Over the LEC rainbow: colour and stability tuning of cyclometallated iridium(III) complexes in light-emitting electrochemical cells

This review surveys the design of cyclometallated iridium(III) complexes for applications in light-emitting electrochemical cells (LECs). Typical iridium-containing ionic transition metal complexes (Ir-iTMCs) are of the general type [Ir(C^N) 2 (N^N)] + where H(C^N) is a cyclometallating ligand and N^N is a 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) or related, chelating ligand. In a more recently introduced series of emitters, the N^N ligand has been replaced by a heterocyclic carbene. Since single component white-light emitting LECs are difficult to design, a more common approach is through colour-mixing. Colour-tuning of [Ir(C^N) 2 (N^N)] + emissions is achieved by manipulation of the C^N and/or N^N units because the HOMO and LUMO are, respectively, localized on the Ir/C^N or N^N domains. This review surveys synthetic strategies to Ir-iTMCs and illustrates the way in which ligand design can address emission colour and LEC stability. We consider the different approaches used to obtain blue-emitters; highly efficient and stable deep-blue emitters are required to complement the wide variety of orange emitting Ir-iTMCs for application in white-light devices. We also review the ways in which deep-red-emitting Ir-iTMCs are accessed. In addition to targeting changes in the Ir-iTMC band-gap and therefore the colour of the emission, ligand-design is critical to improving LEC stability and turn-on times; only a handful of stable deep-red-emitting LECs have been reported. One successful strategy for improving device lifetime is the introduction of intra-cation π-stacking interactions which protect the iridium metal centre in the excited state of the complex; however, a wide range of investigations suggests that the choice of stacked rings is significant and that the involvement of the cyclometallated ring(s) may be important. There are now many data to confirm that the presence of peripheral bulky substituents is beneficial, resulting in greater spatial separation of the Ir-iTMC cations in the solid state which, in turn, reduces self-quenching. Finally, we comment on why it is often difficult to legitimately compare results of different investigations of LEC performances