Aurophilic Interactions in Multi-Radical Species: Electronic-Spin and Redox Properties of Bis- and Tris-[(Nitronyl Nitroxide)-Gold(I)] Complexes with Phosphine-Ligand Scaffolds.

Multinuclear AuI complexes with two or three nitronyl nitroxide-2-ide radical anion and phosphine-ligand scaffolds, (NN-Au)2 -1 o, (NN-Au)2 -1 m, and (NN-Au)2 -1 p, have been synthesized to investigate the influence of AuI -AuI (aurophilic) interactions on the properties of multispin molecular systems. The desired complexes were successfully prepared in moderate yields in a one-pot synthesis from the corresponding phosphine ligand, AuI source, parent NN, and sodium hydroxide. Among the prepared complexes, (NN-Au)2 -1 o, in which an aurophilic interaction was clearly observed by crystal structure analysis, showed characteristic spin-spin interactions, electrochemical properties, and solvatochromic behavior. The results from theoretical calculations also suggested that the differences in properties between complex (NN-Au)2 -1 o and the other complexes are due to intramolecular aurophilic interactions.

Direct imaging of electric field behavior in 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene organic field-effect transistors by sum-frequency generation imaging microscopy.

Sum-frequency generation imaging microscopy combined with compressive-sensing (CS-SFG) is a powerful micro-spectroscopic technique for probing interfaces and surfaces with a spatial resolution where contrast is based on the chemical functional groups. We reported the use of the CS-SFG technique to probe the electric field due to charge accumulation and the internal electric field in operating organic field-effect transistors (OFETs) with the aluminum oxide and octadecylphosphonic acid (ODPA) self-assembled monolayer as the gate dielectric layer and 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT) as the semiconductor layer. In addition, the electric field behavior was discussed by a difference in the electric field induced SFG intensity between the open-circuit and the voltage application conditions. The SFG peak of CH stretching mode derived from methyl groups of ODPA and phenyl groups of DPh-BTBT could be observed at each interface of ODPA/DPh-BTBT or DPh-BTBT/Au, respectively. Moreover, the electric field induced SFG coming from ODPA/DPh-BTBT shows the presence of intense electric field due to charge injection and accumulation near the drain and source electrode edges under the operation of OFETs. Our studies show that the electric field-induced SFG imaging technique is useful for probing the local electric field distribution or charge accumulation behavior in OFETs under operating conditions.

Role of Chemical Functionality in the Adhesion of Aluminum and Isotactic Polypropylene.

In the direct melt bonding of isotactic polypropylene (iPP) to aluminum (Al), the blending of a small amount of maleic anhydride-grafted PP (PPgMA) with iPP was found to induce a dramatic improvement of the strength of adhesion. The effect of blending PPgMA was, however, limited, maximizing at ∼20 wt % PPgMA. Incorporation of larger amounts of PPgMA reduced the strength of adhesion. We studied the mechanism of adhesion between Al and iPP by incorporating chemical functionality to the polymer side. The fracture surfaces produced by peeling off the interfaces were investigated by replicating the surface topographic features on a platinum thin film and analyzing them by scanning transmission electron microscopy (STEM) as well as by reconstructing three-dimensional (3D) surface structures with STEM tomography. The replica-STEM technique enabled us to visualize PP surface crystalline lamellar structures and their deformation upon the failure in 3D. We found that polymer/metal interfaces produced surface features in the failure that were similar to those associated with failure of entanglement-based polymer/polymer adhesion via chain pullout. A fractography study by replica-STEM suggested that the formation of a low-molecular-weight layer with low crystallinity at the interfacial region was responsible for the improvement of adhesion. The adhesion strength depended on the toughness of the "soft layer" and did not depend on the chemical bonding between PPgMA and Al. The interfacial chemical reaction between MA and the Al surface yielded PP with a grafted carboxylic acid (-COOH) group, which may have been excluded from the PP crystalline lamellae. We concluded that chemical bonding was not the primary reason for the improvement of adhesion, but it was necessary to induce the segregation of PPgMA in the interfacial region and the formation of the soft layer.

