Strongly Phosphorescent Palladium(II) Complexes of Tetradentate Ligands with Mixed Oxygen, Carbon, and Nitrogen Donor Atoms: Photophysics, Photochemistry, and Applications

摘要

Transition-metal complexes that show strong phosphorescence in solution at room temperature usually contain a Ru~(II), Os~(II), Ir~(III), or Pt~(II) ion, but luminescent Au~(III) complexes have also been studied increasingly. In contrast to the extensive literature on luminescent Pt~(II) complexes examples of luminescent Pd~(II) complexes that contain non-porphyrin ligands and show decent emission quantum yields (Φ_(em)) are sparse. The crux of this long-standing situation lies in the significantly lower ligand-field splitting of the Pd~(II) ion. Accordingly, the metal-centered d-d states of Pd~(II) complexes are always thermally accessible, which enables facile population of the antibonding 4d_(x~2-y~2) orbital and hence leads to effective nonradiative decay through severe excited-state structural distortion.