Mechanochemical reactivity of a multimodal 2H-bis-naphthopyran mechanophore

Multimodal mechanophores that react under mechanical force to produce discrete product states with uniquely coupled absorption properties are interesting targets for the design of force-sensing polymers. Herein, we investigate the reactivity of a 2H-bis-naphthopyran mechanophore that generates thermally persistent mono-merocyanine and bis-merocyanine products upon mechanical activation in solution using ultrasonication, distinct from the thermally reversible products generated photochemically. We demonstrate that a force-mediated ester C(O)-O bond scission reaction following ring opening establishes an intramolecular hydrogen bond, locking one merocyanine subunit in the open form. Model compound studies suggest that this locked subunit confers remarkable thermal stability to bis-merocyanine isomers possessing a trans exocyclic alkene on the other subunit, implicating the formation of an unusual trans merocyanine isomer as the product of mechanochemical activation. Density functional theory calculations unexpectedly predict a thermally reversible retro-cyclization reaction of the bis-merocyanine species that could explain the mechanochemical generation of the unusual trans merocyanine isomer.

Automated summary

Multi-modal mechanophores that react under mechanical force to produce unique absorption properties are of interest for force-sensing polymer design. In this study, a 2H-bis-naphthopyran mechanophore was investigated for its reactivity in generating thermally stable mono- and bis-merocyanine products upon mechanical activation. The force-mediated ester bond scission reaction followed by intramolecular hydrogen bonding plays a role in locking one merocyanine subunit in the open form, leading to the formation of an unusual trans merocyanine isomer. Unexpectedly, density functional theory calculations suggest a thermally reversible retro-cyclization reaction that may explain the mechanochemical generation of this isomer.