Solid state luminescence of phosphine-EWO ligands with fluorinated chalcone skeletons and their PdX2 complexes: metal-promoted phosphorescence enhancement

Complexes trans-[PdX2L2] (X = Cl and Br), where L is 1-(PR2),2-(CH=CH–C(O)Ph)-C6F4 (R = Ph, Cy, and iPr), display phosphorescent emission in the solid state, whereas due to their substantially lower lifetimes, the free ligands exhibit fluorescent behaviour. Alternatively, structurally identical derivatives with halide replaced by CN− or Pd replaced by Pt are non-emissive. DFT calculations explain this diverse behaviour, showing that the hybridization of orbitals of the MX2 moiety with those of the chalcone fragment of ligands is significant only for the LUMO of the emissive compounds. In other words, in our complexes, only MLMCT processes (LM = Metal-perturbed Ligand-centered orbital) lead to observable luminescence.