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Structural changes at the intra- as well as intermicellar level were induced by the LCST-type collapse transition of poly(N-isopropyl acrylamide) in ABA triblock copolymer micelles in water. The distinct process kinetics was followed in situ and in real-time using time-resolved small-angle neutron scattering (SANS), while a micellar solution of a triblock copolymer, consisting of two short deuterated polystyrene endblocks and a long thermoresponsive poly(N-isopropyl acrylamide) middle block, was heated rapidly above its cloud point. A very fast collapse together with a multistep aggregation behavior is observed. The findings of the transition occurring at several size and time levels may have implications for the design and application of such thermoresponsive self-assembled systems.
This study addresses the interactions of coffee storage proteins with coffee-specific phenolic compounds. Protein profiles, of Coffea arabica and Coffea canephora (var robusta) were compared. Major Phenolic compounds were extracted and analyzed with appropriate methods. The polyphenol-protein interactions during protein extraction have been addressed by different analytical setups [reversed-phase high-performance liquid chromatography (RP-HPLC), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS), and Trolox equivalent antioxidant capacity (TEAC) assays], with focus directed toward identification of covalent adduct formation. The results indicate that C. arabica proteins are more susceptible to these interactions and the polyphenol oxidase activity seems to be a crucial factor for the formation of these addition products. A tentative allocation of the modification type and site in the protein has been attempted. Thus, the first available in silico modeling of modified coffee proteins is reported. The extent of these modifications may contribute to the structure and function of "coffee melanoidins" and are discussed in the context of coffee flavor formation.
Pioneered by Clark's microelectrode more than half a century ago, there has been substantial interest in developing new, miniaturized optical methods to detect molecular oxygen inside cells. While extensively used for animal tissue measurements, applications of intracellular optical oxygen biosensors are still scarce in plant science. A critical aspect is the strong autofluorescence of the green plant tissue that interferes with optical signals of commonly used oxygen probes. A recently developed dual-frequency phase modulation technique can overcome this limitation, offering new perspectives for plant research. This review gives an overview on the latest optical sensing techniques and methods based on phosphorescence quenching in diverse tissues and discusses the potential pitfalls for applications in plants. The most promising oxygen sensitive probes are reviewed plus different oxygen sensing structures ranging from micro-optodes to soluble nanoparticles. Moreover, the applicability of using heterologously expressed oxygen binding proteins and fluorescent proteins to determine changes in the cellular oxygen concentration are discussed as potential non-invasive cellular oxygen reporters.
The fluorescence response of a set of cyclam-triazole-dye ligands is controlled by the appended dye, but simple reversal of the triazole topology affords a novel probe for Zn2+ with a longer fluorescence lifetime and higher fluorescence quantum yield upon Zn2+ binding (<tau t > = 2.0 ns, Phi(f) = 0.76).
A series of new heteroleptic MN2S2 transition metal complexes with M = Cu2+ for EPR measurements and as diamagnetic hosts Ni2+, Zn2+, and Pd2+ were synthesized and characterized. The ligands are N2 = 4, 4'-bis(tert-butyl)-2, 2'-bipyridine (tBu2bpy) and S2 =1, 2-dithiooxalate, (dto), 1, 2-dithiosquarate, (dtsq), maleonitrile-1, 2-dithiolate, or 1, 2-dicyanoethene-1, 2-dithiolate, (mnt). The CuII complexes were studied by EPR in solution and as powders, diamagnetically diluted in the isostructural planar [NiII(tBu2bpy)(S2)] or[PdII(tBu2bpy)(S2)] as well as in tetrahedrally coordinated[ZnII(tBu2bpy)(S2)] host structures to put steric stress on the coordination geometry of the central CuN2S2 unit. The spin density contributions for different geometries calculated from experimental parameters are compared with the electronic situation in the frontier orbital, namely in the semi-occupied molecular orbital (SOMO) of the copper complex, derived from quantum chemical calculations on different levels (EHT and DFT). One of the hosts, [NiII(tBu2bpy)(mnt)], is characterized by X-ray structure analysis to prove the coordination geometry. The complex crystallizes in a square-planar coordination mode in the monoclinic space group P21/a with Z = 4 and the unit cell parameters a = 10.4508(10) angstrom, b = 18.266(2) angstrom, c = 12.6566(12) angstrom, beta = 112.095(7)degrees. Oxidation and reductions potentials of one of the host complexes, [Ni(tBu2bpy)(mnt)], were obtained by cyclovoltammetric measurements.
The minima on the potential energy surface of 1,2-bis(o-carboxyphenoxy)ethane (CPE) molecule in its electronic ground state were searched by a molecular dynamics simulation performed with MM2 force field. For each of the found minimum-energy conformers, the corresponding equilibrium geometry, charge distribution, HOMO-LUMO energy gap, force field, vibrational normal modes and associated IR and Raman spectral data were determined by means of the density functional theory (DFT) based electronic structure calculations carried out by using B3LYP method and various Pople-style basis sets. The obtained theoretical data confirmed the significant effects of the intra- and inter-molecular hydrogen bonding interactions on the conformational structure, force field, and group vibrations of the molecule. The same data have also revealed that two of the determined stable conformers, both of which exhibit pseudo-crown structure, are considerably more favorable in energy to the others and accordingly provide the major contribution to the experimental spectra of CPE. In the light of the improved vibrational spectral data obtained within the "SQM FF" methodology and "Dual Scale Factors" approach for the monomer and dimer forms of these two conformers, a reliable assignment of the fundamental bands observed in the experimental room-temperature IR and Raman spectra of the molecule was given, and the sensitivities of its group vibrations to conformation, substitution and dimerization were discussed.
Density Functional Calculations of the Anisotropic Effects of Borazine and 1,3,2,4-Diazadiboretidine
(2012)
On the basis of the nucleus-independent chemical shift (NICS) concept, the anisotropic effects of two inorganic rings, namely, borazine and planar 1,3,2,4-diazadiboretidine, are quantitatively calculated and visualized as isochemical shielding surfaces (ICSSs). Dissection of magnetic shielding values along the three Cartesian axes into contributions from s and p bonds by the natural chemical shieldingnatural bond orbital (NCSNBO) method revealed that their appearance is not a simple reflection of the extent of (anti)aromaticity.
Based on the nucleus-independent chemical shift (NICS) concept, isotropic magnetic shielding values have been computed along the three Cartesian axes for ethene, cyclobutadiene, benzene, naphthalene, and benzocyclobutadiene, starting from the molecular/ring center up to 10 angstrom away. These through-space NMR spectroscopic shielding (TSNMRS) values, which reflect the anisotropic effects, have been broken down into contributions from localized- and canonical molecular orbitals (LMOs and CMOs); these contributions revealed that the proton NMR spectroscopic chemical shifts of nuclei that are spatially close to the C?C double bond or the aromatic ring should not be explained in terms of the conventionally accepted p-electron shielding/deshielding effects. In fact, these effects followed the predictions only for the antiaromatic cyclobutadiene ring.
The SP-PLP-EPR technique is used to carry out a detailed investigation of the radical termination kinetics of 1H, 1H, 2H, 2H-tridecafluorooctyl methacrylate (TDFOMA) in bulk at relatively low conversion. Composite-model behavior for chain-length-dependent termination rate coefficients, kti,i, is observed. It is found that for TDFOMA, ic approximate to 60 independent of temperature, and as approximate to 0.65 and al approximate to 0.2 at 80 degrees C and above. However, at lower temperatures the situation is strikingly different, with the significantly higher average values of as = 0.89 +/- 0.15 and al = 0.32 +/- 0.10 being obtained at 50 degrees C and below. This makes TDFOMA the first monomer to be found that exhibits clearly different exponent values, as and al, at lower and higher temperature, and that has both a high as, like an acrylate, and a high ic, like a methacrylate.
Magnetotactic bacteria produce chains of magnetite nanoparticles, which are called magnetosomes and are used for navigational purposes. We use these cells as a biological template to prepare a hollow hybrid material based on silica and magnetite, and show that the synthetic route is nondestructive as the material conserves the cell morphology as well as the alignment of the magnetic particles. The hybrid material can be resuspended in aqueous solution, and can be shown to orient itself in an external magnetic field. We anticipate that chemical modification of the silica can be used to functionalize the material surface in order to obtain multifunctional materials with specialized applications, e.g. targeted drug delivery.
Diese Arbeit befasst sich mit der Synthese und Charakterisierung von organolöslichen Thiophen und Benzodithiophen basierten Materialien und ihrer Anwendung als aktive lochleitende Halbleiterschichten in Feldeffekttransistoren. Im ersten Teil der Arbeit wird durch eine gezielte Modifikation des Thiophengrundgerüstes eine neue Comonomer-Einheit für die Synthese von Thiophen basierten Copolymeren erfolgreich dargestellt. Die hydrophoben Hexylgruppen in der 3-Position des Thiophens werden teilweise durch hydrophile 3,6-Dioxaheptylgruppen ersetzt. Über die Grignard-Metathese nach McCullough werden statistische Copolymere mit unterschiedlichen molaren Anteilen vom hydrophoben Hexyl- und hydrophilem 3,6-Dioxaheptylgruppen 1:1 (P-1), 1:2 (P-2) und 2:1 (P-3) erfolgreich hergestellt. Auch die Synthese eines definierten Blockcopolymers BP-1 durch sequentielle Addition der Comonomere wird realisiert. Optische und elektrochemische Eigenschaften der neuartigen Copolymere sind vergleichbar mit P3HT. Mit allen Copolymeren wird ein charakteristisches Transistorverhalten in einem Top-Gate/Bottom-Kontakt-Aufbau erhalten. Dabei werden mit P-1 als die aktive Halbleiterschicht im Bauteil, PMMA als Dielektrikum und Silber als Gate-Elektrode Mobilitäten von bis zu 10-2 cm2/Vs erzielt. Als Folge der optimierten Grenzfläche zwischen Dielektrikum und Halbleiter wird eine Verbesserung der Luftstabilität der Transistoren über mehrere Monate festgestellt. Im zweiten Teil der Arbeit werden Benzodithiophen basierte organische Materialien hergestellt. Für die Synthese der neuartigen Benzodithiophen-Derivate wird die Schlüsselverbindung TIPS-BDT in guter Ausbeute dargestellt. Die Difunktionalisierung von TIPS-BDT in den 2,6-Positionen über eine elektrophile Substitution liefert die gewünschten Dibrom- und Distannylmonomere. Zunächst werden über die Stille-Reaktion alternierende Copolymere mit alkylierten Fluoren- und Chinoxalin-Einheiten realisiert. Alle Copolymere zeichnen sich durch eine gute Löslichkeit in gängigen organischen Lösungsmitteln, hohe thermische Stabilität und durch gute Filmbildungseigenschaften aus. Des Weiteren sind alle Copolymere mit HOMO Lagen höher als -6.3 eV, verglichen mit den Thiophen basierten Copolymeren (P-1 bis P-3), sehr oxidationsstabil. Diese Copolymere zeigen amorphes Verhalten in den Halbleiterschichten in OFETs auf und es werden Mobilitäten bis zu 10-4 cm2/Vs erreicht. Eine Abhängigkeit der Bauteil-Leistung von dem Zinngehalt-Rest im Polymer wird nachgewiesen. Ein Zinngehalt von über 0.6 % kann enormen Einfluss auf die Mobilität ausüben, da die funktionellen SnMe3-Gruppen als Fallenzustände wirken können. Alternativ wird das alternierende TIPS-BDT/Fluoren-Copolymer P-5-Stille nach der Suzuki-Methode polymerisiert. Mit P-5-Suzuki als die aktive organische Halbleiterschicht im OFET wird die höchste Mobilität von 10-2 cm2/Vs erzielt. Diese Mobilität ist somit um zwei Größenordnungen höher als bei P-5-Stille, da die Fallenzustände in diesem Fall minimiert werden und folglich der Ladungstransport verbessert wird. Sowohl das Homopolymer P-12 als auch das Copolymer mit dem aromatischen Akzeptor Benzothiadiazol P-9 führen zu schwerlöslichen Polymeren. Aus diesem Grund werden einerseits Terpolymere aus TIPS-BDT/Fluoren/BTD-Einheiten P-10 und P-11 aufgebaut und andererseits wird versucht die TIPS-BDT-Einheit in die Seitenkette des Styrols einzubringen. Mit der Einführung von BTD in die Hauptpolymerkette werden insbesondere die Absorptions- und die elektrochemischen Eigenschaften beeinflusst. Im Vergleich zu dem TIPS-BDT/Fluoren-Copolymer reicht die Absorption bis in den sichtbaren Bereich und die LUMO Lage wird zu niederen Werten verschoben. Eine Verbesserung der Leistung in den Bauteilen wird jedoch nicht festgestellt. Die erfolgreiche erstmalige Synthese von TIPS-BDT als Seitenkettenpolymer an Styrol P-13 führt zu einem löslichen und amorphen Polymer mit vergleichbaren Mobilitäten von Styrol basierten Polymeren (µ = 10-5 cm2/Vs) im OFET. Ein weiteres Ziel dieser Arbeit ist die Synthese von niedermolekularen organolöslichen Benzodithiophen-Derivaten. Über Suzuki- und Stille-Reaktionen ist es erstmals möglich, verschiedenartige Aromaten über eine σ-Bindung an TIPS-BDT in den 2,6-Positionen zu knüpfen. Die UV/VIS-Untersuchungen zeigen, dass die Absorption durch die Verlängerung der π-Konjugationslänge zu höheren Wellenlängen verschoben wird. Darüber hinaus ist es möglich, thermisch vernetzbare Gruppen wie Allyloxy in das Molekülgerüst einzubauen. Das Einführen von F-Atomen in das Molekülgerüst resultiert in einer verstärkten Packungsordnung im Fluorbenzen funktionalisiertem TIPS-BDT (SM-4) im Festkörper mit sehr guten elektronischen Eigenschaften im OFET, wobei Mobilitäten bis zu 0.09 cm2/Vs erreicht werden.
