A reversible vapor-responsive fluorochromic molecular platform based on coupled AIE-ESIPT mechanisms and its applications in anti-counterfeiting measures

In this work, a series of 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives with an aggregation-induced emission-excited state intramolecular proton transfer (AIE-ESIPT) dual mechanism was synthesized. We systematically investigated their optical properties in solution, aqueous suspension, and the solid state. The fluorescence properties of these five compounds were highly dependent on the solvent environment. Particularly, DMSO and DMF were found to significantly promote deprotonation of the phenolic hydroxyl and favor fluorescence emission from the phenolic anion. The Keto or Enol forms generally could be transformed to phenolic anions with the aid of base in solution. The results from aggregation-induced emission enhancement (AIEE) and red-shifted AIE clearly illustrated the evolution process of the fluorescence spectra, indicating that the three luminescent species in the solution were transformable. The systematic investigation demonstrated that the desired fluorescence could be generated for HBT derivatives by varying the solvents or adding additives, such as base or water, to the solution. Because of the synergistic effect of AIE and ESIPT, these fluorophores exhibited high solid-state quantum yields and large Stokes shifts. These dyes also featured high photostability and tunable emission spectra covering most of the visible light region. Single crystal studies and theoretical calculations elucidated the luminescent properties. When loaded on filter paper, the HBT-based dyes exhibited high-efficiency fluorescence visualization and reversible solid-state luminescence switching under alternating amine and acid vapor treatments. These dyes were used on banknotes for anti-counterfeiting measures, demonstrating the practical applications of these molecules as security inks.