A Multifunctional Stimuli-Responsive AIE-Active Schiff Base for Solvatochromism, Mechanochromism, Acidochromism, Cu2+ Sensing and DNA Binding

Abstract

AIE materials exhibit distinct fluorescence responses to external factors such as heat, solvents, or mechanical force. In this study, a novel Schiff base fluorescent probe, HL, derived from a 9-fluorenone derivative, was designed and synthesized. HL was characterized by utilizing FT-IR, NMR, ESI-MS and SC-XRD analysis. The HL was observed to display a typical AIE behavior, showing strong yellow fluorescence emitted in THF/water mixtures with a high water contents. Interestingly, HL demonstrated multi-stimuli-responsive fluorescence properties.Firstly, it exhibited a pronounced solvatochromic effect, where the emission property changes as the solvent polarity increased from nonpolar hexane to polar DMSO; due to the ICT effect.Secondly, HL showed reversible mechanochromic behavior, the yellow fluorescence of pristine crystalline form changed to orange upon grinding. Remarkably, the original yellow emission could be recovered after THF vapour exposure. PXRD analysis indicated that mechanochromic behavior was due to the variations in molecular packing arrangements. Thirdly, HL exhibits reversible acidochromic behavior, which arises from the protonation of the imine group, leading to a redshifted emission and reduced band gap, which was utilized in a fluorescent anti-counterfeiting system. Additionally, HL selectively detected Cu 2+ ions with an LOD of 23.4 nM and interacted with CT-DNA through an electrostatic binding mode. This work introduced a novel approach for designing AIE-active, stimuli-responsive fluorescent materials, along with their potential applicability in metal ion sensing and DNA binding studies.