Red-Emitting Cyclometalated Platinum(II) Complexes with Pyrrolide and Phenoxide-Based Ancillary Ligands

Phosphorescent platinum(II) complexes often have low photoluminescence quantum yields (Φ_PL) due to the less effective spin-orbit coupling (SOC) in square-planar geometries, leading to relatively small radiative rate constants (<i>k</i>_r). In this work, seven new red-emitting heteroleptic cyclometalated Pt(II) complexes are described, with the goal of investigating ancillary ligands that promote fast <i>k</i>_r values. The heteroleptic four-coordinate complexes include 1-phenylisoquinoline as the cyclometalating ligand, paired with an anionic chelating ancillary ligand with either <i>N</i>^∧<i>N</i>, <i>O</i>^∧<i>O</i>, or mixed <i>N</i>^∧<i>O</i> coordination. More specifically, the ancillary ligands all contain anionic pyrrolide or phenoxide rings, paired with imine, carbonyl, pyridyl, or imidazolyl neutral donors. The seven compounds are structurally characterized by multinuclear NMR and six of the seven by single-crystal X-ray diffraction. A comprehensive study of photophysical properties shows that the compounds all exhibit red phosphorescence, some with a clear vibronic structure indicative of the substantial ³(π → π*) character in the emissive excited state, involving the cyclometalating ligand, while others exhibit a photoluminescence profile suggestive of pronounced charge-transfer character in the triplet state. When measured in poly(methyl methacrylate) films, the highest <i>k</i>_r achieved is 1.2 × 10⁵ s^-1, with photoluminescence quantum yields ranging from 0.14 to 0.38.