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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. 

Charge carrier mobility is a key factor underlying the performance of conjugated polymers as conductive materials for flexible and lightweight electronics. Chemical doping is typically used to improve polymer conductivity by increasing the carrier density. However, doping consequently induces both morphological and electrostatic changes within the polymer that impact charge mobility, the extent to which remains unclear. Using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) polymer films with tuned morphology and controlled ion-carrier distance, we investigated the influence of nanoscale chain ordering on the device-scale mobility of its chemically-induced carriers. Grazing-incidence x-ray diffraction measurements revealed that chemically doping the films resulted in a similar lamellar d-spacing of ~18.5 Å, despite differences in chain ordering within their nanocrystalline domains. Transient absorption (TA) spectroscopy was used to examine the relaxation of hole polarons excited with 0.62 eV (2000 nm) light to study their trapping behavior, and the results were compared with field-effect mobility measurements. Despite a 4-fold difference in hole mobility, the average relaxation times of the mobile and trapped polarons were identically ~0.1 ps and 17 ps, respectively, between the two films. The TA results only showed qualitative differences in the ratio of mobile to trapped polarons, indicating that ordered nanocrystalline domains facilitate the formation of free polarons, which enhance the hole mobility. The results from this study suggest that TA spectroscopy can be used as an electrode-free method of assessing the local mobility of doping-induced charge carriers, and that nanoscale chain ordering – and not just mesoscale structure or ion-carrier distance – is essential to control for improving the device-scale mobility of polarons. 

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. 

Abstract Conjugated polymers (CPs) play a central role in electronic applications due to their easily tuned electronic and ionic conductivities via chemical or electrochemical doping. Although doping improves charge conduction by introducing high densities of carriers into the CP, the accompanying structural changes and their impact on carrier mobility remain elusive. Methods capable of probing carrier distributions and their dependence on polymer morphology are needed to better understand how to improve conductivity. Here, a transient absorption (TA) spectroscopy approach is demonstrated, capable of directly probing mobile and trapped carriers in doped CPs and that is also sensitive to polymer nanostructure by using a model polythiophene system with tuned crystallinity. Exciting polarons in the polymer films produces distinct photoinduced absorption signals in the near‐infrared spectrum that decay during the picosecond timescale in the form of biphasic, stretched exponential kinetics, which reflect a distribution of mobile (free) and trapped polarons. The kinetic analysis provides evidence for mobile polarons irrespective of polymer film crystallinity, whereas polarons located in impure amorphous phases with reduced chain ordering exist within a deeper distribution of trap states. Altogether, these observations suggest a stronger correlation of carrier trapping with local chain ordering (planarity or aggregation) rather than polymer crystallinity. 

Charge carrier mobility is a key factor underlying the performance of conjugated polymers as conductive materials for flexible and lightweight electronics. Chemical doping is typically used to improve polymer conductivity by increasing the carrier density. However, doping consequently induces both morphological and electrostatic changes within the polymer that impact charge mobility, the extent to which remains unclear. Using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) polymer films with tuned morphology and controlled ion-carrier distance, we investigated the influence of nanoscale chain ordering on the device-scale mobility of its chemically-induced carriers. Grazing-incidence X-ray diffraction measurements revealed that chemically doping the films resulted in a similar lamellar d-spacing of ∼18.5 Å, despite differences in chain ordering within their nanocrystalline domains. Transient absorption (TA) spectroscopy was used to examine the relaxation of hole polarons excited with 0.62 eV (2000 nm) light to study their trapping behavior, and the results were compared with field-effect mobility measurements. Despite a 4-fold difference in hole mobility, the average relaxation times of the mobile and trapped polarons were identically ∼0.1 ps and 17 ps, respectively, between the two films. The TA results only showed qualitative differences in the ratio of mobile to trapped polarons, indicating that ordered nanocrystalline domains facilitate the formation of free polarons, which enhance the hole mobility. The results from this study suggest that TA spectroscopy can be used as an electrode-free method of assessing the local mobility of doping-induced charge carriers, and that nanoscale chain ordering – and not just mesoscale structure or ion-carrier distance – is essential to control for improving the device-scale mobility of polarons. 

Two-dimensional infrared (2D IR) spectroscopy, infrared pump–infrared probe spectroscopy, and density functional theory calculations were used to study vibrational relaxation by ring and carbonyl stretching modes in a series of methylated xanthine derivatives in acetonitrile and deuterium oxide (heavy water). Isotropic signals from the excited symmetric and asymmetric carbonyl stretch modes decay biexponentially in both solvents. Coherent energy transfer between the symmetric and asymmetric carbonyl stretching modes gives rise to a quantum beat in the time-dependent anisotropy signals. The damping time of the coherent oscillation agrees with the fast decay component of the carbonyl bleach recovery signals, indicating that this time constant reflects intramolecular vibrational redistribution (IVR) to other solute modes. Despite their similar frequencies, the excited ring modes decay monoexponentially with a time constant that matches the slow decay component of the carbonyl modes. The slow decay times, which are faster in heavy water than in acetonitrile, approximately match the ones observed in previous UV pump–IR probe measurements on the same compounds. The slow component is assigned to intermolecular energy transfer to solvent bath modes from low-frequency solute modes, which are populated by IVR and are anharmonically coupled to the carbonyl and ring stretch modes. 2D IR measurements indicate that the carbonyl stretching modes are weakly coupled to the delocalized ring modes, resulting in slow exchange that cannot explain the common solvent-dependence. IVR is suggested to occur at different rates for the carbonyl vs ring modes due to differences in mode-specific couplings and not to differences in the density of accessible states. 

Abstract Eumelanin is a brown-black biological pigment with sunscreen and radical scavenging functions important to numerous organisms. Eumelanin is also a promising redox-active material for energy conversion and storage, but the chemical structures present in this heterogeneous pigment remain unknown, limiting understanding of the properties of its light-responsive subunits. Here, we introduce an ultrafast vibrational fingerprinting approach for probing the structure and interactions of chromophores in heterogeneous materials like eumelanin. Specifically, transient vibrational spectra in the double-bond stretching region are recorded for subsets of electronic chromophores photoselected by an ultrafast excitation pulse tuned through the UV-visible spectrum. All subsets show a common vibrational fingerprint, indicating that the diverse electronic absorbers in eumelanin, regardless of transition energy, contain the same distribution of IR-active functional groups. Aggregation of chromophores diverse in oxidation state is the key structural property underlying the universal, ultrafast deactivation behavior of eumelanin in response to photoexcitation with any wavelength. 

Here, we investigate the photochemistry of a catechol : o-quinone heterodimer as a model system for uncovering the photoprotective roots of eumelanin.