Sergey I. Bokarev

Tuning Range-Separated Density Functional Theory for Photocatalytic Water Splitting Systems

We discuss the system-specific optimization of long-range-separated density functional theory (DFT) for the prediction of electronic properties relevant for a photocatalytic cycle based on an Ir(III) photosensitizer (IrPS). Special attention is paid to the charge-transfer properties, which are of key importance for the photoexcitation dynamics but cannot be correctly described by means of conventional DFT. The optimization of the range-separation parameter using the ΔSCF method is discussed for IrPS including its derivatives and complexes with electron donors and acceptors used in photocatalytic hydrogen production. Particular attention is paid to the problems arising for a description of medium effects by means of a polarizable continuum model.

Nature of the Chemical Bond of Aqueous Fe2+ Probed by Soft X-ray Spectroscopies and ab Initio Calculations

Aqueous iron(II) chloride is studied by soft X-ray absorption, emission, and resonant inelastic Raman scattering techniques on the Fe L-edge and O K-edge using the liquid-jet technique. Soft X-ray spectroscopies allow in situ and atom-specific probing of the electronic structure of the aqueous complex and thus open the door for the investigation of chemical bonding and molecular orbital mixing. In this work, we combine theoretical ab initio restricted active space self-consistent field and local atomic multiplet calculations with experimental soft X-ray spectroscopic methods for a description of the local electronic structure of the aqueous ferrous ion complex. We demonstrate that the atomic iron valence final states dominate the resonant inelastic X-ray scattering spectra of the complex over the ligand-to-metal charge transfer transitions, which indicates a weak interaction of Fe(2+) ion with surrounding water molecules. Moreover, the oxygen K-edge also shows only minor changes due to the presence of Fe(2+) implying a small influence on the hydrogen-bond network of water.

Chemical bonding in aqueous ferrocyanide: experimental and theoretical X-ray spectroscopic study.

Resonant inelastic X-ray scattering (RIXS) and X-ray absorption (XA) experiments at the iron L- and nitrogen K-edge are combined with high-level first-principles restricted active space self-consistent field (RASSCF) calculations for a systematic investigation of the nature of the chemical bond in potassium ferrocyanide in aqueous solution. The atom- and site-specific RIXS excitations allow for direct observation of ligand-to-metal (Fe L-edge) and metal-to-ligand (N K-edge) charge-transfer bands and thereby evidence for strong σ-donation and π-backdonation. The effects are identified by comparing experimental and simulated spectra related to both the unoccupied and occupied molecular orbitals in solution.

Joint Analysis of Radiative and Non-Radiative Electronic Relaxation Upon X-ray Irradiation of Transition Metal Aqueous Solutions

L-edge soft X-ray spectroscopy has been proven to be a powerful tool to unravel the peculiarities of electronic structure of transition metal compounds in solution. However, the X-ray absorption spectrum is often probed in the total or partial fluorescence yield modes, what leads to inherent distortions with respect to the true transmission spectrum. In the present work, we combine photon- and electron-yield experimental techniques with multi-reference first principles calculations. Exemplified for the prototypical FeCl2 aqueous solution we demonstrate that the partial yield arising from the Fe3s → 2p relaxation is a more reliable probe of the absorption spectrum than the Fe3d → 2p one. For the bonding-relevant 3d → 2p channel we further provide the basis for the joint analysis of resonant photoelectron and inelastic X-ray scattering spectra. Establishing the common energy reference allows to assign both spectra using the complementary information provided through electron-out and photon-out events.

Multi-reference approach to the calculation of photoelectron spectra including spin-orbit coupling.

X-ray photoelectron spectra provide a wealth of information on the electronic structure. The extraction of molecular details requires adequate theoretical methods, which in case of transition metal complexes has to account for effects due to the multi-configurational and spin-mixed nature of the many-electron wave function. Here, the restricted active space self-consistent field method including spin-orbit coupling is used to cope with this challenge and to calculate valence- and core-level photoelectron spectra. The intensities are estimated within the frameworks of the Dyson orbital formalism and the sudden approximation. Thereby, we utilize an efficient computational algorithm that is based on a biorthonormal basis transformation. The approach is applied to the valence photoionization of the gas phase water molecule and to the core ionization spectrum of the [Fe(H2O)6](2+) complex. The results show good agreement with the experimental data obtained in this work, whereas the sudden approximation demonstrates distinct deviations from experiments.

