Azaborine as a Versatile Weak Donor for Thermally Activated Delayed Fluorescence

Extensive research has been devoted to the development of thermally activated delayed fluorescence emitters, especially those showing pure-blue emission for use in lighting and full-color display applications. Toward that goal, herein we report a novel weak donor, 1,4-azaborine (AZB), with complementary electronic and structural properties compared to the widely used dimethylacridan (DMAC) or carbazole (Cz) donors. Coupled with a triazine acceptor, AZB-Ph-TRZ is the direct structural analogue of the high-performance and well-studied green TADF emitter DMAC-TRZ and has ΔEST = 0.39 eV, a photoluminescence quantum yield (ΦPL) of 27%, and λPL = 415 nm in 10 wt % doped mCP films. The shortened analogue AZB-TRZ possesses red-shifted emission with a reduced singlet–triplet gap (ΔEST = 0.01 eV) and fast reverse intersystem crossing (kRISC of 5 × 106 s–1) in mCP. Despite a moderate ΦPL of 34%, OLEDs with AZB-TRZ in mCP showed sky-blue emission with CIE1931(x,y) of (0.22,0.39) and a maximum external quantum efficiency (EQEmax) of 10.5%. Expanding the chemist’s toolkit for the design of blue donor–acceptor TADF materials will enable yet further advances in the future, as AZB is paired with a wider range of acceptor groups.


■ INTRODUCTION
Flat panel displays and lighting based on organic light-emitting diodes (OLEDs) show high efficiencies, are ultrathin, lightweight, and flexible, and can exhibit desirable saturated color coordinates including true black. 1−4 A decade ago, thermally activated delayed fluorescence (TADF) was reintroduced to the organic semiconductor community by Adachi and coworkers, who demonstrated its great potential in harvesting nonemissive triplet excitons and produced OLEDs with efficiencies comparable to those of commercialized versions using organometallic phosphorescent emitters. 5,6 Indeed, TADF OLEDs can achieve up to 100% internal quantum efficiency (IQE), as the emitter is capable of harnessing all the singlet and triplet excitons and converting these to light via emission from the singlet excited state.
In twisted donor−acceptor (D-A) materials, TADF is activated when there is a sufficiently small energy gap (ΔE ST ) between the lowest-lying singlet (S 1 ) and triplet (T 1 ) states, which is achieved by spatially separating the electron densities of the HOMO and LUMO of the molecule on the donor and acceptor moieties, respectively. In addition to the conformation adopted, the electron-donating/-accepting strength of the donor/acceptor moieties strongly influences the degree of localization of the frontier molecular orbitals and hence the emission color of the resulting charge-transfer (CT) emission. 7−9 Blue emitters require a large HOMO−LUMO energy gap, which necessitates the use of both weak donor and acceptor groups. 10,11 A consequence of the combination of weak donor and acceptor groups is extremely weak excited state conjugation across the D-A molecule, which together with the near-perpendicular D-A geometries further hinders the S 1 → S 0 oscillator strength. Simultaneously, low CT character in these excited states can hinder the spin-vibronic coupling mechanism that drives reverse intersystem crossing (RISC). 12,13 Hence, blue D-A TADF emitters can often suffer from low photoluminescence quantum yields (Φ PL ) and sometimes display minimal or even no TADF due to an imbalance in donor and acceptor strengths. 10 As an illustrative example, D-A materials comprising a benzonitrile acceptor and dimethylacridan (DMAC) as the donor display strong (yet green) TADF, 14,15 while analogues using the weaker donor carbazole (Cz) only exhibit blue room-temperature phosphorescence. 16 Therefore, for the continued development of blue TADF emitters, a wider selection of appropriate donors and acceptors is critical.   derivatives are suitably weak to be used in blue TADF emitters ( Figure 1), with Cz alone of these being suitable for deep-blue or UV-emissive D-A TADF emitters. 17−19 However, the compact size of Cz results in compounds adopting less-twisted conformations, leading to emitters with undesirably large ΔE ST and subsequently weak or even inactive TADF. Indeed, the D-A dihedral angles of Cz-containing TADF materials are significantly impacted by their steric environment, for example taking on significantly more planar conformations when attached to compact heterocycles such as pyridine, pyrimidine, or pyrazine, in contrast to more twisted conformations when attached to phenylenes. 20,21 Cz-containing TADF emitters have also been shown to form persistent dimer states that can be detrimental to emission color, color purity, and Φ PL . 22 In contrast, the larger 6-membered central ring of DMAC results in near-perpendicular conformations that are less sensitive to the steric environment, 14,23 although it is simultaneously a much stronger electron donor. 24 It would therefore be appealing to design weak donors akin to Cz but that share the useful steric properties associated with DMAC. 9 Previously we have reported a CF 3 -substituted DMAC derivative that is a weaker electron donor due to inductively electron-withdrawing trifluoromethyl groups, which yielded a blue-shifted emission compared to the reference DMAC-based material; however, this emitter was electrically unstable in an OLED. 9 Here, we envision an embedded boron atom within the donor acting as an electron-withdrawing group, which will similarly subdue the electron-donating strength of the corresponding 1,4-azaborine (AZB).
