Orthogonal di-spiro skeleton engineering on suppressing π–π stacking and spectral broadening for high-performance narrowband electroluminescence

Abstract

It is challenging to establish a universal design strategy to finely suppress exciton quenching and retain color purity for the development of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials. Herein, a molecular design with the steric effect by incorporating an orthogonal di-spiro π-skeleton is proposed to create a weak π–π stacking structure and thus effectively eliminate the formation of detrimental excimers and aggregates. The resulting MR-TADF emitters feature high rigidity, low concentration dependence and high horizontal dipole ratio for efficient electroluminescence. Full-color narrowband organic light-emitting diodes exhibit wideband color tuning from blue to red emission (480–628 nm) and achieve external quantum efficiencies as high as 40.4%. This molecular design strategy offers a viable approach for developing efficient luminescent materials with high color purity.

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New concepts

It is challenging to establish a universal design strategy to finely suppress exciton quenching and retain color purity for the development of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials. Unlike existing approaches, our work presents an orthogonal di-spiro π-skeleton design strategy to develop high-efficiency MR-TADF materials that simultaneously achieve quenching resistance and suppressed spectral broadening. The orthogonal di-spiro strategy significantly increases the intermolecular distance between adjacent MR cores, thereby effectively suppressing intermolecular π–π interactions and preventing both aggregation-induced quenching and spectral broadening. The resulting MR-TADF emitters feature high rigidity, low concentration dependence and high horizontal dipole ratio for efficient electroluminescence. Moreover, full-color narrowband organic light-emitting diodes exhibit wideband color tuning from blue to red emission. This work provides the first demonstration of the crucial role of the fused orthogonal di-spiro π-skeleton in suppressing aggregation and spectral broadening across the full-color MR-TADF spectrum, providing new design principles for developing high-performance, quenching-resistant organic optoelectronic materials.

1. Introduction

The development of organic light-emitting diodes (OLEDs) exhibiting both high efficiency and narrowband emission is crucial for next-generation ultra-high-definition (UHD) displays.^1–6 Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials, which theoretically achieve unity internal quantum efficiency (IQE) and narrow full-width-at-half-maximum (FWHM) emission, have emerged as promising candidates for high-performance OLEDs.^7–12 Consequently, designing and synthesizing three primary-color MR-TADF emitters, blue (B), green (G), and red (R), that combine high efficiency with superior color purity represents a critical research objective. Prototypical MR-TADF molecules based on B,N-doped polycyclic aromatic hydrocarbons (PAHs), first developed by Hatakeyama et al., demonstrate the ability to harvest both singlet and triplet excitons, achieving near-unity photoluminescence quantum yields (Φ_PL ≈ 100%) and remarkably high radiative decay rates (k_r ≈ 10⁸ s⁻¹).^13–15 Importantly, their distinctive MR effect substantially minimizes the bonding/anti-bonding character of frontier molecular orbitals (FMOs), thereby suppressing structural relaxation and vibrational coupling in excited states.^15–21 These combined features ultimately lead to an exceptionally high color purity, as evidenced by low FWHM values.

Benefiting from these superior properties, numerous RGB MR-TADF emitters exhibiting narrowband emission have been developed. The corresponding OLED devices can achieve maximum external quantum efficiencies (EQE_max) exceeding 30% and high color purity through low doping concentration (<5 wt%) methods.^{11,12,22–25} However, at high doping concentrations, the inherent planar molecular framework of most reported MR-TADF materials tends to promote strong π–π stacking interactions, leading to significant aggregation-caused quenching (ACQ) and noticeable emission spectrum red shifts and broadening, accompanied by sacrifices in (degraded) efficiency and color purity.^26–28 Additionally, there are no reports of full-color MR-TADF materials achieving high efficiency, spectral stability, and high color purity under high doping conditions to date.^3,29

To address these problems through molecular engineering, researchers have successfully implemented strategic out-of-plane distortion in MR-TADF frameworks by incorporating either sterically bulky substituents or non-planar structural motifs, effectively suppressing ACQ effects. Several design paradigms have validated this approach, utilizing either sterically hindered peripheral groups or intentionally twisted molecular architectures.^30–33 Notably, Duan and Yang et al. developed a steric wrapping strategy, protecting the chromophore core with carbazole units to create quenching-resistant MR-TADF emitters.^30,31 Wang and Li et al. designed highly distorted molecular systems through auxiliary donor integration, yielding MR-TADF materials with minimal concentration dependence.^33,34 Despite these advances, ACQ-resistant MR-TADF materials remain scarce in the literature.^35–38 Most of these materials demonstrate optimal performance only within specific chromatic ranges (e.g., blue or green emission), while no red emitters have yet achieved comparable performance.^29,39–47 Therefore, developing a universal design strategy for full-color MR-TADF materials that simultaneously achieve high efficiency, high color purity, and doping-concentration-insensitive behavior is highly desirable.

