As a direct consequence, we observed a sizeable decrease in ΔEST from 0.21 to 0.01 eV for the phenyl and naphthalene derivatives respectively, explained by the larger delocalization of the LUMO, which further strengthens the CT character of both S1 and T1. However, starting from the anthracene derivatives, the disproportionate stabilisation of the locally excited triplet compared to the charge-transfer singlet results in a steep increase in ΔEST (Figure ), up to a maximum of 0.96 eV in 1,4-PXZ-Nap-PXZ (A6a). Conversely, there is a stabilization of the 1 CT as the donor position is moved from the terminus of the acene towards the centre, while the 3 LE remains constant, results in a sizeable decrease in ΔEST, and as such the centrally substituted hexacene derivative (6,15-PXZ-Hex-PXZ, A6c) has a noticeably smaller ΔEST of 0.70 eV. With these results in hand, it becomes clear that of all the acenes evaluated naphthalene is the most ideally suited to act as an acceptor for developing TADF emitters. The ΔEST of the centrally substituted anthracene derivative was also minimized, but the inversion of the 3 LE and 3 CT states means this is not guaranteed for other donors, and any blue-shifting of the emission is likely to be accompanied by an increase in ΔEST. In particular, acenes larger than anthracene will likely possess a ΔEST that is too large for TADF, unless either the S1 energy is substantially stabilized and/or the T1 state is sufficiently destabilised. This observation is corroborated by recent reports in the literature of donor-acceptor emitters containing larger acenes such as anthracene, pyrene and perylene (Figure ). The emitters DPAAnCN and CzAnCN have been previously reported by our group and consist of a cyanoanthracene core centrally substituted with diphenylamine and carbazole, respectively.