Anomalous light induced spin state switching for Fe$^\mathrm{II}$ spin-crossover molecules in direct contact with metal surfaces.

The light-induced spin state switching is one of the most attractive properties of the spin-crossover materials. In bulk, low spin (LS) to high spin (HS) conversion via the light-induced excited spin state trapping (LIESST) effect may be achieved with a visible light while the HS-to-LS one (reverse-LIESST) requires an excitation in the near infrared range. Here, we show that those phenomena are strongly modified at the interface with a metal. Indeed, we report an anomalous spin conversion from HS state to LS state under blue light illumination for Fe$^\mathrm{II}$ spin-crossover molecules that are in direct contact with metallic (111) single crystal surfaces (copper, silver and gold). Moreover, we observe a usual LS to HS switching with soft X-rays, allowing some reversible switching between HS and LS states by the subsequent use of blue light and X-rays. To interpret this anomalous spin state switching and the different yields measured on the three samples, we propose a new mechanism for the spin transition, based on the light absorption by the substrate, that would generate low energy valence photo-electrons promoting molecular vibrational excitations and subsequent spin state switching at the molecule-metal interface.