B‒N covalent bond-involved π-extension of multiple resonance emitters enables high-performance narrowband electroluminescence

ABSTRACT Multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters show promise for achieving both high color-purity emission and high exciton utilization efficiency. However, their development is often impeded by a limited synthetic scope and excessive molecular weights, which challenge material acquisition and organic light-emitting diode (OLED) fabrication by vacuum deposition. Herein, we put forward a B‒N covalent bond-involved π-extension strategy via post-functionalization of MR frameworks, leading to the generation of high-order B/N-based motifs. The structurally and electronically extended π-system not only enhances molecular rigidity to narrow emission linewidth but also promotes reverse intersystem crossing to mitigate efficiency roll-off. As illustrated examples, ultra-narrowband sky-blue emitters (full-width at half-maximum as small as 8 nm in n-hexane) have been developed with multi-dimensional improvement in photophysical properties compared to their precursor emitters, which enables narrowband OLEDs with external quantum efficiencies (EQEmax) of up to 42.6%, in company with alleviated efficiency decline at high brightness, representing the best efficiency reported for single-host OLEDs. The success of these emitters highlights the effectiveness of our molecular design strategy for advanced MR-TADF emitters and confirms their extensive potential in high-performance optoelectronic devices.


INTRODUCTION
Narrowband organic emitting materials have recently gained considerable attention to meet the demands of high-definition organic light-emitting diodes (OLEDs) [1 -3 ].A promising approach involves the utilization of multiple resonance thermally activated delayed fluorescence (MR-TADF) materials [4 ].Typically, an MR-TADF emitter features a flat polycyclic aromatic structure with electron-deficient boron (B) and electron-rich nitrogen (N) atoms strategically positioned in an ortho / para manner.This configuration not only promises narrow emission spectra with high photoluminescence quantum yields ( PL s) by reducing vibronic coupling between the ground (S 0 ) and the lowest excited singlet (S 1 ) states, but also facilitates atomically distributed frontier molecular orbitals (FMOs), resulting in a reduced singlettriplet gap ( E ST ).However, creating MR-TADF materials with ultranarrow emission bands remains challenging.Moreover, MR-TADF devices have typically suffered from significant triplet-involved bimolecular quenching processes, mainly due to inefficient reverse intersystem crossing (RISC) of the emitters, leading to substantial efficiency losses at high brightness levels [5 -8 ].
In response to these challenges, recent efforts have focused on incorporating high-order B/N-based MR-TADF motifs containing multiple B atoms.This strategy has been demonstrated to further narrow the emission linewidth and reduce E ST , thereby enhancing color purity and expediting RISC for MR-TADF emitters [9 ].Such improvement is attributed to the promotion of short-range charge transfer (SRCT) and the suppression of structural relaxation and vibronic coupling within π -extended frameworks [10 ].Leveraging this strategy, a wealth of research has aimed to fine-tune the optoelectronic properties of MR emitters, resulting in impressive full-width at half-maximum (FWHM) values, excellent external quantum efficiencies (EQE), and reduced efficiency roll-off at high brightness [11 -21 ].However, the design scope has primarily revolved around double B-embedded MR-TADF structures due to the synthetic challenges associated with multi-borylated products [22 ].Thus far, there have been only a few instances of evaporation-type OLEDs based on MR-TADF emitters featuring three or more B atoms (Scheme 1 ).For instance, V-shaped and ω-shaped MR emitters containing three B atoms, namely V-DABNA-F and ω-DABNA, were reported by Hatakeyama et al ., enabling the fabrication of vacuum-deposited blue and green OLEDs with maximum EQE (EQE max ) values of up to 31.1% and very low efficiency roll-off [23 ,24 ]; linearly extended multi-borylated MR-TADF acene emitters such as α-3BNMes and NOBNacene were reported by Zysman-Colman et al ., both of which enabled deep-blue emission (CIE y ≤ 0.1) featuring high color purity [25 ,26 ].More recently, Hatakeyama et al .developed a oneshot multiple borylation method for constructing acenes possessing four to eight B atoms, delivering ultra-narrowband sky-blue emission with FWHMs as small as 12-16 nm.However, the excessively high molecular weight disabled corresponding device characterization, in spite of the quadruple-borylated derivative CzB4-oPh [27 ].For the construction of long-wavelength emitters, Zhang et al .hybridized para B-π -N and para B-π -B arranged patterns in a quadruple-borylated MR framework, affording orange-red electroluminescence with a high EQE max of 35.8% [28 ].Despite the above progress, it is clear that the conservative approach of introducing B atoms as B-C bonds limited molecular design and increased the synthesis difficulty of multi-borylated MR-TADF emitters.Therefore, it is imperative to explore alternative architectures and synthetic methodologies for incorporating B-based fragments into molecular frameworks.
