Spin crossover materials are switchable transition metal complexes where the high spin (HS) and low spin (LS) states can be reversibly interconverted by external stimuli (temperature, light, pressure, magnetic field, etc.).[1] In the solid state, these complexes may show thermal hysteresis due to the appearance of a crystallographic phase transition triggered by the spin transition as a result of cooperative phenomena. The possibility of realizing memory effect in a single spin crossover molecule was precluded by theory and all current experiments. Thus, although spin crossover materials have been proposed for information storage applications, the memory loss upon nanostructuration has limited their interest.[2]
This state-of-the-art has been finally broken by our work in an iron polyanionic complex that exhibits thermal hysteresis at the single molecule level, upon dilution in the solid state, and even in diluted liquid solution.[3] The origin of this unexpected feature can be assigned to the constrained molecular structure of our polyanionic complex (Figure 1). The charge distribution and intramolecular anion-hydrogen bonding induce an over stabilization of the excited HS state, as confirmed by computational analysis. As a result, the very slow relaxation down to the LS ground state opens a thermal hysteresis, even in diluted liquid solution, demonstrating that spin crossover molecules may also exhibit bistability.
Our results open interesting possibilities to write and read at the single molecule limit by a variety of techniques: thermal, magnetic, spectroscopic, or even mechanical. Furthermore, this occurs around room temperature, when all previous magnetic single-molecule memories required very low temperatures (below liquid nitrogen) to exhibit bistability.
