In twisted donor-acceptor (D-A) materials, TADF is activated when there is a sufficiently small energy gap (DEST) between the lowest-lying singlet (S1) and triplet (T1) 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 colour of the resulting charge-transfer (CT) emission. Blue emitters require a large HOMO-LUMO energy gap, which necessitates the use of both weak donor and acceptor groups. 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 S1→S0 oscillator strength. Simultaneously, low CT character in these excited states can hinder the spin-vibronic coupling mechanism that drives reverse intersystem crossing (RISC). Hence, blue D-A TADF emitters can often suffer from low photoluminescence quantum yields (FPL) and sometimes display minimal or even no TADF due to an imbalance in donor and acceptor strengths. As an illustrative example, D-A materials comprising a benzonitrile acceptor and dimethylacridan (DMAC) as the donor display strong (yet green) TADF, while analogues using the weaker donor carbazole (Cz) only exhibit blue room temperature phosphorescence. Therefore, for the continued development of blue TADF emitters, a wider selection of appropriate donors and acceptors is critical. Some of the most commonly used electron donors in TADF emitter design include Cz (EHOMO = -5.73 eV, 3,6-di-tert-butylcarbazole, dtBuCz, EHOMO = -5.47 eV), diphenylamine (EHOMO = -5.34 eV), DMAC (EHOMO =-5.13 eV), phenoxazine (EHOMO = -4.92 eV), phenothiazine (EHOMO = -4.89 eV), and 5,10-dihydrophenazine (EHOMO =-4.38 eV). Of these, only Cz and DMAC and their derivatives are suitably weak to be used in blue TADF emitters (Figure ), with Cz alone of these being compatible for deep-blue or UV emissive D-A TADF emitters. However, the compact size of Cz results in compounds adopting less-twisted conformations, leading to emitters with undesirably large DEST, and subsequently to weak or even inactive TADF. Indeed, the D-A dihedral angles of Cz-containing TADF materials are significantly impacted by the steric influence of their neighboring donors or acceptor-attachment sites. For example we have shown that Cz takes on significantly more planar conformations when attached via its nitrogen atom to nitrogencontaining heterocycles such as pyridine, pyrimidine or pyrazine, in contrast to more twisted conformations when attached to phenylenes. Cz-containing TADF emitters have also been shown to form persistent dimer states that can be detrimental to emission colour, colour purity, and FPL. In contrast, the presence of the larger 6-member central ring of DMAC results in near perpendicular conformations that are less sensitive to steric environment although it is simultaneously a much stronger electron-donor. 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 CF3-substituted DMAC derivative that is a weaker electron-donor due to the presence of the inductively electron-withdrawing trifluoromethyl groups, and which yielded a blue-shifted emission compared to the reference DMAC-based material; however, this emitter was electrically unstable in the OLED. Here, we envision an embedded boron atom within the donor acting as an electron-withdrawing group, which will similarly subdue the electrondonating strength of the corresponding 1,4-azaborine (AZB). In et al. 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, lPL, of 467 nm and FPL of 99% in 20 wt% PPF (PPF: 2,8-bis(diphenylphosphine oxide)dibenzofuran) doped film. This compound possesses a ∆EST of 0.24 eV and a corresponding delayed lifetime (τd) of 91 µs. The OLED with MFAc-AzB showed a maximum external quantum efficiency (EQEmax) of 18.2% at 473 nm. Tien-Lin et al. nicely demonstrated the impact of the donor strength and torsion angle on the TADF properties in a series of D-A-D materials with AZB acting as the acceptor. Although CzAZB exhibited a FPL of 99%, it did not show TADF in part as the ΔEST value of 0.31 eV is 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 ΔEST values of 0.26 and 0.11 eV for tmCzAZB and dmAcAZB, respectively, and associated FPL 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 EQEmax of 12.4% and 29.9%, respectively, at associated λEL of 464 nm and 469 nm. 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. 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 multi-resonant TADF emitter design (MR-TADF), to date no one has yet explored its alternate use as an electron donor in conjunction with an appropriately matched electron acceptor. We hypothesized and subsequently verified both theoretically and experimentally that the electron-deficient boron atom within the 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 ), 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 ), which provides an ideal comparison to reveal the differences between the AZB and DMAC donors; note that Cz-Ph-TRZ is not TADF-active, 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 lPL of 415 nm and 495 nm, respectively, in 10 wt% doped mCP films. OLEDs using mCP host showed EQEmax of 2.1% and 10.5%, with emission Commission Internationale de l'Éclairage, CIE1931(x,y), coordinates of (0.17, 0.14) and (0.22, 0.39), for AZB-Ph-TRZ and AZB-TRZ, respectively. Sharing desirable electronic and structural features from 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.