Self-Decelerating Relaxation of the Light-Induced Spin States in Molecular Magnets Cu(hfac)2LR Studied by Electron Paramagnetic Resonance

Molecular magnets Cu(hfac)(2)L-R (hfac = hexa-fluoroacetylacetonate) called breathing crystals exhibit thermally and light-induced magnetic anomalies very similar to iron(II) spin-crossover compounds. They are physically different systems, because the spin-state switching occurs in exchange-coupled nitroxide-copper(II)-nitroxide clusters, in contrast to classical spin crossover in d(4)-d(7) transition ions. Despite this difference, numerous similarities in physical behavior of these two types of compounds have been observed, including light-induced excited spin-state trapping (LIESST) phenomenon recently found in the Cu(hfac)(2)L-R family. Similar to iron(II) spin-crossover compounds, the excited spin state in breathing crystals relaxes to the ground state on the time scale of hours at cryogenic temperatures. In this work, we investigate this slow relaxation in a series of breathing crystals using electron paramagnetic resonance (EPR). Three selected compounds represent the cases of relatively strong or weak cooperativity and different temperature of thermal spin transition. They all were studied in a neat magnetically concentrated form; however, sigmoidal self-accelerating relaxation was not observed. On the contrary, the relaxation shows pronounced self-decelerating character for all studied compounds. Relaxation curves and their temperature dependence could be fitted assuming a tunneling process and broad distribution of effective activation energies in these ID materials. A number of additional experimental and theoretical arguments support the distribution-based model. Because self-decelerating relaxation behavior was also found in ID polymeric iron(II) spin-crossover compounds previously, we compared general relaxation trends and mechanisms in these two types of systems. Both similarities and differences of copper nitroxide-based breathing crystals as compared to iron(II) spin-crossover compounds make future research of light-induced phenomena in these new types of spin-crossover-like systems topical in the field of molecule-based magnetic switches.