A novel boron-stereogenic fluorophore with dual-state circular polarization luminescence via a self-dispersing strategy

Abstract

Molecular engineering is a reliable approach for the development of circularly polarized luminescence (CPL) materials for various applications. However, creating dual-state CPL platforms that possess chirality while achieving a delicate balance between molecular rigidity and flexibility remains a formidable challenge. In this study, a novel bisarylboron-anchored pyrrolylsalicylhydrazone (BOPSH) platform was synthesized via a facile “one-pot” condensation. These key aryl-boron substituents not only provide structural rigidity to the fluorophore, enhancing the bright emission and suppressing emission quenching from π–π stacking in solid states due to their twisting and bulky steric effects, but also generate a boron-stereogenic center and enable strong CPL by promoting intramolecular charge-transfer transitions. As a result, these BOPSHs show intense absorption and strong dual-state emissions in both solution and solid states (with Φ_PL value approaching unity), emitting across the visible region with excellent chemical, photostability, and thermal stability. Meanwhile, their enantiomers display dual-state CPL performance, with luminescence dissymmetry factors (g_lum) up to 9.40 × 10⁻³, and CP electroluminescence (EL) with a dissymmetry factor (g_EL) of 3.07 × 10⁻³, along with excellent maximum external quantum efficiencies (η_ext,max) of 5.0%, approaching the theoretical limit for fluorescent molecules. We expect our study to break new ground in the construction of chiral dual-state materials with diverse structures.

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Introduction

A fundamental characteristic of symmetry is chirality; chiral molecules and other chiral objects exist as pairs of non-superimposable mirror images.¹ In molecular design, chirality is typically considered for various applications, including life sciences, biomedicine, and materials science.² Utilizing the polarization of photons and the spin of electrons, chirality offers a novel approach to achieving circular polarization luminescence (CPL) for various applications, including 3D displays, encrypted optical communication, and bioimaging.^3,4

Early research on CPL materials focused on lanthanide or transition metal-based complexes, which have high dissymmetry factors but often suffer from low molar absorption coefficients (ε) and fluorescence quantum yields (Φ_PL) due to the Laporte-forbidden rule.⁵ In contrast, chiral organic small molecules offer several advantages, including large ε, higher emission efficiency, easier chemical structure modification, precise emission wavelength tuning, and simpler fabrication.⁶ As a result, CPL-active organic small molecules have garnered increasing attention.⁷ However, most of these molecules contain polycyclic aromatic structures, which tend to form strong π–π stacking in concentrated solutions or solid states.⁸ This results in aggregation-caused quenching (ACQ), leading to weak or negligible CPL emission, which hinders their use in applications requiring highly emissive solid-state compounds, such as OLEDs or light-emitting electrochemical cells.⁹

A promising alternative is the development of chiral aggregation-induced emission (AIE) fluorophores, which can emit CPL in aggregated states.¹⁰ However, a limitation of these fluorophores is their lack of emission in solution due to fast molecular motions and vibrations, which suppress fluorescence. This raises a key challenge in designing dual-state emissive (DSE) chiral molecules: achieving chirality while maintaining a delicate balance between molecular conformational freedom, which prevents fluorescence quenching in the solid state, and minimizing nonradiative relaxation in solution to ensure strong emission.¹¹

The boron dipyrromethene (BODIPY) chromophore is an attractive platform for constructing organic CPL molecules due to its rich chemistry and tuneable optical and electronic properties, such as intense visible absorption, fluorescence brightness, and good photostability.^12,13 A series of chiral BODIPY derivatives (Fig. 1a) have been reported by incorporating chiral moieties from the periphery, augmenting scaffolds with chiral axes, and integrating helicene units.^14–17 Very recently, several catalytic synthesis methods have been developed for the boron-stereogenic chiral BODIPYs through the desymmetrization reactions of prochiral tetracoordinated boron substrates.^16b,18,19 However, these compounds have failed to demonstrate CPL in the solid state. The rational design of chiral DSE BODIPYs and their analogues as luminescent materials remains a significant challenge that has yet to be addressed. Therefore, there is an urgent need to explore novel chiral DSE BODIPY systems with tuneable CPL for both fundamental research and practical applications.

Fig. 1 (a) Previously reported chiral BODIPY and its analogues. (b) The chiral DSE BOPSH fluorophore designed in this work.

