Utilizing Vibrational Probes to Monitor Polaron-Anion Interactions during Polymer Electrochemical Doping

Understanding the electrostatic interactions between an electrical charge carrier (polaron) and the compensating counterion, and how they evolve during the electrochemical doping of mixed ionic-electronic conducting polymers, is critical for designing new materials to enable applications from bioelectronics to neuromorphic computing. However, direct experimental probes of these interactions remain limited, hindering their development. Here, we introduce an in situ spectroelectrochemistry (SEC) approach that combines visible (vis), near-infrared (NIR), and infrared (IR) spectroscopy with the vibrational Stark effect to directly monitor polaron-ion pairs during the electrochemical doping of mixed conducting polymers. Using poly[3-(ethyl-4-butanoate)thiophene-2,5-diyl] (P3EBT) as a model, we exploit the ester carbonyl in its side chain as an internal vibrational probe of the local electrostatic field created by the polaron and its counterion. Using stepwise vis/NIR/IR SEC, we assign multiple stages of electrochemical doping. Stage 1 occurs at low voltages and features highly delocalized, two-dimensional polarons in aggregates and crystalline domains of the polymer. During stage 2, ions intercalate these domains, causing polarons to localize to single chains due to electrostatic trapping. After this, polarons form in the amorphous regions during stage 3. Calibrating the vibrational probe using a combination of vibrational solvatochromism measurements and molecular dynamic simulations, we quantify doping level-dependent changes in the electrostatic interactions between electronic and ionic charges, with changes in the detected local electric field strength as high as 5 MV/cm. These measurements suggest that polaron-ion distance governs trapping at low voltages, while polymer chain coherence length and bipolaron formation play a larger role at higher voltages. This work establishes the vibrational Stark effect probe as a powerful spectroelectrochemical tool for investigating mixed conducting materials.