In Situ Observation of Degradation by Ligand Substitution in Small-Molecule Phosphorescent Organic Light-Emitting Diodes

Solutions of facial-tris(1-phenylpyrazole)Ir(III) ( fac-Ir(ppz)3),when dissolved in either tert-butyl isocyanide or in solid films of 2-naphthylisocyanide, undergo replacement of a ppz ligand by the isocyanidemolecules after irradiation with UV light as demonstrated by liquid chromatograph mass spectrometer analysis. Similarly, solutions of Ir(ppz)3 and bathophenanthroline (BPhen) in CH2Cl2 or acetone-d6 form a brightly emissive species, [Ir(ppz)2(Bphen)]+ when irradiated with UV light as established by optical, mass, and 1H nuclear magnetic resonance spectroscopy. Electroluminescent data from blocked organic light-emitting diode (OLED) devices demonstrate that both mer- and fac-(Ir(ppz)3) dissociate a ligand and coordinatea neighboring BPhen molecule when the device is operated at moderate to high current levels. These experiments offer direct evidence of the dissociation of a metal-ligand bond and subsequent ligand substitution as a degradation pathway in active OLED devices during operation and provide a route to assay in situ the stability of future dopants