Mechanistic Study of Photocatalytic Hydrogen Generation with Simple Iron Carbonyls as Water Reduction Catalysts

This study provides new insights into light‐driven hydrogen generation using an iridium photosensitizer (IrPS) and simple iron carbonyls as water reduction catalysts (WRCs). Stopped‐flow rapid‐scan FTIR and operando continuous‐flow FTIR spectroscopy as well as time‐dependent density functional theory (TD‐DFT) has been applied to study the reaction. The conversion of the WRC precursor [Fe3(CO)12] into the radicals [Fe3(CO)11].− and [Fe2(CO)8].− as well as [Fe(CO)5] in the absence of light in a solvent mixture of tetrahydrofuran, triethylamine, and water has been studied quantitatively. During light‐induced hydrogen production in the presence of the IrPS, the trimeric [HFe3(CO)11]− and the monomeric [HFe(CO)4]− anion could be identified as major WRC species. The equilibrium between both species can be shifted completely towards [HFe(CO)4]− by increasing the water content of the solvent mixture. Application of other iron(0) carbonyl compounds as WRC precursors also results in the exclusive formation of [HFe(CO)4]−. Kinetic experiments show that the stability of the system is primarily influenced by the applied amount of WRC precursor, whereas the reaction rate is mainly determined by the concentration of the IrPS. At least two loss channels could be identified: light‐induced CO dissociation from the WRC and decomposition of the IrPS at high IrPS/WRC ratios, accompanied by a ligand transfer from the iridium towards the iron center of the WRC. To reveal the nature of the catalytically active complex, binding energies and charge‐transfer probabilities of all coordination geometries of various IrPS⋅⋅⋅WRC complexes have been calculated. These computations indicate an increased probability of charge transfer for dimeric and trimeric iron carbonyl species.

Ultrafast Spin Crossover in [FeII(bpy)3]2+: Revealing Two Competing Mechanisms by Extreme Ultraviolet Photoemission Spectroscopy

Photoinduced spin-flip in FeII complexes is an ultrafast phenomenon that has the potential to become an alternative to conventional processing and magnetic storage of information. Following the initial excitation by visible light into the singlet metal-to-ligand charge-transfer state, the electronic transition to the high-spin quintet state may undergo different pathways. Here we apply ultrafast XUV (extreme ultraviolet) photoemission spectroscopy to track the low-to-high spin dynamics in the aqueous iron tris-bipyridine complex, [Fe(bpy)3 ]2+ , by monitoring the transient electron density distribution among excited states with femtosecond time resolution. Aided by first-principles calculations, this approach enables us to reveal unambiguously both the sequential and direct de-excitation pathways from singlet to quintet state, with a branching ratio of 4.5:1.

Towards an ab initio theory for metal L-edge soft X-ray spectroscopy of molecular aggregates

The Frenkel exciton model was adapted to describe X-ray absorption and resonant inelastic scattering spectra of polynuclear transition metal complexes by means of the restricted active space self-consistent field method. The proposed approach allows to substantially decrease the requirements on computational resources if compared to a full supermolecular quantum chemical treatment. This holds true, in particular, in cases where the dipole approximation to the electronic transition charge density can be applied. The computational protocol was applied to the calculation of X-ray spectra of the hemin complex, which forms dimers in aqueous solution. The aggregation effects were found to be comparable to the spectral alterations due to the replacement of the axial ligand by solvent molecules.

Electronic excitation spectra of the [Ir(ppy)2(bpy)]+ photosensitizer bound to small silver clusters Agn (n = 1–6)

The changes in nature and order of the excited electronic states of the photosensitizer [Ir(ppy)(2)(bpy)](+) upon binding to small silver clusters, Ag(n) (n = 1-6), were studied theoretically using the linear response TDDFT method with the range-separated LC-BLYP functional. Binding energies and localization of HOMO and LUMO orbitals are found to oscillate with the number of silver atoms. Special emphasis is put on the discussion of long-range charge transfer transitions between the photosensitizer and the silver cluster. The energies of these transitions were found to be only slightly dependent on the relative orientations of both fragments, but strongly dependent on the intermolecular distance. The absorption spectrum of the combined system does not show a systematic trend with respect to cluster size, but it is strongly modified by the charge transfer transitions. Possible photophysical processes of the systems containing larger clusters are discussed.

Nuclear Dynamical Correlation Effects in X-ray Spectroscopy from a Theoretical Time-Domain Perspective

To date X-ray spectroscopy has become a routine tool that can reveal highly local and element-specific information on the electronic structure of atoms in complex environments. Here, we focus on nuclear dynamical correlation effects in X-ray spectra and develop a rigorous time-correlation function method employing ground state classical molecular dynamics simulations. The importance of nuclear correlation phenomena is demonstrated by comparison against the results from the conventional sampling approach performed on the same data set for gas phase water. In contrast to the first-order absorption, second-order resonant inelastic scattering spectra exhibit pronounced fingerprints of nuclear motions. The developed methodology is not biased to a particular electronic structure method and, owing to its generality, can be applied to, e.g., X-ray photoelectron and Auger spectroscopies.