Moving Molecules in Crystalline Solids: Gradual Structure Transition and Spin Transition/Crossover

Abstract

Magnetic switches working in solid states are of recent interest for applications to sensor, memory, etc. The author and co-workers have developed various unconventional spin transition/crossover materials. The first example is a supramolecular chemistry consisting of genuine organic nitroxide biradicals with a triplet ground state. Each nitroxide group is located close to each other to form weak covalent bonds in an intermolecular fashion. Biradicals are polymerized/depolymerized stepwise. The second example belongs to 3d-2p heterospin systems where the nitroxide oxygen atom is directly bonded to a nickel(II) or copper(II) ion. The planar/nonplanar chelate structure changes, accompanied by high-/low-spin transition due to a 3d-2p exchange-coupling switch. A novel entropy-driven spin crossover scenario has been established. The third example is a 3d iron(II) spin crossover material carrying a stearyl (C18) group. An order-disorder-type structural transition appears with respect to the alkyl conformation. As these examples show, single-crystal-to-single-crystal structural transitions are often observed. It is because the spin entropy term regulates the atomic dislocation enthalpy, and the entropy change due to the spin multiplicity is basically small. These molecular motions clarified by means of detailed crystallography afford one of the most convincing evidence for the spin transition phenomenon.