Park et al. 25 were the first to report the use of AZB as an acceptor group in a D-A TADF emitter. MFAc-AzB exhibits an emission maximum, λ PL , of 467 nm and a Φ PL of 99% in a 20 wt % PPF (PPF: 2,8-bis(diphenylphosphine oxide)dibenzofuran) doped film, with an ΔE ST of 0.24 eV and a corresponding delayed lifetime (τ d ) of 91 μs (Figure 2a). The OLED with MFAc-AzB showed a maximum external quantum efficiency (EQE max ) of 18.2% at 473 nm. 25 Wu et al. nicely demonstrated the impact of donor strength and torsion angle on TADF properties in a series of D-A-D materials with AZB acting as the acceptor. 26 Although CzAZB exhibited a Φ PL of 99%, it did not show TADF, as the ΔE ST value of 0.31 eV was too large. This is likely due to the too-planar conformation adopted by the donor moiety, leading to too-strong electronic coupling between the donor and acceptor moieties. However, the use of tetramethylcarbazole (tmCz) and DMAC produced compounds with significantly more twisted conformations, leading to reduced ΔE ST values of 0.26 and 0.11 eV for tmCzAZB and dmAcAZB, respectively, and associated Φ PL values of 56% and 95%, respectively, in 10 wt % doped films in 1,3-bis(N-carbazolyl)benzene (mCP). The OLEDs with tmCzAZB and dmAcAZB showed EQE max values of 12.4% and 29.9%, respectively, at associated λ EL values of 464 and 469 nm (Figure 2a). These and similar studies again highlight the appeal of accessing a weak donor that can enable a blue-shifted emission akin to Cz, but also featuring the useful steric properties associated with DMAC. 27 In the aforementioned examples AZB was employed as a weak acceptor group, and although azaborine itself has been reported as a fragment in the context of multiresonant TADF emitter design (MR-TADF), 28 to date its alternate use as an electron donor in conjunction with an appropriately matched electron acceptor has been unexplored. 11 Here we have hypothesized and subsequently verified both theoretically and experimentally that the electron-deficient boron atom within AZB indeed decreases the donating strength of this donor relative to DMAC, while the size of this donor ensures the desired near-orthogonal conformation in D-A TADF emitters.
Two compounds were targeted (Figure 2b), one with AZB directly coupled with chlorodiphenyltriazine (AZB-TRZ) and the other coupled to 4-bromophenyldiphenyltriazine (AZB-Ph-TRZ). These materials have been investigated and their properties and devices contrasted to the literature green TADF emitter DMAC-TRZ (Figure 2b), 29 revealing the differences between the AZB and DMAC donors. Note that the analogous Cz-Ph-TRZ ( Figure 5) is not TADF-active 30,31 and was not investigated. AZB-Ph-TRZ and AZB-TRZ are TADF-active although to significantly different degrees and show deep-blue to sky-blue emission at λ PL values of 415 and 495 nm, respectively, in 10 wt % doped mCP films. OLEDs using the mCP host showed EQE max values of 2.1% and 10.5%, with emission Commission Internationale de l'Éclairage, CIE 1931 (x,y), coordinates of (0.17, 0.14) and (0.22, 0.39) for AZB-Ph-TRZ and AZB-TRZ, respectively. As it shares desirable electronic and structural features with both Cz and DMAC, we anticipate that use of the AZB donor in combination with other acceptors will become widespread and enable further advances in blue TADF emitter discovery.
