Molecular Engineering of Polymers for Optoelectronics.

Polymers are emerging advanced materials for applications in electronics, optoelectronics, and nonlinear optics. For example, pi-conjugated polymers and their charge transfer complexes have been demonstrated in diverse applications ranging from light-emitting diodes, photovoltaic cells, xerographic photoreceptors, optical switching devices, waveguides, and batteries, among others. However, the underlying structure-property relationships needed to improve optoelectronic and nonlinear optical properties are not understood. This thesis explores a molecular engineering approach to optoelectronic and nonlinear optical properties through a systematic study of the effects of molecular structure and morphology on photoluminescence, photocarrier generation, and nonlinear optical properties. Several pi-conjugated rigid -rod polybenzobisazoles were designed, synthesized, characterized, and used to investigate the role of molecular structure and morphology on luminescence. It is shown for the first time that the emission of the pi -conjugated polymers and the relatively low fluorescence quantum efficiency of thin films originate from excimers due to the aggregated chain morphology of the materials. Based on this knowledge, electroactive and photoactive polymer nanocomposites consisting of rod-coil copolymers were synthesized with the aim to control the luminescent properties of pi-conjugated polymers at the supramolecular level. This resulted in a sixfold increase in solid state photoluminescence quantum yield in the nanocomposites compared to the "bulk" pi-conjugated polymers. Photocarrier generation in bilayer assemblies of polybenzobisazole/triarylamine was investigated by photoinduced discharge and picosecond photoinduced absorption spectroscopy. It was shown that charge photogeneration occurs by photoinduced electron transfer at the bilayer interface, forming intermolecular exciplexes which dissociate efficiently in electric field. The measured field dependent quantum efficiency was in accord with Onsager's three-dimensional model for ion-pair dissociation. Structure-third order nonlinear optical susceptibility (chi^{(3)}) studies on the polybenzobisazoles showed that there is no significant effect of the heteroatoms (O, S) on the chi ^{(3)} contrary to previous model compound studies. In the case of the polyanilines, the chi^{(3)} of this class of polymers was found to depend on the oxidation state and the derivatization of the p-phenylene rings.