Multiple Mechanical Behaviors in One Crystal of 2,4-Dichlorophenoxyacetic Acid Form II: Thermomechanical Effect and Elastic Deformation

Mono-component crystals showing multiple stimuli-responsiveness have attracted tremendous interest and remain a big challenge. Herein, we report the discovery of the second polymorph of the famous herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) (Form II), which exhibits multiple mechanical behaviors, including thermomechanical effect and elastic deformation. The relationship between mechanical behaviors and crystal structure was illustrated. The 2,4-D Form II crystals exhibit thermomechanical behavior due to the molecular rearrangement, generating strain within these crystals. The strain energy is developed and accumulated, followed by its rapid release. The elastic behavior requires multiple weak interactions and interlocked molecular packing that prevents sliding between layers. This work can contribute to a better understanding of the design of multifunctional crystal materials.