Molecular design, synthesis and characterization
The chemical structures of the novel MR-TADF emitters, namely BNCz-NPO and BNCz-NPS, are depicted in Fig. 1. We introduced the NPO/NPS unit onto the para-position of the BNCz skeleton with respect to the B atom. NPO/NPS serves as a versatile excited-state modifier with distinct advantages. NPO/NPS exhibits moderate-to-weak electron-donating ability due to the negative inductive effect of the embedded P = O/P = S (Figure S1). We anticipate that the mild CT formed between NPO/NPS and the MR core will compete with the intrinsic short-range CT of the MR structure, leading to blue-shifted emission.34 In addition, The C-N linking style between BNCz and NPO/NPS is designed to minimize undesired π-bonding features, restricting conjugation extension and bond stretching,35 thereby facilitating narrow-spectrum blue light emission. Moreover, by leveraging the thermal pyramidal inversion behaviors commonly observed in arylphosphines,36 the incorporation of NPO/NPS is expected to enhance the degree of conformational freedom.37 The larger atomic radius of phosphorus compared to sulfur hints that the NPO and NPS units exhibit multiple metastable conformers similar to phenothiazine,38 further promoting dense excited state alignment and proper vibration modes. Under these circumstances, a significantly improved SVC-mediated RISC process can be realized, given the small energy differences with coupled vibrational modes in such a dense excited-state system with both LE and CT natures.
The synthetic routes to BNCz-NPO and BNCz-NPS are shown in Scheme S1. The double Br/Li exchange of Boc-protected bis(2-bromophenyl)amine formed a bis-lithiated species and was then treated with dichloro(phenyl)phosphane to afford dihydrophenophosphanizine. Without further purification, the dihydrophenophosphanizine intermediate was oxidized by H2O2 or S8 before Boc detachment by trifluoroacetic acid to yield NPO or NPS, respectively. Finally, BNCz-NPO and BNCz-NPS were synthesized using Hartwig-Buchwald C–N coupling reactions of a brominated BNCz derivative with NPO and NPS, respectively. The target materials were purified by column chromatography and temperature-gradient vacuum sublimation and were characterized by 1H/13C/31P NMR, high-resolution mass spectroscopies, and X-ray crystallographic analyses. According to the cyclic voltammetry results (Figure S2), the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be − 5.6 and − 3.1 eV for BNCz-NPO and − 5.6 and − 3.0 eV for BNCz-NPS, respectively. BNCz-NPO and BNCz-NPS demonstrate high decomposition temperatures (5% weight loss) of 503 and 463°C (Figure S3), respectively, without glass transition points (scanning room temperature to 350°C), thermally stable for the fabrication of vacuum-deposited OLEDs.
Crystallographic analysis
The influences of NPO/NPS substituent on molecular conformation and intermolecular interaction are displayed in single-crystal structures. We obtained two distinct types of single crystals for BNCz-NPO and BNCz-NPS, designated as BNCz-NPO-α/BNCz-NPO-β and BNCz-NPS-α/BNCz-NPS-β, by Employing an anti-solvent vapor diffusion method with various solvent systems. The crystallographic data are summarized in Table S1-S2. The polymorphism of BNCz-NPO and BNCz-NPS confirms their conformational flexibility under ambient conditions. As shown in Fig. 2, the N–P–C angles are 116.08°, 120.01°, 115.60° and 124.97° for BNCz-NPO-α, BNCz-NPO-β, BNCz-NPS-α, and BNCz-NPS-β, respectively. Notably, substantial differences exist in the dihedral angles between the C1–N–C4 and the C2–P–C3 planes (θ). For instance, BNCz-NPO-β exhibits a larger θ of 11.70° when compared with BNCz-NPO-α (0.92°). As sulfur is more sizable than oxygen, BNCz-NPS is expected to exhibit greater conformational variation than BNCz-NPO. Compared to BNCz-NPO, BNCz-NPS shows larger distortion with θ of 3.58° and 22.37° for its α and β type crystals, respectively. The existence of diverse conformers could help to promote multiple excited states and enrich vibration modes in accordance, further supporting our design principle. Equally important, unlike phenothiazine derivatives showing a distorted boat-chair conformation,38 the more crowded configuration of phosphine oxide/sulfide imparts a nearly coplanar geometry to the heterocycle skeleton of NPO (NPS). This characteristic may suppress high-amplitude structural deformation and vibration, mitigating potential spectral broadening. Moreover, the bulky NPO/NPS, with significant steric hindrance, induces a nearly orthogonal conformation with an MR-NPO/NPS dihedral angle exceeding 80°, effectively suppressing strong interchromophore interactions (Figure S4).
