Single molecule studies of structure-property relationships in well-defined conjugated oligomers and push-pull conjugated polymers
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The electronic properties of conjugated polymers are strongly dependent on their morphologies. Despite continuous research efforts in conjugated polymers (CPs) as a promising alternative to traditional inorganic semiconductors for flexible and wearable optoelectronic devices, the morphological and associated electronic properties in CPs still remain ambiguous due to their inherently heterogeneous structures both at the bulk film level and single molecule level. In the studies presented in this dissertation, single molecule spectroscopy techniques were employed to understand the single chain morphology and how it affected photophysics. Structurally well-defined model materials based on bis(2-ethylhexyl)-p-phenylene vinylene (BEH-PPV) oligomers, as a simplified model of CPs, were strategically designed to control folding at the single molecule level. Significant morphological variations were found by incorporating both rigid bent and linear linkers into the backbone, as well as by manipulating the chromophore size. The longest chromophore with the linear linker displayed best single molecule folding order and the film packing was further reinforced by the stronger π-π interactions between the longer conjugated segments. In addition, single chain folding via hydrogen-bonding side chain inclusion was demonstrated on the otherwise isotropic model material composed of a series of BEH-PPV trimers linked with flexible linkers. In comparison with a control oligomer system with non-hydrogen-bonding side chains, the oligomers with hydrogen-bonding motifs showed highly ordered structures and enhanced monomer interactions. These studies were further extended to a new class of emerging push-pull conjugated polymers (also known as donor-acceptor copolymers), specifically poly[N-(1- octylnonyl)-2,7-carbazole]-alt-5,5-[4’,7’-di(thien-2-yl)-2’,1’,3’-benzothiadiazole] (PCDTBT). By probing the polymer chain folding as a function of molecular weight using single molecule techniques, it was found that there are two emissive species in this material. Above a critical molecular weight, the polymer transitions from a non-interacting, more ordered conformation to a self-interacting, more isotropic conformation.
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