Spin cross-over 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 genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers.

International audiencePhotoinduced phase transformations [1,2] occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently directing the functionalities [3,4,5,6,7]. The transient nature of photoinduced states has thus far severely limited the application scope. It is of paramount importance to explore whether structural feedback during the solid deformation has capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons long under scrutiny [8,9,10] , coherently propagating cell deformations over acoustic timescale [11,12,13,14] have not been explored to similar degree, particularly in light of cooperative elastic interactions. Herein we demonstrate experimentally and theoretically a self-amplified responsiveness in a spin-crossover material [15] during its delayed volume expansion. The cooperative response at material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically-driven cooperativity triggered by a light pulse offers a new efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials