The photostability of wool doped with photocatalytic titanium dioxide

Photoyellowing of wool is a serious problem for the wool industry. This study assessed the role of photocatalytic nanocrystalline titanium dioxide (P-25) as a potential antagonist or catalyst in the photoyellowing of wool. Untreated, bleached and bleached and fluorescent-whitened wool slivers were processed into fine wool powders for the purpose of even and intimate mixing with the TiO2 nanoparticles in the solid state. Pure wool and wool/TiO2 mixtures were then compressed into solid discs for a photoyellowing study under simulated sunlight and under UVB and UVC radiations. Yellowness and photo-induced chemiluminescence (PICL) measurements showed that nanocrystalline TiO2 could effectively reduce the rate of photoyellowing by inhibiting free radical generation in doped wool, and that a higher concentration of TiO2 contributed to a lower rate of photooxidation and reduced photoyellowing. Hence nanocrystalline TiO2 acts primarily as a UV absorber on wool in dry conditions and not as a photocatalyst.

Improving the photostability of bleached wool without increasing its yellowness

Bleached wool is rapidly yellowed by exposure to the UV radiation present in sunlight. The conventional application of a water-soluble hydroxyphenyl benzotriazole UV absorber (such as UVFast W) to bleached wool reduces its rate of photoyellowing but has a negative impact on the whiteness of the bleached wool, largely cancelling out the improvements in whiteness achieved during bleaching. However, if the UV absorber is applied to peroxide-bleached wool from a reductive bleach bath, white wool with improved photostability to sunlight and UV radiation can be obtained.

Degradação fotoquímica de hidrocarbonetos da gasolina em efluentes aquosos

Photo-oxidation processes of toxic organic compounds have been widely studied. This work seeks the application of the photo-Fenton process for the degradation of hydrocarbons in water. The gasoline found in the refinery, without additives and alcohol, was used as the model pollutant. The effects of the concentration of the following substances have been properly evaluated: hydrogen peroxide (100-200 mM), iron ions (0.5-1 mM) and sodium chloride (200 2000 ppm). The experiments were accomplished in reactor with UV lamp and in a falling film solar reactor. The photo-oxidation process was monitored by measurements of the absorption spectra, total organic carbon (TOC) and chemical oxygen demand (COD). Experimental results demonstrated that the photo-Fenton process is feasible for the treatment of wastewaters containing aliphatic hydrocarbons, inclusive in the presence of salts. These conditions are similar to the water produced by the petroleum fields, generated in the extraction and production of petroleum. A neural network model of process correlated well the observed data for the photooxidation process of hydrocarbons

Estudo de titanatos nanoestruturados obtidos por tratamento hidrotérmico de óxido de titânio em meio alcalino

TiTanate NanoTubes (TTNT) were synthesized by hydrothermal alkali treatment of TiO2 anatase followed by repeated washings with distinct degrees of proton exchange. TTNT samples with different sodium contents were characterized, as synthesized and after heattreatment (200-800ºC), by X-ray diffraction, scanning and transmission electron microscopy, electron diffraction, thermal analysis, nitrogen adsorption and spectroscopic techniques like FTIR and UV-Vis diffuse reflectance. It was demonstrated that TTNTs consist of trititanate structure with general formula NaxH2−xTi3O7·nH2O, retaining interlayer water in its multiwalled structure. The removal of sodium reduces the amount of water and contracts the interlayer space leading, combined with other factors, to increased specific surface area and mesopore volume. TTNTs are mesoporous materials with two main contributions: pores smaller than 10 nm due to the inner volume of nanotubes and larger pores within 5-60 nm attributed to the interparticles space. Chemical composition and crystal structure of TTNTs do not depend on the average crystal size of the precursor TiO2-anatase, but this parameter affects significantly the morphology and textural properties of the nanostructured product. Such dependence has been rationalized using a dissolution-recrystallization mechanism, which takes into account the dissolution rate of the starting anatase and its influence on the relative rates of growth and curving of intermediate nanosheets. The thermal stability of TTNT is defined by the sodium content and in a lower extent by the crystallinity of the starting anatase. It has been demonstrated that after losing interlayer water within the range 100-200ºC, TTNT transforms, at least partially, into an intermediate hexatitanate NaxH2−xTi6O13 still retaining the nanotubular morphology. Further thermal transformation of the nanostructured tri- and hexatitanates occurs at higher or lower temperature and follows different routes depending on the sodium content in the structure. At high sodium load (water washed samples) they sinter and grow towards bigger crystals of Na2Ti3O7 and Na2Ti6O13 in the form of rods and ribbons. In contrast, protonated TTNTs evolve to nanotubes of TiO2(B), which easily convert to anatase nanorods above 400ºC. Besides hydroxyls and Lewis acidity typical of titanium oxides, TTNTs show a small contribution of protonic acidity capable of coordinating with pyridine at 150ºC, which is lost after calcination and conversion into anatase. The isoeletric point of TTNTs was measured within the range 2.5-4.0, indicating behavior of a weak acid. Despite displaying semiconductor characteristics exhibiting typical absorption in the UV-Vis spectrum with estimated bandgap energy slightly higher than that of its TiO2 precursor, TTNTs showed very low performance in the photocatalytic degradation of cationic and anionic dyes. It was concluded that the basic reason resides in its layered titanate structure, which in comparison with the TiO2 form would be more prone to the so undesired electron-hole pair recombination, thus inhibiting the photooxidation reactions. After calcination of the protonated TTNT into anatase nanorods, the photocatalytic activity improved but not to the same level as that exhibited by its precursor anatase