In this context, we wish to present a B-N covalent bond-involved π -extension strategy that introduces three B atoms into the traditional B-C bond-based MR-TADF framework.Recently, Duan et al .demonstrated that the inclusion of easily accessible B-N/B-O bonds could induce the MR property and achieve narrowband emission with desirable PL in a series of double-boron embedded emitters [29 -31 ].In this work, the B-N bondbased fragments are installed via post-functional modification of the halogenated parent emitters DABNA and BCzBN .This approach offers high flexibility in molecular design, yielding novel highorder B/N-based MR-TADF motifs with compacted structures.The B-N segments not only function as pincers to enhance molecular rigidity, but also act as fusion units to increase electronic delocalization and reduce E ST .The proof-of-concept emitters, DABNA-3B and BCzBN-3B , featuring a compact MR-TADF framework with three B atoms and four N atoms, exhibit significant improvements across various metrics compared to their parent emitters, including increased PL , accelerated RISC rate ( k RISC ), reduced FWHM of emission spectra, and enhanced horizontal orientation factors ( // ).Consequently, OLEDs incorporating DABNA-3B and BCzBN-3B within a binary emitting system exhibit outstanding electroluminescence performance.They achieve high EQE max values of 33.8% and 42.6%, respectively, and exhibit significantly mitigated efficiency roll-offs, along with FWHMs of 25 and 22 nm.This work not only establishes a new benchmark for device performance based on a binary emitting system but also opens up innovative avenues for the design of multi-borylated materials.

RESULTS AND DISCUSSION
The synthetic routes toward the target compounds were established through sequential multiple borylations, as depicted in Fig. 1 A, Schemes S1 and S2 (refer to Supporting Information for detailed synthesis).The initial critical step involved creating a halogenated DABNA or BCzBN core through borylation-annulation reactions.Subsequently, the 2,6-bis(phenylamino)phenyl group was incorporated using consecutive Miyaura borylation and Suzuki coupling reactions.The final products, DABNA-3B and BCzBN-3B , were obtained through a single-step, lithium-free borylationannulation reaction with high yields.Both molecular structures were comprehensively characterized by nuclear magnetic resonance spectroscopy ( 1 H NMR , 13 C NMR , 1 H-1 H COSY and ROESY), high-resolution mass spectrometry (HRMS) and elemental analysis, as shown in Figs S1-S22 of the Supplementary data.The thermal properties of the emitters were investigated by using thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) ( Fig. S23), revealing high decomposition temperatures ( T d , corresponding to 5% weight loss) of 479°C for DABNA-3B and 518°C for BCzBN-3B , as well as glass transition temperatures ( T g ) above 300°C.This study presents a rare instance of post-synthetic annulative π -extension on MR frameworks, which leads to the efficient synthesis of higher-order MR emitters that are anticipated to facilitate the development of boron-doped graphenoid structures [24 ,32 -34 ].