In addition, crystals suitable for X-ray diffraction analysis were obtained directly from the sublimed materials. In the case of AZB-TRZ, there are two independent molecules in the asymmetric unit cell. In both compounds, the sum of bond angles around the boron and nitrogen atoms of the AZB are 360°, indicating that each adopts a trigonal-planar geometry, which imparts planarity to the azaborine ring, unlike phenoxazine and phenothiazine. The AZB donor is strongly twisted about the N−C bond, with dihedral angles of 81.4(2)°a nd 82.5(2)°[87.7(2)°] in AZB-Ph-TRZ and AZB-TRZ, respectively ( Figure 3). This dihedral angle is much larger than  the corresponding angle (45.1°) in an equivalent Cz-Ph-TRZ 30 analogue and is slightly smaller than the 90°angle in the DMAC-TRZ. 34 Quantum Mechanical Calculations. We undertook a DFT study to gain an in-depth understanding of the electronic structures of AZB-Ph-TRZ and AZB-TRZ. The starting geometries of the emitters used for the DFT calculations (PBE0/6-31G(d,p) level of theory in the gas phase) were taken from the X-ray structures. The ground-state optimized geometries of AZB-Ph-TRZ and AZB-TRZ have a torsion angle of ∼90°between the AZB donor and triazine, while the same torsion angles are slightly smaller in the single-crystal structures, which is attributed to packing forces. The other geometric parameters (bond lengths and angles) of the optimized structures closely match those found in the crystal structures (Figures S18 and S19 and Tables S1 and S2).
Excited-state properties were calculated using time-dependent density functional theory (TD-DFT) within the Tamm− Dancoff approximation (TDA-DFT) based on the optimized ground-state geometries. 35 The molecular orbitals and their associated energy levels as well as the energies of the lowestlying excited states are shown in Figure 4. As expected from the perpendicular conformation, the HOMO of each of the two emitters is confined to the AZB moiety while the LUMO is localized on the triazine. In AZB-Ph-TRZ the LUMO is also extended onto the bridging phenylene, which forms part of the nonidentical acceptor electronic system. The HOMO and LUMO energies of AZB-Ph-TRZ are both destabilized compared to those of AZB-TRZ, and considerably more so for the LUMO, corresponding to the distinct diphenyltriazine or triphenyltriazine acceptor units.
Following from recent reports of unexpected multiple conformers in similar DMAC-based emitters, 36,37 the potential energy surfaces (PESs) for the two compounds were analyzed by performing a relaxed dihedral angle scan between the AZB donor and TRZ acceptor. The lowest energy conformer of each compound has a D-A dihedral angle of 90°(Figures S20 and S21). AZB-TRZ has another local minimum energy conformer (with bent donor, dihedral angle ∼190°on the PES). The energy barrier between these two conformers is only 0.088 eV (2.0 kcal mol −1 ), and so rapid interconversion is expected at room temperature. For the bent conformer, the HOMO is mostly located on the mesityl ring attached to the boron with minor contribution from the azaborine ring, while the LUMO is delocalized across both the triazine and azaborine groups ( Figure S21).
The same level of theory was used to additionally compare the HOMO energies of Cz-Ph-TRZ (−5.69 eV), DMAC-TRZ The calculated vertical excited singlet (S 1 ) and triplet (T 1 ) energies of AZB-Ph-TRZ are 3.03 and 2.93 eV, respectively, which are stabilized to 2.86 and 2.84 eV for AZB-TRZ. These CT states are also stabilized when stronger donors are used, as is the case with azasiline-Ph-TRZ (S 1 2.80 eV) and DMAC-TRZ (S 1 2.56 eV). The corresponding calculated ΔE ST values are 0.10 eV for AZB-Ph-TRZ and 0.02 eV for AZB-TRZ, indicating that these compounds should exhibit TADF. These ΔE ST values are similar to those calculated for azasiline-Ph-TRZ and DMAC-TRZ, which is not surprising given the similar perpendicular conformation adopted (both with ΔE ST of 0.01 eV). The ΔE ST of Cz-Ph-TRZ is much larger at 0.38 eV, the result of its less twisted structure. The estimated oscillator strengths ( f) for the S 0 → S 1 transitions (without accounting for vibronic coupling) are 0 and 0.0013 for AZB-Ph-TRZ and AZB-TRZ, respectively, reflecting the very weak electronic coupling between donor and acceptor groups, due to the near-orthogonal conformation in the compounds.
To gain greater insight into the nature of the excited states, natural transition orbitals (NTOs) were analyzed at the ground-state optimized geometries (Figure 4). Both the T 1 hole and particle are localized on the donor for AZB-Ph-TRZ, indicating localized excitonic (LE) character for this state. For the T 2 state, the hole and particle are situated on the AZB and triazine-phenylene bridge, respectively, clearly indicating CT character. For AZB-TRZ the orbital nature of T 1 and T 2 is reversed, and both molecules possess S 1 states with the expected CT character. Spin−orbit coupling (SOC) values were also calculated based on the optimized T 1 geometries. Due to the difference in orbital type between the triplet states and S 1 , the SOC is considerably larger for T 1 (with ⟨S 1 |ĤS OC | T 1 ⟩ = 0.72 cm −1 ) than for T 2 (with ⟨S 1 |ĤS OC |T 2 ⟩ = 0.01 cm −1 ) in AZB-Ph-TRZ, reflecting El-Sayed's rule for direct RISC between T 1 and S 1 . 39 AZB-TRZ behaves in a complementary For AZB-Ph-TRZ, T 1 and T 2 lie just below the S 1 state, with the energy differences between S 1 -T 1 , S 1 -T 2 , and T 2 -T 1 being 0.10, 0.01, and 0.09 eV, respectively. These energy gaps are sufficiently small to support efficient RISC through the spin-vibronic coupling between T 2 and T 1 states to the S 1 state. 40 In the case of AZB-TRZ, T 2 is slightly higher in energy than S 1 and the energy differences between S 1 -T 1 , S 1 -T 2 , and T 2 -T 1 are 0.02, −0.09, and 0.11 eV, respectively. A similar spin-vibronic mechanism is therefore also expected in this compound to support efficient RISC.
