Investigations on formation and specification of neural precursor cells in the central nervous system of the Drosophila melanogaster embryo

Investigations on formation and specification of neural precursor cells in the central nervous system of the Drosophila melanogaster embryoSpecification of a unique cell fate during development of a multicellular organism often is a function of its position. The Drosophila central nervous system (CNS) provides an ideal system to dissect signalling events during development that lead to cell specific patterns. Different cell types in the CNS are formed from a relatively few precursor cells, the neuroblasts (NBs), which delaminate from the neurogenic region of the ectoderm. The delamination occurs in five waves, S1-S5, finally leading to a subepidermal layer consisting of about 30 NBs, each with a unique identity, arranged in a stereotyped spatial pattern in each hemisegment. This information depends on several factors such as the concentrations of various morphogens, cell-cell interactions and long range signals present at the position and time of its birth. The early NBs, delaminating during S1 and S2, form an orthogonal array of four rows (2/3,4,5,6/7) and three columns (medial, intermediate, and lateral) . However, the three column and four row-arrangement pattern is only transitory during early stages of neurogenesis which is obscured by late emerging (S3-S5) neuroblasts (Doe and Goodman, 1985; Goodman and Doe, 1993). Therefore the aim of my study has been to identify novel genes which play a role in the formation or specification of late delaminating NBs.In this study the gene anterior open or yan was picked up in a genetic screen to identity novel and yet unidentified genes in the process of late neuroblast formation and specification. I have shown that the gene yan is responsible for maintaining the cells of the neuroectoderm in an undifferentiated state by interfering with the Notch signalling mechanism. Secondly, I have studied the function and interactions of segment polarity genes within a certain neuroectodermal region, namely the engrailed (en) expressing domain, with regard to the fate specification of a set of late neuroblasts, namely NB 6-4 and NB 7-3. I have dissected the regulatory interaction of the segment polarity genes wingless (wg), hedgehog (hh) and engrailed (en) as they maintain each other’s expression to show that En is a prerequisite for neurogenesis and show that the interplay of the segmentation genes naked (nkd) and gooseberry (gsb), both of which are targets of wingless (wg) activity, leads to differential commitment of NB 7-3 and NB 6-4 cell fate. I have shown that in the absence of either nkd or gsb one NB fate is replaced by the other. However, the temporal sequence of delamination is maintained, suggesting that formation and specification of these two NBs are under independent control.

Characterisation of supramolecular structures by novel recoupling methods in solid-state NMR

In this work, solid-state NMR methods suitable for the investigation of supramolecular systems were developed and improved. In this context, special interest was focussed on non-covalent interactions responsible for the formation of supramolecular structures, such as pi-pi interacions and hydrogen-bonds. In the first part of this work, solid-state NMR methods were presented that provide information on molecular structure and motion via the investigation of anisotropic interactions, namely quadrupole and dipole-dipole couplings, under magic-angle spinning conditions. A two-dimensional 2H double quantum experiment was developed, which is performed under off magic-angle conditions and correlates 2H isotropic chemical shifts with quasistatic DQ-filtered line shapes. From the latter, the quadrupole coupling parameters of samples deuterated at multiple sites can be extracted in a site-selective fashion. Furthermore, 7Li quadrupole parameters of lithium intercalated into TiO2 were determined by NMR experiments performed under static and MAS conditions, and could provide information on the crystal geometry. For the determination of 7Li-7Li dipole-dipole couplings, multiple-quantum NMR experiments were performed. The 1H-13C REREDOR experiment was found to be capable of determining strong proton-carbon dipole-dipole couplings with an accuracy of 500~Hz, corresponding to a determination of proton-carbon chemical-bond lengths with picometer accuracy In the second part of this work, solid-state NMR experiments were combined with quantum-chemical calculations in order to aid and optimise the interpretation of experimental results. The investigations on Calix[4]hydroquinone nanotubes have shown that this combined approach can provide information on the presence of disordered and/or mobile species in supramolecular structures. As a second example, C3-symmetric discs arranging in helical columnar stacks were investigated. In these systems, 1H chemical shifts experience large pi-shifts due to packing effects, which were found to be long-ranged. Moreover, quantum-chemical calculations revealed that helicity in these systems is induced by the propeller-like conformation of the core of the molecules.

