The 'mechanophore' concept has expanded to include units that respond to mechanical forces through noncovalent, physical means such as conformational rearrangements (Fig. ). These noncovalent transformations typically respond to lower-magnitude forces than those required to break covalent bonds (0.24 to several nN on the time scale of 0.1 s, as characterized by single-molecule force spectroscopy). Thus, the exploration of noncovalent mechanical transformations has attracted increasing interest. For example, Weder and Sagara have introduced rotaxane-and cyclophane-based mechanophores where force affects the spatial alignment between chromophores and alters their photoluminescent properties. Saito and co-workers have pioneered 'flapping' mechanophores that undergo conformational planarization under mechanical stimulation that extends the conjugation length. Moreover, the research groups of Matile , Sommer, and Lu have innovated twisted conjugated systems that planarize and gain conjugation efficiency under force. Other strategies for force-induced noncovalent processes include mechanical manipulation of supramolecular interactions in synthetic and biomaterials, and metal-ligand dissociation. However, most noncovalent physical changes provide transient signals that disappear when the force is removed.