Cation–polymer interactions drive water expulsion and deswelling in n-type ladder organic mixed conductors

Controlling ion–polymer interactions in organic mixed ionic-electronic conductors is crucial for optimizing device performance in applications ranging from bioelectronics and energy storage to photonics. Achieving this requires a molecular-level understanding of how ion uptake, solvation and polymer structure evolve during electrochemical doping. Here using a multimodal operando approach, we uncover an unexpected response in the prototypical n-type ladder polymer poly(benzimidazobenzophenanthroline) (BBL) on doping with protic cations such as ammonium. At high doping levels, strong ion–polymer interactions (primarily hydrogen bonding) between cations and the BBL backbone promote charge localization and disrupt ion hydration, leading to a pronounced reduction in mass and thickness. Operando 2H NMR identifies water expulsion, rather than ion removal, as the origin of this deswelling. Our combined experimental and modelling results reveal a previously unobserved regime of ion–polymer coupling in organic mixed ionic-electronic conductors, establishing a framework for material design and applications that span (bio-)electronics to photonics.