In the nascent field of polymer mechanochemistry, force-sensitive molecules called mechanophores are incorporated into polymers and designed to undergo a wide variety of chemical reactions when subjected to mechanical stress. For a number of mechanophores, mechanical force has also been demonstrated to promote reactions that diverge from the more conventional activation pathways achieved with heat or light. Naphthopyran has recently emerged as a versatile mechanophore that undergoes a ring-opening reaction under force to generate a colored merocyanine dye. Force-induced transformations that produce a change in color or luminescence are particularly useful because they enable the direct visualization of stress and/or strain in polymeric materials. Through the development of structure–mechanochemical activity relationships, we have established a library of naphthopyran mechanophores with diverse absorption and fading properties, providing access to polymers with multicolor mechanochromic and complex stimuli-responsive behavior. Our research has also uncovered unusual reactivity of naphthopyran under the bias of mechanical force. For example, we recently discovered a naphthopyran mechanophore that produces a permanent merocyanine dye that is locked through an intramolecular hydrogen-bonding interaction. In addition, the development of mechanophores that are capable of distinguishing between multiple stress states through discrete visual changes remains a grand challenge in the field for being able to quantify stress remotely. To this end, we have designed a bis-naphthopyran mechanophore that exhibits force-dependent changes in color via a unique mechanical activation pathway. Most recently, we have uncovered an anomalous force-promoted dual ring-opening reaction of naphthodipyran, which is historically inaccessible and paves the way for future fundamental studies as well as the development of novel force-responsive materials.

Unraveling the unique mechanochemical reactivity of naphthopyran

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ACS Fall 2022

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