Translational-Symmetry-Breaking Ferroelasticity with Domain Switching and Thermochromism in a Copper(II) Organic–Inorganic Hybrid

Ferroelastic materials, whose bistable orientation states can be switched by external stimuli, hold promise for mechanical applications. Coupling it with thermochromism into multichannel responsive materials, however, remains a compelling challenge, and the underlying electronic mechanism is poorly understood. Here, we report an organic-inorganic hybrid, [ClMe3O]2[CuCl4] (ClMe3O = 1-(chloromethyl)-3-oxoquinuclidin-1-ium), which undergoes a reversible C2/c to P21/n phase transition at 360.9 K, accompanied by reversible thermally driven ferroelastic domain switching and thermochromism. Notably, the point group remains 2/m, with ferroelasticity arising from translational symmetry breaking─outside the classic Aizu rule. Variable-temperature crystallography reveals significant Cl-Cu-Cl angular distortion across the phase transition, which modulates the p-d covalent hybridization within the [CuCl4]2- anion. Density functional theory calculations confirm that the valence band edge is dominated by Cl-3p/Cu-3d hybridized states, while the conduction band minimum receives substantial contributions from the carbonyl π* orbitals of the organic cation. Thus, both the inorganic [CuCl4]2- chromophore and the organic cation participate in governing the thermochromic behavior. This work establishes translational-symmetry-breaking ferroelasticity, which expands the family of ferroelastic materials and provides electronic-structure-level insights into the coupling between domain switching and thermochromism, offering guiding principles for the design of multichannel responsive hybrids.