meso-Dichloropyrimidinyl substituted expanded porphyrins

Condensation of 4,6-dichloro-2-phenylpyrimidine-5-carbaldehyde with pyrrole enables the synthesis of a number of expanded porphyrins of which the [26]hexaphyrin was easily isolated and successfully converted into the decasubstituted doubly N-fused hexaphyrin variant.     

Bis(mu-oxo)dicopper in Cu-ZSM-5 and its role in the decomposition of NO: a combined in situ XAFS,UV-vis-near-IR,and kinetic study

In situ XAFS combined with UV-vis-near-IR spectroscopy are used to identify the active site in copper-loaded ZSM-5 responsible for the catalytic decomposition of NO. Cu-ZSM-5 was probed with in situ XAFS (i) after O(2) activation and (ii) while catalyzing the direct decomposition of NO into N(2) and O(2). A careful R-space fitting of the Cu K-edge EXAFS data is presented, including the use of different k-weightings and the analysis of the individual coordination shells. For the O(2)-activated overexchanged Cu-ZSM-5 sample a Cu.Cu contribution at 2.87 A with a coordination number of 1 is found. The corresponding UV-vis-near-IR spectrum is characterized by an intense absorption band at 22 700 cm(-1) and a relatively weaker band at 30 000 cm(-1), while no corresponding EPR signal is detected. Comparison of these data with the large databank of well-characterized copper centers in enzymes and synthetic model complexes leads to the identification of the bis(mu-oxo)dicopper core, i.e. [Cu(2)(mu-O)(2)](2+). After dehydration in He, Cu-ZSM-5 shows stable NO decomposition activity and the in situ XAFS data indicate the formation of a large fraction of the bis(mu-oxo)dicopper core during reaction. When the Cu/Al ratio of Cu-ZSM-5 exceeds 0.2, both the bis(mu-oxo)dicopper core is formed and the NO decomposition activity increases sharply. On the basis of the in situ measurements, a reaction cycle is proposed in which the bis(mu-oxo)dicopper core forms the product O(2) on a single active site and realizes the continuous O(2) release and concomitant self-reduction.     

Three-coordinate aluminum in zeolites observed with in situ x-ray absorption near-edge spectroscopy at the Al K-edge: flexibility of aluminum coordinations in zeolites

Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions. A unique, previously not observed, pre-edge feature is detected in zeolites H-Mordenite and steamed and unsteamed H-Beta at temperatures above 675 K. Spectra simulations using the full multiple scattering code Feff8 identify the unique pre-edge feature as three-coordinate aluminum. The amount of three-fold coordinated aluminum is a function of temperature and pretreatment of a zeolite: a steamed zeolite Beta contains more three-coordinate aluminum than an unsteamed sample. No clear differences between zeolites H-Mordenite and H-Beta were observed. Octahedrally coordinated aluminum forms in zeolites H-Mordenite and H-Beta at room temperature in a stream of wet helium. This octahedrally coordinated aluminum is unstable at temperatures higher than 395 K, where it quantitatively reverts to the tetrahedral coordination.     

Synthesis of n-type perylene bisimide derivatives and their orthogonal self-assembly with p-type oligo(p-phenylene vinylene)s

Four different (chiral) electron-deficient (n-type) perylene bisimides containing aliphatic, aromatic, or ethyleneoxide side chaines have been synthesized and fully characterized. All of them form supramolecular stacks in apolar methylcyclohexane (MCH) solution as demonstrated by concentration- and temperature-dependent absorption, circular dichroism, and fluorescence studies. One derivative was investigated in more detail in the solid state and proven to be liquid crystalline and capable of forming nanometer-sized fiberlike networks when drop-cast from MCH. Optical spectroscopy techniques show that perylene bisimide and an oligo(p-phenylene vinylene) (p-type) derivative orthogonally self-assemble into separate nanosized p-and n-type stacks in MCH. In contrast in toluene only molecularly dissolved species are present. In films deposited from MCH as well as from toluene photoinduced electron transfer takes place from the p-type material to the n-type material. As a result of the orthogonal self-assembly process, in films from MCH an ordered network of fibers was formed, whereas in films from toluene no ordering was observed. However, probably due to the lateral orientation on the surface and the presence of long aliphatic chains pointing toward the electrodes, efficient bulk heterojunction solar cells could not be constructed.     

Polarized emission of individual self-assembled oligo(p-phenylenevinylene)-based nanofibers on a solid support

We have prepared 5 nm diameter, micrometer long tetra(p-phenylenevinylene) (OPV)-based nanofibers on a graphite surface. The fluorescence emission of an individual fiber shows a profound polarization over its entire length that directly corresponds to its orientation on the substrate. Quantitative analysis of the fluorescence polarization, including the depolarizing effect of the underlying graphite, evidences the high degree of organization within chiral fibers with the OPV molecules perpendicular to the fiber axis. The control of the internal order within self-assembled fibers, and the ability to measure it, is a crucial step to obtain uniform organic fibers that can be applied in nanosized electronics at room temperature.     

