The possibility of controlling the properties of individual molecules using external fields has exciting implications for electronics and data storage. Such systems could allow much higher densities of memory elements to be stored in a memory device, and could introduce previously unachievable functionalities related to the quantum nature of molecules.

Tunable molecular magnets are one example of such externally controllable molecular systems, having magnetic properties that can be modified through external stimuli such as light, heat or pressure. These types of molecules have potential applications in nanoscale sensors and switches. In some cases, it is also possible to vary the spin topology by structural transformation. Now, Osamu Sato and colleagues1 at Kyushu University in Japan have demonstrated a solid-state transformation between molecules with different magnetic properties.

Fig. 1: Solid-state transformation from achiral, antiferromagnetic molecules (top) to chiral, ferrimagnetic molecules (bottom). The dark red arrows represent the spin orientation of iron ions.

The researchers used a starting structure consisting of a chain of hexanuclear clusters with antiferromagnetic spin topology and no chirality (Fig. 1, top). Heating the crystal at 150 °C in a nitrogen atmosphere resulted in the breakage and reformation of coordination bonds, affording a one-dimensional polymer chain with chirality and a dominantly ferromagnetic character (Fig. 1, bottom).

As expected, the magnetic properties of the two molecules were different, as determined from measurements of magnetic susceptibility and magnetization over a wide temperature range. The transformation was also fully reversible: the original structure and magnetic properties were recovered by exposing the molecules to a mixture of water and acetonitrile vapors for one week.

“The reversible polymerization using coordination bond formation and breakage in solid states is a new technique to control structure and magnetic properties,” says Sato. Furthermore, this reversible mechanism could have direct practical applications in advanced switching devices. “Chiral magnets show magneto-chiral dichroism, hence, optical properties, that is, magneto-chiral dichroism, can be switched by chemical stimuli, which has never been reported.”