Anionic cyclometallated Pt(ii) square-planar complexes: new sets of highly luminescent compounds

Two series of novel NBu₄⁺ salts of anionic cyclometallated Pt(ii) complexes were synthesized and fully characterized. These highly luminescent compounds (NBu₄[(C^N)Pt(O^N)] and NBu₄[(C^N)Pt(O^O)]) are incorporated as testing examples of cyclometallating ligands 2-phenylpyridine (PhPy), 2-(2,4-difluorophenyl)-pyridine (F₂PhPy) and 2-thienylpyridine (ThPy), and a benzo[h]quinoline (Bzq) fragment. All complexes display a square-planar coordination sphere, wherein the "(C^N)Pt" fragment is completed either by an O^N orotate (Ort) or an O^O tetrabromocatecholate (Cat) ligand. The HOMO and LUMO levels of all complexes were estimated by cyclic voltammetry and a comprehensive electrochemical and photophysical study was performed. The new complexes are emissive in solution at 298 K and the NBu₄[(ThPy)Pt(Ort)] complex displays good photosensitizing properties (Φ = 28% in deaerated solution vs. Φ = 1.4% in the presence of O₂). Both series of NBu₄[(C^N)Pt(Ort)] and NBu₄[(C^N)Pt(Cat)] complexes are highly luminescent in the solid state (emission quantum yields from 10 to 85%). Remarkably, the square-planar Pt(ii) anionic complexes showed an important increase in luminescence quantum yields on changing from the dilute solution to the solid state (the most significant from 0.13% to 85% for the NBu₄[(PhPy)Pt(Ort)] complex, an ideal candidate as an active species for LEECs).