Crystallization Pathway-Dependent Polymorphism and Photoluminescence in a Schiff Base Hydrazone

Two polymorphic forms of the Schiff base hydrazone (E)-4-amino-N′-(1-(3-aminophenyl)ethylidene)benzohydrazide (compound 1), named 1α and 1β, were synthesized via distinct crystallization pathways using 3′-aminoacetophenone and 4-aminobenzoic acid hydrazide. Polymorph 1α was obtained by direct condensation, whereas 1β was formed under template-assisted crystallization using zinc tetrafluoroborate as a structure-directing agent. The crystal structures, determined by single-crystal and powder X-ray diffraction, reveal different packing motifs: 1α comprises two symmetry-independent molecules (Z′ = 2) united in double-chains by hydrogen bonds, while 1β contains three symmetry-independent molecules (Z′ = 3) linked in triple-chains by hydrogen bonds and N–H···π interactions. Computational analysis (lattice energy, Hirshfeld surface, and energy framework calculations) indicates stronger intermolecular cohesion within the crystal lattice of 1β, whereas thermal analysis shows that 1α exhibits greater thermal robustness under heating conditions, as reflected by its higher melting point. These differences are attributed to crystallization pathway-dependent packing effects rather than a direct thermodynamic stability hierarchy. Photoluminescence measurements show significantly higher emission intensity for 1β, which correlates with differences in crystal packing and electronic structure, as supported by density functional theory calculations. Overall, these results demonstrate how crystallization pathway-dependent polymorphism can strongly influence supramolecular organization and photoluminescence behavior in molecular solids.