In this work, we introduce an orthogonal di-spiro π-skeleton design strategy to develop high-efficiency MR-TADF materials that simultaneously achieve quenching resistance and suppressed spectral broadening. Through strategic fusion of diverse donor moieties to modulate HOMO levels, we demonstrate full-color narrowband emission spanning the visible spectrum: blue (DSBO, 480 nm), green (DSBN, 510 nm), and red (DSBNS, 628 nm). The orthogonal di-spiro strategy significantly increases the intermolecular distance between adjacent MR cores, thereby effectively suppressing intermolecular π–π interactions and preventing both aggregation-induced quenching and spectral broadening. These emitters exhibit high Φ_PLs of ∼100% and enhanced horizontal dipole orientation (Θ_‖ > 85%). The corresponding OLEDs achieve an EQE_max as high as 34.5% (blue), 40.4% (green), and 38.0% (red), representing the state-of-the-art performance for narrowband OLEDs. In addition, all devices maintained narrow emission bandwidths, with nearly unchanged FWHM values at 28 nm (blue), 25 nm (green), and 58 nm (red) across a broad doping concentration range of 5–50 wt%, demonstrating exceptional spectral stability. This work provides the first demonstration of the crucial role of the fused orthogonal di-spiro π-skeleton in suppressing aggregation and spectral broadening across the full-color MR-TADF spectrum, providing new design principles for developing high-performance, quenching-resistant organic optoelectronic materials.

2. Results and discussion

2.1. Molecular synthesis and characterization

Fig. 1 illustrates the molecular design strategy, which employs a B,N-doped orthogonal di-spiro π-skeleton core as the fundamental motif. Photophysical modulation is primarily facilitated by the varying strengths of electron donation, which are synergistically combined with the effects of extended π conjugation. This combination allows for precise color tuning of MR-TADF emission across the blue (DSBO), green (DSBN), and red (DSBNS) spectral regions. DSBO and DSBN were synthesized in three steps from commercially available starting materials via a two-step spiro-cyclization reaction followed by electrophilic borylation (Scheme S1). DSBNS was subsequently obtained through sulfurization of DSBN using sulfur powder with iodine as the catalyst. All final products were purified by temperature-gradient vacuum sublimation and thoroughly characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, time-of-flight mass spectroscopy (TOF-MS), and single-crystal X-ray diffraction (see Fig. S1–S9). The 5% weight loss decomposition temperatures (T_d) of the three materials, DSBO, DSBN, and DSBNS, reach as high as 453, 483, and 494 °C, respectively (Fig. S10a). Differential scanning calorimetry (DSC) analysis indicated the absence of observable glass transition temperature (T_g) in the range up to 300 °C (Fig. S10b). These thermal properties indicate that the three materials ensure sufficient stability against thermal degradation and morphological aggregation during high-temperature vacuum deposition, which is critical for reproducible OLED fabrication, particularly at high doping concentrations.

Fig. 1 Molecular design strategy for modulating emission colors in the orthogonal di-spiro π-skeleton-based parent core systems. Molecular stacking schematics of (a) planar 2D and (b) bulky 3D configurations. (c) Chemical structures of DSBO, DSBN and DSBNS.

To estimate the energy levels of these narrowband MR-TADF materials, we employed cyclic voltammetry (CV) and differential pulse voltammetry (DPV). As shown in Fig. S11, the HOMO energy levels of DSBO, DSBN, and DSBNS are progressively shallower at −5.44, −5.32, and −4.94 eV, respectively. The significant difference in the HOMO energy levels arises from the varying electron-donating capabilities of the externally attached groups. Concurrently, the LUMO energy levels demonstrate an inverse progression, measuring −2.86, −2.89, and −2.95 eV for the same series. The deepening of the LUMO levels is attributed to the extended π-conjugation present within these molecular frameworks.

