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

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 40K 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. © 2012 American Physical Society.