility, and synthetic accessibility, making it one of the most favored chiral elements in the creation of chiral molecules. Among the commonly encountered centrally chiral CPL-SOMs, the chiral center is predominantly based on an sp³-hybridized carbon atom, whereas systems featuring a boron, phosphorus or sulfur atom as the chiral center are comparatively rare.

Achieving precise regulation of CPL signals remains a significant challenge in this field.^47,48 In 2026, Yamaguchi and co-workers reported a class of chiral boron-doped polycyclic aromatic hydrocarbons 1–4 (Fig. 1), which possessed an asymmetric boron atom that was intramolecularly coordinated by a P O unit.⁴⁹ Upon photodissociation of the resulting P O→B bond, the chirality was switched from point chirality localized at the boron center to C–B axial chirality. The ¹¹B NMR spectra of all compounds were consistent with the general characteristics of tetracoordinate boranes, which was ascribed to the presence of intramolecular P O→B coordination bonds. In all tested solvents, the absorption maxima (λ_abs) of compounds 1–4 were blue-shifted by about 50 nm relative to their tricoordinate counterparts, a result of the reduced contribution from the boron atom's vacant p-orbital upon coordination bond formation. In toluene, the emission wavelength of compound 1 (λ_em = 446 nm) was similar to that of the corresponding tricoordinate species 1-Tip (λ_em = 430 nm). The excellent agreement observed between the excitation and absorption spectra of 1 provided strong evidence that the P O→B bond underwent excited-state photodissociation, leading to emission exclusively from the tricoordinate borane. Compound 2 exhibited absorption and emission characteristics in toluene analogous to those of 1, indicating that its emission also originated from the tricoordinate borane. However, due to its more extended π-conjugated system, the emission wavelength of 2 was slightly red-shifted. For compound 3, the emission maximum in toluene was observed at 466 nm, which was significantly blue-shifted relative to that of its tricoordinate congener 3-Tip (λ_em = 553 nm). The appearance of distinct vibronic features in their fluorescence spectra suggested that the emission arose from the tetracoordinate boron species. Additionally, an additional emission band of compound 3 appeared in the long-wavelength region (λ_em = 570–580 nm), whose intensity gradually increased as the solvent was changed from toluene to dichloromethane, chloroform, and methanol. In methanol, the emission spectrum (λ_em = 577 nm) closely resembled that of its tricoordinate analogue 3-Tip (λ_em = 557 nm), indicating that this emission band originated from the tricoordinate boron species produced by photodissociation of the P O→B bond. However, in acetonitrile, a solvent with polarity comparable to that of methanol but without hydrogen bond-donating capability, the emission band was observed at 463 nm, almost identical to that in toluene. Accordingly, the emission in acetonitrile was also attributed to the tetracoordinate species. These results indicated that the photodissociation of the P O→B bond was facilitated primarily by hydrogen-bonding interactions with the solvent, rather than by polarity effects. For compound 4, the emission in toluene (λ_em = 472 nm) was significantly blue-shifted relative to that of its tricoordinate congener 4-Tip (λ_em = 575 nm), indicating that the emission originated from the tetracoordinate species. In all other solvents tested, however, the tricoordinate species predominated. This observation suggested that the D–π–A architecture of compound 4 further promoted photodissociation, which could be achieved not only by hydrogen-bonding interactions but also by increased solvent polarity. Theoretical calculations revealed that for compound 1, the tricoordinate species was 0.02 eV more stable than its tetracoordinate counterpart. In contrast, the π-extended compound 3 exhibited the opposite behavior. These results suggested that the boron-embedded polycyclic aromatic hydrocarbons with higher degrees of π-conjugation were less susceptible to photodissociation. This was attributed to the reduced electronic perturbation of the boron center, which made the energetic gain from tetra-to-tricoordinate conversion less significant. Furthermore, this optical switching did not compromise the fluorescence quantum yield. For compound 3, the Φ_F value remained above 0.65 under all tested conditions. The optical resolution of compounds 1, 3, and 4 were successfully achieved by chiral high-performance liquid chromatography (HPLC). For enantiomer 1, no loss of enantiopurity was detected upon heating at 100 °C, irradiation at 330 nm, or exposure to alcohol-containing solvents. This finding differed from previously reported triarylboranes featuring C–B axial chirality, which underwent racemization in the presence of basic alcohols.⁵⁰ Furthermore, all compounds exhibited CPL activity, with each pair of enantiomers showing mirror-image CPL spectra, and the tricoordinate species also displayed CPL features. This study provided the first example of a chiral triarylborane bearing a stereogenic C–B bond axis exhibiting CPL. In the case of compound 3, both the point chirality of the tetracoordinate form and the axial chirality of the tricoordinate form exhibited CPL activity. In toluene, the tetracoordinate species exhibited a CPL band at 460–480 nm; in methanol, the tricoordinate species dominated the CPL spectrum at 570–600 nm; and in dichloromethane, both forms generated dual CPL emission. Such dual CPL emitters that respond to environmental stimuli at room temperature are uncommon. The g_lum values of compounds 1 and 4 were on the order of 10⁻³, while that of compound 3 was on the order of 10⁻⁴, indicating moderate CPL intensities (Table 1). The larger g_lum value observed for the tricoordinate specie of compound 3 was rationalized by a slightly increased θ between m and µ relative to the tetracoordinate specie. In this work, a reversible coordination/dissociation equilibrium was exploited to confer solvent-responsive control over the emission centers and the corresponding chiroptical characteristics. This non-covalent strategy differed from traditional covalent chiral molecular systems and thereby provided a new path toward environmentally responsive CPL materials.

Fig. 1. (a) Schematic illustration of the molecular design, (b) the chemical structures of 1-Tip, 3-Tip, 4-Tip and 1–4, and (c) CPL spectra of 3. Reproduced with permission from ref. 49. Copyright 2026, John Wiley and Sons under a Creative Commons 4.0 License.

Table 1. Photophysical and chiroptical parameters of CPL-SOMs with central chirality.

Molecule	λ em (nm)	Φ F	g lum (λmon (nm))	Conditions	Ref.
1	446	0.29	The order of 10−3	Toluene	49
2	521	0.27	—	Toluene	49
3	466	0.83	The order of 10−4	Toluene	49
4	472	0.56	The order of 10−3	Toluene	49
(S)-5	400	0.11	+1.1 × 10−3 (410)	Crystalline	53
(S)-6	448	0.21	+2.2 × 10−3 (440)	Crystalline	53
(S)-7	462	0.87a	−1.0 × 10−3 (−)	Toluene	55
(S)-8	498	0.93a	−3.9 × 10−3 (−)	Toluene	55
(S)-9	497	0.91a	−2.4 × 10−3a (−)	Toluene	58
(S)-10	505	0.87a	−4.7 × 10−3a (−)	Toluene	58
(S)-11	495	0.92a	−1.9 × 10−3a (−)	Toluene	58
(S)-12	503	0.93a	+1.7 × 10−3a (−)	Toluene	58
(S)-13	509	0.91a	−5.4 × 10−3a (−)	Toluene	58
(S)-14	496	0.93a	+0.7 × 10−3a (−)	Toluene	58
15	503	—	—	Film	61
16	551	—	Front: −4.5 × 10−2 (545)	Film	61
			Back: 2.5 × 10−2 (545)
17	—	—	—	Film	61
18	512	—	Front: −5.0 × 10−2 (505)	Film	61
			Back: 9.0 × 10−2 (505)
22	513 550	—	1.7 × 10−2 (510)	Film	62
(S,S)-23	564 606	0.25	+2.0 × 10−2 (−)	Chloroform	63

^aIn doped films.

While significant progress had been made in the field of CPL, mechanochromic luminescence (MCL), characterized by reversible emission color changes in solid-state materials under mechanical stimuli, had also witnessed rapid development.^51,52 The exploitation of mechanochromic CPL held considerable potential for applications such as mechanosensors and security technologies. Nonetheless, the exploration of mechanochromic CPL in SOMs had remained scarce. In 2025, Ito, Imai, and their co-workers designed and synthesized a series of chiral pyrenylprolinamide derivatives, 5 and 6, which exhibited mechanochromic CPL properties (Fig. 2).⁵³ These compounds contained either a tert-butoxycarbonyl (Boc) or 2,2,2-trichloroethoxycarbonyl (Troc) group, respectively. The molecular design employed an amino acid as the chiral unit, utilized the pyrene moiety to enable switching between monomer and excimer emission, incorporated an amide group as an intermolecular hydrogen-bonding site, and leveraged the substituent (Boc/Troc) to modulate the arrangement in the crystalline state. Crystalline (S)-5 exhibited blue-violet monomer emission (λ_em = 400 nm, Φ_F = 0.11), attributable to the spatial separation of its pyrene rings. After grinding, the collapse of the crystalline structure caused the pyrene rings of neighboring molecules to approach each other, giving rise to blue-green excimer emission (λ_em = 478 nm, Φ_F = 0.32). Powder X-ray diffraction (PXRD) analyses revealed that (S)-5 underwent a crystalline-to-amorphous transition after grinding. Crystalline (S)-6 exhibited blue emission, with emission maxima at 448 nm (Φ_F = 0.21). Upon grinding, (S)-6 transformed into amorphous states that emitted blue-green light, characterized by peaks at 474 nm (Φ_F = 0.38). The blue excimer emission observed in crystalline (S)-6 was attributed to hydrogen bonding between adjacent amide groups and the consequent stacking of pyrene rings. Accordingly, three emission types were distinguished for compounds 5 and 6 in the solid state: crystalline monomer emission ((S)-5), crystalline excimer emission ((S)-6), and amorphous excimer emission. The CPL spectra reflected the changes in emission wavelength between monomer and excimer emission for compound 5, and between two distinct excimers for compound 6. For (S)-5, the (+)-CPL spectra were recorded at maximum wavelengths of 410 nm (crystalline) and 460 nm (ground), with identical |g_lum| values of 1.1 × 10⁻³ for both states. For (S)-6, the crystalline state exhibited a (+)-CPL maximum at 440 nm, with |g_lum| values of 2.2 × 10⁻³, while the ground state showed a red-shifted peak at 460 nm, with |g_lum| values of 4.5 × 10⁻³. According to the excimer chirality rule,⁵⁴ the (+)-CPL observed in the crystalline state was attributed to the clockwise stacking of the pyrene groups. The (+)-CPL detected in the amorphous state further indicated the presence of a similar clockwise stacking arrangement. The analytical approach of using CPL to probe excited-state structures in the amorphous state was not limited to pyrene rings but could be extended to other luminescent aromatic systems. Moreover, the stabilization of pyrene stacking through intermolecular hydrogen bonding enabled the realization of amorphous-state CPL in SOMs. A distinctive feature of this strategy was that it directly linked the MCL and the variation in the g_lum value to the disruption and reconfiguration of molecular packing, rather than merely to changes in the emission color. These results established new guidelines for designing mechanochromic CPL systems and facilitated the advancement of practical solid-state CPL materials.