2.2. Single-crystal X-ray diffraction analysis

To elucidate the impact of the orthogonal di-spiro π-skeleton on the molecular geometry and intermolecular interactions, single-crystal X-ray diffraction analysis was performed on all three compounds, which were grown via slow solvent evaporation from a CH₂Cl₂/C₂H₅OH mixed solvent system. The single crystals and packing modes of DSBO (CCDC: 2464387), DSBN (CCDC: 2464390), and DSBNS (CCDC: 2464393) are presented in Fig. 2 and Fig. S12. The crystallographic data are summarized in Tables S1–S3. All three molecules generate significant steric hindrances through their orthogonal di-spiro geometries, effectively suppressing strong intermolecular interactions and excimer formation. As expected, the MR chromophores show a quasi-planar rigid conformation with small dihedral angles of the A/B (9.22–15.60°) and C/D (6.68–7.68°) rings. This pronounced structural rigidity is conducive to reducing nonradiative vibrational losses, thereby improving Φ_PL. Additionally, it minimizes excited-state structural relaxation, which directly contributes to the narrow emission bandwidths observed in photophysical measurements. In the crystal packing structure, all three molecules adopt a head-to-tail arrangement. While only C–H⋯π interactions are observed in DSBO, both DSBN and DSBNS exhibit π-stacked structures with small overlap areas and stacking distances of approximately 3.5 Å, which are larger than the typical stacking distance in planar MR-TADF materials.²⁵ These results reveal that the orthogonal di-spiro framework effectively inhibits π–π stacking in the solid state, thereby mitigating the ACQ effect and emission broadening.

Fig. 2 Top view, side view, dihedral angles, packing dimers and packing modes for the single crystals of DSBO (CCDC: 2464387), DSBN (CCDC: 2464390) and DSBNS (CCDC: 2464393).

2.3. Theoretical calculation

To gain insight into the characteristics of the ground states and excited states of DSBO, DSBN, and DSBNS, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed at the B3LYP/6-31G (d,p) level. The HOMO and LUMO orbitals of the three compounds show alternating distributions on the planar chromophores, exhibiting typical MR characteristics (Fig. S13). The HOMO distribution extends partially onto the spiro-junction carbons, indicating its participation in conjugation. In contrast, neither the HOMO nor the LUMO shows significant distribution on other carbon atoms of the orthogonal di-spiro backbone, indicating its limited electronic activity and negligible influence on the emission characteristics of the MR core. This further confirms that the di-spiro backbone primarily functions as a steric barrier rather than an electronic participant. By increasing the intermolecular distance between MR cores, the introduction of the orthogonal di-spiro structure effectively suppresses aggregation-induced exciton annihilation. The different HOMO energy levels among the three materials originate from the distinct electron-donating capacities of the heteroatoms (O, N, S), while the differences in LUMO energy levels result from the extended π-conjugation. The enhanced intramolecular charge transfer (ICT) character reduces both the lowest singlet (S₁) and triplet (T₁) excited state energies, enabling a wide emission tunability. Therefore, the three emitters exhibit progressively decreasing bandgaps (E_g): 3.36 eV (DSBO) > 3.20 eV (DSBN) > 2.78 eV (DSBNS). The calculated natural transition orbitals (NTOs), triplet spin-density distribution (SDD) and root-mean-square deviation (RMSD) are illustrated in Fig. 3. The S₁ and T₁ states exhibit similar distributions to the FMOs, demonstrating MR features. The oscillator strengths (f) of DSBO, DSBN, and DSBNS were calculated to be 0.2093, 0.3479, and 0.2551, respectively, which facilitates the radiative decay and increases the quantum yield. The spin density distribution (SDD) calculated triplet states are located on the MR core. The structural variations between ground (S₀) and S₁ states were analyzed using the RMSD, with the corresponding values of 0.0686, 0.0651, and 0.0932 for DSBO, DSBN, and DSBNS, respectively. The low RMSD values demonstrate the molecular structural rigidity of the orthogonal di-spiro π-skeleton, which significantly suppresses vibrational relaxation and emission broadening.

Fig. 3 Summary of theoretical S₁ and T₁ excitation energies, oscillator strengths (f), hole–electron distribution (hole: green, electron: blue, isovalue = 0.04), triplet spin-density distributions and RMSD (S₀: blue, S₁: red) of DSBO, DSBN and DSBNS.

