High-performance TADF-OLEDs utilizing copper(I) halide complexes containing unsymmetrically substituted thiophenyl triphosphine ligands

Highly efficient OLEDs fabricated with thermally activated delayed fluorescent Cu(I) complexes have attracted significant attention. However, achieving both high quantum efficiency and short decay lifetimes remains a considerable challenge. Herein, we reported the successful synthesis and characterization of two rigid triphosphine ligands, each containing two unsymmetrically substituted thiophenyl rings, and their corresponding mononuclear copper(I) halide complexes, CuX(L1) and CuX(L2) [L1 = (2-PPh₂-C₄H₂S)₂(3-PPh), X = I (1), Br (2), Cl (3); L2 = (2-PPh₂-5-SiMe₃-C₄HS)₂(3-PPh), X = I (4), Br (5), Cl (6)]. The structures and photophysical properties of these complexes were thoroughly investigated. At room temperature, the powder samples of complexes 1–6 exhibited intense delayed fluorescence, ranging from yellow-green to yellow in color (λ_em = 553–581 nm, τ = 3.8–9.4 μs, and Φ = 0.19–0.29 for 1–3; λ_em = 565–589 nm, τ = 2.2–7.6 μs, and Φ = 0.36–0.61 for 4–6). The incorporation of two trimethylsilyl groups into the unsymmetrically substituted thiophenyl rings significantly improved the photoluminescence quantum yield (PLQY) and allowed for fine-tuning of the light-emitting color of the complexes. Among them, complex 4 displayed a high PLQY of 0.61 and a short decay lifetime of 2.2 µs. The radiative decay rate (k_r) was 2.8 × 10⁵ s⁻¹, comparable with that of Ir(III) complexes. Vacuum-deposited organic light-emitting devices incorporating complex 4 exhibited yellow emission, achieving a maximum external quantum efficiency (EQE) of 14.57% and a current efficiency of 33.44