In dieser Arbeit wurden sphärische Gold Nanopartikel (NP) mit einem Durchmesser größer ~ 2 nm, Gold Quantenpunkte (QDs) mit einem Durchmesser kleiner ~ 2 nm sowie Gold Nanostäbchen (NRs) unterschiedlicher Länge hergestellt und optisch charakterisiert. Zudem wurden zwei neue Synthesevarianten für die Herstellung thermosensitiver Gold QDs entwickelt werden. Sphärische Gold NP zeigen eine Plasmonenbande bei ~ 520 nm, die auf die kollektive Oszillation von Elektronen zurückzuführen ist. Gold NRs weisen aufgrund ihrer anisotropen Form zwei Plasmonenbanden auf, eine transversale Plasmonenbande bei ~ 520 nm und eine longitudinale Plasmonenbande, die vom Länge-zu-Durchmesser-Verhältnis der Gold NRs abhängig ist. Gold QDs besitzen keine Plasmonenbande, da ihre Elektronen Quantenbeschränkungen unterliegen. Gold QDs zeigen jedoch aufgrund diskreter Energieniveaus und einer Bandlücke Photolumineszenz (PL). Die synthetisierten Gold QDs besitzen eine Breitbandlumineszenz im Bereich von ~ 500-800 nm, wobei die Lumineszenz-eigenschaften (Emissionspeak, Quantenausbeute, Lebenszeiten) stark von den Herstellungs-bedingungen und den Oberflächenliganden abhängen. Die PL in Gold QDs ist ein sehr komplexes Phänomen und rührt vermutlich von Singulett- und Triplett-Zuständen her. Gold NRs und Gold QDs konnten in verschiedene Polymere wie bspw. Cellulosetriacetat eingearbeitet werden. Polymernanokomposite mit Gold NRs wurden erstmals unter definierten Bedingungen mechanisch gezogen, um Filme mit optisch anisotropen (richtungsabhängigen) Eigenschaften zu erhalten. Zudem wurde das Temperaturverhalten von Gold NRs und Gold QDs untersucht. Es konnte gezeigt werden, dass eine lokale Variation der Größe und Form von Gold NRs in Polymernanokompositen durch Temperaturerhöhung auf 225-250 °C erzielt werden kann. Es zeigte sich, dass die PL der Gold QDs stark temperaturabhängig ist, wodurch die PL QY der Proben beim Abkühlen (-7 °C) auf knapp 30 % verdoppelt und beim Erhitzen auf 70 °C nahezu vollständig gelöscht werden konnte. Es konnte demonstriert werden, dass die Länge der Alkylkette des Oberflächenliganden einen Einfluss auf die Temperaturstabilität der Gold QDs hat. Zudem wurden verschiedene neuartige und optisch anisotrope Sicherheitslabels mit Gold NRs sowie thermosensitive Sicherheitslabel mit Gold QDs entwickelt. Ebenso scheinen Gold NRs und QDs für die und die Optoelektronik (bspw. Datenspeicherung) und die Medizin (bspw. Krebsdiagnostik bzw. -therapie) von großem Interesse zu sein.
Metals are often used in environments that are conducive to corrosion, which leads to a reduction in their mechanical properties and durability. Coatings are applied to corrosion-prone metals such as aluminum alloys to inhibit the destructive surface process of corrosion in a passive or active way. Standard anticorrosive coatings function as a physical barrier between the material and the corrosive environment and provide passive protection only when intact. In contrast, active protection prevents or slows down corrosion even when the main barrier is damaged. The most effective industrially used active corrosion inhibition for aluminum alloys is provided by chromate conversion coatings. However, their toxicity and worldwide restriction provoke an urgent need for finding environmentally friendly corrosion preventing systems. A promising approach to replace the toxic chromate coatings is to embed particles containing nontoxic inhibitor in a passive coating matrix. This work presents the development and optimization of effective anticorrosive coatings for the industrially important aluminum alloy, AA2024-T3 using this approach. The protective coatings were prepared by dispersing mesoporous silica containers, loaded with the nontoxic corrosion inhibitor 2-mercaptobenzothiazole, in a passive sol-gel (SiOx/ZrOx) or organic water-based layer. Two types of porous silica containers with different sizes (d ≈ 80 and 700 nm, respectively) were investigated. The studied robust containers exhibit high surface area (≈ 1000 m² g-1), narrow pore size distribution (dpore ≈ 3 nm) and large pore volume (≈ 1 mL g-1) as determined by N2 sorption measurements. These properties favored the subsequent adsorption and storage of a relatively large amount of inhibitor as well as its release in response to pH changes induced by the corrosion process. The concentration, position and size of the embedded containers were varied to ascertain the optimum conditions for overall anticorrosion performance. Attaining high anticorrosion efficiency was found to require a compromise between delivering an optimal amount of corrosion inhibitor and preserving the coating barrier properties. This study broadens the knowledge about the main factors influencing the coating anticorrosion efficiency and assists the development of optimum active anticorrosive coatings doped with inhibitor loaded containers.
We investigate the vibrational relaxation of a Morse oscillator, nonlinearly coupled to a finite-dimensional bath of harmonic oscillators at zero temperature, using two different approaches: Reduced dynamics with the help of the Lindblad formalism of reduced density matrix theory in combination with Fermi's Golden Rule, and exact dynamics (within the chosen model). with the multiconfiguration time-dependent Hartree (MCTDH) method. Two different models have been constructed, the situation where the bath spectrum is exactly resonant with the anharmonic oscillator transition frequencies, and the case for which the subsystem is slightly off-resonant with the environment. At short times, reduced dynamics calculations describe the relaxation process qualitatively well but fail to reproduce recurrences observed with MCTDH for longer times. Lifetimes of all the vibrational levels of the Morse oscillator have been calculated, and both Lindblad and MCTDH. results show the same dependence of the lifetimes on the initial vibrational state quantum number. A prediction, which should be generic for adsorbate systems is a striking, sharp increase of lifetimes of the subsystem vibrational levels close to the dissociation This is contradictory with harmonic/linear extrapolation laws, which predict a monotonic decrease of the lifetime with initial vibrational quantum number.
Regioselective synthesis of alkylarenes by two-step ipso-substitution of aromatic dicarboxylic acids
(2012)
A strategy for the regioselective alkylation of arenes was developed, starting from commercially available and inexpensive terephthalic acid or naphthalene-1,4-dicarboxylic acid. The method entails a formal ipso-substitution of the carboxylate groups by a sequence of reductive alkylation under Birch conditions and subsequent acid-mediated rearomatization with loss of carbon monoxide and carbon dioxide. More than 20 different arenes with various side-chains were synthesized. With naphthalene-1,4-dicarboxylic acid as starting material, we were able to control the degree of alkylation by choosing the appropriate electrophile in the Birch reduction. Thus, bisalkylated naphthalenes and naphthoic acids became available chemoselectively. All reactions afforded a single regioisomer exclusively in high yields. Overall, aromatic dicarboxylic acids are suitable substrates for a two-step ipso-substitution that allows the selective synthesis of alkylated benzenes and naphthalenes.
We report the synthesis of free 1,6,7,12-tetraazaperylene (tape). Tape was obtained from 1,1'-bis-2,7-naphthyridine by potassium promoted cyclization followed by oxidation with air. Mono-and dinuclear ruthenium(II) 1,6,7,12-tetraazaperylene complexes of the general formulas [Ru(L-L)(2)(tape)](PF6)(2), [1] (PF6)(2)-[5](PF6)(2), and [{Ru(L-L)(2)}(2)(mu-tape)](PF6)(4), [6](PF6)(4)-[10](PF6)(4), with{L-L = phen, bpy, dmbpy (4,4'-dimethyl-2,2'-bipyridine), dtbbpy (4,4'-ditertbutyl-2,2'-bipyridine) and tmbpy (4,4' 5,5'-tetramethyl-2,2'- bipyridine)}, respectively, were synthesized. The X-ray structures of tape center dot 2CHCl(3) and the mononuclear complexes [Ru(bpy)(2)(tape)](PF6)(2)center dot 0.5CH(3)CN center dot 0.5toluene, [Ru(dmbpy)(2)(tape)] (PF6)(2)center dot 2toluene and [Ru(dtbbpy)(2)(tape)](PF6)(2) center dot 3acetone center dot 0.5H(2)O were solved. The UV-vis absorption spectra and the electrochemical behavior of the ruthenium(II) tape complexes were explored and compared with the data of the analogous dibenzoeilatin (dbneil), 2,2'-bipyrimidine (bpym) and tetrapyrido [3,2-a:2',3'-c:3 '',2''-h:2''',3'''-j] phenazin (tpphz) species.
Ruthenium(II) complexes [Ru(L-N4Me2)(dape)](PF6)2 {[1](PF6)2}, [Ru(L-N4Me2)(tape)](PF6)2 {[2](PF6)2}, and [{Ru(L-N4Me2)}2(mu-tape)](PF6)4 {[3](PF6)4} were synthesized in two reaction steps by first reacting [Ru(DMSO)4Cl2] with tetraazamacrocyclic ligand N,N'-dimethyl-2,11-diaza[3.3](2,6)-pyridinophane (L-N4Me2) in ethanol under microwave irradiation to the intermediate [Ru(L-N4Me2)Cl2], which was subsequently, without further isolation, reacted with 1,12-diazaperylene (dape) or 1,6,7,12-tetraazaperylene (tape). X-ray structures of [Ru(L-N4Me2)(dape)](PF6)2, [Ru(L-N4Me2)(tape)](PF6)2.acetone, and [{Ru(L-N4Me2)}2(mu-tape)](ClO4)4.MeCN were determined. The UV/Vis absorption spectra of [1](PF6)2, [2](PF6)2, and [3](PF6)4 in acetonitrile display intense low-energy dp(Ru)?p* (dape or tape) MLCT absorption bands centered at 579, 637, and 794 nm, respectively. Reversible metal oxidations for the bimetallic complex [{Ru(L-N4Me2)}2(mu-tape)]4+ ([3]4+) are detected at 1.69 and 1.28 V vs. SCE. The potential difference ?E = 410 mV and the intervalence-charge-transfer (IVCT) transition at 2472 nm indicate a high degree of electronic interaction between the two ruthenium ions mediated through the tape bridging ligand. All three complexes, [1]2+, [2]2+, and [3]4+, were characterized by UV/Vis spectroelectrochemistry. The monooxidized and monoreduced states, [1]3+, [2]3+, [3]5+, and [1]+, [2]+, [3]3+, are accessible by reversible one-electron oxidation and one-electron reduction processes, respectively, as documented by the observation of several stable isosbestic points in the spectral progressions. The second reduction in each complex and the second oxidation in [3]4+ prove to be irreversible in these spectroelectrochemical experiments. Monoreduced species [1]+, [2]+, and [3]3+ yield EPR signals indicating that the unpaired electron is mainly centered on the large surface ligands dape or tape.