2.4. Photophysical characterization

Fig. 4 shows the photophysical properties of the orthogonal di-spiro π-skeleton-based MR-TADF emitters DSBO, DSBN, and DSBNS, and the key data are summarized in Table 1. In dilute solutions, the absorption/emission bands (λ_abs/λ_em) exhibited bathochromic shifts in the order of DSBO (463/474 nm), DSBN (492/503 nm), and DSBNS (578/616 nm), demonstrating blue, green, and red PL emissions, respectively (Fig. 4a–c). The respective sharp and strong absorption bands primarily originate from the MR-induced short-range CT transitions. Moreover, DSBO, DSBN, and DSBNS showed Stokes shifts of 11, 11, and 38 nm, respectively, indicating small molecular geometrical changes between the S₀ and S₁ states, which resulted in extremely low spectral FWHM values (DSBO: 23 nm/0.12 eV, DSBN: 23 nm/0.11 eV, DSBNS: 48 nm/0.15 eV). The significant FWHM broadening of DSBNS arises from the synergistic effect of two factors: (1) structural modification (S-heteroatom-induced ICT character) and (2) intrinsic photophysical properties (vibronic coupling and energy gap law associated with narrow bandgap).⁴⁸ These mechanisms collectively rationalize the spectral difference between DSBNS and its blue/green analogues. Based on the onset of low-temperature fluorescence and phosphorescence (Fig. S14a), the S₁/T₁ energy levels of DSBO, DSBN, and DSBNS were determined to be 2.616/2.531, 2.457/2.447, and 2.041/2.036 eV, respectively, yielding low ΔE_ST values of 85, 10, and 5 meV, which facilitate exciton transfer from the T₁ to S₁ state. The FWHM variations of these emitters in different solvents (n-hexane to CH₂Cl₂) reveal that intramolecular CT is influenced by the polarity of the environment (Fig. S14b). Fig. 4d presents the PL spectra of DSBO, DSBN, and DSBNS in doped films, covering the visible region from blue to red with emission peaks at 478 nm (FWHM = 29 nm), 508 nm (FWHM = 27 nm), and 623 nm (FWHM = 51 nm), respectively. Notably, all three emitters maintain stable emission spectra in doped films without excimer emission, and the peak wavelengths and FWHM remain almost unchanged over the 5–50 wt% doping concentration range (Fig. S15). The orthogonal di-spiro π-skeleton geometry in these emitters disrupts close molecular packing, thereby reducing excimer formation while suppressing both ACQ and spectral broadening in solid-state films. All three emitters in 5 wt%-doped films showed near-unity photoluminescence quantum yields (Φ_PL = 98–99%), with transient PL decays (Fig. 4e and Fig. S16) clearly displaying both prompt (τ_p) and delayed (τ_d) components that confirm TADF behavior. The measured τ_p/τ_d values were 5.9 ns/62.7 µs for DSBO, 3.7 ns/14.2 µs for DSBN, and 10.1 ns/16.6 µs for DSBNS. The estimated radiative rate constants (k_r) of these compounds exceeded 10⁷ s⁻¹, significantly surpassing their nonradiative rate constants (k_nr < 10⁶ s⁻¹, Table S4), consistent with their large f values and rigid molecular structures. Additionally, the reverse intersystem crossing rate constants (k_RISC) are estimated to be 0.8 × 10⁵ s⁻¹ for DSBO, 5.6 × 10⁵ s⁻¹ for DSBN, and 2.7 × 10⁵ s⁻¹ for DSBNS, respectively. It can be observed that the k_RISC value of DSBO is much lower than that of DSBN and DSBNS, due to its largest ΔE_ST among the three emitters. Furthermore, the horizontal dipole ratios (Θ_‖) of DSBO, DSBN, and DSBNS reached up to 85%, 89%, and 92%, respectively, indicating high light outcoupling efficiencies that are beneficial for device performance enhancement (Fig. 4f).

Fig. 4 UV-visible absorption and fluorescence spectra of (a) DSBO, (b) DSBN and (c) DSBNS in toluene. (d) Fluorescence spectra of three emitters in doped films with doping concentrations of 5 wt%. (e) Transient photoluminescence decay curves of 5 wt%-doped films. (f) Angle-dependent PL spectra and simulation curve for doped films.

