ESIPT-AIE active Schiff base based on 2-(2′-hydroxyphenyl)benzo-thiazole applied as multi-functional fluorescent chemosensors

Abstract

Three AIE (aggregation-induced emission)-ESIPT (excited-state intramolecular proton transfer) active 2-(2-hydroxyphenyl)benzothiazole derivatives, HL¹, HL² and HL³ with one, two and three rotatable phenyl groups, were obtained and characterized. Their AIE properties in THF/HEPES solution were investigated in detail. HL² shows the best AIE performance with 71-fold fluorescence enhancement, while HL³ only shows a 9-fold enhancement. With the AIE property, HL¹ and HL² could act as fluorescence chemosensors to detect Cu²⁺ ions via the "turn off" mode in THF/HEPES media. With the ESIPT property, HL¹ and HL² could also detect Zn²⁺ ions via the "turn on" mode in EtOH/HEPES media. During the detection process, both demonstrate rapid response and high contrast before and after the addition of metal ions. The species formed in the detection system were investigated. The results of X-ray single-crystal diffraction confirm that Zn²⁺ is coordinated with the oxygen atom and Schiff base nitrogen atom instead of the benzothiazole nitrogen atom in the tetrahedron geometry. Moreover, the chemosensors were successfully constructed into handy fluorescence test papers for Cu²⁺ and Zn²⁺ detection.