The full results from this investigation are available in the supporting information (table ). In between diminishing ΔEST and maintaining SOC has been described previously in the literature. Hence, three of the four emitters have an effectively equivalent first-order mixing coefficient of around 10 -4 , with only 1,4-PXZ-Nap-PXZ having a significantly smaller mixing coefficient of around 10 -6 due to the much smaller HSO in this emitter. This highlights that the S1 and T1 excited states remain largely pure spin states. In all cases, the small ΔEST values can be attributed to excellent confinement of the HOMO and LUMO onto the PXZ donor and naphthalene acceptor moieties, respectively, as evidenced from the orbital distributions of the DFT-optimised structures (Figure ).