Electrochemistry. Cyclic and differential pulse voltammetry (CV and DPV) in dichloromethane were conducted to ascertain experimentally the HOMO and LUMO levels ( Figure  6). At a scan rate of 100 mV s −1 , both compounds showed reversible reduction waves and irreversible oxidation waves. The oxidation potentials obtained from the maxima of the DPVs are 1.46 and 1.51 V, while the reduction potentials are at −1.65 and −1.55 V for AZB-Ph-TRZ and AZB-TRZ, respectively. The HOMO levels were thus estimated to be −5.79 and −5.85 eV and the LUMO levels were 2.69 and 2.79 eV, respectively. The estimated redox gaps of 3.10 and 3.06 eV reflect the calculated trends in the HOMO−LUMO gaps of 3.68 (AZB-Ph-TRZ) and 3.59 eV (AZB-TRZ). In comparison, the reported oxidation potentials of Cz-Ph-TRZ (also known as Cz-TRZ 27 in the literature), DMAC-TRZ, 41 and azasiline-Ph-TRZ (also known as DTPDDA 38 in the literature, Figure 5) are 1.33, 0.98, and 1.14 V, respectively, which are all significantly cathodically shifted with respect to the two AZB-containing compounds and clearly evidence the weak nature of the azaborine donor and its poor conjugation with the rest of the compound (Table 1).
Photophysical Properties. Progressing to the optical properties of AZB-Ph-TRZ and AZB-TRZ, the absorption and emission spectra in toluene are shown in Figure 7a. Absorption maxima at 389 nm (ε = 22 × 10 3 M −1 cm −1 ) and 382 nm (ε = 16 × 10 3 M −1 cm −1 ) for AZB-Ph-TRZ and AZB-TRZ, respectively, are assigned to π → π* transitions on the AZB fragment, as they match the absorption spectrum of the AZB donor itself ( Figure S22). CT absorption bands are practically absent due to the strong electronic decoupling of donor and acceptor moieties in the ground state, although there is a noticeable low-energy foot in the absorption spectrum of AZB-TRZ (Figure 7a), which we suggest arises from the greater electronic coupling in this compound due to the reduced distance between the donor and acceptor moieties, a picture that aligns with the DFT study. 45,46 In toluene AZB-Ph-TRZ emits at λ PL = 400 nm along with a weak shoulder band around 417 nm, indicating emission from an LE state that matches that of the donor ( Figure S22). AZB-TRZ instead exhibits a broad and red-shifted CT emission at λ PL = 494 nm. In toluene, Cz-Ph-TRZ, DMAC-TRZ, and azasiline-Ph-TRZ emit at 422, 498, and ∼460 nm, respectively, all of which are red-shifted compared to the λ PL of AZB-Ph-TRZ. 27,34,38 TD-DFT calculations on the S 1optimized geometries predict λ PL of 472 nm (f = 0.000) and 573 nm ( f = 0.001) for AZB-Ph-TRZ and AZB-TRZ in the gas phase, respectively, which align with these experimental results. The emission spectra of AZB-Ph-TRZ and AZB-TRZ both show typical bathochromic shifts with increasing solvent polarity (positive solvatochromism), indicating that the S 1 state is CT in nature in sufficiently polar media ( Figure S23). Comparing the emission onset wavelengths of the structurally analogous series in more polar THF solvent (required to elicit only CT emission), we identify that Cz-Ph-TRZ has the shortest wavelength (highest energy) onset at 400 nm, followed by AZB-Ph-TRZ (425 nm) and then azasilane-Ph-TRZ (450 nm). 27,38 This comparison of emission onsets largely reveals the order of donor strengths, although the distinct donor steric environment and consequently lower CT  character of Cz-Ph-TRZ makes it less sensitive to host polarity and blue-shifts its emission more than might be expected based on the strength of the donor from calculations alone ( Figure  1). Similar comparisons of reported emission onsets between azasilane-Ph-TRZ and DMAC-TRZ in DCM place the donor strength of DMAC as yet stronger than azasilane. The Φ PL values of AZB-Ph-TRZ and AZB-TRZ in degassed toluene solutions are 48% and 13%, respectively, which decreased to 40% and 6% upon exposure to oxygen. The time-resolved emission decay for AZB-Ph-TRZ consists of only a prompt component (∼4 ns lifetime), consistent with its LE emission character and oxygen-insensitive Φ PL in this solvent. AZB-TRZ in toluene instead decays with biexponential kinetics, with a prompt lifetime, τ p , of 28 ns (60%) and a very short delayed lifetime, τ d , of 117 ns (40%). Upon exposure to oxygen, the τ p shortened to 13 ns (62%) while the τ d decreased to 52 ns (38%) ( Figure S24). Because the delayed lifetime for AZB-TRZ is unusually short and is accompanied by a very low Φ PL , this implies significant nonradiative decay in solution as τ d and Φ PL increase significantly in rigid hosts. Measurements in rigid hosts already reveal very small ΔE ST values, and so TADF should remain comparably efficient. We thus hypothesize that one possible source of the additional nonradiative decay is the conformational interconversion between the bent and the orthogonal conformers in the excited states of AZB-TRZ.
