From blue to red : emission colour tuning in iridium(III) complexes with sulfur-functionalized ligands for LEECs

Abstract

Light-emitting electrochemical cells (LEECs) are a promising class of solid state lighting devices. Their working mechanism is based on the presence of mobile ionic species within the device, making them independent of the work function of the electrodes and layer thickness. Compared to multilayer organic light emitting diodes (OLEDs), the architecture and assembly is much simpler, resulting in reduced manufacturing costs. Cationic iridium(III) complexes of the type [Ir(C^N)2(N^N)]+, where C^N is a cyclometallating ligand (e.g. 2-phenylpyridine (ppy)) and N^N is an ancillary ligand (e.g. 2,2'-bipyridine (bpy)), have been extensively used in LEECs due to their excellent properties. High quantum yields and stability make them well suited for application in light emitting devices. Furthermore, the emission colour can be tuned easily, spanning the whole visible region. The frontier orbitals are spatially separated in this type of complex and by careful design of the ligands, desired blue- or red-shifts in the emission maximum are possible. In this thesis, the synthesis, characterization and LEEC performance of iridium(III) complexes containing sulfur-functionalized ligands are described. Thioether- and sulfone-based cyclometallating ligands are introduced in Chapter I, resulting in complexes with yellow to green emission. In Chapter II, the influence on photophysical, electrochemical and device properties of iridium complexes with regioisomeric cyclometallating ligands containing a methylsulfonyl group is investigated. Based on the results of the previous chapters, the combination of sulfone-substituted cyclometallating ligands with electron-rich ancillary ligands to obtain blue emitting complexes is described in Chapter III. Chapter IV summarizes a series of iridium complexes with benzothiazole-based ancillary ligands, yielding red emitters with exceptionally long LEEC lifetimes. Ongoing work in the field of glycosylated iridium complexes is reported in Chapter V