Here, we report two boron-based deep-blue emitters containing a mixed O/N donor atom system, one of which is MR-TADF. 1B-DTACrs and 2B-DTACrs were synthesized in five steps starting from 3,5-difluorobromobenzene. The key borylation reaction proceeded in 62% and 58% yield, respectively, for 1B-DTACrs and 2B-DTACrs from the common intermediate 4. Site selective borylation was achieved by controlling the reaction temperature and the amount of BBr3 in the reaction (Fig. ). The identity and purity of both emitters were confirmed by a combination of 1 H NMR (Figs. and), C NMR (Figs. and) spectroscopy, melting point determination, high-resolution mass spectroscopy (HRMS) (Figs. Crystals for 1B-DTACrs were obtained by slow diffusion of hexane into a saturated solution of the emitter in THF. Crystals of 2B-DTACrs were obtained directly from a temperature gradient train sublimation method (Fig. ). The ditolylamine (DTA) unit adopts a propeller structure. In the case of 1B-DTACrs, where there is only one boron in the structure, the MR-TADF core remains nearly planar. The o-tolyl unit is approximately perpendicular to the MR-TADF core in both emitters. Yasuda and co-workers reported BSBS-N1, a structurally analogous sulfur-containing compound to 2B-DTACrs, which was described as having a highly distorted structure, whereas 2B-DTACrs possesses a nearly planar geometry at the B,O substituted pentacene core with an out-of-plane alignment of two aryl rings of the DTA unit. There are no significant intermolecular π-π interactions in the solid state for either compound, which may be a consequence of the orthogonally disposed o-tolyl unit. The ground and excited-state properties of 1B-DTACrs and 2B-DTACrs were modelled using DFT and spin-component scaling second order approximate coupled-cluster (SCS-CC2) methods, respectively (Fig. ). Both compounds possess difference density distributions between the ground and excited-state reminiscent of other MR-TADF emitters. Addition of the second boron atom in 2B-DTACrs resulted in a small stabilization in the HOMO and LUMO energy levels compared to those of 1B-DTACrs and a corresponding small decrease (0.12 eV) in the band gap (Fig. ). The S1 state is stabilized from 3.38 eV in 1B-DTACrs to 3.27 eV in 2B-DTACrs, while the T1 states are essentially isoenergetic. Thus, there is a much smaller ΔEST value of 0.08 eV for 2B-DTACrs compared to 0.20 eV of 1B-DTACrs. The oscillator strength (f) for the S0-S1 transition is higher at 0.36 for 2B-DTACrs compared to 0.23 for 1B-DTACrs likely for the apparent largest delocalization of the S1 excited state. Both the S2 and T2 states of 2B-DTACrs are also stabilized compared to those of 1B-DTACrs. The small energy difference between S1 and T2 (ΔEST2) is of particular interest as many reports have highlighted that RISC occurs via intermediate triplet states. The ΔEST2 drops from 0.34 eV in 1B-DTACrs to 0.02 eV in 2B-DTACrs. For each compound SOC is modest between S1 and T1, calculated to be 0.07 cm -1 and 0.04 cm -1 for 1B-DTACrs and 2B-DTACrs, respectively. This increases to 0.81 cm -1 and 0.98 cm -1 between S1 and T2 for 1B-DTACrs and 2B-DTACrs, respectively (Table ). These results suggest that RISC likely proceeds from T2 to S1, particularly for 2B-DTACr. Tables S1-S3 provide a full summary of the excited state energies, transitions and SOC values. The electrochemical properties were measured using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deaerated dichloromethane (DCM) with 0.1 M tetran-butylammonium hexafluorophosphate as the supporting electrolyte (Fig. and Table ).