Spatiotemporal Investigations on Light-Driven High-Spin–Low-Spin Interface Dynamics in the Thermal Hysteresis Region of a Spin-Crossover Single Crystal

We have investigated by optical microscopy the thermal spin transition of a single crystal of the spincrossover compound [{Fe(NCSe)(py)(2)}(2)(m-bpypz)] under various shining intensities, far from the light-induced spinstate trapping region. We found evidence of photoheating on the thermally induced hysteretic response of the crystal, leading to the control of the transition temperature and the hysteresis width as a function of the light intensity. The inspections of the spatiotemporal behaviors of the spin-crossover transition, on heating and cooling, have also evidenced a significant dependence of the propagation speed of the high-spin-low-spin interface on the intensity of light. In particular, for strong shining intensities, a slowing of the interface speed at the transition is obtained, and an unprecedented dynamical two-step-like transition was observed in the thermal hysteresis. These results are analyzed theoretically using a spatiotemporal approach based on reaction-diffusion equations including the spin-state propagation and the heat transfer between the crystal and the thermal bath. The obtained results are in good agreement with experimental observations and lead to identification of the key factors governing the interface velocity and the thermal hysteresis behaviors under the light excitation in spin-crossover materials.