Condensed Phenoxazine Dimer and Its Radical Cation.

A condensed phenoxazine dimer was synthesized and characterized. X-ray crystallographic analysis of the dimer shows a double-butterfly structure, in which the nitrogen atoms are located above and below the molecular plane. A radical cation salt of the dimer was obtained using tris(4-bromophenyl)aminium hexafluoroantimonate as the oxidant. The salt is air-stable in solid and solution states. The cation structure was evaluated by X-ray crystallographic analysis, showing that the phenoxazine units were converted to a planar structure upon oxidation.

Why Can Water Droplets Move Smoothly Even on Statically Hydrophilic Surfaces?

Studies on hydrophilic surfaces showing excellent water sliding properties are very rare, despite the numerous practical advantages they offer. One of the authors has recently developed a smooth, transparent and sufficiently-thick hydrophilic film with low contact angle hysteresis (5°) and small water tilt angles (6 ± 2°) through a simple sol-gel reaction of 2-[methoxy (ethyleneoxy)10propyl]trimethoxysilane (PEG-M) and tetraethoxysilane (TEOS). However, the origins of these unusual water sliding properties have not been clearly identified. As such, the impact of the addition of TEOS acting as a "nanospacer" or the identity of PEG terminal functional groups on the final static/dynamic wetting properties needs to be explored. In this study, we investigated the orientational/conformational states of PEG chains in PEG-M/TEOS and [hydroxy(ethyleneoxy)8-12propyl]triethoxysilane (PEG-OH)/TEOS hybrid films under wet and dry conditions using sum-frequency generation (SFG) spectroscopy. We found that PEG-M/TEOS hybrid film surfaces had no marked differences in the conformational states of PEG chains under wet or dry conditions, resulting in excellent water sliding properties as there was no energy barrier for water droplet motion. In contrast, PEG chains were completely disordered after contact with water in the PEG-OH/TEOS hybrid films due to hydration effects. This large conformational change between the liquid/solid and gas/solid interface at the three-phase contact line resulted in an unfavorable energetic barrier for water droplet motion, leading to the poor water sliding properties. TEOS did not physically work as a "nanospacer," but chemically worked as a binder to endow practically useful properties, such as good adhesion and versatility in substrates used, to our hybrid films.

Evidence of chemical-bond formation at the interface between an epoxy polymer and an isocyanate primer.

For several decades, hydroxyl function containing polymer surfaces have been modified using isocyanate primers to improve the adhesion properties. In this study, sum frequency generation spectroscopy was applied to study epoxy polymer/isocyanate solution interfaces, confirming the presence of chemical bonds at the joining interfaces.

Why can organic liquids move easily on smooth alkyl-terminated surfaces?

The dynamic dewettability of a smooth alkyl-terminated sol-gel hybrid film surface against 17 probe liquids (polar and nonpolar, with high and low surface tensions) was systematically investigated using contact angle (CA) hysteresis and substrate tilt angle (TA) measurements, in terms of their physicochemical properties such as surface tension, molecular weight/volume, dielectric constant, density, and viscosity. We found that the dynamic dewettability of the hybrid film markedly depended not on the surface tensions but on the dielectric constants of the probe liquids, displaying lower resistance to liquid drop movement with decreasing dielectric constant (ε < 30). Interfacial analysis using the sum-frequency generation (SFG) technique confirmed that the conformation of surface-tethered alkyl chains was markedly altered before and after contact with the different types of probe liquids. When probe liquids with low dielectric constants were in contact with our surface, CH3 groups were preferentially exposed at the solid/liquid interface, leading to a reduction in surface energy. Because of such local changes in surface energy at the three-phase contact line of the probe liquid, the contact line can move continuously from low-surface-energy (solid/liquid) areas to surrounding high-surface-energy (solid/air) areas without pinning. Consequently, the organic probe liquids with low dielectric constants can move easily and roll off when tilted only slightly, independent of the magnitude of CAs, without relying on conventional surface roughening and perfluorination.