Fig. 2. (a) Chemical structures, and (b) CPL and fluorescence spectra of 5, 6. Reproduced with permission from ref. 53. Copyright 2025, John Wiley and Sons under a Creative Commons 4.0 License.

Considering the similar hybrid orbital configurations of phosphorus and carbon atoms, exploring the potential advantages of phosphorus central chirality for improving CP-OLED performance is highly desirable. In 2024, Zheng and co-workers incorporated a 5-phenylbenzo[b]phosphindole-5-oxide moiety at the para-positions of two multiple resonance thermally activated delayed fluorescence (MR-TADF) scaffolds, thereby constructing the first two pairs of phosphorus chiral MR-TADF enantiomers, 7 and 8 (Fig. 3).⁵⁵ In toluene, the absorption spectra of the two compounds were found to be similar to those of the previously reported MR-TADF molecules based on NBO⁵⁶ or NBN.⁵⁷7 and 8 exhibited nearly pure blue (462 nm) and blue-green (498 nm) emission, respectively, with narrow full width at half maximum (FWHM) values of only 25 nm and 24 nm. Their singlet–triplet energy gaps (ΔE_ST) were determined to be 0.21 eV and 0.13 eV, respectively. In the doped films, (R)-7 and (R)-8 showed emission peaks at 468 nm and 502 nm, with Φ_F of 87% and 93%, respectively. Additionally, both compounds exhibited nanosecond-scale prompt fluorescence lifetimes (τ_p = 6.7/4.9 ns) and delayed fluorescence lifetimes (τ_d = 200/41 µs) in doped films, confirming their TADF behavior. Furthermore, both pairs of (R/S)-7 and (R/S)-8 enantiomers exhibited mirror-image CD and CPL spectra in toluene, with |g_lum| values of 1.18 × 10⁻³ and 4.30 × 10⁻³, respectively. Theoretical calculations afforded calculated g values (|g_cal|) of 0.27 × 10⁻³ for (S)-7 and 1.28 × 10⁻³ for (S)-8, showing a trend that fully matched the experimental observations. The CP-OLED devices fabricated based on the two pairs of enantiomers exhibited strong circularly polarized electroluminescence (CPEL) signals, with |g_EL| values up to 1.4 × 10⁻³, and achieved maximum external quantum efficiencies (EQE_maxs) of nearly 30%. Both compounds exhibited g_lum and g_EL values on the order of 10⁻³, which were comparable to those of most chiral CPL-SOMs. This finding suggested that incorporating a phosphorus-containing chiral unit into CP-MR-TADF materials held promise for application in CP-OLEDs.

Fig. 3. Chemical structures of 7, 8.

Similar to chiral carbon compounds, chiral sulfur compounds are of great importance across diverse domains, notably in drug synthesis and biochemical reactions within living organisms. Despite this, the exploration of sulfur-centered chiral compounds within CP-TADF materials remains markedly underexplored. In 2025, Zheng and co-workers prepared six CP-MR-TADF molecules, 9–14, by covalently attaching N-substituted sulfoximine fragments bearing chiral sulfur center to an MR core (Fig. 4).⁵⁸ In toluene, the intense absorption bands below 350 nm for these molecules arose from n–π* and π–π* transitions, whereas the peaks at 475–481 nm originated from the short-range charge transfer (SRCT) transitions typical of MR-TADF system. These molecules exhibited blue-green to green fluorescence, characterized by maximum wavelengths in the range of 495–509 nm. The FWHMs were consistently less than 30 nm, and the ΔE_ST were determined to be between 0.03 and 0.11 eV. In doped films, nanosecond-scale prompt fluorescence (τ_p = 6.05–11.83 ns) and microsecond-scale delayed fluorescence (τ_d = 47.45–138.31 µs) were observed, confirming their TADF nature. Moreover, high Φ_F values of 86.5–92.8% were achieved in the doped films. All six pairs of enantiomers exhibited symmetric CD spectra in dichloromethane, with |g_abs| values ranging from 0.34 × 10⁻⁴ to 3.87 × 10⁻⁴. Unfortunately, the CPL signals of all enantiomers in toluene were very weak. In contrast, these six enantiomers displayed mirror-image CPL signals in doped films, with |g_lum| values ranging from 0.6 × 10⁻³ to 5.4 × 10⁻³. Only small changes in the key photophysical parameters were observed for the molecules reported in this work. This result revealed that the chiral sulfur-containing moiety caused a very limited perturbation to the electronic structure of the MR core. Consequently, the emission color and efficiency of this system could not be easily and effectively tuned through substituent engineering. Nonetheless, this work provides valuable insights for the development of sulfur-centered chiral CP-MR-TADF materials.

Fig. 4. Chemical structures of 9–14.

The presence of macroscopic anisotropy in solid state, which often caused substantial interference with measured electronic circular dichroism (ECD) and CPL spectra, made the study of chiroptical properties in thin films more complex than that in solution.⁵⁹ Based on the Mueller matrix model of Shindo et al.,⁶⁰ the experimental chiroptical signals of thin films in absorption (ECD) and emission (CPL) were represented as a sum of contributions, of which two were the most significant: the intrinsic component and the non-reciprocal component. In 2024, Di Bari and co-workers synthesized two chiral alkylthio-substituted molecules (15 and 16) based on a 1,4-phenylene/thiophene backbone, where the π-conjugated chromophore was connected to the enantiopure fragment through a sulfur bridge (Fig. 5).⁶¹ The photophysical and chiroptical properties of these compounds in solution and in thin films were then compared with those of the corresponding alkoxy-substituted analogues 17 and 18 reported earlier by the same research group. In chloroform, compound 15 showed a π–π* absorption band centered at 359 nm (ε = 3.98 × 10⁴ M⁻¹ cm⁻¹), an emission peak at 488 nm, and a Stokes shift of 129 nm. Owing to increased molecular planarity, compound 16 exhibited a remarkable red-shift in its absorption spectrum; its lowest-energy band was located at 427 nm, representing a 68 nm red-shift relative to 15. However, its ε value decreased by about one order of magnitude (ε = 0.56 × 10⁴ M⁻¹ cm⁻¹). Compound 16 emitted at 489 nm with a Stokes shift of 62 nm. In the bithiophene system, the absorption of 15 was significantly blue-shifted. The most intense band of 17 appeared at 411 nm, which was red-shifted by 52 nm relative to the corresponding band of 15. In the propanone system, 16 exhibited a slight red-shift: the lowest-energy band moved from 410 nm for the alkoxy-substituted dye 18 to 427 nm for the alkylthio-substituted 16, while its ε value decreased by approximately one order of magnitude. The most significant differences emerged in the thin film. For compound 15, freshly prepared thin films exhibited red-shifted absorption and emission relative to its chloroform solution; the low-energy absorption band appeared at 385 nm, and the emission maximum at 503 nm. Furthermore, the solid-state photoluminescence of 15 was greatly enhanced, whereas that of 17 was completely quenched by aggregation caused quenching (ACQ). However, freshly prepared thin films of 17 displayed strong non-reciprocal ECD signals: the front side gave a maximum g_abs value of −2.7 × 10⁻² at 436 nm, and the back side gave +2.8 × 10⁻². In contrast, no ECD signal was ever observed for 15, and neither solvent annealing nor thermal annealing induced any chiroptical signal. This was attributed to the large dihedral angle (about 50°) between the benzene and thiophene rings in 15, which rendered the core non-planar. This non-planarity simultaneously suppressed π-stacking interactions and led to intrinsic disorder by broadening the conformational space of each monomer. Freshly prepared thin films of 16 exhibited only weak chiroptical signals, with a maximum g_abs value below 10⁻³. The ECD spectrum remained unchanged upon sample rotation and flipping, indicating the presence of a purely intrinsic ECD signal. Interestingly, the sample showed a highly uniform texture in optical microscopy images taken with unpolarized light, and adding a linear polarizer did not produce any discernible alteration, which suggested that no domains possessing local linear anisotropy were present. This behavior closely resembled that observed for thin films of compound 18. One hour of thermal annealing at 60 °C induced strong non-reciprocal ECD signals in the thin films of 16. In the ECD spectra, the most intense band on the front side peaked at 302 nm (g_abs = −2.24 × 10⁻²), and that on the back side peaked at 298 nm (g_abs = 4.04 × 10⁻²). Smaller bands appeared in the 400–550 nm region, and the bands on the two sides were approximately mirror images of each other. Crucially, the front- and back-side ECD profiles were not perfectly inverted, revealing a non-negligible contribution from the intrinsic component, even though it remained clearly smaller than the non-reciprocal one. This behavior was in striking contrast to that of the corresponding alkoxy-substituted analogue 18, whose thermally annealed thin films displayed a nearly pure non-reciprocal ECD. Emission measurements also revealed the strong non-reciprocal chiroptical characteristics of 16. The CPL spectrum exhibited a prominent band centered at 545 nm, which appeared negative on the front side and positive on the back side. At the wavelength of maximum CPL intensity, the g_lum values were measured as −4.50 × 10⁻² and 2.51 × 10⁻² for the front and back sides, respectively. Despite these already sizeable values, they were still roughly one order of magnitude smaller than those obtained for thin films of 18 (−0.15 on the front, 0.09 on the back). Moreover, the strongest chiral ordering was observed at 60 °C; heating beyond that to the melting point (75 °C) led to a complete loss of the signal (dynamic disordered state). No signal was recovered upon slow cooling, but it could be restored by annealing again at 60 °C. This demonstrated that the two-dimensional chiral microdomains giving rise to the strong non-reciprocal chiroptical features at 60 °C were the thermodynamically most stable solid-state arrangement. This discovery offered a straightforward and efficient strategy to tune the chiroptical properties of π-conjugated materials solely by changing the bridging heteroatom.

