Reversible Piezochromism of Platinum(II) and Palladium(II) Dimers in Molecular Single Crystals

Transition metal complexes are well-known for their efficient light emission and are promising for applications ranging from bioimaging to light-emitting diodes. In solution, interactions between the metal centers of two complexes become possible and drastically change the photophysical properties. For real-world devices, solid-state materials consisting of these molecules are preferable. Recently, the ligand-controlled aggregation of platinum(II) and palladium(II) complexes into molecular single crystals and the controlled formation of metal–metal contacts have been demonstrated. Here, we show how the metal–metal distance can be tuned in a controlled way by exerting pressure on the molecular crystal. Using optical spectroscopy inside a diamond anvil cell, we find strong and reversible piezochromism up to 18 GPa. Using time-dependent density functional theory, we attribute the wavelength shift to a reduction in the metal–metal distance and enhanced π orbital overlap in the dimers.