Carrier Tunneling from Charge Transfer States in Organic Photovoltaic Cells

Charge transfer (CT) states play a key role in the functioning of organic solar cells; however, understanding the mechanism by which CT states dissociate efficiently into free charges remain a conceptual challenge. Here, the electric field dependent dynamics of charge generation in planar cyanine/fullerene photovoltaic cells is probed over a wide temperature range using time-resolved Stark effect experiments, transient absorption, and photocurrent measurements. Results indicate that dissociation of thermalized CT states is the rate-limiting step for all temperatures. The dissociation rate strongly depends on the field, but is temperature independent. The results also suggest that the yield of generated charges is temperature independent. Model electrostatic calculations illustrate that specific orientations of the cyanine crystal relative to C₆₀ create a repulsive potential for an electron near the interface that is largely due to the quadrupole moment of the unit cell. In combination with the electron-hole coulomb attraction and the electric field-induced barrier lowering, a high-energy potential barrier forms with a narrow width of a few nanometers. It is proposed that charge separation occurs via a field-dependent electron tunneling mechanism through that barrier, which is temperature independent. The results support a thus far overlooked pathway for CT state dissociation via carrier tunneling.     

Versuchsanlage zur ermittlung der dreiachsigen festigkeit von beton mit ersten versuchsergebnissen

The testing machine for investigations on the triaxial stress-strain-behaviour of concrete consists of a rigid prestressed concrete frame with jacks of 2000 kN (compression) and 300 kN (tension) respectively. To avoid lateral restraint brush-bearing platens were used. The dimensions of the specimens were 10/10/10 cm. The first tests were carried out to gain experience in operating this apparatus. The results were as follows: The deformation under triaxial compression was much higher than that under uni- or biaxial loading. The ultimate strength increases when the stress-relation almost reaches the hydrostatic pressure (6 times or more than the uniaxial strength). The ultimate strength under repeated loading is much less than the ultimate strength under the first loading.     

Dynamics of conformational Ca2+-switches in signaling networks detected by a planar plasmonic device

Ca(2+)-sensor proteins regulate a variety of intracellular processes by adopting specific conformations in response to finely tuned changes in Ca(2+)-concentration. Here we present a surface plasmon resonance (SPR)-based approach, which allows for simultaneous detection of conformational dynamics of four Ca(2+)-sensor proteins (calmodulin, recoverin, GCAP1, and GCAP2) operating in the vertebrate phototransduction cascade, over variations in Ca(2+) concentration in the 0.1-0.6 μM range. By working at conditions that quantitatively mimic those found in the cell, we show that the method is able to detect subtle differences in the dynamics of each Ca(2+)-sensor, which appear to be influenced by the presence of free Mg(2+) at physiological concentration and by posttranslational modifications such as myristoylation. Comparison between the macroscopic Ca(2+)-binding constants, directly measured by competition with a chromophoric chelator, and the concerted binding-conformational switch detected by SPR at equilibrium reveals the relative contribution of the conformational change process to the SPR signal. This process appears to be influenced by the presence of other cations that perturb Ca(2+)-binding and the conformational transition by competing with Ca(2+), or by pure electrostatic screening. In conclusion, the approach described here allows a comparative analysis of protein conformational changes occurring under physiologically relevant molecular crowding conditions in ultrathin biosensor layers.     

Why do we need numerical methods for constrained fermion systems?

Approximative methods such as perturbation theory, mean-field caluclations or variational treatments are not reliable for many strongly interacting fermion models. Numerical methods are thus essential in the investigation of these strongly correlated fermion systems. A strong Coulomb repulsion present in these systems can often be replaced by a constraint to no-double occupancy, leading to an effective model for the low-energy excitations. These constrained systems are often easier to investigate. We review the exact algorithms mainly used to simulate constrained fermion systems, exact diagonalization, quantum Carlo and the quantum transfer matrix algorithm (QTM). We report on a new improved QTM algorithm, obtained by a combination of the QTM with the recently developed look-ahead Lanczos algorithm for non-hermitian matrices. These methods give reliable results and can answer many open questions in the field of strongly correlated electron systems.     

Spannungsoptische Untersuchung rechtwinkelig ausgeklinkter sowie lochbelasteter Biegeträger

Ohne Zusammenfassung Mitteilung aus dem Institut für Holzforschung der Universit?t München     

Citronensäure-Fermentation — biotechnologische Probleme und Möglichkeiten der Rechnersteuerung

Citric acid fermentation – biotechnological problems and possibilities of computer control . Citric acid today is produced almost exclusively by fermentation with the mold Aspergillus niger according to the surface or submerged process. The application of complex raw materials, like molasses, gives rise to considerable waste water burdens as well as to problems with microbiological process control. In order to gain qualitative and quantitative ideas about the metabolic activities of the mold during citric acid fermentation according to the submerged process production fermentors were equipped with several measuring points for the acquisition of physico-chemical changes in the medium and with devices for waste air analysis. The measured values were requested in rotation by an on-line computer and converted to characteristic derivative data. Together with additional analytical laboratory values these data were appropriate for getting a deeper insight into the biochemical events during fermentation. In this way mathematical models could be prepared which supported a better control and optimization of the fermentation course. The knowledge derived from these investigations permitted the extension of the data acquisition and computer system for monitoring and control of all production fermentors.     

Protein GB1 Folding and Assembly from Structural Elements

Folding of the Protein G B1 domain (PGB1) shifts with increasing salt concentration from a cooperative assembly of inherently unstructured subdomains to an assembly of partly pre-folded structures. The salt-dependence of pre-folding contributes to the stability minimum observed at physiological salt conditions. Our conclusions are based on a study in which the reconstitution of PGB1 from two fragments was studied as a function of salt concentrations and temperature using circular dichroism spectroscopy. Salt was found to induce an increase in β-hairpin structure for the C-terminal fragment (residues 41 – 56), whereas no major salt effect on structure was observed for the isolated N-terminal fragment (residues 1 – 41). In line with the increasing evidence on the interrelation between fragment complementation and stability of the corresponding intact protein, we also find that salt effects on reconstitution can be predicted from salt dependence of the stability of the intact protein. Our data show that our variant (which has the mutations T2Q, N8D, N37D and reconstitutes in a manner similar to the wild type) displays the lowest equilibrium association constant around physiological salt concentration, with higher affinity observed both at lower and higher salt concentration. This corroborates the salt effects on the stability towards denaturation of the intact protein, for which the stability at physiological salt is lower compared to both lower and higher salt concentrations. Hence we conclude that reconstitution reports on molecular factors that govern the native states of proteins.