In turn, this localization is the result of the large torsion angles between the donor(s) and acceptor, which is close to perpendicular in all cases, and, to a lesser extent, the high donating strength of the PXZ moiety, as reflected by the computed HOMO energies. The four molecules have a predicted S1 excitation energy ranging from 433 nm for 2-PXZ-Nap to 479 nm for 1,4-PXZ-Nap-PXZ. This trend is naturally mirrored in the calculated HOMO-LUMO gap (ΔEHOMO-LUMO), being largest in 2-PXZ-Nap (3.66 eV) and decreasing to 3.46 eV in 1,4-PXZ-Nap-PXZ since the S1 excited states of all molecules are dominated by a HOMO to LUMO transition. As might be expected, the largest change in both the S1 energy and ΔEHOMO-LUMO was observed between the singly and doubly donorsubstituted examples; however, this change was driven primarily by a stabilisation of the LUMO level as opposed to a destabilisation of the HOMO, despite the addition of a second donor unit. Finally, in all cases the predicted oscillator strength for the S0 → S1 transition is negligible. However, this is typical in charge-transfer type emitters such as those investigated here, in which the HOMO-LUMO overlap in the ground state-optimised structure is intentionally minimised and the probability of the transition becomes small. These emitters can still possess high efficiencies in the real world so long as emission of the S1 is coupled to vibrational modes inducing either: (1) a reduced donor-acceptor torsion and hence greater HOMO-LUMO overlap or (2) contributions from electronic transitions between deeper occupied molecular orbitals and higher-lying unoccupied molecular orbitals. 18