Luminescent Copper(I) Complexes with Dual Bridging Ligands and Excitation-Dependent Emission

Copper(I) complexes exhibit efficient luminescent properties, such as thermally activated delayed fluorescence (TADF), making them attractive for applications in organic light-emitting diodes or sensors. While copper(I) complexes with 2-pyridyl phosphine ligands have been well studied, this work explores the effect of positional isomerism by using 3-pyridyl phosphine instead of 2-pyridyl phosphine, leading to a previously unreported double-bridged distorted tetrahedral structure with an unusually long average of the Cu···Cu distance of 6.238 Å. This new structural motif, enabled by the unique coordination behavior of the 3-pyridyl ligand, leads to excitation-dependent emission behavior with typical TADF behavior and photoluminescence quantum yields ranging from 0.11 to 0.83. Quantum mechanical calculations indicate that the excited-state transitions exhibit primarily metal-to-ligand and halide-to-ligand charge transfer character or a combination of both, which enhances the TADF response. This study highlights how small changes in the ligand topology can lead to fundamentally different coordination modes and photophysical response. The results offer structure-property relationships for copper(I)-based emitters and the development of future ligand derivatives for tunable optical properties.