Synthesis and Photophysical Properties of New Di- and Mononuclear Phosphorescent Iridium(III) Complexes

Abstract

Cyclometallated Ir(III) complexes have attracted significant attention as luminescent materials due to a range of favourable properties, such as their high photoluminescence quantum yields, microsecond phosphorescence lifetimes, thermal stability, robust electrochemistry and synthetic versatility, which has enabled their emission to be tuned across the visible spectrum from the near-UV to the near-IR. They have been applied to various applications, such as in bioimaging, sensing, photocatalysis, and as sensitisers for singlet oxygen and as emitters in organic light emitting devices (OLEDs).
While mononuclear Ir(III) complexes have been extensively studied, dinuclear derivatives have received less attention. This is likely related to their high molecular weights, which limits their application to solution-processed rather than vacuum-processed OLEDs. Historically, diiridium complexes have also been noted to exhibit poorer luminescence efficiency than their mononuclear analogues. Nevertheless, a number recent of studies have indicated that correctly designed dimers can indeed be highly emissive and interest in dinuclear Ir(III) complexes has increased. They are particularly interesting as they introduce a bridging ligand, which can be used to modify the electronic communication between the Ir centers as well as the various photophysical and physical properties of the complex.
In this thesis a range of new diiridium complexes bridged by hydrazide (N^O) chelates will be discussed. In Chapter 2 further structural variation of the peripheral cyclometallating and bridging ligands of the prototypical complex 34 is explored through complexes 35–38. Results indicated that functionalisation of either the bridging or peripheral ligands can facilitate colour tuning, and the matrix dependent photophysical properties of 37 and 38 were explained. Intramolecular π–π interactions were also observed between the peripheral and bridging ligands of the complexes. In Chapter 3, such interactions were enhanced through fluorination of aryl moieties on the bridging ligands, and were utilised to modify the photophysical properties of diiridium complexes (complexes 62–66). The first examples of sky-blue emitting diiridium complexes are also presented (complexes 68–70). In Chapter 4 the series was extended through the application of topical bulky 1,2-diarylimidazole cyclometallating ligands, leading to the first example of sky-blue aggregation-induced phosphorescent emission (AIPE) from a diiridium complex. Inspired by the findings in Chapter 3, in Chapter 5 mononuclear Ir(III) diasteromers featuring chiral oxazoline ancillary ligands were investigated as a platform for a deeper fundamental study into the effect of intramolecular π–π interactions on the photophysical properties of Ir(III) phosphors.