Polymorphism-Dependent Herbicidal Activity of Pyraquinate

Manipulation of solid states is a promising method to enhance the performance of contact insecticides. However, its applicability to herbicides, which control weeds instead of insects, remains unknown. Realizing that herbicides exist as microcrystals on the leaf after application, we are prompted to investigate the polymorphic landscape of the same herbicides with the purpose of boosting herbicidal performance. Herein, we report for the first time that the bioactivity of herbicides is highly dependent on polymorphism, using a newly developed herbicide pyraquinate as a model substance. Five novel polymorphs of pyraquinate were discovered. Their crystal structures, thermodynamic stabilities, and solubilities were evaluated. Bioassays in greenhouse and field trials show that the newly developed metastable Form II is significantly more effective than commercialized Form I against resistant barnyard grass (Echinochloa crusgalli). Moreover, Form II exhibits significant kinetic stability in both water- and oil-based suspension concentrates, promising its practical application. This work emphasizes the necessity of polymorphism investigation for the development of herbicide products and highlights the great potential of crystal engineering to enhance herbicidal efficacy.