Electron Paramagnetic Resonance of Three-Spin Nitroxide−Copper(II)−Nitroxide Clusters Coupled by a Strong Exchange Interaction

The complex of Cu2+ hexafluoroacetylacetonate with two pyrazol-substituted nitronyl nitroxides represents an unusual exchange-coupled three-spin system. The antiferromagnetic exchange coupling, which already at T < 150 K is larger than the thermal energy kT, induces the transition from a total spin state S = 3/2 to a state S = 1/2 and produces static spin polarization. Anomalous electron paramagnetic resonance (EPR) spectra of an S = 1/2 state were detected experimentally and described theoretically. The effective g factor of the three-spin system is smaller than 2, despite the fact that all the individual components have g > 2. The observed signals with g < 2 are highly informative and can be employed for determination of the sign and value of the exchange interaction in three-spin nitroxide−copper−nitroxide clusters.