Parallel synthesis and study of fluorous biphasic partition coefficients of 1H,1H,2H,2H-perfluoroalkylsilyl derivatives of triphenylphosphine: a statistical approach

A library of fluorous, (1H,1H,2H,2H-perfluoroalkyl)silyl-substituted derivatives of triphenylphosphine, Ph(3-a)P[C(6)H(5-y)[SiMe(3-b)(CH(2)CH(2)C(x)F(2x+1))(b)](y)-pos](a) [a = 1-3; b = 1-3; x = 4, 6, 8, or 10; pos = 3, 4 (y = 1) or 3,5 (y = 2)], was prepared using parallel synthetic techniques. Upon variation of these four parameters, a total of 108 different fluorous phosphines can be synthesized. Using factorial design, 37 phosphines were selected and their partition coefficients in the typical fluorous biphasic solvent system PFMCH/toluene (PFMCH = perfluoromethylcyclohexane) determined. By fitting of the partition coefficient data to linear functions of the parameters a, b, and x, the partition coefficients of the remaining 71 fluorous phosphines, which were not prepared, could be predicted. Using this approach, some unexpected trends in the dependence of the partition coefficient on variations of the four parameters became clear, resulting in a better understanding of the optimum fluorous substitution pattern for obtaining the highest partition coefficient (P). In this way, the partition coefficient was increased by 2 orders of magnitude, i.e., from the initial value P = 7.8 for 1(3, 2, 6, C4) to P > 238 for 1(2, 3, 6, C3C5). Para- and 3,5-substituted phosphines showed irregular behavior in the sense that elongation or increase of the number of perfluoroalkyl tails did not necessarily lead to higher partition coefficients. Particularly high values were found for phosphines containing a total of 72 fluorinated carbon atoms on the meta position(s) of the aryl rings. Linear relationships were found between the predicted log P of 1(a, b, x, C4) and the experimentally determined log P values of fluorous diphosphines [CH(2)P[C(6)H(4)(SiMe(3-b)(CH(2)CH(2)C(6)F(13))(b))-4](2)](2) and monophosphines Ph(3-a)P(C(6)H(4)(CH(2)CH(2)C(6)F(13))-4)(a). One of the most fluorophilic phosphines, i.e., 1(3, 1, 8, C3C5), was applied and efficiently recycled in rhodium-catalyzed, fluorous hydrosilylation of 1-hexene by HSiMe(2)Ph using PFMCH as the fluorous phase and the substrates as the organic phase. It was demonstrated that a higher partition coefficient of the ligand in PFMCH/toluene at 0 degrees C indeed resulted in less leaching of both the catalyst and the free ligand during phase separation.     

Competing C-4 and C-5-Acyl Stabilization of Uronic Acid Glycosyl Cations

Uronic acids are carbohydrates carrying a terminal carboxylic acid and have a unique reactivity in stereoselective glycosylation reactions. Herein, the competing intramolecular stabilization of uronic acid cations by the C-5 carboxylic acid or the C-4 acetyl group was studied with infrared ion spectroscopy (IRIS). IRIS reveals that a mixture of bridged ions is formed, in which the mixture is driven towards the C-1,C-5 dioxolanium ion when the C-5,C-2-relationship is cis, and towards the formation of the C-1,C-4 dioxepanium ion when this relation is trans. Isomer-population analysis and interconversion barrier computations show that the two bridged structures are not in dynamic equilibrium and that their ratio parallels the density functional theory computed stability of the structures. These studies reveal how the intrinsic interplay of the different functional groups influences the formation of the different regioisomeric products.     

Small-angle neutron scattering study of microemulsion-polymer mixtures in the protein limit

Contrast variation small-angle neutron scattering (SANS) has been employed to study complex fluids comprising model microemulsions and polymers. The systems are water-in-oil microemulsions with added non-adsorbing polymer, under good polymer solvency conditions and semidilute polymer concentrations. The polymer/colloid size ratio was q approximately 11, which is well within the "protein limit". Four scattering contrasts were produced by selective deuteration of the dispersed and continuous phases and also the surfactant. In this way, the separate partial structure factors (PSF) for colloid-colloid (c-c), polymer-polymer (p-p), and colloid-polymer (c-p) have been obtained. The c-c PSF has been compared with theoretical predictions, allowing determination of a polymer correlation length. This is compared with a similar correlation length obtained from the p-p PSF, which is shown to increase with colloid concentration. In this sense, adding microemulsion has a similar effect on the dissolved polymer as reducing the solvent quality, and an effective Flory-Huggins chi parameter has been calculated. The cross-term PSF shows a distinct anti-correlation. This is the first time such structure factors have been determined experimentally for colloid-polymer systems in the protein limit and these allow a more detailed understanding of the structural interactions in these systems.