Isomeric π‐Extension Strategy Enables Efficient Copper(I) Emitters with High Electroluminescence Efficiency

Abstract Luminescent copper(I) complexes are emerging as sustainable and cost‐effective alternatives to conventional iridium(III) and platinum(II) phosphors in organic light‐emitting diodes (OLEDs). However, achieving electroluminescence efficiencies comparable to those of noble metal‐based emitters remains a formidable challenge. Here, an isomeric π‐extension strategy is adopted to develop two new Cu(I) complexes, Cu‐23BF and Cu‐43BF, by precisely tuning the fusion site of a benzofuran subunit on a carbazole ligand within a carbene‐metal‐amide (CMA) motif. Compared to the prototype complex Cu‐12BF, these new complexes exhibit more coplanar geometries, elongated molecular structures, and increased electron‐hole separation in their excited states, resulting in near‐unity photoluminescence quantum yields (up to 97%) and relatively high horizontal dipole ratios (up to 78%) in thin films. Notably, Cu‐43BF delivers a short thermally activated delayed fluorescence lifetime of 0.65 µs and a fast radiative rate on the 10⁶ s⁻¹ order, attributed to its well‐separated frontier molecular orbitals and favorable excited state ordering. As a result, the Cu‐43BF‐based OLED achieves a high external quantum efficiency of 29.4%, among the highest reported for Cu(I)‐based OLEDs. This work not only provides a practical and effective strategy for designing highly efficient Cu(I) emitters but also highlights the future direction for Cu(I)‐based OLEDs.