Fluorinated quinoxaline-dithienopyrrole donor-acceptor copolymers: Probing structure-property relationships via integrated experimental and theoretical analysis

Abstract

Two donor-acceptor (D-A) semiconducting copolymers, PDTPQx0F and PDTPQx2F, were synthesized via the Stille coupling reaction. The copolymers comprise dithienopyrrole (a strong electron-donating moiety) paired with quinoxaline derivatives that are either non-fluorinated or fluorinated (electron-accepting units). Such materials demonstrate significant potential for optoelectronic device applications due to their tailored electronic and optical properties. A comprehensive characterization technique, including UV-Vis spectroscopy, ¹H NMR, CV, GPC, TGA and XRD, were employed to examine their thermal stability, electrochemical behaviour, optical properties, and molecular organization in the solid-state. The impact of incorporating two fluorine atoms into the quinoxaline unit was systematically investigated. Both copolymers exhibited excellent thermal stability, with decomposition temperatures exceeding 410 °C. The non-fluorinated compound exhibited an optical bandgap (E_g^opt) of 1.77 eV and a HOMO energy level of −5.0 eV. Interestingly, the fluorinated compound (PDTPQx2F) exhibited a narrower E_g^opt of 1.60 eV and a comparable HOMO level, attributed to its higher molecular weight (12100 Da) compared to PDTPQx0F (8500 Da). XRD analysis revealed that fluorination significantly enhances molecular organization by promoting stronger π-π stacking interactions between polymer chains in the solid state. This enhanced ordering highlights the critical role of fluorine substitution in modulating the structural and electronic properties of D-A copolymers for advanced optoelectronic applications. Additionally, DFT calculations were performed to predict the optical properties of the studied compounds. Advanced theoretical analyses, including RDG, ELF, and QTAIM-NCI, were utilized to investigate the interactions within these compounds.