Highly Efficient Organic Light‐emitting Diodes Based on Spatially Compacted π‐Stacked MR‐TADF Emitters

Abstract Organic fluorophores with through‐space donor/acceptor interaction have garnered significant attention for their unique charge transfer properties and advanced applications. However, their use in luminescent applications is hindered by challenges such as low luminescence efficiency, and broad emission spectra. Herein, a series of highly emissive thermally activated delayed fluorophores combining a boron‐based multi‐resonance acceptor and an arylamine donor within a spatially compacted structure is designed and synthesized. By strategically varying spatial compactness, donor/acceptor interactions are fine‐tuned, enabling precise control over high‐lying excited states with charge transfer characteristics while preserving narrow‐spectrum thermally activated delayed fluorescence. Key structural optimizations, including a planar acceptor and a space‐compacting methyl group, resulted in a highly compact configuration, boosting reverse intersystem crossing rates by over 20‐fold compared to the parent fluorophore. This design minimizes concentration‐induced quenching and spectral broadening, yielding superior solid‐state luminescence. The resulting organic light‐emitting diodes achieved ultranarrow electroluminescence (full width at half‐maximum of 22 nm) and a remarkable external quantum efficiency of 31.1%, along with reduced efficiency roll‐offs.