Unlocking the Potential of Organic Semiconductors: Synergistic Alkylthio-Halogen Substitutions for Superior Photoelectrochemical Cathodic Protection

In this study, seven organic polymer semiconductors based on benzodithiophene and fluorinated thiophene conjugated backbones were designed and synthesized. These included the unmodified polymer, polymers singly substituted with alkoxy, alkylthio, fluorine, or chlorine, as well as polymers with synergistic substitutions of alkylthio and F (or Cl). The synergistic substitution not only optimized the geometric and electronic structures at the molecular level but also enabled high-performance photoelectrochemical cathodic protection in macroscopic devices. The synergistically substituted polymers exhibited HOMO/LUMO energy levels of −5.51/–3.54 eV (bandgap 1.97 eV) and −5.47/–3.52 eV (bandgap 1.95 eV). Intermolecular F···S and Cl···S interactions reduced the π–π stacking distance to 4.48 Å, resulting in well-defined microphase separation and an interpenetrating network in the bulk heterojunction. These polymers showed extended carrier lifetimes of up to 4.47 and 4.10 ns, prolonged exciton recombination times of 214.8 and 209.9 μs, and shortened charge transport times of only 41.7 and 31.9 μs─superior to their singly substituted and unmodified counterparts. Moreover, their double-layer capacitance was significantly enhanced, indicating a higher density of electrochemically active sites. In protecting Q235 carbon steel, the synergistically substituted polymers reduced the open-circuit potential to 0.822 and 0.831 V in the presence of a hole scavenger, maintained high photocurrent densities of 70.7 and 62.5 μA cm^–2, significantly decreased interfacial charge transfer resistance, and minimized overall impedance. Polarization tests further demonstrated the highest steady-state currents and the lowest polarization resistances, reflecting excellent polarization behavior and operational stability. Even in a 3.5 wt % NaCl environment, these polymers maintained cathodic protection capabilities due to their low HOMO levels, which facilitated oxygen evolution reactions to supply electrons. This strategy markedly improved carrier separation/transport efficiency and corrosion resistance, offering a new avenue for the development of high-performance organic photoelectrochemical cathodic protection materials.