Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective

doi:10.1016/j.cplett.2020.137681

Chemical Physics Letters

Volume 754, September 2020, 137681

https://doi.org/10.1016/j.cplett.2020.137681 Get rights and content

Highlights

•
First quantitative soft X-ray absorption cross sections of aqueous iron cyanides.
•
Establishing transient soft X-ray signatures of the photo-oxidation.
•
Quantification of photo-oxidized species at two different absorption edges.

Abstract

It is demonstrated for the case of photo-excited ferrocyanide how time-resolved soft X-ray absorption spectroscopy in transmission geometry at the ligand K-edge and metal L₃-edge provides quantitatively equivalent valence electronic structure information, where signatures of photo-oxidation are assessed locally at the metal as well as the ligand. This allows for a direct and independent quantification of the number of photo-oxidized molecules at two soft X-ray absorption edges highlighting the sensitivity of X-ray absorption spectroscopy to the valence orbital occupation of 3d transition metal complexes throughout the soft X-ray range.

Graphical abstract

Introduction

Changes in oxidation state are common steps during photo-chemical reactions involving transition metal complexes. For an unambiguous characterization of the involved transient species, it is therefore crucial to spectroscopically follow the underlying changes in orbital population. The photo-oxidation of aqueous ferrocyanide ( ${[Fe {(CN)}_{6}]}^{4 -}$ ) constitutes an ideal model process that allows to test the capabilities of different spectroscopies in that regard. After an optical excitation in the ultraviolet regime, a Fe 3d electron is ejected into the solvent [1]. Thereby, the Fe(II) complex ${[Fe {(CN)}_{6}]}^{4 -}$ is oxidized resulting in the formation of the Fe(III) species ferricyanide ( ${[Fe {(CN)}_{6}]}^{3 -}$ ). Timescales and yield of the photo-oxidation have been initially established by time-resolved optical spectroscopies [1], [2], [3], [4]. However, in order to acquire information on the electronic structure and in particular valence orbital occupation core-level spectroscopies provide enhanced sensitivity.

With respect to the photo-oxidation of aqueous ${[Fe {(CN)}_{6}]}^{4 -}$ , time-resolved X-ray absorption spectroscopy at the Fe K-edge was used by Reinhard et al. to investigate the formation of ${[Fe {(CN)}_{6}]}^{3 -}$ from the metal perspective on a picosecond time-scale. Due to their quadrupole character, the analyzed pre-edge features are sensitive to the Fe 3d orbital occupation. This allowed Reinhard et al. to study the yield of photo-oxidation in the ultraviolet regime as well as competing relaxation pathways [5]. Complementary information from the ligand perspective can be acquired using time-resolved valence-to-core X-ray emission spectroscopy. March et al. used the technique to follow changes of the overlap between ligand and metal-centered orbitals for the case of the photo-oxidation of ${[Fe {(CN)}_{6}]}^{4 -}$ [6].

In this work, we demonstrate how time-resolved X-ray absorption spectroscopy in the soft X-ray regime combines the capabilities of different spectroscopies in the hard X-ray regime by providing equivalent information regarding valence orbital occupation as seen from the metal as well as the ligand perspective. A liquid flatjet setup (see Materials and methods) is employed allowing for combined transmission experiments in the range of 3d metal L-edges [7] as well as K-edges of light elements [8]. This approach gives unique access to the unoccupied density of states locally at the Fe center through the Fe 2p $\to$ 3d excitation of L-edge spectroscopy [9], [10], [11], [12], [13]. At the same time, information from the CN⁻ ligand perspective is given by the N 1s $\to$ 2p excitation [14], [11], [15], [12], [16]. This allows to access equivalent valence orbitals using ligand- as well as metal-centered core-excitations, which provides experimental flexibility when targeting more complex systems. Furthermore, due to the transmission scheme of the experiment, quantitative absorption cross sections are recorded, which allow to directly infer the number of photo-oxidized species as seen from the metal as well as the ligand perspective.

Section snippets

Results and discussion

In order to rationalize the soft X-ray signatures of the photo-induced interconversion of ${[Fe {(CN)}_{6}]}^{4 -}$ into its oxidized counterpart ${[Fe {(CN)}_{6}]}^{3 -}$ , we first introduce the steady-state features of the two species, which have been analyzed previously [14], [17], [11]. Fig. 1 shows the X-ray absorption spectra of aqueous ${[Fe {(CN)}_{6}]}^{4 -}$ and ${[Fe {(CN)}_{6}]}^{3 -}$ measured at the N K-edge. The spectra are displayed as the measured absorbance A, which is retrieved by comparing the transmitted intensity I to the direct