Surface layer formation in polymer melts and in solutions

In this work the surface layer formation in polymer melts and in polymer solutions have been investigated with the atomic force microscope (AFM). In polymer melts, the formation of an immobile surface layer results in a steric repulsion, which can be measured by the AFM. From former work it is know, that polydimethyl siloxane (PDMS) forms a stable surface layer for molecular weights above 12 kDa. In the present thesis, polyisoprene (PI) was investigated apart from PDMS, by a)measuring the steric surface interactions and b)measuring the surface slip in hydrodynamic experiments. If a polymer flows over a surface, the flow velocity at the surface is larger then zero. If case of a surface layer formation the flow plane changes to the top of the adsorbed layer and the surface slip is reduced to zero. By measuring the surface slip in hydrodynamic experiments, it is therefore possible to determine the presence of a stable surface layer. The results show no stable repulsion for PI and only a small decrease of the surface slip. This indicates that PI does not form a stable surface layer, but is only adsorbed weakly to the surface. Furthermore for 8 kDa PDMS the timescale of the formation of a surface layer was investigated by changing themaximal force the tip applied to the surface. With a repulsive force present, applying a higher force than 15 nN resulted in a destruction of the surface layer, indicated by attractive forces. Reducing the applied force below 15 nN then resulted in an increase of the repulsion to the former state during one minute, thus indicating that a surface layer can be formed within one minute even under the influence of continuous measurements. As a next step, mixtures of two PDMS homopolymers with different chain lengths have been investigated. The aim was to verify theoretical predictions that shorter chains should predominate at the surface due to their smaller loss in conformational entropy. The measurements where done in dependence of the volume fractions of short and long chain PMDS. The results confirmed the short chain dominance for all mixtures with less then 90 vol.% long chain PDMS. Surface layer formation in solution was investigated for superplasticizers which are industrially used as an additive to cement. They change the surface interaction between the cement grains from attractive to repulsive and the freshlymixed cement paste therefore becomes liquid. The aimin this part of the thesis was, to investigate cement particle interactions in a close to real environment. Therefore calcium silicate hydrate phases have been precipitated onto an AFM tip and onto a calcite crystal and the interaction between these surfaces have beenmeasured with and without addition of superplasticizers. The measurements confirmed the change from attraction to repulsion upon addition of superplasticizers. The repulsive steric interaction increased with the length of the sidechain of the superplasticizer, and the dependence of the range of the steric interactions on the sidechain length indicated that the sidechains are in a coiled conformation.

Structural transformations related to organic solid-state reactions: correlation studies of NMR and X-ray analysis

Eine zielgerichtete Steuerung und Durchführung von organischen Festkörperreaktionen wird unter anderem durch genaue Kenntnis von Packungseffekten ermöglicht. Im Rahmen dieser Arbeit konnte durch den kombinierten Einsatz von Einkristallröntgenanalyse und hochauf-lösender Festkörper-NMR an ausgewählten Beispielen ein tieferes Verständnis und Einblicke in die Reaktionsmechanismen von organischen Festkörperreaktionen auf molekularer Ebene gewonnen werden. So konnten bei der topotaktischen [2+2] Photodimerisierung von Zimt-säure Intermediate isoliert und strukturell charakterisiert werden. Insbesondere anhand statischer Deuteronen- und 13C-CPMAS NMR Spektren konnten eindeutig dynamische Wasserstoffbrücken nachgewiesen werden, die transient die Zentrosymmetrie des Reaktions-produkts aufheben. Ein weiterer Nachweis gelang daraufhin mittels Hochtemperatur-Röntgen-untersuchung, sodass der scheinbare Widerspruch von NMR- und Röntgenuntersuchungen gelöst werden konnte. Eine Veresterung der Zimtsäure entfernt diese Wasserstoffbrücken und erhält somit die Zentrosymmetrie des Photodimers. Weiterhin werden Ansätze zur Strukturkontrolle in Festkörpern basierend auf der molekularen Erkennung des Hydroxyl-Pyridin (OH-N) Heterosynthon in Co-Kristallen beschrieben, wobei vor allem die Stabilität des Synthons in Gegenwart funktioneller Gruppen mit Möglichkeit zu kompetetiver Wasserstoffbrückenbildung festgestellt wurde. Durch Erweiterung dieses Ansatzes wurde die molekulare Spezifität des Hydroxyl-Pyridin (OH-N) Heterosynthons bei gleichzeitiger Co-Kristallisation mit mehreren Komponenten erfolgreich aufgezeigt. Am Beispiel der Co-Kristallisation von trans--1,2-bis(4-pyridyl)ethylen (bpe) mit Resorcinol (res) in Gegenwart von trans-1,2-bis(4-pyridyl)ethan (bpet) konnten Zwischenprodukte der Fest-körperreaktionen und neuartige Polymorphe isoliert werden, wobei eine lückenlose Aufklärung des Reaktionswegs mittels Röntgenanalyse gelang. Dabei zeigte sich, dass das Templat Resorcinol aus den Zielverbindungen entfernbar ist. Ferner gelang die Durchführung einer seltenen, nicht-idealen Einkristall-Einkristall-Umlagerung von trans--1,2-bis(4-pyridyl)ethylen (bpe) mit Resorcinol (res). In allen Fällen konnten die Fragen zur Struktur und Dynamik der untersuchten Verbindungen nur durch gemeinsame Nutzung von Röntgenanalyse und NMR-Spektroskopie bei vergleichbaren Temperaturen eindeutig und umfassend geklärt werden.

