Here, we also study Cz-BDT-SO 2 where dearomatisation of the fused heterocycles will serve to further reduce the LUMO (and to a lesser extent the HOMO) through inductive effects and remove the influence of the S lone pairs on the electronic properties of the molecule. Oxidation of S to SO 2 serves to stabilise the LUMO and red-shift emission while it is also expected to enhance PLQYs and modulate singlet and triplet energies, ∆E ST , spin-orbit coupling effects, and solvatochromism. Finally, Cz-BDF is studied in order to identify the impact of light vs heavy heteroatoms on molecular geometries, excited state energies, and characters.