Fig. 5. Chemical structures of 15–18.

In 2020, Di Bari and co-workers designed and synthesized four novel chiral oligothiophenes, 19–22, which were distinguished by (i) the use of an alkyne linker to connect the aromatic central ring to the oligothiophene units, and (ii) a carbazole (Cz) unit as the central core (Fig. 6).⁶² The alkyne group both red-shifted the absorption spectra and guaranteed that the π-conjugation between neighboring units remained independent of the dihedral angle. Meanwhile, the Cz moiety imparted outstanding optical properties in solid state, high carrier mobility, and favorable thermal/photochemical stability to the material. The four molecules bearing chiral alkyl chains, were intrinsically achiral in their isolated form. Nevertheless, upon thin film deposition, they aggregated into chiral supramolecular architectures, giving rise to pronounced chiroptical activity. Compound 19 showed a relatively modest CPL, while 20 gave a maximum g_lum value of 1.3 × 10⁻². For compound 22, the CPL spectrum featured a strong positive band in the 475–575 nm region (peak at 510 nm, g_lum = 1.7 × 10⁻²) accompanied by a shoulder near 550 nm, together with a broad weak positive band between 575 and 675 nm. Crucially, the CP-OLEDs were fabricated using neat π-conjugated oligomers. This strategy not only rendered device fabrication exceptionally straightforward but also maintained the chiral supramolecular architecture, a feature that proved crucial for achieving a strong CPEL signal. The multilayer CP-OLED device constructed from 22 exhibited a g_EL value of up to 5.4 × 10⁻². This study reported the first CP-OLED whose active layer was composed exclusively of a single SOM.

Fig. 6. Chemical structures of 19–22.

To obtain chiral 1,4-diketo-3,6-diarylpyrrolo[3,4-c]pyrrole (DPP)-based materials with enhanced emission and chiroptical properties in the solid state. In 2023, Avarvari and co-workers synthesized fluorescent molecule 23, which is based on a DPP core, bears tetraphenylethylene (TPE) units at both ends, and features chiral sec-phenethyl groups on the nitrogen atoms (Fig. 7).⁶³ Single-crystal structure analysis revealed that the two TPE units adopt complementary M and P propeller conformations; however, owing to intramolecular and intermolecular CH–π interactions, the two conformations are not perfect mirror images. In chloroform, compound 23 displayed its longest-wavelength absorption maximum at 486 nm, with a fluorescence emission peak at 564 nm accompanied by a shoulder at 606 nm, and a Φ_F of 25%. In the solid state as a film, the absorption maximum red-shifted to 495 nm, the emission peak shifted to 579 nm with a shoulder at 621 nm, and the Φ_F increased to 36%. This behaviour is characteristic of aggregation-enhanced emission (AEE): the fluorescence was not completely quenched in solution and became stronger upon aggregation. In THF/water mixtures, when the water fraction reached 70%, the fluorescence intensity increased 2-fold, and the Φ_F rose from 17% to 28%. A further increase in water content caused a slight decrease in both parameters, which was attributed to precipitation. Regarding the chiroptical properties, the enantiomers of compound 23 exhibited clear ECD signals. More importantly, compound 23 exhibited CPL signals in chloroform, with |g_lum| values of approximately 2 × 10⁻⁴, positive for the (S,S)-enantiomer and negative for the (R,R)-enantiomer. Its B_CPL value was estimated to be about 0.6 M⁻¹ cm⁻¹. Therefore, this study leveraged the steric hindrance of the TPE moieties and the location of the chiral center on the chiral N-substituent to realize enhanced solid-state luminescence together with CPL. Nevertheless, the resulting compound exhibited only low g_lum and B_CPL values.

Fig. 7. Chemical structures of 23.

3. Axial chirality

Axial chirality was recognized as an important class of chiral configuration, where chirality originated from the restricted rotation of two aromatic rings or π-systems around a single bond axis, leading to a specific spatial arrangement. Among the commonly used axially chiral CPL materials, the binaphthyl and biphenyl skeletons were the most widely employed.

White circularly polarized luminescence (WCPL) materials have garnered widespread attention due to their extensive applications in optoelectronic devices and related fields. Nevertheless, achieving single-molecule WCPL in the solid state, particularly in electroluminescent devices, remains a significant challenge. Consequently, reports on white circularly polarized electroluminescence (WCPEL) remain scarce to date. In 2024, Chen and co-workers introduced a pair of WCPL enantiomers, (R/S)-24, constructed via a single-molecule strategy that combines conformational isomerization donors with a chiral acceptor (Fig. 8).⁶⁴ Interestingly, two distinct crystalline forms of 24 were isolated by simply varying the dichloromethane/n-hexane volume ratio. Blue-fluorescent 24-B crystals were obtained at a ratio of 1/5, whereas yellow-emitting 24-Y crystals were isolated at a ratio of 1/10. In 24-B, both phenothiazine (PTZ) donor units adopted crook quasi-axis (ax) conformation, with dihedral angles between each donor and the chiral acceptor measured at 9°. Such small dihedral angles were unfavorable for the spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Conversely, in 24-Y, the two PTZ units presented ax and quasi-equator (eq) conformations, with dihedral angles of 5° and 92°, respectively. The larger dihedral angle of 92° facilitated spatial separation between the HOMO and LUMO. Furthermore, compared with 24-B, adjacent molecules in 24-Y were packed through a greater number of intermolecular interactions. According to theoretical calculations, 24-B exhibited a relatively large ΔE_ST of 0.62 eV, whereas 24-Y showed a much smaller ΔE_ST of 0.02 eV, which was sufficiently small to enable an efficient reverse intersystem crossing (RISC) process. Regarding their photophysical properties, 24-B displayed blue fluorescence emission peak at 417 nm with no detectable delayed lifetime. In contrast, 24-Y exhibited yellow emission band centered at 555 nm, accompanied by second-order exponential decay with a τ_d of 2.1 µs, confirming TADF characteristics. In acetone, white photoluminescence was realized with CIE coordinates of (0.30, 0.33), featuring dual emission bands at 445 nm and 531 nm. This dual emission originated from excited-state dynamic conformational change, specifically the transition of the ax–ax conformer into ax–eq and eq–eq structures. Moreover, white photoluminescence was also achieved in doped films, with CIE coordinates of (0.33, 0.35). These films exhibited distinct dual-emission bands, and the relative intensity of the emission near 560 nm was found to increase substantially with rising dopant concentration. This observation was attributed to stronger intermolecular interactions of 24, which increased the proportion of the ax–eq conformer. In toluene, the pair of enantiomers exhibited mirror-image CD signals. Their CPL spectra were measured in doped films, yielding |g_lum| values of 2.1 × 10⁻³ at 446 nm and 3.0 × 10⁻³ at 575 nm (Table 2). The corresponding B_CPL values were determined to be 0.61 × 10⁻³ M⁻¹ cm⁻¹ and 0.86 × 10⁻³ M⁻¹ cm⁻¹, respectively. Furthermore, OLED devices fabricated with (R/S)-24 as emitters achieved WCPEL with CIE coordinates of (0.32, 0.37) and an EQE_max of 4.7%. Further device structure optimization led to warm WCPEL devices featuring a |g_EL| value of 2.8 × 10⁻³, CIE coordinates of (0.37, 0.48), and a significantly higher EQE_max of up to 15.6%. This work demonstrated that the intramolecular conformational equilibrium acted as a switch that interconverted non-TADF and TADF, as well as single-color and white emission. Consequently, this finding offered an efficient strategy for the construction of single-molecule WCPL materials, and further opened up new perspectives for the realization of single-molecule CP-WOLEDs.

Fig. 8. Chemical structure of 24.

Table 2. Photophysical and chiroptical parameters of CPL-SOMs with axial chirality.