Solid-state NMR investigation of phosphonic acid based proton conducting materials

In this work, new promising proton conducting fuel cell membrane materials were characterized in terms of their structure and dynamic properties using solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction. Structurally different, phosphonic acid (PA) containing materials were systematically evaluated for possible high-temperature operation (e.g. at T>100°C). Notably, 1H, 2H and 31P magic angle spinning (MAS) NMR provided insight into local connectivities and dynamics of the hydrogen bonded network, while packing arrangements were identified by means of heteronuclear dipolar recoupling techniques.rnThe first part of this work introduced rather crystalline, low molecular weight ionomers for proton conducting membranes, where six different geometries such as line, triangle, screw, tetrahedron, square and hexagon, were investigated. The hexagon was identified as the most promising geometry with high-temperature bulk proton conductivities in the range of 10-3 Scm-1 at a relative humidity of 50%. However, 2H NMR and TGA-MS data suggest that the bulk proton transport is mainly due to the presence of crystal water. Single crystal X-ray data revealed that in the tetrahedron phosphonic acids form tetrameric clusters isolating the mobile protons while the phosphonic acids in the hexagon form zigzag-type pathways through the sample.rnThe second part of this work demonstrates how acid-base pairing and the choice of appropriate spacers may influence proton conduction. Different ratios of statistical copolymers of poly (vinylphosphonic acid) and poly (4-vinylpyridine) were measured to derive information about the local structure and chemical changes. Though anhydrous proton conductivities of all statistical copolymers are rather poor, the conductivity increases to 10-2 S cm-1 when exposing the sample to relative humidity of 80%. In contrast to PVPA, anhydride formation of phosphonic acids in the copolymer is not reversible even when exposing the sample to a relative humidity of 100%.rnIn addition, the influence of both spacers and degree of backbone crystallinity on bulk proton conductivity was investigated. Unlike in systems such as poly benzimidazole (PBI), spacers were inserted between the protogenic groups along the backbone. It was found that dilution of the protogenic groups decreases the conductivity, but compared to PVPA, similar apparent activation energies for local motions were obtained from both variable temperature 1H NMR and impedance spectroscopy data. These observations suggest the formation of phosphonic acid clusters with high degrees of local proton motion, where only a fraction of motions contribute to the observable bulk proton conductivity. Additionally, it was shown that gradual changes of the spacer length lead to different morphologies.rnIn summary, applying advanced solid-state NMR and X-ray analysis, structural and dynamic phenomena in proton conducting materials were identified on a molecular level. The results were discussed with respect to different proton conduction mechanisms and may contribute to a more rational design or improvement of proton conducting membranes.rn

