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The hexanuclear thioguanidine mixed-valent copper complex cation [Cu6 (NGuaS)6 ]+2 (NGuaS = o-SC6 H4 NC(NMe2 )2 ) and its oxidized/reduced states are theoretically analyzed by means of density functional theory (DFT) (TPSSh + D3BJ/def2-TZV (p)). A detailed bonding analysis using overlap populations is performed. We find that a delocalized Cu-based ring orbital serves as an acceptor for donated S p electrons. The formed fully delocalized orbitals give rise to a confined electron cloud within the Cu6 S6 cage which becomes larger on reduction. The resulting strong electrostatic repulsion might prevent the fully reduced state. Experimental UV/Vis spectra are explained using time-dependent density functional theory (TD-DFT) and analyzed with a natural transition orbital analysis. The spectra are dominated by MLCTs within the Cu6 S6 core over a wide range but LMCTs are also found. The experimental redshift of the reduced low energy absorption band can be explained by the clustering of the frontier orbitals. © 2017 Wiley Periodicals, Inc.

A series of novel hybrid molecules containing sulfur and nitrogen donor functions simultaneously has been synthesized and characterized. Designed for use in biomimetic coordination chemistry, these molecules are expected to act directly or in specific precursor roles as polyfunctional Lewis bases towards metal-based Lewis acids to form complexes with biological relevance. Their principal architecture is based on aliphatic or aromatic backbones connecting guanidine moieties with sulfur-containing thioether or disulfide portions. Based on acyclic tetramethylguanidino units and their cyclic dimethylethyl-substituted counterparts, a panel of forty-six members of this class of compounds are presented here. The associated denticity extends from bidentate (N/S donor sets) and tridentate (N/N/S donor sets) via tetradentate (N/N/S/S donor sets) to pentadentate (N/N/N/S/S donor sets). Five of these novel systems were structurally characterized and subjected to geometry optimizations using density-functional theory (DFT).

Constrained density-functional theory (DFT) calculations show that the recently observed optically induced insulator-metal transition of the In/Si(111)(8×2)/(4×1) nanowire array (Frigge et al., Nature 544:207, 2017) corresponds to the non-thermal melting of a charge-density wave (CDW). Massively parallel numerical simulations allow for the simulation of the photo-excited nanowires and provide a detailed microscopic understanding of the CDW melting process in terms of electronic surface bands and selectively excited soft phonon modes. Excited-state molecular dynamics in adiabatic approximation shows that the insulator-metal transition can be as fast as 350 fs.

International audience; The structural dynamics of charge-transfer states of nitrogen-ligated copper complexes has been extensively investigated in recent years following the development of pump-probe X-ray techniques. In this study we extend this approach towards copper complexes with sulfur coordination and investigate the influence of charge transfer states on the structure of a dicopper(I) complex with coordination by bridging disulfide ligands and additionally tetramethylguanidine units [CuI2(NSSN)(2)](2+). In order to directly observe and refine the photoinduced structural changes in the solvated complex we applied picosecond pump-probe X-ray absorption spectroscopy (XAS) and wide-angle X-ray scattering (WAXS). Additionally, the ultrafast evolution of the electronic excited states was monitored by femtosecond transient absorption spectroscopy in the UV-Vis probe range. DFT calculations were used to predict molecular geometries and electronic structures of the ground and metal-to-ligand charge transfer states with singlet and triplet spin multiplicities, i.e. S-0, (MLCT)-M-1 and (MLCT)-M-3, respectively. Combining these techniques we elucidate the electronic and structural dynamics of the solvated complex upon photoexcitation to the MLCT states. In particular, femtosecond optical transient spectroscopy reveals three distinct timescales of 650 fs, 10 ps and 4100 ps, which were assigned as internal conversion to the ground state (Sn -> S-0), intersystem crossing (MLCT)-M-1 -> (MLCT)-M-3, and subsequent relaxation of the triplet to the ground state, respectively. Experimental data collected using both X-ray techniques are in agreement with the DFT-predicted structure for the triplet state, where coordination bond lengths change and one of the S-S bridges is cleaved, causing the movement of two halves of the molecule relative to each other. Extended X-ray absorption fine structure spectroscopy resolves changes in Cu-ligand bond lengths with precision on the order of 0.01 angstrom, whereas WAXS is sensitive to changes in the global shape related to relative movement of parts of the molecule. The results presented herein widen the knowledge on the electronic and structural dynamics of photoexcited copper-sulfur complexes and demonstrate the potential of combining the pump-probe X-ray absorption and scattering for studies on photoinduced structural dynamics in copper-based coordination complexes.