By analyzing the core of the BODIPY chromophore, which generally assembles via BF₂ complexation of an N,N-bidentate dipyrromethene ligand, we propose the preparation of a suitable ligand of pyrrolylsalicylhydrazone featuring two independent boron coordination sites (N,N,O-tridentate coordination site for B1 and N,O-bidentate coordination site for B2, Fig. 1b). This ligand, when subjected to direct electrophilic aryl-boron substitutions, is expected to yield a novel bis-boron-chelated fluorophore with the desired structure rigidity and essential steric hindrances to promote successful self-dispersion of the fluorophore, ensuring strong DSE performance. Additionally, the aryl-boron substituents on the N,N,O-tridentate coordination B1 site will also generate a boron-stereogenic center in situ. Moreover, these aryl-boron substituents may facilitate intramolecular charge-transfer (ICT) transitions due to their electron deficiency, resulting in enhanced CPL luminescence.²⁰ Based on this reasoning, we design a self-dispersing triarylboron-anchored pyrrolylsalicylhydrazone (BOPSH) fluorophore featuring two independent boron coordination sites. This can be directly accessed via “one-pot” electrophilic aryl-boron substitutions on the pyrrolylsalicyhydrazone ligand by condensing the readily accessible 2-formylpyrrole with salicylhydrazines, followed by subsequent in situ complexation with commercial arylboronic acids. Expectedly, they exhibit DSE emission (Φ_PL value approaching unity) across the visible region. Additionally, these configurationally stable enantiomers display good solid-state CPL and CP electroluminescence (CPEL) with corresponding dissymmetry factors up to 9.40 × 10⁻³ and 3.07 × 10⁻³, respectively.

Results and discussion

Syntheses and characterization

The synthesis involves a condensation reaction between readily available formylpyrrole/isoindole and salicylhydrazine, followed by on-site complexation with commercial arylboronic acids. The condensation of 2-formylpyrrole with salicylhydrazine in the presence of a catalytic amount of glacial acetic acid (HOAc, Scheme 1a) and subsequent complexation with phenylboronic acid generated two products. Among those, the weakly fluorescent product was characterized as a mono-boron coordinated compound, while the bright fluorescent one, confirmed via X-ray analysis was a bisboron-anchored BOPSH 1a (Scheme 1a). Following further optimization of the reaction conditions by screening solvents, temperature, base, and reagent ratios, BOPSH 1a was ultimately obtained as the sole product in an 89% isolated yield.

Scheme 1 Syntheses of BOPSHs 1a–k and 2a–e.

To enrich structural diversity, various commercial arylboronic acids were applied in this reaction, successfully yielding the desired BOPSHs 1b–g in 65–82% yields (Scheme 1a). We further extended this optimized reaction condition to the condensation of 2-formylpyrrole/isoindole derivatives with salicylhydrazide. After subsequent complexation with phenylboronic acid, BOPSHs 1h–j were generated in 63–84% yields. Additionally, the one-pot condensation of 4-chloro-2-hydroxybenzohydrazide and 2-formylpyrrole, followed by phenylboronic acid also provided BOPSH 1k in 76% yield.

We found that the boron complexations occurred in a stepwise fashion. By reducing the ratio of phenylboronic acid to one equivalent, no formation of BOPSH 1a was obtained and only mono-boron coordinated BPSH was obtained (Scheme 1b). This indicates that the boron complexation selectively first took place with the nitrogen of pyrrole and the oxygen of the phenol moiety, forming a four-aromatic-ring-fused BPSH structure. This provided us the opportunity to install two different aryl moieties onto the boron atoms of the BOPSH chromophore to achieve BOPSHs 2a and 2b (Scheme 1b) via further complexation of BPSH with 4-tert-butyl and 4-trifluoromethyl-substituted phenylboronic acids. Similarly, BOPSHs 2c and 2d were prepared using BF₃·OEt₂ to coordinate with the second unilateral boron site, achieving yields of 54% and 50%, respectively. Through the complexation of BPSH with p-phenyl-1,4-diboronic acid, a face-to-face four-boron coordinated dimeric BOPSH 2e was also isolated in a 45% yield (Scheme 1b). These novel BOPSHs were fully characterized using ¹H, ¹³C, ¹¹B NMR, and HRMS.

