Full Conjugation in a Polymer with Non‐conjugated Piperazine‐2,5‐dione Units via Energy‐minimized Lactam‐to‐Lactim Tautomerization Enables Water‐gated Transistor Fluoride Sensors

Abstract Piperazine‐2,5‐dione (glycine anhydride, GA) has recently emerged as a valuable precursor for high‐performance π‐conjugated polymer semiconductors in organic electronics. We utilized GA to design a novel bisindolin‐dihydropiperazine (IDHP)‐based conjugated polymer, PIDHPTT, for aqueous chemical sensing. In the isatin‐flanked monomer, GA exists as a non‐conjugated lactam (DHP‐NH) but converts to a conjugated lactim (DHP‐OH) form within the polymer. Density functional theory (DFT) calculations show that this conversion is driven by energy minimization via extended π‐conjugation. Neighboring DHP units in the lactim form facilitate this process through π‐bridges, demonstrating a vinylogous effect, which has previously only been observed in small molecules. This is the first study to report such a long‐range vinylogous effect in a polymer due to the collective synergy of numerous functional groups. The OH groups in the lactim DHP interact more strongly with fluoride ions than other halides. PIDHPTT exhibits significant changes in optical absorption, electrochemical impedance, and charge transport in response to fluoride ions, which differ from responses to other halides. A water‐gated organic field‐effect transistor based on PIDHPTT shows excellent sensitivity and selectivity for fluoride ions, demonstrating the potential of this polymer design for chemical sensing applications.