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 [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.

Slow Dynamics of the Spin‐Crossover Process in an Apparent High‐Spin Mononuclear FeII Complex

“…These studies have also been completed for the [Fe(etz) 6 ] 2+ complex (etz: ethyltetrazole), which shows a remarkable light‐induced bistability of the HS sites. More recently, comparable bistability has been reported in a two‐dimensional (2D) coordination polymer, {[Fe HS (bbtr) 3 ](BF 4 ) 2 } ∞ (bbtr: 1,4‐di(1,2,3‐triazol‐1‐yl)butane), as well as in 2D HS:LS Hofmann‐like frameworks and an Fe III SCO system …”

A critical review of the T(LIESST) temperature in spin crossover materials − What it is and what it is not

“…However, HS compounds, with HS ground states all along the temperature range from 0 to 400 K could also exhibit the reverse-LIESST process and a related hidden phase. This is probably the case of compounds reported by Chakraborty et al [51] or Ndiaye et al [52]. Making the distinction between these two categories of HS compounds requires knowledge of the ground state in the temperature range of the observation of the hidden phase through theoretical calculations for example.…”

“…Finally, photoswitching between long-lived states can be accessible as soon as the SCO can be inhibited. To do so, overlap between the T(LIESST) and T 1/2 can be useful but one needs also to work on elastic frustration [52,53].…”

The First Observation of Hidden Hysteresis in an Iron(III) Spin‐Crossover Complex

“…[23] Irradiation with near-IR light allowed access to the full LS state with the HS-LS state restored with green light, again showing bidirectional switching. Thea uthors were also able to show that warming,a nd subsequent cooling of the sample reveals ahidden hysteresis of 25 K. In af ollow-up paper, the related compound, [Fe(3PhOH-trz) 2 {Pt(CN) 4 }]•2 H 2 O{ 3PhOH-trz = 4-(3-hydroxy-phenyl)-1,2,4-triazole} was found to be HS even at low temperature but optical switching this time revealed ah idden hysteresis of 37 K. [24] Herein we report the first example of an iron(III) SCO complex to exhibit hidden hysteresis and show that this phenomenon is possible in molecular systems as well as in the coordination polymers previously reported.…”