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 P 4 3 and P 4 1 . The P 4 3 form ( 1L ) has left-handed stacking of the molecules, giving the helical chirality in a crystalline solid. In the other form of P 4 1 ( 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 T C = 1.1 K. The ferromagnetic ordering has been confirmed by heat capacity measurements. The magnetic heat capacity exhibits a λ-shaped peak at T C = 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.