Synthesis of fluorene-flanked diketopyrrolopyrrole-based semiconducting polymers with thermocleavable side chains and their application in organic field effect transistors

This study introduces the first fluorene-flanked diketopyrrolopyrrole (FDPP)-based semiconducting polymers, named Boc-FDPP-CoMs (CoM = TT or BT), synthesized via copolymerization of tert-butoxycarbonyl (t-Boc)-substituted FDPP as the acceptor unit with thieno[3,2-b]thiophene (TT) or bithiophene (BT) as the donor comonomers. The incorporation of t-Boc groups addresses challenges related to poor solubility and polymer backbone twisting associated with the FDPP building block. While Boc-FDPP-CoMs demonstrate solubility in common organic solvents, facilitating favourable solution-processability for uniform film fabrication, they exhibit significant backbone twisting, leading to poor charge transport properties. Post-deposition thermal annealing at a mild temperature as low as 170°C conveniently removes t-Boc groups. The resulting t-Boc-free copolymers, NH-FDPP-TT and NH-FDPP-BT, exhibit hole mobilities up to 5.0 × 10⁻³ and 2.2 × 10⁻³ cm² V⁻¹ s⁻¹ in organic field effect transistors (OFETs), respectively, representing a substantial increase compared to their counterparts with t-Boc groups. This study underscores a meticulously designed strategy for achieving solution solubility and backbone coplanarity through side-chain engineering for FDPP and potentially other sterically demanding building blocks to construct high-performance semiconducting polymers for OFETs and other applications.