n-Type semiconducting polymers based on an bithiophene imide-bridged isoindigo for organic field-effect transistors

n-Type polymer semiconductors are essential for enabling high-performance organic electronic devices; however, their development still remains a challenge. Herein, we reported a new electron-deficient building block, i.e. bithiophene imide (BTI)-bridged isoindigo (IID), BTIID, through a Knoevenagel condensation reaction between the bisaldehyde BTI and the alkylated indolinones. Three donor-acceptor (D-A) copolymers were then successfully synthesized via standard Stille coupling reaction based on this novel building block, which exhibit highly planar backbones and low-lying lowest unoccupied molecular orbitals (LUMO) energy levels. When integrated into organic field-effect transistors (OFETs), these polymers demonstrated unipolar n-type transport characteristics with a highest electron mobility of 5.79 × 10⁻⁴ cm² V⁻¹ s⁻¹. Our research results indicate that the design of strong electron-withdrawing units via integrating two different electron-acceptor units into one single structure represents a promising strategy for developing n-type polymer semiconductors with low-lying LUMO energy levels.