Transitioning to solid hosts, the PL spectra ( Figure 7b) and decays (Figures 7c,d) of AZB-Ph-TRZ and AZB-TRZ were investigated in Zeonex and two high triplet energy OLEDcompatible hosts with contrasting polarity: mCP and DPEPO. In 1 wt % doped Zeonex films, AZB-Ph-TRZ at 300 K shows very long-lived and weak delayed fluorescence (DF), having two main components with the same spectrum across all of the DF lifetime in addition to the short-lived prompt fluorescence (PF, individual spectra shown in Figure S26). The emission is itself very narrow, indicative of 1 LE excitons as was observed in toluene. In comparison, AZB-TRZ shows much stronger DF with considerably shorter DF lifetimes, along with a longerlived PF (individual spectra shown in Figure S27 and fitted exponential lifetimes and rate constants in Table S3). The DF for AZB-TRZ in Zeonex is considerably longer than in toluene solution though (τ d of 12 μs versus 117 ns), which we attribute to the host matrix preventing interconversion between the bent quenching conformer that otherwise accelerates nonradiative triplet decay.
The steady-state PL spectrum for AZB-TRZ is broad but slightly structured, while being Gaussian-shaped in the timeresolved spectra, which we interpret as a mixture of molecules in the film contributing to the steady-state emission with slightly different D-A dihedral angles: some with predominantly 1 CT DF alongside others emitting with 1 LE character PF. Slight differences in emission color and lifetimes between different molecules also explains the various red shifts/blue shifts of the overall emission at different delay times ( Figure  S26). 47,48 The mixture of LE character emission in both materials reflects the weakness of the AZB donor as well as the nonpolar Zeonex host being unable to strongly stabilize CT states. To account for this mixture, experimental singlet energies are taken from the onsets of the relaxed PF emission spectra ( Figures S26 and S27), allowing us to avoid the LE emission component in determining these values (Table S3). As seen in Zeonex, the TADF behavior of both AZB-Ph-TRZ and AZB-TRZ varies considerably when dispersed in small-molecule hosts mCP and DPEPO. The emission decays of each emitter:host combination is strikingly different, both when comparing the two emitters in the same host and when comparing a single emitter in each of the three different environments. For both materials, the DF intensity is lower and the DF emission is longer-lived in DPEPO than in mCP. There is also a notable long-lived secondary DF component for AZB-TRZ in mCP, distinct from the initial rapidly decaying DF. We have recently demonstrated how the flexibility of some D-A bridging bonds can lead to large conformational distributions of molecular geometries in solid films, with subsets of molecules possessing different TADF properties (especially decay times). Similar to the different contributions to the steady-state emission in Zeonex, we suggest that this secondary DF component arises from a subset of AZB-TRZ molecules in an unfavorable conformation in the film, with corresponding long-lived emission. 47 To reveal the nature of the triplet states in these two derivatives, time-dependent emission measurements were performed at 80 K to measure the phosphorescence emission (PH) and determine the energy of the T 1 state. We observed a substantial red shift in the delayed emission spectra of AZB-Ph-TRZ at long delay times (∼80 ms) in 1 wt % doped Zeonex films, which we interpret as the emergence of the PH spectrum ( Figure S26). The microsecond-regime delayed emission is fully suppressed at this low temperature, consistent with a TADF mechanism. Significantly reduced delayed emission intensity was also observed for AZB-TRZ, along with PH emission at very long delay times at 80 K ( Figure  S27). The PH spectrum of AZB-TRZ is only subtly different from the emission of the PF or DF, but the emergence of the differently structured emission allows us to attribute it as belonging to an 3 LE state nonetheless. The AZB-TRZ PH can also have a higher energy onset than the steady-state PL (Figure 7b), a consequence of host polarity or polarizability red-shifting the CT-character singlet states while the LE triplet states are unimpacted. 48−50 Consistent with these decay kinetics, the experimental ΔE ST of AZB-TRZ in 1 wt % doped Zeonex films (0.01 eV, from the onsets of relaxed PF emission at RT and 80 K PH, corresponding to a 1 CT-3 LE gap), is substantially smaller than that of AZB-Ph-TRZ (0.25 eV). These ΔE ST values qualitatively reproduce the ordering of the DFT-calculated ΔE ST gaps, although they are substantially larger for AZB-Ph-TRZ. The observed k RISC ((5.7 ± 0.2) × 10 6 s −1 , from kinetic decay fitting 51 ) for AZB-TRZ indicates its potential as an emitter in OLEDs (Table S3). Similar trends in ΔE ST were found in mCP and DPEPO films, having gaps ≥0.15 eV for AZB-Ph-TRZ and ≤0.02 eV for AZB-TRZ in both hosts (Table S3). This results in a similarly large k RISC of 4.9 ± 0.3 × 10 6 s −1 for AZB-TRZ in the 10 wt % doped mCP film. Finally, the Φ PL of the films was found to vary within experimental error, in the moderate range of 30−40% for each material in the two OLED hosts (Table S3). For AZB-TRZ, the significant increase in film Φ PL compared to toluene solution again supports the hypothesis that access to the bent conformer may act as a quenching pathway, but only in fluid media where conformational interconversion is feasible.
Device Characterization. Deployed into OLEDs using a structure of ITO (anode)|NPB (HIL/HTL, 40 nm)|mCP (EBL, 10 nm)|emitter:host 10% or 20% (EML, 30 nm)|T2T (HBL, 10 nm)|T2T:LiQ 50% (EIL/ETL, 35 nm)|LiQ (0.7 nm)|Al (cathode, 100 nm), the performance of the devices with AZB-Ph-TRZ and AZB-TRZ conforms broadly to the trends established by the optical results and Φ PL values ( Figure  8). Reflecting its stronger DF and faster k RISC , AZB-TRZ in mCP achieves a higher EQE max of 10.5% and reduced efficiency roll-off compared to the device using DPEPO host. This EQE max compares favorably to the equivalent film Φ PL (34% in mCP), indicating highly efficient exciton harvesting in the emissive layer based on unassisted optical outcoupling. Increasing the emitter doping to 20 wt % from 10 wt % makes minimal difference to the electroluminescence (EL) spectrum ( Figure S30); however, although the EQE max is attenuated (6.3%), much higher luminance nearing 10000 cd m −2 can be attained. The device with AZB-TRZ in DPEPO exhibits very similar EL and the same performance trends as in mCP, although with a generally lower EQE max (8.9%) and severe efficiency roll-off, limiting maximum brightness to just below 1000 cd m −2 (Figure 8d). These results in different hosts are consistent with their similar Φ PL and worse DF emission intensity and kinetics in DPEPO.
