Narrowband Emission in Pt(II) Complexes via Ligand Engineering for Blue Phosphorescent Organic Light-Emitting Diodes

In this study, three stable tetradentate Pt(II) complexes are synthesized and characterized, namely, Pt-biPh, Pt-biPh5tBu, and Pt-biPh4tBu, tailored for blue phosphorescent organic light-emitting diodes to realize high-efficiency and narrowband emissions via ligand engineering. Biphenyl (Pt-biPh) or tert-butyl-modified biphenyl (Pt-biPh5tBu and Pt-biPh4tBu) is introduced into the carbene unit of the ligand to control the intermolecular interactions between the Pt(II) phosphors. Pt-biPh, Pt-biPh5tBu, and Pt-biPh4tBu exhibit high photoluminescence quantum yields of 74%, 84%, and 92% with exciton lifetimes of 2.2, 2.3, and 2.5 µs, respectively, demonstrating rapid and efficient light emission. Furthermore, Pt-biPh, Pt-biPh5tBu, and Pt-biPh4tBu show maximum external quantum efficiency (EQE) values of 18.1%, 19.0%, and 21.8%, respectively. Pt-biPh5tBu and Pt-biPh4tBu exhibit narrowband emission with a full width at half maximum of 21 nm owing to the small vibrational emission because of their sterically hindered and bulky ligand structures. Moreover, phosphor-sensitized thermally activated delayed fluorescence devices employing a Pt-biPh4tBu sensitizer achieve a high EQE of 28.6%. In particular, Pt-biPh4tBu performs better than the state-of-the-art phosphor as the sensitizer of the blue phosphor-sensitized thermally activated delayed fluorescence devices in terms of the EQE.