Highly Efficient Mechanochromic Thermally Activated Delayed Fluorescence in the Deep Red to Near‐Infrared in Copper(I) [2.2]Isoindolinophanyl‐Carbene Carbazolates

ABSTRACT Low energy triplet emitters are highly relevant for the development of OLEDs and fiber optics‐based IT applications, but typically suffer from nonradiative decay due to the energy gap law (EGL). Excited state deactivation can be limited by enhancing the radiative decay rate via thermally activated delayed fluorescence (TADF), bypassing spin‐forbidden phosphorescence. We report on linear copper(I) complexes bearing a recently reported [2.2]isoindolinophanyl‐carbene (iPC) ligand as potent excited state π‐acceptor. The compounds show efficient TADF from ligand‐to‐ligand charge transfer ( 1/3 LLCT) states with reverse intersystem‐crossing (RISC) of k RISC = 0.6–21·10 9 s −1 , quantum yields of up to 0.8 and k TADF of 0.8‐1.9·10 6 s −1 that are among the fastest for Cu I emitters, outcompeting traditional triplet emitters based on Ir III and Pt II as well as organic deep red to near‐IR TADF emitters. While yellow to red emission is observed in single crystals, embedding the complexes into polymers or grinding shifts the luminescence into the deep red to near‐IR. The mechanochromism is due to disruption of C─H⋯π interactions between the ligands, reducing the energy gap between the ground state and 1/3 LLCT states. The Cu I iPC complexes bear potential for devices operating under electroluminescent conditions as demonstrated by a proof‐of‐concept deep‐red OLED application.