Halogen Substitution-Controlled Phase Transition, Dielectric Anomaly, and SHG Switching in Ga-Based Hybrid Molecular Ferroelectrics

Abstract Halogen engineering provides a versatile strategy for modulating intermolecular interactions and structural flexibility, enabling precise control over phase transition behaviors and functional responses. In this work, using 1,5-diazabicyclo[3.3.0]nonane (1,5−3.3.0-dabcn) as the organic scaffold, three organic−inorganic hybrid switchable phase-change materials, [1,5−3.3.0-dabcn-CH2Br][GaCl4] (1), [1,5−3.3.0-dabcn-CH2Br][GaBr4] (2), and [1,5−3.3.0-dabcn-CH2Br][GaI4] (3), were successfully synthesized via reaction with GaX3 (X = Cl, Br, I) in aqueous HX solutions. Compounds 1 and 2 exhibit reversible phase transitions, distinct dielectric anomalies, temperature-triggered second-harmonic generation (SHG) switching, and room-temperature ferroelectricity. In contrast, compound 3 shows only phase transition behavior without ferroelectricity or SHG activity. Comparative structural and Hirshfeld surface analyses reveal that halogen substitution systematically alters the strength and distribution of H···X hydrogen bonds, as well as the packing symmetry, thereby governing the observed functional evolution from polar (1, 2) to centrosymmetric (3) structures. This study demonstrates an effective molecular design strategy for constructing gallium halide hybrid materials with tunable physical properties.