Abstract: In polycyclic heteroaromatic (PHA) emitters, which possess great potential for application in ultrahigh-definition displays, the origin of a small singlet–triplet energy gap (ΔES1T1) and its relationship with the molecular structure still remain poorly established. Here we derive an effective expression for ΔES1T1, in which ΔES1T1 positively depends on 2KHL (where KHL is the exchange energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO)) and on the energy gap between LUMO and LUMO + 1 (ΔELUMO–LUMO+1). This expression for ΔES1T1 is validated over a series of 100 reported PHA emitters. It allows us to easily identify various molecular design approaches for managing ΔES1T1 by synergistically regulating 2KHL and ΔELUMO–LUMO+1. The proof-of-concept PHA molecules were synthesized and characterized to further confirm the validity of this expression for ΔES1T1. Overall, our work provides a physical picture to not only modulate ΔES1T1 in emerging PHA emitters but also design and screen such materials with small ΔES1T1.
