Programming Structural Symmetry and Emission in Low‐Dimensional Hybrid Perovskites via Time and Temperature

Layered hybrid perovskites offer exceptional opportunities for tuning optoelectronic properties through structural design. However, achieving reproducible and controllable functionality requires a detailed understanding of how their structures evolve during synthesis and processing. Here, we demonstrate programmable structural symmetry and emission control in benzylammonium-based layered lead bromide perovskites through coupled time- and temperature-dependent transformations. Under continuous shaking, the material evolves from orthorhombic platelets to a lower-symmetry monoclinic phase composed of elongated, grooved microcrystals, accompanied by the emergence of broadband emission associated with exciton self-trapping in the distorted monoclinic structure. By combining in situ and ex situ structural and optical characterization, we show that this transformation involves both lattice reorganization and redistribution of the organic component, consistent with a combination of distortion-driven and solution-mediated mechanisms. Thermal activation induces a reverse pathway toward a higher-symmetry configuration resembling the original orthorhombic phase, accompanied by recovery of sharp excitonic emission. These results establish a direct link between processing history, metastable structural states, and emission behavior, providing a strategy for programming functional states in structurally soft layered perovskites toward adaptive and reconfigurable optoelectronic materials.