Light-induced spin-state switching in heterospin complexes of Cu(hfac)₂ with pyridine-based nitroxides

Copper(II)-nitroxide based molecular magnets exhibit spin-crossover-like spin state switching, which is topical in field of molecular magnetism. However, establishing reliable structure-property relationships in these systems is still challenging, especially regarding the light-induced switching of spin states. In this paper, we report the investigation of photoswitching and relaxation in a series of heterospin Cu(hfac)₂L^R complexes with pyridine-based nitroxide ligands (L^R), which belong to this family of materials. Using electron paramagnetic resonance (EPR) for detection, we demonstrate very long lifetimes of photoinduced spin states at liquid helium temperatures (<15 K), where relaxation to the ground state does not exceed 15% within two hours. At the same time, the efficiency of photoswitching strongly depends on the structure of the radical ligand in this series: the bulkier the ligand, the smaller the fraction of heterospin clusters that undergo photoswitching. These findings expand the understanding of mechanisms and factors behind photoswitching and relaxation in copper(II)-nitroxide molecular magnets and aid in further research aiming to optimize their functional properties.