Luminescent Cyclometalated Platinum Acyclic Diaminocarbene Complexes as PhOLED Dopants

Abstract

There is continuing demand for the discovery of phosphorescent organometallic complexes for applications in phosphorescent organic light-emitting diodes (PhOLEDs), one of the most efficient and well-developed color display technologies. This study explores a class of phosphorescent cationic cyclometalated platinum complexes, using a range of cyclometalating (C^N) ligands paired with 2,6-dimethyphenyl isocyanide (CN dmp ) and acyclic diaminocarbene (ADC) ligands. The compounds are prepared via a two-step procedure. The intermediate bis-isocyanide complexes, with the general formula [Pt(C^N)(CN dmp ) 2 ](PF 6 ), are converted to the final mixed ADC/isocyanide compounds, [Pt(C^N)(ADC)(CN dmp )](PF 6 ), by nucleophilic addition of diethylamine to one of the isocyanides. Structural characterization by single-crystal X-ray diffraction reveals that the isocyanide trans to the neutral N-heterocycle of the C^N ligand is always the one converted to an ADC. Conversion of one isocyanide into an ADC brings about notable shifts in redox potentials and the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Photophysical studies reveal that the ADC compounds have blue-shifted photoluminescence relative to the isocyanide precursors, with higher photoluminescence quantum yields and longer lifetimes, particularly for complexes emitting in the blue region. Solution-processed PhOLEDs using two of the best-performing bis-isocyanide and ADC platinum complexes as the emitters were successfully fabricated and display sky-blue electroluminescence (EL).