Theoretical investigation
Theoretical calculations based on density functional theory (DFT) have been conducted to elucidate the electronic structures. Geometry optimizations in the ground states were adapted from crystallographic data. As illustrated in Figure S5, the LUMOs of BNCz-NPO and BNCz-NPS closely resemble that of BNCz, exhibiting a regular distribution in accordance with the MR effect. However, their HOMO distributions exhibit variations between cases. The HOMOs of BNCz-NPO-α, BNCz-NPO-β, and BNCz-NPS-β align with BNCz’s HOMO distribution, while that of BNCz-NPS-α delocalizes to the NPS unit. Despite its minimal impact on HOMOs, the electron-donating effect of NPO/NPS causes the HOMO-1 to shift from the MR core to the NPO/NPS units. This underscores the notable influence of conformational changes on electronic configuration imparted by the CFDI strategy.
Natural transition orbital (NTO) analyses were conducted to provide insights into the excited-state nature. As shown in Fig. 3, the S1 and T1 states of BNCz-NPO and BNCz-NPS are localized on their MR core, these excited states can be assigned as originating from LE characters (1LE and 3LE). They also exhibit high oscillator strength values of up to ~ 0.4 owing to the great electron − hole overlapping within the MR chromophore. Meanwhile, the S2 states of BNCz-NPO and BNCz-NPS display prominent long-range CT characters, mixing with NPO (NPS) and MR characters. Such excited state characteristics are in contrast to the BNCz, in which a predominating LE excited-state feature has been reported.11 The triplet harvesting of MR-TADF emitters via the SVC mechanism is considered inefficient because of the vast energy gaps between excited states.29 In contrast, the T2 – T4 states of BNCz-NPO and BNCz-NPS are close lying-in energy and exhibit obvious long-range CT or hybridized local and charge-transfer (HLCT) characteristics. With the different conformers having subtle differences in excited-state energies, BNCz-NPO and BNCz-NPS actually demonstrate triplet excited-state ligaments with high density. In addition, the conformational-flexible NPO/NPS would provide appropriate vibrational overlap between the nearly degenerate excited states that facilitate reverse internal conversion. In this case, even though the same spatial orbital occupation (LE) of the S1 and T1 excitations make direct SOC vanished, the close-lying upper CT (or HLCT) excited states of BNCz-NPO and BNCz-NPS provide a feasible channel for efficient RISC, in which the higher-order SOC involving 3LE (or 3CT) and 1CT (or 1LE) in thermal equilibrium is crucial.39 Taking BNCz-NPS-α as an example, its T3 (3LE)/T4 (3CT) states and S1 (1LE)/S2 (1CT) are close in energy, where second-order SVC is valid. The SOC matrix element values of < S1|ĤSOC|T3 > and < S1|ĤSOC|T4 > were calculated to be 5.30 and 1.47 cm–1, respectively, over 0.83 cm–1 of < S1|ĤSOC|T1> (direct SOC). These SOC values notably surpass those of most MR-type TADF emitters.40
Photophysical properties
The absorption and photoluminescence (PL) spectra of BNCz-NPO and BNCz-NPS in 10− 5 M toluene are shown in Fig. 4a. BNCz-NPO and BNCz-NPS exhibit intense short-range CT absorption bands peaking at ca. 462 nm. Compared to the case of BNCz (λabs = 468 nm, Figure S6), hypochromatic shifts in absorption were observed. Following this trend, BNCz-NPO and BNCz-NPS display shorter-wavelength blue emissions with a peak at 476 nm and 475 nm, respectively (BNCz: λem = 483 nm), resulting from the reduced short-range CT characteristic. The PLQYs of BNCz-NPO and BNCz-NPS in dilute toluene solution are as high as 98.3% and 91.8%, respectively. It is worth noting that the PL spectra of the new MR-TADF emitters are unstructured, with a very narrow FWHM of ca. 20 nm for BNCz-NPO and BNCz-NPS. Such small FWHM values were roughly the same as the BNCz MR-core (23 nm) without spectral shoulders, reflecting that the NPO/NPS subunit does not add detrimental vibrations. Polarity-sensitive long-range CT character involvement in the S1 state often impairs emission color purity.18 Negligible solvatochromism was observed in absorption (Figure S7) and PL spectra (Figure S8), implying weak long-range CT characters in the ground and S1 states. The emission peak redshifts from nonpolar n-hexane to polar acetonitrile are 17, 13, and 13 nm for BNCz, BNCz-NPO, and BNCz-NPS, respectively (Table S3), indicating reduced short-range CT character upon NPO/NPS modification, which is in good accordance with the theoretical results.
