Across the entire series, the S1 state retains a large CT character and is mostly described by a HOMO to LUMO transition, except in the hexacene derivatives where it is best described by a HOMO-1 to LUMO transition. Correspondingly, a significant red-shift in S1 is observed as the acene size increases due to the increasing stabilization of the LUMO (Figure ), with 1,4-PXZ-Hexa-PXZ (S1 = 1.59 eV) and 6,15-PXZ-Hexa-PXZ (S1 = 1.30 eV) exhibiting the smallest S1 energies for their respective substitution patterns. We also observed that the magnitude of the S1 energy red-shift is larger when the donors are placed closer to the centre of the acene core, consistent with the trend in the electronic band gap, and this effect is more prominent in the larger acenes. As such, the S1 of the terminally substituted hexacene derivative (1,4-PXZ-Hexa-PXZ, A6a, S1 = 1.59 eV) is higher in energy than that of the centrally substituted pentacene derivative (6,13-PXZ-Pent-PXZ, A5a, S1 = 1.45). The evolution of the T1 energy differs significantly from that of the S1 because of the competing contributions from states possessing locally excited ( 3 LE) and charge-transfer ( 3 CT) character ( Figure ). For the phenyl and naphthalene derivatives, the T1 state is predominantly CT in character. However, starting from the anthracene derivative, the locally excited triplet state localized on the acene core becomes more stabilised than the 3 CT. This arises because the energy ordering of the triplet states of donor-acceptor compounds is ruled by the interplay between the magnitude of the electronic interaction between the electron-donating and the electron-accepting units, which tends to stabilize the 3 CT, and the exchange interaction which favours the 3 LE as the lowest triplet energy state. As a result, the energy of the T1 state decreases significantly with increasing ring size but is essentially invariant across different substitution patterns where the acene acceptor remains constant since the T1 transition is essentially localized on the acene bridge. The corresponding locally-excited singlet state ( 1 LE), meanwhile, remains higher in energy than the charge-transfer state ( 1 CT) because the exchange interaction energy does not affect singlets.