Direct Access by Mechanochemistry or Sonochemistry to Protonated Merocyanines: Components of a Four-State Molecular Switch

Direct access to the protonated merocyanine forms of two substituted spiropyrans by mechanosynthesis or sonochemistry was explored. The compounds were formed by the condensation reaction of the methyleneindolium iodide salt with salicylaldehyde derivatives. X-ray crystallography, ¹H NMR spectroscopy, ab initio geometry optimization, and absorption spectroscopy were combined to provide a better understanding of the four-state molecular switch system in which the newly synthesized protonated merocyanines were found to play a central role. The results of this study suggest that the stability of the protonated merocyanines requires acidic conditions, as treatment with base led to the corresponding unprotonated merocyanines, which in turn spontaneously converted into photochromic closed spiropyrans.