X-ray crystal structure analysis

To confirm the boron-stereogenic center architecture, single crystals of racemic BOPSHs 1a, 1e, and 1h–k, as well as 2e, suitable for X-ray analysis were obtained by the slow diffusion of hexane into their dichloromethane solutions (Tables S1–S4†). As shown in Fig. 2 and S1–S7,† the rigid heteroatom BOPSH core consists of five fused aromatic rings, including one pyrrole, one phenyl, one newly formed five-membered ring, and two newly formed six-membered rings. The dihedral angle between C1–N3–B1 and O1–N3–B1 is 55.9°, and the half-bowl conformation, characterized by a large torsional angle (23.5° for 1a and 32.2° for 1j) between the aromatic groups on both sides of the B1 atom, indicates that the BOPSH core is distorted.

Fig. 2 (a)–(c) X-ray crystal, dimmer, and packing structures of 1a. (d) and (e) X-ray crystal and dimer structures of 1j. (f) NICS(1) values for 1a. Peripheral substituents and hydrogen atoms were removed for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths in 1a: B1–N1, 1.572 Å; B1–N3, 1.545 Å; B2–N2, 1.597 Å; B1–O1, 1.464 Å; B2–O2, 1.574 Å. C, gray; N, light blue; B, pink; O, red; S, yellow.

The bowls display a relatively shallow depth; for example, the bowl depth for 1a is 1.24(3) Å. The B–O and B–N bond lengths in the five-membered ring (around 1.57(2) and 1.61(2) Å) are longer than those observed in the six-membered ring (1.47(2) and 1.56(2) Å). The tetrahedral character (THC_DA) of the B1 and B2 atoms in 1a was calculated as 76.7% and 68.5%, respectively, according to Höpfl's equation,²¹ based on the account of the six bond angles of the boron-related bonds, as shown in Fig. 2a and S8.† The packing structures of 1a reveal two enantiomers without π–π stacking, and multiple C–H⋯π interactions (ca. 2.8 Å, Fig. 2c) are observed. The nucleus-independent chemical shift (1) (NICS(1)) values, calculated using density functional theory (DFT), for the B-containing rings, fall within the −1.06 to 0.22 ppm range, indicating a non-aromatic character.

Spectroscopic properties

These BOPSHs exhibit broad absorption and ultra-bright emission (Fig. 3, S9–S24, Tables 1 and S5–S7†). They absorb light with high molar extinction coefficients (ε_max up to 21 900 M⁻¹ cm⁻¹) and emit light with Φ_PL of up to 97%. These results show that luminous intensity and spectral wavelength can be tuned through minor structural modifications. Importantly, they exhibit high brightness (up to 19 400 M⁻¹ cm⁻¹). In comparison to common BODIPY dyes,^12,13 these BOPSHs demonstrate relatively larger Stokes shifts (2100–4500 cm⁻¹, Tables S5 and S6†) and longer fluorescence lifetimes (6.16–8.07 ns).

Fig. 3 Photophysical and chiroptical properties of 1a and 1h–j in dichloromethane and solid states. (a) Fluorescence photos of respective 1a and 1h–j taken under a 365 nm lamp. (b) Normalized UV-vis absorption and (c) emission spectra in dichloromethane. (d)–(f) CD (dash lines) and CPL spectra (solid lines) of (S/R)-1a (d) and (g), 1i (e) and (h), and 1j (f) and (i) in dichloromethane and solid states.

