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BACKGROUND Photosystem I (PS I) is a plastocyanin-ferredoxin oxidoreductase within the photosynthetic electron transport chain and represents a key target for herbicide development. RESULTS Pyridinium salt compounds A and B were designed and synthesized with active substructural insertion and combination strategies. Bioassays revealed that most compounds exhibited > 70% inhibition of growth in Portulaca oleracea and Echinochloa crusgalli when applied to germinated seeds at a concentration of 10 μg mL-1. Compound B8 demonstrated 90-100% postemergence herbicidal activity on E. crusgalli, Eleusine indica, Setaria viridis, Amaranthus retroflexus, P. oleracea and Abutilon theophrasti. Meanwhile, B8 not only demonstrated electrochemical characteristics associated with PS I inhibitory activity and a negatively charged surface, but also promoted the accumulation of reactive oxygen species in Portulaca oleracea roots. At a concentration of 0.25 mM, B8 exhibited superior safety profiles in zebrafish embryos compared to paraquat. Furthermore, density functional theory calculations corroborated the redox reactivity and molecular orbital properties of B8, showed strong consistency with experimental observations. CONCLUSION Compound B8 had herbicidal activity similar to paraquat with lower toxicity to zebrafish embryos. Therefore, B8 represents a promising lead candidate for new PS I targeted herbicides. © 2025 Society of Chemical Industry.