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The Δ2,2′-Bi(2H-1,4-benzothiazine) Structural Motif of Red Hair Pigments Revisited: Photochromism and Acidichromism in a Unique Four-State System


  • Dedicated to the memory of Ernesto Fattorusso


Complete spectral and computational characterization of Δ2,2′-bi(2H-1,4-benzothiazine) (BBTZ) demonstrated that the structures of the stable yellow form and the photogenerated red form were assigned incorrectly and should be revised to the cis (Z) and trans (E) isomers, respectively. A blue elusive dication generated upon further protonation of the violet cationic form was also identified by UV spectroscopy and DFT calculations. On the basis of these data, BBTZ can be appreciated as a robust four-state system for various applications.