Molecule	λ em (nm)	Φ F	g lum (λmon (nm))	Conditions	Ref.
(R)-24	436 565	0.67	±3.0 × 10−3 (575)	Doped film	64
			±2.1 × 10−3 (446)
(S)-25	484	0.86a	−5.4 × 10−3 (−)	Toluene	66
(S)-26	467	0.91a	−2.5 × 10−3b (−)	Toluene	66
27	504 534	0.03	1.2 × 10−3c (−)	Dichloromethane	70
28	506 542	0.06	7.9 × 10−3c (−)	Dichloromethane	70
29	512 548	0.32	1.9 × 10−3c (−)	Dichloromethane	70
30	528 552	0.47	2.7 × 10−3c (−)	Dichloromethane	70
31	544	0.19	1.1 × 10−2c (−)	Dichloromethane	70
32	614 672	0.59	6.0 × 10−3c (−)	Dichloromethane	70
(R,R)-33	—	0.05	+6.7 × 10−2 (−)	Acetone	71
(R,R)-34	—	0.08	+7.7 × 10−2 (−)	Acetone	71
(R,R)-35	—	0.08	+5.5 × 10−2 (−)	Acetone	71
(S)-36	433	0.79	−0.6 × 10−3 (−)	Toluene	72

^aIn doped films.

^bIn hexane.

^cAbsolute g_lum value.

In most cases, increasing the contribution of chiral moieties to the frontier molecular orbital (FMO) distribution enhances the |m| value, thereby improving the dissymmetry factor (g) of the corresponding chiral materials. In contrast, the role of the angle θ has received comparatively little attention. Considering that |µ| is generally significantly larger than |m| for most organic compounds, θ plays a critical role in improving g values. Notably, previous reports have indicated that molecules with θ approaching 180° typically possess a smaller dihedral angle (α) of approximately 60°.⁶⁵ This observation implies a possible link between θ and α in chiral molecules; however, a systematic investigation of this relationship has yet to be conducted. Following this concept, in 2025, Zheng and co-workers constructed two pairs of axially chiral molecules, (R/S)-25 and (R/S)-26, employing 5H,5′H-6,6′-bibenzo[b]carbazole and 7,7′,8,8′,9,9′,10,10′-octahydro-5H,5′H-6,6′-bibenzo[b] carbazole as the chiral units (Fig. 9).⁶⁶ Single-crystal structure analysis confirmed that (R)-25 and (S)-25 possessed closely matched α of 60.7° and 60.0°, respectively. This structural feature was expected to drive the θ values close to 0° (or 180°), thereby improving the g value. Unfortunately, no single crystals could be obtained for (R/S)-26. Theoretical calculations demonstrated that the direct attachment of boron atoms to the chiral moieties enabled the full participation of the entire chiral units in the FMO distributions. Specifically, the chiral units contributed 35.6%/29.9% and 45.2%/39.0% of the distributions of the HOMO and LUMO, respectively. In toluene, the emission peaks of (S)-25 and (S)-26 were located at 484 nm and 467 nm, with FWHMs of 30 nm and 27 nm, and ΔE_ST values of 0.33 eV and 0.10 eV, respectively. In doped films, the two molecules displayed sky-blue emission with peaks at 489 nm and 481 nm, and absolute Φ_F values of 86% and 91%, respectively. Notably, (S)-26 showed a τ_p of 6.3 ns and a τ_d of 40.7 µs, alongside a delayed proportion (η_DF) of 19%, indicating the presence of TADF characteristics. According to theoretical calculations, a smaller α was predicted to yield a larger g value. However, extremely small α values were found to destabilize the molecule due to steric hindrance. The optimum α of approximately 60° was generally realized by balancing intramolecular steric hindrance and conjugation. For (S)-25, the theoretical calculations yielded a θ value of 17.7° and a g_cal of 1.1 × 10⁻². In toluene, the molecule showed excellent CPL performance with a |g_lum| value of 5.8 × 10⁻³, marking one of the highest g values ever recorded for MR materials. For (R/S)-26, which combined MR-TADF and CPL properties, a |g_lum| value of 2.5 × 10⁻³ was observed. The CP-OLED devices based on (R/S)-25 and (R/S)-26 displayed sky-blue emission and achieved high EQE_maxs of 24.7% and 33.3%, respectively. Moreover, these devices exhibited good CPEL performance, with |g_EL| values of 3.4 × 10⁻³ and 1.2 × 10⁻³, respectively. This work demonstrated that modulating the dihedral angle (α ≈ 60°) of bibenzo[b]carbazole rendered the θ angle close to 0°, thereby enabling a high g_lum value in an MR system. This finding thus provided an effective strategy for the future design of high-efficiency CP-MR-TADF materials.

Fig. 9. Chemical structures of 25, 26.

The construction of axially chiral systems with CPL has predominantly relied on biaryl scaffolds as chiral sources. However, the heavy dependence on these conventional frameworks has limited both structural diversity and functional expandability of such materials. Although recent studies have reported axially chiral systems based on azoles, thiophene, and related analogues,^67–69 they generally exhibit low g_lum values and modest racemization barriers, reflecting the challenge of achieving configurational stability in axially chiral architectures derived from five-membered heteroaromatic rings. Thiophene derivatives are widely employed as building blocks in materials chemistry, but axially chiral systems incorporating thiophene-based skeletons have remained scarce. In 2026, Xu and co-workers designed and synthesized a series of novel axially chiral compounds, 27–32, based on the 3,3′-bidithieno[2,3-b:3′,2′-d]thiophene scaffold (Fig. 10).⁷⁰ All 27–32 molecules displayed two absorption bands located in the regions of 220–300 nm and 310–400 nm. In degassed dilute dichloromethane solution, each compound 27–32 emitted blue fluorescence. Specifically, 27, 28, 29, 30, 31, and 32 showed emission maxima at 400, 403, 408, 445, 358, and 446 nm, with corresponding Φ_F of 0.03%, 0.06%, 0.32%, 0.47%, 0.19%, and 0.59%, respectively. In the solid state, as the size of the aryl groups gradually increased, the Φ_F values of 27–32 were enhanced due to the strengthening of intermolecular interactions. Due to the heavy-atom effect of sulfur atoms, all samples showed an afterglow phenomenon that persisted for several seconds at 77 K, providing clear evidence of phosphorescence emission. The enantiomers of 27–32 were obtained by chiral HPLC. Subsequent racemization studies revealed that as the size of the substituents increased, the racemization barriers of 27–32 gradually rose. Notably, 31 displayed the highest racemization barrier, reaching 29.73 kcal mol⁻¹, which surpasses that of BINOL (23.9 kcal mol⁻¹). In addition, these molecules exhibited mirror-image CD and CPL signals. Different substitution positions of the same substituent led to variations in the g_lum values of the enantiomers, with g₂₈ > g₂₉ and g₃₁ > g₃₀. This behavior was rationalized by the steric-effect-induced variations in the dihedral angles of 27–32 and m and µ arising from different substitution positions. Among all the compounds examined, 31, which possessed the largest steric hindrance, displayed the highest g_lum value, reaching 1.1 × 10⁻². This work demonstrated that a non-benzenoid, sulfur-rich fused heterocyclic system could serve as an axially chiral scaffold with a high g_lum value, thereby expanding the design space for axially chiral materials and providing a new platform for chiroptical research.

Fig. 10. Chemical structures of 27–32.

Developing novel SOMs that achieve a |g_lum| value exceeding 0.1 remains a significant challenge. Furthermore, endowing CPL materials with stimuli-responsive properties has become a crucial direction for expanding their application potential. In 2026, Ema and co-workers constructed a class of multibridged azapyrenophanes, (R,R)-33, (R,R)-34, and (R,R)-35, which exhibited highly functional CPL characteristics (Fig. 11).⁷¹ For (R,R)-33, a longer-wavelength absorption maximum at 330 nm, arising from the pyrene units, was observed across various solvents. Correspondingly, (+)-CD signals in the longest-wavelength region were recorded, which corresponded to a right-handed twisted pyrene conformation. The binaphthyl sandwich architecture resulted in exclusive observation of pyrene excimer emission across most solvents. All solutions of (R,R)-33 exhibited (+)-CPL. According to the excimer chirality rule, this sign indicated the presence of a right-handedly twisted excimer. Acetone gave the most intense CPL signal, with a g_lum value of +0.067. Notably, cooling further boosted the CPL performance, yielding a g_lum of +0.10 at −95 °C. Concurrently, the Φ_F was markedly enhanced upon cooling, rising from 0.05 at 20 °C to 0.43 at −95 °C. (R,R)-34 exhibited stronger CPL, with a g_lum value of +0.077 at 20 °C. (R,R)-35 displayed weaker CPL, which strongly indicated that the xylylene bridge was essential for the CPL enhancement. Cooling proved equally beneficial for (R,R)-34, in acetone at −95 °C, the g_lum and Φ_F values reached +0.12 and 0.78, respectively. On this basis, its B_CPL attained an exceptionally high value of 2600 M⁻¹ cm⁻¹. In addition, the CPL signal of (R,R)-34 disappeared along with a decrease in fluorescence intensity upon addition of trifluoroacetic acid (TFA). Subsequent addition of triethylamine (Et₃N) restored the signal, achieving effective switching of the CPL signal. An exceptionally large Δg_lum of 0.08 was achieved, and this switching was repeatable. The same behavior was also observed for (R,R)-33. This phenomenon was attributed to CPL quenching caused by protonation of the nitrogen atoms. Solvent-dependent studies further revealed that (R,R)-33 in (S)- and (R)-limonene produced excimer (−)-CPL at 20 °C, with g_lum values of −0.012 and −0.022, respectively. This behavior was ascribed to specific solvation at 20 °C, which induced a left-handed excimer in the excited state. The weak (+)-CPL signal at 400 nm, arising from monomer emission, was also detected. Upon heating to 90 °C, the (S)- and (R)-limonene solutions displayed sign inversion and turn-off, respectively, and this behavior was reversible. This thermal response was explained by the weakening of the specific solvation at elevated temperature, which increased the proportion of the right-handed excimer contributing for (+)-CPL. In contrast to (R,R)-33, the limonene solution of (R,R)-34 maintained (−)-CPL at 90 °C, which was most likely attributable to its sufficient rigidity. This work revealed three independent CPL modulation mechanisms based on temperature, acid/base, and chiral solvents. Moreover, it surpassed the typical g_lum value upper limit of 10⁻² for conventional SOMs by reaching values on the order of 10⁻¹. Consequently, this finding opened up a new route toward high-performance CPL emitters.

