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

Abstract
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 (1-3) comprising triply bridged triarylboryl acceptors, the so-called B-heterotriangulenes, which differ depending on the identity of one of the bridging groups: methylene (1), dimethylmethylene (2), or oxo (3). The X-ray crystal structures of 2 and 3 reveal a highly twisted D-A connectivity and a completely planar geometry for the B-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 (k(RISC) approximate to 10(6) s(-1)) with short-lived delayed fluorescence (tau(d) approximate to 2 mu s), which is found to be promoted by the strong spin-orbit coupling between the local triplet excited state ((LE)-L-3, T-2) and singlet (S-1) states. Using compounds 1-3 as the emitters, highly efficient blue TADF-OLEDs are realized. The devices based on the emitters with B-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 (2) 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(2).

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