Single crystals of DABNA-3B and BCzBN-3B were obtained through the gradual evaporation of a toluene solution at ambient temperature [CCDC 2307956, 2307957].X-ray crystallography revealed that both compounds crystallized in the triclinic P 1 space group (Fig. 1 B, Tables S1 and S2).Due to the presence of multiple [4]helicene moieties, DABNA-3B exhibited a quasi-planar configuration w ith partial tw ist at its molecular edges-the dihedral angle between the terminal phenyl rings of the N, B, N-based [4]helicene was 47.5°, while for the B, N, B-based [4]helicenes, the angles were 56.5°a nd 55.8°, respectively.According to previous studies, this twisted geometry was postulated to be conducive to an increased spin-orbit coupling (SOC) effect between singlet and triplet excited states, potentially enhancing the k RISC [17 ,35 ,36 ].Conversely, BCzBN-3B possessed a dihedral angle of only 2.7°b etween the terminal phenyl rings of its N, B, N-based [4]helicene, indicating a much flatter structure.For both emitters, the overall quasi-planar shaped molecular structures made them tend to align parallel on the substrate, which were conducive to higher // factors.In addition, they exhibited substantial dihedral angles between the suspended phenyl rings and the MR plane (70.5°-81.8°),which may weaken the intermolecular interactions between luminescent cores.As a result, we observed relatively large face-to-face distances for both DABNA-3B (4.60 Å) and BCzBN-3B (4.15 Å).Overall, these expansive intermolecular distances may reduce aggregation-induced quenching (ACQ) in the solid state for both DABNA-3B and BCzBN-3B [37 -40 ].
To get a deep insight into the structural and electronic properties of the emitters, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed at B3LYP-D3(BJ)/6-31G(d, p) level ( Tables S3-S11). Figure 2 A i l lustrates the electron distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) wave functions, which remained spatially alternative on the entire skeletons for DABNA-3B and BCzBN-3B , indicating their distinct MR feature.Since the non-bonding character hindered the effective conjugation length, the π -extension only slightly decreased the HOMO-LUMO energy gaps ( E g ) compared with those of the parent molecules (3.36 eV for DABNA-3B vs. 3.66 eV for DABNA ; 3.25 eV for BCzBN-3B vs. 3.35 eV for BCzBN ).The well-preserved short-range charge separation along with extended electronic delocalization jointly led to smaller E ST values of DABNA-3B and BCzBN-3B (0.33 and 0.31 eV) than those of parent molecules (0.49 and 0.42 eV), as revealed by TD-DFT calculations.Besides, it is noted that both DABNA-3B and BCzBN-3B presented high T 2 states lying between S 1 and T 1 states.Despite the similar orbital parentage between S 1 and T 1 , the natural transition orbital (NTO) distribution analysis suggested obvious orbital angular momentum variation in T 2 states ( Fig. S24), offering higher SOC matrix elements Therefore, the RISC should benefit from the second-order spin-vibronic coupling process.These results aligned well with recent theoretical studies that emphasized the importance of the high-lying triplet excited state in facilitating the energy upconversion of MR-TADF emitters [41 ,42 ].Notably, the < S 1 | ˆ H SOC |T 2 > values of DABNA-3B and BCzBN-3B (1.217 and 0.460 cm −1 ) were much higher than those of DABNA and BCzBN (0.910 and 0.124 cm −1 ), which would give rise to enhanced k RISC values.To further investigate the impact of B-N bondbased fusion heterocycles on the structural relaxation during the excitation-emission process, the reorganization energy ( ) of DABNA-3B and BCzBN-3B were calculated and analyzed using the Molecular Materials Property Prediction Package (MOMAP), and compared with those of DABNA and BCzBN .As shown in Fig. 2 B, the B-N bondinvolved π -extension leads to a remarkable decrease of (1117 cm −1 for DABNA-3B vs. 1939 cm −1 for DABNA ; 833 cm −1 for BCzBN-3B vs. 1032 cm −1 for BCzBN ), indicative of the suppressed geometry changes during the transitions.The relationships of w ith v ibrational modes for the S 1 → S 0 transition are further depicted in Fig. S25.Obviously, the incorporation of supplemental fusing moieties diminished the vibration strength at high-frequency regions (above 500 cm −1 ), which represents the suppressed structural deformation on a molecular level as well as atomic level.These above results indicate that the fusion of B-N bond-based heterocycles into the parent cores could effectively enhance molecular rigidity and consequently narrow the emissive bandwidth.
