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 Φ_PL of 0.74 and a short τ_TADF of 1.9 μs, which indicates a fast enough k_r,TADF of 3.9×10⁵ s⁻¹ and a depressed k_nr of 1.4×10⁵ s⁻¹. Such outstanding luminescent properties are attributed to the proper overlap of HOMO and LUMO on Cu^I d orbitals that guarantees not only small ΔE_ST but also sufficient transition oscillator strength for fast (Formula presented.). 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 Cu^I TADF emitters.