Light-induced excited spin state trapping effect on [Fe(mepy)3tren](PF6)2 solvated crystals.

A spin-crossover solvated compound [Fe(mepy)3tren](PF6)2·C7H8·C2H3N has been prepared and its switching properties have been compared to those reported for the non-solvated solid. The thermal spin transition occurs at 88 K with the opening of a 3.5 K wide hysteresis loop, while a fairly steep transition at 215 K without hysteresis has been previously reported for the non-solvated analogue. This feature has been rationalized by the analysis of the high-spin (HS) and low-spin (LS) structures, evidencing a relative stabilization of the high-spin state, as well as strong intermolecular interactions in the solvated compound. The photoswitching of the solvated solid, based on the light-induced excited spin state trapping effect, leads to a quantitative transformation from the low-spin to the high-spin state at 10 K. The long lifetime of the metastable HS state at 10 K allows the measurement of the photo-induced HS structure, where the cooperative interactions are enhanced, compared to those of the thermally populated HS structure. Then,the HS-to-LS relaxations have been studied between 45 and 60 K. They are sigmoidal in shape and can be well fitted in the frame of the mean-field approximation. The relative stability of the photo-induced HS state in this family of spin crossover compounds is not directly related to their thermal spin transition temperature. This unexpected observation is rationalized by a careful analysis of their structural characteristics.