Improved Quantum Yields in Blue-Phosphorescent Cyclometalated Platinum(II) Complexes Supported by Acyclic...

Blue phosphorescence in organometallic compounds remains a significant challenge due to the thermal accessibility of triplet metal-centered (³MC) states. We have previously shown that both nucleophilic and electrophilic ligand-based functionalization strategies on cyclometalated platinum complexes can be combined to destabilize ³MC states and optimize blue phosphorescence. Whereas our previous work used exclusively 4-methoxyphenyl isocyanide as the starting point for nucleophilic addition, here we include a bulkier aryl isocyanide, 2,6-dimethylphenyl isocyanide, as well as the alkyl isocyanides tert-butyl isocyanide and 1-adamantyl isocyanide. In total, four sets of cyclometalated platinum complexes with the cyclometalating ligands 2-(2,4-difluorophenyl)pyridine (F₂ppy, sky-blue phosphorescence) and 1-methyl-3-phenylimidazol-2-ylidene (pmi, deep-blue phosphorescence) were successfully synthesized, with all four distinct synthetic steps tolerating the substituent variations on the isocyanides. A comprehensive photophysical analysis of all compounds was performed, and we show that the 1-adamantyl substituent leads to suppressed nonradiative decay rates and substantially improved photoluminescence quantum yields (Φ_PL), achieving values of Φ_PL = 0.89 in the sky-blue region and Φ_PL = 0.73 in the deep-blue region in the final products that involve both nucleophilic and electrophilic functionalization. These values represent 1.2-fold and 1.8-fold improvements, respectively, over previous designs.