Phosphine oxide balanced charge transfer hybridization in multi-resonance emitters: towards ∼40 % external quantum efficiency and preserved color purity

Introducing long-range charge transfer (LRCT) effect to optimize thermally activated delayed fluorescence (TADF) properties without reducing color purities is one of the key challenges for developing high-performance multi-resonance (MR) emitters. Herein, we construct two B,N-doped polycyclic emitters of MR-Acceptor (A) type DPPO-BN and the MR-π-A type TPPO-BN through incorporating P=O groups into BCz-BN skeleton. Compared to the parent skeleton, the insulating feature of P=O averts the color purity reduction. However, the strongest LRCT markedly enhances TADF properties of DPPO-BN, making its rate constants of prompt and delayed fluorescence, singlet radiation and reverse intersystem crossing (RISC) doubled. But, intermolecular charge transfer of DPPO-BN is simultaneously strengthened. In contrast, TPPO-BN reveals the balanced MR and LRCT and enhanced vibrational coupling, leading to ∼100 % RISC efficiency and two orders of magnitude decreased nonradiative rate constants. As consequence, TPPO-BN-based devices realized the state-of-the-art external quantum efficiency up to 39.8 % among nonsensitized MR diodes to present.