Regulation of Reaction Pathways in Coordinated Chains by Directional Mechanical Force

Mechanochemistry refers to chemical reactions induced by mechanical forces. Due to different reaction mechanisms, products obtained through mechanochemistry can be distinct from those produced by thermochemistry and photochemistry. Scanning probe microscopy is a powerful tool for studying single-molecule mechanochemical processes. Mechanical force is a vector that has both magnitude and direction. Previous studies have focused on triggering reactions by forces and measuring their magnitude. In this work, we use the direction of the force to regulate the reaction pathway in a spin-crossover coordinated chain. The chains are prepared via the dehydrogenated coordination reaction between tetrahydroxybenzene molecules and Ni atoms on Au(111). The Ni atoms in the chain alternate between a high-spin state and a low-spin state. By altering Ni-O bond lengths and O-Ni-O angles through the directional mechanical force, a chemical process occurs, and the spin state of Ni undergoes a transition. With the attraction from a Au tip, the Ni atom is pulled from high-spin to low-spin state. With the repulsion from a C₆₀-functionalized tip, the low-spin Ni atom is pushed to the high-spin state. The force to induce the reaction is measured by qPlus atomic force microscopy. This study provides an approach for regulating chemical pathways.