Table 1 Photophysical data of BOPSHs 1a–k and 2a–e

Dyes	Dichloromethane		Solid powder
	λ^maxabs^a (nm) (10^−4ε^b)	λ^maxem^a (nm) (Φ^c, τ^d (ns))	λ^maxem (nm) (Φ^c, τ^d (ns))
a Data correspond to the strongest absorption or emission wavelength maxima. b Molar extinction coefficient (ε) in M^−1 cm^−1. c Absolute fluorescence quantum yields. d Fluorescence lifetime.
1a	349 (4.15), 412 (4.08)	464, 489 (90%, 7.12)	489 (80%, 5.47)
1b	347 (3.99), 411 (3.92)	465, 492 (73%, 7.12)	501 (82%, 11.33)
1c	347 (4.13), 412 (4.05)	460, 489 (81%, 7.09)	492 (80%, 6.21)
1d	350 (4.16), 412 (4.09)	462, 488 (77%, 6.79)	513 (65%, 6.21)
1e	351 (4.08), 413 (3.92)	460, 486 (75%, 6.25)	492 (78%, 8.42)
1f	348 (4.16), 411 (4.09)	467, 489 (90%, 8.07)	494 (47%, 4.57)
1g	342 (4.14), 412 (3.97)	466, 489 (75%, 7.70)	493 (60%, 5.03)
1h	338 (4.17), 424 (4.23)	483, 507 (96%, 7.09)	523 (59%, 4.74)
1i	332 (4.16), 433 (4.21)	495, 521 (97%, 6.98)	532 (56%, 4.37)
1j	357 (4.07), 494 (4.29)	560 (77%, 6.16)	602 (21%, 3.71)
1k	358 (4.06), 420 (4.06)	474, 502 (65%, 6.79)	528 (89%, 6.76)
2a	347 (3.98), 411 (3.90)	463, 487 (79%, 7.28)	492 (45%, 5.14)
2b	349 (4.19), 412 (4.11)	463, 490 (77%, 6.87)	503 (37%, 3.58)
2c	348 (4.04), 416 (4.01)	468, 490 (59%, 5.57)	492 (15%, 3.89)
2d	337 (4.18), 428 (4.23)	476, 506 (62%, 5.69)	503 (17%, 1.83)
2e	346 (4.41), 412 (4.34)	465, 491 (78%, 7.60)	526 (81%, 4.61)

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As demonstrated by 1a, these newly developed BOPSHs show excellent stability in commonly used organic solvents. For example, no aggregation was observed with the increasing concentration of 1a in toluene (from 2 μM to 40 μM), as indicated by the good linear correlation between the absorbance maxima and the concentrations of the solution (Fig. S25†). Negligible variations in absorbance were observed with changes in pH values in 1% acetonitrile PBS. Additionally, they exhibit excellent photostability in toluene, superior to that of fluorescein under the same light irradiation conditions. For instance, after 40 min of light irradiation using a 50 W LED lamp with an irradiance light dose of 84 J cm⁻², no detectable decrease in absorbance was observed for 1a, while an approximate 60% decrease in absorbance was noted for fluorescein (Fig. S26†). Furthermore, the respective BOPSH dyes demonstrate excellent thermal stability, with a thermal decomposition temperature of 326 °C for 1a and 278 °C for 1d, as evidenced by thermogravimetric analysis (Fig. S27†).

BOPSHs 1a–k and 2a–c show very bright solid-state fluorescence (solid-state Φ_PL up to 89%, Table 1, Fig. 3a and S28–S38†) and can be easily tuned via simple variations of substituents. For example, BOPSH 1a emits bright green light with a wavelength maximum of at 489 nm, with an absolute Φ_PL of 80%. The emission spectra are red-shifted to 532 nm for 1i (Φ_PL = 56%, yellow light) and 622 nm for 1j (Φ_PL = 21%, red light) in the powder states (Tables 1 and S7†). Additionally, the solid-state fluorescence emission spectra of these BOPSHs span from the sky-blue, green, and yellow regions to the red region (Fig. 3a). Most of their solid-state fluorescence lifetimes fall between 3.58 and 11.33 ns (Table 1). The long solid-state lifetimes for 1b and 1e (11.33 ns and 8.42 ns) may be attributed to the bulky substituents on the boron atom, which enhance the stability of the excited state and consequently prolong the fluorescence lifetime. It is worth noting that the absolute solid Φ_PL of 2c and 2d decreased, possibly due to the reduced steric hindrance of the BF₂ unit. Compared to the non-fluorescent mono-boron BPSH, the structural rigidity of BOPSHs enhanced by dual boron coordination significantly improves their bright emissions in solutions and solid states. The results reveal that the two phenyl rings of the BPh₂ group are essential for enhancing the solid-state fluorescence of these BOPSH dyes. Additionally, no obvious intermolecular π–π stacking between these BOPSHs from adjacent molecules is observed. This is reflected in the increased π–π distance (ranging from 6.35 to 10.32 Å) and the lower degree of π–π overlap, which is similar to the recently developed AIE-type 1,4-bis(diphenylamino)-2,5-disubstituted benzene fluorophores exhibiting high solid-state fluorescence efficiency reported by Tang's group.²² Additionally, they generally exhibit coplanar, inclined “staircase-like” arrangements with pitch angles ranging from 20.3° to 50.1° along the long molecular axis (Fig. S1–S7†), contributing to the enhanced solid-state Φ_PL for these BOPSHs. This is in good accordance with Kasha's exciton model.²³

