Facile B-N Covalent Bond Fusion in N,N'-Diaryldihydrophenazines: Achieving Efficient Narrowband Electroluminescence...

Covalently fusing multiple B-N units into redox-active polycyclic aromatic hydrocarbons (PAHs) offers a powerful strategy for creating π-extended systems with novel functionalities, but it remains a formidable challenge. Here, we report a facile, one-pot, and lithium-free NH-directed borylation to construct a series of 5,10-dihydro-5,10-diphenylphenazine (DPPA) derivatives fused by two or four B-N covalent bonds. Such a multiple B-N locking is found not only to enforce molecular rigidity and suppress excited-state structural relaxation, but also to profoundly modulate the electronic structure and antiaromaticity of the central DPPA core. Interestingly, the resultant quadruply fused system (4BN-Ph) can function as an unprecedented narrowband orange-red thermally activated delayed fluorescence (TADF) emitter, enabling efficient electroluminescence with a record-high external quantum efficiency of 31.2% and a notably small full-width at half-maximum of 32 nm at an emissive peak of 595 nm. Also, 4BN-Ph displays intriguing redox-controlled properties, since a stepwise oxidation generates near-infrared-absorbing open-shell radical cations and closed-shell dications. This work establishes a modular route to PAHs incorporating multiple B-N covalent bonds, with exceptional optoelectronic and spintronic properties.