Substrate induced catalysis: Deciphering the weak acid triggered bleaching of an angular terthiazole photochromic dye

The thermal bleaching of the ring-closed colored isomer of a photochromic triangle terthiazole induced by a weak acid in acetonitrile was investigated from a kinetic point of view. A realistic model was proposed involving not less than nine species and thirteen elementary processes. Our studies highlight the importance of the transient monoprotonated species in the carbon–carbon bond weakening and confirm the role of the proton as catalyst for the ring opening. The origin of the sigmoid shape of the Absorbance vs time traces is discussed.