Hidden Hysteretic Behavior of a Paramagnetic Iron(II) Network Revealed by Light Irradiation

The paramagnetic coordination polymer [Fe(3phOH‐trz){Pt(CN) 4 }] · 2H 2 O [3phOH‐trz = 4‐(3‐hydroxyphenyl)‐1,2,4‐triazole] was obtained through the reaction of an iron(II) salt with the 3phOH‐trz ligand and a [Pt(CN) 4 ] 2– anion. Its structure consists of two‐dimensional {FePt(CN) 4 } layers linked by π‐stacking interactions and a strong H‐bonding network between water molecules and the hydroxy groups of the organic neutral ligand. From the paramagnetic high‐spin (HS) state at 10 K, irradiation at λ = 830 nm led to the fully low‐spin (LS) state, as determined through the reverse light‐induced excited spin‐state trapping (LIESST) process. As the compound is warmed up, this LS state undergoes a thermally induced spin transition to high spin at ca. 105 K. This photoswitching process is reversible, and the paramagnetic state can be recovered through light irradiation at λ = 510 nm below 105 K. Moreover, permanent irradiation ( λ = 830 nm) revealed a hidden hysteresis loop with a width of 37 K. Photocrystallographic experiments did not detect any structural phase transition upon excitation but underlined that elastic frustration might be responsible for the inhibition of the spin crossover in this compound and allowed the observation of the hidden hysteresis loop.