Surface structure of sulfuric acid solution relevant to sulfate aerosol: molecular dynamics simulation combined with sum frequency generation measurement.

Surface structure of aqueous sulfuric acid solution at a typical atmospheric concentration (0.2x, x: mole fraction) is investigated by close collaboration of molecular dynamics (MD) simulation and sum frequency generation (SFG) measurement. The SFG spectra of both O-H and S-O stretching vibrations are provided with different sets of polarization combination. These sets of experimental spectra are consistently elucidated by the MD calculations. In modeling the surface structure, there exists a major uncertainty about local ion composition at the surface region. To address this uncertainty, we performed MD simulations with various assumptions on the local dissociation constants of sulfuric acid, and searched for the condition to be consistent with the experimental spectra. We have thereby concluded that the first acid dissociation of sulfuric acid is almost complete at the surface, while the second dissociation is more strongly suppressed than in the bulk liquid. The present MD simulation elucidates the ion distribution and molecular orientation at the sulfuric acid solution surface, and also the concentration dependence of the SFG spectrum.

Two-color sum frequency generation study of poly(9,9-dioctylfluorene)/electrode interfaces.

The air/PFO and the buried electrode/PFO interfaces have been investigated by two-color SFG spectroscopy. Due to the interface confinement effects, the planes of PFO rings are nearly parallel to the surface plane, and the optical band gaps become smaller at the interfaces than those of the bulk.

Anion configuration at the air/liquid interface of ionic liquid [bmim]OTf studied by sum-frequency generation spectroscopy.

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF3-symmetric stretching (ss) mode and SO3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF3-ss peak of the OTf anion has the opposite sign with respect to that of the SO3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF3 group points away from the bulk into the air, whereas the SO3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.

First acid dissociation at an aqueous H2SO4 interface with sum frequency generation spectroscopy.

The vibrational sum frequency generation (SFG) spectra of the air-liquid interface of H2SO4-H2O solutions over a wide range of concentrations are measured in the SO stretching region (1000-1300 cm(-1)). The analogy of the concentration dependence of Raman and SFG is indicative of a nearly identical behavior of the first acid dissociation at the air-liquid interface as in the bulk.

Alkyl chain conformation and the electronic structure of octyl heavy chalcogenolate monolayers adsorbed on Au(111).

Adsorption states of dioctyl dichalcogenides (dioctyl disulfide, dioctyl diselenide, and dioctyl ditelluride) arranged on Au(111) have been studied by X-ray photoelectron spectroscopy (XPS), infrared-visible sum-frequency generation (SFG), and ultraviolet photoelectron spectroscopy (UPS). XPS measurements suggest that dioctyl dichalcogenides dissociatively adsorbed on Au(111) surfaces to form the corresponding monolayers having chalcogen-gold covalent bonds. The elemental compositions of octanechalcogenolates on Au(111) indicate that the saturation coverages of the octyl heavy chalcogenolate (OcSe, OcTe) monolayers are lower than that of the octanethiolate (OcS) self-assembled monolayers (SAMs). The SFG observations of the CH(2) vibrational bands for the heavy chalcogenolate monolayers strongly suggest that a discernible amount of gauche conformation exists in the monolayers, while OcS SAMs adopt highly ordered all-trans conformation. The intensity ratio of the symmetric and asymmetric CH(3) stretching vibration modes measured by SFG shows that the average tilt angle of the methyl group of the OcSe monolayers is greater than that of the OcS SAMs. The larger tilt angle of the methyl group and the existence of a discernible amount of gauche conformation in the OcSe monolayers are due to the lower surface coverage of the OcSe monolayers compared with the OcS SAMs. The smaller polarization dependence in the angle-resolved UPS (ARUPS) spectra of the OcSe monolayers than that of the OcS SAMs is caused by the more disordered structures of the alkyl chain in the former. XPS, SFG, and ARUPS measurements indicate a similar tendency for the OcTe monolayers. The density of states (DOS) observed by UPS at around 1.3 eV for OcS adsorbed on Au(111) is considered to be the antibonding state of the Au-sulfur bond. Similar DOS is also observed by UPS at around 1.0 eV for the OcSe monolayers and at approximately 1.6 eV for the OcTe monolayers on Au(111).

