Structural Investigation of the Photoinduced Spin Transition in the [Fe(PM-BiA)₂(NCS)₂] Compound

Abstract

We investigated the thermo- and photo-induced phase transitions between low spin (LS) and high spin (HS) states of the molecular crystal of [Fe(PM-BiA)2(NCS)2] in the orthorhombic form, by using X-ray diffraction. The structure of the photoinduced HS state, generated from the LS state at low temperature, is compared to the structures of the HS and LS phases at thermal equilibrium and to the thermally trapped HS state. The preliminary results presented here show that the structural reorganization is similar in the different HS states.

Molecular dynamics of spin crossover: The (P,T) phase diagram of [Fe(PM-BIA)2(NCS)2]

“…with a S-shape, more pronounced when Ω ≥ 2RT 1/2 . 6 At the same time, cell parameter variations versus temperature may be directly compared to experiments [3]. Isotherm and isobar MD runs are also performed on the LS state (I-phase) in order to study its instability, the resulting I  II transition in both spin states and to assess the experimental (P,T) phase diagram [13,24].…”

“…One of theses complexes, [Fe(PM-BIA) 2 (NCS) 2 ] 1 , is characterized by iron and surrounded by a distorded octahedron involving two N-(2'-pyridylmethylene) (PM) and two 4-aminophenyl (BIA) ligands and completed by two thiocyanate anions. It is known to undergo a gradual SCO at T 1/2 = 190 K in the monoclinic polymorph (II-phase, Z = 4, space group P2 1 /c) [1][2][3][4][5][6][7][8]. A very abrupt transition is observed for the orthorhombic polymorph (Iphase, Z = 4, space group Pccn) with T 1/2 = 173 K and T 1/2  = 168 K between the low spin (LS, S = 0) to the high spin state (HS, S = 2) of the Fe 2+ ion [4].…”

“…A very abrupt transition is observed for the orthorhombic polymorph (Iphase, Z = 4, space group Pccn) with T 1/2 = 173 K and T 1/2  = 168 K between the low spin (LS, S = 0) to the high spin state (HS, S = 2) of the Fe 2+ ion [4]. Among its interesting magneto-optical properties, this SCO may be light-induced, the limit temperature above which a photomagnetic effect in a material is erased -the so-called T LIESST (for « Light-Induced Excited Spin-State Trapping ») depending strongly of on the structural distortion of the 3d 6 Fe 2+ environment [9] and/or the nature of the ligand [10]. As often [8,11,12], pressure plays a role in the opposite sense of temperature in as far the starting spin state is the high spin: by increasing pressure, both I and II polymorphs of FePMBIA yield a HS  LS transition [13].…”

“…-Mulliken charges of atoms in the Fe-(N)6 octahedron (in e).Table II.-Parameters for Morse potentials describing the 2-body Fe-N i interactions and harmonic potentials of 3-body Fe-N i -C interactions deduced from the DFT potential energy surface. Table III.-Octahedral geometry of the Fe 2+ environment: comparison between experiments and molecular dynamics.Table IV.-Fe-N and NCS group frequencies (in cm -1 ): comparison between experiments of Hoefer [30], molecular dynamics and DFT.…”