Structure-Property Relationships in Acylhydrazone-Based Molecules: Photophysics, Aggregation, and Metal-Binding...

This study investigates structure-property and structure-limitation relationships in a series of newly synthesised low-molecular-weight acylhydrazone derivatives obtained via ultrasonic-assisted condensation of heterocyclic aldehydes (pyrimidine, morpholine, and imidazole derivatives) with 4-methyl-1,2,3-thiadiazole-5-carboxylic acid hydrazide and 4-hydroxybenzhydrazide. The primary objective was to determine how structural variation within the heterocyclic and acyl fragments influences optical, electrochemical, thermal, and aggregation behaviour. Electrochemical studies revealed a single irreversible reduction process, mainly associated with the electron-withdrawing fragment, with LUMO orbital localization on the thiadiazole or hydrazone linkage, as supported by theoretical calculations. UV-Vis absorption maxima were largely solvent-independent across seven solvents of varying polarity. All compounds exhibited blue emission, except for the molecule containing the morpholine-thiadiazole derivative. However, the fluorescence quantum yields were low (Φ ≤ 4.1%), constituting a major photophysical limitation. Metal-complexation experiments showed weak optical responses to the tested metal cations, indicating limited binding affinity. Aggregation studies showed divergent behaviour depending on molecular structure, including aggregation-enhanced emission in the pyrimidine-thiadiazole derivative. Overall, the results demonstrate that although structural modification modulates electronic distribution and aggregation behaviour, the low emission efficiency and weak metal coordination significantly limit the potential of these materials for sensing or imaging applications.