Date of Award

Spring 2010

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

Glen P Miller

Abstract

The computational and experimental studies of acenes, starphenes and [60]fullerene derivatives is described. Acenes are promising organic semiconductors. Hybrid density functional theory applying a triple-zeta valence basis set (B3LYP/6-311+G**) has been used to approximate the HOMO - LUMO excitation gap in various substituted pentacenes. This technique may, in part, synthetically guide substitution patterns of pentacenes and larger acenes. Previous predictions of an open-shell singlet diradical ground state for acenes larger than hexacene have been reproduced and sulfur substitution has been found to alter the electronic ground state to a closed-shell configuration. Substituents that may engage in non-bonding interactions are not well described by the same DFT model, however these shortcomings can be overcome by implementing an appropriate effective-core potential on carbon atoms.

Like acenes, starphenes are a class of polycyclic aromatic hydrocarbon that exhibit interesting electronic characteristics, though synthetically have not been as widely accessable. The synthesis of a soluble precursor to a substituted [4.4.4]tridecastarphene is described. Upon exposure to an excess of [60]fullerene in the presence of a chemical oxidant, the [4.4.4]tridecastarphene PAH is generated and captured in situ by Diels-Alder cycloaddition of the fullerene. Both syn,syn- and syn,anti- diastereomers of the fullerene trisadduct are generated with the syn,syn- formed as the dominant species. Fullerene pi -- pi interactions are believed to drive the formation of the syn,syn- diastereomer.

Understanding intermolecular interactions is critical to the rational design of organic devices. Both electrically conducting and insulating properties are required for the fabrication of such devices. Among these interactions, pi -- pi and CH -- pi interactions dominate the crystal packing in PAHs. Several [60]fullerene derivatives designed to study CH -- pi interactions by variable temperature NMR (VT-NMR) are described. Diels-Alder cycloadducts of [60]fullerene were generated from substituted o-quinodimethane derivatives to give compounds that may or may not engage in an intramolecular CH -- pi interactions. Evaluation of the rate-constant associated with the first-order transition state of a cyclohexene "boat-to-boat" inversion for each fullerene derivative provides evidence for a CH -- pi interaction estimated to be 0.95 kcal/mol.

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