Controlling the Electronic Properties and Morphology of Semiconducting Polymers Through the Incorporation of 1,6-Methanoannulene
Streifel, Benjamin Christopher
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Conducting polymers have been known since polyacetylene was discovered in the 1970s. Since then, myriad new small molecules and polymeric structures have been synthesized to optimize the properties of conducting organic materials. These materials are characterized by their highly delocalized pi-orbitals and semiconductor-like band structure in their neutral state. The vast majority of these conjugated small molecules and structures are based around benzenoid Hückel 4n+2 structures where n=1. Common examples include poly-paraphenylene, polythiophene, polyaniline, and many others. These subunits perform well in certain applications and have been studied extensively, but much less attention has been focused on aromatics with larger pi-electron systems. 1,6-methanoannulene (M10A) is one such aromatic with a 10-pi electron system, a non-planar structure, and a bridging methylene carbon. The non-planarity of the ring leads to low resonance energy, offering significant polyolefin character and lower energy barriers to oxidation, electrophilic addition, and other processes that require breaking of aromaticity. This polyolefin character also extends the effective conjugation length of polymers, leading to reduced bandgap and more effective delocalization of charge. These electronic features of M10A allow it to stabilize reactive species under oxidative electrochemical polymerization conditions, as well as render furan containing polymers resistant to environmental degradation. The curved geometry of the annulene has the ability to prevent torsional strain arising from steric clashes between alkyl chains and aromatic subunits along the polymer backbone. This too leads to extended effective conjugation length and good materials properties. The bridging methylene prevents ordered aggregation, resulting in M10A-containing polymers being amorphous with a variety of comonomers. These highly disordered conducting materials are useful as transistor and thermoelectric materials, exhibiting reasonable hole mobilities (ca. 10^-4 cm^2/Vs) and high Seebeck coefficients (ca. 10^3 uV/K).