A new physical picture to achieve small singlet-triplet energy gaps in high-efficiency polycyclic heteroaromatic emitters

The manipulation of the singlet-triplet energy gap (ΔES1T1) is an active area of research in the field of organic semiconductor materials. In polycyclic heteroaromatic emitters, which possess a great potential for application in ultra-high-definition displays, the origin of a small ΔES1T1 and its relationship with molecular structure still remain poorly established. Here, based on configuration-interaction theory, we derive a new effective expression for ΔES1T1, in which ΔES1T1 positively depends both on 2KHL (where KHL is the exchange energy between the HOMO and LUMO levels) and on the energy gap between the LUMO and LUMO+1 levels (ΔELUMO-LUMO+1). This physical picture of ΔES1T1 goes well beyond the simple HOMO→LUMO description that has been routinely applied in conventional donor-acceptor-type molecules and is validated over a series of more than sixty reported polycyclic heteroaromatic emitters. It allows us to suggest that an appropriate introduction of meta-substituted molecular fragments, which have small KHL and ΔELUMO-LUMO+1 values, is an effective way of designing molecules with smaller ΔES1T1. Importantly, a proof-of-concept molecule, IV-DABNA, was synthesized and demonstrates an extremely small ΔES1T1 (0.02 eV); its rate of reverse intersystem crossing is as fast as 4.4×106 s-1, the highest value reported in such systems. Sky-blue electroluminescent devices exploiting IV-DABNA as an emitter exhibit a remarkable maximum external quantum efficiency (EQE) of 32.1% with a largely suppressed roll-off (5.1%) at 1000 cd m-2. Overall, our work provides a new physical picture to reach a small ΔES1T1 in emerging polycyclic heteroaromatic emitters, which will accelerate the development of high-efficiency systems.