Substitution Effects on Exchange Anisotropy in Heavy Atom Radicals

In the field of organic materials, bisdithiazolyl (and related) radicals have become attractive systems to study since their magnetic properties can be carefully tuned by varying and substituting their chalcanogen and halogen content. In this work, we focus specifically on heavy halide substitution in two families of molecule: IBSSEt (1) and IBBO (2). Structural differences result in a ferromagnetic ground state in 1, while 2 orders as a spin canted antiferromagnet. Herein, we report ferromagnetic resonance (FMR) measurements on 1 which show that substitution of I for Cl has no effect on the exchange anisotropy (EA). However, antiferromagnetic resonance (AFMR) measurements show that substituting I for F has a dramatic effect on the EA. Our analysis demonstrates that the sensitivity to the halide’s larger spin-orbit coupling is dictated by the molecular orbitals that dominate the exchange, in which the symmetry of the SOMO permits no sizeable spin density at the halide’s position, while the opposite is true for the LUMO. As shown by previous DFT studies, we are able to directly show that the lower energy LUMO hybridizes with the SOMO in 2, resulting in an augmentation of the EA upon heavy atom substitution.