Understanding the impact of both the organic semiconducting polymer design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable a range of devices. Synthesis of conjugated aromatic polymers typically involves carbon coupling polymerisations utilising transition metal catalysts and metal containing monomers. This polymerisation chemistry creates polymers where the aromatic repeat units are linked by single carbon-carbon bonds along the backbone. One consequence of this single bond link is that the aromatic repeat units have the freedom to twist with respect to each other, introducing energetic disorder. We present design and synthesis strategies to restrict or even eliminate this tortional disorder through enhancement of non-covalent interactions. We will take a close look at the molecular features responsible for the high charge carrier mobility of the indacenodithiophene-co-benzothiadiazole copolymer, a benchmark polymer for organic transistors. We will highlight the influence of conformational coplanarity and side-chain assisted self-assembly to create efficient one-dimensional transport along conjugated polymer backbones, with sterically free “crossing points” allowing interchain hopping. Additionally, an aldol condensation reaction was explored, in which a bisisatin monomer reacts with a bisoxindole monomer to create an isoindigo repeat unit that is fully fused along the polymer backbone. This aldol polymerization requires neither metal containing monomers or transition-metal catalysts, opening up new synthetic possibilities for conjugated aromatic polymer design, particularly where both monomers are electron deficient. Polymers with very large electron affinities can be synthesized by this method, resulting in air stable electron transport.
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McCulloch et. al. J. “Modification of Indacenodithiophene-Based Polymers and Its Impact on Charge Carrier Mobility in Organic Thin-Film Transistors” J. Am. Chem. Soc. 2020, 142, 2, 652–664.