Acceptor Backbone Cationization via B ← N Functionalization Enables High‐Performance Porous n‐Type Organic Mixed Ionic‐Electronic Conductors for Biosensors

The advancement of high‐performance n‐type organic mixed ionic‐electronic conductors (OMIECs) is pivotal to advancing organic electrochemical transistors (OECTs) for next‐generation bioelectronics. While current design strategies predominantly center on non‐ionic conjugated polymers, their inherently hydrophobic backbones lead to suboptimal ionic transport characteristics. To address this challenge, we introduce an ionic acceptor design strategy of backbone cationization via B ← N functionalization within bipyridine and bipyrazine frameworks. Leveraging these cationic acceptors, we synthesized two n‐type ionic polymer OMIECs, PBPyBF 3 , and PBPzBF 3 , which exhibit low‐lying LUMO levels as low as −4.0 eV, elevated backbone torsional barriers, and ordered microstructures. Most critically, backbone cationization via B ← N coordination significantly enhances hydrophilicity by inducing a distinct porous film morphology, facilitated by hydrogen bonding with processing solvents. This structural evolution translates to a dramatically improved volumetric capacitance of 581 F cm −3 . Consequently, OECTs based on cationic polymer PBPzBF 3 exhibit an exceptional normalized transconductance of 38.9 S cm −1 and figure of merit of 215.9 F cm −1 V −1 s −1 , ranking among the highest values reported for n‐type OMIECs to date. Notably, electrocardiogram sensors integrated with PBPzBF 3 ‐OECTs exhibit high signal‐to‐noise ratios and superior sensitivity. This work establishes fundamental structure‐property relationships governing ion‐electron coupled transport in conjugated polymers.