The devices with AZB-Ph-TRZ show much lower EQE max and more severe efficiency roll-off than those with AZB-TRZ. However, the performance of the devices using AZB-Ph-TRZ varies more significantly depending on the host, in terms of the EL spectrum, EQE max , and efficiency roll-off. Based on the shapes and positions of the EL spectra, we consider the mCP:AZB-Ph-TRZ device to be emitting from the 1 LE state (as in toluene and Zeonex), which frustrates efficient triplet harvesting by RISC and leads to lower efficiency. Indeed, comparing the maximum efficiency (2.1%) with the film Φ PL (27%) reveals that triplet harvesting is hardly active in these devices, consistent with the very large ΔE ST of 0.39 eV in the mCP host. Conversely, in the DPEPO host the EL spectrum is red-shifted and broader, indicative of 1 CT emission; this switchover is attributed to the larger polarity/polarizability of the DPEPO host. 49,50,52 This difference in exciton character together with reduced ΔE ST (0.15 eV) results in more efficient RISC and a higher EQE max of 5.6% despite a lower film Φ PL of 20%. The efficiency roll-off is still considerably strong in DPEPO though, a consequence of its well-documented electrical instability. 53,54 ■ CONCLUSIONS We have introduced azaborine as a very weak donor moiety for the construction of new D-A TADF emitters. As an electron donor AZB is weak like carbazole but due to its size and shape adopts a perpendicular conformation in D-A systems similar to DMAC-containing compounds. Together, these properties lead to blue-shifted PL while retaining TADF activity. This donor can also adopt a bent conformer that quenches emission, although this is suppressed in solid hosts. In the presented example materials, AZB-TRZ exhibits overall better TADF properties and OLED performance than those of AZB-Ph-TRZ because of its smaller ΔE ST and good Φ PL . Yet greater performance in blue TADF materials may be discovered using this new donor in conjunction with other acceptors in the future. ■ EXPERIMENTAL SECTION General Methods. The following starting materials were synthesized according to literature materials: bis(2-bromophenyl)amine 55 and dimethyl mesitylboronate. 56 Air-sensitive reactions were performed under a nitrogen atmosphere using Schlenk techniques; no special precautions were taken to exclude air or moisture during workup and crystallization. HPLC analysis was conducted on a Shimadzu Prominence Modular HPLC system. HPLC traces were performed using an ACE Excel 2 C18 analytical column. Melting points were measured using open-ended capillaries on an Electrothermal 1101D Mel-Temp apparatus and are uncorrected. Highresolution mass spectrometry (HRMS) was performed at the University of Edinburgh. Elemental analyses were performed by the School of Geosciences at the University of Edinburgh.  [1,4]azaborinine (387 mg, 1 mmol, 1 equiv) in mixed solvent of dichloromethane (40 mL). ethanol (30 mL). and ethyl acetate (20 mL) was added Pd on charcoal (400 mg, 10%(w/w), 0.37 mmol of Pd, 0.37 equiv), and the resulting mixture was stirred at room temperature in the presence of hydrogen gas for 2 days. The reaction mixture was passed through a Celite bed and washed with dichloromethane. The reaction mixture was then concentrated in vacuo and purified by column chromatography on silica gel using hexane:DCM = 2:3 as eluent to afford the desired compound as a white solid. The unreacted material was recovered. Yield: 20%. R f : 0.52 (hexane:DCM = 1:1 on silica gel). Mp: 217−219°C (crystals). Lit.: 230−233°C. 57 57 General Procedure for the Synthesis of Azaborine-Based