Photodynamics in ferricyanide revisited

In continuation of our collaboration with Prof. Emad F. Aziz and Dr. Igor Yu. Kiyan from Helmholtz-Zentrum in Berlin, a new investigation has been recently published. In this study, we address the early photodynamics of ferricyanide ion in solution applying transient XUV photoelectron spectroscopy in tandem with theoretical modeling.

Light-induced relaxation dynamics of the ferricyanide ion revisited by ultrafast XUV photoelectron spectroscopy Phys. Chem. Chem. Phys., 2017,19, 14248-14255

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This combination has been already applied by us to unravel peculiarities of spin crossover in [Fe(bpy)3]2+ complex. Here, we have addressed the problem of charge localization and symmetry-breaking in the simple prototypical coordination compound – ferricyanide. Upon absorption of UV light, it is excited to the charge-transfer state, which can undergo non-radiative relaxation to the ground state or be involved in further chemical reactions. This is a usual trait of coordination and organometallic compounds, which is often used by nature and chemists in, e.g., photosynthesis or photocatalytic retrieval of ecologic fuels.

In previous UV pump – IR probe spectroscopic study of the photochemical fate of ferricyanide, it was concluded that the initially populated charge-delocalized state relaxes to the localized one and the process is driven by the reorganization of the polar solvent. However, we obtained strong evidence for the spin crossover followed by geometrical distortions due to Jahn–Teller effect, rather than localization/delocalization dynamics, as suggested previously. Remarkably, our interpretation also consistently explains the transient features observed in UV-IR pump-probe experiments along with transient XUV PES.

Direct or sequential? Actually both! – Spin crossover in iron compounds

With the immense growth of the amount of information, the development of new principles in high-density data storage comes into the foreground. Exploiting spin magnetic moment for the storage of data is one of the perspective directions. Remarkably, the orientation of spin can be changed by absorption of light with high selectivity and this new magnetic state can live relatively long. Thus, studies of transition metal materials undergoing so-called spin crossover are growing in number. We have also put our two cents in the discussion closing down the debates whether the mechanism of spin transition is direct or sequential.

Usual tools to study spin flips comprise transient optical spectroscopic methods such as pump-probe spectroscopies. Our colleagues from Free University in Berlin have addressed spin crossover in classical material iron(II) tris-bipyridine with a new type of spectroscopy where photoionization is used as a probe and we have provided theoretical support for the interpretation of their entangled data. This study has been recently published on pages of ChemPhysChem journal:

A. Moguilevski, M. Wilke, G. Grell, S.I. Bokarev, S.G. Aziz, N. Engel, A.A. Raheem, O. Kühn, I.Yu. Kiyan, E.F. Aziz Ultrafast Spin Crossover in [FeII(bpy)3]2+: Revealing Two Competing Mechanisms by Extreme Ultraviolet Photoemission Spectroscopy ChemPhysChem 18 (2017) 465-469.

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There has been formulated no unambiguous opinion in the community on the mechanism of light-driven spin flip in iron(II) tris-bipyridine. In this process, the singlet spin configuration (no unpaired electrons) is changing to the long-living quintet (four unpaired electrons with parallel spins) within sub-100 fs time upon excitation with 400-580 nm light. After a long discussion, it has been concluded that direct spin flip of two electrons is highly improbable and a manifold of triplet intermediate states needs to be involved in a sequential process. However, recently an indication of direct mechanism has been found.

In our study, we have observed both pathways and even determined the branching ratio between both direct and sequential channels. To our opinion, our study resolves the cognitive dissonance involving mutually exclusive assumptions and adds to the profound mechanistic understanding of the spin crossover.