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Naphthopyran molecular switches undergo a ring-opening reaction upon external stimulation to generate intensely colored merocyanine dyes. Their unique modularity and synthetic accessibility afford exceptional control over their properties and stimuli-responsive behavior. Commercial applications of naphthopyrans as photoswitches in photochromic ophthalmic lenses have spurred an extensive body of work exploring naphthopyran–merocyanine structure–property relationships. The recently discovered mechanochromic behavior of naphthopyrans has led to their emergent application in the field of polymer mechanochemistry, enabling advances in the design of force-responsive materials as well as fundamental insights into mechanochemical reactivity. The structure–property relationships established in the photochemical literature serve as a convenient blueprint for the design of naphthopyran molecular force probes with precisely tuned properties. On the other hand, the mechanochemical reactivity of naphthopyran diverges in many cases from the conventional photochemical pathways, resulting in unexpected properties and opportunities for deeper understanding and innovation in polymer mechanochemistry. Here, we highlight the features of the naphthopyran scaffold that render it a powerful platform for the design of mechanochromic materials and review recent advances in naphthopyran mechanochemistry. 

Understanding structure–mechanochemical reactivity relationships is important for informing the rational design of new stimuli-responsive polymers. To this end, establishing accurate reaction kinetics for mechanophore activation is a key objective. Here, we validate an initial rates method that enables the accurate and rapid determination of rate constants for ultrasound-induced mechanochemical transformations. Experimental reaction profiles are well-aligned with theoretical models, which support that the initial rates method effectively deconvolutes the kinetics of specific mechanophore activation from the competitive process of nonspecific chain scission.