Modulated Symmetry Breaking of Hybrid Metal Halides Driven by Disulfide Conformational Adaptation for Strong Second Harmonic Generation

ABSTRACT Organic–inorganic hybrid metal halides (OIHMHs) possess highly tunable chemical and electronic structures and are promising platforms for the development of functional materials. Among them, OIHMHs with non‐centrosymmetric structures are particularly attractive for their polar characteristics. Disulfide‐containing A‐site organic cations represent promising symmetry‐directing units owing to their dynamic conformation and potential axial chirality. However, their intrinsic flexibility often leads to disorder or racemization, weakening their capability in directing stable non‐centrosymmetric OIHMHs. Herein, we rationally developed a series of OIHMHs featuring rotatable disulfide bonds and uncovered a symmetry‐regulation mechanism in which dynamically adaptive disulfide‐containing cations actively promote asymmetric and non‐centrosymmetric arrangements of inorganic metal‐halide polyhedra. The formamidine disulfide dication (FDD 2+ ), with adaptive conformational adjustment driven by dihedral‐angle rotation, enables divergent structural evolution through adaptive coupling with ns 2 metal centers of different electronic character. The resulting four non‐centrosymmetric OIHMHs exhibit second harmonic generation performances several times those of the benchmark KH 2 PO 4 , promising their nonlinear optical photonic applications. This work establishes dynamic‐bond‐containing cations as effective symmetry regulators in OIHMHs for the construction of non‐centrosymmetric and polar hybrid materials with advanced optoelectronic functionalities.