The Halogenation Effect Induces a Variety of Switchable Phase Transition and Second-Harmonic-Generation Materials

Halogen engineering offers a means of enhancing the physical properties of materials by fine-tuning the rotational energy barrier and dipole moment, which proved to be effective in achieving switchable phase transitions and optical responses in materials. In this work, by substituting the methyl group in ligand N-ethyl-1,5-diazabicyclo[3.3.0]octane (CH₃CH₂-3.3.0-Dabco) with halogen atoms X (Cl or Br) and then contining to react it with FeBr₃ in a HBr aqueous solution, we successfully synthesized three kinds of organic-inorganic hybrid switchable phase-change materials, [CH₃CH₂-3.3.0-Dabco]FeBr₄ (1), [ClCH₂-3.3.0-Dabco]FeBr₄ (2), and [BrCH₂-3.3.0-Dabco]FeBr₄ (3), which were fully characterized by single-crystal X-ray diffraction and variable-temperature powder X-ray diffraction. Compared to compound 1, compounds 2 and 3 show two pairs of reversible phase transitions, dielectric anomalies, and a second-harmonic-generation effect, which are successfully induced due to the halogen substitution. This study offers an effective molecular design strategy for the exploration and construction of iron halide organic-inorganic hybrid materials with temperature-adjustable physical properties.