Electron paramagnetic resonance (EPR) signatures, more specifically the elements of the electronic $g$ tensor, are calculated within density functional theory for hydrogenated Si(111), Si(001), Si(113), Si(114), $\mathrm{Si}(11\overline{2})$, and Si(110) surfaces. Thereby both perturbation theory and a more sophisticated Berry phase technique are applied. Specific defects on different surface orientations are shown to reproduce the resonances at $\overline{g}=2.0043$ and $\overline{g}=2.0052$ measured for hydrogenated microcrystalline silicon: The latter value is argued here to originate from regions with low hydrogen coverage; the resonance at $\overline{g}=2.0043$ is shown to appear in positions with dihydride environment, where an H atom is directly bound to the silicon dangling-bond atoms. A third group of EPR signals with considerably larger $\overline{g}$ values between 2.006 and 2.009 is predicted for highly symmetric dangling bonds resembling single dangling-bond defects in silicon bulk material. As the exact value depends strongly on local strain, this type of defect can explain a less intense signal with large $g$ strain observed in microcrystalline as well as in amorphous material.

Based on the equilibrium geometries of [Cu2(dbdmed)2O2] 2þ and [Cu2(en)2O2] 2þ obtained within density-functional theory, we investigate their molecular electronic structure and optical response. Thereby results from occupation-constrained as well as time-dependent DFT (DSCF and TDDFT) are compared with Green’s function-based approaches within many-body perturbation theory such as the GW approximation (GWA) to the quasiparticle energies and the Bethe-Salpeter equation (BSE) approach to the optical absorption. Concerning the groundstate energies and geometries, no clear trend with respect to the amount of exact exchange in the DFT calculations is found, and a strong dependence on the basis sets is to be noted. They affect the energy difference between bis-l-oxo and l-g 2 :g 2 peroxo complexes by as much as 0.8 eV (18 kcal/mol). Even stronger, up to 5 eV is the influence of the exchange-correlation functional on the gap values obtained from the Kohn-Sham eigenvalues. Not only the value itself but also the trends observed upon the bis-l-oxo to l-g 2 :g 2 -peroxo transition are affected. In contrast, excitation energies obtained from DSCF and TDDFT are comparatively robust with respect to the details of the calculations. Noteworthy, in particular, is the near quantitative agreement between TDDFT and GWAþBSE for the optical spectra of [Cu2(en)2O2] 2þ . V C 2013 Wiley Periodicals, Inc.

High-energy-resolution-fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy is shown to be a sensitive tool to investigate the electronic changes of copper complexes induced by geometric distortions caused by the ligand backbone as a model for the entatic state. To fully exploit the information contained in the spectra gained by the high-energy-resolution technique, (time-dependent) density functional theory calculations based on plane-wave and localized orbital basis sets are performed, which in combination allow the complete spectral range from the prepeak to the first resonances above the edge step to be covered. Thus, spectral changes upon oxidation and geometry distortion in the copper N-(1,3-dimethylimidazolidin-2-ylidene)quinolin-8-amine (DMEGqu) complexes [CuI(DMEGqu)2](PF6) and [CuII(DMEGqu)2](OTf)2·MeCN can be accessed.

The hexanuclear thioguanidine mixed-valent copper complex cation [Cu

Ab initio calculations of the electronic g -tensor of paramagnetic states at surfaces are presented taking the adsorption of hydrogen atoms at silicon surfaces as an example. We show that for silicon surfaces with different hydrogen coverages, the g -tensor is by far more characteristic than the hyperfine splitting of the Si dangling bonds or the adsorbed H atoms. This holds also in the case of powder spectra (e.g. amorphous or microcrystalline material) where only the angular average of the spectra is available from experiments. Hence, the ab initio calculation of the g -tensor should be in general a basic key to a better understanding of the microscopic structure of paramagnetic surfaces or interfaces (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Rare earth induced silicide phases of submonolayer height and 5 × 2 periodicity on the Si(111) surface are investigated by density functional theory and ab initio thermodynamics. The most stable silicide thin film consists of alternating Si Seiwatz and honeycomb chains aligned along the [1\(\overline{1}\) 0] direction, with rare earth atoms in between. This thermodynamically favored model is characterized by a minor band gap reduction compared to bulk Si and explains nicely the measured scanning tunneling microscopy images.

