(3) the delayed lifetime in solution was extremely short. We therefore contend that the absence of TADF in the film state of 2PXZ-Nap and 1-PXZ-Nap is largely driven by quenching of the triplet states through non-radiative decay pathways, which simultaneously explains the low FPL and short/absent delayed emission. Considering the results obtained from the device based on 1,4-PXZ-Nap-PXZ we next chose to investigate in silico whether an even larger acene acting as an electron-acceptor would also yield compounds likely to show TADF. To this end we designed an additional 11 molecules based on the PXZ-acene-PXZ structure where acene is one of either benzene, anthracene, tetracene, pentacene or hexacene and the two donors were placed in all symmetrical combinations on either side of the ring system (Figure ). The results from this computational study are shown in Figures and. As the size of the acene part of the compound is increased, the energy of the LUMO is substantially stabilized because the LUMO is localized to the acene moiety (Figure ). Meanwhile, the HOMO is almost unchanged since it remains largely localized to the phenoxazine units, except in the hexacene derivatives where the HOMO is instead localized to the acene (the HOMO-1 is now localized on the phenoxazine unit), which is accompanied by a minor destabilization effect. Between the different substitution patterns, there is a simultaneous stabilization of the LUMO and a slight destabilization of the HOMO as the phenoxazines are placed closer to the centre, but the change in the LUMO is greater, resulting in a decrease of the electronic band gap. Hexacene again acts as an exception, with both the HOMO and LUMO stabilized simultaneously as the donors retreat from the periphery of thr acene. In all cases, the stabilization of the LUMO is associated with a stronger electronic interaction between the LUMO of the phenoxazine and the acene when the donors are placed near the centre of the acene core. A small coefficient of the LUMO density is therefore located on the phenoxazine donor (Figure ) and this coefficient is greater in the centrally substituted case, thus leading to a stronger stabilization of the LUMO. This stronger electronic interaction is a consequence of the larger LUMO density near the centre of the acene core as opposed to the outer rings.