Angle-resolved two-photon photoemission and high-resolution electron energy loss spectroscopy are employed to derive the electronic structure of a subnanometer atomically precise quasi-one-dimensional graphene nanoribbon (GNR) on Au(111). We resolved occupied and unoccupied electronic bands including their dispersion and determined the band gap, which possesses an unexpectedly large value of 5.1 eV. Supported by density functional theory calculations for the idealized infinite polymer and finite size oligomers, an unoccupied nondispersive electronic state with an energetic position in the middle of the band gap of the GNR could be identified. This state resides at both ends of the ribbon (end state) and is only found in the finite sized systems, i.e., the oligomers.
The self-assembly of double-hydrophilic poly(ethylene oxide)-poly(2-methyl-2-oxazoline) diblock copolymers in water has been studied. Isothermal titration calorimetry, small-angle X-ray scattering, and analytical ultracentrifugation suggest that only single polymer chains are present in solution. In contrast, light scattering and transmission electron microscopy detect aggregates with radii of ca. 100 nm. Pulsed field gradient NMR spectroscopy confirms the presence of aggregates, although only 2% of the polymer chains undergo aggregation. Water uptake experiments indicate differences in the hydrophilicity of the two blocks, which is believed to be the origin of the unexpected aggregation behavior (in accordance with an earlier study by Ke et al. [Macromolecules 2009, 42, 5339-5344]). The data therefore suggest that even in double-hydrophilic block copolymers, differences in hydrophilicity are sufficient to drive polymer aggregation, a phenomenon that has largely been overlooked or ignored so far.
The structure of naturally-occurring cinerin C [systematic name: (7S,8R,3'R,4'S,5'R)-Delta(8')-4'-hydroxy-5,5',3'-trimethoxy-3,4-methylenedioxy-2',3',4',5'-tetrahydro-2'-oxo-7.3',8.5'-neolignan], isolated from the ethanol extract of leaves of Pleurothyrium cinereum (Lauraceae), has previously been established by NMR and HRMS spectroscopy, and its absolute configuration established by circular dichroism measurements. For the first time, its crystal strucure has now been established by single-crystal X-ray analysis, as the monohydrate, C22H26O7 center dot H2O. The bicyclooctane moiety comprises fused cyclopentane and cyclohexenone rings which are almost coplanar. An intermolecular O-H center dot center dot center dot O hydrogen bond links the 4'-OH and 5'-OCH3 groups along the c axis.
Syntheses and conformational analyses of new naphth[1,2-e][1,3]oxazino[3,2-c] quinazolin-13-ones
(2012)
The syntheses of naphth[1,2-e][1,3]oxazino[3,2-c]quinazolin-13-one derivatives (3a-f) were achieved by the solvent-free heating of benzyloxycarbonyl-protected intermediates (2a-f) with MeONa. For intermediates 2a-f, prepared by the reactions of substituted aminonaphthols with benzyl N-(2-formylphenyl)carbamate, not only the expected trans ring form B and chain form A(1), but also the rearranged chain form A(2) as a new tautomer were detected in DMSO at room temperature. The quantity of A(2) in the tautomeric mixture was changed with time.
Conformational analyses of the target heterocycles 3a-f by NMR spectroscopy and accompanying theoretical calculations at the DFT level of theory revealed that the oxazine ring preferred a twisted chair conformation and the quinazolone ring was planar. Besides the conformations, both the configurations at C-7a and C-15 and the preferred rotamers of the 1-naphthyl substituent at C-15 were assigned, which allowed evaluation of the aryl substituent-dependent steric hindrance in this part of the molecules. Configurational assignments were corroborated by quantifying the ring current effect of 15-aryl in terms of spatial NICS.
The structural and dynamical properties of oil-in-water (O/W) microemulsions (MEs) modified with telechelic polymers of different functionality (e.g., number of hydrophobically modified arms, f) were studied by means of dynamic light scattering (DLS), small-angle neutron scattering (SANS), and high frequency rheology measurements as a function of the polymer architecture and the amount of added polymer. For this purpose, we employed tailor-made hydrophobically end-capped poly(N,N-dimethylacrylamide) star polymers of a variable number of endcaps, f, of different alkyl chain lengths, synthesized by the reversible addition-fragmentation chain transfer method. The addition of the different end-capped polymers to an uncharged ME of O/W droplets leads to a large enhancement of the viscosity of the systems. SANS experiments show that the O/W ME droplets are not changed upon the addition of the polymer, and its presence only changes the interdroplet interactions. The viscosity increases largely upon addition of a polymer, and this enhancement depends pronouncedly on the alkyl length of the hydrophobic sticker as it controls the residence time in a ME droplet. Similarly, the high frequency modulus G(0) depends on the amount of added polymer but not on the sticker length. G(0) was found to be directly proportional to f - 1. The onset of network formation is shifted to a lower number of stickers per ME droplet with increasing f, and the network formation becomes more effective. Thus, the dynamics of network formation are controlled by the polymer architecture. The effect on the dynamics seen by DLS is even more pronounced. Upon increasing the polymer concentration, slower relaxation modes appear that become especially pronounced with increasing number of arms. The relaxation dynamics are correlated to the rheological relaxation, and both are controlled by the polymer architecture.
In this work the adsorption of CO2 and CH4 on a series of isoreticular microporous metal-organic frameworks based on 2-substituted imidazolate-4-amide-5-imidates, IFP-1-IFP-6 (IFP Imidazolate Framework Potsdam), is studied firstly by pure gas adsorption at 273 K. All experimental isotherms can be nicely described by using the Toth isotherm model and show the preferred adsorption of CO2 over CH4. At low pressures the Toth isotherm equation exhibits a Henry region, wherefore Henry's law constants for CO2 and CH4 uptake could be determined and ideal selectivity (alpha CO2/CH4) has been calculated. Secondly, selectivities were calculated from mixture data by using nearly equimolar binary mixtures of both gases by a volumetric-chromatographic method to examine the IFPs. Results showed the reliability of the selectivity calculation. Values of (alpha CO2/CH4) around 7.5 for IFP-5 indicate that this material shows much better selectivities than IFP-1, IFP-2, IFP-3, IFP-4 and IFP-6 with slightly lower selectivity (alpha CO2/CH4) = 4-6. The preferred adsorption of CO2 over CH4 especially of IFP-5 and IFP-4 makes these materials suitable for gas separation application.
A number of ionogels - silica-ionic liquid (IL) hybrid materials - were synthesized and studied for their ionic conductivity. The materials are based on a sulfonated IL, 1-methyl-3-(3-sulfopropyl-)-imidazolium p-toluenesulfonate, [PmimSO(3)H][PTS], which contains a sulfonic acid/sulfonate group both in the IL anion and in the side chain of the IL cation. By way of the sulfonate-sulfonic acid proton transfer, the IL imparts the ionogel with a high ionic conductivity of ca. 10(-2) S cm(-1) in the as-synthesized state at 120 degrees C and 10(-3) S cm(-1) in the dry state at 120 degrees C. The ionogels are stable up to ca. 150 degrees C in dynamic thermogravimetric analysis. This suggests that these materials, which are relatively cheap and easily fabricated, could find application in fuel cells in intermediate temperature ranges where many other membrane materials are not suitable.
Suppression of the near-infrared OH night-sky lines with fibre Bragg gratings - first results
(2012)
The background noise between 1 and 1.8 ?mu m in ground-based instruments is dominated by atmospheric emission from hydroxyl molecules. We have built and commissioned a new instrument, the Gemini Near-infrared OH Suppression Integral Field Unit (IFU) System (GNOSIS), which suppresses 103 OH doublets between 1.47 and 1.7?mu m by a factor of 1000 with a resolving power of 10?000. We present the first results from the commissioning of GNOSIS using the IRIS2 spectrograph at the Anglo-Australian Telescope. We present measurements of sensitivity, background and throughput. The combined throughput of the GNOSIS fore-optics, grating unit and relay optics is 36?per cent, but this could be improved to 46?per cent with a more optimal design. We measure strong suppression of the OH lines, confirming that OH suppression with fibre Bragg gratings will be a powerful technology for low-resolution spectroscopy. The integrated OH suppressed background between 1.5 and 1.7 mu m is reduced by a factor of 9 compared to a control spectrum using the same system without suppression. The potential of low-resolution OH-suppressed spectroscopy is illustrated with example observations of Seyfert galaxies and a low-mass star. The GNOSIS background is dominated by detector dark current below 1.67 mu m and by thermal emission above 1.67 mu m. After subtracting these, we detect an unidentified residual interline component of 860 +/- 210 photons s-1 m-2?arcsec-2?mu m-1, comparable to previous measurements. This component is equally bright in the suppressed and control spectra. We have investigated the possible source of the interline component, but were unable to discriminate between a possible instrumental artefact and intrinsic atmospheric emission. Resolving the source of this emission is crucial for the design of fully optimized OH suppression spectrographs. The next-generation OH suppression spectrograph will be focused on resolving the source of the interline component, taking advantage of better optimization for a fibre Bragg grating feed incorporating refinements of design based on our findings from GNOSIS. We quantify the necessary improvements for an optimal OH suppressing fibre spectrograph design.
The need to reduce humankind reliance on fossil fuels by exploiting sustainably the planet renewable resources is a major driving force determining the focus of modern material research. For this reason great interest is nowadays focused on finding alternatives to fossil fuels derived products/materials. For the short term the most promising substitute is undoubtedly biomass, since it is the only renewable and sustainable alternative to fossil fuels as carbon source. As a consequence efforts, aimed at finding new synthetic approaches to convert biomass and its derivatives into carbon-based materials, are constantly increasing. In this regard, hydrothermal carbonisation (HTC) has shown to be an effective means of conversion of biomass-derived precursors into functional carbon materials. However the attempts to convert raw biomass, in particular lignocellulosic one, directly into such products have certainly been rarer. Unlocking the direct use of these raw materials as carbon precursors would definitely be beneficial in terms of HTC sustainability. For this reason, in this thesis the HTC of carbohydrate and protein-rich biomass was systematically investigated, in order to obtain more insights on the potentials of this thermochemical processing technique in relation to the production of functional carbon materials from crude biomass. First a detailed investigation on the HTC conversion mechanism of lignocellulosic biomass and its single components (i.e. cellulose, lignin) was developed based on a comparison with glucose HTC, which was adopted as a reference model. In the glucose case it was demonstrated that varying the HTC temperature allowed tuning the chemical structure of the synthesised carbon materials from a highly cross-linked furan-based structure (T = 180oC) to a carbon framework composed of polyaromatic arene-like domains. When cellulose or lignocellulosic biomass was used as carbon precursor, the furan rich structure could not be isolated at any of the investigated processing conditions. These evidences were indicative of a different HTC conversion mechanism for cellulose, involving reactions that are commonly observed during pyrolytic processes. The evolution of glucose-derived HTC carbon chemical structure upon pyrolysis was also investigated. These studies revealed that upon heat treatment (Investigated temperatures 350 – 900 oC) the furan-based structure was progressively converted into highly curved aromatic pre-graphenic domains. This thermal degradation process was observed to produce an increasingly more hydrophobic surface and considerable microporosity within the HTC carbon structure. In order to introduce porosity in the HTC carbons derived from lignocellulosic biomass, KOH chemical activation was investigated as an HTC post-synthesis functionalisation step. These studies demonstrated that HTC carbons are excellent precursors for the production of highly microporous activated carbons (ACs) and that the porosity development upon KOH chemical activation is dependent on the chemical structure of the HTC carbon, tuned by employing different HTC temperatures. Preliminary testing of the ACs for CO2 capture or high pressure CH4 storage yielded very promising results, since the measured uptakes of both adsorbates (i.e. CO2 and CH4) were comparable to top-performing and commercially available adsorbents, usually employed for these end-applications. The combined use of HTC and KOH chemical activation was also employed to produce highly microporous N-doped ACs from microalgae. The hydrothermal treatment of the microalgae substrate was observed to cause the depletion of the protein and carbohydrate fractions and the near complete loss (i.e. 90%) of the microalgae N-content, as liquid hydrolysis/degradation products. The obtained carbonaceous product showed a predominantly aliphatic character indicating the presence of alkyl chains presumably derived from the lipid fractions. Addition of glucose to the initial reaction mixture was found out to be extremely beneficial, because it allowed the fixation of a higher N amount, in the algae derived HTC carbons (i.e. 60%), and the attainment of higher product yields (50%). Both positive effects were attributed to Maillard type cascade reactions taking place between the monosaccharides and the microalgae derived liquid hydrolysis/degradation products, which were in this way recovered from the liquid phase. KOH chemical activation of the microalgae/glucose mixture derived HTC carbons produced highly microporous N-doped carbons. Although the activation process led to a major reduction of the N-content, the retained N-amount in the ACs was still considerable. These features render these materials ideal candidates for supercapacitors electrodes, since they provide extremely high surface areas, for the formation of electric double-layer, coupled to abundant heteroatom doping (i.e. N and O) necessary to obtain a pseudocapacitance contribution.
