Tetraboron MR framework with ortho-B-π-B pattern enables high-efficiency yellow narrowband OLEDs

Multiple resonance (MR) frameworks with ortho-boron-π-boron (B-π-B) patterns are promising alternatives to those with para-B-π-B ones for realizing bathochromic narrowband emission, but such cases have been rarely reported, and suffer from significant synthetic challenges. Here, a novel tetraboron MR framework ICZ4B incorporating an ortho-B-π-B pattern was constructed by using indolo[2,3-c]carbazole as the central building block. The well-defined borylation active sites facilitate an efficient one-shot quadruple borylation. ICZ4B not only exhibits bright yellow emission with a small full width at half maximum of 24 nm, but also achieves an improved reverse intersystem crossing rate of 4.7 × 10⁴ s^-1. The non-sensitized device realizes an outstanding external quantum efficiency of 36.5%, representing one of the highest results at long wavelengths. This study not only presents a feasible pathway for realizing MR frameworks with ortho-B-π-B patterns via delicate selection of key precursors, but also offers insights for optimizing synthetic strategies for complex MR frameworks.