To assess functionalization effects on ∆E ST , we created variants of a standard INVEST emitter, 2T-7N 32 , as shown in Figure . Our dataset comprises 15 mono-substituted heptazine molecules with electron-donating and electron-withdrawing groups, which we designate HEPTA-INVEST15. Vertical energy gaps between S 0 , S 1 , and T 1 states were obtained with EOM-CCSD. We subsequently expanded this to HEPTA-INVEST46, which includes doubly and triply substituted systems.