Current design approaches for organic mixed ionic electronic conductors (OMIECs) have predominantly centered on the ethylene glycol component, potentially limiting the range of applications. As an emerging OMIEC alternative, we introduce a series of polythiophenes featuring a hybrid carboxyl-alkyl side chain. Through systematic variation in the alkyl chain length, the impact of carboxylic acid functionality and spacer dimensions were investigated. The findings reveal that COOH-functionalization imparts the polymers with both intrinsic low-swelling properties and water-processibility, yielding solvent-resistant conjugated polyelectrolytes while preserving substantial electroactivity in aqueous environments. Leveraging advanced in situ techniques, including time-resolved spectroelectrochemistry and Raman spectroscopy, a comprehensive molecular-level understanding of microstructure, ionic-electronic coupling, and operational stability was attained. Furthermore, by benchmarking the materials through planar and interdigitated organic electrochemical transistors (OECTs) and comparing with state-of-the-art OMIECs, the carboxyl-butyl polythiophene (P3C(Bu)T) is shown to exhibit superior electrochemical performance and faster doping kinetics, achieving a record-high OECT performance among conjugated polyelectrolytes ( [μC*]= 107±4 F cm^-1V^-1s^-1). These results contribute to an enhanced understanding of structure-property relationships in conjugated polyelectrolytes operating in aqueous environments, expanding the materials landscape for future OMIEC development. The results underscore the potential of conjugated polymers featuring alkyl-COOH side chains as a promising avenue for robust OMIEC designs, with the potential to enable facile (bio)chemical functionalization for diverse (bio)sensing applications.

Christopher J Takacs

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