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.

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More on spin state dynamics

Recently, I wrote about our ultrafast spin dynamics project. The first publication in Phys. Rev. Lett. was a proof of concept article where we have shown the possibility of soft X-ray light to trigger an unprecedentedly fast change of a spin state. A follow-up article presenting the theoretical method used in this investigation in detail also appeared recently in the issue of Molecular Physics devoted to the anniversary of Andre D. Bandrauk:

Huihui Wang, Sergey I. Bokarev, Saadullah G. Aziz, Oliver Kühn Density matrix-based time-dependent configuration interaction approach to ultrafast spin-flip dynamics Mol. Phys. (2017) 1-10.

In this article, we reformulate the problem in the form of a density matrix which allows one to treat general open quantum systems with energy dissipation. In addition to the more thorough study of the influence of different parameters of an excitation pulse on the dynamics, we also discuss a regime where the strong electron correlation plays a decisive role. It was shown that core-excited electronic states may demonstrate entangled dynamics both due to the strong spin-orbit coupling and electron correlation. This makes them interesting objects for the future studies of the ultimate limits of the ultrafast electron motion in atoms, molecules, and extended systems.

16th International Congress of Quantum Chemistry

It is time already to make plans for the next year. I have recently got the first info message from the 16th ICQC organizing committee. Next year it will be held in Menton, France. I have attended the previous congress in Beijing in 2015 and even won the best poster prize.

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There is only one session, what is convenient since one has an opportunity to attend all talks. However, this means that only invited talks are included into the program. Other participants are assumed to present posters. Because of this peculiarity, the poster sessions looked quite impressive. I have never seen such amount of eminent professors standing near their posters. It has been a nice opportunity to talk to some of them in an informal atmosphere. Summarizing, I can definitely recommend this event.

Photosensitizers for photocatalytic water splitting

I suppose that you have noticed the hype around new energy sources during last decades even if you are not working in this field. As a continuation of the Light2Hydrogen project, we have also addressed the properties of quite popular iridium-based photosensitizers – molecules which absorb light and transfer energy or electrons further to catalysts. The idea behind this project is to use sunlight to split water into hydrogen and oxygen. The former can be then used as an ecologically clean fuel since upon combustion it produces only water.

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In collaboration with the group of Stefan Lochbrunner, we have studied the influence of chemical derivation onto the absorption spectra as well as the nature of the most important electronic states.  The “bright” absorbing states are important to acquire as much solar energy as possible. For this purpose, it is of interest to shift them to lower energies to increase the overlap with the sun spectrum. In turn, the character of the lowest “dark” state is essential for the efficiency of further reactions, for instance, electron transfer to water reduction agents. We have looked at a series of organic ligands bound to iridium ions and formulated recommendations which can help for the rational design of the photocatalysts.

The article is published in a themed issue under the editorship of Matthias Bauer:

O.S. Bokareva, T. Möhle, A. Neubauer, S.I. Bokarev, S. Lochbrunner, O. Kühn Chemical Tuning and Absorption Properties of Iridium Photosensitizers for Photocatalytic  Applications Inorganics 2017, 5(2), 23.


 

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.

Nuclear correlation effects viewed by X-ray spectroscopy

Continuing the developments of theoretical approaches to X-ray spectroscopy in our group we have recently published an article on the interplay (correlation) of nuclear motions and its implications for absorption (XAS) and resonant inelastic scattering spectra (RIXS):

S. Karsten, S.D. Ivanov, S.G. Aziz, S.I. Bokarev, O. Kühn Nuclear Dynamical Correlation Effects in X‑ray Spectroscopy from a Theoretical Time-Domain Perspective J. Phys. Chem. Lett., 2017, 8 (5), pp 992–996.

Despite working with high-energy electronic transitions and very short lifetimes, X-ray spectroscopy demonstrates remarkable sensitivity to nuclear motions which are characterized by much smaller energies and larger timescales. Given the prominent place of X-rays in material science, it is of importance since it broadens the scope of the effects which can be studied.

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Until now the coupling between electronic and nuclear degrees of freedom in core-level spectra has been analyzed following two strategies: performing numerically exact wave-packet quantum dynamics and applying analytic Franck-Condon model. The former being actively promoted by F. Gel’mukhanov’s group from Stockholm is in general too complicated for large molecules and needs reduction of complexity which could be a non-trivial task. The latter one, in turn, is too simplistic to recover nuclear effects beyond the harmonic approximation for nuclear vibrations.

In our article, we suggest a trajectory-based approach of intermediate complexity, where a system “decides” itself which regions of the phase space to explore and, thus, saving substantial computational effort for large molecules in comparison to exact quantum dynamics. Moreover, our protocol allows disentangling correlated and uncorrelated nuclear dynamics that opens new perspectives in the analysis of vibrational motion with the help of X-rays. Remarkably, we have demonstrated that second-order RIXS spectroscopy should be much more sensitive to nuclear correlation than the first-order XAS.

However, this is only the first proof-of-the-concept step to establishing a robust and versatile tool. In the process of derivation, implementantion, and discussion with colleagues, we have realized the key points, where the protocol needs to be improved.  Now we have a roadmap how to systematically approach the exact dynamics and the development is to be continued.

WATOC 2017

I would like to pay your attention to the 11th triennial congress on theoretical chemistry taking place in Münich this year.  The early bird registration is closed already but you can go for the normal one. I have attended the previous WATOC in Santiago de Chile and my wife has taken part in the congress before that in Santiago de Compostela. This is definitely an event worth visiting. It will give you an opportunity not only to listen to the cutting edge developments in quantum chemistry in multithreaded regime (several sessions are running simultaneously)  but also learn about the research of those people who win medals and prizes. For instance, in Chile Arieh Warshel (Nobel Prize in Chemistry 2013) read a lecture. Hope to see you in Münich this summer!

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