Phenyl- and Pyrazolyl-Functionalized Pyrimidine: Versatile Chromophore of Bis-Tridentate Ir(III) Phosphors for Organic Light-Emitting Diodes

There is growing interest in the bis-tridentate Ir(III) emitters as they are expected to display both improved emission efficiency and improved photostability. Herein, we turned to the new emitters m2h-1–3 and m6h-1–3, bearing a pincer carbene ancillary and a chromophoric chelate derived from judiciously selected phenyl-pyrimidine-pyrazole entities (pzm2hF)H2 and (pzm6hF)H2, which differ in terms of the location of phenyl and pyrazole substituents on the central pyrimidine. Density functional theory calculations revealed a notable change in the spin density distribution from the pyrimidine-pyrazolate entity in m2h to the pyrimidine-phenyl fragment in m6h. As a consequence, the m6h emitters exhibited both shortened emission lifetimes and improved stabilities during extensive photolysis in solution, while corresponding organic light-emitting diodes (OLEDs) doped with green-emitting m6h-1 and sky-blue-emitting m6h-2 and m6h-3 exhibited external quantum efficiencies of 17.6, 15.9, and 17.6%, respectively, superior to those of all of their m2h counterparts at a practical luminance of 103 cd/m2. This finding suggests a new methodology for fine-tuning the electronic transition that is important to high-performance and durable phosphorescent OLEDs.