Asymmetric B←N Functionalized Benzothiadiazoles for High-Performance n-Type Semiconducting Polymers

B←N containing polymers have emerged in organic electronics due to their fascinating optical and electronic properties. Despite these advantages, the development of B←N-based n-type polymers for high-performance organic transistors remains a significant challenge, primarily due to the scarcity of effective B←N containing acceptor units. In this work, we address this challenge through the rational design and synthesis of two asymmetric half-fused B←N functionalized benzothiadiazole derivatives, BTBN and FBTBN. These compounds leverage the unique electronic properties of the B←N motif, enabling the development of two new n-type polymers, PBTBN and PFBTBN. Notably, the lowest unoccupied molecular orbital (LUMO) levels of PBTBN and PFBTBN are significantly lowered by 0.2-0.3 eV compared to their counterparts without B←N functionalization, with PFBTBN achieving a LUMO of ∼ -4.0 eV. Importantly, PFBTBN exhibits exceptional unipolar n-type transistor performance with a high electron mobility (µ_e) of 3.85 cm² V^-1 s^-1. The asymmetric half-fused B←N molecular backbone not only stabilizes the electronic structure but also induces a near-amorphous morphology, thereby enabling PFBTBN-based flexible transistors to retain a high µ_e of 3.16 cm² V^-1 s^-1 even after 1000 bending cycles. This work demonstrates the transformative potential of incorporating asymmetric B←N functionalized acceptors for high-performance n-type semiconducting polymers.