The effect of oxygen impurities and structural imperfections on the coherent on-chain quantum conductance of poly(3-hexylthiophene) is calculated from first principles by solving the scattering problem for molecular structures obtained within density functional theory. It is found that the conductance drops substantially for polymer kinks with curvature radii smaller than 17 A and rotations in excess of about 60∘. Oxidation of thiophene group carbon atoms drastically reduces the conductance, whereas the oxidation of the molecular sulfur barely changes the coherent transport properties. Also isomer defects in the coupling along the chain direction are of minor importance for the intrachain transmission.

Density functional theory (DFT) calculations of the equilibrium geometry, vibrational modes, ionization energies, electron affinities, and optical response of [Cu2(btmgp)2(μ-O)2]2+ (oxo) and [Cu2(btmgp)2(μ-η2:η2-O2)]2+ (peroxo) are presented. Comprehensive benchmarking shows that the description of the oxo–peroxo energetics is still a torture track for DFT, but finds the molecular geometry to be comparatively robust with respect to changes in the exchange-correlation functionals and basis sets. Pure functionals favor the oxo core found experimentally, whereas hybrid functionals shift the bias toward the peroxo core. Further stabilization of peroxo core results from relaxing the spin degrees of freedom using the broken-symmetry (BS) approach. Dispersion effects, conversely, tend to favor the oxo configuration. Triple-zeta basis sets are found to represent a sensible compromise between numerical accuracy and computational effort. Particular attention is paid to the modification of the electronic structure, optical transitions, and excited-state energies along the transition path between the oxo and peroxo species. The excited-state potential energy surface calculations indicate that two triplet states are involved in the transition that stabilize the BS solution. Charge decomposition and natural transition orbital analyses are used for obtaining microscopic insight into the molecular orbital interactions. Here, the crucial role of guanidine π-interactions is highlighted for the stabilization of the Cu2O2 core. © 2015 Wiley Periodicals, Inc.

The geometry of the polar LiNbO3 (0001) surface is strongly temperature dependent. In this work the surface charge associated to various surface terminations is estimated from first-principles calculations. All stable terminations are found to lower the polarization charge, showing that the surface charge compensation is a major driving force for surface reconstruction.

We report a comprehensive computational benchmarking of the structural and optical properties of a bis(chelate) copper(I) guanidine-quinoline complex. Using various (TD-)DFT flavors a strong influence of the basis set is found. Moreover, the amount of exact exchange shifts metal-to-ligand bands by 1 eV through the absorption spectrum. The BP86/6-311G(d) and B3LYP/def2-TZVP functional/basis set combinations were found to yield results in best agreement with the experimental data. In order to probe the general applicability of TD-DFT to excitations of copper bis(chelate) charge-transfer (CT) systems, we studied a small model system that on the one hand is accessible to methods of many-body perturbation theory (MBPT) but still contains simple guanidine and imine groups. These calculations show that large quasiparticle energies of the order of several electronvolts are largely offset by exciton binding energies for optical excitations and that TD-DFT excitation energies deviate from MBPT results by at most 0.5 eV, further corroborating the reliability of our TD-DFT results. The latter result in a multitude of MLCT bands ranging from the visible region at 3.4 eV into the UV at 5.5 eV for the bis(chelate) complex. Molecular orbital analysis provided insight into the CT within these systems but gave mixed transitions. A meaningful transition assignment is possible, however, by using natural transition orbitals. Additionally, we performed a thorough conformational analysis as the correct description of the copper coordination is crucial for the prediction of optical spectra. We found that DFT identifies the correct conformational minimum and that the MLCTs are strongly dependent on the torsion of the chelate angles at the copper center. From the results, it is concluded that extensive benchmarking allows for the quantitative analyses of the CT behavior of copper bis(chelate) complexes within TD-DFT.