Conclusion

In summary, we have presented first quantitative solution-phase soft X-ray transmission data on aqueous ${[Fe {(CN)}_{6}]}^{4 -}$ and ${[Fe {(CN)}_{6}]}^{3 -}$ . The photochemistry of ${[Fe {(CN)}_{6}]}^{4 -}$ following a 257 nm photo-excitation was investigated using time-resolved X-ray absorption spectroscopy at the N K-edge as well as Fe L₃-edge. Thereby, signatures of photo-oxidation of ${[Fe {(CN)}_{6}]}^{4 -}$ were detected and used to independently evaluate the number of oxidized molecules from the ligand as well as metal perspective. Our study

Materials and methods

K₄Fe(CN)₆·(H₂O)₃ and K₃Fe(CN)₆ were purchased from Sigma–Aldrich and used without further purification. Samples were prepared as 150 mM solutions in deionized water for the case of ${[Fe {(CN)}_{6}]}^{4 -}$ , while for the measurements of ${[Fe {(CN)}_{6}]}^{3 -}$ , 100 mM solutions were used.

The experimental data was measured at the UE52-SGM beamline [26] of the BESSY II synchrotron using a setup dedicated to soft X-ray absorption measurements in transmission mode. Details of the experimental setup can be found elsewhere

CRediT authorship contribution statement

Raphael M. Jay: Conceptualization, Formal analysis, Investigation, Writing - original draft. Sebastian Eckert: Investigation, Software, Writing - review & editing. Rolf Mitzner: Investigation, Writing - review & editing. Mattis Fondell: Investigation, Writing - review & editing. Alexander Föhlisch: Project administration, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

R.M.J., S.E. and A.F. acknowledge funding from the ERC-ADG-2014 - Advanced Investigator Grant No. 669531 EDAX under the Horizon 2020 EU Framework Program for Research and Innovation. We thank the Helmholtz-Zentrum Berlin for allocation of synchrotron radiation beamtime and Christian Weniger for technical support during the measurements. We thank Vinícius Vaz da Cruz for fruitful discussions about the quantum chemical simulations.

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¹: Present address: Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Straße 2A, 12489 Berlin, Germany.

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Chemical Physics Letters

Research paperQuantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Results and discussion

Conclusion

Materials and methods

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

Excited state chemistry of the ferrocyanide ion in aqueous solution. I. Formation of the hydrated electron

J. Chem. Phys.

Picosecond study of photoionization in aqueous solution

Il Nuovo Cimento B Series

Ultrafast events in the electron photodetachment from the hexacyanoferrate(II) complex in solution

Chem. Phys. Lett.

Ultrafast mid-IR detection of the direct precursor to the presolvated electron following electron ejection from ferrocyanide

Chem. Phys. Lett.

Photooxidation and photoaquation of iron hexacyanide in aqueous solution: A picosecond X-ray absorption study

Struct. Dyn.

Feasibility of valence-to-core X-ray emission spectroscopy for tracking transient species

J. Phys. Chem. C

Direct determination of absolute absorption cross sections at the L-Edge of dilute Mn complexes in solution using a transmission flatjet

Inorg. Chem.

A liquid flatjet system for solution phase soft-x-ray spectroscopy

Struct. Dyn.

L-edge X-ray absorption spectroscopy of non-heme iron sites: experimental determination of differential orbital covalency

J. Am. Chem. Soc.

Photo-induced spin-state conversion in solvated transition metal complexes probed via time-resolved soft X-ray spectroscopy

J. Am. Chem. Soc.

Viewing the valence electronic structure of ferric and ferrous hexacyanide in solution from the Fe and cyanide perspectives

J. Phys. Chem. B

The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(II) complexes

PCCP

Covalency-driven preservation of local charge densities in a metal-to-ligand charge-transfer excited iron photosensitizer

Angew. Chem. Int. Ed.

Observation of back-donation in 3d metal cyanide complexes through N K absorption spectra

J. Electron. Spectrosc. Relat. Phenom.

Valence orbitals and local bond dynamics around N atoms of histidine under X-ray irradiation

PCCP

T1 Population as the driver of excited-state proton-transfer in 2-thiopyridone

Chem. - A Eur. J.

Fe L-Edge XAS Studies of K4[Fe(CN)6] and K3[Fe(CN)6]: A direct probe of back-bonding

J. Am. Chem. Soc.

Disentangling transient charge density and metal-ligand covalency in photoexcited ferricyanide with femtosecond resonant inelastic soft X-ray scattering

J. Phys. Chem. Lett.

Nature of bonding in metal cyanide complexes as related to intensity and frequency of infrared absorption spectra

Inorg. Chem.

Time-resolved X-ray spectroscopy in the water window: elucidating transient valence charge distributions in an aqueous Fe(II) Complex

J. Phys. Chem. Lett.

Research paper
Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective

$T_{1}$ Population as the driver of excited-state proton-transfer in 2-thiopyridone

Fe L-Edge XAS Studies of K₄[Fe(CN)₆] and K₃[Fe(CN)₆]: A direct probe of back-bonding