Stacking Of Phenyl Rings Research Articles

Molecular dimer junctions forming: Role of disulfide bonds and electrode‐compression‐time

AbstractThanks to their excellent bond strength, phenyl‐based molecules with thiol anchoring groups are extensively employed to form stable single‐molecule junctions. However, two critical questions are still not answered which seriously hinder high‐yield establishing reliable molecular functional devices: (1) Whether molecular dimer junctions will be formed, and if this is the case, whether the dimerization is caused by intermolecular disulfide bonds or π–π stacking of phenyl rings; (2) Upon a mechanical‐compression force, is it possible that both anchoring groups of the molecule bond to the same electrode instead of bridging two opposite electrodes, which would drastically reduce the yield of the molecular junctions. Here, combining UV‐Vis/Raman spectroscopy of bulk molecules and conductance/flicker‐noise measurements of single molecules, we give compelling evidence that molecular dimers naturally form under ambient conditions, primarily via disulfide bonds rather than by π–π stacking. We further proposed a technique, named electrode‐compression‐hold‐on (ECHO), and reveal that the two thiol groups of phenyl‐backboned molecules will bond to the same electrode upon a compression force with a prolongated ECHO time. In contrast, the compression‐time‐dependent phenomenon is not observed for alkyl‐backboned molecules. The underlying mechanism for these unprecedented observations is elucidated, shedding light on the yield of molecular junctions.

Self-assembly of dodecaphenyl POSS thin films

The self-assembly abilities of Dodecaphenyl Polyhedral Oligomeric Silsesquioxane thin films on Si(1 0 0) surfaces were studied. Due to their thermal properties – relatively low sublimation temperature and preservation of molecular structure – cage type silsesquioxanes are ideal material for the preparation of a thin films by Physical Vapor Deposition. The Ultra-High Vacuum environment and the deposition precision of the PVD method enable the study of early stages of thin film growth and its molecular organization. X-ray Reflectivity and Atomic Force Microscopy measurements allow to pursuit size-effects in the structure of thin films with thickness ranges from less than a single molecular layer up to several tens of layers. Thermal treatment of the thin films triggered phase change: from a poorly ordered polycrystalline film into a well-ordered multilayer structure. Self-assembly of the layers is the effect of the π−stacking of phenyl rings, which force molecules to arrange in a superlattice, forming stacks of alternating organic-inorganic layers.

Investigating Miscibility of Polymers and Its Impact on the Morphology, Thermal, and Mechanical Properties of Polymer Blends

The miscibility of polystyrene (PS)/poly(styrene-co-acrylonitrile) (PSAN) blend films, prepared by the solution casting technique using tetrahydrofuran as a common solvent, was characterized by scanning electron microscopy, differential scanning calorimetry (DSC), thermogravimetry, Fourier transform infrared (FTIR) spectroscopy, and tensile testing. Morphological observations revealed partial miscibility of the blend. FTIR measurements also ascertained partial miscibility of the blend as slight variations in the spectra for various blend compositions were observed. DSC studies also confirmed the semicompatible nature of the examined blend by displaying a single \(T_{\mathrm{g}}\) for the composition, 25/75, and two \(T_{\mathrm{g}}\)’s for compositions, 50/50 and 75/25. The enhancement in thermal stability and mechanical properties which were quite pronounced for the composition, 25/75, also favored partial miscibility of the blend. The partial miscibility of the PS/PSAN blend may be attributed to the intramolecular repulsive effect, characteristic of a homopolymer/copolymer blend system, and the Pi–Pi stacking of phenyl rings of the blend components due to some structural similarities.

Substituent Effect on the Intermolecular Arrangements of One-Dimensional Molecular Assembly on the Si(100)-(2×1)-H Surface

The effect of methyl substitution in styrene molecules on the spatial arrangement of molecules in a one-dimensional (1-D) molecular assembly on the Si(100)-(2×1)-H surface has been studied using a scanning tunneling microscope (STM) at 300 K. Styrene molecules form well-defined 1-D molecular assemblies through a chain reaction mechanism along the dimer row direction, where the phenyl rings are separated by distances equal to that of the interdimer distance in a row and aligned parallel to each other. We observed that the substitution in a phenyl ring has no observable effect on the adsorption sites, configurations, and stacking of phenyl rings along the dimer row. In contrast, the methyl substitution at α site (α-methylstyrene) results in a 1-D assembly where the adsorption sites are similar to that of styrene but the adsorbed molecules are arranged in alternate geometrical configurations along the dimer row. In the case of β-methylstyrene, the adsorption sites (diagonal silicon atoms in a dimer row) and ...

An enhanced electronic state in ZnTPP induced by interaction with solvation: a multiple-scattering calculation of x-ray absorption near-edge spectra

Metalloporphyrins (MP) are important for a wide variety of biological processes and photosynthesis and have been intensively investigated by x-ray absorption spectroscopy. The typical metal K-edge XANES spectra of the square-planar MTPP show a very sharp peak on the rising edge of the main absorption peak, which was assigned to 1s → 4pz electronic transition. We carried out a series of multiple-scattering calculations of porphyrin molecules and found that this peak can be greatly enhanced due to the π-π stacking of phenyl rings below and above the porphyrin plane. Density of state calculations confirmed that the emergence of this electronic state is due to the interaction of the 4pz orbital of the metal with π orbitals of an aromatic solvent molecule with its plane parallel with the macrocycle. This XANES feature of metalloporphyrins can be used to investigate the solvent-solute interactions with presumed non-ligating solvent.