The frequency-dependent dielectric function and the second-order polarizability tensor of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained from density-functional theory. The subsequent application of the GW approximation to account for quasiparticle effects and the solution of the Bethe–Salpeter equation yield a dielectric function for the stoichiometric material that slightly overestimates the absorption onset and the oscillator strength in comparison with experimental measurements. Calculations at the level of the independent-particle approximation indicate that these deficiencies are at least partially related to the neglect of intrinsic defects typical for the congruent material. The second-order polarizability calculated within the independent-particle approximation predicts strong nonlinear coefficients for photon energies above 1.5 eV. The comparison with measured data suggests that self-energy effects improve the agreement between experiment and theory. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular for the 21 and 31 tensor components, further improving the agreement with measured data.

Combining orientation dependent electrically detected magnetic resonance and g tensor calculations based on density functional theory we assign microscopic structures to paramagnetic states involved in spin-dependent recombination at the interface of hydrogenated amorphous silicon crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. We find that (i) the interface exhibits microscopic roughness, (ii) the electronic structure of the interface defects is mainly determined by c-Si, (iii) we identify the microscopic origin of the conduction band tail state in the a-Si:H layer, and (iv) present a detailed recombination mechanism.

The effect of ferroelectric poling on the water adsorption characteristics of lithium niobate Z-cut surfaces is investigated by ab initio calculations. Thereby we model the adsorption of H2O monomers, small water clusters and water thin films. The adsorption configuration and energy are determined as a function of the surface coverage on both the positive and negative LiNbO3(0001) surfaces. Thereby polarization-dependent adsorption energies, geometries and equilibrium coverages are found. The different affinity of water to the two surfaces is explained in terms of different bonding scenarios as well as the electrostatic interactions between the substrate and the polar molecules. Surface phase diagrams for the Z-cuts in equilibrium with water are predicted from atomistic thermodynamics.

Ab initio calculations of the electronic g-tensor of paramagnetic states at surfaces and solar cells are presented, whereby special emphasis is given onto the influence of relativistic effects. After discussing the numerical requirements for such calculations, we show that for silicon surfaces the g-tensor varies critically with the hydrogen coverage, and provides an exceptionally characteristic property. This holds also in the case of powder spectra where only the isotropic part g av is available from experiments. Extending our calculations onto microcrystalline 3C-SiC, our study explains why sol-gel grown undoped material can serve as an excellent acceptor material for an effective charge separation in organic solar cells: Due to an auto-doping mechanism by surface-induced states it fits excellently into the energy level scheme of this kind of solar cell and has the potential to replace the usually used rather expensive fullerenes.

Calculations based on (occupation constrained) density functional theory using local as well as hybrid functionals to describe the electron-electron exchange and correlation are combined with many-body perturbation theory in order to determine the electronic and optical excitation properties of 5-(pentafluorophenyl)pyrimidin-2-amine, 5-(4-methoxy-2,3,5,6-tetrafluorophenyl)pyrimidin-2-amine, and 5-(4-(dimethylamino)-2,3,5,6-tetrafluorophenyl)pyrimidin-2-amine. Large quasiparticle shifts and exciton binding energies of about 4 eV are found. They cancel each other partially and thus allow for a meaningful description of the molecular optical response within the independent-particle approximation. We find a surprisingly strong influence of local-field effects as well as resonant-nonresonant coupling terms in the electron-hole Hamiltonian on the optical properties.

The polymerization of tetraazaperopyrene (TAPP) molecules on a Cu(111) substrate, as observed in recent STM experiments, has been investigated in detail by first principles calculations. Tautomerization is the first step required for the formation of molecular dimers and polymers. The substrate is found to catalyze this tautomerization.

Density functional theory (DFT) calculations are performed to determine the mechanism and origin of the intensively debated (4×1)–(8×2) phase transition of the Si(111)-In nanowire array. The calculations (i) show the existence of soft phonon modes that transform the nanowire structure between the metallic In zigzag chains of the room-temperature phase and the insulating In hexagons formed at low temperature and (ii) demonstrate that the subtle balance between the energy lowering due to the hexagon formation and the larger vibrational entropy of the zigzag chains causes the phase transition.