Date of Award
Doctor of Philosophy (PhD)
The unifying theme of most of optoelectronic devices revolves around the charge carrier mobility of the organic materials used in the conductive layers, which is a measure of how easily the electron/hole moves in a particular pi conjugated organic material. When pi conjugated materials are incorporated in these devices, molecules are generally layered in random orientation. Consequently, the efficiency of charge transport in the conducting layers of these devices is governed not only by the intramolecular electron/hole transport through the backbone of the molecule but also by the intermolecular electron/hole transport between the molecules and hence packing of these molecules plays a critical role in the efficiency of these devices. Although significant progress has been made in understanding the charge transport mechanisms in various polycyclic aromatic hydrocarbons (PAHs), the usefulness of such materials in functional devices remains limited; hence design and synthesis of new PAHs to better understand the charge transport mechanisms remains an active area of research.
A novel series of methoxydibenzochrysenes was designed and synthesized utilizing ferric chloride mediated oxidative cyclodehydrogenation as a key step. We have also shown that numerous 9,10-diarylphenanthrenes and dibenzo[g,p]chrysenes can be prepared from a readily available tetraarylethylenes using 1 and 2 equivalents of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), respectively. A similar oxidative cyclodehydrogenation strategy was used for synthesizing a highly soluble, larger derivative of hexa-peri-hexabenzocoronene (HBC), where twelve carbon-carbon bonds are formed in a single step. Deployment of fluorenes at the periphery of the HBC core not only imparts solubility to the structure, but also allows the new PAHs to be functionalized further to make bigger PAHs.
X-ray crystal structure determination of octamethoxydibenzochrysene cation radical and 1,4,5,8-tetramethoxyanthracene dimer cation radical, in addition to their neutral structures, allowed us to delineate the effect of electron removal on their bond length changes. Definitive X-ray crystallographic evidence is obtained for a single hole to be uniformly distributed on the three equivalent 1,2-dimethoxybenzenoid (or veratrole) rings in the hexamethoxytriptycene cation radical. This conclusion is further supported by electrochemical analysis and by the observation of an intense near-IR transition in its electronic spectrum, as well as by comparison of the spectral and electrochemical characteristics with the model compounds containing one and two dimethoxybenzene rings. A new class of intervalance cation radicals from p-diaryloxybenzenes was also studied.