Nanoscale manipulation of CdSe quantum dots in layer-by-layer films: influence of the host polyelectrolyte on the luminescent properties

Thioglycolic acid-capped Use quantum dots (QDs) were assembled on glass substrates with two distinct polyelectrolytes, viz poly(allylamine hydrochloride) (PAH) and poly(amidoamine) (PAMAM), generation 4 dendrimer, via the layer-by-layer (LbL) technique. Films containing up to 30 polyelectrolyte/QD bilayers were prepared. The growth of the multilayers was monitored with UV-vis spectroscopy, which showed an almost linear increase in the absorbance of the 2.8 nm QDs at 535 nm with the number of deposited bilayers. AFM measurements estimated a film thickness of 3 nm per bilayer for the PAH/Cdse films. The adsorption process and the optical properties of the PAMAM/CdSe LbL films were further analyzed layer-by-layer using surface plasmon resonance (SPR), from which a thickness of 3.2 nm was found for a PAMAM/CdSe bilayer. Photoluminescence measurements revealed higher photooxidation of the quantum dots in PAH/CdSe than in PAMAM/CdSe films. (c) 2004 Elsevier B.V. All rights reserved.

Photo-oxidation of polyphenylenevinylene chains in Langmuir-Blodgett and cast films

Poly(p-phenylene vinylene) (PPV) derivatives are well known for their applications in polymer light emitting diodes (PLEDs). These derivatives are highly susceptible to photooxidation though, which is mainly caused by the scission of the vinyl double bond on the polymer backbone. In this work, we show that Langmuir-Blodgett (LB) films are less degraded than cast films of a PPV derivative (OC1OC6-PPV). Both films had similar thickness (∼50 nm) to allow for a more realistic comparison. Photodegradation experiments were carried out by illuminating the films with white light from a halogen lamp (50W, 12 V), placed at a fixed dstance from the sample. The decay was monitored by UV-Vis and FTIR spectroscopies. The results showed that cast films are completely degraded in ca. 300 min, while LB took longer times, ca. 1000 min, i.e. 3 times the values for the cast films. The degradation process occurs in at least two stages, the rates of which were calculated assuming that the reaction follows a first order kinetics. The characteristic times for the first stage were 3.6×10-2 and 1.3×10-3 min-1 for cast and LB films, respectively. For the second stage the characteristic times were 5.6×10-2 and 5.0×10 -3 min-1. The differences can be attributed to the more compact morphology in the LB than in the cast films. With a compact morphology the diffusion of oxygen in the LB film is hampered and this causes a delay in the degradation process.

Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media

This review focuses on the heterogeneous photocatalytic treatment of organic dyes in air and water. Representative studies spanning approximately three decades are included in this review. These studies have mostly used titanium dioxide (TiO2) as the inorganic semiconductor photocatalyst of choice for decolorizing and decomposing the organic dye to mineralized products. Other semiconductors such as ZnO, CdS, WO3, and Fe2O3 have also been used, albeit to a much smaller extent. The topics covered include historical aspects, dark adsorption of the dye on the semiconductor surface and its role in the subsequent photoreaction, semiconductor preparation details, photoreactor configurations, photooxidation kinetics/mechanisms and comparison with other Advanced Oxidation Processes (e.g., UV/H2O2, ozonation, UV/O3, Fenton and photo-Fenton reactions), visible light-induced dye decomposition by sensitization mechanism, reaction intermediates and toxicity issues, and real-world process scenarios. © 2008 Elsevier B.V. All rights reserved.

