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© We report the discovery of a 2H–naphtho[1,2–b]pyran mechanophore that produces a permanent merocyanine dye upon mechanochemical activation, in contrast to the reversible product generated photochemically. Experiments suggest that the irreversibility of the mechanically generated merocyanine is due to a unique reaction in which the scission of an ester C–O bond reveals a β–hydroxy ketone that locks the merocyanine through an intramolecular H–bonding interaction. In addition to demonstrating the reactivity using solution–phase ultrasonication, permanent merocyanine generation is also achieved in solid polymeric materials. The permanent coloration achieved with the naphthopyran mechanophore affords unique opportunities for sensing and force–recording applications as well as fundamental studies limited by the reversibility of typical colorimetric force probes.
