Electrochemical Generation and Spectroscopic Characterization of Charge Carriers within Isolated Planar Polythiophene

In order to unveil the nature of charge carriers in a doped polythiophene, a sterically isolated polythiophenene, poly(1EDOT), was electrochemically synthesized on electrodes. Generation of charge carriers was induced upon controlled electrochemical doping and investigated through various spectroscopic methods; the charge carriers were identified based on spin concentration (ESR spectra), aromatic character (Raman spectra), and electronic transition (UV–vis–NIR absorption spectra) of the polythiophene. Peculiarity of this study lies in the fact that the electrochemistry of the poly(1EDOT) reflects the p-doping process of a single polythiophene wire because interwire interaction (i.e., π–π stacking) is effectively prevented; therefore, the results should be essential and informative to understand polythiophene-based materials and devices. Upon electrochemical doping, ESR active polarons were generated. Further doping concentrated the polarons, which led to the formation of polaron pairs. Eventually, the polaron pairs merged into bipolarons at the doping level of about 30–35%. Such a transformation of charge carriers under different doping levels has been extrapolated from studies using oligomeric model compounds. To the best of our knowledge, this is the first example addressing the nature of the charge carriers generated in a single polythiophene wire by exploiting the unique structure of the isolated polythiophene. Importantly, the comparison of poly(1EDOT) with common polythiophenes such as poly(3,4-ethylenedioxythiophene) (i.e., polyEDOT) implied that π–π stacking strongly affects the generation and stability of charge carriers. Furthermore, we found that the polaron pair plays an important role in charge hopping transport in the conduction mechanism.