Interestingly, upon gentle grinding of crystalline 1a with a spatula, a 28 nm blue shift (from 512 to 484 nm) was observed in the solid-state emission spectra, accompanied by an increased solid Φ_PL (Fig. S39a and Table S7†). Upon heating the ground powder, the fluorescence emission band reverted to 489 nm with a decreased Φ_PL. Similar results were also observed for 1i (Fig. S39b and Table S7†). This red shift may be closely associated with a conformational change from a flat plate conformation in the crystalline state to a folded conformation upon grinding, as indicated by the X-ray powder diffraction analysis results. Sharp diffraction peaks were observed for these molecules in the crystalline state, demonstrating a highly ordered molecular arrangement. Upon grinding, some of these peaks (for example, signals a–c, Fig. S39c and d†) decreased in intensity or disappeared, suggesting a transition from a crystalline to an amorphous packing arrangement.

Electrochemical properties

Electrochemical properties were studied using cyclic voltammetry. Respective 1a, 1i, and 1j displayed reversible one-electron oxidation and reduction processes (Fig. S40 and Table S8†). The oxidation potential shifted negatively when the core ring moieties became electron-rich (1a: 1.26 V, 1h: 0.91, 1i: 0.81 V, and 1j: 0.58 V), while the reduction potential experienced minimal change. This indicates that the installation of electron-donating substituents on the pyrrole moiety effectively increases the HOMO energy levels, resulting in a narrowed HOMO–LUMO energy band gap. DFT calculations reveal that the calculated HOMOs for 1a, 1i, and 1j are mostly distributed over the electron-rich pyrrole segments, with limited distribution over the BPh₂ and salicylhydrazide segments (Fig. S41†). In contrast, their LUMOs are distributed over the BOPSH core, indicating a classic ICT feature. Time-dependent DFT analysis assigns the main absorption peak to S₀ → S₁ transitions with HOMO → LUMO contributions (Table S9†).

Chiroptical properties

The sp³ hybridized boron atom (B1, Fig. 2a) bonds to four different atoms, inducing chirality in the fluorophore. These BOPSHs were crystallized as a 1 : 1 racemic mixture, with the axial phenyl moiety (on B1) pointing in opposite directions for enantiomers in each unit cell (Fig. S1–S7†). Using BOPSHs 1a, 1i, and 1j as model compounds, we performed chiral separation and analysis using high-performance liquid chromatography (HPLC, Fig. S42–S50†). The optically resolved conformational enantiomers exhibited perfect mirror images of each other with opposite Cotton effects (Fig. 3 and S51–S53†). The chiral signals are ascribable to the distorted conformations of substituents on the pyrrole segment, as shown in the structures optimized by DFT. The absolute configurations of the respective enantiomers were elucidated by comparing experimental data with TD-DFT simulation results. Specifically, the Cotton effects for the main absorption band were found to be positive for the (R) configuration and negative for the (S) configuration (Fig. S54†). BOPSH 1a exhibits |g_abs| in dichloromethane (1.51 × 10⁻³, Table S10†), while 1i and 1j exhibited similar CD characters with |g_abs| values of 1.25 × 10⁻³ and 0.58 × 10⁻³, respectively (Fig. S55† and Table 2). To understand why 1a has relatively large dissymmetry factors, the dissymmetry factor (g_abs) is calculated using the formula: g = 4 cos θ|m|/|μ|, where θ is the angle between m and μ. The TD-DFT data listed in Table 2 reveals that 1a has slightly smaller |μ| values but larger cos θ values than those of respective BOPSH 1i. The increased cos θ values indicate the effective coupling of electric and magnetic transition moments of 1a (Fig. 4 and Table 2). These findings suggest that subtle electronic modifications can significantly impact the coupling of electric and magnetic transition moments.

Table 2 Chiroptical propertis of BOPSHs 1a and 1i

	g abs 10^−3	g lum 10^−3	g lum 10^−3	B CPL M^−1 cm^−1	|μ|^e 10^−18	|m|^e 10^−20	cos θ^e	g calc 10^−3
a Corresponding gabs of the (R)-configuration calculated as: gabs = ellipticity/(32 980 × absorbance). b g lum = 2(IL − IR)/(IL + IR), dichloromethane. c g lum obtained in powder solid states. d B CPL = ε × ΦPL × glum/2. e Calculated at the CAM-B3LYP/6-311+G(d,p)/(IEFPCM, dichloromethane) level. |μ| unit: [esu cm], |m| unit: [erg G ^−1]. f Calculated using: g = 4 cos θ|m|/|μ|.
1a	1.51	0.82	0.91	5.98	5.03	3.43	0.079	2.16
1i	1.25	0.50	1.00	3.93	5.84	3.34	0.055	1.26

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Fig. 4 Calculated magnetic (blue) and electronic (red) transition moment densities for BOPSHs 1a (a)–(c) and 1i (d)–(f) with an iso-value of 0.005.

