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

Abstract

Solutions of facial-tris­(1-phenylpyrazole)­Ir­(III) (<i>fac</i>-Ir­(ppz)₃), when dissolved in either <i>tert</i>-butyl isocyanide or in solid films of 2-naphthylisocyanide, undergo replacement of a ppz ligand by the isocyanide molecules 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-<i>d</i>₆ 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 <i>mer</i>- and <i>fac</i>-(Ir­(ppz)₃) dissociate a ligand and coordinate a 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