Boron and Phosphorus Complexes of π-Conjugated Ligands

This work describes the synthesis and characterization of boron and phosphorus complexes of π-conjugated ligands. The fundamental properties of these compounds, such as bonding characteristics, optical properties, and electronic structure, are explored. The impact of ligand design on the coordination environment of the boron or phosphorus centre is considered, as well as the effect of that heteroatom on the compound’s optoelectronic properties. The synthesized compounds often possess noteworthy optical and electronic properties, such as reversible electrochemical redox and photoluminescence. Chapter two is concerned with the isolation of a phosphenium cation supported by a conjugated 2-pyridylhydrazone ligand. Density functional theory calculations and solid-state metrics found a trigonal pyramidal phosphorus centre with a weak interaction with the pyridine moiety. Treatment of these heterocycles with [Rh(COD)Cl]2 demonstrated divergent reactivity based on substitution at phosphorus. The third and fourth chapters deal with boron complexes of tridentate acyl pyridylhydrazones with N,N′,O and N,N′,N′′ coordination modes, respectively. These ligands allowed for facile boron incorporation through treatment with arylboronic acids. Boron complexes of N,N′,O ligands were weakly fluorescent and can be reversibly reduced. By moving to an N,N′,N′′ coordination mode, distinct optical properties were realized. While the neutral compound was not emissive, protonation of the methylamine donor turned on efficient fluorescence by preventing quenching of the excited state via photoinduced electron transfer. Chapter five focuses on boron complexes of asymmetric pyridyl ethers. In the solid-state these compounds exhibit both fluorescence and phosphorescence, visible as a bathochromically shifted afterglow. Phosphorescence in organic systems at room temperature is uncommon, especially without heavy atoms or phosphorgenic functional groups, and the underpinnings of this unexpected phenomenon were considered. The sixth and final chapter summarizes the key findings of each chapter and proposes avenues of future exploration. Overall, this thesis provides fundamental insight into the construction and design of main group complexes of π-conjugated ligands. The significance of a thorough understanding of the relationship between chemical structure and optoelectronic properties will be highlighted. By elucidating this relationship, avenues for the practical application of these materials in sensing, biological imaging, and organic electronics emerge.