Blue TADF Emitters Based on -Heterotriangulene Acceptors for Highly Efficient OLEDs with Reduced Efficiency Roll-Off.

The design of robust boron acceptors plays a key role in the development of boron-based thermally activated delayed fluorescence (TADF) emitters for the realization of efficient and stable blue organic light-emitting diodes (OLEDs). Herein, we report a set of donor (D)-acceptor (A)-type blue TADF compounds (-) comprising triply bridged triarylboryl acceptors, the so-called -heterotriangulenes, which differ depending on the identity of one of the bridging groups: methylene (), dimethylmethylene (), or oxo (). The X-ray crystal structures of and reveal a highly twisted D-A connectivity and a completely planar geometry for the -heterotriangulene rings. All compounds exhibit blue emissions with the unitary photoluminescence quantum yields and small singlet-triplet energy splitting (<0.1 eV) in their doped host films. The compounds exhibit a fast reverse intersystem crossing rate ( ≈ 10 s) with short-lived delayed fluorescence (τ ≈ 2 μs), which is found to be promoted by the strong spin-orbit coupling between the local triplet excited state (LE, T) and singlet (S) states. Using compounds - as the emitters, highly efficient blue TADF-OLEDs are realized. The devices based on the emitters with -heterotriangulenes exhibit better performances than the device incorporating a singly bridged reference emitter over the whole luminance range. Notably, the device based on the fully dimethylmethylene-bridged emitter () achieves the highest maximum external quantum efficiency (EQE) of 28.2% and the lowest efficiency roll-off, maintaining a high EQE value of 21.2% at 1000 cd/m.