Monofluoride substitution modulates molecular symmetry in a 2D copper-based perovskite, inducing a reversible phase transition at 351 K and triggering dual dielectric switching and thermochromism.

Abstract

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.