Hybridizing Three-coordinated and Tetra-coordinated Boron Structures for Multi-Resonance Emitters with Narrowed Emission, Steric Hindrance and Accelerated Reverse Intersystem Crossing

Tri-coordinated and tetra-coordinated boron derivatives form the backbone of boron-containing organic materials, where sp²/sp³ hybridized boron centers endow distinct spatial and electronic features that dictate their chemical/physical behaviors. Herein, we report a rational design of hybrid boron emitters integrating sp²-hybridized boron cores with peripheral sp³-hybridized spiro-boron skeletons. This architecture innovatively merges multiple-resonance (MR) emission with tetracoordinated spiro-boron units, offering a dual-function strategy: steric hindrance from spiro-boron mitigates aggregation-caused quenching, while the synergy of MR intrinsic short-range charge transfer (SRCT) and spiro-boron-induced long-range charge transfer (LRCT) enhances reverse intersystem crossing (RISC) in narrowband emitters. The sulfur-incorporated spiro-boron moiety further amplifies RISC via a heavy-atom effect. Photophysical analyses reveal that BN-SBO, BN-SBS, and BN-SB exhibit narrowband thermally activated delayed fluorescence (TADF) at 508 nm (FWHM = 24 nm), maintaining emission purity under high doping concentration, with RISC rates up to 1.05×10⁵ s⁻¹. OLED devices achieve a peak external quantum efficiency (EQE) of 35.3% and low roll-off (32.4% at 1000 cd m⁻²; 23.0% at 10,000 cd m⁻²). This work establishes a general molecular engineering paradigm via sp³/sp² boron hybridization, enabling concurrent optimization of color purity and exciton utilization for next-generation high-performance optoelectronics.