We first optimized the geometry of each emitter (Figure ), starting from structures drawn in silico. In all cases, the emitters adopt a planar geometry, with a 0.1° or smaller dihedral angle across the central indole nitrogen (Table ), leading to complete conjugation of the π-system throughout each emitter. In general, all five molecules have a low-lying HOMO at around -7.9 eV that is relatively invariant with structure, varying between -7.78 eV for 6NICz and -7.95 eV for 6,10NICz. No trend between HOMO energy and pyridine nitrogen position can be ascertained, with the structurally analogous 6NICz and 6,10NICz having the highest and lowest HOMO levels, respectively. By contrast, the LUMOs of these compounds are very high lying, with a mean predicted energy of 1.5 eV, and showed greater variation. 6,10NICz has the most stabilized LUMO (1.17 eV), followed by 6NICz (1.49 eV), 4NICz (1.65 eV), 5NICz (1.68 eV), and 7NICz (1.73 eV). Here, a structureproperty relationship emerges that we will see repeated throughout this study and that can be empirically derived based on the relative position of the pyridine nitrogen to that of the central indole nitrogen. Namely, structures in which the pyridine N is situated in an ortho-or para-like position (7NICz and 5NICz, respectively) tend to have higher energies (LUMO, DEHOMO-LUMO, S1 and DEST) than those in a meta-like position (4NICz, 6NICz, and 6,10NICz). Among the meta-substituted emitters, 6NICz and 6,10NICz, which have a nitrogen at the 6-position, have more stabilised LUMOs than 4NICz, where the nitrogen is instead at the 4-position. As the HOMO energies are insensitive to structural variation, this pattern is naturally reproduced in the calculated HOMO-LUMO gap (DEHOMO-LUMO). The DEHOMO-LUMO are very large overall, but the greatest occurs in 5NICz and 7NICz (9.61 and 9.56 eV, respectively) and smallest in both 6NICz and 6,10NICz (9.27 and 9.13 eV, respectively), while the value for 4NICz is intermediate at 9.61 eV. c The mean dihedral angle between atoms C7, C7a, N8, C8a, and C7a, N8, C8a, C9. All calculations were performed using the RI-SCS-ADC(2)/cc-pVDZ level of theory, in the gas phase. See Table for extended excited state calculation results.