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Molecular Design, Characterization, and Implementation of Organic Semiconducting Oligothiophenes

Abstract
This work describes new self-assembly strategies and realizes new directions for rational side chain design in organic semiconductors. I examined the synthesis and structure property relations of monomers and dimers of a benchmark organic semiconductor system using a variety of linear alkyl side chains. I observed critical onsets for packing trends based on alkyl side chain length in both monomers and dimers. Monomer systems exhibited a pronounced even-odd effect manifesting directly from side chain length. In the dimer systems, I observed spontaneous bimolecular crystal formation with PC61BM which undergoes an order-to-disorder transition at small side chain lengths. Combining these systematic structure-property studies, I designed and synthesized a set of BTTT dimers with variable substitution of long and short side chains. These systems displayed solid-state packing behaviors unique to their side chain pattern, both in neat films and in blends with fullerene derivatives. As these oligomers provide excellent model systems for their polymer analogues, the translation of the structure-property lessons here will benefit small molecule systems and polymer systems alike. More broadly, this work identifies the utility and application of variable substitution for imparting order and ‘smart’ self-assembly in organic semicondutors or other organic crystalline materials.
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