Synthesis and charge-transport properties of novel π-conjugated polymers incorporating core-extended naphtho[2,1-b:3,4-b′]dithiophene diimides

We report the synthesis and characterization of a core-extended acceptor building block, 2,7-dibromobenzo[2,1-b:3,4-b′]dithiophene diimides (abbreviated as 2Br–NTI), by extending the mellophanic diimide core with a 2,7-dibromobenzo[2,1-b:3,4-b′]dithiophene unit. This design strategy makes it possible to achieve high electron affinity and tunable solubility enabled by two dicarboxylic acid imide groups and imide-N alkylation, respectively. Moreover, a large-size, near-coplanar π-conjugated structure allows access to 2Br–NTI and their conjugated copolymers (CPSs). Consequently, alternative embedding of the NTI acceptor and electron-rich species (e.g., bithiophene, difluorobithiophene, and biselenophene) into the polymer backbones endows the target D–A type CPSs with near-linearly π-extended conjugation and intriguing photophysical/electrochemical properties. Experimental investigations combined with theoretical calculation reveal that three NTI-based CPSs show a near-coplanar main-chain, impressive decomposition temperature (T_d > 450 °C), a low-lying highest occupied molecular orbital level (HOMO < −5.45 eV), strong light-capturing capacities in the visible light area, and compact π–π packing with a preferred face-on orientation. All the polymers behave as p-type semiconductors in solution-processed field-effect transistors, with the best performance for the copolymer containing a fluorinated bithiophene donor. This work offers a facile core-extended strategy to construct novel aromatic diimides and demonstrates its promising potential in engineering organic/polymeric semiconductors.