The photophysical properties of the emitters were examined in dilute toluene solution (1.0 × 10 −5 mol L −1 ), as shown in Fig. 3 .Compared to their respective parent emitters, the absorption spectra of DABNA-3B and BCzBN-3B exhibited more pronounced n-π */ ππ * transitions below 420 nm.However, they maintained similarly intense short-range charge transfer (SRCT) bands above 420 nm, peaking at 455 and 470 nm for DABNA-3B and BCzBN-3B , respectively.The optical band gaps ( E g ) were estimated at 2.62 eV for DABNA-3B and 2.55 eV for BCzBN-3B , based on the absorption threshold.These values were slightly lower compared to those of DABNA and BCzBN , which aligned with theoretical predictions.The fluorescence spectra showed sky-blue emissions with peak wavelengths at 470 and 482 nm for DABNA-3B and BCzBN-3B , respectively.The relatively small Stokes shifts, 15 nm for DABNA-3B and 12 nm for BCzBN-3B , suggested minimal molecular conformational relaxation between S 1 and S 0 , corroborating the calculated results mentioned above.Additionally, both DABNA-3B and BCzBN-3B exhibited extremely narrow FWHM values of 19 nm/0.11eV and 16 nm/0.09eV, respectively.These narrowed emission spectra, compared to the parent molecules (29 nm/0.17eV for DABNA and 23 nm/0.12eV for BCzBN ), implied significantly reduced structural vibrations due to enhanced rigidity.When different solvents were used, ranging from n -hexane to acetonitrile, the new emitters demonstrated similar solvatochromic effects with only slight bathochromic shifts in emission and a modest increase in FWHM, as shown in Fig. S26 and Table S12.This indicated that the introduction of the B-N bond-based fusing unit did not alter the MR characteristics.Of particular note, the spectral FWHM of BCzBN-3B in an n -hexane solution was as narrow as 8 nm/0.05eV (Fig. 3 B), representing one of the narrowest emission bands observed for pure organic emitters.
Based on the fluorescence and phosphorescence maxima at 77 K, the E ST values for DABNA-3B and BCzBN-3B were determined to be 0.13 and 0.10 eV, respectively, which were smaller than those of the parent molecules (0.15 eV for DABNA and 0.14 eV for BCzBN ).Such small E ST s, along with the potential for spin-vibronic coupling mechanisms typical of organoboron emitters, were expected to promote the exciton upconversion from T 1 to S 1 .
