Even cumulenes, odd behaviour: Synthesis, reactivity, and model compounds toward the concept of helical molecular orbitals.
Creators: Johnson, Matthew
Publisher: name: University of Alberta Library
Year: 2024

<p>Even [n]cumulenes are one class of linear molecular based on sp-carbon hybridized carbon, along with odd [n]cumulenes and polyynes. The synthesis and properties of even [n]cumulenes are needed to establish the boundaries of the elusive sp-carbon allotrope, carbyne, and this goal is the focus of this thesis.<br>
Chapter 1 provides an introduction to even [n]cumulenes. Previous studies of synthesis, characterization, and electronic structure are described, particularly as they relate to the work reported in this thesis.<br>
Chapter 2 addresses the synthesis and reactivity of [4]cumulenes. Due to lack of characterization in earlier reports, the aim of this work has been to synthesize [4]cumulenes known in the literature and explore their chemical behaviour. On the basis of work with known [4]cumulenes, the development of novel [4]cumulenes based on improved electronic and steric protection motifs is then established. Evaluation of the reactivity of [4]cumulenes is done by obtaining characterization of several products of decomposition. The properties of [4]cumulenes are evaluated by UV-Vis spectroscopy and X-ray crystallographic analysis. <br>
Chapter 3 focuses on the development of a synthetic protocol that can provide the first isolable [6]cumulene. Several synthetic pathways were investigated toward [6]cumulenes. In general, acid-catalyzed pathways are accompanied by several decomposition reactions, and thus, are not suitable. Performing carbene homologation from a [5]cumulene to a [6]cumulene is established as a promising approach, and preliminary studies have generated crude samples of a [6]cumulene. Unfortunately, however, the resulting [6]cumulene is prone to dimerization. <br>
Chapter 4 explores the synthesis of [n]cumulenes and polyynes with chiral endgroups. Odd [n]cumulenes and polyynes with chiral endgroups are successfully synthesized, and their chiroptical behaviour is examined to compare with theoretical data in search of helical molecular orbitals. These molecules display typical chiroptical behaviour with no presence of helical molecular orbitals. The synthesis of [4]cumulenes with chiral endgroups was unsuccessful, but theoretical analysis suggests that they are promising candidates toward supporting or refuting electrohelicity. <br>
Chapter 5 focuses on the alkali metal reduction of [4]cumulenes. The radical anion and dianion of a sterically bulky [4]cumulene are obtained by reaction with cesium metal. Solid-state crystallographic analysis of the radical anion and dianion allow direct comparison with previously established radical anion and dianions of [3]cumulenes. [3]Cumulenes do not undergo significant skeletal deformations upon reduction. [4]Cumulenes, however, experiences significant deformations including twisting of the endgroups and bending of the cumulene core. Theoretical analysis suggests that the [4]cumulenes face significant energetic penalties if they do not distort upon reduction, while [3]cumulenes have relatively low barriers to deformation.<br>
Chapter 6 includes various side projects that are at various stages of completion. In one project, [5]cumulenes are substituted with hexaphenylbenzene and ethynyl groups to explore the properties of [5]cumulenes with extended conjugation. It is found that incorporation of large polycyclic aromatic hydrocarbon (PAH) fragments has minuscule effects on the properties of [5]cumulenes, while ethynylation led to significantly reduced rotation barriers compared to a tetraaryl[5]cumulene. In the second project, [n]cumulenes (n = 3, 4, 5, and 7) are found to undergo aggregation-induced emission, a phenomenon in which increased aggregation results in significant increased emission intensities. In the final project, theoretical analysis has been done to compute the mechanism for dimerization of [n]cumulenes (n =3, 5, and 7).<br>
Chapter 7 is a summary and outlook that described potential research directions for each of the main topics.</p>