Acylhydrazone Bond: A Pivotal Bridge for Modifying Julolidine Derivative with Pyrazole Group as MultiStimuli‐Responsive Fluorescent Materials

Abstract

To screen julolidine derivatives containing a pyrazole group with excellent fluorescence properties and promising applications in multistimuli‐responsive fluorescent materials, three novel compounds were designed and synthesized by connecting pyrazoles through typical conjugated bonds: imine (T1), dihydrazone (T2), and acylhydrazone (T3). The molecular structure of the title compounds was characterized by 1H NMR, 13C NMR, and HR‐MS. The representative compound T3 was verified by single crystal X‐ray diffraction. Although the three compounds have roughly similar conjugated structures, dissimilarity in their fluorescence properties was apparent. The results of fluorescence studies showed that the compound T3, which was connected to julolidine and pyrazole by an acylhydrazone bond, exhibited excellent fluorescence performance compared to the other compounds T1 and T2. Compound T3 can also respond to external stimuli, such as solvent viscosity, organic acids/bases, and mechanical forces. In addition, T3 could also specifically detect Cu2+ ions, and the fluorescence titration data indicated that the low limit of detection (LOD) of compound T3 for Cu2+ is 0.26 µM within the range from 0 to 50 µM. Therefore, this work provides a preliminary basis for the development of multistimuli‐responsive fluorescent molecular materials containing acylhydrazone fragment.