Evaluation of weather influence on mechanical and viscoelastic properties of polyetherimide/carbon fiber composites

In order to investigate how environmental degradation affects the mechanical and thermal performance of polyetherimide/carbon fiber laminates, in this work different weathering were conducted. Additionally, dynamic mechanical analysis, interlaminar shear strength tests and non-destructive inspections were performed on this composite before and after being submitted to hygrothermal, UV radiation and thermal shock weathering. According to our results, hygrothermally aged samples had their glass transition temperature and elastic and storage moduli reduced by plasticization effect. Photooxidation, due to UV radiation exposure, occurred only on the surface of the laminates. Thermal shock induced a reversible stress on the composite's interface region. The results revealed that the mechanical behavior can vary during weather exposure but since this variation is only subtle, this thermoplastic laminate can be considered for high-performance applications, such as aerospace. © The Author(s) 2013.

Avaliação do método de fotooxidação do DiI por diodo emissor de luz (led): aspectos morfológicos de células horizontais da retina humana

A análise da morfologia celular é um aspecto crucial da neurobiologia, pois a relação entre forma e função pode definir os processos fisiológicos na saúde e na doença. Um dos principais métodos para avaliar a morfologia celular é por meio da fotooxidação de marcadores fluorescentes intracelulares, dentre os quais se tem o percloreto de 1,1’-dioctadecil-3,3,3’,3’-tetrametil-indocarbocianina (DiI), uma carbocianina lipofílica que pode marcar células vivas ou fixadas. O DiI foi escolhido para este trabalho devido, dentre outros fatores, à sua importante utilização para estudos de morfologia celular. Como modelo para avaliar a qualidade da fotooxidação do aparelho construído para essa finalidade, elegeu-se as células horizontais da retina humana com o intuito de prosseguir com estudo morfométrico posterior dessas células, tendo em vista a escassez de trabalhos com essa abordagem em retina humana. Assim, este estudo teve como objetivo avaliar a qualidade do método de fotooxidação do DiI através de LED e utilizando como modelo células horizontais da retina humana. O material foi obtido do Banco de Olhos do Hospital Ophir Loyola e, em sequência dissecado, marcado com cristais de DiI e fotooxidado com o aparelho de LED. As imagens que resultaram do novo método de iluminação para fotooxidação de traçador fluorescente apresentou elevada qualidade de detalhes da morfologia neuronal, similares aos resultados obtidos em reações de fotoconversão convencional com microscópio, o que permite concluir que o aparelho mantém a eficiência da fotooxidação por revelar detalhes finos da morfologia celular, inclusive com as vantagens de processar áreas maiores de tecido e considerável redução de custo por dispensar o emprego de microscópio para o processo.

Within-plant isoprene oxidation confirmed by direct emissions of oxidation products methyl vinyl ketone and methacrolein

Isoprene is emitted from many terrestrial plants at high rates, accounting for an estimated 1/3 of annual global volatile organic compound emissions from all anthropogenic and biogenic sources combined. Through rapid photooxidation reactions in the atmosphere, isoprene is converted to a variety of oxidized hydrocarbons, providing higher order reactants for the production of organic nitrates and tropospheric ozone, reducing the availability of oxidants for the breakdown of radiatively active trace gases such as methane, and potentially producing hygroscopic particles that act as effective cloud condensation nuclei. However, the functional basis for plant production of isoprene remains elusive. It has been hypothesized that in the cell isoprene mitigates oxidative damage during the stress-induced accumulation of reactive oxygen species (ROS), but the products of isoprene-ROS reactions in plants have not been detected. Using pyruvate-2-13C leaf and branch feeding and individual branch and whole mesocosm flux studies, we present evidence that isoprene (i) is oxidized to methyl vinyl ketone and methacrolein (iox) in leaves and that iox/i emission ratios increase with temperature, possibly due to an increase in ROS production under high temperature and light stress. In a primary rainforest in Amazonia, we inferred significant in plant isoprene oxidation (despite the strong masking effect of simultaneous atmospheric oxidation), from its influence on the vertical distribution of iox uptake fluxes, which were shifted to low isoprene emitting regions of the canopy. These observations suggest that carbon investment in isoprene production is larger than that inferred from emissions alone and that models of tropospheric chemistry and biotachemistryclimate interactions should incorporate isoprene oxidation within both the biosphere and the atmosphere with potential implications for better understanding both the oxidizing power of the troposphere and forest response to climate change.

