( Invited ) Beyond the Backbone: How Side-Chains Control Redox Properties and Transport in Conjugated Polymers

Abstract

Redox-active conducting conjugated polymers enable a wide range of electrochemical technologies, including bioelectronics, electrochromic devices, and energy‐storage systems, where their performance is governed by the efficiency of electrochemical doping and charge transport. While backbone design defines the electronic structure, side-chain chemistry plays a critical in regulating ion uptake, swelling, redox energetics, microstructure, carrier mobility, and processability. In this presentation, we demonstrate how side-chain polarity, length, and substitution pattern decisively govern electrochemical doping behavior and mixed ionic–electronic transport in 3,4-propylenedioxythiophene (ProDOT)-based polymers, without requiring significant changes to the conjugated backbone. By systematically varying side chains from aliphatic and oligoether to short hydroxyl and carboxylic acid functionalities, we establish direct correlations between molecular structure and oxidation onset, ion uptake, volumetric capacitance, conductivity, swelling, and mechanical stability under operando conditions. For example, in situ spectroscopic, structural (GIWAXS), and gravimetric (EQCM-D) measurements reveal that side-chain substitution alone can toggle conductivity across four orders of magnitude, while tuning side-chain length, and polarity suppresses mechanical swelling from ~300% to below 10%, drastically improving cycling stability without compromising performance. Lastly, we will show how these design principles influence how the polymers perform as active materials in organic electrochemical transistors, electrochromic devices, and as sensors for detecting extracellular action potentials from cardiomyocytes and hippocampal neurons. The results presented aim to demonstrate that judicious side-chain engineering enables precise control over electrochemical doping and mixed conduction, providing a rational framework for designing high-performance polymers for bioelectronic sensing, electrochromism, and energy-related applications.