Phosphorescence enhancement by close metal–metal interaction in T1 excited state in a dinuclear copper(I) complex

The dinuclear copper(I) complex [Cu2(µ-dppm)2(lact)(µ-lact)] (1) (dppm = bis(diphenylphosphino) methane; lact = L-(+)-lactate) was synthesized and fully characterized both in solution and solid state. Variable temperature NMR experiments (1H and 31P), conductivity measurements and infrared spectroscopy, suggest the occurrence of a fluxional behavior in solution involving the lactate anion. The crystal structure shows the presence of both monodentate and bridged lactate in the complex. In the solid state, 1 shows green phosphorescent emission characterized by a very large Stokes shift (161 nm, 1.09 eV) and a good absolute quantum yield (0.43). Calculations performed at the Density Functional Theory level demonstrate that the electronic transition responsible for the emission originates from a triplet excited state where the shortening of the Cu⋯Cu distance plays a crucial role.