Generation of an Elusive Permanent Merocyanine via a Unique Mechanochemical Reaction Pathway

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.