Moving organic molecules in crystalline solids: Gradual structural transition and spin transition/crossover

Solid-state magnetic switches are of increasing interest for future application to sensing, memory, display, etc. The authors and co-workers have developed unconventional spin transition/ crossover materials, where the sense of organic chemistry has competently been adopted. The first example is a supramolecular chemistry of genuine organic nitroxide biradicals with a triplet ground state, which undergo stepwise polymerization/depolymerization in a crystal. The second example belongs to 2p-3d heterospin systems where the nitroxide-nickel(II) or -copper (II) bond rotates and switches exchange coupling. The third example is an iron (II) spin crossover material carrying a stearyl group and displaying an order-disorder-type structural transition. As these examples show, single-crystal-to-single-crystal structural transitions are often recorded, thanks to small atomic dislocation. Molecular motion in crystalline solids is observed more frequently than expected. Attentive crystallographic study affords sure proof for the spin transition phenomenon.