Olga S. Bokareva

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.

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.

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.

Tuned Range-separated Density Functional Theory and Dyson Orbital Formalism for Photoelectron Spectra

Photoelectron spectroscopy represents a valuable tool to analyze structural and dynamical changes in molecular systems. Comprehensive interpretation of experimental data requires, however, involvement of reliable theoretical modeling. In this work, we present a protocol based on the combination of well-established linear-response time-dependent density functional theory and Dyson orbital formalism for the accurate prediction of both ionization energies and intensities. Essential here is the utilization of the optimally tuned range-separated hybrid density functionals, improving the ionization potentials not only of frontier but also of the deeper lying orbitals. In general, the protocol provides accurate results as illustrated by comparison to experiments for several gas-phase molecules, belonging to different classes. Further, we analyze possible pitfalls of this approach and, namely, discuss the ambiguities in the choice of optimal range-separation parameters, the influence of the stability of the ground state, and the spin contamination issues as possible sources of inaccuracies.

Optimized Long-Range Corrected Density Functionals for Electronic and Optical Properties of Bare and Ligated CdSe Quantum Dots

The reliable prediction of optical and fundamental gaps of finite size systems using density functional theory requires to account for the potential self-interaction error, which is notorious for degrading the description of charge transfer transitions. One solution is provided by parametrized long-range corrected functionals such as LC-BLYP, which can be tuned such as to describe certain properties of the particular system at hand. Here, bare and 3-mercaptoprotionic acid covered Cd33Se33 quantum dots are investigated using the optimally tuned LC-BLYP functional. The range separation parameter, which determines the switching on of the exact exchange contribution, is found to be 0.12 bohr-1 and 0.09 bohr-1 for the bare and covered quantum dot, respectively. It is shown that density functional optimization indeed yields optical and fundamental gaps and thus exciton binding energies, considerably different compared with standard functionals such as the popular PBE and B3LYP ones. This holds true, despite the well established fact that the leading transitions are localized on the quantum dot and do not show pronounced long-range charge transfer character.