Date of Award
Doctor of Philosophy (PhD)
Organic photovoltaics will play an important role in supplementing the energy needs of the twenty first century in a most cost-effective way to convert the solar energy into usable forms of energy. One of the bottlenecks in promoting widespread use of photovoltaic devices for solar energy storage is the inefficiency of the devices, which in part arises due to the inefficient charge separation and long-range charge transport. Design and synthesis of efficient charge-transfer materials would require one to first identify and establish the structural features necessary in a given molecular wire, which may promote effective charge transfer to long distances. Accordingly, herein, we describe the syntheses and study of the electrochemical and optoelectronic properties a number of different series of poly-p-phenylene based oligomers in order to probe the mechanism and extent of hole delocalization in molecular wires containing a large number of p-phenylenes. In order to develop an understanding of hole delocalization in π-conjugated oligomers/polymers, we have synthesized and systematically studied well-defined series of oligomers of π-conjugated poly-p-phenylene based molecular wires with varying inter-ring dihedral angles ( ) between the monomer units (e.g. RPPn, ~ 33º; oligofluorenes, ~ 37º and 0º; or planarized angular polyfluorenes, ~ 0º) together with the effect of end capping groups, such as alkyl (iAPP) and alkoxy (ROPP). The experimental evaluation of redox and optical properties of various molecular wires studied herein showed that the HOMO density distribution extends over the entire chain as expected based on the observed linear cos[π/(n + 1)] trend albeit gravitating toward the center of the molecule, whereas the hole distribution, which determines optoelectronic properties of the oligomer cation radicals, is found to be limited to 7-8 phenylene units. The study of poly-p-phenylene oligomers with electron-donating end-capping groups (i.e. alkyl- or alkoxy-groups) showed that the hole distribution is strongly impacted by end-capping groups and it gravitates from the central position to the end of the oligomeric chain while the distribution of HOMO density remains in the middle of the π-conjugated wires. Such migration of the hole toward the end of the chain due to the electron-donating alkoxy groups has significant experimental consequences, i.e. the optical and redox properties saturated at five phenylene units in case of alkoxy-capped poly-p-phenylene wires. These results were further reconciled by DFT calculations and by a recently developed multistate parabolic model.