Steering chemical reactions with force

Chemical reactivity underlies our fundamental understanding of many physical and biological phenomena. Chemical reactions are typically initiated by heat, electric current or light. Albeit far less studied, mechanical force is yet another way to orthogonally catalyse chemical reactions. An applied force can substantially reduce the reaction energy barrier, thus enabling reaction pathways that are too slow (or even forbidden) according to the laws of thermodynamics. Single-molecule nanomechanical techniques, including optical and magnetic tweezers and atomic force microscopy, offer the possibility to apply a directional force on an individual chemical bond. In non-covalent (or soft) mechanochemistry, low, sub-nN forces trigger bond rotation or hydrogen-bond rupture. By contrast, in covalent mechanochemistry, higher forces typically result in the breaking and re-forming of individual bonds. This Review focuses on the advances in our mechanistic understanding of single-bond mechanochemistry resulting from single-molecule measurements, as well as on the exciting new perspectives that we envision for this burgeoning field in the near future. Mechanical forces can be used as an alternative source of energy to increase chemical reactivity. This Review reports on the latest single-molecule measurements and how they have improved the current understanding of single-bond mechanochemistry.