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

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.