Recent advances in using strong-field ancillary ligands to support blue-emitting iridium(iii) and platinum(ii) complexes

Abstract

The design of blue-phosphorescent metal compounds with high photoluminescence (PL) quantum yields, good color purity, short PL lifetimes, and good photostability has long been a significant challenge. Strong-field ancillary ligands are widely employed to destabilize the deleterious metal-centered (³MC) states and access high-performing blue-phosphorescent iridium and platinum complexes. These materials are attractive for various optoelectronic applications, most prominently organic light-emitting diodes (OLEDs). This review highlights our work and other groups’ recent research on blue-emitting Ir(III) and Pt(II) phosphors incorporating strong-field ligands, and provides an outlook on the future of this field. Isocyanides, cyanide (free or terminated by a Lewis acid), and N-heterocyclic carbenes (NHCs) are all highlighted as common classes of strong-field ligands used to improve blue phosphorescence. Each of these ligand classes offers distinct advantages in the design of blue phosphors, which are highlighted in this work. NHCs are inherently strong donors with well-established synthetic chemistry with 5d metals, and when used as cyclometalating ligands they impart larger HOMO–LUMO gaps than most other classes of cyclometalating ligands. Cyanides and isocyanides are reasonably strong σ donors and good π acceptors, and both can be functionalized to further improve blue-phosphorescence metrics. In particular, isocyanides can be converted to acyclic diaminocarbenes by nucleophilic addition, even stronger σ-donating ligands than NHCs, and our group's efforts to install ADCs and further enhance the PL quantum yields of blue emitters are described.