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

Sébastien Pillet (1) , El-Eulmi Bendeif (1) , Sylvestre Bonnet (2) , Helena Shepherd (3) , Philippe Guionneau (4)

1 CRM2 - Cristallographie, Résonance Magnétique et Modélisations
2 Leiden Institute of Chemistry
3 Department of Chemistry
4 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux

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.

Mots clés: Photoinduced switching; Relaxation; Cryogenic temperatures; Spin-crossover compound

Domaines: Matériaux; Chimie de coordination

Sébastien Pillet, El-Eulmi Bendeif, Sylvestre Bonnet, Helena Shepherd, Philippe Guionneau. Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeII spin-crossover compound. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2012, 86 (6), pp.064106. ⟨10.1103/PhysRevB.86.064106⟩. ⟨hal-00731243⟩