Cu─Cu Dimerization Suppresses Pseudo‐Jahn–Teller Distortion to Enhance Electroluminescence in Tricoordinate Clusters

ABSTRACT Copper(I) emitters are promising low‐cost and high‐efficiency electroluminescent (EL) materials for large‐scale applications, but their performances are limited by severe nonradiative quenching due to excited‐state structural relaxation. Tricoordinate Cu(I) systems with trigonal geometries are believed to be superior in excited‐state structural stability, but still suffer from pseudo‐Jahn–Teller distortion. Here we report tricoordinate Cu 2 clusters with Cu─Cu dimerization to alleviate the pseudo‐Jahn–Teller effect and enhance thermally activated delayed fluorescence (TADF). Through further introduction of electron‐donating and rigid phenoxazine (PXZ) groups, [DPXZCzDP] 2 Cu 2 exhibits ligand‐centered excited states and multiple reverse intersystem crossing (RISC) channels, giving rise to the outstanding photoluminescence quantum yield of 91% and a record external quantum efficiency of ∼24.6% among tricoordinate Cu(I) emitters, accompanied by reduced efficiency roll‐off. This work validates combined ligand and coordination engineering as a robust strategy to boost luminescence performance of Cu(I) systems and advances low‐cost, high‐efficiency cluster light‐emitting devices for practical display and lighting applications.