Table 1 Summary of photophysical data of DSBO, DSBN and DSBNS

Compound	λ abs [nm]	λ em [nm]	FWHM^b [nm]	S1^c [eV]	T1^c [eV]	ΔEST^d [meV]	τ p/τd^e [ns/µs]	Φ PL [%]	HOMO^g [eV]	LUMO^g [eV]	T d [°C]
a Measured in 10^−5 M toluene at room temperature. b Measured in toluene/doped films at room temperature. c Calculated from the onset wavelength of the fluorescence and phosphorescence spectra, respectively. d ΔEST = S1 − T1. e Lifetime of the prompt component (τp) and delayed component (τd) as determined from the transient PL. f Absolute PLQY of mCBP-doped films measured using an integrating sphere under a nitrogen atmosphere. g Determined by cyclic voltammetry curves. EHOMO = −e[4.8 + EOX − EOX(Fc/Fc+)]V, ELUMO = Eg + EHOMO. h 5% weight loss determined from TGA curves.
DSBO	463	474/478	23	2.616	2.531	85	5.9/62.7	98	−5.44	−2.86	453
DSBN	492	503/508	23	2.457	2.447	10	3.7/14.2	99	−5.32	−2.89	483
DSBNS	578	616/623	48	2.041	2.036	5	10.1/16.6	99	−4.94	−2.95	494

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2.5. Device performance

To evaluate the electroluminescence (EL) performance of this family of MR-TADF emitters, vacuum-deposited OLEDs were fabricated with the following configuration (Fig. 5a): ITO/HAT-CN (10 nm)/TAPC (40 nm)/TCTA (10 nm)/emitting layer (EML, 20 nm)/TmPyPb (40 nm)/LiF (1 nm)/Al (100 nm). The EML was configured as follows: (i) Device B: mCBP : x wt% DSBO, (ii) Device G: mCBP : x wt% DSBN, and (iii) Device R: DMIC-TRZ : x wt% DSBNS. The host materials were chosen to align with the device architecture, with mCBP utilized for blue/green emitters and DMIC-TRZ for red emitters. Their energy levels, with mCBP having HOMO/LUMO levels of −6.0 eV/−2.4 eV and DMIC-TRZ having HOMO/LUMO levels of −5.76 eV/−3.08 eV, are specifically optimized to match each corresponding emitter.^49,50 The detailed EL data for all devices are presented in Fig. S17–S19 and Table S5. As shown in the Commission Internationale de l’Éclairage (CIE) chromaticity diagram (Fig. 5b), devices B, G, and R achieved blue (0.13, 0.30), green (0.13, 0.71), and red (0.67, 0.32) coordinates, respectively. The CIE color coordinates of devices G and R closely approach the National Television Standard Committee (NTSC) gamut standards for green and red emissions. With increasing doping concentrations from 5 to 50 wt%, the devices based on DSBO, DSBN, and DSBNS exhibit nearly invariant EL characteristics, maintaining stable emission peaks at 480, 510, and 628 nm with almost constant FWHM values of ∼28, ∼25, and ∼58 nm, respectively (Fig. 5c–e). This exceptional spectral stability originates from the orthogonal di-spiro π-frameworks that effectively isolate the MR cores, suppressing intermolecular interactions (particularly dimer formation) and associated concentration quenching effects. Furthermore, the 5 wt%-doped devices demonstrate optimal EL performance, achieving an EQE_max, a maximum power efficiency (PE_max), and a maximum current efficiency (CE_max) of 34.5%, 51.7 lm W⁻¹ and 55.9 cd A⁻¹ for DSBO, 40.4%, 112.8 lm W⁻¹ and 125.7 cd A⁻¹ for DSBN, and 38.0%, 43.1 lm W⁻¹ and 35.7 cd A⁻¹ for DSBNS (Fig. 5f–h and Table 2). Compared to planar-type MR-TADF molecules (Fig. S20 and Table S6), the orthogonal di-spiro π-skeleton derivatives exhibit red-shifted emission resulting from enhanced ICT and extended π-conjugation, while simultaneously achieving improved FWHM and color purity due to increased molecular rigidity.^25,51,52 Moreover, the devices maintain EQE up to 20% even at high doping concentrations (30 wt%), a performance rarely achieved in conventional MR-TADF systems.^{29–35,37–42,45} These outstanding results highlight their potential for high-resolution OLED display applications. Future work could extend this strategy to multi-boron molecular systems to further enhance color purity toward the BT.2020 standard.

