Twisted vibration of Aza-azulene promotes ultra-fast triplet-singlet up-conversion for extremely stable red electroluminescence

The fast triplet-singlet up-conversion based on thermally activated delayed fluorescence (TADF) molecules is a unique effort to improve the utilization of triplet excitons. We construct two TADF molecules using non-benzenoid aromatic aza-azulene as donor and triazine as acceptor. The distortion structural resonance of aza-azulene enhances the reverse intersystem crossing (RISC) process, resulting in efficient inter/intramolecular charge transfer (CT) emission. The rate constant of RISC (k(RISC)) is 9.0 x 10(6) and 8.4 x 10(6) s(-1) in solid states. Efficient Forster Energy Transfer (FRET) and/or Dexter Energy Transfer (DET) between these molecules and dopants make them suitable host materials to improve the performances of red phosphorescent organic light-emitting diodes (OLEDs). Especially, the top-emitting device achieves an external quantum efficiency (EQE) of 51.3% and a current efficiency of 68.2 cd/A at 1000 cd m(-2) with a long device lifetime of LT95 = 52,350 and LT50 = 536,200 h.