In Pursuit of Extraordinarily Twisted Acenes
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Longitudinally twisted acenes are a topologically interesting class of polycyclic aromatic hydrocarbons (PAHs), and it could be argued that they have quite beautiful chemical structures, too. X-ray crystal analysis of many of these acenes has found them to adopt a helical conformation where the axis of propagation runs parallel to their linearly fused benzene rings. The measured end-to-end twists of many of those currently reported range from a modest 28.1° to an extreme of 143.6°. The helical nature of longitudinally twisted acenes implies chirality, and compared to their racemic mixtures, the pure enantiomers display chiroptical properties, which make them interesting for study and potentially useful in materials applications. The deformation in these compounds is induced by the placement of bulky substituents at the peri positions of the acene, adjacent to one another, which forces the acene to twist out of plane in order to alleviate the steric strain. Because of the substitution requirement, longitudinally twisted acenes can be quite challenging synthetic targets. The current and by far most successful synthetic approach to these compounds is by what we refer to as the “benzyne” approach, which requires the design of a benzyne precursor and its subsequent trapping by a diene compatible with the reaction conditions necessary to generate the aryne. Although quite successful, this approach suffers from many drawbacks including low overall yields, difficulty in derivatization, and lack of scalability. The focus of the dissertation is the application of an alternative synthetic approach to these compounds, which we found gave good overall yields, allowed for easy derivatization, and could be scaled-up to produce gram quantities if desired. Combining an unusually reactive thiophene with a novel reduction method, we were able to synthesize a diverse number of longitudinally twisted acenes. Among these are included the now most twisted anthracene, the most twisted true pentacene, and the most twisted acene as determined by X-ray crystallography.
Table of Contents
Introduction to longitudinally twisted acenes -- The benzyne synthesis of longitudinally twisted acenes derived from a polycyclic thiophene -- An alternative synthetic approach to longitudinally twisted acenes -- Synthesis of exceptionally twisted acenes -- Application of a novel reduction method -- Experimental details and crystal data
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Ph.D.