π-Extended Naphthalene Diimide Derivatives for n-Type Semiconducting Polymers

The electron-transporting properties of n-type polymers strongly depend on the chemical and electronic structure of the electron-deficient building blocks. Here, we design and develop a novel electron-deficient unit, a thienopyridine-fused naphthalene amide (TPNA), via π-conjugative extension at the diagonal position of the widely investigated naphthalene diimide (NDI) moiety to address the severe π-backbone twisting of typical NDI-based polymers. The TPNA-based homopolymer (Homo-TPNA), donor-acceptor copolymer (TPNA-T, T = thienyl), and all-acceptor copolymer (TPNA-BTz, BTz = benzothiadiazole) were synthesized and characterized. Compared to the NDI unit, TPNA promotes greater π-conjugation, increased electron delocalization, and a nearly planar backbone conformation due to the reduced steric demands of the TPNA-aryl connecting points. 2D GIWAXS indicates that all of the TPNA-based polymers are significantly crystalline and exhibit well-ordered microstructures with distinctive lamellar diffractions (h00) in the out-of-plane direction and close π-πstacking distances of 3.5-3.6 Å. Benefiting from the homopolymer and all-acceptor molecular design strategy, the Homo-TPNA and TPNA-BTz polymers exhibit lower frontier MO energy levels than TPNA-T, which promote electron injection while blocking hole accumulation. Thin-film transistors (TFTs) based on the TPNA polymers exhibit n-channel performance with an optimal electron mobility of 0.19 cm2 V-1 s-1 achieved for TPNA-BTz. These results demonstrate that TPNA is an effective building block for constructing n-type polymers with good backbone planarity and promising electron-transport properties.