light-emitting diodes (OLEDs) due to their low production-cost as well as the capability to harvest "dark" triplet excitons via a reverse intersystem crossing (RISC) mechanism. [1][2][3][4] Facilitating of RISC is made by reduction of the energy gap (ΔE ST ) between the lowest excited singlet and triplet (S 1 and T 1 ) states, typically achieved in twisted donor-acceptor (D-A) typed molecules with separated frontier molecular orbitals (FMOs) at the expense of low oscillator strength (f) and photoluminescence quantum yield (Φ PL ). [5][6][7][8] This contradiction undoubtedly sets an obstacle to obtain highly efficient emitter, and the large structural reorganization occurred in the excited state would cause broad emission spectra and increased non-radiative decay channels, eventually weakening the performances of corresponding devices. [9][10][11][12][13][14][15] Instead of the conventional D-A configured emitters, a unique category of multi-resonance TADF (MR-TADF) molecules based on fused polycyclic aromatics was lately proposed by Hatakeyama et al. to mitigate the aforementioned issues. [16][17][18][19][20][21][22] The complementary resonance effects of electrondeficient B and electron-rich N/O atoms within the framework separates the FMOs to induce short-range charge-transfer (SR-CT), concurrently offering a small ΔE ST and a high radiative decay rate (10 7 -10 8 s −1 ) from S 1 to ground (S 0 ) state. [17,23] The planar nature with high rigidity guaranteed narrow full width at half maximum (FWHM) emissions, and could also induce favorable horizontal dipole orientation of the emitters to boost optical out-coupling efficiency. [24] Thus far, the majority of the blue MR-TADF emitters were constructed from the basic motif DABNA (Scheme 1) due to synthetic feasibility and their decent quantum efficiencies. [25] Replacement of the diphenylamino subunits into carbazolyl-derived BN-CZ as a new core skeleton with bathochromic emission, which was adopted to realize full-color electroluminescence (EL) with high color purity and maximum external quantum efficiencies (EQE max s) above 20% by peripheral electronic modulation. [24,[26][27][28][29] Nonetheless, most MR-TADF devices encountered severe triplet-related efficiency loss at high luminance/current density stemmed from the intrinsic structural planarity and long-delayed lifetime, and relied on the involvement of sensitizing host to avoid triplet Multi-resonance thermally activated delayed fluorescence (MR-TADF) offers an exceptional solution for narrowband organic light-emitting diode devices in terms of color purity and luminescence efficiency, while the development of new MR skeleton remains an exigent task. It is hereby demonstrated that a simple modification of the B (boron)−N (nitrogen) framework by sp 3 -carbon insertion will significantly bathochromic shift the short-range charge-transfer emission, boost the reverse intersystem crossing process, and improve the device performances. The bis(acridan)phenylene-based skeleton developed in this contribution presen...
