Atomic‐Scale Structural Distortion Drives Exciton Localization for Near‐Unity Photoluminescence in Copper–Iodide Clusters

Abstract Engineering structural distortions presents a powerful strategy for tailoring the optoelectronic properties of luminescent materials, while a fundamental understanding of how atomic‐scale distortions govern photoluminescence in copper‐iodide clusters has remained elusive. Herein, we report a model van der Waals solid based on copper–iodide clusters, where two vertically oriented and alternately arranged [Cu 2 I 4 ] 2− clusters are assembled via protection and interaction afforded by peripheral long‐alkyl‐chain cetyltrimethylammonium bromide ligands. This unique architecture affords a cooperative distortion response and exceptional buffering capacity, enabling precise control and direct probing of atomic‐scale structural distortions under pressure. We demonstrate that hydrostatic pressure induces controlled atomic distortions and an isostructural phase transition, which collectively enhance exciton localization. This leads to a dramatic amplification of self‐trapped emission, boosting the photoluminescence quantum yield from 32.25% to near‐unity (99.82%). Our work establishes atomic‐distortion engineering as a general principle for achieving ultimate control over light emission in hybrid semiconductors.