Optimizing the energy level alignment for achieving record-breaking efficiency in hot exciton deep red OLEDs.

To effectively compete with the quenching process in long-wavelength regions like deep red (DR) and near-infrared (NIR), rapid radiative decay is urgently needed to address the challenges posed by the "energy gap law". Herein, we confirmed that it is crucial for hot exciton emitters to attain a narrow energy gap (Δ) between the lowest singlet excited (S) state and second triplet excited (T) state, while ensuring that T slightly exceeds S in the energy level. Two proofs-of-concept of hot exciton DR emitters, namely αT-IPD and βT-IPD, were successfully designed and synthesized by coupling electron-acceptors ,-diphenylnaphthalen-2-amine (αTPA) and ,-diphenylnaphthalen-1-amine (βTPA) with an electron-withdrawing unit 5-(4-(-butyl) phenyl)-5H-pyrazino[2,3-]indole-2,3-dicarbonitrile (IPD). Both emitters exhibited a narrow Δ, with T being slightly higher than S. Additionally, both emitters showed significantly large Δ. Moreover, due to their aggregation-induced emission characteristics, J-aggregated packing modes, moderate strength intermolecular CN⋯H-C and C-H⋯π interactions, and unique, comparatively large center-to-center distances among trimers in the crystalline state, both αT-IPD and βT-IPD emitters exhibited remarkable photoluminescence quantum yields of 68.5% and 73.5%, respectively, in non-doped films. Remarkably, the corresponding non-doped DR-OLED based on βT-IPD achieved a maximum external quantum efficiency of 15.5% at an emission peak wavelength of 667 nm, representing the highest reported value for hot exciton DR-OLEDs.