Zeitaufgelöste Messung der Faltung und Pigmentbindung des Lichtsammlerproteins LHCIIb anhand verschiedener spektroskopischer Monitore

Über die Biogenese des Lichtsammelkomplexes des Photosystems II höherer Pflanzen (LHCII) in der Thylakoidmembran der Chloroplasten existieren wenige Daten. Deswegen soll die Aufklärung des Faltungsmechanismus in vitro anhand von zeitaufgelösten Messungen der Rückfaltung des Komplexes Rückschlüsse auf die Situation in vivo ermöglichen.Zur Beobachtung der Rückfaltung wurden Methoden der Fluoreszenz- und CD-Spektroskopie verwendet. Die Pigmentbindung und die Ausbildung von α-helikaler Sekundärstruktur erfolgt in einem schnelleren und einem langsameren apparenten Schritt (Sekunden und Minuten); beide Vorgänge sind eng gekoppelt und limitiert durch die Bindung der Carotinoide. In der schnelleren Phase ist die Bindung von Chl a und Lutein ausreichend für die Zunahme an α-helikaler Struktur. Ein thermodynamisch stabiler Komplex erfordert die Bindung von Chl b und Carotinoiden. In der schnellen Phase bindet Chl a vor Chl b und Lutein mindestens so schnell wie Chl b; beide Pigmente limitieren die Bindung von Chl b. Chl b ist notwendig für die Ereignisse der langsameren Phase.Bzgl. der Situation in vivo deuten die Daten auf (1) eine aktive Rolle der Pigmentbindung für die Membraninsertion des Proteins, (2) einen Schutz vor Photooxidation der Chlorophylle durch die obligatorische Carotinoidbindung und (3) die Möglichkeit der Umsetzung von LHCII-gebundem Chl a zu Chl b.

