Substituents’ Effect on the Photophysics of Trinuclear Copper(I) and Silver(I) Pyrazolate–Phosphine Cages

A series of structurally similar trinuclear macrocyclic copper(I) and silver(I) pyrazolate complexes bearing various short-bite diphosphine R2PCH(R′)PR2 ligands are reported. Upon diphosphine coordination, the planar geometry of the initial complexes undergoes bending along the line between two metal atoms coordinated to the phosphorus moieties. The complexes based on dcpm ligands (R = cyclohexyl, R′ = H, Ph) do not exhibit dynamic behavior in solution at room temperature on the 31P NMR time scale as it was previously observed for similar trinuclear copper complexes bearing the dppm (R = Ph, R′ = H) scaffold. All copper(I) complexes exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. Importantly, the use of aliphatic substituents on the phosphorus atoms instead of aromatic ones leads to an almost double increase in the quantum efficiency (ΦPL) of photoluminescence by eliminating nonradiative decay from the 3LCPh states of the dppm aromatic rings. The higher donating ability of the substituents in the pyrazolate ligand (CF3 vs CH3) lowers the energy of the metal-centered excited state, allowing for a significant metal impact on the T1 state. Finally, the Ag(I) complex displays an emission efficiency of approximately 14%, being the highest among known trinuclear silver(I) pyrazolate homometallic derivatives.