Fig. 5 (a) Energy-level diagram and chemical structures of the functional layer materials for the devices based on DSBO (device B), DSBN (device G) and DSBNS (device R) as MR-TADF emitters. (b) EL images and their color coordinates in the CIE chromaticity diagram. (c)–(e) EL spectra with various doping concentrations of 5–50 wt%. (f)–(h) EQE–luminance curves.

Table 2 Summary of the EL performance of 5 wt%-doped DSBO, DSBN and DSBNS in different host materials

Emitter	L max [cd m^−2]	EQEmax^b/100^c/1000^d [%]	PEmax^b/100^c/1000^d [lm W^−1]	CEmax^b/100^c/1000^d [cd A^−1]	λ EL [nm]	FWHM^e [nm]	CIE^e (x, y)
a Maximum luminance. b Maximum EQE, PE and CE. c EQE, PE and CE at 100 cd m^−2 luminance. d EQE, PE and CE at 1000 cd m^−2 luminance. e EL peak, FWHM, and CIE coordinates.
DSBO	6890	34.5/19.4/8.8	51.7/25.2/9.7	55.9/32.1/14.8	480	28	(0.13, 0.30)
DSBN	22870	40.4/30.7/11.6	112.8/79.5/23.7	125.7/98.7/37.0	510	25	(0.13, 0.71)
DSBNS	15020	38.0/35.0/11.7	43.1/33.0/7.4	35.7/33.6/11.8	628	58	(0.67, 0.32)

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3. Conclusion

In summary, a novel class of MR-TADF emitters has been designed by incorporating an orthogonal di-spiro π-skeletal core that addresses the long-standing challenge of ACQ and spectral shift in high-doping conditions for narrowband emitters. Through strategic FMO engineering, ICT characteristics and extended π-conjugation were precisely modulated to achieve narrowband emission spanning the full visible spectrum. The unique steric constraints imposed by the orthogonal di-spiro architecture have been demonstrated to effectively suppress interchromophore interactions and spectral broadening. These full-color MR emitters exhibit near-unity Φ_PL (∼100%) with exceptional Θ_‖ (>85%), which collectively contribute to their exceptional device performance. The optimized red, green and blue devices achieved EQE_max values of 38.0%, 40.4%, and 34.5%, respectively, with remarkably narrow FWHMs of 58, 25, and 28 nm, representing the state-of-the-art performance for narrowband MR-TADF emitters. Furthermore, the CIE coordinates of green (0.13, 0.71) and red (0.67, 0.32) OLEDs closely approach the NTSC standard. The full-color devices exhibited almost unchanged EL spectra and high performance with increasing doping concentrations from 5 to 50 wt%. This orthogonal di-spiro π-skeleton functionality opens new opportunities in molecular design for realizing ideal high efficiency and color purity emitters.

4. Experimental section

4.1. Synthesis of molecules

All the reagents and solvents were obtained from commercial sources and directly used without any further purification. ¹H NMR and ¹³C NMR spectra were recorded with a Germany Bruker AVANCE III type NMR Spectrometer in CDCl₃ solution. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy was carried out on an UltrafleXtreme MALDI-TOF mass spectrometer with a 1 kHz smart beam-II laser. An HCT-2 instrument was employed to carry out thermal gravimetric analysis (TGA) ranging from 25 °C to 800 °C with a heating rate of 10 °C min⁻¹ under nitrogen flushing. Differential scanning calorimetry (DSC) was performed from 25 °C to 300 °C at a heating rate of 10 °C min⁻¹ under nitrogen flow. Cyclic voltammograms (CV) were obtained in dichloromethane at room temperature with a CHI600 electrochemical workstation at 25 °C and a scan speed of 50 mV s⁻¹. The electrochemical oxidation potential values were collected by cyclic voltammetry measurements via a CHI660 electrochemical workstation (Chenhua, China), and ferrocenium/ferrocene (Fc/Fc⁺) was used as the internal reference, and tetrabutylammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte. A platinum plate electrode was utilized as the working electrode, and a platinum wire was utilized as the counter electrode and Ag/AgCl as the reference electrode. The reduction potentials were calculated from E_ox−E_g, and the optical bandgaps (E_g) were estimated from the onset of the absorption spectra.

4.2. X-ray crystallography

Data collections of single crystal X-ray diffraction were performed on a Bruker D8-Venture diffractometer with a Turbo X-ray Source (Mo Kα radiation) adopting the direct drive rotating anode technique and a CMOS detector. The data frames were collected using the program APEX2 and processed using the program SAINT routine in APEX2. The structures were solved by direct methods and refined by the full-matrix least squares on F2 using the SHELXTL-2014 program. The SQUEEZE approach was not used in the final refinement.