Table 1
Photophysical data and kinetic parameters of BNCz-NPO and BNCz-NPS in toluene (1 × 10− 5 M).
Dopant | λabsa [nm] | λemb [nm] | FWHMc [nm/eV] | ΔESTd [eV] | PLQYe [%] | τPFf [ns] | τDFg [µs] | krh [107 s− 1] | knrh [106 s− 1] | kISCh [107 s− 1] | kRISCh [105 s− 1] |
BNCz-NPO | 463 | 476 | 20/0.10 | 0.15 | 98.3 | 4.8 | 13.9 | 6.5 | 2.0 | 10.4 | 2.25 |
BNCz-NPS | 462 | 475 | 21/0.11 | 0.14 | 91.8 | 4.7 | 9.7 | 5.3 | 5.9 | 15.4 | 3.71 |
a)Peak of absorption spectrum. b)Peak of fluorescence spectrum. c)Full width at half-maximum (FWHM) of fluorescence spectrum. d)S1–T1 energy gap determined from fluorescence and phosphorescence spectra at 77 K. e)Absolute photoluminescence quantum yield. f)Lifetime of prompt fluorescence, g)Lifetime of delayed fluorescence. h) Rate constants of singlet radiative decay (kr), non-radiative decay (knr), intersystem crossing (kISC), reverse intersystem crossing (kRISC).
From the fluorescence and phosphorescence spectra in toluene measured at 77 K (Figure S9), the ΔESTs of BNCz-NPO and BNCz-NPS are determined to be 0.15 and 0.14 eV, respectively. These values are sufficiently small to support the exciton upconversion from the T1 to the S1 state, indicative of TADF. PL decays of BNCz-NPO and BNCz-NPS consist of ns-scale prompt fluorescence and µs-scale delayed fluorescence components (Fig. 4b). The corresponding prompt (τPF) and delayed (τDF) lifetimes were fitted to be 4.8 ns/13.9 µs for BNCz-NPO and 4.7 ns/9.7 µs for BNCz-NPS, respectively. Compared to conventional MR-TADF emitters, τDFs of BNCz-NPO and BNCz-NPS are dramatically reduced, indicating a much more efficient RISC process. Temperature-dependent transient PL measurements (Figure S10) unambiguously confirm the involvement of triplet excitons in light emission through an endothermic RISC process. It is noteworthy that BNCz-NPO and BNCz-NPS exhibit efficient TADF in solution states without the aid of host materials, distinguishing them from most reported MR chromophores with LE-featured singlet and triplet excited states.5,12,41 The photophysical results demonstrate that the CFDI strategy not only prevents the involvement of NPO/NPS in the S1 states to retain the excellent photophysical properties of the MR core but also simultaneously induces a mild electron push-pull effect to accurately modulate long-range CT features of the high-lying singlet and triplet excited states for an allowed RISC process. The fluorescence radiative decay rate constants (kF) of BNCz-NPO and BNCz-NPS are notably high at 6.5 × 107 and 5.3 × 107 s–1, respectively (Fig. 4c, Table 1). These values exceed the corresponding knr values (2.0 × 106 and 5.9 × 106 s–1), indicating negligible energy loss during the S1→S0 transition controlled by the rigid MR core. Importantly, compared to the small kRISC of the prototypical BNCz (1.34 × 104 s–1),42 those of BNCz-NPO and BNCz-NPS are markedly increased by 16.8 and 27.7 folds, reaching 2.25 × 105 and 3.71 × 105 s–1, respectively.
