Design, synthesis, and properties of pyridyl endcapped cumulenes
Creators: Cameron, David
Publisher: name: University of Alberta Library
Year: 2024

<p>Several prototypes for organic molecular wires have been reported in the literature. From these studies, guiding directives have emerged for the design of molecular wires with improved performance. [n]Cumulenes are excellent candidates for these applications, and, at the same time, pyridyl anchoring groups show good performance in single-molecule conductance measurements. Despite these features, pyridyl cumulenes have not been evaluated as molecular wires, owing to their instability, and a lack of general synthetic routes for their synthesis. <br>
    In this thesis, the design, synthesis, and analysis of pyridyl cumulenes as a function of structural isomers, lengths, and substitution patterns is evaluated. Chapter 1 introduces selected examples of heteroaryl and heterocycle-endcapped cumulenes, with special attention paid to methods for their synthesis and descriptions of kinetic stability (persistence). This chapter also provides a brief description of single-molecule conductance, particularly with respect to cumulenes and oligoynes. Furthermore, the motivation for the thesis is introduced, and the factors associated with designing pyridyl cumulenes are discussed. <br>
Chapter 2 covers the use of bulky aryl endgroups as stabilizing moieties for pyridyl endcapped cumulenes. The design leads to mesityl and 3,5-di-tert-butyl endgroups for the stabilization of pyridyl cumulenes. The preparation of several precursors to pyridyl cumulenes is described, while it is determined that reactions conditions involving Sn(II) do not afford pyridyl cumulenes. <br>
Chapter 3 demonstrates the development of a general method for the synthesis of pyridyl cumulenes using P2I4. The optimization of the reaction is briefly described, and a mechanism is proposed. The scope of the reaction is evaluated with respect to 2-, 3-, and 4-pyridyl endgroups. Using this method, preliminary efforts toward longer (n &gt; 5) [n]cumulenes are described. <br>
Chapter 4 involves the application of the developed reactions conditions to the synthesis of longer (n ≥ 5) pyridyl [n]cumulenes. Pyridyl endgroups with bulky substituents have a stabilizing effect on cumulenes, as evidenced by the synthesis of a [5]cumulene with 4-pyridyl endgroups. The development of the synthesis of a pyridyl [9]cumulene is described, with access to a precursor to this molecule.<br>
Chapter 5 describes the analysis of pyridyl [n]cumulenes including UV-vis in neutral and pH dependent environments, rotational barriers, X-ray crystallographic analysis, a comparison of 13C NMR chemical shifts, and kinetic stability. The UV-vis data for neutral pyridyl cumulenes as a function of length and pyridyl isomer are described. 13C NMR spectroscopic and X-ray crystallographic analyses of the pyridyl cumulenes are mostly unremarkable. Rotational barriers for [3]- and [5]cumulenes with pyridyl endgroups are compared. The unique characteristics of pyridyl cumulenes are observed in their protonated forms, in which absorption profiles show diminishing optical gaps as a function of pyridyl isomer. <br>
Chapter 6 describes a summary and outlook for pyridyl cumulenes. The factors that influence kinetic stability of pyridyl cumulenes are further described. Preliminary examples of metal organic frameworks and the preparation of discrete metal complexes bearing pyridyl cumulenes as ligands shows that they have potential for use in supramolecular systems. Cross-conjugated molecular wires and conductive polymeric structures are proposed as future objectives for the expansion of this research.<br>
    Chapter 7 provides the synthetic procedures as well as the spectroscopic characterization and physical data of the compounds discussed in this thesis.</p>