Large‐Scale Synthesis of Helicene‐Like Molecules for the Design of Enantiopure Thin Films with Strong Chiroptical Activity

Helicenes are fascinating molecules owing to their unusual properties and applications in many fields from catalysis to organic electronics. Herein, we report a straightforward pathway for the synthesis of helicene‐like molecules on a gram scale in an enantiopure form. Thin‐film materials with good propagating optical properties and very high chiroptical responses have been grown by using pulsed laser ablation without altering the structure or the enantiopurity of the molecules. Moreover, electronic and vibrational circular dichroism spectroscopies coupled with theoretical calculations enabled some dependences of the chiroptical properties with the structure to be highlighted, for example, effects of rigidification, aromatization, or the state of matter (liquid versus solid).     

The need for new isotope reference materials

Isotope reference materials are needed to calibrate and validate analytical procedures used for the determination of isotope amount ratios, procedurally defined isotope ratios or so-called δ values. In contrast to the huge analytical progress in isotope ratio analytics, the production of isotope reference materials has not kept pace with the increasing needs of isotope analysts. Three representative isotope systems are used to explain the technical and non-technical difficulties and drawbacks, on one hand, and to demonstrate what can be achieved at its best, on the other hand. A clear statement is given that new isotope reference materials are needed to obtain traceable and thus comparable data, which is essential for all kinds of isotope research. The range of available isotope reference materials and δ reference materials should be increased and matrix reference materials certified for isotope compositions or δ values, which do not exist yet, should be provided.     

Influence of Charge Density of Anionic Polyelectrolytes on Structure Formation in Liquid Crystalline Systems

The influence of different amounts of anionic copolymers of N-Methyl N-vinyl acetamide (NMVA) and acrylic acid with various charge densities on the formation of the lamellar liquid crystal formed by sodium dodecyl sulfate (SDS)/decanol/water was investigated by means of polarization microscopy, small angle x-ray scattering (SAXS), transmission electron microscopy and rheology. On the contrary to the incorporation of poly(acrylic acid)     

Cycloaddition mit ungesättigten Zuckern,VI Die Synthese Diastereomerer 4H-Pyrano [3, 4-d] Isoxazol-4-One

Abstract

By 1, 3-dipolar cycloaddition of benzonitrile oxide to 4, 6-di-O-acetyl-2, 3-dideoxy-D-erythro-hex-2-enono-1, 5-lactone (1), [3aR- (3aα, 6β,7α, 7aα)] - (2) and [3aS-(3aβ, 6β, 7α, 7aα)] -7- (acetyloxy) -6- (acetyloxymethyl) -3a, 6, 7, 7a-tetrahydro-3-phenyl-4H-pyrano [3, 4-d] isoxazole-4-one (3) were prepared in 58 and 7% yield respectively. From 2, [1′ R, 3aR-(3aα, 6β, 6aα)] -6-(1′, 2′-dihydroxymethyl)-6, 6a - dihydro-3-phenyl-furano [3, 4-d] isoxazole-4 (3aH) -one (5) was prepared by deacetylation. The structure of 3 was determined by X-ray analysis.     

Diastereo‐ and Enantioselective Synthesis of Organometallic Bis(helicene)s by a Combination of C?H Activation and Dynamic Isomerization

Homochiral and heterochiral cis‐bis‐cycloplatinated‐[6]helicene derivatives 1 b^{1, 2} , as representative examples of platina[6]helicenes that share a common platinum center, have been prepared. A diastereo‐ and enantioselective synthesis, which combines CH activation and dynamic isomerization from heterochiral structure 1 b² into homochiral structure 1 b¹ , is also described. Overall, this isomerization process results in the transfer of chiral information from one helicene moiety to the other one. The chiroptical properties of homochiral (P)‐ and (M)‐ 1 b¹ were greatly modified upon oxidation into their corresponding (P)‐ and (M)‐diiodo‐Pt^IV complexes ( 5 ). The changes were also analyzed by performing theoretical calculations. C? H activation in the synthesis of organometallic helicenes is further demonstrated by the preparation of cis‐bis‐cycloplatinated‐[8]helicene 1 c .     

[2+3] Amide Cages by Oxidation of [2+3] Imine Cages – Revisiting Molecular Hosts for Highly Efficient Nitrate Binding

The pollution of groundwater with nitrate is a serious issue because nitrate can cause several diseases such as methemoglobinemia or cancer. Therefore, selective removal of nitrate by efficient binding to supramolecular hosts is highly desired. Here we describe how to make [2+3] amide cages in very high to quantitative yields by applying an optimized Pinnick oxidation protocol for the conversion of corresponding imine cages. By NMR titration experiments of the eight different [2+3] amide cages with nitrate, chloride and hydrogen sulfate we identified one cage with an unprecedented high selectivity towards nitrate binding vs. chloride (S=705) or hydrogensulfate (S>13500) in CD₂Cl₂/CD₃CN (1 : 3). NMR experiments as well as single-crystal structure comparison of host-guest complexes give insight into structure-property-relationships.     

Simultaneous Raman and fluorescence spectroscopy of single conjugated polymer chains

Simultaneous surface enhanced Raman scattering (SERS) and fluorescence is demonstrated from single conjugated polymer chains. As resonance enhancement of SERS depends on the spectral overlap of the polymer's absorption and the incident laser, resonance Raman and fluorescence effectively probe the absorbing and emitting part of the polymer, respectively. The optical phonon energies change along the polymer chain, providing a window to spatially track excited state relaxation. Whereas a mean spatial redistribution of the excitation is witnessed by a change in vibronic fingerprint following interchromophoric energy transfer, intrachromophoric exciton self-trapping leaves the vibrations unchanged.     

Molecular mobility and transport in polymer membranes and polyelectrolyte multilayers

Polyelectrolyte multilayers prepared by the layer-by-layer technique provide an efficient way to generate planar structures of tailored surface charge and hydrophobicity, which are used as membranes for pervaporation. The use of polyelectrolyte multilayers to form the membrane permits tailoring the surface charge of the membrane and, thus, selectivity; at the same time, it reduces fouling of the membrane by adsorption of organic matter. Pulsed field gradient (PFG) nuclear magnetic resonance has been used to investigate the diffusion of probe molecules into polymer systems. Evaluation of the apparent diffusion coefficient in porous poly(amide) results in a pore size of 4 microm, as found in electron micrographs. For the pore size obtained for polyelectrolyte multilayers, no equivalent pores could be found in microscopy. Propagators for the diffusion of propanol and propanol-water mixture into multilayers reveal that there might be selective interaction of probe molecules with the polyelectrolyte system.