Design, Synthesis and Study of Redox and Optoelectronic Properties of Aromatic Oxidants and Polycyclic Aromatic Hydrocarbons
Abstract
Organic materials play a significant role for the next generation photovoltaic devices that convert solar energy into usable forms of energy. In this regard, polycyclic aromatic hydrocarbons (PAHs) are fundamental tools in the developing area of molecular electronics and photovoltaics as they show excellent optical/electronic properties and are well-suited for applications in such developing areas as flexible display devices, field effect transistors and solar cell panels. Design and synthesis of novel materials for photovoltaics applications would require the proper understanding the mechanism of charge transport and identification of the structural features necessary in a particular molecular wire or PAH. To understand the charge transport mechanism and the hole delocalization one needs to generate the cation radical of a given electron donor in solution by using robust aromatic oxidants. Among these oxidants, magic blue has been widely used as an aromatic oxidant for the one electron oxidation due to its commercial availability and a reasonable oxidizing power. However, a modest stability of the magic blue salt leads to a slow decomposition to produce unknown impurities, which have been named “blues brothers”. Importantly, these impurities produce a noticeable band in the near-IR region—that is the same region where one usually expects to see an intervalence band of the cation radical with extensive hole delocalization. In this work a rational approach to synthesis of novel analogue of the magic blue that does not undergo degradation has been demonstrated. Furthermore, in the course of the rational design of novel molecular wires with enhanced redox and optical properties, one usually considers various geometrical factors in order to control the mechanism of charge delocalization. For example, a relatively small interplanar dihedral angle between adjacent units in poly-p-phenylene wire leads to a significant interchromophoric electronic coupling and thereby to extensive hole delocalization. However, it remains unclear how change in the interplanar angle would impact the redox and optical properties of the wire as well the mechanism of the hole delocalization in its cation radical. Accordingly, in this work it has been described the syntheses and study of the electrochemical and optoelectronic properties of a number of different series of biaryls connected by different numbers of methylene group to vary the dihedral angle in order to probe the mechanism and extent of hole delocalization in biaryls. 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. An oxidative cyclodehydrogenation strategy was used for synthesizing a highly soluble, fluorene based larger derivative of hexa-peri-hexabenzocoronene (FHBC), 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 to tune its desirable electronic properties.