Toward Highly Planar d⁸-Configured Metal Complexes with Tunable Phosphorescence, ¹O₂ Photogeneration, and Glass-Forming Abilities

The synthesis, characterization, and photophysical properties of 16 d8-configured metal complexes featuring Pt(II) or Pd(II) centers with N*N∧C-coordinated ligands is presented. Key findings include the discovery that distortion of the coordination plane improves solubility, as observed for compounds with a pyridine-N-donor instead of a thiazole unit. In addition, we found that substitution of the chlorido coligand by a monodentate cyanido unit enhances the emitters' performance by decreasing the nonradiative decay rate. The switch from Pt(II) to Pd(II) resulted in weaker ligand field splitting and reduced spin–orbit coupling, leading to longer average lifetimes without luminescence at room temperature. Interestingly, complexes with a cyclometalated thiophenyl moiety exhibited a red-shifted luminescence and dual emission. Finally, we observed that the photoinduced generation of singlet dioxygen (1O2), a highly reactive oxygen species, is critically influenced by the chelated metal centers and by the monodentate coligands, as they control the 1O2 photoproduction quantum yields. These results have significant implications for photocatalysis, optoelectronics, and biomedical applications.