Two Acylhydrazone Compounds with AIE Properties: Gelation Behaviors, Crystal Structures, Gelation Mechanism, and the Trace Detection of Al³⁺

Two acylhydrazone derivatives with long-chain alkyl groups, DHS (salicylaldehyde-based) and DHN (naphthaldehyde-based), were designed and synthesized. Both compounds exhibited supramolecular gelation abilities, and DHN was a supergelator for benzene. The single-crystal X-ray diffraction analysis revealed that DHS forms dimers via intermolecular hydrogen bonding. The powder X-ray diffraction (PXRD) studies showed that DHS adopts different repeating units in benzene and propanol gels. Based on the results obtained from single-crystal structure, PXRD, and mass spectrometry measurements, a plausible self-assembly evolution of gelator molecules was proposed. This work demonstrated that the single-crystal structures could provide critical insights into the molecular packing of the gel state. The fluorescence spectra and in situ monitoring of the gelation process confirmed that both DHS and DHN possess typical aggregation-induced emission (AIE). Notably, DHS functions as a highly sensitive optical chemosensor for Al3+, achieving quantitative detection with a remarkable minimum detection limit of 4.79 nM.