Structural investigation of the photoinduced spin conversion in the dinuclear compound {[Fe(bt)(NCS)2]2(bpym)}:: toward controlled multi-stepped molecular switches

The photocrystallographic investigation of the light-induced excited spin-state trapping effect in the dinuclear spin-crossover compound {[Fe(bt)(NCS)(2)](2)(bpym)} is reported. In this system, each of the two Fe sites may be either in the high-spin (HS) or in the low-spin (LS) state, so that the molecule corresponds to a three-state system (LS-LS, HS-LS and HS-HS). At low temperature, the laser excitation wavelength controls the photoswitching from the stable LS-LS state to one of the metastable excited states (HS-LS or HS-HS), and also between these two excited states. Significant changes in the crystalline structure associated with the photoinduced change of spin state are detailed here. The low-temperature photoinduced states look similar to the corresponding states observed at thermal equilibrium within the unit-cell thermal contraction.