Light-induced thermal hysteresis and high-spin low-spin domain formation evidenced by optical microscopy in a spin-crossover single crystal

The low-temperature photoinduced effects of the spin-crossover [{Fe(2-pytrz)(2)[Pd(CN)(4)]}]center dot 3H(2)O single crystal have been investigated by means of a cryogenic optical microscopy technique down to 10 K from which the imaging and quantitative analysis of the spatiotemporal transformation are derived. The magnetic investigations revealed that this compound exhibits an incomplete spin transition between a full high-spin (HS) state at high temperature and an intermediate HS and low-spin (LS) state, where HS and LS species coexist, as a result of the existence of an elastic frustration at the molecular scale, most likely caused by the rigidity of the interconnected [Pd(CN)(4)] [Fe(II)/Pd(II)] two-dimensional network. At low temperature (10 K), thanks to reverse light-induced excited spin-state trapping effect, we could switch the system from the intermediate HS-LS state to the fully photoinduced LS state by irradiating the sample in near-infrared region, revealed by photomagnetic and optical microscopy studies. Optical microscopy images showed monotonous and homogeneous transformation of the crystal color along this process, corresponding to a gradual change of the spin state under light. In contrast, the thermal relaxation in the dark of this photoinduced LS-LS state shows a transition to the intermediate HS-LS state at similar to 90 K with domain formation, which is characteristic of a first-order transition at equilibrium. Interestingly, the same behavior is also obtained during the heating process of the reverse light-induced thermal hysteresis cycle with a heating branch almost unchanged, confirming that light does not act on this transition. It is then concluded that the transition from the full LS to the intermediate HS-LS transition is of first order and therefore the LS state reached by light is a hidden stable state of the system until similar to 90 K. These experimental results are modeled using an adapted version of the electroelastic model including photoexcitation effects and solved by Monte Carlo method. Both thermal equilibrium and light-induced thermal hysteresis are reproduced, in fair qualitative agreement with the experimental data of photomagnetism and optical microscopy.