Naphthopyrans are molecular switches that produce highly colored merocyanine dyes upon photochemical or mechanochemical activation in polymers. The mechanochromic behavior of these molecular force probes enables the straightforward visualization of stress and/or strain in materials. To date, research on the mechanochemistry of naphthopyran has largely focused on the 3H‐naphtho[2,1‐b]pyran (3H) scaffold, whereas isomeric 2H‐naphtho[1,2‐b]pyrans (2H) exhibit complementary properties as suggested from their photochemical reactivity. Here we directly compare the reactivity of two isomeric 2H‐ and 3H‐naphthopyran mechanophores in solution‐phase ultrasonication experiments and in crosslinked polydimethylsiloxane elastomers subjected to uniaxial tensile deformation. The prototypical 3H‐naphthopyran mechanophore produces a yellow merocyanine dye that reverts quickly, while the 2H‐naphthopyran mechanophore generates a red merocyanine dye that reverts significantly slower. The trends in absorption and reversion measured in solution are also reflected in solid polymeric materials activated in tension. Building on recent research into substituent effects, this study identifies naphthopyran isomerism as a simple lever for modulating the mechanochromic properties of the naphthopyran mechanophore used in the development of force‐responsive polymers.

Multimodal mechanophores that exhibit complex mechanochromic behavior beyond the typical binary response are capable of distinguishing between multiple stress states through discrete changes in color. Naphthodipyran photoswitches contain two pyran rings fused to a central naphthalene core and represent a potentially promising framework for multimodal reactivity. However, the concurrent ring opening of both pyran moieties has previously proven inaccessible via photochemical activation. Here, we demonstrate that mechanical force supplied to naphthodipyran through covalently linked polymer chains generates the elusive dual ring-opened dimerocyanine product with unique near-infrared absorption properties. Trapping with boron trifluoride renders the merocyanine dyes thermally persistent and reveals apparent sequential ring-opening behavior that departs from the reactivity of previously studied mechanophores under the high strain rates imposed by ultrasound-induced solvodynamic chain extension.

A 2H-bis-napthopyran (2H-BNP) mechanophore generates an unusually stable bis-merocyanine product upon mechanical activation.

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.

Multimodal mechanophores that exhibit complex mechanochromic behavior beyond the typical binary response are capable of distinguishing between multiple stress states through discrete changes in color. Naphthodipyran photoswitches contain two pyran rings fused to a central naphthalene core and represent a potentially promising framework for multimodal reactivity. However, the concurrent ring opening of both pyran moieties has previously proven inaccessible via photochemical activation. Here, we demonstrate that mechanical force supplied to naphthodipyran through covalently bound polymer chains generates the elusive dual ring-opened dimerocyanine product with unique near-infrared absorption properties. Trapping with boron trifluoride renders the merocyanine dyes thermally persistent and reveals unusual sequential ring-opening behavior that departs from the reactivity of previously studied mechanophores under the high strain rates imposed by ultrasound-induced solvodynamic chain extension.