Fig. 11. Chemical structures of 33–35.

The stringent demand for both a wide bandgap and strong chiroptical activity makes deep-blue CP-OLEDs exceedingly rare. In 2026, Li and co-workers reported a C₂-symmetric axially chiral emitter, 36, whose molecular design integrated the dimeric carbazole donor unit with the triphenyltriazine acceptor unit to construct a donor–acceptor (D–A) bicarbazole framework (Fig. 12).⁷² Single-crystal structure analysis revealed that significant steric hindrance between the ^tBu groups and the triphenyltriazine units effectively constrained free rotation around the biaryl axis, generating a rigid axially chiral conformation. In toluene, 36 exhibited deep-blue emission with a peak at 433 nm, an FWHM of 54 nm, and a high Φ_F of 79%. In the doped film, the emission peak slightly red-shifted to 442 nm, an FWHM of 54 nm, while the Φ_F remained high at 70%. In the crystal state, the emission peaked at 445 nm, an FWHM of 43 nm, but the Φ_F dropped markedly to 15%. Such behavior was ascribed to the densely packed molecular arrangement in the crystal, which hindered intramolecular vibrations while also promoting intermolecular π–π stacking interactions. The enantiomers of 36 were separated by HPLC with ee greater than 99%. The (R)- and (S)-isomers displayed clear mirror-image CD spectra in the 280–420 nm region, confirming a well-defined axially chiral molecular framework. In toluene, both enantiomers exhibited mirror-symmetric CPL signals with |g_lum| value of 0.6 × 10⁻³. The B_CPL value was 98 M⁻¹ cm⁻¹. In the doped film, the |g_lum| values significantly increased to 2.8 × 10⁻³, representing an approximately 5-fold enhancement. This enhancement was attributed to the rigid solid-state environment, which restricted the conformational flexibility, while ordered local aggregation potentially induced intermolecular chiral coupling. Ultimately, CP-OLEDs based on (R)- or (S)-36 achieved deep-blue electroluminescence characterized by CIE coordinates of (0.14, 0.07), with an EQE_max of 4.58% and |g_EL| values of 2.7 × 10⁻², which are the highest reported for blue CP-OLEDs employing SOMs. Consequently, this work establishes a valuable molecular design strategy for developing high-performance deep-blue CPL materials.

Fig. 12. Chemical structures of 36.

4. Helical chirality

Helicenes, characterized by the helical arrangement of ortho-fused aromatic rings, serve as the primary scaffold for constructing helical chiral CPL materials.^73–75 However, early-stage research on these materials was relatively limited due to challenges such as difficult synthesis, high resolution costs, and low luminescence efficiency. In recent years, significant breakthroughs in enhancing the performance of helicene-based CPL materials have been achieved through strategies including heteroatom doping^76,77 and multiple resonance^78,79 approaches.

In recent years, several 1,4-azaborine-embedded helicenes featuring excellent luminescent properties have been reported. While most of these compounds exhibit high Φ_F and narrowband fluorescence, their CPL remains comparatively broad. In 2023, Ravat and co-workers reported a helicene-based CPL-active molecule, 37, which exhibited remarkably narrow CPL characteristics, with FWHM of 24 nm in toluene and 21 nm in 2-methylpentane (Fig. 13).⁸⁰ Quantum chemical calculations revealed that the narrowband emission of 37 originated from the relative localization of natural transition orbitals, combined with small geometric differences between the ground (S₀) and the first excited (S₁) states. Notably, unlike 37, previously reported molecules incorporated the 1,4-azaborine ring directly within the helical framework.^79,81,82 Accordingly, in 2024, Ravat and co-workers modulated both the number and the positional distribution of B and N atoms. Through this approach, they first rigidified and extended the central framework of compound 37, and constructed the helical graphene derivatives 38, 39, and 40 (Fig. 13).⁸³ In toluene, the S₁ ← S₀ transition gave rise to the lowest-energy absorption bands at 501 nm for 38 and at 479 nm for 40. These bands were blue-shifted relative to that of 37 (505 nm). The shift was rationalized as a consequence of the attenuated electron-withdrawing ability of the boron atoms, arising from para B–π–N conjugation, which resulted in a widened HOMO–LUMO gap. The emissions of 38 and 40 also exhibited blue-shifted bands at 508.5 nm and 484.5 nm, respectively, relative to that of 37 (520 nm). As expected, upon rigidification of the molecular structure, the FWHM of the emissions of 38 and 40 significantly decreased to 17.5 nm and 17.0 nm, respectively, compared to that of 37 (28 nm). This reduction is attributed to diminished structural reorganization during the emission process. In 3-methylpentane, 38 displayed an even narrower emission profile, with an FWHM of 10.5 nm. And in deoxygenated toluene, 38 exhibited a Φ_F of 81%, comparable to that of 37 (85%), while 40 gave a lower Φ_F of 59%. Relative to 38, 39 displayed a similar Φ_F (85%) and an FWHM of 16 nm. Density functional theory calculations revealed that, in 38, 39, and 40, the HOMO and LUMO were distributed over alternating atoms. Notably, the extended π-framework in 38 and 39 led to delocalization of the FMOs over a larger structure, whereas in 40, the FMOs remained largely localized near the helicene moiety. This localization accounted for the reduced Φ_F and the larger blue shifts in absorption and emission observed for 40. With respect to chiroptical properties, the CPL spectra of 38, 39, and 40 in toluene showed FWHM values (18–19 nm) that closely matched those of their fluorescence emission and were markedly narrower than the 24 nm FWHM of 37. More importantly, 38 showed a CPL band with an FWHM of 13 nm in 3-methylpentane, this level of narrowband CPL had previously been observed almost exclusively in chiral lanthanide complexes.^84,85 Relative to the dissymmetry factors of 37 (g_abs = 1.3 × 10⁻³, g_lum = 1.1 × 10⁻³), both 38 and 40 yielded slightly higher values: 2.1 × 10⁻³ (g_abs) and 1.7 × 10⁻³ (g_lum) for 38, and 4.1 × 10⁻³ and 2.3 × 10⁻³ for 40 (Table 3). In contrast, 39 showed a decrease in both values (1.0 × 10⁻³ and 0.75 × 10⁻³). In the case of the S₁ ← S₀ transition, 39 exhibited increases in both µ and m, but its cosθ value dropped markedly. This drop led to a lower g_abs value for 39 relative to 38 and 40. For 40, the enhancement in g_lum value arose from an increased m and a decreased µ during the S₀ ← S₁ transition. In addition, the computed vibrationally resolved CPL spectra indicated that rigidification of the conjugated scaffold, together with control over vibrational effects, constituted a viable route to narrowband emission. It was reasonably inferred that embedding a helicene unit within the backbone served as an effective design strategy for controlling vibrational factors. This inference was grounded in the observation that the vibrational transitions were predominantly associated with vibrational modes localized on the helicene moiety, whereas the azaborine moieties remained free from distortion during the transition, thus suppressing band broadening caused by geometric alterations. This work provided valuable guidance for future molecular design toward narrowband CPL-SOMs.

Fig. 13. Chemical structures of 37–40.

Table 3. Photophysical and chiroptical parameters of CPL-SOMs with helical chirality.

Molecule	λ em (nm)	Φ F	g lum (λmon (nm))	Conditions	Ref.
(P)-37	520	0.85	−1.1 × 10−3 (−)	Toluene	80
(P)-38	509	0.81	−1.7 × 10−3 (−)	Toluene	83
(P)-39	500	0.85	−0.75 × 10−3 (−)	Toluene	83
(P)-40	485	0.59	−2.3 × 10−3 (−)	Toluene	83
41	470	0.20	2.9 × 10−3a (−)	Dichloromethane	89
42	480	0.39	4.2 × 10−3a (−)	Dichloromethane	89
43	530	0.24	5.7 × 10−3a (−)	Dichloromethane	89
44	528	0.90	−3.3 × 10−3 (−)	Toluene	92
45	610	0.018	1.0 × 10−2a (613)	Chloroform	94
46	560	0.058	5.0 × 10−3a (563)	Chloroform	94
(P,P)-47	480	0.09	−2.3 × 10−2 (480)	Toluene	96
(P,P)-49	656	0.02	+2.7 × 10−3 (656)	Toluene	96

^aAbsolute g_lum value.