Adsorption states of dialkyl ditelluride autooxidized monolayers on Au(111).

Organic ditellurides (R2Te2 where R = n-butyl (C4), n-octyl (C8), and n-cetyl (C16)) were synthesized, and their adsorption states after oxidation on Au(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), theoretical analyses, near-edge X-ray absorption fine structure (NEXAFS) measurements, contact angle measurements, and atomic force microscopy (AFM). The obtained results show that dialkyl ditellurides form autooxidized monolayers (AMs) on the surfaces under ambient conditions and that the oxidation process is accelerated by ambient light. XPS, UPS, and theoretical analyses suggest that the autooxidized ditelluride species consist of polymers or oligomers with Te-O-Te-O network structures stabilized by oxygen bridges between tellurium molecules following the cleavage of tellurium-gold bonds. NEXAFS and contact angle measurements indicate that the average tilt angles of the alkyl chains from the surface normal are smaller for the AMs of dialkyl ditellurides having longer alkyl chains. AFM measurements show defects and roughness features around a few angstroms in height on the surfaces of the dialkyl ditelluride AMs.

Simulation study of angle-resolved photoemission spectra and intramolecular energy-band dispersion of a poly(tetrafluoroethylene) oligomer film.

Theoretical simulations of the angle-resolved ultraviolet photoemission spectra (ARUPS) for the oligomer of poly(tetrafluoroethylene) [(CF(2))(n); PTFE] were performed using the independent-atomic-center approximation combined with ab initio molecular orbital calculations. Previously observed normal-emission spectra for the end-on oriented sample (with long-chain axis perpendicular to the surface) showed the incident photon-energy (hnu) dependence due to the intramolecular energy-band dispersion along the one-dimensional chain, and the present simulations successfully reproduced this hnu dependence of the observed spectra. We employed the experimentally observed helical structure for PTFE oligomers for the simulations. We also calculated the density of states (DOS) for the planar zigzag structure, and examined the changes in the electronic structure due to the difference in the molecular structure by comparing the DOS for the helical and planar zigzag structures. Only a small change in the DOS was found between these structures, showing little change of the electronic structure between these conformations. We also evaluated the inner potential V(0), which is the parameter defining the energy origin of the free-electron-like final state, and checked the validity of the value of -10 eV estimated in our previous study using the experimentally observed hnu dependence of the peak intensity. The estimation of V(0) was performed by pursuing the best agreement between the energy-band dispersion [E=E(k)] relation along the chain direction obtained from the simulated spectra and the experimentally deduced one. An excellent agreement in the topmost band was achieved when the assumed inner potential V(0) was set at about zero. This value of V(0) is much different from the value of V(0)=-10 eV in the previous study, suggesting the invalidity of the previous assumption at the estimation of V(0) from the peak intensity variation with hnu. Using the presently obtained V(0), we could derive more reliable E=E(k) dispersion relation from the observed ARUPS spectra. The comparison of this newly derived relation gave good agreement with theoretically calculated E=E(k) relations, in contrast to the poor agreement for the previous results with V(0)=-10 eV.

Effective insulating properties of autooxidized monolayers using organic ditellurides.

Dialkyl ditellurides adsorb on Au(111) surfaces by wet deposition to form highly resistive autooxidized monolayers (AMs) due to the automatic formation of oxidized tellurium species after the adsorption of ditellurides on the surfaces under air in contrast to the case of the lighter dichalcogenides such as disulfides and diselenides. The ditelluride AMs could be applied to the selective fabrication of effective resistance, ferroelectric layers, piezoelectric parts, and/or new imaging systems using the feature of tellurium oxide in small device circuits.