On Thermo- and Photo-Induced Symmetry-Broken Transformation in Spin–Crossover Complex: Cooperative Activation

Abstract

We study energetics of a cooperative low-spin (L) to high-spin (H) transformation in a crystal with two sublattices, where thermo-and photo-induced symmetry-broken phases have been recently observed. The model takes into account an intra-dimer antiferromatic coupling and inter-dimer couplings. Cooperative activation processes for switching between LH and HL states and that from LL to HH states of a dimer are analyzed. It is shown that locally (within a dimer) preferred symmetry-broken, LH and HL, states compete with inter-dimer couplings, which stimulates dynamical disorder via a decrease in the energy barrier for a flip between asymmetric configurations. On the other hand, a locally non-preferred symmetric state (HH) is supported by inter-dimer interaction via a cooperative activation mechanism. We conclude that symmetry-broken states can form an intermediate thermo-induced phase or can be observed as a photo-induced one at low temperatures as a result of local equilibrium. The global equilibrium follows from cooperative interaction and governs the thermo-induced re-entrant phase transition.

Interplay of Spin Conversion and Structural Phase Transformations: Re‐Entrant Phase Transitions in the 2‐Propanol Solvate of Tris(2‐picolylamine)iron(II) Dichloride

“…[23,24] It was suggested that solvent disorder in the [Fe II (2-pic) 3 ]Cl 2 ethanol solvate modifies the elastic properties and therefore affects spin conversion. [25] We note that the role of the solvent molecules is more complex, at least in the series of [Fe II (2-pic) 3 ]Cl 2 solvates. The diffraction study of 1 shows that the OH···Cl and the NH···Cl hydrogen bonds may change by as much as 0.1 across the phase transitions.…”

Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

“…30,44,45 A theoretical description of two-step transitions may be provided using Ising-like models or thermodynamic models based on one-or two-sublattice schemes. Two sublattices coupled antiferroelastically with intrasublattice ferroelastic interactions [46][47][48][49] may stabilize an ordered intermediate state, corresponding to the plateau of the two-step thermal transition and characterized by a uniform order of HS or LS state within each sublattice and opposite spin state between sublattices. On the other hand, the one-sublattice approach establishes a competition between antiferroelastic short-range interactions, favoring HS-LS neighbors, and ferroelastic longrange interactions, 50,51 or a competition between first-and second-neighbor short-range antagonistic interactions.…”

Theory of Photoinduced Phase Transitions

“…The Hanamura-Nagaosa theory is extended to systems with internal degrees of freedom, e.g., spin-crossover complexes that are composed of dimers and show two-step transitions. 6) In general, stochastic dynamics can be treated in classical models with master equations [7][8][9][10] and Monte Carlo simulations. 7,[10][11][12] Fig.…”