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Reversible Photoswitching of a Spin-Crossover Molecular Complex in the Solid State at Room Temperature

In recent years a considerable effort has been devoted toward the development of molecule-based devices; in this context Spin-Crossover (SCO) metallic complexes are showing great promises especially for building molecular magnetic switches. SCO metal complexes are an already well-known class of molecules with the possibility of switching their magnetic state and thus other related physical properties such as conductivity or colour by the application of an external perturbation (pressure, irradiation, temperature, etc.). In that framework, the ideal switching molecule for future applications should present two magnetic states easily and reversibly switchable at room temperature and ambient pressure; for this reason the discovery of a SCO complex which can be switched by light irradiation in solid state is a major step forward in the direction of building molecular devices.
The main interest of SCO molecules is their ability to switch the spin state of the metal complex; soft X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (XPS) are ideal tools for the observation of these phenomena. We performed XAS experiments at APE beamline at Elettra to probe in-situ the switching process of a light-sensitive SCO Fe(II) complex (Figure 1). Because of ligand field interaction within the molecule, the d-orbitals of the metal complex are divided in two levels eg and t2g. The switching process in SCO metallic complexes induces a change in the orbital occupancy of these orbitals of the metallic atoms passing from high-spin state to low spin state and this process defines the modification of the lineshape of the absorption spectrum (see Figure 2).


Figure 1. Schematic illustration of the experiment: XAS spectra of the L2,3 absorption edges of Fe embedded in SCO iron(II) complex were recorded after irradiation of visible or UV light on the sample.
A pristine sample of a molecular SCO iron(II) complex featuring a photoisomerizable diarylethene ligand in solid state was measured by means of XAS at the Fe L3 edge (Figure 2). In the obtained spectrum two peaks corresponding to t2g and eg levels are clearly visible. An in-situ irradiation with a UV light during 50 min changed the shape of the XAS spectrum up to describe the electronic configuration of mainly low-spin state. A subsequent visible light irradiation 5 min long induced the reverse reaction toward high-spin state. These results obtained at room temperature are the first experimental proof of a reversible photoswitching at the molecular level in solid state. Further XPS measurements were focusing on the chemical state of sulphur atoms in the molecule as the different chemical environments of this atom in the two switched states can give a fingerprint of the molecular state. The comparison of these experimental results with magnetic measurements gave quantitative information on the photoresponse of the molecular complex as well as time-dependence of the switching process and stability in the two different spin states.

The discovery of this molecule showing two stable magnetic state reversibly switchable by light irradiation at room temperature and in solid state at the molecular level is clearly a step forward in the  development of devices based on molecular metal complexes. 

Figure 2. (Top Left) Schematic change in the molecular bonds due to irradiation. (Bottom Left) The correspondent High Spin and Low spin electronic state in Fe ions. (Right) The absorption spectra of the of the L2,3 of the Fe ion as prepared and irradiated with different wavelengths (blue UV and visible green).


This research was conducted by the following research team:

  • B. Rösner, M. Milek, A. Witt, R. H. Fink and M. M. Khusniyarov, Department of Chemistry and Pharmacy, Friedrich-Alexander-University Erlangen-Nürnberg, Erlangen Germany

  • B. GobautElettra - Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy

  • P. Torelli, CNR, Istituto Officina Materiali, Trieste, Italy

Contact person:

Piero Torelli, Email: piero.torelli@elettra.eu
Marat Khusniyarov, Email: marat.khusniyarov@fau.de 


B. Rösner, M. Milek, A. Witt, B. Gobaut, P. Torelli, R.H. Fink, and M.M. Khusniyarov, “Reversible Photoswitching of a Spin-Crossover Molecular Complex in the Solid State at Room Temperature” Angew. Chem. Int. Ed. 54, 1 (2015), doi: 10.1002/anie.201504192


Last Updated on Thursday, 01 October 2015 11:34