Covalency control of photomagnetic relaxation in a manganese(II) photoswitch

Switching the electronic spin state in molecules by light is achieved via chemical bond reshuffling or light-induced excited spin state trapping. These procedures either require large-amplitude nuclear motions or cryogenic temperatures limiting their application in data storage or quantum computing devices. Here we show that a carbene manganese(II) complex undergoes light-induced excited spin state trapping at 141 K in solution and 112.5 K in the solid state. Irradiation quantitatively switches from low- to high-spin in solution and in the solid. The lifetime of the metastable high-spin state extrapolated from experimental data amounts to over 1 million years at 77 K. Key to success for the very long lifetime is the high reorganization energy due to high metal-ligand bond covalency, and hence, a large barrier height and width for the spin relaxation, paving the way for photo-switchable magnetic materials operating at ambient temperature.