Molecular scale dynamics of light-induced spin crossover in a two-dimensional layer*

Spin crossover molecules show the unique ability to switch between two spin-states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of molecular devices in electronics and spintronics. If the behavior of such molecules is well documented in bulk, little is known on their properties at the molecular scale and in direct contact with an electrode. We focus our study on Fe^II pyrazolyl borate molecules adsorbed on an Au(111) substrate. The combination of scanning tunneling microscopy measurements and ab initio calculations allows us discriminating between both states by local vibrational spectroscopy. We thus evidence the formation of a long range ordered mixed spin-state phase and confirm their magnetic properties by x-ray absorption spectroscopy. We also show that a single layer of spin-crossover molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct time scales dynamics [1]. The thermal spin crossover is also evidence with the opening of a hysteresis. These results open a way to molecular scale control of two-dimensional spin crossover layers.

[1] K. Bairagi et al., Nat. Comm., 2016, 7, 12212.