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The complexes of Cu2hexafluoroacetylacetonate with two pyrazol-substituted nitronyl nitroxides are the choice systems to study the spin dynamics of strongly exchange-coupled spin triads. The large values of exchange coupling (ca. 100 cm-1) and high-resolution electron paramagnetic resonance (EPR) at Q- and W-bands (35 and 94 GHz) allowed us to observe and interpret specific characteristics of these systems. An electron spin exchange process has been found between different multiplets of the spin triad, which manifests itself as a significant shift of the EPR line position with temperature. We propose that the spin exchange process is caused by the modulation of exchange interaction between copper and nitroxides by lattice vibrations. The estimations of the rate of exchange process and model calculations essentially support the observed phenomena. The studied characteristics of strongly coupled spin triads explain previously obtained results, agree with literature, and should be accounted for in future investigations of similar spin systems. [ABSTRACT FROM AUTHOR]

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 ju, 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. [ABSTRACT FROM AUTHOR]