The use of CO2 as a massive and polarizable drift gas is shown to greatly improve peak-to-peak resolution (Rp-p), as compared with N2, for the separation of disaccharides in a Synapt G2 traveling wave ion mobility cell. Near or baseline Rp-p was achieved for three pairs of sodiated molecules of disaccharide isomers, that is, cellobiose and sucrose (Rp-p?=?0.76), maltose and sucrose (Rp-p?=?1.04), and maltose and lactose (Rp-p?=?0.74). Ion mobility mass spectrometry using CO2 as the drift gas offers therefore an attractive alternative for fast and efficient separation of isomeric disaccharides.
The present thesis is to be brought into line with the current need for alternative and sustainable approaches toward energy management and materials design. In this context, carbon in particular has become the material of choice in many fields such as energy conversion and storage. Herein, three main topics are covered: 1)An alternative synthesis strategy toward highly porous functional carbons with tunable porosity using ordinary salts as porogen (denoted as “salt templating”) 2)The one-pot synthesis of porous metal nitride containing functional carbon composites 3)The combination of both approaches, enabling the generation of highly porous composites with finely tunable properties All approaches have in common that they are based on the utilization of ionic liquids, salts which are liquid below 100 °C, as precursors. Just recently, ionic liquids were shown to be versatile precursors for the generation of heteroatom-doped carbons since the liquid state and a negligible vapor pressure are highly advantageous properties. However, in most cases the products do not possess any porosity which is essential for many applications. In the first part, “salt templating”, the utilization of salts as diverse and sustainable porogens, is introduced. Exemplarily shown for ionic liquid derived nitrogen- and nitrogen-boron-co-doped carbons, the control of the porosity and morphology on the nanometer scale by salt templating is presented. The studies within this thesis were conducted with the ionic liquids 1-Butyl-3-methyl-pyridinium dicyanamide (Bmp-dca), 1-Ethyl-3-methyl-imidazolium dicyanamide (Emim-dca) and 1 Ethyl 3-methyl-imidazolium tetracyanoborate (Emim-tcb). The materials are generated through thermal treatment of precursor mixtures containing one of the ionic liquids and a porogen salt. By simple removal of the non-carbonizable template salt with water, functional graphitic carbons with pore sizes ranging from micro- to mesoporous and surface areas up to 2000 m2g-1 are obtained. The carbon morphologies, which presumably originate from different onsets of demixing, mainly depend on the nature of the porogen salt whereas the nature of the ionic liquid plays a minor role. Thus, a structural effect of the porogen salt rather than activation can be assumed. This offers an alternative to conventional activation and templating methods, enabling to avoid multiple-step and energy-consuming synthesis pathways as well as employment of hazardous chemicals for the template removal. The composition of the carbons can be altered via the heat-treatment procedure, thus at lower synthesis temperatures rather polymeric carbonaceous materials with a high degree of functional groups and high surface areas are accessible. First results suggest the suitability of the materials for CO2 utilization. In order to further illustrate the potential of ionic liquids as carbon precursors and to expand the class of carbons which can be obtained, the ionic liquid 1-Ethyl-3-methyl-imidazolium thiocyanate (Emim-scn) is introduced for the generation of nitrogen-sulfur-co-doped carbons in combination with the already studied ionic liquids Bmp-dca and Emim-dca. Here, the salt templating approach should also be applicable eventually further illustrating the potential of salt templating, too. In the second part, a one-pot and template-free synthesis approach toward inherently porous metal nitride nanoparticle containing nitrogen-doped carbon composites is presented. Since ionic liquids also offer outstanding solubility properties, the materials can be generated through the carbonization of homogeneous solutions of an ionic liquid acting as nitrogen as well as carbon source and the respective metal precursor. The metal content and surface area are easily tunable via the initial metal precursor amount. Furthermore, it is also possible to synthesize composites with ternary nitride nanoparticles whose composition is adjustable by the metal ratio in the precursor solution. Finally, both approaches are combined into salt templating of the one-pot composites. This opens the way to the one-step synthesis of composites with tunable composition, particle size as well as precisely controllable porosity and morphology. Thereby, common synthesis strategies where the product composition is often negatively affected by the template removal procedure can be avoided. The composites are further shown to be suitable as electrodes for supercapacitors. Here, different properties such as porosity, metal content and particle size are investigated and discussed with respect to their influence on the energy storage performance. Because a variety of ionic liquids, metal precursors and salts can be combined and a simple closed-loop process including salt recycling is imaginable, the approaches present a promising platform toward sustainable materials design.
Design and development of novel three color-FRET systems in synthetic peptides and oligonucleotides
(2012)
With ongoing miniaturization of electronic devices, the need for individually addressable, switchable molecules arises. An example are azobenzenes on surfaces which have been shown to be switchable between trans and cis forms. Here, we examine the "direct" (rather than substrate-mediated) channel of the trans -> cis photoisomerization after pi pi* excitation of tetra-tert-butyl-azobenzene physisorbed on surfaces mimicking Au(111) and Bi(111), respectively. In spirit of the direct channel, the electronic structure of the surface is neglected, the latter merely acting as a rigid platform which weakly interacts with the molecule via Van-der-Waals forces. Starting from thermal ensembles which represent the trans-form, sudden excitations promote the molecules to pi pi*-excited states which are non-adiabatically coupled among themselves and to a n pi*-excited and the ground state, respectively. After excitation, relaxation to the ground state by internal conversion takes place, possibly accompanied by isomerization. The process is described here by "on the fly" semiclassical surface hopping dynamics in conjunction with a semiempirical Hamiltonian (AM1) and configuration-interaction type methods. It is found that steric constraints imposed by the substrate lead to reduced but non-vanishing, trans -> cis reaction yields and longer internal conversion times than for the isolated molecule. Implications for recent experiments for azobenzenes on surfaces are discussed.
Purpose: Polymer networks with adjustable properties prepared from endgroup-functionalized oligoesters by UV-crosslinking in melt have evolved into versatile multifunctional biomaterials. In addition to the molecular weight or architecture of precursors, the reaction conditions for crosslinking are pivotal for the polymer network properties. Crosslinking of precursors in solution may facilitate low-temperature processes and are compared here to networks synthesized in melt.
Methods: Oligo(epsilon-caprolactone)-(z) methacrylate (oCL-(z) IEMA) precursors with a linear (z = di) or a four-armed star-shaped (z = tetra) architecture were crosslinked by radical polymerization in melt or in solution with UV irradiation. The thermal, mechanical, and swelling properties of the polymer networks obtained were characterized.
Results: Crosslinking in solution resulted in materials with lower Young's moduli (E), lower maximum stress (sigma(max)), and higher elongation at break (epsilon(B)) as determined at 70 degrees C. Polymer networks from 8 kDa star-shaped precursors exhibited poor elasticity when synthesized in the melt, but can be established as stretchable materials with a semi-crystalline morphology, a high gel-content, and a high elongation at break when prepared in solution.
Conclusions: The crosslinking condition of methacrylate functionalized precursors significantly affected network properties. For some types of precursors such as star-shaped telechelics, synthesis in solution provided semi-crystalline elastic materials that were not accessible from crosslinking in melt.
An experimental and computational study on the impact of functional groups on the oxidation stability of higher acenes is presented. We synthesized anthracenes, tetracenes, and pentacenes with various substituents at the periphery, identified their photooxygenation products, and measured the kinetics. Furthermore, the products obtained from thermolysis and the kinetics of the thermolysis are investigated. Density functional theory is applied in order to predict reaction energies, frontier molecular orbital interactions, and radical stabilization energies. The combined results allow us to describe the mechanisms of the oxidations and the subsequent thermolysis. We found that the alkynyl group not only enhances the oxidation stability of acenes but also protects the resulting endoperoxides from thermal decomposition. Additionally, such substituents increase the regioselectivity of the photooxygenation of tetracenes and pentacenes. For the first time, we oxidized alkynylpentacenes by using chemically generated singlet oxygen (O-1(2)) without irradiation and identified a 6,13-endoperoxide as the sole regioisomer. The bimolecular rate constant of this oxidation amounts to only 1 X 10(5) s(-1) M-1. This unexpectedly slow reaction is a result of a physical deactivation of O-1(2). In contrast to unsubstituted or aryl-substituted acenes, photooxygenation of alkynyl-substituted acenes proceeds most likely by a concerted mechanism, while the thermolysis is well explained by the formation of radical intermediates. Our results should be important for the future design of oxidation stable acene-based semiconductors.
Selective excitation of molecule-surface vibrations in H2 and D2 dissociatively adsorbed on Ru(0001)
(2012)
In this contribution we report about the selective vibrational excitation of H2 and D2 on Ru(0001) as an example for nonadiabatic coupling of an open quantum system to a dissipative environment. We investigate the possibility of achieving state-selective vibrational excitations of H2 and D2 adsorbed on a Ru(0001) surface using picosecond infrared laser pulses. The systems behavior is explored using pulses that are rationally designed and others that are optimized using a time-local variant of Optimal Control Theory. The effects of dissipation on the laser-driven dynamics are studied using the reduced-density matrix formalism. The non-adiabatic couplings between adsorbate and surface are computed perturbatively, for which our recently introduced state-resolved anharmonic rate model is used. It is shown that mode- and state-selective excitation can be achieved in the absence of dissipation when using optimized laser pulses. The inclusion of dissipation in the model reduces the state selectivity and the population transfer yield to highly excited states. In this case, mode activation is most effectively realized by a rational pulse of carefully chosen duration rather than by a locally optimized pulse.
Time-resolved emission data (fluorescence decay and fluorescence depolarization) of two three-color Forster resonance energy transfer (tc-FRET) systems consisting of a carbostyril donor (D), a ruthenium complex (Ru) as relay dye, and a Cy5 derivative (Cy) or, optionally, an anthraquinone quencher (Q) were carefully analyzed using advanced distribution analysis models. Thereby, it is possible to get information on the flexibility and mobility of the chromophores which are bound to double stranded (ds) DNA. Especially the distance distribution based on the analysis of the fluorescence depolarization is an attractive approach to complement data of fluorescence decay time analysis. The distance distributions extracted from the experimental data were in excellent agreement with those determined from accessible volume (AV) simulations. Moreover, the study showed that for tc-FRET systems the combination of dyes emitting on different time scales (e.g., nanoseconds vs microseconds) is highly beneficial in the distribution analysis of time-resolved luminescence data in cases where macromolecules such as DNA are involved. Here, the short lifetimes can yield information on the rotation of the dye molecule itself and the long lifetime can give insight in the overall dynamics of the macromolecule.
Aim: The hydrolytic degradation behavior of degradable aliphatic polyester-based polymers is strongly influenced by the uptake or transport of water into the polymer matrix and also the hydrolysis rate of ester bonds.
Methods: We examined the volumetric swelling behavior of poly[(rac-lactide)-co-glycolide] (PLGA) and PLGA-based polyurethanes (PLGA-PU) with water contents of 0 wt%, 2 wt% and 7 wt% water at 310 K using a molecular modeling approach. Polymer systems with a number average molecular weight of M-n = 10,126 g.mol(-1) were constructed from PLGA with a lactide content of 67 mol%, whereby PLGA-PU systems were composed of five PLGA segments with M-n = 2052 g.mol(-1), which were connected via urethane linkers originated from 2,2,4-trimethyl hexamethylene-1,6-diisocyanate (TMDI), hexamethyl-1,6-diisocyanate (HDI), or L-lysine-1,6-diisocyanate (LDI).
