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 (oxydi- 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 (TCSPC). 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.