Computational characterization of novel solar light-harvesting dyes and electronic-transfer system

Light-harvesting devices are of fundamental importance in solar energy conversion. By mimicking natural photosynthetic systems, artificial photosynthetic adaptations are usually based on using organic dyes and transition metal complexes as light-harvesting antennas. To this aim, donor-p-acceptor dyes are potentially used as antennas in dye sensitized solar cells (DSSCs). The advantages of organic dyes are their adjustable optical properties coupled with low production costs. Metal complexes, in particular ruthenium(II) polypyridine complexes, are widely studied because of their unique combination of chemical and physical properties. The present thesis is a theoretical investigation of the photophysical and photochemical properties of several light-harvesting antennas and photosensitisers. The first part of the thesis is devoted to study a series of donor-p-acceptor dyes, based on 4-methoxy-thiazole chromophores and ruthenium(II) polypyridine complexes with 4H-imidazole ligands. Quantum chemical and TDDFT methods have been applied to investigate photophysical properties of the dyes, special mention deserve the performed simulation of resonance Raman (RR) intensities. Based on the calculated RR spectra, protonation effects and the character of the involved excited states could be unraveled. Substitution as well as anchoring was found to be of substantial influence for the photophysical properties, such as excitation energies and excited states characters, of the ruthenium(II) complexes. To allow for applications of the dyes, as e.g. in DSSCs, knowledge of electron transfer (ET) processes occurring at the dye-semiconductor interface is necessary. Such processes can be studied by means of semi-classical Marcus theory. To this aim, a model system of a ruthenium(II) dye linked to a titanium dioxide cluster was constructed. Quantum mechanical/molecular mechanical simulations coupled with molecular dynamics have been performed in order to get the ET rate

Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways

Electron transfer reactions play a key role for artificial solar energy conversion, however, the underlying reaction mechanisms and the interplay with the molecular structure are still poorly understood due to the complexity of the reaction pathways and ultrafast timescales. In order to investigate such light-induced reaction pathways, a new spectroscopic tool has been applied, which combines UV-vis and resonance Raman spectroscopy at multiple excitation wavelengths with electrochemistry in a thin-layer electrochemical cell to study [RuII(tbtpy)2]2+ (tbtpy = tri-tert-butyl-2,2′:6′,2′′-terpyridine) as a model compound for the photo-activated electron donor in structurally related molecular and supramolecular assemblies. The new spectroscopic method substantiates previous suggestions regarding the reduction mechanism of this complex by localizing photo-electrons and identifying structural changes of metastable intermediates along the reaction cascade. This has been realized by monitoring selective enhancement of Raman-active vibrations associated with structural changes upon electronic absorption when tuning the excitation wavelength into new UV-vis absorption bands of intermediate structures. Additional interpretation of shifts in Raman band positions upon reduction with the help of quantum chemical calculations provides a consistent picture of the sequential reduction of the individual terpyridine ligands, i.e., the first reduction results in the monocation [(tbtpy)Ru(tbtpy•)]+, while the second reduction generates [(tbtpy•)Ru(tbtpy•)]0 of triplet multiplicity. Therefore, the combination of this versatile spectro-electrochemical tool allows us to deepen the fundamental understanding of light-induced charge transfer processes in more relevant and complex systems

Deep-Red Luminescent Molybdenum(0) Complexes with Bi- and Tridentate Isocyanide Chelate Ligands

In octahedral complexes, molybdenum(0) has the same 4cr valence electron configuration as ruthenium(II), which is beneficial for establishing energetically low-lying metal-to-ligand charge transfer (MLCT) excited states. Those MLCT states often show luminescence, and they can furthermore undergo photoinduced electron and energy transfer reactions that are of interest in the context of solar energy conversion, sensing, or photocatalysis. Molybdenum is roughly 100 times more abundant than ruthenium, and it seems desirable to increase our fundamental understanding of the photophysical properties of complexes made from non-precious metals. We report here on the luminescence behavior of two new homoleptic molybdenum(0) isocyanide complexes, one with three bidentate, the other with two tridentate chelate ligands. The key novelty is the incorporation of thiophene units into the ligand backbones, causing strongly red-shifted photoluminescence with respect to comparable molybdenum(0) isocyanides with phenylene units in the ligand backbones. Combined experimental and computational studies provide detailed insight into the photophysical properties of this compound class. This work is relevant for the development of new luminescent compounds with possible applications in lighting and sensing, and it complements current research efforts on photoactive complexes with other abundant transition metal and main group elements

MRSA in a large German University Hospital: Male gender is a significant risk factor for MRSA acquisition

Background: The continually rising number of hospital acquired infections and particularly MRSA (Methicillin-resistant Staphylococcus aureus) colonization poses a major challenge from both clinical and epidemiological perspectives. The assessment of risk factors is vital in determining the best prevention, diagnosis and treatment strategies

