Synthesis and Characterisation of Cyclometallated Iridium (III) Tris-triazole Bidentate Complexes and Exploratory Attempts Towards Chromium (III) Analogues

Makin, Amber and Fletcher, Nick (2026) Synthesis and Characterisation of Cyclometallated Iridium (III) Tris-triazole Bidentate Complexes and Exploratory Attempts Towards Chromium (III) Analogues. Masters thesis, Lancaster University.

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Abstract

Cyclometallated iridium(III) complexes containing N-heterocyclic carbene (NHC) ligands have gained significant research interest due to their highly tuneable photophysical properties and potential applications in areas such as organic light-emitting diodes (OLEDs), photocatalysis, and bioimaging. Despite this, controlling emission behaviour, quantum efficiency, and geometric structure through ligand design remains an important challenge in the development of high-performance phosphorescent materials. In addition, extending these design principles to earth-abundant first-row transition metals such as chromium(III) could provide access to more sustainable luminescent systems. This research investigates how the modification of cyclometallating NHC ligands influences the structure, geometric isomer distribution, and photophysical behaviour of luminescent iridium(III) complexes and further goes on to explore whether these ligand design strategies can be transferred to chromium(III) systems to produce analogous complexes. A range of substituted 1,2,4-triazolium precursors were synthesised and subsequently functionalised to produce six cyclometallating NHC ligands with one of three electronic phenyl substituents (H, CF₃, NO₂) and one of two different steric substituents on the triazole (methyl or benzyl). These ligands underwent complexation with iridium(III) to yield six novel tris-cyclometallated complexes, with isolable mer and fac isomers obtained for four complexes and pure mer isomers obtained for the other two. The observed mer/fac ratios were found to be strongly dependent on ligand substitution. Increasing steric bulk was found to favour the formation of the mer isomer under kinetic control, while installing electron-withdrawing substituents promoted fac isomer formation in the less sterically hindered systems. Single-crystal X-ray diffraction showed a clear correlation between ligand substitution, geometric distortion, and crystal packing, including evidence of stabilising π- π stacking interactions in the benzyl-substituted mer isomers. Photophysical studies on these complexes demonstrated that while electron-withdrawing substituents significantly influence absorption characteristics, emission maxima remained largely unchanged when modifying the non-coordinating ligand substituents. This is consistent with what is expected of emission from a triplet metal to ligand charge transfer (MLCT) state. Emission efficiencies, however, were found to be strongly affected by steric bulk and geometric isomerism through their influence on non-radiative decay pathways. Attempts to extend these ligand frameworks to chromium(III) complexes were generally unsuccessful, but highlighted fundamental differences in coordination chemistry between Ir(III) and Cr(III), including slower ligand substitution kinetics, hard-soft acid–base mismatch, and extreme air sensitivity. Overall, this work establishes clear relationships between the structure and properties of Ir(III) NHC emitters and defines the key challenges faced in extending these ligand design strategies to the synthesis of analogous NHC-based earth-abundant metal systems.