Photoinduced ROS Production by Poly(3‐hexylthiophene) (P3HT) is due to Autoxidation Rather Than Catalysis

ABSTRACT Poly(3‐hexylthiophene) (P3HT) is widely reported to generate reactive oxygen species (ROS) under illumination and is often implicitly treated as a photocatalyst in aqueous and biological environments. Here, we demonstrate that ROS formation by P3HT proceeds at the expense of irreversible polymer photooxidation. Systematic irradiation studies of P3HT thin films and nanoparticles under controlled photon flux reveal a strong wavelength dependence of both hydrogen peroxide generation and polymer degradation, with higher photon energies accelerating both processes. Quantitative analysis shows that H 2 O 2 formation is coupled to loss of conjugation and polymer consumption, and is not suppressed by sacrificial donors such as formate or glutathione. Comparative studies of different types of P3HTs indicate that increased interfacial accessibility as well as low regioregularity enhance degradation, and thus also ROS production. Together, these findings demonstrate that photoexcited P3HT acts as an efficient photosubstrate rather than a true photocatalyst, undergoing self‐sacrificial autoxidation during light‐driven oxygen reduction. This distinction is critical for interpreting ROS‐related phenomena in emerging biological applications.