Thesis Format
Integrated Article
Degree
Doctor of Philosophy
Program
Chemistry
Supervisor
Gilroy, Joe B.
2nd Supervisor
Ragogna, Paul J.
Joint Supervisor
Abstract
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.
Summary for Lay Audience
Some materials have important properties, such as the ability to emit light or conduct electricity, which are a result of their chemical structure. By furthering an understanding of the relationship between the chemical structure of a material and these desirable properties we can understand how to design better materials. The molecules prepared in this thesis are composed of conjugated ligands, which have alternating single and double bonds that enable desirable properties like fluorescence or conductance. These ligands are bound to phosphorus or boron atoms to affect change in these properties. How the phosphorus or boron atoms interact with the ligand, as well as the impact of changes to the design of the conjugated ligand, were studied and then detailed herein.
The second chapter of this thesis deals with the coordination of phosphorus into a nitrogen-based ligand, which is a rarely encountered bonding arrangement. Routes to prepare these unusual phosphorus complexes are described, and their reactions with transition metals are explored. Chapters three and four involve the design of a ligand that bonds to a boron atom. Small changes in the chemical structure of the ligand, such as replacing oxygen with a nitrogen atom, significantly influence the molecule’s ability to absorb and re-emit light, or how it reacts with acid. The fifth chapter involves the replacement of a carbon-carbon bond with a boron-nitrogen bond in a well-studied framework that other colourful or fluorescent molecules are based on. Methods to prepare these compounds and the impact of incorporating these new elements are considered. Of particular note is that these compounds emit light through phosphorescence, a mechanism which is usually reserved for compounds that have metal atoms, whereas these contain only nonmetallic elements.
By exploring the fundamental properties of molecules using elements like boron or phosphorus, an understanding of how they react and what properties they impart to a resultant compound is gained. If enough of an understanding of the properties of these types of compounds is accumulated then they can see use in real-world applications, as some have in phone screens, in medical diagnoses, or in the sensing of certain chemicals.
Recommended Citation
Watson, Alexander E. R., "Boron and Phosphorus Complexes of π-Conjugated Ligands" (2024). Electronic Thesis and Dissertation Repository. 10540.
https://ir.lib.uwo.ca/etd/10540
