Influence of Sulfur Oxidation State and Substituents on Sulfur-Bridged Luminescent Copper(I) Complexes Showing Thermally Activated Delayed Fluorescence

Copper­(I) complexes are seen as more sustainable alternatives to those containing metal ions such as iridium and platinum for emitting devices. Copper­(I) complexes have the ability to radiatively decay via a thermally activated delayed fluorescence (TADF) pathway, leading to higher photoluminescent quantum yields. In this work we discuss six new heteroleptic Cu­(I) complexes of the diphosphine–diimine motif. The diphosphine ligands employed are (oxidi-2,1-phenylene)­bis­(diphenylphosphine) (DPEPhos), and the diimine fragments are sulfur-bridged dipyridyl ligands (DPS) which are functionalized at the 6,6′-positions of the pyridyl rings (R = H, Me, Ph) and have varying oxidation states at the bridging sulfur atom (S, SO2). The proton (Cu-DPS, Cu-DPSO2) and phenyl (Cu-Ph-DPS, Cu-Ph-DPSO2) substituted species are found to form monometallic complexes, while those with methyl substitution (Cu-Me-DPS, Cu-Me-DPSO2) are found to have a “Goldilocks” degree of steric bulk leading to bimetallic species. All six Cu­(I) complexes show emission in the solid state, with the photophysical properties characterized by low temperature steady-state and time-resolved spectroscopies and variable temperature time-correlated single photon counting. Cu-DPS, Cu-DPSO2, Cu-Me-DPS, Cu-Me-DPSO2, and Cu-Ph-DPSO2 were shown to emit via a TADF mechanism, while Cu-Ph-DPS showed photoluminescence properties consistent with triplet ligand-centered (3LC) emission