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

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 [Cu₂I₄]^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.