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

We report the ferromagnetic ordering phenomena occurring in organic molecular crystals with structural chirality. Achiral radical 1 has been found to crystallize in two enantiomorphs with chiral space groups of P43 and P41. The P43 form (1L) has left-handed stacking of the molecules, giving the helical chirality in a crystalline solid. In the other form of P41 (1R), the right-handed stacking corresponds to a mirror image of 1L. Magnetic susceptibility measurements show that both the crystals undergo a ferromagnetic phase transition at TC = 1.1 K. The ferromagnetic ordering has been confirmed by heat capacity measurements. The magnetic heat capacity exhibits a λ-shaped peak at TC = 1.1 K with an entropy change of R ln 2, as expected for S = 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.