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

Abstract

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.

The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome

“…32 2 and 8 in PBS buffer, and instead only absorption of the protonated E-form at 422 nm is observed. 104 The higher pK a and concomitant greater thermal stability of the E-form, which has been isolated, 110 is consistent with the infrequent observation of the Z-form. The lower pK a of the Z-merocyanine has, however, enabled their application as photoacids 23 since the first report on photochemical deprotonation in 1967.…”

Synthesis, Photochromic Properties, and Crystal Structures of Salts Containing a Pyridinium-Fused Spiropyran: Positive and Negative Photochromism in the Solution and Solid State

“…Examples of the isolation and structural determination of both isomers of photochromic molecules have been reported for stilbene derivatives, fulgides, and diarylethenes . However, this is the first example of spiropyran compounds except for protonated MCs − and partially isomerized MCs by UV irradiation …”

“…Several examples of the structural analysis of crystals containing the MC form have been reported. However, most MC forms have been protonated with acids − or stabilized by intermolecular interactions . The [ 6′-MC ]­X is a rare example of an intrinsically stable MC structure without such intermolecular interactions in the crystal.…”

Utilizing Yellow Compounds to Introduce π-Conjugated Molecular Structures: A Harmony with Blue Compounds

“…The colored product exhibits solvatochromism: A blue solution is obtained when chloroform is used as a solvent. In addition, the presence of acids leads to a protonated form; , a yellow solution is obtained using ethanol/acetone solvent with the addition of citric acid.…”