Results: The calculated densities of the dry PLGA-PU systems were found to be lower than for pure PLGA. The obtained volumetric swelling of the PLGA-PU was depending on the type of urethane linker, whereby all swollen PLGA-PUs contained larger free volume distribution compared to pure PLGA. The mean square displacement curves for dry PLGA and PLGA-PUs showed that urethane linker units reduce the mobility of the polymer chains, while an increase in backbone atoms mobility was found, when water was added to these systems. Consequently, an increased water uptake of PLGA-PU matrices combined with a higher mobility of the chain segments should result in an accelerated hydrolytic chain scission rate in comparison to PLGA.
Conclusions: It can be anticipated that the incorporation of urethane linkers might be a helpful tool to adjust the degradation behavior of polyesters.
Aim: Multifunctional polymer-based biomaterials, which combine degradability with a shape-memory capability and in this way enable the design of actively moving implants such as self-anchoring implants or controlled release systems, have been recently introduced. Of particular interest are approved degradable polymers such as poly(L-lactide) (PLLA), which can be easily functionalized with a shape-memory effect. In the case of semicrystalline PLLA, the glass transition can be utilized as shape-memory switching domain.
Methods: In this work we applied a fully atomistic molecular dynamics simulation to study the shape-memory behavior of PLLA. A heating-deformation-cooling programming procedure was applied to atomistic PLLA packing models followed by a recovery module under stress-free conditions allowing the shape recovery. The recovery was simulated by heating the samples from T-low = 250 K to T-high = 500 K with different heating rates beta of 125, 40 and 4 K.ns(-1).
Results: We could demonstrate that the obtained strain recovery rate (R-r) was strongly influenced by the applied simulation time and heating rate, whereby R-r values in the range from 46% to 63% were achieved. On its own the application of a heating rate of 4 K.ns(-1) enabled us to determine a characteristic switching temperature of T-sw = 473 K for the modeled samples.
Conclusions: We anticipate that the atomistic modeling approach presented should be capable of enabling further study of T-sw with respect to the molecular structure of the investigated SMP and therefore could be applied in the context of design and development of new shape-memory (bio) materials.
Carbon-based ionogels tuning the properties of the ionic liquid via carbon-ionic liquid interaction
(2012)
The behavior of two ionic liquids (ILs), 1-ethyl-3-methylimidazolium dicyanamide [Emim][DCA] and 1-ethyl-3-methylimidazolium triflate [Emim][TfO], in (meso) porous carbonaceous hosts was investigated. Prior to IL incorporation into the host, the carbon matrix was thermally annealed between 180 and 900 degrees C to control carbon condensation and surface chemistry. The resulting materials have an increasing "graphitic'' carbon character with increasing treatment temperature, reflected in a modified behavior of the ILs when impregnated into the carbon host. The two ILs show significant changes in the thermal behavior as measured from differential scanning calorimetry; these changes can be assigned to anion-pi interaction between the IL anions and the pore wall surfaces of these flexible carbonaceous support materials.
Für den Einsatz in Autobatterien gibt es besondere Anforderungen an den Elektrolyten im Bereich der Energie- und Leistungsdichten, um beispielsweise thermische Verluste gering zu halten. Hochleitfähige Elektrolyte mit Leitfähigkeiten im Millisiemensbereich sind hier ebenso notwendig wie auch sichere, d.h. möglichst nicht brennbare und einen niedrigen Dampfdruck besitzende Materialien. Um diese Vorgaben zu erreichen, ist es notwendig, einen polymeren Separator zu entwickeln, welcher auf brennbare organische Lösungsmittel verzichtet und damit eine drastische Steigerung der Sicherheit gewährleistet. Gleichzeitig müssen hierbei die Leistungsvorgaben bezüglich der Leitfähigkeit erfüllt werden. Zu diesem Zweck wurde ein Konzept basierend auf der Kombination von einer polymeren sauerstoffreichen Matrix und einer ionischen Flüssigkeit entwickelt und verifiziert. Dabei wurden folgende Erkenntnisse gewonnen: 1. Es wurden neuartige diacrylierte sauerstoffreiche Matrixkomponenten mit vielen Carbonylfunktionen, für eine gute Lithiumleitfähigkeit, synthetisiert. 2. Es wurden mehrere neue ionische Flüssigkeiten sowohl auf Imidazolbasis als auch auf Ammoniumbasis synthetisiert und charakterisiert. 3. Die Einflüsse der Kationenstruktur und der Einfluss der Gegenionen im Bezug auf Schmelzpunkte und Leitfähigkeiten wurden untersucht. 4. Aus den entwickelten Materialien wurden Blendsysteme hergestellt und mittels Impedanzspektrometrie untersucht: Leitfähigkeiten von 10-4S/cm bei Raumtemperatur sind realisierbar. 5. Die Blendsysteme wurden auf ihre thermische Stabilität hin untersucht: Stabilitäten bis 250°C sind erreichbar. Dabei wird keine kristalline Struktur beobachtet.
The new 3,4-propylenedioxythiophenes (ProDOT) bearing hydroxy- or chloro-functionalized side chains of varying length and polarity were synthesized and electropolymerized on single carbon fiber microelectrode (SCFME) using cyclo-voltammetry. Electrochemical impedance spectroscopy (EIS) revealed highest capacitance values for the hydroxy-functionalized Poly 5 carrying a side chain of medium length. The EIS data were fitted with an equivalent electrical circuit giving a good correlation. AFM analysis of the topography showed higher roughness values for Poly 5 than for the two other polymers bearing longer side chains. Due to their reactive end groups the polymers should be useful for post-polymerization functionalization of the electrode surface.
From the stem bark of Platycelphium voense (Leguminosae) four new isoflavanones were isolated and characterized as (S)-5,7-dihydroxy-2 ',4 '-dimethoxy-3 '-(3 ''-methylbut-2 ''-enyl)-isoflavanone (trivial name platyisoflavanone A), (+)-5,7,2 '-trihydroxy-4 '-methoxy-3 '-(3 ''-methylbut-2 ''-enyl)-isoflavanone (platyisoflavanone B), 5,7-dihydroxy-4 '-methoxy-2 ''-(2 '''-hydroxyisopropyl)-dihydrofurano-[4 '',5 '':3 ',2 ']-isoflavanone (platyisoflavanone C) and 5,7,2 ',3 ''-tetrahydroxy-2 '',2 ''-dimethyldihydropyrano-[5 '',6 '':3 ',4 ']-isoflavanone (platyisoflavanone D). In addition, the known isoflavanones, sophoraisoflavanone A and glyasperin F; the isoflavone, formononetin; two flavones, kumatakenin and isokaempferide; as well as two triterpenes, betulin and beta-amyrin were identified. The structures were elucidated on the basis of spectroscopic evidence. Platyisoflavanone A showed antibacterial activity against Mycobacterium tuberculosis in the microplate alamar blue assay (MABA) with MIC = 23.7 mu M, but also showed cytotoxicity (IC50 = 21.1 mu M) in the vero cell test.
Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the most important enzyme for the assimilation of carbon into biomass, features a well-known isotope effect with regards to the CO2 carbon atom. This kinetic isotope effect alpha = k (12)/k (13) for the carboxylation step of the RuBisCO reaction sequence, and its microscopic origin, was investigated with the help of cluster models and quantum chemical methods [B3LYP/6-31G(d,p)]. We use a recently proposed model for the RuBisCO active site, in which a water molecule remains close to the reaction center during carboxylation of ribulose-1,5-bisphosphate [B. Kannappan, J.E. Gready, J. Am. Chem. Soc. 130 (2008), 15063]. Alternative active-site models and/or computational approaches were also tested. An isotope effect alpha for carboxylation is found, which is reasonably close to the one measured for the overall reaction, and which originates from a simple frequency shift of the bending vibration of (CO2)-C-12 compared to (CO2)-C-13. The latter is the dominant mode for the product formation at the transition state.
Silver nanoparticles (SNPs) are among the most commercialized nanoparticles worldwide. Often SNP are used because of their antibacterial properties. Besides that they possess unique optic and catalytic features, making them highly interesting for the creation of novel and advanced functional materials. Despite its widespread use only little data exist in terms of possible adverse effects of SNP on human health. Conventional synthesis routes usually yield products of varying quality and property. It thus may become puzzling to compare biological data from different studies due to the great variety in sizes, coatings or shapes of the particles applied. Here, we applied a novel synthesis approach to obtain SNP of well-defined colloidal and structural properties. Being stabilized by a covalently linked small peptide, these particles are nicely homogenous, with narrow size distribution, and form monodisperse suspensions in aqueous solutions. We applied these peptide-coated SNP in two different sizes of 20 or 40 nm (Ag20Pep and Ag40Pep) and analyzed responses of THP-1-derived human macrophages while being exposed against these particles. Gold nanoparticles of similar size and coating (Au20Pep) were used for comparison. The cytotoxicity of particles was assessed by WST-1 and LDH assays, and the uptake into the cells was confirmed via transmission electron microscopy. In summary, our data demonstrate that this novel type of SNP is well suited to serve as model system for nanoparticles to be tested in toxicological studies in vitro.
In the body, nanoparticles can be systemically distributed and then may affect secondary target organs, such as the central nervous system (CNS). Putative adverse effects on the CNS are rarely investigated to date. Here, we used a mixed primary cell model consisting mainly of neurons and astrocytes and a minor proportion of oligodendrocytes to analyze the effects of well-characterized 20 and 40 nm silver nanoparticles (SNP). Similar gold nanoparticles served as control and proved inert for all endpoints tested. SNP induced a strong size-dependent cytotoxicity. Additionally, in the low concentration range (up to 10 mu g/ml of SNP), the further differentiated cultures were more sensitive to SNP treatment. For detailed studies, we used low/medium dose concentrations (up to 20 mu g/ml) and found strong oxidative stress responses. Reactive oxygen species (ROS) were detected along with the formation of protein carbonyls and the induction of heme oxygenase-1. We observed an acute calcium response, which clearly preceded oxidative stress responses. ROS formation was reduced by antioxidants, whereas the calcium response could not be alleviated by antioxidants. Finally, we looked into the responses of neurons and astrocytes separately. Astrocytes were much more vulnerable to SNP treatment compared with neurons. Consistently, SNP were mainly taken up by astrocytes and not by neurons. Immunofluorescence studies of mixed cell cultures indicated stronger effects on astrocyte morphology. Altogether, we can demonstrate strong effects of SNP associated with calcium dysregulation and ROS formation in primary neural cells, which were detectable already at moderate dosages.
Extration von Hexachloroplatinat(IV) mit phoshorylierten Merrifield-Harzen aus salzsauren Lösungen
(2012)
A series of diols (diethylene glycol, triethylene glycol, butane-1,4-diol and hexane-1,6-diol) were immobilized onto Merrifield resin and subsequently phosphorylated with dialkyl chlorophosphate (alkyl = Me, Et, Bu). The resins bearing hexane-1,6-diyl groups exhibited very good extraction abilities in regard to precious metal chloro complexes like platinum(IV), palladium(II) and rhodium(III). In batch experiments, more than 98% of Pt(IV) is extracted even when the metal and the hydrochloric acid concentration is enhanced significantly. Elution can be achieved with a solution of 0.5 mol L-1 thiourea in 0.1 mol L-1 hydrochloric acid. In the presence of other noble metals, platinum(IV) is preferentially bound. The extraction yield decreases in slightly acidic solution in the following order: Pt(IV)approximate to Pd(II)>Rh(III) and changes with increasing hydrochloric acid concentration to Pt(IV)>Pd(II)>> Rh(III). At different ratios of metal and acid, the temperature has nearly no influence on the platinum extraction. On slightly acidic media, the extraction of rhodium decreases by 30% when the temperature is increased from 10 degrees C to 40 degrees C. When the acid and metal concentration is enhanced, the palladium extraction decreases by 7-9%, depending on the resin.