Solid-state NMR on columnar supramolecular structures

The present work deals with the characterisation of three columnar self-assembled systems, that is, benzene-1,3,5-tricarboxamides, a peripherally thioalkyl-substituted phthalocyanine, and several oligo-(p-phenylenevinylene)s. In order to probe the supramolecular organisation solid-state NMR has been used as the main technique, supported by X-ray measurements, theoretical methods, and thermal analysis. rnrnBenzene-1,3,5-tricarboxamides (BTAs) turned out to be well suited model compounds to study various fundamental supramolecular interactions, such as π-π-interactions, hydrogen bonding, as well as dynamic and steric effects of attached side chains. Six BTAs have been investigated in total, five with a CO-centred amide group bearing different side chains and one with an inverted N-centred amide group. The physical properties of these BTAs have been investigated as a function of temperature. The results indicated that in case of the CO-centred BTAs the stability of the columnar mesophase depends strongly on the nature of the side chains. Further experiments revealed a coplanar orientation of adjacent BTA molecules in the columnar assembly of CO-centred BTAs, whereas the N-centred BTA, showed a deviating not fully coplanar arrangement. These differences were ascribed to distinct hydrogen bonding schemes, involving a parallel alignment of hydrogen bonds in case of CO-centred BTAs and an antiparallel alignment in case of the N-centred counterpart.rnrn The fundamental insights of the supramolecular organisation of BTAs could be partially adapted to an octa-substituted phthalocyanine with thiododecyl moieties. Solid-state NMR in combination with chemical shift calculations determined a tilted herringbone arrangement of phthalocyanine rings in the crystalline phase as well as in the mesophase. Moreover, 1H NMR measurements in the mesophase of this compound suggested an axial rotation of molecules, which is inhibited in the crystalline phase.rnrnAs a third task, the supramolecular assembly of oligo-(p-phenylenevinylene)s of varying length and with different polar head groups have been investigated by a combined X-ray and solid-state NMR study. The results revealed a columnar structure formation of these compounds, being promoted by phase separation of alkyl side chains and aromatic rigid rods. In this system solid-state NMR yielded meaningful insight into the isotropisation process of butoxy and 2-S-methylbutoxy substituted oligo-(p-phenylenevinylene) rods.rn

Interplay of pi–pi stacking and hydrogen bonding in conjugated supramolecular systems studied by solid-state NMR

In this work supramolecular organic systems based on rigid pi-conjugated building blocks and flexible side chains were studied via solid-state NMR spectroscopy. Specifically, these studies focussed on phenylene ethynylene based macrocycles, polymer systems including polythiophenes, and rod-coil copolymers of oligo(p-benzamide) and poly(ethylene glycol). All systems were studied in terms of the local order and mobility. The central topic of this dissertation was to elucidate the role of the flexible side chains in interplay of different non-covalent interactions, like pi-pi-stacking and hydrogen bonding.Combining the results of this work, it can be concluded that the ratio of the rigid block and the attached alkyl side chains can be crucial for the design of an ordered pi-conjugated supramolecular system. Through alkyl side chains, it is also possible to introduce liquid-crystalline phases in the system, which can foster the local order of the system. Moreover in the studied system longer, unbranched alkyl side chains are better suited to stabilize the corresponding aggregation than shorter, branched ones.The combination of non-covalent interactions such as pi-pi-stacking and hydrogen bonding play an important role for structure formation. However, the effect of pi-pi-stacking interaction is much weaker than the effect of hydrogen bonding and is only observed in systems with a suitable local order. Hence, they are often not strong enough to control the local order. In contrast, hydrogen bonds predominantly influence the structural organization and packing. In comparison the size of the alkyl side chains is only of minor importance. The suppression of certain hydrogen bonds can lead to completely different structures and can induce a specific aggregation behavior. Thus, for the design of a supramolecular ordered system the presence of hydrogen bonding efficiently stabilizes the corresponding structure, but the ratio of hydrogen bond forming groups should be kept low to be able to influence the structure selectively.

A soft-tetramer model for diblock copolymer melts: structure formation and geometric characterization

The ability of block copolymers to spontaneously self-assemble into a variety of ordered nano-structures not only makes them a scientifically interesting system for the investigation of order-disorder phase transitions, but also offers a wide range of nano-technological applications. The architecture of a diblock is the most simple among the block copolymer systems, hence it is often used as a model system in both experiment and theory. We introduce a new soft-tetramer model for efficient computer simulations of diblock copolymer melts. The instantaneous non-spherical shape of polymer chains in molten state is incorporated by modeling each of the two blocks as two soft spheres. The interactions between the spheres are modeled in a way that the diblock melt tends to microphase separate with decreasing temperature. Using Monte Carlo simulations, we determine the equilibrium structures at variable values of the two relevant control parameters, the diblock composition and the incompatibility of unlike components. The simplicity of the model allows us to scan the control parameter space in a completeness that has not been reached in previous molecular simulations.The resulting phase diagram shows clear similarities with the phase diagram found in experiments. Moreover, we show that structural details of block copolymer chains can be reproduced by our simple model.We develop a novel method for the identification of the observed diblock copolymer mesophases that formalizes the usual approach of direct visual observation,using the characteristic geometry of the structures. A cluster analysis algorithm is used to determine clusters of each component of the diblock, and the number and shape of the clusters can be used to determine the mesophase.We also employ methods from integral geometry for the identification of mesophases and compare their usefulness to the cluster analysis approach.To probe the properties of our model in confinement, we perform molecular dynamics simulations of atomistic polyethylene melts confined between graphite surfaces. The results from these simulations are used as an input for an iterative coarse-graining procedure that yields a surface interaction potential for the soft-tetramer model. Using the interaction potential derived in that way, we perform an initial study on the behavior of the soft-tetramer model in confinement. Comparing with experimental studies, we find that our model can reflect basic features of confined diblock copolymer melts.