Further investigation of the photophysical properties in doped films (3 wt% in 9-(2-(9-phenyl-9H-carbazol-3-yl) phenyl)9H-3,9′-bicarbazole, PhCzBCz) is detailed in Table S4. As shown in Figure S11a and b, the BNCz-NPO and BNCz-NPS films exhibit pure-blue emission, peaking at 478 and 481 nm, respectively. The FWHMs are slightly broadened to 26 nm. The doped films do not show broadband emission from excimer or exciplex. The PLQYs of the films are as high as 95.5% and 92.0% for BNCz-NPO and BNCz-NPS, respectively. The τPFs of BNCz-NPO and BNCz-NPS are 2.8 and 3.0 ns, while their τDFs are 28.5 and 21.9 µs, respectively (Figure S11c,d). The kRISCs of BNCz-NPO and BNCz-NPS in the doped film state were estimated to be 1.6 × 105 and 2.7 × 105 s− 1, respectively. Although these values are slightly lower than those measured in solution due to the reduced effectiveness of the SVC mechanism in the rigid solid state, they still surpass the kRISC values of conventional MR-TADF emitters, which typically fall below 105 s− 1.
It is widely recognized that introducing flexible units in a conjugated emitter typically results in pronounced structural deformation during electronic transitions, leading to spectral broadening and reduced luminescence efficiency.43 Therefore, it is intriguing to observe that incorporating the flexible NPS/NPS moiety into the MR chromophore manages to preserve the merits of narrowband emission and a high PLQY. Structural changes during the S1→S0 transitions were evaluated using root-mean-square deviations (RMSDs). The RMSDs for BNCz-NPO and BNCz-NPS were calculated to be 0.093 and 0.139 Å, respectively, compared to 0.084 Å for BNCz (Figure S12). As anticipated, the larger RMSD values originate from the fluctuation of the flexible NPO and NPS groups at the peripheral terminal. Quantitative analysis of the intramolecular motions of BNCz, BNCz-NPO, and BNCz-NPS has been conducted through reorganization energy calculation (Fig. 4d). Surprisingly, BNCz-NPO and BNCz-NPS exhibit remarkably small total reorganization energies of 425.5 and 442.7 cm–1, respectively, even surpassing the value for BNCz without NPO/NPS (559.5 cm–1). This indicates that the CFDI strategy may increase the number of vibration modes, but the effect on the total reorganization energies is limited since the triphenylphosphine backbone of NPO/NPS is immobilized by a C–N–C locking and strong intramolecular motions of Ph-P = O(S) units (refer also to Figure S12). The flexibility introduced by the NPO/NPS modification increases the share of low-frequency modes related to bond angle changes from 37.8% (BNCz) to 44.8%/43.7%. In contrast, high-frequency modes associated with changes in bond length, which are closely linked to spectral broadening and the appearance of shoulder peaks, are effectively suppressed.
The emission spectra of BNCz-NPO and BNCz-NPS were simulated by the Frank-Condon analysis for S1→S0 transition, and Huang–Rhys factors (S) of the vibrational modes were calculated to elucidate the spectral progression. The simulated emission wavelengths and profiles are in accordance with the experimental results (Figure S13). The principal vibrational modes of BNCz-NPO and BNCz-NPS are identified at frequencies of 11.49 and 3.38 cm− 1, respectively, primarily arising from the twisting and rotation vibrations of the NPO and NPS peripheral units. A more detailed inspection might even allow suggesting that the additional vibrations introduced by NPO/NPS are predominantly located in the low-frequency region with wavenumbers less than 250 cm–1, while the high-frequency vibrational modes are limited (refer to Figure S14 and Table S5). The restrained high-frequency stretching vibrations, coupled with the structural reorganization between S0 and S1, contribute to the preservation of small overall reorganization energies responsible for the ultrasmall FWHM values. It has been emphasized that concurrently enhancing low-frequency vibrations and reducing high-frequency vibrations is pivotal for achieving narrow-spectrum emission in organic emitters.43 The experimental and calculation results collectively illustrate that the CFDI strategy can not only enrich the excited states and vibration modes for realizing efficient SVC-mediated RISC, but also retain highly efficient narrow-spectrum emissions.
