We have developed a robust small cationic copper cluster featuring a tetranuclear [Cu]₄ unit encapsulated by three bulky carbanionic diphosphine ligands [Cu₄(PCP)₃]⁺. The cluster exhibits one of the shortest Cu-Cu distances reported until today (d_Cu-Cu = 2.316(1) Å, compared to 2.56 Å in metallic Cu).
It also shows intense green photo- and electro-luminescent emission under ambient conditions. With increasing pressure over a range of more than 10 GPa, the spectrum shifts significantly from green over yellow, orange, red to pink.

This work was carried out in the group of PD Dr. Simon Grabowsky in collaboration with the groups of Prof. Dr. Jens Beckmann (University of Bremen), Dr. Anna Makal (University of Warsaw) and Dr. Tomasz Poreba (ESRF - European Synchrotron Radiation Facility).