Two-Dimensional Tetrathienonaphthalenes-Based Donor-Acceptor Copolymers: Synthesis, Isomeric Effect, and Organic Field-Effect Transistors

Two-dimensional alkylated alpha beta-TTN and beta alpha-TTN were designed and synthesized. The cross-shaped alpha beta-TTN and beta alpha-TTN moieties were polymerized to afford two isomeric polymers, P alpha beta-TTNFBT40 and P beta alpha-TTNFBT40. P alpha beta-TTNFBT40 using an alpha-aNDT unit in the main chain exhibited more red-shifted absorption and a smaller HOMO/LUMO band gap than P beta alpha-TTNFBT40 using beta-aNDT in the main chain. Compared to P beta alpha-TTNFBT40 showing a curved backbone, density functional theory (DFT) calculations revealed that P alpha beta-TTNFBT40 exhibited a quasi-linear polymeric backbone that induces stronger intermolecular interactions and higher molecular ordering in the solid state. P alpha beta-TTNFBT40 exhibited an organic field-effect transistor (OFET) hole mobility of 1.12 x 10(-2) cm(2) V-1 s(-1), which outperformed P beta alpha-TTNFBT40 by 1 order of magnitude (1.60 x 10(-3) cm(2) V-1 s(-1)). Through side-chain engineering and optimization, P alpha beta-TTNFBT24 using a shorter 2-butyloctyl side chain further strengthened the intermolecular interactions. From grazing incidence wide-angle X-ray scattering (GIWAXS), P alpha beta-TTNFBT24 adopted an edge-on pi-pi stacking orientation in a thin film, leading to an improved mobility of 1.75 x 10(-1) cm(2) V-1 s(-1), which has exceeded the mobility of the one-dimensional alpha-aNDT-based PaNDTDTFBT (4.70 x 10(-2) cm(2) V-1 s(-1)). This research demonstrated that the two-dimensional structure is an effective strategy to achieve higher OFET mobility, and the geometry of the main chain played a more significant role than that of the side chain in determining the physical and packing properties of polymers.