Structural insights into gelation dynamics of vanillin-based N-acylhydrazones

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In this work, a series of N -acylhydrazones based on 4- O -alkylated vanillin and 2-substituted benzohydrazides (2-F, 2-Cl and 2-H) were synthesized through ultrasound-assisted condensation and evaluated as organogelators. In contrast to most reported systems that rely on large planar condensed aromatic rings to promote gelation through π-π stacking, the present structures contain a discrete yet highly tunable aromatic scaffold, providing a modular platform to systematically investigate structure-gelation relationships. Among the twenty-one compounds prepared, fifteen formed stable gels in a wide range of organic solvents, with minimum gelation concentrations between 0.7% and 2.5% (w/w). Morphological analyses revealed self-assembled fibrillar networks responsible for solvent immobilization, while FT-IR, UV–vis, X-ray diffraction and rheology indicated a lamellar supramolecular organization stabilized by hydrogen bonding, dipolar interactions, π-π stacking and van der Waals forces. Systematic structural variations showed that the presence of an aliphatic chain significantly enhances gel stability, with optimal performance observed for chains containing six to ten carbon atoms. Furthermore, ortho -halogen substitution strongly influences gelation behavior by modulating intermolecular interactions within the supramolecular assemblies. The presence of halogen atoms enhances dipolar interactions and promotes π-π stacking between the discrete aromatic units, while also affecting the balance between intra- and intermolecular hydrogen bonding. In particular, chlorine-substituted derivatives produced the highest number of effective gelators and formed gels with enhanced thermal stability, whereas fluorinated analogues exhibit additional hydrogen-bonding capability that can either strengthen or weaken the gel network depending on the solvent environment. Hansen solubility parameter analysis indicated that alcohols are particularly favorable solvents for these systems. These results demonstrate how subtle electronic modulation of a minimal aromatic scaffold can govern supramolecular assembly and organogel formation.

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