A theoretical study was first undertaken to obtain insight into the optoelectronic properties of these compounds and to rationalize the proposed design. To simplify the calculations, we modelled analogues of these compounds that do not contain the tert-butyl groups, MeCzCO2H, 2GMeCzCO2H, DCzB, MeCzCO2HDCzB and 2GMeCzCO2HDCzB (Figure ). The calculations were carried out using Density Functional Theory at the M06-2X/6-31G(d,p) level of theory. The ground states of the reported emitting core DCzB, and the two new MR-TADF emitters MeCzCO2HDCzB and 2GMeCzCO2HDCzB were first optimized starting from a Chem3D generated structure. For MeCzCO2HDCzB, the HOMO is located on the DCzB core while the LUMO is distributed both on the DCzB moiety and the benzoate. For 2GMeCzCO2HDCzB, the HOMO and the degenerate HOMO-1 are located on separate donor dendrons, while as with MeCzCO2HDCzB the LUMO is located on both the DCzB and the benzoate moieties. The HOMO of MeCzCO2HDCzB and 2GMeCzCO2HDCzB are predicted to be -6.18 and -6.20 eV, respectively, which are destabilized compared to the predicted HOMO for DCzB (-6.42 eV). This is due to the presence of the electron-withdrawing benzoate in the former two compounds. The LUMOs are stabilized to -1.21 and -1.39 eV, respectively, compared to -1.14 eV for DCzB (Figure ), which is attributed to the presence of the electron-withdrawing benzoate in the former two compounds. The S1/T1 states of MeCzCO2HDCzB and 2GMeCzCO2HDCzB are predicted to be 3.16/2.71 and 3.15/2.69 eV, both of which are stabilized compared to DCzB (3.40/2.91 eV), resulting in a large DEST of 0.46/0.46 eV of similar magnitude to that of DCzB (0.49 eV). At the ADC(2)/cc-pVDZ level of theory, which we had previously shown to be accurately predict excited state energies, the DEST of DCzB is 0.13 eV (Figure ), which matches the experimental DEST of 0.13 eV for DtBuCzB in (Figure ). The size of MeCzCO2HDCzB and 2GMeCzCO2HDCzB preclude the use of this higher level of theory, but we can infer from the similar DFT calculated DEST values to DCzB that these two compounds will likewise have comparably small singlettriplet values. The natural transition orbitals at the optimized T1 geometry for the S1 and T1