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Molecular switches based on the 2H-1-benzopyran (chromene) scaffold have been widely developed for their desirable photochromic and mechanochromic properties. Extended π-conjugation is necessary to stabilize the ring-opened merocyanine dye at room temperature leading to efficient switching under ambient conditions. To this end, naphthopyrans represent a special class of benzo-annulated benzopyrans that have been studied extensively as both photoswitches and more recently as mechanophores, generating intensely colored merocyanine dyes upon exposure to ultraviolet light or mechanical force, respectively. Alternative annulation strategies with judicious heteroatom substitution have also been studied in the photochemistry literature, but the mechanochemistry of 2H-1-benzopyrans has yet to be explored. Here, we report the mechanochemical activation of an indole-fused 2H-1-benzopyran mechanophore that generates a yellow-colored merocyanine dye in polymers that is subsequently transformed to a purple-colored dye upon treatment with acid. Neutralization with base recovers the yellow-colored merocyanine isomer with trans exocyclic alkene geometry through an unusual acid-mediated alkene isomerization. This study expands the repertoire of mechanochromic mechanophores based on (hetero)annulated benzopyrans to enable multicolor chromomorphic behavior in response to both mechanical force and acid for applications in stimuli-responsive polymeric materials with complex switching properties.
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Naphthopyran molecular switches and their emergent mechanochemical reactivity
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.
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- Award ID(s):
- 2145791
- PAR ID:
- 10491100
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 14
- Issue:
- 37
- ISSN:
- 2041-6520
- Page Range / eLocation ID:
- 10041 to 10067
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D3SC03729K
