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

Abstract

Copper(II)-nitroxide based molecular magnets exhibit spin-crossover-like spin state switching topical in 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 investigation of photoswitching and relaxation in a series of heterospin Cu(hfac)2LR complexes with pyridine-based nitroxide ligands (LR), 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 radical ligand in this series: the bulkier the ligand, the smaller fraction of heterospin clusters undergoes photoswitching. These findings expand 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.