Electroluminescence properties
To evaluate EL properties of the proposed emitters, a set of OLEDs were first prepared with a structure of indium tin oxide (ITO)/dipyrazino[2,3-f:2′,3′-h] quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN, 5 nm)/4,4'-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC, 50 nm)/ 4,4',4''-tris(carbazol-9-yi)triphenylamine (TCTA, 5 nm)/1,3-bis(carbazol-9-yl)benzene (mCP, 5 nm)/ 1–5 wt% BNCz-NPO or BNCz-NPS: PhCzBCz (EML, 20 nm)/2,8-bis(diphenyl-phosphoryl)-dibenzo[b,d]furan (PPF, 5 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB, 30 nm)/LiF (1 nm)/Al (120 nm), as displayed in Fig. 5a. The EL performances are depicted in Fig. 5, and relevant key parameters are summarized in Table 2. The 1 wt% doped OLEDs based on BNCz-NPO and BNCz-NPS respectively exhibit pure-blue emission at 478 and 474 nm with an FWHM of ~ 26 nm (Fig. 5b), and the corresponding Commission Internationale de I’Éclairage (CIE) coordinates are (0.11, 0.17) and (0.11, 0.14). This human-friendly blue light, with minimal photon energy in wavelengths < 455 nm, aligns with the Bio-Blue display concept proposed by Samsung,44 reducing the risk of retinal damage (blue light hazard).45
All devices exhibited an onset voltage of approximately 3.2 V (Von at 1 cd m–2), indicative of efficient carrier injection and transport (Fig. 5c). A marginal redshift in the emission maxima was observed with an increase in doping concentration, while the FWHM remained nearly unchanged, owing to the bulky orthogonal molecular geometries of BNCz-NPO and BNCz-NPS. Both emitters achieved their optimal EL performance at a 3 wt% doping concentration. The EQEmax of the BNCz-NPO-based device is as high as 32.1%, while that of the BNCz-NPS-based device is 29.6% (Fig. 5d). The performances of the reported blue MR-TADF OLEDs are summarized in Table S6, and the EQEmax of 32.1% is among the highest values for all non-sensitized blue MR-TADF OLEDs.46–48 More importantly, these OLEDs exhibit much-reduced efficiency roll-offs under high exciton density. The EQEs of BNCz-NPO and BNCz-NPS maintain a high level of 21.7% and 23.5% (EQE100) at a display relevant luminance of 100 cd m–2, over 12.9% of BNCz without functionalization.49 The alleviated efficiency roll-offs in our devices can be attributed to the enhanced RISC of BNCz-NPO and BNCz-NPS, effectively mitigating triplet-involved exciton quenching.
Table 2
Summary of the EL performances based on BNCz-NPO and BNCz-NPS.
Emitter | Conc. [wt%] | Vona) [V] | Lmaxb) [cd m− 2] | CEmaxc) [cd A− 1] | PEmaxd) [lm W− 1] | EQEe) [%] | λELf) [nm] | FWHMg) [nm] | CIEh) [x, y] |
BNCz-NPO | 1 | 3.2 | 10510 | 32.6 | 32.0 | 30.4/22.2/12.1 | 478 | 25 | (0.11, 0.17) |
BNCz-NPO | 3 | 3.0 | 10960 | 43.0 | 45.0 | 32.1/21.7/9.7 | 480 | 26 | (0.11, 0.22) |
BNCz-NPO | 5 | 3.0 | 10170 | 40.5 | 42.4 | 27.8/19.5/7.8 | 480 | 26 | (0.11, 0.25) |
BNCz-NPO (HF-I) | 3 | 2.8 | 22660 | 40.6 | 45.5 | 30.2/28.0/20.7 | 480 | 26 | (0.12, 0.22) |
BNCz-NPO (HF-II) | 3 | 2.8 | 13850 | 44.7 | 50.1 | 37.6/30.2/12.0 | 480 | 26 | (0.11, 0.18) |
BNCz-NPS | 1 | 3.2 | 4906 | 23.7 | 23.3 | 28.5/20.0/10.8 | 474 | 26 | (0.12, 0.14) |
BNCz-NPS | 3 | 3.2 | 6491 | 30.6 | 30.0 | 29.6/23.5/13.5 | 476 | 26 | (0.11, 0.16) |
BNCz-NPS | 5 | 3.2 | 6154 | 30.3 | 29.7 | 27.4/23.3/12.6 | 478 | 26 | (0.11, 0.17) |
BNCz-NPS (HF-I) | 3 | 3.0 | 26330 | 50.1 | 52.4 | 31.9/31.6/26.4 | 480 | 30 | (0.14, 0.24) |
BNCz-NPS (HF-II) | 3 | 3.0 | 14640 | 40.2 | 39.5 | 32.2/29.2/17.3 | 476 | 30 | (0.13, 0.18) |
a) Voltage measured at 1 cd m− 2; b) maximum luminance; c) maximum current efficiency; d) maximum power efficiency; e) external quantum efficiency at maximum, 100, 1000 cd m− 2, respectively; f) EL peak; g) full width at half maximum of EL spectrum; h) recorded at 4 V.