Pyrene-based chiral molecules and their complexes were established as a promising class of CPL-active materials.^86–88 Intramolecular or intermolecular stacking interactions were found to frequently induce the formation of asymmetrically twisted excimers, and such excimers were responsible for the generation of intense CPL. In 2025, Ema and co-workers constructed a series of azahelicenes, 41, 42, and 43, by introducing electron-accepting groups into phenanthrenyl- or pyrenyl-substituted carbazoles (Fig. 14).⁸⁹ Single-crystal structure analysis revealed that the two phenanthrene units in 42 and the two pyrene units in 43 stacked with plane-to-plane distances of 3.61 Å and 3.34 Å, respectively. The enantiomers of all compounds were successfully obtained via chiral HPLC, and each pair exhibited mirror-image CD and CPL signals. Both the g_abs and g_lum values followed the order 41 < 42 < 43: the g_abs values were 3.3 × 10⁻³, 5.1 × 10⁻³, and 9.1 × 10⁻³, respectively, and the corresponding g_lum values were 2.9 × 10⁻³, 4.2 × 10⁻³, and 5.7 × 10⁻³. 42 and 43 exhibited high B_CPL values of 113 M⁻¹ cm⁻¹ and 65 M⁻¹ cm⁻¹, respectively. Upon addition of tetrabutylammonium fluoride (TBAF), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), or 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) to 41–43, a distinct color change was observed. All compounds formed tight complexes via their NH sites with fluoride ions from TBAF, with binding constants (K_a) in the range of 2.6 × 10⁵ to 3.2 × 10⁵ M⁻¹, which were comparable to those of calix[4]pyrrole derivatives containing four or more NH groups.⁹⁰ After fluoride complexation, the CPL spectra were dramatically red-shifted by more than 100 nm into the red region, with the |g_lum| values still maintained at the 10⁻³ order of magnitude. Moreover, the original spectra were recovered upon the addition of acid to these complexes, demonstrating that the neutral and anionic states were interconvertible. This work integrated anion recognition with the photophysical properties of chiral helicenes, affording reversible and color-switchable CPL properties.

Fig. 14. Chemical structures of 41–43.

The rapidly advancing field of multi-helicene chemistry has prompted extensive investigations into their unique structure–property relationships.^75,91 In this context, the introduction of heteroatoms not only offers new design strategies but also imparts tunable optical properties, thereby paving the way for the development of chiral optoelectronic devices. In 2026, Duan and co-workers designed and synthesized a novel double aza[7]helicene, 44, based on the nitrogen-containing indolocarbazole (ICz) framework (Fig. 15).⁹² This structure incorporated four consecutively para-positioned nitrogen atoms, achieved through the alternating fusion of five- and six-membered rings. X-ray crystallographic analysis identified only the (P,P)- and (M,M)-enantiomeric configurations within the unit cell, with no evidence of the (P,M)-configuration, confirming that 44 existed as a racemate under ambient conditions rather than as a meso compound. In toluene, the molecule exhibited two characteristic absorption peaks at approximately 451 nm and 518 nm, assigned to the nth singlet state (S_n) and S₁ states, respectively. The molecular design strategy enabled pure-green emission with an emission maximum at 528 nm, an FWHM of only 18 nm, and a Φ_F value of 90%. Importantly, the enantiopure (P,P)- and (M,M)-44 were efficiently separated using conventional sodium carboxymethyl cellulose thin-layer chromatography (TLC), avoiding the need for costly chiral chromatography columns. In toluene, (M,M)- and (P,P)-44 displayed mirror-image CD and CPL spectra, with |g_abs| value of 5.0 × 10⁻³ and a |g_lum| value of 3.3 × 10⁻³. Benefiting from its narrowband emission and exceptionally high Φ_F, the corresponding OLED devices achieved pure-green electroluminescence with an EQE_max of 30.2%, which remained above 20% even at an extremely high luminance of 10⁵ cd m⁻². This work provided a novel and cost-effective approach for resolving multi-helicenes, thereby not only accelerating the development iteration cycle of chiral helicene materials but also facilitating the exploration of their practical applications.

Fig. 15. Chemical structure of 44.

The synthesis of helicenes has long been challenged by tedious routes, low yields, and difficulties in precise stereochemical control. A major breakthrough in this field came in 2015, when Murase and Fujita reported the synthesis of [16]helicene, which set a record as the longest single-stranded carbohelicene at that time.⁹³ In 2026, Jin and co-workers achieved the concise and high-yielding synthesis of two D–A [17]helicenes (45 and 46) in just five steps, with an overall yield of up to 55% (Fig. 16).⁹⁴ The key cyclization step employed carefully designed precursors characterized by bond-order control and strategic substitutions, which enabled the efficient and precise construction of these elongated helical frameworks. In carbo[n]helicenes, the intersystem crossing (ISC) rate increases with the number of ortho-fused benzene rings, which results in low Φ_F (Φ_F < 1%) for long [n]helicenes (n > 9).⁹⁵ In chloroform, D–A [17]helicenes 45 and 46 exhibited Φ_F of 1.8% and 5.8%, respectively, which were higher than that recorded for unsubstituted [14]helicene (0.65%). This enhancement originated from their significantly larger fluorescence emission rate constants (k_f). Specifically, k_f values for 45 and 46 were determined to be 2.4 × 10⁶ s⁻¹ and 11.2 × 10⁶ s⁻¹, respectively, which were one to two orders higher than that of [14]helicene (0.75 × 10⁶ s⁻¹). Theoretical calculations revealed that the oscillator strength (f) of compound 46 was approximately 30 times greater than that of unsubstituted [17]helicene, leading to increased k_f and consequently enhanced Φ_F. Chiral resolution of compounds 45 and 46 was achieved by chiral HPLC. In chloroform, both enantiomers showed mirror-image CD signals, with |g_abs| values of 0.028 for 45 and 0.031 for 46, which are in the top tier of the helicene frameworks. In addition, the enantiomers of both compounds exhibited CPL signals in the wavelength range of 500–750 nm, with |g_lum| values of 1 × 10⁻² for 45 and 5 × 10⁻³ for 46. For SOMs in solution, the |g_lum| value of compound 45 is relatively large. This work proved that the D–A strategy could not only partially suppress the luminescence quenching of long carbo[n]helicenes arising from fast ISC, but also enhance the fluorescence efficiency by increased oscillator strength. Consequently, this strategy paved the way for the design of long-conjugated chiral helicenes exhibiting CPL properties.

Fig. 16. Chemical structures of 45, 46.

In 2024, Zhang and co-workers constructed three quasi-oval-shaped macrocyclic compounds, 47, 48, and 49, via the introduction of p-xylenyl, terephthaloyl, and isophthaloyl bridging moieties, respectively, into the cove regions of the double hetero[4]helicene core SPZ (Fig. 17).⁹⁶ The cove-regions bridging strategy successfully granted persistent chirality to the multi-helicenes and allowed the photophysical properties to be modulated. The quasi-oval-shaped macrocyclic structures featured a narrow three-dimensional cavity. This blocked the racemization of the enantiomers, namely the flipping of the bridging group over the phenazine (PZ) unit, in other words, any configurational inversion would inevitably require bond breaking. Thus, persistent chirality was realized. This strategy broke through the limitation of conventional helicenes, where configurational stability was typically improved by increasing the number of fused rings or by increasing the steric hindrance between the head and tail moieties. Consequently, it fundamentally removed the risk of racemization. The bridging groups also exerted a pronounced effect on the electronic structure and luminescence behavior. In toluene, the absorption edges of these compounds showed a red-shift trend: 47 at 436 nm, 49 at 495 nm, and 48 at 612 nm. The originally non-emissive SPZ became luminescent after bridging; 47 displayed sky-blue emission in toluene (peak at 480 nm, Φ_F = 0.09), while 49 exhibited deep-red emission (peak at 656 nm, Φ_F = 0.02). The generally low Φ_F values were ascribed to the small S₁ → S₀ oscillator strengths (<0.006). While 48 was practically non-emissive. Both 47 and 49 showed prompt and delayed emission in oxygen-free toluene, and the proportion of the delayed component was found to increase at higher temperatures, which suggested TADF behavior. It was noteworthy that 47 possessed TADF properties although it lacked a typical D–A architecture. This was due to the increased rigidity from the bridging, the lowered ΔE_ST, and concentrated triplet states lying below the S₁ state. All compounds yielded their enantiomers with ee values above 99% via chiral HPLC. Moreover, under nitrogen, the 49 enantiomers retained ee values > 99% after annealing at 300 °C for 5 min, confirming their high configurational stability. The enantiomers of 47 and 49 displayed sky-blue and red CPL, respectively, and the CPL spectra were also mirror-images of each other. For (P,P)- and (M,M)-49, the g_lum values were determined to be about +2.7 × 10⁻³ and −1.7 × 10⁻³ at 656 nm, while for (P,P)- and (M,M)-47, the corresponding values were −2.3 × 10⁻² and +2.5 × 10⁻² at 480 nm, which are significantly higher. This was the first time that the combination of red CPL and TADF had been achieved in a [4]helicene system. Thus, the cove-regions bridging strategy not only provided a general solution for achieving persistent chirality in multi-helicenes but also enabled effective modulation of their electronic structures and luminescence properties.

Fig. 17. Chemical structures of 47–49.

5. Planar chirality

Planar chiral SOMs are typically macrocyclic compounds with rigid molecular structures, with the [2.2]paracyclophane skeleton representing the most frequently encountered scaffold.^97–99 Substituents on the macrocyclic skeleton break the molecular symmetry plane and inversion center, thereby endowing the molecule with planar chirality. Benefiting from this inherent chiral configuration as well as their cyclic cavity structure, this class of planar chiral SOMs exhibits broad application prospects in fields such as chiroptical materials and asymmetric catalysis.