Bidentate Rh(I)-phosphine complexes for the C-H activation of alkanes: computational modelling and mechanistic insight

The C-H activation and subsequent carbonylation mediated by metal complexes, i. e., Rh(I) complexes, has drawn considerable attention in the past. To extend the mechanistic insight from Rh complexes featuring monodentate ligands like P(Me)3 towards more active bisphosphines (PLP), a computationally derived fully conclusive mechanistic picture of the Rh(I)-catalyzed C-H activation and carbonylation is presented here. Depending on the nature of the bisphosphine ligand, the highest lying transition state (TS) is associated either to the initial C-H activation in [Rh(PLP)(CO)(Cl)] or to the rearrangement of the chloride in [Rh(PLP)(H)(R)(Cl)]. The chloride rearrangement was found to play a key role in the subsequent carbonylation. A set of 20 complexes of different architectures was studied, in order to fine tune the C-H activation in a knowledge-driven approach. The computational analysis suggests that a flexible ligand architecture with aromatic rings can potentially increase the performance of Rh-based catalysts for the C-H activation

Coupling of photoactive transition metal complexes to a functional polymer matrix**

Conductive polymers represent a promising alternative to semiconducting oxide electrodes typically used in dye-sensitized cathodes as they more easily allow a tuning of the physicochemical properties. This can then also be very beneficial for using them in light-driven catalysis. In this computational study, we address the coupling of Ru-based photosensitizers to a polymer matrix by combining two different first-principles electronic structure approaches. We use a periodic density functional theory code to properly account for the delocalized nature of the electronic states in the polymer. These ground state investigations are complemented by time-dependent density functional theory simulations to assess the Franck-Condon photophysics of the present photoactive hybrid material based on a molecular model system. Our results are consistent with recent experimental observations and allow to elucidate the light-driven redox chemical processes – eventually leading to charge separation – in the present functional hybrid systems with potential application as photocathode materials

Tailored Charge Transfer Kinetics in Precursors for Organic Radical Batteries: A Joint Synthetic‐Theoretical Approach **

Abstract The development of sustainable energy storage devices is crucial for the transformation of our energy management. In this scope, organic batteries attracted considerable attention. To overcome the shortcomings of typically applied materials from the classes of redox‐active conjugated polymers (i. e., unstable cell voltages) and soft matter‐embedded stable organic radicals (i. e., low conductivity), a novel design concept was introduced, integrating such stable radicals within a conductive polymer backbone. In the present theory‐driven design approach, redox‐active (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyls (TEMPOs) were incorporated in thiophene‐based polymer model systems, while structure‐property relationships governing the thermodynamic properties as well as the charge transfer kinetics underlying the charging and discharging processes were investigated in a systematical approach. Thereby, the impact of the substitution pattern, the length as well as the nature of the chemical linker, and the ratio of TEMPO and thiophene units was studied using state‐of‐the‐art quantum chemical and quantum dynamical simulations for a set of six molecular model systems. Finally, two promising candidates were synthesized and electrochemically characterized, paving the way to applications in the frame of novel organic radical batteries.Radical approach : Molecular models of stable organic radicals incorporated in a conjugated backbone, with application in the field of organic radical batteries, are investigated by means of multiconfigurational methods. The theory‐guided design allows to tune the charge transfer kinetics as well as the underlying thermodynamics. Auspicious systems are synthesized and characterized electrochemically. imag

An extremely fast halo hot subdwarf star in a wide binary system

New spectroscopic observations of the halo hyper-velocity star candidate SDSS J121150.27+143716.2 (V = 17.92 mag) revealed a cool companion to the hot subdwarf primary. The components have a very similar radial velocity and their absolute luminosities are consistent with the same distance, confirming the physical nature of the binary, which is the first double-lined hyper-velocity candidate. Our spectral decomposition of the Keck/ESI spectrum provided an sdB+K3V pair, analogous to many long-period subdwarf binaries observed in the Galactic disk. We found the subdwarf atmospheric parameters: T_(eff) = 30 600 ± 500 K, log g = 5.57 ± 0.06 cm s^(−2), and He abundance log (nHe/nH) = - 3.0 ± 0.2. Oxygen is the most abundant metal in the hot subdwarf atmosphere, and Mg and Na lines are the most prominent spectral features of the cool companion, consistent with a metallicity of [Fe/H] = - 1.3. The non-detection of radial velocity variations suggest the orbital period to be a few hundred days, in agreement with similar binaries observed in the disk. Using the SDSS-III flux calibrated spectrum we measured the distance to the system d = 5.5 ± 0.5 kpc, which is consistent with ultraviolet, optical, and infrared photometric constraints derived from binary spectral energy distributions. Our kinematic study shows that the Galactic rest-frame velocity of the system is so high that an unbound orbit cannot be ruled out. On the other hand, a bound orbit requires a massive dark matter halo. We conclude that the binary either formed in the halo or was accreted from the tidal debris of a dwarf galaxy by the Milky Way