Novel magnetic interactions in organic polyradical crystals based on nitronyl nitroxide

Abstract

Magnetic properties of a stable organic biradical, meta-phenylene bis(α-nitronyl nitroxide) (abbreviated as m-BNN) were investigated. The molecular ground state of m-BNN is found to be triplet ( S=1) from the solution EPR measurements. The temperature dependence of the product of paramagnetic susceptibility and temperature, ξ p T, of the single crystal of m-BNN exhibits anomalous stationary behavior with decreasing temperature as a result of intermolecular exchange interactions. From the analysis based on the structure of magnetic energy spectrum in a solid state, the formation of unusual magnetic energy gaps in the crystal is deduced. An organic triradical, the 1,3,5-tri-substituted form (TNN), was also studied. By solution EPR measurements, this molecule is shown to have the quartet ( S= 3 2 ground state. A polycrystalline sample of the triradical exhibits a broad susceptibility maximum around 16 K. The value of the susceptibility extrapolated to 0 K is nonzero. This indicates the gapless nature of the triradical solid, in contrast to the gap formation in the biradical.