Tailoring Monofluoride Substitution in a 2D Copper Chloride Perovskite Showing Thermochromism and High-Temperature...

2D hybrid perovskites with diversified species and tunable physical properties have emerged as versatile platforms for applications in many fields such as photovoltaics, light-emitting diodes, ferroics, and so on. However, reports of thermally stable dielectric switching behavior in molecularly tailored 2D lead-free copper chloride perovskites are limited. In this work, we employed monofluoride substitution to successfully regulate the crystal symmetry to achieve high-temperature structural phase transition and switchable dielectric responses in (2-FEA)₂CuCl₄ (2-FEA = 2-fluoroethylaminium). Fluorine substitution establishes intermolecular C─H⋯F hydrogen bonding interactions among the organic cations, driving a symmetry-breaking transition from the parent tetragonal (EA)₂CuCl₄ (EA = ethylaminium) to a monoclinic phase in (2-FEA)₂CuCl₄ but it maintains a 2D perovskite structure. Consequently, (2-FEA)₂CuCl₄ undergoes an evident order-disorder structural transition (P2₁/c-Cmce) around 351 K accompanied by dielectric switching behavior. Besides, eye-catching thermochromic behavior has been evidenced by variable-temperature UV-vis spectra revealing a temperature-dependent bandgap redshift. This finding establishes fluoride substitution as a powerful strategy for the molecular engineering of multifunctional materials while providing valuable insights for the rational design of stimuli-responsive hybrid systems.