Dynamic mixing processes in spin triads of “breathing crystals” Cu(hfac)2LR: a multifrequency EPR study at 34, 122 and 244 GHz

Spin triads of copper(II) with two nitroxides are responsible for the magnetic anomalies in a new family of molecular-magnetic compounds called “breathing crystals”. We have shown previously that electron paramagnetic resonance (EPR) spectroscopy allows one to investigate the peculiarities of these systems and obtain valuable information on exchange interactions governing the magnetic anomalies. One of the key processes revealed is the dynamic mixing between different spin multiplets that leads to a coalescence of individual EPR lines at high temperatures. The rates of the mixing were found to be fast at EPR frequencies between 9 and 94 GHz. In the present work, we expose the spin triads to higher microwave frequencies of up to 244 GHz in order to reach the conditions of intermediate or slow mixing rates. Three representatives of the family of breathing crystals have been studied. Based on the simulations of EPR data at 34, 122 and 244 GHz, the rates of the mixing processes have been estimated and conclusions on their character and temperature dependence have been drawn. The insights from high-field EPR clarify previously obtained results and aid in the further development of EPR approaches for studying these and similar systems. It is suggested that the static and dynamic Jahn–Teller effects may play an important role in the mechanisms governing the observed spin exchange effects.