Halogen-Electronegativity Tuning Induces Symmetry Breaking and Polarity Activation in Chiral Zinc Halide Hybrids

Chiral hybrid metal halides (CHMHs) have recently emerged as promising platforms for optoelectronic and nonlinear optical applications. Yet, most reported systems crystallize in nonpolar space groups, precluding macroscopic polarization and thereby limiting their applicability in polarization-dependent functionalities. Here, a halogen-electronegativity tuning strategy enabled the synthesis of chiral (R/S-3AP)ZnBr₄ (P2₁2₁2₁) and chiral-polar (R/S-3AP)ZnCl₄ (P2₁). Halogen-dependent size effects and the resulting H···X (X = Cl, Br) hydrogen-bonding interactions between the (R/S-3AP)²⁺ cations and [ZnX₄]^2- tetrahedra drive the symmetry evolution and property modulation in (R/S-3AP)ZnX₄. Both compounds exhibit intrinsic chirality and nonlinear optical activity, with second-harmonic generation (SHG) responses that are 0.5 and 1.2 times that of KDP for (R/S-3AP)ZnBr₄ and (R/S-3AP)ZnCl₄, respectively. Owing to its polar crystal structure, (S-3AP)ZnCl₄ further displays a discernible room-temperature ferroelectric response. Theoretical calculations indicate that their optical properties arise from the synergistic interplay between the (R/S-3AP)²⁺ cations and [ZnX₄]^2- tetrahedra. This work presents a simple yet effective approach for inducing structural symmetry breaking, establishing macroscopic polarity, and tuning the multifunctional properties of flexible CHMHs.