Design, Synthesis, and Biological Activity Evaluation of Herbicidal Ionic Liquids Based on Quinclorac Anion for Potential Rice Protection

Quinclorac is a widely used selective herbicide that targets plant growth hormones. However, it exhibits poor water solubility, limited plant permeability, and dependence on adjuvants. To address these limitations, we synthesized a series of novel herbicidal ionic liquids (HILs) by pairing the quinclorac anion with various cations, including quaternary ammonium, imidazolium, morpholinium, and 4-hydroxypiperidinium. Compared with quinclorac, the resulting HILs exhibited markedly improved physicochemical properties, including enhanced solubility, reduced surface tension, and increased lipophilicity. Thermal analysis confirmed excellent stability, with decomposition temperatures above 200°C and vaporization enthalpies exceeding 120 kJ/mol. Surface activity measurements indicated lower critical micelle concentrations (CMC) and reduced contact angles (CA), promoting superior wetting and adhesion on plant surfaces. Greenhouse bioassays demonstrated enhanced herbicidal activity against barnyard grass, achieving inhibition rates above 80% at 225 g AI/ha without phytotoxic effects on rice. Notably, at low concentrations (225-375 g AI/ha), all HILs promoted rice growth, while at higher concentrations (525-675 g AI/ha), HIL3 and HIL4 (containing long-chain quaternary ammonium cations) and HIL5 (containing an imidazolium cation) exhibited significant growth-promoting effects. This study reports the first quinclorac-based HILs and highlights their potential as sustainable, efficient, and environmentally friendly alternatives to conventional herbicide formulations for rice cultivation.