Typische assoziative Verdicker für wässrige Systeme basieren auf linearen, doppelt hydrophob endmodifizierten Poly(ethylenglykolen) (PEGs). Diese Polymere aggregieren aufgrund ihrer Struktur in wässriger Lösung und bilden ein Netzwerk aus verbrückten Polymer-Mizellen. Dabei kann ein Polymer-Molekül maximal zwei Mizellen miteinander verbinden. Bisher ist unklar, wie die Anzahl der Endgruppen eines verzweigten, mehrfach hydrophob endmodifizierten hydrophilen Polymers die Struktur und Dynamik solcher Netzwerke beeinflusst. Die Synthese verzweigter Polymere auf PEG-Basis erfolgt mittels lebender ionischer Polymerisation und ist experimentell aufwändig. Das Einführen hydrophober Endgruppen erfordert zusätzliche Synthese-Schritte. In dieser Arbeit wurden hydrophile Sternpolymere mit hydrophoben Endgruppen in einem Schritt hergestellt. Dazu wurde die Technik der radikalischen Polymerisation unter Kettenübertragung durch reversible Addition und anschließende Fragmentierung (reversible addition-fragmentation chain transfer, RAFT) genutzt. Die Synthese der Sternpolymere erfolgte von einem multifunktionalen Kern, der die R-Gruppe der RAFT-Kettenüberträger (chain transfer agents, CTAs) bildete. Die dazu benötigten CTAs wurden so konzipiert, dass mit ihrer Hilfe sowohl die Anzahl der Arme des Sternpolymers (von 2 bis 4), als auch die Länge der hydrophoben Endgruppe (C4, C12, C18) variiert werden konnte. Der große Vorteil der RAFT-Polymerisation ist, dass sie viele polare Monomere für die Synthese der hydrophilen Arme des Sternpolymers toleriert. In dieser Arbeit wurden als Modell-Monomere Oligo(ethylenglykol)methylether-acrylat (OEGA) und N,N-Dimethylacrylamid (DMA) eingesetzt. Beide Monomere bilden nicht-ionische hydrophile Polymere. Poly(OEGA) ist ein Kammpolymer, das auf PEG basiert. Poly(DMA) besitzt dagegen eine deutlich kompaktere Struktur. Die erhaltenen amphiphilen Sternpolymere wurden umfassend molekular charakterisiert. Die Molmassen wurden mit verschiedenen GPC-Systemen bestimmt und der Grad der Endgruppenfunktionalisierung wurde mittels UV/Vis- und 1H-NMR-Spektroskopie überprüft. Die Polymerisation von OEGA zeigt mit den CTAs einige Charakteristika der Polymerisation mit reversibler Deaktivierung (RDRP, auch „kontrollierte radikalische Polymerisation“), wird aber durch Kettenübertragung zum Monomer bzw. Polymer gestört. Diese Nebenreaktion ist auf die Struktur des Monomers als Oligoether zurückzuführen. Bei allen untersuchten Polymerisationen von DMA mit den multifunktionalen CTAs steigt die Molmasse linear mit dem Umsatz. Die erhaltenen Polymere zeigen durchweg monomodale und enge Molmassenverteilungen (PDI ≤ 1,2). Die Molmassen lassen sich in einem weiten Bereich von 25 kg/mol bis 150 kg/mol einstellen und die Endgruppen der Polymere bleiben zu 90 % erhalten. Während die Polymerisation von DMA sowohl mit den di- als auch den trifunktionalen CTAs innerhalb von 3 h zu quantitativen Umsätzen verläuft, wird der quantitative Umsatz des Monomers bei der Polymerisation mit tetrafunktionalen CTAs erst nach 4 h erreicht. Diese Verzögerung ist auf eine Retardierung in der Anfangsphase der Polymerisation zurückzuführen, die sich aus der besonderen Struktur der tetrafunktionalen CTAs erklärt. Auf dem System zur Polymerisation von DMA aufbauend ließen sich Gradienten-Block-Copolymere in Eintopfreaktionen herstellen. Dazu wurde nach Erreichen des quantitativen Umsatzes von DMA ein zweites Monomer zur Reaktionsmischung gegeben. Mit Ethylacrylat (EtA) wurden so lineare amphiphile symmetrische Triblock-Copolymere erhalten. Dabei wurde die Länge des hydrophoben Blocks durch unterschiedliche Mengen an EtA variiert. Mit N,N-Diethylacrylamid (DEA) wurden lineare symmetrische Triblock-Copolymere sowie 3-Arm Stern-Diblock-Copolymere hergestellt, die über einen thermisch schaltbaren zweiten Block verfügen. Bei diesen Polymeren lässt sich die Länge des hydrophoben Teils in situ durch Veränderung der Temperatur variieren. Das Verhalten der amphiphilen Sternpolymere in wässriger Lösung und in Mikroemulsion wurde im Rahmen einer Kooperation an der TU Berlin mit Hilfe von Kleinwinkel-Neutronenstreuung (SANS), dynamischer Lichtstreuung (DLS) und Rheologie untersucht. Die Polymere wirken durch Assoziation der hydrophoben Endgruppen als effektive Verdicker sowohl allein in wässriger Lösung als auch in Mikroemulsion. Die Struktur des gebildeten Netzwerks hängt dabei von der Konzentration des Polymers in der Lösung und der Länge der Endgruppe (Hydrophobie) ab. Die dynamischen Eigenschaften der Lösungen werden außerdem durch die Anzahl der Arme der Polymere bestimmt.
Multifunctional chain transfer agents for RAFT polymerisation were designed for the one-step synthesis of amphiphilic star polymers. Thus, hydrophobically end-capped 3- and 4-arm star polymers, as well as linear ones for reference, were made of the hydrophilic monomer N,N-dimethylacrylamide (DMA) in high yield with molar masses up to 150 000 g mol(-1), narrow molar mass distribution (PDI <= 1.2) and high end group functionality (similar to 90%). The associative telechelic polymers form transient networks of interconnected aggregates in aqueous solution, thus acting as efficient viscosity enhancers and rheology modifiers, eventually forming hydrogels. The combination of dynamic light scattering (DLS), small angle neutron scattering (SANS) and rheology experiments revealed that several molecular parameters control the structure and therefore the physical properties of the aggregates. In addition to the size of the hydrophilic block (maximum length for connection) and the length of the hydrophobic alkyl chain ends (stickiness), the number of arms (functionality) proved to be a key parameter.
Radical addition fragmentation chain transfer (RAFT) polymerization of ferrocenyl (meth)acrylates
(2012)
We report on the controlled free radical homopolymerization of 1-ferrocenylethyl acrylate as well as of three new ferrocene bearing monomers, namely 4-ferrocenylbutyl acrylate, 2-ferrocenylamido-2-methylpropyl acrylate, and 4-ferrocenylbutyl methacrylate, by the RAFT technique. For comparison, the latter monomer was polymerized using ATRP, too. The ferrocene containing monomers were found to be less reactive than their analogues free of ferrocene. The reasons for the low polymerizability are not entirely clear. As the addition of free ferrocene to the reaction mixture did not notably affect the polymerizations, sterical hindrance by the bulky ferrocene moiety fixed on the monomers seems to be the most probable explanation. Molar masses found for 1-ferrocenylethyl acrylate did not exceed 10,000 g mol(-1), while for 4-ferrocenylbutyl (meth) acrylate molar masses of 15,000 g mol(-1) could be obtained. With PDIs as low as 1.3 in RAFT polymerization of the monomers, good control over the polymerization was achieved.
Applications based on Forster resonance energy transfer (FRET) play an important role for the determination of concentrations and distances within nanometer-scale systems in vitro and in vivo in many fields of biotechnology. Semiconductor nanocrystals (Quantum dots - QDs) possess ideal properties for their application as FRET acceptors when the donors have long excited state lifetimes and when direct excitation of QDs can be efficiently suppressed. Therefore, luminescent terbium complexes (LTCs) with excited state lifetimes of more than 2 ms are ideal FRET donor candidates for QD-acceptors. This chapter will give a short overview of theoretical and practical background of FRET, QDs and LTCs, and present some recent applications of LTC-QD FRET pairs for multiplexed ultra-sensitive in vitro diagnostics and nanometer-resolution molecular distance measurements.
Two new naphthoquinones, 5-hydroxy-3,6-dimethoxy-2-methylnaphthalene-1,4-dione and 5,8-dihydroxy-3-methoxy-2-methylnaphthalene-1,4-dione, were isolated from the roots of Aloe secundiflora together with the known compounds chrysophanol, helminthosporin, isoxanthorin, ancistroquinone C, aloesaponarins I and II, aloesaponols I and II, laccaic acid D methyl ester and asphodelin. The structures were elucidated based on spectroscopic evidence. This appears to be the first report on the occurrence of naphthoquinones in the genus Aloe. Aloesaponarin I and 5-hydroxy-3,6-dimethoxy-2-methylnaphthalene-1,4-dione showed anti-bacterial activity against Mycobacterium tuberculosis with MIC values of 21-23 mu g/mL in the Microplate Alamar Blue Assay (MABA) and Low Oxygen Recovery Assay (LORA); 5-hydroxy-3,6-dimethoxy-2-methylnaphthalene-1,4-dione also showed cytotoxicity against the Vero cell line (IC50 = 10.2 mu g/mL).
When a hydrogen (H) atom approaches a deuterium (D-2) molecule, the minimum-energy path is for the three nuclei to line up. Consequently, nearly collinear collisions cause HD reaction products to be backscattered with low rotational excitation, whereas more glancing collisions yield sideways-scattered HD products with higher rotational excitation. Here we report that measured cross sections for the H + D-2 -> HD(v' = 4, j') + D reaction at a collision energy of 1.97 electron volts contradict this behavior. The anomalous angular distributions match closely fully quantum mechanical calculations, and for the most part quasiclassical trajectory calculations. As the energy available in product recoil is reduced, a rotational barrier to reaction cuts off contributions from glancing collisions, causing high-j' HD products to become backward scattered.
Porous materials (e.g. zeolites, activated carbon, etc.) have found various applications in industry, such as the use as sorbents, catalyst supports and membranes for separation processes. Recently, much attention has been focused on synthesizing porous polymer materials. A vast amount of tailor-made polymeric systems with tunable properties has been investigated. Very often, however, the starting substances for these polymers are of petrochemical origin, and the processes are all in all not sustainable. Moreover, the new polymers have challenged existing characterizing methodologies. These have to be further developed to address the upcoming demands of the novel materials. Some standard techniques for the analysis of porous substances like nitrogen sorption at 77 K do not seem to be sufficient to answer all arising questions about the microstructure of such materials. In this thesis, microporous polymers from an abundant natural resource, betulin, will be presented. Betulin is a large-scale byproduct of the wood industry, and its content in birch bark can reach 30 wt.%. Based on its rigid structure, polymer networks with intrinsic microporosity could be synthesized and characterized. Apart from standard nitrogen and carbon dioxide sorption at 77 K and 273 K, respectively, gas sorption has been examined not only with various gases (hydrogen and argon) but also at various temperatures. Additional techniques such as X-ray scattering and xenon NMR have been utilized to enable insight into the microporous structure of the material. Starting from insoluble polymer networks with promising gas selectivities, soluble polyesters have been synthesized and processed to a cast film. Such materials are feasible for membrane applications in gas separation. Betulin as a starting compound for polyester synthesis has aided to prepare, and for the first time to thoroughly analyse a microporous polyester with respect to its pores and microstructure. It was established that nitrogen adsorption at 87 K can be a better method to solve the microstructure of the material. In addition to that, other betulin-based polymers such as polyurethanes and polyethylene glycol bioconjugates are presented. Altogether, it has been shown that as an abundant natural resource betulin is a suitable and cheap starting compound for some polymers with various potential applications.
