Changes induced by electrochemical oxidation of poly(9,9-dioctylfluorene-alt-thiophene): towards a correlation between charge transport, molecular structure modifications and degradation

In this work, an alternating copolymer of fluorene and thiophene, the poly(9,9-dioctylfluorene-alt-thiophene) (PDOF-alt-Th), was synthesized by the Suzuki coupling reaction. To confirm its molecular structure, the copolymer was characterized by means of photoelectron, vibrational and optical spectroscopies. Raman scattering and infrared absorption spectra were fully interpreted, with the support of DFT modeling. It was found that the incorporation of thiophene units along the poly(9,9-dioctylfluorene) chain caused a red shifted emission, and thus the UV–Vis spectroscopy was also used to determine the value of the optical band gap and electron affinity. Electrochemical and electrical properties were then investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The values of ionization potential (Ip) and the charge transfer resistance (R_CT) at different potentials were determined. The EIS results showed an increase of conductivity at the first oxidation potential peak of the compound which was related with the formation of the thiophene radical cation. Consequently, the electroactive behavior of the copolymer was complemented by means of UV–Vis ex situ and Raman-spectroelectrochemical analysis. Finally, electrical, electrochemical and optical changes induced by electrochemical oxidation were presented, and the relationship between molecular structure modifications and charge generation are discussed.