Reversible single crystal photochemistry and spin state switching in a metal-cyanide complex

Manipulating the physical properties of solid matter using only photons is a major challenge in materials science. However, achieving such control over a chemical reaction in the solid state is even more challenging. Here we demonstrate the reversible photochemistry occurring in a single crystal of a simple cyanide complex, K₄[Mo^III(CN)₇]·2H₂O. Upon exposure to visible light at different wavelengths, a reversible breaking and reformation of dative bonds is triggered, resulting in a photoswitching of the Mo^III coordination geometry between 6- and 7-coordinate. This transformation, in turn, induces a spin state change. The observed solid-state photochemical reactivity is robust, quantitative and occurs at a record-high temperature. It paves the way for the development of new photo-switchable high-temperature magnets and nanomagnets.