Soft X-ray absorption spectroscopy study of spin crossover Fe-compounds: Persistent high spin configurations under soft X-ray irradiation

Abstract

Metal-organic complex exhibiting spin crossover (SCO) behavior has drawn attention for its functionality as a nanoscale spin switch. The spin states in the metal ions can be tuned by external stimuli such as temperature or light. This article demonstrates a soft X-ray?induced excited spin state trapping (SOXEISST) effect in Hofmann-like SCO coordination polymers of FeII (4-methylpyrimidine)2 [Au(CN)2 ]2 and FeII (pyridine)2 [Ni(CN)4 ]. A soft X-ray absorption spectroscopy (XAS) study on these polymers showed that the high spin configuration (HS; S = 2) was prevalent in Fe2+ ions during the measurement even at temperatures much lower than the critical temperatures (>170 K), manifesting HS trapping due to the X-ray irradiation. This is in strong contrast to the normal SCO behavior observed in FeII (1,10-phenanthroline)2 (NCS)2, implying that the structure of the ligand chains in the polymers with relatively loose Fe-N coordination might allow a structural adaptation to stabilize the metastable HS state under the soft X-ray irradiation. (C) 2018 by the authors. Licensee MDPI, Basel, Switzerland