4.3. Theoretical calculations

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed at the B3LYP/6-31G(d,p) level by using the Gaussian 09 program package. Based on the single-crystal structures and optimized geometric configurations, the energy levels, the dihedral angles of these molecules, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO), as well as natural transition orbitals (NTO) and RMSD were obtained logically.

4.4. Photophysical characterization

A Perkin–Elmer Lambda 750 UV-Vis spectrophotometer was used to record UV-vis spectra at 25 °C. Photoluminescence (PL) spectra were determined on a FM-4 type fluorescence spectrophotometer (JY company, French) at 25 °C. Phosphorescence spectra (Phos) were recorded on a FLS 920 spectrometer (Edinburgh Corporation) at 77 K. The transient PL decay curves were obtained by using a Quantaurus-Tau fluorescence lifetime spectrometer (C11367-32, Hamamatsu Photonics) under a vacuum atmosphere, where low-temperature measurements were conducted using a cryostat (Oxford Optistat DN). The photoluminescence quantum yields (PLQYs) were achieved by a C9920-02G type fluorescence spectrophotometer (HAMAMASTU, Japan), and the integrating sphere was purged with dry argon to maintain an inert atmosphere.

4.5. Device fabrication and characterization

To evaluate the EL performance of DSBO, DSBN, and DSBNS as emitter materials, we fabricated multilayered TADF OLEDs. The ITO-coated glass substrates with a sheet resistance of 15 Ω square⁻¹ were ultrasonically cleaned with acetone/ethanol and dried with nitrogen gas flow, followed by 20 min ultraviolet light-ozone (UVO) treatment in a UV-ozone surface processor (PL16 series, Sen Lights Corporation). Then the sample was transferred to the deposition system. All organic layers were deposited at a rate of 1 Å s⁻¹, and subsequently LiF was deposited at a rate of 0.1 Å s⁻¹ and then capped with Al (ca. 5 Å s⁻¹) through a shadow mask in a vacuum of 2 × 10⁻⁵ mbar. For all the devices, the emitting areas were determined by the overlap of two electrodes as 10 mm². The as-fabricated devices were measured in ambient environment without any encapsulation. Current density–voltage–luminance (J–V–L) characteristics and EL spectra of the devices were measured simultaneously with a source meter (Keithley model 2400) and a luminance meter/spectrometer (PhotoResearch PR670). The CIE 1931 colour coordinates were obtained from the EL spectra. The EQE values were calculated by assuming an ideal Lambertian emission profile, which was verified by the independent measurements of luminous flux with an integrating sphere (Hamamatsu Photonics K.K. C9920-12).

Materials' availability

All materials generated in this study are available from the lead contact without restriction.

Author contributions

Yan-Qing Li and Jian-Xin Tang supervised the whole work. Hao-Ze Li, Yu-Tao Yang and Yi-Cheng Zhao conducted the synthesis and photophysical characterizations of the materials and fabricated the OLEDs. Feng-Ming Xie conducted the theoretical calculations. The manuscript was written by Hao-Ze Li and was further revised by Yan-Qing Li and Jian-Xin Tang. All authors discussed the results and contributed to the manuscript.

Conflicts of interest

The authors declare no competing interests.

Data availability

The authors declare that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information includes the synthesis method of the compounds, the characterization of electrochemical and photophysical properties, and crystal structures, the theoretical calculations of molecular geometries, the device fabrication method, and the electroluminescent data of the devices. See DOI: https://doi.org/10.1039/d5mh01864a.

Additional details that support the findings of this study will be made available by the corresponding author upon request.

CCDC 2464387 (DSBO), 2464390 (DSBN), and 2464393 (DSBNS) contain the supplementary crystallographic data for this paper.⁵³

Acknowledgements

The authors acknowledge the financial support from the Advanced Materials-National Science and Technology Major Project (2025ZD0616000), the National Natural Science Foundation of China (No. T2425024, 62274117, and 52303244), the Science and Technology Development Fund (FDCT), Macao SAR (No. 0008/2022/AMJ), the Bureau of Science and Technology of Suzhou Municipality (No. SYC2022144), the Collaborative Innovation Center of Suzhou Nano Science & Technology, and Fundamental Research Funds for the Central Universities.

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