Naphthobispyrazine Bisimide: A Strong Acceptor Unit for Conjugated Polymers Enabling Highly Coplanar Backbone, Short π–π Stacking, and High Electron Transport

The development of acceptor units that ensure high electron transport in π-conjugated polymers is imperative to advance the field of printable electronics. We have designed and synthesized naphthobispyrazine bisimide (NPI) as a novel strong acceptor unit by chemical conversion from naphthobispyrazine tetraester (NPE). We have also synthesized NPI-based polymers using bithiophene derivatives with and without fluorine groups as co-monomers. Notably, the NPI polymer-based transistors demonstrated ambipolar characteristics with hole and electron mobilities of up to 0.8 and 0.7 cm2 V–1 s–1, respectively, despite the unfavorable face-on orientation, and those values were more than 2 orders of magnitude higher than the hole and electron mobilities of their NPE counterpart. We ascribed the greatly enhanced mobilities to the highly coplanar and rigid backbone and the quite high crystallinity along with the short π–π stacking distances of the order of 3.4 Å, which resulted in efficient intra- and interchain charge carrier transport. We also found that the fluorine atom can act as a conformation-directing group, depending on the substitution position, which led to a more regular backbone structure with reduced energetic disorder while achieving higher solubility. We showed that NPI is a promising acceptor unit for high-performance electron-transporting π-conjugated polymers.