Pressure‐Induced Single‐Molecule Deformation Enables 208‐Fold Photoluminescence‐Intensity Enhancement in Zero‐Dimensional Copper Iodide Hybrids

Hybrid metal halides exhibit diverse excited‐state behaviors, yet the molecular‐level origin of pressure‐induced emission enhancement (PIEE) remains elusive. Here, we develop a rational molecular design strategy to isolate pressure‐induced single‐molecule deformation as the key factor of PIEE in 0D copper iodide hybrids. By systematically tuning the alkyl‐chain length of quaternary phosphonium cations, we construct a series of single crystals, (EtPPh 3 ) 2 Cu 2 I 4 , (PrPPh 3 ) 2 Cu 2 I 4 , and (BuPPh 3 ) 2 Cu 2 I 4 , which exhibit distinct crystal‐packing modes and distortions while preserving a [Cu 2 I 4 ] emissive core. In this work, “single‐molecule deformation” refers to the pressure‐induced geometric evolution of an individual discrete [Cu 2 I 4 ] 2− emissive unit within the crystal lattice. Pressure‐dependent structural, spectroscopic, kinetic, and theoretical analyses show that crystal packing and intermolecular interactions govern pressure transmission and suppress nonradiative relaxation, while deformation of the initially pre‐distorted [Cu 2 I 4 ] 2− unit in (PrPPh 3 ) 2 Cu 2 I 4 drives excited‐state reconfiguration and activates radiative decay. Consequently, its photoluminescence (PL) intensity reaches 208‐fold the ambient‐pressure value at 2.6 GPa, and the absolute photoluminescence quantum yield (PLQY) rises from 0.68% to 84.59%. These results identify single‐molecule deformation as the predominant structural origin of its high PIEE. Moreover, we demonstrate a power‐free preload‐sensing gasket and multilevel pressure‐encoded optical encryption, underscoring the application potential of pressure‐responsive luminescence.