Achieving small singlet–triplet energy gaps in polycyclic heteroaromatic emitters

In polycyclic heteroaromatic (PHA) emitters, which possess great potential for application in ultrahigh-definition displays, the origin of a small singlet–triplet energy gap (ΔE_S1T1) and its relationship with the molecular structure still remain poorly established. Here we derive an effective expression for ΔE_S1T1, in which ΔE_S1T1 positively depends on 2K_HL (where K_HL 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 (ΔE_{LUMO–LUMO+1}). This expression for ΔE_S1T1 is validated over a series of 100 reported PHA emitters. It allows us to easily identify various molecular design approaches for managing ΔE_S1T1 by synergistically regulating 2K_HL and ΔE_{LUMO–LUMO+1}. The proof-of-concept PHA molecules were synthesized and characterized to further confirm the validity of this expression for ΔE_S1T1. Overall, our work provides a physical picture to not only modulate ΔE_S1T1 in emerging PHA emitters but also design and screen such materials with small ΔE_S1T1.