Quinacridone-based semiconducting polymers: Implication of electronic structure and orientational order for charge transport property

We report the synthesis, characterization, and field-effect transistor properties of novel semiconducting polymers, PQA2T and PQA3T, incorporating a quinacridone unit, and discuss the structure-property relationships. Comparison of the optical and electrochemical properties between the monomer, repeat unit, and polymer suggests that the effective π-conjugation and the delocalization of HOMO along the backbone are relatively limited. X-ray diffraction studies revealed that the polymers form a π-π stacking with a short distance of 3.6 Å and that the orientational order was enhanced by an increase of molecular weight. The hole mobilities are found to be around 0.2 cm ² V ^-1 s ^-1 and are, interestingly, insensitive to the molecular weight and to the orientational order; the randomly oriented low molecular weight polymer showed similar mobilities to the edge-on oriented high molecular weight polymer. We speculate that the relatively localized HOMO might hinder the charge transport along the backbone, and thus the longer polymer chain is not necessary to facilitate the charge transport. The locally but strongly π-π interacted polymer crystallites seem to be sufficient for the effective charge transport in the QA-based polymer system. These features in the present polymers offer great interest of using QA moieties as the building block for semiconducting polymers and give new insight into the design of a new class of semiconducting polymers.