A series of new monocationic iridium(III) complexes [Ir((CN)-N-boolean AND)(2)((NN)-N-boolean AND)]PF6 with "large-surface" alpha,alpha'-diimin ligands (NN)-N-boolean AND (dap = 1,12-diazaperylene, dmedap = 2,11-dimethyl-1,12-diazaperylene, dipdap = 2,11-diisopropyl-1,12-diazaperylene) and different cyclometalating ligands (CN)-N-boolean AND (piq = 1-phenylisoquinoline, bzq = benzo[h]quinoline, ppz = 1-phenylpyrazole, thpy = 2-(2-thienyl)pyridine, ppy = 2-phenylpyridine, meppy = 2-(4-methylphenyl)pyridine, dfppy = 2-(2,4-difluorophenyl)pyridine) were synthesized. The solid structures of the complexes [Ir(piq)(2)(dap)]PF6, [Ir(bzq)(2)(dap)]PF6, [Ir(ppy)(2)(dipdap)]PF6, [Ir(piq)(2)(dmedap)]PF6, [Ir(ppy)(2)(dap)]PF6 and [Ir(ppz)(2)(dap)]PF6 are reported. In [Ir(piq)(2)(dap)]PF6, the dap ligand and one of the piq ligands of each cationic complex are involved in pi-pi stacking interactions forming supramolecular channels running along the crystallographic c axis. In the crystalline [Ir(bzq)(2)(dap)]PF6 pi-pi stacking interactions between the metal complexes lead to the formation of a 2D layer structure. In addition, CH-pi interactions were found in all compounds, which are what stabilizes the solid structure. In particular, a significant number of them were found in [Ir(piq)(2)(dap)]PF6 and [Ir(bzq)(2)(dap)]PF6. The crystal structures of [Ir(ppy)(2)(dipdap)]PF6 and [Ir(ppy)(2)(dmedap)]PF6 are also presented, being the first examples of bis-cyclometalated iridium(III) complexes with phenanthroline-type alpha,alpha'-diimin ligands bearing bulky alkyl groups in the neighbourhood of the N-donor atoms. These ligands implicate a distorted octahedral coordination geometry that in turn destabilized the Ir-N-N boolean AND N bonds. The new iridium (III) complexes are not luminescent. All compounds show an electrochemically irreversible anodic peak between 1.15 and 1.58 V, which is influenced by the different cyclometalated ligands. All of the new complexes show two reversible successive one-electron "large-surface" ligand-centred reductions around -0.70 V and -1.30 V. Electrospray ionisation mass spectrometry (ESI-MS) and collision induced decomposition (CID) measurements were used to investigate the stability of the new complexes. Thereby, the stability agreed well with the order of the Ir-N-N boolean AND N bond lengths.
We present quantum simulations of a vibrating hydrogen molecule H-2 and address the issue of electron correlation. After appropriately setting the frame and the observer plane, we were able to determine precisely the number of electrons and nuclei which actually flow by evaluating electronic and nuclear fluxes. This calculation is repeated for three levels of quantum chemistry, for which we account for no correlation, Hartree-Fock, static correlation, and dynamic correlation. Exciting each of these systems with the same amount of energy, we show that the electron correlation can be revealed with the knowledge of quantum fluxes. This is evidenced by a clear sensitivity of these fluxes to electron correlation. In particular, we find that this correlation remarkably enhances more electronic yield than the nuclear one. It turns out that less electrons accompany the nuclei in Hartree-Fock than in the correlation cases.
In the present study, the effects of ionic liquids (ILs) on the stability of nanoparticles in several IL compositions were investigated. In this context, we examined the primary role of ILs in the synthesis of metal/metal oxide nanoparticles and their dispersions extensively. However, the focus of the discussion in this communication centers mainly on the effect of EMIM Ethyl Sulfate on growth and stability of nanoparticles. The dispersion properties of ILs based on their ability to aid the synthesis of uniformly dispersed nanoparticles have been further explored to produce nanoparticles of an effective catalyst useful in water purification, soil remediation and battery applications. Two independent protocols were developed for the synthesis of nanoparticles, namely (a) one pot process via chemical reduction (b) dispersion of the inorganic material in ILs. The protocols are simple, sustainable and environmentally friendly because the processes are conducted in ILs as harmless non-toxic green solvent materials. The catalysts were analyzed by x-ray diffraction, electron microscopy, UV visible spectroscopy and dynamic light scattering as the main methodologies.
We report on a new three-color FRET system consisting of three fluorescent dyes, i.e., of a carbostyril (=quinolin-2(1H)-one)-derived donor D, a (bathophenanthroline)ruthenium complex as a relay chromophore A1, and a Cy dye as A2 (FRET=Forster resonance-energy-transfer) (cf. Fig. 1). With their widely matching spectroscopic properties (cf. Fig. 2), the combination of these dyes yielded excellent FRET efficiencies. Furthermore, fluorescence lifetime measurements revealed that the long fluorescence lifetime of the Ru complex was transferred to the Cy dye offering the possibility to measure the whole system in a time-resolved mode. The FRET system was established on double-stranded DNA (cf. Fig. 3) but it should also be generally applicable to other biomolecules.
The conformational analysis of the first representative of the Si-alkoxy substituted six-membered Si,N-heterocycles, 1,3-dimethyl-3-isopropoxy-3-silapiperidine, was performed by low-temperature 1H and 13C NMR spectroscopy and DFT theoretical calculations. In contrast to the expectations from the conformational energies of methyl and alkoxy substituents, the Meaxi-PrOeq conformer was found to predominate in the conformational equilibrium in the ratio Meaxi-PrOeq : Meeqi-PrOax of ca. 2 : 1 as from the 1H and 13C NMR study. The thermodynamic parameters obtained by the complete line shape analysis showed that the main contribution to the barrier to ring inversion originates from the entropy term of the free energy of activation.
Die Photophysik und Photochemie von Flavinen sind aufgrund ihrer biologischen Funktion, inbesondere von Flavoproteinen, von großen Interesse. Flavoproteine spielen eine große Rolle in einer Vielzahl von biologischen Prozessen, z.B. Biolumineszenz, Entfernung von Radikalen, die bei oxidativem Stress entstehen, Photosynthese und DNA-Reparatur. Die spektroskopischen Eigenschaften des Flavin-Cofaktors machen diesen zu einem natürlichen Reporter für Veränderungen innerhalb des aktiven Zentrums. Deshalb sind die Flavoproteine eine der am meisten untersuchten Enzymfamilien. Eine biologische Aktivität des Flavins führt über einen elektronisch angeregten Zustand, wo dann, abhängig von der Aminosäureumgebung, ein bestimmter Mechanismus zu einem biologischen Prozess führt (Photozyklus). Ein wichtiges Analysetool zum Verständnis des anfänglichen Photoanregungsschritts der Flavine sind die elektronische und die Schwingungsspektroskopie. In dieser Arbeit wurden die Prozesse von Riboflavin (RF) während und nach optischer Anregung mit theoretischen Mitteln beleuchtet. Dazu wurden quantenchemische Berechnungen für Schwingungsspektren (vibratorische) von Riboflavin, auch Laktoflavin oder Vitamin B2 genannt, dem Grundmolekül der Chromophore biologischer Blaulichtrezeptoren, in dessen elektronischem Grundzustand und dessen niedrigsten angeregten Zustand durchgeführt. Weiterhin wurden vibronische (vibratorische+elektronische) Absorptionsspektren und ein vibronisches Emissionsspektrum berechnet. Die so berechneten Schwingungs- und elektronischen Spektren sind in guter qualitativer wie quantitativer Übereinstimmung mit gemessenen Werten, und helfen so, die experimentellen Signale der Photoanregung von Flavinen zuzuweisen. Unmittelbar nach der Photoanregung wurde ein Verlust des Doppelbindungscharakters im polaren Bereich des Ringssystems beobachtet, was zu der vibronischen Feinstruktur im elektronischen Absorptions- und Emissionsspektrum führte. Hier zeigte sich zudem, dass neben den vibronischen Effekten auch die Lösungsmitteleffekte wichtig für das quantitative Verständnis der Photophysik der Flavine in Lösung sind. Um Details des optischen Anregungsprozesses als initialen, elementaren Schritt zur Signalweiterleitung zu entschlüsseln, wurden ultraschnelle (femtosekundenaufgelöste) Experimente durchgeführt, die die Photoaktivierung des Flavins untersuchen. Diese Arbeit soll zu einem weiteren Verständnis und der Interpretation dieser Experimente durch das Studium der Post-Anregungsschwingungsdynamik von Riboflavin und mikrosolvatisiertem Riboflavin beitragen. Dazu wurde eine 200 fs lange Molekulardynamik in angeregten Zuständen betrachtet. Durch die Analyse charakteristischer Atombewegungen und durch die Berechnungen zeitaufgelöster Emissionsspektren fand man heraus, dass nach der optischen Anregung Schwingungen im Ringssystem des Riboflavins einsetzen. Mit Hilfe dieser Berechnungen kann die Umverteilung der Energie im angeregten Zustand beobachtet werden. Neben den theoretischen Untersuchungen zu Riboflavin in der Gasphase und auch in Lösung wurde ein Modell für eine BLUF (Blue-Light Photoreceptor Using Flavin) Domäne, ein Flavin benutzender Photorezeptor, erstellt. Hierbei zeigt sich, dass man die in dieser Arbeit angewendeten Analysemethoden auch auf biologisch relevante Systeme anwenden kann.
Flavins are chromophores in light-gated enzymes and therefore central in many photobiological processes. To unravel the optical excitation process as the initial, elementary step towards signal transduction, detailed ultrafast (femtosecond) experiments probing the photo-activation of flavins have been carried out recently [Weigel et al., J. Phys. Chem. B, 2011, 115, 3656-3680.]. The present paper contributes to a further understanding and interpretation of these experiments by studying the post-excitation vibrational dynamics of riboflavin (RF) and microsolvated riboflavin, RF center dot 4H(2)O, using first principles non-adiabatic molecular dynamics. By analyzing the characteristic atom motions and calculating time-resolved stimulated emission spectra following pi pi* excitation, it is found that after optical excitation C-N and C-C vibrations in the isoalloxazine rings of riboflavin set in. The Franck-Condon (vertically excited) state decays within about 10 fs, in agreement with experiment. Anharmonic coupling leads to Intramolecular Vibrational energy Redistribution (IVR) on the timescale of about 80-100 fs, first to (other) C-C stretching modes of the isoalloxazine rings, then by energy spread over the whole molecule, including low-frequency in-plane modes. The IVR is accompanied by a red-shift and broadening of the emission spectrum. When RF is microsolvated with four water molecules, an overall redshift of optical spectra by about 20 nm is observed but the relaxation dynamics is only slightly affected. For several trajectories, a tendency for hydrogen transfer from water to flavin-nitrogen (N-5) was found.
The minima on the potential energy surface of 1,2-bis(o-carboxyphenoxy) ethane (CPE) molecule in its electronic ground state were searched by a molecular dynamics simulation performed with MM2 force field. For each of the found minimum-energy conformers, the corresponding equilibrium geometry, charge distribution, HOMO-LUMO energy gap, force field, vibrational normal modes and associated IR and Raman spectral data were determined by means of the density functional theory (DFT) based electronic structure calculations carried out by using B3LYP method and various Pople- style basis sets. The obtained theoretical data confirmed the significant effects of the intra- and inter-molecular hydrogen bonding interactions on the conformational structure, force field, and group vibrations of the molecule. The same data have also revealed that two of the determined stable conformers, both of which exhibit pseudo-crown structure, are considerably more favorable in energy to the others and accordingly provide the major c ntribution to the experimental spectra of CPE. In the light of the improved vibrational spectral data obtained within the "SQM FF" methodology and "Dual Scale Factors" approach for the monomer and dimer forms of these two conformers, a reliable assignment of the fundamental bands observed in the experimental room-temperature IR and Raman spectra of the molecule was given, and the sensitivities of its group vibratb20s to conformation, substitution and dimerization were discussed.
Based on the nucleus-independent chemical shift (NICS) concept, isotropic magnetic shielding values have been computed along the three Cartesian axes for ethene, cyclobutadiene, benzene, naphthalene, and benzocyclobutadiene, starting from the molecular/ring center up to 10;Å away. These through-space NMR spectroscopic shielding (TSNMRS) values, which reflect the anisotropic effects, have been broken down into contributions from localized- and canonical molecular orbitals (LMOs and CMOs); these contributions revealed that the proton NMR spectroscopic chemical shifts of nuclei that are spatially close to the C=C double bond or the aromatic ring should not be explained in terms of the conventionally accepted ;-electron shielding/deshielding effects. In fact, these effects followed the predictions only for the antiaromatic cyclobutadiene ring.
Novel piperidine-fused benzoxazino- and quinazolinonaphthoxazines-synthesis and conformational study
(2012)
The reactions of 1-(amino(2-hydroxyphenyl)methyl)-2-naphthol (3) and 1-(amino(2-aminophenyl)methyl)-2-naphthol (6) with glutardialdehyde resulted in the formation of piperidine-fused benzoxazinonaphthoxazine 4 and quinazolinonaphthoxazine 7, respectively, both in diastereopure form. The full conformational search protocols of 4 and 7 were successfully carried out by NMR spectroscopy and accompanying molecular modelling; the global minimum-energy conformers of all diastereomers were computed, and the assignments of the most stable stereoisomers, Gtct1 for 4 and Gtct1 for 7, were corroborated by spatial NOE information relating to the H7a-H10a-H15b and H,H coupling patterns of the protons in the flexible part of the piperidine moiety. Additionally, mass spectrometric fragmentation was investigated in collision-induced dissociation experiments. The elemental compositions of the ions were determined by accurate mass measurements.
