Hidden hysteretic behavior of a paramagnetic iron(II) network revealed by light irradiation

Abstract : The paramagnetic coordination polymer [Fe(3phOH-trz){Pt(CN)4}].2H2O was obtained by reaction of iron(II) salt with the 4-(3-hydroxyphenyl)-1,2,4-triazole (3phOH-trz) ligand and a [Pt(CN)4]2- salt. Its structure consists in two dimensional {FePt(CN)4} layers linked by π-stacking interactions and a strong H-bonds network between water molecules and the hydroxyl group from the ligand. From the paramagnetic high spin (HS) state at 10 K, irradiation at 830 nm led to the fully low spin (LS) state, according to the reverse-LIESST process. Upon warming, this LS state undergoes a thermally induced spin transition to high spin around 105 K. This photoswitching process is reversible and the paramagnetic state can be recovered by a 510 nm light irradiation below 105 K. Moreover, a permanent irradiation (830 nm) revealed a hidden hysteresis loop of 37 K width. Photocrystallographic experiments did not evidence any structural phase transition upon excitation but underlined that the elastic frustration might be responsible for the inhibition of the spin crossover in this compound and allowed the observation of the hidden hysteresis loop.