UC Berkeley

This dissertation describes the development and application of zirconocene-mediated alkyne coupling to the synthesis of conjugated nanocarbons. This work comprised the development of fundamentally new patterns of reactivity as well as new conceptual strategies for the assembly of such structures. A particular effort was placed upon the development of practical, selective, and scalable syntheses, with the goal of accessing structurally complex nanocarbons on scales that could facilitate the interrogation of their materials properties. Chapter 1 provides a general introduction to the motivation behind the synthesis of conjugated nanocarbons, namely for their unique electronic properties and value as model systems for carbon nanomaterials such as graphene and carbon nanotubes. Other major ring-fusion strategies are surveyed, and a history of the development of zirconocene-coupling within the Tilley lab is provided. Each subsequent chapter is placed within this context and summarized in greater detail. Chapter 2 describes the development of a new, dual-cycloaddition strategy for the synthesis of π-extended polycyclic aromatic hydrocarbons (PAH) from zirconacycle precursors. This approach offers several advantages over existing strategies for PAH synthesis. It is high yielding and divergent, enabling the efficient synthesis of diverse nanographenes from a common precursor. The photophysical properties of the resulting PAHs are thus highly tunable, making this strategy of broad potential use. Chapter 3 describes the development of new copper transmetalation chemistry to access a highly antiaromatic cyclobutadiene from a zirconacycle precursor. The cyclobutadiene is shown to be a small HOMO-LUMO energy gap system with readily accessible reduced and oxidized states. The mechanism of this transformation is also of broad interest, as it involves the reductive elimination of a rare, highly oxidized copper species. Chapter 4 describes the synthesis of pentacene-containing conjugated macrocycles via a reversible form of zirconocene coupling, which affords dimeric and trimeric macrocycles selectively on gram scale via judicious monomer design. The macrocycles act as rigid scaffolds that arrange the pentacene units in distinct conformations, directly controlling their singlet fission dynamics in solution. This strategy enables the independent modulation of singlet fission rate without compromising triplet lifetime. Chapter 5 describes the synthesis of topologically complex nanocarbons using zirconocene coupling. This strategy represents a fundamentally new way to approach the synthesis of interlocked and interwoven nanocarbons, illustrating that dynamic covalent chemistry can enable higher yielding, more scalable syntheses. The resulting nanocarbons represent a fundamentally new topological class, opening a new frontier in topological molecular design. Chapter 6 describes the efficient synthesis of a carbon nanobelt, a molecular cutout of a carbon nanotube, using zirconocene coupling. This report illustrates a new conceptual strategy for nanobelt synthesis, that the use of pre-fused building blocks results in a more streamlined synthesis. This enabled an order of magnitude improvement in reaction scale, and the resulting nanobelt displayed supramolecular binding of guests due to its unusual, elongated shape.