Syntheses and conformational analyses of new naphth[1,2-e][1,3]oxazino[3,2-c]quinazolin-13-ones
(2012)
The syntheses of naphth[1,2-e][1,3]oxazino[3,2-c]quinazolin-13-one derivatives (3a-f) were achieved by the solvent-free heating of benzyloxycarbonyl-protected intermediates (2a-f) with MeONa. For intermediates 2a-f, prepared by the reactions of substituted aminonaphthols with benzyl N-(2-formylphenyl)carbamate, not only the expected trans ring form B and chain form A1, but also the rearranged chain form A2 as a new tautomer were detected in DMSO at room temperature. The quantity of A2 in the tautomeric mixture was changed with time. Conformational analyses of the target heterocycles 3a-f by NMR spectroscopy and accompanying theoretical calculations at the DFT level of theory revealed that the oxazine ring preferred a twisted chair conformation and the quinazolone ring was planar. Besides the conformations, both the configurations at C-7a and C-15 and the preferred rotamers of the 1-naphthyl substituent at C-15 were assigned, which allowed evaluation of the aryl substituent-dependent steric hindrance in this part of the molecules. Configurational assignments were corroborated by quantifying the ring current effect of 15-aryl in terms of spatial NICS.
The esters of 4-hydroxy-cyclohexanone and a series of carboxylic acids R-COOH with R of different electronic and steric influence (R=Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, sec-Bu, t-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, and CBr3) were synthesized and the conformational equilibria studied by 1H and 13C NMR spectroscopy at 103 K and at 295 K, respectively. The geometry of optimized structures of the axial/equatorial chair conformers was computed at the ab initio MO and DFT levels of theory. Only one preferred conformation was obtained for the axial and the equatorial conformer as well. When comparing the conformational equilibria of the cyclohexanone esters with those of the corresponding cyclohexyl esters a certain polarity contribution of the cyclohexanone framework was revealed, which is independent of the substituent effects and increases the stability of the axial conformers by a constant amount.
The esters of 4-hydroxy-cyclohexanone and a series of carboxylic acids R-COOH with R of different electronic and steric influence (R=Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, sec-Bu, t-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, and CBr3) were synthesized and the conformational equilibria studied by H-1 and C-13 NMR spectroscopy at 103 K and at 295 K, respectively. The geometry of optimized structures of the axial 'equatorial chair conformers was computed at the ab initio MO and DFT levels of theory. Only one preferred conformation was obtained for the axial and the equatorial conformer as well. When comparing the conformational equilibria of the cyclohexanone esters with those of the corresponding cyclohexyl esters a certain polarity contribution of the cyclohexanone framework was revealed, which is independent of the substituent effects and increases the stability of the axial conformers by a constant amount.
The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of benzenoid and quinoid tautomeric structures such as benzodifurantrione and phenazine-type molecules have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Rague Schleyer and visualized as iso- chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to quantify and visualize the partial aromaticity of the studied compounds. In the case of the surprisingly more stable quinoid tautomers, the aromaticity-synonymous with stability due to the conjugation of p electrons and lone pairs-was not found to be particularly reduced.
The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of benzenoid and quinoid tautomeric structures such as benzodifurantrione and phenazine-type molecules have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Rague Schleyer and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to quantify and visualize the partial aromaticity of the studied compounds. In the case of the surprisingly more stable quinoid tautomers, the aromaticity-synonymous with stability due to the conjugation of p electrons and lone pairs-was not found to be particularly reduced.
The spatial magnetic properties (through-space NMR shieldings, or TSNMRSs) of the antiaromatic 9-oxaanthracene anion 12(-) and of the corresponding 9-dimeric dianion 11(2-) have been calculated by the gauge-invariant atomic orbitals (GIAO) perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs) of various size and direction. The TSNMRS values, thus obtained, can be employed to indicate antiaromaticity by paratropic ring currents of the anionic compounds of 11(2-) and 12(-) studied and other neutral and ionic antiaromatic molecules from previous studies because anisotropic effects of functional groups in H-1 NMR spectra have quantitatively proven to be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS).
A simple and efficient method for the conversion of alcohols and phenols to primary O-thiocarbamates and S- thiocarbamates in the absence of solvent (solvent-free condition) using silica sulfuric acid (SiO2OSO3H) as a solid acid is described. The products are easily distinguished by IR, NMR and X-ray data. X-ray data of the compounds reveal a planar trigonal orientation of the NH2 nitrogen atom with the partial C,N double-bond character and the CS or CO groups in synperiplanar position with CarylO and CalkylS moieties, respectively. Moreover, the OCSNH2 group which is perpendicular to the plane of the benzene ring in 1c and the central thiocarbamate SCONH2 group in 2b are essentially planar.
1,3-Dimethyl-3-phenyl-1,3-azasilinane was synthesized and its conformational behavior was studied by the low temperature NMR spectroscopy and quantum chemical calculations. The compound was shown to exist as an equilibrium mixture of the PhaxMeeq and PheqMeax chair conformers with the N-methyl substituent in equatorial position. The barrier to ring inversion was also determined.
Syntheses of thiazolidine-fused heterocycles via exo-mode cyclizations of vinylogous N-acyliminium ions incorporating heteroatom-based nucleophiles have been examined and discussed. The formation of (5,6)-membered systems was feasible with all nucleophiles tried (O, S and N), while the closing of the five-membered ring was restricted to O- and S-nucleophiles. The closure of a four-membered ring failed. Instead, the bicyclic (5,6)-membered acetal derivative and the tricyclic system with an eight-membered central ring were obtained from the substrates containing O and S nucleophilic moieties, respectively. The reaction outcome and stereochemistry are rationalized using quantum chemical calculations at B3LYP/6-31G(d) level. The exclusive cis-stereoselectivity in the formation of (5,6)- and (5,5)-membered systems results from thermodynamic control, whereas the formation of the eight-membered ring was kinetically controlled.
Asymmetric gold nanoparticles synthesized in the presence of maltose-modified poly(ethyleneimine)
(2012)
A self-assembled tube-like network, spontaneously formed by adding maltose-modified poly(ethyleneimine) (mal-PEI5000) to mixed phospholipid vesicles, can be used as a template for the formation of gold nanoparticles. High resolution TEM indicates that the growing process leads not only to the formation of spherical gold nanoparticles with an absorption maximum at 520 nm, but also very flat triangles, hexagons, and long bent rods are formed, revealing an absorption maximum in the NIR at about 850 nm.
One can conclude that nanorods, nanotriangles and nanohexagons are predominantly formed in the tubular network structure.
The synthesis of ultrafine gold nanoparticles in presence of maltose-modified hyperbranched poly(ethyleneimines) (PEI) is described. The polymer acted as both a reducing and stabilising agent in the particle formation process. The nanoparticles were characterized by means of dynamic light scattering (DLS), transmission electron microscopy (TEM), analytical ultracentrifugation (AUC), small-angle x-ray scattering (SAXS), and small-angle neutron scattering (SANS). The mechanism of nanoparticle formation can be described in two steps. The reduction process of the Au3+ ions located in the inner coil region of the hyperbranched PEI led to the formation of a compact gold core, and is accompanied by a collapse of the polymer coil. Therefore, in the subsequent reduction process a gold-polymer hybrid shell is formed. By using the PEI of higher molar mass, core-shell gold nanoparticles of about 3.6 nm size with a more narrow size distribution and special fluorescence behavior could be synthesized.
Water soluble polymers can be incorporated into reverse microemulsion droplets without leaving the isotropic phase region. When the polymer is attached to the surfactant film the bending elasticity is changed, and droplet-droplet interactions are influenced. Different methods are available for studying the droplet-droplet interactions in more detail, e.g. SANS. SAXS and DLS. Conductometric measurements are very useful for detecting exchange processes between the droplets. In presence of polyampholytes a pH dependent tuning of the membrane properties becomes possible, experimentally detectable by conductometry.
Structurally diverse polyamides obtained from monomers derived via the Ugi multicomponent reaction
(2012)
The combination of the Ugi four-component reaction (Ugi-4CR) with acyclic diene metathesis (ADMET) or thiolene polymerization led to the formation of poly-1-(alkylcarbamoyl) carboxamides, a new class of substituted polyamides with amide moieties in the polymer backbone, as well as its side chains. 10-Undecenoic acid, obtained by pyrolysis of ricinoleic acid, the main fatty acid of castor oil, was used as the key renewable building block. The use of different primary amines, as well as isonitriles (isocyanides) for the described Ugi reactions provided monomers with high structural diversity. Furthermore, the possibility of versatile post-modification of functional groups in the side chains of the corresponding polymers should be of considerable interest in materials science. The obtained monomers were polymerized by ADMET, as well as thiolene, chemistry and all polymers were fully characterized. Finally, ortho-nitrobenzylamide-containing polyamides obtained by this route were shown to be photoresponsive and exhibited a dramatic change of their properties upon irradiation with light.
Modeling of a violaxanthin-chlorophyll b chromophore pair in its LHCII environment using CAM-B3LYP
(2012)
Collecting energy for photosystem II is facilitated by several pigments, xanthophylls and chlorophylls, embedded in the light harvesting complex II (LHCII). One xanthophyll, violaxanthin (Vio), is loosely bound at a site close to a chlorophyll b (Chl). No final answer has yet been found for the role of this specific xanthophyll. We study the electronic structure of Vio in the presence of Chl and under the influence of the LHCII environment, represented by a point charge field (PCF).
We compare the capability of the long range corrected density functional theory (DFT) functional CAM-B3LYP to B3LYP for the modeling of the UV/vis spectrum of the Vio + Chl pair. CAM-B3LYP was reported to allow for a very realistic reproduction of bond length alternation of linear polyenes, which has considerable impact on the carotenoid structure and spectrum. To account for the influence of the LHCII environment, the chromophore geometries are optimized using an ONIOM(DFT/6-31G(d):PM6) scheme.
Our calculations show that the energies of the locally excited states are almost unaffected by the presence of the partner chromophore or the PCF. There are, however, indications for excitonic coupling of the Chl Soret band and Vio. We propose that Vio may accept energy from blue-light excited Chl.
Predicting accurate bond-length alternations (BLAs) in long conjugated molecular chains has been a major challenge for electronic-structure theory for many decades. While Hartree-Fock (HF) overestimates BLA significantly, second-order perturbation theory and commonly used density functional theory (DFT) approaches typically underestimate it. Here, we discuss how this failure is related to the many-electron self-interaction error (MSIE), which is inherent to both HF and DFT approaches. We use tuned long-range corrected hybrids to minimize the MSIE for a series of polyenes. The key result is that the minimization of the MSIE alone does not yield accurate BLAs. On the other hand, if the range-separation parameter is tuned to yield accurate BLAs, we obtain a significant MSIE that grows with chain length. Our findings demonstrate that reducing the MSIE is one but not the only important aspect necessary to obtain accurate BLAs from density functional theory.
Materials for biomedical applications are often chosen for their bulk properties. Other requirements such as a hemocompatible surface shall be fulfilled by suitable chemical functionalization. Here we show, that linear, side-chain methylated oligoglycerols (OGMe) are more stable to oxidation than oligo(ethylene glycol) (OEG). Poly(ether imide) (PEI) membranes functionalized with OGMes perform at least as good as, and partially better than, OEG functionalized PEI membranes in view of protein resistance as well as thrombocyte adhesion and activation. Therefore, OGMes are highly potent surface functionalizing molecules for improving the hemocompatibility of polymers.
Recent developments in the synthesis of polyelectrolytes are highlighted, with respect to the nature of the ionic groups, the polymer backbones, synthetic methods, and additional functionality given to the polyelectrolytes. In fact, the synthesis of new polyelectrolytes is mostly driven by material aspects, currently. The article pays particular attention to strong polyelectrolytes, and the new methods of controlled polymerization. These methods and the so-called click reactions have enabled novel designs of polyelectrolytes. Nevertheless, the polymerization of unprotected ionic monomers is still challenging and limits the synthetic possibilities. The structural aspects are complemented by considerations with respect to the aspired uses of the new polyelectrolytes.