A Configurationally Confined Thermally Activated Delayed Fluorescent Two‐Coordinate Cu^I Complex for Efficient Blue Electroluminescence

Thermally activated delayed fluorescence (TADF) from linear two-coordinate coinage metal complexes is sensitive to the geometric arrangement of the ligands. Herein we realize the tuning of configuration from coplanar to orthogonal gradually by variation of substituents. In a complex with confined twist configuration, its blue emission peaking at 458 nm presents a high ΦPL of 0.74 and a short τTADF of 1.9 μs, which indicates a fast enough kr,TADF of 3.9×105 s-1 and a depressed knr of 1.4×105 s-1 . Such outstanding luminescent properties are attributed to the proper overlap of HOMO and LUMO on CuI d orbitals that guarantees not only small ΔEST but also sufficient transition oscillator strength for fast kr,S1${{k}_{{\rm r},{{\rm S}}_{1}}}$ . Vacuum-deposited blue OLEDs with either doped or host-free emissive layer present external quantum efficiencies over 20 % and 10 %, respectively, demonstrating the practicality of the configurationally confined strategy for efficient linear CuI TADF emitters.