Engineering Intermolecular Packing Mode of Oxygen-Bridged Cyclized Boron-Based MR-TADF Emitters Enables High-Efficiency Deep-Blue Narrowband OLEDs.

The development of high-efficiency and low-cost multi resonance thermally activated delayed fluorescence (MR-TADF) emitters especially in the deep-blue region is critically limited due to intrinsic excimer quenching of planar π-extended frameworks. Herein, a novel design strategy is reported for realizing high-efficiency oxygen-bridged cyclized boron-based MR-TADF emitters via engineering intermolecular packing mode. Three organic donor-acceptor (D-A) molecules with different molecular configurations are designed and synthesized, which can readily form modulated packing patterns with fastidiously regulating intermolecular charge transfer (CT) in crystalline states. Experimental and theoretical investigations expose that the intermolecular D-A packing modes could be formed in relatively planar molecular architecture, which cannot only fix the intermolecular CT excited-state configuration, but the multiple conversion channels of triplet excitons can also be involved synergistically to accelerate the spin-flip, and thus achieving near-unity PLQY and excellent reverse intersystem crossing rate of 6.7 × 105 s-1 in solid states. The optimized OLEDs devices achieve an attractive EQE value of 31.75%, which is at a record high for MR-TADF OLEDs with deep-blue emission. Our strategy boosts the luminescence efficiency of MR-TADF emitters through enabling the participation of multiple triplet states and the confined excited-state conformations induced by intermolecular CT interaction in aggregation state.