The design of CP-TADF molecules can be categorized into two main approaches.^100,101 One is the intrinsic chirality approach, which involves designing molecules with an inherently chiral TADF skeleton. The other is the chiral perturbation strategy, wherein chiral units are attached to an achiral TADF moiety. In 2024, Zysman-Colman and coworkers covalently attached MR-TADF units to two distinct chiral groups, synthesizing two pairs of enantiomers, (R/S)-50 and (R/S)-51 (Fig. 18).¹⁰² In toluene, the emission maxima of 50 and 51 were located at 469 nm and 505 nm, with FWHMs of 37 nm and 66 nm, respectively. The fluorescence spectrum of 50 displayed a weak solvatochromism, which was attributed to its SRCT excited-state character. In contrast, the fluorescence spectrum of 51 exhibited a much stronger solvatochromism, indicating that its excited state possessed significant long-range charge transfer (LRCT) character. In tetrahydrofuran and ethyl acetate, 51 exhibited dual emission arising from the SRCT and LRCT states, respectively. In nitrogen atmosphere, 50 and 51 exhibited high Φ_F of 93% and 99%, respectively, in doped films. Under ambient conditions, these values decreased to 66% and 73%, respectively. The emission maxima of the two compounds in doped films were located at 477 nm for 50 and 501 nm for 51, with FWHMs of 44 nm and 66 nm, respectively. The ΔE_ST were determined to be 0.19 eV for 50 and 0.17 eV for 51. Under vacuum, τ_p of 6.4 ns and 11.7 ns, and τ_d of 179 µs and 140 µs were measured for 50 and 51, respectively. Moreover, the delayed emission increased with rising temperature, confirming TADF characteristics. In toluene, both compounds exhibited chiroptical properties in S₀ state, whereas only 51 showed chiroptical activity in the excited state, with |g_lum| value of 4 × 10⁻⁴ (Table 4). The OLEDs fabricated with 50 and 51 exhibited electroluminescence maxima at 489 nm and 518 nm, EQE_maxs of 25.7% and 29.2%, and FWHMs of 53 nm and 69 nm, respectively. Although only compound 51 exhibited CPL activity, with a g_lum value on the order of 10⁻⁴, this work demonstrated that the electronic nature of the chiral groups impacted both the photophysical properties of the emitter and the resulting device performance.

Fig. 18. Chemical structures of 50, 51.

Table 4. Photophysical and chiroptical parameters of CPL-SOMs with planar chirality.

Molecule	λ em (nm)	Φ F	g lum (λmon (nm))	Conditions	Ref.
50	469	0.36	—	Toluene	102
51	505	0.37	4.0 × 10−2a (507)	Toluene	102
(Rp,Rp)-52	422	0.66	−4.7 × 10−4 (413)	Dichloromethane	104
(Rp,Rp)-53	430	0.67	−3.2 × 10−4 (416)	Dichloromethane	104
(Sp)-54	461	0.93	−1.7 × 10−2 (−)	Cyclohexane	105
(Sp)-55	487	0.43	−3.4 × 10−3 (−)	Cyclohexane	106
(Sp,Sp)-56	489	0.46	−2.7 × 10−3 (−)	Cyclohexane	106
(Sp,Sp,Sp)-57	491	0.49	−2.3 × 10−3 (−)	Cyclohexane	106
58	526	0.94	1.2 × 10−3a (526)	Toluene	111
59	480	0.039	2.5 × 10−2a (−)	Dichloromethane	113
60	480	0.017	2.3 × 10−2a (−)	Dichloromethane	113
61	480	0.074	3.4 × 10−2a (−)	Dichloromethane	113
62	500	0.341	2.8 × 10−2a (−)	Dichloromethane	113

^aAbsolute g_lum value.

Researchers have found that π-conjugated units can be utilized as efficient chiral dopants for non-chiral luminescent polymers via π⋯π interactions. In recent years, the g_lum value of π-conjugated polymers can be enhanced by a chiral induction method that involves doping them with helicene derivatives.^59,103 In 2024, Hasegawa and co-workers introduced four phenylcarbazole units into [2.2]paracyclophane, designing and synthesizing two radially extended chiral π-conjugated carbazole derivatives, 52 and 53, with [2.2]paracyclophane as the core (Fig. 19).¹⁰⁴ Their chiroptical properties, as well as their capacity to induce chirality in an achiral π-conjugated polymer, were subsequently investigated. In dichloromethane, the maximum emission wavelengths of 52 and 53 were 422 nm and 430 nm, respectively, again with a modest red shift for 53. The absolute Φ_F values of 52 and 53 in dichloromethane were measured to be 66% and 67%, respectively, which originated from the intrinsic emission behavior of the carbazole chromophore. In dichloromethane, the chiroptical properties of the compounds showed that the CPL sign was consistent with the CD sign: the R_p-enantiomer gave a negative signal, whereas the S_p-enantiomer gave a positive signal. The corresponding |g_lum| values were 4.7 × 10⁻⁴ for (R_p,R_p)-52 and 3.2 × 10⁻⁴ for (R_p,R_p)-53. Theoretical calculations on the S₁ state revealed that 52 possessed a large µ but a small m, which accounted for its high Φ_F and small |g_lum|. To explore the chiral induction capability and chiroptical behavior of the two compounds in the solid state, 52 and 53 were individually blended with the polymer F8BT (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)]) in toluene to form thin films, 52-F8BT and 53-F8BT, respectively. F8BT possessed a low-lying LUMO originating from the benzothiadiazole unit, which was capable of interacting with the carbazole units of 52 and 53. All these films exhibited both CD and CPL signals. The CPL spectrum of (R_p,R_p)-52-F8BT was opposite to that of (R_p,R_p)-52 in dichloromethane, suggesting that the chiroptical properties originated from the induced chirality of F8BT rather than 52 itself. The 53-F8BT film showed a similar inversion. At a doping ratio of 3 wt% of 52, the 52-F8BT film exhibited the highest |g_lum| value, reaching 0.01, which was substantially larger than that of 52 in solution, and the emission was assigned to the locally excited (LE) state of F8BT. However, the 53-F8BT film containing 3 wt% 53 yielded a much lower |g_lum| value of 2.6 × 10⁻³. This was attributed to the fact that the tert-butyl groups in 53 hindered the interaction between the carbazole moieties and the π-backbone of F8BT, which limited the ability of 53 to act as an effective dopant. This work employed a doping strategy to transform chiral SOMs possessing a low g_lum value (10⁻⁴) into polymer films achieving a g_lum value as high as 0.01, thereby providing a successful paradigm for g_lum amplification.

Fig. 19. (a) Chemical structures of 52, 53, (b) CPL spectra in dichloromethane of 52 and 53, and CPL spectra of 52-F8BT film. Reproduced with permission from ref. 104. Copyright 2024, John Wiley and Sons.

By introducing the electron-donating dimethylamino (NMe₂) group onto the [2.2]paracyclophane scaffold, an electron-donating planar chiral dimethylamino[2.2] paracyclophane was constructed. Subsequent incorporation of an electron-withdrawing group enabled the design of D–π* (chiral π)-A type CPL-SOMs with unique properties. In 2021, Zhao and co-workers prepared a CPL-SOM, 54, which exhibited CPL signal inversion (Fig. 20).¹⁰⁵ This molecule demonstrated excellent CPL performance, with a Φ_F of 0.93 and a |g_lum| of 1.73 × 10⁻² in cyclohexane. More importantly, the compound exhibited solvent-induced CPL signal inversion due to the solvent-dependent excited-state dynamics. Specifically, (R_p)-54 displayed a (+)-CPL signal in cyclohexane, a (−)-CPL signal in tetrahydrofuran, and bisignate CPL signals in chloroform. Upon protonation with TFA, the (−)-CPL signal in tetrahydrofuran underwent sign inversion to a (+)-CPL signal, accompanied by a pronounced blue shift that brought it into close agreement with the signal observed in cyclohexane (Δλ = 5 nm). Combined experimental and theoretical calculations revealed that the origin of this behavior lay in the solvent-dependent emissive states: the emission in cyclohexane originated from the second excited state (S₂) with a LE character, whereas the emission in tetrahydrofuran originated from the S₁ state with an intramolecular charge-transfer (CT) character. In chloroform, contributions from both states were present. In a subsequent development, Zhang and co-workers reported in 2025 the synthesis of three triarylborane compounds functionalized with one, two, or three S_p-dimethylamino[2.2]paracyclophane groups, (S_p)-55, (S_p,S_p)-56, and (S_p,S_p,S_p)-57, respectively (Fig. 20).¹⁰⁶ In cyclohexane, the emission spectra of the three compounds were nearly completely overlapped, and their Φ_F values increased only slightly from (S_p)-55 (0.43) to (S_p,S_p)-56 (0.46) and (S_p,S_p,S_p)-57 (0.49). In tetrahydrofuran, all molecules displayed well-separated temperature-dependent dual fluorescence, offering great potential for the development of ratiometric fluorescent thermometers.^107–109 The long-wavelength emission band was assigned to a CT state, whereas the short-wavelength band originated from a LE state. In cyclohexane, (S_p)-55, (S_p,S_p)-56, and (S_p,S_p,S_p)-57 each exhibited two (−)-CD signals, with g_abs values at the longest wavelength of −1.55 × 10⁻³, −1.56 × 10⁻³, and −1.35 × 10⁻³, respectively. In contrast to 54, all three compounds displayed a single (−)-CPL signal of the same sign in both cyclohexane and tetrahydrofuran. The g_lum values in cyclohexane were −3.40 × 10⁻³ for (S_p)-55, −2.68 × 10⁻³ for (S_p,S_p)-56, and −2.34 × 10⁻³ for (S_p,S_p,S_p)-57, indicating that the number of planar chiral [2.2]paracyclophane units had no significant influence on g_lum. Theoretical calculations revealed that, regardless of the number of planar chiral units incorporated, only one of them was operative in the excited state. This finding accounted for the nearly identical emission spectra of the three compounds and the observed independence of their g_lum values from the number of chiral [2.2]paracyclophane units. This work demonstrated a lack of cooperativity among multiple chiral centers, which provided a basis for simplifying the design of chiral molecules.

Fig. 20. Chemical structures of 54–57.

