Spin-dimer networks: Engineering tools to adjust the magnetic interactions in biradicals

Abstract

Magneto-structural correlations in stable organic biradicals have been studied on the example of weakly exchange coupled models with nitronyl nitroxide and imino nitroxide spin-carrying entities. Here, heteroatom substituted 2,2′-diaza- and 3,3′-diaza-tolane bridged biradicals were compared with the hydrocarbon analogue, while a biphenyl model with its 2,2′-bipyridine counterpart. For a 3,3′-diazatolane bridge the torsional angle between the nitronyl nitroxides and the pyridyl rings increased heavily (∼52-54°) leading to a smaller theoretical intra-dimer exchange coupling value. However, a very large antiferromagnetic coupling was obtained experimentally. This could be appropriately explained by the presence of dominating inter-dimer exchange between the molecules. For the bis(imino nitroxide) with tolane bridge a field induced ordered state between 1.8 to 4.3 T in AC-susceptibility measurements was observed. In terms of a Bose Einstein condensate (BEC) of triplons this phenomenon could be described as a magnetic field induced ordered phase with 3D character.