Molecular compounds with switchable electronic states have great potential for information technology in very high-density devices. Systems with the necessary electronic bistability can be found among coordination compounds of transition metal ions. For instance, temperature variation can trigger a redistribution of 3d electrons between t2g and eg orbitals in compounds with medium ligand-field strength, leading to thermal spin state transition between the low-spin (LS) ground state and the close-lying high-spin (HS) state. A number of these spin-crossover complexes were also found to exhibit light-induced spin-state switching phenomena at temperatures far below their transition temperature. The photo-induced excited HS state often displays a very long lifetime at low temperatures (vide infra). The phenomenon of populating this metastable low-temperature HS state by irradiation with visible light was called light-induced excited spin-state trapping (LIESST) [1]. The switching of such compounds proceeds through excited states; the excitation and relaxation mechanisms are determined by the strongly-coupled electron, magnetic, and structural dynamics. Light excitation has proven extremely valuable in the study of such processes. However, investigation of alternative switching mechanisms can offer further insight into the related electron structure and dynamics.

Recently, we reported studies on different molecular compounds of transition metals, and showed that X-ray emission spectroscopy (XES) can probe the 3d spin momentum quantitatively [2]. The investigated systems included the spin-crossover complex [Fe(phen)2(NCS)2], which has a 3d6 central ion. This compound shows a 1A1g (t2g6eg0, LS) to 5T2g (t2g4eg2, HS) transition around 180 K, which we monitored with Kß XES. However, further measurements carried out at ID26 revealed an anomalous behaviour below 55 K, as shown in Figure 15. While a rapid scan at 30 K still reflects the LS ground state, a second, longer scan shows that the majority of the sample (72%) has transformed to the metastable HS state in the beam after several minutes of exposure. Systematically, upon increasing the temperature, a lower X-ray induced metastable HS fraction is observed: 42% is measured at 45 K, and it completely disappears at 55 K. The observed temperature dependence is fully reversible and reproducible, which excludes artefacts due to a local heating or decomposition caused by the X-ray beam.

Fig. 15: a) Low-temperature Kß spectra of [Fe(phen)2(NCS)2]. Reference HS/LS spectra, taken at 80 and 295 K, respectively, are shown as red/blue dashed curves for each spectrum. b) X-ray data (circles with symbol X) compared to conventional magnetisation measurements (solid line). The distribution of the six 3d electrons on the t2g and eg levels in the different spin states is depicted in blue (LS) and red (HS) circles.

Observations of an anomalous metastable HS state in [Fe(phen)2(NCS)2] upon irradiation with green light and below 50 K were interpreted as LIESST [1]. Excited spin-state trapping emerges because the decay of the excited HS state is impeded at low temperatures. The non-adiabatic multiphonon relaxation, which could bring the system back to the LS ground state, is hindered below 50 K as the relevant vibrational modes are inactive; this leads to a very small tunnelling rate (typically 10–6 s–1) [1]. At higher temperatures, where the higher vibrational states of the HS molecules are populated, the relaxation becomes fast due to the larger overlap of the higher vibrational wave functions of the two spin states, as indicated in Figure 16. The remarkably similar temperature dependence of the metastable HS population created with light and X-ray irradiation suggest identical underlying relaxation mechanisms. Consequently, we can conclude that we observed hard X-ray induced excited spin state trapping (HAXIESST).

Fig. 16: Schematical electronic structure of [Fe(phen)2(NCS)2]. Vertical arrows indicate optical transitions (ligand field or metal to ligand charge transfer exitations (MCLT)), green curly arrows show rapid intersystem crossings. (Back-switching with red light, not discussed in the text, is also indicated.) The curved golden arrow symbolises the observed X-ray switching. Relaxation (tunnelling) is determined by the overlap of the vibrational wavefunctions.

While the low-T HS state and its decay conditions are the same in LIESST and HAXIESST, the excitation is obviously very different: visible light can make the necessary ligand-field excitations; however, the energy of hard X-rays is far too high for this. We propose that the spin-state trapping occurs in the relaxation processes that follow electronic excitations caused mostly by secondary electrons. Thus the majority of the complexes experiences only a valence excitation before the probing quantum arrives as the excitation here is made by secondary electrons originating from a remote ionisation.

This result is of the utmost importance for the community, which has to be aware of the non-innocent nature of low-temperature hard X-ray investigations, where spectroscopy or diffraction experiments might lead to excitations similar to visible light-induced ones. The HAXIESST effect can be exploited as an alternative excitation source of high efficiency and large penetrating power for dark-coloured samples or non-transparent sample environments.


[1] A. Hauser, Top. Curr. Chem. 234, 155 (2004), and references therein.
[2] G. Vankó, T. Neisius, G. Molnár, F. Renz, S. Kárpáti, A. Shukla, F.M.F. de Groot, J. Phys. Chem. B 110, 11647 (2006).

Principal publication and authors

G. Vankó (a,b), F. Renz (c), G. Molnár (d), T. Neisius (e), S. Kárpáti (f), Angew. Chem. Int. Ed. 46, 5306 (2007).
(a) ESRF
(b) KFKI Research Institute for Particle and Nuclear Physics, Budapest (Hungary) 
(c) Johannes Gutenberg University, Mainz (Germany)
(d) LCC-CNRS Toulouse (France)
(e) Univ. Paul Cézanne, CP2M Marseille (France)
(f) Eötvös Loránd University, Budapest (Hungary)