As SOC is approximately proportional to the fourth power of the nuclear charge of the atoms involved in the emissive transition, a strategy that has been effectively used to enhance kRISC in TADF molecules is to introduce heavy atoms to enhance the SOC. Halogenation of donoracceptor (D-A) TADF molecules (e.g. with Cl, Br, and I) is a common strategy to enhance SOC. Bunz et al. first reported the exploitation of the internal heavy atom effect (HAE) in TADF compounds by decorating halogen atoms onto carbazole donors in multi-carbazole TADF emitters. With increasing numbers of bromine and iodine substituents, the excited-state lifetimes were found to be considerably shortened. Octaiodo derivatives 3f and 5f exhibited the shortest lifetimes of 1.2 and 0.4 µs respectively (Figure ). Although the photoluminescence quantum yields (FPL) were slightly decreased, halogen substituents were effective at accelerating both ISC and RISC. resulting in multiple spin-flip cycles between S1 and T1 rather than direct emission following RISC. Further examples of HAE investigations of D-A TADF materials are summarised in the SI. In contrast to D-A TADF materials, multi-resonant TADF emitters (MR-TADF) are rigid polycyclic aromatic compounds containing a combination of p-and n-dopants, which usually show larger ΔEST (>0.10 eV) and slower kRISC (∼10 4 s -1 ). RISC is typically enabled through an upper-state crossing mechanism rather than the vibronic coupling of the lower energy triplet states in D-A TADF emitters. Introducing heavy atoms into MR-TADF emitters is therefore a promising strategy to address their slower kRISC. Indeed, Chen et al. reported the boron/sulfur-based MR-TADF emitter BSS (Figure ), which possesses a large SOCME of 0.77 cm -1 between the S1 and T1 states and a faster kRISC of 1.18×10 5 s -1 compared to oxygencontaining analogues BOS (SOCME = 0.62 cm -1 and kRISC = 6.1×10 4 s -1 ) and BOO (SOCME =0.01 cm -1 and kRISC = 1.1×10 4 s -1 ). Additional examples of HAE investigations for MR-TADF emitters are included in the SI, although we note that in all cases these involve heavy atom substitution directly within the MR-TADF core. We previously reported the MR-TADF emitter DiKTa and its mesitylated congener, Mes3-DiKTa (Figure ), which showed reduced aggregation-caused quenching and aggregate formation due to the presence of the bulky mesityl groups. . The presence of homo-intermolecular interactions (likely induced by the halogen atoms) and the weaker C-Br bonds led to more severe device roll-off at these higher current densities, counterbalancing the higher performance they can unlock at lower driving current. These results therefore demonstrate both the advantageous and detrimental features of peripheral halogen decoration on the performance of MR-TADF OLEDs.