The CPL spectra showed mirror–image relationships stemming from S₁ → S₀ transitions with g_lum of up to 0.82 × 10⁻³ for (R)-1a and 1.39 × 10⁻³ for (S)-1a in dichloromethane, which are obviously higher than those outstanding bipyrrole-derived tetra-BF₂ flag-hinge chromophores reported by Ono.²⁴ The CPL maxima coincided with the emission maxima. Notably, the g_lum values for BOPSH 1a in common organic solvents with different polarities and in solid states remained consistently around 10⁻³ (Fig. 3d–f, S56,†Table 2 and S11†). Additionally, the B_CPL values of 1a and 1i were determined to be 5.98 and 3.03 M⁻¹ cm⁻¹ using the formula: B_CPL = ε × Φ_PL × g_lum/2, which is high for chiral BODIPY molecules. The B_CPL values, excellent photostability, and finely tuned emission properties suggest promising prospects for CPL-related biomedical and optoelectronic applications. Moreover, the CPL signals in solid states are stronger than those in solutions, with higher g_lum values of 0.91 × 10⁻³ (500 nm) for 1a and 1.00 × 10⁻³ (530 nm) for 1i, respectively. Furthermore, these BOPSHs are configurationally stable with a unique boron-stereogenic scaffold, and their enantiomers 1j display g_lum values up to 9.40 × 10⁻³ (550 nm) and 4.52 × 10⁻³ (605 nm) in solid states. The increased chiroptical features may be attributed to the orderly or chirality-organized supramolecular array of chromophores within the crystalline lattice. To the best of our knowledge, these BOPSHs represent the first high-performance chiral DSE BODIPY analogues.

Electroluminescent performance

Motivated by the impressive chiral DSE characteristics and favourable HOMO/LUMO energy levels, we proceeded to study the electroluminescence (EL) behaviours of the enantiomers (R/S)-1a as chiral emitters within circularly polarized organic light-emitting diodes (CP-OLEDs).²⁵ The structure of the fabricated devices via vacuum deposition is as follows: ITO/MoO₃ (5 nm)/mCBP (25 nm)/EML (20 nm)/PO-T2T (5 nm)/TmPyPB (25 nm)/LiF (1 nm)/Al, in which indium tin oxide (ITO) serves as the transparent anode, 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) functions as the hole-transporting layer, 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) acts as the exciton-blocking layer, 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) serves as the electron-transporting layer, and LiF/Al acts as the cathode. The emitting layer (EML) is composed of 3 wt% (R)-1a: 48.5 wt% PO-T2T: 48.5 wt% mCBP for device A1, and 3 wt% (S)-1a: 48.5 wt% PO-T2T: 48.5 wt% mCBP for device A2, wherein PO-T2T and mCBP form an exciplex host for both chiral molecules. The device configuration diagram and the molecular structures of the functional layer are displayed in Fig. 5a and S57,† respectively. Notably, devices A1 and A2 can be turned on at low voltages of 3.1 and 3.0 V, which is due to the good injection and balanced transport of holes and electrons in the EML. Importantly, both devices radiate sky-blue light with EL peaks at 492 nm and maximum luminance (L_max) of 9559 and 9493 cd m⁻² (Table S12†), respectively, demonstrating high brightness that is suitable for practical applications in solid-state displays (generally 100–1000 cd m⁻²). The bright OLED emission of BOPSH 1a can be attributed to the synergistic effect of dual boron anchoring and twisted molecular architecture. More importantly, both devices attain excellent maximum external quantum efficiencies (η_ext,max) of 5.0% (Table S12†), which approaches the theoretical η_ext,max limit for fluorescent molecules. This also indicates high exciton recombination efficiencies in both devices. Reliability tests (Tables S13 and S14†) on multiple devices confirm the consistent and reproducible performance of BOPSH-based OLEDs. Owing to their CPL nature, (R)-1a and (S)-1a exhibit good CP-EL properties, displaying mirror-image CP-EL spectra with good dissymmetry factor (g_EL) values of +1.86 × 10⁻³ and −3.07 × 10⁻³, respectively (Fig. 5e and f). These results suggest that these new chiral molecules exhibit comparable g_EL to those of the recently developed CP-OLED devices²⁶ (Table S15†), and could serve as potential light-emitting materials for CP-OLEDs.

