Topological structure optimization of B,N-doped nanographenes for deep-blue emitters

B,N-doped nanographenes are promising blue OLED dopants owing to their narrowband emission and triplet-exciton harvesting capability, but their inefficient reverse intersystem crossing (RISC) remains a bottleneck for practical applications. Although π-extension is widely used to accelerate RISC and improve color purity, the role of molecular topology remains unclear. Here we report three deep-blue, quadruple-borylated nanographenes with isomeric skeletons and topology-dependent conformations ranging from negatively curved to quasi-planar. Combined theoretical and experimental studies reveal that enhanced planarity can facilitate the resonance effect, promote charge-transfer delocalization, and increase structural rigidity. Consequently, the most planarized emitter achieves an ultranarrow emission bandwidth of 13 nm/0.07 eV and a high RISC rate constant of 2.7 × 10⁶s^-1, outperforming the curved analogues. The corresponding OLED delivers an external quantum efficiency of 30.4% at 1000 cd m^-2 with color coordinates of (0.127, 0.078), establishing conformation-guided design principles for high-performance narrowband emitters toward ultrahigh-definition displays.