To delve into the solid-state photophysical characteristics, we prepared doped films by blending the emitters into a host matrix of 3,3'-bis(carbazol-9y l)bipheny l (mCBP) with an optimized doping concentration of 2 wt%.This ratio ensured efficient energy transfer while minimizing bimolecular collisions.The fluorescence spectra of these films, as depicted in Fig. S27, exhibited a modest redshift and slight broadening in FWHM, 28 nm/0.15eV for DABNA-3B and 24 nm/0.12eV for BCzBN-3B , compared to the spectra in solution, likely due to the increased polarity of the host matrix.Notably, the films containing B-N-embedded emitters demonstrated higher PL compared to their parent (94% for DABNA-3B vs 88% for DABNA ; 99% for BCzBN-3B vs 96% for BCzBN ).Such increase was attributed to reduced nonradiative decay from suppressed structural vibrations.The temperature-dependent transient photoluminescence (PL) spectra of the doped films demonstrated pronounced TADF behavior of the emit-ters, with delayed fluorescence components intensifying as temperatures rose ( Fig. S28).At 300 K, the delayed fluorescence lifetimes for DABNA-3B and BCzBN-3B were significantly reduced to 6.10 μs and 25.4 μs, respectively, compared to their parent molecules DABNA (93.7 μs) and BCzBN (67.0 μs) (Fig. 3 C, Table S13).Based on the PL s and transient PL decay profiles, the key rate constants were estimated and listed in Table 1 .The emitters displayed notably high radiative decay rates ( k r ), within the range of 3.80 × 10 7 to 1.50 × 10 8 s −1 , in alignment with the desirable attributes of MR-TADF emitters.Concurrently, the non-radiative decay rates for DABNA-3B and BCzBN-3B were substantially lower than their parent compounds, attributed to increased molecular rigidity and lower reorganization energy.More importantly, the calculated k RISC rates for DABNA-3B and BCzBN-3B were 8.63 × 10 5 s −1 and 1.46 × 10 5 s −1 , respectively.These rates signified improvements of 77-fold for DABNA-3B and 7-fold for BCzBN-3B over their parent molecules (0.11 × 10 5 s −1 for DABNA ; 0.20 × 10 5 s −1 for BCzBN ).The enhanced spin-flipping exciton conversion can be credited to the combined effect of the small E ST and large SOC effects, corroborated by theoretical predictions.Remarkably, the k RISC of DABNA-3B approached 10 6 s −1 , which surpassed the benchmark deep-blue emitter ν-DABNA (2.0 × 10 5 s −1 ) and represented one of the highest reported values for MR-TADF molecules without heavy atom effect ( Table S14) [9 ,43 ].This rapid triplet dynamics underscored the impact of molecular geometry distortion and significant SOC values, as supported by crystal structure analysis and theoretical outcomes.In Fig. 3 D, the radar diagram succinctly compared the photophysical properties of the emitters, where a more extensive outer area corresponded to better performance.The properties charted included FWHM, PL , k RISC , and // (discussed further below).The radar diagram clarified that DABNA-3B and BCzBN-3B outclassed DABNA and BCzBN in all measured aspects, confirming that the photophysical superiority was a direct result of the B-N covalent bond-involved πextension.
Benefitting from the balanced carrier transporting ability of the host materials, all the devices exhibited low turn-on voltages ( V on s, 3.0-3.2V for DABNA and DABNA-3B ; 2.4-3.2V for BCzBN and BCzBN-3B ) and fairly high luminance of over 29 0 0 0 cd m −2 .The EL spectra displayed high color stability with minimal change in the range between 3.2 V and 8.0 V ( Fig. S31), confirming the efficient host-to-emitter energy transfer and the fixation of radiative transition excitons on the emitters.Consistent with the corresponding PL spectra ( Fig. S29), the devices displayed EL peaks at 475 nm and 493 nm with FWHM of 25 nm/0.14eV and 22 nm/0.11eV for DABNA-3B a nd BCzBN-3B , respectively, narrower than their parent molecules ( DABNA , 35 nm/0.20 eV; BCzBN , 30 nm/0.15 eV).Notably, devices based on DABNA-3B and BCzBN-3B achieved high EQE max values of 33.8% and 42.6%, power efficiency (PE max ) of 37.2 and 103.8 lm W −1 , and current efficiency (CE max ) of 78.7 and 79.3 cd A −1 , respectively.These metrics indicated a significant enhancement over their corresponding parent emitters (EQE max of 25.3% and 33.0%, PE max of 33.2 and 86.6 lm W −1 , and CE max of 33.8 and 66.2 cd A −1 for DABNA and BCzBN , respectively).In particular, BCzBN-3Bbased devices portrayed the highest efficiency for TADF emitters in the binary emitting system, to the best of our knowledge ( Table S15).To gain further insight into the excellent EL performance, angle-dependent PL spectra of the EMLs were measured to probe the orientation of the emitters.As displayed in Fig. 4 G and H, the DABNA-3B and BCzBN-3B manifested higher // values than those of the parent molecules (84% for DABNA-3B vs 54% for DABNA ; 94% for BCzBN-3B vs 88% for BCzBN ).The preferential horizontal dipole orientation could be attributed to the enlarged molecular planarity ( Fig. S32) and the higher T g values that stabilized the molecular/dipole orientation during film deposition [46 ,47 ].These factors, combined with the near-unity PL (99%) of the emitter, as well as the potential sensitizing abilit y of the DMIC -TRZ host, under pinned the ex traordinary efficiency of over 40% achieved by BCzBN-3B -based devices.
