Discotic materials for organic electronics

Die Kontrolle der ausgeprägten Aggregationsfähigkeit von alkylsubstituierten Hexa-peri-hexabenzocoronenen (HBC) wurde durch die Reduktion der intermolekularen Wechselwirkungen erreicht. Sterisch anspruchsvolle, verzweigte Alkylketten, mit einem Verzweigungspunkt naher des aromatischen Kerns, wurden in die Corona der aromatischen Scheiben eingebracht und verleihen den Derivaten Schmelzbarkeit ohne thermische Zersetzung. Dies erlaubte eine kostengünstige Verarbeitungstechniken direkt aus der Schmelze wie z.B. Zonenschmelzen, um uniaxial organisierte makroskopische Filme zu erhalten. Abhängig von dem sterischen Anspruch, der durch die Seitenkette erzeugt wird, wurden unterschiedliche molekulare Orientierungen auf Oberflächen erhalten, was eine wichtige Voraussetzung ist, um diskotische Materialien in elektronische Bauteile zu implementieren. Eine weitere Voraussetzung sind hohe Ladungsträgerbeweglichkeiten und Ladungsträgerlebenszeiten in den Halbleitermaterialien, die mit time-resolved pulse-radiolysis microwave conductivity (TR-PRMC) und time-of-flight (TOF) auch für die synthetisieren Materialien bestimmt wurden. Die neuen Materialien zeigten bereits in organischen Solarzellen gute Leistungen. Den Einfluss des Perimeters auf die elektronischen Eigenschaften der polyzyklischen aromatischen Kohlenwasserstoffe (PAKs) wurde theoretisch vorhergesagt und in dieser Arbeit durch die Synthese einer homologe Serie von PAKs experimentell bestätigt. Geht man von der „arm-chair“ Peripherie des HBC sukzessive zu einer partiellen „zickzack“ Peripherie, so findet man eine Abhängigkeit der elektronischen Banden von Symmetrie und Größe des aromatischen Systems. Die spontan ausgebildete Überstruktur dieser Derivate zeigte eine Abhängigkeit von Substitutionsmuster und der Natur der Alkylketten. Zusammenfassend wurden neben der Synthese von neuartigen Materialien für den Einsatz in der organischen Elektronik Synthesen entwickelt, die eine vereinfachte Funktionalisierung von ausgedehnten PAKs ermöglicht. Diese Konzepte erlauben eine Justierung der molekularen und supramolekularen Eigenschaften, eines der wichtigsten Voraussetzungen für den Einsatz von Materialien in elektronischen Bauelementen.

On the acidity and reactivity of HNO in aqueous solution and biological systems

The gas phase and aqueous thermochemistry and reactivity of nitroxyl (nitrosyl hydride, HNO) were elucidated with multiconfigurational self-consistent field and hybrid density functional theory calculations and continuum solvation methods. The pKa of HNO is predicted to be 7.2 ± 1.0, considerably different from the value of 4.7 reported from pulse radiolysis experiments. The ground-state triplet nature of NO− affects the rates of acid-base chemistry of the HNO/NO− couple. HNO is highly reactive toward dimerization and addition of soft nucleophiles but is predicted to undergo negligible hydration (Keq = 6.9 × 10−5). HNO is predicted to exist as a discrete species in solution and is a viable participant in the chemical biology of nitric oxide and derivatives.

Conformational dependence of electron transfer across de novo designed metalloproteins.

Flash photolysis and pulse radiolysis measurements demonstrate a conformational dependence of electron transfer rates across a 16-mer helical bundle (three-helix metalloprotein) modified with a capping CoIII(bipyridine)3 electron acceptor at the N terminus and a 1-ethyl-1'-ethyl-4,4'- bipyridinium donor at the C terminus. For the CoIII(peptide)3-1-ethyl-1'-ethyl-4,4'-bipyridinium maquettes, the observed transfer is a first order, intramolecular process, independent of peptide concentration or laser pulse energy. In the presence of 6 M urea, the random coil bundle (approximately 0% helicity) has an observed electron transfer rate constant of kobs = 900 +/- 100 s-1. In the presence of 25% trifluoroethanol (TFE), the helicity of the peptide is 80% and the kobs increases to 2000 +/- 200 s-1. Moreover, the increase in the rate constant in TFE is consistent with the observed decrease in donor-acceptor distance in this solvent. Such bifunctional systems provide a class of molecules for testing the effects of conformation on electron transfer in proteins and peptides.

