Inhibition of Aggregation‐Caused Quenching in “Half‐Sandwich” Red‐PhosphorescentCyclometalated Platinum(II) Complexes

Aggregation‐caused quenching (ACQ) is a significant challenge encountered in luminescent square‐planar Pt(II) complexes, especially in condensed phases such as neat or doped films. Herein, five red‐emitting heteroleptic bis‐bidentate cyclometalated Pt(II) complexes are described, supported by sterically encumbering “half‐sandwich” β ‐ketoiminate or 2‐picolinamide ancillary ligands. The electron‐rich nature of the ancillary ligands promotes fast radiative decay, and the “half‐sandwich” design places steric bulk directly over the metal center and inhibits ACQ. As a result, the photoluminescence color profiles and quantum yields of the complexes are largely preserved at different concentrations in doped films. All five presented complexes exhibit red phosphorescence with moderate Φ PL up to 0.45, obtained in poly(methyl methacrylate) films at 2 wt%. These values gradually drop by only 0.06–0.12 when increasing the concentration to 5 and 10 wt%, with no significant changes in the photoluminescence emission profile observed. The structures of all five complexes were characterized by multinuclear NMR, with two of them by single‐crystal X‐ray diffraction (XRD). This work enriches the library of red‐phosphorescent cyclometalated Pt(II) complexes, highlighting the effectiveness of “half‐sandwich” ligand architectures in ACQ inhibition.