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The dual emission (DE) characteristics of atomically precise copper nanoclusters (Cu NCs) are of significant theoretical and practical interest. Despite this, the underlying mechanism driving DE in Cu NCs remains elusive, primarily due to the complexities of excited state processes. Herein, a novel [Cu4(PPh3)4(C≡C−p−NH2C6H4)3]PF6 (Cu4) NC, shielded by alkynyl and exhibiting DE, was synthesized. Hydrostatic pressure was applied to Cu4, for the first time, to investigate the mechanism of DE. With increasing pressure, the higher‐energy emission peak of Cu4 gradually disappeared, leaving the lower‐energy emission peak as the dominant emission. Additionally, the Cu4 crystal exhibited notable piezochromism transitioning from cyan to orange. Angle‐dispersive synchrotron X‐ray diffraction results revealed that the reduced inter‐cluster distances under pressure brought the peripheral ligands closer, leading to the formation of new C−H⋅⋅⋅N and N−H⋅⋅⋅N hydrogen bonds in Cu4. It is proposed that these strengthened hydrogen bond interactions limit the ligands′ vibration, resulting in the vanishing of the higher‐energy peak. In situ high‐pressure Raman and vibrationally resolved emission spectra demonstrated that the benzene ring C=C stretching vibration is the structural source of the DE in Cu4. [ABSTRACT FROM AUTHOR]

Two‐dimensional (2D) lead halide perovskites (LHPs) have shown great promises for light‐emitting applications and excitonic devices. Fulfilling these promises demands an in‐depth understanding on the relationships between the structural dynamics and exciton‐phonon interactions that govern the optical properties. Here, we unveil the structural dynamics of 2D lead iodide perovskites with different spacer cations. Loose packing of an undersized spacer cation leads to out‐of‐plane octahedral tilting, whereas compact packing of an oversized spacer cation stretches Pb−I bond length, resulting in Pb2+ off‐center displacement driven by stereochemical expression of the Pb2+ 6s2 lone pair electrons. Density functional theory calculations indicate that the Pb2+ cation is off‐center displaced mainly along the direction where the octahedra are stretched the most by the spacer cation. We find dynamic structural distortions associated with either octahedral tilting or Pb2+ off‐centering lead to a broad Raman central peak background and phonon softening, which increase the non‐radiative recombination loss via exciton‐phonon interactions and quench the photoluminescence intensity. The correlations between the structural, phonon, and optical properties are further confirmed by the pressure tuning of the 2D LHPs. Our results demonstrate that minimizing the dynamic structural distortions via a judicious selection of the spacer cations is essential to realize high luminescence properties in 2D LHPs. [ABSTRACT FROM AUTHOR]