Liquid crystalline oligofluorenes and their derivatives

Homo-oligofluorenes (OFn), polyfluorenes (PF2/6) and oligofluorenes with one fluorenenone group in the center (OFnK) were synthesized. They were used as model compounds to understand of the structure-property relationships of polyfluorenes and the origin of the green emission in the photoluminescence (after photooxidation of the PFs) and the electroluminescence (EL) spectra. The electronic, electrochemical properties, thermal behavior, supramolecular self-assembly, and photophysical properties of OFn, PF2/6 and OFnK were investigated. Oligofluorenes with 2-ethylhexyl side chain (OF2-OF7) from the dimer up to the heptamer were prepared by a series of stepwise transition metal mediated Suzuki and Yamamoto coupling reactions. Polyfluorene was synthesized by Yamamoto coupling of 2,7-dibromo-9,9-bis(2-ethylhexyl)fluorene. Oligofluorenes with one fluorenone group in the center (OF3K, OF5K, OF7K) were prepared by Suzuki coupling between the monoboronic fluorenyl monomer, dimer, trimer and 2, 7-dibromofluorenone. The electrochemical and electronic properties of homo-oligofluorenes (OFn) were systematically studied by several combined techniques such as cyclic voltammetry, differential pulse voltammetry, UV-vis absorption spectroscopy, steady and time-resolved fluorescence spectroscopy. It was found that the oligofluorenes behave like classical conjugated oligomers, i.e., with the increase of the chain-length, the corresponding oxidation potential, the absorption and emission maximum, ionization potential, electron affinity, band gap and the photoluminescence lifetime displayed a very good linear relation with the reciprocal number of the fluorene units (1/n). The extrapolation of these linear relations to infinite chain length predicted the electrochemical and electronic properties of the corresponding polyfluorenes. The thermal behavior, single-crystal structure and supramolecular packing, alignment properties, and molecular dynamics of the homo-oligofluorenes (OFn) up to the polymer were studied using techniques such as TGA, DSC, WAXS, POM and DS. The OFn from tetramer to heptamer show a smectic liquid crystalline phase with clearly defined isotropization temperature. The oligomers do show a glass transition which exhibits n-1 dependence and allows extrapolation to a hypothetical glass transition of the polymer at around 64 °C. A smectic packing and helix-like conformation for the oligofluorenes from tetramer to heptamer was supported by WAXS experiments, simulation, and single-crystal structure of some oligofluorene derivatives. Oligofluorenes were aligned more easily than the corresponding polymer, and the alignability increased with the molecular length from tetramer to heptamer. The molecular dynamics in a series of oligofluorenes up to the polymer was studied using dielectric spectroscopy. The photophysical properties of OFn and PF2/6 were investigated by the steady-state spectra (UV-vis absorption and fluorescence spectra) and time-resolved fluorescence spectra both in solution and thin film. The time-resolved fluorescence spectra of the oligofluorenes were measured by streak camera and gate detection technique. The lifetime of the oligofluorenes decreased with the extension of the chain-length. No green emission was observed in CW, prompt and delayed fluorescence for oligofluorenes in m-THF and film at RT and 77K. Phosphorescence was observed for oligofluorenes in frozen dilute m-THF solution at 77K and its lifetime increased with length of oligofluorenes. A linear relation was obtained for triplet energy and singlet energy as a function of the reciprocal degree of polymerization, and the singlet-triplet energy gap (S1-T1) was found to decrease with the increase of degree of polymerization. Oligofluorenes with one fluorenone unit at the center were used as model compounds to understand the origin of the low-energy (“green”) emission band in the photoluminescence and electroluminescence spectra of polyfluorenes. Their electrochemical properties were investigated by CV, and the ionization potential (Ip) and electron affinity (Ea) were calculated from the onset of oxidation and reduction of OFnK. The photophysical properties of OFnK were studied in dilute solution and thin film by steady-state spectra and time-resolved fluorescence spectra. A strong green emission accompanied with a weak blue emission were obtained in solution and only green emission was observed on film. The strong green emission of OFnK suggested that rapid energy transfer takes place from higher energy sites (fluorene segments) to lower energy sites (fluorenone unit) prior to the radiative decay of the excited species. The fluorescence spectra of OFnK also showed solvatochromism. Monoexponential decay behaviour was observed by time-resolved fluorescence measurements. In addition, the site-selective excitation and concentration dependence of the fluorescence spectra were investigated. The ratio of green and blue emission band intensities increases with the increase of the concentration. The observed strong concentration dependence of the green emission band in solution suggests that increased interchain interactions among the fluorenone-containing oligofluorene chain enhanced the emission from the fluorenone defects at higher concentration. On the other hand, the mono-exponential decay behaviour and power dependence were not influenced significantly by the concentration. We have ruled out the possibility that the green emission band originates from aggregates or excimer formation. Energy transfer was further investigated using a model system of a polyfluorene doped by OFnK. Förster-type energy transfer took place from PF2/6 to OFnK, and the energy transfer efficiency increased with increasing of the concentration of OFnK. Efficient funneling of excitation energy from the high-energy fluorene segments to the low-energy fluorenone defects results from energy migration by hopping of excitations along a single polymer chain until they are trapped on the fluorenone defects on that chain or transferred onto neighbouring chains by Förster-type interchain energy transfer process. These results imply that the red-shifted emission in polyfluorenes can originate from (usually undesirable) keto groups at the bridging carbon atoms-especially if the samples have been subject to photo- or electro-oxidation or if fluorenone units are present due to an improper purification of the monomers prior to polymerization.

Das wasserlösliche Chlorophyll-Protein (WSCP): biochemische Charakterisierung und Untersuchungen zur biologischen Funktion