Cellular localization and mechanism of amyloid precursor protein (APP) homodimer formation in an oxidizing environment

The amyloid precursor protein (APP) is a type I transmembrane glycoprotein, which resembles a cell surface receptor, comprising a large ectodomain, a single spanning transmembrane part and a short C-terminal, cytoplasmic domain. It belongs to a conserved gene family, with over 17 members, including also the two mammalian APP homologues proteins APLP1 and APLP2 („amyloid precursor like proteins“). APP is encoded by 19 exons, of which exons 7, 8, and 15 can be alternatively spliced to produce three major protein isoforms APP770, APP751 and APP695, reflecting the number of amino acids. The neuronal APP695 is the only isoform that lacks a Kunitz Protease Inhibitor (KPI) domain in its extracellular portion whereas the two larger, peripheral APP isoforms, contain the 57-amino-acid KPI insert. rnRecently, research effort has suggested that APP metabolism and function is thought to be influenced by homodimerization and that the oligomerization state of APP could also play a role in the pathology of Alzheimer's disease (AD), by regulating its processing and amyloid beta production. Several independent studies have shown that APP can form homodimers within the cell, driven by motifs present in the extracellular domain, as well as in the juxtamembrane (JM) and transmembrane (TM) regions of the molecule, whereby the exact molecular mechanism and the origin of dimer formation remains elusive. Therefore, we focused in our study on the actual subcellular origin of APP homodimerization within the cell, an underlying mechanism, and a possible impact on dimerization properties of its homologue APLP1. Furthermore, we analyzed homodimerization of various APP isoforms, in particular APP695, APP751 and APP770, which differ in the presence of a Kunitz-type protease inhibitor domain (KPI) in the extracellular region. In order to assess the cellular origin of dimerization under different cellular conditions, we established a mammalian cell culture model-system in CHO-K1 (chinese hamster ovary) cells, stably overexpressing human APP, harboring dilysine based organelle sorting motifs at the very C-terminus [KKAA-Endoplasmic Reticulum (ER); KKFF-Golgi]. In this study we show that APP exists as disulfide-bound, SDS-stable dimers, when it was retained in the ER, unlike when it progressed further to the cis-Golgi, due to the KKFF ER exit determinant. These stable APP complexes were isolated from cells, and analyzed by SDS–polyacrylamide gel electrophoresis under non-reducing conditions, whereas strong denaturing and reducing conditions completely converted those dimers to monomers. Our findings suggested that APP homodimer formation starts early in the secretory pathway and that the unique oxidizing environment of the ER likely promotes intermolecular disulfide bond formation between APP molecules. We particularly visualized APP dimerization employing a variety of biochemical experiments and investigated the origin of its generation by using a Bimolecular Fluorescence Complementation (BiFC) approach with split GFP-APP chimeras. Moreover, using N-terminal deletion constructs, we demonstrate that intermolecular disulfide linkage between cysteine residues, exclusively located in the extracellular E1 domain, represents another mechanism of how an APP sub-fraction can dimerize within the cell. Additionally, mutational studies revealed that cysteines at positions 98 and 105, embedded in the conserved loop region within the E1 domain, are critical for interchain disulfide bond formation. Using a pharmacological treatment approach, we show that once generated in the oxidative environment of the ER, APP dimers remain stably associated during transport, reaching the plasma membrane. In addition, we demonstrate that APP isoforms, encompassing the KPI domain, exhibit a strongly reduced ability to form cis-directed dimers in the ER, whereas trans-directed cell aggregation of Drosophila Schneider (S2)-cells was isoform independent, mediating cell-cell contacts. Thus, suggesting that steric properties of KPI-APP might be the cause for weaker cis-interaction in the ER, compared to APP695. Finally, we provide evidence that APP/APLP1 heterointeractions are likewise initiated in the ER, suggesting a similar mechanism for heterodimerization. Therefore, dynamic alterations of APP between monomeric, homodimeric, and possibly heterodimeric status could at least partially explain some of the variety in the physiological functions of APP.rn

A photophysical study of excited state dynamics in donor-acceptor copolymer photovoltaic blends

