Structural Investigation of the HS to LS Relaxation Dynamics in Spin Crossover compounds

Spin-crossover compounds are very topical, because of their possible applications in electronic and optical devices [1, 2]. Typically, d4-d7 transition metal ions can be converted from the low-spin (LS) to the high-spin (HS)... [ view full abstract ]

Spin-crossover compounds are very topical, because of their possible applications in electronic and optical devices [1, 2]. Typically, d4-d7 transition metal ions can be converted from the low-spin (LS) to the high-spin (HS) state and vice versa with different external stimuli such as temperature, pressure, light or guest adsorption. At cryogenic temperatures, a photo-induced conversion from the LS stable state to the HS metastable state can be induced through the Light-Induced Excited Spin State Trapping (LIESST) effect [3]. Spin transitions are associated with large structural changes [1]. The metal-ligand bond lengths difference in iron(II) complexes of ~0.2 Å creates elastic interactions between the centres, resulting in cooperative effects that influence the thermal and light-induced spin crossover [2]. Therefore, structural analysis is very important for studying the thermal- and photo-induced spin-crossover phenomena. The investigation of the LS to HS photo-excitation and subsequent relaxation dynamics in the solid state is of particular importance for understanding the underlying physics associated with cooperative effects.
In our studies Synchrotron X-Ray powder diffraction has been used to follow the structural changes of different compounds after photo-induced LS-to-HS conversion based on Light-Induced Excited Spin State Trapping (LIESST) at 10 K. In the case of the Hofmann clathrate [Fe(pz)Pt(CN)4] a complete study on the relaxation behaviour has been performed for six different particle sizes and a strong dependence of the relaxation mechanism with the particle size has been found [4]. In the case of the [Fe(n-Bu-im)3(tren)](PF6)2, for which two different spin crossover behaviours have been observed in the thermal transition depending on the sweeping rate of the temperature [5], spectroscopic studies show a plateau during the relaxation after LIESST when approximately half of the centres have relaxed back to the LS state indicating a specific structural feature at this composition. Synchrotron X-Ray powder diffraction reveals the occurrence of an order/disorder transition. In addition two different HS phases have been obtained by photoexcitation of the LS state at 10 and 80 K giving rise to different relaxation time scales.