Das WSCP (water-soluble chlorophyll protein) der Brassicaceen ist das einzig bekannte Chlorophyll-bindende Protein, welches keine Carotinoide bindet. Es ist ein wasserlösliches, ca. 80 kDa großes Homotetramer mit 1-4 gebundenen Chlorophyllen. Das Protein ist äußerst stabil und vermag die gebundenen Chlorophylle vor Photooxidation zu schützen. Seine Funktion in der Pflanze ist bis heute ein Rätsel und sollte in dieser Arbeit zusammen mit seinen biochemischen Eigenschaften weiter aufgeklärt werden. Es wurden Versuche durchgeführt mit nativem und rekombinantem WSCP aus Blumenkohl (BoWSCP bzw. BoWSCPhis) und aus Arabidopsis thaliana (AtWSCP bzw. AtWSCPhis). Die Expressionsausbeute von BoWSCPhis konnte verbessert werden und zusätzlich wurde die Rekonstitutionsmethode für das rekombinante WSCP optimiert, sodass das pigmentierte Protein mit hoher Ausbeute und großer Reinheit gewonnen werden konnte. Zudem wurde ein neuer WSCP-Klon hergestellt, mBoWSCPhis, der in seiner Sequenz dem maturen nativen BoWSCP entspricht und weitaus weniger Aggregationsprobleme zeigte als BoWSCPhis. Weiterführende Versuche zur Stabilität und dem Oligomerisierungsgrad von WSCP haben die neue Erkenntnis erbracht, dass die Phytolschwänze der von WSCP gebundenen Chlorophylle zwar essentiell sind für die Stabilität von WSCP-Oligomeren, nicht aber für die Oligomerisierung selbst, wie es in der Literatur bislang postuliert wurde. Zusätzlich zu ihrer außerordentlichen Hitzestabilität erwiesen sich die Chl-WSCP-Komplexe als stabil in einem breiten pH-Spektrum. AtWSCPhis besaß eine vergleichbare Stabilität, und auch das Oligomerisierungsverhalten zeigte Ähnlichkeiten zu BoWSCPhis. Im Rahmen einer Forschungskooperation mit dem Institut für Optik und Atomare Physik der TU Berlin wurden zeitaufgelöste Absorptionsspektren sowie Tieftemperatur-Fluoreszenzspektren an Chl-WSCP-Komplexen gemessen. Die Ergebnisse zeigten deutlich, dass die WSCP-gebundenen Chlorophylle excitonisch gekoppelt sind und wiesen zudem auf unterschiedliche Chl-Bindungsmodi hin. Aufgrund seines einfachen Aufbaus und seines geringen Chlorophyllgehalts hat sich WSCP bei diesen Versuchen als sehr geeignetes Modellsystem erwiesen, um Messungen zur Chlorophyllbindung mit Vorhersagen aus theoretischen Modellen zu vergleichen. Bei den Experimenten zur biologischen Funktion wurden einerseits Arabidopsis thaliana WSCP-„knock-out“-Pflanzen unter verschiedenen Bedingungen charakterisiert, andererseits wurden Experimente mit rekombinantem WSCP durchgeführt, um eine mögliche Interaktion mit anderen Proteinen zu detektieren. Die vegetativen Stadien der Mutante zeigten keinen Phänotyp; das native Arabidopsis-WSCP konnte später bei der Wildtyp-Pflanze ausschließlich in jungen Schoten lokalisiert werden, was eine Erklärung hierfür lieferte. Rekombinantes WSCP konnte Chlorophylle aus nativem LHCII entfernen, eine Interaktion mit Chlorophyllase konnte jedoch nicht nachgewiesen werden; daher konnte auch die Hypothese, WSCP sei ein Chl-Carrier beim Chl-Abbau, nicht untermauert werden. Bei den durchgeführten Enzym-Assays wurde eine geringfügige Inhibition der Cysteinprotease Papain beobachtet, aber keine Inhibition der Serinprotease Trypsin, obwohl Blumenkohl-WSCP N-proximal das Motiv der Künitz-Proteaseinhibitoren besitzt. Die Frage nach der biologischen Funktion von WSCP bleibt also weiterhin offen.

Einzelmolekülspektroskopie an einer homologen Reihe von Rylendiimiden und an einem bichromophoren Modellsystem für elektronischen Energietransfer

