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Organic mixed ionic-electronic conducting polymers continue to emerge as promising next-generation materials for electrochemical applications ranging from bioelectronics to energy storage. However, we lack a clear understanding of how electrostatic and nanostructural changes in the polymer, which evolve during electrochemical device operation, influence charge and ion conductivity. In this work, we applied ultrafast near-infrared pump-probe spectroscopy, which is sensitive to the local nanostructure and electronic environment of charge carriers (polarons), to an electrochemically doped poly(3-hexylthiophene-2,5-diyl) [P3HT] model polymer system. The impact of electrolyte infiltration on carrier trapping was tested by comparing its photoexcited polaron dynamics to those measured in a chemically doped P3HT film lacking electrolyte and containing the same polaron mole fraction (~15%) and ClO4 − counterions. The transient absorption spectra revealed the presence of Coulombically free and trapped (ion-bound) polarons in both the electrochemically and chemically doped P3HT films, with a greater relative number of free polarons in the latter. However, the ion-bound polarons were less deeply trapped on average in the electrochemically doped film, suggesting that charge trapping was suppressed due to electrostatic screening by the electrolyte. This study highlights how fundamental knowledge gained from past chemically doped polymer studies cannot directly be applied to understand conductivity in mixed conducting polymers, encouraging future spectroscopic studies of charge trapping behavior in their electrochemically active states. 

While the photophysics of closed-shell organic molecules is well established, much less is known about open-shell systems containing interacting radical pairs. In this work, we investigate the ultrafast excited state dynamics of a singlet verdazyl diradical system in solution using transient absorption (TA) spectroscopy for the first time. Following 510 nm excitation of the excitonic S0 → S1 transition, we detected TA signals in the 530–950 nm region from the S1 population that decayed exponentially within a few picoseconds to form a vibrationally hot S0* population via internal conversion. The dependence of the S1 decay rate on solvent and radical–radical distance revealed that the excited state possesses charge-transfer character and likely accesses the S0 state via torsional motion. The ultrafast internal conversion decay mechanism at play in our open-shell verdazyl diradicals is in stark contrast with other closed-shell, carbonyl-containing organic chromophores, which exhibit ultrafast intersystem crossing to produce long-lived triplet states as the major S1 decay pathway. 

Organic mixed ionic–electronic conducting polymers remain at the forefront of materials development for bioelectronic device applications. During electrochemical operation, structural dynamics and variations in electrostatic interactions in the polymer occur, which affect dual transport of the ions and electronic charge carriers. Such effects remain unclear due to a lack of in situ spectroscopic methods capable of capturing these dynamics, which hinders the rational design of higher-performance polymers. Herein, we present the first in situ transient absorption spectroelectrochemical measurement of a conducting polymer in the near-infrared, where photoexcited charge carrier dynamics are used to directly probe their nanoscale environment and trapping behavior in working electrodes. In this method, voltage is applied to charge or discharge the polymer, and the picosecond relaxation dynamics of directly photoexcited charge carriers are spectroscopically monitored to relate their location within the heterogeneous polymer nanostructure to their transport behavior. Applying this technique to working PEDOT:PSS electrodes, we investigated the impacts of voltage-induced changes in polymer chain packing and ion–carrier interactions on charge trapping. At lower voltages, carriers initially form within J-aggregated PEDOT chains that are deeply trapped due to strong electrostatic coupling to PSS− counterions. At higher voltages, the PEDOT lamellae expand and charge–ion pairs enter the PEDOT-rich domains, where trapping is decreased and carriers delocalize among the more tightly stacked, H-aggregated PEDOT chains. Further, this in situ spectroscopic method can also be more broadly applied to study electrochemical dynamics in accumulation-mode and n-type polymer electrodes and electrochemical transistors.