Multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters show promise for achieving both high color-purity emission and high exciton utilization efficiency. However, their development is often impeded by a limited synthetic scope and excessive molecular weights, which challenge material acquisition and organic light-emitting diode (OLED) fabrication by vacuum deposition. Herein, we put forward a B−N covalent bond-involved π-extension strategy via post-functionalization of MR frameworks, leading to the generation of high-order B/N-based motifs. The structurally and electronically extended π-system not only enhances molecular rigidity to narrow emission linewidth but also promotes reverse intersystem crossing to mitigate efficiency roll-off. As illustrated examples, ultra-narrowband sky-blue emitters (full-width at half-maximum as small as 8 nm in n-hexane) have been developed with multi-dimensional improvement in photophysical properties compared to their precursor emitters, which enables narrowband OLEDs with external quantum efficiencies (EQEmax) of up to 42.6%, in company with alleviated efficiency decline at high brightness, representing the best efficiency reported for single-host OLEDs. The success of these emitters highlights the effectiveness of our molecular design strategy for advanced MR-TADF emitters and confirms their extensive potential in high-performance optoelectronic devices.

中文翻译：

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BN共价键涉及的多个共振发射器的π延伸可实现高性能窄带电致发光

多硼嵌入多重共振热激活延迟荧光（MR-TADF）发射器有望实现高颜色纯度发射和高激子利用效率。然而，它们的发展往往受到合成范围有限和分子量过高的阻碍，这对材料获取和真空沉积有机发光二极管（OLED）制造提出了挑战。在此，我们通过 MR 框架的后功能化提出了一种 B−N 共价键涉及的 π 延伸策略，从而产生了基于 B/N 的高阶基序。结构和电子扩展的π-系统不仅增强了分子刚性以缩小发射线宽，而且还促进反向系间窜越以减轻效率滚降。如图所示，超窄带天蓝色发射体（在正己烷中半峰全宽小至 8 nm）已开发出来，与前体发射体相比，其光物理性能得到了多维改进，从而使窄带 OLED 成为可能外量子效率 (EQEmax) 高达 42.6%，同时高亮度下的效率下降得到缓解，代表了单主体 OLED 的最佳效率。这些发射器的成功凸显了我们先进 MR-TADF 发射器分子设计策略的有效性，并证实了它们在高性能光电器件中的广泛潜力。