This thesis deals with the investigation of charge generation and recombination processes in three different polymer:fullerene photovoltaic blends by means of ultrafast time-resolved optical spectroscopy. The first donor polymer, namely poly[N-11"-henicosanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), is a mid-bandgap polymer, the other two materials are the low-bandgap donor polymers poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT) and poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT). Despite their broader absorption, the low-bandgap polymers do not show enhanced photovoltaic efficiencies compared to the mid-bandgap system.rnrnTransient absorption spectroscopy revealed that energetic disorder plays an important role in the photophysics of PCDTBT, and that in a blend with PCBM geminate losses are small. The photophysics of the low-bandgap system PCPDTBT were strongly altered by adding a high boiling point cosolvent to the polymer:fullerene blend due to a partial demixing of the materials. We observed an increase in device performance together with a reduction of geminate recombination upon addition of the cosolvent. By applying model-free multi-variate curve resolution to the spectroscopic data, we found that fast non-geminate recombination due to polymer triplet state formation is a limiting loss channel in the low-bandgap material system PCPDTBT, whereas in PSBTBT triplet formation has a smaller impact on device performance, and thus higher efficiencies are obtained.rn

Excited-state dynamics in donor-acceptor systems for energy conversion

In dieser Arbeit werden die Dynamiken angeregter Zustände in Donor-Akzeptorsystemen für Energieumwandlungsprozesse mit ultraschneller zeitaufgelöster optischer Spektroskopie behandelt. Der Hauptteil dieser Arbeit legt den Fokus auf die Erforschung der Photophysik organischer Solarzellen, deren aktive Schichten aus diketopyrrolopyrrole (DPP) basierten Polymeren mit kleiner Bandlücke als Elektronendonatoren und Fullerenen als Elektronenakzeptoren bestehen. rnEin zweiter Teil widmet sich der Erforschung von künstlichen primären Photosynthesereaktionszentren, basierend auf Porphyrinen, Quinonen und Ferrocenen, die jeweils als Lichtsammeleinheit, Elektronenakzeptor beziehungsweise als Elektronendonatoren eingesetzt werden, um langlebige ladungsgetrennte Zustände zu erzeugen.rnrnZeitaufgelöste Photolumineszenzspektroskopie und transiente Absorptionsspektroskopie haben gezeigt, dass Singulettexzitonenlebenszeiten in den Polymeren PTDPP-TT und PFDPP-TT Polymeren kurz sind (< 20 ps) und dass in Mischungen der Polymere mit PC71BM geminale Rekombination von gebundenen Ladungstransferzuständen ein Hauptverlustkanal ist. Zudem wurde in beiden Systemen schnelle nichtgeminale Rekombination freier Ladungen zu Triplettzuständen auf dem Polymer beobachtet. Für das Donor-Akzeptor System PDPP5T:PC71BM wurde nachgewiesen, dass die Zugabe eines Lösungsmittels mit hohem Siedepunkt, und zwar ortho-Dichlorbenzol, die Morphologie der aktiven Schicht stark beeinflusst und die Solarzelleneffizienz verbessert. Der Grund hierfür ist, dass die Donator- und Akzeptormaterialien besser durchmischt sind und sich Perkolationswege zu den Elektroden ausgebildet haben, was zu einer verbesserten Ladungsträgergeneration und Extraktion führt. Schnelle Bildung des Triplettzustands wurde in beiden PDPP5T:PC71BM Systemen beobachtet, da der Triplettzustand des Polymers über Laungstransferzustände mit Triplettcharakter populiert werden kann. "Multivariate curve resolution" (MCR) Analyse hat eine starke Intensitätsabhängigkeit gezeigt, was auf nichtgeminale Ladungsträgerrekombination in den Triplettzustand hinweist.rnrnIn den künstlichen primären Photosynthesereaktionszentren hat transiente Absorptionsspektroskopie bestätigt, dass photoinduzierter Ladungstransfer in Quinon-Porphyrin (Q-P) und Porphyrin-Ferrocen (P-Fc) Diaden sowie in Quinon-Porphyrin-Ferrocen (Q-P-Fc) Triaden effizient ist. Es wurde jedoch auch gezeigt, dass in den P-Fc unf Q-P-Fc Systemen die ladungsgetrennten Zustände in den Triplettzustand der jeweiligen Porphyrine rekombinieren. Der ladungsgetrennte Zustand konnte in der Q-P Diade durch Zugabe einer Lewissäure signifikant stabilisiert werden.