Bipyridine Quaternary Ammonium Salts as Potential Photosynthesis Inhibitors with Excellent Safety

For the development of photosystem inhibitors, a series of bipyridine quaternary ammonium salts were designed and synthesized based on a structural modification strategy. Herbicidal evaluation demonstrated that compounds C21 and C30 exhibited outstanding inhibitory activity, with 100% control against Echinochloa crusgalli, Setaria viridis, Eleusine indica, Amaranthus retroflexus, Portulaca oleracea, and Abutilon theophrasti at 75 g a.i./ha, as well as providing 100% control against 19 weed species at 150 g a.i./ha. The evaluation of the redox behavior of the compounds revealed that C30 (−15.95 mV) exhibited a low negative zeta potential, similar to the pyridine salt (−15.94 mV), which confirmed the surface’s anionic nature. Oxidative stress investigation revealed that both C21 and C30 induced oxidative stress, thereby promoting cellular damage. Furthermore, electrical conductivity and chlorophyll content assays demonstrated that the compounds disrupted cell membrane integrity and diminished chlorophyll content. Density functional theory calculations indicated that both C21 (3.69 eV) and C30 (3.20 eV) possess high chemical reactivity. Notably, C21 exhibited the lowest acute toxicity, showing negligible mortality in zebrafish at 1 mM (compared to 100% lethality for paraquat), demonstrated an IC50 of 259.3 μM in BEAS-2B cells, and maintained 100% survival in earthworms after 48 h of exposure. Overall, the present work indicates that C21 interferes with photosynthetic processes with excellent safety, offering valuable insights for the development of a novel and nonselective herbicide.