Annellated Benzenoid Macrocycles and Polymerizable Discotic Liquid Crystals
AuthorButtrick, Jonathan Charles
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This dissertation details the efforts towards novel polycyclic aromatic hydrocarbons (PAHs). It begins with an overview of PAHs, liquid crystals, and membrane technologies introducing key concepts pertinent to later discussions, followed by a review on cycloarenes. Then begins discussion on the three projects of my graduate research: the synthesis of septulene, synthetic efforts of towards aromatic belts, and the synthesis and characterization of two polymerizable discotic liquid crystals (DLCs). Cycloarenes are annelated benzenoid macrocycles that contain C-H bonds pointing into their center cavity. Major interest in this class of compounds stemmed from differing theories about the delocalization of their π electrons. Kekulene, the prototypical cycloarene, was synthesized by Stabb and Diederich after over a decade of attempts. Characterization showed that its π electrons remain delocalized into smaller benzenoid units and not across the entire molecule like with annulenes. This provided proof to the electron delocalization theory of McWeeny and gave support for the Clar bonding model. The synthesis septulene and a hexaazakekulene corroborated these conclusions. Our synthesis of septulene, achieved using ring-closing metathesis (RCM) to "stitch up" bridges on a propenyl substituted cyclometaphenylene, was a particularly important addition as it possesses many fundamental structural differences yet similar physical properties. Extensive computational studies tried to quantify the amount of thermodynamic stabilization cycloarenes gain from macrocyclic conjugation, a concept coined as superaromaticity, that ultimately concluded this effect to be negligible. Aromatic belts are another type of annelated benzenoid macrocycle that possess benzenoid rings oriented perpendicular to the macrocyclic ring, and have yet to be rationally synthesized. Interest in aromatic belts comes from their supramolecular properties and particularly their potential to provide a means for the bottom-up synthesis of carbon nanotubes. We attempted to synthesize an aromatic belts my using RCM on propenyl substituted cycloparaphenylenes (CPPs), in the same manner as our septulene synthesis. While we were successful in making the required small molecule building blocks, macrocyclization attempts using established methods for CPP synthesis proved unsuccessful. We attempted to synthesize a nanoporous membrane templated by the columnar order within a discotic liquid crystal (DLC) mesophase. This work stemmed from previous work on boronic ester containing DLCs made for solar cell applications. The parent DLCs were found to organized in a columnar hexagonal fashion, just like that needed for the desired membranes. Our monomers were tailored to possess a hydrolytically labile aromatic core, functionalized by peripheral alkyl chains terminated by polymerizable end groups. Triphenylene cores induce crystallinity while the alkyl chains induce fluidity within the liquid crystal mesophase. Acrylate end groups provide a means to lock-in the order of the DLC mesophase while boronic ester linkages provide a means to remove the triphenylene cores post-polymerization. Our first monomer was found to produce a polymerized film that possessed no columnar order, leading us to synthesize a second monomer unit. This second monomer possesses shorter alkyl chains, in hopes to reduce the fluidity of the mesophase and increase stacking of the triphenylene cores. Polarized optical microscopy of our second monomer showed Schlieren textures indicative of a nematic mesophase, and X-ray diffraction of both monomers showed patterns characteristic of a discotic nematic mesophase. While nematic mesophases possess none of the columnar order needed for the desired nanoporous membranes, they are quite rare for discotic mesogens and have LCD applications worth investigating.