Emitters. An oven-dried Schlenk flask held under a nitrogen atmosphere was charged with dry toluene (50 mL), 10-mesityl-5,10dihydrodibenzo[b,e] [1,4]azaborinine (1 equiv), triazine derivative (1.2 equiv), XPhos (0.32 equiv), palladium acetate (0.22 equiv), and sodium tert-butoxide (3 equiv). The reaction mixture was then heated at 100°C for 12 h. After cooling, the mixture was passed through a Celite pad and concentrated in vacuo. The combined organic layer was dried with anhydrous sodium sulfate and concentrated in vacuo. The resulting mixture was purified by silica gel column chromatography to yield the desired compound. 3,phenyl)- 10-mesityl-5,10dihydrodibenzo[b,e] [1,4]azaborinine (AZB-Ph-TRZ). The quantities used for the reaction are as follows: 10-mesityl-5,10-dihydrodibenzo-[b,e] [1,4]azaborinine (500 mg, 1.68 mmol, 1 equiv), 2-(4bromophenyl)-4,6-diphenyl-1,3,5-triazine (784 mg, 2.02 mmol, 1.2 equiv), XPhos (257 mg, 0.54 mmol, 0.32 equiv), palladium acetate (83 mg, 0.37 mmol, 0.22 equiv), NaO t Bu (485 mg, 5.05 mmol, 3 equiv). White solid. Yield: 70%. Mp: 314−317°C. R f : 0.26 (hexane:DCM = 2:1, silica gel). The target compound was then purified by silica gel column chromatography (hexane:DCM = 4:1, silica gel). 1  Theoretical Calculations. All ground-state optimizations were carried out at the density functional theory (DFT) level with Gaussian16 58 using the PBE0 functional and the 6-31G(d,p) basis set. Excited-state calculations have been performed at time-dependent DFT (TD-DFT) within the Tamm−Dancoff approximation (TDA) 59,60 using the same functional and basis set as for ground state geometry optimization. Spin−orbit coupling matrix elements (ξ) were calculated based on the optimized triplet excited state geometry. Molecular orbitals were visualized using GaussView 6.0. 61 Calculations were automated using an in-house designed software package, Silico, which uses a number of third party libraries and programs, including extraction and processing of results cclib, 62 generations of 3D images VMD 63 and Tachyon. 64 Electrochemistry Measurements. Cyclic voltammetry (CV) analysis was performed on an electrochemical analyzer potentiostat, Model 620E from CH Instruments, at a sweep rate of 100 mV/s. Differential pulse voltammetry (DPV) was conducted with an increment potential of 0.004 V and pulse amplitude, width, and period of 50 mV, 0.05, and 0.5 s, respectively. Samples were prepared as DCM solutions, which were degassed by sparging with MeCNsaturated argon gas for 5 min prior to measurements. All measurements were performed using a 0.1 M DCM solution of tetra-n-butylammonium hexafluorophosphate ([ n Bu 4 N]PF 6 ]). An Ag/ Ag + electrode was used as the reference electrode, while a platinum electrode and a platinum wire were used as the working electrode and counter electrode, respectively. The redox potentials are reported relative to a saturated calomel electrode (SCE) with a ferrocenium/ ferrocene (Fc/Fc + ) redox couple as the internal standard (0.46 V vs SCE). 65 Photophysical Measurements. Optically dilute solutions of concentrations on the order of 10 −5 or 10 −6 M were prepared in spectroscopic or HPLC grade solvents for absorption and emission analysis. Absorption spectra were recorded at room temperature on a Shimadzu UV-2600 double-beam spectrophotometer with a 1 cm quartz cuvette. Molar absorptivity determination was verified by a linear regression analysis of values obtained from at least four independent solutions at varying concentrations with absorbance ranging from 0.036 to 0.173 for AZB-Ph-TRZ and from 0.034 to 0.146 for AZB-TRZ.