Hyperfluorescence (HF) OLEDs using 2,3,4,5,6-pentakis-(3,6-di-tert-butyl-9H-carbazol-9-yl) benzonitrile (5TCzBN) as a TADF sensitizer in EML were fabricated to further optimize the EL performances.50 Great overlap between the PL of 5TCzBN and absorption of BNCz-NPO/BNCz-NPS guarantees efficient Förster energy transfer (Figure S15). The OLEDs were constructed with the configuration of ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/mCP (5 nm)/3 wt% BNCz-NPO or BNCz-NPS: 10 wt% 5TCzBN: PhCzBCz (20 nm)/PPF (5 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al (120 nm) (HF-I type). The EL characteristics are depicted in Figs. 6a and S16, with key parameters summarized in Table 2. Compared with the non-sensitized devices, the efficiency roll-offs of the devices are significantly suppressed. Impressively, the EQE retained as high as 31.6% and 26.4% at high luminance of 100 and 1000 cd cm–2, respectively, for BNCz-NPS (Fig. 6a). Given the relatively high concentration of BNCz-NPO/BNCz-NPS in the codoped EML, which could induce unfavorable Dexter energy transfer between TADF sensitizers and emissive dopants, further optimization was performed using an interlayer sensitization structure with a more efficient TADF sensitizer 9-(5′-(4,6-diphenyl-1,3,5-triazin-2-yl) [1,1′:3′,1′′-terphenyl]-2′-yl)-3,6-diphenyl-9H-carbazole (PPCz-Trz).51 Another set of HF OLEDs was fabricated with a device configuration of ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/3 wt% BNCz-NPO (BNCz-NPS): PhCzBCz (10 nm)/10 wt% PPCz-TRZ: PPF (2 nm)/PPF (5 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al (120 nm) (HF-II). The device structure and characteristics are shown in Figure S17. In HF-II OLEDs, exciton quenching is alleviated as the EML and the sensitizing layer are separately aligned to suppress the short-range Dexter energy transfer.52 As shown in Fig. 6b and Table 2, an ultrahigh EQEmax (EQE100) was achieved with a value of 37.6% (30.2%) for the BNCz-NPO-based HF-II device with the corresponding CIE coordinates of (0.11, 0.18) and with a narrow FWHM of 26 nm.
The EL performances of representative narrow-spectrum blue MR-TADF emitters are summarized in Fig. 6c and Table S6. Notably, the highest efficiencies of narrow-spectrum blue OLEDs are based on multiple-boron MR-TADF motifs that often need intricate synthetic procedures, and only a small fraction of MR-TADF emitters meet the requirements of pure-blue narrow-spectrum emission (CIEy < 0.25, FWHM < 40 nm) and high efficiency (EQE > 30%) at the same time. The molecular design presented here, involving a simple conformationally-flexible donor modulation on a mono-boron MR core, allows our new MR-TADF emitters to fulfill these demands, enabling the development of robust pure-blue OLEDs. Moreover, BNCz-NPO OLEDs achieve record-setting efficiencies among mono-boron MR-TADF OLEDs and prove competitive with the most efficient multiple-boron counterparts.53,54