An Enantiopair of Organic Ferromagnet Crystals Based on Helical Molecular Packing of Achiral Organic Radicals

Abstract

We report the ferromagnetic ordering phenomena occurring in organic molecular crystals with structural chirality. Achiral radical <b>1</b> has been found to crystallize in two enantiomorphs with chiral space groups of <i>P</i>4₃ and <i>P</i>4₁. The <i>P</i>4₃ form (<b>1L</b>) has left-handed stacking of the molecules, giving the helical chirality in a crystalline solid. In the other form of <i>P</i>4₁ (<b>1R</b>), the right-handed stacking corresponds to a mirror image of <b>1L</b>. Magnetic susceptibility measurements show that both the crystals undergo a ferromagnetic phase transition at <i>T</i>_C = 1.1 K. The ferromagnetic ordering has been confirmed by heat capacity measurements. The magnetic heat capacity exhibits a λ-shaped peak at <i>T</i>_C = 1.1 K with an entropy change of <i>R </i>ln 2, as expected for <i>S</i> = 1/2 spins. This is the first example of genuinely organic molecule-based ferromagnetism associated with the structural chirality based on the helical molecular packing in the crystalline solid