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For emission studies, aerated solutions were bubbled by compressed air for 5 min and spectra were taken using the cuvette for absorption analysis. Degassed solutions were prepared via three freeze−pump−thaw cycles, and spectra were taken using a homemade Schlenk quartz cuvette. Steady-state emission, excitation, and timeresolved emission spectra were recorded at 298 K using an Edinburgh Instruments FS5 instrument. Samples were excited at 340 nm for steady-state measurements and at 375 nm for time-resolved measurements. Photoluminescence quantum yields for solutions were determined using the optically dilute method in which four sample solutions with absorbances of ca. 0.11, 0.075, 0.054, and 0.029 at 358 nm were used. The Beer−Lambert law was found to remain linear at the concentrations of the solutions. For each sample, linearity between absorption and emission intensity was verified through a linear regression analysis with the Pearson regression factor (R 2 ) for the linear fit of the data set surpassing 0.9. Individual relative quantum yield values were calculated for each solution, and the values reported represent the slope obtained from the linear fit of these results. The quantum yield of the sample, Φ PL , can be determined by the equation where A stands for the absorbance at the excitation wavelength (λ exc = 358 nm), I is the integrated area under the corrected emission curve, and n is the refractive index of the solvent with the subscripts "s" and "r" representing sample and reference, respectively. Φ r is the absolute quantum yield of the external reference quinine sulfate (Φ r = 54.6% in 1 N H 2 SO 4 ). 67 The experimental uncertainty in the emission quantum yields is conservatively estimated to be 10%, though we have found that statistically we can reproduce Φ PL s to 3% relative error.
To prepare the 10 wt % doped films of emitters in a host matrix, 90% w/w (90 mg) of the host was dissolved in 1 mL of solvent and to this was added 10% w/w (10 mg) of emitter. Thin films were then spin-coated on a quartz substrate using a spin speed of 1500 rpm for 60 s. An integrating sphere (FS5) was employed for quantum yield measurements for thin film samples. The steady-state fluorescence of the doped solid-state films was measured using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. Time-resolved measurements were performed using a spectrograph (Horiba Triax) and a Stanford Computer Optics 4Picos ICCD camera, where samples were excited with a Nd:YAG laser (EKSPLA, 10 Hz, 355 nm) either under vacuum at room temperature or under a stream of dry temperature-controlled nitrogen gas (Janis VNF-100 cryostat).
Freshly evaporated devices were transferred into either a calibrated 6 in. integrating sphere in a glovebox or a calibrated 10 in. sphere under ambient conditions. Electrical properties were measured using a source meter (Keithley 2400) simultaneously with emission spectrum and intensity with a calibrated fiber coupled spectrometer (Ocean optics USB4000). In the 6 in. sphere an additional silicon photodiode was used to monitor very low luminance. All devices were evaluated at 293 K.

■ ASSOCIATED CONTENT Data Availability Statement
The research data supporting this publication can be accessed at 10.17630/e2d1fc63-f102-44ef-ba9c-10a168e9fd92.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.3c05409. 1 H and 13 C NMR spectra, HRMS, HPLC, EA, and single-crystal XRD structures (CCDC: 2219210− 2219212) of the target compounds, additional film spectra and decays, and OLED comparisons at higher loading (PDF) Optimized geometry coordinates (TXT)