Effect of pyridyl position on constructing acid-base dual-responsive fluorophores with ESIPT and AIE characteristics

Exceedingly large Stokes shift solid-state fluorescent materials with acid/base dual responses are promising for potential practical applications. However, their realization is extremely challenging because of the difficulty in designing molecules possessing these functionalities simultaneously. Herein, three pyridyl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) isomers (o-Py-HBT, m-Py-HBT, and p-Py-HBT) were rationally designed with a pyridyl group located at ortho, meso, and para positions relative to the phenolic hydroxyl (-OH), respectively. These compounds are both excited-state intramolecular proton transfer (ESIPT)-and aggregation-induced emission (AIE)-active. The intrinsic -OH and introduced pyridyl moiety were exploited as base-and acid-responsive units, respectively. Among the three HBT isomers, m-Py-HBT exhibited the most significant emission shift upon acidic stimuli. Theoretical calculations confirmed that the prominent intramolecular charge transfer of the protonated keto-tautomer of m-Py-HBT accounts for the trifluoroacetic acid-induced bath-ochromic shift of similar to 33 nm. In addition, it was concluded that intramolecular H-bonding strength and -OH acidity jointly affected the base response. Finally, m-Py-HBT was successfully applied for the fabrication of trichromatic fluorescent patterns upon acid or base stimulation, exhibiting great application potential for encrypted pattern production. The exploration of structure-property correlations provides experimental and theoretical guidance for designing and developing multi-stimuli responsive solid-state molecular switches.