Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters have garnered considerable attention due to their potential to deliver wide color gamut and high-resolution displays based on organic light-emitting diode (OLED) technology. These molecules have a distinct molecular structure characterized by the incorporation of alternatively-positioned electron-accepting (e.g., boron or ketone) and electron-donating (e.g., nitrogen or oxygen) groups that are embedded within planar and rigid fused polycyclic aromatic structure. This distinctive framework effectively separates the frontier molecular orbitals (FMOs) involved in the emissive excited state, relying on complementary mesomeric behavior of the electron acceptor and electron donor motifs. Consequently, there is a moderately small singlet-triplet splitting energy (ΔEST) to trigger the reverse intersystem crossing (RISC) process that enables TADF. By virtue of their rigid structures, these compounds have a remarkably low excitedstate reorganization energy, and so the emission is very narrow and there is a very small Stokes shift. However, unlike the donor-acceptor (D-A) type TADF emitters, whose FMOs are spatially separated with ΔEST approaching 0 eV, MR-TADF emitters usually have ΔEST of around 0.15 eV and consequently show slow reverse intersystem crossing rate constants (kRISC), typically around 10 4 s -1 . Such slow kRISC leads to significant competitive bimolecular quenching, such as from triplet-triplet annihilation (TTA) or triplet-polaron annihilation (TPA) in the OLEDs. A key way to address the slow kRISC would be to incorporate heavy atoms in order to increase the spin orbital coupling (SOC) matrix element between low-lying singlet and triplet states, as quantified within Fermi's golden rule (Equation ): 𝑘