Cyclometalated Pt(II)C^N phosphors bearing a bis(diphenylphorothioyl) amide ligand: synthesis and photophysical properties

Six Pt(II) complexes bearing a primary phenylpyridine (ppy) ligand or a ppy derivative and two Pt(II) complexes bearing a primary 8-hydroxyquinoline (hqnl) or 7-bromo-8-quinolinol ligand have been prepared via coordination with an auxiliary bis(diphenylphosphothionyl) amide (HStpip) ligand. All eight complexes were characterized by NMR (¹H,¹³C, and ³¹P), HRMS, and elemental analysis. In addition, complexes 2, 4, 5, and 8 were characterized by single crystal XRD. The eight complexes were not emissive in solution under N₂ at room temperature (r.t.). However, they all exhibited an intense emission in the solid state and in 2 wt.% polymethyl methacrylate (PMMA) films. In particular, 5 bearing a primary 2-([1,1'-biphenyl]-3-yl)-4-phenylpyridine ligand exhibited a triplet emission at 525 and 552 nm with a quantum yield of 80% in pure solid state. Density functional theory (DFT) and time dependent (TD) DFT calculations were performed for optimization and predicting the excitation properties of 2, 4, 5, and 8. The computational results reveal that the intrinsic triplet emissions of 2, 4, and 5 are the result of combinations of ligand-to-metal charge transfer (LMCT) (π*(ppy) → d(Pt)) and ligand-to-ligand charge transfer (LLCT) (π*(ppy) → p(Stpip)), whereas the triplet emission of 8 shows a π*–π character with a small LMCT (π*(hqnl) → d(Pt)) component.