In dieser Arbeit wurden Untersuchungen zur Fluoreszenzdynamik und zum Mechanismus des Photobleichens einzelner Farbstoffmoleküle einer homologen Reihe von Rylentetracarbonsäurediimiden durchgeführt. Mit der Erweiterung des elektronischen π-Systems verringert sich die HOMO/LUMO-Energiedifferenz, so dass strahlungslose Relaxationsprozesse gemäß des Energielückengesetzes zunehmen. Die konfokale Einzelmolekülspektroskopie in Kombination mit zeitkorrelierter Einzelphotonenzählung ermöglicht es, Fluktuationen der inneren Konversionsrate zu detektieren. Der limitierende Faktor jedes Einzelmolekülexperiments ist die Photostabilität der Moleküle. Für die homologe Reihe konnten mindestens zwei Photobleichmechanismen identifiziert werden. Wenn Singulett-Sauerstoff durch Selbstsensibilisierung erzeugt werden kann, ist unter Luft die Photooxidation der wahrscheinlichste Mechanismus. Unter Ausschluss von Luftsauerstoff spielt die Bildung langlebiger Dunkelzustände eine entscheidende Rolle, die bevorzugt über höher angeregte Triplett- und Singulett-Zustände abläuft. Es wird angenommen, dass es sich hierbei um einen reversiblen Ionisierungsprozess handelt, bei dem das Radikal-Kation der Rylendiimide gebildet wird. Es konnte gezeigt werden, dass durch eine geeignete Wahl der Anregungsbedingungen die Dunkelzustandspopulierung verhindert und zugleich die Photostabilität der Fluorophore deutlich erhöht wird. Durch die Verknüpfung der beiden niedrigsten Homologen erhält man ein Donor-Akzeptor-Modellsystem, bei dem die Anregungsenergie mit hoher Effizienz vom Donor- auf den Akzeptor-Chromophor übertragen wird. In der Fluoreszenz einzelner Bichromophore wurden bei selektiver Anregung des Donors kollektive Auszeiten beobachtet, die durch effiziente Singulett-Triplett-Annihilation verursacht werden.

Studies of degradation of organic solar cell materials using electrical scanning probe microscopy

Intense research is being done in the field of organic photovoltaics in order to synthesize low band-gap organic molecules. These molecules are electron donors which feature in combination with acceptor molecules, typically fullerene derivarntives, forming an active blend. This active blend has phase separated bicontinuous morphology on a nanometer scale. The highest recorded power conversionrnefficiencies for such cells have been 10.6%. Organic semiconductors differ from inorganic ones due to the presence of tightly bonded excitons (electron-hole pairs)resulting from their low dielectric constant (εr ≈2-4). An additional driving force is required to separate such Frenkel excitons since their binding energy (0.3-1 eV) is too large to be dissociated by an electric field alone. This additional driving force arises from the energy difference between the lowest unoccupied molecular orbital (LUMO) of the donor and the acceptor materials. Moreover, the efficiency of the cells also depends on the difference between the highest occupied molecular orbital (HOMO) of the donor and LUMO of the acceptor. Therefore, a precise control and estimation of these energy levels are required. Furthermore any external influences that change the energy levels will cause a degradation of the power conversion efficiency of organic solar cell materials. In particular, the role of photo-induced degradation on the morphology and electrical performance is a major contribution to degradation and needs to be understood on a nanometer scale. Scanning Probe Microscopy (SPM) offers the resolution to image the nanometer scale bicontinuous morphology. In addition SPM can be operated to measure the local contact potential difference (CPD) of materials from which energy levels in the materials can be derived. Thus SPM is an unique method for the characterization of surface morphology, potential changes and conductivity changes under operating conditions. In the present work, I describe investigations of organic photovoltaic materials upon photo-oxidation which is one of the major causes of degradation of these solar cell materials. SPM, Nuclear Magnetic Resonance (NMR) and UV-Vis spectroscopy studies allowed me to identify the chemical reactions occurring inside the active layer upon photo-oxidation. From the measured data, it was possible to deduce the energy levels and explain the various shifts which gave a better understanding of the physics of the device. In addition, I was able to quantify the degradation by correlating the local changes in the CPD and conductivity to the device characteristics, i.e., open circuit voltage and short circuit current. Furthermore, time-resolved electrostatic force microscopy (tr-EFM) allowed us to probe dynamic processes like the charging rate of the individual donor and acceptor domains within the active blend. Upon photo-oxidation, it was observed, that the acceptor molecules got oxidized first preventing the donor polymer from degrading. Work functions of electrodes can be tailored by modifying the interface with monomolecular thin layers of molecules which are made by a chemical reaction in liquids. These modifications in the work function are particularly attractive for opto-electronic devices whose performance depends on the band alignment between the electrodes and the active material. In order to measure the shift in work function on a nanometer scale, I used KPFM in situ, which means in liquids, to follow changes in the work function of Au upon hexadecanethiol adsorption from decane. All the above investigations give us a better understanding of the photo-degradation processes of the active material at the nanoscale. Also, a method to compare various new materials used for organic solar cells for stability is proposed which eliminates the requirement to make fully functional devices saving time and additional engineering efforts.