Tuning chiral-inversion temperature of protonated cystamine in perovskite compounds through halogen substitution

Tuning molecular chirality through external stimuli stands at the forefront of chemical science. Here, we systematically examine the stereodynamic properties of a series of Cu(II)-based perovskite single crystals, CystaH₂[CuCl_(4-x)Br_x] (CystaH₂ = protonated cystamine cation, x = 0.74, 0.80, 1.05, 1.11, 1.18), which undergo a polar-to-nonpolar ferroelectric phase transition. In these compounds, the symmetry breaking structural change is governed by an unusually thermally driven P ↔ M helical inversion of the helical CystaH₂²⁺ organic cations. The conformational inversion can be substantially modulated by bromide doping. As the bromide content increases, the phase-transition temperature progressively shifts closer to room temperature, decreasing by up to 49 K. Detailed structural analyses reveal that the chiral inversion of CystaH₂²⁺ is highly sensitive to local chemical environments, and this doping strategy offers an effective route for precisely governing temperature-mediated chiral transformations, thereby expanding the design space for advanced chiral materials.