Nanopatterns of molecular spoked wheels as giant homologues of benzene tricarboxylic acids

Molecular spoked wheels with D_3h and C_s symmetry are synthesized by Vollhardt trimerization of C_2v-symmetric dumbbell structures with central acetylene units and subsequent intramolecular ring closure. Scanning tunneling microscopy of the D_3h-symmetric species at the solid/liquid interface on graphite reveals triporous chiral honeycomb nanopatterns in which the alkoxy side chains dominate the packing over the carboxylic acid groups, which remain unpaired. In contrast, C_s-symmetric isomers partially allow for pairing of the carboxylic acids, which therefore act as a probe for the reduced alkoxy chain nanopattern stabilization. This observation also reflects the adsorbate substrate symmetry mismatch, which leads to an increase of nanopattern complexity and unexpected templating of alkoxy side chains along the graphite armchair directions. State-of-the-art GFN-FF calculations confirm the overall structure of this packing and attribute the unusual side-chain orientation to a steric constraint in a confined environment. These calculations go far beyond conventional simple space-filling models and are therefore particularly suitable for this special case of molecular packing.

Scanning tunneling microscopy investigations of phenylene-based molecular spoked wheels with D_3h and C_s symmetries on graphite show the competitive or complementary effects of carboxylic acid groups and alkoxy chains on the nanopattern formation.     

Shape-persistent linear, kinked, and cyclic oligo(phenylene-ethynylene-butadiynylene)s: self-assembled monolayers

Shape-persistent rigid phenylene-ethynylene-butadiynylenes form lamellar self-assembled monolayers (SAMs) at the HOPG/TCB interface, which were studied by scanning tunneling microscopy (STM) with submolecular resolution. Substitution of the terminating acetylene functions with polar cyanopropyldimethylsilyl groups leads to 2D phase separation and defined rod-rod interactions, which determine the packing distances between the rigid rods. The results stimulated the connection of rigid rods via septiarylene clamp units. They covalently link two rigid rod units and define the intramolecular rod-rod distance that matches the alkoxy substituent chain lengths. The systems can be described as half-ring structures of two rigid rods connected via a rotatable joint unit. These acetylene-terminated half-ring structures were also oligomerized under Cu and Pd catalysis to yield defined acyclic and cyclic oligomers. Detailed STM studies decoded the molecular origin of the surface patterning of such systems. The dodecyloxy side chains are adsorbed along the HOPG main axes and, together with the alkoxy backbone angle, determine the adsorption direction of the adlayers.     

Cn films (n=50, 52, 54, 56, and 58) on graphite: cage size dependent electronic properties

Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected Cn+ (50< or =n<60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, Cn films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that Cn aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C60. The apparent activation energies for intact Cn sublimation range from 2.2 eV for C58 to 2.6 eV for C50 as derived from thermal desorption spectra. All Cn films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (EB approximately 2.5 eV). This feature has been assigned to Cn units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all Cn studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C60. C58 exhibited the lowest HOMO (6.5 eV). Band gaps of Cn films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C52) and 1.8 eV (C50) were observed, compared to 1.5 eV for solid C60.     

Distribution patterns of firearm discharge residues as revealed by neutron activation analysis

A systematic investigation using a variety of handguns has revealed the existence of distinguishable distribution patterns of firearm discharge residues on surfaces below the flight path of a bullet. The residues are identifiable even at distances of 12 meters from the gun using nondestructive neutron activation analysis. The results of these investigations show that the distribution pattern for a gun is reproducible using similar ammunition and that there exist two distinct regions to the patterns developed between the firearm and the target—one with respect to the position of the gun and the other in the vicinity of the target. The judicious applications of these findings could be of significant value in criminal investigations.     

Neutron Activation Procedures Used for the Calibration of a Nitrogen-16 Reactor Power Monitor

Personnel from the Pennsylvania State University's Breazeale Nuclear Reactor assisted NRC (Nuclear Research Corporation) personnel in calibrating a new MSL/¹⁶N Nitrogen-16 Monitor. This paper describes the neutron flux calibration procedures utilized and the results obtained for the production of a nitrogen-16 source of known activity for a BGO detector calibration.     

Fast Oxidative Cyclooligomerization towards Low‐ and High‐Symmetry Thiophene Macrocycles

Macrocycles with quaterthiophene subunits were obtained by cyclooligomerization by direct oxidative coupling of unsubstituted dithiophene moieties. The rings were closed with high selectivity by an α,β′‐connection of the thiophenes as proven by NMR spectroscopy. The reaction of the precursor with terthiophene moieties yielded the symmetric α,α′‐linked macrocycle in low yield together with various differently connected isomers. Blocking of the β‐position of the half‐rings yielded selectively the α,α′‐linked macrocycle. Selected cyclothiophenes were investigated by scanning tunneling microscopy, which displayed the formation of highly ordered 2D crystalline monolayers.     

Treatment of citrate-capped Au colloids with NaCl, NaBr and Na2SO4: a TEM, EAS and EPR study of the accompanying changes

In the present study, effects of the treatment of citrate-reduced Au sols with NaCl, NaBr and Na₂SO₄ are described. The particles are characterized by transmission electron microscopy (TEM) and spectroscopic methods, suggesting an exchange of the citrate capping by the anions of the Na salts. Under electron beam exposure the capping of the NaCl- and NaBr-treated particles disappears. The specific electronic properties of the Na salt-treated particles are studied by electron absorption spectroscopy (EAS) and electron paramagnetic resonance (EPR). A discussion of the results in comparison with the spectroscopic responses from the original particles is given. A correlation between the data of EAS and EPR is found. The respective electron-withdrawing effect of the capping anions towards the Au core seems to be of considerable significance regarding the orbital situation around the Fermi level.     

Hierarchical self-assembly of polycyclic heteroaromatic stars into snowflake patterns

Seeing stars: The two-dimensional patterns of the polycyclic heteroaromatic star molecules 1 on graphite vary with the side chain length. For n=12, frustrated self-assembly leads to hierarchically organized superstructures: up to 10?molecules form triangular aggregates which pack densely into hexagonal patterns with very large (15.5?nm) lattice constants.