Fig. 5 (a) Configuration diagram of the fabricated devices. (b) EL spectra measured at 1000 cd m⁻², (c) external quantum efficiency curves with the change of luminance, and (d) luminance and current density curves with the change of voltage of devices A1 and A2. (e) CPEL spectra were collected under 5 V and (f) plots of g_ELversus CP-EL wavelength of devices A1 and A2.

Conclusions

We present a facile approach to a dual-state-emissive boron stereogenic BOPSH fluorophore via a self-dispersing strategy. Our overall strategy involves the preparation of a suitable configurationally locked ligand featuring two independent boron coordination sites, followed by their subsequent in situ electrophilic aryl-boron substitutions to generate the target BOPSH fluorophore. The synthesis of these BOPSHs is achieved in a diversity-oriented one-pot fashion from commercial arylboronic acids, and easily accessible 2-formylpyrroles/indoles and salicylhydrazides. These BOPSHs show strong absorptions and DSE emissions in both solution (Φ approach to the unity) and solid states (Φ up to 89%), with a large Stokes shift, high photostability, and mechano-fluorescence. Additionally, they exhibit bright CPL with B_CPL values of up to 5.98 M⁻¹ cm⁻¹. The fabricated CP-OLED devices achieved a high external quantum efficiency (η_ext,max = 5.0%) with promising dissymmetry factors (g_EL up to 3.07 × 10⁻³). These self-dispersing BOPSH fluorophores, along with their diversity-oriented one-pot synthesis, excellent DSE performance, and good CPL and CPEL properties, may provide a new perspective for constructing chiral DSE materials with diverse structures and functions.

Experimental

General synthetic procedure for boron-stereogenic BOPSHs 1

To a solution of pyrrole-2-carboxaldehyde derivative (PA, 1.0 mmol) and salicylhydrazide (167 mg, 1.1 mmol) in dried chlorobenzene (20 mL), several drops of glacial acetic acid (HOAc) were added slowly. The reaction mixture was refluxed in an oil bath at 105 °C for 6 h, and TLC was used to monitor the reaction. After the pyrrole-2-carboxaldehyde derivative disappeared on TLC, K₃PO₄ (690 mg, 3.0 mmol, 3 equiv) and the corresponding arylboronic acid (9.0 mmol, 9 equiv) were added into the mixture. The reaction was then stirred in a sealed tube at 140 °C overnight. After completion of the reaction, the mixture was poured into water (150 mL) and extracted with CH₂Cl₂ (3 × 60 mL). The organic layers were combined, dried over anhydrous Na₂SO₄, filtered, and evaporated under a vacuum. The crude product was purified by silica gel column chromatography and the yellow (or red) powders were obtained through recrystallization from dichloromethane and hexane.

Data availability

The ESI† includes all experimental details, including TD-DFT calculations, spectroscopic measurements, X-ray analysis, and the synthesis and characterization of all products reported in this study. NMR spectra of all products are also included.

Author contributions

C. Y., H. L., Z. Z., E. H., and L. J. supervised the work. C. Y. and C. C. designed the chemical synthetic route and conducted the experiments. Z. N. performed the calculations. Z. L conducted the electroluminescent measurements. C. Y. wrote the manuscript. All authors were involved in checking the data and have approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the financial support provided by the National Natural Science Foundation of China (22271002, 22478005, and U23A20594), the Anhui Provincial Natural Science Foundation (2008085QB67, 2308085J14, and 2408085MB043), the Anhui Provincial University Scientific Research Project (2023AH050149), and the Interdisciplinary Research Project of Hangzhou Normal University (2024JCXK01) for supporting this work. We also thank Prof. Tianyu Yang from The Ninth Medical Center of the Chinese PLA General Hospital for generously sharing the CP-OLED data.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, NMR, HRMS, photophysical, electrochemical, theoretical data, chiroptical data, and additional supplementary figures. CCDC 2116412, 2389168, 2116411, 2116409, 2116410, 2251717 and 2251718. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc01025j

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