Room-Temperature Magnetic Bistability in Organic Crystals: A Comparison with Spin Crossover Transitions

The way in which chirality is manifested in the solid state in a variety of phenyl -nitronyl nitroxides is discussed in the context of the preferred conformations of this family of compounds. The preparation of chiral phenyl -nitronyl nitroxides apparently guarantees the formation of just one conformational diastereomer in the crystals, even if prediction of which is presently unfeasible: of four chiral radicals with the same stereogenic center all four gross conformers are represented. Circular dichroism spectra give useful information regarding the interplay between molecular conformation and optical activity. A variety of both strong and weak hydrogen bonds are at play, leading to the formation of chains and sheets of molecules. The magnetic exchange interactions between the radicals is always weak and antiferromagnetic, and can be attributed to a small face-to-face overlap of the SOMOs.