Rational design of a magnetic material (nickel nitroprusside) at the molecular level allowed switching of a spin coupling by means of photoinduced metal to ligand charge transfer (MLCT). Before irradiation, spins on Ni2+ are in a disordered state. An illumination at 375 nm induces a MLCT from Fe to NO and forms new spins on Fe and NO. The newly appearing spin on Fe orders the spins on the nearest Ni2+ ions around Fe. In other words, one MLCT aligns five spin sources resulting in the formation of magnetic clusters with S = 5. Furthermore, such a photoinduced spin ordering can be rerandomized by thermal treatment.
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