Two helical Schiff bases with “AIE+ESIPT” characteristics exhibiting selective ion recognition properties

Two racemic Schiff bases (H4L1 and H4L2) were synthesized and characterized, and the crystal structures indicated their helical configurations with equal amounts of P- and M-helicity. The aggregation-induced emission (AIE) and selective ion recognition properties of H4L1 and H4L2 were investigated using UV–vis absorption and fluorescence spectra. The results showed that both H4L1 and H4L2 exhibited obvious “AIE + ESIPT” characteristics in THF/H2O solution upon increasing the water fraction. Both H4L1 and H4L2 displayed an obvious turn-off fluorescence response to Cu2+ and F−, and a slight turn-off response to Fe3+ in DMSO, while they displayed an efficient selective ion recognition of Cu2+ in THF/H2O (1: 1, V/V). The corresponding stoichiometry ratio, association constant, Stern-Volmer quenching constant, and the detection limits were determined via Job's plots, UV–vis spectra titration as well as fluorescence spectra titration experiments. The crystal structures of the complexes obtained from the interactions of H4L1 with Cu2+ and Fe3+ illustrated their binding modes and binding differences, which helped us interpret the ion recognition mechanism of H4L1 and H4L2. In addition, the application of H4L2 on test strips was performed, indicating that H4L2 can be used for the rapid detection of Cu2+ and F− in THF. The acid-responsive fluorescence behavior of H4L2 showed that the white light emission can be obtained upon the addition of hydrochloric acid or aldehyde c into H4L2.