Chain scission and branching in irradiated poly(tetrafluoroethylene-co-hexafluoropropylene)

The radiation chemistry of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) with a TFE mole fraction of 0.90 has been studied under vacuum using Co-60 gamma-radiation over a range of temperatures and absorbed doses. The radiolysis temperatures were 300, 363, 423, 523 and 543 K. New structure formation in the copolymers was analysed by solid-state F-19 NMR spectroscopy. The new structures formed in the copolymers have been identified and the G-values for the formation of new chemical structures have been investigated at 363 and 523 K. These two temperatures are just above and just below the polymer T-g and T-m, respectively. At the lower temperature, there was no evidence for any chain branching and an estimate of G(S) of 1.0 was obtained. A value of G(S) of 1.3 and a minimum value of G(X)(Y) of 1.3 were obtained at 523 K. (C) 2003 Society of Chemical Industry.

FT-Raman studies of a range of polyimides subjected to high-energy Radiations at room and elevated temperatures

A range of polyimides have been subjected to electron beam radiolysis at different temperatures. These polyimides were chemically designed to suit space applications, being either transparent or having groups which provide oxidation resistance. The structural changes that occur in the polyimides, when subjected to electron beam irradiation doses up to 18.5 MGy and up to temperatures close to their glass transition temperatures, were studied using FT-Raman spectroscopy. The range of polyimides studied included a series of perfluoropolyimides, a silicon-modified polyimide, and Ultem. The changes in the Raman peak intensities of the different groups indicated scission reactions involving the imide rings and ether linkages. (c) 2006 Wiley Periodicals, Inc.

Effect of polymorph derived oxidants on IgG in relation to rheumatoid factor binding

Immunoglobulin G from rheumatoid patients is denatured around the hinge region. This has been proposed as an explanation for the presence of circulating autoantibodies to IgG in these patients. It has previously been suggested that oxygen radicals (OR) derived from activated polymorphs may play a role in denaturation in vivo. Using sera from rheumatoid patients and age-matched controls in a modified ELISA technique, we have investigated the potential for polyclonal rheumatoid factors (RF) to bind to OR denatured IgG. Three model systems were used to generate OR in vitro: (a) purified PMN s activated by the cell surface stimulant PMA, (b) radiolysis of IgG in solution to generate specifically the superoxide radical and, in a separate system, the hydroxyl radical, (OH.), (c) purified myeloperoxide in the presence of H2O2 and halide ions. Results: 1. The binding of both IgA and IgM RF s to PMN denatured IgG increased dose dependently for seropositive sera only. 2. The OH. radical but not the superoxide radical significantly increased the binding of IgA and M RF, again only for seropositive sera. 3. The myeloperoxidase enzyme system did not increase RF binding. 4. IgG incubated with elastase was not found to be a better antigen than native IgG. These results indicate that IgG is denatured by OR released from activated PMN, thereby producing an antigen for polyclonal RF s.

Advanced oxidation of arsenic species in aqueous solutions

Ingestion of arsenic from contaminated water is a serious problem and affects the health of more than 100 million people worldwide. Traditional water purification technologies are generally not effective or cost prohibitive for the removal of arsenic to acceptable levels (≤10 ppb). Current multi-step arsenic removal processes involve oxidation, precipitation and/or adsorption. Advanced Oxidation Technologies (AOTs) may be attractive alternatives to existing treatments. The reactions of inorganic and organic arsenic species with reactive oxygen species were studied to develop a fundamental mechanistic understanding of these reactions, which is critical in identifying an effective and economical technology for treatment of arsenic contaminated water. ^ Detailed studies on the conversion of arsenite in aqueous media by ultrasonic irradiation and TiO2 photocatalytic oxidation (PCO) were conducted, focusing on the roles of hydroxyl radical and superoxide anion radical formed during the irradiation. ·OH plays the key role, while O2 -· has little or no role in the conversion of arsenite during ultrasonic irradiation. The reaction of O2-· does not contribute in the rapid conversion of As(III) when compared to the reaction of As(III) with ·OH radical during TiO2 PCO. Monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) are readily degraded upon TiO2 PCO. DMA is oxidized to MMA as the intermediate and arsenate as the final product. For dilute solutions, TiO2 also may be applicable as an adsorbent for direct removal of arsenic species, namely As(III), As(V), MMA and DMA, all of which are strongly adsorbed, thus eliminating the need for a multi-step treatment process. ^ Phenylarsonic acid (PA) was subjected to gamma radiolysis under hydroxyl radical generating conditions, which showed rapid degradation of PA. Product analysis and computational calculation both indicate the arsenate group is an ortho, para director. Our results indicate · OH radical mediated processes should be effective for the remediation of phenyl substituted arsonic acids. ^ While hydroxyl radical generating methods, specifically AOTs, appear to be promising methods for the treatment of a variety of arsenic compounds in aqueous media, pilot studies and careful economic analyses will be required to establish the feasibility of AOTs applications in the removal of arsenic. ^

