Light-Driven Intraoctahedral Halide Isomerization in Two-Dimensional Mixed Halide Perovskites

Two-dimensional metal halide perovskites are emerging materials for quantum light emission and neuromorphic computing owing to their quantum-confined structures and tunable optoelectronic properties. Beyond structural dimensionality, the presence of multiple crystallographically distinct halide sites within a single metal halide octahedron presents a unique opportunity to engineer functionality at the subunit-cell level. Here, we report a light-driven, reversible halide-ion isomerization in single-crystalline BA₂PbBr_<i>x</i>I_4-<i>x</i> (BA = butylammonium, <i>x</i> = 1-3), where ions switch between distinct local configurations within individual PbX₆^4- octahedra, without long-range migration or macroscopic phase segregation. Through a combination of hyperspectral imaging, in situ X-ray diffraction, and first-principles calculations, we demonstrate that this intraoctahedral halide site switching modulates the optical bandgap by ∼0.1 eV and enables an estimated reversible electronic bandgap shift of up to ∼0.5 eV. Density functional theory reveals that these changes stem from a redistribution of valence band character, effectively creating chemically distinct optoelectronic isomers that can be activated by light. These results uncover a mechanism of structurally encoded, site-selective photoisomerization in 2D perovskites, offering a new strategy for reconfigurable optoelectronic devices, nonvolatile optical memory, and quantum photonics.