Solid-state phosphorescence colour changes in a tetranuclear Cu(I)-pyrazolate cluster achieved by the conformational flexibility of peripheral alkyl chains.

Polymorph-dependent luminescence is usually governed by intermolecular packing in the crystal, often causing quenching and limited colour control. Here we show that a tetranuclear Cu(I)-pyrazolate cluster achieves solid-state phosphorescence colour switching through intramolecular Cu4 core distortion induced by conformationally flexible peripheral alkyl chains.