Mechanophores are molecular units that undergo specific structure and property changes under mechanical force, allowing for molecular-level insights into mechanical behaviors. This 'mechanophore' concept traditionally refers to mechanically induced chemical transformations, such as the cleavage of azo moieties or ring-opening of spiropyrans (Figure ). Those covalent mechanochemical processes typically involve homolytic cleavage, heterolytic cleavage, pericyclic reactions, or metal-coordinate bond cleavage. Kauzmann and Eyring extended the transition state theory to cases where a constant external force acts along the reaction pathway, shedding light on the fundamentals of covalent mechanochemistry. It is understood that the application of force decreases the activation energy (Eact) of a reaction by coupling mechanical work to the nuclear movements that occur along the reaction coordinate. 29- Bell's approximate theory further states that an applied force F changes a reaction's energy barrier by ∆Eact = -F∆R, where ∆R is the change in the distance between the atoms (to which F is applied) from the reactant state to the transition state along the reaction coordinate. This approximate theory assumes the force only linearly reduces the activation energy, without otherwise distorting structures or altering reaction pathways. More comprehensive theoretical treatments are now well-established, but the Kauzmann/Eyring/Bell framework remains a useful construction for qualitatively interpreting many mechanochemical structure-activity effects.