Vibration-Dependent Dual-Phosphorescent Cu₄ Nanocluster with Remarkable Piezochromic Behavior

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.