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

Abstract

The photoinduced switching and subsequent relaxation regime at cryogenic temperatures of the two-step spin-crossover compound [Fe(bapbpy)(NCS) 2 ] has been investigated by time-dependent photocrystallography. Upon photoexcitation from the low-spin (LS) state, a direct population of the metastable high-spin (HS) state occurs, without involving any intermediate structural state. The relaxation from the metastable HS state in isothermal conditions at 40 K proceeds in two successive steps associated with two symmetry breaking processes. The first step corresponds to the cooperative transformation to an intermediate superstructure, characterized by a long-range-ordered [HS-LS-LS] motif coupled to a commensurate displacive modulation, and concomitant with a tripling of the c axis of the unit cell (C2/c space group). The stabilization of the intermediate state is driven by strong molecule-lattice coupling. In the second stage, the intermediate state undergoes a transformation twinning triggered by lattice strain towards the LS state. The two-step relaxation is reminiscent of the two-step thermal transition of [Fe(bapbpy)(NCS) 2 ] and evidences multimetastability in the light-induced or relaxation regime. The long-range-ordered [HS-LS-LS] superstructure has also been trapped by rapid quench cooling to very low temperature, and has been structurally characterized.

Effect of Metal Dilution on the Thermal Spin Transition of [FexZn1–x(bapbpy)(NCS)2]

“…In fact, these authors support the idea that when the temperature limit for information storage after light irradiation, known as T LIESST , [15,28] is higher than T c , then the relaxation of the HS state becomes slow at temperatures lower than T c , which leads to high residual HS fraction upon cooling. [33,34] In the case of the present study, for all diluted samples [Fe x Zn 1-x (bapbpy)(NCS) 2 ] the transition temperatures T c remain higher than 150 K. By comparison, the T LIESST of compound 1 is 56 K, [9,35] and on the basis of previous work [28] the T LIESST of the metal-diluted samples is expected to be close to that recorded for 1. Thus, all transition temperatures T c of the zinc-diluted samples (a) to (i) are significantly higher than their expected T LIESST .…”

Multiscale Experimental and Theoretical Investigations of Spin Crossover FeII Complexes: Examples of [Fe(phen)2(NCS)2] and [Fe(PM-BiA)2(NCS)2]

“…Through these decades, the structural approach has allowed a more deep understanding of the SCO phenomenon in the crystalline state but also reveals the richness of the associated SCO mechanisms. Many published reviews are focusing on the latter, including structure-properties relationships [39,40], pressure effects [41,42], structural and phase transitions interplay [43], time-dependence mechanisms [44] or non-periodic situations [45,46] for instance. The important point to highlight here is that this experimental approach gives rise to a multi-scale description of the SCO phenomenon, from the atomic to the macroscopic physical-scales (Figure 11).…”

Competing Phases Involving Spin-State and Ligand Structural Orderings in a Multistable Two-Dimensional Spin Crossover Coordination Polymer

“…However, even if there is a unique SCO center in the crystal, inequivalence between SCO centers may spontaneously originate as a consequence of crystallographic symmetry breaking. Although, crystallographic symmetry breaking giving intermediate steps was previously observed for the mononuclear SCO complexes [Fe 27-29 So far, a relatively small number of complete two-step or incomplete (only one step of two expected) SCO events with symmetry breaking have been observed in mononuclear, [30][31][32][33][34][35][36][37][38][39] polynuclear, [40][41][42][43] and polymeric [44][45][46][47][48][49][50][51][52][53] Fe II SCO complexes, and reviewed recently. 54,55 This phenomenology has also been described for a reduced number of Fe III , [56][57][58][59][60] Co II 61 and Mn III 62 complexes.…”