Two AIE-active Schiff base fluorescent probes for highly selective recognition of Cu²⁺ ions

Two helical Schiff base compounds (H₄TPA and H₄TPE) containing a triphenylamine (TPA) or tetraphenylethylene (TPE) scaffold were successfully synthesized and characterized. Both H₄TPA and H₄TPE exhibited typical aggregation-induced emission characteristics in the mixed solvent of THF/H₂O. The two compounds also showed high selectivity and sensitivity for the recognition of Cu²⁺ over other ions in THF/HEPES (1:4, V/V, pH = 7.4, 2.0 × 10^-5 M), and could be used as turn-off fluorescent probes for Cu²⁺. The stoichiometric ratios and association constants were estimated via Job's plots and UV-vis spectra titration, and the detection limits of H₄TPA and H₄TPE toward Cu²⁺ were calculated to be 2.41 × 10^-7 M and 1.38 × 10^-7 M, respectively. Besides, the crystal structure of the complex obtained from the interaction of H₄TPA with Cu²⁺ well illustrated the binding modes, which helped us understand the Cu²⁺ recognition mechanism of H₄TPA and H₄TPE. Moreover, the detection of Cu²⁺ and spiked recovery experiments were carried out, which indicated that the two probes can be applied to Cu²⁺ detection in real samples with satisfactory recoveries.