We initiated a DFT computational screen of 36 naphthalene-based compounds containing one of six different donor moieties with a range of donating strengths and sizes to simultaneously probe the effect of both electronics and sterics. These donors were diphenylamine (DPA), carbazole (Cz), 3,6di-tert-butylcarbazole (DTBCz), 1,8-dimethylcarbazole (DMCz), 9,9-dimethyldihydroacridine (DMAC) and phenoxazine (PXZ). These donors decorate the naphthalene in either the 1-, 1,4-, 2-or 2,6-configurations where in the doubly substituted compounds, the second substituent was either another equivalent of the same donor or alternatively 2,4-diphenyl-1,3,5-triazine (TRZ) (Figure ), the latter being a stronger acceptor than naphthalene and serving to further stabilise the LUMO of the emitter. coupling between S1 and T1 at the ground state geometry using the PySOC program. 15 e first-order mixing coefficient between S1 and T1. f dihedral angle between the donor and acceptor moieties, measured as the average of the four absolute dihedral angles between the two groups where angles > 180° have been normalised according to the formula 𝜃 (%)* = 𝜃 -180.