According to the symmetry-matching rule,¹¹⁰ it was established that when the symmetry condition was satisfied for an electronic transition, the θ could in principle be optimized to 0° or 180°, leading to theoretical maximization of the g factor. However, even under optimal θ values, the imbalance between |µ| and |m| persists, which still limits the values of |g_abs| and |g_lum|. Therefore, how to balance |µ| and |m| through structure–property relationships emerged as a key challenge that urgently demanded a solution in the field. In 2025, Wang and co-workers introduced B–π–B and O–π–O units onto both sides of a chiral space-conjugated [2.2]paracyclophane scaffold, thereby constructing a pair of C₂-symmetric MR-TADF CPL-SOMs, (R/S)-58, which achieved excellent chiroptical properties and high color purity (Fig. 21).¹¹¹ Natural transition orbital (NTO) analyses revealed that the wavefunctions of (S)-58 were either symmetric or antisymmetric with respect to the C₂ axis. The HONTO and LUNTO for the S₀ → S₁ transition were antisymmetric, whereas those for the S₀ → S₂, S₀ → S₄, and S₀ → S₇ transitions were symmetric. According to the symmetry-matching rule, transitions with the same symmetry between the initial and final states, such as the S₀ → S₂ transition, experienced cancellation of the dipole moment components perpendicular to the C₂ axis, thereby achieving an optimal θ value (θ = 0/180°). When the m component was aligned parallel to the C₂ axis and the µ component was aligned perpendicular to it, the total |m| was maximized while the total |µ| was minimized, thereby enabling a large g value. For instance, upon separating 58 into two symmetrical halves, the S₀ → S₂ transition was found to take advantage of the superposition of the m components along the Z direction and the cancellation of the µ components along the X/Y direction. Consequently, the |m| was enhanced and the |µ| was reduced, leading to strong chiroptical activity as a result of the balanced |m| and |µ|. Based on this rationale, the calculated g_cal value for the S₀ → S₁ transition was 0.0006, whereas those for the S₀ → S₂, S₀ → S₄, and S₀ → S₇ transitions were significantly larger, amounting to 0.050, 0.060, and 0.067, respectively. In toluene, 58, which possessed a through-space conjugation effect, exhibited pure green emission with a peak at 526 nm, an FWHM of 26 nm, and CIE coordinates of (0.26, 0.70). Consequently, the incorporation of a through-space conjugation effect proved to be an effective approach to narrow the emission bandwidth and maintain high color purity. In a doped film, the emission peak red-shifted to 531 nm with an FWHM of 33 nm, and the Φ_F reached as high as 0.99. (S)-58 exhibited a τ_p of 7.1 ns and a τ_d of 6.8 µs. The τ_d value increased markedly with rising temperature, indicating the presence of MR-TADF characteristics. Regarding the chiroptical properties, the m-forbidden S₀ → S₁ transition exhibited a |g_abs| of 1.2 × 10⁻³ at 512 nm. In contrast, the m-assisted S₀ → S₂, S₀ → S₄, and S₀ → S₇ transitions showed much larger |g_abs| values, reaching 1.5 × 10⁻² at 439 nm and 2.1 × 10⁻² at 363 nm, respectively. Both enantiomers displayed bright green and extremely narrow-band CPL signals between 500 and 600 nm, which corresponded to their CP-MR-TADF characteristics. Because of the small geometrical relaxation between the S₀ and S₁ states, the |g_lum| value (1.2 × 10⁻³ at 526 nm) was consistent with the |g_abs| value of the S₀ → S₁ transition. The calculated B_CPL was 35 M⁻¹ cm⁻¹, a value comparable to the reported CP-MR-TADF emitters.^81,112 CP-OLEDs based on (R/S)-58 displayed narrowband green electroluminescence featuring a peak at 535 nm, an FWHM of 31 nm, an EQE_max of 29.4%, and a low efficiency roll-off under high brightness. In addition, these devices showed obvious CPEL, with a |g_EL| of 1.2 × 10⁻³. Although the g_lum value of this molecule remained on the order of 10⁻³, this work nevertheless established a correlation between the symmetry-matching rule and the g value, thereby providing a theoretical basis for the development of high-performance CPL materials.

Fig. 21. Chemical structure of 58.

In 2025, Wang and co-workers designed and synthesized a series of planar chiral pyrenophanes, 59–62, which were rigidly locked by methylene, disulfide, disulfoxide, and disulfone bridges (Fig. 22).¹¹³ In dichloromethane, the absorption spectra of all these pyrenophanes showed the characteristic absorption bands of a pyrene dimer. The fluorescence spectra of 59, 60, and 61 showed an intense emission peak at 480 nm, attributed to the pyrene excimer, and no monomer emission was detected, indicating that the excimer emission resulted from the formation of the rigid cyclophane skeleton. For 62, the excimer emission was red-shifted to 500 nm due to its shorter bridge and consequently higher framework rigidity, which enabled stronger through-space conjugation between the two pyrene moieties. It was noteworthy that 62, which did not contain a heavy sulfur atom, afforded a Φ_F of 34.11%, which was markedly higher than those of the sulfur-bearing analogues 59 (3.89%), 60 (1.70%), and 61 (7.42%). These differences highlighted the significant influence of the bridging structure on both the emission behavior and the Φ_F value. The excimer state of Py exhibited a lifetime exceeding 8 ns. In contrast, the sulfur-containing pyrenophanes showed much shorter excimer lifetimes, ranging from 1.6 to 5.5 ns, due to stronger spin–orbit coupling (SOC) between singlet and triplet states, which led to rapid intersystem crossing. This difference accounted for the highest Φ_F value observed for 62. All molecules displayed clear CD spectra that were in good agreement with their UV absorption, demonstrating that the chirality stemmed from the pyrene stacking itself. In addition, all enantiomers emitted strong CPL signals; the highest |g_lum| value was 0.034 for 61, followed by 0.028, 0.025, and 0.023 for 62, 59, and 60, respectively. Notably, 62 possessed the greatest B_CPL value in the series, reaching 90.34 M⁻¹ cm⁻¹. Remarkably, the g_lum values of these compounds remained almost unchanged across ten different solvents, a temperature range from −10 °C to 70 °C, and a concentration interval of 1 × 10⁻⁶–5 × 10⁻⁴ mol L⁻¹, demonstrating excellent stability. According to theoretical calculations, the θ values in the excited states of all pyrenophanes were found to be very small (2.33–9.90°), making cosθ approach 1. This accounted for the large g_lum values observed. Furthermore, the sulfur-free pyrenophane 62 exhibited a very high-performance CPEL emission, with |g_EL| reaching 0.014. This work demonstrated that rigid locking constituted an effective method to achieve a high and environmentally insensitive g_lum value.

Fig. 22. Chemical structure of 59–62.

6. Conclusions

In recent years, CPL-SOMs have attracted considerable attention owing to their unique advantages and broad application prospects. With advances in organic synthetic methodology and a deeper understanding of structure–property relationships, significant progress has been achieved in this field. Researchers have not only developed various multifunctional CPL-SOMs systems but also established effective strategies for modulating chiroptical properties, and have successfully enabled the fabrication of high-performance CP-OLEDs. This review systematically summarizes the latest advances in high-performance CPL-SOMs over the past three years, covering molecular design concepts, photophysical and chiroptical properties, and device performance.

Despite the remarkable achievements made in CPL-SOMs, this system still faces several critical challenges that urgently need to be addressed. These challenges are manifested in the following four aspects. First, the synthesis involves multiple steps and enantiomer separation remains formidable. The construction of most CPL-SOMs, especially helical chiral SOMs, requires multi-step organic reactions. These synthetic routes often involve cyclization steps, suffer from tedious procedures, high catalyst costs, and generally low overall yields. More importantly, most CPL-SOMs are obtained as racemic mixtures after synthesis and must be separated by chiral HPLC. This process is not only costly and time-consuming but also difficult to produce on a large scale, which severely limits the industrial translation of these materials from laboratory research to practical applications. Second, the trade-off between Φ_F and g_lum is difficult to overcome. Many SOMs can overcome aggregation-caused quenching (ACQ) and exhibit high emission efficiency. However, the g_lum values of most CPL-SOMs remain in the range of 10⁻³–10⁻⁵, far below the theoretical maximum. A potential strategy for achieving higher g_lum values involves increasing the proportion of chiral units in the FMO distribution, which may in turn enhance the m of the molecule. Third, compared with blue and green light, research on red and white CPL-SOMs is relatively limited. Red and white CPL are of great significance in bioimaging and display technology. However, red and white CPL-SOMs impose stringent requirements on molecular structures. Attaching red and white chromophores to chiral frameworks may represent a feasible strategy for constructing high-performance red and white CPL-SOMs. Fourth, color purity is generally low. The electroluminescence spectra of most CPL-SOMs exhibit a large FWHM, making it difficult to meet the requirements of high color purity display applications. Recently developed MR-TADF materials have demonstrated advantages in narrowband emission. Introducing MR-TADF units into chiral skeletons may significantly improve color purity while maintaining chiroptical activity, representing one of the important development directions in this field.

In conclusion, despite recent progress, CPL-SOMs research remains constrained by several key limitations that hinder their practical production and application. Consequently, the development of high-performance CPL-SOMs and the further implementation in efficient optoelectronic devices remains a significant challenge. We hope that this review will offer valuable guidance and inspiration for the future molecular design, performance optimization, and device application of chiral optoelectronic functional materials.

Author contributions

M. Zhang: conceptualization, investigation, funding acquisition and writing – review and editing. S. Hou: investigation and writing – original draft. C. Xin: writing – original draft. K. Li: writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.