Additionally, the devices incorporating DABNA-3B a nd BCzBN-3B e mitte rs de mons trated lower efficiency declines at high brightness, with EQE 10 0 0 values (EQE at 10 0 0 cd m -2 ) of 25.1% and 30.5%, respectively.This performance translated to a reduction in efficiency of 25.7% and 28.4%, contrasting sharply with the 63.6% and 60.3% reductions observed in devices with DABNA and BCzBN at the same luminance level.To elucidate the underlying mechanism of the efficiency rol l-off, we modeled the EQE-current density behavior using theories of triplet-triplet anni hi lation (TTA) and triplet-polaron anni hi lation (TPA), as shown in Fig. S33.The models highlighted TTA as a significant factor affecting the efficiency roll-off.The critical current density ( J 0 , the point at which efficiency drops to half due to TTA), was significantly higher in the DABNA-3B device (20.1 mA cm −2 ) compared to those with DABNA (4.1 mA cm −2 ).A similar trend was seen with BCzBN-3B (9.8 mA cm −2 ) vs BCzBN (2.5 mA cm −2 ).Hence, the diminished efficiency roll-off was clearly associated with the fast depletion of triplet excitons, benefitting from the efficient RISC process.These findings were a testament to the efficacy of our molecular design strategy.

CONCLUSION
In summary, we put forward a B-N covalent bondinvolved π -extension strategy to construct multi-boron-embedded MR-TADF emitters.Synthetically, our approach marks a noteworthy example of postsynthetic annulative π -extension applied to MR frameworks.In terms of emission properties, our method enhances the spin-flip process via extended electronic delocalization and achieves narrower emission linewidth due to increased molecular rigidity.Utilizing this design, the sky-blue MR-TADF emitters DABNA-3B and BCzBN-3B exhibited outstanding photophysical performance, including FWHMs narrower than the state-of-the-art quantum dots (as small as 8 nm/0.05eV in non-polar solvent), nearly perfect PL , accelerated RISC process, and predominantly horizontal orientation of emitting dipoles.The exceptional property was further evidenced by their impressive electroluminescence efficiencies and reduced efficiency roll-off in OLEDs with binary EMLs, achieving EQE max/10 0 0 of 33.8%/25.1% and 42.6%/30.5%,respectively, alongside with small FWHM values.This study not only paves the way for innovative molecular design approaches but also holds promise for further advancements in MR-TADF emitters and the continued evolution of high-performance narrowband OLEDs.

METHODS
The detailed preparation and characterization methods of materials are available as Supplementary data.

B <S 1 l 1 2<S 1 l 1 TFigure 2 .
Figure 2. (A) Distributions of FMOs with energy gap ( E g ) values, as well as singlet/triplet state energy levels and SOC matrix elements.(B) Theoretical estimation of the total reorganization energies ( ).

Table 2 .
Summary of OLED performances.Turn-on voltage at 1 cd m −2 .b Electroluminescence peak wavelength.c Full-width at half-ma ximum.d Ma ximum luminance.e External quantum efficiency, power efficiency and current efficiency: maximum, values at 100 and 10 0 0 cd m −2 . a