Biomimetic modeling of the nitrogen-centered radical postulated to occur during the inhibition of ribonucleotide reductases by 2'-azido-2'-deoxynucleotides

Ribonucleotide reductases (RNR) are essential enzymes that catalyze the reduction of ribonucleotides to 2'-deoxyribonucleotides, which is a critical step that produces precursors for DNA replication and repair. The inactivation of RNR, logically, would discontinue producing the precursors of the DNA of viral or cancer cells, which then would consequently end the cycle of DNA replication. Among different compounds that were found to be inhibitors of RNR, 2'-azido-2'-deoxynucleotide diphosphates (N3NDPs) have been investigated in depth as potent inhibitors of RNR. Decades of investigation has suggested that the inactivation of RNR by N3NDPs is a result of the formation of a nitrogen-centered radical (N·) that is covalently attached to the nucleotide at C3' and cysteine molecule C225 [3'-C(R-S-N·-C-OH)]. Biomimetic simulation reactions for the generation of the nitrogen-centered radicals similar to the one observed during the inactivation of the RNR by azionuclotides was investigated. The study included several modes: (i) theoretical calculation that showed the feasibility of the ring closure reaction between thiyl radicals and azido group; (ii) synthesis of the model azido nucleosides with a linker attached to C3' or C5' having a thiol or vicinal dithiol functionality; (iii) generation of the thiyl radical under both physiological and radiolysis conditions whose role is important in the initiation on RNR cascades; and (iv) analysis of the nitrogen-centered radical species formed during interaction between the thiyl radical and azido group by electron paramagnetic resonance spectroscopy (EPR). Characterization of the aminyl radical species formed during one electron attachment to the azido group of 2'-azido-2'-deoxyuridine and its stereospecifically labelled 1'-, 2'-, 3'-, 4'- or 5,6-[2H 2]-analogues was also examined. This dissertation gave insight toward understanding the mechanism of the formation of the nitrogen-centered radical during the inactivation of RNRs by azidonucleotides as well as the mechanism of action of RNRs that might provide key information necessary for the development of the next generation of antiviral and anticancer drugs.

Analysis of radiation induced DNA damage by LC-MS/MS in isolated and cellular DNA

Abstract: It is well established that ionizing radiation induces a variety of damage in DNA by direct effects that are mediated by one-electron oxidation and indirect effects that are mediated by the reaction of water radiolysis products, e.g., hydroxyl radicals (•OH). In cellular DNA, direct and indirect effects appear to have about an equal effect toward DNA damage. We have shown that ϒ-(gamma) ray irradiation of aqueous solutions of DNA, during which •OH is the major damaging ROS can lead to the formation several lesions. On the other hand, the methylation and oxidative demethylation of cytosine in CpG dinucleotides plays a critical role in the gene regulation. The C5 position of cytosine in CG dinucleotides is frequently methylated by DNA methyl transferees (DNMTs) and constitutes 4-5% of the total cytosine. Here, my PhD research work focuses on the analysis of oxidative base modifications of model compounds of methylated and non methylated oligonucleotides, isolated DNA (calf-thymus DNA) and F98 cultured cell by gamma radiation. In addition, we identified a series of modifications of the 2-deoxyribose moiety of DNA arising from the exposure of isolated and cellular DNA to ionizing radiation. We also studied one electron oxidation of cellular DNA in cultured human HeLa cells initiated by intense nanosecond 266 nm laser pulse irradiation, which produces cross-links between guanine and thymine bases (G*-T*). To achieve these goals, we developed several methods based on mass spectrometry to analyze base modifications in isolated DNA and cellular DNA.