Relationship between band gap and bond length alternation in organic conjugated polymers

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Abstract
We describe calculations of the evolution of the band gap as a function of geometry in conjugated polymers based on aromatic rings: polyparaphenylene, polypyrrole, polythiophene. Our results demonstrate that the gap decreases as a function of increasing quinoid character of the backbone and is thus not minimal in the case of zero bond length alternation, in contrast to the situation found in polyacetylene‐like compounds. We stress the consequences of these results for the understanding of the effects of doping and for the design of new organic polymers with small gaps.