Construction of Blue Thermally Activated Delayed Fluorescence Materials Based on the Heptagonal Triarylamine Donor

The development of efficient blue light-emitting materials is a crucial yet challenging task in organic light-emitting diodes (OLED). Two novel blue light-emitting molecules, TRZCz-TPA and TRZ-TPA, have been designed and synthesized based on the heptagonal 9H-tripheno[b,d,f]nitrozoafluorene (TPA) donor. Their thermal stability, electrochemical properties, photophysical properties, and device performances were characterized. TRZCz-TPA and TRZ-TPA both exhibit blue emission in toluene solution with emission peaks at 455 and 440 nm, respectively. In contrast to TRZ-TPA, which exhibits through-bond charge transfer (TBCT) from the TPA donor to the diphenyltriazine (TRZ) acceptor, TRZCz-TPA, featuring two adjacent 3,6-di-tert-butylcarbazole (tBuCz) donors near the TRZ acceptor, demonstrates through-space charge transfer (TSCT) from the tBuCz donors to the TRZ acceptor. This leads to a remarkable reduction in the singlet-triplet energy gap (Delta EST) from 0.53 eV to 0.04 eV, thereby displaying thermally activated delayed fluorescence (TADF) properties. Additionally, tBuCz donor with large steric hindrance effectively suppresses aggregation-induced quenching in the aggregated state. Consequently, TRZCz-TPA achieves a photoluminescence quantum yield (PLQY) of 42.0% and a maximum external quantum efficiency (EQE(max)) of 10.4% in OLED devices, significantly outperforming TRZ-TPA (PLQY=6.5%, EQE(max)=3.7%).